Surface geometry of tryptophan adsorbed on gold colloidal nanoparticles

• Tryptophan shows two distinct surface-enhanced Raman spectra with gold colloids.
• Indole ring, carboxyl, and protonated amine participate in surface adsorption.
• Perpendicular and flat orientations of indole ring to gold nanosurfaces.
• Several indole ring vibrations are highly sensitive to the surface orientation.

Two distinct surface-enhanced Raman (SER) spectra of tryptophan depending on the surface adsorption geometry were obtained by using colloidal gold nanoparticles reduced by borohydride and citrate ions. According to the vibrational assignments based on DFT simulations, the SER spectra of tryptamine and 3-indolepropionic acid, and the pH dependence of tryptophan SER spectrum, we found that some indole ring vibrations are very sensitive to the surface adsorption geometry of the molecules. With citrate-reduced gold colloids, tryptophan and related molecules mainly adsorb via the protonated amine group while maintaining a perpendicular geometry of the indole ring to the surface. However, a flat geometry of the indole ring to the surface is preferred on the borohydride-reduced gold colloids where the surface adsorption occurs mainly through the indole ring π electrons. By comparing our results with previous reports on the SER spectra of tryptophan on various silver and gold surfaces, we propose a general adsorption model of tryptophan on metal nanosurfaces.

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