Colorimetric Sensing of Benzoyl Peroxide Based on the Emission Wavelength-Shift of CsPbBr 3 Perovskite Nanocrystals

: Using the ionic salt characteristics of CsPbBr 3 perovskite nanocrystals (CsPbBr 3 NCs), the ﬂuorescence wavelength of CsPbBr 3 − x I x NCs could be changed by the halogen exchange reaction between CsPbBr 3 NCs and oleylammonium iodide (OLAM-I). Under the excitation of a 365 nm UV lamp and the increase of OLAM-I concentration, the content of iodine in CsPbBr 3 − x I x NCs increased, and the ﬂuorescence emission wavelength showed a redshift from 511.6 nm to 593.4 nm, resulting in the ﬂuorescence color change of CsPbBr 3 NCs from green to orange-red. Since OLAM-I is a mild reducing agent and easily oxidized by benzoyl peroxide (BPO), a novel colorimetric sensing approach for BPO based on the ﬂuorescence wavelength shift was established in this study. The linear relationship between the different wavelength shifts ( ∆ λ ) and the concentration of BPO (C BPO ) is found to be in the range of 0 to 120 µ mol L − 1 . The coefﬁcient of alteration (R 2 ) and the detection limit are 0.9933 and 0.13 µ mol L − 1 BPO, respectively. With this approach, the determination procedure of BPO in ﬂour and noodle samples can be achieved in only a few minutes and exhibit high sensitivity and selectivity.


