Biology-Oriented Design Strategies of AIE Theranostic Probes

Abstract

Theranostic probes integrate both diagnostics and therapy in one platform that offers direct visualization of biochemical processes and facile treatment for various diseases. Organic fluorogens with aggregation-induced emission (AIEgens) have shown tremendous potentials in meeting the requirement of a multifunctional theranostic probe. Different from traditional fluorogens, AIEgens could be designed to produce strong fluorescence and efficient reactive oxygen species or even high photothermal conversion in the aggregate state, which makes them excellent options for building activatable theranostic probes. So far, AIEgens have been deliberately engineered to empower intelligent targeting, bioimaging, and therapeutic functions, which could influence specific biological behavior and further achieve image-guided treatment of various diseases, such as tumors or bacterial infections. In this review, we mainly focus on how new AIE multifunctional probes have been designed based on the latest advances in biomedicine and chemistry. We summarize the recent biology-oriented design strategies and the corresponding representative examples of AIE probes for diagnosis and treatment of disease. AIE theranostic probes offer a useful platform for developments in biomedicine, showing an exciting future for clinical translation. Theranostic probes integrate sensing, imaging, and therapy functions, which not only allow direct visualization of biochemical processes occurring in complex biological systems, but also offer facile therapeutic functions for various diseases. In this regard, organic fluorogens with aggregation-induced emission (AIEgens) are ideal materials to satisfy the needs of multifunctional theranostic probes. Under the guidance of specific biological principles, AIEgens can be deliberately engineered to have simultaneous optical and therapeutic functions, such as disease targeting, responsive drug release, and photosensitizing/photothermal capability, which could influence their interactions with biological microenvironments and further enable image-guided treatment of various diseases. In this perspective, we mainly focus on how new AIE multifunctional probes have been designed based on the latest advances in medicine and biology. We first summarize the biology-oriented design strategies for AIE probes for diagnosis and treatment of disease. Next, specific examples have been demonstrated to illustrate the probe design strategies. Finally, current challenges and future perspectives for development of AIE theranostic probes are presented in the conclusion section. Theranostic probes integrate sensing, imaging, and therapy functions, which not only allow direct visualization of biochemical processes occurring in complex biological systems, but also offer facile therapeutic functions for various diseases. In this regard, organic fluorogens with aggregation-induced emission (AIEgens) are ideal materials to satisfy the needs of multifunctional theranostic probes. Under the guidance of specific biological principles, AIEgens can be deliberately engineered to have simultaneous optical and therapeutic functions, such as disease targeting, responsive drug release, and photosensitizing/photothermal capability, which could influence their interactions with biological microenvironments and further enable image-guided treatment of various diseases. In this perspective, we mainly focus on how new AIE multifunctional probes have been designed based on the latest advances in medicine and biology. We first summarize the biology-oriented design strategies for AIE probes for diagnosis and treatment of disease. Next, specific examples have been demonstrated to illustrate the probe design strategies. Finally, current challenges and future perspectives for development of AIE theranostic probes are presented in the conclusion section. Theranostic probes refer to small molecules or nanomaterials possessing bioimaging and therapeutic capability, which can serve as a useful tool for exploration and monitoring of biological processes in complex biological environments.1Gao M. Tang B.Z. Aggregation-induced emission probes for cancer theranostics.Drug Discov. Today. 