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Anal. Chem. 1988, 60,72-76
Bioaccumulation and Measurement of Copper at an Alga-Modified Carbon Paste Electrode Jorge Gardea-Torresdey, Dennis Darnall,* and Joseph Wang* Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A new strategy for electrode modification with a mlcroorganlom for the bloaccwnulatkn and subsequent voltammetrlc quantitation of metal ions is described. Alga Is used as the surface modifier, and the resulting electrode exhibits preferential uptake of copper( 11) from dilute solutions (using open-circuit conditions). The voltammetrlc response is evaluated with respect to preconcentration time, pH, ionic strength, Cu( I I ) concentration, electrode composition, voitammettic waveform, and other variables. Rapid and convenient add renewal allows use of a dngk modllted electrode in multiple analytical determinatlons over a 4-week perlod. For 16 accumulatlon/measurement/renewal cycles, the response could be reproduced with 4.9% relative standard deviation. Short preconcentration times permit convenient measurements down to the micromolar concentration level. No major interference was observed when the analysis was petformed in the presence of excess of varlous metals. These developments should lead to a new class of modified electrodes, with different patterns of reactivity, utfflzing the unlque Interactions of metals with biological systems.
The design of appropriate chemically modified electrodes (CMEs) should allow development of new voltammetric measurement schemes with enhanced selectivity and sensitivity. One approach of using CME9 to increase the sensitivity and selectivity of a voltammetric procedure is to use a surface capable of preconcentrating the analyte from solution. Most popular schemes used to trap analytes on the electrode surface are based on complexation (1-3) and electrostatic attractions (4-6); covalent attachment can also be used (7). The preconcentration agent (ligand, ion exchanger) is commonly introduced into the surface as part of an appropriate polymeric film or by mixing with the carbon paste matrix. Such nonelectrolytic avenues for the preconcentration step not only extend the scope of stripping techniques toward numerous analytes but mainly offer the potential for designing a new type of CMEbased sensors that couple the inherent sensitivity of voltammetry with the selectivity provided by the chemical requirement of the modifier-analyte interaction. Hence, the success of this concept for practical analytical applications depends primarily upon the choice of the modifier. For example, carbon electrodes modified with dimethylglyoximeor triotcylphosphine oxide have been useful for the determination of nickel(I1) and uranium(VI),respectively, in various matrices (1, 8).
This paper describes a new concept for the preconcentration/voltammetric approach based on bioaccumulation of metal ions at a microorganism-modified carbon-paste electrode. It has been known for sometime that heavy metal ions are accumulatedby microorganisms such as algae, fungi, yeast, or bacteria (+11). High binding capacities have been observed under controlled conditions. Such binding or biosorption of metal ions to algae has been exploited recently in our laboratory for effective removal and recovery of metal contaminants from water and in the selective recovery of valuable metals (12-16). Biosorption of metal ions to the algae cell wall
is not dependent upon a living organism, and thus metal ion binding is often observed under conditions that would normally be detrimental to a living organism. We have found that metal ions can be divided into three classes depending upon the pH dependence of the biosorption (16). The first class is comprised of metal ions that are tightly and rapidly bound at pH 2 5 and that can be stripped (or are not bound) at pH 52. Many metal ions fall into this class: Al(III), Cu(II), Pb(II), Cr(III), Cd(II), Co(II), Zn(II), Fe(III), UO,(VI). The second class is comprised of metallic anions that display the opposite behavior of class I metal ions, i.e., they are strongly bound at pH 1 2 and weakly bound or not bound at all at pH near 5. Ions in class 11include PtC1,2-, Cr042-/Cr2072-, MOO:-, and SeO:-. The third class of metal ions includes those metal ions for which there is no discernible pH dependence between pH 1 and 7, which includes Ag(I), Hg(II), and Au(II1). These three ions are the most strongly bound of all metal ions tested. While the exact mechanism of binding of all metal ions is not clear at this time, it is clear that the biopolymers in the cell walls of algae are responsible for metal ion sorption. Nitrogen and sulfur have recently been shown to be ligating atoms for gold(II1) and gold(1) (17). Biosorption of metal ions in class I appears to occur via an ion-exchange process with metal cations competing with protons for negatively charged binding sites on the cell wall. The ability of algae to rapidly trap metal ions from dilute solutions and the different affinities of metal ions for algae surfaces have prompted this investigation into the development of electrochemical sensors based on algae-modified electrodes. The surface of the electrode thus becomes a mosaic of metal-ion