Hongyan Guo

Facile fabrication and growth mechanism of 3D flower-like Fe3O4 nanostructures and their application as SERS substrates

A template-free solvothermal combined with precursor thermal transformation method has been developed for the preparation of flower-like Fe3O4 nanostructured hollow microspheres. The reaction mechanism and the self-assembly evolution process were studied, and it was found that the synthetic conditions for the precursor such as reaction time, urea concentration and non-aqueous media are all crucial for the formation of the flower-like hierarchical precursors. The flower-like Fe3O4 microspheres obtained by calcining the precursor in Ar gas exhibit superparamagnetic behavior and show relative high saturation magnetization at room temperature. To endow them with SERS activity, silver coating was conducted by magnetron sputtering. The obtained Fe3O4/Ag hybrid microflowers make a positive influence on the high sensitivity of SERS to 4-pyridinethiol (4-Mpy) and Rhodamine 6G (R6G) molecules when compared with the silver film substrates. More importantly, the detection limit of Fe3O4/Ag hybrid microflowers for R6G dye can reach up to 10−15 M, which meets the requirements of ultratrace detection of analytes using SERS. Thus, the SERS-active magnetic hybrids prepared in this work may possibly be used as an optical probe with magnetic function for application in high-sensitivity bioassays.

Rapid, large-scale, sonochemical synthesis of 3D nanotextured silver microflowers as highly efficient SERS substrates

This paper reports a simple, sonochemical, surfactant-free synthesis of three-dimensional (3D) silver microflowers with high yield and good size distribution at ambient temperature. The shape, size and surface structure (controlled roughness of the surface topography) of the Ag microflowers could be tuned by controlling the experimental parameters. The special structural features with nanoscale corrugations of 3D silver microflowers make a significant contribution to the high sensitivity of SERS to Rhodamine 6G (R6G) and 4-mercaptobenzoic acid (MBA) molecules. The rougher Ag microflowers display a higher SERS activity compared to the smooth ones, which demonstrates the crucial role of nanoscale surface texturing in the plasmonic response of Ag microflowers. Raman results show that the detection limits for R6G and MBA of the optimized Ag microflower substrate are as low as 10−17 M and 10−12 M, respectively, which can meet the requirements for ultratrace detection of analytes. Furthermore, when the Ag microflower substrate is applied to detect biomolecules of bovine serum albumin (BSA), it also exhibits a high detection sensitivity and the detection concentration of BSA is as low as 10−12 M. This facile, large-scale, low-cost, and green chemistry synthesized Ag substrate with high SERS activity and sensitivity make it a perfect choice for practical SERS detection applications.

Step terrace tuned anisotropic transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on vicinal SrTiO3 substrates.

Highly epitaxial LaBaCo2O5.5+δ (LBCO) thin films were grown on different miscut (001) SrTiO3 substrates (miscut angle of 0.5°, 3.0°, and 5.0°) to study the substrate surface step terrace effect on the in-plane electrical transport properties. The microstructure studies by X-ray diffraction and transmission electron microscopy indicate that the as-grown films are A-site disordered cubic perovskite structures with the c-axis highly oriented along the film growth direction. The four-probe scanning tunneling microscopy (STM) studies show that the LBCO thin films grown on the vicinal SrTiO3 substrates have a typical semiconductor behavior with the substrate surface terrace step inducing anisotropic electronic transport properties. These results indicate that in highly epitaxial thin films the surface terrace step induced local strains can play an important role in controlling the electronic transport properties and the anisotropic nature.

Green synthesis of rosettelike silver nanocrystals with textured surface topography and highly efficient SERS performances.

3D rosettelike silver nanocrystals were synthesized in a large scale via a green synthetic method at room temperature without using any surfactants. The rosettelike silver nanocrystals exhibit a special textured surface morphology with a wedge-shaped architecture and higher specific surface area. The shape, size and surface morphology of the silver nanostructures can be well controlled by changing the reaction parameters. Further investigation shows different surface morphologies of silver lead to different surface plasmon resonance (SPR) and resulting surface-enhanced Raman scattering (SERS) properties. And the rosettelike silver with textured surface morphology displays much higher SERS enhancement ability than hollow sphere silver and stringlike silver, which demonstrates the crucial role of nanoscale surface texturing on the plasmonic response of silver particles. Importantly, the detection limit of rosettelike silver for Rhodamine 6G (R6G) dye can reach up to 10−14 M, which meets the requirements of ultratrace detection of analytes using SERS.

