Mild and novel electrochemical preparation of β-cyclodextrin/graphene nanocomposite film for super-sensitive sensing of quercetin

Zhen Zhang a,b, Shuqing Gu c, Yaping Ding a,n, Mingju Shen a, Lin Jiang a

a Department of Chemistry, Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, School of Materials Science and Engineering, ShanghaiUniversity, Shanghai 200444, PR Chinab School of Chemistry & Chemical Engineering, Linyi University, Linyi 276005, PR Chinac Technical Center for Animal Plant and Food Inspection and Quarantine, Shanghai Entry-Exit Inspection and Quarantine Bureau, Shanghai 200135, China

a r t i c l e i n f o

Article history:Received 11 November 2013Received in revised form5 February 2014Accepted 6 February 2014Available online 19 February 2014

Keywords:Electro-deposited graphemePolymerized β-cyclodextrinQuercetinNanocomposite filmDifferential pulse voltammetry

a b s t r a c t

A mild and novel preparation tactics based on electrochemical techniques for the fabrication of electro-deposited graphene (E-GR) and polymerized β-cyclodextrin (P-βCD) nanocomposite filmwere developed.The structure and morphology of GR-based nanocomposite were investigated by scanning electronmicroscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Simultaneously,the electrochemical properties of this nanocomposite were characterized by cyclic voltammetry (CV) anddifferential pulse voltammetry (DPV). Based on the synergistic effect of E-GR and P-βCD, a super-sensitiveelectrochemical sensor for quercetin was successfully fabricated. Under optimum conditions, the deter-mination range for quercetin was from 0.005 to 20 mM with a low detection limit of 0.001 mM (S/N¼3).Moreover, this sensor also displays excellent sensitivity, fine reproducibility and stability. To furtherstudy the practical applicability of the proposed sensor, the determination of real samples was carriedout with satisfactory results.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Graphene (GR), a two-dimensional sheet of sp2 conjugatedatomic carbon, has attracted fascinating interests across differentdisciplines because of its ultrahigh specific surface area, uniqueelectronic features (Novoselov et al., 2004; Kim et al., 2008), greatmechanical strength (Zhao et al., 2010), and good biocompatibility(Matthew et al., 2010). Benefiting from these advantages, GR hasbeen widely used for immobilizing organic and inorganic mole-cules (Ohno et al., 2009) and for fabrication of electrochemicalbiosensors (Shan et al., 2009; Wang et al., 2009; Zhou et al., 2009).β-Cyclodextrin (β-CD), a cyclic oligosaccharide consisting of sevenglucose units, presents a toroidal form with a hydrophobic innercavity and a hydrophilic outer side (Rekharsky and Inoue, 1998).It is well known that β-CD has high molecular selectivity andenantioselectivity. Various inorganic, organic and biological guestmolecules can be bound selectively in the inner cavities of β-CD toform stable host–guest inclusion complexes or nanostructuresupramolecular assemblies (Freeman et al., 2009; Rekharsky andInoue, 1998). Therefore, β-CD and its derivatives have been

extensively used as molecular recognition systems and particularlyapplied in the field of mimetic enzymes, molecular recognitionsensors, discrimination of enantiomers and drug encapsulation(Almirall et al., 2003; Fragoso et al., 2002; Martín et al., 2003; Nget al., 2002). The nanocomposites based on β-CD can be an idealsubstrate material for molecular recognition sensors. Based on theexcellent performances of GR and β-CD respectively, a facile effectiveand efficient preparation strategy of GR–β-CD composite is ofconsiderable significance (Chen et al., 2013; Zhang et al., 2014).

Electrochemical technique was as an effective tool for rapid,simple and mild synthesis of various materials. Cyclic voltammetry(CV) (Malitesta et al., 1999; Feng et al., 2004), potentiostaticdeposition (Lattach et al., 2012), electrochemical potential pulsetechnique (Menaker et al., 2009; Choong and Milne, 2010) and thegalvanostatic method (Syritskia et al., 2008) have been applied formaterial preparations. Among them, potentiostatic deposition andCV can ensure similar film thicknesses (Österholma et al., 2012).Until now, there have been no reports concerning electrochemicalsensors based on electro-deposited GR (E-GR) and polymer nano-composite film manufactured by a cooperation of potentiostatictechnique and CV. Thus, there is a substantially promising devel-opment space in this booming field.

