Toward Upconversion Luminescence Enhancement with Dye Sensitization

Abstract

Upconversion nanoparticles (UCNPs) containing lanthanide have attracted considerable attention in the applications of bioimaging, phototherapy, biosensing, solar cells, etc., because of their unique large anti-Stokes luminescence. However, the low upconversion luminescence (UCL) efficiency due to the narrow and weak absorption of lanthanide ions fundamentally restricts their applications. Sensitized UCNPs with dyes are expected to break this limitation, which could yield to 5 orders of magnitude UCL enhancement. The strategy of dye-sensitized UCNPs has broadened upconversion excitation bands, where the dyes efficiently harvest near-infrared light and transfer the energy to lanthanide ions. Benefiting from the sensitizing process of dyes, the brightness was enhanced by 1000 times. In this paper, we summarize the most recent advances on sensitized UCNPs with an antenna effect from different dyes, with an emphasis on the sensitization mechanism and the geometric classification of the dyes. In addition, the perspectives and challenges of sensitized UCNPs are also discussed.

Recent Advances in Triplet Energy Transfer between Organic Dyes and Lanthanide-Doped Luminescent Nanoparticles

“…Particularly, the organic dye-LnLNP triplet energy transfer can start and terminate at different sites following distinct pathways (Figure a). Organic dyes can serve as the energy donor to transport (or transform, when the singlet fission process is involved) harvested photonic energy from the singlet excited state to the triplet excited state and then to lanthanide ions in LnLNPs (pathway 1 in Figure a). , Organic dyes can also funnel photonic energy to lanthanide ions straight through their triplet excited states that receive energy from elsewhere (pathway 2 in Figure a) . Alternatively, LnLNPs can function as the energy harvester to transfer photonic energy to the triplet excited state of organic dyes directly or with the assistance of the singlet excited state of corresponding dyes (pathways 3 and 4 in Figure a). , Notably, other processes that can influence the triplet energy transfer between organic dyes and LnLNPs, such as photon pumping, inner conversion, and energy transfer from the singlet excited state of organic dyes to LnLNPs, are omitted for simplicity in the above depictions. ,, …”

“…The transfer of photonic energy among different materials is of significant importance to a variety of fields, ranging from photosynthesis, which is the key to life on earth, to lighting and displays that bring brightness to us. , As a result, continuous efforts have been devoted to energy transfer between materials since the first discovery of energy transfer over distances larger than collision radii in 1922. − Recently, an emerging type of energy transfer between organic dyes and inorganic nanomaterials, namely triplet energy transfer, has received increasing research enthusiasm because such energy transfer can merge the optical tunability of inorganic nanomaterials and the versatility of molecular design. ,,− Indeed, triplet energy transfer between organic dyes and inorganic nanomaterials has shown enormous potential in various photonic applications, including solar energy conversion, light-emitting devices, photocatalysis, and photodynamic therapy. ,,− For example, triplet energy transfer from quantum dots to organic dyes can initiate intense triplet–triplet annihilation photon upconversion, and a reversed process (i.e., triplet energy transfer from organic dyes to quantum dots) enables the harvest of molecules’ triplet energy for efficient solar cells. ,− …”

“…Triplet energy transfer between organic dyes and lanthanide-doped luminescent nanoparticles (LnLNPs) is a unique occurrence as compared to triplet energy transfer between organic dyes and other inorganic nanomaterials. , Specifically, triplet energy transfer between organic dyes and LnLNPs is appealing to biological applications due to the good stability and low toxicity of LnLNPs. − Moreover, this type of triplet energy transfer can be triggered by penetrative near-infrared (NIR) light, lowering the background noise and radiation damage to biospecies during bioapplications. ,,, Importantly, the organic dye-LnLNP triplet energy transfer can counteract limitations associated with both organic dyes and LnLNPs. For instance, dyes’ triplet energy can sensitize LnLNPs to overcome the intrinsic weak light absorption of LnLNPs, combating the unideal brightness of LnLNPs. ,,,− Meanwhile, LnLNPs are capable of directly activating dark triplet states of organic dyes, bypassing high-lying excited singlet states and energy-wasting intersystem crossing . To date, some progress in the triplet energy transfer between organic dyes and LnLNPs has been made. ,,, …”

“…For instance, dyes’ triplet energy can sensitize LnLNPs to overcome the intrinsic weak light absorption of LnLNPs, combating the unideal brightness of LnLNPs. ,,,− Meanwhile, LnLNPs are capable of directly activating dark triplet states of organic dyes, bypassing high-lying excited singlet states and energy-wasting intersystem crossing . To date, some progress in the triplet energy transfer between organic dyes and LnLNPs has been made. ,,, …”

High-Yb3+-Doped NaYbF4:10% Er@NaYF4 Alloyed Nanoparticles for Brightening Upconversion and Multi-Colorizing Information Encryption

“…In order to overcome the above limitation of the relatively low luminescence intensity, some kinds of methods by adding external factors have been reported, such as introducing a gold nanotriangle array to interact with UCNPs, doping Li + into UCNPs to modify the crystal lattices, and compounding fluorescent dyes on the surface of UCNPs to sensitize their emission. − However, these methods complicated the synthesis of UCNPs. Increasing the doped ions’ concentration (Er 3+ or Yb 3+ ) in core–shell UCNPs was an effective way .…”