*2.5.2 Nitrogen and phosphorus co-doped graphene quantum dots (N,P-GQDs) as nanosensors*

Liu and co-workers prepared N,P-GQDs as fluorescence sensor for the detection of nitrite with high sensitivity and high selectivity [54]. The N,P-GQDs were synthesized by hydrothermal method using tetrakis(hydroxymethyl)phosphonium chloride and ethylenediamine endcapped polyethylenimine as phosphorus, carbon, and nitrogen source, respectively. The N,P-GQDs were prepared by using different temperatures (230° and 250°) and showed higher oxygen, nitrogen, and phosphorus levels at 230° compared to the those at 250°. The absolute quantum yield of N,P-GQDs was 9.4%. The N,P-GQDs showed a fast response to NO2 <sup>−</sup> with high sensitivity and high selectivity. The fluorescence quenching efficiency exhibited a good linear relationship with concentration of NO2 <sup>−</sup> within 5–30 nM. The detection limit was 2.5 nM. The results of MTT assays displayed over 90% cell viability by incubating N,P-GQDs with T24 cells for 24 h, suggesting good biocompatibility and imaging nitrite in live cell.

Ananthanarayanan et al. used carbonization strategy for the preparation of N,P-GQDs from biomolecule adenosine triphosphate (ATP) as nitrogen and phosphorus source [55]. Firstly, adenosine triphosphate (ATP) was carbonized for 1 h at 90° and got carbonized ATP; then the carbonized ATP was exfoliated in HNO3 for 24 h and got final product N,P-GQDs. The results of Raman spectrum characterization of carbonized ATP exhibited prominent D and G bands, indicative of the presence of sp3 carbon with graphitic nature. The N,P-GQDs have many advantages, such as excellent biocompatibility, good photostability, high fluorescence quantum yield (QY ∼ 27.5% by calculation and ∼53.0% after chemical reduction using NaBH4), and low molecular weight (∼1.4 kDa). The doping proportions of N and P are

6.2 (C/N = 7.0) and 6.9 (C/P = 6.3), respectively. The N,P-GQDs exhibited good two-photon upconversion properties. The strong upconverted photoluminescence phenomenon was showed with maximum emission at ∼560 nm upon excitation at 800 nm. The lifetime measurements of N,P-GQDs exhibited τ1 (*A*1) and τ2 (*A*2) to be 320 ps (0.44) and 1.62 ns (0.56), respectively, where τ1 and τ2 are time constants and *A*1 and *A*2 are the corresponding amplitudes. The imaging and real-time tracking of transferrin receptors in human cervical cancer cells came true upon conjugating N,P-GQDs with a transferrin.

Mahyari and Gavgani designed and constructed cobalt porphyrin-supported N,P-GQDs/graphene (CoPP@N,P-GQDs/G) complex as noble metal-free photocatalysts [56]. Firstly, the N,P-GQDs were synthesized by carbonization of adenosine triphosphate as nitrogen source and phosphorous source; secondly, the N,P-GQDs were embedded on graphene oxide; and thirdly, the cobalt porphyrins with photoactive property were loaded through ionic interaction. The resultant product CoPP@N,P-GQDs/G showed good dispersion in the reaction medium (water). The CoPP@N,P-GQDs/G complex as recyclable photocatalysts showed high efficiency with the aerobic oxidation reaction of alcohols by using visible-light irradiation. Furthermore, CoPP@N,P-GQDs/G complex displayed the good selectivity for various alcohols by mild and green ways.
