Material Design for Optical Sensor Applications

Fiona Regan School of Chemical SciencesDublin City University

Outline

Sensors and their applicationsPrinciples of ATR-FTIR spectroscopyUV-Visible sensingExamples of materials for sensingAnalytes determined using polymer-based sensorsNew materialsChallenges

Design of a sensor for a particular application:

Dictated by the nature and requirements of that application.

Ideally adequate sensitivity broad dynamic rangeHigh selectivity towards the species of interestImmunity to sample-matrix interferences should also exist. Suitable for multicomponent measurement, have fast and

reversible responseexcellent long-term stability. The ideal sensor robust, reliable, simple, economical to

fabricate, of small size and with self-calibration capabilities.

Sensor principle

The active surface of a chemical sensor contains the detection component e.g. a polymer layer

Interaction between the active layer and the analyte (s) being measured that is detected by the transducer.

The change in the transducer due to the active surface event is expressed as a specific signal.

Principle of ATR spectroscopy

Infrared light propagating in a crystal of high refractive index is internally reflected.

Some of the light penetrates into the sample region outside the crystal in the form of an evanescent wave.

Analyte absorption spectra can therefore be recorded

Evanescent wave

ZnSe crystal

Radiation from IR source To

detector

CHC

Evanescentwave

Aqueous phase containing CHC

~5 m

Figure

Evanescent field sensing (EFS)

EFS with optical fibres is an extension of the established spectroscopic measurement ATR.

Routinely applied to the measurement of aqueous systems / analytes.

IR radiation is coupled to an ATR element (fibre/crystal) which is non-absorbing and has a higher refractive index than the surrounding medium.

The medium is a thin film or the absorbing analyte.

Polymer-ATR spectroscopy

Removal of background water absorption is required to enable the detection of weaker signals at very low concentrations of analyte.

This can be overcome by coating the internal reflection element (crystal or fibre) with a hydrophobic polymer.

The polymer also serves to enrich the analytes within the penetration depth.

Role of the polymer

PIB film

ZnSe crystalFrom IR source

To detector

CHC

Evanescentwave

Aqueous phase

Penetration depth

Fibre optic evanescent wave sensor - FEWS

Polymer selection criteria

No, or only weak, intrinsic polymeric IR bands in the region of interest;

Substances to be analysed must be reversibly absorbed in the film;

The time constant for the enrichment process should be low; The polymer should be easily prepared and be chemically

inert with respect to the analyte components; The polymer material must be resistant against water and

organic compounds; Must adhere well to the internal reflection element.

Example 1: Teflon AF

window

Teflon

Regan et al. Vibrational spectroscopy 14 (1997) 239-246

FEWS - Effect of film thickness on analyte diffusion rates

0 200 400 600 8000.000

0.001

0.002

0.003

0.004

90% Saturation (T 90)

2.4 m

1.4 m

Abs

orba

nce

Time / seconds

Teflon sensor reproducibility

FEWS - Simultaneous analysis of 6 chlorinated hydrocarbons

0

.005

.01

950 900 850 800 750

Wavenumbers cm-1

Abs

TCE

TeCE

TCE TCB

TeCE

Cf

DCB

CB

10 mins enrichment time, 32 scans, 60 ppm each standard.

Direct aqueous analysis using Teflon-coated ATR crystal (stopped flow)

Compound LOD/ppm

TCE 5

TeCE 1

Cf 10

CB 5

1,2-DCB 2

Real sample analysis - Tolka River, Dublin

0

.001

.002

.003

.004

950 900 850 800 750

Abs.

Wavenumber cm-1

Real sample

60 ppm TCE standard

935 cm-1

Example 2: PVC - plasticised

A. Adipic acid derivatives (1, 3,5,6,7,10,26,27)B. Azelaic acid derivatives (2,4,8,9)C. Epoxy derivatives (11,22)D. Lauric acid derivatives (12)E. Mellitates (13,14)F. Palmitic acid derivatives (15,16G. Phthalic acid derivatives (17,30)H. Sebaic acid derivatives (18,19)I. Stearic acid derivatives (20,21)J.Oleic acid derivatives (24)K. Linoleic acid derivatives (29) L. Isophthalic acid derivatives (28) M. Isobutyrate derivative (25)

Pesticide determinations

Walsh et al. Analyst 121 (1996) 789-792

Regan et al. Anal. Chim. Acta 334 (1996) 85-92

Plasticisers

Plasticisers

Determination of BTEX compounds

Xylene isomers

Multicomponent analysis

Individual MixtureAnalytes t 90 (mins)Absorbance t 90 (mins) AbsorbanceBenzene 2.7 0.178 3.375 0.21Toluene 5.4 0.23 8.775 0.289Ethylbenzene 1.35 0.495 11.475 0.322o-xylene 2.7 0.495 20.25 0.463m-xylene 2.025 0.584 18.9 0.28p-xylene 1.35 0.466 20.25 0.385

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1 2 3 4 5 6 7 8 9 10 1112131415161718192021222324252627282930

Plasticiser

t 90

(min

s)

TolueneTeCE

Comparison of enrichment time for TeCE and toluene into 30 plasticised PVC

phases (65% plasticiser)

Example 3: Poly (isobutylene)

Gas-flow system for analysis of aqueous samples by sparging over PIB coated ATR crystal

ZnSe crystal

PIB film

Steel air-flowcell (Volume = 1 ml)

Gas out

Gas in

Sparging system for aqueous analysis

Why ?Aqueous solutions have proven to have deleterious

effect on polymer films. Ingress of water causes a shift in baseline thereby

lowering sensitivity due to increased noiseWater -OH bands interfere with measurements in the

fingerprint region even if hydrophobic polymers are used.

