Chinese Chemical Letters  2014, Vol.25 Issue (04):505-510   PDF    
Electrochemical determination of quercetin by self-assembled platinum nanoparticles/poly(hydroxymethylated-3,4-ethylenedioxylthiophene) nanocomposite modified glassy carbon electrode
Yuan-Yuan Yaoa,b, Long Zhanga, Zi-Fei Wanga,b, Jing-Kun Xua , Yang-Ping Wenb    
* Corresponding authors at:a Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China;
b College of Science, Jiangxi Agricultural University, Nanchang 330045, China
Abstract: A simple and sensitive platinum nanoparticles/poly(hydroxymethylated-3,4-ethylenedioxylthiophene) nanocomposite (PtNPs/PEDOT-MeOH) modified glassy carbon electrode (GCE) was successfully developed for the electrochemical determination of quercetin. Scanning electron microscopy and energy dispersive X-ray spectroscopy results indicated that the PtNPs were inserted into the PEDOTMeOH layer. Compared with the bare GCE and poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes, the PtNPs/PEDOT-MeOH/GCE modified electrode exhibited a higher electrocatalytic ability toward the oxidation of quercetin due to the synergic effects of the electrocatalytic activity and strong adsorption ability of PtNPs together with the good water solubility and high conductivity of PEDOT-MeOH. The electrochemical sensor can be applied to the quantification of quercetin with a linear range covering 0.04-91 μmol L-1 and a low detection limit of 5.2 nmol L-1. Furthermore, the modified electrode also exhibited good reproducibility and long-term stability, as well as high selectivity.
Key words: Poly(3,4-ethylenedioxylthiophene     methanol)     Platinum nanoparticles     Electropolymerization     Differential pulse voltammetry     Quercetin    

1. Introduction

Quercetin (3,30,40,5,7-penta hydroxyl flavones),one of the most abundant natural flavonoids,has been widely distributed in various vegetables and fruits,especially in traditional Chinese herbs. Its average human daily intake is estimated to be 16-25 mg per person. Over the past years,quercetin has drawn much attention because of its various beneficial effects on human health, including anti-cancer,anti-inflammatory,anti-tumor,anti-ulcer, anti-allergy,anti-viral,and anti-oxidant effects [1, 2],and quercetin also can protect human DNA from oxidative attack in vitro [3]. Hence,it is extremely important to determine and study it.

Several techniques have been utilized in the determination of quercetin,for example,HPLC-UV [4],spectrophotometry [5],liquid chromatography with mass spectrometry [6],and solid phase extraction [7]. These methods are highly sensitive and effective, but often require some complicated and time consuming sample pretreatment. Due to the advantages of time-saving,simple operation,sensitivity,low-cost,and on-field detection,some electrochemical methods have been proposed for the electrochemical study of quercetin. Several materials have been used to fabricate the modified electrode,such as carbon nanotubes [3,8- 10],copper microparticles [11],and graphene nanosheets [12]. However,to date,the application of conductive polymer with metal nanoparticles for the electroanalytical detection of quercetin has not been reported.

operation,sensitivity,low-cost,and on-field detection,some electrochemical methods have been proposed for the electrochemical study of quercetin. Several materials have been used to fabricate the modified electrode,such as carbon nanotubes [3,8- 10],copper microparticles [11],and graphene nanosheets [12]. However,to date,the application of conductive polymer with metal nanoparticles for the electroanalytical detection of quercetin has not been reported.attention for the application of chemo/bio-sensors materials because of the high conductivity,good stability,friendly biocompatibility, low toxicity,and relatively low band gap [20, 21, 22, 23, 24]. Hydroxymethylated-3,4-ethylenedioxylthiophene (EDOT-MeOH), a polar derivative of EDOT,exhibited better water-solubility and a lower onset oxidation potential compared with EDOT monomer [25, 26, 27, 28, 29]. In addition,our group has reported a new and efficient synthetic route to obtain EDOT-MeOH monomer and the fabricated PEDOT-MeOH chemo/bio-sensors were successfully used in the electrochemical determination of the anticancer herbal drug, shikonin [27],and VC in commercial fruit juice [27, 28]. Therefore, it is very meaningful to construct an electrochemical sensor for the determination of quercetin by incorporating the merits of EDOTMeOH and PtNPs.

