Host-guest chemistry of mesoporous silicas: precise design of location, density and orientation of molecular guests in mesopores

Catalytically Active Site Mapping Realized through Energy Transfer Modeling

The demands of a sustainable chemical industry are a driving force for the development of heterogeneous catalytic platforms exhibiting facile catalyst recovery, recycling, and resilience to diverse reaction conditions. Homogeneous-to-heterogeneous catalyst transitions can be realized through the integration of efficient homogeneous catalysts within porous matrices. Herein, we offer a versatile approach to understanding how guest distribution and evolution impact the catalytic performance of heterogeneous host-guest catalytic platforms by implementing the resonance energy transfer (RET) concept using fluorescent model systems mimicking the steric constraints of targeted catalysts. Using the RET-based methodology, we mapped condition-dependent guest (re)distribution within a porous support on the example of modular matrices such as metal-organic frameworks (MOFs). Furthermore, we correlate RET results performed on the model systems with the catalytic performance of two MOF-encapsulated catalysts used to promote CO2 hydrogenation and ring-closing metathesis. Guests are incorporated using aperture-opening encapsulation, and catalyst redistribution is not observed under practical reaction conditions, showcasing a pathway to advance catalyst recyclability in the case of host-guest platforms. These studies represent the first generalizable approach for mapping the guest distribution in heterogeneous host-guest catalytic systems, providing a foundation for predicting and tailoring the performance of catalysts integrated into various porous supports.

SBA-Pr-Is-TAP Functionalized Nanostructured Silica as a Highly Selective Fluorescent Chemosensor for Fe3+ and Cr2O72- Ions in Aqueous Media

SBA-Pr-Is-TAP was synthesized via functionalization of SBA-15. The synthesized hybrid nanomaterial was characterized by various techniques including FT-IR, TGA, XRD, SEM, and BET. SBA-Pr-Is-TAP could precisely bind Fe3+ and Cr2O72- ions among a range of different species in aqueous media, consequently acting as a nanoporous chemosensor of Fe3+ and Cr2O72- ions. An excellent linear relation was observed between the nanoporous chemosensor and ion concentrations, with acceptable detection limits of 2.43 × 10-6 M and 3.96 × 10-7 M for Fe3+ and Cr2O72- ions respectively.

Adsorchromism: Molecular Nanoarchitectonics at 2D Nanosheets-Old Chemistry for Advanced Chromism

Chromism induced by changes in the electronic states of dye molecules due to surface adsorption is termed "adsorchromism" in this article. These changes of molecular electronic states are induced by protonation, aggregation, intramolecular structural changes, and other processes, depending on the surface environment. Intramolecular structural changes, such as co-planarization and decreased molecular motion are the most characteristic and interesting behavior of dye molecules at the surfaces, resulting in spectral shift and/or emission enhancement. In this review, adsorchromism at the surfaces of layered materials are summarized since their flexibility of interlayer distance, surface flatness, and transparency is suitable for a detailed observation. By understanding the relationship between adsorchromism and the electronic states of molecules on the surfaces, it will be possible to induce some desired functions which can be realized simply by adsorption, instead of complicated organic syntheses. Thus, adsorchromism has potential applications such as effective solar energy harvesting systems, or biological/chemical sensors to visualize environmental changes.

Applications of Cu(0) encapsulated nanocatalysts as superior catalytic systems in Cu-catalyzed organic transformations

Recently, Cu nanoparticles (NPs) encapsulated into various materials as supports (e.g., zeolite, silica) have attracted much devotion due to their unique catalytic properties such as high catalytic activity, intensive reactivity and selectivity through highly protective properties. Nowadays, the superior catalytic activity of Cu-NPs, encapsulated onto zeolite, silica and different porous systems, is extensively investigated and now well-established. As a matter of fact, Cu-NPs are protected from deactivation by this kind of encapsulation. Thus, their exclusion proceeds smoothly, and their recyclability is significantly increased. Cu-NPs have been used as potential heterogeneous catalysts in different chemical transformations. In this review, we try to show the preparation and applications of Cu(0) encapsulated nanocatalysts in zeolite and silica as superior catalytic systems in Cu-catalyzed organic transformations. In addition, the catalytic activity of these encapsulated Cu-NPs in different important organic transformations (such as hydrogenation, oxidation and carbon-carbon bond formations) are compared with those of a variety of organic, inorganic and hybrid porous bearing a traded metal ion. Moreover, the results from the TGA/DTA analysis and optical properties of Cu-complexes are demonstrated. The inherited characteristic merits of the encapsulated Cu-NPs onto zeolite and silica, such as their low leaching, catalytic activity, reusability economic feasibility and originality are critically considered.

