A Light Addressable Photoelectrochemical Cyanide Sensor

These dark currents range from 0.1 mA/cm2 in low dark current samples up to 1 .... of the ferrocyanide-stabilized cell remains affected by the presenc...
0 downloads 0 Views 197KB Size
Download PDF
Anal. Chem. 1996, 68, 954-959

A Light Addressable Photoelectrochemical Cyanide Sensor Stuart Licht,* Noseung Myung, and Yue Sun

Department of Chemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610

A sensor is demonstrated that is capable of spatial discrimination of cyanide with use of only a single stationary sensing element. Different spatial regions of the sensing element are light activated to reveal the solution cyanide concentration only at the point of illumination. In this light addressable photoelectrochemical (LAP) sensor the sensing element consists of an n-CdSe electrode immersed in solution, with the open-circuit potential determined under illumination. In alkaline ferro-/ferricyanide solution, the open-circuit photopotential is highly responsive to cyanide, with a linear response of (120 mV) log [KCN]. LAP detection with a spatial resolution of (1 mm for cyanide detection is demonstrated. The response is almost linear for 0.001-0.100 m cyanide with a resolution of 5 mV. An extensive variety of spectroscopic and electrochemical methodologies are available for quantitative analysis of solutionphase species. Hafemen et al. have presented an additional unusual hybrid spectroscopic-electrochemical technique to provide spatial and temporal analyte detection with a single sensing element.1 That technique, light addressable photoelectrochemical (LAP) sensing, was demonstrated for the detection of H+ and K+. In this study, a second demonstration of the LAP methodology is provided to determine the solution-phase concentration of cyanide. The detection and treatment of solution-phase cyanide is of considerable toxicological and environmental importance2-4 and provides an impetus for continued studies on the fundamental interfacial and electrolytic behavior of cyanide.5-11 Cyanide waste is generated by ore extraction, synthesis, and metal finishing industries.11,12 Methodologies continue to be developed for the analysis of solution-phase cyanide. These include atomic absorp* Author to whom correspondence should be addressed. Current address: Department of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel. (1) Hafemen, D. G.; Parce, J. W.; McConnell, M. Science 1988, 240, 11821185. (2) Bhakta, D.; Shukla, S. S.; Chandrasekharaiah, M. S. Environ. Sci. Technol. 1992, 26, 625-626. (3) Meeussen, J. C. L.; Keizer, M. G.; van Riemsdijk, W. H. Environ. Sci. Technol. 1992, 26, 1832-1838. (4) Guo, R.; Chakrabarti, C. L.; Subramanian, K. S. Water Environ. Res. 1993, 65, 640-644. (5) Kim, C. S.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 9784-9787. (6) Li, J.; Wadsworth, M. E. J. Electrochem. Soc. 1993, 140, 1921-1927. (7) Hofseth, C. S.; Chapman, T. W. J. Electrochem. Soc. 1992, 139, 2525-2529. (8) McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7410-7416. (9) Kelsall, G. H. J. Electrochem. Soc. 1991, 138, 108-116. (10) Kelsall, G. H.; Savage, S.; Brandt, D. J. Electrochem. Soc. 1991, 138, 117124. (11) Tissot, P.; Fragniere, M. J. Appl. Electrochem. 1994, 24, 509-512. (12) Palmet, S.; Breton, M. A.; Nunno, T. J.; Sullivan, D. M.; Surprenant, N. F.; Metal/Cyanide Containing Wastes: Treatment Technologies; Noyes Data Corp.: Park Ridge, NJ, 1988.

