Renqiang Yang

Colorimetric detection of DNA, small molecules, proteins, and ions using unmodified gold nanoparticles and conjugated polyelectrolytes

We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly “universal” biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise “folded” DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient—picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.

Origin of the enhanced open-circuit voltage in polymer solar cells via interfacial modification using conjugated polyelectrolytes

It has been previously shown that incorporating alcohol/water-soluble conjugated polyelectrolytes (CPEs) as cathode interlayers can substantially enhance the open-circuit voltage (Voc) of bulk heterojunction polymer solar cells (PSCs). This improvement is attractive since there are only a few solution-based processing technologies that can enhance the Voc. In this article, the Voc enhancement effect of five distinct CPE materials was studied on three typical PSC device configurations, each using different donor materials. Significant Voc enhancement was observed in devices that contain a polyfluorene copolymer as the donor material. The origin of the Voc enhancement was investigated by examination of the dark currents.

Comparing the Properties of Electrochemical-Based DNA Sensors Employing Different Redox Tags

Many electrochemical biosensor approaches developed in recent years utilize redox-labeled (most commonly methylene blue or ferrocene) oligonucleotide probes site-specifically attached to an interrogating electrode. Sensors in this class have been reported that employ a range of probe architectures, including single- and double-stranded DNA, more complex DNA structures, DNA and RNA aptamers, and, most recently, DNA-small molecule chimeras. Signaling in this class of sensors is generally predicated on binding-induced changes in the efficiency with which the covalently attached redox label transfers electrons with the interrogating electrode. Here we have investigated how the properties of the redox tag affect the performance of such sensors. Specifically, we compare the differences in signaling and stability of electrochemical DNA sensors (E-DNA sensors) fabricated using either ferrocene or methylene blue as the signaling redox moiety. We find that while both tags support efficient E-DNA signaling, ferrocene produces slightly improved signal gain and target affinity. These small advantages, however, come at a potentially significant price: the ferrocene-based sensors are far less stable than their methylene blue counterparts, particularly with regards to stability to long-term storage, repeated electrochemical interrogations, repeated sensing/regeneration iterations, and employment in complex sample matrices such as blood serum.

Improved Injection in n-Type Organic Transistors with Conjugated Polyelectrolytes

To improve injection in n-type organic thin film transistors (OTFTs), a thin conjugated polyelectrolyte (CPE) layer was interposed between electrodes and the semiconductor layer. OTFTs were fabricated with [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) and Au source and drain contacts. We demonstrate that the insertion of CPEs beneath top-contact Au source/drain electrodes can be a very effective strategy for improving the carrier injection and reducing turn-on threshold voltages of n-channel OTFTs. Ultraviolet photoemission spectroscopy (UPS) indicates that the decrease of the electron injection barrier is consistent with organized dipoles at the metal/organic interface.

Fluorine substitution enhanced photovoltaic performance of a D–A1–D–A2 copolymer

A new alternating donor-acceptor (D-A1-D-A2) copolymer containing two electron-deficient moieties, isoindigo and quinoxaline, was synthesized. The photovoltaic performance of this polymer could be improved by incorporating fluorine atoms into the quinoxaline units, resulting in an efficiency of 6.32%. This result highlights the attractive promise of D-A1-D-A2 copolymers for high-performance bulk heterojunction solar cells.

Electron injection into organic semiconductor devices from high work function cathodes

We show that polymer light-emitting diodes with high work-function cathodes and conjugated polyelectrolyte injection/transport layers exhibit excellent efficiencies despite large electron-injection barriers. Correlation of device response times with structure provides evidence that the electron-injection mechanism involves redistribution of the ions within the polyelectrolyte electron-transport layer and hole accumulation at the interface between the emissive and electron-transport layers. Both processes lead to screening of the internal electric field and a lowering of the electron-injection barrier. The hole and electron currents are therefore diffusion currents rather than drift currents. The response time and the device performance are influenced by the type of counterion used.

Conjugated Oligoelectrolyte Electron Transport/Injection Layers for Organic Optoelectronic Devices

The function of a conjugated oligoelectrolyte (COE) as the electron transport layer (ETL) in polymer light emitting diodes is demonstrated. Current density−luminance−voltage characteristics demonstrate that insertion of the COE adjacent to an Al electrode yields higher device efficiencies than those obtained using the low work function cathode Ba. Furthermore, the temporal response of the electroluminescence is faster than that observed when using conjugated polyelectrolytes as the ETL. That COEs have better defined structures than their polymeric counterparts may make them more attractive materials for the fabrication of efficient PLEDs prepared by spin coating methods.

