Talanta 68 (2006) 1469–1473 Wide range pH measurements using a single H + -selective chromoionophore and a time-based flow method Afsaneh Safavi ∗ , Abolfazl Rostamzadeh, Saeed Maesum Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran Received 2 June 2005; received in revised form 12 July 2005; accepted 3 August 2005 Available online 20 October 2005 Abstract A hydrophilic transparent triacetyl cellulose membrane was adopted as a pH optode by immobilizing highly selective and sensitive Nile blue indicator on the membrane. Contrary to the common procedure for determinations using optodes, in which a steady state response is measured, a new approach is introduced in which the dynamic response of the optode is used as the analytical signal. While in common procedures, pH optodes exhibit limited linear dynamic range (often 2–4 pH units only), it is shown that in a time-based flow method, an optode with only one acid-base indicator can be used for measurement in the pH range of 0–10. The procedure is simple, inexpensive and rapid. © 2005 Elsevier B.V. All rights reserved. Keywords: Wide range pH measurement; Dynamic method; Optode 1. Introduction Optodes have been actively investigated for their potential in practical uses, such as clinical analysis, environmental anal- ysis, biotechnology and process control. They are suitable for remote sensing and applications where conventional electrodes cannot be used because of their high cost, instability in aggres- sive or high pressure environments, large size, breakable nature, or the risk of electric shock during the in vivo measurements. Also, optical sensors have been developed for chemical analytes or physical parameters for which electrodes are not available [1]. The pH optodes mostly utilize pH indicator dyes in which several kinds of dye immobilization methods in the bulk or sur- face optode sensing phase were reported with unique techniques, such as trapping in dialysis tubing, adsorbing in polymer beads, covalently immobilizing on to porous glass or a cellulose mem- brane and ionically immobilizing on to an anion-exchange resin or sulfonated polystyrene surface [1,2]. Some pH optomem- branes based on cellulose have been prepared for much useful applications [3–5]. In contrary to others, these hydrophilic mem- branes do not need any modification for response time or stability improvement. ∗ Corresponding author. Tel.: +98 711 2284822; fax: +98 711 6305881. E-mail address: safavi@chem.susc.ac.ir (A. Safavi). A major disadvantage of using these types of sensors is that their responses exhibit limited linear dynamic range (often 2–4 pH units only). Several approaches have been proposed or employed in order to extend the pH range of these sensors. These include co-immobilizing indicators having complemen- tary pH responses and multiplexing several optical pH probes [6]. The extension of pH response range of a sensor from its narrow linear range (pH 5–7.25) to the full calibration range (pH 2.51–9.76) has been performed using artificial neural net- work (ANN) [7]. A multilayer feed-forward (ANN) was used to model the input–output data of an optical-fiber pH sensor [8]. Recently, we described the development of an optical pH sensor based on immobilization of a mixture of two dyes on a triacetyl- cellulose membrane [9]. The sensor has a useful pH range at low and high pH values, where glass electrodes show acidic and alkaline errors, respectively. Application of a back-propagation artificial neural network model extended the measuring range of the proposed optode to the whole pH range. While in batch assays, it is necessary to operate via steady state conditions, flow analysis methods have proved that this is not essential, allowing the treatment of optodes with samples in a controlled manner and under precise conditions gives repro- ducible, accurate and precise analytical results [2]. Although there are some reports on the dynamic model of the pH optodes [2,10], the quantitative studies of dynamic pH response of optodes have not been fully discussed in the past. 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.08.039