Srinivasan Kesavan

Fabrication, characterization and application of a grafting based gold nanoparticles electrode for the selective determination of an important neurotransmitter

β-D-Glucose capped gold nanoparticles (Glu-AuNPs) were synthesized and then immobilized on an aminophenyl grafted GC electrode for the selective determination of norepinephrine (NEP) in the presence of uric acid (UA). 1,4-Diaminobenzene (DAB) was used to graft the GC surface and the resulting grafted GC electrode was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and impedance spectroscopy. Glu-AuNPs were immobilized on a grafted GC electrode (Glu-AuNPs electrode) and it was confirmed by SEM, UV-visible spectroscopy and electrochemical techniques. Impedance studies show that the electron transfer reaction of [Ru(NH3)6]3+/2+ was higher at the Glu-AuNPs electrode (7.49 × 10−4 cm s−1) than at bare (5.64 × 10−4 cm s−1) and grafted (1.67 × 10−6 cm s−1) GC electrodes. The Glu-AuNPs electrode shows an excellent electrocatalytic activity towards NEP and UA when compared to grafted and bare GC electrodes. Further, the Glu-AuNPs electrode was successfully used for the selective determination of NEP in the presence of 50-fold excess UA at physiological pH.

Spontaneous grafting: A novel approach to graft diazonium cations on gold nanoparticles in aqueous medium and their self-assembly on electrodes☆

The spontaneous grafting of aminophenyl groups on gold nanoparticles (AuNPs) by reaction with in situ generated 4-aminophenyl diazonium cations (APD) in an aqueous medium was described. The spontaneous grafting was likely to proceed by transfer of electrons from AuNPs to the APD cations to form an aminophenyl radical and subsequent attachment with AuNPs. The aminophenyl (AP) functionalized gold nanoparticles (AP-AuNPs) were characterized by UV-visible spectroscopy, high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction, FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS). The absence of characteristic vibrational bands corresponding to diazonium group in the FT-IR spectrum confirmed the reduction of the aminophenyl diazonium cations at the surface of AuNPs. The spontaneous attachment of AP on AuNPs was confirmed by XPS from the observed binding energy values for -NH2 at 399.4 eV and -N=N- at 400.2 eV. The SERS spectrum reveals the presence Au-C (437 cm(-1)) bond on AP-AuNPs. Further, the AP-AuNPs were self-assembled on GC/ITO electrode (AP-AuNPs modified electrode) with the aid of free amine groups present on the surface of AP-AuNPs via Michaels nucleophilic addition reaction. The AP-AuNPs modified electrode was characterized by cyclic voltammetry, impedance spectroscopy, UV-visible spectroscopy and scanning electron microscopy. Impedance studies show that the electron transfer reaction of [Fe(CN)6](3-/4-) was higher at the AP-AuNPs modified electrode (1.81×10(-4) cm s(-1)) than at bare (3.77×10(-5) cm s(-1)) GC electrode. Finally, the electrocatalytic activity of the AP-AuNPs modified electrode was demonstrated by studying the oxidation of dopamine (DA).

Formation of heteroaromatic diazonium grafted layers on gold nanoparticles and their electrocatalytic activity towards an important purine derivative

