A coral-like macroporous gold–platinum hybrid 3D electrode for enzyme-free glucose detection

Sensors and Actuators B 155 (2011) 134–139

Contents lists available at ScienceDirect

Sensors and Actuators B:

Chemical

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s n

b

A coral-like macroporous gold–platinum hybrid 3D electrode for enzyme-free glucose detection

Yi-Jae Lee,Jae-Yeong Park ?

Department of Electronic Engineering,Kwangwoon University,447-1,Wolgye-Dong,Nowon-Gu,Seoul 139-701,Republic of Korea

a r t i c l e i n f o Article history:

Received 16July 2010

Received in revised form 7November 2010Accepted 22November 2010

Available online 26 November 2010Keywords:

Macroporous Au Au–Pt hybrid Pt nanoparticles Electroplating Enzyme-free Glucose

a b s t r a c t

We are introducing a macroporous Au–Pt hybrid 3D electrode to be used for enzyme-free glucose detec-tion.The proposed hybrid electrode was fabricated with a three dimensional structure by electroplating platinum nanoparticles onto the surface of the coral-like macroporous Au.It was then physically analyzed by using ?eld emission scanning electron microscopy (FESEM).The porosity and window pore size of the macroporous Au electrode were 50%and 100–300nm,respectively.The diameters of the Pt nanoparticles ranged from 10to 15nm.Through cyclic voltammograms in a 1M sulfuric acid solution,we con?rmed that the hybrid electrode exhibited a much larger surface activation area with a roughness factor (RF)of 2024.7than the macroporous Au electrode with a roughness of 46.07.The highly improved surface activation area was caused by the electroplated Pt nanoparticles.The hybrid electrode exhibited a much stronger electrocatalytic activity due to glucose oxidation than the macroporous Au electrode.At 0.4V,it responded linearly to the glucose up to 20mM in a neutral media with a detection limit of 0.025mM and detection sensitivity of 39.53?A mM ?1cm ?2without being affected by interfering species.It also showed a stable recovery response to the step changes of the glucose concentration.

? 2010 Elsevier B.V. All rights reserved.

1.Introduction

Highly sensitive and selective determination of blood glucose has been widely investigated in view of its clinical importance,particularly for patients that have diabetes.The rising demands for a glucose sensor with a high sensitivity,a fast response,good stability,and a low cost have driven tremendous academic and commercial efforts for decades [1].In particular,electrochemi-cal analysis methods have been evaluated as effective approaches for the determination of glucose,due to the simplicity of the instrumentation and operation [2].Therefore,various research efforts have been performed in order to develop amperometric glucose sensors,due to their high sensitivities,easy miniaturiza-tion,and speedy monitoring of blood glucose levels [3].The most well known amperometric sensors utilize enzyme (GOx)based electrodes,which were introduced in 1962[4].Although these enzymatic sensors demonstrate good selectivity and sensitivity,the enzyme is easily denatured during its immobilization processes.The most serious problem is a lack of stability due to the intrinsic nature of enzymes [5,6].From this viewpoint,the development of enzyme-free glucose detection still remains a challenge.

?Corresponding author.Tel.:+8229405113;fax:+8229421502.E-mail address:jaepark@kw.ac.kr (J.-Y.Park).Recently,many studies have attempted to explore enzyme-free glucose sensors with high sensitivity and selectivity.The direct electrochemical oxidation of glucose on bulk noble metals,such as platinum and gold,has been studied in the hope that an effec-tive enzyme-free sensor could be developed.However,even the state of the art technology used for glucose sensing with platinum electrodes is not free from poisoning by adsorbed intermediates,resulting in both a low sensitivity and a poor selectivity [7].Efforts have been made to attempt to overcome these drawbacks by mod-ifying the electrode substrate with ad-metals [8]and nanoparticles [9]or by using different kinds of electrode materials [10,11],since the key factor that affects both the sensitivity and selectivity of glucose detection is the electrocatalytic activity of the electrode material.A gold single crystal electrode modi?ed with Ag ad-atoms could improve the eletrocatalytic activity towards glucose oxidation by effectively reducing the overpotential and increas-ing the peak current [8].In addition,since nano-structured Pt or Au electrodes possess a very large surface activation area,thereby favoring kinetically controlled reactions (i.e.,the electrocatalytic oxidation of glucose)more than diffusion controlled reactions (i.e.,the common electroactive species),high sensitivity for glu-cose detection can be expected.Macroporous platinum [12]and platinum-nanotubule array [10]electrodes with a high electrode surface roughness were developed to enhance the amperometric response of glucose oxidation and effectively lower the interfer-ences coming from the electroactive species.

