The develop of persistent luminescence nanoparticles with excellent performances in cancer targeted bioimaging and killing: a review

Abstract

The use of fluorescent nanomaterials in tumor imaging and treatment effectively avoids the original limitations of traditional tumor clinical diagnostic methods. The PLNPs emitted persistent luminescence after the end of excitation light. Owing to their superior optical properties, such as a reduced laser irradiation dose, spontaneous fluorescence interference elimination, and near-infrared imaging, PLNPs show great promise in tumor imaging. Moreover, they also achieve excellent anti-tumor therapeutic effects through surface modification and drug delivery. However, their relatively large size and limited surface modification capacity limit their ability to kill tumors effectively enough for clinical applications. Thus, this article reviews the synthesis and modification of PLNPs and the research progress in targeted tumor imaging and tumor killing. We also discuss the challenges and prospects of their future applications in these fields. This review has value for accelerating the design of PLNPs based platform for cancer diagnosis and treatment.

Graphical abstract

Introduction

Because of the increasing cancer incidence worldwide, the realization of accurate diagnosis and treatment of cancer needs extensive research [1–4]. The difficulty lies in how to accurately distinguish between normal cells and tumor cells. Traditional clinical methods include computed tomography (CT), positron emission tomography (PET) and magnetic resonance imaging (MRI) [5–7]. Among them, CT has the advantages of good resolution, but its limitation lies in radiation problems and high cost [8]. The safety of MRI is a considerable advantage, but the lack of sensitivity and artifacts caused by motion are its disadvantages [9]. PET shows higher sensitivity and quantitative detection than both methods do, but the problems of cost and limited instruments need to be solved [10]. In this context, fluorescence imaging technology enables tumor monitoring and real-time guidance during surgery with ideal imaging effect, which reduces ionizing radiation and radiation damage and is helpful for overcoming the problems of high technical cost and limited instrument use, especially in terms of sensitivity and specificity at the cellular level [11, 12]. To date, a variety of fluorescent nanomaterials have been applied in fields such as biological imaging, tumor diagnosis and tumor treatment [13–15]. Traditional agents, including porphyrins, phthalocyanines and other organic dyes, are limited by easy photobleaching and long-term photoactivation [16–18]. Thus, novel fluorescent nanomaterials of carbon quantum dots and gold nanorods with sufficient optical stability have been developed for anti-tumor treatment [19, 20]. However, their limited luminescence time makes eliminating the interference of spontaneous fluorescence background in practical applications difficult, which limits the further application of long-term and real-time imaging [21]. Fortunately, persistent luminescence nanoparticles (PLNPs), which can store the energy generated by the excitation source and continue to release the energy gradually after the excitation has stopped, show great promise for application as fluorescent probes for biological imaging [22]. The application of PLNPs can reduce the laser irradiation dose and eliminate the interference of real-time excitation from external light sources, which is conducive to achieving safer and more sensitive long-term in vivo imaging [23–25]. In addition, the emergence of near-infrared (NIR)-emitting PLNPs further enhances the in vivo penetration depth of fluorescent probes, indicating extremely ideal clinical and research application prospects in tumor cell killing [26, 27].

At present, although some PLNPs based materials have been reported for tumor diagnosis and treatment, a comprehensive review related to anti-tumor applications is still relatively lacking. Moreover, there is no systematic explanation for the shortcomings of PLNPs in tumor diagnosis and treatment research, including the need for nanometer-scale PLNPs, further improvement in tumor killing efficiency, and consideration of their combination with other bioimaging methods. Therefore, this review summarizes the development status of the preparation methods, optical properties and biocompatibility of PLNPs, with a focus on related applications in tumor imaging and treatment through modifications and drug delivery, including photothermal therapy (PTT), photodynamic therapy (PDT), guided surgery-targeted anti-tumor drug delivery treatment and fluorescence-guided surgery. We also discuss corresponding prospects, aiming to provide a systematic viewpoint and propose new ideas for related research. Our work provides necessary theoretical support and new research directions for further studies of the anti-tumor applications of PLNPs.

Synthesis methods and surface functionalization of PLNPs

Synthesis methods of PLNPs

Conventional persistent phosphors are synthesized by high-temperature solid-state reactions using powdered raw materials. This traditional method has the advantages of high crystallinity, strong luminescence and a long afterglow time. However, owing to the uncontrollability of mechanical grinding during the synthesis process, the particle size is usually approximately tens of micrometers with uneven size and irregular morphology, which is not conducive to its wide application in biomedical fields [28].

The synthesis of PLNPs with a uniform and regular morphology is essential for the application of persistent phosphors in biomedical fields, especially in medical oncology [29]. These properties help to improve the quality and sensitivity of tumor imaging, and PLNPs with homogeneous sizes and regular morphologies are more conducive to surface functionalization to construct PLNPs for diagnostic and therapeutic integrated nanoplatforms. Thus, there is an urgent need for researchers to explore new synthesis methods to obtain PLNPs. To overcome the drawbacks of traditional synthesis methods, many studies have been conducted for this purpose. In recent years, the emergence of so-called “bottom-up” nanoscale synthesis methods, including sol–gel method, templated method and hydrothermal method, has greatly increased the prospects for the application of PLNPs in biomedical fields.

Sol–gel method

The “sol–gel method” refers to the hydrolysis of the precursor into a sol, which is then converted into a gel for subsequent calcination (800–1100 °C), resulting in high yields of small-sized and homogeneous PLNPs. In this process, generally speaking, in addition to the use of metal or alcohol salts as precursors, citric acid as a chelating ligand and alcohols as cross-linking agents are needed. At higher synthesis temperatures, small nanocrystals are squeezed into the discrete space provided by the complexing agent, limiting the growth of the nanocrystals and resulting in PLNPs [28].

Early in 2007, Shermont et al. developed the first sol–gel method to synthesize NIR PLNPs of Ca_0.2 Zn_0.9 Mg_0.9 Si₂O₆:Eu²⁺, Dy³⁺, Mn²⁺ with a size distribution of 50–100 nm for in vivo imaging [30]. They acidified the chloride and nitrate solutions with concentrated nitric acid, followed by the rapid addition of ethyl orthosilicate (TEOS) and stirring, which allowed the sol–gel transition to occur through heating, and ultimately, the particles were isolated by selective precipitation. At a pH of 2, the obtained particles have a broad excitation band in the ultraviolet (UV) region, a large emission spectrum with a maximum intensity of 690 nm, and an in vitro afterglow time of up to 24 h. Subsequently, improved synthesis methods, such as the sol–gel-microwave route, sol–gel-combustion processing and sol–gel-template technology, were developed to inhibit the growth of nanocrystalline grains [29].

Templated method

The template method is a commonly used and well-established method for synthesizing PLNPs. In this method, mesoporous silica nanoparticles (MSNs) or carbon nanorods are used as templates, which are combined with precursor ions and calcined at low temperatures, thereby obtaining PLNPs with controllable particle sizes, regular morphologies and monodispersities [31]. In addition, the presence of silica provides more possibilities for subsequent functionalization to modify PLNPs.

In 2015, Zhang et al. used MSNs as templates for the in situ synthesis of NIR PLNPs, SiO₂ / ZnGa₂O₄:Cr³⁺ (mZGC). After UV (254 nm) irradiation for 5 min, NIR fluorescence (696 nm) could still be detected after 5 h [31]. Because the conventional solid-state annealing temperature at 750 °C may lead to structural collapse of the MSNs, they reduced the reaction temperature to be 600 °C to maintain the nanostructure of the MSNs. A schematic of the synthesis process and in vivo deep tissue imaging of the synthesized PL-functionalized MSNs are shown in Fig. 1. In 2018, they subsequently used purgeable and highly dispersed carbon spheres as templates to synthesize NIR PLNPs of ZnGa₂O₄:Cr³⁺with a large cavity, which can be loaded with doxorubicin (DOX) or photosensitizers to achieve an effective tumor suppression effect [32]. The synthesized hollow PLNPs are approximately 50 nm in size and produce intense NIR emission at 696 nm under 254 nm excitation, with an afterglow duration of more than 30 min.

Fig. 1

Copyright 2015, Wiley

Illustration of the synthesis of PL-functionalized MSNs and their in vivo imaging application. Reproduced under terms of the CC-BY license [31].

More importantly, the high calcination temperature destroys the surface functional groups of PLNPs, which can lead to poor dispersion and undesirable stacking of PLNPs, thus limiting their application in biomedical fields to a certain extent [22].

