New Electrochemical Sensor Based on a Silver-Doped Iron Oxide

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A New Electrochemical Sensor Based on Silver Doped Iron Oxide Nanocomposite Coupled with Polyaniline and Its Sensing Application for Picomolar Level Detection of Uric Acid in Human Blood and Urine Samples Sathish Kumar Ponnaiah, Prakash Periakaruppan, and Balakumar Vellaichamy J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11504 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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The Journal of Physical Chemistry

A New Electrochemical Sensor Based on Silver Doped Iron Oxide Nanocomposite Coupled with Polyaniline and its Sensing Application for Picomolar Level Detection of Uric Acid in Human Blood and Urine Samples Sathish Kumar Ponnaiah, Prakash Periakaruppan*, Balakumar Vellaichamy Department of Chemistry, Thiagarajar college, Madurai – 625 009, Tamil Nadu, India.

ABSTRACT A simple and very sensitive electrochemical sensor for the detection of uric acid (UA) has been developed based on polyaniline (PANI) merged into silver doped iron oxide (Ag-Fe2O3) nanocomposite modified glassy carbon electrode. The synthesized ternary composite material (Ag-Fe2O3@PANI) was characterized by UV-visible spectroscopy, FT-IR spectroscopy, EDX, HR-TEM, XRD and TGA analysis. The nanocomposite modified electrode shows an exceptional electrocatalytic activity and reversibility to the oxidation of UA in 0.1M phosphate buffer solution (PBS, pH 7.0) compared to PANI and Ag-Fe2O3. Detection limit of UA is found to be 102 pM with a linear dynamic range of 0.001-0.900 µM. The fabricated UA sensor also exhibits good selectivity, reproducibility, and long-time stability. The LOD and linear range attained for the synthesized composite is much greater compared to any other composite materials reported in the literature. The proposed method has been successfully applied for the selective detection of UA in various real samples such as human serum and urine with good recoveries. This platform which assimilates such electrocatalytic ternary nanocomposite with high performance can be very much employed for fabricating diverse sensors.

Corresponding Author: * E-mail: [email protected] (P. Prakash); Fax: +91 4522312375; Tel: +91 9842993931 1 ACS Paragon Plus Environment

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1. INTRODUCTION Uric acid (UA/urate/2, 6, 8-trihydroxypurine), an end product of purine metabolism, plays a critical role in human beings and higher primates.1 UA is found in biological fluids, such as urine, saliva, and serum.2 The physiological range of UA in serum and urine is about 300-500 µM and 1400-4400 µM, respectively.3 It has been demonstrated that high concentration of UA is indicative of certain diseases, such as arthritis, high cholesterol, high blood pressure, gout, hyperuricemia, Lesch–Nyhan syndrome, kidney lesion, metabolic syndrome, heart disease, hypertension, and cardiovascular disease. Unusual low level of UA in serum leads to Parkinson's disease or multiple sclerosis.4-7 An accurate assessment of UA level is crucial in early diagnosis and therapy. Recently, many methods such as HPLC,8 colorimetry (optical detection),9 capillary electrophoresis,10 chemiluminescence,11 enzymatic,12 and fluorescence sensing methods,13 have been explored for the detection of UA. However, these reported methods are usually complicated, time-consuming and requires expensive instruments. In comparison to these methods, the electrochemical approach has many benefits such as operational speed, high sensitivity and low maintenance costs.14-15 Moreover, accurate determination of UA using conventional electrodes is difficult due to their electrochemical oxidation, poor kinetics and overlapping oxidation potential to that of the bare electrode. Hence, it is imperative to fabricate electrodes which must be selective and sensitive to UA analyte. Electrochemical UA sensors have been designede based on chemically modifying the electrodes such as carbon based materials / metallic or bimetallic nanoparticles / biological molecules / polymer based materials etc.4, 14, 16 Polyaniline (PANI) has been found to have many advantages among the polymer matrix, such as low-cost synthesis, corrosion protection, nontoxicity, high conductivity and high instinct redox 2 ACS Paragon Plus Environment

