Understanding signal amplification strategies of nanostructured electrochemical sensors for environmental pollutants

Available online at www.sciencedirect.com

ScienceDirect

Current Opinion in

Electrochemistry

Review Article

Understanding signal amplification strategies of nanostructured electrochemical sensors for environmental pollutants Xiaorong Gan1 and Huimin Zhao2 Abstract

Nanomaterials used in electrochemical sensors can significantly improve the analytical performance to environmental pollutants. This review mainly discusses the strategies for signal amplification by the rational design of nanoelectrode materials from the perspective of mass and electron transfer processes of electrode/solution interface. First, the advantages and features of nanostructured electrochemical sensors for environmental pollutants are summarized. Then, the detailed discussions are focused on the signal amplification strategies by regulating dimensionality, atomic arrangement, and composition of electrode materials. This review gives a unique insight about the influences of electrode material design on mass and electron transfer processes of electrochemical sensors. Finally, on the basis of the current achievements in the field of nanomaterials, the perspectives on the challenges and opportunities for the exploration of nanostructured electrochemical sensors are put forward. Addresses 1 Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China 2 Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China Corresponding author: Zhao, Huimin ([email protected])

Current Opinion in Electrochemistry 2019, 17:56–64 This review comes from a themed issue on Environmental Electrochemistry Edited by Xing-Jiu Huang For a complete overview see the Issue and the Editorial Available online 25 April 2019 https://doi.org/10.1016/j.coelec.2019.04.016 2451-9103/© 2019 Elsevier B.V. All rights reserved.

Introduction Environmental pollutants, mainly including inorganic, synthetic organic, and microbiological pathogens, are a serious threat to human health and Earth’s ecosystems [1,2]. To lower the environmental risks of pollutants, it is of great importance to develop simple but reliable Current Opinion in Electrochemistry 2019, 17:56–64

analytical methods with high sensitivity and selectivity. Compared with traditional analytical methods based on optical and mass spectra, electrochemical sensors are low cost, easily miniaturized, sensitive, selective, rapid, and facile techniques [3]. On the basis of the mechanism of selectively detecting environmental pollutants, electrochemical sensors can be divided into two types: direct redox process and indirect redox process [4]. The former is involved in electron gain and loss for recognition events without the addition of signaling transducers (for the latter) that were frequently used in electrochemical biosensors (e.g. [Ru(NH3)6]3þ cations [5]). In the case, electrode materials play the dual role of sensing probe and signal amplification; moreover, the sensitivity is strongly dependent on the mass and electron transfer between the electrode/solution interface. From the perspective of structureeproperty relationships, low-dimensional nanomaterials are among the first choices for constructing high-performance electrochemical sensors because of their unique properties beneficial for signal amplification and other performance parameters, such as selectivity and processability [3,6]. The main advantages that have driven the impressive success of using nanomaterials for electroanalytical applications lie in four aspects. First, nanomaterials have the potential as non-invasive diagnostic tools and the capacity for combining multiple modalities within a single probe. This enables far higher sensitivities (e.g. single molecule detection) to be achieved, which leads to further clarity and deeper insights into in vivo processes, such as the detection of heavy metal ions in rat brain [7]. Second, owing to the Ohmic (iR) drop distortion, small time constants (RsCdl, Rs is the solution resistance and Cdl for the double layer capacitance), and miniaturized electrode dimensions, nanostructured electrochemical sensors can be used to detect the analytes in poorly conducting media and the ultrafast electron-transfer kinetics that are often too fast to investigate with conventional electrodes [8]. The high specific surface area of nanomaterials can load more sensing elements, enlarge the electrode/electrolyte junction area, and offer abundant active sites available for adsorption and electrochemical reactions [9e11]. As a result, it improves the sensitivity or the detection limit of electrochemical sensors, as Faradaic current usually www.sciencedirect.com

Nanostructured electrochemical sensors for environmental pollutants Gan and Zhao

scales linearly with electrode area. Third, as the electrode (ensembles) size decreases, the rate of mass transport to and from the electrode increases. Finally, the properties of nanomaterials, such as wettability and conductivity, can be flexibly regulated in many ways, for example, Lewis acid-base chemistry and click reactions [12e16]. Therefore, the introduction of nanomaterials in electrochemical sensors can significantly improve the analytical performance. In past few years, our group did a series of research projects about low-dimensional nanomaterials used for constructing highly sensitive and selective chemical sensors or biosensors to detect trace environmental pollutants [16e24]. On the basis of our experimental data and some related work available, the discussions of this review will highlight the fundamental strategies about the rational design of nanomaterials for improving analytical performance of electrochemical sensors, in terms of signal amplification or the sensitivity of detecting environmental pollutants, without considering influences of electrochemical measurement methods. Although several review articles about nanostructured electrochemical sensors were given or divided on the basis of either the type of electrode materials (e.g. nanoparticles or nanosheets) [4,25e29] or analytes (e.g. heavy metal ions or organic pollutants) [30,31], this review, from another perspective of the interfacial mass and electron transfer behaviors, gives a more general summarization about signal amplification using nanomaterials which can be controllable by changing dimensionality, atomic

57

arrangement, and composition (Figure 1). Finally, some personal opinions on the development direction and potential challenge of nanostructured electrochemical sensors will be given. It is reasonable to believe that this review will offer some useful guide to constructing ultrasensitive electrochemical sensors for environmental pollutants.

