1 Introduction
Long afterglow phosphors(PLNPs)are a kind of special materials that can produce persistent luminescence after being excited by energy.the emission spectrum and luminescence time of PLNPs can be controlled by changing The synthesis conditions and elemental composition
[1⇓~3]。 The first generation of long afterglow materials for modern applications is a long afterglow material：SrAl
2O
4:Eu
2+,Dy
3+reported by Matsuzawa et al.In the 1990s,which can emit bright green luminescence for 30 H after being excited by ultraviolet light
[4]。 Since then,more and more attention has been paid to long afterglow materials,and various long afterglow materials have been developed,up to hundreds at present.long afterglow materials can be divided into inorganic long afterglow materials and organic long afterglow materials according to their chemical composition,among which inorganic long after-glow materials are widely used because of their good afterglow luminescent properties。
Most of the inorganic long afterglow materials have semiconductor properties and thus have photocatalytic properties.the unique energy storage and luminescence properties of long afterglow materials make them show advantages in the field of photocatalysis that other materials do not have,because they can store excess energy when the light source is present,and when the light source is removed,the stored energy is released in the form of light to continue to drive photocatalysis
[5,6]。 As one of the typical long-afterglow host materials,Zn
2GeO
4is considered to be a promising photocatalyst due to its wide band gap of 4.68 eV and a large number of oxygen vacancies
[7⇓-9]。 Li et al.Reported the work of doping Eu
3+,Mn
2+into Zn
2GeO
4to make it have the functions of photocatalytic degradation of RhB and afterglow luminescence
[10]。 Similarly,ZnGa
2O
4based long afterglow materials are also often reported for photocatalytic studies
[11]。 However,there are some problems such as low specific surface area and high electron-hole recombination rate when a single long afterglow material is used for photocatalysis,so it is necessary to combine other compatible materials to form a composite material to further improve its performance。
For biomedical applications,PLNPs show unique advantages in biomedicine due to their ability to eliminate autofluorescence interference and tissue scattering interference
[12]。 PLNPs can be used as bioprobes to monitor and identify changes in the biological environment during biomarker sensing and monitoring.At present,researchers have developed many bioprobes based on PLNPs,which have high sensitivity,signal-to-noise ratio and excellent detection performance
[13]。 Wu et al.Used PLNPs to construct a fluorescence resonance energy transfer probe for ratiometric luminescence detection of prostate specific antigen
[14]。 This work paves the way for the design of PLNPs biosensors that manipulate energy transfer.PLNPs have the advantages of in situ excitation,high signal-to-noise ratio and high penetration,and show great potential in in vivo imaging
[15]。 In this study,Cr
3+and Pr
3+doped zinc gallium germanate(ZGGO:Cr,Pr)were synthesized,and the peptide with tumor-targeting ability was transplanted to ZGGO:Cr,Pr.The modified long afterglow material can be used for in vivo tumor-targeting imaging
[16]。 Similarly,PLNPs can be used as an internal persistent light source for in vivo therapy to perform deep tissue therapy and avoid skin tissue damage
[17,18]。 Liu et al.Combined ZnGa
1.996O
4:Cr
0.004and photosensitizer into a photodynamic therapeutic nanoplatform.Under photothermal activation conditions,PLNPs can not only act as an internal light source to continuously excite the photosensitizer and produce reactive oxygen species to kill tumor cells,but also provide high-resolution imaging
[19]。 Jiang et al.Coated mesoporous SiO
2shell on the surface of PLNPs to load drugs,and coupled with hyaluronic acid to achieve tumor-targeting effect and controlled drug release,the multifunctional composite nanoplatform can be used for imaging-guided cancer chemotherapy
[20]。
PLNPs also have unique advantages in anti-counterfeiting applications due to their unique spatiotemporal dynamic spectral properties.Ma et al.Reported a novel near-infrared two-zone emission（NIR-Ⅱ）PLNPs（Zn
2Ga
2Sn
0.5O
6:Yb
3+-Ni
2+）for anti-counterfeiting
[21]。 The material can show strong NIR-II luminescence after being excited,and has the characteristics of high concealment,flexibility and high signal-to-noise ratio.Zhang et al.Designed a long afterglow material Ca
2Sb
2O
7:Pr
3+with dual anti-counterfeiting effects
[22]。 The material can adjust the luminous intensity and color by adjusting the doping concentration of the Pr
3+,thereby realizing double anti-counterfeiting effect.In addition,the luminescent pattern can be different with the change of concentration and time,forming a dynamic anti-counterfeiting 。
Although long afterglow materials are widely used in photocatalysis,biomedicine,anti-counterfeiting and other fields because of their unique advantages,there are still some shortcomings of single long afterglow material,such as single function,low specific surface area and catalytic activity,and poor stability.Therefore,researchers use various strategies to combine long afterglow materials with other functional materials to construct long afterglow composites to make up for their shortcomings,so as to further improve their performance and function.Therefore,in this paper,the preparation methods of long afterglow composite materials are summarized,and the research and application progress of long afterglow composite materials in photocatalysis(pollutant decomposition,hydrogen production),biomedicine(biosensing,bioimaging,cancer therapy),anti-counterfeiting encryption and so on are emphatically reviewed。
2 Preparation of Long Afterglow Composite
