| Summary: | 博士 === 國立中正大學 === 物理系研究所 === 106 === Lanthanide (Ln3+) doped upconversion nanoparticles (UCNPs) possess unique upconversion luminescence (UCL) properties which can generate ultraviolet-visible (UV-VIS) luminescence under continuous-wave near-infrared (NIR) excitation via a stepwise multiphoton absorption process. Up to date, UCNPs have been studied for diverse research applications in display, security, photovoltaic and biomedicine. Compared with traditional downconversion fluorophores, such as organic dyes, quantum dots and gold nanomaterials, UCNPs exhibits numerous advantages to serve as a biomarker for biomedical applications, including photostability, no photobleaching, no blinking, no autoluminescence, low scattering, high signal-to-noise ratio, narrow emission spectra band, large anti-Stokes shifts, long luminescence lifetimes, biocompatibility, low toxicity, and deep penetration depth in biotissues. Various types of highly efficient UCNPs have been developed; with a typical one comprises sodium yttrium fluoride (NaYF4) host co-doped with ytterbium (Yb3+) sensitizer ions and activator ions, for example, erbium (Er3+), thulium (Tm3+) or holmium (Ho3+). However, up to now UCL quantum yields of Ln3+ ions doped UCNPs are rather low, especially in aqueous solution, which limits them to be widely used in bio-imaging and bio-sensing applications. One of the greatest trends and major challenges in the field of UCNPs research is the quest to enhance UCL efficiency. In this thesis, we have developed some strategies for enhancing UCL of UCNPs such as surface passivation (that is, core@shell structure), boardband sensitization (that is, neodymium doing), photonic crystal engineering (that is, 1D resonant waveguide grating (RWG)). Besides, we have synthesized a novel multifunctional nanocomposite combining gold nanorod (AuNR) and NaYF4:Yb3+,Er3+ UCNP for simultaneous bioimaging, temperature sensing and in vitro photothermal therapy of oral cancer cells.
Firstly, we have utilized the core@shell structure and broadband sensitization approaches with two major goals: (i) To suppress the surface defect-induced UCL quenching by growing a crystalline shell around core nanocrystal, leading to enhance UCL intensity of UCNPs, (ii) To tune excitation wavelength to 795 nm by doping of neodymium (Nd3+), avoiding the overheating effect associated with the use of 980 nm excitation. A variety of core@shell structured UCNPs has been synthesized through thermal decomposition technique, including: NaYF4:Yb3+,Er3+@NaYF4 core@shell UCNPs, and NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+@NaYF4 core@shell@shell UCNPs. We demonstrate that UCL intensity of core@shell and core@shell@shell nanoparticles is one order of magnitude higher than those of corresponding core counterparts, indicating the important role of the shell as a shield from UCL quenching. Besides, the introduction of Nd3+ as sensitizer for UCNPs has ability to generate UCL emission under 795 nm excitation, overlapping with the first biologically transparent window in the range of wavelengths from 650 to 950 nm. This permits deep penetration depth into biotissues as well as low overheating effects. It is noteworthy that all the UCL emission bands of Nd3+-sensitized UCNPs under the excitation at 795 nm are enhanced about 3 to 4 times compared to those under the excitation at 976 nm, due to to the larger absorption cross section of Nd3+ at 795 nm.
We have also employed a RWG (a kind of photonic crystal substrate) comprised of a low-refractive index (low-n) mesoporous silica (n=1.22) sinusoidal grating layer and a thin high-n TiO2 waveguide layer to enhance UCL of NaYF4:Yb3+,Tm3+ core and NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+@NaYF4 core@shell@shell UCNPs with more than 104 times in aqueous solution. The structure parameters of the low-n RWG are optimized through rigorous coupled-wave analysis (RCWA) simulation to build up strong local electric field near the interface between TiO2 and aqueous solution under dual-wavelength excitations (976 and 795 nm). When the low-n RWG is illuminated by NIR laser with an incident angle matching with the guided mode resonance (GMR) angle of the low-n RWG, UCL emission is dramatically enhanced thanks to the build-up of strong local field on the surface of the low-n RWG. Besides, the UCL emissions can be further enhanced 2-4 times when the UCL emission wavelengths coincide with their associated GMR wavelengths. Then, we have found that the streptavidin-conjugated UCNPs bioprobes can be used to detect biotin molecules on the surface of the low-n RWG. The results confirm that the low-n RWG is feasible for UCL biosensing and bioimaging applications.
Finally, we develop a hybrid nanostructured material based on the combination of UCNPs and AuNRs having the same absorption at 980 nm. For photoluminescence bioimaging probes, the UCL of Ln3+-doped UCNPs using NIR light excitation have many advantages to be served as bioimaging or therapy agents. Photothermal therapy (PTT) is widely used as a treatment protocol for cancer therapy. Among various PTT agents, AuNRs are especially attractive because their high efficient of absorbing NIR light and converting heat energy through surface plasmon resonance (SPR). Besides, UCNPs can play a role as an nanothermometer to determine local temperature generated from AuNRs. Herein, we design novel multifunctional hybrid nanocomposites based on the conjugation of AuNRs with NaYF4:Yb3+,Er3+ UCNPs to combine UCL, temperature-dependence and SPR properties for fulfilling both luminescence labeling, temperature sensing and photothermal functions under a single 976nm excitation source. The nanocomposites with the help of antibody conjugation can effectively label Her2 marker on the surface of OML-1 oral cancer cells with good specificity. Because of strong absorption at 976nm excitation wavelength, AuNR-UCNP nanocomposites result in high efficiency of PTT effect to kill cancer cells dramatically under 976 nm laser irradiation. Our AuNR-UCNP nanocomposites exhibit simultaneous diagnostic in vitro bioimaging, temperature sensing and PTT, which is feasible for multimodal imaging guided PTT applications.
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