Implementing large arrays of gold nanowires as functional elements of a plasmonic biosensor is an important task for future medical diagnostic applications. Here we present a microfluidic-channel-integrated sensor for the label-free detection of biomolecules, relying on localized surface plasmon resonances. Large arrays (∼1 cm) of vertically aligned and densely packed gold nanorods to receive, locally confine, and amplify the external optical signal are used to allow for reliable biosensing. We accomplish this by monitoring the change of the optical nanostructure resonance in the presence of biomolecules within the tight focus area above the nanoantennas, combined with a surface treatment of the nanowires for a specific binding of the target molecules. As a first application, we detect the binding kinetics of two distinct DNA strands as well as the following hybridization of two complementary strands (cDNA) with different lengths (25 and 100 bp). Upon immobilization, a redshift of 1 nm was detected; further backfilling and hybridization led to a peak shift of additional 2 and 5 nm for 25 and 100 bp, respectively. We believe that this work gives deeper insight into the functional understanding and technical implementation of a large array of gold nanowires for future medical applications.
Gold nanorod antenna arrays provide ultra strong plasmonic field enhancement and a superb broadband spectral tunability, as needed, for example, for label -free biosensing, surface -enhanced Raman spectroscopy, or optical filter design. The key issue when integrating such nanorod substrates is the question whether the plasmonic properties are superior to be applied in reflection or transmission geometry, which clearly needs a thorough analysis. Here we provide a complete and fundamental experimental and theoretical investigation of such gold nanorod antenna arrays embedded in an anodized aluminum oxide matrix. We show that the excitation of individual and coupled plasmonic eigenmodes in such nanorod arrays under an oblique angle of incidence provides an efficient tool for tuning the photonic response for both reflection and transmission applications. Moreover, simultaneous recording of the transmitted and reflected intensities under s- and p -polarization for angles ranging from 0 to 80 degrees over the whole visible wavelength range allows us, for the first time, to quantify also the absorptive losses in these embedded gold nanorod antenna arrays
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