binding sites, with different specificity and affinity. With a proper choice of the alga (and solution conditions) it may be possible to manipulate the binding properties of the electrode and hence its voltammetric response. The chemical and physical environments experienced by metals incorporatedonto algae could thus lead to new patterns of reactivity, not found in other conventional or modified electrodes. Because of its foundation on metal uptake by a biosurface, this concept should be extremely useful for metal-speciation studies (where such resemblance to biological systems is desirable). Biocatalytic properties of microorganisms have been exploited recently for the design of microbial electrodes (with operation similar to that of conventional enzyme electrodes (18))but not for preconcentration/voltammetric measurements of metal ions. The brown alga Eisenia bicyclis is employed in the present work for the preferential uptake of copper. The alga is incorporated into a carbon paste electrode in a simple and controllable manner and allows multiple analytical determinations using the same surface. We report the behavior and analytical advantages of this novel modified electrode in the following sections. Even though the concept is presented in the context of algaemodified electrodes, a number of other microorganism-based electrode sensors (suitable for the bioaccumulation/voltammetric scheme) can be visualized.
EXPERIMENTAL SECTION Apparatus and Reagents. Chemically modified carbon paste electrodes (11%alga by weight) were prepared by hand-mixing
0003-2700/88/0360-0072$01.50/0@ 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988 73 I
Table I. Apparent Copper(I1) Binding Capacity for Various Algae Speciesa algal species Chlorella pyrenoidosa Spirulina platensis Rhodymenia palmata Cyanidium caldarium Eisenia bicyclis
mg of Cu(I1) bound/g of dry cell 4.8
3.5 3.2 4.6 18.8
"Conditions: Washed algae were suspended at 4 mg/mL for 20-min intervals in a solution containing 0.31 mM copper(I1) in 0.05 M sodium acetate at pH 5.0. Reaction mixtures were then centrifuged at 3000 rpm for 5 min and decanted, and supernatant solutions were saved for copper analysis. This procedure was repeated with the same algal pellet until no significant copper binding could be detected. 0.5 g of rehydrated alga and 1.5 g of mineral oil (Aldrich); subsequently, 2.5 g of graphite powder was added to the a l g a 4 slurry and mixed thoroughly. The common brown alga, Eisenia bicyclis, waa purchased, in a sun-driedform, from Westbrae Natural Foods (Berkeley, CA). The alga was ground in a mortar and pestle to particles of around 150 pm diameter. For rehydrating the alga, 2 g of the Eisenia was immersed in 10 mL of water for 1h, after which the excess water was removed by centrifugation (3000rpm for 3 min). A portion of the modified carbon paste was packed into the end of a glass tube (4-mm id., 5-mm o.d.), and its inner end was connected to a copper wire. Three 100-mL cells were used. The preconcentration cell contained the sample solution under test; the measurement cell contained the supporting electrolyte solution; the cleaning cell contained 0.1 M hydrochloric acid. The Ag/AgCl reference electrode and the carbon rod auxiliary electrode were placed in the measurement cell through holes in ita cover. Differential pulse voltammograms were recorded with a Sargent-WelchModel 4001 polarograph. The experimental settings were used 1.0 V/min scan rate, 50 mV amplitude, and 1 s repetition time. Deionized water was used to prepare all solutions. Copper ion M) were prepared by dissolving CuSstock solutions (1.5 X O4.5H2Oin 0.01 M hydrochloric acid and stored in a polyethylene container. The supporting electrolyte was 0.05 M sodium acetate solution adjusted to pH 5 with sulfuric acid. Procedure. All experiments were performed using the preconcentration/medium exchange/voltammetry/cleaning procedure. For the preconcentration step, the modified carbon paste electrode was immersed in a stirred 100-mLsolution for a given period of time; the preconcentration proceeded at open circuit. The electrode was then removed from the preconcentration cell, briefly rinsed with deionized water, and placed in the electrochemical cell containing the supporting electrolyte solution. The initial potential (+0.5 V) was then applied and after 30 s the voltammogram was recorded by scanning the potential to -0.9 V. After the scan, the electrode was transferred to the cleaning cell for 2 min. A subsequent voltammetric run (performed in the measurement cell) was used to verify the absence of trapped copper ion on the surface. The same electrode surface was used throughout this study, with storage in a 0.1 M hydrochloric acid between experiments. The cleaning solution was exchanged daily with a fresh one to avoid potential buildup of copper(I1). Solution-phasecopper binding profiles were obtained by placing the alga directly in contact with a copper-containing solution and shaking for a given time, removing the biomass by centrifugation, and analyzing the supematant by atomic absorption spectroscopy. The copper binding to the alga was determined by the difference between the copper concentrationsinitially and that obtained after contact with the alga.
