S. Abraham John

Simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine using a nanostructured polymer film modified electrode

This paper describes the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and xanthine (XN) using an ultrathin electropolymerized film of 2-amino-1,3,4-thiadiazole (p-ATD) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 5.0). Bare GC electrode failed to resolve the voltammetric signals of AA, DA, UA and XN in a mixture. On the other hand, the p-ATD modified electrode separated the voltammetric signals of AA, DA, UA and XN with potential differences of 110, 152 and 392 mV between AA-DA, DA-UA and UA-XN, respectively and also enhanced their oxidation peak currents. The modified electrode could sense 5 microM DA and 10 microM each UA and XN even in the presence of 200 microM AA. The oxidation currents were increased from 30 to 300 microM for AA, 5 to 50 microM for DA and 10 to 100 microM for each UA and XN, and the lowest detection limit was found to be 2.01, 0.33, 0.19 and 0.59 microM for AA, DA, UA and XN, respectively (S/N=3). The practical application of the present modified electrode was demonstrated by the determination of AA, UA and XN in human urine samples.

Electropolymerized film of functionalized thiadiazole on glassy carbon electrode for the simultaneous determination of ascorbic acid, dopamine and uric acid

The present study reports the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) in 0.20 M phosphate buffer solution (pH 5.0) using electropolymerized ultrathin film of 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) on glassy carbon (GC) electrode. The bare GC electrode does not separate the voltammetric signals of AA, DA and UA. However, electropolymerized AMT (p-AMT) modified GC electrode not only resolved the voltammetric signals of AA, DA and UA but also dramatically enhanced their oxidation peak currents when compared to bare GC electrode. The enhanced oxidation currents for AA, DA and UA at p-AMT modified electrode are due to the electrostatic interactions between them and the polymer film. Using amperometric method, we achieved the lowest detection of 75 nM AA, 40 nM DA and 60 nM UA at p-AMT modified electrode. The amperometric current was linearly increased from 200 nM to 0.80 mM for each AA, DA and UA and the lowest detection limit was found to be 0.92, 0.07 and 0.57 nM, respectively (S/N=3). The practical application of the modified electrode was demonstrated by the determination of DA in dopamine hydrochloride injection.

Determination of nanomolar uric and ascorbic acids using enlarged gold nanoparticles modified electrode.

Individual and simultaneous determination of 50nM uric acid (UA) and ascorbic acid (AA) using enlarged, citrate-stabilized gold nanoparticles (AuNPs) self-assembled to 2,5-dimercapto-1,3,4-thiadiazole (DMT) monolayer modified Au (Au/DMT) electrode by an amperometric method is described for the first time. Self-assembly of AuNPs on the electrode surface was confirmed by atomic force microscopy (AFM), attenuated total reflectance FT-IR and diffuse reflectance spectral measurements. The electron transfer reaction (ETR) of [Fe(CN)(6)](3-/4-) was blocked at Au/DMT electrode, whereas it was restored with a peak separation of 200mV after the attachment of AuNPs on the Au/DMT (Au/DMT/AuNPs) electrode, which was confirmed from the ETR of the [Fe(CN)(6)](3-/4-) redox couple. When the self-assembled AuNPs were enlarged by hydroxylamine seeding, the ETR of [Fe(CN)(6)](3-/4-) was improved significantly with a peak separation of 100mV. Tapping mode AFM showed that the average size of the enlarged-AuNPs (E-AuNPs) was 50-70nm. The E-AuNPs modified electrode catalyzes the oxidation of AA and UA, separates their voltammetric signals by 200mV, and has excellent sensitivity towards AA and UA with a detection limit of 50nM. The practical application of the modified electrode was demonstrated by measuring the concentration of UA in blood serum and urine.

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.

Selective electrochemical sensor for folic acid at physiological pH using ultrathin electropolymerized film of functionalized thiadiazole modified glassy carbon electrode.

