ac050791t

8/9/2019 ac050791t

1/8

Electrochemical Sensor for OrganophosphatePesticides and Nerve Agents Using ZirconiaNanoparticles as Selective Sorbents

Guodong Liu and Yuehe Lin*

Pacific Northwest National Laboratory, Richland, Washington 99352

An electrochemical sensor for detection of organophos-

phate (OP) pesticides and nerve agents using zirconia

(ZrO2) nanoparticles as selective sorbents is presented.

Zirconia nanoparticles were electrodynamically deposited

onto the polycrystalline gold electrode by cyclic voltam-

metry. Because of the strong affinity of zirconia for the

phosphoric group, nitroaromatic OPs strongly bind to the

ZrO2 nanoparticle surface. The electrochemical charac-terization and anodic stripping voltammetric performance

of bound OPs were evaluated using cyclic voltammetric

and square-wave voltammetric (SWV) analysis. SWV was

used to monitor the amount of bound OPs and provide

simple, fast, and facile quantitative methods for nitroaro-

matic OP compounds. The sensor surface can be regener-

ated by successively running SWV scanning. Operational

parameters, including the amount of nanoparticles, ad-

sorption time, and pH of the reaction medium have been

optimized. The stripping voltammetric response is highly

linear over the 5-100 ng/mL (ppb) methyl parathion

range examined (2-min adsorption), with a detection limit

of 3 ng/mL and good precision (RSD ) 5.3%, n ) 10). The detection limit was improved to 1 ng/mL by using

10-min adsorption time. The promising stripping vol-

tammetric performances open new opportunities for

fast, simple, and sensitive analysis of OPs in environ-

mental and biological samples. These findings can lead

to a widespread use of electrochemical sensors to detect

OP contaminates.

Organophosphates (OPs) are known to be highly neurotoxic;

they disrupt the cholinesterase enzyme that regulates acetyl-

choline,1-5 a neurotransmitter needed for proper nervous system

function. Because of their high neurotoxicity, the OPs are widely

used as pesticides and as nerve agents as part of chemical andbiological warfare agents. OP residuals in crop, livestock, and

poultry products are clearly dangerous to human health. The

related clinical signs include negative effects on the visual system,

sensory function, cognitive function, and nervous system. Specif-

ically, exposure to OPs has been shown to cause headache,

dizziness, profuse sweating, blurred vision, nausea, vomiting,

reduced heart beat, diarrhea, loss of coordination, slow and weak

breathing, fever, coma, and death.6 Infants and children may be

especially sensitive to health risks posed by pesticides: an

estimated 74 000 children were involved in common household

pesticide-related poisonings or exposures in the United States in

1994.7 Because of the high toxicity of OPs, the rapid detection of

these toxic agents in the environment, public places, or workplaces

and the monitoring of individual exposures to chemical warfare

agents have become increasingly important for homeland security

and health protection.8-11 Early detection of OPs may give an

indication of terrorist activity, allowing proper procedures to be

followed to mitigate dangers. It is still an extremely difficult

challenge to detect low concentrations of OPs accurately in

environmental samples. Soil and water samples are very likely to

contain OPs because of heavy urban and rural use of these

compounds. Military and terrorist activities may result in air, water, and soil contamination with different chemical warfare

agents. Analysis of OPs in environmental and biological samples

is routinely carried out using analytical techniques, such as gas

or liquid chromatography and mass spectrometry.12 Such analysis

is generally performed at centralized laboratories, requiring

extensive labor and analytical resources, and often results in a

lengthy turnaround time. These analysis methods have a number

of disadvantages that limit their applications primarily to laboratory

settings and prohibit their use for rapid analyses under field

conditions. Biological methods, such as immunoassay, have also

been reported.13 Long analysis time and extensive sample handling

with multiple washing steps limit the applications. In recent years,

OP pesticide kits have become commercially available that offeradvantages, including portability, rapid turnaround time, and cost-

effectiveness.14 Drawbacks of these test kits include the compli-* Corresponding author. Tel.: 01-509-376-0529. Fax: 01-509 376-5106.

E-mail: [email protected].

(1) Rosenberry, T. L. Advances in enzymology and related areas of molecular

biology; John Wiley & Sons: New York, 1975.

(2) Zhang, S.; Zhao, H.; John, R. Biosens. Bioelectron. 2001, 16, 1119-1126.

(3) Fennouh, S.; Casimiri, V.; Burstein, C. Biosens. Bioelectron. 1997, 12, 97-

104.

