We prepare a solid and green film of carbon-dot-loaded alginate gels with a pore structure. Compared to carbon dot suspension, the film exhibits stronger blue light emission. The porous structure of the film enables ion diffusion and contact with the CDs incorporated in the gel network, and thus the photoluminescence (PL) behavior of the film can be influenced by ions. The PL of the film shows a sensitive and selective quenching effect to Cu2+, and it can be repeatedly used as a fluorescent probe to recognize Cu2+ with a detection limit of 5 ppm. A band bending mechanism is proposed to understand the effects of surface/interface states and metal ions on the PL behavior of carbon-dot-loaded alginate gels, and it has been supported by our further experimental results. This band bending mechanism provides a clear physical insight into ion detection by PL behavior.