Catalytic activity of Ag/Pd bimetallic nanoparticles immobilized on quartz surfaces

Stability and SERS signal strength of laser-generated gold, silver, and bimetallic nanoparticles at different KCl concentrations

Noble metal nanoparticles, specifically gold and silver, are extensively utilized in sensors, catalysts, surface-enhanced Raman scattering (SERS), and optical-electronic components due to their unique localized surface plasmon resonance (LSPR) properties. The production of these nanoparticles involves various methods, but among the environmentally friendly approaches, laser ablation stands out as it eliminates the need for toxic chemicals during purification. However, nanoparticle aggregation poses a challenge in laser ablation, necessitating the addition of extra materials that contaminate the otherwise clean process. In this study, we investigate the effectiveness of a biocompatible material, potassium chloride (KCl), in preventing particle aggregation. Although salt is known to trigger aggregation, we observed that certain concentrations of KCl can slow down this process. Over an eight-week period, we examined the aggregation rate, extinction behavior, and stability of gold, silver, and hybrid nanoparticles generated in different KCl concentrations. Extinction spectra, SEM images, SERS signal strength, and zeta potential were analyzed. Our results demonstrate that laser ablation in water and salt solutions yields nanoparticles with a spherical shape and a negative zeta potential. Importantly, we identified the optimal concentration of potassium chloride salt that maintains solution stability and SERS signal strength. Adsorbed chloride ions on silver nanoparticles were evidenced by low-frequency SERS band near 242 cm-1. A better understanding of the effect of KCl concentration on the properties of noble metal nanoparticles can lead to improved generation protocols and the development of tailored nanoparticle systems with enhanced stability and SERS activity.

Synthesis and Antibacterial Activity of (AgCl, Ag)NPs/Diatomite Hybrid Composite

. In the present research, hybrid (AgCl, Ag)NPs/diatomite composites were synthesized by direct impregnation with aqueous silver nitrate solutions. The silver chloride nanoparticles (AgCl-NPs) were formed as an effect of the exchange reaction when silver interacted with the diatomite mineral impurity halite. Nanoparticles of metallic silver (AgNPs) were created by the reduction of silver ions under the influence of hydrogen peroxide. The content of silver chloride nanoparticles in the (AgCl, Ag)NPs/diatomite composite was limited by the content of the halite in the used diatomite. Samples of natural diatomite and synthesized (AgCl, Ag)NPs/diatomite composites were examined by using scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy and thermogravimetric analysis. Moreover, the antibacterial potential of synthesized composites was also studied using the MIC (minimal inhibitory concentration) method against the most common drug-resistant microorganisms in the medical field: Gram-positive Staphylococcus aureus and Gram-negative Klebsiella pneumoniae. The obtained hybrid (AgCl, AgNPs)/diatomite composites were shown to have antimicrobial potential. However, widespread use requires further study by using various microorganisms and additional cytotoxic studies on eukaryotic systems, e.g., cell lines and animal models.

Yolk-Shell Fe/Fe4 N@Pd/C Magnetic Nanocomposite as an Efficient Recyclable ORR Electrocatalyst and SERS Substrate

A yolk-shell Fe/Fe₄ N@Pd/C (FFPC) nanocomposite is synthesized successfully by two facile steps: interfacial polymerization and annealing treatment. The concentration of Pd²⁺ is the key factor for the density of Pd nanoparticles (Pd NPs) embedded in the carbon shells, which plays a role in the oxygen reduction reaction (ORR) and surface-enhanced Raman scattering (SERS) properties. The ORR and SERS performances of FFPC nanocomposites under different concentrations of PdCl₂ are investigated. The optimal ORR performance exhibits that onset potential and tafel slope can reach 0.937 V (vs reversible hydrogen electrode (RHE)) and 74 mV dec^-1 , respectively, which is attributed to the synergistic effects of good electrical conductivity, large electrochemically active areas, and strong interfacial charge polarization. Off-axis electron holography reveals that interfacial charge polarization could facilitate the ORR of Pd NPs and defective carbon simultaneously and the shell with low density of Pd NPs is easier to form strong interfacial charge polarization. Moreover, FFPC-3 with maximum EF of 2.3 × 10⁵ results from more hot-spots, local positive charge centers to attract rhodamine 6G molecules, and magnetic cores. This work not only offers a recyclable multifunctional nanocomposite with excellent performance, but also has instructional implications for interfacial engineering for electrocatalysts design.