Introduction
In halide perovskite ABX 3 , X is the halogen anion (Cl − , Br − , or I − ), A is the monovalent cation Cs + CH 3 NH 3 + (MA) or CH(NH 2 ) 2 + (FA), and B is a divalent metallic cation, such as Pb 2+ , Sn 2+ , or Ge 2+ . Although the discovery of halogen perovskites can be traced back to 1958 [1], they did not attract much attention until the study of Miyasaka et al. using MAPbI 3 for solar cells in 2009; the research upsurge of lead halide perovskites (LHPs) attracted wide attention in relevant fields [2]. The photogenerated electrons and holes of LHPs can not only be separated to produce an electric current, but can also be combined to produce light through radiation. LHPs have the characteristics of narrow emission peak, high color purity, and accurate adjustment of emission wavelength through halogen composition in the whole visible region, which indicates application potential in lighting [3][4][5], optical display [6][7][8], laser [9,10], and optical sensing and detection [11][12][13]. However, the fluorescence quantum yield (PLQY) of the macro-sized LHPs is generally not very high, since (a) LHPs have a low generation energy of ion defects (especially halogen defects) and are prone to produce a large number of ion vacancy defects during crystallization; and (b) crystallization; and (b) LHPs have low exciton binding energy, resulting in low electronhole radiative recombination efficiency. Therefore, researchers have turned their attention to lead halide perovskite nanocrystals (CsPbBr3 NCs), in an attempt to improve the exciton binding energy and radiation recombination efficiency of LHPs by enhancing the quantum confined effect [14]. In 2014, Perez-Prieto et al. first reported the synthesis of MAPbBr3 NCs, whose PLQY could be closer to 20% [15]. In 2015, Kovalenko et al. synthesized and obtained 90% all-inorganic CsPbX3 NCs for the first time [16], which started the vigorous development of CsPbBr3 NCs in the field of luminescence. As shown in Figure 1, the halogen exchange characteristics of CsPbX3 NCs cause the wavelength shift of their fluorescence emission [17]. Generally, the redshift of fluorescence wavelength occurs with the increase of X ion radius. For example, iodide can exchange Br in CsPbBr3 PNCs, and result in the corresponding generation of CsPbBrxI3−x (0 ≤ x ≤ 3). The fluorescence emission wavelength of CsPbBrxI3−x gradually redshifts with the increase of iodine concentration, and the apparent color changes from green to red. The wavelength change has a linear relationship with the concentration of I -, which is veryuseful in colorimetric sensing for an oxidative substance due to the reduction property of iodide (I2 + 2e − →2I − , with E°-0.54 V vs. SHE). CsPbBr3 NCs reveal their anion exchange within tens of seconds due to the ionic salt characteristics in which I − or Cl − ions produce CsPbBrxI3−x NCs and CsPbClxBr3−x NCs after the exchange with CsPbBr3 NCs is completed [17]. After the halogen exchange, the fluorescence emission wavelength of the CsPbBr3 NCs will shift significantly and result in the obvious change of the emission color, which is conducive to colorimetric sensing. In addition to halogen exchange, cations in CsPbBr3 NCs can also be exchanged; for example, in the presence of Hg 2+ , Pb 2+ in CsPbBr3 NCs could gradually be replaced by Hg 2+ [18]. The different fluorescence responses of MAPbBr3 NCs could be obtained if their crystal structure changes. A typical example is the generation of MAPbBr3•H2O by CsPbBr 3 NCs reveal their anion exchange within tens of seconds due to the ionic salt characteristics in which I − or Cl − ions produce CsPbBr x I 3−x NCs and CsPbCl x Br 3−x NCs after the exchange with CsPbBr 3 NCs is completed [17]. After the halogen exchange, the fluorescence emission wavelength of the CsPbBr 3 NCs will shift significantly and result in the obvious change of the emission color, which is conducive to colorimetric sensing. In addition to halogen exchange, cations in CsPbBr 3 NCs can also be exchanged; for example, in the presence of Hg 2+ , Pb 2+ in CsPbBr 3 NCs could gradually be replaced by Hg 2+ [18]. The different fluorescence responses of MAPbBr 3 NCs could be obtained if their crystal structure changes. A typical example is the generation of MAPbBr 3 •H 2 O by MAPbBr 3 NCs and water molecules in a certain humidity range [19]. CsPbBr 3 NCs are one of many superficial defects, and fluorescence sensing methods can be established by passivation of these superficial defects. For example, O 2 can significantly enhance the fluorescence of CsPbBr 3 nanosheets, CsPbBr 3 nanowires, and CsPbBr 3 single crystals due to their many surface defect states [20] which can be used for O 2 sensing. Common energy transfer in fluorescence sensing also is applied by using CsPbBr 3 NCs. Fluorescence resonance energy transfer is formed between CsPbBr 3 NCs/PS FM and Rhodamine 6G (Rh 6G), and thus a highly sensitive sensing method for Rh 6G was established [21]. By dopingMn 2+ into CsPbBr 3 NCs, the O 2 sensing can be realized with good responses [22].
Colorimetric sensing has been applied in chemical analysis for its simplicity, rapid and direct readout analysis, lack of large complicated scientific instruments, and ability to achieve real-time and in-situ analysis. Ordinarily, the naked eye is insufficient in the resolving of intensity change in homochromatism (ca. 64 grades), but it has a high sensitivity in the identification of color change (ca. 10 million color types). Recently, as shown in Table S1, there have been several reports on the studies and applications of colorimetric sensing based on the wavelength-shift of the halogen exchange characteristics of CsPbX 3 NCs. Chen et al. developed a novel colorimetric sensing approach for the peroxide number determination of edible oil, in which the edible oil sample underwent redox reactions with OLAM-I, and then a halogen exchange occurred between the added CsPbBr 3 NCs and the iodide ions from the residual OLAM-I [11]. A colorimetric sensing approach for the determination of Cl − in sweat by the heterogeneous halide exchange between CsPbBr 3 NCs in n-hexane and Cl − in an aqueous solution has also been presented [23]. Additionally, a sensing approach for methylamine (MA) gas has been set up with the fluorescence turnon and wavelength shift in the formation of MAPbBr 3 NPs in hollow SiO 2 nanospheres triggered by the reaction between MA gas and HPbBr 3 /PbBr 2 @SiO 2 nanospheres [24]. In 2019, Lin et al. designed a simple device for the separation and sensing of H 2 S with high sensitivity and selectivity using CsPbBr 3 NCs [20]. The fluorescence intensity was linearly related to H 2 S concentration (in the range of 0~100 µM), and the detection limit was 0.18 µM. This approach was used to detect H 2 S in rat brain microdialysis. In 2020, Yun et al. [25] synthesized a CsPbBr 3 PQDs/cellulose composite material and used it as an effective colorimetric sensing material for the real-time monitoring of chlorine, iodine, and other trace elements in tap water. The results showed an excellent sensitivity for the detection of iodide ions in the range of 0.0001 to 1 M, and the detection limit (LOD) was 2.56 mM. Similarly, the detection limit for chloride ions was 4.11 mM. Additionally, in 2021, the Jacek group [26] reported the sensing results for methane using the halogen exchange characteristics of CsPbX 3 NCs.
Flour is one of the most important foods in our diet. Generally, its color presents slightly yellow due to carotenoids, lutein, and other naturally colored nutrients. To improve the flour color, flour brightener, mainly containing benzoyl peroxide (BPO), has been used. BPO has strong oxidization characteristics and can improve the flour's apparent color, maturity period, and yield. However, due to the strong oxidation of BPO, the addition of BPO can damage the natural nutrients in flour. In addition, BPO causes a strong irritation to the skin and upper respiratory tract. An excess of BPO in the body will cause vertigo, vomiting, neurasthenia, and a variety of diseases, resulting in serious damage to the liver and other organs. Although the addition of BPO in the production process of all kinds of flour is prohibited [27], the illegal addition is still occasionally found due to its low cost and obvious bleaching effect.
As shown in Table S2, there have been a variety of determination methods for BPO such as gas chromatography (GC) [28], high-performance liquid chromatography (HPLC) [29][30][31], capillary electrophoresis (CE) [32], the electrochemical method [33,34], chemiluminescence [35,36], and the other optical methods [37][38][39][40][41]. To discover the presence of BPO easily and quickly, further efforts are necessary to develop real-time methods, including colorimetric determination. In this study, the halogen exchange reaction between CsPbBr 3 NCs and the iodine in oleylammonium iodide (OLAM-I) was studied. The relationship between the concentration of iodine in the solution and the fluorescence wavelength shift has been observed and investigated. Using the redox reaction of BPO and iodine, we have developed a colorimetric sensing method based on the wavelength-shift for BPO using CsPbBr 3 NCs and applied the convenient and low-cost approach towards the determination of BPO in flour and noodle samples.

