In recent years, the whispering gallery mode (WGM) technique has received considerable attention as a novel and extremely sensitive technique for use in sensors. The technique is able to detect target molecules at very low levels and in real time, a capability which cannot be matched by any other detection technique currently in use. With this potential rarely found in common sensors, WGM is becoming one of the most widely used. The WGM set-up is simple and inexpensive. Light generated by a tunable laser, circumnavigates the surface of a resonator through a tapered waveguide. This light is strongly confined inside the microresonator by total internal reflection (TIR). Energy is extracted from the fibre, resulting in a negative peak. The surface of the resonator needs to be functionalised for reacting with the target molecule. If a chemical or biological analyte is to be bound on the surface of the resonator, the negative peak must be shifted. This shift can be used for measuring the amount of the analyte. In view of its exciting potential, it is not surprising that WGM is establishing itself as the detection method of choice, especially in chemical and biomedical applications. The work reported in this thesis is in two sections. In the first part, the use of the WGM technique integrated self-assembled glutathione (GSH) modified gold nanoparticles (Au NPs) on an optical microsphere resonator in an ultrasensitive chemical detection assay for Pb(II) (down to 10 ppt or 0.05 nM) is described. This satisfies the demanding sensitivity required for monitoring the maximum Pb(II) exposure limits set by both International Agency for Research on Cancer (IARC) and the United States Environmental Protection Agency (EPA). The second section presents an example of the use of WGM in a biosensor to study the interactions between small molecules and G-quadruplex DNA which is well known to be active targets for anticancer treatments. Currently methods typically used to study such systems have proven to be valuable; however, they have limitations, such as low sensitivity, time-consuming monitoring and lack of real time analysis. To circumvent these problems, a novel platform based around WGM is developed. The sensor offers a real time, fast and sensitive analysis. In addition, kinetic data such as dissociation equilibrium constant (KD ) as well as association and dissociation constant (kon and koff , respectively) can be easily obtained.