Quercetin, as a natural pentahydroxyflavone, is widely distrib-uted in the medicinal plants, flowers, fruits, leaves and variety ofbeverages as their important constituent (Crozier et al., 1997;

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http://dx.doi.org/10.1016/j.bios.2014.02.0140956-5663 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: School of Materials Science and Engineering, Iron andSteel Metallurgy Key Laboratory, Shanghai University, Shanghai 200444, PR China.Tel.: þ86 021 66134734; fax: þ86 21 66132797.

E-mail address: wdingyp@sina.com (Y. Ding).

Biosensors and Bioelectronics 57 (2014) 239–244

Hertog et al., 1992). It plays an important role in the science ofnutrition and medical science. Quercetin has many biologicalactivities by acting as an effective radical-scavenger againstoxidative cell damage, such as antitumor activity (Verma et al.,1998), cardiovascular protection (Scalbert et al., 2005), antiinflam-matory, estrogenic and antiallergy activity (Boesch-Saadatmandiet al., 2012). However, an overdose of may lead to kidney cancer(Hsieh et al., 2010). The excess quercetin decreased glutathioneS-transferase activity, resulting in DNA damage (Yen et al., 2003).Thus, the low detection of quercetin in the science of nutrition andmedical science has attracted an increasing interest.

In this work, aiming to explore the unrevealed sensor potential,a mild and novel method has been employed to prepare theelectro-deposited graphene/polymerized β-cyclodextrin (E-GR/P-βCD) nanocomposite film by a cooperation of potentiostatictechnique and CV. Quercetin was selected to explore the practicalperformance of the proposed sensor. The work presented hereindicated the effectiveness and feasibility of the proposed strategyand the great potential of E-GR/P-βCD nanocomposite film inelectroanalytical fields.

2. Experimental

2.1. Apparatus and reagents

All electrochemical experiments were performed on a CHI660D electrochemical workstation (Shanghai Chenhua Co. Ltd.,China). A conventional three-electrode system was employed witha glassy carbon electrode (GCE, 3 mm in diameter) as the workingelectrode, a platinum electrode as the counter electrode, and asaturated calomel electrode as the reference electrode. The pHvalue was determined with a pHS-3C acidity meter. Scanningelectron micrographs operated on the field emission scanning

electron microscope (Hitachi S4800, Japan) was applied for char-acterizing the modified materials. Transmission electron micro-scopy was carried out using a transmission electron microscope(JEOL JEM-200CX working at 160 kV).

Graphene was purchased from XFNANO, INC (Nanjing, China).Quercetin and β-cyclodextrin were purchased from SinopharmChemical Reagent Co., Ltd., (Shanghai, China). Ascorbic acid anddopamine were purchased from Aladdin Chemical Reagent Co.,Ltd. (Shanghai, China). All chemicals employed in this work wereanalytical grade. All experiments were performed in 0.1 M NaAc-HAc buffer solution and double distilled water was used through-out. All electrochemical experiments were carried out at roomtemperature at about 25 1C.

2.2. Preparation of modified electrodes

Prior to use, a bare GCE of 3 mm diameter was firstly polishedon the chamois leather with alumina slurries until a mirror-likesurface was acquired. Then the GCE was ultrasonicated in HNO3

solution (v/v¼1:1), ethanol and double distilled water for 3 minrespectively. The electro-deposited GR/polymerized β-cyclodextrinnanocomposite film (E-GR/P-βCD) modified on GCE was gained viatwo steps. The first step was the electro-deposition in 10 mL 0.1 MKCl aqueous solution containing 300 μL GR (1 mg/mL dispersed inwater) at a deposited voltage of þ1.7 V, then the second step waselectro-polymerization in the range from �2.0 to 2.5 V for 6 seg-ments at 100 mV s�1 in 0.1 M phosphate buffer solution (pH 6.0)containing 2.0 mM β-CD. Eventually, the as-prepared electrodewas rinsed carefully with double distilled water for further use.At the same time, another two modified electrodes, E-GR/GCE andP-βCD/GCE were also prepared following the same proceduredescribed above for comparison.

Fig. 1. (A) SEM images of graphene; (B) TEM images of graphene; (C) SEM images of E-GR; and (D) Raman spectra of GR (a), E-GR (b).

