## 3 Results and Discussion
### Detection of the Concentration of Fe (III) by N-CQDs  
Figure 5a demonstrates that the fluorescence peak in the spectra of N-CQDs solutions reduces with increased concentrations of Fe (III). The fluorescence peak in the Fe (III) concentration range of 200 to 1000 \(\upmu\)mol/L exhibits reduced peak intensities and shorter emission wavelengths compared to the range of 0.1 to 100 \(\upmu\)mol/L, along with a weakened correlation between fluorescence intensity and Fe (III) concentration. The peak emission wavelength in the fluorescence spectra across the Fe (III) concentration range of 0.1 to 100 \(\upmu\)mol/L remains mostly invariant, whereas the fluorescence intensity exhibits a notable correlation with the Fe (III) concentration. Figure 5b demonstrates that the ratio \(F_{0}/F\) (with \(F_{0}\) represents the fluorescence peak value of a blank N-CQDs solution at a concentration of 0.25 mg/mL, and \(F\) represents the fluorescence peak values of N-CQDs solutions at varying Fe (III) concentrations) exhibits a strong linear correlation with Fe (III) concentrations ranging from 0.1 to 100 \(\upmu\)mol/L. As shown in the inset of Figure 5b, the linear equation is expressed as \(y=0\). 0073\(x\)+1.0040, with the \(R^{2}\) value of 0.9893 (where \(y\) represents the ratio of \(F_{0}\) to \(F\), \(x\) represents the concentration of Fe (III), and \(R^{2}\) represents the standard deviation of the linear equation). By combining Formula (2) with the detection limit calculation formula \(D=3\delta/k\), the detection limit of Fe (III) is determined to be \(D=4.1\)\(\upmu\)mol/L.  
It can be observed from Figure 5c(1) that when Fe (III) concentration ranges from 200 to 1000 \(\upmu\)mol/L, flocculation begins to appear in the originally suspension, and colorless transparent supernatant begins to appear in the solution after standing for 30 min. In Figure 5c(2), it can be observed that N-CQDs solution has no significant change when the concentration of iron ion ranges from 0.1 to 100 mol/L. However, its green fluorescence under ultraviolet light is weaker than that of blank samples. As shown in Figure 5c(3)(UV), the fluorescence effects of some 200 to 1000 mol/L samples are similar. When exposed to natural light and ultraviolet light (Figure 5c(3)), the colorless and transparent supernatant of the former displayed a distinct blue luminescence under ultraviolet light, whereas the latter continued to emit green light, with its fluorescence intensity being weaker than that of the blank sample shown in Figure 4d. In accordance with Scheme 1b, Fe (III) not only inhibits the fluorescence performance of N-CQDs but also induces aggregation among them. Based on the impact of N-CQDs size on fluorescence performance, it can be inferred that  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Serial Number** & **Precursor** & **Synthesis Method** & **Mass Yield/\%** & **LOD/\(\upmu\)mol/L** & **Reference** \\ \hline
1 & 2,7- & & One-step hydrothermal & 70.9 & none & [59] \\  & & Citric acid and & & & & \\
2 & 4,7,10-trioxa-1,13- & Microwave assisted & 66.4 & none & [60] \\  & & Acid pretreatment/hydrothermal & 45.8 & none & [61] \\  & & synthesis & & & \\
4 & Citric acid & Straightforward & 45 & none & [62] \\
5 & Glucose & Hydrothermal & 44.3 & 1.3–106.7 & [20] \\
6 & Lignin, cellulose & Solvothermal & 42.5 & 0.085 & [63] \\
7 & Ammonium citrate & Hydrothermal & 34 & none & [64] \\
8 & Poplar leaf & Hydrothermal & 30 & none & [65] \\
9 & Cellulose & Hydrothermal & 16.1 & 1.14 (Fe\({}^{3+}\)) & [19] \\
10 & Durian pulp & Hydrothermal & 6.8 & 3.5 & [11] \\
11 & Food waste, ethanol & Hydrothermal & 0.12 & none & [66] \\
12 & Poplar wood & Hydrothermal & 42 & 4.1 (Fe\({}^{3+}\)) & This paper \\ \hline \end{tabular}
\end{table}
Table 2: The summary of carbon sources, preparation methods and mass yield of CQDs.  
Fe (III) preferentially aggregates larger-sized N-CQDs. Conversely, solutions containing smaller-sized N-CQDs that remain unaggregated exhibit stronger blue fluorescence than those doped with a lower concentration of Fe (III).