Organic-Inorganic Supramolecular Gels and Contra - ACS Publications

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Chapter 11

Organic-Inorganic Supramolecular Gels and Contrast Agents for Magnetic Resonance Imaging Based on the Surfactant-Covered Polyanionic Clusters Bao Li and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Qianjin Avenue 2699, Changchun 130012, Jilin, People’s Republic of China *E-mail: [email protected].

The progress of a recently developed supramolecular strategy for organizing nano-sized fine particles into soft material systems was reviewed and discussed. By covering inorganic polyanionic clusters bearing uniform topologic architecture and well-defined chemical composition with cationic surfactants through electrostatic interactions, a type of ionic complex was prepared for the amphiphilic building blocks of self-assemblies in soft materials systems. Because of the core-shell composite structure, these complexes can be regarded as a supramolecular reversed micelle or supramolecular surfactant, depending on the phase separation capability, and thus be used for the fabrication of new hybrid organic-inorganic self-assembled structures and bio-applications. These inorganic clusters introduced in these complexes not only play a role as a structural unit, but also as a functional group for the functionalization of obtained soft materials. In the present chapter, we summarized synergistic interactions and correlations between two incompatible components in these complexes during the self-assembly for responsive supramolecular gels and magnetic resonance imaging contrast agents.

© 2015 American Chemical Society In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Introduction The close cross connection with material science, nano science, biology, medicine, and so forth has been one of the attractive aspects in colloids and interface chemistry in recent years, and thereby creates new exciting crossing borders among those areas. The combination of classic surfactants with fine particles and corresponding physicochemical behaviors are of the fundamental interests from among multidisciplinary fields. Polyoxometalates (POMs) belong to an interesting family of metal-oxygen polyanionic clusters bearing well-defined topologic architectures that are usually comprised of early transition metals such as vanadium, niobium, molybdenum, and tungsten (1–5). POMs can be prepared as an isolated dispersion, or endless linear and network crystalline solids, in which discrete single clusters are in uniform sizes from less than one to several nanometers. Depending on the composition and framework, POMs possess diverse behaviors in solid acidity, redox activities, catalysis, optics, and magnetism, and they have been applied for clinical studies due to their antibacterial and antiviral activities (6–9). Although there are inspiring merits for these self-assemblies due to their beautiful topological architectures, POMs still have not yet become common building blocks in solution systems in the past decades, toward soft materials. Recently some giant polyanions were found to form fantastic hollow spherical aggregations called “blackberries” (10), but the dynamic time scale of their ordered organization and their structural instability requires improvements to facilitate the POMs’ use in further convenient applications in colloids and interface chemistry. Main problems lying in this area are the cluster’s stiffness, the difficulty of chemical modification by organic groups, the lack of necessary components (not for crystals), and the multiple surface charge-induced repulsion between clusters in solution and at interfaces. Some effective methods have been developed to tune these POMs’ colloidal behaviors, including covalent modifications of organic groups (11, 12), surface charge controlling, as well as the selection of solvents (13). However, most of the known strategies were designed for specific clusters and a universal strategy suitable for water soluble POMs is highly desired. On the other hand, all POMs have multiple surface charges and can be used as the binding force bewteen organic components bearing countercharges via electrostatic interactions. Some early efforts in transferring POMs into organic phases for catalysis demonstrated the feasibility of this route by simply replacing the surface counterions with cationic surfactants. Further progress to spread cationic surfactant-covered giant POMs on the water surface provided typical examples of POMs supported by organic components which can be compared with results obtained from in-situ two phase mixing methods (14, 15). Through detailed evaluations for the composition of organic cation/POM complexes, these hybrid building blocks were extended from air/water interface to solution systems, and the regularity in getting diverse self-assembly behaviors targeting the functional applications of POMs in soft materials was discovered. Herein, we would like to summarize representative achievements in the research of gelation behavior and magnetic resonance imaging capability of this kind of POM complexes prepared following the strategy. 200 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Experimental Section


