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Sensors and Actuators B 203 (2014) 471–476

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

lucose-assisted synthesis of Cu2O shuriken-like nanostructures andheir application as nonenzymatic glucose biosensors

izwan Khana,1, Rafiq Ahmadb,1, Prabhakar Raia, Lee-Woon Janga, Jin-Hyeon Yuna,eon-Tae Yua, Yoon-Bong Hahnb, In-Hwan Leea,∗

School of Advanced Materials Engineering and Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 561-756,epublic of KoreaSchool of Semiconductor and Chemical Engineering, and Nanomaterials Processing Research Center, Chonbuk National University, 567 Baekje-daero,eokjin-gu, Jeonju 561-756, Republic of Korea

r t i c l e i n f o

rticle history:eceived 5 April 2014eceived in revised form 25 June 2014ccepted 28 June 2014vailable online 5 July 2014

a b s t r a c t

We have successfully synthesized high quality cuprous oxide shuriken-like nanostructures by low tem-perature hydrothermal process, using glucose as reducing agent. The resulting nanostructures werefurther characterized for the fabrication of nonenzymatic glucose biosensors. The glucose sensors exhib-ited excellent performances, giving a wide linear detection range (from 0.01 �M to 11.0 mM), an ultra-lowdetection limit (0.035 �M) and a high sensitivity (0.933 mA/mM cm2). Furthermore, the proposed biosen-

eywords:u2Oydrothermal methodonenzymatic biosensorlucoseyclic voltammetry

sor showed high selectivity and favorable reproducibility along with long-term performance stability.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Accurate determination of glucose concentrations in blood isrucial for the diabetes diagnosis and treatment. An abnormallood glucose level activates several metabolic pathways relatedo inflammation and apoptosis events [1,2]. Long-term manifesta-ion of diabetes is the leading cause of mortality and several humanealth complications, such as complications to retina, circulatoryystem, and kidney. According to the recent study, people with dia-etes worldwide were approximately 370 million in 2012, and thisumber will continue to increase due to aging population, growthf population size, urbanization and secondary lifestyle changesssues [3–6]. Thus, having means to expressly, accurately, and reli-bly measure the glucose content is very important, especially in

linical chemistry/medicine, where glucose concentration is useds a clinical indicator of diabetes [7]. There exists, of course, aide variety of clinically tested and approved glucometers that are

∗ Corresponding author. Tel.: +82 63 270 2292.E-mail address: ihlee@jbnu.ac.kr (I.-H. Lee).

1 These authors contributed equally to this work.

ttp://dx.doi.org/10.1016/j.snb.2014.06.128925-4005/© 2014 Elsevier B.V. All rights reserved.

commercially available. However, the sensitivity of these sensorsis relatively low which precludes their use in monitoring glu-cose, for example, in saliva or tears, something highly desirablein non-invasive out-patient health control and self-testing. Hencethe ongoing efforts to develop glucose sensors focus on alterna-tive sensing approaches with higher sensitivity, good linearity andreproducibility, easiness to use even by untrained personnel, andsuitability for mass production both in terms of throughput andcost.

One of the breakthroughs in that respect has been the demon-stration of the enzyme electrode suitability for measuring glucose[8–11]. Among various techniques, amperometric enzyme-basedsensors have played a leading role for the determination of glucosedue to their high sensitivity, repeatability and simple operation.Here, enzyme glucose oxidase (GOx) is the crucial constituentto catalyze the oxidation of glucose to gluconolactone. Despitepositive attributes of enzymatic biosensors, the main drawbacksassociated with their applications are the insufficient stability (eas-

ily affected by the temperature, humidity, and pH value) originatingfrom the nature of the enzymes and the complexity of enzymeimmobilization process, regardless of the employment of artifi-cial mediator or direct electron transfer [12,13]. Hence, a growing

4 Actuators B 203 (2014) 471–476

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72 R. Khan et al. / Sensors and

nterest is in the development of new-generation electrodes with-ut using enzyme for detecting glucose with high accuracy, lowost, simplicity in sampling and testing, portability, and reliability.

