Fluorescence Turn-On, Specific Detection of Cystine in Human Blood Plasma and Urine Samples by Nitrogen-Doped Carbon Quantum Dots

[Image: see text] Determination of cystine in blood and urine is very important to monitor and maintain the bio metabolism, immune systems, and prevent the tissue/DNA damage from free radicals, diagnosis of cystinuria disease, cancer, and related autoimmune diseases. Among the various detection methods, fluorometric detection is simple, rapid, and sensitive to cystine using nontoxic, inexpensive, highly fluorescent, stable carbon quantum dots (CQDs). The CQDs are prepared from p-phenylenediamine by the hydrothermal method to get the inherent optical features of pH-dependent and excitation wavelength-independent fluorescence emission along with high aqueous stability due to pre-eminent nitrogen content. The red emission of CQDs originates from the intrinsic core that is associated with photoinduced electron transfer (PET). The turn-on fluorescence observed in presence of cystine is due to decrease in the PET by oxidation of CQDs. On the basis of this observation, we have developed an assay for the determination of cystine with a concentration range of 10 nM to 10 μM and the limit of detection is 0.4 nM. Additionally, our assay shows good recoveries (93–105%) for the spiked blood plasma and urine samples using the standard addition method.