As the World is moving through the fourth industrial revolution, the utilization of nanomaterials has gradually revolutionized the field of sensing and opened up numerous possibilities for advanced sensing applications. Among many rising technologies, localized surface plasmon resonance (LSPR) sensing stands out as a promising phenomenon with largely untapped potential. By applying a strongly confined excited electromagnetic nearfield effect on the metal nanoparticle surface, LSPR sensing is extremely sensitive to the change of local refractive index (RI), enabling it to detect small analytes with high sensitivity. Gold (Au) is a well-known plasmonic material that can provide excellent plasmonic resonance located at the visible range region. Its unique nanostructures and properties have always been the subject of intense research in this field, and so novel approaches continue to appear to bring new insights and discoveries. Although LSPR sensing is now widely used for various detection applications, several limitations still exist, and the search for ever-increasing sensitivity remains the primary priority challenge in this field. Thus, in this thesis, novel plasmonic nanomaterials comprising the alloy of Au and the use of two-dimensional materials on Au nanoislands were performed to explore their LSPR performance to enhance sensitivity.

Firstly, alloy nanoislands comprising Au and platinum (Pt) were fabricated and characterized by using x-ray diffraction analysis, x-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscope and transmission electron microscope to study their nanostructure and characteristics. Their sensitivity toward the change of RI was evaluated through sodium chloride solution testing by comparing Au nanoislands and different ratios of Au-Pt alloy nanoislands. Au-Pt alloy nanoislands with Au0.94-Pt0.06 composition was found to have the optimum composition with the best sensitivity toward the change of refractive index; it has double the sensitivity than that of the traditional Au nanoislands with excellent stability.

To further demonstrate the feasibility of the fabricated Au-Pt chip, the development of a highly sensitive Au-Pt LSPR immunosensor for glioma brain cancer diagnosis was presented. By detecting PD-L1 proteins on cancerous exosomes, the immune invasion can be tracked accurately corresponding to the level of PD-L1 detection. Various methods of antibody immobilization were carried out on the Au-Pt sensing chips and methods with the best performance were chosen. A series of experiments involving different cancerous samples was performed to study and calibrate the immunosensor. An excellent limit of detection was achieved to be 2.77 ng/ml for the detection of U87 GBM blood-derived exosomes. Similarly, an Au-Pt-based LSPR immunosensor was also proposed for coronavirus disease diagnosis. As spike protein is highly conserved among coronavirus and its exosomes, the determination of spike exosomes has been recognized to be a way for coronavirus disease diagnosis. Using anti-spike antibodies functionalized Au-Pt sensing chip, the detection of spike protein in endothelial-derived exosomes was achieved with a good limit of detection of 0.501 ng/ml in mouse blood-derived exosomes.

The chemical sensing application is presented with the detection of a heavy metal, antimony(III) (Sb(III)) ions, by using the nitrotyrosine functionalized Au-Pt LSPR sensor. The electronic properties of the interaction between nitrotyrosine and Sb during the biosensing were explored using density functional theory (DFT) simulation to explain their strong adsorption. The sensor achieved a limit of detection of 0.0504 ppb and this showed its high selectivity while testing among 14 other common elements. In summary, Au-Pt alloy nanoislands was found to be highly feasible for LSPR applications in chemical and biomedical sensing.

Furthermore, the use of two-dimensional materials has shown promising performance in many sensing applications. In this thesis, the sensing of formaldehyde in water was presented by developing a titanium-doped molybdenum disulfide monolayer on the surface of Au nanoislands. Formaldehyde is a common air pollutant that is toxic and highly soluble; it can contaminate water sources and poses a serious health risk to humans and aquatic life. Due to the unique characteristics of two-dimensional material including large surface area, high electrical conductivity, and direct band gap, molybdenum disulfide is an excellent material to be applied in many sensor applications. Experiment investigation and characterizations were conducted to develop a highly sensitive LSPR sensor for the detection of formaldehyde in water, achieving an outstanding limit of detection of 13.2 ppb. The sensing results have demonstrated a wide range detection covering 5 orders of magnitude of concentration from 13.2 ppb to 2000 ppm. To support the research results, DFT simulation was performed to study the adsorption behavior in aqueous environment, and that the outcome further confirmed the feasibility of the sensor.

There has been a significant increase in the focus on healthcare and environmental health issues after the global pandemic, leading to the growing need for precise detection in these fields. The unique properties of Au alloy nanoislands and two-dimensional materials have brought many advantages to the discovery of new plasmonic sensor applications including its improved sensitivity and specificity. Alongside the benefits of LSPR technique, the present LSPR system also has its great potential to perform label-free detection, low cost, and simple operation, thus positioning it as a promising tool for advanced chemical and biosensing applications.