Hydrothermal method

Unlike other synthesis methods, hydrothermal synthesis usually prepares PLNPs via chemical reactions in a dedicated high-pressure reactor, which provides a high-pressure and hermetically sealed environment throughout the reaction process, thus facilitating the reactions between the solid precursors [28]. The reaction time and temperature can be well controlled by this method, which allows the synthesis of highly crystalline PLNPs under relatively mild conditions. Importantly, the PLNPs synthesized via this method are ultrasmall in size and easy to surface modify. Successful synthesis via the hydrothermal method is influenced by several important factors, such as high processing temperatures, organic additives, surfactants, etc., with specific functional groups, including oleic acid, polyethylenimine (PEI), ethylenediaminetetraacetate (EDTA), and cetyltrimethylammonium bromide (CTAB).

Han et al. reported a direct hydrothermal method for the synthesis of NIR PLNPs at 220 °C in 2015 [33]. The synthesized ZnGa₂O₄Cr_0.004 PLNPs are less than 10 nm in diameter and can form stable colloidal solutions in aqueous solution with an in vivo afterglow duration of more than 30 min. In addition, the size of the synthesized PLNPs can be controlled by adjusting the Zn/Ga molar ratio in the solution. In 2017, Yuan et al. reported Zn_1+xGa_2-2xGe_xO₄:Cr via a hydrothermal method, and the size and afterglow duration of PLNPs could be adjusted by adjusting the amount of Ge [34]. Although hydrothermal methods have great advantages in terms of particle size, dispersion, and functionalization, they still suffer from the drawbacks of weak afterglow intensity and short afterglow duration, which need to be improved upon by further research [28]. Finally, a comparison of the various synthesis methods in terms of luminescence intensity, afterglow duration, particle size, and morphology is given in Table 1.

Table 1 Comparison of synthesis methods of PLNPs in terms of luminescence intensity, afterglow duration, particle size, and morphology

The emerging synthesis methods

In addition to these previously introduced methods, researchers have successively proposed several novel synthesis methods to obtain smaller-sized and brighter PLNPs. A co-synthesis method was used by Richard et al. PLNPs precursors were synthesized via a hydrothermal method and subsequently calcined at 750 °C to prepare ZnGa₂O₄:Cr³⁺ with enhanced optical and physical properties [21]. In addition, the tuning of PLNP traps, size and water dispersibility by simple ethylenediaminetetraacetate (EDTA) etching was explored by Zhang et al. [35]. Cr_0.004³⁺:ZnGa₂O₄ (ZGO) was used as a PLNP model, and EDTA etching of the sintered ZGO effectively reduced the size and greatly improved the degree of water dispersion. Besides, microwave-assisted heating has utilized for the synthesis of PLNPs, which greatly reduces the reaction time compared with the conventional heating in autoclave. The principle lies in the use of microwaves to heat the medium that can absorb waves in the reaction system to warm the reaction system. This synthesis method is not only simple and fast but also results in PLNPs with good physical properties. Pedroso et al. reduced the reaction time from 48 h to 30 min, which greatly improved the synthesis efficiency [36].

Organic PLNPs that do not rely on a limited number of rare earth elements are also a hot topic of current research. They are well known for their ability to be easily synthesized and modified, as well as their advantages of biocompatibility, long luminescence lifetime, and easy synthesis. However, organic PLNPs suffer from severe nonradiative leaps of excitons in the trilinear state and serious spin-forbidden blocking between the single and triplet excited states. Researchers usually inhibit the non-radiative leaps through crystal engineering, the introduction of rigid polymer matrices and cross-linking strategies; or promote intersystem crossing and increase spin‒orbit coupling through the introduction of heavy atoms, heteroatoms and aromatic carbonyls [37].

Surface functionalization of PLNPs

Owing to limitations in synthesis methods and raw materials, PLNPs generally lack modifiable groups on their surfaces. The surface properties of nanomaterials have an important impact on their biomedical applications; thus, surface functionalization of PLNPs is essential to obtain better biocompatibility, lower cytotoxicity, stronger stability stabilization and facilitating further attachment of functional groups.

The use of silicon dioxide, especially MSNs coating is an important method for the surface functionalization of PLNPs. In addition to the template synthesis method described previously to obtain MSNs-coated PLNPs, Yan et al. added the synthesized Zn_1.2Ga_1.6Ge_0.2O₄:Cr³⁺ (approximately 20 nm in diameter) together with sodium salicylate and CTAB into a triethanolamine solution and then stirred it with TEOS. Finally, silica-coated PLNPs were obtained after the addition of aminopropyltriethoxysilane [38]. Other methods of surface functionalization have also been reported. Fu et al. reported mesoporous polyacrylic acid (PAA)/calcium phosphate (CaP)-coated Zn_1.25Ga_1.5Ge_0.25O₄:0.5%Cr, 2.5%Yb, 0.25%Er for bioimaging of bacterial infections and chemotherapeutic treatments [39]. The passivation of the surface defects of the PLNPs by the PAA shell layer and the energy transfer between the shell layer and the PLNPs effectively prolonged the decay time of the PLNPs. As a result, the sustained luminescence intensity of the PLNPs was enhanced. In 2020, Chan et al. grafted polyaniline (PANI) and glycol chitosan (GCS) on the surface of synthesized PLNPs to synthesize PLNP@PANI-GCS, which is a pH switchable nanoplatform for in vivo persistent luminescence (PersL) imaging and precise PTT of bacterial infection. The pending positively charged nanoplatform responds to acidic regions of bacterial infection and electrostatically binds to bacteria in vivo, ensuring spatial precision of NIR light irradiation. It has great potential for providing a targeted, nonautofluorescence imaging-guided, precisely visualized therapeutic approach for the treatment of multidrug-resistant bacterial infections [40]. In 2021, Li et al. used neutrophil delivery to penetrate the blood‒brain barrier for ultrasound-enhanced chemotherapy/immunoglioblastoma treatment. ZnGa₂O₄:Cr³⁺ (ZGO) was coated with a hollow acoustic-sensitive TiO₂ shell with the ability to generate reactive oxygen species (ROS) and control drug release. Moreover, an immune checkpoint inhibitor of anti-PD-1 antibodies was loaded and further loaded into liposomes encapsulated with paclitaxel (PTX) [41].

Besides, the combination of multiple modification methods can also enable PLNPs to obtain more complex and flexible biological functions. For example, in 2023, Zhang et al. deposited ultrasound-excited mechanoluminescent nanodots of Sr Al₂O_4:Eu²⁺ (SAOE) and PersL nanodots of ZnGa₂O₄:Cr³⁺(ZGC) on mesoporous silicate, followed by modification with NO donors loaded with polydopamine (PDA). Ultrasonication caused mechanoluminescence from SAOE to excite the ZGC PLNPs to emit NIR, which excited the PDA, generating a sustained autothermophoretic force to facilitate nanoparticle uptake. Moreover, NIR-stimulated PTT and PersL-triggered NO release achieve anti-tumor effects (Fig. 2) [42].

Fig. 2

Copyright 2023, American Chemical Society

Preparation and Treatment Mechanism: a SAOE nanodots deposited on MSNs, followed by ZGC nanodots to obtain mSZ NPs. PDA layers with NO donors selectively capped on one side of mSZ NPs to form Janus mSZ@PDA-NO NPs. b Ultrasonication of NPs generates ML green emission, which excite ZGC nanodots to emit NIR PL. c Internal NIR excitation of PDA caps produces persistent self-thermophoretic force to propel NP motion. Reproduced with permission [42].

Optical properties of PLNPs

Persistent luminescence (PersL)

The most prominent optical property of PLNPs is PersL, which means that the material can quickly absorb the excitation energy and continue to emit luminescence for a certain period of time from minutes to hours and even days after the end of excitation [43]. The generation of PersL is now widely recognized to be associated with two important constituent elements and four processes. These include the generation of charge carriers, trapping of charge carriers, release of trapped charge carriers, and recombination of the released charge carriers to produce emission (Fig. 3) [44]. Emitters and traps play important roles in these four processes. Emitters usually consist of transition metal ions or lanthanide ions doped in a host material, which can form a ground state energy level and a series of excited state energy levels [45]. However, traps usually consist of intrinsic crystal defects and codopants that can trap and store charge carriers. Specifically, the mechanism of the PersL process is the energy transfer process of luminescence centers and trap centers formed by doping elements in the matrix during the preparation of PLNPs. When PLNPs are excited by an outside light source, the electrons in the luminescence centers absorb the energy generated by the excitation and transition to the excited state, and some of the electrons are captured by the trap centers. When the excitation stops, the electrons captured by the trap centers escape from the trap to the excited state and release energy to emit light [28]. Upon effective excitation (electron beams, X-rays, and UV light), the emitters absorb photons, are excited to a higher energy level and produce charge carriers such as electrons and holes [23, 46]. The charge carriers are then captured by traps in the crystal and can be stored for long periods. With thermal motion, optical stimulation, or other physical irritations, such as mechanical irritation, these captured charge carriers are slowly released from the trap to the ground state of the emitters, resulting in the phenomenon of PersL [47].