RESULTS AND DISCUSSION We have chosen the copperlEisenia bicyclis metal/alga combination to demonstrate the new preconcentration/voltammetric strategy. This alga was selected because of its strong affinity toward copper, compared to other species of algae. Values of the copper binding capacity for several algae species,
0.3
-0.1
-0.5
-0.9
Potcntlal ,V
Flgure 1. Voltammograms at an alga-modified carbon paste electrode that had been immersed for different periods (0.5 (A), 1 (B), 3 (C), 5 (D), and 7 min (E)) in a stirred 1 X lo3 M Cu(I1) solution, washed, and placed In the blank electrolyte solution: scan rate, 1 V/min; ampliiude, 50 mV; electrolyte, 0.05 M sodium acetate adjusted to pH 5 with sulfuric acid.
are given in Table I. The superior uptake a t the Eisenia bicyclis is apparent. A series of voltammograms demonstrating the incorporation of copper(I1) onto the Eisenia bicyclis modified electrode is shown in Figure 1. The modified electrode had been immersed in a stirred 1 X M copper(I1) solution for increasing periods of time, rinsed with water, and replaced in the blank electrolyte solution where the voltammogram was recorded. Open-circuit conditions were used during the preconcentration period. A well-defined peak is observed at -0.33 V upon scanning the potential between +0.5 and -0.9 V. The longer the preconcentration time, the more copper(I1) is accumulated onto the surface, and the larger the peak current. Analogous measurements a t an unmodified carbon paste electrode did not yield any copper response. The latter yielded a response only when dipped in the copper solution (EP= -0.41 V; bIl2 = 208 mV). To illustrate the uptake of copper by the alga modified electrode, all data reported in this paper were obtained following a similar transfer to the pure supporting electrolyte solution. Renewal of the surface is easily accomplished by immersing the electrode in 0.1 M hydrochloric acid for 2 min; the subsequent voltammetric run shows no copper peak. The effective cleaning and reproducible accumulation are illustrated by the precision obtained during a continuous 80-min period of operation. A series of 16 repetitions with 1 X M copper yielded a mean peak current of 6.9 FA with a range of 6.2-7.0 FA and a relative standard deviation of 4.9% (conditions, as in Figure 1B). This experiment indicates also that the electrode preparation results in a stable surface, with no leaching of the alga to the surrounding solution. Indeed, a single electrode surface was able to be used for periods of several weeks, performing several hundred sorption/stripping cycles with no noticeable loss of stability. Such performance is of great promise for the operation of alga-based electrochemical sensors, as tedious and often irreproducible regeneration schemes (e.g., manual resurfacing) are eliminated and long-term operation is possible. (In t h e event of electrode failure, the new surface yielded a response similar to that of the previous one, indicating homogeneous dispersion of the alga within the carbon paste
74
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
8-
4:
1
L
E
4-
/'
a
/'
V
/O
5
10
Alga Content
0.4
15 ,%(w/wl
Q.8
( NaC104J,M
Flgure 4. Effect of electrode compositlon (A) and ionic strength (B) upon the voltammetric response of the CME: copper(I1)concentratkm, 5 X lo4 M; preconcentration time, 90 s; other condltlons are as in
Figure 1. 2
4
6
Tirne,rnin
Flgure 2. Response of the alga-modified carbon paste electrode as a function of accumulation time at different Cu(I1) concentrations: 1 X lo-' M (A), 5 X lo-" M (B), 1 X M (C). Other conditions are
as in Figure 1.
