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.

On the binding of cationic, water-soluble conjugated polymers to DNA: electrostatic and hydrophobic interactions.

Water-soluble, cationic conjugated polymer binds single-stranded DNA with higher affinity than it binds double-stranded or otherwise "folded" DNA. This stronger binding results from the greater hydrophobicity of single-stranded DNA. Upon reducing the strength of the hydrophobic interactions, the electrostatic attraction becomes the important interaction that regulates the binding between the water-soluble conjugated polymer and DNA. The different affinities between the cationic conjugated polymer and various forms of DNA (molecular beacons and its open state; single-stranded DNA and double-stranded DNA and single-stranded DNA and complex DNA folds) can be used to design a variety of biosensors.

Semiconducting polymer photodetectors with electron and hole blocking layers: high detectivity in the near-infrared.

Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Photodetectors with broad spectral response, from 300 nm to 1,100 nm, were fabricated using a narrow-band gap semiconducting polymer blended with a fullerene derivative. By using both an electron-blocking layer and a hole-blocking layer, the polymer photodetectors, operating at room temperature, exhibited calculated detectivities greater than 10(13) cm Hz(1/2)/W over entire spectral range with linear dynamic range approximately 130 dB. The performance is comparable to or even better than Si photodetectors.

Conductive conjugated polyelectrolyte as hole-transporting layer for organic bulk heterojunction solar cells.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively used as the hole-transporting layer (HTL) in bulk heterojunction (BHJ) solar cells, however, its anisotropic electrical conduction and intrinsic acidic nature generally limit the device performance. Here we demonstrate the application of a water/alcohol soluble CPE (CPE-K) as HTLs in BHJ solar cells, achieving a PCE up to 8.2%. The more superior and uniform vertical electrical conductivity found in CPE-K reduces the series resistance and provides efficient hole extraction.

"Liquid-liquid-solid"-type superoleophobic surfaces to pattern polymeric semiconductors towards high-quality organic field-effect transistors.

Precisely aligned organic-liquid-soluble semiconductor microwire arrays have been fabricated by "liquid-liquid-solid" type superoleophobic surfaces directed fluid drying. Aligned organic 1D micro-architectures can be built as high-quality organic field-effect transistors with high mobilities of >10 cm(2) ·V(-1) ·s(-1) and current on/off ratio of more than 10(6) . All these studies will boost the development of 1D microstructures of organic semiconductor materials for potential application in organic electronics.

Molecular doping enhances photoconductivity in polymer bulk heterojunction solar cells.

Addition of low concentrations (<1:100, dopant:donor) of a fluorinated p-type dopant, F4-TCNQ leads to a considerable enhancement of the photocurrent in PCDTBT:PC70 BM bulk heterojunction solar cells. As a result, the power conversion efficiency increases from 6.41% to 7.94 %.

Efficient solution-processed small-molecule solar cells with inverted structure.

We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.

High-efficiency polymer solar cells enhanced by solvent treatment.

A significant enhancement of efficiency in thieno[3,4-b]-thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC70 BM) solar cells can be achieved by methanol treatment. The effects of methanol treatment are shown in an improvement of built-in voltage, a decrease in series resistance, an enhanced charge-transport property, an accelerated and enlarged charge extraction, and a reduced charge recombination, which induce a simultaneous enhancement in open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) in the devices.

Enhanced efficiency of single and tandem organic solar cells incorporating a diketopyrrolopyrrole-based low-bandgap polymer by utilizing combined ZnO/polyelectrolyte electron-transport layers.

Power conversion efficiency up to 8.6% is achieved for a solution-processed tandem solar cell based on a diketopyrrolopyrrole-containing polymer as the low-bandgap material after using a thin polyelectrolyte layer to modify the electron-transport ZnO layers, indicating that interfacial engineering is a useful approach to further enhancing the efficiency of tandem organic solar cells.

Manifestation of carrier relaxation through the manifold of localized states in PCDTBT:PC60BM bulk heterojunction material: the role of PC84BM traps on the carrier transport.

The transport and relaxation of photogenerated carriers in a bulk heterojunction (BHJ) material made of a blend of PCDTBT and PC(60) BM are studied as a function of the concentration of PC(84)BM impurities. For low concentrations of PC(84)BM, the increasing activation energy with delay time indicates transport dominated by trap-limited carrier drift while the photocarriers relax through a manifold of disorder-induced localized states near the band edge. In the BHJ material with high concentration of PC(84)BM, transport is dominated by carrier hopping within the PC(84)BM impurity band.

