Selective chemosensors for determination and quantification of various types of molecular target analyte are very important in many fields including chemistry, medicine, and biology. This dissertation describes the efforts of chemosensors for the sensing of metal ions (Hg(II), Fe(III) & Pd(II)) and pharmaceuticals (cephradine & pefloxacin mesylatye). First chapter describes general introduction about chemosensors and nanoparticles. While in the second chapter, synthesis of two new sulfonate and sulfonamide based fluorescent chemosensors, their characterization by EI-MS and 1HNMR and synthesis of gold nanoparticles and silver nanoparticles stabilized Schiff bases have been explained. The synthesized nanoparticles have been characterized by UV-vis spectrophotometric, FTIR and AFM techniques. The average size of synthesized silver nanoparticles were found to be 20-30 nm, and were polydispersed nanoparticles as evidenced by atomic force microscopy. The average size of synthesized gold nanoparticles were found to be 11 nm and were polydispersed. To ascertain the potential for in vivo application, the stability of all synthesized nanoparticles was investigated as a function of pH, temperature and salt concentration. The water suspension of gold nanoparticles were found to be stable for several days at a temperature up to 100 0C, a pH range of 2-13 and salt (NaCl) concentration 5mM-0.01mM. For chemosensing study metals salts and pharmaceuticals were used. The main goal was to achieve sensing in water, which is a prerequisite for application to real blood and tap water samples. The first analyte of choice was heavy metal ion i.e. Hg. The already synthesized probe 218 exhibited marked selectivity for Pb+2 and Hg+2 over 10 other metal ions under physiological buffer condition. Owing to Hg+2 undesirable effects on the environment and the health concerns associated with Hg exposure, this fluorescent probe represents an appealing target and efficient chemosensor for Hg+2. The fluorescence of each solution was measured and the resultant intensity is plotted against concentration of Hg+2 added which shows linear relationship from 10 to 6 μM with a limit of detection of Hg+2 was 0.05 μM and a regression coefficient of 0.907. Another bis-triazol-based fluorescent chemosensor, used as a best sensor for pefloxacin, the chemosensor showed marked quenching among 10 other drugs of interest in aqueous solution, with maximum quenching in intensity at pH 6-8. A novel supramolecular molecular tweezers based on a biphenyl bis-triazole hexahydroquinoline system was used for highly sensitive and selective fluorescent probe for recognizing and detecting cephradine in the presence of other drugs at pH of 7.7. The detection limit was calculated to be 1 μM with a regression coefficient of 0.99. The competivity study, pH sensitivity of the sensor was also studied. The chemosensor allowed the detection of cephradine in tape water too. Among the nanoparticles synthesized, the main attention was paid to the gold nanoparticles chemosensing properties. A pyrazinium thioacetate stabilized gold nanoparticles have been synthesized and were found an excellent sensor for heavy metal Fe(III) and Pd(II) ions in water, without any particular pretreatment. The detection method for Fe(III) by using gold nanoparticles was elegantly applicable over a wide range of pH (2-13) and concentrations (1-100 μM). The regression constant (R2) calculated 0.9813, while the limit of detection (LOD) and the limit of quantification (LOQ) for Fe+3 ions was found to be 4.3 μM and 13.19 μM, respectively. The same pyrazinium thioacetate stabilized gold nanoparticles showed colorimetric change from win-red to grey in the presence of Pd(II). LOD and LOQ for Pd+2 ions were found to be 4.23 μM and 12.83 μM, respectively. Schiff base stabilized gold nanoparticles displayed great selectivity and exhibited best chemosensor properties for pefloxacin in aqueous solution, A linear relationship was almost found when the concentration of pef was between 80 μM to 0.01 μM with a linear regression equation of y=0.0015x + 0.0373 with R2 = 0.9839. LOD was calculated 12.1 μM. The competivity study, pH sensitivity of the sensor was studied. These gold nanoparticles were found to be potent colorimetric sensor and display a very high selectivity for Fe(III), Pd(II) and pefloxacin. The nanoparticle used for the drug sensing allowed the detection of pefloxacin in human serum by simple UV-vis spectroscopic measurements.