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Sensors and Actuators B 260 (2018) 78–85

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

n interdigitated ISFET-type sensor based on LPCVD grown grapheneor ultrasensitive detection of carbaryl

ao Thi Thanh a,b, Nguyen Hai Binh a, Nguyen Van Tu a, Vu Thi Thu c,h, Maxime Bayle d,e,atthieu Paillet d, Jean Louis Sauvajol d, Phan Bach Thang f,g, Tran Dai Lam b,h,

han Ngoc Minh a,b,h, Nguyen Van Chuc a,b,∗

Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet NamGraduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet NamUniversity of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet NamLaboratoire Charles Coulomb (L2C), Univ. Montpellier, CNRS, Place Eugène Bataillon – CC026, Montpellier, F-34095, FranceInstitut des Matériaux Jean Rouxel, UMR 6502 CNRS/Université de Nantes 2, rue de la Houssinière, BP 32229, 44322, Nantes Cedex 3, FranceCenter for Innovative Materials and Architectures, Vietnam National University-Ho Chi Minh City, Linh Trung Ward, Thu Duc Dist., Ho Chi Minh City, VietamFaculty of Materials Science and Laboratory of Advanced Materials, University of Science, Vietnam National University, HoChiMinh city, Viet NamCenter for High Technology Development, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam

r t i c l e i n f o

rticle history:eceived 4 June 2017eceived in revised form0 November 2017ccepted 29 December 2017vailable online 30 December 2017

eywords:

a b s t r a c t

Consider advances are being made to develop new technologies capable of fast in-situ tracing of agricul-tural toxins. In this work, we reported a simple carbaryl sensor with very high sensitivity using grapheneinterdigitated ion selective field effect transistor (ISFET). The graphene films were first prepared on poly-crystalline copper foil by a low-pressure chemical vapor deposition method, and then transferred ontothe interdigitated electrodes of ISFETs by a chemical etching technique. The biorecognition is based on theenzymatic inhibition of carbaryl towards urease. As expected, the weaker enzymatic activity of urease inaddition of carbaryl would result in the weaker current response. It was demonstrated that the interdigi-

−8 −1

raphene

SFETensornterdigitated electrode arrayarbaryl

tated ISFET sensor could achieve high sensitivity to detect carbaryl as low as 10 �g mL and the currentresponse of the device showed a good linearity against the logarithm of carbaryl concentration Ccarbarylwith a regression equation �Ids = −0.4 − 0.6 logCcarbaryl (mA) (R2 = 0.99) in the concentration range from2.58 × 10−7 to 2.58 × 10−2 �g mL−1. In conlusion, such convenient graphene-based ISFET configurationcould be advantageously extended for on-line screening of other pesticide and herbicide agents.

© 2017 Elsevier B.V. All rights reserved.

. Introduction

It was well-known that the excess use of carbaryl (1-naphthyl--methyl carbamate) for pest control in crops can cause itsioaccumulation in food or water, and then lead to bioconcen-ration through the food chain [1]. Up to now, many analytical

ethods have been developed for the determination of carbaryl

n food and water, such as high performance liquid chromatogra-hy (HPLC) [2–4], high performance liquid chromatography-masspectrometry [5], and gas chromatography-mass spectrometry

∗ Corresponding author at: Institute of Materials Science, Vietnam Academy ofcience and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam.

E-mail address: chucnv@ims.vast.vn (N. Van Chuc).

ttps://doi.org/10.1016/j.snb.2017.12.191925-4005/© 2017 Elsevier B.V. All rights reserved.

(GC-MC) [6]. Although these above mentioned methods are accu-rate and selective, they require an extraction step using highly toxicorganic solvents and expensive equipment. Moreover, they canonly be performed by highly trained technicians and are not conve-nient for on-site detection [7]. Recently, biosensors have emergedas low-cost, easy operation, fast response, and ultra-sensitive toolsfor tracing pesticide residue [8–10].

