Xin
Li
a,
Zhao
Wang
a,
Jing
He
a,
Haitham
Al-Mashriqi
a,
Jia
Chen
*a and
Hongdeng
Qiu
*ab
aResearch Center for Natural Medicine and Chemical Metrology, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: jiachen@licp.cas.cn; hdqiu@licp.cas.cn
bKey Laboratory of Rare Earths, Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341119, China
First published on 10th October 2024
AIE luminogens (AIEgens) are a class of unique fluorescent molecules that exhibit significantly enhanced luminescence properties and excellent photostability in the aggregated state. Recently, it has been found that some AIEgens can produce reactive oxygen species, which means that they may have potential enzyme-like activities and are thus termed “AIEzymes”. Consequently, the discovery and design of novel AIEgens with enzyme-like properties have emerged as a new and exciting research direction. Additionally, AIEgens can enhance the catalytic efficiency of traditional nanozymes by direct combination, thereby endowing the nanozymes with multifunctionality. In this regard, nanozymes with aggregation-induced emission (AIE) properties, which represents a win–win integration, not only take full advantage of the low cost and stability of nanozymes, but also incorporate the excellent biocompatibility and fluorescence properties of AIEgens. These synergistic compounds bring about new opportunities for various applications, making AIEzymes of interest in biomedical research, food analysis, environmental monitoring, and especially imaging-guided diagnostics. This review will provide an overview of the latest strategies and achievements in the rational design and preparation of AIEzymes, as well as current research trends, future challenges and prospective solutions. We expect that this work will encourage and motivate more people to study and explore AIEzymes to further promote their applications in various fields.
In the field of catalysis, enzymes play a pivotal role due to their ability to significantly accelerate chemical reactions.13,14 However, although the use of enzymes expanded in the early 20th century, their inherent instability and limitations within specific reaction environments gradually became apparent.15,16 Initially, research switched to the development of artificial enzymes, which are made from tiny organic molecules such as cyclodextrins and used to enhance or replicate the functions of natural enzymes.17–22 In recent years, with the rise of nanotechnology, the concept of nanozymes has garnered significant attention since 2007, when Yan et al. initially identified that Fe3O4 nanoparticles exhibit peroxidase-like activity.23 This groundbreaking finding demonstrated that some nanomaterials possess both physicochemical properties and distinctive enzyme-like catalytic capabilities.24,25 In the last 17 years of research, metals, metal oxides, and carbon compounds have been shown to possess catalytic activities similar to peroxidase (POD), catalase (CAT), oxidase (OXD), and superoxide dismutase (SOD).26–31 It has been reported in the literature that nanozymes possess properties such as high stability, low cost, easy preparation and easy modification. In addition, as the selectivity problem has been gradually overcome and improved, nanozymes have been tailored to specific substrates or environmental conditions, endowing them with a high degree of versatility in various applications.32–37 Besides, nanozymes stand out from natural enzymes due to their possession of physical and chemical properties unique to nanomaterials, endowing them with bifunctional or multifunctional characteristics.38–41 Recently, the integration of nanozyme catalytic activity with other physicochemical properties has become a major hotspot.42–45 This synergistic approach aids in discovering new functions of nanozymes and broadening their applications.46–48 If nanozymes are endowed with AIE properties, it will open up a new field of great significance.
Nanozymes with aggregation-induced luminescence (AIE) properties, known as AIEzymes, combine the catalytic activity of nanozymes with the optical properties of AIE materials. This fusion not only retains the traditional advantages of nanozymes, such as their design flexibility, ease of preparation and regulation, and excellent stability, but also gains additional properties from AIEgens, including low background noise, good photostability, a substantial Stokes shift, and favorable biocompatibility, forming a powerful combination. This synergy has led to a thriving research direction. Specifically, AIEzymes have broad applications in the field of biomedicine. Their unique AIE properties enable them to exhibit low background noise and good photostability in biological systems, aiding in the real-time tracking and monitoring of biomarkers. Furthermore, AIEzymes can produce ROS in response to light, killing cancer cells or pathogens in a way similar to enzyme catalysis, providing novel approaches to cancer treatment and infection management. Additionally, AIEzymes demonstrate great potential in the field of sensing and analysis. Their high sensitivity and selectivity allow them to detect trace amounts of biomolecules and chemical pollutants, providing effective tools for environmental monitoring and food safety analysis.49,50
To date, luminescent materials with AIE properties have received great attention in various research fields.51,52 Despite comprehensive reviews on the combination of AIE luminogens and their applications in the fields of nanomedicine and photodynamic therapy (PDT),53,54 research into the enzyme-like activities of AIE materials remains lacking in systematic and in-depth exploration. This research gap has limited our comprehensive understanding of the potential applications of AIE materials and created obstacles in optimizing their practical use. Currently, there is a lack of a comprehensive review on the progress of nanozymes with AIE properties; hence, the publication of this review is both crucial and urgently needed. This work summarizes two research strategies for designing new AIEzymes (Fig. 1): first, the synthesis of novel nanozymes utilizing the enzyme-like activity of AIE luminogens, and second, the regulation of the aggregation state of AIE luminogens through nanozyme catalysis to construct functional integrated systems, providing a comprehensive view of the latest research trends in AIEzymes. Additionally, this review thoroughly introduces the application of AIEzymes in key areas such as biomedical diagnostics, treatment, food safety analysis, and environmental monitoring, revealing their vast potential in various industries. We hope that this review will become a valuable resource for researchers in related fields, inspiring new research ideas and providing theoretical foundations and practical guidance.
