AmericanChemicalSociety.com

Overall, two major projects were performed during the last year. First is Atomic Force Microscopy (AFM) study of photoreversible nanoscale surface relief gratings on side chain dendritic polyester thin films.  Specifically, low molecular weight photoresponsive side chain dendritic polyesters (SCDPE) were synthesized by stoichiometric addition of diacyl chloride azobenzene with a unique 3^rd generation dendritic wedge diol.  The symmetric design of the dendrimers was added to introduce additional free volume and inhibit aggregation for optimization of trans-cis-trans azobenzene isomerization.  The polymer was spin coated onto quartz slides and UV light was irradiated through a photo mask placed onto the polymer surface to form a nanoscale surface relief grating.  These patterns were characterized using AFM.  We have prepared films of unique photoresponsive polyester in which surface relief gratings were patterned onto the film surface without the conventional use of high powered laser sources. These specifically designed polymer systems contained PAMAM dendritic side groups that increased the free volume and inhibited formation of aggregates for the attached azobenzene chromophore to efficiently photorespond. The resulting model employed non polarized UV light to effectively image the nanoscale surface relief gratings onto a photoresponsive film.  The systems were also shown to be photoreversible by irradiation with visible light.  This suggests a potential simple and cost effective system for developing reversibly generated gratings for use in photonic and optical applications.

Second, our group undertook a significant effort directed towards developing a novel approach to probe redox-transitions at nanoscale using bias-dependent adhesion force measurements by Conductive Probe Atomic Force Microscopy (CP-AFM).  As a case study the focus was on mixed self-assembled monolayers of redox-active ferrocenylundecanethiols and redox passive alkanethiols on gold substrate.  Mixed monolayers with different deposition ratios between these two (ranging between 0 to 100%) were prepared and bias-dependent adhesion force measurements performed between a Pt-coated AFM tip and samples.  Two major effects were observed.  First, the adhesion force was increasing with an increase in the concentration of the ferrocene groups until ~50% surface fraction is reached.  Second, further increase in the concentration of redox-active groups resulted in a concomitant and continuous decrease in the adhesion force.  The effect was highly reproducible providing a way to modulate the adhesion force response by varying the concentration of redox-groups inserted in alkanethiols redox-passive matrix.  Such nonlinear response was modeled using the capacitance force model that accounted charge delocalization effects by ferrocene groups when they come to close proximity to each other, i.e. for the surface fraction above 50%.  Additionally, we demonstrated the sensitivity of such measurements to small variations in the number of redox-groups in the molecular junction, where studies indicate a potential for the single molecule level sensitivity. The first paper that outlines this approach is currently in preparation.  We next plan to extend our studies on other types of electro-active systems, for example ferritin protein nanoparticles with controlled composition of artificial cores (Ni, Co, Mn, Fe₃O₄).  The overall goal is to first understand fundamentals of redox transitions at nanoscale and then to develop a nanodevice (molecular transistor, for example) with desired nanomechanical properties.