a
Table 11. Linear Scan Data for the Bioeorptive Accumulation of Copper(I1) at Alga-Containing CMEs" time for alga content, satura5% (w/w) tion, min 5 11
10
20
15
5
rCU,
Qcu, C 9.5 x 10-5 1.2 x 10-4 2.5 x 10-4
mol/cm2 total amt, ng 4.0
x 10-9
5.0 x 104 1.0 x 10-8
32 41 84
"Conditions: as in Figure 1, except that linear scan (1 V/min) was employed aftm accumulation to steady state.
U
-a C
t
3,
1
3
5
PH
Flgure 3. Effect of pH on the preconcentration/voltammetric response of the alga-modified electrode (A) and on the blnding of copper(I1)to the natural Eisenie alga (8). The electrochemical profile (A) was M copper(I1)solution, 3-min preconobtained by using a 5 X centration, and other conditions as in Figure 1. The solution profile (B) was obtained by using a solution containing 2.5 mglmL alga, 5 X M copper(II),and 30-min contact time: other details are given in the text.
matrix.) Different portions of the same batch of modified carbon paste yielded reproducible response over a period of 6 months (with storage between 0 and 5 O C ) . Figure 2 shows the dependence of the peak current on the accumulation time at different copper(I1j levels. As the accumulation time increases, the response rises rapidly at first and then more slowly. More than 60% of the final response is generated within the first minute of accumulation. Such profiies are observed at the different concentrations employed and appear to represent the kinetics of the copper(I1)uptake. This behavior of the algae-modified electrode is in agreement with common models for the rates of metal accumulation in
algae: initial rapid uptake and subsequent slow uptake (9). Because of the fast binding process, realization of the full preconcentration advantage is obtained following relatively short preconcentration times (compared to some other preconcentration steps at CMEs (5, 19)). The effect of pH on the voltammetric peak height of copper(I1) closely parallels the solution-phase pH dependence of copper binding to the natural alga (Figure 3). In both cases, the copper binding and/or response increased slowly at first (between pH 1 and 3) and then more rapidly; a leveling-off is observed tit pH higher than 5. The decreased binding a t low pH values is attributed to protonation of weakly basic coordinating groups on the algal surface (12). The good correlation between the profiles shown in Figure 3 indicates that binding trends exhibited in solution can be extrapolated to an electrode surface (and may serve as a guide in selecting optimal conditions). For example, batch experiments with various natural algae indicate that different metals exhibit different pH profiies (13, 16). Such variations should enable one to improve the selectivity via pH control, as will be illustrated below. Further information regarding the bioaccumulation process was obtained by studying the effects of the paste composition and ionic strength. The voltammetric peak height increases linearly with the percentage of the alga in the carbon paste matrix (Figure 4A), as expected from the increased binding capacity of the electrode. In all cases, well-defined copper peaks were observed, with the 20% alga containing electrode exhibiting a relatively large cathodic bump in the base line at ca. -0.1 V. The 11% alga CME was employed in all subsequent work. Analogous linear scan measurementswere used to estimate the extent of the binding at the different algabased CMEs. The steady-state (saturation) response was employed to calculate the quantity of charge consumed by the surface process and the surface coverage. These data are
ANALYTICAL CHEMISTRY, VOL. 60, NO. l, JANUARY l , 1988
4:
a 11
t
d 4-
V Y v
i
1 2
4
6
Flgure 5. Differential pulse voltammetric response of the CME as a function of Cu(I1) concentration for 30-9 (A) and 180-9 (B) preconcentration times. Other condkions are as in Figure 1.
