Pore condensation and freezing is responsible for ice formation below water saturation for porous particles.

Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth's climate. An accepted ice nucleation pathway, known as deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below -40 °C and the absence of ice nucleation below water saturation at -35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.

Synthesis of Advanced Mesoporous Materials by Partial Pseudomorphic Transformation.

The structure of porous silica particles can be reorganized without alteration of the particle size and shape by the process of pseudomorphic transformation. Partial pseudomorphic transformation leads to ordered mesoporous silica with bimodal pore size distributions and bottleneck pores. Compared to the classical pathways of mesoporous silica synthesis, pseudomorphic transformation implies less compromise between pore structure and particle shape, while enabling the preparation of complex pore architectures.

Supramolecular Organization of Dye Molecules in Zeolite L Channels: Synthesis, Properties, and Composite Materials.

Sequential insertion of different dyes into the 1D channels of zeolite L (ZL) leads to supramolecular sandwich structures and allows the formation of sophisticated antenna composites for light harvesting, transport, and trapping. The synthesis and properties of dye molecules, host materials, composites, and composites embedded in polymer matrices, including two- and three-color antenna systems, are described. Perylene diimide (PDI) dyes are an important class of chromophores and are of great interest for the synthesis of artificial antenna systems. They are especially well suited to advancing our understanding of the structure-transport relationship in ZL because their core fits tightly through the 12-ring channel opening. The substituents at both ends of the PDIs can be varied to a large extent without influencing their electronic absorption and fluorescence spectra. The intercalation/insertion of 17 PDIs, 2 terrylenes, and 1 quaterrylene into ZL are compared and their interactions with the inner surface of the ZL nanochannels discussed. ZL crystals of about 500 nm in size have been used because they meet the criteria that must be respected for the preparation of antenna composites for light harvesting, transport, and trapping. The photostability of dyes is considerably improved by inserting them into the ZL channels because the guests are protected by being confined. Plugging the channel entrances, so that the guests cannot escape into the environment is a prerequisite for achieving long-term stability of composites embedded in an organic matrix. Successful methods to achieve this goal are described. Finally, the embedding of dye-ZL composites in polymer matrices, while maintaining optical transparency, is reported. These results facilitate the rational design of advanced dye-zeolite composite materials and provide powerful tools for further developing and understanding artificial antenna systems, which are among the most fascinating subjects of current photochemistry and photophysics.

Incorporation of a FRET dye pair into mesoporous materials: a comparison of fluorescence spectra, FRET activity and dye accessibility.

Fluorescein and rhodamine B modified mesoporous silica particles were synthesized by post-grafting and co-condensation approaches. The materials exhibited different pore size distributions, particle shapes and sizes. The materials were characterized by nitrogen sorption, scanning electron microscopy and fluorescence spectroscopy. The Förster resonance energy transfer between the selected dye pair was explored for the different materials by exposure to various concentrations of gaseous ammonia. A logarithmic increase in rhodamine B emission with increasing ammonia concentration was observed for both post-grafted and co-condensed materials. The dye accessibility by ammonia gas in the silica framework of mesoporous materials was evaluated by using a flow cell gas sensor setup built in-house. Response to ammonia gas and recovery with nitrogen gas are explained by comparing the structure properties and dye loading of the materials. The post-grafted dye modified silica showed better performance in terms of reversibility and recovery.

Host-guest interactions and orientation of dyes in the one-dimensional channels of zeolite L.

A combined experimental and modeling study of methylacridine (MeAcr(+)) dye-zeolite L composites unravels the microscopic origin of their functional properties. The anisotropic orientation of the cationic dye inside the ZL channel is unambiguously determined and understood. The most stable orientation of MeAcr(+), which features both its long and short molecular axes nearly perpendicular to the channel axis, is mainly determined by dye-ZL electrostatic interactions but also depends on the cosolvent water. In ZL, MeAcr(+) is not hydrogen bonded to water or ZL framework oxygens and is hydrophobically solvated by water molecules. These findings further support the hypothesis that the cosolvent can importantly influence properties of dye-zeolite composites. Of relevance for a deeper comprehension of the physical chemistry of these hybrids is the observation that trivial energy transfer processes (self-absorption) are often playing a significant role in the optical properties of the composites.

Energy-related chemical research at the Universities of Applied Sciences.

An overview of current activities in the field of energy-related chemical research at the Swiss Universities of Applied Sciences is presented.

Indigo-A New Tribological Substance Class for Non-Toxic and Ecological Gliding Surfaces on Ice, Snow, and Water.

