Abstract
Eu3+-modified carbon dots (C-dots), 3–5 nm in diameter, were prepared, functionalized, and stabilized via a one-pot polyol synthesis. The role of Eu2+/Eu3+, the influence of O2 (oxidation) and H2O (hydrolysis), as well as the impact of the heating procedure (conventional resistance heating and microwave (MW) heating) were explored. With the reducing conditions of the polyol at the elevated temperature of synthesis (200–230 °C), first of all, Eu2+ was obtained resulting in the blue emission of the C-dots. Subsequent to O2-driven oxidation, Eu3+-modified, red-emitting C-dots were realized. However, the Eu3+ emission is rapidly quenched by water for C-dots prepared via conventional resistance heating. In contrast to the hydroxyl functionalization of conventionally-heated C-dots, MW-heating results in a carboxylate functionalization of the C-dots. Carboxylate-coordinated Eu3+, however, turned out as highly stable even in water. Based on this fundamental understanding of synthesis and material, in sum, a one-pot polyol approach is established that results in H2O-dispersable C-dots with intense red Eu3+-line-type emission.
Highlights
Carbon dots (C-dots) have recently attracted considerable attention due to their unique properties (e.g., inexpensive nature, chemical stability, adaptable surface functionalization, high biocompatibility, intense photoluminescence (PL)) and a wide range of potential applications [1,2,3,4,5,6]
Following our previous work [13], we started with Eu3+ -modified C-dots that were prepared by conventional resistance heating of solutions of EuCl3 × 6H2 O in PEG400 (1 h, 230 ◦ C)
C-dots show variable blue and/or red emission depending on the conditions of heating
Summary
Carbon dots (C-dots) have recently attracted considerable attention due to their unique properties (e.g., inexpensive nature, chemical stability, adaptable surface functionalization, high biocompatibility, intense photoluminescence (PL)) and a wide range of potential applications (e.g., bioimaging/biosensing, optoelectronics, catalysis) [1,2,3,4,5,6]. Eu3+ - and Tb3+ -modified C-dots were prepared via the polyol method [15] by in situ thermal decomposition of the solvent (e.g., polyethylene glycol 400/PEG400) and showed excellent quantum yields for line-type red (75%) and green (85%) emission [13]. Eu3+ -modified C-dots is highly promising, several restrictions limit their use : (i) the PL is of limited reproducibility; (ii) the emission is rapidly quenched by humidity, which is not acceptable for biomedical application; and (iii) the achievable yield of C-dots is very low. Zhou synthesis et al havetechniques modified C-dots chemistry were suggested [17,18,19]. With this knowhow, a one-pot, MW-mediated polyol synthesis of Eu3+C-dots showing stable red emission water isinrealized.
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