954 Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

tion13,14 and other spectroscopic methods,14,15 chromatographic,16 modified electrode and amperometric techniques,17-20 and voltammetric14,21 and potentiometric methods.14,22 Sufficiently energetic illumination incident upon semiconductors immersed in solution can generate significant electrochemical potentials. This photoelectrochemical effect has been used to investigate surface treatment and preparation of semiconductor materials, environmental detoxification, and solar cells.23 Photoelectrochemistry has also been used to probe semiconductor surface defects and photoassisted chemical reactions.24,25 In 1988, Hafemen et al. used optical scanning of the semiconductor as an ion-selective photoelectrochemical probe. In that study, the semiconductor employed was silicon coated with a silicon oxynitride overlayer and was demonstrated as a pH probe to detect enzymatic activity.1 A conventional sensor can be physically displaced to achieve spatial discrimination of an analyte. However, this can induce convective currents in an electrolyte. Figure 1 compares a scheme for a LAP sensor to that for a conventional potentiometric or amperometric sensor. For the conventional sensors, convective disruption due to sensor displacement will not occur if an array of sensors, as shown on the right-hand portion of the figure, each including a separate sensing element, are used to achieve spatial discrimination of the analyte. Alternatively, as shown on the lefthand portion of the figure, with a LAP sensor, a single wide area sensing element may be utilized which is scanned with light to determine the response in different spatial elements. An advantage of the LAP is the simplicity provided by a single sensor, and spatial sensing is achieved without electrolyte displacement and with fewer electrical contacts compared to a conventional array scheme. In this study, we introduce a light addressable photoelectrochemical sensor capable of spatial and temporal resolution of cyanide in solution. A single scanning beam, rather than the array (13) Rosentreter, J. J.; Skogerboe, R. K. Anal. Chem. 1991, 63, 682-688. (14) do Nascimento, P. C.; Schwedt, G. Anal. Chim. Acta 1993, 283, 755-761. (15) Kurzawa, J.; Janowicz, K.; Kurzawa, Z. Anal. Chim. Acta 1992, 263, 155. (16) Bond, A. M.; Heritage, I. D.; Wallace, G. G. Anal. Chem. 1982, 54, 582585. (17) Langaier, J.; Janata, J. Anal. Chem. 1992, 64, 523-527. (18) Smit, M. H.; Cass, A. E. G. Anal. Chem. 1990, 62, 2429-2436. (19) Groom, C. A.; Luong, J. H. T. J. Biotechnol. 1991, 21, 161-172. (20) Nikolic, S. D.; Milosavljevic, E. B.; Hendrix, J. L.; Nelson, J. H. Analyst 1992, 117, 47-50. (21) Galyamedinov, G. Y.; Budnikov, K. G. J. Anal. Chem. (Moscow) 1993, 48, 731. (22) Baranowski, R.; Kubik, T. Talanata 1993, 40, 1465. (23) Miller, B.; Licht, S.; Orazem, M. E.; Searson, P. C. Crit. Rev. Surf. Chem. 1993, 3, 29-47. (24) Furtak, T. E.; Canfield, D. C.; Parkinson, B. A. J. Appl. Phys. 1980, 51, 60186021. (25) Casillas, N.; James, P.; Smyrl, W. H. J. Electrochem. Soc. 1995, 142, L16L18. 0003-2700/96/0368-0954$12.00/0

© 1996 American Chemical Society

Potentiostatic measurements were conducted in a threeelectrode cell with a large Pt mesh counter electrode and a Pt reference. Freshly prepared Ar-saturated electrolytes were used in the cell. Indoor illumination was provided by a 75 W tungstenhalogen light (Gilway L6408). The illumination passed through both a ground glass diffuser and a water filter and, in slit-collimated light experiments, was displaced using an Edmund XY stage movement. Illumination intensity was varied by changing the location of the light source. Potential was controlled by a PINE AFRDE4X1 bipotentiostat, and the cell current and/or potential output was monitored by a Houston Instruments Model 2000 X-Y recorder.

Figure 1. Schematic comparison of a light addressable photoelectrochemical sensor (left side) and a conventional sensor (right side). To achieve spatial discrimination within an analyte, the conventional sensor requires a separate sensing element with connecting wire for every separate spatial area. In a LAP sensor, a single wide-area sensing element may be utilized which is scanned with light to determine the response in different spatial elements, necessitating fewer electrical contacts.