At present,the preparation process of long afterglow composite materials needs to synthesize a single long afterglow material first,and then consider using various means to combine to form composite materials.the configurations of the prepared long afterglow composites mainly include supported type,core-shell type and dumbbell type(Fig.1)。
2.1 Load type
Supported long afterglow composites are mainly formed by loading PLNPs on the surface of other functional materials or other functional materials on the surface of PL NPs.Feng et al.Used the sol-gel method to wrap TiO
2nanoparticles on the surface of CdSiO
3:Gd
3+,Bi
3+and designed CdSiO
3:Gd
3+,Bi
3+@TiO
2composites
[23]。 The composite material has excellent performance of photocatalytic degradation of methylene blue,and the composite material can still carry out photocatalytic degradation work after the light source is turned off due to the afterglow emission of the CdSiO
3:Gd
3+and the Bi
3+,which is not available in a single TiO
2.In 2022,our group successfully dispersed ZnGa
2O
4:Cr on the surface of I
2O
3by hydrothermal method,and synthesized a flower-like I
2O
3/ZnGa
2O
4:Cr heterojunction composite(Figure 2A )
[24]。 The heterostructure formed between the two improves the photocatalytic activity of the I
2O
3/ZnGa
2O
4:Cr composite.The degradation efficiency of I
2O
3/ZnGa
2O
4:Cr composite for RhB was 98.8%after 80 min of UV irradiation,which was higher than that of I
2O
3and ZnGa
2O
4:Cr alone.In addition,it also exhibits a long afterglow luminescence of more than 72 H,demonstrating great potential for all-weather photocatalytic applications.The preparation method of supported long afterglow composite is relatively simple,which can be obtained by physical mixing,but the proportion and composition of the material can not be accurately controlled,and the stability needs to be improved 。
2.2 Core-shell type
the preparation of core-shell long afterglow composites generally uses PLNPs as the core,and then grows or assembles other functional materials on its surface to form a core-shell structure.The researchers constructed a novel core-shell multifunctional nanoplatform with PLNPs with NIR luminescence as the core and ZIF-8 as the shell(Figure 2B)
[25]。 the afterglow luminescence of PLNPs and the large specific surface area of ZIF-8 enable the composite to have dual functions of good infrared fluorescence imaging as well as pH-responsive drug delivery,which has high potential In tumor therapy.in 2023,Yan et al.Used mesoporous poly(acrylic acid)(PAA)/calcium phosphate(CaP)shell coated PLNPs(PLNPs@PAA/CaP)for improved bioimaging and drug delivery
[26]。 the encapsulation of the PAA/CaP shell effectively prolongs the decay time of the PLNPs and improves their persistent luminescence time by about 3 times,which is due to the passivation of the surface defects of the PLNPs by the shell and the energy transfer between the shell and the PLNP.the preparation of core-shell long afterglow composites generally requires multi-step operations,and the preparation process is slightly complex,but the core-shell structure endows the composites with good stability and close contact interface,which is more conducive to the interaction between components and energy transfer。
2.3 Dumbbell type
Dumbbell-shaped long afterglow composites are generally prepared by PLNPs and other functional materials with similar size and shape,which are formed in dumbbell-shaped composites.Han et al.Connected the blocky long afterglow material with the spherical up-conversion materialβ-NaYF
4:Yb and Er@NaYF
4by heating and stirring with the assistance of solvent to form a dumbbell-shaped composite material
[27]。 The NIR-excited green luminescence of upconversion materials can activate the red persistent luminescence of PLNPs.With this strategy,the unique optical properties of PLNPs and UCNPs can be optimally synergized,resulting in efficient upconversion,photoluminescence,and UCPL simultaneously.In 2023,Viana et al.Combined the bulkβ-NaGd
0.8Yb
0.17Er
0.03F
4upconversion material with the spherical long-afterglow material Zn
1.33Ga
1.335Sn
0.33Cr
0.005O
4by dry impregnation method(Fig.2C),and continuously emitted light at 700 nm under 980 nm laser irradiation,proposed a mechanism to explain this energy transfer process,and demonstrated the ability of the hybrid material as a rechargeable persistent nanoprobe for in vivo applications
[28]。 Dumbbell-type long afterglow composites are generally bonded by chemical bonds,and the preparation process is slightly cumbersome and needs to be completed in several steps,but the stability and energy transfer between components are not as good as those of core-shell type。
3 Application of Long Afterglow Composite
in this paper,The applications of long afterglow composites are discussed from three aspects of photocatalysis,biomedicine and anti-counterfeiting,and some applications of long afterglow composites are classified and summarized.the results are shown In Table 1。
3.1 Photocatalysis
Conventional photocatalytic materials(PCMs)require an external light source to maintain their photocatalytic activity,which limits their application.In the past few years,people have been exploring alternative strategies to maintain the photocatalytic reaction without external light source,and the construction of composite photocatalysts by combining PLNPs with PCMs has attracted more and more attention.the working principle of self-luminescence of PLNPs@PCMs composites is mainly based on the long afterglow luminescence of PLNPs as an internal light source,which can produce continuous self-induced photocatalytic activity when the external light source is turned off
[29]。 However,only a small number of PCMs combined with PLNPs have been reported for photocatalysis.Up to now,the photocatalytic applications of PLNPs@PCMs hybrid composites mainly focus on photocatalytic degradation and photocatalytic hydrogen evolution。
3.1.1 Degradation of pollutant
