Surface Assembly and Redox Dissolution of Silver Nanoparticles Monitored by EvanescentWave Cavity Ring-Down Spectroscopy

Mathias Schnippering,† Hayley V. Powell,† Meiqin Zhang,† Julie V. Macpherson,†Patrick R. Unwin,*,† Mikhail Mazurenka,‡ and Stuart R. Mackenzie*,‡

Department of Chemistry, UniVersity of Warwick, Gibbet Hill Road, CoVentry CV4 7AL, United Kingdom, andDepartment of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom

ReceiVed: May 25, 2008; ReVised Manuscript ReceiVed: July 24, 2008

The adsorption kinetics of Ag nanoparticles on a silica surface modified with poly-L-lysine (PLL) have beenmeasured in situ by following the interfacial optical absorbance at 405 nm by evanescent wave cavity ring-down spectroscopy (EW-CRDS). Sensitivity toward nanoparticle detection is enhanced due to the plasmonresonance of the Ag nanoparticles. The redox-dissolution kinetics of the immobilized nanoparticles havebeen investigated by two distinct approaches. First, IrCl6

2- was generated electrochemically from IrCl63- by

a chronoamperometric potential step in a thin-layer cell configuration formed between the silica surface anda Pt macroelectrode. The oxidative dissolution kinetics were obtained by monitoring the EW-CRDS signal asthe nanoparticles dissolved. The reaction kinetics were extracted by complementary finite element modelingof diffusional and reaction processes. The second method of dissolution investigated involved the injectionof IrCl6

2-(aq) directly at the surface by means of a microcapillary located close to the evanescent field.

Introduction

Immobilized arrays of metal nanoparticles continue to attractconsiderable attention for electronic and sensing applications,1-3

including biosensing and the study of protein interactions.4-8

In the latter context, Ag nanoparticles have proven useful forstaining applications, which enable the visualization of proteinsand DNA, and as an integral component of antibacterialcoatings. Ag nanoparticles can be functionalized with oligo-nucleotides for DNA sensing applications9 or act as bridgematerials for electron transfer reactions from proteins toelectrodes.10 It has also been shown that Ag nanoparticles canbe used as substrates for surface-enhanced Raman scattering(SERS).11-13

For these applications and others, understanding reactions atthe nanoscale, especially electron transfer at nanostructures andmetal nanoparticles, is very important. The oxidative etchingkinetics of metal surfaces were probed in earlier scanningelectrochemical microscopy (SECM) work by us14 usingtris(2,2′-bipyridyl)ruthenium(III) and Br2 as electrogeneratedoxidants. Only a few studies have investigated the dissolutionof nanoparticles, either in solution by varying the pH15-17 andthrough O2 purging18 or embedded in a glass matrix. In the lattercase, the particles were dissolved by use of heat,19 ultrafast laserpulses20 or intense electric fields.21,22 In all these examples,analysis was performed either ex situ or in bulk by conventionalUV-vis spectroscopy. To date there have been no time-resolvedstudies of the dissolution kinetics of metal nanoparticles oninsulating surfaces. In order to monitor and rationalize theseprocesses, rather sensitive analytical techniques are needed.

The redox mediator [IrCl6]2-/3- has been employed in severalprevious studies to dissolve Ag nanoparticles. For example, ithas been applied to protein detection via SECM (generation of

IrCl62- from IrCl6

3- at an ultramicroelectrode) after Ag stain-ing23 and for Ag-enhanced fingerprint determination.24 Theformation of electrochemically generated Ag nanoparticles hasbeen investigated by Rodriguez-Sanchez et al.25 In all thesestudies, the Ag nanoparticle dissolution/growth was eitherdetected indirectly by surface-sensitive techniques, such asscanning electron microscopy and X-ray scattering, or time-resolved by bulk UV-vis spectroscopy. It has not been possibleto combine both approaches to produce a time-resolved surfacesensitive technique.

Evanescent wave cavity ring-down spectroscopy (EW-CRDS)is rapidly emerging as a highly sensitive technique for thedetermination of interfacial kinetics in condensed-phasesystems.26-28 Several groups have used EW-CRDS to study theadsorption kinetics of different dyes, as well as biologicallyinteresting molecules, in real time.26-31 We recently reportedthe use of EW-CRDS for measurement of gold colloid deposi-tion on poly-L-lysine (PLL) -functionalized silica.32 The presentpaper illustrates the use of EW-CRDS as a powerful techniqueto study the kinetics of processes on insulating surfaces suchas adsorption and dissolution, which has been challenging inthe past because the introduction/removal of the adsorbents intoa solution has to occur on a rapid time scale. A key attribute ofEW-CRDS is its surface sensitivity, which arises due to thesmall penetration depth (typically ∼100 nm) of the evanescentfield beyond the boundary of interest. For condensed phases,this opens up a wide range of applications for either adsorptionstudies or nanoscale reaction kinetics, coupled with an impres-sive temporal resolution.

