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Sensors and Actuators B 210 (2015) 190–196

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

elf-supported porous CoOOH nanosheet arrays as a non-enzymaticlucose sensor with good reproducibility

i Zhang, Chunli Yang, Guangyu Zhao, Jianshuai Mu, Yan Wang ∗

cience Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China

r t i c l e i n f o

rticle history:eceived 5 November 2014eceived in revised form9 December 2014ccepted 27 December 2014vailable online 3 January 2015

a b s t r a c t

Porous CoOOH nanosheet arrays are constructed on Ti foils by an electrochemical assistant method. Thenanosheet arrays exhibit hierarchical porous structures with a specific surface area of 470 m2 g−1. Theas-prepared CoOOH nanosheet arrays are used for glucose detection as a non-enzymatic sensor. Theamperometric detection of glucose is carried out at 0.52 V (versus Ag/AgCl). The amperometric signalsare linearly proportional to glucose concentration from 3 �M to 1.109 mM (R = 0.995), showing a lowdetection limit (S/N = 3) of 1.37 �M and a high sensitivity of 526.8 �A mM−1 cm−2. The high sensitiv-

eywords:on-enzymatic glucose sensoranosheet arrayslectrodepositionelf-supported

ity towards glucose is mainly attributed to the freestanding and self-supported features of hierarchicalporous CoOOH nanosheet arrays on a conducting substrate. The most attractive feature of the array elec-trodes is the reproducibility, which helps the relative standard deviation of the response currents toglucose from five electrodes is 3.21%.

© 2015 Elsevier B.V. All rights reserved.

eproducibility

. Introduction

Glucose biosensors have become an intense topic, because fastnd easy detection is important in lots of fields such as clinical diag-ostics, food industry, and biotechnology [1–6]. Glucose biosensorsan be mainly classified into two categories: glucose oxidase (GOD)ased sensors [7–9] and non-enzymatic glucose sensors [10–13].OD, owing to its high sensitivity and selectivity to glucose, haseen widely applied to construct various amperometric biosen-ors for glucose detection. However, GOD has limitations such asack of stability, the need of an enzyme immobilization process,he high cost of enzymes, the requirement of low temperaturetorage and the narrow operation temperature window etc. Non-nzymatic glucose sensors are expected to have advantages such asccessible, good stability, and free from oxygen restriction [14–18].ecently, various metals, metal alloys, and transition metal oxideanomaterials or the composites of carbon nanotube or graphaneith them have been used for non-enzymatic glucose detection

19–37]. Among of them, the oxides of cobalt attract substantialttention due to their various valences, such as Co3O4 [28–30] and

oOOH [27]. High oxidizing state Co can oxidize glucose in alka-

ine solution, which leads the cobalt oxides promising candidatesf non-enzymatic glucose sensors. Generally, the electrooxidation

∗ Corresponding author.E-mail address: wangy msn@hit.edu.cn (Y. Wang).

ttp://dx.doi.org/10.1016/j.snb.2014.12.113925-4005/© 2015 Elsevier B.V. All rights reserved.

of glucose is considered mediating by CoOOH/CoO2 in the alkalinemedium both on Co3O4 and CoOOH surface [27,30]. Therefore, theattempt using CoOOH as the glucose sensor directly is meaningful[27]. However, in most of non-enzymatic glucose sensor studies,the general preparation strategy of electrodes is casting the as-prepared materials onto the surface of glassy carbon electrode orgold electrode, and then entrapping them with Nafion to obtainmodified electrodes. Together with the pretreatment steps, such aspolishing, modification and drying, the electrode preparation pro-cess is fussy, time-consuming, and weakly reproducible. Otherwise,electrochemical performances are affected by the electrode modi-fication processes constantly, and the active materials also sufferedfrom falling off from the electrodes easily. Therefore, exploringreproducible and tough electrodes to detect glucose accurately isurgent. Preparing self-supported electrode on conductive substratefor non-enzymatic glucose detection is an attractive strategy, inwhich, active materials grow on conductive matrix directly, provid-ing an accessible means without the pretreatment and modificationof the electrode [27].

