M. Amal Raj

Simultaneous determination of uric acid, xanthine, hypoxanthine and caffeine in human blood serum and urine samples using electrochemically reduced graphene oxide modified electrode

This paper describes the fabrication of graphene on glassy carbon electrode (GCE) by electrochemical reduction of graphene oxide (GO) attached through 1,6-hexadiamine on GCE and the simultaneous determination of structurally similar four purine derivatives using the resultant electrochemically reduced GO (ERGO) modified electrode. The electrocatalytic activity of ERGO was investigated toward the oxidation of four important purine derivatives, uric acid (UA), xanthine (XN), hypoxanthine (HXN) and caffeine (CAF) at physiological pH. The modified electrode not only enhanced the oxidation currents of the four purine derivatives but also shifted their oxidation potentials toward less positive potentials in contrast to bare GCE. Further, it successfully separates the voltammetric signals of the four purine derivatives in a mixture and hence used for the simultaneous determination of them. Selective determination of one purine derivative in the presence of low concentrations other three purine derivatives was also realized at the present modified electrode. Using differential pulse voltammetry, detection limits of 8.8×10(-8)M, 1.1×10(-7)M, 3.2×10(-7)M and 4.3×10(-7)M were obtained for UA, XN, HXN and CAF, respectively. The practical application of the modified electrode was demonstrated by simultaneously determining the concentrations of UA, XN, HXN and CAF in human blood plasma and urine samples.

Assembly of gold nanoparticles on graphene film via electroless deposition: spontaneous reduction of Au3+ ions by graphene film

In this paper, an electrochemically reduced graphene oxide–gold nanoparticle (ERGO–AuNP) composite film was fabricated on a glassy carbon electrode (GCE) by a simple electroless deposition method using a solution containing HAuCl4 and NH2OH. The deposition of AuNPs on the ERGO film was achieved via two different approaches. The first approach involves the electroless deposition of AuNPs on the graphene oxide (GO) modified GCE followed by the electrochemical reduction. The second approach is the electroless deposition of AuNPs on the ERGO film modified GCE which was fabricated by self-assembling GO on a 1,6-hexadiamine modified GCE followed by the electrochemical reduction. The particle coverage estimated from cyclic voltammetry (CV) showed that the particle coverage of AuNPs deposited on the ERGO film (22%) was higher than that of the GO film (17%) and bare GCE (7%) under identical conditions. The obtained higher coverage is attributed to the ERGO films ability to spontaneously reduce Au3+ ions. Although AuNP deposition was observed at the ERGO surface in the absence of NH2OH, the particle coverage was much less (2%) and hence the electroless deposition was carried out in the presence of NH2OH. SEM and CV studies showed that the particle coverage and density of AuNPs were increased while increasing the electroless deposition time.

Selective determination of 3,4-dihydroxyphenylacetic acid in the presence of ascorbic acid using 4-(dimethylamino)pyridine capped gold nanoparticles immobilized on gold electrode.

4-(Dimethylamino)pyridine capped gold nanoparticles (DMAP-AuNPs) were synthesized in aqueous medium and then immobilized on 1,6-hexanedithiol (HDT) modified Au electrode for the selective determination of 3,4-dihydroxyphenylacetic acid (DOPAC) in the presence of ascorbic acid (AA). The synthesized DMAP-AuNPs were characterized by UV-visible spectroscopy and high resolution-transmission electron microscopy (HR-TEM). The HR-TEM images showed that the nanoparticles are spherical in shape with a diameter of ∼12 nm. The DMAP-AuNPs immobilized on HDT modified electrode was characterized by cyclic voltammetry and impedance spectroscopy. Impedance spectra show that the electron transfer reaction was more facile at the AuNPs modified electrode when compared to bare and HDT modified Au electrodes. The application of DMAP-AuNPs modified electrode was demonstrated by selective determination of DOPAC in the presence of high concentration of AA at pH 4. Using amperometry method, 40 nM detection of each AA and DOPAC was achieved. The current response was increased linearly with increasing AA and DOPAC in the concentration range of 40×10(-9) to 10×10(-5) M and a detection limit was found to be 5.6×10(-10) M and 3.7×10(-10) M (S/N=3) for AA and DOPAC, respectively. The present modified electrode was also successfully used for the determination of 40 nM DOPAC in the presence of 2500-fold excess of common interferents such as Na(+), Mg(2+), Cu(2+), Ca(2+), NH(4)(+) urea and glucose.

Graphene layer modified glassy carbon electrode for the determination of norepinephrine and theophylline in pharmaceutical formulations

In this paper, electrochemical determination of norepinephrine (NE) and theophylline (TP) was demonstrated using an electrochemically reduced graphene oxide (ERGO) modified glassy carbon electrode (GCE). The ERGO was fabricated on a GCE by self-assembling graphene oxide (GO) on 1,6-hexadiamine (HDA) which was preassembled on the GCE, followed by the electrochemical reduction of GO. SEM imaging shows a flake-like structure for the GO modified substrate, whereas an aggregated structure was observed for the ERGO modified substrate. Then, the ERGO modified GCE was used for the determination of NE and TP. It was found that the ERGO modified electrode not only enhanced the oxidation currents of NE and TP, but also prevented a surface fouling effect caused by their oxidation products. In contrast, the oxidation currents of both NE and TP were low at bare and GO modified GCEs and very importantly, their oxidation signals were not stable due to the surface fouling effect. The ERGO modified GCE separated the oxidation signals of NE and TP in a mixture and was hence used for their simultaneous determination. Selective determination of NE in the presence of a higher concentration of TP was also achieved at an ERGO modified GCE. Using amperometry, a detection of 40 nM NE and 50 nM TP was obtained. The current responses of NE and TP were increased linearly while increasing their concentrations from 4 × 10−8 to 1 × 10−4 M and 5 × 10−8 to 4 × 10−5 M and the detection limits were found to be 8.4 × 10−10 M and 2.9 × 10−9 M (S/N = 3), respectively. The practical application of the present modified electrode was demonstrated by measuring the amounts of NE and TP in injections and tablets.

