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Cyclodextrins (CDs) form a family of naturally abundant, inexpensive, cyclic oligosaccharides, consisting of truncated cone-shaped molecules, with a hydrophobic internal cavity and a hydrophilic external surface. Through the coordination of CDs with alkali metal ions (Na+, Rb+, Cs+, K+,…) biocompatible, carbohydrate CD-based, porous metal-organic frameworks (CD-MOFs) can be obtained. These CD-MOFs are attractive materials for practical applications as they can be considered as biocompatible, biodegradable, inexpensive, and sustainable nanoporous MOF alternatives. Hence, CD-MOFs have already been found in a wide range of applications, such as molecular adsorption, separation and recognition, drug delivery, as host template for nanoparticle synthesis, and light emission and sensing. In this work, we first designed macro- and nanosized core, as well as core-shell, γ-cyclodextrin metal-organic frameworks (γ-CD-MOFs) and used them as platforms for the encapsulation of dye molecules to develop the first CD-MOF-based ratiometric optical thermometer materials. A novel dye combination was employed for this purpose, i.e., the duo rhodamine B (RhB) and fluorescein (FL). RhB is highly temperature-sensitive, whereas FL is less temperature-sensitive, and its luminescence emission peak is used as a reference. Promising results in terms of thermometric properties were obtained for a whole series of dye-encapsulated γ-CD-MOF materials based on this dye combination, with high relative sensitivities, even up to 5%K-1, for the dye-encapsulated 75%RhB-25%FL nanosized γ-CD-MOF, among the highest performance values reported so far for luminescent dual thermometers. In our study, we have additionally developed a simple yet effective preparation method for core−shell γ-CD MOFs, allowing effective manipulation of the γ-CD-MOF shell growth. The proposed method allows incorporation of the FL and RhB dyes in the γ-CD-MOFs in a more controlled manner, enhancing the efficiency of the developed ratiometric (macro) γ-CD-MOF thermometers. In recent years, the incorporation of metal or metal oxide nanoparticles (NPs) into porous MOFs to form NP-MOF structures has attracted great interest. Although procedures for the deposition of Ag and/or Au NPs (AgNPs, AuNPs) in MOFs have already been reported, their application with CD-MOFs has not been investigated thoroughly yet. Both AgNPs and AuNPs are plasmonic, which means they could be exploited to concentrate light at a certain wavelength, providing local heating and, thus, be employed for photothermal therapy and can play a beneficial role when it comes to CD-MOFs used as drug delivery systems, as local heating can speed up the drug release. In this work, we additionally investigated different routes of depositing AuNPs and AgNPs in a quick and easy manner into nanosized CD-MOFs. The loading of the nanoparticles can be easily steered by increasing or decreasing the amount of used metal precursors. The nanoparticles can be synthesized at low temperatures which do not affect the structure nor crystallinity of the nano CD-MOFs. It was also shown that small and very uniform AuNPs and AgNPs can be obtained upon dissolving the CD-MOF template. Therefore, this is also a route to yield well water-dispersible metal nanoparticles without the use of any ligands or surfactants. Analogously to the first employed dye combination of RhB and FL, the thermometry properties of the nano CD-MOFs with deposited AuNPs and AgNPs were obtained upon embedding a mixture of a different dye combination, i.e. the RhB and rhodamine 110 (Rh110) pair in the pores of the CD-MOFs. This is the first work where CD-MOFs are proposed for the development of luminescent thermometric materials, and in combination with plasmonic AuNPs and AgNPs. By employing this approach, various fluorescent dyes can be combined, which opens new possibilities toward the future development of a wide range of ratiometric biological (plasmonic) nanothermometers.

Metal-organic frameworks (MOFs) are crystalline porous molecular materials, which consist of organic molecules (linkers) and metal centers, for example lanthanide (Ln) ions. In recent years, these materials have emerged as a new class of materials for photonics applications such as solid-state lighting, lasing and nonlinear optics.[1,2] In this work, we present a series of LnMOFs doped with terbium (Tb3+) and europium (Eu3+) ions with 1,3,5-benzenetricarboxylic acid (H3BTC) as linker. The crystallization process was monitored in situ using nonlinear scattering techniques and showed that the Ln-BTC MOFs crystallize in non-centrosymmetric space groups. The compounds crystallize in tetragonal space groups P4122 (Tb-BTC) and P4322 (Eu-BTC), as well as the mixed doped Tb100-xEuxBTC compounds with different molar ratios (x = 0.01, 0.05 and 0.1). The structures were characterized with single-crystal X-ray diffraction (XRD), powder XRD, FTIR and CHN analysis. The crystallization process was monitored in situ using nonlinear scattering techniques and showed that the Ln-BTC MOFs crystallize in non-centrosymmetric space groups. Additionally, the optical response of the LnBTC MOFs could be revealed by the combined power of linear and nonlinear spectroscopy (two-photon, SHG). The results obtained from the nonlinear optical study will be used as an input for the quantum spectroscopy investigation that is in progress, and it’s based on the interaction of MOFs with entangled photons.

