Photoluminescence Sensing of Lead Halide Perovskite Nanocrystals and Their Two-Dimensional Structural Materials

Abstract

In recent years, the development of new efficient, fast, and intuitive materials and methods for photoluminescence (PL) sensing has become a research hotspot in analytical chemistry. Lead halide perovskite (LHP) materials have the characteristics of adjustable PL properties, high PL efficiency, and a variety of synthesis methods. Their PL is also sensitive to the change in specific factors in the environment. Based on these characteristics, LHP has shown good application prospects in the field of optical sensing. The study of the structural dimension, organic composition, or doped ions of LHP is helpful in exploring its sensing potential and proposing new sensing mechanisms, which have important research significance to promote sensing applications. In this review, the PL characteristics and sensing mechanisms, as well as their sensing applications of two- and three dimensional LHP, are discussed and summarized.

1. Introduction

Perovskite materials were originally referred to as calcium titanate (CaTiO₃) and were named by a German mineralogist, Gustav Rose, in 1839, in honor of the Russian mineralogist Lev Perovski. Actually, if the crystal structure of a material is similar to CaTiO₃, it is also collectively referred to as a perovskite material. In 1958, Møller [1] reported a metal halide, CsPbX₃, where X represents a halogen element such as Cl, Br, or I, with a perovskite structure. Weber [2] obtained the first metal halide perovskite CH₃NH₃PbX₃ containing organic cations in 1978, which evolved into a generalized lead halide perovskite (LHP) family [3]. The chemical formula of perovskite is ABX₃, in which the common cation at the A position is mainly represented by Cs⁺, K⁺, Ca²⁺, CH₃NH₃⁺, or HC(NH₂)₂⁺, as well as C₆H₅CH₂CH₂NH₃⁺, etc. The common cation at the B position is represented by Pb²⁺, Sn²⁺, or Ti⁴⁺, etc. X is an anion, which is mainly divided into an O²⁻ or halogen anion (Cl⁻, Br⁻, I⁻) [3,4]. Generally, perovskite materials include a three-dimensional (3D) structure of ABX₃ (X = Cl⁻, Br⁻, I⁻) and a low-dimensional structure of A₂BX₄, ABX₄, and AB₂X₅, as well as A₄BX₆, etc.

Since the lead halide perovskite nanocrystal (LHP NC) of CH₃NH₃PbX₃ was used in solar cells by Kojima in 2009 [5], it has aroused great interesting for the researchers to study the fully inorganic perovskite, organic/inorganic hybrid perovskite in photovoltaic laser and scintillator conversion fields [4,6,7], with the different morphology as large single crystal, polycrystalline thin films or nanocrystals. In addition to Kojima, Snaith and Park et al. also reported solar cells using CH₃NH₃PbI₃ as a photosensitive material in 2009 and 2012, respectively [5,8,9]. Schmidt et al. [10] first obtained MAPbBr₃ nanomaterials emitting green light with a narrow band in 2014. In 2015, Protesescu et al. [11] synthesized CsPbX₃ nanocrystals (X = Cl⁻, Br⁻, I⁻) with an emission wavelength covering the full visible region, and their photoluminescence (PL) quantum yield (QY) reached 90%. These breakthroughs became the beginning of a new wave of LHP NC study. Today, LHP NCs have attracted wide attention in applications such as light-emitting diode, solar cell, laser, photodetector, scintillator, and optical sensing [12,13,14,15,16] due to their low material cost and superior photoelectric properties such as a high PLQY, high defect tolerance, wide absorption spectrum, easy adjustment of emission wavelength, and long carrier diffusion length.

The development of new materials with a high sensitivity and fast response for PL sensing is a hotspot in the field of analytical chemistry. In addition to the above advantages, LHP NCs still reveal a good application prospect in the field of PL sensing due to their characteristics of various synthesis methods, rich composition, high PL efficiency, and easy adjustment of PL properties. Additionally, compared with the traditional inorganic semiconductor PL materials, the soft ion lattice of LHPs makes their PL more sensitive to the change in specific factors in the environment. Meanwhile, LHP with a different dimensional structure also shows a high sensitivity to the change in some environmental parameters, which could be employed for PL sensing. At present, three-dimensional (3D) LHP and derived two-dimensional (2D) layered perovskite materials have realized the sensing of ions, small molecules, and metabolites, as well as physical parameters such as humidity and temperature.

2. Composition and Structure of Lead Halide Perovskite Materials and Their Sensing Characteristics

2.1. Composition and Structure of Lead Halide Perovskite Materials

Generally, LHP material can be divided into 3D nanocubeset, 2D nanosheet, one-dimensional (1D) wire, and zero-dimensional (0D) quantum dot in morphology. Despite their different dimensions, all LHPs have the same crystal structure at the molecular level [17] and exhibit size-related properties such as quantum limiting effects at the nanoscale [18]. Low-dimension LHP is a kind of organic–inorganic hybrid material. The suitable selection of an organic component in LHP results in a change in its crystal structure and LHP dimension at the molecular level [19,20]. Interestingly, the macro-crystal of low-dimension LHP exhibits the same characteristics as the crystal unit structure. If the size of organic cations in LHP is too large, the cations cannot be integrated into the 3D framework. Therefore, 2D LHP material with layered or corrugated inorganic layers can be obtained, and the organic cations are in the inorganic layers. For 1D perovskite material, the inorganic octahedrons form a chain structure in a co-angular, co-sided, or co-planar manner, and this is surrounded by organic cations. For 0D LHP material, the isolated inorganic octahedrons are completely isolated by organic cations. The crystal structures of typical 3D, 2D, 1D, and 0D LHP materials are shown in Figure 1.

As shown in Figure 2, a standard 3D LHP, ABX₃, is composed of A, monovalent cation (Cs⁺, CH₃NH₃⁺ as MA⁺, CH(NH₂)₂⁺ as FA⁺ in the figure); B, a bivalent metal cation Pb²⁺; and X, a monovalent halogen anion (Cl⁻, Br⁻, I⁻) [21]. The lead halide octahedron [PbX₆]₄, formed by the coordination of Pb²⁺ and six halide anions, forms a spatially angular shared 3D framework with monovalent cations located in the void of the lead halide octahedron framework. LHP can be divided into total inorganic perovskite and organic–inorganic hybrid perovskite according to the different cation at the A position. In order to determine the combination of cations and anions in LHP, Goldschmidt proposed the tolerance factor t based on the oxide perovskite in 1926, which is now widely used to measure the structural stability of an LHP. The calculation formula can be shown as t = (RA + RX)/√2(RB + RX). In the formula, RA, RB, and RX are the ionic radii of the A, B, or X position, respectively. When f is in the range of 0.81 < t < 1.11, an LHP with the ABX₃ type maintains a stable 3D structure. If the f value is beyond this range, the crystal structure of an LHP will decrease its dimension [22,23].

