Omowunmi A. Sadik

Status of biomolecular recognition using electrochemical techniques

The use of nanoscale materials (e.g., nanoparticles, nanowires, and nanorods) for electrochemical biosensing has seen explosive growth in recent years following the discovery of carbon nanotubes by Sumio Ijima in 1991. Although the resulting label-free sensors could potentially simplify the molecular recognition process, there are several important hurdles to be overcome. These include issues of validating the biosensor on statistically large population of real samples rather than the commonly reported relatively short synthetic oligonucleotides, pristine laboratory standards or bioreagents; multiplexing the sensors to accommodate high-throughput, multianalyte detection as well as application in complex clinical and environmental samples. This article reviews the status of biomolecular recognition using electrochemical detection by analyzing the trends, limitations, challenges and commercial devices in the field of electrochemical biosensors. It provides a survey of recent advances in electrochemical biosensors including integrated microelectrode arrays with microfluidic technologies, commercial multiplex electrochemical biosensors, aptamer-based sensors, and metal-enhanced electrochemical detection (MED), with limits of detection in the attomole range. Novel applications are also reviewed for cancer monitoring, detection of food pathogens, as well as recent advances in electrochemical glucose biosensors.

Trends and challenges in biochemical sensors for clinical and environmental monitoring

Biochemical sensors have emerged as a dynamic technique for qualitative and quantitative analysis of different analytes in clinical diagnosis, environmental monitoring, and food and process control. The need for a low-cost, reliable, ultra-sensitive, and rapid sensor continues to grow as the complexity of application areas increases. New biosensing techniques are emerging due to the need for shorter sample preparation protocols. Such novel biosensor designs make field and bed-site clinical testing simpler with substantial decrease in costs per sample throughputs. In this paper, we will review the recent trends and challenges in clinical and environmental biosensors. The review will focus on immunological, nucleic acid, and cell-based clinical and biological sensors. Special emphasis will be placed on the approaches used for immobilization or biological reagents and low-cost electrochemical biosensors. The promising biosensors for rapid diagnosis of cancer or HIV are also discussed.

Pulse damperometric detection of proteins using antibody containing conducting polymers

Abstract The use of pulsed potential waveforms to control antibody-antigen interactions on conducting polymer surfaces has been demonstrated. This enables detection of proteins at low levels and the use of antibody containing electrodes for multiple analyses.

First comparative reaction mechanisms of β-estradiol and selected environmental hormones in a redox environment

Abstract This work describes the first comparative electrochemical behavior of β-estradiol and selected important environmental hormones, specifically alkylphenols (APs). Most of the concerns about environmental hormones in humans center on their interference with estrogens, an ovarian steroid. APs are believed to have high affinity for biological receptors by generating conformational changes that can be detected by other macromolecules. However, no mechanistic or molecular evidence has been established for these studies. Here we show, an electrochemical mechanism of estradiol and selected APs including bisphenol-A (BPA), nonylphenol (NP), and diethylstilbestrol (DES) using cyclic voltammetry (CV), rotating disk electrode (RDE) and controlled potential coulometry (CPC). We observed a striking similarity in the electrochemistry of these synthetic xenobiotics and estradiol, a natural estrogen. The complete reaction mechanisms for the oxidation of NP, DES and estradiol were found to follow EC–EC scheme. Unlike most aromatic hydroxy phenols, our results showed that using bulk electrolysis with CPC, both estradiol and DES generated a ketone and quinone intermediate products, respectively. These products were isolated and confirmed using mass spectrometry. This study may provide insights into the origin of the in vivo molecular recognition since there is little steric or electronic hindrance expected at reactor sites between the APs and estradiol.

Monitoring antibody-antigen reactions at conducting polymer-based immunosensors using impedance spectroscopy

Abstract The mechanisms of antibody–antigen (Ab–Ag) interactions at conducting polypyrrole electrodes have been investigated using impedance spectroscopy techniques. The effects of the variation in ion exchange, solution composition, and the condition of the synthesis have been used to study the capacitive behavior of antibody-containing polypyrrole electrodes in the presence of the antigen. The theory of charge generation and transportation in the heterogeneous polymeric domains is proposed as the predominant basis for the analytical signals observed at these electrodes. The significant difference observed in the impedance response at different potentials confirmed that the Ab–Ag interaction was largely influenced by the applied potential.

