Benjamin Sierra-Martin

A water soluble diruthenium-gold organometallic microgel.

Selection of the combination of metal, ligand set, and spacer groups that are most appropriate to form a coordination complex with a desired function are of paramount importance in supramolecular chemistry. In particular, the establishment of reproducible methods to accomplish controlled selforganization of molecules to form polymers and homoor heterometallic coordination aggregates is an important field of research. Although important advances have already been made, few metallopolymers, which are one of the most exciting classes of functional materials, are water soluble. An important example of a water-soluble polymer is the polyferrocenylsilane-b-polyaminomethacrylate copolymer described by Manners and co-workers, as part of their ongoing study on metallocene-based polymers. Recent examples also include the water-soluble metallopolymer obtained by reaction of bipyridyl-appended poly(p-phenyleneethynylene) (PPEs) with metal ions in organic and aqueous solution. Other examples of ligands that afford coordination polymers with various topologies and applications are ferrocenyl groups bearing bipyridine (bpy) or carboxylate moieties. A recent example of a non-water-soluble multimetallic polymer is [Sm(H2O)5][Ru2(bpy)2(CN)7], in which the CN ligands bridge the samarium and ruthenium metal centers. The first air-stable water-soluble multimetallic polymer that includes mixed P,N ligands as metal-coordinating spacers has been recently reported by us. This heterobimetallic complex is based on two metal-containing moieties, [CpRu] (Cp= cyclopentadienyl) and [AgCl2] , and is bridged by the cagelike water-soluble monodentate phosphine 1,3,5-triaza-7phosphaadamantane (pta) in an unprecedented P,N coordination mode. More recently, the synthesis of silver coordination polymers containing pta bridging molecules in a tridentate P,N,N’ coordination mode has been reported and several examples of pta N coordination have been presented. Therefore the pta molecule could be an excellent ligand from which to obtain water-soluble Ru–Au polymers which could have interesting and useful properties for a variety of applications such as magnetism, nonlinear optics, electrocatalysis, photocatalysis, photovoltaic, template formation of ordered networks, advanced electrode materials, and conjugated coordination polymers. Herein, we describe the first water-soluble, air-stable heterobimetallic polymeric structure based on two metalcontaining moieties [CpRuCNRuCp] and [Au(CN)4] , bridged by pta in the P,N coordination mode. Interestingly, this complex display gel-like properties in water, specifically a thermally controlled volume transition. To the best of our knowledge, this is the first example of an coordination polymer network that is sensitive to its environment. The physical and chemical properties of this complex make it a promising material for industrial and biological applications, for example, smart catalysis, drug delivery, or chemical sensing. The first strategy we attempted to obtain a water soluble Ru–Au polymer was similar to that used for the synthesis of [{CpRu(pta)2(DMSO-kS)}{AgCl2}]1. [8] The complex [CpRuCl(pta)2] (1) was reacted in a straightforward manner with AuCl3 in water, EtOH, and DMSO, and in the presence of NaBF4 and NaCF3SO3. This resulted in the swift formation of a stoichiometric amount of metallic gold. A second strategy, in which the [Au(CN)4] moiety (which is very stable under a wide variety of conditions) was used, was then tested. The reaction of 1 with K[Au(CN)4] led to the partial reduction of gold together with the formation of several new species. A careful analysis of the reaction products suggested that the presence of the chloride ion in 1 could give rise to the formation of unstable gold species which finally decompose to metallic gold. To avoid the presence of the chloride ion in the reaction, the complex [RuCp(CN)(pta)2] (2) was prepared by substitution of 1 with KCN at room temperature in water (Scheme 1). Slow crystallization of a diluted H2O solution of [Ru(CNkC)Cp(pta)2] (2) in air give colorless crystals suitable for Xray diffraction. The X-ray crystal structure of complex 2 shows that it is a mononuclear complex in the solid state and is similar to 1, except that the chloride ligand has been

Coupled Deswelling of Multiresponse Microgels

In this article, we study the response of a thermosensitive and ionic microgel to various external stimuli where coupling between different contributions to the total osmotic pressure is needed to describe the observations. We introduce a new Flory solvency parameter chi ( T, Q, n) with strong dependence on the network charge, Q, and salt concentration, n. The scaling exponent for the salt-induced deswelling of the microgel is the signature of the coupling between the mixing and ionic osmotic pressures.