Instruments
The absorption spectra were characterized by a Hitachi UV-Vis 2550 spectrophotometer. Fluorescence spectra of CsPbBr 3 NCs were collected by an F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan). The in-situ halogen exchange between Br and I was observed using a Renishaw Invita Raman (Renishaw, London, UK) spectrometer with a laser excitation source (457 nm). The morphology of CsPbBr 3 NCs was characterized by JEOL-1400 transmission electron microscopy (TEM, Tokyo, Japan), and the relevant acceleration voltage was set at 100 kV.

Preparation of Cesium Oleate
In the preparation of cesium oleate [16], 0.814 g of CS 2 CO 3 , 40 mL of octadecene solution, and 2.5 mL of oleic acid were added and mixed into a 50 mL three-necked flask. The CS 2 CO 3 was completely dissolved and reacted at 120 • C in a vacuum resulting in a brownish-yellow solution. This product was prepared for the subsequent experiments.

Preparation and Purification of CsPbBr 3 NCs
The CsPbBr3 NCs were prepared per the report with some adaptations [16]. A preparation of 0.069 g of PbBr 2 , 5 mL of octadecene solution, 0.6 mL of oleic acid, and 0.5 mL of oleic amine were added into a 25 mL three-necked flask. The flask was then placed under a vacuum and heated to 120 • C. The temperature was held constant until the PbBr 2 was completely dissolved, then the temperature was increased to 150 • C. A total of 0.4 mL of prepared productand cesium oleate, was quickly injected into the three-necked flask at 150 • C in a nitrogen environment. After the injection, the flask was quickly placed in an ice water bath and shaken continuously to obtain a yellow-green colloidal product, which was removed and kept at room temperature.
The yellow-green liquid of the obtained product was centrifuged at 10,000 rpm for 10 min. The bottom precipitates were then retained and the supernatant discarded. A total of 2 mL n-hexane was added to the precipitate to disperse the CsPbBr 3 NCs. The final product was stored in a refrigerator for subsequent experiments.