The introduction of large organic amine cations at the A position of a 3D LHP can be regarded as splitting the 3D structure along specific crystal faces, resulting in <100>, <110>, and <111>-oriented 2D LHP. As shown in Figure 3, the most common 2D LHP with <100> orientation can be further divided into the Ruddlesden–Popper (RP) phase, Dion–Jacobson (DJ) phase, and alternating cations in the interlayer (ACI) phase [17]. The general chemical formula of the RP phase is A’₂A_n−1B_nX_3n+1, which has a 2D (n = 1) or quasi−2D (n = 2–5) layer structure [24]. The A’-site monovalent organic ammonium ions spatially separate the adjacent inorganic layers by hydrogen bonding and the electrostatic interaction with the lead halide octahedron. The interlaced organic cations are connected by van der Waals forces, causing the inorganic layers to stagger half of the octahedral units. The general chemical formula of the DJ phase is A A_n−1B_nX_3n+1, where the bivalent organic ammonium ions of the A’ position connect the two adjacent inorganic layers, and the compact organic cations cause the inorganic layers to overlap on top of each other without displacement. The general chemical formula of the ACI phase is A’A_n−1B_nX_3n+1, which is relatively rare compared to the first two types. Soe et al. [25] first reported (C(NH₂)₃)(CH₃NH₃)_nPb_nI_3n+1 in 2017, in which CH₃NH₃⁺(MA⁺) not only exists in the void of the inorganic octahedral framework, but also alternates between inorganic layers with another large-sized organic cation, C(NH₂)₃⁺(GA+). At present, the ACI phase exists only in 2D perovskites containing GA⁺.

The study of 2D LHP materials can be dated back to 1986 when Dolzhenko et al. [26] first reported a 2D LHP of (C₉H₁₉NH₃)₂PbI₄. Subsequently, Ishihara [27,28], Papavassiliou [29,30], and Calabrese et al. [31] studied the structural and optical properties of 2D LHP materials. It was also proved that (RNH₃)₂(MA)_n−1Pb_nI_3n+1 between MAPbI₃ and (RNH₃)PbI₄ is not a discrete LHP material, but belongs to the same series [31]. The study results show that 2D LHP materials have a richer chemical composition and electronic structure than their 3D counterparts. The characteristics of an ionic lattice, low dimensional structure, and organic cations not limited by an inorganic framework improve the compositional and structural flexibility of 2D LHP in the functional design. The organic and inorganic layers of an alternating 2D LHP by self-assembly form an ordered quantum well electronic structure, which can produce effective excitonic PL at room temperature [32]. The regulating free excitons, bound excitons, and self-trapped excitons affect the PL properties of materials [33]. In addition, van der Waals interactions between organic cations increase the formation energy of 2D LHP [34], and some hydrophobic organic components further improve the stability of 2D LHP.

For LHP materials, their composition and structure are easy to control. The unique electronic and band structure give them efficient PL and easy to adjust their PL properties. In addition, the soft ion lattice makes their PL more sensitive to the change in specific factors in the environment. These characteristics reveal the potential of LHPs as new PL-sensing materials. At present, based on the sensing mechanisms of crystal structure or phase change, defect introduction or passivity, component ion exchange, and electron or energy transfer of LHPs, researchers have realized the sensing of various chemical substances in the gas, organic, and water phases, as well as the environmental physical parameters such as temperature or humidity by their PL intensity change or their PL wavelength shift [35,36].

2.2. Photoluminescence Sensing of 3D Lead Halide Perovskite Nanocrystal

2.2.1. Sensing Mechanism of 3D Lead Halide Perovskite Nanocrystal

The PL-sensing method is intuitive and convenient in sensing applications. The high-efficiency PL and easily regulated PL properties of LHP materials give them a good potential for PL-sensing applications. In the research and application of the PL sensing of 3D LHPs, The ionic salt characteristic of lead perovskite nanocrystals (LHP NCs), typically CsPbBr₃ nanocrystals (CsPbBr₃ NCs), gives them an ion exchange property (both cation and cation ions can be exchanged quickly). Generally, the wavelength shift of 3D LHP NCs is caused by the electronic structure change in 3D LHP NCs after halogen exchange. When the halogen changes from Cl to Br to I, the valence band changes from 3p–4p–5p, which reduces the ionization potential [37]. In July 2015, Nedelcu [38] and Akkerman [39] reported that halogen exchange regulates the optical properties of CsPbBr₃ NCs. They found that the post-regulation of CsPbBr₃ NCs by halogen exchange can accurately regulate the PL wavelength in the range of 410 to 700 nm. Owing to the good defect tolerance of LHP NCs, after regulation, the LHP NCs still maintain the original crystal structure and keep their excellent PL properties (Figure 4). It is difficult to carry out halogen exchange between CsPbI₃ and Cl⁻. Its crystal structure is unstable and easily collapses after exchange due to the large difference in ionic radius between Cl⁻ and I⁻. At present, the main halogen sources carrying out halogen exchange are OAmX, MeMgX, halogenated salts (PbX₂, ZnX₂, KX, MnCl₂, etc.) [38,39,40,41,42,43], and halogenated organics. Parobek et al. [44] found that ultraviolet light can catalyze the photoelectronic reduction of dichloromethane to Cl⁻ by CsPbBr₃ NCs, and Cl⁻ can undergo halogen exchange with CsPbBr₃ NCs. Similarly, CsPbCl₃ NCs and dibromomethane are also feasible. The halogen exchange property of CsPbX₃ NCs helps to provide a new means of efficient preparation of CsPbX₃ NCs [45,46], and it also provides application potential in the fields of pattern engraving [47] and sensing [48].

Many researchers are interested in the study of the halogen exchange of LHP NCs at the solid–liquid or solid–solid interface of CsPbBr₃ NCs. As shown in Figure 5, Nikolai et al. [49] first dispersed CsPbBr₃ NCs into an organic solvent, added KX (X = Cl⁻, Br⁻, I⁻) salt to the solution, and then removed the solvent. The halogen exchange occurred on the KX interface of CsPbBr₃ NCs, then yielded CsPbCl_xBr_3−x and CsPbBrxI_3−x. The products are of the emission wavelength covering the entire visible region. Our research group adopted a similar strategy to discuss the halogen exchange between CsPbBr₃ NCs and MnCl₂ as well as the exchange of Pb²⁺/Mn²⁺. CsPbCl₃−Mn NCs could be obtained, and the stability was significantly improved [50].

