In this thesis, cadmium telluride quantum dots (CdTe QDs) were used in order to determination of different chemical species. In the first study, a simple turn-on fluorescent assay was developed for the determination of trace amounts of the lethal poison, cyanide, based on glutathione-capped CdTe QDs using of silver nanoparticles. In this method, the fluorescence intensity of glutathione-capped CdTe QDs increased with increasing concentration of cyanide. Using this optical sensor under optimum conditions, cyanide was measured in the range of 0.01 – 2.5 µg mL–1. Investigation of the effects of potential interfering anions showed the high selectivity of the sensor for the detection of cyanide in real samples. High sensitivity, superior selectivity, low detection limit, and ease of production are the most important advantages of the present sensor. In the second study, a novel fluorescent sensor has been developed, based on glutathione-capped CdTe QDs aggregation, for the determination of trace amount of an important drug, protamine. In this method fluorescence quenching upon aggregation was accrued with increasing the concentration of protamine. Different parameters affect the sensitivity, such as pH and amount of the quantum dots were optimized. Using this optical osensor, under optimized conditions protamine could be measured in the range of 2.0 – 200 ng mL–1. The optical biosensor was successfully used for the determination of protamine in real samples. In the third study, a new approach for developing a fluorimetric aptasensor has been described and applied for determination of a highly toxic cation, As(III). In this method an aptamer was used to aggregate cationic cysteamine-stabilized CdTe/ZnS core/shell QDs, as a result fluorescence quenching was accrued. In the presence of As(III), the aptamer and As(III) make a complex, which prevents aggregation of the QDs. Thus, the fluorescence intensity of the QDs was enhanced upon the de-aggregation, which depends on the concentration of As(III). This method has a low detection limit of 1.3 pmol L–1 As(III). The interference effect of a wide variety of cations were investigated. The present assay was successfully applied for the determination of As(III) in several water samples. In the fourth study, a sensitive optical sensor, using a molecularly imprinted polymer (MIP) on CdTe QDs, is developed for the selective detection of prilocaine. At the first step of this work, thioglycolic acid capped CdTe QDs (TGA capped-CdTe QDs) were prepared using refluxing method. At the next step, a thin film of silica was formed, by reverse microemulsion technique, on the surface of CdTe QDs. Finally, MIP embedded CdTe QDs were obtained. In this step, 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) were applied as a functional monomerand a cross linker, respectively. Different variables affecting the optical signal were optimized. Under optimal conditions, the dynamic range of the optical sensor was 5.5–260 nmol L?1 prilocaine. The response of the optical sensor on common speciesin biological media was investigated. The results confirmed the good selectivity of the sensor for the measurement of prilocaine. In the final study, a MIP has been developed for the determination of an important drug, propranolol (PROP). For this purpose, water-soluble TGA capped-CdTe QDs as a support for plastic antibody was synthesis via refluxing method. Following, reverse micro emulsion technique was applied to stabilize a thin silica shell on the surface of QDs. In the next step, molecularly imprinted polymer embedded CdTe QDs were obtained using APTES (as monomer) and TEOS (as cross linker) in the present of PROP. Finally, the obtained MIPs was used for PROP sensing. At optimized conditions, dynamic range was calculated 3.0 to 139 ?mol L-1