Synthesis of Oleylammonium Iodide
Oleylammonium iodide (OLAM-I) was prepared per the report [42]. In a nitrogen environment, 8 mL oleylamine and 3 g I 2 were added to a 50 mL three-neck flask with magnetic stirring at 2500 rpm. Then the flask was heated to 140 • C under nitrogen protection. When the solution became clear, the heating temperature was raised to 180 • C. A yellow-brown solution of OLAM-I was then obtained andstored in a nitrogen environment in a refrigeratorfor the subsequent experiments.

Fluorescence Wavelength-Shift with the Different OLAM-I Concentration
Different 2 mL concentrations (0 to 260 µmol L −1 ) of the OLAM-I n-hexane solution were added to 5 mL centrifuge tubes followed by 50 µL CsPbBr 3 PNCs . After a 2 min reaction, their fluorescence spectra were measured. Meanwhile, their apparent colors under the excitation of a 365 nm UV lamp were collected by a camera.

BPO Concentration and the Corresponding Wavelength-Shift
Different 1 mL concentrations of a BPO (0~120 µmol L −1 ) toluene solution were added into 5 mL centrifuge tubes followed by 2 mL of 240 µmol L −1 OLAM-I and allowed to react for 5 min. Afterwards, 50 µL CsPbBr 3 PNCs were added for the fluorescence spectra measurement. The fluorescence wavelength of the solution with 0 µmol L −1 BPO was taken as λ 0 , and those of different concentrations of BPO were λx. Their difference was considered as ∆λ = λ 0 − λx. A relationship curve between the concentration of BPO and ∆λ was obtained, which was used as the standard working curve for BPO.
In the sample pre-preparation for the BPO determination of flour or noodle samples, the noodle samples were first ground, and then the noodle and flour samples were filtered through a 600-mesh filter. After filtering, 1.000 g of flour or noodle powder was added into a 10 mL centrifuge tube with 5 mL toluenethen added. BPO in the sample was extracted using ultrasonic means for 30 min, and then the tube was centrifuged at 10,000 rpm for 10 min. The supernatant solution was used for the BPO determination as referred to in the above experimental procedures. Then 1mL of supernatant solution from the sample was added into a 5 mL centrifuge tube containing 1 mL of 240 µmol L −1 OLAM-I solution. The solution was shaken for 10 min, and then 50 µL CsPbBr 3 PNCs in n-hexane were added. Their fluorescence spectra were recorded after the solution was shaken for 1 min. The BPO concentration was then obtained based on the wavelength shift of the corresponding fluorescence emission.

Spectroscopy and Structural Characterization of CsPbBr 3 PNCs
Sensing reproducibility and accuracy are greatly dependent on the fluorescence characteristics of CsPbBr 3 PNCs and are affected by their size and uniformity. As shown in Figure

Fluorescence Wavelength-Shift in the Halogen Exchanges of CsPbBr3 PNCs
In this study, the halogen exchange process of CsPbBr3 PNCs with Iin homogeneous and heterogeneous phases was investigated. CsPbBr3 PNCs and OLAM-I were selected for the study of the homogeneous halogen exchange. In this study, heterogeneous halogen exchange on a water/organic or a solid/liquid interface was considered. NaI aqueous so-

