Spectroscopy on Single Metallic Nanoparticles Using Subwavelength Apertures

The scattering of single nanoparticles (NPs) is used in biosensing as molecular label or as a sensor transducer, indicating the adsorption of molecules on the nanoparticles. Currently, a dark-field microscopy setup is used to separate the scattered light signal of the NP from background noise, requiring precise optical elements and cumbersome use of immersion oil. Here we present an alternative, simpler method of suppressing the background noise by using subwavelength apertures. The nanoparticle spectroscopy is carried out in a transmission configuration with an additional chromium mask with apertures that is placed in front of the nanoparticles. Because the detected transmitted light passing through the aperture is spatially restricted to the comparable size of a single metallic nanoparticle, a nanoparticle placed into the aperture sufficiently changes the spectral properties of the transmitted light. Initially in this work the size of the apertures for an optimal detection of the transmitted signal is investigated. It is followed by the measurement of transmitted light through the aperture in the presence of a varying number of nanoparticles. These spectroscopic results correlated with topological characterization techniques (scanning electron microscopy, atomic force microscopy) show that the number of the nanoparticles can be determined based on the spectral characteristics. Furthermore, a spectral shift of scattered light from metal nanoparticle upon binding molecules is observed, which can be utilized as a sensoric effect. This is demonstrated by binding DNA molecules to the nanoparticles in the apertures and measuring the change of the transmitted light. The experimental measurements are supported by theoretical calculations. The transmission spectra through subwavelength apertures with and without nanoparticles were simulated and qualitatively confirmed the experimental spectra. The results of this work represent a proof-of-principle toward biosensors based on single metallic nanoparticle utilizing a novel readout (light transmission through subwavelength apertures) without the need for a dark-field microscopy configuration.