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.     

The photoluminescence,field emission and femtosecond nonlinear absorption properties of Al-doped ZnO nanowires,nanobelts, and nanoplane-cone morphologies

Al-doped ZnO (AZO) nanowires, nanobelts and nanoplane-cone nanostructures have been successfully synthesized. The structural, photoluminescence (PL) and field emission (FE) properties of AZO nanowires have been characterized. The dependence of the PL properties of AZO nanostructures versus excitation laser power in the range from 1 to 12 mW and temperature in the range of 10–273 K was discussed. The PL measurement results demonstrated that the ultraviolet emission came from a near band edge emission, and two peaks in visible light region were due to deep-level emission. Moreover, the AZO nanowires have a relatively stronger ultraviolet emission than other kinds of samples. The FE measurements indicate that the turn-on field for the nanoplane-cone structure is 2.52 V μm⁻¹, which is smaller than 4.42 V μm⁻¹ for nanowires and 5.28 V μm⁻¹ for nanobelts. In addition, the nonlinear absorption properties of AZO nanowires were measured using a femtosecond Z-scan technique. The effect of morphology on the nonlinear optical absorption properties of AZO nanowires was studied. From the results, the AZO nanowires show reverse saturable absorption (RSA) behavior. Furthermore, the results show that the order of magnitude of the nonlinear absorption coefficient for AZO nanowires is ∼10⁻² cm³ GW⁻². Our results show that AZO films are a promising candidate in further optoelectronic device applications.

Al-doped ZnO (AZO) nanowires, nanobelts and nanoplane-cone nanostructures have been successfully synthesized.     

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 antibacterial properties of biocompatible titanium via electrochemically deposited Ag/TiO2 nanotubes and chitosan–gelatin–Ag–ZnO complex coating

A novel double-layered antibacterial coating was fabricated on pure titanium (Ti) via a simple three-step electrodeposition process. Scanning electronic microscopy (SEM) images show that the coating was constructed with the inner layer of TiO₂ nanotubes doped with silver nanoparticles (TNTs/Ag) and the outer layer of chitosan–gelatin mixture with zinc oxide and silver nanoparticles (CS–Gel–Ag–ZnO). In comparison, we also investigated the composition, structure and antibacterial properties of pure Ti coated with TNTs, TNTs/Ag or TNTs/Ag + CS–Gel–Ag–ZnO, respectively. The TNTs was about 100 nm wide and 240 nm to 370 nm tall, and most Ag nanoparticles (Ag NPs) with diameter smaller than 20 nm were successfully deposited inside the tubes. The CS–Gel–Ag–ZnO layer was continuous and uniform. Antibacterial activity against planktonic and adherent bacteria were both investigated. Agar diffusion test against Staphylococcus aureus (S. aureus) shows improved antibacterial capacity of the TNTs/Ag + CS–Gel–Ag–ZnO coating, with a clear zone of inhibition (ZOI) up to 14.5 mm wide. Dead adherent bacteria were found on the surface by SEM. The antibacterial rate against planktonic S. aureus was as high as 99.2% over the 24 h incubation period.

A novel complex antibacterial coating fabricated via a simple three-step electrodeposition process shows high antibacterial rate of 99.2%.     

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.     

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.     

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.     

Insights into non-noble metal based nanophotonics: exploration of Cr-coated ZnO nanorods for optoelectronic applications

Herein, the room temperature photoluminescence and Raman spectra of hydrothermally grown ZnO nanorods coated with Cr are investigated for optoelectronic applications. A thorough examination of the photoluminescence spectra of Cr coated ZnO nanorods showed the suppression of deep level emissions by more than twenty five times with Cr coating compared to that of pristine ZnO nanorods. Moreover, the underlying mechanism was proposed and can be attributed to the formation of Schottky contacts between Cr and ZnO resulting in defect passivation, weak exciton–plasmon coupling, enhanced electric field effect and formation of hot carriers due to interband transitions. Interestingly, with the increase in sputtering time, the ratio of the intensities corresponding to the band gap emission and deep level emission was observed to increase from 6.2 to 42.7, suggesting its application for UV only emission. Further, a planar photodetector was fabricated (Ag–ZnO–Ag planar configuration) and it was observed that the dark current value got reduced by more than ten times with Cr coating, thereby opening up its potential for transistor applications. Finally, Cr coated ZnO nanorods were employed for green light sensing. Our results demonstrated that ZnO nanorods decorated with Cr shed light on developing stable and high-efficiency non-noble metal based nanoplasmonic devices such as photodetectors, phototransistors and solar cells.

