A novel “on–off–on” fluorescent probe based on carbon dots for dual-detecting rifampicin and levofloxacin

Abstract

In this work, carbon dots (CDs) with blue fluorescence emission were synthesized using citric acid and cysteine through simple one-pot hydrothermal method. Under the optimal experimental conditions, the fluorescence intensity of the CDs could be quenched by rifampicin (RIF), with a good linear relationship (R² = 0.9925), and the limit of detection (LOD) was determined to be 0.687 μg mL⁻¹. Furthermore, the quenched fluorescence of CDs-RIF system could be restored by levofloxacin (LEV), the fluorescence intensity of CDs-RIF was continuously enhanced with the increase of the LEV concentration, which also showed a good linear relationship (R² = 0.9929), and the LOD was 0.965 μg mL⁻¹. Based on this discovery, a low-cost fluorescent probe with dual-detection performance was constructed and applied for the rapid and accurate detection of RIF and LEV residues in actual samples.

1 Introduction

As effective bactericidal drugs, antibiotics are widely used in medical and aquaculture industries. The phenomenon of overuse and abuse of antibiotics are gradually increasing, however, this can cause serious food safety problems and environment pollution problems (Kovalakova et al., 2020; Wang et al., 2025). For example, the antibiotic presence in water can result in direct harm to animals (Bawa-Allah and Ehimiyein, 2022) and plants (Carballo et al., 2022), but more significantly, they can induce antibiotic-resistant bacteria or genes, which increases health and ecological hazards through food chains, even at low concentrations (Hassoun-Kheir et al., 2020; Wang et al., 2017). In this sense, the detection and quantification of antibiotics are important to the health of the ecosystem and human.

Rifampicin (RIF) and levofloxacin (LEV) are common broad-spectrum antibiotics. RIF exhibits strong antimicrobial activity against Mycobacterium tuberculosis, and also is effective against Gram-positive or negative bacteria, as well as viruses (Singh et al., 2017); while LEV is a new fluoroquinolone drug, which has expanded activity against Gram-positive bacteria and atypical intracellular pathogens (Sitovs et al., 2021; Li et al., 2023). Until now, the common detection methods for antibiotics such as RIF and LEV mainly include high-performance liquid chromatography (HPLC) (Jalaludin and Kim, 2021), liquid chromatography-mass spectrometry (LC–MS) (Ye et al., 2025; Yin et al., 2024; Peng et al., 2024), nuclear magnetic resonance (NMR) (Arachchi et al., 2023), and electrochemical method (Zhou et al., 2022). Although these methods can detect targets with high sensitivity and selectivity, they require complex instruments, are time-consuming, costly and involve complicated sample pre-treatment. Therefore, the development of a rapid, efficient, convenient, and low-cost assay for the detection of RIF and LEV is extremely urgent.

Carbon dots (CDs) are a new type of zero-dimensional nano-luminescent materials with spherical or quasi-spherical shape and size less than 10 nm. They were discovered accidentally discovered by Xu et al. (2004) during the purification of single-unit carbon nanotubes by gel electrophoresis separation and purification using arc discharge method. In 2006, Sun et al. (2006) obtained nanomaterials with fluorescence properties by using the laser ablation method and passivation, and named them as “CDs” for the first time. Compared with semiconductor quantum dots (QDS), CDs are widely used in the fields of biosensing and fluorescence imaging due to the superior performance, such as easy preparation, low cost, low toxicity, easy modification, high biocompatibility, excellent optical properties and good fluorescence characteristics. A wide variety of biological/chemical sensors have been developed based on the fluorescence properties of CDs and their abundant surface functional groups, including detection of pesticide residues (Xie et al., 2026), heavy metal (Fu et al., 2022), glucose (Zou et al., 2021), and DNA (Zhang et al., 2020). Moreover, the biocompatibility of CDs can be improved through modifying the surface groups, which further enhance the potential functional applications (Zhou et al., 2017).

