Surface-Enhanced Raman Spectroscopy of Acetil-neuraminic Acid on Silver Nanoparticles: Role of the Passivating Agent on the Adsorption Efficiency and Amplification of the Raman Signal

Abstract

We present a combined experimental and theoretical study dedicated to analyzing the surface-enhanced Raman spectra of solutions of citrate-covered silver nanoparticles (NPs) in the presence of acetil-neuraminic acid (Neu5Ac). The Raman signals from Neu5Ac (particularly the bands located at 1002 and 1237 cm −1 ) can easily be detected for concentrations as low as 1 mg/dl, providing outstanding molecular sensing properties for our synthesized Ag-NPs. When compared to its solid phase, Neu5Ac adsorption on citrate-covered Ag particles leads to enhanced Raman intensities; many vibrational frequencies are shifted; and relative intensities undergo significant changes. These variations in the spectra complicate molecular identification, especially in mixed overlayers as the ones considered in this work. Consequently, experimental results are discussed on the basis of extensive density functional theory (DFT) calculations on model citrate-covered silver clusters with coadsorbed Neu5Ac species. Several citrate−Neu5Ac-cluster complexes are optimized, and the Raman spectra of every stable structure are calculated. The theoretical data reveal a complex interplay between the chemical nature and distribution of the adsorbates. Citrate molecules prefer to aggregate on the silver surface, leaving thus sizable regions of the Ag cluster exposed for Neu5Ac adsorption. The simulated Raman spectra of citrate−Neu5AcAg complexes show acceptable similarities with experimental SERS measurements, allowing for a clear identification of the different vibrational bands. By changing the functional groups attached to the Ag cluster it is possible to modulate the adsorption properties and Raman response of Neu5Ac. DFT calculations reveal that propanethiol adsorption on model Ag clusters allows the formation of a more uniform molecular overlayer, effectively protecting the silver surface from the surrounding media. We predict that Ag-NPs functionalized with propanethiol species are not efficient substrates for Raman detection of Neu5Ac. The previous predictions are corroborated by performing SERS measurements on propanethiolcovered Ag-NPs exposed to different concentrations of Neu5Ac from which the absence of Neu5Ac spectral features is observed. The here-used DFT scheme provides an adequate theoretical frame to predict and understand the physical origin of the Raman response of adsorbed molecules in Ag-NPs, and we expect that this predictability may extend to other metallic NPs.

Pattern Formation in Precipitation Reactions: The Liesegang Phenomenon

“…Some papers exclude K 1 (NG) and construct the reaction–diffusion equations by taking only K 2 (NG) into account. , The assumption that K 1 (NG) is not essential to the appearance of Liesegang patterns was validated in an experiment based on a reduction reaction that essentially excludes K 1 (NG) . The important role of K 2 (NG) in this system was clarified through experimental and numerical control of the nucleation energy based on surface chemistry in which the surface energy of nuclei or small clusters is controlled by an appropriate surface ligand . In such systems, the decrease in the spacing coefficient in the presence of the ligand is reproduced by the reaction-diffusion equation with reduced K 2 (NG) .…”

“…55 The important role of K 2 (NG) in this system was clarified through experimental and numerical control of the nucleation energy 56 based on surface chemistry in which the surface energy of nuclei or small clusters is controlled by an appropriate surface ligand. 57 In such systems, the decrease in the spacing coefficient in the presence of the ligand is reproduced by the reaction-diffusion equation with reduced K 2 (NG) . This behavior can be explained through an interaction between ligand and nuclei in which the presence of the ligand reduces E c and K 2 (NG) and kinetically promotes the nucleation process during fluctuations between loosely interacting molecules and defined nuclei.…”

On-Site Detection of SARS-CoV-2 Antigen by Deep Learning-Based Surface-Enhanced Raman Spectroscopy and Its Biochemical Foundations

“… 14 The use of SERS in biological fields has been widely reported. 15 , 16 Recently, SERS has been employed to detect the influenza virus and hepatitis B virus in human plasma. 17 − 19 However, the absence of clinical research makes it difficult for the practical applications of SERS in large-scale screening for SARS-CoV-2.…”

Antioxidant‐modulated cytotoxicity of silver nanoparticles

“…It should be noted that trisodium citrate was applied as an additional stabilizing agent during the synthesis of AAgNPs (Section 2). Hernández‐Arteaga et al (2017) have discussed the Raman bands for the AgNPs covered by this reagent. According to the cited authors, the most intense signals due to citrate vibrations are observed at 924, 1125, 1399, and 1606 cm −1 .…”

“…Table S1. The wavenumbers and suggested band assignments for the Raman and SERS spectra of AA, GA and AAgNPs and GAAgNPs, respectively (Alvarez‐Ros et al, 2003; Boca et al, 2012; Garrido et al, 2016; Hernández‐Arteaga et al, 2017; YohannanPanicker et al, 2003). Table S2 .…”