Synthesis of Ni–Ag–ZnO solid solution nanoparticles for photoreduction and antimicrobial applications

ZnO is one of the most promising and efficient semiconductor materials for various light-harvesting applications. Herein, we reported the tuning of optical properties of ZnO nanoparticles (NPs) by co-incorporation of Ni and Ag ions in the ZnO lattice. A sonochemical approach was used to synthesize pure ZnO NPs, Ni–ZnO, Ag–ZnO and Ag/Ni–ZnO with different concentrations of Ni and Ag (0.5%, 2%, 4%, 8%, and 15%) and Ni doped Ag–ZnO solid solutions with 0.25%, 0.5%, and 5% Ni ions. The as-synthesized Ni–Ag–ZnO solid solution NPs were characterized by powdered X-ray diffraction (pXRD), FT-IR spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), UV-vis (UV) spectroscopy, and photoluminescence (PL) spectroscopy. Ni–Ag co-incorporation into a ZnO lattice reduces charge recombination by inducing charge trap states between the valence and conduction bands of ZnO and interfacial transfer of electrons. The Ni doped Ag–ZnO solid solution NPs have shown superior 4-nitrophenol reduction compared to pure ZnO NPs which do not show this reaction. Furthermore, a methylene blue (MB) clock reaction was also performed. Antibacterial activity against E. coli and S. aureus has inhibited the growth pattern of both strains depending on the concentration of catalysts.

The synergic effect of Ni and Ag in Ni–Ag–ZnO solid solutions has tuned the optoelectronic properties of ZnO for photoreduction reactions.     

Antibacterial activity of AgNPs–TiO2 nanotubes: influence of different nanoparticle stabilizers

Enhanced antibacterial properties of nanomaterials such as TiO₂ nanotubes (TNTs) and silver nanoparticles (AgNPs) have attracted much attention in biomedicine and industry. The antibacterial properties of nanoparticles depend, among others, on the functionalization layer of the nanoparticles. However, the more complex information about the influence of different functionalization layers on antibacterial properties of nanoparticle decorated surfaces is still missing. Here we show the array of ∼50 nm diameter TNTs decorated with ∼50 nm AgNPs having different functionalization layers such as polyvinylpyrrolidone, branched polyethyleneimine, citrate, lipoic acid, and polyethylene glycol. To assess the antibacterial properties, the viability of Gram-positive (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) has been assessed. Our results showed that the functional layer of nanoparticles plays an important role in antibacterial properties and the synergistic effect such nanoparticles and TiO₂ nanotubes have had different effects on adhesion and viability of G⁻ and G⁺ bacteria. These findings could help researchers to optimally design any surfaces to be used as an antibacterial including the implantable titanium biomaterials.

Synergictic antibacterial effect of AgNPs–TiO₂ nanotubes is influenced by different nanoparticle stabilizers.     

Preparation of mussel-inspired silver/polydopamine antibacterial biofilms on Ti–6Al–4V for dental applications

The properties of osseointegration and antibacterial ability is vital import for dental materials. Herein, we designed the multilayer TC4–Ag–polydopamine coatings, to provide TC4 with slow-release antibacterial properties whilst maintaining cytocompatibility. In brief, thickness of Ag inner layer can be easily controlled by magnetron sputtering technology. The resulting top polydopamine layer protected the Ag well from corrosion and gave a sustained release of Ag⁺ up to one month. In addition, the prepared TC4–Ag–polydopamine samples with Ag thickness of 20 and 30 nm, showed high hydrophilic performance with the contact-angle less than 20°, low cytotoxicity and good cytocompatibility. Expectedly, it could become a prospective candidate for future slow-release antibacterial dental materials.

The properties of osseointegration and antibacterial ability is vital import for dental materials.     

Fabrication,characterization and high photocatalytic activity of Ag–ZnO heterojunctions under UV-visible light

Pure ZnO and Ag–ZnO nanocomposites were fabricated via a sol–gel route, and the obtained photocatalysts were characterized by XRD, SEM, TEM, BET, XPS, PL and DRS. The results showed that Ag⁰ nanoparticles deposit on the ZnO surface and Ag modification has negligible impact on the crystal structure, surface hydroxyl group content and surface area of ZnO. However, the recombination of photogenerated electrons and holes was suppressed effectively by Ag loading. The photocatalytic activity was investigated by evaluating the degradation of MB under xenon lamp irradiation as the UV-visible light source, and the results show that the photocatalytic activity of ZnO significantly improved after Ag modification. Ag–ZnO photocatalysts exhibit higher photocatalytic activity than commercial photocatalyst P25. The degradation degree of MB for 1%Ag–ZnO was 97.1% after 15 min. ˙O₂⁻ radicals are the main active species responsible for the photodegradation process, and Ag–ZnO heterojunctions generate more ˙O₂⁻ radicals, which is the primary reason for the improved photocatalytic performance.

Ag–ZnO heterojunction promotes the separation of photogenerated pairs and thus exhibits high catalytic activity under UV-visible light.     

