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

Abstract

processes in the liquid phase. In general, monolayer nanosheets are extracted only from exfoliated materials through purification. The unique properties of monolayers have been extensively studied in previous works. [1,2,4,6,[8][9][10] In contrast, other exfoliated nanosheets thicker than monolayers, such as few-layers or multiple-layers, were not widely used for applications, such as sensors and catalysts. At the same time, such materials also have potential applications. [16][17][18][19][20] Therefore, the size, surface chemistry, and assembly state of nanosheets, including monolayers and few-layers, are important for device fabrication. Our group has previously studied the exfoliation of layered transition metal oxides in organic dispersion media through the formation of layered composites with the intercalation of organic guests. [21,22] Surface-modified nanosheets were obtained by exfoliation in an organic medium. The prediction models for the yield, size, and size distribution were constructed through the extraction of the descriptors by a combination of machine learning and chemical insight on small experimental data. [23][24][25][26][27][28] These predictors contributed to the efficient exploration of appropriate experimental conditions, such as host-guest-medium combinations that yield the monodispersed and polydispersed nanosheets. [26] In the present work, surface-modified nanosheets with polydispersed lateral sizes and thicknesses were used to prepare an uneven thin film exhibiting an angle-independent structural color (Figure 1). The corresponding host-guest-medium combinations were selected based on the prediction model.Exfoliated nanosheets have been used as building blocks for the production of composites, [29][30][31][32][33][34][35] such as layer-by-layer assemblies, gels, and organic-inorganic composites. However, nanosheets are not easy to organize into the aligned states, such as thin films and liquid crystals. Specific methods and optimized conditions are required to align the nanosheets because the evaporation of the dispersion media causes random aggregation. Thin films and liquid-crystalline assemblies exhibiting structural colors have been previously prepared by homogeneous stacking of monolayered nanosheets. [35][36][37][38][39][40][41] Structural colors have been applied in the detection of chemical vapors and mechanical stresses. [37][38][39]41] However, self-assembled thin films with homogeneous thickness exhibit angle-dependent structural colors. In previous works, the angle-independent structural color was