Study of the surface-enhanced Raman spectroscopy of residual impurities in hydroxylamine-reduced silver colloid and the effects of anions on the colloid activity

Direct and Water-Mediated Adsorption of Stabilizers on SERS-Active Colloidal Bimetallic Plasmonic Nanomaterials: Insight into Citrate-AuAg Interactions from DFT Calculations

In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au₂₀) and bimetallic (Au₁₉Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid's pH and surrounded by a water and sodium first solvation shell. A population of stable cluster-citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔG_ads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.

Simultaneous and rapid detection of polychlorinated phenols in water samples by surface-enhanced Raman spectroscopy combined with principal component analysis

In this work, a simple, high-throughput, and sensitive analytical method based on surface-enhanced Raman spectroscopy (SERS) and principal component analysis (PCA) was fabricated for simultaneous and rapid determination of three polychlorinated phenols (PCPs) including 2,4-dichlorophenol (2,4-DCP), 2,4,5-trichlorophenol (2,4,5-TCP), and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP). The aggregated Ag nanoparticles (AgNPs) induced by inorganic salt ions were used as sensitive SERS substrate, and the electromagnetic field distribution of AgNPs with different distances was simulated by finite difference time domain (FDTD) to verify the theory feasibility. The high throughput and rapid detection can be achieved by commercial 96-pore plate. Under the optimum conditions, the linear relationship between the Raman intensity and the concentrations of PCPs was established with satisfied correlation coefficient. The limits of detection (LOD) for 2,4-DCP, 2,4,5-TCP, and 2,3,4,6-TeCP are 0.27 mg L-1, 0.09 mg L-1, and 0.10 mg L-1 by rules of 3σ, respectively. The simultaneous quantitative analysis can be achieved thanks to the independent Raman characteristic peaks of three PCPs. Afterwards, the PCA method was used to eliminate the limitations of overlapping of characteristic Raman peaks in structural analogues of 2,4-DCP, 2,4,5-TCP, and 2,3,4,6-TeCP. The recovery experiments including single PCPs and mixed PCP samples show satisfied recoveries ranging from 85.0 to 113.9% and 80.4 to 114.0% with RSDs in range of 0.4-9.5% and 1.1-10.7%, respectively. The proposed method shows great potentials in rapid, high-throughput, and sensitive monitoring of the contaminants in water and pesticide samples with similar structure. Here, we introduced aggregated Ag nanoparticles (AgNPs) induced by inorganic salt ion for simultaneous, rapid, and sensitive determination of polychlorinated phenols (PCPs) including 2,4-dichlorophenol (2,4-DCP), 2,4,5-trichlorophenol (2,4,5-TCP), and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP) by surface-enhanced Raman spectroscopy (SERS) combined with principal component analysis (PCA). The AgNPs induced by inorganic salt ions were used as sensitive SERS substrate, and the electromagnetic field distribution of AgNPs with different distances was simulated by finite difference time domain (FDTD) to verify the theory feasibility. The PCA method was used to eliminate the limitations of overlapping of characteristic Raman peaks in structural analogues of 2,4-DCP, 2,4,5-TCP, and 2,3,4,6-TeCP. The proposed method shows great potentials in rapid, high-throughput, and sensitive monitoring of the contaminants in water and pesticide samples with similar structure.

Fast and Low-Cost Surface-Enhanced Raman Scattering (SERS) Method for On-Site Detection of Flumetsulam in Wheat

The pesticide residues in agri-foods are threatening people’s health. This study aims to establish a fast and low-cost surface-enhanced Raman scattering (SERS) method for the on-site detection of flumetsulam in wheat. The two-step modified concentrated gold nanoparticles (AuNPs) acted as the SERS substrate with the aid of NaCl and MgSO₄. NaCl is served as the activator to modify AuNPs, while MgSO₄ is served as the aggregating agent to form high-density hot spots. The activation and aggregation are two essential collaborative procedures to generate remarkable SERS enhancement and achieve the trace-level detection of flumetsulam. This method exhibits good enhancement effect with an enhancement factor of 10⁶ and wide linear range (5–1000 μg/L). With simple pretreatment, the flumetsulam residue in real wheat samples can be successfully detected with the limit of detection (LOD) down to 0.01 μg/g, which is below the maximum residue limit of flumetsulam in wheat (0.05 μg/g) set in China. The recovery of flumetsulam residue in wheat ranges from 88.3% to 95.6%. These results demonstrate that the proposed SERS method is a powerful technique for the detection of flumetsulam in wheat, which implies the great application potential in the rapid detection of other pesticide residues in various agri-foods.

