Fe(III) Cycle Promoted Ultra-Highly Sensitive

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Surface Fe(II)/Fe(III) Cycle Promoted Ultra-Highly Sensitive Electrochemical Sensing of Arsenic(III) with Dumbbell-like Au/Fe3O4 Nanoparticles Shan-Shan Li, Wen-Yi Zhou, Min Jiang, Zheng Guo, Jinhuai Liu, Lizhi Zhang, and Xing-Jiu Huang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Analytical Chemistry

Surface Fe(II)/Fe(III) Cycle Promoted Ultra-Highly Sensitive Electrochemical Sensing of Arsenic(III) with Dumbbell-like Au/Fe3O4 Nanoparticles

Shan-Shan Li,†,‡,§ Wen-Yi Zhou,†,‡,§ Min Jiang,†,‡ Zheng Guo,†,‡ Jin-Huai Liu,†,‡ Lizhi Zhang║,* 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

║

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute

of Environmental Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China

§

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

*

Correspondence should be addressed to L.Z.Zhang and X.J.Huang

E-mail: [email protected] (L.Z.Z) and [email protected] (X.J.H). Tel.: +86-551-65591167. Fax: +86-551-65592420.

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Developing a new ultra-sensitive interface to detect As(III) is highly desirable because of its seriously toxic and low concentration in drinking water. Recently, Fe3O4 nanoparticles of high adsorption toward As(III) become very promising to be such an interface, which is still limited by the poor understanding of their surface physicochemical properties. Herein, we report that dumbbell-like Au/Fe3O4 nanoparticles, when being modified the screen-printed carbon electrode, can serve as an efficient sensing interface for As(III) detection with an excellent sensitivity of 9.43 µA ppb-1 and a low detection limit of 0.0215 ppb. These outstanding records were attributed to the participation of Fe(II)/Fe(III) cycle on Fe3O4 surface in the electrochemical reaction of As(III) redox, as revealed by X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. This work provides new insight into the mechanism of electroanalysis from the viewpoint of surface active atoms, and also helps to predict the construction of ultra-highly sensitive electrochemical sensors for other heavy metal ions with nonprecious redox active materials.

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INTRODUCTION Arsenic [As(III)] content in drinking water is very low but very toxic (upper limit of 10 ppb recommended by the World Health Organization (WHO) and the Environmental Protection Agency.1-2 Thus, achieving ultrasensitive detection of As(III) in electro-analysis fields is important.3-4 Many electrode nanomaterials, including metal oxides, carbon nanotubes (CNTs), graphene, and noble metals (gold, silver, and platinum), have been reported for the measurement of As(III).5-16 These electrodes include gold-based electrodes with superior catalytic ability toward As(III). Jena and Raj fabricated an Au nanoelectrode ensemble (Au-NEE) to investigate As(III) in 1 M HCl with a sensitivity of 3.14 µA ppb−1 and a limit of detection (LOD) of 0.02 ppb.17 Kounaves et al. developed portable equipment for the rapid and reliable on-site analysis of As(III) in groundwater with a measured LOD of 0.05 ppb using Au ultramicroelectrode disk arrays (Au-UMEA).6 The Au(111)-like polycrystalline Au electrode detected As(III) in 0.1 M PBS (pH = 1) with an LOD of 0.28 ppb without any interference from Cu(II).8 More recently, Au nanoparticle (Au NP)-embedded carbon films were formed with high sensitivity and excellent electrode stability for repetitive measurements of As(III) in water.18 Noble metal-free electrodes also have been used. For example, a reduced graphene oxide–lead dioxide composite has been applied for low level detection of As(III) with an LOD of 0.75 ppb.19 Salimi et al. developed novel and well-defined cobalt oxide (CoOx) nanoparticles (NPs) for the detection of trace amount of As(III) with a sensitivity of 1.5 nA ppb−1 and an LOD of 0.83 ppb.20 They ascribed the outstanding analytical performance to the excellent 3



