Introduction

There is an increasing amount of evidence that supports the association between the uptake of ziram and certain neurological diseases [1]. Therefore, the detection of ziram residues is of great significance for public health security. Traditional methods for the detection of ziram, including gas chromatography (GC), absorption spectrophotometry, and Fourier transform infrared (FTIR) spectroscopy, mainly rely on the use of large-scale analytical instruments [2]. These techniques show advantages in the detection of ziram owing to their high precision and sensitivity. However, these methods generally require sophisticated analytical procedures, which usually involve the initial acid hydrolysis of ziram to yield carbon disulfide (CS2) followed by instrument detection [3]. One alternative method consists of extraction and enrichment and then subsequent detection by liquid chromatography. Although this detection method enables the direct detection of ziram, derivatization is usually required because of the instability of ziram. Flame atomic absorption spectrometry, voltammetry, and inductively coupled plasma mass spectrometry can also be used for the detection of ziram. However, substances containing zinc easily interfere with these detection methods, because their detection mainly relies on the indirect analysis of zinc content in ziram. Therefore, it is highly desirable to develop inexpensive, rapid, and convenient detection methods for use in the on-site monitoring of ziram.

In the last few years, photochemical methods have been widely used for the detection of analytes due to their advantages of easy operation, low cost, and suitability for portable warning apparatus [4]. Up to now, photochemical methods based on different enzymes have been successfully applied in the analysis of organophosphorus and carbamate pesticide residues [5]. Nevertheless, the strong non-specific chelating properties of ziram largely restrict the applications of these optical enzyme chemosensors in the detection of ziram [6, 7]. Recently, photochemical methods based on gold nanoparticles for the analysis of pesticide residues have gained significant attention [8]. However, the extent of the aggregation of gold nanoparticles enables changes in the produced optical properties, which may lead to inaccurate determination. Copper nanoparticles are an alternative fluorometric probe for the analysis of pesticide residues [9]. However, their instability greatly limits the practical application of copper nanoparticles. Thus, it is necessary to seek new fluorometric probes with high selectivity and considerable sensitivity for ziram assay.

Metal halide perovskites have become highly sought-after versatile materials over the past few years, owing to their ease of synthesis, structural tunability, and appealing potential applications in various domains [10]. In effect, metal halide perovskites have become possible candidates for fluorescent probes owing to their excellent optical properties, such as broad color tunability, high absorption coefficients, and high quantum efficiencies [11]. Previous studies have indicated that most metal halide perovskites may undergo reversible compositional and structural changes that reduce their fluorescence after exposure to external stimuli. This feature makes them potentially useful as fluorescent probes [12,13,14]. Very recently, metal halide perovskites have been used as an indicator for the assays of temperature, humidity, various gases, solvents, and metal ions [15,16,17,18,19,20,21,22]. For example, Dong’s group reported a novel photochemical method based on CsPbX3 (X = Cl, Br, or I) quantum dots (QDs) for the detection of ammonia [23]. Their photochemical method, which works using turn-on fluorescence, showed high selectivity and sensitivity toward ammonia with a limit of detection as low as 8.85 ppm. Despite these successes, the application of metal halide perovskites for the detection of pesticides or fungicides is rare.

In this work, we described the initial use of CsPbBr3 QDs for the detection of pesticides and fungicides. The highly luminescent CsPbBr3 QDs, which adopted didodecyldimethylammonium bromide (DDAB) and octanoic acid as ligands, were used to detect ziram with high selectivity and sensitivity. The limit of detection was calculated to be 0.086 ppm for ziram with a linear quantitation range of 0.10 to 50.0 ppm. The determination of ziram in spiked fruit samples was also performed, and satisfactory recoveries (100–107%) were achieved. Furthermore, the mechanism of ziram detection was investigated and tentatively explained by the increase of surface defects and structural changes of the CsPbBr3 QDs.

Experimental

Chemicals and materials

PbBr2 (99%), Cs2CO3 (99.99%), tetraoctylammonium bromide (98%), didodecyldimethylammonium bromide (DDAB, 98%), and octanoic acid (99%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China, https://www.aladdin-e.com/). Pesticide and fungicide standards, including ziram (> 97.0%), chlorsulfuron (98%), fipronil (≥ 98%), cartap (98%), chlorpyrifos (99%), dimethoate (98%), molosultap (98%), fenobucarb (99%), maneb (> 97.0%), zineb (> 97.0%), propineb (98%), and imidacloprid (98.5%), were also bought from Aladdin Reagent Co., Ltd. (Shanghai, China, https://www.aladdin-e.com/). All of the chemicals were used as received without further purification.

