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).     

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.     

Silver nanoribbons achieved by picosecond ablation using cylindrical focusing and SERS-based trace detection of TNT

We report the fabrication of silver nanoribbons by picosecond laser ablation of bulk silver (Ag) targets submerged in double distilled water (DDW) using a cylindrical focusing geometry. The laser ablation was performed by ∼2 picosecond laser pulses and the corresponding light sheet engendered by a cylindrical lens of focal length ∼4.5 cm. The input pulse energies employed at a wavelength ∼800 nm in the experiments were ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ. In contrast to the case of ablation with spherical lenses, cylindrical lens ablation produced nanoparticles (NPs) and nanostructures (NSs) in 20% less time. The data obtained from the optical characterizations exemplify that localized surface plasmon resonance (LSPR) was observed at 406 nm, 408 nm, and 410 nm for the input energies of ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ, respectively. Interestingly, it was observed that the ablation performed at an input energy of ∼1200 μJ demonstrated the fabrication of Ag nanoribbons rather than the formation of Ag NPs. Selected area electron diffraction (SAED) data of the nanoribbons recorded revealed their crystalline phase and linear morphology. Ag nanomaterials (NPs and ribbons) synthesized in these experiments were employed to detect the explosive molecules of 2,4,6-trinitrotoluene (TNT) at a concentration 25 nM using the technique of surface enhanced Raman scattering. The enhancement factor in the case of Ag nanoribbons (width of ∼20–30 nm, length of ∼0.6–2 μm), obtained using the cylindrical focussing geometry at input pulse energies of ∼1200 μJ, was estimated to be ∼10⁷ for the 1362 cm⁻¹ mode, corresponding to the symmetric NO₂ stretch of TNT.

We synthesised silver nanomaterials by laser ablation of Ag in DDW with ∼2 ps pulses using cylindrical focussing geometry. Ag nanoribbons were obtained at ∼1200 μJ input pulse and were utilized to detect TNT via surface enhanced Raman scattering studies.     

Highly efficient core–shell Ag@carbon dot modified TiO2 nanofibers for photocatalytic degradation of organic pollutants and their SERS monitoring

In the present study, a novel hybrid nanomaterial composed of core–shell structured Ag@carbon dot (CD) modified TiO₂ nanofibers (NFs) was successfully fabricated via a simple two-step strategy for the first time. Herein, the Ag@CDs–TiO₂ NFs are demonstrated to be an efficient SERS substrate. The strong LSPR-induced electromagnetic enhancement (EM) by Ag@CDs NPs and efficient charge transfer (CT) effect between Ag@CDs and TiO₂ NFs synergistically contribute to the excellent SERS enhancement. In addition, the Ag@CDs–TiO₂ NFs exhibit enhanced photocatalytic activity regarding the organic pollutant degradation under visible light irradiation because of the enhanced light absorption and improved separation of photo-generated electron–hole pairs. Thus, this new nanocomposite can be used as a sensitive SERS substrate for determining the catalytic activity and reaction kinetics during the photodegradation of methylene blue (MB). Compared with UV-vis spectroscopy, the SERS technique enables more accurate monitoring of the changes of adsorption molecules and actual catalytic process on the surface of the catalyst. These results are significant for the development of metal or semiconductor-based catalysts for ensuring optoelectronic, energy and environmental applications.

The possible mechanism of enhanced photocatalytic performance of Ag@CDs–TiO₂ hybrid NFs.     

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.     

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.     

Facile synthesis of silver/gold alloy nanoparticles for ultra-sensitive rhodamine B detection

