Article pubs.acs.org/IECR
Flexible Electrospun PVdF-HFP/Ni/Co Membranes for Efficient and Highly Selective Enzyme Free Glucose Detection Nangan Senthilkumar,†,⊥ Kaliyamoorthy Justice Babu,†,⊥ Georgepeter Gnana kumar,*,† Ae Rhan Kim,‡ and Dong Jin Yoo*,§ †
Department of Physical Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India Department of Chemistry, Chonbuk National University, Jeonju 561-756, Republic of Korea § Department of Energy Storage/Conversion Engineering, R&D Education Center for Specialized Graduate School of Hydrogen and Fuel Cells Engineering, and Hydrogen and Fuel Cell Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea ‡
ABSTRACT: Highly selective and efficient nonenzymatic electrochemical glucose sensors were fabricated by using electrospun polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) composite nanofiber membranes. The homogeneous, smooth and strongly interconnected nanofibers were identified for the bare PVdF-HFP membrane and beads in the PVdF-HFP nanofibers were observed for the PVdF-HFP/Ni and PVdF-HFP/Ni/Co nanofiber membranes. The face-centered cubic structure of Ni nanoparticles doped in the PVdF-HFP nanofibers has not been altered, even after the alloy formation with Co. The electrochemical oxidation of glucose in an alkaline medium was chosen as a probe for the detection of glucose and was achieved through the fabricated nanofiber membranes. Among the fabricated nanofiber membranes, the PVdF-HFP/Ni/Co membrane exhibited an excellent sensing behavior toward glucose with the low limit of detection and linear range of 0.26 μM and 1 μM to 7 mM, respectively. Furthermore, the fabricated nanofiber membrane exhibited good selectivity, high stability and reproducibility, which promises its applications in nonenzymatic glucose sensors.
1. INTRODUCTION The disposable, portable, cost-effective, highly efficient and selective detection of glucose has become essential in pharmaceuticals, environmental monitoring, textiles and food industries, owing to its endangerment in increasing cholesterol content, weakening eye sight, food allergies and diabetes.1,2 Hence, a number of methods including optical,3 thermometric,4 fluorescent5 and electrochemical sensors6 have been developed for the efficient detection of glucose. Among the above, the electrochemical method has been considered as the foremost fascinating technique due to its unique features such as simplicity, reliability, high sensitivity, affordable cost, lower limit of detection (LOD) and compatibility for miniaturization.6 In comparison with enzymatic glucose sensors, nonenzymatic glucose sensors have received considerable attention owing to the inclination of enzymes utilization, stress-free electrode fabrication, stability, simplicity, reproducibility and low cost.7 The overall development of electrochemical nonenzymatic glucose sensors is purely dependent upon the cardinal part of the electrochemical system, i.e., the electrode. Hence, a variety of materials such as glassy carbon (GC),8 indium tin oxide (ITO),9 fluorine doped tin oxide (FTO),10 gold,11 platinum12 and carbon cloth13 were utilized as electrodes in electrochemical sensors.7−13 However, the aforementioned bare electrodes experienced high over potential, less electron transference and lower sensitivity in the presence of other interfering species, which are not feasible for the development of nonenzymatic sensors. It urges the modification of electrodes with the conductive materials, in particular, catalytic nanoparticles.14 Hence, Pt,15 Au16 and Ag17 nanoparticles were extensively exploited for the modification of electrode materials © 2014 American Chemical Society
applicable for nonenzymatic glucose sensors. However, the precious metal nanoparticles based glucose sensors exhibited certain constraints including high cost, low sensitivity, narrow linear range and poor selectivity due to the surface etching or surface poisoning processes,15−18 which directed the research efforts toward nonprecious metal nanoparticles.19,20 Although the improved performances were reported for the nonprecious metals modified electrodes, the tedious steps involved in the electrode modification process such as electrode cleaning, proper binder selection, slurry preparation, catalyst loading, etc. obstructed the large scale applications of electrochemical glucose