Enhanced visible-light photodegradation of fluoroquinolone-based antibiotics and E. coli growth inhibition using Ag–TiO2 nanoparticles

Antibiotics in wastewater represent a growing and worrying menace for environmental and human health fostering the spread of antimicrobial resistance. Titanium dioxide (TiO₂) is a well-studied and well-performing photocatalyst for wastewater treatment. However, it presents drawbacks linked with the high energy needed for its activation and the fast electron–hole pair recombination. In this work, TiO₂ nanoparticles were decorated with Ag nanoparticles by a facile photochemical reduction method to obtain an increased photocatalytic response under visible light. Although similar materials have been reported, we advanced this field by performing a study of the photocatalytic mechanism for Ag–TiO₂ nanoparticles (Ag–TiO₂ NPs) under visible light taking in consideration also the rutile phase of the TiO₂ nanoparticles. Moreover, we examined the Ag–TiO₂ NPs photocatalytic performance against two antibiotics from the same family. The obtained Ag–TiO₂ NPs were fully characterised. The results showed that Ag NPs (average size: 23.9 ± 18.3 nm) were homogeneously dispersed on the TiO₂ surface and the photo-response of the Ag–TiO₂ NPs was greatly enhanced in the visible light region when compared to TiO₂ P25. Hence, the obtained Ag–TiO₂ NPs showed excellent photocatalytic degradation efficiency towards the two fluoroquinolone-based antibiotics ciprofloxacin (92%) and norfloxacin (94%) after 240 min of visible light irradiation, demonstrating a possible application of these particles in wastewater treatment. In addition, it was also proved that, after five Ag–TiO₂ NPs re-utilisations in consecutive ciprofloxacin photodegradation reactions, only a photocatalytic efficiency drop of 8% was observed. Scavengers experiments demonstrated that the photocatalytic mechanism of ciprofloxacin degradation in the presence of Ag–TiO₂ NPs is mainly driven by holes and ˙OH radicals, and that the rutile phase in the system plays a crucial role. Finally, Ag–TiO₂ NPs showed also antibacterial activity towards Escherichia coli (E. coli) opening the avenue for a possible use of this material in hospital wastewater treatment.

Ag nanoparticles decorated-TiO₂ P25 are a viable alternative for the degradation, through a rutile-mediated mechanism, of fluoroquinolone-based antibiotics under visible light irradiation and, at the same time, for bacteria inactivation in water.     

Synergistic effect of Ni–Ag–rutile TiO2 ternary nanocomposite for efficient visible-light-driven photocatalytic activity

P25 comprising of mixed anatase and rutile phases is known to be highly photocatalytically active compared to the individual phases. Using a facile wet chemical method, we demonstrate a ternary nanocomposite consisting of Ni and Ag nanoparticles, decorated on the surface of XTiO₂ (X: P25, rutile (R)) as an efficient visible-light-driven photocatalyst. Contrary to the current perspective, RTiO₂-based Ni–Ag–RTiO₂ shows the highest activity with the H₂ evolution rate of ∼86 μmol g⁻¹ W⁻¹ h⁻¹@535 nm. Together with quantitative assessment of active Ni, Ag and XTiO₂ in these ternary systems using high energy synchrotron X-ray diffraction, transmission electron microscopy coupled energy dispersive spectroscopy mapping evidences the metal to semiconductor contact via Ag. The robust photocatalytic activity is attributed to the improved visible light absorption, as noted by the observed band edge of ∼2.67 eV corroborating well with the occurrence of Ti³⁺ in Ti 2p XPS. The effective charge separation due to intimate contact between Ni and RTiO₂via Ag is further evidenced by the plasmon loss peak in Ag 3d XPS. Moreover, density functional theory calculations revealed enhanced adsorption of H₂ on Ti₈O₁₆ clusters when both Ag and Ni are simultaneously present, owing to the hybridization of the metal atoms with d orbitals of Ti and p orbitals of O leading to enhanced bonding characteristics, as substantiated by the density of states. Additionally, the variation in the electronegativity in Bader charge analysis indicates the possibility of hydrogen evolution at the Ni sites, in agreement with the experimental observations.

Robust photocatalytic activity of Ni–Ag–RTiO₂ is attributed to the improved visible light absorption and effective charge separation due to intimate contact between Ni and RTiO₂via Ag, as evidenced by Ti³⁺ in Ti 2p XPS and energy dispersive mapping.     

Photocurable acrylate epoxy/ZnO–Ag nanocomposite coating: fabrication,mechanical and antibacterial properties

In this study, a UV-curable acrylate epoxy nanocomposite coating has been prepared by incorporation of ZnO–Ag hybrid nanoparticles. For this purpose, firstly ZnO–Ag hybrid nanoparticles were fabricated by a seed-mediated growth method. Then, these ZnO–Ag hybrid nanoparticles (2 wt%) were added into the UV-curable acrylate resin matrices. The photocuring process of nanocomposite was evaluated by various factors, such as the conversion of acrylate double bonds, pendulum hardness and gel fraction. Under the 4.8 s UV-exposure time for full crosslinking, the obtained data indicated that incorporation of ZnO–Ag nanohybrids into the coating matrix changed the crosslinking process of coating significantly. A mechanical teat indicated that the presence of nanohybrids in photocurable coating matrix enhanced its abrasion resistance from 98.7 to 131.6 L per mil (33.3%). The antibacterial test against E. coli over 7 h indicated that E. coli bacteria were killed totally by nanocomposite coating, whereas it was 2.6 × 10⁴ CFU mL⁻¹ for the neat coating without nanoparticles.

ZnO-Ag hybrid nanoparticles were fabricated by seed-mediated growth method and incorporated into the UV-curable acrylate resin matrice to form a composite. This improved the mechanical property of UV-cured coating and exhibited high antibacterial activity against E. coli.     

