The controlled fabrication of upconverting hollow structures, enhanced effect on photoluminescence and applications

Abstract

Lanthanide-doped upconverting photoluminescence nanomaterials which exhibit excellent photostability, long luminescent lifetimes, large anti-Stokes shifts and sharp-band emissions have attracted considerable attention in various research domains, including lasing, displays, anti-counterfeiting, biological systems. Hollow structures possess attractive physicochemical characteristics such as low density, high loading capacity. More importantly, they also possess multiple adjustable shells numbers, shell thickness and inter-shell spaces. Hollow structures can also generate multiple reflection and scattering of incident light by the adjustable inner structures, which is beneficial to the enhancement of light harvesting efficiency and modulation of electric field density. Therefore, it is of great importance to do intense research on developing lanthanide-doped photoluminescence hollow structures (LPHS). Hollow structures with various morphologies and compositions have been fabricated by many approach, such as hard-template method, soft-template strategy, self-template tactics and other techniques. However, the controllabl esynthesis of hollow structures remained a great challenge by using soft-template strategy, which impede the synthesis of hollow structures. With the good repeatability and generality, carbonaceous spheres hard-template method has been demonstrated a straightforward and versatile technique to prepare uniform LPHS. By using carbonaceous spheres hard-template method, the diversity of LPHS has been flourished in both geometrical morphology and composition. Benefiting from the enrichment of synthetic method, various LPHS have been prepared and employed in many domains. However, enhanced effect on photoluminescence remained indistinct, which impede the design of excellent LPHS. Herein, we focus on efforts on synthesis, enhanced effect on photoluminescence and application of LPHS. We first review efforts in geometrical and compositional manipulation based on carbonaceous spheres hard templates approach. After reviewing the compositional and geometrical manipulation, we consider that how hollow structures influence upconversion emission and review recent developments on modulating upconversion emission. Finally, we systematically discuss the typical examples to highlight applications in various areas, especially for imaging, drug delivery and therapy. Due to the preeminent characteristic, lanthanide-doped upconverting nanomaterials with hollow multi-shelled structures (LUHoMSs) demonstrate excellent upconversion emissions. Upconversion emission spectra states that with the increase of shell numbers, LUHoMSs show improved luminous intensities and tunable green/red ratio. This enhanced upconverting luminescence might be attributed to the enhanced NIR excitation light harvesting efficiency. Lanthanide-doped 1D upconverting microrod comprising isolated holes can modulate the upconverted emission intensity because of the intensified electric field density, which originated from NIR excitation light reflection and scattering by inner walls. As the understanding of light reflection and scattering and plasmonic effect on upconversion process has improved, hollow structures have been testified available for boosting upconversion emission. In the plasmonic system, emission enhancement of LUHoMSs indicates that it mainly derived from the combined effects of surface plasmon with excitation electromagnetic field. Considering the unique features of LUHoMSs, they are extremely suitable for applications in multimodal bioimaging, drug delivering and phototherapy. LUHoMSs with large drug loading can boost local drug concentrations and promote drug efficiency. LUHoMSs with high signal/noise ratio and excellent imaging function, which includes magnetic resonance imaging and X-ray computed tomography, can achieve precise diagnosis and benign therapeutic effect. By making use of the combination of phototherapy with drug release can achieve the enhancement of synergistic disease diagnosis and therapy. We believe that with in-depth understandings of enhanced effect on photoluminescence, hollow structures can play important role in guiding the fabrication of LPHS, light conversion and further improve the application performance.