Photoexcited state properties of carbon dots from thermally induced functionalization of carbon nanoparticles

Abstract

Carbon dots are small carbon nanoparticles with various surface passivation schemes, in which more effective has been the deliberate chemical functionalization of the nanoparticles for brighter fluorescence emissions, though the synthesis method is more tedious and subject to some limitations in the selection of functionalization molecules. Another more popular synthesis method has been the carbonization of organic species, with the method being more efficient and versatile, but less controllable in the synthesis and for the desired dot structure and performance. In this work, a hybrid approach combining the advantageous characteristics of the two synthesis methods was applied to the preparation of carbon dots with polyethyleneimine (PEI) for surface passivation, where pre-processed and selected small carbon nanoparticles were functionalized with PEI in microwave-induced thermal reactions. The optical absorption and fluorescence emission properties were evaluated, and the results suggested that the carbon dots thus prepared shared the same photoexcited state characteristics with those from the deliberate chemical functionalization, including comparable fluorescence colors and other properties. A further demonstration on the similarity in photoexcited state properties was based on the same visible light-activated bactericidal functions of the PEI-carbon dots as those found in carbon dots from the deliberate chemical functionalization. The advantages and potential limitations of the hybrid approach for more controllable yet versatile and efficient syntheses of carbon dots are highlighted and discussed.

Correlation of carbon dots' light-activated antimicrobial activities and fluorescence quantum yield

“…These fractions are very stable aqueous solutions, without any precipitation. As shown in Figure 1, the absorption and fluorescence spectra of the different fractions are similar, consistent with what have generally been observed in the fractionation of carbon dots (22, 23). However, the observed fluorescence quantum yields (Φ F ) were significantly different, 7.5%, 17%, and 27% for the three fractions at 400 nm excitation, again consistent with what have found generally in other studies concerning the fractionation of carbon dots (22).…”

Modified facile synthesis for quantitatively fluorescent carbon dots

“…However, the PEI/CA-CDots obtained from the hydrothermal processing in this work exhibit different absorption features from those of the recently reported PEI-CDots prepared by the oligomeric PEI functionalization of pre-processed and selected small carbon nanoparticles (Figure 3). 38 Interestingly, the extra absorption contributions of the PEI/CA-CDots in the blue spectral region are similar to those found in absorption spectra of CDots prepared by the deliberate chemical functionalization of small carbon nanoparticles with oligomeric poly(ethylene glycol) diamine (PEG 1500N -CDots) 25 and 3-ethoxypropylamine (EPA-CDots, Figure 3). 39 In fact, as also reported recently, 39 the extra absorption band in the blue for EPA-CDots (and similarly for PEG 1500N -CDots) is correlated with the high fluorescence quantum yields in these CDots.…”

“…Similar mixtures were found for as-prepared samples of CDots from other syntheses. 25,26,38–40 For the EPA-CDots and PEG 1500N -CDots as examples, the separation of the as-synthesized samples on an aqueous gel column (Sephadex G-100) yielded fractions of significantly higher fluorescence quantum yields. 25 The same gel column and separation protocol were applied to the fractionation of the as-prepared samples of PEI/CA-CDots, with some of the collected fractions reaching fluorescence quantum yields of 85–90%.…”

“…27,33 Also for comparison are the spectra of the PEICDots (-.-.-.-) 38 and the absorption of aqueous dispersed small carbon nanoparticles (·······). 37 …”

“…39 Also for comparison is the absorption of the PEI-CDots (-.-.-.-) 38 (the corresponding fluorescence spectrum already shown in Figure 2). Inset: Photos of a dilute aqueous solution of the PEI/CA-CDots under ambient room light (left) and under sunlight (right).…”