0925-4005/$–see front matter ? 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.snb.2010.11.037

Y.-J.Lee,J.-Y.Park/Sensors and Actuators B155 (2011) 134–139

135

Fig.1.The schematic drawings for the fabrication process of(A)the macroporous Au by the templating and dealloying method based on the sol–gel process and(B)the macroporous Au–Pt hybrid electrode(macroporous Au-/nPts)using the electroplating technique.

In this paper,we introduce a macroporous Au–Pt hybrid electrode to be used for an ultra-high sensitivity and selective enzyme-free glucose detection.The open interconnected porous structures of the three-dimensional Au electrode were utilized to ef?ciently enlarge the surface area and ensure the accessibility of the reactants to the surface active sites of the electrode[12].The Pt nanoparticles were applied in order to improve the surface rough-ness and the electrochemical catalytic activity.The macroporous Au electrode was?rst fabricated by using a templating method. The Pt nanoparticles were then formed onto the surface of the macroporous Au electrode by using a nonionic surfactant and an electroplating technique.The surface morphology of the fabricated hybrid electrode was?rst analyzed by using a?eld emission scan-ning electron microscope(FESEM).It was then characterized in a sulfuric acid solution to check its surface roughness using cyclic voltammetry.In order to evaluate its electrochemical catalytic characteristics and applicability,amperometric measurements of the hybrid electrode were performed in a hydrogen peroxide and a glucose solution with interfering species,respectively.Finally,its physical and electrochemical characteristics were compared to the macroporous Au and other previously reported electrodes.

2.Experiment

2.1.Reagents

All of the solutions were prepared using deionized water (resistivity≥18M cm).The aluminum precursor(aluminum sec-butoxide),surfactants(stearic acid and magnesium stearate),gold precursor(HAuCl4),acid etchant(mixture of11.8M H3PO4and 0.6M HNO3),and sodium tetrahydridoborate(NaBH4)were pre-pared to make the coral-like macroporous Au electrode[13].The electroplating mixture for the Pt nanoparticles was comprised of42%(w/w)C16EO8(octaethylene glycol monohexadecyl ether, 98%purity,Fluka),29%(w/w)deionized water(18M cm)and 29%(w/w)HCPA(hexachloroplatinic acid hydrate,99.9%purity, Aldrich)[14].

The fabricated hybrid electrode was measured to check its sur-face roughness factor(RF)in a1M sulfuric acid(H2SO4,95–98%, ACS,Sigma–Aldrich)solution using cyclic voltammetry.The1M H2SO4was prepared by dilution in deionized water(100mL vol-ume).The0.1M ascorbic acid(AA,98%,Sigma),acetaminophen(AP, 99%,Sigma),uric acid(UA,99%,Sigma),urea(98%,Sigma),sucrose (99.5%,Sigma),and0.5M hydrogen peroxide(H2O2,30–32%,Dong-Woo Fine Chem,Korea)solutions were prepared by diluting them in a phosphate buffered saline(PBS,0.1M,pH7.4)solution.The?-d(+)glucose(99.5%,Sigma)stock solution was prepared by diluting it in a0.1M PBS(pH7.4)solution and allowing it to stand for24h before use,in order to allow equilibration.