In this report, the adsorption of Ag nanoparticles onto PLL-modified silica was investigated, extending the work on othermetal nanoparticles reported previously.33,34 The kinetics werefollowed in real time, permitting the extraction of key informa-tion about the rate-controlling processes. This part of the paperhighlights the sensitivity of the EW-CRDS technique in probingthe assembly of disperse films of particles.

* Corresponding authors: (P.R.U.) tel + 44 24 7652 3264, e-mailP.R.Unwin@warwick.ac.uk; (S.R.M.) e-mail srm49@cam.ac.uk.

† University of Warwick.‡ University of Cambridge.

J. Phys. Chem. C 2008, 112, 15274–1528015274

10.1021/jp804615m CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/10/2008

The kinetics of the redox dissolution of Ag colloidal particleswere also obtained by combining EW-CRDS with electrochem-istry. The nanoparticles were dissolved by use of the redoxcompound IrCl62- which was electrochemically generated fromthe reduced (nonreactive) form, IrCl63-, at a strategically locatedelectrode just above the silica surface. IrCl6

2- oxidizes Ag0 toAg+, causing dissolution of the nanoparticles. Studies werecarried out for various concentrations of IrCl6

3- and differentelectrogeneration times, by use of a potential step method. Theexperimental data, in the form of interfacial absorbance-timeplots, were used to calculate the rate constant of the dissolutionprocess after solution of the underlying mass transport problem.

A second method of Ag colloid dissolution was also tested,in which a simple micropipette was used to dispense the reagent(IrCl6

2- solution) directly in the region of the immobilizednanoparticles. The dissolution process was followed as afunction of IrCl6

2- concentration and flow rate. These initialstudies provide a platform for the further use of this technique.

Theory

In order to extract kinetic parameters from the experimentalobservations, we fitted the data obtained to a simple model,encapsulating the diffusion of the electrogenerated species andthe kinetics of dissolution. The model was developed for theelectrochemical arrangement shown in Figure 1 and describedbelow. A 2 mm diameter disk electrode is positioned 180 ( 20µm above a prism surface bearing the Ag nanoparticles and isused to generate the oxidant, IrCl6

2-, from IrCl63-, local to the

surface. Experiments show that the Ag nanoparticles on thesurface are highly dispersed, but as the interparticle spacing ismuch smaller than the electrode-prism separation d, it isreasonable to treat the reactive prism surface as uniform (forthe purpose of modeling) and diffusion between the electrodeand prism as planar. The model considers the diffusion of the

mediator within the thin-layer cell coupled to the dissolutionof the Ag layer at the prism boundary and calculates theabsorbance of the Ag layer with time, as described later.

The Diffusional Problem. The diffusion of species i, withconcentration ci(z, t), in a one-dimensional domain is given by

∂ci(z, t)

∂t-Di

∂2ci(z, t)

∂z2) 0 (1)

for z ∈ [0, d], where d is the distance between the prism andthe electrode (z ) 0 corresponding to the electrode surface),and Di is the diffusion coefficient of species i (IrCl6

3- orIrCl62-). If it is assumed that IrCl62- can oxidize only the atomson the surface of the nanoparticle, the boundary conditions forthe reduced (IrCl6

3-) and oxidized (IrCl62-) forms of the

mediator, denoted CR(z, t) and CO(z, t), respectively, in this caseare given by

cR(0, t)) 0

DO

∂cO

∂z(0, t))-DR

∂cR

∂z(0, t)

-DO

∂cO

∂z(d, t))DR

∂cR

∂z(d, t)) kcONNPAp(t)

(2)

for 0 < te tstep, where tstep is the potential step time (the durationof the electrogeneration period), and

cO(0, t)) 0

DO

∂cO

∂z(0, t))-DR

∂cR

∂z(0, t)

-DO

∂cO

∂z(d, t))DR

∂cR

∂z(d, t)) kcONNPAp(t)

(3)

for tstep < t. Here k is the Ag dissolution rate constant, NNP isthe nanoparticle surface concentration (per square centimeter)on the prism, and AP(t) is the surface area of a nanoparticle,which decreases as the nanoparticle is dissolved. Our focus ison initial rate measurements, which allow us to neglect the smallbuildup of Ag+ that will occur in the thin-layer cell during thedissolution process. Additionally, because the change in particlesize during dissolution is small, the rate constant was assumedto be independent of the nanoparticle size. These boundaryconditions were coupled to an ordinary differential equation forAp(t) describing the dissolution of Ag at the prism surface:

dAp(t)

dt)- 4Mπ1/2

FkcO(d, t)[Ap(t)]

1/2 (4)

where F is the density of Ag (10.5 g cm-3) and M is themolecular weight of Ag (107.67 g mol-1). The initial valuesfor Ap(t) were obtained from the nanoparticle size determinedby tapping-mode atomic force microscopy (TM-AFM).

