Electrowetting on conductors: Anatomy of the phenomenon

We have recently reported that reversible electrowetting can be observed on the basal plane of graphite, without the presence of a dielectric layer, in both liquid/air and liquid/liquid configurations. The influence of carbon structure on the wetting phenomenon is investigated in more detail here. Specifically, it is shown that the adsorption of adventitious impurities on the graphite surface markedly suppresses the electrowetting response. Similarly, the use of pyrolysed carbon films, although exhibiting a roughness below the threshold previously identified as the barrier to wetting on basal plane graphite, does not give a noticeable electrowetting response, which leads us to conclude that specific interactions at the water–graphite interface as well as graphite crystallinity are responsible for the reversible response seen in the latter case. Preliminary experiments on mechanically exfoliated and chemical vapour deposition grown graphene are also reported.</p

Photocatalytic Mineralization of Organic Acids over Visible-Light-Driven Au/BiVO 4

Au/BiVO4 visible-light-driven photocatalysts were synthesized by coprecipitation method in the presence of sodium dodecyl benzene sulfonate (SDBS) as a dispersant. Physical characterization of the obtained materials was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), UV-Vis diffuse reflectance spectroscopy (DRS) and Brunauer, and Emmett and Teller (BET) specific surface area measurement. Photocatalytic performances of the as-prepared Au/BiVO4 have also been evaluated via mineralizations of oxalic acid and malonic acid under visible light irradiation. XRD and SEM results indicated that Au/BiVO4 photocatalysts were of almost spherical particles with scheelite-monoclinic phase. Photocatalytic results showed that all Au/BiVO4 samples exhibited higher oxalic acid mineralization rate than that of pure BiVO4, probably due to a decrease of BiVO4 band gap energy and the presence of surface plasmon absorption upon loading BiVO4 with Au as evidenced from UV-Vis DRS results. The nominal Au loading amount of 0.25 mol% provided the highest pseudo-first-order rate constant of 0.0487 min−1 and 0.0082 min−1 for degradations of oxalic acid (C2) and malonic acid (C3), respectively. By considering structures of the two acids, lower pseudo-first-order rate constantly obtained in the case of malonic acid degradation was likely due to an increased complexity of the degradation mechanism of the longer chain acid

A new portable toluidine blue/aptamer complex-on-polyethyleneimine-coated gold nanoparticles-based sensor for label-free electrochemical detection of alpha-fetoprotein

The quantification of alpha-fetoprotein (AFP) as a potential liver cancer biomarker which is generally found in ultratrace level is of significance in biomedical diagnostics. Therefore, it is challenging to find a strategy to fabricate a highly sensitive electrochemical device towards AFP detection through electrode modification for signal generation and amplification. This work shows the construction of a simple, reliable, highly sensitive, and label-free aptasensor based on polyethyleneimine-coated gold nanoparticles (PEI-AuNPs). A disposable ItalSens screen-printed electrode (SPE) is employed for fabricating the sensor by successive modifying with PEI-AuNPs, aptamer, bovine serum albumin (BSA), and toluidine blue (TB), respectively. The AFP assay is easily performed when the electrode is inserted into a small Sensit/Smart potentiostat connected to a smartphone. The readout signal of the aptasensor derives from the electrochemical response of TB intercalating into the aptamer-modified electrode after binding with the target. The decrease in current response of the proposed sensor is proportional to the AFP concentration due to the restriction of the electron transfer pathway of TB by a number of insulating AFP/aptamer complexes on the electrode surface. PEI-AuNPs improve SPE’s reactivity and provide a large surface area for aptamer immobilization whereas aptamer provides selectivity to the target AFP. Consequently, this electrochemical biosensor is highly sensitive and selective for AFP analysis. The developed assay reveals a linear range of detection from 10 to 50000 pg mL−1 with R2 = 0.9977 and provided a limit of detection (LOD) of 9.5 pg mL−1 in human serum. With its simplicity and robustness, it is anticipated that this electrochemical-based aptasensor will be a benefit for the clinical diagnosis of liver cancer and further developed for other biomarkers analysis

