Magnetically-assembled multifunctional magnetic-plasmonic SERS substrate for low-concentration analyte detection.

Multifunctional particles with combined magnetic and optical properties are promising materials for applications such as sensing and detection of analytes, and contrast agents for imaging techniques such as MRI, and photocatalysis. While the magnetic property allows for non-contact manipulation of the nanoparticles, optical properties can be harnessed for such sensing applications. We present the synthesis and large-scale assembly of inter-layered magnetic-plasmonic nanoparticles with graphene oxide (GO) spacer (Fe3O4@GO@Ag). The multifunctional composite particles were prepared using simple chemical methods and had an average size of 225 nm. The prepared samples were characterized using different techniques including powder XRD, FT-IR, Raman scattering, SEM, and TEM imaging. By using an external magnetic field, it is possible to form an assembly of these multifunctional particles on a large scale. Due to the chain-like formation in the presence of a magnetic field, such assemblies are good substrates for surface-enhanced Raman scattering (SERS). Here, we demonstrate the application of these magnetically-assembled particles for the detection of very low concentrations of analyte molecules (4-mercaptopyridine) using SERS. These multifunctional composite particles are good candidates for potential applications involving chemical detection, photocatalytic reactions, optoelectronic devices, and photothermal effects.

Optically-assisted thermophoretic reversible assembly of colloidal particles and E. coli using graphene oxide microstructures.

Optically-assisted large-scale assembly of nanoparticles have been of recent interest owing to their potential in applications to assemble and manipulate colloidal particles and biological entities. In the recent years, plasmonic heating has been the most popular mechanism to achieve temperature hotspots needed for extended assembly and aggregation. In this work, we present an alternative route to achieving strong thermal gradients that can lead to non-equilibrium transport and assembly of matter. We utilize the excellent photothermal properties of graphene oxide to form a large-scale assembly of silica beads. The formation of the assembly using this scheme is rapid and reversible. Our experiments show that it is possible to aggregate silica beads (average size 385 nm) by illuminating thin graphene oxide microplatelet by a 785 nm laser at low intensities of the order of 50-100 µW/µm2. We further extend the study to trapping and photoablation of E. coli bacteria using graphene oxide. We attribute this aggregation process to optically driven thermophoretic forces. This scheme of large-scale assembly is promising for the study of assembly of matter under non-equilibrium processes, rapid concentration tool for spectroscopic studies such as surface-enhanced Raman scattering and for biological applications.

Dielectrophoresis-Assisted Raman Spectroscopy of Intravesicular Analytes on Metallic Pyramids.

Chemical analysis of membrane-bound containers such as secretory vesicles, organelles, and exosomes can provide insights into subcellular biology. These containers are loaded with a range of important biomolecules, which further underscores the need for sensitive and selective analysis methods. Here we present a metallic pyramid array for intravesicular analysis by combining site-selective dielectrophoresis (DEP) and Raman spectroscopy. Sharp pyramidal tips act as a gradient force generator to trap nanoparticles or vesicles from the solution, and the tips are illuminated by a monochromatic light source for concurrent spectroscopic detection of trapped analytes. The parameters suitable for DEP trapping were optimized by fluorescence microscopy, and the Raman spectroscopy setup was characterized by a nanoparticle based model system. Finally, vesicles loaded with 4-mercaptopyridine were concentrated at the tips and their Raman spectra were detected in real time. These pyramidal tips can perform large-area array-based trapping and spectroscopic analysis, opening up possibilities to detect molecules inside cells or cell-derived vesicles.

Template-Stripped Tunable Plasmonic Devices on Stretchable and Rollable Substrates.

We use template stripping to integrate metallic nanostructures onto flexible, stretchable, and rollable substrates. Using this approach, high-quality patterned metals that are replicated from reusable silicon templates can be directly transferred to polydimethylsiloxane (PDMS) substrates. First we produce stretchable gold nanohole arrays and show that their optical transmission spectra can be modulated by mechanical stretching. Next we fabricate stretchable arrays of gold pyramids and demonstrate a modulation of the wavelength of light resonantly scattered from the tip of the pyramid by stretching the underlying PDMS film. The use of a flexible transfer layer also enables template stripping using a cylindrical roller as a substrate. As an example, we demonstrate roller template stripping of metallic nanoholes, nanodisks, wires, and pyramids onto the cylindrical surface of a glass rod lens. These nonplanar metallic structures produced via template stripping with flexible and stretchable films can facilitate many applications in sensing, display, plasmonics, metasurfaces, and roll-to-roll fabrication.