Synthesis, characterization and modification of functionalized pyrimidine stabilized gold nanoparticles on ITO electrode for the determination of tannic acid

This paper describes the synthesis of 4-amino-6-hydroxy-2-mercaptopyrimidine capped gold nanoparticles (AHMP-AuNPs) in aqueous medium and their immobilization on indium tin oxide (ITO) electrode modified with (3-mercaptopropyl)trimethoxysilane (MPTS) sol-gel for the determination of tannic acid (TA). The high resolution transmission electron microscopy (HR-TEM) images show that the particles are spherical in shape with a diameter of ~6 nm. The heterogeneous electron transfer rate constant (k(et)) of [Fe(CN)₆]³⁻/⁴⁻ at ITO/MPTS/AHMP-AuNPs electrode was found to be 1.14×10⁻⁷ m/s. This value was much higher than the values obtained at ITO/MPTS (4.94×10⁻⁹ m/s) and bare ITO (8.79×10⁻⁸ m/s) electrodes, indicating that the electron transfer reaction was faster at AuNPs modified electrode. Further, the ITO/MPTS/AHMP-AuNPs electrode shows excellent electrocatalytic activity toward TA oxidation when compared to bare ITO electrode. This was understood from the obtained higher heterogeneous rate constant (k(s)) value at AuNPs modified electrode (7.35×10⁻⁵ m/s) than at bare ITO electrode (5.45×10⁻⁶ m/s). Using the amperometry method, detection of 20 nmol/L TA was achieved. The practical application of the present method was demonstrated by determining the concentration of TA in commercial beer samples.

Growth of gold nanorods in solution and on ITO and Au substrates using non-peripheral amine functionalized nickel(II) phthalocyanine capped gold nanoparticles as a seed solution

In this paper, we report the synthesis of gold nanorods (GNRs) both in solution and on ITO and Au substrates using non-peripheral amine functionalized nickel(II) phthalocyanine capped gold nanoparticles (4α-NiIITAPc-AuNPs) as a seed solution. The GNRs formed in solution show transverse and longitudinal waves at 526 and 761 nm, respectively. The high-resolution transmission electron microscopy (HR-TEM) studies show the presence of GNRs in addition to few spherical AuNPs. The average aspect ratio of GNRs was found to be 4.5 ± 0.3. Further, the GNRs were also grown on the Au and ITO substrates by seed mediated growth method using (3-mercaptopropyl)-trimethoxysilane sol–gel as a linker. The 4α-NiIITAPc-AuNPs were first self-assembled on ITO or Au surface modified with MPTS sol–gel film and then immersed in a GNR growth solution, to form surface grown GNRs. The growth of GNRs on the MPTS sol–gel film modified ITO surface was monitored by UV-vis spectroscopy. The 4α-NiIITAPc-AuNPs attached on ITO surface shows a surface plasmon resonance band at 550 nm. The intensity of this band increases and an additional shoulder band around 680 nm was observed while increasing the immersion time of the 4α-NiIITAPc-AuNPs modified ITO surface into the growth solution. After 24 h, the 4α-NiIITAPc-AuNPs modified ITO surface shows a broad wave around 680 nm along with a band at 550 nm. Similarly, Au modified electrode also showed characteristic transverse and longitudinal absorption bands for GNRs which confirm the successful growth of GNRs on Au surface. The surface grown GNRs were characterized by atomic force microscopy, ATR-FT-IR, cyclic voltammetry and impedance spectral studies. Finally, the electrocatalytic activity of GNRs grown Au electrode was examined by studying the oxidation of L-tyrosine.

Highly sensitive interference-free electrochemical determination of pyridoxine at graphene modified electrode: Importance in Parkinson and Asthma treatments

To reduce the side effects in the medication of Parkinson and Asthma, pyridoxine (PY) is administered along with l-3,4-dihydroxyphenyl alanine (l-dopa) and theophylline (TP), respectively. However, excessive dosage of PY leads to nervous disorder. Thus, a sensitive and selective electrochemical method was developed for the determination of PY in the presence of major interferences including TP, l-dopa, ascorbic acid (AA) and riboflavin (RB) using electrochemically reduced graphene oxide (ERGO) film modified glassy carbon electrode (GCE) in this paper. The ERGO fabrication process involves the nucleophilic substitution of graphene oxide at basic pH on amine terminal of 1,6-hexadiamine which was pre-assembled on GCE followed by electrochemical reduction. The electrocatalytic activity of the ERGO modified electrode was examined towards the oxidation of PY. It greatly enhanced the oxidation current of PY in contrast to bare and GO modified GCEs due to facile electron transfer besides π-π interaction between ERGO film and PY. Since TP and l-dopa drugs antagonize the drug action of PY, ERGO modified GCE was also used for the simultaneous determination of PY and l-dopa and PY and TP. Further, the selective determination of PY in the presence of other water soluble vitamins such as ascorbic acid and riboflavin was also demonstrated. Using amperometry, detection of 100nM PY was achieved and the detection limit was found to be 5.6×10(-8)M (S/N=3). The practical application of the present method was demonstrated by determining the concentration of PY in human blood serum and commercial drugs.