summarized in Table 11. The trend in the copper surface coverage is in agreement with the increased binding capacity of the electrode. Typical coverages range from 4 X lo4 to 1 x mol/cm2 for the 5% and 20% alga containing electrodes, respectively. The dependence of the copper(I1) peak current upon the ionic strength was examined by using sodium perchlorate as the preconcentrating media (Figure 4B). As the ionic strength increases, the response decreases rapidly at first and then more slowly. A similar profile, but with ca. 20% lower copper(I1) peaks, was observed with a potassium nitrate solution. Such behavior is attributed to the effect of ionic strength on the binding rate of cationic copper to weakly basic groups (such as carboxylates)on the algal cell wall. The data in Figure 4B were collected by using a preconcentration time of 90 s. Figure 2 (curve B) shows that for 5 X M Cu 90 s is considerably less than the time required for the system to reach equilibrium. Thus we would expect the effect of increasing ionic strength on a reaction involving positively and negatively charged reactants to be manifested by a decrease in reaction rate. In addition, the activity of free copper ion is considerably decreased with increasing ionic strength. A decreased reaction rate (i.e,, a decrease in rate of copper binding) in turn would correspond to a decreased peak current. We attribute the slightly lower peak current in the nitrate solution (as compared to perchlorate) as being due to the weakly coordinating nature of nitrate to copper ion which would further decrease the activity of free copper ion. Figure 5 shows the dependence of the CME response on the copper(I1)concentration using different preconcentration times. For 30-9 (A) and 180-8 (B) preconcentration, the response is linear up to 6 X and 4 x M, respectively (slope of the initial linear portion, 0.72 and 1.29 bA/104 M; correlation coefficients, 0.980 and 0.996, respectively). Deviations from linearity are observed at higher concentrations, as expected from the nature of the preconcentration process (i.e., saturation of binding sites). Possible means to extend the linear range include the use of unstirred solutions, shorter preconcentration times, or electrodes with higher alga content. Other voltammetric schemes based on nonelectrolytic accumulation at CMEs exhibit similar nonlinear concentration dependence (e.g., ref 3 and 5). By analogy to conventional stripping measurements, the quantity of the metal accumulated at the modified surface is generally dependent on two experimental parameters: the preconcentration time and analyte preconcentration, keeping the other conditions (stirring, pH, electrode composition, etc.) constant. Mea-
* 75
surements of 2.5 X M copper(I1) after 10-min preconcentration were used to estimate the detectability (other conditions as in Figure 1). The detection limit, calculated from 3 times the noise, was found to be 2 X 10” M. Even lower values may be obtained by using longer preconcentration periods or after removal of oxygen. All data reported were obtained with nondeaerated solutions; a small cathodic bump on the base line at -0.1 V, that affects the quantitation of dilute solutions, is attributed to the reduction of oxygen. Nondeaerated solutions, preferred for routine sensor operation, can be used without interference over the 5 X to 1X M range. The use of linear scan voltammetry, instead of differential pulse measurements, yielded inferior detectability due to substantially larger background current. Coexisting metal ions (whether electroactive or not) can interfere with the determination of copper if they compete for binding sites on the electrode surface. Nonaccumulated reducible metal ions do not interfere because of the use of the medium-exchange approach. Ions tested at the 5 X M level and found not to interfere in the determination of 2.5 x 10“‘ M copper(I1) were nickel(II), cadmium(II), calcium(II), magnesium(II), iron(II), and zinc(I1) (90 8, preconcentration; other conditions, as in Figure 1). Additions of 5 X M manganese(II), lead(II), chromium(III), and aluminum(II1) resulted in 11%, 24%, 29%, and 32%, respectively, depressions of the 2.5 X 10“‘ M copper(I1) peak. A separation step would be required for samples “rich” in interfering metals. Additions of 5 X 10“‘M silver(I), mercury(II), and gold(II1) yielded large overlapping peaks, ca. 180, 110, and 20 mV negative to the copper peak. The copper peak height, however, was not affected by the presence of silver and mercury. In contrast, the copper and gold peaks were poorly resolved to allow quantitation of copper. The relatively good selectivity toward copper is consistent with the fact that, in general, copper is bound more strongly to various algal species than are most other metal ions in class I (13, 16). Metal ions in class I display a sigmoidalpH dependence of binding, similar to that seen in Figure 3B. The pH profiles for metal binding are shifted to lower pH for the more strongly bound metal ions and to higher pH for the more weakly bound metals. In the case of copper, maximum binding to the alga occurs near pH 5 and above. The interference studies clearly show that at pH 5 there is some competition for copper binding sites by manganese(II), lead(II), gold(III), silver(I), chromium(III), mercury(II), and aluminum(II1) when these ions are present in concentrations twice that of copper. One approach to improve the selectivity of the electrode for copper (at some expense of sensitivity) would be to preconcentrate at a pH of 4-4.5. The extent of interference would depend not only upon the concentration ratio of the analyte and interferent but also on their levels. Lower concentrations (Le., lower surface coverage of competing metals) would result in decreased interference. Studies are under way to investigate these possibilities. Convenient operation and possible automation are expected utilizing flow systems that minimize the number of manipulations associated with the preconcentration/medium-exchange/voltammetry/cleaning scheme (20). Work in this laboratory is continuing with the goal of extending this approach in different directions.
