ИСТИНА

Carbon nanoparticles (CNPs) – nanodiamonds (NDs) and carbon dots (CDs), particularly their subspecies graphene oxides – have a characteristic broadband fluorescence. Due to this fluorescence coupled with mechanical stability, non-toxicity, and easily modifiable surface, CNPs can be used as fluorescent biomarkers, drugs carriers, and adsorbents for biomedicine [1]. Results of many studies indicate the surface origin of CNPs’ fluorescence, but the mechanisms of which yet is not fully known. For both types of nanoparticles – NDs and CDs – the dependence on surface functionalization [2,3], “red edge” effect [4] and pH-dependence [5,6] of their fluorescence was shown. Based on the similarity of the mentioned characteristic and the fact that on the surfaces of NDs some sp2-hybridized carbon is almost always present, it has been suggested that this non-diamond phase of carbon is the source of surface fluorescence of NDs. However, the clear connection between the carbon phase and fluorescence of carbon nanoparticles has not been established. The aim of this work was the verification of the theory of the surface-“graphite” origin of the nanodiamonds’ fluorescence. To do this, the fluorescence of the aqueous suspensions of the following carbon nanoparticles with different sp2/sp3 carbon hybridization ratio were studied: Carbon dots (graphene oxides) (CD); Nanodiamonds decorated with carbon dots (CDND); CDNDs purified from sp2 -hybridized carbons by different methods; NDs as much as possible cleared of graphite carbon. The surface properties of CNPs and their relations with fluorescence were studied by the methods of vibrational spectroscopy – confocal Raman and IR adsorption spectroscopies. The changes of the fluorescence of CNPs with the change of the pH of the suspensions were observed, the quantum yields of their fluorescence were calculated. The obtained results support the proposed hypothesis. This study has been performed at the expense of the Russian Science Foundation (grant number 17-12-01481) (A.L., T.D. – data acquisition, data analysis), Basis Foundation (A.V. – data processing).