1714
Anal. Chem. 1881, 63,1714-7719
obtain a series of determinations as a function of the extraction parameter (time or subphase composition). This determination would thereby generate a two-dimensional signal, perhaps allowing the determination and identification of several species within a sample on the basis of the differing solubilities of components in the subphase. The HT voltammetric method might also be useful following extraction of samples (e.g., polymers) with low polarity solvents, for example, in the determination of antioxidants.
ACKNOWLEDGMENT We thank Paul McCord for the synthesis of the Ru(DPP)t+ and R U ( D P B ) ~complexes ~+ and assistance in the ECL experiments. We also appreciate samples provided to us by H. Horowitz and W. Riley of Exxon Corp. and acknowledge important preliminary experiments in this area by Xun Zhang. LITERATURE CITED Lord. S. S.; ONeiil, R. C.; Rogers, L. B. Anel. Chem. 1952, 24, 209. Barendrecht, E. In €bcWonna&tkal Chemlsby: Bard, A. J., Ed.; Marcel Dekker: New York, 1967; Voi. 2 and references therein. Wang, J. In €~troenelybrce/Cbemisby;Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 16 and references therein. Oyama, N.; Anson, F. C. J . Am. Chem. Soc. 1979, 101, 3450. Quadalupe, A. R.; Abruk, H. D. Anel. CY". 1985, 57, 142. Zhang, X.; Bard, A. J. J . Am. Chem. Soc. 1989, 1 7 1 , 8098.
Gaines. G. L. Insobbk Monoleyers at UqUld-Ges Interfaces; WileyInterscience: New York. 1966. Blodgett, K. 8.; Langmuk. I.phvs. Rev. 1937. 51. 964. Langmuir, I.; Schaefer, V. J. J . Am. Chem. Soc. 1938, 60, 1351. Iwahashi, M.;Naito, F.; Watanabe, N.; Selmiya, T.; Morlkawa, N.; Nogawa, N.; Ohshim, T.; Kawakami, H.; Ukai. K.; Sugai, I.; Shibata, s.; Yasuda, T.; Shoji, Y.; Suruki, T.; Nagafuchi. T.; Taketanl, H.; Matsuda, T.; Fukushima, Y.; Fujioka, M.; Hisatake, K. Bull. Chem. Soc. Jpn. 1985, 58, 2093. Fujihira, M.;Araki, T. Chem. Lett. 1986, 921. Zhang, X.; Bard, A. J. J . phvs. Chem. 1988, 92, 5566. Anderson, S.; Seddon, K. R. J . Chem. Res. ( S ) 1979, 74. Burstail, F. H.;Nyhoim, R. S. J . Chem. Soc. 1952, 3570. Obeng, Y. S.; Bard, A. J. Langmuk 1991, 7 , 195. Chang, M-M.;Saji, T.; Bard, A. J. J . Am. Chem. SOC. 1977, 99. 5399. Hammerich, 0 . In Organic Electrochemistry; Baizer. M. M.,Lund, ti., Eds.; Marcel Dekker: New York, 1983; pp 485-496. Yang, H.; Bard, A. J. J . Electroanel. Chem. I n t e r f a c i a l E k t . , in press. Berkenkotter. P.; Nelson, R. F. J . Electrochem. Soc. 1973, 720, 346. Roulkr, L.; Lavkon, E. J . Ebciroenal. Chem. Intsffaclal Elecbpchem. 1982, 134, 181. Wightman, R. M.; Wipf. D. 0. In ElecboanelyNcel Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15.
RECEIVED for review Januray 11,1991.Accepted May 24,1991. The support of this research by grants from the National Science Foundation (CHE 8901450) and Exxon Corp. is gratefully acknowledged.
Electrocatalysis and Amperometric Detection Using an Electrode Made of Copper Oxide and Carbon Paste Youqin Xie and Calvin 0. Huber* Department of Chemistry, University of Wisconsin-Milwaukee, P.O. Box 413,Milwaukee, Wisconsin 53201
A m p e m t r k flow measurements were made at + O M V vs Ag/AgCl In 0.1 M NaOH background electrolyte wlth Cu20/ C-paste ektrochs. Stable and sendttve electrocatalytk responses for oxldatlons of carbohydrates, amlno aclds, allphatlc dlols, simple alcohols, amlnes, and alkyl polyethoxy akohol detergents were o h w e d . Linear responses at low concentrath yielded detectkn lhntts In the picomole range. Typical relatlve standard devlatlons were 1.2%. Llnear plots Indlcated Langmulr-type adsorption of of 1//, vs l/C-, anatytes on the electrode wtface. I t was concluded that the actlve sltes could be represented as CuO'OH. CuO, CuCI, and Cu20 were examined as startlng materlals, and the last was found to be the best. A proposed mechanism for 0x1datlons of these anatytes requlred both hydroxyl radlcal active dtes (CUO'OH) and nelghborlng adsorbed analyte Snw. The rate-determlnlng step Involved the formatlon of a brldged cyclic lntermedlate and the abstractlon of a hydrogen from the atarbon by the adsorbed hydroxyl radical attack.
