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. 2025 Feb 28;5(3):1350–1358. doi: 10.1021/jacsau.4c01220

Control Synthesis of Multicolor Emitting Carbonized Polymer Dots Using Different Dihydroxynaphthalene Isomers

Xingzhong Chen 1, Fang Yan 1, Jiurong Li 1, Xiao Gong 1,*
PMCID: PMC11938013  PMID: 40151271

Abstract

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Carbonized polymer dots (CDs) with sizes smaller than 10 nm have gained significant interest, particularly in the pursuit of multicolor fluorescence, which is a fascinating field of research. Here, we report the control synthesis of multicolor CDs with tunable emissions via the rational selection of isomer-based reaction precursors. Using cost-friendly dihydroxynaphthalene (DHN) isomers (1,7-DHN, 1,6-DHN, or 2,7-DHN) and l-methionine as the precursors, multicolor CDs with blue, green, and red fluorescence were obtained under ethanol-thermal conditions. It can be observed that structural changes in the coordination of the hydroxyl group within the aromatic compound DHN lead to a significant redshift in the emission of the synthesized CDs. Notably, the red-emitting CDs (r-CDs, 10 μM) at very low concentrations exhibit high sensitivity to tetracycline (TC), and the ratiometric fluorescent probe based on r-CDs enables quantitative and visual detection of TC. Compared with ratiometric fluorescent probes containing metal and rare earth elements, CDs-based ratiometric fluorescent probes are more cost-effective and environmentally friendly.

Keywords: carbonized polymer dots, isomer, multicolor, tetracycline, fluorescent probes

Introduction

Tetracycline (TC) is widely applied in the treatment of human and animal diseases owing to its antimicrobial effectiveness against both gram-positive and gram-negative bacteria.1−4 Consequently, antibiotics like TC have been used as broad-spectrum antimicrobial agents in livestock, poultry, and human medical industries for a long time.5−7 Due to this, overuse has led to the detection of various antibiotics in processed waters and aquatic environments. TC contamination in these environments can promote bacterial resistance, disrupt ecological water balance, and pose health risks to humans through drinking water sources.8 Thus, developing effective methods for detecting TC in wastewater is essential.

Commonly used methods for the detection of TC include high-performance liquid chromatography, microbiological methods, and mass spectrometry.9,10 However, these methods are more suitable for laboratory validation of results rather than on-site collection and analysis, as they include time-consuming, expensive, and cumbersome instrumentation drawbacks.11 Therefore, establishing a simple, accurate, and visualizable method to detect TC is valuable. Fluorescent sensors are considered to be an ideal method for in situ detection of TC due to their ease of handling, visualization, and high sensitivity.12−16

Carbonized polymer dots (CDs), as an emerging carbon material with excellent fluorescent properties, offer advantages such as good biocompatibility, cost-effective raw materials, and easy fabrication.17−19 Notably, CDs are characterized by a special surface state, abundant surface functional groups, and binding sites, enabling them to be widely used in many fields such as fluorescent nanoprobes and sensing.20−24 Current research on CDs primarily focuses on tuning intrinsic energy levels, which are related to particle sizes, and the regulation of the surface states by doping associated with ligands and defects.25 However, there are a few reports on the controllable synthesis of multicolor CDs by isomer-based reaction precursors. Therefore, it remains an interesting task to synthesize multicolor CDs using different isomers.

In this work, the emission of CDs could be modulated by altering the isomeric structure of aromatic compounds. Dihydroxy naphthalenes (DHNs) were selected as the precursor due to their abundant hydroxyl functional groups on conjugated structural naphthalene rings, which favor the synthesis of CDs with diverse surface states. Under consistent reaction conditions, multicolor CDs were obtained by mixing cost-friendly 1,7-DHN, 1,6-DHN, or 2,7-DHN with l-methionine and ethanol. It was observed that the structural differences among the DHN precursors influenced the degree of oxidation on the surface of the CDs, leading to CDs with distinct fluorescence properties. Among them, the red CDs (r-CDs) exhibit obvious bimodal emission, whose peaks display varying degrees of reactivity in response to different TC concentrations. Given this, rapid visual detection and quantification of trace TC occur through changes in the intensity ratio of different peaks and the color shift of the r-CDs/TC solution.

