Fluorescent sensors with advantages of excellent sensitivity, rapid response, and easy operation are emerging as powerful tools in environmental monitoring, biological research, and disease diagnosis. However, conventional fluorophores featured with π-planar structures usually suffer from serious self-quenching in the aggregated state, poor photostability, and small Stokes' shift. In contrast to conventional aggregation-caused quenching (ACQ) fluorophores, the newly emerged aggregation-induced emission fluorogens (AIEgens) are featured with high emission efficiency in the aggregated state, which provide unique opportunities for various sensing applications with advantages of high signal-to-noise ratio, strong photostability, and large Stokes' shift. In this review, we will first briefly give an introduction of the AIE concept and the turn-on sensing principles. Then, we will discuss the recent examples of AIE sensors according to types of analytes. Finally, we will give a perspective on the future developments of AIE sensors. We hope this review will inspire more endeavors to devote to this emerging world.

Activatable photosensitizers (PSs) have been widely used for the simultaneous fluorescence imaging and photodynamic ablation of cancer cells. However, the ready aggregation of traditional PSs in aqueous media can lead to fluorescence quenching as well as reduced phototoxicity even in the activated form. We have developed a series of PSs that show aggregation-enhanced emission and phototoxicity and thus the exact opposite behavior to that of previously reported PSs. We further developed a dual-targeted enzyme-activatable bioprobe based on the optimized photosensitizer and describe simultaneous light-up fluorescence imaging and activated photodynamic therapy for specific cancer cells. The design of smart probes should thus open new opportunities for targeted and image-guided photodynamic therapy.

Subcellular organelle-specific reagents for simultaneous tumor targeting, imaging, and treatment are of enormous interest in cancer therapy. Herein, we present a mitochondria-targeting probe (AIE-mito-TPP) by conjugating a triphenylphosphine (TPP) with a fluorogen which can undergo aggregation-induced emission (AIE). Owing to the more negative mitochondrial membrane potential of cancer cells than normal cells, the AIE-mito-TPP probe can selectively accumulate in cancer-cell mitochondria and light up its fluorescence. More importantly, the probe exhibits selective cytotoxicity for studied cancer cells over normal cells. The high potency of AIE-mito-TPP correlates with its strong ability to aggregate in mitochondria, which can efficiently decrease the mitochondria membrane potential and increase the level of intracellular reactive oxygen species (ROS) in cancer cells. The mitochondrial light-up probe provides a unique strategy for potential image-guided therapy of cancer cells.

Efficient synthesis of poly(enamine)s has been a great challenge because of their poor stability, poor solubility, and low molecular weights. In this work, a spontaneous amino-yne click polymerization for the efficient preparation of poly(enamine)s was established, which could proceed with 100% atom efficiency under very mild conditions without any external catalyst. Through systematic optimization of the reaction conditions, several soluble and thermally stable poly(β-aminoacrylate)s with high molecular weights (M up to 64400) and well-defined structures were obtained in excellent yields (up to 99%). Moreover, the polymerization can perform in a regio- and stereospecific fashion. Nuclear magnetic resonance spectra analysis revealed that solely anti-Markovnikov additive products with 100% E-isomer were obtained. The reaction mechanism was well demonstrated with the assistance of density functional theory calculations. In addition, by introducing the tetraphenylethene moiety, the resulting polymers exhibit unique aggregation-induced emission characteristics and could be applied in explosives detection and bioimaging. This polyhydroamination is a new type of click polymerization and opens up enormous opportunities for preparing functional polymeric materials.

Activatable photosensitizers (PSs) have been widely used for the simultaneous fluorescence imaging and photodynamic ablation of cancer cells. However, the ready aggregation of traditional PSs in aqueous media can lead to fluorescence quenching as well as reduced phototoxicity even in the activated form. We have developed a series of PSs that show aggregation-enhanced emission and phototoxicity and thus the exact opposite behavior to that of previously reported PSs. We further developed a dual-targeted enzyme-activatable bioprobe based on the optimized photosensitizer and describe simultaneous light-up fluorescence imaging and activated photodynamic therapy for specific cancer cells. The design of smart probes should thus open new opportunities for targeted and image-guided photodynamic therapy.

Fluorescent light-up probe with “AIE + ESIPT” characteristics for specific detection of lysosomal esterase.

Amines play vital roles in agricultural, pharmaceutical, and food industries, but volatile amine vapors are serious threats to human health. Previously reported fluorescent sensors for amine vapor detection usually suffer from aggregation-caused quenching (ACQ) effect and need to be dispersed in solution or matrix materials. Herein, based on the fluorogen of 2-(2-hydroxyphenyl)quinazolin-4(3H)-one (HPQ) with aggregation-induced emission (AIE) properties, we have developed a fluorescent sensor HPQ-Ac for light-up detection of amine vapors through aminolysis reaction. The portable HPQ-Ac sensor can be easily prepared by directly depositing on filter paper, and it can only light up via exposure to amine vapors among various volatile organic compounds. Taking advantage of its portability and high sensitivity for amine vapors, HPQ-Ac sensor can also be used for food spoilage detection and fluorescent invisible ink.

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