Article pubs.acs.org/ac
Dithizone Modified Gold Nanoparticles Films for Potentiometric Sensing Emilia Woźnica, Michał M. Wójcik, Marcin Wojciechowski, Józef Mieczkowski, Ewa Bulska, Krzysztof Maksymiuk, and Agata Michalska* Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland S Supporting Information *
ABSTRACT: For the first time, application of a membrane composed of gold nanoparticles decorated with complexing ligand for potentiometric sensing is shown. Gold nanoparticles drop cast from a solution form a porous structure on a substrate electrode surface. Sample cations can penetrate the gold nanoparticles layer and interact with ligand acting as a charged ionophore, resulting in Nernstian potentiometric responses. Anchoring of complexing ligand on the gold surface abolishes the necessity of ionophore application. Moreover, it opens the possibility of preparation of potentiometric sensors using chelators of significantly different selectivity patterns further enhanced by the absence of polymeric membrane matrix. This was clearly seen, for example, for gold nanoparticles stabilizing the applied ligand−dithizone−thiol conformation leading to a high potentiometric selectivity toward copper ions, much higher than that of ionophores typically used to induce selectivity for polymeric ion-selective membranes.
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thiophene leading to polymer nanoparticles, was shown to result in a layer of potentiometric sensitivity toward potassium ions.21 Also gold nanochannels were used to prepare potentiometric sensors.22 The latter approach, similar to linking ionophore to gold nanoparticles11 takes advantage of the immobilization of ligand (ionophore) on the gold surface. However, this demanding approach requires synthetic modification of the (traditional) ionophore to have a thiol side chain anchoring the molecule on the gold surface (ideally without alteration of the ionophore properties). This is certainly disadvantageous, especially acknowledging the requirements related to purity and cost of even unmodified starting compounds. An alternative to this approach can be application of old-school chemical ligands that due to the presence of a thiol sulfur atom in the molecule can be used directly, i.e., without synthetic modification, provided that they anchor on the gold surface. Thus not only significant simplification of sensor preparation procedure is possible but also, in principle, this approach can lead to analytical devices benefiting from application of compound of complexing characteristic other than that of conventional ionophores. Moreover, this can open new possibilities of potentiometric exploration of excellent ligands, that in classical systems cannot be easily immobilized within the traditional (PVC or polyacrylate based) membranes. A good example of such compound is dithizone (diphenylthiocarbazone) complexing agent used primarily for spectrophotometric determination of, for example, Cu2+ or
ne of the alternatives for potentiometry development is the search for novel materials that can be applied as receptor layers or can be used to improve existing sensors. Recently this trend has manifested itself through interest in micro- or nanomaterials (e.g., refs 1−10) in potentiometric sensors construction. Different materials have been studied, ranging from conducting polymer micro- and nanostructures7−9 to gold nanoparticles6 applied as transducers (solid contact) under the plastic, solvent polymeric membranes. Alternatively, polymeric microstructures,10 platinum nanoparticles5 or ionophore modified gold nanoparticles,11 were introduced to the ion-selective membrane phase. Carbon nanotubes were also successfully applied for constructional improvements serving as transducer layers (e.g., refs 1−4 and 12) of excellent properties also for biosensors.13 Also other carbon based materials like nanostructured three-dimensionally ordered macroporous carbon,14 fullerene,15 or recently graphene16 have been used as transducers in potentiometric sensors. Nanomaterials are also applied as alternative (to polymeric, poly(vinyl chloride) (PVC) or polyacrylate based) receptor phases. Carbon nanotubes were used directly as receptor layers for detection of aromatic hydrocarbons.17,18 Modification of nanostructures resulting in ion potentiometric sensitivity/selectivity is also of interest. Novel type potentiometric sensors of high selectivity for lead ions were obtained due to covalent linking of ionophore to carbon nanotubes.19 Graphene layers noncovalently modified with complexing ligand were recently proposed for Zn2+ sensors.20 Spontaneous organization of the thiophene derivative of crown ether ligand, prior electropolymerization together with © 2012 American Chemical Society
Received: January 16, 2012 Accepted: April 17, 2012 Published: April 17, 2012 4437
dx.doi.org/10.1021/ac300155f | Anal. Chem. 2012, 84, 4437−4442
Analytical Chemistry
Article
Zn2+ ions.23,24 Nevertheless so far, published potentiometric sensor related applications of dithizone do not seem promising.25 Dithizone molecules can exist in different tautomeric forms,26 including both the thione and thiol forms, which are in tautomeric equilibrium, Figure 1.
