Mapping the Inhomogeneity in Plasmonic Catalysis on Supported Gold Nanoparticles Using Surface-Enhanced Raman Scattering Microspectroscopy

The characterization of a catalyst often occurs by averaging over large areas of the catalyst material. On the other hand, optical probing is easily achieved at a resolution at the micrometer scale, specifically in microspectroscopy. Here, using surface-enhanced Raman scattering (SERS) mapping of larger areas with micrometer-sized spots that contain tens to hundreds of supported gold nanoparticles each, the photoinduced dimerization of <i>p</i>-aminothiophenol (PATP) to 4,4′-dimercaptoazobenzene (DMAB) was monitored. The mapping data reveal an inhomogeneous distribution of catalytic activity in the plasmon-catalyzed reaction in spite of a very homogeneous plasmonic enhancement of the optical signals in SERS. The results lead to the conclusion that only a fraction of the nanostructures may be responsible for a high catalytic activity. The high spot-to-spot variation in catalytic activity is also demonstrated for DMAB formation by the plasmon-catalyzed reduction from <i>p</i>-nitrothiophenol (PNTP) and confirms that an improvement of the accuracy and reproducibility in the characterization of catalytic reactions can be achieved by microspectroscopic probing of many positions. Using SERS micromapping during the incubation of PATP, we demonstrate that the reaction occurs during the incubation process and is influenced by different parameters, leading to the conclusion of dimerization in a gold-catalyzed, nonphotochemical reaction as an alternative to the plasmon-catalyzed process. The results have implications for the future characterization of new catalyst materials as well as for optical sensing using plasmonic materials

Ultrasensitive Visual Sensing of Molybdate Based on Enzymatic-like Etching of Gold Nanorods

Here, we have developed a novel approach to the visual detection of molybdate with high sensitivity and selectivity in aqueous media based on the combination of catalytic formation of iodine and iodine-mediated etching of gold nanorods. In weak acid solution, like peroxidase, molybdate can catalyze the reaction between H₂O₂ and I^– to produce I₂, a moderate oxidant, which then etches gold nanorods preferentially along the longitudinal direction in the presence of hexadecyltrimethylammonium bromide. The etching results in the longitudinal localized surface plasmon resonance extinction peak shifts to short wavelength, accompanied by a color change from blue to red. Under optimal conditions, this sensor exhibits good sensitivity with a detection limit of 1.0 nM. The approach is highlighted by its high selectivity and tolerance to interference, which enables the sensor to detect molybdate directly in real samples, such as tap water, drinking water, and seawater. In addition, perhaps the proposed sensing strategy can be also used for other targets that can selectively regulate the formation of I₂ under given conditions

Highly Sensitive Visual Detection of Copper Ions Based on the Shape-Dependent LSPR Spectroscopy of Gold Nanorods

We have developed a novel approach to the rapid visual detection of Cu²⁺ in natural samples based on the copper-mediated leaching of gold nanorods (GNRs). In the presence of hexadecyltrimethylammonium bromide, which can reduce the redox potential of Au­(I)/Au, the GNRs are catalytically etched by Cu²⁺ preferentially along the longitudinal direction. And as a result, the localized surface plasmon resonance extinction peak shifts to short wavelength, accompanied by a color change from blue to red. The leaching mechanism has been carefully discussed in a series of control experiments. Under optimal conditions, this sensor exhibits good sensitivity (LOD = 0.5 nM). Most importantly, the approach is highlighted by its high selectivity for and tolerance of interference, which enables the sensor to detect Cu²⁺ directly in a complex matrix, especially in seawater. Moreover, such a nanoparticle-based sensor is also successfully applied to test paper for the visual detection of Cu²⁺

Tailored Pore Size and Hydrophilicity: Advancing Poly(vinyl formal) Sponges for Efficient Emulsion Separation

