Nanosheet‐Based Uneven Thin Film Exhibiting Angle‐Independent Structural Color and Its Application to Detection of Biogenic Amines

Abstract Anisotropic ultrathin 2D materials are used as building blocks to obtain higher‐order structures. Tailoring the size, surface chemistry, and assembly state of exfoliated nanosheets is required for the design of functional materials. However, these factors are not easily controlled during the down‐sizing exfoliation process, which is a general route for obtaining 2D materials. In the present work, polydispersed nanosheets of a transition metal oxide are prepared based on prediction model for the size distribution constructed by a combination of machine learning and chemical perspective on small data. Self‐assembly of the exfoliated nanosheets results in the uneven thin films exhibiting an angle‐independent structural color. As the nanosheets vary in thickness and lateral size, their assembled thin films exhibit structural color independent of the incident angle. The structural color is changed by casting only 20 µL of an aqueous solution containing biogenic amines such as histamine within a sensing time of 5 min. The biogenic amines are intercalated into the interlayer space of the stacked nanosheets and the resulting increase in the thickness changes the color of the thin‐film device. The new design strategies can be applied to constructing other thin‐film sensing devices based on nanosheets.