Development of a Nanostructured Thin Film Device and Optics System for Protein Detection

• Author / Creator

  Sweet, Hillary M.

• Although various medical diagnostics exist to test biologics for biomarkers, diseases, or proteins of interest; there is currently no point-of-care platform test that is quick, inexpensive, and easy to use. The aim of this work was to further develop a stand-alone, point-of-care medical diagnostic based on thin film visible interference. The use of different anodization parameters (i.e. electrolyte, voltage, length of time) created devices with drastically different structures and properties. Analyzing the reflections and interference off these surfaces demonstrated that the surface porosity influences the reflectance and in turn the optimal viewing conditions. Highly porous films formed in oxalic and sulfuric acids generated optimal interference colours when viewed through a p-polarizing filter, whereas less porous films created in phosphoric acid generated optimal interference colours when s-polarized. The device structure proved to be highly tunable during the anodization process. Varying the sputtered aluminum thickness changed the length of anodization, and ultimately the refractive index of the resulting alumina layer. Current density provided a viable method to monitor and regulate the oxidation process. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy analysis of devices oxidized in phosphoric acid at 5 and 8 V potentials showed that the phosphate integration from phosphoric acid was equal at these potentials, as well as the nitrogen to aluminum ratio on the surface of both with protein exposure. Secondary ion mass spectroscopy was used to detect CN- ions from the proteins, and monitored the signal strength and depth, showing greater protein penetration in the 5 V samples compared to the 8 V samples. Furthermore, surface analysis techniques showed the importance of consistent storage and handling of devices, as it can lead to a noticeable difference in the surface chemistry. Tunability was further explored using phosphoric acid and a constant potential of 8 V. The importance of tantalum oxide generation and thickness was demonstrated, as without this layer, low interference colours result. Furthermore, without the tantala layer ellipsometry models contain large standard error and prevent accurate alumina parameters from being obtained. With the adsorption of an antigen and subsequently antibody pair the optimal changes in optical path lengths were observed using colour coordinates from the surface interference generated. Changes in the deposition energy during tantalum layer formation created different tantalum phases, α-Ta and β-Ta. These layers possessed different optical constants and interference colours, while also impacting the resulting alumina thicknesses in the anodized device. In some instances, devices created on α-Ta showed delamination during the oxidation process. Various vitamin K-dependent proteins were exposed to the alumina device surface in solutions of constant molarity, yet the size of the molecule was the main factor contributing to the limit of detection. The addition of calcium and magnesium ions to prothrombin solutions showed drastic increases in the optical path length, and colour shift, that were though to result from changes to the protein structure and potentially the binding efficiency. Radiolabelled prothrombin studies showed that ~0.2 mg/ml prothrombin solutions were required to form a monolayer on the surface in 15 minutes. Furthermore, binary solutions of prothrombin and competing proteins were analyzed as well as subsequent wash solutions, some of which demonstrated prothrombin removal. Comparing the visible colour shifts resulting on the device surface to radiolabelled data, it was found that 34 to 38% prothrombin surface coverage was required to generate a detectable colour shift, and the base colour of the device was deemed highly relevant to the sensitivity of protein detection. Detection of an antigen-antibody prototype of prothrombin and anti-prothrombin showed colour quantification is possible with this device. The limit of detection and analytical sensitivity were determined from this work for the antigen-antibody complex. Overall, the thin film interference device showed quantification with colour, and detection after 15 minutes of exposure, which demonstrates its potential as a point-of-care diagnostic in a variety of applications.

• Subjects / Keywords

  • nanostructured thin films
  • anodic alumina-tantala
  • porous anodic alumina
  • barrier anodic tantala
  • protein detection
  • thin-film device

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3PV6BP5G

• License

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