Green pomelo, navel orange, and tangerine were purchased from a fruit shop in Beijing, China. CoCl2, Ba(NO3)2, AgCl, MgCl2, CuSO4, NaCl, CaCl2, ZnCl2, CdCl2, NiCl2, and FeSO4·6H2O were acquired from Tianjin Chemical Reagent (Tianjin, China). MnCl2, FeCl3·6H2O, and AlCl3·6H2O were obtained from Aladdin. The 15 amino acids used in the study were obtained from Alfa Aesar (Tianjin) Chemical. 1-Allyl-3-methylimidazolium chloride (AmimCl) ionic liquid was acquired from Sigma-Aldrich. Dialysis bags (1000 Da molecular weight cutoff, MWCO) were supplied by MYM Biological Technology. All reagents and chemicals used in the experiments were of analytical grade and not further purified. Tap water sample was obtained from our laboratory.
All fluorescence spectra were obtained by Synergy H1 full-function microplate reader (BioTek, USA). Transmission electron microscopy (TEM, FEI, USA) images were obtained using a Talos F200X electron microscope. Fourier transform infrared (FTIR, Nicolet Instruments, USA) spectra (400–4000 cm−1) were obtained on IS10 spectrometer. X-Ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA) was carried out by ESCALAB 250Xi spectrometer. Fluorescence lifetime was recorded via FLS1000 fluorescence spectrometer (Edinburgh Instruments, UK). X-ray diffraction (XRD, Bruker, Germany) analysis using a D8 ADVANCE instrument using Cu Kα (λ = 0.15405 nm).
Synthesis of CQDs
The CQDs were derived via one-step hydrothermal method using waste green pomelo peel as the carbon source. Dried green pomelo peel was ground into powder in the mortar and 0.5 g of powder was dispersed in deionized water of 50 mL. Transfer the mixture to the polytetrafluoroethylene-lined stainless steel autoclave of 100 mL. After heating for 5 hours at 180 °C, then cool to ambient temperature naturally and the suspension was centrifuged at 10000 rpm for 10 min to obtain a yellow-brown supernatant. It was then filtered through a 0.22 um cellulose filter membrane and dialyzed for 24 hours using deionized water (MWCO: 1000 Da), deionized water was changed every 8 hours. Subsequently, the CQDs were obtained by evaporation of the solvent and placed in the vacuum drying oven for further drying.
For comparison, navel orange peel and tangerine peel were selected as alternative raw materials to synthesize CQDs. The same experimental steps were repeated, and the corresponding tests of fluorescence intensity were performed.
QY of CQDs
The QY of the obtained CQDs were determined by comparison method with a reference substance of quinine sulfate (QY = 56% in 0.1 M H2SO4) (Jiang et al. 2021). The QYCQDs was calculated according to equation (1):
where F represents the integral of the fluorescence intensity,A is the absorbance at the excitation wavelength, η represents the refractive index of the solvent(1.33 for water), and subscripts R represents the reference substance quinine sulfate.
Fluorescence stability of CQDs
To evaluate fluorescence stability of the CQDs in different solvents, the CQDs were dispersed in deionized water, ethanol, acetic acid, and PBS buffer, respectively. Add aliquots of these solutions (200 μL) to 96-well plates to measure the fluorescence intensity. For a continuous investigation of fluorescence stability, the fluorescence intensitiy of CQDs was measured every 3 days for 30 days at ambient temperature. The effect of different irradiation time (5, 10, 20, 30, 40, 50, 60, 70, 80, 90 min) with ultraviolet (UV) light was performed in a UV light box at 356 nm. The effect of different pH values on fluorescence intensity was investigated from 3 to 10. Aliquots (100 μL) NaCl solutions of different concentrations (10, 20, 50, 100, 150, 200, 300, 400, 500 mM) were added into 100 μL of CQD solution, respectively, and the fluorescence intensities were recorded.
Selectivity and interference measurements of CQDs
In the selectivity experiment, 100 μL of CQDs (0.01 mg·mL−1) solution was added to the 96-well plate. Aliquots of individual metal ion solution (Mg2+, Ag+, Zn2+, Cd2+, Co2+, Na+, Al3+, Fe2+, Fe3+, Cu2+, Ni2+, Ca2+ , Mn2+, and Ba2+, 100 μL, 1 mM) were respectively mixed with CQDs solution, then the fluorescence intensity was measured, add 100 μL of Fe3+ solution to the CQDs solution, and measured the fluorescence intensity again. The interference of metal ions on Fe3+ detection by CQDs was determined by the relative fluorescence intensities of CQDs solution containing different metal ions before and after adding Fe3+ solution.
Sensitive detection of Fe3+ and L-Cys
In the Fe3+detection experiment, 100 μL of Fe3+ solutions of different concentrations (0.1–160 µM) were added into aqueous CQDs (100 μL, 0.01 mg·mL−1) solutions, then 100 μL aliquots of different metal ions (1 mM) were added into the CQDs solutions for comparison. Furthermore, the states of the CQDs in Fe3+ solutions with different concentrations were recorded under sunlight and UV light.
Added 100 μL of L-Cys solutions (0.4–1 mM) to the mixed solutions of CQDs (100 μL, 0.01 mg·mL−1) and Fe3+ (100 μL, 4 ×10−4 mol·L−1) for L-Cys detection. After sufficent mixing, recorded the fluorescence intensities immediately (equilibration time 10 s). The fluorescence intensities of all samples in the experiment were recorded in the wavelength range of 360–700 nm, and the excitation wavelength was 330 nm. The optical behavior of the CQDs in the presence of the mixture was recorded under sunlight and UV light.
Tests were also performed with other amino acids in the mixture of CQDs and Fe3+ under the same conditions. Measurements of sensitivity and selectivity were repeated three times.
Fe3+ and L-Cys detection in real samples
Fe3+ was dissolved into the tap water to give different concentrations of 50, 100, and 400 µM for detection (excitation at 360 nm). For L-Cys detection, the amino acid beverage sample was diluted 100 times, and then mixed with CQDs solution containing Fe3+. Aliquots (200 μL) of 10, 30, and 60 μM mixtures of amino acid beverage sample were added into 96-well plates for detection (excitation at 360 nm). Preparation of cellulose/CQDs composite hydrogels
The Cotton pulp (2.5 g) was mixed into 100 g of AmimCl and stirred vigorously rapidly at 80 °C to obtain a homogeneous solution, then placed in a vacuum oven for 4 hours to remove air bubbles. The solution was then poured slowly into 12-well plate, which was subsequently put into an AmimCl/H2O (v/v = 1/1) coagulation bath for 24 hours. Completely remove AmimCl ionic liquid by ethanol/H2O (v/v=1/1). The prepared cellulose hydrogels were soked in CQDs aqueous solution for 24 hours to obtain CQDs composite hydrogels.
Preparation of CQDs-based test papers
The aqueous solution of CQDs was brushed onto strip-type filter papers, which were previously treated with dimethylformamide to remove the luminescent substances. The fluorescein-free filter paper was soaked overnight in the CQDs aqueous solution. The solvent was evaporated thoroughly to obtain the CQDs-based test papers. Fe3+ solution and L-Cys were dripped onto the filter papers and the digital photo of the test papers was acquired in a UV light box (365 nm).