On‐Line Monolithic Enzyme Reactor Fabricated by Sol‐Gel Process for Elimination of Ascorbic Acid While Monitoring Dopamine

Catecholamine, a well‐known neurotransmitter, is released to restore heart function after the occurrence of cardiac ischemia. Endogenous ascorbic acid interferes considerably with the monitoring of neurotransmitters such as catecholamine and glutamate. In this work we have successfully developed a nanoliter volume monolithic enzyme reactor and integrated it with a carbon film electrode for monitoring dopamine in order to scavenge such electroactive interferents as ascorbic acid. A monolithic silica support prepared by the sol‐gel process has a large through‐pore structure and a nanoporous surface. The low back pressure resulting from the large though‐pore structure makes it possible to immobilize the enzyme and introduce fluid into the flow monitoring system by using a microsyringe pump. Ascorbate oxidase (AOx) was immobilized on the nanoporous surface of a monolithic matrix by physical adsorption and used for converting the ascorbic acid into its electrochemically inert form, dehydroascorbic acid, while monitoring dopamine. We showed that more than 99.8% of 100 μM of ascorbic acid could be converted to its oxidized form in the monolithic enzyme reactor. We succeeded in monitoring dopamine at a concentration of less than 100 nM in the presence of 50 μM of ascorbic acid. Therefore, this work demonstrated that a monolithic silica support offers excellent potential in regards to realizing a highly selective enzymatic reactor for biosensors.     

紫外分光光度法快速测定浓缩黑加仑果汁中维生素C含量

本文研究了一种用紫外分光光度法快速测定黑加仑浓缩果汁中维生素C含量的新方法。此法采用室温下Cu~(2 )催化样品中的维生素C做空白样品,在267纳米和pH值为6的条件下测定其吸光度。解决了维生素C分解速度快,影响测定结果的问题。实验结果表明,用紫外分光光度法测定浓缩黑加仑果汁中维生素C的含量,方法简单、快速、准确,且具有较好的准确度和精密度。     

Self‐Assembled Redox System for Bioelectrocatalytic Assay of L‐Ascorbylphosphate and Alkaline Phosphatase Activity

Ferrocene‐terminated self‐assembled monolayer (Fc‐SAM) on gold was used as an electron‐transfer mediator in the electrochemical assay of L ‐ascorbic acid 2‐phosphate (AAP). The assay is based on the enzymatic action of alkaline phosphatase (ALP), which triggers the release of vitamin C (L ‐ascorbic acid, AA) from AAP. The latter is easily oxidized on the Fc‐SAM under the diffusion limiting conditions that favors quantitative measurement of the AA concentration on a rotating disk electrode. We demonstrate the utility of the electrochemically active Fc‐SAM to probe the mechanism and to determine the kinetic parameters of an enzymatic reaction. The electrochemical technique was compared to a conventional spectrophotometric method of ALP activity detection using p‐nitrophenylphosphate (p‐NPP) as a substrate. We demonstrate that our new technique is also suitable for the analytical determination of ALP activity. The detection limits for both AAP and ALP were found to be 13 μM and 2 pM, respectively.     

Selective Electrochemical Determination of Uric Acid in the Presence of Ascorbic Acid Using a Carbon Paste Electrode Modified with β‐Cyclodextrin

Uric acid (UA) was determined in the presence of ascorbic acid (AA) by using a carbon paste electrode modified superficially by a β‐cyclodextrin film (CPE/β‐CD). The surface carbon paste electrode was prepared applying a 30 cycles potential program and using a 1 M HClO₄+0.01 M β‐CD electrolytic solution. The UA and AA solutions were used to evaluate the electrode selectivity and sensitivity by cyclic voltammetric and amperometric methods. In these experiments the detection limit for UA was (4.6±0.01)×10^?6 M and the RSD calculated from the amperometric curves was 10%. From the data obtained it was possible to quantify UA in the urine and saliva samples. Selective detection of UA was improved by formation of an inclusion complex between β‐CD and UA. The results show that the CPE/β‐CD is a good candidate due to its selectivity and sensitivity in the UA determination in complex samples like the biological fluids.     

Fabrication,Characterization, and Application of ‘Sandwich‐Type’ Electrode Based on Single‐Walled Carbon Nanotubes and Room Temperature Ionic Liquid

The much‐enhanced electrochemical responses of potassium ferricyanide and methylene blue (MB) were firstly explored at the glassy carbon electrode modified with single‐walled carbon nanotubes (SWNT/GCE), indicating the distinct electrochemical activity of SWNTs towards electroactive molecules. A hydrophobic room temperature ionic liquid (RTIL), 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆), was used as electrode modification material, which presented wide electrochemical windows, proton permeation and selective extraction ability. In consideration with the advantages of SWNTs and RTIL in detecting target molecules (TMs), a novel strategy of ‘sandwich–type’ electrode was established with TMs confined by RTIL between the SWNT/GCE and the RTIL membrane. The strategy was used for electrochemical detection of ascorbic acid (AA) and dopamine (DA), and detection limits of 400 and 80 fmol could be obtained, respectively. The selective detection of DA in the presence of high amount of AA could also be realized. This protocol presented many attractive advantages towards voltammetric detection of TMs, such as low sample demand, low cost, high sensitivity, and good stability.     

