Introduction

Lead is a significant water pollutant. Lead in aquatic systems is harmful and injurious to human health and well-being. Industries such as battery manufacturing, lead-tetra acetyl, ceramic processing, and iron smelting, to mention a few, are significant examples of sectors that disperse lead-polluted water into water bodies (Chooto 2020). In the list of priority pollutants, lead had been categorized as priority number two according to the Agency for Toxic Substances and Disease Registry (ATSDR) (United States Environmental Protection Agency 2021). Lead has little biodegradability and is highly soluble in water, thereby increasing the risk of absorption by living organisms, especially seafood, and subsequently into human systems through ingestion of seafood (Kinuthia et al. 2020; Vardhan et al. 2019). Lead exposure can damage the brain and central nervous system, causing coronary heart disease, coma, and even death. In children, exposure to lead can cause permanent intellectual disability and disordered behaviour. Fluoride ion, in the right amount, is essential in potable water. The presence of fluoride in water prevents tooth decay and the formation of cavities. At the same time, excess fluoride can lead to health issues such as dental fluorosis, brain impairment, and nervous system disorder. Industries that produce glass, fertilizer, semiconductors, and precious metals require fluoride as a processing chemical.

The availability of potable water is a global challenge; about one-third of the world’s population accesses drinking water through rivers, lakes, streams, and canals. The primary cause of water pollution is the discharge of untreated or inadequately treated industrial effluents into surface water. In developing countries, most wastewater is not treated before being released into water bodies. Sadly, most of the population in developing countries still rely on untreated surface water as their primary source of domestic water. Chances are high that wastewater from such industries will contain fluoride pollutants (El Diwani et al. 2022; Wang et al. 2021). Fluoride ion has been described as a significant source of groundwater pollution (Duggal et al. 2022; Podgorski et al. 2022).

Therefore, the need to adequately monitor the level of Pb2+ and F− priority pollutants in wastewater, seafood, and the environment is of utmost importance to the well-being of humans. Traditional analytical methods have been employed to determine Pb2+ and F− (Gao et al. 2015; Sisay et al. 2019; Zhang et al. 2017). Despite the reliability and sensitivity of these techniques, disadvantages exist, such as expensive operation costs, tedious procedures, and complex and time-consuming sample preparation. Hence, exploring other cheaper, more rapid, and sensitive methods for detecting Pb2+ and F− is inevitable.

Recently, the use of carbon quantum dots as a nanoprobe for metal ion sensing and other applications has garnered widespread interest because of their superior properties such as, cost-effectiveness and facile synthetic methods, photostability, non-toxic nature, and biocompatibility, to mention a few (Atabaev et al. 2018; Yoo et al. 2019). Numerous methods have been employed in the synthesis of carbon quantum dots (CQDs); they include electrochemical synthesis (An et al. 2021), microwave-assisted (de Medeiros et al. 2019), hydrothermal (Zhu et al. 2022), pyrolysis carbonation method (Ye et al. 2017), etc. However, the hydrothermal technique provides the following merits: ease of operation, eco-friendliness, and the CQDs obtained are mono-dispersible (Shabbir et al. 2021; Yadav et al. 2023). Several approaches have been utilized to improve CQDs fluorescence performance and optical properties, including doping or co-doping with metal and non-metallic heteroatoms. However, metallic doping or co-doping has been discouraged mainly due to the toxicity and non-uniformity of the metallic dopant.

Consequently, non-metallic heteroatoms such as nitrogen, sulphur, and phosphorus have been widely applied as dopants or co-dopants in the synthesis of CQDs for metal ion detection (Chaghaghazardi et al. 2023; Mohandoss et al. 2023; Pei et al. 2023; Yang et al. 2018). As indicated by numerous reports, co-doping with N and S is the most intriguing form of doping. Co-doping with these two results in improved quantum yield, fluorescent intensity, and red shift in emission wavelength (Sarkar et al. 2016). Three sensing strategies are involved in using CQDs for the fluorescent detection of analytes (Kainth et al. 2022). They are (i) direct interaction between the analyte and the CQDs leading to a change in fluorescence (Aladesuyi and Oluwafemi 2023a), (ii) post-synthesis functionalization of the CQDs through conjugation with specific receptors to ensure selective and sensitive fluorescent response of the analyte (Zou et al. 2020), and (iii) combination of CQDs with a sensory material that acts as a fluorescent quencher which can interact with the target analyte and lead to fluorescent recovery (Bai et al. 2021). In this study, the principles of quenching and fluorescence recovery were employed to create a nanoprobe for identifying and measuring Pb2+ and F− in wastewater and fish species. Analyte detection via luminescence ‘off–on’ technique using CQDs has been reported (Raja et al. 2023; Li et al. 2021). Boruah et al. (2020) developed a CQDs fluorescent probe for detecting Eu3+ and F− via an ‘off–on’ using biomass waste material as the precursor. The luminescence of the developed CQDs was quenched (off) upon adding Eu2+ and restored (on) when F− was introduced. The as-synthesized nanoprobe was sensitive and selective in detecting F− in aqueous solution. In another study, Liu et al. (2022) developed an  ‘on–off’ and ‘off–on’ fluorescence nanoprobe for the detection and quantification of Hg2+ and I− using polyethyleneimine and anhydrous citric acid as precursors. The obtained nanoprobe selectively detected Hg2+ and I− in river water with low detection limits and satisfactory accuracy. To the best of our knowledge, the determination of Pb2+ and F− in wastewater and consumed fish using nanoprobe such as CQDs has not been reported.

