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Biosensors and Bioelectronics 23 (2007) 130–134

Short communication

Immobilization of acetylcholinesterase on gold nanoparticlesembedded in sol–gel film for amperometric detection

of organophosphorous insecticide

Dan Du a,∗, Shizhen Chen a, Jie Cai b, Aidong Zhang a,∗a Key Laboratory of Pesticide & Chemical Biology of Ministry of Education,

Central China Normal University, Wuhan 430079, PR Chinab The Technology Center of Wuhan Iron & Steel Company, Wuhan 430080, PR China

Received 15 October 2006; received in revised form 12 January 2007; accepted 19 March 2007Available online 25 March 2007

bstract

A simple method to immobilize acetylcholinesterase (AChE) on silica sol–gel (SiSG) film assembling gold nanoparticles (AuNPs) was proposed,hus a sensitive, fast and stable amperometric sensor for quantitative determination of organophosphorous insecticide was developed. The largeuantities of hydroxyl groups in the sol–gel composite provided a biocompatible microenvironment around enzyme molecule and stabilized itsiological activity to a large extent. The immobilized AChE could catalyze the hydrolysis of acetylthiocholine chloride (ATCl) with a K

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f 450 �M to form thiocholine, which was then oxidized to produce detectable single with a linear range of 10–1000 �M. AuNPs catalyzed thelectro-oxidation of thiocholine, thus increasing detection sensitivity. Based on the inhibition of organophosphorous insecticide on the enzymaticctivity of AChE, using monocrotophos as a model compound, the conditions for detection of the insecticide were optimized. The inhibition

f monocrotophos was proportional to its concentration ranging from 0.001 to 1 �g/ml and 2 to 15 �g/ml, with the correlation coefficients of.9930 and 0.9985, respectively. The detection limit was 0.6 ng/ml at a 10% inhibition. The developed biosensor exhibited good reproducibilitynd acceptable stability, thus providing a new promising tool for analysis of enzyme inhibitors.

2007 Elsevier B.V. All rights reserved.

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eywords: Amperometric sensor; Sol–gel; Gold nanoparticles; Acetylcholines

. Introduction

Organophosphorous (OP) compounds have been used exten-ively for pest control due to their high insecticidal activityKumar et al., 2006; Carloa et al., 2004.). OP compounds exhibitcute toxicity and can irreversibly inhibit acetylcholinesteraseAChE) that is essential for the functioning of central nervousystem, often causing respiratory paralysis and death (Kim et al.,005). Therefore, rapid determination and reliable quantification

f trace level of OP compounds are important. Traditional ana-ytical methods, i.e., gas chromatography or high-performanceiquid chromatography which are often coupled with mass-elective detectors have been widely used for the determinationf OP compounds (Chen and Huang, 2006; Rotiroti et al., 2005;

∗ Corresponding authors. Tel.: +86 27 67867953.E-mail addresses: dudan@mail.ccnu.edu.cn (D. Du),

dzhang@mail.ccnu.edu.cn (A. Zhang).

paMptrtty

956-5663/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2007.03.008

; Monocrotophos

eandro et al., 2006). Although these methods for pesticideetermination in water samples are accurate, they are ratherime-consuming and beyond the analytical capacities of smallerater works as they require expensive instrumentation. The

omplicated sample pretreatments together with highly trainedersonnel are not suitable for the field conditions.

A combination of enzymatic reactions with the electro-hemical method allowed developing different enzyme basedlectrochemical biosensors for determination of environmentalollutants due to the good selectivity, sensitivity, rapid response,nd miniature size (Hashimoto et al., 2006; Yu et al., 2006;aly et al., 2005). A variety of enzymes, such as organophos-

horous hydrolase, alkaline phsosphatase, ascorbate oxidase,yrosinase and acid phosphatase have been employed for prepa-

ation of pesticide biosensors (Trojanowicz, 2002). Amonghese, amperometric AChE biosensors based on the inhibitiono AChE have shown satisfactory results for pesticides anal-sis (Schulze et al., 2003; Kok and Hasirci, 2004; Shi et al.,

D. Du et al. / Biosensors and Bioelectronics 23 (2007) 130–134 131

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Scheme 1. Principle of AChE biosenso

006) in which the enzymatic activity was employed as anndicator of quantitative measurement of insecticides. WhenChE was immobilized on the working electrode surface, its

nteraction with the substrate of acetylthiocholine obtained anlectro-active product of thiocholine, which produced an irre-ersible oxidation peak. The inhibition of OP on AChE wasonitored by measuring the oxidation current of thiocholine.