(4) Cremisini, C.; Disario, S.; Mela, J.; Pilloton, R.; Palleschi, G. Anal. Chim.

Acta 1995, 311, 273-280.

(5) Guerrieri, A.; Monaci, L.; Quinto, M.; Palmisano, F. Analyst2002, 127, 5-7.

(6) http://www.beyondpesticides.org/.

(7) http://www.epa.gov/pesticides/.

(8) Wang, J. Anal. Chim. Acta 2003, 507, 3-10.

(9) Sadik, O. A.; Land, W. H.; Wang, J. Electroanalysis 2003, 15, 1149-1159.

(10) Lin, Y.; Lu, F.; Wang, J. Electroanalysis 2004, 16, 145-149.

(11) Wang, J.; Pumera, M.; Collins, G.; Mulchandani, A.; Lin, Y.; Olsen, K. Anal.

Chem. 2002, 74, 1187-1191.

(12) Sherma, J. Anal. Chem. 1993, 65, 40R-54R.

(13) Miller J. K.; Lenz D. J. Appl. Toxicol. 2001, 21, S23-S26.

Anal. Chem. 2005, 77, 5894-5901

5894 Analytical Chemistry, Vol. 77, No. 18, September 15, 2005 10.1021/ac050791t CCC: $30.25 2005 American Chemical SocietyPublished on Web 08/09/2005

8/9/2019 ac050791t

2/8

cated handling procedure and often a lack of sensitivity (ppm level)

and precision. Moreover, in most cases, these tests are qualitative

or semiquantitative and show false positive and negative results.

To meet the requirements of rapid warning and field deploy-

ment, more-compact low-cost instruments, coupled to smaller

sensing probes, are highly desirable for facilitating the task of

on-site monitoring of OP compounds. Various inhibition andnoninhibition biosensor systems, based on the immobilization of

acetylcholinesterase or OP hydrolase onto various electrochemical

or optical transducers, have been proposed for field screening of

OP neurotoxins.15-19 Specific antibodies against OP pesticides have

been recently developed for enzyme-linked immunoassay and

immunosensors.20,21Although acetylcholinesterase is commercially

available, OP hydrolase and antibodies against OPs are still only

produced in laboratories, which limits wide applications of bio-

sensors. To avoid the use of enzymes and antibodies, molecular

imprint technologies with high selectivity toward specific OP

species have been developed and applied to the detection of

pesticides in environmental samples.22,23 Nitroaromatic OPs, such

as paraoxon, methyl parathion, and fenitrothion (Figure 1), exhibit

good redox activities at the electrode surface.11 Electrochemical

detection of nitroaromatic OPs showed great promise when it was

coupled with different separation technologies, such as high-

performance liquid chromatography24 or capillary electrophore-

sis.25 Surprisingly, little attention has been given to direct

electrochemical sensing of nitroaromatic OP compounds, despite

their inherent redox activity and the compact nature of electro-

chemical instruments.

Zirconia (ZrO2) is an inorganic oxide with thermal stability,

chemical inertness, and lack of toxicity.26-29 Researchers have

demonstrated that zirconia has a strong affinity for the phosphoric

group. This has been used to prepare multiple films by self-

assembly,27-29 or a DNA probe30,31was attached with the phosphate

group at the 5 end to develop a DNA biosensor. Zirconia films

or microcrystals were prepared by electrodeposition of ZrOCl2 at

bare or functionalized gold surfaces.32-34

In this paper, we describethe electrochemical sensing nitroaromatic OPs based on a gold

electrode modified with zirconia nanoparticles (Figure 2). The new

ZrO2 nanoparticle-based electrochemical sensing protocol involves

electrodynamically depositing ZrO2 nanoparticles onto a gold

electrode surface (A), followed by OP adsorption (B), and

electrochemical stripping detection of adsorbed electroactive OPs

(C). The electrochemical characterization and anodic stripping

voltammetric performance of bound nitroaromatic OP compounds

were evaluated using cyclic voltammetric and square-wave vol-

tammetric (SWV) analysis. The promising stripping voltammetric

performances open new opportunities for fast, simple, and sensi-

tive analysis of OPs. A disposable screen-printed gold electrode

and portable electrochemical instrument would benefit the fieldmonitoring of OPs.