Structure and polymer dynamics within PNIPAM-based microgel particles

The synthesis of temperature-responsive microgels of poly(N-isopropylacrylamide) (PNIPAM) was first reported in 1986 and, since then, there have been hundreds of publications describing the preparation, characterization and applications of these systems. This paper reviews the developments concerning the study of the structure of PNIPAM-based microgels performed over the last years using small angle neutron scattering (SANS) and also the investigations of the polymer-chain dynamics within the microgels carried out with incoherent elastic and quasielastic neutron scattering, and pulse field gradient nuclear magnetic resonance (PFG-NMR) techniques. Furthermore, the self-diffusion coefficient of the water molecules within the microgel, determined by means of solvent relaxation NMR, is also discussed as a function of the polymer volume fraction of the microgels.

Structural study of poly (N-isopropylacrylamide) microgels interpenetrated with polypyrrole

Thermosensitive cross-linked poly(N-isopropylacrylamide)[poly(NIPAM)] microgels in D2O present a continuous volume phase transition from swollen to deswollen states at a temperature within 32–34°C and the process is thermoreversible without hysteresis. The poly(NIPAM) microgels with 0.25% (w/w) cross-linker content were used as a matrix to entrap polypyrrole. Pyrrole was dissolved in the colloidal microgel dispersion and polymerisation was performed at three selected temperatures: 20°C in the swollen state of the microgel, at 37°C having the microgel collapsed and at 32.5°C corresponding to the beginning of the volume phase transition of the microgel. Dynamic light scattering, differential scanning calorimetry, small angle neutron scattering and transmission electron microscopy have been used to study the structural modifications induced by polypyrrole in these microgels. The small angle neutron scattering pattern was analysed using an expression that consists of solution-like concentration fluctuations given by an Orstein–Zernike type function, a solid-like concentration fluctuations described by a Gauss function and a contribution arising from the surface of the colloidal microgel. It seems that polymerisation of pyrrole, when the microgel particles are in the swollen state (20°C), results in a uniform distribution of polypyrrole within the particles. However, when the polymerisation is carried out at the beginning of the volume phase transition (32.5°C) an outer shell of polypyrrole could be formed. Finally, polymerisation of pyrrole at 37°C produced non-stable colloidal particles.

Structural Properties of Thermoresponsive Poly(N-isopropylacrylamide)-poly(ethyleneglycol) Microgels

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.

Phase and non-equilibrium behaviour of microgel suspensions as a function of particle stiffness

We investigate the phase and non-equilibrium behavior of suspensions comprised of swollen, ionic microgels as a function of particle stiffness. We use visual inspection of the samples, rheology and UV-visible spectroscopy to determine how the system phase changes with particle concentration for particles with different crosslinker content. Our results indicate that stiff particles exhibit all three phases observed in hard sphere suspensions: liquid, crystal and glass; however, the boundaries separating one phase from the other are different from the corresponding boundaries in hard sphere suspensions. In particular, the width of the liquid–crystal phase-coexistence region increases with decreasing particle stiffness. For particles with intermediate stiffness, the crystal phase disappears and the microgel suspension transitions from a liquid to a glassy state at certain particle concentration. For even softer particles, no glassy state is observed. Instead the system remains liquid within the experimentally accessed concentration range. These results emphasize the richness in behaviour that is brought about when deformable and compressible objects are considered instead of the more usual hard colloidal particles.