Basically, one biosensor is consisted of one biological recog-nition element to sense targeted molecule and one transducingelement to convert that biological recognition into measurable sig-nals. To recognize carbaryl (one popular carbamate compound),enzymatic inhibition effect is often used. Several enzymes have

been used to develop biosensors for carbaryl detection such asacetylcholinesterases (AChE) [8–13], butyrylcholinesterase (BChE)[14], organophosphorus hydrolase (OPH) [15], alkaline phos-

https://doi.org/10.1016/j.snb.2017.12.191http://www.sciencedirect.com/science/journal/09254005http://www.elsevier.com/locate/snbhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.snb.2017.12.191&domain=pdfmailto:chucnv@ims.vast.vnhttps://doi.org/10.1016/j.snb.2017.12.191

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tric pads were protected from GA vapor by using a parafilm. Theincubation process was conducted at room temperature for 60 min.After that, the electrodes were washed several times to remove the

C.T. Thanh et al. / Sensors an

hatase (ALP) [16], tyrosinase [17], and urease [18]. To translate thenhibition of carbaryl towards the above enzymes into measureableignals, various transducing techniques such as optical, electro-hemical, FET, and so on have been employed. Several researchroups have reported the optical biosensors based on quantum dots19,20] and nanoparticles (gold, silver) [21,22] for carbaryl detec-ion. For instance, Yinhui Yi and co-workers [19] have reported aovel carbaryl optical biosensor based on silicon quantum dots.he proposed sensor is able to detect carbaryl at concentrationsanging from 7.49 × 10−6 to 7.49 × 10−1 �g mL−1 with detectionimit of 7.25 × 10−6 �g mL−1. Many other works have demonstratedhe possibility to use electrochemical biosensors based on hybridlms constructed from various conductive materials such as con-ucing polymers, novel metal nanoparticles, and carbonaceousanomaterials. Abdolhamid and co-workers [23] have developedn electrochemical carbaryl biosensors based on the covalentmmobilization of two enzymes (acetylcholinesterase (AChE) andholineoxidase (ChO)) on self-assembled monolayer attached to aolycrystalline gold electrode. The linear range for the determi-ation of carbaryl was found to be ∼2 × 10−3–2 × 10−1 �g mL−1nd detection limit was estimated to be ∼1.2 × 10−3 �g mL−1 [23].lthough the detection limit of the above mentioned carbaryl sen-ors (optical and electrochemical) is below the maximum residueevel (5 × 10−2 �g mL−1) established by European Union [24], theirtability and reproducibility are still limited. To overcome this prob-em, field effect transistor (FET) has become a rising star to achieve

uch improved biosensing performances of carbaryl sensors.Graphene (Gr), a single layer of carbon atoms assembled

n a honey comb lattice, has attracted increasing attention ofhysicists, chemists, biologists and electronic engineers due to

ts extraordinary mechanical, thermal and electrical behaviors.ith a large specific surface ∼2630 m2/g, an extremely highobility ∼105 cm2 V−1 s−1 and a facile surface functionalization to

mmobilize various bio-molecules (antibodies, antigens, enzymes,ells, peptides, etc), this material is widely studied in the field of bio-ogical sensors [25–32]. For graphene-based FET biosensors, Gr isften utilized as conduction channel that allows the mobile chargeso flow between drain and source electrodes in solution-gating con-guration, whereas the biological processes on that graphene layerre monitored by characteristics of the FET (i.e., Dirac point) [33]. Itas anticipated that the adsorption of ions (either OH− or H+) onto

raphene channel can modulate channel conductance by dopingoles or electrons, thus shifting position of charge neutrality pointDirac point) in the right or left direction, respectively [34]. Forhis reason, various research groups have utilized graphene and itserivatives as conduction channel of FETs for pH sensing as well asiosensing applications [34–36]. In fact, Gr and reduced graphenexide (rGO) were the most often used carbonaceous nanomaterialsor constructing these devices. Until now, it is still questionablef Gr-based FETs or rGO-based FETs can provide better biosens-ng performances. It was sometimes reported that the presence oftructural defects and oxygen moieties in imperfect Gr and rGO lat-ices might even create active sites to attract these ions. Otherwise,everal works demonstrated lower performances in terms of filed-ffect channel mobility in rGO-based FETs compared to Gr-basedETs [35].

In this study, a simple configuration of a graphene based on ISFETensor is presented for the on-site monitoring of carbaryl. The few-ayered graphene films prepared by LPCVD have been transferrednto a prepatterned interdigitated microelectrode array (IDA) ofource (S) and drain (D) electrodes. The surface of these graphene-ased electrodes was subsequently functionalized with urease. The

ensing of carbaryl was recorded by the variation in position andntensity of Dirac point. The development of this simple sensing

uators B 260 (2018) 78–85 79

technology could probably lower the time and cost to control foodsafety in future.

