Carbon Supported Cobalt and Nickel Based Nanomaterials for Direct Uric Acid Determination

Carbon Supported Cobalt and Nickel Based Nanomaterials forDirect Uric Acid Determination

Baljit Singh,a Fathima Laffir,b Calum Dickinson,b Timothy McCormac,c Eithne Dempsey*a

a Centre for Research in Electroanalytical Technologies (CREATE), Institute of Technology Tallaght, Tallaght, Dublin 24, Irelandtel: 00353140428562; fax 0035314042404

b Materials Surface Science Institute, University of Limerick, Co. Limerick, Irelandc Electrochemistry Research Group, Dundalk Institute of Technology, Dundalk, Co. Louth, Ireland*e-mail: [email protected]; [email protected]

Received: July 15, 2010;&Accepted: October 13, 2010

AbstractNovel Co-Ni based catalysts on activated carbon support were prepared using NaBH4 as a reducing agent in aque-ous conditions and examined with respect to direct amperometric uric acid detection. The surface morphology andcomposition of the synthesised materials were examined using transmission electron microscopy (TEM), thermogra-vimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was employed inorder to confirm the typical metallic electrochemical response of the Co, Ni and Co-Ni based materials. The combi-nation of metal hydroxides/oxides and nanoparticles in the carbon supported Co-Ni based materials were found toplay a key role in uric acid determination. Upon surface confinement of the Co1Ni1/C material, uric acid sensitivity248.2 mA mM�1 cm�2 and limit of detection 0.08 mM at Eapp =+0.4 V vs. Ag/AgCl was found by hydrodynamic am-perometry over the range 0–250 mM (r2 =0.9992). The sensor provided linear and reproducible behaviour over awide range (25–575 mM) of uric acid. The composite materials showed excellent selectivity with respect to common-ly found interferences with no response even at 10-fold concentration of urea, glucose and oxalate, and minimal in-fluence of ascorbic acid (2 fold concentration). Overall, these materials are excellent candidates for direct uric aciddetection in a stable, sensitive and very specific fashion over relevant physiological ranges, eliminating the pH, tem-perature sensitivity and lifetime issues associated with enzyme based systems. The materials are very promising fora range of applications including wound care/management and as non-enzymatic disposable uric acid test strips.

Keywords: Uric acid detection, Nonenzymatic sensor, Carbon supported catalysts

DOI: 10.1002/elan.201000444

Presented at the 13th International Conference on Electroanalysis, ESEAC 2010, Gij�n, Spain

1 Introduction

Uric acid (UA, 2, 6, 8-trihydroxypurine), is the primaryend product of purine metabolism (of exogenous or en-dogenous purine bases), and is an important biomoleculepresent in blood serum and urine [1]. For a healthyperson, the normal concentration of uric acid is 240–520 mM in serum and typically 1.4–4.4 mM in urine [2].Hyperuricemia (elevated concentrations of UA) enhancesthe uric acid level and results in gout, due to depositionof high concentrations of sodium urate crystals in the softtissues, joints and tendons. The level and determinationof uric acid in blood, serum and urine is very importantand is a strong marker for cardiovascular [3], hyperten-sion [4], metabolic syndrome [5] and diabetic complica-tions [6]. Therefore, monitoring uric acid levels is re-quired in order to avoid and control such disorders. Themost commonly employed methods include colorimetric[7], enzymatic [8] and electrochemical approaches [9–13].However, all of these methods are not suitable for accu-

rate determination of uric acid and are affected by somecommon issues such as specificity, stability, cost and inter-ferences. The colorimetric method is unreliable for accu-rate determination of uric acid due to interference by as-corbic acid. Reductive and enzymatic determination ofuric acid, are two acceptable methods applied in routineclinical analysis. The reductive methods involve the oxi-dation of uric acid with phosphotungstate reagent to al-lantoin with the resultant blue color of the tungstate solu-tion and are non-specific in nature. The enzymaticmethod is specific and involves the catalytic oxidation ofuric acid with the enzyme uricase to allantoin associatedwith the formation of hydrogen peroxide. These methodsare slow and also expensive, making them unsuitable forreal time monitoring.

Although enzymatic methods are promising due totheir high level of selectivity, they are expensive and theassociated detection limits may not be sufficient. The ap-plication of enzymatic biosensors has been hindered bythe thermal and chemical deformations of the enzymes as

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well as complex immobilisation procedures that may fur-ther decrease stability. The response of enzymatic sensorsalso depends upon pH, humidity, toxic chemicals and O2

concentrations. Electrochemical methods for the determi-nation of uric acid are more selective, less expensive andless time-consuming relative to other methods and offeran analytical platform which can exhibit higher sensitivityand specificity. Several studies based on non-enzymaticmethods have also been made to detect uric acid, e.g. useof carbon paste or activated glassy carbon electrodes[14], carbon nanotubes [15] and redox mediators modi-fied electrodes. [10]. A methylene blue based uric acidbiosensor is capable of reducing the effects of O2 concen-tration and interference by tuning the redox potential andkinetic parameters [10] and was employed for measure-ments in human serum or urine. There is very limitedwork regarding direct detection of uric acid in humanwhole blood [10,11,16] and a reliable, accurate and pre-cise uric acid detection using a disposable test strip couldbe a useful development.

