Nanostructured fe2 o3 platform for the electrochemical sensing of folic acid

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Nanostructured fe2 o3 platform for the electrochemical sensing of folic acid

  1. 1. Analyst View Article Online PAPER View Journal | View Issue Nanostructured a-Fe2O3 platform for the electrochemical sensing of folic acid† Cite this: Analyst, 2013, 138, 1779 Thandavarayan Maiyalagan,‡a J. Sundaramurthy,‡bc P. Suresh Kumar,bc Palanisamy Kannan,*d Marcin Opallo*d and Seeram Ramakrishna*bcPublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B a-Fe2O3 nanofibers are synthesized by a simple and efficient electrospinning method and the selective Downloaded by University of Texas Libraries on 22 February 2013 determination of folic acid (FA) is demonstrated in the presence of an important physiological interferent, ascorbic acid (AA), using the a-Fe2O3 nanofiber modified glassy carbon (GC) electrode at physiological pH. Bare GC electrode fails to determine the concentration of FA in the presence of a higher concentration of AA due to the surface fouling caused by the oxidized products of AA and FA. However, modification with a-Fe2O3 nanofibers not only separates the voltammetric signals of AA and FA by 420 mV between AA and FA, but also enhances higher oxidation current. The amperometric current response is linearly dependent on FA concentration in the range of 60–60 000 nM, and the a-Fe2O3 nanofiber modified electrode displayed an excellent sensitivity for FA detection with an Received 24th November 2012 Accepted 17th January 2013 experimental detection limit of 60 nM (1.12 Â 10À10 M (S/N ¼ 3)). Furthermore, the a-Fe2O3 nanofiber modified electrode showed an admirable selectivity towards the determination of FA even in the DOI: 10.1039/c3an00070b presence of a 1000-fold excess of AA and other common interferents. This modified electrode has been www.rsc.org/analyst successfully applied for determination of FA in human blood serum samples. 1 Introduction nanostructures can be used for both efficient transport of elec- trons and optical excitation, and these two factors make them In recent years, the synthesis and fabrication of nanomaterials critical to the function and integration of nanoscale devices, and with tailoring their size, morphology, and porosity have been have been the focus of intensive research for many potential intensively pursued not only for fundamental scientic interest applications in electronics, photonics, drug delivery, medical but also for many technological applications.1–3 Nanoparticles diagnostics, and magnetic materials.8–11 (zero-dimensional (0-D)) and nanowires/nanorods (one-dimen- Hematite (a-Fe2O3) is the most stable iron oxide with n-type sional (1-D)) with controlled size and shape are of key importance semiconducting properties (Eg ¼ 2.2 eV) under ambient condi- because their electrical, optical, and magnetic properties are tions. It has been intensively investigated because of its wide strongly dependent on their size and shape.1–3 Currently, one- applications in catalysts, pigments, magnetic materials, gas dimensional (1-D) nanomaterials such as silicon nanowires sensors, and lithium ion batteries.4,12–15 Fe2O3 was generally (SiNWs), carbon nanotubes (CNTs), and conducting polymer considered to be biologically and electrochemically inert, and its nanowires (CP NWs) have opened the possibility to fabricate electrocatalytic functionality has been rarely realized directly in electrochemical sensors and biosensors.4–7 Their high sensitivity the past,16 whereas Fe2+ ions (instead of Fe3+) play the dominant and new sensing mechanisms are related to intrinsic properties role in the oxidation reaction.17–19 Meanwhile, Fe2O3 was also associated with a high surface-to-volume ratio.4–7 Further, 1-D demonstrated to show both reversible reduction and reversible oxidation of Fe(III) in a basic carbonate buffer solution.17 Never- a School of Chemical and Biomedical Engineering, Nanyang Technological University, theless, in contrast with interests focusing on synthetic and 62 Nanyang Drive, Singapore, 637459s catalytic applications of Fe3O4, reports on the electrochemical b Centre for Nanobers and Nanotechnology, National University of Singapore, characterization of Fe2O3 nanoparticles are rather rare, and little Singapore 117576. E-mail: seeram@nus.edu.sg; Fax: +65-6872 5563; Tel: +65-6516 attention has been paid to the detailed study of their sensing 6593 c Department of Mechanical