Preparation of Surfactant-Covered POM Complexes The preparation of electrostatically binding complexes undergoes simple but complete procedures depending on the hydrophilicity of organic components. For the two phase mixing route, the detailed preparation includes following process: mixing the aqueous solution of polyanion cluster and the weakly polar organic solution of surfactant together, stirring the incompatible mixture for full charge interaction, then transferring the POM component into the organic phase, and finally separating the product from the organic phase and carrying out a purification (16). Elemental analysis, TGA, NMR and IR spectroscopy, and Mass spectrometry are necessary tools for the determination of the chemical composition. Preparation of Supramolecular Gels The preparation of supramolecular gels is similar to the general route for the preparation of organic gels. In most cases, solids of complexes were dissolved in organic solvents. For the mixture solvents, the complexes were usually dissolved first in an easily soluble solvent to which the other one was added. Gels often form quickly, but in some cases, solutions need to be heated and then cooled to room temperature slowly. Relaxivity Measurement Sample solutions at the investigated concentration were prepared through sonication of the complex in water for a short period of time. Then the solution was left to stand for 24 h at room temperature before the following characterization. For the preparation of sample Mn-12-C18 mixing with C18EO10, a stable emulsion was obtained by sonicating the mixture of Mn-12-C18 in n-hexane and C18EO10 in water in a suitable ratio. The organic solvent was evaporated during vigorous stirring, yielding the C18EO10 encapsulated Mn-12-C18 aggregates. The prepared Mn-12-C18/C18EO10 aggregates were well-dispersed in aqueous solution and underwent a centrifugation to remove insoluble residues. The relaxivity measurements were carried out at 25 °C on a Bruker Ultrashield 500 MHz spectrometer. For each sample, longitudinal 1H relaxation time (T1) was measured using the inversion recovery method. Slopes of plots of longitudinal relaxation rate r1 (1/T1) versus the concentration of Gd-POM complexes were used to calculate r1 values.

Results and Discussion Fundamental Properties of Surfactant-Covered POM Complexes After the charge exchange and coupling, the formed POM complexes no longer dissolve in water, but become soluble in weakly polar organic media instead because the cationic hydrophilic head of surfactant molecules have anchored on the POM surface while the hydrophobic tail faces toward the outside 201 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

environment. After this process, initial counterions of both POMs and surfactants have been removed upon the separation of the aqueous phase. Therefore, in these complexes, the cationic surfactant and the polyanionic cluster become counterions of each other. The negative charges are delocalized on the surface of the POM, facilitating the formation of ion-pairs where surfactant cations are available.


