Charles M. Hosten

Plasmonic nanopillar arrays for large-area, high-enhancement surface-enhanced Raman scattering sensors.

Efforts to create reproducible surface-enhanced Raman scattering (SERS)-based chemical and biological sensors has been hindered by difficulties in fabricating large-area SERS-active substrates with a uniform, reproducible SERS response that still provides sufficient enhancement for easy detection. Here we report on periodic arrays of Au-capped, vertically aligned silicon nanopillars that are embedded in a Au plane upon a Si substrate. We illustrate that these arrays are ideal for use as SERS sensor templates, in that they provide large, uniform and reproducible average enhancement factors up to ∼1.2 × 10(8) over the structure surface area. We discuss the impact of the overall geometry of the structures upon the SERS response at 532, 633, and 785 nm incident laser wavelengths. Calculations of the electromagnetic field distributions and intensities within such structures were performed and both the wavelength dependence of the predicted SERS response and the field distribution within the nanopillar structure are discussed and support the experimental results we report.

The Role of Propagating and Localized Surface Plasmons for SERS Enhancement in Periodic Nanostructures

Periodic arrays of plasmonic nanopillars have been shown to provide large, uniform surface-enhanced Raman scattering (SERS) enhancements. We show that these enhancements are the result of the combined impact of localized and propagating surface plasmon modes within the plasmonic architecture. Here, arrays of periodically arranged silicon nanopillars of varying sizes and interpillar gaps were fabricated to enable the exploration of the SERS response from two different structures; one featuring only localized surface plasmon (LSP) modes and the other featuring LSP and propagating (PSP) modes. It is shown that the LSP modes determine the optimal architecture, and thereby determine the optimum diameter for the structures at a given incident. However, the increase in the SERS enhancement factor for a system in which LSP and PSP cooperatively interact was measured to be over an order of magnitude higher and the peak in the diameter dependence was significantly broadened, thus, such structures not only provide larger enhancement factors but are also more forgiving of lithographic variations.

Investigation of chemically modified barium titanate beads as surface-enhanced Raman scattering (SERS) active substrates for the detection of benzene thiol, 1,2-benzene dithiol, and rhodamine 6G.

SERS active surfaces were prepared by depositing silver films using Tollens reaction on to barium titanate beads. The SERS activity of the resulting surfaces was probed using two thiols (benzene thiol and 1,2-benzene dithiol) and rhodamine 6G. The intensity of the SERS signal for the three analytes was investigated as a function of silver deposition time. The results indicate that the SERS intensity increased with increasing thickness of the silver film until a maximum signal intensity was achieved; additional silver deposition resulted in a decrease in the SERS intensity for all of the studied molecules. SEM measurement of the Ag coated barium titanate beads, as a function of silver deposition time, indicate that maximum SERS intensity corresponded with the formation of atomic scale islands of silver nanoparticles. Complete silver coverage of the beads resulted in a decreased SERS signal and the most intense SERS signals were observed at deposition times of 30 min for the thiols and 20 min for rhodamine 6G.

Normal‐mode analysis of the Raman‐active modes of the anti‐tumor agent 6‐mercaptopurine

The Raman spectra of the antitumor agent 6-mercaptopurine (6MP) in the solid and solution state were recorded in the 600–1700 cm−1 spectral region. The normal modes of 6MP were calculated using a Urey–Bradley empirical force field (UBFF) and with the PM3 semi-empirical method. Prior to performing the normal-mode calculations on 6MP, similar calculations were performed on methanethiol and thiophenol. The force constant parameters obtained from these calculations were then transferred to the 6MP calculations. The results of the normal-mode calculations were compared with the observed spectra and an assignment of the vibrational modes is proposed. Based on changes in the wavenumbers of the Raman spectra between the solution and solid form 6MP and on normal-mode calculations, it is proposed that in the solid form 6MP crystallizes in the tautomer form whereas in solution the molecule ionizes by losing the hydrogen at the N-1 position. Copyright

Determination of the orientation of azathioprine adsorbed on a silver electrode by SERS and ab initio calculations

We report the SERS spectrum of azthioprine (AZA) on a silver electrode surface and the results of normal mode calculations using empirical and ab initio calculations of the 6-mercaptopurione (6-MP) component of AZA. The empirical calculations were done with a Urey-Bradley force field (UBFF) and the ab initio calculations with the STO-3G basis set using the UHF, MP2 and BLYP methods. From the difference between the SERS and solid spectra, we determined that AZA attaches edge-on to the surface through the N3 site on the 6-MP component of the molecule. The UBFF calculation on an Ag adatom-molecule model reproduced most of the main observed frequency shifts in the SERS spectrum. With a similar model, the ab initio calculations yielded frequency shifts in the same direction as the one observed for the in-plane normal modes, but they yielded opposite shifts for the out-of-plane normal modes. This phenomenon may be attributed to a face-on interaction of the 6-MP component with a neighboring adatom made possible by an inclination of the molecule on the surface.