Fig. 2 (A) Illustration of AIEgens with enzyme-like activity; (B) possible mechanism for the sustained chemiluminescence of the AIEzyme/LUM system. Reproduced from ref. 59 with permission from American Chemical Society, copyright 2023. |
For example, combining metal-based supramolecular systems with AIEgens can endow nanozymes with unique spectral properties. In this context, Zhang and colleagues made a breakthrough by incorporating AIEgens as ligands in the synthesis of novel nanozymes, such as organic molecular cages (OMCs) like TPE-Cages. These cage structures not only exhibited exceptional luminescence properties but also showed potential applications in gas detection, storage, and biological imaging.69,70 Additionally, Zhang's team introduced ZnDPA (Zn coordinated dipicolylamine) units into the TPE-Cages, creating a positively charged supramolecular cage, ZnDPA-TPE-Cage.71 As shown in Fig. 3A, this structure not only enhances the AIE characteristics but also demonstrates outstanding oxidase-like activity, generating ROS under light conditions to selectively recognize and eradicate Gram-positive bacteria. This research has opened new avenues for integrating AIEgens with enzyme-like functions. Nevertheless, there are limitations to the broad application of these materials. For instance, although ZnDPA-TPE-Cage has demonstrated the ability to produce ROS under light exposure, effectively targeting Gram-positive bacteria for recognition and eradication, its efficacy against a wider range of bacterial species and other microorganisms is yet to be fully explored. Additionally, the potential for broad-spectrum antimicrobial activity may require further investigation.
Fig. 3 (A) Process for the preparation of novel organic molecular cages with oxidase properties and AIE properties. Reproduced from ref. 71 with permission from Royal Society of Chemistry, copyright 2021. (B) A Pt(II)-based molecular cage with aggregation-induced emission for enzymatic photocyclization of alkynylaniline. Reproduced from ref. 72 with permission from John Wiley and Sons, copyright 2022. |
In addition, although the above-mentioned materials can utilize the ROS-generating properties of AIEgens to achieve synergistic catalysis, the catalytic activity of the existing AIEzymes still needs to be further enhanced. Duan and colleagues have proposed an innovative strategy by incorporating triphenylphosphine ligands to construct AIE-active platinum-based cages, which successfully activate amino and alkyne groups through π-acidic Pt(I) complexes to facilitate photocyclization reactions.72Fig. 3B illustrates how the AIE-active cage activates amino groups via photoinduced electron transfer (PET) in the AIE excited state, while simultaneously activating alkyne groups through π-acidic Pt(I) complex formation, thus promoting the photocyclization reaction. Notably, the presence of water enhances the luminescence of AIE, increases the material's substrate affinity, and improves photocatalytic efficiency. This research, by integrating AIE fluorophores with π-acidic active sites, addresses the solubility issues of metal–organic cages in aqueous media and points to a new direction for developing more efficient enzyme mimics. Furthermore, Rana et al. developed a Pt(II)-centric supramolecular composite, named NanoPtA, which exhibits enhanced AIE in aqueous media. This complex demonstrates exceptional light driven oxidase-like activity and has the potential to degrade environmental pollutants.73 However, before practical applications, these composite materials must undergo evaluation regarding their stability, toxicity, cost, and feasibility for large-scale production.
Fig. 4 (A) Schematic diagram of several strategies for integrating nanozymes with AIEgens. (B) Dual enzyme cascade catalytic system AOX/CeO2@TPE + OPD that realizes ratiometric fluorescence–colorimetric dual-mode detection of CH3SH. Reproduced from ref. 74 with permission from American Chemical Society, copyright 2023. (C) NSs@DCPy nanohybrid and their application in oxygen self-contained photodynamic therapy. Reproduced from ref. 75 with permission from Elsevier, copyright 2022. (D) An AIEgen-based COF nanozyme for tumor cell-specific imaging and therapy. Reproduced from ref. 76 with permission from Elsevier, copyright 2023. |
One simple and effective approach to integrating these two components is encapsulating AIEgens within the cavity of nanozymes. This results in composites with several benefits: (1) protection of AIEgens. Encapsulation shields AIEgens from degradation by enzymes or other biomolecules, thus improving their stability and effectiveness in the body. (2) Targeted delivery. The strategic design of nanozymes enables targeted delivery to specific cells or tissues, facilitating the localized activation of AIEgens and allowing for precise imaging or therapy. (3) Synergistic effects. AIEgens with ROS-generating capabilities can enhance the catalytic functions of nanozymes, amplifying their overall effect. In this regard, various inorganic hollow spheres, including metals77 and metal oxides,74,78 have been engineered to encapsulate AIEgens.
