Molecularly imprinted polymer modified TiO2 nanotube arrays for photoelectrochemical determination of perfluorooctane sulfonate (PFOS)

Sensors and Actuators B 190 (2014) 745–751

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Molecularly imprinted polymer modified TiO2 nanotube arrays for photoelectrochemical determination of perfluorooctane sulfonate (PFOS) ThanhThuy Tran.T a,b , Jiezhen Li a , Hui Feng a , Jin Cai a , Lijuan Yuan a , Niya Wang a , Qingyun Cai a,∗ a b

State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan University, Changsha 410082, People’s Republic of China Department of Chemistry, Industrial University of Ho Chi Minh City, Ho Chi Minh, Viet Nam

a r t i c l e

i n f o

Article history: Received 13 April 2013 Received in revised form 10 August 2013 Accepted 8 September 2013 Available online 18 September 2013 Keywords: TiO2 Nanotube Molecularly imprinted polymer Photoelectrochemical Sensor Perfluorooctane sulfonate

a b s t r a c t A novel photoelectrochemical sensor is fabricated by a convenient surface modification of molecularly imprinted polymer on highly ordered and vertically aligned TiO2 nanotube arrays. As-prepared sensor (denoted as MIP@TiO2 NTAs) is highly sensitive to perfluorooctane sulfonate in water samples, with a linear range from 0.5 to 10 ␮M and a limit of detection of 86 ng mL−1 (S/N = 3). Moreover, the perfluorooctane sulfonate MIP@TiO2 NTA photoelectrochemical sensor exhibits outstanding selectivity. Twenty times 2,4-dichorophenoxyacetic acid, anthracene-9-carboxylic acid, and pentachlorophenol, or two times perfluoroheptanoic acid and perfluorooctanoic do not interfere the determination of perfluorooctane sulfonate. The practical application of the photoelectrochemical sensor is also realized in the selective determination of perfluorooctane sulfonate in water samples. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Perfluorooctane sulfonate (PFOS) and related compounds have been widely used in industry as various large scale technical applications, including to stain- and water-resistant coatings for fabrics and carpets, oil-resistant coatings for paper products approved for food contact, fire-fighting foams, mining and oil well surfactants, floor polishes, and insecticide formulations [1,2]. It is not only persistent, bioaccumulative and toxic to mammalian species but also persistent in environment and has been shown to be bioconcentrated in a number of species of wildlife, including marine mammals [3–5]. In addition, PFOS also is a water soluble and stable chemical substance. The widespread prevalence of PFOS in environment raises human health concerns [6]. Due to the ubiquity and high toxicity, it is important to monitor PFOS. The current major analytical technique for PFOS determination is high-performance liquid chromatography (HPLC)–mass

∗ Corresponding author at: 107 Chemistry Building, State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan University, Changsha 410082, China. Tel.: +86 731 88821848. E-mail addresses: [email protected], [email protected] (Q. Cai). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.09.048

spectrometry (HPLC–MS) [7–9]. Although good sensitivity has been achieved, this protocol requires expensive, complex instrumentation, complicated sample pretreatment, and highly trained technical personnel. Electrochemical analysis is a widely applied method for determination of environmental pollutants [10]. Nevertheless, PFOS is hard to be analyzed by direct electrochemistry on most electrodes owing to its lack of electrochemical activity [2,11]. Photoelectrochemical method can solve this problem perfectly, because both electroactive and nonelectroactive species can be photocatalytically oxidized at electrode generating photocurrent under certain bias voltage [12–14]. However, the photocatalytic oxidation may be lack of selectivity due to the nature of the oxidants which are generally hydroxyl radicals or photogenerated holes. In this work, high selectivity was achieved by using molecularly imprinted acrylamide as sensing film in the determination of PFOS. Besides the high stability and environmental compatibility [15,16], the acrylamide compound is with functional groups that can bind the target molecules [17–19]. TiO2 nanotube arrays (NTAs), an outstanding photochemical semiconductor material [20,21] with high surface area, was used as the photoelectrode. The molecularly imprinted acrylamide cannot only selectively capture the target molecules, but also can enhance the photocurrent [22] due to its fast electron-transfer capability. All these characteristics would lead

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to an excellent photoelectrochemical sensor with high selectivity, sensitivity, and stability.

