Hybrid Analytical Techniques
Spectroelectrochemistry The Combination of Optical and Electrochemical Techniques William R. Heineman University of Cincinnati, Cincinnati, OH 45221 Two quite different techniques, electrochemistry and spectroscopy, can he combined for studying the redox chemistry of inorganic, organic and biological molecules (1-8). Oxidation states are chaneed electrochemicallv " -hv.addition or removal of electrons at an electrode while spectral measurements on the solution adiacent to the electrode are made simultaneously. Such "spec~roelectrochemical" techniques are a convenient means for ohtainine sDectra and redox uo-
Optically Transparent Electrodes The initial development of spectroelectrochemistry was stimulated by the availability of optically transparent electrodes (OTEs), which enable light to he passed directly through the electrode and adjacent solution (9). Electrode transuarencv is necessarv for several snectroelectrochemical tech&pes. " One tvoe of OTE consists of a verv thin film of conductive materiaikch as Pt, Au, SnOy, C, or Hg-coated P t which is de~ositedon a t r a n s ~ a r e nsuhstrate t such as elass or nlastic (vikhle), quartz (uV-visible) or Ge (infraredjdepenling on the s ~ e c t r areeion l of interest. The transDarencv (20-85'3) of these electrodes is due to the thinness i100-5000 A) of the conducting film.
SCATTERING SPECTROSCOPIES
Figure 1. SpenroelectrochemicaI techniques. [Adapted from (4). wilh permission.]
The transparency of a second type of OTE is due to small holes in the electrode. Two examples are the minigrid electrode, which consists of a metal (Au, Ni, Ag, or Hg-coated Au) micromesh of 100 to 2000 wires per inch, and porous reticulated vitreous carbon.
Representative Applications of Spectroelectrochernistry Technique Electrochemical
Optical
Chemical System
Reference
P',n, spectra of technetium complexes p', n, spectra of blue copper proteins
28 29 30
Controlled potential, thin-layer
UV-vis absorption
Controlled potential, semi-infinite diffusion
Hydrolysis reaction of a diimine electrogenerated from 5.6diaminouracil Spectroelectrochemistry in melts CO, UV-vis absorption, fluorescence Circular dichroism. light induced absorption changes. and flUOreScence yield changes of photosynthetic electrontranspon components UV-vis-IR absorption Spectra of ninhydrin reduction products Fluore~cence Fluorescence of o-talidine UV-vis-IR absorption by specular Spectra of 9.10-diphenyl anthracene and barbital electrolysis reflectance products UV-vis absorption Dimerimtion of electrogenerated 4.4'-dimethonystibene cation radical
Derivative UV-vis absorption Internal reflectance, UV-vis Resonance Raman
Controlled potential coulometry, Stirred solution (indirect coulometric titration)
Vis absorption
31 23
32 33 43 34
radical Heterogeneouselectron transfer of cytochrome c Homogeneous electron transfer reactions Spectra of electrogenerated tetracyanoethylene radical anion Surface enhanced spectra of cytochrome c and myoglobin adsorbed on Ag electrode f?'. n for complex IV in intact mitochondria and submitochondrial panicles P ' ,n for cytochrome c oxidase components
Volume 60 Number 4
April 1983
305
Optical Absorption Spectroelectrochemistry
The most frequently used optical technique in spectroelectrochemistry is absorption spectroscopy in the ultraviolet, visible, or infrared region. Absorption spectroscopy can he combined with electrochemistry in a number of ways as described below. Thin-Layer Spectroelectrochemistry. One of the most generally useful spectroelectrochemical techniques involves observation of a thin layer of solution ( 5 0.2 mm) that is confined next to a transwarent electrode as shown in Figure 1A (10). The optical beam of the spectrophotometer is passed directly through the transparent electrode and the solntion. Thin-layer electrochemistry offers a simple way of controlling the oxidation state of species in a very small volume of solution for simultaneous spectral observation. The redox potential of the thin layer of solution is adjusted precisely by the applied potential as determined for the reversible system O+eaR
(1)
by the Nernst equation 0.059
E = E O.R ' +-log-
[0]
n LRI Although the applied potential, E, controls the ratio [[O]/[R]] a t the electrode surface, the ratio in the thin solution layer quickly adjusts to the same ratio by electrolysis. A spectropotentiostatic technique has been developed for obtaining spectra, formal reduction potentials (Eo'), and electron stoichiometries (n-values) of redox couwles (11.12). The redox couple is converted incrementally from one oxidation state to another by a series of applied potentials for which each corresponding value of [O]/[R] is determined spectrally. Each potential is maintained until electrolysis ceases so that the equilibrium value of [O]/[R] is established as defined hv the Nernst eauatiou. A nernstian plot can then he made from the values of E and the corresponding values of [O]l[R]determined spectrally. An example of this technique is its application to the complex [Tc(III)(dmpe)2Br2]+where dmpe is 1,2-bis(dimethy1phosphino)ethane(13). Spectra for a series of potentials are shown in Figure 2. A nernstian plot
of this data (Fig. 3) gives values for the Tc(III)/Tc(lI) couple of E"' = -108 mV versus AgIAgC1 and n = 0.98 from the intercept and slope, respectivdy.. Many biological redox species such as heme proteins and ferredoxins undergo electron transfer at an electrode at a slow or negligible rate. Such behavior is often attributable to insulation of the redox center from the electrode by the surrounding protein structure. In such cases another redox agent, a mediator-titrant. can he added to convev electrons between electrode and biocomponent, i.e., couple the solution potential to the electrode potential. Reductionloxidation of the biocomponent (B) is thus indirect through the mediator-titrant (M) as shown below for a reduction.
