Tannic Acid-Reduced Graphene Oxide Deposited with Pt

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3805−3814

pubs.acs.org/journal/ascecg

Tannic Acid-Reduced Graphene Oxide Deposited with Pt Nanoparticles for Switchable Bioelectronics and Biosensors Based on Direct Electrochemistry Bilge Akkaya,† Bekir Ç akiroğlu,‡ and Mahmut Ö zacar*,†,‡ Science & Arts Faculty, Department of Chemistry, and ‡Biomedical, Magnetic and Semiconductor Materials Research Center (BIMAS-RC), Sakarya University, 54187 Sakarya, Turkey

ACS Sustainable Chem. Eng. 2018.6:3805-3814. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 07/28/18. For personal use only.

†

S Supporting Information *

ABSTRACT: In this study, we reported a novel biosensor based on direct electrochemistry of glucose oxidase (GOx) on a tannic acid-reduced graphene oxide nanocomposite modified glassy carbon electrode deposited with Pt nanoparticles. Tannic acid (TA) was utilized for the simultaneous green reduction of Pt4+ and graphene oxide (GO), along with modifying the reduced GO for the GOx immobilization, thus constructing a switchable surface with pH and temperature alterations. Upon the electrochemically oxidation of TA to quinone, enhanced electron transfer was obtained, and a third generation biosensor was fabricated using π−π interaction between GO and TA and Schiff-base assisted hydrogen bonds between GOx and TA interactions. The redox peaks were observed at a formal potential of −0.462 V with a peak separation (ΔEp) of 56 mV, which reveals the fast electron transfer. The linear response to glucose oxidation was in the range of 2−10 mM with a limit of detection of 1.21 μM and a sensitivity of 27.51 μA mM−1 cm−2. Upon the deposition of poly(Nisopropylacrylamide) (PNIPAAm) onto the constructed biosensor via hydrogen bonds, an on−off biosensor was fabricated with zipperlike interfacial properties upon the formation of the shrunken and compact globule PNIPAAm structure and varying surface charge. Therefore, this study confirmed the versatile aspects of natural TA without using complicated methods. KEYWORDS: Reduced graphene oxide, Green chemistry, Glucose oxidase, Smart interfaces, pH controlled surface

■

INTRODUCTION Blood glucose measurement, in vitro or in vivo, has been paid much attention with respect to producing reliable methods and devising electrode materials for the improvement of responses and enzyme-based electrochemical biosensors since the development of the first glucose biosensor. Graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), have recently emerged as advanced materials in various application areas owing to their unique mechanical and electronic properties, which enable us to employ these superior materials in biosensors.1 Graphene based materials display excellent electrical conductivity, high surface area to volume ratio, biocompatibility, lower cost compared with other carbonaceous materials, and mechanical strength,2 and are employed in various biosensors, such as metal ion sensors and immunosensors.3,4 Despite such advanced electrochemical capabilities of graphene derivatives, there still has been significant interest in the development of new graphene-based nanomaterials to give impetus to electroanalytical applications. Recently, third generation biosensors (TGB) have been fabricated based on the direct electrochemistry between oxidoreductases (e.g., glucose oxidase (GOx)) and electrode.5 Graphene-derivatives have been used as an electrode material in TGB,6 and this material can be modified to enhance surface © 2018 American Chemical Society

area, electrical conductivity, catalytic activity, and biocompatibility, resulting in improved enzyme performance. The noble nanoparticles (NPs), especially platinum NPs, can lead to a synergistic effect on electrocatalytic activity by catalyzing hydrogen peroxide, which releases from the oxidoreductase biocatalytic reactions, thus enhancing the sensitivity of the enzyme immobilized electrochemical biosensor. Also, Pt NPs can expand the surface area by allowing the fast diffusion of target analytes.7,8 There are green reduction approaches of GO in the literature, and one of the important reducing agents is natural polyphenols (e.g., tannic acid (TA)), which modify rGO surface by binding through π−π interactions.9,10 Also, polyphenols can bind to protein molecules via mainly hydrogen bonds, and this phenomenon has been used for the enzyme immobilization using TA modified surfaces.11 TA can bind to the protein molecules through covalent Schiff base bonds only by basic pretreatment of TA to gain quinone moieties.12 Recently, on−off biosensors have been gaining much attention with their interesting switchable surfaces with temperature and pH alteration. The stimuli-responsive polymer poly(NReceived: November 10, 2017 Revised: December 29, 2017 Published: January 11, 2018 3805

DOI: 10.1021/acssuschemeng.7b04164 ACS Sustainable Chem. Eng. 2018, 6, 3805−3814

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. TGB and On−Off Biosensor for the Biosensing of Glucose

platinum standard solution and 6 μL of 0.5 M sodium hydroxide solution were added into the suspension at room temperature. The reduction of GO was enhanced by keeping the resulting mixture at 90 °C for 30 min. Then, the black rGO was centrifuged and washed twice with deionized water (DW) to eliminate the residual TA and redispersed in 1 mL DW for the GCE coating. Fabrication of the rGO-Pt NPs-GOx Modified GCE. Before nanocomposite deposition, the GCE (3 mm in diameter) were polished with alumina powder and subsequently rinsed and sonicated in DW. Subsequently, glassy carbon electrodes were modified with 8 μL of the composite (0.5 mg mL−1) by drop-casting and dried in air. The enzyme immobilization was realized by immersing the nanocomposite-coated electrodes into 250 μL of GOx solution (40 mg mL−1) at 4 °C for 4 h. Finally, the modified GCE was rinsed with PBS to remove the loosely bound GOx and was stored at 4 °C in a refrigerator under dry conditions when not in use. PNIPAAm Covering on rGO-Pt NPs-GOx/GCE. The rGO-Pt NPs-GOx nanocomposite on the GCE was covered with PNIPAAm to investigate the temperature and pH sensitive behavior. A 50 μL portion of PNIPAAm (20 mg mL−1) solution in pH 7.4 PBS was dropcasted on the nanocomposite modified GCE, and the obtained sensitive bioconjugate (PNIPAAm-rGO-Pt NPs-GOx) was left to dry at room temperature for 30 min.

isopropylacrylamide) (PNIPAAm) has been used extensively for the thermosensitive behavior in the drug delivery and biosensor applications. According to the literature, TA can establish interaction with PNIPAAm and other polyamides via hydrogen and hydrophobic bonds under lower critical solution temperature (LCST) of around 31−32 °C.13,14 However, there is no work using this phenomenon in the switchable biosensor surfaces. In this work, we report a novel Pt NPs deposited and TAreduced graphene oxide nanocomposite for GOx immobilization and its application for glucose sensing with a decent sensitivity based on direct electrochemistry. To obtain a good selectivity, and catalytic activity, GOx, mostly used enzyme in glucose biosensors, was chosen owing to its high catalytic activity and specificity to glucose. Also, Pt NPs have synergistic effect on the electrochemical response and improve the sensitivity. GO and Pt4+ reduction were carried out in a facile and low-cost way using natural reducing agent, TA. Also the deposited TA on the rGO via π−π interactions provided the biocompatible surface for GOx immobilization through hydrogen bonds. The synthesized nanocomposite was drop-casted on a glassy carbon electrode (GCE). Also, a pH and temperature controlled switchable biosensor surface was obtained by using the TA-PNIPAAm interactions. The electrochemical measurements were carried out with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques.

