Nanocomposites and nanomaterials

Luminescent carbon nanodots stabilized by a polymer matrix for pH-sensing
Diyuk N. V., Zaderko O. M., Nadtoka O. M., Kutsevol N. V., Lisnyak V. V., Keda T. Ye.
Taras Shevchenko National University of Kyiv, Volodymyrs’ka str. 64/13, Kyiv-01601, Ukraine. E-mail: nvdiyuk@gmail.com

Carbon nanodots (CNDs) combine luminescent properties with photostability and low toxicity. They are of high promise for catalysis, optics, sensor systems, and medicine. The indisputable advantage of CNDs is their low cost and wide availability of raw materials for their synthesis.
In this work, we will focus on the characteristics of the CNDs used in analytical applications. We synthesized these unique CNDs from citric acid and urea at medium-high temperature and applying a differential route. The response of CNDs towards pH change was investigated by the PL method. We used the modification of polymer hydrogels to get composites for pH-sensing applications. Black, voluminous powders of CNDs were examined with thermal analysis and FTIR ATR, while their aqueous solutions were studied with IR, PL, and NMR spectroscopies.
It was shown, there is a band from 300 to 400 nm in the absorption spectra and a band with maximum at 450 nm in emission spectra of the CND solutions. The blue luminescence intensity increases with changing of CNDs concentration in solution from 1.5 to 9.010–7 %. The calculated quantum yield of CNDs is 26%. Commonly used indifferent electrolytes, e.g., the Krebs electrolytic solution, do not affect a luminescence signal. The luminescence of CNDs solutions enhances with acidity decreasing and reversibly diminishes in the range of pH from 2 to 7. 
Polyacrylamide-based polymeric (PP) hydrogels are good candidates for wound dressings in modern medicine. In our work PP hydrogels were impregnated with CNDs. It was shown, that electrolyte and water solutions could not remove the CNDs from the hydrogels matrixes. Immobilized CNDs keep their luminescent properties. Hence, proposed composites permit to visualize the changes in the acidity of the solutions and biological fluids.