Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles

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Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles
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        A      R      T      I      C      L      E Copyright © 2012 by American Scientific Publishers All rights reserved.Printed in the United States of America Science of Advanced Materials  Vol. 4, pp. 994–1000, 2012 (www.aspbs.com/sam) Non-Enzymatic Glucose Sensor Based onWell-Crystallized ZnO Nanoparticles Kulvinder Singh 1 , Ahmad Umar  2,3, ∗ , Arun Kumar  1 , G. R. Chaudhary 1 ,Sukhjinder Singh 1 , and S. K. Mehta 1, ∗ 1 Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India 2 Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran 11001,Kingdom of Saudi Arabia 3 Faculty of Arts and Sciences, Department of Chemistry, Najran University, P.O. Box 1988, Najran 11001,Kingdom of Saudi Arabia ABSTRACT In this paper, well-crystallized ZnO nanoparticles were rapidly synthesized by facile simple solution process atlow temperature and used as an efficient electron mediator for the fabrication of non-enzymatic glucose sensor.The synthesized nanoparticles were characterized in terms of their morphological, structural and compositionalproperties. The detailed morphological characterizations confirmed the large-scale synthesis of well-crystallinewurtzite hexagonal phase ZnO nanoparticles with the typical sizes of 35 ± 5 nm. The as-synthesized nanopar-ticles were effectively used as an electrode material for the fabrication of efficient, highly sensitive and repro-ducible non-enzymatic glucose sensor. The fabricated glucose sensor shows a high and reproducible sensitivityof 38.133    A/cm 2 mM with a response time less than 5 s. A linear dynamic range from 1 ∼ 10 mM and cor-relation coefficient of   R  = 0.9988 were observed for the fabricated sensor. The presented work demonstratedthat the synthesized ZnO nanomaterials could act as effective electron mediators for the fabrication of efficientnon-enzymatic glucose sensors. KEYWORDS:  ZnO Nanoparticles, Non-Enzymatic, Glucose Sensor, Cyclic Voltammetry. 1. INTRODUCTION Glucose is one of the essential molecules in physiologi-cal processes and a primary energy source of human body.Even though glucose is essential and beneficial for thehuman health in an optimum amount, however, the unreg-ulated amount of glucose in the blood stream could causeseveral serious health problems such as diabetes melli-tus, heart disease, kidney malfunction, blindness, etc. 1–3 Not only the glucose is important in clinical diagnos-tics, it is also useful in the environment and food indus-tries, bioprocess monitoring and so on. 1–3 Due to the highimportance of glucose, there is a need to search for areliable, robust, high-sensitive and cost effective techniqueto fabricate efficient glucose sensors. In this regard, vari-ous methods have been developed to investigate and eval-uate the glucose in clinical applications, food processing,biotechnology, bio-processing monitoring and so on and ∗ Authors to whom correspondence should be addressed.Emails: skmehta@pu.ac.in; ahmadumar786@gmail.comReceived: 23 March 2012Accepted: 17 July 2012 reported in the literature. 4–10 The methods include, spectro-scopic methods, 4–6 wireless magneto-elastic sensor, 7 FETbased glucose sensors, 8 potentiometric sensor, 9 current–voltage ( I  – V  ) technique based sensor, 10  11 electrochemicalsensors, 12–16 and many more. Among various detectiontechniques, the electrochemical technique is one of theeasy, robust and simple techniques which provide a highsensitive and selective detection of glucose. 16–17 To fabri-cate the electrochemical biosensors sensors, generally arti-ficial electron mediators are used which helps to transferthe electrons from the substrate to the analyte. Becauseof the exotic properties of different biocompatible nano-materials, it was observed that nanomaterials could be thepromising electron mediators for electrochemical glucosesensors and hence variety of nanomaterials were used forthis purpose and reported in the literature. 