- Email: [email protected]
Screen-printed voltammetric sensor for TNT
Talanta 46 (1998) 1405 – 1412
Screen-printed voltammetric sensor for TNT Joseph Wang a,*, Fang Lu a, Douglas MacDonald a, Jianmin Lu a, Mehmet E.S. Ozsoz a, Kim R. Rogers b a
b
Department of Chemistry and Biochemistry, New Mexico State Uni6ersity, Las Cruces, NM 88003, USA US En6ironmental Protection Agency, National Exposure Research Laboratory, Charaterization Research Di6ision, P.O. Box 93478, Las Vegas, NV 89193 -3478, USA Received 6 October 1997; received in revised form 8 December 1997; accepted 9 December 1997
Abstract Screen-printed carbon electrodes have been developed as disposable voltammetric sensors for 2,4,6-trinitrotoluene (TNT). Thick-film electrodes based on various conventional and modified inks have been compared for this task. The operation is based on placing the selected thick-film carbon sensor in the non-deaerated/quiescent sample and using a fast (B 1 s) and sensitive square-wave voltammetric scan. Different experimental variables have been optimized to yield a detection limit of 200 ppb TNT and a wide linear range. The high selectivity, demonstrated in assays of various untreated environmental samples, is attributed to the facts that the reducible nitro group is rare in nature and that most electroactive organic compounds require higher potentials. The new single-use sensor strips should facilitate the on-site environmental screening of TNT. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical sensor; Explosive detection; Screen-printed electrode; Square-wave voltammetry; TNT
1. Introduction Because of upsurge in terrorist and criminal activity, there are urgent needs for reliable methods for the detection of explosives such as 2,4,6trinitrotoluene (TNT) [1]. In addition to forensic or security considerations, such methods should address the environmental and toxicological significance of various explosives. In particular, with the end of the Cold War, the environmental community is confronted with problems of water and * Corresponding author. Tel.: + 1 505 6462505; fax: +1 505 6462649; e-mail: [email protected]
soil on military sites contaminated with explosives. Cumbersome and expensive instruments based on X-ray imaging or thermal neutral analysis have been introduced recently for the detection of hidden explosives [1,2]. In addition, bench-top instruments based primarily on chromatographic techniques are commonly used for laboratory measurements of explosives in water samples [3,4]. More compact low-cost instruments, coupled to smaller sensing probes, are highly desired for facilitating the task of on-site monitoring of explosives. Although sniffing dogs can provide high portability and selectivity, they are unpredictable
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(98)00005-8
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J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 1. SWV response for 1 ppm TNT (b), along with blank voltammograms (a) at different screen printed electrodes: carbon (A), gold (B), iridium-dispersed carbon (C) and Ru-dispersed carbon (D). Conditions: amplitude, 25 mV; frequency, 50 Hz; step, 4 mV; quiet time, 2 s; electrolyte, 0.05 M phosphate buffer (pH 6.5).
and require special training. Recent activity at the US Naval Research Laboratory resulted in a portable fiber-optic competitive immunosensor for rapid on-site detection of TNT [5]. Surprisingly, little attention has been given to electrochemical sensing of nitroaromatic explosives, despite their inherent redox activity [4] and the compact nature of electrochemical instruments. In this paper we describe the characterization and performance of a disposable sensor strip for on-site electrochemical measurements of TNT. The screen-printing (thick-film) technology represents an ideal route for large-scale fabrication of highly reproducible and yet inexpensive electrochemical sensors [6]. While most of
the commercial activity in this direction has focused on ‘one-shot’ glucose sensors [7], screenprinted electrodes have been shown to be useful for the amperometric biosensing of other metabolites [8] or toxic enzyme inhibitors [9], for decentralized stripping measurements of trace metals [10], or for the specific detection of DNA sequences [11]. In the following sections we will compare various conventional and modified thick-film electrodes for on-site screening of TNT, and will demonstrate the attractive analytical performance accrued from the coupling of disposable carbon strips with rapid square-wave voltammetric (SWV) detection of this important explosive.