EXPERIMENTAL SECTION

Reagents. Paraoxon, methyl parathion, and fenitrothion were

purchased from Sigma-Aldrich, and their 10 000 mg/L stock

solutions were prepared in acetonitrile. Stock solutions of 5 mg/L

trinitrotoluene (TNT) were prepared from a 1000 mg/L standard

solution of TNT in acetonitrile (Cerilliant, Austin, TX) in 0.1 M

potassium chloride, which was used as the supporting electrolyte

and also served as the adsorption medium during the adsorption

experiments. Zirconium oxychloride (ZrOCl2), nitrobenzene, and

p-nitrophenol were obtained from Sigma-Aldrich (St. Louis, MO)

and used without further purification. Other reagents were

commercially available and were of analytical reagent grade.

Solutions were prepared with ultrapure water from a Millipore

Milli-Q water purification system (Billerica, MA).

Instruments. Cyclic voltammetric and SWV measurements

were performed using an electrochemical analyzer CHI 660 (CH

Instruments, Austin, TX) connected to a personal computer. A

three-electrode configuration was employed, consisting of a

zirconia nanoparticle-modified gold electrode (3-mm diameter)

serving as a working electrode, while Ag/AgCl/3 M KCl and

platinum wire served as the reference and counter electrodes,

respectively. Electrochemical experiments were carried out in a

(14) The EnviroLogix Cholinesterase Screening Test (EP 014). EnviroLogix Inc.,

www.envirologix.com.

(15) La Rosa, C.; Pariente, F.; Hernandez, L.; Lorenzo, E. Anal. Chim. Acta 1994,

295, 273-282.

(16) Mulchandani, A.; Kaneva, I.; Chen, W. Anal. Chem. 1998, 70, 5042-5046.

(17) Wang, J.; Mulchandani, A.; Chen, L.; Mulchandani, P.; Chen, W. Anal. Chem.

1999, 71, 2246-2249.(18) Mulchandani, A.; Mulchandani, P.; Kaneva, I.; Chen, W. Anal. Chem. 1998,

70, 4140-4145.

(19) Wang, J.; Mulchandani, A.; Chen, L.; Mulchandani, P.; Chen, W. Elec-

troanalysis 1999, 11, 866-869.

(20) Hu, S.; Xie, J.; Xu, Q.; Rong, K.; Shen, G.; Yu, R. Talanta 2003, 61, 769-

777.

(21) Marty, I.-L.; Leca, B.; Noguer, T. Analusis Mag. 1998, 26, M144-M149.

(22) Turiel, E.; Matin-Esteban, A.; Femandez, P.; Perez-Conde, C.; Camara, C.

Anal. Chem. 2001, 73, 5133-5141.

(23) Muldoon, M. T.; Stanker, L. H. Anal. Chem. 1997, 69, 803-808.

(24) Martinez, R. C.; Gonzalo, E. R.; Garc a, F. G.; Mendez, J. H. J. Chromatogr.

1993, 644, 49-58.

(25) Wang, J.; Chatrathi, M.; Mulchandani, A.; Chen, W. Anal. Chem. 2001,

73, 1804-1808.

(26) Thomas Buscher, C.; McBranch, D.; Li, D. J. Am. Chem. Soc. 1996, 118,

2950-2953.

(27) Fang, M.; Kaschak, D. M.; Sutorik, A. C.; Mallouk T. E. J. Am. Chem. Soc.

1997, 119, 12184-12191.

(28) Lee, H.; Kepley, L. J.; Hong, H.; Mallouk T. E. J. Am. Chem. Soc. 1988,

110, 618-620.

(29) Hong, H.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521-527.

(30) Zhu, N.; Zhang A.; Wang, Q.; He, P.; Fang, Y. Anal. Chim. Acta 2004, 510,

163-168.

(31) Liu, S.; Xu, J.; Chen, H. Bioelectrochemistry2002, 57, 149-154.

(32) Yu, H.; Rowe, A.; Waugh, D. M. Anal. Chem. 2002, 74, 5742-5747.

(33) Aslam, M.; Pethkar, S.; Bandyopadhyay, K.; Mulla, I. S.; Sainkar, S. R.;

Mandale, A. B.; Vijayamohanan, K., J. Mater. Chem. 2000, 10, 1737-1743.

(34) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir1998, 14, 6924-6929.

Figure 1. Structure of nitroaromatic OP compounds. (A) Paraoxon;(B) fenitrothion; (C) methyl parathion.