2017; 22: 1288-1294Crossref PubMed Scopus (41) Google Scholar Up to now, many design and synthetic strategies for theranostic probes have been developed, which combine different materials with various functionalities in one theranostic system.2Kim J. Piao Y. Hyeon T. Multifunctional nanostructured materials for multimodal imaging, and simultaneous imaging and therapy.Chem. Soc. Rev. 2009; 38: 372-390Crossref PubMed Scopus (904) Google Scholar,3Park K. Lee S. Kang E. Kim K. Choi K. Kwon I. New generation of multifunctional nanoparticles for cancer imaging and therapy.Adv. Funct. Mater. 2009; 19: 1553-1566Crossref Scopus (354) Google Scholar However, the increased availability of components makes the theranostic system complicated and difficult to reproduce for large-scale preparation. In this regard, organic fluorescent materials could simultaneously allow real-time visualization of biochemical activities and offer light-induced therapeutic effects, which further could circumvent undesirable obstacles in multicomponent theranostic systems.4Zhou Z. Song J. Nie L. Chen X. Reactive oxygen species generating systems meeting challenges of photodynamic cancer therapy.Chem. Soc. Rev. 2016; 45: 6597-6626Crossref PubMed Google Scholar,5Guo Z. Park S. Yoon J. Shin I. Recent progress in the development of near-infrared fluorescent probes for bioimaging applications.Chem. Soc. Rev. 2014; 43: 16-29Crossref PubMed Google Scholar Unfortunately, the performance of conventional fluorophores is largely restricted by fluorescence efficiency and stability due to the well-known problem of aggregation-caused quenching (ACQ), which can cause fluorescence quenching and reduced therapeutic function when molecules aggregate.6Li X. Kim C.y. Lee S. Lee D. Chung H.-M. Kim G. Heo S.-H. Kim C. Hong K.-S. Yoon J. Nanostructured phthalocyanine assemblies with protein-driven switchable photoactivities for biophotonic imaging and therapy.J. Am. Chem. Soc. 2017; 139: 10880-10886Crossref PubMed Scopus (227) Google Scholar This problem limits the performance of traditional fluorescent probes on biological application, leading to unsatisfactory theranostic effects.7Hong Y. Lam J.W. Tang B.Z. Aggregation-induced emission.Chem. Soc. Rev. 2011; 40: 5361-5388Crossref PubMed Scopus (4395) Google Scholar Aggregation-induced emission (AIE) probes can be broadly classified into small molecular probes and nanoparticle (NP) probes. Both rely on organic fluorogens with aggregation-induced emission (AIEgens) to serve as the foundation and core theranostic element.8Ding D. Li K. Liu B. Tang B.Z. Bioprobes based on AIE fluorogens.Acc. Chem. Res. 2013; 46: 2441-2453Crossref PubMed Scopus (1359) Google Scholar Different from traditional ACQ molecules, AIEgens (e.g., tetraphenylethene [TPE]) are almost non-emissive as single molecules, while they can be activated to emit strong fluorescence in the solid or aggregate state due to reduced π–π stacking interaction and restriction of intramolecular motion.9Mei J. Leung N.L. Kwok R.T. Lam J.W. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (4474) Google Scholar These special features make them excellent optical materials for the design of activatable AIE molecular probes and AIE NP probes, which have been applied to the visualization of biological systems, such as cell imaging, disease detection, and image-guided surgery.10Liang J. Tang B.Z. Liu B. Specific light-up bioprobes based on AIEgen conjugates.Chem. Soc. Rev. 2015; 44: 2798-2811Crossref PubMed Google Scholar, 11Li K. Liu B. Polymer-encapsulated organic nanoparticles for fluorescence and photoacoustic imaging.Chem. Soc. Rev. 2014; 43: 6570-6597Crossref PubMed Google Scholar, 12Wang Y.F. Zhang T. Liang X. Aggregation-induced emission: lighting up cells, revealing life!.Small. 