The combination of carbon dots (CDs) and rare-earth ions has attracted increasing attention due to their unique fluorescence properties. Herein, CDs are incorporated with NaYF4:Eu to form stable NaYF4:Eu@CDs nanocomposites by a simple sol-gel route. It has been demonstrated that CDs are successfully embedded in the silica layer outside NaYF4:Eu nanoparticles. These nanocomposites exhibit characteristic emissions of both CDs and NaYF4:Eu under a single excitation wavelength. By using their dual-emissive properties, the nanocomposites can act as ratiometric fluorescent sensor for selective detection of Fe3+ ions in the range of 0-350 mu M. The temperature-dependent luminescent property of the nanocomposites makes them also suitable as thermometers in the range from 377 to 467 K. In addition, the luminescent color can be regularly adjusted by tuning the mass ratio of NaYF4:Eu to CDs. This combing strategy enables us to achieve dual emission of CDs and Eu3+ ions in nanocrystals, opening an avenue towards multifunctional applications in widespread fields. (C) 2019 Elsevier Inc. All rights reserved.