Linkoping Studies in Science and TechnologyDissertation No. 1206

Electric Fields for Surface Design and

Chemical Analysis

Christian Ulrich

Department of Physics, Chemistry and BiologyLinkopings universitet, SE-581 83 Linkoping, Sweden

Linkoping 2008

The cover picture shows SPR response as a function of electrode potential, thick-ness variations of different surface gradients, and the variations in potential andcurrent density in a solution containing a bipolar electrode.

During the course of the research underlying this thesis, Christian Ulrich wasenrolled in Forum Scientium, a multidisciplinary doctoral programme at LinkopingUniversity, Sweden.

c© Copyright 2008 Christian Ulrich, unless otherwise noted.All rights reserved.

Christian UlrichElectric Fields for Surface Design and Chemical AnalysisISBN 978–91–7393–819–8ISSN 0345–7524Published online at www.ep.liu.seLinkoping Studies in Science and TechnologyDissertation No. 1206

Printed in Sweden by LiU-Tryck, Linkoping 2008

Work. Finish. Publish.†

– Michael Faraday(1791-1867)

†Advice to the young William Crookes.

Abstract

This thesis deals with the use of electric fields for evaluation and control of chem-ical systems. An electric field can result in the flow of charge across an interfacebetween a metal and a solution, by means of chemical reactions. This interplaybetween electricity and chemistry, i.e. electrochemistry, is a field of crucial impor-tance both within research and industry. Applications based on electrochemicalprinciples encompass such diverse areas as batteries and fuel cells, pH electrodes,and the glucose monitor used by people suffering from diabetes.

A major part of the present work concerns the use of static electric fields insolutions containing a non-contacted metal surface. In such a setup it is possibleto control the extent of electrochemical reactions at different positions on themetal. This allows the formation and evaluation of various types of gradients onelectrodes, via indirectly induced electrochemical reactions. This approach is anew and simple way of forming for instance molecular gradients on conductingsurfaces. These are very advantageous in biomimetic research, because a gradientcontains a huge amount of discrete combinations of for example two molecules. Thebasis for the technique is the use of bipolar electrochemistry. Briefly, a surface canbecome a bipolar electrode (an electrode that acts as both anode and cathode)when the electric field in the solution exceeds a certain threshold value, therebyinducing redox reactions at both ends. In our experiments, the driving forcefor these reactions will vary along the electrode surface. Since the result of anelectrochemical reaction can be the deposition or removal of material from anelectrode, bipolar electrochemistry can be used to create gradients of that materialon a surface. In order to gain a deeper understanding of these processes, thepotential and current density distributions at bipolar electrodes were investigatedwith different methods. Especially the use of imaging techniques was importantfor the visualization and analysis of the gradients. Using this knowledge, theformation of more complex gradients was facilitated, and the results were furthercompared to simulations based on simple conductivity models. These simulationsalso provided us with means to predict the behavior of new and interesting setupgeometries for pattering applications.

The other major part is more application driven and deals with the use of alter-nating electric fields for chemical analysis, a technique known as electrochemicalimpedance spectroscopy (EIS). In this work, EIS has been applied for the analy-

v

sis of engine oils and industrial cutting fluids. Emphasis was placed on practicalaspects of the measurement procedure, and on the evaluation of the results usingstatistical methods. It was for example shown that it was possible to simultane-ously determine the amount of different contaminants in low conducting solutions.Generally, EIS is used to measure the impedance of a solution or a solid, oftenas a function of the frequency of the alternating electric field. The impedanceof a system is closely correlated to its complex dielectric constant, and EIS cantherefor be used to examine many chemical and physical processes. It is furtherwell suited for characterizing low conducting media with little or no redox-activespecies. The evaluation of impedance data is often a quite complex task, whichis why we have made use of statistical methods that drastically reduce the effortand quickly reveal significant intrinsic parameters.

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Popularvetenskaplig

sammanfattning

Denna avhandling handlar i grunden om hur elektriska falt kan anvandas for attutvardera och kontrollera olika kemiska system. Omradet inom kemin som ror sam-bandet mellan elektricitet och kemiska foreningar kallas elektrokemi, och anvandsflitigt inom bade industriella sammanhang och inom forskning. Det finns mangaexempel pa kommersiella produkter som ar baserade pa elektrokemiska principer,till exempel batterier, pH-elektroder och glukosmatare som anvands av diabetiker.

En stor del av det har arbetet ror anvandandet av statiska elektriska falt ien losning innehallande en okontakterad metallyta. Med en sadan uppstallningar det mojligt att kontrollera omfattningen av elektrokemiska reaktioner pa olikapositioner pa ytan. Pa det har sattet ar det mojligt att skapa och utvardera olikakemiska gradienter. Tillvagagangssattet bygger pa bipolar elektrokemi, och harhar for forsta gangen utnyttjats till att skapa en proteingradient pa en bipolarguldyta. En sadan ytgradient kommer att uppvisa en gradvis variation i en ellerflera egenskaper. En av de stora fordelarna ar alltsa att en gradient innehalleren stor mangd kombinationer av till exempel tva kemiska egenskaper. Metoden arsnabb och enkel, och gor det mojligt att skapa avancerade gradienter av molekyler,som sedan skulle kunna anvandas inom forskning och utveckling av exempelvisbiosensorer.

En bipolar elektrod, det vill saga en elektrod som agerar som bade anod och ka-tod, kan skapas genom att placera en elektriskt ledande yta i ett parallellt elektrisktfalt. Detta kommer att ske under forutsattning att det elektriska faltet overstigerett visst troskelvarde och darmed inducerar redoxreaktioner vid ytans bada andar.Drivkraften for dessa reaktioner kommer att variera utmed ytan, parallellt meddet elektriska faltet. Eftersom resultatet av en elektrokemisk reaktion kan varabade borttagande och deponering av material pa en elektrod, kan bipolar elek-trokemi anvandas till att skapa gradienter av detta material. For att fa en djupareforstaelse for fenomenet har matningar gjorts for att utvardera hur potentialenoch strommen fordelar sig kring en bipolar elektrod. Sarskilt kombinationen avelektrokemi och avbildande optiska tekniker, huvudsakligen ellipsometri och yt-plasmonresonans (SPR), har ocksa varit en viktig del av arbetet. Detta eftersomolika processer som sker vid en elektrod da kan avbildas och studeras i realtid.

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Resultaten har vidare jamforts med simuleringar baserade pa grundlaggande kon-duktivitetsmodeller.

Den andra stora delen av arbetet ror mer direkta analytiska applikationerav elektrokemi, och speciellt anvandandet av varierande elektriska falt. Dennamatteknik kallas for elektrokemisk impedansspektroskopi (EIS), och har har ut-nyttjats for analys av motoroljor och industriella skarvatskor. Tonvikt har lagtsvid att optimera mattekniken och att utvardera resultaten med hjalp av statistiskaverktyg. Resultaten visar till exempel pa mojligheten att samtidigt bestamma hal-ter av olika fororeningar i lagledande losningar. Generellt anvands EIS for att mataimpedansen for bade fasta och flytande amnen, ofta som funktion av frekvensenfor det applicerade elektriska faltet. Denna impedans ar tatt forknippad med sys-temets komplexa dielektricitetskonstant, och EIS kan anvandas till att undersokamanga kemiska och fysikaliska processer. Darmed ar tekniken till exempel mycketlampad for att karaktarisera lagledande medier. Utvarderingen av impedansdatakan dock vara mycket komplex, nagot som vi visat kan underlattas genom attanvanda statistiska verktyg.

viii

List of Publications

This thesis is based on six papers (included at the end). In the text, they will bereferred to by their roman numerals (Papers I–VI).

Paper I

Formation of Molecular Gradients on Bipolar ElectrodesChristian Ulrich, Olof Andersson, Leif Nyholm, and Fredrik BjoreforsAngewandte Chemie International Edition, 47:3034–3036, 2008.

Author’s contributionWas responsible for the planning, and performed all experimental work to-gether with Olof Andersson. Wrote the manuscript.

Paper II

Potential and Current Density Distributions at Electrodes In-tended for Bipolar PatterningChristian Ulrich, Olof Andersson, Leif Nyholm, and Fredrik BjoreforsSubmitted to Analytical Chemistry

Author’s contributionWas responsible for the planning, and performed all experimental work andsimulations. Olof Andersson was responsible for the iSPR experiments.Wrote the manuscript.

Paper III

Imaging SPR for Detection of Local Electrochemical Processeson Patterned SurfacesOlof Andersson, Christian Ulrich, Fredrik Bjorefors, and Bo LiedbergSensors and Actuators B, doi:10.1016/j.snb.2008.05.042, 2008.

Author’s contributionContributed during the planning, evaluation, and writing.

ix

Paper IV

Current Oscillations During Chronoamperometric and CyclicVoltammetric Measurements in Alkaline Cu(II)-citrate SolutionsJonas Eskhult, Christian Ulrich, Fredrik Bjorefors, and Leif NyholmElectrochimica Acta, 53:2188–2197, 2008.

Author’s contributionWas responsible for the ellipsometric experiments, and performed theoptical simulations. Contributed to the writing.

Paper V

Simultaneous Estimation of Soot and Diesel Contamination inEngine Oil Using Electrochemical Impedance SpectroscopyChristian Ulrich, Henrik Petersson, Hans Sundgren, Fredrik Bjorefors, andChristina Krantz-RulckerSensors and Actuators B, 127:613–618, 2007.

Author’s contributionResponsible for the planning, performance, and evaluation of the experi-mental work, including the cell design. Wrote the manuscript.

Paper VI

Quality Evaluation of Industrial Cutting Fluids Using Electro-chemical Impedance SpectroscopyChristian Ulrich, Dan Louthander, Per Martensson, Andre Kluftinger,Michael Gawronski, and Fredrik BjoreforsIn manuscript

Author’s contributionResponsible for the planning, performance, and evaluation of the experi-mental work, including the cell design. Wrote the manuscript.

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Acknowledgements

It is with a sense of sadness and sentimentality I write this acknowledgement,because in thanking all the people who have helped me through the years, I realizethat my time at IFM is nearing its end. It has been an inspirational and rewardingtime, but now new things await.

I am forever grateful to my supervisor Fredrik Bjorefors. You have given meinvaluable help through the years, and further managed to provide both excel-lent supervision and support. My co-supervisor Ingemar Lundstrom is, by hismere presence, a source of inspiration. Special thanks go to my former supervisorTina Krantz-Rulcker, who gave me the opportunity to explore the mysteries ofexperimental research. My former co-supervisor Fredrik Winquist was also the su-pervisor on my diploma work, which started this journey. Thanks to Bo Liedberg

for welcoming me into his group. Hans Sundgren has been essential to many ofmy undertakings. I could always knock on his door to discuss things, big as wellas small.

Bo Thuner and Ingemar Grahn are acknowledged for valuable assistance withall things practical; Stefan Klintstrom for guidance whenever needed and for man-aging Furum Scientium; Agneta Askendal for the laboratory assistance; the ad-ministrative staff at IFM for all the help, and especially Susann Arnfelt, Kerstin

vestin, Pia Blomstedt, and Anna Maria Uhlin.All the people I have collaborated with, who have contributed to this thesis.

In no particular order I would like to thank the following. Olof Andersson for thesuccessful team work, his positive attitude, and nice company; Henrik Petersson

for the data evaluations and general support; Jonas Eskhult and especially Leif

Nyholm for the successful cooperation and rewarding discussions; representativesof the industrial partners through the years, Jaco Visser, Claes Frennfelt, Per

Martensson, Michael Gawronski, and Andre Kluftinger; Dan Louthander, for thecollaboration in the end, but foremost for all the Friday morning breakfasts atLanemos.

Without naming anyone in particular, I have very much valued the companyof my former room-mates, the coffee-group, the lunch-group, all the colleagueswithin Forum Scientium, and all former and present members of S-SENCE andthe Sensor Science and Molecular Physics group.

xi

Last but not least, I wish to thank my friends and family for their never endinglove and support, which has made it possible for me to cope with all the hardshipsof life, especially towards the end.

Christian Ulrich, Linkoping, 2008.

xii

Contents

1 General introduction 1

2 Electrochemistry 32.1 Introduction to electrochemical reactions . . . . . . . . . . . . . . . 32.2 The electrical double layer . . . . . . . . . . . . . . . . . . . . . . . 82.3 Electrochemical cells and cell resistance . . . . . . . . . . . . . . . 102.4 Mass transfer and the diffusion layer . . . . . . . . . . . . . . . . . 112.5 Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6 Electrochemical impedance spectroscopy . . . . . . . . . . . . . . . 14

2.6.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.6.2 General applications . . . . . . . . . . . . . . . . . . . . . . 18

2.7 Bipolar electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . 182.8 Limitations of electrochemistry . . . . . . . . . . . . . . . . . . . . 20

3 Imaging optical methods and electrochemistry 213.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2 Surface plasmon resonance . . . . . . . . . . . . . . . . . . . . . . . 21

3.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.2 SPR and electrochemistry . . . . . . . . . . . . . . . . . . . 22

3.3 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.3.2 Ellipsometry and electrochemistry . . . . . . . . . . . . . . 24

4 Electrode surface design and analysis 274.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . 274.3 Surface gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5 Alternating electric fields for chemical analysis 355.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.2 Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . 365.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

xiii

6 Future outlook 41

Bibliography 43

Paper I 51

Paper II 59

Paper III 71

Paper IV 79

Paper V 91

Paper VI 99

xiv

1General introduction

In 1834, Michael Faraday published his two laws of electrolysis, relating the amountof charge passed in a circuit to the extent of electrochemical reactions taking placeat the interface between an electrode and a solution. This fundamental discoverymight seem somewhat self-evident today, but one has to remember that Faraday’swork was done in a time when electricity was poorly understood, and the natureof the atom was still unknown. His contributions to this field credited him asone of the founders of modern electrochemistry. Electrolysis was, however, onlya small part of his collected work. Faraday also introduced the idea of electricfields into 19th century science. Furthermore, he defined the dielectric constantas a result of his studies on capacitances. He is regarded as one of history’sgreatest experimentalists and, needless to say, his impact on modern science isunmistakable.

A more recent pioneer was Jaroslav Heyrovsky, who has been called the fa-ther of electroanalytical chemistry. He received the Nobel Prize in chemistry in1959 “for his discovery and development of the polarographic methods of anal-ysis”. Polarography is an electroanalytical technique where a dropping mercuryelectrode is used as the working electrode in voltammetric experiments. Basically,a current response is recorded when a potential difference (and hence a resultingelectric field) is applied across the mercury/solution interface. This, for example,facilitates the control and evaluation of the amount of electrochemical reactions at

1

2 General introduction

an electrode. Similar techniques can also be used to establish a significant electricfield in the entire solution between two electrodes. These types of electric fieldshave been extensively used in the work described in this thesis.

Electric fields can be utilized to affect, control, and analyze different physicaland chemical parameters in a material or solution. This is the basis of electrochem-istry, which deals with the interaction between electricity and chemical compounds.What separates electrochemistry from many other techniques or methods is thefact that it allows both the manipulation and probing of a sample at the sametime. Voltammetry is perhaps the most well known example, where informationabout an analyte is obtained as it is oxidized or reduced at an electrode/solutioninterface under potential control. Important parameters such as reaction kinetics,adsorption processes, as well as mass transfer characteristics can be studied.

Many of the electrochemical techniques employed today are based on the useof static electric fields. It is also possible to make use of alternating electric fields,which for example allows the possibility to evaluate the impedance of the systemof interest. This technique is known as electrochemical impedance spectroscopy(EIS), and is widely used in science and industrial applications. Impedance data of-ten contains information on many different parameters of the investigated system,and EIS has therefore been applied in many diverse areas. Some examples includecorrosion studies on pipelines and reinforcements in concrete, body impedancemeasurements, and fundamental research related to sensor science and batteries.The evaluation of the data can, however, be quite a complex task. In this work,statistical methods were therefore used to more easily extract relevant information.

2Electrochemistry

2.1 Introduction to electrochemical reactions

A very simple electrochemical setup can consist of a battery, two metal plates, anda solution (i.e. an electrochemical cell). The battery is of course also an electro-chemical cell, but is here treated as an arbitrary voltage source. The metals areimmersed in the solution and connected to the battery as in Figure 2.1, resultingin a current in the circuit if the voltage is sufficiently high. The metals (and wires)are in this case electronic conductors and charge is here transported by electrons.In the solution, charge is carried by the movement of charged species. An imper-ative demand for the passage of a current in this circuit is that charge can crossfrom the metal to the solution and vice versa. This is in most cases enabled bythe uptake or loss of electrons by species in the solution. These electrochemicalreactions are one of the most fundamental connections between electricity andchemistry. It is also possible to make use of spontaneous electrochemical reactionsto convert chemical energy into electrical energy.

In electrochemistry, as in most other fields, a well defined terminology is es-sential. In his publication from 1834,1 Faraday (after discussions with colleagues)defined many of the electrochemical terms we still use today. First of all, theelectronic conductors in contact with the solution were called electrodes. Typicalelectrode materials today include solid metals (e.g. Pt and Au), carbon (graphite),

3

4 Electrochemistry

i

+-

Figure 2.1: A simple electrochemical setup.

and semiconductors (e.g. indium-tin oxide and Si). The solution was called an elec-trolyte, and the most common electrolytes contain charged species such as H+,Na+, and Cl – , in order to increase the conductivity, in either water or organic sol-vents. Faraday defined the substances that can pass a current in a solution as ions,and divided them into negative anions and positive cations. Further, he termedthe electrode at which oxidation occurs the anode and that where reduction occursthe cathode. Oxidation (Red −−→ Ox + ne – ) is the removal of electrons from aspecies and reduction (Ox + ne – −−→ Red) is the addition of electrons. A redoxreaction is consequently a reaction in which there is a transfer of electrons fromone species to another. The main concern in electrochemical systems is processesand factors that affect the transport of charge across the interface between twophases, e.g. between an electrode and an electrolyte.2 To chemists, Faraday isperhaps best known for his work on electrolysis, which involves passing a currentthrough a solution and thereby inducing reactions at the metal/solution interfaces.

To proceed with a real example, Figure 2.2 shows an electrochemical setupwhere two electrodes are immersed in an electrolyte. This electrolyte contains1 M of Cl – and equal concentrations of ferri and ferrocyanide ions. One elec-trode is a piece of platinum and the other is a silver wire coated with solid silverchloride. The composition of this cell allows two reactions to occur; Fe(CN) 3 –

6 +e – −−−− Fe(CN) 4 –

6 at the platinum electrode and AgCl + e – −−−− Ag + Cl – at thesilver electrode. These are called the half-reactions of the cell, and together theywill constitute the overall chemical reaction. If the power supply is disconnected,the high impedance voltmeter will show the open-circuit of the cell. This poten-tial, measured in volts (V), is related to the energy available in the cell to forcean external current in the circuit. By using an amperemeter, this current can berecorded.