Among them, mesoporous CeO2 nanospheres are considered as important candidates for the design of multifunctional and intelligent optical/electro-sensing platforms due to their excellent stability, good biocompatibility, and loading capacity.79 Driven by this, Wu et al. synthesized mesoporous cerium oxide nanospheres using hydrothermal techniques and embedded TPE into hollow CeO2 nanospheres to synthesize a novel blue fluorescent nanozyme CeO2@TPE with high peroxidase activity.74Fig. 4B illustrates the design of a dual-enzyme cascade fluorescence–colorimetric dual-mode system, AOX/CeO2@TPE + OPD, which is used for the detection of CH3SH in the presence of ethanol oxidase (AOX) and o-phenylenediamine (OPD). In the absence of CH3SH, AOX/CeO2@TPE emitted blue fluorescence at 441 nm. However, in the presence of CH3SH, AOX is activated to produce H2O2, which then triggers the catalytic activity of CeO2@TPE, generating ·OH. This results in the oxidation of non-fluorescent OPD to 2,3-diaminophenazine (DAP), emitting yellow fluorescence at 550 nm. Simultaneously, the internal filtering effect (IFE) led to a gradual reduction in the blue fluorescence of AOX/CeO2@TPE at 441 nm, yielding a characteristic ratio fluorescence response. Moreover, the enzyme cascade exhibited an increase in absorbance at 425 nm due to the transformation of colorless OPD to dark yellow DAP, visible to the naked eye, in the presence of CH3SH. Therefore, this dual-mode strategy, which combines ratiometric fluorescence and colorimetry, provides an effective means for the detection of CH3SH. By integrating smartphone technology, this method makes it possible to reliably and accurately detect CH3SH in field environmental samples. Although this study highlights the potential of integrating nanozymes and AIEgens to create efficient field sensors for environmental monitoring and pollution source analysis, further research is needed to validate and optimize their performance for various environmental monitoring and pollution source analysis tasks under different conditions.
Besides encapsulation, adsorbing or modifying AIEgens onto the surface of nanozymes is another common integration strategy. As depicted in Fig. 4C, Tang and colleagues drew inspiration from bonsai to synthesize ultra-thin nanosheets (NSs) as a “potting soil” from vermiculite, and subsequently “planted” DCPy (an AIEgen) onto the NSs' surface via electrostatic attraction, creating an AIE-vermiculite nano-hybrid photosensitizer (NSs@DCPy) for PDT.75 When NSs@DCPy is absorbed by hypoxic tumors and exposed to light radiation, NSs@DCPy, as an enzyme mimic, not only produces 1O2 and ·OH, but also catalyzes H2O2 to O2, thus providing “nutrients” for AIEgens to alleviate hypoxia and significantly improving the efficacy of photodynamic therapy. Remarkably, NSs@DCPy also demonstrated glutathione oxidase activity, capable of inducing cell ferroptosis. Hence, this study introduces an AIEgen-vermiculite nanohybrid platform that enables “self-sufficient” photodynamic cancer therapy, augmented by ferroptosis and oxygen generation. Liao et al. utilized bioorthogonal reactions to modify AIEgens onto the multifunctional Zn@MOF surface, yielding a nanozyme (Zn@MOF-TPD) with AIE properties, and applied this to enhance recovery after spinal cord injury.80 However, despite the great potential of these integration strategies, there are still some challenges in practical applications. For example, further research needs to address whether the production process for adsorbing AIEgens onto nanozymes is simple and cost-effective and whether these conjugates are able to maintain stable adsorption and avoid detachment or degradation in the complex physiological environments in living organisms.
Meanwhile, AIEgen-based covalent organic frameworks (COFs) offer new avenues for the integration of nanozymes.81,82 Utilizing their substantial specific surface area, the integration of AIEgens and nanozymes can also be achieved by modifying the nanozymes onto the surface of AIEgen-based COFs.76,83Fig. 4D illustrates a bionic multifunctional COF nanozyme reported by the Zhang group, which consists of an AIEgen-based COF (TPE-s COF) and gold nanoparticles (Au NPs) immobilized on its surface.76 This nanozyme was co-cultured with HepG2 cells, leading to lipophilic TPE-s COF-Au@cisplatin fusion with the cell membrane. Subsequently, an inactivated nanozyme (M@TPE-s COF-Au@cisplatin) was prepared by the freezing-shaking method and endocytosed by HepG2 cells, losing its proliferation and pathogenicity. After that, laser irradiation triggered the release of TPE-s COF-Au nanozymes and cisplatin for photothermal and drug therapy, respectively. This research introduces a novel strategy for synthesizing tumor-derived fluorescent TPE-s COF-Au nanozymes for efficient, synergistic, and targeted chemotherapy–photothermal combination therapy in hepatocellular carcinoma (HCC). Overall, there are several key factors that require special attention when integrating nanozymes with AIEgens by surface modification or adsorption: (1) the binding density must be finely tuned to avoid hindering the nanozyme's active sites and impacting its catalytic performance. Hence, optimizing the binding density is vital. (2) AIEgens' aggregation-induced emission is central to high-sensitivity detection and treatment. Ensuring that the AIEgens aggregate appropriately on the nanozyme surface for efficient luminescence is paramount. (3) The stability of the modified layer is essential for the long-term use of nanozyme-AIEgen integrations. The modified layer needs to be stable enough to resist various environmental conditions, such as pH changes and temperature fluctuations.
