Nicolas Pazos-Perez

Gold nanorods 3D-supercrystals as surface enhanced Raman scattering spectroscopy substrates for the rapid detection of scrambled prions

Highly organized supercrystals of Au nanorods with plasmonic antennae enhancement of electrical field have made possible fast direct detection of prions in complex biological media such as serum and blood. The nearly perfect three-dimensional organization of nanorods render these systems excellent surface enhanced Raman scattering spectroscopy substrates with uniform electric field enhancement, leading to reproducibly high enhancement factor in the desirable spectral range.

Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media

Gold nanoparticles are very interesting because of their potential applications in microelectronics, optical devices, analytical detection schemes, and biomedicine. Though shape control has been achieved in several polar solvents, the capability to prepare organosols containing elongated gold nanoparticles has been very limited. In this work we report a novel, simplified method to produce long, thin gold nanowires in an organic solvent (oleylamine), which can be readily redispersed into nonpolar organic solvents. These wires have a characteristic flexible, hairy morphology arising from a small thickness (<2 nm) and an enormous length (up to several micrometers), with the possibility of adjusting the dimensions through modification of the growth conditions, in particular, the gold salt concentration. Despite their extreme aspect ratio, the wires are stable in solution for long periods of time but easily break when irradiated with high-energy electron beams during transmission electron microscopy.

Organized Plasmonic Clusters with High Coordination Number and Extraordinary Enhancement in Surface- Enhanced Raman Scattering (SERS)**

Noble metal nanoparticles exhibit optical excitations known as surface plasmons that produce large enhancement of the local light intensity under external illumination, particularly when the nanoparticles are arranged in closely spaced configurations.1 The interparticle gap distance2 plays a critical role in the generation of hotspots with high electromagnetic fields, and thus such assembled nanoparticles find application to ultrasensitive detection, for example through surface-enhanced Raman scattering3 (SERS) and nonlinear optics, among other feats.4 Controlled assembly using colloidal chemistry is an emerging and promising field for high-yield production of metal nanoparticle clusters with small interparticle gaps.5 However, most of the reported methods rely on the use of nucleic acids or other organic molecules as linking elements,6 which yield long separation distances and thus weak plasmon coupling. Additionally, only simple clusters, such as dimers and trimers, have been efficiently synthesized. Herein, we report the controlled assembly of gold nanospheres into well-defined nanoparticle clusters with large coordination numbers (up to 7) and high symmetry. We further demonstrate ultrasensitive direct and indirect SERS sensing, thus corroborating the outstanding optical performance of these clusters with robust enhancement factors that are over three orders of magnitude higher than those of single particles.

Growth of Sharp Tips on Gold Nanowires Leads to Increased Surface-Enhanced Raman Scattering Activity

We report the formation of gold nanoparticles with a novel and useful morphology, comprising nanowires fully covered with sharp tips (thorned nanowires). The synthesis is based on a seeded-growth approach based the rapid overgrowth of ultrathin gold wires in N,N-dimethylformamide, in the presence of poly(vinylpyrrolidone). The process allows a fine control over the thickness of the final wires, as well as the tunability of the number and sharpness of the thorns. These new plasmonic nanostructures display extremely strong optical enhancing properties and can be readily used as platforms for SERS and for integration in ultrasensitive optical devices.

Macroscale Plasmonic Substrates for Highly Sensitive Surface-Enhanced Raman Scattering

The fabrication of macroscale optical materials from plasmonic nanoscale building blocks is the focus of much current multidisciplinary research. In these macromaterials, the nanoscale properties are preserved, and new (metamaterial) properties are generated as a direct result of the interaction of their ordered constituents.1 These macroscale plasmonic assemblies have found application in a myriad of fields, including nanophotonics, nonlinear optics, and optical sensing.2 Owing to their specific requirements in terms of size and shape, their fabrication is not trivial and was until recently restricted to the use of lithographic techniques, especially those based on electron- or ion-beam patterning.3 However, these techniques are not only expensive, time-consuming, and demanding but are also restricted to small simple and solid geometries, which are good for proof of concepts but less suitable for real-life applications. Approaches based on colloidal chemistry are gaining relevance as an alternative. During the past few years, several examples of the fabrication of organized particles have been reported, including the preparation of complex colloidal particles4 and the use of preformed colloids to create large crystalline organized entities known as supercrystals.5 The latter approach provides optical platforms with unprecedented plasmonic properties that can be exploited for the design of cheap ultrasensitive and ultrafast sensors with surface-enhanced Raman scattering (SERS)6 spectroscopy as the transducer.