catalytic activity of CoOx NPs.20 Our group recently found some efficient nanomaterials for the determination of As(III) based on metal oxides such as SnO2 and 400 nm Fe3O4.10, 21 Because of the poor conductivity of these noble free materials, the high adsorptive capacity toward As(III) is responsible for the performance of As(III) analysis. As(III) is first adsorbed onto the surface of these nanomaterials and then is released to the electrode for redox reaction giving electrochemical signal. This adsorb-release model uses nonconductive materials for the detection of heavy metal ions in electrochemistry.22 Combining the 400 nm Fe3O4 with 5 nm Au NPs, we constructed the Au@Fe3O4 nanomaterials for detection of As(III).11 The nano-Fe3O4 materials are attractive in the detection of As(III) because of their excellent adsorption toward As(III).23 Galina’s group proposed a sensitive interface based on a Fe-modified carbon composite electrode.24 They stated that the most probable role of Fe was the formation of an alloy or intermetallic compound with As(III) on electrode surface.24 Although Fe-based metal oxide nanomaterials have been used widely and some progress has been made, the vague detection mechanism of nano-Fe3O4 limits its further development for ultrasensitive detection. Royce Murray also commented on this trend in analytical chemistry stating that nanocomposite-modified electrodes have many questions as to whether the increased sensitivity are simply reflections of an increased microscopic surface area and not electrocatalytic activity.25 Indeed, many of these works are based on empiricism. They improve results but do so without scientific understanding.25 Empirical and mechanistic studies can help develop a new sensitive electrode, but 4


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direct and convincing evidence is still needed about the mechanism. The ferrous ions [Fe(II)]-activated and -catalyzed iron oxide systems offer rapid redox transformation in the presence As(III) and As(V).26-27 The Fe(II)/ferric ions [Fe(III)] cycle also can be a highly efficient catalyst in environmental contaminant remediation and detoxification.28-34 Various representative works have been reported by the Zhang group using the aqueous Fe(III)/Fe(II) cycles to accelerate the process of Fenton system.28, 30-38 However, many studies have found that Fe(II) adsorbed on (or associated with) iron oxide surfaces degrades oxidized contaminants much faster than aqueous Fe(II).39-45 In particular, Silvester et al. observed an amplification of the ferrocene oxidation peak in cyclic voltammetry. They attributed this increase to the catalytic process between ferrocenium ion and adsorbed Fe(II) species.46 Furthermore, the small size of the iron oxide results in more activity in the electrical contact with the electrode relative to the big size of the iron oxide. This leads to high electrode exchange current.46 Although many attempts have been made to explain the organic digestion mechanism of these nanomaterial, it remains a major challenge in electrochemical measurements to obtain a thorough understanding of the redox processes between the working electrode and the modified redox-active minerals.47 In this work, sub-20 nm dumbbell-like Au/Fe3O4 nanoparticle were synthesized and applied for As(III) detection under nearly neutral conditions with square wave anodic stripping voltammetry (SWASV). We found excellent performance of Au/Fe3O4 screen-printed carbon electrode (SPCE) for As(III) analysis by combining the catalytic properties of Au NPs with the adsorption ability of the ~10 nm Fe3O4 5



NPs—as well as the enhanced redox activity by Fe(II)/Fe(III) cycling on the surface of Au/Fe3O4 nanoparticle. Most important, As(III) redox by Fe(II)/Fe(III) cycle is distinctly demonstrated by XPS, XANES, and EXAFS. We identified a clear redox processes between the working electrode and the modifier of the Au/Fe3O4. In detail, we characterized the prepared Au/Fe3O4 nanoparticle by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), and XPS. We also investigated the interference measurements of coexisting ions (Cd(II), Pb(II), Hg(II), Cu(II) and Zn(II)) and the detection of real water samples via the Au/Fe3O4 SPCE with good performance.

EXPERIMENTAL SECTION Chemical Reagents. HAuCl4·4H2O was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Tertbutylamine: borane complex (TBAB, 95%), Fe(acac)3, oleylamine (OAm, C18 content: 80%–90%), oleic acid (OLA, 99.99%), and hexadecyl trimethyl ammonium bromide (CTAB) were purchased from Alfa Aesar, Beijing, China. We used all chemicals were used as received without further purification. Synthesis of Dumbbell-like Au/Fe3O4 Nanoparticle. We prepared OLA-covered Au/Fe3O4 dumbbell-like nanoparticles according to previous reports with minor modification.48-49 First, we synthesized Au NPs as follow: 0.2 g HAuCl4·4H2O was added to 20 mL chloroform and 4 mL OAm. After stirring at 30 °C for 10 min, we 6