Engineering, National University of Singapore, Singapore performance.20–22 In principle, Fe2O3 nanoparticles may effi- 117576 ciently mediate the nal heterogeneous chemical oxidation or d Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 ul. Kasprzaka, 01- reduction of the target agent, while the converted iron oxides can 224 Warszawa, Poland. E-mail: ktpkannan@gmail.com; mopallo@ichf.edu.pl; Tel: be continuously and simultaneously recovered by electro- +48-223 433 375. Fax: +48-223 433 333 chemical oxidation or reduction due to their high surface to † Electronic supplementary information (ESI) available. See DOI: volume ratio. From this key point, an electrocatalytic study of 10.1039/c3an00070b nanostructured Fe2O3 in biocompatible environments may not ‡ Both authors contributed equally to this work. This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 1779–1786 | 1779
  2. 2. View Article Online Analyst Paper only be of scientic interest, but could also produce real benets (FA), were purchased from Merck chemicals and were used as such as their substitution for noble metals/enzymes for practical received. All other chemicals used in this investigation were of enzyme-free biosensor applications. Due to its low cost, good analytical grade. The phosphate buffer solution (PBS; pH ¼ 7.2) stability, and reversibility, a-Fe2O3 has been proven to be an was prepared using Na2HPO4 and NaH2PO4. Double distilled important semiconductor nanomaterial for electrochemical water was used to prepare the solutions in this investigation. sensors.23 However, so far, there have been only a few reports on bio-sensing properties of 1-D nanostructural a-Fe2O3. Generally, 2.2 Preparation of a-Fe2O3 nanobers the properties of a biosensor are strongly dependent on its Firstly, 1 g of PVP was dissolved in 10 mL of ethanol solution surface area. The relatively low surface to volume ratio of and homogeneously stirred at room temperature for 1 h for conventional bulk a-Fe2O3 materials leads to their poor bio- complete dissolution. Then, 0.4 g of Fe(acac)3 precursor was sensing properties. Hence, developing 1-D nanostructure a-Fe2O3 added to the PVP solution and continuously stirred for 6 h with a high surface area is very important for increasing their followed by addition of 1 mL acetic acid. Finally, 5 mL of applications in sensors.4,14 Recently, as prepared a-Fe2O3 nano- Fe(acac)3–PVP precursor solutions were loaded in a 5 mL plasticPublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B material has been proven to be a successful electrode material syringe with a hypodermic needle (dia. 27 G). The hypodermic due to the intrinsic peroxidase-like catalytic activity.24 The needle was then connected to a high-voltage supply capable of a-Fe2O3 nanowire array modied glucose sensor exhibited an Downloaded by University of Texas Libraries on 22 February 2013 generating direct current (DC) voltages up to 30 kV. Electro- excellent biocatalytic performance towards the oxidation of spinning was carried out by applying a power supply of around glucose with a detection limit of 6 mM (S/N ¼ 3).24 In the present 16.5 kV at the needle in a controlled electrospinning set-up study, we have synthesized a-Fe2O3 nanober by a simple elec- (Electrospunra, Singapore). Aluminum foil was used as the trospinning method and used it to improve the bio-sensing counter electrode, and the distance between the needle and the performance towards the oxidation of FA for the rst time. collector was maintained at 15 cm. The as-spun Fe(acac)3–PVP Folic acid, known as a widely used water soluble vitamin, is composite nanober mats were placed in an advanced vacuum reported to be a very signicant component for human health oven at room temperature for 12 h to remove the solvent which relates to a series of diseases such as gigantocytic anemia, residuals. Finally, the nanobers were calcined at 500 C for 5 h leucopoenia, mental devolution, heart attack and congenital in air at a heating rate of 5 C minÀ1, and nally a-Fe2O3 malformation.25–28 FA is one of the important coenzymes of the nanobers were obtained and stored carefully. haematopoietic system that controls the generation of ferro- haeme.28 The dosage of FA is associated with the treatments of 2.3 Instrumentation hyperhomocysteinemic coronary artery disease, hypertension, depression, hypercholesterolemia, mammary tumor, vascular The crystallographic information of the prepared a-Fe2O3 disease and neural tube defects of