Near-infrared mechanoluminescence crystals: a review

Summary

Due to the in situ, real-time, and non-destructive properties, mechanoluminescence (ML) crystals have been considered as intelligent stress sensors, which demonstrate potential applications such as in inner crack visualization, light source, and ultrasonic powder recording. Thereinto, it is highly expected that near-infrared (NIR) MLs can realize the visualization of inner biological stress because mechanically induced signals from them can penetrate biological tissues. However, such an energy conversion technique fails to work in biomechanical monitoring due to the limited advances of NIR ML materials. Based on those, some research groups have begun to focus on this field and initially realized this idea in vitro while related advances are still at the early stage. To advance this field, it is highly desirable to review recent advances in NIR ML crystals. In this review, to our knowledge, all the NIR ML crystals have been included in two main groups: oxysulfides and oxides. Besides, the present and emerging trends in investigation of such crystals were discussed. In all, the aim is to advance NIR ML crystals to more practical applications, especially for that of biomechanical visualization in vivo.

Graphical Abstract

Medical Imaging; Optical Materials; Biomedical Material

Introduction

Mechanoluminescence (ML) is an amazing physical phenomenon of a material (usually a solid), which emits corresponding to external mechanical stimuli such as friction, impact, compression, and bending (Wang et al., 2019; Zhang et al., 2019). It exists in both inorganic and organic compounds (Feng and Smet, 2018; Xie and Li, 2018). Generally, it is recognized that Francis Bacon made the first documented ML in 1605: “sugar shineth only while in scraping” (Bacon, 1863), which is now classified as fractoluminescence accompanied by the structure destruction of the material leading to limited applications. In 1999, Xu et al. developed two classic elastic ML materials (ZnS:Mn²⁺ and SrAl₂O₄:Eu²⁺ [only monoclinic phase SrAl₂O₄]) whose ML intensities can be fully restored keeping complete crystal structure (Xu et al., 1999a, 1999b). From then on, the development of ML has advanced a big step during the past two decades (Figure 1) (Notably, the word “ML” in this review only consists of elastic ML materials.). Thereinto, visible ML based on various hosts was reported such as in aluminates (Akiyama et al., 1998; Xu et al., 2000), gallates (Matsui et al., 2000), titanates (Wang et al., 2005), silicates (Zhang et al., 2010), and stannates (Kamimura et al., 2012). Typically, the nature of visible ML optical centers can be lanthanide (Eu²⁺, Sm³⁺, Pr³⁺, etc) or transition metal (Cu⁺ and Mn²⁺) ion. Besides, visible ML has covered the whole visible spectral range (380–760 nm). Based on those, ML materials have found huge potential applications such as in crack visualization (Kundu et al., 2012), photo-catalysis (Terasaki et al., 2013), smart display (Jeong et al., 2014), energy storage and conversion (Zhuang et al., 2020), and photonic skin (Qian et al., 2018). While as for the applications in bioimaging, to our knowledge, limited progress has been made.

Figure 1.

Recent advances in the development of ML materials (year: 1998–2021)

Remarkably, as a novel non-destructive, in situ, real-time, and non-contact mechano-opto-responsive technique, ML is highly expected to find practical applications in bioimaging to help study the complex biomechanical behaviors of our body or corresponding artificial implants. Compared to traditional bioimaging ones, the demands in biological imaging urge such ML materials to possess stronger intensities, shorter response times, and lower costs with flexible wavelengths (Huang et al., 2017a). Indeed, researchers have tried to develop such biocompatible ML-based materials and devices, while few researchers have focused on the in vivo biomechanical detection (Larson et al., 2016; Tan et al., 2020). Thereinto, the difficulty in non-contact exciting ML probes from outside of the body should be one reason. However, if one has been aware that acoustic shock or pulsed infrared laser (both can penetrate tissues) also can induce ML, this restriction may not exist (Eddingsaas and Suslick, 2006; Hardy et al., 1979). Also, very recently, Wu et al. used the brain-penetrant focused ultrasound to excite the local nano ML light sources (ZnS:Ag⁺, Co²⁺), such a method may provide a new idea to non-contact ML excitation mode in bioimaging (Wu et al., 2019). After solving the difficulty in ML exciting modes, detecting ML signals of biomechanical behaviors becomes the main problem. To avoid invasive delivery of detection equipment demonstrated such as by Zhang et al. (Zhang et al., 2020b), such ML signals are required to penetrate tissues (namely, near-infrared [NIR] MLs located within the tissue penetration biowindows), especially for implants such as an artificial bone which is deeply covered by other soft tissues. Although dual-energy X-ray absorptiometry or sonography can be used to assess the biomechanical quality of bones, they are expensive and require bulky devices (Liu and Webster, 2016). Other biomechanical sensors were developed to study the bone strength; however, they either require complex external equipment or they are not sensitive enough to enable in situ and real-time detection (Afsarimanesh et al., 2018). Hence, developing efficient NIR ML materials which can penetrate tissues for potential inner biomechanical visualization is urgently required. So, how is the development of NIR ML crystals? Disappointingly, even dating back to 1605, no more than 11 documented NIR ML crystals have been reported (Table 1). Obviously, the number of visible ML crystals far exceeds that of NIR. It is until the last 3 years that researchers began to show more interest in NIR MLs. The reasons behind this are as follows: First, one may easily mention that this may be because of the difference in common spectral detectors between visible (Si-CCD [charge-coupled device]) and NIR (usually InGaAs-CCD) band, which makes it “harder” to obtain NIR ML signals (Huang et al., 2017b). Second, no convincing NIR ML mechanisms have been established. Without theoretical guidance, all material exploration will only be trial and error, which has costed too much effort. Third, it is precisely because there is no direct application requirement related to NIR ML that the research development of this field is so slow.

