There are various essential anions and metal cations in the human body, such as F⁻, Ca²⁺, Zn²⁺, and so on. An imbalance of ions in cells can lead to diseases. The detection of these ions in the living cells might indicate whether there has been a pathological change in cells, which can play a crucial role in monitoring people’s physical condition. Recently, more and more studies have reported on biosensor materials. However, traditional organic fluorescent materials have bright fluorescence levels when they are in a high dispersion state in a good solvent. While in the aggregated state, the fluorescence becomes weak or even quenching, which is called “aggregation-induced quenching” (ACQ). Scholars have adopted many methods to solve the ACQ phenomenon, such as reducing the doping concentration of the fluorescent material, and in turn reducing the aggregation degree. However, the aggregation of molecules is a spontaneous process in thermodynamics, which is difficult to inhibit using various physical methods.

The organic materials with strong luminescence in the solid or aggregated state are more favorable, since the fluorescence can be easily detected using equipment. In 2001, Tang’s group [1] found that hexaphenylsilole (HPS) does not emit light in the highly dispersed state, but exhibits extremely shining fluorescence under the aggregated state, which presents totally unusual optical phenomena. The authors found that the six benzene rings in the molecule can rotate or vibrate very freely, so that the energy transferred to them by ultraviolet light can be consumed via rotation or vibration without fluorescent radiation in the dispersed state.

However, under the aggregate state, through the restriction of intramolecular rotation or vibration [2], the system cannot undergo non-radiative decay. Thus, the energy needs to find another way to dissipate in terms of producing bright fluorescence. The author named such materials aggregation-induced emission (AIE) materials. It seems that the restriction of the intra-molecular motion is a simple and useful method for developing AIE materials. Later, Yang’s group [3] designed a compound using the phenyl groups to substitute the 9,10-position of anthracene, which exhibits a strong blue emission under the aggregate state. Consequently, more and more different AIE materials have been developed by the scientific community, such as tetrastyrene materials ^{[4][5][6][7][8][9]}[4,5,6,7,8,9], distilbene anthracene materials ^{[10][11][12][13][14]}[10,11,12,13,14], triphenylamine materials ^{[15][16][17][18][19][20]}[15,16,17,18,19,20], hexaphenylsilole materials ^{[21][22][23][24][25]}[21,22,23,24,25], pyrene materials ^{[26][27][28][29][30]}[26,27,28,29,30], and so on. These are important functional groups, which have recently found wide applications not only for bioimaging but also for detecting other environmental responses. Among these kinds of materials, the most investigated application is in biofluorescent probes for use in organisms.

AIE materials such as biofluorescent probes are widely used to detect the contents of various ions in organisms, such as Zn²⁺, Hg⁺ Ca²⁺, and so on ^{[31][32][33][34]}[31,32,33,34], which can be used to characterize the health of an organism and predict disease. In addition, these kinds of materials are also used in drugs to label the diseased organism for targeted drug release ^[35][36][37][35,36,37]. Based on the requirements of biology and medicine and the development of computational science ^{[38][39][40][41][42][43][44][45]}[38,39,40,41,42,43,44,45], researchers have developed a number of biocompatible AIE materials for fluorescent probes and monitored various ions in living organisms, which is very meaningful. However, to obtain such high-performance AIE materials as fluorescent probes, the materials should exhibit not only strong fluorescence under the aggregated state but also biocompatibility, non-toxic properties, metabolic effects, and so on. This field is still in its early infancy and several open challenges urgently need to be addressed.

Many researchers have combined electron-rich and electron-deficient units to form donor–acceptor (D-A) fluorescent probes ^[46][47][48][46,47,48]. The D-A strategy can adjust the material emission color and wavelength in the optical spectrum, meaning that the materials can probe the cells more easily and clearly. Moreover, introducing a hydrogen-bonding system in the molecules is another useful and simple molecular design strategy for molecular conformation control ^[49][50][51][49,50,51]. For one thing, hydrogen bonds can reduce the intra-molecular motion in the aggregate state, which can lead to a high emission intensity. Additionally, hydrogen bonds can boost the links or inter-molecular interaction between fluorescent probe molecules. Therefore, the recognition of specific molecules or ions will be strengthened by the restriction of the diffusion phenomenon.

