Preparation and characterization of pH-responsive films of cresol red/starch/carboxymethyl cellulose induced by boric acid

The rapid advancement of smart agriculture has created a growing demand for innovative materials capable of real-time environmental monitoring. In this study, starch/carboxymethyl cellulose/cresol red (SCR, CMCCR, SCMCCR) composite films were developed using a casting method to explore the effects of pH-responsive reagents at 0.3 wt% content on the films' physicochemical properties. The SCMCCR film showed a 20.26 % reduction in thickness compared to the SCR film, while its density increased by 26.6 %. Mechanical testing revealed that the tensile strength improved by 17.38 %, and the elongation at break remained at 52.56 %. Notably, the SCMCCR film achieved a Young's modulus of 552.96 MPa, which is over 6 times higher than that of the CMCCR film and 23 times greater than that of the SCR film. Water solubility of 27.77 % and enhanced thermal stability, with a final residual content of 49.57 % at 600 °C, were observed for the SCMCCR film. Characterization by FT-IR, XRD, and SEM confirmed structural and morphological modifications. The films raw materials exhibited exceptional responsiveness to humidity variations and film demonstrated rapid and distinct color transitions from yellow to purple across specific pH value. The environmentally friendly composition, low cost, and high sensitivity of these films make them promising candidates for real-time soil pH monitoring. • CR was effectively immobilized within starch-carboxymethyl cellulose composite film. • Young's modulus of SCMCCR film increased by 23 times compared to that of SCR film. • PH-responsive film appeared yellow in acidic soil and purple in alkaline condition. • The composite film effectively monitored soil pH through visible color changes.