INTRODUCTION The detection of aliphatic compounds in flow injection analysis (FIA) and high-performance liquid chromatography (HPLC) has been of great interest to analytical chemists for the past decade. Aliphatic compounds do not have strong chromophores in the UV-visible range. Low-wavelength UV and refractive index techniques are often limited in specificity 0003-2700/91/0363-1714$02.50/0
and sensitivity. The development of electrode materials for direct amperometric detection of aliphatic compounds is a growing area of electroanalytical chemistry. The commonly used carbon amperometric electrodes, however, exhibit no response for aliphatic compounds. Metallic electrodes have thus been utilized in place of carbon electrodes. At present, gold (1-61,platinum (7-9),nickel (1&16), and copper (17-19) have been reported useful for detection of aliphatic compounds. The inherent instability and nonreactivity of noble metallic electrodes, such as gold and platinum, have been overcome by pulse-amperometric detection (PAD), which combines cleaning and activation steps with detection (20,21). For active metallic electrodes, such as nickel and copper, the electrode processes in alkaline solutions involve the electrochemical formation of metal oxide or hydroxide films of higher oxidative states, e.g., NiO(0H) and CuO(OH), which have been proposed to behave aa the redox mediators associated with analyte oxidations. In addition, chemically modified electrodes (CMEs) (22),containing surface-bound inorganic redox species have been described,which demonstrate unusual catalytic stability and reactivity for aliphatic compounds. For example, copper deposits on glassy-carbon (23-25) and Ru02-mixed carbon paste (26)greatly enhanced the amperometric detection of carbohydrates and other related compounds at the picomole level under constant applied potentials. Continuing investigations in this laboratory involving transition-metal oxide electrode responses suggested that copper oxide particles in a carbon-paste matrix warranted 0 1991 American Chemical Society
““TFVmo ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
I
to ,000
tO.800
T
1715
T
SOD
I
E,V vs. AgIAgCI Flgwe 1. Voltammetrlc response of Cu,O CME In 0.1 M NaOH blank solution: (A) lnltlal scan: (B) second scan. Scan rate: 50 mVls.
examination for amperometric applications. Preliminary studies have shown that copper oxide particles in carbon paste exhibited a highly stable electrocatalytic effect and extremely high current density for the oxidations of aliphatic compounds. In this paper, the electroanalytical chemistry and applications of the copper oxide/carbon-paste electrode for the amperometric detection of saccharides, alditols, amino acids, alcohols, and amines, at constant applied potentials, in flow systems is reported.
