Noble-Metal-Free Co0.6Fe2.4O4 Nanocubes Self-Assembly

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Noble Metal-Free Co0.6Fe2.4O4 Nanocubes Self-Assembly Monolayer for Highly Sensitive Electrochemical Detection of As(III) based on Surface Defects Shan-Shan Li, Wen-Yi Zhou, Yi-Xiang Li, Min Jiang, Zheng Guo, Jinhuai Liu, and Xing-Jiu Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04025 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Analytical Chemistry

Noble Metal-Free Co0.6Fe2.4O4 Nanocubes Self-Assembly Monolayer for Highly Sensitive Electrochemical Detection of As(III) based on Surface Defects

Shan-Shan Li,†,‡,§ Wen-Yi Zhou,†,‡,§ Yi-Xiang Li,†,‡ Min Jiang,†,‡, Zheng Guo,† Jin-Huai Liu,†,‡ and Xing-Jiu Huang†,‡,*

†

Key Laboratory of Environmental Optics and Technology, and Institute of

Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

‡

University of Science and Technology of China, Hefei 230026, People’s Republic of

China

§

S.-S.L. and W.-Y.Z. contributed equally to this work.

*

Correspondence should be addressed to X.J.Huang

E-mail: [email protected] (X.J.H); Tel.: +86-551-65591167; fax: +86-551-65592420.

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ABSTRACT Nanocrystals generally suffer from the agglomeration because of the spontaneous reduction of the system surface energy, resulting in blocking the active sites from reacting with target ions and then severely reducing the electrochemical sensitivity. In this article, a highly ordered self-assembled monolayer array is successfully constructed using ~14 nm Co0.6Fe2.4O4 nanocubes uniformly and controllably distributing on the surface of working electrode (glass carbon plate). The large area and high exposure of the surface defects on Co0.6Fe2.4O4 nanocubes are clearly characterized by high resolution transmission electron microscopy (HRTEM) and atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Expectedly, a considerable sensitivity of 2.12 µA ppb-1 and a low limit of detection of 0.093 ppb are achieved for As(III) detection on this highly homogeneous sensing interface; this excellent electroanalysis performance is even better than that of noble metals electrodes. Most importantly, this approach of uniformly distributing the small-sized defective nanoparticles on the electrode surface provides a new opportunity for modifying the electrodes, as well as the realization of their applications in the field of environmental electroanalysis for heavy metal ions.

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INTRODUCTION Sensitive electrochemical sensing of As(III) has been fruitfully achieved with the explosive application of nanomaterials, such as gold based, metal oxide, carbon-based nanomaterials and so on.1-10 Among them, gold based nanomaterials are extensively used because of their excellent catalytic property in the electrochemical detection of arsenic.2, 4, 11-13 For example, Au nanoelectrode ensemble (Au-NEE) electrode was fabricated by the colloidal chemical approach to investigate As(III) with a high sensitivity of 3.14 µA ppb−1.13 Niwa et al. prepared Au nanoparticle (Au NP)-embedded carbon films with high sensitivity and excellent stability for repetitive detection of As(III) in water.12 The Au(111)-like polycrystalline electrode was fabricated for As(III) determination in 0.1 M PBS (pH = 1) and a LOD of 0.28 ppb was obtained without any interference from Cu(II).4 Reviewing a series of articles, it is found that the same gold modified electrodes have extremely various detection performances; the well dispersed Au-NEE13 (3.14 µA ppb−1 ) even shows more than 2 orders of magnitude higher sensitivity than that of Au NPs modified glass carbon electrode (0.024 µA ppb−1).14 The electrochemical performances of the nanomaterials modified electrodes display a significant dependence on the dimensionality and distribution of the nanomaterials on the surface of electrodes. Decreasing the size of nanoparticles is an effective method to improve the sensitivity of nanomaterials due to its unique property compared with a bulk one. Sub-10 nm Au nanoparticles have higher electrocatalytic activity for As(III) detection than bulk Au electrode.11 However, the easy agglomeration of these small size 3