In this contribution,in view of the merits of conducting polymer PEDOT-MeOH and metal nanoparticles PtNPs,a simple and sensitive PtNPs/PEDOT-MeOH nanocomposite modified glassy carbon electrode (GCE) was constructed for the electrochemical determination of quercetin. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) results showed that the PtNPs were inserted into the PEDOT layer and formed a porous 3D structure,which exhibited large surface area and excellent electrocatalytic activity for the oxidation of quercetin. Moreover,the electrochemical properties of the PtNPs/PEDOTMeOH/ GCE modified electrode for the voltammetric detection of quercetin were studied.

2. Experimental 2.1. Chemicals

3,4-Dibromothiophene (EDOT) (98%,Shanghai Bangcheng Chemical Co.,Ltd.),Quercetin (>98%) and platinum powder (99.99%) were obtained from Aladdin Reagent Co.,Ltd. All other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. with analytical grade and were used as received without any further purification. All solutions were prepared using doubledistilled deionized water.

2.2. Apparatus

Electrochemical measurements were performed on a CHI660B electrochemicalworkstation(Chenhua InstrumentCompany,Shanghai, China) with a conventional electrochemical cell containing a three-electrode system. Themodified electrode or bare glassy carbon electrode (GCE) (F = 3mm) was served as a working electrode,a platinum wire (F = 1mm) was used as a counter electrode,and a saturated calomel electrode (SCE) as the reference electrode. The addition of samples was performed with a microsyringe (Shanghai Gaoge Industry & Trade Co.,Ltd.,China). The pH values of PBS were measured with a PHB-5 portable pH meter (Hangzhou Qiwei Instrument,China). SEMmeasurementswere taken with a JEOL JSM- 6701F scanning electron microscope (Tokyo,Japan).

2.3. Preparation of modified electrode

Prior to modification,the working electrode bare GCE was carefully polished with 0.05 mm alumina slurry until a mirrorshine surface was obtained,followed by successively sonicating in doubly distilled deionized water and ethanol,and then dried in air. The PEDOT-MeOH/GCE modified electrode was prepared facilely by one-step electropolymerization of EDOT-MeOH (10 mmol L-1) in 0.1 mol L-1 LiClO4. The deposition potential was 1.1 vs. SCE and the deposition time is 70 s. After drying at room temperature under a clean environment,the PEDOT-MeOH/GCE was electrodeposited in 5 mmol L-1 HPtCl6 solution by cyclic voltammetry (CV) at the scan rate of 100 mV s1 from -0.3 V to +1.0 V vs. SCE for 10 cycles. Then,the resulting PtNPs/PEDOT-MeOH/GCE modified electrode was washed repeatedly with double-distilled deionized water to remove the electrolyte and monomer from electrode surface and naturally dried at room temperature.

2.4. Experimental measurements

Five milliliters of 0.1 mol L-1 PBS (pH 7.0) with a specific amount of quercetin solution was transferred into the sealed electrochemical cell by microsyringe. The voltammetric detection of quercetin was studied by differential pulse voltammetry (DPV). The DPV conditions were as follows: potential increase,0.004 V; amplitude,0.05 V; pulse width,0.05 s; and pulse interval,0.2 s. Prior to each experiment,all solutions were deoxygenated by purging with nitrogen for 10 min. The fabrication process of the modified electrode and the working mechanism for the electrochemical analysis of quercetin are shown in Scheme 1.

Scheme 1.The process of construct PtNPs/PEDOT-MeOH/GCE and trace determination of quercetin.

3. Results and discussion 3.1. Characterization of modified electrodes

The surface morphology of the PEDOT-MeOH film and the PtNPs/PEDOT-MeOH composite deposited on the indium-tin oxide (ITO) transparent electrode was investigated by SEM. As shown in Fig. 1A,The PEDOT-MeOH film obtained from the aqueous solution was rough and compact structure,while the morphology of PtNPs/ PEDOT-MeOH film exhibited a three-dimensional (3D) morphological structure (Fig. 1B). The 3D structure of PtNPs/PEDOT-MeOH modified electrode could produce a large surface area structure, which is beneficial to maintaining large electroactive area on the electrode surface. Moreover,the 3D structure may give a high sensitivity for the detection of analytes. In addition,energy dispersive spectrum (EDS) analysis was performed to further confirm the components of the PtNPs/PEDOT-MeOH film shown in Fig. 1C. The presence of C,S,O,and Pt come from the component elements of the PEDOT-MeOH-PtNPs composite. Other elements mainly originated from electrolysis solution and ITO. The content of PtNPs is shown in Table 1. All the results confirm the presence of Pt on PEDOT-MeOH-PtNPs composite.

