Researchers have developed a colorimetric pH sensor capable of delivering a uniform, non-sigmoidal linear response across ten pH units, addressing long-standing limitations in optical pH detection. The study, published in Microsystems & Nanoengineering, introduces a leaching-free nanopigment system that combines covalent dye immobilization with a carefully engineered microenvironment to achieve stable, wide-range sensing.
The pigments are created from a blend of two sulfonephthalein dyes, Bromocresol Green (BCG) and Phenol Red (PR), each covalently attached to raspberry-like silica nanoparticles (RSNs). Together, they form a single sensing platform capable of operating from pH 1 to 10 with consistent linear calibration.
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Colorimetric pH sensors are widely used in environmental monitoring, food safety, and medical diagnostics because they offer visual simplicity and design flexibility compared with electrochemical probes. However, certain issues hold them back from larger success.
Many systems are limited to a narrow dynamic range of roughly three pH units. Attempts to broaden this range often introduce sigmoidal response curves, making accurate calibration difficult. Dye leaching further undermines reliability, particularly in long-term or high-temperature applications.
Sulfonephthalein dyes are attractive indicators because of their distinct colour transitions and low cost. However, physically trapping them within polymer matrices rarely prevents dye migration. Ensuring chemical stability without sacrificing responsiveness has remained a central challenge.
The Covalent Nanopigment
To address this, the researchers chemically activated six sulfonephthalein dyes by converting their sulfonic acid groups into reactive sulfonyl chlorides, enabling covalent bonding to amine-functionalised raspberry-like silica nanoparticles. The reaction forms stable sulfonamide linkages, preventing dye release while preserving pH sensitivity.
The resulting nanopigments were embedded within an agarose/polyethylene oxide (PEO) hydrogel matrix and cast into thin films. The hydrated network supports proton transport while maintaining structural integrity. Colour changes were quantified by converting digital RGB values into CIELAB colour space, with ΔE*ab values used to define visually distinguishable differences and calibration behaviour.
Achieving Linearity Across pH 1-10
Individually, dye-attached nanoparticles still exhibited limited dynamic ranges. The key advance came from combining two dyes with complementary sensitivity windows: BCG, effective in acidic conditions from pH 1 to 5, and PR, responsive from pH 6 to 8.
When integrated within the RSN-agarose-PEO microenvironment, the blended system achieved colour differentiation over pH 1-9 and a linear calibration response over pH 1-10, with optimal performance at a 1:2 BCG:PR ratio.
The extended linearity is not simply the result of mixing two indicators. The heterogeneous matrix plays a decisive role. Dye molecules confined within the porous silica structure experience varying electrostatic and hydrogen-bonding interactions, creating a pKa gradient within the nanoparticles. This multi-stage protonation and deprotonation behaviour produces the observed non-sigmoidal, uniform response over the full sensing window.
Stability Under Demanding Conditions
A central claim of the study is the elimination of dye leaching. Compared with commercial pH paper, which showed significant dye release, the nanopigment film exhibited negligible leaching after prolonged exposure to buffer.
The films remained stable at temperatures up to 100 °C and in solutions with ionic strength up to 1 M NaCl. Photostability under ambient light was confirmed, and the films maintained stable performance for 2 weeks at room temperature. Tests in artificial sweat further suggested compatibility with wearable sensing formats.
To demonstrate practical applicability, the researchers placed the mixed BCG–PR film inside sealed shrimp packaging. As spoilage progressed, protein degradation generated alkaline compounds such as trimethylamine, increasing the local pH. Over 24 hours at 25 °C, the film visibly shifted from yellow to brown, tracking spoilage in real time.
The nanopigments were also successfully coated onto cellulose filter paper, cotton fabric, and cotton swabs, maintaining consistent pH responsiveness across substrates.
A Broader Platform For Optical Sensing
By combining covalent dye immobilisation with a structurally engineered microenvironment, the researchers achieved a rare combination of wide-range sensing, non-sigmoidal linear response, and long-term non-leaching stability in a single colorimetric platform. The approach addresses the fundamental weaknesses of conventional dye-based pH sensors while remaining compatible with portable, image-based analysis.
Journal Reference
Kim M. J., Shaban S.M., et al. (2026). A leaching-free nanopigment for extended linear colorimetric pH sensing over 10 pH units. Microsystems & Nanoengineering 12, 54. DOI: 10.1038/s41378-026-01163-