2.2. Fabrication of the PFOS MIP@TiO2 NTA photoelectrochemical sensor

2. Methods/experimental procedure

The fabrication of the PFOS MIP@TiO2 NTA photoelectrochemical sensor is illustrated by Scheme 1A. Polished Ti substrate was fist used for anodic oxidation to obtain highly ordered and vertically aligned TiO2 NTAs. Anodization was performed in a twoelectrode configuration in an electrolyte containing 0.1 M NaF and 0.5 M NaHSO4 under 20 V at room temperature for 2 h using a platinum cathode. The TiO2 NTAs formed on Ti substrate were then annealed in air at 500 ◦ C for 3 h for crystallization in the anatase phase. As-prepared TiO2 NTAs were pretreated in 0.5 M NaOH aqueous for 30 min [23] to hydrolyze the TiO2 surface to form Ti-OH groups. After being rinsed by distilled water and dried, the TiO2 NTAs were chemically modified to obtain the double bonds and aminopropyl monomer-capping TiO2 NTAs. The immobilization was carried out as below: a 3.0 cm × 0.5 cm TiO2 NTA film was immersed into a 3 mL

2.1. Materials Titanium foil (99.8% purity, 0.25 mm thick) was purchased from Aldrich (Milwaukee, WI). Perfluorooctane sulfonate (PFOS), perfluoroheptanoic acid (PFHA), perfluorooctanoic acid (PFOA), and acrylamide were obtained from Sigma–Aldrich Chemie Inc. Ethylene glycol dimethacrylate (EGDMA), 2,2 -azobis (2(AIBN), 3-aminopropyltriethoxylsilane methylpropionitrile) (APTS), 3-methacryloxypropyl trimethoxysilane (MPS) were all purchased from Shanghai Chemical Corporation of China. All other reagents of analytical grade were obtained from commercial sources and used as received. Deionized water was used for preparation of all aqueous solution.

Scheme 1. Schematic illustration of the fabrication of (A) the PFOS photoelectrochemical sensor and (B) the molecularly imprinted polymer process.

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anhydrous toluene solution containing 30 ␮L of APTS and 30 ␮L MPS. The mixture was purged for 15 min under dry nitrogen and then heated at 70 ◦ C in 2 h. The resulting APTS- and MPS-modified TiO2 NTAs were washed with toluene and acetonitrile respectively, and dried in pure nitrogen gas [24]. The APTS/MPS-modified TiO2 NTAs were ready for the molecularly imprinted polymerization. The molecularly imprinted polymerization was conducted based on the reported methods [24–27]. Acrylamide, EGDMA, and AIBN were used as the functional monomer, crosslinker and initiator agent of the imprinting polymerization, respectively. 3 mL methanol/acetonitrile (1/1) (v/v) solution containing 10 mM PFOS, 0.2 M acrylamide, 1.4 M EGDMA, and 40 mM AIBN was added in a columniform quartz tube. After sonication for 5 min, the APTS/MPSmodified TiO2 NTA electrode was fixed in the columniform quartz tube which was then sealed and purged with nitrogen for 15 min while cooled in ice bath. Afterwards, polymerization was carried out under UV light irradiation for 10 h. The template was removed by immersing electrode in 1/1 (v/v) methanol/deionized water until no PFOS was detected in the eluate. Non-molecularly imprinted polymerization on TiO2 NTAs (NIP@TiO2 NTAs) was prepared as a reference using the same reaction mixture without addition of the template. 2.3. Characterization and photoelectrochemical measurements of the PFOS photoelectrochemical sensor Field emission scanning electron microscope (FESEM, Hitachi S4800) was used for studying the morphologies of MIP@TiO2 NTAs. Photoluminescence (PL) spectrum was recorded using Hitachi F-4600 fluorescence spectrophotometer at an excitation wavelength of 270 nm. Fourier transforms infrared (FTIR) spectrum was recorded on a Thermo Nicolet 5700 (Thermo Electron Corporation) with 2 cm−1 resolution in the range of 500–4000 cm−1 using KBr pellets. Photoelectrochemical measurements were conducted using an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Co., Ltd.) in a standard three electrode configuration with the MIP@TiO2 NTAs or NIP@TiO2 NTAs (3.0 cm × 0.5 cm) as the working electrode; a Pt wire counter electrode, and a SCE reference electrode. The electrolyte used is 0.1 M PBS (pH = 7) solution containing 0.1 M KCl. A 300 W Xenon lamp (Beijing Changtuo Co., Ltd.) was used as the light source, filtered to 100 mW cm−2 AM 1.5 G as determined by a radiometer (OPHIR, Newport, USA). The electrode was dipped into the electrolyte and purged with nitrogen for 15 min. After 30 s equilibration time, the I–t curve was recorded in the init potential 0 V at a sensitivity of 10−4 A/V. I–t curve was used for characterization of all sensitivity, selectivity, and determination of samples experiment.