Z. Zhang et al. / Biosensors and Bioelectronics 57 (2014) 239–244240

3. Results and discussion

3.1. Characterization of the modified electrode

The TEM and SEM images of graphene are shown in Fig. 1, fromwhich we can see that graphene was sheet-like, glossy andcrimple shape. As shown in Figs. 1C and 2A, the morphologies ofE-GR and E-GR/P-βCD on the surface of GCE were investigated bySEM. E-GR film with honeycombed layers acquired from theelectro-deposited method can be well modified on the surface ofelectrode. Compared with GR film on the surface of electrode,specific surface area of E-GR film was increased, which is based onthe impetus of the electrochemical power to the changes of Fermienergy level of electrode materials surface (Guo et al., 2009).

Raman spectroscopy is a powerful nondestructive tool todistinguish ordered and disordered crystal structures of carbon.The Raman spectra of GR and E-GR are shown in Fig. 1D. Asexpected, GR (a) displays two prominent peaks at 1345.6 and1592.9 cm�1, which correspond to the well-documented G and Dbands, respectively. By the electro-deposited method, the G and Dbands of E-GR (b) shifted to 1352.1 and 1601.1 cm�1. The G bandrepresents the first-order scattering of the E2g vibrational modewhile the D band has been attributed to the reduction in size ofthe in-plane C sp2 atoms. The D/G band intensity ratio expressesthe atomic ratio of sp2/sp3 carbons, and is a measure of the extentof disordered graphite. The D/G intensity ratios of GR and E-GRwere calculated to be 1.4 and 1.2, respectively. It is anticipated thatE-GR film with a distinctive morphology from GR and otherreported electrochemical synthesized GR (Zhang et al., 2013;Yang et al., 2012) will support the sensitivity of electrode as anelectrochemical sensor.

Compared with E-GR film, a relatively flat and smooth surfacewas obtained after the modification of E-GR/P-βCD nanocompositefilm, as showed in Fig. 2A. As seen from the transection images,

β-CD was well implanted in E-GR film with nanosize, implying afavorable distribution with compact multilayer composite-films.These could be the wrinkled E-GR film which offered large surfaceareas and excellent mechanical durability. It is also expected thatthe displayed cooperation of β-CD and E-GR will promote thesensitivity and stability of GCE in electrochemical sensing.

3.2. Electrochemical behavior of quercetin on the GCEs

The electrochemical oxidations of quercetin on different mod-ified GCEs by differential pulse voltammetry (DPV) were carriedout in 0.1 M NaAc-HAc buffer solution (pH 7.0) containing 20 μMquercetin in order to make a comparison of the electrochemicalproperties, as shown in Fig. 2B. The electrochemical response ofE-GR/P-βCD/GCE to quercetin was investigated and the corre-sponding voltammogram was shown in Fig. 2B, curve d. As acomparison, a similar set of experiments were also performedusing the bare GCE (a), E-GR/GCE (b) and P-βCD/GCE (c), shown inFig. 2B. Compared with the bare GCE, about 20-fold larger currentsignal was obtained, which demonstrated that the E-GR/P-βCD/GCE could greatly facilitate the oxidation of quercetin. It is alsoexpected that the displayed cooperation of E-GR and P-βCD willenhance the sensitivity and stability of GCE in electrochemicalsensing. E-GR displayed the excellent conductivity and a largesurface area which provided more active sites and enhanced thetransfer of charge between the electrode and the reagent. Mean-while, P-βCD improved the uniform distribution, catalytic activityand durability of catalysts. Based on the synergic electrocatalyticeffect of E-GR and P-βCD, this E-GR/P-βCD nanocomposite filmincreased the electrocatalytic active area and promoted the elec-tron transfer between quercetin and the surface of GCE.

To investigate the reaction mechanism, CV experiments werecarried out at the E-GR/P-βCD/GCE to realize how the scan rate (υ)affected the oxidation of 20 μM quercetin in 0.1 M NaAc-HAc

Fig. 2. (A) SEM images of ED-GR/P-βCD. The inset is the cross-section image of the nanocomposite. (B) DPVs of bare GCE (a), E-GR/GCE (b), P-βCD/GCE (c) and E-GR/P-βCD/GCE (d) with 20 μM quercetin in 0.1 M NaAc-HAc buffer solution (pH¼7.0). (C) CVs of E-GR/P-βCD/GCE with 20 μM quercetin in 0.1 M NaAc-HAc buffer solution (pH¼7.0) atscan rate ranging from 10 to 500 mV s�1. Inset: plots of peak currents vs. scan rates.