Analytical Chemistry, Vol. 77, No. 18, September 15, 2005 5895

8/9/2019 ac050791t

3/8

2-mL voltammetric cell at room temperature (25 C). All potentials

are referred to the Ag/AgCl reference electrode (CH Instru-

ments). Scanning electron microscopy (SEM) was carried outusing a JEOL JSM-5900 LV machine. All samples were imaged

under vacuum conditions using secondary electron imaging. The

typical accelerating voltage of the electron beam used was 10 kV.

All samples were grounded with a piece of copper tape to curtail

specimen charging.

Preparation of Gold Electrode Modified with Zirconia

Nanoparticles. A gold electrode (3-mm diameter) from CH

instruments was polished carefully to a mirrorlike surface with

0.3- and 0.05-m alumina slurry and sequentially sonicated for 2

min in 6 M nitric acid, acetone, and water. Before the experiment,

the bare gold electrode was cyclic-potential scanned within the

potential range 0.5-1.5 V in freshly prepared 0.2 M H 2SO4 until

a voltammogram characteristic of the clean polycrystalline goldwas established. Then it was washed with distilled water and dried

by nitrogen. Zirconia nanoparticles were deposited onto bare gold

electrodes in an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M

KCl by cycling the potential between -1.1 and + 0.7 V (versus

Ag/AgCl) at a scan rate of 20 mV/s for 10 consecutive scans.32

The gold electrodes modified with zirconia nanoparticles (ZrO2/

Au) were rinsed with water and dried with N2 for further

experiments.

Electrochemical Stripping Detection.A ZrO2/Au electrode

was dipped into a stirring 0.1 M KCl solution containing the

desired concentration of OP pesticides for 2 min, washed with

distilled water carefully, and transferred to a 2-mL electrochemical

cell containing 0.1 M KCl solution. Before electrochemical

measurements, the electrolyte solution was purged with nitrogen

for 5 min. SWV measurements were performed from-0.4 to +0.3

V with a step potential of 4 mV, an amplitude of 20 mV, and a

frequency of 25 Hz (unless otherwise stated). Baseline correction

of the resulting voltammogram was performed using the linear

baseline correction mode of the CHI 660 (CH Instruments,

Austin, TX) software. Cyclic voltammetric measurements were

performed under batch conditions. The cyclic voltammogram was

recorded between -0.8 and +0.5 V at a scan rate of 100 mV/s.

All measurements were performed at room temperature.

Regeneration of Electrode Surface. After the electrochemi-

cal stripping measurement, multiple successive SWV scanning was

used to remove the bound OPs until the anodic stripping peakdisappeared. The electrode was washed with distilled water for

the next measurement.

Safety Considerations. OP pesticides are highly toxic and

should be handled in a fumehood. Skin and eye contact and

accidental inhalation or ingestion should be avoided.

RESULTS AND DISCUSSION

In the current study, the ZrO2 nanoparticles were electrody-

namically deposited onto a cleaned gold electrode surface in an

aqueous electrolyte of 5.0 mM ZrOCl2 and 0.5 M KCl by cycling

the potential scanning between -1.1 and+0.7 V (versus Ag/AgCl)

for 10 consecutive scans at a scan rate of 20 mV/s (unless

otherwise stated). Figure 3A shows a representative cyclicvoltammogram of the formation processes of ZrO2 nanoparticles

on the cleaned gold electrode surface (curve a, red line). A normal

electropolymerization growth, with increasing current upon repeti-

tive scanning, is observed during the electrodeposition processes.

The steep rise in the cathodic and anodic current at the potential

range of -0.6 to -1.1 V corresponds to the complex redox

behavior of ZrOCl2 on gold.32 Such redox behavior was not

observed in the absence of ZrOCl2 (curve b, blue line). Note that

the increasing cathodic and anodic current is different from the

results reported by Yu et al. The cathodic current decreased with

a thiol self-assembled monolayer modified gold electrode.32 The

observed difference may come from electrode material or a

different electrodeposition mechanism of zirconia. Different cy-

cling potential ranges (between -1.1 V to varying high potential

from 0.7 to 1.2 V) were used to prepare ZrO2 nanoparticles on

the gold electrode surface. Experimental results showed there

was no significant difference observed including the shape, density

of formed ZrO2 nanoparticle, and stripping voltammetric charac-

teristics of bound OPs. A cycling potential range between -1.1 V

and +0.7 V was used to prepare the ZrO2/Au electrode. A SEM

image (Figure 3B) confirms the distinct ZrO2 nanoparticle forma-

tion on the gold electrode surface. The ZrO2 nanoparticles formed

by 10 consecutive potential cycling possess an average size of 50-

Figure 2. Scheme of electrochemical sensing nitroaromatic OP compounds. (A) Electrodeposition ZrO2 nanoparticle to gold electrode surface;(B) nitroaromatic OP compounds adsorb to ZrO2 nanoparticle surface; (C) electrochemical stripping detection of nitroaromatic OP compounds;

X ) O or S and R ) nitroaromatic OP group.