nanoparticles is a huge problem that researchers must face during the modification. Drip coating is the easiest and most widely used method of modifying the electrode with nanomaterials. However, it is also the easiest way to cause agglomeration due to the existence of surface tension on bare electrode.15 The agglomeration of nanomaterials could lead to a serious lack of reproducibility and stability of the fabricated electrode. Controllable assembly has been considered a promising method to fabricate an electrode with modifiers dispersed uniformly.13 Undoubtedly, achieving the controllable and uniform distribution of the nanomaterials with small size and good morphology on the surface of electrodes remains a great challenge. In addition to the small size of nanoparticles, the presence of defects on the surface of nanocatalysts, which usually perform as an active site, plays an important role in the enhancement of the adsorption ability and catalysis activity as well as detection performance. Introducing a large number of specific surface defect sites to regulate the electronic structure of nanomaterials is a promising approach of enhancing the electrochemical sensitivity.16-22 Typically, Dey et al. designed a reusable heavy metal ions (HMIs) sensor by the introduction of dopant (Cu) into the NiO lattice, further enhancing the effective number of surface defects which can act as adsorption sites.20 Long et al. achieved ultrasensitive NO2 detection by changing the defect state of the MoS2 aerogel tunable through creating more sulfur vacancies.21 Introducing carboxylic groups to the defect sites of MWCNTs that can highly improve the adsorption of 2,4,6-trichlorophenol and Cu(II) was reported by Chen et al.23 However, the easy agglomeration of nanomaterials usually makes active sites (such as 4


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surface defects) lose their specific function and interact incompletely with target ions. In a word, agglomeration seriously limits the development of modifying electrode for sensitive analysis. Considering this, the achievement of stable and surface dispersible nanomaterials for the detection of As(III) is extremely desirable. Very recently, some metal oxides have also been widely reported due to their electrocatalytic property through redox mediation.24-25 The well-defined cobalt oxide (CoOx) nanoparticles with excellent electrocatalytic activity have been produced for the detection of trace amount of As(III).24 The prepared nanorod-shaped Co3O4, with (110) planes and active Co3+ species predominantly exposed, shows their efficient oxidation catalytic activity to CO.25 The metal oxide nanomaterials, especially Fe3O4 nanoparticles, are well known for their excellent adsorption properties for arsenic.7-8, 26

The combination of the excellent catalytic properties of noble metal with the good

adsorption ability of the metal oxide, Au@Fe3O4 and Au/α-MnO2 nano-composites were constructed and achieved excellent performance in the detection of arsenic.8, 27 Redox-active iron based minerals are ubiquitous presented in soil and water environment; their extraordinary redox properties have been widely applied in accelerating the electron transfer of redox reaction such as organic pollutant digestion and electrochemical process.28-31 Furthermore, the Fe2+ in Fe3O4 could be partial or complete replacement by other divalent transition ions (such as Co2+, Ni2+, Mn2+) and have got good electrocatalytic performance.32-36 Among these various complex ferrites, the extensively studied CoFe2O4 exhibited its unique advantages including good adsorption capacity toward As(III).37-40 5



Given the adsorption ability and the well redox-active performance of Fe-based and Co-based oxide nanomaterials, in this work, sub-20 nm Co0.6Fe2.4O4 nanocubes with surface defects are prepared. Self-assembly nanocubes monolayer on glass carbon plate (GCP) is successfully constructed. The morphology and the structure of synthesized Co0.6Fe2.4O4 nanocubes self-assembly monolayer are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The fabricated electrodes are successfully used to detect As(III) using square wave anodic stripping voltammetry (SWASV) method. The current change of As(III) stripping after the addition of Fe(II) and Co(II) is observed. The XPS is used to study the interaction between ions and electrode during detection process. The interferences of HA, Cu(II) and other common anions (Cl-, NO3-, SO42-, CO32-) and cations (Cd2+, Pb2+, Hg2+, Zn2+) in water are all investigated. The practical value of this electrode including stability, reproducibility and the analysis of real water are also evaluated with good results.

EXPERIMENTAL SECTION Chemical Reagents. Co(acac)2, Fe(acac)3, oleic acid (OA), sodium oleate (SO) and benzyl ether were purchased from Alfa Aesar, China. N2 (99.999%) was purchased from Chenghong Gas, Nanjing, China. All chemicals were used as received without further purification. Fabrication of Co0.6Fe2.4O4 Nanocubes Self-Assembly Monolayer. Co0.6Fe2.4O4 6


