Effect of pH and ionic strength on the photoluminescence of size-fractionated AgInS$_2$/ZnS quantum dots

Background and Objectives: Cellular labeling with fluorescent molecules appears to be one of the key methods of cell biology that continues to evolve with the advent of new fluorescent probes possessing unique properties. Ternary AgInS$_2$/ZnS quantum dots occupy a special position compared to other fluorescent molecules due to their size-adjustable photoluminescence combined with broadband excitation and long emission lifetime. For the use of quantum dots of AgInS$_2$/ZnS composition as a fluorescent probe in in vitro applications, they should have low physiological toxicity and good stability in physiological pH range. The objective of this work is therefore to evaluate the change of photoluminescent properties of AgInS$_2$/ZnS quantum dots with changing pH of the medium and ionic strength. Materials and Methods: To evaluate the effect of pH and ionic strength on the photoluminescence properties of AgInS$_2$/ZnS quantum dots, a size-selective precipitation procedure was carried out and the photoluminescence and absorption spectra of the quantum dot fractions were analyzed. Results: Ternary photoluminescent AgInS$_2$/ZnS quantum dots stabilized in water by thioglycolic acid have been obtained by direct synthesis. Size-selective precipitation allowed to discriminate of 11 AgInS$_2$/ZnS quantum dots fractions from the initial ensemble, revealing distinctly various optical properties. The effect of different pH and ionic strengths on the photoluminescent properties of AgInS$_2$/ZnS quantum dots fractions has been studied. While in strong acidic and basic media the dramatic changes have been observed, the pH and ionic strength range corresponding to the biological fluids has shown no significant influence on the photoluminescent properties of all quantum dots fractions. Conclusion: This indicates the potential application of these nanoobjects as photoluminescent probes in various bioapplications.