in Spectroelectrochemical
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© 2008 American Chemical Societ y
Measurements: Optically Transparent Carbon Electrodes Given the useful electroanalytical properties of carbon electrodes, having an optically transparent material could lead to new measurement paradigms that combine electrochemical and spectroscopic measurements.
A
Yingrui Dai
nalytical methods that provide qualitative information Greg M. Swain about the presence and structure of an analyte and quantitative information about its concentration are Michigan State needed for many of today’s complex chemical and bioUniversity logical measurements. Electrochemical methods generally provide minimal information about the identity Marc D. Porter of a redox system. Qualitative insight can be gained, however, if the electrochemical measurement is comUniversity of Utah bined with a spectroscopic one (e.g., transmission) in Jerzy Zak a hybrid strategy known as spectroelectrochemistry (SEC; 1–3). Silesian University In this dual detection and sensing method, the electrode of Technology serves as a source or a sink for electrons and as an optically (Poland) transparent window (1, 2). If the incident radiation is in the UV–vis region of the electromagnetic spectrum, then details about the electronic structure of a redox system and/or about its reaction products may be gleaned. If IR radiation is used, then molecular-level information about local bonding is potentially accessible. In all cases, the successful application of this information-rich, hybrid technique requires an electrode that effectively integrates four key ingredients: electric conductivity, optical transparency, robustness, and inertness. Since its start with Kuwana’s work in the early 1960s, UV–vis SEC has become an essential tool in the study of the mechanistic aspects of redox reactions (4 –10). Transmission is the simplest mode of measurement and requires an optically transparent electrode (OTE). Over the years, OTEs have been prepared by coating thin films of metals (e.g., gold and platinum), carbon, and metal oxides onto glass or quartz substrates. Of these, indium tin oxide (ITO) films have been the most widely used (11, 12). ITO is a Sn(IV)-doped, In 2O3based n-type semiconductor, which, along with ZnO, has been extensively applied for >20 years to electrooptical devices and photovoltaics (13). To a lesser extent, carbon materials have been used as OTEs. Carbon materials possess four attractive features: a wide window of working potential in aqueous media, good electrochemical activity for a range of redox systems, chemical stability under strongly acidic and alkaline conditions, and a wide array of easy strategies for surface modification. The design of a film-based OTE involves a trade-off between optical transparency and electric resistivity, both of which are functions of the film thickness (14). Usually, film-based OTEs must be tens of nanometers thin to be optically transparent. However, at such thicknesses most electrode materials have large internal ohmic resistances, which can distort the electrochemical response because of variations in the applied potential distribution across the electrode surface (14). Thin-film electrodes may also be mechanically fragile and should be handled by supporting the thin carbon film directly on an optical window. In early reports on the fabrication of carbon OTEs, resistive sputtering and vapor deposition of carbon or pyrolysis of organic thin films was used (15, 16). This was followed by a long dry spell during which there were few reports on the preparation and use of such materials. In this article, we describe some recent progress in the preparation, characterization, and application of optically transparent sp3- and sp2-bonded carbon electrodes. We discuss Ja nua ry 1, 2 0 0 8
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thin films of boron-doped diamond and pyrolyzed photoresist. In some respects, these two materials are similar in terms of electric conductivity and electrochemical behavior, but they are very different in terms of ease of preparation, microstructure, thickness, and optical window. SEC with carbon OTEs represents a new analytical measurement paradigm that could prove useful for addressing some of today’s complex chemical and biological detection and sensing challenges as well as for studies of structure–function relationships in electron-transfer reactions, investigations of molecular adsorption, and dual detection in microchip analysis.
