The paper presents the results of a study of the interaction of heavy metal cations Fe³⁺, Cr³⁺, Cu²⁺, Pb²⁺, Zn²⁺, Ni²⁺, Ag⁺ with carbon dots synthesized by the hydrothermal method in water. It was found that all the studied cations quench the fluorescence of nanoparticles to one degree or another. The series of cations were determined according to the degree of their influence on the fluorescence of carbon dots. The mechanisms of quenching the fluorescence of carbon dots upon interaction with cations have been investigated. It is shown that the quenching of the luminescence of nanoparticles in the presence of the investigated cations of heavy metals has a complex character. The observed changes in the intensity of fluorescence of carbon dots are the result of a superposition of several factors—electrostatic interactions of metal cations with surface fluorophores of carbon dots, aggregation of nanoparticles, and the effect of an inner filter for iron ions.

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.

Photopharmacology is a booming research area requiring a new generation of agents possessing simultaneous functions of photoswitching and pharmacophore. It is important that any practical implementation of photopharmacology ideally requires spatial control of the medicinal treatment zone. Thus, advances in the study of substances meeting all the listed requirements will lead to breakthrough research in the coming years. In this study, CQDs@phosphonate nanohybrids are presented for the first time and combine biocompatible and nontoxic luminescent carbon quantum dots (CQDs) with photoactive phosphonate enabling inhibition of butyrylcholinesterase (BChE), which is a prognostic marker of numerous diseases. The conjunction of these components in hybrids maintains photoswitching and provides enhancement of BChE inhibition. After laser irradiation with a wavelength of 266 nm, CQDs@phosphonate hybrids demonstrate a drastic increase of butyrylcholinesterase inhibition from 38% up to almost 100% and a simultaneous luminescence decrease. All the listed hybrid properties are demonstrated not only for in vitro experiments but also for complex biological samples, i.e., chicken breast. Thus, the most important achievement is the demonstration of hybrids characterized by a remarkable combination of all-in-one properties important for photopharmacology: (i) bioactivity toward butyrylcholinesterase inhibition, (ii) strong change of inhibition degree as a result of laser irradiation, luminescence as an indicator of (iii) bioactivity state, and of (iv) spatial localization on the surface of a sample.