Justin Abell

Fabrication and characterization of a multiwell array SERS chip with biological applications.

Uniform, large surface area substrates for surface-enhanced Raman spectroscopy (SERS) are fabricated by oblique angle deposition. The SERS-active substrates are patterned by a polymer-molding technique to provide a uniform array for high throughput biosensing and multiplexing. Using a conventional SERS-active molecule, 1,2-di(4-pyridyl)ethylene (BPE) >or=98%, we show that this device provides a uniform Raman signal enhancement from well to well with a detection limit of at least 10(-8)M of the BPE solution or 10(-18)mol of BPE. The SERS intensity is also demonstrated to vary logarithmically with the log of BPE concentration and the apparent sensitivity of the patterned substrate is compared to previous reports from our group on non-patterned substrates. Avian influenza is analyzed to demonstrate the utility of SERS multiwell patterned substrates for biosensing. The spectra acquired from patterned substrates show better reproducibility and less variation compared to the unpatterned substrates according to multivariate analysis. Our results highlight potential advantages of the patterned substrate.

Label-Free Detection of Micro-RNA Hybridization Using Surface-Enhanced Raman Spectroscopy and Least-Squares Analysis

Label-free surface-enhanced Raman spectroscopy (SERS) detection of nucleic acid hybridization is impeded by poor spectral reproducibility and the fact that the chemical signatures of hybridized and unhybridized sequences are highly similar. To overcome these issues, highly reproducible silver nanorod SERS substrates along with a straightforward least-squares (LS) technique have been employed for the quantitative determination of the relative ratios of the four nucleotide components A, C, G, and T/U before and after hybridization using a clinically relevant micro-RNA sequence.

Gold-modified silver nanorod arrays: growth dynamics and improved SERS properties

Only a few remaining technical hurdles currently prevent the implementation of SERS as a mainstream detection technology. Although oblique-angle deposited silver nanorod arrays provide superior analytical figures of merit for SERS sensing, stability issues of silver surfaces can impede their use for real-world sensing applications within certain environments. To circumvent this issue, silver nanorod arrays are modified with a straight-forward, inexpensive Au-coating via a galvanic replacement reaction. The morphological, structural, compositional, and optical properties of the Au-modified Ag nanorod arrays are studied by multiple ex situ morphological characterization techniques and in situ optical absorbance spectroscopy. Depending on the reaction time, the Au coating experiences five different stages of the morphological and compositional changes. The porosity of the Au layer and the content of Ag decrease with reaction time. The optical measurements show that the representative localized plasmon resonance peak of the nanorod red-shifts as the reaction proceeds. The surface enhanced Raman scattering (SERS) intensity, tested using 4-mercaptophenol, decreases exponentially with reaction time, due to the compositional evolution of the nanostructure from pure Ag to a Au–Ag alloy with increasing Au content. We show that the Au-modified Ag nanorod is very stable in NaCl solution compared to the as-deposited Ag nanorod, and the 20 or 30 minute Au-modified Ag nanorod substrate shows an improved SERS sensitivity for air contamination detection. Such an improved SERS substrate can be used in more hostile environments where a pure Ag nanorod substrate cannot be used, and is good for practical sensing applications.

Ag–SiO2 Core–Shell Nanorod Arrays: Morphological, Optical, SERS, and Wetting Properties

Using the hydrolysis of tetraethylorthosilicate, a uniform and conformal layer of porous SiO(2) with controlled thickness has been coated onto the oblique angle deposited Ag nanorod (AgNR) array to form an aligned AgNR-SiO(2) core-shell array nanostructure. The morphology, optical property, SERS response, and surface wettability of the AgNRs with different SiO(2) shell thicknesses have been obtained by multiple characterization techniques. The morphological characterization shows that each AgNR on the array is coated with a uniform and porous silica shell independently and the growth of shell thickness follows a linear function versus the coating time. Thickening of the shell induces a monotonic decrease of the apparent contact angle, red-shift of the transverse mode of the localized surface plasmon resonance peak, and makes the SiO(2) shell more compact. The SERS response of 4-Mercaptophenol on these substrates exhibits an exponential decay behavior with the increasing coating time, which is ascribed to the decreasing Ag surface coverage of core-shell nanorods. Under the assumption that the Ag surface coverage is proportional to the SERS intensity, one can estimate the evolution of SiO(2) coverage on AgNRs. Such coverage evolution can be used to qualitatively explain the LSPR wavelength change and quantitatively interpret the contact angle change based on a double Cassies law.