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nanocubes monolayer was constructed using the self-assembly approach at the water-air interface.33 Before assembly, a clean teflon through (20 * 5 cm) with two movable barriers was poured with appropriate deionized water. Firstly, the prepared nanocubes dispersed in hexane were drop-wise (120 µL, 5 mg/ml) into the surface of water. Due to the incompatibility of the oil and water phase, the OA coated nanocubes spread rapidly on the surface of water. After hexane evaporating, a floating ordered monolayer of nanocubes is formed on the water surface with large-area. Slowly move the two barriers to the middle of the through to make the nanocubes arrange more dense. At last, the large-area ordered nanocubes monolayer was transferred onto GCP or Cu TEM grids for further detection and characterization. Multilayer self-assembly nanocubes can be easily got by repeating the process of monolayer. Co0.6Fe2.4O4 nanocubes were synthesized according to a previous work.33 In general, 560 mg Fe(acac)3, 268 mg Co(acac)2, 600 mg SO and 4 mL OA were mixed with 20 mL benzyl ether. The mixture was heated to 120 oC for 1 h and then heated to 290 oC for another 1 h under N2 flow. After cooling down to room temperature, the product was precipitated out by addition of ethanol and then re-dispersed in hexane for using. Verification the Redox Activity of Surface Defects on Nanocubes. The mediation effect of Fe(II)/(III) and Co(II)/(III) cycles on Co0.6Fe2.4O4 nanocubes is proved by SWASV through the addition of Fe(II) and Co(II) as well as the humic acid (HA) in the presence of As(III) during the detection. Adsorption experiments are performed to investigate the valence changes of As(III) on Co0.6Fe2.4O4 nanocubes by XPS during the detection process, including adsorption without potential (proof of the 7



good adsorption ability), adsorption with potential (preconcentration) and stripping (evidence of excellent redox activity).

Scheme 1. Fabrication of Co0.6Fe2.4O4 Nanocubes Self-Assembly Monolayer at Water-Air Interface and the Strategy for Detection of As(III) based on As(III) Adsorption and Redox Mediation with Fe(II)/Fe(III) and Co(II)/Co(III) Cycles on the Surface of Co0.6Fe2.4O4 Nanocubes.

RESULTS AND DISCUSSION Strategy for Detection of As(III). Prior to detection, the Co0.6Fe2.4O4 nanocubes monolayer was annealed at 300 oC for 1 h in N2 to eliminate the electronic blocking effect of surface organic functional groups of OA. Then it is used as working electrode to detect As(III) via classic three-electrode system. The detection strategy of As(III) on Co0.6 Fe2.4 nanocubes is illustrated in Scheme 1 (lower part). As(III) in the solution is firstly adsorbed on the surface of nanocubes due to the excellent adsorption property of the nanocubes toward As(III). Then the adsorbed As(III) can be reduced to As(0) with the help of the mediation of Fe(II) and 8


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Co(II); that is the redox cycle between Fe(II) and Fe(III) or Co(II) and Co(III) (left cycle in the Scheme 1). This is the process of As(III) preconcentration under a negative potential for a while before detection. It should be pointed out that the oxidation of Fe(II)sol and Co(II)sol in solution are impossible because the applied potential is more negative than the reduction potential of Fe(III)sol and Co(III)sol. The reduction potential values of the Fe(III)/Fe(II) cycle on surface of an iron oxide are reported to be much lower than that of aqueous Fe(III)/Fe(II).41 In this work, the Fe(II) and Co(II) that must be adsorbed or formed on the nanocubes surface have good redox activity to promote the detection of As(III). It is believed that Fe(III) and Co(III) on the surface of nanocubes is possible to exist in the form of intermediates and part of them then rapidly reduce to Fe(II)sur and Co(II)sur by obtaining the electrons from the electrode during preconcentration process. However, the oxidation potentials of Fe(II)sur and Co(II)sur on the surface of nanocubes are difficult to get due to the absence of in-situ and accurate quantitative studies on the monitor of surface Fe(II)sur and Co(II)sur. Much more studies are necessary to clarify it. After preconcentration, the reduced As(0) can be oxidized into As(III), giving stripping peak signal. Meanwhile, the Fe(III) and Co(III) can be reduced to Fe(II) and Co(II) again, completing the cycle (right cycle in the Scheme 1). The redox can be described as follows: M(II)sur + As(III)ads → M(III)sur + As(0)ads

(Pre-concentration)

M(III)sur + e- (from electrode) → M(II)sur

(Pre-concentration)

M(III)sur + As(0)ads → M(II)sur + As(III)ads

(SWASV)

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Where M represents Fe and Co. It is clearly that the Co(II) and Fe(II) works as catalyst to mediate electron transfer between electrode and As(III). The mediation of Co(II)/(III) and Fe(II)/(III) cycles could highly enhance the analytic performance of As(III). The monolayer dispersion of the nanocubes could facilitate the electron and mass transfer between electrode and As(III), which can further accelerate the rate of electrochemistry related reaction. It is significant for the electroanalysis of As(III).