Koscher et al. [51] conducted an in-depth discussion on the halogen exchange process of CsPbBr₃ NCs. They found that CsPbBr₃ NCs have a different mechanism of halogen exchange with I⁻ and Cl⁻. As soon as reaction with I⁻ occurs, CsPbBr₃ NCs is alloyed rapidly. The reaction process is a uniexponential surface-reaction-limited exchange process. When Cl⁻ is exchanged, it is carried out exponentially. This intermediate conversion presents a diffusion-limited exchange process, which is more complicated than the halogen exchange of I⁻ due to the different ionic bond energy and ion migration rate. Compared with halogen anion exchange, the study on the cation exchange started relatively late and is less studied, but it is still of great significance [52,53]. Stam et al. [54] systematically studied the process and mechanism of M cation exchange between Pb²⁺ and MBr₂ (M = Sn²⁺, Cd²⁺, and Zn²⁺) in CsPbBr₃ NCs. Compared with the halogen exchange process, the activation energy of Pb²⁺ cation exchange (2.31 eV) is much higher than that of vacancy-induced halogen anion exchange (0.58 eV in the case of I⁻). Therefore, halogen anion exchange can be completed in a few minutes at room temperature, while the cation exchange process takes 16 h at room temperature. The product, CsPb_1−xM_xBr₃ (0 < x ≤ 0.1), can maintain the cubic crystal structure of the original CsPbBr₃ NCs after the cation exchange. Its PL peak shows a significant blue shift, which is independent of the type of M, and only linearly correlated with the lattice vector of PNCs (Figure 6a,b). In addition, the CsPb_1−xM_xBr₃ is of the blue light emission with a higher PLQY and narrower half-peak width. It shows better optical properties than the products after Cl⁻ exchange. Based on this interesting phenomenon, they conducted a profound analysis of its mechanism, and concluded that the main reason for this phenomenon is the lattice contraction of PNCs. Generally, the entire exchange process can be divided into four processes (Figure 6c). Firstly, oleylamine (OLAM) interacts with CsPbBr₃ NCs to generate OLAM−Br and form a Br vacancy on the surface of PNCs. Subsequently, OLAM−MBr₂ enters the Br vacancy and releases energy, which breaks the bond between PbBr₂ and PNCs, and the formed PbBr₂ vacancy is occupied by MBr₂. The energy releases in this process is basically equivalent to the energy of the broken bond, completing the exchange of M with Pb²⁺, and diffusing inward along the Br vacancy. With the increase in inward exchange and diffusion M, the lattice shrinkage of CsPbBr₃ NCs is caused by the difference of ionic radius (r (Pb²⁺) = 119 pm, r (Cd²⁺) = 95 pm, r (Zn²⁺) = 74 pm, and r (Sn²⁺) = 118 pm). As a result, the lattice stress inside CsPbBr₃ NCs continues to increase, counteracting the increase in entropy of the system, and finally the exchange stops. This is a self-limited diffusion process. In addition, due to the effect of lattice shrinkage, the increase in the Pb−Br effect and the energy of the anti-bonding orbital composed of Pb (6p) and Br (4p) s results in the upward shift of the bottom end of the conduction band. The small influence of the Br (4p⁶) orbit and Pb (6s²), as well as the valence band change, increases the band gap width, resulting in the blue shift of the emission peak [54]. These results are important in understanding the cation exchange of LHP NCs.

2.2.2. Sensing Applications of 3D Lead Halide Perovskite Nanocrystal

Since the halogen exchange products of CsPbBr₃ NCs are of different emission wavelength, they can be employed in PL sensing. It has been reported [38,39] that the formation of CsPbBr_xI_3−x NCs and CsPbCl_xBr_3−x NCs by the anion exchange between I⁻ and Cl⁻ using CsPbBr₃ NCs, respectively, is extremely fast. The anion exchange can be completed in tens of seconds. The halogen exchange in LHP NCs will cause the PL wavelength to shift significantly, which is conducive to colorimetric sensing. In terms of ion determination, Sheng et al. [55] realized the highly selective determination of Cu²⁺ in an organic phase based on the PL quenching caused by the introduction of Cu²⁺ defects to the surface of CsPbBr₃ NCs. Liu et al. [56] believed that CsPbBr₃ NCs have a photogenic electron transfer with Cu²⁺, which also achieves PL quenching of Cu²⁺. Wang et al. [57] used a sulfhydryl modified porous alumina template to capture Pb²⁺. This PL-enhanced method reveals less interference in the sensing application from the co-existing metal ions in samples, and the linear range of PL response to Pb²⁺ concentration in water is found to be 0.01–1.0 μg/mL. The detection limit reaches at 5 × 10⁻³ μg/mL.

Using the redox reaction between peroxides in food samples and I⁻, Zhu et al. [58] designed a PL-wavelength shift method for colorimetric sensing of the peroxide number in oil products based on the halogen exchange reaction between CsPbBr₃ NCs and I⁻. As shown in Figure 7, the peroxide number of edible oil could be determined indirectly by CsPbBr₃ NCs. With the increase in peroxide content, the content of I⁻ decreases due to the reaction between I⁻ and peroxide in the sample. The PL wavelength of the solution gradually shifts from 638 nm to 532 nm with the color change from red to yellow and then to green. Using this sensing method, it is easy to identify whether the peroxide number of oil samples exceeds the standard or not. When the peroxide number is lower than the National Standard Limit value (0.25 g/100 g), the presented color is red, but it changes to yellow when the peroxide number is near the limit value, and even becomes green if the peroxide number is higher than the limit value. A light-assisted method was proposed by Feng et al. [59]. They used CsPbBr₃ NCs as a host material to immobilize Y atoms (Y−SA). Using CsPbBr₃ NCs as anchors, Y−SA/CsPbBr₃ NCs could be obtained by forming two Y−O and two Y−Br bonds. The material presents an excellent PL stability. I⁻ could be obtained by the alkylation reaction between and CH₃I and oleoamide (OLA), then making a halogen exchange with Y−SA/CsPbB_r3 NCs, resulting in a color change and colorimetric sensing. The method was applied to the determination of CH₃I in sweet potato samples.