Results and Discussion

The DHN isomer is characterized by a large aromatic conjugated structure and an abundance of hydroxyl groups, which helps to modulate the emission color of CDs, thereby enabling emission at different wavelengths. Meanwhile, as a dopant, l-methionine not only contains amino and carboxyl groups, which facilitate surface modification and functionalization of CDs during synthesis, but also introduces sulfur atoms into the structure of the CDs, altering their electronic properties. As illustrated in Scheme 1, x-CDs (x = b, g, r) with stable blue, green, and red emissions were synthesized by using 1,7-DHN, 1,6-DHN, or 2,7-DHN as precursors and l-methionine as a dopant, which were dispersed in ethanol and then reacted at 180 °C for 12 h. Notably, the r-CDs exhibit dual emission peaks, enabling the visual detection of trace amounts of TC.

Scheme 1. Synthetic Routes for x-CDs (x = b, g, r) and Their Applications.

Scheme 1

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The morphology of the multicolored CDs was revealed by transmission electron microscopy (TEM). As shown in Figure 1a–c, the x-CDs (x = b, g, r) exhibit quasi-spherical particles with a uniformly dispersed distribution. High-resolution TEM (HR-TEM) images in the upper right corner indicate that the x-CDs (x = b, g, r) have clearly visible lattice stripes with a lattice spacing of 0.21 nm, suggesting a graphite-like crystal structure.26 The average particle sizes of the x-CDs (x = b, g, r) were determined to be 8.04, 3.70, and 5.01 nm (Figure 1d–f), respectively.

Figure 1.

Figure 1

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TEM and HR-TEM (inset) images of (a) b-CDs, (b) g-CDs, and (c) r-CDs. Particle size distributions of (d) b-CDs, (e) g-CDs, and (f) r-CDs.

The crystal structures of the CDs were analyzed by using X-ray diffraction (XRD). As shown in Figure S1a, the XRD patterns of x-CDs (x = b, g, r) are nearly identical, exhibiting diffraction peaks at 23° and 42°, respectively. These peaks correspond to the (002) and (100) crystal planes of graphitic carbon.27 Raman spectra of x-CDs (x = b, g, r) present similar characteristic D bands at ∼1365 cm–1 and characteristic G bands at ∼1575 cm–1 (Figure S1b–d). The D band, a low-frequency peak in the Raman spectra, arises from defects or impurities in the carbon material and serves as an indicator of the defect density. In contrast, the G band, a high-frequency peak, arises from the vibrations of sp2-hybridized carbon. The intensity of the G band reflects the size of the sp2 structural domains within the carbon material.28,29 The graphitization degree of carbon materials is often evaluated using the ratio of IG/ID, where a higher ratio indicates a higher degree of graphitization.30 For x-CDs (x = b, g, r), the ratios of IG/ID were calculated to be 0.94, 1.03, and 1.1, respectively. With the change in precursor structure, the ratio of IG/ID for the synthesized b-CDs to r-CDs gradually increased and the degree of graphitization increased significantly. Notably, as the graphitization degree of CDs increases, their emissions undergo a redshift.31,32

The Fourier transform infrared spectroscopy (FT-IR) patterns of x-CDs (x = b, g, r, s) are displayed in Figure S2, where s-CDs are the control samples obtained from l-methionine without precursor DHN alone under the same conditions. In the FT-IR spectra, the peak at 1520 cm–1 corresponds to the stretching vibration of the C=C bond. Notably, x-CDs (x = b, g, r) exhibit sharp peaks at this position, whereas s-CDs display smoother curves, indicating that the incorporation of DHN has resulted in the synthesized CDs having an aromatic conjugated ring structure. The broad peaks at ∼3370 cm–1 and the peak at ∼2960 cm–1 are attributed to the stretching vibration of the O–H/N–H and the C–H bonds, respectively.31 Additional peaks near 1380, 1100, and 670 cm–1 are assigned to the stretching vibration of the C–N, the C–O/S=O, and the C–S, respectively.33 Furthermore, the peak at ∼1670 cm–1 represents the stretching vibration of the C=O bond, and the intensity of the C=O absorption peaks increases progressively from s-CDs to b-CDs, g-CDs, and r-CDs, suggesting a gradual increase in the surface C=O functional group content across the series.33