Figure 1. (a), (b) TEM and (c) HRTEM of as-synthesized nanocubes monolayer assembled on GCP with annealing at 300 oC for 1 h in N2 flow; (d), (e) TEM and (f) HRTEM of as-synthesized nanocubes assembled on GCP without annealing. Insets in a and d are corresponding XRD patterns; inset in b is the particle size distribution of nanocubes; insets in c and f are the corresponding diffraction pattern of nanocubes. Morphologic and Structure Characterization of Nanocubes Monolayer. The TEM image of large scale nanocubes self-assembly monolayer (after annealing at 300 o

C for 1 h in N2 flow) is clearly shown in Figure 1a, indicating that the nanocubes

monolayer is successfully deposited on a solid substrate via the self-assembly 10


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approach. The cobalt ferrite nanoparticles are not perfect cubes but rather a bit deformed with a slightly concave shape (Figure 1 b). The mean diameter of this refer nanocubes is 14.3 nm (~14 nm) (inset in Figure 1b). It is worth noting that the concave structure is benefit for the introduction of a large number of multi-tips, edges and high-energy step atoms that can result in a high electrocatalytic activity.42 The XRD pattern of the nanocubes self-assembly monolayer (inset in Figure 1a) can be perfectly indexed to the diffraction of CoFe2O4 (No. JCPDS 02-1045). The interplanar spacing of 0.297 nm distinctly (Figure 1c) is the (220) plane of inverse spinel phase CoFe2O4. Corresponding, the characterization of the nanocubes monolayer without annealing are also shown in Figure 1d-f. There is no obvious morphology change after thermal treatment. The XRD pattern of nanocubes monolayer (inset in Figure 1d) further indicates that annealing at 300 oC do not make the variation of crystal phase. The red circles in the Figure 1f are the obvious traces of oleic acid, while no trace can be observed on the surface of the calcined nanocubes (Figure 1c). This difference also suggests that the calcination in N2 effectively removes the organic matter. The full XPS spectra (Figure S1b) and high-resolution XPS spectra of the C1s, O 1s (Figure S1c and S1d) for nanocubes with and without annealing are further investigated. The severe decrease of -CHx (Figure S1c), –COO and Fe-O-C (Figure S1d) all confirm that oleic acid is been successfully removed from the surface of nanocubes after annealing. Furthermore, the FTIR of Co0.6Fe2.4O4 nanocubes with and without annealing are also performed to characterize the vibration of oleic acid (Figure S2). The disappearance of the vibration peak in FTIR clearly shows that oleic 11



acid was fully removed from the surface of Co0.6Fe2.4O4 nanocubes after annealing treatment. The SEM images of nanocubes self-assembly monolayer, two layers and three layers (in Figure S3a-3d) prove that this self-assembly method is greatly controllable. The non-assembly (drip coating) nanocubes are severely agglomerated on the electrode surface (Figure S3e and S3f). In short, the controlled assembly of nanomaterials effectively avoids the coverage of active sites resulting from agglomeration, while the calcination can well prevent the organic matter from blocking electron transport.

Figure 2. Surface defects characterization of Co0.6Fe2.4O4 nanocubes with annealing at 300 oC for 1 h in N2 flow. (a) HAADF-STEM image, corresponding EDS element maps of (b) Fe, (c) Co, and (d) O of Co0.6Fe2.4O4 nanocubes; (e), (f) and (g) are HRTEM images; (h) is the crystal structure of CoFe2O4 built by Materials Studio 7.0; (i) and (j) are the atomic resolution HAADF-STEM images. The dashed ellipses in (e), (f), (g), (i) and (j) are the crystal defects on annealed nanocubes. Insets in i and j are the corresponding atomic arrangement model of crystal face. 12


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Surface Defects Characterization. The HAADF-STEM image and EDS element maps of Fe, Co and O of annealed nanocubes (in Figure 2a-2d) suggest that the Fe, Co and O are evenly distributed in the crystal. The amount of Fe, Co, O is displayed in Figure S4 with an atomic content ratio of Fe:Co about 4:1, which is well consistent with stoichiometric ratio in Co0.6Fe2.4O4 substance. The composition of the nanocubes is also confirmed by inductively coupled plasma-mass spectrometry (ICP-MS) and the same result is obtained (not shown here). Since the Co0.6Fe2.4O4 nanocubes are synthesized through an un-complete crystal growth, and the incorrect proportions of Fe and Co, a bit deformation with a slightly concave shape are formed with a large amount of surface defects. The dashed ellipses in HRTEM (Figure 2e, 2f and 2g) and HAADF-STEM images (Figure 2i and 2j) are typical surface defects on nanocubes. A huge amount of the distortion and dissolution of lattice fringes possibly caused by the O vacancies are clearly observed in Figure 2e-2g. Figure 2i typically displays the edge defects, which are probably indicative of Co vacancies. The elongated or aggregated bright lattice is also presented in the image of Figure 2j. It is easy to find that the surface defects are abundant existence. For comparison, the surface defects on the as-synthesized Co0.6Fe2.4O4 nanocubes without annealing were also studied (Figure S5), indicating that the annealing process would not change the surface defects. These surface defects are also reported by Sun.33 The large area exposure of high-energy defective atoms on the surface of the concave nanocubes could act well as adsorption and reaction sites.