LITERATURE CITED Baldwin, R. P.; Christensen, J. K.; Kryger, L. Anal. Chem. 1988, 58. 1790. Guadalupe, A. R.; Abruna. H. D. Anal. Chem. 1985, 5 7 , 142. Prabhu, S. V.; Baldwin, R. P.; Kryger, L. Anal. Chem. 1987, 5 9 , 1074. Wang, J.; Greene, 8.; Morgan, C. Anal. Ghlm. Acta 1984, 158, 15. Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 5 6 , 1898. Cox, J. A,; Kulesza, P. J. Anal. Chim. Acta 1983, 154, 71. Prlce, J. F.; Baldwin. R. P. Anal. Chem. 1980, 5 2 , 1940. Izutzu, K.; Nakarnura, T.; Takizawa, R.; Hanawa, H. Anal. Chlm . Acta 1983, 149, 147.
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(9) Khummongkd, D.; Canterford. C. S.; Fryer, C. Blotechnol. Bioeng. 1984,24. 2643. (10) NE-, A.; HcrlkosM, T.; Sakaguchi, T. J . Appl. Mlcrobol. Blotechnd. 1982, 18, 88. (11) Les, A.; Walker, R. W. Water, Ah, SdlPollut. 1984,23. 129. (12) Orwns, B.: Henzl, M. T.; Hosea, M.; Darnall, D. W. Biotechnol. Bloeng. 1988, 28,764. (13) Damall, D. W.; Oreene, 8.; Hmzl, M. T.; Hosea. M.; McPherson, R. A,; sneddon, J.; ~ l e mD.~~ , ~ sei. Techno/. ~ h1988. ~20, 206. ~ (14) Greene, 8.; Hosea, M.; McPherson, R.; Henzl, M.; Alexander, M. D.; Darnall, D. W. Environ. Sci. Techno/. 1986,20,627. (15) Hosea, M.; Grwne, B.; McPherson, R.; Henzl, M.; Alexander, M. D.; Damn. D. w. I W ~ . chim.ACB 1986, 123, 161. (16) Darnail, D. W.; Orwne, B.; Hosea, M.; McPherson, R. A.; Henzl, M.; Alexander, M. D. I n Trace &tal Removal from Aqueous Solution; Thompson, R., Ed.; Special Publication No. 61, Royal Society of Chemfstry: London, 1986;p 1.
(17) Watkins, J. W., 11; Elder, R. C.; Greene, 8.; Darnall, D. W. Inorg. Chem. 1907, 26. 1147. (18) Kobos, R. K. Trends Anal. Chem. 1983, 2 , 154. (19) O'Riordan, D. M. T.; Wallace, G. G. Anal. Chem. 1986, 5 8 , 128. (20) Wang, J.; Frelha, 8. A. Anal. Chem. 1983, 55, 1285.
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RECEIVED for review June 1,1987. Accepted September 18, 1987. This work was supported by the National Institutes of Health (Grant No. GM30913-04) and the National Science Foundation (Grant No. CBT-8610461). J.G.T. acknowledges the financial support of the Mexican Council of Science and (CoNACYT) and thanks B. Greene for helpful discussions.
