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Title: Liquid crystal biosensors
Author: Cronin, Thomas
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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The aim of the thesis was to identify and hence investigate the physical properties of liquid crystals that influence their potential as components of biosensor devices. Silicon surfaces presenting photolithographically fabricated arrays of 50nm thick gold spots were used as the model for a biosensor that detects the surface binding of a biological analyte. The spots ranged in diameter from 2μm to 16μm and their spatial separation varied between 5μm to 40μm. A Self Assembled Monolayer (SAM) of the thiol 3-mercaoptopropionic acid was used to control the surface chemistry of the gold. The responses of the nematic liquid crystals 5CB, E7, ZLI 1695, ZLI 1132 and MDA 01-2012 to were measured by optical microscopy. The spots were seen to induce a tilted planar alignment in the liquid crystals in their nematic phase for spot diameters down to 4μm and for all separations. Anchoring transitions between different tilt angles were observed between spots for some arrays. This was linked to a change in anchoring energy at the gold, possibly stemming from the angle of gold deposition. When heated through the nematic to isotropic phase transition cross defects were observed to nucleate on the gold spots for all spot sizes above 4μm. On cooling through the transition grid patterns of defects were observed to nucleate pinned between the spots for arrays of spots with length scales between 10μm and 20μm. The birefringence and elastic constants K11 and K33 of the liquid crystals were measured for temperatures up to their nematic to isotropic transition points. The birefringences of the liquid crystals at the transition were found to range between 0.003 and 0.007. The device thickness was varied between 7μm and 40μm. Values for the elastic constants were found to range between 1pN and 4pN. The intensity of monochromatic light (670nm) reflected from the arrays as the liquid crystals were cooled through the phase transition was found to increase for smaller values of the elastic constants and found to be highest where the grid of defects on the array was observed most clearly. The effect on the intensity of the birefringence and cell thickness was shown to be small compared to the effect of elasticity. Two possible biosensor designs are proposed. The first would identify the presence of a biological analyte at a surface by the change in alignment of a liquid crystal. This type of sensor would be optimised by carefully controlling the anchoring energy of the liquid crystal at the surface to minimise the quantity of surface binding required to induce an anchoring transition. The second would detect the presence at a patterned surface of an analyte by the defects that form over the pattern as the liquid crystal changes between the nematic to isotropic phases. This type of sensor would be optimised by choosing a liquid crystal with small elastic constants at the phase transition and by designing a patterned surface with length scales between 10microns and 20microns.
Supervisor: Gleeson, Helen ; Dierking, Ingo Sponsor: Sharp Laboratories of Europe
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Liquid Crystal ; Biosensors ; Nematic ; Defects ; Surfaces ; Elasticity