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For the highly selective and sensitive sensing of Hg2+ in water, a new design concept was realized where the selectivity of the probe's binding site is amplified by electronic properties of the chromophore. The molecular architecture of this phenoxazinone-type sensor molecule combines two potential coordination sites via an amino-keto conjugative backbone. These structural prerequisites allow only the most preferred mercuric ion to bind to the dithia dioxa monoaza crown unit, while other heavy, transition, and main group metal ions as well as protons are trapped at the keto group, inducing opposite spectral effects due to interaction with either the donor (Hg2+) or the acceptor group (other cations) of the probe. Besides these advantageous features, the probe operates well within the visible range of the spectrum and displays rather intense molar absorptivities as well as fluorescence quantum yields.
Sensory hybrid host materials for the selective chromo-fluorogenic detection of biogenic amines
(2006)
A strategy for the rational design of a new optical sensor material for the selective recognition of long-chain carboxylates in water is presented. The approach relies on the combination of structure-property relationships to single out the optimal molecular sensor unit and the tuning of the sensing characteristics of an inorganic support material. A spacer-substituted 7-urea-phenoxazin-3-one was employed as the signaling moiety and a mesoporous trimethylsilylated UVM-7 (MCM-41 type) material served as the solid support. The sensor material shows the advantageous features of both modules that is absorption and emission in the visible spectral range, a fluorescence red-shift and enhancement upon analyte coordination, and the amplification of noncovalent (binding) and hydrogen-bonding (recognition) interactions in the detection event. Besides these basic results that are related to the design and performance of the sensor material, the paper discusses general aspects of amido-substituted phenoxazinone photophysics and addresses some general features of molecular anion recognition chemistry in aqueous vs nonaqueous media, utilizing steady-state and time-resolved optical as well as NMR spectroscopies. Detailed studies on potentially competing biochemical species and a first access to the schematic model of the response of the sensor material as obtained by a combination of fluorescence lifetime distribution analysis and Langmuir-type fitting of the gross binding constants complement the key issues of the paper.
Design strategies for (bio)chemical systems that are inspired by nature's accomplishments in system design and operation on various levels of complexity are increasingly gaining in importance. Within the broad field of biomimetic chemistry, this article highlights various attempts toward improved and sophisticated sensory materials that rely on the combination of supramolecular (bio)chemical recognition principles and nanoscopic solid structures. Examples range from more established concepts such as hybrid sensing ensembles with improved sensitivity and selectivity or for target analytes for which selectivity is hard to achieve by conventional methods, which were often inspired by protein binding pockets or ion channels in membranes, to very recent approaches relying on target-gated amplified signalling with functionalised mesoporous inorganic supports and the integration of native biological sensory species such as transmembrane proteins in spherically supported bilayer membranes. Besides obvious mimicry of recognition-based processes, selected approaches toward chemical transduction junctions utilizing artificially organized synapses, hybrid ensembles for improved antibody generation and uniquely colour changing systems are discussed. All of these strategies open up exciting new prospects for the development of sensing concepts and sensory devices at the interface of nanotechnology, smart materials and supramolecular (bio)chemistry.
A family of N-crowned 4-p-(aminophenyl)-2,6-diphenylpyridines DA (1-6) has been synthezised, characterized, and studied as potential hosts for the signaling of cationic and anionic guests. The ditopic probes contain two coordination sites, a monodentate 2,6-diphenylpyridine and an anilino group with macrocycles of different ring size, denticity, and type of secondary heteroatom (O and/or S). X-ray structure analysis of aza-oxa-thia-crowned 5 indicated a largely planar chromophore. Optical spectroscopic and electrochemical studies revealed that the anilino-type donor (D) and the 2,6-diphenylpyridine acceptor (A) are strongly -conjugated, entailing intense intramolecular charge-transfer absorption bands at 340 nm. Binding studies with protons and metal ions (M = Cu2+, Zn2+, Hg2+, Fe3+, Pb2+, Ni2+, Cd2+) showed shifts of the band to the visible (440 nm) when coordination at the pyridine group occurs, strengthening its acceptor character. In contrast, no band in the visible is formed if binding takes place at the anilino group. Three different responses were found for various pairs of DA and M: selective metal coordination to D or A as well as coordination at both sites. A selective response was found for 5 and Hg2+. Because of the multitude of coordination-induced effects, the DA-M ensembles were further employed for differential anion sensing. In this protocol, the addition of an anion X to a certain, weakly coordinated DA-M can (i) lead to the formation of a ternary ion pair complex (DA-M-X), (ii) change the preference for A or D coordination, (iii) induce dissociation of the complex, or (iv) can have no effect. Various patterns of absorption changes were obtained as a result of different responses (i)-(iv) of the DA-M's in the presence of various X's. Data analysis yielded recognition patterns for acetate, F- and CN-, demonstrating the potential of simple chromogenic host-guest pairs for differential anion signaling.