One of the main problems with electrochemical meth-ods is the measurement at high overpotentials and thepresence of interferences by electro active compounds.Since uric acid and ascorbic acid co-exist in physiologicalfluids, the development of new methodologies for thesensitive, selective and rapid analysis is required. Bothuric acid and ascorbic acid have similar oxidation poten-tials at most common electrodes, requiring design of newstrategies which allow signal discrimination. To solve thisproblem, attention has been focused on electrode surfacemodification using carbon materials [17] and polymers[18]. Surface modification is also an effective way to im-prove the analytical performance and to enhance the sen-sitivity due to co-catalytic activity by the modifier or me-diator. Several electrochemical approaches have been re-ported for the selective determination of uric acid. Weiet al. [19] used a SWCNT-modified gold electrode for uricacid analysis while Wang et al. [13] proposed the detec-tion of uric acid in the presence of 5 mM dopamine, usinga glassy carbon electrode coated with choline and goldnanoparticles. O�Sullivan et al. [20] have reported the si-multaneous detection of ascorbic acid and uric acid, byusing an o-aminophenol-modified GCE. Ramaraj andSelvaraju used a Cu modified GCE by electro-deposition[12] for the detection of uric acid (between 20 and50 mM).

Nickel electrodes were initially studied by Fleischmannet al. [21] in the early 1970s in an investigation of theelectrocatalytic ability of the Ni(II)/(III) redox couple to-wards the electro-oxidation of small organic molecules.As a result of follow-on activity [22,23], it was deter-mined that the Ni(OH)2, present at the electrode, is ini-tially oxidised to the catalytically active NiOOH species,which subsequently oxidises small organic analytes.Nickel alloys have been frequently used in non-enzymaticdetection, such as Zn-Ni [24] and Ni-Cu [25]. However,to date, there has been no report on the usage of carbonsupported CoNi composites as an electrode modifier for

the direct determination of uric acid. The electrochemicalbehaviour of uric acid at such a modified electrode wasinvestigated here by cyclic voltammetry (CV), rotatingdisc electrode (RDE) and hydrodynamic amperometry.The presence of metallic oxide/hydroxide species alongwith nanoparticles appears to be responsible for the ex-cellent response of the supported catalysts. Results fromthe present study will aid in the development of a plat-form technology for non-enzymatic disposable uric acidtest strips which will avoid interference signals caused bythe common interferants, such as ascorbic acid, urea, oxa-lates and glucose.

2 Experimental

2.1 Reagent and Materials

Cobalt(II) chloride, nickel(II) chloride hydrated, silver ni-trate, sodium borohydride (NaBH4), isopropyl alcohol,activated carbon (Vulcan XC 72), uric acid, glucose, oxa-late, urea and ascorbic acid were used as received fromSigma Aldrich. Potassium hydroxide, Nafion (1 % v/v so-lution, diluted from 5 % solution in a mixture of loweraliphatic alcohols and water) and phosphate buffer solu-tion (PBS, 0.01 M, pH 7.4) were prepared according to re-quirements. Ultra pure water purified with Purelaboption water equipment (resistivity 17 MW cm) was usedin all experiments.

2.2 Synthesis of Carbon-Supported Co, Ni and Co-NiElectrocatalysts

In a typical synthesis, the calculated amount (based upon15% (w/w) calculations) of cobalt(II) chloride was dis-solved and stirred in 100 mL of phosphate buffer solution(PBS, 0.01 M, pH 7.4). Activated carbon (85 mg) in85 mL of deionised water was dispersed by sonicating itfor 50 min to achieve a homogeneous solution, followedby the addition of the metal precursor solution dropwiseto the carbon solution using a dropping funnel, maintain-ing the addition rate 0.5 mL/min with continuous stirring.NaBH4 (0.1 M) in 20 mL of distilled water was main-tained at 0 8C and then added to the solution and stirredfor 30 min. Then solution was heated to reflux at 100 8Cfor 2 hours followed by cooling to room temperature withconstant stirring. The solution was filtered using a0.45 mm pore size Millipore polycarbonate membrane, fol-lowed by washing with deionised water until all chlorideions were removed, as confirmed by the silver nitrate testafter each washing (also supported by XPS). The samplewas then dried under vacuum at 100 8C for 14 hours.

The same procedure was followed for the synthesis ofNi/C and Co1Ni1/C catalysts by maintained the overallmetallic weight (15% w/w) in the carbon matrix (85 %).The composition of the catalysts was controlled via thefeeding of metal precursors in the synthetic solutions andthe resulting composition was labeled based upon the ex-perimental feed i.e. composition 1 :1 indicates 50% Co

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and 50% Ni, based upon the overall metallic weight(15 % w/w) in activated carbon matrix (85 %) and was de-noted as Co1Ni1/C.