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3. Results and discussions 3.1. Fabrication and characterization of the PFOS MIP@TiO2 NTA photoelectrochemical sensor Scheme 1A illustrates the fabrication process of the PFOS MIP@TiO2 NTA photoelectrochemical sensor. The molecularly imprinted polymer was functionalized on the TiO2 NTAs with PFOS as the template molecules. The formation of a precursor between the functional monomer acrylamide and PFOS template molecules via hydrogen bonds, is a prerequisite for the molecular imprinting. In the mixture solution, PFOS was embedded in acrylamide film by strong hydrogen bonds formed between fluorine atoms of PFOS and the amino groups (–NH2 ) [25]. After removal of PFOS, recognition sites complementary to the molecular shape, size, and functionality of PFOS were formed in acrylamide film which could efficiently and selectively rebind PFOS in solution as shown in Scheme 1B [28,29]. The surface morphologies of the MIP@TiO2 NTAs and TiO2 NTAs are shown in Fig. 1. The TiO2 NTAs have an inner pore diameter ranging from 70 to 110 nm and wall thickness of about 15 nm (Fig. 1A). After molecularly imprinted polymerization, the molecular imprinted polymer was uniformly deposited onto the surface of the TiO2 NTAs. The TiO2 NTAs covered with a thin MIP layer still keep the open-top character as shown in Fig. 1B. Fig. 2A shows the photoluminescence emission spectrum, which has been widely used to investigate the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron–hole pairs in semiconductor particles [30]. As shown in Fig. 2A the photoluminescence emission of (a) TiO2 NTAs, (b) NIP@TiO2 NTAs, and (c) MIP@TiO2 NTAs exhibit similar broad PL emission bands with the PL intensity decreased in order of: TiO2 NTAs > NIP@TiO2 NTAs > MIP@TiO2 NTAs. These results imply that the modification of MIP leads to a low electron–hole recombination rate because that low PL intensity indicates a low electron–hole recombination rate. The direct evidence for the incorporation and removal of PFOS in the molecular imprinted titania film was obtained by FTIR spectroscopy. Fig. 2B shows the FTIR spectra of pure TiO2 NTAs, pure acrylamide, pure PFOS, MIP@TiO2 NTAs before and after removing of PFOS, as well as NIP@TiO2 NTAs. The pure TiO2 NTAs show no significant FTIR absorptions except the weak absorptions of water (curve a). With the modification of MIP or NIP, N H bending mode around 1620 cm−1 [31], C O amide stretching mode around 1680 cm−1 , C N stretching mode around 1139 cm−1 , C H and CH2 out-of-plane bending mode around 960 cm−1 , and N H stretching mode around 3370 cm−1 [32–34] are observed in curves e and f. These results are in good agreement with the characteristic absorption bands of acrylamide, curve b. With the PFOS loading

Fig. 1. FESEM top-surface images of (A) TiO2 NTAs and (B) MIP@TiO2 NTAs.

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(B)

(A)

(a)

Transmittance (a.u.)