Supramolecular Gels of POM-Cored Complexes In comparison to polymeric gels, in which the three dimensional network comprised of the fibrous structures of polymer gelators stabilizes the solvent molecules in the aqueous solution, supramolecular gels are derived from the bicontinuous self-assembled structure of small molecules due to intermolecular interactions. Therefore, in principle, most low molecular weight molecular components or composites in solutions that stabilize solvent molecules by generating bicontinuous self-assemblies may be used as the gelator. Because of rich self-assembled properties of surfactant-covered POMs, it is also possible to utilize these amphiphilic complexes to prepare supramolecular gels. We evaluated the basic synergistic effect of organic and inorganic components in hybrid complexes as gelator candidates. Both parts have been demonstrated to be crucial for the formation of the self-assembly structure. The length of the alkyl chain and the number of organic cations covering POMs (17, 18), the terminal modification (19, 20), the shape and the surface charge density of POMs (21) were found to be important in the modulation of interactions between alkyl chains and even between complexes in the assemblies. These factors are decisive in subsequently fabricating various aggregation architectures under selected conditions (22). To understand the effects of alkyl chain density and the shape of POMs on the gelation property, we first employed the cationic surfactant, dioctadecyldimethylammonium bromide (DODA·Br) to wrap up POMs bearing different charges. To make a convenient comparison among the complex gels, we investigated three types of complexes according to the structure of POMs and amount of the surface charge (Figure 1). Complex-1 (a, b), and -2 (a, b, c) correspond to complexes bearing Keggin-type clusters ([XW12O40]n−, X = P, Si and n = 3, 4; a: PW12O403− and b: SiW12O404−) and lacunary Keggin-type POMs ([XW11O39]n−, X = P, Si, B, and n = 7, 8, 9; a: PW11O397−, b: SiW11O398−; c: BW11O399−), respectively, while complex-3 (a, b, c) afford cashew-shaped POMs comprising two lacunary Keggin-type POMs linked by an europium ion ([Eu(XW11O39)2]n−, X = P, Si, B, and n = 11, 13, 15; a: Eu(PW11O39)211−; b: Eu(SiW11O39)213−; c: Eu(XW11O39)215−). The gelation behavior of these complexes was examined in common organic solvents at the same concentration (3 wt%). The collected results revealed that weakly polar solvents were in favor of the fabrication of supramolecular gels. Complexes of less charged POMs were hard to dissolve in weakly polar solvents even upon heating, due to their deficiency in binding cationic surfacants. On the contrary, complexes comprising POMs with appropriate charges dissolved better in weakly polar solvents such as n-hexane, cyclohexane, isooctane, and nonpolar carbon tetrachloride due to the increased amount of organic components. The formation of gels could be 202 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


then observed after slow cooling of the heated solutions to room temperature. However, in benzene the gelation of Complex-2 series was not observed. For complexes of POMs with relatively large numbers of charges, the situation became complicated. On one side, the increased organic groups still could not provide enough hydrophobicity for complexes to dissolve in n-hexane, while Complex-3 (b, c) yielded typical gels in cyclohexane and isooctane, in which it is insoluble. In carbon tetrachloride and benzene, Complex-3 (b, c) existed as a solution and a jelly respectively, while the existing state of Complex-3a was just the opposite in those two solvents. In comparison to non-polar solvents, all these complexes dissolved well in weakly polar solvents such as toluene, dichloromethane, chloroform and strong polar solvents such as ethanol and methanol, but no obvious gel formation has been observed.

Figure 1. The chemical composition of cationic surfactant with different chain lengths, polyanionic clusters bearing different charges, and the formed electrostatic complexes.

To evaluate the role of alkyl chains on the fabrication of hybrid supramolecular gels, beside DODA, we covered the inorganic core, (K9BW11O39)9−, with three other surfactants bearing shorter alkyl chains: dihexadecyldimethylammonium bromide (DHDA· Br), ditetradecyldimethylammonium bromide (DTDA·Br), and didodecyldimethylammonium bromide (DDDA·Br). Among these three prepared complexes, DTDA- and DDDA-hybridized complexes could not form supramolecular gels in general solvents due to the lowered self-assembly capability. Similar to the DODA covered POM complex, the gelation of DHDA-grafted complex took place in n-hexane, cyclohexane, and isooctane due to the relatively higher hydrophobicity and interaction between alkyl chains. However, the gels became no longer stable over time, and the gel state could only be kept at low temperature. The worsened gel behaviors can be well attributed to the weakened van der Waals interaction between shortened alkyl chains (16). 203 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The observed gels are not transparent, especially when the concentration of gelator complexes was high. Polarizing optical microscopy demonstrated that the phenomenon was closely related to the aggregation’s morphologies and sizes in the gel phase. SEM images of xerogels of Complex-2 series in n-hexane and cyclohexane indicated the entangled and tape-like network structure (23). On the contrary, xerogels of Complex-3 series in cyclohexane adopted network structure, which could be attributed to the partial fusion of spherical aggregations into larger ones. Apparently, the enhancement in the size scale of stripe and sphere aggregations increased the turbidity of gels. Those long, thin stripe structures occupied more overlapping areas and cross-linked together stably comparing to dispersed spherical aggregation structures of stable supramolecular gels. XRD diffractions of those xerogels suggested lamellar structures of complexes. As confirmed in previous publications (24), the charge neutralization between organic and inorganic components lead to the attachment of the cationic head of surfactants to the surface of polyanionic clusters at a very close distance instead of being fixed at a certain site in a potential field, due to the delocalization of surface charges on POMs. Because of the charge delocalization and the larger surface area on POM surface, it was proposed that the surfactant cations would move and rearrange in the surface electrostatic field around POMs. Driven by the interfacial energy in the solvent environment, the phase separation of complexes propelled surfactant cations to accumulate into a nonspherically symmetric state. Thus, considering the smaller lateral size of the cationic head, the polarity of the polyanionic core and its tight electrostatic interaction with surfactants, the self-assembly of these complexes into the reverse bilayer structure in organic media became a favorable state for tight packing density and low interfacial energy. Independent investigations have confirmed that the reversed bilayer structure in which the inorganic component located in the middle could induce different morphologies accompanied by the change of solvent polarity (25, 26).