The synthesis of Ag/Au nanoparticles (NPs) in a controlled manner has been a challenge for a long time. The aim of this report is to present a systematic study on the fabrication, characterization of Ag/Au alloy NP-based surface-enhanced Raman spectroscopy (SERS) substrates. Silver (Ag) and gold (Au) colloidal NPs were prepared by chemical reduction route of the corresponding metal salts by trisodium citrate (TSC). Ag/Au alloy nanoparticles with varying molar fractions are prepared in aqueous solution by the simultaneous reduction of AgNO₃ and HAuCl₄ by TSC. The composition of Ag and Au in the alloy samples was controlled by tuning the molar ratio of Ag⁺/Au³⁺ in the mixture solution. The morphologies of the different products were characterized by TEM, and the size of obtained samples was in the range of 40 to 60 nm. The resulting samples were denoted as AgNPs, AuNPs, Ag₃Au, AgAu, and AgAu₃ NPs. In order to compare the optical property of the Ag/Au alloy and Ag/Au mixture, we mixed the pure Ag and Au NPs with different ratios to obtain the aggregated nanoparticles. Ag/Au alloy NPs were demonstrated as an ultrasensitive SERS substrate for the detection of rhodamine B (RhB) molecules. The concentration of RhB ranged from 10⁻¹¹ to 10⁻⁵ M. The effect of the Au content on the optical and SERS properties of the Ag/Au alloys was studied. The obtained results show that the Au content in the Ag/Au alloys play an important role in the physical properties of Ag/Au alloy NPs. The SERS spectra of RhB from the as-prepared Ag/Au alloy NP substrates indicated the superior enhancement with high reproducibility and sensitivity compared to those of Ag or Au samples. Interestingly, the highest SERS activity was achieved for the Ag₃Au sample with an enhancement factor larger than 10¹⁰ for 10⁻¹¹ M RhB and a limit of detection (LOD) at 10⁻¹¹ M, as well as good long-term stability after storage for 1 year. As far as we know, this is the highest sensitivity record of RhB by SERS detection. Furthermore, the composition-dependent SERS activity was explained in detail. These advantages demonstrated the potential for growing Ag/Au alloy NP-based SERS substrates in food safety and bioanalysis.

The synthesis of Ag/Au nanoparticles (NPs) in a controlled manner has been a challenge for a long time.     

Biosynthesis of Ag–Pd bimetallic alloy nanoparticles through hydrolysis of cellulose triggered by silver sulfate

We report a simple but efficient biological route based on the hydrolysis of cellulose to synthesize Ag–Pd alloy nanoparticles (NPs) under hydrothermal conditions. X-ray powder diffraction, ultraviolet-visible spectroscopy and scanning transmission electron microscopy-energy dispersive X-ray analyses were used to study and demonstrate the alloy nature. The microscopy results showed that well-defined Ag–Pd alloy NPs of about 59.7 nm in size can be biosynthesized at 200 °C for 10 h. Fourier transform infrared spectroscopy indicated that, triggered by silver sulfate, cellulose was hydrolyzed into saccharides or aldehydes, which served as both reductants and stabilizers, and accounted for the formation of the well-defined Ag–Pd NPs. Moreover, the as-synthesized Ag–Pd nanoalloy showed high activity in the catalytic reduction of 4-nitrophenol by NaBH₄.

We report a simple but efficient biological route based on the hydrolysis of cellulose to synthesize Ag–Pd alloy nanoparticles (NPs) under hydrothermal conditions.     

Modified graphene supported Ag–Cu NPs with enhanced bimetallic synergistic effect in oxidation and Chan–Lam coupling reactions

Herein, well dispersed Ag–Cu NPs supported on modified graphene have been synthesized via a facile and rapid approach using sodium borohydride as a reducing agent under ambient conditions. Dicyandiamide is selected as an effective nitrogen source with TiO₂ as an inorganic material to form two kinds of supports, labelled as TiO₂–NGO and NTiO₂–GO. Initially, the surface area analysis of these two support materials was carried out which indicated that N-doping of GO followed by anchoring with TiO₂ has produced support material of larger surface area. Using both types of supports, ten nano-metal catalysts based on Ag and Cu were synthesized. Benefiting from the bimetallic synergistic effect and larger specific surface area of TiO₂–NGO, Cu@Ag–TiO₂–NGO is found to be a highly active and reusable catalyst out of other synthesized catalysts. It exhibits excellent catalytic activity for oxidation of alcohols and hydrocarbons as well as Chan–Lam coupling reactions. The nanocatalyst is intensively characterized by BET, SEM, HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS.

