Kinetic studies of glucose oxidase in polyelectrolyte multilayer films by means of scanning electrochemical microscopy (SECM)

Label-Free Assessment of Neuron-Specific Enolase via Polydopamine over a Carbon-Nanotube-Based Flexible Immunosensor

A label-free electrochemical immunosensor was developed for the rapid and sensitive detection of neuron-specific enolase (NSE). The electropolymerization of dopamine in conjunction with highly conductive carbon nanotubes offers a simple and quick platform for the direct anchoring of antibodies without the assistance of any coupling agent as well as a blocking agent. The developed immunosensor exhibited a wider detection range from 120 pM (9 ng mL-1) to 3 nM (200 ng mL-1) for NSE with a high sensitivity of 3.9 μA pM-1 cm-2 in 0.1 M phosphate-buffered saline (PBS) at physiological pH (7.4). Moreover, the short recognition time (15 min) for the antigen enabled the detection to be fast and less invasive. Additionally, the evaluation of a rate constant at various concentrations of NSE via feedback mode of scanning electrochemical microscopy (SECM) explained the profound effect of antigen concentration on the rate of flow of electrons. Therefore, the proposed immunosensor can be a promising tool for the early detection of small cell lung cancer in a very short period of time with consistent accuracy.

Quantitative Feedback Referencing for Improved Kinetic Fitting of Scanning Electrochemical Microscopy Measurements

Scanning electrochemical microscopy (SECM) has matured as a technique for studying local electrochemical processes. The feedback mode is most commonly used for extracting quantitative kinetic information. However, approaching individual regions of interest, as is commonly done, does not take full advantage of the spatial resolution that SECM has to offer. Moreover, fitting of experimental approach curves remains highly subjective due to the manner of estimating the tip-to-substrate distance. We address these issues using negative or positive feedback currents as a reference to calculate the tip-to-substrate distance directly for quantitative kinetic fitting of approach curves and line profiles. The method was first evaluated by fitting simulated data and then tested experimentally by resolving negative feedback and intermediate kinetics behavior in a spatially controlled fashion using (i) a flat, binary substrate composed of Au and SiO₂ segments and (ii) a dual-mediator system for live-cell measurements. The methodology developed herein, named quantitative feedback referencing (QFR), improves fitting accuracy, removes fitting subjectivity, and avoids substrate-microelectrode contact.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

Scanning electrochemical microscopy in the development of enzymatic sensors and immunosensors

Scanning electrochemical microscopy (SECM) is very useful, non-invasive tool for the analysis of surfaces pre-modified with biomolecules or by whole cells. This review focuses on the application of SECM technique for the analysis of surfaces pre-modified with enzymes (horseradish peroxidase, alkaline phosphatase and glucose oxidase) or labelled with antibody-enzyme conjugates. The working principles and operating modes of SECM are outlined. The applicability of feedback, generation-collection and redox competition modes of SECM on surfaces modified by enzymes or labelled with antibody-enzyme conjugates is discussed. SECM is important in the development of miniaturized bioanalytical systems with enzymes, since it can provide information about the local enzyme activity. Technical challenges and advantages of SECM, experimental parameters, used enzymes and redox mediators, immunoassay formats and analytical parameters of enzymatic SECM sensors and immunosensors are reviewed.

Redox competition and generation-collection modes based scanning electrochemical microscopy for the evaluation of immobilised glucose oxidase-catalysed reactions

Redox competition (RC-SECM) and generation-collection (GC-SECM) modes of scanning electrochemical microscopy were applied for the evaluation of a glucose oxidase (GOx)-modified non-conducting poly(methyl methacrylate) surface.

Micropatterned multienzyme devices with adjustable amounts of immobilized enzymes

Multienzyme microstructures of glucose oxidase (GOx) and horseradish peroxidase (HRP) were prepared by layer-by-layer deposition inside microfluidic networks on glass substrates in order to allow both site-specific deposition and control of the amount of immobilized enzymes. The obtained microstructures were characterized by scanning force microscopy for the topography of the deposited layers. The local enzyme activity was characterized by the substrate-generation/tip-collection mode and the enzyme-mediated feedback mode of the scanning electrochemical microscope (SECM). These measurements provided quantitative information about the immobilized enzyme activity as a basis for adjusting enzyme loading for multienzyme structures that realize logical operations based on enzymatic conversions. Information about local HRP activity can also be obtained by optical readout using an Amplex UltraRed fluorgenic substrate and reading with a confocal laser scanning microscope with a much higher repetition rate for image acquisition. Using these principles, a layout with HRP and GOx microstructures was realized that showed the functionality of an OR Boolean logic switch.

Analytical expressions for quantitative scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM), is a recent analytical technique in electrochemistry, which was developed in the 1990s and uses microelectrodes to probe various surfaces. Even with the well-known disc microelectrodes, the system geometry is not as simple as in regular electrochemistry. As a consequence even the simplest experiments, the so-called positive and negative feedback approach curves, cannot be described with exact analytical expressions. This review gathers all the analytical expressions available in the SECM literature in steady-state feedback experiments. Some of them are claimed as general expressions, other are presented as approximate. Their validity is discussed in the light of the current understanding and computer facilities.

Efficiency of a bienzyme sequential reaction system immobilized on polyelectrolyte multilayer-coated colloids

We assembled multilayer films of glucose oxidase (GOx) and horseradish peroxidase (HRP) coimmobilized together with polyelectrolyte layers on the surface of silica microparticles. The influence of different polyelectrolyte combinations on the immobilization and functionality of the enzymes was examined for several multilayer configurations. Precomplexation of the enzymes with a polyvinylpyridine-based polyamine allowed the stable adsorption of enzyme layers without affecting their catalytic activity. The efficiency of the sequential reaction between GOx and HRP on the surface of the colloids was quantitatively analyzed and rationalized in terms of the kinetic parameters of both enzymes and the reaction-diffusion kinetics of the system. In the optimized configuration, with GOx and HRP coimmobilized in the same layer, the overall rate of hydrogen peroxide conversion was around 2.5 times higher than for GOx and HRP in separate layers or for equivalent amounts of both enzymes free in solution.

Optimized preparation and scanning electrochemical microscopy analysis in feedback mode of glucose oxidase layers grafted onto conducting carbon surfaces

An optimized immobilization procedure based on the electroreduction of aryldiazonium salt followed by covalent attachment of a cross-linked hydrogel was used to graft glucose oxidase on a carbon surface. Scanning electrochemical microscopy (SECM) and cyclic voltammetry were used to follow the construction steps of the modified electrode. By adjusting the compactness of the layer through the electrografting reaction, the penetration of the mediator through the layer can be controlled to allow the monitoring of the enzymatic activity by both cyclic voltammetry and SECM in feedback mode. The enzymatic activity of the film is finally characterized by SECM.