Due to mass conservation of the redox couple within the thin-layer cell and the equality of the diffusion coefficients of theoxidized and reduced forms of the mediator, the concentrationsat any point are related by

cR + cO ) cR* (5)

where cR* is the original bulk concentration of IrCl63-. Thus,

it is only necessary to solve the diffusion equation numericallyfor either cO or cR and Ap(t).

Modeling Changes in Interfacial Absorbance. To computethe interfacial absorbance with time, the average nanoparticlesize obtained by TM-AFM measurements was considered. Theinitial Ag nanoparticle surface concentration is known from TM-AFM measurements, and the change in nanoparticle size with

Figure 1. (a) Experimental arrangement for the study of silvernanoparticle dissolution by evanescent wave cavity ring-down spec-troscopy. Home-synthesized silver nanoparticles are immobilized onthe PLL-functionalized hypotenuse of a fused silica prism, which formsone element of an optical ring cavity. (b) IrCl6

2- is electrogeneratedfrom IrCl6

3- in an axisymmetric thin layer electrochemical cell. (c)IrCl6

2- solution is injected in close proximity to the surface via a 300µm diameter micropipette.

Assembly and Dissolution of Silver Nanoparticles J. Phys. Chem. C, Vol. 112, No. 39, 2008 15275

time was computed from the flux at the interface (eqs 2-4),where face-centered cubic (fcc) packing of the atoms withinthe nanoparticles was assumed. The size dependence of theabsorbance was obtained from the experimental data of Cot-tancin et al.,35 which gives the absorption coefficient fornanoparticles of diameters in the range of 1.5-6.7 nm (fournanoparticle sizes), for which there is a linear relationship ofthe absorption coefficient and nanoparticle diameter. As shownlater, the extinction coefficient, εm, of Ag nanoparticles can bederived by use of this UV-vis data and TM-AFM and EW-CRDS measurements. With knowledge of the extinction coef-ficient and the data of Cottancin et al.,35 the size-dependentabsorbance A(a) can be estimated. With A established, the bestfit to the data was found by varying k.

Experimental Section

Chemicals. Silver nitrate salt (stored in the dark), tribasicsodium citrate, sodium borohydrate, methanol (Aldrich, spec-troscopy grade), PLL (MW 30 000-70 000, Sigma) were usedas received.

Synthesis of Ag Nanoparticles and Sample Preparation.Citrate-stabilized Ag nanoparticles were prepared followingestablished methods.34 In order to obtain colloids with a narrowsize distribution, it was crucial to rinse all glassware withcopious amounts of purified water from a Millipore Milli-Qsystem (resistivity >18 MΩ cm) prior to sample preparation.Sodium borohydrate (0.6 cm3 of 0.1 M solution) was added to20 cm3 of a solution containing equal concentrations (0.25 mM)of sodium citrate and silver nitrate under strong stirring. Thereaction was complete after 2 h at room temperature.

The coverage and size distribution of nanoparticles achievedwith deposition from quiescent solution onto PLL-functionalizedsilica surfaces was investigated by TM-AFM of PLL-coatedquartz slides. Quartz slides were ashed in oxygen plasma for 1min at 100 W (Emitech, K1050X) and wiped with methanol.PLL films were deposited from 1 mg cm-3 stock solution ofthe polyelectrolyte at pH 7. The quartz slides were then rinsedwith ultrapure water and dried in an air flow. Ag nanoparticleswere electrostatically adsorbed by exposing a PLL-modifiedquartz slide in a Ag nanoparticle solution for a defined time.For all TM-AFM measurements, a Digital Instruments multi-mode atomic force microscope with a Nanoscope IIIa controller(Veeco) was employed. Commercially available SPIP softwarewas used for statistical grain analysis in order to obtain the sizeand the size distribution of the Ag nanoparticles.