The electrochemistry of antibody-modified conducting polymer electrodes

The modification of conducting polymer electrodes with antibodies (i.e. proteins) by means of electrochemical polymerization is a simple step that can be used to develop an immunological sensor. However, the electrochemical processes involved leading to the generation of analytical signals by the sensor have not been fully investigated. In this work, we report on the characterization of the interaction between an antigen, human serum albumin (HSA) and an antibody-immobilized polypyrrole electrode (such as anti-HSA) using cyclic voltammetry (CV) and impedance spectroscopy. This interaction was monitored using electrochemical impedance spectroscopy at three different potentials. The potentials correspond to the three redox states of the electroconducting polymer (i.e. reduced, doped and overoxidized states). Evidence from the CV experiments confirmed that there was a shift in the potential, which was found to be proportional to the concentration. Both the CV and the impedance experiments indicated that this potential-dependent shift could be attributed to antibody–antigen (Ab–Ag) binding.

A new electrocatalytic mechanism for the oxidation of phenols at platinum electrodes

Electrochemical oxidation of phenolic compounds generally produces unstable phenoxy radicals that readily polymerize to passivate the surface of solid electrodes. In this study, the electrocatalytic oxidation of phenol in the presence and absence of methanol was investigated by cyclic voltammetry on a platinum electrode. The cyclic voltammogram of phenol in a mixture of phosphate buffer/methanol solution showed well-defined peaks at ∼600 mV vs. Ag/AgCl reference electrode, which surprising, gradually increased with repetitive scanning, stabilizing after 50 cycles. This unexpected behavior is in contrast to previous studies involving phenolic compounds, which always show a decrease in intensity during continuous potential scanning. Scanning electrochemical spectroscopy (SEM) was further used to investigate the changes in the surface morphology of the Pt electrode after electrodeposition. A new electrocatalytic mechanism for phenol oxidation on the surface of a Pt electrode is suggested in the presence of methanol. The proposed mechanism is based on the formation of a film of Pt oxide/hydroxides onto which the phenol and the products of its electrochemical oxidation are further deposited. The mechanism was also studied using more complex phenolic compounds including resveratrol, quercetin and bisphenol A. The results emphasized the effect of aryl substituents on the electrochemistry of this particular class of compounds.

Electrochemical DNA biosensor for the detection of specific gene related to Microcystis species

Abstract A new electrochemical biosensor is described for voltammetric detection of gene sequence related to bloom-forming genera of cyanobacteria, Microcystis spp. The sensor involves the immobilization of a 17-mer DNA probe, which is complementary to a specific gene sequence related to Microcystis spp. on a gold electrode through specific adsorption. The DNA probe was used to determine the amount of target gene in solution using methylene blue (MB) and ruthenium bipyridine as the electrochemical indicators. The anodic peak currents (ipa) of Ru(bpy)32+ were linearly related to the concentration of the target oligonucleotide sequence in the range 1.8×10 −10 –9.0×10 −8 M . The detection limit of this approach was 9.0×10−11 M. In addition, these indicators were capable of selectively discriminating against mismatches; a very desirable condition for the detection of disease-related point-mutation in guanine bases of the cyanobacteria.

Catalytic adsorptive stripping voltammetric measurements of trace vanadium at bismuth film electrodes.

Bismuth-coated glassy carbon electrodes have been successfully applied for catalytic adsorptive stripping voltammetric measurements of low levels of vanadium(V) in the presence of chloranilic acid (CAA) and bromate ion. The new protocol is based on the accumulation of the vanadium-chloranilic acid complex from an acetate buffer (pH 5.5) solution at a preplated bismuth film electrode held at -0.35V (versus Ag/AgCl), followed by a square-wave voltammetric scan. Factors influencing the adsorptive stripping performance, including the CAA and bromate concentrations, solution pH, and accumulation potential or time have been optimized. The response compares favorably with that observed at mercury film electrodes. A linear response is observed over the 5-25mug/L concentration range (2min accumulation), along with a detection limit of 0.20mug/L vanadium (10min accumulation). High stability is indicated from the reproducible response of a 50mug/L vanadium solution (n=25; R.S.D.=3.1%). Applicability to a groundwater sample is illustrated.

Pulsed amperometric detection of thaumatin using antibody-containing poly(pyrrole) electrodes

An electrochemical sensor produced by direct incorporation of anti-thaumatin into a polypyrrole film was developed. The use of flow injection analysis and pulsed amperometric detection permitted the development of a sensitive, reversible and rapid electrochemical immunoassay for thaumatin.