In this work, N,S-CQDs was synthesized hydrothermally using citric acid, glutamine, and sodium sulphide as precursors. The photostability of the as-produced nanosensor was examined through continuous exposure to UV light. After 6 h of constant exposure, the change in the fluorescent intensity was insignificant, indicating that the material is photostable. We subsequently assessed the selectivity of the nanosensor towards Pb2+. The as-synthesized nanosensor displayed excellent selectivity for Pb2+ amidst interfering ions. The N, S-CQDs/ Pb2+ complex was used as a nanoprobe for detecting F− ions. The developed nanosensor was further used for measuring the amount of Pb2+ and F− in wastewater and fish species through the ‘turn off’ and ‘turn on’ effects. Satisfactory recoveries of (89.30–116.40%) and (90.22–115.05%) were obtained for Pb2+ and F−, respectively. The method is facile, and the developed nanoprobe can rapidly detect Pb2+ and F− in environmental samples.

Material and methods

Materials and reagents

Citric acid, Na2S, glutamine, H2SO4, n-butanol, NaOH, PbCl2, Cr(NO)3, Fe(NO)3, Co(NO)3, Cd(NO)3, Zn(NO)3, and Pb(NO)3 were acquired from Sigma-Aldrich. Wastewater was collected from the Johannesburg canal, while Tilapia fish were procured in Johannesburg at the Shoprite supermarket. There was no further purification of the purchased materials.

Synthesis of N,S-CQDs

The co-doped CQDs were synthesized following our previous report with slight modifications (Aladesuyi and Oluwafemi 2023b). Citric acid (6mmoles), glutamine (9mmoles), and Na2S (9mmoles) were dissolved in 30 mL of deionized water. A homogenous mixture was obtained by slight stirring and ultrasonication at room temperature. Subsequently, this resulting mixture was placed in a Teflon-lined stainless-steel autoclave and heated at 200 °C for 3 h. Upon cooling, the obtained yellowish solution underwent centrifugation for 15 min at 10,000 rpm and filtered through a 0.02-µm filter paper to eliminate any unreacted particles. Dissolved impurities were removed by washing the solution severally with n-butanol. Afterwards, solid crystals of the developed nanosensor were obtained after freeze-drying.

Characterization of N,S-CQDs

The characterisation of the as-synthesized material was achieved using different techniques, which include JOEL (JEM 2100) high-resolution transmission electron microscope (HRTEM) for the morphology and particle size, Spectrum Two PerkinElmer Fourier transform infrared instrument (FTIR) fitted with an attenuated total reflection sample holder with a scanning range of 400–4000 cm−1 for the surface chemistry. An FS5 spectrofluorometer (Edinburg instrument) was used for lifetime and fluorescence measurement using a 1-cm quartz cuvette at λex of 460 nm. The excitation and emission slit were set at 5 nm, with a visible PMT-980 detector. The data were processed using Flouracle software. A double-beam SP-UV 500 spectrophotometer (Perkin Elmer) was used for absorption spectra measurement, while Brucker D8 advance diffractometer with monochromatic CuKα1 radiation (λ = 1.5418 Å) was used for XRD analysis.