Herein, we electrodeposit porous CoOOH nanosheet arrays onTi substrate for non-enzymatic glucose detection. Various methodshave been applied in preparing CoOOH, including direct solutionprecipitation [38], precursor conversions [39], and electrochemical

deposition [40]. Among of them, to obtain a self-supported CoOOHfilm, electrodeporition is a facial, controllable, and accessiblemethod. When using the CoOOH electrodes obtained by elec-trodeposition method as the glucose detection electrodes, these

Actuators B 210 (2015) 190–196 191

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L. Zhang et al. / Sensors and

ree-standing nanosheets on the conductive substrates can supplydequate ion transmission channels relying on the porous archi-ectures, and provide abundant active sites due to the large specificurface area. These features help the electrodes realizing a detec-ion limit of 1.37 �M and a sensitivity of 526.8 �M mA−1 cm−2.therwise, the self-supported electrodes obtained by the bottom-p process are so tough that preventing active material falling off,

eading the electrodes good reusability, stability and reproducibil-ty, which are significant for practical application.

. Experimental

.1. Synthesis and characterizations of porous CoOOH nanosheetrrays

All chemical reagents were of analytical grade and werebtained commercially. A three-electrode electrochemical cellomprising a stainless foil (1 cm × 2 cm) as counter electrode, aaturation mercury electrode (SCE) as reference electrode, and ai foil (1 cm × 2 cm) as working electrode was assembled for elec-rochemical deposition experiments. The electrolyte solution foreposition was 0.1 M Co(NO3)2. The reduction current of electro-hemical deposition was fixed at 2 mA, and the deposition time is00 s. The resulting films were then carefully washed with deion-

zed water and dried in air.Scanning electron microscope (SEM) images were obtained

n a Hitachi Su-8100. Transmission electron microscope (TEM)mages and selected area electron diffraction (SAED) patterns werebtained on a FEI Tecnai G2. X-ray diffraction (XRD) patterns werebtained by Rigaku D/max-2000 X-ray diffractometer with Cu K�adiation (� = 1.5418 A). The specific surface area of the materi-ls was analyzed by the Brunauer–Emmett–Teller (BET) methodith a Micromeritics Accelerated Surface Area and Porosimetry

ystem (ASAP) 2020. The gas used was N2 with a liquefaction tem-erature of −195.87 ◦C, and the gas desorption time was 6 h. Theotal pore volume and pore size distribution were evaluated by thearrett–Joyner–Halenda (BJH) method.

.2. Electrochemical analysis

A CHI 660 electrochemical workstation was used for electro-hemical experiments. A three-electrode cell was assembled withhe porous CoOOH nanosheet arrays on Ti foil as working electrode,

platinum foil as counter electrode and an Ag/AgCl as the referencelectrode. All potentials were referenced to the Ag/AgCl (sat’d KCl)lectrode. NaOH aqueous solution (0.1 M) was used as the elec-rolyte. Hydrodynamic chronoamperometric measurements werearried out under magnetic stirring. For comparison, CoOOH pow-er scraped from the substrates was also tested as the glucoseensor. CoOOH powder electrodes were prepared using commonrocess for modified electrode, that mixing CoOOH powder withifion, pasting the mixture on glass carbon (GC) electrode and dry

t in room temperature.