The electrochemical experiments on the fabricated electrodes were performed by using an electrochemical analyzer(Model600B series,CH Instruments Inc.,USA)at room temperature.A three electrode system was used,including a fabricated electrode for the working electrode,a?at Pt bar as a counter electrode,and an Ag/AgCl reference electrode with3M NaCl.The surface mor-phology was characterized using a Hitachi S-4300?eld emission scanning electron microscope(FESEM).The amperometric mea-surements were carried out in a0.1M PBS(20mL volume)solution under continuous stirring by diluting the H2O2(0.5M)and glucose (0.5M)with a constant concentration.The response current was recorded after stabilizing the background current at a nearly con-stant value and the prepared H2O2and glucose were consecutively injected at100s regular intervals.

2.2.Preparation of the macroporous gold–platinum hybrid electrode

Fig.1represents the simpli?ed fabrication sequence of the pro-posed coral-like macroporous Au and Au–Pt hybrid electrodes.As shown in Fig.1A,the coral-like macroporous Au was formed using aggregated Au nanoparticles.A similar phase separation was found in the preparation of the monolithic TiO2via a template-free sol–gel process[15].The aluminum precursor and surfactants were sepa-rately dissolved in sec-butyl alcohol.The Au precursor was added to the dissolving surfactant solution.These two solutions were mixed;this was followed by a slow addition of water at the rate of1mL/min.The molar ratio of this reaction mixture was1Al(sec-BuO)3:0.09HAuCl4:0.2surfactant:10sec-BuOH:7H2O.NaBH4was used as the reducing agent for the Au precursor.The mixture was then continuously stirred for24h.Subsequently,the mixture was dried at80?C and calcined at550?C in air.After this calcination step,the surfactant was easily removed;the resulting material had

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155 (2011) 134–139

Fig.2.The FESEM images of (A)the fabricated Au–Pt hybrid electrode and (B)its close up view:the porousity and window pore size are 50%and 100–300nm,respectively.The diameter of the Pt nanoparticles is approximately 10–15nm (white dots).

a nanoporous alumina structure with a sintered Au network.The alumina network with the nanopores was etched selectively with acid etchants (11.8M H 3PO 4and 0.6M HNO 3).After the etching,an additional heating process was conducted at 150?C for 20min in order to remove the defects of the Au nanoparticles.As a result,we obtained the desired macroporous Au,namely,the reverse phase of the alumina network.The macroporous Au is in the form of a powder.In order to form a disk type electrode,the fabricated macroporous Au was pressed at room temperature into a circular mold (10kgf cm ?3applied pressure,7mm dia.,1mm thickness).Fig.1B shows the fabrication sequence of the proposed macro-porous Au–Pt hybrid electrode.The electroplating mixture used for forming the Pt nanoparticles was well mixed and heated to 85?C in a water-jacketed vial;the mixture became transparent and homogeneous.The macroporous Au electrode was dipped into the mixture and the temperature was lowered to 25?C.As a result of this step,the liquid crystalline structure of the nonionic sur-factant C 16EO 8was formed onto the surface of the macroporous Au electrode.The Pt nanoparticles were then electroplated onto the macroporous Au electrode by a constant potential (?0.12V vs.Ag/AgCl)with a charge condition of 70mC.It was subsequently soaked in distilled water for several hours in order to remove the C 16EO 8[16].The ?nal result was the proposed hybrid electrode.

3.Results and discussion 3.1.Morphological analysis

The morphology of the fabricated hybrid electrode was exam-ined using a FESEM.As shown in Fig.2A,it had a coral-like structure and demonstrated a greatly enhanced surface roughness.The pri-

mary branch,at ca.2–3?m,has the shape of a bumpy branch that resembles a one-directional link of spherical particles (ca.200–400nm),which are made up of aggregated Au nanoparticles (ca.10–30nm).The porosity and window-pore size of the fabri-cated macroporous Au–Pt hybrid electrode were approximately 50%and 100–300nm,respectively.Fig.2B shows that the elec-troplated Pt nanoparticles were uniformly well-formed onto the surface of the macroporous Au electrode.Their diameters were approximately 10–15nm (white dots).The density and size of the Pt nanoparticles could also be controlled by varying the electroplat-ing charge conditions,since the electroplating current is strongly dominant in the forming of the metal particles [16,17].3.2.Electrochemical characterization