Fig. 3

Copyright 2021, Springer Nature

Schematic diagrams of the PersL mechanism under different excitation sources. Reproduced under terms of the CC-BY license [28].

The factors affecting the PersL are closely related to two elements: the emitter and the trap. The luminescent properties of PLNPs are usually effectively controlled from these two aspects through electronic structure engineering [45]. The characteristics of the trap affect the duration and intensity of PersL [48]. Improving the trapping efficiency of traps, such as setting the right density of the trap distribution and choosing the right trap depth, can help to increase the number of trapped electrons and thus store more excitation energy, which makes it possible to realize longer-lasting, high-quality luminescence. For example, in 2021, Li et al. replaced Zn²⁺ with Li⁺ in the Zn₂GeO₄ nanocrystals [49]. Li⁺ easily occupies the Zn²⁺ positions in the crystals because of their similar ionic radius (76 pm for Li⁺ and 74 pm for Zn²⁺). The afterglow intensity changed depending on the Li⁺ doping concentration and was positively correlated before the Li⁺ concentration was increased to 30%. More original electron traps and a new type of electron trap with deeper depth were observed, both of which contributed to the enhancement of the PersL. In addition, we can add external defects through the introduction of different phenomena, such as the addition of a dopant. When SrAl₂O₄ is doped with Eu²⁺, the afterglow is short-lasting and weak, but with the use of co-dopant Dy³⁺, both the afterglow time and the intensity of emission increase [50]. The nature of the emitters also influences the PersL phenomenon. Emitters should be able to emit radiation effectively after absorbing energy. Therefore, enhancing the nature of the emitter can help extend the PersL time and increase the luminous intensity. Currently, two main methods are increasing the size of PLNPs and protecting them via shell coating [48]. In terms of increasing size, Jing et al. reported a hydrothermal synthetic route for preparing differently sized ZnGa₂O₄:Cr nanoparticles (NPs). According to the particle size-dependent luminescence and luminescence decay curves, dimensional changes can effectively increase the intensity of the PersL [51]. Similarly, increasing the size of ZnGa₂O₄:Cr³⁺ (ZGO) or zinc gallogermanate (ZGGO) can also lead to an increase in the PersL response of PLNPs [52, 53]. For the method of shell coating, in 2021, Li et al. reported straightforward thermolysis-mediated colloidal synthesis of CaF₂:Dy@NaYF₄ core–shell PLNPs that can enhance PersL. Compared with the bare core CaF₂:Dy, the CaF₂:Dy core was well protected when the shell/core ratio increased to 0.5:1 and higher, accompanied by a 2–2.5 times increase in PersL efficacy [54].

Moreover, PLNPs can maintain luminescence after excitation even for days, a feature that allows imaging without in situ excitation [48]. Therefore, interferences from tissue autofluorescence are eliminated, which allows for an excellent signal-to-noise ratio (SNR) and significantly improved sensitivity, providing background-free signals for long-term bioimaging, such as cell tracing, highly sensitive in vivo imaging, and ultrasensitive detection [28, 55, 56].

Influence of excitation source on PersL

In the process of biomolecular detection and fluorescence imaging, when an external light source excites a fluorescent probe, the excitation light and the fluorescence emitted by the probe are partially scattered and absorbed by biological tissues. If the scattering and absorption increase, then accordingly, the depth of penetration of light into the tissues decreases, and the intensity of the fluorescent signal produced decreases. However, PLNPs can be excited by a variety of excitation sources, such as UV, light-emitting diodes (LEDs), NIR lasers, X-rays, and radiopharmaceuticals, which overcomes the problems of poor imaging quality and penetration depth caused by short-wavelength excitation (Table 2) [28].

Table 2 Comparison of the comparison of the penetration depth of different excitation sources

Excitation sources are usually categorized into UV light, the visible region, the first NIR window (NIR-I, 700–900 nm), and the second NIR window (NIR-II, 1000–1700 nm) regions based on the wavelength [62]. Research has shown that the absorption coefficient of water increases in the NIR region, but at the same time, the scattering coefficient decreases for different types of tissues. Compared with those of visible light, the absorption coefficients of the main light-absorbing substances, such as water, hemoglobin, and lipids, are lower, which greatly increases the penetration depth of NIR light into tissues and avoids the problems of scattering generated by organisms [63]. Therefore, the wavelength range of the NIR region is called the “optically transparent window” of biological tissues, which refers to the wavelength range where the penetration depth of light in biological tissues reaches the maximum value, and it is the key region for obtaining high sensitivity, high resolution, and in vivo imaging effects. When the emission wavelength of the PLNPs is adjusted to the NIR region, the effects of low scattering and high detection depth are obtained.

However, even in the NIR region, penetration into tissues is limited, preventing in vivo excitation. Another approach is to use other excitation sources, such as X-rays. Compared with conventional excitation sources (UV/visible/NIR), X-rays offer the advantages of weaker scattering, deeper tissue penetration, and simplified image reconstruction from tomography. This allows us to achieve deeper tissue imaging and better spatial resolution [59].

Optical properties of novel PLNPs

With the further study of PersL and the rapid development of persistent phosphors, many now PLNP materials have been developed.

Metal–organic frameworks (MOFs) are emerging porous materials constructed via self-assembly of metal ions or clusters and multifunctional organic ligands. Owing to their tunable porosity structure, high specific surface area, low cost, various interaction sites, and well-designed functionalities, they have been widely used in molecular storage, catalysis, separation, optoelectronics, chemical/physical sensing, bioimaging, and oncology therapy [64]. In particular, it is favorable for the encapsulation of a wide range of molecules, including various drugs. When MOFs are combined with PLNPs, precise imaging and drug delivery can be achieved at the same time, which is promising for tumor therapy. For example, in 2019, Zhao et al. produced PLMOF (PLNPs@ZIF-8) via in situ growth of MOF on PLNPs via a surface adsorption-induced self-assembly method. This combination enables PersL without external irradiation, providing deep tissue penetration and background-free interference imaging. Moreover, the remarkable drug-carrying capacity of the porous framework ZIF-8 results in drug release at the tumor site. It provides a good platform for precision tumor treatment [65]. In 2020, Yang et al. coated a PLNP surface with MOF UiO-66 to form a PLNP@UiO-66 core‒shell composite, followed by calcination, and synthesized a macrophage membrane coated with PersL nanoparticle@MOF-derived mesoporous carbon core‒shell nanocomposites (PLMCs) via the pyrolysis of UiO-66 shells. This compound shows great potential in drug encapsulation and luminescence imaging-guided chemotherapy [66]. In 2023, Zhao et al. reported a PersL MOF (PLNPs@MIL-100(Fe)) with a core–shell structure through a layer-by-layer method. Animal studies have shown that the PLNP core is capable of PersL without outside radiation, and the MIL-100(Fe) shell structure demonstrated high drug loading and controllable drug release capacity (Fig. 4) [67]. However, novel PLNP materials based on MOFs still face some challenges that need to be overcome. Controlling their shape and size is still challenging, but the size, morphology and structure of MOFs affect the luminescent properties of materials.

Fig. 4

Copyright 2023, Royal Society of Chemistry

Schematic illustration for the preparation of PLNPs@MIL-100(Fe) NPs for PersL/magnetic resonance (MR) dual-modal imaging and drug delivery. Reproduced with permission [67].