Xiao et al.Uniformly coated CdS on the surface of SrAl
2O
4:Eu
2+,Dy
3+long afterglow materials by sol-gel method
[30]。 The composite has obvious photocatalytic activity for the degradation of methyl orange and hexavalent chromium under ultraviolet and visible light irradiation.When the molar ratio of CdS to SrAl
2O
4:Eu
2+,Dy
3+was 1:2,the degradation rate of methyl orange under visible light was increased by 2.5 times to 96.3%.The enhancement of photocatalytic activity is mainly due to the formation of heterostructure between them,which enhances the transfer efficiency of photogenerated carriers,and the reuse of SrAl
2O
4:Eu
2+,Dy
3+luminescence improves the light utilization efficiency 。
In 2023,our research group uniformly dispersed carboxyl-modified Zn
3Ga
2Ge
2O
10:Mn(PLNPs)on the surface of NH
2-MIL-101(Fe)(MOFs)by hydrothermal method to form PLNPs/MOFs composite–all-weather photocatalyst(Figure 3A),and investigated the photocatalytic RhB degradation performance of the composite.PLNPs/MOFs(1∶1)had the best photocatalytic activity
[31]。 after the introduction of MOFs materials,the composite has a larger specific surface area and more active sites.At the same time,the recombination rate of photogenerated electron-hole pairs is greatly reduced and the photocatalytic efficiency is improved after the PLNPs and MOFs are constructed into a heterojunction.Based on the afterglow luminescence of PLNPs,the composite still exhibited sustained RhB degradation activity after the light source was removed.This work provides a feasible strategy for constructing long afterglow composite all-weather photocatalysts。
3.1.2 Hydrogen evolution
Liu et al.Used SrAl
2O
4:Eu
2+,Dy
3+to form a composite material with g-C
3N
4-xand Au,and designed a novel self-luminous plasmonic photocatalyst composite material(i.e.,g-C
3N
4-Au@SrAl
2O
4:Eu
2+,Dy
3+）)for photocatalytic hydrogen evolution from water splitting
[32]。 As shown in Fig.3(B),those systems without SrAl
2O
4:Eu
2+,Dy
3+cannot show photocatalytic H
2generation when the light source is turned off,thus,SrAl
2O
4:Eu
2+,Dy
3+plays a crucial role in all-weather photocatalytic H
2generation.In 2024,Wu et al.Reported a long afterglow composite Sr
2MgSi
2O
7:（Eu,Dy）/CdS for all-weather photocatalytic hydrogen evolution
[33]。 In the dark,Sr
2MgSi
2O
7:（Eu,Dy）can not only be used as a light source to drive the photocatalytic hydrogen evolution reaction of CdS,but also form a type II heterostructure between Sr
2MgSi
2O
7:（Eu,Dy）and CdS,which effectively inhibits the recombination of photogenerated carriers and improves the photocatalytic hydrogen evolution efficiency.18.07 mmol·g
-1hydrogen was produced in 0.5 H under illumination,and the photocatalytic hydrogen evolution reaction lasted for 4 H under dark conditions,resulting in 6.31 mmol·g
-1hydrogen,which exceeded that of other materials reported before.The hydrogen evolution efficiency of the long afterglow composite is compared with that of some mainstream photocatalytic materials,and the results are shown in Table 2.Compared with other photocatalytic materials,the long afterglow composite material has all-weather photocatalytic characteristics,and has obvious advantages in hydrogen evolution rate and energy utilization 。
Although researchers have done some pioneering work on the application of long afterglow composite materials in photocatalysis,this field is still in its infancy,and its research is mainly focused on the degradation of pollutants and hydrogen evolution,and there are few reports on photocatalytic CO2reduction or H2O2production.In addition,the photocatalytic efficiency has a certain distance from the practical application and needs to be improved 。
3.2 Biomedical application
At present,the biomedical applications of long afterglow composites mainly focus on biosensing detection,bioimaging,cancer therapy and drug delivery。
3.2.1 Biosensing
PLNPs can eliminate the interference of autofluorescence and scattered light by pre-excitation before detection,which solves the problem of high background signal in traditional fluorescence biosensing,thus significantly improving the sensitivity and signal-to-noise ratio of biosensing
[39,40]。 Recently,some optical sensors based on PLNPs have been successively reported,including the detection of tumor markers,food-borne dangerous substances,and active substances
[41,42][43,44][45,46]。
serum AFP levels are usually elevated in the rapid growth of liver cancer cells,cirrhosis,chronic active hepatitis and carbon tetrachloride poisoning,so the detection of Serum AFP can be used as an early diagnosis of liver cancer
[47]。 Yan et al.Conjugated polyimine-coated PLNPs withα-fetoprotein(AFP)antibody-coated gold nanoparticles and developed a highly sensitive AFP probe
[48]。 Because the probe uses the afterglow intensity as a signal for sensing detection,it can effectively eliminate the autofluorescence and scattered light of biological matrix generated by in situ excitation,and can detect AFP in serum samples with high sensitivity.Fibroblast activation protein(FAP-α),as a biomarker of cancer-associated fibroblasts,plays an important role in affecting the proliferation,invasion and metastasis of cancer cells,early diagnosis,real-time monitoring of treatment and prognosis.Wang et al.Reported a PLNPs composite to detect FAP-αby the afterglow resonance energy transfer(ARET)method
[49]。 In this system,PLNPs modified by Au nanorods were used as the donor and Cy5.5-KGPNQC-SH was used as the acceptor,and the ARET between the donor and the acceptor extinguished the afterglow signal of the donor.If the system encounters FAP-α,it causes the cleavage of KGPNQC and the disappearance of ARET,thus restoring the afterglow of the donor.The method has a linear range of 0.1~2.0 mg·L
-1（1.2~22.9 nmol·L
-1）,LOD of 11μg·L
-1（115 pmol·L
-1）,RSD of 3.9%(for 0.5 mg·L