2.3 Preparation of Catalysts on Electrodes

Prior to each experiment, the glassy carbon electrode(geometric area, 0.0707 cm2) was polished with 1.0, 0.3,0.05 micron alumina powder and the electrode was soni-cated in acetone and distilled water, then washed withdistilled water and dried using argon at room tempera-ture. A typical suspension of the catalyst was prepared bysuspending 2 mg in 1 mL of water isopropyl alcohol mix-ture (3 : 1). The calculated amount (catalyst to Nafionratio 8 : 1) of Nafion solution (1 % v/v, diluted from 5 %Nafion solution) was added to the dispersion. The mix-ture was then sonicated for 20 min to achieve uniformdispersion of particles. The suspension was then quantita-tively (6.0 mg in each case) transferred to the surface ofthe polished glassy carbon electrode using a micropipetteto form a thin film on the surface of electrode and driedin air at room temperature. The same procedure wasadopted for all samples. Amperometric experiments werecarried out in phosphate buffer solution (0.01 M, pH 7.4)at Eapp =+0.4 V vs. Ag/AgCl for the direct detection ofuric acid.

2.4 Instrumentation and Measurements

The surface morphology and distribution of the synthe-sized Co/C, Ni/C and Co1Ni1/C catalysts were character-ised using transmission electron microscopy (TEM) witha JEOL 2011 operated at 200 kV using a LaB6 filamentequipped with a Gatan Multiscan Camera 794. For TEMmeasurements, Co/C, Ni/C and Co1Ni1/C samples weresuspended in isopropyl alcohol, sonicated and drop castonto a Cu grid and dried overnight at 40 8C. Thermogravi-metric analysis (TGA) was performed using a ThermalAdvantage Q50 with platinum pan, balance N2 flow40 mL min�1 and sample N2 flow 60 mL min�1. X-rayPhotoelectron Spectroscopy (XPS) was performed in aKratos AXIS 165 spectrometer, using monochromatic AlKa radiations of energy 1486.6 eV. High resolution spec-tra were taken at fixed pass energy of 20 eV. The surfacecharge was efficiently neutralised by flooding the samplesurface with low energy electrons. C 1s peak at 284.8 eVwas used as a charge reference to determine core levelbinding energies. For peak synthesis of high resolutionspectra, a mixed Gaussian-Lorenzian function with a Shir-ley type background subtraction were used. Relative sen-sitivity factors used are from CasaXPS library containingScofield cross-sections. The sample powder was dustedonto double sided adhesive tape for measurements.

All the electrochemical experiments were performedusing an electrochemical work station (CH InstrumentsInc. 900). Experiments were performed in 0.01 M phos-phate buffer solution and KOH (deaerated with high-purity Argon before each measurement) contained within

a conventional three-electrode cell at room temperature.A glassy carbon electrode modified with Co/C, Ni/C andCo1Ni1/C catalysts served as the working electrode, whilea platinum wire and a standard Ag/AgCl electrode wereused as counter and reference electrode respectively. Hy-drodynamic amperometric studies were carried out inphosphate buffer solution (pH 7.4) at an applied potential+0.4 V vs. Ag/AgCl for uric acid detection. Rotating diskelectrode (RDE) measurements were carried out in phos-phate buffer solution (pH 7.4) using a glassy carbon rotat-ing disc electrode (0.2828 cm2) with modulated speed ro-tator with CE mark, E2 series fast speed - PINE Instru-ment Company, at different rotation speeds.

3 Results and Discussion

3.1 Morphological, Compositional and Surface Studies

Co and Ni based catalysts on activated carbon supportwere synthesised using the method described above. Fig-ure 1a–d shows a typical set of TEM micrographs of theCo1Ni1/C and Ni/C catalysts. The lattice fringes of thenanoparticles in Co1Ni1/C, as shown in Figure 1a, b, canbe distinguished from the carbon support and the averagesize of nanoparticles was 3�1 nm. Lattice fringes for themetal nanoparticles for the monometallic catalysts werenot visible against the background of the amorphouscarbon support. This was possibly due to the nanoparti-cles being <5 nm and amorphous, making them difficultto observe on the amorphous carbon with various thick-ness. The presence of the metal in both the Co/C and Ni/C was confirmed by further techniques (XPS and electro-chemical techniques).

Thermo gravimetric analysis (Table 1) confirms themetallic weight % in the synthesised catalysts (theoretical15% loading in all cases). The final wt (%) obtained Co/C (14.7 %), Ni/C (17.7 %) and Co1Ni1/C (15.2 %) were ingood agreement with the synthetic composition (15 % w/w) and verifies the overall metallic content in the compo-site materials, supporting the experimental nominal com-position (Co : Ni, 1 :1), based upon 15 % (w/w) calcula-tions.

X-ray photoelectron spectroscopy (XPS) analysis wasemployed in order to investigate the oxidation states, sur-face species and surface composition in the carbon sup-ported Co, Ni and Co1Ni1 catalysts. The C 1s peak at284.8 eV was used as a charge reference to determine thecore level binding energies for the composites. Table 1summarises the results from XPS analysis of Co and Nibased hybrid materials. Co 2p and Ni 2p photoelectrontransitions were examined and appeared as a doublet(split due to spin-orbit coupling) with an intensity ratio of2 :1 for 2p3/2 : 2p1/2. In the fitting of the Co 2p spectra forthe Co/C sample (Figure 2A), the intensity ratio and thefull width at half maximum (FWHM) of the doublet com-ponent were kept constant. The principle Co 2p3/2 peakappears at a binding energy of 783.2 eV and its satellite at788.0 eV. The presence of intense satellite structures is in-

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dicative of Co+2 [26]. The chemical shift for Co+2 inCo(OH)2 is larger than for CoO with respect to Co0 [27].Additionally, the satellite peak appears at 4.8 eV abovethe principle line, close to a value of ~5.0 eV reported forCo(OH)2 and further from 5.6 eV for CoO by Mclntyreet al. [26]. It has been reported earlier that hydroxidesdisplay strong surface charge effects [26]. Although acharge neutraliser was used in this study to flood thesample surface with low energy electrons, it may be possi-ble that differential charging may have occurred resultingin the observed higher binding energies. Thus the Copeak may have contributions from both Co(OH)2 andCoO.