. Results and discussion

.1. The structure of the porous CoOOH nanosheet arrays

The crystal structure of the CoOOH scraped from the as-preparedoOOH/Ti was characterized by XRD. The XRD patterns in Fig. 1

ndicate a typical orthorhombic system (JCPDS: 26-0408). The

ormation of cubic crystalline CoOOH is revealed by the diffrac-ion peaks at 2� values of 25.3◦, 28.2◦, 43.7◦, 57.7◦ and 63.0◦,orresponding to (1 3 0), (0 4 0), (1 4 0), (2 4 0) and (1 5 1) crys-al planes, respectively. Typical SEM images in Fig. 2a show a

Fig. 1. XRD pattern of the CoOOH nanosheet powder scraped from the substrate.

dense network array of CoOOH nanosheets on Ti foils. The free-standing architectures of the nanosheets let the CoOOH filmspresent a porous structure, which is favorable for mass transportin the electrochemical processes. Fig. 2b show the SEM images ofCoOOH nanosheet powder scraped from the substrate. The scrapingdestroys the freestanding structure of the films, and the preparationprocess of modified electrode causes disordered aggregation of thenanosheets seriously. On the other hand, the CoOOH nanosheets arealso porous, illustrated by the typical TEM images of the isolatedCoOOH nanosheets in Fig. 2c and d. From the TEM images, abundantmesopores (about 3–4 nm) can be observed on the flakes endow-ing the films second grade porous structure (the first grade is thepores surrounded by the flakes). The hierarchical porous structuresendow the CoOOH films not only a large specific surface area butalso adequate mass transport channels. The large specific surfacearea supplies abundant active sites for the electrocatalysis, whilethe adequate mass transport channels enable the reactants andproducts diffusing into/out of the active sites easily. The nanopar-ticles in the TEM image of Fig. 2c demonstrate a polycrystallinestructure, which can be verified by the concentric rings in SAEDpattern, as shown in the inset of Fig. 2c. The SAED pattern of theas-deposited flakes can be indexed to the diffraction of CoOOH(JCPDS: 26-0408), which is consistent with the XRD result in Fig. 1,indicating a diffraction pattern belonging to CoOOH.

A distinct hysteresis loop can be observed from the N2 adsorp-tion/desorption isotherm shown in Fig. 3a, indicating the existenceof a typical mesoporous microstructure. The BJH pore size dis-tribution function calculated from the isotherms (Fig. 3b) showsuniform mesopores with an average diameter of 3–4 nm, whichwas identical with the TEM results. These porous nanosheet net-work arrays directly growing on a conducting substrate can providea large effective specific surface area (around 470 m2 g−1, from theBET results) and adequate channels for mass transport, which arefavorable for electrochemical sensors.

3.2. The porous CoOOH nanosheet arrays as glucose sensors

In order to confirm the point that the electrocatalytic activity onglucose oxidation is from the CoOOH nanosheet arrays but not fromTi substrate, the electrochemical experiments of Ti foil and porousCoOOH nanosheet arrays on Ti foil in the absence and presence ofglucose were carried out. Fig. 4a shows cyclic voltammograms (CVs)

of the bare Ti foil and the CoOOH nanosheet arrays modified Ti foilin the absence and presence of 0.5 mM glucose in 0.1 M NaOH solu-tion. For bare Ti foil, no redox response can be seen in the potentialrange from −0.4 to 0.7 V. While the curves of CoOOH nanosheet

192 L. Zhang et al. / Sensors and Actuators B 210 (2015) 190–196

F t powdi

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ig. 2. SEM images of porous CoOOH nanosheet arrays (a) and the CoOOH nanosheenset in (c) is the SAED image pattern of a CoOOH nanosheet.

rrays show different appearance. CoOOH is known comprisingainly trivalent cobalt, often containing mixture of Co2+ existing

s non-stoichiometric phases [41]. Accordingly, the redox pair inhe region of 0–0.5 V in the CVs can be assigned to the reversiblerocess of Co2+ and Co3+. Thus, the increasing current from 0.5 Vo 0.6 V can be attributed to the oxidation process of Co3+ to Co4+

reduction shoulder at ∼0.55 V), which is similar with the assign-ents suggested in the literatures [41–45]. Compared with the CV

