Polycrystalline Gold Electrodes: A Comparative Study of Pretreatment Procedures Used for Cleaning and Thiol Self-Assembly Monolayer Formation

Probing fundamental film parameters of immobilized enzymes--towards enhanced biosensor performance. Part II-Electroanalytical estimation of immobilized enzyme performance

The method of immobilization of a protein has a great influence on the overall conformation, and hence, functioning of the protein. Thus, a greater understanding of the events undergone by the protein during immobilization is key to manipulating the immobilization method to produce a strategy that influences the advantages of immobilization while minimizing their disadvantages in biosensor design. In this, the second paper of a two-part series, we have assessed the kinetic parameters of thin-film laccase monolayers, covalently attached to SAMs differing in spacer-arm length and lateral density of spacer arms. This was achieved using chronoamperometry and an electroactive product (p-benzoquinone), which was modeled in a non-linear regressional fashion to extract the relevant parameters. Finally, comparisons between the kinetic parameters presented in this paper and the rheological parameters of laccase monolayers immobilized in the same manner (Part I of this two paper series) were performed. Improvements in the maximal enzyme-catalysed current, i(max), the apparent Michaelis-Menten constant, K(m) and the apparent biosensor sensitivity were noted for most of the surfaces with increasing linker length. Decreasing the lateral density of the spacer-arms brought about a general improvement in these parameters, which is attributed to the decrease in multiple points of immobilization undergone by functional proteins. Finally, comparisons between rheological data and kinetics data showed that the degree of viscosity exhibited by protein films has a negative influence on attached protein layers, while enhanced protein hydration levels (assessed piezoelectrically from data obtained in Paper 1) has a positive effect on those surfaces comprising rigidly bound protein layers.

Application of DNA Hybridization Biosensor as a Screening Method for the Detection of Genetically Modified Food Components

An electrochemical biosensor for the detection of genetically modified food components is presented. The biosensor was based on 21-mer single-stranded oligonucleotide (ssDNA probe) specific to either 35S promoter or nos terminator, which are frequently present in transgenic DNA cassettes. ssDNA probe was covalently attached by 5′-phosphate end to amino group of cysteamine self-assembled monolayer (SAM) on gold electrode surface with the use of activating reagents – water soluble 1-ethyl-3(3′-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxy-sulfosuccinimide (NHS). The hybridization reaction on the electrode surface was detected via methylene blue (MB) presenting higher affinity to ssDNA probe than to DNA duplex. The electrode modification procedure was optimized using 19-mer oligoG and oligoC nucleotides. The biosensor enabled distinction between DNA samples isolated from soybean RoundupReady^® (RR soybean) and non-genetically modified soybean. The frequent introduction of investigated DNA sequences in other genetically modified organisms (GMOs) give a broad perspectives for analytical application of the biosensor.

Study of the underlying electrochemistry of polycrystalline gold electrodes in aqueous solution and electrocatalysis by large amplitude fourier transformed alternating current voltammetry

Polycrystalline gold electrodes of the kind that are routinely used in analysis and catalysis in aqueous media are often regarded as exhibiting relatively simple double-layer charging/discharging and monolayer oxide formation/removal in the positive potential region. Application of the large amplitude Fourier transformed alternating current (FT-ac) voltammetric technique that allows the faradaic current contribution of fast electron-transfer processes to be emphasized in the higher harmonic components has revealed the presence of well-defined faradaic (premonolayer oxidation) processes at positive potentials in the double-layer region in acidic and basic media which are enhanced by electrochemical activation. These underlying quasi-reversible interfacial electron-transfer processes may mediate the course of electrocatalytic oxidation reactions of hydrazine, ethylene glycol, and glucose on gold electrodes in aqueous media. The observed responses support key assumptions associated with the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis.

Preparation of electrode-immobilized, redox-modified oligonucleotides for electrochemical DNA and aptamer-based sensing

Recent years have seen the development of a number of reagentless, electrochemical sensors based on the target-induced folding or unfolding of electrode-bound oligonucleotides, with examples reported to date, including sensors for the detection of specific nucleic acids, proteins, small molecules and inorganic ions. These devices, which are often termed electrochemical DNA (E-DNA) and E-AB (electrochemical, aptamer-based) sensors, are comprised of an oligonucleotide probe modified with a redox reporter (in this protocol methylene blue) at one terminus and attached to a gold electrode via a thiol-gold bond at the other. Binding of an analyte to the oligonucleotide probe changes its structure and dynamics, which, in turn, influences the efficiency of electron transfer to the interrogating electrode. This class of sensors perform well even when challenged directly with blood serum, soil and other complex, multicomponent sample matrices. This protocol describes the fabrication of E-DNA and E-AB sensors. The protocol can be completed in 12 h.

Formation of tethered bilayer lipid membranes on gold surfaces: QCM-Z and AFM study

Recently, tethered bilayer lipid membranes (tBLMs) have shown high potential as biomimetic systems due to their high stability and electrical properties, and have been used in applications ranging from membrane protein incorporation to biosensors. However, the kinetics of their formation remains largely uninvestigated. By using quartz crystal microbalance with impedance analysis (QCM-Z), we were able to monitor both the kinetics and viscoelastic properties of tether adsorption and vesicle fusion. Formation of the tether monolayer was shown to follow pseudo-first-order Langmuir kinetics with association and dissociation rate constants of 21.7 M-1 s(-1) and 7.43 x 10-6 s(-1), respectively. Moreover, the QCM-Z results indicate a rigid layer at the height of deposition, which then undergoes swelling as indicated by AFM. The deposition of vesicles to the tether layer also followed pseudo-first-order Langmuir kinetics with observed rate constants of 5.58 x 10(-2) and 2.41 x 10-2 s(-1) in water and buffer, respectively. Differential analysis of the QCM-Z data indicated deposition to be the fast kinetic step, with the rate-limiting steps being water release and fusion. Atomic force microscopy pictures taken complement the QCM-Z data, showing the major stages of tether adsorption and vesicle fusion, while providing a road map to successful tBLM formation.

Electrochemical detection of cysteine in a flow system based on reductive desorption of thiols from gold

A simple strategy for cysteine determination using flow-injection analysis with electrochemical detection is described. The approach is based on the chemisorptions reactions of the sulfur moiety of cysteine upon polycrystalline gold electrodes and its subsequent reductive desorption. The electrochemical measurements were accomplished by the application of differential pulse voltammetry (DPV) for the operational optimization and pulsed electrochemical detection (PED) in combination with flow-injection analysis for the electrochemical detection as time function. The electroactive species could be adsorbed in a potential level (0.1 V versus SCE), at other (-0.6 V versus SCE) occurs their reductive desorption from the electrode, while the analytical current is recorded simultaneously, and a third potential step is applied to the complete regeneration of the gold electrode surface (-1.3 V versus SCE). The linear response range was observed between 1.0 x 10(-6) and 6.0 x 10(-6) mol L(-1) with a good reproducibility (R.S.D.<3.2%) and sensitivity (1.1 microA/microM). The repeatability (a series of 27 continuous FIA peaks of 5. 0 micromol L(-1) of cysteine) was 3.8 % and the limit of detection was 5.0 x 10(-7) mol L(-1). The sample throughput was 23 samples per hour with a very high stability in its voltammetric response. The developed methodology was successfully used for the determination of cysteine in commercial supplementary food sample.