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Molecular Electronics uses molecules sandwiched between two metals as a model system to create tailored solutions for applications such as energy harvesting and sensing. Electrons tunnelling across such a junction are a↵ected by the properties of the molecule itself as well as the interaction between metal and molecule. In particular, charge transport is known to couple to molecular vibrations, which can act both to dissipate energy as well as increase conductance. This thesis therefore applies in-situ vibrational spectroscopies, surface-enhanced Raman scattering and vibrational sum frequency generation, to investigate molecular junctions. As a model system, 4-mercaptopyridine sandwiched between a gold surface and an elec- trochemically deposited second metal layer is used. Four aspects are studied in detail in this thesis. Chapter 3 presents a detailed study of surface enhanced Raman spectra of 4-mercaptopyridine on gold. All experimental vibrational modes are assigned and related to the symmetry of the adsorbed molecule with the help of density functional calculations. In particular, the e↵ect of hydrogen bonding on the ring breathing modes of adsorbed mercaptopyridine is revealed for the first time. In chapter 4, surface-enhanced Raman spectroscopy is used to identify a spectroscopic signature of a successfully formed metal-molecule-metal junction after electrochemical deposition of a tran- sition metal layer. Chapter 5 then addresses the use of surface-enhanced Raman spectroscopy to identify charge transfer states of 4-mercaptopyridine by changing bias potential and excitation wavelength. A charge transfer state is found for protonated 4-mercaptopyridine at about 1.7 eV above the Fermi level, while the corresponding state for unprotonated 4-mercaptopyridine must lie at least 0.8 eV higher. Chapter 6 then explores the use of ultrafast vibrational sum frequency generation. The pyridine ring stretching modes are detected and metallisation of the 4-mercaptopyridine layer is seen to decrease the local order of the molecular layer. The influence of the mercaptopyridine charge transfer state can be seen in ultrafast pump - sum frequency probe spectroscopy of the gold substrate. This opens the prospect of investigating coupling between molecular vibrations and charge transfer in these junctions on a timescale of a picosecond or less.

The current variability and speciation of indoor VOCs are studied by analysing indoor air in UK homes and offices. These measurements were carried out via passive sampling into silica-treated canisters followed by thermal desorption-gas chromatography and high mass accuracy time-of-flight mass spectrometry (TD-GC-Q-TOF/MS). It was found that majority of the homes had d-limonene and α-pinene as the most abundant VOCs, with average concentrations ranging from 18 μg m-3 to over 1400 μg m-3 and 2 μg m-3 to 230 μg m-3 respectively. In these analyses, cyclic volatile methyl siloxanes (cVMS) were frequently detected in high abundances. cVMS are chemicals in high volume production as they are used as solvents in formulations of consumer products. They were found in persistently high background concentrations in our analyses. Hence, a passive sampling method involving sorbents was developed to allow the analysis and quantification of these compounds, with LODs calculated to be 7.2 to 16.8 ng m-3. This method was validated with real indoor air sampling with average D5 and D6 concentrations of about 2480 ng m-3 and 664 ng m-3 respectively. Advancements have also been made in the development of a multispecies sensor for the detection of VOCs. A temperature control method was developed using a Peltier device and a control software programme written in LABVIEW. Attempts were made to manufacture a lab-on-a-chip GC column, but was deemed unsuitable due to leakage and mechanical problems. Instead, a short length of column was wound and placed in a copper enclosure. Tests were conducted using photoionsation detector (PID) as the detection method in this sensor development. The final set-up involved the assembly of the temperature control method, the GC column enclosure and the PID for the detection. Tests were conducted by introducing headspace standards into the set-up, with promising results.

It is the aim of this work to extend the use of phosphorescence as an analytical technique to a number of different classes of drugs and to study any structure-spectral correlations existing between them. Part of the present work is directed towards developing a technique capable of scanning thin-layer chromatograms and measuring the phosphorescence of separated components in situ. The sensitivity of phosphorescence and separatory function of thin-layer chromatography could thus be combined to produce a rapid, sensitive method capable of resolving small quantities of similar compounds, especially drugs and their metabolites, from complex mixtures.

The aim of the present work was to eliminate some of the problems associated with phosphorimetric techniques. Also to extend the use or the application of phosphorescence as an analytical tool to a number of different classes of drugs and determining them in low concentrations. The aim was to extend the application to environmental pollution by studying the characteristics of thirteen polycyclic aromatic hydrocarbons in solutions and on thin layer phosphorimetry. When this work started thin-layer phosphorimetry was already developed, the aim of this work is to examine the usefulness of TLC phosphorimetry for different groups of drugs and related compounds which have not been studied previously by this technique, and to determine them in very low concentrations. The sensitivity of phosphorescence and thin-layer chromatography could be combined to produce a rapid, sensitive and selective method of analysis capable of resolving small quantities of similar compounds, especially drugs and their metabolites, from complex mixtures.

