Quantum size and nonlinear effects in metal nanostructures and bio-applications with SERS

With the help of advances in nanotechnology, metal nanostructures have attracted much interest in various areas. Surface Enhanced Raman Spectroscopy is gaining high interest with new fabrication techniques and may find potential applications in biology and medicine. A possibility of SERS to see the difference between proteins with small structural changes, such as difference between human insulin (Humulin) and its artificial substitute Humalog or phosphorylation detection of 13-mer peptides, is demonstrated using semicontinuous silver film as an active substrate. A good quality spectrum could be obtained for as low as 25 amol of proteins, which makes it more suitable for applications in biology. The detection limit can be further improved by using a mixture of protein and polymer. The high sensitivity is mostly due to the high local fields, which are created in the gaps between particles. An investigation of nonlinear optical properties of colloids of silver nanoparticles using self-rotation of ellipse, Optical Kerr Effect (OKE), and Inverse Faraday Effect (IFE) also suggests that the biggest contribution to the enhancement of local field is coming from small clusters of particles, which are brought close to each other rather then big aggregates (100–1000 particles). A model of a Spherical Quantum Well have been implemented for describing two-photon excited luminescence from metal nanoparticles and shows a good agreement in experiment with colloid of silver nanoparticles. The results suggest that SERS is a powerful tool for numerous biological applications such as protein secondary structure and phosphorylation detection