17–25 Generally,glucose electrochemical sensors are fabricated with enzy-matic approach using glucose oxidase (GOx) due to itshigh selectivity and sensitivity. 22–25 However, due to theintrinsic thermal and chemical instabilities of the enzymesand very tedious fabrication process, the utilization of enzymatic glucose sensors for analytical applications arelimited. 26–31 Therefore, to overcome with the enzymaticglucose sensor problems towards analytical applications, 994  Sci. Adv. Mater. 2012, Vol. 4, No. 9   1947-2935/2012/4/994/007 doi:10.1166/sam.2012.1399  Singh et al.  Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles A RT I     C L  E  much more efforts have been made to directly determinethe glucose without using any enzyme through a non-enzymatic approach.Recently various nanomaterials have been used as effi-cient electron mediators to fabricate the non-enzymatic glu-cose sensors. 32 Previously, Zhuang et al. 32 have used CuOmodified Cu electrode to fabricate the non-enzymatic glu-cose sensor. In another report, Cao et al. 33 have demon-strated that   -Fe 2 O 3  NPs can efficiently be used for thefabrication of glucose sensor with non-enzymatic approach.Recently, Zhang et al. 34 have used NiO nanofibers to fab-ricate the non-enzymatic glucose sensor. Similarly, Zhuet al. 35 demonstrated the utilization of Pt–Au-SWCNTsnanocomposites for the fabrication of non-enzymatic glu-cose sensor. Among various nanomaterials, ZnO presentsas an effective electron mediator to fabricate efficient elec-trochemical sensors due to its own properties, to name afew, wide band gap, high-chemical and thermal stability,biocompatibility, optical transparency, high-specific sur-face area, high-electron communication features, ease of fabrication and so on. 19 Even though ZnO nanomaterialhave various interesting properties, however, their utiliza-tion for non-enzymatic sensor is occasional.In the report, we present the successful synthesis andcharacterizations of ZnO nanoparticles by simple solutionprocess at low-temperature. Further, the as-synthesizednanoparticles have been used as effective electron media-tors for the fabrication of efficient non-enzymatic glucosesensor. The fabricated glucose sensor exhibits a high andreproducible sensitivity of 38.133   A/cm 2 mM. The find-ing exhibit that ZnO nanomaterial synthesized in a simpleway can efficiently be used for the fabrication of high-sensitive non-enzymatic glucose sensor. 2. EXPERIMENTAL DETAILS 2.1. Synthesis of Well-CrystallizedZnO Nanoparticles Well-crystallized ZnO nanoparticles were rapidly syn-thesized by simple and facile solution process at low-temperature (90   C). For the synthesis, zinc acetatedihydrate (Zn(CH 3 COO) 2 · 2H 2 O; Zn(OAc) 2   and sodiumhydroxide (NaOH) were purchased from Sigma-Aldrichand used as received without further purification. In a typ-ical reaction process, 0.01 M Zn(OAc) 2  was dissolved in100 mL of distilled water (DW) in a round bottom flask.Consequently, appropriate amount of NaOH dissolved in50 mL DW, was added drop-wise to the Zn(OAc) 2  solu-tion under stirring for 10 min. This process was done tokeep the Zn + 2  /OH − ratio = 0  09 for controlling the growthof the particles. After vigorous stirring, the resultant solu-tion was heated at 90   C and finally a fluffy solution of Zn(OH) 2  and [Zn(OH) 4 ] − 2 was appeared which was con-verted to ZnO by continuous heating for 30 min. Thechemical reactions involved in the formation of ZnO dur-ing the synthesis process can be depicted as:Zn + 2 + 2OH − → Zn  OH  2 Zn  OH  2 + 2OH − → Zn  OH  − 24 Zn  OH  − 24  → ZnO + H 2 O + 2OH − White colored ZnO products were obtained which werewashed several times with water, ethyl alcohol and acetone,sequentially and dried for few hours at 50   C. The detailedcharacterizations have been done of as-synthesized ZnOmaterial in terms of their morphological, structural, compo-sitional, and scattering properties. Finally, the synthesizedZnO nanoparticles were used as supporting electron medi-ators for the fabrication of efficient non-enzymatic glucosesensors. 