pH-Dependent surface properties of N-Cdots obtained by the hydrothermal method with multicolored emissions

This study addresses the pH-dependent surface properties of hydrophilic nitrogen-doped carbon dots (N-Cdots) elaborated through a hydrothermal method. The samples were obtained using p-phenylenediamine and urea (N-CdotR), and citric acid and ammonium hydroxide (N-CdotB) as precursors, respectively. The structural and morphological properties of the N-Cdots were investigated using TEM, Raman spectroscopy and FTIR. The nanoparticles of both samples exhibited a large number of hydrophilic surface groups. Potentiometric and conductometric titrations were used to determine the pKs of the N-Cdots surface groups (pK1 = 3.4 and pK2 = 6.6 for N-CdotR; pK1 = 7.6 and pK2 = 9.9 for N-CdotB). Measurements of zeta potential and hydrodynamic diameter as a function of pH indicated that the charge modulation and colloidal stability of the N-Cdots are controlled mainly by amine and carboxyl groups in N-CdotR and carboxyl and phenolic groups in N-CdotB. The optical properties of the N-Cdots were characterized using UV-Vis spectroscopy. The photoluminescence results indicated that the acid-base behavior of the surface groups promotes changes in the energy emission states, with blue- and redshifts. Moreover, the N-Cdots presented significant quantum fluorescent yield (QY = 25% for N-CdotR and QY = 30% for N-CdotB) and good photostability. The whole of these results demonstrates that pH is a key parameter to monitor the colloidal stability and optical properties of N-Cdots as well as their potential technological applications.

Automated summary

This study investigated the pH-dependent surface properties of hydrophilic nitrogen-doped carbon dots prepared through a hydrothermal method. The results show that pH plays a crucial role in controlling the colloidal stability and optical properties of N-Cdots.