Si Cheng

Surface-enhanced Raman scattering-active substrates of electrospun polyvinyl alcohol/gold–silver nanofibers

Surface-enhanced Raman scattering (SERS)-active substrates of polyvinyl alcohol/gold-silver (PVA/Au-Ag) nanofibers were prepared using a simple approach involving electrospinning. The tunable surface plasmon resonance (SPR) of gold-silver alloy (Au-Ag alloy) nanoparticles (NPs) was achieved by controlling the feed ratio between gold and silver precursors. A higher concentration of Au-Ag alloy NPs could be obtained than the conventional methods, using 1wt% of PVA as the stabilizer. The Au-Ag alloy structure was demonstrated by HRTEM and STEM-EDX. After the electrospinning, the Au-Ag alloy NPs were successfully embedded in PVA nanofibers, as shown in the SEM and TEM images. Raman spectra displayed an apparent enhancement in the signal of 4-mercaptobenzoic acid (4-MBA), pyridine, and thiophenol molecules pre-absorbed from their ethanol solution onto the PVA/Au-Ag nanofibers. Different SERS effects were achieved by varying the Au content or excitation wavelength.

Galvanic replacement approach for bifunctional polyacrylonitrile/Ag–M (M = Au or Pd) nanofibers as SERS-active substrates for monitoring catalytic reactions

SERS has been applied to monitor different types of catalytic reactions on the surface of metal NPs in recent years, which requires bifunctional metal structures with plasmonic properties as well as catalytic activity. Monitoring catalytic reactions with SERS technology in previous reports was usually performed in metal solution, using metal electrodes or on a glass/silica substrate with an immobilized metal. Here, we report a general approach for preparing novel SERS-active substrates which are used to monitor catalytic reactions. The polyacrylonitrile (PAN)/Ag–M (M = Au or Pd) bimetallic nanofibrous mats are prepared through galvanic replacement reactions of Ag with Au or Pd on the surface of electrospun PAN/Ag nanofibers. The composition of Ag and Au or Pd could be tuned by changing the concentration of the metal solution used for replacement. The PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats exhibit both excellent SERS and catalytic activities. The reduction of 4-nitrothiophenol (4-NTP) to the corresponding to 4-aminothiophenol (4-ATP) on PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats are monitored by SERS technology. The SERS signals of the reaction intermediate, 4,4′-dimercapto-azobenzene(4,4′-DMAB), are observed as the reaction proceeds with time.

A Platform for Preparation of Monodispersed Fluorescent Conjugated Polymer Microspheres with Core-Shell Structures

A strategy to prepare stable monodispersed fluorescent microspheres is developed by modifying the Wessling method to synthesize poly(p-phenylenevinylene) (PPV) on the surface of a highly crosslinked polymer core. The positively charged PPV polymer precursors (pre-PPV) are adsorbed onto the core with negative charges on the surface and then the insoluble fluorescent PPVs form after thermal elimination. Each individual sphere is found to possess a very smooth surface with an even distribution of fluorescence by microscopic techniques. Very small coefficient of variance (CV) values of emission intensity (<4.0%) and size (<2.3%) are realized for microspheres prepared in the same batch. The spheres are demonstrated to have good thermal stability and photostability.

Polydiacetylene-Polymethylmethacrylate/Graphene Composites as One-Shot, Visually Observable, and Semiquantative Electrical Current Sensing Materials

Aiming to develop pH-paper-like current sensing materials, we prepared irreversible electrochromic PDA-PMMA/graphene composites. The composites exhibited an excellent linear relationship between critical responsive currents and the amount of graphene in the system. In these composites, PDA acted as the electrochromic material and graphene as the conductive matrix. The presence of PMMA not only ensured mechanical performance but also made the color change more obvious to be observed by the naked eye.

Colloidal Gold Nanocups with Orientation‐Dependent Plasmonic Properties

Colloidal gold nanocups are synthesized through single-vertex-initiated gold deposition on PbS nanooctahedrons and subsequent selective dissolution of the PbS component. They possess strong magnetic plasmon resonance and exhibit remarkable orientation-dependent plasmonic properties when deposited on flat substrates. They can also effectively couple s-polarized light into the interfacial region between the nanocup and substrate.

Preparation and surface-enhanced Raman performance of electrospun poly(vinyl alcohol)/ high-concentration-gold nanofibers

A facile approach to prepare electrospun poly(vinly alcohol) (PVA) nanofibers with high concentration of gold nanoparticles (Au NPs) on the fibers, had been developed. These PVA/Au nanofibers could be used as flexible surface-enhanced Raman scattering (SERS) substrates. Relatively high concentration of PVA aqueous solution (10 wt %) was used as the stabilizing agent for gold salt precursor, as well as the starting solution for electrospinning. This method was demonstrated to be effective to prepare high-concentration-gold nanoparticles without aggregation and precipitation by reducing high concentration of gold salt in the presence of PVA aqueous solution. SEM and TEM images showed that both the amount and the size of Au NPs which embedded in PVA nanofibers, increased with increasing the gold salt content, while the gap between the adjacent NPs decreased. Raman spectra showed an apparent enhancement in the signal of 4-mercaptobenzoic acid (4-MBA) molecules pre-absorbed from its ethanol solution onto the PVA/Au nanofibers. The high SERS activity to 4-MBA in solution with a relatively low concentration (10−6xa0M), could be mainly attributed to the reduced gap of Au NPs.

