Optical Materials 127 (2022) 112285 0925-3467/© 2022 Elsevier B.V. All rights reserved. Self-assembled phenolphthalein functionalized zincite doped silica-anatase nanocomposite as fast responsive optical pH sensor Shumaila Islam a, b, * , Adil Alshoaibi a, b a Al Bilad Bank Scholarly Chair for Food Security in Saudi Arabia, The Deanship of Scientifc Research, The Vice Presidency for Graduate Studies and Scientifc Research, King Faisal University, Al Ahsa, Saudi Arabia b Department of Physics, College of Science, King Faisal University, Al-Hassa, P.O. Box 400, Hofuf, 31982, Saudi Arabia A R T I C L E INFO Keywords: Sol-gel technique Zinc oxide nanoparticles Silica-titania Phenolphthalein pH sensing ABSTRACT A pH evaluation is required in numerous felds of life such as environmental monitoring, agriculture, and food science research. The chemical properties (acidity and basicity) of aqueous solutions can be determined by their pH values. Therefore, zincite-supported silica-anatase nanocomposite (ZSAC) is synthesized by the sol-gel method. Owing to the broad pH range 1–12 and fast response pH sensing characteristics, phenolphthalein is immobilized in ZSAC. FESEM analysis exhibited that both nanocomposites exhibited porous and self-assembled structures. Thermally stable ZSAC demonstrates low Ra (surface roughness) 1 nm, refractive index (n) 1.48 at 633 nm, and surface area 282 m 2 /g which varied to Ra 4 nm, n around 1.5, and surface area 345 m 2 /g after php immobilization. Both nanocomposites show heterogeneous chemical bonding. PZSAC revealed high pka around 11, fast color response 0.68 s in pH 12, and non-leachable behavior. Experimental fndings suggested that synthesized nanocomposite has the capability for opto-chemical sensing applications. 1. Introduction In scientifc and industrial areas specifcally agriculture, environ- mental monitoring, and food science research, the colorimetric pH sensor has great importance due to its simplicity, visual detection, and strength [1]. Optically active nanocomposites-based pH sensors are signifcant in molecules detection for clinics [2]. In the chemical in- dustry, biomedical engineering, environmental monitoring, food pro- cessing, and in many scientifc felds, glass electrodes, paper strips, and acid-base indicators, are utilized for pH analysis. Though, complex procedures and regular calibration is limited their utilization [2,3]. Hence, optical pH sensors are effective substitutes over conventional sensors, such opto-sensors work through their refectivity, fuorescence, and absorption against pH variations. The dye immobilized silica has considerable attention from researchers due to its high transmission and synthesis simplicity [4]. However, dye immobilized silica suffers from dye leaching from the matrix due to particles agglomeration which caused coating surface damage [5]. For smooth morphology and low roughness, silica is doped with titania which can lessen the sol surface tension, compel the formation of polymerizing gel, and improve sensing abilities [6,7]. Additionally, the sensing properties can be enhanced by mesoporous structures than dense flm due to large surface area and large pores which allow a more effective diffusion. However, titania is not effcient for sensing because of its small surface area and inadequate crystalline structure. The small size anatase crystallites increased the oxidizing and reducing capability and self-assembled the surfactant micelles. The growth of nanocrystalline silica-anatase in such porous structure is sterically limited by the TiO 2 pore walls thickness [8]. Therefore, the zincite doping in silica-titania caused mesoporosity, and crystallite size is homogeneously distributed throughout the matrix structure. The tenability of TiO 2 through doping of transition metal ions like manganese, chromium, cobalt, nickel, iron, zinc, etc. is vastly re- ported [9]. Among these transition metals, Zn 2+ caused effective by doping with TiO 2 because Ti 4+ ionic radii (0.75 Å) are like Zn 2+ ionic radii (0.74 Å), therefore, Zn 2+ ion can certainly substitute Ti 4+ ion in titania matrix without any detrimental of structure [9]. For sensing analysis, phenolphthalein (php) is a color change indicator and is mostly used as a temperature resistant, and optoelectronic photosensor. Moreover, php is a pH-reactive dye that experiences molecular confor- mational variations at a changeover pH i.e., pH1-8.2 (colorless) and pH 8.2–12 (pink). There are various methods reported for the synthesis of SiO 2 , TiO 2 , and ZnO nanoparticles such as hydrothermal [10], * Corresponding author. Al Bilad Bank Scholarly Chair for Food Security in Saudi Arabia, The Deanship of Scientifc Research, The Vice Presidency for Graduate Studies and Scientifc Research, King Faisal University, Al Ahsa, Saudi Arabia. E-mail addresses: siislam@kfu.edu.sa (S. Islam), adshoaibi@kfu.edu.sa (A. Alshoaibi). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2022.112285 Received 22 February 2022; Received in revised form 26 March 2022; Accepted 28 March 2022