Given the characteristics of CDs, in this work, CDs with blue fluorescent emission were simply synthesized through one-pot hydrothermal method using citric acid and cysteine as raw materials. Due to the selectivity and sensitivity response of these CDs to RIF, the obtained CDs were used as fluorescent “turn-off” nanoparticle probes for the rapid detection of RIF. Interestingly, the quenched fluorescent of CDs-RIF system could be restored by LEV, and the enhancement extent of fluorescent presented a significant correlation to the increased LEV concentration. As a result, the synthetic CDs can be used as a fluorescent probe for dual-detecting RIF and LEV through “on–off–on” mode. What’s more, the developed method was successfully applied to detect RIF and LEV in actual samples, thereby offering a novel avenue for residue analysis of antibiotics and presenting promising prospects for broad applications.

2 Materials and methods

2.1 Chemicals and reagents

Citric acid, cysteine, CaCl₂, KCl, MgCl₂·6H₂O, MnCl₂·4H₂O, NiCl₂·6H₂O and quinine sulfate were bought from Hushi Laboratory Equipment Co., Ltd. (Shanghai; China); Cd(NO₃)₂·4H₂O, ZnCl₂, Cu(NO₃)₂, LEV, chloramphenicol, and tetracycline were obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai; China); AgNO₃ and HgCl₂ were purchased from Guizhou Tongren Chemical Reagent Factory (Guizhou; China); RIF was bought from Chengdu Tak Wah Pharmaceutical Co., Ltd. (Sichuan, China); arginine and glutamic acid were purchased from Shanghai Yuanye Biotechnology Co., Ltd.; cysteine and alanine were obtained from Tianjin Fuchen Chemical Reagent Factory; phenylalanine was bought from Tianjin Institute of Fine Restoration; methionine was obtained from Beijing Solaibao Technology Co., Ltd.; glycine was bought from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China); histidine was purchased from Sigma Aldrich, USA. The tap water was obtained from our laboratory. The BR buffer solution was prepared using acetic acid, boric acid and 85% phosphoric acid, and its pH value (pH = 7.4) was adjusted with NaOH solution. HPLC grade acetonitrile was acquired from Fischer Scientific Co., Ltd. (Waltham, USA). The experimental water was ultra-pure with a resistivity of 18.2 MΩ cm. All chemicals and reagents were of analytical grade and used directly without further purification.

2.2 Apparatus

The fluorescence of CDs was measured on a fluorescence spectrophotometer (F-4600, Hitachi, Japan). The UV–Vis spectra of the materials were tested on a UV–Vis spectrophotometer (TU-1900, Beijing PERSEE, China). Fourier transform infrared (FT-IR) spectra of the materials were recorded on an infrared spectrophotometer (FTIR-680, Tianjin Ruian, China). Transmission electron microscopy (TEM) images of CDs were taken by a scanning transmission electron microscope (HT-7800, Hitachi, Japan) with an acceleration voltage of 100 kV. X-ray photoelectron spectroscopy (XPS) data were obtained using an X-ray photoelectron spectrometer (ESCA Lab250Xi, Thermo Fisher, USA), which consists of a monochromatic Al Kα as the X-ray source.

2.3 Experimental methods

2.3.1 Synthesis of CDs

Citric acid and cysteine were used as precursors to synthesize CDs by the one-pot hydrothermal method. The specific steps were as follows: citric acid (0.26 M) and cysteine (0.52 M) were dissolved in 10.0 mL of ultrapure water, and the mixed solution was transferred into a polytetrafluoroethylene high-pressure reactor, which was then placed in a high-temperature oven and heated at 200 °C for 5 h. The resulting solution was a clear yellow liquid, which was filtered 3 times by using 0.22 μm membrane to acquire CDs solution. The obtained CDs were stored in a refrigerator at 4 °C.

It is worth noting that the as-prepared CDs solution was used directly after filtration without any dilution. To ensure reproducibility, all measurements were performed using the same batch of CDs and a fixed volume (100 μL) under identical conditions. Since the synthesis procedure is simple and well-defined, other researchers can readily prepare CDs with similar fluorescence properties by following the same protocol. If necessary, the volume of the CDs solution can be adjusted to achieve the same initial fluorescence intensity at the excitation wavelength (345 nm) as used in our study, thereby normalizing any batch-to-batch variations.

