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Infrared absorption spectroscopy is a powerful biochemical analysis tool as it extracts detailed molecular structural information in a label-free fashion. Its molecular specificity renders the technique sensitive to the subtle conformational changes exhibited by proteins in response to a variety of stimuli. Yet, sensitivity limitations and the extremely strong absorption bands of liquid water severely limit infrared spectroscopy in performing kinetic measurements in biomolecules' native, aqueous environments. Here we demonstrate a plasmonic chip-based technology that overcomes these challenges, enabling the in-situ monitoring of protein and nanoparticle interactions at high sensitivity in real time, even allowing the observation of minute volumes of water displacement during binding events. Our approach leverages the plasmonic enhancement of absorption bands in conjunction with a non-classical form of internal reflection. These features not only expand the reach of infrared spectroscopy to a new class of biological interactions but also additionally enable a unique chip-based technology.

Metamaterial-based perfect absorbers utilize intrinsic loss, with the aid of appropriate structural design, to achieve near unity absorption at a certain wavelength. For most of the reported absorbers, the absorption occurs only at a single wavelength where plasmon resonances are excited in the nanostructures. Here we introduce a dual-band perfect absorber based on a gold nanocross structure. Two bands of maximum absorption of 94% are experimentally accomplished by breaking the symmetry of the cross structure. Furthermore, we demonstrate the two bands can be readily tuned throughout the mid-infrared with their associated resonances giving rise to large near-field enhancements. These features are ideal for multiband surface-enhanced infrared spectroscopy applications. We experimentally demonstrate this application by simultaneously detecting two molecular vibrational modes of a 4 nm thick polymer film utilizing our proposed absorber. Furthermore, in response to variations in the interaction strength between the plasmonic and molecular dipoles, we observe an anticrossing behavior and modification in the spectral line shape of the molecular absorption peak, which are characteristic of the coupling between the two modes.

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF 2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF 2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules. Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods. Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers. We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes. The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields. The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 10(4)-10(5). We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna. The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses. Finally, an advanced model based on nonequilibrium Green's function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance.