6. Applications of AIE polymers.

AIE polymers have various applications ranging from using them as tools in studying reaction mechanisms to different types of sensors to theranostic applications, and so on.[4, 12, 190-193] In this section, some examples will be highlighted with more emphasis on theranostic applications, which briefly describes the importance of AIE polymers and how AIE polymers can improve our daily activities.

6.1 Theranostics

Theranostics is a novel concept which combines diagnosis (cell imaging) and therapy treatment (targeted drug release) in a full drug delivery system where AIE polymers are utilized as drug trackers to monitor the release of drug molecules after entering the targeted cells. Biological imaging techniques have played an important role in the field of biomedical application such as in guiding drug carriers for targeted cell treatments, cancer cell detection and stem cell transplantation. Fluorescence imaging garnered worldwide interest as the “next-generation” technology in high precision imaging at the subcellular level, with strong PL, high sensitivity and versatility in the designing of the fluorescent nanoparticles.
Zhang, Wei and co-workers reported red R-PEG series and red R-F127 series fluorescent organic nanoparticles (FONs) in 2013 and 2014 respectively,[86, 194] with excellent cell compatibility of at least 90% against A549 cells. Ouyang, Zhang, Wei and co-workers in 2020, reported the facile preparation of AIE-active PTH-P(BzMA-MPC) FPNs with good water dispersibility and similar cell compatibility percentage against L02 cells as the previously mentioned FONs.[150] Similar trend patterns in cell penetration ability of both types of FONs and FPNs were confirmed by Confocal Laser Scanning Spectroscopy (CLSM).
Drug delivery systems containing self-guiding carrier molecules for anti-cancer drug treatment became popular in recent years. The use of anti-cancer drugs alone led to an increased possibility of drug resistance development in cancer cells,[195] and the lack of real-time monitoring of the entire delivery system of drug contributed to the limited application in cancer cell treatments. In 2018, Liu, Li and co-workers prepared poly(N 6-carbobenzyloxy-L-lysine)-b -poly(2-methacryloyloxyethyl phosphorylcholine) (TPE-PLys-b -PMPC) capable of spherical core-shell self-assembly with encapsulation of DOX in the micelle core via hydrophobic interactions for intracellular release and tracking (Figure 10A ).[149] CLSM images were taken after incubating DOX-loaded TPE-PLys-b -PMPC with HeLa cells, where the images are taken at timestamps of 2, 4, 6, and 8 h. Red fluorescence pattern trends revealed that DOX entered the cytoplasm during the first 4 h after incubation, and slowly diffuses into the cell nuclei from the 6 h mark onwards, while blue fluorescence pattern trends suggests that TPE-PLys-b -PMPC remains only in the cell cytoplasm. TPE-PLys-b -PMPC improves the endocytosis of DOX and degrades during the first 4 h after incubation through which DOX was released, that will ultimately diffuse into the cell nuclei and inhibit cancer cell growth. This mode of mechanism is one of the ways for designing smart drug delivery systems with triggered biodegradable drug carriers.
Based on similar principles of intracellular drug release, Jia, Tang and co-workers in 2022 synthesized TPE-PEGA-Hyd-DOX with a slightly different drug release mechanism, where the hydrazone bond acts as the linker and conjugated with the anti-cancer drug DOX.[181] Confocal images of TPE-PEGA-Hyd-DOX, TPE-PEGA-Hyd, and pristine DOX where taken after incubation with HeLa cells and NIH3T3 cells respectively with a much stronger fluorescence registered for TPE-PEGA-Hyd-DOX in both cases. The DOX channel image represents the extent of release of the drug into the cells. In both cases, pristine DOX uptake was relatively less readily than TPE-PEGA-Hyd-DOX, where the merged images confirmed the excellent performance of this drug carrier delivery system. In addition, Jia, Tang and co-workers found that drug release was limited in healthy cells compared to cancer cells as the environment in the cancer cells encouraged the hydrazone bond cleavage and subsequent release of DOX in the cell cytoplasm, where it migrated into the cell nuclei to kill the cells. The combined benefits of AIE and targeted drug delivery enabled this drug delivery system to monitor drug movement and in vivocellular responses in real-time which can help to revolutionise traditional methods in direct administration of medicinal drugs.
In another work by Liu, Tang and co-workers in 2012, TPE-functionalized 2-(2,6-bis((E )-4-(phenyl(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)amino)styryl)-4H -pyran-4-ylidene) malononitrile (TPE-TPA-DCM) with strong PL intensity in the far-red/near-infrared (FR/NIR) electromagnetic region, was combined with BSA as the polymer matrix to form uniformly-sized protein nanoparticles (Figure 10B ).[190] These FPNs were then examined for their cell compatibilities against MCF-7 breast cancer cells and murine hepatoma-22 (H22)-tumor-bearing mouse models. In vivo imaging of BSA-loaded FPNs were determined via non-invasive fluorescence imaging of the live animals after injection of the FPNs with images taken at the 3 h, 8 h, 28 h mark for BSA-loaded FPNs and bare FPNs respectively, where fluorescence intensity was twice as high for the mice with tumors than for the mice without any tumors.Ex vivo imaging on the different parts of the mice when it was sacrificed 24 h post-injection helps to confirm the accumulation of the BSA-loaded FPNs in the tumor areas through visualization of the intense coloration in that particular area.

