F. Javier del Campo

Toward Membrane-Free Amperometric Gas Sensors: A Microelectrode Array Approach

Room temperature ionic liquids (RTILs) have been applied to a microelectrode array and been demonstrated to form effective, membrane-free amperometric gas sensors. Determining the RTIL [P(6,6,6,14)][FAP] as the most appropriate choice for extended use, the amperometric quantification of oxygen has been demonstrated. The response of the sensor was quantified by both cyclic voltammetry and chronoamperometry. A range of O(2) contents (2-13% v/v) and RTIL layer thicknesses (from ca. 6 to 125 mum) have been investigated. The combination of microelectrode array and RTIL, as well as the absence of membrane and volatile solvent, results in an elegant, easy to calibrate gas sensor with potential utility in standard and nonstandard conditions.

Emulsion electrosynthesis in the presence of power ultrasound Biphasic Kolbe coupling processes at platinum and boron-doped diamond electrodes

Abstract The electrochemical oxidation of aliphatic carboxylic acids, hexanoic, heptanoic, and lauric acid, under biphasic conditions is studied as a model system for ultrasound enhanced Kolbe electrosynthesis processes. Power ultrasound is used to generate an in situ emulsified medium and to remove reaction products continuously from the electrode surface. A clean and highly efficient process at platinum electrodes with formation of ‘one electron’ products only occurs in marked contrast to processes in monophasic media. For hexanoic acid the Kolbe dimer product R–R is formed in up to 75% yield with 45% current efficiency at 0.18 A cm −2 current density and in the presence of 190 W cm −2 ultrasound. The mechanism is explained in terms of a dynamically modified electrode surface, at which hydrophobic products are immediately ‘trapped’ via partitioning into a non-polar organic phase and transported away into the emulsion system. Kolbe electrosynthesis is undertaken both at platinum electrodes and at free-standing polycrystalline boron-doped diamond electrodes, in order to minimize the surface erosion effect induced by power ultrasound. The type and yield of products obtained from the biphasic Kolbe electrolysis process at diamond electrodes are essentially identical to those observed at platinum and based on this observation, the presence of a biphasic reaction layer at the electrode surface is postulated to govern the process.

Sensitive electrochemical thrombin aptasensor based on gold disk microelectrode arrays

The construction of a sensitive electrochemical aptamer sensor (aptasensor) for thrombin detection is described. Among the advantages of using microelectrode-based devices are the possibility to work with small sample volumes and enjoying faster mass transport rates and lower interfacial capacitance than at macroelectrodes. Therefore, gold disk microelectrode arrays are an attractive transducer option for aptasensors. The morphology of the gold disk microelectrode arrays was inspected by scanning electron microscope. The interaction between a thrombin aptamer and thrombin on gold disk microelectrode arrays was demonstrated by differential pulse voltammetry using methylene blue (MB) as an electrochemical indicator. MB adsorbed to aptamers via their guanine base. When thrombin was introduced, it displaced the MB adsorbed to the aptamers and bound to them. This resulted in a decrease of MB peak current which correlated to the concentration of thrombin over a dynamic range spanning from 10⁻⁵ to 10⁻¹² M. This method was able to linearly and selectively detect thrombin with a detection limit of 0.143 pM.

Membraneless glucose/O2 microfluidic enzymatic biofuel cell using pyrolyzed photoresist film electrodes

Biofuel cells typically yield lower power and are more difficult to fabricate than conventional fuel cells using inorganic catalysts. This work presents a glucose/O2 microfluidic biofuel cell (MBFC) featuring pyrolyzed photoresist film (PPF) electrodes made on silicon wafers using a rapid thermal process, and subsequently encapsulated by rapid prototyping techniques into a double-Y-shaped microchannel made entirely of plastic. A ferrocenium-based polyethyleneimine polymer linked to glucose oxidase (GOx/Fc-C6-LPEI) was used in the anode, while the cathode contained a mixture of laccase, anthracene-modified multi-walled carbon nanotubes, and tetrabutylammonium bromide-modified Nafion (MWCNTs/laccase/TBAB-Nafion). The cell performance was studied under different flow-rates, obtaining a maximum open circuit voltage of 0.54 ± 0.04 V and a maximum current density of 290 ± 28 μA cm(-2) at room temperature under a flow rate of 70 μL min(-1) representing a maximum power density of 64 ± 5 μW cm(-2). Although there is room for improvement, this is the best performance reported to date for a bioelectrode-based microfluidic enzymatic biofuel cell, and its materials and fabrication are amenable to mass production.

Low-temperature sonoelectrochemical processes: Part 1. Mass transport and cavitation effects of 20 kHz ultrasound in liquid ammonia

Abstract Sonoelectrochemical processes in liquid ammonia in a temperature range between −70 and −35°C in the presence of 20 kHz power ultrasound are studied with the aim of improving low temperature electrosynthetic procedures. The one and two electron reductions of nitrobenzene and para-chloronitrobenzene are investigated as model systems. Placing an immersed ultrasonic horn emitter ‘face-on’ to a platinum disc electrode in liquid ammonia is shown to result in extreme mass transport enhancements with a resulting diffusion layer thickness of approximately δ=2 μm. This limit of the diffusion layer thickness is shown to be essentially temperature independent and correspondingly, the highest limiting currents can be observed near the boiling point of liquid ammonia. Cavitation processes are detected even at −70°C and result in a considerable fluctuation in the observed mass transport controlled limiting current. Further, the deposition of ionic products formed in the second reduction step for both nitrobenzene and para-chloronitrobenzene reduction and the associated drop in current, can be shown to be affected by sonication. Ultrasound has been found to be beneficial by (i) causing extremely fast mass transport; (ii) enhancing the mixing and dissolution kinetics at low temperature; and (iii) affecting the formation of solid products at the electrode surface.