setup used is similar to that described earlier,5,31 respectively. The recorded potential values were corrected for the liquid junction potential calculated according to the Henderson approximation. For the LA ICPMS experiments, the following parameters were used: 3 mJ/pulse energy, repetition rate 5 Hz, spot size 100 μm. Reagents. AuCl3, sodium borohydride, methyltrioctylammonium bromide (MTOABr), butanethiol, and chloroform were from Aldrich (Germany); dry solvents, toluene, acetone, ethanol, analytical grade salts, and dithizone were from POCh (Poland). Doubly distilled and freshly deionized water (resistance 18.2 MΩ cm, Milli-Qplus, Millipore, Austria) was used throughout this work. Synthesis of Gold Nanoparticles Modified with Butanethiol (GNP@C4). Synthesis was carried out via a modified Brust method as described previously32,33 using butanethiol (C4) (2.7 mmol) as ligands for nanoparticles preparation in the course of gold reduction with sodium borohydride. Nanoparticles were purified by precipitation/ centrifugation using a mixture of ethanol/acetone, ethanol, and finally GNP@C4 were dispersed in toluene and stored in the dark (TEM pictures of obtained GNP@C4 are presented in Figure S1a in the Supporting Information). Synthesis of Gold Nanoparticles Modified with Dithizone (GNP@Dit). After synthesis, the butanethiol stabilized particles (GNP@C4) were isolated and dissolved in toluene. Addition of dithizone (Dit) to this particle solution led to the partial precipitation of a black sediment within 72 h. This observation indicates significant ligand exchange and a coverage of the particles with dithizone causing precipitation, while particles with partial coverage stayed in the solution. Crude GNP@Dit were washed using acetone, ethanol, and methanol to remove unbound dithizone and butanethiol. The presence of unsubstitued particles in the supernatants as well as in the toluene solution of GNP@Dit was indicated by the thin layer chromatography (TLC). The modification scheme is presented in Figure 2 (TEM pictures of obtained GNP@Dit are presented in Figure S1b in the Supporting Information). Preparation of GNP@Dit or GNP@C4 Modified Electrodes. Glassy carbon (GC) electrodes with a surface area 0.07 cm2 were used. The substrate electrodes surface was polished with Al2O3, 0.3 μm, and rinsed well in water. For the LA ICPMS experiments, the GNPs layers were prepared on
Figure 1. Tautomeric equilibrium of dithizone.
Depending on the cation of choice, complexes can be formed with both or just one form of dithizone, e.g., copper(II) ions are known to form complexes with both tautomers, on the other hand, zinc ions can form complexes only with the thione form of dithizone. It is expected that the thiol form of dithizone would be preferably bound to the gold surface, resulting in stabilization of this tautomeric form. Thus, for gold modified with dithizone, complexation of cations able to react with the thiol form of this compound is expected, increasing the selectivity of the ligand due to conformational stabilization of the complexing reactive tautomer. This system is potentially interesting for analytical applications, including potentiometric sensing. As shown by us recently in the case of a modified gold surface for potentiometric applications, nanoparticles (GNPs) seem to be especially attractive,6 among others due to a high surface area compared to volume. For potentiometric applications, contrary to most typical applications of GNP, formation of a layer instead of application of single nanoparticles is preferred. Conveniently, GNPs can be drop-cast applied on the surface of a substrate electrode. Moreover, GNPs are known to spontaneously form organized structures, clusters or layers,27−29 which have intriguing properties. Formation of films from GNPs, especially if the latter are modified with ligands, can be an attractive alternative to application of gold nanochannels22 for potentiometric sensing. It is known that in gold nanoparticle monolayer films, the interparticle edge-to-edge distances are generally equal to the chain length of the capping agent,29 i.e., typically in the range of a few nanometers. Thus, it seems rational to expect that in a multilayer structure, the distances would be bigger, however, at the most in the range of less than 10 nm, thus much smaller compared to the diameter of nanopores22 but still big enough to enable solution cations to penetrate the layer. The aim of this preliminary work was to verify the possibility of application of GNPs modified with commercially available complexing ligand as potentiometric sensing membranes. As an example, GNPs modified with dithizone were used, leading to novel type copper cation potentiometric sensors. There is a clear interest in copper(II) ion potentiometric sensors that would be free from limits imposed by application of polymeric membrane matrixes.30 Dithizone is widely known for high affinity to copper ions, and its immobilization on the surface of GNPs allows diversion from the ionophores relaying approach typically applied for potentiometric ion-sensing leading to receptor layers of significantly improved selectivity.
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EXPERIMENTAL SECTION Apparatus. In the potentiometric experiments, the electrochemical measurements and the laser ablation with inductively coupled plasma mass spectrometry (LA ICPMS) experimental
Figure 2. Schematic representation of (a,b) exchange of butanethiol ligands present on the surface of GNP for the dithizone molecules and (c,d) of GNPs modified with dithizone film and interactions of this structure with copper ions. 4438
dx.doi.org/10.1021/ac300155f | Anal. Chem. 2012, 84, 4437−4442
Analytical Chemistry
Article
homemade electrodes, glassy carbon disk tips of the above given surface area.34 GNP@C4/GNP@Dit layers were prepared by drop casting toluene solutions of GNP@C4 or GNP@Dit, respectively, in 5 μL portions. Unless otherwise stated, the dry mass of applied GNP@Dit was equal to 0.314 mg, whereas the dry mass of applied GNP@C4 was equal to 0.20 mg. For some experiments, also significantly thinner GNP@Dit membranes were prepared applying (dry mass) 0.063 mg of dithizone modified gold nanoparticles, denoted as GNP@Dit-A. The applied solution was left for GNP solvent evaporation at room temperature. As prepared GNPs layers were used for experiments, unless otherwise stated. If conditioning was applied as for LA ICPMS, following this conditioning step, the layers were thoroughly rinsed with water, tissue dried and then the experiment was performed. In potentiometric experiments, stable potentials were recorded and are reported (change
