Phenolic condensation and facilitation of fluorescent carbon dot formation: a mechanism study

Abstract

Fluorescent carbon dots have received considerable attention as a result of their accessibility and potential applications. Although several prior studies have demonstrated that nearly any organic compound can be converted into carbon dots by chemical carbonization processes, mechanisms explaining the formation of carbon dots still remain unclear. Herein, we propose a seed-growth mechanism of carbon dot formation facilitated by ferulic acid, a widespread and naturally occurring phenolic compound in the seeds of Ocimum basilicum (basil). Ferulic acid triggers the local condensation of polysaccharide chains and forms catalytic core regions resulting in nanoscale carbonization. Our study indicates that carbon dots generated from natural sources might share the similar mechanism of phenolic compound mediated nanoscale condensation followed by core carbonization.

Fig. 1

a) An optical image of basil seeds and their water-swelling behavior. b) Scanning electron image of the hydrocolloid of basil seed. c) Chemical structure of the representative polysaccharide in basil seed (i.e., glucomannan) d) Fluorescence images of the outer layer of an untreated basil seed. e) Fluorescence microscope images and f) optical images of a swelled basil seed cluster (λ_ex = 312 nm).

Fig. 2

a) FPLC analysis of physically extracted polysaccharide solution from basil seed. b) HPLC peaks (P1 and P2) from the collected (by FPLC) solution. MS data at c) P1 and d) P2. e) The direct-MS result of glucomannan solution.

Fig. 3

Optical properties and dimension of the carbon dots from seeds of Ocimum basilicum. a) Macroscopic fluorescent development of the basil seed solution during sulfuric acid-induced carbonization reactions observed under UV projection (λ_ex = 312 nm). b) The UV-Vis absorption spectrum (black line) of the carbonized carbon dots from basil seed polysaccharides and the corresponding photoluminescence spectra: Red (λ_ex = 320 nm), orange (λ_ex = 360 nm), green (λ_ex = 400 nm), blue (λ_ex = 440 nm), purple (λ_ex = 480 nm), pink (λ_ex = 520 nm) c) HPLC chromatogram of the ethanol extracted carbon dot products from basil seeds. The highlighted peak was further collected for TEM analysis. d) Characterization of the purified carbon dots by TEM. e) Statistical analysis of the size distribution of the carbon dots shown in the TEM image.

Fig. 4

a) UV-Vis absorption of natural glucomannan before carbonization (black), after carbonization (red), and the addition of ferulic acid to glucomannan followed by carbonization (blue). b) Photoluminescence (ë_ex = 400 nm) measurement of the carbonized glucomannan in the presence (blue) or absence (red) of ferulic acid. c) The schematic experimental procedures and the illustration in which carbon dots are doubly increased in presence of a trace amount of ferulic acid during carbonization, based on the spectroscopic results from UV-vis and photoluminescence. d) The GPC results of carbonized natural glucomannan in the absence (black) or presence of ferulic acid (red) and e) the corresponding schematic illustration to explain the role of ferulic acid. f) The representative AFM image with corresponding magnified image, and additional images of the carbonized polysaccharides from basil seed. Developed carbon dots from polysaccharides were marked as arrows and circles. g) Proposed seed-growth mechanism of carbon dots from natural polysaccharides.