J. Wang et al. / Talanta 46 (1998) 1405–1412
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Fig. 2. Effect of the SWV frequency (A) and the amplitude (B) upon the voltammetric response of 10 ppm TNT at the screen-printed carbon electrode. Other conditions as in Fig. 1A.
2. Experimental
mesh, 99.9%) or ruthenium powder (Alfa, 325 mesh, 99.95%) to give a 5%wt. metal loading.
2.1. Apparatus 2.3. Screen-printed electrode fabrication Most experiments were performed using a microprocessor-controlled electrochemical analyzer (Model 620; CH Instruments), interfaced with an IBM personal computer. The AutoLab electrochemical system (Eco Chemie) was used for obtaining the background-corrected voltammograms. SWV measurements were carried out in a 5-ml electrochemical cell (Model VC-2; BAS). The screen-printed electrode, the reference electrode (Ag/AgCl (3 M NaCl), Model RE-1; BAS), and platinum wire auxiliary electrode joined the cell through its Teflon cover.
2.2. Ink preparation Four different inks were used and compared. These included unmodified carbon or gold inks from Ercon (Ercon G-449 (I) and Exp 44281, respectively), and metal-dispersed carbon inks. These were prepared by thoroughly mixing (for 30 min with a spatula) 1.9 g of carbon ink (Ercon, G-449 (I)) with 100 mg iridium powder (Alfa, 325
A semi-automatic screen printer (Model TF 100; MPM, Franklin, MA) was used for printing the working electrodes. The inks were printed through a patterned stencil onto 10 cm× 10 cm alumina ceramic plates containing 30 strips (of 3.33 cm× 1.00 cm, as defined by a laser pre/semi cut). The resulting 1 mm× 30 mm printed carbon structures were cured for 30 min at 100°C; the gold ink was cured at 150°C for the same period. An insulating ink (Ercon R-488c1, Green) was subsequently printed on a portion of the plate, to leave 1 mm× 5 mm sections on both ends for defining the working electrode and electrical contact. The insulating layer was cured at 100°C for 30 min.
2.4. Reagents and procedure All solutions were prepared daily with doubledistilled water using reagent-grade chemicals. The TNT stock solution (100 ppm in water) was a gift
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J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 3. Effect of the ionic strength upon the SWV response (peak current (A) and potential (B)) of the carbon strip electrode towards 10 ppm TNT. Other conditions as in Fig. 1A.
from Sandia NL. Potassium phosphates (monobasic and anhydrous) were obtained from Sigma, hydrochloric acid from J.T. Baker and acetic acid (glacial) from Fisher Scientific. A potassium phosphate buffer solution (0.05 M, pH 6.5) was employed in most experiments. The river water (Rio Grande) and groundwater samples were collected at Las Cruces (NM) and the Hanford Site (Richland, A), respectively. All measurements were performed at room temperature, using non-deaerated quiescent solutions and a rapid Osteryoung SWV scan from 0.0 to − 0.70V.
3. Results and discussion Polynitroaromatic compounds, such as TNT, are readily reduced, particularly at various mercury electrodes. Such compounds undergo stepwise reduction processes, involving initial reduction of the nitro groups to a hydroxylamine, followed by conversion to an amine group. While mercury drop [12] or film [4] electrodes have been traditionally used for laboratory measurements of reducible explosive substances, solid-state ‘mercury-free’ devices should be more attractive for
single-use field-screening applications. Accordingly, we evaluated various conventional and modified screen-printed electrodes as disposable TNT sensors. Fig. 1b compares square-wave voltammograms for trace (1 ppm) TNT at carbon (A), gold (B), iridium-dispersed carbon (C) and ruthenium-dispersed carbon (D) screen-printed electrodes. All strip electrodes display a single voltammetric peak (at ca. − 0.45 V), corresponding to the formation of the hydroxylamine moiety. However, only the unmodified carbon strip results in a well-defined response, suitable for such trace measurements. With the other sensors the peak appears as a shoulder on a rising background current (associated with the reduction of dissolved oxygen). Apparently, the metal-dispersed (Ru and Ir) catalytic centers have a more profound effect on the oxygen background than upon the target TNT signal. In view of its favorable signal-to-background characteristics, the screen-printed carbon electrode was used for all subsequent analytical work. The very low oxygen background current of this sensor eliminates the need for a time-consuming deoxygenation step and hence further simplifies practical field operations. Additional reduction peaks were observed at the carbon strip electrode
J. Wang et al. / Talanta 46 (1998) 1405–1412
1409
Fig. 4. Effect of solution pH upon the SWV response (peak current (A) and potential (B)) of the carbon strip electrode towards 10 ppm TNT. Other conditions as in Fig. 1A.
