Xiumei Feng

Synthetically Directed Self-Assembly and Enhanced Surface-Enhanced Raman Scattering Property of Twinned Crystalline Ag/Ag Homojunction Nanoparticles

A synthetically directed self-assembly strategy to the aqueous-phase synthesis of twinned crystalline silver/silver homojunction nanoparticles (Ag/Ag HJNPs) is demonstrated. In the self-assembly, ethylenediamine tetraacetic acid disodium (EDTA) and solution pH values play a crucial role in the formation of Ag/Ag HJNPs while the sizes of Ag nanoparticles (NPs) in the Ag/Ag HJNPs depend on the reductant concentrations of ascorbic acid. Surface-enhanced Raman scattering (SERS) measurements indicate that the SERS intensity acquired from the Ag/Ag HJNP colloidal solution is about 200 times stronger than that obtained from isolated Ag NP colloid solution. The plasmonic and SERS behaviors of Ag/Ag HJNPs were simulated by discrete-dipole approximation (DDA) and three-dimensional finite-difference time domain (3D-FDTD) methods, respectively. Theoretical calculation results disclose that surface plasmon resonance (SPR) properties of the Ag/Ag HJNPs are different from those of isolated Ag nanospheres, and their maximal SERS enhancement is about 2 orders of magnitude higher than that of isolated Ag nanospheres, which is in good agreement with the experimental results. The extra SERS enhancement can be explained by the hot spots at homojunction structures between Ag particles because of near-field coupling effect.

Nanoplated bismuth titanate sub-microspheres for protein immobilization and their corresponding direct electrochemistry and electrocatalysis

A layered inorganic perovskite sub-micrometer-scale material, nanoplated bismuth titanate (Bi(4)Ti(3)O(12)) sub-microspheres (NBTSMs) constructed with tens of Bi(4)Ti(3)O(12) nanoplates, was for the first time synthesized by a facile hydrothermal synthesis strategy. The NBTSMs were employed as a supporting matrix to explore a novel immobilization and biosensing platform of redox proteins through a combined hydrogen bond and electrostatic assembly process. Biocompatibility, stability, reproducibility, and electrochemical and electrocatalytic properties of the resulting NBTSMs-based composite were studied by UV-vis absorption, FTIR, and electrochemical methods. The research results revealed that the NBTSMs-based composite was a satisfying matrix for proteins to effectively retain their native structure and bioactivity. With advantages of the Bi(4)Ti(3)O(12) layered material, facilitated direct electron transfer of the metalloenzymes with an apparent heterogeneous electron transfer rate constant (k(s)) of 20.0+/-3.8s(-1) was acquired on the NBTSMs-based enzyme electrode. The NBTSMs-based biosensor demonstrated significant electrocatalytic activity for the reduction of hydrogen peroxide with an apparent Michaelis-Menten constant (204 microM), wide linear range (2-430 microM), and low detection limit (0.46 microM, S/N=3). These indicated that the nanoplate-constructed Bi(4)Ti(3)O(12) sub-microspheres were one of ideal candidate materials for direct electrochemistry of redox proteins and the construction of the related enzyme biosensors, and may find potential applications in biomedical, food, and environmental analysis and detection.

Sol–gel hydrothermal synthesis and enhanced biosensing properties of nanoplated lanthanum-substituted bismuth titanate microspheres

Nanoplated lanthanum-substituted bismuth titanate (Bi3.25La0.75Ti3O12, BLTO) microspheres constructed with tens of BLTO nanoplates were synthesized by a sol–gel hydrothermal method. Using nanoplated BLTO microspheres, a novel third-generation H2O2 biosensor was fabricated with loading of myoglobin (Mb) and chitosan (Chi). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) measurements reveal that partial Bi ions of bismuth titanate (BTO, Bi4Ti3O12) are successfully substituted with La ions by the sol–gel hydrothermal method. UV-visible (UV-vis) and Fourier-transform infrared (FTIR) spectra show that Mb encapsulated in the Mb–Chi–BLTO film can retain its bioactivity well. Comparative experiments witness that the Mb–Chi–BLTO biosensor, compared with the Mb–Chi–BTO biosensor, not only has enhanced direct electron-transfer capacity (e.g., stronger redox peak currents (approximately 3-fold) and a larger heterogeneous electron-transfer rate constant of 12.8 ± 3.3 s−1), but also exhibits a wider linear response to H2O2 in the concentration range of 2–490 μM, higher sensitivity (88 mA cm−2 M−1), a lower Michaelis–Menten constant (0.55 mM) and detection limit (0.14 μM), a shorter response time (2.8 s), and better reproducibility and stability. These results imply that La doping greatly improves electrochemical and electrocatalytic properties of the Mb–Chi–BLTO biosensor, which will open up a new idea for the design of third-generation electrochemical biosensors, and the BLTO-based biosensors are also expected to find potential applications in many areas such as clinical diagnosis and food and environmental detection.

Synthesis and electron transfer property of sulfhydryl-containing multi-walled carbon nanotube/gold nanoparticle heterojunctions

One-dimensional metal/semiconductor heterojunction nanomaterials have opened many new opportunities for future nanodevices because of their novel structures and unique electrical and optical properties. In this work, sulfhydryl-containing multi-walled carbon nanotube/gold nanoparticle (MWCNT/Au) heterojunctions were synthesized in high yield by a sulfhydryl- functionalized self-assembly strategy. The component, size, structure, morphology and bond mode of the MWCNT/Au heterojunctions thus prepared were investigated and demonstrated by transmission electron microscopy, scanning electron microscopy, x-ray diffraction, energy-dispersive x-ray spectroscopy, Fourier-transform infrared and UV–visible measurements. Cyclic voltammogram and electrochemical impedance spectroscopy studies indicate that the MWCNT/Au heterojunctions have a novel electron transfer property, which retards electron transfer of the horseradish peroxidase or the ferricyanide in the underlying electrodes. We believe that MWCNT/Au heterojunctions with high stability and a unique electrical property are expected to find potential applications for nanodevices.

