Up-conversion process and mechanism of up-conversion luminescent materials

Its principles include excited state absorption (ESA), energy transfer up conversion (ETU), and photon avalanche (PA). Energy transfer refers to the coupling of two excited ions with similar energy through a non-radiative process, one of which transfers energy to the other and returns to a low energy state, and the other ion receives energy and transitions to a higher energy state. Energy transfer upconversion can occur between the same ions, or between different ions. Therefore, energy transfer up-conversion can be divided into two categories: (a) Continuous energy transfer is shown in Figure 2-2, which is a schematic diagram of continuous energy transfer up-conversion. The donor ion in the excited state returns to the ground state through a non-radiative transition, transferring energy to the acceptor ion, so that it transitions to the excited state. The acceptor ion in the excited state can also transition to a higher energy level through this energy transfer, thereby transitioning At the ground state, higher-energy photons are emitted. Figure 2-2 Continuous energy transfer process Upconverted nanoparticles usually consist of an inorganic matrix and rare earth doped ions embedded in it. Although in theory most rare earth ions can be up-converted to emit light, in fact, under low pump power (10W / cm2) excitation, visible light is only observed when and as the active ion, because these ions have more uniform discrete The energy level can promote the processes involved in upconversion such as photon absorption and energy transfer. In order to enhance the up-conversion efficiency, it is usually doped as a sensitizer with an activator, because its near-infrared spectrum shows that it has a wide absorption range. As a rule of thumb, in order to avoid the loss of excitation energy due to cross relaxation as much as possible, in the sensitizer-activator system, the doping concentration of the activator should not exceed 2%. The upconversion process mainly depends on the stepped energy levels of the doped rare earth ions. However, the crystal structure and optical properties of the matrix also play an important role in improving the up-conversion efficiency, so the choice of the matrix is ​​crucial. The energy used to excite the activated ions may be absorbed by the matrix vibration. Differences in the crystal structure of the matrix also cause changes in the crystal field around the activated ions, which can cause changes in the optical properties of the nanoparticles. A high-quality substrate should have the following properties: good light transmission in a specific wavelength range, lower phonon energy, and higher photo-induced damage thresholds. In addition, in order to achieve a high concentration of the doped host and the doped ions, there should be better lattice matching. In summary, inorganic compounds of rare earth metals, alkaline earth metals, and some transition metal ions (such as, and) can be used as the ideal rare earth ion doping matrix. Table 1 lists the matrix of upconversion materials commonly used in biological research. Although there are many methods for synthesizing UC particles at present, in order to obtain efficient UC light-emitting products, many studies are still devoted to exploring and synthesizing UC particles with high crystallinity. Nanoparticles with better crystal structures have stronger crystal fields around their doped ions, and have less energy loss due to crystal defects. Considering the application in the biological field, in order to bind with biological (large) molecules, the nanoparticles should have the characteristics of small size and good dispersibility. In the traditional method of synthesizing up-converted nanoparticles, in order to obtain products with high crystallinity, high dispersion, specific crystal phase and size, there are generally higher requirements on reaction conditions, such as high temperature and long reaction time. This may lead to agglomeration of particles or an increase in particle size. In this regard, we have recently found milder reaction conditions. The nanoparticles synthesized under these conditions have small size and good optical properties. The strict control of the doping concentration can also obtain nanoparticles with different crystal phases and sizes, which has been confirmed in the recent Yu literature. The absorption and emission spectra of rare earth ions mainly come from the 4f electron transition of the inner layer. Under the shielding of the surrounding 5s and 5p electrons, its 4f electrons hardly interact with the matrix, so the absorption and emission spectra of the doped rare earth ions are similar to their free ions, showing extremely sharp peaks (the half-peak width is about 10 ~ 20nm). At the same time, there is a great limitation on the wavelength of the excitation light source. Fortunately, commercial 980nm InGaAs diode laser systems happen to match the absorption, providing an ideal excitation source for up-converted nanoparticles. Lanthanide metal ions usually have a series of sharp emission peaks, so it provides a characteristic map for the analysis of the spectrum and avoids the effects of overlapping emission peaks. The emission peak wavelength is essentially unaffected by the chemical composition and physical size of the matrix. By adjusting the composition and concentration of doped ions, the relative intensity of different emission peaks can be controlled, thereby achieving the purpose of controlling the color of light emission. Different from the traditional anti-Stokes process (such as two-photon absorption and multi-photon absorption processes), the up-conversion luminescence process is based on many intermediate energy levels, so it has higher frequency conversion efficiency. Generally, the up-conversion process can be excited by a low-power continuous-wave laser. In contrast, the “two-photon process” requires an expensive high-power laser to excite. Because the up-conversion luminescence process of the 4f electron transition of the inner layer does not involve the breaking of chemical bonds, the UC nanoparticles have higher stability without photofading and photochemical decay. Many independent studies have shown that rare earth-doped nanoparticles have not changed fundamentally after several hours of UV and IR laser exposure. The up-conversion luminescence of UC nanoparticles is continuous without “flashing”. Although a “flash” will be observed for a single ion, and because a large amount of rare earth ions are contained in the UC nanoparticles, recent experiments have confirmed that the UC nanoparticles will not “flash” under continuous infrared laser excitation. Due to the f-f electron transition prohibition, trivalent rare earth metal ions usually have a long luminescence lifetime. Time-controlled luminescence detection technology uses this optical property to avoid interference of short-lived background fluorescence caused by the multi-photon excitation process of biological tissues, certain organic species or other dopants. Compared with the traditional steady-state luminescence detection technology, the detection sensitivity is greatly improved because the signal / noise ratio is significantly increased.