Anti‐Interferent Properties of Oxidized Nickel Based on Layered Double Hydroxide in Glucose Amperometric Biosensors

An amperometric biosensor based on Pt electrodes modified with a thin film of a Ni, Al layered double hydroxide (LDH), submitted to a preliminary oxidative treatment in order to have the nickel centers at the oxidation state +4, and glucose oxidase (GOx) is presented. The oxidized LDH acts both as a system to support the enzyme and as a barrier to anions since it acquires an overall negative charge, as demonstrated by electrochemical impedance spectroscopy. Even if the biosensor response is due to the detection of H₂O₂ at anodic potentials, glucose can be accurately determined in the presence of ascorbic acid or other interferences, commonly present in real matrices in anionic form, since they can not reach the electrode surface. The effectiveness of the developed biosensor has been demonstrated by measuring glucose in samples of fruit juices containing ascorbic acid at high levels.     

Some Properties of Sodium Dodecyl Sulfate Functionalized Multiwalled Carbon Nanotubes Electrode and Its Application on Detection of Dopamine in the Presence of Ascorbic Acid

A sodium dodecyl sulfate (SDS) functionalized multiwalled carbon nanotubes (MWNTs) electrode (SDS/MWNTs) was successfully constructed in this study. The electrochemical property of the SDS/MWNTs electrode has been characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Nyquist plots suggest that the immersion time of SDS affects the resistances of the MWNTs electrodes. The thickness of adsorbed SDS on MWNTs surface is estimated to be 1.23 nm, which is close to the value of SDS monolayer. CV results demonstrate a 5‐fold enhanced response for dopamine (DA) at the SDS/MWNTs electrode compared to the bare MWNTs one. DPV results illustrate that DA can be selectively determined in the presence of high concentration ascorbic acid (AA) with a linear range from 20 μM to 0.20 mM and a sensitivity of 0.024 μA μM^?1 at the SDS/MWNTs electrode.     

Synthesis and Characterization of MWNTs/Au NPs/HS(CH2)6Fc Nanocomposite: Application to Electrochemical Determination of Ascorbic Acid

In this article, a detailed electrochemical study of a novel 6‐ferrocenylhexanethiol (HS(CH₂)₆Fc) self‐assembled multiwalled carbon nanotubes‐Au nanoparticles (MWNTs/Au NPs) composite film was demonstrated. MWNTs/Au NPs were prepared by one‐step in situ synthesis using linear polyethyleneimine (PEI) as bifunctionalizing agent. HS(CH₂)₆Fc, which acted as the redox mediator, was self‐assembled to MWNTs/Au NPs via Au‐S bond. Transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), Fourier transformed infrared absorption spectroscopy (FT‐IR), UV‐visible absorption spectroscopy, and cyclic voltammetry were used to characterize the properties of the MWNTs/Au NPs/HS(CH₂)₆Fc nanocomposite. The preparation of the nanocomposite was very simple and effectively prevented the leakage of the HS(CH₂)₆Fc mediator during measurements. The electrooxidation of AA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique performance of MWNTs/Au NPs. The nanocomposite modified electrode exhibited excellent catalytic efficiency, high sensitivity, good stability, fast response (within 3 s) and low detection limit toward the oxidation of AA at a lower potential.     

Meldola's Blue Immobilized on a SiO2/SnO2/Phosphate Xerogel,a New Sensor for Determination of Ascorbic Acid in Medicine and Commercial Fruit Juice

A modified carbon paste electrode with SiO₂/SnO₂/Phosphate/Meldola's blue, SSPMelB, was used to study the electrocatalytic oxidation of ascorbic acid by cyclic voltammetry and chronoamperometry. The adsorbed dye mediates ascorbic acid oxidation at an anodic potential of 0.04 V vs. saturated calomel electrode (SCE) at pH 7.0, in 0.5 mol L^?1 solution. The linear range of the sensor is between 4.0×10^?7 and 2.0×10^?3 mol L^?1, with a limit of detection of 4.0×10^?7 mol L^?1. This novel electrode shows good analytical performance for determination of ascorbic acid in medicine and commercial fruit juice.     

Monitoring benzene formation from benzoate in model systems by proton transfer reaction-mass spectrometry

The presence of benzene in food and in particular in soft drinks has been reported in several studies and should be considered in fundamental investigations about formation of this carcinogen compound as well as in quality control.Proton transfer reaction-mass spectrometry (PTR-MS) has been used here for rapid, direct quantification of benzene and to monitor its formation in model systems related to the use of benzoate, a common preservative, in presence of ascorbic acid: a widespread situation that yields benzene in, e.g., soft drinks and fruit juices.Firstly, we demonstrate here that PTR-MS allows a rapid determination of benzene that is in quantitative agreement with independent solid phase micro-extraction/gas chromatography (SPME/GC) analysis. Secondly, as a case study, the effect of different sugars (sucrose, fructose and glucose) on benzene formation is investigated indicating that they inhibit its formation and that this effect is enhanced for reducing sugars. The sugar-induced inhibition of benzene formation depends on several parameters (type and concentration of sugar, temperature, time) but can be more than 80% in situations that can be expected in the storage of commercial soft drinks. This is consistent with the reported observations of higher benzene concentrations in sugar-free soft drinks.