Trends in Environmental Analytical Chemistry xxx (2014) e1–e8 G Model TEAC-12; No. of Pages 8 Review Nanostructured metal oxides/hydroxides-based electrochemical sensor for monitoring environmental micropollutants Xin-Yao Yu a, Zhong-Gang Liu a,b, Xing-Jiu Huang a,b,* aNanomaterials and Environment Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China bDepartment of Chemistry, University of Science and Technology of China, Hefei 230026, PR China Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2. Organic micropollutants detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.1. Non-enzymatic electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.1.1. Adsorptive NMOs/HOs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.1.2. Electrocatalytic NMOs/HOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.1.3. Molecularly imprinted polymer/metal oxides hybrids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.1.4. Photoelectrocatalytic metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 2.2. Enzymatic electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3. Toxic ions detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.1. Heavy metal ions (HMIs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.1.1. Striping method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.1.2. PEC method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.2. Anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.2.1. Arsenite and nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 3.2.2. Iodate, bromate and chlorate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 000 A R T I C L E I N F O Article history: Received 11 June 2014 Received in revised form 7 July 2014 Accepted 7 July 2014 Keywords: Nanostructured metal oxides/hydroxides Electrochemical sensor Micropollutant monitoring Heavy metal ions Organic micropollutants Toxic anions A B S T R A C T Nanostructured metal oxides/hydroxides (NMOs/HOs) with unique optical, electrical and molecular properties, chemical and photochemical stability, electrochemical activity, large surface area along with desired functionalities have recently become important as materials to construct electrochemical sensor for monitoring environmental micropollutants. In this review, we present and discuss the NMOs/HOs- based electrochemical sensor for detection of micropollutants including toxic organic micropollutants, heavy metal ions (HMIs), and anions in water. The analytical performance of a NMOs/HOs-based electrochemical sensor can be improved by tailoring the properties of the NMOs/HOs through engineering of morphology, particle size, exposed crystal facets, effective surface area, functionality, adsorption capability and electron-transfer properties. These interesting NMOs/HOs are expected to find potential applications in a new generation of miniaturized, smart electrochemical environmental monitoring devices. � 2014 Published by Elsevier B.V. Contents lists available at ScienceDirect Trends in Environmental Analytical Chemistry jo u rn al ho m epag e: ww w.els evier .c o m/lo cat e/ teac * Corresponding author at: Nanomaterials and Environment Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China. Tel.: +86 551 65591142; fax: +86 551 65592420. E-mail address:
[email protected] (X.-J. Huang). Please cite this article in press as: X.-Y. Yu, et al., Nanostructured metal oxides/hydroxides-based electrochemical sensor for monitoring environmental micropollutants, Trends Environ. Anal. Chem. (2014), http://dx.doi.org/10.1016/j.teac.2014.07.001 http://dx.doi.org/10.1016/j.teac.2014.07.001 2214-1588/� 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.teac.2014.07.001 mailto:
[email protected] http://dx.doi.org/10.1016/j.teac.2014.07.001 http://www.sciencedirect.com/science/journal/22141588 www.elsevier.com/locate/teac http://dx.doi.org/10.1016/j.teac.2014.07.001 X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8e2 G Model TEAC-12; No. of Pages 8 1. Introduction Recently, there has been an increasing interest in developing nanostructured metal oxides/hydroxides (NMOs/HOs) because of their unique optical, electrical and molecular properties along with desired functionalities and surface charge properties [1,2]. As a consequence, NMOs/HOs have been extensively investigated in view of their applications as gas sensors, catalyst, adsorbent, lithium ion batteries, supercapacitors, energy storage [1,3]. Besides, considering the toxicity of the environmental contaminants and their adverse effects on human health and the environment, NMOs/HOs have been developed to fabricate the efficient electrochemical sensor and to realize their application in environmental science, especially in monitoring micropollutants. Fortunately, great progress has been made on the development of NMOs/HOs-based electrochemical sensors in the monitoring environmental micropollutants, and the number of papers devoted to this topic has grown rapidly in the past few years. Fig. 1. Scheme of electrochemical sensing nitroaromatic OP compounds. (A) Electrodeposition ZrO2 nanoparticle to gold electrode surface; (B) nitroaromatic OP compounds adsorb to ZrO2 nanoparticle surface; (C) electrochemical stripping detection of