2. AIE Material Design Strategy Based on the Functional Groups

Material design concepts play a key role in obtaining high-performance AIE materials. So far, for conjugated AIE materials, the most acceptable strategy is a restriction of the intra-molecular motion. To further increase the emission intensity, the most used method is enhancing the overlap of the π-electron while forming a conjugate system of polyatomic orbitals. However, this method is usually a double-edged sword. Because the π-conjugation extension can cause a high fluorescence efficiency, with the increasing conjugation, the π-conjugation system might induce better π–π stacking interactions ^[52][55]. Therefore, when designing the AIE molecules in fluorescent probes of the conjugated system, the rationality of the conjugated system should be considered. On this basis, scientific researchers have developed many AIE systems in fluorescent probes with high luminous efficiency in the aggregated state. However, most of these fluorescent probes contain some functional cores, through which other units can be substituted to reduce the intra-molecular motion in the aggregate state. These kinds of cores are usually triphenylamine (TPA) or hexaphenylsilole (HPS). In addition, introducing some functional groups, such as tetraphenylethylene (TPE) and biphenylvinyl anthracene (stilbene anthracene, DSA), could also help in obtaining the AIE materials (Figure 12). Tetrastyrene and its derivatives belong to one of the most used and studied AIE materials, which was first found by Tang’s group ^[53][56]. In the TPE system, the benzene ring is connected to a single bond of vinyl and the rotational potential resistance in space is small. This means that the benzene ring can rotate or vibrate very freely in a dispersed state, while the rotation or vibration of the benzene ring is restricted in the aggregated state in terms of producing bright fluorescence. Recently, Jianxin Guan et al. ^[54][57] systematically investigated the AIE mechanism regarding TPE-based materials. They passed the real-time structural evolution and dynamics of the electronically excited state with frequency and polarization-resolved ultrafast UV/IR spectroscopy as well as theoretical calculations. In addition, the luminescence mechanism of the TPE-based AIE materials was conducted in-depth. The author found that the TPE-based materials pass through the cone intersection in the dispersed state; meanwhile, the electron excitation energy quickly becomes non-radiated via attenuation. However, in the aggregate state, these molecules cannot pass through the cone intersection, and the electron excitation energy can be preserved for a long time. This results in a slow transfer of energy or charge between molecules and avoids the ACQ of the combined results. Whether or not the materials pass through a tapered intersection plays a crucial role in determining the ratio of radiated to non-radiative transitions. Based on the above AIE mechanism, a series of novel TPE-based materials were designed and used in fluorescent probes with good performance. Sheng-yu Shi et al. ^[55][58] used a thiol one-click reaction to synthesize the thiol TPE (TPE-SH₄), through which a three-dimensional gel network was built. The three-dimensional gel network had the properties of acid- and redox-switchable aggregation-induced emission characteristics from TPE-SH4 and bis(2-acryloyloxyethyl) disulfide (BDA) via a thiolene click reaction. Thus, the fluorescence emission of the polymer gels can be observed in the absence of dithiothreitol and trifluoroacetic acid, which results in this system’s potential application value in fluorescence sensing. Bo Song’s group ^[56][59] reported on nanowires contained TPE functional groups with AIE properties. The nanowires were successfully applied in labeling HeLa cells with high-viability and high-contrast fluorescence images. Therefore, the nanowires showed potential in the field of biological imaging. Besides the cells, the TPE-based materials could also detect the ions. Wang et al. ^[57][60] synthesized a small molecule, named L, containing the TPE group and the functional units, such as hydroxyl and amide, which can react with the Zn²⁺. The small amount of probe L exhibited almost no fluorescence in the HeLa cells. However, it exhibited strong blue fluorescence in the RPMI-1640 solution containing 100 μM Zn²⁺ and incubated for 15 min. The results indicated that probe L has good membrane permeability and can be used for the imaging of Zn²⁺ in living cells. The pH is a significant factor in the probe, which should be considered by researchers. Wang et al. discussed the pH effect on probe L ^[57][60]. Other studies also reported on the use of TPE-based materials for Zn²⁺ sensors in living cells with good performance. For fluorescent materials, they should be stable in living cells. However, this property is often ignored in most reported studies. Li et al. ^[58][61] presented a TPE-based AIE nanoparticle as a fluorescence material with good stability, even in living cells after one week. In addition, when the materials were incubated with Hela cells, the cells were colorless at the beginning, and then they exhibited bring blue fluorescence after 4 h. However, when they were incubated with the L929 cells, it was found that the cells were colorless at all times. These nanoparticles can be used to image cancer cells, which means the materials have the potential application for in tumor detection. The further development of TPE-based AIE fluorescent materials is important. Recently, besides the functional TPE groups, the stilbene anthracene units have also been popular in the design of AIE materials. In the DSA system, the vinyl benzene ring is connected to a single bond of anthracene. The rotational potential resistance in space between the two groups is small. Thus, the conformation of DSA molecularly can be distorted freely in a dilute solution, which is restricted in the aggregated state. This might result