A wavelength shift method based on the halide exchange of CsPbBr₃ NCs and Cl⁻ has been developed in order to realize the rapid colorimetric sensing of Cl⁻ in sweat [60]. The study indicates that CsPbBr₃ NCs could achieve halide exchange with Cl⁻ in the aqueous phase, accompanying a significant wavelength blue shift and vivid color change. A novel water-dispersed nanocrystalline (W−PNC) with a strong PL and excellent water stability has been synthesized by Chen et. al. [61] using oleamine (OAm) CsPbBr₃@CsPb₂Br₅ by oil–solid–water phase transition. It is not necessary to immobilize W−PNCs in a dense polymer or an inorganic material. As indicated in Figure 8, the analytes could be directly detected in the water phase by chemical reactions. They studied the ion exchange between W−PNCs and halide ions as well as its effect on the PL wavelength of nanocrystals in water. They also developed a novel colorimetric sensing platform for halide ions using a smart phone.

Saikia et al. [62] found that the addition of ethanol triggers the dissociation of cetyltrimethylammonium (CTAB) cations, which competes and interacts with NH₄⁺ on the surface. In addition, ethanol can form strong hydrogen bonds with Br⁻ and pull Br⁻ away from the surface. The separation of NH₄⁺ and Br⁻ leads to the formation of positive and anionic vacancies, resulting in charge recombination and thus the PL quenching at 550 nm. The PL signal at 430 nm is generated by the formation of PbBr₂ due to the degradation of CTAB@CsPbBr₃ in the presence of ethanol. With the gradual addition of ethanol, the concentration of PbBr₂ increases, and the PL intensity of PbBr₂ at 430 nm increases. Ethanol can be identified from other alcohols by its reaction with CTAB@CsPbBr₃, since ethanol induces a PL increase at 430 nm, while methanol induces a CTAB@CsPbBr₃ PL decrease at 550 nm. In the presence of ethanol, the behavior of dynamic quenching reduces the PL lifetime after the hydrogen bond interaction occurs between CTAB@CsPbBr₃ and ethanol, leading to non-radiative decay.

In addition to halogen exchange, cations in CsPbBr₃ NCs could also be changed by other metal cation. In the presence of Hg²⁺, Pb²⁺ in CsPbBr₃ NCs is gradually replaced by Hg²⁺. However, unlike the halogen exchange reaction that causes a significant shift in emission wavelength, the exchange reaction between Hg²⁺ and Pb²⁺ only causes a small wavelength shift, but a significant PL intensity decrease occurs. Therefore, a sensing method for Hg²⁺ based on its PL quenching was established by Lu et al. [63]. In terms of gas sensing, Huang et al. [64] proposed a sensing method of NH₃ gas using the reversible passivation of surface defects of CsPbBr₃ NCs by NH₃. PbS could be generated by the reaction Pb and H₂S, which causes damage of the perovskite structure. H₂S in rat brain microdialysis could be detected in the range of 0–100 μmol/L by Chen et al. [65]. You et al. [66] prepared a composite material of MAPbBr₃ quantum dots and superhydrophobic silica gel. They proposed a reversible sensing method for SO₂ using the PL quenching through the transfer of excited state electrons from MAPbBr₃ to SO₂. Kim et.al. [67] developed a high-performance and ultra-fast-response sensing approach for NO₂ using CsPbI₂Br NCs with an LOD of 2 ppb NO₂. Its remarkable photoelectric properties enable dual-mode sensing including PL and voltaic sensing. As shown in Figure 9, the electron transfer between the sensing layer and NO₂ causes a resistance change in the layer. As a strong electron-absorbing gas, NO₂ tends to adsorb on the surface of CsPbI₂Br NCs to attract electrons. This process leads to the hole number increase in the p-type, and it results in an obvious resistance decrease in the CsPbI₂Br NCs, which can be applied for NO₂ sensing. Additionally, since electrons are captured by NO₂ adsorbed on the surface of CsPbI₂Br NCs, the obvious reduction in carrier recombination causes the PL quenching of CsPbI₂Br NCs.

The MAPbBr₃ PL could be enhanced and its emission wavelength shifted due to in situ restricted growth of MAPbBr₃, which has been applied to analyze methylamine (MA) gas by Huang et al. [68]. As the MA concentration increases, the PL intensity of the sensing film changes from blue to green. Using this method, the MA concentration in air could be effectively detected. This two-mode MA sensing method is expected to play an important role in fast MA analysis.

The great influence of water molecules should be considered regarding the structural stability of CsPbBr₃ NCs [69] in their sensing applications. Thus, some approaches have been developed in order to increase the stability of CsPbBr₃ NCs. Shu et al. [70] used amphiphilic polymer octylamine modified polyacrylic acid as the surface passivating ligand of CsPbBr₃ NCs. The obtained perovskite nanomaterial is of good water dispersion and stability, and it could be applied in the rapid determination of Cl⁻ in sweat through the aqueous halogen exchange reaction. The wavelength shift presents a good linear relationship in the range of 1 to 80 mmol/L Cl⁻. Li et al. [71] used a phospholipid membrane wrapped with CsPbX₃ NCs as peroxide-like enzymes to carry out the self-catalytic PL sensing of H₂O₂ in water. They constructed a cascade catalytic system with four oxidases to achieve the detection of metabolites.

In terms of the colorimetric sensing of oxygen, it has been reported that CsPbBr₃ NCs have various oxygen-sensitive PL behaviors. By analyzing CsPbBr₃ with different morphologies [72,73], researchers found that O₂ molecules can extract the photogenerated electrons from CsPbBr₃ nanocubesets through collision interaction, resulting in the dynamic PL quenching of CsPbBr₃. On the other hand, the surface hole traps of CsPbBr₃ nanowires, nanosheets, or single crystals can be passivated and give a higher PL without a change in the exciton recombination dynamics. The oxidation of O₂ to MAPbI₃ gap iodine eliminates deep defects, resulting in a PL increase in perovskite [74,75]. O₂-assisted photoinduced etching can cause the wavelength blue-shift of MAPbI₃ PL, and eventually the emission disappears completely [76]. In addition, Mn²⁺-doped PL has been observed in Mn²⁺:CsPbCl₃ as a sensing signal of oxygen [77]. It has also been found that there is a quantum cutting process in Mn²⁺:CsPbCl₃, which causes the oxygen quenching of doping-related Mn²⁺ PL and surface defect state PL [78]. These study results demonstrate the good potential of CsPbX₃ as a new optical oxygen sensing material.

