Manipulation of dynamics in a droplet for label-free detection of proteins using surface-enhanced raman scattering and protein melting profiles

Detection and identification of biomacromolecules have critical importance in many fields ranging from biotechnology to medicine. Surface-enhanced Raman scattering (SERS) is an emerging technique for the label-free detection and identification of biological molecules and structures with its fingerprinting properties and high sensitivity. However, there are a number of obstacles for its applications for biological macromolecules due to the complexity of biological samples. In this report, manipulation of microscopic processes in play during the drying of a sessile droplet to influence the nanoparticle-macromolecule packing, which has dramatic effect on SERS performance, before the SERS acquisition is demonstrated. A process known as the ?coffee ring phenomenon? jams all particles and molecular species to the edges of the droplet during drying. This uncontrolled process has dramatic effects on a SERS experiment, using colloidal metal nanoparticles as substrates, by sweeping everything to the edges and influencing the packing of nanoparticles in the droplet area.In this study, the dynamics of silver nanoparticles-protein structures in a drying droplet was explored under two different experimental conditions and the effects of these conditions on SERS performance were investigated. Denaturation profiles of proteins with SERS were also investigated and obtained data was used to detect proteins in multiple protein mixtures. Negatively and positively charged proteins were used as model biomacromolecules in this study. A detection limit of 0.05 ?g/mL is obtained for the model proteins with using one of the presented methods.