AbstractNacre-like graphene films are prepared by evaporation-induced assembly of graphene oxide dispersions containing small amounts of cellulose nanocrystal (CNC), followed by chemical reduction with hydroiodic acid. CNC induces the formation of wrinkles on graphene sheets, greatly enhancing the mechanical properties of the resultant graphene films. The graphene films deliver an ultrahigh tensile strength of 765 ± 43 MPa (up to 800 MPa in some cases), a large failure strain of 6.22 ± 0.19%, and a superior toughness of 15.64 ± 2.20 MJ m−3, as well as a high electrical conductivity of 1105 ± 17 S cm−1. They have a great potential for applications in flexible electronics because of their combined excellent mechanical and electrical properties.

Graphene oxide films are excellent structural and functional materials for their outstanding performance. Recent measurements reported widely scattered mechanical properties, which have been attributed to the difference in the chemistry and complex microstructures that rely on the fabrication process. In this work, we present an experimental study showing that the procedure of mechanical characterization is also critical for measuring intrinsic mechanical properties of graphene oxide films. We find that the specimen geometry and loading conditions could lead to significant variation in the measured stress, strain and toughness. To quantify these effects, we propose a rigid-plasticity shear-lag model to capture the effect of interfacial slippage at gripping ends, which can not only eliminate artifacts in measurements, but also be used to determine interfacial mechanical properties of gripping. Effects of grip pressure, length and loading rate are also discussed, following which suggestions for the experimental setup are provided. These understandings lay the ground for probing intrinsic mechanical properties of graphene oxide films in a reliably way.Download high-res image (163KB)Download full-size image

Artificial nacre-like composite films of graphene oxide (GO) with a variety of commercially available water-soluble polymers were fabricated by a gel–film transformation (GFT) technique. The blending of a polymer into the aqueous dispersion of GO can modulate the interaction between GO sheets. Typically, the attraction force between polymer and GO sheets overcomes the dominant hydration and electrostatic repulsive forces between GO sheets, promoting the gelation of GO. Cast drying the resultant GO hydrogel containing small amounts of polymer (1–20 wt % relative to GO depending on the intrinsic structures of polymers) generates layered GO composite films with tensile strengths over 200 MPa and failure strains larger than 3.0%, which are higher than those of natural nacre and most nacre-like GO films. These results indicate that GO/polymer composite hydrogels are excellent precursors for nacre-like GO films and that the GFT approach is a general route toward the large-scale fabrication of nacre-like GO films with unique combinations of high strength and high toughness.Keywords: artificial nacre; gelation; graphene oxide; hierarchical structure; mechanical properties;

Composites of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and reduced graphene oxide (rGO) have been prepared by solution mixing and applied as electrocatalysts for oxygen reduction reaction (ORR) after treatment with concentrated H2SO4. The blending of rGO induces the conformational change of PEDOT chains from benzoid to quionoid structure and charge transfer from rGO to PEDOT. H2SO4 post-treatment can remove part of insulating PSS from the surface of the PEDOT:PSS/rGO composite film, resulting in a significant conductivity enhancement of the composite. This synergistic effect makes the H2SO4-treated PEDOT:PSS/rGO composite a promising catalyst for ORR. It exhibits enhanced electrocatalytic activity, better tolerance to a methanol crossover effect and CO poisoning, and longer durability than those of the platinum/carbon catalyst.Keywords: composite; oxygen reduction reaction; poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); reduced graphene oxide;

Water-soluble, regioregular polythiophene (PT) derivatives are conformational sensitive to external stimuli, and their spectral responses are determined by conformation changes, π–π interactions and scattering of visible lights originated from PT aggregates. In this paper, we review the synthesis of water-soluble PTs and their photophysical properties, and then highlight our efforts toward the development of PT-based optical sensors for small molecules. Mechanism studies indicate that the interplay between side-chain steric/electrostatic repulsive and interchain attractive interactions is responsible for the assembly and disassembly of water-soluble PTs in aqueous media. Consequently, the planarization/deplanarization of PT backbones provides the possibility for colorimetric and fluorescent dual detection of small bioanalytes and chemical species.Keywords: colorimetric; fluorescent; inorganic ions; sensor; small bioanalytes; water-soluble polythiophene;

A complex between an anionic perylene diimide derivative (PDI-GlyAsp) and cupric ion has been prepared and applied to be turn-on fluorescent probe for the detection of pyrophosphate (PPi) in 100% aqueous solution. The complex formation process and PPi detection have been studied by absorption and emission spectroscopy. It was confirmed that the introduction of cupric ion into PDI-GlyAsp solution resulted in the assembly of PDI-GlyAsp into PDI-GlyAsp/Cu2+ aggregates, leading to the fluorescence quenching of PDI-GlyAsp. Upon addition of PPi into the above solution led to the disassembly of the aggregates due to the competitive binding of PPi with Cu2+ in the PDI-GlyAsp/Cu2+ complex, and a recovery of PDI-GlyAsp emission was observed. Therefore, the PDI-GlyAsp/Cu2+ complex can be applied as a turn-on fluorescent probe for detecting PPi with high selectivity and sensitivity.Keywords: aggregation; biosensor; fluorescence; perylene diimide; pyrophosphate;