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properties.17 But PANI-based sensor experiences certain inherent disadvantages such as poor reproducibility, selectivity, and stability. To overcome these demerits, PANI is often functionalized or incorporated with nanoparticles (NPs) (metallic / bimetallic / metal oxide NPs). 18-19

Recently, various metal oxide NPs such as Co3O4,20 CuO,21 and Fe2O322 NiO,23 have

attracted the attention of researchers in the area of electrochemical studies. Among these metal oxide NPs, Fe2O3 emerges as a prominent one owing to its distinctive electrical and catalytic properties, which has a vast potential application as electrode resources in lithium secondary batteries, catalysts and particularly sensors.24 Moreover, a simple way to enhance the sensing properties of pure metal oxides is considered by merging metals and metal oxides which increases the sensitivity and selectivity, reduces the operating temperature and decreases the response and recovering time.25 In this work, nanostructured Ag-Fe2O3@PANI modified glassy carbon electrode (AgFe2O3@PANI /GCE) has been used for the determination of UA. The fabricated sensor shows a very high and reproducible sensitivity of 128.29 mA mM-1 cm-2 with the detection limit of 102 pM. The method proposed here has been successfully applied to detect the UA level in serum and urine samples. The advantages involve fast detection, extensive detection range and high selectivity. Inherently, the present proposed electrochemical sensor has the potential for rapid quantitative detection of UA in human blood and urine samples of various age groups. 2. EXPERIMENTAL SECTION 2.1 Materials AgNO3, Na2CO3, C6H5NH2, (NH4)2S2O8, NaBH4, FeSO4.7H2O, FeCl3.6H2O, PVP, glucose, sucrose, folic acid, dopamine, ascorbic acid and uric acid were purchased and used as received

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from Sigma-Aldrich. A phosphate buffer solution (PBS) of different pH was prepared with appropriate amounts of NaH2PO4.2H2O, Na2HPO4.2H2O, and adjusted with 0.1 M H3PO4 or NaOH. All other chemicals used were of analytical grade. 2.2 Synthesis of Ag-doped Fe2O3 (Ag/Fe2O3) Ag-doped Fe2O3 was prepared following the method of hydrothermal synthesis. PVP was added in a small quantity to an aqueous solution of 10 mL of (0.1 M) AgNO3 solution. In a separate beaker, (0.4 M) NaBH4 and 0.2 g of sodium hydroxide with stirring and dilute to 100 ml with water, under vigorous stirring. NaBH4/NaOH solution was added to above solution and the resultant solution was stirred continuously for 10 min. The color of the solution gradually became yellow while adding NaBH4/NaOH, indicating the conversion of Ag+ ions to Ag. 10 mL each of FeSO4.7H2O (0.150 M)) and FeCl3.6H2O (0.3 M) was added to the above solution. The pH was adjusted by adding Na2CO3 to the reaction mixture at 80 °C, under vigorous stirring. The mixture was then stirred for 1 h at room temperature which was later transferred to a 100 mL autoclave, sealed, and heated at 160 °C for 3 h wherein, a precipitate of Ag/FeOOH was formed. The precipitate was digested overnight at room temperature, washed with deionized water, dried at 80 °C and calcined at 400 °C for 1 h to obtain Ag/ Fe2O3 with a calculated mass ratio of about 1:2. For comparison, pure Fe2O3 samples were also obtained by adopting the method mentioned above in the absence of AgNO3 and NaBH4. 2.3 Synthesis of Ag-doped Fe2O3@Polyaniline (Ag/Fe2O3@PANI) 10 mg of the as-prepared Ag/Fe2O3 nanocomposite was dispersed in 0.1 M APS [(NH4)2S2O8], to which 5 ml each of 0.2 M aniline and 0.2 M HCl mixture was added. The mixture was stirred for 1 h at room temperature and then transferred to a 100 mL an autoclave, and heated at 160 °C for 4 ACS Paragon Plus Environment

