Ionic Liquids Are Involved In The Construction Of Liquid-liquid Crystal Interface Sensors

Liquid crystals(LCs)sensor based on liquid-LC interface could achieve the goal of detecting targets,which is attributed to the interactions between surfactants and targets to induce the alteration of LCs orientation.This device has received extensive attention due to lots of superiorites,such as easier operation,detection biological or chemical events whenever and wherever possible,easier constructing arrayed and miniaturized sensors,etc.By now the related researches about this device are always restricted to aqueous medium and traditional surfactants modifiers,which is difficult to meet the increasingly higher requirements for the detection of targets and biochemical events.Therefore,it is urgently necessary to find out more suitable non-aqueous solvents and develop more newly structural surfactants to build new-type sensors at the liquid-LC interface.In view of this,this paper aims to utilize green solvent as fluid(viz.ionic liquids,ILs),and surfactants with novel structures as modifiers(viz.stimulus responsive surfactants,surface active ionic liquids(SAILs),Gemini surfactants,ect.)to add into the liquid-LC interface.Then new-style LCs interface sensors will be built through observation of the optical appearances of LCs.There are four main parts in this dissertation as follows.Chapter 1 is a brief introduction of basic background knowledge and recent achievements closely relevant to the LCs interface sensors,surfactants as well as ILs,and the research ideas for this dissertation.In Chapter 2,we synthesized a photoswitchable azobenzene-based ionic surfactant(4-ethylazobenzene-4'-(oxy-hexyl)trimethyl ammonium bromide,azoTAB)to used as modifier at the liquid-LC interface and employed one IL(ethylammonium nitrate,EAN)as medium.A persistent and reversible photoresponsive alignment of LCs at fluid interfaces was achieved.In this Chapter,polarizing microscope and UV/Vis near-infrared spectrophotometer were used as mainly technical methods.When azoTAB with high concentration at the fluid interface underwent UV and visible light irradiations in turn,the alternating dark-to-bright shift of optical responses was observed,indicating the orientation of LCs alternatively changes from homeotropic to planar alignment.It is caused by photoisomerization of the azobenzene group in azoTAB,which changes the structure of the surfactant from trans to cis configuration.In compared with aqueous-LC interface,the LC films were more stable in the presence of EAN during a month ofobservation under the condition of unchanging solvent.The photoresponsive ordering transitions of LCs were reversible for many times.Therefore,the IL-LC interface has better controllability,stability and reproducibility.Moreover,when a small amount of EAN was added into aqueous-LC interface as electrolyte,the threshold concentration under which the response of LCs changed from bright to dark decreased due to screening of electrostatic interactions.It is similar with the results of adding the common electrolytes(such as NaCl).However,upon addition of a lot of EAN into aqueous-LC interface,the threshold concentration increased due to the change of solvent properties.In Chapter 3,effects of a series of imidazolium-based surface active ionic liquids(IM-SAILs)on the alignments of LCs at the aqueous-LC and IL-LC interfaces,respectively,were investigated.We synthesized three short-chained ILs,namely ethylammonium nitrate(EAN),propylammonium nitrate(PAN)and butylammonium nitrate(BAN),and eight IM-SAILs,viz.single-chained IM-SAILs,1-alkyl-3-methylimidazolium bromide([Cnmim]Br,n=12,14,16),1-dodecyl-3-methylimidazolium salicylate([C12mim]Sal),1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate([C12mim]HNC),1-dodecyl-3-methylimidazolium cinnamate([C12mim]CA),1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate([C12mim]PCA);gemini IM-SAIL,1,2-bis(3-dodecylimidazolium-1-yl)ethane bromide([C12-2-C12im]Br2).Polarizing microscope was used to evaluated the influence of the structures of IM-SAILs(e.g.length and number of aliphatic chain,as well as the counterion)on the cross-polarized optical images of LCs at the aqueous-LC interface.It was observed that the threshold concentration reduced with the increase of aliphatic chain length no matter at the aqueous-LC or IL-LC interface.Double-chained[C12-2-C12im]Br2 has a far lower threshold concentration than the single-chained[C12mim]Br.But the alteration of counterions(e.g.Br-and aromatic counterions)scarcely affected the anchoring of LCs at the interface.In addition,compared with aqueous-LC interface,higher threshold concentration and more stable LC films in IL(EAN,PAN or BAN)-LC interface.This can be ascribed to the different internal structures of solvents.In Chapter 4,we investigated two-kinds of cationic Gemini surfactants with different structures decorated at the aqueous-LC interface to induce the change of LCs orientation.We synthesized quaternary ammonium salt-type Gemini surfactants(Gemini QASaa),1,2-dialkyl quaternary ammonium ethane bromide(m-2-n,m=12,14,16;n=8,10,12;m+n=24)and Gemini IM-SAILs,1,2-bis(3-alkylimidazolium-1-yl)ethane bromide([Cn-s-Cnim]Br2,s=2,n=6,8,10,12,16),1,s-bis(3-dodecylimidazolium-1-yl)alkylate bromide([Cn-s-Cnim]Br2,n=12,s=2,4,6,10).It was observed that the threshold concentration reduced with the increase of two aliphatic chains length for Gemini IM-SAILs at the aqueous-LC interface.However,the threshold concentration increased with the increment length of spacer for Gemini IM-SAILs at the aqueous-LC interface.The threshold concentration of three Gemini QASaa with different symmetry and the same total aliphatic chain length was extremely close.Furthermore,the threshold concentration of[C12-2-C12im]Br2 with imidazolium headgroup is smaller than that of 12-2-12 with quaternary ammonium cation.In summary,for Gemini surfactants,the change of two aliphatic chain length,spacer length and the type of headgroup all can convert the orientation of LCs.The research work mentioned above demonstrates that ILs and novel surfactants show good application prospects in construction of liquid-LC interface-type sensors.This article gives a guideline for extending the construction methods and improving their detection performances.