pregnant women.29–33 The nanobers was studied using the powder X-ray diffraction important biomolecules such as AA and FA are present in human technique (XRD, Shimadzu XRD-6000, Cu Ka radiation oper- blood plasma,34,35 urine36,37 and blood serum samples.38,39 Since ating at 30 kV/40 mA). The surface morphologies of the nano- these biomolecules coexist in human uids, their simultaneous structures were characterized using a eld emission-scanning determination is essential to secure the human health from the electron microscope (FE-SEM) JEOL JSM-6301F. Transmission above critical diseases risk. Therefore, the selective and sensitive electron microscopy (TEM), JEM-2010, JEOL USA Inc., was determination of FA is very important from the clinical and employed to study the surface morphology of a-Fe2O3 nano- health viewpoints. In this paper, we will show that the a-Fe2O3 bers. The electron beam accelerating voltage of the microscope nanober modied GC electrode exhibits an excellent electro- was at 200 kV. Electrochemical measurements were performed catalytic activity towards FA, and a detection limit of 60 nM FA in a conventional two compartment three electrode cell with a has been achieved using the amperometry method. The a-Fe2O3 mirror polished 3 mm glassy carbon (GC) as the working elec- nanober array on the GC electrode with a nano-size and coarse trode, Pt wire as the counter electrode and a NaCl saturated surface provides a platform for FA oxidation by contributing both Ag/AgCl as the reference electrode. The electrochemical excess electroactive sites and strong adhesion to the GC electrode measurements were carried out with a CHI Model 660C (Austin, surface, which results in the enhanced sensitivity and long term TX, USA) electrochemical workstation. In cyclic voltammetry, stability of the a-Fe2O3 electrode. The application of the Fe2O3 the electrochemical oxidations of AA and FA were carried out at modied electrode has been successfully demonstrated by a scan rate 50 mV sÀ1. Pulse width ¼ 0.06 s, amplitude ¼ 0.05 V, measuring the concentration of FA in real samples. sample period ¼ 0.02 s and pulse period ¼ 0.20 s were used in differential pulse voltammetry (DPV). For chronoamperometric measurements, pulse width ¼ 0.25 s and potential step ¼ 1 mV 2 Experimental section were used. All the electrochemical measurements were carried 2.1 Materials and methods out under a nitrogen atmosphere at room temperature (27 C). Polyvinylpyrrolidone (PVP; MW ¼ 1 40 000) and iron(III) acety- lacetonate (Fe(acac)3) were purchased from Sigma-Aldrich 2.4 a-Fe2O3 nanober modied electrodes and Fluka, Singapore, respectively. Ethanol (HPLC grade) and The a-Fe2O3 nanostructure modied GC electrode was prepared glacial acetic acid were purchased from Tedia, Singapore and as follows. First, the surface of the glassy carbon electrode used as received. The biomolecules, uric acid (UA) and folic acid for each experiment was mechanically polished with 600 grit 1780 | Analyst, 2013, 138, 1779–1786 This journal is ª The Royal Society of Chemistry 2013
  3. 3. View Article Online Paper Analyst sand-paper and 0.050 mm alumina powders, which was then rinsed with acetone and double distilled water. A 3 mL aliquot of a-Fe2O3 nanobers (dispersed in water, 5 mg mLÀ1, and pH ¼ 7.0) was dropped onto the surface and dried under atmospheric conditions. In addition, a bulk-Fe2O3 electrode (denoted as b-Fe2O3 GC electrode, fabricated by the above mentioned process) was used for comparison. 3 Results and discussion 3.1 Characterization of the as-prepared a-Fe2O3 nanostructure The morphology of as-electrospun Fe(acac)3–PVP compositePublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B nanober before and aer calcination was investigated by per- forming FESEM analysis. Fig. 1a shows the formation of a highly interconnected network of nanobers with an average Downloaded by University of Texas Libraries on 22 February 2013 Fig. 2 XRD pattern of Fe(acac)3–PVP nanofibers calcined at 500 C for 5 h at a ber diameter of 288 nm. Fig. 1a (inset) shows the distribution heating rate of 5 C minÀ1 in air. of bers upon applying a potential of 16.5 kV; the broad distribution of the bers was due to the dominancy of may also result in enhanced long-term stability due to a more Coulombic repulsive forces upon applying such a higher secure attachment to the electrode surface. potential. The formation of nanobers was observed aer Further, the XRD analysis on calcined nanobers has been calcination