Additionally, co-loading AIEgens and nanozymes onto other carriers can further enhance their performance. For example, Luo et al. developed a chloramphenicol (CAP) sensor that combines the properties of aggregation-induced electrochemiluminescence (AI-ECL) and nanozymes, with the AI-ECL serving as the initial signal generator, while the nanozymes amplify the signal.84 As depicted in Fig. 5A, a COF material (COF-AI-ECL) containing AI-ECL groups and Co3O4 nanozymes was synthesized respectively. This material was then crosslinked and immobilized on a gold electrode. Subsequently, molecularly imprinted polymers (MIPs) were formed on the electrode surface using CAP as a template. The inclusion of COF-AI-ECL expanded the MIP's polymerization area, enhancing the MIP's CAP-imprinting effect and adsorption capacity. Additionally, COF-AI-ECL produced a robust and stable ECL signal, while Co3O4 nanozymes amplified the ECL signal in the presence of H2O2. This combination, along with the MIP's selectivity for CAP, effectively suppressed non-specific ECL signals. Consequently, the sensor not only facilitated the sensitive detection of CAP but also provided a model for detecting other analytes. By the same way, they developed a molecularly imprinted ciprofloxacin (CFX) electrochemiluminescence sensor with significantly improved detection sensitivity by utilizing COF-AIECL as a signaling element and Fe3O4@Pt NP nanozymes for signal amplification.87 However, the primary limitations of these kinds of MIP sensors are their small imprint capacity and insufficient membrane stability, which require resolution in future research.
Fig. 5 (A) Novel chloramphenicol sensors based on aggregation-induced electrochemiluminescence and nanozyme signal amplification. Reproduced from ref. 84 with permission from Elsevier, copyright 2021. (B) An injectable nanozyme hydrogel was used as an AIEgen reservoir and release controller for multiple rounds of tumor therapy. Reproduced from ref. 85 with permission from Elsevier, copyright 2021. (C) Preparation process and therapeutic mechanism of Lipo-OGzyme-AIE. Reproduced from ref. 86 with permission from Elsevier, copyright 2019. |
In addition to inorganic carriers, hydrogels have become an ideal choice for loading AIEgens and nanozymes due to their high biocompatibility and photo-controlled reversible phase transition properties.88 Tang et al. developed an injectable nanozyme hydrogel by encapsulating Prussian blue (PB) nanoparticles and AIEgens (CQu) within a hydrogel, serving as an AIEgen reservoir and release controller (ARC).85 As depicted in Fig. 5B, PB NPs convert NIR laser light into heat, triggering hydrogel degradation and the release of CQu. PB nanozymes can then catalyze the decomposition of endogenous hydrogen peroxide to produce O2. Under light irradiation, AIEgens generate high levels of ROS and sufficient O2 to induce cytotoxicity in tumor cells. Due to the hydrogel's extended residence time in the tumor (at least 48 hours), multiple treatment cycles can be achieved with a single injection. This study represents the first use of hydrogels for delivering AIEgens, and the developed ARC system provides a new clinical development tool for therapeutic strategies for cancer patients. However, despite the promising therapeutic potential of this carrier, the storage conditions of hydrogels and their long-term biocompatibility when remaining in the body need to be further evaluated.
Despite the superior properties of the above-mentioned inorganic carriers, their biocompatibility and targeting in biological systems still need to be improved. Consequently, bionic carriers are increasingly becoming a focus for the integration of nanozymes and AIEgens. Huang et al. synthesized manganese dioxide nanoparticles within the hollow cavities of ferritin nanocages (FTn) in a bioinspired manner, creating a hypoxia-tropic nanozyme as an oxygen generator (OGzyme). They further developed a reaction cascade therapeutic nanosystem by integrating the PDT potential of OGzymes with AIEgens.86 As shown in Fig. 5C, AIEgens were encapsulated within phospholipid bilayers, while OGzymes were placed inside the cavity (lipo-OGzyme-AIE). Encapsulating OGzymes within liposomes facilitated their efficient uptake by tumor cells. The inclusion of AIE molecules aided in the delivery of therapeutic substances to the tumor site and their aggregation post-liposome degradation, supporting PDT and enabling multimodal imaging. This proof-of-concept study underscores the multifunctional benefits of biomimetic nanozymes in tumor therapy and validates their potential of modulating tumor hypoxia as a therapeutic approach. Integrating nanozymes with AIEgens on biocarriers can achieve selective treatment of specific cells through targeting, improving the precision of treatment. However, the high production costs of biomimetic carriers, limited loading capacity, and strict conditions required for storage and transportation all limit their feasibility for widespread application.
