New polymeric membrane cadmium(II)-selective electrodes using tripodal amine based ionophores

https://doi.org/10.1016/j.aca.2011.08.005Get rights and content

Abstract

Fabrication of PVC membrane electrodes incorporating selective neutral carriers for Cd2+ was reported. The ionophores were designed to have different topologies, donor atoms and lipophilicity by attaching tripodal amine (TPA) units to the lipophilic anthracene (ionophore I) and p-tert-butylcalix[4]arene (ionophores II, III and IV). The synthesized ionophores were incorporated to the plasticized PVC membranes to prepare Cd(II) ion selective electrodes (ISEs). The membrane electrodes were optimized by changing types and amounts of ionic sites and plasticizers. The selectivity of the membranes fabricated from the synthesized ionophores was evaluated, the relationship between structures of ionophores and membrane characteristics were explored. The ionophore IV which composed of two opposites TPA units on the calix[4]arene compartment showed the best selectivity toward Cd2+. The best membrane electrode was fabricated from ionophore IV (10.2 mmol kg−1) with KTpClPB (50.1 mol% related to the ionophore) as an ion exchanger incorporated in the DOS plasticized PVC membrane (1:2; PVC:DOS). The Cd-ISE fabricated from ionophore IV exhibited good properties with a Nernstian response of 29.4 ± 0.6 mV decade−1 of activity for Cd2+ ions and a working concentration range of 1.6 × 10−6–1.0 × 10−2 M. The sensor has a fast response time of 10 s and can be used for at least 1 week without any divergence in potential. The electrode can be used in the pH range of 6.0–9.0. The proposed electrodes using ionophores III and IV were employed as a probe for determining Cd2+ from the oxidation of CdS QDs solution and the real treatment waste water sample with excellent results.

Highlights

► New four ionophores having tripodal amine (TPA) unit on anthracene and calixarene. ► Synthesis and characterization data were reported. ► Incorporated to the plasticized PVC membranes to prepare Cd-ISEs. ► Two TPA units on calixarene showed the best selectivity toward Cd2+. ► Applied for sensing Cd2+ from the oxidation of CdS QDs solution.

Introduction

Cadmium, one of the transition metals, occurs as a minor component in most zinc ores [1]. It has been used as a pigment and corrosion resistant plating on steel [2]. Cadmium compounds have been used widely in electroplating, battery industry and chemical industry for stabilizing plastics [2]. Cadmium is a component of semiconductors such as cadmium sulfide [3], cadmium selenide [4] and cadmium telluride [5] which are used for light detection or solar cell [6]. Exposure of cadmium can contaminate food and water. Cadmium can accumulate, store in living organisms and may lead to cancer [7]. Therefore, the determination of cadmium has become increasingly important because its toxicity and the increasing level of extended use in industry.

There are many analytical techniques for determination of cadmium in samples such as atomic absorption spectrometry [8], [9], [10], [11], inductively coupled plasma-atomic emission spectrometry [12], [13], [14], inductively coupled plasma-mass spectrometry [15], [16], [17], stripping voltammetry [18], [19], [20], [21], spectrophotometry [22], [23], [24] and ion chromatography [25], [26], [27], [28]. These methods required expensive instruments and time-consuming sample preparation. During the past three decades, ion selective electrodes (ISEs) were developed to use in many fields including clinical chemistry, cosmetics, process control, agriculture and environmental analysis. Membrane ion selective electrodes containing ionophores are successfully used for monitoring and determination of several metal ions [29], [30] with many advantages such as good selectivity, high sensitivity, good reversibility, convenient, inexpensive and rapid for analysis [31].

Extensive efforts were put into the synthesis of ionophores with high selectivity for Cd2+ recognition [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. The sensors were used successfully in various fields such as water monitoring and waste water treatment. Many researchers used crown ethers as ionophores in Cd2+ selective electrodes. Gupta and co-workers prepared membrane potentiometric sensors based on crown ethers that showed good membrane characteristics [32], [33], [34]. The sensors showed selectivity toward Cd2+ upon adding benzene or cyclohexane constituents to the flexible crown ethers leading to their stiffening and altering the binding strength to complex Cd2+ selectively. The selectivity also depended on the size of the crown ether ring. Other crown ethers were also demonstrated as ionophores for Cd2+. Monoaza-18-crown-6 [35] and benzo-15-crown-5 [36] were used as neutral carriers in plasticized PVC membrane with good selectivity toward Cd2+ over other studied interfering ions. Shamsipur and Mashhadizadeh reported the successful use of tetrathia-12-crown-4 in fabrication of a membrane ion selective electrode for cadmium [37]. The electrode displayed low detection limit, fast response, wide pH range and lifetime for at least 6 weeks. A potentiometric sensor for cadmium based on tetrol compound was prepared by the same research group [38]. The ligand tetrol with flexible tri-dimensional structure and four hydroxyl groups can bind Cd2+ to form a stable 1:1 tetrol–Cd2+ complex.