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Figure 3. (a) SWASV responses of 10 ppb As(III) on bare, monolayer, two layers, three layers, non-assembly Co0.6Fe2.4O4 nanocubes; insets are corresponding SEM images of Co0.6Fe2.4O4 nanocubes (screenshot from Figure S3). (b) Typical SWASV response of As(III) from 1-20 ppb and inset is the corresponding linear calibration plots of peak current against As(III) concentrations. Error bars correspond to standard errors were obtained from three independent measurements.

Highly Sensitive Detection of As(III). As known, As(III) is difficult to preconcentrate on a pure carbon electrode surface including GCP due to its chemically inert for As(III) redox.12 So there is no response for As(III) on bare GCP (Figure 3a). Obviously, significant enhancement is achieved on nanocubes self-assembly monolayer which shows an excellent analysis performance (Figure 3a). However, compared with monolayer, the two and three layers nanocubes present lower response current toward As(III); non-assembly nanocubes even display 14


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insignificant response. In detail, the current toward 10 ppb (III) on nanocubes monolayer is about 3 and 5 times than that on two and three layers; as much as 28 times than that on non-assembly nanocubes. This result demonstrates that Co0.6Fe2.4O4 nanocubes self-assembly monolayer with more exposure of surface defects is benefit for the detection of As(III). Uniform dispersion of the nanoparticles on the electrode surface is important for good electrochemical detection. Furthermore, by comparing the cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) of the obtained electrode with the bare GCP (Figure S6), it is found that the conductivity of nanocubes monolayer modified GCP has slightly deteriorated, mainly due to the semiconductive behavior of Co0.6Fe2.4O4. As the layer of nanocubes increases (two and three layers), the current of CV is obviously reduced accordingly, indicating that the multilayer could hinder the electron transfer. The non-assembly nanocubes even show a much poorer conductivity. As discussed above, large tight overlap nanocubes can completely cover the electrode reducing their conductivity. In addition, the coverage of surface defects between each nanocubes can be more serious as the layer increases. Nanocubes self-assembly monolayer is effective for conduction and make the surface defects exposure most. It is believed that these nanocubes monolayer with high density defects is desirable for As(III) detection. By the way, before the detection we optimized the detection conditions: 0.1 M NaAc-HAc (pH=5) with -0.9 V for 120 s (Figure S7) and all the next experiments were carried out under this condition. The well-defined peaks of typical SWASV response toward As(III) are clearly exhibited (Figure 3b). The sensitivity of the detection (inset in Figure 3b) is 2.12 µA 15



ppb-1 and the limit of detection (LOD) is 0.093 ppb. The LOD is calculated using 3σ method i.e. 3SD/S, in which SD and S represent standard deviation of the measurements and the slope of the calibration line, respectively. The linear relationship (R2) of the corresponding linear calibration plots in the range of 1-20 ppb is very high (0.998) implying that the sensing interface has a large linear range and good stability. Comparing with other reported papers for the As(III) detection (Table S1), it is not difficult to find that the nanocubes have higher sensitivity and lower LOD, even better than some noble metal electrodes. Completely free from the high electrocatalytic activity of the noble metal (such as Au nanoparticles), the sensitivity and LOD in this work are especially attractive. The well electroanalytic behavior mainly ascribes to the good adsorption and outstanding redox mediation effect on Co0.6Fe2.4O4 nanocubes monolayer. Mediation Effect of Fe(II)/(III) and Co(II)/(III) Cycle. In order to verify the role of iron and cobalt in the process of detection, the addition of Fe(II) and Co(II) into the solution during detection is performed to observe the current changes of As(III) (Figure 4). With the gradually increasing the concentration of Fe(II) and Co(II), the current of 10 ppb As(III) also grows, indicating that the existence of Fe(II) and Co(II) could promote the detection of As(III). In the process of preconcentration, the added Fe(II) and Co(II) are firstly adsorbed to the surface of the nanocubes under electric field and stirring. Then the surface associated Fe(II) and Co(II) participate in and accelerate the reduction of As(III). It has been reported that the activity of Fe(II) presented on (or associated with) the surface of the mineral is higher than that Fe(II) 16