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3 h. After the completion of polymerization, a green precipitate was obtained which is the emeraldine salt form of PANI. The product was filtered and washed sequentially with deionized water, ethanol and 1 M HCl. Finally, the precipitate was dried overnight in air at 50 °C to obtain Ag/ Fe2O3@PANI. Scheme 1 shows a schematic representation of synthesis of Ag/Fe2O3@PANI nanocomposite and sensing mechanisum of UA. 2.4. Preparation of Electrodes GCE was initially polished with 1, 0.3, and 0.05 µm alumina powder successively and washed ultrasonically in water, ethanol, and acetone. 2 mg of Ag-Fe2O3@PANI nanocomposite was dispersed in 1 mL of ethanol with a sonication for a few minutes to yield a stable suspension. 5 µL of the resultant solution was coated onto the surface of the GCE. For comparison, several electrodes including bare GCE, Ag-Fe2O3, PANI and Ag-Fe2O3@PANI were prepared, and their electrochemical performances were investigated towards UA. 2.5 Instrumental Characterization UV-visible spectra were measured using a Jasco (V-560) model. The FT-IR spectra were measured on a JASCO FT-IR 460 Plus spectrophotometer. XRD analysis was carried out on JEOL JDX 8030 Xray diffractometer. The morphology of the material was ascertained by HRTEM/SAED using TEM, FEI TECNAI T20 G2). Electrochemical measurements were performed with glassy carbon electrode (GCE) as a working electrode, platinum (Pt) wire and KCl saturated Ag/AgCl as counter electrode and reference electrode respectively. Surface mapping analysis was measured by SEM measurements done at VEGA3 TESCAN, USA. The BET (Brunauer‒ Emmett‒Teller) surface area was derived from the N2 adsorption – desorption isotherm by the



Barrer-Joyner-Halenda (BJH) technique. The electrochemical measurement was carried out using CHI electrochemical workstation (Model 660E, Austin, TX, USA). 3. RESULTS AND DISCUSSION 3.1 UV Analysis Figure.1 shows the UV- Vis absorption spectra of the Fe2O3 (a), Ag/Fe2O3 (b), PANI (c) and Ag/Fe2O3@PANI nancomposite (d). Pure Fe2O3 shows a broad peak at 246 nm as shown in Figure.1 (a). A slight red shift is observed with the wavelength shift to 249 nm for Ag/Fe2O3 (Figure. 1 (b)), which may be due to the sensitization of binary Ag/Fe2O3 nanocomposite. The red shift with high intensity reveals that Fe2O3 might form a partial bond with Ag.25 The characterstic peak at 228 nm in the absorption spectra of pure PANI is attributed to the transition of electrons from HOMO to LUMO, that is π– π* electronic transition. The bands at around 411 nm and 700 nm, as shown in Figure. 1 (c) are due to the polaron π* and π polaron benzenoid to quinoid electronic transition.26 For PANI@Ag/Fe2O3 nanocomposite, the indensity of absorption at 700 nm decreases and the band at 411 nm shows a little red shift (Figure. 1 (d)), which may be caused by the strong interaction between PANI and Ag/Fe2O3. 3.2 FT-IR Analysis The bond analysis and chemical composition was examined for the as prepared Fe2O3, Ag/Fe2O3, PANI, Ag/Fe2O3@PANI nanocomposite using FT-IR analysis. The IR spectrum of Fe2O3 exhibits characteristic peaks at 449, 856, 1488 and 2895 cm-1 which can be attributed to the symmetric stretching vibrations of Fe-O, Fe-O-Fe, C=O and C-H owing to the presence of minor amount of PVP stabilized Fe2O3. In the case of Fe2O3, the bands appearing at 1679 cm-1 is attributed to the angular deformation of water δH−OH bond, while the band appearing at 3727 6 ACS Paragon Plus Environment