Signal amplification strategies in nanostructured electrochemical sensors Nanoelectrodes can detect or monitor the fast kinetics of electroactive matters, even have great potential for single-molecule electrochemistry, which cannot be realized via traditional microelectrodes, mainly because of the high electric field (108 V cm1) at the nanometer scale interphase [32]. A general electrode process can be divided into three parts (Figure 1): electronic conductor region, ionic conductor region, and their interface region. For the electronic conductor region, it mainly refers to electron transfer in bulk part of electrode material, which determines conductivity and is related to the electronic band structure of material [33]. In the interface region, the processes are determined by surface properties (e.g. structural defects and wettability [34]) and electrical double layer of electrode for detecting trace or ultra-trace pollutants [8]. In general, the reactivity of redox or electrocatalytic reactions depends largely on the interfacial structure rather than on the bulk [35]; that is, the electrode/electrolyte interface plays the predominant role in the binding, transformation, and transport of surface species, such as

Figure 1

Fundamental strategies of regulating electrode material structures for signal amplification (left) and pathway of a general electrode reaction (right). Whether the charge of electrode is negative or positive with respect to the solution depends on the potential across the interface and the composition of solution. Here, carbon nanomaterials are used as the typical example; IHP and OHP represent inner Helmholtz plane and outer Helmholtz plane, respectively; active sites mean the specific places where the targeting analytes can specifically react with electrode materials through redox reactions. www.sciencedirect.com

Current Opinion in Electrochemistry 2019, 17:56–64

58 Environmental electrochemistry

electrons, adsorbents, and intermediates [36]. Here, our concerns are focused on the interfacial recognition events or processes with electron gain and loss, where the environmental pollutants in solution directly react on the surface or interface of electrode materials. In general, for a given redox reaction, both the electrontransfer kinetics and the mass transport contribute to the overpotential or current density [34]. The following discussions highlight how mass and electron transfer is affected by the intertwined factors, including dimensionality, atom arrangement, and composition of electrode materials, to further realize the signal amplification of electrochemical sensors for high sensitivity because atomic-level microstructures have great influence on the macroscopic properties of electrode materials. Of note, the strategies for signal amplification can be proper for the other type of electrochemical sensors based on indirect redox process. Dimensionality of electrode materials

The changes in dimensionality from bulk form to low dimensionality directly vary the electronic (the electronic conductor region) and chemical properties (interface region), which further affect electrode processes (Figure 1) [4,9,25,27,37,38]. At the nanometer scale, quantum-mechanical effects involved in electron transfer become relevant [35]. With respect to layered materials, as the crystal thickness is reduced from bulk to single layer, quantum confinement effects cause significant changes of the electronic structure [39]. For example, layered transition metal dichalcogenides exhibit the transition from indirect to direct bandgap semiconductor with enlarged band gap [4,40]. Moreover, an abundance of low-coordinated surface atoms will appear, especially on plane edges and defect sites because of energy fluctuation. The lattice point defects, such as vacancies and interstitials, can affect the adsorption/desorption of analytes, the conductivity, surface wettability, and other surface properties of electrode materials, which determine the mass and electron transfer processes. In general, the ultra-thin nature of two-dimensional (2D) materials can amplify the role of these structural motifs in defining their electrode responses [4,13,21,27,35,41]. For example, with the decrease of thickness, the bandgap of 2H-MoS2 nanosheets will enlarge from 1.2 eV (bulk form with indirect bandgap) to 1.9 eV (single layer with direct bandgap); however, more active sites mainly located at the plane edges are available for electrochemical reactions [16,41]. Compared with bulk counterpart, 2HMoS2 nanosheets exhibit the improved electrochemical activity [4,16,26,28,35,41]. For 2H-MoS2 nanosheets obtained by liquid phase exfoliation method (Figure 2a), the influences of thickness on electrochemical activity is more obvious than surface components (e.g. covalent functionalization) (Figure 2b) [16,41]. As the building block for electrochemical sensors, MoS2 nanosheets Current Opinion in Electrochemistry 2019, 17:56–64

obtained in N, N-dimethylformamide by liquid phase exfoliation can obviously enhance the analytical performance (Figure 2c,d) for Cd2þ detection with a measured detection limit of 0.2 nM and a linear range from 2 nM to 20 mM [16]. Similarly, zero-dimensional (0D) nanoparticles can play the role as the electron wire for signal amplification because of unique electrochemical properties, such as strong ability for electron transfer, high catalysis activity, and large contact area [7]. However, 0D nanoparticles are suffered from inactivation or dissolution because of electrode fouling or electrochemical corrosion, because the active sites or crystal planes with high surface energy are easily covered by electrooxidation (e.g. organic pollutants) or electroreduction (e.g. heavy metal ions) resultants or disappeared as applying large redox potential [42]. For example, 0D nanoparticle-based electrochemical sensors for heavy metal ions will result in a certain amount of pure metal covering nanoparticles because of electrodeposition. In addition to the type and number of active sites, the dimensionality can change the microenvironment of active sites (Figure 1), which consequently enhances the electrode kinetics by the confined effects (the geometric constraint and confinement field) so that many electrochemical reactions in nanospaces, such as nanopores and van der Waals (vdWs) gaps, are much faster than in an open system [43e45]. Atom arrangement of electrode materials