-1FAP-α,n=5),high sensitivity,low detection limit and good anti-interference ability.In addition,Yang et al.Reported that PLNPs were coupled with 4-carboxyphenylboronic acid(CPBA)to construct a PLNPs-CPBA composite,which achieved non-invasive and highly selective imaging of tumor metastasis and provided a powerful universal platform for the study of tumor metastasis(Figure 4A )
[50]。
In addition to the detection of tumor markers,PLNP can also be used to detect a variety of other active substances simultaneously.Jiang et al.Reported a fast and sensitive multicolor long afterglow luminescent nanosensor
[51]。 As shown in Fig.4(B),the nanosensor is a composite nanomaterial obtained by combining the modified functional PLNPs with interDNA-modified ferroferric oxide magnetic nanoparticles for the simultaneous determination of aflatoxin B
1and penicillenone in food samples.Two kinds of PLNPs with single excitation wavelength were used as signal units,and aptamers with high affinity and specificity for the corresponding mycotoxins were used as recognition units.The advantages of the nanosensor's autofluorescence-free detection coupled with the aptamer's high specificity and high-speed magnetic separation allow for highly sensitive and selective detection of mycotoxins in food samples.This work provides a new universal optical platform based on PLNPs for the simultaneous detection of multiple contaminants in complex samples 。
Yao et al.Developed an ultrasensitive multifunctional sensing platform,ZGC/HZIF-8,by combining ultra-small size of Cr
3+doped zinc gallate（ZnGa
2O
4:Cr
3+,ZGC)with hierarchical porous zeolite imidazole framework-8(HZIF-8)with high loading capacity and target preconcentration effect to detect dopamine with signal quenching strategy
[52]。 The abundant layered pores in the HZIF-8 matrix can provide considerable attachment points for ZGC,which greatly improves the stability of ZGC/HZIF-8 composites.Under the optimized conditions,the nanoplatform has good sensitivity and selectivity for dopamine within 0.0025~75μmol·L
-1with a detection limit of 0.0010μmol·L
-1.In addition,the biosensor has been used to measure dopamine concentration in human serum samples without in situ excitation,effectively eliminating autofluorescence interference.This new strategy can help to develop practical applications of PLNPs-based biosensors in clinical analysis 。
3.2.2 Biological imaging
3.2.2.1 Optical imaging
PLNPs can effectively avoid the autofluorescence of biological tissues and are one of the ideal bioimaging probes.However,PLNPs also require a short wavelength excitation source during excitation,which limits tissue penetration depth
[53,54]。 Therefore,researchers have tried to combine PLNPs with other long-wavelength excited functional materials for bioimaging.Up-conversion luminescence can convert low-energy light in the NIR region into high-energy visible and ultraviolet light
[55,56]。 Therefore,upconversion NPs(UCNPs)can be used as an ideal wavelength converter,which can effectively charge PL NPs to obtain NIR photoexcited long afterglow luminescence。
In 2017,Qiu et al.Synthesized a hybrid nanocluster(UCPL-NC)composed of UCNPs(β-NaYbF
4:Tm@NaYF
4)and PLNPs,which can be activated by 980 nm laser and display afterglow at 700 nm to achieve near-infrared to near-infrared UCPL bioimaging
[57]。 UCNPs can effectively charge PLNPs,and The luminescence decay and lifetime of PLNPs are no longer the limitation of NIR PL imaging.the PL signal of UCPL-NC can be reactivated even when covered with 10 mm of pork.UCPL-NC can be excited by 980 nm light in the biological transparent window(600~1100 nm),which has deep penetration into biological tissues.As shown in Fig.5(a),UCPL-NC achieved PL bioimaging after in vivo activation due to the increased penetration depth of NIR excitation in UCPL imaging compared with conventional PL imaging.the development of UCPL-NC provides an ideal way for UCPL to monitor biological processes in real time over long periods of time。
In 2023,Zhang et al.Complexed MnO
2with PLNPs through a mesoporous SiO
2shell
[58]。 As shown in Fig.5(B),because PLNPs are quenched by the outer layer of MnO
2in normal tissues and degraded and recovered by MnO
2in the tumor microenvironment,it enables targeted imaging,thus significantly improving the sensitivity of tumor imaging.Combined with the absence of background fluorescence in PLNPs imaging,ultra-high sensitivity was achieved.In the orthotopic breast cancer model,the tumor-to-normal tissue signal ratio during nanoprobe surgery was about 9 times that of the down-conversion nanoprobe.The imaging time is only 1.5 min.An ultrasensitive and convenient imaging method for identifying residual tumor tissue based on long afterglow composite was developed 。
3.2.2.2 Multimodality bioimaging
Multimodality imaging is an imaging method that combines the advantages of different imaging methods,which can make diagnosis with high sensitivity and high resolution at the same time.it has attracted wide attention in biomedical applications because It provides more accurate,complete and reliable diagnostic information
[59⇓~61]。 Each imaging modality has its own advantages and disadvantages In terms of sensitivity,spatial and temporal resolution,and penetration depth.NIR fluorescence imaging has the advantage of high sensitivity,but it is difficult to provide high-resolution tomographic images;in contrast,magnetic resonance imaging(MRI)has the advantage of excellent spatial resolution and unlimited depth penetration,but still has an inherently low sensitivity
[62,63]。 Therefore,it is attractive to combine the unique advantages of PLNPs and the advantages of multimodality imaging to design high-performance PLNPs-based imaging probes。
We present a novel multimodality bioimaging probe based on gadolinium complex functionalized Zn
2.94Ga
1.96Ge
2O
10:Cr
3+,Pr
3+PLNPs for in vivo NIR luminescence imaging and MRI imaging