Some onium compounds react with thiocyanate complex anions to give stable coloured salts, which can be extracted into organic solvents. This generally results in an increase in the apparent molar absorptivity of the thiocyanate complexes and in their stability. Reactions of thiocyanate complexes of molybdenum, niobium, and rhenium with tetraphenylarsonium compounds have been critically studied in an attempt to improve existing methods for determining trace amounts of these metals. A method has been developed for the determination of trace amounts of molybdenum in steels or in other matrices. The optimal conditions for the formation of the thiocyanate complex and for its extraction were determined. Because of the high sensitivity, accuracy and precision of the procedure, it is considered to be superior to other chemical methods for the quantitative determination of molybdenum in steels. Fifteen metals commonly found in steels were shown not to interfere with the determination of molybdenum. Tungsten and niobium however, do interfere, but can readily be masked with oxalic acid. A forty-fold ratio of tungsten to molybdenum can be tolerated.

Determination of a target molecule is of great importance both in industrial applications and human health. For a long time, this has attracted the extensive attention of researchers to develop new strategies. One of the vigorous sub-branches lies in supramolecular field. its ability to mimic a natural occurring event and unearth the mode of interaction for a targeted molecule. By varying the structure of a probe, researchers could rationally design synthetic sensors with selective binding affinity to target molecules and also satisfy different requirements for a certain application by combining different functional groups. The growing incidence of diabetes has promoted scientists to develop new effective strategies for early and quickly monitoring saccharide concentration. Interest in boronic acid-based saccharide probes has dramatically increased in the past several decades due to the unique properties of forming boronate esters with cis-diols. Previous work in James group demonstrated that by the introduction of a proximal amine group, the binding between boronic acid and saccharide can occur at neutral, i.e., physiological pH and the binding between boronic acids and saccharides can induce fluorescent "turn-on" responses. Acrylamide hydrogels with boron-additives have potential applications in separating saccharides and recognising glycation process in electrophoresis. Two monoboronic acid-based gel monomers 27 and 28 with different fluorophores have been synthesised and analysis measurements were carried out both in hydrogel, as well as in solution. As a Lewis acid, the strong binding affinity between boron and fluoride was also extensively explored to develop fluoride sensors. By investigating boron-containing 1,8-naphthalimide derivative 33, it is found that the binding between boron and fluoride could induce a red-shift of absorption (about 20 nm) with obvious colour changes and unique changes of NMR chemical shift. By comparison with control compounds (34 and 35), it was found that the addition of the boronic acid moiety would improve the selectivity, though at the expense of loss of sensitivity towards anions. Traditional molecular recognition is mainly based on host-guest interaction, where a synthetic host molecule could selectively interact with a guest molecule via weak and generally reversible interactions. The reaction-based sensing strategy is a new strategy in designing a high sensitivity probe toward a specific analyte, in which an irreversible reaction is normally utilised. Boronic acid moieties were extensively used both in exploring saccharide sensors and organic synthesis (Suzuki Homocoupling reaction). Here these two aspects were combined together to develop a Suzuki Homocoupling reaction-based saccharide sensing strategy. Different boronic acid substrates were employed and the sensing mechanism was investigated. Furthermore, as a collaboration project, colorimetric enantioselective recognition of chiral secondary alcohols was achieved via hydrogen bonding to a chiral metallocene containing chemosensor (38 and 39).The observed enantioselectivity of 38 could partly be due to the steric effect of the formed hydrogen bond complexes and the distance to the chiral centres.

Electrochemical impedance spectroscopy (EIS) has been one of the most intensively studied electrochemical techniques for biosensing in recent decades. However, despite its great potential, real applications of EIS sensors still remain not as widely spread as other techniques because there are many factors that govern impedimetric signals. In particular, features such as the probe coverage density on the sensor surface, as well as the target size, charge and conformation, can assume significant importance. The aim of this study was to analyse both the limitations and the potential solutions for maximizing the EIS signals in certain possible systems which are characterised by having different types of target to detect. In order to reproduce different conditions, four types of molecular targets have been taken in consideration. These provided example systems where the approach for the development of an EIS sensor varied considerably. In the first system the importance of the surface conditions of an aptasensor for prostate specific antigen (PSA) using Faradaic processes was reported. It was demonstrated that the effect of the target binding combined with the relative surface charge modification can diminish the EIS signals. In the second system, a strategy for improving the sensitivity and selectivity of the assay for drug discovery applications was demonstrated. This was obtained by adopting ferrocene-crowned gold nanoparticles that specifically bind to the molecular targets and induced a transition from non-Faradaic to Faradaic processes. In the last two example systems, particular attention has also been given to non-Faradaic processes which are the least exploited in EIS systems although the technique can be simpler to implement compared to Faradaic EIS. Moreover, with non-Faradaic processes more effective results can be achieved under some specific conditions, such as exploiting the size and charge of molecular targets. On such premises, the current work wants to serve as a reference for overcoming the limitations met in several EIS systems and hopefully contribute as a support for enhancing the fields of applications of EIS sensors.

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