sensors.19,20 In addition, limitations such as less sensitivity, high over potential and poor stability due to the peel of casting material have also been observed. Therefore, the development of a low-price, highly efficient and interference free sensor for nonenzymatic glucose detection is essential. If the modification of electrode materials could be achieved during the preparation of the electrode material itself, high over potential and other limitations of conventional processes could be effectively tackled and the fabrication cost of electrodes could also be limited. However, extra care has to be devoted to the fabrication of electrode materials on the basis of significant requirements such as anticorrosion, free from poisoning, controlled aggregation of metal ions and gas molecules on electrode surface, etc. Recently, polymeric membranes, in particular, electrospun Received: Revised: Accepted: Published: 10347
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their corresponding Ni(0) and Co(0) nanoparticles, the prepared membranes were activated with 0.1 M NaBH4 for 30 min. Then the membrane was repetitively washed with deionized water to remove the excess amount of adsorbed reducing agent, if any, and dried for 6 h at 100 °C. The Ni2+ and Ni2+/Co2+ ions reduced in the PVdF-HFP fibers are represented as PVdF-HFP/Ni and PVdF-HFP/Ni/Co membranes. 2.4. Characterizations. Surface morphological properties of the prepared nanofiber membranes were studied with scanning electron microscopy (SEM) using a JSM5410LV instrument (JEOL, Tokyo, Japan) coupled with an energy dispersive X-ray spectroscopy (EDAX) analyzer. Crystallinity and structural information on the nanofiber membranes were obtained by using X-ray diffraction (XRD, Rigaku). The surface chemical composition and bonding of the prepared of PVdFHFP/Ni/Co membrane was ascertained by using X-ray photoelectron spectroscopy (XPS, Model-Kratos Amicus). 2.5. Electrochemical Characterizations. A PVdF-HFP, PVdF-HFP/Ni or PVdF-HFP/Ni/Co nanofiber membrane was carefully cut into a 1.5 × 1.5 cm sized piece and dried for 30 min to remove the atmospheric moisture content. Six centimeters of copper wire was connected with the membrane, and the membrane was used as a working electrode without further modification. The cyclic voltammetry and amperometric experiments were carried out by using a CHI-650D analytical system at room temperature. The single compartment, three-electrode cell assembly containing a working electrode (PVdF-HFP/PVdFHFP/Ni/PVdF-HFP/Ni/Co membrane), a Ag/AgCl reference electrode and a Pt wire counter electrode was placed in 0.1 M NaOH solution. The cyclic voltammograms of the studied membranes were recorded in 0.1 M NaOH at a scan rate of 20 mV/s in the presence and absence of 5 mM glucose. The amperometric experiments were carried out in 0.1 M NaOH solution with the successive addition of different concentrations of glucose at an applied potential of 0.5 V (vs Ag/AgCl).
nanofiber membranes, have been developed as the rapid subject of enrollment due to the production of novel surfaces with nanoscale fiber diameters with high surface-to-volume ratios, excellent porosity, high interconnectivity, flexibility, etc.21−23 In addition, the high porosity of the nanofibers allows the entrapment of analytes with the minimal diffusion resistance, which may increase the response to electroactive species.24 Although a number of polymers such as poly(vinylidenefluoride), 2 5 poly(vinyl alcohol), 2 6 poly(vinylpyrrolidone),27 poly(vinyl acetate),28 poly(methyl methacrylate),29 etc. were exploited for the preparation of nanofiber membranes, polyvinylidenefluoride-co-hexafluoropropylene (PVdF-HFP) is preferable, owing to its unique features such as piezo and pyroelectric properties, high dielectric constant, thermal stability, semicrystalline nature, excellent membrane forming capability, prompt hydrophobicity, etc.30,31 By combining the specific methods of interest such as electrospun nanocomposite fiber membrane and nonenzymatic sensors, effective results toward the unique structural composition and excellent sensing performances could be obtained. This report is aimed at the modification of PVdF-HFP nanofibers with the Ni and Co nanostructures and explores the influence of aforementioned nanoparticles in nonenzymatic glucose sensor applications.