[64]。 the probe not only has an ultra-long NIR PL,but also has a higher longitudinal relaxation rate than the commercial gadolinium complex.PL has good imaging sensitivity,while its spatial resolution can be easily complemented by MRI.The successful fabrication of Gd(Ⅲ)-PLNPs bimodal nanoprobe offers great potential for in vivo MRI/optical imaging.To simplify the synthesis process and improve the MR imaging capability,Wang et al.Developed a dual-modality nanoprobe for PL and MR imaging
[65]。 The dual-modality imaging nanoprobe PLNPs-Gd
2O
3-HA was fabricated by combining PLNPs and HA-Gd
2O
3.The prepared nanoprobe has reproducible PL and higher T
1relaxivity.Its unique advantages have led to the success of nanoprobes in tumor-targeted PL and MR dual-modality imaging,providing a convenient way to construct targeted PL/MR dual-modality nanoprobes.In addition to this,other bimodal composite nanoprobes based on combining PL imaging and MRI have been developed,such as：Zn
2.94Ga
1.96Ge
2O
10:Cr
3+,Pr
3+@TaO
x@SiO
2,Gd
2O
3@mSiO
2@CaTiO
3:Pr
3+,Gd
2O
3@mSiO
2/ZnGa
2O
4:Cr
3+,Bi
3+,Mn-ZGOCS-PEG
[66][67][68][69]。
In addition to bimodal bioimaging,Liu et al.Also developed a core-shell structured trimodal composite nanoprobe GdAlO
3:Mn
4+,Ge
4+@Au（Fig.5C )
[70]。 In the core-shell composite nanoprobe,the GdAlO
3:Mn
4+,Ge
4+core is used as the NIR persistent luminescence center,and the Au shell is used to improve the persistent luminescence efficiency through plasmon resonance.The Gd element in PLNPs can provide MRI,and the Au element can provide computed tomography.The in vivo bioimaging results show that each mode of probe is comparable to or superior to previously reported probes,making the probe valuable in applications such as disease diagnosis and monitoring of biological processes.The above results show that long afterglow materials have good imaging effects,but their biocompatibility is also a key issue to be considered.At present,the biocompatibility of inorganic long afterglow materials needs to be further improved,which is conducive to the clinical application of inorganic long afterglow composites 。
3.2.3 Treatment
Considering the special advantages of PLNPs in bioimaging,combining PLNPs with other functional materials such as photothermal nanomaterials and photodynamic nanomaterials will become an important field,providing new ideas for novel imaging-guided therapy in the biomedical field.So far,there are different therapeutic approaches such as drug delivery,chemotherapy,photothermal therapy,and photodynamic therapy,among others。
3.2.3.1 Drug delivery
Nanocarriers have been developed as effective drug delivery and diagnostic tools that have received significant attention,particularly in the field of nanomedicine
[71]。 Drug carrier is a tool to deliver drugs to target tissues or organs by changing the route of administration to control the distribution and release rate of drugs in the body
[72]。 drug carriers can leave some defective drugs with high side effects or poor solubility in the carrier,thus reducing the damage to the human body and improving the efficacy of drugs.PLNPs have been integrated with Drug carriers,such as porous silica-loaded antitumor drugs for simultaneous diagnostic imaging for effective cancer therapy
[73⇓~75]。
In 2019,Zhao et al.Reported a PLNPs@ZIF-8 composite with NIR emission for tumor imaging and drug delivery without background interference(Figure 6A)
[76]。 PLNPs in PLNPs@ZIF-8 continuously emit NIR light for hours or even days without external illumination,providing deep tissue imaging without background interference,and are reactivated by LEDs for long-term bioimaging.the shell of porous framework ZIF-8 in PLNPs@ZIF-8 has significant drug loading ability,which successfully achieved acidic tumor site-triggered drug release.Moreover,the acidic microenvironment of the tumor activated the breakdown of PLNPs@ZIF-8,which enhanced the luminescence of PLNPs and illuminated the tumor site,facilitating tumor-specific imaging.Disassembly of the ZIF-8 framework and protonation of doxorubicin(DOX)under the acidic tumor microenvironment enable tumor site-specific drug release for tumor therapy。
In 2023,Shu et al.Prepared PLNPs@MIL-100(Fe)using PL imaging/MR imaging to reduce autofluorescence interference and achieve dual-modality imaging and drug delivery
[77]。 As shown in Fig.6(B),PLNPs provide external radiation-free PL for sensitivity and autofluorescence-free imaging.After bonding with MIL-100(Fe),the composite produced a T
2MR signal with a transverse relaxation r
2of 3.95 mg·mL
-1·s
-1.PLNPs@MIL-100(Fe)combines PL imaging with MR imaging modalities to enhance high spatial resolution imaging of tumors.In addition,the MIL-100(Fe)shell structure has high drug loading capacity and pH-controlled drug release,which is effective for tumor therapy.The bimodal PLNPs@MIL-100(Fe)provide high spatial resolution and sensitive autofluorescence-free imaging,as well as efficient drug loading and pH-controlled drug release for tumor diagnosis and therapy 。
PLNPs have the advantages of eliminating side effects and improving treatment efficiency in the delivery of chemotherapeutic drugs.Therefore,in the past few years,researchers have tried to use PLNPs for imaging-guided chemotherapy
[78⇓~80]。 Chen et al.Reported PLNP@MOF-derived mesoporous carbon(MC)core-shell nanocomposite(PLMC)for chemotherapy without autofluorescence imaging guidance
[81]。 MOF UiO-66 was used as both precursor and template,and PLNP was controllably coated on the surface to form PLNP@UiO-66 core-shell composite.Compared with PLNP@UiO-66,the drug loading capacity of the prepared PLMC was enhanced.the PLMC surface is further coated with a macrophage membrane to obtain the MPLMC,so that the MPLMC has enhanced stealth ability to avoid the mononuclear phagocyte system,and the circulation time and the targeting ability are increased.drug-loaded MPLMC is promising for autofluorescence-free PL imaging-guided drug delivery and tumor therapy。
