The protein and gold nanoparticle (AuNP) interfacial interaction has broad implications for biological and biomedical applications of AuNPs. In situ characterization of the morphology and structural evolution of protein on AuNPs is difficult. We have found that the protein coating layer formed by bovine serum albumin (BSA) on AuNP is highly permeable to further organothiol adsorption. Using mercaptobenzimidazole (MBI) as a molecular probe, it is found that BSA interaction with AuNP is an exceedingly lengthy process. Structural modification of BSA coating layer on AuNP continues even after 2 days’ aging of the (AuNP/BSA) mixture. While BSA is in a near full monolayer packing on the AuNPs, it passivates only up to 30% of the AuNP surfaces against MBI adsorption. Aging reduces the kinetics of the MBI adsorption. However, even in the most aged BSA-coated AuNP (3 days), 80% of the MBI adsorption occurs within the first 5 min of the MBI addition to the (AuNP/BSA) mixture. The possibility of MBI displacing the adsorbed BSA was excluded with quantitative BSA adsorption studies. Besides MBI, other organothiols including endogenous amino acid thiols (cysteine, homocysteine, and glutathione) were also shown to penetrate through the protein coating layer and be adsorbed onto AuNPs. In addition to providing critical new understanding of the morphology and structural evolution of protein on AuNPs, this work also provides a new venue for preparation of multicomponent composite nanoparticle with applications in drug delivery, cancer imaging and therapy, and material sciences.

Determination of the ligand conformation on gold nanoparticle (AuNP) is of fundamental importance in nanoparticle research and applications. Using a combination of surface-enhanced Raman spectroscopy (SERS), density function calculation, and normal Raman spectroscopy, the pH dependence of mercaptobenzimadazole (MBI) adsorption onto AuNP was systematically studied. Structures and conformations of MBI adsorbates on AuNP were determined together with their binding constants, and saturation packing densities were determined at three different pHs (1.4, 7.9, and 12.5). While MBI thione is the predominant tautomer in solution with a pH value lower than 10.3, MBI thiolate is the main adsorbate on AuNP surface in solution with pH > 2. MBI thiones dominate the AuNP surface only in solutions with pH < 2. While MBI thione has a higher saturation packing density (∼632 pmol/cm 2 ) than MBI thiolate (∼540 pmol/cm 2 ), its binding constant (2.14 × 10 6 M -1 ) is about five times smaller than that for MBI thiolate (10.12 × 10 6 M -1 ). Using the MBI footprint deduced from its saturation packing density on AuNP, the conformation of MBI was determined. While the MBI thione binds monodentately to the AuNP with a perfectly upright orientation, MBI thiolate binds bidentately to AuNP with a tilt angle that allows interaction of AuNP with both the sulfur and the nitrogen atoms in MBI thiolate. In addition to the new insights provided on MBI binding onto gold nanoparticle, the methodology employed in this study can be particularly useful for studying AuNP interactions with other imidazole-thiol compounds, a class of heterocylic compounds that can exist in different tautomeric forms.

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage and electronics cooling to thermal processing of materials. In spite of the extensive studies in the literature, a consensus is lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. In this work, an experimental investigation was conducted to study single-phase forced convection of Al2O3-water nanofluid in a circular minichannel with a 1.09 mm inner diameter. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared with those of the base fluid. The Reynolds number (Re) varied from 600 to 4500, covering the laminar, transition, and early fully developed turbulent regions. It was found that in the laminar region, the nanofluids exhibit pronounced entrance region behaviors possibly due to the flattening of the velocity profile caused by the flow-induced particle migration. Three new observations were made for nanofluids in the transition and turbulent regions: (1) The onset of transition to turbulence is delayed; (2) both the friction factor and the convective heat transfer coefficient are below those of water at the same Re in the transition flow; and (3) once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A simple scaling analysis was used to show that these behaviors may be attributed to the variation in the relative size of the nanoparticle with respect to the turbulent microscales at different Re. The results from this work suggest that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, as a heat transfer fluid, nanofluids should be used in either the laminar flow or the fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.

Single-phase convective heat transfer of nanofluids has been studied extensively, and dif ferent degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclu sive whether the same pessimistic outlook should he expected. In this work, an experimen tal study of forced convective flow boiling and two-phase flow was conducted for Al20 3-water nanofluids through a minichannel. General flow boiling heat transfer char acteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-pliase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFl). It was found that the presence of nanoparticles delays ONB and suppresses OFl, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and sin face wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations.ALO^-water nanofluids with two nanoparticle volume concen trations (0.01 vol. % and 0.1 vol. %) were used in this work. They were prepared following the same method described in Ref.[40], Journal of Heat Transfer