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

•N-dots as Fluorescent Probe.•N-dots Synthesized by Microwave-Assisted Carbonization.•Determination of Nitrate/Nitrite using N-dots.•Optical Recognition NOX by Spectrofluorimetry.

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 HClO4 inside acetic acid was determined as the degree of acidity with a linear range between 1.0 and 4.0%. Determination of nitrate (NO3−) and nitrite (NO2−) based on the fluorescence quenching of N-CDs was also evaluated in detail. The linear ranges for NO2− and NO3− species were estimated to be from 1 × 10−7to 7.5 × 10−5 and from 2.5 × 10−6 to 7.5 × 10−4 mol L−1, respectively with RSD of 3.69% (n = 5) for NO2− and 3.54% (n = 5) for NO3−. The LODs (X+3Sb) for both NO2− and NO3− were estimated to be 2.5 × 10−8 and 7.5 × 10−7 mol L−1, respectively. The synthesized N-CDs were also applicable for NOX 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+3Sb) 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.

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