Herein, the room temperature photoluminescence and Raman spectra of hydrothermally grown ZnO nanorods coated with Cr are investigated for optoelectronic applications.     

Spitzer shaped ZnO nanostructures for enhancement of field electron emission behaviors

We observed enhanced field emission (FE) behavior for spitzer shaped ZnO nanowires synthesized via a hydrothermal approach. The spitzer shaped and pointed tipped 1D ZnO nanowires of average diameter 120 nm and length ∼5–6 μm were randomly grown over an ITO coated glass substrate. The turn-on field (E_on) of 1.56 V μm⁻¹ required to draw a current density of 10 μA cm⁻² from these spitzer shaped ZnO nanowires is significantly lower than that of pristine and doped ZnO nanostructures, and MoS₂@TiO₂ heterostructure based FE devices. The orthodoxy test that was performed confirms the feasibility of a field enhancement factor (β_FE) of 3924 for ZnO/ITO emitters. The enhancement in FE behavior can be attributed to the spitzer shaped nanotips, sharply pointed nanotips and individual dispersion of the ZnO nanowires. The ZnO/ITO emitters exhibited very stable electron emission with average current fluctuations of ±5%. Our investigations suggest that the spitzer shaped ZnO nanowires have potential for further improving in electron emission and other functionalities after forming tunable nano-hetero-architectures with metal or conducting materials.

Spitzer shaped ZnO nanowires had a reduced work function providing a significantly smaller barrier for the direct emission of an electron toward the emission site and contributed to a lowest turn-on field of 1.56 V μm⁻¹.     

Optoelectronic,femtosecond nonlinear optical properties and excited state dynamics of a triphenyl imidazole induced phthalocyanine derivative

A novel zinc phthalocyanine derivative [2(3), 9(10), 16(17), 23(24) tetrakis-4-((4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)ethynyl)phthalocyanine zinc(ii) (PBIPC)] was synthesized by incorporating a triphenyl imidazole moiety at its peripheral positions. The detailed mechanisms of absorption, emission, electrochemical, nonlinear optical (NLO) and photophysical (excited state dynamics) properties of PBIPC were explored. The absorption and emission properties of the compound were studied in different solvents. The incorporation of a triphenyl imidazole moiety at the peripheral position of the zinc phthalocyanine slightly broadened the Soret band. The emission studies revealed fluorescence quantum yields to be in the range of 0.11–0.22. The time-resolved fluorescence data established the radiative lifetimes to be in the nanosecond range. The oxidation and reduction processes were found to be ring centered, which were studied using the cyclic voltammetry (CV) technique. The energy optimized structures and HOMO–LUMO levels were calculated using DFT, TD-DFT analysis and were employed by means of hybrid functional theory (B3LYP) at 6-31G (d,p) basis set in the Gaussian 09 package. Two-photon absorption was observed in the NLO studies performed in the visible wavelength range of 600–800 nm while the nonlinear absorption was dominated by three- and four-photon absorption processes in the NIR wavelength range (1.0–1.5 μm). The molecule exhibited self-focusing behavior for all the wavelengths. Finally, the excited state dynamics of the title molecule PBIPC were investigated using femtosecond transient absorption spectroscopy and the results obtained were understood on the basis of a simple three kinetic model, for excitation wavelengths of 400 nm (Soret band) and 650 nm (Q-band). Both the spectra demonstrated a broad positive transient absorption (TA) data which overlapped with the ground state bleach (GSB), which in turn displayed a red shift over a delay of ∼2 ns. The lifetimes revealed a possibility of intersystem crossing (τ > 1 ns) owing to the triplet state transition.

A novel zinc phthalocyanine derivative PBIPC was synthesized by incorporating a triphenyl imidazole moiety at its peripheral positions.     

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.     

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.     