2.2. Characterization of ZnO Nanoparticles The synthesized ZnO nanoparticles were characterized indetail in terms of their morphological, structural, com-positional and scattering properties. The morphologicalproperties of synthesized ZnO nanoparticles were exam-ined by using field emission scanning electron microscopy(FE-SEM JEOL-JSM-7600F) and transmission electronmicroscopy (TEM; JEOL-JEM-2100F) equipped withhigh-resolution TEM (HRTEM). The crystallinity andcrystal phases were characterized by using X-ray diffrac-tion (XRD; PANanalytical Xpert Pro.) measured withCu-K   radiation (  = 1  54178 Å) in the range of 10–65  with scan speed of 8   / min. The elemental compo-sition was examined by using energy dispersive spec-troscopy (EDS) while the chemical composition wasevaluated by Fourier transform infrared (FTIR; PerkinElmer-FTIR Spectrum-100) spectroscopy in the range of 450–4000 cm − 1 . The scattering properties were observedwith Raman-scattering spectroscopy (Perkin Elmer-RamanStation 400 series) at room-temperature. 2.3. Fabrication and Characterizations of Non-Enzymatic Glucose SensorBased on ZnO Nanoparticles For application point of view, the synthesized ZnO NPswere used as electron mediators for the fabrication of high sensitive and reproducible non-enzymatic glucosesensors using modified gold electrode (GE, surface area = 3  14 mm 2   with butyl carbitol acetate (BCA) and ZnOnanoparticles. Prior to the modification, GE surface waspolished with alumina-water slurry on a polishing cloth,followed by rinsing with distilled water thoroughly. For theelectrode surface modification, firstly, an appropriate com-position of functional nanomaterial (ZnO nanoparticles)and BCA were mixed together. The prepared slurry wasthen uniformly coated on the GE and dried at 60 ± 5   C Sci. Adv. Mater., 4, 994–1000, 2012   995  Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles  Singh et al.       A      R      T      I      C      L      E for 4–6 hrs to get a uniform and dry layer over active elec-trode surface. All the electrochemical experiments wereperformed at room-temperature with a   Autolab Type-IIIcyclic voltammeter using three-electrode configuration inwhich the modified ZnO/Au electrode was used as work-ing electrode, a Pt wire as a counter electrode and anAg/AgCl (sat. KCl) as reference electrode. For all the non-enzymatic glucose sensor experiments, 0.1 M NaOH wasused as supporting electrolyte since the glucose easily oxi-dized in basic solution. 36 3. RESULTS AND DISCUSSION 3.1. Morphological, Structural and CompositionalProperties of Synthesized ZnO Nanoparticles The morphologies of as-synthesized ZnO products wereexamined by FESEM and TEM. Figures 1(a) and (b) showtypical FESEM images of as-synthesized ZnO which con-firmed that the synthesized products are nanoparticles. Thenanoparticles have been synthesized in very large quantityas was confirmed by the low-magnification FESEM image(Fig. 1(b)). Due to high density growth, it is seen that thenanoparticles are agglomerated. Most of the nanoparticlespossess spherical shape while some elongated nanoparti-cles can also be seen in the FESEM micrographs. Thetypical diameters of as-synthesized nanoparticles are inthe range of   ∼ 35 ± 5 nm. Moreover, the nanoparticlesexhibit smooth surfaces. To examine in detail the mor-phologies and structural properties, TEM and HRTEM Fig. 1.  