Sensors and Actuators B 149 (2010) 122–128 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb A ﬁbre-optic humidity sensor based on a porous silica xerogel ﬁlm as the sensing element Juncal Estella, Pablo de Vicente, Jesús C. Echeverría, Julián J. Garrido ∗ Departamento de Química Aplicada, Universidad Pública de Navarra, Campus Arrosadía, 31006 Pamplona, Navarre, Spain article info Article history: Received 23 December 2009 Received in revised form 17 May 2010 Accepted 7 June 2010 Available online 17 June 2010 Keywords: Humidity sensor Water vapour Fibre-optic sensor Xerogel Silica porous ﬁlm Sol–gel process abstract We have prepared a ﬁbre-optic (FO) sensor for humidity measurements with a porous silica xerogel ﬁlm as the sensing element. This ﬁlm was synthesised by the sol–gel process and ﬁxed onto the end of the optical ﬁbre by the dip-coating technique. This intrinsic sensor works in reﬂection mode, and the transduction takes place in the light that travels through the core of the ﬁbre. A novel measuring cell, which works under volumetric static conditions, has also been designed and developed to evaluate the analytical sensor response without interference from atmospheric gases or vapours. The sensor was capable of sensing a relative humidity (RH) from 4 to 100%. The response time of the sensor varied from 10 s to less than 2 min, depending on the RH percentage and the measuring procedure. © 2010 Published by Elsevier B.V. 1. Introduction Humidity sensors are widely used in a variety of applica- tions, such as meteorological services, air conditioning, health and medical care, civil engineering, environmental control, chemical and food processing, as well as in paper, textile and automobile industries [1–3]. Humidity sensing devices should fulﬁl several requirements [4], including sufﬁcient sensitivity over a wide range of humidity and temperature values, short response time with small or null hysteresis, long operational life, low temperature effect, resistance to contaminants and low cost and maintenance. In some cases, low weight and compatibility with a microprocessor may also be required. Extensive reviews on humidity sensor meth- ods have been recently published [1,2,5]. These methods either probe the fundamental properties of water vapour, such as dew temperature, latent heat transfer and absorption of optical radia- tion, or use several transduction methods that rely on capacitive, resistive and mechanical approaches. Fibre-optic (FO) sensors offer a new approach to humidity sensing. They are suitable for use in an environment of poten- tially hazardous or explosive nature and also in situations where requirements such as immunity to electromagnetic interference, high resistance to chemical species, multi-sensor operation, and in situ and remote monitoring are required [5–8]. FO-based humidity ∗ Corresponding author. Tel.: +34 948 169601; fax: +34 948 169606. E-mail address: j.garrido@unavarra.es (J.J. Garrido). sensing techniques include direct spectroscopic methods, evanes- cent waves, in-ﬁbre grating and interferometric methods. A variety of chemical reagents have been used as sensing elements to pre- pare FO humidity sensors, such as cobalt chloride (CoCl 2 ) [9], cobalt oxide (Co 3 O 4 ) [10] or crystal violet [11]. In most optical sensors, the sensitive reagents are immobilised in a solid matrix and attached to the ﬁbre. The matrix serves to encapsulate the reagent such that it is accessible to the analyte while being impervious to leaching effects. A variety of polymers are being used in optical sensors, includ- ing silicones, poly(vinylchloride) (PVC), poly(tetraﬂuoroethylene) (PTFE), agarose and cellulose derivatives [12,13]. The choice of the support matrix may inﬂuence the performance of the sen- sor, especially its selectivity and response time and is governed by parameters like mechanical stability, permeability to the analyte and suitability for reagent immobilisation [14]. Sol–gel materials have also been widely used as matrix supports to immobilise sensitive reagents into which analyte molecules may diffuse and interact [13–16]. The porous silica materials synthe- sised by the sol–gel process combine several physical and chemical properties appropriate to sensor preparation, such as chemical and thermal stability, an inert nature and transparency over a wide range of wavelengths. Furthermore, the textural properties can be tuned by properly choosing the experimental parameters of pH, the water:precursor:solvent molar ratio, temperature, drying conditions and aging media [17,18]. Thin ﬁlms may be prepared from homogeneous colloidal suspensions obtained before gelation by means of the deposition techniques of spin and dip-coating. Thin ﬁlms require only small amounts of precursors, can embed 0925-4005/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.snb.2010.06.012