Fig. 6 (A) Enzyme-motivated cross-linking aggregation of AIEgens for selective visualization and inhibition of inflammatory cells for precise detection and treatment. Reproduced from ref. 90 with permission from John Wiley and Sons, copyright 2018. (B) Dual-mode ochratoxin A immunoassay based on Ce4+-induced oxidation of TMB and Ce3+-induced aggregation of AuNCs. Reproduced from ref. 91 with permission from Elsevier, copyright 2022. (C) Colorimetric/fluorescence dual-mode detection of S. typhimurium based on MCOF-CuO/Au@apt multifunctional nanozymes catalyzing the oxidation of AIEgens (TPE-4A). Reproduced from ref. 92 with permission from Elsevier, copyright 2024. |
On the other hand, inorganic materials such as gold nanoclusters (AuNCs) have also attracted widespread attention in the development of AIEgens due to their exceptional fluorescence characteristics, biocompatibility, ease of synthesis, high quantum yield, and robust colloidal stability.91,94–97 Recently, Zhou et al. devised a dual-signal immunoassay for ochratoxin A (OTA) detection, leveraging Ce4+-induced oxidation of TMB and Ce3+-induced aggregation and emission enhancement of AuNCs.91 In this method, as shown in Fig. 6B, alkaline phosphatase (ALP) catalyzes the dephosphorylation of L-ascorbic acid-2-phosphate (AAP) to produce ascorbic acid (AA), which reduces Ce4+ to Ce3+, inducing an AIE effect on AuNCs and enhancing their fluorescence. Simultaneously, unreacted Ce4+ oxidizes TMB to form blue oxTMB. This multi-signal detection strategy leverages the AIE effect of AuNCs and the enzymatic properties of Ce4+, significantly enhancing the sensitivity of detection. However, it is important to note whether the aggregation and luminescence enhancement of AuNCs are susceptible to potential interferences.
In addition, the catalytic products of nanozymes, especially certain fluorescent products, may also quench the luminescence of AIEgens through the internal filtering effect (IFE).98,99 Li's group found that hemin/G-quadruplex with peroxidase-like properties can catalyze the generation of fluorescent dopachrome from dopamine, which quenches the fluorescence of AIEgens (TPE-BTD) through the IFE.100 Based on this, they proposed a TPE-BTD/dopamine system for the sensitive detection of various biomarkers, including H2O2 and G-quadruplex DNA. Eventually, a paper-based biosensor was developed, utilizing TPE-BTD as an emitter. The TPE-BTD has excellent emission characteristics that enable the sensor to detect a wide range of biomarkers in a reliable, device-free manner for visual identification. This development establishes a new platform for the practical implementation and advancement of biosensing technology. Although this development provides a new platform for device-free detection, given that a variety of substances produced during the nanozyme catalytic process can affect the luminescence of AIEgens, identifying the specific factors leading to the quenching of AIEgens in practical applications may become complex and challenging. Overall, the interaction between nanozymes and AIEgens offers a promising platform for multi-signal detection and biosensing. However, its practical application still faces several challenges. Future research should focus on enhancing the stability, sensitivity, and operability of this system within complex practical environments.
Adhering to this approach, TPE-4A can serve as a substrate for enzymatic reactions, generating color/fluorescence signals through nanozymatic catalytic oxidation.92,102 For instance, Wang et al. engineered a multifunctional nanohybrid, a magnetic covalent organic framework (MCOF) functionalized with aptamers and supported by copper oxide and gold nanoparticles (MCOF-CuO/Au@apt), which exhibits strong magnetic separation capabilities, high oxidase-mimicking activity towards TPE-4A, and high recognition abilities. This system was employed for colorimetric/fluorescent dual-mode sensing of Salmonella typhimurium (S. typhimurium).92 As illustrated in Fig. 6C, the nanohybrids' oxidase mimetic activity diminished when bound to MCOF-CuO/Au@apt S. typhimurium, resulting in reduced colorimetric and fluorescent signals due to TPE-4A oxidation, which led to the establishment of a dual-mode detection method. The results showed that the detection ranges for S. typhimurium were 102–106 CFU mL−1 and 101–106 CFU mL−1 in colorimetric and fluorescence detection modes, respectively, with limits of 7.6 CFU mL−1 and 2.1 CFU mL−1. In addition, by combining a smartphone and an analytical system, on-site portable detection was finally realized, while the method was also validated for reagent sample detection. The use of TPE-4A, which possesses catalytic color development and aggregation-induced emission characteristics, as a signal output instead of conventional display reagents brings about benefits such as reduced background noise and dual-reading capability. These advantages help to improve the accuracy of analysis and broaden the application areas of biosensing technology. Nevertheless, this approach faces several technical challenges. For instance, while TPE-4A reduces background noise through catalytic color development and aggregation-induced emission, its chemical stability in complex biological systems remains uncertain. Additionally, although this study demonstrated the system's efficacy under laboratory conditions, its application to real biological samples may introduce potential errors and limitations that must be addressed.
In this regard, Tang et al. developed platelet-mimicking MnO2 nanozyme-AIEgen composites (PMD) for the interventional PDT of hypoxic tumors.103 As shown in Fig. 7A, these biomimetic nanoparticles demonstrated robust stability and tumor targeting capabilities. Additionally, their catalase-like activity enables oxygen generation in the tumor microenvironment. Significantly, interventional PDT using optical fibers in the peritoneal cavity of orthotopic colon tumors in mice achieved favorable photodynamic therapeutic outcomes. The combination of AIEgen-based nanozymes with biomimetic cell membrane coatings represents an optimal therapeutic platform for targeted antitumor photodynamic therapy. Meanwhile, Zhang et al. constructed a tumor exosome-loaded CuSAZ/AIEgen cascade catalytic system (CCS) for sustained production of hydroxyl radicals and effective tumor treatment.104 The CCS fulfills the need for enhanced and sustained ·OH generation, addressing the issue of insufficient hydrogen peroxide in single-atom nanozyme (SAZ)-catalyzed therapies, and holds promise for clinical applications. Overall, AIEzymes provide new ideas and research directions for the future development of tumor therapy.