■

■

RESULTS AND DISCUSSION Preparation and Characterization of Electrodes and Electron Transfer through TA. GO was synthesized according to modified Hummers’ Method15 (Figure S1), followed by chemically reduction, and the electrochemical and on−off biosensors were fabricated as depicted in Scheme 1. Simultaneous green reduction of GO and Pt4+ was carried out with green reducer TA. Also, TA was deposited on the rGO surface via π−π interactions between the phenolic groups of TA, and benzene rings of rGO. GOx was immobilized on TA

MATERIALS AND METHODS

One-Pot Synthesis of Platinum Nanoparticles Deposited Reduced Graphene Oxide Composite (rGO-Pt NPs). The GO was synthesized according to the previously published method.15 Then, 1 mL of GO (0.5 mg mL−1) was dispersed in 2 mL of 5 mg mL−1 TA aqueous solution under vigorous stirring, and 650 μL of 3806


Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (A) TEM image of rGO. (B) rGO-Pt NPs (inset shows the size distributions of Pt NPs). (C) HRTEM image of rGO-Pt NPs. (D) FTIR spectra of GO (a), rGO (b), and rGO-Pt NPs-GOx (c).

The immobilized GOx was determined according to the Bradford Method,16 the immobilized GOx amount was found to be 3.2 ± 0.6 mg (enzyme) mg−1 (nanocomposite). Also, the DNSA glucose determination test demonstrated that immobilized GOx increased the enzyme activity by 140% compared to that of free GOx. According to the literature, enzyme activity can enhance upon immobilization owing to the formation of much favorable 3D structure for the enzymatic activity.17 According to the inductively coupled plasma measurements, it was confirmed that about 5% of the nanocomposite consists of platinum nanoparticles. The morphologies of rGO and rGO-Pt NPs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Figure 1A and B display the SEM and TEM images of rGO with a wrinkled and a high surface area for the

coated rGO via covalent assisted hydrogen bonds by immersing the rGO-Pt NPs-GOx/GCE into the enzyme solution. The effects of different parameters on electrochemical biosensor fabrication were investigated. The optima of GOx immobilization time, platinum amount, and nanocomposite volume for the deposition were determined for the electrochemical biosensor fabrication (Figures S2−S4), and their optimum values were found to be 4 h, 650 μL of 1000 ppm platinum standard solution, and 8 μL of composite solution, respectively. The current response was diminished with the increasing immobilization time. Although, 850 μL platinum solution led to a bit more current response, 650 μL was chosen owing to the cost care, and upon the deposition of different volume of nanocomposites, the redox currents remained constant after 8 μL of nanocomposite deposition. 3807


Research Article


Figure 2. (A) CV curves of GOx/GCE (a), GO-GOx/GCE (b), rGO-GOx/GCE (c), and rGO-Pt NPs-GOx/GCE (d) in deoxygenated 0.1 M PBS (pH 7.4) solution at scan rate 100 mV s−1 (all CV measurements in this study were carried out in the anodic sweep). (B) pH effect of the solution on CV responses. (C) Scan rate effect of the experiment on CV responses. (D) Glucose measurement using DPV technique.

the spectra of rGO for 30 min and 60 min (Figure S8c and d). This finding confirms the π−π stacking interaction between the hydrophobic aromatic regions of GO and that of TA along with hydrogen bond formation (H-type π−π stacking molecular model).18 Thus, TA can attached to rGO surface, leading to modification of surface for enzyme immobilization. In order to confirm the synthesis of graphene oxide, reduction and modification of its surface with TA, and GOx immobilization, FTIR analyses were performed. FTIR spectra of samples measured in the range of 400−4000 cm−1 are demonstrated in Figure 1F. In the GO spectrum, the peak at 1726 cm−1 is assigned to the CO stretching vibration of COOH groups, while the broad bands centered at about 1380 and 3170 cm−1 are associated with the deformed and stretching mode of OH, respectively,19,20 and the peaks at 1039 and 1620 cm−1 are attributed to the C−O and CC stretching vibrations. In the rGO spectrum, the peaks in the GO diminished or nearly disappeared, and peaks attributed to TA appeared, which confirm the reduction and TA deposition on rGO. In the spectrum of rGO, the absorption peaks at 1712 and 1613 cm−1 correspond to the CO stretching and aromatic C−O stretching, respectively.21 Also, the peaks at 1329 cm−1 originated from in plane −OH bending, at 1060 cm−1 associated with aromatic C−H deformation vibrations, at 1444 cm−1 originates from symmetric stretching vibration of C−O, and at 1512 cm−1 assigned to C−C stretching in rings were also observed.11 The peak at 1581 cm−1 can be attributed to the presence of quinone moieties, which result from the oxidation of hydroxyls of TA upon GO and Pt NPs reduction. This peak was also observed in our previous study, in which

enhanced enzyme immobilization. Figure 1C and D show TEM images of rGO-Pt NPs, and the Pt NPs demonstrated a good size distribution in the range of 2.0−4.5 nm (inset of Figure 1C), and uniformly distributed on the rGO, which enhance the reproducibility of the biosensor. The HRTEM image of the rGO-Pt NPs (Figure 1E) demonstrated that the synthesized Pt NPs possess a particle size of about 3.4 nm. The d-spacings of adjacent fringe was found to be 2.2 Å, which corresponds to the {111} plane of face-centered cubic (fcc) Pt and confirms the formation of highly crystalline Pt NPs. Energy dispersive X-ray spectroscopy (EDS) analysis confirmed the Pt NPs presence by yielding a peak of Pt M around 2 keV, as shown in Figure S5. The information about TGA, Zeta potential analysis, and UV−vis measurements of the nanocomposites were given in Figures S6−S8, respectively. UV−vis spectra of the nanocomposites were given in Figure S8 to investigate the time dependent reduction process of GO. Accordingly to Figure S8, TA can attached to the reduced graphene oxide surface via π−π interactions along with modifying the surface for enzyme immobilization using hydrogen bonding between nitrogen containing moieties of enzyme and hydroxyls of TA, which was confirmed by FTIR. In Figure S8b, GO displays two characteristic absorption peaks. An intense absorption band at 230 nm attributed to π−π* transition and the second band at 305 nm corresponding to the n−π* transition of the CO bond were observed. Upon the reduction with TA for 30 min and 60 min, n−π* transition band disappeared, which confirms the reduction of GO. According to the UV−vis spectra, the π−π* transition and n−π* transition of GO was blue-shifted to lower wavelength of 212 nm and ca. 275 nm, respectively, in 3808