EXPERIMENTAL SECTION Reagents. Saccharides, amino acids, aliphatic diols, alditols, amines, and related compounds (Aldrich Chemical Co.,Milwaukee, WI) were used as received without additional purification. Stock solutions were prepared fresh daily in 0.1 M NaOH and were diluted to the desired concentration and pH just prior to measurement. Experimentswere performed by using 0.1 M sodium hydroxide as the background electrolyte unlw otherwise specified. Cuprous oxide, cupric oxide, and cuprous chloride (Aldrich Chemical Co.) were reagent grade materials and were used as received. Apparatus. Cyclic voltammetry was conduded with a BAS100 electrochemicalanalyzer (BioanalyticalSystems, Inc., West Lafayette, IN). A BAS Model VC-2 electrochemical cell was employed in these experiments with a carbon-paste working electrode (BAS,MF-2010),the reference electrode (Ag/AgCI,BAS, MF-2020), and the platinmwire auxiliary electrode. All cyclic voltammograms were obtained with unstirred solutions. The flow injection system consisted of a reservoir that fed the carrier solution via gravity, a sample injection valve (Rheodyne, Model 50,20-pL sample loop), interconnecting Teflon tubing (2 mm i.d.), and a thin-layer electrochepical detsdor (made in-house, BAS Model TI-4 style, with a working electrode area of 0.07 cm?. An in-house opamp potentiostat and a potentiometric stripchart recorder (Houston Instruments,OmniscribeModel B5217-15) were used in the flow injection experiments. Flow rates were determined volumetrically in the waste. Cuprous oxide (CuzO)/carbonpaste (usually 20%:8O% w/w cuprous 0xide:carbon paste) was prepared by mixing weighed amounts of Cu20and carbon paste (Metrohm Ltd., Switzerland, already mixed with mineral oil) thoroughly with a mortar and pestle. Portions of the resulting paste were packed into the cavity of the flow detector. The surface was then smoothed on a piece of weighing paper. The same procedure was used to prepare CuOand CuCl/carbon-paste electrodes. RESULTS AND DISCUSSION Electrochemistryof Cu,O/CPE. The cyclic voltammetry of a 20% Cu,O-modified carbon-paste electrode in 0.1 M NaOH solution in the absence of any analyte is shown in Figure 1. The broad peak a t ca. +0.16 V disappeared after the first scan; its magnitude increased almost linearly with the loading percentage of CuzO in the paste (not shown). This peak current also increased upon increasing concentration of NaOH, while the peak potential shifted cathodic (-0.33 V/ pH). This peak was assigned to the formation of CuO film from CuzO, in agreement with oxidation of metallic Cu
Figwe 2. Cyclic voltammogram for 5 mM gkrcose at ChO CME. Scan rate: 50 mVls.
electrodes in alkaline solutions (27-29). As can be seen from Figure 1,there was no anodic peak in the potential range +0.40 to +0.60 V where Cu(II1) oxides would be expected (27-29); instead, Figure 1revealed a rather large hysteresis between the anodic and cathodic currents obtained for the positive and negative scans, respectively, with a maximum hysteresis a t ca. +0.57 V. This hysteresis was more significant with higher loadings of Cu20,similar to the data reported by Johnson (30) for CuO-film electrodes. As the positive-scan limit was increased from +0.6 to +0.8 V, the cathodic current increased significantly for the subsequent cathodic scan, but further increase in the scan limit to +LO V produced only a very small enhancement of the subsequent cathodic current. The above results indicate that the small net charge for the cathodic process a t ca. +0.57 V was apparently not from reduction of oxygen generated by water discharge, but rather from the reduction of Cu(II1) oxide formed at E 1 +0.5 V. The formal reduction potential in 1 M NaOH for CuO/CuO(OH) was estimated as +0.56 V vs Ag/AgC1(31), in good agreement with the maximum hysteresis potential observed here. After the first cycle, the peak at ca.+0.16 V was not obse~ed,suggeathg that the Cu surface oxidation state is not reduced below 2 in this potential range. The residual voltammograms of CuO/CPE were similar to that of Cu20/CPE, except that for CuzO there was a CuO formation peak a t ca. +0.16 V for the first cycle. The cyclic voltammogram of CuCl/CPE in 0.1 M NaOH blank solution showed a rather large anodic peak With peak potential of ca. +0.55 V for the first scan, which could be assigned to the formation of Cu2(0H)&1 according to a Pourbaix diagram (32),and this peak disappeared for the following scans, which indicates passivation of CuCl/CPE. Addition of 5 mM glucose to the blank solution greatly increased the current in the vicinity of +0.5 V, but no well-defined peak was observed. After a stable cyclic voltammogram of Cu20/CPE was obtained in 0.1 M NaOH, several different analytes were added and the voltammograms were determined. Figure 2 shows an example for 5 mM glucose. Compared to the background electrolyte voltammogram, anodic current enhancements were observed at potentials ranging from +0.40 to +0.75 V. Cyclic voltammetric anodic peak potentials for some analytes are listed in Table I. No cathodic current was observed for the subsequent cathodic scan, instead an anodic peak was obtained with almost the same magnitude as with the anodic scan. Similar phenomena were also observed by using the nickel oxide electrodes with unstirred solutions (33). The anodic peak current exhibited a current versus scan rate linearity up to 100 mV/s for a constant positive-scan limit; the peak potential shifted positive when the scan rate was increased. This is evidence of a slow surface Faradaic process. Peak currents also increased significantly upon increasing the NaOH concentration from 0.05 to 0.4 M (not shown), and the peak potential shifted to more cathodic values (ca. -0.12 V/pH), indicating that hydroxide ion was involved in the
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
Table I. FIA Responses of Cu10 CME to Various Hydroxyl Compounds
CV peak
compd
pot., mV vs Ag/AgCl
lS 1.4
5 1
FIA
LOD,” sensitivity, nA/(cm2.pM) ng
I. Carbohydrates
glucose ribose galactose lactose sucrose
maltose maltotriose maltotetraose maltopentaose
582 560 585 585 590
585 590
595 595 11. Amino Acids 678 680 670 670
ethylene glycol 1,2-propanediol 1,2-butanediol
111. Diols 705 700 660
215 300 200 125 109 110 74 64 56
28.9 5.7 9.2 9.2
0.2 0.1
0.3 0.6 0.7 0.6 1.6 2.4 3.2
0.4 4
3 4
6.5
2 3
5
4
7.5
IV. Amines
ethylenediamine diethylenetriamine
755 760
78.6 93
0.2 0.3
V. Others 0.42 28 ethanol 615 98 0.2 glycerol 7.5 23 Triton-100x a Potential applied, +0.55 V vs Ag/A&l; signallnoise = 3.