urve of the array electrodes in bare NaOH solution, the CoOOHanosheets exhibited obvious electro-oxidation ability toward glu-ose starting from 0.50 V to 0.60 V on the glucose added solution,s seen from the blue curve in Fig. 4a. Fig. 4b is the multi-cycle CVsf the porous CoOOH nanosheet arrays in 0.1 M NaOH without glu-ose. As the scanning proceeding, the curve has almost no variantithin the potential window of 0.4–0.6 V in the initial 25 cycles,emonstrating that porous CoOOH nanosheet arrays are stable andepeatable as the electrocatalyst in alkaline medium. The electro-

atalytic activity of the porous CoOOH nanosheet arrays towardshe oxidation of glucose with different concentration in an alka-ine medium was investigated. Fig. 4c presents the CVs recorded at0 mV s−1 in the presence of glucose (0.2–3.0 mM) in 0.1 M NaOH.

able 1omparison of non-enzymatic glucose sensing performance based on different electrode

Type of electrode Sensitivity (�A mM−1 cm−2) D

Co3O4 nanoparticles 520.7

Electrospun Co3O4 nanofibers 36.25

Cobalt oxide acicular nanorods 571.8

Co/CoOOH 967 1Porous CoOOH nanosheet arrays 526.8

er scraped from the substrates (b); TEM images (c, d) of porous CoOOH nanosheets,

Obviously, the anodic current in the potential window of 0.4–0.6increases with the concentration of glucose.

The electrochemical performances of the porous CoOOHnanosheet array electrodes as the non-enzymatic glucose sensorsin an alkaline medium were investigated. For confirming the detec-tion potential, five different potentials of 0.50, 0.52, 0.54, 0.56and 0.58 V were chosen for non-enzymatic amperometric glucosesensing, as seen in Fig. 5a. The regular increase of the current at0.52 V (vs. Ag/AgCl/KCl sat’d) indicates that the electrodes can beused for glucose detection with high sensitive detection. A well-defined and rapid current response can be detected with adding0.1 mM glucose, as shown in Fig. 5a. The CoOOH nanosheet arraysexhibit a sensitive and rapid current response to the glucose addi-tion, which can be attributed to the hierarchical porous structuresof the electrodes. We suggest that, the abundant mesopores inthe nanosheets and the macropores surrounded by the nanosheetscombine to form facilitated mass transport channels. The abun-

dant channels lead the reactants diffusing to the active sites easily,resulting in a sensitive and rapid current response in the detec-tion. Otherwise, the high specific surface area of the CoOOH filmsendows the electrodes abundant active sites in the electrochemical

materials with different Co oxides.

etection limit (�M) Linear range (mM) Reference

0.13 0.005–0.8 Hou et al. [28]0.97 ∼2.04 Ding et al. [30]0.058 ∼3.5 Kung et al. [29]0.9 ∼0.5 Lee et al. [27]1.37 0.003–1.109 This study

L. Zhang et al. / Sensors and Actuators B 210 (2015) 190–196 193

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Fig. 4. (a) CVs of bare Ti foil and CoOOH nanosheet arrays in 0.1 M NaOH solution inthe absence and presence of 0.5 mM glucose with a 20 mV s−1 scan rate; (b) 25 cyclesconsecutive CV curves of a porous CoOOH nanosheet array electrode in 0.1 M NaOHsolution with a 20 mV s−1 scan rate; (c) CV responses of the porous CoOOH nanosheet

ig. 3. BET results of CoOOH nanosheets scraped from the substrates: (a) N2 adsorp-ion/desorption isotherm. (b) Pore size distribution computed from the isotherm byhe BJH method.

eactions, resulting in a large response current toward the glucosen detection. As seen in Fig. 5a, the response current at 0.52 V ofhe electrode toward 0.1 mM glucose is more than 1.6 mA, whichs larger than the previous reported cobalt oxide glucose sensors27,28,30]. The larger response current is significant to lower theost of the devices in the practical detection.