3.2.3.2 Photothermal therapy
photothermal therapy(PTT)is a treatment method that converts light energy into heat energy around the tumor tissue to kill cancer cells through targeted Photothermal conversion efficiency cation technology under external light source irradiation.Its advantages are that it has little long-term side effects and can be repeated many times to achieve high therapeutic efficiency
[82,83]。 Optical imaging guidance can provide accurate information about the location and area of tumor tissue for highly specific PTT.Despite growing interest,the method is still in its early stages and faces many challenges.in 2021,Meng et al.Constructed a biocompatible PLNP-GNR composite nanoplatform by self-assembly and surface modification of PLNPs and gold nanorods(GNRs)using cetyltrimethylammonium bromide(CTAB)and phosphotungstic acid molecules,using a single-wavelength photoexcitation strategy to monitor autofluorescence-free bioimaging and temperature changes during in vitro and in vivo PTT
[84]。 the photothermal conversion efficiency of The PLNP-GNR composite nanoplatform is about 37%.The biocompatible PLNP-GNR composite nanoplatform has potential application prospects in the precise PTT of deep tissue skin cancer and breast cancer。
In 2022,Meng et al.Employed Bi
2S
3nanoparticles with good photothermal conversion performance and CTAB-coated Zn
2Ga
2.98Ge
0.75O
8:Cr
0.023+(ZGGO:Cr
3+@CTAB)PLNP to construct a PSS molecule-modified PLNP-Bi
2S
3composite nanoplatform through electrostatic adsorption(Figure 6C )
[85]。 The PSS-modified PLNP-Bi
2S
3composite nanoplatform showed good solution dispersibility in blood and normal tissue environment,and could rapidly accumulate in cancer cells in acidic environment due to acid-induced characteristics.The composite nanoplatform can be applied to cancer diagnosis and PTT,and may be applied to the precise diagnosis and treatment of deep tissue tumors in the future 。
3.2.3.3 Photodynamic therapy
Photodynamic therapy(PDT)is an emerging clinical cancer treatment using photosensitizers,which involves irradiating the tumor site with spectral wavelengths to selectively activate photosensitizing drugs that accumulate in the tumor tissue to produce singlet oxygen,thereby killing cancer cells
[13]。 this method requires prolonged exposure to visible or ultraviolet light,which not only limits tissue penetration and penetration strength,but also causes skin and tissue damage.PLNPs have great potential to solve This problem,because the long afterglow property of PLNPs can activate the photosensitizer,that is,continuous light irradiation is not required。
Hu et al.Prepared an NIR rechargeable"optical cell"for radiation-free PDT by embedding UCNPs（NaYF
4:25%Yb,0.5%Tm）,PLNPs（SrAl
2O
4:2%Eu
2+,4%Dy
3+）,and photosensitizer into biocompatible polydimethylsiloxane(Fig.6d )
[86]。 the PDT"photocell"can produce green PL after a rapid short charge of 5 s with a 980 nm NIR laser and generate cytotoxic singlet oxygen for long-term Irradiation-free PDT without external irradiation.Due to the deep tissue penetration and intermittent short exposure of the NIR light source,the PDT"optical cell"can be charged to continuously produce highly effective reactive oxygen species even in deep tissues up to 4 mm,and can reduce the photothermal effect of external irradiation.Irradiation-free PDT implants can effectively inhibit tumor proliferation even in hypoxic tumors.Therefore,this smart rechargeable NIR radiation-free PDT"photocell"provides a new approach for light-responsive biomedicine。
3.2.3.4 Combination therapy
Combination therapy is the treatment of cancer by combining two or more therapies,such as chemotherapy,photothermal therapy,and photodynamic therapy
[87]。 Yu et al.Combined near-infrared PLNPs SiO
2@Zn
1.05Ga
1.9O
4:Cr with MOFs HKUST-1 to form a core-shell composite nanoplatform mSiO
2@Zn
1.05Ga
1.9O
4:Cr@HKUST-1（HSZGO）for in vivo imaging,tumor chemokinetics(CDT),and PTT combination therapy(Figure 6E )
[88]。 The HSZGO nanoplatform can carry Cu
2+,which is effectively enriched in the tumor site,and convert the overexpressed endogenous H
2O
2into·OH through Fenton or Fenton-like reaction,causing harmful oxidative damage to the tumor
[89⇓⇓⇓⇓~94]。 In addition,copper sulfide is formed by the reaction of HSZGO with endogenous disordered hydrogen sulfide in tumor cells,which can be used as a PTT drug to effectively treat tumor
[95].The in vitro photothermal conversion efficiency of HSZGO in the presence of sodium hydrosulfide reached 58.7%,and the nanoplatform could enter mouse tumors within 0.5 H and could be activated by H
2O
2and H
2S in tumor cells,resulting in effective PTT and CDT synergistic therapy to effectively inhibit the growth of hepatoma cells.Therefore,the tumors of tumor-bearing mice can be completely cured by synergistic treatment without causing significant damage to normal tissue cells.Therefore,HSZGO nanoparticles have the potential to penetrate human skin and superficial group for imaging and treatment of superficial tumors 。
3.2.3.5 Bactericidal and antibacterial
Bacterial infection is one of the major threats to human life and health,causing a severe burden on the global economy and public health
[96⇓⇓~99]。 Therefore,different types of antibacterial materials have been developed,such as antibacterial hydrogels,silver,cationic polymers,photocatalysts,and composites with synergistic effects
[100⇓⇓⇓⇓⇓~106]。 Antimicrobial photocatalytic therapy is a promising treatment for wound disinfection.It uses a photocatalyst activated by controlled light to generate active oxygen to destroy bacteria,and the photocatalyst with antibacterial photocatalytic therapy activity is the most effective and safe
[107⇓⇓⇓~111]。
TiO
2is a promising photocatalyst that has been widely used in various fields due to its chemical stability,strong oxidizing property,inexpensive commercial use,and nontoxic properties