Figure 2B shows the XPS spectra for Ni 2p region ofNi/C catalyst. As in the case of Co 2p, Ni 2p is fitted withthe intensity ratio and the FWHM of the doublets wereconstrained. Ni 2p3/2 appears at a binding energy of858.2 eV and a strong satellite peak at 863.6 eV which arecharacteristic of Ni+ 2 and likely to be composed of NiOand Ni(OH)2. However, the satellite appears at 5.4 eVabove the principal line (5.6 eV reported by Mclntyreet al. [26]) and since the principal line was not accompa-nied by a satellite shoulder as in the case of NiO [26], itis reasonable to assign the peak to Ni(OH)2, although

minor contribution from Ni oxides particularly Ni2O3

cannot be ruled out [28]. The peak positions were how-ever higher than the reported, possibly due to surfacecharge effect/differential charging of the hydroxides [26].Figure 2C and D show the XPS spectra of Co 2p and Ni2p regions of the hybrid composite Co1Ni1/C, fitted withdoublet components. Both Co 2p3/2 and Ni 2p3/2 show in-tense satellite structures. Binding energies of Co 2p3/2 andNi 2p3/2 in the bimetallic catalyst show +0.2 and �0.2 eVshifts relative to their respective mono-catalysts. Thesechemical shifts are small, however, the fact that theyshow shifts in opposite directions suggest possible interac-tion between the two metallic ions. The FWHM of theprinciple peak of Co 2p3/2 in Co1Ni1/C is narrower(3.7 eV) than that of Co/C (4.8 eV) and a comparison ofthe intensities with the respective satellite peaks suggest agreater mix of compounds, Co(OH)2 and CoO in themonocatalyst. There is no indication of the metallic state ofeither species in all three catalysts demonstrating that thecatalyst nanoparticles have undergone surface oxidation.

The corresponding O 1s spectra of the Co/C, Ni/C andCo1Ni1/C catalysts (not shown) are broad and can be de-convoluted into more than one component to representthe oxidation of the metallic species and functionalisation

Fig. 1. Transmission electron micrographs (TEM) for (a, b) Co1Ni1/C and (c, d) Ni/C nanocomposites.




of activated carbon. The peak component associated withmetal oxidation appears at binding energies greater than~531.5 eV, also contributed perhaps by differential charg-ing of surface hydroxides and confirms the dominance ofhydroxides in all samples. The atomic ratio of Ni/Co is 2.1as determined by XPS for the catalyst Co1Ni1/C, which ishigher than its nominal composition shown in Table 1 andindicates that the surface layers of the Co1Ni1/C catalystare considerably enriched with nickel. When surface freeenergies are considered in a Co-Ni based system, it ismoderately favorable for Ni to segregate onto the surface[29], however, other factors such as reactivity and strongaffinity towards adsorbates also affect the segregationprocess [30]. Interestingly, the metal to carbon ratio esti-mated from XPS is 0.02, 0.13 and 0.19 for Co/C, Ni/C and

Co1Ni1/C respectively. The presence of Ni appears to in-crease the surface concentration of metallic species.

3.2 Electrochemical Characterisation

Figure 3 shows the cyclic voltammetric comparison of (A)Co/C, (B) Ni/C and (C) Co1Ni1/C modified electrodeswith a bare glassy carbon electrode in 1 M KOH at scanrate 0.1 V/s vs. Ag/AgCl and confirm the presence andtypical redox behaviour for the Co and Ni metals in alka-line media. In the case of Co/C (A), the redox couple ap-peared at E1/2�124 mV vs. Ag/AgCl at a scan rate of0.1 V/s and confirms the presence of cobalt in Co/C com-posite. The observed peaks correspond to the redoxcouple of Co(II)/Co(III). Over a wider potential scan

Fig. 2. XPS spectra for (A) Co 2p regions of Co/C catalyst and (B) Ni 2p regions of Ni/C catalyst verifying the presence of differentoxidation states in Co/C and Ni/C. (C) Co 2p and (D) Ni 2p regions of Co1Ni1/C catalyst and verifies the presence of different oxida-tion states in Co1Ni1/C.

Table 1. Summary of TEM, TGA and XPS results.