nitroaromatic OP compounds; X = O or S and R = nitroaromatic OP group. Reprinted from Ref. [4] with permission of American Chemical Society. As is well known, the performance of the modified electrode is highly dependent on the outstanding properties of sensing materials. Various NMOs/HOs with different morphologies and sizes have been explored as sensing materials, such as ZrO2 nanoparticles [4], SnO2 nanooctahedron [5], TiO2 nanotubes [6] and Ni–Al-LDH nanosheets [7]. Novel NMOs/HOs have played an important role in the construction of electrochemical sensors and the promotion of the electroanalysis performance. Their unique properties provide interesting platforms for interfacing environ- mental elements with transducers for signal amplification. Moreover, as an essential procedure, the construction strategy on the NMOs/HOs-based electrochemical sensors was concerned to optimize the performance of electrochemical sensors [8]. Such methods of drop-coating, electrodeposition, self-assembly, elec- trostatic interactions have been proposed and studied. Some representative applications of NMOs/HOs with various structures as well as the construction methods were summarized in Table 1. To timely evaluate the success and development of NMOs/HOs as sensing materials, in this article, we mainly overview and discuss their role used in the design of electrochemical sensors for environmental micropollutant monitoring within last 6 years. The analysis of toxic organic micropollutants, heavy metal ions (HMIs), and anions with NMOs/HOs-based electrochemical sensors was emphasized. Besides, their key challenges and improvements in environmental analysis are highlighted and assessment. 2. Organic micropollutants detection 2.1. Non-enzymatic electrochemical sensors 2.1.1. Adsorptive NMOs/HOs Nanostructured ZrO2, TiO2, Al2O3, and layered double hydro- xides (LDHs) have been used to construct electrochemical sensors Table 1 Representative construction methods of NMOs/HOs-based sensors. Nanomaterials Solid substance Imm ZrO2 nanoparticles Gold electrode Elect SnO2 nanooctahedron Glassy carbon electrode Drop TiO2 nanotubes Ti foils Poten Ni–Al-LDH nanosheets ITO electrode Elect Fe3O4@Au nanoparticles Glassy carbon electrode Self-a Zn3Al(OH)8Cl/chitosan Glassy carbon electrode Form MgO nanoflower Glassy carbon electrode Drop Mesoporous NiO nanosheets Glassy carbon electrode Drop Co3O4 nanocrystals Glassy carbon electrode Drop WO3 nanofilm Carbon cloth Elect Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. (2014) using their intrinsic large surface areas and high adsorptivity properties. These NMOs/HOs have abundant adsorption sites and strong affinity for organic compounds, which can be used for enrichment and measurement of organic compounds such as electroactive pesticides, herbicides, and phenolic compounds with high selectivity and sensitivity [4,15–24]. Thus such nitroaromatic organophosphates (OPs) as paraoxon, methyl parathion, and fenitrothion could exhibit good redox activities at the electrode surface. Lin et al. described a new protocol for the electrochemical sensing of nitroaromatic OPs based on a gold electrode modified with zirconia nanoparticles [4]. Fig. 1 shows the fabrication of ZrO2 nanoparticle modified electrode and the successful determination toward nitroaromatic OPs, which involved electrodynamically depositing ZrO2 nanoparticles onto a gold electrode surface (A), followed by OP adsorption (B), and electrochemical stripping detection of adsorbed electroactive OPs (C). Due to the strong and specific affinity of ZrO2 nanoparticles for the phosphoric group, nitroaromatic OPs can be selectively detected. Moreover, new morphologies such as TiO2 nanotubes and nanocomposites of noble metal nanoparticles or graphene and ZrO2, TiO2, and LDHs have been made to improve their stability and sensitivity in electroanalysis of organic compounds [15–20,22,23].With the large surface areas and high adsorptivity of NMOs/HOs, good analytic results have been made. However, some limitations can be observed. In the case of adsorptivity, the interaction mechanism between NMOs/HOs and target micropollutants is unclear and lack of scientific understanding and reasonable support from experi- ment. In addition, due to the high adsorptivity of NMOs/HOs, they may have an unnecessary affinity with other interferents (e.g., surfactants, bacteria, humic acid) that exited in many natural water samples, which may therefore cause the surface-fouling matter of these electrochemical sensors and limit their lifetime. obilization strategy Analyte(s) Ref. rodeposition Organophosphates [4] -coating 1-Naphthol [5] tiostatic oxidation Lindane [6] rostatic interactions Catechol [7] ssembly monolayer Parathion-methyl [9] ation of matrix with chitosan Phenol [10] -coating Pb(II), Cd(II) [11] -coating Hg(II) [12] -coating Pb(II) [13] rodeposition IO3 � [14] l oxides/hydroxides-based electrochemical sensor for monitoring , http://dx.doi.org/10.1016/j.teac.2014.07.001 http://dx.doi.org/10.1016/j.teac.2014.07.001 X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8 e3 G Model TEAC-12; No. of Pages 8 2.1.2. Electrocatalytic NMOs/HOs Nanostructured Cu2O, Ni(OH)2, MnO2, CdO and SnO2 usually have