Swee Ngin Tan

Silica sol-gel immobilized amperometric biosensor for hydrogen peroxide

Abstract An amperometric enzyme electrode for hydrogen peroxide was developed via easy and effective enzyme immobilization using the sol-gel technique. The enzyme electrode comprises horseradish peroxidase (HRP) immobilized in the silica sol-gel matrix on a carbon paste electrode (CPE). Hydrogen peroxide was detected in the presence of hexacyanoferrate(II) as a mediator to transfer electrons between the electrode surface and hydrogen peroxide. The effects of operational parameters such as the operating potential of the working electrode, mediator concentration and pH, and thermal stability, were explored for optimum analytical performance by using an amperometric method. Linear calibration for hydrogen peroxide was obtained in the range 2× 10−5 to 2.6× 10−3 M under the optimized conditions. The apparent Michaelis-Menten constant of the enzyme electrode was 4.8mM. The performance of this enzyme electrode was also investigated using flow-injection analysis. The enzyme electrode retained about 60% of its activity after 35 days of storage in a phosphate buffer at 4 °C.

Pressurized hot water extraction (PHWE)

Pressurized hot water extraction (PHWE) has become a popular green extraction method for different classes of compounds present in numerous kinds of matrices such as environmental, food and botanical samples. PHWE is also used in sample preparation to extract organic contaminants from foodstuff for food safety analysis and soils/sediments for environmental monitoring purposes. The main parameters which influence its extraction efficiency are namely the temperature, extraction time, flow rates and addition of modifiers/additives. Among these different parameters studied, temperature is described as the most important one. It is reported that the extraction of certain compounds is rather dependent on pressurized water with different applied temperature. Thus, the stability and reduced solubilities of certain compounds at elevated temperatures are highlighted in this review. With some modifications, a scaled-up PHWE could extract a higher amount of desirable compounds from solid and powdered samples such as plant and food materials. The PHWE extracts from plants are rich in chemical compounds or metabolites which can be a potential lead for drug discovery or development of disease-resistant food crops.

The Chemical Composition and Biological Properties of Coconut (Cocos nucifera L.) Water

Coconut water (coconut liquid endosperm), with its many applications, is one of the world’s most versatile natural product. This refreshing beverage is consumed worldwide as it is nutritious and beneficial for health. There is increasing scientific evidence that supports the role of coconut water in health and medicinal applications. Coconut water is traditionally used as a growth supplement in plant tissue culture/micropropagation. The wide applications of coconut water can be justified by its unique chemical composition of sugars, vitamins, minerals, amino acids and phytohormones. This review attempts to summarise and evaluate the chemical composition and biological properties of coconut water.

Simultaneous analysis of different classes of phytohormones in coconut (Cocos nucifera L.) water using high-performance liquid chromatography and liquid chromatography–tandem mass spectrometry after solid-phase extraction

Coconut (Cocos nucifera L.) water, which contains many uncharacterized phytohormones is extensively used as a growth promoting supplement in plant tissue culture. In this paper, a high-performance liquid chromatography (HPLC) method was developed for the simultaneous determination of various classes phytohormones, including indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), abscisic acid (ABA), gibberellic acid (GA), zeatin (Z), N(6)-benzyladenine (BA), alpha-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) in young coconut water (CW). The analysis was carried out using a reverse-phase HPLC gradient elution, with an aqueous mobile phase (containing 0.1% formic acid, pH adjusted to 3.2 with triethylamine (TEA)) modified by methanol, and solute detection made at 265 nm wavelength. The method was validated for specificity, quantification, accuracy and precision. After preconcentration of putative endogenous phytohormones in CW using C(18) solid-phase extraction (SPE) cartridges, the HPLC method was able to screen for putative endogenous phytohormones present in CW. Finally, the identities of the putative phytohormones present in CW were further confirmed using independent liquid chromatography-tandem mass spectrometry (LC-MS/MS) equipped with an electrospray ionization (ESI) interface.

Amperometric hydrogen peroxide biosensor based on immobilization of peroxidase in chitosan matrix crosslinked with glutaraldehyde

A new enzymatic amperometric biosensor for hydrogen peroxide (H2O2) was developed via an easy and effective enzyme immobilization method using chitosan film crosslinked with glutaraldehyde. Horseradish peroxidase was immobilized on the surface of a carbon paste electrode. Hexacyanoferrate(II) was present in the solution as a mediator. The biosensor exhibited a relatively fast response of less than 10 s and produced currents linearly related to the H2O2 concentration in the range of 4.7 × 10−5 to 2 × 10−3 M. The biosensor had very good stability as it retained ca. 85% activity after 30 d of storage in a phosphate buffer at 4 °C. The effect of the chitosan film thickness and various operational parameters were optimized. The applicability of this biosensor was demonstrated with the analysis of real samples and the results obtained by this biosensor corroborated well with the classical iodometric titration method.

Validation of green-solvent extraction combined with chromatographic chemical fingerprint to evaluate quality of Stevia rebaudiana Bertoni.