The influences of atom arrangement on the electrochemical properties of materials are attributed to polytypic structures, phase transition, and varying exposures of surface atoms, such as the phase transition of 2H-MoS2/1T-MoS2, and bimetallic or multimetallic nanoparticles (e.g. core/shell, segmented), which affect the type and number or density of active sites [42]. Electrode materials with same composition but different atomic arrangements may exhibit completely different electrochemical properties. For example, 2HMoS2 is semiconducting, whereas 1T-MoS2 is metallic with zero band gap and high conductivity [46e 48] because of the localization behavior of the d-band electrons in transition metals. Monolayer 2H-MoS2 exhibits poor capability of charge transfer with carrier mobility of 0.1e10 cm2 V1 s1. In contrast, 1T-MoS2 is hydrophilic and 107 times more conductive than the 2H phase, which is beneficial for applications in electrochemical fields [46]. Another typical example (Figure 3a) is the allotropes of carbon, including but not limited to, diamond (tetrahedral sp3-bond network), graphene (sp2), and amorphous carbon (sp3 and sp2 hybridized bonds). Diamond is insulator, and both graphene and amorphous carbon are excellent conductors. The utility of doped-nanodiamond for electroanalytical applications has been paid more www.sciencedirect.com

Nanostructured electrochemical sensors for environmental pollutants Gan and Zhao

59

Figure 2

(a) 2H-MoS2 nanosheets with decreased and ununiform thickness obtained by liquid phase exfoliation (LPE) method. (b) Cyclic voltammograms plots of glass carbon electrode (GCE) modified by bulk MoS2 and MoS2 nanosheets obtained by LPE in N, N-dimethyl formamide (MoS2-a), formamide (MoS2-b), and 1-methyl-2-pyrrolidinone (MoS2-c). (c) Square wave anodic stripping voltammetry responses of MoS2-a/GCE to varying Cd2+ concentrations (a–j: 0.2 nM–50 mM), and (d) linear relationship of the peak currents versus the logarithm of Cd2+ concentrations. Panel b–d were reproduced from Ref. [16] with permission from Elsevier B.V.

attention because of their excellent mechanical properties, chemical and electrochemical inertness, combined with low background current, and the wide operating potential window between the onset potentials for oxygen and hydrogen evolution [49], especially, its electrode fouling and stability [50]. In terms of mass transfer, as electrode materials amorphous carbon is better than graphene that has superior conductivity [51]. Research also has demonstrated that for hydroquinone detection, nanocarbon-modified glass carbon electrode (nanocarbon-glass carbon electrode [GCE]) exhibited higher current density (Figure 3b), heterogeneous rate constant, and sensitivity (Figure 3c) than those of graphene-GCE and nanodiamond-GCE [50]. Composition of electrode materials

The composition change can have obvious impacts on properties of low-dimensional nanomaterials. Dependent on composition and layer thickness, 2D layered transition metal dichalcogenides can display a wealthy of fascinating properties ranging from semiconductors and semimetals to true metals and superconductors www.sciencedirect.com

[4,52]. In addition, the nanomaterial compositions can be regulated frequently by surface functionalization at the atomic level (elemental doping and component tuning) or at the molecular level (copolymerization and surface covalent functionalization) and by the combination of different nanomaterials, such as 0D/2D, 1D/ 2D, 2D/2D, and 3D hierarchical hybrid materials [4,13,16,20,21,30,31,50]. In general, the nanocomposites can minimize the drawbacks and maximize the advantages of the individual components; more importantly, in addition to the desired performance, some novel functions might also be generated. For example, surface functionalization of carbon black (CB) with b-cyclodextrin can improve the wettability of CB particles and lower the electron-transfer resistance between CB particles [53]. As a result, CB/bcyclodextrin-nanocomposite-modified screen-printed carbon electrodes can simultaneously detect flutamide and 4-nitrophenol (4-NP) with high sensitivity, selectivity, and reproducibility. In addition to surface functionalization, the composition change made by element doping can be used to flexibly regulate electron energy band structures and surface properties of Current Opinion in Electrochemistry 2019, 17:56–64

60 Environmental electrochemistry

Figure 3

(a) Atomic configurations (from left to right) of diamond, graphene, and nanocarbon (amorphous). (b) Cyclic voltammograms of glassy carbon electrode (GCE, black plot), nanodiamond-GCE (orange plot), graphene-GCE (blue plot), and nanocarbon-GCE (red plot) in 0.1 M phosphate buffer and 0.2 mM hydroquinone (HQ). (c) HQ detection based on amperometric experiments of GCE, nanodiamond-GCE, graphene-GCE, and nanocarbon-GCE. Panel b and c were reproduced from Ref. [50] with permission from Nature Publishing Group.

