Introduction

In the past several decades, the control of heavy metal pollution has been the focal point of environmental protection efforts1,2,3,4,5,6. Development of simple and selective sensors is critical for detection of heavy metals. Several methods such as atomic absorption spectrometry (AAS)7, inductively coupled plasma-mass spectrometry (ICP-MS)8, inductively coupled plasma atomic-emission spectroscopy (ICP-AES)9, electrochemical methods10, and fluorescent techniques11 have been devised to detect heavy metals. Compared with other methods, the fluorescence-based methods display many advantages such as low cost, high sensitivity, rapid detection, and ease of use12. As fluorescent materials, lead halide perovskite CH3NH3PbBr3 (MAPbBr3) and CsPbBr3 show excellent luminescent properties including bright photoluminescence (PL), high PL quantum yields (PLQY), and narrow bandwidth13,14,15. Compared with MAPbBr3, the PL emission peaks of MAPbI3 and MAPbCl3 are red and blue light, respectively, which can’t be excited by a UV lamp. Due to these advantages, the MAPbBr3 and CsPbBr3 have been used in light-emitting diodes (LED)16,17,18,19,20,21,22 and fluorescence sensors or detectors12,23. Chinnadurai et al.24 reported that fluorescent MAPbBr3 nanoparticles can be used as an excellent sensor for the detection of 2, 4, 6-trinitrophenol (TNP). Liu et al.25 used CsPbBr3 perovskite quantum dots as photoluminescent probe for selective detection of Cu2+. Zhang et al.12 encapsulated MAPbBr3 perovskite quantum dots in MOF-5 matrix as a stable fluorescent probe for the detection of Al3+, Bi3+, Co2+, Cu2+, Fe3+, and Cd2+. The detection mechanisms of the perovskite fluorescent sensor is mostly related to luminescence-quenching mechanisms. The introduction of metal ions in perovskite solutions will quench the PL performance of perovskite materials. However, the excellent PL properties of MAPbBr3 and CsPbBr3 are due to the Pb2+ ion. The high toxicity of Pb2+ is a considerable concern for the future applications of the lead halide perovskite fluorescent probe. In this work, we report on the use of MABr solution for the selective and sensitive detection of Pb2+. The MABr solution detects the Pb2+ due to the luminescence enhancing effect which is different from the quenching mechanisms of lead halide perovskite fluorescent probes.

Experimental Section

All materials were purchased from Xi’an Polymer Light Technology Corp (China). The MABr solution was prepared by dissolving 0.8 mmol MABr in 1 ml N, N-dimethylformamide (DMF). To detect the Pb2+ concentration, different amounts of PbBr2, PbI2 and PbCl2 powders were added into MABr solutions to form the MABr@PbBr2, MABr@PbI2and MABr@PbCl2 precursor solutions, respectively. After stirring the precursor solutions at room temperature for 30 min, the MABr@PbBr2, MABr@PbI2 and MABr@PbCl2 solutions were transformed into MABr@MAPbBr3, MABr@MAPbBr3−xIx and MABr@MAPbBr3−xClxsolutions that are transparent liquids under room light. The photoluminescence (PL) emission spectra of the MABr@MAPbBr3, MABr@MAPbBr3−xIx and MABr@MAPbBr3−xClx solutions were measured by a photoluminescence system in the reflection mode. The time-resolved PL spectra of MABr@MAPbBr3 solution were measured by an FLS980 time-resolved fluorescence spectrometer (Edinburgh Instrument). To analyze the structures of these solutions, MABr@MAPbBr3 solutions were dropped on the glass substrate and then heated at 100 °C for 30 min in order to evaporate the DMF solvents. After the heat treatment, the precipitates of MABr@MAPbBr3 solutions were formed on the substrate. For all of the samples on the substrates, X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements were carried out to analyze the crystal structure and morphology of the precipitate, respectively. Dynamic light scattering measurement (DLS) was conducted to analyze the size distributions of the particles in MABr and MABr@MAPbBr3 solutions.

