title: Optical sensors based on asymmetric plasmonic nanostructures for environmental monitoring
Creators: name: Mohamad Ali Nasri, Ili Farhana
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Metamaterials, plasmonics, nanostructures, mid-infrared, LSPR, sensors, biosensor, estrogen, aptamers.

description: Optical sensors based on plasmonic nanostructures can be used for high-throughput bio-sensing applications in environmental monitoring, healthcare, food safety etc. Due to their high-sensitivity and flexibility, array metasurfaces have been widely employed as label-free optical sensors for organic compounds and could eventually be used for rapid environmental analysis. Estrogenic hormones have been noted as harmful organic compounds that are often found in water and threaten the quality and security of water supplies. In this thesis, the fabrication and characterization of plasmonic metamaterial nanostructures array based on asymmetric split-H (ASH) resonators. The ASHs have been optimized to produce plasmonic resonances that match molecular vibrations of the biological material in order to increase the sensitivity and specificity of the biosensor. The geometry of ASH nanostructures was formed by a crossbar (with a gap) placed between asymmetric (unequal arm-length) vertical dipoles. ASH and double ASH (D-ASH) were designed on silica substrates and the geometry of the ASHs were tuned produced double plasmonic resonances. The resonances were measured at normal incidence using a microscope coupled Fourier Transform Infrared (FTIR) spectrometer and observed at mid-infrared wavelengths in the range between 2 and 8 µm, both experimentally and in simulation. To illustrate the sensitivity of the D-ASH, a thin layer of PMMA used as analyte was simulated on top of the devices. The sensitivity for the D-ASH was 1297 nm/RIU. Then, the ASHs were fabricated on the zinc selenide (ZnSe) substrates to ensure low-loss transmission operation up to 21 µm. Estrogenic hormones; Estrone (E1), 17β-Estradiol (E2), Estriol (E3) and synthetic estrogen; 17α-Ethinyl Estradiol (EE2) were chosen as analytes for coupling with plasmonic resonances. Two geometry sizes of ASH labelled as ASH1 and ASH2 produced two distinct plasmonic resonances each that were matched with the five molecular bond resonances (for O-H, C-H, C=O, C=C and C≡C-H bonds) observed for the estrogenic hormones. Consequently, it was also possible to differentiate E2 from E1, E3 and EE2 in a mixture of two estrogens. Sensitivities of 363 nm/RIU and 636 nm/RIU were achieved from the deposition of E2 on two ASHs. The amplitudes of the molecular vibrational resonances were also around 500 times greater when matched with the plasmonic resonances of the ASHs, as compared with deposition on bulk ZnSe substrates. Finally, the surface functionalization of ASH with specific thiol-terminated aptamers as a bio-recognition element that binds to E2 was described. The 6-Mercapto-1-hexanol (MCH) was used as a blocking agent to prevent non-specific binding on the gold surface. Aptamers functionalised on ASH1 nanostructures were demonstrated to be effective refractometric sensors for E2 over 5 orders of magnitudes.