Associated with every electrochemical half-reaction is a potential, called the

2.1 Introduction to electrochemical reactions 5

Power supply

Vi

Pt

AgCl

Ag

Power supply

VA

1M Cl-

Fe(CN)6

3-/4-

Figure 2.2: An electrochemical cell with two electrodes immersed in an electrolyte. A

power supply together with the voltmeter and amperemeter provides the possibility to

obtain current-potential curves.

standard reduction potential (denoted E0). For the two reactions

Fe(CN) 3−6 + e− −−−− Fe(CN) 4−

6 and AgCl + e− −−−− Ag + Cl−

E0 is 0.361 V and 0.222 V, respectively. These values are valid under certainconditions (i.e. standardized temperature and pressure, and unit activity). Inpractice, the values of the electrode potentials will depend on the temperatureand the activities of the species present in the solution. It is important to notethat the standard reduction potentials are defined with respect to something calledthe standard hydrogen electrode, which will be further explained below. For non-standard conditions, electrode potentials can be calculated using the Nernst equa-tion, which includes two terms. The first is the standard reduction potential andthe second term involves the activities of the species in the reaction.

For the half-reaction

Fe(CN) 3−6 + e− −−−− Fe(CN) 4−

6

the Nernst equation giving the half-cell potential, E, is

E = E0 − RT

nFlnARed

AOx= E0 − RT

FlnAFe(CN)4−6

AFe(CN)3−6

(2.1)

where R = molar gas constant (8.31447 JK−1mol−1), T = temperature (K), n =number of electrons in the half-reaction, F = Faraday constant (96485.3 C), andAi = activity of species i. The activity can often be approximated by the con-centration of the species. Solids and solvents are normally omitted from equationbecause their activities are unity (or close to unity). When all activities are unity,the logarithmic term in the equation is zero, thus giving E = E0.

6 Electrochemistry

For a complete cell, the voltage (Ecell) is the difference between the potentialsof the cathode and anode:

Ecell = Ecathode − Eanode (2.2)

If the cell voltage is positive, the net cell reaction is spontaneous in the forwarddirection. If the cell voltage is negative, the reaction is spontaneous in the reversedirection.

Going back to the example in Figure 2.2, AgCl and Ag are both solids, andhence their activities are unity. The activity of Cl – can be found from the con-centration in solution. The potential of the Ag/AgCl electrode will in this case be0.235 V due to the concentration of chloride ions (1 M). The potential of the Ptelectrode will be E ≈ E0 = 0.361 V , since the activities of the ferri/ferrocyanideions are approximately equal.

Often in electrochemical experiments, only one of the two half-reactions isof interest. The electrode at which this reaction occurs is called the workingelectrode.2–4 In order to study only one reaction, the other half of the cell isstandardized using a reference electrode, which is made up of phases having es-sentially constant chemical composition regardless of the experimental conditions.The internationally accepted primary reference is the standard hydrogen electrode(SHE), which has all components at unit activity:

Pt/H2(p = 1 bar)/H+(a = 1, aq)

where a slash represents a phase boundary. The potential of the SHE is definedas being equal to zero, i.e. the standard reduction potential (E0) for the reductionof two protons to form hydrogen is 0.00 V. All electrochemical half reactions cannow be assigned a standard reduction potential with respect to the SHE. Potentialsare often measured and quoted with respect to other reference electrodes than theSHE, since the latter is not very convenient from an experimental point of view.A commonly used reference is the Ag/AgCl electrode shown in Figure 2.2, calledthe silver-silver chloride electrode,

Ag/AgCl/KCl (saturated in water)

which for a saturated chloride solution has a potential of 0.197 V vs. SHE.In an electrochemical experiment, the potential of the working electrode is said

to be observed or controlled with respect to the reference. This is equivalent toobserving or controlling the energy of the electrons within the working electrode.The energy can be raised by driving the electrode to more negative potentials.When the electrons reach a level sufficient to transfer into vacant electronic stateson species in the electrolyte, a flow of electrons from electrode to solution (areduction current) occurs, see Figure 2.3. Similarly, the energy of the electronscan be lowered by imposing a more positive potential on the electrode. At some

2.1 Introduction to electrochemical reactions 7

-

+

Potential

Electrode

Reduction

A + e- → A-

A → A+ + e-

Solution

Energy levelof electrons

VacantMO

OccupiedMO

e-

e-

Electrode Solution

-

+

Potential

Electrode

Oxidation

Solution

Energy levelof electrons

VacantMO

OccupiedMO

Electrode Solution

Figure 2.3: Reduction (top) and oxidation (bottom) of species A in solution. Vacant

MO is the lowest vacant molecular orbital of species A and occupied MO is the highest

occupied.

point electrons on species in the electrolyte will find a more favorable energy levelon the electrode, resulting in a flow of electrons from solution to electrode, i.e.an oxidation current. The potentials at which these processes occur are relatedto the standard reduction potentials, E0, for each electrochemical reaction in thesystem.

Continuing with the example in Figure 2.2, we are now ready to change thepotential of the Pt electrode with respect to the Ag/AgCl reference electrode.If the potential is made sufficiently positive, electrons cross from the solutionphase into the electrode, resulting in the oxidation of Fe(CN) 4 –

6 to Fe(CN) 3 –6 .

While this reaction occurs at the Pt electrode, AgCl is reduced to Ag, and Cl – isliberated into the solution. The potential here will be nearly constant because thecomposition of the Ag/AgCl/Cl – interface is almost unchanged with the passageof modest currents, due to the high Cl – concentration. Thus, it works as anexcellent reference electrode in this setup. Indeed, the essential characteristic of

8 Electrochemistry

a reference electrode is that its potential remains practically constant with thepassage of small currents. Hence, when a potential is applied between the twoelectrodes, nearly all of the potential change occurs at the Pt/solution interface.

The number of electrons that cross an interface is related stoichiometrically tothe extent of the chemical reaction, and is measured in terms of the total charge Q.Charge is expressed in units of coulombs (C), where 1 C is equivalent to 6.24 ·1018

elementary charges. The relationship between charge and amount of reactions isgiven by Faraday’s law. This law states that the passage of 96485.3 C causes 1equivalent of reaction (e.g. consumption of 1 mol of reactant or generation of 1mol of product in a one-electron reaction). The current, i, is the rate of flow ofcharges (or electrons), where 1 ampere (A) is equivalent to 1 C/s. If the currentis plotted as a function of the potential, a current-potential (i vs. E) curve isobtained. Such curves are very informative about the nature of both the solutionand the electrodes, and about the reactions that occur at the interfaces.

2.2 The electrical double layer

Charge transfer reactions take place in a region very close to the electrode surface.As described above, the driving force for these reactions is the potential differencebetween the electrode and electrolyte. However, other processes are also importantin this region due to the polarization of the electrodes. If the solution in a setup likethe one in Figure 2.1 only contains an inert supporting electrolyte (e.g. KNO3),no charge transfer reaction will take place if the applied potential difference ismoderate. The left electrode is in this case made negative by the battery andwill hence have an excess of electrons at the surface. This electrode will thusattract positive ions and dipoles present in the solution (see Figure 2.4). If thepotential (and thereby the charge) of the electrode is changed, the compositionof the solution side will also change to maintain electroneutrality. In this respect,the electrode/solution interface behaves like a capacitor, which is governed by theequation

q

E= C (2.3)

where q is the charge stored on the capacitor, E is the potential across the capacitorand C is the capacitance (in farads, F). When the electrode potential is changed,charge will accumulate until q satisfies Equation 2.3. The resulting movementof charged species in the solution will give rise to a current, called the chargingcurrent. A transient current can thus flow in the circuit without any charge transferreactions at the interface. This is a very important effect that has to be consideredin electrochemical experiments whenever the potential of an electrode is changed.

The whole array of charged species at the metal/solution interface is calledthe electrical double layer.2 The interface can be characterized by a double-layercapacitance, Cdl, which typically is in the range of 10–40 µF/cm2. Cdl is, however,

2.2 The electrical double layer 9

Electrode Solution

Figure 2.4: Illustration of a negatively charged electrode and the corresponding abun-

dance of positive charges in the solution.

unlike real capacitors often a function of potential. Normally the existence ofthe double-layer capacitance and the presence of a charging current cannot beneglected in electrochemical experiments. The charging current can in reality bemuch larger than the charge transfer current if the concentrations of electroactivespecies are very low, or if the potential change is very rapid.

The solution side of the double layer is itself made up of at least two layers. Theone closest to the electrode is called the inner layer and contains solvent moleculesand sometimes other species (ions or molecules) that are said to be specificallyadsorbed (Figure 2.5). This inner layer is also called the compact or Helmholtzlayer. The locus of the electrical centers of the specifically adsorbed ions definesthe inner Helmholtz plane (IHP), which is at a distance x1 from the electrodesurface. The inner layer extends out from the electrode to the outer Helmholtzplane (OHP), which is defined as passing through the locus of centers of the nearestsolvated ions, at a distance x2 from the surface. The solvated ions interact withthe charged metal only through long-range electrostatic forces. They are saidto be non-specifically adsorbed and are distributed in a region called the diffuselayer, which extends from the OHP into the bulk of the solution. The thicknessof the diffuse layer is determined by the potential difference and the total ionicconcentration, and for concentrations greater than 10−2 M, the thickness is lessthan ∼100 A.

The double layer capacitance is a very important parameter when evaluatingand modeling results from electrochemical experiments. It can give importantinformation on the structure of the double layer, and also on the composition ofthe electrolyte. If an electrode surface is modified by a layer of some material, thethickness and dielectric properties of that material will greatly affect Cdl.

10 Electrochemistry

IHP OHP

Electrode

Diffuse layer

Solvated cation

= Solvent molecule

= Specifically adsorbedion or molecule

x1

x2

+

+

++

+

+

-------------

Figure 2.5: Schematic representation of the double layer for a negatively charged elec-

trode. IHP is the inner Helmholtz plane and OHP is the outer.