Research Article


the effective GOx working pH range and a strong dependence on medium pH was observed. A decrement in pH leaded to a positive shift in both Epa and Epc, indicating that hydrogen ions are involved in the electrochemical reaction of GOx, and DET potential decreases with decreasing pH. As shown in Figure 2B, the formal potential of GOx has a linear dependence on the pH in the range from 5.0 to 8.6. The slope value for pH vs formal potential is found to be −(56.4 ± 0.4) mV pH−1 (R2 = 0.988; Figure 2B, inset). The observed slope value (56.4 mV pH−1) is very close to the theoretical value (58.6 mV pH−1) for twoproton couple with two-electron transfer of GOx as shown eq 1.29 The observed slope value is comparable to the studies in the literature.6,27 The small decrement in the value compared to the literature can be attributed to the acidic nature of TA, and there is no substantial effect on the efficiency of biosensor. According to the Figure 1B, the maximum current appeared between pH 7.0 and 8.6, and physiologic pH 7.4 was chosen as pH of experiment media for being appropriate for body fluids.

quinones formed with the increment in the medium pH. However, this peak totally disappeared in the rGO-Pt NPs-GOx spectrum, which verifies the Schiff base bond formation.22 The presence of amide II at 1540 cm−1 and amide I at 1646 cm−1 confirmed the immobilization of GOx and the preservation of secondary structure after the immobilization.23 The carbonyl group (CO) stretching vibration in rGO spectrum was shifted from 1697 to 1713 cm−1 in the rGO-Pt NPs-GOx, indicating the hydrogen bonds between TA and GOx.24,25 Therefore, improved enzyme immobilization was carried out to construct the enzyme based biosensor. Direct Electrochemistry of GOx on rGO-Pt NPs/GCE. The nanocomposite modified electrodes were scanned at the potential range of 0.1−0.9 V right before the experiments to oxidize phenolic hydroxyls of TA to obtain quinone moieties,26 which enable electron transfer (Scheme 1). During the electron transfer between the GOx and electrode, formed quinones groups are reduced to hydroquinones at the direct electron transfer (DET) potential. This phenomenon confirms that electrochemically tailored TA can continuously transfer the electrons from the GOx to the electrode. Thus, an electrochemical biosensor, which possess fast electron transfer was fabricated. Additionally, PNIPAAm was coated on the prepared electrochemical biosensor, to impart the switchable interface to the biosensor based on hydrogen bonds, and the pH and thermosensitive surface was comprehensively explained in the section On−Off Switchable Bioelectrocatalysis with a Zipperlike Biointerface. Direct electrochemistry of GOx on rGO-Pt NPs has been investigated by CV. Figure 2A demonstrates the CV curves of GOx/GCE, GO-GOx/GCE, rGO-GOx/GCE, and rGO-Pt NPs-GOx/GCE in deaerated 0.1 M PBS. GOx/GCE (Figure 2A, line a) and GO-GOx/GCE (Figure 2A, line b) demonstrated no redox peak, indicating the insufficient and inappropriate GOx immobilization. On the other hand, the rGO-GOx/GCE (Figure 2A, line c) yielded a pair of redox peaks, which is associated with the electrochemical reaction of FAD. This finding can confirm that TA deposition and GO reduction allow the appropriate enzyme immobilization for the direct electron transfer. However, rGO-Pt NPs-GOx/GCE (Figure 2A, line d) gave a more enhanced and quasi-reversible redox peaks with the anodic peak potential (Epa) at −0.434 V and the cathodic peak potential (Epc) at −0.490 V. The peak to peak separation (ΔEp) is found to be 56 mV. The formal potential (E°) calculated from the average of cathodic and anodic peak potential is −0.462 V. The improved redox peak of rGO-Pt NPs-GOx/GCE can be associated with the enhanced surface area and increased electrical conductivity resulting from the Pt NPs deposition. This value is close to the standard electrode potential of −0.459 V (vs Ag/AgCl) for FAD/ FADH2,27 suggesting that the DET between GOx and the nanocomposite was accomplished. This finding confirms that as a protein binding agent, TA provided a short distance between GOx and rGO for the DET, and the Pt NPs reinforced the redox peaks. According to the literature, DET can be achieved by using adsorption based immobilization or covalent immobilization with the short cross-linking agent to reduce the active site-surface distance.27,28 Enzyme-free electrodes demonstrated no redox peak in the potential region of experiments (data not shown). The effect of pH on the DET of GOx on rGO-Pt NPs/GCE was studied at a scan rate of 100 mV s−1 using CV at GOx working range of pH 5.0−8.6, which is good agreement with

FAD + 2H+ + 2e− ⇌ FADH 2

(1)

Figure 2C illustrates the effect of scan rate on CV characteristics of the DET of rGO-Pt NPs-GOx/GCE. As the scan rate increased from 0.02 to 0.2 V s−1, both anodic (Ipa) and cathodic (Ipc) peak currents increased linearly with scan rate, indicating that the redox reaction is a surface controlled process. Nevertheless, the value of ΔEp was independent of the scan rates, and remained constant. The electron transfer rate (ks) between GOx and rGO-Pt NPs-GOx/GCE was calculated using the Laviron equation (nΔEp < 0.2 V): log ks = α log(1 − α) + (1 − α)log α − log[(RT /nFυ)] − α(1 − α)nF ΔEp/2.303RT