of light emitting diodes utilized in the earlier LAP sensor,1 is used to characterize analyte concentration in solution. In this sensor, ion-selective discrimination of cyanide is based on our earlier observations of the influence of cyanide on the variation of n-CdSe photopotential.26 EXPERIMENTAL SECTION Materials. Electrolytes were freshly prepared from distilled deionized water (Barnstead, E-pure) and were deaerated with argon. K4Fe(CN)6‚3H2O (Aldrich analyzed reagent) and K3Fe(CN)6 (ACS certified Fisher Scientific) were used as received. All other chemicals (KCN and KOH) were analytical grade. Single crystals of n-CdSe (Cleveland Crystals, 1 Ω cm, 1120) were used as photoelectrodes. Instrumentation and Measurement Techniques. n-CdSe samples with the 1120 plane exposed were mounted on a copper wire and sealed with clear epoxy. The area exposed was generally 0.2 cm2. Back contact to the n-CdSe was established by rubbing Ga-In eutectic. Prior to use, the sample was mechanically polished successively with 5, 1, 0.3, and 0.05 µm alumina and chemically polished by dipping in 1.5% Br2 in methanol for 40 s, followed by a 10 s dip in 1 M HCl. Following a thorough wash with deionized water, the electrode was photoelectrochemically etched in 1 M Na2SO4 according to a reported procedure.28 The n-CdSe photocurrent was further improved by ∼25% when the electrode, immersed in 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, 0.5 m KOH, and 0.1 m KCN and maintained with the potentiostat at zero applied bias versus Pt (short circuit condition), was illuminated at 100 mW/cm2 for several hours. (26) Licht, S.; Peramunage, D. Nature 1990, 345, 330-333. (27) Reichman, J.; Russak, M. A. J. Electrochem. Soc. 1984, 131, 796-798. (28) Freeze, K. W., Jr. Appl. Phys. Lett. 1982, 40, 275-277.

RESULTS AND DISCUSSION Variation of CdSe Photopotential with Cyanide. Several semiconductors, including n-CdSe immersed in alkaline polysulfide and ferrocyanide electrolytes, are relatively stable and exhibit low electrochemical dark current over a wide range of applied electrochemical potentials.27,28 In photoelectrochemical cells, the semiconductor photopotential often responds to the concentration of specific ions in solution. Sulfide ion has been shown to specifically adsorb onto cadmium chalcogenide surfaces, and the measured photopotential of cadmium selenide electrodes varies with concentrations of H+ (and OH-), sulfide, and polysulfide.29,30 When specific ion adsorption is evident, several models, including our own,31 have been presented for the variation of photopotential with solution-phase ion concentration. Whether a photopotential shift is due to chemical or physical absorption of specific ions, the resultant potential shift is dominated by the electrode/ electrolyte interfacial region in which the ion is physically present. Therefore, light incident on different regions of a single photoelectrochemical sensing element may discern between different ionic concentrations. Observations of photogenerated potential shifts with cyanide concentration26 are quantified with n-CdSe flat band potential measurements;32 similar trends have been observed with n-CdTe electrodes.32 In conditions of stabilized low semiconductor dark current, a shift in photoelectrochemical photopotential may be accompanied by a shift in the photovoltage measured under open-circuit conditions (voltage at zero net current denoted Voc), which can vary with modifications in the localized ionic environment of the electrolyte.32 This provides the possibility of a single stationary sensing LAP element which can be externally illuminated in different regions to provide both spatial and temporal information of analyte concentration. This contrasts with conventional electrochemical single sensing elements, which provide temporal but not spatial information. The magnitude of the n-CdSe dark current varies with sample choice, mechanical, electrochemical, and photoelectrochemical surface pretreatment, and the perfection of the n-CdSe’s electrical contact’s isolation from the electrolyte. These dark currents range from 0.1 mA/cm2 in low dark current samples up to 1 mA/cm2 in high dark current n-CdSe samples in a 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, and 0.5 m KOH electrolyte at an applied potential of -0.8 V versus the alkaline ferro-/ferricyanide redox couple. The potential of this redox couple, to within a few millivolts, does (29) Hodes, G. In Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983; Chapter 13. (30) Licht, S. Nature 1987, 330, 148-151. (31) Licht, S.; Marcu, V. J. Electroanal. Chem. 1986, 210, 197-204. (32) Licht, S.; Peramunage, D. J. Phys. Chem., submitted.

Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

955

Figure 2. Variation of the n-CdSe open-circuit photovoltage, Voc, under 80 mW/cm2 tungsten halogen illumination in 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, and 0.5 m KOH containing various concentrations of CN- or Cd(CN)42-. (i) Data presented with open or closed circles represent Voc variation with concentrations CN-, added as KCN (no added Cd(CN)42-), and expressed as log [KCN]. The upper curve (b) is measured with a low dark current n-CdSe (dark current 0.1 mA/cm2 at -0.8 V versus Pt in electrolyte without added KCN). The lower curve (O) is measured with a high dark current n-CdSe sample (dark current 1 mA/cm2 at -0.8 V). (ii) Data presented with open squares represent Voc variation at a low dark current n-CdSe electrode in constant 0.1 m KCN, but with varying concentrations of Cd(CN)42(O), added as 4:1 [KCN]:[CdCl2], and expressed as log [Cd(CN)42-].

not change with the addition of KCN to the electrolyte.26 Whereas the redox couple, as measured at a platinum electrode, is invariant to cyanide addition, as shown in Figure 2, the open-circuit voltage for illuminated n-CdSe varies by several hundred millivolts with addition of KCN. The Voc is affected by the magnitude of the n-CdSe dark current. As shown for both low and high dark current n-CdSe electrodes, the variation of Voc is highly linear with variation of log [KCN], particularly in the 0.001-0.1 m KCN concentration domain. In addition to surface pretreatment, illumination source (solar, tungsten halogen, or 632.8 nm laser), and intensity (from 0.1 to 1 mW/mm2), can vary Voc by 0.1 V. However, in comparing Voc in alkaline ferro-/ferricyanide solutions with and without 0.1 m cyanide, the differential voltage shift is consistent at ∆Voc ) 0.22-0.25 V. In alkaline potassium ferrocyanide electrolytes, the n-CdSe photoelectrochemical reaction is dominated by the oxidation of ferrocyanide:31-33

Figure 3. Dark current and partially illuminated photocurrent of n-CdSe immersed in alkaline ferro-/ferricyanide solutions either free of cyanide or containing 0.1 m KCN. The immersed n-CdSe has 20% of its surface area illuminated (exposed to slit-collimated 80 mW/cm2 tungsten halogen illumination), which, as indicated, constitutes a 1 mm × 2 mm region of the total 1 mm × 10 mm electrode area. The alkaline ferro-/ferricyanide electrolyte contains 0.5 m KOH, 0.25 m K4Fe(CN)6, and 0.0125 m K3Fe(CN)6. Dark current and photocurrent are measured by linear voltammetry at a potential sweep rate of 20 mV/s.

variations are consistent shifts in measured Voc with a slope, S, given by the log concentration:

S(CN-) ) (0.12 ( 0.003 V) log [CN-]-1

(3)

S(Cd(CN)42-) ) (0.03 V) log [Cd(CN)42-]

(4)

This is indicated in Figure 2, in which the open-circuit voltage also varies with Cd(CN)42-aq, as well as with CN-aq. Both

In addition to the open-circuit potential measurements performed at zero current galvanostatic conditions, under potentiostatic conditions, the potential may be linearly swept. Figure 3 includes the linear voltammetry for immersed n-CdSe in the dark or under partial illumination, i.e., for a 2 mm2 illuminated region of a 10 mm2 immersed surface area of n-CdSe. The electrode is immersed in alkaline ferro-/ferricyanide electrolyte with concentrations of KOH, K4Fe(CN)6, and K3Fe(CN)6 respectively of 0.5 m, 0.25 m and 0.0125 m. As indicated in the figure, the solutions are either with or without 0.1 m added KCN. The short-circuit photocurrent, ISC, is measured at zero bias and for illuminated CdSe is limited by harvesting of photons of energy greater than the CdSe bandgap of 1.7 eV. In these alkaline ferro-/ferricyanide electrolytes, ISC is measured at 2 × 10-3 mA mW-1 mm-2 and is a linear function of both illumination intensity and exposed surface area. From the figure, the open-circuit photovoltages, Voc, are determined at zero photocurrent. The light-assisted oxidation of ferrocyanide (eq 1) is important for the physical endurance of the n-CdSe photoelectrode and contributes to the stability of the measured photopotential. This has been studied for n-CdSe photoelectrodes in simple alkaline cyanide solutions (in the absence of ferro-/ferricyanide solutions).34 Upon illumination in these solutions, photogenerated holes attack and consume the photoelectrode, as measured by

(33) Licht, S.; Peramunage, D. Solar Energy 1994, 52, 197-204.