2. Experimentals

2.1. Chemical agents

Urease enzyme (EC 3.5.1.5) was purchased from Merck. Phos-phate buffer solution (PBS, pH 7.4) and glutaraldehyde (GA) werepurchased from Sigma-Aldrich. All aqueous solutions were pre-pared in deionized water.

2.2. Fabrication of graphene films

The LPCVD process for the synthesis of graphene films on poly-crystalline Cu foils (25 �m-thick) of 99.8% purity (Alfa-Aesar). First,Cu foils were cleaned in acetone and isopropanol (IPA) to removeall organic contaminants and electrochemically polished in 85%H3PO4 at 1.9 V using another Cu sheet as a cathode. The furnace waspumped down to a base pressure at 60 Torr and then heated fromroom temperature to 1000 ◦C in mixture of argon (Ar) (20 standardcubic centimeter/minute (sccm)/H2 (50 sccm)) for 25 min. Then,the Cu foils were annealed at 1000 ◦C for 30 min in H2 (20 sccm)to reduce the native Cu oxide and to facilitate Cu grain growth.Then, a flow of methane (CH4, 0.3 sccm) was introduced to growthe graphene films. After 30 min, the CH4 flow was turned off andthe Cu foil was rapidly cooled to room temperature under a mixturegas flow of Ar (20 sccm)/H2 (20 sccm). The pressure in the furnacewas maintained at 60 Torr during CVD process.

2.3. FET fabrication

The schematic illustration of FET based on graphene filmsis detailed in ref [28]. In brief, an IDA was patterned on asilicon substrate with a top layer of 100 nm of silicon diox-ide (SiO2/Si) by lithography. The IDA consisted of a pair ofchromium (20 nm)/platinum (100 nm) electrode bands with 19fingers each, in which the bands served as source (S) anddrain (D) electrodes, respectively. The configuration of IDAwas 60 �m × 60 �m × 2.4 mm (width, interfinger gaps, long).Poly(methyl methacrylate) (PMMA) was spin coated on thegraphene surface and baked at 120 ◦C for 1–2 min on top of a hotplate. Then, the PMMA/graphene/Cu film was placed in a baker con-taining 0.3 M (NH4)2S2O8 solvent until all the Cu foil layer wasetched away. Following the etching, samples were rinsed withdeionized (DI) water five times and then transferred onto top ofthe prefabricated IDA. After drying in air, a small amount of freshPMMA was dropped onto the PMMA/graphene/FET electrodes todissolve soak the previously coated dried PMMA. To remove thePMMA layers, the samples were immersed into an acetone bath for2 h and then immersed into isopropanol (IPA) for 1 h. Finally, thegraphene based FET was annealed in Ar gas at 110 ◦C for 30 minto enable the flattening of the graphene film on the substrate andremove the water completely.

2.4. Enzyme immobilization

10U urease was immobilized on the interdigitated electrodesusing glutaraldehyde (GA) vapor as cross-linking agent. The elec-

unbound GA molecules.

80 C.T. Thanh et al. / Sensors and Actuators B 260 (2018) 78–85

sed F

2

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the graphene has adsorbed any chemicals and biological reagents.We can determine the change of graphene surface by observing

Fig. 1. Principle of fabrication of the graphene ba

.5. Detection of carbaryl

In order to test the carbaryl detection, a saturated urea solu-ion (30 mM) and various carbaryl solutions with concentrationsanging from 2.58 × 10−7 �g mL−1 to 2.58 × 10−2 �g mL−1 wererepared in deionized water. For each measurement, the FETensors were incubated for 30 min at room temperature in theesticide solution and then their electrical characterization was

mmediately processed in urea solution. The variations of the drain-ource current as a function of the gate voltage have been recorded

o evaluate the inhibition of carbaryl toward the enzymatic activityf urease.

Fig. 2. (a) Photography and (b, c) SEM images of

ET sensor and of the carbaryl dectection using it.