A family of azo dyes containing amide (1), urea (2), thiourea (3), carbamate (4) or amino (5) hydrogen-bond donating groups were synthesized and their response toward anions was studied. Acetonitrile solutions of 1-5 show bright yellow colours, due to charge-transfer bands in the 375-400 nm region, slightly modulated by the electron donor strength of the group attached to the 4 end of the 4-nitroazobenzene scaffold. Anions of different shape and size (i.e., spherical F-,Cl-, Br- and I-, planar and tetrahedral oxoanions such as NO3-, H2PO4- and HSO4-, carboxylates such as acetate and benzoate and the linear anions cyanide and thiocyanate) were employed in the recognition studies. Two different effects were distinguished: (i) bathochromic shifts of <40 nm to pale orange, due to anion coordination, and (ii) strong red shifts of ca. 200 nm with a concomitant colour change to blue, due to deprotonation. This behaviour was explained as a balance between the deprotonation tendency of the binding sites in the different receptors and the proton affinities of the anions. Semiempirical calculations were carried out to evaluate the hydrogen bond-donating abilities of the anions and the dyes and a good correlation with the experimentally found values was observed. Stability constants for the receptor-anion complexes were determined spectrophotometrically and the different responses toward fluoride and acetate were assessed by NMR titration experiments. Finally, as a special case, we also report the use of this family of compounds as colorimetric carbon dioxide sensors through reaction with the unprotonated forms of the azo dyes.
Die Entwicklung von Hybridmaterialien mit stark verbesserten Funktionen gelingt durch die Kombination geeigneter Nanomaterialien als Träger mit Motiven aus der supramolekularen Chemie. Diese hetero-supramolekularen Konzepte bieten vielfältige Möglichkeiten, die Lücken zwischen der Molekülchemie, Materialwissenschaften und Nanotechnologie zu schließen. Im Hinblick auf funktionelle Aspekte wurden in jüngster Zeit wichtige Fortschritte erzielt, z. B. verbesserte Erkennungs- und Sensoreigenschaften durch die Anordnung von Molekülen auf vororganisierten Oberflächen, der reversible Aufbau von nanometergroßen Netzwerken und dreidimensionalen Strukturen oder biomimetische und gesteuerte chemische Prozesse in Hybrid-Nanomaterialien, die als Grundlage für hoch spezialisierte Protokolle in dreidimensionalen Gerüststrukturen dienen können. Diese Ansätze ermöglichen eine Feinabstimmung der Eigenschaften von Nanomaterialien und eröffnen neue Perspektiven für die Anwendung supramolekularer Konzepte.
The combination of nanomaterials as solid supports and supramolecular concepts has led to the development of hybrid materials with improved functionalities. These hetero-supramolecular ideas provide a means of bridging the gap between molecular chemistry, materials sciences, and nanotechnology. In recent years, relevant examples have been reported on functional aspects, such as enhanced recognition and sensing by using molecules on preorganized surfaces, the reversible building of nanometer-sized networks and 3D architectures, as well as biomimetic and gated chemistry in hybrid nanomaterials for the development of advanced functional protocols in three-dimensional frameworks. This approach allows the fine-tuning of the properties of nanomaterials and offers new perspectives for the application of supramolecular concepts.
