Donald Keith Roper

Enhanced Spectral Sensing by Electromagnetic Coupling With Localized Surface Plasmons on Subwavelength Structures

Existing sensor platforms have limited sensitivity, specificity, and portability. With a new algorithm for the coupled dipole approximation of Maxwells equations, we examine near- and far-field features of electromagnetism (EM) coupled with localized surface plasmons on subwavelength, solid-state nanoparticle (NP) structures measured using spectroscopy, microscopy, and calorimetry. Near-field extinction efficiency, blue/redshifts, and full-width at half-maximum are optimized using a new ¿bottom-up¿ NP assembly method that tunes particle size and spacing to enhance sensitivity and produce molecule-specific ¿tenfold surface-enhanced Raman spectroscopy enhancements. Far-field plasmon-photon resonances are identified, which offer ¿ 106-fold higher sensitivity. Solid-state NP structures increase stability, reduce power consumption, and improve response time and optothermal transduction up to tenfold for better portability and throughput relative to aggregation-prone NP suspensions. Sample rate is increased ¿tenfold by inducing transverse hydrodynamic diffusion adjacent to sensor interfaces. These results guide development of next-generation chemical and biological sensors based on EM-coupled UV, Raman, or terahertz modes that improve sensitivity, biospecificity, stability, and portability to distinguish biological molecules and species at high throughputs.

Enhanced Nanoparticle Response From Coupled Dipole Excitation for Plasmon Sensors

Regular lattices of metallic nanoparticles exhibit extraordinary spectral features that arise from electromagnetic coupling between the dipole component of localized surface plasmons and constructive interference from diffracted far-field radiation. The present work introduces this coupled dipole excitation as an additional method to perform refractive index-based sensing using gold nanoparticle arrays. These arrays exhibit an aggregate sensitivity of 31 nm·RIU-1 using the coupled dipole peak in transmission UV-vis spectroscopy. This aggregate sensitivity is in good agreement with values predicted by three models for coupled dipole excitation: an analytical coupled dipole approximation, a discrete dipole approximation, and finite difference time domain. A particle-based sensitivity, S NP , of 389 nm·RIU-1 was determined for a fabricated array. Plasmon sensing based on the coupled dipole excitation in a gold nanoparticle array was possible even when the local surface plasmon signal from individual nanoparticles was indistinguishable from noise. Further increases in sensitivity and signal-to-noise are predicted as coupled dipole excitation parameters are optimized in high-precision fabrication of nanoparticle arrays.

Silver disposition and dynamics during electroless metal thin film synthesis

Morphological and physicochemical disposition of silver (Ag) during redox-driven self-assembly of metal films on silica surfaces under equilibrium hydraulic conditions has been examined in real time in a novel electroless (EL) metal deposition cell by transmission UV-vis (T-UV) spectroscopy. Optical features due to localized surface plasmon resonance, surface plasmon polaritons, and photoluminescence from Ag and gold (Au) nanoarchitectures such as particles, clusters, and films were attributed by correlating T-UV with time-resolved scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Silver that deposited onto tin-sensitized surfaces in thin films nucleated nanoparticles when exposed to reductant or broadband light. Kinetic changes in plasmon features suggested four previously unrecognized time-dependent physicochemical regimes occur during consecutive EL deposition of Ag and Au onto tin-sensitized silica surfaces: self-limiting Ag activation; transitory Ag nanoparticle formation; transitional Au–Ag alloy formation during galvanic replacement of Ag by Au; and uniform metal deposition under controlled hydraulic conditions. Growth mechanisms at the surface, interior, and interface of the resulting thin metal films inferred from real time T-UV spectra were characterized by depth profile XPS analysis.

Spectrophotometric determination of tin(II) by redox reaction using 3,3′,5,5′-tetramethylbenzidine dihydrochloride and N-bromosuccinimide

A rapid, straightforward spectrophotometric method based on the redox reaction of tin(II) with a mixture of N-bromosuccinimide (NBS) and 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB) was developed for determining low concentrations of tin(II). The redox method improved sensitivity by 2.3-fold relative to the existing spectrophotometric methods by titrating tin(II) into an equimolar solution of colorimetric reagent TMB and oxidant NBS buffered with acetate to pH between 4.0 and 4.4 at 25°C. The spectral absorption at 452 nm was linear with respect to tin(II) concentration between the limit of quantitation (LOQ, 10σ) of 0.05 and 0.34 μg/mL over which a response curve was generated (R2 = 0.9981, n = 7). The limit of detection (LOD) was calculated (3σ) at 0.01 μg/mL. Deviation between actual and measured concentrations varied from 0.014 μg/mL near the LOQ to 0.042 at 0.255 μg/mL.

Diatom frustule photonic crystal geometric and optical characterization

Diatom algae are single-celled, photosynthetic organisms with a cell wall called a frustule—a periodically patterned nano-structure made of silica. Throughout the last decade, diatom frustules have been studied for their potential uses as photonic crystals and biomimetic templates for artificially developed metamaterials. A MATLAB program characterizing their pore structure as a function of angle was developed, potentially giving insight into how their geometric characteristics determine their optical properties.

Complex dielectric and geometry influences on plasmon excitation and energy transfer in nanocomposite systems

Distinguishing contributions of physical and optical characteristics, and their interactions, to complicated features observed in spectra of nanocomposite plasmonic systems slows their implementation in optoelectronics. Use of vacuum, effective medium, or analytic approximations to compute such contributions are insufficient outside the visible spectrum (e.g., in energy harvesting) or for interfaces with complex dielectrics (e.g., semiconductors). This work synthesized discrete dipole computation of local physical/optical interaction with coupled dipole approximation of far-field Fano coupling to precisely distinguish effects of locally discontinuous dielectric environment and structural inhomogeneity on complicated spectra from a square lattice of gold nanospheres supported by complex dielectric substrates. Experimental spectra decomposition of resonant energies/bandwidths elucidated indium tin oxide affected surfaced plasmon resonance while silica affected diffractive coupled resonance features. Energy transport during plasmon decay was examined for each substrate under a variety of physical support configurations with the gold nanospheres. The compact, multi-scale approach can be adapted to arbitrary nanoantenna shapes (e.g., nanorings) interacting with various dielectrics (e.g., dichalcogenides). It offers >104-fold reduction in computation time over existing descriptions to accelerate the design and implementation of functional plasmonic systems.