Template Synthesis of Well-Defined Rutile Nanoparticles by Solid-State Reaction at Room Temperature

Well-defined nanoparticles of rutile (with the size of 5 nm) were successfully prepared by the unusual solid-state transformation of an amorphous precursor in well-defined nanospace of a mesoporous silica template (SBA-15) at room temperature. An aqueous colloidal suspension of the rutile nanoparticles was successfully obtained by dissolution of SBA-15 and subsequent pH adjustment. The isolated rutile nanoparticles were used for H₂ evolution from an aqueous methanol solution by UV irradiation.

Superiority of L-tartaric Acid Modified Chiral Mesoporous Silica Nanoparticle as a Drug Carrier: Structure, Wettability, Degradation, Bio-Adhesion and Biocompatibility

PURPOSE

The purpose of this research was to study the basic physicochemical and biological properties regarding the application of L-tartaric acid modified chiral mesoporous silica nanoparticle (CMSN) as a drug carrier, and to explore the structure-property relationship of silica-based materials.

METHODS

CMSN with functions of carboxyl modification and chirality was successfully synthesized through co-condensation method, and the basic characteristics of CMSN, including morphology, structure, wettability, degradation, bio-adhesion and retention ability in gastrointestinal tract (GI tract) were estimated by comparing with non-functionalized mesoporous silica nanoparticles (MSN). Meanwhile, the biocompatibility and toxicity of L-tartaric modification were systematically evaluated both in vitro and in vivo through MTT cell viability assay, cell cycle and apoptosis assay, hemolysis assay, histopathology examination, hematology analysis, and clinical chemistry examination.

RESULTS

CMSN and MSN were spherical nanoparticles with uniform mesoporous structure. CMSN with smaller pore size and carboxyl functional groups exhibited better wettability. Besides, CMSN and MSN could dissolve thoroughly in simulated physiological fluids during a degradation period of 1-12 weeks. Interestingly, the in vitro and in vivo behaviors of carriers, including degradation, bio-adhesion and retention ability in the GI tract were closely related to wettability. As expected, CMSN had faster degradation rate, higher mucosa-adhesion ability, and longer retention time. Particularly, CMSN improved the bio-adhesion property in both gastric mucosa and small intestinal mucosa, and prolonged the GI tract retention time to at least 12 h, which meant higher probability for absorption. The biocompatibility and toxicity examination indicated that CMSN was a kind of biocompatible bio-material with good blood compatibility and negligible toxicity, which is required for further applications in biological fields.

CONCLUSION

CMSN with functions of carboxyl modification and chirality had superiority in terms of both physicochemical and biological properties. The in vitro and in vivo behaviors of carriers, including degradation, bio-adhesion, and retention were closely related to wettability.

Preferential immobilization of size-controlled anatase nanoparticles in mesopores

Precise structural design of a host-guest complex was carried out from the aspects of the size and the location of the guest (anatase particles), and the remaining open space of the host (mesoporous silica). The size of the anatase particles was successfully controlled (3, 5 and 8 nm) during the preparation, and the size-controlled nanoparticles were preferentially encapsulated into the mesopores with a diameter of 8 nm. Due to the precise control of the anatase particles, size dependent photoluminescence of the anatase quantum dots was observed for the first time. The change in the porosity of the mesoporous silica by the immobilization of the anatase in the pore was followed to find a systematic variation of the porosity corresponding to the loaded anatase amount. This correlation can be useful to estimate the location of the guest in/on the host for the host-guest hybrids.

Acceleration of photochromism and negative photochromism by the interactions with mesoporous silicas

The adsorption of merocyanine dye onto mesoporous silicas with varied pore sizes (5.5, 9.4 and 2.2 nm) from the toluene solution of 1,3,3-trimethylindolino-6'-nitrobenzopyrylospiran under UV irradiation was investigated quantitatively. The photoinduced adsorption of merocyanine onto SBA-15 with the pore diameter of 9.4 nm followed the pseudo-second order kinetics and the rate constant was larger than that observed for MCM-41 (pore size of 2.2 nm) owing to the efficient diffusion of merocyanine. The maximum adsorbed amounts of the merocyanine dye was 152 mg g-1 of SBA-15, which corresponded to the sufficiently high concentration of merocyanine in the pores (0.376 mol L-1 of pore). The resulting red-colored hybrids (SBA-15 containing merocyanine) showed decoloration in the solid-state by visible light irradiation (negative photochromism). The conversion was high (about 80% at the photostationary state) under visible light irradiation at room temperature using a solar simulator (100 W). The red color was re-generated by storing the photochemically formed colorless samples in the dark at room temperature. The half-lives of the thermal coloration process were 2.6, 1.9 and 1.3 h for the MCM-41, SBA-15s with the BJH pore sizes of 5.5 and 9.4 nm, respectively. Since the coloration was affected by the diffusion of the molecules in the pores, larger pores provided the efficient molecular diffusion, leading to faster reactions.