Synthesis of Sn/Ag–Sn nanoparticles via room temperature galvanic reaction and diffusion

Tin (Sn) has a low melting temperature, i.e., 231.9 °C for the bulk, and the capability to form compounds with many metals. The galvanic reaction between Sn nanoparticles (NPs) as the core and silver nitrate at room temperature under argon gas in an organic solvent without any reducing power, was employed for the first time to coat an Ag–Sn intermetallic shell, i.e., Ag₃Sn and/or Ag₄Sn, on Sn NPs. For spherical Sn NPs, the NPs retained a spherical shape after coating. Uniform and Janus structures consisting of a β-Sn core with Ag–Sn shell were observed in the resulting NPs and their population related to the input molar ratios of the metal precursors. The observation of the intermetallic shell is general for both spherical and rod-shape Sn NPs. The formation of the intermetallic shell indicated that two reactions occurred sequentially, first reduction of Ag ions to Ag atoms by the Sn core, followed by interdiffusion of Ag and Sn to form the Ag–Sn intermetallic shell.

Coating of Ag–Sn intermetallic compound on Sn nanoparticles at room temperature.     

Preparation and characterization of Ag–Pd bimetallic nano-catalysts in thermosensitive microgel nano-reactor

Thermosensitive microgels consisting of a solid core of polystyrene and a shell of cross-linked poly(N-isopropylacrylamide) (PNIPA) were synthesized as nano-reactors, in which Ag–Pd bimetallic nanoparticles were prepared through simultaneous in situ reduction reaction. The spatial distribution of metallic nanoparticles in the microgels was analyzed by small angle X-ray scattering (SAXS) and the results indicated that metal nanoparticles were mainly located in the inner layer of microgels. The catalytic activity of Ag–Pd bimetallic nanoparticles was investigated using the reduction of p-nitrophenol to p-aminophenol by NaBH₄ as model reaction. The data demonstrated that Ag–Pd bimetallic nanoparticles showed enhanced catalytic activity compared to each monometallic nanoparticle alone and their catalytic activity was controllable by temperature due to the volume transition of PNIPA microgels.

Thermosensitive microgels with PS core and cross-linked PNIPA shell were synthesized as nano-reactor to prepare Ag–Pd bimetallic nanoparticles.     

Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels: synthesis and their regulated optical and catalytic properties

Herein, we present the synthesis of Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels. The aim is to get both the surface plasmon resonance (SPR) and catalytic performance of the composite material can be changed in response to external stimuli. Ag@poly(N-isopropylacrylamide-co-3-methacryloxypro-pyltrimethoxysilane) (Ag@P(NIPAM-co-MAPTMS)) hybrid microgels were synthesized by seed-emulsion polymerization using Ag nanoparticles (NPs) as the core and NIPAM/MAPTMS as monomers. Ag–Au@P(NIPAM-co-MAPTMS) bimetallic hybrid microgels were prepared by a galvanic replacement (GR) reaction between Ag NPs and HAuCl₄, with the composition and structure of these bimetallic nanocomposites being determined by the amount of added HAuCl₄. The highly porous organic–inorganic microgel layer provided confined space for the GR reaction, effectively preventing the aggregation of Ag–Au NPs. The shell layer of P(NIPAM-co-MAPTMS) three-dimensional network chains not only enhanced nanocomposite dispersity and stability, but also provided highly porous gel microdomains that could increase the diffusion of the substrate and hence enhanced catalytic activity. Additionally, the SPR and catalytic properties of Ag–Au@P(NIPAM-co-MAPTMS) are reversibly sensitive to external temperature. With increase of temperature, the maximum absorption peak of bimetallic nanocomposites shifted to longer wavelengths, and the catalytic activity of these composites for the reduction of 4-nitrophenol by NaBH₄ remarkably increased. The features above mentioned are related to presence of the thermosensitive PNIPAM chains and the highly porous structure constructed by rigid MAPTMS segments intersected between NIPAM chains.

Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels allowed the mass transfer of reactants to be controlled by temperature modulation.     

Microwave-assisted synthesis of Ag/ZnO nanoparticles using Averrhoa carambola fruit extract as the reducing agent and their application in cotton fabrics with antibacterial and UV-protection properties

This is the first time Averrhoa carambola fruit extract has been used as a reducing agent to synthesize Ag/ZnO composites for coating cotton to develop antibacterial activity and UV protection under domestic microwave irradiation. The effects of the molar concentration of silver nitrate solutions, applied power, reaction duration, and pH on the yield of nanoparticles were determined. The treated fabrics were subjected to the investigation of surface morphology and chemical structure using SEM and EDX techniques, respectively. The antibacterial activity of the ZnO NPs and the Ag/ZnO nanocomposite coated on cotton fabric was evaluated against E. coli and S. aureus using the agar well diffusion method. The results revealed good antibacterial activity in the cotton fabric treated with the Ag-doped ZnO composite. The stability of the Ag/ZnO nanocomposite coated fabrics was determined by a wash durability test, the results of which demonstrated that this fabric could retain good antibacterial activity even after 20 wash cycles. The UV-blocking capacity of the treated fabrics was evaluated based on the ultraviolet protection factor (UPF) value determined in the range of 280–400 nm. The UPF value determined for the Ag/ZnO-coated fabric was 69.67 ± 1.53, which indicated an excellent ability to block UV radiation. Collectively, these results demonstrated the Ag/ZnO nanocomposite prepared in the present study as a promising material for preparing textiles with good antibacterial activity and UV protection.

This is the first time Averrhoa carambola fruit extract has been used as a reducing agent to synthesize Ag/ZnO composites for coating cotton to develop antibacterial activity and UV protection under domestic microwave irradiation.     