In this regard, significant efforts have been made to utilize metalnd metal oxide nanomaterials-based nonenzymatic glucose sen-ors such as Ni, Pt, Cu2O, CuO, TiO2, ZnO, and SnO2, for glucoseetection [14–20]. Among them, cuprous oxide (Cu2O), an impor-ant p-type semiconductor with a band gap of about 2.0 eV, haseen widely used in many fields, such as solar/photovoltaic energyonversion, semiconductor electronics, catalysis and gas sensingpplications due to its good electrochemical activity, high specificurface area, proper redox potentials and excellent stability in airnd various solutions. These properties make Cu2O an excellentandidate for the active electrodes materials of the non-enzymaticlectrochemical sensors, where charge transfer and adsorption onhe electrode surface play an important role. In that sense, attain-ng controlled and uniform morphology of the Cu2O nanostructuress extremely important. Previously, attempts to synthesize variousu2O nanostructures using various approaches, such as electrode-osition, thermal relaxation, and sonochemical methods have beeneported [21–23]. However, all these methods not only employn organic directing agent but also necessitate complex synthesisteps, which may inhibit the sensing properties.

Herein, we report a facile, low-cost, and environment-riendly glucose-assisted hydrothermal method to synthesize Cu2Ohuriken-like nanostructures (SLNs), and thus synthesized SLNsere used for the fabrication of electrochemical non-enzymatic

lucose sensor. To our knowledge, the Cu2O SLN/Ag/Si electrodeas not been applied for the effective glucose detection. The fab-icated electrodes showed good linearity, high sensitivity and lowetection limit in glucose concentration measurements. The mod-

fied electrodes exhibited high sensitivity, which was ascribed tohe large surface area of SLNs structures resulting in increasedlectrocatalytic activity and enhanced electron transfer in the elec-rocatalytic process. The prototype glucose sensors based on theLN electrodes demonstrated good prospects in practical medi-al sensing. This is particularly so since no expensive reagentsere involved in the process and chemical reactions were well-

ontrolled and fast, which are pre-requisites for cost-effectiveommercial production.

. Experimental methods

The Cu2O SLNs were synthesized as reported previously [24],ut with some modifications. In a typical procedure, 10 mL cop-er chloride (CuCl2·2H2O, 0.5 M) and 10 mL polyvinylpyrrolidonePVP, 0.1 g) solution were mixed and dissolved into 50 mL of deion-zed water and stirred at room temperature. Then, 5 mL potassiumarbonate (K2CO3, 1 M) and 5–10 mL sodium citrate (Na3C6H5O7)olutions were added into the mixture and kept for further stir-ing. Afterwards, 5–10 mL of glucose (0.5 M) were dropped into therepared solution and finally stirred for 30 min. Subsequently, theesultant solution was transferred into a Teflon-lined stainless steelutoclave with a volume of 100 mL, and then sealed and heated at0 ◦C for 2 h. After the growth, it was allowed to cool down to roomemperature. Brick red precipitate was finally collected and washed–3 times in ethanol and deionized water, respectively. The mor-hology of the as-synthesized products was investigated by fieldmission scanning electron microscopy (FESEM, Hitachi S-4700)nd transmission electron microscopy (TEM, H-7650, Hitachi). Thehemical composition of the product was investigated using elec-

ron dispersive spectra (EDS) analysis and X-ray diffraction analysisXRD; CuK�, � = 1.54178 A).

