Light-addressable potentiometric sensors based on self-assembled organic monolayer modified silicon on sapphire substrates

PhDLight-addressable potentiometric sensors (LAPS) have become attractive in many chemical and biological sensor applications. This thesis introduces the use of self-assembled organic monolayers (SAMs) as the insulator in LAPS and scanning photo-induced impedance microscopy (SPIM) for the first time. Two types of monolayer assemblies with alkenes (1-octadecene or undecylenic acid) and alkynes (1, 8-nonadiyne) were immobilised on hydrogenated silicon on sapphire (SOS) or silicon through thermal hydrosilylation. Further derivations were performed on the 1, 8-nonadiyne monolayers via “click” reactions. The monolayers were characterised by water contact angle, ellipsometry and X-ray photoelectron spectroscopy (XPS). LAPS/SPIM measurements with SAM-modified SOS showed the same good spatial resolution that was previously obtained with a conventional SiO2 insulator on SOS, but also a significant improvement in the accuracy of LAPS and the sensitivity of SPIM. Surface potential imaging using LAPS insulated by SAMs was validated by studying micropatterns of poly(allylamine hydrochloride) (PAH), poly(styrene sulfonate) (PSS) and DNA on a PAH template. Two potential strategies for chemically patterning SAMs on oxide-free SOS or Si substrates were investigated and compared. Microcontact printing (μCP) followed by “click” chemistry is a mild and efficient means of modifying the surface, whereas the combination of photolithography and “click” chemistry is not. LAPS was also shown to be extremely sensitive to surface contamination. LAPS/SPIM insulated by SAMs can also generate impedance images with high resolution and high sensitivity. Microcapsules labelled with gold nanoparticles (AuNPs) integrated with a femtosecond laser were used for the validation. In contrast, capsules without AuNPs showed no SPIM response at all, indicating that the impregnation with AuNPs can significantly increase the impedance of microcapsules. Finally, new instrumentation to integrate two-photon fluorescence microscopy with LAPS/SPIM was proposed. Preliminary results have shown that the new technique is promising to produce two-dimensional electrochemical images and two-photon fluorescent images of the cell-attachment area with subcellular resolutionChina Scholarship Council Queen Mary University of London