SER spectra of acridine and acridinium ions in colloidal silver sols. Electrolytes and PH effects

Extensive surface-enhanced Raman (SER) spectra of acridine and acridinium ions, taken as probes of DNA intercalating agents, were investigated in colloidal silver sols. From comparison of the SER and Fourier transform near-IR Raman spectra, it is confirmed that acridine is adsorbed on the metal surface via its protonatable nitrogen atom with its plane perpendicular to the surface. The effects of the electrolyte composition of silver sols, the laser excitation wavelengths and the solution pH were carefully investigated. When present in the solution, halide ions increase the SER signal intensity and shift the nitrogen protonation towards high pH values. Moreover, SER spectra of acridinium (AH+) show a higher signal-to-noise ratio than those of the neutral species and this is particularly pronounced when halides ions are present in the silver sols. The additional enhancement probably originates from a resonant charge transfer which appears to be more efficient when AH+ species forms an ion pair with halide ions. The pKa of acridine on a silver surface was estimated from titration curves based on the intensity ratios of lines at ca. 1564 and ca. 1584 cm−1, characteristic of the neutral and the protonated species, respectively. The titration curves behave quadratically or linearly according to whether the silver sols do or do not contain halide ions. On the basis of the phenomenological electrochemical model, the pH was found to change more slowly in the double layer than in the bulk solution.