Browsing by Subject "Electrical double layer"
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Item Open Access Investigation of electronic properties of ionic liquid electrochemical devices by X-ray photoelectron spectroscopy(Bilkent University, 2016-12) Camcı, MerveAttention towards electrochemical energy storage devices assembled with innovative solvent-free electrolytes ‘ionic liquids’ (ILs) has been progressively rising over the last two decades. In order to design a particular electrochemical device it becomes crucial to understand the structure of interfacial region and the electrical response of ILs. Accordingly, this thesis focuses on Xray Photoelectron Spectroscopic (XPS) investigations of electrochemical devices containing ILs, that is compatible with ultra high vacuum condition needed for XPS. Towards better understanding the fundamental aspects of certain electrochemical issues, electrochemical devices consisting of two metalelectrodes, which contains N,N – Diethyl -N- methyl -N- (2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide, (DEME-TFSI) IL-electrolyte between them, have been investigated by XPS under external electrical stimuli control, as a novel analytical tool for elucidating; (i) charging/ discharging phenomena, (ii) electrical double layer (EDL) formation and (iii) electrochemical reaction products. In the first part, a co-planar electrochemical device, with two gold electrodes on porous polyethylene membrane (PEM) plus DEME-TFSI impregnated between the electrodes, has been studied using external DC bias, for recording the position dependent electrical potential variations. In addition, AC bias is used to harvest temporal behavior. For the AC bias a square wave excitation is used, for which two frequencies are adopted corresponding to slow (10 mHz) and fast (1 kHz) time scales, for probing the response of the system at infinite- and zero-time onset, respectively. In all cases XP spectra have been recorded at different lateral positions. As a result of these DC and AC applications a new understanding has surfaced. Accordingly, although at the metal-electrolyte interface the EDL formation is limited to lateral dimensions at the nanometer scale, its visualization through the analysis of the XPS-probed voltage transients can be extended to very large distances from the interface, in the millimeters scale. These responses have also been modeled using a simple equivalent circuit with two oppositely polarized electrodes and an ionic conducting medium in between. In the second part, re-arrangement of the DEME-TFSI’s ionic constituents at the Au electrode/IL-electrolyte interface has been monitored by the dynamic-XPS approach under application of electrical pulses in the form of a slow (1 mHz) triangular wave with an amplitude of 5V, while recording the intensity fluctuations of the two N1s peaks corresponding to the anionic and the cationic fragments. In the last part, the externally bias XPS analysis has been used for insitu and in-vacuo monitoring of anodically triggered electrochemical preparation and characterization of Au NPs in both a co-planar and also in a wire-plane electrode electrochemical geometries. The small sized Au NPs’ formation within the DEME-TFSI medium has been confirmed by the characteristic peak around 470 nm in the Visible spectrum and with the spherical and well-dispersed (~4 nm) particles in TEM images.Item Open Access Nanogap based label-free impedimetric biosensors(Bilkent University, 2012) Hanoğlu, OğuzDespite lots of research going on to find a hope, cancer is still a major cause of death in today‘s world. It has been reported that cancer has some biomarkers in human body and detecting these biomarkers timely can pave the way for early detection and successful treatments. Point-of-care biosensors are highly promising for this mission. If these biosensors can achieve sensitivity and reliability with a low-cost and simple platform, they can address a large mass of people who are at the early stages of cancer without any clear symptoms yet. For this purpose, various biosensing mechanisms can be used to convert the signal coming from the recognition elements on the biosensor surface to the digital domain for signal processing. One of these mechanisms, impedimetric (impedance based) sensing is a very appealing electrical biosensing method since this method can offer label-free, low-cost, low-power requirement, miniaturizable, and chip-integrable detection platforms. However, impedimetric sensing in liquid medium is problematic, since during the electrical measurements, ion-based undesired layers (electrical double layers) are formed over the electrodes in the target liquid. Unfortunately, these layers act like a shield against the applied electric field to the liquid and can prevent the detection of the target biomarkers. In this thesis, a nanogap based label-free biosensor structure is designed and using this design impedimetric sensing in liquid medium is demonstrated at low frequencies (1 kHz – 100 kHz). Low frequency platforms are quite amenable to low-cost applications like point-of-care biosensing. The designed structure utilizes nanometer scale electrode separation (nanogap). Theoretical calculations show that nanogap reduces the undesired effect of electrical double layer. Moreover, nanogap also helps in minimizing the volume of the required liquid for the measurement. Design, fabrication, surface functionalization and biotinylation stages of the biosensor are realized in a cleanroom environment and biomimetic materials laboratory. The fabricated biosensor is tested by introducing the target molecules (streptavidin) in a phosphate-buffered saline solution. A parameter analyzer with a capacitance-voltage unit and a probe station are used for the impedance measurements. With these biosensors, label-free detection of streptavidin is observed for 100 µg/mL, 10 µg/mL, 1 µg/mL, 100 ng/mL and 10 ng/mL concentrations. This is, to the best of our knowledge, the first demonstration of streptavidin detection in nanogap based label-free impedimetric biosensors. The above-mentioned concentrations show that these biosensors are promising for commercial applications. Sensitivity to the dielectric constant of the target medium is measured to be 132 pF per unit change in the dielectric constant at 10 kHz measurement frequency. Reliability tests are performed: stable and repeatable operation of the sensors are checked and verified. In conclusion, this proof-of-concept study shows that nanogap based biosensors would be a suitable and appealing choice for sensitive, reliable, simple, low-power and low-cost point-of care biosensing applications. Next step would be utilizing the platform presented in this work in detecting specific cancer biomarkers like PSA or CA125. Thereby, developed further and commercialized, nanogap based label-free impedimetric biosensors can act in the battle of human being against cancer in the future.