Trace detection of polycyclic aromatic hydrocarbons in environmental waters by SERS

Polycyclic aromatic hydrocarbons (PAHs) are among the most hazardous pollutants and have attracted significant attention in the last decades. Up to now, rapid and on-site trace detection of PAHs remains a challenging issue. Here, taking advantage of the high sensitivity and reliable qualification of Surface-enhanced Raman Spectroscopy (SERS), we firstly carried out trace analyses of 16 typical PAHs in water at concentrations as low as 100-0.1 μg/L, depending on the number of aromatic rings of the molecule. Furthermore, owing to the simplicity of the liquid-liquid extraction (LLE) step, the sensitivity was further improved 2-3 orders of magnitude, and the lowest detectable concentrations were 100, 50, and 5 ng/L for anthracene, pyrene, and benzo[a]pyrene (the three PAHs typically found in heavily polluted waters), respectively. The LLE-SERS approach was successfully applied to the qualitative and quantitative analyses of different (ocean and coast) water samples being spiked by these three PAHs, which showed great promise as a trace detection tool of PAHs under water environments having different contaminant matrices.

Development of uncoated near-spherical gold nanoparticles for the label-free quantification of Lactobacillus rhamnosus GG by surface-enhanced Raman spectroscopy

The Surface-enhanced Raman spectroscopy (SERS) method based on gold nanoparticles as SERS substrate was investigated for the label-free detection and quantification of probiotic bacteria that are widely used in various pharmaceutical formulations. Indeed, the development of a simple and fast SERS method dedicated to the quantification of bacteria should be very useful for the characterization of such formulations in a more convenient way than the usually performed tedious and time-consuming conventional counting method. For this purpose, uncoated near-spherical gold nanoparticles were developed at room temperature by acidic treatment of star-like gold nanoparticle precursors. In this study, we first investigated the influence of acidic treatment conditions on both the nanoparticle physicochemical properties and SERS efficiency using Rhodamine 6G (R6G) as "model" analyte. Results highlighted that an effective R6G Raman signal enhancement was obtained by promoting chemical effect through R6G-anion interactions and by obtaining a suitable aggregation state of the nanoparticles. Depending on the nanoparticle synthesis conditions, R6G SERS signals were up to 10²-10³-fold greater than those obtained with star-like gold nanoparticles. The synthesized spherical gold nanoparticles were then successfully applied for the detection and quantification of Lactobacillus rhamnosus GG (LGG). In that case, the signal enhancement was especially due to the combination of anion-induced chemical enhancement and nanoparticle aggregation on LGG cell wall consecutive to non-specific interactions. Both the simplicity and speed of the procedure, achieved under 30 min, including nanoparticle synthesis, sample preparation, and acquisition of SERS spectra, appeared as very relevant for the characterization of pharmaceutical formulations incorporating probiotics. Graphical abstract.

Facile solvothermal synthesis of Ag/Fe3O4 nanocomposites and their SERS applications in on-line monitoring of pesticide contaminated water

The dual-functional substrates of Ag/Fe₃O₄ exhibit excellent SERS performance, and have been successfully applied in real-time on-line monitoring of wastewater.

Influence of various chloride ion concentrations on silver nanoparticle transformations and effectiveness in surface enhanced Raman scattering for different excitation wavelengths

This study reports the effect of six various concentrations of chlorides on the surface enhanced Raman scattering activity of silver nanoparticles.

Label-free detection of serum proteins using surface-enhanced Raman spectroscopy for colorectal cancer screening

Surface-enhanced Raman scattering (SERS) spectra of serum proteins purified from human serum samples were employed to detect colorectal cancer. Acetic acid as a new aggregating agent was introduced to increase the magnitude of the SERS enhancement. High-quality SERS spectra of serum proteins were acquired from 103 cancer patients and 103 healthy volunteers. Tentative assignments of SERS bands reflect that some specific biomolecular contents and protein secondary structures change with colorectal cancer progression. Principal component analysis combined with linear discriminant analysis was used to assess the capability of this approach for identifying colorectal cancer, yielding diagnostic accuracies of 100% (sensitivity: 100%; specificity: 100%) based on albumin SERS spectroscopy and 99.5% (sensitivity: 100%; specificity: 99%) based on globulin SERS spectroscopy, respectively. A partial least squares (PLS) approach was introduced to develop diagnostic models. An albumin PLS model successfully predicted the unidentified subjects with a diagnostic accuracy of 93.5% (sensitivity: 95.6%; specificity: 91.3%) and the globulin PLS model gave a diagnostic accuracy of 93.5% (sensitivity: 91.3%; specificity: 95.6%). These results suggest that serum protein SERS spectroscopy can be a sensitive and clinically powerful means for colorectal cancer detection.

