Jorge Pérez-Juste

Recent Progress on Silica Coating of Nanoparticles and Related Nanomaterials

In recent years, new strategies for silica coating of inorganic nanoparticles and organic nanomaterials, which differ from the classical methodologies, have emerged at the forefront of materials science. Silica as a coating material promises an unparalleled opportunity for enhancement of colloidal properties and functions by using core-shell rational designs and profiting from its synthetic versatility. This contribution provides a brief overview of recent progress in the synthesis of silica-coated nanomaterials and their significant impact in different areas such as spectroscopy, magnetism, catalysis, and biology.

Au@pNIPAM Colloids as Molecular Traps for Surface-Enhanced, Spectroscopic, Ultra-Sensitive Analysis†

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique that allows ultra-sensitive chemical or biochemical analysis. Since the first reported SERS on silver and gold colloids in 1979, they have become one of the most commonly used nanostructures for SERS, both as a testing ground for the most thorough theoretical modeling, and for the achievement of single-molecule detection (SMD). Analytical applications based on average SERS are mature, and current work is focused on specific tuning of the experimental conditions for each particular analyte. For example, the enhancement factors (EF) reported for organic acids and alcohols are several orders of magnitude lower than those achieved for thiols and amines. The main reason for this situation is the different affinity of the functional groups in the analyte toward colloidal gold or silver surfaces, and it is the affinity which determines the analytes retention. To circumvent this problem, various approaches have been proposed, including the functionalization of silver nanoparticles with different surface functional groups (e.g. calixarenes, viologen derivatives), so as to increase their compatibility with polycyclic aromatic compounds. 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. This approach has been reported to consistently enhance the signal for acids and amines, but it hardly helps in the case of alcohols, ethers, and other oxygencontaining groups, as well as for non-functionalized molecules. Therefore, there is a clear need for development of colloidal systems containing a noble-metal component together with a material that can trap a wide variety of molecular analytes. Herein we present the application of a recently developed core–shell colloidal material comprising gold nanoparticles coated with a thermally responsive poly-(N-isopropylacrylamide) (pNIPAM) microgel, which we denote Au@pNIPAM. While the gold cores provide the necessary enhancing properties, the pNIPAM shells can swell or collapse as a function of temperature, this change is expected to serve as a means to trap molecules and get them sufficiently close to the metal core for providing the SERS signal. Although similar systems have been proposed for applications in catalysis, temperature and pH sensing, or light-responsive materials, we propose that our particular configuration, with sufficiently big metal cores, can function as a general sensor for detection of all types of analytes. Apart from the SERS enhancement, this system can also be used to modulate the fluorescence intensity of adsorbed chromophores as a function of temperature. It is important to note that, the porous, protective pNIPAM shell not only enhances the long-term colloidal stability of the system in aqueous solutions, but additionally prevents electromagnetic coupling between metal particles, thus providing highly reproducible SERS signal and intensity, which is crucial for quantitative applications. Through a rational choice of model analytes, we demonstrate the application of these thermoresponsive hybrid materials for surface-enhanced Raman scattering, fluorescence, and resonance Raman scattering (SERS, SEF, and SERRS, respectively). This demonstration includes the first report of the SERS spectrum of 1-naphthol, which had remained elusive to SERS ultra-sensitive analysis until now. 1-Naphthol is a relevant biomarker for quantifying the exposure to polycyclic aromatic hydrocarbons in urine, as well as the presence of carbaryl pesticides in the environment and in fruits. Additionally, chronic exposure of humans to 1-naphthol has been reported to result in genotoxicity. The synthesis of the core–shell Au@pNIPAM colloids has been described in detail elsewhere and involves initial growth of a thin polystyrene (PS) shell on cetyl trimethyl ammonium bromide (CTAB) coated, 67 nm gold nanoparticles, followed by polymerization of N-isopropylacrylamide (NIPAM) and a cross-linker (N,N-methylenebisacrylamide; see Experimental Section for details). NIPAM monomers are polymerized in situ on the Au@PS surfaces using 2,2’azobis(2-methylpropionamidine) dihydrochloride (AAPH) as an initiator (Scheme 1a and Figure 1a). Particles with larger metal cores (116 nm) were prepared by seeded growth of the coated gold cores through addition of HAuCl4 and ascorbic acid (Figure 1 b). The SERS spectrum of Au@PS [*] Dr. R. A. lvarez-Puebla, Dr. I. Pastoriza-Santos, Dr. J. P rez-Juste, Prof. L. M. Liz-Marz n Departamento de Qu mica-F sica and Unidad Asociada CSICUniversidade de Vigo, 36310 Vigo (Spain) http://webs.uvigo.es/coloides/nano E-mail: ramon.alvarez@uvigo.es lmarzan@uvigo.es

Gemini‐Surfactant‐Directed Self‐Assembly of Monodisperse Gold Nanorods into Standing Superlattices

Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research. The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, nanoscale organization of readily accessible spherical gold nanoparticles 4] has been manipulated to produce a diverse range of topologies with interesting optical and electrical properties. 7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant. Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties. The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale. Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes. Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion. This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent–air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies. To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-functionalized phospholipid derivatives have proven excellent binders in nanoparticle–lipid– nanoparticle assemblies, 21] as they can be efficiently supported as bilayers on different substrates. In this respect, gemini surfactants, made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, are currently available as excellent scaffolds for structurally mimicking the bilayer formation of lipids in water. Moreover, gemini surfactants display exceptional amphiphilic properties, such as high adsorptivities on solid surfaces, which make them ideal candidates to influence the growth 25] and assembly of gold nanorods. We present herein the first study in which cationic gemini surfactants, (oligooxa)alkanediyla,w-bis(dimethyldodecylammonium bromide) (12-EOx-12, see the Supporting Information), were used for the reproducible and controlled synthesis of monodisperse gold NRs, focusing on the role of the chemical structure of these surfactants on the self-assembly of highly ordered, robust 2D and 3D gold NR superlattices with directional optical properties. We synthesized gold nanorods using the gemini surfactant that contains a spacer with one ethylene oxide group, 12-EO112 (Gem1-NRs), as a shape-directing agent by Ag-assisted seeded growth on preformed CTAB-capped gold seeds at 27 8C (see details in the Experimental Section). Controlling the molar ratio between seed and gold salt, the aspect ratio and longitudinal surface plasmon (LSP) resonance of the NRs could be readily tuned (see the Experimental Section). Significantly narrower LSP bands were measured for Gem1NRs than for CTAB-NRs, with typical full width at half LSP maximum (FWHM) reduction of 20–30 % (see the Support[*] Dr. A. Guerrero-Mart nez, Dr. J. P rez-Juste, E. Carb -Argibay, Prof. L. M. Liz-Marz n Departamento de Quimica Fisica and Unidad Asociada CSIC Universidade de Vigo, 36310 Vigo (Spain) Fax: (+ 34)986-812556 E-mail: aguerrero@uvigo.es lmarzan@uvigo.es

Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study

The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

Highly Controlled Silica Coating of PEG-Capped Metal Nanoparticles and Preparation of SERS-Encoded Particles†

Thiol-modified poly(ethylene glycol) (mPEG-SH) has been used to replace standard capping agents from the surfaces of gold nanoparticles with different sizes and shapes. Upon PEG stabilization, the nanoparticles can be transferred into ethanol, where silica can be directly grown on the particle surfaces through the standard Stober process. The obtained silica shells are uniform and homogeneous, and the method allows a high degree of control over shell thickness for any particle size and shape. Additionally, Raman-active molecules can be readily incorporated within the composite nanoparticles during silica growth so that SERS/SERRS-encoded nanoparticles can be fabricated containing a variety of tags, thereby envisaging multiplexing capability.

Size Tunable Au@Ag Core–Shell Nanoparticles: Synthesis and Surface-Enhanced Raman Scattering Properties

We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core-shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core-shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core-shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core-shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core-shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core-shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

Optical sensing of biological, chemical and ionic species through aggregation of plasmonic nanoparticles

Plasmonic nanoparticles made of gold and silver have attracted a great deal of research attention in various fields, such as biosensors, imaging, therapy, nanophotonics, catalysis and light harvesting due to their unique optical and electronic properties. Plasmonic nanoparticle colloids may exhibit strong colours in the visible region due to localized surface plasmon resonances, whereas their aggregates exhibit different linear and nonlinear optical properties. Therefore, a smart design of chemical interactions between analytes and the nanoparticles surface may lead to gradual optical changes, which can be probed by various sensing methods, allowing quantitative analyte detection. A significant amount of research has been carried out toward the development of plasmonic sensors based on analyte-induced aggregation of Au or Ag nanoparticles, and the sensitivity and selectivity of such plasmonic biosensors have been greatly improved over the years. In this feature article, we summarize different design strategies that have been employed to induce the aggregation of plasmonic nanoparticles upon the addition of various analytes such as DNA, proteins, organic molecules and inorganic ions. We introduce various optical assays, such as colorimetry, surface-enhanced Raman scattering, two-photon photoluminescence, dynamic light scattering, hyper-Rayleigh scattering and chiroptical activity. From the discussion, it can be concluded that plasmonic sensors based on nanoparticle aggregation offer simple, highly sensitive and selective detection of various analytes. Finally, we discuss some of the future directions of plasmonic nanosensors toward device integration for practical applications.

Multiresponsive Hybrid Colloids Based on Gold Nanorods and Poly(NIPAM-co-allylacetic acid) Microgels: Temperature- and pH-Tunable Plasmon Resonance

This work describes the control and manipulation of the optical properties of multiresponsive organic/inorganic hybrid colloids, which consist of thermo-responsive poly-(NIPAM-co-allylacetic acid) microgel cores and gold nanorods assembled on their surface. These composites are multifunctional, in the sense that they combine the interesting optical properties of the rod-shaped gold particles--exhibiting two well-differentiated plasmon modes--with the sensitivity of the copolymer microgel toward external stimuli, such as temperature or solution pH. It is shown that the collapse of the microgel core, induced by changes in either temperature or pH, enhances the electronic interactions between the gold nanorods on the gel surface, as a result of the subsequent increase of the packing density arising from the surface decrease of the collapsed microgel. Above a certain nanorod density, such interactions lead to remarkable red-shifts of the longitudinal plasmon resonance.

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.

Influence of silver ions on the growth mode of platinum on gold nanorods

Gold nanorods were used as seeds for platinum growth, using a mild reducing agent, ascorbic acid, in the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Highly preferential growth on the tips or complete overcoating can be achieved by manipulation of the reduction conditions, among which the presence of silver ions was found to be highly dominant. In either case, the growth was found to be epitaxial, as demonstrated by high resolution electron microscopy and Fourier transform analysis. Additionally, the deposition mode leads to very different effects on the optical properties of the nanoparticles, with tip growth inducing huge surface plasmon red shifts.