Generally, the PL change in LHP NCs is sensitive to same environmental factors such as humidity and temperature since they may change the crystal structure of LHP NCs and induce a change in the PL. Researchers have studied the interaction between MAPbX₃ and water molecules using conductivity and found that the interaction between perovskite and water is reversible, and MAPbX₃·H₂O is obtained [79]. The effect of humidity on the single-crystal structure of MAPbI₃ has also been studied, and the results reveal that the combination of water molecules into the lattice is driven by hydrogen bond. At the same time, a blue-shift of the PL could be found [80]. Loi et al. found that the surface recombination rate of CH₃NH₃PbBr₃ is caused by the completely reversible physical adsorption change in surface water molecules, which leads to a reduction in PL intensity [81]. Recently, Keonwoo et al. found that moisture affects the formation of phase perovskite nuclei and changes the stoichiometric, thermodynamic stability, and photoelectron transfer of perovskite [82]. It becomes the most direct strategy to construct a sensing system using LHP NCs for these physical parameters. In a certain humidity range, MAPbBr₃ and water molecules produce MAPbBr₃•H₂O, resulting in the change in MAPbBr₃ structure and PL quenching. Based on this phenomenon, Xu et al. [83] established a humidity-sensing method with a humidity response range of 7–98% using MAPbBr₃. By controlling the exposure time of MAPbBr₃, a good reversibility can be obtained (Figure 9).

Figure 9. Gas sensing. (a) Humidity sensing based on the crystalline structure change in MAPbBr₃ [83]. (b) Schematic illustration of the adsorption-charge transfer mechanism between PNCs and NO₂ molecules [67]. (c) (A) PL spectra, (B) enhanced PL intensity, (C) emission wavelength shift, and (D) photographs under 365 nm UV light of the (HPbBr₃)₂PbBr₂ NCs@h-SiO₂ film as a function of MA concentration [68].

Figure 9. Gas sensing. (a) Humidity sensing based on the crystalline structure change in MAPbBr₃ [83]. (b) Schematic illustration of the adsorption-charge transfer mechanism between PNCs and NO₂ molecules [67]. (c) (A) PL spectra, (B) enhanced PL intensity, (C) emission wavelength shift, and (D) photographs under 365 nm UV light of the (HPbBr₃)₂PbBr₂ NCs@h-SiO₂ film as a function of MA concentration [68].

An ultrasensitive temperature-sensing approach was achieved by Huang et al. [64] using the defect in CsPbX₃ (X⁻ = Cl⁻, Br⁻) NCs. Temperature-sensitive CsPbCl_1.2Br_1.8 NCs@h−SiO₂ is rationally designed by the template-assisted ligand-free synthesis and halogen constituent manipulation. CsPbCl_1.2Br_1.8 NCs@h−SiO₂ and KSF integrated into an ethylene-vinyl acetate (CsPbCl_1.2Br_1.8 NCs@h−SiO₂/KSF@EVA) composite film show a relative sensitivity of 13.44%/°C ranging from 30 °C to 45 °C, and a temperature resolution of 0.2 °C is achieved. After this study, they further developed a high-sensitivity ratiometric PL approach for temperature sensing using the microencapsulation of CsPbBr₃ and K₂SiF₆:Mn⁴⁺ phosphor [84]. Recently, Chen et al. [85] found that silanol sites are fatal to the PL of CsPbBr₃ NCs@m−SiO₂. A large number of silanol groups lead to the formation of a CsPb₂Br₅ impurity due to the deficiency of Cs⁺ and Br⁻ caused by silanol sites. After the removal of silanol groups, as shown in Figure 10, the formation of surface traps and CsPb₂Br₅ impurity is greatly suppressed, leading to a remarkable PLQY increase (over two orders of magnitude) in CsPbBr₃ NCs@m−SiO₂. As a section summary, the PL-sensing characteristics of 3D LHP including HLP material, sensing analytes, sensing mechanism, and detection range, as well as the detection limit, are summarized in

Table 1. Photoluminescence sensing of 3D LHP.

LHP Material	Analyte	Sensing Mechanism	Detection Range	Limit of Detection	Reference
	halide ion	Halogen-exchange	--	--	[38]
CsPbBr3 NCs	Cu2+	photogenic electron transfer	0–100 nmol/L	0.1 nmol/L	[56]
CsPbBr3 NCs	Pb2+	In situ perovskite growth	0.01–1.0 μg/mL	5 × 10−3 μg/mL	[57]
CsPbBr3 NCs	edible oil	PLwavelength shift	0–0.6 mg/100 g	0.139 mg/100 g	[58]
CsPbBr3 NCs	CH3I	Halogen-exchange	0.794–22.244 mg/L	0.044 mg/L	[59]
CsPbBr3 NCs	Cl− in sweat	Halogen-exchange	10–130 mmol/L	3 mmol/L	[60]
CTAB@CsPbBr3	ethanol	Ethanol promotes the dissociation of CTAB@CsPbBr3	3.2–11.05 mg/L	7.3 ppb	[62]
CsPbBr3 NCs	Hg2+	Exchange reaction between Hg2+ and Pb2+	0–100 nmol/L	0.124 nmol/L	[63]
CsPbBr3 QDs	NH3	Effectively passivate surface defects of perovskite QDs introduced during purification	25–350 mg/L	8.85 mg/L	[64]
CsPbBr3 NCs	H2S	Damage structure	0–100 μmol/L	0.18 μmol/L	[65]
MAPbBr3	SO2	Transfer of excited state electrons	0–10 mg/L	155 ppb	[66]
CsPbI2Br NCs	NO2	The electron transfer between the sensing layer and NO2 causes change in resistance	1–8 mg/L	1.34 mg/L	[67]
MAPbBr3	methylamine	In situ perovskite growth	1–95 mg/L	70 ppb	[68]
CsPbBr3	O2	O2 molecules extract the photogenerated electrons from CsPbBr3 nanocubesets through collision interaction	--	--	[72]
MAPbBr3	humidity	Humidity changes the crystal structure	7–98%	0.68%	[83]
CsPbCl1.2Br1.8 NCs@h-SiO2	temperature	Temperature change radiative transition	30–45 °C	--	[84]

.