The structural transition of bimetallic Ag–Au from core/shell to alloy and SERS application

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS). In recent years, there have been many studies on the fabrication of bimetallic nanostructures. However, controlling the shape, size, and structure of bimetallic nanostructures still has many challenges. In this work, we present the results of the synthesis of silver nanoplates (Ag NPls), and Ag–Au bimetallic core/shell and alloy nanostructures, using seed-mediated growth under green LED excitation and a gold salt (HAuCl₄) as a precursor of gold. The results show that the optical properties and crystal structure strongly depend on the amount of added gold salt. Interestingly, when the amount of gold(x) in the sample was less than 0.6 μmol (x < 0.6 μmol), the structural nature of Ag–Au was core/shell, in contrast x > 0.6 μmol gave the alloy structure. The morphology of the obtained nanostructures was investigated using the field emission scanning electron microscopy (FESEM) technique. The UV–Vis extinction spectra of Ag–Au nanostructures showed localized surface plasmon resonance (LSPR) bands in the spectral range of 402–627 nm which changed from two peaks to one peak as the amount of gold increased. Ag–Au core/shell and alloy nanostructures were utilized as surface enhanced Raman scattering (SERS) substrates to detect methylene blue (MB) (10⁻⁷ M concentration). Our experimental observations indicated that the highest enhancement factor (EF) of about 1.2 × 10⁷ was obtained with Ag–Au alloy. Our detailed investigations revealed that the Ag–Au alloy exhibited significant EF compared to pure metal Ag and Ag–Au core/shell nanostructures. Moreover, the analysis of the data revealed a linear dependence between the logarithm of concentration (log C) and the logarithm of SERS signal intensity (log I) in the range of 10⁻⁷–10⁻⁴ M with a correlation coefficient (R²) of 0.994. This research helps us understand better the SERS mechanism and the application of Raman spectroscopy on a bimetallic surface.

It is well-known that Ag–Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS).     

Curcumin loaded Ag–TiO2-halloysite nanotubes platform for combined chemo-photodynamic therapy treatment of cancer cells

The use of naturally occurring anticancer materials in combination with doped metal oxide has emerged as one of the most promising ways for improving anticancer treatment efficacy. In this study, the anticancer potential of curcumin-loaded Ag–TiO₂-halloysite nanotubes (curcumin-loaded Ag–TiO₂-HNTs) was examined. Ag–TiO₂-HNTs with different wt% of Ag–TiO₂ were synthesized and characterized using XRD, TGA, FT-IR, UV-Vis spectroscopy, and SEM-EDX. The XRD results revealed the presence of crystalline TiO₂. However, the presence of Ag was detected through the SEM-EDX analysis. Cyclic voltammetry measurements suggested the enhancement of the release of ROS from TiO₂ upon deposition with Ag. FT-IR and TGA analysis confirmed the successful loading of curcumin inside the nanotubes of the halloysite. In vitro drug released studies revealed the release of approximately 80–99% curcumin within 48 hours. Kinetic model studies revealed that the release of curcumin from HNT and Ag–TiO₂-HNT followed the first-order and Higuchi models, respectively. The light irradiated curcumin-loaded Ag–TiO₂-HNTs samples exhibited considerable anticancer potential as compared to the free curcumin, irradiated Ag–TiO₂ NPs samples, and unirradiated curcumin loaded Ag–TiO₂-HNTs samples. The obtained results revealed that combined chemo- and photodynamic therapy using curcumin-loaded Ag–TiO₂-HNTs nanomaterial has the potential as an effective anticancer treatment method.

The synergy between photogenerated reactive oxygen species and the anticancer potency of curcumin was examined by exposing HeLa cancer cells to irradiated curcumin loaded halloysite nanotubes-Ag–TiO₂ nanomaterial. 0% cell viability was obtained.     

A platinum promoted Ag/SBA-15 catalyst effective in selective oxidation of methanol – design and surface characterization

The aim of this study was better understanding of surface properties of bimetallic (silver–platinum) catalysts and to verify if a very small addition of platinum (ca. 0.05 wt%) to silver (ca. 2.0 wt%) loaded on ordered mesoporous silica, SBA-15, would improve the catalytic properties of bimetallic Ag–Pt materials in selective oxidation of methanol to methyl formate. Ag–Pt catalysts were prepared by one-step and step-by-step procedures and the final Ag/Pt molar ratio in the respective samples was equal to 86 and 63. The catalysts were characterized after calcination and different activation treatments (in Ar and O₂). X-ray diffraction, UV-vis and XP spectroscopy confirmed the lack of Ag–Pt alloy crystallites in the samples and also evidenced a higher resistance of silver oxide species to reduction upon activation in Ar flow in the presence of platinum promoter interacting with silver species. Methanol oxidation over the samples activated in Ar flow and in oxidizing flow (O₂ + Ar) helped identify the role of each component in the bimetallic Ag–Pt catalyst in terms of activity and selectivity in the oxidation of methanol to methyl formate. A highly active bimetallic Pt/Ag/SBA-15 catalyst, selective to methyl formate and stable in methanol oxidation was constructed.