2. EXPERIMENTAL METHODS 2.1. Materials. PVdF-HFP (MW = 400 000g/mol), N,Ndimethylformamide (DMF), nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O), cobalt(II) acetate tetrahydrate (Co(CH3COO)2·4H2O), sodium borohydride (NaBH4), glucose, acetaminophen, uric acid, urea, ascorbic acid and dopamine were obtained from Sigma-Aldrich and used without any further purification. 2.2. Preparation of Polymeric Solution. 2.2.1. Preparation of PVdF-HFP Solution. For the preparation of 10 wt % PVdF-HFP solution, an appropriate amount of PVdF-HFP was dissolved in a DMF/acetone (7:3 v/v %) mixture and the solution was magnetically stirred for 1 h at room temperature. 2.2.2. Preparation of PVdF-HFP/Nickel Acetate or Nickel Acetate/Cobalt Acetate Solution. To the 10 wt % of PVdFHFP solution, 20 wt % of nickel acetate or nickel acetate− cobalt acetate mixture in DMF/acetone was gradually added, and the solution was magnetically stirred for 1 h at room temperature. 2.3. Fabrication of Electrospun Nanofiber Membranes. The electrospinning setup consists of a high-voltage supplier, a capillary tube with a stainless steel needle, a syringe pump and a metal collector. A high electric field was generated between the metallic needle and collector through a highvoltage supplier. The produced voltage difference overcame the surface tension of a polymeric solution, which allowed the formation of interconnected fibers along with the continuous evaporation of a solvent under the ambient atmosphere. The prepared polymeric solution (PVdF-HFP or PVdFHFP/Ni or PVdF-HFP/Ni/Co) was transferred into a glass syringe with a 0.4 mm diameter spinneret that was attached to a syringe pump. The nozzle to the grounded state was fixed at 15 cm and a voltage of 20 kV was applied to the tip of the nozzle. The flow rate of a polymeric solution in the syringe pump was adjusted to be 0.3 mL/h. The polymeric fibers were collected over the rotating drum collector with a rotating speed of 1500 rpm. The collected electrospun membrane was dried at 100 °C for 12 h. For the reduction of Ni2+ and Co2+ metallic ions into
3. RESULTS AND DISCUSSION 3.1. XRD Analysis. The XRD patterns observed for the prepared electrospun nanofiber membranes are given in Figure 1. The bare PVdF-HFP membrane exhibited significant
Figure 1. XRD patterns of (a) PVdF-HFP, (b) PVdF-HFP/Ni and (c) PVdF-HFP/Ni/Co nanofiber membranes. 10348
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PVdF-HFP was also confirmed by the formation of beadlike structures (Figure 2c,d). The average diameters of fibers in PVdF-HFP/Ni/Co and PVdF-HFP/Ni were found to be 77 and 115 nm, respectively, which are smaller than that of bare PVdF-HFP (190 nm) nanofibers. The incorporation of Ni and Co metals increased the electrical conductivity of a polymeric solution, which caused a higher elongation of a jet along its axis, resulting in smaller sized nanofibers. It is also clear that the PVdF-HFP/Ni/Co nanofiber membrane exhibited the smaller sized nanofibers over the PVdF-HFP/Ni membrane, owing to the high electrical conductivity of Ni/Co alloy structure. The nanocatalysts doped within the PVdF-HFP nanofibers inclined the particle size increment even under the contact of an aqueous analyte, which may ensure the fast and gradual electron transport. For the confirmation of elements present in the prepared nanostructures and nanocomposites, EDAX analysis was carried out and given in Figure 3. The pure PVdF-HFP membrane exhibited carbon and fluorine elements (Figure 3a), which are the constituents of a host polymer. For the PVdF-HFP/Ni nanofiber membrane, carbon, fluorine and nickel elements were observed (Figure 3b) and the EDAX pattern of PVdF-HFP/ Ni/Co membrane exhibited the carbon, fluorine, nickel and cobalt elements, ensuring the presence of Ni and Co in the PVdF-HFP/Ni/Co membrane (Figure 3c). 3.3. XPS Studies. For further confirmation of surface composition and chemical states of the individual species in the PVdF-HFP/Ni/Co membrane, the X-ray photoelectron spectrum was recorded and is given in Figure 4a−c. Fluorine and carbon are the major constituents of the PVdF-HFP host matrix and are ensured by the peaks found at 688 and 282−288 eV, respectively. The C 1s core-level spectrum of PVdF-HFP is further resolved into two peak components with the binding energies of 282 and 288 eV, representing the CH2 and CF2 units, respectively.30 (Figure 4a). The zero valence state of Co was ensured from the peaks obtained at 778.03 and 794.42 eV, resulting from the 2p electron multiple-splitting of 2p3/2 and 2p1/2, respectively (Figure 4b). The spin−orbit splitting of 2p3/2 and 2p1/2 of Ni 2p are found at 852.89 and 870.72 eV, respectively, ensuring the Ni(0) state in PVdF-HFP nanofibers (Figure 4c).37 In comparison with the pure Ni and Co bulk metals, the binding energies of Ni and Co in the Ni/Co structure were slightly shifted, indicating the Ni/Co alloy formation. 