Lectures on entropy

Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.

Consider advances are being made to develop new technologies capable of fast in-situ tracing of agricultural toxins. In this work, we reported a simple carbaryl sensor with very high sensitivity using graphene interdigitated ion selective field effect transistor (ISFET). The graphene films were first prepared on polycrystalline copper foil by a low-pressure chemical vapor deposition method, and then transferred onto the interdigitated electrodes of ISFETs by a chemical etching technique. The biorecognition is based on the enzymatic inhibition of carbaryl towards urease. As expected, the weaker enzymatic activity of urease in addition of carbaryl would result in the weaker current response. It was demonstrated that the interdigitated ISFET sensor could achieve high sensitivity to detect carbaryl as low as 10−8 μg mL−1 and the current response of the device showed a good linearity against the logarithm of carbaryl concentration Ccarbaryl with a regression equation ΔIds = −0.4 − 0.6 logCcarbaryl (mA) (R2 = 0.99) in the concentration range from 2.58 × 10−7 to 2.58 × 10−2 μg mL−1. In conlusion, such convenient graphene-based ISFET configuration could be advantageously extended for on-line screening of other pesticide and herbicide agents

The complete understanding of the mechanism driving colloid-polyelectrolyte complexation still represents a fundamental problem of great interest in soft matter. Polyelectrolyte adsorption onto oppositely charged surfaces represents the core of this problem and a number of theoretical studies, using different approaches, have been published on this subject.During the past few decades, colloid-multivalent ion complexation has been investigated by using either model systems, such as solid hard colloids, soft colloids of biological interests, or hydrophilic globular proteins6. In all cases two distinct but intimately related phenomena accompany and drive the self-assembly, i.e. charge-inversion and reentrant condensation.Although these phenomena have been observed in a variety of polyelectrolyte-colloid mixtures in different conditions, in all previously reported works the charge density on the colloid surface was fixed, or, at least, it could not be changed without changing the ionic strength or the pH of the suspending medium. Thermoresponsive microgels, whose synthesis is initiated by charged groups and is equivalent to an end-group functionalization, are characterized by a thermodynamic volume phase transition (VPT) that gives the opportunity to tune finely the adsorption of polyelectrolytes simply by changing temperature. Indeed, by controlling the particle volume, the VPT affects dramatically the microgel charge density and hence the polyelectrolyte adsorption.We investigated the complexation of thermoresponsive anionic poly(Nisopropylacrylamide) (PNiPAM) microgels and cationic -polylysine (epsilon-PLL) chains. By combining electrophoresis, light scattering, transmission electron microscopy (TEM) and dielectric spectroscopy (DS) we studied the adsorption of epsilon-PLL onto the microgel networks and its effect on the stability of the suspensions. We show that the volume phase transition (VPT) of the microgels triggers a large polyion adsorption. Two interesting phenomena with unique features occur: a temperature-dependent microgel overcharging and a complex reentrant condensation. The latter may occur at fixed polyion concentration, when temperature is raised above the VPT of microgels, or by increasing the number density of polycations at fixed temperature. TEM and DS measurements unambiguously show that short PLL chains adsorb onto microgels and act as electrostatic glue above the VPT. By performing thermal cycles, we further show that polyion-induced clustering is a quasi-reversible process: within the time of our experiments large clusters form above the VPT and partially re-dissolve as the mixtures are cooled down.Finally we give a proof that the observed phenomenology is purely electrostatic in nature: an increase of the ionic strength gives rise to the polyion desorption from the microgel outer shell. By showing that the VPT of thermoresponsive ionic microgels can be employed to trigger polyion adsorption and tune reentrant microgel condensation, our work lays the foundation for a groundbreaking strategy to tune electroadsorption ruled by temperature and that can be employed in a variety of fields spanning wastewater and soil remediation, nanoencapsulation of small charged nanoparticles, and selective drug delivery.

By combining rheology, light scattering, small angle X-ray scattering and simulations we explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time (SRT) of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard sphere systems [1]. This suggests that softness has no impact on the growth of the SRT on approaching the glass transition, as confirmed by computer simulations. We show that the onset of a finite yield-stress, occurring within the supercooled regime, systematically corresponds to a minimum of the stretching exponent characterizing the decay of the two-time intensity autocorrelation function.Softness becomes relevant only at very large packing fractions, when the system falls out of equilibrium. In this non-equilibrium regime, the SRT depends surprisingly weakly on packing fraction and time correlation functions exhibit compressed exponential decays consistent with internal stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease of local order with increasing density, reminiscent of the reentrant glass transition predicted theoretically in ultrasoft systems [2,3], and corresponds to a weakening of the volume fraction dependence of the plateau modulus of the suspensions. We propose that these peculiar dynamics result from a competition between the non-equilibrium aging dynamics of the glassy state and the tendency of soft systems to refluidize at high packing fractions.