electrode materials [54]. For example, compared with pure diamond with wide band gap, element-doped nanodiamonds exhibit better electrocatalytic activity with high current density and were widely applied in environmental analysis for various trace pollutants such as hormones bisphenol A and 4-nonylphenol [54]. For such hybrid materials as 0D/2D, 2D nanomaterials not only play the role as an electron transfer mediator or electrochemical catalyst for signal amplification but also play the role as co-catalysts or supporting materials that improve the dispersion of 0D nanoparticles [20]. In return, 0D nanoparticles can minimize or prevent the aggregation of 2D nanosheets so that the unique physicochemical properties can be kept [55]. Such synergetic effects of hybrid electrode materials for signal amplification can be well explained by the electrode material of Au nanoparticle-decorated hydrogen-incorporated TiS2 nanosheets (PATP/AuHxTiS2) for Cu2þ detection. Au nanoparticles were not only acted as an excellent electrical catalyst to facilitate electron transfer and as a substrate for immobilizing PATP molecules (the sensing element) but also effectively avoided the aggregation of conductive HxTiS2 nanosheets [20]. Therefore, the electrochemical sensors based on the nanocomposites Current Opinion in Electrochemistry 2019, 17:56–64

generally exhibit better analytical performance than electrochemical sensors based on pure nanomaterials [20,21]. In nanocomposite electrodes, the introduction of conductive nanomaterials, especially carbon materials (e.g. graphene and carbon nanotube), can significantly improve the sensitivity and stability of electrochemical sensors for detecting environmental pollutants such as 2, 4-dichlorophenol and nitrite [56e 58], where the conductive nanomaterials can improve the electron transfer as the “electron wires” and/or minimize electrode surface fouling [57,58]. Relative to 0D and 1D nanomaterials, the composition control of ultrathin 2D nanomaterials by surface modification or functionalization has a more profound effect on the electronic energy band structure, surface active sites, and electrical conductivity because 2D nanomaterials lack bulk volume and only consist of surface atoms [13,59]. Physical and/or chemical adsorption on 2D ultrathin nanosheets can result in p-dopants or ndopant effects. For example, hydric titanium disulfide ultrathin (HxTiS2) nanosheets (Figure 4a) showed exclusively high electrical conductivity of 6.76  104 S/ m (H0.515TiS2) at room temperature because of the reinforcement of the electroneelectron correlations (Figure 4b), which is completely different from bulk www.sciencedirect.com

Nanostructured electrochemical sensors for environmental pollutants Gan and Zhao

TiS2 and layered TiS2 nanosheets. The conductivity of HxTiS2 nanosheets is even better than 2D thin films of graphene (5.5  104 S/m) assembled in the solution [21,60]. By combining HxTiS2 nanosheets with polyaniline, the synergetic effects can be observed in electrode stability, interfacial electron transfer and mass transfer [21]. The electrochemical sensor based on three-dimensional porous HxTiS2 nanosheet with polyaniline nanocomposites exhibited highly sensitive and selective assaying of Cu2þ with a detection limit of 0.7 nM (signal-to-noise ratio = 3) and a linear range from 25 nM to 5 mM, under optimal conditions [21].

Conclusion and outlook In this review, the general strategies for signal amplification used in nanostructured electrochemical sensors for environmental pollutants were discussed. The focus is the influences of electrode material parameters, including dimensionality, atomic arrangement, and composition on mass and electron transfer of nanoelectrode, where the mass transfer as another critical factor was not paid enough attention. For nanostructured electrochemical sensors, a grand challenge is

61

the reproductivity of initial high performance (e.g. sensitivity, selectivity, and stability) even in the complex detection environments or media. In essence, the challenge can be overcome by controlled synthesis or design of nanomaterials for sensing elements and/or signal amplification and by deeply understanding about surface or interface processes of the recognition events. From a long-term view, nanomaterials, especially 2D nanomaterials, will play a critical role for constructing high-performance electrochemical sensors for environmental pollutants because their ultrathin structure where the electrons/holes are confined to a plane makes 2D nanomaterials sensitive (fast response) to the interaction with targeting analytes. In addition to the control synthesis of electrode materials, further efforts may be focused on some in-situ characterizations (e.g. electrochemical surface-enhanced Raman scattering [61] and electrochemical in situ Fourier transform infrared spectroscopy [62]) with better exploring the interface processes. Moreover, the combining theory and experiment for better design of electrode materials and understanding of electrochemical processes need be

Figure 4

(a) Atomic structure of hydrogen-incorporated TiS2 (HxTiS2) ultrathin nanosheets. (b) Temperature-dependent electrical resistivity for hydrogenincorporated TiS2 ultrathin nanosheets with different amounts of hydrogen atoms. Panel a and b were reproduced from [60] with permission from American Chemical Society. (c) Square wave anodic stripping voltammetry responses of three-dimensional porous HxTiS2 nanosheet-polyaniline (PANI) nanocomposite-modified glass carbon electrode (GCE) for different concentrations of Cu2+ (a–h: 0–5 mM). (d) Linear relationship between the peak currents and the logarithm of Cu2+ concentrations. Panel c and d were reproduced from [21] with permission from American Chemical Society.

www.sciencedirect.com

Current Opinion in Electrochemistry 2019, 17:56–64

62 Environmental electrochemistry

paid more attention because density functional theory calculations can monitor the changes of local electronic structure of materials and binding energies of the reaction intermediates at atomic or molecular level [33,63].

