Sensors and Actuators B 142 (2009) 347–354 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb pH sensor based on the crown heteropolyanion K 28 Li 5 H 7 P 8 W 48 O 184 ·92H 2 O immobilized using a layer by layer assembly process Malika Ammam a,b,∗ , Benita Keita a , Louis Nadjo a , Jan Fransaer b a Laboratoire de Chimie Physique, UMR 8000, CNRS, Université Paris-Sud, Bâtiment 350, 91405 Orsay Cedex, France b Department of Metallurgy and Materials Engineering (MTM), K.U. Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium article info Article history: Received 26 June 2009 Received in revised form 7 August 2009 Accepted 17 August 2009 Available online 25 August 2009 Keywords: pH sensor Layer by layer immobilization process Heteropolyanion K28Li5H7P8W48O184·92H2O abstract A new sensitive pH sensor based on immobilization of the crown heteropolyanion K 28 Li 5 H 7 P 8 W 48 O 184 ·92H 2 O (P 8 W 48 ) on a electrode surface through a layer by layer assembly pro- cess is described. The immobilization is based on the electrostatic adsorption of the complex in layers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH). The deposited P 8 W 48 /LBL film was investigated by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. Compared to the electrochemical behavior of dissolved P 8 W 48 , a slight shift in the redox peak towards negative potentials is observed, which have been attributed to a slight decrease in the acidity of the interior of the P 8 W 48 /LBL film compared to the testing buffer solution. The relationship between the peak currents of the deposited P 8 W 48 /LBL film and the number of layers is shown to be linear, which demonstrates that equal amounts of P 8 W 48 are adsorbed in each deposition layer. The P 8 W 48 /LBL modified electrode showed high sensitivities toward pH. Therefore, such electrodes were tested as pH sensors using the titration method. The resulting pH sensor has a detection range of pH 1–13, a sensitivity of 69 ± 2 mV/pH, high repeatability (<3 mV), fast response time (<7 s), low sensitivity toward change in ionic strength and nature of the supporting electrolyte, low internal resistance and a working life time of at least 3 months. Moreover, the sensor is easy to manufacture and can be easily miniaturized for measurements in micro- and nano-systems. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Since most chemical reactions are pH dependent, the accurate measurement of pH is an essential and crucial task. Despite the importance of accurate monitoring of pH in laboratories, clinics and industry, there are not many online techniques available to control the pH of the operational environment. The well-known pH glass electrodes are usually used for this purpose [1,2]. However, the use of these glass electrodes has several disadvantages: they are fragile, expensive, tricky to build, prone to attack by alkaline and fluoride solutions [3], and have a high output impedance and slow response. Wells–Dawson heteropolytungstates are a class of polyoxomet- alates (POMs) with applications in catalysis (acid and oxidation), medicine and in molecular magnetism [4–8]. One of the most important properties of these structurally well-defined early tran- sition metal oxide clusters POMs is their ability to undergo reversible multiple electron transfer reactions [9]. The reduction ∗ Corresponding author at: Department of Metallurgy and Materials Engineering (MTM), K.U. Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium. Tel.: +32 16 321260; fax: +32 16 321991. E-mail address: Malika.Ammam@mtm.kuleuven.be (M. Ammam). of the POM cluster results in an accumulation of negative charge, which increases the basicity of the POM anion. The reduction process may thus be accompanied by concomitant protonation. For these reason, the redox properties of POMs are remarkably pH sensitive. The crown heteropolysalt K 28 Li 5 H 7 P 8 W 48 O 184 ·92H 2 O (P 8 W 48 ) is remarkably stable and inert over a large pH range: litera- ture cites the range from 1 to 8 [10]. It is known from literature that the complex P 8 W 48 shows several quasi-reversible redox waves with peak potentials that are pH dependent [10]. These character- istics, including its sensitivity to pH and stability over a large pH range make this complex a good candidate for pH sensing. The immobilization process based on layer by layer (LBL) assem- bly was first reported by Decher and Hong [11]. The method is based on the alternate deposition of oppositely charged polyelectrolytes. The process is characterized by high stability, ease of manufactur- ing and good reproducibility. Recently, the process based on LBL deposition was used to deposit thin films of POMs for fundamental studies and applications especially for electrocatalysis [12–20]. In this work, we describe the immobilization of the crown het- eropolyanion P 8 W 48 in a LBL film for the manufacturing of a pH sensor. The deposited film was investigated using cyclic voltamme- try, potentiometry and electrochemical impedance spectroscopy. The electrochemical behavior of dissolved and immobilized P 8 W 48 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.036