Nitrogen doping is a widely used method for enhancing the performance of carbon quantum dots (CQD). However, the precise relationship between nitrogen content and emission spectra remains unclear when preparing high-performance nitrogen-doped CQD (N-CQD). This study systematically investigates the effects of nitrogen content on the crystalline structure, optical properties, and electronic band structure of N-CQD. Citric acid was used as the carbon source, and ethylenediamine monohydrate was used as the nitrogen source, with their ratio controlled to hydrothermal synthesized N-CQD with N/C ratios ranging from 0 to 0.4. Notably, when the N/C ratio increases from 0 to 0.2, the N-CQD exhibits redshifted emission with excitation dependence. However, when the N/C ratio rises from 0.2 to 0.4, the N-CQD shows blueshifted emission with excitation-independence. We define it as the dual-phase emission behavior of N-CQD attributed to the transition of doping sites from graphitic nitrogen to pyridine nitrogen with increased nitrogen content. DFT calculations indicate that different doping sites influence electron transfer in N-CQD, resulting in distinct optical behaviors. Importantly, this work comprehensively explains the relationship between nitrogen content and the emission behavior of N-CQD for the first time, providing crucial insights for refining the theoretical framework of N-CQD.

Hepatocellular carcinoma (HCC), characterized by poor early diagnosis, exhibits the second-highest lethality rate among malignancies. This clinical challenge underscores the significance of Golgi protein 73 (GP73) as a promising serum biomarker for HCC detection. Herein, a ratiometric fluorescent aptasensor was constructed employing a dual-signal modulation strategy. The system integrated boron, nitrogen co-doped carbon quantum dots (BNCQDs) as the first fluorescent signal (I445) with a functional copper-based metalorganic framework conjugated with GP73-specific aptamer (Cu-MOF-Apt). The latter served dual functions: target recognition and peroxidase-mimetic catalyst for converting o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP), another fluorescent signal (I560). The peroxidase-like activity of Cu-MOF increased with the increase of GP73Apt attached. In the presence of GP73, target binding induced structural disintegration of Cu-MOF-Apt through GP73-Apt complex formation, thereby suppressing DAP generation. This target-responsive process led to the reduction of the fluorescence intensity of DAP and the increase of the fluorescence intensity of BNCQDs. Under the optimal conditions, the established ratiometric relationship (I445/I560 = 0.0007X + 0.7021, R2 = 0.997) enables quantitative detection of GP73 in the range of 25.00-600.00 ng/mL with the limit of detection (LOD) of 14.06 ng/mL. Clinical validation using serum specimens demonstrated excellent reproducibility (RSD 0.28 %-0.98 %) and recovery rates (99.75 %-107.49 %). The ratiometric fluorescent aptasensor's linear range effectively covers clinically relevant GP73 concentrations in HCC patients, while the robust serum analysis performance confirms its potential for practical diagnostic applications.

In this study, sustainable, low-cost, and environmentally friendly biomass (Terminalia chebula) was employed as a precursor for the formation of nitrogen-doped carbon dots (N-CDs). The hydrothermally assisted Terminalia chebula fruit-derived N-CDs (TC-CDs) emitted different bright fluorescent colors under various excitation wavelengths. The prepared TC-CDs showed a spherical morphology with a narrow size distribution and excellent water dispensability due to their abundant functionalities, such as oxygen- and nitrogen-bearing molecules on the surfaces of the TC-CDs. Additionally, these TC-CDs exhibited high photostability, good biocompatibility, very low toxicity, and excellent cell permeability against HCT-116 human colon carcinoma cells. The cell viability of HCT-116 human colon carcinoma cells in the presence of TC-CDs aqueous solution was calculated by MTT assay, and cell viability was higher than 95%, even at a higher concentration of 200 μg mL^-1 after 24 h incubation time. Finally, the uptake of TC-CDs by HCT-116 human colon carcinoma cells displayed distinguished blue, green, and red colors during in vitro imaging when excited by three filters with different wavelengths under a laser scanning confocal microscope. Thus, TC-CDs could be used as a potential candidate for various biomedical applications. Moreover, the conversion of low-cost/waste natural biomass into products of value promotes the sustainable development of the economy and human society.