Shengli Zou

A Nanoscale Optical Biosensor : The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles

Silver and gold nanotriangles were fabricated by nanosphere lithography (NSL) and their localized surface plasmon resonance (LSPR) spectra were measured by UV -vis extinction spectroscopy. It is demonstrated that the short range (viz., 0 -2 nm) distance dependence of the electromagnetic fields that surround these nanoparticles when resonantly excited can be systematically tuned by changing their size, structure, and composition. This is accomplished by measuring the shift in the peak wavelength, λmax, of their LSPR spectra caused by the adsorption of hexadecanethiol as a function of nanoparticle size (in-plane width, out-of-plane height, and aspect ratio), shape (truncated tetrahedron versus hemisphere), and composition (silver versus gold). We find that the hexadecanethiol-induced LSPR shift for Ag triangles decreases when in-plane width is increased at fixed out-of-plane height or when height is increased at fixed width. These trends are the opposite to what was seen in an earlier study of the long range distance dependence in which 30 nm thick layers were examined (Haes et al. J. Phys. Chem. B2004, 108, 109), but both the long and short range results are confirmed by a theoretical analysis based on finite element electrodynamics. The theory results also indicate that the short range results are primarily sensitive to hot spots (regions of high induced electric field) near the tips of the triangles, so this provides an example where enhanced local fields play an important role in extinction spectra. Our measurements further show that the hexadecanethiol-induced LSPR peak shift is larger for nanotriangles than for hemispheres with equal volumes and is larger for Ag nanotriangles than for Au nanotriangles with the same in-plane widths and out-of-plane heights. The dependence of the alkanethiolinduced LSPR peak shift on chain length for Ag nanotriangles is approximately size-independent. We anticipate that the improved understanding of the short range dependence of the adsorbate-induced LSPR peak shift on nanoparticle structure and composition reported here will translate to significant improvements in the sensitivity of refractive-index-based nanoparticle nanosensors.

Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes

Using electrodynamics calculations, we have discovered one dimensional array structures built from spherical silver nanoparticles that produce remarkably narrow ( approximately meV or less) plasmon resonance spectra upon irradiation with light that is polarized perpendicular to the array axis. The narrow lines require a minimum particle radius of about 30 nm to achieve. Variations of the plasmon resonance wavelength, extinction efficiency and width with particle size, array structure, interparticle distance and polarization direction are examined, and conditions which lead to the smallest widths are demonstrated. A simple analytical expression valid for infinite lattices shows that the sharp resonance arises from cancellation between the single particle width and the imaginary part of the radiative dipolar interaction.

Designing, fabricating, and imaging Raman hot spots

We have developed a probe of the electromagnetic mechanism of surface-enhanced Raman scattering via Au nanodisk arrays generated by using on-wire lithography. In this approach, disk thickness and interparticle gap are precisely controlled from 5 nm to many micrometers. Confocal Raman microscopy demonstrates that disk thickness and gap play a crucial role in determining surface-enhanced Raman scattering intensities. Theoretical calculations also demonstrate that these results are consistent with the electromagnetic mechanism, including the surprising result that the largest enhancement does not occur for the smallest gaps.

Narrow plasmonic/photonic extinction and scattering line shapes for one and two dimensional silver nanoparticle arrays

The interaction of light with silver nanoparticle arrays can in some cases produce mixed plasmonic/photonic bands that have extremely narrow (<1 meV) line shapes in extinction and scattering. In this paper we extend computational electrodynamics results of a recent communication [S. Zou, N. Janel, and G. C. Schatz, J. Chem. Phys. 120, 10871 (2004)] where this effect was first described to study how these narrow bands are influenced by a number of structural factors, and to determine how useful these arrays might be for sensing applications. Included are studies of the effect of disorder in the array structure on plasmon intensity and width, of the effect of orientation of the array relative to the polarization and propagation direction of the incident light, and of the effect of particle shape (comparing results for silver spheres and cylindrical disks). Our results show that the narrow lines are remarkably robust to array disorder, but vacancy defects can easily destroy the effect. The narrowest lines are associated with one dimensional arrays in which both polarization and wave vectors are perpendicular to the array axis. For two dimensional arrays, the narrowest lines are associated with the wave vector perpendicular to the plane of the array and polarization in the plane. Arrays composed of oblate cylinders generate more intense and more redshifted plasmon/photonic peaks than do prolate or spherical particles under comparable conditions. Finally, for sensing applications in which analyte binding is determined by the plasmon wavelength shift associated with change in the surface refractive index, we show that the arrays have greater sensitivity than isolated nanoparticles.

