Semiconductor quantum dots (QDs) are a new generation of inorganic probes with advantageous properties over traditional organic probes for biological applications. A major hurdle in the use of QDs for biology is the inability of the hydrophobically synthesized QDs to interface with aqueous environments. Here we describe the synthesis of water-soluble CdS QDs end-capped with N-hydroxysuccinimide ester groups. These CdS QDs are synthesized in polyol medium using terthiophene dicarboxylic acid as a stabilizer. The carboxylic end groups are transformed into N-hydroxysuccinimidyl ester and are further cross-linked with biotin and avidin. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy results indicate successful modification of CdS QD surfaces.