Andrew Glidle

CHARACTERISING THE FORMATION OF A BIOELECTROCHEMICAL INTERFACE AT A SELF-ASSEMBLED MONOLAYER USING X-RAY PHOTOELECTRON SPECTROSCOPY

Abstract In this paper, we characterise the process of immobilisation of cytochrome c at a gold surface modified with the short chain alkyl-thiol self-assembled monolayer, mercaptopropionic acid. Using high resolution X-ray photoelectron spectroscopy, we reveal detail of the step-by-step construction of the biomolecular structure at a self-assembled monolayer, measured during two different immobilisation protocols involving either a carbodiimide activated intermediate or aminolysis of a succinimide ester by the protein. Finally, we relate the subtle variations in the charged species produced during the immobilisation procedures to differences that we measure in the proteins redox potential.

Dynamic quartz crystal impedance measurements of polyvinylferrocene film deposition

Dynamic crystal impedance measurements were used to study the electrochemically-driven deposition of polyvinylferrocene (PVF) films from dichloromethane solutions of PVF onto Au electrodes of a quartz crystal microbalance. Under all the conditions studied, the PVF films were non-rigid, to an extent that depended upon several experimental factors. Films deposited from solutions containing perchlorate salts of tetraalkylammonium salts departed less from rigidity than those deposited under otherwise identical conditions from tetrafluoroborate media. In contrast, the use of different tetraalkylammonium cations, or the use of a potential sweep vs. a potential step from 0 V to the deposition potential (0.7 V), made no significant difference. The departure from rigid characteristics varied with coverage. For the interval of time during deposition that the coverage was 10–160 nmol cm−2 (of monomeric ferrocene units), the films showed no additional departure from rigidity. On the basis that the upper figure corresponds closely to a “monolayer” of terminally attached chains of mean length, we attribute this behaviour to the presence of a dense film, within which chain motion is considerably restrained.

Glutamate oxidase enzyme electrodes: microsensors for neurotransmitter determination using electrochemically polymerized permselective films

Abstract Enzyme entrapment in electrochemically polymerized membranes is an attractive method for the development of stable reproducible enzyme microsensors with fast response times. In this paper the development of a biosensor for the determination of glutamate, involving the entrapment of the enzyme glutamate oxidase in two different electropolymerization matrices, is described. Subsequently, the kinetics of the immobilized enzyme in the different matrices are compared, and the role of the polymers in providing a permselective barrier for the exclusion of potential electroactive interferents are characterized.

Characterization of electron transfer reactions of microperoxidase assembled at short-chain thiol-monolayers on gold

In this study thiol-monolayers were used in order to modify gold and provide suitable chemical functionalities for the immobilization of the small redox-active haem-containing peptide, microperoxidase (MP-11). Initially, we assembled a variety of thiol-containing species on the gold electrodes and measured a series of electron transfer reactions, each characteristic of the surface-modifier. Using suitable immobilization strategies, we subsequently covalently bound MP-11 to appropriate monolayers and found two characteristic electrochemical responses (i.e. using MP-11 bound to mercaptopropionic acid, redox peaks were seen at E0′ = −315 mV and at +179 mV versus Ag|AgCl, with the former being attributed to the haem and the latter with the thiol monolayer). Exposure of the peptide-thiol surface to UV irradiation resulted in cleavage of the AuS bond, leading to a decrease in the magnitude of both responses. Our work is supported by corroborative evidence showing the immobilization of the peptide, obtained using both X-ray photoelectron and reflectance Fourier transform infra-red spectroscopies. We hypothesize that differences in the ionic charges on the protein backbone account for the shift in E0′ for MP-11, as observed in our system, when compared to that found for MP-11 immobilized by different strategies.

Multiple plasmon resonances from gold nanostructures

Understanding and controlling plasmon resonances from metallic nanoscale structures have been the focus of much attention recently, with applications including local surface plasmon resonance sensing, surface enhanced Raman spectroscopy, and negative refractive index materials. In this letter the authors demonstrate the fabrication of uniform arrays of split rings from gold and show that such structures are capable of supporting multiple plasmon resonances. The authors show that up to five plasmon resonances can be identified and use finite difference time domain modeling and absorption spectroscopy to fully characterize and identify each resonance. The implications of higher order surface plasmon resonances for sensing are discussed.