3.4. Electrochemical Behavior of PVdF-HFP, PVdFHFP/Ni and PVdF-HFP/Ni/Co Membranes. Prior to the implementation of prepared membranes in glucose nonenzymatic sensors, the electrochemical behavior of PVdFHFP, PVdF-HFP/Ni and PVdF-HFP/Ni/Co membranes in NaOH was investigated by using cyclic voltammetry. Figure 5 represents the cyclic voltammograms of PVdF-HFP, PVdFHFP/Ni and PVdF-HFP/Ni/Co membranes in 0.1 M NaOH at a scan rate of 20 mV/s. No obvious redox peaks were observed for the bare PVdF-HFP membrane in a −0.2 to +0.7 V potential window. However, the PVdF-HFP/Ni membrane exhibited a pair of well-defined redox peaks, indicating the catalytic activity of doped Ni nanoparticles in PVdF-HFP nanofibers. Under the alkaline conditions, the metal Ni(0) and Ni(II) were oxidized into Ni(OH)2 and NiOOH,38 respectively and the involved electrochemical oxidation mechanisms of Ni2+/Ni3+ are proposed as follows
characteristic peaks at 18.16, 20.3, 26.6 and 36°, which are ascribed to the (1 0 0), (0 2 0), (1 1 0) and (0 2 1) reflection planes, respectively (Figure 1a).32,33 In addition to the aforementioned significant peaks of PVdF-HFP, a few additional peaks were observed for the PVdF-HFP/Ni membrane at 44.51, 51.85 and 76.37° and are assigned to the (1 1 1), (2 0 0) and (2 2 0) reflection planes of the Ni nanoparticles, respectively (Figure 1b).34,35 The obtained reflection planes ensured the face-centered cubic (fcc) structure of Ni nanoparticles (JCPDS card No. 04-0850).36 The observed characteristic reflections of the Ni/Co alloy in the PVdF-HFP/Ni/Co membrane are very similar to the Ni nanoparticles in the PVdFHFP/Ni membrane (Figure 1c). Because the lattice parameters of Ni (3.5238 Å) and Co (3.5447 Å) closely resemble each other,36 any noticeable differences in the characteristic XRD patterns of Ni monometallic and Ni/Co bimetallic nanoparticles were not observed. 3.2. Surface Morphology of the Membranes. Figure 2 depicts the surface morphology of prepared PVdF-HFP, PVdF-
Figure 2. SEM images of (a) PVdF-HFP, (b) PVdF-HFP/Ni and (c) and (d) PVdF-HFP/Ni/Co nanofiber membranes.
HFP/Ni and PVdF-HFP/Ni/Co membranes. The PVdF-HFP electrospun membrane exhibited the highly interconnected, homogeneously distributed and smooth surfaced nanofibers with an average diameter of 190 nm (Figure 2a). The generated unique porous structures are highly useful for entrapping and retaining the liquid analyte. In general, metal nanoparticles are incorporated in the polymer nanofiber matrix through ex situ and in situ methods. In spite of the specific advantages of the ex situ approach, the constrains including nonhomogeneous distribution of nanoparticles in the polymeric nanofiber matrix, tedious processes in tuning the morphology and size, agglomeration and size increment of nanoparticles under the contact of an aqueous environment faded its extensive applications. To tackle the aforementioned significant issues, in situ generation of Ni and Ni/Co nanostructures in the PVdF-HFP nanofiber matrix was developed and reported in this study. The incorporation of nickel(II) acetate with the PVdF-HFP polymer resulted in the Ni2+ ions, which is further reduced into Ni(0) by using an effective reducing agent, NaBH4. The formation of Ni(0) nanoparticles in PVdF-HFP nanofibers was ensured from the presence of beadlike structures (Figure 2b). Similar to the above, Ni2+ and Co2+ ions coexisting in the PVdF-HFP nanofibers by the incorporation of nickel(II) acetate and cobalt(II) acetate mixture were successfully reduced by a simple chemical reduction method, leading to the doped metal nanostructures in nanofibers. The existence of Ni/Co alloy structures over
Ni(0) + 2OH− → Ni(OH)2 + 2e− 10349
(1)
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Figure 3. EDAX spectra of (a) PVdF-HFP, (b) PVdF-HFP/Ni and (c) PVdF-HFP/Ni/Co nanofiber membranes.
