Design of Stable Plasmonic Dimers in Solution: Importance of Nanorods Aging and Acidic Medium

We describe a simple and effective strategy to couple gold nanorods (GNRs) into end to end dimers and freeze the assembly in water. The assembly is initiated using cysteine and driven by hydrogen bonding between two cysteine. We show that the aging of GNRs samples impacts both the assembly kinetics and the final yield of GNRs dimers. The addition of an appropriate amount of silver nitrate induces the immediate termination of GNRs dimerization and stabilizes the small aggregates in solution for at least 24 h

Discerning the Origins of the Amplitude Fluctuations in Dynamic Raman Nanospectroscopy

We introduce a novel experimental and analytical method for discerning rare surface-enhanced Raman scattering (SERS) events observable at the nanoscale. We show that the kinetics of the Raman activity recorded on an isolated nanostructure is punctuated by intense and rare events of large amplitude and spectral variations. The fluctuations of thousands of SERS spectra were analyzed statistically in terms of power density functions, and the occurrence of the rare events was quantified by a wavenumber statistics. Our analysis enables one to extract valuable and unique spectroscopic signature of Raman variations usually hidden in time-average or space-average measurements. We illustrate our approach using molecular surface dynamics of gold adatoms on nanoparticles

A Scheme for Detecting Every Single Target Molecule with Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of <i>every</i> single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule

Selective Functionalization of the Nanogap of a Plasmonic Dimer

We report a self-developing anisotropic gold/polymer hybrid nanosystem that precisely places dye molecules at the plasmonic hotspot of metal nanostructures for sensing and photonics applications. Unlike conventional molecule–particle configurations, the anisotropic hybrid nanosystem (AHN) introduces an anisotropic spatial distribution of dye-containing active medium. This allows us to precisely overlap the near-field spatial distribution with the active medium and rule out the contribution from the background molecules. This overlap effect selectively highlights the optical response of the molecules of interest, that is, molecules located at the hotspots. Our AHN consists of gold nanodimers whose gaps have been filled with methylene blue molecules. They have been studied by plasmon-enhanced Raman spectroscopy as a probing tool. The AHN opens new doors not only for fundamental studies and photonics applications of molecule–particle interactions, but also for molecular trapping methods at the nanoscale

Engineering Thermoswitchable Lithographic Hybrid Gold Nanorods as Plasmonic Devices for Sensing and Active Plasmonics Applications

In this article, we aim to investigate the sensitivity of regular arrays of hybrid plasmonic nanostructures to variations in properties of the local environment (temperature, refractive index, polymer thickness), in the context of sensing and active plasmonic applications. A proper description and characterization of such hybrid systems is indeed essential in order to provide designed criteria for efficient stimuli-responsive devices. As an ideal model, we introduce a novel kind of hybrid plasmonic core–shell system made of lithographic gold nanorods (GNRs), coated by a thermosensitive polymer shell based on poly­(<i>N</i>-isopropylacrylamide) (pNIPAM). The grafting of the polymer on the GNRs results from a multistep but simple approach in order to confine the pNIPAM brushes on the GNRs and to control the thickness of the polymer coating. We show that the optical response of the plasmonic hybrid structures (GNR@pNIPAM) is strongly modified upon a variation of the external temperature, due to a physical change of the conformation of the polymer coating. These thermo-induced changes of the optical properties can be optimized by changing the aspect ratio of the GNRs and the polymer thickness to obtain very efficient optical reporters of the polymer state in a controlled and reversible manner. This work could provide an important step toward the use of GNR@pNIPAM structures for applications spanning from opto-mechanical modulators to nanoscale adhesion and molecular sensing