of the Fe(acac)3–PVP composite at 500 C for 5 h at a carried out to conrm the a-Fe2O3 phase formation. Fig. 2 rate of 5 C minÀ1 in air (Fig. 1b). The novel morphology of shows the XRD pattern of a-Fe2O3 nanobers aer calcination nanostructures with ellipsoidal shape of a-Fe2O3 nanoparticles of the Fe(acac)3–PVP composite at 500 C for 5 h in air. All the uniformly plaited along the ber directions was observed. This diffraction peaks were well indexed to the rhombohedral morphology was due to the combined effect of phase separation hexagonal phase of hematite (a-Fe2O3) (JCPDS: 33-0664). The (thermodynamic) and electrospinning (electro-hydrody- strong and narrow-sharp diffraction peaks showed the purity namic).40 The phase separation of the polymer and the and high degree of crystallization of synthesized a-Fe2O3 precursor induced the formation of precursor islands, and the nanobers. electrospinning coerced the precursors to plait together with the result of spinning and whirling effects upon applying potential. During calcining, the polymer PVP present all over 3.2 Electrochemical oxidation of FA the brous structure was decomposed and yielded nanorod-like We have examined the electrocatalytic activity of a-Fe2O3 GC, b- structures. The nanobers have a coarse surface due to the Fe2O3 GC and unmodied GC electrodes towards the oxidation of adsorption and assembly of small crystalline nanoparticles FA. We found that the a-Fe2O3 nanober modied GC electrode (Fig. 2b; inset), some of which even have a chain-like showed higher electrocatalytic activity towards AA and FA than morphology. In comparison with the randomly packed particle the b-Fe2O3 GC and unmodied GC electrodes. Fig. 3A shows the counterpart, such arrayed nanowires provide more ordered cyclic voltammograms (CVs) obtained for 0.25 mM FA at bare and spatial orientation and improved structural stability. As a result, a-Fe2O3 nanober modied GC electrodes in a 0.20 M phosphate higher mass transfer and permeation rate, a stable porous volume and less structural corruption can be expected during the electrochemical recycling. In addition, the coarse surface Fig. 3 (A) CVs obtained for 0.25 mM FA at bare and a-Fe2O3 nanofiber modified GC electrodes after the 1st (a and d), 10th (b and e) and 20th (c and f) cycles in a Fig. 1 (a) FE-SEM image of as-electrospun Fe(acac)3–PVP composite nanofibers 0.2 M PB solution at a scan rate of 50 mV sÀ1 and (g) CV obtained for the a-Fe2O3 at a power supply of 16.5 kV (inset: histogram of the nanofiber diameter and nanofiber modified GC electrode in the absence of 0.5 mM FA in a 0.2 M PB distribution) and (b) FE-SEM image of a-Fe2O3 nanofibers after calcining solution. Inset: bulk-Fe2O3 modified GC electrode in the presence of 0.25 mM FA composites at 500 C for 5 h at a ramp rate of 5 C minÀ1. in 0.2 M PB solution. (B) Anodic peak current vs. square root of scan rates. This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 1779–1786 | 1781
  4. 4. View Article Online Analyst Paper buffer (PB) solution (pH ¼ 7.2). At the bare GC electrode, an fast electron transfer reaction of FA at the a-Fe2O3 nanober oxidation peak was observed for FA at 0.95 V in the rst cycle modied GC electrode quantitatively, we have calculated the (curve a). In the subsequent cycles, the FA oxidation peak was standard heterogeneous rate constant (ks) for FA at a-Fe2O3 shied to more positive potential with decreased peak current. nanobers and bare GC electrodes using the Velasco equation42 Aer 20 cycles, the oxidation peak of FA almost disappeared as given below: (curve c), indicating that the bare GC electrode was not suitable for the stable and simultaneous determination of FA. The ks ¼ 1.11Do1/2 (Ep À Ep/2)À1/2n1/2 adsorption of the oxidized product of FA on the electrode surface is the possible reason for the decreased FA oxidation current and where, ks is the standard heterogeneous rate constant; Do is the more positive peak shi in the oxidation potential at the bare GC apparent diffusion coefficient; Ep is the oxidation peak poten- electrode. On the other hand, a well-dened oxidation peak was tial; Ep/2 is the half-wave oxidation peak potential and n is the observed at 0.81 V for FA at the a-Fe2O3 nanober modied GC scan rate. In order to determine ks, it is necessary to nd the electrode (curve d), which was a 140 mV less positive potential diffusion coefficient for FA. The apparent diffusion coefficientPublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B than