2.3.2 Characterization of the CDs

The optical properties of the CDs were characterized with UV–Vis spectroscopy and fluorescence spectroscopy. The fluorescence stability of CDs under different ion concentration, pH, and reaction time conditions was also analyzed. TEM was applied to understand the microstructure and particle size. FT-IR spectroscopy was performed to analyze the composition of surface functional groups of the obtained CDs. XPS was applied for the surface groups and elemental analysis of CDs.

The relative fluorescent quantum yield of the prepared CDs was determined according to the method established by Zhu et al. (2015), in which 0.1 mol L⁻¹ H₂SO₄ was used as solvent and quinine sulfate (QY = 54%) was chosen as fluorescence standard solution. The absolute value was determined using quinine sulfate with a known constant value of fluorescence quantum yield. The fluorescence quantum yield of CDs was calculated according to the method proposed by Han et al. (2018) with fluorescent signal recorded at excitation wavelength of 345 nm:

Where Φ responds to the fluorescence quantum yield, F is the area of the fluorescence curve, and A is the optical density. The subscript s denotes to the reference with known QY, and u represents the test sample.

2.4 Detection methods of RIF and LEV

2.4.1 Fluorescence detection of RIF and LEV

For RIF, 0.0020 g of standard substance was weighed and dissolved in methanol to prepare a 200 μg mL⁻¹ working solution of RIF. In a 1 mL colorimetric tube, CDs (100 μL), BR buffer solution (pH = 7.4, 100 μL) and different concentrations of the RIF (800 μL) were added. After gently shaking and thoroughly mixing the above solutions, they were incubated at room temperature for 5 min. The fluorescence intensities of the resulting solutions were measured on a fluorescence spectrophotometer, and a standard curve was constructed.

For LEV, the CDs-RIF solution was prepared by mixing 100 μL CDs solution with 800 μL working solution of RIF (50 μg mL⁻¹). Then, in a 1 mL colorimetric tube, CDs-RIF (100 μL), BR buffer solution (pH = 7.4, 100 μL) and different concentrations of the LEV (800 μL) were added. After gently shaking and thoroughly mixing the above solutions, they were incubated at room temperature for 5 min. The fluorescence intensities of the resulting solutions were measured on a fluorescence spectrophotometer, and a standard curve was constructed.

2.4.2 Detection of RIF and LEV in actual samples

The tap water and river water were pretreated as follows (Fu et al., 2022): Firstly, they were put into 50 mL beaker, followed by ultrasound for 10 min, and then heated and boiled for 10 min. After cooling to room temperature, the solutions were filtered through 0.22 μm MCE membrane. Subsequently, 0.5 mL pretreated sample was spiked with RIF and LEV, respectively at different concentration levels. Three replicate experiments were performed at each spiked concentration level. The detection procedure was similar to the standard detection procedure for RIF and LEV.

2.4.3 Method validation

High performance liquid chromatography (HPLC) was used to validate the accuracy of the developed method. The HPLC was equipped with a Waters 2,695 separation unit and a Waters 2,998 photodiode array detector (Milford, MA, USA). The chromatographic column was Agilent ZORBAX Eclipse XDB-C18 reversed-phase column (250 mm × 4.6 mm i.d., 5 μm particle size). The column temperature was set to 40 °C, and the photodiode array detector (PAD) collected the absorption spectra in the range of 200–600 nm at a step size of 1 nm and a frequency of 2 Hz. The mobile phase consisted of 0.1% formic acid-water (A) and acetonitrile (B). Gradient elution was performed at a flow rate of 1.0 mL min⁻¹ and the program was as follows: 5%–45% B 0.00–20 min, 45%–60% B 20–25 min, 60%–95% B 25–28 min, 95%–5% B 28–31 min, 5% B 31–40 min. The injection volume was 20 μL.

2.4.4 Data analysis

All data statistics and calculations were analyzed using Microsoft Excel 2016, and plotted using Origin 2018. The fluorescence intensity was fitted using the Stern-Volmer equation F₀/F=K_sv[c] + 1, where F₀ and F are the fluorescence intensities of the blank control and the addition of targets, respectively, K_sv is the Stern-Volmer coefficient, and [c] is the concentration of analyte. The limit of detection was determined using the 3-fold standard deviation rule LOD = 3σ/s, where σ represents the standard deviation of 10 blank group tests and s represents the slope of the calibration curve.