2.3. Photoluminescence Sensing of 2D Lead Halide Perovskite

2.3.1. Influence Factors on the Photoluminescence of 2D Lead Halide Perovskite

The 2D LHP can be regarded as a layered structure formed by cutting along one of the crystal faces of 3D LHP. By cutting along the different crystal face of 3D LHP, 2D LHP with different configurations can be obtained. The rich crystal structure of 2D LHP materials makes them more abundant in structural diversity and design ability. In general, they display more action sites and adjustable optical properties due to their larger specific surface area. The bulk crystals of 2D LHP usually have a better stability and lower trap density, demonstrating the promising potential of 2D LHP for PL sensing. On the other hand, the tighter lattice structure of 3D LHP greatly weakens the phonon–exciton coupling and generally exhibits narrowband emission with a high PLQY, which is conducive to colorimetric sensing [86]. The PL properties of 2D LHP are influenced by the regulation of various organic cations. Firstly, the organic ammonium ion plays a role in separating the inorganic layer and stabilizing the lattice. The band gap width of 2D LHP can be adjusted by the quantum limiting effect. Blancon et al. [87] observed the characteristic peaks of strong excitons in the absorption and emission spectra of BA₂MA_n−1Pb_nI_3n+1. As the inorganic layer number decreases from 5 to 1, the quantum limiting effect increases, and the blue-shift of the characteristic exciton peak can be observed. Yuan et al. [23] combined organic cations with different chain lengths and prepared (C₆H₁₈N₂O₂)PbBr₄ with different components and layers by changing the type and proportion of organic ammonium ions. In their study, the free exciton emission wavelength was continuously adjustable in the region of 403–530 nm (Figure 11a).

Organic ammonium ions can also regulate the band structure of 2D LHP by affecting the degree of lattice distortion. Zhou et al. [88] reported that the chain diamine hybrid 2D perovskite, (C₆H₁₈N₂O₂)PbBr₄, has a narrowband emission of free excitons (403 nm), and the electron-lattice coupling in the deformable lattice also produces wideband emission of self-trapping excitons (555 nm), which reveals a large Stokes shift. The PL lifetime is also longer than that of free excitons. Based on this result, doping Mn²⁺ can further broaden the emission spectrum. Fu [89] et al. regulated the white light emission of 2D LHP with short aromatic diammonium ion with different substituents. When the substituents of diamenite (PDA⁺) are in the meso- and para-position, 2D LHP can be obtained. The distortion of lead halide octahedron results in the wideband emission of PDAPbBr₄. Since the degree of distortion is affected by the stereochemistry of diammonium ion and the hydrogen bond interaction, the displacement with the highest level of bond length distortion in the inorganic layer reveals the widest white light emission (Figure 11b). Luo et al. [90] synthesized 2D LHP, (F_xCH_3−xCH₂NH₃)₂PbBr₄, and introduced different F⁻ for the formation of intermolecular and intramolecular hydrogen bond networks, which affects the configuration, layer spacing, and inorganic layer distortion of 2D LHP. The strong coupling between the exciton and the lattice produces a wideband emission, in which (F₂CHCH₂NH₃)₂PbBr₄ exhibits the largest band gap (3.04 eV) and the highest PLQY (12.3%). In additional, the F−C bond also improves the water stability of the 2D LHP.

Figure 11. (a) PL spectra of layered perovskites (RNH₃)₂MA_n−1Pb_nBr_3n+1 [23], (b) the white light emission of PDAPbBr₄ and the corresponding CIE coordinates [89], and (c) phosphorescence emission spectra of (NMA_xPEA_1−x)₂PbBr₄ films and the corresponding photographs under UV light [91].

Figure 11. (a) PL spectra of layered perovskites (RNH₃)₂MA_n−1Pb_nBr_3n+1 [23], (b) the white light emission of PDAPbBr₄ and the corresponding CIE coordinates [89], and (c) phosphorescence emission spectra of (NMA_xPEA_1−x)₂PbBr₄ films and the corresponding photographs under UV light [91].

The introduction of optically active organic cations can also produce non-intrinsic PL emission of 2D LHP. Hu et al. [18] reported that the direct transfer of exciton energy of a 2D LHP, A₂PbBr₄, to the triplet state of a TTMA⁺ ion containing dithiophene can produce PL at room temperature. Mixing PEA⁺ further inhibits non-radiating recombination, and the PLQY reaches at 11.2%. Then, Hu et al. [17] designed conjugated organic cations with low triplet energy levels to generate room-temperature PL by transferring the triplet exciton energy of the inorganic components of 2D LHP. The Dexter-type energy transfer efficiency in 2D LHP could reach at 80%. The organic ammonium ions with a different triplet exciton energy also realized multi-color PL. Tian et al. [91] reported the room-temperature PL of 2D LHP. They studied the triplex sensitization principle of naphthalene and found that the material achieves efficient triplex sensitization through subpicosecond ultrafast hole transfer from the inorganic layer to naphthalene rather than through the Dexter process. The naphthalene triplet is associated with other ground state molecules to form a triplet excimer, and the triplet and triplet excimer show different PL values. The intensity ratio can be adjusted by changing the percentage of NMA⁺ in the organic layer. The study provides a new way to obtain the single component white light emission of 2D LHP (Figure 11c).

