A. Fernández-Barbero

Multifunctional Microgel Magnetic/Optical Traps for SERS Ultradetection

We report on the fabrication of a SERS substrate comprising magnetic and silver particles encapsulated within a poly(N-isopropylacrylamide) (pNIPAM) thermoresponsive microgel. This colloidal substrate has the ability to adsorb analytes from solution while it is expanded (low temperature) and reversibly generate hot spots upon collapse (high temperature or drying). Additionally, the magnetic functionality permits concentration of the composite particles into small spatial regions, which can be exploited to decrease the amount of material per analysis while improving its SERS detection limit. Proof of concept for the sequestration of uncommon molecular systems is demonstrated through the first SERS analysis of pentachlorophenol (PCP), a chlorinated ubiquitous environmental pollutant.

Growing Au/Ag Nanoparticles within Microgel Colloids for Improved Surface-Enhanced Raman Scattering Detection

The synthesis, characterization, and assembly of different types of nanoparticles, which was established as a necessary prerequisite for the application of nanotechnology, have dramatically advanced over the last 20 years. However, it has recently been realized that the incorporation of multiple functionalities within nanoscale systems would become much more useful for most of the foreseen applications. Thus, the fabrication of multifunctional nanoparticles has become a major challenge. Among these systems, the incorporation of active (optically, catalytically, magnetically...) nanoparticles within so-called “smart” thermosensitive microgels has received significant attention over the last few years. The incorporation of nanoparticles can be accomplished either by in situ formation, by post-synthesis assembly or by direct polymerization on the nanoparticles surface. We have recently reported the growth of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels on the surface of gold nanoparticles, involving several steps, including the formation of a first polystyrene thin layer, followed by pNIPAM polymerization after the required purification process. Although gold nanoparticle growth could be achieved within the microgel shell, this synthesis was restricted to spherical nanoparticle seeds, whereas, for example, nanorods (which display a much more interesting optical response) were not properly incorporated. Thus, there was a need to both simplify the coating process and make it more widely applicable. We report here a novel procedure where butenoic acid is used for the synthesis of gold nanoparticles in aqueous surfactant solutions, in the presence of preformed Au seeds. Apart from the interesting observation that butenoic acid can be used as a reducing agent, this is particularly interesting because it provides the particles with a vinyl functionality, which should be useful for the direct pNIPAM polymerization on the nanoparticles surface and their subsequent encapsulation, while avoiding complicated surface functionalization steps. Although we have only optimized the reduction process for nanosphere growth, we also demonstrate that butenoic acid can replace cetyltrimethylammonium bromide (CTAB) molecules from Au nanoparticle surfaces, including Au nanorods, and adsorb on the metal surface, thereby facilitating the polymerization of pNIPAM on the metal core. The improved stability of the nanocomposites and the porosity of the pNIPAM shell allows subsequent reduction of metal atoms on the metal core, which was exploited for the overgrowth of pNIPAM encapsulated Au spheres and rods with both Au and Ag under mild conditions. In a previous publication we demonstrated the ability of these composite colloids to mechanically trap non-common surface-enhanced Raman scattering (SERS) analytes. However, the use of 60 nm gold spheres as colloidal cores and the impossibility of forming hot spots due to the physical barrier imposed by the pNIPAM shell severely limited the enhancing capability and thus the detection limit. In the present work, the SERS intensity was significantly increased through manipulation of the composition and the morphology of the nanoparticle cores. Thus, the first modification involved the controlled growth of uniform silver shells, a much more efficient plasmonic material, on the gold cores. Second, we exploited the near field concentration at the ends of nanorods to further increase the signal. Finally, we demonstrate that the molecular affinity of the pNIPAM shells toward analytes can be extended by tuning the surface charge, which would allow the electrostatic attraction of charged molecules. [a] R. Contreras-C ceres, Dr. I. Pastoriza-Santos, Dr. R. A. Alvarez-Puebla, Dr. J. P rez-Juste, Prof. L. M. Liz-Marz n Departamento de Qu mica F sica, and Unidad Asociada CSIC Universidade de Vigo, 36310, Vigo (Spain) Fax: (+34) 986812556 E-mail : juste@uvigo.es lmarzan@uvigo.es [b] R. Contreras-C ceres, Prof. A. Fern ndez-Barbero Departamento de F sica Aplicada Universidad de Almer a Almer a (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001261.

