The role of Ag+, Ca2+, Pb2+ and Al3+ adions in the SERS turn-on effect of anionic analytes

In our recent studies we highlighted the role of adsorbed ions (adions) in turning on the surface-enhanced Raman scattering (SERS) effect in a specific mode for anionic and cationic analytes. In this work, we emphasize the role of Ag+, Ca2+, Pb2+ and Al3+ adions in the specific adsorption of anionic analytes such as the citrate capping agent and three organic acids. Our results suggest an adion-specific adsorption mechanism: the adsorption of anionic analytes is facilitated by positively charged adions such as Ag+, Ca2+, Pb2+ or Al3+, which provide adsorption sites specific for the anionic analytes. The turn-on of the SERS effect is explained in the context of the chemical mechanism of SERS. The adions form SERS-active sites on the silver surface enabling a charge transfer between the adsorbate and the silver surface. High-intensity SERS spectra of uric acid, salicylic acid and fumaric acid could be recorded at a concentration of 50 µM only after activation of the colloidal silver nanoparticles by Ca2+, Pb2+ or Al3+ (50 µM). The chemisorption of the three anionic species to the silver surface occurs competitively and is enhanced with the anions of higher affinities to the silver surface as indicated by the SERS spectra of corresponding mixed solutions.


Content
•Additional SERS enhancement of citrate at pH 4 due to Al 3+ adions •UV-vis extinction spectra of the colloidal solutions • Raman and SERS spectra of the organic acids • SERS spectra of salicylic acid in the 6-9 pH range • Additional evidence for the role of adions in the SERS turn-on effect S2 Additional SERS enhancement of citrate at pH 4 due to Al 3+ adions Figure S1. Additional intensity enhancement of the SERS spectrum of citrate at pH 4 in the presence of Al 3+ adions. From bottom to top: SERS spectrum of citrate obtained from cit-AgNPs at pH 4 (pH lowering by addition of HNO3) and SERS spectrum of citrate obtained from cit-AgNPs at pH 4 supplemented with Al 3+ 50 µM (added in form of sulphate salt Al2(SO4)3). The SERS spectra were obtained after the addition of Al 3+ 50 µM to the cit-AgNPs.
For Raman measurements aqueous solutions of salicylic and fumaric acid were obtained by solving the powders in ultrapure water with addition of NaOH in order to ensure a basic pH. All spectra were acquired with a 532 nm laser line.

Additional evidence for the role of adions in the SERS turn-on effect
For this, Ca 2+ , Pb 2+ or Al 3+ (50 µM final concentration) were added to 1 mL cit-AgNPs and the solution was vortexed for 1 minute to homogenize the solution and ensure the complete adsorption of the cations to the silver surface. Next, the solutions were centrifuged at 5800 g for 10 minutes, the supernatant was removed by pipetting and the cit-AgNPs were resuspended in 1 mL ultrapure water, so that the AgNPs concentration remained the same. By doing so, only the adsorbed cationic adions remain on the surface of the cit-AgNPs, whereas the free cations present in the solution were discarded.
As a reference, inactivated, as synthesized cit-AgNPs underwent the same washing process.
Salicylic acid (50 µM final concentration) was added to both, activated and not activated washed colloidal solutions. The SERS spectra presented in Figure S5 show S7 that only the Ca 2+ , Pb 2+ or Al 3+ -activated cit-AgNPs enabled the obtaining of the SERS signal of salicylic acid, whereas by using the washed colloidal solution without any cations added resulted in a blank SERS signal of the colloidal solution. However, very weak intensity bands of citrate were observed in the SERS spectra of inactivated cit-AgNPs due to citrate residues remaining on the silver surface adsorbed to Ag + active sites.
The citrate SERS spectrum is clearly observed when recording the SERS spectrum of the Ca 2+ , Pb 2+ or Al 3+ activated cit-AgNPs indicating that after the washing process, the adsorbed cationic adions remain on the surface of the cit-AgNPs together with a considerable amount of citrate capping agent.