Danny K.Y. Wong

Square wave voltammetry versus electrochemical impedance spectroscopy as a rapid detection technique at electrochemical immunosensors

Square wave voltammetry (SWV) was compared to electrochemical impedance spectroscopy (EIS) in developing a label-free electrochemical immunosensor for the hormone estradiol. The immunosensor consists of a Au electrode anchored with a Au nanoparticle|thiolated Protein G-scaffold to facilitate immobilisation of an enhanced quantity of an almost uprightly aligned anti-estradiol capture antibody. Competitive immunoassays between an estradiol-bovine saline albumin complex and free estradiol in a sample were then promoted at the immunosensor. Next, SWV and EIS of [Fe(CN)(6)](3-/4-) were sequentially conducted at the immunosensor. SWV yielded familiar peak-shaped voltammograms with the peak currents readily employable in establishing calibration. A dynamic range up to approximately 1200 pg mL(-1) and a detection limit of 18 pg mL(-1) estradiol were achieved. In EIS, an electron transfer resistance estimated from the Nyquist plots was used in the calibration experiments. A comparable dynamic range up to approximately 1000 pg mL(-1) and a detection limit of 26 pg mL(-1) estradiol were obtained. However, a significantly 10 times longer analysis time and substantial effort were required to complete the EIS determinations relative to SWV. Moreover, a large amount of EIS data involving phase angle was collected but ignored because they would not contribute any useful information to quantitative determination. Overall, SWV was determined to be a more rapid, efficient, effective and low cost detection technique than EIS at label-free electrochemical immunosensors.

Enantiomeric differentiation of a wide range of pharmacologically active substances by capillary electrophoresis using modified β-cyclodextrins

Abstract This paper shows the versatility of modified charged and uncharged β-cyclodextrins (CDs) in the direct chiral resolution of β-agonists, β-antagonists, phenylethyamines and alcohol stimulants, and thalidomide and its metabolites. A total of 42 compounds were optically resolved using hydroxypropyl-β-CD and 20 with sodium sulfobutyl ether-β-CD. The degree of enantiomeric separation for most substances is dependent on the modified CD concentration. The separation efficiency reaches a maximum at a particular CD concentration. The separation efficiency reaches a maximum at a particular CD concentration, after which further increases in CD concentration causes a progressive decrease in chiral differentiation. Chiral separation of amphetamine enantiomers indicated that a three-point hydrogen bond interaction between the chiral guest molecule and host CD is not necessary for chiral separation under the conditions used.

A kinetic model for the dissolution mechanism of copper in acidic sulfate solutions

Abstract Steady-state polarization curves and impedance data have been obtained for the electrochemical dissolution copper in 1.0 M Na 2 SO 4 solutions at pH 1, 2, 3, 4 and 5. The steady-state polarization curves display only active dissolution in the potential range investigated (from the open circuit potential up to + 100mV vs. sce ) and exhibit a Tafel range of 54.8–55.6mV. Two time constants, over a wide spectrum of frequencies (10 −3 –10 −4 Hz), have been observed in all complex impedance plots. The experimental results have been quantitatively fitted by a reaction mechanism model which provides an excellent fit to the steady state data and is in qualitative agreement with the impedance data.

Electrocatalytic detection of phenolic estrogenic compounds at NiTPPS|carbon nanotube composite electrodes

A Ni(II)tetrakis(4-sulfonatophenyl) porphyrin (NiTPPS)|carbon nanotube composite electrode that shows strong catalytic and antifouling capability was developed to detect a series of phenolic endocrine compounds including bisphenol A, nonylphenol and ethynylestradiol. This electrode was fabricated by electropolymerizing NiTPPS complexes on a carbon nanotube-modified glassy carbon electrode. Optimized experimental parameters including a hydrodynamic potential of 0.7 V for flow injection analysis (FIA) and a NiTPPS surface coverage of 2.2 nmol cm(-2) (standard deviation 0.2 nmol cm(-2); n=6) were obtained for detection of the endocrine disrupting compounds. The sensor responded well to all the tested compounds with limits of detection ranging from 15 nmol L(-1) to 260 nmol L(-1) (based on three times S/N ratio) under FIA conditions. Both carbon nanotubes and NiTPPS account for the excellent performance of the composite modified electrode.

Hydrogen peroxide detection at a horseradish peroxidase biosensor with a Au nanoparticle-dotted titanate nanotube|hydrophobic ionic liquid scaffold.