The aim of this study was better understanding of surface properties of Ag–Pt catalysts and to verify if addition of platinum promoter to silver loaded on SBA-15, would improve the catalytic properties of Ag–Pt materials in selective oxidation of methanol to methyl formate.     

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.     

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.     

Ag–K/MnO2 nanorods as highly efficient catalysts for formaldehyde oxidation at low temperature

A series of Ag–K/MnO₂ nanorods with various molar ratios of K/Ag were synthesized by a conventional wetness incipient impregnation method. The as-prepared catalysts were used for the catalytic oxidation of HCHO. The Ag–K/MnO₂ nanorods with an optimal K/Ag molar ratio of 0.9 demonstrated excellent HCHO conversion efficiency of 100% at a low temperature of 60 °C. The structures of the samples were investigated by BET, TEM, SEM, XRD, H₂-TPR, O₂-TPD and XPS. The results showed that Ag–0.9K/MnO₂-r exhibited more facile reducibility and greatly abundant surface active oxygen species, endowing it with the best catalytic activity of the studied catalysts. This work provides new insights into the development of low-cost and highly efficient catalysts for the removal of HCHO.

Ag–K/MnO₂ nanorods with appropriate K/Ag ratio demonstrated excellent catalytic activity for complete oxidation of formaldehyde.     

D–π–A type conjugated indandione derivatives: ultrafast broadband nonlinear absorption responses and transient dynamics

The ultrafast nonlinear optical response of two 1,3-indandione derivatives (INB3 and INT3) was systematically investigated by the femtosecond Z-scan and pump-probe technique at multiple visible and near infrared wavelengths. Both compounds show strong broadband nonlinear absorption (NLA) and different wavelength-dependent two-photon absorption (TPA) characteristics in the range of 650–1100 nm. The TPA cross section of trithiophene-based compound INT3 was found to be larger than that of triphenylamine-based compound INB3 in the red region (650–800 nm), which is attributed to its longer π-conjugated structure and better molecular planarity. Interestingly, the effective NLA of INB3 was found to be larger than INT3 in the NIR region (800–1100 nm), which is related to the excited state absorption (ESA) induced by TPA. The ultrafast dynamics of both compounds were investigated by femtosecond transient absorption spectroscopy, which revealed a broadband ESA including several relaxation processes. This work extends nonlinear optical research on indandione derivatives, and the results suggest that these derivatives are promising candidates for optical limiting applications.

In the red region (650–800 nm), the nonlinear absorption of trithiophene-based compound INT3 is greater than that of triphenylamine-based compound INB3, while in the NIR region (800–1100 nm), the strength of nonlinear absorption is the opposite.     

Spontaneous alloying of ultrasmall non-stoichiometric Ag–In–S and Cu–In–S quantum dots in aqueous colloidal solutions

The effect of spontaneous alloying of non-stoichiometric aqueous Ag–In–S (AIS) and Cu–In–S (CIS) quantum dots (QDs) stabilized by surface glutathione (GSH) complexes was observed spectroscopically due to the phenomenon of band bowing typical for the solid–solution Cu(Ag)–In–S (CAIS) QDs. The alloying was found to occur even at room temperature and can be accelerated by a thermal treatment of colloidal mixtures at around 90 °C with no appreciable differences in the average size observed between alloyed and original individual QDs. An equilibrium between QDs and molecular and clustered metal–GSH complexes, which can serve as “building material” for the new mixed CAIS QDs, during the spontaneous alloying is assumed to be responsible for this behavior of GSH-capped ternary QDs. The alloying effect is expected to be of a general character for different In-based ternary chalcogenides.

The effect of spontaneous alloying of aqueous glutathione-capped Ag–In–S and Cu–In–S quantum dots (QDs) into quaternary Cu(Ag)–In–S QDs is reported.