S. Sayin, L. Sun, K.D. Benkstein, K.L. Steffens, S. Semancik, M. Zaghloul
The George Washington University,
Keywords: plasmonic sensors, localized surface plasmon sensors, nanobiosensors, diagnostics
Summary:Surface plasmon resonance (SPR) is the collective oscillation of the free electron plasma at boundary of metal and dielectric material. Properly designed nanostructures can provide plasmonic coupling between surface plasmon polaritons to offer localized enhancement that is the basis for localized surface plasmon resonance sensors (LSPR). Optical signals derived from local “hot spots” are dependent on changes in refractive index that occur when molecules interact with these regions. We are developing LSPR-based sensors which utilize such enhanced signals for monitoring low level biomolecular target species of relevance for biomedical diagnostics. Since the optical properties and sensitivity are determined by geometry and dimensional aspects, we have comparatively investigated two types of nanostructured interfaces: nanohole arrays (NHAs) and nanopillar arrays (NPAs). Fabrication parameters for the nanostructures have been guided by finite-difference time domain (FDTD) simulations which indicated maximum electric field enhancements at diameters of 160 nm (NHA) and 80 nm (NPA). NHAs were fabricated by the deposition of 100 nm of silicon nitride on silicon substrates using plasma-enhanced chemical vapor deposition (PECVD), electron-beam lithography (EBL) patterning and ion milling. Using a mask aligner and ion milling on the other side, a membrane window was patterned. NPAs were fabricated using a 200 nm negative photoresist coating, patterning by using EBL. Both plasmonic structures included a top layer produced by depositing 5 nm Ti and 45 nm Au via an e-beam evaporator. The fabricated NHAs and NPAs were imaged by using scanning electron microscopy (SEM). The sensing system consisted of a portable spectrometer, optical fibers, probe station, microscope, light source, and a PDMS microfluidic channel, and measurements were made on the plasmonic platforms by reflectance. Functional surface chemistries are employed on the Au surfaces to introduce target-specific capture probes for biomedical applications, where binding of a target biomolecule yields a local refractive index change and measurable signal (shift in peak wavelength and relative intensity). The processing steps on carried out on clean Au surfaces have been characterized by X-ray photoelectron spectroscopy (XPS). As an example of a diagnostic application, preliminary results relating to the detection of SARS-CoV-2 virus will be presented in which target capture from solution-phase samples occurs due to spike protein (S1) binding to surface-immobilized nanobodies. These NHA- and NPA-based sensors have shown to be advantageous because of their miniaturized features, and potential for high sensitivity, cost-effectiveness, and point-of-care use.