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cm-1 is assigned to the O−H stretching of water as shown in Figure.2 (a). For Ag/Fe2O3, the peaks are appearing at 3747, 2935, 1689, 1497, 1164, 824 and 409 cm-1 as shown in Figure.2 (b). The slight shift observed in the peak position in Figure.2 (b) may be accounted to the influence of Ag on the nanocomposite.25 The backbone of pure PANI is indicated by the characteristic peaks shown in Figure.2 (c) at 3408, 2925, 1558, 1470, 1298, 1234, 1118, and 804 cm−1, harmonizing to the stretching vibrations of the C=C of the quinonoid and benzenoid rings.27 The FT-IR spectrum of Ag/Fe2O3@PANI composite exhibits all the bands of PANI and Ag/Fe2O3. Intensity of some peaks in the composite is lower than that of pure PANI, which further confirms the interactions between PANI and Ag/Fe2O3 in the composite as shown in Figure.2 (d). 3.3 XRD The phase purity and crystal structure of the samples were determined by XRD analysis. As shown in Figure.3, the precursor, pure Fe2O3 phase (JCPDS No. 24-0072), has a rhombohedral structure with the lattice parameters of the 10.50 nm as shown in Figure.3 (a). As presented in Figure.3 (b), the XRD pattern of Ag/Fe2O3 exhibits diffraction peaks indicative of both metallic Ag and Fe2O3, which goes along with the values in literature for JCPDS No. 65-2871 and JCPDS No. 24-0072. One broad peak can be observed for pure PANI as seen in Figure.3 (c) at 2θ =21.4°, which is ascribed to amorphous PANI. Moreover, the diffraction peaks of PANI and Ag/Fe2O3 appear in the curves of Ag/Fe2O3@PANI composites as exposed in Figure.3 (d), signifying that the three components are successfully hybridized. 28-30 3.4 TEM and surface area The morphology and size information of the Ag/Fe2O3@PANI composite was studied precisely by TEM analysis with the images shown in Figure.4. As shown in Figure. 4(a), the TEM image 7 ACS Paragon Plus Environment


of Ag/Fe2O3@PANI reveals that Fe2O3 doped with Ag are homogeneously attached to the surface of the PANI.31 The morphology of nanocomposite is almost having a wrinkle type structure as exhibited in Figure 4 (b), The particle size distribution histogram for Ag/Fe2O3@PANI is shown in Figure. 4 (insert b). The average size of this nanocomposite is found to be 40 nm. where some of the nanocomposite are dispersed and some of them are agglomerated due to magneto–dipole interactions between Ag/Fe2O3 and PANI.32 Figure 4 (c) and (d) shows the particles with dark color which is assigned to Ag/Fe2O3, with the grey area representing PANI and it allows to see closely the Ag0 on the hematite. In surface, the particle size and fringe spacing of 0.23 nm corresponds to {1 1 1} planes of Ag0 while the other distance 0.27 nm is agreed with the successive {1 0 4} planes of hematite. Ag/Fe2O3 nanocomposite is attached to the surface as well as the edge of PANI. Furthermore, the crystallite mean size of Ag and Fe2O3 was calculated by Scherrer equation and found to be 27.3 and 14.2 nm respectively. The SAED pattern of synthesized Ag/Fe2O3@PANI nanocomposite reveals the crystalline nature as shown in Figure 4 (e). Figure 4 (f) shows the chemical analysis by means of EDX of the synthesized Ag/Fe2O3@PANI nanocomposite which confirms the existence of C, O, N, Fe, Ag. The tabulated results provide the elemental composition in the inspection field in units of both weight percent and atomic percent as shown in Figure 4 f (inset). Figure 5 shows the SEM images of the selection area of Ag/Fe2O3@PANI and the elemental mapping analysis. The elemental mapping of the Ag/Fe2O3@PANI image indicates that O, N, Ag and Fe are uniformly distributed, confirming the formation of Ag/Fe2O3@PANI. Isothermal N2 adsorption-desorption BET analysis was used to study the specific surface areas of Ag/Fe2O3@PANI nanocomposite as shown in Figure 6. The surface area of the composite is found to be 28.93 m2 g-1.