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added the solution containing 0.2 g TBAB and 0.2 mL OAm in 4 mL of chloroform to the HAuCl4–OAm mixture and further stirred at 30 °C for 1 h. We then washed the Au nanoparticles with ethanol and redispersed the nanoparticles in hexane. Second, we prepared OLA-covered Au/Fe3O4 dumbbell-like nanoparticles: The 140.0 mg Fe(acac)3, 8.0 mL OAm, and 5.4 mL OLA were heated in a flask to 160 °C and kept for 30 min under a flow of N2. The 10 mg of Au NPs in 2 mL of OAm were injected into the reaction mixture followed by 2.5 mL of OLA. The solution was kept at 160 °C for another 90 min, and then heated to 300 °C for 30 min. The particles were centrifuged and redispersed into hexane. The preparation of Fe3O4 NPs was similar to Au/Fe3O4 but without the addition of Au NPs. The Au/Fe3O4 nanoparticle coated with OLA could be transferred from the organic phase to water as reported.50 Then, 20 mg of OLA-covered Au/Fe3O4 nanoparticles were added into the solution containing 60 mg CTAB in 40 mL water and sonicated for 30 min. After 40 mL H2O2 was injected into the mixture, the material was transferred into a Teflon-lined stainless steel autoclave (100 mL capacity) and heated at 90 °C for 6 h. The resulting black product was washed several times and dried at 60 °C under vacuum for 24 h. Evidence of the Fe(II)/Fe(III) Cycle Experiments. The adsorption experiments of As(III) on ~10 nm Fe3O4, Au/Fe3O4 and 400 nm Fe3O4 NPs are performed for further characterization. We present the changes of Fe(II) and Fe(III) content on adsorption samples by XPS (using VG ESCALAB MKII spectrometer). We used XANES and EXAFS (at BL14W1 beamline of the Shanghai Synchrotron Radiation Facility) analyses to obtain the redox states and the information of As and Fe 7



regarding the local shell structure around the absorbing atom. The detailed adsorption experiments and data analysis are shown in the Supporting Information.

RESULTS AND DISSCUSSION Scheme 1. Ultra-Highly Sensitive Electroanalysis of As(III) Based on the Adsorption of ~10 nm Fe3O4 NPs and the Catalyst of ~ 7 nm Au NPs as well as the Redox Mediation by Surface-Active Fe(II) at Au/Fe3O4 SPCE.

Detection Strategy of As(III). Scheme 1 illustrates the detection mechanism of As(III) on dumbbell-like Au/Fe3O4 nanoparticles. As previously reported by our group, the synergistic effect of the excellent catalytic properties of Au NPs and the good adsorption ability of 400 nm Fe3O4 NPs significantly improve the detection of As(III); the adsorbed As(III) on 400 nm Fe3O4 NPs will be directly reduced and oxidized on the Au surface (left side of Scheme 1).11 As the size of Fe3O4 NPs decreases (~10 nm), the adsorption capacity toward As(III) increases. Furthermore, active Fe(II) exposed on the surface of the Au/Fe3O4 nanoparticles has a role. The concentration of As(III) near the electrode surface gradually increases as a result of the adsorption of ~10 nm Fe3O4 NPs. The surface-activated Fe(II) can denote an electron to form Fe(III) in the reduction of As(III) to As(0) (right side of Scheme 1). The generated Fe(III) will then 8


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get an electron from the electrode or oxidation of As(0) to As(III) during the process of SWASV. This completes the Fe(II)/Fe(III) cycle (right side of Scheme 1). The Fe(II) works as a electrocatalyst to mediate electron transfer between electrode and As(III). This mediation of the Fe(II)/Fe(III) cycle as well as the catalyst of Au NPs will efficiently enhance the electrochemical sensitivity toward As(III). The proof of the Fe(II)/(III) cycle will be discussed in the following section. Morphology and Structure Characterization of Dumbbell-like Au/Fe3O4 Nanoparticle. Figure 1a shows a TEM image of the as-synthesized Au/Fe3O4 nanostructures. A smaller gold nanoparticle (black) is partially embedded in a larger Fe3O4 nanoparticle (light colored). This is defined as a dumbbell-like structure. Without a doubt, the multicomponent nanoparticles are advantageous because they combine all components into one nanoparticle that can lead to better physicochemical properties. In addition, it has been widely reported that the synergistic effects resulting from the heterojunction are beneficial and facilitate various plasmonic effects.51-53 The interplanar spacing at 0.24 and 0.49 nm (Figure 1b) are perfectly indexed to the (111) plane of the Au cubic phase and Fe3O4 cubic inverse spinel phase. The HAADF-STEM image and the STEM-EDS (Energy Dispersive Spectrometer) elemental maps of the as synthesized nanoparticles are shown in Figure 1c. The brighter dots belong to Au NPs because they have higher atomic numbers than the Fe3O4 nanoparticles.54 The EDS maps and the line scan of the Au/Fe3O4 nanoparticles are obvious and show the location of Au and Fe3O4, respectively (Figure 1d). The XRD pattern (Figure 1e) can respectively index the pattern for Au (JCPDS No. 9