EDTA-directed self-assembly and enhanced catalytic properties of sphere-constructed platinum nanochains

A simple and synthetically directed self-assembly approach to the construction of platinum nanochains (PtNCs) has been demonstrated. Micrometre-length PtNCs constructed with spherical Pt nanoparticles (PtNPs) of about 5 nm were synthesized by the reduction of Pt–EDTA (ethylenediaminetetraacetic acid) chelate complex with sodium borohydride (NaBH4). It was found that EDTA played a critical role in the formation of sphere-constructed PtNCs. The PtNPs–Nafion/glassy carbon (GC) and PtNCs–Nafion/GC modified electrodes were fabricated and their corresponding electrocatalytic activities were studied. Comparative studies demonstrated that the PtNCs catalyst had a higher electrochemically active surface area (ECSA, almost 2 times) and better catalytic activity (almost 10 times) in comparison with the PtNPs catalyst. The PtNCs catalyst with a high ECSA and catalyst activity will be a good anode catalyst candidate for direct methanol fuel cells.

Al3+-directed self-assembly and their electrochemistry properties of three-dimensional dendriform horseradish peroxidase/polyacrylamide/platinum/single-walled carbon nanotube composite film.

A novel general methodology for protein immobilization and third-generation biosensor construction is demonstrated, which involves Al(3+)-directed polyacrylamide (PAM) self-assembly into an ordered dendriform structure, easily immobilizing enzymes and nanoparticles. Platinum/single-walled carbon nanotube (Pt/SWCNT) heterojunction nanomaterials were for the first time fabricated via an EDTA-directed synthesis strategy. The Pt/SWCNTs were employed as a supporting matrix to explore a novel immobilization and biosensing platform of redox proteins through cooperating Al(3+)-directed PAM self-assembly. Compared with the almost single-layer horseradish peroxidase (HRP)/PAM film electrode, multilayer HRP/PAM/Pt/SWCNT film electrode exhibited a pair of much stronger redox peaks at -0.22 V (vs. Ag/AgCl). Moreover, with advantages of the ordered multilayer HRP/PAM/Pt/SWCNT film, facilitated direct electron transfer of the metalloenzymes with an apparent heterogeneous electron transfer rate constant (k(s)) of 14.94+/-1.36 s(-1) and smaller peak-to-peak separation (DeltaE(p)) of about 37 mV was acquired on the PAM/Pt/SWCNT-based enzyme electrode. The PAM/Pt/SWCNT-based biosensor demonstrated significant electrocatalytic activity for the reduction of hydrogen peroxide with a small apparent Michaelis-Menten constant (87 microM), wide linear range (1-270 microM), very low detection limit (0.08 microM, S/N=3), and high sensitivity (372 mA cm(-2) M(-1)). Together, these indicated that the Al(3+)-directed HRP/PAM/Pt/SWCNT film was one of ideal candidate materials for direct electrochemistry of redox proteins and the construction of the related enzyme biosensors, and may find potential applications in biomedical, food, and environmental analysis and detection.

Achieving a table-like magnetocaloric effect and large refrigerant capacity in in situ multiphase Gd65Mn25Si10 alloys obtained by crystallization treatment

In situ multiphase structure Gd65Mn25Si10 alloys were fabricated by melt spinning and subsequent crystallization treatment. In the process of crystallization, the α-Gd, GdMn2 and Gd5Si3 phases precipitate in the amorphous matrix in turn. The Curie temperature (T C) values for the α-Gd crystallization phase and amorphous matrix can be tailored by tuning the crystallization treatment time. All three multiphase alloys obtained by crystallization treatment at 637 K for 20, 30 and 40 min, respectively, undergo multiple successive magnetic phase transitions. A table-like magnetic entropy change over a wide temperature range (~90–120 K) and a large full width at half maximum (ΔT FWHM) magnetic entropy change (~230 K) were achieved in the above-mentioned crystallized alloys, resulting in large refrigerant capacities (RCs). The enhanced RCs of the three crystallized alloys for a magnetic field change of 0–5 T are in the range of 541–614 J kg−1. Large ΔT FWHM and RC values and a table-like (−ΔS M)max feature obtained in in situ multiphase Gd65Mn25Si10 crystallized alloys make them suitable for potential application in efficient Ericsson-cycle magnetic refrigeration working in a temperature range from 74 to 310 K.

A straightforward and effective strategy for controllable synthesis of Ag/Ag2S and Ag/CdS heterojunction nanowires

Ag/Ag2S heterojunction nanowires (HJNWs) have been successfully fabricated through one-pot solution-phase method, which were transferred into Ag/CdS HJNWs by cation exchange. The synthesis involved a template-less, non-seed, and one-pot solution-phase process to high-quality Ag/Ag2S HJNWs. The sizes, positions, and spacing distances between the Ag2S or CdS NPs of the growing Ag2S and CdS NPs in the Ag/Ag2S and Ag/CdS HJNWs could be finely tailored by reaction temperatures and PVP concentrations. By varying reaction temperature, the sizes and positions (tip or surface) of the growing Ag2S and CdS NPs in the Ag/Ag2S and Ag/CdS HJNWs could be effectively controlled while PVP concentration could tailor the sizes and spacing distances between the Ag2S or CdS NPs of the growing Ag2S and CdS NPs in the Ag/Ag2S and Ag/CdS HJNWs. We also proposed a primary experimental model to illustrate the growth mechanism of the Ag/Ag2S and Ag/CdS HJNWs.Graphical Abstract