Synthesis of luminescent glass monoliths embedding water-soluble Carbon dots

Carbon dots (CD) are an emerging class of nanomaterials, currently motivating an intense scientific interest because of their bright and characteristically tunable fluorescence, and their possible applications such as sensors, lasers, imaging agents, white light emitting devices [1]. While most studies focused on CDs in liquid phase, a strong effort is being recently devoted to produce fluorescent solids embedding highly dispersed CDs. Many of these procedures are elaborate and require pre-functionalization of the dots [2]. Here we report a novel and very facile route to prepare glass monoliths containing CDs with no need of pre-functionalization of the dots. Our low-cost synthesis method produces solids with high optical quality preserving the fascinating luminescence properties of CDs. We prepared CDs from an aqueous solution of citric acid monohydrate and urea, which was boiled up to the complete evaporation of water, yielding a black powder which was dried in vacuum at room temperature for 90 minutes. The obtained sample is highly soluble in polar solvents, especially water (up to at least 15 gL-1), these solutions being stable and highly fluorescent under near-UV or visible light. CDs were characterized via atomic force microscopy, thus obtaining the statistical distribution of heights (Figure 1). The resulting typical size of CDs (<5 nm) is consistent with previous reports for dots prepared by similar thermal decomposition procedures [1]. We also investigated CD nanopowders by infrared absorption: as expected, the vibrational spectrum around 1600 cm-1 (Figure 2) shows several peaks, which can be ascribed to amide and carboxyl surface functional groups according to the literature [3]. We successfully synthetized silica monoliths embedding CDs by sol-gel preparation at room temperature, whereby an aqueous solution of CDs was mixed with ethanol and TMOS (tetramethyl orthosilicate) to prepare the sol. Before gelification, an aliquot of the sol was moved to a sealed container used as a mold, and opened only after a few days. Then, the gels exhibited spontaneous shrinkage and became glasses (CD@SiO2) with very remarkable optical quality (Figure 3). In Figure 4 we show the typical optical absorption (OA) of CD@SiO2, which closely resembles that found for CDs in solution (not shown). By changing the initial concentration of CDs in the sol, their final amount in SiO2 can be controlled up to 0.4 mg/cm3. The OA of CD@SiO2 samples can be as large as 10 cm-1 (at 400 nm) while remaining highly homogenous (within 6.5%) across the samples, a few millimeter-sized. The main features of the fluorescence, such as the spectral shape and quantum yield (14%), are essentially unchanged in CD@SiO2 with respect to aqueous CDs. Besides the distinctive tunability of CD emission is preserved as well; indeed, as shown in Figure 4, the emission of CD@SiO2 shifts from 420 to 540 nm when the excitation is moved from 355 to 510 nm.