Instrumental set-up for direct sparging method of analysis

Zero-grade air

Flow meter

CHC solution

Steel gas-flow cell

Sorbenttube

Multi-component analysis by spargingSparging @ 50oC, 0.02 L/min, 6 minutes enrichment.

(Concentrations from 2-12 ppm)

Analysis of solvent residues in pharmaceuticals using sparging

Chloroform

Tablet sample batch analysis for chloroform residuesTablets crushed, dissolved in water, sparged

Sparging @ 18oC, 1 L/min

Example 4: Ethylene propylene copolymer

(CH CH) X

CH

(CH CH ) Y (CH CH) N

CH

CH

O Si

CH

CH

Ethylene-propylene co-polymer(E/Pco)

Poly(isobutylene)

Poly(dimethylsiloxane)(PDMS)

2 2

3

3

3

2 2

3

3

(PIB)

Binary Studies

Optical sensing materials for metal ion determinations

Azo DyeIonophorePlasticiserPVCSolvent

Dyes for PVC Films

N N

Br

NN(C2H5)2

OH

OH

CH3

COOH

Cl Cl

SO3H

O

CH3

COOH

C

BrPADAP

CAS

 Figure 3.8 Structure of KTCPB

B

Cl

Cl Cl

Cl

K+

-

Ionophore

Visible spectrum for metal-dye (BrPADAP) complex

Metal-CAS sensing

Real Sample Analysis – Pb(II) determination

Reversibility of metal films

Selective responses

Effect of pH on sensing films

Principal component Analysis (PCA)

New materials based on molecular imprinting

Molecular imprinting: Sensor materials Solid phase extraction materials HPLC packed columns Imprinting process

Functional monomerCross linkerTemplateInitiator

MIP for caffeine

NMR Mole ratio plot

Ibuprofen - MIP

Novel Antifouling CoatingsPVC compositesWhy PVC?Optically transparentLow bacterial adhesion Can be spin coatedDecided to study effect of using

different plasticizers on antifouling properties of PVC

PVC and PlasticizersPlasticizers used to improve the flexibility

of the material. These molecules form secondary bonds

to the polymer chains and thus, spread them apart. (ester group has van der Waals interaction with H-Cl bond in PVC, linear chain acts as buffer between the polymer chains)

This results in increased permeability of the polymer

PublicationCritical ReviewJ. Environ. Monit., 2006, 8, 880 - 886, DOI: 10.1039/b603289cAntifouling strategies for marine and

riverine sensors

Aine Whelan and Fiona Regan

Assessment of FoulingThe change in mass of coated slides was

recorded after removal from tank.

The change in absorbance of films was recorded after removal from tank.

The quantity of biofilm on slide was determined by staining method reported by Tsai et al.

SEM images of the films were recorded after removal from tank.

Ref. C.L. Tsai, D.J. Schurman, R. Lane Smith, J. Orthopaedic Research, 1988, 666.

Characterisation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 300 400 500 600 700 800

w avelength(nm)

abso

rban

ce

UV-Vis spectrum of films prior to exposure

Type of film Contact Angle

PVC/Tridecyl adipate

84.6±2

PVC/Diethyl succinate

78.4±10

PVC/Ethyl hexyl sebacate

83.9±3

PVC/Diisononyl adipate

84.9±2

PVC/Tridecyl adipate PVC/Ethyl hexyl sebacate

PVC/Diisononyl adipate PVC/Diethyl succinate

SEM Images of Films

Optical density of biofilm on coated slides after exposure to river water (14 days)

00.2

0.40.60.8

11.2

1.41.6

type of coating

optic

al d

ensi

ty

Change in mass of PVC/plasticizer coated slides after exposure to sea water (14 days)

0

2

4

6

8

10

12

14

16

18

20

Ethylhexylsebacate

Diethylsuccinate

Diisononyladipate

Tridecyl adipate Glass

type of coating

perc

enta

ge c

hang

e in

mas

s

Change in absorbance of PVC/plasticizer coated slides after exposure to sea water

0

100

200

300

400

500

600

700

800

900

1000

Glass Diethylsuccinate

Ethylhexylsebacate

Diisononyladipate

Tridecyladipate

type of coating

perc

enta

ge c

hange a

bsorp

tion

Conclusions

In the case of the river water, there was no clear difference in the antifouling performance of the different PVC films.

In the case of sea water, the PVC/tridecyl adipate film showed the best antifouling performance.

This plasticizer has the highest molecular weight and is expected to be less easily leached from PVC. It is proposed that this allows the PVC film to retain flexibility and toughness for longer.

It is interesting that for biofilm assay, in river water and sea water, PVC/tridecyl adipate performed best. Perhaps, tridecyl adipate has greater biocidal properties than the other plasticizers.

PVC and plasticizers and surfactant

Add CTAB to polymer film (0.1% w/v)Why CTAB?Quaternary ammonium compounds

are widely employed as disinfectants due to their antimicrobial properties

ConclusionsAs for PVC/plasticizer films, there was no clear

difference in the antifouling performance of the PVC/plasticizer/CTAB composites.

However, for Sea Water, the PVC/ethyl hexyl sebacate/CTAB film showed best antifouling performance.

Perhaps the PVC/ethylhexyl sebacate/CTAB film is more porous than other films, hence, CTAB can be more easily leached.

Porosity measurements are being undertaken at present.

Acknowledgements

Funding: Strand I, Enterprise Ireland Strategic Grant & Basic Grant

Researchers: Eoin O’Donoghue Ambrose Hayden Kathleen O’Malley Fiona Walsh Keith Farrington

Collaboration with:Prof Boris Mizaikoff and Dr Edmond MagnerMSSI & NCSR