Cherian J. Mathai

Experimental realization of epsilon-near-zero metamaterial slabs with metal-dielectric multilayers

Epsilon-near-zero (ENZ) metamaterial slabs at visible frequencies based on metal-dielectric multilayers are experimentally realized. Transmission, reflection, and absorption spectra are measured and used to determine the complex refractive indices and the effective permittivities of the ENZ slabs, which agree with the results obtained from both the numerical simulations and the optical nonlocalities analysis. Furthermore, light propagation in ENZ slabs and directional emission from ENZ prisms are also analyzed. The accurate determination of the ENZ wavelength for metal-dielectric multilayer metamaterial slabs is important for realizing many unique applications, such as phase front manipulation and enhancement of photonic density of states.

Sub-2 nm size and density tunable platinum nanoparticles using room temperature tilted-target sputtering

This paper describes a tilted-target RF magnetron sputter deposition system to grow nanoparticles in a controlled way. With detailed characterization of ultra-high density (up to 1.1 × 10¹³ cm⁻²) and ultra-small size Pt nanoparticles (0.5-2 nm), it explains their growth and crystalline properties on amorphous Al₂O₃ thin films. It is shown that Pt nanoparticle size and number density can be precisely engineered by varying selected experimental parameters such as target angle, sputtering power and time of deposition to control the energy of the metal atoms in the deposition flux. Based on rate equation modelling of nanoparticle growth, three distinct growth regimes, namely nucleation dependent, coalescence dependent and agglomeration dependent regimes, were observed. The correlation between different nanoparticle growth regimes and the consequent crystal structure transformation, non-crystalline clusters → single crystalline nanoparticles → polycrystalline islands, is also discussed.

Transparent electrode materials for simultaneous amperometric detection of exocytosis and fluorescence microscopy.

We have developed and tested transparent microelectrode arrays capable of simultaneous amperometric measurement of oxidizable molecules and fluorescence imaging through the electrodes. Surface patterned microelectrodes were fabricated from three different conducting materials: Indium-tin-oxide (ITO), nitrogen-doped diamond-like carbon (DLC) deposited on top of ITO, or very thin (12-17 nm) gold films on glass substrates. Chromaffin cells loaded with lysotracker green or acridine orange dye were placed atop the electrodes and vesicle fluorescence imaged with total internal reflection fluorescence (TIRF) microscopy while catecholamine release from single vesicles was measured as amperometric spikes with the surface patterned electrodes. Electrodes fabricated from all three materials were capable of detecting amperometric signals with high resolution. Unexpectedly, amperometric spikes recorded with ITO electrodes had only about half the amplitude and about half as much charge as those detected with DLC or gold electrodes, indicating that the ITO electrodes are not as sensitive as gold or DLC electrodes for measurement of quantal catecholamine release. The lower sensitivity of ITO electrodes was confirmed by chronoamperometry measurements comparing the currents in the presence of different analytes with the different electrode materials.

Influence of silver grain size, roughness, and profile on the extraordinary fluorescence enhancement capabilities of grating coupled surface plasmon resonance

Since the development of fluoroimmunoassays, researchers have sought a method of substantially enhancing fluorescence intensity to extend the limits of detection to new levels of sensitivity. Surface plasmon resonance (SPR) and metal enhanced fluorescence has long been a topic of research and has led to the development of prism- and grating-based SPR systems. However, with the wide coupling range and ease of exciting SPR on plasmonic gratings with a simple microscope objective, they have tremendous potential for revolutionizing the fields of plasmonics, fluorescence, and sensors. In an effort to better understand the influence of grating profile and metal film properties on the extraordinary fluorescence enhancement capabilities of plasmonic gratings, a novel microcontact printing process and different metal deposition techniques were used to fabricate silver gratings with varying grain diameters, roughnesses, heights, and duty cycles using thermal evaporation and RF sputtering. The resulting plasmonic gratings exhibited fluorescence enhancements up to 116× that of dye-coated glass slides using an epifluorescence microscope, much higher than more expensive prism-based SPR systems. This silver grating represents an extraordinary opportunity to quickly and easily enhance fluorescence and widen the detection limits of common fluorescence based assays with little to no equipment modification.

Experimental characterization of optical nonlocality in metal-dielectric multilayer metamaterials

The optical nonlocalities in metal-dielectric multilayer metamaterials are characterized as functions of incident angles for different polarizations. The measured epsilon-near-zero frequency shifts due to nonlocal effects agree with the theoretical analysis developed from transfer-matrix method.

Enhanced fluorescence through the incorporation of nanocones/gaps into a plasmonic gratings sensor platform

In this article, a novel plasmonic grating sensor platform was developed and tested for feasibility in sensor applications using a “lights-on” fluorescence based DNA sensor. The sensor platform combined the fluorescence enhancement of a grating-based plasmonic platform with the electric field intensifying effects of nano-scale cones and cavities. The gratings were made through a microcontact printing process that replicated HD-DVD discs in polymethylsilsesquioxane (PMSSQ) and coated in a thin gold film. Nanocavities were incorporated into the sensor platform during the printing process and nanocones were incorporated during the 100 nm gold deposition process. Fluorescently-tagged single-strand (SS) DNA molecules were immobilized onto the surface and were designed such that the molecules would fluoresce when bound to a complementary sequence. Sensor substrates were imaged after exposure to a mismatched and matched oligomer to quantify the fluorescence enhancement of the sensor. Much higher fluorescence intensity was observed on all of the plasmonic structures as compared to flat gold.