The full X-ray photoelectron spectroscopy (XPS) spectra of x-CDs (x = b, g, r, s) are shown in Figure S3a–d. The four main peaks at 532.08, 400.08, 286.08, and 162.08 eV correspond to the characteristic peaks of O 1s, N 1s, C 1s, and S 2p, respectively.34 Obviously, from b-CDs to r-CDs, the O 1s peak gradually increased, and the emission wavelength shifted from blue to red as the oxidation degree of CDs increased.31,35

Figures S4a–d display the high-resolution C 1s spectra of x-CDs (x = b, g, r, and s), with three characteristic peaks at 284.68 eV (C=C/C–C), 285.88 eV (C–O/C–N/C–S), and 289.08 eV (C=O). The high-resolution C 1s spectra of x-CDs (x = b, g, r) are largely similar, whereas the control s-CDs exhibit significant differences. Attention should be drawn to the fact that the C=O group content of the s-CDs is almost zero, indicating that the surface oxidation of the s-CDs is greatly reduced compared to that of the x-CDs (x = b, g, r). Figure S5a,c,e,g shows the high-resolution N 1s spectra of x-CDs (x = b, g, r, s), with three characteristic peaks isolated at 399.68 (pyridine N), 400.58 (pyrrole N), and 402.08 eV (graphite N), respectively. The spectra of x-CDs (x = b, g, r) are nearly identical, while s-CDs exhibit a noticeably lower intensity for the graphite N peak. The high-resolution O 1s spectra of x-CDs (x = b, g, r, s) are shown in Figure S5b,d,f,h; the characteristic peaks located at 531.88 and 532.98 eV correspond to C=O, C–OH/C–O–C groups. As a remark, x-CDs (x = b, g, r) have a significant difference in the content of C=O groups. From b-CDs to r-CDs, the content of C=O groups gradually increases, indicating that the degree of oxidation on the surface of CDs gradually increases. The higher degree of oxidation on the surface of CDs implies that there are more surface oxidation defects, which makes the HOMO–LUMO band gap decrease, leading to the redshift of the photoluminescence (PL) of CDs (Figure S6).36

Figure S7a–d illustrates the high-resolution S 2p spectra of x-CDs (x = b, g, r, and s), revealing four characteristic peaks at 162.88, 163.68, 164.78, and 166.88 eV. The peaks at 162.88 and 163.68 eV correspond to S 2p3/2 and S 2p1/2, respectively, whereas the peaks at 164.78 and 166.88 eV are attributed to C–S and S=O characteristic peaks.37 These results confirm the successful doping of S elements in the CDs.

In Figure 2a–c, the absorption, photoluminescence excitation (PLE), and PL emission spectra of x-CDs (x = b, g, and r) are plotted. Under daylight, x-CDs exhibit light blue, green, and light pink colors, respectively, while under ultraviolet (UV) irradiation, they display bright blue, green, and red fluorescence. For b-CDs, the optimal excitation and emission wavelengths are about 364 and 432 nm, respectively. Both g- and r-CDs exhibit bimodal emission corresponding to different optimal excitations. In the emission spectra of g-CDs, the blue emission peaks have optimal excitation and emission wavelengths at near 353 and 428 nm, while the green emission peaks are at about 470 and 532 nm. The r-CDs possess bimodal peaks including blue emission (excitation/emission at 348 and 430 nm) and red emission (excitation/emission at about 512 and 602 nm). Overall, the blue peaks of the x-CDs (x = b, g, r) have very close excitation and emission wavelengths. It can be found in the UV–vis absorption spectra that there are multiple similar absorption peaks of x-CDs (x = b, g, r) in the range of about 200–300 nm, and the absorption peaks in this region are the core states, which are usually attributed to the π–π* transition of the carbon core.38 Meanwhile, the x-CDs (x = b, g, r) exhibit a distinct absorption peak around 330 nm, which matches well with the optimal excitation center of the blue emission peaks. In addition, there is an absorption peak at 480 and 536 nm for g-CDs and r-CDs, respectively, and the absorption in this range can be attributed to the molecular state transition of CDs.39

