Surface plasmon resonance optical sensors for detection of chemicals

The thesis investigates the realization of all-optical sensors for the detection of chemicals and bio-chemicals by exploiting the properties of surface plasma waves (or plasmons) excited at a dielectric-metal interface, the so called phenomenon of surface plasmon resonance (SPR). SPR occurs for light with suitable wavelength, incidence and polarization and manifests itself as a strong attenuation of the light reflection coefficient at the metal-dielectric interface. Supposing to use a broadband light source for the interrogation, this turns out in a deep and narrow notch in the reflected spectrum, the position of which is strongly dependent on the refractive index of the substance (the analyte) in contact with the metal layer. These sensors are characterized by very high sensitivity and, after proper functionalization, are therefore used in chemical and biochemical analysis to detect ppb (part-per-billion) concentrations of specific substances, such as pollutants in water. A further reason of interest is the possibility to conduct label-free analyses, without the use of other chemicals to make the analyte detectable. The construction parameters (e.g. metal used, layer thickness, and incidence angle, just to name a few) play a key role in determining the performance of these sensors. Therefore, in the first part of the thesis a mathematical model is introduced to allow running parametric simulations to optimize the layout according to the different configurations and applications. Then, based on the indications obtained from the simulations, some prototypes both in bulk optic (i.e. using prisms-based setups) and in optical fiber configurations have been realized and fully characterized to validate the implemented models. Optical fiber based sensors are particularly interesting for their small size that makes them well suited in all those applications where it is required the use of a compact, lightweight and minimally invasive system. For this reason, the fiber optic SPR sensor solution is often taken as the reference sensing system throughout the thesis. In a second part of the thesis, in view of long-term monitoring applications, the most critical parameters affecting the sensor resolution have been identified and analyzed in detail. These are: i) the stability of the optical source used to interrogate the sensor; ii) the influence of misalignments and other mechanical instabilities; iii) the effect of temperature fluctuations. As for the optical source stability, the most common source types used in practical setups (halogen lamp, super-continuum laser and SLED) have been measured, and a method to compare their long-term fluctuations and subsequent influence on the sensor performance has been devised. Similar approach based on the analysis of repeated experimental results has been applied also to evaluate the impact of the main mechanical parameters, such as the light beam angle of incidence, both intentionally and unintentionally variable, respectively in the bulk optic setup or in the fiber-based setup. The latter, in particular, is very relevant in the development of fiber sensing probes that to be disposable need to be low cost, and thus they make use of cheap connectors with poor repeatability. Another key issue is the effect of temperature. Indeed, since the refractive index is also function of the temperature and a SPR sensor basically detects changes in the refractive index, if the whole measuring system is not kept at a constant temperature, its drift induces a shift in the position of the plasmon resonance; and this could be erroneously interpreted as a variation in the concentration of the analyte. The solution widely used to reduce this drawback is to stabilize the temperature (typically using Peltier modules) and insert the whole sensing part in robust chassis; this, however, makes the instrument bulky and suitable for lab use only, and definitely not for portable monitoring systems. To overcome these limitations, a new internally compensated sensor configuration has been studied and its behavior assessed through some experiments. In the final part of the thesis all the analyzed aspects and developed solutions are applied to a feasibility study of SPR fiber sensor probes for Continuous Glucose Monitoring application (CGM), a topic of high relevance considering that diabetes constitutes one of the most common diseases, affecting more than 350 million people worldwide, corresponding to more than 8% of the adult population.