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3.5 TGA Analysis Thermo gravimetric analysis (TGA) is employed to verify the temperature-dependent changes. As shown in Figure 7 (a), raw Fe2O3 presents virtually a minimum mass loss of 5.10 % in the temperature range from 40-160 °C, 180-420 °C and 430-530 °C corresponding to the evaporation of adsorbed water and additional dehydration of [FeOOH)] group followed by saturation leaving their unique mass (~95.16%). The residual weight of the Ag/ Fe2O3 is a bit higher (Figure 7 (b)) than that of pure Fe2O3 due to the presence of metal in the sample with the original mass (~97.21%). In the case of PANI, two steps of weight loss are observed. The first weight loss up to 78 °C is possibly due to the evaporation of water molecules which are physisorbed and the second weight loss in the range 310-580 °C is attributed to the degradation of unsaturated polymer groups as shown in Figure 7 (c). The degradation temperature of the synthesized composite, Ag/Fe2O3@PANI suggests that the interaction between PANI and Ag doped Fe2O3 of the composite helps the late degradation of the polymer chain. Hence, the TGA analysis data clearly reveal that the thermal stability of PANI is greatly enhanced due to the presence of thermally-stable Ag doped Fe2O3 nanocomposite material as given (Figure 7 (d)) monitored by facility leaving their original mass (~80.17%). 3.6 Electrochemical Oxidation of UA The electrocatalytic oxidation of UA was studied by cyclic voltammetry with Ag/Fe2O3@PANI nanocomposite modified electrode at a scan rate of 50 mVs-1 in the potential range, – 0.2 to 0.8 V in 0.1 M PBS. The electrocatalytic activity of Ag/Fe2O3@PANI and unmodified GCE towards the oxidation of UA was examined. No oxidation peak was observed for bare GCE while a substantial oxidation peak of UA was observed in Fe2O3, Ag/Fe2O3, PANI and Ag/Fe2O3@PANI



with a peak potential around 0.50 V as presented in Figure 8a.33 This was greatly enhanced compared with those of the bare GCE, Fe2O3, Ag/Fe2O3, PANI and Ag/Fe2O3@PANI, indicating that the Ag/Fe2O3@PANI composite exhibits an excellent catalytic performance toward UA oxidation due to the high effective surface area. The value of the peak current depends on several factors including the concentration of UA, kinetics of electron transfer and the mass transport of the UA. The large catalytic current obtained can be attributed to the PANI with rough surface and high conductivity acting as a support for Ag/Fe2O3, which provides many anchor sites for Ag and prevents Ag from aggregation, thus preserving the larger surface area for UA molecules adsorption and oxidation.34-39 Hence the Ag/Fe2O3@PANI nanocomposite shows a better performance towards the sensing of UA. Furthermore, the electroactive surface area of Ag/Fe2O3@PANI was evaluated by using the Randles– Sevcik equation as follows: ip = 2:69 × 105n 3/2 AD ½ Cv ½ where D is the diffusion coefficient of [Fe(CN)6]3/4(cm2 s-1), ip is the anodic or cathodic peak current (A), C is the concentration of the [Fe(CN)6]3/4(mol cm-3), A is the electroactive area (cm2), n is the number of transferred electrons and m1/2 is the square root of scan rate (V s-1). The estimated active surface area values are 0.050 and 0.084 cm2 for bare GCE and Ag/Fe2O3@PANI, respectively. The main product of the electrochemical oxidation of aqueous UA is 4, 5-dihydroxyluric acid, which is unstable and mostly decomposes into allantoin, which is why the cathodic peak is not obvious in cyclic voltammogram.40 In Figure. 8b, curve a, shows the CV response of Ag/Fe2O3@PANI modified electrode to the presence of 10 µM concentration of UA and curves b-j shows the CV response after the addition of 10 µM concentration of UA. The oxidation peak current has a linear dependence against the concentration of UA from 10 to 100 µM with the correlation coefficient of 0.9960 as given Figure. 8c. We believe this effect 10 ACS Paragon Plus Environment

