Nanometer-sized fluorescent particles were successfully synthesized. The nanoparticles have a narrow, tunable, symmetric emission spectrum and a broad, continuous excitation spectrum. They are also photochemically stable. A synchronous fluorescence method was developed for the rapid determination of DNA with functionalized CdS as a fluorescence probe, based on the synchronous fluorescence quenching of functionalized CdS in the presence of DNA. Maximum fluorescence is produced at pH 7.0, with maximum excitation and emission wavelengths of 360 and 620 nm, respectively. The maximum emission wavelength of synchronous fluorescence is 354 nm when Δλ = 260 nm. Under optimum conditions, the calibration graphs are linear over the range 0–3.5 μg mL⁻¹ for calf thymus DNA (CT-DNA) and 0.2–3.0 μg mL⁻¹ for fish sperm DNA. The corresponding detection limit is 0.01 μg mL⁻¹ for CT-DNA and 0.02 μg mL⁻¹ for fish sperm DNA. The relative standard deviation of seven replicate measurements is 2.2% for 1 μg mL⁻¹ calf thymus DNA and 2.4% for 1 μg mL⁻¹ fish sperm DNA. The method is simple, rapid and sensitive. The recovery and relative standard deviation are very satisfactory.