Figure 2.

Figure 2

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Absorption spectra, excitation spectra, and emission spectra of (a) b-CDs, (b) g-CDs, (c) r-CDs where in the spectra of g-CDs, PLE-1: monitored at 428 nm, PL-1: excitation at 353 nm, PLE-2: monitored at 532 nm, and PL-2: excitation at 470 nm; while in the spectra of r-CDs, PLE-1: monitored at 430 nm, PL-1: excitation at 348 nm, PLE-2: monitored at 602 nm and PL-2: excitation at 512 nm. PL emission spectra of (d) b-CDs, (e) g-CDs, and (f) r-CDs under different excitation wavelengths.

Figure 2d–f shows the emission spectra of x-CDs (x = b, g, and r). For b-CDs, as the excitation wavelength increases from 350 to 400 nm, the emission wavelength red-shifts from 432 to 496 nm, with emission intensity first increasing and then decreasing. A magnified view of the blue emission peaks in Figure 2e reveals that the blue peaks of g-CDs and r-CDs exhibit red-shifting and intensity changes as b-CDs. Additionally, the green emission peak of g-CDs remains excitation-independent at 516 nm. For r-CDs, the red emission peak slightly red-shifts from 600 to 602 nm as the excitation wavelength increases. Remarkably, as the degree of oxidation and carbonization increases, the synthesized CDs exhibit red-shifted emission spectra along with enhanced PL quantum yields (PLQYs, Figure S8).

The emission spectra of the x-CDs (x = b, g, r) in the eight solvents with different polarities (in order of increasing polarity), such as hexane, toluene, dichloromethane (DCM), ethyl acetate (EA), acetone, ethanol (EtOH), methanol (MeOH), and water, were analyzed (Figure S9). Combining the optical pictures with the emission spectra, it can be seen that the emission of x-CDs (x = b, g, r) is appropriately red-shifted with increasing solvent polarity. This may be due to the influence of the solvent dipole moment on the surface electronic states of the CDs, causing some changes.40,41 As the solvent polarity increases, the surface electronic states are more affected and the energy gap decreases, leading to a red shift in the emission wavelength.42,43 Furthermore, the time-resolved PL (TRPL) spectra of these samples were measured (Figure S10). The average fluorescence lifetimes of x-CDs (x = b, g, and r) were calculated to be 4.43, 4.54, and 7.96 ns after fitting.

DHNs are equipped with two reactive phenolic hydroxyl groups, which are easily oxidized in air. Due to the availability of different binding sites, DHNs undergo varied oxidative self-polymerization pathways. Based on this, possible polymerization pathways involved in the synthesis of multicolor CDs are illustrated in Figure 3.44−46 In this work, DHNs were self-polymerized to form various structural units, which were further cross-linked and polymerized with l-methionine and then carbonized to form CDs. Therefore, it could be hypothesized that controlling the structural changes of DHNs allows for different polymerization routes, thereby regulating the structure of CDs and their emissions.

Figure 3.

Figure 3

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Possible schematic diagram of the CDs formation.

