Improving Surface Adsorption via Shape Control of Hematite Î±-Fe2O3

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Improving Surface Adsorption via Shape Control of Hematite #-Fe2O3 Nanoparticle for Sensitive Dopamine Sensors Anran Chen, Liang Xu, Xiaojing Zhang, Zhimao Yang, and Shengchun Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11088 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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ACS Applied Materials & Interfaces

Improving Surface Adsorption via Shape Control of Hematite

α-Fe2O3

Nanoparticle

for

Sensitive

Dopamine Sensors Anran Chen†, Liang Xu†, Xiaojing Zhang†, ZhimaoYang†,‡, Shengchun Yang*,†,‡ †

School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of

Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China. ‡

Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou Academy of

Xi’an Jiaotong University, 215000, Suzhou, People’s Republic of China.

ABSTRACT: The α-Fe2O3 nanoparticles (NPs) with morphologies varying from shuttle to drum were synthesized through an anion-assisted and surfactant-free hydrothermal method by simply varying the ratios of ethanol and water in solvent. Control experiments show that the structural evolution can be attributed to a small-molecular-induced anisotropic growth mechanism in which the growth rate of α-Fe2O3 NPs along a-, b or c-axis was well controlled. The detailed structural analysis through the high resolution transmission electron microscope (HRTEM) indicated that shuttle-like Fe2O3 NP surface was covered by high density atomic steps, which endowed them the enhanced adsorption ability, so as to the sensor ability toward dopamine (DA). The XPS characterizations indicated that the percentages of the OC component follow the order of

ACS Paragon Plus Environment

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shuttle-like Fe2O3 (S-Fe2O3 for short) > pseudo-shuttle-like Fe2O3 (Ps-Fe2O3 for short) > polyhedron-like Fe2O3 (Ph-Fe2O3 for short) > drum-like Fe2O3 (D-Fe2O3 for short). Benefits from these structural advantages, the S-Fe2O3 NPs/Nafion composite electrode exhibited remarkable electrochemical detection ability with a wide liner range from 0.2 µM to 0.107 mM and a low detection limit of 31.25 nM toward DA in the presence of interferents.

KEYWORDS: Hematite, Shape evolution, Dopamine, Electrochemical biosensor, Chemical absorption ■ INTRODUCTION Dopamine (DA), a kind of catecholamine, is a key marker for excitatory chemical neurotransmitters. It has been proved that DA has important regulatory functions toward mammalian central nervous, renal, cardiovascular and hormonal systems.1-6 Moreover, studies in neurobiology revealed that the systematic functional disorder of DA would also results from a clinical reflection of HIV virus infection7-9 The precise determination of DA level in biological systems is therefore important in above disease diagnoses. Thus there are strong needs for sensitive, selective and dependable approaches for direct DA detection in vitro or in vivo. As for the healthy individuals, the concentration of DA is in the submicromolar range in the brain extracellular fluid, while it will increases when someone suffers the diseases as mentioned above.10-12 Recently, Tang et.al fabricated a Ag shell coated aptamer-mediated dimer to further enhance the SERS signal for DA detection.13 Dan and coworkers found a efficient colorimetric method to detect DA molecular by using Au NPs as optical probes through modifying the natural beta-cyclodextrin.3 Tevhide et.al reported a NMR relaxation time-based approach to detect DA molecular in which the paramagnetic nanoparticles, more precisely, Fe3+ acted as both the