The biogenic substance E-indigo can form supramolecular, hydrophobic structures using self-organization. These structures show a low coefficient of friction as a gliding layer against polar surfaces. The formation of primary particles with platelet morphology based on hydrogen-bonded E-indigo molecules is ideal to produce the gliding layer. Structures with excellent gliding properties can be achieved by means of directed friction and high pressure, as well as through tempering. The resulting hard, thin gliding layer of E-indigo does not easily absorb dirt and, thus, prevents a rapid increase in friction. Field tests on snow, with cross-country skis, have shown promising results in comparison to fluorinated and non-fluorinated waxes. Based on quantitative structure-activity relationship (QSAR) data for E-indigo, and its isomers and tautomers, it has been demonstrated that both the application and abrasion of the thin indigo layers are harmless to health, and are ecologically benign and, therefore, sustainable.

On the significance of the anchoring group in the design of antenna materials based on phthalocyanine stopcocks and zeolite L.

The synthesis of stopcocks based on zinc phthalocyanine for selective adsorption at the channel entrances of zeolite L is reported. The introduction of either an inert SiMe3 moiety, an imidazolium cation or a reactive isothiocyanate (NCS) group allows attachment to the channel entrances of zeolite L through van der Waals interactions, electrostatic interactions, or covalent binding, respectively. Stopcocks that rely on van-der-Waals-driven adsorption require careful selection of the solvent used for the deposition onto the zeolite surface to avoid a nonspecific distribution of the molecules. Regarding the design of photonic antenna systems, a stopcock with a cationic tail was found to be the most convenient, based on the observation that efficient energy transfer from molecules located in the zeolite nanochannels is more readily obtained than in the other cases.

Designing dye-nanochannel antenna hybrid materials for light harvesting, transport and trapping.

We discuss artificial photonic antenna systems that are built by incorporating chromophores into one-dimensional nanochannel materials and by organizing the latter in specific ways. Zeolite L (ZL) is an excellent host for the supramolecular organization of different kinds of molecules and complexes. The range of possibilities for filling its one-dimensional channels with suitable guests has been shown to be much larger than one might expect. Geometrical constraints imposed by the host structure lead to supramolecular organization of the guests in the channels. The arrangement of dyes inside the ZL channels is what we call the first stage of organization. It allows light harvesting within the volume of a dye-loaded ZL crystal and also the radiationless transport of energy to either the channel ends or center. One-dimensional FRET transport can be realized in these guest-host materials. The second stage of organization is realized by coupling either an external acceptor or donor stopcock fluorophore at the ends of the ZL channels, which can then trap or inject electronic excitation energy. The third stage of organization is obtained by interfacing the material to an external device via a stopcock intermediate. A possibility to achieve higher levels of organization is by controlled assembly of the host into ordered structures and preparation of monodirectional materials. The usually strong light scattering of ZL can be suppressed by refractive-index matching and avoidance of microphase separation in hybrid polymer/dye-ZL materials. The concepts are illustrated and discussed in detail on a bidirectional dye antenna system. Experimental results of two materials with a donor-to-acceptor ratio of 33:1 and 52:1, respectively, and a three-dye system illustrate the validity and challenges of this approach for synthesizing dye-nanochannel hybrid materials for light harvesting, transport, and trapping.

Functional group distributions on mesoporous silica.

Most applications of mesoporous silica require some degree of functionalization. The surface of porous materials can be divided into external and internal (pore) surfaces, and in many cases, a selective functionalization of these surface subsections is desired. This short review outlines our recent work in this field and focuses on the postsynthetic functionalization of mesoporous silica with aminopropylalkoxysilanes and on the analysis of the respective functional group distributions by confocal laser scanning microscopy. Methods to obtain an amino-functionalized external surface and functional group gradients on the pore surface are reported. Arrays of silica nanochannels (ASNCs) serve as a model system for mesoporous silica.

Silica particles with fluorescein-labelled cores for evaluating accessibility through fluorescence quenching by copper.

Core-shell particles with fluorescent cores were synthesised by growing silica shells on fluorescein-labelled Stöber-type particles. The porosity of the shell could be altered in a subsequent pseudomorphic transformation step, without affecting the particle size and shape. These core-shell particles constitute a platform for the evaluation of pore connectivity and core accessibility by observing the effect of a quencher on the fluorescence signal emitted by the fluorescein-labelled cores. In combination with argon sorption measurements, quenching experiments with copper provided valuable information on the porosity generated during the shell formation process. It was further observed that the introduction of well-defined mesopores by pseudomorphic transformation in the presence of a structure-directing agent reduces the core accessibility. This led to the conclusion that the analysis by conventional gas sorption methods paints an incomplete picture of the mesoporous structure, in particular with regard to pores that do not offer an unobstructed path from the external particle surface to the core.

Surprising properties of a furo-furanone.