In order to electrochemically characterize the real surface of the fabricated electrodes,a cyclic voltammogram was obtained in a 1M H 2SO 4solution using a scan rate of 50mV s ?1.The applied potential ranged from ?0.4to 1.2V vs.Ag/AgCl.The RF was calculated as laid out in [18]:

Roughness factor (RF):

f r =

A r A g

(1)

where A r is the real (true,actual)surface (interface)area and A g is the geometric surface (interface)area.As shown in Fig.3A,the amount of surface oxide formed is measured by the integra-tion of the Au oxide reduction peak in a cathodic scan (shadow area).The RF of an Au electrode is usually calculated on the basis of the electrochemically deposited oxygen monolayer on the elec-trode surface and the measurement of the charge corresponding to this monolayer [19,20].Since the charge/area ratio is different for the different planes of an Au-crystal,it is usually considered to

be

Fig.3.The cyclic voltammograms (CV)of the fabricated (A)macroporous Au and (B)Au–Pt hybrid electrodes in a 1M sulfuric acid solution at a potential scan rate of 50mV s ?1in order to obtain the surface roughness factor.

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137

Fig.4.The amperometric response of the macroporous Au–Pt hybrid electrode to the successive additions of 1mM H 2O 2in a stirred PBS (pH 7.4);the various applied potentials:200–600mV.

about 0.4mC cm ?2for a regular polycrystalline Au electrode [21].The accumulated charge density (18.42mC cm ?2)in the shadow area of Fig.4A was divided by the aforementioned charge density of the oxygen monolayer (0.4mC cm ?2).As a result,the calculated RF of the fabricated macroporous Au electrode was ?gured to be approximately 46.07.

The determination of the RF of the Pt electrode is based on the formation of the hydrogen monolayer electrochemically

adsorbed

Fig.5.The response currents of the fabricated (A)macroporous Au and Au–Pt hybrid electrodes to various H 2O 2concentrations in a continuously stirred 0.1M PBS (pH 7.4)solution and (B)the amperometric responses to the consecutive addition of the 5mM glucose and the 0.1mM interfering species (e.g.AA,AP)in a stirred PBS (lower left inset:the detection limit test)with an applied potential of 400

mV.

Fig.6.The calibration curve of the amperometric response of the macroporous Au–Pt hybrid electrode to the successive additions of 5mM of glucose in a stirred PBS (pH 7.4)with an applied potential of 0.4V.The inset is the plot of the amper-ometric response to injections of 5mM glucose,0.1mM interferents of AA,AP,UA,urea,and sucrose at the macroporous Au–Pt hybrid electrode at 0.4V.

at the electrode surface [19].In Fig.3B,the ?rst cathodic peak (at the 0.4V areas)is caused by the reduction of oxygen,which is quickly followed by a second dual peak related to the reduction of hydrogen (shadow area).Since a charge is necessary to form the monolayer of adsorbed hydrogen and since the electrode area is covered by each hydrogen atom,the electrochemical RF is easily calculated.The RF of the macroporous Au–Pt hybrid electrode is determined to be the value of the adsorption charge divided by 0.21mC cm ?2[22].The accumulated charge density (425.18mC cm ?2)in the shadow area of Fig.4B was divided by the aforementioned charge density of the adsorbed hydrogen monolayer (0.21mC cm ?2).The calculated RF was 2024.7which is the highest value found in the reported litera-ture.This might be caused by a synergistic effect of the macroporous Au structure and the Pt nanoparticles.This result is also supported by the aforementioned FESEM images.These results imply that the second metal is a crucial factor affecting the electrochemical activity of the bimetallic nanocatalysts.