Fig. 1 shows the schematics of the glucose sensor fabricationrocess, design, and detection principle. The fabricated glucose

Fig. 1. Schematic illustration of measurement setup (a), sensor structure (b) andglucose detection principle (c).

sensor is schematically illustrated in Fig. 1(a). It is comprised ofa working electrode (Nafion/Cu2O SLNs/Ag/Si electrodes), a Pt wireas a counter electrode, and Ag/AgCl (saturated KCl) as a referenceelectrode. The process of the electrode fabrication is depicted inFig. 1(b). First, the electrodes were prepared by sputtering silvermetal on Si substrate. Then, as-synthesized Cu2O SLNs were mixedwith conducting binders (butylcarbitol acetate) and the preparedslurry was cast on the Ag electrode with the area of 0.09 cm2. Priorto the modification, the Ag electrode was polished with the 0.05 �malumina slurry and then ultrasonically cleaned in deionized waterand dried at room temperature. After drying the modified Cu2OSLNs/Ag/Si electrode, a 5 �L Nafion solution was dropped ontothe electrode and dried for 24 h at 4 ◦C to form a net-like film onthe modified electrode. Fig. 1(c) demonstrates a schematic repre-sentation of the glucose detection process by the electrochemicalmethod.

3. Results and discussion

The typical morphology of the as-synthesized products is shownin Fig. 2(a). As can be seen, the average diameters of the obtainedCu2O SLNs are ∼500–700 nm with smooth surface and low spreadin shape and dimensions. Fig. 2(b) shows a transmission electronmicroscopy (TEM) image of the Cu2O SLNs with a higher magnifi-cation. Notably, each Cu2O nanostructure is seen to be composedof four pointed arrows (each with a length ∼200–300 nm) pointingto four orthogonal directions and having fine and clear surfaces.This result is consistent with the FESEM observation. A typical EDSspectrum displayed in Fig. 3(a) clearly demonstrates the domi-nant features attributed to oxygen and copper, thus indicating thesuccessful formation of Cu2O. Fig. 3(b) presents a typical X-raydiffraction (XRD) pattern of the Cu2O SLNs. All the diffraction peakscan be clearly attributed to the cubic Cu2O phase with a lattice

constant of a = 4.267 A (JCPDS Card No. 77-0199).

In order to study the electrochemical properties of the workingelectrodes, the cyclic voltammetry (CV) studies were performed in0.10 M NaOH electrolyte at a scan rate of 100 mV/s and potential

R. Khan et al. / Sensors and Actuators B 203 (2014) 471–476 473

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gests the saturation of electrocatalytic active sites of Cu2O SLNs.The inset of Fig. 5 plots the average response current versus glu-cose concentration for the three fabricated non-enzymatic glucosesensors. The sensor exhibited a good linearity of the response

ig. 2. FESEM image (a) and TEM image (b) of the as-synthesized Cu2O SLNs. Insetn (b) shows TEM image of the single Cu2O SLNs.

ange of −0.20 and +0.80 V with respect to the reference electrodesee Fig. 4). While no peak has been observed in the CV curve in thebsence of glucose, a dramatic change was observed at the electrodehen different concentration of glucose (0.5, 1.0, 1.5 and 2.0 mM)as added. An obvious increase of the oxidation peak at +0.58 V was

bserved from the CV curves of the modified electrode with increas-ng glucose concentrations, which corresponded to the irreversiblelucose oxidation due to the conversion of Cu(II) to Cu(III) [25–27].he electrocatalytic oxidation of glucose in NaOH medium at Cu2Oodified electrode undergoes in several steps. Here, Cu(I) is first

lectrochemically oxidized to Cu(II) species viz. Cu(OH)2 (Eqs. (1)nd (2)). The possible reaction could be explained by the followingquations [28–30].

u2O + 3H2O → 2Cu(OH)2 + H2 (1)

u(OH)2 → CuO + H2O (2)

uO + OH− → CuOOH + e− (3)

uOOH + e− + glucose → CuO + OH− + gluconic acid (4)

Cu (III) is generated on the Cu2O SLNs surface rapidly and thexidized glucose is converted to gluconic acid as represented byqs. (3) and (4). Also, the conversion of Cu (III) into Cu (II) speciesives rise to the increase of the oxidation peak current and theecrease of the reduction peak current. The formation of Cu (III)

pecies not only leads to high catalytic activity but also plays theole of an electron transfer mediator.

