Xinqi Chen

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate)

For the first time, stable aqueous dispersions of polymer-coated graphitic nanoplatelets can be prepared via an exfoliation/in-situ reduction of graphite oxide in the presence of poly(sodium 4-styrenesulfonate).

Aligning single-wall carbon nanotubes with an alternating-current electric field

Single-wall carbon nanotubes (SWCNTs) were highly aligned by an external electric field. The results suggest that the alignment of SWCNTs shows significant dependencies on the frequency and the magnitude of the applied electric field. The electric field with 5 MHz straightened out the SWCNTs and created highly oriented samples with fewer large particles. We also discussed the mechanism and applications.

Mechanics of hydrogenated amorphous carbon deposits from electron-beam-induced deposition of a paraffin precursor

Many experiments on the mechanics of nanostructures require the creation of rigid clamps at specific locations. In this work, electron-beam-induced deposition (EBID) has been used to deposit carbon films that are similar to those that have recently been used for clamping nanostructures. The film deposition rate was accelerated by placing a paraffin source of hydrocarbon near the area where the EBID deposits were made. High-resolution transmission electron microscopy, electron-energy-loss spectroscopy, Raman spectroscopy, secondary-ion-mass spectrometry, and nanoindentation were used to characterize the chemical composition and the mechanics of the carbonaceous deposits. The typical EBID deposit was found to be hydrogenated amorphous carbon (a-C:H) having more sp2- than sp3-bonded carbon. Nanoindentation tests revealed a hardness of ∼4GPa and an elastic modulus of 30–60GPa, depending on the accelerating voltage. This reflects a relatively soft film, which is built out of precursor molecular ions impacting ...

Resonance vibration of amorphous SiO2 nanowires driven by mechanical or electrical field excitation

In this work, we have used the mechanical resonance method to determine the bending modulus of amorphous SiO2 nanowires and to study an electron charge trapping effect that occurs in these nanowires. For uniform amorphous nanowires having diameter ∼100 nm and length over 10 μm, the fit modulus values cluster near 47 GPa; this value is lower than the commonly accepted value of ∼72 GPa for fused silicon oxide (glass) fibers. For some SiO2 nanowires, we observed up to three closely spaced resonances that are a result of the nanowire anisotropy. We have compared the resonance vibration of nanowires driven by mechanical and also ac electrical field loading. All of the measurements were done inside the chamber of a scanning electron microscope where the nanowires were under bombardment of a flux of ∼3 keV energy electrons. By watching the interaction between the ac electrical field and exposed nanowire when driven at resonance frequency, we have observed significant charge trapping in the nanowires. The combina...

Photoexpulsion of surface-grafted ruthenium complexes and subsequent release of cytotoxic cargos to cancer cells from mesoporous silica nanoparticles

Ruthenium(II) polypyridyl complexes have emerged both as promising probes of DNA structure and as anticancer agents because of their unique photophysical and cytotoxic properties. A key consideration in the administration of those therapeutic agents is the optimization of their chemical reactivities to allow facile attack on the target sites, yet avoid unwanted side effects. Here, we present a drug delivery platform technology, obtained by grafting the surface of mesoporous silica nanoparticles (MSNPs) with ruthenium(II) dipyridophenazine (dppz) complexes. This hybrid nanomaterial displays enhanced luminescent properties relative to that of the ruthenium(II) dppz complex in a homogeneous phase. Since the coordination between the ruthenium(II) complex and a monodentate ligand linked covalently to the nanoparticles can be cleaved under irradiation with visible light, the ruthenium complex can be released from the surface of the nanoparticles by selective substitution of this ligand with a water molecule. Indeed, the modified MSNPs undergo rapid cellular uptake, and after activation with light, the release of an aqua ruthenium(II) complex is observed. We have delivered, in combination, the ruthenium(II) complex and paclitaxel, loaded in the mesoporous structure, to breast cancer cells. This hybrid material represents a promising candidate as one of the so-called theranostic agents that possess both diagnostic and therapeutic functions.

