Detection Of Environmental Pollutants Based On Visual Liquid Crystal Sensing Platform/Cholesteric Liquid Crystal Film

Environmental pollutants cause direct and indirect harm to the ecosystem and human society,which can subsequently affect the quality of life,both physical and mental health of human beings.Serious environmental pollutants even trigger major social problems.At present,conventional methods for detecting environmental pollutants(e.g.electrochemical method,fluorescence method and chromatography,etc.)still have problems such as expensive instruments,complex operations and inconvenient to carry.Therefore,it is of great significance to develop a simple,portable,low-cost and visual detection strategy for environmental pollutants.Liquid crystals(LCs)with long-range order,excellent birefringence and surface sensitivity have been applied to the filed of analysis and detection,which provides a powerful material support for exploiting convenient and visualized new detection strategy for environmental pollutants.In this dissertation,biological or chemical signals were transformed into optical signals visible to the naked eye by using Nematic liquid crystal(NLC),namely 4-cyano-4'-pentylbiphenyl(5CB)and cholesteric liquid crystal(CLC)as signal recognition elements to achieve visual detection of environmental pollutants.There are four main parts in this dissertation as follows:Chapter 1 is an introduction of the relevant background knowledge of classification and hazards of environmental pollutants,NLCs sensing mechanism,construction of LCs platforms and CLC photonic crystal sensors,as well as the recent research status at home and abroad for NLCs sensing platform and CLC photonic crystal sensors in the fields of analysis and test.On this basis,the research ideas for this dissertation are put forward.In chapter 2,we construct a high sensitivity and specificity NLCs sensing platform for the detection of Pb2+ by the specific recognition effect of Pb2+ on DNAzyme and in-situ rolling circle amplification(RCA)on magnetic beads(MBs).Firstly,the MBs coated with streptomycin avidin were connected to the biotinylated DNAzyme.In the presence of the target Pb2+,the in-situ RCA reaction on the surface of the MBs was initiated to obtain a long single stranded DNA(ssDNA).The elongated ssDNA on the MBs disrupted the arrangement of self-assembled monolayers of octadecyl trimethyl ammonium bromide(OTAB),resulting in planar orientation of NLCs at the aqueous-LC interface.Thus,NLCs exhibited bright optical appearance.In contrast,in the absence of Pb2+,RCA on the MBs did not occur,in this case NLCs adopted perpendicular orientation and thus showed dark optical image.Therefore,variation in the bright and dark optical morphology of the NLCs can be employed as signals to detect Pb2+ in water.The detection limit of Pb2+ reaches as low as 16.7 pM.This method also shows excellent performance in detecting Pb2+ in tap water and lake water.Since this strategy avoids the interference of impurities in the actual water samples to the NLCs sensing platform,it also opens up a new path for the detection of other heavy metal ions in sewage,which has important implications for the monitoring and treatment of environmental water pollution.In chapter 3,we demonstrate a simple strategy for rapid and highly sensitive detection of dimethyl methyl phosphate(DMMP)gas by Cu(ClO4)2-doped 5CB droplets in the capillary.Originally,the NLCs droplets doped with Cu(ClO4)2 show presented a dark optical morphology.Then the optical response of the NLCs droplets changed from dark to bright appearance after adding DMMP to the capillary,due to the stronger interaction between DMMP and Cu(ClO4)2,which interfered with the anchoring of the LC molecules.Therefore,detection of DMMP gas can be realized by observing the dark-to-bright transformation in the optical morphology of NLCs droplets,and the detection limit reaches 2.0 ppb.In addition,the NLCs droplet sensing platform maintained good performance for the detection of DMMP gas even after 12 times of reuse or storage for two weeks at room temperature,indicating that it has good reusability and stability.In this chapter,a miniaturized,portable and reusable NLCs droplet microarray with high sensitivity was constructed on the capillary substrate,which successfully realized the visual detection of DMMP gas.This strategy has great application potential in the detection of organophosphorus nerve agents.In chapter 4,a simple preparation,low-cost and label-free pH-responsive cholesteric liquid crystal polyacrylic acid(CLC-PAA)photonic crystal hydrogel film immobilized with urease is developed for Hg2+ visual detection in which the activity of urease is inhibited by Hg2+.In the absence of Hg2+,urea was hydrolyzed by urease to produce NH3,which caused an increase in the pH of the microenvironment inside the CLC-PAA photonic crystal hydrogel film,leading to a significant change in the color from green to orange-yellow,and an obvious red shift in the reflected light wavelength for the film.The presence of Hg2+ could inhibit the activity of urease,so that the color variation of the film was not obvious,corresponding to a slight change of the reflected light wavelength.Therefore,Hg2+ can be quantitatively detected by measuring the position modification of the reflected light wavelength of the film.The limit detection of Hg2+ is about 10 nM.This method has a good application prospect in the monitoring of heavy metal ions in environmental water resources.