upon scanning to more negative potentials (up to − 1.1 V). However, due to the high background at these negative potentials all analytical work was focused on the first reduction process at − 0.45 V. The data of Fig. 1 indicate that the combination of carbon strip electrodes with rapid SWV yields an attractive analytical behavior. Besides its high sensitivity and speed, SWV offers higher information content compared to the chronoamperometric measurements commonly employed in connection with screen-printed sensors. The effect of different variables of the SWV waveform was explored for further optimization. Fig. 2A examines the influence of the square-wave frequency upon the response to 10 ppm TNT. The peak current rises sharply with the frequency at first up to 50 Hz, and then decays gradually. The effect of the SWV amplitude (height of square-wave component) is shown in Fig. 2B. The TNT signal increases nearly linearly upon raising the amplitude between 5 and 50 mV, and then more slowly. All subsequent work was thus carried out using an amplitude of 25 mV, a frequency of 50 Hz, and a staircase step height of 4 mV (i.e., less than 5 s for the entire scan). We also attempted to increase the sensitivity by employing a preceding adsorp-
tive accumulation of TNT (i.e., an adsorptive stripping operation). However, only a slight (15%) signal enhancement was observed in connection with a 3 min preconcentration at 0.0 V. Such a stripping approach was abandoned considering also its very slow response time. Field-screening applications will greatly benefit from the minimization of sample pretreatment. Reduced dependence upon solution variables (such as pH or ionic strength) would assure a minimal sample pretreatment and simplified field operations. Fig. 3 displays the dependence of the SWV response upon the solution ionic strength. As desired, the sensor displays a broad ionic strength independence, with minimal changes in the peak current (A) or potential (B) for phosphate buffer concentrations ranging from 10 to 200 mM. As might be expected, a sharp decrease in the current response and large potential shift are observed without any electrolyte. Environmental water samples commonly contain sufficient natural electrolyte, and hence permit convenient and direct monitoring of TNT (see following sections). Fig. 4 examines the effect of the pH upon the SWV TNT response. The peak current increases nearly linearly with the pH over the 3.5–5.5
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J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 5. SWV response of the thick-film carbon sensor to TNT solutions of increasing concentration from 1 ppm to 10 ppm (b–k), along with the background voltammogram (a). Other conditions as in Fig. 1A.
range, and then it starts to level off (A). The nearly pH independence over the 5.5 – 8.5 range indicates great promise for numerous natural water matrices (with quantitation based on calibration in the given matrix). As expected from the
involvement of protons in the redox reaction, the peak potential shifts negatively (in a nearly linear fashion) upon raising the pH from 3.5 to 8.5 (B). Fig. 5 displays SW voltammograms for increasing levels of TNT in 1 ppm steps (b–k), along
Fig. 6. Assays of untreated river water (A) and groundwater (B) samples. Response to the sample (a), as well as subsequent concentration increments of 3 ppm TNT (b–d). Other conditions as in Fig. 1A.
