A study on the synthesis of carbon dots by the bottom-up approach for various color emission

Abstract

Carbon dots that exhibited various color emission including blue, cyan, and multicolor emission (blue, cyan, yellow and red) were successfully synthesized via bottom-up approach. In addition, the as-prepared carbon dots were applied as chemical sensing of heavy metal ion and toxic organic compound. There were also proposed mechanisms for the formation of carbon dots, mechanism for quenching effect, and photoluminescence of multicolor emission.

First, highly fluorescent N and S co-doped carbon dots (CDs) were synthesized in one pot using caffeine as a raw material via a facile, green and cost-effective solid-state method. It is found that when caffeine and a mixture of caffeine and urea were heated, the CDs were not produced. However, when a mixture of caffeine and ammonium persulfate was heated, the CDs were fabricated exhibiting blue fluorescence with a quantum yield of 38%. In addition, the formation of CDs was enhanced with excellent optical properties when urea was added to the mixture of caffeine and ammonium persulfate. Without further chemical modification, the CDs emitted bright blue fluorescence with a high quantum yield as high as 69% due to the enhancement effect of N and S co-dopants on the surface of the CDs. Owing to the strong blue fluorescence, the CDs fabricated in this study could be used effectively as a fluorescence probe for Ag+ ions that exhibited high sensitivity and selectivity.

Then, a green, facile, and one pot hydrothermal process was employed for fabricating highly fluorescent N-doped carbon dots (NCDs) using pyromellitic acid (PA) as a novel precursor. It is found that under hydrothermally treatment process of a solution of PA and ethylenediamine (EDA), the NCDs were successfully fabricated exhibiting brightly pure cyan color emission. Due to the enhancement effect of N dopant on the surface of the NCDs, it exhibited an excellent fluorescent property with a quantum yield of 75%. Moreover, owing to its highly cyan fluorescence, the newly synthesized NCDs could be effectively applied as a fluorescence platform for sensitive and selective determination of 4-nitrophenol (4-NP) without further chemical modification. Our experiments shown that the calibration curve, obtained from a Stern-Volmer semilog plot, was linear within the range 0.1 to 100 μM 4-NP (R2 = 0.9993), and the limit of detection (LOD) was calculated to be 17 nM 4-NP following IUPAC standard (3.3sigma).

Lastly, multicolor emission carbon dots are fabricated via solvothermal method at mild temperature. In this process, pyromellitic acid is employed as a novel precursor and various diamines including aliphatic diamines (ethylenediamine; EDA) and aromatic diamines (ortho, meta, and para phenylenediamine; 2-, 3-, 4-PD) are employed as nitrogen sources. The synthesized carbon dots are exhibited multicolor emission including blue, cyan, yellow and red with a maxima PL emission at 409, 486, 554, and 615 nm when the additives changes from EDA to 3-, 2-, and 4-PD, respectively. The properties of these prepared carbon dots are studied and a hypothesis of their photo-luminescence is proposed.