The working principle of our carbaryl sensors is based onthe inhibition of this carbamate compound toward urease in thehydrolysis reaction:

CO(NH2)2 + 3H2O + urease → CO2 + 2NH4+ + 2OH− (1)With a perfect and contamination -free graphene device, the

Dirac point will be close to zero. The Dirac point of graphenewill shift to positive or negative gate voltage if the surface of

the shift of the Dirac point. When no pesticide is added to thereaction solution and saturated concentration (30 mM) of urea sub-

graphene film on interdigitated electrodes.

C.T. Thanh et al. / Sensors and Actuators B 260 (2018) 78–85 81

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ig. 3. (a) SEM of the graphene film grown on the Cu substrate after CVD process ubstrate to glass substrate. Inset in (b) is its corresponding optical image.

trate exists in solution, the ions (i.e., NH4+) born from enzymaticydrolysis of urea would be adsorbed onto graphene surface, thenause n-doping of the material, and finally shifting Dirac pointowards negative gate voltage. In contrast, the Dirac point will behifted to the right direction in addition of carbaryl since less ionsre born at the surface of the graphene channel compared withhe shift of urea saturated concetration. Additionally, the slopest both right and left sides of ‘V-shape” transfer characteristicurve should be somehow varied because of restricted field-effectobilities of charges. Actually, the most widely used enzyme for

iosensing systems to detect carbamate and/or organophospho-us pesticides is acetylcholinesterse [37] whereas urease-basediosensors are more promising for tracing heavy metals in waternd foods [38–40]. However, it has been also reported in liter-ture that carbamate and/or organophosphorus pesticides mightignificantly inhibit enzymatic reaction of urease. Noticably, thenzymatic inhibition of carbamates was found to be irreversibleor acetylcholinesterse and reversible for urease. In the former case,

arbamate molecules attach to active sites of acetylcholinesterasend binds covalently to them, therefore diminishing the enzymaticctivity. In the latter case, these molecules create a flexible bridgeetween two nickel bi-nuclear active sites. Since the major charac-

ig. 4. (a) AFM topography image and (b) Representative Raman spectrum of the graphehows a height profile between the bare SiO2/Si substrate and the graphene film along them−1) of the main Raman peaks are labeled on the graph.

) UV–vis transmittance spectrum of the graphene film after transferring from Cu

ter of the bonds between this bridge and nickel ions is not covalentbut ionic, the carbamate molecules can be released from this com-plex after washing the electrodes and/or incubating the electrodesin a buffer solution. This enables us to develop a new-generation ofpesticide sensors which can be easily regenerated after use.

It is also worth to notice that both carbaryl and natural sub-strates of urease (i.e. urea) possess amide groups (O C NH )which are able to attach to hydroxy group and other electronega-tive functional groups nearby nickel ions. This structural similarityallows carbaryl to act as a substrate-like inhibitor that reducesenzymatic activity of urease. Other inhibitors such as thiourea,hydroxyurea, hydroxamic acid, phosphazenes also work in thisway [41]. Moreover, the intermediate product formed during thefirst step enzymatic hydrolysis of urea is actually one carbamatemolecule (H2N CO OH) [41]. Clearly, the presence of carbamatemolecules like carbaryl probably reduces the rate of enzymaticreaction.

The inhibition level of carbaryl toward urease was determined

from I–Vg curves as follows:

RI = 100% ∗ ((�Idso − �Idsi/(�Idso) (2)

ne film after transferring from Cu substrate to a SiO2/Si substrate. In (a) the inset back line in the AFM image. In (b) the names and frequencies (in parentheses, unit

8 nd Actuators B 260 (2018) 78–85

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Fig. 5. Electrical properties of the fabricated sensor (a) current-voltage transfercharacteristics of the bare graphene FET at different Vds. (b) Ids versus Vg curvesmeasured after urea immobilization and for different carbaryl concentration atVds = 1.5 V. As the carbaryl concentration increases, from 2.58 × 10−7 �g mL−1 to2.58 × 10−2 �g mL−1, the minimum of Ids is shifted towards higher Vg. (c) The depen-dence of the differential drain-source current (�Ids) on the concentration of carbaryl.

2 C.T. Thanh et al. / Sensors a

here RI is relative inhibition, (�Idso) and (�Idsi) are the outputignals before and after inhibition (as shown in Fig. 2).