(2) −1

where α is the charge transfer coefficient (∼0.5) and υ (V s ) is the scan rate. The constants R, T, and F represent their usual meanings (R = 8.314 J K−1 mol−1, T = 298 K, F = 96,485 C mol−1). At the scan rate of 0.1 V s−1, the ks value for rGO-Pt NPs-GOx/GCE has been estimated to be 3.05 s−1. Although, there are studies, which possess superior rate constant such as, for GOx immobilized on rGO-ZnO (7.55 s−1)30 and for immobilized GOx on rGO-magnetic nanoparticles (13.78 s−1),31 the current study is better or comparable to the values observed for GOx immobilized on ERGO-MWCNTs (3.02 s−1),28 on rGO-Ag NPs nanoparticles (2.0 s−1),32 on graphene/ gold nanoparticles/poly neutral red (1.73 s−1),33 and on graphene and cobalt-phthalocyanine composite (3.57 s−1).34 When compared to poly neutral red and cobalt-phthalocyanine, it can concluded that TA is both appropriate modifying agent for enzyme immobilization, and a decent alternative electrically active linker, which makes the GO a decent conductive material with faster electron kinetics owing to the Pt NPs deposition. The surface amount (Γ, mol cm−2) of electroactive GOx immobilized on the surface of composite can be estimated according to the formula below35 Q = nFA Γ

(3)

where F is the Faraday constant, Q is the charge consumed in the redox reaction, which can be found from the integration of cathodic peak, Γ is the electroactive glucose oxidase amount, n and A stand for the number of transferred electrons, which corresponds to 2 in this reaction, and the area of the GC electrode (0.071 cm2), respectively. The amount of electroactive glucose oxidase at the rGO-Pt NPs-GOx/GCE was 3809


Research Article


Figure 3. (A) CV curves of rGO-Pt NPs-GOx/GCE in O2-saturated 0.1 M PBS at a scan rate of 100 mV s−1 in the presence of different concentrations of glucose (0, 1, 2, 4, 6, 8, 10, 12 mM). (B) Cathodic peak current versus glucose concentration plot. (inset) Linear part of the curve. (C) CV curves of PNIPAAm-rGO-Pt NPs-GOx/GCE in 5 mM Fe [CN]3−/4− and 0.1 M ABS at a scan rate of 100 mV s−1 in the presence of different concentration of glucose (0, 1, 2, 4, 6, 8, 10 mM). (D) Cathodic peak current versus glucose concentration plot for the on−off biosensor. (inset) Linear part of the curve.

Table 1. Comparison of the rGO-Pt NPs-GOx/GCE and Other Direct Electron Transfer Based Biosensors Reported in the Literature for Glucose Determination modified electrode

linear range (mM)

sensitivity (μA mM−1 cm−2)

LOD (μM)

Km (mM)

ref

rGO-Pt Np-GOx/GCE ERGO-MWCNT-GOx-Nf/GCEa GO-DDAB-GOx/GCEb rGO-CD-GOx/GCEc GNS-PEI-Au NPs-GOx/GCEd GO-CNT-GOx/GCE Cs-PGA-GOx/GCEe rGO-PPy-Au NPs-GOx/GCEf G-MWCNTs-AuNPs-GOx/GCE GN-Py-GOx/GCEg GN-PDA-GOx/Au Electrodeh rGO-MNPs/GCEi rGO-ZnOj

2−10 0.01−6.5 up to 0.6 0.05−3 0.001−0.1 2−8 0.5−5.5 0.2−1.2 0.005−0.175 up to 2 0.001−4.7 0.05−1 0.2−6.6

27.51 7.95 0.22 59.74 93 19.31 2.13 123.8 29.72 7.29 × 10−5 28.4 5.9 13.7

1.21 4.7 20 12 0.32

3.43

this study 28 37 38 6 27 39 40 41 42 43 31 30

1.5 1.78

0.12 4.8 50 0.1 0.1 0.2

2.09 6.77 0.16 2.2

a

ERGO electrochemically reduced graphene oxide; MWCNT multiwalled carbon nanotubes; Nf nafion. bDDAB didodecyldimethylammonium bromide. cCD β-cyclodextrin. dGNS-PEI graphene-polyethylenimine. eCs-PGA chitosan-poly(glutamic acid). fPPy polypyrrole. gGN-Py: graphite nanoparticle-pyrene. hGN-PDA graphene-polydopamine. iMNPs magnetic nanoparticles. jZnO zinc oxide.

estimated to be 2.72 ± 0.4 × 10−11 mol cm−2, which is comparable or larger than that observed for the saturated monolayer surface coverage of GOx on the electrode (1.7 × 10−12 mol.cm−2).36 The amount of immobilized GOx was significantly larger than that of monolayer amount and that of rGO-GOx/GCE (4.14 ± 0.2 × 10−12). This finding confirms that enhanced electroactive GOx amount was achieved with Pt NPs deposition,8 and the surface was efficiently employed in terms of GOx binding. Also, the literature confirms the efficient electroactive GOx immobilization and improved electro-

chemical signals with the preserved enzyme structure, upon the utilization of electroactive molecules and metal NPs resulting in expanded surface area and conductivity.6,34 In this context, Liu et al. confirmed that Au NPs deposited rGO facilitates the electron transfer on electrode surface and provides a large surface area for the immobilization of antibody. Therefore, metal nanoparticles deposited graphene derivatives have beneficial properties for the improved sensitivity in biosensor applications.4 3810


Research Article


Figure 4. (A) UV−vis spectra of rGO-Pt NPs-PNIPAAm at two different temperatures and pH values. (B) Nyquist plot of the bare GCE and PNIPAAm-rGO-Pt NPs-GOx at two different temperature values. (C) CV curves of PNIPAAm-rGO-Pt NPs-GOx at different temperature and pH values. (D) Dependence of amperometric currents vs number of cycles at −0.1 V switched between two different temperature values.

Performance of rGO-Pt NPs-GOx/GCE Based Glucose Biosensor. DPV and CV measurements were carried out in O2-saturated PBS (0.1 M, pH 7.4) using rGO-Pt NPs-GOx/ GCE in the absence and presence of 1, 2, 4, 6, 8, 10, 12 mM of glucose with a scan rate of 100 mV s−1. As shown in Figures 2D and 3A, the reduction peaks were diminished with the increasing glucose concentration. The addition of glucose solutions into O2-saturated PBS (0.1 M, pH 7.4) led to an increase in the Ipa intensity and a decrease in the Ipc intensity (Figure 3A). These findings confirmed the bioelectrocatalytic activity of rGO-Pt NPs-GOx/GCE toward glucose through DET mechanism as shown in eqs 4 and 5: GOD(FADH 2) + O2 → GOD(FAD) + H 2O2

