Rui-Xuan Dong

The disruption of bacterial membrane integrity through ROS generation induced by nanohybrids of silver and clay.

Nanohybrids, synthesized via silver nitrate reduction in the presence of silicate clay, exhibit a high potency against bacterial growth. The plate-like clay, due to its anionic surface charges and a large surface area, serves as the support for the formation of silver nanoparticles (AgNPs) approximately 30 nm in diameter. The nanohybrid consisting of Ag/silicate at a 7/93 weight ratio inhibited the growth of dermal pathogens including Staphylococcus aureus (S. aureus), Pseudomonas aeruginosa and Streptococcus pyrogens, as well as the methicillin- and oxacillin-resistant S. aureus (MRSA and ORSA). Scanning electron microscope revealed that these nanohybrids were adherent on the surface of individual bacteria. The thin silicate plates provide a surface for immobilizing AgNPs in one highly concentrated area but prevent them from entering the cell membrane. Subsequent cytotoxicity studies indicated that surface contact with the reduced AgNPs on clay is sufficient to initiate cell death. This toxicity is related to a loss in membrane integrity due to reactive oxygen species (ROS) generation. The hybridization of AgNPs on clay surface is viable for generating a new class of nanohybrids exhibiting mild cytotoxicity but high efficacy for battling drug-resistant bacteria.

A high performance dye-sensitized solar cell with a novel nanocomposite film of PtNP/MWCNT on the counter electrode

An imide-functionalized material, poly(oxyethylene)-segmented polymer, was synthesized from the reaction of poly(oxyethylene)diamine of 2000 g mol−1Mw and 4,4′-oxydiphthalic anhydride and used to disperse hybrid nanomaterials of platinum nanoparticles and multi-wall carbon nanotubes (PtNP/MWCNT). The composite material was spin-coated into film and further prepared as the counter electrode (PtNP/MWCNT-CE) for a dye-sensitized solar cell (DSSC). The short-circuit current density (JSC) and power-conversion efficiency (η) of the DSSC with PtNP/MWCNT-CE were found to be 18.01 ± 0.91 mA cm−2 and 8.00 ± 0.23%, respectively, while the corresponding values were 14.62 ± 0.19 mA cm−2 and 6.92 ± 0.07% for a DSSC with a bare platinum counter electrode (Pt-CE). The presence and distribution of PtNP/MWCNT on the CE were characterized by using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The attachment of PtNPs on MWCNTs was observed by transmission electron microscopy (TEM). Cyclic voltammetry (CV), incident-photo-to-current efficiency (IPCE) and electrochemical impedance spectra (EIS) were correlated to explain the efficacy of this nanocomposite system.

The cellular responses and antibacterial activities of silver nanoparticles stabilized by different polymers

Silver nanoparticles (AgNPs) are known for their excellent antibacterial activities. The possible toxicity, however, is a major concern for their applications. Three types of AgNPs were prepared in this study by chemical processes. Each was stabilized by a polymer surfactant, which was expected to reduce the exposure of cells to AgNPs and therefore their cytotoxicity. The polymer stabilizers included poly(oxyethylene)-segmented imide (POEM), poly(styrene-co-maleic anhydride)-grafting poly(oxyalkylene) (SMA) and poly(vinyl alcohol) (PVA). The cytotoxicity of these chemically produced AgNPs to mouse skin fibroblasts (L929), human hepatocarcinoma cells (HepG2), and mouse monocyte macrophages (J774A1) was compared to that of physically produced AgNPs and gold nanoparticles (AuNPs) as well as the standard reference material RM8011 AuNPs. Results showed that SMA-AgNPs were the least cytotoxic among all materials, but cytotoxicity was still observed at higher silver concentrations (>30 ppm). Macrophages demonstrated the inflammatory response with cell size increase and viability decrease upon exposure to 10 ppm of the chemically produced AgNPs. SMA-AgNPs did not induce hemolysis at a silver concentration below 1.5 ppm. Regarding the antibacterial activity, POEM-AgNPs and SMA-AgNPs at 1 ppm silver content showed 99.9% and 99.3% growth inhibition against E. coli, while PVA-AgNPs at the same silver concentration displayed 79.1% inhibition. Overall, SMA-AgNPs demonstrated better safety in vitro and greater antibacterial effects than POEM-AgNPs and PVA-AgNPs. This study suggested that polymer stabilizers may play an important role in determining the toxicity of AgNPs.