3 Results and discussion

3.1 Construction of fluorescent probe

The construction and detection process of the proposed sensor is demonstrated in Scheme 1. Briefly, citric acid and cysteine were used as precursors to synthesize CDs by one-pot hydrothermal method, which served as fluorescent probe for RIF. The presence of a large number of –NH₂ and –COOH groups on the surfaces of CDs provides sites of chemical interaction with RIF through hydrogen bonding, resulting in a decrease in the fluorescence intensity of CDs at an emission wavelength of 414 nm when RIF is added. It was found that the fluorescence quenching intensity of CDs increased with the increasing of RIF concentration. In contrast, the addition of LEV into the CDs-RIF system allows LEV to compete with CDs for binding to RIF through the electron-withdrawing group –F and –COOH group, thereby facilitating the restoration of CDs fluorescence. Therefore, the prepared CDs can be used as an “on–off–on” fluorescent probe for dual-detecting both RIF and LEV.

SCHEME 1

3.2 Characterization of the CDs

Typical TEM image of the CDs is shown in Figure 1A. The prepared CDs were spherical and well dispersed with an average diameter of 32.72 ± 1.13 nm (Figure 1B). This relatively large size is likely attributable to the relatively high precursor concentrations and the prolonged hydrothermal treatment (200 °C, 5 h), which may promote further polymerization and cross-linking of the precursors, thereby generating larger carbonaceous nanoparticles. The chemical composition and surface functional groups of CDs could be detected by FT-IR spectroscopy. As shown in Figure 1C, a strong broad absorption peak at 3158 cm⁻¹ was due to the O-H/N-H stretching vibration. A strong peak centered at 1724 cm⁻¹ was attributed to characteristic absorption band of C=O stretching vibration, indicating the existence of carboxyl (Hilmi et al., 2023). The peaks at 1525 cm⁻¹ and 1,398 cm⁻¹ were ascribed to characteristic absorption bands of C=C stretching vibration and C-H bending vibration, respectively (Omar et al., 2022; Tschirner et al., 2009). To sum up, FT-IR spectral analysis revealed that the surface of CDs was typically decorated with a large number of hydrophilic groups such as carboxyl, amino, and hydroxyl groups, to enhance water solubility (Zhou et al., 2017). In addition, the surface groups were also characterized by XPS analysis. The XPS results (Figure 1D) showed that CDs contained elements C, O, N, and S in 51.96, 39.32, 3.99, and 4.72%, respectively. In Figure 1E, the characteristic peaks at 284.8 eV, 286.4 eV, and 288.7 eV are attributed to C–C, C–O, and C=O bonds, respectively (Mohammed Ameen et al., 2024). The N 1 s spectrum in Figure 1F displays peaks at 400.5 eV and 401.5 eV, which are assigned to pyridinic N and pyrrolic N functional groups, respectively (Sakhaei Niroumand et al., 2024). In Figure 1G, the peak at 531.8 eV corresponds to the C=O bond and the peak at 533.2 eV corresponds to the C–O bond (Ameen et al., 2024). As shown in Figure 1H, the binding energy peaks corresponding to S 2p consisted of S 2p2/3 and S 2p1/2 at 163.0 eV and 164.2 eV (Ganesha Krishna and Mahesha, 2021).

Figure 1

The optical properties of the prepared CDs were also investigated. As shown in Figure 2A, two obvious UV–Vis absorption peaks at 240 nm and 350 nm were observed, which may be attributed to the π–π* transition of C=C/C=N bonds and n–π* transition of C=O bond, respectively (Huang et al., 2019). In Figures 2B,C, the maximum fluorescent emission wavelength was observed at 414 nm and remained unchanged across different excitation wavelengths, indicating excitation independent emission. The relative quantum yield (QY) of the CDs was calculated about 64.03% according to Equation 1, which exhibits a higher QY than that without doping of nitrogen atoms (Chen et al., 2016; Gedda et al., 2016; Wei et al., 2013).