Metal cations regulate the PL of 2D LHP. The metal ions doping into the B site (general formula ABX₃ for perovskite) of 2D LHP produces new PL centers or affect the self-limited exciton PL process. For example, the PL of Mn²⁺ comes from the exciton–Mn²⁺ exchange coupling. Mn²⁺ impurity does not directly absorb the excitation light but activates the ⁴T₁→⁶A₁ transition process through the photogenerated exciton of the main body of 2D LHP. The overlap degree of the wave function between excitons and d electrons determines the intensity of exchange coupling. The matching degree between the electron and band structure (including the defect level) of the main body of LHP and the Mn²⁺ impurity level has a great influence on the PL efficiency of Mn²⁺ [92]. An effective Mn²⁺ PL is commonly found in Br−based 2D LHP or Cl⁻−based quasi−2D LHP. A small number of I⁻ based 2D LHPs (such as STA₂PbI₄) can also produce Mn²⁺ PL under a non-mixed halogen (BrI) condition [93]. Mn²⁺ doping enhances the 2D LHP PL by defect engineering. Compared with the 3D equivalent, the 2D structure is more conducive to improve the doping efficiency of Mn²⁺. Li et al. [78] introduced a high density shallow defect in the interband of EA₂PbBr₄ through high content doping to trap the photogenerated excitons on the picosecond scale, which inhibits the self-limited exciton PL, and helps to induce the exciton energy transfer to the Mn²⁺−d excited state. As shown in Figure 12a,b, the maximum PLQY of Mn²⁺: EA₂PbBr₄ reaches 78%, and the lifetime of Mn²⁺ luminescence at 616 nm is about 0.75 ms. The co-doping of Mn²⁺/Zn²⁺ or Mn²⁺/Cd²⁺ increases the defect density and promote Mn²⁺ luminescence. Hu et al. [24] reported the existence of interband capture state-assisted exciton energy transfer in Mn²⁺:(C₈H₂₀N₂)PbBr₄. The increase in the doping amount results in more Mn²⁺ luminescence centers and surface defects. The activation energy barrier (~6.72 meV) between the interband trapping state and the Mn²⁺−d excited state is much smaller than that of the 3D counterpart. Therefore, Mn²⁺ impurity fully receives the exciton energy of the 2D LHP, which enhances the PL intensity at 610 nm. At the same time, the decay lifetime of free excitons and self-limited excitons is shortened accordingly. However, with the increase in the doping amount, Gao et al. [94] observed that the PL lifetime of Mn²⁺and the PLQY of Mn²⁺:PEA₂PbBr₄ are affected by Mn–Mn interaction and crystal field change. Sun et al. [95] reported that the PLQY of Mn²⁺:PEA₂PbBr₄ is as high as 97%. The 2D layered structure and the twisted inorganic octahedron effectively reduce the probability of exciton non-radiative recombination, and the wavelength of Mn²⁺ emission is affected by the crystal field intensity. At a high PEA⁺ content, the larger steric hindrance induces PEA⁺ rearrangement and inorganic octahedral distortion, which reduces the field symmetry of the octahedral crystal, resulting in a blue-shift of the Mn²⁺ PL wavelength. With the decrease in PEA⁺ content, the crystal field symmetry increases and red-shifts the PL wavelength of Mn²⁺. However, the lack of PEA⁺ will also lead to the loss of Br⁻ of the inorganic octahedron, and Mn²⁺ is easily photo-oxidized to Mn⁴⁺. At this time, the hydrogen bond between PEA⁺ and the halide containing Mn⁴⁺ is destroyed, resulting in a change in the arrangement of PEA⁺. The crystal field symmetry is finally reduced again, and a 30 nm blue-shift of the Mn²⁺ PL wavelength could be observed. In addition, Cortecchia et al. [96] doped heterovalent metal ions Eu³⁺ into NMA₂PbBr₄ and NMA₂PbCl₂Br₂ to introduce ⁵D₀→⁷F₂ radiation, which generates a maximum PL wavelength at 611 nm under the sensitization of NMA⁺. The PLQY can be enhanced by doping Eu³⁺ complexes (Figure 12c). Lyu et al. [97] found that doping Bi³⁺ in BA₂MA_n−1Pb_nI_3n+1 can generate two different Bi³⁺ sites, namely the non-radiative recombination site and the optically active site. The quasi−2D perovskite with n = 3 can generate nearinfrared radiation (about 943 nm).

The PL properties of 2D LHP can also be regulated by doping some non-luminescent metal ions. As shown in Figure 12d, PEA₂PbI₄ has a narrowband emission of free excitons, but Sn²⁺ doping causes the localization of excitons by the generation of a local potential well, resulting in lattice distortion around Sn²⁺. The strong coupling of excitons and phonons produce a new self-limited state exciton PL. The wideband emission of Sn²⁺: PEA₂PbI₄ includes both the red and near-infrared regions, and the PLQY of the 2D LHP increases from 0.7% before doping to 6.0% after doping [98]. Li⁺ doping can also enhance the self-limited exciton PL of the 2D LHP PEA₂PbBr₄ (450–750 nm) [99].

By means of a mixed halogen source or halogen ion exchange after synthesis, the type and proportion of the anion (Cl⁻, Br⁻, I⁻) of 3D LHP can be adjusted to change its band gap width, so that the PL wavelength of intrinsic free excitons can cover the full visible wavelength [39,52,100]. The free exciton PL of 2D LHP has similar regulatory properties. Yang et al. [101] introduced Br⁻ into PEA₂PbI₄ to obtain a single-phase mixed-halogen 2D LHP. As shown in Figure 13a,b, with the increase in Br⁻ content, the exciton absorption peak and emission peak of 2D LHP gradually shift from 520.5 nm to 404.4 nm, and 524.0 nm to 409.1 nm, respectively, and PEA₂PbBr₄ with a PLQY of 46.5% could be obtained.

The halogen ion change regulates the PL of 2D LHP by forming self-trapping excitons. Mao et al. [102] reported a 2D LHP with adjustable white light emission, EA₄Pb₃Br_10−xCl_x. EA⁺ distorted the inorganic lattice of perovskite, and the degree of distortion of Cl⁻−based perovskite was much greater than that of Br⁻−based perovskite. Therefore, with the increase in Cl⁻ content, 2D LHP produces more self-limiting excitons, and the narrow-band blue emission of EA₄Pb₃Br₁₀ gradually changes into the wideband emission of EA₄Pb₃ Cl₁₀ (white) with a band gap between 2.75 and 3.45 eV. More self-limiting excitons also extend the PL lifetime. Zhou et al. [92] synthesized a 2D LHP, R₂PbBr_4−xCl_x, which contains different organic amine ligands (butylamine, benzylamine, or phenethylamine). The relative energy level positions of the free excitons and self-trapped excitons of 2D LHP could be modified by changing the proportion of halogen ions. The 2D LHP has a double emission of free excitons and self-trapped excitons at the same time (Figure 13c). The mixed LHP with a stoichiometric ratio of R₂PbBr₂Cl₂ achieves a single component of white light emission, and the ultra-wideband emission spectrum includes the full visible region (Figure 13d).

2.3.2. Sensing Applications of 2D Lead Halide Perovskitel

Although there have been many reports on the PL-sensing application of 3D CsPbBr₃ NCs as well as 3D LHP NCs, there is still a lot of exploration space for 2D LHP with a more abundant composition structure. Zhang et al. [103] proposed a method for the colorimetric sensing of Cs⁺ in soybean oil using a 2D LHP, PEA₂PbI₄. The increase in Cs⁺ concentration improves the exchange of PEA⁺ and Cs⁺, resulting in the growth of CsPbI₃ from the 2D structure. The decrease in PEA⁺ content reduces the PL of PEA₂PbI₄, while the PL of 3D CsPbI₃ is enhanced. The overall PL changes from green to yellow and red. In this work, the colorimetric sensing resolution of Cs⁺ reached 5.0 μmol/L. As shown in Figure 14, the PL intensity at 675 nm shows a good linear relationship with the concentration of Cs⁺ in the range of 5–25 μmol/L, and the detection limit is found to be 1.95 μmol/L.