Fluorescent detection of Pb2+ and F−

The following procedure was followed for the fluorescent detection of Pb2+: 100 μL of the nanosensor was introduced into a cuvette containing 3 mL of water and the fluorescence was measured at an excitation wavelength of 460 nm. The sensitivity was determined by placing the developed nanosensor individually in separate cuvettes, each containing 100 μM of various metal salts, including Zn(NO3)2, Co(NO3)2, Pb(NO3)2, Cr(NO3)3, Cd(NO3)2, NaCl, KCl, and Fe(NO3). The spectrofluorometer was used to record the fluorescence of the respective nanosensor/metal ion mixtures, and the fluorescent responses were compared. Subsequently, the fluorescent spectra of the nanosensor amidst various Pb2+ standards were recorded. To investigate the selectivity of the developed material towards Pb2+, the fluorescent spectra of a mixture containing the nanosensor, Pb2+(100 μM), along with the same volume and concentration of each of the individual metal ions such as Co2+, Cd2+, Fe3+, Cr 3+, Zn2+, Na+, and K+ were recorded under the same experimental conditions.

For F− ion detection, N, S-CQDs solution (100 µL) and Pb2+ solution (3 mL, 100 μM) were added in a centrifuge tube. The N, S-CQDs/Pb2+ mixture was combined with various concentrations of F− standards, and the total volume was adjusted to 3 mL. Subsequently, fluorescent spectra were recorded under the same conditions employed to determine Pb2+. In evaluating the selectivity of the N, S-CQDs/Pb2+ nanoprobe towards F−, the fluorescence response of a mixture of F− (100 μM), along with the same volume and concentration of anions such as Cl−, Br−, I−, CO32−, PO4−, NO3−, was recorded under the same experimental conditions.

Actual sample analysis

The detection of Pb2+ and F− in Tilapia fish extracts and environmental wastewater was performed to determine the reliability and precision of the developed sensor in the actual sample. The preparation of fish extracts was carried out using our previously reported method (Aladesuyi and Oluwafemi 2023b). The wastewater samples were obtained from the Johannesburg canal and filtered using 0.22-µm filter paper to remove insoluble waste materials. To detect Pb2+ in the resultant solutions, each sample solution, consisting of 3 mL, was combined with 100 µL of the prepared nanosensor in a cuvette, and their photoluminescence was recorded at 460 nm excitation.

Results and discussion

Characterization and optical properties of N,S-CQDs

The structural and morphological properties of the synthesized N,S-CQDs were examined using HRTEM, FTIR, and XRD techniques. HRTEM analysis (Fig. 1a) revealed that the developed nanosensor exhibited uniform dispersion and a quasi-spherical shape with an average particle diameter of 3.45 ± 0.86 nm. The crystalline nature of the material was confirmed by the presence of lattice fringes with a measured lattice spacing of 0.23 nm, matching the (002) lattice planes of graphite.

Fig. 1
figure 1

a TEM micrograph (inset: size distribution and lattice fringes) and b XRD pattern c FTIR spectrum and d Absorption spectrum of the N,S-CQDs

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XRD analysis (Fig. 1b) showed a broad hump at 2θ = 21.55°, corresponding to the (002) graphite lattice plane, indicating a low graphitic carbon structure. The FTIR spectrum (Fig. 1c) shows a peak at 3214 cm−1 attributed to O–H stretching vibration bands from the carboxylic groups on the surface of the N,S-CQDs, while the peak at 3096 cm−1 corresponded to the N–H stretching from the conjugated amide group. The absorption bands at 1661 cm−1 and 1576 cm−1 represented the C = O and C-NH groups, respectively. The peak at 713 cm−1 confirmed the presence of C-S stretching vibrations (Wu and Tong 2019), while the peaks in the range of 1406–1102 cm−1 were assigned to C–C, C–N, and C–O stretching vibration bands. The existence of these hydrophilic groups on the CQDs’ surface likely contributed to their improved solubility and fluorescence behaviour (Zu et al. 2017).

The optical properties of the synthesized nanosensor were explored through UV–Vis absorption and photoluminescence spectroscopy (Fig. 1d). The absorption spectra exhibited two peaks at 225 and 330 nm. The former corresponded to the π–π* transition of aromatic C = C and C = N bonds, while the latter could be attributed to the n-π* transition of the C = O bond (Hao et al. 2023; Keenan et al. 2019). The quantum yield (QY) significantly increased from 1.59 to 10.35% when co-doping with N and S, indicating that co-doping with electron-rich heteroatoms substantially enhanced the QY. The N,S-CQDs exhibited excitation-dependent emission, a phenomenon influenced by the energy state at the surface (Hu 2016).