2.3 Electrochemical cells and cell resistance

In the electrolyte, charge is transported by the movement of ions (anions in onedirection and cations in the other). The total resistance between the electrodeswill thus depend on the charge, mobility, and concentration of the species presentin the electrolyte. This solution resistance, Rs, will behave as a true resistanceover a wide range of conditions. If a current, i, is passed through a solution,a potential drop equal to iRs will arise. When controlling the potential of anelectrode (with respect to a reference electrode), this potential drop often has tobe taken into account. The actual potential of the electrode will be less than theapplied potential by and amount equal to iRs. This is shown by Equation 2.4

Eappl(vs. SHE) = E(vs. SHE) + iRs (2.4)

where Eappl is the voltage applied by the external power supply and E is thepotential of the working electrode. If both the current and the solution resistanceare reasonably small, the potential drop can often be disregarded. However, insituations where this is not true, the problem can be minimized by an alterationof the electrochemical setup.

When a working electrode and a reference electrode are used to perform electro-chemical measurements, the setup is called a two-electrode cell (see Figure 2.6A).The potential of the working electrode will in this case be given by Equation 2.4.Many electrochemical instruments offer the possibility to use a third electrode,called a counter electrode, resulting in a three-electrode cell. As can be seen inFigure 2.6B, the current will now be passed between the working and the counter

2.4 Mass transfer and the diffusion layer 11

V

iWE RE

Eappl

V

Powersupply

i

V

WE

RE

CE

EWE

vs. RE

V

(A) (B)

Powersupply

A A

Figure 2.6: A two-electrode cell (A) and a three-electrode cell (B). WE is the working

electrode, RE the reference electrode, and CE the counter electrode.

electrodes. The behavior of the working electrode is not affected by the elec-trochemical properties of the counter electrode and it often consists of an inertconducting material. A high input impedance device is used to measure the po-tential difference between the reference and working electrodes, and the currentpassed in this circuit will hence be negligible. The result is thus a very low poten-tial drop in the solution, and the measured voltage will be representative of thetrue potential of the working electrode.

The existence of a potential drop can however be useful in many situations.Large potential drops can for instance be used to perform electrophoresis, and gelseparations of proteins and DNA. We have utilized the potential drop in a solutioncontaining an isolated conducting surface, to form reaction gradients. This will befurther discussed in section 4.3.

2.4 Mass transfer and the diffusion layer

As previously discussed, the electrolyte is far from static during electrochemicalexperiments. The majority of processes occurring here are based on the movementof different species. This movement of material from one location in solution toanother is called mass transfer and can arise from the following:2

1. Migration – movement of charged species in an electric field.

2. Diffusion – movement of species as a result of a concentration gradient.

3. Convection – movement of species caused by stirring or by a density (orheat) gradient.

In electrochemical experiments, it is often desirable to keep one or more of thesecontributions to mass transfer to a minimum. For example, by the addition ofa supporting electrolyte the migration of the redox species can be made negligi-

12 Electrochemistry

COx

x

COx

*

δN

1

2

Figure 2.7: Concentration profiles at two different electrode potentials: (1) where

COx(x = 0) is about half of C∗Ox and (2) where COx(x = 0) ≈ 0.

ble. The diffusion, however, is more complicated to control, and it often has aconsiderable impact on electrochemical experiments.

In an experiment where species Ox is reduced at a cathode, the concentrationat the electrode surface, COx(x = 0), will become smaller than the bulk concen-tration, C∗Ox. This will give rise to a concentration profile that extends into thesolution near the electrode by an amount δN , the Nernst diffusion layer thickness.3

The magnitude of δN is defined by the intersection of the tangent to the concen-tration profile with the horizontal line C∗Ox (Figure 2.7). δN increases with timeuntil it reaches a constant value, termed the stationary diffusion layer thickness.If the solution is unstirred, δN of up to ∼0.5 mm is possible, and reaching steadystate can take minutes. In a stirred solution, δN is well defined and much smaller(in the order of 1 µm), and steady state will usually be reached within seconds.

If the potential is sufficiently high, COx(x = 0) will decrease to an extremelysmall value and, provided that the chemical reactions are rapid compared to themass transfer processes, the current will be limited by the rate at which new speciescan reach the electrode. Increasing the potential further will hence not affect thecurrent, since the diffusion is a function of the concentration gradient, only.

2.5 Voltammetry

There are many ways to use electrochemistry as an analytical tool. One widelyused method is to let the potential of a working electrode vary linearly with timeand record the current. A result from such a linear potential sweep can be seen inFigure 2.8. In this example, the potential of a working electrode was swept be-tween E1 and E2 in an electrolyte containing only the reduced form of an analyte.

2.5 Voltammetry 13

Potential

Current

0E

2E

1E0

Figure 2.8: Current potential curve resulting from a linear potential scan.

In the beginning of the sweep (near E1) no reaction can occur, but as the poten-tial gets closer to E0 an oxidation starts, resulting in the current increase. Thesurface concentration of the reduced analyte now begins to decrease, leading to aflux of species to the surface. When the potential has moved past E0, the surfaceconcentration will be almost zero, and the flux of redox species will reach a max-imum value. This rate of mass transfer can however not be maintained, becausethe diffusion layer continues to grow. The result will be a current-potential curvewith a peak, positioned somewhat positive of E0 for the analyte. This method isreferred to as linear sweep voltammetry, and is just one of many electroanalyticaltechniques. Voltammetry2–5 is actually a collection of techniques where currentis measured as a function of applied potential. Two of these techniques will befurther described, i.e. cyclic voltammetry and pulse voltammetry.

Cyclic voltammetry is an extension of linear sweep voltammetry, and is oftenused to study the redox behavior of electroactive species. The word cyclic comesfrom the fact that when the potential has been swept between E1 and E2, the di-rection of the sweep is reversed. The resulting triangular waveform can be seen inFigure 2.9A. The potentials E1 and E2 are chosen to incorporate the redox behav-ior of the analyte, and is often centered around its E0-value. Another importantparameter is the sweep rate (or scan rate), normally ranging from 10 mV/s to afew V/s. The current-potential curve obtained is called a cyclic voltammogram.Figure 2.9B shows the result of a measurement in a solution containing a reversibleredox couple. During the reversed scan, the redox behavior of the species oxidizedduring the forward scan can be evaluated. The concentration of an analyte can bedetermined, since the current is proportional to this concentration. Also, the valueof E0 can be estimated by the position of the oxidation and reduction peaks. Infact, several important parameters such as reaction kinetics, surface adsorption,and mass transfer are frequently studied with cyclic voltammetry.

In pulse voltammetry, the potential is instead changed in a stepwise manner.There are many pulse parameters that can be changed, naturally giving different

14 Electrochemistry

Time

Potential

E2

E1

Potential

Current

E2

0

E1

E0

Oxidation

Reduction

(A)

(B)

Figure 2.9: (A) Triangular potential waveform applied to the working electrode in cyclic

voltammetry. (B) A schematic cyclic voltammogram for a reversible redox couple.

current responses. This technique provides the same general information as cyclicvoltammetry, but is especially known for low limits of detection, and better sen-sitivity and resolution. This is due to the fact that the influence of the chargingcurrent can be minimized using an appropriate sampling of the current. Also,short potential pulses will counteract the build-up of thick diffusion layers, andredox species close to the electrode will consequently not be depleted. Both ofthese effects will result in a better signal to noise ratio.

2.6 Electrochemical impedance spectroscopy

2.6.1 Theory

Electrochemical impedance spectroscopy (EIS)2,3, 6 is a powerful tool for examin-ing many chemical and physical processes in solutions as well as solid materials.It is a non-destructive technique capable of separating different contributions tothe overall electrochemical processes. The basis for this is that the impedancesassociated with different processes have varying frequency-dependencies. EIS cantherefore be used to study electrode kinetics, adsorption rates, corrosion processes,battery properties, ageing of sensors, and many more.

Impedance (Z) is generally defined as the total opposition a device offers to theflow of an alternating current at a given frequency. Impedance can be measured in

2.6 Electrochemical impedance spectroscopy 15

Phaseshift

Time

E(t) = E0sin(ωt)

I(t) = I0sin(ωt+θ)

Figure 2.10: Sinusoidal potential excitation and current response.

many different ways. Usually, a sinusoidal potential or current excitation signal isapplied to a device and the resulting current or potential is recorded, respectively.

If a potential excitation signal (E(t)) is used, expressed as a function of time,it will have the form

E(t) = E0 sin (ωt) (2.5)

where E0 is the amplitude of the signal and ω (rad/s) is the radial frequency. Theresulting current can be expressed as

I(t) = I0 sin (ωt+ θ) (2.6)

where It is the amplitude and θ is the phase difference between the potential andcurrent, see Figure 2.10. For a pure resistance, the phase difference is zero, andfor a pure capacitor it is 90.

In the time domain, the relation between the system properties and the re-sponse is, however, very complex. If many capacitive and inductive elementsare present, a solution of a system of differential equations is required. By usingFourier transformation of the signals into the frequency domain, the mathematicaltreatment is significantly simplified. The Fourier transforms of the voltage, E(jω),and current, I(jω), become E0π and I0π · exp(θj), respectively. The impedancefunction can thus be defined as

Z(jω) =E(jω)I(jω)

(2.7)

and Z(jω) can be now be related to different circuit elements. For a resistance itis simply equal to R, and for a capacitance and an inductance, Z(jω) is 1/(Cωj)

16 Electrochemistry

Im(Z)

Re(Z)Z’

Z’’

|Z|

θ

Figure 2.11: Vector representation of the complex impedance.

and Lωj, respectively. Usually, Z(jω) is for simplicity written as just Z(ω). Now,the impedance of a system with multiple elements can be calculated in the sameway as for multiple resistors.

Since the complex quantity Z(ω) contains information on both the magnitudeand the phase of the impedance, it is convenient to express it as a complex number.