EW-CRDS Apparatus. The specifications of the CRDspectrometer used in this work (and the alignment procedure)were similar to those described elsewhere32,36 and are shownin Figure 1a. The interface of interest was the hypotenuse of aright angled prism that forms one optical element of a high-finesse ring cavity. The remainder of the optical cavity consistedof two high-reflectance concave mirrors (R 99.997% at 405 nm,Los Gatos Research) with radii of curvature of 1 m. The othertwo surfaces of the fused silica prism (CVI) were coated witha standard antireflection coating (AR; R < 0.5%, λ ) 350-532nm for normal incidence). The total length of the cavity was56.4 cm and, at the geometry used, the penetration depth of theevanescent field was calculated to be 240 nm.37 The cavity wasfilled by light from a pulsed diode laser (Power TechnologyInc., 405 nm, 50 mW maximum output) operating at 5 kHz(TTI pulse generator, TGP110). Light intensity leaking out ofthe cavity due to reflection at the incident prism surface wasmeasured with a photomultiplier tube (Electron Tubes, 9781B).The decay in the light level within the cavity was recorded on

a 12-bit 400 MS/s oscilloscope card (Gage CS12400), analyzedby the fast Fourier transform method in order to extract the ring-down time.38 For the purposes of plotting, the data weresmoothed by use of a 25-point adjacent average. The ring-downtime is characteristic of all optical losses within the cavity, andfrom it the interfacial absorbance per pass within the evanescentfield can be calculated in a straightforward manner.36 Electro-chemical data could be recorded along with the EW-CRDSresponse on a National Instruments PCI-6221 card. Custom-written LabVIEW software ensured optimal data synchroniza-tion. The same cell as described in ref 36 was employed for allEW-CRDS measurements.

UV-Visible Absorption Spectroscopy. UV-vis absorptionspectra were measured with a Lambda 25 UV-vis spectrometer(Perkin-Elmer Instruments).

Ag Nanoparticle Adsorption. PLL (0.2 cm3 of a 1 mg cm-3

solution in ultrapure water) was added to the cell above a cleanfused silica prism and left for 20 min leading to PLL depositionby drop coating. After deposition, the prism was rinsed withultrapure water and dried in a flow of air. The adsorptionbehavior of Ag nanoparticles, at different dilutions, on PLL-modified fused silica prisms was followed by EW-CRDS in asimilar way to that described previously for other metalnanoparticles.32 In brief, 0.2 cm3 of Ag colloid solution wasinjected into the cell above the substrate and the EW-CRDSresponse for the adsorption process was monitored continuously.Where the modified substrate was used for subsequent dissolu-tion studies, adsorption was stopped after a defined time(typically 2 min) by removing the remaining colloidal solutionand rinsing five times with 1.5 cm3 of ultrapure water. By thisstage, a disperse film of nanoparticles had been deposited forwhich the surface coverage was known. The same procedurewas followed prior to all dissolution experiments. After eachexperiment, the prism surface was wiped several times withmethanol and ashed for 1 min in an O2 plasma at 100 W. Ashingfor such short times did not affect the properties of the prism,and the initial ring-down time was always re-established.

Chronoamperometric EW-CRDS. As displayed in Figure1b, the working electrode (2.0-mm-diameter circular platinumelectrode with an overall diameter, including insulation, of 6.35mm) was placed over the region of the prism coincident withthe evanescent field; a 3-axis micropositioner was used to createa thin-layer electrochemical cell. The distance between theworking electrode and the prism surface was chosen to be 200µmswell outside the evanescent fieldsand the electrode itselfdid not affect the EW-CRDS signal. Two Pt wires acted ascounterelectrode and quasi-reference electrode, respectively,with all potentials applied with respect to the latter. Measure-ments were made with a home-built potentiostat controlled bya National Instruments card (NI PCI-6221) for potential controland data acquisition. The dissolution of the nanoparticles wasinduced by oxidizing 1 mM or 0.1 mM IrCl6

3- in 0.1 M KNO3

supporting electrolyte. The potential was stepped from -0.1 to+0.4 V for a defined time during which the oxidation of IrCl63-

to IrCl62- was diffusion-controlled. The potential was then

returned to -0.1 V, where IrCl62- was reduced back to IrCl6

3-.Flow EW-CRDS. Micropipettes with a typical internal

diameter of 300 µm were produced by pulling a 2-mm (o.d.)borosilicate glass capillary (Harvard Apparatus) with a PB7Narishighe micropipette puller and cutting the tapered the endwith a sharp scalpel. The resulting nozzle was polished on ahome-built polishing wheel with a 0.1-µm diamond polishingpad (Buehler). The micropipette was placed over the region ofthe evanescent field by use of a 3-axis micropositioner at a

15276 J. Phys. Chem. C, Vol. 112, No. 39, 2008 Schnippering et al.

distance of ca. 500 µm from the prism surface (Figure 1c). Flowof the solution of K2IrCl6 at concentrations between 1 and 5µM at a constant rate was achieved with a syringe pump(KDS100, KD Scientific) equipped with a 10 cm3 glass syringe(Hamilton). The system delivered flow rates in the range of0.22-35.1 µL s-1. Typical flow rates used were between 1.7and 33.3 µL s-1. All solutions were freshly prepared im-mediately prior to experiments.