Conflict of interest statement Nothing declared.

Acknowledgements This study was supported by the National Natural Science Foundation of China (No. 21777012) and the Programme of Introducing Talents of Discipline to Universities (B13012). Also, the study was supported by the Fundamental Research Funds for the Central Universities (No. 2019B02414 and No. 2019B44214) and PAPD, Open Foundation of Key Laboratory of Industrial Ecology and Environmental Engineering, MOE (KLIEEE-18-02).

References Papers of particular interest, published within the period of review, have been highlighted as: * of special interest * * of outstanding interest 1.

 ndulescu R: Electrochemical Feier B, Florea A, Cristea C, Sa detection and removal of pharmaceuticals in waste waters. Curr Opin Electrochem 2018, 11:1–11. https://doi.org/10.1016/j. coelec.2018.06.012.

2.

Jaffrezic-Renault N, Mousty C: Environmental electrochemical processes: remediation, energy harvesting and monitoring. Curr Opin Electrochem 2018, 11:A1–A4. https://doi.org/10.1016/j. coelec.2018.11.005.

3.

Chen A, Chatterjee S: Nanomaterials based electrochemical sensors for biomedical applications. Chem Soc Rev 2013, 42: 5425–5438. https://doi.org/10.1039/c3cs35518g.

4. **

Gan XR, Zhao HM, Schirhagl R, Quan X: Two-dimensional nanomaterial based sensors for heavy metal ions. Microchim Acta 2018, 185. https://doi.org/10.1007/s00604-018-3005-1. The review discusses the developments, synthesis methods, overall properties of two-dimensional nanomaterials, and the advantages and applications in sensors and assays for heavy metal ions 5.

Gan XR, Zhao HM, Chen S, Quan X: Electrochemical DNA sensor for specific detection of picomolar Hg (II) based on exonuclease III-assisted recycling signal amplification. Analyst 2015, 140:2029–2036. https://doi.org/10.1039/c5an00082c.

6. *

Zhu CZ, Yang GH, Li H, Du D, Lin YH: Electrochemical sensors and biosensors based on nanomaterials and nanostructures. Anal Chem 2015, 87:230–249. https://doi.org/10.1021/ ac5039863. This review provides the reader with a clear and concise view of new advances in areas ranging from electrode engineering, strategies for electrochemical signal amplification, and novel electroanalytical techniques used in the miniaturization and integration of the sensors. 7. **

Shao XL, Gu H, Wang Z, Chai XL, Tian Y, Shi GY: Highly selective electrochemical strategy for monitoring of cerebral Cu2+ based on a carbon dot-TPEA hybridized surface. Anal Chem 2013, 85:418–425. https://doi.org/10.1021/ac303113n. This paper reported an electrochemical sensor based on C-Dot-TPEA hybridized nanocomposites for determination of Cu2+ in the complex brain system. 8.

Pennathur S, Santiago JG: Electrokinetic transport in nanochannels. 1. theory. Anal Chem 2005, 77:6772–6781. https:// doi.org/10.1021/ac050835y.

9. **

Yu XY, Liu ZG, Huang XJ: Nanostructured metal oxides/ hydroxides-based electrochemical sensor for monitoring environmental micropollutants. Trends Environ Anal Chem 2014:3–4. https://doi.org/10.1016/j.teac.2014.07.001; 2014.