Z = Re(Z) + jIm(Z) = Z ′ + jZ ′′ (2.8)

The impedance is easily depicted in a plot of the imaginary part of the impedancevs. the real part, as in Figure 2.11. The impedance is here represented by avector of length |Z|, and the angle between this vector and the x-axis is the phaseshift θ (=argZ). The real and imaginary parts of the impedance are termed theresistance (R) and the reactance (X), respectively. It is worth noticing that theoriginal time variations of the applied and measured signals have disappeared inthe representation of the impedance. Some important relationships are

|Z| =√R2 +X2 and θ = arctan

X

R(2.9)

To display the variation of the impedance with frequency, a Bode plot is oftenused. Here, log |Z| and θ are both plotted against logω or log f . Another repre-sentation is a Nyquist plot, where X is plotted against R for different values of ω.Examples of both these types of plots can for a series and parallel RC circuit beseen in Figures 2.12 and 2.13, respectively.

Sometimes it is mathematically convenient to use the reciprocal of the impedance,in which case

1Z

=1

R+ jX= Y = G+ jB (2.10)

where Y is the admittance, G the conductance, and B the susceptance. The unitof impedance is ohm (Ω), and the unit of admittance is siemen (S). Two more

2.6 Electrochemical impedance spectroscopy 17

R C

log |Z|

log ω log ω

-θ

-Im(Z)

Re(Z)

ω

∞

Figure 2.12: A series RC circuit (top left) and the resulting Nyquist plot (top right).

Bottom left and right are the corresponding Bode plots.

C

R

log |Z| -θ

-Im(Z)

ω∞

log ω

log ω

Re(Z)

Figure 2.13: A parallel RC circuit (top left) and the resulting Nyquist plot (top right).

Bottom left and right are the corresponding Bode plots.

18 Electrochemistry

quantities related to impedance are the modulus function M = jωCcZ = M ′ +jM ′′, and the complex dielectric constant ε = M−1 = Y/(jωCc) = ε′+ jε′′. Here,Cc = ε0Ac/l is the capacitance of the empty cell of electrode area Ac and electrodeseparation length l. ε0 is the dielectric permittivity of free space, 8.854·10−12 F/m.All four of these functions are valuable in EIS due to their different dependenceon, and weighting with, frequency.

An electrochemical cell can often be modeled as an equivalent circuit usingelectrical components. Examples of circuit elements include a solution resistance,a double layer capacitance, a charge transfer resistance, and an impedance termrepresenting mass transfer. Using such an equivalent circuit, impedance data cannow be evaluated in order to determine the physical and chemical properties ofthe electrochemical system.

2.6.2 General applications

During the last couple of decades, EIS has been used for many different applica-tions. Here, a brief summary of some important examples will be given. For athorough insight into the theory and applications of EIS, see the excellent text byBarsoukov and Macdonald.6

The characterization of the interfacial region is very important in optimizationof electroanalytical sensors, and EIS is very useful for evaluating the processes oc-curring at the electrode/solution interface. The sinusoidal excitation will give riseto an alternating charging and discharging of the double layer, as well as influencingredox reactions and mass transfer. Hence, a lot of information may be obtainedsince these processes can have different frequency-dependencies. For example,Janek et al. used EIS to investigate the interfacial impedance of self-assembledmonolayers (SAMs) on gold surfaces.7 Surface modifications with DNA have alsobeen studied, for example the electron transfer through monolayers of thiol-labeledDNA duplexes,8 as well as oligonucleotide-DNA interactions.9 Further, EIS hasbeen used for surface characterization of conductive polymer-modified electrodes,10

and solid polymer electrolytes.11

Corrosion research is an enormous field and EIS has been extensively used togain information on the corrosion processes at metals and metal-coated surfaces.12

Also, reaction mechanisms can be deduced, as shown by Keddam et al. who studiediron dissolution in acidic media.13,14

2.7 Bipolar electrochemistry

If the word bipolar is used to describe an object, the meaning will be that is hastwo poles. In an electrochemical setup the poles are the anode and cathode. Abipolar electrode is hence a single electrode, acting as both anode and cathode.A requisite for this is that an oxidation and a reduction must take place at this

2.7 Bipolar electrochemistry 19

i1+i

2

i1

i2

Oxidation

OxidationReduction

Bipolar electrode

Voltage source

Reduction

E

Feed

er e

lect

rod

e

Feed

er e

lect

rod

e

Figure 2.14: Experimental setup showing the current paths when electrochemical re-

actions are taking place on the bipolar electrode. A variable voltage source is used to

control the total current between two feeder electrodes.

electrode simultaneously. Furthermore, these two reactions must be separated inspace to give rise to the two poles. The driving force for these reactions is anelectric field in the solution parallel or perpendicular to the electrode surface.

In Figure 2.14, an example of a bipolar setup is shown. Here, a voltage (orcurrent) source and two feeder electrodes are used to establish an electric fieldin the solution. It should be noted that the isolated conducting surface has nophysical connection to the power source, and its potential is hence floating. Thepotential difference between a point on the equipotential surface and the solutionwill now vary laterally along the surface. This potential difference can inducecharge transfer reactions at both ends of the surface, provided that the electricfield exceeds a certain threshold value. At this point, the surface becomes a bipolarelectrode. A potential gradient across the electrode is thus induced by the electricfield in the solution. In practice, the bipolar electrode will act as an additional,less resistive, path for the total current.

As apposed to a regular electrochemical setup, it is the applied external electricfield that is used to control the interfacial potentials at the electrode of interest.The overall potential drop across the cell will now be non-linear when a floatingsubstrate acts as a bipolar electrode, and the current density distribution will alsobe affected. Since the potential difference between a point on the bipolar electrodeand the solution will vary laterally along the surface, the rates of the reactions willvary accordingly. Thus, electrochemical reaction gradients are present on bipolarelectrodes. The use of bipolar electrochemistry can be very advantageous, forinstance when connecting stacks of individual cells, as a result of the reducedinternal resistance.

Bipolar electrochemistry is a quite established field. For example, in 1973Eardley et al.15 investigated the conductivity of fixed arrays of particles dispersedin a continuous electrolyte medium. In recent years several new and interest-

20 Electrochemistry

ing applications have been reported. The phenomenon has been used in batteryapplications,16,17 in proton exchange membrane based fuel cells,18 for energy stor-age and load leveling,19,20 as well as to carry out electrochemical reactions inpoorly conducting media.21 Further, Bradley et al. have in several publicationsstudied the use of bipolar electrochemistry for the directional growth of copperwires between micrometer-sized particles,22and the electrodeposition of palladiumcatalysts.23,24 Moreover, Said et al.25 used bipolar electrochemistry to visualizethe magnitude and direction of an electric field in an electrolyte. This type ofelectrochemistry has also been used as an external electric field driven in-channeldetection technique in microfluidic PDMS channels.26 Finally, the use of bipolarelectrochemistry provides the possibility of studying chemical reactions withoutany physical contact to the electrode, for instance in electrochemiluminescenceapplications.27–29

Considerable theoretical contributions to the understanding of bipolar electro-chemistry have been made by Duval et al.30–35 Here, simulations were made ofthe electrochemical behavior of parallel plate electrodes in a lateral electric field,in conjunction with electrokinetic experiments. It was shown how the potentialand current distribution can be evaluated for different setups.

2.8 Limitations of electrochemistry

Even though electrochemical techniques are very versatile, voltammetry and EISare more widely utilized as analytical tools within research than in day-to-dayuse. This is related to some of the inherent limitations with electrochemical meth-ods. First, there is the issue of stability of the electrode surface. The activityand morphology of the electrodes frequently change over time, often as a resultof the adsorption of species from the solution. This is, however, something thatcan be circumvented by the use of disposable electrodes, or by careful polishingand cleaning procedures. Second, the limit of detection is normally in the micro-molar range. Although this is sufficient for many applications, electrochemistrycannot compete with other high-performance techniques. Third, is the issue ofselectivity. It is sometimes difficult to separate the response from a single analytefrom the total current. For instance, if two compounds are to be distinguishablein voltammetry, their standard potentials must be sufficiently separated.

Finally, a rather important limitation is the fact that regular electrochemistryoffers no lateral resolution of the reactions and processes at electrode surfaces.Apart from scanning techniques such as scanning electrochemical microscopy, acollective current response from the entire surface is always obtained. However,this limitation can be overcome by combining electrochemistry with imaging op-tical methods, such as ellipsometry and surface plasmon resonance.

3Imaging optical methods and

electrochemistry

3.1 Introduction

One of the major advantages of electrochemistry is that it is easily combined withother techniques. Successful combinations have been made with, for example,ultraviolet and visible spectroscopy, vibrational spectroscopy, acoustic resonance(i.e. quartz crystal microbalance), mass spectrometry, chromatography, and elec-trophoresis. The combination of imaging optical techniques and electrochemicalmeasurements is a very interesting area, because it allows the detection of electro-chemical processes with lateral resolution in the micrometer range. This chapterfocuses on the combination of electrochemistry with imaging surface plasmon res-onance and imaging ellipsometry.

3.2 Surface plasmon resonance

3.2.1 Theory

Surface plasmon resonance (SPR) spectroscopy is an optical method for measuringthe refractive index of very thin layers of material on a metal. SPR is a very

21

22 Imaging optical methods and electrochemistry

Glass prismMetal filmAmbient

Evanescent field

Figure 3.1: Schematic illustration of the Kretschmann setup.

sensitive technique for real-time, label-free monitoring of events taking place at asurface/liquid interface, and has been exploited for biosensing36 for over 20 years.

