Chemical Sensors and Microinstrumentation - American Chemical

Chapter 19

Construction and Characterization of Optical Sensors with Porous Polymer Film 1

Scott M. Stole, Thomas P. Jones, Lai-Kwan Chau, and Marc D. Porter

Downloaded by CORNELL UNIV on June 12, 2017 | http://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch019

Ames Laboratory-U.S. Department of Energy and Department of Chemistry, Iowa State University, Ames, IA 50011 The range and scope of the applicability of direct dyes as immobilized indicators for the construction of optical pH sensors have been examined. The sensors are fabricated by the immobilization of these indicators at a porous cellulosic film, exhibiting a rapid response time. The rapid response results from the porous microstructure of a cellulose acetate film, which is manipulated by the duration of a base hydrolysis. To gain insights into the optimization of the microstructure of the film, a multifaceted characterization was undertaken. The change in permeability of films coated on a glassy carbon electrode was examined by cyclic voltammetry with electroactive probes of various sizes and shapes. The change in the composition of the film was assessed with infrared external reflection spectroscopy by monitoring the hydrolytic removal of the acetate functional group from the backbone of the cellulosic polymer. In addition to a discussion of the results of these structural studies, the properties of a variety of new pH sensors fabricated from the immobilization of various direct dyes are described. This includes the performance characteristics of a two-component pH sensor - a first step towards the development of an optical device that exhibits a response over a broad pH range. Interest in the properties, structure, chemical modification, and application of thin films of organic polymers has grown enormously in recent years. The impetus for this rebirth derives from the relevance of such materials to adhesion (J.) , microelectronics (2) , lubrication Q) , and biocompatibility (specifically the interface between living and nonliving components as occurs with joint replacements or a r t i f i c i a l hearts (4.5)). These materials have also gained increasing importance in the construction of sensors that are based on electrochemical and Address correspondence to this author.

1

0097-6156/89AM03-0283$06.00/0 c 1989 American Chemical Society Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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spectrochemical d e t e c t i o n schemes. These l a t t e r a p p l i c a t i o n s e x p l o i t the chemical and p h y s i c a l p r o p e r t i e s o f t h i n f i l m s to control the s e l e c t i v i t y and s e n s i t i v i t y of an analysis. For the development and construction o f electrochemical sensors, strategies have focused on manipulating the composition and porosity of "redox" and ion exchange polymer films. Applications include the mediation of the heterogeneous electron transfer of large biomolecules with redox polymers (6) and the preconcentration of metal ions at ion exchange polymers (7). An alternative scheme u t i l i z e s the porosity of c e l l u l o s i c (8.9) and other polymeric materials (10.11) to control the s e l e c t i v i t y and to enhance the s t a b i l i t y of an e l e c t r o a n a l y s i s . Such approaches have been applied to the electroanalysis of H2O2 (8) and low molecular weight organic compounds (9) i n the presence of macromolecules which may adsorb and subsequently poison an electrode surface. Strategies f o r the design and incorporation of polymeric films as components of o p t i c a l sensors have, to a large degree, p a r a l l e l e d those o f t h e i r electrochemical analogs. As i s the case with electrochemical sensors (12), the s e l e c t i v i t y and s e n s i t i v i t y of a chemical analysis with o p t i c a l sensors are governed by a complex mixture of chemical and physical properties and interactions, as w e l l as by i n s t r u m e n t a t i o n performance c h a r a c t e r i s t i c s . Of p a r t i c u l a r p r a c t i c a l and fundamental importance are questions that concern changes i n the r e a c t i v i t y of an immobilized species with respect to i t s solution analog. For instance, how does the mode of attachment a l t e r the r e a c t i v i t y of the immobilized species? What other intermolecular interactions, such as those that are involved i n solvation, require a close examination? To what extent does the molecular a r c h i t e c t u r e of the e l e c t r i c a l double layer influence r e a c t i v i t y ? Answers to such questions, which are also relevant to a substantial number o f surface and material science problems, promise to enhance our a b i l i t y to design sensors with s p e c i f i c performance characteristics. To assess s p e c i f i c a l l y the influence of immobilization on the r e a c t i v i t y of colorimetric reagents used f o r o p t i c a l sensors, i t i s i n s t r u c t i v e to examine the entries i n Table I. This Table, although not exhaustive, l i s t s several c h a r a c t e r i s t i c s of sensors that have r e c e n t l y been developed f o r the determination of pH, metal ions ( M ) , and h a l i d e ions (A"), i n aqueous s o l u t i o n s . Reviews of o p t i c a l sensors f o r gas-phase chemical a n a l y s i s are a v a i l a b l e elsewhere (28). The l i s t i n g includes the scheme for immobilization, the r e a c t i v i t y of the solution and immobilized forms of the ligand, the mode of o p t i c a l analysis, and the response time. Ideally, to f a c i l i t a t e the predictive design of o p t i c a l sensors, the mode of attachment should exert a minimal influence on the r e a c t i v i t y of an immobilized reagent. However, as shown i n Table I, a d i r e c t t r a n s l a t i o n of r e a c t i v i t y from the solution structure to the surface structure i s r a r e l y observed. For example, the pH sensor based on immobilized Congo Red has an acid strength of almost two orders of magnitude greater than the solution form of the indicator. Several of the metallochromic indicators exhibit differences that are even more dramatic; i n f a c t , s e v e r a l completely lose t h e i r chelating a b i l i t y upon i m m o b i l i z a t i o n . P o s s i b l e i n s i g h t s i n t o these differences can be found by an examination of the structure of the n +

Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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immobilized indicator and i t s interactions with the support. For example, the metallochromic azo reagent^ calcichrome (2,8,8 t r i h y d r o x y - 1 , 1 -azonaphthalene-3,6,3 ,6 - t e t r a s u l f o n i c a c i d ) contains four sulfonic- acid functional groups (30), each o f which may p a r t i c i p a t e i n i t s e l e c t r o s t a t i c immobilization at an anion exchange polymer f i l m (24). An examination of a molecular model suggests that the a b i l i t y of calcichrome to adopt a conformation that favors the formation of a metal complex with calcium (II) would be greatly hindered i f the chelate were tethered to the polymeric f i l m by more than one e l e c t r o s t a t i c linkage. This suggests that metallochromic reagents which are tethered through a single linkage may more r e a d i l y undergo a s t r u c t u r a l rearrangement to achieve a conformation that i s favorable to the formation of a metal complex, whereas immobilization through multiple linkages may severely l i m i t the rearrangement requisite for complex formation. Furthermore, the e f f e c t s on the formation of a metal complex a f t e r immobilization should be minimal f o r those ligands, such as q u i n o l i n - 8 - o l - 5 s u l f o n a t e (2_7) , which are n a t u r a l l y f i x e d i n a c o n f i g u r a t i o n r e q u i s i t e f o r complexation. Several other f a c t o r s also merit consideration, including electronic induction e f f e c t s and the ionic c h a r a c t e r i s t i c s of the polymeric support. With the former, the mode of attachment may s u f f i c i e n t l y a l t e r the e l e c t r o n density at a c h e l a t i n g group, t r a n s l a t i n g i n t o a change i n the formation constant, Kf, f o r a complex. For the l a t t e r , the i o n i c charge at a polymer f i l m may influence the p a r t i t i o n i n g or d i f f u s i o n of an ionic species into the f i l m by a charge (Donnan) e f f e c t (31.32). Table I also l i s t s response time as an important performance c h a r a c t e r i s t i c of o p t i c a l sensors. This c h a r a c t e r i s t i c i s p a r t i c u l a r l y relevant i n defining the range and scope of the u t i l i t y of the sensor; that i s , the faster the response, the more widespread its applicability. At present, the response of most sensors i s l i m i t e d by the slow d i f f u s i o n a l mass transport of analyte through an impermeable polymeric support. Recent studies have shown, however, t h a t r e s p o n s e times c a n be d r a m a t i c a l l y d e c r e a s e d by the c o n s t r u c t i o n o f sensors from porous polymeric m a t e r i a l s that minimize b a r r i e r s to the mass transport of an analyte (22). As the previous d i s c u s s i o n s i l l u s t r a t e , the considerations relevant to the optimization of the performance c h a r a c t e r i s t i c s of o p t i c a l sensors represent a complex mixture o f fundamental and p r a c t i c a l issues. Further, as research progresses, i t w i l l become i n c r e a s i n g l y important t o develop d e t a i l e d m o l e c u l a r l e v e l d e s c r i p t i o n s o f the m i c r o s t r u c t u r e o f these sensors. Such d e s c r i p t i o n s w i l l p r o v i d e a d i r e c t c o r r e l a t i o n between the i n t e r f a c i a l structure, r e a c t i v i t y , and performance, enhancing the a b i l i t y to design sensors with new and/or improved performance characteristics. Thus, future progress w i l l demand an increasingly i n t e r d i s c i p l i n a r y research strategy, drawing from s p e c i a l t i e s such as s y n t h e s i s , t h e o r y , i n s t r u m e n t a t i o n , and m a t e r i a l s characterization. In a recent communication (22), we described the f a b r i c a t i o n and preliminary evaluation o f a chemically modified c e l l u l o s e acetate f i l m as an o p t i c a l sensor f o r the selective determination of pH. This sensor was fabricated by the immobilization of Congo Red at a porous c e l l u l o s i c f i l m . Advantageous features of t h i s design