Responsive Gels of Organically Grafted POM Complexes with Additives In comparing to the lateral combination between complexes derived from the van der Waals interaction, by introducing additional agents to control the lateral force between complexes, we realized the response of the supramolecular gels to the external condition. Hasenkernopf and his coworkers reported the initial example of organically modified POM complexes that could form supramolecular gels through coordination with metal ions (27). Via the organic modification of pyridyl groups on both sides of an Anderson-type disc-like cluster, [MnMo6O24H6]3−, we built hybrid supramolecular gels comprising three components (28). By mixing with dicarboxylic acids in acetonitrile, the main gelator component of pyridyl-substituted POM cluster (Py–MnMo6) bearing tetrabutylammonium counterions formed gels immediately. The hydrogen bonding between pyridyl groups and the carboxylic acid additives leads to the generation of supramolecular polymer chains, and then the primary fibrils self-assemble into fibrous bundles, which further entangled with each other to yield cross-linking network structures. The distance between adjacent POMs 204 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


was confirmed to be an important factor for the formation of gels, which was dominated by the interfacial energy of the polymer chains. Only those additives with a proper molecular length supported the construction of gels. Though quantitative estimation on the interfacial energy seemed difficult in the present stage, these influence factors referring to the synergy among components provided useful traces for the design of other POM hybrid gels. Due to the hydrogen bonding feature of the polymer chain, the hybrid gels displayed a quick response to the addition of organic bases due to the competitive disruption of the hydrogen bond. In principle, any actions capable of breaking supramolecular polymer networks could induce the response of the gels (Figure 2).

Figure 2. Schematic drawing of the gelation of TBA-Py-MnMo6 grating pyridyl groups under the existence of dicarboxylic acid through hydrogen bonding and the response to the organic base 4-dimethylaminopyridine (DMAP). 205 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Following the approach of lateral modification, the suitably modified POMs could afford more gelators for hybrid gels. A pair of Anderson clusters [MnMo6O24H6]3− that have been organically grafted with adenine and thymine base groups on both sides, respectively, were synthesized (Figure 3). When the base pair-modified clusters were mixed together, they linked to each other linearly through complementary hydrogen bonds, yielding an alternative POM supramolecular polymer, which enhanced processing properties such as mold casting and electrospinning (29). Though the counterions of the POM had been changed to tetrakis(decyl)ammonium (TDA), the formed aggregation could not afford gelation in solution. As mentioned above, the modification and the substitution of cation changed the amphiphilicity. However, the polyanionic cluster in the main polymer chains also provided electrostatical binding sites for crosslinking, because there is no saturation for electrostatic interaction. After the addition of a bola-form cationic surfactant into the hybrid supramolecular polymer in chloroform solution, a quick gelation was found. The combination of POM cluster in the main chain with cationic additives provided an additional adjustment for the lateral intermolecular interaction when controlling aggregation structures. One could envision that this kind of organic-inorganic hybrid gels can provide another effective method to accommodate inorganic clusters in soft materials systems.