Ma et al. [104] adopted a unique approach to maintain the stability of 2D LHP, PEA₂PbI₄, in aqueous solution containing high concentrations of PEA⁺. This approach is directly applied to the detection of Cu²⁺ in water. When Cu²⁺ interacts with I in PEA₂PbI₄, a new trapping state is generated at the valence band edge of perovskite, and exciton recombination is blocked, resulting in PL quenching. When the concentration of Cu²⁺ in water is 0.5 nmol/L, PL quenching can be detected. At a concentration of 50 mmol/L Cu²⁺, the PL of PEA₂PbI₄ is almost completely quenched without interference from other metal ions of the same concentration (Figure 15).

The study of pressure sensing is important to understand the relationship between the optical properties and the structure of perovskites and then expand their application. The 2D LHP shows a sensing ability for thermodynamic parameters. Pressure can effectively regulate the electronic and crystal structure of 2D LHP. As shown in Figure 16, Liu et al. [105] reported that pressure can induce a reversible shift of the emission wavelength of the 2D LHP PEA₂PbI₄. At a moderate pressure of 0–3.5 GPa, the PL of PEA₂PbI₄ gradually changes from green to red. This phenomenon is attributed to the fact that the organic part of PEA₂PbI₄ is able to contract and stretch flexibly, thus adapting to the different pressure. During the pressurization process, the compressed organic layer absorbs energy and changes the band gap of the 2D LHP. When the pressure is reduced, the organic layer releases the absorbed energy, thus achieving a reversible shift in the PL wavelength. When the pressure exceeds the critical value, the crystal structure is irreversibly destroyed, and the PL completely disappears.

As shown in Figure 17, Niu et al. [106] constructed a temperature-sensing approach based on the PL quenching of (C_nH_2n+1NH₃)₂PbI₄ (n = 4, 12, 16, 18). The 2D LHP undergoes a reversible phase transition under the temperature change. The structural change in the 2D LHP leads to exciton radiant thermal quenching after the temperature increases. The reversible PL wavelength shift is attributed to electron–phonon coupling, which leads to a decrease in band gap width and a red-shift in PL wavelength with the increase in temperature. By adjusting the type of alkyl ammonium ion, (C₁₆H₃₃NH₃)₂PbI₄ exhibits good temperature-sensing responses from 0 to 80 °C. Its sensing reversibility is greater than 500 cycles. This work provides a new idea for the study of the temperature sensing of 2D LHP.

In addition, the PL-sensing characteristics of 2D LHP including different LHP material, sensing analytes, sensing mechanisms, detection ranges, and detection limits are summarized in

Table 2. Photoluminescence sensing of 2D LHP.

LHP Materials	Analyte	Sensing Mechanism	Detection Range	Limit of Detection	Reference
PEA2PbI4	Cs+ in soybean oil	Cs+ concentration improves the exchange of PEA+ and Cs+ and results in the growth of CsPbI3	5–25 μmol/L	1.95 μmol/L	[103]
PEA2PbI4	Cu2+ in water	Cu2+ interacts with I and exciton recombination is blocked	5 × 10−10–5 × 10−2 M	--	[104]
PEA2PbI4	Pressure	Effectively regulate the electronic and crystal structure	0–7.6 GPa	--	[105]
(CnH2n+1NH3)2PbI4 (n = 4, 12, 16, 18)	Temperature	The structural change in the 2D LHP leads to exciton radiant thermal quenching after the temperature increases	0–80 °C	--	[106]

.

3. Conclusions and Outlook

The development of new materials with a high sensitivity and fast response for PL sensing is still a hotspot in the field of analytical chemistry. LHP materials have attracted increasing attention in the field of analytical sensing since they reveal a good application prospect in the field of PL sensing.

The 3D LHP NCs have many excellent characteristics such as the precise modulation of the emission wavelength in the visible region, high PLQY, narrow emission peak, and high defect tolerance, while 2D LHP materials have a richer chemical composition and electronic structure, as well as a larger specific surface area and softer lattice than their 3D counterparts, which can be realized by various synthesis methods to modify their PL properties. The study of the structural dimension, organic composition, or doped ions of LHPs is helpful to explore their PL sensing and propose new sensing mechanisms, which has important research significance.

From the overall perspective of LHP materials, there are still some constraints to be solved for their sensing studies and applications. LHPs are unstable to polar solvents due to their salt nature, which is particularly obvious in 3D LHPs. It limits the sensing applications to the targets in aqueous solution. A variety of strategies have been achieved to solve this problem by the surface modification of 3D LHPs including micellar encapsulation using block copolymers, phospholipids or monolithic. In addition, single-particle coating using hydrophobic polymers or silicon-based materials have also be developed. Encapsulation enhances the stability of LHPs but reduces the contact degree between the LHPs and the target analytes, and therefore may partially reduce the sensing speed and sensitivity. It is necessary to develop suitable encapsulation materials to balance the stability and sensing performance of LHPs. In the future, low-dimensional LHPs with more hydrophobic components can be considered and developed to improve the stability and sensing ability. In addition, the heavy metal toxicity of LHPs limits their sensing applications in the field of biological aspects. LHPs-coated biocompatible materials form water-stable LHPs and ensure their delivery into cells while avoiding biotoxicity. Although lead-free perovskites have been reports using Sn, Ge, Bi, Sb, Ag/Bi, Ag/In, etc., to replace Pb in LHP [107,108,109], the perovskite materials prepared from these elements have the disadvantages of poor stability and low PLQY. Thus, more non-lead halide perovskites with low toxicity will be developed in the future, such as copper, bismuth, tin, and silver indium-based double perovskites. Furthermore, the sensing properties of LHPs are limited by both their composition and the preparation process of the materials. Composite sensing materials based on LHPs will be developed, combining with more advanced materials such as metal–organic skeleton compounds or molecularly imprinted polymers, etc., to enrich the sensing performance from a chemical perspective. From the perspective of technology, in the future, more examples of simple batch synthesis methods of high-quality perovskite materials need to be proposed.

In spite of the abovementioned, there are still a lot of challenges for the sensing applications of LHP materials: (1) The key sensing characteristics such as sensitivity, response time, and reversibility need to be further improved; (2) The mechanism of sensitive PL behavior needs to be further studied; (3) It is necessary to establish PL-sensing methods that are not only efficient but also intuitive. Therefore, at this stage, it is still important to design and synthesize new LHP sensing materials. The further study of the sensing properties and mechanisms of the target analytes will provide more theoretical and experimental guidance. Although PL-sensing studies and applications [35,110], including gas sensing [111] based on metal halide perovskite material have been reviewed, this review summaries the PL properties of 3D and 2D LHP materials. It is helpful to develop new types of PL-sensing materials with high efficiency, convenient synthesis, and low cost, which will further expand the PL-sensing methods and applications.