**3.3 Upconversion fluorescent materials**

Upconversion fluorescent materials are emerging fluorescent nanoparticles that can convert low frequent exciting light into high frequent emitting light by absorbing two-photons or multi-photons. The luminous mechanism of upconversion nanoparticles (UCNPs) is anti-stokes, which is opposite with the most fluorescent materials, including organic fluorescent dyes, quantum dots, fluorescent proteins, metal complexes, etc [84]. Because of the distinctive luminescence mechanism, UCNPs have some unique advantages, which make up the disadvantage of above other dyes. Firstly, UCNPs have improved biological tissue penetration. Secondly, UCNPs can reduce light damage on biological samples. Thirdly, UCNPs can effectively avoid the disturbance of autofluorescence from biological samples. Therefore, UCNPs have wide applications in biosensing and imaging [85]. The sensing principle of UCNPs-based probe is widely based on fluorescence resonance energy transfer (FRET) between UCNPs (donor) and other down-conversion fluorophores (acceptor). The reaction between UCNPs-based probe and the target biomolecules will trigger or hinder the FRET process, causing the quenching and enhancement of fluorescence for detection.
