Minopoli, Antonio (2021) Plasmonic nanostructures for optical biosensing. [Tesi di dottorato]

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In the last few decades, an increasing interest for nanotechnologies is spanning more and more fields of application thanks to the unique properties exhibited by metal nanomaterials if stimulated by external electromagnetic radiations. Indeed, a new research field called plasmonics is emerging and fast growing as a result of the recent technological progress and a deeper understanding of such phenomena. Recently, several types of plasmonic nanostructures are being conceived aiming at improving the performance of plasmon-based devices. For instance, sharp nanostructures exhibit higher field enhancement than smooth surfaces thereby representing a remarkable advantage in applications relying on signal amplification such as surface-enhanced Raman spectroscopy and plasmon-enhanced fluorescence. In addition, when nanostructures are ordered in periodic arrays, collective modes can arise as a result of the field coupling among the surface plasmons so as to promote the occurrence of impressive effects such as lattice resonances. Therefore, the possibility to tune the optical response of a nanostructure by tailoring the nanomaterial shape and size, as well as the structure arrangement, is spurring the researchers to explore new approaches, in terms of both nanofabrication and nano applications, in order to go beyond the current limits of many techniques. The aim of this work is to provide an understanding of this growing field of research and to convey the main features in biosensing applications. To date, several biosensor-based approaches including colorimetric and fluorescence analysis have been explored to effectively work alongside – or even replace – the gold standard methods in a wide variety of applications including environmental pollution monitoring and medical diagnostics. In this regard, optical biosensors offer a rapid, affordable, and practical approach in many fields of applications paving the way for point of care tests and high-throughput analysis. Fluorescence-based techniques are of growing interest since their potential high-throughput analysis, point of care applications, and improvable sensitivity through plasmon-enhanced fluorescence effect. On the other hand, when quickness, practicality, and easiness of use are preferred rather than extremely high sensitivity and accuracy, colorimetric biosensors relying on gold nanoparticles are the ideal candidates since their capability to produce a qualitative response in a few minutes visible by naked eye (a portable and handheld spectrophotometer can be employed if a quantitative measurement is required). The performance of colorimetric biosensors have been tested for detecting small molecules, such as 17β-estradiol in tap water down to picomolar level, and SARS-CoV-2 virions in naso-oropharyngeal swabs from hospital patients, whereas two-dimensional patterns of honeycomb-arranged and randomly positioned gold nanoparticles have been implemented in fluorescence-based malaria apta-immunoassays to effectively amplify the signal intensity through plasmon-enhanced fluorescence effect thereby attaining an ultrasensitive limit of detection at femtomolar level for detecting proteins in human whole blood.

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