Angle-resolved reflectance of obliquely aligned silver nanorods

Arrays of silver nanorods (AgNRs) formed by oblique-angle deposition (OAD) are strongly anisotropic, with either metallic or dielectric characteristics depending on the polarization of incident light, and may be used to enhance Raman scattering and surface plasmon polaritons. This work investigates the polarization-dependent reflectance of inclined AgNR arrays at the wavelengths of 635 and 977 nm. The specular reflectance at various incidence angles and the bidirectional reflectance distribution function were measured with a laser scatterometer, while the directional-hemispherical reflectance was measured with an integrating sphere. The AgNR layer is modeled as an effectively homogenous, optically uniaxial material using the effective medium theory to elucidate the dielectric or metallic response for differently polarized incidence. The thin-film optics formulation is modified considering optical anisotropy and surface scattering. This study helps gain a better understanding of optical properties of nanostructured materials.

Dynamic Rastering Surface-Enhanced Raman Scattering (SERS) Measurements on Silver Nanorod Substrates

Surface-enhanced Raman spectra of a thiol-modified biotin derivative on oblique-angle-deposited silver nanorod (AgNR) array substrates were measured using both static and rotating rastering methods. We find that the rotating rastering method has a strong tendency to decrease the point-to-point relative standard deviation (RSD) compared to static measurements as well as decrease the effects of cumulative excitation exposure. The AgNR substrates treated with the modified biotin typically demonstrate intra-substrate RSDs of <10%, with an average RSD of ∼3% when the rastering radius r=1 mm. The quantitative studies on the relationship between rastering radius, sampling area, and rastering frequency show that only the rastering radius appears to have significant effect on the measured RSD. Our results demonstrate that under the proper measurement and sample preparation conditions, the Ag nanorod substrates are very uniform.

Design and Applications of Nanomaterial-Based and Biomolecule-Based Nanodevices and Nanosensors

This review will highlight recent research underlying the design of novel nanodevices and nanosensors that incorporate graphene, nanodots, nanowires, and biomolecules including DNA aptamers and peptides. The emphasis is on models and theory that guide the design of these nanodevices and nanosensors. In selected cases, research designed to test the usefulness of these designs is highlighted in this chapter.

Anisotropic Diffraction from Inclined Silver Nanorod Arrays on Grating Templates

Inclined silver nanorods (AgNRs) are highly anisotropic, with various applications in energy conversion, plasmonic and photonic devices, and sensing. So far, related studies have focused on the reflection or transmission, whereas studies on the diffraction and scattering from anisotropic AgNRs are still lacking. We investigate the anisotropic radiative properties of novel micro/nanoscale hybrid structures of inclined AgNRs deposited on compact disk (CD) gratings, which were fabricated with an oblique angle deposition technique. The reflectance of each diffraction peak and directional–hemispherical reflectance are characterized with a laser scatterometer and an integrating sphere, respectively, for both polarizations at a wavelength of 635 nm. The ordinary and extraordinary optical constants predicted from the effective medium theory (EMT) are incorporated into a rigorous coupled-wave analysis (RCWA) algorithm to estimate the diffraction efficiencies. The effect of scattering (mainly due to surface roughness) is also examined. The result shows that the orientation of the tilted nanorods with respect to the direction of grooves has a significant effect on both the energy distribution of the diffraction rays and the angular distribution of the scattered light for different polarizations. Such a hybrid structure may offer an alternative solution to engineering the radiative properties of a surface.

Differentiating intrinsic SERS spectra from a mixture by sampling induced composition gradient and independent component analysis

By generating a composition gradient on a highly uniform SERS substrate and applying independent component analysis, we demonstrate that one can extract the intrinsic SERS spectrum of individual components from SERS spectra obtained from a two-component mixture.

Surface-enhanced Raman spectroscopy as a tool for characterizing nanostructures containing molecular components

Raman spectroscopy, which is based on inelastic scattering of light that interacts with phonons or molecular vibrations in the nanostructure, has been widely used to identify chemical and biological molecules. Surface-Enhanced Raman Spectroscopy (SERS) greatly enhanced the sensitivity of conventional Raman spectroscopy by a factor of >106 through the use of a plasmon-generating substrate [1, 2]. This study investigated the use of Raman spectroscopy/SERS to verify that synthesized nanostructures contain active molecular components critical to their functioning. In particular, this study uses SERS to identify the signature spectrum of Methylene Blue (MB) and uses standard Raman spectroscopy to verify the fictionalization of a DNA aptamer terminated with MB.