Recent Advances in Triplet–Triplet Annihilation Upconversion for Bioimaging and Biosensing

Photon upconversion is an anti-Stokes process that converts low-energy photons into high-energy photons. The use of upconversion luminescence can avoid the autofluorescence of biological tissue and realize background-free bioimaging with a high signal-to-noise ratio at a low power density. In addition, the excitation of red or near-infrared light facilitates the reduction of photodamage in biological tissues and subsequent bioimaging of deep tissue features in vivo. Meanwhile, upconversion emission-mediated biosensing offers both high sensitivity and low detection limits for quantitative analysis of the target substances in complicated biological samples. Due to its high upconversion quantum yield, low excitation power density, and tunable absorption and emission wavelengths, triplet–triplet annihilation upconversion (TTA-UC) has garnered considerable interest for bioimaging and biosensing. This review will introduce the fundamental concepts of TTA-UC, the factors that influence TTA-UC materials, and the methodologies for preparing TTA-UC materials. The important progress of TTA-UC in bioimaging and biosensing in recent years will also be discussed in detail in vitro and in vivo. Furthermore, the current challenges of TTA-UC in bioimaging and biosensing will be discussed, along with potential solutions.