electrocatalytic redox on its surface in alkaline or neutral medium, showing enhanced electrocatalytic activity in detection of organic micropollutants in terms of high sensitivity and low LODs [5,25–31]. Najafi et al. synthesized CdO nanoparticles by a controlled mechanochemical reaction and the electrocatalytic behavior of its modified carbon paste electrode toward the reduction of trichloroacetic acid [25]. The detection limit and linear dynamic range of trichloroacetic acid were 2.3 mM and 3.0– 230 mM, respectively, and the proposed modified electrode was promising for measurements of trichloroacetic acid in water samples [25]. Up to now, two strategies were developed to expand the application of electrocatalytic nanostructured metal oxides/ hydroxides. On the one hand, graphene–metal oxides/hydroxides, MWCNTs–metal oxides/hydroxides, conductive polymer–metal oxides/hydroxides, and noble metal–metal oxides/hydroxides nanocomposites have been synthesized [26,28–31]. On the other hand, another strategy based on the engineering the crystal facets of metal oxides/hydroxides have been developed by Huang et al. [5]. In their study, hydrothermally synthesized SnO2 nanooctahe- dron with (2 2 1) high-index facet was used to modify glassy carbon electrode (GCE) for determination of 1-naphthol [5]. Com- pared with conventional SnO2 nanoparticles, the high-index facet SnO2 crystals showed much higher sensitivity and lower LODs [5]. As demonstrated, the favorable electrocatalytic behavior of some nanostructured metal oxides/hydroxides is highly depen- dent on their high surface area and unique structure. They often suffer from low surface area, structural collapse, and the resulted poor electrocatalytic efficiency. To achieve high electrocatalytic activity, it can be generally solved by loading these nanostructured materials onto suitable supports or designing multilevel electro- catalyst, thus leading to highly dispersed surface. In respect to the unique structural electrocatalyst, more work should be explored the electrocatalytic mechanism in combination with other advanced techniques such as computational electrochemistry and simulation. 2.1.3. Molecularly imprinted polymer/metal oxides hybrids The molecularly imprinted polymer (MIP) technique is an attractive method for the generation of polymer-based molecular recognition elements tailor-made for a given target or group of target molecules. Nanostructured metal oxides such as Fe3O4 and TiO2 nanoparticles have been developed to construct novel molecularly imprinting sensor with excellent selectivity and high sensitivity for target organic micropollutants detection [9,32]. To effectively improve the sensitivity and selectivity for parathion- methyl detection, Tang et al. combined the Fe3O4@Au nanosized molecular imprint with the surface self-assembled monolayer systems [9]. Fe3O4 nanoparticles can make the imprinting process faster and more effective due to their magnetic effect on the washing after each self-assembled process. The spherical molecular imprinted monolayer can also give more recognition sites for parathion-methyl due to the high surface area of the spheroidal structure. The proposed sensor realized the highly sensitive and excellent selectivity for parathion-methyl detec- tion with a lower detection limit of 0.1 mM in differential pulse voltammetry mode [9]. Generally, MIPs technique can exhibit a high selectivity in the detection of organic micropollutants in environment, which have some advantages involving their predetermination, specificity and practicability for molecular recognition. However, traditional MIP suffered from some potential deficiencies such as lower capacity, poor site accessibility, incomplete template removal, low-affinity binding and slow mass transfer. Interestingly, MIPs surface materials using surface imprinting technique have been Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. (2014) developed, which enable the template-imprinting to situate at the surface of the nanostructured metal oxides. This can provide the advantages of favorable selectivity and fast association/dissocia- tion kinetics. Meanwhile, to realize the successful detection of micropollutants in complicated conditions, the stability of sensor based on MIPs requires highly concerned because of its complicated assembly. 2.1.4. Photoelectrocatalytic metal oxides Photoelectrochemical (PEC) measurement is a newly developed technique for the sensing platform based on the electron transfer among analyte, semiconductor (such as nanostructured TiO2), and electrode with photoirradiation. Coupling photoirradiation with electrochemical detection, PEC sensors combine the advantages of optical methods with electrochemical sensors. The photoelec- trochemical sensing technology is thus expected to monitor both electroactive and non-electroactive organic pollutants. During the recent years, the fascinating inorganic semiconductor titanium dioxide (TiO2) has attracted extensive attention in the detection of organic pollutants such as pesticides, herbicide, and phenolic compounds [6,33–39]. Li et al. reported the photocatalytic degradation combined with electrochemistry for determination of non-electroactive