reaction mechanism. The oxidation peak currents were also proportional to the CuzO loading in the carbon paste up to 20% by weight. When analyte cyclic voltammograms were obtained with newly packed Cu20/CPE, the anodic peak current increased by almost 35% on the second cycle; only after the f&h cycle were the voltammetric responses stabilized. For example, the oxidation peak current for 5 mM glucose increased by only about 4% over 100 repetitive scans after the fifth scan. This was true for every analyte. Flow Injection Analytical Response. Mixed-paste electrodes using Cu20,CuO, or CuCl with carbon were tested for oxidations of aliphatic compounds. The voltammetric data suggests that these species apparently function only as providers of copper oxide active sites at appropriate potentials and thus should exhibit similar electrocatalytic effects on oxidation of aliphatic compounds. The FIA oxidation current density for 1 mM glucose a t +0.55 V with 20% Cu20/CPE was about 4 times higher than that with 20% CuO/CPE. This 4-fold factor was also observed for other loadings and at other glucose concentration levels. The background current densities for these two electrodes were about the same (ca. 1.7 pA/cm2 at +0.55 V with 20% loadings). Apparently, the differences in electrocatalysis are related to total electrode area and microelectrode array density aspects of particle size, particle shape,and molecular weight effects. The particle size of CuzOwas visually observed to be smaller than that of CuO. The CuCl/CPE did not produce any net oxidative current for 1mM glucose under the same FIA conditions. On the other hand, the background current densities for 20% CuCl/CPE at +0.55 V was about 100-fold larger than that of Cu20/CPE or CuO/CPE. A decrease in CuCl loading to 5% failed to improve the situation. Of the three materials examined here, CuzOwas clearly the best starting material to provide copper oxide active sites. As a point of interest, CuCl has been reported to be superior to other copper species in terms of
0’3 0.2
3 5
15
25
35
Loading (“9 Cu,O) F l g u O 3.
CUZO LOedlng Effect On FIA Of 0.1 mM glucose: (A) without pretreatmnt; (B) with pretrealment at +0.75 V for 1.0 mln. condltkns: swortlng electrolyte,0.1 M NaoH; Fbw rate, 1.0 Wmln;E,, + O M V.
background noise levels and detection limits when it was deposited from solution onto a glassy-carbon electrode (25). The dependence of the FIA peak current for 0.1 mM g l u m at +0.55 V on Cu20 loading is shown in Figure 3. The peak currents increased upon increasing CuzO loading up to about 20% by weight, similar to the cyclic voltammetry results with newly packed electrodes. Pretreating the electrode a t +0.75 V for 1.0 min before any injections of analyte enhanced the analytical currents. The relative enhancement with loading decreased for loadings higher than about 25%, while the net increases in peak currents were about the same (ca. 0.12 f 0.01 PA) for Cu20loadings from 5% to 40%. The mechaniim for this unusual constant net current increase in peak current is still not clear. To optimize the applied potential, the hydrodynamic voltammetry (HDV)results for the oxidation of glucose, ribose, and glycine were studied at 15-25-mV increments between +0.2 and +LO V. When an unmodified carbon-paste electrode was used, none of these anlaytes contributed any significant currents over the entire potential window. With Cu20/CPE, distinctive maxima for current versus applied potential were obtained in the concentration ranges, yielding linear amperometric responses (lower than 1mM). The hydrodynamic voltammetry peak potentials at different concentrations of analyte are listed in Table 11. At applied potentials greater than the optimum, competition by excess hydroxide ion and analyte for sites on the electrode surface apparently limited the current. Adsorption of hydroxide anions would be more sensitive to applied potential than neutral analyte species. At potentials less positive than the optimal value, the population of active sites on the electrode surface is apparently controlled by the applied potential. A t lower concentrations,fewer active sites are needed, hence lower optimum applied potentials were observed. At concentrations higher than about 5 mM, the current versus concentration data showed that a saturation
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, l B B l ~~
1717
~~~~
Table 11. HDV Peak Potentials of Some Hydroxyl Compounds at Different Concentrations HDV peak pot., mV v8 AgJAgC1
compd
conc,mM
glucose
0.01 0.1 1.0 5.0
500 550 650
ribose
0.1
525 625
plateau
1.0
plateau
5.0 0.1
glycine
1.0
625 700
5.0
plateau
71
Y
d
4 {
0 0
o
a4
0.8
1.2
1.6
x)
40
€4
80
1w
2.0
Flow Rate ( mllmin )
Flgure 4. Effect of flow rate on the FIA response of 0.5 mM glucose. Conditions: supporting electrolyte, 0.1 M NaOH E,, +0.55 V.
