Colorimetric Determination of Chloridion in Domestic Water Based on the Wavelength Shift of CsPbBr3 Perovskite Nanocrystals via Halide Exchange

Cubic phase CsPbBr3 perovskite nanocrystals (PNCs) was prepared by a high-temperature hot-injection method. The high photoluminescence quantum yield (PLQY) of as-prepared CsPbBr3 PNCs was 87%, which can be used for the determination of chloridion in domestic water samples based on their wavelength-shift characteristics via halide exchange. The proposal approach for the determination of chloridion reveals a linear correlation ranged from 10 to 200 μM of the chloridion concentration and the wavelength shift of CsPbBr3 PNCs with a correlation coefficient of R2 = 0.9956. The as-mentioned method reveals neglectable responses towards those co-existing ions in the water aside from chloridion, due to the quick exchange between Cl and Br and the outstanding color change caused by wavelength shift. The strategy has been applied to the determination of chloridion in water samples with the recoveries of 98.9–104.2% and the limit of detection (LOD) of 4 μM. These results show that the suggested approach is promising for the development of novel fluorescence detection for chloridion in water.


Introduction
Water resource, an indispensable element in the world, has great relation with the ecological environment and public health. Moreover, the available natural water for exploitation is extremely limited, and always full of bacteria and fungi residual. In hence, the utilization of natural water is restricted and effective treatment to decrease the existence of bacteria and fungi are necessary. Sterilization process in the treatment of natural water has enabled the usability of the target and decrease of the number of bacteria. Chloridion, a familiar anion, has played an essential role and showed better efficiency towards many kinds of bacteria in the sterilization process. The underexercise chloridion will cause potential impact on water quality, too little chloridion will bring about an incomplete inhibit of bacteria, while excessive chloridion cause damage to human skin, respiratory and nervous system [1][2][3]. Therefore, a highly attractive method to detect chloridion in domestic water must be taken into account. Rapidly developing chloridion detection techniques, such as iodometry, colorimetry, electrochemical sensors and ion chromatography, provide potential solutions for the control of chloridion in domestic water. However, these widespread methods mainly require complicated pretreatment and equipment, resulting in longer detection time or expensive cost [4][5][6][7][8]. Fluorescence sensing methods based on nanomaterials are of low cost, easy operation, fast response and high efficiency that serve in the detection of numerous ions and micro-molecule [9][10][11].
In this study, a fluorescence sensing approach for chloridion has been constructed based on the halogen exchange between chloridion and CsPbBr 3 PNCs. In addition to the acceleration of halogen-exchange process through vortexassisted oscillation, the other sensing conditions were also studied and optimized. The method has been applied in the determination of chloridion in domestic water with the characteristics of high efficient and straightforward, which can be completed within half a minute.

Preparation of CsPbBr 3 PNCs
The preparation of CsPbBr 3 PNCs using high-temperature hot-injection was referenced to the report of Kovalenko [12]. In the preparation, 15 mL ODE, 300 mg Cs 2 CO 3 and 1 mL OA were sequentially loaded into a three-necked flask under N 2 atmosphere and the temperature was raised to 150 °C to yield the faint yellow Cs-OA. In another three-necked flask, PbBr 2 precursor was prepared by mixed 10 mL ODE, 0.4 mM PbBr 2 , 1 mL OA and 1 mL OLAM. After completing solubilisation of PbBr 2 , the temperature was raised to 160 °C and 1 mL preheated Cs-OA solution was quickly injected. Finally, the high-quality CsPbBr 3 PNCs can be obtained after purification with hexane/ethyl acetate for several times. The as-prepared CsPbBr 3 PNCs was diluted with hexane to prepare a standard stock solution and stored for further use.

Determination of Chloridion
The complex solution containing 2 mL CsPbBr 3 PNCs and 20 μL TBA-Cl, with H 2 SO 4 used to adjust the pH to 1, was mixed into a centrifuge tube and drastic mechanic vibration at 1000 rpm for 30 s. After that, the color change was immediately observed by naked eyes with the concentrations of chloridion from 10 to 200 μM. The photoluminescence (PL) of CsPbBr 3 PNCs accomplishing the anion exchange could be obtained by a fluorescence spectrophotometer. Subsequently, the method was established by investigating the relationship between the chloridion concentration and the wavelength shift of CsPbBr 3 PNCs, and all experiments were retested three times.

Selectivity and Practical Sample Testing
The interference of co-existing ions in domestic water was studied following the similar procedure as the above. In the study, 50 μL aqueous solution containing 500 μM ion (Br − , F − , ClO − , K + , Na + , Mg 2+ or Fe 3+ ) was tested with the CsPbBr 3 PNCs. Water samples were collected from the university campus, filtered with 0.22 μm filter to remove large particles, and stored for use. 50 μL prepared water samples were used to observe the wave-shifted fluorescence.

Characterization
Powder X-ray diffraction (XRD) patterns were recorded by a Rigaku Ultimate IV (Japan) diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected on a Tecnai-G2-F30 transmission electron microscopes (FEI, USA) under 300 kV. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific™ K-Alpha™ + spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W. UV-Vis absorption spectroscopy was conducted on a UV-2600i UV-Vis spectrophotometer (Shimadzu, Japan). Fluorescence spectra were performed with an F-4500 fluorescence spectrophotometer (Hitachi, Japan), and PL decay curve was collected by the QM-TM spectrofluorometer (PTI, USA).

Results and Discussion
The characterization of CsPbBr 3 PNCs was performed and shown in Fig. 1. The three-dimensional structure of cubic phase CsPbBr 3 PNCs was composed of octahedral structure of [PbBr 6 ] 4− , bromine (I) anions and cesium (I) cation exhibited in Fig. 1a. The octahedral structure of [PbBr 6 ] 4− was composed of lead (II) cation and bromine (I) anions and formed via corner sharing. Cesium (I) cation was filled in the cubic center's vacancy to support the entire threedimensional frame. The crystal phase of CsPbBr 3 PNCs was confirmed by X-ray diffraction and shown in Fig. 1b (PDF#18-0364), respectively. A typical transmission electron microscopy (TEM) image, as presented in Fig. 1c, indicating a cubic morphology CsPbBr 3 PNCs with a mean diameter about 11.09 ± 1.98 nm (Fig. 1d). The high-resolution transmission electron microscopy (HRTEM) revealed the as-prepared CsPbBr 3 PNCs with high crystallinity, and the interplanar spacing was 5.8 Å corresponding to (100) crystal of CsPbBr 3 PNCs. The structure of CsPbBr 3 PNCs changed from uniform to disorder gradually after halide exchanges with chloride in water. With the influence of H 2 O molecules, as shown in the TEM image of Fig. S1, the microstructures of CsPbCl x Br 3-x PNCs were agglomerated, destroyed and the size of some particles became larger. Surface chemical composition and structure analysis of CsPbBr 3 PNCs were carried out using X-ray photoelectron spectroscopy (Fig. 2a). The strong peaks in the XPS survey spectra included Cs3d (~ 725 and 738 eV), O1s (~ 532 eV), Pb4d (~ 413 and 437 eV), C1s (~ 285 eV), Pb4f (~ 138 and 143 eV), Br3d (~ 68 eV) and Pb5d (~ 20 eV). The presence of elemental C and O were probably related to the surface ligand OA. The fluorescence emission and the UV-Vis absorption spectra of CsPbBr 3 PNCs were shown in Fig. 2b. A sharp emission peak at 513 nm, with a narrow full width at half-maximum (FWHM) of 28 nm, was found in the fluorescence emission spectra. Furthermore, a strong absorption peak at 488 nm could be found in the UV-Vis absorption spectra, which belonged to the band-edge absorption of CsPbBr 3 PNCs. The CsPbBr 3 PNCs emitted very bright green fluorescence (as shown in the inset of Fig. 2b, PLQYs ~ 87%) under the excitation at 365 nm. The emission dynamics decay curve of CsPbBr 3 PNCs had been presented in Fig. 2c, and the average PL decay lifetime was 23.7 ns (triple-exponential decay function, τ = (A 1 τ 1 2 + A 2 τ 2 2 + A 3 τ 3 2 )/(A 1 τ 1 + A 2 τ 2 + A 3 τ 3 )). These results indicate that the radiation transition dominates the exciton recombination process, which makes CsPbBr 3 PNCs exhibit excellent optical properties.
Several parameters, in the process of halogen exchange, including the dosage of the aqueous solution, pH, the stirring rate and time were investigated. According to previous reports [29], the phenomenon of ion exchange between Cl − with CsPbBr 3 PNCs was not evident and without external force when it migrated from aqueous phase to organic. On the contrary, the vortex-assisted shock method increased the number of Cl − entering the interface of CsPbBr 3 PNCs due to the aqueous-organic interface uniformly re-dispersed, which remarkably accelerated the ion exchange Furthermore, the chloridion exchange occurred easily in a short time under a condition of strong acidic, and pH 1 was selected as the optimal in our work (Fig. S2). It can be observed that the ion exchange process could be completed within 30 s as displayed in Fig. 3a and b, when the stirring rate of vortex-assisted shock changed from 750 to 1250 rpm (C cl − = 200 μM). However, it could be observed that the ion exchange rate did not increase significantly as the stirring rate over 1000 rpm. In hence, 1000 rpm of stirring rate and 30 s of reaction time were selected as the optimal conditions in our work. After that, the selectivity of the fluorescence sensing approach was explored to confirm the practical application potential. The wavelength shifts of Br − , F − , ClO − , K + , Na + , Mg 2+ and Fe 3+ in the same concentration (500 μM) were displayed in Fig. 3c. Compared with Cl − , the response of this sensing method to these ions is negligible, indicating that it has high specificity towards Cl − .
Under the optimal conditions, the wavelength shift and correlation relationship of CsPbBr 3 PNCs induced by different concentrations of Cl − were studied. As shown in Fig. 4a, the fluorescence spectra of the CsPbBr 3 PNCs presented a continuous blue shift as the Cl − concentration increased, and the central peak wavelength moved from 513 to 483 nm (Δλ = 30 nm). Meanwhile, the band-edge absorption of CsPbBr 3 PNCs shifted from 488 to 463 nm (Fig. 4b). Under the visible light, as shown in Fig. 4c, the distinct color of CsPbBr 3 PNCs changed significantly, from cyan to reseda as the Cl − concentration increased. When exposed at 365 nm UV light, the fluorescence color of CsPbBr 3 PNCs changed from green to blue-green, and finally to blue. The correlation relationship between Cl − concentration in the range of 10-200 μM and the wavelength shift of CsPbBr 3 PNCs was established, and the wavelength shift was enhanced from 4 The concentration of Cl − in different water samples were determined, and the results are listed in Table 1 based on the linear relationship established in above experiment. The recoveries were between 98.9 and 104.2%, and the relative standard deviations (RSDs) were less than 8.8%. Furthermore, the fluorescence wavelength-shift colorimetric approach via halide exchange of CsPbBr 3 PNCs showed a wider linear range and lower LOD than those of reported methods (Table S1). The above experimental data proved that halogen ion exchange based on CsPbBr 3 PNCs could be a practical and straightforward approach to detect the Cl − in domestic water samples.

Conclusion
In the present work, the CsPbBr 3 PNCs with excellent optical properties were prepared via a high-temperature hot-injection approach. The as-prepared CsPbBr 3 exhibited cubic phase structure, high photoluminescence quantum yields and a quick halogen exchange between chloridion and CsPbBr 3 PNCs. The established method Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.