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contrastable agent and the target recognizable element.14 However, these techniques require sample pretreatment, complex equipment, and maybe, high cost materials, thus partially limited their practical applications. As DA is electroactive,11,15-17 the electrochemistry offers an alternative method for its determination because of their simple operation, time savings, fast response and comparable low cost, and more importantly, their excellent selectivity is conductive to overcoming the defaults resulting from the presence of interferents.4,11,18-22 Hematite α-Fe2O3 with a corundum type structure (R3c space group, a = 0.50352 and c = 1.37508 nm) is the most stable structure of iron oxide under ambient conditions.23,24 Recently, nanosized α-Fe2O3 has been extensively explored in the field of catalysis, energy conversion, diagnostics and theranostics, resulting from some of their obvious advantages, such as low cost, high performance in corrosion resistance and ideal band gap (2.1 -2.2eV).25-30 In addition, the precise control of NP morphologies was a crucial issue for improving their performance in above applications, resulting from the difference in the atomic arrangement on different exposed crystal facets. It is thus imperative to construct the NPs sounded with high-energy surface that has been regarded as an efficient way for improving their catalytic activity and selectivity.31 For example, the α-Fe2O3 NPs enclosed with high energy facets have been found to exhibit more enhanced CO oxidation activity and ethanol gas-sensing ability compared with that of the NPs with low surfaceenergy.32 Similarly, Kim et al. reported that the hematite NPs presented a shape-dependent performance in photocatalytic degradation of methyl orange, which was attributed to the difference in the density of surface Fe3+ ions in particular facets.33 Recently, hematite nanocrystals based biomedical sensor has attracted extensive attention ascribing to their relatively low toxicity and high biological tolerance, low cost and environmentally friendly.34 Especially, α-Fe2O3 nanostructures has become the promising


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candidates for constructing the DA or glucose enzyme-free biosensors,35,36 resulting from the variable valence state of iron oxides, which can be in-situ recovered by electrochemically oxidizing or reducing during the sensing reaction, thus triggering the heterogeneous oxidation or reduction of the target molecular.37 These reaction processes are closely related to their type of surface facets or morphologies, because the chemical potential much interrelated with the surface energy for the nanoscaled materials.38,39 Nevertheless, few reports were focused on exploring the relationship between the electrochemical biosensor properties of hematite nanocrystal and their morphologies or facets, which was a key issue for better understanding the facet-dependant electrochemical reaction mechanism, thus improving their performance in biosensors. Hence in current work, single crystalline α-Fe2O3 NPs with shuttle, pseudo-shuttle, polyhedron and drum morphologies were well synthesized on the basis of a simple ethanol assisted hydrothermal method. The DA sensor electrode was then fabricated based on the as-prepared hematite NPs. Due to the surface was enclosed with facets of high surface energy, the shuttle-like NPs exhibited remarkable detection ability with a wide liner range from 0.2 µM to 0.107 mM and a low detection limit of 31.25 nM. And it also presented an excellent anti-interference performance, reproducibility and stability toward DA. Furthermore, the essential effects from the shape of four types of α-Fe2O3 NPs in DA sensing were also discussed. ■ EXPERIMENTAL SECTION Chemicals Hydrated iron nitrate (Fe(NO3)3·9H2O), dopamine (DA) and potassium hydroxide (KOH) were obtained from Aladdin Industrial Corporation, ethylene glycol (EG) and anhydrous ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. Deionized water used in all the


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experiments was 18.25 MΩ·cm and the chemical regents used in the preparation were analytical grade and without any further purification. Syntheses of α-Fe2O3 NPs with of Shuttle, Pseudo-shuttle-, Polyhedron- and Drum-like Morphologies The synthesis of α-Fe2O3 NPs was carried out following a facile hydrothermal procedure. Typically, 0.2425 g of hydrated iron nitrate was dissolved in 20 mL solvent, which was composed of EG (6 mL), distilled water (12 mL) and ethanol (2 mL), followed by vigorous magnetic stirring. After the solutes completely dissolved, 0.123 g of KOH was rapidly injected into the mixture solvent. After further stirring for 40 min, the mixture solution was transferred into a Teflon-lined stainless autoclave and put it to an oven to heat for 8 h at 200 ℃. After the synthesis, the solution was cooled to ambient temperature, and the precipitations were centrifuged and washed with ethanol and deionized water for several times, respectively. Then the final products were dispersed and stored in deionized water for further use. As for the synthesis of the hematite α-Fe2O3 NPs from shuttles to drum-like shapes, the volume ratio of EG, water and ethanol in the mixture solutions was 6:12:2, 6:10:4, 6:6:8 and 6:4:10 in turn, as shown in Table S1. Characterizations The morphology of the as-produced samples was characterized by scanning electron microscope (SEM, FEI Quanta, FEG 250, energy spectrum: EDAX, Apollo XL-SDD) at an acceleration voltage of 20 kV. The structural characterization was further investigated by X-ray diffraction (XRD), which was performed on a Bruker-AXS D8 Advance diffractometer operated at 40 kV voltage and 30 mA current using Cu Kα radiation (λ=1.5418 Å) in the range of 15-90o. The


