X. Zhang, O. Rabin, K. Zhang, R.M. Briber, H. von Bredow, C. Metting, G. Atanasoff
University of Maryland,
Keywords: SERS, nanoparticle, plasmonics, self-assembly, block copolymer, chemical sensing, fiber, high-throughput, in-situ, remote sensing
Summary:Optical metamaterials obtain their unique properties from the combination of nanoscale plasmonic components and dielectric components. Reproducible mass-production of metamaterials is challenging but desirable for applications such as surface-enhanced Raman scattering (SERS) for chemical sensing. A patented technology for the manipulation of inorganic nanoparticles enables us to fabricate metamaterials in a variety of forms (plates, tubes, fibers, etc.). These products were utilized as SERS substrates and have demonstrated large enhancement factor values that are consistent over large areas [1,2]. When incorporated as optical windows or optical fiber probes in a dynamic chemical environment, trace detection of organics with selectivity for particular chemical functionalities is achieved. The key elements of the SERS sensing element are (1) an array of gold nanoparticles with controlled size and periodicity, and (2) a multi-layered film structure that controls both the assembly of the individual nanoparticles into an ordered array and the plasmonic coupling between the nanoparticles in the array. The external layer in the multi-layer film is a block co-polymer film that exhibits nanoscale texturing achieved by self-assembly of its polymeric components. Through coulombic interactions, the block co-polymer film guides the self-assembly of gold nanoparticles, resulting in a surface-adsorbed two-dimensional array of nanoparticles.  Beneath the block co-polymer layer are dielectric and metallic thin-films that modulate the inter-particle plasmonic interactions. Through the modification of the layered structure and the size of the gold nanoparticles, an optimal metamaterial is engineered and tailored to the specific Raman spectroscopy instrumentation and application of the end-user.  Examples of finished metamaterial surfaces on plates are shown in Fig. 1. Engineering solutions were developed to adapt the self-assembly fabrication process and generate SERS-active surfaces as an integrated part of chemical monitoring system. Glass capillaries (d=1mm) were used to monitor chemicals in a flow-cell geometry. The inside walls of the capillary tubes were coated with the metamaterial nanostructure (Fig. 2) and the system was able to detect multiple instances of contamination by a Raman active molecule, opening the way for high-throughput Raman sensing. Optic-fiber probes (d=100micrometer) carrying the surface metamaterial offer further reduction in the size of the Raman sensor probe area as well as the platform for conducting remote sensing experiments. An economic approach for fabricating plasmonic nanostructures, a product of fundamental research in nanoscience at the University of Maryland, enables the integration of SERS active metamaterials in optical windows, capillary tubes and optical fibers. These sensing elements now offer in-situ, remote and high-throughput Raman sensing of trace-level organics with unprecedented reproducibility.  Rabin, Oded; Briber, Robert; Lee, Seung Yong; Lee, Wonjoo; Zhang, Xin, Nanoparticle Array with Tunable Nanoparticle Size and Separation. United States Patent 9,279,759 (Mar 8, 2016)  Lee, W.; Lee, S. Y.; Briber, R. M.; Rabin, O., Self-Assembled SERS Substrates with Tunable Surface Plasmon Resonances. Advanced Functional Materials 2011, 21, (18), 3424-3429.  Lee, W.; Lee, S. Y.; Zhang, X.; Rabin, O.; Briber, R. M., Hexagonally Ordered Nanoparticles Templated Using a Block Copolymer Film Through Coulombic Interactions. Nanotechnology 2013, 24, 045305.