Donna Hohertz

Improved Performance of Nanohole Surface Plasmon Resonance Sensors by the Integrated Response Method

We examine both experimental and simulated data of the optical transmission response of nanohole arrays in metal films to bulk and surface refractive index changes. We compare the signal-to-noise performance of the following three different analysis methods: the conventional peak shift method, a normalized-difference integrated-response method that is commonly used in 3-D plasmonic crystals, and an integrated response (IR) method. Our IR method shows a 40% and 90% improvement in the signal-to-noise ratio (SNR) for bulk and surface binding tests, respectively, compared with the direct measurement of the transmission-peak wavelength shift, promising improved sensing performance for future nanohole-array sensor applications.

Large-Area Low-Cost Flexible Plastic Nanohole Arrays for Integrated Bio-Chemical Sensing

Detection of plasmonic resonance peak shifts of nano-structured metamaterials is a promising method for sensing bio-chemical binding events. Although the concept is widely demonstrated in the laboratory environment using surface nano-structures machined at low-throughput and high-costs, practical solutions for high-volume production of an integrated sensing device are very limited. We present a concept of an integrated architecture that combines a thin layer of plasmonic nanohole sensing arrays, organic light-emitting diode illumination source, and microfluidic chip, for point-of-care, field, or lab applications. We discuss the fabrication of the sensor components. In particular, we present the improved fabrication of master nano-structure replication stamps, and demonstrate outstanding results for producing singular sheets or scale up to roll-to-roll embossing of nanohole arrays on a 2000 ft production roll. We further demonstrate that nanohole arrays embossed on flexible polyethylene terephthalate plastic sheets, when coated with 100 nm thin Au metal film, are capable of generating average plasmonic resonance shifts of 180 nm refractive index unit. Hence, we report the extraordinary transmission and plasmonic resonance shifts of nanohole arrays fabricated on roll-to-roll plastic sheets for the very first time. Our results show that the embossed nano-structures on plastic are suitable as sensor elements in our proposed integrated sensor architecture, and a promising technology for low-cost disposable applications.

Detecting Antibodies Secreted by Trapped Cells Using Extraordinary Optical Transmission

Isolation of monoclonal (M-) antibodies (Abs) from Ab-secreting cells (ASCs) is impeded by difficulties in identifying the minority of ASCs secreting Ab(s) of interest. We present a novel label-free system that traps ASCs into biocompatible microwells and detects the binding of secreted Ab(s), to a surface-immobilized antigen (Ag), as induced shifts in the extraordinary optical transmission (EOT) spectra through milled nanohole arrays. In this paper, a biotinylated target Ag (Bio-HA and Bio-2F5) is exposed to varying known MAb concentrations (17/9 and 2F5). The mean EOT shift in response to MAb concentration is similar in shape to specific MAb-Ag binding as seen in an ELISA. In another experiment, hybridoma cells secreting 17/9 MAb are trapped in wells inset into poly(dimethyl siloxane) (PDMS) (5-20 cells/trap), to which the nanohole arrays of Bio-HA “Sample” and Bio-2F5 “Ag Control” slides are aligned. Simultaneously, a Bio-HA “Media Control” slide is exposed to MAb-free media. The mean “Sample” shift (3.2 ±0.1 nm) is distinguished from the mean “Ag Control” shift (1.2 ±0.4 nm), but indistinguishable from the mean “Media Control” shift (3.6 ±0.3 nm). We have made significant progress in detecting Ab(s) secreted from trapped ASCs. With further development, our device could identify ASCs of interest rapidly, streamlining therapeutic MAb discovery.

Sensing of antibodies secreted by microfluidically trapped cells via extraordinary optical transmission through nanohole arrays

This work presents a multi-component chip that: (i) traps settling cells into poly(dimethyl siloxane) microwells; and (ii) senses the binding of antibodies to antigen immobilized onto a gold surface via the surface plasmon resonance shifts of milled nanohole arrays. We have previously detected the 17/9 antibodies secreted by ∼200 hybridoma cells/trap. This work demonstrates our systems current capabilities and limitations via a follow-up experiment using a reduced ∼5–20 hybridoma cells/trap density.

Hanle measurements of electrodeposited Fe/GaAs spin tunnel contacts

We report spin transport in electrodeposited Fe/n-GaAs tunnel diodes via three-terminal Hanle measurements. For temperatures between 20 K and 150 K, the spin resistance was up to 20 times higher than expected from theoretical calculations and 1000 times larger compared to a vacuum-deposited counterpart. This higher spin resistance was correlated with a higher contact resistance, and a higher concentration of oxygen impurities in the electrodeposited Fe film and interface, as detected via x-ray photoelectron and Auger spectroscopies, and inferred from Fe film nucleation rates. These results can be explained via a small effective tunnel-contact area of 5%, but extra spin filtering via interfacial states or magnetic oxide layers cannot be ruled out. The spin diffusion times (8.5 ± 0.4 ns to 1.8 ± 0.4 ns, for 20 K to 150 K) extracted from Lorentzian fits were in good agreement with values obtained from earlier 4-terminal Hanle measurements (7.8 ± 0.4 ns to 3.2 ± 0.4 ns, for 25 K to 77 K), both 10 times slower than reported vacuum-deposited contacts.

Optical response of large-area aluminum-coated nano-bucket arrays on flexible PET substrates

The high-cost of fabrication of nanohole arrays for extraordinary optical transmission, surface-plasmon-resonance-based sensors, inhibits their widespread commercial adoption. Production typically involves the application of small-area patterning techniques, such as focused-ion-beam milling, and electron-beam lithography onto high-cost gold-coated substrates. Moving to lower-cost manufacturing is a critical step for applications such as the detection of environmental oil-leaks, or water quality assurance. In these applications, the sensitivity requirements are relatively low, and a bio-compatible inert surface, such as gold, is unnecessary. We report on the optical response of aluminum-coated nano-bucket arrays fabricated on flexible polyethylene terephthalate substrates. The arrays are fabricated using an economical roll-to-roll UV-casting process from large sheets of nickel templates generated from master quartz stamps. The nano-featured surface is subsequently coated with 50 nm of thermally-evaporated aluminum. The roll-to-roll production process has a 97% yield over a 600 m roll producing nano-buckets with 240 nm diameters, 300 nm deep, with a 70° taper. When exposed to a series of refractive index standards (glucose solutions), changes in the locations of the resonance transmission peaks result in optical sensitivities as high as 390 ± 20 nm/RIU. The peak transmission is approximately 5% of illumination, well within the sensitivity requirements of most common low-cost detectors.