Figure 3. The schematic illustration of base-pair modified Anderson type POMs, the formation of complementary hydrogen bonds and the supramolecular chain cross-linking via electrostatic interaction with a bola-form cationic cross-linker. 206 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Self-Assembly and Alterable Relaxivity of Gadolinium-Incorporated POM Complexes While the encapsulation of cationic surfactants to POMs through electrostatic interaction changed the surface properties, basic physical and chemical characteristics of these clusters in their free state were maintained (30, 31). Thus, the structural stability and the biocompatibility of POMs could be greatly improved. More importantly, functional properties of POM clusters could be optimized in hybrid organizations due to the synergy of components in aggregation structures. Appealing biomedical applications of some POMs as antiviral, antitumor, and antibiotic agents have been carefully studied (32). Considering these features, it is of significance to combine them with probes or labelings of biomolecules, because phosphotungstic acid and its derivatives are often used as dyes of biomolecules for TEM measurements. The paramagnetic property of Gd(III)-Sandwiched POM complexes as bio-imaging enhancement materials such as contrast agents (CAs) for magnetic resonance imaging (MRI) has received interest recently (33). Compared to normal organic ligands, lacunary POMs as inorganic multidentate ligands serve for the preparation of stable paramagnetic polyanions. The POM’s stiff architecture and high molecular weight could also provide longer rotational correlation time leading to reinforced longitudinal relaxivity (r1) (34). However, naked polyanions often bind strongly to the positively charged biological molecules such as proteins, and may yield unexpected cluster disintegration in the physiological environment (35), thereby inducing toxicity. Therefore, it is important to wrap up Gd-substituted POMs with neutral organic components with the covering layer PROVIDING both protection of the cluster and binding sites for specific recognition to target groups when organic modification is necessary. To meet these purposes, an amphiphilic molecule (EO12BphC10NC12) possessing a poly(ethylene oxide) (PEO) terminal on one end of a double-chain quaternary ammonium connected to a biphenyl group was designed to enwrap the paramagnetic POM, K13[Gd(β-SiW11O39)2]·27H2O (Gd-POM) (Figure 4). The PEO chain locating at periphery of hydrophobic shell provided hydrophilicity, while the hydrophobic part in the middle sustained the electrostatic interaction of the amphiphilic molecule with POM. Thus, the formed Gd-POM complex was not only soluble in aqueous solution, but also spontaneously generated a phase-separation and aggregated into larger assemblies (36). The magnetic moment of the Gd-POM remained after the electrostatic encapsulation, and those assemblies introduced an additional modulating approach for T1 value of water protons as the complex existed in different states with the change of concentrations. For example, at high concentration, relaxivity was lower than that of the free POM, while at lower concentration the relaxivity of the complex was enhanced by several folds to the naked POM. Apparently, aggregation is unfavorable for the MRI quality due to the congestion of water molecule exchange, which was not hindered by the covering layer of the isolated complex. Importantly, the increased molecular weight corresponding to the naked POMs promoted the relaxation time, which increased the relaxivity r1. 207 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 4. Representative drawing for the preparation and aggregation of Gd-POM complex for magnetic resonance imaging contrast agent. Reproduced with permission from reference (36). Copyright 2015 Royal Society of Chemistry.