Fluorescence Wavelength-Shift in the Halogen Exchanges of CsPbBr 3 PNCs
In this study, the halogen exchange process of CsPbBr 3 PNCs with I − in homogeneous and heterogeneous phases was investigated. CsPbBr 3 PNCs and OLAM-I were selected for the study of the homogeneous halogen exchange. In this study, heterogeneous halogen exchange on a water/organic or a solid/liquid interface was considered. NaI aqueous solution and CsPbBr 3 PNCs in a n-hexane solution were focused on for the halogen exchange study of the water/organic phase, while NaI solid powder and CsPbBr 3 PNCs in n-hexane were used for the solid/liquid phase. Following the study of Kamat et al. [43], three steps occur in the halogen exchange between CsPbBr 3 PNCs in n-hexane and I − including: (1) diffusion of I − to the CsPbBr 3 PNCs interface; (2) interfacial halogen exchange; and (3) I − diffusion of CsPbBr 3 PNCs in crystals. In this experiment, to understand the halogen exchange process of CsPbBr 3 PNCs and I − , a constant concentration of CsPbBr 3 PNCs was used to keep the characteristic absorption of CsPbBr 3 PNCs at 335 nm. In this study, the interfacial exchange between water and the organic phase is first considered. After a halogen exchange, the CsPbBr x I 3−x PNCs become more unstable in a polar reagent such as ethyl acetate, so n-hexane was selected as the dispersion solvent of CsPbBr 3 PNCs in the experiment. Due to the great polarity differences between H 2 O and n-hexane, it is difficult to effectively exchange I-in water to n-hexane. Generally, higher concentrations of NaI (60 mmol L −1 ), acidic condition (pH 1), and a quick stir rate (1500 rpm, 5 min) are necessary to realize the exchange. As shown in Figure 3, the fluorescence emission wavelength of CsPbBr 3 PNCs shifted from 518 nm to 549 nm. However, in a neutral medium (pH 7), the whole exchange process became very slow, and it took more than 24 h to finish the reaction process.  The halogen exchange on the solid/liquid interface was also investigated. The process of in-situ halogen exchange between CsPbBr3 PNCs in n-hexane and NaI powder was observed using a Renishaw Invita Raman spectrometer. The schematic diagram of the experiment was shown in Figure 4. As shown in Figure 5, each spectrum was collected at 30 s intervals. Analysis of the spectrum change indicated that the I-and Br-exchange occurred quickly on the NaI crystal interface as soon as the addition of CsPbBr3 PNCs. The initial fluorescence emission at 518 nm of CsPbBr3 PNCs changed to 625 nm due to the exchange between the NaI crystal interface and CsPbBr3 PNCs. The wavelength at 625 nm should be relegated to the exchange product as CsPbBrxI3-x (CsPbBrxI3-x is the existing form on the solid/liquid interface, but CsPbBrxI3-x is the form after the exchange in the solution). When the exchange of CsPbBrxI3-x and NaI crystals in the solution was carried out further, increasingthe iodine content in the CsPbBrxI3-x, the fluorescence peak of CsPbBrxI3-x was continuously redshifted in wavelength and its intensity enhanced. However, with the exchange time increased, the redshift rate of CsPbBrxI3-x on the interface of NaI crystals was greatly reduced, due to the slow diffusion into the crystal interior. Finally, the fluores- The halogen exchange on the solid/liquid interface was also investigated. The process of in-situ halogen exchange between CsPbBr 3 PNCs in n-hexane and NaI powder was observed using a Renishaw Invita Raman spectrometer. The schematic diagram of the experiment was shown in Figure 4. As shown in Figure 5, each spectrum was collected at 30 s intervals. Analysis of the spectrum change indicated that the I-and Br-exchange occurred quickly on the NaI crystal interface as soon as the addition of CsPbBr 3 PNCs. The initial fluorescence emission at 518 nm of CsPbBr 3 PNCs changed to 625 nm due to the exchange between the NaI crystal interface and CsPbBr 3 PNCs. The wavelength at 625 nm should be relegated to the exchange product as CsPbBr x I 3−x (CsPbBr x I 3−x is the existing form on the solid/liquid interface, but CsPbBr x I 3−x is the form after the exchange in the solution). When the exchange of CsPbBr x I 3−x and NaI crystals in the solution was carried out further, increasingthe iodine content in the CsPbBr x I 3−x , the fluorescence peak of CsPbBr x I 3−x was continuously redshifted in wavelength and its intensity enhanced. However, with the exchange time increased, the redshift rate of CsPbBr x I 3−x on the interface of NaI crystals was greatly reduced, due to the slow diffusion into the crystal interior. Finally, the fluorescence wavelength of the interface between the solution and NaI crystal overlaps, indicating that the exchange reached an equilibrium state in about 20 min.     The halogen exchange between CsPbBr 3 PNCs and OLAM-I in n-hexane solution was further studied. The results as shown in Figure 6 indicated that OLAM-I presents good solubility in n-hexane. When different concentrations of OLAM-I were added to CsPbBr 3 PNCs in n-hexane, the halogen exchange could be carried out rapidly, revealing a single exponential process (Figure 6a). The exchange is essentially a surface-restricted exchange process and could generally be completed within 2 min (OLAM-I = 120 µmol L −1 ). In addition, it was found that the reactionwas mainly controlled by the kinetic process, and that temperature had little effect on the halogen exchange in the range of 15 • C to 45 • C, which is convenient for the application at room temperature. The halogen exchange between the different concentrations of OLAM-I and CsPbBr 3 PNCs was further investigated. As shown in Figure 6b, the yellowish-green color of the n-hexane solution containing CsPbBr 3 PNCs changed to orange-red and red rapidly with the concentration increase of OLAM-I due to the generation of CsPbBr x I 3−x . The bandgap width of CsPbBr x I 3−x decreases with the increasing content of I, corresponding to the red-shift of the band-edge absorption and the emission spectrum. The corresponding color changed from green to yellow, and then red (Figure 6c). Experimental results reveal that the proposed method is of high resolution in colorimetric sensing, and the redshift of the wavelength difference (∆λ) and the concentration of OLAM-I presents a good linear relationship in the range of 0 to 240 µmol L −1 (Figure 6d); the corresponding linear equation for ∆λ = 0.563 C OA mI (µmol L −1 ), R 2 = 0.9960. addition, it was found that the reactionwas mainly controlled by the kinetic process, and that temperature had little effect on the halogen exchange in the range of 15 • C to 45 • C, which is convenient for the application at room temperature. The halogen exchange between the different concentrations of OLAM-I and CsPbBr 3 PNCs was further investigated. As shown in Figure 6b, the yellowish-green color of the n-hexane solution containing CsPbBr 3 PNCs changed to orange-red and red rapidly with the concentration increase of OLAM-I due to the generation of CsPbBr x I 3−x . The bandgap width of CsPbBr x I 3−x decreases with the increasing content of I, corresponding to the red-shift of the band-edge absorption and the emission spectrum. The corresponding color changed from green to yellow, and then red (Figure 6c). Experimental results reveal that the proposed method is of high resolution in colorimetric sensing, and the redshift of the wavelength difference (∆λ) and the concentration of OLAM-I presents a good linear relationship in the range of 0 to 240 µmol L −1 (Figure 6d); the corresponding linear equation for ∆λ = 0.563 C OA mI (µmol L −1 ), R 2 = 0.9960.