Results and Discussion

Tapping-Mode Atomic Force Microscopy. Figure 2 showsa typical 1 µm × 1 µm TM-AFM height image of Agnanoparticles on a PLL-modified fused silica slide followingadsorption from stock solution. A clear submonolayer coverageof Ag nanoparticles is evident, from which their size, the sizedistribution and the coverage of the nanoparticles can beestimated. The average size of the Ag nanoparticles was foundto be 7.6 nm ( 2.6 nm and in this case the coverage was 60 (17 nanoparticles per µm-2. As shown below, there is excellentagreement between surface coverages measured by TM-AFMand EW-CRDS.

Ag Nanoparticle Adsorption. The concentration of Agnanoparticles in solution obtained from the synthesis can beestimated by assuming that all Ag+ ions present in the solutionreacted to Ag0. By use of eq 5, the nanoparticle concentrationis given by39

cNP )cAg+

∑ fi

NA

MFVNP

(6)

where cAg+ is the initial concentration of Ag+ (0.25 mM), fi isthe frequency of nanoparticles with radius ai as obtained byTM-AFM, NA is Avogadro’s constant, and VNP is the volumeof nanoparticles with radius ai. The distribution is normalizedso that it satisfies

∑i)1

L

fi ) 1 (7)

In this work, the resulting concentration was calculated to be11.3 nmol dm-3. With this information, the extinction coefficientof the Ag nanoparticle solution can be calculated by plottingthe relative peak absorbance of different dilutions from UV-visspectroscopy as a function of concentration (see Figure 3). As

shown in the inset of Figure 3, the relative absorption valuesshow a good linear dependence on the concentration. Thecalculated molar extinction coefficient for Ag nanoparticles wasεm ) 2.88 × 1011 cm2 mol-1, which is in good agreement withprevious reports.39

The time-dependent surface concentration could be calculatedfrom absorbance values measured from the EW-CRDS data,A(t), and the extinction coefficient:

NNP(t)) A(t)εm

(8)

This equation is valid only if there is negligible absorbance fromthe bulk solution within the evanescent field. Our simulationssupport this assumption: even if a 500 µm effective thickness

Figure 2. (Left) Representative 1 × 1 µm tapping-mode AFM image of Ag nanoparticles on PLL-modified quartz following adsorption for 2 min.(Right) Statistical analysis of particle size from which a mean particle size of 7.6 ( 2.6 nm was obtained from several images.

Figure 3. Bulk UV-visible absorbance spectra for different dilutionsof Ag nanoparticles in water. The large band at 400 nm correspondsto the localized surface plasmon resonance band of Ag. (Inset) Peakabsorbance vs colloid concentration.

Figure 4. Interfacial absorbance transients for the adsorption of severaldilutions of nanoparticles on PLL-modified fused silica. The dilutionsare 1:10, 1:20, 1:30, 1:50, and 1:100. The red lines denote fits wherediffusion-controlled (∝ t1/2) adsorption kinetics is assumed.

Assembly and Dissolution of Silver Nanoparticles J. Phys. Chem. C, Vol. 112, No. 39, 2008 15277

is assumed, simulations show that the bulk absorbance neverexceeds 3% of the interfacial absorbance for any of the resultsreported here.

For a typical absorbance value of 0.002, the correspondingcalculated surface coverage was 42 µm-2, which is similar tothe value of 60 ( 17 nanoparticles µm-2 obtained by TM-AFMmeasurements for the Ag nanoparticle adsorption from the stocksolution. This represents an effective surface coverage of ca.0.2%. This also suggests that the extinction coefficient for Agnanoparticles adsorbed on the surface is very similar to that inbulk solution measured by UV-vis spectroscopy.