Instruments for the characterization of the CsPbBr3 QDs

X-ray diffraction analysis (XRD) of the CsPbBr3 QDs was conducted at room temperature using an X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation operating at 40 kV and 40 mA. Fluorescence spectrophotometry (F-4600) was used to record the emission and excitation spectra. To determine the relative photoluminescence quantum yield (PLQY), sodium fluorescein in 0.1 M NaOH solution (PLQY = 0.95) was used as the reference fluorophore. Luminescence lifetime was measured on a fluorimeter (Edinburgh, FLS980). The FTIR spectra of the samples were recorded using a FTIR spectrometer (Thermo Scientific Nicolet iS10). The ultraviolet–visible light (UV–Vis) absorption spectra were determined at room temperature using a UV–Vis spectrophotometer (UV-2401). X-ray photoelectron spectroscopy (XPS) was measured by a Thermo Scientific ESCALAB 250 XI XPS spectrometer. The surface morphology of the CsPbBr3 QDs was recorded using transmission electron microscopy (TEM, FEI-Tecnai G2 Spirit TWIN) at a voltage of 120 kV.

Synthesis of CsPbBr3 QDs

Refer to the electronic supplementary material for synthesis details.

Determination of ziram and real sample experiments

The obtained CsPbBr3 QDs were diluted 10 times to 4.8 ppm with toluene. Afterward, 1 mL of CsPbBr3 QD dilution (4.8 ppm) was added into 1 mL of various concentrations of ziram (0.1, 5.0, 12.5, 20, 40, and 50 ppm). After shaking and then incubation at 30 °C for 30 min, the fluorescence spectra of all the samples were analyzed using a fluorescence spectrophotometer (Ex/Em = 365/516 nm).

Selective experiments were conducted with 14 pesticides and fungicides, i.e., admire, chlorpyrifos, fipronil, dimethoate, chlorsulfuron, monosultap, cartap, fenobucarb, dazomet, aapirol, maneb, zineb, propineb, and ziram, with concentrations of 125 ppm. Seven metal ions (Al3+, K+, Na+, Ba2+, Ni2+, Mg2+, and Zn2+; 125 ppm) were used for the interference test under the same conditions. First, 1 mL of CsPbBr3 QD dilution (4.8 ppm) was added to 1 mL of each of the above pesticides, fungicides, and metal ion solutions (125 ppm). After shaking and then incubation at 30 °C for 30 min, the fluorescence spectra were recorded (Ex/Em = 365/516 nm).

For real sample experiments, fruits (apples and cherry tomatoes) that were bought from a local supermarket were used as real samples. The real samples (10 g) were washed with toluene (10 mL) to prepare the fruit sample solution. Next, 1 mL of CsPbBr3 QD dilution (4.8 ppm) was added to 1 mL of the above fruit sample solution. After shaking and then incubation at 30 °C for 30 min, the fluorescent spectra of the mixture solution were recorded to obtain the original concentration of ziram in the real samples (Ex/Em = 365/516 nm). Afterward, 1.0, 5.0, and 15 ppm (mg∙kg−1) ziram were separately added onto the surface of the real samples (10 g). The spiked real samples were then washed with toluene (10 mL) to create spiked solutions for further determination. Then, 1 mL of CsPbBr3 QD dilution (4.8 ppm) was added to 1 mL of the spiked solution. After shaking up and subsequent incubation at 30 °C for 30 min, the fluorescence spectra of the above mixture solution were recorded at the excitation wavelength of 365 nm and the emission wavelength of 516 nm.

Results and discussion

Synthesis and characterization of CsPbBr3 quantum dots

The CsPbBr3 QDs were synthesized through a facile and inert gas-free process at room temperature, similar to a previous study [24]. As depicted in Fig. 1, a mixture of PbBr2, tetraoctylammonium bromide, and toluene was taken as the lead bromide precursor, while a mixture of CsCO3 and octanoic acid was used as the cesium precursor. The cesium precursor solution was swiftly added to the lead bromide precursor while stirring, giving rise to a yellow-green turbid liquid. DDAB solution was then added to the obtained turbid liquid to obtain stable CsPbBr3 QDs. The microstructure of the CsPbBr3 QDs was investigated by transmission electron microscopy (TEM). As shown in Fig. 2a, CsPbBr3 QDs exhibit cubic morphology, which is similar to that of CsPbBr3 QDs previously reported elsewhere [24, 25]. The size distribution, shown in the inset of Fig. 2a, reveals that the average diameter of CsPbBr3 QDs is 11.62 nm. X-ray diffraction (XRD) analysis was carried out to further understand the structural properties of the CsPbBr3 QDs. As depicted in Fig. S1, the XRD pattern of the CsPbBr3 QDs matches well with monoclinic CsPbBr3 (PDF# 18–0364), suggesting that the CsPbBr3 QDs had a monoclinic crystal lattice. The formation of CsPbBr3 QDs was further confirmed by X-ray photoelectron spectroscopy (XPS) and IR analyses (Figs. S2–S4).