A facile strategy for obtaining fresh Ag as SERS active substrates

A facile strategy has been reported to obtain on-line fresh Ag as surface-enhanced Raman scattering (SERS) active substrates by making AgCl nanoparticles exposed to the laser beam of Raman spectrometer. The composition and morphology of AgCl nanoparticles were characterized by X-ray diffraction (XRD), UV-visible (UV-vis) spectroscopy and scanning electron microscopy (SEM). The laser-driven evolvement and possible formation mechanism of cubic AgCl nanoparticles to Ag/AgCl composites were also investigated by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM). Raman measurements demonstrate that the fresh Ag nanoparticles with a few defects have a prominent SERS sensitivity to probe molecules, such as the 4-mercaptopyridine (4-Mpy) and 4-aminothiophenol (PATP) molecules. The SERS intensity of 4-Mpy and PATP increases up to the maximum when the laser irradiation time is prolonged to 50s, which corresponds to the defect extent and the proportion of fresh Ag in the Ag/AgCl composites. This work provides a simple, efficient and feasible approach for obtaining on-line fresh Ag as SERS substrates.

Periodic silver nanodishes as sensitive and reproducible surface-enhanced Raman scattering substrates

Highly ordered silver nanodishes which consist of nanorings and a film were fabricated as surface-enhanced Raman scattering (SERS) substrates. Numerical simulation reveals the bottom film can dramatically enhance the local electromagnetic (EM) field in the ring cavity, due to the plasmonic interaction between the nanorings and the film. Raman results show that the nanodishes can produce about sevenfold stronger signal than the nanorings alone, in accordance with the theoretical simulation. The detection limit for Rhodamine 6G (R6G) in the order of 10−12 M and the average relative standard deviation (RSD) of less than 12% indicate the excellent sensitivity and reproducibility of silver nanodishes. The SERS enhancement factor (EF) of R6G on the nanodishes was calculated to be 6.17 × 107. For practical application, the silver nanodishes were also used to detect thiram, one dithiocarbamate fungicide that has been extensively used as a pesticide in agriculture. The detection limit of thiram molecules is as low as 1 × 10−7 M, which can meet the requirements for ultra trace detection of pesticide residues. The resulting substrate with high SERS activity, stability and reproducibility makes it a perfect choice for practical SERS detection applications.

Bioinspired ribbed hair arrays with robust superhydrophobicity fabricated by micro/nanosphere lithography and plasma etching

Inspired by the hierarchical seta arrays on the legs of a water strider, ribbed hair arrays have been fabricated on a polymethylmethacrylate (PMMA) substrate using polystyrene (PS) sphere colloidal lithography, followed by oxygen plasma reactive ion etching. Microscopic analyses of the as-synthesized samples demonstrate that each microhair is sculptured with elaborate nanogrooves, leading to a unique hierarchical micro/nanoscale feature and all the hairs are vertically aligned in a large area. Inspired by the novel hierarchical hair arrays, their static and dynamic wettability has been investigated. It is revealed that after fluorination, the hierarchical hair arrays can present robust superhydrophobicity, and the wettability can be tuned by altering the morphologies of the hair arrays. Based on these remarkable wetting properties, a simple aquatic device has been fabricated by covering both sides of a PMMA plate with the superhydrophobic hair arrays; the device shows both an excellent self-cleaning performance and a large loading capacity. It can carry a load that is 4.6 times heavier than its own weight. Experimental results and theoretical analysis demonstrate that the superhydrophobic properties of the upper and lower surfaces are responsible for the large loading capacity.