From Fig. 5a, obviously, the sensitivity at 0.52 V is the highest inhe five applied potentials. The amperometric response of porousoOOH nanosheet arrays on Ti substrate and porous CoOOHanosheets modified GC electrode at 0.52 V with successive addi-ions of glucose are presented in Fig. 5b and c. For nanosheet arraylectrodes, the regression equation is Ipa (�A) = 0.0348 + 0.5268 Cconcentration, �M), with R = 0.995 for applied potential of 0.52 V.t applied potential of 0.52 V, the sensor displays a linear range up

o 1.109 mM, with a sensitivity of 526.8 �A mM−1 cm−2 and a detec-ion limit of 1.37 �M (signal/noise = 3). For comparison, the CoOOHanosheet powder scraped from the substrates was also tested byasting it on GC with Nifion. The amperometric response of porousoOOH nanosheet powder electrodes is shown in Fig. 5c. Theegression equation is Ipa (�A) = 0.0023 + 0.0667 C, with R = 0.9959or applied potential of 0.52 V. The sensor displays a linear rangef 3 �M–0.659 mM, with a sensitivity of 66.7 �A mM−1 cm−2 and

detection limit of 1.89 �M (signal/noise = 3). Obviously, thelectrocatalytic performances such as sensitivity and detection

imit all decline after scraping the nanosheets from the substrateso use as a powder electrode. Suffering from the damage of thereestanding porous structure, the powder electrodes cannotealize high sensitive and rapid current response due to the lack

array electrodes for various concentrations of glucose (0.2, 0.6, 1.0, 3.0 mM) in 0.1 MNaOH solution with a 20 mV s−1 scan rate.

of the facilitated mass transport channels compared with thenanosheet array electrodes, as seen in Fig. 2b. The comparisonof the analytical performances between CoOOH nanosheet array

electrodes with the other electrodes in the literatures is shown inTable 1. Comparing with the electrochemical glucose sensors basedon Co oxides reported in the literatures, the self-supported CoOOH

194 L. Zhang et al. / Sensors and Actuators B 210 (2015) 190–196

Fig. 5. (a) Amperometric responses of the CoOOH nanosheet array electrodes onthe addition of glucose solution at different potentials. (b) Amperometric responsesof the CoOOH nanosheet array electrodes on the addition of glucose solution ofdifferent concentrations. Inset of top left: amplification of part with low glucoseconcentrations. Inset of bottom right: calibration curve for current density vs. con-centration of glucose. (c) Amperometric responses of the CoOOH nanosheet powderelectrodes on the addition of glucose solution of different concentrations. Inset oftc

naattp

Fig. 6. Amperometric responses of CoOOH nanosheet array electrodes in 0.1 mM

blood serum sample which was obtained from Hospital of HarbinInstitute of Technology. The determined results of the blood serumsample shown in Table 2 are in accordance with those tested by

Table 2Results for determinations of glucose in blood serum sample.

Hospital (mmol L−1) Our biosensor (mmol L−1) RSD

op left: amplification of part with low glucose concentrations. Inset of bottom right:alibration curve for current density vs. concentration of glucose.

anosheet arrays on Ti foils in our work exhibit comparativenalytical performances, as seen in Table 1. Furthermore, the mostttractive features of the CoOOH nanosheet array electrodes are

he good reusability and reproducibility, which are significant forhe practical sensors. The good reusability and reproducibility areresented and discussed in following test.

glucose at different stages with the addition of 0.002 mM DA, 0.01 mM UA, 0.005 mMAA, in the solution of 0.1 M NaOH. Inset: Comparison of sensitivities of the testedanalytes, at an applied potential of 0.52 V vs. Ag/AgCl/sat’d KCl.