reduction peak current intensity of the fabricated electrode decreases linearly at the range of 2−10 mM. Thus, a mediatorfree TGB was fabricated. The linear regression equation can be expressed as Ip (μA) = 74.289 ± 0.646 (μA) − 1.953 ± 0.101 (glucose concentration) (μA mM−1), R2 = 0.989. The linear measurement range observed in this biosensor is comparable or broader compared to those reported for biosensors constructed based on TGB given in Table 1. The calibration plot over the linear concentration range exhibited a sensitivity of 27.51 mA mM−1 cm−2, with a correlation coefficient of 0.9893. The limit of detection (LOD) was determined according to the formula, LOD = 3Sb/S (Sb standard deviation of ten blank measurements and S sensitivity) and was found to be as low as 1.21 μM (S/N = 3). The sensitivity, linear range, and detection limit of the developed biosensor were enhanced and are comparable with recent reports summarized in Table 1. When compared to the literature, decent sensitivity can be attributed to three main reasons. First is the substantial prevention of enzyme conformational changes upon immobilization owing to the biocompatible nature of TA and improved enzyme activity. Second is that Pt NPs can reduce the released H2O2 by synergistic effects on the reduction current for the FAD at the DET potential. The third reason is that metal nanoparticles containing composites can lead to electrochemical signal amplification.3 Therefore, more improved electrochemical response was observed with the Pt NPs deposition. Li et al. fabricated PtPd nanoparticles/reduced graphene oxide based glucose biosensor, and owing to the fact that operation potential was high compared to that of this biosensor and selectivity was low without enzyme, the effect of interfering agents was observed high. Also the obtained sensitivity value of the fabricated biosensor surpasses the value of the non-

(4)

GOD(FAD) + glucose → gluconolactone + GOD(FADH 2)

(5)

In the O2-saturated medium, nearly all GOx molecules are oxidized biocatalytically according to eq 4. Therefore, the electrochemically reducing current required for the oxidized GOx has the maximum value. Upon the addition of glucose, the reduction current for FAD of GOx diminished gradually. This is owing to the fact that with the addition of glucose, more GOx (FAD) was converted to GOx (FADH2) according to the biocatalytic reaction between GOx (FAD) and glucose (eq 5). Therefore, the cathodic current for GOx (FAD) decreased linearly with the increasing of glucose concentration a certain concentration range (Figure 3B). The fabricated biosensor was oxygen sensitive and the glucose measurements were carried out by considering the glucose concentration in merely O2 saturated PBS as 0. The inset of Figure 3B shows that the 3811


Research Article

ACS Sustainable Chemistry & Engineering enzymatic biosensor (1.47 μA mM−1 cm−2).44 When the concentration of glucose was higher than 12 mM, the cathodic peak currents deviated from linearity and leveled off, showing the characteristics of the Michaelis−Menten kinetic mechanism. The apparent Michaelis−Menten constant (Km) is an important parameter for explaining enzyme−substrate reaction kinetics which can be obtained by using the electrochemical version the Lineweaver−Burk equation as follows:45 1/Iss = 1/Imax + (K m/Imax)(1/Cglucose)

can repel the [Fe(CN)6]3−/4− redox couple by leading to an increase in the Rct values. In addition, more small Rct values observed at 40 °C compared to those at 20 °C, at pH 4 and 7 were attributed to the shrunken structure of the polymer with broken inter- and intrahydrogen bonds upon the phase transition above 32 °C, which allows the easy [Fe(CN)6]3−/4− transition.47 Mishra et al. also observed the highest redox current of Fc(COOH)2 on poly(N-isopropylacrylamide-codiethylaminoethylmethyl acrylate) containing a switchable biointerface at 40 °C and pH 5.48 It was concluded that the redox couple conductivity of the thermosensitive PNIPAAmrGO-Pt NPs-GOx depends mostly on the pH of the medium owing to the neutral-negative transition of the surface charge upon pH alteration, which alters the permeability of the [Fe (CN)6]3−/4− to the electrode surface. This finding was also confirmed with CV curves, FTIR, and UV−vis spectra of the nanocomposites. According to the CV curves of the PNIPAAm containing nanocomposites, it was revealed that pH and temperature responsive on−off biosensor could be formed using the TAPNIPAAm interactions and surface charge alteration. Figure 4C shows the CV curves of the polymer coated biosensors. According to the figure, the most enhanced redox peak of the [Fe(CN)6]3−/4− was observed at 40 °C and pH 4. This finding confirms that at higher temperature, the polymer takes the shrunken structure above the lower critical solution temperature (LCST) of 32 °C, which facilitate the transfer of the redox couple to the electrode surface by uncovering the surface with the hydrogen bonds cleavage upon conformational changes.48,49 Therefore, the highest electrochemical response was observed at pH 4 with temperature increment. However, at 20 °C and pH 4, the redox peaks of the redox couple decayed owing to the fact that TA can bind to the polymer via hydrogen bonds, by covering the electrode surface and reducing the peak currents. Nevertheless, nonionized hydroxyls of TA at pH 4 avoid the possible repellence between the redox couple and TA coated surface, and a pair of redox peaks were observed. In the case of pH 7 at 40 and 20 °C, the redox peaks totally disappeared because of the fact that TA is present in its negatively charged form at neutral pH, and a repellence can occur between the TA and negatively charged redox couple by hindering the electron transfer through to the electrode surface and the electrochemical signal disappeared.50 Therefore, 40 °C and pH 4 was the on state and 20 °C and pH 7 was the off state of the biosensor. Consequently, a multi-stimuli-responsive smart biointerface was fabricated, and this phenomenon can be beneficial for the development of versatile materials for electrochemical systems. Repeatability tests of polymer modified electrodes in the on and off states were demonstrated in Figure 4D. A reversible interaction between TA and PNIPAAm led to the decent repeatability of the biosensor. The glucose measurements using the on−off biosensor were carried out at 40 °C, and pH 4 (on state), as shown in Figure 3C. From the Figure 3D, the on−off biosensor exhibited a sensitivity of 43.06 mA mM−1 cm−2 with an R2 of 0.9893, LOD of 1.02 μM, and a linear measurement range of 2−10 mM. Parlak et al.51 have constructed a switchable biosensor using poly(4-vinylpyridine) conjugated graphene oxide for glucose sensing, and at pH 4, the linear measurement range was 0.01−6.0 mM and the sensitivity was 127.57 mA mM−1 cm−2. When compared to this study, a decent sensitivity and linear range were obtained with a facile and straightforward fabrication process, and the biosensor can

