Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Thermally stable Au decorated silica-titania mesoporous nanocomposite for pH sensing evaluation Shumaila Islam a , Hazri Bakhtiar a , Adil Alshoaibi b , Saira Riaz c , Shahzad Naseem c a Physics Department, Faculty of Science, King Faisal University, Al-Hassa, P.O. Box 400, Hofuf 31982, Saudi Arabia b Laser Centre, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, Skudai, Johor 81310 Malaysia c Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan ARTICLE INFO Keywords: Sol-gel method Au nanoparticles Fiber optic sensing Mesoporous composite ABSTRACT Herein, doping/decoration of gold nanoparticles (AuNPs) within mesoporous silica-titania nanocomposite is achieved via a facile and co-assembly sol-gel method. Polyethylene glycol is used as a co-structure-directing- agent. For sensing analysis, a mixture of organic dyes i.e., bromophenol blue, phenol red, and cresol red is encapsulated in the AuST nanocomposite matrix. FESEM/EDX analysis shows a crack-free surface, porous net- work and uniform distribution of Au, Ti, Si, along with dye species. FTIR and XRD suggested the heterogeneous chemical bonding and crystallite size 24 nm after encapsulation. AuST nanocomposite shows thermally stable behavior after 450 °C even after co-dyes encapsulation. High surface area 322 ± 2.5 m 2 /g, pore diameter 30.2 Å, and pore volume 0.24 cm 3 /g, average surface roughness 10 nm, and refractive index 1.29 is advanta- geous for good sensing response at pH 1–12. The sensitivity is measured as 10 counts/pH at 441 nm. Moreover, good reversible response, fast response time 2.1 sec in acidic media, and 0.9 sec in basic media is observed. 1. Introduction Owing to the unique properties of metal-based nanocomposites such as high purity, narrow particle size distribution, small crystallite size, and well-defined morphologies with the miniaturization of the func- tional device make them a most diverse class of materials in emergent technologies: catalyst, sensor, energy conversion, and storage [1]. However, the thermal stability of nanocomposites is a severe appre- hension in numerous aspects including gas-phase reactions at > 250 °C temperatures. At elevated temperatures, the sintering of nanoparticles (NPs) is mostly irreversible due to the nanosized particles fusion gov- erned by Ostwald ripening of surface atoms. This, leads to the decrease of surface area and active sites because of NPs aggregation, as docu- mented in the literature [2]. Nevertheless, coalescence or overgrowth of NPs in the solution can be controlled by the utilization of surfactants as surface ligands. AuNPs with PEG (surfactant) linker carrying two car- boxylic groups enhances the properties of interfaces. Politi et al., [3] reported that functional groups at the AuNPs surface enhanced the nanoparticles interaction ability with target analytes. For 2-D confined surfaces to stabilize the metal nanoparticles (MNPs) under harsh con- ditions is difficult. Therefore, at high temperatures, a fusion of MNPs, irreversible sintering, or migration to the surface of mesoporous fra- meworks mostly occurred. This problem can be overcome by embed- ding the AuNPs either within thermally stable shells or oxide supports. In the case of core-shell nanostructures, shells can isolate the AuNPs but with limited mass transport of reactants to penetrate the shells irre- spective of preventing interparticle coalescence. While, large surface area and well-defined porosity based oxides support to embed the AuNPs and allow the fast mass transfer. The adsorption/incorporation of AuNPs in the oxide matrix regulates the interface and modifies the pathways through which photogenerated charge carriers endure surface reactions or recombination and has potential for optical applications. Moreover, aggregation and reshaping of AuNPs in the porous oxide matrix influenced the characteristics of nanocomposites such as their structures, and phase transformation [4]. The hybrid nanocomposite, i.e. silica-titania has unique characteristics than only pure silica and titania alone. Although, silica is an intrinsically non-toxic material, environmentally friendly, and supporting their use in vivo diagnosis. It is photo physically inert, while, it’s photochemical, photodecomposi- tion, and other characteristics can be described by other doping mate- rial, i.e. titania as additives could control the structure-sorption, optical, and sensing properties [4]. Silica-titania (ST) composite has high sta- bility and a large specific surface area. Hence, due to the thermal sta- bility of ST composite, they can be used in high temperature based materials applications [5]. Furthermore, there are several methods to synthesize the AuNPs: thermal decomposition [6], vapor deposition [7], by radiation [8], electrochemically [9], reduction in microemulsion [10], Turkevish method [11], seeded growth method [12], and pulsed https://doi.org/10.1016/j.apsusc.2020.146329 Received 14 January 2020; Received in revised form 27 March 2020; Accepted 13 April 2020 E-mail addresses: shumaium@yahoo.com (S. Islam), hazri@utm.my (H. Bakhtiar), adshoaibi@kfu.edu.sa (A. Alshoaibi). Applied Surface Science 521 (2020) 146329 Available online 18 April 2020 0169-4332/ © 2020 Published by Elsevier B.V. T