Z. Zhang et al. / Biosensors and Bioelectronics 57 (2014) 239–244 241

buffer solution (pH 7.0). The results are shown in Fig. 2C. The peakcurrent varied linearly with the scan rates from 10 to 500 mV s�1

with a linear regression equation of Ip (μA)¼0.3324 c (μM)þ2.8313 (R¼0.999), indicating a typical adsorption-controlled pro-cess. The oxidation potentials (Ep) shifted positively as the increaseof scan rate (υ) and showed a linear relationship with log υ, whichwas further constructed with the equation: Ep(V)¼0.055 log v

(V s�1)þ0.236 (V). According to Laviron's theory, Ep can berepresented by the following equation (Fei et al., 2005):

Ep ¼ Fþ½2:303RT=ð1–αÞnαF� log v ð1Þwhere α is the electron transfer coefficient, and nα is the number ofelectrons involved in the rate-determining step. R, T and F are thegas constant, temperature and Faraday constant, respectively. Thetransfer coefficient (α) characterizes the effect of electrochemicalpotential on the activation energy of an electrochemical reaction,and its reasonable values range from 0.3 to 0.7 for most systems.For our system, the transferred number of electrons (n) during theoxidation of quercetin was calculated to be 2.1 (α¼0.5), suggestingthat totally two electrons were involved in the oxidation reaction.This result was in accordance with the reported value (Goyal et al.,2012). The probable mechanism was surmised according to thereferences (Chen et al., 2012) and shown in Scheme 1.

3.3. Optimization of quercetin sensor performance by DPV

3.3.1. Effect of the applied potential of electro-deposited GRThe effect of electro-deposited potential is an important factor

affecting the performance of the sensor. The applied potentialsranging from 1.5 V to 1.9 V were investigated for electro-depositedGR. Fig. S1A (Supporting information) shows the current responseby different applied potentials after addition of 10 μM quercetin in0.1 M NaAc-HAc buffer solution (pH 7.0). The maximum peakcurrent towards 10 μM quercetin was obtained on E-GR/GCE withthe applied potential at 1.7 V. Therefore, 1.7 V was chosen forsubsequent experiments.

3.3.2. Effect of polymerized cyclesElectro-polymerized cycle is another important factor affecting

the performance of the sensor. Fig. S1B (Supporting information)exhibits the effect of polymerized cycles of β-CD on the currentresponse of 10 μM quercetin. When increasing the electro-deposited cycles from 2 to 10, it was observed that the oxidationcurrent of quercetin increased and reached the maximum at6 cycles, and then decreased with a higher number of cycles.Hence, 6 cycles were chosen for analytical experiments.

3.3.3. Effect of supporting electrolyte and solution pHTo study the influence of pH on the oxidation process of

quercetin, 0.1 M NaAc-HAc buffer solution was chosen as theelectrolyte to study the influence of pH on the oxidation processof quercetin. An obvious increase of peak current with the pHvalue in the range of 2.0–7.0 was observed (Fig. S1C, Supportinginformation). As described in the figure, a negative shift of Ep withthe increase of pH value was gained and this indicated that there

was a proton at least involved in the oxidation reaction. Ep and pHwere in a good linear relationship described by: Ep¼0.4336–0.0278 pH (V) (R¼0.998). The current reached a peak value atpH 4.0. However, the oxidation potentials shifted negatively to0.249 V at pH 7.0, compared with the oxidation potentials(Ep¼0.316 V) at pH 4.0. The buffer solution of NaAc-HAc at pH7.0 is close to the physiological level (Xu and Wang, 2005).Accordingly, 0.1 M NaAc-HAc buffer solution at pH 7.0 was chosenfor the following experiments.

3.4. Calibration curve and interference

DPV is a powerful electrochemical technique that can beapplied in both electrokinetic and analytical measurements. Inthis study, it was used to determine the concentration of querce-tin. Fig. 3 shows the DPVs of various concentrations of quercetin atthe E-GR/P-βCD/GCE in 0.1 M NaAc-HAc buffer solution (pH 7.0).The current responses are increased linearly with quercetin con-centrations in the range of 0.005–20 μM with a current sensitivityof 24.760 μA μM�1 cm�2 (Fig. 3). The corresponding regressionequation can be described as follows: Ip (μA)¼1.74901 c (μM)þ0.56309 (R¼0.999). The detection limit is 0.001 μM (S/N¼3).Table 1 displays comparisons of the proposed E-GR/P-βCD/GCEperformance with other electrochemical methods reported pre-viously (Chen et al., 2012; Wang et al., 2011; Xu and Kim, 2006;Saber-Tehrani et al., 2013; Gutiérrez et al., 2010; Xiao et al., 2007;Jin et al., 2006). The proposed E-GR/P-βCD/GCE possessed lowerdetection limit and wider concentration range compared withmost reported papers. The excellent performance may be attrib-uted to the synergic electrocatalytic effect of E-GR and P-βCD, thisE-GR/P-βCD nanocomposite film increased the electrocatalyticactive area and promoted the electron transfer between quercetinand the surface of GCE. Hence, E-GR/P-βCD/GCE in this work isfavorable and suitable for practical quercetin detection.