Ni(OH)2 + OH− → NiOOH + H 2O + e−
Figure 6 depicts the cyclic voltammograms of the PVdFHFP/Ni/Co membrane in 0.1 M NaOH as a function of scan rate, and the redox peak currents were linearly increased with an increase in scan rate ranging from 20 to 200 mV/s. The obtained peak currents were linearly proportional to the square root of scan rate with a high correlation coefficient (R) of 0.993 for both Ipa and Ipc, indicating the diffusion controlled electrochemical kinetics with a fast electron transfer rate. 3.5. Electro-Oxidation of Glucose at PVdF-HFP, PVdFHFP/Ni and PVdF-HFP/Ni/Co Membranes. The electrooxidation of glucose at bare PVdF-HFP, PVdF-HFP/Ni and PVdF-HFP/Ni/Co membranes was measured by using cyclic voltammetry in 5 mM glucose and 0.1 M NaOH at a scan rate of 20 mV/s (Figure 7). The bare PVdF-HFP membrane has not exhibited any obvious redox peaks, indicating no contribution to the direct nonenzymatic glucose detection. The existence of Ni and Ni/Co nanostructures in the PVdFHFP nanofibers enhanced the electron transfer efficiency of nanofibers through the excellent contact between the polymer matrix and catalytic nanostructures, allowing its utilization as a single component working electrode system. The PVdF-HFP/ Ni membrane exhibited a significant oxidation peak at 0.48 V, which is attributed to the electro-oxidation of glucose with the participation of Ni(III). In comparison with the cyclic voltammetric responses obtained under the absence of glucose solution, the PVdF-HFP/Ni membrane exhibited an increased oxidation peak current with a slightly decreased cathodic current in the presence of glucose solution, indicating the irreversible electrochemical oxidation process. The oxidation of glucose (C6H12O6) is catalyzed by the high valent oxy hydroxide species into glucolactanone (C6H10O6)41 as follows
(2)
The PVdF-HFP/Ni membrane displayed an anodic peak potential at 0.45 V, owing to the electrocatalytic activity of Ni and it has been reported that Co has exhibited an anodic peak potential of NaOH at less than 0.3 V.39 Meanwhile, the prepared PVdF-HFP/Ni/Co membrane exhibited an anodic peak potential at 0.34 V, which is intermediate between Ni and Co anodic peak potentials. Hence, it is clear that the anodic peak observed at 0.35 V for PVdF-HFP/Ni/Co membrane is attributed to the transformation of different complex oxidation species of Ni/Co in an alkaline medium, i.e., Ni(OH)2, NiOOH, Co(OH) 2 and CoOOH. The electrochemical oxidation process of the PVdF-HFP/Ni/Co membrane is proposed as follows Ni(0) + 2OH− → Ni(OH)2 + 2e−
(3)
Ni(OH)2 + OH− → NiOOH + H 2O + e−
(4)
Co(0) + 2OH− → Co(OH)2 + 2e−
(5)
Co(OH)2 + OH− → CoOOH + H 2O + e−
(6)
At higher potentials, the CoOOH was further oxidized into CoO240 as follows CoOOH + OH− → CoO2 + H 2O + e−
(7)
The obtained prompt electrochemical responses of prepared composite membranes toward NaOH are attributed to the high chemical stability and electrocatalytic activity of PVdF-HFP and Ni or Ni/Co nanoparticles, respectively. 10350
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Figure 4. XPS spectrum of (a) PVdF-HFP/Ni/Co nanofiber membrane and maginified spectra of (b) Co 2p and (c) Ni 2p.
Figure 6. Cyclic voltammograms of PVdF-HFP/Ni/Co membrane in 0.1 M NaOH as a function of scan rate ranging from 20 to 200 mV/s (inset: calibration plot of current vs square root of scan rate).
Figure 5. Cyclic voltammograms of studied membranes in 0.1 M NaOH at a scan rate of 20 mV/s.