Cu@Ag–TiO₂–NGO prepared from modified graphene by simple methodology exhibits enhanced catalytic activity towards oxidation and Chan–Lam coupling due to the synergistic effect between Ag and Cu NPs.     

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 nanoparticles on ZnO nanoplates as a hybrid SERS-active substrate for trace detection of methylene blue

Decorating two-dimensional (2D) nanomaterials with nanoparticles provides an effective method to integrate their physicochemical properties. In this work, we present the hydrothermal growth process of 2D zinc oxide nanoplates (ZnO NPls), then silver nanoparticles (AgNPs) were uniformly distributed on the surface of ZnO NPls through the reduction procedure of silver nitrate with sodium borohydride to create a metal–semiconductor hybrid. The amount of AgNPs on the ZnO NPls'' surface was carefully controlled by varying the volume of silver nitrate (AgNO₃) solution. Moreover, the effect of AgNPs on the surface-enhanced Raman scattering (SERS) property of ZnO NPls was thoroughly investigated by using methylene blue (MB) as the target molecule. After calculation, the maximum enhancement factor value for 10⁻⁴ M of MB reached 6.2 × 10⁶ for the peak at 1436 cm⁻¹ and the limit of detection was 10⁻⁹ M. In addition, the hybrid nanosystem could distinguish MB with good reproducibility over a wide range of concentrations, from 10⁻⁹ to 10⁻⁴ M. The SERS mechanism is well elucidated based on the chemical and electromagnetic mechanisms related to the synergism of ZnO and Ag in the enhancement of Raman signal. Abundant hot spots located at the gap between adjacent separate Ag nanoparticles and ZnO nanoplates which formed a strong local electromagnetic field and electron transfer between ZnO and Ag are considered to be the key factors affecting the SERS performance of our prepared ZnO/Ag substrates. In this research, we found high sensitivity of ZnO nanoplates/Ag nanoparticles in detecting MB molecules. This unique metal–semiconductor hybrid nanosystem is advantageous for the formation of Raman signals and is thus suitable for the trace detection of methylene blue.

Decorating two-dimensional (2D) nanomaterials with nanoparticles provides an effective method to integrate their physicochemical properties.     

Hierarchical Ag nanostructures on Sn-doped indium oxide nano-branches: super-hydrophobic surface for surface-enhanced Raman scattering

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template. ITO nano-branches with tens of nanometer diameter are first fabricated through the vapor–liquid–solid (VLS) growth to provide roughness of the substrate. 10 nm thickness of Ag thin film was deposited and then treated with the post-annealing process to create numerous air-pockets in the Ag film, forming a hierarchical Ag nanostructures. The resulting substrate obtained Cassie''s wetting property with a water contact angle of 151°. Compared to the normal hydrophobic Ag nanoparticle substrate, increase of about 4.25-fold higher SERS signal was obtained for 7 μL of rhodamine 6G aqueous solutions.

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template.     

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.     

Fast photocatalytic degradation of dyes using low-power laser-fabricated Cu2O–Cu nanocomposites

In this study, we report a simple method to controllably synthesize Cu₂O–Cu nanocomposites by using a low-power CO₂ laser, and application of these nanomaterials for photocatalytic degradation of methylene blue (MB). Our experiments demonstrate that efficient tailoring of the Cu₂O–Cu nanocomposites can be realized by accurate control and optimization of the ambient parameters, such as laser energy and NaOH concentration. Compared to hydrothermally fabricated Cu₂O–Cu catalysts, the laser-reduced composites exhibit better visible-light photocatalytic activity for MB degradation, which could be attributed to the formation of special catalytically active structures on the nanocomposite surface. Under the conditions of 10 mA laser irradiation and 5 M NaOH addition, the fabricated Cu₂O–Cu composites had the highest catalytic activity. The degradation rate of MB is 90.10% after visible-light irradiation for 50 min under the optimum conditions. The as-synthesized Cu₂O–Cu composites showed selective dye degradation, and exhibit relatively higher photocatalytic efficiency for positively charged dyes. This work could lead to facile synthesis of high-performance photocatalysts for fast removal of environmentally hazardous dyes from aqueous solution.

