pH-dependent photoluminescence of nanodiamonds and carbon dots: similarities and differences
Both nanodiamonds (NDs) and carbon dots (CDs) exhibit surface photoluminescence (PL). In combination with their properties such as biocompatibility and high dispersibility, stable photoluminesce provide carbon nanoparticles with broad prospects for their use in biomedicine. However, to successfully control the photoluminescence of nanoparticles, it is necessary to know the mechanisms of their surface PL. It is known that the surface photoluminescence of both NDs  and CDs  is caused by trap levels created by surface groups. Moreover, in publications [3,4], a hypothesis was put forward that the regions of sp2-hybridized carbon on the diamond surface, an analog of carbon dots, is the main source of NDs’ PL. This hypothesis implies that the photoluminescence mechanisms of NDs and CDs are common, and, as a result, the effects of external factors on PL of both types of carbon nanoparticles must be the same as well.
In this work, we investigated the similarity of the nature of the surface photoluminescence of NDs and CDs. For this purpose, we studied the changes in their photoluminescence properties with a change in the pH values of the environment. It was shown that the PL of both NDs and CDs depends significantly on pH. However, if in the dependences of the PL intensity of carboxylated nanodiamonds on pH there is a single intensity jump with the pH reaches certain values, such dependences for CDs are more complex and significantly depend on a particular sample. The observed differences are due to the difference in the functional groups of the studied nanoparticles. The stability of the diamond core allows additional cleaning and functionalization of the NDs surfaces, impossible for the CDs. As a result, the NDs surface covers are easier to control and more uniform than that of CDs surfaces; carboxyl groups dominating on the surfaces of the studied NDs are only one of the wide variety of surface groups of the studied CDs. Therefore, the mechanisms of the revealed features of the change in the photoluminescence of carboxylated nanodiamonds with a change in pH are only a part of the mechanisms of similar changes in carbon dots.
This study has been supported by Russian Foundation for Basic Research (Project 20-32-70150).
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Vervald A. M.
Faculty of Physics, M.V. Lomonosov Moscow State University
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