2016; 12: 6451-6477Crossref PubMed Scopus (92) Google Scholar Moreover, through precise chemical design, AIEgens can further be empowered with photosensitization capability and photothermal effect. In this regard, traditional AIEgens can be immediately transformed into versatile theranostic photosensitizers (PSs), which have great potential in diagnosis and treatment of various diseases (Figure 1).13Hu F. Xu S. Liu B. Photosensitizers with aggregation-induced emission: materials and biomedical applications.Adv. Mater. 2018; 30: e1801350Crossref PubMed Scopus (348) Google Scholar Different from conventional organic PSs, such as Chlorin e6 (Ce6), which exhibit compromised brightness and photosensitization in the aggregate state, AIE PSs could concurrently improve fluorescence and production of reactive oxygen species (ROS) as molecular aggregates, revealing unique advantages on light-up bioimaging and disease theranostics.14Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (332) Google Scholar In addition, due to the rotor structure, AIEgens also show tremendous potential in the design of photothermal agents (PTAs). Through the introduction of AIE rotors and spacer groups into planar skeleton molecules, the nonradiative decay of PTAs could be further improved, leading to excellent photothermal conversion efficiency for photoacoustic (PA) imaging and photothermal therapy (PTT). Recently, great efforts have been made to design more innovative and rational AIE theranostic probes for precise medical treatment and personalized care.15Hu Q. Gao M. Feng G. Liu B. Mitochondria-targeted cancer therapy using a light-up probe with aggregation-induced-emission characteristics.Angew. Chem. Int. Ed. 2014; 53: 14225-14229Crossref PubMed Scopus (285) Google Scholar To achieve this goal, understanding the fundamental mechanisms of biological processes in cutting-edge biomedical fields, followed by the rational chemical design of AIE probes, is necessary. Nowadays, increasing designs have been channeled toward developing AIE probes from a biological perspective rather than just a physical (i.e., chemical and material properties) change. For example, inspired by natural metabolic reactions occurring in pathogens and eukaryotes, we have recently developed a series of metabolizable light-up AIE theranostic probes, which showed excellent performance on detection and therapy of ineradicable intracellular bacterial infections.16Hu F. Qi G. Kenry Mao D. Zhou S. Wu M. Wu W. Liu B. Visualization and in situ ablation of intracellular bacterial pathogens through metabolic labeling.Angew. Chem. Int. Ed. 2019; 59: 9288-9292Crossref PubMed Scopus (50) Google Scholar,17Mao D. Hu F. Qi G. Ji S. Wu W. Kong D. Liu B. One-step in vivo metabolic labeling as a theranostic approach for overcoming drug-resistant bacterial infections.Mater. Horizons. 2020; 7: 1138-1143Crossref Google Scholar In another example, given that oxidative damage of intracellular organelles could efficiently evoke immunogenic cell death (ICD), an AIE theranostic probe that specifically targets mitochondria has been synthesized, which could robustly induce ICD of tumor cells to produce a tumor vaccine for immunotherapy.18Chen C. Ni X. Jia S. Liang Y. Wu X. Kong D. Ding D. Massively evoking immunogenic cell death by focused mitochondrial oxidative stress using an AIE luminogen with a twisted molecular structure.Adv. Mater. 2019; 31: 1904914Crossref Scopus (199) Google Scholar We think that the idea of biology-oriented design of AIE theranostic probes will attract more attention in the years to come. It is expected that this shift of design paradigm will stimulate more ideas for the future design of theranostic materials, which will also broaden the potential applications of AIEgens. In this review, we present recent advances in the development of AIE theranostic probes for biomedical applications and future clinical translation, driven by biological principles. We first summarize the biology-oriented design strategies of AIE theranostic probes and discuss their potential advantages for disease theranostics. Next, we introduce recent representative examples of the design of AIE probes for various biomedical applications, including activatable cancer theranostics, anti-immunotherapy, rapid discrimination and killing of bacteria, and intracellular bacterial therapy. Finally, we