Facile synthesis of Cu₂O–Cu nanocomposites by using a low-power CO₂ laser was realized, and the fabricated nanomaterials showed excellent photocatalytic activity for the degradation of various dyes.     

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.     

A novel multifunctional Ag and Sr2+ co-doped TiO2@rGO ternary nanocomposite with enhanced p-nitrophenol degradation,and bactericidal and hydrogen evolution activity

In the present study, a novel multifunctional Sr²⁺/Ag–TiO₂@rGO ternary hybrid photocatalyst was prepared via facile sol–gel and hydrothermal methods. The prepared catalyst was well characterized by UV-vis, XRD, Raman, HRTEM and XPS. The synthesized composite was utilised for p-NP degradation, E. coli disinfection and H₂ generation under visible light. The Sr²⁺/Ag–TiO₂@rGO catalyst showed enhanced photocatalytic H₂ evolution rate (64.3 μmol h⁻¹) compared with Ag–TiO₂@rGO (30.1 μmol h⁻¹) and TiO₂ (no activity). Nearly complete degradation of 15 mg l⁻¹p-NP was achieved over Sr²⁺/Ag–TiO₂@rGO after 3 h, while only 66% and 5% was achieved by Ag–TiO₂@rGO and TiO₂ respectively. Furthermore, TEM analysis was carried out on Escherichia coli (E. coli) before and after visible light irradiation to understand the inactivation mechanism and DNA analysis indicated no fragmentation during inactivation. Radical quantification experiments and ESR analysis suggested that ·OH and O₂˙⁻ were the main ROS in the degradation and disinfection processes. The superior photocatalytic H₂ evolution rate of Sr²⁺/Ag–TiO₂@rGO was attributed to the synergetic effect between the Ag, Sr²⁺ and TiO₂ components on the rGO surface. The localized SPR effect of Ag induced visible light generated charge carriers into the conduction band of the TiO₂ and Sr²⁺ which further transfer to the rGO for the reduction of H⁺ ions into H₂. The results suggest that Sr²⁺/Ag–TiO₂@rGO structures could not only induce separation and migration efficiency of charge carries, but also improve charge collection efficiency for enhanced catalytic activity. Thus, we believe that this work could provide new insights into multifunctional nanomaterials for applications in solar photocatalytic degradation of harmful organics and pathogenic bacteria with clean energy generation during wastewater treatment.

In the present study, a novel multifunctional Sr²⁺/Ag–TiO₂@rGO ternary hybrid photocatalyst was prepared via facile sol–gel and hydrothermal methods.     

Counterion coupled (COCO) gemini surfactant capped Ag/Au alloy and Ag@Au core–shell nanoparticles for cancer therapy

Hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized. Ag/Au alloy and Ag@Au core–shell type NPs were prepared from Ag and Au with various ratios using the COCO gemini surfactant, 1,6-bis (N,N-hexadecyldimethylammonium) adipate (COCOGS), 16-6-16 as a stabilizer. The formation of the Ag/Au alloy and Ag@Au core–shell was confirmed by UV-visible absorption spectroscopy, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED) patterns. Depending on the composition of the Ag/Au alloy NPs, the λ_max values varied from 408 nm to 525 nm. FTIR measurements were used to evaluate the adsorption of the COCO gemini surfactant (16-6-16) on the Ag/Au alloy and Ag@Au core–shell surface. In this present work, we study how to achieve the stability and activity of the COCO gemini surfactant (16-6-16) capped Ag/Au alloy and Ag@Au core–shell NPs for developing novel anti-cancer agents by evaluating their potentials in the Hep-2 cell line model. Thus the developed core–shell NPs were possibly involved in inducing cytotoxicity followed by inhibition of cell proliferation to the cancer cells with apoptosis induction. The developed core–shell NPs might serve as highly applicable agents in the development of next-generation cancer chemotherapeutic agents.

In this work hybrid silver (Ag)–gold (Au) nanoparticles (NPs) with different sizes and compositions were synthesized and applied for anticancer evaluations and which is effectively involved in cancer cell apoptosis through DNA damage.     