Anomalous compression behavior in lanthanum/cerium-based metallic glass under high pressure

In situ high-pressure x-ray diffraction, low-temperature resistivity, and magnetization experiments were performed on a La32Ce32Al16Ni5Cu15 bulk metallic glass (BMG). A sudden change in compressibility at ≈14 GPa and a rapid increase of resistivity at ≈12 K were detected, whereas magnetic phase transformation and magnetic field dependence of the low-temperature resistivity do not occur at temperatures down to 4.2 K. An interaction between conduction electrons and the two-level systems is suggested to explain the temperature and field dependences of resistivity of the BMG alloy. Although the cause of the unusual change in compressibility at ≈14 GPa is not clear, we believe that it could be linked with the unique electron structure of cerium in the amorphous matrix. An electronic phase transition in BMG alloys, most likely a second-order amorphous-to-amorphous phase transition, is suggested.

Design and synthesis of novel carbazole–spacer–carbazole type conjugated microporous networks for gas storage and separation

Two novel conjugated microporous networks, P-1 and P-2, with carbazole–spacer–carbazole topological model structures, were designed and prepared by FeCl3 oxidative coupling polymerization. Monomer m-1 (fluorenone spacer) was modified with a thiophene Grignard to form the fluorenyl tertiary alcohol monomer m-2, and this step can increase the polymerization branches from four to five and incorporate the polar –OH group into the building block. N2 adsorption isotherms show that, after modification, the Brunauer–Emmett–Teller (BET) surface area of P-2 (1222 m2 g−1) is two times that of P-1 (611 m2 g−1), and the total pore volume increases 1.63 times from 0.95 to 1.55 at P/P0 = 0.99. However, the domain pore size (centred at 1.19 nm) and the pore distribution of both networks are not changed. It demonstrates that the domain pore width may be determined by the size of the rigid carbazole–spacer–carbazole backbone, not the degree of crosslinking when the networks were prepared under same polymerization conditions in this system. Hydrogen physisorption isotherms of P-1 and P-2 show that the H2 storage can be up to 1.05 wt% and 1.66 wt% at 77 K and 1.1 bar, and the isosteric heat is 9.89 kJ mol−1 and 10.86 kJ mol−1, respectively. At 273 K and 1.1 bar, the CO2 uptake capacity of P-2 can be up to 14.5 wt% which is 1.63 times that of P-1 under the same conditions. The H2 and CO2 uptake capacities of P-2 are among the highest reported for conjugated microporous networks under similar conditions. The CO2/CH4 and CO2/N2 selectivity results indicate that P-1 exhibits a slightly higher separation ability than P-2. There is often a trade-off between absolute uptake and selectivity in other microporous organic polymers. Fine design and tailoring the topological structure of the monomer can change the adsorption isosteric enthalpy and optimize the gas uptake performance. The obtained networks with the carbazole–spacer–carbazole rigid backbone show promise for potential use in clean energy applications and the environmental field.

Solvent Effects on the Architecture and Performance of Polymer White‐Light‐Emitting Diodes with Conjugated Oligoelectrolyte Electron‐Transport Layers

Multilayered structures improve the function of organic optoelectronic devices. For example, operational voltages can be substantially decreased, and the light output to current ratio increased, upon insertion of electron-injection/transport layers (ETLs) or hole-injection/transport layers at the cathode and anode interfaces, respectively. These layers can reduce the barriers to charge injection from the electrodes into the organic semiconductor, and improve efficiencies via mechanisms that have been summarized in the literature. One practical challenge concerns methods for fabricating multilayered systems via procedures that are efficient and that provide for control of activelayer thicknesses and organic/organic interfaces widths. The fabrication of small-molecule-based devices benefits from the ability of vacuum depositionmethods to sequentially depositmaterials, with excellent precision over the individual layer thickness and without large disruptions of underlying organic coatings. The situation with conjugated polymer-based devices, such as polymer light-emitting diodes (PLEDs), is different. PLEDs offer the possibility of solution-based fabrication options and of tailoring blends that incorporate multiple components in a single layer. However, multilayer fabrication is challenging if one wishes to deposit layers atop of each other when the materials have similar solubility characteristics. Under these circumstances, one observes poorly defined interfaces and a disruption of the underlying polymer layers. It is in the context of PLED preparation that conjugated polyelectrolytes can offer new options to build well-defined multilayered structures. These materials are described as presenting a backbone with an electronically delocalized structure, that is, the semiconducting component, and pendant groups bearing ionic functionalities. Such structural attributes lead to higher solubility