Fig. 1
figure 1

Scheme for the synthesis of CsPbBr3 QDs

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Fig. 2
figure 2

a TEM image of the CsPbBr3 QDs (inset: size distribution of CsPbBr3 QDs). b Photoluminescence properties of the CsPbBr3 QDs (Ex/Em = 365/515 nm)

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Optical properties of CsPbBr3 QDs

Generally, CsPbBr3 QDs have been reported to have excellent photoluminescent properties [26]. In this work, the fluorescence spectra of CsPbBr3 QDs were recorded. As shown in Fig. 2b (blue line), the CsPbBr3 QDs show a characteristic emission peak centered at 515 nm under 365 nm excitation. The narrow emission peak with a full width at half maximum (FWHM) of about 35-nm illustrates that the CsPbBr3 QDs had high color purity. At the same time, the CsPbBr3 QDs display a high luminescence quantum yield of up to 96%. There are two broad peaks at approximately 288 and 365 nm in the excitation spectrum of the CsPbBr3 QDs, as depicted in Fig. 2b (red line). The UV–Vis absorption spectrum of CsPbBr3 QDs was also determined and was plotted, as is shown in Fig. S5. These results differ from those of the lead bromide and cesium precursors, further suggesting that CsPbBr3 QDs have been obtained in this work. It is worth noting that the absorption peaks of CsPbBr3 QDs at 276 and 311 nm are slightly different from those in the excitation spectrum. A biexponential function fits the PL decay curve of the CsPbBr3 QDs well, as depicted in Fig. S6. The lifetimes of the CsPbBr3 QDs were τ1 = 2.76 and τ2 = 19.04 ns, which can be attributed to bound and free excitons, respectively. The average lifetime of the CsPbBr3 QDs (10.9 ns) is much shorter than those of hybrid halide perovskites, indicating the high ratios of radiative to nonradiative transition [27]. The photostability of the CsPbBr3 QDs was also determined. As shown in Figs. S7 and S8, the PL intensity of the CsPbBr3 QDs is maintained at > 95% after heating for 1 h at 60 °C and at > 90% even after exposure to UV light (365 nm) for 7 days. These results indicate the strong stability of the CsPbBr3 QDs.

Determination of ziram and real sample experiments

First, optimization of the reaction temperature, reaction time, and CsPbBr3 QD concentration was achieved. As shown in Figs. S9–S11, the optimal reaction temperature, reaction time, and CsPbBr3 QD concentration are 30 °C, 30 min, and 4.8 ppm (after diluting 10 times), respectively. To demonstrate the detection sensitivity of CsPbBr3 QDs toward ziram, fluorometric titration experiments were performed by adding ziram toluene solution at different concentrations to CsPbBr3 QD solution. As depicted in Fig. 3a, under 365-nm excitation, the photoluminescence (PL) emission intensity gradually decreases as the concentration of ziram increases. The PL emission intensity (Ex/Em = 365/516 nm) of the CsPbBr3 QDs against different concentrations of ziram is plotted in Fig. 3b. The calibration plot illustrates that the CsPbBr3-based method possesses relative accuracy for the determination of ziram at different concentrations ranging from 0.1 to 50 ppm (mg∙kg−1), with a correlation coefficient (R2) of 0.9958. Considering that the LOD = 3σ/k, where LOD, σ, and k are the limit of detection, the standard deviation of the three measurements for blank samples, and the slope of the calibration plot, respectively, the LOD is calculated to be as low as 0.086 ppm. This result is below the permitted maximum concentration listed in the EU pesticides database (0.1 mg∙kg−1), indicating the outstanding sensitivity of CsPbBr3 QDs for the detection of ziram. In comparison with currently available determination methods for ziram, the convenient synthesis and large determination range of CsPbBr3 QDs without the need for pretreatment demonstrate their great potential in the determination of ziram (Table 1) [8, 28,29,30,31]. The low cost and abundance of raw materials make CsPbBr3 QDs a promising candidate for alternative noble metal nanomaterials (such as gold and silver nanoparticles) with potential applications in pesticide assays.

