Rapid reduction of self-assembled monolayers of a disulfide terminated para-nitrophenyl alkyl ester on roughened Au surfaces during XPS measurements

Pd(II)-Imprinted Chitosan Adsorbent for Selective Adsorption of Pd(II): Optimizing the Imprinting Process through Box-Behnken Experimental Design

The ion/molecular imprinting technique is an efficient method for developing materials with high adsorption selectivity. However, it is still difficult to obtain an imprinted adsorbent with desirably high selectivity when the preparation processes are not well designed and optimized. In this present work, a chitosan-based ion-imprinted adsorbent was optimally prepared through Box-Behnken experimental design to achieve desirably high selectivity for Pd anions (PdCl₄ ^2-) from aqueous solutions with high acidity. The dosage of epichlorohydrin (ECH) used in the first and second steps of cross-linking as well as the pH of the imprinting reaction medium is likely one of the key factors affecting the selectivity of the synthesized ion-imprinted chitosan adsorbent, which were selected as factors in a three-level factorial Box-Behnken design. As a result, the effects of these three factors on Pd(II) selectivity were able to be described by using a second-order polynomial model with a high regression coefficient (R ²; 0.996). The obtained optimal conditions via the response surface methodology were 0.10% (v/v) of first cross-linking ECH, an imprinting pH of 1.0, and 1.00% of second cross-linking ECH. Competitive adsorption was performed to investigate the selectivities of the ion-imprinted chitosan adsorbents prepared under the optimal conditions. The selectivity coefficient of Pd(II) versus Pt(IV) (β_Pd/Pt) of the Pd(II)-imprinted adsorbent was 115.83, much greater than that of the chitosan adsorbent without imprinting and various reported selective adsorbents. Therefore, the Box-Behnken design can be a useful method for optimizing the synthesis of ion-imprinted adsorbents with desirably high adsorptive selectivity for precious metals.

Correlating the Influence of Disulfides in Monolayers across Photoelectron Spectroscopy Wettability and Tunneling Charge-Transport

Despite their ubiquity, self-assembled monolayers (SAMs) of thiols on coinage metals are difficult to study and are still not completely understood, particularly with respect to the nature of thiol-metal bonding. Recent advances in molecular electronics have highlighted this deficiency due to the sensitivity of tunneling charge-transport to the subtle differences in the overall composition of SAMs and the chemistry of their attachment to surfaces. These advances have also challenged assumptions about the spontaneous formation of covalent thiol-metal bonds. This paper describes a series of experiments that correlate changes in the physical properties of SAMs to photoelectron spectroscopy to unambiguously assign binding energies of noncovalent interactions to physisorbed disulfides. These disulfides can be converted to covalent metal-thiolate bonds by exposure to free thiols, leading to the remarkable observation of the total loss and recovery of length-dependent tunneling charge-transport. The identification and assignment of physisorbed disulfides solve a long-standing mystery and reveal new, dynamic properties in SAMs of thiols.

pH and Surface Charge Switchability on Bifunctional Charge Gradients

Multifunctionalized pH-sensitive silica gradients containing acidic and basic functional groups have been prepared to evaluate how the spatial arrangement of active sites on a surface influences the surface charge and pH switchability. The gradient surfaces were prepared using controlled rate infusion in such a manner that the individual gradients in the strong acid (sulfonic acid) and in the weak base (propylamine) align, whereas a gradient in the weakly acidic silanol groups opposes them. The relative amounts of the three species were varied by controlling the composition of the deposition solution, whereas the hydrophobicity of the underlying surface was set by using base layer-coated substrates prepared from either tetramethoxysilane or tetramethoxysilane/octyltrimethoxysilane mixtures. Results from X-ray photoelectron spectroscopy confirm that aligned gradients are formed in both amine and sulfonic acid groups, and the relative amounts bound to the surface follow that expected from the solution composition. Water contact angle measurements show a 40°-50° change across the length of the gradient, the exact values being dependent on the hydrophobicity of the base layer. Zeta potential measurements on gradient mimics reveal that there is a pH where the net charge on the gradient surface is predicted to have a constant but nonzero value. Static contact angle measurements and modeling confirm this prediction. At a pH acidic of this value, the gradient in charge runs in one direction, whereas at a pH basic of this value, the gradient in charge runs in the other direction. This point can be strategically moved from acidic values to basic values by changing the relative amounts of acidic and basic functionalities on the surface. The origin of this unique pH switchability can be found in acid-base chemistry. By modeling the charge along the gradient surface using a simple equilibrium model, a distribution of pK_a values were noted in these materials.

Synthesis of a lignin-based phosphorus-containing flame retardant and its application in polyurethane

In this work, new lignin-based flame retardant LHDs were successfully synthesized through the reaction between lignin, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexamethylene diisocyanate (HDI). The chemical structure of LHD was characterized by FTIR, ¹H NMR, ³¹P NMR. The thermal stability of LHD was studied by TGA. The results showed that the residual carbon content of L15HD (15% of lignin in LHD) at 600 °C reached 16.55%, indicating that this prepared flame retardant can be a type of good char forming agent. LHDs were then applied to prepare flame-retardant lignin-based polyurethane (FLPU). Lignin-based polyurethane (LPU) was synthesized by the reaction between lignin, polyethylene glycol 200 (PEG 200) and hexamethylene diisocyanate (HDI). The limiting oxygen index (LOI) value of the FLPU reached 30.2% when the addition content of L15HD (15% lignin in LHD) in L₂₀PU (20% lignin in LPU) was 25%, exhibiting excellent flame-retardant properties. Scanning electron microscopy (SEM) analysis of the FLPU char residual showed that there was a continuous dense outer carbon layer on the residue surface, and the inner carbon layer had many expansion bubbles, indicating the LHDs have an excellent flame retardant effect for PU. In addition, FLPU presented better hardness and adhesion than PU. The hardness of F_L15-25L₂₀PU (lignin content in LPU was 20%, and added content of L15HD in LPU was 25%) reached 4H, and its adhesion was 0. These excellent properties illustrated that the LHDs are ideal flame retardants and reinforcing agents for LPU because of the co-curing and strong interface between LHD and LPU.