A sandwich SERS immunoassay platform based on a single-layer Au–Ag nanobox array substrate for simultaneous detection of SCCA and survivin in serum of patients with cervical lesions

The evaluation of tumor biomarkers in blood specimens is vital for patients with cervical lesions. Herein, an ultrasensitive surface enhanced Raman scattering (SERS) platform was proposed for simultaneous detection of cervical-lesion-related serum biomarkers. Raman reporter labeled Au–Ag nanoshells (Au–AgNSs) acted as SERS tags and an Au–Ag nanobox (Au–AgNB) array substrate prepared by the oil–water interface self-assembly method was used as a capture substrate. This single-layer Au–AgNB array substrate was proved to have exceptional uniformity by atomic force microscopy and SERS mapping. Numerous “hot spots” and specific adsorption surfaces offered by the Au–AgNB array substrate were confirmed by the finite difference time domain method, which could generate a SERS signal in electromagnetic enhancement. Binding of antigens between antibodies on Au–AgNSs and the Au–AgNB array substrate led to the formation of a sandwich-structure by the two metal nanostructures. Consequently, an ultralow detection limit of 6 pg mL⁻¹ for squamous cell carcinoma antigen (SCCA) and 5 pg mL⁻¹ for survivin in a wide linear logarithmic range of 10 pg mL⁻¹ to 10 μg mL⁻¹ was acquired. High selectivity and reproducibility with relative standard deviations of 7.701% and 6.943% were detected. Furthermore, the simultaneous detection of the two biomarkers in practical specimens was conducted, and the results were consistent with those of the enzyme-linked immunosorbent assay. This platform exhibited good robustness in the rapid and sensitive detection of SCCA and survivin, which could be a promising tool in early clinical diagnosis for different grades of cervical lesions.

A novel SERS immunoassay platform for simultaneous detection of SCCA and survivin based on Au–Ag nanoshells (Au–AgNSs) combined with a single-layer Au–AgNB array substrate was proposed in the current study.     

Synthesis and evaluation of the SERS effect of Fe3O4–Ag Janus composite materials for separable,highly sensitive substrates

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method. The optimal growth process was determined by investigating the relationship between the particle morphologies and reaction time. Magnetic and Raman spectroscopic measurements showed that the as-synthesized Janus composites have both good magnetic response and significant surface-enhanced Raman scattering (SERS) effects, as well as reproducibility. The calculated Raman enhancement factor reached an unprecedented magnitude of 10⁹ compared with the values of other Fe₃O₄–Ag compounds. Furthermore, the SERS effect was exhibited even at a concentration of probe molecules as low as 10⁻¹³ M. This demonstrates that the as-synthesized Fe₃O₄–Ag Janus composite particles have promise for application as separable, highly sensitive SERS substrates.

Fe₃O₄–Ag Janus composites were synthesized using a two-step solvothermal method.     

Halogen bond triggered aggregation induced emission in an iodinated cyanine dye for ultra sensitive detection of Ag nanoparticles in tap water and agricultural wastewater

Aggregation induced emission (AIE) has emerged as a powerful method for sensing applications. Based on AIE triggered by halogen bond (XB) formation, an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs) is reported. The dye (CyI) has an iodine atom in its skeleton which functions as a halogen bond acceptor, and aggregates on the Ag NP plasmonic surfaces as a halogen bond donor or forms halogen bonds with the vacant π orbitals of silver ions (Ag⁺). Formation of XB leads to fluorescence enhancement, which forms the basis of the Ag NPs or Ag⁺ sensor. The sensor response is linearly dependent on the Ag NP concentration over the range 1.0–8.2 pM with an LOD of 6.21 pM (σ = 3), while for Ag⁺ it was linear over the 1.0–10 μM range (LOD = 2.36 μM). The sensor shows a remarkable sensitivity for Ag NPs (pM), compared to that for Ag⁺ (μM). The sensor did not show any interference from different metal ions with 10-fold higher concentrations. This result indicates that the proposed sensor is inexpensive, simple, sensitive, and selective for the detection of Ag NPs in both tap and wastewater samples.

Based on AIE triggered by halogen bond (XB) formation, we established an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs).