In this study, the up-conversion luminescence for aqueous suspensions of carbon dots with polyfunctional and carboxylated surfaces synthesized by a hydrothermal method was investigated. The obtained quadratic dependence of the luminescence intensity on the power of the exciting radiation indicates that the up-conversion luminescence of these carbon dots is caused by two-photon absorption. The optimal wavelength of the exciting radiation was determined for the studied samples. The dependences of the signal for the up-conversion luminescence of carbon dots on the pH value of the suspension were obtained. It was shown that these carbon dots can be used as the nanosensor of pH of liquid media in a wide range of pH values. The advantage of this nanosensor is that the excitation of the up-conversion luminescence of carbon dots does not entail excitation of autoluminescence of the biological medium. It expands the possibilities of using this sensor in biomedical applications.
In this study carbon dots synthesized by the hydrothermal method and subsequent functionalization of the surface with carboxyl and hydroxyl groups were explored. As a result of studying the optical and colloidal properties of monofunctionalized carbon dots, a significant effect of the acidity of the nanoparticle environment on them was found. It has been found that for the photoluminescence quantum yield of carbon dots the greatest changes occur in the pH range from 2 to 5 for carbon dots with carboxyl surface groups and from 8 to 12 for carbon dots with hydroxyl groups. The mechanisms of the influence of surface functional groups on the photoluminescence of carbon dots with a change in the pH of the suspension are proposed.
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