Comparative study of antidiabetic,bactericidal, and antitumor activities of MEL@AgNPs,MEL@ZnONPs,and Ag–ZnO/MEL/GA nanocomposites prepared by using MEL and gum arabic

In this study, a variety of nanocomposites, namely, MEL@AgNPs, MEL@ZnONPs, and Ag–ZnO/MEL/GA were biosynthesized using MEL and gum arabic to serve in biomedical applications. The synthesized nanocomposites were examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and FTIR spectroscopy. The physicochemical properties and biomedical activities of the synthesized nanocomposites were investigated. The Ag–ZnO/MEL/GA nanocomposites showed greater antidiabetic activity against α-amylase and α-glucosidase, and higher antibacterial activity compared to MEL@AgNPs and MEL@ZnONPs. Furthermore, HepG2 cells were exposed to MEL@AgNPs, MEL@ZnONPs, and Ag–ZnO/MEL/GA nanocomposites for 24 h and their IC₅₀ values were 63.25, 26.91 and 28.97 μg mL⁻¹ (P < 0.05), respectively. According to this comparative study, it is apparent that the Ag–ZnO/MEL/GA nanocomposites have a great potential to serve as antitumor agents against HepG2, and antidiabetic and antibacterial agents.

MEL@AgNPs, MEL@ZnONPs, and Ag–ZnO/MEL/GA nanocomposites were successfully prepared by using mannosylerythritol lipids (MEL) and gum arabic.     

Synthesis of antimicrobial AlOOH–Ag composite nanostructures by water oxidation of bimetallic Al–Ag nanoparticles

The facile one-step synthesis of AlOOH–Ag nanocomposite has been performed. Bimetallic Al–Ag nanoparticles prepared by electrical explosion of Al and Ag wires were used as a precursor. AlAg nanoparticles consisted of a supersaturated Al–6 at% Ag solid solution and Ag-rich Guinier–Preston zone several nanometer in diameter that were not detected by XRD due to their extremely small size and peculiarities of their crystal structure. An environmentally friendly process of water oxidation at 60 C was used to convert Al–Ag nanoparticles into AlOOH–Ag nanocomposites. In the course of oxidation, chemical dealloying of Al–Ag solid solution took place yielding porous agglomerates with inclusions of very fine 5–30 nm Ag nanoparticles. The agglomerates consisted of 2–5 nm thick crumpled nanosheets of boehmite 200 nm in size. The synthesized AlOOH–Ag nanocomposites possessed high antibacterial activity against both Gram-negative and Gram-positive microorganisms as indicated by the time-kill assay. The presented results open up new processing possibilities of metal-oxide composite nanostructures with attractive properties that can be used in catalysis, water purification and biomedical applications.

The facile one-step synthesis of AlOOH–Ag nanocomposite has been performed.     

Hybrid levan–Ag/AgCl nanoparticles produced by UV-irradiation: properties,antibacterial efficiency and application in bioactive poly(vinyl alcohol) films

Foodborne diseases caused by resistance of microorganisms to multiple antimicrobial agents have emerged as a major public health concern around the world. The search for potential antimicrobials has resulted in the emergence of metal nanoparticles for protection against these infections. In this study an eco-friendly and green approach was used to biosynthesize hybrid Ag/AgCl nanoparticles (NPs), using levan from Bacillus mojavensis as a stabilizing/reducing agent, with a high efficiency against a broad spectrum of foodborne bacteria as well as biofilm formations. The morphology and physicochemical characteristics of levan–Ag/AgCl NPs were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-vis spectroscopy (UV), dynamic light scattering (DLS) and thermogravimetric analysis (TGA). The hybrid levan–Ag/AgCl was evaluated for antibacterial activity against foodborne pathogenic bacteria (Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Pseudomonas aeruginosa, Staphylococcus aureus, Micrococcus luteus, Listeria monocytogenes, Enterococcus faecalis, Bacillus subtilis and Bacillus thuringiensis). The study demonstrated the strong efficiency of hybrid levan–Ag/AgCl NPs as a potent inhibitor against all tested strains, with much higher activity against Gram-negative than Gram-positive bacteria. Furthermore, bacterial strains were found to be highly sensitive to hybrid levan–Ag/AgCl NPs in comparison to the tested antibiotics. As a possible application of levan–Ag/AgCl NPs as an additive in packaging, PVA films with different amounts of hybrid levan–Ag/AgCl NPs were prepared by casting and their antibacterial, mechanical, and optical properties and ability to expand the shelf life of beef meat were explored. Interestingly, the amount of Ag leached out from films was below the permissible limit. This work demonstrates the strong antibacterial action of hybrid levan–Ag/AgCl NPs and their potential use in bioactive packaging material.

Hybrid Ag/AgCl nanoparticles with high antibacterial activity were synthesised using bacterial levan.     

Ultrafast nonlinear optical properties and carrier dynamics of silver nanoparticle-decorated ZnO nanowires

Silver (Ag) nanoparticle-decorated zinc oxide (ZnO) nanowires (Ag–ZnO) have been successfully synthesized by chemical vapour deposition and the magnetron sputtering method. Scanning electron microscopy images indicate that Ag nanoparticles are distributed uniformly on the surface of the ZnO nanowires. The results of room temperature photoluminescence (RTPL) reveal two major emission peaks for the Ag–ZnO nanowires, and the emission peaks in the visible region are stronger than those of the unmodified ZnO nanowires. The mechanism of RTPL and low temperature photoluminescence (LTPL) emission is discussed in detail. Nonlinear optical properties and ultrafast dynamics have been investigated using the Z-scan and two color pump–probe (TCPP) techniques, respectively. The nonlinear absorption properties in the nano-, pico- and femto-second regime have been analyzed using the singlet state three-level and four-level models, respectively. The samples show self-focusing nonlinearity and good two-photon absorption (TPA)-induced ground state saturation absorption as well as excited state reverse saturable absorption behavior. For the nanosecond and picosecond pulses, the reverse saturated absorption in the excited state mainly originates from the absorption at low excited states or deep levels; however, for the femtosecond pulse, it is caused by the absorption at high excited states. The TCPP results show that the ground state or deep level light bleaching (for nano- and pico-second regime) and TPA-induced excited-state absorption (for femtosecond regime) behaviors range from 470 nm to 620 nm. The remarkable nonlinear optical properties reveal that Ag–ZnO nanowires are potential nanocomposite materials for the development of nonlinear optical devices.

