SERS permits identifying the nature of molecules in extremely low concentrations, but it is hindered by poor enhancement or low reproducibility. We demonstrate controllable ∼1010 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.

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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.

SERS is nowadays a well-established ultrasensitive technique with potential to solve many analytical problems, especially those related to biosciences. This Perspective article aims at summarizing the experimental complexities, in particular, those related to the interaction of light with the sample, that the SERS practitioner may find when acquiring a spectrum while providing a general basis for the interpretation of the obtained vibrational features. With such an idea in mind, factors related to the instrumentation, the optical enhancer, and the analyte molecule are discussed to illustrate the effects of the incident light on the absolute and relative intensity, as well as the spectral profile of the SERS spectra.

Advances in nanophotonics have shown the potential of colloidal metal particles with sharp tips, such as nanostars, to focalize the plasmonic electromagnetic fields. We describe in this paper the performance of gold nanostars in common surface-enhanced scattering (SES) experiments, including SERS, SERRS, and SEF. The study was carried out for several analytes and demonstrated the potential of nanostars for ultradetection of nonfunctionalized analytes, by simply depositing nanostars on top of analyte thin films previously assembled onto a smooth gold surface.

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The synthesis, characterization, and assembly of different types of nanoparticles, which was established as a necessary prerequisite for the application of nanotechnology, have dramatically advanced over the last 20 years. [1,2] However, it has recently been realized that the incorporation of multiple functionalities within nanoscale systems would become much more useful for most of the foreseen applications. Thus, the fabrication of multifunctional nanoparticles has become a major challenge. Among these systems, the incorporation of active (optically, catalytically, magnetically…) nanoparticles within so-called "smart" thermosensitive microgels has received significant attention over the last few years. [3,4] The incorporation of nanoparticles can be accomplished either by in situ formation, by post-synthesis assembly or by direct polymerization on the nanoparticles surface. [5,6] We have recently reported the growth of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels on the surface of gold nanoparticles, involving several steps, including the formation of a first polystyrene thin layer, followed by pNIPAM polymerization after the required purification process. [7,8] Although gold nanoparticle growth could be achieved within the microgel shell, this synthesis was restricted to spherical nanoparticle seeds, whereas, for example, nanorods (which display a much more interesting optical response) were not properly incorporated. Thus, there was a need to both simplify the coating process and make it more widely applicable.We report here a novel procedure where butenoic acid is used for the synthesis of gold nanoparticles in aqueous surfactant solutions, in the presence of preformed Au seeds. Apart from the interesting observation that butenoic acid can be used as a reducing agent, this is particularly interesting because it provides the particles with a vinyl functionality, which should be useful for the direct pNIPAM polymerization on the nanoparticles surface and their subsequent encapsulation, while avoiding complicated surface functionalization steps. Although we have only optimized the reduction process for nanosphere growth, we also demonstrate that butenoic acid can replace cetyltrimethylammonium bromide (CTAB) molecules from Au nanoparticle surfaces, including Au nanorods, and adsorb on the metal surface, thereby facilitating the polymerization of pNIPAM on the metal core. The improved stability of the nanocomposites and the porosity of the pNIPAM shell allows subsequent reduction of metal atoms on the metal core, which was exploited for the overgrowth of pNIPAM encapsulated Au spheres and rods with both Au and Ag under mild conditions. In a previous publication [9] we demonstrated the ability of these composite colloids to mechanically trap non-common surface-enhanced Raman scattering (SERS) analytes. However, the use of 60 nm gold spheres as colloidal cores and the impossibility of forming hot spots due to the physical barrier imposed by the pNIPAM shell severely limited the enhancing capabili...