In this thesis, several optical and electrochemical sensors were constructed using cadmium telluride, carbon and graphene quantum dots, and were used to determine several chemical species. In the first part, a new, label-free, highly sensitive, and selective fluorescent “quenching-recovery” model for the detection of DNA was developed through mercaptopropionic acid (MPA)-capped cadmium telluride (CdTe) quantum dots (QDs) aggregation. Under the optimal conditions, the linear dependence of the fluorescence signal and the con-centration of DNA was in the range of 0.04 -14.0 µg mL-1, with a detection limit of 0.013 µg mL-1 .This analytical fluorescent reversible “quenching-recovery” sensor offered a method for the detection of DNA with excellent sensitivity and selectivity. In the second part, molecularly imprinted polymers (MIPs) were used on the CdTe QDs for the simultaneous determination of folic acid (FA) and methotrexate (MTX). For this purpose, two different sizes of CdTe QDs with emission peaks in the yellow (QDY) and orange (QDO) spectral regions were initially synthesized and capped with MIPs. Under optimal conditions, the dynamic range was 0.5-20 mmol L-1 for FA and MTX, and the detection limits for FA and MTX were 32.0 nmol L-1 and 34.0 nmol L-1, respectively. The reproducibility of the method was checked for 12.5 mmol L-1 of FA and MTX to find RSD values of 4.2% and 6.3%, respectively. Finally, the applicability of the method was checked using human blood plasma samples. In the third part, an optical sensor based on carbon quantum dots (CQDs) coated with MIPs was fabricated for selective and sensitive determination of promethazine hydrochloride (PrHy). Water-soluble and fluorescent CQDs were synthesized by a simple, low cost and green approach using orange juice as a carbon source. The fluorescence intensity of CQDs-MIPs exhibited linear response with the concentration of PrHy in the range from 2.0 to 250 µmol L?1 with a detection limit of 0.5 µmol L?1. The reproducibility of the method was checked for 20 µmol L?1 of PrHy, and the results showed RSD% as 5.1. This sensor was used for determination of PrHy in real samples and showed satisfactory results. In the fourth part, an extremely sensitive and selective electrochemical sensor based on a modified glassy carbon electrode (GCE) for metronidazole (MNZ) determination was developed. At first, molecularly imprinted polymers (MIPs) on the surface of graphene quantum dots (GQDs) was synthesized via sol-gel method. Then, it was dropped on the surface of GCE that modified with graphene nanoplatelets (G). Several effective parameters on the sensor response were investigated and optimized. The proposed imprinted electrochemical sensor, under the optimized conditions, showed two linear dynamic ranges from 0.0075 to 0.75 µmol L?1 and 0.75–10.0 µmol L?1 with a low detection limit of 0.5 nmol L?1 for MNZ determination. Finally, the proposed sensor was used for MNZ analysis in human blood plasma samples and provided acceptable results. In the final part, a glassy carbon electrode (GCE) was modified with multiwall carbon nanotubes/ionic liquid/graphene quantum dots (MWCNTs/IL/GQDs) nanocomposite. Then, the nanocomposite was decorated with nickel-cobalt nanoparticles (Ni-Co ), and it was used as a non-enzymatic glucose sensor. The novel amperometric sensor displayed two linear ranges from 1.0 to 190.0 µmol L-1 and 190.0 to 4910 µmol L-1 with a low detection limit of 0.3 µmol L-1 as well as fast response time (2 s) and high stability. Also, the sensor showed good selectivity for glucose determination in the presence of ascorbic acid, citric acid, dopamine, uric acid, fructose, and sucrose, as potential interference species. Finally, the performance of the proposed sensor was investigated for glucose determination in real samples.