Long afterglow luminescent materials for biological applications

Long afterglow luminescence is a phenomenon in which a substance continues to emit light even after the excitation light has stopped. Long afterglow luminescent materials do not contain toxic heavy elements and can be excited prior to detection and imaging, enabling biosensing and imaging under "no-excitation" conditions, thus effectively avoiding background interference from in situ excitation. Despite this advantage, the application of long afterglow materials for biosensing and imaging was not reported until 2007, when Chermont et al. synthesized silicate long afterglow nanomaterials with near-infrared fluorescence at high temperature using the sol-gel method and applied them for in vivo imaging in mice (Figure 6). 

This method successfully avoided the potential harm of excitation light source in traditional fluorescence analysis methods, and achieved the first "excitation-free" fluorescence imaging in organisms.

Figure 6 In vivo imaging results of Ca0.2Zn0.9Mg0.9Si2O6:Eu,Dy,Mn long afterglow nanoparticles (A) The nanoparticles were firstly excited at 2 cm under a 6 W UV lamp for 5 min before injection.(B) The nanoparticles were then injected at three different locations on the dorsum of mice with different concentrations (100, 10 and 1 μg/mL) of nanoparticle suspension ( 20 μl). The fluorescence emitted by two of the larger doses ( 2 μg and 200 ng) was easily monitored using a collection time of 2 min. (C) Nanoparticle diffusion after 90 s of intramuscular injection to mice

Li Zhan-jun and his co-workers reported the use of MCM-41 molecular sieve as a template to prepare long afterglow materials and use them for biological cell imaging (Figure 7), which cleverly utilized MCM as a template, and spherical nanoparticles with good morphology could be cleverly obtained using MCM-41, and although the materials were prone to collapse during sintering at high temperatures for longer periods of time, the size of the materials obtained by sintering was on the nm scale and had good particle size dispersion.

In 2012, Maldiney et al. used PEG-coated PLNP materials and modified biotin on the surface, which had strong interactions between biotin and affin with a Kw dissociation constant below 10. Their group used this material to detect glioma cells with highly expressed affin on the surface, and the results were very sensitive and could be used for imaging of glioma cells.

After a long period of development, long afterglow materials have become self-contained, emerging with their own unique "charm" and showing a wide range of applications. Although, the research in this area is very active. However, in its research and application there are still many problems to be solved. 

The luminescence mechanism of long afterglow materials is not fully studied, and there are still many problems to be explained; the choice of substrate materials and activating ions is relatively small and single; how to replace the high-temperature solid-phase synthesis reaction method with better synthesis methods such as sol-gel method is an urgent problem ...... long afterglow materials due to their in vitro excitation in their biological applications to avoid the effects of in vivo autofluorescence, and are very promising for applications in storage bioimaging. 

It is believed that by controlling the composition and structure of the material and improving the preparation process, long afterglow materials will definitely be used in more and more extensive applications.