NiOOH + C6H12O6 → Ni(OH)2 + C6H10O6
The PVdF/HFP/Ni/Co nanofiber membrane displayed the glucose oxidation peak potential at 0.43 V with a high electrocatalytic current of 30 μA, which is 2-fold higher than
(8)
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the monometallic Ni counterpart, and thereby, the PVdF-HFP/ Ni/Co membrane exhibited a maximum glucose oxidation/ detection among the studied membranes. The maximum electro-oxidation of glucose was observed at a higher pH level of 11, suggesting a significant role of OH− ions in the oxidation of glucose at PVdF-HFP/Ni or PVdF-HFP/Ni/Co nanofiber membranes. The formation of a higher oxidized species such as NiOOH and CoOOH was facilitated at a higher pH level, which significantly influenced the glucose oxidation reaction. In addition, hydrogen bridges formed between the hydroxyl groups of the NiOOH/CoOOH bimetallic catalyst and hydroxyl groups of an analyte44 influenced the maximum adsorption and followed by the maximum electro-oxidation of glucose. The mutarotation of glucose is greatly enhanced at higher pH levels, maintaining the normal equilibrium percentages in the solution, which favored the higher oxidation of both forms at identical rates.45 Figure 9 exhibits the cyclic voltammograms of PVdF-HFP/ Ni/Co in 0.1 M NaOH with the various concentrations of
Figure 7. Cyclic voltammograms of studied membranes in the presence of 5 mM glucose in 0.1 M NaOH at a scan rate of 20 mV/s.
that of the PVdF/HFP/Ni membrane. The resultant NiOOH and CoO2 species of PVdF/HFP/Ni/Co under alkaline conditions effectively oxidized the glucose into glucolactaone, which resulted in the regeneration of Ni(OH)2 and CoOOH and the proposed mechanism for the electrocatalytic oxidation of glucose is given in Figure 8. 2NiOOH + 2CoO2 + C6H12O6 → 2Ni(OH)2 + 2CoOOH + C6H10O6 + H 2O
(9)
Figure 9. Cyclic voltammograms of PVdF-HFP/Ni/Co membrane as a function of glucose concentration in 0.1 M NaOH at a scan rate of 20 mV/s.
glucose. An increase in the concentration of glucose increased the oxidation peak currents, specifying the increment of glucose oxidation process at the solid−solution interface. The elecctrocatalytic peak currents were increased with increasing glucose concentration, indicating that the PVdF-HFP/Ni/Co membrane has exhibited better electrocatalytic activity toward the glucose oxidation. For a better understanding of the electrocatalytic activities of the prepared PVdF-HFP/Ni/Co membrane toward nonenzymatic electrochemical glucose sensor applications, the current response of the PVdF-HFP/Ni/Co membrane in 5 mM glucose as a function of scan rate was measured and is given in Figure 10. The anodic response ascribed to the glucose electrooxidation was increased and positively shifted with an increase in sweep rate. Furthermore, the square root of scan rates and oxidation peak currents displayed good linearity with a high correlation coefficient (R) of 0.997, indicating that the involved electrochemical reaction is a diffusion controlled process. It is also observed that the glucose oxidation peak potential of the PVdF-HFP/Ni/Co membrane was positively shifted, specifying a kinetic limitation in the reaction between the redox sites of Ni/Co in the PVdF-HFP/Ni/Co membrane and glucose. In
Figure 8. Proposed mechanism for the electrocatalytic oxidation of glucose.
The strongly interconnected nanofibers with the high porosity provided a high specific surface area to the prepared electrospun nanofiber membranes. The specific surface area and number of active sites of PVdF-HFP nanofibers is enhanced by the incorporation of Ni nanoparticles and is further enhanced by the Ni/Co bimetal alloy in the fibers. The high porosity of electrospun membrane reduced the diffusion resistance of an analyte, facilitating the rapid diffusion and adsorption/ desorption of an analyte.42,43 The rapid diffusion has increased the intimate contact between the analyte and electroactive species, which favored the electroresponse of prepared composite fibers toward glucose oxidation. The synergetic interaction between Ni and Co in the alloy structure leads to the number of active sites and high electrical conductivity over 10352
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Figure 10. Cyclic voltammograms of PVdF-HFP/Ni/Co membrane in the presence of 5 mM glucose as a function of scan rate ranging from 10 to 100 mV/s in 0.1 M NaOH (inset: corresponding calibration plot of current vs square root of scan rate).