Microelectrode study of single cavitational bubbles induced by 500 kHz ultrasound

Insight is gained into about the processes governing cavitational activity and acoustic streaming induced by high frequency (500 kHz) ultrasound by the use of microelectrodes with short time resolution electrochemical equipment to allow monitoring of the activity of single cavitating bubbles. Current transients are interpreted as showing the flux of solution towards the electrode surface due to microstreaming. In order to explain the current amplitude, a simplified model is produced. Important parameters such as bubble size and shape on the surface as well as the boundary layer thickness for microstreaming are taken into account. This model leads to the amplitude of the oscillations of the cavitating bubble. Introducing realistic bubble sizes, this amplitude is found to be in the order of 1 micron. The conclusions arising from this work allow a further interpretation of previous observations at millimeter scale electrodes.

Dual chronoamperometric detection of enzymatic biomarkers using magnetic beads and a low-cost flow cell

In this work we report on the production of a low cost microfluidic device for the multiplexed electrochemical detection of magneto bioassays. As a proof of concept, the device has been used to detect myeloperoxidase (MPO), a cardiovascular biomarker. With this purpose, two bioassays have been optimized in parallel onto magnetic beads (MBs) for the simultaneous detection of MPO endogenous peroxidase activity and quantification of total MPO. Since the two bioassays produced signals of different magnitude for each concentration of MPO tested, two detection strategies have been compared, which entailed registering steady state currents (Iss) under substrate flow, and measuring the peak currents (Ip) produced in a stopped flow approach. As it will be shown, appropriate tuning of the detection and flow conditions can provide extremely sensitive detection, but also allow simultaneous detection of assays or parameters that would produce signals of different orders of magnitude when measured by a single detection strategy. In order to demonstrate the feasibility of the detection strategy reported, a dual MPO mass and activity assay has been finally applied to the study of 10 real plasma samples, allowing patient classification according to the risk of suffering a cardiovascular event.

Fast electrochemical detection of anti-HIV antibodies: Coupling allosteric enzymes and disk microelectrode arrays

Here a novel electrochemical method for the rapid detection of anti-HIV antibodies in serum is presented. The novelty lies in the combination of allosteric enzymes and coulometry to yield a fast, simple and reliable HIV diagnostic method. We have used a previously developed beta-galactosidase enzyme that is efficiently activated by anti-HIV antibodies directed against a major B-cell epitope of the gp41 glycoprotein. When these antibodies bind the enzyme, the 3D conformation changes positively affecting the performance of the active site and, consequently, the enzyme activity is stimulated. Using 4-aminophenyl beta-D-galactopyranoside (PAPG) as substrate yields p-aminophenol (PAP), which is reversibly oxidised at a very mild potential, ca. 0.37 V vs. Ag/AgCl over a range of electrode materials within the working pH range of beta-galactosidase. In the present case, photolithographically produced microelectrode arrays resulted in a detection limit of 4 microM for 4-aminophenol (PAP). The presence of anti-HIV antibodies results in enzyme activity increases above 50% which, combined with the sensitivity and response time afforded by the microelectrode arrays, allowed for the diagnosis of HIV in sera samples within an hour.

Low-temperature sonoelectrochemical processes Part 3. Electrodimerisation of 2-nitrobenzylchloride in liquid ammonia

Abstract Dinitrobibenzyls are key intermediate species in certain drug syntheses. They can be formed by the electrochemical reduction of nitrobenzyl halides (in this work, 2-nitrobenzylchloride) in various solvents. In liquid ammonia at −60°C, the mechanism involves a one-electron reduction and de-chlorination followed by the coupling of the neutral radical intermediate species. Exhaustive voltammetric studies, including fast scan cyclic voltammetry, of the starting material are presented prior to preparative electrolysis experiments. Electrolysis under both potentiostatic and galvanostatic conditions are compared. Under conditions of severe dryness, potentiostatic reduction at platinum gauze set at a voltage of −0.30 V (vs. Ag wire) in the presence of ultrasound yields the dimer 2,2-dinitrobibenzyl (>95%) and no detectable side products. Water and oxygen have been found to decrease both the current efficiency and product yield of the process. Ultrasound is beneficial by: (i) enhancing the dissolution kinetics of the starting material; (ii) mass transport from the bulk towards the electrode, and vice versa, is greatly enhanced, thus considerably reducing the reaction times and optimising the current efficiency and product yields.

Self-assembled monolayers as a base for immunofunctionalisation: unequal performance for protein and bacteria detection.

Biosensor development strongly depends on the optimisation of surface functionalisation strategies. When gold surfaces are considered, immunofunctionalisation by modification of self-assembled monolayers (SAMs) is one of the preferred approaches. In this respect, SAM-based antibody (Ab) incorporation has shown better performance than Ab physisorption for the detection of proteins and small targets. Reports on bacteria detection are less frequent. In this work, we assess the performance of various SAM-based gold immunofunctionalisation strategies, currently applied to protein detection, in the field of bacteria determination. We present the results for Ab chemical conjugation on mercaptopropanoic acid and mercaptoundecanoic acid SAMs, as well as on a dextranized cysteamine SAM. All the modified surfaces studied were shown to be appropriate for the direct detection of an enzyme-labelled protein, but none succeeded in detecting a bacterial target in a sandwich assay format. Conversely, gold functionalised by Ab physisorption allowed E. coli detection when a sandwich enzyme-linked assay was carried out. The implications of bacteria size and wall complexity are discussed. These results indicate that immunofunctionalisation strategies appropriate for protein detection are not necessarily transferable to work with more complex targets such as bacteria. In this respect, Ab physisorption appears to be a suitable alternative to SAM-based gold functionalisation for bacteria detection.