.6. Sample characterizations

The surface morphologies of graphene films were investigatedy field-emission scanning electron microscopy (FESEM) (Hitachi-4800) and atomic force microscopy (AFM) (XE-100 Park Systems).he transmittance of the graphene film in the 350–800 nm rangeas measured using an UV–vis spectrophotometer (Model Jasco-600 series). Raman spectra were recorded using an Acton spec-

rometer fitted with a Pylon CCD detector and a 600 grooves/mmrating (∼2.5 cm−1 between each CCD pixel). The sample was char-cterized on Si/SiO2 substrate and using a 532 nm (2.33 eV) laser.he full with at half-maximum of the focused laser spot was about00 nm. Electrical measurements of the graphene based FET werearried out using a semiconductor parameter analyzer (Keithley,odel 4200-SCS). The source and drain measurement pads were

rotected from solution by covering the sample with a silicon rub-er well (0.9 mm in radius). The Ag/AgCl reference electrode was

mmersed in urea solution to provide gate control. To evaluate theET characteristics, the gate potential (Vg) was applied betweenhe source electrode and the reference electrode immersed in theaturated urea solution.

. Results and discussion

.1. Graphene film-based field effect transistor elaboration

We have fabricated a layer-by-layer carbaryl FET sensor basedn IDA configuration with graphene films. The whole procedure ofensor fabrication is schematically presented in Fig. 1. The grapheneased FET sensor was fabricated with (i) the graphene film as theottom layer which acts as the transducer material to amplify theetected signal; and (ii) an urease enzyme layer on top which actss the receptor/recognition element. Fig. 2(a) and (b, c) are pho-ography and (b, c) SEM images of graphene film on interdigitatedlectrodes, respectively. From SEM images (Fig. 2(b,c)), we can seehat the graphene film with the size of 1.8 mm x 1.8 mm was suc-essfully transferred onto the surface of interdigitated electrodes.

.2. Characterization of graphene films

Fig. 3(a) shows a FESEM image of a graphene film grown at000 ◦C under flow of CH4 and H2 (CH4/H2 = 20/0.3 sccm) and pres-ure 60 Torr with a CVD duration of 30 min. This image shows theresence of Cu surface steps, and graphene wrinkles (bright lines)hich result from the difference between the thermal expansion

oefficients of graphene and of the Cu substrate. The bright spotslso visible in Fig. 3(a) correspond to rounded particles with diam-ters in the range of 10–40 nm which are attributed to amorphousarbon residues.

Fig. 3(b) shows the optical transmittance spectrum of a simi-ar graphene film transferred on a glass substrate (seen insert inig. 3(b)). The optical transmittance of the graphene film is 96.8%t 550 nm which is compatible with an average number of layerlightly higher than 1 [42].

The graphene films transferred on 100 nm SiO2/Si substrateere characterized by AFM and Raman spectroscopy. A typicalFM image is shown on Fig. 4(a). According to the AFM image, theveraged thickness of the graphene film is about 0.91 nm which is

ompatible with a 1 layer thickness although AFM has been showno be a poor reliability technique for number of graphene layerstimation [43]. This image also shows the presence of graphenerinkles (bright lines) and of PMMA residues (white spots).

C.T. Thanh et al. / Sensors and Act

Table 1Calculated mobility (using expression (3)) and Ion/Ioff ratios of the bare graphene-FETfor the Vds range of 1.3 V–1.55 V.

Vds (V) 1.3 1.35 1.40 1.45 1.50 1.55

Hole mobility (cm2/V.s) 78 76 80 80 81 77

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Electron mobility (cm2/V.s) 77 77 76 76 77 77Ion/Ioff 1.32 1.32 1.31 1.32 1.32 1.31