Before the application of our CDs as fluorescent probes, the three CDs were mixed with TC (Figures S11 and 4), and it can be observed that TC can cause the fluorescence quenching of CDs. After comparison, we found that when TC was mixed with r-CDs, the blue peak fluorescence intensity (IF1) of r-CDs had a different magnitude of change from the red peak fluorescence intensity (IF2), and the fluorescence color changed from red to orange; thus, r-CDs were chosen to be the ratiometric fluorescent probes for the detection of TC. As shown in Figure 4a, the fluorescence spectra of the aqueous solution of r-CDs under different concentrations of TC were recorded. When the TC concentration was gradually increased from 0 to 90 μM, it was calculated that the blue emission peaks of r-CDs decreased faster than the red emission peaks, which led to a change in the fluorescence color of r-CDs (Figure 4b,c). Figure 4d shows the corresponding optical pictures. With the increase of TC concentration, r-CDs gradually tended to become transparent from a light pink color under natural light.

Figure 4.

Figure 4

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(a) Fluorescence emission spectra of r-CDs containing different concentrations of TC (from 0 to 90 μM, Ex. 365 nm). (b) Ratiometric fluorescent probe in the presence of different concentrations of TC (inset: calibration curve of IF2/IF1 and concentrations of TC). (c) Coordinate changes of r-CDs/TC fluorescence colors in CIE 1931 chromaticity coordinates. (d) Optical photographs of r-CD solutions with various TC concentrations under ambient light and 365 nm UV light.

Meanwhile, the r-CDs under UV irradiation gradually changed from red to orange fluorescence, which was consistent with the CIE chromaticity diagram (Figure 4c). The linear relationship between IF2/IF1 and TC concentration was analyzed in Figure 4b. The value of IF2/IF1 gradually increased when the TC concentration increased. In the concentration range of 0–90 μM, there was a good linear relationship between the values of IF2/IF1 and the concentration of TC (R2 = 0.989), and the fitting equation was IF2/IF1 = 0.00797 CTC + 2.08356. The limit of detection (LOD) was calculated based on eq 1.

graphic file with name au4c01220_m001.jpg 1

where b is the slope of the calibration curve and S represents the standard deviation of the blank, i.e., IF2/IF1 without TC (n = 3). The LOD was calculated to be about 200 nM.

The detection types are categorized into two main groups: single emission quenching and ratiometric detection. Comparatively, most of the sensing mechanisms focus on single-emission quenching, and the conditions required for the realization of ratiometric detection are more complex. However, the single-emission quenching detection method has certain drawbacks, and the accuracy is easily affected by the instrument and the surrounding environment.47 Some of the recent ratiometric detection methods are demonstrated in Table S1, which have good detection performance but need to be compounded with rare earth ions, metal ions, and other materials. These materials are expensive and polluting to the environment.48,49 The raw materials used in the preparation of r-CDs in this work are inexpensive, small-molecule carbon sources and have a certain degree of biocompatibility. At the same time, the r-CDs-based probes can achieve good quantitative and visual detection of TC, with easily recognizable color changes under both natural light and UV light and a wide detection range and sensitive detection limit.

The selectivity and anti-interference of r-CDs for TC detection were further tested. As shown in Figure 5a, 200 μM TC, metal ions (Mg2+, Ca2+, Na+), small molecules (l-cysteine, dopamine hydrochloride, l-glutamic acid, dl-aspartic acid), and anions (SO42–, Br–, and NO2–) were added to the r-CDs solution. At the same concentration, r-CDs with only TC added showed an obvious fluorescence quenching phenomenon, and the intensity of the emission peaks was greatly reduced. The r-CDs emission peaks changed very little with the addition of other substances, which showed that r-CDs had excellent selectivity for the detection of TC.

Figure 5.

Figure 5

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(a) Fluorescence emission spectra of r-CDs in the existence of various substances (excitation at 365 nm). (b) Intensity changes of F1 and F2 of r-CDs in the presence of different substances. (c) Fluorescence emission spectra of the r-CDs/TC system when different substances are added to the system (excitation at 365 nm). (d) Intensity changes of F1 and F2 of r-CDs when different substances are added to the r-CDs/TC system.