OPs insecticide residues [33]. However, the wide band gap of TiO2 (�3.2 eV, anatase) only allows it to absorb the ultraviolet light ( X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8e4 G Model TEAC-12; No. of Pages 8 as organophosphorus pesticides and phenolic compounds. Due to their high surface area, high adsorption capability for target organic micropollutants, good stability and biocompatibility, more and more attention has been paid to nanostructured metal oxides/ hydroxides, such as Fe3O4, TiO2, ZnO, ZrO2, and LDHs to fabricate enzyme-metal oxides/hydroxides hybrids-based electrochemical biosensors [7,10,40–42]. Due to their magnetic feature, Fe3O4 nanoparticles enable a rapid separation of the immobilized enzyme in a magnetic field. Another important property of Fe3O4 nanoparticles as electrochemical biosensors is their ability to provide a favorable microenvironment in which biomolecules may exchange electrons directly with an electrode [40]. Gan et al. have developed a novel disposable OPs enzyme biosensor based on Fe3O4/Au nanoparticles modified CNTs/nano-ZrO2/Prussian blue/ Nafion composite electrode. Non-electroactive OPs can be moni- tored by combining zirconia nanoparticles (selective absorbents) with enzyme against OPs (recognition elements). Fe3O4/Au and ZrO2 nanoparticles composite particles have been successfully employed as sensitive membrane matrix to immobilize AChE enzyme on a screen-printed carbon electrode (SPCE) surface to fabricate a disposable OPs biosensor. The surface of the biosensor can be renewed easily due to the easy removal of Fe3O4/Au/AChE from the biosensor surface by applying an external magnetic field. Chauhan et al. have described a unique approach of immobilizing covalently maize acetylcholinesterase onto Fe3O4/MWCNTs modi- fied gold electrode and its application in construction of an amperometric biosensor for determination of pesticides [43]. Be- sides, for detecting phenolic compounds, a biosensor based on Au- modified TiO2 nanotube arrays has been developed, in which TiO2 nanotubes were used to increase the loading of enzyme to improve the sensor performance. The proposed biosensor showed very high sensitivity toward seven different phenolic compounds and a detection limit of 90 nM was obtained in the detection of 3- nitrophenol [42]. Based on the 2D structure, unique ion-exchange properties as well as good biocompatibility, LDHs Zn3Al(OH)8Cl was also explored for the design of amperometric phenol biosensor. Such LDHs materials could be used as host matrices for the storage and release of biomolecules [10]. Besides, positively charged LDH nanosheets were used to construct catechol biosensor with a layer-by-layer technique through alternate assembly of LDH and negatively charged protein [7]. As reported, enzymatic electrochemical sensors based on nanostructured metal oxides/hydroxides were available in the detection of organic micropollutants, and interestingly, highly selective detection could be realized. However, further applica- tions are largely restricted because most of biomolecules can only be adsorbed on the surface of LDHs particles due to their large size, resulting in low loading and serious aggregation. 3. Toxic ions detection 3.1. Heavy metal ions (HMIs) 3.1.1. Striping method 3.1.1.1. NMOs/HOs and NMOs/HOs-nanocarbon composites. Most electrochemical systems for HMIs detection are based on conductive metal or carbon nanomaterials-based platform. However, non-conductive metal oxides used as sensing materials for the determination of HMIs have rarely been reported. Due to their excellent adsorption capacity, NMOs/HOs are expected to effectively accumulate HMIs in solution for electrochemical stripping. Recent years, three-dimensional (3D) hierarchically micro-/ nano-structured metal oxides have been received great research interest. The synergistic effect of their nanometer-sized building Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. (2014) blocks and overall micrometer-sized structure may be desirable for a variety of applications. Their nanometer-sized building blocks provide a high surface area, a high surface-to-bulk ratio, and high surface active sites which can interact with HMIs and their overall micrometer-sized structure provides desirable mechanical strength. In considering the high surface areas of the hierarchical nano-architectures, it has been recognized that the strong adsorption ability might provide new opportunities for improving their sensing performance in practical applications. There are only a few reports on the simultaneous and selective electrochemical detection using nanoparticles with hierarchical nanostructures, partly because of the difficulty in finding a suitable hierarchical unit having an overall deposition potential allowing the simulta- neous measurement of several elements. We have synthesized fried egg jellyfish-like AlOOH@SiO2/Fe3O4 porous magnetic micro- spheres using a simple template-induced method [44]. The synthesized material was found to have ultra-high adsorption capacity toward aqueous Zn(II), Cd(II), Pb(II), Cu(II) and Hg(II). Due to the high adsorption capacity toward HMIs, AlOOH@SiO2/Fe3O4 porous magnetic microspheres were