Fig. 1.SEM image of the PEDOT-MeOH film (A) and PtNPs/PEDOT-MeOH film (B),and EDX of PtNPs/PEDOT-MeOH film (C).

Table 1
Data of elemental analysis of PtNPs/PEDOT-MeOH from EDX.

Electrochemical impedance spectroscopy (EIS) was performed at the potential of 0.23 V and the frequency range from 10 kHz to 100 mHz,using 5 mmol L-1 [Fe(CN)6]3-/4- redox couple (1:1) with 0.1 mol L-1 KCl,as supporting electrolyte. The value of the modified electrode was estimated by the semicircle diameter. Fig. 2A illustrates the EIS of bare GCE (a),PEDOT-MeOH/GCE (b), and PtNPs/PEDOT-MeOH/GCE (c). Obviously,the bare GCE exhibited a semicircle portion and the value of electron-transfer resistance (Rct) was estimated to be 200 V. However,compared to the bare GCE,instead of the semicircle part,the PEDOT-MeOH/GCE and PtNPs/PEDOT-MeOH/GCE were linear curves,implying the PEDOT-MeOH films had a good capacitance behavior [29].

Fig. 2.(A) Nyquist plots of different electrodes in 5 mmol L-1 [Fe(CN)6]3-/4- containing 0.1 mol L-1 KCl: bare GCE (a),PEDOT-MeOH/GCE (b),and PtNPs/PEDOT-MeOH/GCE(c). (B) CVs of different electrodes in 5 mmol L-1 [Fe(CN)6]3-/4- containing 0.1 mol L-1 KCl: bare GCE (a),PEDOT-MeOH/GCE (b),and PtNPs/PEDOT-MeOH/GCE (c). Scan rate:50 mV s-1.

Cyclic voltammetric experiments were further carried out in 5 mmol L-1 [Fe(CN)6]3-/4- containing 0.1 mol L-1 KCl,shown in Fig. 2B. A pair of well-defined anodic and cathodic peaks was observed on all modified electrodes. The potential peak separation (DEp) between the anodic and cathodic potential peaks PEDOTMeOH/ GCE (96 mV,curve b) and PtNPs/PEDOT-MeOH/GCE (92 mV,curve c) were smaller than that at bare GCE (116 mV, curve a),indicating that PEDOT-MeOH films were electropolymerized on the surface of bare GCE and PtNPs were electrodeposited on the surface of PEDOT-MeOH/GCE,and also implying that the reversibility of electrochemical reaction at PEDOT-MeOH films modified electrodes were improved. Meanwhile,the response currents were higher than bare GCE,which was attributed to the good conductivity of PEDOT-MeOH films.

3.2. Electrochemical behavior of quercetin

Fig. 3 shows cyclic voltammograms (CVs) of 50 mmol L-1 quercetin in 0.1 mol L-1 PBS (pH 7.0) at different electrodes: bare GCE (a),PEDOT-MeOH/GCE (b),and PtNPs/PEDOT-MeOH/GCE (c). It can be seen that besides a pair of well-defined redox peaks at PtNPs/PEDOT-MeOH/GCE,quercetin showed an irreversible cathodic peak (II). The oxidation of the catechol moiety,30,40- dihydroxyl groups at ring A,occurs first at very low positive potential corresponding to peak I [8, 30]. The further scan to positive potential displayed an additional oxidation peak,known as the oxidation of other hydroxyl groups (Scheme 2). As also can be seen at the bare GCE,quercetin was reversibly oxidized with an anodic peak potential at around 0.19 V,while the anodic peak potential of quercetin at PtNPs/PEDOT-MeOH/GCE (0.13 V) shifted more negatively than that at PEDOT-MeOH/GCE (0.16 V) and bare GCE (0.19 V),suggesting that PtNPs could reduce the overpotential of quercetin and accelerate the electron transfer rate,which due to their subtle electronic properties [14]. Furthermore,the redox peak currents of quercetin on PtNPs/PEDOT-MeOH/GCE and PEDOTMeOH/ GCE were stronger than that on bare GCE. These results mean that both PEDOT-MeOH and PtNPs had an electrocatalytic activity toward quercetin,and the synergic effect of PEDOT-MeOH and PtNPs as co-catalysts made the PtNPs/PEDOT-MeOH/GCE show higher electrocatalytic activity than PEDOT-MeOH/GCE. PtNPs/PEDOT-MeOH films promoted the electrochemical reaction of quercetin more efficiently.