Carbon nanotube composite architectures for use as a novel electrodes

Electrode platforms based on carbon nanotubes (CNTs) or their composites have been extensively investigated over the last two decades since their discovery. CNT‐based electrode platforms have been intensively researched in the electroanalytical and electronic fields. CNTs offer many excellent properties to create electrochemical devices and also improve the properties of composites made from them. Consequently, the theme of this thesis utilises one kind of CNT architecture, which to date has not been successfully investigated for use in electrochemical sensing applications. This architecture, CNT paper or CNT Buckypaper (BP), can be easily prepared and processable by a vacuum‐assisted filtration of well‐dispersed CNT material. Historically, BP has so far not been suitable as an electrochemical sensing platform, as they generally reveal a high background current and low signal to noise ratio (S/N ratio) or low Faradaic response to background charging current ratio. The advantages of BP are that they are readily prepared (cheap), flexible,highly conducting and all carbon in nature. Therefore the objective of this thesis is to successfully design and develop a novel superior CNT electrochemical platform from a BP architecture with the specific aim to lower the charging effects and therefore improve redox responses. This infers a better S/N ratio suitable for use as a sensing platform. These platforms were fabricated by the intercalation of insulating polymers (IPs) such as poly(styrene‐β‐isobutylene‐β‐styrene) (SIBS), polystyrene (PS), polyisobutylene (PIB), polyurethane‐diol (PU), poly(DL‐lactic acid‐coglycolic acid) copolymers (75:25) (PLA‐PLGA), poly(L‐lactic acid) (PLA), and the inherently conducting polymers (CPs) such as poly(3‐octyl pyrrole) (POP), poly(2‐methoxyaniline‐5‐ sulfonic acid) (PMAS), and poly((E)‐4,4’’‐didecoxy‐3\u27‐styryl[2,2\u27:5\u27,2\u27\u27]terthiophene) (PDSTTP). It was found that significant differences in redox behaviour of the bare and V intercalated BPs were found with five of the polymers tested (SIBS, PS, PIB, PDSTTP, and POP). The details of the thesis were summarised as follows. Firstly, Chapter 3 reveals the screening and investigation of polymer intercalation in terms of the improvements in properties of BP structures which are related to the type of polymer; SIBS, PS, PIB, PLA, PLA‐PLGA and PU improve the mechanical properties; PLA, PLA‐PLGA, PDSTTP, POP and PMAS improve thermal stability; POP and PMAS improve electrical conductivity; SIBS, PS, PIB, POP and PDSTTP improve the electrochemical properties and redox behaviour. The improvement of the electrochemical properties at the novel composite platforms by measurements of reduction of the peak‐to‐peak separation (ΔEP), reduction of double layer capacitance (Cdl), enhancement of the S/N ratio and generation of the highest AC harmonic response resulted in the development of better electrochemical sensing BP platforms. Full details of the electrochemical studies of IP/SWCNT platforms (Chapter 4) and of CP/SWCNT platforms (Chapter 5) show that the novel polymer‐BP structure relates to a randomly ordered micro‐/nano‐electrode array possessing faster electron transfer (ET) and surface heterogeneity, when compared to the raw BP structure, which contributes to the overall enhanced voltammetric response and emphasises the benefit for electroanalysis. Direct current (DC) and large‐amplitude Fourier transform alternating current (FT‐AC) cyclic voltammetric (CV) techniques have been employed in the investigation of solutionphase electrochemistry at the five novel BP platforms. The high power FT‐AC technique shows the systematic measurement of readily accessible components; DC component and fundamental, second, third and higher harmonics. Modelling and simulation have been used with all relevant electrochemical parameters, to compare with the experimental data to elucidate the details of the electrode reaction mechanism and electrode surface structure. Both DC and AC techniques proved these intercalated BP electrodes to be superior sensing materials as compared to raw BP materials. Three standard redox probes (ferricyanide [Fe(CN)6]3‐, ferrocenemonocarboxylic acid FMCA0 and ruthenium (III) hexamine [Ru(NH3)6]3+) were employed to evaluate capability in use of intercalated BPs as electrodes having fast ET rate, significantly improved Faradaic response, reduced background charging current, increased S/N ratio and generated higher AC harmonics. Furthermore, the edge‐plane defects of CNTs would be predominant in the novel polymer‐ SWCNT BP composite electrodes. Finally, for practical use in electrochemical sensing applications, the novel platform (SIBSBP) was successfully demonstrated as a chemical sensor in Chapter 6. This platform requires no further treatment or modifications such as polishing or electrochemical activation which are generally employed for standard electrodes such as glassy carbon electrode (GCE), edge plane pyrolytic graphite electrode (EPPGE), and basal plane pyrolytic graphite electrode (BPPGE) in detection of dopamine (DA), an important neurotransmitter, with the presence of coexistent interferences, such as ascorbic acid (AA) and uric acid (UA). The novel platform reveals selectivity to such analytes with high voltammetric resolution. The knowledge achieved above during the course of this study could form the basis of novel freestanding superior electrode materials for use in the electroanalytical, bio‐sensing or environmental monitoring fields

Integration of heuristic and automated parametrization of three unresolved two‐electron surface‐confined polyoxometalate reduction processes by AC voltammetry

The thermodynamic and electrode kinetic parameters that describe each of the three unresolved proton‐coupled two‐electron transfer processes of surface‐confined Keggin‐type phosphomolybdate, [PMo12O40]3− adsorbed onto glassy carbon electrode in 1.0 M H2SO4 have been elucidated by comparison of experimental and simulated AC voltammmetric data. Modelling of this problem requires the introduction of over 30 parameters, although this may be reduced to about half this number when intelligent forms of data analysis are introduced. Heuristic (i. e., an experimenter based trial and error method) and automated data optimization approaches are integrated in this very extensive parameter estimation exercise. However, obtaining a unique solution remains challenging for reasons that are outlined. In the final analysis and using the automated strategy, estimates of six reversible potentials, lower limits of the six electron transfer rate constants, the double layer capacitance, uncompensated resistance and surface coverage are reported, with others (such as the charge transfer co‐efficient) present in the model being unobtainable for reasons that are provided. The fit to experimental data using parameters obtained by automated data optimisation is excellent and slightly superior to that obtained by heuristic analysis. The parameters obtained by either method account for differences in shapes and current magnitudes of each of the overall two electron processes