This thesis presents original diagnostic investigations of atmospheric pressure gaseous discharges, operating in owing helium and helium with low concentrations (0.1 - 1 %) of gas admixtures, together with novel biomedical surface functionalisations. The initial body of this work focuses on comprehensive electrical and optical diagnostics of the operation of an industrial scale dielectric barrier discharge (DBD), maintained in a 10 l/min w of both helium and helium with 1% admixed oxygen. The experimental results reveal a coupling between the power dissipated in the discharge and the discharge homogeneity which, in turn, correlates to a shift in the power supply operating frequency and species optical emission intensities. The shift in the operating frequency is shown to be dependent upon increased charge deposition on the electrodes, as the input power is increased, thus changing the overall system capacitance through charge-voltage LIssajous gure analyses. Furthermore it is demonstrated that the gas temperature did not exceed approximately 380-410 K over the full parameter space, in the helium discharge, through model tting of the rst negative system of the N+2 band around 391.4 nm. A real-time, PC based monitoring diagnostic system has been developed which is used to perform long term analyses of a laboratory DBD chamber in helium and helium with 0.1% admixtures of both oxygen and nitrogen. Post analysis of the results, through multivariate analysis of the large experimental datasets, show that rapid system characterisations are faciltated using this method, the outcomes of which are compared with both global and uid model outputs within the literature. Passive acoustic diagnostics of a plasma jet system, together with signal analysis in the time, frequency and time-frequency domains are explored. Here it is found that ow induced mode transitions from buoyant, through laminar, to turbulent regimes may be identied using time-frequency scalogram plots. It is shown that the scalogram plots may also be used to identify the point at which power coupling together with the plume length are maximised. The transition into a fully turbulent plume structure is shown to be accompanied by low frequency acoustic signals which modulate the acoustic rst harmonic in the time-frequency domain. Moreover, a non invasive measurement of the power coupled into the discharge is demonstrated through passive acoustic sensing whilst the jet ow is laminar. Finally, surface modications of disposable plastics for biosensor applications are performed. In this work it is shown that the density of packing of both 40 nm and 80 nm gold nanoparticles may be tailored, through variation of the gas input ow rate of a linear eld jet, in order to enhance the optical signal according to the Mie theory for light extinction at a metal nanoparticle interface. The functionalisation using long chain polethyleneglycol has also been demonstrated to provide a protein repellent surface for non-specic protein binding reduction.