Fig. 3.CVs of 50 mmol L-1 quercetin in 0.1 mol L-1 PBS (pH 7.0) at different electrodes: bare GCE (a),PEDOT-MeOH/GCE (b),and PtNPs/PEDOT-MeOH/GCE (c).Scan rate: 50 mV s-1.

Scheme 2.The mechanism of oxidation of quercetin.

3.3. Optimization of conditions

To achieve a highly sensitive detection of quercetin,experimental parameters: preconcentration time and pH value have been optimized.

The effect of pH value on the determination of quercetin at PtNPs/PEDOT-MeOH/GCE over the range of pH 3.0-9.0 was investigated by CVs. As shown in Fig. 4A (solid line),the anodic peak currents of quercetin at PtNPs/PEDOT-MeOH/GCE increased gradually with the increasing pH value until it attained the maximum at pH 7.0. Thus,pH 7.0 was chosen in the following experiments as the best pH condition. The relationship between anodic peak potentials (Ep) and pH value was also demonstrated in Fig. 4A (dotted line). Ep shifted negatively with the increasing of pH values and the equation was Ep (V) = -0.06581 pH + 0.6161 (R2 = 0.99898). The slope (65.8 mV pH-1) of the equation is approximately close to the theoretical value of 58.5 mV pH-1, indicating that the electro-chemical reaction involves equal numbers of proton-transfer and electron-transfer.

Fig. 4B shows the influence of preconcentration time on the current response of quercetin at PtNPs/PEDOT-MeOH/GCE. The PtNPs/PEDOT-MeOH/GCE was dipped into 50 mmol L-1 under different preconcentration times. The anodic peak current increased rapidly with the increasing of preconcentration time and reached a platform at 80 s. The rapid response of quercetin at PtNPs/PEDOT-MeOH/GCE was attributed to the synergic effect of excellent conductivity of PEDOT-MeOH and the catalytic activity of PtNPs. Thus,80 s of preconcentration time was employed in the experiments.

Fig. 4.Effect of (a) the pH value and (b) preconcentration time for the adsorption of quercetin at PtNPs/PEDOT-MeOH/GCE in 0.1 mol L-1 PBS (pH 7.0).

3.4. Effect of scan rates toward quercetin

The effect of scan rates on the electrochemical behavior of quercetin at PtNPs/PEDOT-MeOH/GCE was investigated by CV (Fig. 5A). As shown in Fig. 5B,with the increasing of scan rates from 10 to 300 mV s-1,the reversible anodic peak currents (Ipa) and cathodic peak currents (Ipc) increased linearly with the scan rates (n). The regression equations were Ipa (mA) = 0.54n + 10.78 (n in mV s-1) (R2 = 0.9951) and Ipc (mA) = 0.42n + 2.16 (R2 = 0.9989), indicating that the reaction is an adsorption-controlled process.

Fig. 5.(a) CVs of 50 mmol L-1 quercetin at PtNPs/PEDOT-MeOH/GCE with different scan rates: 10,20,30,50,70,100,150,200,250 and 300 mV s-1 (from a to j). (b) The plot of peak currents vs. scan rate.