Table 1.

Known NIR ML crystals (collected whose MLs have been extended into the NIR range (780–1700 nm); year: 1605–2020)

Category		Host	Crystal system space group	Sites (CN, radii (Å))	Emitter	Bio-penetration tissues	Published year	Reference
I	Oxysulfides	CaZnOS	Hexagonal P63mc	Ca2+ (6, 1.000)	Nd3+	Pork slices	Apr. 2018	(Li et al., 2018b)
		CaZnOS	Hexagonal P63mc	Ca2+ (6, 1.000)	Pr3+, etc	/	Dec. 2018	(Du et al., 2018)
		SrZn2S2O	Orthorhombic Pmn21	Sr2+ (6, 1.180)	Yb3+	/	Nov. 2019	(Chen et al., 2020)
		SrZnSO	Hexagonal P63mc	Sr2+ (6, 1.180)	Nd3+, etc	/	Nov. 2019	(Chen et al., 2020)
		CaZnOS	Hexagonal P63mc	Ca2+ (6, 1.000)	Er3+, etc.	/	Dec. 2019	(Li et al., 2020)
II	Aluminates	SrAl2O4	Monoclinic P21	Sr2+ (6, 1.180)	Er3+	/	/, 2011	(Terasawa et al., 2011)
		SrAl2O4	Monoclinic P21	Sr2+ (6, 1.180)	Nd3+	/	Sep. 2020	(Fujio et al., 2020)
	Gallates	LiGa5O8	Cubic P4332	Li+ (6, 0.760)	Pr3+	Pork	Sep. 2019	(Xiong et al., 2019b)
	Niobates	LiNbO3	Trigonal R3c	Li+ (6, 0.760)	Nd3+	Pork	Apr. 2019	(Xiong and Peng, 2019)
	Stannates	Sr3Sn2O7	Orthorhombic Cmcm	Sr3+ (9, 1.310)	Nd3+	Pork, HAP	Mar. 2019	(Xiong et al., 2019a)
		Sr3Sn2O7	Orthorhombic A21am	Sr3+ (9, 1.310)	Nd3+	Pork, chicken	May 2020	(Tu et al., 2020)

In this review, to our knowledge, recent advances of all the existed NIR ML crystals are collected and are divided into two categories: oxysulfides and oxides (Notably, only ML based on inorganic crystals was discussed in this mini review. For non-crystalline MLs such as in organic systems, one can refer to corresponding review for details [Xie and Li, 2018]. Pure organic MLs have already found applications in health care [Wang et al., 2020], piezo-display [Li et al., 2018c], or impact sensor [Incel et al., 2017].). For known ML systems, only lanthanide ions (Nd³⁺, Pr³⁺, Yb³⁺, Tm³⁺, and so on) were confirmed to function as NIR ML emitters, where Nd³⁺ ion dominates the number in those NIR ML materials. Besides, data concerning crystal system, space group, and the exact chemical environment around NIR ML emitters such as coordination number (CN) and ion radii of the to-be-substituted host cation ions were also carefully discussed. Finally, series of challenges and outlooks were provided, which are highly expected to advance the development of NIR ML crystals. It is hoped that this review could shed light on the exploration of efficient NIR ML biomechanical detection probe materials.

