Upconverting Dvd To 720p Dimensions
Tanesha Prately
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Lanthanide-based, spectrally shifting, and multi-color luminescent upconverting nanoparticles (UCNPs) have received much attention in the last decades because of their applicability as reporter for bioimaging, super-resolution microscopy, and sensing as well as barcoding and anti-counterfeiting tags. A prerequisite for the broad application of UCNPs in areas such as sensing and encoding are simple, robust, and easily upscalable synthesis protocols that yield large quantities of UCNPs with sizes of 20 nm or more with precisely controlled and tunable physicochemical properties from low-cost reagents with a high reproducibility. In this context, we studied the reproducibility, robustness, and upscalability of the synthesis of β-NaYF4:Yb, Er UCNPs via thermal decomposition. Reaction parameters included solvent, precursor chemical compositions, ratio, and concentration. The resulting UCNPs were then examined regarding their application-relevant physicochemical properties such as size, size distribution, morphology, crystal phase, chemical composition, and photoluminescence. Based on these screening studies, we propose a small volume and high-concentration synthesis approach that can provide UCNPs with different, yet controlled size, an excellent phase purity and tunable morphology in batch sizes of up to at least 5 g which are well suited for the fabrication of sensors, printable barcodes or authentication and recycling tags.
Spectrally shifting upconverting nanoparticles (UCNPs), that can convert near infrared (NIR) light to luminescence photons of higher energy via a non-linear optical process, show a multitude of characteristic emission bands in the ultraviolet (UV), visible (vis), and NIR and long luminescence lifetimes, that are ideal for low background optical measurements and a high penetration depth in biological systems1,2,3. Moreover, the remarkable tunability of the upconversion luminescence (UCL) through variations of the host lattice, crystal phase, type(s) and concentrations of the dopant rare earth (RE3+) ions, particle size, and morphology as well as excitation conditions, i.e., excitation wavelength and power density, can be exploited for spectroscopic fingerprints in the color and lifetime domain4,5. This has meanwhile triggered their use as optical reporters for imaging and sensing applications6,7,8 and tags for anticounterfeiting, security, recycling, and food quality control applications9,10. The most frequently used crystalline host matrices for UCL-emissive UCNPs are fluorides such as NaYF4 because of their high transparency, very low phonon energies, and high chemical stability11. Doping is most frequently done with the sensitizer/activator pairs Yb3+/Er3+ and Yb3+/Tm3+, providing efficient green, red, and blue emissive UC materials. Although meanwhile many synthetic concepts for sophisticated core/multi-shell UCNPs of different size with optimized luminescence properties such as a high UCL quantum yield have been reported12, for many sensing, barcoding and tagging applications, simple core-only particle architectures with sizes of 25 nm or larger are completely sufficient. These UCNPs are more easily synthetically accessible and the commercial availability of such UCNPs for a reasonable price could broaden the utilization of the upconversion technology. This calls for simple and up-scalable synthesis methods for UCNPs utilizing relatively harmless and relatively inexpensive precursors that enable the controlled tuning of the UCNP physicochemical properties such as size, shape, and luminescence color.
Control of the application-relevant physicochemical properties of UCNPs, that also determine their luminescence color, intensity, and brightness, requires careful control of all synthesis parameters and an in depth-understanding of the most relevant factors governing UCNP quality. This is particularly important for the up-scaling of UCNP synthesis. Synthesis parameters that can influence nanocrystal size, morphology, and crystal phase include temperature, pressure, capping ligand, precursor composition, heating rate, cooling rate, reaction time, solvent(s), and reagent concentrations. In addition, the complex mass transport dynamics associated with seed formation and nanocrystal growth require the consideration of the reaction temperature and reaction time as well as stirring speed and stirring period of the reaction mixture. This large number of synthesis variables makes the reproducible fabrication of UCNPs with specific features challenging and complicates the up-scaling of batch sizes. Furthermore, although many examples for the synthesis of NaYF4:Yb3+,Er3+ and the influence of different reaction parameters on the morphology of the resulting UCNPs can be found in the literature, a comparison between different studies is difficult as often different procedures including different precursors, precursor ratios, solvent ratios etc. have been employed. This was only recently demonstrated by Jurga et al. who examined the influence of the synthesis route on the spectroscopic and temperature-sensing properties of oleate-capped and ligand-free core/shell UCNPs produced from RE chlorides, acetates, and oleates as well as their cytotoxicity37.
