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 CsPbBr
3 nanocrystals (CsPbBr
3 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 CsPbBr
3 NCs. They found that the post-regulation of CsPbBr
3 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 CsPbI
3 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 (PbX
2, ZnX
2, KX, MnCl
2, 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 CsPbBr
3 NCs, and Cl
− can undergo halogen exchange with CsPbBr
3 NCs. Similarly, CsPbCl
3 NCs and dibromomethane are also feasible. The halogen exchange property of CsPbX
3 NCs helps to provide a new means of efficient preparation of CsPbX
3 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 CsPbBr
3 NCs. As shown in
Figure 5, Nikolai et al. [
49] first dispersed CsPbBr
3 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 CsPbBr
3 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 CsPbBr
3 NCs and MnCl
2 as well as the exchange of Pb
2+/Mn
2+. CsPbCl
3−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 CsPbBr
3 NCs. They found that CsPbBr
3 NCs have a different mechanism of halogen exchange with I
− and Cl
−. As soon as reaction with I
− occurs, CsPbBr
3 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 Pb
2+ and MBr
2 (M = Sn
2+, Cd
2+, and Zn
2+) in CsPbBr
3 NCs. Compared with the halogen exchange process, the activation energy of Pb
2+ 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
xBr
3 (0 < x ≤ 0.1), can maintain the cubic crystal structure of the original CsPbBr
3 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
xBr
3 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 CsPbBr
3 NCs to generate OLAM−Br and form a Br vacancy on the surface of PNCs. Subsequently, OLAM−MBr
2 enters the Br vacancy and releases energy, which breaks the bond between PbBr
2 and PNCs, and the formed PbBr
2 vacancy is occupied by MBr
2. The energy releases in this process is basically equivalent to the energy of the broken bond, completing the exchange of M with Pb
2+, and diffusing inward along the Br vacancy. With the increase in inward exchange and diffusion M, the lattice shrinkage of CsPbBr
3 NCs is caused by the difference of ionic radius (r (Pb
2+) = 119 pm, r (Cd
2+) = 95 pm, r (Zn
2+) = 74 pm, and r (Sn
2+) = 118 pm). As a result, the lattice stress inside CsPbBr
3 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 (6
p) and Br (4
p) s results in the upward shift of the bottom end of the conduction band. The small influence of the Br (4
p6) orbit and Pb (6s
2), 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 CsPbBr
3 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 CsPbBr
3 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 Cu
2+ in an organic phase based on the PL quenching caused by the introduction of Cu
2+ defects to the surface of CsPbBr
3 NCs. Liu et al. [
56] believed that CsPbBr
3 NCs have a photogenic electron transfer with Cu
2+, which also achieves PL quenching of Cu
2+. Wang et al. [
57] used a sulfhydryl modified porous alumina template to capture Pb
2+. 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 Pb
2+ 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 CsPbBr
3 NCs and I
−. As shown in
Figure 7, the peroxide number of edible oil could be determined indirectly by CsPbBr
3 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 CsPbBr
3 NCs as a host material to immobilize Y atoms (Y−SA). Using CsPbBr
3 NCs as anchors, Y−SA/CsPbBr
3 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 CH
3I 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 CH
3I in sweet potato samples.
A wavelength shift method based on the halide exchange of CsPbBr
3 NCs and Cl
− has been developed in order to realize the rapid colorimetric sensing of Cl
− in sweat [
60]. The study indicates that CsPbBr
3 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) CsPbBr
3@CsPb
2Br
5 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 NH
4+ on the surface. In addition, ethanol can form strong hydrogen bonds with Br
− and pull Br
− away from the surface. The separation of NH
4+ 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 PbBr
2 due to the degradation of CTAB@CsPbBr
3 in the presence of ethanol. With the gradual addition of ethanol, the concentration of PbBr
2 increases, and the PL intensity of PbBr
2 at 430 nm increases. Ethanol can be identified from other alcohols by its reaction with CTAB@CsPbBr
3, since ethanol induces a PL increase at 430 nm, while methanol induces a CTAB@CsPbBr
3 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@CsPbBr
3 and ethanol, leading to non-radiative decay.
In addition to halogen exchange, cations in CsPbBr
3 NCs could also be changed by other metal cation. In the presence of Hg
2+, Pb
2+ in CsPbBr
3 NCs is gradually replaced by Hg
2+. However, unlike the halogen exchange reaction that causes a significant shift in emission wavelength, the exchange reaction between Hg
2+ and Pb
2+ only causes a small wavelength shift, but a significant PL intensity decrease occurs. Therefore, a sensing method for Hg
2+ 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 NH
3 gas using the reversible passivation of surface defects of CsPbBr
3 NCs by NH
3. PbS could be generated by the reaction Pb and H
2S, which causes damage of the perovskite structure. H
2S 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 MAPbBr
3 quantum dots and superhydrophobic silica gel. They proposed a reversible sensing method for SO
2 using the PL quenching through the transfer of excited state electrons from MAPbBr
3 to SO
2. Kim et.al. [
67] developed a high-performance and ultra-fast-response sensing approach for NO
2 using CsPbI
2Br NCs with an LOD of 2 ppb NO
2. 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 NO
2 causes a resistance change in the layer. As a strong electron-absorbing gas, NO
2 tends to adsorb on the surface of CsPbI
2Br 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 CsPbI
2Br NCs, which can be applied for NO
2 sensing. Additionally, since electrons are captured by NO
2 adsorbed on the surface of CsPbI
2Br NCs, the obvious reduction in carrier recombination causes the PL quenching of CsPbI
2Br NCs.
