Scaling in Nonstationary Voltammetry Representations

Despite the widespread use of voltammetry for a range of chemical, biological, environmental, and industrial applications, there is still a lack of understanding regarding the functionality between the applied voltage and the resulting patterns in the current response. This is due to the highly nonlinear relation between the applied voltage and the nonstationary current response, which casts a direct association nonintuitive. In this Article, we focus on large-amplitude/high-frequency ac voltammetry, a technique that has shown to offer increased voltammetric detail compared to alternative methods, to study heterogeneous electrochemical reaction-diffusion cases using a nonstationary time-series analysis, the Hilbert transform, and symmetry considerations. We show that application of this signal processing technique minimizes the significant capacitance contribution associated with rapid voltammetric measurements. From a series of numerical simulations conducted for different voltage excitation parameters as well as kinetic, thermodynamic, and mass transport parameters, a number of scaling laws arise that are related to the underlying parameters/dynamics of the process. Under certain conditions, these observations allow the determination of all underlying parameters very rapidly, experiment duration typically ≤1 s, using standard electrode geometries and without any a priori assumptions regarding their value. The theoretical results derived from this analysis are compared to experiments with an outer-sphere electron-transfer species, Ru(NH_3)_6^(2+/3+), on different electrode materials, and the determined parameters are in excellent agreement with published values

Rapid Measurement of Lactate in the Exhaled Breath Condensate: Biosensor Optimization and In-Human Proof of Concept.

Lactate concentration is of increasing interest as a diagnostic for sepsis, septic shock, and trauma. Compared with the traditional blood sample media, the exhaled breath condensate (EBC) has the advantages of non-invasiveness and higher user acceptance. An amperometric biosensor was developed and its application in EBC lactate detection was investigated in this paper. The sensor was modified with PEDOT:PSS-PB, and two different lactate oxidases (LODs). A rotating disk electrode and Koutecky-Levich analysis were applied for the kinetics analysis and gel optimization. The optimized gel formulation was then tested on disposable screen-printed sensors. The disposable sensors exhibited good performance and presented a high stability for both LOD modifications. Finally, human EBC analysis was conducted from a healthy subject at rest and after 30 min of intense aerobic cycling exercise. The sensor coulometric measurements showed good agreement with fluorometric and triple quadrupole liquid chromatography mass spectrometry reference methods. The EBC lactate concentration increased from 22.5 μM (at rest) to 28.0 μM (after 30 min of cycling) and dropped back to 5.3 μM after 60 min of rest

The development of electrochemical sensors for the study of intervertebral disc nutrition

Imperial Users onl

Miniaturised glucose-oxygen biofuel cells

Miniaturized glucose-oxygen biofuel cells are useful to power implantable medical devices such as biosensors. They are small, more biocompatible and run continuously on glucose and oxygen, providing cleaner energy at neutral environment. A typical glucose-oxygen biofuel cell consists of an anode with glucose oxidase (GOx) and a cathode with various oxygen reducing catalysts. This thesis describes experimental investigations of the major issues of such systems, viz.: complex electrode fabrication, enzyme instability and inefficient oxygen reduction. Electrodes were built using the simple and scaleable bulk modification method, where all the material was simply mixed and bound together into composites with epoxy resin. For the anodes, the composite made of 10% GOx with 7:7 TTF-TCNQ was found optimal. The GOx electrodes were modified with various enzyme stabilisers (PEI, DTT, PEG, GLC, FAD and mixture of PEI:DTT and PEI:FAD) and 2% of PEI-DTT (1:1 w/w) was most effective in the presence of O2. Its maximum output current density was 1.8 x 10-2 ± 9.9 x 10-3 A.m-2. It also showed the resistant against O2 electron deprivation and enzyme inhibition. Its KM.was 5 mM. For the cathodes, various oxygen reducing catalysts (metalised carbon, anthroquinone modified carbon, laccase and bilirubin oxidase) were incorporated into graphite composite and the electrodes were pretreated in different media in order to enhance their catalytic activity. None showed four-electron O2 reduction. NaOH-pretreated cobalt (II) salophen composite electrodes showed two-electron O2 reduction and were most catalytic. Its standard catalytic rate constant was 1.2 x 10-5 ± 1.2 x 10-6 m.s-1. Of the catalysts examined, metal complex composites gave the best results for oxygen-reducing cathodes and their pretreatment led to the synergetic effect because it increased the concentration of catalytic surface oxygen groups and enhanced oxygen reduction.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Characterisation and modelling of conducting composite electrodes

Conducting composite electrodes are versatile devices for biomedical applications showing good biocompatibility, low cost, ease of construction. They can be bulk modified with stabilised enzymes to make biosensors and electrodes for biofuel cells. Low sensitivity to flow conditions arises from the microelectrode array like behaviour but this is accompanied by high capacitance and bulk resistance which can blur the voltammetric detail. We have undertaken a systematic investigation of the effects of composition and formulation on the voltammetric behaviour and non-faradaic properties. Monodisperse glassy carbon spherical powders have been used to simplify the modelling and the relative importance of the patterns of surface conductivity and bulk 3-D connectivity has been investigated. Three dimensional numerical models based on percolation theory have been constructed which allow calculation of the distributed resistances in the composite and which enable qualitative prediction of the voltammetric properties. Voltammetric results showed a bias of E1/2 (half-wave potential) and an unstable il (diffusion limiting current) for conducting composite electrodes. A.C. Impedance showed major changes of Rct (charge transfer resistance) and Cd (double layer capacitance) as the electrodes’ ratio and thickness are varied. Comparison with carbon fibre arrays separates the effects of a distributed interface from three dimensional disparities in the conductance. Key findings are: the distributed resistance in electrodes results in bias for E1/2 and the overlap of diffusion layers on electrode surface leads to an uneven il. Numerical results have shown that the bias of E1/2 and il are in good agreement with experimental works. Different geometric configuration allows the investigation of diffusion layer difference caused by electrode array location and different internal resistance.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

In-column electrochemical detection for liquid chromatography

This research focuses on the development of whole column detection (WCD) for liquid chromatography (LC). The WCD uses electrochemical techniques for detecting the analytes passing through the separation column. Electrode array for in-column electrochemical detection (ICED) is fabricated along the separation column to enable whole column separation monitoring and allow better understanding on the affinity of particular analyte to the stationary and mobile phases. Numerical models were built to understand the feasibility and differences of electrochemical detection within an unpacked and packed column. From the simulated results, the surface area of the electrode was not hindered by the presence of the particles in flow condition. An electrochemical microfluidic device has been successfully fabricated on PET (polyethylene terephthalate) substrate using the reverse imprinting technique. The photolithographically produced gold metal electrode lines were imprinted into the PET substrate using a blank mould and produced an inlaid electrode array with overall step residue within 40 nm. The semi-cured thermoset polyester channel was irreversibly thermal bonded on the PET substrate. The devices were able to tolerate pressure in excess of 90 bars. The PET column was packed with 5 μm C18 silica beads to perform reverse phase chromatography separation. The array was electrochemically characterised using standard redox probes in both stagnant conditions and under flow. Both numerical modelling and experimental data show improved sensitivity under flow and a limiting current which scaled linearly with cubic root of volume flow rate. Isocratic and gradient mode chromatographic separations of neurotransmitters and metabolites: serotonin, dopamine, adrenaline, 5- HIAA and DOPAC were conducted in the fabricated device. Separation progress was electrochemically detected at multiple locations along the column. Whole column assessment on separation efficiency and column packing efficiency monitoring were conducted using the ICED.EThOS - Electronic Theses Online ServiceGBUnited Kingdo