A typical Raman spectrum of the transferred graphene films shown on Fig. 4(b). As expected, the two characteristic peaksssigned to G band (at 1587 cm−1) and 2D band (at 2684 cm−1)f graphene/few layer graphene (FLG) material are present on theaman spectrum. The defect related peaks, D and D’ band locatedround 1346 cm−1 and 1625 cm−1, respectively, are also visible. Inrder to estimate the number of layers, we chose to analyze the G-and integrated intensity normalized versus the one of an highlyriented pyrolytic graphite (HOPG) reference sample (AGnorm) sincehe criteria based on the analysis of the 2D band have been showno be misleading in many cases [44]. On about 70% of the probedreas, AGnorm is found between 0.8 and 0.9 which is consistent withhe presence of single layer graphene [45]. On the other part ofhe sample, thicker regions (few layer graphene) are observed byaman and optical microscopy (not shown). Regarding defects, the

ntensity ratio between D and D’ bands, found around 7 on all mea-ured positions, suggest that the defects are mainly vacancies [46].o go further, the analysis of the integrated intensity ratio of D and

bands and of G band full width at half maximum indicates thathe average distance between defects (LD) is between 8 and 20 nmnd the average grain size (La) is larger than 40 nm [47].

.3. Charge mobility of graphene based FET

The hole and the electron mobility were also extracted from theransfer characteristic using [48]:

= mlin(L/W.Vds.Ci) (3)here mlin is the obtained slope from the linear fits in the Ids − Vg

iagram. W and L are the channel width and length, respectively.ds is the applied source-drain voltage and Ci is the specific capac-

tance of top gate = 1.2 (�F/cm2) [28].Fig. 5(a) shows the room temperature transfer characteristics

f the bare graphene liquid gated FET device. From Fig. 5(a), we

erive the mobility (using expression (3)) of holes and electrons

or Vds in the 1.3–1.55 V range (see Table 1). The Ion/Ioff ratiosre also defined and calculated in Table 1 by measuring satu-ated drain-source current (Ids) divided by drain-source current at

able 2omparison of major methods and configurations reported for the determination of carb

Configuration Measurement method

AChE-CAMC/SPCE High performance liquid chromatography (HPLC– HPLC AChE/SCh/ASCh/ITO Electrochemical AChE-e-pGON/GCE Electrochemical impedance AchE/CHIT/IAM Electrochemical impedance spectroscopy AChE-MWCNTs/GONRs/GCE Electrochemical impedance AChE/AgNPs-CGR-NF/GCE Electrochemical NH2-graphene-Nafion Electrochemiluminescence (ECL) – HCA and PCA Graphene-IL-Au/CS-AuPtNPs/GCE Electrochemical AChE/SEMmix/Au Chronoamperometric PDDA-SWCNTs/AChE Amperometric biosensor GA/urease enzyme/graphene/Pt ISFET

otes: AChE, acetylcholinesterase; CAMC, cellulose acetate composite; SPCE, screen-pri-pGON, electrochemically inducing porous graphene oxide network; HCA, hierarchicalanoparticles; IL, liquid-nano; CS, chitosan; MWCNTs, multi-walled carbon nanotubes;EMmix, mixed self-assembled monolayer; PDDA, poly(diallyldimethylammonium chlor

uators B 260 (2018) 78–85 83

the minimum conductance point (charge neutrality point) (Icnp).The calculated results show that, the mobility values of our baregraphene-FET are found around of 80 cm2/V s for both electrons andholes. The Ion/Ioff ratio is around of 1.3. This value is lower than thatof the graphene-based FETs (2–3.5 [25] and ∼2 [49]). This seems tobe due to our ISFETs use single-layer graphene with large area andthe existence of PMMA residues on the surface of graphene aftertransferring from Cu substrate to electrode substrate.

3.4. Carbaryl detection

The current responses of carbaryl intedigitated ISFET sensor atdifferent sample concentrations are shown in Fig. 5(b). Interest-ingly, the carbaryl level can be monitored by either position orintensity of charge neutrality point (Vg at the minimum conduc-tance point or Dirac point). When the concentration of carbarylincreased from 0 to 2.58 × 10−2 �g mL−1, the position of Dirac pointwas shifted towards positive values. On the other hand, an increasein charge concentration was obtained. It must be noticed thatgraphene material should be intrinsically n-doping due to the exis-tence of negatively charged oxygen moieties on its surface. Herein,the increase in charge concentration actually reflects decreasingnumber of positive ions (i.e., NH4+) absorbed on the surface due torestricted hydrolysis of urea in presence of pesticide.

