Multifunctional carbon dots for pH sensing, intracellular imaging, amino acid classification, and test strip...

Intracellular pH is a key indicator of cellular stability and plays a critical role in understanding biological processes and diagnosing various diseases. In this study, yellow-emitting carbon dots (C-dots) were synthesized via hydrothermal treatment of o-phenylenediamine and p-aminobenzoic acid. These C-dots exhibited pH-sensitive fluorescence, with intensity increasing as the pH ranged from 2.0 to 10.0, displaying a pK_a value of 5.14 and a linear response between pH 4.0 and 6.0. The observed pH sensitivity is mainly due to the abundant carboxyl groups on the C-dots' surface, which undergo protonation, deprotonation, and electrostatic transfer in response to pH changes. Additionally, these C-dots effectively detected both acidic and alkaline amino acids in aqueous solutions and on test strips. Notably, they were successfully used for intracellular imaging of amino acids, offering a significant advantage over toxic small-molecule reagents that can alter pH levels in living cells. These pH-sensitive C-dots have the potential to enhance applications in both analytical detection and biological imaging, expanding their use in variety of fields.