Dual-Responsive Carbon Quantum Dots for the Simultaneous Detection of Cytosine and 5-Methylcytosine Interpreted by a Machine Learning-Assisted Smartphone

DNA methylation is an epigenetic alteration that resultsin 5-methylcytosine(5-mC) through the addition of a methyl group to the fifth carbonof a cytosine (C) residue. The methylation level, the ratio of 5-mCto C, in urine might be related to the whole-body epigenetic statusand the occurrence of common cancers. To date, never before have anynanomaterials been developed to simultaneously determine C and 5-mCin urine samples. Herein, a dual-responsive fluorescent sensor forthe urinary detection of C and 5-mC has been developed. This assayrelied on changes in the optical properties of nitrogen-doped carbonquantum dots (CQDs) prepared by microwave-assisted pyrolysis. In thepresence of C, the blue-shifted fluorescence intensity of the CQDsincreased. However, fluorescence quenching was observed upon the additionof 5-mC. This was primarily due to photoinduced electron transferas confirmed by the density functional theory calculation. In urinesamples, our sensitive fluorescent sensor had detection limits forC and 5-mC of 43.4 and 74.4 & mu;M, respectively, and achieved satisfactoryrecoveries ranging from 103.5 to 115.8%. The simultaneous detectionof C and 5-mC leads to effective methylation level detection, achievingrecoveries in the range of 104.6-109.5%. Besides, a machinelearning-enabled smartphone was also developed, which can be effectivelyapplied to the determination of methylation levels (0-100%).These results demonstrate a simple but very effective approach fordetecting the methylation level in urine, which could have significantimplications for predicting the clinical prognosis.