We wish to report a spontaneous grafting of heteroaromatic aminotriazole groups at the surface of gold nanoparticles (AuNPs) by the reduction of in situ generated aminotriazole diazonium cations (ATD) in an aqueous medium. The aminotriazole layer at the surface of the AuNPs (AT-AuNPs) was characterized by UV-visible spectroscopy, high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction, FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS). The absence of characteristic peaks for the diazonium group at 403.8 and 405.1 eV in the XPS confirms the reduction of the aminotriazole diazonium cations at the surface of the AuNPs. SERS shows the presence of an Au–C bond at 433 cm−1 on AT-AuNPs, which suggests the spontaneous grafting of an aminotriazole group on the AuNPs. Interestingly, the aminotriazole grafted AuNPs (AT-AuNPs) showed greater stability than the AuNPs grafted with other diazonium cations. Furthermore, the AT-AuNPs were self-assembled on both glassy carbon (GC) and indium tin oxide (ITO) electrodes (AT-AuNPs modified electrode) and were characterized by cyclic voltammetry, impedance spectroscopy and scanning electron microscopy. The AT-AuNPs modified electrode was successfully utilized for the sensitive determination of an important purine derivative, uric acid (UA). Using amperometry, detection of 30 nM UA was obtained. The current responses of UA increased linearly while increasing their concentrations from 3.0 × 10−8 to 1 × 10−4 M and the detection limit was found to be 3 × 10−11 M (S/N = 3). The practical application of the modified electrode was demonstrated by determining the concentration of UA in human blood serum and urine samples.

A novel approach to fabricate stable graphene layers on electrode surfaces using simultaneous electroreduction of diazonium cations and graphene oxide

The short time fabrication of graphene layers on glassy carbon (GCE) and indium tin oxide (ITO) electrodes by simultaneous electroreduction of graphene oxide (GO) and diazonium salts was described for the first time. Initially, aminotriazine groups were introduced on the electrode surface by an in situ process of a diazotization protocol using melamine. Subsequent electrochemical reduction of diazonium groups immersed in GO solution leads to the formation of graphene layers on the electrode surface. Using this methodology, stable graphene layers were fabricated within 10 min on the electrode surface. The simultaneous reduction of triazine diazonium cations and GO and the formation of graphene layers on the electrode surface were confirmed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and cyclic voltammetry (CV). The absence of characteristic peaks in XPS for the diazonium group (403.7 and 405.6 eV) and oxygen functionalities confirmed the reduction of the triazine diazonium cations and GO on the electrode surface. Raman spectra reveal that the intensity ratio of D and G bands (1340 and 1584 cm−1) was increased after the electrochemical reduction of GO. The SEM image shows the formation of thin layers of graphene. Impedance studies indicate that the electron transfer reaction of [Fe(CN)6]3−/4− was higher at the graphene modified electrode (4.10 × 10−4 cm s−1) than at bare (3.77 × 10−5 cm s−1) GC electrode. The electrocatalytic activity of the graphene modified electrode was examined by studying the oxidation of dopamine (DA). The graphene modified electrode not only increased the oxidation current of DA enormously but also shifted its potential towards a less positive potential when compared to bare and GO modified GC electrodes. The present strategy of modification of graphene is simple and requires a very short time.

Simultaneous determination of neurotransmitters and a neuroprotector in human blood serum and urine samples using a diazonium grafted gold nanoparticle film electrode

An electrode modified with a film of aminophenyl grafted gold nanoparticles (AP-AuNPs) was used for the simultaneous determination of the neurotransmitters dopamine (DA) and adenosine (AD) in the presence of the neuroprotector guanosine (GN) at physiological pH. A bare glassy carbon electrode was unable to resolve the voltammetric signals of DA, GN and AD as a result of surface fouling caused by the oxidation products. However, the electrode modified with an AP-AuNP film separated the voltammetric signals of DA, GN and AD with potential differences of 800 mV between DA and GN and 340 mV between GN and AD; their oxidation peak currents were also enhanced. The simultaneous determination of DA, GN and AD was successfully achieved at the electrode modified with an AP-AuNP film using differential pulse voltammetry. The amperometric current response increased linearly with increasing concentrations of DA, GN and AD in the ranges 4.0 × 10−8 to 1 × 10−4 M, 7.0 × 10−8 to 5 × 10−5 M and 2.0 × 10−7 to 1 × 10−4 M, respectively, and the detection limits were 8.0 × 10−11 M for DA, 2.6 × 10−8 M for GN and 5.1 × 10−8 M for AD (S/N = 3). The practical application of the electrode modified with an AP-AuNP film was demonstrated by simultaneously determining the concentrations of DA, GN and AD in human blood serum and urine samples.