Sam Fong Yau Li

Molecular-scale interface engineering for polymer light-emitting diodes

Achieving balanced electron–hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers. Using this route, we previously incorporated ultrathin (<100 Å) charge-injection interfacial layers in polymer LEDs. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15–20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

Microgravimetric DNA sensor based on quartz crystal microbalance: comparison of oligonucleotide immobilization methods and the application in genetic diagnosis

We report on the study of immobilization DNA probes onto quartz crystal oscillators by self-assembly technique to form variety types of mono- and multi-layered sensing films towards the realization of DNA diagnostic devices. A 18-mer DNA probe complementary to the site of genetic beta-thalassaemia mutations was immobilized on the electrodes of QCM by covalent bonding or electrostatic adsorption on polyelectrolyte films to form mono- or multi-layered sensing films by self-assembled process. Hybridization was induced by exposure of the QCMs immobilized with DNA probe to a test solution containing the target nucleic acid sequences. The kinetics of DNA probe immobilization and hybridization with the fabricated DNA sensors were studied via in-situ frequency changes. The characteristics of QCM sensors containing mono- or multi-layered DNA probe constructed by direct chemical bonding, avidin-biotin interaction or electrostatic adsorption on polyelectrolyte films were compared. Results indicated that the DNA sensing films fabricated by immobilization of biotinylated DNA probe to avidin provide fast sensor response and high hybridization efficiencies. The effects of ionic strength of the buffer solution and the concentration of target nucleic acid used in hybridization were also studied. The fabricated DNA biosensor was used to detect a set of real samples. We conclude that the microgravimetric DNA sensor with its direct detection of amplified products provide a rapid, low cost and convenient diagnostic method for genetic disease.

Spontaneous formation of complex and ordered structures on oxygen-plasma-treated elastomeric polydimethylsiloxane

In this letter, we describe the spontaneous formation of complex and ordered structures on polydimethylsiloxane when subjected to oxygen plasma treatment. Periodicity of wavy structures from submicrometer to micrometer length scales could be generated reproducibly and quantitatively accounted for. The origin of these patterns could be related to the relief of compressive stress by buckling of the silica-like thin film that was formed as a result of the plasma exposures. Atomic force microscope has been used to characterize the varied trends of the modifications. The present approach could be extended to the fabrication of intricate ordered patterns on polymeric surfaces with integrated relief structures obtained by soft lithography and micromolding.

Surface deactivation in protein and peptide analysis by capillary electrophoresis

For the analysis of proteins and peptides in capillary electrophoresis, shielding of the analytes from the active sites of the fused silica capillary wall is often necessary. There are several strategies to avoid undesirable interaction of the analytes with the capillaries. This article reviews the current, most relevant, methods. The different approaches are classified as uncoated capillaries, dynamic coated capillaries and permanent coated capillaries, according to the final effect that is produced on the capillary surface. Permanently bonded coatings are further classified by the characteristics of electroosmotic flow that they generate at the surface of the capillary. Finally, organic polymeric capillaries are discussed as they have recently been introduced successfully for the separation of proteins and peptides. Throughout, attempts have been made to emphasize the potential application and the separation conditions and how these coating strategies might be utilized for the analysis each type of protein or peptide.

Standard Reference Data for the Thermal Conductivity of Liquids

The available experimental liquid‐phase thermal conductivity data for water, toluene, and n‐heptane have been examined with the intention of establishing standard reference values along the saturation line. The quality of available data is such that for toluene and water new standard reference values can be proposed with confidence limits better than ±1.0% for most of the normal liquid range. For n‐heptane there are insufficient reliable experimental data for the system to be treated as a primary reference standard, so a lower quality correlation has been developed which yields a set of secondary reference data with confidence limits of ±1.5% for most of the normal liquid range.