found to be useful for the simultaneous detection of Hg(II), Cu(II), Pb(II), Cd(II) and Zn(II). Similarly, we have developed hierarchical nanostructured flower- like MgO, well-arranged porous Co3O4 microsheets, tube-in-tube SnO2, flower-like Mg–Al-LDHs, and urchin-like NiCo2O4 hollow spheres for HMIs detection based on their adsorption behaviors toward HMIs [11,45–48]. Following our work, Wu et al. synthe- sized mesoporous MgO nanosheets, mesoporous NiO nanosheets, and folding flake-like CuO sub-microstructure for HMIs detection [12,49,50]. To further improve the sensitivity and selectivity of NMOs/ HOs-based electrochemical sensor toward HMIs, we investigated the morphology effect of a-MnO2 nanocrystals on electrochemi- cal detection of toxic HMIs [51]. The three different morphologies (nanobowls, nanotubes, and nanoparticles) of MnO2-modified electrodes offered an obvious regularity in individual electro- chemical determination of Zn(II), Pb(II), Cu(II) and Hg(II): MnO2 nanobowls > MnO2 nanotubes > MnO2 nanoparticles. Further- more, we revealed an interesting facet-dependent electrochemi- cal behavior toward HMIs based the adsorption behaviors of Co3O4 nanocrystals [13]. As shown in Fig. 2, no effective capture on HMIs can be achieved when no sensing materials were immobilized on the working electrode, thus leading a weak stripping response. On the contrary, large amount of HMIs could be adsorbed onto the surface of Co3O4 nanocrystals and then diffused to the electrode surface. The more target HMIs were adsorbed onto the surface of nanoscale materials, the more HMIs diffused to the GCE and the stronger the stripping peak current obtained as a result. Furthermore, as Co3O4 nanoplates enclosed by (1 1 1) facet can adsorb more metal ions and diffuse more easily than Co3O4 nanocubes with (0 0 1) facet, the Co3O4 nanoplates have better electrochemical sensing performance than that of the (0 0 1) facet of Co3O4 nanocubes. Adsorption measurements and density-functional theory (DFT) calculations revealed that adsorption of HMIs was responsible for the difference of electrochemical properties. Our combined experi- mental and theoretical studies provide a solid hint to explain the mechanism of electrochemical detection of HMIs using nano- structured metal oxides. Although NMOs/HOs have been used to detect HMIs, most of these metal oxides/hydroxides are non-conductive, which will not facilitate electron transfer on the surface of electrode. Interesting- ly, CNTs and graphene have extraordinary electronic transport properties, large surface area, and high electrocatalytic activities. In order to improve the conductivity of NMOs/HOs, the nano- composite of metal oxides/hydroxides with carbon nanotubes or graphene have also been developed as sensing materials for HMIs l oxides/hydroxides-based electrochemical sensor for monitoring , http://dx.doi.org/10.1016/j.teac.2014.07.001 http://dx.doi.org/10.1016/j.teac.2014.07.001 Fig. 3. (A) Schematic of light scattering occurring on a ZnO/CdS modified electrode. (B) Electron–hole pairs generation, separation, and transfer between ZnO and CdS at a ZnO/CdS-modified electrode. Reprinted from Ref. [58] with permission of American Chemical Society. Fig. 2. Schematics of how adsorptive nanoscale materials exposed with different crystal facets could be designed to enhance the performance of electrochemical sensing. (A) Bare GCE. (B) Co3O4 nanocubes modified GCE. (C) Co3O4 nanoplates modified GCE. Reprinted from Ref. [13] with permission of Nature Publishing Group. X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8 e5 G Model TEAC-12; No. of Pages 8 [52–54]. We have developed AlOOH nanoplate–graphene and SnO2 nanoparticles–graphene nanocomposites for detection of Cd(II), Pb(II), Cu(II), and Hg(II) with improved analytical performance [53,54]. 3.1.1.2. Organic or biomolecules functionalized NMOs/HOs. Some organic or biomolecules have specific and high affinities toward specific HMIs. However, modified electrodes singly constructed by organic or biomolecules suffered from poor electron transfer and small electrochemical surface area. On the other hand, NMOs/HOs exhibit large surface-to-volume ratio, relatively high conductivity and strong adsorption ability. By combining the advantages of both organic or biomolecules and NMOs/HOs, organic or biomolecules functionalized metal oxides/hydroxides (such as TiO2, Fe3O4, and LDHs) have been used to obtain improved stability and sensitivity for HMIs determination [21,55,56]. An electrochemical Pb(II) sensor was developed on DNA-based vertically aligned conductive carbon hybridized TiO2 nanotube arrays (DNA/C-TiO2 NTs) [55]. The TiO2 tubular structures increased the surface area of the electrodes for DNA immobilization. The controllable carbon hybridization of the TiO2 NTs increased the conductivity, biocompatibility and hydrophilicity of the electrode. The designed DNA/C-TiO2 NTs sensor was superior in determination of lead with high sensitivity, selectivity and repeatability, as well as wide pH adaptability. A wide linear calibration ranging from 0.01 to 160 nM with the detection limitation at a picomole level (3.3 pM) were obtained by this sensor. Electrochemical sensors