Chemically bound gold nanoparticle arrays on silicon: assembly, properties and SERS study of protein interactions

A highly reproducible and facile method for formation of ordered 2 dimensional arrays of CTAB protected 50 nm gold nanoparticles bonded to silicon wafers is described. The silicon wafers have been chemically modified with long-chain silanes terminated with thiol that penetrate the CTAB bilayer and chemically bind to the underlying gold nanoparticle. The silicon wafer provides a reproducibly smooth, chemically functionalizable and non-fluorescent substrate with a silicon phonon mode which may provide a convenient internal frequency and intensity calibration for vibrational spectroscopy. The CTAB bilayer provides a potentially biomimetic environment for analyte, yet allows a sufficiently small nanoparticle separation to achieve a significant electric field enhancement. The arrays have been characterized using SEM and Raman spectroscopy. These studies reveal that the reproducibility of the arrays is excellent both between batches (< 10% RSD) and across a single batch (< 5% RSD). The arrays also exhibit good stability, and the effect of temperature on the arrays was also investigated. The interaction of protein and amino acid with the nanoparticle arrays was investigated using Raman microscopy to investigate their potential in bio-SERS spectroscopy. Raman of phenylalanine and the protein bovine pancreatic trypsin inhibitor, BPTI were studied using 785 nm excitation, coincident with the surface plasmon absorbance of the array. The arrays exhibit SERS enhancements of the order of 2.6 x 104 for phenylalanine, the standard deviation on the relative intensity of the 1555 cm-1 mode of phenylalanine is less than 10% for 100 randomly distributed locations across a single substrate and less than 20% between different substrates. Significantly, comparisons of the Raman spectra of the protein and phenlyalanine in solution and immobilized on the nanoparticle arrays indicates that the protein is non-randomly orientated on the arrays. Selective SERS enhancements suggest that aromatic residues penetrate through the bilayer inducing conformational changes in the protein

The effect of the local refractive index on the spectral properties of gold nanoparticle arrays

Gold nanoparticle arrays exhibit extraordinary spectral properties useful to a wide variety of applications including biomolecular sensing, alternative energy, and informatics. The spectral properties of these arrays depend on the array geometry (particle size and spacing) and the surrounding environment (refractive index). In this thesis, a thorough map of the parameter space describing the spectral properties of square, gold nanoparticle arrays is developed in conjunction with a fundamental theoretical description of the phenomena that govern these properties

Surface Plasmon Enhanced Photoconductance of Gold Nanoparticle Arrays with Incorporated Alkane Linkers

We report on a photoconductive gain effect in two-dimensional arrays of gold nanoparticles, in which alkane molecules are inserted. The nanoparticle arrays are formed by a self-assembly process from alkanethiol-coated gold nanoparticles, and subsequently they are patterned on a Si/SiO2 chip by a microcontact printing technique. We find that the photoconductance of the arrays is strongly enhanced at the frequency of the surface plasmon of the nanoparticles. We interpret the observation as a bolometric enhancement of the conductance of the nanoparticle arrays upon excitation of the surface plasmon resonance

Plasmonic modes of gold nano-particle arrays on thin gold films

Regular arrays of metal nanoparticles on metal films have tuneable optical resonances that can be applied for surface enhanced Raman scattering or biosensing. With the aim of developing more surface selective geometries we investigate regular gold nanoparticle arrays on 25nm thick gold films, which allow to excite asymmetric surface plasmon modes featuring a much better field confinement compared to the symmetric modes used in conventional surface plasmon resonance setups. By optical extinction spectroscopy we identify the plasmonic modes sustained by our structures. Furthermore, the role of thermal treatment of the metal structures is investigated, revealing the role of modifications in the crystalline structure of gold on the optical properties.Comment: 8 pages, 3 figure

Fabrication and characterisation of dielectric nanoparticle arrays - Tuning the light-matter interaction

Nanoparticles ordered in periodic arrays provide electromagnetic resonances called surface lattice resonances (SLRs) leading to a high degree of local field confinement. If dye molecules are added on top of the nanoparticle arrays the system can be in the strong coupling regime. Typically, nanoparticle arrays consist of metals which, however, provide very high losses. In contrast, high-index dielectrics are low-dissipative and can therefore lead to narrower SLRs and therefore to longer lifetimes. In this work, we fabricate dielectric nanoparticle arrays made out of germanium and amorphous silicon. The SLRs of germanium and amorphous silicon nanoarrays are measured and compared to the SLRs of gold nanoarrays. A quality factor of up to 1.27 higher is provided by the SLRs of dielectric nanoarays compared to the SLRs of the gold nanoarrays. In addition, we measure the dispersion of the nanoarrays with fluorescent dye molecules on top and observe an indication of strong coupling

Electromagnetic energy transport below the diffraction limit in periodic metal nanostructures

We investigate the possibility of using arrays of closely spaced metal nanoparticles as waveguides for electromagnetic energy below the diffraction limit of visible light. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission through chain networks such as corners and tee structures is possible at high efficiencies. Although radiation losses into the far field are negligible due to the near-field nature of the coupling, resistive heating leads to transmission losses of about 3 dB/500 nm for gold and silver particles. We confirmed the predictions of this analytical model using numeric finite difference time domain (FDTD) simulations. Also, we have fabricated gold nanoparticle arrays using electron beam lithography to study this type of electromagnetic energy transport. A modified illumination near field scanning optical microscope (NSOM) was used as a local excitation source of a nanoparticle in these arrays. Transport is studied by imaging the fluorescence of dye-filled latex beads positioned next to the nanoparticle arrays. We report on initial experiments of this kind

Collective resonances in gold nanoparticle arrays

Baptiste Auguié and William L. Barnes, Physical Review Letters, Vol. 101, article 143902 (2008). Copyright © 2008 by the American Physical Society.We present experimental evidence of sharp spectral features in the optical response of 2D arrays of gold nanorods. A simple coupled dipole model is used to describe the main features of the observed spectral line shape. The resonance involves an interplay between the excitation of plasmons localized on the particles and diffraction resulting from the scattering by the periodic arrangement of these particles. We investigate this interplay by varying the particle size, aspect ratio, and interparticle spacing, and observe the effect on the position, width, and intensity of the sharp spectral feature