Silver nanoparticles formation on fibres and potential for optical sensing

Silver nanoparticles (AgNP) formation mediated by fibres in paper can be a promising process for optical sensing applications with high sensitivity and low cost. Thus, the aim of this thesis was to develop a paper-based sensor to quantify the concentration of analyte. The work focused on investigating the role of interactions between silver ions-cellulosic hydroxyl groups and analyte molecules in AgNP formation. A multilayer sensor module that included layers supporting silver ion precursor and ascorbic acid reducing agent was constructed. The effect of positional order of the sensor layers on AgNP formation was studied. UV-vis spectroscopy and SEM were used to characterize the AgNP plasmon effect and morphology, respectively. Initially, cationic (DDAB), anionic (SDS), and nonionic (Triton X) surfactants were used as model analytes carrying different electrostatic charges. Further investigation was performed with biomolecules including cholesterol, BSA, gamma-globulin. UV-vis absorption results of sensor modules with varied layer-order confirmed the pivotal role of cellulosic hydroxyls in AgNP nucleation and growth processes, which were characterized by the two typical plasmonic peaks located at short and long wavelengths (~420 and 500~600 nm, respectively). Compared with the nonionic and the surfactant-free conditions, the cationic and anionic surfactants yielded more uniform and reproducible results in terms of AgNP formation and respective plasmon signal. The investigation with cholesterol confirmed main findings with surfactants by revealing distinct blue shifts in the long wavelength plasmonic peaks. However, the results with BSA and gamma-globulin proteins were inconclusive due to the possible the interference of impurities or other causes not yet identified. The findings in this thesis not only are consistent with previous studies, but also highlight opportunities for sensor design based on AgNP formation process instead of traditional platforms based on pre-synthesized AgNP. Further work will be needed to understand the mechanism of blue shift caused by the analyte and to optimize its correlation with concentration in the presence of interfering substances