Facile synthesis of nitrogen-doped carbon dots with robust fluorescence in a strongly alkaline solution and a reversible fluorescence ‘off–on’ switch between strongly acidic and alkaline solutions

Abstract

In this study, hydrophilic nitrogen-doped carbon dots (N-CDs) have been hydrothermally prepared using citric acid and various concentrations of ammonium hydroxide. The N-CDs exhibit an excitation-independent blue emission and an optimal quantum yield of 24.2%. Under strongly alkaline conditions adjusted by various alkaline solutions, the N-CDs exhibit robust fluorescence performance, even at an extreme pH value of 13. Although the fluorescence is quenched in strongly acidic solutions (pH = 1), the N-CDs exhibit reversible and repeatable ‘off–on’ fluorescence on modulating the pH value between 1 and 13, resulting from the protonation and deprotonation of the fluorescence-related surface states. Moreover, the N-CDs maintain a bright and stable emission in simulated body fluid (SBF). Thus, the N-CDs have promising applications for dynamic detection of pH switching caused by strongly acidic and strongly alkaline conditions and fluorescence bio-imaging of living organisms even under extreme pH conditions.