Study of Label-Free Detection and Surface Enhanced Raman Spectroscopy (SERS) of Fibrinogen Using Arrays of Dielectric Core-Metal Nanoparticle Shell

Label-free detection using arrays of SiO2 nanospheres (of diameters ~500 nm) core and gold nanoparticle seeding (~4 nm) shell, with additional gold electroplated films were used as biosensors for adsorption of human fibrinogen. Also, surface enhanced Raman spectroscopy (SERS) of fibrinogen was studied as a function of the thickness of the electroplated film. It is observed that the extinction spectra of these arrays show multiple extinction peaks resulting from the interference of beams reflected between the flat substrate and the surface of the dielectric spheres as reported in the literature. There is an increase in the peak intensity and a red shift with increasing plating time. The sensitivity of these arrays to the adsorption of fibrinogen was obtained from the further red shift in the extinction spectra. SERS of fibrinogen shows an increase in the Raman bands with the time of electroplating up to a surface roughness of mean value ~1.35 nm (measured by scanning tunneling microscopy (STM)). This was followed by a decrease in the intensity of peaks with more increasing electroplating. Similar SERS results were obtained of the inorganic dye cresyl violet (CV). These results are explained in terms of interplay of the surface field enhancement due to the growth in the nanoparticles size and the resulting hotspot effects and the reduction due to the increase in the metal shell thickness.