Typical (a) low and (b) high magnification FESEM images; (c) low and (d) high-resolution TEM images of as-synthesized ZnO nanoparticles. analysis have been done. From the TEM image (Fig. 1(c)),it is clear that the nanoparticles are grown in high den-sity and due to that agglomeration in the nanoparticlescan be seen. The TEM result also confirms the spheri-cal shape of nanoparticles with some elongated nanoparti-cles. The typical sizes of the nanoparticles are ∼ 35 ± 5 nmbut some bigger sized nanoparticles were also observed.These TEM observations reveal the full consistency withthe FESEM results. Figure 1(d) shows the typical HRTEMimage of as-synthesized ZnO nanoparticles. From theimage, the calculated distance between two lattice fringesis  ∼ 0.52 nm which corresponds to the  d -spacing of [0001] crystal planes of the wurtzite hexagonal ZnO.This clearly confirms that the synthesized nanoparticlesare well-crystalline and possess pure wurtzite hexagonalphase ZnO.To determine the elemental composition of as-synthesized ZnO nanoparticles, EDS analysis was done.Figure 2(a) exhibits the typical EDS spectrum of as-synthesized ZnO nanoparticles. It shows well defined peaksfor zinc and oxygen only. No other peaks related to anyimpurity are detected in the EDS spectrum, up to thedetection limit of EDS instrument which confirms thatthe prepared nanoparticles are made by zinc and oxygenonly without any noticeable impurity. To find out the crys-tallinity and crystal structures, the ZnO nanoparticles wereexamined by XRD pattern. Figure 2(b) shows typical XRDpattern of as-synthesized ZnO nanoparticles. Several well-defined diffraction reflections were seen in the observedpattern which is fully corresponds to the wurtzite hexag- 996  Sci. Adv. Mater., 4, 994–1000, 2012   Singh et al.  Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles A RT I     C L  E  Fig. 2.  Typical (a) EDS spectrum, (b) XRD pattern, (c) FT-IR spectrum and (d) Raman-scattering spectrum of as-synthesized ZnO nanoparticles. onal phase of ZnO. The observed result is well matchedwith the reported JCPDS Card no. 36-1451 and henceconfirms that the synthesized nanoparticles are pure ZnOand possessing wurtzite hexagonal phase. 37 Except welldefined reflections related with ZnO, no other reflectionrelated with other impurities was observed in the patternwhich further verified that the synthesized ZnO nanoparti-cles are pure.FT-IR spectra (Fig. 2(c)) have been recorded for as-synthesized ZnO nanoparticles for determining the chem-ical composition. Several well-defined absorption peakswere observed in the obtained FTIR spectrum. The appear-ance of a sharp peak at 473 cm − 1 correspond to the metaloxygen (M O) bond and is assigned as    (Zn O). Thepresence of this band further confirms that the synthe-sized product is ZnO. The appearance of a short peak at 897 cm − 1 is most probably due to the acetate ion. 38 Two other short bands at 1631 and 3421 cm − 1 were alsoobserved in the spectrum. The appearance of a very shortband at 1631 cm − 1 is observed due to the bending vibra-tion of absorbed water and surface hydroxyl, and a broadpeak 3421 cm − 1 is due to the O–H stretching mode. 38  39 For the vibrational properties, the ZnO nanoparticleswere analyzed by Raman-scattering properties. Figure 2(d)exhibits typical Raman-scattering spectra of ZnO nanopar-ticles. The obtained Raman-scattering spectrum exhibitsvarious phonon peaks at 332, 438 and 557 cm − 1 . The pres-ence of a strongest peak appeared at 438 cm − 1 which isassigned to  E  high2  mode and attributes to ZnO non-polaroptical phonon. 40 The presence of   E  high2  mode confirmsthe wurtzite hexagonal phase for the as-synthesized ZnOnanoparticles. Moreover, the srcination or a small peak at 332 cm − 1 is attributed as  E  2 H   − E  2 L  (multi-phononprocess). In addition to this, the presence of a sup-pressed and broad peak appearing at 557 cm − 1 , could beassigned as  E  1 L  mode. Finally, the presence of strong andsharp Raman-active  E  high2  mode further confirms that thesynthesized nanoparticles are well-crystalline and possesswurtzite hexagonal phase. 