We also investigated the interferences from Dopamine (DA),Uric Acid (UA), ascorbic acid (AA) toward the detection of glu-cose, which commonly exist together with glucose in real samples(human blood). The normal physiological level of glucose is about3–8 mM, which is much higher than the concentrations of inter-fering species like DA, UA and AA. In this work, we evaluated theinterfering effect of 0.002 mM DA, 0.01 mM UA, and 0.005 mM AAcompared to 0.1 mM glucose at the potential of 0.52 V. As shown inFig. 6, there is no obvious current response observed with the addi-tion of DA, UA and AA. On the contrary, an obvious current responsewith the addition of 0.1 mM glucose was appeared. So, the currentresponse of the three common biomolecules DA, UA and AA causednegligible interference to the response of glucose on the CoOOHnanosheet array electrodes [20].

The most attractive features of the CoOOH nanosheet array elec-trodes are the good reusability and reproducibility, as shown inFig. 7. The reusability of an electrode was investigated by inspect-ing the amperometric responses on the addition of 1.0 mM glucosesolution for respective five times, as shown in Fig. 7a. The resultsdemonstrate the five response currents are very close, that theirrelative standard deviation is 2.14%. Another significant featureof the electrodes in practical application is the repeatability. Westudied the amperometric responses of five electrodes preparedwith the same method on the addition of 0.2 mM glucose solu-tion, as shown in Fig. 7b. The five electrodes realize outstandingconsistency, since the relative standard deviation of their responsecurrents is 3.21%. The good reusability, stability and reproducibil-ity of the CoOOH nanosheet array electrode lead them becominga promising candidate as the practical non-enzymatic glucosesensors.

In order to explore the practicability of the glucose sensor, itwas applied to determine the concentration of glucose in human

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Fig. 7. The reusability, stability and reproducibility of CoOOH nanosheet array elec-trodes: (a) amperometric responses of an electrode on the addition of 1.0 mMglucose solution for respective five times, the relative standard deviation is 2.14%;(t

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b) amperometric responses of five electrodes prepared with the same method onhe addition of 0.2 mM glucose solution, the relative standard deviation is 3.21%.

ospital, indicating that the glucose sensor has a great potentialor practical application for the analysis of glucose in real clinicalamples.

. Conclusion

Porous CoOOH nanosheets are electrodeposited on Ti waferss the electrodes to detect glucose. The macropores surroundedy the nanosheets and the mesopores in the nanosheets lead thelectrodes to show hierarchical mass transport channels and ade-uate active sites for detecting glucose, which endow the electrodes

arge response currents and high sensitivity as the glucose sensors.urthermore, the tough electrodes prepared by the in-situ bottom-p approach demonstrate a series of outstanding characteristics,

ncluding reusability, stability and reproducibility. These excellentdvantages lead this kind of simply prepared enzymeless glucoseensor to be a promising candidate for practical application.

cknowledgements

This work was supported by the National Science Foundation ofhina (NSFC) (no. 21203044) and Fundamental Research Funds forhe Central Universities (HIT. IBRSEM. A. 201407).

[

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Biographies

Li Zhang is a Lecturer of Academy in Fundamental and Interdisciplinary Sciences,Harbin Institute of Technology, China. She obtained her Ph.D. degree in 2009 fromLanzhou University in physical chemistry. Her research interest lies in electroana-lytical chemistry.

Chunli Yang earned a master degree in 2014 in Academy of Fundamental and Inter-disciplinary Sciences, Harbin Institute of Technology, China. Her MS.D. researchconcentrates on the non-enzymatic glucose sensors.

Guangyu Zhao is an Associate Professor in Academy of Fundamental and Interdisci-plinary Sciences, Harbin Institute of Technology, China. He obtained his Ph.D. degreein 2008 from Lanzhou University in Analytical Chemistry. His interest lies in Lithiumion batteries.

Jianshuai Mu is currently Ph.D. student majoring in analytical chemistry inAcademy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Tech-nology, China. He mainly engages in the study of nanomaterials as artificial

Yan Wang is a Professor of chemistry in Harbin Institute of Technology, China.Her research interests are biotechnology, nanotechnology and sensing technol-ogy, especially as applied to the development of electrochemical sensors andbiosensors.