Insights into π-conjugated small molecule neat films and blends as determined through photoconductivity.

Spectrally dependent steady-state photoconductivity is a convenient method to gain insight into the charge generation and transport processes within a given material. In this work, we report on the photoconductive response of solution-processed neat films and blends of the fullerene, PC(71)BM, and the donor-acceptor small-molecule, p-DTS(PTTh(2))(2), as function of the processing additive, diiodooctance (DIO). The results, when considered in the context of their structural, optical, and electronic properties give insight into the dominant carrier generation and charge transport mechanisms in each of these molecular systems.

"Columnlike" structure of the cross-sectional morphology of bulk heterojunction materials.

The cross-sectional morphology of the bulk heterojunction (BHJ) films comprising regio-regular poly(3-hexylthiophene) (rrP3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) was observed with transmission electron microscopy (TEM). The cross-sectional TEM images of the BHJ film provide information on the pathways for charge transport through the film thickness. The length scale of the phase separation was obtained from spatial Fourier transform analysis of the TEM images and from calculations of the autocorrelation function.

Beyond the metal-insulator transition in polymer electrolyte gated polymer field-effect transistors.

We have studied the carrier transport in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) field-effect transistors (FETs) at very high field-induced carrier densities (10(15) cm(-2)) using a polymer electrolyte as gate and gate dielectric. At room temperature, we find high current densities, 2 x 10(6) A/cm(2), and high metallic conductivities, 10(4) S/cm, in the FET channel; at 4.2 K, the current density is sustained at 10(7) A/cm(2). Thus, metallic conductivity persists to low temperatures. The carrier mobility in these devices is approximately 3.5 cm(2).V(-1).s(-1) at 297 K, comparable with that found in fully crystalline organic devices.

Comparison of the signaling and stability of electrochemical DNA sensors fabricated from 6- or 11-carbon self-assembled monolayers.

We have characterized the solution-phase and dry storage stability of electrochemical E-DNA sensors fabricated using mixed self-assembled monolayers (SAMs) composed of 6- or 11-carbon (C6 and C11, respectively) alpha,omega-thiol alcohols and the analogous C6- or C11-thiol-terminated stem-loop DNA probe. We find that the solution-phase and dry storage stability of C6-based E-DNA sensors are limited and poorly reproducible. The use of stabilizing agents bovine serum albumin plus either glucose or trehalose significantly improves the dry storage shelf life of such sensors: when using these preservatives, we observe only 7-9% sensor degradation after 1 month of storage in air at room temperature. In comparison, the stability of C11-based E-DNA sensors is significantly greater than that of the C6-based sensors; we observe only minor (5-8%) loss of signal upon storing these sensors for a week under ambient solution conditions or for more than a month in air in the presence of preservatives. Moreover, whereas the electron-transfer rate through C11 SAMs is slower than that observed for C6 SAMs, it is rapid enough to support good sensor performance. It thus appears that C11 SAMs provide a reasonable compromise between electron-transfer efficiency and sensor stability and are well suited for use in electronic DNA-sensing applications.

An electronic, aptamer-based small-molecule sensor for the rapid, label-free detection of cocaine in adulterated samples and biological fluids.

Whereas spectroscopic and chromatographic techniques for the detection of small organic molecules have achieved impressive results, these methods are generally slow and cumbersome, and thus the development of a general means for the real-time, electronic detection of such targets remains a compelling goal. Here we demonstrate a potentially general, label-free electronic method for the detection of small-molecule targets by building a rapid, reagentless biosensor for the detection of cocaine. The sensor, based on the electrochemical interrogation of a structure-switching aptamer, specifically detects micromolar cocaine in seconds. Because signal generation is based on binding-induced folding, the sensor is highly selective and works directly in blood serum and in the presence of commonly employed interferents and cutting agents, and because all of the sensor components are covalently attached to the electrode surface, the sensor is also reusable: we achieve >99% signal regeneration upon a brief, room temperature aqueous wash. Given recent advances in the generation of highly specific aptamers, this detection platform may be readily adapted for the detection of other small molecules of a wide range of clinically and environmentally relevant small molecules.