Results and Discussion

Figure 1 shows the photographs of MABr@MAPbBr3 solutions under 365 nm UV lamp in a darkroom. As shown in Fig. 1(a–h), the solutions consisted of 0.8 M MABr and different amounts of PbBr2 (0–2 × 10−1 M). All of the solutions were transparent under ambient light. The MABr solution without PbBr2 only reflects the purple color of the UV light under the UV lamp illumination, as shown in Fig. 1(a), indicating that the MABr solution is nonfluorescent under the UV lamp illumination. Introduction of a small amount of PbBr2 to the MABr solution leads to the formation of the MABr@MAPbBr3 solution, and the MABr@MAPbBr3 solution emits very pale yellow color under UV light illumination. As the Pb2+ concentration of the MABr@MAPbBr3 solutions increased from 1.6 × 10−3 to 2 × 10−1 M, the emission colors of these solutions changed quickly from pale yellow to bright green, as shown in Fig. 1(b–h). The dependence of the photoluminescence (PL) of the MABr@MAPbBr3 solutions on Pb2+ concentration is displayed in Fig. 2(a). All of the solutions were measured at room temperature with an excitation wavelength of 400 nm. The MABr solution does not show any florescence signal and the MABr@MAPbBr3 solutions exhibit a green emission peak centered at 557 nm. However, the green emission peaks of the solutions display a large full-width-at-half-maximum (FWFM). For the MABr@MAPbBr3 solution with 2 × 10−1 M Pb2+, the FWFM of emission peak is 60 nm, which is larger than that of the MABrPb3 thin film and powder26,27,28,29,30. Hence, a yellow green color emission is observed from the MABr@MAPbBr3 solutions under the 365 nm UV lamp illumination in a darkroom (Fig. 1). The PL intensity of the MABr solution was significantly increased by the addition of Pb2+ ion in a concentration-dependent manner (Pb2+ concentration ranging from 0 to 2 × 10−1 M). The influence of MABr concentration on the sensitivity of Pb2+ detection was studied (Fig. S1). If the Pb2+ concentration is greater than 1 × 10−1 M, then for the same Pb2+ concentration, the PL intensity of the 0.8 M MABr solution is nearly the same as that of the 0.4 M MABr solution (Fig. S1). However, upon a further decrease in the Pb2+ concentration, the PL intensity of the 0.4 M MABr solution is much smaller that of the 0.8 M MABr solution. Hence, increasing the concentration of MABr will enhance the sensitivity for detection of Pb2+ ions. However, if the MABr concentration is larger than 0.8 M, the MABr powder is insoluble in the DMF solution at room temperature. Therefore, we choose the 0.8 M MABr solution as the fluorescent probe for the detection of Pb2+ ions.

Figure 1
figure 1

Photographs of MABr@MAPbBr3 solutions under illumination by a 365 nm UV lamp; Pb2+ concentration: (a) 0 M (b) 1.6 × 10−3 M (c) 3.1 × 10−3 M (d) 6.2 × 10−3 M (e) 2.5 × 10−2 M (f) 5 × 10−2 M (g) 1 × 10−1 M (h) 2 × 10−1 M.

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Figure 2
figure 2

(a) PL emission spectra of MABr@MAPbBr3 solutions for different Pb2+ concentrations for an excitation wavelength of 400 nm. (b) Fitting curve for the PL intensity of the MABr@MAPbBr3 solutions as a function of Pb2+ concentration.

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To clarify the general applicability of MABr for Pb2+ detection, we replace the PbBr2 powders with PbI2 and PbCl2 powders to form the MABr@PbI2 and MABr@PbCl2 precursor solutions, respectively. Figure S2 shows the PL emission spectra of MABr@PbI2 and MABr@PbCl2 solutions for different Pb2+ concentrations. Compared with MABr solutions, both MABr@PbI2 and MABr@PbCl2 solutions display a green emission peak under UV light illumination. For MABr@PbI2 or MABr@PbCl2 solutions, with I− or Cl− concentrations increase, the center of the PL emission peak gradually changed. Based on the PL measurement of MABr, MABr@PbBr2, MABr@PbI2 and MABr@PbCl2 solutions, we can get the conclusion that MABr solutions exhibit luminescent response to Pb2+ ions. To obtain the quantitative relationship, we plotted the PL intensity of the solutions as a function of Pb2+ concentration (Fig. 2(b)). The relationship can be described by the following equation:

$$I/{I}_{0}=A+K[{\rm{C}}]$$

where I and I0 are the PL intensities of the solution in the presence and absence of Pb2+ ions, respectively. A and K (1/M) are the intercept and sensitivity (slope), respectively, and [C] (M) represents the Pb2+ concentration. The I/I0 − Pb2+ concentration curve can be fitted to I/I0 ± Δ (I/I0) = 1.43 ± 0.59 + (137.00 ± 7.22) [C], with the correlation coefficient R2 of 0.98, as shown in Fig. 2(b). The Δ(I/I0), 0.59 and 7.22 are the standard error of I/I0, A and K respectively. It was reported that perovskite fluorescent materials for the selective detection of metal ions or 2, 4, 6-trinitrophenol (TNP) are based on the quenching mechanism and the Stern-Volmer relationship31. However, the PL emission intensity of the MABr solution was enhanced with the addition of Pb2+, which is different from the quenching mechanism. To evaluate the selective detection ability of the MABr solution for Pb2+ ions, the PL response of the MABr solution to different metal ions was explored, as shown in Fig. 3(a). The PL intensity (I) of the MABr solutions (0.8 M) after the addition of different metal ions in the same concentration (2 × 10−1 M), including Pb2+, Ga2+, Co2+, Cu2+, Fe3+, Mg2+, Ni2+, Sn2+, Sr2+, and Zn2+, Cs1+ were measured. Analysis of the PL intensity ratios I/I0 (I0 is the PL intensity of the MABr solution without the metal ions) of the MABr solutions with different ions showed that only Pb2+ gave rise to a clear PL effect for the MABr solutions, while other cations exhibit almost no PL behavior for an excitation wavelength of 400 nm or under illumination by a 365 nm UV lamp.

Figure 3
figure 3

(a) PL response of the MABr solution (0.8 M) to different metal ions. (b) X-ray diffraction patterns of MABr@MAPbBr3 solutions with different Pb2+ concentrations.

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In order to get a better insight into the role of MABr on selective fluorescence detection of Pb2+ ions, the effect of interference by other metal ions were studied. Some equimolar mixtures of PbBr2 (0.1 M) along with XBr (X = different metal ions) were added to the MABr solutions. The PL response of the MABr solution to equimolar mixtures of Pb2+ with different metal ions is shown in Fig. S3. For transparent solutions (Pb2+@Ca2+, Pb2+@Mg2+, Pb2+@Sn2+, Pb2+@Sr2+, Pb2+@Zn2+, Pb2+@Cs+), the presence of interfering ions has little impact on the luminescent response of the MABr solution to Pb2+. For semitransparent solutions Pb2+@Co2+ and Pb2+@Ni2+, their PL intensities are greatly lower than that of MABr@PbBr2 solutions. However, compared with MABr, the MABr@PbBr2@CoBr2 and MABr@PbBr2@NiBr2 solutions still exhibit a green PL emission peak. The MABr@PbBr2@CoBr2 solution emits green color under UV light illumination as shown in Fig. S4. Hence, the MABr can be used to selectively detect the Pb2+ in the presence of Ca2+, Mg2+, Sn2+, Sr2+, Zn2+, Cs+, Co2+, Ni2+. For opaque solutions Pb2+@Cu2+ and Pb2+@Fe3+, the MABr@PbBr2@CuBr2 and MABr@PbBr2@FeBr3 solutions do not show any florescence signal. Researchers also reported that Cu2+ leaded to dramatic quenching of the PL of perovskite materials11,12,25. Therefore, the MABr can’t detect the Pb2+ in Pb2+@Cu2+ or Pb2+@Fe3+ solutions.

To explain the significant selective luminescent response of MABr solutions to Pb2+, the structure of MABr@MAPbBr3 solutions with different Pb2+ concentrations should be elucidated. For all of the solutions, the DMF solvents were evaporated on the glass substrates, and then we obtained MABr@MAPbBr3 films that were analyzed by XRD. Figure 3(b) shows the XRD patterns of these films with different Pb2+ concentrations from top to bottom at room temperature. The XRD patterns of the MABr powder and MABr thin films were measured (Fig. S5). For the MABr solutions without PbBr2 (top spectrum in Fig. 3(b)), all of the diffraction peaks are the characteristic peaks of MABr. As Pb2+ concentration increases, the MABr peaks intensities gradually decrease and new diffraction peaks of MAPbBr3 indicated by squares appear in the XRD patterns presented in Fig. 3(b). The evolution of MAPbBr3 peaks with Pb2+ concentration is shown in Fig. 4(a,b). The XRD results (Fig. 4) show that the two peaks’ intensities gradually increase with increasing Pb2+ concentration, corresponding to the increasing crystalline characteristics of MAPbBr3 with a preferential orientation in the (100) and (200) directions. Our previous study also found that MAPbBr3 thin films prepared with high MABr concentration exhibit partial preferential orientation along the (100) and (200) directions28,29. The (100) and (200) peaks shift toward larger angles with increasing Pb2+ concentration, indicating that the lattice constant of MAPbBr3 is decreasing. Due to the preferential orientation, we calculated the lattice parameters of MAPbBr3 from the (100) and (200) diffraction peaks. For the film with 2 × 10−1 M Pb2+, the lattice constant of MAPbBr3 is 6.10 Å which is larger than the previously reported value28,29. However, our research indicated that when Pb2+ concentration increased to 1 × 10−1 M, the lattice constant of MAPbBr3 was 5.93 Å which is closer to the values reported by other researchers26,27. To further confirm the MAPbBr3 phase in MABr@MAPbBr3 films, we studied the morphology of these films with SEM measurements.

Figure 4
figure 4

XRD peaks of MAPbBr3 in MABr@MAPbBr3 solutions with different Pb2+ concentrations (a) (100) and (b) (200) peak.

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Figure 5 shows the morphologies of the films prepared with MABr@MAPbBr3 solutions with different Pb2+ concentrations. An examination of Fig. 5(a,b) shows that the surface exhibits two different nonuniform aggregation morphologies. The first type of aggregation shows a shapeless morphology which is characteristic of the MABr organic compound. The other kind of aggregation is composed of cubic-shaped crystals, which is the crystalline morphology of MAPbBr3. The SEM results are consistent with the XRD analyses that indicate that this film is composed of MABr and MAPbBr3 phases as shown in Figs 3(b) and 4. When the PbBr2 concentration in the precursor solution is reduced, the number of MAPbBr3 crystals formed in the films decreased, as shown in Fig. 5(c,d). For the solution with 1.6 × 10−3 M Pb2+, the morphology of the film displays highly dense MABr aggregation with high coverage and only a small amount of MAPbBr3 crystals are observed on top of the MABr as shown in Fig. 5(e,f). We did not find any phases other than MABr and MAPbBr3 in the XRD patterns and SEM images of the films prepared by evaporating the DMF solvents from the MABr@MAPbBr3 solutions. To further exploring the interactions between MABr and PbBr2, the size distributions of the particles formed in MABr and MABr@MAPbBr3 solutions were measured by Dynamic Light Scattering (DLS), as shown in Fig. S6. The high MABr concentration (0.8 M) tends to form a gel-like solution and its hydrodynamic particle diameter is 650.1 nm, indicating the formation of MA+ organic aggregates. Adding PbBr2 in MABr solution increases the particles size. With Pb2+ concentration increased to 0.2 M, the diameter of the particles in MABr@MAPbBr3 solutions increased from 650.1 to 783.5 nm. The added PbBr2 might quickly react with Br− and MA+ to generate PbBr6 octahedron inorganic frame and then self-assemble into MAPbBr3 perovskite lattice. Adding PbBr2 in MABr solutions would provide more nucleating sites and growth spaces, resulting in the formation of bigger aggregates in MABr@MAPbBr3 solutions. Hence, addition of a small amount of PbBr2 to a high MABr concentration solution and stirring of this mixture could lead to a rapid chemical reaction to form MAPbBr3.

Figure 5
figure 5

Low-magnification and high-magnification SEM images of the MABr@MAPbBr3 solutions with different Pb2+ concentrations; Pb2+− 2 × 10−1 M (a,b), Pb2+− 5 × 10−2 M (c,d), Pb2+− 1.6 × 10−3 M (e,f).