In this work, a novel sensing scaffold, consisting Au nanoparticle (GNP)-dotted TiO(2) nanotubes (TNTs) as the rigid material and the hydrophobic ionic liquid (HIL), 1-decyl-3-methylimidazolium tetrafluoroborate, as the entrapping agent, was applied to facilitate the electron transfer of horseradish peroxidase (HRP) on a glassy carbon electrode. GNPs were immobilised on the TNTs in our work using a one-step reduction of HAuCl(4)·3H(2)O by sodium borohydride in the presence of sodium citrate as a stabilising reagent. The morphology and composition of the as-synthesised composite materials were characterised by transmission electron microscopy, scanning electron microscopy, X-ray diffraction and Fourier-transform infrared spectroscopy. Cyclic voltammetry of HRP at the modified electrode presented a pair of reproducible, quasi-reversible redox peaks with a peak-to-peak separation of 69 mV, indicating electron transfer between HRP and composite electrode. The GNP-TNT|HIL|HRP electrode was then applied to the detection of H(2)O(2) in a pH 7.0 phosphate buffer using chronoamperometry. The biosensor exhibited a linear response in the 15-750 μM range, and a limit of detection of 2.2 μM. The biosensor also exhibited stability with 90% of the detection signal retained over a two-week duration.

Picogram-detection of estradiol at an electrochemical immunosensor with a gold nanoparticle|Protein G-(LC-SPDP)-scaffold.

Low picograms of the hormone 17beta-estradiol were detected at an electrochemical immunosensor. This immunosensor features a gold nanoparticle|Protein G-(LC-SPDP)(1)-scaffold, to which a monoclonal anti-estradiol capture antibody was immobilised to facilitate a competitive immunoassay between sample 17beta-estradiol and a horseradish peroxidase-labelled 17beta-estradiol conjugate. Upon constructing this molecular architecture on a disposable gold electrode in a flow cell, amperometry was conducted to monitor the reduction current of benzoquinone produced from a catalytic reaction of horseradish peroxidase. This current was then quantitatively related to 17beta-estradiol present in a sample. Calibration of immunosensors in blood serum samples spiked with 17beta-estradiol yielded a linear response up to approximately 1200 pg mL(-1), a sensitivity of 0.61microA/pg mL(-1) and a detection limit of 6 pg mL(-1). We attribute these favourable characteristics of the immunosensors to the gold nanoparticle|Protein G-(LC-SPDP) scaffold, where the gold nanoparticles provided a large electrochemically active surface area that permits immobilisation of an enhanced quantity of all components of the molecular architecture, while the Protein G-(LC-SPDP) component aided in not only reducing steric hindrance when Protein G binds to the capture antibody, but also providing an orientation-controlled immobilisation of the capture antibody. Coupled with amperometric detection in a flow system, the immunosensor exhibited excellent reproducibility.

Gold Nanoparticle Encapsulated-Tubular TIO2 Nanocluster As a Scaffold for Development of Thiolated Enzyme Biosensors

In this work, a highly sensitive and stable sensing scaffold consisting of gold nanoparticle-encapsulated TiO2 nanotubes, the hydrophilic ionic liquid, 1-decyl-3-methylimidazolium bromide, and Nafion was developed for the fabrication of electrochemical enzyme biosensors. A significant aspect of our work is the application of 12-phosphotungstic acid as both a highly localized photoactive reducing agent to deposit well-dispersed gold nanoparticles on TiO2 nanotubes and an electron mediator to accelerate the electron transfer between an enzyme and the electrode. After characterizing the nanocomposite component of the scaffold by Fourier transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy, thiolated horseradish peroxidase (as a model enzyme) was immobilized on the scaffold and the biosensor was applied to the detection of H2O2. The direct electron transfer between the enzyme and the electrode was promoted by the excellent biocompatibility and conductivity of the scaffold. In addition, a thiolated enzyme has significantly improved the stability and direct electron transfer of horseradish peroxidase on the biosensor, which could be ascribed to the strong affinity between the sulfhydryl group on the enzyme and gold nanoparticles on the biosensor surface. Cyclic voltammetry, chronoamperometry, and square wave voltammetry were used to study the electrochemistry and analytical performance of the biosensor. A dynamic range from 65 to 1600 μM, a limit of detection of 5 μM, and a sensitivity of (18.1 ± 0.43) × 10(-3) μA μM(-1) H2O2 were obtained. The sensing scaffold based on the nanocomposite was demonstrated to be effective and promising in developing enzyme biosensors.