Later on Gupta and coworker prepared Cd2+-ISE using Schiff base ionophores [39]. The selectivity of the Cd2+ electrode systems increased to a larger extent with the increase of amounts of anionic additives in the membrane. Moreover, they reported the use of two neutral ionophores, Schiff base-based o-phthalaldehyde (L1) and 4-hydroxynaphthalaldehyde (L2), for fabrication of Cd-ISE. The electrodes fabricated from ionophore L1 exhibited a remarkable low detection limit in nanomolar concentration level with a fast response time (11 s) [40]. Ensafi el al. employed 4-hydroxy salophen, a good complexing ligand for metal ions, as an ionophore in a Cd2+ PVC membrane potentiometric sensor [41]. This Cd2+-ISE showed a good characteristic of the membrane sensor in term of reproducible results and good selectivity. Rezaei and co-workers explored N,N′-(4-methyl-1,2-phenylene)diquinoline-2-carboxamide as a new neutral carrier for the recognition site of cadmium(II)-PVC membrane electrode [42]. The prepared Cd2+-ISEs exhibited a response for Cd2+ over a wide concentration range with good selectivity. Ionophores based on three dimensional lipophilic compounds such as calixarene were also reported. Gupta and colleague reported the used of t-butyl thiacalix[4]arene and thiacalix[4]arene as Cd2+ ionophores. The electrode membrane incorporated thiacalix[4]arene showed better characteristics than the other. The sensor gave a linear potential response for Cd2+ over a wide activity range with Nernstian compliance (29.5 mV decade−1 of activity) and a fast response time of ∼8 s [43].

Thus far, there are a few Cd2+ ionophores based on coordination bonding from nitrogen donating ligands. To construct ISEs for Cd2+, choosing the appropriate recognition unit to recognize Cd2+ selectively is very challenging. Recently, tripodal ligands such as tris(2-pyridylmethyl)amine (TPA) have been employed as chemosensors for Zn2+ [44], [45]. The selectivity of this type of ligand toward metal ions depends on the preorganization of its structures. Both Zn2+ and Cd2+ are d10 metal ions and have similar coordination chemistry [46]. However, Cd2+ is a bigger ion than Zn2+, and the metal center can accommodate 4–6 ligands. Therefore, we have designed 4 new ionophores containing a different number of the TPA units. Ionophore I contained one TPA group linked to the anthracene unit. Ionophores II, III and IV have been designed to have a different number of the TPA unit connected to the calix[4]arene framework. Ionophores IIIV are expected to form complexes with Cd2+ to a different extent.

In this work, electrode characteristics such as the formation constant of each ionophore toward Cd2+, membrane composition optimization, selectivity, working pH range, reversibility of the sensor and the electrode life time were fully examined. Moreover, the relationship between the membrane characteristics and the topology of the ligands, the number of the donor atoms and the lipophilicity of ionophores was explored. The use of the proposed electrode as the sensor for Cd2+ releasing from nanocrystalline CdS quantum dots solution and Cd2+ in the real treatment water sample was also carried out.

Section snippets

Reagents

Anthracene-9-carbaldehyde was purchased from Aldrich. The precursor compounds, 2-(bis(2-pyridylmethyl)aminomethyl)aniline (1), 25-2(2-ethyloxybenzaldehyde)-26,27,28-triihydroxy p-tert-butylcalix[4]arene (2), 25,27-(4,4′-bis(ethoxybenzaldehyde))-26,28-dihydroxy p-tert-butylcalix[4]arene (3) were synthesized using the previously published procedure [44], [47], [48]. High molecular weight poly(vinylchloride) (PVC), tetrakis[3,5-bis(trifluoro-methyl)phenyl]borate (NaTFPB), potassium tetrakis (p

Design and synthesis of ionophores

Recently, many researchers have employed calix[4]arene as a three-dimensional molecular building block to attach particular functional units to their skeleton for selective binding with different metal ions. Reinhoudt and co-workers reported the use of calix[4]arenes substituted with dithioamide functionalities to prepare chemically modified field effect transistors (CHEMFETs) possessing high selectivity toward Cd2+ [52], [53]. Therefore, we attached the TPA unit to our modified anthracene and

Conclusion

New four neutral carriers based on tripodal amine units were successfully incorporated to plasticized PVC membranes for detection of Cd2+. The membrane compositions such as type of plasticizer and ionic additive, percentage of ionic additive, were optimized to obtain the best characteristic Cd-ISE. The optimized membrane showed response to the change of the activity of Cd2+ with Nernstian's slope except the membrane fabricated from ionophore I which is not a calix[4]arene based ionophore. The

Acknowledgements

This research was financially supported by the Thailand Research Fund (RTA5380003 and MRG5380064), the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University and the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education. SW is a Ph.D. student under support of the Royal Golden Jubilee Ph.D. Program

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