The response currents of the hybrid electrode to the applied potential were investigated at various H 2O 2concentrations (see Fig.4).The current increased with the H 2O 2concentration,caused by the increase of the electron transfer force.However,in order to avoid a high working potential causing the AA and AP to directly oxidize on the electrode,a working potential of 0.4V was ?nally selected and applied.

Fig.5A illustrates the electrocatalytic effect of the macroporous Au and the Au–Pt hybrid electrodes towards the oxidation of H 2O

2

Fig.7.The current responses of 8different macroporous Au–Pt hybrid electrodes to 5mM of glucose in PBS (pH 7.4)with an applied potential of 0.4V.

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Table1

The comparison of the analytical parameters of the macroporous Au–Pt hybrid electrode to other glucose electrodes.

Reference Electrode Detection limit(?M)Linear range(mM)Sensitivity(?A mM?1cm?2)

[35]PtRu/MWCNTs modi?ed GCE251–1528.26

[36]Nanoporous PtPb networks–Up to1610.8

[37]PtPb nanowires8Up to1111.25

[38]Mesoporous Pt52–1011.8

[39]PtM(M=Ru,Pd and Au)/MWNT50Up to1510.6

This work Macroporous Au–Pt hybrid251–2039.53

in PBS at a pH of7.4.The H2O2has a direct relationship with numerous biological processes because H2O2is a very important intermediary or product of these biochemical reactions[23].The measurement was performed at a potential of0.4V after stabiliz-ing the background current,done at regular100s intervals.The 1mM H2O2was consecutively injected at regular100s intervals. The measured raw data was plotted against the response cur-rent to the applied H2O2concentration at the saturation time. As shown in Fig.5A,the response currents gradually increased when the concentration of the H2O2increased.The calculated sen-sitivities of the macroporous Au and Au-/nPts electrodes were 78.75?A mM?1cm?2and258.87?A mM?1cm?2,respectively.

Fig.5B shows the amperometric responses of the fabricated macroporous Au and Au–Pt hybrid electrodes to consecutive addi-tions of the5mM glucose and the0.1mM interfering species(e.g. AA,AP)in a stirred PBS.One of the most important analytical factors in amperometric bio-molecule detection is the ability of the elec-trode to discriminate between the interfering species,which has electroactivities similar to the target analyte,and the target ana-lyte itself.The oxidizable compounds,such as AA and AP,normally co-exist with the glucose in real samples.The normal physiological level of glucose is about3–8mM,which is much higher than the concentrations of the interfering species like AA(0.1mM)and AP (0.1mM)[24–26].As shown in Fig.5B,the response current of the macroporous Au-/nPts was stable and linear without being affected by the interfering species.A more speci?c anti-interferents study was also performed with0.1mM UA,urea,and sucrose.Consider-ing that the concentration of glucose in human blood is about30 times that of AA,AP,UA,urea,and sucrose[27,28],the amperomet-ric response currents of the hybrid electrode towards the addition of5mM of glucose,0.1mM of AA,AP,UA,urea,and sucrose were examined in a0.1M PBS(pH7.4)solution at a potential of0.4V. Fig.6and its inset show their amperometric responses and compar-isons.The current response for the electroactive interfering species to that of the glucose shown by the hybrid electrode was below10%. In addition,upon injection of the glucose at regular intervals,a rapid increase in the currents was also observed.However,the response of the macroporous Au electrode to the glucose concentration was relatively slow and affected by the interfering species.The lower left trace in Fig.5B shows the amperometric response of the hybrid electrode to the successive injections of low concentration glucose. As indicated,a response with95%of the steady state current was observed at between14s and45s in the various glucose detections ranging up to25mM.When the glucose concentration increased, the response time decreased,since the current response gradu-ally saturated at the high glucose concentration range.The limit of detection was0.025mM,which was comparable to the previously reported ones[29,30].

The current density vs.the glucose concentration relationship is shown in Fig.6.The response current is linear to the glucose con-centration in the up to20mM range.The glucose sensitivity of the macroporous Au–Pt hybrid electrode was39.53?A mM?1cm?2, which is much higher than the one reported in our previous work [31,32].In recent reports,the sensitivities of the glucose sensors reached only17.8and11.8?A mM?1cm?2[33,34]

.Fig.8.The current–time recordings for the recovery characteristics of the fabricated Au–Pt hybrid electrode with step changes in the glucose concentration.