effect occurred, i.e., the currents were limited by available adsorption sites, rather than by reaction kinetics. The response of CuzO/CPE is strongly dependent on the NaOH concentration, typical for anodic electrode systems for aliphatic compound detections. The FIA peak current for 1 mM glucose increased dramatically with NaOH concentration to 0.1 M, after which it remained almost constant, similar to the cyclic voltammetry results. Similar results were observed for ribose and glycine. The dependence of the anodic current upon the concentration of hydroxide ion may result from several processes that enhance oxidations of aliphatic compounds in the presence of high alkalinity. This will be discussed in detail below. The effect of flow rate on the CuzO/CPE response for glucose is illustrated by Figure 4. A decrease in peak current was observed for increasing flow rate between 0.05 and 2.2 mL/min. The dispersion and hydrodynamic effects alone would predict the opposite pattern. This behavior can be caused by slow adsorption of reactant or slow desorption of the product, as well as decreased residence time of the anal@ at high flow rate. The effects of hydrodynamic conditions on glucose oxidation in batch mode were studied by using a rotated disk CuzO/CPE. It is obvious that the KouteckyLevich plot ( l / i vs not shown) had only one slope in the rotation speed range 50-2000 rpm. This means that the number of electrons involved in glucose oxidation at the CuzO/CPE surface is the same at low and high rotation speed regions, respectively. The reactant adsorption shows Langmuir-like properties (see below), so that slow adsorption of reactant is unlikely. Apparently, therefore, the adsorption of the electrode reaction product would limit the electrode process. Significant peak broadening was observed at low flow rates, and this can be attributed to dispersion of the sample plug and a slow desorption process of the product. The long-term stability was examined by measuring the flow injection peak current for 0.1 mM glucose. Without pretreatment of the electrode at +0.75 V for 1.0 min, the response
1/Concentration ( 1/mM ) Flgure 5. Reciprocal plots of FIA response vs concentration for (A) sucrose, (B) glucose, and (C) ribose. condltkns: supporting electrolyte, 0.1 M NaOH; flow rate, 1.0 mL/min; E-, + O S V.
usually increased by ca,10% during the first 20-30 injections. When the electrode was pretreated, the relative standard deviation for the first 100 consecutive injections was only 1.2% and remained constant throughout the remainder of the 24-h test period. Longer term observation of the Cu,O/CPE responses indicated decreases for analytical currents of only 4% from day to day with a completely continuous flow stream, even when the flow rate wm slowed and the potentiostat was disconnected between working sections. These results indicate analytically useful catalytic stability. The lifetime of this electrode is at least 1 week. The amperometric measurement characteristics for carbohydrates, amino acids, aliphatic diols, amines, and related compounds are shown in Table I. Linear calibration plots (peak current vs concentration) were found for all of these analytes at an applied potential of +0.55 V, with correlation coefficients of at least 0.997. The linear range was about 2 orders of magnitude for ethanol, which showed the lowest sensitivity. Other analytes yielded larger linear ranges, e.g., that for ribose is about 5 orders of magnitude. The detection limits were calculated from their sensitivities (slopes of the calibration plots) and based on the signal-to-noise level of 3. All these values are comparable to those with pulse-amperometric detection at metallic surfaces (21). The CuzO/CPE provides higher sensitivities for carbohydrates and simple alcohols when compared to other chemically modified electrodes. For example, the sensitivity for glucose with 20% CuzO/CPE was 215 pA/(cm2.mM), which was significantly greater than that reported for RuOz CME (26) and Cu CME (23) (estimated from Figure 4 therein). Baldwin et al. reported that no response was observed for methanol and ethylene glycol with the Cu CME (H), whereas the sensitivities reported here were 0.45 and 7.5 pA/(cm2-mM),respectively. Plota of l/i, vs l/Cdm showed linear analytical relationships for these analytes. Figure 5 illustrates several examples with glucose, ribose, and sucrose. These data indicate Lang-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
:
10
I
I
A m
1 No. of Monosaccharide Rings
-0.2 0.18
9 L.
5 Y
m
a, a
1
i
radical (*OH)&generated by anodic oxidation of water. The kinetic information obtained for the oxidation of organic compounds at a nickel oxide electrode in basic solution has been interpreted by Fleischmann et al. (31,35) to suggest that the nickel oxide surface contains “trapped” hydroxyl radicals and the rate-controlling step in the anodic oxidation is the abstraction of a hydrogen atom from the carbon in the aposition to the functional group. The general mechanism proposed for the oxidation of aliphatic compounds at the CupO surface must be consistent with the experimental observations reported in the previous sections. The mechanism must account for the following: (a) optimal applied potentials are observed, which shift positive with increased analyte concentrations; (b) FIA peak currents increase directly with hydroxide concentration below 0.1 M (c) analytes adsorb onto the CuO surface before their oxidations, and the adsorption conforms to a Langmuir model; (d) the rate-determining step is the abstraction of a hydrogen atom from the a-carbon according to Fleischmann (31,35). Equation 1 includes the formation of a CuO layer from Cu20oxidation. Equation 2 shows the chemisorption of hydroxide ions on the CuO surface lattice followed by oxidation of the hydroxide to a hydroxyl radical. Meanwhile, at adCuzO
0.12
-
CuO
+
HzO
+
-
0.M
1
0.02 0.04
I
01
2CuO + HzO
cuo
+
H+--
0 0
0.10.08
-
20H-
I
CUO +
OH RCHzOH
2e
+
CuO
+ e
I
*OH
+
cr-
RCHZOH I
1
2
3
4
5
I
No. of Monosaccharide Rings Flguro 6. Effect of molecular size on FIA response of some saccharldes: (A) 1 mM, (B) 0.01 mM. Other conditions are the same as
CUO
I
*OH
+
CUO
t
RCH20H
-cu-0-cu-0
HCH
for Figure 5.
muir-type analyte adsorption on the electrode surface. This relationship can also be used to extend the analytical response range. A common characteristics for the analytes listed in Table I was that the FIA responses decreased with the increase in molecular sizes within the same classification group. Similar observationsfor carbohydrates were reported for Ru02/CME (26).The relationship between FIA response with CuzO/CPE for saccharides and the number of monosaccharide rings in their structure is shown in Figure 6 at two concentration levels. It can be seen from Figure 6A that from glucose to maltotriose a t the 1 mM level the FIA signal was about inversely proportional to the molecular size. As stated earlier, those analytea adsorbed onto the electrode surface before the oxidation process and the adsorption obeyed the Langmuir isotherm. For still larger molecules (more than four rings), the cleavage of ring-to-ring bonds yielding smaller oxidizable species would tend to offset the above trend. A t the lower concentration level (10 pM, Figure 6B), the oligosaccharidesignals are apparently not regulated by surface availability, but rather by ringto-ring cleavage release of electroactive functionalities. Comparison of the signals for glucose and maltose suggests that the ring-to-ring cleavage for the oligosaccharidesis not complete in this electrolyte. Reaction Mechanisms. Copper(II1)oxide, or CuO(OH), generated at positive potentials has been proposed as the redox mediator for the oxidation of several amines a t copper electrodes (31, 34). Johnson et al. (30) proposed an 0-transfer mechanism for CN- oxidation at the CuO film electrode that required both CuO(0H) sites and the adsorbed hydroxyl
t
I
OH
I
R 2CuO + RCHOH RCHOH
+
40H-
-
RCOII
+
+
Hz0
3Hz0 + 3e
0