Known NIR mechanoluminescent crystals

Oxysulfides

CaZnOS

Generally, to design an NIR ML material, two aspects should be considered. One is to confirm a suitable host for a dopant with desired NIR emission and another is to regulate related luminescence quenching and energy-level mismatch, which determine whether NIR ML will appear. Based on the known fact that CaZnOS can accommodate for most rare-earth (RE) ions to have visible MLs, in 2018, Li et al. reported that Nd³⁺ ion-doped CaZnOS could emit under external force (compression/friction) (Figure 2) (Li et al., 2018b). ML signals span from the first (650–950) to the second (950–1400) NIR tissue-penetrating bioimaging window. For this regard, when the force reaches up to 5 kN, the NIR ML emission can penetrate tissues (pork) as deep as 22 mm. Besides, NIR ML signals from the sample bitten by human teeth with mouth open or closed were also observed, demonstrating that such NIR MLs can be generated by biomechanically teeth biting and have good tissue penetration ability. Moreover, such force-response behavior is highly reproducible after proper recharging by UV irradiation. This work has expanded the potential applications of ML into biomedical fields. This should be attributed to such high ML sensitivity of CaZnOS (d₃₃ = 38 pm/V). This is reasonable from a certain point of view, and CaZnOS has indeed been proven to be one of the most classic ML hosts after ZnS with many published works within the visible range (Tu et al., 2014, 2015; Wang et al., 2017; Zhang et al., 2013, 2015). Except for this, another impactful work was also reported based on CaZnOS in 2018, which realized multicolor luminescence from visible to NIR range (∼270–1000 nm, Figure 3) (Du et al., 2018). In this work, Du et al. proposed a lithium-assisted (LiNO₃) annealing method for effectively incorporating nearly all the RE ions into CaZnOS. Notably, this method is of great significance for improving the ML intensity because it has helped realize the NIR ML signals of Pr³⁺, Ho³⁺, and Dy³⁺ (in CaZnOS), and these NIR MLs were originally ignored. However, although many NIR MLs had been focused on in this compound, no related application demonstrations to NIR ML had been provided in this work. Afterward in 2019, Li et al. extended the NIR ML to ∼1540 nm (Er³⁺ ions) at the third tissue biowindow (Li et al., 2020). Here, multiple NIR ML spectral bands from Er³⁺ such as 845–880 nm, 960–1000 nm, and 1450–1700 nm were demonstrated. At the material synthesis stage, Li₂CO₃ was added to regulate its ML performances (namely, CaZnOS:Er³⁺, Li⁺ is better than CaZnOS:Er³⁺ in both NIR ML sensitivity and intensity). Piezoresponse force microscopy was used to firstly confirm that both samples have good piezoelectric performances, and Li⁺ codoping led to a higher piezoelectric coefficient, whose behind reason is attributed to the preferred crystal orientation after the addition of Li₂CO₃. Worth noting, in situ high-pressure study was also introduced to study inorganic ML powders, which aims to in situ probe both the luminescence properties and band structure. Finally, observations tend to indicate that the oriented migration of electrons which can be modulated or accelerated in a strain-induced piezoelectric field matters in the ML process. Compared with other NIR MLs, the “shortest” NIR ML wavelength (Er³⁺: ∼1540 nm) located within the so-called third biowindow (1500–1800 nm) was obtained. One should note that the Rayleigh scattering (varies as λ⁻⁴, where λ is wavelength) will decrease as wavelength increases (Clays and Persoons, 1991); hence, NIR ML of Er³⁺ should give the lowest scattering coefficient to ensure an improved resolution quality and deeper penetration depth. However, authors may just have ignored such possible applications since we find no such demonstrations here.

Figure 2.

Demonstration of bioimaging with CaZnOS:1% Nd³⁺

(A) The schematic diagram for imaging and spectroscopic detection of the NIR EML after passing through the tissue.

(B) Bioimaging of tissues by detecting NIR EML light passing through the tissues with different thicknesses.

(C) Emission spectra of NIR EML passing through the tissues with different thicknesses. Inset: emission spectra corresponding to tissue thickness of 20 and 22 mm.

(D) Tissue thickness-dependent log of integrated transmitted emission intensity. Red dash line shows the noise level, whereas red solid line shows the fitted results using a linear equation, log(I) = −0.124T + 3.864, where I and T denote the integrated intensity and tissue thickness, respectively.

(E) NIR ML signal from the sample bitten by human teeth with mouth open (left) or mouth closed (right), showing that the NIR ML signal can be generated due to teeth biting and still be detected with closed mouth due to good tissue penetration capability. Reproduced with permission from (Li et al., 2018b). Copyright 2018 American Chemical Society.