The MAPbBr
3 PL could be enhanced and its emission wavelength shifted due to in situ restricted growth of MAPbBr
3, 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 CsPbBr
3 NCs [
69] in their sensing applications. Thus, some approaches have been developed in order to increase the stability of CsPbBr
3 NCs. Shu et al. [
70] used amphiphilic polymer octylamine modified polyacrylic acid as the surface passivating ligand of CsPbBr
3 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 CsPbX
3 NCs as peroxide-like enzymes to carry out the self-catalytic PL sensing of H
2O
2 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 CsPbBr
3 NCs have various oxygen-sensitive PL behaviors. By analyzing CsPbBr
3 with different morphologies [
72,
73], researchers found that O
2 molecules can extract the photogenerated electrons from CsPbBr
3 nanocubesets through collision interaction, resulting in the dynamic PL quenching of CsPbBr
3. On the other hand, the surface hole traps of CsPbBr
3 nanowires, nanosheets, or single crystals can be passivated and give a higher PL without a change in the exciton recombination dynamics. The oxidation of O
2 to MAPbI
3 gap iodine eliminates deep defects, resulting in a PL increase in perovskite [
74,
75]. O
2-assisted photoinduced etching can cause the wavelength blue-shift of MAPbI
3 PL, and eventually the emission disappears completely [
76]. In addition, Mn
2+-doped PL has been observed in Mn
2+:CsPbCl
3 as a sensing signal of oxygen [
77]. It has also been found that there is a quantum cutting process in Mn
2+:CsPbCl
3, which causes the oxygen quenching of do**-related Mn
2+ PL and surface defect state PL [
78]. These study results demonstrate the good potential of CsPbX
3 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 MAPbX
3 and water molecules using conductivity and found that the interaction between perovskite and water is reversible, and MAPbX
3·H
2O is obtained [
79]. The effect of humidity on the single-crystal structure of MAPbI
3 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 CH
3NH
3PbBr
3 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, MAPbBr
3 and water molecules produce MAPbBr
3•H
2O, resulting in the change in MAPbBr
3 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 MAPbBr
3. By controlling the exposure time of MAPbBr
3, a good reversibility can be obtained (
Figure 9).
Figure 9.
Gas sensing. (
a) Humidity sensing based on the crystalline structure change in MAPbBr
3 [
83]. (
b) Schematic illustration of the adsorption-charge transfer mechanism between PNCs and NO
2 molecules [
67]. (
c) (
A) PL spectra, (
B) enhanced PL intensity, (
C) emission wavelength shift, and (
D) photographs under 365 nm UV light of the (HPbBr
3)
2PbBr
2 NCs@h-SiO
2 film as a function of MA concentration [
68].
Figure 9.
Gas sensing. (
a) Humidity sensing based on the crystalline structure change in MAPbBr
3 [
83]. (
b) Schematic illustration of the adsorption-charge transfer mechanism between PNCs and NO
2 molecules [
67]. (
c) (
A) PL spectra, (
B) enhanced PL intensity, (
C) emission wavelength shift, and (
D) photographs under 365 nm UV light of the (HPbBr
3)
2PbBr
2 NCs@h-SiO
2 film as a function of MA concentration [
68].
An ultrasensitive temperature-sensing approach was achieved by Huang et al. [
64] using the defect in CsPbX
3 (X
− = Cl
−, Br
−) NCs. Temperature-sensitive CsPbCl
1.2Br
1.8 NCs@
h−SiO
2 is rationally designed by the template-assisted ligand-free synthesis and halogen constituent manipulation. CsPbCl
1.2Br
1.8 NCs@
h−SiO
2 and KSF integrated into an ethylene-vinyl acetate (CsPbCl
1.2Br
1.8 NCs@
h−SiO
2/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 CsPbBr
3 and K
2SiF
6:Mn
4+ phosphor [
84]. Recently, Chen et al. [
85] found that silanol sites are fatal to the PL of CsPbBr
3 NCs@
m−SiO
2. A large number of silanol groups lead to the formation of a CsPb
2Br
5 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 CsPb
2Br
5 impurity is greatly suppressed, leading to a remarkable PLQY increase (over two orders of magnitude) in CsPbBr
3 NCs@
m−SiO
2. 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.