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The significant luminescent response of the MABr solution to Pb2+ arises from the outstanding photoluminescence properties of MAPbBr3. The detection limit is an important indicator of the fluorescence detector performance. As the PbBr2 concentration was reduced to 1.6 × 10−3 M, the PL emission peak and diffraction peaks associated with the MAPbB3 can no longer be observed in the PL emission spectra (Fig. 2) and XRD patterns (Figs 3(b) and 4), respectively. When the PbBr2 concentration is lower than 1.6 × 10−3 M, only the diffraction peak of MABr could be clearly observed in the XRD pattern. However, the MABr solution with 1.6 × 10−3 M PbBr2 shows a pale yellow color under UV light illumination and we can still find the MAPbBr3 crystals in SEM images, unlike for the MABr solution without PbBr2. Therefore, the detection limit of the 0.8 M MABr solution for Pb2+ is at least as low as 1.6 × 10−3 M. Compared with other methods7,8,9,10 whose detection limits for Pb2+ are μM, the sensitivity of the MABr is not high. However, the MABr fluorescent sensor also displays many advantages such as low cost, rapid detection and ease of use. For the MABr fluorescent sensor, to selectively detect the Pb2+ from other metal ions, we only need a UV lamp equipment which is very cheap and easy to use. Moreover, the high MABr concentration solutions can quickly react with Pb2+ to form MAPbBr3, which can be used to extract Pb2+ from liquid waste in recycling applications. Other heavy metal detectors can’t extract Pb2+ from liquid waste. We also justify the luminescent response of MABr to Pb2+ on paper strips, as shown in Fig. S7. The letters “BJTU” were written with PbBr2 solution (0.1 M) on paper strips. The “BJTU” are invisible on paper strips under ambient light. However, after loading of MABr solutions (0.8 M) on these paper strips, the “BJTU” show bright green emission pattern under UV light illumination.

Based on these results, the fluorescence sensing mechanism can be schematically represented as shown in Fig. 6. To obtain excellent performance in a photovoltaic device, it is necessary to lower the rate of the chemical reaction between MABr and PbBr2 to form uniform MAPbBr3 films with good surface coverage32,33,34. However, for fluorescence sensors or detectors with a short response time, we seek to make MABr react with PbBr2 to form MAPbBr3 as quickly as possible. Excess MABr contributes to speeding up the transformation from PbBr2 to MAPbBr3. On the other hand, a MABr-rich environment gives rise to MABr residue that encompasses the MAPbBr3 crystal after the reaction. Thus, the use of excess MABr leads to the formation of a high amount of defects. The recombination lifetimes of the MABr@MAPbBr3 solution with 2 × 10−1 M Pb2+ is only 1.33 ns (Fig. S8). For perovskite materials, the recombination lifetime is related to crystallite dimension, the larger crystallites (e.g. single crystal) present longer photoluminescence lifetime. However, in our research, the DLS and SEM studies indicate that the MABr@MAPbBr3 solutions display big aggregates (783.5 nm) which consist of organic aggregates and MAPbBr3 crystals. Because of the organic aggregates and numerous defects, the MABr@MAPbBr3 solutions display a short-lived PL lifetime. In fact, the spin-coating method could evaporate some of the used MABr. We used a MABr@MAPbBr3 solution with 2 × 10−1 M Pb2+ as the precursor solution and spin-coated it on the glass substrate to obtain the MABr@MAPbBr3 thin film. The SEM images exhibit that the number of MABr residues of the MABr@MAPbBr3 thin film prepared by the spin-coating method is smaller than that of the MABr@MAPbBr3 film prepared by evaporating the DMF solutions (Figs 5(a,b) and S9). For MAPbBr3 films, a previous study also indicated that the introduction of Cl was conducive to the evaporation of the excess MABr during the spin-coating process32,33,34. If the MABr in MABr@MAPbBr3 solutions can be removed completely, we can obtain MAPbBr3 material with excellent photovoltaic performance. Therefore, the MABr solution not only can be used to detect the Pb2+ heavy metal but also may extract the Pb2+ from the liquid waste for reuse.

Figure 6
figure 6

Schematic representation for the luminescent response of the MABr solution to Pb2+.

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Conclusion

In summary, our research indicates that MABr can be used as a new platform for selective fluorescence detection of Pb2+ ions. The incorporation of PbBr2 into a MABr solution formed MAPbBr3@MABr solutions that exhibit significant luminescent responses under UV light illumination. The significant color changes of the MABr solutions before and after the addition of PbBr2 under UV lamp illumination can be observed by the naked eye. The PL intensity of the MABr sensor increases with increasing Pb2+ concentration, exhibiting a linear relationship. The fluorescence sensing mechanism of MABr for Pb2+ is due to the excellent PL performance of MAPbBr3 in MAPbBr3@MABr solutions. Some MABr in MAPbBr3@MABr solutions can be evaporated by the spin-coating method, enabling the extraction of Pb2+ from the liquid waste for recycling use. These findings may contribute to the development of new applications for luminescent perovskite materials.