The electronic absorption, fluorescence, and excitation spectra of furo[3,4-c]furanone (1) have been measured in different solvents at different concentrations. We observed a complex dependence of absorption and excitation spectra as a function of the concentration in CH(2)Cl(2) and THF due to aggregate formation. Interestingly, the fluorescence spectra were not affected. Resolving the puzzle was made possible by the fact that 1 fits perfectly into the channels of zeolite L (ZL) microcrystals to form 1-ZL guest-host composites. The geometry of the ZL channel system ensures a well-defined orientation of the embedded dye molecules, thereby leading to a preferred orientation of their electronic transition dipole moment (ETDM) and thus to objects with pronounced optical anisotropy properties. This enabled us to understand that in solution the monomers that are present at low concentration form an aggregate in which the molecules sit on top of each other and arrange into a J-type aggregate configuration at higher concentrations. The signature of the latter is observed in the 1-ZL composites. This seems to be the first example in which the insertion of molecules into a nanochannel microcrystal has helped in understanding the weak intermolecular interactions that take place in solution.

Hollow Silica Cubes with Customizable Porosity.

Hollow silica cubes were synthesized by a deposition of a thin silica shell onto micrometer-sized hematite cubes. Ordered mesopores with well-defined pore diameters of 2.8 nm and 3.8 nm were introduced into the silica shell by means of pseudomorphic transformation after removal of the hematite core. The particles retained their cubic morphology upon pseudomorphic transformation, allowing for the preparation of close-packed layers of the hollow mesoporous silica cubes by drop-casting and the visualization of the hollow core by focused ion beam scanning electron microscopy.

Novel phthalocyanine-based stopcock for zeolite L.

We report the first phthalocyanine-based stopcock for selective adsorption to the channel entrances of zeolite L and realisation of a new electronic dipole moment coupling situation.

A novel (99m)Tc labelling strategy for the development of silica based particles for medical applications.

The reaction of high-valent fac-{(99m)TcO3}(+) complexes with alkenes ((3 + 2)-cycloaddition) has been used to develop a novel labelling procedure for silica based particles. This strategy can help to gain more detailed in vivo data of mesoporous silica by non-invasive radioimaging in the future. By applying norbornene as a coupling unit the (3 + 2)-cycloaddition reaction proceeds stereoselectively and with high reaction rates.

Correlation of Nitrogen Sorption and Confocal Laser Scanning Microscopy for the Analysis of Amino Group Distributions on Mesoporous Silica.

Aminopropylalkoxysilanes are frequently used for the functionalization of mesoporous silica. The analysis of amino group distributions on arrays of silica nanochannels by a combination of nitrogen sorption and confocal laser scanning microscopy provides valuable insight into the mechanisms underlying the interaction of these silanes with mesoporous silica surfaces. Tendencies towards external surface functionalization, non-uniform distribution in the pores, and hydrolysis of the silica framework are shown to depend to a large extent on the mobility of the aminopropylalkoxysilane molecules, which can be adjusted by the number and type of alkoxy groups.

Synthesis of subphthalocyanines as probes for the accessibility of silica nanochannels.

The synthesis of a new subphthalocyanine is reported. Its structural and photophysical properties are ideal for probing the accessibility of arrays of silica nanochannels.

Microspectroscopic analysis of green fluorescent proteins infiltrated into mesoporous silica nanochannels.

The infiltration of enhanced green fluorescent protein (EGFP) into nanochannels of different diameters in mesoporous silica particles was studied in detail by fluorescence microspectroscopy at room temperature. Silica particles from the MCM-41, ASNCs and SBA-15 families possessing nanometer-sized (3-8 nm in diameter) channels, comparable to the dimensions of the infiltrated guest protein EGFP (barrel structure with dimensions of 2.4 nm × 4.2 nm), were used as hosts. We found that it is necessary to first functionalize the surfaces of the silica particles with an amino-silane for effective encapsulation of EGFP. We demonstrated successful infiltration of the protein into the nanochannels based on fluorescence microspectroscopy and loading capacity calculations, even for nanochannel diameters approaching the protein dimensions. We studied the spatial distributions of the EGFPs within the silica particles by confocal laser scanning microscopy (CLSM) and multimode microscopy. Upon infiltration, the fluorescence lifetime drops as expected for an emitter embedded in a high refractive index medium. Further, the spectral properties of EGFP are preserved, confirming the structural integrity of the infiltrated protein. This inorganic-protein host-guest system is an example of a nanobiophotonic hybrid system that may lead to composite materials with novel optical properties.

Postsynthetic functionalization of mesoporous silica.

Functionalized mesoporous silica offers promising possibilities for numerous applications, including drug delivery, catalysis, and adsorption. This minireview focuses on recent developments related to the postsynthetic positioning of functional groups on mesoporous silica. After briefly introducing the reagents that are commonly used for this purpose, methods to control and to analyze the distribution of the grafted functional groups are discussed, with particular emphasis on concepts that allow the placement of the groups at specific distances from each other, as well as on approaches towards the selective functionalization of the external particle surface.