Claus Feldmann

Ionic Liquids: New Perspectives for Inorganic Synthesis?

Ionic liquids are credited with a number of unusual properties. These include a low vapor pressure, a wide liquid-phase range, weakly coordinating properties, and a high thermal/chemical stability. These properties are certainly of great interest for inorganic synthesis and the creation of novel inorganic compounds. On the other hand, the synthesis repertoire for preparing inorganic compounds has always been broad, ranging from syntheses in solutions and melts to solid-state reactions, and from crystal growth in the gas phase to high-pressure syntheses. What new aspects can ionic liquids then add to the synthesis of inorganic compounds? This Minireview uses some early examples to show that the use of ionic liquids indeed provides access to unusual inorganic compounds.

Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements

Since the first description by Fievet, Lagier and Figlarz in 1989, the synthesis of nanoparticles in high-boiling, multivalent alcohols – so-called polyols – has been developed into a widely applied strategy, and nowadays belongs to the standard repertoire for preparing high-quality nanomaterials. The polyols take advantage of several features such as: (i) water-comparable solubility of simple metal-salt precursors; (ii) high boiling points (up to 320 °C); (iii) reducing properties for the instantaneous synthesis of metals; (iv) coordinating properties for surface functionalization and colloidal stabilisation of nanoparticles; (v) wide adaptability of the polyols ranging from low-weight ethylene glycol (EG) to high-weight polyethylene glycols (PEGs). This review summarises the status and perspectives on nanoscaled elemental metals, metal oxides, metal chalcogenides, and non-metal elements that were prepared via the polyol synthesis. Moreover, we summarize our results and concepts to expand the limits of the polyol synthesis. This includes strategies for less-noble metal synthesis, phase transfer reactions, photochemical reduction, NMR-based characterisation of polyol-functionalised nanoparticles, realisation of phase-pure and readily crystalline metal tungstates, stabilisation of low-melting elements, and controlled thermal decomposition of polyols to obtain high-quality, lanthanide-modified carbon dots (C-dots).

Multifunctional Phosphate-Based Inorganic–Organic Hybrid Nanoparticles

Phosphate-based inorganic-organic hybrid nanoparticles (IOH-NPs) with the general composition [M](2+)[Rfunction(O)PO3](2-) (M = ZrO, Mg2O; R = functional organic group) show multipurpose and multifunctional properties. If [Rfunction(O)PO3](2-) is a fluorescent dye anion ([RdyeOPO3](2-)), the IOH-NPs show blue, green, red, and near-infrared fluorescence. This is shown for [ZrO](2+)[PUP](2-), [ZrO](2+)[MFP](2-), [ZrO](2+)[RRP](2-), and [ZrO](2+)[DUT](2-) (PUP = phenylumbelliferon phosphate, MFP = methylfluorescein phosphate, RRP = resorufin phosphate, DUT = Dyomics-647 uridine triphosphate). With pharmaceutical agents as functional anions ([RdrugOPO3](2-)), drug transport and release of anti-inflammatory ([ZrO](2+)[BMP](2-)) and antitumor agents ([ZrO](2+)[FdUMP](2-)) with an up to 80% load of active drug is possible (BMP = betamethason phosphate, FdUMP = 5-fluoro-2-deoxyuridine 5-monophosphate). A combination of fluorescent dye and drug anions is possible as well and shown for [ZrO](2+)[BMP](2-)0.996[DUT](2-)0.004. Merging of functional anions, in general, results in [ZrO](2+)([RdrugOPO3]1-x[RdyeOPO3]x)(2-) nanoparticles and is highly relevant for theranostics. Amine-based functional anions in [MgO](2+)[RaminePO3](2-) IOH-NPs, finally, show CO2 sorption (up to 180 mg g(-1)) and can be used for CO2/N2 separation (selectivity up to α = 23). This includes aminomethyl phosphonate [AMP](2-), 1-aminoethyl phosphonate [1AEP](2-), 2-aminoethyl phosphonate [2AEP](2-), aminopropyl phosphonate [APP](2-), and aminobutyl phosphonate [ABP](2-). All [M](2+)[Rfunction(O)PO3](2-) IOH-NPs are prepared via noncomplex synthesis in water, which facilitates practical handling and which is optimal for biomedical application. In sum, all IOH-NPs have very similar chemical compositions but can address a variety of different functions, including fluorescence, drug delivery, and CO2 sorption.

Microemulsions: Options To Expand the Synthesis of Inorganic Nanoparticles

Microemulsions (MEs) are ideal for obtaining high-quality inorganic nanoparticles. As thermodynamically stable systems with a nanometer-sized droplet phase that serves as a nanoreactor, MEs have obvious advantages for the synthesis of nanoparticles. MEs also have disadvantages, such as their complexity as multicomponent systems, the low amount of obtainable nanoparticles, their limited thermal stability, the fact that hydrolyzable or oxidizable compounds are often excluded from synthesis, the partly elaborate separation of nanoparticles, as well as the removal of surface-adhered surfactants subsequent to synthesis. This Review presents some strategies to further expand the options of ME-based synthesis of inorganic nanoparticles. This comprises the crystallization of nanoparticles in high-temperature MEs, the synthesis of hollow nanospheres, the use of hydrogen peroxide or liquid ammonia as the polar droplet phase, and the synthesis of base metals and nitrides in MEs.

Organic melt, electride, and CVD induced in situ deposition of luminescent lanthanide imidazolate MOFs on nanostructured alumina

The highly luminescent MOFs 3∞[Ce(Im)3ImH]·ImH, 3∞[Tb(Im)3], and the dense framework 3∞[Sr0.95Eu0.05(Im)2] (Im− = imidazolate-anion, C3N2H3−) were grown on nanostructured macroporous aluminium oxide (AAO) membranes. Thereby, luminescent coatings on the membranes were achieved. Three different growth processes unusual for MOFs were investigated and compared: a film growth process in situ to MOF formation from the linker melt, electride induction with solvated electrons, and chemical vapour deposition (CVD) to additionally utilize the gas phase. Deposition from the organic melt has proved to be a fast approach to achieve various film thicknesses of the luminescent frameworks. The electride-based approach offers excellent homogenization at an atomic level for the highest quantum yields of QY > 90% for 3∞[Sr0.95Eu0.05(Im)2] including the formation of barite rose analogous crystals prior to growth of a complete film on AAO membranes. For 3∞[Tb(Im)3] and 3∞[Ce(Im)3ImH]·ImH, deposition of bundles of crystals by CVD on AAO is possible while also maintaining the luminescence of the original MOFs but without complete layers. In order to elaborate the divalent character of europium, being the basis of the high efficiency of the luminescence, EPR studies were carried out on 3∞[Sr1−xEux(Im)2], x = 0.01, 0.05 and 1.