The robust gallium carboxylate Ga(OH)(btec)·0.5H₂O (MIL-61) is selected as a parent MOF to prepare Ln-MIL-61 by PSM. This work investigates the luminescence of Ln-doped MIL-61 (Ln = Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺) in the visible light region. Upon 314 nm excitation, the emission spectra of Eu-MIL-61, Tb-MIL-61 and Eu/Tb-MIL-61 all exhibit their respective strong sharp emission bands. However, Sm-MIL-61 and Dy-MIL-61 show very similar emission to MIL-61 and almost no luminescence of Sm³⁺ and Dy³⁺. In this work, the weak fluorescence of Sm³⁺ or Dy³⁺ doped MIL-61 is effectively sensitized by Ag⁺. Besides, single-phase white-light emitters based on the resulting 4d–4f heterometallic co-doped MOFs can be realized. Furthermore, Sm-MIL-61 shows more highly sensitive and selective sensing towards Ag⁺, and is a promising optical sensor for Ag⁺ detection in a few daily water samples.