<p>As demand for sustainable and biocompatible technologies grows, low-toxicity mid-infrared materials, such as silver chalcogenides, have attracted significant interest. Herein, we report mid-wavelength infrared tunable Ag₂Te colloidal quantum dots through a post-growth method starting from short-wavelength infrared Ag₂Te colloidal quantum dots. Using the synthesized Ag₂Te colloidal quantum dots, we successfully fabricate a photodetector covering the full mid-wavelength infrared spectral range (3–5 μm) with an onset wavelength extending to 6.9 μm. At 78 K, the photodetectors exhibit a photoresponse time of 230 ns (rise) and 576 ns (fall). Responsivity varies from 1.9 × 10^-3A W⁻¹ at 0.02 V to 1.1 A W⁻¹ at 0.5 V, depending on the applied bias, and the specific detectivity of the device is 1.2 × 10⁹ Jones at 0.02 V. The measured noise-equivalent temperature difference of 0.3 K enables us to reliably distinguish temperature variations between 37 °C and 40 °C, directly enabling the diagnosis of fever-level body temperatures.</p>