Confined Sensing Systems: Principles and Applications from Bulk to Nanoscale

In this thesis, the study of molecules and biomolecules confined to the surfaces of both bulk carbon electrodes and within nanopipettes have been investigated, and the use of these sensing systems for the detection of pH and pathogenic bacterial DNA explored. Initially, carbon electrodes in the form of sp2 based glassy carbon (GC), and sp3 based boron-doped diamond (BDD), were modified with the redox active probe, anthraquinone. Following modification, properties including electron transfer rate (GC: 4.5  0.4 s-1; BDD: 0.41  0.01 s-1) and the pKa of the immobilised anthraquinone moiety (GC: pKa = 9.1; BDD: pKa = 6.6) were determined. A shift in the apparent pKa value of anthraquinone from the bulk value (pKa = 7.7) was observed when immobilised on the surface of the two different carbon substrates. The apparent pKa was found to shift in different directions relative to bulk solution, to more basic values on GC electrodes and more acidic values on BDD electrodes. The shift in pKa values was attributed to the circa ten-fold increase in dielectric constant of the immobilized anthraquinone film on GC compared to BDD. The use of ion current rectifying nanopipettes for the detection of pathogenic bacterial DNA was also explored. A surface modification strategy to immobilise a sequence selective probe DNA strand was optimised. Significant increases in the reproducibility and throughput of sensors fabricated was achieved compared to previous strategies in the group. Studies were carried out to differentiate complementary (cDNA) sequences, from those containing single nucleotide polymorphisms (SNPs). The rate of DNA hybridisation inside the nanopipette was shown to differ in the presence of the mismatch base pair (cDNA  = 0.5  0.1 hr; C:T  = 1.0  0.2 hr; C:A  = 0.9  0.2 hr; C:C  = 1.6  0.2 hr). Based on the differences in the rate of DNA hybridisation inside a nanopipette, a lab-cultured sample of wild type Klebsiella pneumoniae was differentiated from a sample which contained a SNP in response to exposure to the antimicrobial agent glyphosate (WT  = 0.5  0.1 hr; SNP = 1.0  0.2 hr). Overall, the work presented demonstrates that through understanding the fundamental properties associated with materials, and through careful control of any surface modifications made to them, the sensing ability of macro- and nano-scale sensors can be optimized for a variety of useful applications.