Fig. 7 (A) Principles of interventional photodynamic therapy for colon cancer using AIEgen-based bionic nanozymes. Reproduced from ref. 103 with permission from American Chemical Society, copyright 2022. (B) Coordination polymer nanoparticles (CPNPs) with AIEzyme properties are employed in combating biofilms. Reproduced from ref. 12 with permission from John Wiley and Sons, copyright 2023. |
In order to enhance the therapeutic effect, researchers have adopted a multi-therapy synergistic strategy, which achieves a significant increase in therapeutic efficacy by combining two or more different therapeutic modalities. For example, Xia et al. developed a GSH-responsive MnO2 nanosheet combining an AIE-active photosensitizer and a DNAzyme.105 This composite is capable of not only killing tumor cells through PDT, but also intervening in tumor cells at the gene level through gene therapy, thus achieving dual therapy and significantly improving the therapeutic effect. Similarly, Zhang and colleagues used UiO-66 as a carrier, loaded AIEgens to form A-NUiO, and further encapsulated it in copper-doped hydrated ZIF-8 (ZIF-Cu) to create A-NUiO@DCDA@ZIF-Cu. This material can efficiently accumulate in the tumor microenvironment and exhibit fluorescence imaging capabilities, along with the combined effects of PDT and chemodynamic therapy (CDT), providing a new strategy for cancer treatment.106 Although the multiple therapies introduced above theoretically demonstrate promise in enhancing treatment outcomes, their therapeutic specificity and potential side effects still require thorough research to ensure the safety and effectiveness of their clinical applications.
As a result, the integration of AIEgens and nanozymes offers a novel approach to tumor imaging and therapy, particularly in enhancing the efficacy and targeting of PDT. This combination has demonstrated significant potential. However, ensuring the clinical safety and effectiveness of these technologies remains a key focus for future research.
With this regard, Liu and his team further advanced the application of AIEzymes in bacterial detection by developing a chemiluminescence-based system that does not require hydrogen peroxide or an external light source.59 This research not only holds significance for antibacterial therapy but also broadens the practical applications of AIEzymes. The newly developed AIEzyme can catalyze luminol (LUM) to emit blue luminescence at 425 nm in the absence of hydrogen peroxide (H2O2). Upon incubation with E. coli and lysozymes, the adenosine triphosphate (ATP) released from bacterial lysis facilitates the dissolution of ZIF-8 nanoparticles. This dissolution, in turn, releases fluorescent molecules (FL), which leads to an enhancement of the fluorescence signal. Additionally, through the mechanism of chemiluminescence resonance energy transfer (CRET), the chemiluminescence of LUM is diminished, enabling ratiometric bacterial detection. This system boasts long-lasting luminescence, making it particularly suitable for point-of-care testing (POCT), while also improving detection accuracy and ease of use. Notably, compared to traditional single-signal POCT systems, this ratiometric detection method exhibits greater resistance to interference and enhanced visual detection sensitivity. Moreover, the system leverages smartphones as signal readers and analyzers, facilitating on-site quantitative bacterial detection. This innovative detection approach offers new perspectives for the future development of biosensing technologies.
In addition to their various applications, these multifunctional AIEzymes have demonstrated novel antibacterial mechanisms, particularly against resistant biofilms and pathogens. For instance, some nanozymes mimic the activity of DNA-cleaving enzymes.113,114 Han's research team developed an AIE nanomaterial with DNase-like properties, specifically utilizing zirconium-based coordination polymer nanoparticles.12 These nanoparticles possess key advantages such as low activation energy, robust structure, and stable fluorescence. As depicted in Fig. 7B, AIEzymes effectively hydrolyze extracellular DNA within biofilms, disrupting their structure and exhibiting significant penetration capabilities. Remarkably, a single dose of this AIEzyme was sufficient to accelerate the healing of wounds infected by superbugs. This study introduces a method of endowing AIEgens with DNase-like activity and anti-biofilm effects, significantly enhancing their therapeutic value. Moreover, the inherent AIE properties of these nanozymes facilitate easy visualization, enabling researchers and clinicians to track their location and persistence in the wound healing process. This dual functionality, both as a treatment and a diagnostic tool, makes AIEzymes a promising innovation in combating drug-resistant infections and promoting tissue regeneration.