Fig. 5 shows a typical steady-state amperometric responsef the modified Nafion/Cu2O SLNs/Ag/Si electrodes with the

Fig. 3. EDS spectrum (a) and XRD pattern (b) of the as-synthesized Cu2O SLNs.

successive addition of glucose in 0.10 M NaOH electrolyte at anapplied potential of +0.58 V under continuous stirring condition.A rapid and sensitive response to each injection of glucose wasobtained for the Cu2O SLNs-modified electrode to reach 97% steadystate current with response time of ∼3 s, indicating a good electro-catalytic oxidation as well as a fast electron conduction behaviorof the modified electrode. It is also clearly seen from Fig. 5that the response current increases as the glucose concentrationincreases and saturates at high glucose concentrations, which sug-

Fig. 4. CV plots of the glucose sensor electrode in the absence and presence ofdifferent concentration of glucose (0.5–2.0 mM) showing the strong redox peak at+0.58 V.

474 R. Khan et al. / Sensors and Actuators B 203 (2014) 471–476

Fig. 5. Steady-state amperometric response of the modified Nafion/Cu2O SLNs/Ag/Sielectrodes with the successive addition of glucose in 0.10 M NaOH electrolyte. Insetsd

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Fig. 6. Amperometric response curve upon subsequent injection of 1.0 mM glu-cose and then interfering chemical species (each 0.10 mM) such as fructose, lactose,

cent recovery of the added glucose was determined after deducting

TS

hows a calibration curve with the error bars indicating the deviation for threeifferent electrodes.

n the concentration range of 0.01–10 mM (R2 = 0.9994) with aensitivity of 0.933 mA/mM cm2 and an ultra-low detection limitf 0.035 �M (S/N = 3). The obtained sensitivity, detection concen-ration range, and the detection limit are better than for thereviously reported glucose sensors based on other Cu2O modi-ed electrodes [31–34]. (For instance, sensitivity, linear range, andetection limits were 0.629 mA/mM cm2, up to 6 mM, and 2.4 �M

n Ref. [31]; 0.00653 mA/mM cm2, up to 10 �M, and 0.05 �M inef. [32]; 0.19 mA/mM cm2, 0.05–1.1 mM, and 47.2 �M in Ref. [33];.2 mA/mM cm2, 0.3–3.3 mM, and 3.3 �M in Ref. [34]). The highensitivity of Cu2O modified electrodes can be mainly attributedo the two aspects: the shuriken like structure and the tight adhe-ion of Cu2O nanostructures on the surface of the electrode. Theormer provides the higher surface area and the latter enables fastnd direct conduction of electrons. Therefore, the Cu2O SLNs areromising and effective as working electrodes for the detection oflucose.

The selectivity of sensor is investigated by amperometric mea-urements. Fig. 6 presents the amperometric response curve uponubsequent injection of 1.0 mM glucose and then interfering chem-cal species (each 0.10 mM) such as fructose, lactose, mannose,scorbic acid (AA), and uric acid (UA), followed by successiveddition of 1.0 mM glucose in 20 mL NaOH (0.10 M) solution at aotential of +0.58 V (vs. Ag/AgCl). It is clear that the Cu2O SLNs mod-

fied electrode produces negligible current signals for all interferingpecies, yet still gives out significant response to glucose injectedfter adding interfering species, demonstrating that the modifiedlectrode has an effective selectivity to glucose.

The repeatability and long-term stability of the electrodes werelso tested by storing them in air at ambient conditions and mea-uring the sensors response after various storage time periods. Thectivity of the electrodes exhibited almost no change within therst 45 days, and after 80 days the electrodes retained ∼94% of their

riginal response to glucose. Three successive amperometric mea-urements of glucose oxidation at one modified electrode yielded aeproducible current with the relative standard deviation (RSD) of

able 1ensor response in human serum samples.

