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. [Display omitted] •N-doped carbon dots prepared via hydrothermal route with different precursors.•Titrations allowed the evaluation of the acid-base behavior of N-doped carbon dots.•Colloidal stability of N-doped carbon dots can be monitored by the pH of the medium.•Optical properties of N-doped carbon dots are highly pH-dependent.•N-doped carbon dots showed large quantum fluorescent yield and good photostability.