present the current challenges of developing AIE theranostic probes for clinical translation and propose future perspectives based on the analysis of recent advances in biology and materials. We hope to stimulate more collaborative studies to facilitate the practical translation of AIE probes. A brief summary of the classification of AIEgen-based theranostic probes is presented in this section. The design and synthesis of AIE theranostic materials are introduced in the following order: design of AIEgens, AIE PSs, and AIE PTAs, synthesis of small molecular/NPs theranostic probes, and the design paradigm of AIE theranostic probes, respectively, with strong focus on the designs behind the synthesis and application of AIE theranostic probes. The properties of AIEgens largely determine the performance of AIE theranostic probes. Fluorescent molecules with a propeller-shaped structure, such as TPE and tetraphenylsilol (TPS), are two typical AIEgens, which can be regarded as an example of building units for the construction of more complicated AIEgens with different functions.9Mei J. Leung N.L. Kwok R.T. Lam J.W. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-11940Crossref PubMed Scopus (4474) Google Scholar Different from ACQ molecules, TPE and TPS show negligible fluorescence emission in good solvents but, upon aggregation, they emit strong fluorescence. Some traditional ACQ fluorogens can be endowed with AIE activity through the chemical conjugation of TPE onto the ACQ structure.19Shih P.I. Chuang C.Y. Chien C.H. Diau E.G. Shu C.F. Highly efficient non-doped blue-light-emitting diodes based on an anthrancene derivative end-capped with tetraphenylethylene groups.Adv. Funct. Mater. 2007; 17: 3141-3146Crossref Scopus (233) Google Scholar,20Shen X.Y. Wang Y.J. Zhang H. Qin A. Sun J.Z. Tang B.Z. Conjugates of tetraphenylethene and diketopyrrolopyrrole: tuning the emission properties with phenyl bridges.Chem. Rev. 2014; 50: 8747-8750Google Scholar Meanwhile, through introduction of electron donor and electron-withdrawing groups into building units to form donor–acceptor structures, the absorption/emission spectra of AIEgens can cover the visible to near-infrared (NIR) range.21Feng G. Tay C.Y. Chui Q. Liu R. Tomczak N. Liu J. Tang B.Z. Leong D.T. Liu B. Ultrabright organic dots with aggregation-induced emission characteristics for cell tracking.Biomaterials. 2014; 35: 8669-8677Crossref PubMed Scopus (82) Google Scholar, 22Ding D. Mao D. Li K. Wang X. Qin W. Liu R. Chiam D.S. Tomczak N. Yang Z. Tang B.Z. et al.Precise and long-term tracking of adipose-derived stem cells and their regenerative capacity via superb bright and stable organic nanodots.ACS Nano. 2014; 8: 12620-12631Crossref PubMed Scopus (121) Google Scholar, 23Liu J. Chen C. Ji S. Liu Q. Ding D. Zhao D. Liu B. Long wavelength excitable near-infrared fluorescent nanoparticles with aggregation-induced emission characteristics for image-guided tumor resection.Chem. Sci. 2017; 8: 2782-2789Crossref PubMed Google Scholar More importantly, through molecular design, some AIEgens can also achieve the capability for photosensitization to become AIE theranostic molecules. It is known that most conventional PSs, such as Ce6, generally possess a planar structure, which usually leads to decreased fluorescence intensity and self-quenching of photosensitization in the aggregate state via nonradiative decay.24Cai X. Liu B. Aggregation-induced emission: recent advances in materials and biomedical applications.Angew. Chem. Int. Ed. 2020; 132: 9952-9970Crossref Google Scholar Since AIEgens generally show low nonradiative decay in the aggregate state, the potential of developing AIE PSs with enhanced emission and the capability for robust photosensitization in the aggregate state is promising. To obtain such AIE PSs, the triplet-singlet excited state (S1–T1) energy gap (ΔEst) should be reduced, thereby promoting the intersystem crossing process to produce more ROS (Figure 2A).25Wu W. Mao D. Xu S. Ji S. Hu F. Ding D. Kong D. Liu B. High performance photosensitizers with aggregation-induced emission for image-guided photodynamic anticancer therapy.Mater. Horizons. 