J. Wang et al. / Talanta 46 (1998) 1405–1412
1411
Fig. 7. Conventional (a) and background-corrected (b) square-wave voltammograms for untreated tap water (A) and river water (B) samples containing increasing levels of TNT in 200 ppb steps, along with the sample blank voltammogram. Amplitude, 10 mV; frequency, 30 Hz; step, 2 mV.
with the blank voltammogram (a). Well-defined peaks, proportional to the explosive concentration, are observed, along with the low background response. The resulting calibration plot (also shown) is highly linear (slope, 4.144 mA/ppm; corellation coefficient, 0.999) Besides their promise for single-use field applications, the screen-printed electrodes can be used as reusable TNT detecting devices. For example, a prolonged series of 16 repetitive measurements of 10 ppm TNT using the same carbon strip resulted in a highly stable peak current, with a relative standard deviation of 0.6% (not shown). High selectivity is expected in environmental samples because the nitro group is rare in nature and few other organic compounds are as easily reduced [13]. The screen-printed carbon electrodes
thus holds great promise for direct analysis of relevant water samples, without any prior separation or pretreament. Fig. 6 displays typical SW voltammograms, illustrating such assays of untreated river water (A) and groundwater (B) samples. A low (nearly flat) baseline is observed for both samples (a), indicating the absence of interfering electroactive species. Three subsequent additions of 3 ppm TNT resulted in well-defined peaks, proportional to the explosive concentration, and similar to those common in synthetic samples. Note again that neither pH nor ionic strength adjustments were required to obtain these high-quality voltammetric data. We also tested the effect of potential organic, inorganic and surface-active interfering materials. The response for 5 ppm TNT was not affected by the presence of 50 ppm of the
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J. Wang et al. / Talanta 46 (1998) 1405–1412
similar nitroaromatic compounds nitrophenol and nitrobenzene, by the presence of 1000 ppm of nitrite or nitrate ions, by 1 ppm of the reducible copper, iron(III), lead or zinc ions, or the presence of 10 ppm Triton X-100 or sodium dodecyl sulphate. Yet, it is strongly recommended to employ the new screen-printed voltammetric electrodes as early screening/warning devices (in connection to a high-resolution laboratory confirmation technique). It should be pointed out that even the use of antibodies for imparting selectivity into TNT sensing suffers from cross-reactivity interferences from structurally similar nitroaromatic compounds [5]. Extremely low detection limits can be achieved in connection with a computerized backgroundcorrection operation. Fig. 7 compares conventional (a) and background-subtracted (b) square-wave voltammograms for untreated tap (A) and river (B) water samples, containing increasing levels of TNT in 200 ppb steps (along with the blank voltammogram). Such a background-correction operation offers a detection limit of about 100 ppb. Note again the absence of electroactive interferences in these untreated water samples, and the well-defined concentration dependence. In conclusion, the results presented above demonstrate that the coupling of rapid SWV with screen-printed electrodes results in effective disposable sensors for TNT. As desired for field operations, such coupling leads to a fast, sensitive, simple and low-cost detection of TNT. Although the present work deals with the measurement of TNT, other nitro-containing explosives (e.g. RDX, HMX, picric acid) may be similarly detected on the basis of differences in their peak potentials [4], and in connection with chemometric or on-chip separation schemes. The coupling of these single-use devices with hand-held, battery-operated, instruments should facilitate on-site
.
.
environmental screening of TNT. Remote (submersible) electrochemical sensors, based on the rapid SWV strategy, can also be envisioned for continuous real-time monitoring applications.
Acknowledgements This work was partially supported by a contract from Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy under Contract DE-AC04-94AL85000. K.R. acknowledges EPA support for an IPA assignment at NMSU. Discussions with C. Renschler (Sandia NL) are acknowledged.