A surface plasmon is a p-polarized surface bound electromagnetic wave prop-agating at the interface between a plasma and a dielectricum. Since a metal hasnearly free electrons, it can be regarded as a plasma. Associated with the sur-face plasmon is an evanescent field, which is a non-propagating wave that decaysexponentially in the direction orthogonal to the direction of the interface. A sur-face plasmon can, under certain conditions, be excited by photons. In Figure 3.1

a common SPR setup, called the Kretschmann configuration, is shown. Here, alight beam is totally reflected at the glass/metal interface, exciting a surface plas-mon and an evanescent field. If the metal film is thin enough, the evanescentfield will penetrate into the ambient on the other side. The optical propertiesof the interface region will affect the excitation resonance condition. Thereby, alink is established between the ambient and the reflected light. Examples of SPRbiosensing applications include the study of biomolecular interactions and bindingproperties, environmental monitoring, and medical diagnostics.37

Imaging SPR38 (also known as SPR microscopy) is attained when the detectoris a replaced by a charge-coupled device (CCD) camera. The lateral resolution,typically in the µm range, is a function of the wavelength of the light, the objective,and the CCD resolution. See Brockman et al.39 for an extensive review on thetheory and applications of imaging SPR.

3.2.2 SPR and electrochemistry

If the metal substrate in SPR also acts as a working electrode in an electro-chemical experiment, these techniques can be employed simultaneously. Manyelectrochemical reactions will change the optical properties of the electrode sur-face and the nearby solution, and hence change the SPR response. This fact hasbeen used for several different investigations of surface interactions. Iwasaki etal.40 used the combination of SPR and cyclic voltammetry to investigate oxide

3.3 Ellipsometry 23

film formation and ion adsorption on gold electrodes. They also showed that itwas possible to separate the diffusion and adsorption processes. Further, Xiang etal.41 showed that SPR in conjunction with scanning electrochemical microscopycould be used to determine local variations in thin film thicknesses. The setup wasalso used to study conformational changes of cytochrome c molecules attached toa surface. Electropolymerization is a convenient way of forming a polymer filmon an electrode. This has been investigated by Knoll and co-workers for poly(3,4-ethylenedioxythiophene) (PEDOT) films of different thicknesses.42,43 Finally, Ertland co-workers used imaging SPR to obtain images of the potential distributionat electrodes in order to study spatiotemporal patterns.44 The same setup wasalso used to study Turing-type patterns on electrode surfaces.45

In this work Papers I, II, and III are based on the use of imaging SPR andelectrochemistry. It is especially the visualization of electrochemical reactions withlateral resolution that has been explored.

3.3 Ellipsometry

3.3.1 Theory

Ellipsometry is an optical technique for characterization of surfaces, thin films, andmultilayers.46 The basic idea is to analyze the polarization changes when light isreflected by a surface or film. Ellipsometry is non-destructive, making it suitablefor real-time in situ measurements. Furthermore, it is essentially insensitive to driftin light intensity, since it is the polarization change that is evaluated. Ellipsometryis very sensitive to surface changes, and can have sub-Angstrom resolution in filmthickness determinations.

The electric field, E, associated with a wave of light, can be divided into twocomponents, Ep and Es. Ep is the p-polarization (or TM-polarization) and is thecomponent of the electric field that lies in the plane of incidence. Es is the s-polarization (or TE-polarization) and its component is perpendicular to the planeof incidence. It is the correlation between Ep and Es that determines the state ofpolarization of the light.

In order to perform an ellipsometric measurement, an incident light beam withknown polarization is required. After reflection at a sample, the change in thepolarization of the light is determined. In Figure 3.2, a common setup, the PCSA(polarizer compensator sample analyzer) ellipsometer, can be seen. The lightsource is often a laser, emitting monochromatic, collimated light. The polarizerand analyzer are both linear polarization filters, and the compensator is a quarterwave plate, with which an arbitrary elliptical polarization can be obtained byinducing a phase shift between the components of the light. As the beam travelsthrough the polarizer, it becomes linearly polarized. The compensator, whichis fixed at ±45, will then produce elliptically polarized light. The reflection

24 Imaging optical methods and electrochemistry

Light source

Rotating polarizer

Fixed compensator Rotating analyzer

Detector

Sample

Figure 3.2: Schematic representation of a PCSA ellipsometer.

at the sample will induce an additional phase shift, denoted 4, along with arelative amplitude change, denoted tan Ψ. The beam then passes through theanalyzer before reaching the detector, where the intensity of the transmitted lightis measured. The strategy is to adjust the optical components so that the light isextinguished at the detector. This will be possible if the light is linearly polarizedafter the reflection at the surface, which means that it can be quenched by theanalyzer at a certain angle. By rotating the polarizer and the analyzer until zerointensity is detected,4 and tan Ψ can be determined using the angles of the opticalcomponents. Now, using the two parameters together with a model of the system,different surface characteristics can be estimated. For instance, the thickness ofa film on the surface can be calculated with the McCrackin algorithm,47 if therefractive indices of the substrate and the film are known.

Imaging ellipsometry48 is an interesting development of ellipsometry, where theregular detector is replaced by a CCD camera and a focusing objective. This allowsthe simultaneous determination of for instance film thicknesses over the entiresurface area observed with the CCD. The lateral resolution will be determined bythe wavelength of the light, the CCD resolution, and the objective. Typical valuesfor the resolution are in the µm range. Imaging ellipsometry has for example beenused for biosensor applications, as shown by Jin et al.,49 where antigen-antibodyinteractions were visualized.

3.3.2 Ellipsometry and electrochemistry

In combination with electrochemistry, ellipsometry is perhaps most widely used toevaluate film formation and growth on electrode surfaces. This in situ combinationwas pioneered by Tronstad50 in 1931, and has since gained popularity, see forinstance Kruger51 and Kruger and Calvert.52 Here, the thickness and opticalproperties of passive iron films were evaluated, as the electrochemical potentialwas either controlled or monitored.

3.3 Ellipsometry 25

In a review by Christensen and Hamnett,53 the in situ evaluation of electro-deposited polymer films was discussed. The influence of potential, and hence thefilm thickness, on the ellipsometric angles and reflected intensity was shown. An-other example is the in situ study of a self-assembled ferrocenylalkylthiol monolayeron a gold electrode performed by Abrantes et al.54 They showed that the mono-layer thickness increased 1 to 2 A as the ferrocene was oxidized to ferricinium, dueto a reversible orientation change. Further, Yu and Jin55 used imaging ellipsome-try to investigate the effect of the potential of a gold electrode on the adsorptionof fibrinogen.

In this thesis, imaging ellipsometry was used in Paper I to obtain thicknessmaps of a protein gradient, formed via bipolar patterning. It proved to be avery convenient way of visualizing the results from different preparation steps. InPaper IV, ellipsometry was used in situ during the electrodeposition of nanostruc-tured Cu and Cu2O materials. This gave valuable information on the depositionprocess, and the structure of the resulting film.

26 Imaging optical methods and electrochemistry

4Electrode surface design and

analysis

4.1 Introduction

Enormous efforts have been put into the modification of electrodes to alter orproduce specific functionalities. Such modifications can be very advantageous, andexamples of application areas include fuel cells and batteries, the protection fromcorrosion, and the use of modified electrodes as analytical sensors. Some 30 yearsago, interest arose regarding the modification of electrode surfaces by covalentattachment of monolayers of different species.2,56 Later, thicker polymeric filmsand inorganic layers started to be used, as well as conducting polymers and organicmetals. This chapter focuses on the use of electrochemistry for both the designand evaluation of modified electrodes.

4.2 Materials and methods

In this section, a summary of some important methods and materials used tomodify electrodes will be given, with some emphasis on self-assembly of alkane-based thiols on gold surfaces.

27

28 Electrode surface design and analysis

Polymers. A polymer is a macromolecule built up by repeating monomer units.There is an abundance of different polymers available, and new ones can read-ily be synthesized. Polymer films can be formed on an electrode from a solu-tion of dissolved polymer by, e.g., cast or dip coating, spin coating, covalentattachment or electrodeposition. If the solution contains the monomer, poly-merization on the surface can be induced via different means, e.g., thermal,photochemical, or electrochemical. Conducting polymers contain partiallydelocalized electrons, and are typically classified as semiconductors. Elec-troactive polymers contain electroactive components linked to the polymerbackbone, while ion-exchange polymers (polyelectrolytes) can attract ionsfrom the solution to charged sites in the film via ion-exchange processes.

Some electrochemical applications include work by Smela et al.,57 who useda conducting polymer (polypyrrole) to fabricate microactuators. Further,the ellipsometric characterization of electropolymerized polypyrrole was per-formed by for instance Kim et al.58 For an extensive review on conductingpolymers, see Simonet and Rault-Berthelot.59

Inorganic films. The most common inorganic material formed on electrodes aremetal oxides. By anodization of a metal electrode, an oxide film can easilybe grown, where the thickness is influenced by the potential and the time.Interesting work in this area has been done by Iwasaki et al.40 and Stevensonet al.60 In Paper IV, electrodeposition was used to form nanostructured Cuand Cu2O layers. Electrochemistry was here combined with a gravimetrictechnique (quartz crystal microbalance, QCM) and ellipsometry to investi-gate the potential and local pH dependence on the nature of the depositedmaterials.

Self-assembled monolayers (SAMs). The formation of a self-assembled mono-layer is a spontaneous process, leading to a molecular film with an oftenlarge degree of order, as a result of the lateral interactions between the ad-sorbed molecules. One of the most investigated systems is the self-assemblyof organosulfur (thiol) compounds. In 1983, Nuzzo and Allara showed thatgold surfaces could be easily functionalized by the spontaneous self-assemblyof organic disulfides.61 The tail-group of a thiol can be chosen to display dif-ferent physical and chemical properties at the ambient interface. SAMs canalso be used as linkers for the attachment of biomolecules to surfaces, whichis usually done in order to prepare electrochemical sensors. The most de-veloped application is a surface modified with an enzyme, such as glucoseoxidase.62 Further, patterns of SAMs can be formed by microcontact print-ing,63 giving linewidths ranging from 30 nm to 500 µm. Informative reviewson the subject are given by Ulman64 and Whitesides and co-workers.65

Extensive work has been done in order to characterize and optimize self-

4.3 Surface gradients 29

assembly of thiol based monolayers. The most commonly used substrateis gold, but SAMs can also be formed on other metals.66 Self-assemblyfrom a solution containing two different thiol species is also possible. Suchco-adsorbtion will result in SAMs having various degrees of phase separa-tion.67–73 One important application of SAMs is the formation of proteinresistant surfaces. Li et al.74 concluded that SAMs of oligo(ethylene gly-col) thiols with different lengths displayed different surface morphology andthereby different protein resistance.