Figure 3.

ML spectra of CaZnOS doped with different rare earth ions

ML spectra of CaZnOS doped with (A) Pr³⁺, (B) Ho³⁺, (C) Er³⁺, (D) Dy³⁺, (E) Sm³⁺, (F) Eu³⁺, (G) Tm³⁺, (H) Nd³⁺, and (I) Yb³⁺. Inset: photographs of the corresponding samples under single-point dynamic pressure of a ballpoint pen. Reproduced with permission from (Du et al., 2018). Copyright 2018 WILEY.

Witnessing the development, researchers deeply realize that the applications of (NIR) ML are mostly limited by the weak and non-recoverable ML emissions. For this regard, numerous efforts have been paid to enhance the ML intensity such as by addition of fluxing agent (Zhang et al., 2020a), design of array structures (Bao et al., 2019), textile-assisted emission (Park et al., 2019), and regulation of reaction conditions (Wu et al., 2018). However, till now, quite limited MLs can be self-recoverable (no need of pre-irradiation to generate ML). For the self-recovery of ML, doped ZnS seems to have always stood out, which was attributed to the trapping of drifting charge carriers in the presence of piezoelectric field (Chandra et al., 2013). Following this idea, stable ML intensities over thousands loading cycles were obtained, but they are all limited to the visible ML range (Jeong et al., 2013; Moon Jeong et al., 2013; Wang et al., 2015; Zhou et al., 2020). One should eagerly expect that NIR ML (which can penetrate tissues) can also be self-recoverable since this will insure the in vivo biomechanical monitoring wherein repeated recharging by high-energy UV lights fails to work. Interestingly, in 2020, Peng et al.’s research work has reached this goal to some extent. Herein, a new class of heterojunctioned ZnS/CaZnOS piezophotonic system with remarkable ML performance in both intensity and reproducibility was demonstrated (Figure 4) (Peng et al., 2020). ZnS and CaZnOS were prepared by controlling the ratio of CaCO₃ and ZnS, and they accommodate Mn²⁺ and lanthanide ions (such as Tm³⁺, Yb³⁺, and Pr³⁺ NIR luminescent ions), respectively. Here, highly stable heterojunction was reported and was confirmed by X-ray powder diffraction, a high-resolution transmission electron microscope, and the fast Fourier transform techniques. Under a sliding force (10 N) for 20 cycles, only a marginal intensity decay was demonstrated. Notably, the ML intensity self-recovery performance here is based on the red emission of Mn²⁺, while whether this performance is applicable to the ML in the NIR range remains to be discussed. In all, CaZnOS is one of the best NIR ML hosts, and corresponding biomechanical imaging based on this system may be possible in the near future.

Figure 4.

Scheme of proposed mechanism for mechanical-to-optical energy conversion in the piezophotonic semiconductor heterojunction

Scheme of proposed mechanism for mechanical-to-optical energy conversion in the piezophotonic semiconductor heterojunction: electronic transition in the ML process is facilitated by band offsets at the heterojunction interface region (indicated by the red frame). Reproduced with permission from (Peng et al., 2020). Copyright 2020 WILEY. VB, valence band; CB, conduction band; E-charging, electric field-induced charge separation; ETU, energy transfer upconversion; Ln³⁺, trivalent lanthanide ion; orange balls, neutral defect centers; red balls, electrons; blue balls, holes.

SrZn₂S₂O and SrZnSO

Indeed, one cannot deny that developing novel ML host has always been a research hotspot, which is limited to the systematical research strategies. Since MZnOS (M = Ba²⁺ or Ca²⁺ has been confirmed as one of the best ML hosts, one may easily note that another ML host can be obtained if M is replaced by Sr²⁺ cations. Following this idea, in 2019, Chen et al. developed novel MLs with super broad visible to NIR (470–1600 nm) spectra by doping lanthanide ions or transition ions into mixed anion compounds SrZn₂S₂O and SrZnSO (Figure 5) (Chen et al., 2020). NIR MLs were obtained both from lanthanide ions such as Tm³⁺, Nd³⁺, Yb³⁺, and Er³⁺ ions in SrZnSO and Yb³⁺ ion in SrZn₂S₂O. Here, mixed anion compound is defined as one kind of solid-state material consisting of multiple anion species in a single phase and such were expected to be another species of ML materials. Indeed, before this work, Smet et al. have already confirmed green (∼498 nm while not NIR) ML from Eu²⁺-doped mixed anion compounds BaSi₂O₂N₂ and SrSi₂O₂N₂ (Botterman et al., 2012). However, more ML systems based on such mixed anion compounds should be exploited. As demonstrated, NIR MLs based on Er³⁺-doped SrZnSO can be used to sense bright-field stress with the help of an NIR camera. It is a promising approach for stress sensing since the least interference of ambient light (most visible lights) can be guaranteed. To address this advantage, Xiong et al. have firstly developed an NIR signature system previously (Xiong et al., 2019a). A glass rod was used to “write” on the surface of an ML pellet, and the personal abundant information (e.g. signing speed and local pressure) was recorded by NIR ML signals of an NIR camera. This work will definitely improve the anti-counterfeiting level because such NIR “signature information” cannot be read out directly by naked eyes while signing.