Salt effects over the swelling of ionized mesoscopic gels

In this work, the effects of salt concentration over the swelling of ionic mesoscopic gels will be studied theoretically and verified with experiments. We will restrict ourselves to the weak screening limit where the Debye screening length is larger than the mesh size of the gel. Under this condition, direct electrostatic interactions are negligible and the swelling is driven by the osmotic pressure of the ions. The swelling response of a mesoscopic gel is strongly dependent on the ionization degree of the gel. In particular, a maximum in the size–salt concentration curve appears for a partially ionized gel, when the salt concentration equals the network charge concentration. This maximum is removed for a totally ionized polymer network. Despite these facts, a charge independent asymptotic behavior between size and salt concentration becomes apparent, at sufficiently high values of the latter. The Flory–Huggins mean-field approach together with the Donnan relations describe the observed swelling adequatel...

Osmotic de-swelling of ionic microgel particles

In this work, we study experimentally the effect of an external osmotic pressure πext on the swelling of ionic mesoscopic gels in the weak screening limit, where the Debye screening length is larger than the mesh size of the gel. Variations in the osmotic pressure were induced by adding dextran to the solution. The results show that ionic microgels do not respond to πext below a given value of the normal stress; above this value the system de-swells with increasing osmotic pressure. The start of de-swelling is set by the gel charge density. The Flory thermodynamic theory for ionic gels captures the essential characteristics of the de-swelling behavior; in particular, it predicts with fairly good accuracy the value of πext at which de-swelling begins. Finally, due to the colloidal character of the gels, we observe that the system flocculates by a depletion interaction mechanism at high dextran concentrations.

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

The aggregation behaviour of protein-coated particles: a light scattering study

The different mechanisms involved in the aggregation of spherical latex particles coated with bovine serum albumin (BSA) have been studied using static and dynamic light scattering. These techniques assess the fractal dimension of the aggregates and their mean hydrodynamic radius. Particles with different degrees of surface coverage have been prepared. The net charge of the covered particles has been modified by varying the pH of the aqueous phase. The aggregation rate was measured and used to determine the importance of the different aggregation mechanisms that are responsible for these types of flocculation processes. At low and intermediate degrees of surface coverage, bridging flocculation is the principal aggregation mechanism irrespective of the electrical state of the protein-particle complexes. At high degree of surface coverage, however, weak flocculation is important only when the BSA molecules are at their isoelectric point.

Synthesis of thermosensitive microgels with a tunable magnetic core.

In this work, we describe a new methodology for the preparation of monodisperse and thermosensitive microgels with magnetic core. In order to produce such a material, hydrophobic magnetic Fe(3)O(4) nanoparticles were prepared by two methods: thermal decomposition and coprecipitation. The surface of these nanoparticles was modified by addition of 3-butenoic acid, and after that these nanoparticles were dispersed in water and submitted to free radical polymerization at 70 °C in the presence of N-isopropylacrylamide (NIPAM) and bisacrylamide. The result of this reaction was monodisperse microgels with a magnetic core. By varying the amount of 3-butenoic acid, it was possible to obtain hybrid microgels with different magnetic core sizes and different architectures.

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.

Colloidal aggregation induced by attractive interactions

Colloidal aggregation induced by attractive interactions is tackled experimentally and by Brownian dynamics simulations using a mixture of positive and negative particles. The structure of the aggregates and the aggregation kinetics are used to characterize the aggregation behavior. The clusters show uniform internal structures, with a fractal dimension lower than that of clusters formed in diffusion, indicating a more branched architecture. The aggregation kinetics also differs from the diffusive one, slowing down as time proceeds. Both results are totally confirmed by simulation. The transition from the attractive driven to the diffusion controlled regimes is studied varying the range of interaction. Continuous transitions are observed both for the aggregation kinetics and cluster structure.

Motion of microgel particles under an external electric field

The influence of the swelling of charged microgel particles on their motion under an external electric field has been studied. The selected experimental observable was the electrophoretic mobility of the particles, which was measured as a function of the pH since its value controls the electrical charge of the particles. The mobility-pH curve presents a maximum and a minimum as a consequence of the competition between charge-density and friction coefficient variations during swelling. Ohshimas theory for polyelectrolyte-coated particles was employed, describing qualitatively the experimental results. Quantitative discrepancies suggest that charge renormalization should be considered.