Fluorescence-Only Readable Micro-QR Codes Achieved by Hydrogel Encapsulation.

Quick response (QR) codes are widely used for information storage and authentication; however, their macroscopic visibility and ease of replication limit their security performance in anticounterfeiting applications. Reducing QR patterns to the microscale can increase replication difficulty, yet the encoded structures remain optically detectable under conventional microscopy. In this study, we present a hydrogel encapsulation strategy that transforms microscale QR codes into covert tags that are indistinguishable under bright-field observation while remaining selectively readable through fluorescence imaging. Micro-QR patterns were fabricated using a photocurable hydrogel system and subsequently encapsulated within a secondary hydrogel matrix with distinct optical properties, resulting in effective suppression of visible contrast while preserving fluorescence signal transmission. The encapsulated structures exhibited no recognizable pattern under bright-field conditions while enabling reliable decoding under fluorescence microscopy. This approach combines optical masking and physical isolation to enhance resistance against visual inspection and direct replication, providing a simple yet effective platform for covert information encoding. The proposed hydrogel-based encapsulation strategy offers potential applications in anticounterfeiting, secure labeling, and microscale authentication technologies.