in DSA-based molecules with AIE properties. In 2006, Prasad’s ^[59][62] research group first found the phenomenon of the enhanced fluorescence of anthracene compounds in the aggregate state. The authors analyzed the DSA-based materials’ AIE mechanism. They found that the internal steric hindrance after conjugation can cause a partially distorted structure, which results in the DSA molecules exhibiting a strong AIE phenomenon, similar to the TPE molecules; this was the prelude to DSA-based AIE material research ^{[60][61][62][63][64]}[63,64,65,66,67]. In the past few years, numerous novel DSA-based materials have been designed; meanwhile, various applications have also been developed. Among these, the most used application is for fluorescent sensors in living cells. Han et al. ^[65][68] prepared two symmetrical DSA-based AIE molecules, namely NDSA and CNDSA fluorescent nanoparticles. The authors found great cellular uptake and long-term (even after 9 days) bioimaging results for NDSA and CNDSA in A549 cells with strong blue fluorescence. For the sensors, the materials should exhibit non-toxic properties as well as good biocompatibility. In this work, the authors found that more than 90% of the A549 cells were alive with concentrations of up to 30 μM of NDSA and CNDSA for 24 h, which means that the cytotoxicity of NDSA and CNDSA toward A549 cells is negligible. Additionally, Han et al. used the ultrasound-aided nanoprecipitation method to enhance the stability of fluorescent sensors. This method has been used for many excellent probes. This research revealed that DSA-based materials have potential applications in sensors. The TPE and DSA groups are often the side chain in the design of AIE molecules. However, introducing some functional cores can also help in obtaining good AIE materials, such as triphenylamine and hexaphenylsilole. The two molecules belong to a non-planar helical molecule. The core is connected by a single C-C bond to the multi-side benzene rings with a large torsional angle between these side benzene rings and the central core. As previously described, when the molecules are in a dispersed state, the benzene ring rotates around the core through a single bond in the isolated molecule. The light energy is converted into thermal energy under a disappearing state due to molecular vibration or rotation. However, the movement of the phenyl is limited under the aggregate state, which means that light cannot be converted into thermal energy; meanwhile, the AIE phenomenon occurs. Thus, these two molecules are often designed as the core to obtain high-performance AIE biosensors. In 2022, Chen’s group ^[66][69] designed a molecule named TPA-3NBA, which contains the TPA as the core and the styrene with nitro and carboxyl units as the arm chain. They found that TPA-3NBA can selectively and rapidly stain (within 0.5 min) and effectively kill Gram-positive cocci without affecting bacilli under white light irradiation. In addition, this selective bacterial imaging approach enhances the material’s potential to quickly identify the local bacterial infection. Interestingly, the antibacterial activity can be further improved, since the normal concentration of divalent ions (Ca²⁺) in the human body can significantly increase the absorption of TPA-3NBA by Gram-positive cocci. TPA-3NBA is a potential application prospect in targeted antimicrobial therapy. There are many probes that can kill the cocci. However, most of reported probes cannot exhibit the process when it comes to killing cocci. As a consequence, it is crucial for cocci-killing photosensitizers to combine the indicating and cocci-killing functions. Zhang et al. ^[67][70] employed TPA as the core and 2-cyanoethylene-thiophene as the arm when designing TTM and MeO-TTM, which can be used for the lipid-droplet-specific bioimaging of cells and atherosclerosis plaques due to their strong AIE properties. The authors used both molecules to stain the ApoE^-/- mice. They found that the probes can not only brighten the lipid droplets (LDs) in AS plaques with low background noise but can also identify the spatial distribution of the LDs and can be used to image the depths of AS plaques. TPA has also been used in functional groups introduced as side chains in sensor designs. Ding’s group ^[68][71] reported on small molecules containing TPA, which had a significant germicidal effect on E. faecalis suspensions and 21-day-old biofilms in human root canals. Except for the TPA, the HPS is also popularly introduced when designing high-performance AIE materials. The pH of the human body will fluctuate with the changes in body temperature. This factor restricts many fluorescent molecules from probing the body’s temperature. Gao et al. ^[69][72] reported on a nano-thermometer containing HPS as the AIE dye and household butter as the matrix. The author used these nano-thermometers for temperature sensing in living HUVEC cells with fluorescence lifetime imaging microscopy. They found that the nanosensors both inside and outside the HUVEC cells showed a long fluorescence lifetime (1.45 ns) at 24 °C (low temperature), with a redder color. However, once the temperature was increased to 38 °C, a shorter lifetime of 0.83 ns with a greener color was detected. The fluorescence responses upon temperature changes are reversible and are independent of the environmental pH. Later, Wu et al. ^[70][73] used HPS as the AIE dye and obtained nanobeads, which were applied in the biosensor to detect the carcinoembryonic antigen concentrations in the cells. Therefore, early-age cancer can be easily detected. In addition, the long-term stability and toxicity are both essential factors in cancer detection. Introducing functional groups with the ability to restrict intra-molecular motion is a simple and useful strategy in the design of AIE biosensors. Developing novel groups or units with these kinds of functionality is desirable and challenging, which is important for the further development of AIE biosensors.