(34) Licht, S.; Peramunage, D. J. Electrochem. Soc. 1992, 139, L23-L26.

KFe(CN)63- + (hν) f KFe(CN)62- + e-

(1)

In addition, we have shown that the photovoltage shift with cyanide is consistent with a decomposition reaction:

CdSe + 4CN- f Cd(CN)42- + Se + 2e-

956 Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

(2)

mass loss of the n-CdSe electrode and by concurrent increase in the solution-phase cadmium concentration, consistent with eq 2. Alternatively, the photoelectrode is highly stabilized when a compatible kinetically facile redox couple such as ferro-/ferricyanide is added to the solution, and the kinetics of ferrocyanide oxidation, eq 1, rather than the oxidation of the photoelectrode dominates. The n-CdSe photopotential of the ferrocyanidestabilized cell remains affected by the presence of 0-1 m added KCN and varies by several hundred millivolts in a manner consistent with eq 2. The (dark) potential of the alkaline ferro/ferricyanide redox couple (at platinum) is observed to be invariant (to within a few millivolts) to this added cyanide in the electrolyte. Thus, incorporation of CN- in the electrolyte had no significant effect on the formal potential of 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, and 0.5 m KOH electrolyte, and the potential, as measured at platinum, of this redox couple remains at 0.35 V versus the standard hydrogen electrode (SHE). Similar to the fully illuminated n-CdSe results in Figure 2, the observed photopotential in Figure 3 for the partially illuminated cell is substantially affected by added cyanide. The partially illuminated cell Voc depends on cyanide concentration (Voc ) -0.82 V without cyanide or Voc ) -1.04 V with 0.1 m cyanide in the electrolyte). Although the n-CdSe electrode employed exhibited relatively low dark current (0.2 mA/cm2 at -0.8 V versus Pt), the measured Voc is smaller than that for the comparable fully illuminated electrode. Whereas this dark current is low compared to the ISC of the fully illuminated cell, it is proportionally higher with regard to the small surface area in the partially illuminated cell, as 80% of this partially electrode remains in the dark. Hence, the observed Voc voltage shift is consistent with a larger relative dark current, which acts to positively shift Voc for solutions either with or without added cyanide. Despite this dark currentinduced Voc shift, the differential voltage (∆Voc ) Voc(no cyanide) - Voc[KCN]) is similar in both the partially and the fully illuminated cell. The upper portion of Figure 3 includes a schematic of a onedimensional cyanide LAP using a rectangular 1 mm × 10 mm n-CdSe single crystal. Of this 10 mm2 surface, a 2 mm2 portion of the crystal (1 mm × 2 mm) may be incrementally illuminated. The Voc was observed to vary by (0.004 V as measured over the five distinct 2 mm2 segments of the CdSe detector. As with the case of the fully illuminated compared to the partially illuminated electrode, the impact of this small variation is minimized by comparing the surface-specific potential variation with and without cyanide, and this differential voltage parameter, ∆Voc, is employed throughout the remainder of this study. Data presented in Figure 2 suggest that the linear effective range for cyanide LAP response will be from 0.001 to 0.1 m KCN. The transient response to varying cyanide concentrations is also of interest. To probe this effect, the n-CdSe photoelectrode was situated in the lower 2 mL of a cell containing 10 mL of electrolyte. Under zero current galvanostatic conditions, the cell Voc is continuously monitored, and the upper 8 mL of cell electrolyte can be replaced to effect a desired KCN concentration change. Figure 4 presents the n-CdSe Voc potential jump which occurs in the cell in which the electrolyte cyanide is rapidly increased (to 0.1 m KCN) and then rapidly returned to the original cyanide concentration (0.02 m KCN). The resultant potential jump with an observed magnitude of ∆Voc ) 88 mV is consistent with log [CN-] in eq 3. Consistent with the potential determining reaction,