Figure 4 shows the time dependence of the interfacialabsorbance as Ag nanoparticles adsorb irreversibly on the PLL-modified fused silica. As the figure shows, the experimentalabsorbance data fit satisfactorily to a simple square-rootdependence (t1/2), which is expected for diffusion-controlledadsorption.40 Since the adsorption process is completely diffu-sion-controlled, the diffusion coefficient of Ag nanoparticles,DNP, can be calculated:41

A(t)) 2εcNPπ-1⁄2DNP1⁄2t1⁄2 (9)

where A(t) represents the time-dependent optical absorption,which relates to the amount adsorbed on the surface. Thediffusion coefficient was calculated to be (1.92 ( 0.22) × 10-6

cm2 s-1 from all the sets of data in Figure 4. This value shouldbe treated as an upper limit since we make no attempt to accountfor the effects of natural convection, which could impact themeasurement at longer times.

The fact that the adsorption of nanoparticles can be followedfrom a solution that contains such a low concentration of Agnanoparticles is testament to the sensitivity of EW-CRDS,coupled with the strength of the localized surface plasmon peakof Ag nanoparticles, which enables the detection of very lowcoverages of nanoparticles. As shown in Figure 4, a dilution ofthe stock solution of 1:100 (i.e., 0.18 nmol dm-3) gives rise toan absorbance value of 0.000 45 after 120 s, which correspondsto only 18 particles µm-2. For the dissolution experiments thatfollow, surfaces were prepared from either the 1:20 or the 1:100dilutions. The adsorption was typically stopped after ca. 120 s.The corresponding absorbance value of the transient at this timewas used to determine the surface coverage of Ag nanoparticles.

Electrochemical Dissolution. The electrochemical dissolu-tion of Ag nanoparticles can be summarized by the followingredox reaction:

IrCl62-+Ag(0)f IrCl6

3-+Ag(I) (10)

In these experiments, IrCl62- was electrogenerated from IrCl63-

at an electrode situated ca. 200 µm above the prism surface.The electrogeneration process occurred for a defined time beforethe potential of the working electrode was switched back to avalue that reduced IrCl62- to IrCl63-. The current-time behaviordisplayed a Cottrellian response. Figure 5 shows typicalexperimental EW-CRDS transients for the electrochemicaldissolution process for two different step times of 5 s (panel a)and 10 s (panel b), together with transients simulated by use ofthe model outlined earlier. The diameter of the nanoparticlesdecreases from 7.6 to 5.8 nm after 80 s for the 10 s transient.The dissolution rate constant per nanoparticle, k, was calculatedto be 1.0 × 10-3 cm s-1 for the 5-s step and 0.9 × 10-3 cm s-1

for the 10-s step.After an initial delay period of a few seconds, during which

IrCl62- diffuses from the electrode toward the prism, the

interfacial absorbance slowly decreases with time. This can beattributed to the dissolution of Ag nanoparticles within the

evanescent field. Since the plasmon resonance of the Agnanoparticles is the main contribution to the extinction at 405nm, the decrease in the absorbance due to the dissolution processwill be determined by the number of particles within theevanescent field as well as their size.

Figure 6 shows the absorbance transient for a potential steptime of 600 s but at a lower IrCl6

3- concentration of 0.1 mM.The absorbance decreases much more gradually for this lowconcentration, which can be explained by the fact that thedissolution process is limited with the amount of IrCl62- presentin the thin-layer cell. The rate constant, k, for this transient wasdetermined to be 1.1 × 10-3 cm s-1, which is in very goodagreement with the rate constants obtained above. The meanrate constant for the dissolution per nanoparticle from severalruns was calculated to be (1.0 ( 0.1) × 10-3 cm s-1.

Figure 5. Interfacial absorbance transients for the electrochemicaldissolution of Ag nanoparticles by use of 1 mM IrCl6

3- oxidized for(a) 5 s and (b) 10 s at the electrode. The red lines indicate fits obtainedby use of the model described in the text, from which rate constantsfor the dissolution of 1.0 × 10-3 cm s-1 for the 5-s potential step and0.9 × 10-3 cm s-1 for the 10-s potential step were extracted.

Figure 6. Interfacial absorbance transient for the electrochemicaldissolution of Ag nanoparticles by use of 0.1 mM IrCl6

3- oxidized for600 s. The red line indicates a fit obtained by use of the model describedin the text, from which a rate constant for the dissolution of 1.1 ×10-3 cm s-1 was extracted.

15278 J. Phys. Chem. C, Vol. 112, No. 39, 2008 Schnippering et al.

Convective Dissolution. An alternative strategy to investigatethe dissolution behavior of Ag nanoparticles was to introducea micropipette close to the prism surface, through which IrCl62-

was directly injected onto the colloidal particles. Figure 7a showsthe concentration dependence of the dissolution rate for aconstant flow rate of 8.3 µL s-1, in which all transients havebeen normalized with respect to their initial absorbance:

Arel )Aexp(t)

A(t) 0)(11)

where Aexp is the absorbance transient and A(t ) 0) is theinterfacial absorbance of Ag nanoparticles before dissolutionbegins. The sensitivity of this approach, with a constant supplyof IrCl6

2-, even at very low concentrations of IrCl62- is clear.