NIR PLNPs are emerging optical materials. They can emit NIR light for a long time after excitation and prevent autofluorescence. Furthermore, as mentioned above, the wavelength range of the NIR is within the “optically transparent window” of biological tissues, which allows for greater depth of tissue penetration, thus improving the sensitivity of biosensing and bioimaging. For example, in 2017, Zeng et al. proposed a new type of X-ray-activated ZnGa₂O₄:Cr PLNPs (X-PLNPs) which exhibited long-lasting up to 6 h NIR emission after excitation. Through the injection or oral administration of X-PLNPs after in situ activation, long-term deep-tissue in vivo bioimaging capabilities were demonstrated, which provided a new approach for overcoming the short decay time of PLNPs [57]. Cr³⁺-doped ZnGa₂O₄ (ZGC) NPs are among the most representative NIR PLNPs. They have attracted widespread attention for their excellent PersL properties, high chemical stability, and variable excitation light sources [68]. In 2017, Huang et al. reported SiO₂@ZnGa₂O4:Cr³⁺@SiO₂ PLNPs with high luminescence intensity and a long afterglow time through a silica template method. The PLNPs not only enhanced the NIR PersL properties but could also be repeatedly activated in situ by soft X-rays and continue to emit NIR for up to 12 days [69]. In 2020, Zhao et al. prepared PLNPs (Zn_1.1Ga_1.8Ge_0.1O4:Eu_0.009, Cr_0.09) via a hydrothermal method and combined them with alginate hydrogel to create PLNPs-containing hydrogels (PL-gel) for tumor-specific labeling and tracking of tumor metastases without autofluorescence. When combined with 4-carboxyphenylboronic acid, they can target MBA-MD-231 breast cancer cells, providing an effective way of revealing the tumor metastasis process [70]. Many current NIR PLNPs are limited to single-wavelength emission, and the PersL of PLNPs can be applied only to bioimaging or PDT; most of the time, the two cannot be effectively realized at the same time. Therefore, the design of nanoplatforms with multifunctionality for both bioimaging and PDT is important for the precision treatment of tumors. In 2022, Sun et al. proposed X-ray-excited PLNPs (Zn₃Ga₂Ge₂O10:Cr³⁺, Mn²⁺, ZGGCM) with dual functions for in vivo PersL imaging and PDT. These results indicated that Cr³⁺ ions in ZGGCM provided good NIR PersL imaging performance, whereas the green PersL emission peak of Mn²⁺ ions effectively promoted the generation of ROS to achieve significant antitumor effects. This nanomaterial facilitates the development of an integrated tumor diagnostic and therapeutic approach [71]. In 2024, Abdukayum et al. combined PLNPs (LiGa_4.99O₈:Cr_0.01, LGO: Cr) with iridium oxide NPs (IrO₂ NPs) covalently to produce multifunctional nanoplatforms with simultaneous NIR PersL, PDT and PTT effects (Fig. 5) [72].

Fig. 5

Copyright 2024, Royal Society of Chemistry

Schematic construction of a multifunctional nanoplatform of LGO:Cr/IrO2 with NIR sustained luminescence, and “afterglow” PDT and PTT capabilities. Reproduced with permission [72].

Biocompatibility of PLNPs

At present, PLNPs for biomedical applications are biocompatible because of their prolonged retention in normal tissue. Thus, a good understanding of the pharmacokinetics and biosafety of PLNPs in biological systems can greatly promote the biomedical applications of PLNPs for future clinical translation. To date, multiple cell lines have been used to evaluate the in vitro cytotoxicity of different PLNPs. Most of the results indicated that the PLNPs had no obvious cytotoxicity. Jing-Min Liu et al. incubated three cell types, Balb/3T3, HeLa and MCF-7, with PLNPs at concentrations ranging from 50 to 1000 μg·mL⁻¹ for 24 h. The viability was determined to be greater than 85% after incubation, confirming the relatively low toxicity of the PLNP nanocarriers [73].

Biomembrane-modified PLNPs can effectively enhance the biocompatibility of PLNPs [22]. Li et al. studied the in vitro cytotoxicity of CCM-coated PLNPs, and the PDT-controlled cytotoxicity of DSPLNPs@hSiO₂@CCM was evaluated in 4T1 cells. CCM, SPLNPs@hSiO₂, and SPLNPs@hSiO2@CCM up to 50 μg·mL^–1 showed negligible cytotoxicity and did not cause pathological changes in organs [74].

In 2021, Wang et al. synthesized ultrasmall PLNPs with an average diameter of 2.5 nm using MSNs as the template [75]. After further coating with polyethylene glycol (PEG), the diameter of the PLNPs-PEG increased to 5.0 nm and exhibited excellent PersL performance under 254 nm UV light irradiation. After injection into live nude mice via the tail vein, PLNPs-PEG was effectively metabolized by the kidneys within 24 h and did not cause damage or inflammatory changes in the main organs (heart, liver, spleen, lung, kidney, or intestines) of the mice after 14 days.

The possible mechanisms of tissue damage caused by NPs include cell morphology and cytoskeleton changes, oxidative stress, genotoxicity, and tissue damage caused by the illumination requirements of NPs application [76, 77]. Tissue damage caused by long-term in situ excitation light sources can be avoided because of the long-term luminescence of PLNPs after excitation. The mechanism of oxidative stress involves the recognition and response of NPs as foreign substances by immune system cells, resulting in the production of ROS, and high concentrations of ROS for a long period of time mediate damage to or death of various types of cells [78, 79]. There are lots of studies on the effect of PLNPs on biological safety via ROS-related mechanisms, and ROS levels can be assessed via the use of fluorescent probes such as dihydrofluorescein and its derivatives. The process of NPs uptake by cells and the occupation of the intracellular volume may cause changes in cell morphology and the cytoskeletal network [80]. Gene level changes are based on mechanisms such as high levels of ROS or induced cellular stress induced by NPs [81]. There are relatively few studies on the genotoxicity and morphological effects of PLNPs, and further research is needed.

In order to reduce tissue damage, surface modification is used to increase the biocompatibility of PLNPs. In 2017, Ramírez-García et al. evaluated the acute (24 h), short-term (30 days) and long-term (6 months) toxicological effects of PLNPs in vivo in terms of oxidative stress, cell and tissue morphology, and DNA levels, aiming to verify the effects of surface modification on the biocompatibility of PLNPs [82]. They modified ZnGa_1.995Cr_0.005O₄ by hydroxylation and PEGylation to obtain ZGO-OH and ZGO-PEG, respectively. They were injected into BALB/c mice via the tail vein. An extremely high concentration of 8 mg/mouse caused weight loss, a significant increase in white blood cells, and liver tissue damage caused by oxidative stress in the mice. All other concentrations did not result in abnormal parameters throughout the 6 months. These results demonstrated the protective effect of PEG functionalization of PLNPs and the safety of the application of PLNPs in vivo.

Application of PLNPs in tumor imaging

Tumor imaging of PLNPs based on cellular and biomolecular levels

PLNPs exhibit excellent optical properties, high sensitivity and good biocompatibility, thus offering unique advantages in the selective imaging of tumor cells in vitro. To target tumor cells, various ligands, such as biotin (BT), folic acid (FA), antibodies, aptamers and peptides, have been anchored on the surface of PLNPs [83–86]. In 2020, Yang et al. prepared NIR-emitting PLNPs (Zn_1.1Ga_1.8G_e0.1O₄:Eu_0.009, Cr_0.09) through a hydrothermal method and then modified them with 4-carboxyl phenylboronic acid, which is specific to the sialic acid receptor of MDA-MB-231 metastatic breast cancer cells. Compared with smooth muscle cells and macrophages, tumor cells exhibit significantly stronger luminescence [70]. In 2021, aptamer-containing DNA was conjugated with Au-coated PLNPs to target the overexpressed nucleolin of HeLa cells (Fig. 6a-c) [87]. In 2022, Song et al. synthesized zinc-doped silica nanospheres (Zn@SiNSs) with ultralong phosphorescence via a hydrothermal method. RGD (Arg-Gly-Asp) peptides were functionalized on Zn@SiNSs, which could target highly expressed integrins of human glioblastoma cells and achieve a high signal-to-background ratio (≈69) imaging [88]. Introducing dual-targeting ligands to PLNPs is attractive for improving their specificity toward tumor cells. For example, ZnGa₂O₄:Cr,B PLNPs synthesized via a hydrothermal method were modified with hyaluronic acid (HA) and FA to provide synergistic targeting effects, which led to brighter luminescence signals than single- or nontargeting ligand-functionalized PLNPs in the NIR region after the imaging of MCF-7 breast cancer cells [89].