Samples Concentration of glucose inhuman serum sample (mM)

RSD (n = 3) (%)

1 1.22 1.9

2 2.45 0.8

3 3.67 2.7

mannose, ascorbic acid (AA), and uric acid (UA), followed by successive addition of1.0 mM glucose in 20 mL NaOH (0.10 M) solution at a fixed potential. Inset showsFESEM image of the Cu2O SLNs taken after electrochemical measurements.

2.7%, where the RSD was estimated by dividing the standard devia-tion by the average and was expressed as a percentage. This low RSDvalue indicates that the sensor was stable and could be used repeat-edly for the detection of glucose without poisoning by the oxidationproducts. To confirm the reproducibility of the sensor, three elec-trodes were fabricated and their responses were investigated at+0.58 V. In this case, the RSD value was obtained to be 3.2%, imply-ing a high repeatability and yield rate together with good stability.These characteristics are thought to be attributed to the chemi-cal stability of Cu2O SLNs in alkaline solution. The inset of Fig. 6shows the FESEM image of the Cu2O SLNs taken after electrochem-ical measurements. No notable deformation is seen, suggesting thestability of the synthesized Cu2O SLNs electrode. This result is ingood agreement with the stability tests reported previously [35].

Moreover, the proposed sensor was applied to detect the glucosein a sample of human serum. The Cu2O SLN/Ag/Si working elec-trodes with a reference and counter (Pt) electrodes were immersedin NaOH buffer solution containing different concentration ofhuman serum samples at a constant potential of +0.58 V. To deter-mine the accuracy and precision of the sensor in human serum,we examined the sensor response in known concentration of glu-cose in human serum samples (4.89 mM; Sigma–Aldrich, H4522)after dilution in NaOH (0.10 M) buffer solution (samples 1–3). Eachsample was analyzed three times. The reliability and repeatabil-ity of the fabricated sensor were checked by standard addition ofpure glucose to the known concentration samples of human serum[36,37]. First, a stock solution of pure glucose (100 mM) was pre-pared. Then, 20 �L (1.0 mM), 40 �L (2.0 mM) and 80 �L (4.0 mM),respectively, from stock solution were added into human serumsample having glucose concentration of 1.22, 2.45, and 3.67 mM in20 mL buffer solution. The concentrations of the standard additionswere compared to the measured values by our sensor. The per-

the concentration of glucose in human serum sample from thetotal measured value (after adding pure glucose); then obtainedvalue was multiplied by 100 and divided by added pure glucose

Added pure glucose intohuman serum sample (mM)

Recovery of addedglucose (%)

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oncentration. The obtained results are summarized in Table 1. Theata are in good agreement with the known concentration of glu-ose, demonstrating that the nonenzymatic glucose sensors studiedn this work exhibit good repeatability and reliability.

. Conclusions

We have successfully synthesized high quality Cu2O SLNs inarge quantity by low temperature hydrothermal process, usinglucose as reducing and polyvinylpyrrolidone as capping agento tailor the shape of nanostructures. The resulting productsere further characterized for the fabrication of Cu2O SLNs-basedonenzymatic biosensor. Cyclic voltammetry and amperomet-ic methods were used to investigate the catalytic properties ofhe fabricated electrode with Cu2O SLNs modified for glucoselectro-oxidation in alkaline media. Under optimized conditions,he nonenzymatic biosensor exhibited excellent performance forlucose detection, giving a wide linear range (from 0.01 �M to1.0 mM), an ultra-low detection limit (0.035 �M) and a high sen-itivity (0.933 mA/mM cm2). Furthermore, the proposed biosensorhowed high selectivity and favorable reproducibility along withong-term performance stability.

cknowledgment

This research was supported by National Research Foundationf Korea (NRF) funded by Ministry of Science, ICT & Future Planning2013R1A2A2A07067688, 2010-0019626).

