LI Hu，YUAN Changzhou 

School of Materials Science and Engineering， University of Jinan， Jinan 250022， China

Abstract

Objective Excessive Fe³⁺ causes environmental degradation and a decline in human health. Therefore， developing a simple and sensitive method for Fe³⁺ detection is crucial. As a new material for fluorescence-based methods， carbon dots have attracted significant attention due to their high sensitivity， selectivity， and low cost. Flos puerariae， a legume， contains abundant hydroxyl and amino molecules， making it an ideal precursor for carbon dot preparation.

Methods Flos puerariae carbon dots were synthesized via a hydrothermal method. The physicochemical properties and optical performance of these carbon dots were characterized in detail using transmission electron microscopy， X-ray diffraction， Fourier-transform infrared spectroscopy， X-ray photoelectron spectroscopy， ultraviolet-visible （UV-vis） spectrophotometry， and fluorescence spectrophotometry. The morphology， particle size， crystal structure， elemental composition， surface functional groups， photoluminescence properties， and optical stability of these carbon dots were studied. Their application potential in metal ion detection， invisible fluorescent ink， and implication logic gates was systematically investigated. In addition， their fluorescence quenching and reversible recovery mechanisms were studied.

Results and Discussion The flos puerariae carbon dots exhibited an uneven quasi-spherical shape， with a particle size ranging from 1 to 3 nm and an average diameter of 2.52 nm. X-ray diffraction analysis revealed high disorder in these carbon dots. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that these carbon dots contained abundant carboxyl， hydroxyl， carbonyl， and amino groups， with elemental composition of 67.78% C， 8.41% N， and 23.81% O. Flos puerariae carbon dots exhibited excellent photoluminescence properties and optical stability. The optimal excitation wavelength was 365 nm， and the optimal emission wavelength was 444 nm. The carbon dots maintained good fluorescence stability under long-term 365 nm UV irradiation， varying temperatures， high-ion concentrations， or long-term room temperature storage. Invisible fluorescent ink was prepared using the carbon dot aqueous solution. This ink was used to write words and sailboat patterns， and to spray bird patterns. The handwritten and spray-painted patterns were invisible under normal sunlight but emitted bright blue light and showed clear patterns under 365 nm ultraviolet light. The carbon dots showed obvious concentration dependence and good resistance to pH changes. The fluorescence intensity of the solution was most stable at pH 7 with a 20-fold dilution， which was suitable for metal ion detection. When 13 metal ions and 12 interference components were added to the solution， only Fe³⁺ caused significant fluorescence quenching. The carbon dots exhibited a sensitive concentration response to Fe³⁺ with a limit of detection （LOD） of 1.897 9 μmol/L， which was lower than the national minimum standard in China. Using the Stern-Volmer equation， the interaction between carbon dots and iron ions was found to follow the pattern of static quenching. Moreover， the carbon dots showed strong resistance to interference， with PO₄^3- and ascorbic acid （AA） causing fluorescence recovery. AA completely reduced Fe³⁺ to Fe²⁺， leading to full fluorescence recovery. An implication logic gate was designed based on the reversible fluorescence switching characteristics of flos puerariae carbon dots， demonstrating good reversibility.Conclusion Nitrogen-doped blue fluorescent flos puerariae carbon dots are successfully synthesized using flos puerariae as the precursor. Flos puerariae carbon dots exhibit a quasi-spherical shape and good dispersion， with abundant functional groups on the surface. These carbon dots possess good optical properties and stability， making them suitable for preparing invisible fluorescent ink. Flos puerariae carbon dots demonstrated good selectivity， sensitivity， and anti-interference ability in Fe³⁺ detection. The static quenching caused by Fe³⁺ can be fully restored by AA. Flos puerariae carbon dots have been successfully used to design implication logic gates due to their reversible fluorescence switching characteristics.

Keywords： carbon quantum dot； fluorescent material； metal ion detection； fluorescent ink； logical gate