Fig. 6

Copyright 2021, Elsevier. d Illustration of sensing exosomes using afterglow signal of the nanocomplex (ASPNC). e Fluorescence and afterglow spectra of ASPNC with and without exosomes. Reproduced with permission [90]. Copyright 2019, Wiley

a Confocal laser scanning microscope (CLSM) images of the HeLa and 3T3 cells treated with Au/PLNP, DNA-Au/PLNP and tetrakis (4-carboxyphenyl) porphyrin (TCPP) and G-quadruplex structure (GDNA) -Au/PLNP (TCPP-GDNA-Au/PLNP). b Relative luminescence intensity of the HeLa and 3T3 cells treated with Au/PLNP, DNA-Au/PLNP and TCPP-GDNA-Au/PLNP (The detailed red signal intensity was calculated by Image J software with the same amount of cells and the luminescence intensity of the cells treated with Au/PLNP was taken as 1). c Cell viability of the HeLa treated with Au/PLNP (a), DNA-Au/PLNP (b) and TCPP-GDNA-Au/PLNP (c) with or without 650 nm LED irradiation. (D) CLSM images of HeLa cells incubated with Au/PLNP, DNA-Au/PLNP and TCPP-GDNA-Au/PLNP with or without 650 nm LED irradiation. Reproduced with permission [87].

PLNPs can also serve as nanoprobes for imaging signal molecules in tumor cells. H₂S has been proven to be an important endogenous signaling molecule that plays a role in tumor development. In 2020, Suzuki et al. reported NIR organic PLNPs for imaging H₂S in hepatic cancer cells (HCCs). They employed EM F1²⁺ as a H₂S-responsive chromophore and integrated it with the photosensitizers NIR775 and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]. After specific ligands are introduced on the surface, this probe can efficiently enter HCCs, and the afterglow intensity increases with increasing intracellular H₂S levels [91]. Epidermal growth factor receptor (EGFR) activation is related to various tumor biological processes, such as tumor cell proliferation, angiogenesis and metastasis. In 2022, Sun et al. developed a PLNP-based probe to observe EGFR-related signaling pathways in lung cancer cells. The electron transfer from the ligand of EGFR to ZnGa₂O₄ PLNPs mediated by the ferrocene-DNA polymer chain could trigger afterglow signals, and this probe allowed dynamic monitoring of the EGFR signaling pathway during tumor cell division [92].

Many sensors based on PLNPs have also been developed for in vitro detection of tumor markers because they show great advantages in eliminating the background fluorescence noise of complex samples. These biosensors often involve fluorescence resonance energy transfer (FRET) from PLNPs to the receptor. Quenchers are commonly used as receptors to quench luminescence. When tumor markers are present as competitors, the FRET system is destroyed, and the luminescence signal is restored. For example, in 2020, Zhao et al. doped Mn²⁺ in Zn₂GeO₄ to prepare green-emitted ZGO:Mn PLNPs and doped Cr³⁺ in ZnGa₂O₄ to prepare NIR-emitted ZGC PLNPs. Aptamers of prostate-specific antigen (PSA) and carcinoembryonic antigen (CEA) were subsequently functionalized on the surfaces of the ZGO:Mn and ZGC PLNPs, respectively. The luminescence signals of the two PLNPs were quenched by PDA NPs and recovered in the presence of PSA and CEA, which provided a new method for high-throughput tumor marker detection in serum samples [93]. In another study, Yuan et al. presented a PLNPs-based biochip for the sensitive detection of bladder cancer-related miRNA-21 (miR-21). The ZGO:Mn PLNPs were conjugated with single-stranded DNAs (cDNAs) complementary to miR-21 and further hybridized with black-hole-quencher-labeled (BHQ) DNAs (BHQ-DNAs). When samples containing miR-21 were added, BHQ-DNAs were separated from the PLNPs because of the specific binding between the cDNAs and miR-21, thus turning on the afterglow signals. In addition, this luminescence probe was linked to the photonic crystal substrate to amplify luminescence signals, which could achieve a detection limit of 26.3 fM in human urine [94]. Interestingly, Pu et al. also applied PLNPs for the detection of tumor exosomes. They synthesized a poly(p-phenylene viny-lene)-based NIR afterglow semiconducting polyelectrolyte was passed and employed a BHQ-tagged aptamer as the quencher. Similarly, exosomes could trigger the recovery of luminescence signals, and the limit of detection was nearly two orders of magnitude lower than that of common fluorescence detection (Fig. 6d–e). This sensor was able to distinguish exosomes from different cells and could be used for the identification of tumor cells [90]. In addition to the above quenchers, fluorescent molecules can also be used as receptors for PLNPs. In 2015, Yan’s group used a PSA antibody (PS6)-modified PLNP as the energy donor and another PSA antibody (8A6)-modified rhodamine B (RhB) as the acceptor to construct a fluorescent probe. The specific binding of PSA with antibodies mediated FRET from PLNP-PS6 to RhB-8A6 and enabled the PLNPs to excite RhB, thus realizing sensitive ratiometric photoluminescent detection of PSA in serum and cell extracts [95].

Tumor imaging of PLNPs in vivo

Direct modification of relevant targeting molecules on PLNPs can result in direct targeting of tumor cells, and the NPs used for tumor labeling must possess specific characteristics and be considered for practical application in vivo. The appropriate dimensions and morphology significantly affect their distribution and uptake within the tumor tissue. Surface modification is crucial to ensure targeting ability in the tumor microenvironment. Optical properties are also important for high-resolution imaging and precise localization within tumor tissue. Biocompatibility is essential for toxicity or immune reactions. The targeting capability can be achieved through the incorporation of molecules that specifically bind to imaging specificity. Moreover, achieving efficient conversion of light into heat while maintaining thermal stability is necessary. Finally, it is crucial for NPs to exhibit favorable solubility and stability under physiological conditions to ensure prolonged circulation time and accurate imaging capabilities within the tumor tissue [96].

PLNPs are capable of absorbing and retaining excitation energy, subsequently releasing it as PersL, which persists for an extended duration following cessation of excitation. They allow in vivo imaging without autofluorescence, with a high target-to-background ratio and no need for in situ excitation. Thus, it has significant advantages in in vivo tumor imaging. In 2024, Liu et al. unveiled a multifunctional nanoplatform based on LiGa_4.99O₈:Cr_0.01/IrO₂, which exhibits diverse functionalities, including NIR PersL, photodynamics, and PTT. This platform is formed through covalent bonding between a persistent luminescent nanoparticle LiGa_4.99O₈:Cr_0.01 and an iridium oxide nanoparticle IrO₂, enabling the generation of a laser-induced photothermal effect at 808 nm as well as sustained photodynamic effects without in situ excitation due to persistent energy transfer (Fig. 7) [72].

Fig. 7

Copyright 2024, Royal Society of Chemistry

Transmission Electron Microscope images of the a LGO:Cr, b IrO₂ NPs, and c LGO:Cr/IrO₂ composites. d High Resolution Transmission Electron Microscope images of LGO:Cr/IrO₂. e–j energy dispersive spectroscopy (EDS) elemental mapping of LGO:Cr/IrO₂. X-Ray Diffraction patterns of k LGO:Cr, IrO₂ NPs, and LGO:Cr/IrO₂ composites; and l EDS analysis of LGO:Cr/IrO₂. m Fourier Transform Infrared Spectroscopy spectra of LGO:Cr–Si–Cl, IrO –NH, and LGO:Cr/IrO. n Zeta potentials of LGO:Cr and IrO NPs with different surface modifications. Reproduced with permission [72].

For the majority of NPs, in the absence of a specific surface coating, they undergo rapid opsonization and are subsequently captured by the liver following systemic administration in small animal models. In 2020, Liu et al. focused on the coating of PLNPs with hydrophilic polymers for in vivo imaging. A highly hydrophilic polymer of poly(N-2-hydroxypropyl) methacrylamide (pHPMA) has the ability to delay capture by the liver and prolong circulation in the bloodstream. Finally, preliminary in vivo imaging was performed in healthy mice injected with PLNPs encapsulated with pHPMA, where luminescent signals could be detected throughout the animal for up to 60 min, demonstrating the ability of the material to circulate in the bloodstream (Fig. 8) [97]. The excitation bands of previous PLNPs were typically located during UV region, leading to significant tissue damage and limited tissue permeability [45]. As mentioned above, NIR PLNPs have shown low scattering and absorption coefficients in penetrating biological organs or tissues and do not require continuous excitation light irradiation, which can effectively reduce the damage caused by light irradiation to tissues. Additionally, the reactivation of PersL in NIR PLNPs can be achieved via red or NIR light. This characteristic enables the utilization of NIR-emitting PLNPs for long-term bioimaging without being constrained by the limited lifetime of PersL [32, 98]. In 2013, Abdukayum et al. prepared Zn_2.94Ga_1.96Ge₂O₁₀:Cr³⁺, Pr³⁺ PLNPs with NIR luminescence through co-doping of Pr³⁺ and Cr³⁺. Additionally, they improved the biocompatibility and water solubility of PLNPs through the incorporation of modified PEG. These PLNPs demonstrate an exceptionally prolonged afterglow duration, making them highly promising for extended in vivo targeted tumor imaging when combined with peptides. By chemically linking the RGD peptide of c through covalent conjugation, the increased binding affinity of PLNPs leads to an extended period of targeted tumor imaging in vivo [55]. HA receptors are commonly overexpressed on the surface of certain cancer cells. Similarly, FA receptors are similarly overexpressed in various types of cancers [99, 100]. In 2016, Zhao et al. modified ZnGa₂O₄ NPs with chromium (Cr³⁺) and boron (B³⁺) ions and leveraged the specific binding affinities of ligands to target both HA and FA receptors on tumor cell surfaces. The combination of a dual-targeting strategy with the prolonged luminescence properties provided by the Cr³⁺ and B³⁺ co-doping system achieved high-resolution imaging of tumor cells with enhanced specificity and reduced false positives. The NPs exhibited excellent biocompatibility, and their hydrophilic surface modification improved the circulation time, thereby increasing their uptake in the target tumor tissue [22].

