Mikella E. Hankus

Highly Sensitive Surface Enhanced Raman Scattering Substrates Based on Filter Paper Loaded with Plasmonic Nanostructures

We report a novel surface enhanced Raman scattering (SERS) substrate platform based on a common filter paper adsorbed with plasmonic nanostructures that overcomes many of the challenges associated with existing SERS substrates. The paper-based design results in a substrate that combines all of the advantages of conventional rigid and planar SERS substrates in a dynamic flexible scaffolding format. In this paper, we discuss the fabrication, physical characterization, and SERS activity of our novel substrates using nonresonant analytes. The SERS substrate was found to be highly sensitive, robust, and amiable to several different environments and target analytes. It is also cost-efficient and demonstrates high sample collection efficiency and does not require complex fabrication methodologies. The paper substrate has high sensitivity (0.5 nM trans-1,2-bis(4-pyridyl)ethene (BPE)) and excellent reproducibility (~15% relative standard deviation (RSD)). The paper substrates demonstrated here establish a novel platform for integrating SERS with already existing analytical techniques such as chromatography and microfluidics, imparting chemical specificity to these techniques.

SERS based immuno-microwell arrays for multiplexed detection of foodborne pathogenic bacteria

A novel surface enhanced Raman scattering (SERS)-based immuno-microwell array has been developed for multiplexed detection of foodborne pathogenic bacteria. The immuno-microwell array was prepared by immobilizing the optical addressable immunomagnetic beads (IMB) into the microwell array on one end of a fiber optic bundle. The IMBs, magnetic beads coated with specific antibody to specific bacteria, were used for immunomagnetic separation (IMS) of corresponding bacteria. The magnetic separation by the homemade magnetic separation system was evaluated in terms of the influences of several important parameters including the beads concentration, the sample volume and the separation time. IMS separation efficiency of the model bacteria E.coli O157:H7 was 63% in 3 minutes. The microwell array was fabricated on hydrofluoric acid etched end of a fiber optic bundle containing 30,000 fiber elements. After being coated with silver, the microwell array was used as a uniform SERS substrate with the relative standard deviation of the SERS enhancement across the microwell array < 2% and the enhancement factor as high as 2.18 x 107. The antibody modified microwell array was prepared for bacteria immobilization into the microwell array, which was characterized by a sandwich immunoassay. To demonstrate the potential of multiplexed SERS detection with the immuno-microwell array, the SERS spectra of different Raman dye labeled magnetic beads as well as mixtures were measured on the mircrowell array. In bead mixture, different beads were identified by the characteristic SERS bands of the corresponding Raman label.

SERS nano-imaging probes for characterizing extracellular surfaces

Across extracellular surfaces, lipid rafts are believed to be an important organizing membrane microdomain component, facilitating specific protein-protein interactions by selectively excluding or including proteins into them. The lipid-based sorting mechanism of these microdomains has been implicated in many cellular processes including; membrane trafficking, signal transduction and cell growth regulation. However, since individual rafts are estimated to range in size from the nanoscale to the microscale, in many cases, they cannot be easily monitored by conventional imaging techniques. We have developed surface enhanced Raman scattering (SERS)-based nanoimaging probes for nanoscale imaging of biochemical species on and within extracellular environments. These probes synergistically combine the qualitative and quantitative information of SERS with the nanoscale imaging capabilities of tapered fiber optic bundles, potentially allowing for chemical imaging of extracellular components and chemical exchange events across cellular surfaces. These probes are fabricated from coherent fiber optic bundles containing 30,000 individual fiber elements that have been tapered to have diameters as small as 140 nm, thus allowing for image magnification and submicron spatial resolution. Due to the uniformly roughened surface features across the probes imaging surface onto which silver island arrays are fabricated, these probes exhibit less than 3% RSD in SERS signal across the imaging area. In this work, tunability, multiplex detection capabilities and an application of these SERS nanoimaging probes to biological systems are demonstrated.

A nanoscale probe for dynamic-chemical imaging

Abstract : Sight has long been one of mankind s most relied-upon senses for attempting to understand the natural world. However, the optical diffraction limit of visible light ordinarily limits the spatial resolution of conventional imaging techniques, such as white-light microscopy, to approximately 250 300nm. This fundamentally restricts our ability to visualize objects at the nanometer scale while retaining spectroscopic (i.e., molecular or atomic) information about the sample. A number of imaging techniques confocal fluorescence and scanning-probe microscopy, as well as others1 3 have been developed to overcome this limitation. While these methods have been powerful tools for studying the nanoscale world, they have their own limitations. They are generally restricted to imaging fluorescent or fluorescently labeled samples, or they require lengthy imaging times hours or days to produce high-quality images of microscopic areas. These long imaging times are driven by the need to move a tiny probe tip over all parts of a microscopic specimen.