Single-molecule Conductance Studies Of Cucurbituril Host-guest Complexes

Moore’s Law predicted that electronic devices designed with the"top to down"ideas will slowly reach the physical limit of their size,which means that the further improvement of electronic device performance will face great challenges.The concept of molecular electronics proposed by Professor Feynman aims to use molecules or atoms as basic building blocks in a"bottom-up"design approach to form functionally complexes molecular devices.The construction of metal-molecule-metal junctions at the nanoscale and precise characterization of their charge transport properties are the focus of molecular electronics research.Cucurbit[n]uril（CB[n]）molecules have a unique nano-cavity structure and can form host-guest complexes with guest molecules through intermolecular interactions.In addition,its molecular ports have a negatively charged carbonyl oxygen group,which can not only bind with cationic guest molecules through ion-dipole interaction,but also couple with Au through electrostatic interaction,thus making it possible to introduce cucurbit[n]uril molecules into the field of molecular electronics.Due to their unique chemical structures and excellent molecular recognition properties,cucurbit[n]uril molecules have great prospects for applications in the fields of physical chemistry,analytical chemistry,biosensing and gene sequencing.Based on the background,this thesis accomplished the following research work:（1）Through the STM fixed-junction and theoretical calculations,it demonstrated that the complexes formed by the interaction of Lewis base DBP and Lewis acid can effectively enhance the charge transfer efficiency without affecting its binding configuration.On the basis,the reversible molecular switch with 7.5 times of current switching ratio can be achieved by adding Lewis bases and Lewis acids to the Au-DBP-Au molecular junction sequentially.（2）The Au/CB7/Au molecular junction was successfully constructed and characterized electrically by STM-FJ method.Subsequently,the electrical properties of the CB7-AFC and CB7-Am were investigated.The results showed that single-molecule conductance of the host-guest complexes was slightly reduced compared with that of the CB7 molecular junction,but the molecular junction lifetime was significantly enhanced.The reason is due to the contributions of the amino group in the guest molecule and the carbonyl oxygen in host CB7.（3）Based on the above work,we prepared functionalized CB7 nanoelectrodes with CB7 as the reader molecule,and realized the single molecule electrical recognition of DNA base（A/T/G/C）.Compared with the no-reader functionalized Au electrodes,the CB7-functionalized nanoelectrodes can effectively improve read accuracy and frequency of each nucleobase.This is due to the unique confinement effect of the CB7 cavity that inhibits the molecular conformation of DNA bases in the nanogap.（4）Combined with the above research experience of regulating conductance by external stimuli and CB7-functionalized electrodes,we constructed a nano-electrical platform for dynamic monitoring of biochemical molecules by modifying CB7 molecules on both ends of Au electrodes.The protonation/deprotonation process of the camptothecin（CPT）was captured between the CB7 electrode pair and the process was detected in real time.On the other hand,the conductance distribution of CPT at different p H showed a sigmoid titration curve,which p K_a was consistent with the chemical characterization result of about 6.3.In addition,we achieved the repeated and controllable binding and release of CB7 and the drug molecule CPT by externally adding salt stimuli such as Na⁺or Ca²⁺.