Portable Colorimetric Sensor Array Based on a Porous Single-Atom Fe Nanozyme with Different Surface Sites for Identifying Artificially Ripened Fruits

Single-atom nanozyme materials have demonstrated exceptional specific catalytic activity due to the atomic-level dispersion of their active centers. However, the exploration of catalytic mechanisms for single-atom catalysts is so far limited to the 2D surfaces of nanozymes. In this study, porous single-atom Fe nanozyme (psaFeN) is successfully prepared through a straightforward coordination-assisted polymerization-assembly strategy. The psaFeN composite nanospheres are uniformly sized, exhibiting excellent dispersibility with well-organized pore channels extending from the center to the surface. Density functional theory calculations reveal that in the psaFeN nanozyme, the (010) facets serve as the primary active surface, where Fe atoms form tri-coordinated or tetra-coordinated structures with doped nitrogen atoms. The (100) facets act as auxiliary reactive surfaces with tetra-coordinated Fe─N as the active center. psaFeN exhibits excellent POD-like activity (K_m = 1.77 mM; V_max = 173.53 × 10⁻⁸ M s⁻¹). Given this exceptional bioactivity, a portable colorimetric biosensor is constructed for distinguishing artificially ripened fruits from naturally ripened ones. The sensor achieves precise discrimination with a detection limit as low as 310 nmol L⁻¹. This study is anticipated to offer valuable insights into understanding the 3D catalytic mechanisms of single-atom nanozymes, promoting their application in the development of robust biosensors for food safety.