Enhanced sensing of molecular optical activity with plasmonic nanohole arrays

Abstract

Prospects of using metal hole arrays for the enhanced optical detection of molecular chirality in nanosize volumes are investigated. Light transmission through the holes filled with an optically active material is modeled and the activity enhancement by more than an order of magnitude is demonstrated. The spatial resolution of the chirality detection is shown to be of a few tens of nanometers. From comparing the effect in arrays of cylindrical holes and holes of complex chiral shape, it is concluded that the detection sensitivity is determined by the plasmonic near field enhancement. The intrinsic chirality of the arrays due to their shape appears to be less important.

Investigating the nature of chiral near-field interactions

“…It is well accepted that the concentrated electric fields around plasmonic particles can increase the excitation rate of molecules, increasing the sensitivity with which they are detected in general [31]. Recent works have also claimed large enhancements in the circular dichroism measured from chiral molecules placed on achiral plasmonic structures [32], [33], proposing that the enhanced circular dichroism is due to strong absorption in the metal at the plasmon frequency [34]. In such circumstances the plasmonexciton interaction is key, and the spatial structure of chiral evanescent fields around the particles themselves is likely to play little, if any, role.…”

Enantio-specific Detection of Chirality at Nanoscale Using Photo-induced Force

“…The advantage of creating a chirality reporter using a plasmonic particle can be understood by observing that the chiral response of most biomolecules peaks in UV regime, however, in the vicinity of a plasmonic particle, this signature can be brought to plasmonic band, making the measurement easier in the visible region. Although this phenomena has been already reported in [46][47][48][49][50], here we have provided a rigorous analytic representation of it which allows to design proper antennas in order to maximize magnetoelectric coupling and hence improving chirality detection. Using this equivalent approach, we can show that the differential induced force on the tip at the close vicinity of the sample reads (see Appendix C) (13) in which / z  represents partial derivative with respect to the position on the z-axis.…”

“…Our goal is to enantiospecify samples with nanoscale size (microscopic chirality identification), despite the weak chiral response of their matter constituent, by using plasmonic material for the tip. It should be noted that the tip shape could also be engineered to provide even higher electric polarizability compared to a simple plasmonic sphere [11,29,[41][42][43][44][45][46]] (e.g. a triangular prism or truncated tetrahedron).…”

Force Detection of Electromagnetic Chirality of Tightly Focused Laser Beams

“…Light can exist in a chiral state, and this property makes it possible to study chiral objects, such as molecules and nanostructures, in an optical manner. − Optical detection of chirality is important in a variety of scientific fields, with key applications in molecular biology and pharmacology. − In common applications, the specimen is illuminated sequentially with left-handed and right-handed circularly polarized light (LCP and RCP), and the chiral information is inferred from the differential response of the material. The chiral state of circularly polarized light is well understood, which facilitates the analysis of such measurements.…”