A SAM on a surface will have a thickness of the same order of magnitudeas the diffuse layer. This means that the total capacitance of a modifiedelectrode will be greatly influenced by the dielectric properties of the surfacefilm. Therefore, electrochemical techniques are well suited for studying thinself-assembled systems. An early example is an investigation by Porter etal.,75 focusing on the influence of alkanethiol chain length on heterogeneouselectron transfer and interfacial capacitance. Voltammetry has further beenused to characterize a mixed monolayer of thiol analogues of cholesterol andfatty acids on gold,76 as well as to examine the chemistry of the bound alka-nethiol head group.77 In the latter work, it was also shown that the adsorbedthiols could be desorbed from the surface by both oxidation and reduction.In a recent study, Chan and Yousaf78 demonstrated that the ligand activ-ity of an electroactive monolayer could be controlled by the electrochemicalpotential.

The degree of organization of a SAM on electrode surfaces has a huge im-pact on its optical and electrical properties. Different packing densities andporosities will for instance affect the ability of the SAMs to impede chargetransfer reactions. This was investigated in Paper III, where a patternof microcontact printed SAMs of thiocholesterol and 1-hexadecanethiol wasused to investigate the possibility of detecting local electrochemical processesin situ on chemically modified electrodes.

4.3 Surface gradients

A surface with a gradually changing chemical or physical property is said to containa surface gradient. Many types of gradients are found in nature, for instanceconcentration gradients across cell membranes and biomolecular gradients guidingthe motility of cells. The mimicking of these in vivo gradients in the lab canbe a valuable tool to better understand biological processes. Also, by havinggradually changing properties on a biosensor surface, it is possible to perform high-throughput and cost effective analysis. Since all electroanalytical devices based onbiomolecular interactions have to be thoroughly optimized and investigated, theaccess to a surface gradient of the interacting species would be very advantageous.

30 Electrode surface design and analysis

i2

i1+i

2

i1 Oxidation

Oxidation

Cathodic side

Reduction

Reduction

Figure 4.1: Illustration of alkanethiol desorption from the cathodic side of a bipolar

electrode.

Two good examples of the use of molecular gradients are the study of surfacetension effects79,80 and protein adsorption.81,82 Recent reviews on the subject aregiven by Kim et al.,83 Morgenthaler et al.,84 and Genzer and Bhat.85

Several gradient-forming approaches have been demonstrated to date; theseapproaches are based, for example, on diffusion,86,87 electric fields88,89 microflu-idic systems,90 and immersion procedures.91,92 Bohn and co-workers proposed anelectrochemical method to form gradients on very thin metal substrates (about50 nm) by using in-plane electrochemical potentials.93–105 In that method, a po-tential gradient was generated across the surface by passing a current throughthe resistive film using a bipotentiostat. Gradients with varying widths and po-sitions could thus be created on the surface. This method has also been usedto deposit copper gradients in nanoporous alumina membranes,106 to manipulatedroplet transportation by the spatiotemporal control of a wetting gradient,107 andto electrodeposit polymer films in various patterns.108

Based on the use of bipolar electrochemistry, we have developed a new tech-nique to produce surface gradients, which we call bipolar patterning. Here, weuse the fact that the driving force for electrochemical reactions will vary laterallyacross a bipolar electrode. If the result of such reactions on either end of theelectrode is the adsorption or desorption of a specific molecule, a gradient of thatmolecule can be created on the surface. An example of this is seen in Figure 4.1.

In Paper I, this technique was used to form a biomimetic gradient on a goldsurface. Briefly, self-assembled monolayers and protein immobilization procedureswere used to realize a protein gradient. The first step involved the reductive des-orption of a SAM based on methoxy terminated poly(ethylene glycol)-containingalkanethiols. After backfilling with carboxyl terminated poly(ethylene glycol)-

4.3 Surface gradients 31

00.5

1

0 0.2 0.4 0.6 0.8

2.5

3

3.5

4

4.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

1

2

3

4

5

1

2

3

(A)

~0.6 nm

x (mm)

Thic

knes

s (n

m)

x (mm)y (mm)

Thic

knes

s (n

m)

(B)

~2.4 nm

Figure 4.2: (A) Line profiles obtained from imaging ellipsometry measurements, in

which the thicknesses of the gradients after the different preparation steps are shown.

Line 1 shows the result of the desorption of HS−C2H4−(O−C2H4)6−OCH3, line 2 shows

that obtained after backfilling with HS−C2H4−(O−C2H4)8−COOH, and line 3 repre-

sents the resulting protein gradient. (B) Thickness map of the protein gradient (the solid

line shows the region from which the line profile was taken). See Paper I for further

details.

containing alkanethiols and activation of the carboxyl groups, the surface wasincubated in a lysozyme solution. By employing imaging ellipsometry, the resultsof the preparation steps and the protein gradient could be evaluated (Figure 4.2).The resulting protein gradients formed with this specific setup were about 0.5–1mm wide. Bipolar patterning is a fast and straightforward technique with manypossibilities for improvements, for instance the generation of gradients of differentgeometries. The substrate can basically consist of any conducting material andthe technique does not require access to advanced laboratory equipment.

Gradients with widths in the millimeter range are sufficient for many applica-tions, but sometimes wider gradients and other spatial geometries are required. Aclear knowledge of the effects of the electric field in a solution containing a con-ducting substrate will make it possible to optimize this gradient forming technique.The distributions of the potential and current densities in a bipolar setup can beevaluated by rather simple methods involving the use of ordinary reference elec-trodes. Also, the possibilities offered by simple conductivity models was evaluatedto perform simulations of new and exciting geometries. In Paper II, the potential

32 Electrode surface design and analysis

Position (mm)

Rela

tive

cu

rren

t d

ensi

ty

Solu

tio

n p

ote

nti

al (m

V)

Position (mm)-20 -10 0 10 20 30 40

0.4

0.6

0.8

1

1.2

1.4

1.6

(A) (B)

-30 -30

-20

-10

0

10

20

30

-20 -10 0 10 20 30 400.4

0.6

0.8

1

1.2

1.4

1.6

-20

-10

0

10

20

30Re

lati

ve c

urr

ent

den

sity

Solu

tio

n p

ote

nti

al v

s. s

urf

ace

(mV

)

Figure 4.3: Solution potentials and relative current densities for different positions with

respect to the bipolar electrode (0.5 mm above the surface). The shaded areas represent

the position of the surface. (A) Experimental results as 1 mA was passed in a solution

consisting of 10 mM [Fe(CN)6] 4 – and 10 mM [Fe(CN)6] 3 – in 500 mM KNO3. (B)

Simulated results using COMSOL Multiphysics. A simple conductive media DC model

of the experimental setup was utilized. See Paper II for further details.

and relative current density distributions in a solution containing a bipolar elec-trode was investigated, see Figure 4.3A. It is clear that the presence of a bipolarelectrode has changed both distributions, which would otherwise be linear. As canbe seen from the current density, the amount of electrochemical reactions varieslaterally across the surface. In Figure 4.3B, results from a simulation of the setupused to obtain the experimental results are shown. The important result is thata good qualitative agreement was obtained between the measured and simulatedvalues. This demonstrates the value of the model to predict important parametersin bipolar setups.

Further, the use of imaging SPR provided the possibility to evaluate the amountof reactions occurring at the bipolar electrode. Since the extent of these reactionsare governed by the potential difference between the bipolar electrode and thesolution, the SPR response will be a function of this potential difference. Bycomparing the SPR responses with those obtained in a regular three-electrodesetup (including a standard reference electrode), the potential distribution veryclose to the surface can be determined (Figure 4.4).

To demonstrate the relevance of bipolar patterning for biomimetic and biosen-sor applications, a spherical electric field was used to selectively remove thiolsfrom a surface. Here, one feeder electrode was a platinum rod, placed above themonolayer. The resulting effective layer thickness is shown in Figure 4.5. Thismethod could now be further used to create an array of such gradient regions ona single substrate.

4.3 Surface gradients 33

-1.5

-1

-0.5

0

0.5

1

1.5

i (m

A)

-0.1 0.1 0.3 0.5E vs. Ag/AgCl (V)

(B)

2 2.5 3 3.5 4-0.05

0

0.05

Position (mm)

∆RTM

/RTE

10 mA

(A)

5 mA

1 mA

5 mA (in KNO3)

Figure 4.4: (A) The SPR response for different currents passed through the electrolyte,

simultaneously showing the reduction (left side) and oxidation (right side). The elec-

trolyte consisted of 10 mM [Fe(CN)6] 4 – and 10 mM [Fe(CN)6] 3 – in 500 mM KNO3.

(B) SPR response for different potentials when the sensor surface acted as a working

electrode in a three-electrode setup. The solid line was calculated using the Nernst equa-

tion. The scan rate used to obtain the cyclic voltammogram was 50 mV/s. See Paper II

for further details.

0

0.5

00.5

11.5

2

1.5

2

2.5

x (mm)y (mm)

d (n

m)

Figure 4.5: Results from imaging null-ellipsometry measurements, showing the effective

layer thickness, d, of the patterned thiol layer. The inset shows a larger image and the

region from where the thickness map was taken. See Paper II for further details.