Figure 1. (a) TEM, (b) HRTEM and (c) HAADF-STEM images and STEM-EDS elemental map of two Au/Fe3O4 nanoparticle. (d) Line scan of Au/Fe3O4 nanoparticles. (e) XRD pattern of Au/Fe3O4 nanostructures. The particle size distribution of (f) ~7 nm Au and (g) ~ 10 nm Fe3O4 in Au/Fe3O4 NPs, respectively. Inset in panel b is the diffraction pattern of Au/Fe3O4 nanostructures.

040784) and Fe3O4 (JCPDS No. 65-3107). The Au NPs have a mean diameter of 6.8 nm (~7 nm, Figure 1f), and Fe3O4 NPs is 11.4 nm (~10 nm, Figure 1g). The Fe3O4 NPs are face-centered cubic (fcc) with an average crystal domain size of ∼11.8 nm as estimated from (311) peak broadening analysis and the Sherrer equation. This is similar to the size obtained from the TEM image. The average crystal domain size of Au NPs is calculated to be ∼6.5 nm, which is consistent with TEM. The characterization of OLA-covered Au/Fe3O4 nanoparticles is shown in Figure S1. The morphology and structure of the Au/Fe3O4 nanoparticles transferred to the aqueous 10


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phase do not change significantly. The Fourier Transform infrared spectroscopy (FTIR, Figure S1d) and high-resolution XPS spectra of the C1s, O 1s (Figures S1e and S1f) for hydrophobic Au/Fe3O4 (coated with OLA) and hydrophilic Au/Fe3O4 indicate the successful removal of surface organics.

Figure 2. (a) Typical SWASV responses of As(III) at Au/Fe3O4 SPCE across different concentration ranges. (b) Corresponding linear calibration plots of peak current against As(III) concentrations from 0.1 to 10 ppb. Insets in panels a and b are the enlarged views that correspond to a range of 0.1 to 2 ppb. Electrochemical Detection of As(III). The electrochemical characterizations of Au/Fe3O4 modified SPCE can probe the influence of nanoparticles on electron-transfer kinetics after modification as discussed in the Supporting Information (Figures S2 and S3 in Supporting Information). The experimental parameters were optimized (Figure S4) before detecting As(III) at Au/Fe3O4. Finally, 11



all detection of As(III) was accomplished under optimal experimental conditions (0.1 M, pH = 5 HAc-NaAc: deposition potential of −0.9 V, deposition time of 120 s, and modified volume of 7 µL). We then measured the As(III) at Au/Fe3O4 SPCE. Figures 2a and 2b are typical SWASV responses and corresponding linear calibration plots of peak current against As(III) concentrations at Au/Fe3O4 SPCE in different concentration ranges from 0.1 to 10 ppb. The peak shape of the analysis is well defined, and the detection range (0.1-10 ppb) with one consistent sensitivity of 9.43 µA ppb-1 is very attractive. The analysis of As(III) at high concentrations (10-600 ppb) with the preconcentration time of 60 s is also studied (Figure S5). As seen, two different sensitivities are observed: one is at the concentrations of 10-200 ppb that can achieve the quantitative analysis of As(III) (sensitivity of 0.87 µA ppb-1); another is at 200-600 ppb that the increase rate of the current drops and eventually tends to stabilize. At low As(III) levels (10-200 ppb), the synergistic effects of the electrocatalytic activity of Au NPs and the strong adsorptivity of ~10 nm Fe3O4, as well as the surface Fe(II)/(III) cycle, is responsible for the linear plot. In contrast to the case of lower concentration range (0.1-10 ppb), the sharply decreased sensitivity might be due to the shorter concentration time and/or weaker synergistic effect mentioned above. As the concentration continues to increase (200-600 ppb), the large amount of As(III) directly diffuse to the surface of Au/Fe3O4, and the synergistic effects almost could be ignored. This turning point of sensitivity at different concentrations has also been similarly reported.11 In summary, the stronger adsorption ability of the small Fe3O4 NPs (~10 nm) toward As(III),23 and the 12