References [1] P. Kolla, Angew Chem. Int. Ed. Engl. 36 (1997) 801. [2] A. Fainberg, Science 255 (1992) 1531. [3] T. Jenkins, D. Leggett, C. Grant, C. Bauer, Anal. Chem. 58 (1986) 170. [4] K. Bratin, P.T. Kissinger, R. Briner, C. Bruntlett, Anal. Chim. Acta 130 (1981) 295. [5] L. Shriver-Lake, K. Brestin, P. Charles, D. Conrad, J. Golden, F.S. Ligner, Anal. Chem. 67 (1995) 2431. [6] J. Hart, S. Wring, Electroanalysis 6 (1994) 617. [7] M. Green, P. Hilditch, Anal. Proc. 28 (1991) 374. [8] I. Rohm, M. Gengrich, W. Collier, U. Bilitewski, Analyst 121 (1996) 877. [9] J. Wang, V. Nascimento, S. Kane, K. Rogers, M. Smyth, L. Angnes, Talanta 43 (1996) 1903. [10] J. Wang, Analyst 119 (1994) 763. [11] J. Wang, G. Rivas, X. Cai, Electroanalysis 9 (1997) 95. [12] K. Conley, W. Mikucki, Migration of explosives and chlorinated pesticide in a simulated sanitary landfill, Technical Report N-8, US Army Construction Engineering Research Laboratory, Champaign, IL. [13] P. Kissinger and W. Heineman (Eds.), Laboratory Techniques in Electroanalytical Chemistry, 2nd ed., Dekker, New York, 1996, p. 842.
Screen-printed voltammetric sensor for TNT Joseph Wang a,*, Fang Lu a, Douglas MacDonald a, Jianmin Lu a, Mehmet E.S. Ozsoz a, Kim R. Rogers b a
b
Department of Chemistry and Biochemistry, New Mexico State Uni6ersity, Las Cruces, NM 88003, USA US En6ironmental Protection Agency, National Exposure Research Laboratory, Charaterization Research Di6ision, P.O. Box 93478, Las Vegas, NV 89193 -3478, USA Received 6 October 1997; received in revised form 8 December 1997; accepted 9 December 1997
Abstract Screen-printed carbon electrodes have been developed as disposable voltammetric sensors for 2,4,6-trinitrotoluene (TNT). Thick-film electrodes based on various conventional and modified inks have been compared for this task. The operation is based on placing the selected thick-film carbon sensor in the non-deaerated/quiescent sample and using a fast (B 1 s) and sensitive square-wave voltammetric scan. Different experimental variables have been optimized to yield a detection limit of 200 ppb TNT and a wide linear range. The high selectivity, demonstrated in assays of various untreated environmental samples, is attributed to the facts that the reducible nitro group is rare in nature and that most electroactive organic compounds require higher potentials. The new single-use sensor strips should facilitate the on-site environmental screening of TNT. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical sensor; Explosive detection; Screen-printed electrode; Square-wave voltammetry; TNT
1. Introduction Because of upsurge in terrorist and criminal activity, there are urgent needs for reliable methods for the detection of explosives such as 2,4,6trinitrotoluene (TNT) [1]. In addition to forensic or security considerations, such methods should address the environmental and toxicological significance of various explosives. In particular, with the end of the Cold War, the environmental community is confronted with problems of water and * Corresponding author. Tel.: + 1 505 6462505; fax: +1 505 6462649; e-mail: [email protected]
soil on military sites contaminated with explosives. Cumbersome and expensive instruments based on X-ray imaging or thermal neutral analysis have been introduced recently for the detection of hidden explosives [1,2]. In addition, bench-top instruments based primarily on chromatographic techniques are commonly used for laboratory measurements of explosives in water samples [3,4]. More compact low-cost instruments, coupled to smaller sensing probes, are highly desired for facilitating the task of on-site monitoring of explosives. Although sniffing dogs can provide high portability and selectivity, they are unpredictable
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(98)00005-8
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J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 1. SWV response for 1 ppm TNT (b), along with blank voltammograms (a) at different screen printed electrodes: carbon (A), gold (B), iridium-dispersed carbon (C) and Ru-dispersed carbon (D). Conditions: amplitude, 25 mV; frequency, 50 Hz; step, 4 mV; quiet time, 2 s; electrolyte, 0.05 M phosphate buffer (pH 6.5).