Current Opinion in Electrochemistry 2019, 17:56–64

This review discusses the metal oxides/hydroxides-based electrochemical sensor for detection of micropollutants including toxic organic micropollutants, heavy metal ions (HMIs), and anions in water. 10. Hua M, Zhang SJ, Pan BC, Zhang WM, Lv L, Zhang QX: Heavy metal removal from water/wastewater by nanosized metal oxides: a review. J Hazard Mater 2012, 211:317–331. https:// doi.org/10.1016/j.jhazmat.2011.10.016. 11. Wei D, Bailey MJ, Andrew P, Ryhanen T: Electrochemical bio* sensors at the nanoscale. Lab Chip 2009, 9:2123–2131. https:// doi.org/10.1039/b903118a. This review is focused on the interface between nanotechnology and biosensors. 12. Mistry H, Varela AS, Kühl S, Strasser P, Cuenya BR: Nanostructured electrocatalysts with tunable activity and selectivity. Nat Rev Mater 2016, 1. https://doi.org/10.1038/natrevmats. 2016.9. 13. Lei SD, Wang XF, Li B, Kang JH, He YM, George A, Ge LH, Gong YJ, Dong P, Jin ZH, Brunetto G, Chen WB, Lin ZT, Baines R, Galvao DS, Lou J, Barrera E, Banerjee K, Vajtai R, Ajayan P: Surface functionalization of two-dimensional metal chalcogenides by Lewis acid-base chemistry. Nat Nanotechnol 2016, 11:465–471. https://doi.org/10.1038/nnano.2015. 323. 14. Presolski S, Pumera M: Covalent functionalization of MoS2. Mater Today 2016, 19:140–145. https://doi.org/10.1016/j.mattod. 2015.08.019. 15. Voiry D, Goswami A, Kappera R, e Silva CDCE, Kaplan D, Fujita T, Chen MW, Asefa T, Chhowalla M: Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering. Nat Chem 2015, 7:45–49. https://doi.org/10. 1038/nchem.2108. 16. Gan XR, Zhao HM, Wong KY, Lei DY, Zhang YB, Quan X: Co* * valent functionalization of MoS2 nanosheets synthesized by liquid phase exfoliation to construct electrochemical sensors for Cd (II) detection. Talanta 2018, 182:38–48. https://doi.org/10. 1016/j.talanta.2018.01.059. This paper explored the possibility of the covalent functionalization of MoS2 nanosheets by liquid phase exfoliation, and the influences of surface functional groups on the recognition performances towards heavy metal ions. 17. Zhao HM, Li YX, Tan B, Zhang YB, Chen X, Quan X: PEGylated molybdenum dichalcogenide (PEG-MoS2) nanosheets with enhanced peroxidase-like activity for the colorimetric detection of H2O2. New J Chem 2017, 41:6700–6708. https://doi.org/ 10.1039/c7nj00899f. 18. Wang A, Zhao H, Chen X, Tan B, Zhang Y, Quan X: A colorimetric aptasensor for sulfadimethoxine detection based on peroxidase-like activity of graphene/nickel@palladium hybrids. Anal Biochem 2017, 525:92–99. https://doi.org/ 10.1016/j.ab.2017.03.006. 19. Tan B, Zhao HM, Wu WH, Liu X, Zhang YB, Quan X: Fe3O4AuNPs anchored 2D metal-organic framework nanosheets with DNA regulated switchable peroxidase-like activity. Nanoscale 2017, 9:18699–18710. https://doi.org/10.1039/ c7nr05541b. 20. Gan XR, Zhao HM, Quan X, Zhang YB: An electrochemical sensor based on p-aminothiophenol/Au nanoparticledecorated HxTiS2 nanosheets for specific detection of picomolar Cu (II). Electrochim Acta 2016, 190:480–489. https://doi. org/10.1016/j.electacta.2015.12.145. 21. Gan XR, Zhao HM, Chen S, Yu HT, Quan X: Three-dimensional * * porous HxTiS2 nanosheet-polyaniline nanocomposite electrodes for directly detecting trace Cu(II) ions. Anal Chem 2015, 87:5605–5613. https://doi.org/10.1021/acs.analchem.5b00500. This paper demonstrated a rapid, sensitive, and specific detection of 2+ Cu based on the synergetic effects of HxTiS2 nanosheet and polyaniline (PANI). 22. Rong R, Zhao HM, Gan XR, Chen S, Quan X: An electrochemical sensor based on graphene-polypyrrole nanocomposite for the specific detection of Pb (II). Nano 2017, 12. https://doi.org/10.1142/S1793292017500084.

www.sciencedirect.com

Nanostructured electrochemical sensors for environmental pollutants Gan and Zhao

23. Tan B, Zhao HM, Du L, Gan XR, Quan X: A versatile fluorescent biosensor based on target-responsive graphene oxide hydrogel for antibiotic detection. Biosens Bioelectron 2016, 83: 267–273. https://doi.org/10.1016/j.bios.2016.04.065. 24. Dang XM, Zhao HM, Wang XN, Sailijiang T, Chen S, Quan X: Photoelectrochemical aptasensor for sulfadimethoxine using g-C3N4 quantum dots modified with reduced graphene oxide. Microchim Acta 2018, 185. https://doi.org/10.1007/s00604-0182877-4. 25. Rassaei L, Marken F, Sillanpää M, Amiri M, Cirtiu CM, Sillanpää M: * * Nanoparticles in electrochemical sensors for environmental monitoring. TrAC Trends Anal Chem (Reference Ed) 2011, 30: 1704–1715. https://doi.org/10.1016/j.trac.2011.05.009. This review summarizes the methods for preparing nanoparticles and modifying electrode surfaces with NPs used in electrochemical analysis of environmental pollutants.

63

electrochemical sensor for the sensitive detection of cadmium. Anal Chim Acta 2014, 851:43–48. https://doi.org/10.1016/ j.aca.2014.08.021. 39. Bonaccorso F, Bartolotta A, Coleman JN, Backes C: 2D-crystalbased functional inks. Adv Mater 2016, 28:6136–6166. https:// doi.org/10.1002/adma.201506410. 40. Gan XR, Lei DY, Wong KY: Two-dimensional layered nanomaterials for visible-light-driven photocatalytic water splitting. Mater Today Energy 2018, 10:352–367. https://doi.org/10. 1016/j.mtener.2018.10.015. 41. Wang X, Nan F, Zhao J, Yang T, Ge T, Jiao K: A label-free ultrasensitive electrochemical DNA sensor based on thin-layer MoS2 nanosheets with high electrochemical activity. Biosens Bioelectron 2015, 64:386–391. https://doi.org/10.1016/j.bios. 2014.09.030.