Single-Molecule Surface Plasmon-Coupled Emission with Plasmonic Gratings

The ability to image single molecules (SM) has been the dream of scientists for centuries, and because of the substantial recent advances in microscopy, individual fluorescent molecules can now be observed on a regular basis. However, the development of such imaging systems was not without dilemmas, such as the detection and separation of individual fluorescence emissions. One method to solve this problem utilized surface plasmon resonance (SPR) to enhance the emission intensity of SMs. Although enhancing the SM emission intensity has yielded promising results, this method does not fully utilize the unique plasmonic properties that could vastly improve the SM imaging capabilities. Here, we use SPR excitation as well as surface plasmon-coupled emission from a high-definition digital versatile disc grating structure to image and identify different fluorophores using the angular emission of individual molecules. Our results have important implications for research in multiplexed SM spectroscopy and SM fluorescence imaging.

Highly sensitive plasmonic grating platform for the detection of a wide range of infectious diseases

We report the cost-effective fabrication of a plasmonic grating for improved light coupling in a fluorescence-based sensor platform by a simple micro-contact printing technique. The fluorescence of Rhodamine 6G (R6G) film on gratings was enhanced by up to 239-fold with respect to glass using a fluorescence microplate reader. The silver gratings are made suitable for use in biological buffers by a protective alumina layer. The platform has been optimized to detect Interferon-gamma (IFNδ), a commonly used biomarker for M. tuberculosis infection and other autoimmune diseases, with an immunofluorescence assay. The platform demonstrates 500 fg/ml limit of detection (LOD) while commercial enzyme-linked immunosorbent assays (ELISA) report 2 pg/mL LOD. The extraordinary fluorescence enhancement was compared with simulation result to predict and support the experimental analysis. These results can be used to expand the platform for numerous fast, robust, sensitive biosensors.

Fluorescence-based temperature sensor for in-situ imaging local temperature of aluminum nanoparticles on plasmonic gratings

We developed a novel plasmonic grating platform as a fluorescence-based temperature sensor for in-situ imaging of localized temperature and dynamic mapping of temperature in nanoscale due to photothermal heating of aluminum nanoparticles. Al/polymer nanoenergetics films with temperature sensitive dyes (Rhodamine 6G) were prepared and calibrated to obtain temperature-dependent dye fluorescence intensity. A tunable laser heating setup was developed for microscope imaging. We monitored the in-situ laser heating on Al/polymer/R6G systems by microscope and then constructed the spatial thermal mapping over time. Based on the fluorescence microscopic imaging, we can monitor Al nanoparticles movement and morphology changes for Al/polymer/Rhodamine 6G systems induced by laser heating. Al nanoparticles play an important role in laser heating due to the plasmonic, interband and intraband absorption characteristics of Al nanoparticles. Plasmonic grating platforms can not only significantly enhance the photothermal heating of Al compared with glass platforms, but also act as superlensing for sub-diffraction limited imaging of nanoparticles.

Patterning Single Cell-Electrode Pairs for Electrochemical Measurement of Quantal Exocytosis on Microchips

We are developing transparent multi- electrochemical electrode arrays on microchips in order to automate measurement of quantal exocytosis. Design goals are that one and only one cell be positioned directly over each electrode and working electrodes have μm-scale dimensions in order to resolve pA-level currents. Patterning of cell-adhesion molecules in register with electrodes using conventional photolithographic approaches is problematic because organic solvents can disable sensitive biomolecule films. We report the parylene “dry liftoff” approach pioneered by Ilic and Craighead (Biomed Microdev 2: 317, 2000) can be used to pattern single cell-electrode pairs on the chip. A 1 μm-thick parylene C film is deposited on the multi-electrode array and S1813 photoresist is spin coated onto the device and patterned. The unprotected parylene over the electrodes is then removed using Reactive Ion Etch. Poly-l-lysine (PLL) is then added to promote cell attachment. Chromaffin cells are loaded on the chip in standard culture media and left in an incubator overnight. Finally, the parylene film is peeled off to remove excess cells and PLL, leaving tightly adhered chromaffin cells at the desired locations. Importantly, we find that promoting cell attachment with PLL films does not passivate the electrochemical electrodes. Experiments are in process to explore an alternative approach whereby PLL is patterned using the dry liftoff approach but cells are added after peel off of the parylene. With this approach, cell attachment to inactive areas of the chip is blocked by using “cytophobic” materials such as Teflon AF. This alternative approach may allow efficient targeting of cells at lower cell densities as cells migrate from cytophobic areas to the electrode binding sites (Supported by NIH BRP grant RO1 NS048826).