Basics of transmission measurements
The measurement is made with an electrochemical cell in which one wall is an optically transparent window and the
Thin solution layer
Ox
I0
Optical window
I
+ e– hν
Red
molecule on the timescale of the measurement . D (cm/s) is the diffusion coefficient and t (s) is the timescale of the measurement. Measurements can also be made with a “thick” solution layer in which semi-infinite linear diffusion of the analyte can occur. In this arrangement, the thickness of the solution layer is greater than the diffusional distance. Typical volumes for thin-layer cells are 2–200 µL, with optical path lengths of 10–100 µm. The redox reaction can be driven by either a potential step or a potential sweep perturbation, usually by cyclic voltammetry (CV). In this method, when a thin-layer cell is used, the applied potential is changed at a low sweep rate (1–5 mV/s), beginning at a value at which the redox analyte is stable. For example, if the redox system is initially in its reduced form R, then the potential is swept linearly in time from a more negative toward a more positive potential; the forward reaction converts the analyte to its oxidized form O. For a well-behaved, reversible redox system, the peak CV current in a thin-layer cell is n2F 2VC R*ν (1) ip= 4RT in which C R* is the bulk concentration of the redox molecule (mol/cm3), V is the cell volume (cm3), and ν is the potential sweep rate (V/s); n, F, R (gas constant), and T have their usual meanings. It can be seen that the peak current in a thin-layer 2.303RT CR* – CR 2.303RT C should sweep(4) rate. log Oincrease log the Evoltammogram = E 0´+ = E 0´+ linearly with nFto oxidize all nFof coulombs CR Q required CRthe R in the The number thin-layer volume is given by Faraday’s law, which is Q = nFVC R* = nFALC R*
(2)
in which A is the electrode (cm 22.303RT ) and L is theAthickness 2.303RT Amax,R – Aarea 0 0 R ´+ solution log ´+ faradaic charge log Opassed (5) is E =ofEthe = E layer (cm). The total nF nF AR AR predicted to be invariant with the sweep rate. Because R is confined within a thin solution layer in which exhaustive electrolyFIGURE 1. General design of an OTE and a thin-layer electrochemical sis can occur, then at any time during the electrolysis, Transparent substrate
Carbon film
cell for transmission SEC.
other is the OTE (Figure 1; 1, 2, 17). A solution layer containing the redox system under study is confined between the OTE and the optical window. Electromagnetic radiation of intensity I0 is passed through the cell, and changes in the spectroscopic signature of the redox system are recorded as a function of the applied potential. The counterelectrode and reference electrode are placed in contact with the solution layer but outside the optical path. The electrode transparency enables potential-dependent spectra of electrogenerated species or of species produced via homogeneous chemical reactions to be recorded at equilibrium when a relatively thin solution layer is used, or as a function of time on the approach to equilibrium when a thick solution layer is used. Equilibrium concentrations of a reactant R and product O are achieved in 1 mA/cm 2); possess electrooptical properties that are stable in a variety of chemical environments; are resistant to polar molecular ad-
sorption and fouling because of a nonpolar, hydrogen-terminated surface (31, 32); and are 50–80% transparent in the UV–vis and midand far-IR regions of the electromagnetic spectrum (22–28). The disadvantages are the high temperatures required for synthesis and the lengthy time (1–4 hours [h]) needed for film deposition. Also, the types of substrates that can be coated are more limited because of the high growth temperatures (>600 °C) and the thermal stress that can result in the diamond overlayer. Diamond OTEs can be fabricated in two architectures—a thin film supported on an optically transparent substrate or a freestanding film. The freestanding diamond disk electrodes are clearly useful but are laborious to prepare because of the lengthy growth time needed to produce a film thickness >300 µm and the mechanical polishing required to smooth the polycrystalline surface (22, 23). In the other architecture, a thin film is deposited on an optically transparent substrate; for example, a diamond/quartz OTE (24, 26). This OTE is analogous to a thin film of ITO on quartz and has an electric resistivity of ~10 S/cm, an optical transparency of ~50% at 300–800 nm, a working-potential window of ≥3V in aqueous media, stable voltammetric background currents and optical properties over a wide potential range, and relatively rapid electron-transfer kinetics for a number of redox systems without conventional pretreatment. The transparency of diamond on quartz in the visible region is lower than that for ITO on quartz, which is ~85% (24 –26). A sharp reduction in transparency occurs at wavelengths 800 nm. The band-gap absorption edge is at ~225 nm, so this represents the low-wavelength cutoff. Throughput reductions at 300–225 nm are due to absorbance by nitrogen impurities (24 –26, 33). Transparency reductions at >800 nm are caused by the boron doping. Reflection is the main cause of loss at 300–800 nm, because of diamond’s high refractive index of 2.41 at 590 nm (24 –26, 33). For example, the calculated intensity loss due to reflection of light perpendicularly incident upon the diamond/quartz OTE is ~27%. The remainder of the light loss is attributed to a combination of scattering and absorption by impurities in the film. This means that this visible transparency is about the best that can be achieved for diamond, even with the relatively high boron doping level. By way of comparison, the transparency of a piece of polished “white” diamond (i.e., optically clear and freestanding) is only ~65% in this region (22, 25). Though diamond is not as transparent as ITO on quartz, the throughput and exceptional electric and optical stability are more than adequate to enable quality SEC measurements.