The corresponding bar graph in Figure 5b demonstrates the intensity ratio of F1 and F2, further illustrating the selectivity of r-CDs for detecting TC. The anti-interference of r-CDs for TC detection was tested using the same substances. As shown in Figure 5c, when r-CDs coexisted with 200 μM metal ions (Mg2+, Ca2+, Na+), small molecules (l-cysteine, dopamine hydrochloride, l-glutamic acid, dl-aspartic acid), and anions (SO42–, Br–, and NO2–), respectively, they were still able to detect the 200 μM TC without any influence, which indicates that the presence of other substances would not interfere with the detection of TC. The corresponding bar graph in Figure 5d also demonstrates this excellent anti-interference property.

In addition, it is well-known that there are many types of antibiotics, and representative antibiotics from the tetracycline, nitroimidazole, and β-lactam groups were selected to evaluate the susceptibility of r-CDs to different classes of antibiotics (Figure S12). The emission spectra of r-CDs under 365 nm excitation were monitored after mixing with varying concentrations of amoxicillin (AMX) and metronidazole (MNZ). It was found that the effect of these two antibiotics on r-CDs was not significant compared to TC. Since tetracycline antibiotics share similar biochemical structures, oxytetracycline (OTC) and chlortetracycline (CTC) were also tested. Significant changes in the double emission peaks of r-CDs were observed with varying concentrations of these antibiotics, showing a good linear relationship within a specific range. Comparing the bimodal ratios of r-CDs/TC mixed with AMX, MNZ, OTC, and CTC, it was evident that AMX and MNZ had minimal impact on the r-CDs/TC bimodal ratio, whereas r-CDs showed a higher sensitivity to tetracycline antibiotics.

Inner filter effect (IFE) refers to the overlap between the excitation or emission spectrum of a fluorophore and the absorbance spectrum of a quencher.21 As shown in Figure S13a, TC has a clear absorption band in the wavelength range 250–400 nm, and there is a large region of overlap between the fluorescence excitation spectrum of r-CDs and the absorbance spectrum of TC. This indicates that the quenching of r-CDs is related to the IFE of TC on r-CDs. Figure S13b shows the UV–vis spectra of r-CDs and TC-containing r-CDs. The two absorption curves showed essentially identical absorption peaks, indicating that no new complexes were formed between r-CDs and TC. In contrast, only spectral overlap is required for the IFE effect to occur, without the formation of new compounds.22 In addition, the average fluorescence lifetimes of r-CDs were measured in the absence and presence of TC (Figure S13c). Before and after the addition of TC, the average fluorescence lifetime of r-CDs decreased from 7.96 to 5.05 ns. The change in fluorescence lifetime indicated that the fluorescence quenching mechanism of r-CDs belonged to dynamic quenching.23

Conclusions

In summary, this work explored the effect of the structural properties of aromatic compounds on the luminescence of CDs. b-CDs, g-CDs, and r-CDs with bright fluorescence were obtained by a simple solvothermal method using different DHN isomers as the precursor. Results revealed that the gradual increase in the content of C=O groups was the key reason for the redshift in the fluorescence of CDs. The positional changes of the hydroxyl group on the naphthalene ring of precursor DHN effectively regulate the degree of oxidation on the surface of CDs. A higher degree of oxidation of the CDs surface leads to more surface oxidation defects and a smaller HOMO–LUMO band gap, which results in a redshift of CDs PL. Importantly, the luminescence of r-CDs is very sensitive to TC and can be applied to TC sensing detection. The ratiometric probes based on r-CDs enable good visual detection of TC with a wide detection range and a sensitive detection limit.