Receptor-Free Poly(phenylenevinylene) Fibrous Membranes for Cation Sensing: High Sensitivity and Good Selectivity Achieved by Choosing the Appropriate Polymer Matrix

Poly(phenylenevinylene)/polyimide (PPV/PI) and poly(phenylenevinylene)/ polymethylmethacrylate (PPV/PMMA) fibrous membranes without any deliberately introduced receptors were prepared as fluorescence sensing materials through electrospinning, followed by thermal treatment. Both of these membranes displayed higher sensitivity toward most cations compared to the corresponding spin-coated films. PPV/PMMA membranes were more sensitive than PPV/PI membranes toward Cu(2+) and Fe(3+). About 4.5 fold of intensity enhancement upon 20 nM of Cu(2+), 80% of quenching upon 20 nM of Fe(3+) with fast response and simple regeneration were realized for PPV/PMMA membrane. The preliminary investigation into the mechanism revealed that the properties of the polymer matrix and thermal treatment of the membrane played important roles in the sensing performance.

Preparation of electrospun luminescent polyimide/europium nanofibers by simultaneous in situ sol–gel and imidization processes

Luminescent polyimide (PI)/europium nanofibers have been successfully prepared by electrospinning combined with an in situ sol-gel technique. The possible reaction mechanism of the simultaneous imidization of polyamide acid and gelation of europium phase was analyzed by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The results showed that chemical coupling and noncovalent interaction existed between the polymer and the europium which formed during the preparative process. Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electronic spectroscopy (SEM) studies also indicated the successful incorporation of europium into PI matrix and various morphologies could be achieved by controlling the calcination temperature and the europium content. Nanofibers with necklace-like structures were obtained after calcination under high temperatures. These PI/europium nanofibers were further demonstrated to have strong fluorescence emission. The intensity ratio for the PI/europium nanofibers, labeled as ((5)D(0)→(7)F(2))/((5)D(0)→(7)F(1)), which is well known as the asymmetry parameter, was lower than that in pure Eu(2)O(3) powder, indicating that there were highly symmetric coordination spheres around europium in the nanofibers.

2D PdAg Alloy Nanodendrites for Enhanced Ethanol Electroxidation

The development of highly active and stable electrocatalysts for ethanol electroxidation is of decisive importance to the successful commercialization of direct ethanol fuel cells. Despite great efforts invested over the past decade, their progress has been notably slower than expected. In this work, the facile solution synthesis of 2D PdAg alloy nanodendrites as a high-performance electrocatalyst is reported for ethanol electroxidation. The reaction is carried out via the coreduction of Pd and Ag precursors in aqueous solution with the presence of octadecyltrimethylammonium chloride as the structural directing agent. Final products feature small thickness (5-7 nm) and random in-plane branching with enlarged surface areas and abundant undercoordinated sites. They exhibit enhanced electrocatalytic activity (large specific current ≈2600u2009mA mgPd-1) and excellent operation stability (as revealed from both the cycling and chronoamperometric tests) for ethanol electroxidation. Control experiments show that the improvement comes from the combined electronic and structural effects.

Amplified Detection of Iron Ion Based on Plasmon Enhanced Fluorescence and Subsequently Fluorescence Quenching

A facile and rapid approach for detecting low concentration of iron ion (Fe3+) with improved sensitivity was developed on the basis of plasmon enhanced fluorescence and subsequently amplified fluorescence quenching. Au1Ag4@SiO2 nanoparticles were synthesized and dispersed into fluorescein isothiocyanate (FITC) solution. The fluorescence of the FITC solution was improved due to plasmon enhanced fluorescence. However, efficient fluorescence quenching of the FITC/Au1Ag4@SiO2 solution was subsequently achieved when Fe3+, with a concentration ranging from 17xa0nM to 3.4xa0μM, was added into the FITC/Au1Ag4@SiO2 solution, whereas almost no fluorescence quenching was observed for pure FITC solution under the same condition. FITC/Au1Ag4@SiO2 solution shows a better sensitivity for detecting low concentration of Fe3+ compared to pure FITC solution. The quantized limit of detection toward Fe3+ was improved from 4.6xa0μM for pure FITC solution to 20xa0nM for FITC/Au1Ag4@SiO2 solution.