Development Of Atomic Force Microscope Techniques And Their Applications In Studying Macromolecular Structure And Properties

In this dissertation,novel AFM-based lithographic techniques were developed to fabricate the patterned structures of functional polymer brushes and biomacromolecules.In addition,magnetic force modulation atomic force microscopy(MFM-AFM)was established to investigate the rheological properties of biofilms.The main results are as follows:(1)Liquid-mediated scanning probe nanosculpting(LSPN)was developed to fabricate 3D functional polymer brushes.LSPN employs an AFM cantilever tip which is immersed in a good solvent of the polymer to “sculpt” a thin layer of surface-tethered polymer brushes.Subjected to higher forces,polymer chains are scratched away from the substrate by the tip to form desirable 2D and 3D patterned structures.Afterwards,lower forces are utilized for in situ imaging and repairing of the structures that have been made.LSPN is capable of fabricating sub-50 nm 2D and 3D patterned structures.(2)Liquid-mediated non-contact scanning probe lithography(L-NCSPL)was developed for patterned-structural fabrication of protein.L-NCSPL employs an AFM cantilever tip operated in the non-contact mode to scan a substrate in a protein solution.During the noncontact scanning process,protein molecules in the solution are caught and thrown onto the scanned area by the vibrating tip,forming patterned structures of protein within the scanning area.Utilizing L-NCSPL,sub-100 nm dots array,sub-50 nm lines array and even 3D structures of protein were produced.(3)MFM-AFM was established to study the microrheology of bacterial biofilms during growth.This technique produces a harmonic deformation onto the Escherichia coli(E.coli)biofilms using a magnetic cantilever which is driven by an oscillating magnetic field with frequency range from 5 to 200 Hz.In this way,MFM-AFM is able to monitor the rheological variation during the process of biofilm development,i.e.,from individual cells to matured biofilms.It was found that E.coli biofilms show rheological properties identical to those of soft glassy materials.As incubation time increases,E.coli biofilms are fluidized and become homogeneous in structure.In addition,the shear stress influences the growing biofilms,but shows little impact on the stiffness of the upper layer of biofilms once they are mature.