Experiments at higher IrCl62- concentrations demonstrated

complete dissolution of the nanoparticles.Studies at different flow rates of IrCl6

2- solution were alsocarried out and the results are displayed in Figure 7b. Higherflow rates lead to faster dissolution, indicating at least partialcontrol of the kinetics by mass transport of IrCl62- to the surface.For future studies, we plan to establish a finite element model,building onto earlier investigations42 in order to describe thistype of hydrodynamic system more rigorously.

Conclusions

The time-dependent adsorption of Ag nanoparticles on PLL-modified silica and the corresponding surface coverage has beenmeasured accurately by use of EW-CRDS. This technique hasproven very useful as a detection tool for very low surfaceconcentrations of Ag nanoparticles (typically 20-60 particlesµm-2). Additionally, EW-CRDS has been applied successfullyto study the chemical kinetics of nanoparticle oxidative dis-solution. This sensitive technique has been coupled with two

complementary approaches to dissolve immobilized colloidalparticles on functionalized surfaces: electrochemical dissolu-tion via a redox mediator, and convective dissolution with amicrocapillary setup to deliver the oxidant. The heterogeneousrate constant for dissolution was established as 1.0 × 10-3 cms-1.

These measurements illustrate the ability of EW-CRDS toobtain kinetic information, especially on inert supportingsurfaces such as silica or polymer-modified silica, which aretraditionally challenging to study. The combination of EW-CRDS with both chronoamperometric and flow methods showthat the dissolution process can be monitored even with verylow fluxes and low concentrations of reagents, making bothapproaches excellent tools for potential sensing applications.

Acknowledgment. This work was generously supported bythe Engineering and Physical Sciences Research Council underGrant EP/C00907X. H.V.P. is grateful for a studentship underthe MOAC Doctoral Training Centre, and S.R.M. is furthergrateful to the EPSRC for his Advanced Research Fellowship.M.Z. gratefully acknowledges a Marie Curie Incoming Inter-national Fellowship (040126).

References and Notes

(1) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455.(2) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature

2000, 408, 67.(3) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18.(4) Bauer, G.; Pittner, F.; Schalkhammer, T. Mikrochim. Acta 1999,

131, 107.(5) Habicht, W.; Behrens, S.; Wu, J.; Unger, E.; Dinjus, E. Surf.

Interface Anal. 2004, 36, 720.(6) Kohut, A.; Voronov, A.; Peukert, W. Part. Part. Syst. Charact.

2006, 22, 329.(7) Lu, H. C.; Yi, G. S.; Zhao, S. Y.; Chen, D. P.; Guo, L. H.; Cheng,

J. J. Mater. Chem. 2004, 14, 1336.(8) Maxwell, D. J.; Taylor, J. R.; Nie, S. M. J. Am. Chem. Soc. 2002,

124, 9606.(9) Lee, J. S.; Lytton-Jean, A. K. R.; Hurst, S. J.; Mirkin, C. A. Nano

Lett. 2007, 7, 2112.(10) Liu, T.; Zhong, J.; Gan, X.; Fan, C. H.; Li, G. X.; Matsuda, N.

ChemPhysChem 2003, 4, 1364.(11) Chen, Y. C.; Young, R. J.; Macpherson, J. V.; Wilson, N. R. J.

Phys. Chem. C 2007, 111, 16167.(12) Li, X. L.; Xu, W. Q.; Zhang, J. H.; Jia, H. Y.; Yang, B.; Zhao, B.;

Li, B. F.; Ozaki, Y. Langmuir 2004, 20, 1298.(13) Doering, W. E.; Nie, S. M. J. Phys. Chem. B 2002, 106, 311.(14) Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. J. Chem. Soc.,

Faraday Trans. 1996, 92, 3799.(15) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 7764.(16) Rimer, J. D.; Trofymluk, O.; Navrotsky, A.; Lobo, R. F.; Vlachos,

D. G. Chem. Mater. 2007, 19, 4189.(17) Shi, H. Z.; Bi, H. J.; Yao, B. D.; Zhang, L. D. Appl. Surf. Sci.

2000, 161, 276.(18) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481.(19) Chen, S. H.; Akai, T.; Kadono, K.; Yazawa, T. Appl. Phys. Lett.

