Advanced biosensors are highly demanded for accurate biological detection and clinical diagnostics. Fluorescence (FL) is an essential signal for in situ visualization of bioanalytes at the molecular level and monitoring complex biological processes in real time. The red to near-infrared fluorescence could offer minimized autofluorescence interference in living systems. However, the performance of most traditional fluorophores is still limited by the photobleaching effect and moderate signal-to-noise ratio, and their applicability for in vivo imaging is restricted to a superficial region. Although inorganic nanoparticles such as quantum dots or upconverting nanoparticles possess bright fluorescence and good photostability, their heavy metal components would cause further toxicity concerns.
Unlike conventional organic fluorophores, luminogens with aggregation-induced emission (AIEgens) with propeller-shaped structures provide a superior choice for light-up fluorescence sensing. As isolated molecules, the rotor-containing AIEgens undergo low-frequency motions and dissipate exciton energy, leading to fast nonradiative decay of the excited states and weak emission. In the aggregated form, the radiative pathway is predominant for strong emission via the restriction of intramolecular rotation, vibration and motion. The AIEgen aggregates exhibit large absorptivity, robust luminosity, strong photobleaching resistance, no random blinking, and excellent biocompatibility. They have been widely applied for in vitro and in vivo biosensing and imaging, including specific biomolecular analysis (DNA, protein, enzyme, antigen, etc), micro-environment sensing (intracellular pH, membrane potential, viscosity, ROS, etc), real-time organelle or cellular imaging, highly sensitive pathogen detection, long-lasting drug delivery tracking and high-resolution biological process visualization (protein fibrillation, cell apoptosis, mitophagy, proliferation, etc).