Enzymes are essential biological elements that play vital roles in many key cellular events and physiological processes. The dysregulation of enzyme activity broadly occurs in a large number of diseases ranging from inflammation to neurodegenerative disorders to tumors. Molecular imaging allows accurate and noninvasive visualization of biological analytes/physiological processes of interest closely linked to human health at different levels. Among various imaging modalities, optical imaging stands out benefited from its high sensitivity, excellent spatiotemporal resolution, real‐time mode, and facile accessibility. Diverse optical probes specifically activatable by disease‐relevant enzymes have sprung up. In comparison to the “always‐on” counterparts, the “off‐on” imaging probes activated by enzymes hold great promise for precise diagnosis of diseases at early stage with high target‐to‐background ratio, dramatically improved specificity, and significantly enhanced sensitivity. Herein, the most recent advances in optical probes activatable by enzymes for biosensing and bioimaging are briefly reviewed emphasizing their molecular design, working mechanism, and biomedical applications. Besides, some important prospects and the current challenges to fully implement the potential of enzyme‐activatable probes for precise and efficient theranostics in life science are also pointed out to hopefully arouse new insights into the development of new generation of theranostics.

Ratiometric thermometry with ultra-high sensitivity and tunable response range has been realized in a single molecular system by making full use of the excited-state configuration transformation of a dihydrophenazine derivative (dibenzo[a,c]phenazine-9,14-diylbis(4,1-phenylene))bis(methylene) bis(icosanoate), (DPC). By facilely manipulating the disaggregation and aggregation of DPC, the excited-state configuration transformation could be controlled, thus affording a ratiometric response to temperature change. By altering the composition of the ethanol/glycerol mixtures, the temperature response region could be finely tuned and the overall linear range is as broad as 49.1 °C (−11.4–37.7 °C). Surprisingly, apart from the relative sensitivity as high as around 2000% per °C, which is the highest among all reported luminescent thermometers, and the good repeatability (stability), the present thermometry scheme can even allow the temperature to be read out accurately from the fluorescence (FL) colour since the precise functional relationship between the CIE coordinates of the fluorescence colour and temperature was established. This is unprecedented for luminescent thermometers, meaning that there is no need for the luminescence-based thermometer to rely on a spectrometer. In this way, the application scope of luminescence-based thermometers could be significantly enlarged. The strategy proposed here solves the conflict between high sensitivity and wide temperature response range masterfully.

Exploration of high-performance fluorescent materials, especially those with two-photon absorption and aggregation-induced emission (AIE) properties, is of great significance to both fundamental research and practical applications. In the present work, a series of triphenylamine–dibenzo[a,c]phenazine adducts (Q1–Q5) with triphenylamine (TPA) moieties decorated by substituents ranging from nil to alkyl (methyl/octyl) and finally to alkoxy (methoxyl/octyloxy) groups were elaborately designed and facilely synthesized. Their photophysical properties including one- and two-photon absorption properties have been systematically investigated to clarify the relationships between their structures and properties and to see how a small change in the structure makes big differences in their performances. The proterotype triphenylamine–dibenzo[a,c]phenazine (TPA–DBP) adduct Q1 and the alkyl-substituted TPA–DBP adducts (Q2 and Q3) show intramolecular charge transfer (ICT) plus aggregation-enhanced emission (AEE) features while the alkoxy-decorated TPA–DBP adducts, i.e., Q4 and Q5, exhibit typical AIE behaviors. The differences in their photophysical properties can be mainly ascribed to the substituent effects, which are closely associated with the RIM (restriction of intramolecular motion) mechanism. Moreover, the AIE-active red luminogen Q5 with the largest two-photon absorption cross-section (σ = 801 GM) and high brightness has been further fabricated into nanoparticles via a simple and well-established method to satisfy the requirements of in vivo two-photon fluorescence imaging of blood vessels. The water-dispersible and biocompatible PEG-modified nanoparticles of Q5 performed well as an effective contrast agent for the visualization of blood vasculature with high signal-to-noise ratios, low photodamage and deep-tissue penetration capability (100 μm).

Single-molecular white-light emissions with CIE coordinates of (0.33, 0.37) and (0.33, 0.31) were achieved with DPAC-p-TPE in viscous solution and with DPAC-Tri(o1,2) in the aggregation state through the combination of vibration-induced emission (VIE) and aggregation-induced emission (AIE) mechanisms.