In the field of emulsion separation, the application of porous hydrophilic sponge materials is of paramount importance; however, the conventional preparation process of hydrophilic poly(vinyl formal) (PVF) restricts the simultaneous enhancement of separation efficiency and water flux. In this work, by introducing two poly(vinyl alcohol) (PVA) and aldehyde feedstocks with different molecular weights and functional group contents into the reaction system, the differences in the mobility and reactivity of the feedstocks were exploited to successfully achieve tailoring of the pore size and hydrophilicity. The PVF sponge prepared by using this method exhibited high separation efficiency and water flux in emulsion separation. This preparation method not only avoided the requirement for pore-forming agents in traditional preparations but also significantly reduced the amount of formaldehyde by adding trace amounts of glutaraldehyde, ensuring the hydrophilicity of the sponge and markedly reducing the pore size. The experimental results showed that the addition of 1.5 × 10–3 mol/L glutaraldehyde could effectively reduce the average pore size of the PVF sponge from 26.5 to 7.2 μm while maintaining excellent hydrophilicity. The preparation mechanism of PVF sponges was thoroughly explored, and the effects of different concentrations of formaldehyde and glutaraldehyde on the sponge properties were extensively investigated. Experimental results demonstrated that the prepared PVF sponge achieved a separation efficiency of up to 98.5% for the OP-10-stabilized oil-in-water emulsion and exhibited outstanding recyclability. This cost-effective and easily scalable method for PVF sponge preparation could be employed to produce a range of highly efficient filtration materials, effectively separating target oil-in-water emulsions

Role of Metal Cations in Plasmon-Catalyzed Oxidation: A Case Study of <i>p</i>‑Aminothiophenol Dimerization

The mechanism of the plasmon-catalyzed reaction of <i>p</i>-aminothiophenol (PATP) to 4,4′-dimercaptoazobenzene (DMAB) on the surface of metal nanoparticles has been discussed using data from surface-enhanced Raman scattering of DMAB. Oxides and hydroxides formed in a plasmon-catalyzed process were proposed to play a central role in the reaction. Here, we report DMAB formation on gold nanoparticles occurring in the presence of the metal cations Ag⁺, Au³⁺, Pt⁴⁺, and Hg²⁺. The experiments were carried out under conditions where formation of gold oxide or hydroxide from the nanoparticles can be excluded and at high pH where the formation of the corresponding oxidic species from the metal ions is favored. On the basis of our results, we conclude that, under these conditions, the selective oxidation of PATP to DMAB takes place via formation of a metal oxide from the ionic species in a plasmon-catalyzed process. By evidencing the necessity of the presence of the metal cations, the reported results underpin the importance of metal oxides in the reaction

Coordination-Induced Assembly of Coordination Polymer Submicrospheres: Promising Antibacterial and in Vitro Anticancer Activities

Spheres-like coordination polymer architectures in submicro regimes have been synthesized from the hydrothermal reaction of transition metal ions and 3,5-bis­(pyridin-3-ylmethylamino)­benzoic acid (L1). The size of the final coordination polymer was dependent on the concentrations of reactants. Scanning electron microscopy studies monitored at numerous stages of growth reveal that coordination-induced morphology changes from uncoordinated flowerlike ligands to sphere-like coordination polymer particles. Moreover, variations of luminescent and antibacterial profiles are associated with coordination environments or the size of as-obtained coordination polymer samples. In addition, the newly synthesized Cu-based polymer particles may act as novel metal-based anticancer drugs in the future because of their potent in vitro anticancer activities against three chosen cancer lines MCF-7, HeLa, and NCI-H446

Catalysis by Metal Nanoparticles in a Plug-In Optofluidic Platform: Redox Reactions of <i>p-</i>Nitrobenzenethiol and <i>p-</i>Aminothiophenol

The spectroscopic characterization by surface-enhanced Raman scattering (SERS) has shown great potential in studies of heterogeneous catalysis. We describe a plug-in multifunctional optofluidic platform that can be tailored to serve both as a variable catalyst material and for sensitive optical characterization of the respective reactions using SERS in microfluidic systems. The platform enables the characterization of reactions under a controlled gas atmosphere and does not present with limitations due to nanoparticle adsorption or memory effects. Spectra of the gold-catalyzed reduction of <i>p</i>-nitrothiophenol by sodium borohydride using the plug-in probe provide evidence that the borohydride is the direct source of hydrogen on the gold surface, and that a radical anion is formed as an intermediate. The in situ monitoring of the photoinduced dimerization of <i>p</i>-aminothiophenol indicates that the activation of oxygen is essential for the plasmon-catalyzed oxidation on gold nanoparticles and strongly supports the central role of metal oxide species