Figure 12. Amperometric current−time responses of PVdF-HFP/Ni/ Co membrane with successive addition of different concentrations of glucose to 0.1 M NaOH at an applied potential of 0.5 V (inset: the amperometric responses of PVdF-HFP/Ni/Co membrane toward a 1 to 50 μM concentration of glucose solution).
addition, a linear relationship was obtained between the oxidation peak potential (Epa) and natural logarithm of the scan rate (ln ν) (Figure 11) with the linear regression equation
membrane reached the dynamic equilibrium state and generated a steady state current within 10 s. It indicates the good electrocatalytic oxidation and fast electron transfer properties of the PVdF-HFP/Ni/Co nanofiber membrane. The calibration curves for the electrochemical responses of the PVdF-HFP/Ni/Co nanofiber membrane toward glucose at 0.5 V (vs Ag/AgCl) for the concentrations ranging from 1 μM to 7 mM is shown in Figure 13. A wide linear range from 1 μM to 7
Figure 11. Anodic peak potential (Epa) dependence on logarithm of scan rate (log) for the oxidation of glucose at PVdF-HFP/Ni/Co.
of Epa (V) = 0.089 log ν + 0.327 (R = 0.990) in the range from 10 to 100 mV/s, indicating the electroxidation of glucose is an irreversible electrode process. According to Laviron theory, the slope of the plot of Epa versus logarithm of the scan rate for the anodic reaction is 2.3RT/(1 − α)nF and the charge transfer coefficient (α) is found to be 0.33. 3.6. Amperometric Determination of Glucose at PVdF-HFP-Ni/Co Membrane. Amperometry is a sensitive and reliable technique to evaluate the electroactivities of catalysts applicable for electrochemical glucose sensors. The amperometric responses obtained for the PVdF-HFP/Ni/Co membrane in 0.1 M NaOH solution for the successive addition of different concentrations of glucose at an applied potential of 0.5 V (vs Ag/AgCl) are given in Figure 12. The PVdF-HFP/ Ni/Co nanofiber membrane exhibited a sensitive response toward the different concentrations of glucose and an obvious increase in the current was noted for the increased glucose concentration. From Figure 12, it is clear that the electrochemical response of the PVdF-HFP/Ni/Co nanofiber
Figure 13. Calibration plot of PVdF-HFP/Ni/Co membrane’s amperometric responses as a function of glucose concentration.
mM with a high correlation coefficient of 0.998 (Figure 13) was observed for the prepared PVdF-HFP/Ni/Co membrane. It has also exhibited a low level limit of detection (LOD) of 0.26 μM with a moderate sensitivity of 7.56 μA/mM·cm2. The obtained nonenzymatic glucose sensor performances of the PVdF-HFP/Ni/Co membrane were suitably compared with the previously reported nonenzymatic glucose sensors (Table 1). The fabricated membrane exhibited superior properties including a wide linear range and low level limit of detection over the electrospun Ni−nanoparticle loaded carbon nanofiber paste electrode,46 Ni−nanoparticles/titania nanotube arrays loaded titanium foil,47 nickel oxide nanofibers anchored 10353
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Table 1. Comparison of Electroanalytical Performances of Nonenzymatic Glucose Sensors sensing materials
LODa(μm)
linear range (mM)
response time (s)
ref
NiCFP electrode Ni/TiO2 NiONFb-rGOc Co3O4 nanofibers CuO nanoparticles NiO/OMCd Ni−Co NSs/rGOc Cu−Co NSs/CHITe-rGOc NiO−Pt nanofibers Ni(II)−Quf-MWCNTg-ILh Ti/TiO2 nanotube arrays/Ni dendritic Cu−Ni/TiO2 film Ni(OH)2/rGOc Pt/Ni nanowire arrays NiNP/SMWNTsi CuO/TiO2 Nf/Pt HNPCs/AuNPs-CS2 Cu NPs/graphene PVdF-HFP/Ni/Co
1.0 2.0 0.77 0.97 ∼0.5 0.65 3.9 10 0.313 1.0 4.0 0.35 0.6 1.5 0.5 1.0 1.0 0.2 0.26
0.002−2.5 0.004−4.8 0.002−0.60 up to 2.04 0.05−18.45 0.002−1.0 0.01−2.65 0.0105−6.9 up to 3.67 0.005−2.8 0.1−1.7 0.001−0.5 0.002−3.1 0.002−2 0.001−1.0 up to 2.0 3.0−7.7 0.005−1.4 0.001−7.0
5
