Effect of pH on phenolphthalein immobilized gold nanoparticles/nanostructures for pH sensing evaluations: sol–gel method

Sol–gel-based phenolphthalein immobilized AuNPs (phph-AuNPs) of different shapes such as spherical, dendrites, and flower are observed by varying pHs 1–12. pH 1 induced the Au cages, and pH 2–6 revealed the different shapes of dendrites. Moreover, pH 7–8 assisted phph-AuNPs exhibited the spherical shape which transformed into truncated dendrites such as flowers at pH 9–11, and again spheres at pH 12. FTIR spectra exhibited the bonding vibration of AuNPs and phenols. pH 12 assisted phph-AuNPs are observed highly porous (79%) compared to pH 1 assisted phph-AuNPs (60%) porosity and low (1.1) refractive index. Furthermore, pH 1 assisted phph-AuNPs are found to possess RMS 11 nm and thickness 56 nm which increased up to 23 and 88 nm, respectively, by increasing the pH up to 12. The pKa ~9.4 of the pH 1–12 assisted phph dye is observed at 559 nm which increased up to 9.9 at 556 nm for pH 1–12 assisted phph-AuNPs, suggesting a wide range of sensitivity. Fast response time ~0.6 s is measured at pH 12 assisted phph-AuNPs. At acidic pH, higher photoluminescence (PL) intensity indicates a high charge carrier’s recombination. Experimental findings confirmed that pH has a great influence on the morphology of phph-AuNPs.