CH3NH3Br solution as a novel platform for the selective fluorescence detection of Pb2+ ions

The development of a simple fluorescent sensor for detecting the Pb2+ heavy metal is fundamentally important. The CH3NH3PbBr3 perovskite material exhibits excellent photoluminescence properties that are related to Pb2+. Based on the effects of Pb2+ on the luminescent properties of CH3NH3PbBr3, we design a novel platform for the selective fluorescence detection of Pb2+ ions. Herein, we use a CH3NH3Br solution at a high concentration as the fluorescent probe. Incorporation of PbBr2 into the CH3NH3Br solution results in a rapid chemical reaction to form CH3NH3PbBr3. Hence, the nonfluorescent CH3NH3Br material displays a sensitive and selective luminescent response to Pb2+ under UV light illumination. Moreover, the reaction between CH3NH3Br and PbBr2 could transform Pb2+ into CH3NH3PbBr3, and therefore, CH3NH3Br may also be used to extract Pb2+ from liquid waste in recycling applications.

In the past several decades, the control of heavy metal pollution has been the focal point of environmental protection efforts [1][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 methods 10 , and fluorescent techniques 11 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 use 12 . As fluorescent materials, lead halide perovskite CH 3 NH 3 PbBr 3 (MAPbBr 3 ) and CsPbBr 3 show excellent luminescent properties including bright photoluminescence (PL), high PL quantum yields (PLQY), and narrow bandwidth [13][14][15] . Compared with MAPbBr 3 , the PL emission peaks of MAPbI 3 and MAPbCl 3 are red and blue light, respectively, which can't be excited by a UV lamp. Due to these advantages, the MAPbBr 3 and CsPbBr 3 have been used in light-emitting diodes (LED) [16][17][18][19][20][21][22] and fluorescence sensors or detectors 12,23 . Chinnadurai et al. 24 reported that fluorescent MAPbBr 3 nanoparticles can be used as an excellent sensor for the detection of 2, 4, 6-trinitrophenol (TNP). Liu et al. 25 used CsPbBr 3 perovskite quantum dots as photoluminescent probe for selective detection of Cu 2+ . Zhang et al. 12 encapsulated MAPbBr 3 perovskite quantum dots in MOF-5 matrix as a stable fluorescent probe for the detection of Al 3+ , Bi 3+ , Co 2+ , Cu 2+ , Fe 3+ , and Cd 2+ . 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 MAPbBr 3 and CsPbBr 3 are due to the Pb 2+ ion. The high toxicity of Pb 2+ 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 Pb 2+ . The MABr solution detects the Pb 2+ 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 Pb 2+ concentration, different amounts of PbBr 2 , PbI 2 and PbCl 2 powders were added into MABr solutions to form the MABr@ PbBr 2 , MABr@PbI 2 and MABr@PbCl 2 precursor solutions, respectively. After stirring the precursor solutions at room temperature for 30 min, the MABr@PbBr 2 , MABr@PbI 2 and MABr@PbCl 2 solutions were transformed into MABr@MAPbBr 3 , MABr@MAPbBr 3−x I x and MABr@MAPbBr 3−x Cl x solutions that are transparent liquids under room light. The photoluminescence (PL) emission spectra of the MABr@MAPbBr 3 , MABr@MAPbBr 3−x I x and MABr@MAPbBr 3−x Cl x solutions were measured by a photoluminescence system in the reflection mode. The time-resolved PL spectra of MABr@MAPbBr 3 solution were measured by an FLS980 time-resolved fluorescence spectrometer (Edinburgh Instrument). To analyze the structures of these solutions, MABr@MAPbBr 3 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@MAPbBr 3 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@MAPbBr 3 solutions. Figure 1 shows the photographs of MABr@MAPbBr 3 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 PbBr 2 (0-2 × 10 −1 M). All of the solutions were transparent under ambient light. The MABr solution without PbBr 2 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 PbBr 2 to the MABr solution leads to the formation of the MABr@MAPbBr 3 solution, and the MABr@MAPbBr 3 solution emits very pale yellow color under UV light illumination. As the Pb 2+ concentration of the MABr@MAPbBr 3 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@MAPbBr 3 solutions on Pb 2+ 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@MAPbBr 3 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@MAPbBr 3 solution with 2 × 10 −1 M Pb 2+ , the FWFM of emission peak is 60 nm, which is larger than that of the MABrPb 3 thin film and powder [26][27][28][29][30] . Hence, a yellow green color emission is observed from the MABr@MAPbBr 3 solutions under the 365 nm UV lamp illumination in a darkroom (Fig. 1). The PL intensity of the MABr solution was significantly  www.nature.com/scientificreports www.nature.com/scientificreports/ increased by the addition of Pb 2+ ion in a concentration-dependent manner (Pb 2+ concentration ranging from 0 to 2 × 10 −1 M). The influence of MABr concentration on the sensitivity of Pb 2+ detection was studied (Fig. S1). If the Pb 2+ concentration is greater than 1 × 10 −1 M, then for the same Pb 2+ 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 Pb 2+ 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 Pb 2+ 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 Pb 2+ ions.