Small-Molecule Detection in Thiol–Yne Nanocomposites via Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is generally performed on planar surfaces, which can be difficult to prepare and may limit the interaction of the sensing surface with targets in large volume samples. We propose that nanocomposite materials can be configured that both include SERS probes and provide a high surface area-to-volume format, i.e., fibers. Thiol-yne nanocomposite films and fibers were fabricated using exposure to long-wave ultraviolet light after the inclusion of gold nanoparticles (AuNPs) functionalized with thiophenol. A SERS response was observed that was proportional to the aggregation of the AuNPs within the polymers and the amount of thiophenol present. Overall, this proof-of-concept fabrication of SERS active polymers indicated that thiol-yne nanocomposites may be useful as durable film or fiber SERS probes. Properties of the nanocomposites were evaluated using various techniques including UV-vis spectroscopy, μ-Raman spectroscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and transmission electron microscopy.

Bis(1,10-phenanthroline-5,6-dione-κ2N,N′)silver(I) tetra­fluoridoborate

In the structure of the title compound, [Ag(C12H6N2O2)2]BF4 or [AgL 2]BF4 (L = phendione), the Ag and B atoms are located on twofold rotation axes. The dihedral angle between the two phendione ligands is 36.7 (2)°. The coordination about the AgI center is distorted tetrahedral (τ4 = 0.546). The crystal structure is consolidated by weak C—H⋯O(phendione) and C—H⋯F(BF4 −) interactions. The BF4 − counter-anion is strongly disordered and was modelled with two sets of idealized F atoms.

A surface-enhanced Raman study of N-methylquinolinium tricyanoquinodimethanide adsorbed on Ag nanospheres: Determination of molecular orientation and order

Quinolinium tricyanoquinodimethanides are among the most promising molecules for electronic applications. Disorder can be detrimental to the desired electronic properties of a monolayer, and as such, a reliable method to characterize a monolayer without destroying or creating defects is paramount to determining potential applications. Here, the normal and surface-enhanced Raman scattering spectra of N-methylquinolinium tricyanoquinodimethanide (CH₃Q-3CNQ) on silver coated nanosurfaces have been obtained and analyzed. Theoretical treatment of CH₃Q-3CNQ was performed. Optimization and frequency search was conducted using the B3LYP functional with the 6-31G(d) basis set. A complete list of frequencies and assignments for the molecules are presented. The spectroscopic evidence points to the fact that a monolayer of CH₃Q-3CNQ can be formed through the self-assembly process, and the SERS data indicate that the monolayer attaches to the silver surface through the nitrile groups.

Determination of molecular orientation and order of N-(6-Mercaptoacetylhexyl)quinolinium tricyanoquinodimethanide adsorbed on Ag nanoparticles

The surface-enhanced and tip-enhanced Raman scattering spectra of N-(6-Mercaptoacetylhexyl)quinolinium tricyanoquinodimethanides on silver coated nanosurfaces have been obtained, analyzed using Density Functional Theory Calculations, and a complete list of frequencies and assignments for the molecules are presented. The spectroscopic evidence points to the fact that monolayers of the molecule can be formed through the self-assembly process and the SERS data indicate that the monolayer attach to the silver surface through the nitrile groups. SERS spectroscopy was useful in determining the orientation of the monolayer as well as estimating its order. Deprotection the thiol group thereby terminating the tail of the molecule with a sulfur atom allowed for a selectively oriented monolayer to be formed which permanently bound the molecules to the surface preventing rearrangements. This orientation of AcSC6H12Q-3CNQ on silver a surface allowed the electron pairs of the nitrogen to be available for interaction with a second contact. Based on trigonometric tangent function calculations the tilt angle was calculated to be 38° for the protected molecule and 70° for the deprotected alkane thiol monolayer.

Surface-enhanced Raman scattering of a Ag/oligo(phenyleneethynylene)/Ag sandwich.

α,ω-Dithiols are a useful class of compounds in molecular electronics because of their ability to easily adsorb to two metal surfaces, producing a molecular junction. We have prepared Ag nanosphere/oligo(phenyleneethynylene)/Ag sol (AgNS/OPE/Ag sol) and Ag nanowire/oligo(phenyleneethynylene)/Ag sol (AgNW/OPE/Ag sol) sandwiches to simulate the architecture of a molecular electronic device. This was achieved by self-assembly of OPE on the silver nanosurface, deprotection of the terminal sulfur, and deposition of Ag sol atop the monolayer. These sandwiches were then characterized by surface-enhanced Raman scattering (SERS) spectroscopy. The resulting spectra were compared to the bulk spectrum of the dimer and to the Ag nanosurface/OPE SERS spectra. The intensities of the SERS spectra in both systems exhibit a strong dependence on Ag deposition time and the results are also suggestive of intense interparticle coupling of the electromagnetic fields in both the AgNW/OPE/Ag and the AgNS/OPE/Ag systems. Three previously unobserved bands (1219, 1234, 2037 cm(-1)) arose in the SER spectra of the sandwiches and their presence is attributed to the strong enhancement of the electromagnetic field which is predicted from the COSMOL computational package. The 544 cm(-1) disulfide bond which is observed in the spectrum of solid OPE but is absent in the AgNS/OPE/Ag and AgNW/OPE/Ag spectra is indicative of chemisorption of OPE to the nanoparticles through oxidative dissociation of the disulfide bond.