Nucleation – the initial step of the crystallization – is still rather unexplored, while this step has a major impact on the outcome [1]. One of the possible systems for studying nucleation are organic polymers, namely polythiophenes, where nucleation and particle growth has recently been examined by non-linear optics (NLO) [2]. In this research, dynamic small-angle X-ray scattering (SAXS) experiments are used for nanoscale information about size, structure and morphology of monomers/aggregates in the initial stages of the particle growth. In total, 3 different poly(3-alkylthiophenes) were studied by sealing various concentrations of the polymer in a mixture of a THF solvent and MeOH anti-solvent. The polymer mixtures were dissolved by heating, and the aggregation with subsequent particle formation was observed during the cooling period. In-house trials were succeeded by synchrotron experiments at the ESRF BM26 beamline. Results are showing differences in particle formation between the types of the polymers, based on the concentration, side-chain, presence of defects in the backbone of the polymer or the ratio of solvent/anti-solvent in the mixture. Aggregation of polymers is also observed prior to the precipitation of particles. In conclusion, these experiments, together with work in the field of NLO, are providing deeper insight in particle formation of organic materials. [1] Davey, Roger J. et al. Crystal engineering – nucleation, the key step. CrystEngComm. 4, 257 – 264 (2002). [2] Moris, Michèle et al. Harmonic light scattering study reveals structured clusters upon the supramolecular aggregation of regioregular poly(3-alkylthiophene). Communications Chemistry. 2:130 (2019).

The most efficient upconversion (UC) nanophosphors are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with lanthanide ions (Yb3+/Er3+) is among the most efficient upconversion phosphors. However, its low thermal stability limits its use for certain applications. Oxide hosts, on the other hand, typically have much better thermal stability, despite the fact that these matrices have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect.[1, 2] Rare earth oxyfluoride (YOF) matrixes have been developed to combine the benefits of low phonon energy of fluorides with the benefits of oxides for active optical ions.[3] The ongoing challenge is to improve the luminescence properties of lanthanide doped rare earth oxyfluoride materials in order to boost their future applications, such as for example for temperature sensing. In this presentation, we demonstrate the synthesis of water dispersible core-shell SiO2@NaYF4: Yb3+/Er3+ beads, which were then converted into NaYF4: Yb3+/Er3+ hollow capsules after the silica template was removed. Further, a simple high-temperature annealing step induced an in-situ ion-exchange reaction between O2 and NaYF4: Yb3+/Er3+, resulting in pure YOF: Yb3+/Er3+. By introducing different metal ions, such as Gd3+, Li+ and Mn2+, at different co-doping concentration into the YOF host matrix, green UC luminescence of Er3+(2H11/2 and 4S3/2 states) issuccessfully strengthened when excited by a 980 nm laser. Strong green emission is especially important for luminescence thermometry applications.

Doping, in general, is a widely used strategy in materials design that entails incorporating atoms or ions into host lattices to achieve desired properties and functions. Rare earth (RE3+) activated phosphors are gaining popularity due to their efficient luminescence, which can be used in cathode ray tubes (CRT), tricolor lamps, white light-emitting diodes (WLEDs), and field emission display (FED) devices, as well as in biomedical applications. In addition, such materials have been studied as potential temperature probes for use in luminescence thermometry. Trivalent erbium (Er3+) is a remarkable ion having upconversion properties due to its ladder-like energy levels, and it has been regarded as one of the most promising RE3+ candidates for luminescence thermometers due to its intense green emissions from thermally coupled 4S3/2 and 2H11/2 emitting levels. Furthermore, the RE3+ ion of choice is frequently combined with Yb3+ ion to benefit from its high absorption cross-section in the NIR, which favors energy transfer-induced upconversion processes to their RE3+ ions. Moreover, some of such phosphors are useful for drug delivery, due to their exceptional properties such as low density, large specific surface area, and good permeability, as well as their high loading capacity and diffusibility for drug molecules. Here we describe the fabrication of Y2O3 hollow spheres doped with Yb3+ and Er3+ ions abide by the sequential templating process accompanying a simple hydrothermal treatment followed by heat treatment. In addition, we demonstrate the preparation of Y2O2SO4 doped with Yb3+ and Er3+ using a simple one-pot hydrothermal method. In this work these two different inorganic hollow phosphors were extensively investigated and showed to have excellent upconversion luminescence properties and could be effectively used as optical temperature sensors in the biological range (288.15 – 333.15 K). Additionally, they could be used for simultaneous drug delivery, employing doxorubicin (DOX) as a model cancer drug.