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Figure 4. Current change of the SWASV response of 10 ppb As(III) at Co0.6Fe2.4O4 nanocubes in the presence of (a) 0−220 ppb Fe(II) (prepared from FeCl2) and (b) 0-260 ppb Co(II) (prepared from CoCl2) in 0.1 M HAc-NaAc solution (pH 5.0), respectively. Error bars correspond to standard errors were obtained from three independent measurements. ions in the aqueous solution.28-29 The electrochemical detection process is an interfacial process where the electrons are gain and loss on the surface of electrode. A rapid redox transformation between As(III) and As(V) has been found in the Fe(II) activated and catalyzed iron oxide system.43-44 This result confirmed that the Fe(II)/(III) and Co(II)/(III) cycles can facilitate the redox of As(III) as an efficient electron shuttle between GCP and As(III), further enhancing the response of electrochemical analysis. Besides the added Fe(II) and Co(II) to promote the cycle, the experiments that 17



hindering the cycle were also carried out to verify the mediation effect. HA (humic acid) is considered to be major disturbance in the actual environmental detection of As(III). It can occupy the adsorption and active sites on the surface of nanomaterials

Figure 5. (a) Current change of 10 ppb As(III) in the presence of 0−10 ppm HA (humic acid) at Co0.6Fe2.4O4 nanocubes monolayer electrode. Inset is the corresponding SWASV response of As(III). Error bars correspond to standard errors were obtained from three independent measurements. (b) Schematic of Fe(III) role in preventing the interference from HA for As(III) detection at the low level (0-4 ppm) and the high level (5-10 ppm) of HA.

and electrodes, then reducing the effect of detection.45-47 Figure 5a is the SWASV response of 10 ppb As(III) in the presence of 0−10 ppm HA and its corresponding current change of As(III). When the concentration of HA is relatively low (0-4 ppm), 18


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the current value of As(III) almost shows no change, indicating that the HA do not interfere the detection. In contrast, as the concentration of HA continues to grow (5-10 ppm), the interference causes obvious current drop. The complex between Fe(III) and HA has been extensively studied.10, 48-53 In our group’s previous works, the introduction of Fe(III) into the solution is successfully applied to reduce the interference of HA and improve the analysis of As(III).10 As shown in Figure 5b, the HA (0-4 ppm) is first combined with the dissolved Fe(III) in the solution without adsorbing on the surface of nanocubes. This would not cause the decrease of As(III) current; in other word, the dissolved Fe(III) in the solution could effectively exclude the interference of HA. When the concentration of HA grows to 5-10 ppm, as a result, the excessive HA would not only combined with Fe(III) in the solution, but also covers the Fe(III) on the surface of nanocubes. After covering with HA on nanocubes, the iron cycle is blocked, and further reduces the response of As(III). This decrease clearly reveals that the Fe(II)/(III) cycle seriously affect the detection process of As(III). The mediation based on Fe and Co on the surface of Co0.6Fe2.4O4 nanocubes is truly enhanced the sensitivity. Furthermore, the amount of HA in groundwater and drinking water is generally no more than 5 ppm which does not exceed the range of anti-interference in this system. The more added Fe(III) can even eliminate the interference of HA in surface water (about 10 ppm). Evidence of High Adsorption Ability and Redox Activity Based on XPS. Monitoring the valence state of As(III) in the detection process with XPS is performed

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Figure 6. High-resolution XPS spectra of As(III) on Co0.6Fe2.4O4 nanocubes self-assembly monolayer without (top curve) and with (middle curve) applying negative potential, as well as with applying negative potential and then stripping (bottom curve); inset (magenta dashed line box) is the cycle and the mediation of Fe and Co during the detection of As(III); different detection processes correspond to different background colors. to support the high adsorption ability and redox activity of nanocubes. Considering the low detection sensitivity of XPS, As(III) at a concentration (10 ppm) higher than the detection range were chose to perform the adsorption experiments in order to clearly observe the XPS signal of As. As shown in Figure 6, the adsorption of 10 ppm As(III) on Co0.6Fe2.4O4 nanocubes without and with applying potential, as well as the stripping after electro-adsorption are all studied. In the absence of a negative potential, a lot of As(III) adsorbed on the surface of the Co0.6Fe2.4O4 nanocubes keeps its original valence state, demonstrating the good adsorption property toward As(III) (top curve in Figure 6). It is well known that the glassy carbon has no adsorption capacity for As(III). With the assistance of electrons and the redox mediation of the Fe(II)/Fe(III) and Co(II)/Co(III) cycles, more As(III) adsorbed on nanocubes is immediately reduced to As(0) (middle curve in Figure 6). This fast reduction reveals 20


