Fiber-optic cation determination using crown ether dyes immobilized on polymer membranes

Abstract

This study describes the determination and comparison of some cations such as Na⁺, K⁺, Ba²⁺ and Cu²⁺ in aqueous media by using crown ether dyes immobilized onto thin (0.04 mm) sulfonated kraton polymeric membranes. The studies have been carried out in aqueous solutions first, and in dye–immobilized membrane systems later. The flow-cell system has been found to be more practical in application and had lower response times (30–60 s) as well as detection limits (2.24×10⁻¹⁰–1.19×10^-8 mol l⁻¹). The optimum pH range for the formation of ion-dye complexes have been found to be 5 and 9 for the ions above. Since the complexation reactions were reversible it was possible to regenerate the membranes by using HCl solutions and reuse them many times. The same membrane showed high reproducibility and low relative standard deviation. In addition, the interferences of the studied cations to each other and of other various cations during the determinations have also been investigated.

Introduction

Fiber optic chemical sensors consisting of immobilized indicators coupled to a spectrometer provide continuous, low cost, in situ measurements of metal ions. Recently, there has been growing interest in low cost continuous analysis using chemical sensors while the use of commercially available ion sensitive sensors recommended for many metal ions have many disadvantages, such as the requirement for measurements under well-defined conditions for such parameters as pH, temperature, ionic strength and interfering ions.

In newly developed sensors high strength, stability, and long-operational life and such properties as short response time low detection limit, high reproducibility and low cost are highly desirable. The quantitative determination of alkali, alkaline earth and heavy metal ions, is particularly important in clinical and environmental analysis. For the determination of alkali, alkaline earth and heavy metal ions, a variety of methods for immobilizing indicator dyes and attaching them to optical fibers are available [1], [2], [3], [4].

Among the compounds with rapidly increasing application in analytical chemistry are the crown ethers and related compounds [5]. These dyes, with added chromophoric functional groups within the molecule, are designed to bring about specific color changes on the interaction with metal cations, as alkali, alkaline earth and heavy metals [6] and exhibit high ion-selectivities according to the pH of the medium [7], [8], [9], [10]. Therefore, they can serve as probes or photometric reagents for these metal ions. The chromophoric groups of these dyes can bear either dissociable protons or can be non-ionic.

A recent publication concerning a Cu²⁺ ion-selective sensor has reported a detection limit of 1.0×10⁻⁵ M Cu²⁺ ion in non-buffered medium [11]. The desirable detection limit of our sensors is expected to be comparable to that for the atomic absorption spectrometry [12]. In that case, the sensors developed in this study will present fewer problems in matrices associated with environmental samples when compared with AAS. Hence, the pretreatment of the samples will usually not be necessary.

The aim of this study is to develop new fiber optic sensors using recently synthesized crown ether dyes immobilized on kraton polymer membranes for the determination of Na⁺, K⁺, Ba²⁺ and Cu²⁺. Among the desired characteristics for the sensor to be developed are low detection limit, high reproducibility, short response time, ease of use, high longevity, as well as, better remote sensing capability when compared with sensors reported in recent literature. The capability of the sensor to monitor and measure with both flow and non-flow systems is also among the aims. In addition, the interferences of various cations during the analysis have also been investigated.

For these purposes, two crown ether dyes, one of which is ionic and the other neutral have been synthesized and immobilized on the separate membranes individually. They are: (4-Nitrophenyl) [4-(4,7,10,13,16-pentaoxa-1-azacyclooctadecyl) phenyl] diazen (NCED) and 1-methyl-{2-[4-(4,7,10,13,16 pentaoxa-1-azacyclooctadecyl) phenyl] ethenyl}pyridinium iodide (ICED) [13].

The polymer system used in this study was thin kraton membranes (sulfonated hydrocarbon-based block copolymer ionomer) which offer attractive properties as sensors for various metal ions based on absorbance or reflectance measurements as well as swelling-shrinking characteristics. The dye-immobilized membranes were suitable for various cation determinations through complexation.