Modification of a SERS-active Ag surface to promote adsorption of charged analytes: effect of Cu²⁺ ions

Abstract

This work studies the impact of the electrostatic interaction between analyte molecules and silver nanoparticles (Ag NPs) on the intensity of surface-enhanced Raman scattering (SERS). For this, we fabricated nanostructured plasmonic films by immobilization of Ag NPs on glass plates and functionalized them by a set of differently charged hydrophilic thiols (sodium 2-mercaptoethyl sulfonate, mercaptopropionic acid, 2-mercaptoethanol, 2-(dimethylamino)ethanethiol hydrochloride, and thiocholine) to vary the surface charge of the SERS substrate. We used two oppositely charged porphyrins, cationic copper(II) tetrakis(4-N-methylpyridyl) porphine (CuTMpyP4) and anionic copper(II) 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (CuTSPP4), with equal charge value and similar structure as model analytes to probe the SERS signal. Our results indicate that the SERS spectrum intensity strongly, up to complete signal disappearance, correlates with the surface charge of the substrate, which tends to be negative. Using the data obtained and our model SERS system, we analyzed the modification of the Ag surface by different reagents (lithium chloride, polyethylenimine, polyhexamethylene guanidine, and multicharged metal ions). Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher dye. Only the addition of copper ions into the analyte solution yielded a good SERS signal. Considering the strong interaction of copper ions with the oligonucleotide molecules, we suppose that inversion of the analyte charge played a key role in this case, instead of a change of charge of the substrate surface. Changing the charge of analytes could be a promising way to get clear SERS spectra of negatively charged molecules on Ag SERS-active supports.

Experimental and Theoretical Screening of Core Gold Nanoparticles and Their Binding Mechanism to an Anticancer Drug, 2-Thiouracil

“…Additionally, negligible peaks were observed in all three colloidal models if aged longer than one week. This is probably due to the formation of small AuNP aggregates such as dimers or trimers (“hot spots”), which can enhance the ordinary Raman signal of absorbed or nearby citrate ions (e.g., 834, 1378, and 1643 cm −1 overlapping with water at 1641 cm −1 ) via the SERS effect [ 22 , 23 , 24 ]. Citrate ions are known to experience very weak interactions with noble metal NPs, and thus are easily displaced by active functional agents.…”