5896 Analytical Chemistry, Vol. 77, No. 18, September 15, 2005

8/9/2019 ac050791t

4/8

150 nm with a small interparticle distance that is much smaller

than the ZrO2 microcrystalline (7-15 m), which has a 17-m

interparticle distance, formed on the dithiol functionalized gold

surface or bare vacuum-deposited 2000- gold surface.34 The

formed nanosize ZrO2 particles in our experiments may benefit

from the polycrystalline gold seeds that were formed during the

electrochemical cleaning step in the 0.2 M H2SO4 solution. The

amount of zirconia nanoparticle on the gold electrode surface

increases with the increase in the number of potential cycles. An

average of 300 zirconia nanoparticles/m2 was observed on the

prepared ZrO2/Au electrode surface (based on counting at six

different locations) after 10 potential cycles.

Zirconia has a strong affinity to the phosphoric group and

provides a facile method to attach OPs to an electrode surface.Nanosize ZrO2 particles offer a large electrode surface area and

increase the interacting opportunities of OPs. Figure 4 shows the

cyclic voltammograms of methyl parathion/ZrO2/Au electrode (a)

and ZrO2/Au (b) in a 0.1 M potassium chloride solution. A pair

of rather well-defined redox peaks (Epa, 0.093 V and Epc, 0.037)

and an irreversible reduction peak (Epc, -0.61 V) were observed

with a methyl parathion/ZrO2/Au electrode in the potential range

of-0.8 to +0.5 V (Figure 4 a). The irreversible reduction peak

corresponds to the reduction of the nitro group to the hydroxyl-

amine group (reaction 1), and the reversible redox peaks are

attributed to a two-electron-transfer process (reactions 2 and 3),

as shown below:

These profiles are consistent with those described elsewhere

for nitroaromatic OP pesticides and nitrophenyl derivates.25,35-37

A control experiment (Figure 4b) was performed under the same

conditions with the ZrO2/Au electrode in the absence of methyl

parathion; no redox peak appeared at the selected potential range

(the standard reduction potential of ZrO2 is -1.544 V). SWV

analysis has a higher sensitivity than other electrochemical

technologies, such as cyclic voltammetry and differential pulse

voltammetry. The inset of Figure 4 shows corresponding SWV

voltammograms of methyl parathion /ZrO2/Au electrode (a) andZrO2/Au electrode (b) in 0.1 M KCl. There is no anodic stripping

peak observed at the ZrO2/Au electrode (inset, curve b). The

methyl parathion/ZrO2/Au electrode exhibits a very sharp and

well-defined stripping peak at the potential range from -0.8 V to

+0.4 V (inset, curve a). The peak potential of the oxidation peak

(0.06 V) shifts 20 mV to a negative potential direction compared

with that in the cyclic voltammogram.

The attracting voltammetric characteristics of adsorbed methyl

parathion on the ZrO2/Au show that ZrO2 nanoparticles have a

strong affinity to the OP compound, which possesses a phosphate

group. To confirm that the affinity occurred between ZrO2 and

methyl parathion instead of the nonspecific adsorption between

the exposed gold surface and methyl parathion, Figure 5 shows

a comparison of the SWV signals of a cleaned bare gold electrode

(a) and a ZrO2/Au electrode (b) after incubating 2 min in 0.1 M

KCl containing 200 ng/mL methyl parathion. A substantially

smaller signal (35 times less compared to the ZrO2/Au electrode)

is observed for a bare electrode. Such a big difference in SWV

signals is attributed to the specific adsorbing between ZrO2

(35) Lin, Y.; Zhang, R. Electroanalysis 1994, 6, 1126-1131.

(36) Roston, D. A.; Kissinger, P. T. Anal. Chem. 1982, 54, 429-434.

(37) Kastening, B., Zuman, P., Meites, L., Kolthoff, I. M, Eds. Progress in

Polarography; Wiley-Interscience: New York, 1972; Vol. 3, p 259.