Phase Engineering of Hydrophobic Meso-Environments in Silica Particles for Technical Performance Enrichment

Hexadecyltrimethylammonium bromide (CTAB) was utilized to template the growth of mesoporous silica particles via ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in the reaction solutions with varied volume fractions of ethanol ( f_R). The use of 9,10-bis(phenylethynyl) anthracene (BPEA) as a fluorescence probe unraveled a clear difference in interior structure between the CTAB micelles confined at different f_R. At f_R of 0.3, the confined CTAB micelles consisting of regularly and densely packed alkane chains, which created crystalline interiors, in which the doped BPEA molecules were effectively isolated in the monomeric form and well protected against aggressive attack from the surrounding environment. At f_R of 0.4 or 0.5, the confined CTAB micelles consisting of less regularly but densely packed alkane chains created glassy interiors, which enabled reversible aggregation of the doped BPEA in response to the surrounding environmental change, for instance, the ethanol content in the particle dispersion. At f_R of 0.6 or 0.7, the confined CTAB micelles consisting of loosely packed alkane chains created amorphous interiors, which offered sufficiently large free spaces to facilitate the material exchange with the surrounding environment, as evidenced by noticeable intake of the Pyronin Y molecules present in the particle dispersion. The revealed phase modulation of the interiors of surfactant micelles, confined in the pores of mesoporous particles, from crystalline to glassy and amorphous structures, which were scarcely reported in literature, will inspire rational design of mesoporous silica particles with desired technical performance according to the purposes of the practical application.

Formation and optical properties of metal/10-hydroxybenzo[h]quinolone complexes in the interlayer spaces of magadiite by solid-solid reactions

Intercalation and in situ formation of three fluorescent complexes, Al(III)-, Cr(III)- and Cu(II)-10-hydroxybenzo[h]quinolone (M-HBQ, M = Al, Cr and Cu), in the interlayer spaces of magadiite (mag) were studied by solid-solid reactions between metal ions exchanged mags (M-mag, M = Al, Cr and Cu) and HBQ. Results show that the basal spacings of the intercalated composites increase after the intercalation of HBQ into M-mags. The amount of HBQ in the intercalated compounds is different due to the amount of metal ions and the diversification of coordination ability of metal ions, and the order of the coordination ability of these three metal ions is Cu²⁺ > Cr³⁺ > Al³⁺. The amount of the metal cations in the interlayer of mag is enough for the in situ complex formation of M-HBQ complexes. The slight shift of the absorption and luminescence bands of the complexes suggests the different microstructures, including molecular packing of the complexes in the interlayer spaces of mags, resulting that the host-guest interactions are formed. These findings show that the intercalation and in situ formation of M-HBQ complexes (M = Al, Cr and Cu) in the interlayer space of mag are successfully achieved in the current work.

Intercalation of calcein into layered silicate magadiite and their optical properties

Calcein-Ca (II), Zn (II) and Al (III) complexes were successfully intercalated into interlayer surfaces of layered silicate magadiite and fluorescence properties of organic metal-chelates in the confined spaces were investigated. Structures, compositions and morphologies of the intercalated magadiites were adequately studied by tests, including X-ray diffraction, energy-dispersive X-ray spectrometer, elemental mapping, X-ray photoelectron spectroscopy, inductively coupled plasma atomic emission spectroscopy, Fourier-transform infrared spectra, ultraviolet-visible spectroscopy, thermo-gravimetric analysis, differential thermal analysis and scanning electron microscopy. Results confirmed that metal-organic chelate species were immobilized onto the silicate sheets via solid-state interaction. Basal spacings between silicate layers decreased by exchanged metal ions and increased after intercalation of calcein into the interlayer spaces of cation-exchanged magadiites. The encapsulation was obtained by a flexible solid-solid reaction, and the present reaction and products had a potential of application to industrial uses. A speculative mechanism was proposed for reaction by solid-state intercalation. Furthermore, it was found that the complexes in the interlayer space also exhibited special fluorescence properties. The significance of this current work was that it provided a possible route for synthesizing metal-organic complexes that encapsulated in phyllosilicate.

Photoinduced structural changes of cationic azo dyes confined in a two dimensional nanospace by two different mechanisms

By UV irradiation, the interlayer space of a dried phenylazonaphthalene–magadiite diminished, while that of the phenylazobenzene-form expanded under high humidity.

Photoinduced adsorption of spiropyran into mesoporous silicas as photomerocyanine

UV irradiation to a spiropyran-containing suspended mesoporous silica in toluene, the photochemically formed photomerocyanine was adsorbed into the mesoporous silica to give a very stable red-colored product.