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the good redox activity of nanocubes and a fast electron transfer rate between nanocubes and GCP. After stripping, the As(0) is oxidized to As(III) completely (bottom curve in Figure 6), suggesting the very effective oxidation of As(III) on Co0.6Fe2.4O4 nanocubes. This result perfectly matches with the electrochemical detection process. In other words, As(III) is firstly adsorbed on the surface of nanocubes, then reduced to As(0) at the process of pre-concentration, at last oxidized to As(III) at stripping (Figure 6). All these redox reactions are believed to operate on the surface of Co0.6Fe2.4O4 nanocubes not on GCP. For metal oxide semiconductor nanomaterials, our group proposed an approvable adsorb-release model. That is, the analyte (HMI) is first adsorbed to the surface of nanomaterials with large surface area and then the adsorbed analyte releases from the nanomaterial to the surface of electrode where the redox reaction of the analyte is happened because of the poor conductivity of these metal oxide material.7, 54-56 These two process (adsorb and release) reduce the efficiency of the redox rection and thus decrease the sensitivity of the detection. The constructed Au@Fe3O4 and Au/α-MnO2 nano-composite are intended to directly oxidize the adsorbed analyte (As(III)) on Au; without release process, excellent electroanalysis of As(III) are indeed achieved.8, 27 In this system, the self-assembly monolayer are beneficial for the electron transfer from the surface of GCP to Co0.6Fe2.4O4 nanocubes; the surface defects are good for the adsorption and the redox of As(III). As a result, the redox of As(III) directly happens on the surface of Co0.6Fe2.4O4 nanocubes rapidly and do not need to release to GCP, further increasing the detection sensitivity. 21



Figure 7. (a) Current change of 10 ppb of As(III) for 20 consecutive and repetitive stripping voltammetric measurements on the Co0.6Fe2.4O4 nanocubes; inset is the corresponding SWASV response. (b) The response of 10 ppb As(III) at three nanocubes GCPs after storage at room temperature for 30 days; inset is corresponding SEM of Co0.6Fe2.4O4 nanocubes after detection. Error bars correspond to standard errors were obtained from three independent measurements. Evaluation of Stability. The stability and reproducibility are the important standards for evaluating the practical application value of an electrode. The relative standard deviation (RSD) of the 20 consecutive and repetitive response to 10 ppb As(III) at the nanocubes is only 1.02% (Figure 7a), suggesting the high stability of such an electrode. The response toward 10 ppb As(III) on three nanocubes GCPs after storing at room temperature for 30 days have ignorable change compared with newly made electrode (Figure 7b). Furthermore, the corresponding SEM of Co0.6Fe2.4O4 22


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nanocubes after detection As(III) (inset in Figure 7b) also shows that no morphological change is observed, but only insignificant loss of the nanocubes from GCP. The control and repeatability of the modification approach have a critical role in promoting the stability and reproducibility of electrode such constructed electrodes, leading to a great advantage over other common coated electrodes. Selectivity Measurement in Real Water. Achieving the analysis of the actual water sample is one important evaluation criterion for the development of new electrodes. There is a large amount of interfering ions in the actual water such as other HMIs. The interference measurements of some common cations (Cd2+, Pb2+, Hg2+, Zn2+) and anions (Cl-, NO3-, SO42-, CO32-) toward 10 ppb As(III) are studied in Figure S8. These ions exhibit unobvious effect to As(III) detection, similar as the reported the previous work,8 suggesting the anti-interference and selectivity of the fabricated nanocubes monolayer. This is possible due to that the As(III) preferentially occupies the adsorption site and not forms intermetallic compounds like Cu(II).57 Typically, Cu(II) and As(III) can form Cu3As2 compounds depositing on the surface of nanomaterials and electrodes, and then affect the effective detection of As(III).8, 13, 58-60

The sensitivity of 0-9 ppb As(III) (2.23 µA ppb-1, Figure S9a) in the presence of