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primarily stems from the large surface areas of the PANI and the excellent conductivity of the Ag/Fe2O3 nanocomposite. Figure. 9a shows the CVs of Ag/Fe2O3@PANI modified GCE with different scan rates (50-400 mV/s) in PBS (pH 7.0) containing 10 µM of UA. With the scan rate increase, the oxidation peak current (Ipa) increases linearly with the peak potential shifting to a positive direction. Figure. 9b shows a plot of logarithm of the peak current for UA versus square root of the scan rate, which exhibits a linear relationship with the correlation coefficient of 0.9842, suggesting that diffusion controls the electro-oxidation process. 3.7 Optimization of pH The effect of pH on the response of UA at Ag/Fe2O3@PANI modified GCE was investigated and the results are shown in Figure. 9c. When the pH of the solution increases from 3.0 to 11.0, the maximum peak current is attained at the pH of 7.0. In acidic pH, the synthesized is protonated and the amine group in UA is not ionized, which decreases the adsorption capacity of UA thereby producing a high overpotential. UA is unstable in alkaline solution, and can be easily oxidized; thus, affecting the detection of target molecules.4 Further increase of pH leads to a decrease in the peak current. Therefore, pH 7.0 PBS was used as the electrolyte in the subsequent experiments. 3.8 Differential Pulse Voltammograms The differential pulse voltammetry (DPV) of the as-prepared sensor was observed under optimal conditions. As shown in Figure. 10a, it is noted that upon increasing the concentration of UA (0.001-0.900 µM) in 0.1 M PBS, the oxidation peak current observed at 0.45 V exhibits a remarkable linear enhancement (Figure. 10a (curve a-j)) suggesting the application of Ag/Fe2O3@PANI modified GCE in the quantitative determination of UA. The oxidation peak 11 ACS Paragon Plus Environment


current (Ipa) of UA has a good linear relationship with the UA concentration in the range from 0.001-0.900 µM (Figure. 10b) with the correlation coefficient of 0.9970. Limit of detection (LOD) stipulates the minimum detectable amount of analyte using the developed sensor. LOD is calculated using the formula 41 LOD = 3.3S/b Where S is the standard deviation of the lowest concentration of UA, and b is the slope of calibration curve obtained from the DPV. The value of LOD for the proposed sensor is calculated to be 102 pM which is lower than the corresponding values reported in the literature. The proposed Ag/Fe2O3@PANI nanocomposite has an outstanding sensing performance, good linear relationship, higher sensitivity and lower LOD as illustrated in Table 1. 42, 4, 43-47 3.9 Interference Studies and Stability of the Sensor Interference was evaluated in the presence of various substances on the determination of UA. For this study, the interfering species of various concentrations (a 20 fold) higher than the concentration of UA (1 × 10−6 M) were added. The addition of filler materials namely glucose, sucrose, folic acid, dopamine and ascorbic acid cause no significant effect on the DPV response as seen in Figure. 11. The π–π stacking interaction between UA and modified electrode surface may accelerate the electron transfer whereas weakens the other biological interferences on this modified electrode system.48 Although folic acid has a similar structure and oxidation potential as that of UA, the oxidation current intensity of folic acid is much lower than that of UA due to the inability of the folic acid to bind to the sites tightly. The oxidation current intensities of glucose, sucrose, dopamine and ascorbic acid are also small.


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The storage stability of the sensor system was determined by measuring the current response of 50 µM UA after a storing period of 30 cycles as shown in Figure. 12. The peak current decrease of the electrode after the storage period of 15 days at 4 °C is just less than 4 ± 0.03%. These results indicate that the electrode containing the Ag/Fe2O3@PANI nanocomposite was strongly attached to the surface of the glassy carbon electrode and that the modified electrode possessed excellent reproducibility repeatability and stability of the present electrode. 3.10 Validation of Proposed Biosensor using Real Samples The ability of the proposed sensor on the detection of UA in real samples was also investigated in human blood and urine samples. The samples were diluted 100 times with PBS (pH 7.0), and different amounts of UA were spiked in them without further treatment. The well-defined oxidation peak of UA at proposed Ag/Fe2O3@PANI modified electrode is observed at 0.45 V. The human serum and urine samples were spiked with known concentration of UA. Favorable recoveries are obtained thereby indicating the reliability of the proposed sensor for real sample analysis as shown in Table. 2 4.