Conducting polypyrrole films as a potential tool for electrochemical treatment of azo dyes in textile wastewaters

In this paper, we demonstrate conducting polypyrrole films as a potential green technology for electrochemical treatment of azo dyes in wastewaters using Acid Red 1 as a model analyte. These films were synthesised by anodically polymerising pyrrole in the presence of Acid Red 1 as a supporting electrolyte. In this way, the anionic Acid Red 1 is electrostatically attracted to the cationic polypyrrole backbone formed to maintain electroneutrality, and is thus entrapped in the film. These Acid Red 1-entrapped polypyrrole films were characterised by electrochemical, microscopic and spectroscopic techniques. Based on a two-level factorial design, the solution pH, Acid Red 1 concentration and polymerisation duration were identified as significant parameters affecting the entrapment efficiency. The entrapment process will potentially aid in decolourising Acid Red 1-containing wastewaters. Similarly, in a cathodic process, electrons are supplied to neutralise the polypyrrole backbone, liberating Acid Red 1 into a solution. In this work, following an entrapment duration of 480 min in 2000 mg L(-1) Acid Red 1, we estimated 21% of the dye was liberated after a reduction period of 240 min. This allows the recovery of Acid Red 1 for recycling purposes. A distinctive advantage of this electrochemical Acid Red 1 treatment, compared to many other techniques, is that no known toxic by-products are generated in the treatment. Therefore, conducting polypyrrole films can potentially be applied as an environmentally friendly treatment method for textile effluents.

Recent developments in electrochemical immunoassays and immunosensors

Publisher Summary This chapter focuses on the various developments in electrochemical immunoassays and immunosensors after 2002. Electrochemical immunoassay is a solid phase system in which an antibody–antigen reaction takes place but the corresponding electrochemical detection is carried out elsewhere. However, an electrochemical immunosensor is a stand-alone device, with the immunoreaction and electrochemical detection occurring within the same device. There are several strategies for immobilizing a captured antibody on a solid phase, which include covalent attachment, physical adsorption, and electrostatic/physical entrapment in a polymer matrix. The (strept)avidin–biotin interaction technique is used to immobilize various types of biomolecules such as nucleic acids, polysaccharides, and proteins, including the capture antibody in immunoassay/immunosensor systems. Another commonly used affinity-based immobilization technique for capture antibodies in immunoassay systems involves a bacterial antibody-binding protein, the two most common of which are Protein A and Protein G. These proteins bind specifically to antibodies through their nonantigenic (Fc) regions, which allow the antigen binding sites of the immobilized antibody to be oriented away from the solid phase and be available to bind the target analyte. The application of conducting polymers such as polyaniline, polypyrrole, and polythiophene for immobilizing capture antibodies in immunoassay systems is widespread and they may be used in amperometric, potentiometric, and impedimetric immunoassay systems. Self-assembled monolayers (SAMs) are another attractive method for immobilizing the capture antibody in immunoassay systems, which takes advantage of the spontaneous chemisorption of alkanethiols to metals such as gold or silver to assemble highly ordered monolayers. Recently, interdigitated array (IDA) microelectrodes have gained popularity as an alternative transducer in electrochemical immunoassays in which a simple design of an IDA consists of a pair of interdigitated microelectrode fingers.

Recent strategies to minimise fouling in electrochemical detection systems

Abstract Electrode fouling is a phenomenon that can severely affect the analytical characteristics of a technique or a sensor, such as sensitivity, detection limit, reproducibility, and overall reliability. Electrode fouling generally involves the passivation of an electrode surface by a fouling agent that forms an increasingly impermeable layer on the electrode, inhibiting the direct contact of an analyte of interest with the electrode surface for electron transfer. Some potential fouling agents include proteins, phenols, amino acids, neurotransmitters, and other biological molecules. Various antifouling strategies have been reported to reduce or eliminate electrode fouling. Most antifouling strategies exploit a protective layer or barrier on an electrode substrate to prevent the fouling agent from reaching the electrode surface. Although such strategies can be quite effective, they are inappropriate for systems in which the analyte itself is also the fouling agent. In such cases, other strategies must be used, including electrode surface modification and electrochemical activation. In this review, recent strategies to minimise and efforts to overcome electrode fouling across a diverse range of analytes and fouling agents will be presented.