jacent lattice site, the analyte also adsorbs onto the electrode surface as shown in eq 3. The rate determining step (eq 4) involves the formation of a bridged cyclic intermediate and the abstraction of a hydrogen atom from the carbon in the a-position to the functional group. After the a-hydrogen abstraction the analyte radical is further rapidly oxidized to carboxylate or other product. The standard potential for ‘OH/OH- is reported as +1.33 V vs Ag/AgC1(36). The adsorption process probably enhances the stability of the radical, allowing its formation at the applied potential used here. This mechanism is consistent with the observation of an optimum applied potential. A t insufficiently positive potentials, the number of CuO’OH sites is limiting, whereas at excessively positive potentials, the population of adjacent adsorbed analyte sites to yield the necessary bridged cyclic intermediate is limiting. The population of CuO’OH (eq 2) is more dependent upon applied potential than adsorption of analyte at adjacent sites (eq 3), corresponding to more positive optimum applied potential with increasing analyte concentrations, as shown in Table 11. The observation reported above that analytical currents are first order with respect to hydroxide concentrations below 0.1 M is consistent with the proposed mechanism. Hydroxide ions are required
Anal. Chem. 1991, 63, 1719-1727
to neutralize the protons generated during the formation of active sites, CuO'OH, through discharge of water (eq 2). At low hydroxide concentrations, formation of active sites would be expectad to proceed at a slower rate, which directly affects the formation of the bridged intermediate. Further oxidation of the analyte radical, RCHOH, is dependent upon the concentration of hydroxide. Thus, the anodic process reaches a limiting current when hydroxide is in excess.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14)
Rodtlln, R. D.; Pohl, C. A. J . Llq. chrometog.1083, 6, 1577. Edwards, P.; Haak, K. K. Am. Lab. 1083, Aprll, 78. N e W g e r , G.; Johnson, D. C. Anal. Chem. 1087, 59, 203. Johnson, D. C. Netwe 1086, 321, 451. Welch, L. E.; LaCourse, W. R.; Mead, D. A., Jr.; Johnson, D. C. Anal. Chem. 1980, 6 1 , 555. LaCourse, W. R.; Jackson, W. A.; Johnson, D. C. Anal. Chem. 1089, 6 1 , 2466. Huges, S.; Johnson, D. C. J . A g k . FoodChem. 1082, 30, 712. Pdte. J. A.; Johnson, D. C. J . Llq. Chrometog. 1083. 6 , 1727. Austin, D. S.;Polta, J. A.; Polta, T. 2.; Tang, A. P.C.; Cabelka, T. D.; Johnson. D. C. J . Electroenel. Chem. InterfeclelElecb.ochem.1884, 108, 227. Schick, K. G.; Magearu, V. G.; Huber, C. 0. Clh. Chem. 1078, 2 4 , 448. Morrison. T. N.; Schick, K. G.; Huber, C. 0. Anal. Chlm. Acta 1080, 120, 75. Hul. B. S.; Huber, C. 0. Anal. Chlm. Acta 1082, 134, 211. Yuan, C. J.; Huber, C. 0. Anal. Chem. 1085, 5 7 , 180. Kafil, J. 6.; Huber, C. 0. Anal. Chlm. Acta 1985, 176, 275.
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(15) Botros, N.; Huber, C. 0. Anal. Chkn. Acta 1088, 208, 247. (16) Arkawa, Y.; Huber. C. 0. Anal. CMm. Acta 1000, 229, 191. (17) Kok, T. Th.; Brlnkman, U. A. Th.; Frei, R. W. J . clwometog. 1083, 256, 17. (18) Kok, W. Th.; Hanekamp, H. 6.; Bos, P.; Frei, 0. W. AMI. Chlm. Acta 1082, 142, 31. (19) Stulik. K.; Pacakova, K. Le; Hennissen, B. Talenta 1088, 3 5 , 455. (20) Huges, S.; Johnson, D. C. Anal. Chim. Acta 1081, 132, 11. (21) Johnson, D. C.; LaCourse, W. R. Anal. Chem. 1000, 6 2 , 589A. (22) Wang. J. Anal. C h h . Acta 1090, 234, 41. (23) Prabhu, S. V.; Baldwin, R. P. Anal. Chem. 1080, 67. 852. (24) Prabhu, S. V.; Baldwln, R. P. Anal. Chem. 1080, 61, 2258. (25) Luo, P.; Prabhu, S. V.; Baldwln. R. P. Anal. Chem. 1000, 62, 752. (26) Wang, J.; Taha, 2. Anal. Chem. 1000, 62, 1413. (27) Miller, B. J . Electrochem. Soc. 1080. 716, 1675. (28) Pyun, C.-H.; Park, S.M. J . Electrochem. Soc. 1088, 133, 2024. (29) Burke, L. D.; Ahem. M. J. G.; Ryan, T. 0. J . E k t t u d " . Soc. 1000, 137. 553-61. (30) Wels, 6.; Johnson, D. C. J . Electrochem. Soc. 1990, 137, 2785. (31) Flelschmann, M.: Korlnek, K.; Pietcher, D. J . Chem. Soc., M/n Trans. 1072, 2 , 1396. (32) Bard, A. J. Encyclopedia of Electrochemlsby of the E&ments; Marcel Dekker, Inc.: New York, 1974; Voi. 11, p 393. (33) Enea. 0. E l d " . Acta 1000. 3 5 , 375-378. (34) Meyersteln, D.; Hawkridge, F. M.; Kuwana, T. J . Electroenal. Chem. Interfacie1 Ekctrochem. 1072, 40, 377. (35) Fleischmann, M.; Korinek, K.; Pletcher, D. J . Electroanal. Chem. I n terfacialEfsctrochem. 1971, 3 1 , 39. (36) Bard, A. J.; Parsons, R.; Jordan, J. Standard PotenHels In Aqueocrs Sdutions; Marcel Dekker, Inc.: New York, 1985; p 63.