[112]。 Therefore,Liu et al.Developed a photocatalyst LiLuGeO
4:Bi
3+/TiO
2（LLG/T）with antibacterial activity by combining PLNPs（LiLuGeO
4:Bi
3+）with excellent UV PL lasting more than 100 H with TiO
2[109]。 Since the ultraviolet PL of LLG perfectly matches the light absorption of TiO
2,LLG was used as the excitation light source for TiO
2.As shown in Fig.6(f),after the material is excited,the ultraviolet light emitted by LLG is effectively absorbed by the TiO
2,so that the TiO
2continuously generates active oxygen,and after the excitation is stopped,active oxygen can also be generated,which can effectively kill and inhibit the growth of bacteria.In addition,a thermoresponsive hydrogel-loaded LLG/T(H@LLG/T)with sol-gel transition ability at body temperature was further prepared on this basis,which can effectively control bacterial growth and promote wound healing,while showing appropriate biosafety.In addition to this,in 2023,Wang et al.Reported an antibacterial nanoplatform PLNPs-Cs-Cu
2+w ith high-sensitivity sustained luminescence imaging and simultaneous treatment of bacterial infections
[113]。 Among them,PLNPs play an imaging role to monitor the treatment process in real time,and the Cu
+produced by Cu
2+catalyzes H
2O
2to generate hydroxyl radical（$HO^{-·}$）to kill bacteria.The prepared antibacterial nanoplatform is expected to be used for highly sensitive continuous luminescence imaging and simultaneous treatment of bacterial infection 。
PLNPs have particular advantages in eliminating autofluorescence interference without constant in situ excitation,which is an ideal material for long-term biological applications.in order to promote the application of PLNPs in biomedicine,people have explored more mature modification methods,which are conducive to further composite other functional materials to enhance the effect of diagnosis and treatment,improve biocompatibility,enhance stability and so on.in this section,we comprehensively introduce the major advances in the diagnostic and therapeutic applications of long afterglow composites,including biosensing,bioimaging,and imaging-guided therapy.Several pioneering works have been reported on the biosensing applications of long afterglow composites.PL-based biosensing technology has superior detection signal-to-noise ratio,accuracy and stability,which can realize high-sensitivity biosensing of tumor biomarkers,bioactive molecules,etc.for bioimaging and imaging-guided therapy,PL-based molecular imaging,cellular imaging,and tumor imaging play an important role in diagnosis and treatment.While long afterglow composites have been widely explored for biomedical applications,the collection of persistent luminescence signals,whether for sensing,imaging,or therapy,relies on a variety of large and expensive instruments,in which case further efforts are needed to improve detection efficiency and reduce costs.At the same time,in order to meet people's demand for more intelligent,lower toxicity and better selectivity,the research on long afterglow composite materials needs to be further improved。
3.3 Anti-counterfeiting encryption
At present,anti-counterfeiting technology mainly includes watermarking technology,laser holography technology,bar code technology and Fluorescent anti-counterfeiting technology.fluorescent anti-counterfeiting technology shows unique advantages because of its high efficiency,easy implementation,difficult replication,easy concealment,low cost and large synthesis scale
[114⇓⇓~117]。 However,PLNPs have shown great application potential in anti-counterfeiting due to their unique spatiotemporal dynamic spectral properties.At present,the anti-counterfeiting function of PLNPs can be further enhanced by changing the afterglow color and increasing the stability of the excitation light source。
the combination of perovskite QDs and PLNPs not only maintains the performance advantages of each component material,but also can obtain comprehensive properties that cannot be achieved by a single component material through the complementary and related properties of each component
[118,119]。 Therefore,Hai et al.Uniformly dispersed inorganic cesium lead halide（CsPbBr
3）PeQDs on the surface of Sr
2MgSi
2O
7:Eu
2+,Dy
3+（SMS）,and developed a composite CsPbBr
3/SMS@SiO
2with adjustable afterglow luminescent color
[120]。 On the basis of this experiment,in order to achieve the full band afterglow in the visible region,CsPbI
3-CsPbBr
3/SMS@SiO
2composite materials are designed
[121]。 SMS is used to store light energy and PQDs is used to control the color of afterglow emission;Through energy transfer,the afterglow color can be adjusted in the visible region;As shown in Fig.7(a),the composite exhibits different luminescent colors and afterglow colors under different excitation environments.Under ultraviolet light and dark conditions,the double anti-counterfeiting of fluorescence and afterglow is realized.It is of great significance to prevent fake and inferior products from damaging market interests,and has great application potential in the field of double anti-counterfeiting of fluorescence and afterglow.In addition to perovskite quantum dots,the composite of carbon dots(CDs)and PLNPs can also achieve the control of luminescence color.Lei et al.Reported the synthesis of a series of CDs grafted CaAl
12O
19:Mn
4+(CAO)tunable hybrid phosphors by sol-gel method
[122]。 Through the hydrolysis and condensation reaction of silica precursor,CDs can be combined with CAO In silica gel network to form highly flexible hybrid materials,which can be used to manufacture flexible displays and multicolor light-emitting materials.By adjusting the mass ratio of CDs/CAO and the excitation wavelength,the luminescence color customization from purplish red(0.504,0.250)to greenish blue(0.238,0.192)can be systematically achieved.in addition,the temperature-dependent luminescence of CDs/CAO phosphors can also be used as a thermometer with a wide linear temperature sensing range of 80~300 K。
