Application of nanomaterials in sensors

Abstract

Metal nanoparticles are of strong interest for their unusual optical and electronic properties with potential for new and improved device applications. Specifically, the scattering enabled by Surface Plasmon Resonance on metal nanoparticles open up the potential for the implementation of very sensitive sensors. It has been demonstrated that Surface Plasmon resonance can be used for the detection of a single molecule. Gold (Au) nanoparticles (NPs) are free electron metals which shows strong resonance peak in the visible spectral region and their biocompatibility making them suitable for biosensors. Au NPs that are soluble in aqueous solution over a broad range of pH and ionic strength values and that are capable of selective uptake by folate receptor positive (FR+) cancer cells. The method of sensing relies on the change in the absorbance spectrum of a self-assembled monolayer of colloidal gold on glass, as a function of biomolecular binding to the surface of the immobilized colloids. In this work, Au NPs are conjugated with ligands DUPA and Pteroate, which have component functional groups compatible with disease cells (eg: prostate-specific membrane antigen, folate cells etc). The conjugated Au NPs are characterized using various spectroscopic techniques including UV-Visible, FTIR. The analysis of UV-Visible data is performed using classical electrodynamic theory (Mie theory). Theoretical calculation of various physical parameters such as dielectric function of the ligand and embedded medium, thickness of ligand layer over Au nanoparticle, size of the Au NPs, are calculated from the theoretical analysis and by fitting the experimental data with the theoretical model. The dependence of the optical properties of spherical Au NPs on particle size and wavelength were analyzed theoretically using Mie scattering theory, where the complex refractive index of Au was corrected for the effect of a reduced mean free path of the conduction electrons in small particles. To compare these theoretical results to experimental data, gold nanoparticles of the size 5 nm and 100 nm were characterized with SEM and UV-Vis spectroscopy. Excellent agreement is found between the theoretical model and experiment.