A highly fluorescent nitrogen and sulfur codoped carbon nanoparticles (N,S-CNP) sample was obtained by microwave-assisted pyrolysis of citric acid and L-cysteine. After being purified by dialysis, the complexity and chemical composition of N,S-CNP were evaluated by ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS) as well as by UPLC coupled with ultraviolet (UV) absorption and fluorescence detection (UPLC-UV/FLD) methods. By using the high-resolution UPLC separation, the N,S-CNP were well fractionated into six fractions within 3.5 min. Based on high-accuracy MS and tandem (MS/MS) analyses, the N,S-CNP species were revealed to display various chemical formulas, including (C12H16N2O7S2) n , (C9H13NO8S) n , (C18H20N2O14S2) n , (C18H20N2O12S2) n , (C9H11NO5S) n , and (C9H11NO7S) n . More importantly, our study disclosed unambiguously for the first time that the N,S-CNP species exist as supramolecular clusters with their individual monomer units linked together through noncovalent bonding forces. By using UPLC-UV/FLD analysis, the spectral characteristics of each N,S-CNP species were revealed. Each individual CNP species possesses its unique absorption and PL properties with absorption bands that are redshifted, whereas its emission bands are blueshifted with its elution order. This work highlights the merit of UPLC-MS together with UPLC-UV/FLD to investigate the chemical composition and the spectral properties of each individual N,S-CNP species. It is anticipated that our proposed methodology will open up a new venue in optimizing experimental conditions for producing specific N,S-CNP species of desired composition. Graphical Abstract Carbon nanoparticles synthesized by microwave-assisted pyrolysis of citric acid and L-cysteine exist as supramolecular clusters with their individual monomer units linked together by noncovalent interactions.

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