26. Su S, Chao J, Pan D, Wang LH, Fan CH: Electrochemical sensors using two-dimensional layered nanomaterials. Electroanalysis 2015, 27:1062–1072. https://doi.org/10.1002/elan. 201400655.

42. Pattadar DK, Sharma JN, Mainali BP, Zamborini FP: Anodic stripping electrochemical analysis of metal nanoparticles. Curr Opin Electrochem 2019, 13:147–156. https://doi.org/10. 1016/j.coelec.2018.12.006.

27. Wang TY, Du KZ, Liu WL, Zhang JX, Li MX: Electrochemical sensors based on molybdenum disulfide nanomaterials. Electroanalysis 2015, 27:2091–2097. https://doi.org/10.1002/ elan.201500117.

43. Li HB, Xiao JP, Fu Q, Bao XH: Confined catalysis under twodimensional materials. PANS (Pest Artic News Summ) 2017, 114:5930–5934. https://doi.org/10.1073/pnas.1701280114.

28. Wang ZY, Mi BX: Environmental applications of 2D molyb* * denum disulfide (MoS2) nanosheets. Environ Sci Technol 2017, 51:8229–8244. https://doi.org/10.1021/acs.est.7b01466. This critical review presents the latest advances in the use of MoS2 nanosheets for important water-related environmental applications, such as contaminant adsorption, photocatalysis, membrane-based separation, sensing, and disinfection. 29. Chandran GT, Li XW, Ogata A, Penner RM: Electrically trans* * duced sensors based on nanomaterials (2012-2016). Anal Chem 2017, 89:249–275. https://doi.org/10.1021/acs.analchem. 6b04687. This review highlights recent advances in chemical and biological sensors that are based upon nanowires, nanotubes, nanoparticles, and other types of nanostructures. 30. Badihi-Mossberg M, Buchner V, Rishpon J: Electrochemical biosensors for pollutants in the environment. Electroanalysis 2007, 19:2015–2028. https://doi.org/10.1002/elan.200703946. 31. Cui L, Wu J, Ju HX: Electrochemical sensing of heavy metal * * ions with inorganic, organic and bio-materials. Biosens Bioelectron 2015, 63:276–286. https://doi.org/10.1016/j.bios.2014. 07.052. This review introduces briefly the recent achievements in electrochemical sensing of heavy metal ions with inorganic, organic and biomaterials modified electrodes. 32. Martínez-Hincapié R, Climent V, Feliu JM: New probes to surface free charge at electrochemical interfaces with platinum electrodes. Curr Opin Electrochem 2019, 14:16–22. https://doi. org/10.1016/j.coelec.2018.09.012. 33. Seh ZW, Kibsgaard J, Dickens CF, Chorkendorff I, Norskov JK, Jaramillo TF: Combining theory and experiment in electrocatalysis: insights into materials design. Science 2017, 355. https://doi.org/10.1126/science.aad4998. 34. Friedl J, Stimming U: Determining electron transfer kinetics at porous electrodes. Electrochim Acta 2017, 227:235–245. https://doi.org/10.1016/j.electacta.2017.01.010. 35. Schorr NB, Hui JS, Rodriguez-Lopez J: Electrocatalysis on ultra-thin 2D electrodes: new concepts and prospects for tailoring reactivity. Curr Opin Electrochem 2019, 13:100–106. https://doi.org/10.1016/j.coelec.2018.11.003. 36. Zhang ZC, Xu B, Wang X: Engineering nanointerfaces for nanocatalysis. Chem Soc Rev 2014, 43:7870–7886. https://doi. org/10.1039/c3cs60389j. 37. He B, Morrow TJ, Keating CD: Nanowire sensors for multiplexed detection of biomolecules. Curr Opin Chem Biol 2008, 12:522–528. https://doi.org/10.1016/j.cbpa.2008.08.027. 38. Wu LD, Fu XC, Liu H, Li JC, Song Y: Comparative study of graphene nanosheet- and multiwall carbon nanotube-based