UV–vis measurements
The application of diamond OTEs for SEC in the UV–vis spectral region has been demonstrated for aqueous (22, 24 –26) and organic redox systems (23). The concept was first demonJa nua ry 1, 2 0 0 8
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Absorbance
potentials during the oxidation reaction. Spectra were collected after a 1 min equilibration period at each potential; this was adequate to achieve a constant concentration ratio of ferricenium to ferrocene (Equation 4). Each difference spectrum (ΔAbsorbance) was generated by subtracting the reference spectrum for ferrocene (measured at 0.10 V) from the spectrum recorded at each potential. As the potential is made more positive (upward arrows), progressive oxidation of ferrocene to ferricenium occurs. Ferrocene is optically inactive at 230–350 nm, but ferricenium is optically active. These two maxima have been assigned to ligand-to-metal charge-transfer modes (34). The absorbance increases at 252 and 285 nm are consistent with an increasing equilibrium ferricenium concentration. The absorbance was constant beyond 0.60 V, indicating complete electrolysis of ferrocene in the thin-layer cell. The spectral features were completely revers(a) 0.6 (b) ible with cycling; that is, the absorbance peaks 0.15 252 0.4 disappeared with the reduction of ferricenium back to ferrocene. 0.2 285 0.10 Ox In addition to obtaining the absorption 0.6 V 0.0 spectrum for ferricenium, we can extract n and Red 0.05 –0.2 values of E 0´ from a Nernst plot of log[AO/ 0.2 V (A max,O − AO)] (i.e., log[AO/A R]) versus the –0.4 0.00 electrode potential. The linear response that –0.6 225 250 275 300 325 350 0.2 0.3 0.4 0.5 0.6 was observed indicates that equilibrium conWavelength (nm) Potential (V vs Ag wire) centrations of R and O are established at each potential on the timescale of the measureFIGURE 2. Ferrocene. ment. The slope of the plot yields a value of 1.1, which is the number of electrons being (a) Thin-layer CV i–E curve for 0.1 mM ferrocene at a freestanding diamond disk OTE at a transferred per redox molecule. The potential scan rate of 0.002 V/s. (b) UV–vis difference absorbance spectra for 1 mM ferrocene as a function of the applied potential of 0.2–0.6 V. (Adapted with permission from Ref. 23.) at which log(AO/A R) = 0 (i.e., C R = C O) is E 0´ for the redox analyte. E 0´ = 0.397 V is deterthese potentials, ferrocene at the electrode is oxidized to ferri- mined from the plot and is in excellent agreement with the cenium. Postpeak, the current quickly decays back to the pre- value of 0.394 V calculated by CV. A O is the absorbance at 252 electrolysis level as the ferrocene in the thin solution layer is (or 285) nm at a given potential, and A max,O is the maximum exhaustively oxidized. absorbance at 252 (or 285) nm observed when the applied Upon scan reversal at 0.60 V, reduction current begins to potential E appl > E 0´ (e.g., E appl = 0.60 V). These results clearly flow at 0.45 V, reaching a maximum at 0.38 V. At these po- demonstrate the utility of the diamond OTE for SEC of both tentials, the ferricenium molecules generated on the forward aqueous and nonaqueous redox systems in the UV–vis range. sweep are reduced back to ferrocene. The current then rapidly The electrode and technique are not only useful for studydecays back to pre-electrolysis levels. The oxidation and reduc- ing simple chemical systems; they can also be used to investion peaks are symmetric in shape and increase in magnitude tigate more complicated ones. UV–vis difference absorption linearly with a scan rate of 2–10 mV/s, as expected for the spectra are presented (reduced minus oxidized forms) for 500 thin-layer voltammetric behavior predicted by Equation 1 (23). µM horse heart cytochrome c (Figure 3; 25). The spectra were The peak charge is independent of the scan rate, as expected generated by measuring the absorption of the progressively for the thin-layer behavior predicted by Equation 2. From the reduced form of the protein and subtracting from each the peak charge, a cell volume of 2.5 µL was calculated, which