Strategies towards functionalised electronically conducting organic copolymers

Here we describe the synthesis and electrochemical polymerisation of 2,5-di(2-thienyl)-3-(3-cyanopropyl)pyrrole, 2,5-di(2-thienyl)-3-(3-cyanopropyl)furan, and 3′-(3-cyanopropyl)-2,2′:5′,2″-terthiophene. We report a synthetic methodology to these important conducting polymer precursor compounds that is facile, convenient and flexible. The key precursor to this study is the functionalised diketone 1,4-bis(2-thienyl)-2-(3-cyanopropyl)butane-1,4-dione. This molecule undergoes convenient ring closure to the terthiophene and dithienylpyrrole and dithienylfuran derivatives, all of which are, to our knowledge, new compounds. Importantly, this approach provides a flexible route to a range of heterocyclic polymer precursors because the cyanoalkyl functionality is grafted to the diketone before ring closure. Subsequently the nitrile group provides synthetic utility either by reduction to the amine, or hydrolysis to the carboxylic acid. The new compounds described here undergo electrochemical polymerisation leading to fixed ratio copolymers of functionalised pyrrole, thiophene and furan with thiophene itself. We describe the characterisation of these polymers using FT-IR and X-ray photoelectron spectroscopies.

Tuneable visible resonances in crescent shaped nano-split-ring resonators

Electron beam lithography was used to fabricate gold crescent shaped split-ring resonators with 30 nm minimum feature size. By varying the crescent’s arc length over a range of nanometer-scale dimensions the authors demonstrate the tuneability of visible resonances within such structures. Results, which correlate closely with those predicted using finite-difference time-domain modeling, open the way for these devices to be used in near-field biological sensing.

Electron-beam-induced densification of Ge-doped flame hydrolysis silica for waveguide fabrication

Experimentally we compare the densification induced by electron beam irradiation of Ge-doped silica, produced by flame hydrolysis deposition with the densification of thermally produced SiO2. By comparing these results to the predictions made by elasticity theory, we find good agreement for the thermal SiO2 by considering a single region of electron beam damage. For the Ge-doped flame-hydrolysis-deposited silica, we need to include in the model a second, shallow region, which densifies to a greater extent. X-ray photoelectron spectroscopy measurements suggest that the thickness of this additional region is comparable to a layer that was found to be depleted of Ge.

Microrheology with Optical Tweezers: Measuring the relative viscosity of solutions ‘ at a glance ’

We present a straightforward method for measuring the relative viscosity of fluids via a simple graphical analysis of the normalised position autocorrelation function of an optically trapped bead, without the need of embarking on laborious calculations. The advantages of the proposed microrheology method are evident when it is adopted for measurements of materials whose availability is limited, such as those involved in biological studies. The method has been validated by direct comparison with conventional bulk rheology methods, and has been applied both to characterise synthetic linear polyelectrolytes solutions and to study biomedical samples.

Gradient Microfluidics Enables Rapid Bacterial Growth Inhibition Testing

Bacterial growth inhibition tests have become a standard measure of the adverse effects of inhibitors for a wide range of applications, such as toxicity testing in the medical and environmental sciences. However, conventional well-plate formats for these tests are laborious and provide limited information (often being restricted to an end-point assay). In this study, we have developed a microfluidic system that enables fast quantification of the effect of an inhibitor on bacteria growth and survival, within a single experiment. This format offers a unique combination of advantages, including long-term continuous flow culture, generation of concentration gradients, and single cell morphology tracking. Using Escherichia coli and the inhibitor amoxicillin as one model system, we show excellent agreement between an on-chip single cell-based assay and conventional methods to obtain quantitative measures of antibiotic inhibition (for example, minimum inhibition concentration). Furthermore, we show that our methods can provide additional information, over and above that of the standard well-plate assay, including kinetic information on growth inhibition and measurements of bacterial morphological dynamics over a wide range of inhibitor concentrations. Finally, using a second model system, we show that this chip-based systems does not require the bacteria to be labeled and is well suited for the study of naturally occurring species. We illustrate this using Nitrosomonas europaea, an environmentally important bacteria, and show that the chip system can lead to a significant reduction in the period required for growth and inhibition measurements (<4 days, compared to weeks in a culture flask).