Silver (Ag) nanoparticle-decorated zinc oxide (ZnO) nanowires (Ag–ZnO) have been successfully synthesized by chemical vapour deposition and the magnetron sputtering method.     

Efficient photocatalytic degradation of dyes using photo-deposited Ag nanoparticles on ZnO structures: simple morphological control of ZnO

In this work, morphology-controlled ZnO structures were prepared via a hydrothermal method by simple adjustments in the NaOH concentration. The NaOH concentration variation from 0.2 to 1.2 M resulted in the formation of ZnO structures in shapes such as walnut, spherical flower, flower, rod, and urchin-like. The extent of OH⁻ ions is the main factor influencing the growth of ZnO structures. Well-defined morphologies, good crystallinity, and optical properties were obtained for all ZnO structures. Among these ZnO structures, ZnOsf (spherical flower-like) structure showed a greater percentage of photodegradation of methyl orange and rhodamine B dyes. Surface plasmon resonance was achieved by modifying the surface of ZnO with Ag nanoparticles. ZnOsf was loaded with Ag nanoparticles by a facile photo-deposition method. Ag–ZnOsf showed superior photoactivity and recyclability for the degradation of methyl orange and rhodamine B. Therefore, modification of different ZnO structures can help realize potential catalysts for future environmental applications.

Morphology control of ZnO structures were fabricated by hydrothermal method with simple adjustments of NaOH concentration and Ag–ZnO composite showed superior photoactivity and recyclability for the degradation of MO and RhB.     

Au–Ag and Pt–Ag bimetallic nanoparticles@halloysite nanotubes: morphological modulation,improvement of thermal stability and catalytic performance

In this study, Au–Ag and Pt–Ag bimetallic nanocages were loaded on natural halloysite nanotubes (HNTs) via galvanic exchange based on Ag@HNT. By changing the ratio of Au to Ag or Pt to Ag in exchange processes, Au–Ag@HNT and Pt–Ag@HNT with different nanostructures were generated. Both Au–Ag@HNT and Pt–Ag@HNT systems showed significantly improved efficiency as peroxidase-like catalysts in the oxidation of o-phenylenediamine compared with monometallic Au@HNT and Pt@HNT, although inert Ag is dominant in the composition of both Au–Ag and Pt–Ag nanocages. On the other hand, loading on HNTs enhanced the thermal stability for every system, whether monometallic Ag nanoparticles, bimetallic Au–Ag or Pt–Ag nanocages. Ag@HNT sustained thermal treatment at 400 °C in nitrogen with improved catalytic performance, while Au–Ag@HNT and Pt–Ag@HNT maintained or even had slightly enhanced catalytic efficiency after thermal treatment at 200 °C in nitrogen. This study demonstrated that natural halloysite nanotubes are a good support for various metallic nanoparticles, improving their catalytic efficiency and thermal stability.

Bimetallic Au–Ag@HNT and Pt–Ag@HNT nanocages showed significantly improved efficiency in the oxidation of o-phenylenediamine as peroxidase-like catalyst compared with corresponding monometallic nanoparticles.     

Synthesis and characterization of tannic acid–PEG hydrogel via Mitsunobu polymerization

Tannic acid (TA) based materials have received significant interest owing to their broad spectrum of chemical and biological properties. Herein, a novel tannic acid based hydrogel, TA–PEG hydrogel, was synthesized via Mitsunobu polymerization/polycondensation, in which TA and polyethylene glycol (PEG) were simply crosslinked together by ether linkages. This method was performed in one pot, straightforward, metal free and robust, ignoring the strong ionic/hydrophobic interactions between tannic acid and PEG. Bearing catechol and pyrogallol units from TA, TA–PEG hydrogel did not only reduce the silver and gold precursor, but also served as a capping agent and stabilizer for the in situ formed Au and Ag nanoparticles (NPs). Furthermore, the antioxidant activity of the hydrogel was excellent (94%) in the case of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging. TA–PEG hydrogel also showed antibacterial activity against Staphylococcus aureus and Escherichia coli. This work suggested a new method leading to polyphenol based soft materials rather than a complex coacervated microstructure. The resulting TA–PEG hydrogel has potential application in biomedical materials.

Mitsunobu polymerization was involved to construct bulk hydrogel via direct PEGylation of tannic-acid under mild condition.     

Organic–inorganic hybrid liquid crystals of azopyridine-enabled halogen-bonding towards sensing in aquatic environment

In this study, organic–inorganic hybrid mesogens of silver nanoparticles (Ag NPs) and azopyridines (AzoPys) enabled by halogen bonding were prepared. Triple functions of the degree of orientation change, metal-enhanced fluorescence, and surface-enhanced Raman scattering were observed in Ag⋯Br–Br⋯AzoPy nanoparticles (12Br–Ag), which were induced by the in situ synthesis of Ag NPs in AzoPy. The bromine molecules were then linked by halogen bonding and electrostatic interaction resulting in the smectic A phase of 12Br–Ag. To demonstrate the potential of Br–Br⋯AzoPy (12Br) as a practical sensor, we used the 12Br compound to detect silver in an aqueous condition, and significant signals of the halogen-bonded complex-silver system were observed in the X-ray diffraction pattern and Raman spectra. Herein, we provide a novel perspective and design principle for the practical applications of organic–inorganic hybrid liquid crystals in environmental monitoring.