A novel polymer gel electrolyte for highly efficient dye-sensitized solar cells

A structurally interconnected block copolymer was facilely prepared by the oligomerization of poly(oxyethylene)-segmented diamine and 4,4′-oxydiphthalic anhydride, followed by a late-stage curing to generate amide-imide cross-linked gels. The gel structure, with multiple functionalities including poly(oxyethylene) segments, amido-acid linkers, amine termini, and amide cross-linker was characterized by Fourier transform infrared spectroscopy. The gel-like copolymer was used to absorb a liquid electrolyte; formation of 3D interconnected nanochannels, as could be observed by field emission scanning electronic microscopy has confirmed this absorption of the liquid electrolyte by the copolymer. This elastomeric copolymer was used as the matrix of a polymer gel electrolyte (PGE) for a dye-sensitized solar cell (DSSC), which shows extremely high photovoltaic performance (soaking for 1 h in the electrolyte). In particular, the PGE containing 76.8 wt% of the liquid electrolyte renders a power conversion efficiency of 9.48% for its DSSC, with a short-circuit photocurrent density of 19.50 mA cm−2, an open-circuit voltage of 0.76 V, and a fill factor of 0.64. The outstanding performance of the gel-state DSSC, superior to that (8.84%) of the DSSC with the liquid electrolyte, is mainly ascribed to the suppression of the back electron transfer through the PGE. Electrochemical impedance spectra, and dark current measurements were used to substantiate the explanations of the photovoltaic parameters.

Polymer-dispersed MWCNT gel electrolytes for high performance of dye-sensitized solar cells

A hybrid of polymer-dispersed multi-walled carbon nanotubes (MWCNT) was utilized in networking with the conventional composition of gel electrolyte in dye-sensitized solar cells (DSSCs) to purposely enhance the cell efficiency. The requisite polymer as the dispersant is structurally tailored for its functionalities consisting of poly(oxyethylene)-segmented amides and imides. The existence of the dispersant is multi-functional for first de-bundling the originally aggregated MWCNT and subsequently networking with the conventional gel electrolyte, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP)/LiI system. The gel electrolyte comprised of only 0.25 wt% MWCNT/POEM in the finely dispersed state was fabricated into a quasi-solid-state DSSC which showed high power-conversion efficiency (η) of 6.86% and short-circuit current density (JSC) of 15.3 mA cm−2 at the test of 100 mW cm−2 irradiation. The DSSC efficiency was significantly improved from the use of the unmodified gel electrolyte having the values of JSC = 9.6 mA cm−2 and η = 4.63%. The enhancement was further confirmed by the electrochemical impedance spectra analyses for the lowest Warburg resistance (Rw). The fine dispersion of MWCNT in the polymeric dispersant was characterized by UV-Vis, TEM, FT-IR and DSC. The finding indicates the role of MWCNT for homogenizing the amorphous PVDF–HFP and facilitating the diffusion state of I−/I3− ion pairs in this electrolyte system.

Synthesis of immobilized silver nanoparticles on ionic silicate clay and observed low-temperature melting

Silver nanoparticles (AgNPs) of narrow size distribution and low melting point were synthesized from the reduction of silver nitrate in the presence of inorganic silicate clays. The natural clays with a lamellar geometric shape provided a high surface area for immobilizing AgNPs with nanometer diameter in the range of 17–88 nm. At a 1/1 equivalent ratio of Ag+ to clay counter ions, the generated particles had a narrow size distribution (polydispersity of Dw/Dn = 1.2 at 26 nm Dn by SEM) and a UV absorption at 420 cm−1. Without organic dispersants, the colloidal clays could complex with Ag+ in the initial stage of mixing and subsequently stabilized the generated Ag0 particles. It seems that the high surface area stabilizes the clay rather than the Ag metal intercalation into the layered structure since the basal spacing was only slightly enlarged (12.0 A versus 13.9 A by XRD). The resulting AgNPs were highly stable and maintained their particle size after several cycles of drying at 80 °C and re-dispersion in water. Moreover, the AgNPs on the clay surface melted at a low temperature (110 °C), observed by SEM. Such AgNPs may have potential applications for fabricating silver arrays or conductors at low temperature.