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redox-active of surface Fe(II)/Fe(III) cycle on Fe3O4 could both dramatically facilitate the response of electrochemical analysis. Consequently, the sensitivity exhibits excellent; the LOD is as low as 0.0215 ppb, which is much lower than WHO standards (10 ppb) in drinking water. For contrast, the response of As(III) at hydrphobic Au/Fe3O4 SPCE is investigated (Figure S6). The peak shape of As(III) exhibits more wide and the sensitivity (0.49 µA ppb-1) is much smaller than that of hydrophilic Au/Fe3O4 SPCE. Otherwise, the actual detection limit concentration (15 ppb) is also significantly higher than the hydrophilic Au/Fe3O4 SPCE (0.1 ppb). These results all demonstrate that the organic matter covered the surface of the nanoparticle can highly lead to the block of mass transfer and electron transport, further decrease the signal of detection. We also investigated the electrochemical behavior toward As(III) of Au NPs and ~10 nm Fe3O4 (Figures 3a and 3b). Figure 3c is a systematic comparison of sensitivities and LODs for bare, Au NPs, ~10 nm Fe3O4, and Au/Fe3O4 SPCE. The sensitivities of Au NPs and ~10 nm Fe3O4 SPCE are 0.97 and 2.89 µA ppb-1, which is only 1/10 and 1/3 of Au/Fe3O4 SPCE, respectively. The bare SPCE has almost no response to low levels of As(III),11 whereas the better performance of Au SPCE is due to the excellent catalytic activity of nanosized Au. The ~10 nm Fe3O4 SPCE is much better than Au NPs—even better than that of 400 nm Fe3O4 SPCE in our previous work.10 The extraordinary synergistic effect of Au NPs and ~10 nm Fe3O4 NPs can lead to the higher sensitivity of Au/Fe3O4 SPCE seen here relative to other studies (Table S1).

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Figure 3. Typical SWASV response of As(III) at different concentration ranges for (a) ~7 nm Au NPs and (b) ~10 nm Fe3O4 SPCE. (c) Comparison of sensitivities for SWASV detection of As(III) at ~7 nm Au, ~10 nm Fe3O4, and Au/Fe3O4 SPCE. Insets in panels a and b are the corresponding linear calibration plot of peak current against As(III) concentrations, respectively. Inset in panel c compares LODs.

Validating the Fe(II)/Fe(III) Cycle through Addition of Fe(II). The influence of Fe(II) on the anodic peak currents of As(III) is shown in Figure 4. As the 14


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concentration of Fe(II) increases, the peak currents of 1 ppb As(III) increases (Figure 4a). The existence of 40 ppb Fe(II) can further enhance the analytical sensitivity (11.32 µA ppb-1, Figure 4b). The added Fe(II) ions are first adsorbed on the surface of Au/Fe3O4 NPs under electric field resulting in an improved concentration of surface Fe(II). These results indicate that surface Fe(II) is beneficial to the detection of As(III), supporting the mediated effect of the Fe(II)/Fe(III) cycle in electrochemical detection. Compared with 400 nm Fe3O4 NPs, the ~10 nm Fe3O4 NPs have a larger surface area and more active surface iron. This enhances the adsorption of As(III) on ~10 nm Fe3O4 NPs and mediates the redox of As(III) by surface Fe(II)/Fe(III) cycle. Furthermore, the addition of Au NPs forming dumbbell-like Au/Fe3O4 nanoparticle can accelerate the As(III) redox electrocatalysis. The electrochemical detection is an interfacial reaction in which the Fe(II)/Fe(III) cycle occurs in this system. Thus, the mediation of Fe(II) occurs on the nanomaterial surface rather than in the solution. Sodium tetrapolyphosphate (Na6TPP) is a superior ligand to coordinate with Fe(II) and can generate soluble Fe(II)-TPP complex to accelerate the Fe(III)/Fe(II) cycle in the Fenton reaction.31, 35-36 Figure S7a is the SWASV response of 1 ppb As(III) at Au/Fe3O4 SPCE in the presence of 0.5–2.5 µM Na6TPP. The addition of Na6TPP did not increase the current of As(III), but rather it slightly decreased the current. This demonstrates that the active Fe(II) improves the electrochemical analysis via Fe(II) on the surface of the Fe3O4 nanoparticles rather than Fe(II) in solution. Protocatechuic acid,30 hydroxylamine,55 and ascorbic acid33, 37 were reduced and promoted the Fe(III)/Fe(II) cycles seen here. This proved the 15