The reproducibility of the hybrid electrode was also evaluated by comparing it to the currents of different electrodes.The ampero-metric response of8different electrodes to a5mM glucose solution was independently tested,providing a relative standard deviation (RSD)value of8.19%(see Fig.7).As listed in Table1[35–39],the detection limit,the linear calibration range,and the sensitivity to glucose of the hybrid electrode are comparable to,and sometimes better than,that obtained from several recently studied electrodes.

Fig.8shows a current–time recording illustrating the recovery characteristics of the hybrid electrode towards stepwise changes in the glucose concentration.This test was performed using different concentrations of glucose(0,5,and10mM).All of the measure-ments were performed using a stepwise change of the glucose concentration at regular100s intervals.As shown in Fig.8,the cur-rent response was very stable regarding both the increments and decrements of the glucose concentration.The recovery rates for the 0,5,and10mM states were90.5%,126%,and94.5%,respectively.

4.Conclusions

We developed and characterized a highly roughened macro-porous Au–Pt hybrid electrode to be used for highly sensitive enzyme-free glucose sensor applications.The hybrid electrode was fabricated using a templating method and an electroplating tech-nique.It was then characterized by using a FESEM and a cyclic voltammetric method.It showed a highly improved surface rough-ness factor;the highest compared to the previously reported ones. The highly roughened electrode surface of the hybrid electrode exhibited an extremely high sensitivity for H2O2without any redox mediator or enzymes.Therefore,it was evaluated to check its appli-cability to enzyme-free glucose detection.It exhibited an extremely high glucose sensitivity,due to the enlarged activation surface area, without being affected by the interfering species,such as AA and AP.The electrochemically deposited Pt nanoparticles on the macro-porous Au electrode led to a signi?cant improvement in the real surface activation area and so exhibited a highly sensitive glucose

Y.-J.Lee,J.-Y.Park/Sensors and Actuators B155 (2011) 134–139139

detection.In addition,it provided a number of other attractive fea-tures,such as a fast response and a stable recovery.Therefore,we believe it is strongly applicable for enzyme free glucose sensors or for various biosensors used to detect biologically important ana-lytes.An anti-fouling packaging method for practical applications is currently being investigated.

Acknowledgements

This research was supported by the Seoul Research and Business Development Program(Grant no.10583).The authors are grateful to MiNDaP(Micro/Nano device&Packaging Lab.)group members of Department of Electronic Engineering and Prof.Y.H.Kim of Depart-ment of Chemical Engineering in Kwangwoon University for their technical support.

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Biographies

Yi-Jae Lee received his B.S.and M.S.E.E.degrees in Electronic Engineering from Kwangwoon University,Seoul,Korea in2007and2009,respectively.Now he is a doctoral student at Kwangwoon University.His research focuses on high catalytic electrode based in vivo or in vitro electrochemical non-enzymatic biosensors for micro-TAS,and dissolved oxygen sensors for bio/environmental applications.

Jae-Yeong Park received his Ph.D.degree in Electrical and Computer Engineering from the Georgia Institute of Technology,Atlanta,GA,USA in1997.After graduation, he worked at the Packaging Research Center of the Georgia Institute of Technology as a research engineer for2years.He has also worked for the Microsystem group at the LG Electronics Institute of Technology as a Team leader of RF MEMS research for 6years.In September2004,he joined the faculty of the department of Electronics Engineering in Kwangwoon University in Seoul,Korea.He has published more than 150journal articles and conference proceedings and?led more than105patents. His current research interests include vibration based MEMS energy harvesting devices,microactuators for optical and RF applications,micro-packaging(wafer level packaging and micro-solder transferring),non-enzymatic electrochemical bio and environmental sensors,and PCB embedding RF components and modules.