Surface-Enhanced Raman Difference between Human Insulin and Insulin Lispro Detected with Adaptive Nanostructures

A protein sensor based on nanostructured adaptive silver films (ASFs) is developed for soft protein adsorption and detection with surface-enhanced Raman scattering (SERS). Under protein deposition, the ASFs modify their local nanostructure so that the conformational state of proteins is preserved and SERS is optimized. The sensor is used to examine differences in Raman spectra of two insulin isomers, human insulin and its analogue insulin lispro. These two insulins differ only in the interchange of two neighboring amino acids; specifically, the propyl−lysyl sequence at the C-terminus of the B-chain in insulin lispro is inverted as compared to human insulin. This switch leads to conformational changes at the C- and N-termini and has an important clinical effect for diabetes treatment. The difference in SERS spectra for the two insulins was detected at a submonolayer density, 80 fmol/mm2, with only 25 amol in the probed area, and macroscopic enhancement factor 3 × 106. The performed studies indicate that SERS detection using ASFs can lead to efficient proteomic sensing technology.