In the environmental, industrial, and biomedical fields, pH monitorization is of the most importance. However, the most used type of pH sensors, glass pH-electrodes, still present limitations in their application in low volume samples and in cellular pH sensing, due to their size and invasive nature. Fluorescence-based sensors present a solution to such issues, providing a non-invasive solution to pH sensing [1].

Herein, we report the rational development of carbon dots (CDs) as a pH nanosensor, via an active surface preservation (ASP) method. Carbon dots (CDs) are carbon-based fluorescent nanoparticles with valuable properties such as high aqueous solubility, low cost, low toxicity, and good biocompatibility, with remarkable fluorescence performance, been increasingly used as fluorescent nanosensors [2].

By employing ASP strategies, the CDs will be prepared by using precursors with known active functional features. The ASP method allows the nanoparticles to retain the structural features of precursors, thus retaining their properties, without the need for costly and time-consuming post-synthesis functionalization procedures [2].

In this work, as proof-of-concept, the known pH-sensitivity of fluorescein is used to provide to the CDs a pH-based response. The resulting CDs presented reversible response by fluorescence enhancement in the range of pH from 4 to 12. The nanoparticles exhibited excellent photostability, in different pH solutions. The studied CDs were also unaffected by, either variation of ionic strength or the presence of interferent species, while being compatible with human cells. Finally, CDs were able to determine the pH of real samples. Thus, a selective pH fluorescent CDs-based nanosensor was developed.

Acknowledgments: Projects PTDC/QUI-QFI/2870/2020 and UIDB/00081/2020.

References:
[1] Shamsipur M, Barati A, Nematifar Z (2019). J Photochem Photobiol C Photochem Rev 39:76–141
[2] Hou P, Yang T, Liu H, et al (2017). Nanoscale 9:17334–17341