Sensitive detection of hydroquinone using exfoliated graphene-Au/glassy carbon modified electrode.

Graphene nanosheets (EGr) were electrochemically prepared through one-step exfoliation of a graphite rod in a mixture of H2SO4:HNO3 (3:1) at low bias (4 V). Subsequently, gold nanoparticles were attached to the graphene surface (EGr-Au) by the reduction of the metal precursor (HAuCl4) in aqueous solution containing dispersed graphene sheets. According to the XRD investigation, the synthesized material consists of a mixture of few-layer (86%) and multi-layer (14%) graphene. The interlayer distance was found to be in the range of 0.466-0.342 nm, which is larger than the interlayer distance in graphite (0.335 nm). The average size of gold nanoparticles in the EGr-Au sample was 24 nm, in excellent agreement with the TEM results. The synthesized material was then employed to modify a glassy carbon (GC) substrate, in order to obtain a modified electrode (GC/EGr-Au). Next, the electrochemical behavior of hydroquinone (HQ) in the presence and absence of interfering species, catechol (CAT) and bisphenol A (BPA) was studied and the corresponding calibration curves were plotted. Thus, in solutions without interfering species, the GC/EGr-Au electrode has a wide linear range (3 × 10-7-10-4 M), high sensitivity (0.089 A M-1) and low detection limit (LOD = 10-7 M; S/N = 3). The presence of either catechol or bisphenol A leads to the increase of LOD to 2 × 10-7 M, and in addition changes the electrode sensitivity, up to 0.146 A M-1.

New approach application of data transformation in mean centering of ratio spectra method.

Most of mean centering (MCR) methods are designed to be used with data sets whose values have a normal or nearly normal distribution. The errors associated with the values are also assumed to be independent and random. If the data are skewed, the results obtained may be doubtful. Most of the time, it was assumed a normal distribution and if a confidence interval includes a negative value, it was cut off at zero. However, it is possible to transform the data so that at least an approximately normal distribution is attained. Taking the logarithm of each data point is one transformation frequently used. As a result, the geometric mean is deliberated a better measure of central tendency than the arithmetic mean. The developed MCR method using the geometric mean has been successfully applied to the analysis of a ternary mixture of aspirin (ASP), atorvastatin (ATOR) and clopidogrel (CLOP) as a model. The results obtained were statistically compared with reported HPLC method.

Simultaneous determination of L- and D-T4 using a sequential injection analysis/sensors system.

A sequential injection analysis system was selected for the simultaneous determination of L- and D-T4 in raw material and pharmaceutical products, because of the highest precision and accuracy, the lower consumption of sample and buffer, and the flexibility of selecting different buffers accordingly with the type of sensors utilized as detectors. An amperometric biosensor based on D-amino acid oxidase was used as detector for D-T4 and an immunosensor based on anti-L-T4 was used as detector for L-T4. The SIA/biosensors system can be used reliably on-line in synthesis process control, for the simultaneous assay of L- and D- T4 with a frequency of more than 30 samples per hour.

Utilization of an enantioselective surface plasmon resonance electrode for the selection of the best C(70) fullerene as chiral selector for the enantioanalysis of L-cysteine.

Two novel enantioselective surface plasmon resonance (SPR) sensors based on self-assembled monolayer of (5-6)-fullerene-C(70) and diethyl (1,2-methanofullerene C(70))-71-71-dicarboxylate as chiral selectors are proposed. Binding assay, apparent affinity constant, and apparent dissociation binding constant have been used to analyze and study the enantioselectivity of C(70) fullerene-octadecanethiol film for L-cysteine which was chosen as model analyte. The apparent affinity constant for the complex formed by L-cysteine with (5-6)-fullerene-C(70) and diethyl (1,2-methanofullerene C(70))-71-71-dicarboxylate films were 1 x 10(10) L/mol, and 6.7 x 10(8) L/mol, respectively. Accordingly, the chiral selector of choice will be (5-6)-fullerene-C(70). No binding was recorded between any of the tested C(70) fullerenes and D-cysteine, this proving the enantioselectivity of the proposed SPR sensors.