Reducing Interferences from Organic Matter during Optical Environmental Detection using SERS-Silent Region Nanosensors: A Case of Nitrite Detection

Optical detection techniques are frequently used in environmental monitoring due to their high sensitivity and stability; however, to decrease the optical interferences from environmental matrix remains challenging. Herein, to decrease the optical interferences from dissolved organic matter (DOM), a surface-enhanced Raman scattering (SERS)-silent region (1800–2800 cm–1) sensor has been studied for the detection of NO2– (a model pollutant) in the presence of DOM. 4-Ethynylaniline (4-EA) was selected as the sensing molecule. The signal band (2205 cm–1) and the internal standard band of 4-EA (1985 cm–1) in the SERS-silent region are used for the quantitative analysis of NO2–. Since the SERS-silent region can well avoid the overlapping with the DOM signal, the dual-silent band ratio I2205/I1985 exhibits outstanding anti-interference ability toward different common organic matter. Compared with the traditional UV–vis detection, this method maintains a higher sensitivity (with a detection limit of 10–7 M) and a better accuracy (detection recovery ranges from 89.9 to 109.7%) for NO2– detection in real samples. Furthermore, this SERS dual-silent region sensor has been successfully employed in an environmental survey of a local river. These findings imply that the SERS-silent nanosensor provides a way to develop accurate optical detection techniques for environmental monitoring

Sulfonimide-Containing Triblock Copolymers for Improved Conductivity and Mechanical Performance

Ion-containing block copolymers continue to attract significant interest as conducting membranes in energy storage devices. Reversible addition–fragmentation chain transfer (RAFT) polymerization enables the synthesis of well-defined ionomeric A–BC–A triblock copolymers, featuring a microphase-separated morphology and a combination of excellent mechanical properties and high ion transport. The soft central “BC” block is composed of poly­(4-styrene­sulfonyl­(trifluoro­methyl­sulfonyl)­imide) (poly­(Sty-Tf₂N)) with −SO₂–N^––SO₂–CF₃ anionic groups associated with a mobile lithium cation and low-<i>T</i>_g di­(ethylene glycol)­methyl ether methacrylate (DEGMEMA) units. External polystyrene A blocks provide mechanical strength with nanoscale morphology even at high ion content. Electrochemical impedance spectroscopy (EIS) and pulse-field-gradient (PFG) NMR spectroscopy have clarified the ion transport properties of these ionomeric A–BC–A triblock copolymers. Results confirmed that well-defined ionomeric A–BC–A triblock copolymers combine improved ion-transport properties with mechanical stability with significant potential for application in energy storage devices

Platinum Nanoparticles Encapsulated in MFI Zeolite Crystals by a Two-Step Dry Gel Conversion Method as a Highly Selective Hydrogenation Catalyst

A unique and well-controllable synthesis route to encapsulate metallic nanoparticles in the interior of MFI zeolite crystals has been developed. In the first step, hierarchical micro-mesoporous ZSM-5 zeolite was obtained by alkali treatment, and the platinum was deposited mainly in the pores. Then the precursor was covered with a gel similar in composition to silicalite-1 zeolite, which was structurally converted as whole to the Pt-encapsulated MFI zeolite employing the dry gel conversion method. With this method, the metal species, content, size, and encapsulation in the zeolite are easily controllable. The highly thermally stable Pt nanoparticles encapsulated in MFI zeolites kept their original size after a high-temperature catalytic test for CO oxidation. Because of the size selectivity of the MFI zeolite, the current Pt@MFI catalyst was highly active for hydrogenation of nitrobenzene but inert for hydrogenation of 2,3-dimethylnitrobenzene. Also, the Pt@MFI catalyst is highly selective for the hydrogenation of 4-nitrostyrene, whereas impregnated Pt/ZSM-5 is totally nonselective under the same conditions. The high performance of the Pt nanoparticles encapsulated within MFI crystals should bring about opportunities for new catalytic reactions