Fig. 8

In vivo biodistribution of coated ZGO in healthy mice. Reproduced under terms of the CC-BY license [97]. Copyright 2020, Frontiers Media S.A

Finally, Table 3 the applications of the share of PLNPs mentioned above for re-tumor imaging, including their materials, absorption wavelengths, emission wavelengths, and imaging applications.

Table 3 A summary of published works about PLNPs-based tumor imaging

Application of PLNPs in cancer treatment

Photodynamic therapy based on PLNPs

At present, the applications of PLNPs in tumor therapy mainly include PDT, PTT, fluorescence-guided surgery and targeted drug delivery [22, 101]. Among them, PDT refers to the use of photosensitizers to generate ROS under specific wavelengths of irradiation, thereby killing tumor cells without damaging normal cells [102]. Currently, commonly used photosensitizers for PDT include porphyrins, chlorins, and phthalocyanines [103]. However, the application of traditional photosensitizers requires long-term light activation, which may lead to cell damage due to overheating [104]. Therefore, PLNPs with continuous luminescence after light excitation can reduce the applied dose of laser and minimize the adverse effects of light irradiation, which makes PLNPs ideal for application in PDT [23]. In addition, PLNPs show ideal biocompatibility, as mentioned above, which means that PLNPs do not significantly affect the viability of normal cells or tumor cells in the absence of light excitation. Under laser irradiation, PLNPs exhibit ideal anti-tumor properties. In addition, some PLNPs exhibit NIR optical properties. In biological imaging and PDT applications, NIR light nanomaterials have obvious advantages, as described above [105]. In 2016, Abdurahman et al. prepared Cr³⁺-doped ZGGO (ZGGO:Cr³⁺) as a NIR-excited PLNPs via a hydrothermal method and combined it with the photosensitizer silicon phthalocyanine (Si-Pc) (Fig. 9) [106]. Si-Pc-PLNPs showed the ability to continuously generate ROS at an excitation wavelength of 808 nm, resulting in a killing effect on HepG2 cells at 200 µg·ml⁻¹. In 2017, Fan et al. prepared ZGC-PLNPs via a one-pot hydrothermal method and then combined them with poly(lactic-co-glycolic acid)/N-methylpyrrolidone oleosol to obtain injectable PersL implants, which further improved the PersL intensity and lifetime [107]. In subsequent in vivo, the PersL implants changed from liquid to solid, which prolonged the retention time of the material in the tumor area of U87MG tumor-bearing mice. In addition, the PersL implants not only showed better imaging effects in vivo than ZGC but also produced ROS under LED irradiation. In 2018, Wang et al. prepared NIR PLNPs with hollow structures using a carbon sphere template as a template to carry the chemical drug DOX and the photosensitizer Si-Pc, which showed good in vitro and in vivo tumor killing effects via the production of singlet oxygen in PDT and the release of DOX [32].

Fig. 9

Copyright 2016, Royal Society of Chemistry

Schematic illustration for the design of the 808 nm NIR light renewable PersL sensitized PDT platform. Reproduced with permission [106].

Photothermal therapy based on PLNPs

Like PDT, PTT is also a common antitumor method that uses photoabsorbers to convert laser energy into heat under light irradiation for tumor cell killing (Table 4) [28]. In addition, the application of NIR in PTT has the advantages of strong tissue penetration and harmlessness. In 2017, Chen et al. combined NIR PLNPs of ZnGa₂O₄:Cr³⁺ (ZGC) and indocyanine green (ICG) into MSNs (Fig. 10) [108]. Among them, ZGC emits NIR light under UV light excitation for in vivo imaging. Moreover, the NIR light was absorbed by ICG and converted into heat, indicating an ideal antitumor effect of PTT in vivo. In 2021, Liu et al. used a hydrothermal method to prepare ZnGa₂O₄:Cr PLNP with two dual emission peaks at 508 nm and 714 nm for 1 h [109]. In addition, the authors used the strong adhesion of PDA and the electrostatic adsorption of 2,3-dimethylmaleic anhydride (DMMA) to functionalize PEG for the grafting of DOX. Under 808 nm laser irradiation, PLNP@PDA@DMMA/DOX showed strong tumor-killing effects both in vivo and in vitro, exerting dual anti-tumor effects of chemotherapy and PTT. In addition, the construction of dual-tumor effects of PDT and PTT based on PLNPs has also attracted widespread attention. In 2021, Zhao et al. prepared Zn_1.25Ga_1.5Ge_0.25O₄:0.5%Cr³⁺, 2.5%Yb³⁺, and 0.25%Er³⁺ PLNPs via a hydrothermal method combined with the pH-reversibly responsive photosensitizer brominated asymmetric cyanine (BAC) and BT-functionalized PEG to prepare a multifunctional nanoplatform [110]. Under conditions of pH 6.0 and 808 nm laser irradiation for 10 min, obvious temperature changes and significant ¹O₂ generation contributed to the effective in vitro and in vivo killing of human lung adenocarcinoma A549 cells and human cervical cancer HeLa cells.

Table 4 Comparison of the mechanism of PLNPs in PDT/PTT

Fig. 10

Copyright 2017, Elsevier

A schematic illustration shows applications of the NPs for persistent luminescent imaging-guided PTT in vivo. In this schematic illustration, the ICG and PersL phosphors co-loaded mesoporous silica nanoparticle. Reproduced with permission [108].

Fluorescence-guided surgery based on PLNPs

With the increasing demand for tumor diagnosis and treatment, in order to achieve high treatment efficiency and few side effects, a biomedical system that integrates biological imaging and treatment into a single nanoplatform has been constructed for imaging-guided treatment. Bioimaging probes are among the most important parts of a treatment system and are responsible for identifying the location of tumor tissue, monitoring the biological distribution of nanoplatforms, and evaluating treatment effectiveness. Therefore, fluorescence-guided surgery (FGS), a surgical procedure guided by real-time fluorescence images of diseases, provides a less expensive and simpler method for precise tumor resection [111–113].

In recent years, PLNPs have been positively involved in theranostic studies, as PersL can be used to determine the accurate position and time of therapy, allegedly “imaging-guided therapy”. Owing to the versatile surface functions of PLNPs, photothermal agents and photosensitizers can be easily loaded onto PLNP nanoplatforms for PersL imaging-guided therapy, demonstrating outstanding advantages in this field. In addition, PLNPs can be excited by different excitation sources, including UV, LED, NIR laser, X-ray and radiopharmaceutical. The application of different excitation sources is helpful for overcoming the problems of poor imaging quality and penetration depth caused by shortwave excitation [114]. From multiple perspectives, the manufacturing of PLNPs based nanoprobes for biological applications should follow the following standards: (i) strong and stable initial luminescence, long afterglow, and repeatable regeneration; (ii) suitable size for in vivo applications (< 200 nm); (iii) Target trigger signal changes for biosensing; (iv) The passive or active targeting ability of biological imaging; (v) The specific multifunctionality of therapeutics [29, 44, 115]. In 2011, Sun et al. prepared PLNPs nanoprobe based on target-induced FRET system interruption for the detection of α-fetal protein (Fig. 11) [116]. PLNPs are also suitable for fluorescence-guided surgery. For example, in 2019, Ni et al. synthesized a NIR afterglow luminescent nanoparticle with aggregation-induced emission (AIE) characteristics of rapid PersL signal quenching in normal tissues and an ultrahigh tumor-to-liver signal ratio [117]. These fascinating features make AIE PLNPs excellent imaging-guided probes for the resection of peritoneal cancer. In 2018, Tian et al. used ZnGa₂O₄:Cr³⁺ for long-term image-guided surgery of hepatocellular carcinoma (HCC). Interestingly, ZnGa₂O₄:Cr³⁺ can be taken up by normal liver tissue but not by HCC tumor tissue, allowing for precise localization after more radical resection [118]. These findings indicate that PLNPs can further contribute to areas such as the study of complex molecular networks and the construction of guiding systems for surgery.