Figure 2

3.3 Fluorescence stability of the CDs and optimization of detection conditions

It’s well known that the fluorescence stability of CDs is an important foundation for their use as fluorescent probes. The effects of ion concentration and pH on the fluorescence intensity of CDs and CDs-RIF systems were studied. Figure 3A showed the fluorescent change trend of CDs and CDs-RIF systems in the presence of different concentrations of NaCl. It could be found that the fluorescence intensities of the CDs and CDs-RIF systems did not change significantly even when the ion concentration of the solution was as high as 1.6 mol L⁻¹, enabling their use in high ionic strength environments. Additionally, as shown in Figure 3B, the fluorescence intensities of CDs and CDs-RIF systems gradually increased with the pH ranging from 2.0 to 5.0, but exhibited a plateau in the pH range of 6.0 to 12.0. The rise in fluorescence intensity with pH observed from 2.0 to 5.0 could be ascribed to the protonation mechanism of carboxylate groups located on the surface of CDs. As a result, pH = 7.4 was chosen as the optimal pH for the detection of RIF. Finally, the change of fluorescence intensity of the CDs and CDs-RIF systems with reaction time was also detected, so as to clarify the response time of CDs to RIF. As shown in Figure 3C, when RIF was added at 50 μg mL⁻¹, the fluorescence intensity of CDs was obviously quenched and reached a stable state after 5 min. What’s more, with the increase of reaction time, it did not lead to further fluorescence quenching of the CDs, so 5 min was chosen as the optimal incubation time for the detection of RIF in order to achieve a lower detection limit.

Figure 3

3.4 RIF detection

3.4.1 Selectivity and interference experiments

Selectivity is one of the important indicators for evaluating performance of a sensor. In a buffer solution of pH 7.4, the effects of Ag⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Zn²⁺, Cu²⁺, Cd²⁺, typical organic compounds in foods including amino acids, sugars and analogues (tetracycline, chloramphenicol) on the fluorescence intensity of CDs were studied. As shown in Figure 4A, metal ions had no significant effects on the fluorescence intensity of CDs, and the fluorescence quenching effect of RIF on CDs was significantly greater than that of the selected metal ions. Similarly, other substances had much less effects on the fluorescence intensity of the CDs, which confirmed that the synthetic CDs could selectively recognize RIF (Figures 4B,C). To further verify the specificity of the synthetic CDs in the detection of RIF, the effects of these interferents coexisting on the fluorescence quenching of CDs by RIF were observed. As shown in Figures 4D–F, the coexistence of these interferents has no obvious impact on the detection of RIF. These observations offered evidences for the superior selectivity and commendable resistance to interference exhibited by the CDs.

Figure 4

3.4.2 Fluorescence detection of RIF using CDs as fluorescent probe

As shown in Figure 5A, the addition of RIF could gradually decrease the fluorescence intensity of the CDs as its concentration increased, indicating that the CDs are sensitive to RIF. The change in fluorescence intensity ratio F/F₀ was linear with the added concentrations of RIF ranging from 10 μg mL⁻¹ to 90 μg mL⁻¹ (Figure 5B). The linear regression equation is F/F₀ = −7.2250C_RIF + 0.9651 R² = 0.9925, indicating a good linear relationship. The limit of detection (LOD) of RIF was calculated to be 0.687 μg mL⁻¹. These results indicated that the CDs have potential for the detection of RIF.

Figure 5

3.5 LEV detection

3.5.1 Fluorescence stability of the CDs-RIF system and optimization of detection conditions

The effects of pH and ion concentration of the solution on the fluorescence intensity of CDs-RIF and CDs-RIF-LEV systems were investigated. As shown in Figure 6A, the fluorescence intensity of CDs-RIF and CDs-RIF-LEV systems was the highest around pH = 7.4, and therefore, pH 7.4 was chosen as the optimal pH for the detection of LEV. Similar to Section 3.3, the fluorescence stability of CDs-RIF and CDs-RIF-LEV systems in the presence of different concentrations of NaCl was studied. As shown in Figure 6B, the fluorescence intensities of CDs-RIF and CDs-RIF-LEV systems at different ionic strengths did not change considerably even when the ion concentration of the solution reached up to 1.4 mol L⁻¹, which allowed the CDs-RIF system to be used in high ionic strength environments.

