Self-supporting graphene films and their applications

The self-supporting monolayer material which is graphene has excited enormous interest over the ten years since its discovery due to its remarkable electrical, mechanical thermal and chemical properties. In this paper we describe our work to develop chemical vapour deposition methods to grow monolayer graphene on copper foil substrates and the subsequent transfer process. Raman microscopy, scanning electron microscopy and atomic force microscopy (AFM) are used to examine the quality of the transferred material. To demonstrate the process we describe transfer onto patterned SiO2/Si substrates which forms freely suspended graphene with focus on circular wells forming graphene drums. These show interesting mechanical properties which are being explored as nanomechanical resonators.UK NMS Programme, the EU EMRP (European Metrology Research Programme) projects MetNEMS and GraphOh

On-chip integrated graphene aptasensor with portable readout for fast and label-free COVID-19 detection in virus transport medium

Graphene field-effect transistor (GFET) biosensors exhibit high sensitivity due to a large surface-to-volume ratio and the high sensitivity of the Fermi level to the presence of charged biomolecules near the surface. For most reported GFET biosensors, bulky external reference electrodes are used which prevent their full-scale chip integration and contribute to higher costs per test. In this study, GFET arrays with on-chip integrated liquid electrodes were employed for COVID-19 detection and functionalized with either antibody or aptamer to selectively bind the spike proteins of SARS-CoV-2. In the case of the aptamer-functionalized GFET (aptasensor, Apt-GFET), the limit-of-detection (LOD) achieved was about 103 particles per mL for virus-like particles (VLPs) in clinical transport medium, outperforming the Ab-GFET biosensor counterpart. In addition, the aptasensor achieved a LOD of 160 aM for COVID-19 neutralizing antibodies in serum. The sensors were found to be highly selective, fast (sample-to-result within minutes), and stable (low device-to-device signal variation; relative standard deviations below 0.5%). A home-built portable readout electronic unit was employed for simultaneous real-time measurements of 12 GFETs per chip. Our successful demonstration of a portable GFET biosensing platform has high potential for infectious disease detection and other health-care applications

Microwave Whispering-Gallery Resonators for Nanolitre Liquid Sensing

Within this thesis, a novel approach for the investigation of aqueous solutions interacting with electromagnetic radiation at micro-to-millimetre wave frequencies was developed and employed with relevance to biological applications. Whispering-gallery modes in cylindrically shaped dielectric disks machined from low-loss single crystalline materials such as sapphire or quartz allow for very high quality factors, because the electromagnetic fields are strongly confined inside the dielectric disk such that radiation losses can be neglected. Based on this resonator type, three different liquid sensing approaches were developed, analysed and optimized with emphasis on the determination of the complex dielectric permittivity of liquids. As a first method, whispering-gallery sapphire resonators at 35-37 gigahertz were developed and optimized for the investigation of small droplets of biochemical liquids with volumes ranging from 100 picolitres up to about a few nanolitres. Employing a micrometer controlled microinjection system, droplets could be spotted at arbitrary positions on the surface of the sapphire disk. At the surface of the disk, an evanescent field emerges, such that the droplet induces a slight change of the resonance frequency and a reduction of the inverse quality factor depending on complex dielectric permittivity of the liquid under test. The optimum spotting position on the resonator surface corresponding to the maximum electric field was determined experimentally. Two types of resonator excitation schemes were investigated and optimized for the Ka frequency band (26.5 – 40 Gigahertz). At first, resonators were embedded in a semi-open metal shielding cavity and excited in a standing wave regime by coaxial loop antennas fixed at the walls of the cavity. Secondly, dielectric disks without shielding cavity were investigated, comprising excitation of whispering-gallery running waves by dielectric image guides. In contrast to standing wave loop excitation, the electric field strength of the running whispering gallery waves is nearly independent of the azimuthal angle, which is of great advantage for the positioning of very small samples. In addition, this scheme can be extended up to the terahertz frequency range. As a first demonstration, whispering gallery resonances were excited in a quartz disk resonator at 170 Gigahertz with quality factors of order of 30000. The results on droplet induced changes of resonant frequency and inverse quality factor were found to be in quantitative agreement with perturbation theory in conjunction with numerical field simulation of the unperturbed resonator. In particular it was found, that the effect of real and imaginary part of permittivity could be separated. Both measured quantities were found to be nearly linear dependent on the droplet volume, as expected by perturbation theory. Due to the very high quality factor, droplets down to 100 picoliters volume could be analysed by this technique. Droplets of distilled water were compared with aqueous solutions of ethanol, sodium chloride, glucose and albumin. For all solutes, the measured results were clearly different from water for concentrations above 5%. Within perturbation theory it was found, that the ratio of droplet induced change of inverse quality factor and droplet induced frequency shift,[...

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Microwave whispering-gallery resonators for nanolitre liquid sensing Dissertation zur Erlangung des akademischen Doktorgrades in Physik vorgelegt der Fakultät für Physik der Technischen Universität Dortmund vo