From paper to polymers: Substrate engineering in point-of-care colorimetric sensors for ionic species detection

Colorimetric sensors are attractive platforms for point-of-care testing of ionic species due to their simplicity, low-cost, and suitability for decentralized analysis. However, their translation from laboratory concepts to robust, field-deployable devices is often hindered by limitations in signal uniformity, reproducibility, and environmental stability. In this context, substrate engineering has emerged as a key design strategy for enhancing colorimetric sensing performance. This review examines recent advances in solid-substrate colorimetric sensors for ionic species detection based on molecular dyes, natural pigments, and nanostructured colorimetric agents, including metal nanoparticles and nanozymes, with emphasis on both conventional paper-based materials and engineered polymeric substrates. Particular attention is given to how substrate properties, surface modification, and immobilization strategies influence analyte transport, colorimetric-agent distribution, and optical response. The integration of digital image analysis is discussed alongside current limitations, including reproducibility and validation challenges in field conditions. Overall, the review highlights substrate engineering as a unifying design framework for developing robust colorimetric devices while identifying key gaps that must be addressed for reliable real-world deployment. • Substrate engineering plays a key role in point-of-care colorimetric sensors. • Organic dyes, metal nanoparticles, and nanozymes enable diverse colorimetric POC mechanisms. • Co-design of substrates and chromogenic systems is essential for reliable field-deployable sensors. • The transition from paper to engineered polymers enables improved robustness and reliability. • Sustainability, translational readiness, and commercialization depend on rational substrate design.