Fig. 3
figure 3

a Emission spectra of CsPbBr3 QDs after reacting with different concentrations of ziram (0–50 ppm) (Ex = 365 nm). b Relationship between the PL emission intensity (Ex/Em = 365/516 nm) of CsPbBr3 QDs and the concentration of ziram (0.1–50 ppm) pesticides (n = 3). c Selectivity study of CsPbBr3 QDs in the presence of other pesticides (125 ppm). Ex/Em = 365/516 nm (n = 3). d PL intensity changes (Ex/Em = 365/516 nm) of CsPbBr3 QDs in the presence of various interfering metal cations (125 ppm)

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Table 1 Performance comparison of different optical methods for the determination of ziram
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To investigate the selectivity of CsPbBr3 QDs toward ziram, 14 pesticides and fungicides (admire, chlorpyrifos, fipronil, dimethoate, chlorsulfuron, monosultap, cartap, fenobucarb, dazomet, aapirol, maneb, zineb, propineb, and ziram) were used for comparative experiments. The structural formulas of the relevant pesticides and fungicides are depicted in Fig. S12. As shown in Fig. 3c, fluorescence quenching induced by ziram is far more remarkable than that of admire, chlorpyrifos, fipronil, dimethoate, chlorsulfuron, monosultap, cartap, or fenobucarb. Dithiocarbamates (DTCs), including dazomet, aapirol, maneb, zineb, and propineb, have a certain degree of influence on the fluorescence intensity of the CsPbBr3 QDs. It is worth noting that the fluorescence quenching induced by DTCs with zinc (zineb, propineb, and ziram) was stronger than that of other DTCs (dazomet, aapirol, and maneb). This result is in agreement with the detection mechanism, which can be tentatively explained by the interactions between the Cs and Br of the CsPbBr3 QDs and the S and Zn of ziram. Therefore, the CsPbBr3 QDs may also be used for the determination of zineb and propineb. As shown in Figs. S13 and S14, the experimental data confirm this speculation. The interference of common metal cations was also evaluated in this work. As depicted in Fig. 3d, samples containing Zn2+ influence the fluorescence intensity of the CsPbBr3 QDs to a certain extent. However, interference from other metal cations is almost negligible.

To investigate the practicality of CsPbBr3 QDs for ziram detection, the analysis of apple samples, cherry tomato samples, and recoveries tests (1.0, 5.0, and 15 ppm) were performed. The measurement conditions were identical with the standard experiment except for the replacement of toluene with the sample solution. It is worth noting that the original concentrations of ziram in the apple and cherry tomato samples were 0.081 and 0.075 ppm, respectively, which were determined with molecular absorption spectrometry (GBT5009.218–2008, Fig. S15). This original concentration was below the LOD of our method. Therefore, the found concentrations were obtained from the difference between the data calculated by the calibration plot (y =  − 40.305x + 4741.8) and the original concentrations. The recoveries were calculated based on the ratio of the found and spiked concentrations. As shown in Table 2, the found concentrations are close to the concentrations obtained by molecular absorption spectrometry. The small deviation between the found and spiked concentrations can be tentatively explained by the interference of impurities on the surface of the fruit samples. Recoveries ranging from 100 to 107% were also obtained. According to the Codex Alimentarius Commission (CAC) pesticide maximum residue limit (MRL) database, the permitted MRL of ziram in apples and tomatoes is 2 ppm (mg∙kg−1) [32]. The good recoveries of the spiked samples (5.0 and 1.0 ppm) indicate that the CsPbBr3 QDs can be considered a novel fluorescent probe capable of detecting ziram. Despite this success, the application of our method in the analysis of biomatter (such as blood, serum, cells, and marine water) is restricted due to its strong background UV absorption and fluorescence.

Table 2 Recoveries tests of ziram residues on the peels of apple and cherry tomatoes (n = 3)
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Reaction mechanism