PL Intensity (a.u.)

a 3000

b c

2000

(b) (c) (d) (e) (f)

1000

4000

320

380

440

500

Wavelength (nm)

3500

3000

2500

2000

1500

1000

500

-1 Wavenumber ( cm )

Fig. 2. (A) PL spectrum of (a) TiO2 NTAs; (b) NIP@TiO2 NTAs; and (c) MIP@TiO2 NTAs, the excitation wavelength is 270 nm. (B) The FTIR spectrum of (a) TiO2 NTAs; (b) pure acrylamide; (c) pure PFOS; (d) MIP@TiO2 NTAs before removing the PFOS; (e) MIP@TiO2 NTAs after removing the PFOS; and (f) NIP@TiO2 NTAs.

in the MIP@TiO2 NTAs, there appear three characteristic absorption bands at 1153, 1207, and 1251 cm−1 which are assigned to the C C C bending vibration, S O stretch mode, and C F stretch mode of PFOS molecule (curve d) [35]. These characteristic absorption peaks are as same as the FTIR absorption spectrum of pure PFOS (curve c), confirming the loading of PFOS in MIP@TiO2 NTAs. After rinsing the MIP@TiO2 NTAs with CH3 OH/H2 O solution for 60 min, these characteristic peaks disappear (curve e) indicating that PFOS are removed. Carefully comparing the absorption bands of MIP@TiO2 NTAs before and after removal of PFOS (curves d and e), one can see that the absorption band of acrylamide at 3370 cm−1 (N H stretch) becomes stronger after removing the target molecule PFOS, implying there form hydrogen bonds between PFOS and amide as shown in Scheme 1. The formation of hydrogen bonds results in a decrease in the bond strength of N H, which leads to the decrease in absorption strength and red shift in frequence [36–38]. Fig. 3A shows the photocurrent response of (a) TiO2 NTAs, (b) NIP@TiO2 NTAs, and (c) MIP@TiO2 NTAs in 0.1 M PBS (pH 7) solution containing 0.1 M KCl under AM 1.5 G illumination. The measured photocurrents of MIP@TiO2 NTAs, NIP@TiO2 NTAs, and TiO2 NTAs are 0.198, 0.187, and 0.138 mA cm−2 , respectively. The measured photocurrents of MIP@TiO2 NTAs and NIP@TiO2 NTAs are significantly higher than that of TiO2 NTAs, indicating that the modification of conductive polymer favors the transfer of charges. As it is known, acrylamide is a conjugated monomer with good conductivity. For TiO2 NTAs, photogenerated carriers will be easily recombined, lowering the photoelectric conversion efficiency, whereas for the acrylamide modified TiO2 NTAs film, acrylamide will promote the separation of electrons-holes, enhancing the conversion efficiency [28].

Fig. 3B shows values of (I − I0 )/I0 on TiO2 NTAs, NIP@TiO2 NTAs and MIP@TiO2 NTAs electrodes, where I0 and I are photocurrent responses in 0.1 M PBS (pH 7) solution containing 0.1 M KCl without and with addition of 5 ␮M PFOS under AM 1.5 G illumination. The values of (I − I0 )/I0 are 2.59 × 10−2 , 7.44 × 10−2 and 17.12 × 10−2 for TiO2 NTAs, NIP@TiO2 NTAs and MIP@TiO2 NTAs electrodes, respectively, which indicates that the photocurrent response on MIP@TiO2 NTAs electrode is much higher than that on TiO2 NTAs and NIP@TiO2 NTAs electrodes. With the capture of PFOS, the formed network as shown in Scheme 1B favors the transfer of charges resulting in the enhancement in photocurrent. That is why the photocurrent of MIP@TiO2 NTAs is higher than that of NIP@TiO2 NTAs. This assumption was further confirmed via the PFOS concentration-dependent photocurrent responses. As shown in Fig. 4, the photocurrent gradually increases with increasing the PFOS concentration. 3.2. Detection of PFOS The response of the PFOS MIP@TiO2 NTA photoelectrochemical sensor to PFOS was investigated in 0.1 M PBS (pH 7) solution containing 0.1 M KCl. As shown in Fig. 4A, the photocurrent increases with increasing the PFOS concentration. Fig. 4B shows that the relative photocurrent change is linear dependent on the PFOS concentration in the range of 0.5–10 ␮M with a correlation coefficient of 0.9869. The limit of detection (LOD) (S/N = 3) was calculated to be 86 ng mL−1 . The applicability of the sensor was further investigated with the recovery rate test. Water samples collected from Xiangjiang river, Yuelu mountain and tap system, which then filtered 0.22 ␮m filter

Fig. 3. Photocurrent responses measured on (a) TiO2 NTAs; (b) NIP@TiO2 NTAs; and (c) MIP@TiO2 NTAs in 0.1 M PBS (pH 7) solution containing 0.1 M KCl without (A) or with (B) 5 ␮g/L PFOS.