Figure 3. (A) Cyclic voltammograms of gold electrode (curve a,

red line) during electrodeposition process in 5.0 mM ZrOCl 2 and 0.1M KCl aqueous solution at a scan rate of 20 mV/s. Potential cycles,

10; curve b (blue line) is the cyclic voltammograms of gold electrodein 0.1 M KCl aqueous solution in the absence of ZrOCl 2 under the

same conditions. (B) Typical SEM image of zirconia nanoparticlesformed with 10 consecutive potential cycling on a gold electrode.


8/9/2019 ac050791t

5/8

nanoparticles and methyl parathion. Also to be noted is that the

peak potentials for the stripping voltammograms shift to positive

potential direction with the increase of concentration of methyl

parathion on the ZrO2/Au electrode surface. The peak potential

for the stripping voltammogram (Figure 5, 200 ng/mL methyl

parathion) is shifted around 100 mV from that observed in Figure

4 (800 ng/mL methyl parathion). Similar behavior was observed

in the stripping voltammograms of different concentrations of

methyl parathion (see Figure 9).

Another two electroactive OPs, paraoxon and fenitrothion,

which possess a structure similar to that of methyl parathion

(Figure 1), exhibit similar voltammetric characteristics after they

are adsorbed on the ZrO2/Au electrode surface. The mixture of

three identical concentrations of OP compounds shows a big

stripping response, which almost equals the sum of the individual

OPs. To confirm that the affinity occurred between ZrO2 and the

phosphate group instead of the nitro group, trinitrotoluene was

used to perform the comparison experiment. A negligible signal

was obtained even though the concentration of TNT (1 g/mL)is 5 times more than methyl parathion (not shown).

One of the most important issues in the development of a

chemical biosensor is the regeneration of the sensor surface.

Electrochemical stripping analysis includes built-in preconcentra-

tion and stripping steps. The target analyte is normally ac-

cumulated on the working electrode by applying a constant

potential followed with a stripping step, which can also be regarded

as a cleaning step to remove the target from the electrode surface.

In our experiments, the adsorption process of methyl parathion

corresponds to the stripping step to obtain the electrochemical

stripping signal of the analyte, which automatically removes the

adsorbed OPs. Figure 6 presents a typical successive SWVvoltammogram of a methyl parathion/ ZrO2/Au electrode. It was

found that the stripping peak currents decreased rapidly with the

increase of scanning times, and the anodic stripping peak

disappeared completely after multiple scanning, indicating that

the methyl parathion-ZrO2 complex is dissociated. The electrode

was washed carefully with distilled water and measured again in

fresh 0.1 M KCl solution; no stripping peak was obtained. Also to

be noted is that the SWV scanning times depended on the

concentration of adsorbed OPs. More scanning times are neces-

sary for a higher amount of bound OPs.

Figure 5. Stripping voltammograms of bare gold electrode (a) andZrO2/Au electrode (b) after 2-min adsorption in stirring 0.1 M of KCl

solution containing 200 ng/mL methyl parathion. Potential scanningpotential. -0.4 to +0.3 V; other conditions, same as Figure 4.

Figure 4. Cyclic voltammograms of methyl parathion/ZrO2/Au (a) and ZrO2/Au electrode (b) in 0.1 M KCl solution (pH 7.0). Potential scanningrate, 100 mV/s. Methyl parathion/ZrO2/Au electrode was prepared by dipping the ZrO2/Au electrode in stirring 0.1 M of KCl solution containing800 ng/mL methyl parathion for 2 min and carefully washing with distilled water before electrochemical measurement. Inset is corresponding

stripping voltammograms. SWV conditions: scanning potential range, -0.8 to +0.4 V; frequency, 25 Hz; increasing potential, 4 mV.


8/9/2019 ac050791t

6/8

The attracting stripping voltammetric characteristics of methyl

parathion on the ZrO2/Au provide a facile electrochemical

quantitative method for analyzing OPs. Parameters of the assay

procedure would affect the stripping response of OPs. The amount

of ZrO2 nanoparticles influences the amount of OPs bound to the

surface of the ZrO2/Au electrode. The cycles of cyclic-potential

scanning were used to control the amount of ZrO2 nanoparticle

on the gold electrode surface. Figure 7A shows the effect of the

cycles of cyclic-potential scanning on the adsorption of methyl

parathion. The stripping current of methyl parathion rises with

the cycles at first up to 10 cycles and then decreases. The increaseof the stripping current indicates that the amount of adsorbed

methyl parathion is increasing with the increase of the amount of

zirconia nanoparticle on the electrode surface. The decrease of

the stripping current can be understood by considering the

continual buildup of zirconia nanoparticles, which consequently

cause aggregation of zirconia nanoparticles and generate higher

resistance for the electrochemical stripping processes, leading to

a change of sensing characteristics of the electrode. The change

of electrochemical sensing characteristics of ZrO2/Au was inves-

tigated by cyclic voltammetry of a 5 mM Fe(CN) 63-/0.1 M KCl.