3.5. Detection of quercetin

DPV method was selected for determination trace amounts of quercetin in 0.1 mol L-1 PBS (pH 7.0). Fig. 6 shows DPV responses of different quercetin concentrations at PEDOT-MeOH/ GCE and PtNPs/PEDOT-MeOH/GCE under the optimum experimental conditions,which were preconcentrated in quercetin solutions with different concentrations for 80 s. As can be seen from Fig. 6A,the linear response ranges of quercetin at PEDOTMeOH/ GCE were 0.1-1.0 mmol L-1 and 1.0-31.0 mmol L-1. The detection limit is found to be 0.015 mmol L-1 (S/N = 3). For PtNPs/ PEDOT-MeOH/GCE (Fig. 6B),the peak currents were proportional to the concentration of quercetin in three ranges,0.04- 1.0 mmol L-1,1.0-19.6 mmol L-1 and 19.6-90.9 mmol L-1. The linearization equations were I1 (mA) = 48.48 + 9.51C (mmol L-1) (R2 = 0.9452),I2 (mA) = 56.86 + 1.10C (mmol L-1) (R2 = 0.9912) and I3 (mA) = 71.18 + 0.31C (mmol L-1) (R2 = 0.9992),respectively, with a detection limit of 5.2 nmol L-1 (S/N = 3). Compared with PEDOT-MeOH/GCE,PtNPs/PEDOT-MeOH/GCE has a lower detection limit,indicating that the introduction of PtNPs can greatly improve the electrocatalytic activity toward quercetin. A comparison of linear range,detection limit,and preconcentration time at PtNPs/PEDOT-MeOH/GCE with other quercetin sensors reported in the literature are shown in Table 2. The preconcentration time at PtNPs/PEDOT-MeOH/GCE exhibited much lower time than other previously reported electrodes [8, 9, 10, 12, 31], revealing the rapid response of quercetin at PtNPs/PEDOT-MeOH/ GCE. Meanwhile,the detection limit was lower than some other previously reported electrodes [8, 9]. In combination of the electrocatalytic activity and strong adsorption ability of nanoparticles PtNPs with the advantages of conducting polymer (good water solubility and high conductivity),the higher catalytic efficiency and stronger adsorptive ability of PtNPs/PEDOT-MeOH/ GCE exhibited excellent performances for the trace determination of quercetin.

Fig. 6.DPVs of PEDOT-MeOH/GCE in different concentrations of quercetin (0.1,0.4,0.8,1.0,3.0,5.5,9.0,13.8,19.0,and 31.0 mmol L-1) (A). DPVs of PtNPs/PEDOT-MeOH/GCE in different concentrations of quercetin (a-o: 0,0.04,0.06,0.1,0.4,0.8,1.0,3.0,5.5,9.0,13.8,19.0,39.1,65.6,and 90.9 mmol L-1) (B). Inset: Plot of peak current vs.concentration of quercetin. Supporting electrolyte: 0.1 mol L-1 PBS (pH 7.0); accumulation time: 80 s; potential increment: 4 mV.

Table 2
Comparison of the proposed electrode with other reported electrodes for the determination of quercetin.

3.6. Reproducibility,stability and selectivity of PtNPs/PEDOT-MeOH/ GCE

The reproducibility of PtNPs/PEDOT-MeOH/GCE for the electrochemical response of quercetin was estimated by 30th successive measurements with the relative standard deviation (RSD) of 0.67%. The RSD of five individual determination of MP was 3.5%,indicating an excellent reproducibility of PtNPs/PEDOTMeOH/ GCE.

The stability of PtNPs/PEDOT-MeOH/GCE was also explored. After 10 days of storage,the response to quercetin was tested each day with the response of the sensor decreasing only 5% compared to the initial response,which confirms long-term stability.

Moreover,under the optimal experimental conditions,interference studies were carried out by adding various foreign species to a fixed amount of quercetin (50 mmol L-1). The results showed that ions,such as K+,Ca2+,Na+,Mg2+,Pb2+,Ni2+,Zr2+,Fe3+,Cu2+,SO42-,PO43-,NO3-,and NO2- at a 100-fold concentration,glucose, l-tyrosine,glycine,folic acid,and citric acid in a 50-fold concentration,ascorbic acid at a 20-fold concentration had no effect on the detection of quercetin. The change of the peak currents of quercetin was less than 5% (i.e.,96.6%-104.3%), suggesting that the proposed electrode has good selectivity.

4. Conclusion

A simple and sensitive quercetin electrochemical sensor was successfully constructed based on conducting polymer PEDOTMeOH and nanoparticles PtNPs. The synergic effect of the electrocatalytic activity and strong adsorption ability of PtNPs with the advantages of PEDOT-MeOH (good water solubility and high conductivity) exhibited high catalytic efficiency and strong adsorptive ability for the trace determination of quercetin. The sensitivity of the electrochemical sensor was 9.51 mA mol L-1 cm-2 and the limit of detection was 5.2 nmol L-1. Furthermore,the modified electrode also exhibited good reproducibility and long-term stability,as well as high selectivity.


This work was supported by the National Natural Science Foundation of China (Nos. 51263010 and 51272096),Jiangxi Provincial Department of Education (No. GJJ11590),Natural Science Foundation of Jiangxi Province (No. 2010GZH0041) and Graduate Student Innovation Foundation of Jiangxi Province (No. YC2012-S123).

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