Thiol end-functionalized polystyrene chains have been introduced onto the surface of gold nanoparticles via a two-step grafting-to method. This simple grafting procedure is demonstrated to be efficient for gold nanoparticles of different sizes and for particles initially dispersed in either aqueous or organic media. The method has been applied successfully for a relatively large range of polystyrene chain lengths. Grafting densities, as determined by thermogravimetric analysis, are found to decrease with increasing chain length. In all cases, the grafting density indicates a dense brush conformation for the tethered chains. The resulting functionalized nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solution. Furthermore, the distance between the gold cores in the dried monolayer is controlled by the molecular weight of the grafted polystyrene. Optical absorption spectra recorded for the organized monolayers show the characteristic plasmon absorption of the gold particles. Importantly, the plasmon resonance frequency exhibits a distinct dependence on interparticle separation that can be attributed to plasmon coupling between neighboring gold cores.

A low-cost methodology to prepare highly sensitive sensors is reported with different nanoparticles for LSPR and SERS measurements. The LSPR and SERS sensor was self-assembled as an array of polystyrenecapped gold nanoparticles (Au NPs), produced by drop-coating a Au NP suspension on a glass surface followed by plasma etching of the polystyrene. The procedure was applied to different sizes and shapes of Au NPs, primarily 60 nm gold nanospheres and 80 nm gold nanoraspberries, created with precise control on the packing density of the array. The LSPR sensitivity and figure of merit (FOM) for the arrays of gold nanoraspberries were found to be almost 2-fold better than for the nanospheres and had a shorter penetration depth, a consequence of their small-sized surface asperities. The average measured SERS enhancement factor (EF) for the raspberries was also higher than for spheres and control over the EF was achieved using different packing densities which allowed control of the interparticle coupling enhancing the SERS response. The localized EF (hot spot) in dense arrays of nanoraspberries was measured to be in the order of a typical EF required for single molecule SERS, as demonstrated from the general agreement of experimental measurements with simulations performed using Comsol Multiphysics. This simple LSPR and SERS sensor could potentially serve as an effective platform for the identification and quantification of various analytes.

The plasmonic properties of single Au triangular nanoprisms are investigated using photoemission electron microscopy with particular emphasis on polarization dependence. Two localized surface plasmon resonances (LSPRs) are studied, namely, the in-plane dipolar and quadrupolar plasmon excitations. Experimental maps of the near-field spatial distribution upon polarization and wavelength of the exciting field are interpreted in the framework of a group theory description and finite difference time domain simulations. This work demonstrates the selective excitations, by lifting of degeneracy, of the different LSPR eigenmodes at the single object level and opens ways for the active control of the angular radiation patterns of optical nanoantennas. This approach is general and applies to any nano-object, whatever its initial shape symmetry.

Abstract. We describe a technology for the fabrication of inexpensive and versatile mirrors through the use of a new type of nanoengineered optical material composed by the spreading of a self-assembling reflective colloidal film spread at the surface of a liquid. These new reflecting liquids offer interesting possibilities for astronomical instrumentation. For example, they can replace mercury in conventional rotating liquid mirrors. The main advantages offered include extremely low cost and, by coating a viscous liquid, the possibility of tilting the mirror by a few tens of degrees. We also have coated ferromagnetic liquids with these reflecting films. The resulting surfaces can be shaped by the application of a magnetic field, yielding reflecting surfaces that can have complicated shapes that can rapidly shift with time. These inexpensive and versatile optical elements could have numerous scientific and technological applications. Among possible astronomical applications, they could be used to make large inexpensive adaptive mirrors exhibiting strokes ranging from nanometers to several millimeters.