Mid-infrared surface plasmon polariton sensors resonant with the vibrational modes of phospholipid layers

In this work we employed micrometric subwavelength hole arrays on thin metal films as surface plasmon-based biochemical sensors. This allowed for determining, from a single mid-infrared measurement, both the thickness and the absorption spectrum of liposome adsorbed phospholipid monolayers and trilayers. Improved sensitivity is observed due to anti-crossing behavior when mid-infrared molecular vibrations frequency matches surface plasmon mode resonance. The development of optical sensors of few molecular layers is attracting considerable interest because they ensure non-contact interaction of the probing system with the target and portability for on-site biodiagnostics. Surface Plasmon Polaritons (SPP) propagating at the interface between a low-loss metal film and a dielectric medium have been exploited for increasing the sensitivity to few-molecule-thick layers, as the probing electric field is strongly confined by SPPs in the near-field region of the interface (Adato et al., P Natl Acad Sci U S A 106(46):19227-19232, 2009; Wu et al., Nat Mater 11(1):69-75, 2012; Limaj et al., Appl Phys Lett 98(9):091902, 2011). In the mid-infrared (IR) range (wavelengths ? between 2 and 10?m) many classes of biomolecules naturally display a specific vibrational spectrum which could be used for a label-free identification of the target. In this work, we present the modeling, fabrication, and spectroscopic characterization of SPP sensors working in the mid-IR range. Here, in addition, we have set by design the SPP frequency at the vibrationalmode of the target layer, in this case phospholipid mono- and multilayers representing simplified models of the cell plasma membrane.The sensor prototype was a hole array of square apertures with varying micrometric period in an Al film on Silicon substrate. The sensor was fabricated through electron beam lithography, but the ease of the design allows also for more cost effective nanoimprint techniques to be used. Finite-difference time-domain (FDTD) simulations have been performed to analyze the electric field spatial profile associated to SPP excitation (Limaj et al., Plasmonics 8:851-858, 2013). The phospholipid layers were prepared by first depositing a monolayer on the sensor surface with a Langmuir-Blodgett trough (estimated thickness was 2.6 nm). Then the sensor has been immersed in a solution of liposomes and left for incubation, so to allow the liposomes to adsorb on the surface and fuse with the lipid monolayer. The optical response of the sensors was investigated through Fourier Transform Infrared Spectroscopy transmittance (T) measurements. The SPP resonance profile has been fitted through a Fano lineshape and the SPP resonance frequency shift upon deposition of the phospholipid layers has been calculated. Within the same single measurement, it was possible to discriminate whether liposome fusion took place or not and identify the target fingerprints through the absorption spectrum (Fig. 22.1).