(6)

where Iss is the steady-state response current after the addition of substrate, Imax is the maximum current under saturated substrate conditions, and Cglucose is the bulk concentration of glucose. The enzymatic reaction velocity increases with the substrate concentration, and it deviates from linearity at higher concentration. Km is an indicator of enzyme catalyzed reaction kinetics, and the smaller Km value demonstrates that immobilized enzyme has a higher affinity to its substrate, and decent enzymatic activity is observed. Km value of the GOxrGO-Pt NPs/GCE was calculated as 3.43 mM, which was compared to those of other GOx immobilized biosensors in Table 1. The lower Km value confirms the preservation of GOx native structure in rGO-Pt NPs/GCE, resulting in higher affinity toward the glucose in enzymatic reaction. On−Off Switchable Bioelectrocatalysis with a Zipperlike Biointerface. Upon the deposition of PNIPAAm on the as-fabricated biosensor, the on−off biosensor was obtained, and the characterization and biosensing in the on state were carried out. UV−vis, and FTIR spectra of the obtained PNIPAAm containing bioconjugates demonstrated a pH sensitive behavior of the switchable interfaces. According to Figures 4A, S9, and S10A and B, the UV bands and FTIR peaks switched substantially with the pH alteration rather than temperature change. In Figure 4A, the UV−vis spectra of the PNIPAAmrGO-Pt NPs-GOx demonstrate clearly the TA-PNIPAAm interactions. At pH 4, π−π and n−π transitions of rGO revealed the blue-shifted behavior confirming the hydrophobic bonds promoted hydrogen bonding. However, these bands disappeared at pH 7, indicating the diminishing interactions between PNIPAAm and TA. Also, in Figure S10, FTIR measurements demonstrated the mainly pH responsive behavior of the PNIPAAm-rGO-Pt NPs-GOx.46 EIS can give information on the impedance changes of the electrode surface during the modification process. The Nyquist plots of the modified electrodes consist of a semicircle part at higher frequencies corresponding to the electron transfer limited process and a linear part at lower frequencies corresponding to the diffusion process. The electron transfer resistance (Rct) at the PNIPAAm-rGO-Pt NPs-GOx modified electrodes can be obtained from the diameter of the semicircle. Figure 4B displays the EIS curves of the electrodes. The Rct value of the sensitive PNIPAAm-rGO-Pt NPs-GOx at 40 °C and pH 4 (1290 Ω) is larger than that of the bare GCE (675 Ω) indicating that PNIPAAm containing nanocomposite reduced the conductivity. When the temperature was decreased to 20 °C at the same pH, the Rct value increased to 2875 Ω, indicating that conductivity of the biosensor surface was diminished with the decreasing temperature toward the redox couple of [Fe(CN)6]3−/4−. Upon pH increase to 7, Rct values were even more increased to 5060 and 9240 Ω at 40 and 20 °C, respectively. This can be explained by the fact that, at pH 7, ionization of hydroxyl groups of TA can occur much more compared to pH 4, and negatively charged TA on the surface 3812


ACS Sustainable Chemistry & Engineering switch the measurement response with medium pH and temperature.

CONCLUSION In conclusion, simultaneous GO and Pt4+ reduction were carried out in a facile and low-cost way by using a natural reducing agent, TA. We have synthesized a novel rGO-Pt NPs nanocomposite in a one-pot manner, which combined the catalysis of releasing H2O2 by Pt NPs, biocompatibility and modifying role of TA, excellent electron transfer properties of TA with the electrochemically formed quinone groups, and enhanced surface area of rGO. The obtained nanocomposite enabled the DET between FAD and modified electrode. By using DET, the electrochemical detection of glucose was carried out based on a TGB system, and a wide linear measurement range was achieved with high sensitivity and stability. Also, the fabricated biosensor yielded high selectivity as the operation potential was set to a low potential, with a decent accuracy for the glucose measurement in serum samples, which offers a new approach for a developing novel type of electrochemical biosensors. The chemically and electrochemically oxidized TA provided the electron transfer between GOx and electrode. It is confirmed that TA is a suitable natural product for green reduction processes, enzyme immobilization, and electron transfer in bioelectronics related areas. Also, by using the interactions between TA and PNIPAAm, a smart surface coating was carried out to fabricate an on−off biosensor. At 40 °C and pH 4, upon the hydrogen bond cleavage, and appropriate surface charge formation for redox couple diffusion, a switchable biosensor determined the glucose concentration with a decent sensitivity. In this study, we achieved a novel nanocomposite using the electrochemical behavior of TA, and we hope that this paper will be a pioneering work for the electrochemical systems that use polyphenols, in the future. A rapid, environmentally friendly, low-cost, synthesized rGO-Pt NPs nanocomposite is suitable for an electrochemical glucose biosensor, which can be extended to be an on−off biosensor, and other enzyme based TGBs.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Osikoya, A. O.; Parlak, O.; Murugan, N. A.; Dikio, E. D.; Moloto, H.; Uzun, L.; Turner, A. P.; Tiwari, A. Acetylene-sourced CVDsynthesised catalytically active graphene for electrochemical biosensing. Biosens. Bioelectron. 2017, 89, 496−504. (2) Wu, H.; Wang, J.; Kang, X.; Wang, C.; Wang, D.; Liu, J.; Aksay, I. A.; Lin, Y. Glucose biosensor based on immobilization of glucose oxidase in platinum nanoparticles/graphene/chitosan nanocomposite film. Talanta 2009, 80, 403−406. (3) Wang, N.; Lin, M.; Dai, H.; Ma, H. Functionalized gold nanoparticles/reduced graphene oxide nanocomposites for ultrasensitive electrochemical sensing of mercury ions based on thyminemercury-thymine structure. Biosens. Bioelectron. 2016, 79, 320−326. (4) Liu, N.; Ma, Z. Au-ionic liquid functionalized reduced graphene oxide immunosensing platform for simultaneous electrochemical detection of multiple analytes. Biosens. Bioelectron. 2014, 51, 184−190. (5) Jensen, U. B.; Ferapontova, E. E.; Sutherland, D. S. Quantifying protein adsorption and function at nanostructured materials: Enzymatic activity of glucose oxidase at GLAD structured electrodes. Langmuir 2012, 28, 11106−11114. (6) Rafighi, P.; Tavahodi, M.; Haghighi, B. Fabrication of a thirdgeneration glucose biosensor using graphene-polyethyleneimine-gold nanoparticles hybrid. Sens. Actuators, B 2016, 232, 454−461. (7) Zhong, H.; Yuan, R.; Chai, Y.; Li, W.; Zhong, X.; Zhang, Y. In situ chemo-synthesized multi-wall carbon nanotube-conductive polyaniline nanocomposites: Characterization and application for a glucose amperometric biosensor. Talanta 2011, 85, 104−111. (8) Lu, J.; Do, I.; Drzal, L. T.; Worden, R. M.; Lee, I. Nanometaldecorated exfoliated graphite nanoplatelet based glucose biosensors with high sensitivity and fast response. ACS Nano 2008, 2 (9), 1825− 1832. (9) Singh, C.; Ali, M. A.; Sumana, G. Green Synthesis of Graphene Based Biomaterial Using Fenugreek Seeds for Lipid Detection. ACS Sustainable Chem. Eng. 2016, 4, 871−880. (10) Lei, Y.; Tang, Z.; Liao, R.; Guo, B. Green Chemistry Hydrolysable tannin as environmentally friendly reducer and stabilizer for graphene oxide. Green Chem. 2011, 13, 1655−1658. (11) Altun, S.; Ç akiroğlu, B.; Ö zacar, M.; Ö zacar, M. A facile and effective immobilization of glucose oxidase on tannic acid modified CoFe2O4 magnetic nanoparticles. Colloids Surf., B 2015, 136, 963− 970. (12) Atacan, K.; Ç akiroğlu, B.; Ö zacar, M. Covalent Immobilization of Trypsin onto Modified Magnetite Nanoparticles and its Application for Casein Digestion. Int. J. Biol. Macromol. 2017, 97, 148−155. (13) Wu, D.; Gao, Y.; Li, W.; Zheng, X.; Chen, Y. G.; Wang, Q. Selective Adsorption of La3+ Using a Tough Alginate-Clay-Poly(nisopropylacrylamide) Hydrogel with Hierarchical Pores and Reversible Re-Deswelling/Swelling Cycles. ACS Sustainable Chem. Eng. 2016, 4, 6732−6743. (14) Takemoto, Y.; Ajiro, H.; Akashi, M. Hydrogen-Bonded Multilayer Films Based on Poly(N -vinylamide) Derivatives and Tannic Acid. Langmuir 2015, 31, 6863−6869. (15) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Layer-by-Layer Assembly of Ultrathin Composite Films from Micron-Sized Graphite Oxide Sheets and Polycations. Chem. Mater. 1999, 11, 771−778.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04164. Apparatus and reagents, TGA and zeta potential characterizations of biosensor, electrochemically active surface area of Pt NPs, repeatability, reproducibility, stability of biosensor, chronoamperometric measurements, interference study, and glucose analysis in serum (PDF)

■

■

This investigation has been supported by the Scientific Research Projects Commission of Sakarya University (Project number: 2017-02-04-024 and 2017-50-01-079). M.O. thanks the Turkish Academy of Sciences (TUBA) for partial support. We also thank to Dr. Soner Ç akar for TGA stud;, Prof. Dr. Necmettin Tozlu, Dr. Engin Şahin, Dr. Iḃ rahim Hakkı Karataş, and Mr. Süleyman Tekmen for TEM study; Dr. Atıf Koca and Miss. Duygu Akyüz for the chronoamperometric measurements; and Mr. Celal Caner for ICP study.

■

■

Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 264 295 60 41. Fax: +90 264 295 59 50. E-mail address: [email protected] (M.O.). ORCID

Mahmut Ö zacar: 0000-0002-1783-7275 Notes

The authors declare no competing financial interest. 3813


Research Article

ACS Sustainable Chemistry & Engineering (16) Bradford, M. M. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 1976, 72, 248−254. (17) Franssen, M. C. R.; Steunenberg, P.; Scott, E. L.; Zuilhof, H.; Sanders, J. P. M. Immobilised enzymes in biorenewables production. Chem. Soc. Rev. 2013, 42 (42), 6491−6533. (18) Ma, M.; Gu, J.; Yang, M.; Li, Z.; Lu, Z.; Zhang, Y.; Xing, P.; Li, S.; Chu, X.; Wang, Y.; et al. Controllable self-assemblies of sodium benzoate in different solvent environments. RSC Adv. 2015, 5, 70178− 70185. (19) Su, S.; Wang, J.; Vargas, E.; Wei, J.; Martínez-Zaguilán, R.; Sennoune, S. R.; Pantoya, M. L.; Wang, S.; Chaudhuri, J.; Qiu, J. Porphyrin Immobilized Nanographene Oxide for Enhanced and Targeted Photothermal Therapy of Brain Cancer. ACS Biomater. Sci. Eng. 2016, 2, 1357−1366. (20) Grosse, W.; Champavert, J.; Gambhir, S.; Wallace, G. G.; Moulton, S. E. Aqueous dispersions of reduced graphene oxide and multi wall carbon nanotubes for enhanced glucose oxidase bioelectrode performance. Carbon 2013, 61 (61), 467−475. (21) Liu, Y.; Yu, D.; Zeng, C.; Miao, Z.; Dai, L. Biocompatible graphene oxide-based glucose biosensors. Langmuir 2010, 26 (9), 6158−6160. (22) Atacan, K.; Ç akiroğlu, B.; Ö zacar, M. Efficient protein digestion using immobilized trypsin onto tannin modified Fe3O4 magnetic nanoparticles. Colloids Surf., B 2017, 156, 9−18. (23) Tadepalli, S.; Hamper, H.; Park, S. H.; Cao, S.; Naik, R. R.; Singamaneni, S. Adsorption Behavior of Silk Fibroin on Amphiphilic Graphene Oxide. ACS Biomater. Sci. Eng. 2016, 2, 1084−1092. (24) Shin, M.; Kim, K.; Shim, W.; Yang, J. W.; Lee, H. Tannic Acid as a Degradable Mucoadhesive Compound. ACS Biomater. Sci. Eng. 2016, 2, 687−696. (25) Xiong, K.; Qi, P.; Yang, Y.; Li, X.; Qiu, H.; Li, X.; Shen, R.; Tu, Q.; Yang, Z.; Huang, N. Facile immobilization of vascular endothelial growth factor on a tannic acid-functionalized plasma-polymerized allylamine coating rich in quinone groups. RSC Adv. 2016, 6, 17188− 17195. (26) Ç akar, S.; Güy, N.; Ö zacar, M.; Findik, F. Investigation of Vegetable Tannins and Their Iron Complex Dyes for Dye Sensitized Solar Cell Applications. Electrochim. Acta 2016, 209, 407−422. (27) Terse-Thakoor, T.; Komori, K.; Ramnani, P.; Lee, I.; Mulchandani, A. Electrochemically Functionalized Seamless ThreeDimensional Graphene-Carbon Nanotube Hybrid for Direct Electron Transfer of Glucose Oxidase and Bioelectrocatalysis. Langmuir 2015, 31, 13054−13061. (28) Mani, V.; Devadas, B.; Chen, S. M. Direct electrochemistry of glucose oxidase at electrochemically reduced graphene oxide-multiwalled carbon nanotubes hybrid material modified electrode for glucose biosensor. Biosens. Bioelectron. 2013, 41, 309−315. (29) Jose, M. V.; Marx, S.; Murata, H.; Koepsel, R. R.; Russell, A. J. Direct electron transfer in a mediator-free glucose oxidase-based carbon nanotube-coated biosensor. Carbon 2012, 50, 4010−4020. (30) Dey, R. S.; Raj, C. R. A hybrid functional nanoscaffold based on reduced graphene oxide−ZnO for the development of an amperometric biosensing platform. RSC Adv. 2013, 3, 25858−25864. (31) Pakapongpan, S.; Poo-arporn, R. P. Self-assembly of glucose oxidase on reduced graphene oxide-magnetic nanoparticles nanocomposite-based direct electrochemistry for reagentless glucose biosensor. Mater. Sci. Eng., C 2017, 76, 398−405. (32) Palanisamy, S.; Karuppiah, C.; Chen, S. M. Direct electrochemistry and electrocatalysis of glucose oxidase immobilized on reduced graphene oxide and silver nanoparticles nanocomposite modified electrode. Colloids Surf., B 2014, 114, 164−169. (33) Mazar, F. M.; Alijanianzadeh, M.; Molaeirad, A.; Heydari, P. Development of Novel Glucose oxidase Immobilization on Graphene/ Gold nanoparticles/Poly Neutral red modified electrode. Process Biochem. 2017, 56, 71−80. (34) Mani, V.; Devasenathipathy, R.; Chen, S. M.; Huang, S. T.; Vasantha, V. S. Immobilization of glucose oxidase on graphene and