f

d

C

e

Phenol red/ acrylamide Bromophenol blue/ XAD Bromocresol purple/ XAD Bromothymol blue/ XAD Thymol blue/XAD Phenolphthalein/XAD Thymolphthalein/XAD HPTS /SG HCC /SG Fluorésceinamine/SG/ c e l l u l o s e acetate Fluoresceinamine/ acrylamidemethyl acrylamide HOPSA/anion exchangeS Phenol red/ acrylamidemethylacrylamide Fluor e see ine isocyanate/glass Congo red/cellulose acetate

b

1.1

£0.15

reflectance reflectance reflectance reflectance fluorescence fluorescence fluorescence fluorescence

7.7 9.6 10.2 11.0 6.8 6.9

— —

7.1 8.9 9.6 9.3 7.3 7.0

—

—

H+ H+ H H H H H

0.8 2.8

—

+

H

adsorption

covalent

0.15 fluorescence

5.9

6.5

+

H

0.005

covalent

-7.4

7.9

+

H

22

21

20

19 0.4 0.03

fluorescence fluorescence fluorescence

7.6

7.3

+

electrostatic

14,15 14 14 14 16 16 17

14

14

13

18

H

transmission

, and A")

Reference

n

covalent

H

0.8 0.8 0.3

+

+

+

+

+

+

adsorption

—

reflectance

5.8

6.1

+

H

adsorption adsorption adsorption adsorption covalent covalent covalent

adsorption

—

reflectance

3.7

4.1

+

H

— — —

adsorption

0.7

reflectance

7.6

Mode of Immobilization

7.9

Estimated Response Time (min.)

+

Optical Mode of Analysis

H

Reactivity Sensor Composition (Indicator/Support) Analyte Solution Immobilized Analog

0

Table I. C h a r a c t e r i s t i c s o f O p t i c a l Sensors f o r the Chemical Analysis of Aqueous Solutions (Η , M



H

h

i

i

Zn(II)

Cu(II)

F"

j

i

Hg(II)

4.6

h

h

3.7

j

j

j

i

—

7.0

—

h

Hg(II)

3.1 11.8 5.3 10.7 4.0 4.5 4.9 6.5 8.0 7.1 1.5

10.6

3.9

7.2 11.5 5.4 6.3 4.0

—

10.3

Be(II) Al(III) Mg(II) Al(III) Zn(II) Cd(II) Ca(II)

+

+

H

+

H

reflectance

reflectance

reflectance

reflectance

reflectance

fluorescence fluorescence fluorescence fluorescence fluorescence fluorescence reflectance

reflectance

transmission

transmission

electrostatic electrostatic

— —

adsorption

electrostatic

—

-12

electrostatic

—

electrostatic

electrostatic

covalent

electrostatic

— 1.3 0.7 -1 -1 -1 ~1 0.06

adsorption

adsorption

—

—

28

24

24

24

24

25 26 27 27 27 27 24

24

23

23

a

f

a)Reactivity indicators f o r hydrogen ion given as pK . Those for metal i o n indicators given as log K . b) Estimated from the l i t e r a t u r e as the time required f o r the o p t i c a l change to reach 63% of i t s e q u i l i b r a t i o n value, i e . 1-1/e. c) XAD - styrene-divinylbenzene co-polymer, d) HPTS - l-hydroxypyrene-3,6,8-trisulfonate, e) SG - sintered glass, f ) HCC - 7-hydroxycoumarin-3-carboxylic acid, g) HOPSA - 8-hydroxyl1,3,6-pyrenetrisulfonic acid, h) Calculated f o r dissolved 1:1 metal-ligand complex from formation constants at an i o n i c strength of 0.1 (31). i ) Observable reaction, j ) No observable reaction.