mmobilization of enzyme to solid electrode surface is a cru-ial step for the fabrication of biosensors (Amine et al., 2006).sual methods include direct physical adsorption onto a solid

upport (Sotiropoulou and Chaniotakis, 2005), encapsulationnto a hydrogel (Yadavalli et al., 2004), cross-linking (Solnat al., 2005), and covalent binding (Lin et al., 2004). A keyequirement of enzyme immobilization is attachment withouthe bioactivity being sacrificed (Gill and Ballesteros, 2000).ol–gel technology provides an attractive way for the immobi-

ization of biological entitles including full cell, enzyme, proteinnd antibody or antigen due to the inert low temperature pro-ess (Du et al., 2003; Yu and Ju, 2002; Yu et al., 2003). Thettractive features have led to an intensive research in designf biosensors. Recently, nanoparticles, particularly the goldanoparticles (AuNPs) have received considerable attentionn analytical electrochemistry due to their high conductivitiesSanz et al., 2005). Willner’s group have extensively studiedhe electrochemical and optical applications of AuNPs and thebility to promote the electron transfer (Willner et al., 2005,006; Xiao et al., 2005; Pardo-Yissar et al., 2003; Zayats et al.,005).

This work is motivated by assembling AuNPs on a sol–gel-erived silicate network (AuNPs-SiSG) for immobilization ofChE. Here, a sensitive, fast and cheap method for rapidetermination of monocrotophos quantitatively (Scheme 1) wasroposed.

. Experiments

.1. Reagents

Acetylthiocholine chloride (ATCl) and AChE (Type3389, 500 U/mg from electric eel) were purchased fromigma–Aldrich (St. Louis, USA) and used as received.etrathoxysilane (TEOS) was obtained from international labo-atory (USA). Monocrotophos and gold(III) chloride hydrate

3.9 g/ml) were purchased from Treechem Co. (Shanghai,hina). The 17-nm AuNPs was prepared according to litera-

ure (Xiao et al., 1999) and stored in a brown bottle at 4 ◦C.hitosan (95% deacetylation), phosphate buffer solution (PBS,

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d for determination of OP compound.

H 7.0) and other reagents were of analytical reagent grade. Allolutions were prepared with double distilled water.

.2. Instruments

Electrochemical measurements were performed on a Bio-nalytical System (BAS, cv-50w, USA) with a conventionalhree-electrode system comprising platinum wire as auxiliarylectrode, saturated calomel electrode (SCE) as reference andChE combined to AuNPs-SiSG modified glass carbon elec-

rode (AChE-AuNPs-SiSG/GCE) as working electrode.

.3. Preparation of AuNPs-SiSG composite

A homogenous AuNPs-SiSG composite was prepared byixing 20 �l of TEOS, 60 �l of ethanol, 150 �l of AuNPs,

00 �l of 0.5 mg/ml chitosan solutions (final concentration.3%). This mixture was stirred for 1 h until a clear sol–gelomposite was formed and its pH was adjusted to 4.0–6.0 using70 �l 0.1 M NaOH solution. The obtained composite can betored for several months when refrigerated at 4 ◦C.

.4. Preparation of the biosensor

GCE was polished to mirror finish using the BAS-polishingit with 0.3 and 0.05 �m Al2O3 paste. After sonicated withthanol and water, the electrode was applied to a potentialf +1.75 V under string in pH 5.0 PBS for 300 s and thencanned from +0.3 to +1.25 V and +0.3 to −1.3 V until ateady-state current–voltage curve was obtained (Du et al.,005).

Three microliters of the above AuNPs-SiSG composite wasoated on a pretreated GCE and allowed for reaction at 20 ◦C forh. The obtained electrode was then coated with 4.0 �l AChE

olution, which was incubated at 25 ◦C for 30 min to obtain theChE-AuNPs-SiSG/GCE.

.5. Measurement procedure

Desired volume of standard ATCl was added to 1.0 ml PBSnd the current–time response curve was recorded.

For the measurements of monocrotophos, the obtainedChE-AuNPs-SiSG/GCE was first immersed in PBS solution

ontaining different concentrations of standard monocrotophosor 10 min, and then transferred to the electrochemical cell of.0 ml pH 7.0 PBS containing 1.0 mM ATCl to study the electro-hemical response by cyclic voltammetry (CV). The inhibition

132 D. Du et al. / Biosensors and Bioel

Fig. 1. Cyclic voltammograms of GCE (a) and AChE-AuNPs-SiSG/GCE (b)in pH 7.0 PBS; AChE-AuNPs-SiSG/GCE (c), AuNPs-SiSG/GCE (d) andAChE-SiSG/GCE (e) in pH 7.0 PBS containing 1.0 mM ATCl. Inset: CyclicvAs

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oltammograms of AChE-AuNPs-SiSG/GCE in pH 7.0 PBS containing 1.0 mMTCl after immersed in 0 (a), 0.01 (b), 0.2 (c) and 5 (d) �g/ml monocrotophosolution, respectively, for 10 min.

f monocrotophos was calculated as follows:

nhibition (%) = 100% × (iP,control − iP,exp)

iP,control

here iP, control is the peak current of ATCl on the AChE-AuNPs-iSG/GCE, iP, exp is the peak current of ATCl on the AChE-uNPs-SiSG/GCE with monocrotophos inhibition.