constructed by Fe3O4 nanoparticles or LDHs with organic or biomolecules have also been used to determine Cd(II), Pb(II), and Cu(II) with good analytical results [21,56]. The introduction of carbon-based nanomaterials and organic or biomolecules can improve the efficiency of nanostructured metal oxide/hydroxides, which could facilitate electron transfer or stabilize the sensors in detection of HMIs. There is still much room for the exploration of NMOs/HOs-based electrochemical sensors and challenges remain in the understanding and develop- ing of NMOs/HOs in the area of electrochemistry. For example, an on-going research to obtain NMOs/HOs with the desired morphol- ogy, structure and properties should be developed. Besides, in combination with other high-activity nanomaterials or function- alized materials, it would contribute to expanding the research on NMOs/HOs-based nanomaterials in the robust, sensitive and selective determination of HMIs, especially existed in environ- mental or complicated conditions. Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. (2014) 3.1.2. PEC method PEC sensing methods have been used to detect HMIs in order to further improve the sensitivity. Most of these researches are based on the increase or decrease of the photocurrent when HMIs react with the electrodes surface [57]. Wang et al. developed a highly sensitive and selective PEC sensor for Cu(II) based on the selective interaction between CdS quantum dots and Cu(II) in a triethano- lamine solution to form CuxS-doped CdS quantum dots, which disrupted the electron transfer from the conduction band of CdS to indium tin oxide (ITO) electrode surface and resulted in a decrease of photocurrent [57]. To enhance light scattering and the effective separation and transportation of electron–hole pairs, Shen et al. prepared ZnO/CdS hierarchical nanospheres for Cu(II) sensing [58]. As shown in Fig. 3, large-band-gap semiconductor ZnO coupled with small-band-gap CdS nanocrystals could facilitate charge separation through the fast electron transfer, and thus the light absorption and charge separation were significantly en- hanced, resulting in the improvement of photocurrent intensity and the ideal candidate material for photoelectrochemical determination toward Cu(II). The detection limit was found to be 0.01 mM. In addition, the PEC sensor based on ZnO/CdS hierarchical nanospheres exhibited good selectivity for Cu(II). Interestingly, the selective and sensitive detection of Cd(II) was also realized based on photocurrents obtained at CdSe clusters which were in situ electrodeposited on TiO2 nanotubes with the gradual addition of Cd(II) [59]. It is well known that almost no absorption in the visible region can be observed on the bare TiO2, because of the wide band gap (3.2 eV) of TiO2. However, CdSe clusters with a narrow band gap show clear absorption peaks in the l oxides/hydroxides-based electrochemical sensor for monitoring , http://dx.doi.org/10.1016/j.teac.2014.07.001 http://dx.doi.org/10.1016/j.teac.2014.07.001 Fig. 4. (A) Scanning electronic microscope image of Fe3O4 sub-microspheres. (B) Typical SWASV response of Fe3O4-[C4dmim][NTf2] modified SPE for analysis of As(III). ([C4dmim][NTf2], 1-butyl-2,3-dimethylimidazolium bis-(trifluoromethane sulfonyl)imide). Reprinted from Ref. [62] with permission of American Chemical Society. X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8e6 G Model TEAC-12; No. of Pages 8 visible region. As such, electrodeposition of CdSe clusters on TiO2 nanotubes to form CdSe/TiO2 heterostructures generated by electrochemical scanning with the gradual addition of Cd(II) into H2SO4 solution containing SeO2 added in advance produced the photocurrent under visible light irradiation [59]. The developed method showed a very broad linear range (1 nM–10 mM) for Cd(II), and the detection limit was achieved at 0.35 nM. Although better sensitivity and selectivity have been made, these photoelectro- chemical-based methods were confined to certain HMIs. Since the PEC sensors for the detection of HMIs are still in their infancy, some challenges still exist. As mentioned, the successful detection of HMIs is mainly based on the interaction between of such quantum dots and HMIs. Therefore, it is highly necessary to further expand by designing more specific quantum dots-based system or constructing of other novel chemical reactions on the surface of PEC electrodes for sensing other formal HMIs. Considering the toxicity issues of Cd-based quantum dots that limit their application in nature water samples, more work will have to be carried out to explore the large array of materials or less toxic QD materials. 3.2. Anions Anionic species such as arsenite, nitrite, iodate, bromate and chlorate are electroactive. However, the oxidation/reduction of them requires relatively high over potential at bare carbon electrode and exhibits poor response. An effective method is to modify the electrode surface with suitable electrocatalyst to lower the overpotential and improve sensitivity and selectivity. It is well known that numerous transition metals or their oxide species show interesting electrocatalytic properties for oxida- tion/reduction of these electroactive anions. Although conven- tional metal oxide modified electrodes have been successfully employed for monitoring these anions, they usually have many disadvantages such as reduced stability under physiological conditions, high detection limit, poor long-term stability, slow kinetics and complicated multi-step preparation methods. To overcome these limitations, some nanostructured metal oxides have been synthesized and their modified electrodes showed enhanced electrochemical oxidation/reduction performance [60–62]. 