2001, 79, 3687.(20) Jiang, X. W.; Qiu, J. R.; Zeng, H. D.; Zhu, C. S.; Hirao, K. Chem.

Phys. Lett. 2004, 391, 91.(21) Carvalho, I. C. S.; Mezzapesa, F. P.; Kazansk, P. G.; Deparis, O.;

Kawazu, M.; Sakaguchi, K. Mater. Sci. Eng., C 2007, 27, 1313.(22) Podlipensky, A.; Abdolvand, A.; Seifert, G.; Graener, H.; Deparis,

O.; Kazansky, P. G. J. Phys. Chem. B 2004, 108, 17699.(23) Zhang, M. Q.; Wittstock, G.; Shao, Y. H.; Girault, H. H. Anal.

Chem. 2007, 79, 4833.(24) Zhang, M.; Becue, A.; Prudent, M.; Champod, C.; Girault, H. H.

Chem. Commun. 2007, 3948.(25) Rodriguez-Sanchez, M. L.; Rodriguez, M. J.; Blanco, M. C.; Rivas,

J.; Lopez-Quintela, M. A. J. Phys. Chem. B 2005, 109, 1183.(26) Fan, H. F.; Li, F.; Zare, R. N.; Lin, K. C. Anal. Chem. 2007, 79,

3654.(27) Fisk, J. D.; Batten, R.; Jones, G.; O’Reilly, J. P.; Shaw, A. M. J.

Phys. Chem. B 2005, 109, 14475.(28) Everest, M. A.; Black, V. M.; Haehlen, A. S.; Haveman, G. A.;

Kliewer, C. J.; Neill, H. A. J. Phys. Chem. B 2006, 110, 19461.(29) Fan, H. F.; Hung, C. Y.; Lin, K. C. Anal. Chem. 2006, 78, 3583.

Figure 7. (a) Interfacial absorbance transients showing the dissolutionof Ag nanoparticles in the micropipette setup with IrCl6

2- solutionconcentrations of (A) 0.1, (B) 0.5, or (C) 1 µM. A constant flow ofIrCl6

2-, 16.6 µL s-1, was used for all measurements. (b) Dissolutionof Ag nanoparticles in the micropipette setup with 1 µΜ IrCl6

2- solutionflow rates of (A) 1.7, (B) 8.3, or (C) 33.3 µL s-1.

Assembly and Dissolution of Silver Nanoparticles J. Phys. Chem. C, Vol. 112, No. 39, 2008 15279

(30) Hallock, A. J.; Berman, E. S. F.; Zare, R. N. Anal. Chem. 2002,74, 1741.

(31) Pipino, A. C. R.; Hudgens, J. W.; Huie, R. E. Chem. Phys. Lett.1997, 280, 104.

(32) Mazurenka, M.; Hamilton, S. M.; Unwin, P. R.; Mackenzie, S. R.J. Phys. Chem. C 2008, 112, 6462.

(33) Decher, G. Science 1997, 277, 1232.(34) Schnippering, M.; Carrara, M.; Foelske, A.; Kotz, R.; Fermin, D. J.

Phys. Chem. Chem. Phys. 2007, 9, 725.(35) Cottancin, E.; Celep, G.; Lerme, J.; Pellarin, M.; Huntzinger, J. R.;

Vialle, J. L.; Broyer, M. Theor. Chem. Acc. 2006, 116, 514.(36) Mazurenka, M.; Wilkins, L.; Macpherson, J. V.; Unwin, P. R.;

Mackenzie, S. R. Anal. Chem. 2006, 78, 6833.

(37) de Fornel, F. EVanescent WaVes: From Newtonian Optics to AtomicOptics; Springer-Verlag: Berlin, 1997.

(38) Mazurenka, M.; Wada, R.; Shillings, A. J. L.; Butler, T. J. A.;Beames, J. M.; Orr-Ewing, A. J. Appl. Phys. B: Lasers Opt. 2005, 81, 135.

(39) Liu, X. O.; Atwater, M.; Wang, J. H.; Huo, Q. Colloids Surf., B2007, 58, 3.

(40) Crank, J. The Mathematics of Diffusion; Oxford University Press:Oxford, U.K., 1975.

(41) Adamczyk, Z. J. Colloid Interface Sci. 2000, 229, 477.(42) Bitziou, E.; Rudd, N. C.; Edwards, M. A.; Unwin, P. R. Anal. Chem.

2006, 78, 1435.

JP804615M

15280 J. Phys. Chem. C, Vol. 112, No. 39, 2008 Schnippering et al.