Manganese Cluster Single-Molecular Magnets as MRI CAs Mn12O12(OOCCH3)16 (Mn-12) clusters are known as a type of single molecular magnets (SMMs) due to their large ground spin (S) state and magnetic anisotropy derived from the parallel array of partial Mn (III) ions in the Jahn–Teller axis (37). The Mn-12 SMM possesses a precise chemical composition and electronic configuration, and it is susceptible for chemical modifications on both the framework and periphery. Based on the particle size and the paramagnetic property, Mn-12 clusters were hopefully to be developed into a CA system in connection to paramagnetic and superparamagnetic nanomaterials. However, the instability of Mn-12 SMMs in aqueous solution became one of the major drawbacks of applications. In a highly concentrated acetic acid solution, Mn-12 retained its structure and performed as a negative CA. By anchoring onto the carboxylic acid surface of polystyrene spheres, Mn-12 was found to have an improved stability to some extent, and the effect of negative contrast was observed (38). However, more convenient methods to stabilize clusters and a much closer environment to the physiological condition are still required when attempting to apply the clusters as CAs. Under the illumination of the paramagnetic POM complex, we proposed a quite mild approach for transferring and stabilizing stearic acid (C18)-coordinated Mn-12 (Mn-12-C18) cluster in an aqueous solution for CAs through an emulsion-supported method without obvious structure decompositions (39). A nonionic surfactant, C18H37(OCH2CH2)10OH (C18EO10) was used as the emulsifier. By mixing the Mn-12-C18 cluster in n-hexane and the nonionic surfactant in water, we prepared an emulsion. Controlling the evaporation speed of organic solvent gave the surfactant-encapsulated cluster complex in the microenulsion aggregates in which the Mn-12 clusters were located in the center (Figure 5). Due to the protection of nonionic surfactant that covered the surface via the van der Waals interaction between alkyl chains, the Mn-12 cluster could be dispersed in water, and its magnetization hysteresis loop at 2 K exhibited characteristic SMM performance. Within the multicomponent system, the alkyl chains of carboxylic acid dispersing on the surface of Mn-12-C18 accommodated hydrophobic carbon chains of C18EO10, which greatly improved the structural stability of the inorganic cluster aggregates. In the meantime, the PEO chain 208 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


locating outside of the cluster aggregates promoted the water solubility and especially the biocompatibility. It should be noted that, while improving the application of Mn-12 derivatives as negative CAs, the intermediate alkyl chain layer between Mn-12-C18 and C18EO10 blocked the water proton relaxation by the “inner-sphere” mechanism (40). Apparently, a further modification on terminal groups of the carboxylic acid ligand is promising to simplify the system so that the nonionic surfactant becomes no longer necessary.

Figure 5. Formation process of Mn-12-C18/C18EO10 aggregates through emulsion-assisted self-assembly at room temperature in aqueous solution. Reproduced with permission from reference (39). Copyright 2015 Royal Society of Chemistry.

Concluding Remarks and Perspectives The electrostatic combination of cationic surfactant with inorganic polyanionic cluster yields new type of amphiphilic complexes. Being different from general surfactants, the interfacial energy-propelled polarity separation keeps the hydrophilic part and hydrophobic part of complex away from each other and self-assembles into diverse aggregation structures, relying on the occupation of the organic component on the surface of each inorganic cluster. The selection of suitable solvents not only modulated the aggregation morphology and structure, but also triggered the gelation of the complexes in solution. The introduction of additives and the interaction control between components yielded the sensitive response of supramolecular gels. On the other side, electrostatic covering could greatly improve the surface property of inorganic clusters and thereby directed the enhancement of the stability and biocompatibility, toward the application of POMs as the CAs. The present method can be apparently applied in the functionalization of other charged nanoparticle systems. 209 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, Chengshan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


As a general approach, the collection of surfactant molecules by POM cluster provides a possibility to build single complex reverse micelles. The electrostatic interaction increased the stability of the small complex aggregation, so that we can use it to fabricate more complicated hybrid assemblies. The present method could be also used for other similar systems and the realizations of functions that regular surfactants and micelles do not possess, for example, the simple phase transfer of graphene from an aqueous solution to the organic phase, the large accommodation of guest insolubles and so forth.

Acknowledgments The authors acknowledge financial support from the National Basic Research Program (2013CB834503) and the National Natural Science Foundation of China (NSFC) (51203059, 91227110, 21221063) and MOE (20120061110047).

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Organic-Inorganic Supramolecular Gels and Contra - ACS Publications

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