is spectrum for the fully oxidized form recorded at 0.40 V. close to the volume estimated from geometric dimensions. The spectra were obtained with a freestanding diamond The peak potential separation ΔE p = 33 mV indicates that the disk OTE after a 1 min equilibration time at each potential in redox reaction proceeds with relatively rapid electron-transfer a thin-layer cell. The potential was changed in increments of 50 kinetics. By way of comparison, ΔE p = 0 would be expected for mV from 0.30 to −0.20 V versus Ag/AgCl. a reversible redox system undergoing rapid electron transfer in A series of positive-going (reduced form of the protein) and a cell devoid of significant ohmic resistance. negative-going (oxidized form) peaks is observed at the differFigure 2b shows a series of difference absorption spectra ent potentials. The positive-going peaks increase in amplitude for ferrocene recorded at 225–350 nm at different positive as the potential is made more negative. The UV–vis spectra Current (µA)
strated on the optically active redox systems ferri/ferrocyanide and methyl viologen with a freestanding diamond disk OTE (22). This was followed by work on the optically active redox molecule chlorpromazine with a diamond/quartz OTE (24). Transmission SEC data were obtained for the organic redox system ferrocene by using a freestanding disk (~350 µm thick; 23). Ferrocene can be electrooxidized via a one-electron redox reaction to ferricenium cation, and the oxidation reaction product can be spectroscopically monitored in the UV (λmax = 252 and 285 nm, 1.6 × 10 4 and 1.1 × 10 4 M−1cm−1). Figure 2a shows a CV i–E curve for ferrocene recorded in the transmission cell at 2 mV/s. Well-defined curves are seen for both the oxidation and reduction reactions, and they are stable with cycling. On the forward potential scan, oxidation current begins to flow at 0.30 V, reaching a maximum at 0.40 V. At
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Absorbance
for heme-containing proteins, such as cytochrome c, contain two major bands, the Soret and α bands. 414 0.10 The most intense is the Soret band, which occurs at 409 400–450 nm, depending on the nature of the heme 0.08 iron and the oxidation state of the protein (35). Both bands are attributed to the π→π* transitions of the 550 0.06 heme iron and are, therefore, a very sensitive indicator 300 400 500 600 of the heme iron oxidation state. The spectra between Wavelength (nm) 0.04 400 and 600 nm are the same as those reported previously (36) and indicate that the direct electrochemical 0.02 titration of this protein is possible. It is important to note that the response is stable and reproducible with 0.00 repeated cycling at the diamond OTE. The stable isosbestic point further indicates that –0.02 no protein denaturation occurred. The absorbance intensity at 550 nm, obtained from the direct redox –0.04 titration of cytochrome c as a function of the elec300 350 400 450 500 550 600 trode potential, was found to be a near-perfect fit to Wavelength (nm) the Nernst equation. The formal reduction potential determined from the plot was 0.072 V, which is in FIGURE 3. Horse heart cytochrome c. good agreement with the accepted value of 0.06 V (vs UV–vis reduced minus oxidized (at 0.40 V) difference absorbance spectra Ag/AgCl; 37). The final benefit is the ability to record collected at 0.050 V intervals from 0.30 to –0.20 V at a diamond disk OTE. The inset depicts the absolute reduced (gray) and oxidized (black) spectra. transitions below 400 nm. Two absorption peaks, the (Adapted with permission from Ref. 25.) N and L bands, are seen in the spectra; these, like the Soret and α bands, are attributed to π→π* transitions of the heme iron (38). Many characteristic amino acid absorp- reveal subtle changes in the bonding of a redox system upon a tion transitions occur below 400 nm, such as tyrosine absorp- change in oxidation state. tion at 280 nm. The diamond OTE provides the possibility of Figure 4b shows an example of a mid-IR SEC data set detecting such transitions. with a diamond/silicon OTE. Difference