To determine the mechanism for the reaction of CsPbBr3 QDs with ziram, a variety of dynamic studies, including UV–Vis absorption, TEM, XPS, and IR analyses, were conducted. First, the UV–Vis absorption spectra of the various pesticides and fungicides were determined to investigate the fluorescence resonance energy transfer (FRET) behavior between CsPbBr3 QDs and ziram. As shown in Fig. S16, the absorption spectrum of ziram exhibits almost no overlap with the emission spectrum of CsPbBr3 QDs, which indicates that there was no effective FRET [33]. The lifetime of the CsPbBr3 QDs was calculated to further explain the fluorescence quenching mechanism. As shown in Fig. S17, after reacting with ziram, there is only a neglectable change in the lifetime of the CsPbBr3 QDs (from τ1 = 2.76 and τ2 = 19.04 to τ1 = 1.78 and τ2 = 18.34 ns). This further indicates that there was no effective FRET. Previous reports have demonstrated that ion exchange, surface defects, and structural transformations are common photoluminescence sensing mechanisms for CsPbBr3 QDs [34]. According to previous reports, ion exchange between Zn2+ and Pb2+ leads to shifts in the emission wavelength [15]. Nevertheless, the emission wavelength of CsPbBr3 QDs showed negligible shifts after reacting with ziram, indicating that the substitution of Zn2+ ions with Pb2+ in the CsPbBr3 QDs did not occur. Hence, surface defects and structural transformations were suspected to be involved in the reaction mechanism of CsPbBr3 QDs and ziram. To examine this speculation, TEM images were recorded. As depicted in Fig. 4a, the CsPbBr3 QDs after reacting with ziram has a spherical morphology with an average diameter of 32.46 nm. The TEM results imply that an interaction between CsPbBr3 QDs and ziram may have contributed to the structural changes of the CsPbBr3 QDs.

Fig. 4
figure 4

a TEM images of the CsPbBr3 QDs after reacting with ziram (inset: size distribution of the CsPbBr3 QDs). b, c XPS spectra of Cs 3d and Br 3d in CsPbBr3 QDs before and after reacting with ziram. d, e XPS spectra of S 2p and Zn 2p in ziram before and after reacting with CsPbBr3 QDs

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To obtain more information about the interaction between CsPbBr3 QDs and ziram, the XPS spectra of the CsPbBr3 QDs before and after reacting with ziram were recorded and plotted (Fig. S18). The Cs:Pb:Br atomic ratio of the CsPbBr3 QDs before and after reacting with ziram was 0.84:1:3.38 and 1.16:1:3.94, respectively. The Pb 4f spectrum of CsPbBr3 QDs showed negligible changes after reacting with ziram, indicating that Pb did not directly interact with ziram (Fig. S19). These results further rule out the existence of ion exchange between Zn2+ and Pb2+. The Cs 3d peaks at 724.22 eV shifted slightly to 724.60 eV after reacting with ziram (Fig. 4b), while the Br 3d peaks shifted from 67.79 to 68.46 eV after reacting with ziram (Fig. 4c). These results suggested that a reaction may have occurred between the Cs and Br in the CsPbBr3 QDs and ziram. With this in consideration, the XPS spectra of S 2p and Zn 2p were also recorded. As shown in Fig. 4d and e, the S 2p peak at 161.65 eV shifts slightly to 161.38 eV and the Zn 2p peak at 1021.8 eV is found to have shifted slightly to 1021.62 eV. These results indicate an interaction between the Cs and Br in the CsPbBr3 QDs and the S and Zn of ziram. Such an interaction can be further confirmed via IR spectra (Fig. S20). In the IR spectra of ziram, the peaks at 1390 and 1514 cm−1 were attributed to the C–N stretching vibrations with partial double-bond character. These two peaks almost completely disappeared after reacting with CsPbBr3 QDs [35]. The interactions between the Cs and Br of the CsPbBr3 QDs and the S and Zn of ziram are in good agreement with the results obtained from the selectivity and interference experiments. Notably, the interaction between the CsPbBr3 QDs and ziram suggests that the added ziram was attached to the surface of CsPbBr3 QDs and had replaced the original ligands. This replacement would increase the surface defects of the CsPbBr3 QDs, further leading to a structural change of the CsPbBr3 QDs from cubic morphology to spherical morphology. Thus, the sensing mechanism of the CsPbBr3 QDs toward ziram is supposedly the interaction between ziram and CsPbBr3 QDs that gives rise to increased surface defects, structural changes, and the fluorescence quenching of CsPbBr3 QDs.

Conclusions

In this work, a novel and facile fluorometric strategy for the determination of ziram was developed using CsPbBr3 quantum dots. After reacting with ziram, increased CsPbBr3 QD surface defects and structural changes led to a decrease in the fluorescence intensity of the CsPbBr3 QDs. The strategy was successfully applied in the analysis of fruit samples, and satisfactory recoveries were achieved. Overall, this method is simple, accurate, and can be generalized for various zinc DTCs, indicating its broad potential applications in food safety. However, the application of our method in the analysis of complex biological samples is yet to be resolved, due to their strong background UV absorption and fluorescence.