Optimisation and characterisation of biosensors based on polyaniline

With lower limits of detection and increased stability constantly being demanded of biosensor devices, characterisation of the constituent layers that make up the sensor has become unavoidable, since this is inextricably linked with its performance. This work describe the optimisation and characterisation of two aspects of sensor performance: a conductive polymer layer (polyaniline) and the immobilised protein layer. The influence of the thickness of polyaniline films deposited electrochemically onto screen-printed electrode surfaces is described in this work in terms of its influence on a variety of amperometric sensor performance characteristics: time to reach steady state, charging current, catalytic current, background current and signal/background ratios. The influence of polymer film thickness on the conductivity and morphology of finished films is also presented. An electrostatic method of protein immobilisation is used in this work and scanning electron microscopy in conjunction with gold-labelled antibodies and back-scattered electron detection has enabled the direct visualisation of individual groups of proteins on the sensor surface. Such information can provide an insight into the performance of sensors under influence of increasing protein concentrations

Understanding and optimising the packing density of perylene bisimide layers on CVD-grown graphene

The non-covalent functionalisation of graphene is an attractive strategy to alter the surface chemistry of graphene without damaging its superior electrical and mechanical properties. Using the facile method of aqueous-phase functionalisation on large-scale CVD-grown graphene, we investigated the formation of different packing densities in self-assembled monolayers (SAMs) of perylene bisimide derivatives and related this to the amount of substrate contamination. We were able to directly observe wet-chemically deposited SAMs in scanning tunnelling microscopy (STM) on transferred CVD graphene and revealed that the densely packed perylene ad-layers adsorb with the conjugated {\pi}-system of the core perpendicular to the graphene substrate. This elucidation of the non-covalent functionalisation of graphene has major implications on controlling its surface chemistry and opens new pathways for adaptable functionalisation in ambient conditions and on the large scale.Comment: 27 pages (including SI), 10 figure

Electronic structure of tetra(4-aminophenyl)porphyrin studied by photoemission, UV–Vis spectroscopy and density functional theory

The valence and conduction bands of a thin film of tetra(4-aminophenyl)porphyrin (TAPP) are investi-gated by direct and inverse photoemission as well as by comparison to density functional theory (DFT)calculations. By projecting the electronic eigenfunctions onto the molecular framework it was possibleto interpret the origin of each spectroscopic feature. Although the majority of the photoemission spec-trum is attributed to the unsubstituted tetraphenylporphyrin (TPP) parent molecule, several featuresare clearly due to the amino substitution. Substitution also has important consequences for the energypositions of the frontier orbitals and therefore on the low-energy electronic excitations. The measuredelectronic transport energy gap (Eg= 1.85 eV) between the highest occupied molecular orbital (HOMO)and lowest unoccupied (LUMO) in TAPP is found to be significantly reduced with respect to TPP. More-over, an increased energy separation between the two highest occupied states (HOMO and HOMO−1) isfound both experimentally and by DFT calculations. Such evidence is attributed to an increased HOMOorbital destabilization due to an enhanced electron-donor character of the phenyl substituents uponamino functionalization. Finally, the above findings together with further time-dependent DFT calcula-tions are used to interpret the effect of the amino groups on the UV–Vis absorption spectrum, namely anoverall red-shift of the spectrum and remarkable intensity changes within the Q band.© 2017 Elsevier B.V

Absence of long-range ordered reconstruction on the GaAs(311)A surface

We have investigated the decapped GaAs(311)A surface using both scanning tunneling microscopy and synchrotron-radiation photoemission. While our data are in broad agreement with the structural model of GaAs(311)A proposed in a recent study [Wassermeier et al., Phys. Rev. B 51, 14 721 (1995)], we find considerable differences in the surface order. In particular, the As dimer rows are unbroken over much shorter length scales and are highly kinked. We observe a correspondingly lower degree of anisotropy in the surface roughness than that previously reported. An (8×1) reconstruction was not observed. An analysis of As 3d and Ga 3d core-level photoemission spectra suggests that surface As atoms are in only one bonding configuration while surface Ga adopts two different bonding states. We discuss possible origins for the core-level spectra surface components

Sulphur overlayers on the Au(110) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)–(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)–(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S2 molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal

Correlation of Raman and X-Ray Diffraction Measurements of Annealed Pulsed Laser Deposited ZnO Thin Films.

Raman spectroscopy, X-ray diffractometry and atomic force microscopy have been used to characterise ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. The results show substantial enhancement and broadening of certain Raman features which correlate excellently with the change in width of the X-ray diffraction peaks. The 570 cm[-1] Raman feature showed pronounced asymmetry and enhanced intensity in the unannealed sample. An increase in grain size observed after subsequent annealing produced a substantial reduction in both the asymmetry and intensity of this peak. Our experimental data suggest that electric fields, due to charge trapping at grain boundaries, in conjunction with localised and surface phonon modes are the cause of the intensity enhancement and asymmetry of this feature