Light-induced Antibacterial Properties In P-MSB Few Layers

Following the outbreak of novel coronavirus,more than 100 milliom people were diagnosed and the death toll is near 2.22 million,dated on the 31 st of January,2021.it has made a significant impact on global economy and public health.As a result,the demand for disposable medical masks and other protective goods has increased greatly.However,most of the personal health protective equipments only have the effect of preventing virus but not antibacterial activity and the one-time use will cause serious waste of resources and secondary environmental pollution.So it is particularly critical to develop medical protective materials with antibacterial properties.In recent years,many antimicrobial materials have been developed,but there are still some defects.For example,the traditional antimicrobial materials is unstable and difficult to prepare,and easy to form drug resistance.Inorganic and metal antimicrobial materials have poor biocompatibility and retrievability.The strategy of using photocatalytic effect to produce reactive oxygen groups such as superoxide ions to kill bacteria has broad application prospect because of its simple preparation,high efficiency and stability,low cost and environment friendly.In this thesis,the light-induced structural transformation of two-dimensional(2D)molecular crystals and its effectively antibacterial mechanism are studied.A 6-10 nm thin2 D molecular crystal,1,4-bis(4-methylstyrene)benzene(p-MSB),a highly ordered single crystal structure,has been designed and prepared.Under illumination of a ultraviolet light,it translates to cis from trans,resulting in high photocatalytic antibacterial activity.Main results in this thesis are described as follows:1.A seed-epitaxial drop-casting method is used to grow p-MSB molecular crystals based on Gibbs-Curie-Wulff law.Scanning electron microscope(SEM),optical microscope,atomic force microscope(AFM)were used to characterize the morphology of the p-MSB.It shows that the p-MSB molecular crystals is deposited on the Si O2/Si substrate in a regular and uniform pattern.The single crystal is about 50-200 μm long and 10 nm(3-5 layers)thick.X-ray diffraction(XRD)and Raman spectroscopy(Raman spectra)show the p-MSB crystal is layered and long-term orderly.The connection between adjacent molecules is byπ-π interaction,which greatly improves the separation of photogenerated carriers.The strong conjugation between benzene rings can lead to a rapidly transfer of charge to oxygen active sites,which enhances the photocatalytic antibacterial activity.2.In order to research the influence of ultraviolet illumination,nuclear magnetic resonance spectroscopy(NMR),UV-Vis absorption spectroscopy(UV)and fluorescence spectroscopy(PL)have been used.Under the ultraviolet light(395 nm),p-MSB few layers transform from trans to cis.This leads to molecular structure bending and some low-power active sites are produced in bending locations.These sites can localize the photoexcited electrons to more efficiently and fastly react with the surface O2 to creat ·O2-.In addition,in the cis structure,the molecular symmetry is destroyed,which causes the increase of dipole moment.The methyl group is negative charged and the π bond is positively charged.Such a large molecular dipole promotes carrier separation.3.The absorption spectral response of few-layer p-MSB is found to expand to visible light range(350-800 nm)compared to bulk material(350-550 nm).The band gap decreases from 2.9 to 2.54 e V for few-layer crystal with 6-10 nm.The more suitable band structures help electrons and holes to participate in oxidation-reduction reaction at the same time.More active oxygen groups are produced to efficiently sterilize.Different degrees of molecular deformations occur under ilumination,resulting in changes of distance between adjacent moleculars.Compared with bulk materials,the arrangement of moleculars is closer in few layers and electrons more easily transfer to the surface active sites,reducing their recombination in the body.4.The results of electron spin resonance spectroscopy(EPR)and fluorescence probe test show that a large amount of ·O2-and ·OH are produced in the few-layer p-MSB under the illumination.The Oxford cup and anti-bacterium-circles tests also confirm the high antibacterial performance of p-MSB few layers.When the volume of aqueous solution of few-layer p-MSB molecular crystal with a density of 5×10-6 g/ ml is greater than or equal to200 μl,obvious antibacterial property can be observed.The minimum antibacterial dose is only 1.0×10-6 g.Based on the above results,we can expect few-layer p-MSB crystal to have important and favorable applications in water disinfection,biological medicine,flexiblewearable devices and so on.