Detailed Atomic Force Microscopy Study Research Articles

Atomic force microscopy investigation of smart superabsorbent hydrogels based on carboxymethyl guar gum: Surface topography and swelling properties

Atomic force Microscopy (AFM) is commonly used to study various mechanical, functional, and electrical properties of materials at the nanoscale, as well as for characterizing the topography and surface properties. Up to our knowledge, this is the first investigation to study the surface properties of some carboxymethyl guar gum-based smart superabsorbent hydrogels before and after swelling. In this regard, three sets of modified carboxymethyl guar gum (CMGG) hydrogels were prepared via reaction with different weight ratios of acrylate monomers. The chemical composition of these hydrogels was assured via the IR spectroscopy and the morphology of the dry and swelled gels was studied via the ESEM. The swelling data were correlated with the chemical composition of the prepared hydrogels together with a detailed AFM study of their maps. The chemical modifications were also confirmed by a recent model of AFM. The data reveal that the highest swelling was achieved by C2 in which the weight ratio of acrylate monomers is twice that of guar gum (Qmax = 676 g g after 150 min) It was also found that the swelling capacity of all the prepared hydrogels decreases slightly from the first to the third swelling cycle. For instance, the swelling performance for C2 is as following: 767, 750 and 734 g g for the first, second and third cycles respectively. The data extracted from the AFM showed that surface roughness increases from 15.62 nm to 22.47 and the height rises from 11.2 nm to 43.9 nm as a result of chemical modification for guar gum into carboxymethyl guar gum.

Inferring Aggregation Mechanisms of Polyglutamine Through Quantitative Studies of Phase Behavior

Many proteins associated with neurodegenerative diseases are intrinsically disordered, i.e. they lack a stable, folded structure under physiological conditions. It is currently believed that the aggregation of these proteins plays a causative role in neurodegenerative disease pathogenesis. Therefore, understanding how and why these proteins aggregate is crucial to understanding and possibly suppressing disease.Proteins are polymers and protein aggregation is akin to phase separation. We employ techniques and theories borrowed from polymer physics to help understand the driving forces and mechanisms of protein aggregation. Our system of interest is polyglutamine, as expansions of polyglutamine are thought to be causally linked to the development of at least nine different neurodegenerative diseases. In this work, we measure the saturation concentrations of aqueous polyglutamine solutions containing 30 and 40 glutamines and either 2 or 4 lysines. We use classical polymer physics theory to construct the entire (soluble-insoluble) phase diagram from the measured saturation concentrations. The low-concentration arm of the phase diagram provides a thermodynamic basis for assessing aggregation propensity. For a given chain length of polyglutamine, we find that aggregation propensity increases with fewer lysines, and, for a fixed number of lysines, the aggregation propensity increases with increasing chain length. Although the phase diagrams are thermodynamic in nature, they still provide insights regarding the kinetic mechanisms of phase separation. For the concentrations used in most in vitro experiments, the phase diagrams predict that intrinsically disordered monomers first form disordered, higher-order oligomers or clusters which then undergo a nucleated structural conversion into an ordered, insoluble form. These predictions are supported by detailed atomic force microscopy studies. This work highlights the prominence of intrinsic disorder even in multimolecular complexes and its role in facilitating conformational conversion to ordered supramolecular aggregates.

3D Coordination Framework with Uncommon Two‐Fold Interpenetrated {33⋅59⋅63}‐lcy Net and Coordinated Anion Exchange

Unusual {3(3)5(9)6(3)}-lcy topology has been found in an uncommon 3D six-connected, two-fold interpenetrated {3(3)5(9)6(3)}-lcy net (illustrated). The coordinated SO(4) (2-) anions in this framework can undergo a full exchange with Cl(-) anions, in the course of which the crystals change color as shown. The process has a solvent-mediated rather than a solid-state mechanism.Reaction of tetrazole-1-acetic acid with CuSO(4)5H(2)O produces two novel 3D coordination frameworks: ([Cu(3)(mu(3)-OH)(eta(1):eta(1):eta(1):mu(3)-SO(4))(tza)(3)]3 CH(3)OHH(2)O}(n) (1; Htza=tetrazole-1-acetic acid) and {[Cu(4)(mu(2)-OH)(eta(2):eta(2):mu(4)-SO(4))(tta)(5)]3 H(2)O}(n) (2; Htta=tetrazole). Framework 1 is constructed from a trinuclear copper cluster [Cu(3)(mu(3)-OH)(eta(1):eta(1):eta(1):mu(3)-SO(4))] and displays a six-connected framework with uncommon {3(3)5(9)6(3)}-lcy topology. Framework 2 presents a complicated five-nodal (3,4,6)-connected net with (4(2)6)(34(2)5(2)7)(4(3)6(2)8)(3(2)4(8)5(3)6(2))(4(6)8(6)10(3)) topology. The eta(2):eta(2):mu(4)-SO(4) bridging mode in 2 has not been found in the reported coordination polymers. The coordinated SO(4) (2-) anions in 1 can be replaced completely when the solid polymer is treated with an aqueous methanolic solution containing Cl(-) anions. Detailed atomic force microscopy studies indicate a solvent-mediated rather than a solid-state mechanism for the exchange process.