Universal One-Pot and Scalable Synthesis of SERS Encoded Nanoparticles

Encoded particles are one of the most powerful approaches for multiplex high-throughput screening. Surfaceenhanced Raman scattering (SERS) based codification can, in principle, avoid many of the intrinsic limitations due to conventional alternatives, as it decreases the reading time and particle size while allowing for almost unlimited codification. Unfortunately, methods for the synthetic preparation of these particles are tedious; often subjected to limited reproducibility (associated with large fluctuations in the size distributions of the polymers employed in the standard protocols); and to date, limited to a small amount of molecules. Herein, we report a universal, one-pot, inexpensive, and scalable synthetic protocol for the fabrication of SERS-encoded nanoparticles. This synthetic strategy is highly reproducible, independent of the chemical nature and size of the Raman code used (31 different codes were tested) and scalable in the liter range without affecting the final properties of the encoded structures. Furthermore, the SERS efficiency of the fabricated encoded nanoparticles is superior to that of the materials produced by conventional methods, while showing a remarkable reproducibility from batch to batch. This encoding strategy can easily be applied to nanoparticles of different materials and shapes.

Controlling inter-nanoparticle coupling by wrinkle-assisted assembly

This highlight focuses on recent advances in controlling inter-nanoparticle coupling effects by template-assisted organization of colloidal particles. We show that the use of templates formed by wrinkling allows circumventing drawbacks of classical lithographic approaches for template formation like the large number of processing steps and poor scalability. Subsequently, we illustrate that confinement effects can be used for creating particle assemblies with excellent short and long range order. This allows controlling inter-particle coupling effects. As an example, we focus on plasmonic coupling in gold nanoparticle arrays. We demonstrate that these arrays can be applied to develop efficient and homogenous substrates for surface enhanced Raman scattering (SERS).

From nano to micro: synthesis and optical properties of homogeneous spheroidal gold particles and their superlattices.

Iodide ions have been used as an additive to fabricate homogeneous gold spheres with a la carte dimensions, ranging from the nano- (50 nm) to the microscale (ca. 1 μm). Due to the high uniformity and surface functionalization of the produced materials, they undergo spontaneous assembly into organized superlattices upon solvent drying. Thus, optical properties of the particles including localized surface plasmon resonances and surface enhanced Raman scattering (SERS) response, both in solution and organized into superlattices, are also reported.

Ultrasensitive Direct Quantification of Nucleobase Modifications in DNA by Surface-Enhanced Raman Scattering: The Case of Cytosine.

Recognition of chemical modifications in canonical nucleobases of nucleic acids is of key importance since such modified variants act as different genetic encoders, introducing variability in the biological information contained in DNA. Herein, we demonstrate the feasibility of direct SERS in combination with chemometrics and microfluidics for the identification and relative quantification of 4 different cytosine modifications in both single- and double-stranded DNA. The minute amount of DNA required per measurement, in the sub-nanogram regime, removes the necessity of pre-amplification or enrichment steps (which are also potential sources of artificial DNA damages). These findings show great potentials for the development of fast, low-cost and high-throughput screening analytical devices capable of detecting known and unknown modifications in nucleic acids (DNA and RNA) opening new windows of activity in several fields such as biology, medicine and forensic sciences.

Gold encapsulation of star-shaped FePt nanoparticles

We present a seeded-growth method for the encapsulation of star-like FePt nanoparticles with gold shells in aqueous solution, which allows not only further functionalization (such as silica coating) but also the organization of the synthesized core-shell nanoparticles into mesoscopic one-dimensional nanostructures under the influence of an externally applied magnetic field.