High-resolution microspectroscopy of plasmonic nanostructures for miniaturized biosensing

In this article, we demonstrate how to perform microscale spectroscopy of plasmonic nanostructures in order to minimize the noise when determining the resonance peak wavelength. This is accomplished using an experimental setup containing standard optical components mounted on an ordinary light microscope. We present a detailed comparison between extinction spectroscopy in transmission mode and scattering spectroscopy under dark field illumination, which shows that extinction measurements provide higher signal-to-noise in almost all situations. Furthermore, it is shown that rational selection of nanostructure, hardware components, and data analysis algorithms enables tracking of the particle plasmon resonance wavelength from a 10 microm x 50 microm area with a resolution of 10(-3) nm in transmission mode. We investigate how the temporal resolution, which can be improved down to 17 ms, affects the noise characteristics. In addition, we show how data can be acquired from an area as small as 2 microm x 10 microm (approximately 240 particles) at the expense of higher noise on longer time scales. In comparison with previous work on macroscopic sensor designs, this represents a sensor miniaturization of 5 orders of magnitude, without any loss in signal-to-noise performance. As a model system, we illustrate biomolecular detection using gold nanodisks prepared by colloidal lithography. The microextinction measurements of nanodisks described here provide detection of protein surface coverages as low as 40 pg/cm(2) (<0.1% of saturated binding). In fact, the miniaturized system provides a detection limit in terms of surface coverage comparable to state of the art macroscopic sensors, while simultaneously being as close to single protein molecule detection as sensors based on a single nanoparticle.