Sequence-defined polypeptoid brush The polypeptoid 21-mers used in this study have molecular weights between 2400 g mol -1 and 3200 g mol -1 (Table ), which would have a radius of gyration (Rg) ~1 nm based on previous smallangle neutron scattering (SANS) measurements of similar polypeptoids in near-𝜃 conditions. With the potentially very thin brush monolayers, multiple surface characterization techniques are employed to confirm the surface coverage of Si substrates with grafted polypeptoids. With attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), the presence of grafted polypeptoids on substrates is confirmed by the amide C=O stretching peak ~1670 cm -1 in ATR-FTIR, and the N1s peak between 390-400 eV in XPS (Figure ). Yet these two macroscopic characterization techniques do not provide direct evidence on whether homogeneous modification of the substrate surface on nanoscopic length scales is achieved. In order to examine the surface coverage at nanoscale resolution, we further use infrared photoinduced force microscopy (IR PiFM) to directly visualize the surface coverage of Si substrates by grafted polypeptoids. IR PiFM is a nano-scale microscopy and spectroscopy technique that measures photoinduced thermal response and polarizability of samples in the near field by detecting the force between the tip and the sample. This technique enables simultaneous spatial mapping of topographical and chemical information from two mechanical eigenmode resonances of the cantilever at the resolution of an atomic force microscope (AFM). The prepared polypeptoid brush monolayers are topographically smooth, with a root-mean-square roughness, Rq » 0.2 nm, comparable to bare Si substrates. Importantly, the chemical map collected simultaneously at 1648 cm -1 (characteristic band of polypeptoid backbone amide C=O stretching)