Figure 4. Voc variation occurring when cyanide concentration is rapidly enhanced or diminished at an illuminated n-CdSe photoelectrode. The n-CdSe was situated in the lower 2 mL of a cell with 10 mL of alkaline ferro-/ferricyanide electrolyte containing 0.5 m KOH, 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, and 0.02 m KCN. The upper 8 mL of electrolyte is replaced with electrolyte of the same alkaline ferro-/ferricyanide concentrations but different KCN concentration to effect the cyanide concentration change. The “potential overshoots” in the figure are electrical noise, which appears to occur when the upper portion of the cell electrolyte is rapidly replaced.

eq 2, there is some sensitivity to mass transport. The rate of potential convergence is more rapid than shown in Figure 4 when the solution is also stirred, but the present experimental cell precluded a quantitative assessment of this effect. Light Addressable Cyanide Photoelectrochemical Sensors. As schematically represented in Figure 5, the onedimensional LAP configuration introduced in Figure 3 is capable of probing five distinct 2 mm2 regions along a 1 mm × 10 mm surface area of a single n-CdSe detector. In testing this device, the CdSe is immersed in the solution to be investigated, and an X-Y movement is utilized to direct which portion of the electrode is illuminated. In this study, slit-collimated white light (tungsten halogen) of comparable intensity to AM1 insolation (∼1 mW/ mm2) was used to illuminate the different portions of the rectangular n-CdSe LAP probe. Figure 6 presents the differential open-circuit variation at three different LAP regions as a localized, but concentrated, source of cyanide is introduced into the LAP configuration illustrated in Figure 5. After 15 min, the 1.2 µL/min source of 7.7 m KCN is sufficient to raise the average cyanide concentration in the 10 mL cell to ∼0.01 m. As seen as the upper curve of Figure 6, the sensor region adjacent to the cyanide source exhibits a differential voltage which rises to a near maximum within minutes. The magnitude of the observed voltage shift in the LAP region adjacent to the cyanide source is higher than expected from eq 3 with the resultant average 0.01 m cyanide concentration. However, this value gradually diminished to the expected value when the cyanide source was removed. This is consistent with a localized above-average cyanide concentration established near the concentrated cyanide source. Shown in the figure by the lower curve farthest from the source at ∂x ) 8 mm, the ∆Voc change in time occurs approximately an order of magnitude slower but is approaching the expected voltage shift consistent with the average Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

957

Figure 5. Schematic of the LAP demonstration for in situ spatial measurement of the variation of cyanide concentration in solution. The one-dimensional LAP sensor consists of a rectangular n-CdSe electrode immersed in solution. At one end of the sensor, a tube filled with cyanide electrolyte and ending in a porous frit serves as a cyanide source and introduces concentrated cyanide into solution. The magnitude of the cyanide concentration is determined by illuminating one of five different regions of the sensor situated at the specified increasing distances from the cyanide source.

Figure 6. Differential open-circuit voltage variation in time as concentrated (7.7 m) KCN electrolyte is introduced at a flow rate of 1.2 µL/min into a 10 mL cell configured as described in Figure 3. ∆Voc is measured at 0 (top curve), 4 (middle curve), or 8 mm (lower curve) from the cyanide source, as illustrated in Figure 5.

increase in cyanide concentration. For the probing light beam centered at ∂x ) 0, 4, or 8 mm from the cyanide source, perturbations comparable to a resolution of 5 mV can be observed in each of the three resultant ∆Voc curves. These perturbations are attributed to several secondary factors, including uncontrolled convection within the cell, the stability at each electrode surface, and instability of the illuminating beam. The combined sum of 958 Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

Figure 7. Spatial and temporal variation of cyanide concentration measured by the LAP sensor at 0-5 min (lower curve, 9), 6-11 min (middle curve, O), and 15-20 min (upper curve, b) while a concentrated cyanide source is added into the electrolyte. The beam of light is moved approximately onece per minute onto regions of the LAP sensor located 8 mm, then 6 mm, then 4, mm, then 2 mm, and then 0 mm from the cyanide source. This process is repeated three times. As indicated, while the light illuminates a specified LAP region, several measurements are made during each minute interval. Further details of the experimental configuration are given in the legends of Figures 5 and 6. Inset: Measured cyanide spatial concentration profile at 0.5, 5, 10, 15, and 30 min following introduction of 1.2 µL/min, 7.7 m KCN into 10 mL of electrolyte.