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0.25

0.26

(A) 0.24

R2 = 0.9869

(B)

10 µM

0.2

0.22

(I-I0)/I0

Photocurrent (mA/cm2 )

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0.5 µM

0.2

0.15

0.1 0.18

0.05

0.16 0

50

100

150

0

2.5

5

7.5

10

Concentration of PFOS ( µM)

Time (s)

Fig. 4. Photocurrent responses of (A) MIP@TiO2 NTA electrode in 0.1 M PBS (pH 7) solution containing 0.5, 1, 2.5, 5, 7.5, 10 ␮M PFOS (from bottom to top). (B) Linear calibration curve. All photocurrents were recorded after the electrodes were immersed in the PFOS solution for 15 min.

Table 1 Recovery study for PFOS using MIP@TiO2 NTAs photoelectrochemical sensor with various water samples. Sample

PFOS added (␮M)

Found (␮M)

Recovery (%)

RSD (%)

Tap water

0 2.500 5.000

0 2.401 5.090

– 95.81 101.9

– 4.560 2.741

Xiangjiang river water

0 2.500 5.000

0 2.622 5.891

– 104.8 117.8

– 6.072 5.094

Yuelu mountain water

0 2.500 5.000

0 2.514 4.982

– 100.2 99.54

– 3.453 2.024

membranes, were spiked with PFOS at 2.5, 5 ␮M concentrations and analyzed by the proposed method. As shown in Table 1, good recovery rates of 95.81–117.83% are achieved, indicating the applicability of the method in analysis of real water samples. The selectivity of the PFOS MIP@TiO2 NTA photoelectrochemical sensor was investigated with 2,4-D, 9-AnCOOH, PCP, PFHA, and PFOA as the interfering substances. These compounds represent different kinds of pollutants while PFHA and PFOA are the analogs of PFOS with high similarity in structure. Fig. 5A shows the responses of the sensor toward 5 ␮M of each these compounds. The relative change in photocurrent are: 17.12, 6.59, 4.92, −1.15, −2.09, and

−1.55% for PFOS, PFHA, PFOA, 2,4-D, 9-AnCOOH, and PCP, respectively. The photocurrent response to PFOS is much higher than to those investigated interferences, indicating the high binding selectivity toward PFOS. Although these interferences all exhibit anionic property in aqueous solution, they are different in molecular structure. 2,4-D, 9-AnCOOH, and PCP have benzene ring in the molecular structure. Due to the existing of the benzene rings, the binding of these compounds would result in a decrease in the conductivity of the MIP film, consequently leading to the decrease in photocurrent. PFHA and PFOA have structures as nearly the same as PFOS except the end carboxylic group. Even toward such high similar compounds, the responses of the sensor are pretty low in comparing with PFOS, indicating the high selectivity of the MIP-coated sensor. This identification to PFOS on the MIP@TiO2 NTAs sensor is mainly due to the shape identification and the hydrogen bond identification [25]. The acrylamide monomer coordinates with PFOS target molecules and polymerizes together. When the target molecules are removed leaving a special polymer with voids in particular shape and size, which is a good complement to target molecules [39–41]. So when rebinding, the PFOS molecules can recognize the imprinted sites and adsorb on the sites very quickly. The selectivity of the MIP@TiO2 NTA photoelectrochemical sensor was further investigated by determining the responses of the sensor to PFOS in the presence of these interferences. In this way the effects of these interferences on the PFOS rebinding can be measured. Fig. 5B shows the responses of the sensor to PFOS alone

Fig. 5. The responses of MIP@TiO2 NTA electrode to (A) 5 ␮M PFOS or interferents, and (B) to 5 ␮M PFOS alone (S0), or 5 ␮M PFOS containing 100 ␮M 2,4-dichlorophenoxyacetic acid (S1), 100 ␮M anthracene-9-carboxylic acid (S2), 100 ␮M pentachlorophenol (S3), 10 ␮M PFHA (S4), or 10 ␮M PFOA (S5) in 0.1 M PBS (pH 7) solution containing 0.1 M KCl.

