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Plasmonics involves the control of light at the nanoscale by using surface plasmons. Localized surface plasmon resonance LSPR, which imparts unique optical properties to metal nanostructures, involves the collective and coherent oscillation of dielectrically confined conduction electrons of metal nanostructures. Owing to the unique combination of physical, chemical and biological properties such as large absorption and scattering cross-section, high sensitivity to local dielectric environment and enhanced electric field at the surface, plasmonic nanostructures are emerging as an important class of materials for nanomedicine. High sensitivity of plasmonic nanostructures and their assemblies to the change in refractive index of the surrounding medium allows them to serve as sensitive nanotransducers for the detection of chemical and biochemical analyte.

1. The integration onto the optical fiber tip of metallic nanostructures supporting LSPR provides the opportunity to perform remote label-free biological sensing in a novel and exciting fashion. The development of this advanced platform also requires the identification and implementation of reliable and effective fabrication techniques able to integrate functional materials defined at micro and nano-scale onto optical fiber. Specifically, advanced LSPR based optical fiber devices are developed in order to realize a label free biosensor probe able to perform assays for detecting and quantifying cancer biomarkers and mycotoxins of interest. In addition to metallic structures supporting LSPR, also metasurface could be integrated on the optical fiber tip (Fig.1) .
[ACS NANO,2012, DOI: 10.1021/nn204953e; ACS PHOTONICS 2014, DOI: 10.1021/ph400075r, ISSN 2330-4022, ANALYST 2015 8068-8079, DOI:10.1039/c5an01241d, LIGHT: SCIENCE & APPLICATIONS 2017, doi: 10.1038/lsa.2016.226, SENSORS AND ACTUATORS B: CHEMICALS 2018, https://doi.org/10.1016/j.snb.2018.07.030]


Fig.1 SEM image of a fiber optic sensor based on gold nanostructures (pillar diameter 400 nm; periodicity 1 mm).

 

2. Metasurfaces based on two dimensional 2D-arrays of plasmonic NAs are used in the Surface Enhanced InfraRed Absorption (SEIRA) technique to highly increase the sensitivity in the detection of chemical and biological substances. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area, therefore they are particularly suitable for the construction of bio-molecular sensors, given the random orientation of the dipole moments of molecules. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs. We developed large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range (Fig.2a). The cross-shaped NAs we have designed exhibit a LSPR which is very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds a small molecule (molecular weight < 500 Da) containing a nitrile group resonating at about 2100 cm-1 and a large polymer (molecular weight ~ 950000 Da) possessing carbonyl groups resonating at wavenumbers at about 1700 cm-1. We show a sensitivity of 600 nm/RIU at the different wavelengths and at a maximum amount of immobilized small molecule of 2 fmoles and a SEIRA enhancement factor of 15000 (Fig 2b). We also show the device potential to reveal chemical reactions occurring at the same scale on the sensor surface by converting the nitrile group into a triazole ring. 

Fig 2 a) SEIRA chips. A macrophotograph of a 1 cm2 chip containing a 9 mm2 NAs array. ; b) Resonant SEIRA spectra.  Reflectance curve of the gold NAs array in air (black curve) and of the gold NAs array covered with a 50 nm PMMA layer (red curve),in the inset s scanning electron micrograph of NAs with L = 1100 nm and W = 190 nm with periodicity 2 μm..

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