Gated hybrid delivery systems: En route to sensory materials with inherent signal amplification
(2013)
Hybrid nanoparticles with a large specific surface area are a particularly exciting vehicle for delivery applications. Such highly porous or container-like structures, usually prepared from silica and frequently from gold, can incorporate a large number of chemical substances such as drug and/or indicator molecules. When equipped with a chemically or physically addressable gating function at the openings of the voids, the release of the cargo can be controlled at will. Because many more molecules can be stored as cargo in the pores of the support than there are functional groups as anchoring sites for the gating entities attached to the outer surface (for efficient pore capping), the systems possess inherent features of (signal) amplification. The present article will introduce various design strategies for different types of physical (light, temperature, magnetism) and chemical (pH, metal ions, anion, small organic molecules, enzymes) stimuli in connection with drug and indicator release. We will highlight exciting aspects of combining both features in theranostic applications and will stress which requirements still have to be met by many of the systems to be readily applicable in a sensory context.
Set them free: Brightly fluorescent indicators that are loaded into mesoporous silica nanoparticle carriers, capped with bulky antibodies, are released into the lateral flow of a test strip upon analyte arrival. Integration of the system into a rapid, simple flow test with fluorescence readout is applied for the selective and sensitive determination of the presence of triacetone triperoxide (TATP) as a prototype small-molecule analyte (see figure).
The combination of suitable nanoscopic inorganic supports with supramolecular concepts and bioconjugation strategies opens up exciting perspectives for the development of bioinorganic and organicinorganic hybrid materials with improved functionalities. Many of the examples from the period 2004 to mid-2007 reviewed here bridge the gap between molecules, materials sciences, biochemistry, and nanotechnology. The synergistic use of these approaches has led to the advancement of various functions such as enhanced recognition, amplified signaling, the controlled assembly and disassembly of aggregates and 3-D architectures. Many of the implemented active functions can be carried out in a reversible and directed fashion on purpose by triggering with an external stimulus. These ideas bring tunability to properties of inorganic solids and new perspectives of application to biochemical and supramolecular concepts. Classic inorganic chemistries such as metal or semiconductor nanoparticles, silica or carbon materials, and coordination chemistry are thus elevated to another level of sophistication and contribute significantly to frontier research areas in directed transport and delivery, nanoelectronics and mechanics, chip technology, smart materials, memory devices, and bioanalytics.
The article discusses representative examples published in the field of functional biomimetic chemistry of hybrid materials between 2004 and mid-2007. First, we focus on the modification of morphological aspects of (bio)organicinorganic hybrid materials to create sophisticated gating and switching mechanisms. The architecture of the systems is presented and the elements of control by external stimuli that have been established in recent years, such as optical, electrochemical, chemical, thermal, or magnetic means, are described. Various functions ranging from (site-specific) delivery to indication at the nanometric level are reported. Next we develop the idea of utilizing hybrid frameworks for biomimetic signaling inspired by the way nature has created binding pockets and the concept of induced-fit. Such a merging of inorganic scaffolds with supramolecular concepts results in enhanced molecular recognition and signaling. Finally selected recent advances in the field of hybrid biomimetic motors both of linear and rotary types are presented. The contribution shows how novel biomimetic functions can arise from synergisms between specifically designed hybrid materials and the implementation of supramolecular and/or bioorganic concepts.
Supramolecular interactions are vital processes in many chemical sensors, biochemical assays, and other analytical detection schemes. On the background of the current, rapidly developing, and constantly changing requirements for (bio)analytical techniques, the classical molecular host is increasingly overburdened to accomplish a specific analytical task with the desired selectivity and sensitivity. Instead, other signaling strategies have to be conceived that overcome the limits in signal output, dynamic range, or compound targeting, which are imminent to various traditional methods. A very promising approach is the combination of concepts of supramolecular chemistry with nanostructured or nanoscopic inorganic materials, that is, the development of hybrid sensor materials. Such hybrids often lead to improved functionality and enhanced performance, and the present chapter discusses the contributory features in relation to the major aspects, which are related to the enhanced coordination by the preorganization of binding sites, signal amplification by the preorganization on surfaces, aggregation‐mediated signaling, surface‐modification‐based signaling, the tuning of selectivity through polarity and size, and gated signaling.