The effect of artificial seawater on SERS spectra of amino acids-Ag colloids: an experiment of prebiotic chemistry

The large enhancement of signal observed in surface enhanced Raman spectroscopy (SERS) could be helpful for identifying amino acids on the surface of other planets, in particular for Mars, as well as in prebiotic chemistry experiments of interaction minerals/amino acids. This paper reports the effect of several substances (NaCl, MgCl2, KBr, CaSO4, K2SO4, MgSO4, KI, NH4Cl, SrCl2, CaCl2, Na2SO4, KOH, NaOH, H3BO3) on the SERS spectra of colloid of sodium citrate-CSC and colloid of sodium borohydride-CSB. The effect of four different artificial seawaters and these artificial seawaters plus amino acids (α-Ala-alanine, Gly-glycine, Cys-cysteine, AIB-2-aminoisobutiric acid) on SERS spectra using both CSC and CSB was also studied. For CSC, the effect of water, after dilution of the colloid, was the appearance of several absorption bands belonging to sodium citrate in the SERS spectrum. In general, artificial seawaters enhanced several bands in SERS spectra using CSC and CSB and CSC was more sensitive to those artificial seawaters than CSB. The identification of Gly, α-Ala and AIB using CSC or CSB was not possible because several bands belonging to artificial seawaters, sodium citrate or sodium borohydride were enhanced. On the other hand, artificial seawaters did not interfere in the SERS spectra of Cys using CSC or CSB, although the interaction of Cys with each colloid was different. For CSC the band at 2568 cm(-1) (S-H stretching) of Cys vanished and for CSB the intensity of this band decreased, indicating the -SH of Cys was bonded to Ag to form -S-Ag. Thus SERS spectroscopy could be used for Cys detection on Mars soils using Mars land rovers as well as to study the interaction between Cys and minerals in prebiotic chemistry experiments.

Influence of protein size on surface-enhanced Raman scattering (SERS) spectra in binary protein mixtures

The size-dependent interactions of eight blood proteins with silver nanoparticles (AgNPs) in their binary mixtures were investigated using surface-enhanced Raman scattering (SERS). Principal component analysis (PCA) was performed on the SERS spectra of each binary mixture, and the differentiation ability of the mixtures was tested. It was found that the effect of relative concentration change on the SERS spectra of the binary mixtures of small proteins could be detected using PCA. However, this change was not observed with the binary mixtures of large proteins. This study demonstrated that the relative interactions of the smaller proteins with an average size of 50 nm AgNPs smaller than the large proteins could be monitored, and this information can be used for the detection of proteins in protein mixtures.

A novel surface-enhanced Raman scattering nanosensor for detecting multiple heavy metal ions based on 2-mercaptoisonicotinic acid functionalized gold nanoparticles

A novel, effective and simple surface-enhanced Raman scattering (SERS) nanosensor for selectively and sensitively detecting heavy metal ions in aqueous solution has been developed in the form of 2-mercaptoisonicotinic acid (2 MNA)-modified gold nanoparticles (AuNPs). Multiple heavy metal ions can be identified and quantified by using relative peak intensity ratios of selected vibrational bands in the SERS spectra of 2 MNA. Especially, concentration of Hg(2+) and Pb(2+) ions are determined by comparing the intensity ratios of the bands 1160/1230 cm(-1) for Hg(2+) and 861/815 cm(-1) (or 815/1392 cm(-1)) for Pb(2+), with detection limits of 3.4×10(-8) and 1.0×10(-7)M, respectively. 2 MNA-AuNPs sensors show a high selectivity for Hg(2+) without masking reagent, and they can also be highly selective for Pb(2+) when using sodium thiosulphate and l-cysteine as masking reagents. These results demonstrate that these 2 MNA-AuNPs nanosensors are promising candidates for in situ heavy metal ions detection and quantification, maybe even inside living cells.