Figure 7B presents cyclic voltammograms of a bare Au electrode,

and different amounts of ZrO2 nanoparticles modified the Au

electrodes. We can see the redox peak currents of Fe(CN) 63-

decrease with the increase of potential cycle times (from top to

bottom, 0, 2, 4, 6, 8, 10, 15, and 20 cycles). Although more potential

cycles increase the amount of ZrO2 nanoparticles on the electrode

surface, aggregations of nanoparticles increase the electron-

transfer distance, which leads to the decrease of redox peak

current of Fe (CN)63- and decreases the sensitivity of the

electrode. So 10 potential cycles were used to prepare the ZrO2nanoparticle modified gold electrode.

The effect of adsorption time on the stripping peak current

was investigated (Figure 8A). The peak currents increase rapidly

Figure 6. Stripping voltammograms (without baseline correction) of the regeneration process of methyl parathion/ZrO2 /Au electrode; other

conditions, same as Figure 5.

Figure 7. (A) Effect of the amount of zirconia nanoparticle on methylparathion adsorption. ZrO2/Au electrodes were prepared by different

potential scanning cycles (2, 4, 6, 8, 10, 15, 20). The adsorptionexperiments were performed for 2 min in 0.1 M KCl solution containing

200 ng/mL methyl parathion. Stripping detection conditions, same asFigure 5. (B) Cyclic voltammograms of corresponding electrodes in

5 mM Fe(CN)63-/0.1 M KCl solution (from top to bottom, 0, 2, 4, 6, 8,10, 15, 20 cycles), potential scanning rate. 100 mV/s.


8/9/2019 ac050791t

7/8

with the accumulation time at first and then more slowly from 2

min. The resulting current versus time plot displays a curvature

consistent with adsorption processes. No such surface adsorption

is indicated in analogous measurements at the cleaned bare gold

electrode surface (not shown). We also observed the adsorption

of methyl parathion under constant potential conditions (not

shown); there is no significant increase of the stripping peak

current. Two minutes of adsorption time under open-circuit

conditions was thus employed.

An additional parameter that affected the adsorption of methyl

parathion was the pH of the adsorption medium. The pH of the

adsorption solution (0.1 M KCl) was adjusted with 1.0 M NaOH

or HCl solution and varied from 3.0 to 9.0. Figure 8B presents

the pH effect of the adsorption solution on the adsorption ofmethyl parathion. We can see that the stripping signal increases

with an increase of pH up to 7.0, and then it decreases at higher

pH. It indicates that ZrO2 has the maximum adsorption to methyl

parathion in a neutral environment. The loss of signal at acidic or

basic environment may be attributed to the effect of H+ or OH-

on adsorption. The mechanism of the pH effect is under investiga-

tion in our laboratory. So a pH 7.0 of 0.1 M KCl solution was used

as the adsorption medium in most experiments.

Analytical Performance. Figure 9 displays the SWV response

of the ZrO2/Au electrode incubated in increasing concentrations

of methyl parathion solution under optimum experimental condi-

tions. Well-defined peaks, proportional to the concentration of the

corresponding methyl parathion, were observed. A linear relation-

ship between the stripping current and the methyl parathion

concentration was obtained covering the concentration range from

5 to 100 ng/mL, the linear regression equation being I (nA) )

1.0696C+ 5.4453, with a correlation coefficient of 0.9939. A wide

linear range will be realized by increasing the amount of zirconia

nanoparticle on the gold electrode surface. A detection limit of 3

ng/mL (based on signal-to-noise ratio equal to 3) was obtained

under the optimum experimental conditions. The detection limit

was improved significantly by increasing the accumulation time.

A detection limit of 1 ng/mL was estimated on the basis of a

s/n ) 3 characteristic of the 3 ng/mL data points in connection

with a 600-s incubating time. The detection limit obtained is

Figure 8. Effect of adsorption time (A) and the pH of adsorption

medium (B) on the adsorption of methyl parathion. The concentrationof methyl parathion in adsorption medium was 200 ng/mL. The

adsorption experiments of (A) were performed in a pH 7.0 of 0.1 MKCl. Electrochemical stripping detection conditions, same as

Figure 5.