Fluorescent detection of Pb 2+

The ability of the N,S-CQDs to accurately detect Pb2+ was investigated under optimized experimental parameters at pH 6 and an incubation time of 5 min to ensure proper interaction between the nanosensor and the quencher molecule (Pb2+). The fluorescent intensity of the as-prepared material decreased gradually in the presence of Pb2+ standards. The degree of reduction in fluorescent intensity was concentration-dependent (Fig. 2a). The decrease in fluorescent intensity had been attributed to the coordination of the Pb2+ ion with the carboxyl or sulfhydryl group on the CQDs’ surface (Dewangan et al. 2022; Shi et al. 2022). The formation of complexes involving metal ions, hydroxyl, and carboxyl groups gives rise to changes in the electronic structure of the N, S-CQDs, which, in turn, disrupts the activity and arrangement of the electrons. This adjustment in electron distribution can lead to non-radiative recombination of electrons and holes and, consequently, a reduction in the fluorescent intensity of the CQDs. The reduced fluorescent intensity at different Pb2+ standards (0 to 25 µM) has a linear relationship, as illustrated in Fig. 2b. This linear correlation conforms to the Stern–Volmer equation, with an R2 value of 0.9913.

$$\frac{{Fo}}{F} = {\text{ }}K_{{sv}} C{\text{ }} + {\text{ }}1$$
Fig. 2
figure 2

a PL spectra of N,S-CQDs in the presence of Pb2+ standards and b Stern–Volmer plot (R2 = 0.9913). Selectivity assessment of fluorescence response of the nanosensor towards Pb2+ c and metal ions d amid interfering metal ions

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Ksv—Stern–Volmer constant, C—concentration of quencher ion (Pb2+), F and Fo are the respective PL intensities of nanosensor with and without Pb2+ ions.

The calculated limit of detection (LOD) of 13.13 nM was obtained based on \(\frac{3SD}{S}\) (SD is the standard deviation, S is the slope of the calibration curve). The as-synthesized material can detect Pb2+ with selectivity and sensitivity comparable to previous reports (Table 1).

Table 1 Comparison of the as-synthesized fluorescent probe with reported methods for the detection of Pb2+
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Sensors’ selectivity and anti-interference ability are vital tools in determining the efficacy and suitability of the material, especially their potential applications in complex systems. As a result, common metal ions such as Co2+, Cd2+, Fe3+, Cr 3+, Zn2+, Na+ and K+ were used in investigating the selectivity (Fig. 2c) and the anti-interference (Fig. 2d) ability of the as-synthesized nanosensor. The result shows that the presence of the other metal ions has little or no impact on the fluorescence response of the nanosensor to Pb2+. This indicates that the as-synthesized material is highly selective towards Pb2+ ions compared to other interfering ions.

Fluorescent detection of F− using N,S-CQDs-Pb2+ nanoprobe

The affinity between lead and fluoride motivates the design of an ‘off–on’ fluorescent nanoprobe for the determination of fluoride in actual samples. The ability of the N, S-CQDs-Pb2+ to detect F− was investigated by adding fluoride standards (0-12 µM) under optimized experimental conditions (pH 6 and incubation time of 5 min) at 460 nm excitation. The result (Fig. 3a) indicates that the as-synthesized nanosensor transited from the ‘off’ mode due to the quenching effect of Pb2+ to the ‘on’ mode (fluorescence recovery) due to F− standards. The extent of fluorescent recovery is concentration-dependent, a clear indication that the N,S-CQDs-Pb2+ can be used to detect and quantify F−. The fluorescent recovery could be attributed to the ionic bond formation between lead and fluoride to give PbF2. In addition, lead, a relatively hard acid, will have an affinity for a hard base in water, such as fluoride ions. The linear relationship between the change in fluorescent intensity and F− standards (0–12 µM) is shown in Fig. 3b; the detection limit was calculated to be 43.17 nM. The selectivity studies (Fig. 3c) show that the N-S-CQDs + Pb2+ system was selective towards F− compared to other non-metal ions tested (Cl−, Br−, I−, CO32−, NO−3, PO3−4). There was no significant recovery in PL intensity in the presence of other anions. Furthermore, a mixture of F− and these anions was added to the solutions of the N-S-CQDs + Pb2+ system under the same experimental conditions (Fig. 3d). The PL recovery was similar to that obtained using F− ion alone as the interfering ions posed only insignificant change in the intensity of the recovered fluorescence. The result shows that the as-synthesized N-S-CQDs + Pb2+ nanosensor displayed an outstanding selectivity towards F− ion in the presence of interfering ions. A comparison of various nanosensors for determining F− ion is shown in Table 2.