3.2. Electrochemical Non-Enzymatic GlucoseSensor Based on ZnO NanoparticlesModified Gold Electrodes To fabricate the non-enzymatic glucose sensor, the as-synthesized ZnO nanoparticles were used as an efficientelectron mediator for the modification of gold elec-trode (GE). Finally, the modified ZnO/GE was used as aworking electrode to determine the electrocatalytic behav-iors of the modified electrode towards the oxidation of glucose at a scan rate of 50 mV/s.Figure 3 shows the typical cyclic voltammogram (CV)sweep curve of the modified ZnO/GE electrode in absence(black line) and presence (red line) of glucose (1 mM)in 0.1 M NaOH at the scan rate of 50 mV/s in thepotential range of 0.0 to 0.9 V (vs. Ag/AgCl). It can beseen that no oxidation/reduction peak has been observedfrom the obtained CV graph in the absence of glucose in Sci. Adv. Mater., 4, 994–1000, 2012   997  Non-Enzymatic Glucose Sensor Based on Well-Crystallized ZnO Nanoparticles  Singh et al.       A      R      T      I      C      L      E Fig. 3.  Typical cyclic voltammogram (CV) sweep curve of the modi-fied ZnO/GE electrode in absence (black line) and presence (red line)of glucose (1 mM) in 0.1 M NaOH at the scan rate of 50 mV/s in thepotential range of 0.0 to 0.9 V (vs. Ag/AgCl). 0.1 M NaOH while in the presence of 1 mM glucose solu-tion a pair of redox peaks with the anodic and cathodicpeak potentials positioned at 0.55 and 0.08 V have beenobserved. The appearance of reduction peak in the CVmight be due the reduction of Glucono-  -lactone. 41 How-ever, in the presence of glucose, a significant redox peak was observed which reveal the electrochemical response of the ZnO/GE electrode in glucose. Moreover, this confirmsthat the ZnO NPs efficiently acts as electron mediator forthe electrochemical sensing of glucose.To investigate the electron transport mechanism of glu-cose with the modified electrode, effect of scan rates havebeen studied on 1 mM glucose solution in 0.1 M NaOHsolution using ZnO/GE electrode. Figure 4(a) depicts theCV sweep curve of the ZnO/GE electrode at different scanrates (i.e., 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600,700, 800 mV/s respectively). Increasing the scan rates, thepeak current increases which confirms a typical diffusioncontrolled electrochemical behavior. Also it can been seenthat the catalytic oxidation peak potential shifts positivelywith increasing scan rates, indicating a kinetic limitationin the reaction between the redox site of ZnO/GE andglucose. 42 Figure 4(b) shows a plot for the anodic peak currents versus the square root of the scan rates (  1 / 2 ) inthe same solution. The anodic peak current show a lineardependence with square root of scan rates which furtherconfirms the typical diffusion-controlled electrochemicalbehavior.The irreversibility and number of electrons involved inthe electrochemical reaction of glucose has also been ana-lyzed. Figure 5 depicts the tafel plot (i.e., Peak potential vs.log scan rate (log  )). The linearity in peak potential versuslog   reveals that the electrochemical behavior of glucoseis totally irreversible diffusion control process. By usingTafel 43 Eq. (1), Tafel slope ( b ) has been calculated E  p = b/ 2  log  + constant (1) Fig. 4.  (a) Cyclic voltammogram (CV) sweep curve CV sweep curvesat different scan rates (i.e., 50, 60, 70, 80, 90, 100, 200, 300, 400, 500,600, 700 and 800 mV/s) in 0.1 M NaOH solution and (b) anodic peak current ( I  a   versus square root of scan rate (  1 / 2  . Figure 6 shows a plot of   E  p  versus log  . The value of   b comes out to be 0.07 V. Devanathan 44 has calculated thetafel slope from the following equation:Tafel slope = 59 /n  mV (2)Where    is the symmetry factor and is generally 0.5and  n  is number of electron involved electrochemicalbehavior. 44 From this effective number of electron involvedin the oxidation of glucose comes out to  ∼ 2 which indi-cates that two electron process should be rate limitingstep. These results reveal that the total number of electronsinvolved in oxidation of glucose is estimated to be 2. Thechemical equations which satisfy the calculation can bewritten as mentioned in the literature: 45  46 Glucose → Glucono-  -lactone + 2e − + 2H + Glucono-  -lactone hydrolysis −−−−−→ Gluconic acidFor detailed electrochemical response of the modifiedZnO/GE electrode towards glucose, various experiments 998  Sci. Adv. Mater., 4, 994–1000, 2012 
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