Synthesis of gold nanoparticles on diazonium-generated heteroaryl films and their electrocatalytic activities
Introduction
Hydrazine, which is an oxygen scavenger, has wide applications in many industrial fields, including agricultural chemicals, rocket fuels, pharmaceuticals, foaming agents and fuel cells [1]. However, hydrazine is a highly toxic chemical, and it causes serious health problems [2]. Additionally, Environmental Protection Agency (EPA) has classified this compound as a priority pollutant due to its carcinogenicity to humans [3]. Similarly, nitrobenzene is also a potential carcinogenic compound and is considered to be an environmental pollution [4]. Nitrobenzene is widely used for the production of explosives, dyes, pesticides, herbicides and insecticides [5]. Every day, large amounts of these toxic chemicals are released into the environment by industrial processes. Therefore, the quantitative determination of hydrazine and nitrobenzene is of importance to address the issues related public health challenges and environmental threats. Various methods have been developed for the detection of hydrazine and nitrobenzene such as spectrophotometry, chemiluminescence, chromatography and electrochemistry [[6], [7], [8], [9], [10], [11]]. Among them, electrochemical methods are widely employed for the direct determination of target compounds due to the low cost, short analysis time and mild reaction conditions [[12], [13], [14], [15]]. However, the redox responses of hydrazine and nitrobenzene at unmodified electrodes are limited due to their sluggish electrochemical kinetics and large over-potentials. To overcome this drawback, a common approach is the use of chemically modified electrodes, incorporating catalytic sites at the electrode surface [16].
Metal nanoparticles have received considerable attention due to their unique optical and electronic features compared to those of bulk materials [17]. Among these materials, gold nanoparticles (AuNPs) are probably the most extensively investigated nanostructures owing to their remarkable chemical stability, excellent electrical conductivity and good biocompatibility as well as large surface area to volume ratio [18]. Recently, a wide range of research interest has been focused on the preparation of inorganic/organic composite materials consisting of gold nanoparticles, since they exhibit very interesting synergistic properties arising through combining the components [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. These materials are extensively used in the catalytic applications [[42], [43], [44]]. The AuNPs found in the composite structure show high catalytic capabilities for many redox reactions including hydrogenation or oxidative dehydrogenation of small organic molecules. Thus, the composite materials as efficient electrocatalysts play a crucial role in electrochemical sensor technology.
So far, two basic strategies have been developed to generate inorganic nanoparticle/organic film composite structures on solid substrates. The first strategy involves the immobilization of metallic nanoparticles onto the surfaces which have previously been modified with small organic molecules or macromolecules [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. In this method, traditionally, nanoparticles have been presynthesized by chemical reduction of metal salts using reducing agents such as sodium borohydride, hydrazine hydrate and ascorbic acid. In order to prevent agglomeration, particles are also stabilized by organic capping agents. A good capping agent hinders the growth of AuNPs, however, it passivates the surface of nanoparticle, resulting in the loss of catalytic activity. In this case, the second strategy has proven to be an effective way to overcome this drawback. The second strategy is based on a three step procedure [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. In the first step, the surface is coated with an organic film, and then the coated substrate is immersed in the solution containing gold ions. In the last step, the reduction of the metal ions bound to the substrate are performed via electrochemical or chemical methods, which results in the formation of bare nanoparticles collapsed on top of the film. In both procedures, nanoparticles are attached to the surface through hydrophobic or ionic interactions. In this context, the size, structure and distribution of AuNPs on the surfaces are strongly dependent on the functional groups/atoms present in organic film. For this purpose, the molecules with –NH2 or –SH terminal groups are commonly used due to the strong interaction between metal nanoparticles and terminal groups, and thus, these groups offer an efficient and versatile way to prepare nanoparticles on the solid surface [[19], [20], [21], [22], [23], [24], [25], [26],[29], [30], [31]].
Many successful techniques have been developed to build-up organic films on the substrates, such as vapor deposition [45], Langmuir-Blodgett (LB) [46], spin-coating [47], dip-coating [48], drop-casting [49], self-assembly [50] and diazonium reduction method [51]. In particular, the modification of the electrodes by electrochemical reduction of diazonium salts has recently become a popular method for the surface modification, as it allows to produce stable organic layers on conductive or semiconductive substrates. This surface modification procedure, first developed by Pinson et al. [51], involves the formation of highly reactive diazonium radicals which irreversibly bind to the electrode surface. Using diazonium reduction method, the substrates such as carbon, silicon, iron, gold and indium tin oxide (ITO) can be covered with a wide variety of aromatic/heteroaromatic molecules bearing different functional groups [[52], [53], [54], [55], [56], [57]].
In the present study, we aim the electrochemical preparation of AuNPs on glassy carbon (GC) substrates functionalized through diazonium modification method (Scheme 1). Accordingly, the electrode surfaces were first modified with pyridine and phenanthroline rings through electrochemical reduction of the corresponding in situ generated diazonium salts. These heteroaromatic rings are particularly preferred because they contain at least one nitrogen atom that can coordinate to the target metal ion [[58], [59], [60], [61], [62]]. After the covalent functionalization, the electrodes were immersed into HAuCl4 aqueous solution for overnight, and then AuNPs were obtained by electrochemical reduction of metal ions adsorbed on the modified surfaces in metal-ion-free solution without the need of any other reducing reagent. The electrocatalytic properties of the resulting composite films containing AuNPs were evaluated for both hydrazine hydrate oxidation reaction and nitrobenzene reduction reaction in neutral solution by cyclic voltammetry and differential pulse voltammetry. The results showed that stable composite structures with high catalytic activity could be produced by a facile approach without using harmful reducing agents.
Section snippets
Materials
4-aminopyridine (4-APy), 5-amino(1,10-phenanthroline) (5-APhen), aniline, butyl nitrite (BuNO2), tetrafluoroboric acid (HBF4, 48%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O, 99.999%), ferrocene, nitrobenzene and hydrazine hydrate were purchased from Sigma-Aldrich and used without further purification. Acetonitrile (Sigma-Aldrich) was distilled over calcium hydride and stored over activated molecular sieves. Tetrabutylammonium tetrafluoroborate (TBABF4, Sigma-Aldrich) was used
Preparation and characterization of gold nanoparticles on heteroaromatic films
The synthesis of AuNPs on heteroaromatic films was carried out by a three-step procedure, as shown in Scheme 1: (i) covalent attachment of pyridine and phenanthroline rings to GC surfaces through electrochemical reduction of the corresponding in situ generated diazonium cations; (ii) adsorption of HAuCl4 onto the surface of GC electrodes coated with heteroaromatic rings through coordination via nitrogen donor atoms; (iii) formation of nanoscale gold particles by electrochemical reduction of
Conclusion
In conclusion, we demonstrated that the covalently grafted pyridine and phenanthroline rings on GC electrode could be used as template for the formation of AuNPs. The presence of AuNPs on the surfaces was characterized by various spectroscopic and electrochemical techniques. The density and distribution uniformity of the nanoparticles were controlled with thickness and composition of the organic films. The results demonstrated that the Au nanoparticles were homogeneously distributed on the
Acknowledgement
The authors are grateful to Atatürk University for the financial and technical support.
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