Lanthanide metal–organic frameworks (Ln-MOFs) have demonstrated great potential in luminescence sensing and optical anti-counterfeiting. High-security anti-counterfeiting technology is of great importance and requires the development of universal luminescent materials with multiple modes of emission and adjustable photoluminescence. Herein, a 3D red light emission microporous europium(III) metal–organic framework [Eu₃(OH)(1,3-db)₂(H₂O)₄]·3H₂O (1) (1,3-db = 1,3-di(3′,5′-dicarboxylpheny) benzene) was constructed from a zigzag [Eu₃(COO)₈] chain and π-electron-rich terphenyl-tetracarboxylate. Notably, the quenched fluorescence of 1 under hydrogen chloride vapor could be recovered upon fuming by a vapor of Et₃N. Most strikingly, the strong blue light emission by nitrogen and sulfur co-doped carbon dots (N,S-CDs) could be encapsulated in 1 to generate a dual-emission composite, namely, N,S-CDs@Eu-MOF, which shows solvent-dependent photoluminescence: N,S-CD-related blue luminescence in water and Eu-MOF-related red emission in organic solvents. Taking advantage of the above unique reversible fluorescent behavior, Eu-MOF and N,S-CDs@Eu-MOF are prepared as fluorescent high-security inks to achieve data encryption and decryption on specific flower patterns.