The detection of analytes that are not interacting with (or adsorbing to) plasmonic surfaces remains an important practical task. It is a common practice to demonstrate the SERS effectivity of a SERS-active substrate by using analytes that are strongly adsorbed at the plasmonic metal surfaces and located at the “hot spots”. Thus, a principal challenge in using SERS for sensitive and nondestructive detection is to localize the molecules of interest at the plasmonic surface but at a proper distance (2–4 nm). The SERS in the “hot spots” suffers from those undesired effects even more because the analyte molecules have to be localized in a small volume in gaps between the NPs. However, close proximity is not optimal because of possible quantum tunneling effects. Since the LSPR-enhanced electromagnetic field decays exponentially with the distance from the metal surface, the analyte molecules should be located near the surface of the SERS substrate to achieve maximum enhancement. Thus, we suppose EM is dominating in our work. Consequently, the CE mechanism should be accompanied by a change of spectral properties of the analyte, which was not observed in this study. The CE mechanism is supposed to be caused by a charge transfer between the plasmonic surface and the chemically adsorbed analyte molecules, which introduces new states in the electronic structure of the metal–adsorbate complex leading to an increase in the Raman scattering cross section of the analyte. CE is at least two orders of magnitude weaker than EM. The basic mechanism is EM through localized surface plasmon resonances (LSPRs) on the metal surface. There are two generally recognized mechanisms responsible for the SERS enhancement, namely electromagnetic enhancement (EM) and chemical enhancement (CE). The design of SERS substrates commonly aims at maximizing the plasmonic effect of Raman enhancement. Due to the progress in nanotechnology, a large number of highly sensitive SERS substrates has been synthesized. Significant attention has been devoted to the development of formation methods of metallic NP arrays with controllable parameters such as size, shape, interparticle distance, and ordering degree, with a focus on plasmonic structures with a high density of “hot spots”. Raman signal surface enhancement uses so-called SERS-active substrates that are mainly inorganic or hybrid nanostructured materials. However, despite the promising potential, it turned out that a lot of practical, theoretical, and even technical tasks need to be solved for practical applications of the method. It became popular in the scientific community during the last decades due to great prospects for practical solutions of, particularly, analytical problems. Surface-enhanced Raman scattering (SERS) with its advantages of extreme sensitivity, high selectivity, and non-destructive nature has demonstrated great potential for the quick detection of chemicals in different samples. Changing the charge of analytes could be a promising way to get clear SERS spectra of negatively charged molecules on Ag SERS-active supports. 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. Only the addition of copper ions into the analyte solution yielded a good SERS signal. Finally, all those surface modifications were tested using a negatively charged oligonucleotide labeled with Black Hole Quencher dye. 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). 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. 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. 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. 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).
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