Surface potential of ferroelectric thin films investigated by scanning probe microscopy

Scanning probe microscopy was used to form local polarized domains in ferroelectric thin films by applying a voltage between the gold-coated cantilever and the conductive substrate in contact mode. Two methods of visualizing the poled areas are described. The first is to detect the piezoelectric response of the films by applying a small oscillating voltage between the probe tip and the substrate. This measurement determines the local ferroelectric polarity and domain structure directly. The second method is to measure the surface potential of the poled films using scanning Maxwell stress microscopy. This does not directly address the ferroelectric behavior of the film, but rather the potential due to surface charge. We determined the surface potential dependence on pulse voltage and duration applied to the ferroelectric film. The results demonstrate that the charged area will increase rapidly, but the surface potential will saturate as the pulse voltage and duration are increased. The resultant stable loc...

Strong substrate effect in local poling of ultrathin ferroelectric polymer films

By locally poling and measuring ultrathin films of the ferroelectric copolymer vinylidenefluoride/trifluoroethylene (VDF/TrFE), we show that the substrates on which these films are adsorbed play a strong role in orienting and poling the films. We carried out both poling and measurements using the Au-coated probe tip of the cantilever of an atomic force microscope. For ultrathin films of about 20 nm of the copolymer spin-coated on graphite, only a fraction of the thickness of the copolymer film can be repoled, so that the net local piezoelectric effect is enhanced or reduced, depending upon whether poling is parallel or antiparallel to that induced by the substrate, respectively. For sufficiently thick films, this substrate effect is negligible. The apparent lateral resolution with which information can be stored by poling in ultrathin films depends strongly on the film thickness and poling direction with respect to the substrate effect.

Electrocatalytic Drug Metabolism by CYP2C9 Bonded to A Self-Assembled Monolayer-Modified Electrode

Cytochrome P450 (P450) enzymes typically require the presence of at least cytochrome P450 reductase (CPR) and NADPH to carry out the metabolism of xenobiotics. To address whether the need for redox transfer proteins and the NADPH cofactor protein could be obviated, CYP2C9 was bonded to a gold electrode through an 11-mercaptoundecanoic acid and octanethiol self-assembled monolayer (SAM) through which a current could be applied. Cyclic voltammetry demonstrated direct electrochemistry of the CYP2C9 enzyme bonded to the electrode and fast electron transfer between the heme iron and the gold electrode. To determine whether this system could metabolize warfarin analogous to microsomal or expressed enzyme systems containing CYP2C9, warfarin was incubated with the CYP2C9-SAM-gold electrode and a controlled potential was applied. The expected 7-hydroxywarfarin metabolite was observed, analogous to expressed CYP2C9 systems, wherein this is the predominant metabolite. Current-concentration data generated with increasing concentrations of warfarin were used to determine the Michaelis-Menten constant (Km) for the hydroxylation of warfarin (3 μM), which is in good agreement with previous literature regarding Km values for this reaction. In summary, the CYP2C9-SAM-gold electrode system was able to carry out the metabolism of warfarin only after application of an electrical potential, but in the absence of either CPR or NADPH. Furthermore, this system may provide a unique platform for both studying P450 enzyme electrochemistry and as a bioreactor to produce metabolites without the need for expensive redox transfer proteins and cofactors.

Mechanical resonance of quartz microfibers and boundary condition effects

We have measured the mechanical resonance of microscale quartz fibers to qualify the method of obtaining the Young’s modulus of nanowires from their resonance frequency and geometry. An equation for a circular beam with a linearly varying cross-section is derived and used to calculate the resonance frequency shift. We have established a model to discuss the boundary condition effect on the resonance frequency. The Young’s modulus of the quartz fibers has been determined by measuring the resonance frequency, and the geometry, and by applying the model that treats the influence of the type of clamp. The mean value from measurements of the fundamental resonance on 14 different microfibers is 70±6 GPa. This mean value is close to 72 GPa, the Young’s modulus of bulk fused quartz. Four resonance modes were observed in high vacuum and air. The mechanical resonance in high vacuum is linear at the fundamental vibration mode, and nonlinear for higher modes.