these effects appear small, and consistent voltage trends over time are evident in Figure 6. The results in Figure 6 are from three repeat experiments in which the illuminating beam was only moved between runs in order to probe one of three specified regions of the cyanide LAP sensor. Additionally, as summarized in Figure 7, the illumination probe may be scanned over the LAP surface during a single experiment to provide spatial and temporal in situ cyanide measurement during a single experiment. The vertical axis of this figure presents measured cyanide values determined from the five calibrated spatial regions of the LAP sensor. In this calibration, each of the electrode regions is incrementally illuminated, and Voc is measured at two cyanide concentrations to yield the slope, S(CN-). From the measured open-circuit potential shift, ∆Voc, the in situ cyanide concentration is then determined in accordance with 10-∆Voc/S(CN-). As in the experiments summarized in Figure 6, a confined concentrated cyanide source is again introduced into the LAP cell. The probe beam is then moved once per minute to illuminate a different region of the cell. During each minute interval, several LAP measurements are performed at the indicated region of the cell. Figure 7 summarizes the resultant spatial and temporal variation of cyanide concentration measured by the LAP sensor at 0-5 min (lower curve, 9), 6-11 min (middle curve, O), and 15-20 min (upper curve, b) and at locations 8, 6, 4, 2, or 0 mm from the cyanide source. As seen in the lower curve of the figure, during the first 5 min, the majority of cyanide diffusion occurs in the region next to the cyanide source. During minutes 6-11, the cyanide concentration grows rapidly in the region 2 mm from the cyanide source, and some cyanide is observed 4 mm from the source. During minutes 15-20, cyanide is detectable at the farthest sensor region (∂x ) 8 mm), the most rapid

Table 1. Effects of Various Ions on LAP Cyanide Detectiona ions, X

[X], m

∆EX, mV

KCN-,X

Cl-

0.2 0.2 0.2 0.2 0.2 0.002 0.02

-2 ( 3 -3 ( 3 8(3 8(3 -12 ( 3 -2 ( 3 off scale

|K| < 0.006 |K| < 0.006 0.08 ( 0.006 0.017 ( 0.006 0.021 ( 0.006

NO3SO42SCNIS2S2-

off scale

a Variation of the n-CdSe open-circuit photovoltage, V , under 80 oc mW/cm2 tungsten halogen illumination in 0.25 m K4Fe(CN)6, 0.0125 m K3Fe(CN)6, 0.5 m KOH, and 0.02 m KCN PECs and containing a variety of ions, X. Selectivity coefficient determined from the conventional Nikolsky equation, given by KCN-,X ) (10∆EX/S - 1)[CN-]/[X]zX, where zX is the absolute value of charge on X, the cyanide concentration is 0.02 m, and the slope, S, is 120 mV.

cyanide buildup occurs 4 mm from the source, and the concentration of cyanide adjacent to the electrode has reached a plateau. From these measurements, cyanide concentration profiles may be mapped using a single LAP sensor, as shown in the inset to Figure 7. The degree of convectional force contribution to the cyanide concentration profile in the figure inset can be sensitive to small changes in the geometry of the cell configuration. Although the specific point at which convection dominates over diffusion in the profile is not evident, the effect of convection is evident in the 30 min profile in the figure inset, in which a level concentration of ∼0.011 m cyanide occurs beyond 4 mm from the cyanide source. Competitive Ion Interference. Table 1 summarizes the interference effects due to the presence of additional ions on LAP cyanide determination. As shown in the table, most anions such as NO3-, Cl-, SO42-, SCN-, and I- show little significant interference with cyanide determination. When these anions are added at up to 10 times a cyanide concentration of 0.02 m, interferent selectivity coefficients of less than 0.1 are exhibited in each case. Sulfide is a substantial interferent with traditional potentiometric methods for the determination of cyanide.35 Similarly with the CdSe LAP, the presence of sulfide substantially interferes with cyanide detection. At very low concentrations of sulfide (0.002 m), there is no significant interference, but, as seen in Table 1, at high concentrations, cyanide is no longer determinable. At low sulfide concentrations ([S2-] , [Fe(CN)63-]), the chemical oxidation of sulfide by ferricyanide appears to protect the LAP from sulfide interference, but at higher concentrations, sulfide must be removed from solution prior to cyanide determination. CONCLUSIONS Prior to the landmark 1988 light addressable photoelectrochemical (LAP) sensor study demonstrating photoelectrochemical detection of pH to locate enzymatic activity sites, pioneering work was reported in the application of photoelectrochemistry for surface imaging36 and in 1992 for nonselective detection of organic species in solution.37 The significance of the current study is that LAP methodologies can be expanded to provide specific species (35) Handbook of Electrode Technology; Orion Research Co.: Cambridge, MA, 1982. (36) Butler, M. A. J. Electrochem. Soc. 1983, 130, 2359. (37) Brown, G. N.; Birks, J. W.; Koval, C. A. Anal. Chem. 1992, 64, 427-434. (38) Smith, D. K.; Tender, L. M.; Lane, G. A.; Licht, S.; Wrighton, M. S. J. Am. Chem. Soc. 1989, 111, 1099-1105.