To explain the significant selective luminescent response of MABr solutions to Pb 2+ , the structure of MABr@ MAPbBr 3 solutions with different Pb 2+ concentrations should be elucidated. For all of the solutions, the DMF solvents were evaporated on the glass substrates, and then we obtained MABr@MAPbBr 3 films that were analyzed by XRD. Figure 3(b) shows the XRD patterns of these films with different Pb 2+ 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 PbBr 2 (top spectrum in Fig. 3(b)), all of the diffraction peaks are the characteristic peaks of MABr. As Pb 2+ concentration increases, the MABr peaks intensities gradually decrease and new diffraction peaks of MAPbBr 3 indicated by squares appear in the XRD patterns presented in Fig. 3(b). The evolution of MAPbBr 3 peaks with Pb 2+ concentration is shown in Fig. 4(a,b). The XRD results (Fig. 4) show that the two peaks' intensities gradually increase with increasing Pb 2+ concentration, corresponding to the increasing crystalline characteristics of MAPbBr 3 with a preferential orientation in the (100) and (200) directions. Our previous study also found that MAPbBr 3 thin films prepared with high MABr concentration exhibit partial preferential orientation along the (100) and (200) directions 28,29 . The (100) and (200) peaks shift toward larger angles with increasing Pb 2+ concentration, indicating that the lattice constant of MAPbBr 3 is decreasing. Due to the preferential orientation, we calculated the lattice parameters of MAPbBr 3 from the (100) and (200) diffraction peaks. For the film with 2 × 10 −1 M Pb 2+ , the lattice constant of MAPbBr 3 is 6.10 Å which is larger than the previously reported value 28,29 . However, our research indicated that when Pb 2+ concentration increased to 1 × 10 −1 M, the lattice constant of MAPbBr 3 was 5.93 Å which is closer to the values reported by other researchers 26,27 . To further confirm the MAPbBr 3 phase in MABr@MAPbBr 3 films, we studied the morphology of these films with SEM measurements. Figure 5 shows the morphologies of the films prepared with MABr@MAPbBr 3 solutions with different Pb 2+ 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 MAPbBr 3 . The SEM results are consistent with the XRD analyses that indicate that this film is composed of MABr and MAPbBr 3 phases as shown in Figs 3(b) and 4. When the PbBr 2 concentration in the precursor solution is reduced, the number of MAPbBr 3 crystals formed in the films decreased, as shown in Fig. 5(c,d). For the solution with 1.6 × 10 −3 M Pb 2+ , the morphology of the film displays highly dense MABr aggregation with high coverage and only a small amount of MAPbBr 3 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 MAPbBr 3 in the XRD patterns and SEM images of the films prepared by evaporating the DMF solvents from the MABr@MAPbBr 3 solutions. To further exploring the interactions between MABr and PbBr 2 , the size distributions of the particles formed in MABr and MABr@MAPbBr 3 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 PbBr 2 in MABr solution increases the particles size. With Pb 2+ concentration increased to 0.2 M, the diameter of the particles in MABr@MAPbBr 3 solutions increased from 650.1 to 783.5 nm. The added PbBr 2 might quickly react with Br − and MA + to generate PbBr 6 octahedron inorganic frame and then self-assemble into MAPbBr 3 perovskite lattice. Adding PbBr 2 in MABr solutions would provide more nucleating sites and growth spaces, resulting in the formation of bigger aggregates in MABr@MAPbBr 3 solutions. Hence, addition of a small amount of PbBr 2 to a high MABr concentration solution and stirring of this mixture could lead to a rapid chemical reaction to form MAPbBr 3 .