CONCLUSION

A PANI supported Ag/Fe2O3 nanocomposite was synthesized by hydrothermal method. The unique properties of Ag/Fe2O3@PANI nanocomposites offer new aspirants to construct better electrodes for environmental monitoring studies. The sensor fabricated using this nanocomposite exhibits an ultra-high selectivity, sensitivity, very low detection limits and good stability. We believe that the high-performance achieved by Ag/Fe2O3@PANI based UA sensor shows a great potential for various applications in the determination of UA concentration in human blood and



urine samples. This work can be successfully used for the fabrication of sensing devices which may provide valuable solution for clinical diagnosis. REFERENCES 1. Asami, H.; Saigusa, H. Multiple Hydrogen-Bonding Interactions of Uric Acid/9-Methyluric Acid with Melamine Identified by Infrared Spectroscopy. J. Phys. Chem. B 2014, 118, 4851– 4857. 2. Latosinska, J. N.; Latosinska, M.; Seliger, J.; Zagar, V.; Kazimierczuk, Z. An Insight into Prototropism and Supramolecular Motifs in Solid-State Structures of Allopurinol, Hypoxanthine, Xanthine, and Uric Acid. A 1H–14N NQDR Spectroscopy, Hybrid DFT/QTAIM, and Hirshfeld Surface-Based Study. J. Phys. Chem. B 2014, 118, 10837– 10853. 3. Leon-Carmona J. R.; Galano, A. Uric and 1-Methyluric Acids: Metabolic Wastes or Antiradical Protectors. J. Phys. Chem. B 2011, 115, 15430–15438. 4. Zhang, C.; Si, S.; Yang, Z. A Highly Selective Photoelectrochemical Biosensor for Uric Acid Based on Core–Shell Fe3O4@C Nanoparticle and Molecularly Imprinted TiO2. Biosens. Bioelectron. 2015, 65, 115-120. 5. Arora, K.; Tomar, M.; Gupta, V. Reagentless Uric Acid Biosensor Based on Ni MicrodiscsLoaded NiO Thin Film Matrix. Analyst 2014, 139, 4606-4612. 6. Garcia-Esquinas, E.; Rodriguez-Artalejo, F. Association between Serum Uric Acid Concentrations and Grip Strength: is there Effect Modification by Age? Clinical Nutrition 2017, In press, DOI: https://dx.doi.org/10.1016/j.clnu.2017.01.008. 7. Singh, H. P. In-situ Generation of Au Nanostructures during Enzyme Free Oxidation of Uric Acid: A New Recognition at an Old Problem. Colloid Interface Sci. Commun. 2017, 19, 5–8.


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Scheme, Figure and Table Captions Scheme 1 The schematic representation of synthesis of Ag/Fe2O3@PANI nanocomposite and sensing mechanism of UA. Figure. 1 UV-Vis spectra of Fe2O3 (a), Ag/Fe2O3 (b), PANI (c), Ag/Fe2O3@PANI nanocomposite (d). Figure. 2 FT-IR spectra of Fe2O3 (a), Ag/Fe2O3 (b), PANI (c), Ag/Fe2O3@PANI nanocomposite (d). Figure. 3 XRD patterns of Fe2O3 (a), Ag/Fe2O3 (b), PANI (c), Ag/Fe2O3@PANI nanocomposite (d). Figure. 4 (a-d) HR-TEM with different magnifications, (insert b) Particle size distribution histogram, (e) SAED pattern of Ag/Fe2O3@PANI nanocomposite and (f) EDX image of Ag/Fe2O3@PANI nanocomposite. Figure. 5 Selection area of the Ag/Fe2O3@PANI for elemental mapping of C, O, N, Ag and Fe, (a), element mapping of C (b), O (c), N (d), Ag (e) and Fe (f). Figure. 6 N2 adsorption/desorption measurement results of synthesized Ag/Fe2O3@PANI nanocomposite. Figure. 7 TGA analysis of Fe2O3 (a), Ag/Fe2O3 (b), PANI (c) Ag/Fe2O3@PANI nanocomposite (d).