Simplex Optimization of a Fiber--Optic Ammonia Sensor Based on Multiple Indicators Timothy D. Rhines and Mark A. Arnold* Department of Chemistry, University of Iowa, Iowa City, loova 52242
An optlmlzatlon strategy for a flberoptlc ammonia sensor is presented. Thk rtrategy condderr the v e r b experlmcHltai parameters that affect both steadystate and dynamic rerpol#rr ch8ractwktlCr. The wo ot multiple lncHcators to enhance the deadpatate perfomance of the sensor Is demonstrated. A simplex OptknlzaUOn routine has been developed to identify the best comblnatlon of two lndlcators for a particular analyte commtrat!on range. OptMZatlon strategy k WhstrateU by developing a smsor for wastewater analysls. I n addftbn, respOme characterlstlcs of the resufnng fiberoptic sensor are dlrectly canpared to those of the conventional potentlometrlc ammonla electrode. The fiber-optic msor k W a M e for wastewater analyses, and it compares wen wRh the conventional ekctrode system. In comparleon to the electrode, the fiberoptlc sensor possesms equivalent sensltivlty, similar response times, and superlor recovery times.
-
Several different approaches toward the development of fiber-optic ammonia sensors have been presented (1-4). David, Willson, and Ruffin (1) were the first to report a fiber-optic ammonia sensor. Their system involved a quartz rod coated with a polymer/ninhydrin reagent, and ammonia vapor was detected colorimetrically. Jarvis and co-workers (2) reported a reversible fiber-optic sensor for vaporous ammonia Their sensor was baaed on a thin solid film of oxazine perchlorate dye on a glass capillary. Wolfbeis and Posch (3) have recently described a fluorometric-based fiber-optic sensor for aqueous ammonia determinations. In their system, a fluorescent pH indicator was trapped in a polymeric membrane which was held a t the tip of a bifurcated fiber-optic bundle. Diffusion of ammonia from solution into this membrane was detected and the resulting fluorescence signal was related to the sample ammonia concentration. We have introduced an ammonia-gas sensor based on the entrapment of an indicator solution behind a gas-permeable membrane (4).This indicator solution is composed of ammonium chloride and a suitable chromophoricor fluorophoric pH indicator. Ammonia diffuses across this membrane from 0003-2700/88/0360-0076$01.50/0
the sample into the indicator solution. This ammonia influx continues until the ammonia partial pressure is equal on both sides of the membrane and a steady-state ammonia concentration is established in the indicator solution. The pH of the indicator solution varies according to this steady-state ammonia Concentration. By selection of pH indicators with appropriate acid dissociation constants, the change in pH can be monitored optically. A set of optical fibers is positioned in the indicator solution to measure changes in the relative concentrations of the protonated and nonprotonated forms of the indicator. An equation has been developed which relates absorbance by the nonprotonated form of the indicator to the sample ammonia concentration (4). It must be recognized that a sensor based on the titration of a single indicator will be limited with respect to dynamic range of response. Response of a single pH indicator will be restricted to a narrow pH range which corresponds to a narrow dynamic range of detection for ammonia. This limitation is particularly evident in comparison to potentiometric-based sensors which typically provide a dynamic range of several orders of magnitude (5). This problem is not limited to the fiber-optic ammonia sensor described here but is a general problem with all fiber-optic sensors that are based on a single indicator. Although it is neither practical nor necessary to develop fiber-optic gas sensors that respond over several orders of magnitude, it is desirable to develop sensors which respond over the entire concentration range of interest for a particular analysis. One approach is to use several pH indicators in the indicator solution. By selection of indicators with complementing acid dissociation constants and by use of appropriate relative concentrations of these indicators, sensors with the required dynamic response range can be developed. The primary goal of our investigation has been to develop a practical optimization strategy for our fiber-optic ammonia sensor. Such a strategy is presented here where sensor response is optimized over a particular concentration range of interest. In this paper, we demonstrate for the first time the use of multiple indicators for the development of a fiber-optic ammonia-gas sensor. Application of this sensor for the determination of ammonia in wastewater samples is used to 0 1987 Amerlcan Chemlcai Society
