Cholesterol is an integral component of eukaryotic cell membranes and a key molecule in controlling membrane fluidity, organization, and other physicochemical parameters. It also plays a regulatory function in antibiotic drug resistance and the immune response of cells against viruses, by stabilizing the membrane against structural damage. While it is well understood that, structurally, cholesterol exhibits a densification effect on fluid lipid membranes, its effects on membrane bending rigidity are assumed to be nonuniversal; i.e., cholesterol stiffens saturated lipid membranes, but has no stiffening effect on membranes populated by unsaturated lipids, such as 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). This observation presents a clear challenge to structure–property relationships and to our understanding of cholesterol-mediated biological functions. Here, using a comprehensive approach—combining neutron spin-echo (NSE) spectroscopy, solid-state deuterium NMR (2H NMR) spectroscopy, and molecular dynamics (MD) simulations—we report that cholesterol locally increases the bending rigidity of DOPC membranes, similar to saturated membranes, by increasing the bilayer’s packing density. All three techniques, inherently sensitive to mesoscale bending fluctuations, show up to a threefold increase in effective bending rigidity with increasing cholesterol content approaching a mole fraction of 50%. Our observations are in good agreement with the known effects of cholesterol on the area-compressibility modulus and membrane structure, reaffirming membrane structure–property relationships. The current findings point to a scale-dependent manifestation of membrane properties, highlighting the need to reassess cholesterol’s role in controlling membrane bending rigidity over mesoscopic length and time scales of important biological functions, such as viral budding and lipid–protein interactions.

We have presented systematic cross-plane thermal conductivity (λ) data for the undoped strain-symmetrized Si/Ge superlattices grown on Si(111) with superlattice (SL) period thickness varying from 3.6 to 16 nm. In thin SL period (L⩽7 nm) samples, the data have shown considerable reductions of λ, by more than 50% and 30% compared to the SiGe alloy and to the earlier reported values in (100)-oriented Si/Ge superlattice structures (SLS), respectively. For the thick SL period samples (L>10 nm), λ has shown a tendency to saturate at the SiGe alloy value. This is understood as, with increasing L, the SLS breaks and the SiGe alloying starts to grow. This structural behavior is clearly observed in the cross-plane transmission electron microscope images as well. In addition to these, for the thin SL period (L⩽7 nm) samples, the data have shown a shallow minimum which is attributed to the competing behavior of the wave nature and the classical particle nature of the localized phonons. Nevertheless, the present study of thermal conductivity on undoped strain-symmetrized Si/Ge SLs in (111) orientation suggests that an enhancement of thermoelectric figure-of-merit Z is possible.

Immobilization of colloidal gold nanoparticles (AuNPs) on metal oxide supports is often done to enhance stability and recoverability over multiple reaction cycles but with a reduction in available surface area. It has been theorized that colloidal AuNPs are catalytically more active and selective than the supported counterparts on a gold mass basis. Direct comparison of catalytic activity and available surface area of 5 nm diameter colloidal gold nanoparticles (AuNP) dispersed in solution and supported on titania has been conducted in this work. A versatile spectroscopic organothiol ligand adsorption method was used to measure available surface area of both colloidal and supported catalysts in aqueous media for direct comparison, circumventing phase limitations of other characterization techniques like chemisorption. Turnover frequency measurements on a gold mass basis showed a reduction in catalytic activity for supported AuNP compared to colloidal AuNP, whereas turnover frequency normalized to available surface area was increased for supported AuNP. This suggests enhancement of AuNP catalytic activity by the metal oxide support for this particular model reaction and led to quantification of the synergistic support effect. An induction time was observed, which increased with increasing reactant concentration with respect to available surface area. The effect of the reactant addition sequence and reactant concentration on the induction time was explored and suggests competitive adsorption of reactant species on the AuNP surface and also depicts a threshold reactant to available surface area ratio, above which emergence of induction time is observed.

Thin semiconducting thermoelectric films with narrow energy band gaps are considered to be very promising for future microdevice applications (sensors and generators). The polycrystalline BiSbTe alloys (V–VI semiconductors) are examples. In this report, the detailed temperature dependence of electrical resistivity [ρ(T)], n- and p-type carrier concentration [n(T) and p(T)], and Hall mobility [μ(T)] of n-type Bi2Te3, p-type Sb2Te3, and p-type (Bi1−xSbx)2Te3 (x=0.73 and 0.77) alloy films prepared by metalorganic chemical vapor deposition are presented in the range of 100–500 K. From the room temperature measurement of the Seebeck coefficient (α), the values of α for Bi2Te3, Sb2Te3, and (Bi1−xSbx)2Te3 with x=0.73 and 0.77 are found to be −220, +110, +240, and +210 μV/K, respectively, which are optimal in these types of film materials. The carrier concentration of these films at 300 K is found to be around (1019–1020) cm−3. The ρ(T) data show an exponential increase with increasing temperature irrespective of the carrier types. For the temperature dependence of the Hall mobility, the lattice contribution is found to be predominant for all the films. Also, we have fabricated a simple micromodule Peltier device (MMP) using the n-type Bi2Te3 and the p-type (Bi1−xSbx)2Te3 (x=0.77) films where a maximum cooling of 2.6 °C was obtained with a low input current of 2.5 mA.

Mercaptoundecanoic acid (MUA) functionalized gold nanoparticles (AuNP-MUA) were synthesized and demonstrated to possess pH-triggered aggregation and re-dispersion, as well as the capability of phase transfer between aqueous and organic phases in response to changes in pH. The pH of aggregation for AuNP-MUA is consistent with the pKa of MUA (pH ~4) in solution, while AuNP-MUA phase transition between aqueous and organic phases occurs at pH ~9. The ion pair formation between the amine group in octadecylamine (ODA), the carboxylate group in MUA, and the hydrophobic alkyl chain of ODA facilitates the phase transfer of AuNP-MUA into an organic medium. The AuNP-MUA were investigated as a reusable catalyst in the catalytic reduction of 4-nitrophenol by borohydride—a model reaction for AuNPs. It was determined that 100% MUA surface coverage completely inhibits the catalytic activity of AuNPs. Decreasing the surface coverage was shown to increase catalytic activity, but this decrease also leads to decreased colloidal stability, recoverability, and reusability in subsequent reactions. At 60% MUA surface coverage, colloidal stability and catalytic activity were achieved, but the surface coverage was insufficient to enable redispersion following pH-induced recovery. A balance between AuNP colloidal stability, recoverability, and catalytic activity with reusability was achieved at 90% MUA surface coverage. The AuNP-MUA catalyst can also be recovered at different pH ranges depending on the recovery method employed. At pH ~4, protonation of the MUA results in reduced surface charge and aggregation. At pH ~9, ODA will form an ion-pair with the MUA and induce phase transfer into an immiscible organic phase. Both the pH-triggered aggregation/re-dispersion and aqueous/organic phase transfer methods were employed for catalyst recovery and reuse in subsequent reactions. The ability to recover and reuse the AuNP-MUA catalyst by two different methods and different pH regimes is significant, based on the fact that nanoparticle-catalyzed reactions may occur under different pH conditions.