Localized surface plasmon resonance (LSPR) is one of the signature optical properties of noble metal nanoparticles. Since the LSPR wavelength λmax is extremely sensitive to the local environment, it allows us to develop nanoparticle-based LSPR chemical and biological sensors. In this work, we tuned the LSPR peaks of Ag nanotriangles and explored the wavelength-dependent LSPR shift upon the adsorption of some resonant molecules. The induced LSPR peak shifts (Δλmax) vary with wavelength and the line shape of the LSPR shift is closely related to the absorption features of the resonant molecules. When the LSPR of the nanoparticles directly overlaps with the molecular resonance, a very small LSPR shift was observed. An amplified LSPR shift is found when LSPR of the nanoparticles is at a slightly longer wavelength than the molecular resonance of the adsorbates. Furthermore, we apply the "amplified" LSPR shift to detect the substrate binding of camphor to the heme-containing cytochrome P450cam protiens (CYP101). CYP101 absorb light in the visible region. When a small substrate molecule binds to CYP101, the spin state of the molecule is converted to its low spin state. By fabricating nanoparticles with the LSPR close to the molecular resonance of CYP101 proteins, the LSPR response as large as ~60 nm caused by the binding of small substrate has been demonstrated.
The extinction spectra of silver nanoparticle arrays are studied using the couple dipole (CD) method, with emphasis on determined the array pattern and particle spacing which produces the narrowest plasmon resonances. All calculations refer to one and two dimensional arrays of spherical particles having radii 50 nm (or of nonspherical particles with the equivalent effective volume), and only particle spacings much larger than the particle radius are considered so that the dipole approximation is accurate. The narrowest lines in all cases occur when the incident wave vector is perpendicular to the plane of the array while the polarization vector is in the plane and along a symmetry axis which depends on the array structure. We find that the narrowest plasmon bands for square and hexagonal arrays have about the same width (about 100 meV), but that the array spacing for the square array where this occurs is smaller than that for the hexagonal array. The comparison at constant array density is closer. Much smaller widths (20 meV) occur for one dimensional arrays than for two dimensional arrays. For rectangular lattices, we find that the array spacings perpendicular to the polarization vector play a much more important role in determining the plasmon wavelength and width than do spacings parallel to the polarization vector. The evolution of spectra from a two dimensional array to a one dimensional chain is studied by considering rectangular arrays in which one spacing is very large. We find that when the large spacing is 5000 nm or more, the interactions between rows of particles is weak and extinction spectrum has a narrow peak that matches what is seen for the equivalent one dimensional chain.
The optical properties of silver nanoparticle arrays are studied by T-matrix and discrete dipole approximation (DDA) methods. Arrays of spherical silver particles with a radius of 30 nm are investigated with particular emphasis on the influence of array disorder on optical response. We find that the dipole peak intensities decrease when the array becomes disordered and the plasmon resonance wavelengths generally exhibit smaller blue shifts compared to perfectly ordered arrays. Using an extended DDA method, we calculate the extinction spectra of ordered nanodisk arrays, examining the variation of plasmon resonance wavelength with interparticle spacing (at fixed particle size). The calculated blue shift of the plasmon wavelength with decreasing interparticle spacing is found to be in excellent agreement with recent experiments.