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Biographies

Rizwan Khan received his doctoral degree in chemical engineering in 2012 fromChonbuk National University, Korea. He is currently working as postdoctoral fellowat Chonbuk National University, Korea. His current research interests are the syn-thesis of metal and metal oxides nanostructures and chemical functionalization ofsemiconductor oxide nanowire and their chemical and biosensor applications.

Rafiq Ahmad received his BSc (Honors) in zoology from Aligarh Muslim University(AMU) and MSc degree in biotechnology from KIIT Bhubaneswar, India, and Ph.D. inDepartment of BIN Fusion Technology Chonbuk National University, South Korea.

He is now working as a postdoctoral fellow in semiconductor and chemical engi-neering from Chonbuk National University, South Korea. He is currently engaged inthe synthesis of different metal and metal oxide nanomaterials by solution processand development of amperometric, potentiometric and field-effect transistor basedchemical, biological and hybrid sensors.

4 Actua

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76 R. Khan et al. / Sensors and

rabhakar Rai received his doctoral degree in information electronic materialsngineering in 2012 from Chonbuk National University, Korea. He is currentlyorking as Research Assistant Professor at Chonbuk National University. His

esearch interests are synthesis of nanoscale metal and metal oxides for gas sensorpplications.

ee-Woon Jang received his doctoral degree in information electronic materi-ls engineering in 2013 from Chonbuk National University, Korea. He is currentlyorking as postdoctoral fellow at Chonbuk National University, Korea. His current

esearch interests are the GaN based light-emitting diodes, applications of localizedurface plasmon and graphene quantum dot.

in Hyeon Yun is currently working as master’s course in information electronicaterials engineering at Chonbuk National University, Korea. His current research

nterests are the GaN based light-emitting diodes and applications of localized sur-ace plasmon.

eon-Tae Yu received his engineering diploma in department of metallurgy engi-eering in 1988 from Chonbuk National University, Korea and a Doctoral degree

1993) from Tohoku University, Japan. From 1993 until 2003, he was a senioresearcher for Korea Institute of Geoscience and Mineral Resources. In 2004, heoved to Division of Material Science Engineering of Chonbuk National University.is main research interests are of core-shell structure composite nanoparticles and

heir application in gas sensor.

tors B 203 (2014) 471–476

Yoon-Bong Hahn is a WCU Professor in Department of BIN Fusion Technology,School of Semiconductor and Chemical Engineering, Chonbuk National University,Korea. He is also the director of National Leading Research Laboratory for HybridGreen Energy Research. He received his Ph.D. in 1988 from University of Utah, USA.His current research activities focus on the synthesis and characterization of metaland metal oxides nanostructures and their applications for optoelectronic devices,solar cells, chemical and biosensors, etc.

In-Hwan Lee received the B.S., M.S., and Ph.D. degrees in materials science andengineering from Korea University, Korea, in 1991, 1993, and 1997, respectively.During 1997–1999, he was a postdoctoral fellow at the Northwestern University.He joined Samsung Advanced Institute of Technology, where he led an epitaxialteam and developed a world-top-class high-power [50 mW, continuous wave (CW)]InGaN/GaN violet laser diode for optical storage source (Blu-ray). Since 2002, he hasbeen a faculty member in the School of Advanced Materials Engineering, ChonbukNational University, Korea. With the sabbatical research grant from LG Foundation,he was at Yale University, New Haven, CT (2008–2009). His current research inter-ests include the growth and characterizations of GaN-based optoelectronic materials

and devices. He is also interested in the development of nanocrystal embedded noveloptoelectronic devices including solid-state lighting devices, photovoltaic devices,and sensors. He has authored or coauthored over 180 peer-reviewed research arti-cles in major scientific journals, and presented over 50 invited seminars and talksaround the world, and holds over 15 patents at various stages of the process.