Figure 6

Subsequently, the change trend of fluorescence intensity with reaction time was detected, so as to clarify the response time of CDs-RIF to LEV. As shown in the Figure 6C, when LEV was added at 50 μg mL⁻¹, the fluorescence intensity of CDs-RIF system was increased significantly and reached a stable state after 5 min. What’s more, with the increase of reaction time, it did not lead to further fluorescence enhancement of the CDs-RIF system. So, 5 min was chosen as the optimal incubation time for the detection of LEV in order to achieve a lower detection limit.

3.5.2 Fluorescence detection of LEV using CDs-RIF as fluorescent probe

Interestingly, it was found that the addition of LEV to the CDs-RIF system can restore the fluorescence of CDs. As shown in Figure 7A, the fluorescence intensity of the CDs-RIF system was continuously enhanced with the increase of LEV concentration. The change of fluorescence intensity ratio F/F₀ was linear with the added concentrations of LEV ranging from 1 μg mL⁻¹ to 50 μg mL⁻¹. The linear regression equation is F/F₀ = 12.0305C_LEV + 1.0364 (R² = 0.9929), showing a good linear relationship (Figure 7B). The LOD of LEV was calculated to be 0.965 μg mL⁻¹. These results indicated that the CDs-RIF system has potential for the detection of LEV. The sensor exhibited satisfactory sensitivity and linear range that compared favorably with other methods reported for the detection of RIF and LEV in Table 1.

Figure 7

3.6 Investigation of detection mechanism

The preliminary inference on the detection mechanism of RIF was that there were -NH₂ and -COOH groups on the surface of the CDs, while a large number of -OH existed in the structure of the added RIF, which could combine through hydrogen bonding. Therefore, the electron arrangement of the surface of RIF will be affected when RIF was close to the CDs, quenching the fluorescence of the CDs. As shown in the Figure 8, the K_sv decreases with the increase of temperature, which suggests that RIF causes static fluorescence quenching in CDs. In contrast, the fluorescence recovery in the CDs-RIF system after the addition of LEV may be attributed to the electron-withdrawing groups –F and –COOH in LEV, which interacted with the hydroxyl group in LEV through hydrogen bonding, resulting in the fluorescence recovery after CDs detachment.

Figure 8

3.7 Detection of RIF and LEV in actual samples

To verify the feasibility and practicality of the developed method for analyzing of RIF and LEV in actual samples, it was employed to determine RIF and LEV in tap water and river water. Recovery experiments were carried out on tap water and river water by adding a certain amount of RIF and LEV. The recoveries of RIF in actual samples ranged from 92.2% to 110.7% with relative standard deviation (RSD) ranging from 0.2% to 4.7%. In addition, the reliability of the analytical results was confirmed by a conventional HPLC method (Table 2). It was demonstrated that the detection results of RIF are in agreement with those of the HPLC method. Similarly, the analytical results of LEV in actual samples were summarized in Table 3. The recoveries of LEV in actual samples ranged from 99.0% to 106.2% with RSD ranging from 2.3% to 5.0%, which are in agreement with the results of HPLC method. All these results demonstrated the good accuracy and stability in determining RIF and LEV using the developed method in actual samples.

4 Conclusion

In summary, CDs were successfully synthesized via one step hydrothermal treatment of citric acid and cysteine. The prepared CDs exhibited good photostability and fluorescence quantum yield up to 64.03%. The fluorescence of CDs was efficiently quenched by RIF and other coexisting substances have little influences. A good linearity between quenched fluorescence intensity and the concentration of RIF was obtained in the range of 10 μg mL⁻¹ to 90 μg mL⁻¹ with a LOD of 0.687 μg mL⁻¹ by utilize the fluorescence “turn-off” mode of the CDs. When LEV was added to the CDs-RIF system, the fluorescence intensity of the system was continuously enhanced with the increase of LEV concentration. This observation demonstrated a robust linear correlation within the concentration range of 1 μg mL⁻¹ to 50 μg mL⁻¹, with a LOD determined as 0.965 μg mL⁻¹. What’s more, the developed fluorescent probe were applied for dual-detecting RIF and LEV in actual samples, and acceptable recoveries (92.2% to 110.7%) with small RSDs (0.2%–5.0%) were obtained, indicating the good accuracy of the developed detection method. These findings demonstrated the application prospects of the CDs-based “on–off–on” sensor for monitoring RIF and LEV residues in actual samples.

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