Millimeter-Sized Suspended Plasmonic Nanohole Arrays for Surface-Tension-Driven Flow-Through SERS.

We present metallic nanohole arrays fabricated on suspended membranes as an optofluidic substrate. Millimeter-sized suspended nanohole arrays were fabricated using nanoimprint lithography. We demonstrate refractive-index-based tuning of the optical spectra using a sucrose solution for the optimization of SERS signal intensity, leading to a Raman enhancement factor of 107. Furthermore, compared to dead-ended nanohole arrays, suspended nanohole arrays capable of flow-through detection increased the measured SERS signal intensity by 50 times. For directed transport of analytes, we present a novel methodology utilizing surface tension to generate spontaneous flow through the nanoholes with flow rates of 1 µL/min, obviating the need for external pumps or microfluidic interconnects. Using this method for SERS, we obtained a 50 times higher signal as compared to diffusion-limited transport and could detect 100 pM 4-mercaptopyridine. The suspended nanohole substrates presented herein possess a uniform and reproducible geometry and show the potential for improved analyte transport and SERS detection.

Template-stripped asymmetric metallic pyramids for tunable plasmonic nanofocusing.

We demonstrate a novel scheme for plasmonic nanofocusing with internally illuminated asymmetric metallic pyramidal tips using linearly polarized light. A wafer-scale array of sharp metallic pyramids is fabricated via template stripping with films of different thicknesses on opposing pyramid facets. This structural asymmetry is achieved through a one-step angled metal deposition that does not require any additional lithography processing and when internally illuminated enables the generation of plasmons using a Kretschmann-like coupling method on only one side of the pyramids. Plasmons traveling toward the tip on one side will converge at the apex, forming a nanoscale "hotspot." The asymmetry is necessary for these focusing effects since symmetric pyramids display destructive plasmon interference at the tip. Computer simulations confirm that internal illumination with linearly polarized light at normal incidence on these asymmetric pyramids will focus optical energy into nanoscale volumes. Far-field optical experiments demonstrate large field enhancements as well as angle-dependent spectral tuning of the reradiated light. Because of the low background light levels, wafer-scale fabrication, and a straightforward excitation scheme, these asymmetric pyramidal tips will find applications in near-field optical microscopy and array-based optical trapping.

Controlling the optical near field of nanoantennas with spatial phase-shaped beams.

We report on a novel approach, based on sub-wavelength spatial phase variations at the focus of high-order beams, to reconfigure the optical near field distribution near plasmonic nanostructures. We first show how the introduction of phase jumps in the incident field driving a gap nanoantenna strongly affects its near field response. Beyond, we demonstrate the feasibility of exploiting this approach to selectively switch on and off hot-spots sites within a complex antenna architecture.

Local observation of plasmon focusingin Talbot carpets.

We present a detailed experimental and theoretical study of plasmon Talbot effect. A theoretical model based on simple scattering theory is developed to describe the Talbot self-imaging pattern generated by a linear arrangement of cylindrical nanostructures forming a periodic array. We first show the experimental observation of plasmon Talbot carpets created by propagating surface plasmon polaritons (SPP) interacting with cylindrical nanostructures positioned on a thin Au film using leakage radiation microscopy. Such images provide information on the distribution of the plasmon intensity close to the nanostructures. Next, heterodyne interferometer based near-field imaging is carried out to extract information on the plasmonic modes forming the Talbot carpet deployment. We report the experimental observation of Talbot focal spots with dimensions down to lambda/4.

Spectroscopic mode mapping of resonant plasmon nanoantennas.

We present spatially resolved spectral mode mapping of resonant plasmon gap antennas using two-photon luminescence microspectroscopy. The obtained maps are in good agreement with 3D calculations of the antenna modes. The evolution of the modal field with wavelength, both in the gap and along the two coupled gold nanowires forming the antenna, is directly visualized. At resonance, the luminescence for the gap area is enhanced at least 80 times and a comparison with the antenna extremities shows a dynamical charge redistribution due to the near-field coupling between the two arms.