Morphology dependent catalysis and surface enhanced Raman scattering (SERS) studies using Pd nanostructures in DNA, CTAB and PVA scaffolds

Palladium nanoparticles (Pd NPs) of three different morphologies viz., nanocubes with cetyltrimethylammonium bromide (CTAB), nanowires with polyvinyl alcohol (PVA) and Pd NPs with deoxyribonucleic acid (DNA) scaffolds were synthesized by UV-irradiation. Catalysis and surface enhanced Raman scattering (SERS) studies were done with the synthesized morphologically distinct Pd nanostructures for the very first time. The catalytic rate was extremely high with Pd nanowires templated with PVA and the order of the catalytic rate was Pd nanowires in PVA > Pd nanocubes in CTAB > DNA-Pd wire-like assemblies. The highest catalytic rate was observed for PVA capped Pd nanowires which is a few hundred fold higher than other metal NP catalysts. Methylene blue (MB) was used as a Raman analyte for the SERS study and the largest EF of 1.9 × 10⁵ at a peak position of 1391 cm^-1 was observed with Pd nanowires in the DNA scaffold as a SERS substrate. The order of the SERS EF values was DNA-Pd wire-like assemblies > Pd nanocubes in CTAB > Pd nanowires in PVA. Beyond everything, the present synthesis route is easy, faster, candid, highly reproducible and cost-effective. In the near future, the same protocol could be applied to synthesize other materials for various applications.

Antibacterial activity of silver nanoparticles obtained by pulsed laser ablation in pure water and in chloride solution

Silver nanoparticles (AgNPs) have increasingly gained importance as antibacterial agents with applications in several fields due to their strong, broad-range antimicrobial properties. AgNP synthesis by pulsed laser ablation in liquid (PLAL) permits the preparation of stable Ag colloids in pure solvents without capping or stabilizing agents, producing AgNPs more suitable for biomedical applications than those prepared with common, wet chemical preparation techniques. To date, only a few investigations into the antimicrobial effect of AgNPs produced by PLAL have been performed. These have mainly been performed by ablation in water with nanosecond pulse widths. We previously observed a strong surface-enhanced Raman scattering (SERS) signal from such AgNPs by "activating" the NP surface by the addition of a small quantity of LiCl to the colloid. Such surface effects could also influence the antimicrobial activity of the NPs. Their activity, on the other hand, could also be affected by other parameters linked to the ablation conditions, such as the pulse width. The antibacterial activity of AgNPs was evaluated for NPs obtained either by nanosecond (ns) or picosecond (ps) PLAL using a 1064 nm ablation wavelength, in pure water or in LiCl aqueous solution, with Escherichia coli and Bacillus subtilis as references for Gram-negative and Gram-positive bacteria, respectively. In all cases, AgNPs with an average diameter less than 10 nm were obtained, which has been shown in previous works to be the most effective size for bactericidal activity. The measured zeta-potential values were very negative, indicating excellent long-term colloidal stability. Antibacterial activity was observed against both microorganisms for the four AgNP formulations, but the ps-ablated nanoparticles were shown to more effectively inhibit the growth of both microorganisms. Moreover, LiCl modified AgNPs were the most effective, showing minimum inhibitory concentration (MIC) values in a restricted range of 1.0-3.7 µg/mL. An explanation is proposed for this result based on the increased surface reactivity of the metal surface due to the presence of positively charged active sites.

Investigation of chemically modified barium titanate beads as surface-enhanced Raman scattering (SERS) active substrates for the detection of benzene thiol, 1,2-benzene dithiol, and rhodamine 6G

SERS active surfaces were prepared by depositing silver films using Tollen's reaction on to barium titanate beads. The SERS activity of the resulting surfaces was probed using two thiols (benzene thiol and 1,2-benzene dithiol) and rhodamine 6G. The intensity of the SERS signal for the three analytes was investigated as a function of silver deposition time. The results indicate that the SERS intensity increased with increasing thickness of the silver film until a maximum signal intensity was achieved; additional silver deposition resulted in a decrease in the SERS intensity for all of the studied molecules. SEM measurement of the Ag coated barium titanate beads, as a function of silver deposition time, indicate that maximum SERS intensity corresponded with the formation of atomic scale islands of silver nanoparticles. Complete silver coverage of the beads resulted in a decreased SERS signal and the most intense SERS signals were observed at deposition times of 30 min for the thiols and 20 min for rhodamine 6G.