3.2.1. Arsenite and nitrite Most reports on electrochemical detection of As(III) still highly depend on noble metals (predominantly Au) in a strong acid condition, thus increasing the cost and hampering the widespread application. Recently, nanostructured metal oxides, such as Fe3O4, Co3O4, and IrO2 with higher adsorption capacity and electrocatalytic activities have been used to detect As(III) in mild solutions [60–62]. Both electrooxidation and electroreduc- tion methods have been investigated for As(III) determination using nanostructured metal oxides modified electrodes. Mafa- kheri et al. synthesized iridium oxide nanotubes using a template method for electrooxidation of arsenite at pH 5 using differential pulse voltammetry. Unfortunately, the nanotubes modified electrode can only be used for detection of arsenite at micromole level [60]. Salimi et al. electrodeposited Co3O4 nanoparticles on GCE electrode for electrooxidation of arsenite over the pH range 5–11 and a wide range of concentration [61]. The modified electrode can be used for micromolar or nanomolar concentra- tion range of arsenite detection using voltammetry or hydrody- namic amperometric techniques, respectively. Recently, we have reported a disposable platform completely free from noble metals for electrochemical detection of As(III) in drinking water under nearly neutral condition by square-wave anodic stripping voltammetry (Fig. 4) [62]. By combining the high absorptivity of Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. (2014) Fe3O4 nanospheres toward arsenite and the advantages of room temperature ionic liquid (RTIL), the Fe3O4-RTIL composite modified SPCE showed even better electrochemical performance than commonly used noble metals. Among various Fe3O4-RTIL composite modified SPCEs, Fe3O4-[C4dmim][NTf2] showed the best performance. In contrast to gold-based and nonprecious metal systems, the obtained sensitivity (4.91 mA ppb�1) in this work was the highest and the corresponding LOD was the lowest [62]. In addition, in combination of noble metal nanoparticles, the metal oxides-based nanocomposites have also been devel- oped for arsenite determination based on the high adsorption ability of metal oxides nanoparticles and the excellent electro- catalytic ability of noble metal nanoparticles. Cui et al. developed a novel nanocomposite composed of magnetic Fe3O4 nanopar- ticles and gold nanoparticles for determination of trace amounts of arsenite [63]. The obtained results showed that the sensitivity of arsenite was significantly improved on nanocomposite modified GCE in comparison with Au nanoparticles modified GCE. Under optimal conditions, the detection limit of 0.00097 ppb was achieved. Apart from As(III), nitrite determination is also significant and necessary for environment security and public health. Recently, determination of nitrite based on electrochemical oxidation has attracted great attention by offering several advantages. CuO- based nanostructures have been regarded as the most promising electrocatalyst for determination of nitrite in water [64,65]. Some novel nanostructured CuO have been synthesized to improve their sensitivity and selectivity. Cao et al. reported the synthesis of hierarchically nanostructured CuO chains assembled with small nanorods [64]. Due to their intrinsic structural stability, ordered porosity and high specific surface area, the CuO nanochains provided larger contact area and well distributed porosity between sensing materials and sensed species than the dispersed nanoparticle or bulk CuO powders. A wide linear range (0.004–3.7 mM), low LOD (0.3 mM), highly reproducible response (RSD of 2.0%), and an excellent long-term stability were achieved by CuO nanochains modified electrode. Similarly, Zhang et al. synthesized nanoflake-assembled flower like CuO by a hydro- l oxides/hydroxides-based electrochemical sensor for monitoring , http://dx.doi.org/10.1016/j.teac.2014.07.001 http://dx.doi.org/10.1016/j.teac.2014.07.001 X.-Y. Yu et al. / Trends in Environmental Analytical Chemistry xxx (2014) e1–e8 e7 G Model TEAC-12; No. of Pages 8 thermal method and used them to determine nitrite by amperometric method [65]. Except for CuO, Fe3O4@Au core- shell nanoparticles have also been used for nitrite determination [66]. The gold shell prevented Fe3O4 nanoparticles from aggregation. The modified electrode presented higher peak current and lower oxidation potential for nitrite, which can be explained by the synergetic effect of Au and Fe3O4 nanoparticles in electrocatalytic oxidation of nitrite. 