The effect of hydrostatic pressure over the swelling of microgel particles

We review our recent results on the use of hydrostatic pressure to change the size and the structure of microgel particles based on poly-(N-isopropylacrylamide). These changes are brought about through changes in the miscibility of the polymer in the solvent. Swelling induced by hydrostatic pressure can thus be thought of as an alternative to swelling induced by temperature, an interesting fact that can be exploited in the study of fundamental problems in soft condensed matter.

Inorganic/polymer hybrid nanoparticles for sensing applications

This paper reviews a wide set of sensing applications based on the special properties associated with inorganic/polymer composite nanoparticles. We first describe optical sensing applications performed with hybrid nanoparticles and hybrid microgels with special emphasis on photoluminescence detection and imaging. Analyte detection with molecularly imprinted polymers and HPLC-based sensing using hybrid nanoparticles as stationary phase is also summarized. The final part is devoted to the study of ultra-sensitive molecule detection by surface-enhanced Raman spectroscopy using core-shell hybrid materials composed of noble metal nanoparticles and cross-linked polymers.

Surface-Enhanced Raman Scattering Sensors based on Hybrid Nanoparticles

Surface-enhanced Raman scattering (SERS) is a powerful vibrational spectroscopic technique that allows ultra-sensitive chemical or biochemical analysis (Kneipp, Kneipp et al. 1999). It works by increasing the Raman signal of analyte molecules located nearby the surface of metallic nanostructures that can undergo localized surface plasmon resonance. Among these nanostructures, gold and silver nanoparticles are the dominant substrates, for both experimental and theoretical perspectives (Kneipp, Wang et al. 1997; Nie and Emery 1997), since they can support plasmon resonance properties able to increase the Raman signal up to 14 or 15 orders of magnitude, high enough to detect single molecules (Nie and Emery 1997; Qian and Nie 2008). Since the first report concerning the enhanced Raman signal of pyridine molecules adsorbed on a roughened silver electrode (Fleischm, Hendra et al. 1974), considerable efforts have been made in understanding the SERS mechanisms (Schatz 1984; Campion and Kambhampati 1998). Nowadays, analytical applications have centred the attention, and research is devoted to optimize the specific conditions for detecting each particular analyte (Porter, Lipert et al. 2008). Interestingly, the enhancement factor is found to depend on the different affinity of the functional groups in the analyte toward gold or silver surfaces because it is the affinity which determines the analyte retention (Pearson 1963; Pearson 1966). To improve the surface-analyte interaction, various approaches have been developed, including the functionalization of nanoparticle surface (Guerrini, Garcia-Ramos et al. 2006; Guerrini, Garcia-Ramos et al. 2008); however, a problem inherent to this alternative is that usually the assembled molecules provide strong SERS signals that overlap and screen those corresponding to the analyte. Another alternative relies on controlling the surface charge of the nanoparticles to promote the electrostatic attraction of the analyte onto the particle surface (Alvarez-Puebla, Arceo et al. 2005; Aroca, Alvarez-Puebla et al. 2005). This approach has been reported to consistently enhances the signal for acids and amines, but it hardly helps in the case of alcohols, ethers, and other oxygen containing groups, as well as for non-functionalized molecules. Thereby, there is a clear need for development of new nanocomposites, based on noble-metals, containing a sensitive material that enables the physical trapping of a wide variety of analyte molecules. Herein we present the synthesis and applications of novel core-shell nanocomposites comprising Au and Au-Ag bimetallic cores, with spherical or rod-shaped morphology,

Ion-Specific and Reversible Wetting of Imidazolium-Based Minigels

Cross-linked imidazolium-based [poly(ViEtIm +Br -)] microparticles were synthesized, and their wetting properties were studied by optical microscopy, after addition of aqueous solutions of sodium halides. Particle wetting showed ion specificity due to counterion binding, described by Desnoyers model. The interaction between anions and the microparticles allowed exchanging halogenides between them in a reversible way. A salt-independent characteristic wetting time was found as well as a decreasing power law with salt concentration, for the network diffusion coefficient. It modified the polymer network elasticity as ion concentration increased, making the network softer.