Silver nanoparticle-mesoporous oxide nanocomposite thin films: a platform for spatially homogeneous SERS-active substrates with enhanced stability.

We introduce a nanoparticle-mesoporous oxide thin film composite (NP-MOTF) as low-cost and straightforward sensing platforms for surface-enhanced Raman Spectroscopy (SERS). Titania, zirconia, and silica mesoporous matrices templated with Pluronics F-127 were synthesized via evaporation-induced self-assembly and loaded with homogeneously dispersed Ag nanoparticles by soft reduction or photoreduction. Both methods give rise to uniform and reproducible Raman signals using 4-mercaptopyridine as a probe molecule. Details on stability and reproducibility of the Raman enhancement are discussed. Extensions in the design of these composite structures were explored including detection of nonthiolated molecules, such as rhodamine 6-G or salicylic acid, patterning techniques for locating the enhancement regions and bilayered mesoporous structures to provide additional control on the environment, and potential size-selective filtration. These inorganic oxide-metal composites stand as extremely simple, reproducible, and versatile platforms for Raman spectroscopy analysis.

Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces.

Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.

From single to multiple Ag-layer modification of Au nanocavity substrates: a tunable probe of the chemical surface-enhanced Raman scattering mechanism.

We present experimental and computational results that enlighten the mechanisms underlying the chemical contribution to surface-enhanced Raman scattering (SERS). Gold void metallic arrays electrochemically covered either by a Ag monolayer or 10-100 Ag layers were modified with a self-assembled monolayer of 4-mercaptopyridine as a molecular Raman probe displaying a rich and unexpected Raman response. A resonant increase of the Raman intensity in the red part of the spectrum is observed that cannot be related to plasmon excitations of the cavity-array. Notably, we find an additional 10-20 time increase of the SERS amplification upon deposition of a single Ag layer on the Au substrate, which is, however, almost quenched upon deposition of 10 atomic layers. Further deposition of 100 atomic Ag layers results in a new increase of the SERS signal, consistent with the improved plasmonic efficiency of Ag bulk-like structures. The SERS response as a function of the Ag layer thickness is analyzed in terms of ab initio calculations and a microscopic model for the SERS chemical mechanism based on a resonant charge transfer process between the molecular HOMO state and the Fermi level in the metal surface. We find that a rearrangement of the electronic charge density related to the presence of the Ag monolayer in the Au/Ag/molecule complex causes an increase in the distance between the HOMO center of charge and the metallic image plane that is responsible for the variation of Raman enhancement between the studied substrates. Our results provide a general platform for studying the chemical contribution to SERS, and for enhancing the Raman efficiency of tailored Au-SERS templates through electrochemical modification with Ag films.

Redox molecule based SERS sensors.

We describe a general framework to design nanobiosensors based on a wired enzyme coupled to a redox molecule and integrated with SERS Au core-shell nanoparticles and ordered nanocavities. The response of the proposed sensor is based on the different electronic resonant Raman behavior of the oxidized or reduced electronic states of the molecular wire, and on the surface plasmon amplification induced by the tailored metallic substrate. The nanobiosensors can be interrogated remotely through the resonant Raman scattering intensity recovery or spectral variation of the redox molecule, an Os-complex, when the latter varies its oxidation state. Alternatively, we show through two-color spectro-electrochemistry that Raman scattering is also finely sensitive to oxidation state changes of flavin, a biomimetic system that mimics the active center of many flavoprotein enzymes. We show that multiple sample spectroscopic ellipsometry gives access to the spectral dependence of the optical constants of single redox-molecule layers, and through it to the electronic resonances of the system. All the components for selective molecular recognition and for the generation of an optical amplified signal, are self-contained in the proposed biosensor. As proof of concept a compact SERS sensor responsive to glucose with millimolar concentration in solution is demonstrated.

Ag-modified Au nanocavity SERS substrates.

The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies from the near infrared to the ultraviolet through the tuning of the void height and diameter, and the selection of the appropriate material. Typically Au nanocavity substrates can be grown with better reproducibility, homogeneity, and stability, while Ag structures display significantly larger SERS enhancements. To exploit these two apparently excluding aspects, quality and enhancement, we report a detailed study of 500 nm Au-nanocavity templates modified by the controlled electrochemical deposition of 100 Ag layers, a thickness similar to the visible light skin-depth of bulk Ag. The SERS amplification of the ordered cavity-arrays is determined using 4-mercaptopyridine as a non-electronic resonant molecular probe. The ultrathin Ag layer modification of the Au substrates results in a strong amplification of the SERS signal both in the red and the green part of the spectrum, and in a spectral shift of the Raman resonance scans. These observations are assigned to Ag-induced changes in the plasmon-polariton response of the nanostructure. The reported results provide a general platform for the preparation of renewable SERS-active substrates that combine the durability and higher quality of Au nanotemplates, with the enhanced SERS amplification factors of Ag.

Phospholipid bilayers supported on thiolate-covered nanostructured gold: in situ Raman spectroscopy and electrochemistry of redox species.

Thiol-covered nanostructured gold has been tested as a platform for the preparation of high-area phospholipid bilayer systems suitable for optical and electrochemical sensing. In situ and ex situ Raman spectroscopy and electrochemical measurements are made to study methylene blue (MB) and flavin-adenine dinucleotide (FAD) incorporation into dimyristoylphosphatidylcholine (DMPC) bilayers prepared by vesicle fusion on dithiothreitol (DTT)-covered nanostructured gold. Results show that lipophilic positively charged MB molecules are incorporated in the bilayer reaching the DTT-gold interface. On the other hand, the negatively charged FAD molecules are immobilized at the outer part of the phospholipid bilayer and cannot be electrochemically detected. Our results demonstrate that DTT-covered nanostructured gold provides a suitable high-area platform for phospholipid membranes that are able to separate and sense different kinds of molecules and biomolecules.

Immobilization of methylene blue on self-assembled iodine monolayers on gold.

The immobilization of methylene blue (MB) on iodine-covered Au(111) is studied by electrochemical techniques, scanning tunneling microscopy (STM), Auger Electron Spectroscopy (AES), and Raman spectroscopy. Results show that MB species are efficiently adsorbed on the square root of 3 x square root of 3 R30 degrees I lattice on Au(111). The electrochemical behavior of the adsorbed MB molecules is reversible, indicating a relatively fast electron transfer from the Au(111) surface to the immobilized MB species through the iodine layer. STM images with molecular resolution are consistent with adsorption of MB dimers on a square root of 3 x square root of 3 R30 degrees I lattice placed atop of the Au(111) substrate. Results are compared to those obtained for MB immobilized on Au(111) covered by S(n) (n = 3-8) surface structures.

Exploring three-dimensional nanosystems with Raman spectroscopy: methylene blue adsorbed on thiol and sulfur monolayers on gold.

Resonant Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl- and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.