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UV-vis spectrum of the samples was recorded on Hitachi UV-4100 UV/Vis spectroscopy. The surface structure of the samples was investigated through the X-ray photoelectron spectroscopy (XPS, Kratos Axis UL equipped with monochromatic Al Ka radiation, 150 W, 5 kV at 1486.6 eV, chamber pressure, 10-9 Torr), by referencing the spectra to the C 1s peak for the C-C bond at a binding energy of 284.8 eV. TEM image of the products was recorded on a JEOL JEM-2100 transmission electron microscope. The operation was performed at an accelerating voltage of 200 kV. Zeta potential of the NPs was measured using a Zeta sizer Nano ZS (Malvern Instruments). Electrochemical Measurements of Samples The sensor electrode was prepared by dispersing 20 µL suspension of α-Fe2O3 (1 mg/mL) onto a polished glassy carbon electrode (GCE) and dried at room temperature. Then, 10 µL of Nafion solution

(0.02

%,

Sigma-Aldrich)

was

dropped

on

the

electrode

to

prepare

a

Nafion/α-Fe2O3/GCE sensor electrode. The electrochemical measurements were performed on the Pine AFCBP1 with a three-electrode system. The α-Fe2O3 NP modified electrodes were used as the working electrode, platinum foil was used as counter electrode and the saturated calomel electrode (SCE) was used as the reference electrode. For comparison, a blank electrode was prepared by coating 10 µL of above mentioned Nafion solution on the bare GCE electrode and dried at room temperature. All the experiments were carried out at room temperature. ■ RESULT AND DISCUSSIONS Microstructures and Characterization The morphology of α-Fe2O3 NPs was well controlled by changing the volume ratio of ethanol and water in the presence of ethylene glycol (EG). Observed in detail, Figure 1a and a1 show the


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SEM and TEM images of the S-Fe2O3 NP synthesized at a volume ratio of water and ethanol 6:1, respectively. The average apex to apex length (L) of S-Fe2O3 NP is around 330 nm, and the average edge length (D) is about 170 nm. The HRTEM image in Figure 1a2 is taken from a single NP pointed out by a red cycle in Figure 1a1, and a 0.36 nm interval of the lattice fringes agrees well with the spacing of (012) plane of α-Fe2O3 (three-index notation). It is worthwhile to note that the S-Fe2O3 NP is covered by a highly roughened surface, which implies that it possesses a high surface energy compared with that of the particles with smooth surface. The corresponding selected area electron diffraction (SAED) of a single NP reveals that the product has a good crystal and the external surfaces were different from the primary {110} and {100} facets of the hexagonal hematite (Figure 1a3). When the volum ratio of water/ethanol was decreased to 5:2, as shown in Figure 1b, the L to D ratio of the NPs decreases from around 1.9:1 to 1.8:1, indicating that the tip growth of the as-prepared S-Fe2O3 NPs was blocked and formed a pseudo shuttle-like NPs. In addition, as shown by the HRTEM image in Figure 1b2, the edges of the NPs became smoother than the S-Fe2O3 NPs, indicating that the increase of ethanol ratio was conducive to reduce the density of surface atomic steps, defects and voids, so as to decrease the surface energy of the α-Fe2O3 NPs. When the volume ratio of water/ethanol was further decreased to 3:4, Ph-Fe2O3 NPs still with a shuttle-like morphology was synthesized. As one can see from Figure 1c1, two terminals, as well as the cambered surfaces of the nanopartiles obviously became small facets. A 0.36 nm lattice space measured in Figure 1c2 can be ascribed to the spacing of (012) planes. This observation indicates that the hindrance function become stronger by increasing the ethanol ratios, and thus inducing the tip growth of the polyhedron nanocrystals being blocked and forming a facet. Figure 1d-1d1 shows that the D-Fe2O3 NPs synthesized by decreasing the water/ethanol volume ratio to 2:5 present an average diameter and