Calcichrome/anion exchange Beryllon II/anion exchange Congo Red/anion exchange Zincon/anion exchange Fast Sulphon Black F/anion exchange Alizann Complexone Ce(III)/XAD

Quinolin-8-ol-5sulfonate/anion exchange

Direct Blue 8/ c e l l u l o s e acetate Direct Orange 8/ c e l l u l o s e acetate Calcichrome/anion exchange Morin/cellulose


^

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included: 1) a rapid response time ( 4 pH u n i t s ) , 3) a s e l e c t i v e response, and 4) ease of fabrication. The r a p i d response r e s u l t e d from the porous microstructure of the polymeric support, which was achieved by a c a r e f u l l y controlled base hydrolysis. This porous microstructure minimized the b a r r i e r s to the mass transport of the analyte to the immobilized indicator. The large dynamic range resulted from both the p o l y p r o t i c acid-base r e a c t i v i t y of Congo Red and the high o p t i c a l absorptivity of i t s various ionic forms. A d d i t i o n a l l y , t h i s sensor was e a s i l y fabricated by the immersion of the hydrolyzed f i l m i n t o an e l e c t r o l y t i c dye bath which contained Congo Red. Furthermore, this sensor exhibited a response that was s e l e c t i v e only to hydrogen ion. This was a consequence of the immobilization chemistry which apparently prohibits the amine groups of Congo Red from p a r t i c i p a t i n g i n the formation of a metal complex. As part of our on-going e f f o r t s to develop novel design and f a b r i c a t i o n schemes to enhance sensor performance, we have continued to explore the u t i l i t y of porous polymeric f i l m s as support materials for immobilized colorimetric reagents. In t h i s paper, we report the r e s u l t s from a multifaceted c h a r a c t e r i z a t i o n of the porosity and composition of t h i n films of c e l l u l o s e acetate as a f u n c t i o n o f the d u r a t i o n o f the b a s e h y d r o l y s i s . This characterization was undertaken to correlate the microstructure and composition of the f i l m with changes i n i t s permeability, thereby providing insights for the optimization and the extension of the a p p l i c a b i l i t y of this material as a support for sensor f a b r i c a t i o n . The change i n permeability was characterized by c y c l i c voltammetry (CV) with electroactive probes of various sizes and shapes. This was f a c i l i t a t e d by coating the f i l m onto a glassy carbon electrode. The change i n f i l m composition was examined with i n f r a r e d external r e f l e c t i o n spectroscopy (IR-ERS). These measurements focused on monitoring the hydrolytic removal of the acetate functional groups from the backbone of the c e l l u l o s i c polymer. In addition to these s t r u c t u r a l studies, the results from an exploration of the range and scope of dyes s i m i l a r to Congo Red, v i z . other d i r e c t dyes, as immobilized c o l o r i m e t r i c reagents f o r o p t i c a l sensors w i l l be described. A goal of this l a t t e r e f f o r t was to i d e n t i f y and to test dyes that could be incorporated as part of a multi-component sensor which would e x h i b i t a response over a broad pH range, i . e . something akin to " o p t i c a l l y transparent litmus paper". EXPERIMENTAL Preparation of Cellulose Acetate Films. Cellulose acetate films ( A l d r i c h , Inc., Milwaukee, WI) were c a s t onto e i t h e r GC-20 electrodes (Tokai Carbon Co., Tokyo, Japan) or microscope s l i d e s by spin-coating techniques. This polymer has a 39.8% acetyl content (by weight) which represents an average of 2.45 a c e t y l groups per glucosidic u n i t . Sheets of GC were cut to provide plates that were approximately 2.5 by 2.5 cm. The plates were successively polished with s l u r r i e s of 600-grit s i l i c o n powder and 1 μιη, 0.3 μπι, and 0.05 μτα alumina (Buehler Ltd., Evanston, IL) u n t i l a m i r r o r l i k e f i n i s h was obtained. Between each p o l i s h i n g step, the substrates were