Organic–inorganic hybrid liquid crystals of azopyridines enabled by halogen bonding with the triple functions of degree of orientation change, MEF, and SERS towards sensing silver in aquatic environment.

Halogen bonding is an attractive intermolecular interaction for adjusting the interaction strength by choosing suitable halogen atoms that participate in the bond formation without significantly changing the electronic structure of the compound.¹ Azobenzene-containing composites are of great interest, having promising applications in various fields, such as, optical data storage, owing to the reversible trans–cis photoisomerisation of azobenzene chromophores.^2–5 The halogen-bonded liquid crystals of azobenzene were first reported in 2012.⁶ Currently, many studies describe halogen bonding using azobenzene in the field of organic-liquid crystals^7,8 and photoresponsive nanocomposites;^9–12 however, few studies have investigated the metal-containing azobenzene mesogens. Recently, azobenzene hybrid mesogen-capped thiolated ligands of gold and silver nanoparticles with lamellar and columnar superstructures were achieved and changed the phase transitions significantly; they can behave as intriguing photo-switched temperature sensors with storage and erase functionality with well-organised hybrid systems.¹³ The coupling of the azobenzene mesogen with inorganic nanoparticles has recently become an attractive approach as it can give rise to novel hybrid materials in which the properties of the two components are mutually enhanced.¹⁴ However, studies have not explored organic–inorganic hybrid liquid crystals of azobenzene-containing composites with intermolecular interactions for this purpose, not to mention exploring their applications for the next generation of liquid crystal sensors.Herein, we report azopyridine (AzoPy) mesogens embedded with silver nanoparticles (Ag NPs) obtained by halogen-bonding. Although the texture or degree of orientation remained unchanged in Ag⋯Br–Br⋯AzoPy (12Br–Ag) compared to that in Br–Br⋯AzoPy (12Br), a clear birefringence change was detected, which was attributed to the variation in the electronic properties of 12Br–Ag with a shorter d-spacing. Meanwhile, the halogen bonding intensified the metal-enhanced fluorescence (MEF) of the AzoPy ligand by inserting Br₂ between the Ag NPs and ligand molecules in the solution along with the Surface-enhanced Raman (SER) scattering but not in the condensed phases (crystal or mesophase). By taking advantage of the assisted enhancement of MEF and SER factors of halogen bonding, we report a new approach for the sewage detection by identifying the trace metals using a halogen-bonded liquid crystal as a sensor in water owing to its high sensitivity; it may possess the potential to transform the waste metal pollution into treasure.The organic–inorganic hybrid liquid crystals of azopyridines are shown in Scheme 1. Generally, there are two approaches for preparing nanoparticles: ex situ and in situ (direct growth). The in situ synthesis induces the morphology-controlled growth of nanoparticles.^15,16 Thus, we adopted the in situ synthesis of Ag NPs (10 wt%) with the ligand AzoPy, followed by a treatment with molecular Br₂ to obtain the organic–inorganic hybrid liquid crystals of Ag NPs and AzoPy. The synthesis protocols are given in the ESI.^† The resulting Ag⋯Br–Br⋯AzoPy nanoparticles were characterized by scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), UV-vis absorption, and Raman spectroscopy.Open in a separate windowScheme 1Schematic illustration of organic–inorganic hybrid liquid crystals (Ag⋯Br–Br⋯AzoPy).The STEM, TEM, and EDS elemental mapping were performed on the hybrid Ag NPs. As shown in Fig. 1(a), the Ag NPs were covered by an organic corona of the halogen-bonded complex Br–Br⋯AzoPy, which is a result of the interactions between Ag NPs and a pyridyl moiety in AzoPy linked molecular Br₂ by halogen bonding and electrostatic interactions. Moreover, the EDS mapping data support the presence of Ag and Br atoms in the hybrid nanoparticles (Fig. 1(b)). Fig. 1(c) shows an organic layer surrounding the Ag NPs with a small thickness, and the EDS mapping demonstrates the presence of Ag (Fig. 1(d)) and Br (Fig. 1(e)) in the hybrid nanoparticles. The HR-TEM images also showed distinct lattice fringe patterns, indicating the highly crystalline nature of the Ag nanocrystals. The obtained lattice spacing was 0.25 nm, which agrees with the (111) plane for the face-centred cubic (fcc) crystal structure of bulk Ag.¹⁷ The collective EDS mapping and STEM/TEM results provide a definitive evidence for the stabilised halogen-bonded complex of Ag NPs. However, a structure of Ag⋯AzoPy nanoparticles devoid of halogen bonding was not identified (Fig. S1(a)).^†Open in a separate windowFig. 1The STEM image (a), EDS Ag and Br elemental mapping image (b), and HR-TEM image (c) of the di-noncovalent bonded Ag⋯Br–Br⋯AzoPy nanoparticle compared to EDS elemental mapping images of Ag (d) and Br (e). Mapping of the Ag region is depicted in yellow, while the Br region is shown in red.The powder XRD was performed to confirm the structure of nanoparticles. The d-spacing, determined by the deconvolution of a specific diffraction peak in the XRD pattern following Vegard''s law, predicts a linear relationship between the crystal lattice parameter and the concentration of the constituent elements.