Abstract
Gold nanoparticles have long been explored and used throughout history for many purposes, from decoration in ancient times to biomedical applications nowadays. Recent progress in the field of nanotechnology has enabled us to better understand these materials and provided necessary technological tools for controlling their synthesis and functionalisation. As a result, it has been observed that nanomaterials possess many interesting properties not found in bulk materials, deriving from the quantum size effect. Among them, their nanozyme activity and their Surface-Enhanced Raman Spectroscopy (SERS) properties have been thoroughly studied and characterised in this thesis. For that purpose, gold nanostars have been synthesised following a seed-mediated bottom-up approach, which enables to control the optical and morphological properties of the resulting nanoparticles. Gold nanostars have shown potential to replace biological enzymes as catalysts while behaving as excellent substrates for Raman enhancement studies due to the high presence of Raman hot spots within their anisotropic structure. However, it has been revealed that differences in the morphological properties or in the surface nature of gold nanostars strongly affect both their nanozyme and SERS properties, boosting or hindering these activities. These findings are of special relevance for certain applications, such as the development of diagnostic devices based on these mechanisms.On the other hand, smartphones have emerged as ideal readout systems which, in combination with gold nanostars conjugated with specific biorecognition elements define a new type of point-of-care (POC) sensors barely explored until now, satisfying many conditions required for on-site applications of diagnostic devices. A direct and obvious application where gold nanostar-based sensors can play an important role is found in the food safety field, a field that still relies, sometimes exclusively, on hygiene protocols instead of on sensing devices. This is due to many factors, including the lack of portable, user-friendly and robust sensing assays, making virtually not possible to test food for contaminants everywhere where necessary. In this line, almost 400 contamination cases of milk products have been notified in the last decade in the EU, being the dairy sector of special interest in Northern Ireland as it represents 25.7% of its farm output and comes second in terms of gross turnover. Among these contamination events, microbiological cases are the most worrying, given that they can cause both spoilage of the foodstuff and a wide range of human diseases if consumed. Together with the necessity of producing more food than ever before as the world population grows, it has become essential to test food to discard any contamination event before food reaches the consumers.
Prior to the biosensor development, a literature review was performed leading to the creation of a novel online sensors classification system. Over 900 sensors retrieved form the literature and market were analysed to build an interactive platform providing state-of-the-art information on sensors targeting various food contaminants. They have been classified according to their portability, capability to quantify the analyte, user-friendliness or capability to perform multiplex analyses. Not only this has served as an interesting tool to be used by any person involved in the field, but also has provided us with a clear picture of the benefits and drawbacks of each analytical method. Besides, it has further strengthened the idea of incorporating gold nanomaterials to enhance the sensor capabilities.
Along this thesis, gold nanostars have proved that new strategies for sensing are possible, which eventually may lead to POC sensors that replace costly lab-based analyses. Several assays targeting food contaminants (such as Mycobacterium bovis or Hg2+ ions) are herein introduced. The former combines the use of antibodies as biorecognition elements and the peroxidase-like activity of gold nanostars as signal transducers, leading to a colourimetric signal that can be read and quantified using a smartphone camera to predict the presence of M. bovis in the concentration range 104 – 106 CFU/mL. Hg2+ ions detection, on the other hand, relies on the SERS properties of gold nanostars, which can be further enhanced in combination with their nanozyme activity or exploiting the synergistic effect observed using gold-silver bimetallic nanoparticles. Sensitivities of 0.2 and 0.1 ppb, respectively, have been achieved using these approaches.
Overall, the latest findings related to the optical and spectroscopic properties of gold nanostars are reported in this thesis, together with a thorough study of current methodologies being used for sensing food contaminants. This has resulted of paramount importance for successfully developing diagnostic devices targeting food contaminants. The implications and extent of these findings are yet to be evaluated, considering that the full potential of anisotropic gold nanoparticles and bimetallic nanostructures is only beginning to be understood. However, the incorporation of smartphones as readout systems for POC sensing seems to have just undertaken a path of no return.
| Date of Award | 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Cuong Cao (Supervisor) & Christopher Elliott (Supervisor) |