Novel Polymer Gel Electrolyte with Organic Solvents for Quasi-Solid-State Dye-Sensitized Solar Cells

A cross-linked copolymer was previously synthesized from poly(oxyethylene) diamine (POE-amine) and an aromatic anhydride and cured to generate an amide-imide cross-linking structure. The copolymer containing several chemical groups such as POE, amido acids, and imide, enabled to absorb liquid electrolytes in methoxypropionitrile (MPN) for suitable uses in dye-sensitized solar cells. To establish the advantages of polymer gel electrolytes (PGE), the same copolymer was studied by using different electrolyte solvents including propylene carbonate (PC), dimethylformamide, and N-methyl-2-pyrrolidone, and shown their long-term stability. The morphology of the copolymer after absorbing liquid electrolytes in these solvents was proven the same as a 3D interconnected nanochannels, evidenced field emission-scanning electron microscopy. Among these solvents, PC was selected as the optimized PGE, which demostrated a higher power conversion efficiency (8.31%) than that of the liquid electrolyte (7.89%). In particular, the long-term stability of only a 5% decrease in the cell efficiency after 1000 h of testing was achieved. It was proven the developed copolymer as PGE was versatile for different solvents showing high efficiency and long-term durability.

Controlling Formation of Silver/Carbon Nanotube Networks for Highly Conductive Film Surface

Flexible polymer films with high electrical conductivity were prepared through a simple coating of well-dispersed silver nanoparticle (AgNP) and multiwalled carbon nanotube (CNT) solution. The hybrid film with surface resistance as low as 1 × 10(-2) Ω/sq was prepared by controlling the annealing temperature in air and by using a suitable composition of silver nitrate/CNT/poly(oxyethylene)-oligo(imide) (POE-imide) in the ratio 20:1:20 by weight. During the heating, color of the film surface changed from black to golden to milky white, indicating the accumulation of AgNPs through surface migration and melting into CNT-connected networks. Thermogravimetric measurements showed that the transition temperature of 170 °C was responsible for the POE-imide degeneration and the subsequent Ag melting with a decrease in the surface resistance from 2.1 × 10(5) to 2.0 × 10(-1) Ω/sq, which was able to illuminate light-emitting diode lamps because of the formation of a continuous Ag network.

A dual-functional Pt/CNT TCO-free counter electrode for dye-sensitized solar cell

A nanohybrid of platinum and carbon nanotubes (Pt/CNT) with dual functions of catalytic activity and conductivity was synthesized. The selection of the poly(oxyethylene)-backboned polyimide dispersant was essential for preparing the dispersion with the de-bundled CNTs and platinum salts in ethanol/water. Subsequent solution casting and annealing at the optimized temperature of 390 °C led to the in situ reduction of platinum salts and the formation of a thin film of Pt/CNT nanohybrids on the glass substrate. The film was used directly as the counter electrode (CE) in a dye-sensitized solar cell (DSSC), which exhibited a short-circuit current density of 16.6 ± 0.2 mA cm−2 and a power conversion efficiency of 6.96 ± 0.09% at 100 mW cm−2 illumination. This performance is comparable with a DSSC with a conventional Pt-CE, and feasible for replacing the conventional transparent conductive oxide (TCO) conductive layer in DSSCs.

Transparent graphene–platinum nanohybrid films for counter electrodes in high efficiency dye-sensitized solar cells

A dispersion of platinum-on-graphene was prepared essentially by a two-step process, involving uniform distribution of graphene nanoplatelets in a cosolvent of ethanol–water in the presence of a polymeric dispersant and subsequent in situ reduction of dihydrogen hexachloroplatinate to metallic platinum on the graphene surface. The process generated platinum nanoparticles (PtNPs) of ca. 4.0–10 nm in diameter on the graphene surface. The platinum-on-graphene dispersion was coated on an FTO glass to prepare a counter electrode (CE) for a dye-sensitized solar cell (DSSC). The hybrid film of platinum nanoparticles and graphene nanoplatelets (PtNP/GN) showed a transparency of 70% at 550 nm, indicating its suitability as a CE material for a rear-illuminated DSSC. The DSSC with the CE having the film of PtNP/GN exhibited a power conversion efficiency (η) of 8.00%, superior to 7.14% of the DSSC with a conventional sputtered platinum (s-Pt) CE. In the case of rear-illumination the DSSC showed an η of 7.01%, while the DSSC with the conventional s-Pt showed an η of only 2.36%. HRTEM and FE-SEM were used to observe the dispersion of the hybrid material in the solvent, UV-vis spectroscopy and cyclic voltammetry were used to characterize the films, and IPCE spectra and electrochemical impedance spectra were used to explain the photovoltaic parameters of the DSSCs.