Regular ArticleWorm-like gold nanowires assembled carbon nanofibers-CVD graphene hybrid as sensitive and selective sensor for nitrite detection
Graphical abstract
Introduction
Nowadays, the overuse of nitrite compounds as important additives for food and drink production causes serious influences on the quality of ecosystem and human health. The World Health Organization (WHO) has recognized a fatal nitrite ion (NO2−) concentration of (8.7–28.3) μM; therefore, a sensitive technique for nitrite detection is urgently required. Until now, some analytical routes have been established for NO2– detection, such as spectrophotometry [1], chromatography [2], chemiluminescence [3], spectrofluorimetry [4], mass spectrometry [5], and electrochemistry [6]. Among these, the electrochemical route has attracted wide interest, due to its promising advantages, which include cost-effectiveness, fast response, reliability, sensitivity, simplicity, portability, and easy operation [7], [8], [9]. The electrochemical mechanism kinetics relating to redox reaction of NO2− depend on the modulation of the electrode’s surface through the employment of nanocatalysts. While electrochemical NO2− reduction suffers from the significant interference of co-existent dissolved reagents, such as O2 and NO, the oxidation of NO2− to NO3− occurs perfectly, without effect from other species (elements or anions). Thus, the development of electrode modified by a suitable catalyst to boost the NO2− oxidation has been highly preferred to not only enhance nitrite response at low limit of detection (LOD), but also to expand the dynamic range detection in analytical purposes. For this purpose, a variety of nanocatalysts used for nitrite oxidation have been developed in recent years [10], [11]. In this regard, gold (Au) nanomaterials has attracted huge attention due to its distinctive features, such as unique catalytic activity, excellent conductivity, and outstanding chemical/electrochemical stability [12], [13]. Recent developments of Au nanomaterials with various morphologies and structures have demonstrated their outstanding potential for employment in electrocatalysis [14], [15]. In particular, worm-like Au nanowires (Au WNWs) has been among the most advanced nanostructures, because it offers the intrinsic exposed defects, large open surface area, and numerous active corners that are advantageous to improve the catalytic behavior and strength towards electrochemical reactions [16], [17]. In addition, the WNWs can contact supporting substrate more efficiently than the spherical or polyhedral nanocrystals; thereby not only effectively improving the charge transfer between WNWs and their support, but also avoiding their agglomeration and Ostwald ripening [18]. Upon their advantages, different synthetic approaches, such as thermal decomposition, seed-mediated growth, and polyol methods [16], [19], [20], have been employed to prepare high-quality Au WNWs; however, the use of surfactants, stabilizers, and reduction agents for the oriented formation mechanism and protection of the resulting material from aggregation, has been revealed to create impurities, which limit the exposed electroactive sites and conductivity. Therefore, the rational design of an effective route to easily control the fabrication of high-quality Au WNWs with the improved activity and stability is of great necessity. It was demonstrated that the assembly of catalyst on a reasonable supporting material could lead to an impressive increase of its catalytic performance [21], [22]. Recently, the growth of Au nanostructures on carbon-based materials, such as graphene (Gr), carbon nanofibers (CNFs), and carbon nanotubes, has been proposed as a solution to achieve their small size, uniformity, good dispersion, and high density suitable in various applications [23], [24], [25]. Among various carbon allotropes, Gr, a wonderful one-atom-thick 2D graphitic sheet owning special structure and properties, has exhibited remarkably potential [26], [27]. In this regard, chemical vapor deposition (CVD)-grown Gr has been emerging as a promising nanomaterial in the development of high-quality sensors rather than chemically synthesized Gr, which faces serious concerns of complicated and environmentally unfriendly synthesis procedure, and insufficient uniformity. However, a facile process for transferring large-scale CVD–grown Gr onto a flat substrate without significant damage to its structure is still a challenge. In order to solve this issue, in-situ growth of Gr during graphitization of carbon nanofibers (CNFs) to form a hybrid with high-mechanical network and excellent interfacial strength has been demonstrated as an effective solution [28]. Such a combination of Gr and continuous CNFs into a heterostructured network could produce a big difference in improving the properties of the resulting materials, in terms of mechanical strength, large surface area, enhanced conductivity, and easy portability [28]. Recently, the Gr-embedded CNFs hybrid has been obtained using a two-step process relating to electrospinning and carbonization for supporting Pt catalyst [29]. In another study, the Gr-CNFs paper based on the use of graphite oxide and CNFs have also been synthesized to deposit MnO2 nanosheets [30]. However, these nanohybrids involved to multiple synthesis steps as well as applied chemically synthesized Gr, a non-ideal candidate. Motivated by above concerns, we believed that the combination of the advantages from ultrasmall Au WNWs and binary CNFs/CVD–grown Gr network could produce further enhancements in both activity and stability for electrochemical catalytic reactions. Therefore, in this study, we developed a sustainable synthesis procedure to fabricate a high-quality hybrid based on Au WNWs and thin CNFs–CVD Gr network with precise controlling of crystal morphology, shape, and surface composition, without using any reducing agent, stabilizer, or capping agents. The hybrid was directly used as a non-binder electrocatalyst to detect nitrite content in neutral phosphate buffer solution (PBS). The Au WNWs/CNFs–Gr exhibited excellent sensing performance, suggesting a potential electrochemical sensor with high sensitivity, large detection range, low LOD, long-term stability, and good reproducibility for nitrite sensing application.
Section snippets
Materials
Polyacrylonitrile (PAN), gold (III) chloride hydrate (HAuCl4), ethanol, sodium nitrite (NaNO2, ≥97%), ascorbic acid (AA, ≥99%), sodium dihydrogen phosphate (NaH2PO4, ≥99%), uric acid (UA, ≥99%), dopamine (DA, ≥99%), glucose (≥99.5%), disodium hydrogen phosphate (Na2HPO4, ≥99%), and urea (NH2CONH2, ≥99%) were purchased from Sigma-Aldrich Chemicals Co. (USA). Acetic acid (CH3COOH, 99.7%), sodium sulfate (Na2SO4, ≥99%), magnesium nitrate (Mg(NO3)2, ≥99%), acetone (CH3COCH3, 99.9%), sodium
Results and discussion
Fig. 1a shows the preparation strategy of the Au WNWs/CNFs–Gr hybrid material. Initially, a specific electrospinning step was applied to produce uniform PAN NFs on Cu substrate, which was then thermally treated to graphitize PAN NFs to CNFs and grow Gr to form a thin CNFs–Gr hybrid network. The fabrication of Au WNWs on surface of the CNFs–Gr was subsequently conducted via an electroless deposition, in which the standard reduction potential of Au higher than that of Cu metal led to the Cu
Conclusions
In this study, we developed a novel CNFs–Gr network, which was large surface area and excellent conductivity to support high-density of uniform Au WNWs. The formation of the unique Au WNWs/CNFs–Gr hybrid offered intrinsic alternation of physicochemical properties and the enhanced electroactive sites. As a result, the Au WNWs/CNFs–Gr hybrid demonstrated as an effective binder-free sensor with great performance for nitrite detection in terms of excellent sensitivity of 836 μA cm−2 mM−1, wide linear
CRediT authorship contribution statement
Huu Tuan Le: Methodology, Investigation, Validation, Formal analysis. Duy Thanh Tran: Methodology, Writing - original draft, Visualization. Nam Hoon Kim: Conceptualization, Data curation, Writing - review & editing, Supervision. Joong Hee Lee: Conceptualization, Writing - review & editing, Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Program for Fostering Next-Generation Researchers in Engineering of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2017H1D8A2030449) and Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20184030202210).
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