Optimization of laser deposited silver nanoparticle substrates for surface-enhanced raman spectroscopy.

Creating sensitive and reproducible substrates for surface-enhanced Raman spectroscopy (SERS) has been a challenge in recent years. While SERS offers significant benefits over traditional Raman spectroscopy, certain hindrances have limited their commercial use, especially in settings where low limits of detection are necessary. We studied a variety of laser-deposited silver microstructured SERS substrates with different morphology as a means to optimize analyte detection. We found that using a 405 nm laser to deposit lines of silver nanoparticles (AgNPS) from a 2 mM silver nitrate and sodium citrate solution offered not only the best enhancement, but also the most consistent and reproducible substrates. We also found that the probability of deposition by laser was wavelength dependent and that longer wavelengths were less likely to deposit than shorter wavelengths. This work offers a better understanding of the laser deposition process as well as how substrate shape and structure effect SERS signals.

Particle-substrate interactions in the laser deposition process.

We theoretically study particle-substrate interactions under laser irradiation. Van der Waals, electrostatic double layer and a laser induced dipole in the nanoparticle and an image dipole in the substrate were considered to be the major components of the total interaction potential. It was shown that laser-induced attractive potential energy between the particle and substrate reduces the potential barrier which increases the probability for metal nanoparticles to be deposited onto the substrate.

Laser-Fabricated Plasmonic Nanostructures for Surface-Enhanced Raman Spectroscopy of Bacteria Quorum Sensing Molecules.

We used a laser-directed fabrication to create silver nanostructures on glass cover slips via photo-reduction. The resulting silver films exhibited plasmonic properties which show promise in application towards surface enhanced Raman spectroscopy (SERS). The enhancement factor calculated for the deposits was approximately ~106 using the standard thiophenol, which is comparable to other SERS-active plasmonic nanostructures fabricated through more complex techniques, such as electron beam lithography. The silver nanostructures were then employed in the enhancement of Raman signals from N-butyryl-L-homoserine lactone, a signaling molecule relevant to bacteria quorum sensing. In particular, the work presented here shows that the laser-deposited plasmonic nanostructures are promising candidates for monitoring concentrations of signaling molecules within biofilms containing quorum sensing bacteria.

Angle dependent collective surface plasmon resonance in an array of silver nanoparticles.

Theoretical analysis of the scattering efficiency of an equidistantly spaced regular array of spherical silver nanoparticles reveals a nonmonotonic shift of the collective SPR wavelength and its bandwidth depending on the distance between the particles and the angle of the incidence of the linear polarized electromagnetic wave. The far-field electromagnetic coupling between the particles in the chain exhibits the largest range of angular tuning of the collective SPR band when the distance between the particles in the chain approaches that of the collective SPR wavelength. The dependence of the SPR wavelength and its bandwidth on the angle of the incidence of the linear polarized electromagnetic wave and the distance between the particles in the chain provides an additional flexibility for the development of optical biochemical sensors and subwavelength waveguides.

Methods for describing the electromagnetic properties of silver and gold nanoparticles.

This Account provides an overview of the methods that are currently being used to study the electromagnetics of silver and gold nanoparticles, with an emphasis on the determination of extinction and surface-enhanced Raman scattering (SERS) spectra. These methods have proven to be immensely useful in recent years for interpreting a wide range of nanoscience experiments and providing the capability to describe optical properties of particles up to several hundred nanometers in dimension, including arbitrary particle structures and complex dielectric environments (adsorbed layers of molecules, nearby metal films, and other particles). While some of the methods date back to Mie's celebrated work a century ago, others are still at the forefront of algorithm development in computational electromagnetics. This Account gives a qualitative description of the physical and mathematical basis behind the most commonly used methods, including both analytical and numerical methods, as well as representative results of applications that are relevant to current experiments. The analytical methods that we discuss are either derived from Mie theory for spheres or from the quasistatic (Gans) model as applied to spheres and spheroids. In this discussion, we describe the use of Mie theory to determine electromagnetic contributions to SERS enhancements that include for retarded dipole emission effects, and the use of the quasistatic approximation for spheroidal particles interacting with dye adsorbate layers. The numerical methods include the discrete dipole approximation (DDA), the finite difference time domain (FDTD) method, and the finite element method (FEM) based on Whitney forms. We discuss applications such as using DDA to describe the interaction of two gold disks to define electromagnetic hot spots, FDTD for light interacting with metal wires that go from particle-like plasmonic response to the film-like transmission as wire dimension is varied, and FEM studies of electromagnetic fields near cubic particles.

Metamaterials with gradient negative index of refraction.

We propose a new metamaterial with a gradient negative index of refraction, which can focus a collimated beam of light coming from a distant object. A slab of the negative refractive index metamaterial has a focal length that can be tuned by changing the gradient of the negative refractive index. A thin metal film pierced with holes of appropriate size or spacing between them can be used as a metamaterial with the gradient negative index of refraction. We use finite-difference time-domain calculations to show the focusing of a plane electromagnetic wave passing through a system of equidistantly spaced holes in a metal slab with decreasing diameters toward the edges of the slab.

Electrostatic aggregation and formation of core-shell suprastructures in binary mixtures of charged metal nanoparticles.

Electrostatic aggregation of oppositely charged silver and gold nanoparticles leads to the formation of core-shell clusters in which the shell is formed by the nanoparticles, which are in excess. Arguments based on Debye screening of interactions between like-charged particles help explain why these clusters are stable despite possessing net electric charge. The core-shell aggregates exhibit unusual optical properties with the resonance absorption of the shell particles enhanced by the particles in the core and that of the core suppressed by the shell. Experimental UV-vis absorption spectra are faithfully reproduced by Mie theory. The modeling allows for estimation of the numbers of particles forming the shell and of the shell's effective thickness. These theoretical predictions are substantiated by experiments using nanoparticles covered with different combinations of charged groups and performed at different values of pH.