(ion-selective) detection. In this study, an ion-selective sensor has been demonstrated to be capable of spatial discrimination of cyanide with use of only a single stationary sensing element. This LAP sensor consists of an n-CdSe electrode immersed in solution and the potentiometric (open-circuit potential) determined under illumination. Different spatial regions of the sensing element are light activated to reveal the solution cyanide concentration only at the point of illumination. In alkaline ferro-/ferricyanide solution, the sensor open-circuit photopotential is highly responsive to cyanide, linearly responding to (120 mV) log [KCN]. Response is most linear from 0.001 to 0.100 m cyanide, with a resolution of 5 mV. A spatial resolution of (1 mm for cyanide detection is also demonstrated. The lower concentration limit detection by this LAP sensor appears to be ∼0.0007 m cyanide (∼50 ppm); below this level, little photopotential variation was observed. S2- interferes with the sensor, whereas NO3-, Cl-, SO42-, SCN-, and I- do not. The study has demonstrated a proof of concept of a cyanide detection by LAP but has not pursued the minimum spatial limits of that detection, which might be further reduced with improved optics for the light addressable component. The present spatial resolution of (1 mm is excellent compared to those possible with commercial potentiometric sensors but is poor compared to that which might be achieved with microfabricated sensing arrays. Similarly, the time response for LAP cyanide detection is on the order of seconds, typical of fast potentiometric sensor response time. Throughout this study, the CdSe current and open-circuit voltage were probed with a three-electrode cell (CdSe working, Pt counter, and Pt reference electrodes) to isolate CdSe electrode behavior. As shown in Figure 2, the ferro-/ferricyanide potential on platinum and other surfaces is highly independent of added cyanide. Furthermore, polarization losses at the counter electrode can be minimized. Hence, although not investigated in this study, in principle, a two-electrode cell configuration may also be functional. The present LAP cyanide sensor demonstrates spatial and temporal detection of cyanide by an illuminated sensor. However, it is limited to media which contain sufficient levels of ferro-/ferricyanide to stabilize the n-CdSe photoelectrochemistry. A second generation cyanide LAP sensor, in which the ferro-/ ferricyanide or other supporting photoelectrolyte is immobilized near the electrode surface, will be of interest. For example, we have been interested in a polycationic polymer based on viologen and quinone subunits which confines the ferro-/ferricyanide redox couple near a platinum electrode surface.38 Future studies will be required to determine if a similar configuration at a semiconductor surface can be used to extend the operational domain of a LAP sensor. ACKNOWLEDGMENT S.L. is grateful to the U.S. Department of Energy National Renewable Energy Laboratory (NREL) and the Petroleum Research Fund (PRF) for partial support of this work and to suggestions by Dharmasena Peramanuge and Truman S. Light.

Received for review July 25, 1995. Accepted January 2, 1996.X AC9507449 X

Abstract published in Advance ACS Abstracts, February 1, 1996.

Analytical Chemistry, Vol. 68, No. 6, March 15, 1996

959