The significant luminescent response of the MABr solution to Pb 2+ arises from the outstanding photoluminescence properties of MAPbBr 3 . The detection limit is an important indicator of the fluorescence detector performance. As the PbBr 2 concentration was reduced to 1.6 × 10 −3 M, the PL emission peak and diffraction peaks associated with the MAPbB 3 can no longer be observed in the PL emission spectra (Fig. 2) and XRD patterns (Figs 3(b) and 4), respectively. When the PbBr 2 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 PbBr 2 shows a pale yellow color under UV light illumination and we can still find the MAPbBr 3 crystals in SEM images, www.nature.com/scientificreports www.nature.com/scientificreports/ unlike for the MABr solution without PbBr 2 . Therefore, the detection limit of the 0.8 M MABr solution for Pb 2+ is at least as low as 1.6 × 10 −3 M. Compared with other methods 7-10 whose detection limits for Pb 2+ 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 Pb 2+ 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 Pb 2+ to form MAPbBr 3 , which can be used to extract Pb 2+ from liquid waste in recycling applications. Other heavy metal detectors can't extract Pb 2+ from liquid waste. We also justify the luminescent response of MABr to Pb 2+ on paper strips, as shown in Fig. S7. The letters "BJTU" were written with PbBr 2 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 PbBr 2 to form uniform MAPbBr 3 films with good surface coverage [32][33][34] . However, for fluorescence sensors or detectors with a short response time, we seek to make MABr react with PbBr 2 to form MAPbBr 3 as quickly as possible. Excess MABr contributes to speeding up the transformation from PbBr 2 to MAPbBr 3 . On the other hand, a MABr-rich environment gives rise to MABr residue that encompasses the MAPbBr 3 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@MAPbBr 3 solution with 2 × 10 −1 M Pb 2+ is only 1.33 ns (Fig. S8). For perovskite materials, the recombination lifetime is related to crystallite dimension, the larger  www.nature.com/scientificreports www.nature.com/scientificreports/ crystallites (e.g. single crystal) present longer photoluminescence lifetime. However, in our research, the DLS and SEM studies indicate that the MABr@MAPbBr 3 solutions display big aggregates (783.5 nm) which consist of organic aggregates and MAPbBr 3 crystals. Because of the organic aggregates and numerous defects, the MABr@ MAPbBr 3 solutions display a short-lived PL lifetime. In fact, the spin-coating method could evaporate some of the used MABr. We used a MABr@MAPbBr 3 solution with 2 × 10 −1 M Pb 2+ as the precursor solution and spin-coated it on the glass substrate to obtain the MABr@MAPbBr 3 thin film. The SEM images exhibit that the number of MABr residues of the MABr@MAPbBr 3 thin film prepared by the spin-coating method is smaller than that of the MABr@MAPbBr 3 film prepared by evaporating the DMF solutions (Figs 5(a,b) and S9). For MAPbBr 3 films, a previous study also indicated that the introduction of Cl was conducive to the evaporation of the excess MABr during the spin-coating process [32][33][34] . If the MABr in MABr@MAPbBr 3 solutions can be removed completely, we can obtain MAPbBr 3 material with excellent photovoltaic performance. Therefore, the MABr solution not only can be used to detect the Pb 2+ heavy metal but also may extract the Pb 2+ from the liquid waste for reuse.

conclusion
In summary, our research indicates that MABr can be used as a new platform for selective fluorescence detection of Pb 2+ ions. The incorporation of PbBr 2 into a MABr solution formed MAPbBr 3 @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 PbBr 2 under UV lamp illumination can be observed by the naked eye. The PL intensity of the MABr sensor increases with increasing Pb 2+ concentration, exhibiting a linear relationship. The fluorescence sensing mechanism of MABr for Pb 2+ is due to the excellent PL performance of MAPbBr 3 in MAPbBr 3 @MABr solutions. Some MABr in MAPbBr 3 @MABr solutions can be evaporated by the spin-coating method, enabling the extraction of Pb 2+ from the liquid waste for recycling use. These findings may contribute to the development of new applications for luminescent perovskite materials.

Data availability
The datasets analysed during the current study are available from the corresponding author on reasonable request.