Figure 4. (a) SWASV response of 1 ppb As(III) at Au/Fe3O4 SPCE in the presence of 20−80 ppb Fe(II) in 0.1 M HAc-NaAc solution (pH 5.0). (b) SWASV response of 0.1−1 ppb As(III) at Au/Fe3O4 SPCE in the presence of 40 ppb Fe(II). Inset in panel a is the corresponding increase in As(III) signal as a function of Fe(II) concentration. Inset in panel b is the corresponding linear calibration plot of peak current against As(III) concentrations. The addition of Fe(II) can markedly enhance the response of As(III) proving the mediating effect of the Fe(II)/Fe(III) cycle.

existence of Fe(III)/Fe(II) cycles. We also performed SWASV response of 1 ppb As(III) at Au/Fe3O4 SPCE in the presence of 1-5 µM NH2OH (Figure S7b). The peak current of As(III) decreased, and a new peak of NH2OH appears upon the addition of NH2OH. We also investigated the influence of protocatechuic acid and ascorbic acid were also investigated and achieved similar results (not shown here). These results are the opposite of what has been reported in the literature. It is possible that these reductive can directly reduce As(III) to As(0) in the solution, leading to a decrease in 16


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As(III) adsorbed on the surface of Fe3O4 NPs. Change in As(III) Valence during Adsorption as Presented by XPS. We conducted physical adsorption experiments without applying negative potential (ANP) to study the adsorption ability of ~10 nm Fe3O4 and Au/Fe3O4 NPs. For comparison, we studied the adsorption of As(III) on 400 nm Fe3O4 NPs followed by simple mixing of Fe(II) solution (Fe(II)sol) with As(III). We further studied the detailed detection process by in-situ electro-adsorption with ANP followed by stripping. Experiments included adsorption of As(III) without ANP, with ANP, and stripping on ~10 nm Fe3O4 NPs. These can be described via the following equations: (1) Fe(II)sur + As(III) → Fe(III)sur + As(0)

without ANP

(2) Fe(II)sur + As(III) → Fe(III)sur + As(0) Fe(III)sur + e- (from electrode) → Fe(II)sur (3) Fe(III)sur + As(0) →Fe(II)sur + As(III)

with ANP (Fe cycle) stripping

The XPS spectra of As and Fe after adsorption are shown in Figure 5 and Figure S8. After simply mixing the Fe(II) in solution with As(III), the As(III) does not change its valence, suggesting that the Fe(II)sol has no effect in the redox of As(III) (Figure 5a). Interestingly, the As(III) adsorbed on the surface of ~10 nm Fe3O4 NPs (Figure 5b) and Au/Fe3O4 (Figure 5d) are reduced to be As(0), while keeping As(III) on 400 nm Fe3O4 (Figure 5c). This indicates that the Fe(II) on ~10 nm Fe3O4 NPs and Au/Fe3O4 have higher reactivity in the redox property leading to a direct reduction of As(III) to As(0). As described in reaction (1), the activated Fe(II) on the surface of ~10 nm Fe3O4 is oxidized to Fe(III). This oxidation accompanies the reduction of As(III). 17



Thus, a decreased ratio of Fe(II) in ~10 nm Fe3O4 NPs is observed after adsorption (Table S2). We performed the electro-adsorption of As(III) with ANP and stripping to model the specific reaction between As(III) and Fe(II) during detection (Figure 5). After holding a negative potential for some time, the As(III) will be enriched and then reduced to As(0) on the surface of the modified SPCE. This process is

Figure 5. High-resolution XPS spectra of As (a) with 10 ppm Fe(II) in solution, on (b) ~10 nm Fe3O4 NPs, (c) 400 nm Fe3O4 NPs, and (d) Au/Fe3O4 NPs after adsorption of 10 ppm As(III). The adsorption including physical adsorption, electro-adsorption with applying negative potential (ANP), and stripping after electro-adsorption. Insets in panels a-c correspond to the schematics of adsorption. The chemical information of As(0) is clearly seen on ~10 nm Fe3O4 and Au/Fe3O4 NPs indicating the redox activity of the Fe(II)/Fe(III) cycle.

