Polymerization-based signal amplification (PBA) has been regarded as a promising colorimetric detection method for diagnostic tests in resource-limited settings because it is affordable, rapid, and quantitative in terms of analyte levels when combined with cell-phone based imaging. PBA produces phenolphthalein-entrapped hydrogel in the presence of analytes, so the red detection signal can be seen with the naked eye when the hydrogel is exposed to a basic solution. Since no hydrogel forms in the absence of analytes, PBA achieves high contrast between positive and negative results in diagnostic tests. Thus, PBA can reduce subjectivity in interpretation of the results by untrained users compared to other competing methods based on enzymes and gold nanoparticles. However, the red signal generated by PBA completely disappears in several minutes if the test is not laminated and basic solution is allowed to evaporate. Because phenolphthalein is not covalently bonded to the hydrogel, it can diffuse out of the polymeric network upon drying and rehydration. To address this problem, polymerizable phenolphthalein (N-(2-hydroxy-5-(1-(4-hydroxyphenyl)-3-oxo-1,3-dihydroisobenzofuran-1-yl)benzyl)-acrylamide) was prepared and copolymerized under visible light with 1-vinyl-2-pyrrolidone (VP) and poly(ethylene glycol) diacrylate (PEGDA) in an air-saturated aqueous solution. Covalent attachment of the polymerizable phenolphthalein to cross-linked polymer is demonstrated by colorimetric intensities of phenolphthalein-entrapped and phenolphthalein-conjugated hydrogels that were measured over time. Furthermore, using Plasmodium falciparum histidine-rich protein 2 as an example, we demonstrate that the covalent attachment of phenolphthalein to the polymer network provides permanent red signal while continuing to achieve a clinically-relevant limit of detection with reaction times of less than two minutes.