. Results and discussion

.1. Cyclic voltammetric behavior ofChE-AuNPs-SiSG/GCE

The cyclic voltammograms of ATCl on different electrodesre shown in Fig. 1. No peak was observed at GCE (curve a) andChE-AuNPs-SiSG/GCE (curve b) in pH 7.0 PBS. However,hen 1.0 mM ATCl was added into PBS, the cyclic voltam-ograms of AChE-AuNPs-SiSG/GCE showed an irreversible

xidation peak at 689 mV (curve c), while no detectable signalas observed at AuNPs-SiSG/GCE without immobilization ofChE (curve d). Obviously, this peak came from the oxidationf thiocholine, hydrolysis product of ATCl, catalyzed by themmobilized AChE. Furthermore, this peak current was muchigher and the peak potential shifted negatively compared tohose on the silica sol–gel modified electrode without AuNPsAChE-SiSG/GCE) (curves e and c). This phenomenon is dueo the presence of AuNPs, which possessed inherent conductiveroperties and catalytic behavior, thus they provide a conductiveathway to electron transfer and promote electron transfer reac-

ions at a lower potential. If AuNPs were directly coated on thelectrode surface instead of entrapment in cross-linked sol–gel,uch higher background current was observed. Thus, the AChE-uNPs-SiSG/GCE was used in following experiments.

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ectronics 23 (2007) 130–134

With increasing scan rate, the peak current increased andhe peak potential shifted slightly (Fig. S1). The peak currentsxhibited a linear dependence on the scan rates ranging fromto 200 mV/s (inset in Fig. S1), indicating a typical surface-

ontrolled electrode process (Liu and Lin, 2005).

.2. Effect of monocrotophos on response ofChE-AuNPs-SiSG/GCE

The produced current is related to the activity of immobilizednzyme, which can be used as an indicator for quantitative mea-urement of OP compounds. As shown from inset of Fig. 1, afterChE-AuNPs-SiSG/GCE was immersed in the standard solu-

ion of monocrotophos at a known concentration for 10 min,he produced current decreased drastically (curves b–d) com-ared with the control (curve a). The decrease in peak currentncreased with the increasing concentration of monocrotophos.his was because monocrotophos as one of the OP compoundsxhibited acute toxicity and involved in the irreversible inhibi-ion action on AChE, thus reduced the enzymatic activity to itsubstrate. Due to the notable change in voltammetric signal ofhe AChE-AuNPs-SiSG/GCE, simple method for determinationf monocrotophos was established.

.3. Optimization of AChE-AuNPs-SiSG/GCE preparation

The effect of AuNPs content on response was studied betweenand 30% (vAu/v). The anodic current gradually increased with

ncreasing the amount of AuNPs and reached a maximum at5% (vAu/v) (Fig. S2A). Further increase of the AuNPs ledo the decrease of peak current. This phenomenon was possi-ly attributed to the increase of the resistance and double layerapacitance of the modified electrode. So 15% (vAu/v) AuNPsas used for preparation of the biosensor.The peak current increased with the increasing tempera-

ure, and the optimal temperature range occurred between 20nd 50 ◦C (Fig. S2B). No obvious decrease of the responseas observed during this temperature interval, indicating an

xcellent activity of the immobilized enzyme and no denatura-ion occurred. The stability was attributed to the biocompatible

icroenvironment provided by sol–gel matrix, which stabilizesnzymatic activity to a large extent.

The amount of enzyme loading on electrode surface wasnother important parameter. Fig. S2C displays the effectf enzyme loading on amperometric response. The currentncreased with the increasing volume of AChE and then trendedo a constant value. More than 4.0 �l enzyme adsorbing were notnough stable, owing to imitation of electrode area, indicating aaturation of enzyme loading.