Under the optimized experimental conditions described above,the influence of possible interfering substances was investigatedwith concentration of quercetin fixed at 20 μM. The tolerance limitwas taken as the maximum concentration of interfering sub-stances that caused an approximately 75% relative error in thedetermination of quercetin. Several substances such as glucose,ascorbic acid and uric acid, etc. could possibly co-exist withquercetin in natural samples are studied. The result of interferenceis shown in Table S1 (Supporting information). Experiment resultsillustrated that 50-fold of D-glucose, tryptophan, phenylalanineand leucine, 25-fold of ascorbic acid and dopamine, 50-fold ofCu2þ , Pb2þ , Zn2þ , Al3þ , Ca2þ and Mg2þ , 250-fold of Naþ and Kþ

OH

HO O

OOH

OH

OH

OH

HO O

OOH

OO

-2H+-2e-

+2H++2e-

Scheme 1. The reaction mechanism of electrooxidation of quercetin.

Fig. 3. DPV of E-GR/P-βCD/GCE at different concentrations of quercetin. Inset: thecalibration curve for the determination of quercetin. Each was measured for threerepetitive determinations. (RSDo5%).

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together with 500-fold citric acid hardly caused interference(Table S1, Supporting information).

3.5. Reproducibility and stability of the sensor

The long-term storage and operational stability of the electrodeare essential for the continuous monitoring of quercetin. Duringthe test, 10 successive measurements were performed in NaAc-HAc (pH 7.0) solution containing 20 μM quercetin by using oneE-GR/P-βCD/GCE. The relative standard deviation was 4.05%, con-firming that the modified electrode for quercetin sensing wasstable. The long-term stability was explored by measuring 10 μMquercetin solutions intermittently. The results show that thecatalytic current response maintains 93.6% of its initial value evenafter storing the modified electrode at room temperature for2 weeks, reflecting the good stability of the quercetin sensor.

3.6. Real samples analysis

To further verify the practical performance of the proposedelectrochemical sensor, E-GR/P-βCD/GCE was applied to determinethe content of quercetin in tea and honeysuckle samples by thestandard addition method. Firstly, tea and honeysuckle wererespectively soaked in the double deionized water for about 2 h.Then the solutions were respectively centrifuged for 0.5 h and thesupernatants were taken out as the two samples to use for themeasurement. The tea and honeysuckle samples were diluted with0.1 M NaAc-HAc buffer solution, and the final concentration was1.0–10.0 μM which is in the linear range of the proposed sensorand can be detected sensitively in 10 mL 0.1 M NaAc-HAc buffersolution (pH 7.0). The DPV curve was recorded by using one E-GR/P-βCD/GCE under optimum conditions, and the results were listedin Table 2. Each sample was determined by three parallel detec-tions, and the RSD was lower than 5%, revealing excellent preci-sion. Moreover, the recovery of quercetin standard was performedto testify the accuracy of this method. The value of recovery rangesfrom 95.6% to 104.5%, suggesting that this method is effective andreliable.

4. Conclusion

A facile and novel E-GR/P-βCD nanocomposite film was suc-cessfully synthesized by potentiostatic deposition and cyclic vol-tammetry techniques. The synergistic effect produced by E-GR and

P-βCD nanocomposite film facilitated the electrocatalytic activity,electron transfer and mass transport. As a result, a super-sensitiveelectrochemical sensor for detecting quercetin based on E-GR/P-βCD nanocomposite film was fabricated with ideal results in realsamples. The present strategy provides a way to prepare graphene/β-cyclodextrin nanocomposite film, thus providing a novel andpromising platform for the study of the analytical application of E-GR nanocomposite films.

Acknowledgements

This research is supported by the National Natural ScienceFoundation of China (Nos. 21271127 and 61171033), the Nano-Foundation of Science and Techniques Commission of ShanghaiMunicipality (12nm0504200), Leading Academic Discipline Projectof Shanghai Municipal Education Commission (J50102).

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.bios.2014.02.014.

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