Synthesis and Properties of Carbon Nanodots Obtained from Glucose and Urea by Microwave-Assisted Hydrothermal Carbonization

DARÍO D. FERREYRA; HERNÁN B. RODRÍGUEZ; MÓNICA C. GONZALEZ

Villa Carlos Paz

Encuentro; XIII Encuentro Latinoamericano de Fotoquímica y Fotobiología (XIII ELAFOT); 2017

Grupo Argentino de Fotobiología

Carbon nanodots (C-dots) comprise discrete, quasiespherical nanoparticles with sizes bellow 10 nm. Recently, great advances had been achieved on the application of C-dots as luminescent probes for biosensing, bioimaging, drug delivery and optoelectronic devices [1]. C-dots can be prepared from numerous simple organic precursors and renewable natural products through economical, user-friendly and large-scale processes [2]. However, there is still debate about the functional groups, structure, and morphology of these particles whose variations determine their fundamental properties. The understanding of the correlation between these factors is further complicated by the strong dependence of C-dots properties with the nature of precursor materials and synthesis methods.In the present work we explore a facile C-dots synthesis method based on hydrothermal carbonization of glucose via microwave pyrolisis. First, a glucose aqueous solution is mixed with a small amount of concentrated sulfuric acid and heated in a commercial microwave oven. The product (from yellow to dark brown solution to black residues, depending on power and reaction time) is then diluted/suspended in water, sonicated, and the C-dots extracted using 1-butanol as extraction solvent. After separation of the organic phase, the solvent is vacuum-evaporated and the particles resuspended in water and 0.45microm-filtered. Different reaction conditions, including variations in power, reaction time and concentrations of glucose and acid were tested. The incorporation of urea as a N-rich additive in the reaction mixture was also evaluated and the photophysical properties of the obtained nanoparticles studied as a function of the molar fraction of urea (Xu = nurea / (nurea + nGlu)). The nanoparticles were characterized by TEM, ATR-FTIR, UV-visible absorption, steady-state and time-resolved fluorescence spectroscopies, including determination of photoluminescence quantum yields (FiPL), and laser flash photolysis (LFP).Quasiespherical C-dots with sizes bellow 25 nm were obtained in all cases. They form stable aqueous suspensions showing photoluminescence in the blue-green region with an excitation wavelength dependent behavior, and decays in the order of ns. In the absence of urea (Xu = 0), FTIR evidenced the presence of carbon doble bonds, carbonyl, carboxyl, alcohol and ether groups, while FiPL ~ 1% was obtained, independent of the excitation wavelength (320-380 nm) and synthesis conditions. Including urea, and before Xu = 0.7, FiPL shows a maximum of about 2% at Xu = 0.2, with no significant changes on excitation-emission matrices. However, after Xu = 0.7, a large increase in FiPL was observed, peaking in Xu = 0.9 with FiPL ~ 10%, showing hypsochromic shifts both on excitation and emission spectra. For Xu = 0, a broad transient absorption spectrum, decaying in the μs time scale with maximum around 500 nm, appears both in the presence of molecular oxygen and after degassing with argon, while similar but less intense results were obtained for Xu = 0.9. The origin of the complex photophysical behavior of the C-dots obtained on increasing the urea content in the reaction mixture will be analyzed and discussed.[1] S. N. Baker, G. A. Baker, Angew. Chem. Int. Ed. 2010, 49, 6726.[2] K. Hola, Y. Zhang, Y. Wang, E. P. Giannelis, R. Zboril, A. L. Rogach, Nano Today 2014, 9, 590.