Date of Award
Doctor of Philosophy
Department of Applied Physics and Instrumentation
Dr Liam McDonnell
The aim of this work was to develop a non-contact mode scanning force microscope (SFM) for application to biological samples within liquid environments. Given that biological samples are particularly soft in their natural state and that a liquid is the most difficult operating environment for non-contact mode scanning force microscopy, the development of an instrument that is optimised to image biological samples in vitro presents significant challenges.
Firstly, the SFM tip to sample interaction has to be minimised either, in the worst case, to prevent the sample from being detached from its support or to maximise spatial resolution by minimising sample distortion. Given such concerns, it is not surprising that in vitro SFM imaging is carried out, more often than not, using non- contact mode, rather than by using contact mode with very soft cantilevers. Secondly, the classic method used to oscillate the cantilever for non-contact SFM, i.e. piezoelectric actuation using either the SFM scanner or a specific piezoelectric dither transducer, is not ideal for liquid applications. Piezoelectric actuation generates multiple peaks in the frequency spectrum, often referred to as a “forest of peaks”, that arise from resonances in the liquid cell and elsewhere in the instrument, making it difficult to locate the cantilever resonance. Furthermore, viscous damping of the oscillating cantilever by the liquid degrades the quality factor of the cantilever and thus its sensitivity.
A prototype scanning force microscope, capable of operation in contact and non- contact modes in air or liquid, was designed and constructed as a development platform. By using a modular approach to the hardware, electronics and software,modifications could be made to the prototype without serious consequences. Alternative methods of cantilever excitation were investigated and magnetic excitation was found to provide a single cantilever resonance. Furthermore, implementing active-Q control boosted the effective quality factor of the cantilever, thereby returning its sensitivity in liquid to a level comparable to that obtained in air.
Incorporating magnetic excitation and active-Q control of the cantilever enabled the performance of the prototype microscope to be optimised for imaging biological samples in liquid. Samples of immobilised red blood cells, nanogold conjugates and three different antibodies were successfully imaged in water by the prototype. The performance of the prototype was validated by imaging monolayers of immobilised antibodies in liquid and achieving image qualities comparable to those achieved in air for such samples. Images of such quality were not possible without incorporating magnetic excitation and active-Q control within the prototype.
Grant, Anthony, "Development of Non-Contact Scanning Force Microscopy for Imaging in Liquid Environments" (2001). Theses [online].
Available at: https://sword.cit.ie/allthe/340