www.sciencedirect.com

44. Pan XL, Bao XH: The effects of confinement inside carbon nanotubes on catalysis. Acc Chem Res 2011, 44:553–562. https://doi.org/10.1021/ar100160t. 45. Deng DH, Novoselov KS, Fu Q, Zheng NF, Tian ZQ, Bao XH: Catalysis with two-dimensional materials and their heterostructures. Nat Nanotechnol 2016, 11:218–230. https://doi.org/ 10.1038/nnano.2015.340. 46. Gan XR, Lee LYS, Wong KY, Lo TW, Ho KH, Lei DY, Zhao HM: 2H/1T phase transition of multilayer MoS2 by electrochemical incorporation of S vacancies. ACS Appl Energy Mater 2018. https://doi.org/10.1021/acsaem.8b00875; 2018. 47. Acerce M, Voiry D, Chhowalla M: Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nat Nanotechnol 2015, 10:313–318. https://doi.org/10.1038/nnano.2015.40. 48. Wang DZ, Zhang XY, Bao SY, Zhang ZT, Fei H, Wu ZZ: Phase engineering of a multiphasic 1T/2H MoS2 catalyst for highly efficient hydrogen evolution. J Mater Chem A FEB 14 2017, 5: 2681–2688. https://doi.org/10.1039/c6ta09409k. 49. Manivannan A, Seehra MS, Tryk DA, Fujishima A: Electrochemical detection of ionic mercury at boron-doped diamond electrodes. Anal Lett 2002, 35:355–368. https://doi.org/10.1081/ al-120002535. 50. Jiang LY, Santiago I, Foord J: Nanocarbon and nanodiamond * * for high performance phenolics sensing. Commun Chem AUG 6 2018, 1. UNSP 43, https://doi.org/10.1038/s42004-018-0045-8. This paper demonstrated a potential solution based on environmentally friendly and biocompatible carbon nanomaterials to detect monophenols (phenol and cresol) and biphenols (hydroquinone and catechol). 51. Gan XR, Zhao HM: A review: nanomaterials applied in graphene-based electrochemical biosensors. Sensor Mater 2015, 27:191–215. https://doi.org/10.18494/SAM.2015.1059. 52. Gan XR, Zhao HM, Quan X: Two-dimensional MoS2: a prom* * ising building block for biosensors. Biosens Bioelectron 2017, 89:56–71. https://doi.org/10.1016/j.bios.2016.03.042. This review mainly discusses the synthesis and characteristic methods of two-dimensional MoS2, the relationships between structure and property, and recent advances of two-dimensional MoS2-based biosensors. 53. Kubendhiran S, Sakthivel R, Chen SM, Mutharani B, Chen TW: Innovative strategy based on a novel carbon-black-betacyclodextrin nanocomposite for the simultaneous determination of the anticancer drug flutamide and the environmental pollutant 4-nitrophenol. Anal Chem 2018, 90: 6283–6291. https://doi.org/10.1021/acs.analchem.8b00989. 54. Yuan XX, Gao N, Gao X, Qiu DC, Xu R, Sun ZL, Jiang ZG, Liu JS, Li HD: Nanopyramid boron-doped diamond electrode realizing nanomolar detection limit of 4-nonylphenol. Sensor Actuator B Chem 2019, 281:830–836. https://doi.org/10.1016/j. snb.2018.11.011.

Current Opinion in Electrochemistry 2019, 17:56–64

64 Environmental electrochemistry

55. Si YC, Samulski ET: Exfoliated graphene separated by platinum nanoparticles. Chem Mater 2008, 20:6792–6797. https:// doi.org/10.1021/cm801356a. 56. Peleyeju MG, Idris AO, Umukoro EH, Babalola JO, Arotiba OA: Electrochemical detection of 2,4-dichlorophenol on a ternary composite electrode of diamond, graphene, and polyaniline. ChemElectroChem 2017, 4:1074–1080. https://doi.org/10.1002/ celc.201600621. 57. Zhou SF, Han XJ, Fan HL, Huang J, Liu YQ: Enhanced electrochemical performance for sensing Pb (II) based on graphene oxide incorporated mesoporous MnFe2O4 nanocomposites. J Alloy Comp 2018, 747:447–454. https://doi.org/ 10.1016/j.jallcom.2018.03.037. 58. Annalakshmi M, Balasubramanian P, Chen SM, Chen TW: Amperometric sensing of nitrite at nanomolar concentrations by using carboxylated multiwalled carbon nanotubes modified with titanium nitride nanoparticles. Microchim Acta 2018, 186:8. https://doi.org/10.1007/s00604-018-3136-4.

dichalcogenide nanosheets. Nat Chem 2013, 5:263–275. https://doi.org/10.1038/Nchem.1589. 60. Lin CW, Zhu XJ, Feng J, Wu CZ, Hu SL, Peng J, Guo YQ, Peng LL, Zhao JY, Huang JL, Yang JL, Xie Y: Hydrogenincorporated TiS2 ultrathin nanosheets with ultrahigh conductivity for stamp-transferrable electrodes. J Am Chem Soc 2013, 135:5144–5151. https://doi.org/10.1021/ja400041f. 61. Willets KA: Probing nanoscale interfaces with electrochemical surface-enhanced Raman scattering. Curr Opi Electrochem 2019, 13:18–24. https://doi.org/10.1016/j.coelec.2018.10.005. 62. Chen W, Yu A, Sun ZJ, Zhu BQ, Cai J, Chen YX: Probing complex eletrocatalytic reactions using electrochemical infrared spectroscopy. Curr Opi Electrochem 2019, 14: 113–123. https://doi.org/10.1016/j.coelec.2019.01.003. 63. Garlyyev B, Liang Y, Xue S, Watzele S, Fichtner J, Li WJ, Ding X, Bandarenka AS: Theoretical and experimental identification of active electrocatalytic surface sites. Curr Opi Electrochem 2018. https://doi.org/10.1016/j.coelec.2018.09.002.

59. Chhowalla M, Shin HS, Eda G, Li LJ, Loh KP, Zhang H: The chemistry of two-dimensional layered transition metal

Current Opinion in Electrochemistry 2019, 17:56–64

www.sciencedirect.com