Catalyst d [a] (nm) wt [b] (%) B. E. [c] (eV) B. E. [d] (eV) Co :Ni [e] Co : Ni [f]

Co/C 3�1 14.7 783.2 788.0 1 : 0 –Ni/C 3�1 17.7 858.2 863.6 0 :1 –Co1Ni1/C 3�1 15.2 783.4 787.9 1 : 1 1 :2.1

858.0 863.6

[a] is the size (d) of nanoparticles in nanometers obtained from TEM analysis; [b] (%) wt obtained from TGA analysis (experimentalwt (%) is 15% (w/w) in all the catalysts; [c, d] are the binding energy (eV) values for principle and satellite Co 2p3/2 and Ni 2p3/2 [e] isthe experimental Co:Ni ratio based upon 15% (w/w) calculation; [f] is the Co : Ni ratio obtained from XPS analysis.




(�0.25 V to +0.65 V), an additional oxidation peak wasobserved at approximately 500 mV vs. Ag/AgCl with scanrate 0.1 V/s, which suggested the possible conversion be-tween different cobalt oxidation phases (e.g. CoO, CoO2,Co(OH)2, Co3O4, CoOOH) which are stable in alkalinepH [31,32]. In the case of Ni/C (B), a redox couple wasobserved at E1/2�356 mV vs. Ag/AgCl, corresponding toNi2+/Ni3+ in alkaline media. These peaks in the initialcycles appeared at 445 and 302 mV vs. Ag/AgCl and thenupon cycling become stable at 412 and 300 mV. It is wellknown in the literature that there are four phases of Ni,namely, b-Ni(OH)2, a-Ni(OH)2, b-NiOOH and g-NiOOH[33]. These phases can be interconverted and transforma-tions occur slowly and are generally incomplete, such thata/g and b/b systems can coexist under steady state condi-tions [21,34]. Therefore, there is a possibility thatNi(OH)2 is converted to NiOOH and the enrichment ofNi3+ species on the surface [21,34, 35] occurs in alkalineconditions in the forward scan. The reverse scan shows areduction wave corresponding to the reduction ofNiOOH to Ni(OH)2.

Figure 3C shows the cyclic voltammetric comparison ofthe Co1Ni1/C modified glassy carbon electrode with thebare glassy carbon electrode in alkaline solution. The

redox couple was shifted to a lower potential at E1/2

�310 mV vs. Ag/AgCl in comparison to Ni/C and ap-pears between Co/C and Ni/C, verifying the ensemble/electronic effect between nickel and cobalt. There is apossibility that the Co1Ni1/C modified electrode surface ismore abundant with Ni hydroxides (as the surface is en-riched with Ni according to XPS analysis) in alkaline so-lution (KOH), which are then transformed into oxy-hy-droxides along with the Co oxide/hydroxide species. It isalso well known that the inclusion of the cobalt atom intothe nickel hydroxide lattice leads to a significant increasein the amount of structural defects [36]. This can providemore active sites during catalytic performance and can in-crease sensitivity. The Co1Ni1/C modified electrodeshowed a stable response and upon redox cycling, thepeak potentials are invariable, which may be an indica-tion that the phase transformation from b to g is inhibitedfor Ni oxyhydroxide [34]. Figure 3D shows the stabilityperformance for the Co1Ni1/C modified glassy carbonelectrode in 1 M KOH at 0.1 V/s vs. Ag/AgCl over thepotential window (0.0 to 0.65 V) with a 3.0 % decrease ininitial response upon redox cycling (25 cycles).

Fig. 3. Comparison of cyclic voltammetric response for (A) Co/C, (B) Ni/C and (C) Co1Ni1/C modified electrodes with bare glassycarbon electrode in alkaline solution (1 M KOH) at scan rate 0.1 V/s vs. Ag/AgCl. (D) Stability performance (25 cycles) for Co1Ni1/Cmodified glassy carbon electrode in 1 M KOH at scan rate 0.1 V/s vs. Ag/AgCl.




3.3 Rotating Disk Measurements for Uric AcidElectrooxidation

Rotating disk electrode (RDE) measurements were per-formed for confirmation of direct electro catalytic oxida-tion of uric acid in phosphate buffer solution (pH 7.4)using the Co1Ni1/C modified glassy carbon electrode. Thecatalytic currents were recorded over rotation speeds 0–7000 rpm for different uric acid concentrations (50 mM–300 mM). Figure 4A shows the polarisation curves record-ed at different electrode rotation speeds (numbered 1–9),for the oxidation of uric acid at the Co1Ni1/C modifiedelectrode, in the absence (a) and presence (b) of 150 mMuric acid with electrode rotation rates: (1) 52.36 rad s�1,(2) 104.72 rad s�1, (3) 157.08 rad s�1, (4) 209.44 rad s�1, (5)314.16 rad s�1, (6) 418.88 rad s�1, (7) 523.60 rad s�1, (8)628.32 rad s�1 and (9) 733.04 rad s�1 at scan rate of1 mVs�1. This experiment was repeated with differentconcentrations of uric acid (50 mM, 100 mM, 150 mM,200 mM and 300 mM) and in each case, the current in-creased with rotation speed and result in a limiting cur-rent plateau for each concentration. This confirms the

electrooxidation of uric acid by the modified electrodesand the kinetic limitation at higher rotation speeds.

Figure 4B shows the resulting Levich plots obtained forthe Co1Ni1/C modified electrode at different concentra-tions of uric acid in phosphate buffer solution (pH 7.4) re-corded at different electrode rotation rates at scan rate of1 mVs�1. The Levich plots shown demonstrate the kineticlimitation at high rotation speeds for all concentrationsand enable kinetic information to be determined via theassociated Koutescky–Levich plots (Figure 4C) for differ-ent concentrations of uric acid. It is well known that Kou-tecky–Levich equation can be used to determine the rateconstant, kobs as [37]:

l�1 ¼ ðnFAkobsG ½UA�Þ�1

þðð0:62 nFAD2=3 v�1=6 ½UA�Þ�1 w�1=2Þð1Þ

Where n is the number of electrons, F is the Faradayconstant, A is the electrode area (cm2), G is the total sur-face coverage of the catalyst (mol cm�2), w is the elec-trode rotation speed (rad s�1), D is the diffusion coeffi-

Fig. 4. Polarisation curves (A) recorded at different electrode rotation rates for oxidation of uric acid (150 mM) on Co1Ni1/C modi-fied electrode, in (a) absence and (b) presence of uric acid with electrode rotation rates: (1) 52.36 rad s�1, (2) 104.72 rad s�1, (3)157.08 rad s�1, (4) 209.44 rad s�1, (5) 314.16 rad s�1, (6) 418.88 rad s�1, (7) 523.60 rad s�1, (8) 628.32 rad s�1 and (9) 733.04 rad s�1, atscan rate of 1 mVs�1, with apparent surface coverage 2.41�10�8 mol cm�2. (B) Levich plot for the steady-state electrocatalytic re-sponse for a Co1Ni1/C modified rotating disc electrode (RDE) at different concentrations (labelled) of uric acid and (C) is the Kou-tecky–Levich plots for the experimental data shown in (B). (D) Cyclic voltammetric response for Co1Ni1/C modified glassy carbonelectrode (GCE) in the presence of (a) 0 mM, (b) 1 mM, (c) 2 mM and (d) 3 mM uric acid in phosphate buffer solution (pH 7.4) atscan rate 0.1 V/s vs. Ag/AgCl.




cient (cm2 s�1), [UA] is the uric acid concentration (inmM) used during RDE experiment, u is the kinematic vis-cosity of the solution (cm2 s�1). The experimental surfacecoverage (G) was calculated using the equation [38].

G ¼ Q=nFA ð2Þ

Where Q is the charge obtained by integrating theanodic wave at slow scan rate (1 mV s�1), F is the Faradayconstant (96485 C) and A is the geometric area(0.2828 cm2) of the rotating disc electrode. The calculatedsurface coverage (G) using Equation 2 was 2.41� 10�8 molcm�2 at 1 mV/s. The slope and intercept obtained fromthese plots over mM uric acid range were used to find thediffusion coefficient (D) and rate constant (kobs) (Equa-tion 1). The resulting values of diffusion coefficient, D=5.1 �10�6 cm2 s�1 and rate constant, kobs =3.61�103

mol�1L s�1 are in agreement with previous reports[7, 17,39]. More detailed kinetic studies are required tounderstand the nature and role of such materials on thekinetics of the oxidation reaction.

3.4 Electrochemistry of Uric Acid

In order to verify the catalytic activity of the Co1Ni1/C to-wards non-enzymatic electro-oxidation of uric acid and tounderstand the electrochemical processes occurring at themodified electrode surface, cyclic voltammetric studieswere carried out in phosphate buffer solution (0.01 M,pH 7.4) in the absence and presence of uric acid. Forcomparison, studies were also carried out with bare andactivated carbon modified glassy carbon electrodes. Theanodic current (ipa) increased upon addition of uric acidto the phosphate buffer solution. Figure 4D shows the re-sponse of the Co1Ni1/C modified glassy carbon electrodeat different concentrations (labeled 0–3 mM) of uric acidin 0.01 M phosphate buffer solution (pH 7.4) at scan rate0.1 V/s vs. Ag/AgCl. There was a slight shift in the anodicpeak potential with increasing concentration of uric acidwhich may be attributed to a possible surface foulingeffect caused by the adsorption of the oxidised productsof uric acid. Cyclic voltammetry was employed to assessthe direct response of uric acid on both activated carbonmodified glassy carbon electrodes and bare glassy carbon(data not shown). A broad peak was observed at the bareglassy carbon electrode surface in the presence of uricacid indicating slow electron transfer kinetics due to thenature of the electrode and surface fouling by oxidation

products [40, 41]. The Co1Ni1/C modified electrodeshowed a much higher current response than activatedcarbon modified glassy carbon electrode, demonstratingthat the nanoparticles-hydroxides/oxides combinationplayed a role in the enhanced response towards the elec-trochemical reaction occurring at the electrode surface.This also signifies the role of surface structures and theensemble effect towards catalytic activity. The electrooxi-dation of uric acid at Co1Ni1/C modified electrode wasrelatively well defined with an enhanced anodic peak cur-rent by more than 4-fold compared to the unmodifiedglassy carbon electrode. The other factor that could facili-tate electrooxidation of uric acid is the improved/in-creased electrode surface area provided by the nanocom-posite catalyst. It can also be concluded that the surfacestructure and composition are very important and plays akey role in controlling the catalytic response.

The oxidation of uric acid is known to occur by a 2e�/2H+ transfer [18,41] to form an unstable bis-imine, whichsubsequently hydrolyzes, primarily to imine-alcohol andthen to uric acid-4, 5 diol. The latter is unstable and de-composes to allantoin. The overall reaction is summarizedas seen in Scheme 1.

This oxidation process also depends upon the pH ofthe solution. The uric acid exists mostly in anionic format pH higher than 5.4, which is the pKa value of uric acidat 25 8C [18,40].

3.5 Amperometric Studies

3.5.1 Amperometric Determination of Uric Acid

Figure 5A shows the comparison of the amperometric re-sponse for (a) Co1Ni1/C, (b) Ni/C, (c) Co/C and (d) ACmodified glassy carbon electrodes in phosphate buffer so-lution (pH 7.4) at Eapp =+0.4 V over the concentrationrange 25–250 mM (250 s time intervals for additions, withresponse time of 10 seconds). The sensitivity values (s,mA mM�1 cm�2), for all the electrocatalysts were calculat-ed from the calibration plot (Figure 5B) between back-ground corrected current responses and uric acid concen-tration and follows the order Co1Ni1/C (s=248.2, r2 =0.9992)>Ni/C (s=213.1, r2 =0.9992)>Co/C (s=193.0,r2 =0.9990). The observed order for LOD (limit of detec-tion in mM) values by the catalysts towards uric acid sens-ing was Co1Ni1/C (0.08)>Ni/C (0.21)>Co/C (0.32).These values are among the best obtained in literature todate (0.08 mM for Co1Ni1/C) as shown in Table 2.

Scheme 1.




Co1Ni1/C showed a relatively superior performanceunder similar conditions, demonstrating the ensembleeffect, surface modification and associated stability of theCo1Ni1/C catalyst, though the metal content in Co1Ni1/C(15.2 %) is lower than Ni/C (17.7 %) according to TGAanalysis. The catalyst also showed good reproducibility(n=3) with average sensitivity, 248.2 mA mM�1 cm�2 and% RSD=3.58 %, r2 =0.9992 and limit of detection0.08 mM at Eapp =+0.4 V vs. Ag/AgCl over the linearrange 25–250 mM.

The excellent performance is possibly due to the opti-mum dispersion, modification and utilisation of the sur-face structures. It is also well known that the inclusion ofcobalt atom into the nickel hydroxide lattice leads to asignificant increase in the amount of structural defects[36]. The synergism by cobalt and the surface composi-tion (hydroxide/oxide-nanoparticle combination) seemsto provide more active sites, utilising the surface effec-tively, enhancing the catalytic performance and hencesensitivity. The ensemble effect, along with the surfacecomposition, is very effective and shows good reproduci-ble performance for the non-enzymatic uric acid detec-tion at an applied potential (Eapp =+0.4 V) in phosphatebuffer solution. The surface behaviour and composition

of carbon supported catalysts are very important becausethey have a significant effect on the response and are cru-cial in controlling the catalytic activity. The presence ofNi provides stability and enhances catalytic performancein addition to the capability of Co to regenerate theactive sites from unwanted adsorbed species/intermediateon electrode surface during catalysis.

3.5.2 Interference Studies

The avoidance of endogenous interfering species is achallenge in non-enzymatic uric acid detection as some ofthe electroactive species such as ascorbic acid, glucose(Glu), urea (Ur) and oxalate (Ox), normally co-exist withuric acid in real samples and may be simultaneously oxi-dised along with the uric acid at the electrode surface.The Co1Ni1/C catalyst exhibited strong and very specificamperometric response to uric acid, even in the presenceof common interfering species in phosphate buffer solu-tion (pH 7.4). Figure 6A presents the results of theCo1Ni1/C catalyst for two consecutive additions of uricacid (50 mM) followed by one addition of glucose(500 mM), urea (500 mM) and oxalate (500 mM) each, atan Eapp =+0.4 V in phosphate buffer solution. When uric

Fig. 5. (A) is the comparison of amperometric response by (a) Co1Ni1/C, (b) Ni/C, (c) Co/C and (d) AC modified glassy carbon elec-trodes in phosphate buffer solution at Eapp =+0.4 V over the concentration range (25–250 mM) of uric acid at 250 s time interval. (B)Calibration plot for the amperometric response obtained for different (labelled) nanocomposite modified electrodes.

Table 2. Comparison of different modified electrodes for uric acid sensing. N. A.: not applicable; PAN: polyaniline nanonetworks;ABSA: aminobenzensulfonic acid; OMC: ordered mesoporous carbon; GC: glassy carbon; SWCNT: single-walled carbon nanotubes;CCA: calconcarboxylic acid; TPP: tetrakisphenylporphyrin; MB: methylene blue; CNF: carbon nanofiber; CP: carbon paste.

Electrode Ref. electrode Epa (mV) DE [a] (mV) pH (PBS) LR [b] (mM) LoD [c] (mM) Interference by AA Refs.