Test strips that in combination with a portable fluorescence reader or digital camera can rapidly and selectively detect chemical warfare agents (CWAs) such as Tabun (GA), Sarin (GB), and Soman (GD) and their simulants in the gas phase have been developed. The strips contain spots of a hybrid indicator material consisting of a fluorescent BODIPY indicator covalently anchored into the channels of mesoporous SBA silica microparticles. The fluorescence quenching response allows the sensitive detection of CWAs in the μg m−3 range in a few seconds.
The design of comparatively simple and modularly configurable artificial systems able to communicate through the exchange of chemical messengers is, to the best of our knowledge, an unexplored field. As a proof-of-concept, we present here a family of nanoparticles that have been designed to communicate with one another in a hierarchical manner. The concept involves the use of capped mesoporous silica supports in which the messenger delivered by a first type of gated nanoparticle is used to open a second type of nanoparticle, which delivers another messenger that opens a third group of gated nanoobjects. We believe that the conceptual idea that nanodevices can be designed to communicate with one another may result in novel applications and will boost further advances towards cooperative systems with complex behavior as a result of the communication between simple abiotic individual components.
The article discusses representative examples published in the field of functional biomimetic chemistry of hybrid materials between 2004 and mid-2007. First, we focus on the modification of morphological aspects of (bio)organicinorganic hybrid materials to create sophisticated gating and switching mechanisms. The architecture of the systems is presented and the elements of control by external stimuli that have been established in recent years, such as optical, electrochemical, chemical, thermal, or magnetic means, are described. Various functions ranging from (site-specific) delivery to indication at the nanometric level are reported. Next we develop the idea of utilizing hybrid frameworks for biomimetic signaling inspired by the way nature has created binding pockets and the concept of induced-fit. Such a merging of inorganic scaffolds with supramolecular concepts results in enhanced molecular recognition and signaling. Finally selected recent advances in the field of hybrid biomimetic motors both of linear and rotary types are presented. The contribution shows how novel biomimetic functions can arise from synergisms between specifically designed hybrid materials and the implementation of supramolecular and/or bioorganic concepts.
The combination of suitable nanoscopic inorganic supports with supramolecular concepts and bioconjugation strategies opens up exciting perspectives for the development of bioinorganic and organicinorganic hybrid materials with improved functionalities. Many of the examples from the period 2004 to mid-2007 reviewed here bridge the gap between molecules, materials sciences, biochemistry, and nanotechnology. The synergistic use of these approaches has led to the advancement of various functions such as enhanced recognition, amplified signaling, the controlled assembly and disassembly of aggregates and 3-D architectures. Many of the implemented active functions can be carried out in a reversible and directed fashion on purpose by triggering with an external stimulus. These ideas bring tunability to properties of inorganic solids and new perspectives of application to biochemical and supramolecular concepts. Classic inorganic chemistries such as metal or semiconductor nanoparticles, silica or carbon materials, and coordination chemistry are thus elevated to another level of sophistication and contribute significantly to frontier research areas in directed transport and delivery, nanoelectronics and mechanics, chip technology, smart materials, memory devices, and bioanalytics.
This review highlights how the combination of supramolecular principles and nanoscopic solid structures enables the design of new hybrid sensing ensembles with improved sensitivity and/or selectivity and for the targeting of analytes for which selectivity is hard to achieve by conventional methods. Such ideas are bridging the gap between molecules, materials sciences and nanotechnology. Relevant examples will be detailed, taking into account functional aspects such as (1) enhanced coordination of functionalized solids, (2) enhanced signalling through preorganization, (3) signalling by assemblydisassembly of nanoscopic objects, (4) biomimetic probes utilizing discrimination by polarity and size and (5) distinct switching and gating protocols. These strategies are opening new prospects for sensor research and signalling paradigms at the frontier between nanotechnology, smart materials and supramolecular chemistry.