at the bare GC electrode. It can be seen from Fig. 3A that the (Do) value was determined using a single potential chro- oxidation potential of FA remained stable even aer 20 repeated noamperometry technique based on the Cottrell slope obtained by plotting current versus 1/Otime. Chronoamperometry Downloaded by University of Texas Libraries on 22 February 2013 potential cycles (curve f), indicating that the oxidation of FA was highly stable at the a-Fe2O3 nanober modied GC electrode. measurements were carried out for FA both at bare and a-Fe2O3 The a-Fe2O3 nanober modied GC electrode did not show any nanober modied GC electrodes aer 20 potential cycles. The oxidation response in the absence of FA (curve g). These results Do of 1.98 Â 10À6 cm2 sÀ1 was obtained for FA. The estimated ks indicated that the a-Fe2O3 nanobers are excellent candidates values for the oxidation of FA at bare and a-Fe2O3 nanober toward the electrochemical oxidation of FA. modied GC electrodes were found to be 1.43 Â 10À5 cm sÀ1 For comparison, we have also modied the electrode with and 2.91 Â 10À4 cm sÀ1, respectively. The obtained higher ks bulk Fe2O3 (b-Fe2O3 GC) as an electrocatalyst, and we observed value for FA at the a-Fe2O3 nanober modied GC electrode that the electrochemical oxidation of FA is almost the same as indicated that the oxidation of FA was faster at the a-Fe2O3 the electrochemical response of the bare GC electrode (Fig. 3A; nanober modied GC electrode than at the bare GC electrode. inset). Unlike bare GC and b-Fe2O3 modied GC electrodes, the Further, we have investigated whether the oxidation of FA at FA oxidation peak is highly stable at the a-Fe2O3 nanober the a-Fe2O3 nanober modied GC electrode is due to diffusion modied GC electrode. This indicated that a-Fe2O3 nanobers control or adsorbed species by varying the scan rates. The effectively prevent the fouling caused by the oxidized products oxidation current of FA was increased while increasing the scan of FA. The observed oxidation peak for FA in Fig. 3A is due to the rates (Fig. 3B). A good linearity between the anodic peak current two electron oxidation of FA to dehydrofolic acid,41 as shown in and the square root of the scan rate was obtained within the range Scheme 1. The oxidation process can be deduced through an from 100 to 1000 mV sÀ1 with a correlation coefficient of 0.995 electrocatalytic mechanism involving the Fe(III)/Fe(II) ion for FA, as shown in the inset of Fig. 3B. This indicated that the centers, and the catalytic mechanism of the a-Fe2O3 to folic acid electrode reaction process was controlled by the diffusion of FA. oxidation can be explained by the following scheme; the vol- Further, we have studied the optimization of pH for the tammetric response of FA at the a-Fe2O3 electrode is due to two present FA sensor. ESI, Fig. S1A,† shows the DPVs obtained for steps, viz., an electrochemical process followed by a chemical 100 mM FA at the a-Fe2O3 nanober modied GC electrode from reaction. In the rst step, Fe(II) was electrochemically oxidized pH 5.2–10.2 PB solution. It can be clearly visualized that as the to Fe(III) (eqn (1)) and in the second step FA was chemically pH value increases, the Epa of FA shis towards negative oxidized to dehydrofolic acid by Fe(III) (eqn (2)). potential, which conrms that during electrochemical oxida- tion of FA not only electrons but also protons are involved. The 2Fe(II) / 2Fe(III) + 2eÀ (1) plot of Epa vs. pH shows good linearity in the pH range of 5.2– 10.2. The linear regression equation of Epa/V ($0.032 V) vs. pH 2Fe(III) + folic acid / 2Fe(II) + dehydrofolic acid + H2O (2) was obtained with a correlation coefficient r ¼ 0.990, indicating that the number of protons and electrons involved is equal. Fig. S1B† also reveals that the Ipa increases with an increase in The a-Fe2O3 nanober does not show any oxidation peak in pH up to 7.2, and a further increase of pH results in the decrease the absence of FA (curve e). Further, in order to understand the of the anodic peak current. Since the present modied electrode shows a higher current for FA at pH 7.2 and it is also close to the physiological pH value, we have chosen pH 7.2 for the deter- mination of FA in this work. 3.3 Selective determination of FA in the presence of AA Further, we have investigated the determination of FA in the presence of very high concentrations of AA. It is well known that Scheme 1 Electrochemical oxidation of FA at the a-Fe2O3 nanofiber modified AA is an important interferent compound which coexists with GC electrode. FA in our body uids, and further its concentration is always 1782 | Analyst, 2013, 138, 1779–1786 This journal is ª The Royal Society of Chemistry 2013