3.2.2. Iodate, bromate and chlorate Modification of electrode surfaces with electron transfer mediators such as tungsten oxide and iridium oxide have been used to determine iodate, bromate and chlorate in water. For example, Salimi et al. developed the electrodeposited iridium oxide nanoparticles to modify electrodes for nanomolar detection of iodate and periodate by amperometric method [67]. To enhance the stability and electrocatalytic activity of WO3 for iodate reduction, a composite nanofilm of polyaniline and WO3 was fabricated [14]. Amperometric experiment results revealed a good linear relationship with concentration of iodate from 20 to 500 mM, with a high sensitivity of 0.54 mA mM�1 and a detection limit of 2.7 mM for the determination of iodate. The detection of such anions as arsenic, nitrite and iodate is of great importance in the environmental analysis. Since that limited NMOs/HOs materials have been explored in the detection of anions, it is expectable that the research in developing more NMOs/HOs is required and it would be benefit to realize the detection of more anions. Furthermore, the mechanism in the response of anions is incomplete understanding to date. The related work should be carried out on this subject. 4. Conclusions and outlook Nanostructured metal oxide/hydroxide-based electrochemi- cal sensors provide a new horizon for novel functions with a variety of applications in environmental monitoring. Various NMOs/HOs have already made a major impact on determination of micropollutants in water, ranging from organic micropollu- tants to toxic ions. The useful properties of NMOs/HOs suggest that future interdisciplinary research is likely to lead to a new generation of environmental electrochemical sensors. Highly effective micropollutants detection protocols based on NMOs/ HOs open up the possibility of creating electrochemical sensors for sensitive, selective, and fast determination of micropollutants. Various NMOs/HOs with different size, morphology, exposed crystal facets, functionality and NMOs/HOs-based nanocomposites have been extensively developed to enhance the sensitivity or selectivity of environmental electrochemical sensor. Such improvements related to tuning the physical or chemical proper- ties of NMOs/HOs still need to be better clarified so as to understand the obtained responses and to design better detection strategies. It is important to understand nano-chemistry, reactivi- ty, and possible mechanisms involved in the interaction between NMOs/HOs and micropollutants in water. And also, a wide range of new NMOs/HOs is expected to further expand the area of environmental electrochemical sensor. New strategies for the synthesis of novel nanostructured NMOs/HOs are likely to result in new sensing interfaces. Nevertheless, environmental electrochemical sensors are rarely tested in real or industrial samples, which usually show considerable analytical complexity. Issues related to the repro- ducibility and stability of these systems have rarely been studied. The NMOs/HOs-based electrochemical sensors need to be inte- grated into in situ devices to test their performance in detection of multiple and complex micropollutants system. Please cite this article in press as: X.-Y. Yu, et al., Nanostructured meta environmental micropollutants, Trends Environ. Anal. Chem. 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Acta 661 (2010) 28. l oxides/hydroxides-based electrochemical sensor for monitoring , http://dx.doi.org/10.1016/j.teac.2014.07.001 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0270 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0270 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0275 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0275 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0280 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0280 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0285 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0290 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0290 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0295 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0300 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0300 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0305 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0305 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0310 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0315 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0320 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0325 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0330 http://refhub.elsevier.com/S2214-1588(14)00031-2/sbref0335 http://dx.doi.org/10.1016/j.teac.2014.07.001 Nanostructured metal oxides/hydroxides-based electrochemical sensor for monitoring environmental micropollutants Introduction Organic micropollutants detection Non-enzymatic electrochemical sensors Adsorptive NMOs/HOs Electrocatalytic NMOs/HOs Molecularly imprinted polymer/metal oxides hybrids Photoelectrocatalytic metal oxides Enzymatic electrochemical sensors Toxic ions detection Heavy metal ions (HMIs) Striping method NMOs/HOs and NMOs/HOs-nanocarbon composites Organic or biomolecules functionalized NMOs/HOs PEC method Anions Arsenite and nitrite Iodate, bromate and chlorate Conclusions and outlook Acknowledgements References