Fig. 11

Copyright 2018, American Chemical Society

Engineering strategies of PLNPs for biological applications. Reproduced with permission [116].

In addition, in contrast to traditional fluorescent agents such as semiconductor quantum dots, upconversion nanoparticles, and organic dyes, the optical imaging of PLNPs can eliminate real-time excitation from external light sources, completely avoiding interference from their own fluorescence and achieving high sensitivity in vivo imaging. In 2023, Lin et al. constructed mZGS@Mn-AMD, an image‐guided tumor resection. The MnO₂ shell quenches PersL in normal tissue and is degraded in the TME. Owing to the improved sensitivity of tumor imaging, multiple types of residual tumor tissue can be successfully recognized via PersL imaging [119].

Targeted anti-tumor drug delivery based on PLNPs

Chemotherapy is widely used in clinical cancer treatment, but its poor targeting ability can result in severe toxic side effects, such as dose-limiting side effects. The clinical manifestations of chemotherapy-induced peripheral neuropathy include pain, numbness, proprioceptive deficits, muscle weakness, falls, dizziness, headache, and even paralysis [120]. Fortunately, PLNPs have been integrated into various drug delivery systems to achieve precise drug release and real-time monitoring, avoiding dose-dependent side effects.

Liposomes have been extensively used as nanocarriers owing to their excellent biocompatibility, low toxicity and biodegradability.

In 2017, Yan et al. used liposome-coated PLNPs as carriers of the drug paclitaxel (PTX) to inhibit the growth of human breast cancer (MCF-7) with simultaneous luminescence signals [121]. MSNs give PLNPs an easy-to-modify surface, which allows PLNPs to bind with various functional molecules, metal nanomaterials and cell membranes, which enables the excellent targeting ability and immune escape of PLNPs. MSNs with high pore volume are another commonly used drug carrier. A Lipo-PLNP-PTX nano-system constructed on the basis of this mechanism achieved an ideal PTX encapsulation efficiency as high as 69.2%, with excellent PTX release. Lipo-PLNP-PTX achieved a high PTX release efficiency of 98.9% within 24 h in 50% human serum, while the drug release rate of Lipo-PLNP-PTX in PBS at 4 °C was only 10.5%, which indicates the good stability of Lipo-PLNP-PTX under storage conditions. Richard et al. embedded ZnGa_1.995Cr_0.005O₄ PLNPs in an MSN shell and then loaded them with the antitumor drug DOX. This drug-loaded probe can release DOX in a pH-sensitive manner, and its distribution can be easily monitored in vivo [122]. To further improve the targeting ability, Yan et al. conjugated the pH-low-insertion peptide to MSN-coated PLNPs via the GFLG peptide and disulfide bonds to target the acidic tumor microenvironment. In addition, GFLG and disulfide bonds are two switches that can be specifically opened by highly expressed CB and GSH in tumor cells. Because of this principle, the MSPLNPs triggered the release of loaded DOX in a PBS solution containing CB and GSH, achieving a drug release efficiency of more than 90% within 8 h to effectively inhibit the growth of tumors [123].

In 2019, Wang et al. synthesized MSN-coated zinc gallogermanate PLNPs to load the colorectal cancer chemotherapeutic drug 5-FU and then coated them with Lactobacillus reuteri biofilms, which helped target the colorectum tumor and prevent the digestion of gastric acid [124]. Moreover, the porous framework ZIF-8 also shows great potential as a carrier for anti-tumor drugs. In 2019, Zhao et al. prepared PLNP@ZIF-8 via a surface adsorption-induced self-assembly strategy, which presented a high DOX loading capacity. More importantly, the acidic tumor microenvironment activated the disassembly of ZIF-8, thus promoting DOX release and enhancing the luminescence of the PLNPs [65].

Synergistic effects of imaging and therapy based on PLNPs

Combination of PLNPs imaging and PDT application

Many studies have investigated the application of PLNPs in tumor imaging and antitumor therapy. In addition, the combination of the performance of PLNPs in imaging and PDT-induced tumor killing has been used to achieve ideal tumor diagnosis and treatment effects. In 2017, Chen et al. used hydrothermal method to prepare LiGa₅O₈: Cr (LGO: Cr) PLNPs, which were combined with the photosensitizers 2,3-naphthalocyanine (NC) and mesoporous silica nanoparticles to obtain NC-LGO: Cr@mSiO₂ [125]. Under X-ray excitation, NC-LGO: Cr@mSiO₂ exhibited 2 h of NIR-PersL excitation with a penetration depth more than 1.5 cm. In vivo, long-term repeatable imaging in nude mice was confirmed. The overlap of the excitation wavelengths of the NCs and the emission wavelengths of LGO: Cr caused the material to generate ¹O₂ under X-ray excitation, thus realizing the effective killing of H1299 human non-small cell lung cancer cells in vitro and the PDT treatment of lung cancer in vivo. In 2020, Shi et al. combined tin-doped ZGSPLNPs with the photosensitizer ZnPC₄ to prepare NIR-PersL ZnPcS₄ [126]. It showed an ideal imaging function for deep tissues in vivo and could induce ZnPC₄ to produce ¹O₂ under the excitation of a 659 nm LED, suggesting the construction of an excellent NP system for killing tumors by combining deep imaging with PDT.

Combined application of PLNPs imaging and chemotherapy

In addition to PDT applications, research on the combination of chemotherapy and PLNPs imaging has made remarkable progress. In 2019, Su et al. modified Zn_1.1Ga_1.8Ge_0.1O₄:0.5%Cr³⁺, 0.5%Eu³⁺ PLNPs with dopamine (PDA), polyethyleneimine (PEI), folic acid (FA) and doxorubicin (DOX) and realized siRNA loading [127]. The prepared PPP-FA-DOX-siRNA could target the DOX-resistant human breast cancer cell line MCF-7 in vitro and effectively release DOX in an acidic environment. They effectively killed tumor cells via PTT under irradiation with an 808 nm laser, thus realizing the triple collaborative diagnosis and treatment of targeted imaging labeling, drug delivery and PTT. In 2021, Zou et al. prepared NIR-PersL ZnGa₂O₄:Cr³⁺, Sn⁴⁺ PLNPs via the template method and modified them with mesoporous silica, DOX and PEG [128]. The DOX-Mn-ZGOCS-PEG had good imaging ability for tumor-specific external magnetic resonance (MR) in vivo and in vitro and achieved excellent DOX release and anticancer effects in the tumor microenvironment, thus achieving effective chemotherapy under the guidance of tumor microenvironment-targeted MR/NIR-PersL imaging.

Future perspectives

This review summarizes the synthesis, modification and research progress of PLNP in targeted tumor imaging and tumor killing (Table 5). At present, some progress has been made in the application of PLNPs in targeted biological imaging and killing of tumors, but many aspects of further research are needed in the future.

Table 5 Comparison of PersL duration and Application of PLNPs

Perspectives in the synthesis and modification of PLNPs

This review first explains the necessity of synthesizing nanoscale PLNPs for antitumor applications and the advantages of high imaging sensitivity and ease of surface functionalization over micrometer-sized PLNPs. The three main methods used to prepare PLNPs include the sol‒gel method, the precisely controlled templated method, and surface modification, which is a convenient hydrothermal method. Surface functionalization plays a pivotal role in enhancing the biocompatibility and functionality of PLNPs. Coatings of mesoporous silica or PAA/CaP shells improve stability and enable targeted therapeutic applications. Additionally, PANI/GCS grafting creates pH-responsive platforms for precise biomedical imaging and therapy, and the integration of multiple modification strategies promises to further enhance the functionality of PLNPs. As a result, in terms of antitumor applications, PLNPs are used as optical carriers to track cancer cells, graft functional groups with antitumor effects, and undergo surface modification by coatings such as PDA, liposomes, and silica. Thus, nanosystems with long-lasting bioimaging and anti-tumor effects were obtained. In addition, various tumor markers, mainly immune checkpoint inhibitors of PD-1, have been utilized as effective tumor targets. However, continued research into advanced synthesis and surface functionalization methods will drive the development of PLNPs for increasingly sophisticated biomedical applications, including imaging, therapy, and theranostics.