This paper demonstrated the selective determination of folic acid (FA) in the presence of important physiological interferents, ascorbic acid (AA) and uric acid (UA) at physiological pH using electropolymerized film of 5-amino-2-mercapto-1,3,4-thiadiazole (p-AMT) modified glassy carbon (GC) electrode. Bare GC electrode fails to determine the concentration of FA in the presence of AA and UA due to the surface fouling caused by the oxidized products of AA and FA. However, the p-AMT film modified electrode not only separates the voltammetric signals of AA, UA and FA with potential differences of 170 and 410 mV between AA-UA and UA-FA, respectively but also shows higher oxidation current for these analytes. The p-AMT film modified electrode displays an excellent selectivity towards the determination of FA even in the presence of 200-fold AA and 100-fold UA. Using amperometric method, we achieved the lowest detection of 75 nM UA and 100 nM each AA and FA. The amperometric current response was increased linearly with increasing FA concentration in the range of 1.0 x 10(-7)-8.0 x 10(-4)M and the detection limit was found to be 2.3 x 10(-10)M (S/N=3). The practical application of the present modified electrode was successfully demonstrated by determining the concentration of FA in human blood serum samples.

Simultaneous determination of epinephrine, uric acid and xanthine in the presence of ascorbic acid using an ultrathin polymer film of 5-amino-1,3,4-thiadiazole-2-thiol modified electrode

This paper describes the simultaneous determination of epinephrine (EP), uric acid (UA) and xanthine (XN) in the presence of ascorbic acid (AA) using electropolymerized ultrathin film of 5-amino-1,3,4-thiadiazole-2-thiol (p-ATT) modified glassy carbon (GC) electrode in 0.2 M phosphate buffer solution (pH 5). Although bare GC electrode resolves the voltammetric signals of AA and XN, it fails to resolve the voltammetric signals of EP and UA in a mixture. However, the p-ATT modified electrode not only separates the voltammetric signals of AA, EP, UA and XN with potential difference of 150, 120 and 400 mV between AA-EP, EP-UA and UA-XN, respectively but also shows higher oxidation current for these molecules. The p-ATT modified electrode exhibits excellent selectivity towards the oxidation of EP, UA and XN in the presence of 40-fold higher concentration of AA. Further, the p-ATT modified electrode was also used for the selective determination of EP in the presence of 40-fold higher concentrations of AA, UA and XN. Using amperometric method, we achieved the lowest detection of 40 nM EP and 60nM each UA and XN. The amperometric current response was increased linearly with increasing EP concentration in the range of 4.0 x 10(-8) to 4.0 x 10(-5) M and the detection limit was found to be 27 x 10(-11) M (S/N = 3). The practical application of the present modified electrode was demonstrated by determining the concentration of EP in epinephrine tartrate injection and XN in human urine samples.

Multielectrochromic properties of methylene blue and phenosafranine dyes incorporated into Nafion ® film

Abstract The electrochemical and spectroelectrochemical properties of phenosafranine (a phenazine dye) (PS + ) and methylene blue (a phenothiazine dye) (MB + ) immobilized into Nafion® (Nf) film were studied in 0.5 M H 2 SO 4 . The PS + dye was readily incorporated into Nf film containing MB + or vice versa, presumably by electrostatic interaction with the free sulfonate (SO 3 − ) groups and hydrophobic interaction with the fluorocarbon moieties of the Nf film. The interaction of these dyes with the hydrophobic fluorocarbon moieties could be understood from their observed higher concentration than the available sulfonate groups in the Nf film and also from their stable redox behavior. On the other hand, thionine (TH + ), another member of the phenothiazine family, was partially leached from the Nf film when the incorporation of PS + was followed because of its weak interaction with the hydrophobic fluorocarbon moieties of the Nf film. The PS incorporated Nf film containing MB + (Nf/PS + /MB + ) showed two well-separated reversible redox waves at 0.22 and −0.05 V (SCE) due to the redox process of MB + and PS + , respectively. The amount of dye molecules in the Nf film could be varied by changing the concentration of dye molecules in the immersion solution. The Nf/PS + /MB + film showed highly stable reversible color changes upon repeated cycles of applied potentials when compared to the Nf film with incorporated TH + and PS + dyes (Nf/TH + / PS + ). The changes in the absorbance of Nf/PS + /MB + film were accompanied with visible reversible changes of film color, ranging from blue-red at 0.7 V to red at 0.0 V and colorless at −0.5 V(SCE) due to the reversible redox process of the MB + and PS + dyes in the Nf film.