absorption spectra recorded for 10 mM Fe(CN)63−/4− + 1 M KCl at two different IR measurements potentials are presented. These simple difference spectra clearly Another unique feature of diamond is its optical transparency reveal the bonding changes that occur around the metal center in the mid- and far-IR (25, 27, 28). Again, either freestanding with a change in the oxidation state of the iron. A spectrum for or film-supported OTEs can be used. In our group, thin films the fully reduced form of the redox couple Fe(CN)6 4− was first of diamond supported on undoped silicon are the norm; they recorded at 0 V. The potential was then stepped to 0.5 V for 1 are useful down to ~800 cm−1. Such films are 1–2 µm thick min, and a spectrum was recorded for the fully oxidized form and have a well-faceted, polycrystalline morphology (28). Fe(CN)63− of the couple. Figure 4a shows a typical IR transmission spectrum for a Reduced minus oxidized and oxidized minus reduced boron-doped diamond/silicon OTE deposited for 4 h. The spectra are presented. The spectra are mirror images of one introduction of boron to make the material conductive breaks another, indicating that the reaction is reversible and complete the lattice symmetry, resulting in the broad absorption cen- within the thin-layer cell. The CN stretching mode at 2039 tered at 1290 cm−1 in the one-phonon vibronic region (25, 28, cm−1 shifts to 2116 cm−1 upon oxidation to ferricyanide (39– 33). A progressive decrease in transparency from ~50% to 20% 41). The molar extinction coefficients for these two transitions is evident above 2000 cm−1. The high doping level produces are obviously different. The greater the positive charge on the an acceptor band, so the reduction in transparency is caused central metal, the less the metal can back-bond electrons into by a broad absorption continuum, which, in turn, is caused by the π* orbitals of the CN ligands. Less back-bonding means a electronic transitions from the valence band to numerous states higher bond order and a larger CN bond energy. Far-IR SEC in the acceptor band. The higher the boron doping level is, the measurements of ferrocene and mid-IR SEC measurements of more opaque the material is at these wavenumbers. cytochrome c with diamond OTEs have also been reported Moderately boron-doped films transmit ~40–60% of the (25, 28). light in the range 2000–500 cm−1. As is the case for the diamond/quartz OTE, the optical and electric properties of the sp2-Bonded electrodes diamond/silicon OTE are reproducible from film to film and Another intriguing development is the preparation of sp2are stable during anodic and cathodic polarization and expo- bonded carbon-based OTEs (C-OTEs) by the pyrolysis of thin sure to various organic solvents. Such stability is essential for photoresist films (42, 43). Recent advances in other laboratomaking high-quality difference SEC measurements that can ries (42–45) can be extended to fabricate amorphous, optically Ja nua ry 1, 2 0 0 8
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(b)
% Transmittance
∆Absorbance
ing for film thickness, can be used to calcu0.04 late the nominal conRed–ox 80 ductivity of ~400 S/ 0.02 cm for 35–80 nm C60 OTEs. This conductiv0.00 ity compares well with 40 the value reported for – 0.02 thick, nontransparent 20 films of pyrolyzed phoOx–red toresist (~175 S/cm; – 0.04 0 43). The conductivity 2200 2100 2000 1900 3000 2500 2000 1500 1000 trend parallels that for Wavenumber (cm–1 ) Wavenumber (cm–1 ) sputter-deposited carbon films and ultrathin FIGURE 4. (a) IR transmission spectrum for a boron-doped diamond thin film deposited on undoped metal and semiconducsilicon. The spectrum was acquired with an air background. (b) IR SEC difference absorbance spectra tor films (46). of Fe(CN) 6 3−/4− at the diamond/silicon OTE with a CaF 2 window. The optical path length was 7.5 µm. These data, coupled The potential was stepped from 0 (Red) to 0.5 V (Ox) vs Ag/AgCl. (Adapted from Ref. 28.) with the smoothness of the postpyrolyzed COTE films, point to an transparent carbon coatings at thicknesses of
