Joana Maria

Nanostructured Plasmonic Sensors

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Seeing Molecules by Eye: Surface Plasmon Resonance Imaging at Visible Wavelengths with High Spatial Resolution and Submonolayer Sensitivity

a device form factor that can enable powerfulnew forms of multispectral, spectroscopic, and multiplexedimaging-mode assays.For example, we recently demonstrated one-dimensionalplasmonic imaging with micrometer spatial resolution andmonolayer sensitivity using quasi-three-dimensional (quasi-3D) plasmonic crystals as sensing optic.

Multispectral Thin Film Biosensing and Quantitative Imaging Using 3D Plasmonic Crystals

This work provides plasmonic crystal platforms for quantitative imaging mode biosensing and multispectral immunoassays, establishing and validating both the optical and equilibrium bases for their operation. We investigated the distance-dependent refractive index sensitivity of full 3D plasmonic crystals to thin polymeric films formed using layer by layer (LbL) assembly of polyelectrolytes as a model system. LbL was also used to determine the preferred gold thickness and plasmonic crystal design rules (nanowell diameter and periodicity) for improved thin-film sensitivity, and full 3D finite-difference time-domain (FDTD) calculations were used to quantitatively model and confirm the experimentally observed thin film sensitivities. The integrated multispectral response of the crystals increases approximately linearly with film thickness for values <70 nm, which enables the use of molecular rulers with known thicknesses (such as self-assembled monolayers of alkanethiols on gold) to calibrate these optics for quantitative detection and speciation of surface binding events in a multiplexed imaging format. The utility of these sensors and multispectral analysis for applications in quantitative biosensing was further demonstrated by measuring the equilibrium response curve of an antibody/antigen pair (rabbit antigoat IgG/goat IgG) at increasing antigen concentrations. Fitting the integrated response to a Langmuir isotherm yielded a calculated binding constant on the order of approximately 10(7) M(-1), which is in agreement with the affinity constants reported in the literature for anti-IgG/IgG binding pairs and provides intrinsic detection limits of approximately 400 pM for such unamplified assays.

Nanopost plasmonic crystals

We describe a class of plasmonic crystal that consists of square arrays of nanoposts formed by soft nanoimprint lithography. As sensors, these structure show somewhat higher bulk refractive index sensitivity for aqueous solutions in the visible wavelength range as compared to plasmonic crystals consisting of square arrays of nanowells with similar dimensions, with opposite trends for the case of surface bound layers in air. Three-dimensional finite-difference time-domain simulations quantitatively capture the key features and assist in the interpretation of these and related results.

Soft Embossing of Nanoscale Optical and Plasmonic Structures in Glass

We describe here soft nanofabrication methods using spin-on glass (SOG) materials for the fabrication of both bulk materials and replica masters. The precision of soft nanofabrication using SOG is tested using features on size scales ranging from 0.6 nm to 1.0 μm. The performance of the embossed optics is tested quantitatively via replica patterning of new classes of plasmonic crystals formed by soft nanoimprinting of SOG. These crystals are found to offer significant improvements over previously reported plasmonic crystals fabricated using embossed polymeric substrate materials in several ways. The SOG structures are shown to be particularly robust, being stable in organic solvent environments and at high temperatures (∼450 °C), thus extending the capacities and scope of plasmonic crystal applications to sensing in these environments. They also provide a stable, and particularly high-performance, platform for surface-enhanced Raman scattering. We further illustrate that SOG embossed nanostructures can serve as regenerable masters for the fabrication of plasmonic crystals. Perhaps most significantly, we show how the design rules of plasmonic crystals replicated from a single master can be tuned during the embossing steps of the fabrication process to provide useful modifications of their optical responses. We illustrate how the strongest feature in the transmission spectrum of a plasmonic crystal formed using a single SOG master can be shifted precisely in a SOG replica between 700 and 900 nm for an exemplary design of a full 3D plasmonic crystal by careful manipulation of the process parameters used to fabricate the optical device.

Experimental and computational studies of phase shift lithography with binary elastomeric masks

This article presents experimental and computational studies of a phase shifting photolithographic technique that uses binary elastomeric phase masks in conformal contact with layers of photoresist. The work incorporates optimized masks formed by casting and curing prepolymers to the elastomer poly(dimethylsiloxane) against anisotropically etched structures of single crystal silicon on SiO2∕Si. Scanning optical microscopy and full-vector finite element computations reveal the important near field and proximity optical effects. Representative structures fabricated with this technique, including several that exploit subtle features in the intensity distributions, illustrate some of the capabilities.