and require special training. Recent activity at the US Naval Research Laboratory resulted in a portable fiber-optic competitive immunosensor for rapid on-site detection of TNT [5]. Surprisingly, little attention has been given to electrochemical sensing of nitroaromatic explosives, despite their inherent redox activity [4] and the compact nature of electrochemical instruments. In this paper we describe the characterization and performance of a disposable sensor strip for on-site electrochemical measurements of TNT. The screen-printing (thick-film) technology represents an ideal route for large-scale fabrication of highly reproducible and yet inexpensive electrochemical sensors [6]. While most of
the commercial activity in this direction has focused on ‘one-shot’ glucose sensors [7], screenprinted electrodes have been shown to be useful for the amperometric biosensing of other metabolites [8] or toxic enzyme inhibitors [9], for decentralized stripping measurements of trace metals [10], or for the specific detection of DNA sequences [11]. In the following sections we will compare various conventional and modified thick-film electrodes for on-site screening of TNT, and will demonstrate the attractive analytical performance accrued from the coupling of disposable carbon strips with rapid square-wave voltammetric (SWV) detection of this important explosive.
J. Wang et al. / Talanta 46 (1998) 1405–1412
1407
Fig. 2. Effect of the SWV frequency (A) and the amplitude (B) upon the voltammetric response of 10 ppm TNT at the screen-printed carbon electrode. Other conditions as in Fig. 1A.
2. Experimental
mesh, 99.9%) or ruthenium powder (Alfa, 325 mesh, 99.95%) to give a 5%wt. metal loading.
2.1. Apparatus 2.3. Screen-printed electrode fabrication Most experiments were performed using a microprocessor-controlled electrochemical analyzer (Model 620; CH Instruments), interfaced with an IBM personal computer. The AutoLab electrochemical system (Eco Chemie) was used for obtaining the background-corrected voltammograms. SWV measurements were carried out in a 5-ml electrochemical cell (Model VC-2; BAS). The screen-printed electrode, the reference electrode (Ag/AgCl (3 M NaCl), Model RE-1; BAS), and platinum wire auxiliary electrode joined the cell through its Teflon cover.
2.2. Ink preparation Four different inks were used and compared. These included unmodified carbon or gold inks from Ercon (Ercon G-449 (I) and Exp 44281, respectively), and metal-dispersed carbon inks. These were prepared by thoroughly mixing (for 30 min with a spatula) 1.9 g of carbon ink (Ercon, G-449 (I)) with 100 mg iridium powder (Alfa, 325
A semi-automatic screen printer (Model TF 100; MPM, Franklin, MA) was used for printing the working electrodes. The inks were printed through a patterned stencil onto 10 cm× 10 cm alumina ceramic plates containing 30 strips (of 3.33 cm× 1.00 cm, as defined by a laser pre/semi cut). The resulting 1 mm× 30 mm printed carbon structures were cured for 30 min at 100°C; the gold ink was cured at 150°C for the same period. An insulating ink (Ercon R-488c1, Green) was subsequently printed on a portion of the plate, to leave 1 mm× 5 mm sections on both ends for defining the working electrode and electrical contact. The insulating layer was cured at 100°C for 30 min.
2.4. Reagents and procedure All solutions were prepared daily with doubledistilled water using reagent-grade chemicals. The TNT stock solution (100 ppm in water) was a gift
1408
J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 3. Effect of the ionic strength upon the SWV response (peak current (A) and potential (B)) of the carbon strip electrode towards 10 ppm TNT. Other conditions as in Fig. 1A.
from Sandia NL. Potassium phosphates (monobasic and anhydrous) were obtained from Sigma, hydrochloric acid from J.T. Baker and acetic acid (glacial) from Fisher Scientific. A potassium phosphate buffer solution (0.05 M, pH 6.5) was employed in most experiments. The river water (Rio Grande) and groundwater samples were collected at Las Cruces (NM) and the Hanford Site (Richland, A), respectively. All measurements were performed at room temperature, using non-deaerated quiescent solutions and a rapid Osteryoung SWV scan from 0.0 to − 0.70V.