RECEIVED for review January 22,1991.Accepted May 6,1991.
Surface Acoustic Wave Vapor Sensors Based on Resonator Devices Jay W. Grate* and Mark Klusty' Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000
Surface acourtk wave (SAW) devlces fabrlcated In the resonator conflguratlon have been utlllzed as organlc vapor sensors and compared wlth delay line devlces more commonly used. The expwhwntaHy detennkred mass semMvMs of 200-, 300-, and 400-MHz resonators and 158-MHz delay the8 coated wHh Langmulr-Blodgett (LB) fllms of poly(vlny1 tetradecanal) are In excellent agreement wlth theoretical predlctknr. The responses of L E and spray-coated sensors to varlow organk vapors were determlned, and scaling laws for mass sendtlvltles, vapor benslttlvltles, and detectlon llmlts are dlscussed. The 200-MHz resonators provide the lowest nolse levels and detectlon llmlts of all the devlces examlned.
INTRODUCTION Interest in the use of surface acoustic wave (SAW) devices as chemical sensors increased rapidly during the past decade. Wohltjen first reported this method of chemical detection in 1979,and now several general articles and reviews have appeared (1-8). The SAW device responds to changes in the mass on its surface with a shift in frequency and is most frequently used in gas-phase sensing applications. A thin overlayer of a chemically selective material serves to collect and concentrate analyte molecules at the sensor's surface by sorption. The sensitivity of this type of sensor is dependent
*
To whom all correspondence should be addressed. 'Present address: Microsensor Systems, Inc., 6800 Versar Center, Springfield, VA 22151.
on the amount of vapor sorbed and the SAW transducers inherent ability to respond to the physical changes in the overlay film caused by vapor sorption. We define sensitivity as the incremental change in signal occurring in response to an incrementalchange in analyte concentration. The detection limits achievable depend on the vapor sensitivity and on the noise of the sensor's signal. The vast majority of investigations of SAW chemical sensors have used SAW devices fabricated in the delay line configuration. Indeed, a review by DAmico and Verona specifies the types of SAW devices utilized, and all those for chemical sensing were delay lines (4). An alternative SAW device configuration is the resonator. Whereas the delay line consists of metallized interdigital transducers on the ends of a planar piezoelectric substrate, the resonator has metallized interdigital transducers placed centrally on the planar device inside a resonant cavity defined by microfabricated reflectors. These two configurations are compared in Figure 1. We became interested in SAW resonator devices because of the possibility that chemical sensors based on these devices might have lower noise levels and hence yield lower detection limits than similarly coated devices based on delay linea. Bare SAW resonator devices have narrower bandwidths and higher Q values than delay lines, which results in lower noise levels (7,9).It remained to be established, however, whether resonator devices would tolerate soft organic coatings on their surfaces and whether lower noise levels would be maintained after a coating had been applied. To our knowledge, the first use of a SAW device in the resonator configuration for chemical detection was reported by Martin et al. (IO). These authors prepared ZnO-on-Si
Thls article not subject to U.S. Copyrlght. Publlshed 1991 by the Amerlcan Chemical Soclety