Figure 5.

Design route and applications of ML materials in mixed anion compounds

(A-i) Formation of local asymmetry, e.g., C₄ or C₃ through replacing one and three ligands by other anion species. (A-ii) An overall noncentrosymmetric structure constructed by periodically aligned mixed anion polyhedrons. (A-iii) Creation of multi-color ML by doping various luminescent centers in mixed anion compounds.

(B) Multiband ML spectra covering the whole VIS-NIR region in Ln³⁺- or Mn²⁺-doped SrZnSO.

(C) Various applications of multi-color ML materials. The VIS ML materials can be applied to a conventional dark-field stress sensing. The NIR ML materials offer an advanced stress sensing approach operable under the natural light (bright-field sensing). Reproduced with permission from (Chen et al., 2020). Copyright 2019 Elsevier.

Oxide compounds

Aluminates

Undeniably, Eu²⁺-doped SrAl₂O₄ aluminate has been one of the most famous ML systems for decades. However, it was only until 2011, Terasawa et al. declared the first NIR ML (peaked at ∼1530 nm) by codoping Er³⁺ ions into SrAl₂O₄:Eu²⁺ (Terasawa et al., 2011). Since both Eu²⁺ or Er³⁺ singly doped SrAl₂O₄ failed to emit ML in the NIR range, energy transfer from Eu²⁺ to Er³⁺ ions and subsequent transition ⁴I_13/2→⁴I_15/2 were considered to attribute to such NIR ML emission. Unfortunately, one cannot find a clear NIR ML spectrum. In this work, only a visible light cut filter was set right in front of the photomultiplier tube to ensure that only NIR ML signals were recorded. Except for this, very recently, in 2020, Fujio et al. demonstrated that NIR ML (750–1000 nm) from Nd³⁺ was also obtained in SrAl₂O₄:Eu²⁺, Cr³⁺, and Nd³⁺, where Eu²⁺ and Cr³⁺ ions were considered to be the sensitizer ions (Fujio et al., 2020). The NIR ML powders were synthesized by metal nitrates and malic acid, where an acid-aided method was believed to better regulate the target samples to obtain higher ML intensity. For the mechanical excitation mode, tensile testing was adopted. However, no NIR ML spectra data were also provided. On the one hand, unless by energy transfer from Eu²⁺ to Er³⁺ and Nd³⁺, it seems “impossible” to obtain NIR MLs by singly doping in aluminate. On the other hand, one can also note NIR photoluminescence by Nd³⁺ or Er³⁺ singly doped aluminate SrAl₂O₄ (Ayvacıkli et al., 2013; de Herval et al., 2015). These facts indicate that NIR ML may be physically different from photoluminescence or visible ML of Eu²⁺ in SrAl₂O₄. The authors are eager to see more NIR MLs (at least with corresponding spectral data) in aluminates.

Gallates

(Inverse) spinel structure compound (such as LiGa₅O₈) has been confirmed to persistently emit after doping optically active centers such as transition metal ions (Chen et al., 2018; De Clercq et al., 2018). However, no lanthanide ion-doped LiGa₅O₈ had been found to show persistent luminescence or ML until 2019. Xiong et al. realized visible to near infrared persistent luminescence by incorporation of Pr³⁺ ions into the 6 coordinated Li⁺ sites. More interestingly, ML spectra (300–1000 nm extending into the NIR range similar to persistent luminescence) were obtained when quickly sliding a borosilicate rod on the surface of these powders (Figure 6). By normalizing the intensity of the peak at 862 nm, a comparison of peak positions and corresponding intensities between ML and persistent luminescence spectra was provided. Conclusion from this work is that mechanical rather than thermal effect plays the dominant role in the reduction of possibility of ³P₁→³H_J of Pr³⁺. Besides, due to its persistent luminescence, one can find biological tissue penetration of the pork skin, fat, or lean in this work. Further biomechanical imaging was then expected since ML has a similar visible to NIR luminescence profile, and a larger proportion of the NIR part that can be used to penetrate tissues.