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washed i n an ultrasonic bath with deionized water ( M i l l i p o r e Corp., Bedford, MA). Glass substrates were c a r e f u l l y degreased with acetone. Films were prepared by flooding a surface with a solution of the polymer and spinning at 1000-4000 rpm. Thicknesses were c o n t r o l l e d by the v a r i a t i o n of the concentration of c e l l u l o s e acetate i n cyclohexanone (1-10% w/v) and by the a l t e r a t i o n of r o t a t i o n rate. A f t e r casting, the films were stored under a watch glass and allowed to dry for 24 h. Film thicknesses were measured p r i o r to h y d r o l y s i s with an Alphastep surface p r o f i l e r (Tencor Instruments, Mountainview, CA). A k n i f e edge was used to remove several portions of the f i l m from the s u b s t r a t e with the r e s u l t i n g step used f o r the thickness measurements. Thicknesses are reported as the average from at least three d i f f e r e n t locations on the f i l m ; the uncertainty i s reported as the range of these measurements. Base-Hydrolysis of Cellulose Acetate Films. The c e l l u l o s e acetate f i l m s were h y d r o l y z e d by immersion i n 0.070 M KOH at 25°C. Immersion times varied from intervals of a few minutes up to 24 h. Upon removal from the hydrolysis bath, the samples were immersed i n a c o l d water bath to quench the r e a c t i o n . A 24 h h y d r o l y s i s r e s u l t e d i n -47% decrease i n the mass of the f i l m and a -53% decrease i n thickness. Dve Immobilization. The acid-base indicators were immobilized at the hydrolyzed c e l l u l o s e acetate films v i a a conventional dye bath recipe (33). This consisted of immersing a hydrolyzed f i l m into an e l e c t r o l y t i c dye bath of approximately 2 mM Na2S0^ and 2 mM K2CO3 f o r 10 min. Dye concentrations were 0.1-1.0 mM. In addition to Congo Red (Direct Red 28), several other dyes were tested including the Direct dyes: Blue 8, Brown 2, Orange 8, Red 1, Red 2, Black 38, Red 13, Yellow 4, Yellow 12, Yellow 44, V i o l e t 1, Red 75, Red 126, Blue 78, Orange 6, and Green 6. A chemical and s t r u c t u r a l description of these and other d i r e c t dyes can be found i n reference 34. along with their IUPAC names. Based on the length of this l i s t i n g , further d e t a i l s regarding the structure of these dyes w i l l be described only when germane to the discussion. Colorimetric indicators such as Congo Red are known as d i r e c t dyes (33.35). This terminology derives from the a b i l i t y of these dyes to adsorb s t r o n g l y to c e l l u l o s e - b a s e d polymers simply by immersing the polymer into a hot dye s o l u t i o n at a high ionic strength. The factors that govern the formation and adsorption strength of such dyes are a complex mixture of chemical, physical, and s t r u c t u r a l e f f e c t s . For Congo Red, i t i s apparent that the chemical i n t e r a c t i o n s between i t s amine groups and the hydroxyl groups of the c e l l u l o s i c support play an important role i n the f o r m a t i o n o f the immobilized s t r u c t u r e s (3j6) . The r e l a t i v e importance of the molecular planarity (flatness) and s o l u b i l i t y , both of which have been found to contribute at d i f f e r i n g degrees to the binding strength of such dyes, remains the subject of extensive controversy (35.37).


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Instrumentation. Infrared spectra were obtained with a Nicolet 740 Fourier transform IR spectrometer ( N i c o l e t Inst., Madison, WI). R e f l e c t i o n measurements employed a m o d i f i e d , v a r i a b l e angle r e f l e c t i o n accessory (Harrick S c i . , Ossining, NY) set to an angle of incidence of 60°. The fundamental considerations f o r performing IRERS measurements at materials with a low IR r e f l e c t i v i t y , such as GC, have been previously described (38) · An aluminum wire g r i d p o l a r i z e r on KRS-5 (Cambridge P h y s i c a l Sciences) was p l a c e d immediately before the accessory to provide s e l e c t i o n of ρ-polarized light. S p e c t r a were o b t a i n e d i n e i t h e r a r e f l e c t i o n or a transmission mode with a l i q u i d nitrogen-cooled, narrow-band HgCdTe detector. After Happ-Genzel apodization, the spectral resolution was nominally 4 cm . The spectrometer and sample chamber were purged with b o i l o f f from l i q u i d N2. Spectra are presented as the r a t i o of 1024 sample to 1024 reference scans. For the IR-ER s p e c t r a , the y - a x i s i s defined as -log(R/R ), where R i s the r e f l e c t i v i t y of the coated electrode and R i s that f o r a bare electrode. A l l samples were c a r e f u l l y rinsed and dried under a stream of dry N2 gas before placement i n the spectrometer. The UV-VIS data were obtained with a DMS-200 spectrometer ( V a r i a n Instruments Co., Palo A l t o , CA) . A flow c e l l (22), configured i n a conventional transmission mode, was mounted to the base plate of the spectrometer. At a solution flow rate of 1.0 mLs , the time required to e f f e c t a complete change (99.9%) i n the o p t i c a l signal was 1.22 ± 0.18 s. C y c l i c voltammetric measurements were made with a BAS CV-27 potentiostat (Bioanalytical Systems, West Lafayette, IN). A Ag/AgCl electrode (saturated with KC1) served as the reference. Q