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pre-concentration. The As(III) on the ~10 nm Fe3O4 NPs (Figure 5b) and the Au/Fe3O4 (Figure 5d) with ANP transformed into As(0). Larger amounts of As(III) are adsorbed and reduced to As(0) on ~10 nm Fe3O4 NPs under a negative potential relative to those without ANP. Otherwise, the addition of Au NPs, as electronic conductors and catalysts, can further enhance the reduction of As(III) to As(0) on Au/Fe3O4 NPs (Figure 5d). As shown in Figure 5c, almost no XPS signal of As(0) could be observed under applying negative potential, it is believed that the redox of As(III) does not occur on the surface of 400 nm Fe3O4 nanoparticle because their poor conductivity. It is specially to point out that a pure carbon electrode is not good to reduce As(III) to As(0) because of its chemically inert for As(III) redox.18, 56 However, the excellent adsorption effect of 400 nm Fe3O4 is used to concentrate more As(III) from the solution and desorb/diffuse to the surface of bare SPCE, resulting in a higher electrochemical signal.10-11 Reaction (2) shows that the Fe(III) on ~10 nm Fe3O4 will be reduced to Fe(II) via electrons from SPCE. This completes the Fe(II)/Fe(III) cycle. As a result, the Fe(II) content increases (40.1%, Figure S8; Table S2). Pure Au/Fe3O4 (Figure S8d) has 41.7% Fe(II), which is more than that of ~10 nm Fe3O4 NPs (35.4%) possibly because of the addition of Au NPs. The intercalation of Au will vary with the electronic structure of the metal oxides.57-58 The Fe3O4 nucleates preferentially on the gold seeds and free electrons in the Au NPs. These also can catalyze the Fe3O4 nucleation.49, 59 After adsorption without ANP, the ratio of Fe(II) to Fetotal at Au/Fe3O4 nanostructures decreases from 41.7% to 38.3%. It increases to 53.4% with ANP (Table S2). This confirms the Fe(II)/Fe(III) cycle. The redox of As(III) could be 19



accelerated during the adsorption as well as the electrochemical detection with Au NPs catalysis. As(III) on 400 nm Fe3O4 NPs, however, keep its valence both with and without ANP (Figure S8b). The content variation of Fe(II) on 400 nm Fe3O4 NPs (Figure S8b) is also smaller than that of ~10 nm Fe3O4 and Au/Fe3O4 NPs. Stripping is the oxidation of As(0) to As(III), giving electrochemical signal at SWASV. As expected, the adsorbed and reduced As(0) on ~10 nm Fe3O4 and Au/Fe3O4 NPs oxidize to As(III) (Figures 5b and 5d, Table S2, reaction (3)) with stripping. This indicates that the oxidation of As(0) to As(III) occurs on the surface of ~10 nm Fe3O4 and Au/Fe3O4 NPs, demonstrating that the catalysis activity originates from the surface Fe(II). A larger amount of As(0) is oxidized to As(III) on Au/Fe3O4 compared with ~10 nm Fe3O4, suggesting that the presence of Au NPs effectively enhances the oxidation of As(III), which is consistent with the electrochemical response in Figure 3. As As(0) is stripped to As(III), the Fe(III) on ~10 nm Fe3O4 and Au/Fe3O4 NPs also is reduced to Fe(II), leading to an increase in the Fe(II) content (Table S2). Surface Active Fe(II) Confirmed via XANES and EXAFS. Because of the high sensitivity to redox and coordination atomic states, XANES analyses can offer quantitative information regarding the As and Fe after 10 ppm As(III) is adsorbed on ~10 nm Fe3O4, Au/Fe3O4, and 400 nm Fe3O4 NPs without ANP. The normalized XANES spectra (Figure S9a) represent the oxidation state of As after adsorption—this is consistent with XPS. A small shift in the Fe profile toward high energy (Figure S9b) indicates increasing Fe(III) content, which agrees with XPS. 20


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As(III) adsorbed on the surface of ~10 nm Fe3O4 and Au/Fe3O4 would be reduced to As(0) via the Fe(II)/Fe(III) cycle, but As(III) on 400 nm Fe3O4 remains unchanged. The reduction of As(III) is accompanied by a change in the Fe chemical valence. As mentioned earlier, the insertion of Au into ~10 nm Fe3O4 raises the question about that the distortion of oxidation state in Fe3O4 NPs. This is also obvious in Figure S9.