^17,18 As shown in Fig. 2(a), the diffraction peaks of Ag (111), (200), (220), and (311) planes of Ag NPs were located at 2θ = 38°, 45°, 64°, and 77°, respectively. These distinct peaks are the structural features of Ag NPs, which also appear in the XRD pattern of Ag⋯AzoPy and Ag⋯Br–Br⋯AzoPy nanoparticles, confirming the successful formation of organic–inorganic hybrid complexes of Ag NPs.^19–21Open in a separate windowFig. 2The X-ray diffraction patterns (a), UV-vis absorption spectra of Ag NPs, AzoPy, Ag⋯AzoPy, Br–Br⋯AzoPy, and Ag⋯Br–Br⋯AzoPy in acetone (dashed line: predicted diffraction peaks for Ag metal) (b), and the Raman spectra of AzoPy, Br–Br⋯AzoPy, Ag⋯AzoPy, and Ag⋯Br–Br⋯AzoPy nanoparticles in the solid state (c).The UV-vis absorption spectra of Ag NPs, AzoPy, Ag⋯AzoPy nanoparticles, Br–Br⋯AzoPy, and Ag⋯Br–Br⋯AzoPy nanoparticles in acetone are shown in Fig. 2(b). The surface plasmon resonance (SPR) band of pristine Ag NPs is barely detectable because of the insolubility of Ag NPs in acetone. The absorption maximum (λ_max) of AzoPy and Ag⋯AzoPy nanoparticles are located in the same band at 352 nm, and the SPR band of Ag in Ag⋯AzoPy nanoparticles is absent because of the weak interaction between the electronic doublet of the nitrogen atom of the pyridyl moiety and the Ag surface.^22–24 However, compared to the Br–Br⋯AzoPy, the Ag⋯Br–Br⋯AzoPy nanoparticles showed a large redshift with λ_max at 393 nm because of the incorporation of Ag NPs to AzoPy, demonstrating a stronger interaction of Br–Br⋯AzoPy with Ag NPs than with Ag⋯AzoPy nanoparticles, leading to the formation of the Ag⋯Br noncovalent bond composites.In addition, the UV-vis absorption spectra of Ag⋯Br–Br⋯AzoPy nanoparticles at different concentrations of Ag NPs in acetone and a mixture of surface-stabilised Ag NPs and the AzoPy ligand in the solution were evaluated (Fig. S2^†).Further investigation of the Ag⋯Br–Br⋯AzoPy nanoparticles was corroborated by the Raman spectra. The Raman spectra of Ag⋯AzoPy nanoparticles showed a strong band at 249 cm⁻¹, characteristic of adsorbed pyridine groups compared to AzoPy (Fig. 2(c)), which is similar in frequency to the strong bands reported for pyridine adsorbed on Ag electrodes²⁵ and Ag sols.²⁶ However, the selective Raman enhancement by excitation at 785 nm was observed, corresponding to the characteristic Ag⋯Br vibrations at 151.8 cm⁻¹ in Ag⋯Br–Br⋯AzoPy nanoparticles, which is a result of the complexation between halogen-bonded azodye molecules Br–Br⋯AzoPy and Ag NPs after adding Br₂ to Ag⋯AzoPy nanoparticles.This extraordinary enhancement was caused by the metal–molecule interaction, which involves the exchange mixing of Ag electron–hole pairs and the HOMO–LUMO excited state of bromine molecules.^27,28 These results confirm that Ag⋯Br–Br⋯AzoPy nanoparticles were successfully assembled based on the interaction of Br₂ with the N atom of the pyridyl moiety by bromine bonding and the surface of Ag NPs by the electrostatic interaction. Furthermore, the XPS and ¹H NMR spectra were recorded to further ascertain the bonding between Br–Br⋯AzoPy ligand molecules and Ag NPs (Fig. S3–S5^†) Fig. 3(a) shows the mesophase produced by the organic–inorganic hybrid complexes via the in situ synthesis of Ag NPs in AzoPy, and then linked by the halogen bonding, which stabilised the mesophase with an increase of 10.3 °C in the liquid crystal to isotropic transition compared with 12Br in heating cycle owing to the chemisorption of mesogens on nanoparticles.²⁹ This is different from the physical mixture of liquid crystals and nanoparticle hybrid systems, which lower the temperature (destabilisation) of the liquid crystal phases with the metal nanoparticles. The POM images of 12Br–Ag show focal conic fan textures between plain glass slides (Fig. 3(b)) and a uniform dark image in a vertical-orientation cell at their liquid crystal temperatures (Fig. 3(c)), which is similar to the smectic A phase of the pristine materials (12Br).⁸ However, the photochemical phase transition in 12Br–Ag induced by UV irradiation was unlike that of the previously reported⁸ brominated compounds (12Br; Fig. 3(d)).Open in a separate windowFig. 3The DSC measurements of composites 12Br and 12Br–Ag in heating and cooling cycle (a), POM images of 12Br–Ag at 120 °C on cooling from the isotropic phase (b), in vertical orientation cell in liquid crystal temperature (c), and upon UV irradiation (d).After the annealing process, the SAXS experiments were employed to confirm the type of the smectic mesophase, as shown in Fig. 4. Two peaks with q values of 1.36 and 2.78 nm⁻¹ were detected for 12Br–Ag, similar to those observed for 12Br. The ratio of the scattering vectors of these two peaks was 1 : 2, indicating a smectic packing of 12Br–Ag. Using the MM2 force field method, it was determined that the d-spacings of the first diffraction peaks of 12Br and 12Br–Ag were 5.10 and 4.64 nm approximately, which is 1.67 and 1.41 times the length of the calculated molecular 12Br (l = 3.06 nm) and 12Br–Ag complexes (l = 3.26 nm, the length includes the non-interaction distance between the peripheral bromine atom and the Ag NPs), respectively. Thus, the structures of 12Br and 12Br–Ag could be interdigitated smectic A phase, in which parts of the 12Br–Ag molecules overlap (see the inset picture in Fig. 4). The d/l ratio of 12Br–Ag decreases compared to that of 12Br, probably because of the distinct electronic properties between them, which were induced by the in situ synthesis of Ag NPs in AzoPy.Open in a separate windowFig. 