Adsorption of chitosan onto carbonaceous surfaces and its application: atomic force microscopy study

Morphological Characteristics of Biopolymer Thin Films Swollen-Rich in Solvent Vapors

Biopolymers exhibit a large variety of attractive properties including biocompatibility, flexibility, gelation ability, and low cost. Therefore, especially in more recent years, they have become highly suitable for a wider and wider range of applications stretching across several key sectors such as those related to food packaging, pharmaceutic, and medical industries, just to name a few. Moreover, biopolymers' properties are known to be strongly dependent on the molecular arrangements adopted by such chains at the nanoscale and microscale. Fortunately, these arrangements can be altered and eventually optimized through a plethora of more or less efficient polymer processing methods. Here, we used a space-confined solvent vapor annealing (C-SVA) method to subject various biopolymers to rich swelling in solvent vapors in order to favor their further crystallization or self-assembly, with the final aim of obtaining thin biopolymer films exhibiting more ordered chain conformations. The results obtained by atomic force microscopy revealed that while the gelatin biopolymer nucleated and then crystallized into granular compact structures, other biopolymers preferred to self-assemble into (curved) lamellar rows composed of spherical nanoparticles (glycogen and chitosan) or into more complex helix-resembling morphologies (phytagel). The capability of the C-SVA processing method to favor crystallization and to induce self-assembly in various biopolymeric species or even monomeric units further emphasizes its great potential in the future structuring of a variety of biological (macro)molecules.

Unusual DNA structures formed on bare highly oriented pyrolytic graphite surfaces studied by atomic force microscopy

It is important to know the detailed DNA structure on carbonaceous surfaces for further application of DNA-functionalized carbonaceous materials in diverse research areas. In this study, the topographic and structural characteristics of the separated single DNA molecules and their assembly on highly oriented pyrolytic graphite (HOPG) surfaces have been investigated by atomic force microscopy (AFM). AFM results indicate that both circular and linear DNA molecules tend to form hexagonal patterns along with some unusual structures that include node, protrusion, cruciform, parallel single-stranded DNA (ssDNA), and compact zigzag. Furthermore, parallel ssDNA patterns and their crossed structures have been obtained under high-temperature conditions. Our AFM results reveal that a bare HOPG surface can induce DNA molecules to form various unusual structures. This finding is helpful for understanding the adsorption behavior of DNA on other carbonaceous surfaces such as carbon nanotubes and graphene. In addition, the hexagonal DNA patterns in this study are similar to those formed on the alkylamine-modified HOPG surface, which implies that a bare HOPG, without any chemical modification, has a strong ability to align biomolecules. This study could expand our knowledge of the diversities of DNA structures and the aligning ability of carbonaceous surfaces.

Assembly of single-stranded DNA onto HOPG surface at different temperature: atomic force microscopy study

Assembly of long single-stranded DNA (ssDNA) and short oligodeoxynucleotides onto bare highly oriented pyrolytic graphite (HOPG) at different temperature has been studied. It was indicated that both long ssDNA and oligodeoxynucleotides can sequentially form network, straight chains, and layer structures when the adsorption temperature was changed from room temperature, 37-55°C. High-resolution atomic force microscopy (AFM) imaging of the layer structures revealed that they are composed of parallel ssDNA chains with relatively higher height and tend to form patterns with three-fold symmetry. These new findings are significantly important for understanding assembly characterization of ssDNA. In addition, this assembly method for ssDNA is expected to be used for preparation of DNA structures in biosensing and DNA-based nanodevices.

Influence of Mg²⁺, Ni²⁺, and Cu²⁺ on DNA assembly on HOPG surfaces: atomic force microscopy study

Adsorption of circular DNA onto bare highly oriented pyrolytic graphite (HOPG) surfaces by the addition of Mg²⁺, Ni²⁺, and Cu²⁺ has been investigated by atomic force microscopy (AFM). AFM results revealed that the topography and height of DNA on HOPG surface by the addition of different metal ions are quite different. After the addition of Mg²⁺ for incubation, DNA molecules tend to form many loops on HOPG surfaces, which are derived from the crossover of intramolecular and intermolecular chains. After the addition of Ni²⁺, DNA molecules can form network on HOPG surfaces, and the density of DNA network was significantly increased with increasing DNA concentration. Consequently, dense DNA network can be obtained by using relatively low concentration of DNA and Ni²⁺. As for the addition of Cu²⁺, angular DNA loops composed of flat chains were observed. The observed flat DNA chains with an average height of 0.52 nm can be ascribed to Cu²⁺ insert into the site between bases and phosphate group of DNA inducing denaturation of DNA molecules. This study is very helpful for understanding the interactions of metal ions and DNA molecules, and for constructing various DNA structures on the carbonaceous surfaces.