An approach that combined green-solvent methods of extraction with chromatographic chemical fingerprint and pattern recognition tools such as principal component analysis (PCA) was used to evaluate the quality of medicinal plants. Pressurized hot water extraction (PHWE) and microwave-assisted extraction (MAE) were used and their extraction efficiencies to extract two bioactive compounds, namely stevioside (SV) and rebaudioside A (RA), from Stevia rebaudiana Bertoni (SB) under different cultivation conditions were compared. The proposed methods showed that SV and RA could be extracted from SB using pure water under optimized conditions. The extraction efficiency of the methods was observed to be higher or comparable to heating under reflux with water. The method precision (RSD, n = 6) was found to vary from 1.91 to 2.86% for the two different methods on different days. Compared to PHWE, MAE has higher extraction efficiency with shorter extraction time. MAE was also found to extract more chemical constituents and provide distinctive chemical fingerprints for quality control purposes. Thus, a combination of MAE with chromatographic chemical fingerprints and PCA provided a simple and rapid approach for the comparison and classification of medicinal plants from different growth conditions. Hence, the current work highlighted the importance of extraction method in chemical fingerprinting for the classification of medicinal plants from different cultivation conditions with the aid of pattern recognition tools used.

Cyclic Voltammetry Studies of Sputtered Nitrogen Doped Diamond-Like Carbon Film Electrodes

The conductive nitrogen-doped diamond-like carbon film (N-DLC) of about 0.14 m in thickness was deposited on highly conductive silicon wafer (111) with DC magnetron sputtering system. The electrochemical characteristics of the film have been studied with cyclic voltammetry (CV). The N-DLC film electrodes compare very favorably with conventional carbon based electrodes such as glassy carbon. The N-DLC film electrode exhibits a low double-layer capacitance, a large electrochemical potential window, and a relatively high electrochemical activity toward ferricyanide reduction. In addition, the electrode exhibits catalytic activity for Cl2/Cl as well as durability to high anodic potential, and a high signal for the trace analysis of Pb 2 . These characteristics demonstrate great promise of the N-DLC film as a novel electrode material for electrochemical analysis.

Evaluation of the extraction efficiency of thermally labile bioactive compounds in Gastrodia elata Blume by pressurized hot water extraction and microwave-assisted extraction.

Our earlier work showed that the stability of the bioactive compounds gastrodin (GA) and vanillyl alcohol (VA) in Gastrodia elata Blume behaved differently with varying compositions of water-ethanol using pressurized liquid extraction (PLE) at room temperature. To have a better understanding of the extraction process of these thermally labile compounds under elevated temperature conditions, pressurized hot water extraction (PHWE) and microwave-assisted extraction (MAE) methods were proposed. PHWE and MAE showed that GA and VA could be extracted using pure water under optimized conditions of temperature and extraction time. The extraction efficiency of GA and VA by the proposed methods was found to be higher or comparable to heating under reflux using water. The marker compounds present in the plant extracts were determined by RP-HPLC. The optimized conditions were found to be different for the two proposed methods on extraction of GA and VA. The method precision (RSD, n=6) was found to vary from 0.92% to 3.36% for the two proposed methods on different days. Hence, PHWE and MAE methods were shown to be feasible alternatives for the extraction of thermally labile marker compounds present in medicinal plants.

Renewable silica sol-gel derived carbon composite based glucose biosensor

Abstract An amperometric mediated glucose biosensor has been developed based on a sol–gel derived carbon composite material. Glucose oxidase and the mediator vinylferrocene have been immobilised within the porous, rigid and organically modified silicate network in the composite material. The organic group in the silicate network controls the hydrophobicity of the electrode surface and thus limits the wettability of the electrode surface. Various important fabrication factors controlling the biosensor performance have been investigated systematically. The glucose biosensor can be renewed easily in a reproducible manner by a simple polishing step and it has a long operational lifetime. Applicability of the biosensor has been demonstrated in real samples and the results obtained by this biosensor corroborate well with a classical UV spectrophotometric technique.

Stripping Voltammetric Analysis of Heavy Metals at Nitrogen Doped Diamond-Like Carbon Film Electrodes

Conductive nitrogen doped diamond-like carbon (N-DLC) film electrodes were used to investigate the possibility of detecting heavy metals such as lead, copper and cadmium by differential pulse anodic stripping voltammetry (DPASV) in the absence of mercury film. The preconcentration conditions (deposition potential, deposition time) and solution pH were optimized for the determination of lead in aqueous solution. A linear dependence of lead stripping current peak within the concentration (5 10 7 to 2 10 6 MP b 2 ) and deposition time (30 to 300 s at 1.00 V vs. SCE) was obtained. A multi-elemental analysis (Pb 2 ,C d 2 and Cu 2 ) illustrated that the N-DLC film electrode provided a significant stripping response for determination of multi-metals simultaneously. The present novel electrode showed great promise for the analysis of heavy metals.