Chemical Communications Blog

Written by Tianyu Liu, University of California, Santa Cruz

Chitosan is a polysaccharide derived from chitin, a second most abundant bio-polymer on earth that consists of the shells of crustaceans (such as shrimps and crabs). It is a commonly used bio-compatible material for bio-medical applications such as drug delivery.

Previous studies have shown that chitosan is pH sensitive in water: it forms a viscous solution at low pH values and becomes insoluble at high pH values. Preliminary investigations suggest that such pH responsiveness is associated with the protonation and de-protonation processes of the amine groups on the chitosan polymer chain (the red box in Figure a). However, there still lacks fundamental understanding on how the pH affects the behavior of chitosan.

Now writing in Chem. Commun., Xu and Matysiak from University of Maryland, USA provided us new insights on the chitosan’s pH responsiveness at the molecular level. They adopted a method called “coarse-grained molecular simulation” to illustrate the self-assembly behaviors of chitosan polymer chains at different pH values. Unlike atomistic molecular simulations that focus on individual atoms of a molecule, the coarse-grained molecular simulation treats a group of atoms as one ensemble and probes the collective behavior of each ensemble (Figure a and b). This simulation technique demands less time than the atomistic counterparts without significantly reducing the simulation accuracy. It is suitable for characterizing polymers composed of thousands of atoms, such as chitosan.

The key discovery of this work is that the chitosan polymer chains can adopt different configurations at different pH values. At high pH values, each chain tends to crosslink perpendicularly with adjacent chains. The crosslinking reaction propagates and eventually builds up a three-dimensional dense chain network (Figure c). At low pH values, the protonated amine groups favor parallel crosslinking. Thus, each chain aligns in parallel with each other, which leads to a loosely-packed structure (Figure d). The perpendicularly cross-linked configuration reduces the solubility of chitosan in water but renders robustness and elasticity of the chitosan networks. The parallel cross-linked morphology increases water solubility but decreases the elasticity of the chitosan assembly. These conclusions obtained by the simulation are consistent with experimental results.

To find out more please read:
Effect of pH on Chitosan Hydrogel Polymer Network Structure
Hongcheng Xu and Silvina Matysiak
DOI: 10.1039/C7CC01826F

About the author:
Tianyu Liu is a Ph.D. in chemistry from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/

Written by Tianyu Liu, University of California, Santa Cruz

Supramolecular smart materials are a family of materials composed of several molecules. They have the ability to change their configurations in response to external stimuli such as the presence of enzymes, light irradiation, and changes in pH. This property can be manipulated for a variety of applications including drug delivery and tissue engineering.

In recent years, pH-responsive supramolecular smart materials have been intensively investigated due to the simplicity of pH alteration. However, adjusting pH can have undesired consequences. First, chemical species other than the supramolecular materials (e.g., acid and base) are needed for tuning pH. The involvement of external reagents hinders the readiness of operation. Additionally, the use of acid and base inevitably introduces waste products, which could eventually suppress the stimulus-response activity of the smart materials. Therefore, developing alternative ways to initiate the configuration modification of the supramolecular smart materials is highly desirable.

In a recent ChemComm. publication, Professor Heng-Yi Zhang, Professor Yu Liu and coworkers from Nankai University, China have developed supramolecular smart nanorods consisting of β-cyclodextrin (β-CD) and 4,4’-bipyridine-coordinated zinc ions. In the presence of protonated merocyanine (MEH) in water, the nanorods are able to dissociate upon visible light illumination and reconstruct themselves when placed in the dark (Figure above).

The method by which these structures can reconfigure involves a light-driven proton transfer process (Figure below). MEH molecules absorb energy from visible light and subsequently release their protons to the surroundings. These free protons then combine with the 4,4’-bipyridine (DPD). The protonated DPD molecules lose their coordination ability and disassemble with zinc ions. As a result, the entire nanorod structure collapses. When no light is present, the aforementioned proton transfer process is reversed and the nanorods are reformed. Such a process is highly reversible with no observable light-responsive activity loss for at least five cycles.

The demonstrated light-responsive supramolecular nanorods enable facile operations with no additional chemicals. This technology opens up endless new opportunities in remote control of light-responsive processes.

To find out more please see:

Light-controlled reversible self-assembly of nanorod suprastructures

Jie Guo, Heng-Yi Zhang, Yan Zhou and Yu Liu

DOI:10.1039/C7CC03280C