Nanoplasmonic molecular sensing Mikael Svedendahl, Lianming Tong, Si Chen, Björn Brian, Andreas Dahlin, Linda Gunnarsson, Alexander Dmitriev, Fredrik Höök and Mikael Käll Applied Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden e-mail: kall@chalmers.se Label-free molecular detection based on plasmons in metal nanostructures can utilize several different principles, the two most important being surface-enhanced Raman scattering (SERS) and refractive index contrast. In this presentation, some of our recent achievements in these areas will be discussed. Biosensing based on the refractive index sensitivity of surface plasmons is still the most influential and widespread application area in plasmonics. Traditional sensing schemes based on surface plasmon polaritons in flat gold films, so-called Surface Plasmon Resonance (SPR) sensing, has been developed and commercialized since the 80’s while sensing schemes based plasmons in nanostructured metals, so-called Localized Surface Plasmon Resonance (LSPR) sensing, is more recent. However, the actual pros and cons of the two methodologies, in terms of sensitivity and other critical parameters, are poorly understood. We have now performed direct comparisons of the two sensing techniques using the same illumination and detection conditions. The results indicate that LSPR biosensing is highly competitive in terms of sensitivity and simplicity, despite the much higher bulk refractive index sensitivity of classical SPR sensors. Examples of biosensing of relevant target molecules, such as PSA and EAP, will be given, and the ultimate detection limit will be discussed [1]. Finally, a few examples of how to improve the sensitivity of LSPR sensors via nanostructure tuning and substrate choice will be described [2]. Aggregation of metal nanoparticles strongly affects their optical response, including the magnitude of induced fields, position and widths of localized plasmons, etc. A particularly important example of this near-field coupling effect is SERS, which is typically strongest for molecules situated in gaps between interacting particles. Hence, control of the aggregation state of metal nanoparticles is a prerequisite for bio/chemo sensors based on SERS and other surface-enhanced spectroscopies. Here, we use optical tweezers to trap, aggregate and manipulate colloidal Ag nanoparticles for the purpose of lab-on-a-chip based SERS sensing [3]. The Ag colloid and the Raman probe solution were injected separately from two input tubings and mixed at a cross before flowing through the microfluidic channel, where Ag nanoparticles were trapped by a NIR laser and generated an intense SERS signal (Fig. 2(a) and 2(b)). Elastic scattering spectra showed a red-shift during a trapping process and a blue-shift while the trapping laser was blocked (Fig. 2(c)), demonstrating near-field coupling due to optical aggregation. Figure 1 Biomolecular adsorption kinetics as measured by the shift of the plasmon wavelengths as molecules are inserted into the flow cell. SPR sensing was performed using fixed angle white light illumination in Kretschmann geometry for gold films with thickness ~50 nm on glass. The LSPR sensor utilized layers of gold nanodisks made by colloidal hole mask lithography. Figure 2. (a), (b) Dark-field (DF) images and temporal SERS spectra recorded during a trapping process in a microfluidic channel. (c) DF scattering spectra measured during trapping between two glass slides. References [1] M. Svedendahl et al, in ms.; S. Chen et al, subm. ms.; A. Dahlin et al., in ms. [2] B. Brian et al., Opt. Exp. 17, 2015 (2009); A. Dmitriev et al., Nano Lett. 8, 3893 (2008). [3] L. Tong et al., Lab-on-a-Chip, 9, 193-195(2009). View publication stats View publication stats