Insights into Plasmon-Induced Dimerization of Rhodanine-A Surface-Enhanced Raman Scattering Study

Plasmon-mediated chemical reactions (PMCRs) have attracted considerable interest in recent times. The PMCR initiated by hot carriers is known to be influenced by the type of metals and the excitation wavelength. Herein, we have carried out the surface-enhanced Raman scattering (SERS) investigation of rhodanine (Rd), an important pharmacologically active heterocyclic compound, adsorbed on silver and gold nanoparticles (AgNP and AuNP) using 514.5 and 632.8 nm lasers. The prominent Raman band at 1566 cm^-1 observed in the SERS spectra is attributed to the characteristic ν(C═C) stretching vibration of the Rd dimer and not of Rd tautomers. The chemical transformation of Rd to Rd dimer on metal surfaces is plausibly triggered by the indirect transfer of energetic hot electrons generated during the non-radiative decay of plasmon. The mechanism involved in the dimerization of Rd via the indirect transfer of hot electrons is also presented. The effect of wavelength on the dimerization of Rd is also observed on the AgNP surface, which indicates that the dimerization occurs more efficiently on the AgNP surface with excitation at 514.5 nm wavelength.

Aptamer-Adjusted Carbon Dot Catalysis-Silver Nanosol SERS Spectrometry for Bisphenol A Detection

Carbon dots (CDs) can be prepared from various organic (abundant) compounds that are rich in surfaces with –OH, –COOH, and –NH₂ groups. Therefore, CDs exhibit good biocompatibility and electron transfer ability, allowing flexible surface modification and accelerated electron transfer during catalysis. Herein, CDs were prepared using a hydrothermal method with fructose, saccharose, and citric acid as C sources and urea as an N dopant. The as-prepared CDs were used to catalyze AgNO₃–trisodium citrate (TSC) to produce Ag nanoparticles (AgNPs). The surface-enhanced Raman scattering (SERS) intensity increased with the increasing CDs concentration with Victoria blue B (VBB) as a signal molecule. The CDs exhibited a strong catalytic activity, with the highest activity shown by fructose-based CDs. After N doping, catalytic performance improved; with the passivation of a wrapped aptamer, the electron transfer was effectively disrupted (retarded). This resulted in the inhibition of the reaction and a decrease in the SERS intensity. When bisphenol A (BPA) was added, it specifically bound to the aptamer and CDs were released, recovering catalytical activity. The SERS intensity increased with BPA over the concentration range of 0.33–66.67 nmol/L. Thus, the aptamer-adjusted nanocatalytic SERS method can be applied for BPA detection.

Plasmon-Induced Dimerization of Thiazolidine-2,4-dione on Silver Nanoparticles: Revealed by Surface-Enhanced Raman Scattering Study

Surface-enhanced Raman scattering (SERS) study carried on thiazolidine-2,4-dione (TZD), a pharmacologically active heterocyclic compound, points to the presence of TZD dimer formed by plasmon-induced dimerization reaction of TZD on the surface of silver nanoparticles (Ag NP) at TZD concentrations of 10^-3 M and above. The evidence for the presence of dimer was obtained from the appearance of a prominent band at 1566 cm^-1 corresponding to the ν(C═C) band (a characteristic vibrational band observed for the Knoevenagel condensation reaction products) which is absent in the normal Raman scattering (NRS) spectra of TZD solid/solution. The observed spectrum compares well with the calculated spectrum of dimer obtained using density functional theory (DFT) calculations. The dimerization reaction is plausibly induced by the transfer of hot electrons generated by the nonradiative plasmon decay of Ag NP, and the proposed reaction mechanism is discussed. However, at lower concentrations (10^-4-10^-6 M), the characteristic dimer peak (1566 cm^-1) is absent and the SERS spectra resemble more the NRS spectrum of TZD with a few changes. The spectral analysis supported by DFT calculations showed that TZD molecules undergo deprotonation and get adsorbed on the Ag NP surface as enolate forms. The proximity of TZD molecules on the surface of Ag NP is a necessary factor for the dimerization to occur. At lower concentrations, most molecules lie apart and reactions between molecules become less feasible, and they remain as monomers on the surface, while at higher concentrations the molecules are closer to each other on the Ag NP surface favoring the dimerization reaction to take place, leading to the formation of the dimer.