50 ppb Cu(II) shows almost no change but the variation of peak shape compared with that in the absence of Cu(II) (2.12 µA ppb-1). When fixed As(III) at 10 ppb, as the increase of the content of Cu(II) (0-600 ppb), the peak current of As(III) also increases (Figure S9b). There is an obvious new peak appearing at the right and near the peak of As(III) that known as copper shoulder;57, 61 the new peak is possibly due to 23



the formation of Cu3As2 after the As(III) deposited on Cu. As the peaks of these two are very close, the growth of the new peak leads to the slowly increase of As(III). This interference is negligible, on the one hand the concentration of Cu(II) in the drinking water contains lower levels of Cu(II) than our test range, on the other hand EDTA as a masking agent is successfully used to remove the interference of Cu(II)12, 62 The application of this electrode for the analysis of real water has been evaluated with good results (Figure S10, Table S2, and discussion in Supporting Information). Sensing Behavior Based on Surface Defects. It is expected to find that Co0.6Fe2.4O4 nanocubes self-assembly monolayer shows an excellent electrochemical sensitivity towards As(III) associated with the surface defects formation. We strongly believe that this high exposure of large area surface defects paves a powerful way to design a novel sensing interface. While some success have been achieved in anti-interference of HA and some coexistence of ions in our investigation, the ultra-high selective detection of As(III) on bare Co0.6Fe2.4O4 nanocubes without any interference is difficult to come true because of the intrinsic indiscriminate adsorption and catalysis activity of surface defects toward other HMIs. In fact, the complete elimination of mutual interference between different HMIs is impossible for the researchers in electrochemical detection field. The selectivity and anti-interference of Cu(II) for the electrode in this work is undoubtedly a challenging task which could be effectively improved by the assistance of masking agent or pretreatment means. Much more efforts like modification of functional groups on nanomaterials to have specific capture of As(III) is also ongoing to be an important attempt to improve its selectivity, 24


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which has been commonly used in some reported papers.63-66

CONCLUSIONS In summary, we have facilely fabricated an effectively sensitive interface on GCP by self-assembly monolayer of Co0.6Fe2.4O4 nanocubes with surface defects for the analysis of As(III). This thin monolayer dispersed nanocubes on GCP provides a stable sensing platform for As(III) transportation and shortens the distance of As(III) diffusion. Monolayer array also improves the electrical contact between bare GCP and modified nanocubes, leading to high electrode exchange current. Furthermore, the monolayer perfectly overcomes the agglomeration of nanocrystals during electrode modification, ensuring the exposure of active surface area on nanocubes to the largest extent. Owing to the exist of surface defects, Co0.6Fe2.4O4 nanocubes exhibit high adsorption to As(III) and the fast redox of As(III) which has been demonstrated by XPS. The increased stripping current as the addition of Fe(II) and Co(II) suggests that the mediation effect of Fe(II)/(III) and Co(II)/(III) cycles is benefit for As(III) analysis. All these results confirm that the dramatically enhanced response toward As(III) is attributed to the excellent adsorption property and redox activity of defective Co0.6Fe2.4O4 nanocubes monolayer. The anti-interference of HA, good stability, and practical application in actual water all indicate the proposed electrode is suitable for applying in complex real sample.

25



Acknowledgements This work was supported financially by the National Natural Science Foundation of China (21735005, 21475133, 61573334, 61474122 and U1532120) and the youth project of National Natural Science Fund of China (11405256). X.-J.H. acknowledges the CAS Interdisciplinary Innovation Team and the CASHIPS Director’s Fund (YZJJ201701), for financial support.

Supporting Information Available The experimental section including apparatus, electrochemical detection of As(III); The analysis of real water; XRD pattern of the Co0.6Fe2.4O4 nanocubes powder and XPS of the nanocubes with and without annealing (Figure S1); The FTIR of Co0.6Fe2.4O4 nanocubes on Si substrate with and without annealing (Figure S2); The SEM of as-synthesized Co0.6Fe2.4O4 nanocubes including monolayer, two layers, three layers and non-assembly (Figure S3); EDS pattern of the Co0.6Fe2.4O4 nanocubes (Figure S4); HRTEM images of as-synthesized Co0.6Fe2.4O4 nanocubes without annealing (Figure S5); CV and EIS of the bare, monolayer, two layers, three layers, non-assembly Co0.6Fe2.4O4 nanocubes modified GCP (Figure S6); Optimum the experimental conditions (Figure S7); Interference studies of Co0.6Fe2.4O4 nanocubes GCP at 10 ppb As(III) in the presence of 100 ppb anions and cations (Figure S8); The interference of Cu(II) during the detection of As(III) (Figure S9); The detection of As(III) in real water (Figure S10); A comparison of electrochemical performance of electrodes for voltammetric detection of inorganic arsenic (Table S1); Detection of 26


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Noble-Metal-Free Co0.6Fe2.4O4 Nanocubes Self-Assembly

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