R.L. Cromartie, Y. Zhao, K.D. Benkstein, K.L. Steffens, S. Semancik
National Institute of Standards and Technology,
Keywords: Plasmonic, Nanopillars, LSPR, Au, Biosensors
Summary:Nanosized structures are actively being investigated for a wide variety of potential high-impact application areas. The flexibility that nanostructures provide, particularly for high-throughput nano-optical devices, will impact diverse fields, including defense technology, drug discovery and point-of-care monitoring. Efforts are underway at NIST to develop optimized methods for optical biosensing based on nanoengineered interfaces which support localized surface plasmon resonance (LSPR). This work describes investigations on nanopillar arrays – including plasmonic modeling that guides nanostructure geometrics/dimensions, fabrication procedures, surface functionalization chemistry, and characterization of the produced interfaces. The overall goal of this work is to demonstrate an exploratory sensing platform that will enable more efficient research in areas such as bioanalytical and chemical sensing. Fabrication of preferred structures based on the Finite-Difference Time-Domain (FDTD) simulations has been achieved using electron beam lithographic (EBL) processing on 100 mm diameter silicon wafers. To define patterns of differing densities for the nanopillars, the wafers were covered with a 150 nm thickness of PMMA resist and EBL was scripted to write the various nanopillar arrays. Fabricated nanostructures consisted of pillars aligned in a square arrangement with nominal diameters and heights of 80 nm, and inter-pillar gaps which ranged from 120 nm to 420 nm. After writing multiple plasmonic platforms within wafer sectors dedicated to each of the selected inter-pillar gaps to be studied (120 nm, 220 nm, 320 nm and 420 nm), a 5 nm layer of Cr followed by a 50 nm layer of Au were deposited onto the surface of a wafer by e-beam metal-vapor deposition. Following deposition, lift-off processing was performed to remove excess metal from the substrate to further define uniform plasmonic nanopillars. SEM and AFM imaging were used to assess the actual dimensions and feature quality of the nanostructured platforms, and XPS measurements were performed to obtain a baseline composition of the Au-coated devices. The nanopillar plasmonic interfaces serve as the platform bases for biosensing investigations of responses that arise from changing surface chemistry (sensing target capture by an immobilized surface probe). The signal quality and spectral shifts for sensing are dependent on both the optical enhancement of the Au-coated nanopillar arrays and the efficiency with which molecular targets can access and bind to surface probes, which are bound to the Au. While we note that the nanopillar platforms can also be used for gas-phase samples, we describe here solution-phase studies involving DNA surface interactions. To examine the relative performances of platforms with different inter-pillar gaps and DNA-probe densities, 12-mer DNA full-match hybridization was employed as the model sample system for sensing. XPS was used to characterize the interfacial chemistry of thiol-based DNA immobilization, as well as the deposition of alkane thiolate self-assembled monolayers (SAMs) that minimized nonspecific binding. Smaller gaps generally correlate with higher probe density, but diffusional and steric effects can limit target capture at the near-pillar hot spots for such constructs. To examine such phenomena, optical properties of the various functionalized plasmonic nanostructures were evaluated via LSPR measurements made with a portable spectrometer.