The introduction of upconversion nanoparticles can achieve the purpose of broadening the excitation light source of long afterglow composite materials in the field of anti-counterfeiting.In 2024,Gao et al.Reported the study of combining upconversion nanoparticles NaYF
4:Yb,Tm with Zn
2GeO
4:Mn
2+,Li
+for anti-counterfeiting applications,and the introduction of upconversion nanoparticles allowed the composite to be charged using a 980 nm laser,resulting in an emission peak at 540 nm
[123]。 The composite phosphor was further processed into a two-dimensional film,which was then used as a rewritable"paper".Optical information can be written on UCPL paper at room temperature by using a near-infrared laser as a"pen".Combining the NIR writing and PSL performance(the energy stored in the deep trap cannot be released under normal environment,but it will be released under the light stimulation of UCPL material,resulting in luminescence),the data recording device is specially constructed to realize the encryption and decryption of data.As shown in Fig.7(B),the UCPL paper was charged with ultraviolet light for 5 min,and the words"XAUAT"were written on the green UCPL paper with a 980 nm laser.Due to the strong color contrast,the signal could be observed by the naked eye within 2 min.Therefore,NIR-writable UCPL materials with UP performance and PersL characteristics can be used for data encryption and decryption.In addition,Wu et al.Reported the development of a new type of spectral fingerprint anti-counterfeiting fiber Sr
2ZnSi
2O
7:Eu
2+,Dy
3+/PAN,using Sr
2ZnSi
2O
7:Eu
2+Dy
3+and polyacrylonitrile(PAN)as the main raw materials by wet spinning
[124]。 The introduction of PAN does not change the luminescent color of the material,but forms a composite spectral fingerprint anti-counterfeiting fiber with Sr
2ZnSi
2O
7:Eu
2+,Dy
3+,which is more stable for anti-counterfeiting applications and has high anti-counterfeiting ability,so it is a potential new anti-counterfeiting fiber 。
At present,the research on the application of long afterglow composite materials in anti-counterfeiting encryption mainly focuses on the control of fluorescence/afterglow color of materials by time,temperature,excitation light source and other parameters.Compared with other fields,the research on anti-counterfeiting encryption of long afterglow composite materials is still in the initial stage,and the future research direction should be inclined to develop multi-protection anti-counterfeiting encryption materials with multi-wavelength excitation,multi-peak emission,multi-stimulus response and dynamic anti-counterfeiting effect。
4 Conclusion and prospect
PLNPs have shown great application potential in different fields due to their unique afterglow luminescence advantages.in this paper,the main synthesis methods of long afterglow phosphorescent composites are summarized,and the applications of long afterglow phosphorescent composites in the fields of photocatalysis,biomedicine and anti-counterfeiting security in recent years are discussed in detail.Although substantial progress has been made in the preparation and application of long afterglow composites,the development of long afterglow composites in photocatalysis,biomedicine and anti-counterfeiting security is still in its early stage and faces great challenges。
(1)preparation.there have been many reports on the preparation of long afterglow composites,but these strategies are not yet mature.the ideal strategy is to synthesize long afterglow composite materials with small size,afterglow performance,good stability,biocompatibility,recyclability,low or even non-toxicity through simple operations,and at the same time,the construction of long afterglow composite materials can realize the complementary advantages of the two materials and achieve the effect of"1+1>2".Therefore,There is still much room for improvement in the preparation of long afterglow composites。
(2)application.First,the long afterglow composite in photocatalytic application,although the PLNPs@PCM composite can maintain photocatalytic activity without external irradiation source,the intensity of afterglow gradually decreases with time,resulting in the weakening of photocatalytic reaction.Therefore,there is still a need To continue to explore and optimize them to have longer duration,stronger intensity and more ideal persistent spectral region,which can match the absorption range or optical band gap of PCM,and develop efficient all-weather photocatalysts.Secondly,although great progress has been made in the research of long afterglow composites in the field of biomedicine,whether it is sensing,imaging or therapy,it is still mainly at the laboratory level,lacking of clinical application attempts.Therefore,the clinical application of long afterglow composites in biomedicine is the most important problem to be solved.Finally,for anti-counterfeiting applications,it is necessary to conduct more in-depth research on the afterglow mechanism and luminescent anti-counterfeiting,and to design and synthesize luminescent anti-counterfeiting materials with high cost performance,high efficiency and high brightness,so as to design more advanced luminescent anti-counterfeit technology.to sum up,it is believed that the multifunctional long afterglow composite materials with unique properties have potential development prospects and far-reaching application value,and will become a new generation of optical materials,causing a major innovation in the fields of photocatalysis,biomedicine,and anti-counterfeiting security。
Natural Science Foundation of Xinjiang Uygur Autonomous Region(2022D01E16)
National Natural Science Foundation of China(22364016)
Tianshan Innovation Team Plan of Xinjiang Uygur Autonomous Region(2023D14002)
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