Despite significant progress in the synthesis of PLNPs, there are still some issues with the morphological regulation of PLNPs. In terms of biomedical applications, more advanced synthesis methods are needed to precisely control the morphology, particle size, surface properties, PersL strength, and PersL time of PLNPs, in order to construct PLNPs nanoplatform with high-efficiency [129]. Synthesizing PLNP materials with uniform structures can enhance the active region of the sensing interface, and the use of microfluidic platforms with advantages such as high throughput and reproducibility may provide a superior solution to these challenges. In addition to the co-synthesis method, which combines several synthesis methods, the synthesized PLNPs also combine the advantages of various synthesis methods in terms of properties such as afterglow properties, particle size, and morphological aspects. This also provides a solution for synthesizing PLNPs with balanced properties. BesidesIn addition, EDTA etching of sintered PLNPs can effectively reduce their size and greatly improve water dispersion. Microwave-assisted heating effectively improves the efficiency of PLNPs synthesis, and the emergence of organic PLNPs also makes them promising for biomedical applications. However, problems such as the serious spin-forbidden barrier between single and triplet excited states and the serious non-radiative leaps of excitons in triplet states still seriously hinder the development of organic PLNPs, and researchers are gradually exploring the prospects of organic PLNPs through the introduction of heavy atoms, crystal engineering and other strategies.

Perspectives in the optical properties and tumor imaging application of PLNPs

PLNPs avoid tissue autofluorescence and greatly improve the SNR, which has attracted great attention in tumor imaging. Considering the short afterglow time of UV preexcitation and the short penetration depth of visible light, increasing research has focused on NIR and X-ray excitation. NIR light results in improved deep tissue permeation and reduced photodamage, and it can release precharged energy in deep traps to achieve reactivation. However, during the NIR-recharging process of carrier redistribution from deeper traps to shallower traps, the depletion of carriers leads to decreased afterglow intensity. By creating “upconversion-like” trap-energy storage, carriers can be captured first by deep traps and then transferred to shallow traps, and such opposite energy transfer can result in more stable persistent luminescence [130]. To achieve repeated charging, novel systems should be designed to pump carriers from the luminescent center into shallow traps under NIR excitation. X-rays have also been used as an excitation source for deep penetration, but the radiation dose of X-rays must be carefully set to avoid potential safety risks. Radiopharmaceuticals such as 18F-fluorodeoxyglucose can efficiently excite fluorescence and afterglow luminescence via Cerenkov resonance energy transfer and ionizing radiation. Combining tumor-selective radiopharmaceuticals with PLNPs can provide a new way to image tumors with high sensitivity and long decay times.

To obtain novel PLNP materials with higher excitation efficiency and better PersL properties, future studies should focus on improving the response efficiency to red light, NIR light, and X-rays. Finally, although imaging with novel PLNP materials has high sensitivity, it cannot provide all the information needed for disease diagnosis. Therefore, it is necessary to combine these methods with other imaging modalities to provide more accurate and reliable information. In addition, the development of highly hydrophilic PLNPs that can target specific tissues or tumor cells through ligand modification represents a significant advancement in noninvasive and long-term in vivo imaging. This evolution from UV to NIR luminescence, along with the ability to reactivate PersL without continuous excitation, opens new avenues for safer and more effective imaging techniques that could greatly benefit medical diagnostics and research.

Design of multifunctional PLNPs with excellent tumor-killing potential for clinical application

In addition, in anti-tumor research, PLNPs have advantages such as ideal optical performance, PersL after removing excitation light and the exclusion of background fluorescence interference from in situ excitation. PLNPs also provide good antitumor performance via PTT, PDT, surgery, drug delivery, and other methods, with great progress in related fields. However, these methods also have certain limitations and need further research. First, most studies on the biocompatibility of PLNPs are based on the cytotoxicity level, and the elimination of toxic side effects on normal tissues at the gene, protein, and metabolic levels still needs to be verified. Second, PLNPs have superior optical performance, but the diagnostic information provided by imaging with PLNPs is limited, and the migration, transformation, and distribution of PLNPs during in vivo imaging still need to be studied. Hopefully, more studies could be conducted to develop PLNPs with excellent optic properties and explore their wide application in tumor diagnosis and treatment. In addition, the role of PLNPs in PTT and PDT therapy is usually to provide light excitation for photosensitizers. There are relatively few reports on the ability of PLNPs to generate ROS or heat energy under photoexcitation. Therefore, current nanomaterial systems based on PLNPs rely heavily on the performance of the photosensitizer to achieve PDT and PTT. Moreover, upconversion PLNPs, which emit high-energy light under low-energy light excitation conditions, exhibit superior deep tissue penetration ability [131, 132]. However, these methods are lacking in tumor imaging and treatment reports; thus, further research is urgently needed. In summary, tumor-targeted bioimaging and killing materials based on PLNPs are making rapid progress, but there is still much room for improvement in the preparation of this emerging field. It is hoped that PLNPs related materials can be further developed for precise and effective treatment of cancer in the future.

Current research on near-infrared PLNPs in the field of tumor imaging and therapy is promising but still faces multiple challenges. Limitations in material properties are the primary issue, as most PLNPs have a narrow dynamic range of afterglow signals. In addition, traditional PLNPs rely on in vitro UV excitation, making it difficult to achieve in vivo in situ cyclic charging, and the emission wavelengths of existing materials are mostly concentrated in the NIR-I window (650–950 nm), which still has a limited penetrating imaging effect on deep tumors. In terms of synthesis and biological application, inorganic PLNPs are complicated to synthesize and prone to large particles, whereas organic PLNPs can be tailored to the active site but have poor stability and low afterglow efficiency. Moreover, most PLNPs lack specific targeting modifications and rely on passive enrichment, whereas surface modifications may interfere with luminescence performance or trigger an immune response. Finally, the difficulty of integrating multimodal therapeutics further restricts their development: existing studies focus on a single function (e.g., long afterglow imaging or PTT/PDT) and lack the synergistic design of therapeutic integration, and the long-term toxicity, metabolic pathway, and standards for large-scale production of PLNPs have yet to be clarified, which leads to a significant barrier to clinical translation.

To address the aforementioned challenges, future studies should break through conventional paradigms and pursue interdisciplinary integration. The AI-guided design of PLNPs represents a critical direction of machine learning models; for example, generative adversarial networks can predict rare-earth ion doping ratios and surface modification schemes to optimize afterglow intensity and emission wavelengths. By integrating tumor-specific metabolic profiles (e.g., glycolytic activity), AI-driven platforms may further enable the customization of pH-responsive or enzyme-activated PLNPs while dynamically adjusting photothermal therapy parameters.

Another strategic breakthrough lies in multimodal theranostic platforms. For example, Wu et al. demonstrated the integration of long-afterglow materials with polypyrrole for dual-modal afterglow/photoacoustic imaging and photothermal therapy in breast cancer [133].

In situ self-energizing technologies could minimize the reliance on external excitation sources. Inspired by “wormhole-inspired” material design, in vivo cyclic recharging of PLNPs can be achieved via low-energy near-infrared light (740 nm) [130]. Concurrently, leveraging Fenton-like reactions to harness tumor microenvironment metabolites (e.g., H₂O₂) as excitation energy sources for PLNPs may increase the energy conversion efficiency.

Finally, advancing is imperative. Green synthesis approaches such as aqueous-phase reactions, clinical translation and standardization of o-templated protocols could reduce production costs. Coupling metabolomics with immunohistochemical profiling to establish comprehensive long-term toxicity evaluation frameworks will accelerate the transition to clinical trials.

Conclusion

In summary, PLNPs have been used for tumor-targeted imaging and killing, showing great application prospects in the field of tumor diagnosis and treatment. In terms of imaging, PLNPs have shown an ideal imaging effect and low degree of radiation damage, which is obviously superior to traditional tumor diagnosis methods, while avoiding the in situ excitation interference of commonly used fluorescent nanoimaging materials. Among them, NIR-emitting PLNPs show excellent penetration depth. In terms of tumor killing, PLNPs have achieved outstanding antitumor effects via PDT, PPT, FGS, drug delivery and their combined application. However, further modifications, the preparation of safer and more effective imaging PLNPs and the design of multifunctional PLNPs with excellent tumor killing potential for clinical application still need further study. It is hoped that more efforts will be made in the future to develop PLNPs with excellent imaging performance and explore their wide application in tumor diagnosis and treatment.