Highly Efficient Removal of Cr(VI) from Aqueous Solutions by Polypyrrole/Monodisperse Latex Spheres.

Pyrrole (Py) is easily agglomerated during the polymerization process, affecting its performance. In this paper, polypyrrole/monodispersed latex sphere (PPy/MLS) composites were prepared using in-situ polymerization for the adsorption of hexavalent chromium (Cr(VI)). The specific surface area of PPy/MLS (39.30 m2/g) was increased relative to that of PPy (24.82 m2/g), thus providing more effective adsorption sites. In addition, the adsorption properties of Cr(VI) under different conditions, including Py content, pH of the aqueous solution, and PPy/MLS dosage, were investigated to reveal the adsorption mechanism. The results showed that PPy/MLS possessed high Cr(VI) adsorption capacities when the Py content was 50 wt %. The maximum adsorption capacity was 343.64 mg/g at pH 2.0 and 25 °C. Remarkably, the adsorbents exhibited an excellent removal rate of Cr(VI) after three cycles of adsorption-desorption (over 99%), suggesting that the adsorbents had exceptional recyclability. Furthermore, the adsorption process followed quasi-second-order kinetics and Langmuir isothermal adsorption model. The high adsorption performance, sustainability, and cost-efficiency make this adsorbent a promising candidate for large-scale Cr(VI) contaminant removal.

Effect of La Doping on the Structure and Lithium Storage Performance of V2O5.

La-doped vanadium pentoxide (V2O5) was prepared by sol-gel method and subsequent heat treatment in air at 400 °C. The influence of La-doping on the crystalline structure, surface morphology, and lithium storage performance of V2O5 were investigated by XRD, SEM, CV, EIS, and discharge-charge tests. The results demonstrated that La doping did not change the phase structure of orthorhombic V2O5, but significantly refined the crystal grain of V2O5; La doping obviously improved the cycling stability and rate capability of V2O5 due to the decreased electrode polarization, smaller electron-transfer resistance, and enhanced lithium diffusivity. For example, the La-doped V2O5 delivered a reversible capacity of 131 mAh/g after 50 cycles at a current density of 200 mA/g, while the pure V2O5 showed a reversible capacity of only 63 mAh/g under the same condition. At the high rate of 500 mA/g, the La-doped V2O5 still displayed a reversible capacity of 106 mAh/g, which is two times higher than that (45 mAh/g) of pure V2O5 sample.

Fabrication of paper devices via laser-heating-wax-printing for high-tech enzyme-linked immunosorbent assays with low-tech pen-type pH meter readout.

In this work, a new method named laser-heating-wax-printing (LHWP) is described to fabricate paper devices for developing sensitive, affordable, user-friendly paper-based enzyme-linked immunosorbent assays (P-ELISAs) that initially use common pen-type pH meters for portable, quantitative readout. The LHWP enables a rapid patterning of wax in paper via one step of heating the wax layer coated on the paper surface using a mini-type CO2 laser machine. Wax-patterned paper microzones created in this way are utilized to conduct the pen-type pH meter-based P-ELISAs with enzyme-loaded SiO2 microbeads for highly efficient signal amplification of each antibody-antigen binding event. The results show that this new P-ELISA system is quantitatively sensitive to the concentrations of a model protein analyte in buffer samples ranging from 12.5 to 200 pg mL-1, with a limit of detection of ca. 7.5 pg mL-1 (3σ). Moreover, the satisfactory recovery results of assaying several human serum samples validate its feasibility for practical applications.

Naked-eye quantitative aptamer-based assay on paper device.

This work initially describes the design of low-cost, naked-eye quantitative aptamer-based assays by using microfluidic paper-based analytical device (µPAD). Two new detection motifs are proposed for quantitative µPAD measurement without using external electronic readers, which depend on the length of colored region in a strip-like µPAD and the number of colorless detection microzones in a multi-zone µPAD. The length measuring method is based on selective color change of paper from colorless to blue-black via formation of iodine-starch complex. The counting method is conducted on the basis of oxidation-reduction reaction between hydrogen peroxide and potassium permanganate. Their utility is well demonstrated with sensitive, specific detection of adenosine as a model analyte with the naked eye in buffer samples and undiluted human serum. These equipment-free quantitative methods proposed thus hold great potential for the development of more aptamer-based assays that are simple, cost-efficient, portable, and user-friendly for various point-of-care applications particularly in resource-constrained environments.

Timing readout in paper device for quantitative point-of-use hemin/G-quadruplex DNAzyme-based bioassays.

This work describes a quantitative point-of-use hemin/G-quadruplex DNAzyme-based assay that integrates a simple timing detection motif with low-cost, portable microfluidic paper-based analytical devices (µPADs). The timing readout is based on the selective DNAzyme-mediated wettability change of paper from hydrophilicity to hydrophobicity. Its utility is well demonstrated with sensitive, specific detection of K(+) ion as a model analyte in artificial samples as well as real human serum samples. This new method only requires a ubiquitous cheap timer (or a cell phone with a timing function) to provide quantitative results. It could offer new opportunities for the development of more peroxidase-mimicking DNAzyme-based bioassays that are simple, affordable, portable, and operable by minimally-trained users for broad point-of-use applications especially in resource-poor settings.

Equipment-free quantitative measurement for microfluidic paper-based analytical devices fabricated using the principles of movable-type printing.

Microfluidic paper-based analytical devices (µPADs) are a growing class of low-cost chemo/biosensing technologies designed for point-of-use applications. In this article, we describe MTWP (movable-type wax printing), a facile method for the fabrication of µPADs. MTWP is inspired by the Chinese movable-type printing and requires only a hot plate and homemade small iron movable components. It is able to pattern various wax microstructures in paper via a simple adjustment of the number, patterning forms or types of the metal movable components. This inexpensive and versatile method may thus hold great potential for producing wax-patterned µPADs by untrained operators at minimized cost in developing countries. In addition, two novel equipment-free assay methods are further developed to render µPAD measurements straightforward and quantitative. They use the flow-through time of a detection reagent in a three-dimensional paper device and the number of colored detection microzones in a 24-zone paper device as the detection motifs. The timing method is based on the selective wettability change of paper from hydrophilic to hydrophobic that is mediated by enzymatic reactions. The counting method is carried out on the basis of oxidation-reduction reactions of a colored substance, namely iodine. Their utility is demonstrated with quantitative detection of hydrogen peroxide as a model analyte. These methods require only a timer or a cell phone with a timing function and the abilities of seeing color and of counting for quantitative µPAD measurement, thus making them simple, cost-efficient, and useful sensor technologies for a great diversity of point-of-need applications especially in resource-poor settings.

One-step patterning of hollow microstructures in paper by laser cutting to create microfluidic analytical devices.

In this paper, we report a simple, low-cost method for rapid, highly reproductive fabrication of paper-based microfluidics by using a commercially available, minitype CO(2) laser cutting/engraving machine. This method involves only one operation of cutting a piece of paper by laser according to a predesigned pattern. The hollow microstructures formed in the paper are used as the 'hydrophobic barriers' to define the hydrophilic flowing paths. A typical paper device on a 4 cm × 4 cm piece of paper can be fabricated within ∼7-20 s; it is ready for use once the cutting process is finished. The main fabrication parameters such as the applied current and cutting rate of the laser were optimized. The fabrication resolution and multiplexed analytical capability of the hollow microstructure-patterned paper were also characterized.