Recently, they developed a dual-mode biosensor that relies on smartphone readout processing for the detection of the bioactive component hypoxanthine (Hx) in food.115 As shown in Fig. 8A, the core of this sensor is a multi-enzyme cascade catalytic system that includes amorphous Fe-doped phosphotungstate nanozyme Fe-Phos@TPE, xanthine oxidase (XOD), and a dual chromogenic substrate OPD. When Hx is present, XOD catalyzes the production of H2O2, activating the peroxidase activity of Fe-Phos@TPE to generate hydroxyl radicals, which further oxidize non-fluorescent OPD to form yellow fluorescent DAP. During this process, the intrinsic blue fluorescence of Fe-Phos@TPE at 440 nm is attenuated due to the internal filter effect of DAP, enabling ratiometric fluorescence analysis. Additionally, OPD is transformed into dark yellow DAP with enhanced absorbance at 435 nm, enabling a colorimetric analysis. By leveraging smartphones, the sensor is capable of outputting Hx-dependent ratiometric fluorescence and colorimetric images, achieving simple, rapid, portable, and on-site intelligent detection. Moreover, the nanozyme Fe-Phos@TPE and XOD in this system exhibit optimal catalytic activity at near-neutral pH 6.5, avoiding the potential effects of acidic conditions on food components, making this method more advantageous in practical applications.
Fig. 8 (A) Diagram of the smartphone-assisted Hx-activated multienzyme cascade biosensor for on-site visual fluorescence and colorimetric dual-mode detection. Reproduced from ref. 115 with permission from Elsevier, copyright 2024. (B) Scheme of PTDNPs@MnO2 preparation and its application in an AIE-PAD for AChE and OP sensing. Reproduced from ref. 78 with permission from Elsevier, copyright 2021. (C) Illustration of Ce-Au NC fabrication and use in a fluorescence and colorimetric dual-mode paper-based sensor for Hg2+ detection. Reproduced from ref. 116 with permission from Elsevier, copyright 2024. |
Despite the sensor system demonstrating good detection performance, it still faces technical challenges. For instance, while Fe-Phos@TPE and XOD exhibit optimal catalytic activity at near-neutral pH 6.5, avoiding potential damage to food components from acidic conditions, the stability and anti-interference capabilities of the system in complex food matrices still need to be further validated. Moreover, the sensor relies on a multi-enzyme catalytic system, which increases operational complexity and may pose limitations on its application. Therefore, future research should aim to further simplify the system design while ensuring the detection sensitivity and specificity, in order to enhance its practicality and operability.
Building on this foundation, Han et al. further developed a bifunctional AIEzyme that combines the organophosphorus hydrolase (OPH)-like catalytic properties and fluorescence properties of AIE materials for the detection of nerve agents and organophosphorus pesticides.118 This AIEzyme capable of fluorescence and colorimetric detection without ROS production ensures system stability and enables specific, self-reporting detection. This dual-mode method enhances efficiency and detection scope, advancing AIEzymes for environmental monitoring. Separately, Feng's team synthesized a bright red-orange fluorescent probe, CeO2@PDA@AuNCs-MIPs, with high phosphatase-like activity for methyl paraoxon detection.119 Its hollow structure and MIPs enhance activity and specificity, while Ce(III) boosts fluorescence by inducing the AIE of AuNCs, offering exceptional analytical performance suitable for on-site testing. This nanozyme-based fluorescent assay demonstrates exceptional analytical performance with a wide linear range and a low detection limit, making it suitable for on-site testing and showcasing its promising practical application capabilities.
Similarly, heavy metal pollution, especially Hg2+, poses a serious threat to the natural environment and human health. Despite the potential of gold nanoclusters (Au NCs) as fluorescent probes or chromogenic nanozymes in Hg2+ detection, the development of fluorescent nanozymes with integrated multifunctionality remains a technical challenge. In this regard, You et al. innovatively reported Ce-alloyed gold nanoclusters (Ce-Au NCs), which combine three functions: intense fluorescence emission, excellent peroxidase activity, and highly specific recognition of Hg2+.116 Using Ce-Au NCs, they developed a portable dual-mode sensing device that combines fluorescence and colorimetric detection techniques, which is suitable for rapid and intuitive analysis of Hg2+ in the field. As shown in Fig. 8C, in the presence of Hg2+, the quenching of the fluorescence signal and the change in the color of the paper-based chip (from green to black) provide dual validation of the detection, which significantly improves the reliability and accuracy of the assay. The study introduces a simple synthesis method for multifunctional fluorescent nanozymes and presents the significant potential of the resulting portable detection device for rapid environmental heavy metal ion analysis, inspiring further research and assay diversification. However, their stability in complex environments, detection sensitivity, and cost-effectiveness still need to be further optimized.
(1) Relatively few AIE molecules with enzyme-like properties. In recent years, AIEgens with high luminescence and efficient generation of ROS under light excitation have found extensive application in antimicrobial, tumor imaging, photodynamic therapy, and biomedical engineering. In the study by Liu et al., a novel AIE molecule was designed, synthesized, and evaluated for its enzyme-like properties.59 This type of AIEzyme is simple in structure, easy to synthesize, and highly designable, and has great potential in practical applications. However, the discovery and utilization of AIEgens with enzyme-like properties are limited, largely due to the complexity and subtlety of natural enzyme catalysis, which poses challenges in the design and synthesis of AIEgens. The development of new AIE molecules with enzyme-like activities necessitates a profound understanding of AIE and enzyme catalysis mechanisms, as well as innovative molecular design and synthesis approaches. Therefore, future research should focus on a deep understanding of the molecular mechanisms of AIE and enzyme catalysis processes and the design and synthesis of AIE molecules with stronger enzyme activity.