A Fluorescent Turn-On Carbazole-Rhodanine Based Sensor for Detection of Ag+ Ions and Application in Ag+ Ions Imaging in Cancer Cells

Carbazole - Rhodanine conjugate is an effective fluorescent host for silver ions through fluorometric transformation from green to red color with a hyperchromic emission. An intramolecular charge transfer process derived from carbazole towards rhodanine favors interaction of thiocarbonyl S and carboxylic acid O of the rhodanine moiety towards Ag⁺ ion. Carbazole - rhodanine dyad accomplishes the lowest detection limit of 12.8 × 10^-9 M and high quantum efficiency. A fluorescence reversibility of the probe with I^- ion surges reutilization of sensor molecule as an Ag⁺ ion probe with minimal loss in the fluorescent efficiency. This fluorescent ligand is a biocompatible probe and is also a proficient candidate for fluorescent imaging of Ag⁺ ion in live cells.

Tunable and Linker Free Nanogaps in Core-Shell Plasmonic Nanorods for Selective and Quantitative Detection of Circulating Tumor Cells by SERS

Controlling the size, number, and shape of nanogaps in plasmonic nanostructures is of significant importance for the development of novel quantum plasmonic devices and quantitative sensing techniques such as surface-enhanced Raman scattering (SERS). Here, we introduce a new synthetic method based on coordination interactions and galvanic replacement to prepare core-shell plasmonic nanorods with tunable enclosed nanogaps. Decorating Au nanorods with Raman reporters that strongly coordinate Ag⁺ ions (e.g., 4-mercaptopyridine) afforded uniform nucleation sites to form a sacrificial Ag shell. Galvanic replacement of the Ag shell by HAuCl₄ resulted in Au-AgAu core-shell structure with a uniform intra-nanoparticle gap. The size (length and width) and morphology of the core-shell plasmonic nanorods as well as the nanogap size depend on the concentration of the coordination complexes formed between Ag⁺ ions and 4-mercaptopyridine. Moreover, encapsulating Raman reporters within the nanogaps afforded an internal standard for sensitive and quantitative SERS analysis. To test the applicability, core-shell plasmonic nanorods were functionalized with aptamers specific to circulating tumor cells such as MCF-7 (Michigan Cancer Foundation-7, breast cancer cell line). This system could selectively detect as low as 20 MCF-7 cells in a blood mimicking fluid employing SERS. The linking DNA duplex on core-shell plasmonic nanorods can also intercalate hydrophobic drug molecules such as Doxorubicin, thereby increasing the versatility of this sensing platform to include drug delivery. Our synthetic method offers the possibility of developing multifunctional SERS-active materials with a wide range of applications including biosensing, imaging, and therapy.

Label-free SERS study of galvanic replacement reaction on silver nanorod surface and its application to detect trace mercury ion

It is significant to explore a rapid and highly sensitive galvanic replacement reaction (GRR) surface enhanced Raman scattering (SERS) method for detection of trace mercury ions. This article was reported a new GRR SERS analytical platform for detecting Hg(II) with label-free molecular probe Victoria blue B (VBB). In HAc-NaCl-silver nanorod (AgNR) substrate, the molecular probe VBB exhibited a strong SERS peak at 1609 cm(-1). Upon addition of Hg(II), the GRR occurred between the AgNR and Hg(II), and formed a weak SERS activity of Hg2Cl2 that deposited on the AgNR surfaces to decrease the SERS intensity at I1609 cm(-1). The decreased SERS intensity was linear to Hg(II) concentration in the range of 1.25-125 nmol/L, with a detection limit of 0.2 nmol/L. The GRR was studied by SERS, transmission electron microscopy and other techniques, and the GRR mechanism was discussed.