Figure 9. Stripping voltammograms of increasing methyl parathion concentration, from bottom to top, 5, 10, 20, 40, 60, 80, 100, and 200ng/mL, respectively. The inset shows the calibration curve. Electrochemical stripping detection conditions, same as Figure 5.


8/9/2019 ac050791t

8/8

comparable with that reported so far with an enzyme-based

biosensor.18 A series of 10 repetitive measurements of a solution

containing 20 ng/mL yielded reproducible peak currents withrelative standard deviations of 5.3.

Interferences arising from the other electroactive nitrophenyl

derivatives and oxygen-containing inorganic ions (PO43-, SO42-,

NO3- ) that are expected to coexist in solution were used to

evaluate the selectivity of the ZrO2/Au electrode to nitroaromatic

OPs. Separate adsorbing experiments were performed with 100

ng/mL methyl parathion in 0.1 M KCl solution in the absence

and presence of 100 ng/mL ofp-nitrophenol, 100 ng/mL nitroben-

zene, 100 ng/mL TNT, 0.1 M of PO43-, 0.1 M of SO42-, and 0.1 M

of NO3-. Figure 10 shows the electrochemical stripping signals

of methyl parathion at different experimental conditions. One can

see that electroactive nitrophenyl derivatives and oxygen-contain-

ing inorganic ions do not interfere with the adsorption of methylparathion, and the stripping peak current varies slightly. Also note

that it was reported in the literature that zirconia has a good affinity

to PO43-,33, 34 but in this case, it does not interfere with the

adsorption of methyl parathion. The reason may be that the

adsorption capability of methyl parathion to zirconia is much

stronger than PO43-. Further experiments are being conducted

in our laboratory. The stripping peak potential of OPs is 0 mV,

which also avoids the interferences from other phenol compounds

and electroactive species, whose oxidation potentials are more

than 0.3 V. An electrochemical stripping analysis used in conjunc-

tion with a zirconia nanoparticle modified gold electrode thus

holds great promise for direct analysis of relevant water samples

without any prior separation or pretreatment.

CONCLUSION

We have demonstrated a sensitive electrochemical sensing

protocol for nitroaromatic OPs based on the use of zirconia

nanoparticles as selective sorbents. The strong affinity of zirconiananoparticles for the phosphoric group and the promising SWV

characteristics of nitroaromatic OPs provide a facile quantitative

method for a group of electroactive OPs. Other electroactive

nitrophenyl derivatives do not interfere with the adsorption of OPs.

An anodic stripping analysis with a very low stripping peak

potential avoids the interferences from other electroactive species.

The results obtained from this work imply that the combination

of a disposable screen-printed gold electrode with a portable

electrochemical instrument would benefit the field monitoring of

OPs. Current methods are limited to the detection of a group of

nitroaromatic OPs. Nonelectroactive OPs can be monitored by

combining zirconia nanoparticles (selective absorbents) with

enzyme or metal nanoparticle-labeled antibodies against OPs(recognition elements) and electrochemically measuring the

enzymatic product or dissolved metal ions.

The proposed electrochemical sensing technology is thus

expected to open new opportunities for detecting OP pesticides

and nerve agents in the environment, public places, or workplaces

and for monitoring the exposures of individuals to chemical

warfare agents.

ACKNOWLEDGMENT

The work is supported by a laboratory-directed research and

development program at Pacific Northwest National Laboratory

(PNNL). The research described in this paper was performed at

the Environmental Molecular Sciences Laboratory, a national

scientific user facility sponsored by the U.S. Department of

Energys (DOEs) Office of Biological and Environmental Research

and located at PNNL. PNNL is operated by Battelle for DOE under

Contract DE-AC05-76RL01830.

Received for review May 6, 2005. Accepted July 12, 2005.

AC050791T

Figure 10. Electrochemical stripping signals of methyl parathion/ZrO2/Au electrodes. Adsorption experiments were performed with pH

7 0.1 M KCl containing 100 ng/mL methyl parathion in the absenceand presence of 100 ng/mL p-nitrophenol, 100 ng/mL nitrobenzene,

100 ng/mL TNT, 0.1 M PO43-, 0.1 M SO42-, and 0.1 M NO3-,

respectively. Electrochemical stripping detection conditions, same asFigure 5.


ac050791t

Documents

Transcript of ac050791t