While a number of colorimetric sensors are available for the detection of Hg2+, lack of sensitivity or selectivity, complicated synthetic routes and difficulty in adaptation for field application, are common drawbacks. Consequently, this thesis describes the development of readily synthesised, robust, selective, sensitive, cost-effective, small molecule colorimetric sensors for Hg2+ detection in water.The reaction between Hg2+ and off-the-shelf chromogenic reagents, cuprous iodide (CuI), diphenylcarbazone (DPC) and rhodamine 6G (R6G), was studied in solution; in spot test mode (as wet reagents and dried onto filter papers); and in a Hg-bubbler apparatus. Both DPC and R6G produced a colorimetric response in solution and wet spot test mode, but bleaching and oxidation affected the response in dry spot mode. The Hg-bubbler apparatus was found applicable to R6G and CuI. However, reduction of reagents in the field is proved difficult.Rhodamine B thiolactone (RBT) was synthesised. It exhibited high selectivity for Hg2+, with reasonable visual and spectrophotometric detection limits. A disadvantage was that the solid was undergo ring-opening reaction unless stored at low temperature away from light. The RBT was immobilised in sol-gel and agar-agar matrixes with filter paper substrate. Pumping Hg2+ solutions through these membranes changed their colour. Images of the membranes were recorded and analysed using a flatbed scanner and ImageJ software. Based on the grayscale intensity in the green channel, the detection limits were 0.4 and 0.2 μg/L for the sol-gel and agar-agar membranes, respectively. The method was used to quantify Hg2+ in real water samples. Interference due to the presence of Cl- was noted.Finally, a portable platform was developed. Images of RBT doped agar-agar membranes were captured using the camera of a mobile phone, together with a homemade lightproof box, and a bespoke 'Hg-Sense' app. Results were generally acceptable and can be applied by regulation authorities.