  5. 5. View Article Online Paper Analyst higher concentrations of AA is very important. Fig. 4 shows the DPVs obtained for the increment of 10 mM FA in the presence of 2500 mM AA. The concentration of FA was varied from 10 to 50 mM (curves b–f). A very clear signal was observed for 10 mM FA in the presence of 2500 mM AA in Fig. 4 (curve b), which revealed that detection of a very low concentration of FA is possible even in the presence of 250-fold AA. On increment of 10 mM FA to a PB solution containing 2500 mM AA, the oxidation current of FA was increased linearly with a correlation coefficient of 0.9995. However, the oxidation peak current of AA was almost unchanged in each addition of FA. These results demonstrated that the a-Fe2O3 nanober modied GC electrode is more selective towards FA even in the presence of very high concen-Published on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B trations of AA. Downloaded by University of Texas Libraries on 22 February 2013 Fig. 4 DPVs obtained for the increment of 10 mM FA to 2500 mM AA in a 0.2 M 3.4 Amperometric determination of FA along with AA PB solution at the a-Fe2O3 nanofiber modified GC electrode. Pulse width ¼ 0.06 s, The amperometric method was used to examine the sensitivity amplitude ¼ 0.05 V, sample period ¼ 0.02 s and pulse period ¼ 0.2 s. of the a-Fe2O3 nanober modied GC electrode towards the detection of FA individually and also along with AA. Fig. 5A shows the amperometric i–t curve for FA at the a-Fe2O3 nano- much higher than that of FA.43 For example, the concentrations ber modied GC electrode in a homogeneously stirred 0.20 M of AA and FA in human blood serum are 53.8 Æ 36.6 mmol LÀ1, PB solution by applying a potential of 0.90 V. The modied and 34.4 Æ 10.4 nmol LÀ1, respectively.43–45 Therefore, from a electrode shows the initial current response due to 600 nM FA. clinical point of view, the determination of FA in the presence of The current response increases and a steady state current is Fig. 5 (A) Amperometric i–t curve for the determination of FA at the a-Fe2O3 nanofiber modified GC electrode in a 0.2 M PB solution. Each addition increases the concentration of 60 nM of FA. Eapp ¼ 0.90 V. (B) Calibration plot obtained for conc. of FA vs. amperometric current. (C) (a) 60, (b) 300, (c) 900, (d) 1500, (e) 4500, (f) 9000, (g) 15 000, (h) 20 000, (i) 30 000, (j) 40 000 and (k) 60 000 mM addition of FA at the a-Fe2O3 nanofiber modified electrode. (D) Amperometric i–t curve response obtained for the addition of 60 nM FA (a–c) and a mixture of 60 nM each of FA and AA (d–f) using the a-Fe2O3 nanofiber electrode in a 0.2 M PB solution at a regular interval of 50 s. This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 1779–1786 | 1783
  6. 6. View Article Online Analyst Paper Table 1 Comparison of different chemically modified electrodes for the deter- mination of FA with the a-Fe2O3 nanostructure modified electrode Modied electrodes Detection limit Ref. Single-walled carbon 1 Â 10À9 M 46 nanotube-ionic liquid paste electrode Single-walled carbon nanotube 1 Â 10À9 M 47 lm modied glassy carbon electrode Lead lm modied glassy 7 Â 10À10 M 49 carbon electrode Poly(5-amino-2-mercapto-1,3,4- 2.3 Â 10À10 M 50 thiadiazole) lm modiedPublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B glassy carbon electrode 3-Amino-5-mercapto-1,2,4-triazole 2.5 Â 10À7 M 51 Fig. 6 Amperometric i–t curve for 60 nM addition of FA at the a-Fe2O3 nanofiber polymerized lm modied modified GC electrode (a–c, g–i), and the addition of 60 mM of Na+, Ca2+, SO42À Downloaded by University of Texas Libraries on 22 February 2013 glassy carbon electrode (d–f), glucose, urea and oxalate (j–l), in a homogeneously stirred 0.2 M PB a-Fe2O3 nanostructure modied 1.12 Â 10À10 M This work solution. glassy carbon electrode such as Na+, Ca2+, and SO42À, and some physiological interfer- ents such as glucose, urea and oxalate using the amperometric attained within 3 s for further addition of 60 nM FA in each step method (Fig. 6). Furthermore, no change in the amperometric with a sample interval of 50 s. The dependence of the response current response was observed for 60 nM FA in the presence of current with respect to the concentration of FA was linear from 60 mM of MgSO4, CaCl2, NaCl, K2CO3, NaF, ClÀ, FÀ, and