4The SAXS patterns of 12Br and 12Br–Ag after the annealing process with intensity in log scale. The inset picture is the schematic drawing of the smectic packing of 12Br–Ag.The directing abilities of the Ag NPs are selective to surpass those of the alignment layers of the cell according to the temperature condition, which implies that doping with Ag NPs in the liquid crystal plays a dominant role in changing the birefringence by decreasing the d-spacing and inhibiting the orientation, as shown in Fig. S6 and S7.^†Furthermore, when dyes are localised near either free or immobilised metal particles, their luminescence (i.e., fluorescence) intensifies, which is known as metal-enhanced fluorescence (MEF).^30–32 To investigate the occurrence and intensification of MEF in AzoPy, the fluorescence emission of the free AzoPy ligands and their corresponding complexes with Ag NPs were evaluated, as shown in Fig. S8(a).^† It is important to note that the fluorescence intensity of Ag⋯Br–Br⋯AzoPy nanoparticles was approximately three times higher than that of Ag⋯AzoPy nanoparticles. This implies that the insertion of Br₂ between the AzoPy ligand and Ag nanoparticles intensifies the MEF of the ligand. This interesting characteristic is attributed to the increased distance between the azodye molecule and the Ag NP surface due to the presence of the intervening Br₂. As shown in Fig. S9,^† the MM2 molecular mechanics calculations support this hypothesis.The influence of the concentration of Ag NPs, different excitation wavelengths, and doping method on the fluorescence enhancement effect was evaluated in detail (Fig. S8(b–d) and S10^†). The fluorescence quantum yield and fluorescence lifetime of the azodye functionalised Ag NPs were determined, as shown in Fig. S11, S12 and Table S1.^†To investigate the viability of halogen-bonded liquid crystals for application as sensors in metal pollution, inspired by the above-mentioned intensification phenomenon of the MEF and SERS of the organic and inorganic hybrid liquid crystal, 12Br was used for detecting silver components under realistic conditions, such as liquid waste produced in the preparation of conductive silver ink and tap water. As shown in Fig. 5(a), the diffraction peaks of Ag (200), (220), and (311) planes of Ag NPs in liquid waste are located at 45°, 64°, and 74°, respectively; thus, recovering silver from the waste liquid. Thereafter, trace amounts of silver were further detected in tap water. Although the content of silver in the tap water was extremely low, the detection signals of the Ag (111) and (200) planes at 2θ = 38° and 45° could also be detected by 12Br due to the interaction between 12Br and silver in water with high sensitivity.Open in a separate windowFig. 5The XRD pattern and Raman spectra of 12Br in liquid waste produced from the preparation of conductive silver ink and tap water (100 mg mL⁻¹), respectively.In addition to XRD characterisation, we studied the detection of silver in liquid waste and tap water via Raman spectroscopy. Compared to 12Br–Ag, the Raman peak of the bromine stretching vibration frequency of 12Br appeared at 222.4 cm⁻¹ and 166.8 cm⁻¹, but no SERS signals were detected because of weak interaction obtained by mixing them directly, as shown in Fig. 5(b). Instead, the N N stretching and azobenzene quadrant stretching at 1414 cm⁻¹ and 1452 cm⁻¹ became active due to the interaction of the azobenzene ring and N atom with silver. These results confirm that the 12Br halogen-bonded liquid crystal is capable of detecting silver in a water environment and has the potential to transform metal pollution into valuable organic–inorganic hybrid mesogenic materials.In conclusion, Br₂ was successfully introduced as a bridge between Ag NPs and the AzoPy ligand by electrostatic interaction and halogen bonding at the Ag NP surface and azodye end, respectively, thereby displaying liquid crystalline properties with the intensification of the MEF and SERS. The 12Br–Ag was difficult to orientate by the alignment layers of the cell in the liquid crystal temperature but could orientate after the annealing treatment at room temperature. More importantly, the bromine-bonded complex was used as a sensor for detecting silver in aqueous fluids and was capable of transforming waste metal pollution into valuable organic–inorganic hybrid liquid crystals for potential ultrasensitive detection in the water-environment monitoring and analytical chemistry.     

Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation. Initially, pure Ag and Cu NPs were produced individually using the laser ablation in liquid technique (with ∼50 femtosecond pulses at 800 nm) which was followed by laser irradiation of the mixed Ag and Cu NPs in equal volume. These Ag, Cu, and Ag–Cu NPs were characterised by UV-visible absorption, HRTEM and XRD techniques. The alloy formation was confirmed by the presence of a single surface plasmon resonance peak in absorption spectra and elemental mapping using FESEM techniques. Furthermore, the results from surface enhanced Raman scattering (SERS) studies performed for the methylene blue (MB) molecule suggested that Ag–Cu alloy NPs demonstrate a higher enhancement factor (EF) compared to pure Ag/Cu NPs. Additionally, SERS studies of Ag–Cu alloy NPs were implemented for the detection of explosive molecules such as picric acid (PA – 5 μM), ammonium nitrate (AN – 5 μM) and the dye molecule methylene blue (MB – 5 nM). These alloy NPs exhibited superiority in the detection of various analyte molecules with good reproducibility and high sensitivity with EFs in the range of 10⁴ to 10⁷.

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation.