Ultrasensitive and Tunable Achiral Metamaterial Substrates as Nanobiosensors for Enantiomer Detection

Distinguishing the chirality of biomolecules is of fundamental importance in biophysics and pharmaceutics. Circular dichroism (CD) spectroscopy provides a noninvasive approach to distinguish right- and left-handed enantiomers and can offer valuable insight into the structure of the investigated molecules. However, the intrinsic CD signal of biomolecules is often weak and typically resides in the ultraviolet spectral range, for which optical components are costly. Therefore, a tunable platform that boosts the CD signal of analytes in the optimal spectral range for the desired application would be an ideal solution. We combine tilted plasmonic nanohole arrays with ultrathin photonic cavities in our metamaterial substrates to achieve very high CD signal enhancement. In this approach, the spectral region with a high CD signal can be tailored by the geometric parameters of the array and cavity. In a tilted geometry, achiral structures mimic chiral behavior, offering an interesting alternative to inherently chiral structures. Our work highlights the role of near-field optical chirality and of the chirality enhancement factor, χ, in boosting the CD signal. To combine plasmonic and photonic effects for CD enhancement, we integrate the nanohole array as a top layer in a metal-dielectric-metal cavity structure. This metamaterial design strongly amplifies the electromagnetic near field and, in particular, its asymmetry. From both experimental and numerical results, we obtain another 10-fold increase in the χ factor, leading to a 50-fold enhancement compared to the bare biolayer on glass. With our robust and intrinsically achiral plasmonic and photonic metamaterial structures, we introduce a versatile platform for enantiomer discrimination and nanobiosensing applications that allows for spectral tuning of the operational resonance band via the geometry of the lattice and cavity.