Schematic overview of the screening parameters (red, left) and the physio-chemical properties of the resulting UCNPs (blue, right) assessed for the basic synthetic approach with respect to reproducibility, robustness, and up-scaling potential (green, middle); OA, oleic acid, ODE 1-octadecene, Robustness reproducibility of UCNP properties as a function of or response to changes in reaction parameters such as temperature and reaction time.
In the next step, we assessed the robustness of our UCNP synthesis for the optimized standard precursor and solvent ratios regarding the influence of the reaction temperature and reaction time, again utilizing the size, size distribution, and morphology of the resulting UCNPs as measures. These two reaction parameters are the ones which are most frequently slightly modified by chance between different reactions, often even unnoticed. As follows from the TEM images shown in Fig. 2b, also a slight increase in reaction temperature from 325 to 335 C and a prolongation of the reaction time from 30 to 60 min barely affected UCNP size. The well reproducible size of the resulting UCNPs of about 20 nm (see entries for STD-1_60min and STD-1_335C in Fig. 2e) underlined the robustness of our synthesis. Additionally performed ICP-OES and spectroscopic measurements assessing the impact on UCNP elemental composition and luminescence features confirmed the results of the TEM measurements. Only for sample STD 1_60min slightly longer decay kinetics were obtained. This is ascribed to a slight reduction in the number of crystal defects by the increased reaction time.
In summary, our basic synthetic procedure enabled the reproducible synthesis of up to 5 g oleate-capped UCNP in a single batch (resulting from the 25 mmol batch STD-1_5x) of about 20 nm sized core UCNPs with size and shape control, a narrow size distribution, and matching optical properties. Aiming for the synthesis of larger UCNPs with tunable size and morphology and then variation of the luminescence features, in the next sections, subsequently we stepwise modified this synthesis approach. Then, we analogously examined the influence of synthesis parameters solvent composition, dopant concentration, and precursor concentration on the size, morphology, and optical properties of the resulting UCNPs.
The observed Yb3+-induced size and shape evolution of the UCNPs, that was also reported by other groups58, is attributed to the Yb3+-controlled crystal growth rate involving the modification of the electron charge density on the nanoparticle surface. The opposite trend, i.e., a reduction in particle size by replacing Y3+ ions (ion radius of 0.893 ) in the NaYF4 matrix by larger Gd3+ ions (ion radius of 0.938 ) has been more frequently examined. Therefore, Gd3+ ions are not only exploited for the design of multimodale probes for theranostic applications detectable with optical and magnetic methods59, but also for the preparation of small and ultrasmall UCNPs with sizes below 10 nm60. Based on density functional theory (DFT)-calculations, the particle growth behavior in the presence of higher amounts of Gd3+ ions, yielding an increased electron density of the crystal surface and hence a more negative charge of the UCNPs, is ascribed to the electrostatic repulsion of the fluoride anions. This favors the formation of smaller UCNPs61. When Y3+ ions (ion radius of 0.893 ) are substituted by smaller Yb3+ ions (ion radius of 0.868 ), the electron density on the UCNP surface is reduced. This facilitates the interaction with fluoride- anions and results in the growth of larger UCNPs.
Subsequently, the crystalline phases of the as-prepared UCNP samples of the Yb3+ dopant concentration series were examined by powder XRD. The diffractograms are shown in the SI in Fig. S2 in comparison to hexagonal phase NaYF4 (JCPDS 16-0334) and hexagonal phase NaYbF4 (JCPDS 27-1427) UCNPs. These measurements confirmed the formation of a hexagonal crystal phase for all prepared samples. With increasing Yb3+ concentration, the XRD peaks shift to high angles. This is indicative of a decrease of the unit cell volume caused by the replacement of Y3+ by the smaller Yb3+ ions. Apparently, the hexagonal phase NaYF4 crystalline host matrix is gradually transformed to a hexagonal phase NaYbF4 crystalline host matrix with elevated Yb3+ doping concentrations.
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