As shown in Fig. 5(c), the differential drain-source current(�Ids) decreases (from 3.58 mA to 0.56 mA) when the carbarylconcentration increases (from 2.58 × 10−7 to 2.58 × 10−2 �g mL−1).This response curve shows a good linearity against the loga-rithm of carbaryl concentration Ccarbaryl with a regression equation:�Ids = −0.4 − 0.6 logCcarbaryl (mA) (R2 = 0.99). From the decrease of�Ids, the enzyme urease inhibition level of carbaryl for the dif-ferent concentrations was calculated (using expression (2)). Thehighest inhibition level is up to 86% with a carbaryl concentrationof 2.58 × 10−2 �g mL−1.

The analysis of the sensor response as a function of the carbarylconcentration (Fig. 5 (c)) also shows a good inhibition rate witha LOD of 10−8 �g mL−1 carbaryl. This LOD is far below the maxi-mum residue level (5 × 10−2 �g mL−1) established by the EuropeanUnion [24]. This value is also lower than detection limit of carbarylbiosensors reported in previous works (see Table 2) which rangesfrom 10−7 �g mL−1 to 10−3 �g mL−1 [2,3,9–11,13,50–53]. Only fewstudies have demonstrated the ability for carbaryl detection with a

detection limit as low as our system [8,54].

Additionally, the charge mobilities (both electron and hole)were decreased as pointed out by decreasing slopes of both sides of‘V-shape’ curve. As the carbaryl concentration increases, i.e., from 0

aryl.

Linear range (�g/mL) Limit of detection (�g/mL) Reference

) 10−2 to 0.5 4 × 10−3 [2]5 × 10−3 to 10 2 × 10−3 [3]– 6 × 10−8 [8]3 × 10−4 to 6.1 × 10−3 1.5 × 10−4 [9]10−3 to 0.1 7.8 × 10−4 [10]1 × 10−3 to 1 3.4 × 10−4 [11]∼2 × 10−7 to 2 × 10−3 ∼1.1 × 10−7 [13]5 × 10−4 to 10 2 × 10−4 [50]– 2.5 × 10−3 [51]6 × 10−3 to 1.2 1.6 × 10−3 [52]0–0.35 6.9 × 10−7 [53]10−8–10−3 10−9 [54]2.58 × 10−7 to 2.58 × 10−2 10−8 This work

nted carbon electrode; IAM, interdigitated array microelectrodes; CHIT, chitosan; clustering analysis; PCA, principal component analysis; GA, glutaraldehyde; NPs,

GONRs, graphene oxide nanoribbons; SCh, thiocholine; ASCh, acetylthiocholine;ide); SWCNTs, single walled carbon nanotubes; CGR, carboxylic graphene.

84 C.T. Thanh et al. / Sensors and Act

Fig. 6. The dependence of the inhibition level and of the charge carrier mobilityon concentration of carbaryl. Measurements were conducted in PBS 1×, pH 7.4,ati

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nd 30 mM urea. (b) UV–vis transmittance spectrum of the graphene film afterransferring from Cu substrate to glass substrate. Inset in (b) corresponding opticalmage.

o 2.58 × 10−2 �g mL−1, the mobility (using expression (3)) of holesnd electrons decreases, from 72 cm2/V s to 9 cm2/V s for holes androm 62 cm2/V s to 4 cm2/V s for electrons (see Fig. 6). The decreasef the carrier mobility of the graphene based ISFET when increas-

ng of the carbaryl concentration can be explained as following. Theobility of carrier in graphene-ISFET depends on the differential

ate voltage. When the carbaryl molecules absorb onto the surfacef the graphene electrode, the ions concentration (NH4+, OH−) inhe solution will be decreased, so the differential gate voltage andhe carrier mobility will decrease.

. Conclusion

In this study, we have demonstrated the realization of carbaryl sensor using a LPCVD-grown graphene basedSFET. This sensor is able to detect carbaryl in the range of.58 × 10−7–2.58 × 10−2 �g mL−1 with low limit of detectionf 10−8 �g mL−1, which is among the lowest reported so far.part from the pesticide detection, the used sensor design cane extended for other biosensing applications to achieve highensitivity and specificity.

cknowledgments

This research was financially supported mainly by the Viet-am National Foundation for Science and Technology DevelopmentNo.103.99-2016.19). Raman measurement was done with the sup-orting by PICS-CNRS no 6457.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at https://doi.org/10.1016/j.snb.2017.12.191.

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