An inkpen-based reversible biodegradable sensor (IRBS) has been developed for the detection of mercuric ions (Hg²⁺). The colour response can be tuned to allow detection with the naked eye based on Hg²⁺-induced opening of the rhodamine spirocycle, leading to an increase in fluorescence and other colour changes. After dipping in water, the colour change in the IRBS paper can be quantified by visual comparison with standard images of IRBS papers, allowing the concentration of Hg²⁺ to be determined over the concentration range 4.0 × 10⁻⁸ M to 1.0 × 10⁻⁶ M. Reusability has been established by repeatedly dipping and rinsing the paper in aqueous Hg²⁺ and EDTA solutions. This approach provides a sensitive and accurate method for the estimation of Hg²⁺ in environmental and biological applications.