DNA Directed Self-Assembly of Anisotropic Plasmonic Nanostructures

Programmable positioning of one-dimensional (1D) gold nanorods (AuNRs) was achieved by DNA directed self-assembly. AuNR dimer structures with various predetermined inter-rod angles and relative distances were constructed with high efficiency. These discrete anisotropic metallic nanostructures exhibit unique plasmonic properties, as measured experimentally and simulated by the discrete dipole approximation method.

Distance-dependent interactions between gold nanoparticles and fluorescent molecules with DNA as tunable spacers

Using stoichiometrically controlled 1:1 functionalization of gold nanoparticles with fluorescent dye molecules in which the dye molecule is held away from the particle surface by a rigid DNA spacer allows precise determination of the distance-dependent effect of the metal nanoparticles on fluorescence intensity. Two dyes were studied, Cy3 and Cy5, with two sizes of nanoparticles, 5 and 10 nm. The larger the particle, the more quenching of the photoluminescence (PL) intensity, due to increased overlap of the dyes emission spectrum with the Au surface plasmon resonance. Fluorescence is quenched significantly for distances somewhat larger than the particle diameter, in good agreement with the predictions of an electrodynamics model based on interacting dipoles. The distance dependence of surface energy transfer behavior, i.e. quenching efficiency, is proportional to 1/d(4), which involves no consideration of the size of the particle and the spectral overlap of the dye and AuNp. This surface energy transfer model is found qualitatively and agrees with the electrodynamic model, though the exponent is greater than 4 for the smaller nanoparticles (5 nm), and smaller than 4 for the larger nanoparticles (10 nm).

Removal of Molecular Adsorbates on Gold Nanoparticles Using Sodium Borohydride in Water

The mechanism of sodium borohydride removal of organothiols from gold nanoparticles (AuNPs) was studied using an experimental investigation and computational modeling. Organothiols and other AuNP surface adsorbates such as thiophene, adenine, rhodamine, small anions (Br(-) and I(-)), and a polymer (PVP, poly(N-vinylpyrrolidone)) can all be rapidly and completely removed from the AuNP surfaces. A computational study showed that hydride derived from sodium borohydride has a higher binding affinity to AuNPs than organothiols. Thus, it can displace organothiols and all the other adsorbates tested from AuNPs. Sodium borohydride may be used as a hazard-free, general-purpose detergent that should find utility in a variety of AuNP applications including catalysis, biosensing, surface enhanced Raman spectroscopy, and AuNP recycle and reuse.

Fluorescence Quenching of Quantum Dots by Gold Nanoparticles: A Potential Long Range Spectroscopic Ruler

The dependence of quantum dot (QD) fluorescence emission on the proximity of 30 nm gold nanoparticles (AuNPs) was studied with controlled interparticle distances ranging from 15 to 70 nm. This was achieved by coassembling DNA-conjugated QDs and AuNPs in a 1:1 ratio at precise positions on a triangular-shaped DNA origami platform. A profound, long-range quenching of the photoluminescence intensity of the QDs was observed. A combination of static and time-resolved fluorescence measurements suggests that the quenching is due to an increase in the nonradiative decay rate of QD emission. Unlike FRET, the energy transfer is inversely proportional to the 2.7th power of the distance between nanoparticles with half quenching at ∼28 nm. This long-range quenching phenomena may be useful for developing extended spectroscopic rulers in the future.

Theoretical studies of plasmon resonances in one-dimensional nanoparticle chains : narrow lineshapes with tunable widths

In this paper we describe a new configuration for producing narrow extinction lineshapes for light scattering from one-dimensional arrays of silver nanoparticles. In this configuration, which is specifically concerned with an array with a finite number of relatively large (radius greater than around 30 nm) nanoparticles, the wavevector of the light is chosen to be parallel to the array axis, while the polarization direction is perpendicular to the array axis. This leads to narrow plasmon/photonic lineshapes when the particle spacing is half the incident wavelength. This effect stands in contrast to the narrow lines previously found for wavevector and polarization vector perpendicular to the array axis, where the optimum spacing is close to the wavelength. The results are rationalized using a semi-analytical evaluation of the coupled dipole interaction, and it is demonstrated that the parallel and perpendicular chains have very different dependence on the number of particles in the chain. Results as a function of chain orientation relative to the wavevector are also considered, as is the possibility of sensing using an array configuration that combines the parallel and perpendicular chain directions.

A new ab initio potential energy surface describing acetylene/vinylidene isomerization

Abstract A new potential energy surface for C 2 H 2 that describes acetylene/vinylidene isomerization is reported. The surface is an accurate, least-squares fit to nearly 10,000 symmetry-equivalent, ab initio electronic calculations obtained at the CCSD(T) level of theory, with an aug-cc-pVTZ basis. The ab initio geometries and normal-mode frequencies of the acetylene and vinylidene minima, and saddle point are reproduced very well by the fitted potential energy surface. Full-dimensional calculations of low-lying acetylene vibrational energies are also reported and compared to experiment.