cobalt phthalocyanine composite and its application for the determination of glucose. Enzyme Microb. Technol. 2014, 66, 60−66. (35) Bard, A. J.; Faulkner, L. R. Electrochemical methods; Wiley, 2000. (36) Liu, Y.; Zhang, X.; He, D.; Ma, F.; Fu, Q.; Hu, Y. An amperometric glucose biosensor based on a MnO2/graphene composite modified electrode. RSC Adv. 2016, 6, 18654−18661. (37) Castrignano, S.; Valetti, F.; Gilardi, G.; Sadeghi, S. J. Graphene oxide-mediated electrochemistry of glucose oxidase on glassy carbon electrodes. Biotechnol. Appl. Biochem. 2016, 63, 157−162. (38) Palanisamy, S.; Devasenathipathy, R.; Chen, S. M.; Ajmal Ali, M.; Karuppiah, C.; Balakumar, V.; Prakash, P.; Elshikh, M. S.; AlHemaid, F. M. A. Direct electrochemistry of glucose oxidase at reduced graphene oxide and beta-cyclodextrin composite modified electrode and application for glucose biosensing. Electroanalysis 2015, 27, 2412−2420. (39) Zhou, X.; Tan, B.; Zheng, X.; Kong, D.; Li, Q. Interfacial electron transfer of glucose oxidase on poly(glutamic acid)-modified glassy carbon electrode and glucose sensing. Anal. Biochem. 2015, 489, 9−16. (40) Xue, K.; Zhou, S.; Shi, H.; Feng, X.; Xin, H.; Song, W. A novel amperometric glucose biosensor based on ternary gold nanoparticles/ polypyrrole/reduced graphene oxide nanocomposite. Sens. Actuators, B 2014, 203, 412−416. (41) Yu, Y.; Chen, Z.; He, S.; Zhang, B.; Li, X.; Yao, M. Direct electron transfer of glucose oxidase and biosensing for glucose based on PDDA-capped gold nanoparticle modified graphene/multi-walled carbon nanotubes electrode. Biosens. Bioelectron. 2014, 52, 147−152. (42) Piao, Y.; Han, D. J.; Seo, T. S. Highly conductive graphite nanoparticle based enzyme biosensor for electrochemical glucose detection. Sens. Actuators, B 2014, 194, 454−459. (43) Ruan, C.; Shi, W.; Jiang, H.; Sun, Y.; Liu, X.; Zhang, X.; Sun, Z.; Dai, L.; Ge, D. One-pot preparation of glucose biosensor based on polydopamine-graphene composite film modified enzyme electrode. Sens. Actuators, B 2013, 177, 826−832. (44) Li, M.; Bo, X.; Zhang, Y.; Han, C.; Guo, L. One-pot ionic liquidassisted synthesis of highly dispersed PtPd nanoparticles/reduced graphene oxide composites for nonenzymatic glucose detection. Biosens. Bioelectron. 2014, 56, 223−230. (45) Li, J.; Tan, S. N.; Ge, H. Silica sol-gel immobilized amperometric biosensor for hydrogen peroxide. Anal. Chim. Acta 1996, 335, 137− 145. (46) Chen, G.; Niu, C. H.; Zhou, M. Y.; Ju, X. J.; Xie, R.; Chu, L. Y. Phase transition behaviors of poly(N-isopropylacrylamide) microgels induced by tannic acid. J. Colloid Interface Sci. 2010, 343, 168−175. (47) Zhang, X.; Wang, Y.; Zhao, J.; Xiao, M.; Zhang, W.; Lu, C. Mechanically Strong and Thermally Responsive Cellulose Nanofibers/ Poly(N-isopropylacrylamide) Composite Aerogels. ACS Sustainable Chem. Eng. 2016, 4, 4321−4327. (48) Mishra, S.; Ashaduzzaman, M.; Mishra, P.; Swart, H. C.; Turner, A. P. F.; Tiwari, A. Stimuli-enabled zipper-like graphene interface for auto-switchable bioelectronics. Biosens. Bioelectron. 2017, 89, 305−311. (49) Parlak, O.; Turner, A. P. F.; Tiwari, A. On/off-switchable zipperlike bioelectronics on a graphene interface. Adv. Mater. 2014, 26, 482− 486. (50) Parlak, O.; Turner, A. P. F. Switchable bioelectronics. Biosens. Bioelectron. 2016, 76, 251−265. (51) Parlak, O.; Turner, A. P. F.; Tiwari, A. pH-induced on/offswitchable graphene bioelectronics. J. Mater. Chem. B 2015, 3 (37), 7434−7439.

3814


Tannic Acid-Reduced Graphene Oxide Deposited with Pt

Recommend Documents