Sensors and Actuators B 230 (2016) 485–492 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb Surface plasmon resonance based fiber optic ethanol sensor using layers of silver/silicon/hydrogel entrapped with ADH/NAD Vivek Semwal a , Anand M. Shrivastav a , Roli Verma b , Banshi D. Gupta a,∗ a Physics Department, Indian Institute of Technology Delhi, New Delhi 110016, India b School of Chemistry, Reymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Israel a r t i c l e i n f o Article history: Received 30 October 2015 Received in revised form 21 January 2016 Accepted 18 February 2016 Available online 23 February 2016 Keywords: Surface plasmon resonance Hydrogel entrapment Ethanol Sensor Optical fiber Enzyme a b s t r a c t In this study an approach for the fabrication and characterization of a surface plasmon resonance (SPR) based fiber optic sensor for the detection of ethanol in 0–10 mM concentration range has been reported. The sensing probe has been prepared by coating films of silver and silicon over the unclad portion of an optical fiber followed by immobilization of enzyme (ADH) and coenzyme (NAD) entrapped hydrogel layer. The sensor works on spectral interrogation mode of operation. The resonance wavelength has been found to decrease with the increase in ethanol concentration. For the concentration range 0–10 mM, a 30 nm shift in resonance wavelength has been observed. The maximum sensitivity of the sensor has been found to be 21.70 nm/mM near zero concentration of ethanol solution. The calculated values of limit of detection (LOD) and limit of quantification (LOQ) of the sensor are 15.34 M and 29.63 M respectively. In addition, the sensor has high selectivity, repeatability and stability. Further, the sensor has various other advantages such as ease of operation, low cost, and fast response. Since the probe has been fabricated on optical fiber, it can work for online monitoring and remote sensing of analyte without any interference of electromagnetic fields. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Ethanol is an alcoholic compound which is mostly found in food, drinks and beverages. The fermentation of fruit sugar by yeast gives the ethanol and this is the natural source for its pro- duction. It plays a very important role in clinical and forensic analysis in order to analyze human body fluids, e.g., blood, urine, breath, serum, saliva, sweat etc. The concentration range of ethanol in ripe fruits is 8.68–156.25 mM. Similarly the concentrations of endogenous ethanol in healthy human blood as well as in vari- ous metabolic disorders patient like diabetes, hepatitis, cirrhosis range from 0–0.017 mM [1,2]. These concentrations are very small to have any medical and forensic implication. Hence the detec- tion of ethanol in above concentration range is important. For the detection of ethanol, a number of techniques/methods have been reported in the literature such as amperometric, gas chromatogra- phy and liquid chromatography [3–5]. These techniques/methods are slow, expensive and take lot of time for analysis. To overcome ∗ Corresponding author. E-mail addresses: bdgupta@physics.iitd.ernet.in, banshigupta@yahoo.co.in (B.D. Gupta). the problems of various ethanol sensors, development of a fast, cheap and having low response time sensor is required. In the last few years, hydrogels have been used in various biosensing applications because hydrogel provides admirable envi- ronment for enzymes, coenzymes and other biomolecules and maintains their activity [6]. Hydrogels are the cross-linked poly- mer networks which have the ability of swelling/shrinking in an aqueous medium. The swelling/shrinking of hydrogels depends upon its cross-linking mechanism. According to its mechanism, it is divided into two categories: (i) physically cross-linked hydrogels, and (ii) chemically cross-linked hydrogels. Physically cross-linked hydrogels are formed by non-covalent network between the polymer and cross-linking agent, while chemically cross-linked hydrogels are formed by covalent network. With the help of inter- action between polymer and cross-linking agent, one can find the swelling/shrinking ratio of the hydrogel. Highly cross-linked hydrogels have small void size into their matrix while loosely cross- linked hydrogels have large void size. The swelling ability in highly cross-linked hydrogels is less as compared to loosely cross-linked hydrogels [7]. In the past few decades, surface plasmon resonance (SPR) has emerged a very promising and useful tool for the detection of var- ious analytes, biological substances, bacterial detection and food safety etc. [8,9], due to its advantage of rapid and label free sensing. http://dx.doi.org/10.1016/j.snb.2016.02.084 0925-4005/© 2016 Elsevier B.V. All rights reserved.