Figure. 8 Cyclic voltammetry curve for GCE with different electrodes in the presence of UA (10 µM) (a), different addition of UA (from 10-80 µM) in 0.1 M PBS (pH 7.0) at scan rate 50 mV s-1 (b), and the calibration plot for the linear dependence of UA vs I/ µA(c) Figure. 9 Cyclic voltammetry curve for Ag/Fe2O3@PANI modified GCE with different scan rates (from 50-400 mV s-1) in the presence of 10 µM concentration of UA (a), the calibration plot for the linear dependence of oxidation and reduction peak current vs squre root of scan rate (b), and effect of pH on the peak current (c) Figure. 10 (a) DPV response of Ag/Fe2O3@PANI modified GCE in 0.1 M PBS (pH=7.0) with different concentrations of UA: 0.001-0.900 µM, (b) The calibration plot of current response vs. concentration of UA Figure. 11 DPV response of the interference test for Ag/Fe2O3@PANI modified GCE to successive additions of 0.10 µM UA and 20 fold excess of glucose, sucrose, folic acid, dopamine and ascorbic acid with stirring in 0.1 M PBS (pH 7.0) Figure. 12 Cyclic voltammograms (30 cycles) confirming the stability of the sensor system with the addition of 50 µM UA. Table 1: Detailed comparison of the electrocatalytic oxidation of UA using various electrode materials Table 2: Determination of UA levels in various human urine and blood samples using the proposed Ag/Fe2O3@PANI modified electrode.


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Scheme 1



Figure. 1


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Figure. 2



Figure. 3


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Figure. 4



Figure. 5


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Figure. 6



Figure. 7


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Figure. 8



Figure. 9


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Figure. 10



Figure. 11


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Figure. 12



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Table 1

Sensing materials

pH

Fe3O4@SiO2/MWCNT

Method of detection SWV

Fe3O4@C@TiO2

PEC

7.0

Au/PANI Fe-Meso-PANI

DPV LSV

7.0 3.5

CoFe2O4-LSA/CT

AMP

7.0

CU/PPy

DPV

7.0

SiO2@AuNPs@PANI

DPV

6.0

Pd@Fe3O4

SWV

7.4

ZnO/PANI/RGO

DPV

4.0

Ag-Fe2O3@PANI

DPV

7.0

6.0

Linear range (µM) 0.60100.0

LOD

0.300 34.0 29–720 10–300

0.02 µM

1.998.0 0.0010.1 51100 0.96107 0.5100.0

0.004 µM 2 µM

Sensitivity Samples (µA mM-1 analyzed cm-2) Human 0.303 serum, urine Human urine 0.2621 Human serum, urine 5140 Human urine Human urine -

0.41 µM

-

0.12 µM

-

0.13 µM

0.17 µM 5.3 µM

0.3 µM

0.0010.900

102.0 pM 128.29


Human serum Human urine Human serum, urine

Ref.

42

4 43 44

33 41 45 46 47 Present work

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Table 2

Samples (age group) Urine

Added (µM)

Found (µM)

Recovery (%)

(15)

0.5

0.501 ± 0.002

100.2 ± 0.40

(35)

0.5

0.502 ± 0.001

100.4 ± 0.20

(55)

0.5

0.504 ± 0.002

100.8 ± 0.40

(15)

0.5

0.502 ± 0.001

100.4 ± 0.20

(35)

0.5

0.503 ± 0.002

100.6 ± 0.40

(55)

0.5

0.506 ± 0.002

101.2 ± 0.40

Serum



Graphic for Manuscript


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New Electrochemical Sensor Based on a Silver-Doped Iron Oxide

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