Oxide Coated Noble Metal Nanoparticles in Biosensors: Analytical Modeling and Discrete Dipole Approximation Method

Noble metal nanoparticles (NMNPs), such as gold and silver, have been studied extensively in various fields in science and technology due to their peculiar properties, including high stability, easy chemical synthesis, tuneable surface functionalization and plasmonic property. Researchers have used them to fabricate biosensors. Indeed, biosensors have received a lot of attention because they enable the production of small, portable devices. The biosensor industry has grown; design attempts to improve and strengthen their detection characteristics and reduce their volumes. Enzymes are generally used to provide high selectivity and sensitivity; however, their short shelf life becomes a major drawback. Scientists have tried to find other materials to replace enzymes; having long-term stability and suitability for biosensors. Nanoparticles and metal oxides substituting enzymes in sensing devices represent the best candidate to achieve high selectivity and sensitivity. Herein, coated noble metal nanoparticles of various shapes and sizes, including nanospheres, nanowires, nanocubes and nanocylinders, are dispersed in surrounding media with different refractive indices to study, via the discrete dipole approximation (DDA) method, the response of their surface plasmon peaks. For this, a simulation model is proposed for the calculations of the plasmonic properties of the considered NPs, and analytical formulas are presented. The refractive index sensitivities (RISs) have been found to depend on the shape, size, core material, shell thickness and shell composition of the nanoparticles. LSPR sensors based on gold nanoparticles (AuNPs) exhibit the lowest RISs compared to the Ag and Al based nanosensors with a value of 93.33 nm/RIU (Ag) > 46 nm/RIU (Al) > 26 nm/RIU (Au), X = 5 nm. Numerical data clearly explain why silver is the plasmon material of choice for sensing applications.