Figure 6. Fourier transforms (FTs) of EXAFS data for (a–c) As and (d–f) Fe K-edge on ~10 nm Fe3O4, 400 nm Fe3O4, and Au/Fe3O4 NPs after adsorption with 10 ppm As(III), respectively. Dotted lines are their fitting paths. Fits to spectra are uncorrected for phase shift. Distorted local coordination on ~10 nm Fe3O4 and Au/Fe3O4 NPs are obvious. This benefits the production of surface defects and leads to a higher adsorption and redox activity. XANES offers information about the oxidation state, whereas the EXAFS region of the data shows information regarding the local shell structure around the absorbing atom. The background subtracted k3-weighted χ(k) functions of As and Fe K-edge (Figure S10) are k-space oscillations after sorption on samples. For further confirmation, the radial structure functions of these samples were Fourier transformed and plotted in R-space (Figure 6). It is well known that there are different Fe−O and Fe−Fe bonds in Fe3O4 with cubic inverse spinel structure due to the diverse tetrahedral and octahedral sites. 21



Although the Fe3+ crystallizes in one of the eight tetrahedral interstices—and Fe2+/3+ crystallizes in half of the octahedral interstices—the fitting data and the components of ~10 nm Fe3O4 and 400 nm Fe3O4 (Figures 6d and 6e) are similar and agree with earlier reports about Fe3O4. This indicates that the size of the Fe3O4 does not sharply affect the local structure of the nanocrystals.60-61 Because of their extremely small size, an essential part of the Fe–O and Fe–Fe near the surface of ~10 nm Fe3O4 NPs become different with distorted local coordination. This disorder on the surface of ~10 nm Fe3O4 NPs produces surface defects leading to higher adsorption and catalytic ability.62-64 In addition, the presence of Au in Fe3O4 causes a highly distorted local coordination of Fe–O2 and Fe–Fe compared with pure Fe3O4 NPs. Figures 6a–6c indicates that the As(III) adsorbed on ~10 nm Fe3O4 NPs and Au/Fe3O4 are similar to each other while exhibiting different coordination numbers and Fe bond lengths on the 400 nm Fe3O4 NPs. This difference is attributed to the diversity of surface atoms that are active in these NPs. This could lead to diverse adsorption and catalysis properties.

Figure 7. Typical SWASV responses with standard additions of As(III) into a real water sample diluted with 0.1 M HAc-NaAc solution (pH 5.0) at a volume ratio of 1:9. Inset is the corresponding linear calibration plot of peak current against As(III). 22


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Real Sample Analysis. The detection of real water is also investigated to demonstrate the potential application of the Au/Fe3O4 SPCE. The real water is took from Dongpu Reservior in Hefei City, Anhui, China. The water simple treated with a filtered to remove insoluble substances and then diluted with 0.1 M HAc−NaAc buffer solution in a volume ratio of 1:9. Figure 7 shows the detection of real water spiked with 0.1 ppb As(III). The corresponding linear relationship (R2=0.997) and the sensitivity both exhibit good results. Furthermore, Table S3 concludes the spiked As(III) in real water at different concentration and the recovery. As shown, the recovery was calculated to be 90−110% indicating that the Au/Fe3O4 SPCE has a great practical application.

CONCLUSIONS In summary, we have designed an effective sensitive interface to detect As(III) with using dumbbell-like Au/Fe3O4 nanoparticles. A dramatically enhanced electrochemical response for the detection of As(III) can be obtained by combining the adsorption of ~ 10 nm Fe3O4 NPs, the Au NPs catalyst, and the mediation of surface-active Fe(II). The characterization results have demonstrated that the Fe(II)/Fe(III) cycle is involved in the As(III) detection on the interface of dumbbell-like Au/Fe3O4 nanoparticles. Meanwhile, a more distorted local coordination of Fe–O and Fe–Fe in 10 nm Fe3O4 NPs offer their higher adsorption and surface atom activity. We do believe that modulating the surface defects of nanomaterials is a powerful technique to design excellent sensing interface for 23



analytical community in the future.

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ACKNOWLEDGEMENTS This work was supported financially by the National Natural Science Foundation of China (21735005, 21475133, U1532123, 61573334, 61474122 and U1532120), Natural Science Funds for Distinguished Young Scholars (Grant 21425728), and National Natural Science Fund of China (11405256). The authors thank Shanghai Syn-chrotron Radiation Facility (beam line BL14W1) for providing measurement time. 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 and discussion including apparatus, fabrication of Au/Fe3O4 SPCE, electrochemical detection of As(III), adsorption experiments, XPS of As, XANES and EXAFS measurement and data analysis, optimization of experimental conditions, and interference measurements are available in Supporting Information.

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Fe(III) Cycle Promoted Ultra-Highly Sensitive

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