The inclusion of squaraine (SQ) dye I into the supercages of zeolite Y (structure code FAU) with a SiO2/Al2O3 ratio of 30 (Z30) through a two-step ship-in-the-bottle approach is reported, resulting in the squaraine-containing hybrid material Z30-SQ. Attempts to use the same synthetic procedure on sodium and ammonium Y zeolites were unsuccessful, stressing the importance of the presence of Brønsted acid centres in the Z30 zeolite. The reflectance spectra of Z30-SQ show a maximum in the typical squaraine monomer region at 618 nm and a shoulder at 570 nm. Spectral analyses revealed that the major species are SQ dimers and (tightly adsorbed) SQ monomers and SQ aggregates are only formed to a minor degree. The emission spectrum of Z30-SQ shows a single band at 669 nm. The sensory performance of this material was tested in the presence of volatile propylamine and propylthiol and other volatile organic compounds (acetone, ethanol, dichloromethane, hexane, acetonitrile, ethyl acetate, ethyl ether and acetic acid). The blue Z30-SQ material reacts with the thiol and amine derivatives, resulting in the bleaching of the solid and disappearance of the absorption band in the visible region, but does not undergo spectroscopic changes in the presence of other volatile organic compounds (VOCs). Whereas the reaction with propylamine is irreversible, a partial reversibility was found in the presence of the thiol. Theoretical investigations of the reactivity of I with several VOC molecules at the semi-empirical level reproduced well the experimentally observed reactivity.
Probe 1, which contains an anilinopyridine chromophore and an azaoxa macrocyclic subunit, presented an absorption band centered at 340 nm in acetonitrile. Addition of Fe(III), Cr(III) and Hg(II) induced the growth of a new absorption band at 430 nm (with color change from colorless to yellow), whereas in the presence of Cu(II), Zn(II) and Pb(II), less marked changes were observed. The color changes observed upon addition of Fe(III), Cr(III) and Hg(II) were ascribed to the formation of 1:1 stoichiometry complexes with probe 1. Coordination of Fe(III), Cr(III) and Hg(II) with the pyridine fragment of 1 induced an enhancement of the charge transfer character accompanied with a marked bathochromic shift that was reflected in a color change from colorless to yellow. The strength of the interaction between probe 1 and Fe(III) cation was modulated upon interaction with anions. Of all the anions tested, only cyanide was able to induce the bleaching of the yellow 1·Fe(III) complex solution. This bleaching was ascribed to
the formation of 1·Fe(III)-CN complex that restored, to some extent, the optical features of the free probe allowing the chromogenic sensing of cyanide. Besides, 1·Fe(III) complex was used to detect
cyanide in acetonitrile-water 90:10 v/v mixtures with good recoveries.
A novel mesoporous silica material containing boron–dipyrromethene (BODIPY) moieties (I) is employed for the detection of nerve agent simulants (NASs) and the organophosphate nerve or chemical warfare agents (CWAs) Sarin (GB), Soman (GD), and Tabun (GA) in aqueous environments. The reactive BODIPY dye with an optimum positioned hydroxyl group undergoes acylation reactions with phosph(on)ate substrates, yielding a bicyclic ring. Due to aggregation of the dyes in water, the sensitivity of the free dye in solution is very low. Only after immobilization of the BODIPY moieties into the silica substrates is aggregation inhibited and a sensitive determination of the NASs diethyl cyanophosphonate (DCNP), diethyl chlorophosphate (DCP) and diisopropyl fluorophosphate (DFP) possible. The signaling mode is a strong quenching of the fluorescence, reaching LODs in the pM range. The best performing hybrid material was singled out from a library of hybrid silicas varying in morphology and surface functionalization. The response to actual CWAs such as GB, GD, and GA has also been tested, offering similar behavior as for the simulants. The proposed reaction mechanism has been verified by investigation of other model materials, containing for instance BODIPY moieties without an optimum hydroxyl group (III) or a BODIPY dye with an all-aliphatic counterpart (IV). The latter can only form a monocyclic reaction product, showing much less reactivity as I. Assays with other possible competitors have been additionally carried out, showing favorably low cross-reactivities. Finally, the determination of NASs in several natural waters has been demonstrated.