Materials and Methods

Materials

1,7-Dihydroxynaphthalene (1,7-DHN, 98%), 1,6-dihydroxynaphthalene (1,6-DHN, 99%), 2,7-dihydroxynaphthalene (2,7-DHN, 97%), l-methionine (99%), magnesium chloride hexahydrate (MgCl2 6H2O, AR), calcium chloride (CaCl2, AR), sodium chloride (NaCl, AR), l-cysteine (Cys, 99%), l-glutamic acid (Glu, 99%), dl-aspartic acid (Asp, 98%), sodium sulfate (Na2SO4, ACS), sodium bromide (NaBr, ACS) and sodium nitrite solution (NaNO2, ACS) were supplied by Aladdin Chemistry Co., Ltd., China. Tetracycline hydrochloride (TC, 96%), dopamine hydrochloride (DA, 98%), oxytetracycline hydrochloride (OTC, 95%), chlorotetracycline hydrochloride (CTC, USP), Amoxicillin (AMX, 98%), and Metronidazole (MNZ, 99%) were obtained from Macklin Biochemical Technology Co., Ltd. Ethanol (EtOH), methanol (MeOH), acetone, EA, dichloromethane (DCM), toluene (Tol), and n-hexane (Hxn) were bought from Sinopharm Holding Chemical Reagent Co., Ltd., China.

Synthesis of Multicolor CDs

Synthesis of blue CDs (b-CDs): 10 mL of ethanol was added to a beaker, followed by weighing 0.149 g of l-methionine into the beaker and 0.16 g of 1,7-DHN, and stirring for 10 min until well mixed. The mixture was transferred to a 25 mL PTFE reactor liner and loaded into the reactor. Subsequently, the closed reactor was placed in an oven and kept at 180 °C for 12 h. After the reaction, the cooled product was filtered through a polycarbonate membrane filter with a pore size of 0.45 μm to obtain a b-CDs solution.

Synthesis of green CDs (g-CDs): the preparation was the same as that of b-CDs except that the precursor 1,7-DHN was replaced by 1,6-DHN.

Synthesis of red CDs (r-CDs): the preparation was the same as that of b-CDs except that the precursor 1,7-DHN was replaced by 2,7-DHN.

Synthesis of control CDs (s-CDs): the preparation was the same as that of b-CDs except that the precursor 1,7-DHN was removed.

TC Ratiometric Fluorescence Detection

First, an aqueous solution of r-CDs with a concentration of 100 μL/mL was configured. 0.5 mL of r-CDs solution was taken, different concentrations of TC (or antibiotics) aqueous solution were added, and finally the volume of the solution was kept at 5 mL with ultrapure water. After the mixtures were incubated for 1 min at room temperature, the PL spectra of the mixtures were recorded under 365 nm UV excitation.

Anions (SO42–, Br–, and NO2–), molecules (l-cysteine, dopamine hydrochloride, l-glutamic acid, and dl-aspartic acid), and cations (Na+, Mg2+, and Ca2+) were utilized to validate the selectivity of the r-CDs probes for TC. The concentrations of these substances were all 200 μM. The anti-interference properties of r-CDs against TC were also investigated by mixing TC (200 μM) and the above substances (200 μM).

Characterizations

A JEM-1400Plus 120 kV transmission electron microscope was used to characterize the morphology, dispersion, and particle size of the samples. The diffraction pattern was recorded by an Empyrean X-ray diffractometer (XRD) of null, The Netherlands, which was used to characterize the crystal structure. The structure and chemical composition of the samples were characterized by a Fourier transform infrared (FT-IR) spectrometer of Nicolet6700 and a laser confocal microscopic Raman spectrometer (Raman) of LabRAM Odyssey. The X-ray photoelectron spectrometer of an ESCALAB 250Xi was used to characterize the surface elements and chemical composition of the sample. A Lambda 750 S UV–visible near-infrared (UV–vis) spectrophotometer was used to characterize the optical properties of the samples. The luminescence spectra of LED were characterized by a Spectrascan PR-650 spectrophotometer with an integrating sphere.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 21774098) and the 111 project (no. B18038).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.4c01220.

  • Experimental method, measurement and characterization, XRD spectra, Raman spectra, FTIR spectra, optical data, and calculation of PL lifetime (PDF)

The authors declare no competing financial interest.

Supplementary Material

au4c01220_si_001.pdf (2.2MB, pdf)

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