(2) The mechanism of the interaction between nanozymes and AIEgens is uncertain. AIEgens usually exhibit enhanced luminescence properties in the aggregated state, which may be intrinsically related to the catalytic activity of many nanozymes. When integrating nanozymes with AIEgens, the mechanism of their interaction is a complex issue that has not been fully elucidated in current studies. Most of the current AIEzyme compounds are obtained by some simple methods such as direct encapsulation, surface modification, and co-loading on other carriers. Although the AIEzyme compounds thus obtained can exhibit their respective characteristics, different interactions and spatial arrangements will affect the catalytic activity and AIE properties of the materials. Therefore, future research needs to further explore the relationship between the structure and performance of AIEzymes and use them to guide the design of novel AIEzymes with multifunctionality in the future.
(3) The resolution of AIEzymes needs enhancement. The fluorescence color emitted by most existing AIEzymes is dominated by blue and green, which has limitations in imaging applications, especially in the analysis of deep tissues in vivo. Consequently, expanding the fluorescence wavelength range of AIEzymes into the near-infrared (NIR) spectrum is expected to address these limitations. NIR-emitting AIEzymes, with their superior tissue penetration and reduced cellular background interference, hold promise as practical and high-resolution tools for in situ living cell imaging.
(4) Practical application challenges with false positives. Prior to activation, AIEzymes may exhibit self-aggregation and be susceptible to environmental factors, potentially leading to unintended interfering signals and elevated background noise, thereby diminishing detection sensitivity. To address this issue, it is necessary to consider more comprehensively the influence of environmental factors on the detection results and to design AIEzymes capable of avoiding self-aggregation prior to enzyme activation. With such a strategy, when applying AIEzymes for imaging or detection, false-positive signals can be effectively reduced, the specificity and sensitivity of detection can be improved, and more reliable results can be obtained.
(5) Biocompatibility and toxicity assessment. The biocompatibility and potential toxicity of AIEzymes are issues that need to be carefully considered when applying them in the fields of medical diagnosis, cellular imaging and biosensing. Current studies have shown that most AIEzymes show good biocompatibility during assays, but their long-term presence in the cellular microenvironment, degradation after participation in cellular metabolism, and possible cytotoxicity are still not fully understood. Therefore, future research should establish more comprehensive animal models for long-term biosafety assessments to ensure their safety in clinical applications.
(6) Continuously expanding the range of applications. Currently, AIEzymes reported in the literature are predominantly employed in biosensing, disease diagnosis, and antibacterial applications, using their high sensitivity, selectivity, and visual tracking capabilities. However, their application potential extends far beyond these areas. Research indicates growing interest in employing AIEzymes for environmental and food safety assessments, such as detecting pollutants and monitoring harmful substances in food. In addition, with advances in materials science and nanotechnology, AIEzymes may be further optimized for a wider range of applications. It is foreseeable that in the future, AIEzymes will show their unique value and potential in more fields and become a new focus of research and application.
Overall, this review outlines effective approaches to integrating nanozymes with AIE, surveys the latest advances in this research field, analyzes the current problems faced by AIEzymes, and discusses possible strategies for their solution. At the same time, we also predict the future development trend. By summarizing and analyzing the latest research results in this field, we help researchers understand the potentials and limitations of AIEzymes, and at the same time provide guidance for the rational design and development of AIEzymes for biosensing, disease diagnosis, and disease therapy. We expect that this review will stimulate the interest of more researchers and attract them to join this promising field, in order to promote the wide application of AIEzyme technology in clinical therapy and practical applications.
AA | Ascorbic acid |
AAP | Ascorbic acid-2-phosphate |
AChE | Acetylcholinesterase |
AIE | Aggregation-induced emission |
AI-ECL | Aggregation-induced electrochemiluminescence |
AIEgens | AIE luminogens |
ALP | Alkaline phosphatase |
AOX | Ethanol oxidase |
ARC | AIEgen reservoir and release controller |
ATP | Adenosine triphosphate |
CAP | Chloramphenicol |
CAT | Catalase |
CCS | Cascade catalytic system |
CDT | Chemodynamic therapy |
CFX | Ciprofloxacin |
CL | Chemiluminescence |
COFs | Covalent organic frameworks |
CRET | Chemiluminescence resonance energy transfer |
DAP | 2,3-diaminophenazine |
ESR | Electron spin resonance |
HCC | Hepatocellular carcinoma |
HRP | Horseradish peroxidase |
IFE | Internal filtering effect |
LUM | Luminol |
MCOF | Magnetic covalent organic framework |
MIPs | Molecularly imprinted polymers |
MPO | Myeloperoxidase |
NSs | Nanosheets |
OMCs | Organic molecular cages |
OPD | O-phenylenediamine |
OPs | Organophosphorus pesticides |
OTA | Ochratoxin A |
OXD | Oxidase |
PB | Prussian blue |
PDT | Photodynamic therapy |
PET | Photoinduced electron transfer |
POCT | Point-of-care testing |
POD | Peroxidase |
RIM | Intramolecular motion |
ROS | Reactive oxygen species |
SAZ | Single-atom nanozymes |
SOD | Superoxide dismutase |
TME | Tumor microenvironment |
TPE | Tetraphenylethylene |
ZnDPA | Zn coordinated dipicolylamine |
This journal is © The Royal Society of Chemistry 2024 |