2017; 4: 1110-1114Crossref Google Scholar For example, due to the large ΔEst, TPE cannot really generate ROS. However, introducing donor–acceptor pairs and spacers can significantly reduce highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) overlap, leading to smaller ΔEst and higher ROS production efficiency of the obtained AIE PSs, such as TPEDC (Figure 2C). In addition, a benzo-2,1,3-thiadiazole (BT) moiety as an auxiliary acceptor can be conjugated onto TPEDC to further broaden the absorption spectrum for stronger light harvesting and tissue penetration. TPEBTDC has a stronger ROS production than TPEDC, with an absorption of around 600 nm. In addition, once a strong electron-withdrawing group (e.g., TCAQ) was introduced into TPE, the emission of TPE could be further red-shifted up to 820 nm.26Wu W. Mao D. Hu F. Xu S. Chen C. Zhang C. Chen X. Yuan Y. Ding D. Kong D. Liu B. A highly efficient and photostable photosensitizer with near-infrared aggregation-induced emission for image-guided photodynamic anticancer therapy.Adv. Mater. 2017; 29: 1700548Crossref Scopus (257) Google Scholar TPETCAQ has a robust photosensitizing effect and NIR fluorescence emission simultaneously, revealing potential advantages on image-guided photodynamic therapy (PDT). On the other hand, as a new type of the theranostic molecules, AIE PTAs have also been developed for PA imaging and PTT in recent years.29Wu W. Li Z. Nanoprobes with aggregation-induced emission for theranostics.Mater. Chem. Front. 2020; https://doi.org/10.1039/D0QM00617CCrossref PubMed Google Scholar Because of the rotor structures, AIEgens have strong intramolecular motions for nonradiative decay of excitation, thus showing a good photothermal effect in the molecular state.29Wu W. Li Z. Nanoprobes with aggregation-induced emission for theranostics.Mater. Chem. Front. 2020; https://doi.org/10.1039/D0QM00617CCrossref PubMed Google Scholar However, intramolecular motion of AIEgens can be largely inhibited when they aggregate in poor solvents, which reduce their photothermal effect. Therefore, it is anticipated that if the intramolecular motion of AIEgens can be sustained upon aggregation, the molecule would have a good photothermal conversion efficiency in NP-based probes (Figure 2B). Recently, several approaches to reduce the steric effect of AIEgens in the aggregation state have been proposed.27Cai X. Liu J. Liew W.H. D Y. Geng J. Thakor N. Yao K. Liao L. Liu B. Organic molecules with propeller structures for efficient photoacoustic imaging and photothermal ablation of cancer cells.Mater. Chem. Front. 2017; 1: 1556-1562Crossref Google Scholar, 30Liu S. Zhou X. Zhang H. Ou H. Lam J.W. Liu Y. Shi L. Ding D. Tang B.Z. Molecular motion in aggregates: manipulating TICT for boosting photothermal theranostics.J. Am. Chem. Soc. 2019; 141: 5359-5368Crossref PubMed Scopus (112) Google Scholar, 31Liu S. Li Y. Zhang H. Zhao Z. Lu X. Lam J.W. Tang B.Z. Molecular motion in the solid state.ACS Mater. Lett. 2019; 1: 425-431Crossref Scopus (45) Google Scholar Through conjugation of a skeleton molecule with planar structures and D–A structures into AIEgens, the light-harvesting ability of PTAs could be further enhanced in the NIR region. More importantly, isolation molecular spacers are also conjugated on the PTAs, which could ensure the free motion of rotors in the aggregated state, promoting nonradiative decay of PTAs for high photothermal conversion efficiency. In an initial design, BTPETTQ was firstly reported by our group through combination of TTQ with the spacer group hexyloxyphenyl and TPE (Figure 2D).27Cai X. Liu J. Liew W.H. D Y. Geng J. Thakor N. Yao K. Liao L. Liu B. Organic molecules with propeller structures for efficient photoacoustic imaging and photothermal ablation of cancer cells.Mater. Chem. Front. 