Analytica Chimica Acta 692 (2011) 109–115 Contents lists available at ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca Used gold nano-particles as an on/off switch for response of a potentiometric sensor to Al(III) or Cu(II) metal ions Mohammad Hossein Mashhadizadeh ∗ , Rasoul Pourtaghavi Talemi Faculty of Chemistry, Tarbiat Moallem University, Tehran, Iran article info Article history: Received 3 November 2010 Received in revised form 7 February 2011 Accepted 12 February 2011 Available online 18 February 2011 Keywords: Gold nanoparticle Mercaptosuccinic acid Potentiometric sensor Carbon paste electrode Hard soft metal ions abstract The potentiometric response of a carbon paste electrode modified with silica sol–gel and mercap- tosuccinic acid (MSA) in the presence and absence of gold nano-particles was studied. The results showed that the electrode with gold nano-particles was responded to Al 3+ ions as a hard metal ion. On the other hand, the electrode without gold nano-particles was responded to copper ions as a soft metal ion. The electrodes without and with gold nano-particles exhibits a Nernstian slope of 29.1 and 19.2 mV decade -1 for copper and aluminum ions over a wide concentration range of 4.3 × 10 -7 –1.0 × 10 -2 and 4.5 × 10 -7 –1.6 × 10 -3 mol L -1 , respectively. The detection limits of electrodes were 4.0 × 10 -7 and 1.6 × 10 -7 mol L -1 for copper and aluminum ions, respectively. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Ion selective electrodes offer direct and selective detection of ionic activities in samples. Such potentiometric devices are sim- ple, rapid, inexpensive, and compatible with on-line analysis. The inherent selectivity of these devices is attributed to highly selec- tive interactions between the membrane materials and the target ion. Many ion selective electrodes are commercially available and routinely used in various fields [1–7]. Natural carriers type ion-selective electrodes have been develop mainly with respect to alkali and alkaline earth metals; however, continuing investigations into the role of transition metals in bio- logical systems have created an interest in the use of transition metal ion-selective membranes [8]. A large number of ionophores, especially a wide variety of neutral macrocyclic polyethers, have been developed and found widespread applications in potentiometric sensors for the deter- mination of respective ions in real samples [9]. The selectivity behavior is obviously one of the most important characteristics of a sensor, as it often determines whether a reliable measurement in the target sample is possible. The potentiometric selectivity of these sensors among different ions is dictated mainly by the com- plexation specificity of the carrier molecules involved, but it may also be influenced to some extent by the membrane composition. ∗ Corresponding author. Tel.: +98 21 88848949; fax: +98 21 88820993. E-mail addresses: mashhadizadeh@tmu.ac.ir, mashhadizadeh@yahoo.com (M.H. Mashhadizadeh). The question of which metal ions prefer to form complex ions with which ligands was answered partly by Pearson during a critical review of hundreds of naturally occurring minerals and synthetic coordination compounds [10–13]. The general rule of thumb is that hard (Lewis) acids tend to combine with hard (Lewis) bases, and soft acids with soft bases. This is known as the hard–soft acid/base (HSAB) principle. In many reports the ligands with hard donor atoms (include the hydroxide and oxo anions, the phenolate side chains of ligands, phosphate groups, alkoxide groups (e.g., citrate, sugars), and car- boxylate groups) were used as ionophore for hard ions such as K + , Al 3+ and Na + , on the other hand [14–18], the ligands with soft donor atoms (for example with S atoms) were used as ionophore for soft ions such as Ag + , Hg 2+ , Cd 2+ and Cu 2+ [19–22]. The aluminum ion, with its relatively small radius and high ionic charge, is a prototypical hard metal ion, and thus is expected to form stable complexes with hard donors. In biological sys- tems, the most readily available hard donors include the hydroxide and oxo anions, the phenolate side chains of tyrosine, phosphate groups, alkoxide groups (e.g., citrate, sugars), and carboxylate groups. Copper is an essential element and is also toxic at high con- centration. Reactivity and biological uptake of copper are strongly influenced by its free ion concentration. Copper is both vital and toxic for many biological systems [23], so that its determination in water samples is warranted by the narrow window of concentra- tion between essentiality and toxicity [24]. Thus, the determination of trace amounts of copper from different matrices is of great importance. 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.02.028