RESUMO
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.
RESUMO
Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.
RESUMO
A highly sensitive method for the detection and quantitative evaluation of antimony(III) using the surface-enhanced Raman scattering (SERS) technique is demonstrated. The method is based on the analysis of SERS spectra intensity of antimony bound to phenylfluorone (Sb-PhF). Phenylfluorone is widely used as an organic reagent for the spectrophotometric determination of some heavy metals. For the SERS experiment a Sb-PhF complex was adsorbed onto the silvered porous silicon substrate. The significant degradation of the SERS signal was observed during measurements in the air. The time evolution of SERS spectra at ambient and degassed conditions was investigated to find an optimal regime for SERS measurements. The limit of Sb detection in degassed samples was determined to be near 1 ng/mL, which is one order of magnitude less than that attainable by the photometric approach. The linear range of the method to Sb(III) was found to a mass concentration range of 1-10 ng/mL. This approach permits an absolute quantity of Sb(III) to be detected at the picogram level (â¼50 pg). It is remarkable that a very small sample volume (50 µL) is required for SERS analysis. Moreover this technique offers high selectivity owing to the distinctive vibrational features for the metallorganic complex and to the resonance character of Raman spectra. The proposed SERS-based detection of Sb is a fast and highly sensitive method for use in environmental and industrial waste monitoring as well as for forensic science to determine gunshot residue. We expect that the approach reported herein can be further extended to develop new detection techniques for other heavy metals.
