A hydroxylamine electrochemical sensor based on electrodeposition of porous ZnO nanofilms onto carbon nanotubes films modified electrode
Introduction
Hydroxylamine, NH2OH (abbreviated as HA), a derivative of ammonium, is an intermediate in two important microbial processes of the nitrogen cycle: it is formed during nitrification as well as during anaerobic ammonium oxidation [1], [2]. Although it is a well-known mutagen, moderately toxic and harmful to human, animals, and even plants [3], which has been known to cause both reversible and irreversible physiological changes [4], it is available commercially and frequently used industrially widely in pharmaceutical intermediates and final drug substances synthesis, nuclear fuel reprocessing and the manufacturing of semiconductors [5]. In recent years, chemists became aware of the potentials of hydroxylamine as a result of two major accidents, one occurred in the USA in February 1999, which killed five people, and the other occurred in Japan in June 2000, which killed four people [6], [7]. Therefore, from the industrial, environmental and health viewpoints, development of a sensitive analytical method for the determination of low levels of hydroxylamine is of significant importance.
The reported methods of the hydroxylamine determination include spectrophotometry [8], high performance liquid chromatography [9], gas chromatography [10], potentiometry [11], polarography [12] and biamperometry [13]. However, the processes involved in many of these methods are extremely complex, and the linear ranges are relatively narrow and have low precision. Fortunately, electrochemical techniques offer the opportunity for portable, cheap and rapid methodologies. However, hydroxylamine cannot be electrooxidized at bare carbon electrodes. One promising approach is the use of chemically modified electrodes (CMEs) containing specifically selected redox mediators immobilized on conventional electrode materials. Recently, various chemically modified electrodes (CMEs) have been prepared and applied in the determination of hydroxylamine [14], [15], [16], [17], [18], which can significantly lower the overpotentials and increase the oxidation current response.
In recent years, the II–VI semiconductor zinc oxide (ZnO) nanostructures have drawn many attentions in the application of efficient amperometric sensors with many extraordinary properties, including nontoxicity, biological compatibility, chemical and photochemical stability, high electrochemical activities and easy preparation, and so on [19], [20], [21], [22], [23], [24]. For example, the use of ZnO nanostructures to fabricate electrochemical sensor have been reported in the literature [25], [26], [46], [47], [48]. Among various fabrication strategies of nano- or microscaled ZnO, such as precipitation [27], thermal decomposition [28] and electrodeposition [29], the one-step electrochemical deposition method by treatment of the reactant in different solvents seems to be the simplest and most effective way to prepare nicely crystallized ZnO at relatively low temperatures, exempted from further calcination. However, as for the electrodeposition, the template strategy (anodic alumina membrane or porous polycarbonate membrane) was often used and it is difficult to dissolve template completely and undissolved film will influence the capability of biosensor, so there are still few studies concerning the electrodeposition of nanostructure ZnO applied to biosensor [30]. In order to overcome these drawbacks, a new strategy was introduced by Hrapovic et al. [31] who deposited the Pt nanoparticles onto CNT film successfully. Because CNTs have the high accessible surface area, low electrical resistance, extremely high mechanical strength and stiffness, outstanding charge-transport characteristics, high chemical stability, and doughty sorption [32], [33], [34], [35], [36], [37], there have been a series of investigations to use CNTs for biosensor interface fabrication [38], [39].
In this work, we present, for the first time, a novel hydroxylamine sensor fabricated based on electrodeposition of the loose and porous ZnO nanofilms onto multi-walled carbon nanotubes films using potentiostatic electrodeposition technique. The advantages of this architecture are as follows: (1) the MWCNTs have strong physical adsorption ability. What's more, –COOH and –OH are existent in hydrophilic carboxyl MWCNTs, the H of the –COOH and –OH may form oxygen–hydrogen bonds with O from ZnO. The porous ZnO films can be adsorbed on the surface of the GCE by MWCNT films very firmly due to the strong physical adsorption ability and oxygen–hydrogen bonds; (2) MWCNTs not only help to moor the ZnO films onto the electrode surface, but also to increase the electron transfer rate; (3) in this paper, the ZnO we electrodeposited is a low density, loose and porous material with good dispersion and uniformity, and higher accessible surface area, which is favorable to a catalytic application.
Section snippets
Materials
Hydrophilic multi-walled carbon nanotubes (MWCNTs) were purchased from Chengdu Institute of Organic Chemistry (China). Zn(NO3)2·6H2O (99.5%) was obtained from Shantou Xilong Chemical Factory (China). All other chemicals were obtained from Chemical Reagent Company of Shanghai (China), such as hydroxylamine, KCl, NaH2PO4, Na2HPO4. All chemicals were of analytical grade and used as received without further purification. Doubly distilled water was used throughout this research. Phosphate buffer
Characterization of the ZnO/MWCNT films
Fig. 1(A) and (B) shows typical SEM images of the MWCNTs and of the films consisting of ZnO and MWCNTs. The pristine MWCNTs are curved and twisted with each other and have very much smooth surfaces (Fig. 1(A)). The SEM image shown in Fig. 1(B) confirmed the co-existence of MWCNTs and porous ZnO thin films, and the ZnO is a low density, loose and porous material that is favorable to a catalytic application. ZnO films strongly adhere onto the network structure provided by MWCNTs. The strong
Conclusions
In this work, we have successfully fabricated an electrochemical hydroxylamine sensor based on electrodeposition of the loose and porous ZnO films onto carbon nanotubes films. An elegant chemical architecture to construct electrodes using MWCNTs as a binder to stabilize the ZnO films onto the surface of a GCE was presented. The ZnO is a low density, loose and porous material with higher accessible surface area, which is favorable to a catalytic application. And MWCNTs not only help to bind the
Acknowledgements
We appreciated the support of the National Natural Science Foundation of China (No. 20675001), Science Foundation of Office of Anhui Province (No. KJ2009B013z) and the Program for Innovative Research Team in Anhui Normal University.
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