Synthesis of a novel nitrogen-doped carbon dot by microwave-assisted carbonization method and its applications as selective probes for optical pH (acidity) sensing in aqueous/nonaqueous media, determination of nitrate/nitrite, and optical recognition of NO gas.

A novel nitrogen-doped carbon dot (N-CD) was synthesized via carbonization of citric acid in the presence of triethylenetetramine as a nitrogen source. The average size of the N-doped CDs and also the quantum yield of the synthesized N-doped CDs were both estimated to be 9 ± 2 nm and 39.5%, respectively. The applications of the synthesized carbon nanostructure as a high quantum yield fluorescence probe were initially adopted in the fabrication of a novel optical pH (acidity) sensor in both aqueous and nonaqueous environments. Two optimum dynamic intervals were obtained with the ranges of1.5-5.0 and 7.0-10.0. for the fabricated pH sensor with a standard deviation of 0.09 pH (n = 4). The quantity of HClO inside acetic acid was determined as the degree of acidity with a linear range between 1.0 and 4.0%. Determination of nitrate (NO) and nitrite (NO) based on the fluorescence quenching of N-CDs was also evaluated in detail. The linear ranges for NO and NO species were estimated to be from 1 × 10to 7.5 × 10 and from 2.5 × 10 to 7.5 × 10 mol L, respectively with RSD of 3.69% (n = 5) for NO and 3.54% (n = 5) for NO. The LODs (X+3S) for both NO and NO were estimated to be 2.5 × 10 and 7.5 × 10 mol L, respectively. The synthesized N-CDs were also applicable for NO recognition in the gaseous form at part per thousand (ppt) levels with linear ranges of 3.77-36.51 and 27.67-43.77 ppt, LOD (X+3S) of 1.41 ppt (n = 4) and RSD of 4.37% (n = 5). The reliability of these methods was also evaluated via the analyses of different forms of gaseous, water and rumen samples.