Three distinct adsorbed states of adenine on gold nanoparticles depending on pH in aqueous solutions

Based on critical concentration for flocculation (ccf), zeta potential and surface enhanced Raman scattering (SERS) spectra, we found three distinct adsorbed states of adenine on AuNPs that are protonated cations in pH < pKa1 (4.2), neutral molecules in pKa1 < pH < pKa2 (9.8), and deprotonated anions in pH > pKa2. Experimental SERS spectra of adenine and various methyl adenine derivatives in addition to vibrational analysis using DFT calculations indicated that adenine adsorbs on AuNP in a perpendicular orientation via lone pair electrons at N3 and N9 atoms in acidic, neutral and basic pH solutions at the concentration of 10−5–10−6 M, in contrast to a slightly-tilted orientation via NH2 and N1 (or N7) atoms at lower concentration close to the ccf.