A complex between an anionic polythiophene derivative (PT−COO−) and a cationic surfactant, myristoylcholine, has been prepared and applied to be colorimetric probe for acetylcholinesterase (AChE) assays. The complex formation process, AChE activity assay and inhibitor screening has been studied by absorption spectroscopy. It was confirmed that the introduction of myristoylcholine into PT−COO− phosphate buffer solution resulted in the disassembly of PT−COO− aggregates, and further addition of AChE into the above solution led to the reassembly of PT−COO− due to the catalyzed hydrolysis of myristoylcholine and the collapse of the complex. The colorimetric assay for AChE can be readily realized with the concentration of AChE as low as 0.2 U/mL. The results also demonstrate that the colorimetric approach can be applied for screening inhibitors of AChE.Keywords: acetylcholinesterase; aggregation; biosensor; myristoylcholine; polythiophene

Colorimetric and fluorescent dual detection of cysteine (Cys) and homocysteine (Hcy) has been realized based on the ionic self-assembly of a cationic polythiophene derivative with aromatic derivatives of Cys and Hcy formed in situ.

A colorimetric strategy based on conjugated polyelectrolyte aggregates has been applied to determine and distinguish anionic, cationic and non-ionic surfactants.

The vital role of the charged moieties of monovalent aromatic anions in regulating the aggregation of a cationic conjugated polyelectrolyte was exploited in terms of the hard–soft acid–base principle and its application to colorimetric sensing of taurine was examined.

A sensitive colorimetric and fluorescent probe based on a water-soluble cationic conjugated polyelectrolyte was applied to detect glutathione through an in situ premodification technique.

A sensor based on an array of wavelengths of a cationic poythiophene derivative was applied to discriminate fifteen nucleotides along with monophosphate and pyrophosphate with 100% confidence limits.

Insulated molecular wires of poly(phenylenevinylene) (PPV) were prepared by wrapping the conjugated backbones with dendrons through a noncovalent approach. It was found that electrostatic interaction between the quaternary ammonium groups of PPV-1 and the carboxylate moieties in dendrons induced the packing of dendrons along PPV-1 conjugated backbones. Absorption and emission spectroscopic examinations in solution and solid film indicated that the PPV-1 backbones adopted a more planar and isolated conformation in the complexes. Furthermore, interchain interactions in the complexes could be greatly reduced, improving the quantum yield of PPV-1.

A water-soluble cationic conjugated oligomer, oligo(2-(4-(1-pyrenyl)butanoyloxy)ethyltrimethylammonium bromide) (OPBEAB) was synthesized by the combination of chemical and electrochemical synthesis techniques. The oligomer has an average repeat unit of 4 and a high quantum yield (Ф) of 0.7 in an aqueous solution containing 5.91 × 10−3 mol L−1 sodium dodecyl sulfate (SDS). In this medium, surfactant molecules formed shielding layers along the OPBEAB chains and prevented aggregation of the oligomer and also improved its fluorescence stability. The fluorescence quenching of cationic oligomer (OPBEAB) depends strongly on the mixing sequence of the probe molecule (OPBEAB), analyte (TNT), and anionic surfactant (SDS). A molecule probe based on OPBEAB can rapidly detect ultra-trace TNT in both pure aqueous solution and environmental ground water with high sensitivity. The Stern–Volmer constant (KSV) of the probe in aqueous solution was measured to be as high as 5.30 × 105 mol−1 L and the limit of detection was about 7.0 × 10−8 mol L−1 (70 ppb) under optimized conditions.

Optically active supramolecular complexes of water-soluble achiral polythiophene (PT) derivatives, PMTPA or PMTEA (Chart 1), and folic acid have been prepared; and the complex formation processes have been studied by absorption, emission, and circular dichroism (CD) spectroscopies. The complexes exhibited unique split-type induced CDs in the π−π* transition region of PTs, indicating that the molecular chirality of the glutamic acid moiety in folic acid was expressed in PT backbones. The influences of temperature, solvent composition, and the structures of the inducing molecules on the chirality induction to PTs were also investigated, and a possible mechanism for the formation of chiral superstructures was proposed. Furthermore, it was found that, upon addition of folic acid into aqueous solution of PTs (PMTPA or PMTEA), a dramatic color change from yellow to purple along with the emission quenching of PT derivatives was observed. PMTEA, having one fewer carbon in the hydrophobic side chain relative to PMTPA, showed better selectivity toward folic acid sensing over ATP because of its higher solubility in water and the appropriate hydrophilic/hydrophobic balance in the complex. Therefore, it can be applied as a colorimetric and fluorescent probe for detecting folic acid with high selectivity and sensitivity. Besides naked-eye detection of folic acid, the detection limit can be extended to be 10−8 M by using fluorometry and PMTEA as the probing molecule.

A colorimetric strategy based on conjugated polyelectrolyte aggregates has been applied to determine and distinguish anionic, cationic and non-ionic surfactants.

The vital role of the charged moieties of monovalent aromatic anions in regulating the aggregation of a cationic conjugated polyelectrolyte was exploited in terms of the hard–soft acid–base principle and its application to colorimetric sensing of taurine was examined.

A sensor based on an array of wavelengths of a cationic poythiophene derivative was applied to discriminate fifteen nucleotides along with monophosphate and pyrophosphate with 100% confidence limits.

Colorimetric and fluorescent dual detection of cysteine (Cys) and homocysteine (Hcy) has been realized based on the ionic self-assembly of a cationic polythiophene derivative with aromatic derivatives of Cys and Hcy formed in situ.

A sensitive colorimetric and fluorescent probe based on a water-soluble cationic conjugated polyelectrolyte was applied to detect glutathione through an in situ premodification technique.