Application Of Dark-Field Light Scattering Imaging Technique In The Study Of Photoinduced Crystal Structure Transformation Of Porphyrin Organic Framework

Dark-field light scattering imaging is a technique in which the light source of the dark-field microscope（DFM）irradiates the material through a condenser in an oblique way to obtain a dark background scattering image.And on this basis,the scattering spectra can be obtained by combining with a dark-field spectrometer,and the scattering signal of the material can be analyzed and studied.The scattering signal of the substance is generally affected by its own material,shape and size,medium microenvironment and coupling between particles,etc.Therefore,the physical and chemical reaction process of the substance can be monitored according to the change of the scattering signal of the substance.Dark-field light scattering imaging technology can be used to characterize the scattering signals of individual particles,which can be applied to analytical detection,biosensing and real-time monitoring of reactions at the single particle level.Due to the unique localized surface plasmon resonance（LSPR）properties,noble metal nanomaterials can more sensitively reflect the changes of their own light scattering signals than other materials,and have become the main probes for dark-field light scattering imaging applications.The application of other materials probes in DFM is also slowly emerging,metal-organic frameworks（MOFs）,as a kind of porous crystal material,have been reported the relationship between the changes of dark-field light scattering signal and reaction kinetics during gas adsorption,thermal dehydration and other processes.The crystal structure of MOFs is a key factor in determining their catalytic activity,so it is important to study and characterize the crystal structure.Powder X-ray diffraction（PXRD）analysis is an important method to study the crystal structure,but it usually requires a large amount of sample,at the same time,this method of analyzing the overall properties of a large number of samples cannot accurately reveal the local structure information and differences.And the low flux of single crystal X-ray diffraction analysis cannot be used for the measurement of dynamic changing processes.DFM can analyze the dynamic process at the single-particle level.However,the application of DFM in the study of the crystal structure of single-particle MOFs is rarely reported,mainly owing to the correlation between the optical signal and the crystal structure has not been elucidated.Therefore,it is expected to realize the dynamic reaction process of the crystal structure transition of MOFs monitored by DFM at the single particle level,and reveal the relationship between the crystal structure characteristics of MOFs and their scattering signals.However,due to the lack of LSPR properties,dark-field imaging strategy cannot simply and effectively obtain useful optical information directly even after weak changes of MOFs.In recent years,Artificial Intelligence（AI）technology,such as machine learning and convolutional neural network and other algorithms,has been widely used to enhance the imaging effect of dark-field microscopy imaging system in the field of biomedical imaging analysis with noble metal nanomaterials as probes.However,these algorithms are usually only applicable to specific application scenarios and require a large number of model samples for training and learning in advance.Some digital image processing technologies based on other algorithms can process optical images more quickly and efficiently,and obtain images with high resolution and clearer.Therefore,it is of great significance to develop a new algorithm to enhance the dark-field imaging effect of MOFs and other materials.In this study,zirconia porphyrin metal-organic framework（ZrTCPP）was used as the research object.On the basis of study its dark-field light scattering imaging technique and the dynamic bistable stochastic resonance（DBRS）algorithm.Study details are as follows:1.DFM Monitored the photoinduced crystal structure transformation process of ZrTCPP.Based on the dark-field light scattering imaging technology,we established a platform to analyze the relationship between the dark-field light scattering signal and the crystal structure information,and revealed that the change of crystal structure will change the scattering signal of ZrTCPP.ZrTCPP nanoparticles with a size of 100-200 nm were synthesized by a one-pot method.Based on dark field light scattering imaging,scanning electron microscopy imaging and powder X-ray diffraction analysis,it was found that under continuous illumination,the scattering spots of ZrTCPP changed from blue-white spots,cyan spots and blue spots to green spots,and the corresponding scattering spectra also changed from multi-peak to unimodal,accompanied by the decrease of peak intensity.On this basis,the reason for the change of scattering signal of ZrTCPP was investigated.It was confirmed that the morphology of ZrTCPP was unchanged,the size of ZrTCPP decreased slightly,and the internal crystal structure changed greatly under continuous illumination,which led to the change of scattering signal of ZRTCPP.Then,the reason for the crystal structure transformation of ZrTCPP was explored,and it was found that under continuous illumination,the reactive oxygen species produced by ZrTCPP were mainly singlet oxygen（¹O₂）,which led to the decrease of the bond energy of Zr-O bond in ZrTCPP and the loosening of Zr-O bond,which was the cause of the crystal transformation.Moreover,the crystal structure change of ZrTCPP has an impact on its catalytic activity.The study of the crystal structure transformation and scattering signal change of ZrTCPP under continuous illumination revealed the relationship between scattering signal and crystal information,provideda simple and effective platform for obtaining structure information by studying optical signals.2.Algorithm assisted dark-field microscope to enhance the weak signal and enlarge the difference of signal change.The scattering signal of ZrTCPP has an important relationship with its crystal structure,but it needs a long time of continuous illumination to observe obvious scattering signal change in the DFM.Due to the lack of LSPR property of ZrTCPP,the dark-field light scattering technology cannot sensibly show the weak scattering signal change of ZrTCPP in the early stage of illumination.In this chapter,we established a dynamic bistable stochastic resonance（DBRS）algorithm assisted dark-field microscopy to enhance the scattering image brightness of ZrTCPP,which magnified the difference in the scattering signal changes of ZrTCPP in the illumination process.Firstly,the DBRS algorithm was established according to the principle of stochastic resonance combined with the bistable system of double well potential.The DFM images of ZrTCPP at early period of illumination were obtained by the DFM imaging system.The scattering images of ZrTCPP were input into the computer and processed iteratively by DBRS algorithm to obtain two images with enhanced scattering signals.Image-Pro Plus（IPP）software was used to perform intensity statistics on the images before and after the algorithm processing.The results show that the algorithm can not only enhance the brightness of ZrTCPP DFM images,but also the difference of the scattering signal in the early stage of ZrTCPP illumination is amplified twice.In order to verify the universality of the algorithm,the traditional noble metal nanoprobes,silver nanoparticles（Ag NPs）as the contrast object,and observed the etching process of Ag NPs by Fenton reagent under DFM.The DFM images at different reaction time points were also selected and processed by DBRS algorithm to obtain the enhanced DFM images of Ag NPs.Finally,the intensity statistics of the scattering images before and after the algorithm processing were performed.The results show that this algorithm can magnify the signal variation difference of Ag NPs to about 10 times.In addition,the scattering signal of Ag NPs was further enhanced by the combination of the algorithm and the CFM.The establishment of DBSR algorithm enhanced the imaging effect of dark-field light scattering imaging technology successfully,and extended the application range of dark-field light scattering imaging technology on other materials.In summary,we took ZrTCPP nanoparticles as the research object,and established a new platform to link scattering signals with crystal information by using dark field light scattering imaging technology.Then,based on computer technology and stochastic resonance principle,the analysis method of scattering signal changes in the process of amplifying weak reactions was established.It provided a new idea and method to expand the application range of dark field light scattering imaging technology.