Sensors and Actuators A 238 (2016) 8–18 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l ho me page: www.elsevier.com/locate/sna Sol–gel based fiber optic pH nanosensor: Structural and sensing properties Shumaila Islam a , Noriah Bidin a,∗ , Saira Riaz b , Ganesan Krishnan a,c , Shahzad Naseem b a Laser Centre, Ibnu-Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia b Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan c Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai 81310, Malaysia a r t i c l e i n f o Article history: Received 18 September 2015 Received in revised form 2 December 2015 Accepted 2 December 2015 Available online 4 December 2015 Keywords: Sol–gel method Fiber optic Encapsulated indicators pH sensing Silica–titania a b s t r a c t Evanescent wave absorption based PCS (plastic clad silica) fiber optic pH sensor is constructed by encap- sulation of indicator dyes in the heterogeneous inorganic hybrid silica–titania nanomatrix in the presence of cetyl trimethyl ammonium bromide (CTAB) surfactant by sol–gel method. A nanoporous multilayers (triple layer and six layers) of silica–titania hybrid is deposited on 5 cm uncladed middle portion of the fiber by dip-coating sol–gel technique. The CTAB surfactant expressively improved the host matrix structure and enhanced its porosity. Thermally stable, adhesive and smooth thin nanocladding has been examined using microscopic and thermal analysis. Indicators encapsulated nanomatrices are highly sen- sitive to pH and show optical output signals in terms of intensity. Therefore, it provides confirmation that interactions between the host matrix and pH-indicator molecules are in good assistance. Furthermore, experimental findings reveal that the sensing species are probably better encapsulated in the matrix and show no leaching of sensing molecules. The prepared nanosensors exhibit a much faster response time because of the material to be sensed diffuses faster into the triple-layer sensors <2 s. These pH sensors are found to have 60 days (2 months) age stability and high reproducibility. A repeatable response over a wide range of pH values between 3 and 11 is obtained. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Modern society has attracted considerable attention towards the exploitation of advanced chemical detection systems. Optical sensors relying on the various optical detection principles such as absorbance, reflectance, luminescence, fluorescence and excita- tions as a mode of transduction have proven to be highly effective in this regard [1–4]. The development of optical pH sensors has gained attraction because in several scientific research studies and industrial applications conventional pH glass electrodes are con- sidered unsuitable such as in chemical, biomedical, clinical and in environmental areas [5]. Moreover, optic fiber pH sensors can be implemented in all sorts of risky and hazardous environments such as deep-water analysis. Optical sensing would be most advan- tageous due to the feasibility of miniaturization, associated with exciting possibilities for remote sensing and in-situ measurement. ∗ Corresponding author. Fax: +60 75566162. E-mail addresses: shumaium@yahoo.com (S. Islam), noriah@utm.my, noriah.utm@gmail.com (N. Bidin), saira cssp@yahoo.com (S. Riaz), kganesan34@yahoo.com (G. Krishnan), shahzad naseem@yahoo.com (S. Naseem). Other reasons for the development of optical pH sensors include the lack of requirement of a reference sensor, resistance from electrical interference and improvements in electrical safety con- cerning clinical applications [2,6]. Hence, there is numerous efforts that have been focused towards the development of fiber optic pH sensors, in which pH indicator is physically and chemically immobilized into a polymer matrix [7–9]. Some of these are sum- marized in Table 1. Furthermore, San, Dantan, Dong, and Werner developed the pH chemical sensors by using the uncladed sur- face of the fiber which was coated with a material and was used as a sensing region for corrosion detection [10–13]. Zaggout stud- ied different dyes for sensing such as methyl orange [14], thymol phtalein [15] and a-naphtholphthalein [16]. These pH-indicator dyes physically get entrapped in the inorganic matrix by sol–gel method. Goicoechea [17] fabricated evanescent wave optical fiber pH sensors using layer-by-layer (LbL) electrostatic self-assembly technique. However, one major problem associated with many of these studies is leaching. Moreover, leaching of indicator leads to reduce fluorescence and absorption properties with loss of dye sensitivity, which makes impractical long-term use of the sen- sors [18]. However, monitoring the changes in intensity based signals can often be inconvenient or unreliable due to limitations in http://dx.doi.org/10.1016/j.sna.2015.12.003 0924-4247/© 2015 Elsevier B.V. All rights reserved.