6.2 pH level fluctuation sensor

By incorporating stimuli-responsivenes into AIE polymers, it can respond to environmental variations such as temperature, light, pH and many more. One of the important modifications performed to AIE polymers is the ability to detect environmental pH changes, which becomes important when dealing with biological applications such as intracellular drug delivery and carrier systems.[196]
Recently, a study was conducted by Li, Sun and co-workers in 2021, where block copolymer poly(ethylene glycol)-b -poly(L-lysine) (PEG-b -PLys) was synthesized and modified with TPE-CHO group to form PEG-b -P(Lys-TPE) bearing reversible pH-responsive fluorescence properties.[4] PEG-b -P(Lys-TPE) forms spheres with a core-shell structure when added to a solvent system comprising DMF/H2O, and forms vesicles when added to a solvent system comprising THF/H2O. For the THF/H2O solvent system, fluorescence intensity dropped drastically upon reducing pH level from 10.7 to 1.4, postulated to be the detaching of the TPE moiety from the imine bond, causing the polymer to lose the AIE characteristics, while regaining strong fluorescence after increasing the pH level to 12.6, indicating the re-attachment of the TPE moiety to the polymer via the imine bond (Figure 10C ). The authors also found that this reversible behavior is only possible in a mixed solvent system as the polymer exhibited irreversible pH fluorescence behavior when pH variations were performed in pure water solvent systems due to the precipitation of TPE residues after detachment from the polymer. Nevertheless, such a polymer can find potential use as a pH probe in mixed solvent systems but has limited applications in single solvent systems.

6.3 Metal ion selective sensor

Metal ion pollution is a major environmental concern and it is crucial that these metal ion can be readily detected through the use of probes that interact with them and provide sensing capabilities. Fluorogenic probes have the ability to interact with the metal ions via complexation and other chemical reactions to change their fluorescence properties, which can be detected by fluorescence measurements.[197] AIE-based polymer probes can be designed to take advantage of the metal ion-induced aggregation effect to detect a single type or multiple types of metal ions by registering a change in fluorescence intensity.
Metal ion probes can also be designed to detect a single type of metal ion instead of multiple metal ions. Bai, Zhang and co-workers in 2018, facilely constructed a hyperbranched AIE poly(acrylamide) HPEAM-TPEAH to be used as a probe for the detection of Zn2+specifically.[5] An aqueous mixture of fluorescence HPEAM-TPEAH and different metal ions were prepared to determine which metal ion is responsive towards HPEAM-TPEA, and the authors discovered amongst the many metal ions tested such as Zn2+, Mn2+, Na+, Ca2+, Mg2+, Fe2+and K+, only Zn2+ ion provided a significant decrease in fluorescence intensity when mixed with HPEAM-TPEAH in water and in simulated body fluid. Zn2+ion remained detectable even at low concentration of\(2\times 10^{-5}\ M\), indicating the highly selective and sensitive “turn-off” response of HPEAM-TPEAH towards Zn2+ ion.

7. Summary and perspective.

This review summarizes some of the many interesting AIE polymer end product design from a wide range of monomers and some important applications that AIE polymers can bring about. The unique discovery of the AIE phenomenon manages to solve problems associated with the ACQ phenomenon as aggregation is highly encouraged for AIE polymers to be useful. Due to the versatility of RDRP, various strategies can be used to incorporate AIE components into polymers such as direct polymerization of non-AIE monomers and AIE monomers, surface-initiated polymerization, AIE monomers containing more than one vinyl bond acting as crosslinkers, AIE components as pendent groups which can be found in hyperbranched-type polymers, AIE core-functionalized multi-arm star polymers, AIE end-functionalized polymers, direct linkage of AIE monomers, and through the unusual AIE fluorescence behavior exhibited after polymerizing non-AIE monomers. More efforts are being invested in discovering other possible combinations of monomers and initiators/crosslinkers to produce unique AIE polymers possessing multi stimuli-responsive properties for high-throughput new applications or improving upon currently known applications including their use as cell imaging agents and drug delivery systems in theranostics applications, pH sensors, and metal ion selective sensors. An emerging trend in the AIE polymer field is the shift towards simpler fabrication processes where multicomponent reactions and one-pot reactions assisted by microwave or ultrasonic irradiation are favoured over tedious multi-step preparations. Another exciting area of AIE polymers is the use of carbohydrate-based monomers and unusual monomers without phenyl groups but still able to possess AIE characteristics after polymerization such as acrylonitrile, and epoxide-containing branched monomers, which may find application for imaging and biological related purposes.
The possibility to combine artificial intelligence (AI) and machine learning (ML) to AIE polymers fabrication and application opens up exciting future directions for high-end technologies such as incorporation of AIE polymers into AI systems with complex logic gates as multi-sensors, advanced ML models that can rapidly predict structure-property relationships (SPRs) of AIE/ACQ polymers, ML tools with the ability to generate fast and accurate information on pathogens through AIE responsiveness to environmental variations, and so on. In addition, AI and ML can also be applied to automate polymerization techniques on the benchtop to quickly screen and identify different SPRs in a large chemical space for high throughput experiments and high throughput screening, which would otherwise require laborious work by researchers. Even though AIE polymers became popular more than a decade ago, it can only be considered in the infancy stage of development as many of the applications are being constantly developed and improved upon. With the unwavering efforts of many researchers around the world, AIE polymers will become even better and more useful in the future.