A new functional paper-based sensor was developed to analyze Fe3+ ions based on its capability to catalyze Au leaching from gold nanoparticles. The colorimetric sensor realized droplet detection of aqueous Fe3+ samples with good selectivity in the solid/liquid interface. The sensor showed high resistance to leaking of immobilized AuNPs in aqueous solutions as well as significant reproducibility in batch experiments. The sensor was applied to detecting Fe3+ in spiked serum samples.