In this work, new lignin-based flame retardant LHDs were successfully synthesized through the reaction between lignin, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexamethylene diisocyanate (HDI).

Cooperative Effects in Aligned and Opposed Multicomponent Charge Gradients Containing Strongly Acidic, Weakly Acidic, and Basic Functional Groups

Bifunctionalized surface charge gradients in which the individual component gradients either align with or oppose each other have been prepared. The multicomponent gradients contain strongly acidic, weakly acidic, and basic functionalities that cooperatively interact to define surface wettability, nanoparticle binding, and surface charge. The two-step process for gradient formation begins by modifying a siloxane coated silicon wafer in a spatially dependent fashion first with an aminoalkoxysilane and then with a mercapto-functionalized alkoxysilane. Immersion in hydrogen peroxide leads to oxidation of the surface immobilized sulfhydryl groups and subsequent protonation of the surface immobilized amines. Very different surface chemistries were obtained from gradients that either align with or oppose each other. X-ray photoelectron spectroscopy (XPS) data show that the degree of amine group protonation depends on the local concentration of sulfonate groups, which form ion pairs with the resulting ammonium ions. Contact angle measurements show that these ion pairs greatly enhance the wettability of the gradient surface. Finally, studies of colloidal gold binding show that the presence of both amine and thiol moieties enhance colloid binding, which is also influenced by surface charge. Cooperativity is also revealed in the distribution of charges on uniform samples used as models of the gradient surfaces, as evaluated via zeta potential measurements. Most significantly, the net surface charge and how it changes with distance and solution pH strongly depend on whether the gradients in amine and thiol align or oppose each other. The aligned multicomponent gradients show the most interesting behavior in that there appears to be a point at pH ∼ 6.5 where surface charge remains constant with distance. Setting the pH above or below this transition point leads to changes in the direction of charge variation along the length of the substrate.

Silver nanoparticle based coatings enhance adipogenesis compared to osteogenesis in human mesenchymal stem cells through oxidative stress

Silver nanoparticle (AgNP) based antibacterial surfaces were fabricated using plasma polymerization technology and their effects on differentiation of human bone-marrow derived mesenchymal stem cells (hMSCs) were investigated in this study. The results showed that AgNP coated surfaces do not affect the initial adhesion, spreading and proliferation of hMSCs. Furthermore, the silver coated surface promoted adipogenic differentiation of hMSCs as demonstrated by more accumulation of lipid droplets and upregulation of adipogenesis-related genes such as peroxisome proliferator activated receptor gamma (PPARγ), adipocyte determination and differentiation factor (ADD1) and CCAAT/enhancer binding protein alpha (C/EBPα). In addition, silver incorporation activated the expression of antioxidant enzymes as a consequence of the accumulation of intracellular reactive oxygen species (ROS) in adipogenic induced cells, which was correlated with the enhanced adipogenic capacity of hMSCs. ROS generation was enhanced due to silver ion release and consequently reduced osteogenesis at the early stage after 7 days of osteogenic induction as a result of reducing alkaline phosphatase (ALP) activity. However, the differentiation and mineralization capacity of osteoblasts were restored after 14 days of osteogenic induction, which indicated that adipogenesis favors intracellular ROS accumulation mediated by silver coatings compared to osteogenesis. None of the osteogenic related genes was affected by ROS mediated by AgNP dissolution. The findings in this work are instructive for the use of silver as an antibacterial agent in the areas of tissue engineering, stem cell therapies and implantable biomedical devices.

UV/vis and NIR light-responsive spiropyran self-assembled monolayers

Self-assembled monolayers of a 6-nitro BIPS spiropyran (SP) modified with a disulfide-terminated aliphatic chain were prepared on polycrystalline gold surfaces and characterized by UV/vis absorption, surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopies (XPS). The SAMs obtained are composed of the ring-closed form (i.e., spiropyran) only. Irradiation with UV light results in conversion of the monolayer to the merocyanine form (MC), manifested in the appearance of an N(+) contribution in the N 1s region of the XPS spectrum of the SAMs, the characteristic absorption band of the MC form in the visible region at 555 nm, and the C-O stretching band in the SERS spectrum. Recovery of the initial state of the monolayer was observed both thermally and after irradiation with visible light. Several switching cycles were performed and monitored by SERS spectroscopy, demonstrating the stability of the SAMs during repeated switching between SP and MC states. A key finding in the present study is that ring-opening of the surface-immobilized spiropyrans can be induced by irradiation with continuous wave NIR (785 nm) light as well as by irradiation with UV light. We demonstrate that ring-opening by irradiation at 785 nm proceeds by a two-photon absorption pathway both in the SAMs and in the solid state. Hence, spiropyran SAMs on gold can undergo reversible photochemical switching from the SP to the MC form with both UV and NIR and the reverse reaction induced by irradiation with visible light or heating. Furthermore, the observation of NIR-induced switching with a continuous wave source holds important consequences in the study of photochromic switches on surfaces using SERS and emphasizes the importance of the use of multiple complementary techniques in characterizing photoresponsive SAMs.