Single-Molecule Optical Tweezers And Adsorption Free Energy Measurement At Solid-Liquid Interferce With Mehcanically Controllable Break Junction Technique

The single-molecule study helps to discover novel phenomena that have never exposed to macroscale objects,which thus leads to in-depth understandings of physical,chemical,and biological processes.The prerequisite for the study of single molecules is single-molecule manipulation technology.In the past decades,optical tweezers have attracted much interest in single-molecule science,which is mainly motivated by the non-contact feature and high resolution.However,due to the diffraction limit,conventional optical tweczers are unable to manipulate the molecules with nano-size directly.In recent years,when the plasmon effect was introduced,the optical tweezers can manipulate particles or even molecules on the scale of tens to hundreds of nanometers.The plasmonic optical tweezers rely on the fine structure of the substrate.However,for an ultra-small particle likes a molecule,it is still impossible to generate enough optical force to overcome the Brownian motion by using the state-of-art micro/nano-fabrication and detection techniques.To date,there is no report for the successful trapping and manipulation of single molecules.This thesis introduces a new experimental technique named as single-molecule plasmonic optical tweezers.With this method,for the first time the capture and release of single molecules(?2 nm)in solution at room temperature were realized.In particular,mechanically controllable break junction(MCBJ)was coupled with optical tweezers.MCBJ is able to generate precisely controllable nano-gap with a sufficiently strong plasmon-enhanced effect,while the coupled laser system serves for the manipulation of single molecules in solution.The manipulation process was in-situ monitored and confirmed by using the single-molecule measurement.In the process for developing this single-molecule plasmonic optical tweezers,a new method for single-molecule adsorption free energy measurement at the solid-liquid interface was developed based on the MCBJ and single-molecules counting method.Significantly,it is the unique method capable of measuring the molecular adsorption at the solid-liquid interface when the solid remains an intrinsic state.The main contents and conclusions included in this paper are as follows:1.This thesis reports a new method for measuring molecular adsorption at the solidliquid interface.With this method,for the first time,the Gibbs free energy of molecular adsorption at the solid-liquid interface was experimentally measured when the solid remains an intrinsic state.Based on MCBJ and single-molecule counting method,the charge transport through three molecules with different terminal groups of methylthio(SMe),pyridyl(Py)and amino(NH2)was measured at different concentration.It was found that the relationship between the formation probability of molecular junction and concentration was consistent with the Langmuir adsorption function.Based on that,the adsorption free energies were found to be 32.5 KJ/mol,33.9 KJ/mol,28.3 KJ/mol,which are consistent with theoretical calculation and results reported in previous literature.2.This thesis reports a new method named as single-molecule plasmonic optical tweezers.With this method,for the first time,the manipulation of a single molecule in solution at ambient conditions was realized in experiment.The instrument for single-molecule manipulation was set up successfully.With this homemade instrument,the oligophenylacetylene(OPE3-SMe)molecules were successfully trapped into the pre-determined location.A mechanism was proposed with the aid of theoretical simulation.Particularly,the probe molecules were trapped by the localized plasmonic effect.Then the single-molecule plasmonic tweezers experiments were performed in variable experimental conditions,such as different laser wavelengths,different laser polarizations,different laser intensities.The findings in these experiments further confirmed the plasmon-based mechanism.Moreover,as insphed by the volume-dependent feature of optical force,the selective single-molecule manipulation in a mixed solution was realized.This demonstration shows the enormous potential of the single-molecule plasmonic optical tweezers in biological,physical and chemical science.