PAN/ABSA/GC SCE 290 120 6.8 50–250 12.0 Yes* [45]OMC/GC SCE 330 220 7.4 7–150 5.0 Yes* [46]SWCNT/Au Ag/AgCl 360 – 7.4 2.5–17.5 2.5 Yes* [7]CCA/GC Ag/AgCl 382 78 6.0 1–200 0.5 Yes* [47]Co(II)TPP/GC SCE 320 – 6.5 2–100 0.5 Yes* [48]MB/SGGC SCE 460 – 6.5 0.001–50 0.001 Yes* [10]CNF/CP Ag/AgCl 475 53 4.5 0.8–16.8 0.2 Yes* [49]g-Zn3Ni/CG Ag/AgCl 280 110 7.0 1–400 0.2 No [24]Fe Nps/GC SCE 425 – 5.0 0.5–20.0 0.15 N. A. [50]CoNiC/GC Ag/AgCl 350 100 7.4 25–575 0.08 No This work

[a] DE is the negative shift in potential compared to the bare electrode. [b] LR is the linear range (mM) and [c] LoD is the limit of de-tection (mM). * indicates interference with separated peak.




acid (50 mM) was added to the phosphate buffer solution,a significant increase in current response was observed.However no change in current was observed followingaddition of glucose, oxalate and urea. These results indi-cated that even in the presence of 10-fold excess ofcommon physiological interferents, Co1Ni1/C modifiedelectrode can be effectively used for uric acid determina-tion.

Ascorbic acid (AA) has an oxidation potential veryclose to that of uric acid (UA) and the bare electrodevery often suffers from fouling effects [41]. The interfer-ence by ascorbic acid was also studied on Co1Ni1/C modi-fied glassy carbon electrode to check the specificity of thecatalyst and it was found that the electrode is highly se-lective towards uric acid determination. The cyclic vol-tammetry response for individual and mixtures of ascor-bic acid and uric acid (1000 and 500 mM), in phosphatebuffer solution (pH 7.4) at scan rate 0.02 V/s, verified theinsensitivity of the electrode towards ascorbate (data notshown), demonstrating its capacity for highly selective de-termination of uric acid. Under physiological conditions(phosphate buffer solution, pH 7.4), ascorbic acid existsas ascorbate anion at pH>4.17 [41], which is the firstpKa value of ascorbic acid. It can be concluded that theCo1Ni1/C electrode is relatively passive towards the ascor-bate species for radical formation [42–43] in a similar wayas reported for Zn, Ni nanoalloy. It is also reported thatZn, Ni and their alloy are not capable of oxidizing ascor-bic acid (AA) [44]. Similar behaviour was reported re-cently by Tehrani and Ghani for Zn-Ni based nanoalloymodified electrode [24].

The effect of ascorbic acid (AA) on uric acid (UA)measurement was investigated further through hydrody-namic amperometry. Figure 6B shows the amperometricresponse for the Co1Ni1/C modified electrode, in phos-phate buffer solution (pH 7.4) at Eapp =+0.4 V vs. Ag/AgCl, towards successive additions of (1) 25 mM uric acid,(2) 25 mM ascorbic acid, (3) 50 mM uric acid and (4)

50 mM ascorbic acid. The results demonstrated that theelectrode is highly selective for uric acid (UA) determina-tion, as no/negligible response was obtained for ascorbicacid (AA).

4 Conclusions

A novel and controlled synthetic route was applied forthe synthesis of CoNi/C based nanocomposites, which isvery effective for the optimum utilisation of surface struc-tures. The role of such materials was investigated for thefirst time as an electrode modifier for the direct electro-chemical uric acid determination. The synthesised cata-lysts exhibit a very specific amperometric response to uricacid in comparison to some common interfering speciessuch as ascorbate, glucose, urea and oxalate in phosphatebuffer solution. Other important features include lowmetal loading, the ability to respond at neutral conditions(PBS), cost effectiveness, stability and selectivity of thesematerials. This study demonstrated their strong potentialin effective non-enzymatic uric acid sensing.

Acknowledgements

We thank and acknowledge Technological Sector Re-search Strand III, Postgraduate Research and Develop-ment for funding. Dr. Calum Dickinson and Dr. FathimaLaffir acknowledge the INSPIRE programme, funded bythe Irish Government’s Programme for Research in ThirdLevel Institutions, Cycle 4, National Development Plan2007–2013. We also acknowledge Dr. Helen Hughes atthe Waterford Institute of Technology, Ireland, for ther-mogravimetric analysis.

Fig. 6. Interference study I: Amperometric response (A) for Co1Ni1/C modified glassy carbon electrode in phosphate buffer solutionwith two successive additions of (1) 50 mM uric acid followed by additions of (2) 500 mM glucose, (3) 500 mM urea and (4) 500 mM oxa-late, at 250 sec. time interval at Eapp =+0.4 V vs. Ag/AgCl. Interference study II: Amperometric response (B) by Co1Ni1/C modifiedglassy carbon electrode in phosphate buffer solution (pH 7.4) towards repeated additions of (1) 25 mM uric acid, (2) 25 mM ascorbicacid, (3) 50 mM uric acid and (4) 50 mM ascorbic acid at 250 s. time interval at Eapp =+0.4 V vs. Ag/AgCl.




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Carbon Supported Cobalt and Nickel Based Nanomaterials for Direct Uric Acid Determination

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Transcript of Carbon Supported Cobalt and Nickel Based Nanomaterials for Direct Uric Acid Determination