Studies of Optical and Photocatalytic Properties of Lanthanide-Doped Upconversion Nanoparticle-Based Hybrid Nanoplatforms

Abstract

Lanthanide-doped upconversion nanoparticles have the ability to convert near-infrared incident light into higher-energy ultraviolet or visible light photons. From this, a great interest has recently arisen in utilizing a generally untapped part of the spectrum. A limiting factor in the use of upconversion is the low efficiency of the energy transfer, resulting in low upconverted photoluminescence. A new one-pot successive epitaxial layer-by-layer formation strategy based on ion layer adsorption and Ostwald ripening is reported to synthesize high-quality, monodisperse, multi-shelled upconversion nanoparticles with narrow size distribution and increase photoluminescence in order to overcome the low-efficiency bottleneck. Up to 30 layers of uniform shells were successfully deposited by successive introduction of the shell precursor solution resulting in a 200-300-fold increase in upconverted emission. Using these more efficient particles in conjunction with fabricated down-conversion nanoparticles, a facile and cost-effective strategy based on dual-modal manipulation of luminescence was developed for anticounterfeiting to provide extra high-level security protection. The genuine pattern was able to be viewed with a near-infrared laser with the naked eye, due to the increased photoluminescence. Meanwhile, the false information was revealed under ultraviolet light. Additionally, latent fingerprinting recognition with low background interference is achieved by taking advantage of the improved brightness of the multi-shelled upconversion nanoparticles. To expand the use of these particles into near-infrared driven photocatalysis, several post-synthetic modifications were done, based on the application. The photocatalytic evolution of hydrogen from ammonia borane at ambient temperature was demonstrated by using a novel core-shell upconversion-semiconductor hybrid nanostructure. The activity of such hybrid nanoparticles was remarkably higher than that of the bare upconversion nanoparticles under the same conditions. The upconverted photoluminescence was efficiently reabsorbed by the semiconductor to promote charge separation and facilitate the formation of photoinduced electrons and hydroxyl radicals generated via the reaction between the aqueous medium and the positively charged holes resultant from the electron promotion. Both serve as reactive species on the dissociation of the weak boron-nitrogen bond, to produce hydrogen gas at a rate of 5.7*10-4 μmol/hr. Moreover, these hybrid nanoparticles suffered no loss of efficacy after several cycles. Upconversion nanoparticles may also be used in conjunction with complimentary plasmonic nanoparticles to facilitate dual plasmonic action for photocatalysis. When exposed to white light, the plasmonic particles can absorb the incident higher energy light for activation, while the upconversion particles can absorb the lower energy light, upconvert it, and transfer it to the plasmonic particles, for further activation. Here, the upconversion nanoparticles are coated in polyethylene glycol to attract the hydrophilic plasmonic nanoparticles. The proximity encourages efficient energy transfer of the upconverted light. These upconversion-plasmonic superstructures were then tested by the photothermal effect and the increased activity compared to the solo particles is undeniable. The combined particles increased the temperature of the surroundings at a rate of 0.55 degrees/minute as opposed to 0.28 and 0.27 degrees/minute for the pure dual plasmonic particles or the upconversion nanoparticles, respectively. Furthermore, in an attempt to perfect the above proof-of-concept design, and demonstrate increased energy transfer efficiency, a secondary approach was taken. Rather than synthesizing the upconversion nanoparticles and the plasmonic nanoparticles separately and conjoining the two, here, the syntheses were performed subsequently on the same sample. Upconversion nanoparticles were first synthesized via the established protocols, coated in polyvinylpyrrolidone, which serves as a reducing agent and nucleation site for plasmonic silver nanoparticles to be grown. With additional modification, the silver nanoparticles can then be activated by the upconverted light from the lanthanide-doped nanoparticles. This work may lead to a rational design of near-infrared-activated photocatalysts in the fields of green energy, security, and environmental protection.