Recently, there has been a shift in the global healthcare paradigm, which is prioritizing a more patient-centric approach causing an increase in the demand for rapid and point-of-care (PoC) biomolecular detection. Electrochemical (EC) signal transduction has been used to great effect to meet some of this demand by constructing biosensors with high sensitivity and low limit-of-detection (LOD). However, signal generation in EC biosensors requires input bias potentials to activate electrochemical redox reactions. This means EC systems are inherently built-in with high background noise that limits the performance of biosensors. Biosensors with photoelectrochemical (PEC) signal transduction have recently shown great promise in being able to deliver biomolecular detection on par with, if not better than, EC biosensors. PEC biosensing directly improves upon EC signal transduction by combining EC signal readout with optical excitation as the bias input, and generally being able to achieve similar performance with simpler bioassay designs. In this scheme, the input and output of the signal transduction are decoupled from each other, significantly reducing background signal in biosensors to enhance their sensitivity. Despite being highly effective, PEC biosensors have yet to find commercial breakthrough as they have so far only shown quantitative analysis on a limited set of biomarkers and have not shown to be PoC-capable. In this thesis, we developed new strategies to improve PEC signal transduction so that it could be applied to build robust ultrasensitive PoC biosensors with high dynamic range, simple operation, and low LOD for detecting a wide variety of different disease biomarkers.
The most popular photoactive materials used in the fabrication of PEC biosensors are TiO2 nanomaterials on account of their availability, chemical stability, high catalytic efficiency, tunable morphology, and ideal band energy levels for driving useful EC reactions. However, unmodified TiO2 suffers from several drawbacks that limit its photocurrent generation efficiency, such as poor visible range absorbance due its wide bandgap and fast charge carrier recombination. Alongside the additional difficulty of biofunctionalization, PEC biosensors fabricated from TiO2 nanomaterials are limited in their bioanalytic performance. In order to make improvements on PEC biosensors, we modified the surface of TiO2 nanomaterials by chelating them with catecholate molecules. The surface modification with catecholates formed charge transfer complexes on TiO2, which resulted in enhanced photoexcitation due to enhanced electron injection attributable to intermolecular orbital excitations in the catecholate molecules. The catecholate ligands also added improved colloidal stability and additional functional groups that aided with biofunctionalization. This resulted in multifunctional TiO2 nanoparticles with improved photocurrent signal generation and enhanced visible range photoabsorption. We took this one step further by taking advantage of the high binding affinity of catecholates on TiO2 surfaces to create novel synthesis methods that created high surface area nanostructures. Photoelectrodes fabricated from these new TiO2 nanostructures had nanoporous morphology and were able to capture biomolecules more efficiently. Using our novel TiO2 nanomaterials, we fabricated signal-off biosensors that were able to detect DNA biomarkers and IL-6 protein (cancer and inflammatory biomarker) in urine with an LOD of 1.38 pM and 3.6 pg mL-1, respectively.
We further explored hybrid semiconductor structures by combining TiO2 nanomaterials with other materials such semiconductors with different bandgaps or plasmonic metal nanoparticles (NP). Using the aforementioned catechol-assisted synthesis techniques, we were able to produce different morphologies of TiO2 nanomaterials with distinct phases: anatase TiO2 nanorod assemblies and rutile TiO2 NP. The two different TiO2 nanomaterials have different bandgaps and can be used to form semiconductor heterostructures. By combining rutile TiO2 NPs with DNAzymes, a type of synthetic functional nucleic acid, we created a photoactive molecular switch that worked by making and breaking heterostructures between the two TiO2 nanomaterials. We used DNAzymes specific to E. coli bacteria to develop a highly sensitive signal-on bacterial detection platform that was able to detect E. coli in lake water samples with an LOD of 18 CFU mL-1. Using catecholate-assisted photoreduction synthesis, we developed an efficient and novel method for decorating TiO2 NP with silver (Ag) NP. The resultant nanomaterial featured TiO2 NP surfaces modified with Hematoxylin (HTX) dyes and covered with sub-nanometer sized silver NP. The band structure of TiO2/HTX/Ag NP hybrid material involved high energy electron generation through decay of surface plasmons in the Ag NP and then enhancing the photoelectron injection process between HTX and TiO2. This significantly enhances the photoexcitation and photoabsorbtion, resulting in the material with the highest photocurrent generation as presented in this thesis. By taking advantage of thiol-metal bonds, we used the TiO2/HTX/Ag NP material system in the fabrication of a highly sensitive signal-off microRNA (prostate cancer biomarker) sensor with an LOD of 172 fM in urine.
Special attention was paid to the design of PEC bioassays in this work so that they are miniaturized and easy to use, and thus suitable for PoC applications. Because PEC signal transduction generates ultrahigh signals compared to other transduction methods, it allows bioassay designs to remain simple without sacrificing performance. This allowed us to create bioassays with very few operational steps, that excel in reliability and ease-of-use. To further improve PoC capability, we explored multiplexing with the biosensor made from TiO2/HTX/Ag NP. Here we were able to demonstrate multiplexing with PEC signal transduction for the first time. Another major barrier to PEC biosensors becoming widespread is the requirement of large benchtop instrumentation such as potentiostats and light sources. To address this challenge, we designed a portable smartphone-interfacing potentiostat with a built-in LED light source to support PEC biosensing. This device, named the PECsense was as versatile as any commercial potentiostats, having features such as adjustable recording periods, variable illumination periods, automatic data processing and being able to record both anodic and cathodic photocurrents. The PECsense was demonstrated to be used successfully as a signal reader in a PEC DNA detection assay.
Ultimately, we designed several ultrasensitive PEC biosensors used for the detection of four different diagnostic biomarkers. Combined with the exploration of miniaturized design, multiplexing and portable signal-reading, our designed PEC biosensors were made PoC-capable. The work in this thesis presented innovations in areas of nanotechnology, material synthesis, solid-state physics, biotechnology and embedded systems for the advancement of biomolecular detection and PoC diagnostics. / Thesis / Doctor of Philosophy (PhD) / Biosensors show great promise for use in point-of-care diagnostics and health monitoring systems. Such deceives combine biorecongition with signal transduction for analyzing biologically relevant targets. Photoelectrochemical (PEC) mode of signal reading, particularly those based on TiO2 nanomaterials, have shown great promise in delivering point-of-care biosensors that have excellent diagnostic performance. In this thesis, our goal was to develope new techniques for creating low-cost, easy-to-use and ultrasensitive photoelectrochemical biosensors. To achieve this goal, our work here can broadly be split into three objectives. Firstly, we focused on developing new material synthesis methods to improve traditional TiO2 nanomaterials so they can be more useful in PEC biosensors. These methods involved combining TiO2 with organic molecules known as catecholates and metal nanoparticles. This work created material systems that are able to generate high signals and more easily interface with biomolecules for improving PEC biosensor sensitivity. For the second objective, we used our newly developed enhanced TiO2 nanomaterials as the foundation for designing various bioassays for the detection of a wide range of different biological targets such as DNA, RNA, proteins and bacteria. This served to demonstrate the robustness of PEC signal reading as a tool for various markers of diseases. Despite PEC biosensors being a powerful tool in healthcare, they have seen very little commercial breakthrough, which can primarily be attributed to needing bulky benchtop instruments and light sources for signal reading. For the last objective, we worked on designing a handheld smartphone-operated signal-reader for PEC biosensing with its own built-in light source.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/28294 |
| Date | January 2023 |
| Creators | Sakib, Sadman |
| Contributors | Soleymani, Leyla, Zhitomirsky, igor, Engineering Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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