R.L. Cromartie, Y. Zhao, K.D. Benkstein, K.L. Steffens, S. Semancik
National Institute of Standards and Technology,
United States
Keywords: plasmonic, nanopillars, LSPR, Au, biosensors, microfluidic
Summary:
Optical sensing based on nanoengineered interfaces has had a growing impact in a variety of application areas, particularly those related to biological research and biomedical diagnostics. However, the ultimate measurement capabilities for such techniques can only be attained when one optimizes the optical enhancement and chemical interactions associated with the target detection process. In this work we have investigated nanopillar arrays (NPAs) as localized surface plasmon resonance (LSPR)-based biosensors for use on low-volume, solution-phase samples. After developing a fabrication approach to prepare NPAs of varied spacings on a single sensing substrate for systematic studies, we focused on exploring the possible tradeoffs occurring between nanostructure-induced optical enhancement and the efficiency of biomolecular target capture in the localized surface volumes at nanopillar “hot spots” where dominant signal contributions from refraction index change arise. Finite Difference Time Domain (FDTD) simulations were used as a guide in determining optically favorable nanostructural dimensions (nanopillar diameter, height, and approximate gap spacing within the array). These simulations provided both resonance frequencies and expected optical enhancement values at localized positions. To examine effects on target capture efficiency, four types of Au-coated NPAs were fabricated on a thermally oxidized wafer - with nanopillars having fixed heights of 80 nm and fixed diameters of 80 nm, but varied interpillar spacings of 120 nm, 220 nm, 320 nm, and 420 nm. Electron-beam lithography (eBL) was used to write the pillar structures in a square arrangement at the desired pitches, and the eBL was followed by reactive-ion-etching process to etch the patterned SiO2 NPAs. The SiO2 NPA patterns were then coated with 5 nm of Cr and 50 nm of Au using physical vapor deposition by sputtering. A lift-off process was performed to remove excess metal and better define uniform plasmonic nanopillars. The actual dimensions and feature quality of the platforms were assessed by FIB-SEM imaging and AFM. Testing of these four NPA platforms on solution-phase samples was performed using a microfluidic setup which consisted of a PDMS channel for fluid inlet and outlet, and a glass-surface lid to properly control the optical-path interfaces down to the plasmonic surfaces during LSPR measurements. Initial feasibility studies with the four nanostructured platforms and microfluidics were performed via measurements on water-ethanol mixtures of varied concentration ratios. Peak shift changes were correlated to known refractive index values for the water-ethanol mixtures. To investigate the combined measurement capabilities arising from optical enhancement and biochemical capture efficiency, we utilized 12-mer DNA hybridization as a model system to evaluate the impact of varied NPA nanoscale densities in biochemical sensing. Spacing between pillars can affect the inherent optical properties but also influence diffusional transport in a way which can limit target capture near the pillar hot spots. We note that the adaptability of this platform along with our approach for optimizing the interfacial properties allows the use of such LSPR biosensors, with appropriate surface functionalization, for a range of biochemical applications involving biomolecules of different sizes.