Label-free plasmonic nanostar probes to illuminate in vitro membrane receptor recognition

Protein-ligand recognition is a key activity where chemical signals are communicated to cells to activate various biochemical pathways, which are important for understanding membrane signaling and drug interactions. Gold nanostars are highly attractive for biological applications due to their readily modified surface chemistry, facile synthesis and optical properties. The increase in electromagnetic field at their branches increases the surface enhanced Raman scattering (SERS) making them ideal candidates as label free in vitro probes that can be used to detect a variety of cellular activities. However, the use of particles in vitro is complicated by the adsorption of proteins, which forms the protein corona. In this paper we demonstrate gold nanostars as label free in vitro probes to study the interaction between α_vβ₃ integrin and RGD. Nanostars functionalized with cyclic-RDGFC reduced the formation of the protein corona, due to its zwitterionic nature, indicating a small peptide approach to minimizing protein absorption. Additionally, the functionalized nanostars evince a SERS response from their interaction with α_vβ₃ integrin representative of the amino acids present at the binding site which is also retained in a complex biological matrix. The nanostars were used in vitro to selectively detect α_vβ₃ integrin on the membrane of human metastatic colon cancer cells. By exploiting the intense SERS and tunable plasmon resonance properties of gold nanostars functionalized with cyclic RGDFC, we have demonstrated a label free approach to investigate the chemical interactions associated with protein-ligand binding from both purified proteins and membrane bound receptors in cells.