Fig. 3
figure 3

a PL spectra of N,S-CQDs/ Pb2+ under the influence of F− standards and b Stern–Volmer plot (R2 = 0.9913). Selectivity assessment of fluorescence response of the nanosensor towards F− c and non-metal ions d amid interfering non-metal ions

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Table 2 Comparison of the as-synthesized fluorescent probe with reported methods for the detection of F−
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Detection mechanism

The common fluorescent quenching mechanisms associated with CQDs include static quenching, dynamic quenching, inner field effect (IFE), and Forster energy resonance transfer (FRET) (Tian et al. 2023; Wang et al. 2022). Static quenching occurs due to the formation of a non-fluorescent ground-state complex between the quencher and the CQDs, thereby forming a new substance (Giordano et al. 2023). Dynamic quenching, on the other hand, occurs when the CQDs return to the ground state from the excited state due to collision with the quencher molecule (Molaei 2020). This is achieved via energy or charge transfer between the duo. In FRET, the quenching mechanism occurs via energy transfer from a luminescent donor to an acceptor when the distance is within 10 nm (Gopal et al. 2024; Miao et al. 2020). This results in the donor’s fluorescence loss and the enhancement of the acceptor’s fluorescence. The IFE mechanism occurs when an overlap occurs between the acceptor’s absorption spectrum and the CQD’s excitation or emission spectra (Molaei 2020; Rahal et al. 2021). Time-resolved fluorescence lifetimes measurement and the UV absorption spectra are essential tools used to evaluate the quenching mechanism of CQD-based nanoprobe (Liu et al. 2021; Liu et al. 2024). The fluorescence lifetime and the absorption spectra of N, S-CQDs, N, S-CQDs + Pb2+, and N, S-CQDs + Pb2+  + F− are shown in Fig. 4a and b. The average lifetimes of N, S-CQDs, N,S-CQDs + Pb2+, and N,S-CQDs + Pb2+  + F− were measured to be 3.605 ns, 5.495 ns, and 5.105 ns, respectively (Fig. 4a). The lifetime increased on the addition of Pb2+ and F−; the difference in the lifetime is an indication of excited state electron or energy transfer between the N,S-CQDs and the quencher molecules (Pb2+), a characteristic of dynamic quenching (Long et al. 2021; Molaei 2019). The strong interaction between Pb2+ and F− led to the displacement of Pb2+ from the surface of the N,S-CQDs, consequently leading to the recovery of fluorescent intensity. As depicted in Fig. 4b, no new absorption peaks were observed in the UV absorption spectra of N,S-CQDs, N,S-CQDs + Pb2+, and N,S-CQDs + Pb2+  + F−. This indicates that no new substance was formed, as there was no ground-state interaction between the nanosensor and the quencher molecules; this further suggested a dynamic quenching mechanism.

Fig. 4
figure 4

a Fluorescence lifetime decay curve of N,S-CQDs, N,S-CQDs + Pb2+ and N,S-CQDs + Pb2+  + F−. b Absorption spectra of N,S-CQDs, N,S-CQDs + Pb2+ and N,S-CQDs + Pb2+  + F−

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Validation of method using real samples

Wastewater and fish samples were used to determine the viability of the as-synthesized nanosensor for detecting Pb2+ and F− in real samples (Table 3). Spiked samples displayed excellent recoveries with the nanosensor. Actual spiked samples showed remarkable recoveries for both Pb2+ (89.30%-116.40%) and F− (90.22–115.05%) with low RSD (< 5). These results demonstrated that the developed turn ‘off–on’ fluorescent nanosensor is reliable and accurate for quantifying Pb2+ and F− in aquatic systems.

Table 3 Determination of Pb2+ and F− in spiked samples of wastewater and fish extract using the developed nanosensor
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Conclusions

In summary, we synthesized N- and S-doped carbon quantum dots using CA, glutamine, and Na2S as precursors through a facile hydrothermal method. The resulting nanosensor was employed as an ‘on–off’ and ‘off–on’ fluorescent sensor for precisely detecting Pb2+ and F− ions with outstanding sensitivity and selectivity. A LOD of 13.35 nM and 43.17 nM was obtained for Pb2+ and F−, respectively. The quenching mechanism, as evidenced by fluorescence decay lifetime and UV–Vis spectrum analysis, indicated a dynamic quenching mechanism. Furthermore, this developed material was effectively employed as a fluorescent probe for detecting Pb2+ and F− ions in wastewater and fish samples, respectively, yielding accurate measurements and excellent recovery rates. Thus, the developed nanosensor is a promising tool for the rapid assessment of Pb2+ and F− ions in aquatic systems. Based on this study and previous literature, CQD-based nanosensors have shown exemplary performance regarding accuracy, selectivity, and low LODs. However, performance improvement in specific areas, such as longer wavelength emission and higher fluorescent quantum yield, still needs to be achieved. In addition, the design of CQDs that will be useful for adequately sensing gaseous pollutants is highly desirable.