Controlled synthesis of Au–Fe3O4 hybrid hollow spheres with excellent SERS activity and catalytic properties

Au-Fe3O4 hybrid hollow spheres have been successfully synthesized by a one-pot process via the hydrothermal treatment of FeCl3, HAuCl4, citrate, urea, and polyacrylamide (PAM). The amount of Au nanoparticles located in the hybrid hollow spheres can be tuned by changing the molar ratio of Au/Fe precursors. A possible synthetic mechanism of the Au-Fe3O4 hybrid hollow spheres has been proposed. The obtained hybrids exhibit not only a superior surface-enhanced Raman scattering (SERS) sensitivity, but also an excellent catalytic activity. The detection limit of the Au-Fe3O4 hybrid hollow spheres (the Au/Fe molar ratio is 0.2, Au-Fe3O4-0.2) for R6G can reach up to 10(-10) M, which can meet the required concentration level for ultratrace detection of analytes using SERS. Furthermore, the catalytic experiments of the Au-Fe3O4-0.2 hybrid hollow spheres demonstrate that the model of 4-nitrophenol (4-NP) molecules can be degraded within 3 min and the catalytic activity can be recovered without sharp activity loss in six runs, which indicates their superior catalytic degradation activity. The reason may be due to the highly efficient partial charge transfer between Au and Fe3O4 at the nanoscale interface. The results indicate that the bifunctional Au-Fe3O4 hybrid hollow spheres can serve as promising materials in trace detection and industrial waste water treatment.

Generalized green synthesis of diverse LnF3–Ag hybrid architectures and their shape-dependent SERS performances

This paper reports the generalized green synthesis of a series of LnF3–Ag (Ln = Nd, Sm, Eu, Tb) hybrid architectures with tunable shape, surface features, and composition. These intriguing hybrid structures are formed through the uniform magnetron sputtering of silver (Ag) nanoparticles on LnF3 micro-supporters. The LnF3–Ag surfaces are corrugated with high-density and numerous Ag nanogaps (which can serve as Raman active ‘hot spots’, to amplify the Raman signal), providing the sound reliability and reproducibility of Raman detection. We find that the special spindle structure of TbF3–Ag particles display the highest Raman enhancement efficiency compared to disk-, pancake-, peanut-, and rice-like structures. These experimental observations are in good agreement with the theoretical calculation by using the three-dimensional finite difference time domain (3D-FDTD) method. It is found that the produced LnF3–Ag composites are robust and efficient SERS substrates for high sensitivity detection of molecular adsorbates. Raman results show that the limit of detection (LOD) for crystal violet (CV), p-aminothiophenol (PATP) and Rhodamine 6G (R6G) of the optimized TbF3–Ag spindles substrate are as low as 10−11 M, 10−10 M and 10−14 M, respectively, which meets the requirements for ultratrace detection of analytes. In addition, the LnF3–Ag substrates are stable and can be produced with high reproducibility, which shows great potential applications for universal SERS substrates in practical SERS detection.

Uniform mesoporous 3D hematite superstructures: phase transition and their magnetic properties

Uniform mesoporous superparamagnetic 3D hematite superstructures self-assembled using nanoparticles as building blocks have been successfully obtained in large quantities via an oleic acid-assisted one-pot solvothermal route. The as-obtained products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and N2 absorption-desorption isotherm. Some factors influencing the phase and morphology of the products were systematically investigated. Reaction time plays an important role in phase transition of the products from magnetite (microspheres), to hematite (mesoporous 3D superstructures), to maghemite (magnetic chains). Notably, ultrafine nanoparticles or cubic superstructures can also be obtained without oleic acid or instead by other additives. Oleic acid-assisted self-assembly and dissolution-recrystallization process is proposed based on experimental investigation and analysis. The magnetic study reveals a surprising phenomenon that the mesoporous 3D hematite superstructures exhibit a unique superparamagnetic behavior with a high saturation magnetization at room temperature, which is seldom reported thus far for the mesoporous 3D hematite superstructures. Uniform mesoporous superparamagnetic 3D hematite superstructures probably have potential application in magnetorheology, catalysis and water treatment.