Analytical Performance of the Sensing Approach
Before the applications using the wavelength-shift-based sensing of the CsPbBr 3 PNCs for the determination of BPO, the validation method included linear range, the limit of detection (LOD), the limit of quantitation (LOQ), precision, and accuracy under optimized conditions. BPO has high oxidization characteristics and a good solubility in organic solvents. As shown in Figure 7a, BPO oxidizes OLAM-I, which reduced the halogen

Analytical Performance of the Sensing Approach
Before the applications using the wavelength-shift-based sensing of the CsPbBr 3 PNCs for the determination of BPO, the validation method included linear range, the limit of detection (LOD), the limit of quantitation (LOQ), precision, and accuracy under optimized conditions. BPO has high oxidization characteristics and a good solubility in organic solvents. As shown in Figure 7a, BPO oxidizes OLAM-I, which reduced the halogen exchange between iodine and CsPbBr 3 PNCs, resulting in the decrease of wavelength redshift in the reaction. The following experimental phenomena would verify the reaction process as 1 mL of 240 µmol L −1 OLAM-I reacted with different concentrations of BPO (0~120 µmol L −1 in toluene). After the reaction was complete, CsPbBr 3 PNCs in an nhexane solution was then added to observe the halogen exchange. The results showed that BPO reacted completely with OLAM-I within 5 min and directly reduced the halogen exchange of CsPbBr 3 PNCs. In the absence of BPO, as shown in Figure 7b, the apparent color of CsPbBr 3 PNCs changed from the original green to the red of CsPbBr x I 3−x . With the increase of BPO concentration, although the wavelength redshift of the fluorescence emission could be observed in the reaction of OLAM-I and CsPbBr 3 PNCs, the apparent color changed from red to orange, yellowish-green, and finally green, indicating the concentration decrease of OLAM-I due to the addition of BPO.   In the presence of 120 µmol L −1 BPO, OLAM-I was completely oxidized, and no halogen exchange could be found for CsPbBr 3 PNCs. The experimental results as shown in the inset of Figure 8b reveal that 0~120 µmol L −1 BPO could be detected with this method. When the concentration of BPO exceeded 120 µmol L −1 , the BPO would further oxidize CsPbBr 3 PNCs, resulting in poor stability and interference in the determination due to the strong oxidation characteristics of BPO. The wavelength shift difference, ∆λ, presented a good linear relationship with the concentration of BPO (C BPO ) in the range of 0~120 µmol L −1 ; the linear equation for ∆λ = 1.122 C BPO (R 2 = 0.9933). Based on the limit of detection (LOD) = 3σ/K (where σ is the relative standard deviation of 12 blank measurements and k is the slope of the linear equation), the LOD and LOQ of 0.13 µmol L −1 and 0.43 µmol L −1 BPO could be obtained, respectively. The experimental results revealed that the alteration of the intraday coefficient for five duplicate samples of 10 µmol L −1 BPO was less than 4.2%. Meanwhile, the alteration of the interday coefficient was less than 5.6% for the same concentration of BPO during three days.

Determination of BPO in Samples
In the determination of BPO in flour and noodle samples, pretreatment is the key step. The BPO sample extraction procedures and timing were selected and optimized. Ultrasonic extraction was selected since the ultrasonic process is simple, highly efficient, and can effectively remove the dissolved oxygen in the solvent thus reducing the interference caused by the reaction of dissolved oxygen and OLAM-I. In the separation process, a direct filtration treatment method was applied in the National Standard Method (GB/T 18415-2001 Determination of Benzoyl Peroxide in Wheat Flour). However, flour particles easily block the pores of filter paper, which leads to problems such as irregular filtration, loss of extract, and incomplete extraction. In addition, the volatilization of organic extract in the filtration process can cause deviation of the test results and environmental pollution. In the experiment, the centrifugal precipitation separation method was adopted to achieve higher separation efficiency, more thorough extraction, and a smaller impact on the determination results. At the same time, using the centrifugal separation method significantly reduces the harm to the experimenter and the environmental pollution. The ultrasonic time was optimized as such: 1 g of flour or noodle powder was collected (BPO was added to the flour to make the mass concentration of BPO 24.2 mg kg −1 ), and 5 mL of n-hexane was used for the extraction. Generally, with the increase of extraction time, the extraction yield of BPO increased continuously, and so the concentration of BPO would be 120 μmol·L −1 if the extraction efficiency was 100%. Based on the redshift of the fluorescence emission wavelength, the experimental results as shown in Figure 8, indicate that 99.7% BPO in the sample could be extracted after 30 min. Thus, the optimal extraction time of 30 min was selected.
The effect of possible co-existing substances' influence on the sensing was investigated, and the results are shown in Figure 9. The experimental results show that the influence of common cations on the sensing determination was very small, revealing the selection of n-hexane as an ideal extracting solvent for BPO. BPO can be dissolved in nhexane, while other co-existing substances (such as KI, MgSO4, etc.) had poor solubility in n-hexane, which reduces their possible effect, and thus greatly improves the selectivity. The influence of sample humidity on the results was also studied. In the test environment with a humidity of 75%, the samples were allowed to fully absorb water in the air for one night. At the same time, another flour or noodle sample was dried using a vacuum to