Highly sensitive determination of hydroxylamine using fused gold nanoparticles immobilized on sol–gel film modified gold electrode

We are reporting the highly sensitive determination of hydroxylamine (HA) using 2-mercapto-4-methyl-5-thiazoleacetic acid (TAA) capped fused spherical gold nanoparticles (AuNPs) modified Au electrode. The fused TAA-AuNPs were immobilized on (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film, which was pre-assembled on Au electrode. The immobilization of fused TAA-AuNPs on MPTS sol-gel film was confirmed by UV-vis absorption spectroscopy and atomic force microscopy (AFM). The AFM image showed that the AuNPs retained the fused spherical morphology after immobilized on sol-gel film. The fused TAA-AuNPs on MPTS modified Au electrode were used for the determination of HA in phosphate buffer (PB) solution (pH=7.2). When compared to bare Au electrode, the fused AuNPs modified electrode not only shifted the oxidation potential of HA towards less positive potential but also enhanced its oxidation peak current. Further, the oxidation of HA was highly stable at fused AuNPs modified electrode. Using amperometric method, determination of 17.5 nM HA was achieved for the first time. Further, the current response of HA increases linearly while increasing its concentration from 17.5 nM to 22 mM and a detection limit was found to be 0.39 nM (S/N=3). The present modified electrode was also successfully used for the determination of 17.5 nM HA in the presence of 200-fold excess of common interferents such as urea, NO(2)(-), NH(4)(+), oxalate, Mn(2+), Na(+), K(+), Mg(2+), Ca(2+), Ba(2+) and Cu(2+). The practical application of the present modified electrode was demonstrated by measuring the concentration of HA in ground water samples.

Picomolar melamine enhanced the fluorescence of gold nanoparticles: Spectrofluorimetric determination of melamine in milk and infant formulas using functionalized triazole capped goldnanoparticles

We wish to report a simple and sensitive method to determine the melamine in milk and infant formulas using 3-amino-5-mercapto-1,2,4-triazole capped gold nanoparticles (AMTr-AuNPs) as fluorophore. The AMTr-AuNPs were synthesized by a wet chemical method and were characterized by high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction, UV-visible and fluorescence spectroscopic techniques. The AMTr-AuNPs show the absorption maximum at 520 nm and emission maximum at 759 nm (λ(ex)=520 nm). While adding 10 μM melamine, the wine red color of AMTr-AuNPs was changed into purple and the absorption band at 520 nm was decreased. The observed changes were ascribed to the hydrogen bonding interaction between melamine and AMTr-AuNPs, which led to the aggregation of the nanoparticles. This was confirmed by dynamic light scattering and HR-TEM measurements. No appreciable absorption change was observed for AMTr-AuNPs in the presence of less than micromolar concentrations of melamine. But, the emission intensity of AMTr-AuNPs was enhanced even in the presence of picomolar concentration of melamine. Based on the enhancement of emission intensity, the concentration of melamine was determined. The present fluorophore showed an extreme selectivity towards the determination of 100 nM melamine in the presence of 500-fold common interferents. The good linearly was observed from 1×10⁻⁹ to 100×10⁻¹² M melamine and a detection limit was found to be 10 fM/L (S/N=3). The proposed method was successfully applied to determine melamine in cow milk and infant formulas. The obtained results were validated with HPLC.

Ultrasensitive fluorescence-quenched chemosensor for Hg(II) in aqueous solution based on mercaptothiadiazole capped silver nanoparticles

This manuscript describes a highly selective and ultra sensitive determination of Hg(II) in aqueous solution using functionalized mercaptothiadiazole capped silver nanoparticles (AgNPs) by spectrofluorimetry. We have synthesized 2,5-dimercapto-1,3,4-thiadiazole (DMT), 2-mercapto-5-methyl-1,3,4-thiadiazole (MMT) and 2-mercapto-5-amino-1,3,4-thiadiazole (AMT) capped AgNPs by wet chemical method. Among these AgNPs, DMT capped AgNPs (DMT-AgNPs) were more stable and highly fluorescent than the other two AgNPs. DMT-AgNPs show the emission maximum at 677 nm while exciting at 400 nm. After the addition of Hg(II), the emission intensity was decreased at 677 nm. The observed decreased emission intensity was ascribed to the aggregation of AgNPs and it was confirmed by TEM. Based on the decrease in emission intensity, the concentration of Hg(II) was determined. The lowest detection limit (LOD=3S/m) of 1.0 pg L(-1) was achieved for the first time using DMT-AgNPs by spectrofluorimetry. The quantum yield (φ(F)), Stern-Volmer constant (K(SV)), Gibbs free energy changes (ΔG°), association constant (K(f)) were calculated and the quenching mechanism also was discussed. Finally, the proposed method was successfully utilized for the determination of Hg(II) in river water, industrial effluent water and tap water samples. The obtained results were fairly matches with the ICP-AES method.