Nando Gartmann

Nanochannels for supramolecular organization of luminescent guests

Zeolites and mesoporous silica are versatile host materials for the supramolecular organization of a large variety of guests. The inclusion of luminescent molecules, complexes, or nanoclusters into ordered one-dimensional channel systems is particularly intriguing, as the resulting host–guest compounds may exhibit unique properties such as optical anisotropy, efficient energy transfer, and enhanced stability. Several levels of organization have been realized for zeolite L, extending from the interior of a given crystal to the channel entrances and the external surface as well as from the microscopic to the macroscopic scale. The diverse chemistry that is involved in the development of a highly organized and functional host–guest material is illustrated by the design of molecules which are able to selectively adsorb at the channel entrances and establish communication between included guests and external objects. The importance of phthalocyanines as chromophores for this particular concept is exemplified. Based on the extensive research on zeolite L, possibilities and recent developments in the field of mesoporous silica hosts are discussed, revealing that despite the obvious differences between microporous and mesoporous host–guest materials, many similarities and analogies exist.

Controlling and Imaging the Functional-Group Distribution on Mesoporous Silica†

The control of the distribution of functional groups on mesoporous silica is essential for applications of these materials in various fields including catalysis, drug delivery, and sensing. In order to define the interaction of the mesoporous silica particles with their surrounding medium, the selective modification of the external surface is of particular importance. Externally grafted functional groups can, for example, regulate the cellular uptake or provide targeting ability of mesoporous-silica-based drug-delivery systems. The introduction of functional groups by grafting to a preformed mesoporous material (often referred to as postsynthetic functionalization) is a versatile modification method, as the desired pore-size distribution, pore system dimensionality, particle size, and particle morphology can be obtained in a straightforward manner. However, the control of the functional-group distribution poses a particular challenge. A recently reported concept employs fluorenylmethoxycarbonyl(Fmoc)-modified organosilanes which are grafted to the external and internal (pore) surfaces of mesoporous silica. Under certain reaction conditions, the external surface groups can be deprotected selectively and subsequently functionalized further, whereas the groups located on the pore surface remain protected by Fmoc. A frequently used general method for modifying the external surface is based on the reaction of chloro-, methoxy-, or ethoxysilanes with as-synthesized mesoporous silica, in other words, mesoporous silica still containing the structure-directing agent (SDA). We show herein that considerable grafting to the pore surface can occur despite the presence of the SDA, and we describe a convenient postsynthetic functionalization method with a high selectivity for the external surface. Confocal laser scanning microscopy (CLSM) has been used to visualize the spatial distribution of fluorescent guests in mesoporous and microporous host materials. The distribution of functional groups covalently bound to mesoporous silica can be similarly imaged after coupling with appropriate fluorescent labels. Large particles of defined morphology are ideal for this purpose. We have been working with hexagonal particles, also known as arrays of silica nanochannels (ASNCs), as well as with spherical particles of the SBA-15 type (SBA-s) featuring a less ordered pore system and a larger average pore size than the ASNCs (Figure 1, Table 1). In both cases, functionalization reactions were carried out either before or after removal of the SDA. Apart from the frequently employed 3-aminopropyltriethoxysilane (APTES), 3-aminopropyltris(methoxyethoxyethoxy)silane (APTMEES) and bis(triethoxysilylpropyl)amine (BTESPA) were used as reactants (Figure 2). The surface-grafted amino groups were subsequently labeled with fluorescein isothiocyanate (FITC) or Texas Red sulfonyl chloride (TR). Deposition of the silanes from hexane at room temperature and curing at 80 8C led to the remarkably different distributions shown in Figure 2. The following can be concluded: 1) As a result of the comparatively large pore diameter, reaction with calcined SBA-s leads to a high degree of pore-surface grafting for all investigated silanes. The uniformity of the functional-group distribution decreases in the series APTES>BTESPA>APTMEES. As a consequence of the narrower channels, this tendency is more pronounced when grafting to calcined ASNCs. In the case of APTMEES, excellent selectivity for the external surface is obtained. The observation that BTESPA produces a less uniform distribution than APTES is in agreement with results obtained from a systematic study of the pore-size distribuFigure 1. Pore-size distribution of ASNCs (*) and SBA-s (*). The morphology of the particles is evident in the corresponding electron micrographs. The image of the ASNCs shows two particles (one particle is standing on its hexagonal base).

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.

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.

Direct synthesis and fluorescent imaging of bifunctionalized mesoporous iodopropyl-silica

The cocondensation of 3-iodopropyltrimethoxysilane and tetraethoxysilane with an additional substituted trimethoxysilane (RTMS) in the presence of Pluronic P123 and hydrogen iodide yields bifunctionalized mesoporous silica. The pore-size distribution of these materials depends on the nature of the RTMS additive. Excellent results in terms of a narrow pore-size distribution were obtained with methyltrimethoxysilane. A particularly interesting bifunctionalized mesoporous material is formed by the coinclusion of iodopropyl and aminopropyl moieties. The complementary reactivity of these two functional groups is demonstrated by the selective labeling of the amino-iodo-functionalized mesoporous silica with 2-hydroxy-substituted Nile red and fluorescein isothiocyanate, allowing further characterization of the functional group distribution by confocal laser scanning microscopy.