Building novel materials by designing and engineering the basic nanosized building blocks like molecules or supramolecular assemblies is the foundational goal of modern nanotechnology. Nanomaterials may have a huge number of applications such as in medicine, consumer products and electronics. Very promising candidates are provided by chiral self-assembly-driven liquid crystals, where chiral particles reversibly self-assemble into supramolecular structures before forming liquid crystal phases. Self-assembly-driven liquid crystals can be obtained from water suspensions of short/long DNA duplexes, chromonics, G-quadruplexes, amyloid fibrils, nanocellulose and ad-hoc DNA origami. Liquid crystal materials are in a thermodynamic state in between the disordered liquid phase and the fully ordered crystal phase. If constituent particles are chiral they may form cholesteric liquid crystals, which have spatial correlations which extend to dozens of nanometers. In these systems, on which we will focus within this project, all-atom computer simulations are highly demanding, not to say impracticable, because the number of simulated atoms is very large. To tackle this problem we plan to improve an existing theoretical approach, which at present neglect electrostatic interactions and account for particle flexibility too simplistically, to calculate thermodynamic, elastic and chiral properties and we will resort on highly coarse-grained modelling for carrying out computer simulations. Main goal of this proposal is to provide a quantitative description of macroscopic elastic and thermodynamic properties of chiral self-assembly-driven liquid-crystal systems through a suitable microscopic theoretical treatment which will fill the huge gap between microscopic and macroscopic levels in these systems. A stringent validation of the theory will be provided by comparing theoretical predictions and experimental results for suspensions of short DNA duplexes.
The microscopic structure of the constituent particles determines the handedness and pith of cholesteric helix. However, the relationship between the chiral microscopic conformation of constituent particles and the mesoscopic helical ordering is far from being fully understood. In particular, if electrostatic interactions play a role and chain-like aggregates are rather flexible a general theoretical treatment is still lacking. Development of a general theory is rather challenging because a variety of behaviors have been reported in literature. Use of computer simulations to establish a connection between microscopic and macroscopic chirality is also rather challenging as formerly discussed.
Therefore, a general theoretical framework, which fills the gap between computer simulations and experiments, would be a significant step forward in understanding the physical mechanisms behind the formation of chiral self-assembly-driven liquid crystals. Theories developed for simple models have highlighted general features of the chirality propagation in lyotropic systems, like the non-unique relationship between particle and phase helicity and the significant role played by self-assembly. In the specific case of short DNA duplexes for sequences which exhibit an isotropic-cholesteric transition at high concentration, where electrostatic interactions can be neglected and chain-like aggregates are rather stiff, a direct comparison between theoretical and experimental data has been already done. Nevertheless, in the general case of chiral particles interacting via non-negligible electrostatic interactions which self-assemble into very flexible chain a theoretical framework is still lacking.
Within this project we aim to develop a general theoretical treatment for chiral self-assembly-driven liquid crystals which will provide an estimate for isotropic-cholesteric transition phase boundaries as well as thermodynamic, elastic and chiral properties of self-assembling mesogenic polyelectrolytes, such as amyloid fibrils [Mezzenga10], G-quadruplexes [Mariani09], short and long DNA duplexes, chromonics [Lydon11] and cellulose nanocrystals [Lagerwaal14]. This general theoretical framework will make it possible, for the first time, to have a quantitative thermodynamic description (including elastic and chiral properties) of cholesteric phases of these systems, which will take explicitly into account electrostatic interactions and self-assembly.
By clarifying aspects of helix formation in chiral self-assembly-driven liquid crystals which have not been completely understood yet, we will expand the ability to tune the cholesteric pitch of these cholesteric systems. Without a full control of liquid-crystalline self-assembly and of the resulting helical ordering it will be impossible indeed to exploit self-assembly-driven liquid crystals for applications. Biological liquid crystals based on DNA, for example, could ease the fabrication of novel biocompatible nanomaterials which can be exploited for their birefringence, such as in liquid crystal displays. Even more promising is the case of cellulose nanocrystals (CNCs) which exhibit a cholesteric phase with a pitch which can be as short as few hundred nanometers. A pitch in this range makes CNCs suitable for applications as paper-like photonic materials, self-assembled template for the synthesis of inorganic multifunctional mesoporous materials with photonic crystal properties, mirrorless lasers, enantioselective sensors or mimetics high-performance and low production cost biomaterials [Shopsowitz10]. In particular, the use of CNC suspensions as templates to produce inorganic films such as metal organosilica, helical silica, nitride/carbon composite, TiO2 and hydrogels [Lagerwall14] which exhibit an internal left-handed helical structure, has recently received much attention since the seminal works of MacLachlan and co-workers, who produced CNCs templated films of silica [Kelly12]. These photonic mesoporous inorganic films that are a cast of cholesteric CNCs could lead to the development of new materials for applications in tuneable reflective filters and sensors. Hence, for these photonic applications it is again crucial to control the pitch by tuning the parameters on which it depends, such as concentration, temperature, ionic-strength, pH, CNC conformation and CNC surface negative charge. Indeed, without understanding and controlling CNCs self-assembly and resulting helical ordering the potential for fabricating multifunctional mesoporous materials would be tremendously weakened.
REFERENCES
[Kelly12] J. A. Kelly et al. Langmuir 28, 17256 (2012).
[Lagerwaal14] J. P.F. Lagerwall et al. NPG Asia Materials 6, e80 (2014).
[Lydon11] J. Lydon, Liq. Cryst. 38, 1663 (2011).
[Mariani09] P. Mariani et al., J. Phys. Chem. B 113, 7934 (2009).
[Mezzenga10] R. Mezzenga et al., Langmuir 26, 10401 (2010).
[Shopsowitz10] K. E. Shopsowitz et al. Nature 468, 422 (2010).