X-ray photoelectron spectroscopy of surface-treated indium-tin oxide thin films

Angle-resolved X-ray photoelectron spectroscopy and photothermal deflection spectroscopy are used to study the oxygen-plasma or aquaregia treated indium-tin oxide (ITO) anodes for organic light-emitting diodes. Detailed analysis of the Ols core-level spectra and their dependence on photoemission angle was performed. The results indicate the presence of different chemical forms of oxygen atoms (two types of O2-, OH-, organic oxygens and H2O) which evolve with surface treatment. We find that the treatments lead to a modification of the surface chemical states and therefore of the physico-chemical properties of ITO, which in turn control the performance of organic light-emitting diodes

1, 3-Dialkylimidazolium-based room-temperature ionic liquids as background electrolyte and coating material in aqueous capillary electrophoresis

The 1-ethyl-3-methylimidazolium (EMIM) cation was found to have constant mobility of 4.5 x 10(-4) cm2 V(-1) s(-1) over the pH range of 3 to 11. The electroosmotic flow of bare silica capillary was reversed by the covalently bonded room-temperature ionic liquid (RTIL) coating. With run buffer of 5 mM EMIM (pH 8.5), NH4+ in human urine was separated from the K+ matrix and was detected to be 0.37 +/- 0.012%. K+, Na+, Li+, Ca2+, Mg2+ and Ba2+ were baseline separated in RTIL-coated capillary with run buffer of 10 mM EMIMOH-acetic acid at pH 5, and the concentration of the above ions in a red wine were detected to be 907, 27.9, 0, 71.0, 83.4 and 31.1 microg/ml, respectively. The RTIL-coated capillary showed stable electroosmotic flow for at least 80 h in the run buffer.

High-performance chiral separation of fourteen triazole fungicides by sulfated β-cyclodextrin-mediated capillary electrophoresis

In this paper, sulfated beta-cyclodextrin-mediated capillary electrophoresis (CE) is evaluated as a new approach for the chiral separation of triazole-type fungicides. The 14 fungicides investigated were bitertanol, cyproconazole, difenoconazole, diniconazole, flutriafol, hexaconazole, myclobutanil, paclobutrazol, penconazole, propiconazole, tebuconazole, tetraconazole, triadimefon and triadimenol. Under the optimal conditions, excellent enantioseparation was achieved for all the 14 fungicides, including those fungicides containing two chiral centers. To our knowledge, this is the only system to date that offers outstanding enantiodiscrimination towards all triazole-type fungicides. The impact of the molecular structures of the triazole compounds on their migration behavior was studied. Similar to other chemical systems involving host-guest complexation, the interaction between sulfated beta-cyclodextrin and the triazole compounds was found to be affected by a variety of factors, including electrostatic force, hydrogen bonding, steric effect and hydrophobicity. These factors, coupled with the countercurrent electroosmotic flow (EOF), were believed to be the major forces behind the exceptional chiral selectivity.

Green synthesis of gold nanoparticles using palm oil mill effluent (POME): a low-cost and eco-friendly viable approach.

The present study reports the synthesis of gold nanoparticles (AuNps) from gold precursor using palm oil mill effluent (POME) without adding external surfactant, capping agent or template. The biosynthesized AuNps were characterized by using UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). According to the image analysis performed on a representative TEM micrograph by counting 258 particles, the obtained AuNps are predominantly spherical with an average size of 18.75 ± 5.96 nm. In addition, some triangular and hexagonal nanoparticles were also observed. The influence of various reaction parameters such as reaction pH, concentration of gold precursor and interaction time to the morphology and size of biosynthesized AuNps was investigated. This study shows the feasibility of using agro waste material for the biosynthesis of AuNps which is potentially more scalable and economic due to its lower cost.

Potential of plant as a biological factory to synthesize gold and silver nanoparticles and their applications

Green synthesis of metallic nanoparticles has become a promising field of research in recent years. Syntheses of gold and silver nanoparticles by various chemical and physical methods as well as the biosynthetic approach mediated by numerous microorganisms have been actively researched. A more scalable and economic route to produce these metallic nanoparticles would be through the plant-mediated synthetic approach. Owing to the biodiversity of plant biomasses, the mechanism by which bioconstituents of plants have contributed to the synthetic process is yet to be fully understood. Nevertheless, the feasibility of controlling the shape and size of nanoparticles by varying the reaction conditions has been demonstrated in many studies. This paper provides an overview of the plant-mediated syntheses of gold and silver nanoparticles, possible compounds and mechanisms that might be responsible for the bioreduction process as well as the potential applications of biosynthesized nanoparticles in different fields. The challenges and limitations of this plant-mediated biosynthetic approach are also discussed.