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.

Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics.

Inorganic light-emitting diodes and photodetectors represent important, established technologies for solid-state lighting, digital imaging and many other applications. Eliminating mechanical and geometrical design constraints imposed by the supporting semiconductor wafers can enable alternative uses in areas such as biomedicine and robotics. Here we describe systems that consist of arrays of interconnected, ultrathin inorganic light-emitting diodes and photodetectors configured in mechanically optimized layouts on unusual substrates. Light-emitting sutures, implantable sheets and illuminated plasmonic crystals that are compatible with complete immersion in biofluids illustrate the suitability of these technologies for use in biomedicine. Waterproof optical-proximity-sensor tapes capable of conformal integration on curved surfaces of gloves and thin, refractive-index monitors wrapped on tubing for intravenous delivery systems demonstrate possibilities in robotics and clinical medicine. These and related systems may create important, unconventional opportunities for optoelectronic devices.

Functional nanostructured plasmonic materials.

Plasmonic crystals fabricated with precisely controlled arrays of subwavelength metal nanostructures provide a promising platform for sensing and imaging of surface binding events with micrometer spatial resolution over large areas. Soft nanoimprint lithography provides a robust, cost-effective method for producing highly uniform plasmonic crystals of this type with predictable optical properties. The tunable multimode plasmonic resonances of these crystals and their ability for integration into lab-on-a-chip microfluidic systems can both be harnessed to achieve exceptionally high analytical sensitivities down to submonolayer levels using even a common optical microscope, circumventing numerous technical limitations of more conventional surface plasmon resonance techniques. In this article, we highlight some recent advances in this field with an emphasis on the fabrication and characterization of these integrated devices and their demonstrated applications.

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.

Fabrication of non-close-packed arrays of colloidal spheres by soft lithography.

Ordered 2D non-close-packed sphere arrays with controllable lattice structures have been fabricated by using soft lithography based on the solvent-swelling and mechanical deformation behaviors of PDMS film. The figure shows an SEM image of the ordered quasi-one-dimensional parallel wires of silica spheres on a polymer-coated substrate.

Microcontact printing of colloidal crystals.

Patterned two-dimensional (2D) colloidal crystals have been transferred by a modified mucp technique that was based on the use of polymer film as "glue" to provide an efficient interaction between the microsphere "ink" and substrate. The versatility of this method has been demonstrated by the patterning of colloidal crystal on a nonplanar substrate and heterogeneously structured colloidal crystal film. The table of contents graphic shows an SEM image of the ordered parallel lines of 2D colloidal crystals on a polymer-coated glass tube with a 3.7 mm radius of curvature.