Isabel Pastoriza-Santos

Modelling the optical response of gold nanoparticles

This tutorial review presents an overview of theoretical methods for predicting and understanding the optical response of gold nanoparticles. A critical comparison is provided, assisting the reader in making a rational choice for each particular problem, while analytical models provide insights into the effects of retardation in large particles and non-locality in small particles. Far- and near-field spectra are discussed, and the relevance of the latter in surface-enhanced Raman spectroscopy and electron energy-loss spectroscopy is emphasized.

High-yield synthesis and optical response of gold nanostars

Multipod Au nanoparticles (nanostars) with single crystalline tips were synthesized in extremely high yield through the reduction of HAuCl(4) in a concentrated solution of poly(vinylpyrrolidone) (PVP) in N,N-dimethylformamide (DMF), in the presence of preformed Au nanoparticle seeds, but with no need for external energy sources. Nanostar dispersions display a well-defined optical response, which was found (through theoretical modeling) to comprise a main mode confined within the tips and a secondary mode confined in the central body. Calculations of the surface enhanced Raman scattering (SERS) response additionally show that this morphology will be relevant for sensing applications.

Zeptomol Detection Through Controlled Ultrasensitive Surface-Enhanced Raman Scattering

SERS permits identifying the nature of molecules in extremely low concentrations, but it is hindered by poor enhancement or low reproducibility. We demonstrate controllable approximately 10(10) signal amplification reaching the zeptomol detection limit for a nonresonant molecule by sandwiching the analyte between the tips of star-shaped gold nanoparticles and a planar gold surface using a simple synthetic procedure. This unprecedented control over light-intensity amplification opens a new avenue toward high-yield, fully reproducible, SERS-based, zeptomol detection and holds promise for nonlinear optics applications at the single-particle level.

Tuning Size and Sensing Properties in Colloidal Gold Nanostars

Gold nanostars are multibranched nanoparticles with sharp tips, which display extremely interesting plasmonic properties but require optimization. We present a systematic investigation of the influence of different parameters on the size, morphology, and monodispersity of Au nanostars obtained via seeded growth in concentrated solutions of poly(vinylpyrrolidone) in N,N-dimethylformamide. Controlled prereduction of Au(3+) to Au(+) was found to influence monodispersity (narrower plasmon bands), while the [HAuCl(4)]/[seed] molar ratio significantly affects the morphology and tip plasmon resonance frequency. We also varied the size of the seeds (2-30 nm) and found a clear influence on the final nanostar dimensions as well as on the number of spikes, while synthesis temperature notably affects the morphology of the particles, with more rounded morphologies formed above 60 °C. This rounding effect allowed us to confirm the importance of sharp tips on the optical enhancing behavior of these nanoparticles in surface-enhanced raman scattering (SERS). Additionally, the sensitivity toward changes in the local refractive index was found to increase for larger nanostars, though lower figure of merit (FOM) values were obtained because of the larger polydispersity.

Colloidal silver nanoplates. State of the art and future challenges

This Feature Article provides an overview of current research on the synthesis and properties of silver nanoplates. Starting from a brief description of the origin of the optical properties of Ag nanoparticles and the factors affecting them, we introduce the numerical methods used for modelling—discrete dipole approximation (DDA) and boundary element method (BEM)—and present a comparative study between theoretical predictions and experimental results. Then, the principal physical and wet-chemical synthetic protocols that have been used to obtain Ag nanotriangles/nanoplates in high yield are described, with a discussion of the formation mechanisms proposed by the different authors. Subsequently, the structural characterization of the particles is described, followed by a section devoted to the reactivity and stability of silver nanoplates. We conclude with a description of potential applications in the field of biological and chemical sensors and surface enhanced Raman spectroscopy (SERS).

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

On the temperature stability of gold nanorods: comparison between thermal and ultrafast laser-induced heating

The response of gold nanorods to both thermal and ultrafast laser-induced heating has been examined. The thermal heating experiments show structural changes that occur on timescales ranging from hours to days. At the highest temperature examined (250 degrees C) the nanorods are transformed into spheres within an hour. On the other hand, no structural changes are observed in the laser-induced heating experiments up to temperatures of 700 +/- 50 degrees C. This is attributed to thermal diffusion in the laser experiments. Measurements of the period of the extensional mode of the nanorods using time-resolved spectroscopy show a significant softening at high pump laser powers. However, the decrease in the period is less than expected from bulk Youngs modulus vs. temperature data.

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.

All-In-One Optical Heater-Thermometer Nanoplatform Operative From 300 to 2000 K Based on Er3+ Emission and Blackbody Radiation

A single nanoplatform integrating laser-induced heat generation by gold nanoparticles and temperature sensing up to 2000 K via (Gd,Yb,Er)2 O3 nanorods is demonstrated, which presents considerable potential for nanoscale photonics and biomedicine. Blackbody emission is ascertained from the temperature increment with AuNP concentration, emission color coordinates as a function of the laser pump power, and Plancks law of blackbody radiation.

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.