Q

Reagents. A l l reagents, except Congo Red, were used as received. The Congo Red was p u r i f i e d by r e c r y s t a l l i z a t i o n i n absolute ethanol, and was determined to be 97X pure by potentiometric t i t r a t i o n . The pH of the s o l u t i o n s was c o n t r o l l e d with HC1 or NaOH or with citrate/phosphate buffers. The ionic strength of the solutions was maintained with 0.5 M KC1. RESULTS AND DISCUSSION Characterization of Microstructure and Permselectivitv of Cellulose Acetate Films at Glassy Carbon. In t h i s section, the results from a s t r u c t u r a l characterization of t h i n films of c e l l u l o s e acetate at GC are presented. Cyclic voltammetric measurements were used to characterize the change i n permeability as a function of hydrolysis time. The corresponding compositional changes of the f i l m were examined with IR-ERS. A goal of t h i s study was to correlate the changes i n the permeability of the f i l m with those of i t s composition, providing insights into optimizing t h i s material as a support f o r sensor f a b r i c a t i o n . A second goal was to determine the hydrolysis time required to remove exhaustively the acetate groups from the f i l m . E a r l i e r studies (23) had indicated that the s t a b i l i t y of the response of these sensors i n strongly a c i d i c solutions was dependent on the duration of the base hydrolysis. Since an acid hydrolysis represents another pathway f o r


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the removal of an acetate moiety from the f i l m , i t was apparent that the exhaustive depletion o f t h i s moiety was r e q u i s i t e f o r the attainment of a stable sensor response. Electrochemical Characterization. F i l m s o f impermeable, e l e c t r i c a l l y i n s u l a t i n g materials represent a substantial b a r r i e r to heterogeneous electron-transfer (39.40). Ideally, electron-transfer through a pinhole-free f i l m o f such a material should proceed through a highly nonadiabatic pathway with the k i n e t i c s e x h i b i t i n g an exponential dependence on the separation between the electron donor and electron acceptor. Cellulose acetate, however, i s well known f o r i t s permselective p r o p e r t i e s , which have l e d to i t s widespread a p p l i c a t i o n as a d i a l y s i s membrane (41.42). Recent studies have also shown that a controlled hydrolysis of such films regulates their permselectivity based on the s i z e and shape of a solute (41.43). By employing electrochemical probes of d i f f e r e n t s i z e s and shapes, Wang and Hutchins (9) have characterized the change i n porosity of these films as a function of hydrolysis time. In our studies, the permeability of c e l l u l o s e acetate films was monitored as a function o f hydrolysis time with c y c l i c voltammetry (CV). The electrochemical probes were 1,4-hydroquinone (HQ), the cofactor nicotinamide adenine dinucleotide (0-NADH), and Fe(CH)g . Table II provides estimates of the molecular dimensions f o r each of these probes.

Table I I . Probes

Estimated Molecular Dimensions of the Electrochemical

Dimensions ( À ) Probe Species Hydroquinone 4

Fe(CN) " 6

0-NADH

a)

Length

Width

a

Thickness

Molecular Volume(Â ) 3

Molecular Weight

8.4

6.6

3.4

190

110.1

9.4

9.4

9.4

830

212.0

18.2

10.7

5.4

1050

709.4

Dimensions estimated from tabulations of Van der Waals and covalent r a d i i (44) where the length equals the a axis, width the b axis, and thickness the ç axis (45).

Figure 1A shows the normalized electrochemical charge, Q ' t as a function of hydrolysis time f o r a 170 ± 10 nm f i l m of c e l l u l o s e acetate a t GC. The ordinate, Q' , i n Figure 1A equals K'(Q /n), where K' equals the r a t i o of the concentrations o f 0-NADH or Fe(CN)g " to that of HQ, and η i s the number of electrons generated by the oxidation reaction. The η value f o r HQ and 0-NADH equals two, whereas that f o r Fe(CN)g " equals one. The change i n Q' i n Figure 1A v e r i f i e s the size discrimination of mass transport through the hydrolyzed films. At short hydrolysis times (

Chemical Sensors and Microinstrumentation - American Chemical

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