NH4Cl, 60 nM to 600 nM at the a-Fe2O3 nanober modied GC elec- indicating that the present modied electrode is highly selective trode with a correlation coefficient of 0.9991 (Fig. 5B). The towards the determination of FA even in the presence of a 1000- current response for 60 nM FA was found to be 39.2 nA. Further, fold excess of these interferents. the amperometric current response was increased linearly with increasing FA concentration in the range of 60–60 000 nM (Fig. 5C) with a correlation coefficient of 0.9901, and the a-Fe2O3 3.6 The stability and reproducibility of the a-Fe2O3 nanober modied electrode displayed an excellent sensitivity nanober modied electrode for FA detection with an experimental detection limit of 60 nM In order to investigate the stability of the a-Fe2O3 nanober (1.12 Â 10À10 M (S/N ¼ 3)). The linear range and the lowest modied GC electrode, the DPVs for 0.20 mM FA in a 0.20 M PB detection limit for FA at a-Fe2O3 nanobers were compared with solution were recorded for every 5 min interval. It was found the recently reported chemically modied electrodes.46–51 Thus, that the oxidation peak current remained the same with a the present modied electrode shows the lowest detection limit relative standard deviation of 2.1% for 20 repetitive measure- for FA (60 nM (1.12 Â 10À10 M (S/N ¼ 3)) when compared to the ments, indicating that the electrode has a good reproducibility reported FA detection limits (see Table 1).46–51 As mentioned and does not undergo surface fouling. Aer voltammetric above, the normal level of FA in blood serum is 34.4 Æ 10.4 nmol measurements, the electrode was kept in a pH ¼ 7.2 PB solution LÀ1. Therefore, the a-Fe2O3 nanober modied GC electrode is at room temperature. The current response decreased about more suitable for the determination of FA in real (blood serum) 1.24% in one week and 5.54% in about two weeks. To ascertain samples even in the presence of 53.8 Æ 36.6 mmol LÀ1 AA. The the reproducibility of the results, three different GC electrodes amperometric method was also performed to determine the were modied with the a-Fe2O3 nanobers and their response concentration of FA along with AA. The amperometric current towards the oxidation of 0.50 mM AA and FA was tested by 20 response for the alternative addition of AA and FA in the repeated measurements. The separation between the voltam- mixture is shown in Fig. 5D. The a-Fe2O3 nanober modied GC metric peaks of AA–FA was the same at all the four electrodes. electrode showed the initial current response due to the addi- The peak current obtained in the 20 repeated measurements of tion of 60 nM AA (Fig. 5D; curves a–c) into a PB solution with a three independent electrodes showed a relative standard devi- sample interval of 50 s, and the current response was increased. ation of 1.48%, conrming that the results are reproducible. Further, the addition of a mixture of 60 nM AA and 60 nM FA to The above results showed that the present modied electrode a stirred solution of 0.2 M PB showed a two-fold enhanced was very much stable and reproducible towards these analytes. amperometric oxidation current at the same applied potential It is worthy to compare the determination of FA at the a-Fe2O3 (Fig. 5D; curves d–f). The two-fold amperometric oxidation nanober modied GC electrode with other chemically modi- current obtained was due to the oxidation of both AA and FA. ed electrodes. In the reported papers, the procedures adopted for the modication of electrode surfaces are very tedious, more 3.5 Anti-interference ability of the a-Fe2O3 nanobers time consuming and further reproducible results cannot be The anti-interference ability of the a-Fe2O3 nanobers was obtained.47,48,52–54 In the case of a carbon paste electrode, rst tested towards the detection of FA from various common ions the carbon paste was mixed with the palmitic and stearic acids 1784 | Analyst, 2013, 138, 1779–1786 This journal is ª The Royal Society of Chemistry 2013
