Surface Charge Imaging And Determination Based On The Optical Microscopy

As a basic physical property of matter,charge plays a vital role at micro-nano scales in electrochemical reactions and molecular processes including molecular recognition,cell signaling,enzyme reaction and protein aggregation.A simple and sensitive measurement of charge on a surface will contribute to the understanding of the related chemical and biological processes.In addition,mapping local surface charge distribution can reveal the dynamic process and internal mechanism of surface interaction,and further expand its applications in biomolecular binding dynamics.At present,there are relatively few techniques that can directly perform charge imaging analysis and measurement on the surface interface,and most of the existing approaches process some deficiencies.In this dissertation,imaging approaches and dynamic changes determination of the single nanoparticles,indium tin oxide（ITO）,gold electrode as well as two-dimentional materials are developed based on both optical transmission and total internal reflection microscopy.The main contents are as follows:1.Surface charge measurement of individual nanoparticles and its applications based on bright-field imagingA precise charge measurement plays an important role for individual nanoparticles and even single molecules,and it contributes to our comprehension of the various chemical and biological processes involving charge change.Here we propose a method to measure the surface charge of an individual nanoparticle using a conventional bright field optical microscope combined with a fast Fourier transform（FFT）imaging processing algorithm.The nanoparticle is tethered to an ITO coated glass slide surface with a polymer chain and driven into oscillation with an alternating electric field,the process of which was captured by a camera.The weak scattered light is separated from the intense bright field background with a FFT filter.The surface effective charge of the nanoparticle can be obtained from the changes of the image contrast with precision of a few elementary electron charges.The high-sensitivity charge detection method can be used to monitor the surface charge of an individual nanoparticle in real time.Using this method,we compared the surface charge of nanoparticles with different sizes,and further apply it to detect the binding of DNA to individual nanoparticles,thus demonstrating a new tool for the imaging at single-molecule level.2.Surface charge measurement of individual nanoparticles based on total internal reflection imagingThe reflection mode total internal reflection microscope（Reflection-TIRM）is developed from total internal reflection fluorescence microscope,total internal reflection dark field microscope and surface plasma resonance（SPR）microscope.Herein we constructed a single silica@gold core-shell structure（SiO₂@Au）nano-oscillator,imaged it with a Reflection-TIRM,and determined the surface charge of the individual SiO₂@Au nanoparticle by combining with two-dimentional FFT algorithm.In addition,we compared the background noise of gold film,ITO and graphene substrate with the present experimental setup,and revealed the potential application of graphene as a transparent electrode in this imaging mode by constructing a similar nano-oscillator on the graphene substrate.3.Optical imaging of surface charge distribution on indium tin oxide and gold electrode The surface charge distributions of the electrodes reflect their electrical property directly,which allows us a better understanding of the electrochemical reaction process on their surface.By applying an electrochemical gate modulation,we first performed simultaneous SPR and optical transmission imaging of the surface charge on a gold film coated glass slide.The photoelectric signal conversion mechanism of both SPR and optical transmission imaging to surface charge was demonstrated by analyzing the optical changes at different modulated potential amplitudes,different modulated frequencies,and different gate voltages,respectively.In addition,we mapped the surface charge of an ITO coated glass slide with optical transmission technique,and obtained its optical sensitivity to the surface charge.From the results,SPR response of the gold film coated glass slide,optical transmission response of the gold film coated glass slide,and optical transmission response of the ITO coated glass slide show the sequential decrease optical sensitivity to the surface charge.This laid the foundation for further visual research of the charge distribution and dynamic at the micro-level.4.Optical imaging of surface charge distribution with monolayer molybdenum disulfideThe observation of local surface charge distribution of the nanomaterials is helpful for our comprehension of the charge related dynamic reaction process and internal mechanism,and it is useful for the detection of the molecules binding to the surface.The optical absorption of monolayer molybdenum disulfide（MoS₂）is highly sensitive to charge change,based on which we demonstrate optical imaging of local surface charge distribution and charge change with this ultrathin material.We first validate the optical response of the monolayer MoS₂ and perform charge sensitivity calibration with an electrochemical gate voltage.With the current experimental setup,the charge detection limit for an imaging area defined by optical diffraction is a few tens of elementary electron charges,while the detection limit due to shot noise is only a few elementary electron charges.We have shown that a high-contrast charge images can be obtained with a modulated voltage by using two-dimensional FFT algorithm,which reflects the local charged impurity distribution in the material.We further show that binding of charged molecules to the monolayer MoS₂ leads to a large change in the image contrast,allowing us to determine charge of the adsorbed molecules.This capability opens possibilities for impurities and defects characterization in two-dimensional materials and for label-free optical detection and imaging analysis of charged molecules.