34 Electrode surface design and analysis

5Alternating electric fields for

chemical analysis

5.1 Introduction

It is often desirable to monitor the state, or quality, of a liquid. This is sometimesa difficult task when relying on techniques based on static electric fields, due tothe often varying conductivity of the samples and the complexity of the solution.These problems can be circumvented by using EIS, which is based on alternatingelectric fields. We have utilized EIS in two different applications; the simultaneousdetermination of soot and diesel contamination in engine oil, and the determina-tion of concentration and pH in an industrial cutting fluid. Both of these liquidscontain very low amounts of electroactive species, and they can further have verydifferent conductivities. Evaluations of these can however still be performed withEIS due to the possibility to separate different processes based on their frequency-dependencies. Since this technique is very sensitive, it is of crucial importance touse a carefully designed measurement setup.

35

36 Alternating electric fields for chemical analysis

5.2 Practical considerations

When performing EIS measurements, several practical aspects have to be consid-ered. A lot can be gained by using a correctly optimized experimental setup. Thereare several commercially available test fixtures and cells, but the prices are oftenquite high. Sometimes it is hence worth the time and effort to design and optimizean application-specific measurement setup. When evaluating a solid sample in or-der to gain information on the bulk properties, the role of the electrodes is simplyto provide electrical contact to the material. For liquids, electrochemical reactionsand other surface interactions can take place at the electrode/solution interfaces.In these cases a parallel plate setup is often used, where the plates consist of aninert metal. The electrode area and separation can be varied depending on theapplication. Listed below are some other important parameters that need to beconsidered.

System. Most often a potentiostat is used together with a frequency responseanalyzer (FRA) for EIS measurements. When potentiostatic control is notrequired, as is the case for most materials impedance tests, a self-containedimpedance analyzer can be used instead. Many different systems are com-mercially available, with varying specifications regarding sensitivity and fre-quency range.

Terminal configuration. There are several connection configurations available,and the choice depends on the expected impedance range and the frequenciesused. Most modern equipments have four terminals; high and low potentialand high and low current. It is between the high and low current terminalsthat the excitation signal is applied, and the resulting signal is measuredbetween the high and low potential terminals. If the two high and the twolow terminals are shorted, a two-terminal configuration is obtained, whichis sufficient for many high impedance measurements. A configuration pro-viding a very wide measurement range is the four-terminal configuration.This ensures that only the potential drop across the cell is measured, andeliminates the effects of the cables connected to the high and low currentterminals.

Cables. The cables for connecting the cell to the measuring equipment need tobe carefully designed. Preferably a short, static, and well shielded cableassembly should be used. It is also very important to make sure that allconnections are good in order to not introduce resistive loads outside thesample volume. Otherwise these will be included in the total impedance.

Shielding. For sensitive measurements, it is always recommended to shield thecell from external interference by placing it in a Faraday cage. This is true

5.3 Applications 37

Electrode 2

Electrode 1

Sample ~200µm

Shielding boxHc

Hp

Lp

Lc

Triaxialcable

Coaxialcable

Figure 5.1: Schematic representation of the measurement set-up showing the impedance

meter, the four-terminal cable configuration, the shielding box, and the electrodes. Hc:

High current, Hp: High potential, Lp: Low potential, Lc: Low current.

for both low and high impedances. The cage should be connected to theshield of the instrument input terminals.

Temperature. The impedance is generally a function of temperature. In orderto compare results from different samples, it is very important to be able tocontrol or at least to measure the temperature of the sample.

A schematic representation of the setup used in Papers V and VI can beseen in Figure 5.1. In both cases, the electrodes were separated by approximately200 µm, and consisted of stainless steel in a parallel plate cell configuration. Afour-terminal configuration was employed together with a coaxial/triaxial cableassembly, to be able to perform measurements over a large impedance range.

5.3 Applications

General applications of EIS were given in section 2.6. Here, the focus will be onthe chemical analysis of lubricants and cutting fluids. These liquids are abun-dant in industrial applications and there is presently a huge demand for on-linemeasurement techniques.

Smiechowski and Lvovich have made several interesting EIS studies on in-dustrial lubricants. Some examples include the detection of water leaks in engineoil,109 the analysis of colloidal dispersions,110 and the investigation of the relation-ship between lubricant chemical composition and EIS data.111 Further, Allaharet al.112 studied the impedance of steels in new and degraded jet engine oil, whileWang and Lee113 focused on the glycol contamination in regular engine oil. Wangand co-workers have also employed fast scan voltammetry to determine the condi-tion of similar samples.114–117 Here, the maximum current output was measuredand correlated to degradation processes in the oil. The obvious drawback was theuse of only one parameter to evaluate the condition of the sample.

The quality of the engine oil greatly affects the performance of an engine, andthe need for on-board sensors for real-time monitoring is increasing. In Paper V,

38 Alternating electric fields for chemical analysis

1 1.5 2 2.5 3 3.5 4 4.5 5-1

0

1

2

3

4

5

6

7

8

Soot (%)

Die

sel (

%)

1S1D4S1D3S3D1S5D4S5D

Figure 5.2: The result from soot and diesel predictions made with impedance data and

PLS modeling. The cross-hair markers represent the true concentrations of soot and

diesel for the five samples. The samples are labeled according to their levels of soot (S)

and diesel (D). See Paper V for further details.

the simultaneous estimation of soot and diesel in engine oil was shown to befeasible. In these experiments, a carefully planned measurement cell was employed,consisting of two stainless steel electrodes in a parallel plate configuration. Realengine oil samples with varying amounts of soot were spiked with diesel and thenevaluated using a frequency range of 600 kHz to 20 Hz. Due to the low conductivityof the samples, the voltage amplitude was set to 20 Vrms. The data was thenevaluated with multivariate data analysis, namely partial least squares (PLS).The use of statistical tools for the interpretation of impedance data can be veryinformative, since it quickly provides information on the correlation between thedata and the sought parameters. It also allows the classification of the samples intocategories. In Figure 5.2, the results of predictions made with two PLS models areshown, one for soot and one for diesel. As can be seen, samples with different sootconcentrations are easily classified, with good accuracy. The diesel variations wereharder to predict. The analysis would most likely be improved if lower frequenciescould have been used.

One drawback of the cell used in Paper V was the lack of temperature control.Therefore, a new cell was designed, including heaters and a temperature sensor.To simplify the sample application procedure, it was further designed as a flowcell. In Paper VI, this cell was used for quality evaluation of industrial cuttingliquids. In this study, the frequency range was expanded (1 MHz–10 mHz), andthe amplitude was much lower (40 mVrms). The correlation between EIS data

5.3 Applications 39

-1

-0.5

0

0.5

1

NitriteOilpH

Co

rrel

atio

n

Frequency (Hz)

|Z| θ

10-2

106

10-2

106

Figure 5.3: Correlation of EIS data to cutting fluid (oil) concentration, pH, and nitrite

level. The left part represents |Z| and the right part θ. See Paper VI for further details.

5

10

15

8

9.5

11

Co

nce

ntr

atio

n (%

(w/w

))

Samples Samples

pH

(A) (B)

Figure 5.4: PLS training data for the cutting fluid concentration (A) and the pH level

(B). Real values (no markers) are also shown. See Paper VI for further details.

and the cutting fluid concentration, the pH, and the nitrite level is shown inFigure 5.3. It is evident that the concentration and the pH correlate well withthe measured variables, and more importantly, that both parameters have differentfrequency dependencies. Interesting is also the fact that when the correlation forone parameter is zero, the correlation for the other is often quite high. This shouldprovide the means to separate their individual contributions to the impedance.The nitrite level, however, correlated poorly to the impedance and was thereforenot further evaluated. In Figure 5.4, the PLS training data for the concentrationand pH is shown. The figures indicate the possibility of predicting both parameterssimultaneously. If more samples were evaluated, model stability would most likelyimprove. This would also permit validations to be performed. However, the nextstep should in our opinion be an on-line analysis of real samples.

40 Alternating electric fields for chemical analysis

6Future outlook

One of the major conclusions reached during the work underlying this thesis, is thatelectric fields in their various forms are excellent tools for surface design. Staticand alternating, they also allow the analysis of surface and interfacial properties,as well as chemical analysis of the bulk of a solution.

The use of bipolar electrochemistry for the formation of surface gradients isa new and exciting method. In fact, in the last decade or so, many interestingapplications of bipolar electrochemistry have been reported. It is especially thelack of an electrical contact to the electrode that makes this technique quite unique.We have shown that bipolar patterning can be used to create surface gradientsof many different materials. Interestingly, the width, position, and geometry ofthe gradients can be controlled. The possibility to predict the bipolar behavior bysimulations also allows the optimization of the experimental setup. This knowledgecan be used to form new and exciting gradients, with varying functionalities andgeometries. Ongoing work hence deals with bipolar patterning with sphericalelectric fields. Here, one of the feeder electrodes is placed above a substrate,inducing a circular reaction region on the bipolar electrode. With this, it is possibleto create several local gradients on one surface. This approach could for instancebe used to form an array of circular gradient spots on a single substrate.

There are several ways to increase the widths of the gradients. If the bipolarelectrode, for instance, is placed at an angle relative to the feeder electrodes,

41

42 Future outlook

the gradient will be widened. Another interesting concept is to use two bipolarelectrodes, connected to an external voltage source. We have also worked with theelectrodeposition of polymers, and initial experiments show the possibility to formgradients in the degree of doping of polypyrrole.

In this thesis, EIS has been extensively employed for quality determination inindustrial applications. We have explored the combination of EIS and statisticaltools, which proved to be an easy way to quickly obtain information on the state ofa sample. The evaluation of EIS data to gain insight on intrinsic parameters, canhowever be a quite complex task. It would be very interesting to use one of thesoftwares available to parameterize data and extract physical quantities. Finally,the implementation of an on-line test setup would really put the method to thetest. This would also require optimization of the setup and electronics, togetherwith a model based on the interactions present.

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