  7. 7. View Article Online Paper Analyst samples, indicating that the adopted method could be effi- ciently used for the determination of FA in real samples in the presence of possible interferents. 4 Conclusions We have demonstrated the synthesis of a-Fe2O3 nanobers by a simple electrospinning method and their application in vol- tammetric determination of FA in the presence AA (pH 7.2). The a-Fe2O3 nanober modied electrode not only separates the voltammetric signals of AA and FA with a potential difference of 420 mV between AA and FA, but also shows a higher oxidationPublished on 31 January 2013 on http://pubs.rsc.org | doi:10.1039/C3AN00070B current than the bulk-Fe2O3 and unmodied electrodes. The amperometric current response is linearly dependent on FA Fig. 7 DPVs obtained for blood serum (green line) and after the addition of concentration in the range of 60–60 000 nM, and the a-Fe2O3 Downloaded by University of Texas Libraries on 22 February 2013 10 mM commercial FA (blue line) to blood serum at the a-Fe2O3 nanofiber nanober modied GC electrode displayed an excellent sensi- modified GC electrode in a 0.2 M PB solution. Pulse width ¼ 0.06 s, amplitude ¼ tivity for FA detection with an experimental detection limit of 0.05 V, sample period ¼ 0.02 s and pulse period ¼ 0.2 s. 60 nM (1.12 Â 10À10 M (S/N ¼ 3)). The practical application of the present modied electrode was successfully demonstrated by determining the concentration of FA in human blood serum in the presence of carbon tetrachloride and then dried over- samples. The excellent analytical performance and low cost night at room temperature.52 Similarly, for the fabrication of a nanomaterials are not only scientically signicant for the multi-walled carbon nanotube coated Au electrode, the multi- development of effective biosensors, but also could produce real walled carbon nanotubes (MWNTs) were reuxed in the mixture benets such as energy and cost savings in comparison with of concentrated H2SO4 and HNO3 for 4–5 h, then washed with other noble metals or enzymes for a wide range of potential water and dried in vacuum at room temperature.53 When applications in medicine, catalysis, and biosensing. compared to the reported procedure for the electrode modi- cations, the procedure for the deposition of a-Fe2O3 nanobers on the GC electrode in the present study is very easy, less time Acknowledgements consuming (12 min), highly stable and reproducible. The authors thank the National University of Singapore and Nanyang Technological University for providing excellent 3.7 Determination of FA in human blood serum samples research facilities to carry out this work. Palanisamy Kannan The practical application of the a-Fe2O3 nanober modied GC and Marcin Opallo thank NanOtechnology, Biomaterials and electrode was tested by measuring the concentration of FA in aLternative Energy Source for the ERA Integration [FP7- human blood serum samples. The human blood serum samples REGPOT-CT-2011-285949-NOBLESSE] Project from the Euro- were collected from a local hospital (Muthu clinic and X-rays, pean Union. Dindigul district, India). The standard addition technique was used for the determination of FA in serum samples. The DPV of References blood serum in a PB solution (pH ¼ 7.2) shows two oxidation peaks at 0.35 and 0.81 V as shown in Fig. 7, green line, and these 1 J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, peaks may be due to the oxidation of AA and FA, respectively. To K. Cho and H. Dai, Science, 2000, 287, 622–625. conrm the observed oxidation peak at 0.81 V for FA in Fig. 7, 2 P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and green line, we have added a known concentration of FA into the J. M. Tarascon, Nature, 2000, 407, 496–499. same blood serum solution, the oxidation current at 0.81 V was 3 D. Yu and V. W.-W. Yam, J. Am. Chem. Soc., 2004, 126, 13200– further enhanced (Fig. 7; blue line) and recovery results are 13201. given in Table 2. The enhanced oxidation peak current at 0.81 V 4 G. Neri, A. Bonavita, S. Galvagno, P. Siciliano and S. Capone, indicated that the peak corresponds to the oxidation of FA. The Sens. Actuators, B, 2002, 82, 40–47. proposed method shows a better recovery of spiked FA in serum 5 Y. Cui, Q. Wei, H. Park and C. M. Lieber, Science, 2001, 293, 1289–1292. Table 2 Determination of FA in human blood serum samples 6 K. Ramanathan, M. A. Bangar, M. Yun, W. Chen, N. V. Myung and A. Mulchandani, J. Am. Chem. Soc., 2004, 127, 496–497. Human 7 R. J. Chen, S. Bangsaruntip, K. A. Drouvalakis, N. Wong Shi blood Original Added Found Recovery Kam, M. Shim, Y. Li, W. Kim, P. J. Utz and H. Dai, Proc. serum (mM) (mM) (mM) (%) Natl. Acad. Sci. U. S. A., 2003, 100, 4984–4989. Sample 1 50.10 10 59.80 99.5 8 Y. Ding, P. X. Gao and Z. L. Wang, J. Am. Chem. Soc., 2004, Sample 2 25.40 10 35.16 99.3 126, 2066–2072. This journal is ª The Royal Society of Chemistry 2013 Analyst, 2013, 138, 1779–1786 | 1785
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