Figure 6.

PersL and ML of Li_0.995Ga₅O₈:0.005Pr³⁺

(A) Comparison between ML and persistent luminescence (PersL) spectra of Li_0.995Ga₅O₈:0.005Pr³⁺. PersL was detected at the delay time of 2 s after pre-irradiation by a 254 nm lamp for 60 s.

(B) Energy level diagrams of Pr³⁺ ion and corresponding emission wavelengths. Reproduced with permission from (Xiong et al., 2019b). Copyright 2019 WILEY.

Niobates

As for niobates, in 2017, Tu et al. achieved red-emitting ML by doping Pr³⁺ ions into the piezoelectric host LiNbO₃ (Tu et al., 2017). It was considered the “first” piezo multifunctional material that presents both piezoelectricity and intense ML. From this work, one may find that ML signal appears once loaded, and this is indeed of great importance since a “zero threshold” ML is highly desired for practical applications. However, the underlying reason for such an efficient ML host was not fully understood at that time. For this, Huang et al. conducted a case study on alkaline niobates to deeply explain the intrinsic energy conversions for photongeneration in piezophototronic materials (Huang et al., 2018). ML mechanism was then becoming clear from the native point defect levels of LiNbO₃. Thanks to this work, series of ML work on niobate hosts were reported (Pan et al., 2018; Zhang et al., 2018). However, it was not until 2019 that Xiong and Peng firstly realized NIR ML (peaked at ∼895 nm) from niobate (LiNbO₃:Nd³⁺) (Xiong and Peng, 2019). Under compressive load from 500 to 3000 N, NIR ML intensity increases to a maximal value at 2500 N, and further, ML intensity is limited by the mechanical strength of the resin sample. Besides, both the start and stop of the loading were confirmed to generate ML. When the load is stable, ML intensity still continuously decreases, and finally, the stop of loading can produce only much weaker ML intensity. Importantly, ML intensity is highly trap dependent, such as shallow traps may result in lower ML intensity; for more details, one can refer to previous work on BaSi₂O₂N₂ (Botterman et al., 2012). Finally, as a proof of concept of biopenetration, tissues such as the pork skin, fat, and meat were used, and the best penetration depth reaches 22 mm (Figure 7). Hence, in situ, real-time, and in vivo biomechanical sensing was further expected to realize. Also, since stable load generates no ML, we also believe that only pulsed biomechanical behaviors can be visualized using ML techniques. For this, one can find more evidence in the heartbeat-sensing ML device (Wang et al., 2020).

Figure 7.

Biological tissue penetration experiments

(A and B) (A) The thickest tissue used in this test; (B) ML spectra under 1000 N after passing through different tissue samples: (top) 2 mm skin; (middle) 2 mm skin +8 mm fat; and (bottom) 2 mm skin +8 mm fat +12 mm meat. Reproduced with permission from (Xiong and Peng, 2019). Copyright 2019 Royal Society of Chemistry.

Stannates

Regarding the possible ML hosts, layered Ruddlesden-Popper perovskite such as stannates Sr_n+1Sn_nO_3n+1 (n = 1, 2, …, ∞) should be one of the best efficient ML hosts. Temperature switchable polarization of bulk crystals has been confirmed, and such nature is closely related to piezoelectricity that desired for ML (Oh et al., 2015). Generally, ML performances of such layered structure materials are strongly dependent on the number of the layers. For example, Sm³⁺-doped Sr₃Sn₂O₇ has the best ML intensity compared to that of Sr₂SnO₄ and SrSnO₃ (Kamimura et al., 2012). The ML performances also can be largely improved such as by tailoring the band gap and trap distribution (Li et al., 2018a). However, only visible MLs were reported before. Afterward, by incorporating Nd³⁺ ions into Sr₃Sn₂O₇, Xiong et al. confirmed its NIR ML emission in 2019 (Xiong et al., 2019a). Under compressive load or friction, same NIR ML spectra (peaked at ∼900 nm) located within the first biowindow were observed. Moreover, such emission is recoverable as ML intensity can be fully restored if recharged by high-energy 254-nm light irradiation. Except for NIR ML centers of Nd³⁺ ions, it was particularly meaningful that NIR ML from intrinsic defects (700–850 nm) was observed. And such intrinsic NIR MLs should be enhanced by doping Nd³⁺ ions because nearly no such signals appear from the blank sample. Namely, the incorporation of Nd³⁺ has bridged the energy transfer from external force to Sr₃Sn₂O₇. Results from thermoluminescence indicated an easy way to selectively quench the NIR ML of the host by just prolonging the detection time. Finally, to demonstrate its biomechanical bioimaging possibility, pigskin and hydroxyl apatite (HAP, which is the main component of human bones) were chosen to simulate “muscles” and “bones”, respectively. Interestingly, such NIR ML emissions can penetrate both of them, and the deepest penetration depth for pigskin reaches ∼30 mm (Figure 8). These facts may have provided a possible way to realize in situ biomechanical behavior visualization.