2017; 1: 1556-1562Crossref Google Scholar After encapsulation into NPs, BTPETTQ could produce more obvious photothermal effects than TTQ only, with a photothermal conversion efficiency of 40%, confirming the important role of molecular motions in PTAs. Subsequently, more AIE PTAs, such as NIRb14 and 2TPE-NDTA, were developed by conjugating planar core molecules with different AIE moieties and larger-size spacer groups, which could further decrease fluorescence emission and improve photothermal conversion efficiency of PTAs due to the enhanced nonradiative decay process, showing improved PA imaging and PTT performance.30Liu S. Zhou X. Zhang H. Ou H. Lam J.W. Liu Y. Shi L. Ding D. Tang B.Z. Molecular motion in aggregates: manipulating TICT for boosting photothermal theranostics.J. Am. Chem. Soc. 2019; 141: 5359-5368Crossref PubMed Scopus (112) Google Scholar,28Zhao Z. Chen C. Wu W. Wang F. Du L. Zhang X. Xiong Y. He X. Cai J. Kwok R.T. Lam J.W. et al.Highly efficient photothermal nanoagent achieved by harvesting energy via excited-state intramolecular motion within nanoparticles.Nat. Commun. 2019; 10: 1-11PubMed Google Scholar According to different synthetic approaches, AIE PSs or PTAs can be further fabricated into two types of theranostic probes.14Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (332) Google Scholar The first type is the AIE molecular probe, which mainly employs AIE PSs as the main structure to construct small molecular probes for pathogen and tumor cell therapy. It is well known that AIE PSs tend to aggregate and generate fluorescence in aqueous solution because of their hydrophobic structures. Therefore, AIE probes are usually designed to have a hydrophilic group, such as a phosphate group, which ensures good dispersibility of the probe with negligible background signal under physiological conditions.32Hu F. Yuan Y. Wu W. Mao D. Liu B. Dual-responsive metabolic precursor and light-up AIEgen for cancer cell bio-orthogonal labeling and precise ablation.Anal. Chem. 2018; 90: 6718-6724Crossref PubMed Scopus (23) Google Scholar,33Yuan Y. Xu S. Cheng X. Cai X. Liu B. Bioorthogonal turn-on probe based on aggregation-induced emission characteristics for cancer cell imaging and ablation.Angew. Chem. Int. Ed. 2016; 55: 6457-6461Crossref PubMed Scopus (127) Google Scholar In addition, AIE probes also contain targeting or reactive groups to improve their cell or tissue targeting specificity. Once specifically recognized and bound to biomolecules though biochemical reactions, AIE probes could be induced to produce light-up fluorescence and photosensitization due to restriction of intramolecular motions. Moreover, the high specificity causes photothermal and photosensitization of probes on the targeted cells, leading to more precise manipulation of the biological process and effective photo treatment, while avoiding potential problems of phototoxicity. Meanwhile, many approaches have been developed to form homogeneous AIE NPs with different size and surface functionalities for various biomedical applications, such as cell tracking, microvascular imaging, and image-guided PDT/PTT therapy.34Chen S. Wang H. Hong Y. Tang B.Z. Fabrication of fluorescent nanoparticles based on AIE luminogens (AIE dots) and their applications in bioimaging.Mater. Horizons. 2016; 3: 283-293Crossref Google Scholar, 35Wang Y. Chen M. Alifu N. Li S. Qin W. Qin A. Tang B.Z. Qian J. Aggregation-induced emission luminogen with deep-red emission for through-skull three-photon fluorescence imaging of mouse.ACS Nano. 2017; 11: 10452-10461Crossref PubMed Scopus (113) Google Scholar, 36Li K. Zhu Z. Cai P. Liu R. Tomczak N. Ding D. Liu J. Qin W. Zhao Z. Hu Y. et al.Organic dots with aggregation-induced emission (AIE dots) characteristics for dual-color cell tracing.Chem. Mater. 2013; 25: 4181-4187Crossref Scopus (109) Google Scholar A common and important method is to physically encapsulate AIE PSs/PTAs into NPs using the nanoprecipitation process, which involves transferring AIE PSs/PTAs and an amphipathic polymer (e.g., DSPE-poly(ethylene glycol) [PEG]) from organic solvents into aqueous media to induce the self-assembly process.37Li K. Qin W. Ding D. Tomczak N. Geng J. Liu R. Liu J. Zhang X. Liu H. Liu B. Tang B.Z. Photostable fluorescent organic dots with aggregation-induced emission (AIE dots) for noninvasive long-term cell tracing.Sci. Rep. 2013; 3: 1-10Google Scholar Such AIE PS-based NPs generally show excellent biocompatibility and theranostic properties, such as bright fluorescence, anti-bleaching effects, and good ROS generation capability. On the contrary, when AIE PTAs are encapsulated into NPs, AIE NPs show very weak fluorescence emission due to an enhanced nonradiative decay process, leading to stronger photothermal conversion for PA imaging and PTT of diseases.29Wu W. Li Z. Nanoprobes with aggregation-induced emission for theranostics.Mater. Chem. Front. 