Preparation And Properties Of DNA Thermotropic Liquid Crystal Materials

Biological macromolecules are combined with positively charged surfactants through electrostatic force,and then subjected to freeze-drying and dehydration treatment,a series of biological macromolecule thermotropic liquid crystal(TLCs)composite materials can be obtained,the kind of TLCs is amphiphilic.DNA can be combined with surfactants to produce DNA TLCs.The order of the liquid crystal(LC)is determined by the three-dimensional structure of the molecules,the rigid and flexible parts,the weak intermolecular forces.So they can respond to external stimulus.At present,the transformation behavior of DNA-surfactant TLCs is regulated mainly by changing the length of the surfactant alkyl chain.However,light is one of the best physical and chemical stimulus,and there are few studies on the regulation of material properties through light stimulation.Therefore,the preparation of photoresponsive TLCs using photoisomerization and photocontrollable orientation has been the focus of research.In this paper,we design and synthesize a variety of DNA thermotropic liquid crystal materials,and realize the control of the phase state of the materials through light stimulation and different temperatures.The main research contents are as follows:(1)In order to investigate the influence of the counterions of ammonium surfactant on the phase transition properties of DNA-surfactant materials.The ammonium surfactant DDA⁺X^-(X=counterions)with different counterions(halide,BF₄^-,metal halide,Ts O^-)was synthesized by using didodecyldimethylammonium bromide(DDAB)for the preparation of DNA-DDA(X)materials.Due to the electrostatic interaction between the ammonium and counterions,the counterions were inevitably introduced during the preparation of DNA TLC.The LC state provides greater freedom of movement for counterions.The movement of counterions in the surfactant layer will destroy the interaction between surfactant molecules,leading to the dissociation of the ordered structure.At the same time,the bulky inorganic anion has a low Coulombic force against the cationic ammonium,which reduces the stability of the LC phase.However,the introduction of the planar counterion Ts O^-leads to the strong?-?interaction in the surfactant molecular layer,which increases the phase stability.Therefore,the powerful regulation of counterions results from their different interactions with the ammonium surfactant layer.(2)In order to obtain photoresponsive anhydrous DNA materials through the isomerization of trans?cis azobenzene,we synthesized two different azobenzene-type ammonium surfactants,which have huge"head"groups and long alkyl chain,used to prepare DNA materials.However,the high packing density of azobenzene surfactants will lead to a low efficiency of UV penetration into the material,so the introduction of a certain amount of DDAB will destroy the high packing density surfactants,making the material prone to the transition of phase state.With the assistance of the azo-isomerization,the two DNA TLCs achieved reversible photoresponsive on their Cr?IL phase transition.Besides the loose packing density among surfactants,low melting point and an immediately following LC?IL transition is critical for gaining the objective.The presence of the benzyl group also makes the related DNA TLC gain fast self-assembly property from IL state after ceasing UV light.POM analysis and self-healing tests also confirmed that both DNA materials were excellent self-healing biomaterials.(3)CTAB-Au was synthesized based on the seed growth method and used to prepare two DNA-surfactant complexes containing Au NPs,Au NPs/DNA-DDAB(1:5)and Au NPs/DNA-DDAB(1:2).In order to study the light response characteristics of the two DNA materials,the visible light was irradiated at 520 nm and POM analysis was performed,during which obvious boundary motion could be observed.By adjusting the charge ratio of DNA:DDAB and the amount of Au NPs in sample preparation,the physical properties of the obtained DNA materials(including the phase state at room temperature,clearing point and photocurrent properties)were characterized.In the controlled experiments,the visible light-induced mechanical property changes at room temperature only worked on the DNA-surfactant TLCs prepared by Au NPs,but not on the prepared DNA-DDAB TLC and DNA-Au NPs complexes.(4)Based on the above-mentioned CTAB-stabilized Au NPs(CTAB-Au)synthesized by seed growth method,we further improved the seed growth method synthesis strategy and prepared a dialkyl chain quaternary ammonium salt surfactant stabilized Au NPs(DDAB-Au).In the presence of CTAB-Au and DDAB-Au,uranine is reduced with Na BH₄at room temperature.Regardless of incubation or unincubation conditions,the presence of uranine on the active site of Au NPs makes the adsorption of BH₄^-possible,and then metal hydrides begin to form and accumulate,thereby reducing uranine.The catalytic process is monitored by changes in fluorescence intensity.Compared with CTAB containing only a single alkyl chain,DDAB is more flexible,less polar,more sterically hindered,and has a lower charge-to-molecular weight ratio,all of which lead to a weaker binding affinity to the active site of Au NPs,However,the binding affinity between CTAB and the active sites of Au NPs was strong,which resulted in a longer induction time of CTAB-Au catalytic reaction.It should be noted that high concentrations of Na BH₄ will greatly increase the chance of BH₄^-adsorbing to the active site,thus accelerating the reduction of sodium fluorescein by rapidly supplying H^-.This property favors the adsorption of BH₄^-and generation of metal hydride on the active sites,and thus DDAB-Au catalyzed reduction of uranine could give shorter induction time,compared to that of CTAB-Au catalyzed reaction.