Mechanisms of Carbon Nanotube Aggregation and the Reversion of Carbon Nanotube Aggregates in Aqueous Medium

Single-walled carbon nanotubes (SWCNTs) dispersed in aqueous medium have many potential applications in chemistry, biology, and medicine. Reversible aggregation of SWCNTs dispersed in water has been frequently reported, but the mechanisms behind are not well understood. Here we show that SWCNTs dispersed into aqueous medium assisted by various charged molecules can be reversibly aggregated by a variety of electrolytes with two distinct mechanisms. Direct binding of counterions to SWCNTs leads to aggregation when the surface charge is neutralized from 74 to 86%. This aggregation is driven by electrostatic instead of van der Waals interactions, thus showing similarity to that of DNA condensation induced by multivalent cations. Sequestration of counterions by chelating reagents leads to the redispersion of SWCNT aggregates. In contrast to various metal ions, polyelectrolytes have the unique ability to induce SWCNT aggregation by bridging between individual SWCNTs. Aggregation through the latter mechanism can be engineered to be reversible by exploiting various mechanisms of chain breaking, including reduction of disulfide bond in the polymer chain, and the cleavage action of proteolytic enzymes. These findings clarify the mechanisms of SWCNT aggregation, and have broad implications in various applications of SWCNTs in water

Evaporation-Induced Self-Assembly of Ultrathin Tungsten Oxide Nanowires over a Large Scale for Ultraviolet Photodetector

Self-assembly of inorganic nanowires on a large scale directly on a substrate represents a great challenge. Starting from colloidally stable dispersions of ultrathin tungsten oxide nanowires, we successfully assemble the nanowires on a centimeter scale on flat or patterned substrates by a simple evaporation-induced self-assembly method. The capillary flow generated during the evaporation is responsible for the assembly of the nanowires. The concentration of the nanowire dispersion has a significant influence on the self-assembly behavior. Well-aligned tungsten oxide nanowire thin films are achieved when the concentration of the dispersion is in the range from 0.5 to 3.0 mg/mL. While at higher concentrations disordered nanowire thin films with cracks are formed, lower concentrations do not result in the formation of a continuous thin film. A macroscopic device based on the self-assembled tungsten oxide nanowires is fabricated, exhibiting good performance for UV light detection. Our results may pave the road for integrating aligned ultrathin semiconductor nanowires into macroscopic devices for optoelectronic applications

Correlated noise in single-molecule real time trajectories.

<p>The autocorrelation function (ACF) and the partial autocorrelation function (PACF) for AR noise of order 7 as observed in a typical RNA unwinding trace (A and C). The plots from simulated Gaussian noise were also shown for comparison (B and D). The horizontal lines indicate the 95% confidence intervals under the null hypothesis of no correlation. For the AR(7) noise, the ACF shows exponentially decay while the PACF gradually cuts off, i.e. goes to zero after lag 7. For Gaussian noise, the ACF is 1 at lag 0 and zero for other lags while the PACF is zero for all lags.</p

Summary of false positive and true negative steps from 100 realizations of the simulated traces.

<p>The traces contained correlated noise and the steps were identified using GLS method to take the noise structure explicitly into account.</p

Histogram of the number of steps detected from 100 realizations of the simulated traces.

<p>(A), (B) and (C) show the results from GLS method, GLS method but ignoring the correlation in the noise and KERS method, respectively.</p

Efficiency of step detection using GLS method.

<p>The proportion of the traces in which a given step was detected was plotted as a function of the step index. The red dashed line indicates 90%.</p

Over- and underestimates of step numbers in the test simulated traces.

<p>(A) A representative best fit by the GLS method for data that contains correlated noise; the 2.5 kHz test trace is shown in grey and the fit is shown in red. The original step function is shown in blue dashed line for comparison. (B) One of the best fits obtained from GLS method by ignoring correlation for data that contains correlated noise; the 2.5 kHz test trace is shown in grey and the fit is shown in red. (C) and (D) Fit and S-Statistic distribution from KERS method. (C) One of the best fits from KERS method; the 2.5 kHz test trace shown in grey and the fit is shown in red. (D) Distribution of S-statistic as a result of fitting using KERS method. The crosses are the S-statistic from the known step function and the red dots from the best fit for each trace.</p

Precision of identified step location using GLS method.

<p>Deviation of a step from its true location as a percentage of the plateau length was plotted as a function of the step index in a box plot. The cyan boxes indicate the middle 75% of the data or the interquartile range (IQR). The extended lines or whiskers mark the 1.5×IQR distance. Any point greater or less than this value is an outlier and are shown by the dots.</p

Summary of number of steps detected from 100 realizations of the simulated traces.

<p>Four different procedures were used to detect steps, where Procedure (a), (b) and (d) are for traces with correlated noise analyzed with GLS method (a), ignoring noise correlation and assuming Gaussian noise (b) and the KERS method (d); Procedure (c) was done for traces with Gaussian white noise that were analyzed using the GLS method.</p

Three-Dimensional Assembly of Yttrium Oxide Nanosheets into Luminescent Aerogel Monoliths with Outstanding Adsorption Properties

The preparation of macroscopic materials from two-dimensional nanostructures represents a great challenge. Restacking and random aggregation to dense structures during processing prevents the preservation of the two-dimensional morphology of the nanobuilding blocks in the final body. Here we present a facile solution route to ultrathin, crystalline Y₂O₃ nanosheets, which can be assembled into a 3D network by a simple centrifugation-induced gelation method. The wet gels are converted into aerogel monoliths of macroscopic dimensions <i>via</i> supercritical drying. The as-prepared, fully crystalline Y₂O₃ aerogels show high surface areas of up to 445 m²/g and a very low density of 0.15 g/cm³, which is only 3% of the bulk density of Y₂O₃. By doping and co-doping the Y₂O₃ nanosheets with Eu³⁺ and Tb³⁺, we successfully fabricated luminescent aerogel monoliths with tunable color emissions from red to green under UV excitation. Moreover, the as-prepared gels and aerogels exhibit excellent adsorption capacities for organic dyes in water without losing their structural integrity. For methyl blue we measured an unmatched adsorption capacity of 8080 mg/g. Finally, the deposition of gold nanoparticles on the nanosheets gave access to Y₂O₃–Au nanocomposite aerogels, proving that this approach may be used for the synthesis of catalytically active materials. The broad range of properties including low density, high porosity, and large surface area in combination with tunable photoluminescence makes these Y₂O₃ aerogels a truly multifunctional material with potential applications in optoelectronics, wastewater treatment, and catalysis