The origin and reduction of dislocations in Gallium Nitride

Two methods for GaN growth on sapphire by metal-organic vapour phase epitaxy are discussed. The first involves only two-dimensional (2D) growth, and results in a high dislocation density, but also a high electrical resistivity. The second involves initial growth of three-dimensional (3D) islands employing a low V:III ratio, followed by island coalescence at a high V:III ratio. It is often assumed that threading dislocations (TDs) form via the coalescence of 3D islands, but detailed atomic force microscopy studies on partially coalesced samples find no evidence of an increased TD density at coalescence boundaries, suggesting that other possible origins for TDs should be considered. The 3D–2D growth method allows TD densities as low as 1.1 × 108 cm−2 to be achieved, but unlike the 2D growth samples these layers are not highly resistive. Scanning capacitance microscopy is used to demonstrate the presence of an unintentionally doped layer close to the GaN/sapphire interface. To simultaneously achieve a reduced TD density compared to 2D growth samples and a high resistivity, a high temperature AlN buffer layer may be employed.

Controlling Relative Fundamental Crystal Growth Rates in Silicalite: AFM Observation

A detailed atomic force microscopy study has been performed on the open-framework, microporous material silicalite. Emphasis has been placed on determining the effect of supersaturation on the crystal growth process. The relative rates of fundamental crystal growth processes can be substantially altered by tuning the supersaturation. In this manner, it is possible to, for instance, switch on and off surface nucleation while retaining terrace spreading. This offers a potential mechanism by which it might be possible to control important crystal aspects such as defect density and intergrowths.

Inside Front Cover: Oxidation Conditions for Octadecyl Trichlorosilane Monolayers on Silicon: A Detailed Atomic Force Microscopy Study of the Effects of Pulse Height and Duration on the Oxidation of the Monolayer and the Underlying Si Substrate (Adv. Funct. Mater. 6/2005)

Schubert and co-workers have performed a detailed atomic force microscopy study to establish and characterize the oxidation conditions of self-assembled monolayers of octadecyl trichlorosilane on silicon substrates. The cover image illustrates different examples of surfaces that were structured with different patterning conditions, as reported on p. 938. The graph in the background depicts three observed oxidation regimes depending on applied voltage and oxidation time. In current scanning-probe nanolithography research, substrates consisting of octadecyl trichlorosilane monolayers on silicon are often used. On one hand, the presence of an organic monolayer can be used as a passive resist, influencing the formation of silicon dioxide on the substrate, whereas in other cases the monolayer itself is patterned, creating local chemical functionality. In this study we investigate the time scales involved in either process. By looking at friction and height images of lines oxidized at different bias voltages and different pulse durations, we have determined the parameter space in which the formation of silicon dioxide is dominant as well as the region in which the oxidation of the monolayer itself is dominant.

Oxidation Conditions for Octadecyl Trichlorosilane Monolayers on Silicon: A Detailed Atomic Force Microscopy Study of the Effects of Pulse Height and Duration on the Oxidation of the Monolayer and the Underlying Si Substrate

In current scanning-probe nanolithography research, substrates consisting of octadecyl trichlorosilane monolayers on silicon are often used. On one hand, the presence of an organic monolayer can be used as a passive resist, influencing the formation of silicon dioxide on the substrate, whereas in other cases the monolayer itself is patterned, creating local chemical functionality. In this study we investigate the time scales involved in either process. By looking at friction and height images of lines oxidized at different bias voltages and different pulse durations, we have determined the parameter space in which the formation of silicon dioxide is dominant as well as the region in which the oxidation of the monolayer itself is dominant.

A Comparative Langmuir−Blodgett Study on a Set of Covalently Linked Porphyrin-Based Amphiphiles: A Detailed Atomic Force Microscopic Study

A set of covalently linked phenyl-amidophenyl-substituted porphyrin amphiphiles with n-C15H31 tails have been synthesized and completely characterized. These amphiphiles form good Langmuir-Blodgett (LB) films at the air/water interface. Mean molecular areas for the series were measured from the isotherms and found to increase as the number of aliphatic chains increased from one to four. No influence of the subphase pH was observed on the isotherms. LB films can be transferred successfully onto different solid surfaces. The LB films were characterized using tapping mode atomic force microscopy (AFM). Bis-, tris-, and tetra-substituted porphyrins were found to be fairly good film-forming amphiphiles, whereas irregular aggregates were seen in the case of the monosubstituted porphyrin amphiphile. Multilayers were also formed with tetra-substituted amphiphiles on mica. Detailed AFM studies of tetra-substituted amphiphiles have been carried out to investigate the effect of preparation procedure and solid substrates on film formation and transfer. The absorption and fluorescence spectra for the amphiphiles in solution and LB films deposited onto mica and glass were recorded, which demonstrated the successful transfer of LB films onto the substrates and provided more information about the arrangement of porphyrin molecules within the LB films. For comparison, self-assembled monolayers (SAMs) and the cast thin films of the amphiphiles were prepared and characterized.