Figure 8.

Tissue penetration experiments

(A) and (B) are the experimental detection images of penetrating pigskin and HAP, respectively.

(C) ML spectra after passing through different thicknesses of pigskins, which increases 5 mm each time from 0 to 30 mm. The inset is the integrated ML intensities over the spectral range of 870–970 nm of these spectra shown in (C).

(D) ML spectra passing through HAP pellet and the background signal. Reproduced with permission from (Xiong et al., 2019a). Copyright 2019 The American Ceramic Society.

After this work, in 2020, Tu et al. also reported NIR ML emissions from Nd³⁺-doped Sr₃Sn₂O₇ (Tu et al., 2020). Totally, Sr₃Sn₂O₇ reported from both researchers belong to the same orthorhombic crystal system, while the space groups are reported to be A2₁am by Tu et al. and Cmcm by Xiong et al. Such differences made Sr₃Sn₂O₇:Nd³⁺ (A2₁am) a “true” elastic deformation-induced ML with improved sustainability and ultrasensitivity. Namely, 800–1500 nm NIR MLs were induced at micro-strain levels, which are enhanced by the ferroelectrically polarized charges in Sr₃Sn₂O₇ (A2₁am). Notably, as a proof of biomechanical imaging, fresh chicken skin and pork (∼5 mm in thickness) were used to cover the surface of ML pellets, and the obtained NIR ML intensity was believed sufficient for biomechanical imaging of implanted materials within small animals. NIR ML signals were recorded in situ using an InGaAs CCD camera with a spectral sensitivity between 900 and 1700 nm (Figure 9). It is worth emphasizing that such NIR ML crystals will accelerate the application for bioscience in situ under both light and dark conditions.

Figure 9.

Tissue penetration experiments for SSN

(A) CCD images for SSN (Sr₃Sn₂O₇:Nd³⁺) pellet set.

(B) Sequence of images recorded using an InGaAs-CCD camera for SSN applied during the load cycle of 350 N at a rate of 3 mm min-1 showing the real-time images of NIR light distribution.

(C) The distribution along vertical component of the NIR piezoluminescence in the SSN pellet with and without a chicken skin under a compressive load of 77 N.

(D) CCD image for Sr₃Sn₂O₇:Nd³⁺ (SSN) flm.

(E) Images recorded with an InGaAs-CCD camera for SSN flm by tensile load applied (nominal strain: 600 μst).

(F) The distribution at the horizontal position along the center of a circular hole of the NIR piezoluminescence of the SSN flm outside and embedded 5 mm in a sample of pork under a tensile load (maximum of nominal strain: 600 μst). Reproduced with permission from (Tu et al., 2020). Copyright 2020 WILEY.

Conclusions and outlook

Although ML crystals (used as mechanical sensor probes) have be widely known around our daily life for a long time, their use as mechanical sensor probes was only refocused on until ∼1999. The elastic ML intensities of ZnS:Mn and SrAl₂O₄:Eu²⁺ linearly correspond to external mechanical stimuli while keeping complete crystal structure and advanced this field to a big step since then. Notably, lots of ML crystals have been found and corresponding potential applications have been demonstrated. However, their use in bioimaging has achieved limited progress, which is mostly limited to the number of such NIR ML crystals. One can easily note that no more than 11 ML crystals can emit NIR emissions (Table 1). Then, a detailed review of these crystals has been provided, concerning the crystal system, space group, and chemical environments. It is suggested that NIR ML crystals should better be non-centrosymmetric as hexagonal, orthorhombic, monoclinic, or cubic. High piezoelectric oxysulfide semiconductor such as CaZnOS still dominates the number of NIR ML hosts. Besides, not all NIR optical centers can be used as NIR ML dopants, and Nd³⁺ ion should always have been the best choice. To demonstrate the tissue penetrating ability of those NIR ML crystals in vitro, soft tissue pork is always chosen, as well as chicken sometimes. Sclerous tissues such as human teeth and HAP have also been confirmed not to prevent from penetration by those NIR ML emissions. Besides, no in vivo bioimaging has been provided and this should be of importance in next step development. Except for those, other outlooks also should be addressed, and further directions may include the following:

• (1)Gaining more insights into the behind NIR ML mechanism

Even today, controversies over the mechanism of ML remain, let alone that of NIR ML. For example, ML mechanism of SrAl₂O₄:Eu and ZnS:Mn mostly consisted of such processes: detrapping carriers at shallow traps under mechanical stimuli and, afterward, activator recombination occurs with the local piezoelectric field. However, Sohn et al. insisted on that the triboelectricity-induced luminescence works in ZnS:Cu-polydimethylsiloxane composite due to its poor piezoelectric properties (Sohn et al., 2016). Generally, most ML materials can be excited by friction, while not all of them can easily emit under compression (e. g. LiGa₅O₈: Pr³⁺). Nowadays, researchers tend to simply attribute them to a so-called piezoelectric field-induced trap detrapping model, while no corresponding piezoelectric parameters have been provided. Hence, it is suggested that researchers pay more attention to their statements in explaining ML mechanisms. Besides, does NIR ML really share the same ML mechanism with that of visible ML?

• (2)Establishing the possible relation between the possibilities of generating ML and its emission centers

One can easily note that Eu²⁺ dominates the number of visible MLs, while Nd³⁺ accounts for the vast majority of NIR MLs (Figure 1 and Table 1). So, one may propose that are Eu²⁺ and Nd³⁺ more sensitive to external mechanical stimuli than other ions? Hence, can ML be the intrinsic properties of these luminescent ions and their response to mechanical stimuli is intrinsically inclined? In a word, researchers who are good at luminescence should participate in this field to help solve this problem.

• (3)Exploring more practical applications for NIR ML

Remarkably, NIR ML can penetrate tissues and realize in situ, real-time, and non-destructive visualization of internal biostress. One can expect that such technique is promising for monitoring the mechanical strength of an implanted component. However, although several NIR ML crystals have been confirmed, the tissue penetrations have only been demonstrated in vitro (e.g. Sr₃Sn₂O₇:Nd [Tu et al., 2020; Xiong et al., 2019a], LiNbO₃:Nd [Xiong and Peng, 2019], and CaZnOS:Nd [Li et al., 2018b]). Regarding the use in vivo, it is suggested that those NIR ML crystals should be smaller in size and having them nanosized (which is usually more biocompatible) is thus worth expecting. Besides, except for biomechanical sensor of NIR ML, other applications such as dynamic pressure mapping (Xiong et al., 2019a) and bright-field stress visualization (Chen et al., 2020) should be focused on since such NIR ML signals cannot be observed directly by naked eyes.

• (4)Developing diversified NIR ML centers. Notably, most known NIR ML centers are from lanthanide ions

Exceptionally, Xiong et al. once attributed the NIR ML within 700–850 nm of Sr₃Sn₂O₇:Nd³⁺ to the host, which was confirmed since similar photoluminescence was observed. Although no clear evidence has been provided, this finding has certain guiding significance for widening existed understanding of the nature of NIR ML centers.

• (5)Organisms have complex biomechanical behaviors, and these biostresses are varied in tissues

Thus, it is of importance to evaluate the required pressure levels of mechanical stimuli for bioimaging such as in vivo, in vitro, electronic skins, prosthetics, false tooth, etc. For this, professional medical researchers are suggested to be invited in. From another perspective, it is quite important to develop NIR ML materials with ultra-low or even zero threshold. This low threshold NIR ML crystal may come from ML material itself such as high piezoelectric system of LiNbO₃:Pr³⁺, whose “zero” ML threshold has been reported, since immediate ML signal can be observed once load appears (Tu et al., 2017) or else designing modification architecture to enhance the sensitivity of ML devices. Following this idea, a skin-driven ML (namely, ML induced by the movement by lips corner, canthus, and cheek) has been demonstrated (Qian et al., 2018). Both strategies should be focused on in developing ultra-low/even zero threshold NIR MLs.

Finally, we want to encourage researchers to think ML with physics. If force causes deformation and produces light and this effect is reciprocal, that is, using light to achieve stress deformation, even this deformation may be small. Then, if you can measure this tiny deformation, you may reveal its physical ML mechanism. How to realize the measurement of such tiny deformations still needs light realization. Researchers who focus on ultrafast lasers, especially those who do Brillouin nonlinear effects, may provide professional suggestions. In all, NIR ML is expected to bring out other promising applications due to the combination of ML and NIR spectral techniques. It will open path toward the realization of encouraging applications in vivo bioimaging and fundamental research.