Large-scale self-assembly of hydrophilic gold nanoparticles at oil/water interface and their electro-oxidation for nitric oxide in solution

Abstract

Mono-/bi-layer Au nanoparticle films with large areas were prepared by the assembly of Au nanoparticles in aqueous colloid at toluene/water interfaces, which can be transferred onto the hydrophilic solid surface and adhere strongly to the substrate without any binding agent. The transferred Au nanoparticle films exhibited satisfactory catalytic performance for electro-oxidizing nitric oxide (NO) in solution, and had a low detection limit (2.7 × 10⁻⁸ mol/L), a rapid response time (less than 0.5 s) and a wide linear range (5.0 × 10⁻⁸–1.0 × 10⁻⁵ mol/L) for the detection of NO in solution. UV–vis spectra, cyclic voltammetry and chronoamperometry were conducted to characterize the prepared Au nanoparticle films.

Introduction

Nanoparticle films have attracted extensive interest because of their wide applications in catalysis [1], electronic device [2], and biological sensing [3], [4]. Generally, nanoparticle films are prepared through self-assembly of metal nanoparticles from organic media and then transferred onto solid substrates, which often requires the addition of strong stabilizers to prevent the aggregation of nanoparticles [5], [6]. Consequently, the surface activity of nanoparticles is suppressed seriously and cannot be fully exhibited, and such films usually have poor bio-compatibility. Despite that, some interesting physical and chemical properties of nanoparticle films have been explored [5], [7], [8]. For example, Zhong and coworkers have demonstrated that the ordered monolayer film of gold nanoparticles (2 nm or 5 nm) displayed high catalytic activity for CO electrochemical oxidation [1], [9], and gold nanoparticle films for NO electro-oxidization also had been reported [10], [11].

Nitric oxide has a wide range of biological roles, such as serving as a neurotransmitter or neuromodulator in the neuronal system [12], acting as a major defense molecule in the immune system [13], marking some tissue damage [14], and preventing platelet aggregation [15]. However, nitric oxide is an active molecule with a short half-life of ∼6 s in physiological solution, and can be easily oxidized by O₂ to form ${\text{NO}}_2^{-}$ or ${\text{NO}}_3^{-}$ ions [16], which makes NO detection difficult. Electrochemical techniques are of significance in NO detection due to not only their high selectivity, good sensitivity, fast response time, and long-term calibration stability, but also their suitability for applications in vivo [17], [18]. So far, NO electro-oxidation and -detection have been widely carried out based on modified electrodes with different types of porphyrin and phthalocyanine films [17], [19], [20], vinylterpyridine complexes [18], and nanomaterials [10], [11], [21]. However, hydrophilic nanoparticles with good bio-compatibility were difficult to be assembled into a dense nanostructured film without other chemical mediators. Zhu et al. made Au nanoparticles on Pt electrodes modified with cysteine by covalent bonds of –NH₂ and –SH [11], with surface coverage of less than 30% [22]. Caruso and coworkers [10] prepared polyelectrolyte (PE)/Au nanoparticle hybrid films by incorporating 4-(dimethylamino)pyridine-stabilized gold nanoparticles into PE multilayers for NO electrochemical detection, and obtained a detection limit of 10 μmol/L, which was lower than hemoglobin-based NO sensors using the same source of NO (NaNO₂) [23], [24].

Recently, we have developed a simple and efficient method to assemble hydrophilic nanoparticles into ordered monolayers at a two-phase interface by ethanol-mediated inducement [25]. Addition of ethanol into an aqueous colloid could change the surface charge of hydrophilic nanoparticles, causing a change of the contact angle of nanoparticles with the interface so that nanoparticles are trapped at the interface [26]. A close-packed nanoparticle monolayer film can be obtained depending on the interfacial tension [25]. It is of significance that this method does not use any chemical mediators, which not only retains the surface activity of nanoparticles, but also makes nanoparticles have excellent biocompatibility.

Here, we prepared Au nanoparticle monolayer films by ethanol-mediated inducement, and the as-prepared monolayers can in turn be transferred onto glassy carbon (GC) surface to assemble bilayer films for electrochemical and catalytic studies. The transferred nanoparticle film can strongly adhere onto GC surface without any linkers, resistant to a stirring solution in the process of electrochemical experiments. In conventional methods, a solid surface modified with nanoparticles must be constructed by modifying the solid surface with some proper binding linkers. The existence of binding linkers usually delays the electron-transfer process between substrates and particles, thus limiting the detection sensitivity of target species. In this study, gold nanoparticle film exhibits a satisfying sensitivity toward NO detection.