Over the past few decades, one principle research area has focused on the quantification of toxic heavy metals within the organism, the environment and in industrial waste. Amongst these metals, mercury is considered as one of the most harmful due to its devastating effects on the environment and human health. To minimize the dramatic consequences of this highly toxic metal, it seems essential and urgent to design a novel fluorescent detection method to monitor Hg2+ ions based on highly sensitive and selective, fast, miniturisable, reusable and also portable optical devices which can be used to perform field measurements in-situ. Therefore, this thesis project aims at designing optical nanosensors based on fluoroionophores immobilised within mesoporous silica matrices for the detection of Hg2+ ions. To achieve the aim of this project, different fluorescent probe molecules (fluoroionophores), each responding by an enhancement in fluorescence intensity after complexation with Hg2+ ions (due to the modification of their photophysical properties), have been successfully synthesised and tested for their ability to quantify Hg2+ ions with sensitivity and selectivity. The optimisation phase of the molecular structure of the fluoroionophore revealed to be fundamental not only in order to reinforce the interaction (sensitivity and selectivity) between the ion and the recognition site of the fluorescent molecule but also to fill the immobilisation conditions of the probe molecule onto the porous support. To do this, structural changes have been made to both the chelating part (ionophore) and the fluorescent part (fluorophore) of the molecule. Sulfur and selenium have been selected as chelating atoms to the Hg2+ ion and thus incorporated into the ionophore structure and withdrawing groups (bromine and methyl ester) have been grafted onto the fluorophore, the anthracene. Through this study, it was shown that sulfur based ionophores complexed with sensitivity and selectivity both Cu2+ and Pb2+ ions whereas selenium based ionophores enabled a slight enhancement of the sensitivity and selectivity towards the Hg2+ ion. The modification of the fluorophores has also enabled to improve the sensitivity and selectivity of the fluorescent molecule. Hence the grafting of the selenide ionophore onto an anthracene derivative (bromo or methyl ester anthracene) has led to the design of extremely sensitive and selective fluoroionophores towards the Hg2+ ion. With the view to immobilising the most efficient fluoroionophore to obtain nanosensors adapted to the very sensitive and selective detection of Hg2+ ions, various highly structured mesoporous silica materials (viz. CMI-1, SBA-15 and SBA16) were synthesized and characterised. The remarkable properties that these porous matrices possess, which are ideal for the design of very efficient nanosensors, were highlighted by a suite of different characterisation techniques. These materials exhibited highly accessible specific surface areas close to 800 - 1000 m2/g and significant porous volumes of around 1 cm3/g. These materials were identified as having pores with either a bidimensional hexagonal arrangement (CMI-1 and SBA-15) or a tridimensional cubic arrangement (SBA-16), whose size varied from 2.8 to 9.0 nm, making them ideal candidates for the immobilisation of the fluoroionophore. Finally, the nanosensors were obtained by the immobilisation of the fluorescent molecule through direct impregnation and were shown to be efficient in terms of sensitivity, selectivity and response time.