Sensing of silver ions by nanotubular polyaniline film deposited on quartz-crystal in a microbalance
Introduction
Noble-metal nanoparticles have unique physical, chemical and biological properties compared to their macro-scaled counter-parts [1] and therefore have been used in many applications such as catalysis, electronics, optics, medicine, and environmental protection for many years [2], [3], [4], [5], [6], [7], [8]. Silver exhibits the highest electrical and thermal conductivity among metals. It is also known for its antibacterial properties and its ability to promote surface-enhanced Raman scattering [9]. Silver nanoparticles in particular showed catalytic activity, photonics and optoelectronics phenomena, and unique optical properties, and hence have been used in a variety of applications such as optical and electrical nanodevices and nanosensors [10], [11].
An interest in the development of metal nanoparticle–polymer composites was growing rapidly due to wide range of potential applications. The composite polymeric films combine the attractive functional properties of metal nanoparticles with the materials properties of polymers [12], [13], [14]. Many of these composites exhibited electrocatalytic activity and increased gas sensing properties [14], [15], [16]. In addition, the composites of this type were used for fuel-cell design [17], proton-exchange membrane-cell electrocatalysts [18] and are candidates for electrodes in supercapacitors [19]. Many chemical and physical methods have been used to incorporate silver nanoparticles into polymer films [20]; however, homogeneous dispersion into the polymer matrix is difficult as the suspensions or colloidal dispersions of silver nanoparticles tend to aggregate during processing [21].
A group of conducting polymers, represented especially by polyaniline (PANI) and polypyrrole is particular interest for the preparation of composites, due to low monomer cost and ease of preparation, high environmental and chemical stability, large conductivity range, and unique redox properties. Recently, self-doped polyaniline nanotubes with silver nanoparticles assembled along them have been prepared [22]. Khanna et al. have also synthesized PANI–silver nanocomposites by a photochemical reaction [23]. The ability of conducting polymers to reduce noble metal ions and to be excellent hosts for trapping of resulting metal nanoparticles has been demonstrated [24], [25]. Polyaniline has recently been used for the reduction of silver nitrate to produce PANI–silver composites with silver nanoparticles of about 50 nm size [26], [27]. The feasibility of the metal deposition inside PANI nanotubes has also been discussed [28]. The use of PANI powders, however, could be limited by the polymer surface area and complicated by diffusion processes. We assume that the PANI nanofibers or nanotubes [29] constituting thin films would provide a profound improvement to the metal reduction efficiency due to the high surface area of the polymer.
In the present paper, nanotubular PANI film was grown by the chemical oxidation of aniline in mildly acidic medium [27], [28], [29] on the electrode of quartz-crystal microbalance (QCM). The resulting set-up was then used to sense the uptake and reduction of silver ions from an aqueous solution of silver nitrate. Such process may simulate the recover of noble metals from waste waters produced by industries using silver-containing compounds. Bearing in mind that the silver nanotechnology is increasingly contributing to a variety of consumer products, that release of silver ions into the environment is becoming a reality. On the other hand, the resulting PANI–silver composite film could also have the possibility of combining the properties of both PANI and silver nanostructures. Such composites, especially when combined with fibres and textiles, may find application as antimicrobial substrates. The generation of silver metal in the PANI film was monitored in situ by using QCM. The morphology and structure of the resulting composite was examined by using UV–vis spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and X-ray diffraction (XRD).
Section snippets
Preparation of nanotubular PANI films and deposition of silver
Aniline was purified by vacuum distillation, ammonium peroxydisulfate (APS), acetic acid, and silver nitrate were from BDH Chemicals (UK) and used as received. Both aniline and APS were stored at 4 °C. Recently, PANI nanotubes were prepared in the bulk solution by several research groups [30], [31], [32], [33], [34], [35]. Typical concentrations of reactants used for the preparation used also in the present communication were 0.2 mol L−1 aniline and 0.25 mol L−1 APS in an aqueous medium of 0.5 mol L−1
The deposition of PANI film
It has been demonstrated that the emeraldine form of PANI acts as an efficient reductant of silver salts [26], [27], [28] to metallic silver (Scheme 1). The ability of PANI film to uptake silver ions from aqueous silver nitrate solution was examined by QCM. Initially, the PANI film was grown on the electrode of QCM. Fig. 2 shows the variation of frequency during the PANI film deposition, which is directly proportional to the mass increase, on the polymerization time. The polymerization was
Conclusions
The present study demonstrates the use of QCM with deposited PANI film as a sensor for the presence of silver ions in aqueous media. Silver ions are reduced with PANI and the corresponding increase in mass would cause the easily detectable crystal-frequency change. The sensor of this type is cumulative, i.e. it would respond to low-term exposure to low concentration of silver ions by gradual decrease of frequency. The monitoring of industrial waste waters or the search for the new resources of
Acknowledgments
The authors thank the Academy of Scientific Research and Technology (Egypt) Grant Agency of the Academy of Sciences of the Czech Republic (IAA 400500905) and the Japan Society for the Promotion of Science (JSPS) for financial support.
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