3. Results and discussion Polynitroaromatic compounds, such as TNT, are readily reduced, particularly at various mercury electrodes. Such compounds undergo stepwise reduction processes, involving initial reduction of the nitro groups to a hydroxylamine, followed by conversion to an amine group. While mercury drop [12] or film [4] electrodes have been traditionally used for laboratory measurements of reducible explosive substances, solid-state ‘mercury-free’ devices should be more attractive for
single-use field-screening applications. Accordingly, we evaluated various conventional and modified screen-printed electrodes as disposable TNT sensors. Fig. 1b compares square-wave voltammograms for trace (1 ppm) TNT at carbon (A), gold (B), iridium-dispersed carbon (C) and ruthenium-dispersed carbon (D) screen-printed electrodes. All strip electrodes display a single voltammetric peak (at ca. − 0.45 V), corresponding to the formation of the hydroxylamine moiety. However, only the unmodified carbon strip results in a well-defined response, suitable for such trace measurements. With the other sensors the peak appears as a shoulder on a rising background current (associated with the reduction of dissolved oxygen). Apparently, the metal-dispersed (Ru and Ir) catalytic centers have a more profound effect on the oxygen background than upon the target TNT signal. In view of its favorable signal-to-background characteristics, the screen-printed carbon electrode was used for all subsequent analytical work. The very low oxygen background current of this sensor eliminates the need for a time-consuming deoxygenation step and hence further simplifies practical field operations. Additional reduction peaks were observed at the carbon strip electrode
J. Wang et al. / Talanta 46 (1998) 1405–1412
1409
Fig. 4. Effect of solution pH upon the SWV response (peak current (A) and potential (B)) of the carbon strip electrode towards 10 ppm TNT. Other conditions as in Fig. 1A.
upon scanning to more negative potentials (up to − 1.1 V). However, due to the high background at these negative potentials all analytical work was focused on the first reduction process at − 0.45 V. The data of Fig. 1 indicate that the combination of carbon strip electrodes with rapid SWV yields an attractive analytical behavior. Besides its high sensitivity and speed, SWV offers higher information content compared to the chronoamperometric measurements commonly employed in connection with screen-printed sensors. The effect of different variables of the SWV waveform was explored for further optimization. Fig. 2A examines the influence of the square-wave frequency upon the response to 10 ppm TNT. The peak current rises sharply with the frequency at first up to 50 Hz, and then decays gradually. The effect of the SWV amplitude (height of square-wave component) is shown in Fig. 2B. The TNT signal increases nearly linearly upon raising the amplitude between 5 and 50 mV, and then more slowly. All subsequent work was thus carried out using an amplitude of 25 mV, a frequency of 50 Hz, and a staircase step height of 4 mV (i.e., less than 5 s for the entire scan). We also attempted to increase the sensitivity by employing a preceding adsorp-
tive accumulation of TNT (i.e., an adsorptive stripping operation). However, only a slight (15%) signal enhancement was observed in connection with a 3 min preconcentration at 0.0 V. Such a stripping approach was abandoned considering also its very slow response time. Field-screening applications will greatly benefit from the minimization of sample pretreatment. Reduced dependence upon solution variables (such as pH or ionic strength) would assure a minimal sample pretreatment and simplified field operations. Fig. 3 displays the dependence of the SWV response upon the solution ionic strength. As desired, the sensor displays a broad ionic strength independence, with minimal changes in the peak current (A) or potential (B) for phosphate buffer concentrations ranging from 10 to 200 mM. As might be expected, a sharp decrease in the current response and large potential shift are observed without any electrolyte. Environmental water samples commonly contain sufficient natural electrolyte, and hence permit convenient and direct monitoring of TNT (see following sections). Fig. 4 examines the effect of the pH upon the SWV TNT response. The peak current increases nearly linearly with the pH over the 3.5–5.5
1410
J. Wang et al. / Talanta 46 (1998) 1405–1412
Fig. 5. SWV response of the thick-film carbon sensor to TNT solutions of increasing concentration from 1 ppm to 10 ppm (b–k), along with the background voltammogram (a). Other conditions as in Fig. 1A.
range, and then it starts to level off (A). The nearly pH independence over the 5.5 – 8.5 range indicates great promise for numerous natural water matrices (with quantitation based on calibration in the given matrix). As expected from the
involvement of protons in the redox reaction, the peak potential shifts negatively (in a nearly linear fashion) upon raising the pH from 3.5 to 8.5 (B). Fig. 5 displays SW voltammograms for increasing levels of TNT in 1 ppm steps (b–k), along
Fig. 6. Assays of untreated river water (A) and groundwater (B) samples. Response to the sample (a), as well as subsequent concentration increments of 3 ppm TNT (b–d). Other conditions as in Fig. 1A.