E"' and n for the biocomponent are determined from a nernstian plot of the electrode potentials and the corresponding values of the ratio [O]/[R]for B determined from the spectra of the hiological species recorded for each potential, as described above for the technetium complex. This technique has been used with biological redox components such as cytochrome c (11) The thin-laver electrode is also useful for studvina homoabout 30 sec to convert completely all of species 0 in the thin solution laver to species R. The electrode is then disconnected and the reactionif R monitored optically by light passing through the electrode and solution. Semi-infinite Diffusion Spectroelectrochemistry. The cell for this technique (Figs. lB, LC, ID) is similar to a conventional electrochemical cell in that the electrode is in contact with an electrolyte solution that is much thicker than the diffusion layer adjacent to the electrode such that a condition of semi-infinite diffusion exists for the electroactive species exists. Such a cell is analogous to a standard l-cm cuvette with one of the optical faces being a transparent electrode. This spectroelectrochemical technique has been used pri-
WAVELENGTH, nrn Figure 2. Spectra recorded during thin-layer spectropotentiostatic experiment on 1.04 mM [Tc(lll)(dmpe)2Br,]+, 0.5 MTEAP in DMF for various applied potentials, mV versus AglAgCl.
306
Journal of Chemical Education
POTENTIAL, mV vs.Ag/AgCI Figure 3. Neinstian plot of data at 499 nm in Figure 2.
marily to measure fast homogeneous chemical reactions of an electrogenerated species. Consider the situation in which species 0 is present in an unstirred solution that is contacting an optically transparent electrode through which a light beam is nassed as shown in Fieure 1B. Suecies R is electroeenerated
These techniques are capable of monitoring extremely fast reactions with rates up to the diffusion-controlled limit. A variety of chemical reactions have been studied by the various spectroelectrochemical techniques. Indirect Coulometric Titration with Optical Monitoring. The titration of molecules hy reductant or oxidant that is quantitatively generated at an electrode is commonly known as coulometry. When the generating electrode is optically transuarent. the course of the titration can be monitored ODsurlvcu~ert+lwta I hi: gt u t ~ ; ~ tni dt l 1 >IT ,I r;ut(;i I t . t t rn>ineilh? ticdll) by l ~ y l pas4i11g ~t tllrtmgh 111, ('lt,ctr~~It, i~ndthr i r i r r ~ d the diitmim u i 0 I V t l ~ e l r c ~ w d e.iurlJce. 'rll., d h w h ~ ~ ~ w e - v l ~ ~ t i tut ~' i i~ 1. HI. .I tirmrim nlrve I* uhtuined I N uluttiur the time behavior is described by absorbance-at the monitored wavelength as t b t i o i of charge passed through the cell. The charge is proportional to the amount of titrant generated. The shape of the titration curve is determined by the optical properties, redox potentials, where ERis the molar absorptivity of the optically monitored and n values of both the molecule being titrated and the species R and C; and Do are the solution concentration and electrogenerated titrant. I t has been shown that redox podiffusion coefficient. resnectivelv. of the suecies 0 from which tentials of complex biological molecules can be obtained hy It i ? Iwing t le~~!ru&+ner.a~ed. ('on,iiler nun r h ? airuari,ln i n careful measurement of the shape of titration curves (2-4,15, which ,~nt.tht,rs ~ w c i st Z I.; iiddt~lt o the r u l ~ ~ f Snwie. im i! is 16). capable of rapid homogeneous redox reaction with R to reFluorescence Spectroelectrochemistry generate 0 at a rate characterized by the second order rate A beam of excitation light can he passed through an elecconstant k trochemical cell and the resulting fluorescence (Fig. IF) of electrogenerated species observed (17,18). The greater sensitivity of fluorescence enables measurements to be made at lower concentrations of fluorescent species than is possible 'The wmplctv I I W ~IIAIIISIU ULSV \Y,IISIS~S uf:twtnti