Highly-reproducible Raman scattering of NaYF4:Yb,Er@SiO2@Ag for methylamphetamine detection under near-infrared laser excitation

NaYF4:Yb,Er@SiO₂@Ag displays highly-reproducible Raman enhancement ability for methylamphetamine detection under near-infrared excitation.

Synthesis and biological evaluation of new rhodanine analogues bearing 2-chloroquinoline and benzo[h]quinoline scaffolds as anticancer agents

Several rhodanine derivatives (9-39) were synthesized for evaluation of their potential as anticancer agents. Villsmeier cyclization to synthesize aza-aromatic aldehydes, rhodanine derivatives preparation and Knoevenagel type of condensation between the rhodanines and aza-aromatic aldehydes are key steps used for the synthesis of 31 compounds. In vitro antiproliferative activity of the synthesized rhodanine derivatives (9-39) was studied on a panel of six human tumor cell lines viz. HGC, MNK-74, MCF-7, MDAMB-231, DU-145 and PC-3 cell lines. Some of the compounds were capable of inhibiting the proliferation of cancer cell lines at a micromolar concentration. Six compounds are found to be potent against HGC cell lines; compound 15 is found to be active against HGC - Gastric, MCF7 - Breast Cancer and DU145 - Prostate Cancer cell lines; compound 39 is potent against MNK-74; four compounds are found to be potent against MCF-7 cell lines; three compounds are active against MDAMB-231; nine compounds are found to be potent against DU-145; three compounds are active against PC-3 cell lines. These compounds constitute a promising starting point for the development of novel and more potent anticancer agents in future.

When the signal is not from the original molecule to be detected: chemical transformation of para-aminothiophenol on Ag during the SERS measurement

Surface-enhanced Raman spectroscopy (SERS) has long been considered as a noninvasive technique that can obtain the fingerprint vibrational information of surface species. We demonstrated in this paper that a laser with a power level considered to be low in the traditional SERS measurement can already lead to a significant surface reaction. para-Aminothiophenol, an important probe molecule in SERS, was found to be oxidized to form 4,4'-dimercaptoazobenzene (DMAB) on a roughened silver surface during the SERS measurement. The assumption was confirmed experimentally by surface mass spectroscopy and SERS as well as electrochemistry of the synthesized DMAB, which agrees well with theoretical calculations. A defocusing method was used to avoid the laser induced surface reaction and perform reliable SERS characterization and identification, which can effectively avoid erroneous interpretation of the distorted experimental result.

Silver ion incorporation and nanoparticle formation inside the cavity of Pyrococcus furiosus ferritin: structural and size-distribution analyses

Highly symmetrical protein cage architectures from three different iron storage proteins, heavy and light human ferritin chains (HuHFt and HuLFt) and ferritin from the hyperthemophilic bacterium Pyrococcus furiosus (PfFt), have been used as models for understanding the molecular basis of silver ion deposition and metal core formation inside the protein cavity. Biomineralization using protein cavities is an important issue for the fabrication of biometamaterials under mild synthetic conditions. Silver nanoparticles (AgNPs) were produced with high yields within PfFt but not within HuHFt and HuLFt. To explain the molecular basis of silver incorporation, the X-ray crystal structure of Ag-containing PfFt has been solved. This is the first structure of a silver containing ferritin reported to date, and it revealed the presence of specific binding and nucleation sites of Ag(I) that are not conserved in other ferritin templates. The AgNP encapsulated by PfFt were further characterized by the combined use of different physical-chemical techniques. These showed that the AgNPs are endowed with a narrow size distribution (2.1 +/- 0.4 nm), high stability in water solution at millimolar concentration, and high thermal stability. These properties make the AgNP obtained within PftFt exploitable for a range of applications, in fields as diverse as catalysis in water, preparation of metamaterials, and in vivo diagnosis and antibacterial or tumor therapy.