J. Wang et al. / Talanta 46 (1998) 1405–1412
1411
Fig. 7. Conventional (a) and background-corrected (b) square-wave voltammograms for untreated tap water (A) and river water (B) samples containing increasing levels of TNT in 200 ppb steps, along with the sample blank voltammogram. Amplitude, 10 mV; frequency, 30 Hz; step, 2 mV.
with the blank voltammogram (a). Well-defined peaks, proportional to the explosive concentration, are observed, along with the low background response. The resulting calibration plot (also shown) is highly linear (slope, 4.144 mA/ppm; corellation coefficient, 0.999) Besides their promise for single-use field applications, the screen-printed electrodes can be used as reusable TNT detecting devices. For example, a prolonged series of 16 repetitive measurements of 10 ppm TNT using the same carbon strip resulted in a highly stable peak current, with a relative standard deviation of 0.6% (not shown). High selectivity is expected in environmental samples because the nitro group is rare in nature and few other organic compounds are as easily reduced [13]. The screen-printed carbon electrodes
thus holds great promise for direct analysis of relevant water samples, without any prior separation or pretreament. Fig. 6 displays typical SW voltammograms, illustrating such assays of untreated river water (A) and groundwater (B) samples. A low (nearly flat) baseline is observed for both samples (a), indicating the absence of interfering electroactive species. Three subsequent additions of 3 ppm TNT resulted in well-defined peaks, proportional to the explosive concentration, and similar to those common in synthetic samples. Note again that neither pH nor ionic strength adjustments were required to obtain these high-quality voltammetric data. We also tested the effect of potential organic, inorganic and surface-active interfering materials. The response for 5 ppm TNT was not affected by the presence of 50 ppm of the
1412
J. Wang et al. / Talanta 46 (1998) 1405–1412
similar nitroaromatic compounds nitrophenol and nitrobenzene, by the presence of 1000 ppm of nitrite or nitrate ions, by 1 ppm of the reducible copper, iron(III), lead or zinc ions, or the presence of 10 ppm Triton X-100 or sodium dodecyl sulphate. Yet, it is strongly recommended to employ the new screen-printed voltammetric electrodes as early screening/warning devices (in connection to a high-resolution laboratory confirmation technique). It should be pointed out that even the use of antibodies for imparting selectivity into TNT sensing suffers from cross-reactivity interferences from structurally similar nitroaromatic compounds [5]. Extremely low detection limits can be achieved in connection with a computerized backgroundcorrection operation. Fig. 7 compares conventional (a) and background-subtracted (b) square-wave voltammograms for untreated tap (A) and river (B) water samples, containing increasing levels of TNT in 200 ppb steps (along with the blank voltammogram). Such a background-correction operation offers a detection limit of about 100 ppb. Note again the absence of electroactive interferences in these untreated water samples, and the well-defined concentration dependence. In conclusion, the results presented above demonstrate that the coupling of rapid SWV with screen-printed electrodes results in effective disposable sensors for TNT. As desired for field operations, such coupling leads to a fast, sensitive, simple and low-cost detection of TNT. Although the present work deals with the measurement of TNT, other nitro-containing explosives (e.g. RDX, HMX, picric acid) may be similarly detected on the basis of differences in their peak potentials [4], and in connection with chemometric or on-chip separation schemes. The coupling of these single-use devices with hand-held, battery-operated, instruments should facilitate on-site
.
.
environmental screening of TNT. Remote (submersible) electrochemical sensors, based on the rapid SWV strategy, can also be envisioned for continuous real-time monitoring applications.
Acknowledgements This work was partially supported by a contract from Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy under Contract DE-AC04-94AL85000. K.R. acknowledges EPA support for an IPA assignment at NMSU. Discussions with C. Renschler (Sandia NL) are acknowledged.
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