2020; https://doi.org/10.1039/D0QM00617CCrossref PubMed Google Scholar In addition, the size of AIE NPs could be fine-tuned by changing the ratio of AIE PSs/PTAs to matrix and control of the sonication parameter.14Feng G. Liu B. Aggregation-induced emission (AIE) dots: emerging theranostic nanolights.Acc. Chem. Res. 2018; 51: 1404-1414Crossref PubMed Scopus (332) Google Scholar,38Wang D. Qian J. Qin W. Qin A. Tang B.Z. He S. Biocompatible and photostable AIE dots with red emission for in vivo two-photon bioimaging.Sci. 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Targeted and imaging-guided in vivo photodynamic therapy for tumors using dual-function, aggregation-induced emission nanoparticles.Nano Res. 2018; 11: 2756-2770Crossref Scopus (19) Google Scholar AIE NPs could also co-capsulate other components, such as chemodrugs, traditional fluorogens, and reactive molecules, for combination therapy, responsive theranostic probes, and amplified optical imaging.40Geng J. Zhu Z. Qin W. Ma L. Hu Y. Gurzadyan G.G. Tang B.Z. Liu B. Near-infrared fluorescence amplified organic nanoparticles with aggregation-induced emission characteristics for in vivo imaging.Nanoscale. 2014; 6: 939-945Crossref PubMed Scopus (74) Google Scholar, 41Yuan Y. Xu S. Zhang C.-J. Liu B. Light-responsive AIE nanoparticles with cytosolic drug release to overcome drug resistance in cancer cells.Polym. Chem. 2016; 7: 3530-3539Crossref Google Scholar, 42Wu W. Mao D. Cai X. Duan Y. Hu F. Kong D. Liu B. ONOO- and ClO-responsive organic nanoparticles for specific in vivo image-guided photodynamic bacterial ablation.Chem. Mater. 2018; 30: 3867-3873Crossref Scopus (45) Google Scholar Early reporting of AIE probes for biomedical applications generally followed the trend of chemistry-oriented design, synthesis, and validation with biological experiments to demonstrate the material properties. The final theranostic performance mainly relied on the optical and physical properties of AIEgens. However, such straightforward design philosophy has the risk of yielding poor outcomes, and even the risk of side effects in the complex and in the dynamic biological systems. With in-depth research, many advanced AIE probes designed from a biological perspective have been developed in recent years, which are capable of visualizing and modulating biological species more intelligently and effectively.24Cai X. Liu B. Aggregation-induced emission: recent advances in materials and biomedical applications.Angew. Chem. Int. Ed. 2020; 132: 9952-9970Crossref Google Scholar These biology-oriented AIE probes could interact with specific biological molecules that exist in the surrounding environment to activate imaging and therapeutic functions. Thus far, according to different biological design paradigms, AIE theranostic probes can be designed and formulated based on three different modes (Figure 3). Firstly, according to the different surface properties and structures of targeted biomolecules, AIEgens or NPs can be chemically conjugated with different targeting groups, which facilitates the recognition and binding of the yielded AIE probes to biomolecular targets. For example, as most bacteria carry a negative surface charge, introduction of a positively charged group into AIEgens can lead to enhanced non-specific binding with various bacteria due to an electrostatic interaction, which causes the activation of the theranostic function of AIEgens.43Kwok R.T. Leung C.W. Lam J.W. Tang B.Z. Biosensing by luminogens with aggregation-induced emission characteristics.Chem. Soc. Rev. 2015; 44: 4228-4238Crossref PubMed Google Scholar In addition, positively charged AIEgens and NPs can a