Utilization of Simple Scaling Laws for Modulating Tip-Sample Interaction Forces in Aqueous Environment AFM Characterization: Application to the Self-Assembly of Protein Polymers

Abstract

We have recently reported on experimental observations of silk-elastin-like protein polymers (SELPs) that self-assembled into 1-dimensional nanofibers on mica surfaces upon application of a mechanical stimulus with atomic force microscopy (AFM) in water. SELPs are genetically engineered block co-polymers made of silk-like blocks (Gly-Ala-Gly-Ala-Gly-Ser) from Bombyx mori (silkworm) and elastin-like blocks (Gly-Val-Gly-Val-Pro) from mammalian elastin. The experiment consisted of adsorbing the protein polymer onto a freshly cleaved mica surface, followed by AFM characterization under different sets of imaging parameters, each of which led to different nanofiber coverage rates. In order to gain further understanding of the factors governing the self-assembly process, we utilized multimodal AFM simulation to formulate and guide the implementation of a suitable force modulation strategy, which allowed us to observe trends of the surface coverage rate as a function of the applied peak forces. The simulations suggest that a nearly linear control of the peak tapping forces can be achieved by following simple scaling laws based on the harmonic oscillator model.