Determination of BPO in Samples
In the determination of BPO in flour and noodle samples, pretreatment is the key step. The BPO sample extraction procedures and timing were selected and optimized. Ultrasonic extraction was selected since the ultrasonic process is simple, highly efficient, and can effectively remove the dissolved oxygen in the solvent thus reducing the interference caused by the reaction of dissolved oxygen and OLAM-I. In the separation process, a direct filtration treatment method was applied in the National Standard Method (GB/T 18415-2001 Determination of Benzoyl Peroxide in Wheat Flour). However, flour particles easily block the pores of filter paper, which leads to problems such as irregular filtration, loss of extract, and incomplete extraction. In addition, the volatilization of organic extract in the filtration process can cause deviation of the test results and environmental pollution. In the experiment, the centrifugal precipitation separation method was adopted to achieve higher separation efficiency, more thorough extraction, and a smaller impact on the determination results. At the same time, using the centrifugal separation method significantly reduces the harm to the experimenter and the environmental pollution. The ultrasonic time was optimized as such: 1 g of flour or noodle powder was collected (BPO was added to the flour to make the mass concentration of BPO 24.2 mg kg −1 ), and 5 mL of n-hexane was used for the extraction. Generally, with the increase of extraction time, the extraction yield of BPO increased continuously, and so the concentration of BPO would be 120 µmol·L −1 if the extraction efficiency was 100%. Based on the redshift of the fluorescence emission wavelength, the experimental results as shown in Figure 8, indicate that 99.7% BPO in the sample could be extracted after 30 min. Thus, the optimal extraction time of 30 min was selected.
The effect of possible co-existing substances' influence on the sensing was investigated, and the results are shown in Figure 9. The experimental results show that the influence of common cations on the sensing determination was very small, revealing the selection of n-hexane as an ideal extracting solvent for BPO. BPO can be dissolved in n-hexane, while other co-existing substances (such as KI, MgSO 4 , etc.) had poor solubility in n-hexane, which reduces their possible effect, and thus greatly improves the selectivity. The influence of sample humidity on the results was also studied. In the test environment with a humidity of 75%, the samples were allowed to fully absorb water in the air for one night. At the same time, another flour or noodle sample was dried using a vacuum to compare the sensing response with those samples in 75% humidity. The results indicated that there was no significant difference between the two conditions. This indicates that the water adsorption in the samples did not affect the sensing results because it is very difficult to dissolve the high polarity molecule, H 2 O, into n-hexane, resulting in the insignificant effect of humidity on the sensing results.
Using the above optimal conditions, the colorimetric sensing of BPO in flour and noodle samples was carried out. As shown in Table 1, the sensing results of the recovery ranged from 97.0% to 112.0%, indicating that the method has good accuracy and application potential.  Using the above optimal conditions, the colorimetric sensing of BPO in flour and noodle samples was carried out. As shown in Table 1, the sensing results of the recovery ranged from 97.0% to 112.0%, indicating that the method has good accuracy and application potential.

Conclusions
The sensing studies and applications of CsPbX 3 NCs in analytical chemistry mainly benefit from its excellent optical and photoelectric properties such as high quantum yield, narrow half-peak width, and ion exchange, which has excellent application prospects in colorimetric sensing. Through the ratio of halogen ions in CsPbX 3 NCs, the emission wavelength of their fluorescence spectra can be regulated to cover the whole visible region and produce significant color change from blue to red, which is very beneficial in colorimetric sensing with high visual sensitivity based on wavelength-shift. However, CsPbX 3 NCs are unstable in water due to their intrinsic ionic salt properties, low surface energy generation, and limited ion radius tolerance factor, which creates challenges for their application in an aqueous medium. In this study, we have discussed the halogen exchange Br − in CsPbBr 3 PNCs and I − in OLAM-I under three homogeneous phases: solid/liquid (organic phase), liquid (water phase)/liquid (organic phase), and liquid (organic phase)/liquid (organic phase). The experimental results show that Br − in CsPbBr 3 PNCs and I − in OLAM-I causes rapid halogen exchange, and results in a fluorescence emission wavelength shift. Based on these results, we have realized the feasibility of the colorimetric sensing of BPO, in which the reaction between OLAM-I and BPO caused a concentration change of OLAM-I. Using the halogen exchange, we have successfully developed an indirect approach for the colorimetric sensing of BPO with high visualization resolution, fast response, and high sensitivity, and attained the determination of BPO in flour and noodle samples. Based on this consideration, we can realize the discovery of other oxides in the organic system, such as hydrogen peroxide, or introduce a microemulsion system and enzymatic catalytic oxidation to achieve the colorimetric sensing in a biological system, which will further broaden the application of CsPbBr 3 PNCs in analytical sensing.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/chemosensors9110319/s1, Table S1: The sensing applications of perovskite material, Table S2: Comparison among the results from the present work and others found in the reference.