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(S0) and the mixture solution containing PFOS and an individual interference, (2,4-D, 9-AnCOOH, PCP, PFHA, or PFOA represented as S1 to S5). The responses in the mixture were normalized to that in 5 ␮M PFOS (S0) which was set as 100%. One can see that the photocurrent deviations are all less than 6% even the concentrations of the interfering substances are 20 times (S1, S2, S3) or 2 times (S4, S5) that of PFOS, which adequately reveals that this MIP@TiO2 NTAs electrode presents outstanding selectivity. The repeatability of the PFOS MIP@TiO2 NTA photoelectrochemical sensor was evaluated by the photocurrent response in 0.1 M PBS (pH 7) solution containing 0.1 M KCl and 5 ␮M PFOS. The photocurrent response remained stable after 10 continuous measurements with a relative standard deviation of 2.8%. The long-term stability revealed that after storage of 1 month, the photocurrent response decreased only 3.7%. The lifetime of the PFOS MIP@TiO2 NTA photoelectrochemical sensor was test after 3 months, the photocurrent response decreased 5.1%. These results suggest that the developed PFOS MIP@TiO2 NTA photoelectrochemical sensor is with satisfactory repeatability and stability. 4. Conclusions In summary, a new selectively PFOS MIP@TiO2 NTA photoelectrochemical sensor was fabricated on highly ordered and vertically aligned TiO2 NTAs, utilizing molecularly imprinted acrylamide film as recognition element. The sensor showed good sensitivity and selectivity toward PFOS, and was used to detect PFOS with a low detection limit. Determination of PFOS in real water samples was also performed with satisfactory results. The work will open a new way to the development of high selective and sensitive photoelectrochemical sensor toward environmental pollutants.

[10]

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Acknowledgements We gratefully acknowledge the National Basic Research Program of China (Grants No. 2009CB421601), and the National Science Foundation of China (Grant No. 21175038, 21235002) for financial support. We thank the editor and reviewers for helpful comments and suggestions.

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Biographies ThanhThuy Tran.T is a lecture in the Department of Chemistry of Industrial University of Ho Chi Minh City, Ho Chi Minh, Viet Nam. She received her B.Sc. and MSc. from Vietnam National University-HoChiMinh City, University of Natural Sciences in 2002 and 2009, respectively. She has just earned her PhD in analytical chemistry in 2013 from Hunan University. Her current interests are (nano) materials, chemical sensor, electrochemistry and environmental chemical engineering. Jizhen Li received her B.Sc. in School of science in 2010 from South China of Agricultural University, Guangzhou, China. He is a master course student

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in Hunan University. His current interests are material chemistry and electrochemistry. Hui Feng is a senior experimentalist in the Department of Chemistry and Chemical Engineering, Hunan Arts and Sciences College, Changde, Hunan Province 415000, China. She received her B.Sc. from Hunan Normal UniversityChangsha City in 1997, and MSc. from Hunan University, Changsha, in 2006, respectively. She is now a Doctor of Science course student in Hunan University. Her current interests are (nano) materials, chemical sensor, photoelectrochemistry and environmental chemical engineering. Jin Cai received her B.Sc. in School of Chemical Engineering and Pharmacy in 2011 from Wuhan Institute of Technology, Wuhan, China. She is a master course student in Hunan University. Her current interests are biosensors and electrochemistry. Lijuan Yuan received her B.Sc. in School of science in 2010 from Jiangxi Agricultural University, Nanchang, China. She is a master course student in Hunan University. Her current interests are biosensors and electrochemistry. Niya Wang received her B.Sc. in School of Pharmaceutical Engineering in 2011 from Hunan Normal University, Changsha, China. She is a master course student in Hunan University. Her current interests are biosensors and electrochemistry. Qingyun Cai received his B.Sc. degree in 1983 and MSc. degree in 1986, both in chemistry from Hunan University, PR China. Since then he has been on the faculty at Hunan University. He earned his PhD in chemistry in 1996 from Hunan University. From 1997 to 2001, he left to the University of Michigan and the University of Kentucky as a visiting scholar. He is currently a full-time professor in the Department of Chemistry at Hunan University, PR China. His primary research interests concern the chem/biosensor and functional (nano) materials.