.4. Effect of pH on response of AChE-AuNPs-SiSG/GCE

Fig. S2D shows the relation between the peak current and

olution pH. Obviously, the maximum value was obtained at pH.0 indicating that AuNPs-SiSG matrix did not alter the optimalH value for catalytic behavior of enzyme and the microenvi-onment surrounding the enzyme in the sol–gel pores was easily

Bioelectronics 23 (2007) 130–134 133

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D. Du et al. / Biosensors and

ccessed by substrate. Therefore, pH 7.0 was used in detectionolution.

.5. Calibration curve of ATCl

The typical current–time response curve of the biosensor wasbtained by successive additions of the substrate into a stirredell. With the increasing concentration of ATCl the amperomet-ic response increased linearly in the range of 10.0–1000 �Mith correlation coefficient of 0.9989 and then tended to alateau value (Fig. S3), showing a typical Michaelis–Mentenrocess. The apparent Michealis–Menten constant (Kapp

m ) wasalculated to be 0.45 mM (Kamin and Willson, 1980). This valueas lower than those of 0.66 mM for AChE adsorbed on AuNPs-OL composite film (K.A. Joshi et al., 2005), and 1.5 mM forChE adsorbed on polyethyleneimine modified electrode (P.P.

oshi et al., 2005). Thus, the immobilized AChE on AuNPs-iSG matrix exhibited a higher affinity to ATCl. The detection

imit was 1.0 �M at a signal-to-noise ratio of 3. The biosensorchieved 95% of the steady-state current in 10 s, indicating aast response. Thus, 1000 �M ATCl was selected for detectionf monocrotophos.

.6. Effect of inhibition time on response ofChE-AuNPs-SiSG/GCE

In pesticide analysis one of the most influential parame-ers is inhibition time. With an increase of immersing timen the monocrotophos solution, the peak current of ATCl onhe AChE-AuNPs-SiSG/GCE decreased greatly. As shown inig. S4, monocrotophos displayed increased inhibition on AChEith increased immersing time. When the immersing time was

onger than 10 min the curve tended to a stable value, indicat-ng the binding interaction with active target groups in enzymeould reach saturation. This change tendency of peak currenteflected the alteration of enzymatic activity, which resulted inhe change of the interactions with its substrate. However, the

aximum value of inhibition was not 100%, which was likelyo attribute to the binding equilibrium between pesticide andinding sites in enzyme.

.7. Detection of monocrotophos

The response decreased with an increasing monocro-ophos concentration. The inhibition of monocrotophos wasroportional to its concentration from 0.001 to 1 �g/ml androm 2 to 15 �g/ml. The linearization equation were inhi-ition (%) = 36.07c + 6.506 (%) and inhibition (%) = 3.482c +3.82 (%), with the correlation coefficients of 0.9930 and.9985, respectively (Fig. 2). The detection limit was calculatedo be 0.6 ng/ml at a 10% inhibition (Kandimalla and Ju, 2005).

.8. Precision, reproducibility and stability of

ChE-AuNPs-SiSG/GCE

The inter-assay precision was estimated by determining theesponse of 1.0 mM ATCl at six different electrodes immersed

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ig. 2. Relationship between peak currents and concentrations of monocro-ophos. Insets: Calibration curves for monocrotophos determination.

n 0.1 and 5.0 �g/ml monocrotophos for 10 min, respectively.he coefficients of variation was 2.3 and 1.7%, respectively,

ndicating acceptable fabrication reproducibility. The intra-ssay precision of the sensors was evaluated by assaying onenzyme electrode for six replicate determinations, and the rel-tive standard deviations was 0.8% at the ATCl concentrationf 1.0 mM.

When the enzyme electrode was not in use, it was storedt 4 ◦C. The sensor retained 90% of its initial current responsefter a 30-day storage period. The large quantities of hydroxylroups in the sol–gel substrate were able to form strong hydrogenonds. These hydrogen bonds and the intermolecular interac-ions between enzyme molecule and specific sites of the SiSGrevented enzyme from leaking out of the film.

. Conclusions

This work proposed a simple and efficient method for immo-ilization of AChE, as a model enzyme, on electrode surface andeveloped a sensitive, fast and cheap sensor to detect both ATClnd OP compound. Based on the inherent conductive propertiesf AuNPs, the immobilized AChE exhibited a higher affinityo it substrate and produced detectable and fast response. Toum up, the constructed biosensor processing good fabricationeproducibility, acceptable stability, fast response and lowetection limit has potential application in detection of otheroxic compounds against to AChE. We are currently facinghe challenges of semi-automated procedures, which could bechieved through further improvement in miniaturization oflectrochemical system.

cknowledgements

The authors are gratefully acknowledging the financial sup-ort of the Natural Science Foundation of Hubei Province (No.

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006ABA183) and the National Natural Science Foundation ofhina (No. 20672043).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.bios.2007.03.008.

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