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a thickness of around 340 nm and 270 nm, respectively. The HRTEM image (Figure 1d2) shows that the lattice fringes in the up facet were perfectly aligned and their distance was measured to be 0.25 nm, which can be ascribed to (110) facet of α-Fe2O3. Additionally, the SAED pattern of all the samples shows discrete but regular reflecting spots, indicating the single crystalline property of the as-prepared α-Fe2O3 NPs. Meanwhile, a series of weak spots from other diffraction directions can be observed from the S-Fe2O3 NPs (Figure 1a3), which is due to the existence of the nanovoids, further indicating that the S-Fe2O3 NPs was enclosed by a more roughened surface compared to other samples.30

Figure 1. Wide-field low magnification SEM images of the hydrothermally synthesized α-Fe2O3 NPssamples:(a) S-Fe2O3, (b) Ps-Fe2O3, (c) Ph-Fe2O3and (d) D-Fe2O3. Scale bar in the images is 500 nm. (a1-d1) TEM image of α-Fe2O3 NPs, (a2-d2) HRTEM images pointed out by red cycles in a1-d1, the scale bar is 5 nm, (a3-d3) SAED pattern of single particle, (a4-d4) corresponding ideal geometrical models of individual α-Fe2O3 NPs.


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The main X-ray powder diffraction peaks from the α-Fe2O3 NPs (Figure 2a) can be ascribed to the rhombohedral α-Fe2O3 (JCPDS No. 33-0664). The appearance of the sharp and narrow peaks suggests that the as-synthesized α-Fe2O3 NPs have good crystallinity. At the same time, the relative intensity of some specific diffraction peaks in samples are quite different, and the (110), (116) and (300) diffraction peaks of the drum Fe2O3 NPs is obviously higher than the others (Figure S1), which is consistent with that of the plate-like Fe2O3 NPs in previous reports,40 indicating that the α-Fe2O3 develops better in the crystal growth along the a- than in the cdirection, which is consistent with that of the two dimensional property of the plate- or sheet-like NPs.41,42 Generally, the growth rates of different planes of NPs would play a decisive role in its final morphology grown in specific conditions. The formation of D-Fe2O3 NPs indicated that the growth of NPs along a-, b-axis is improved, while their vertical growth along c-axis is restrained.41 The EDAX analysis was also used to obtain more information about composition of the all of the samples (Figure S2), which indicates the presence of iron, oxygen and silicon. The signals from silicon result from the substrate. The Raman spectra of α-Fe2O3 NPs revealed their typical seven Raman active vibration modes, as shown in Figure 2b. The typical A1g mode at 220 and 495 cm-1 and Eg mode at 286, 403 and 602 cm-1 are presented in the spectra of the all samples. At larger wave numbers, overtones or second order scattering processes of the first order scattering can be observed. The band at 1305 cm-1 results from the magnon scattering,43,44 and the peak at 652 cm-1 could be ascribed to the Si-O band, which may come from the glass-substrate.45 Additionally, owing to the difference in the exposed crystal facets, an obviously enhanced light absorption from UV to near-infrared (600-1200 nm) was observed and shown in Figure S3, the enhanced absorption in near-infrared range suggested that the Ph- and D-Fe2O3 NPs might have potential applications for


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photocatalysts and photothermal therapy. The magnetic moment as a function of magnetic field was also detected, as shown in Figure S4, which reveals a typical weak ferromagnetic behaviour at room temperature for all samples.

Figure 2. (a) XRD patterns and (b) Raman spectra of the as-prepared α-Fe2O3 NPs, S-, Ps-, Phand D-like Fe2O3 NPs (top to bottom). Cyclic Voltammetric Behaviour of DA on the Fe2O3/GCE Electrode


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The electrochemical responses of DA on the S-Fe2O3/GCE (shuttle-like Fe2O3 NPs modified GCE),

Ps-Fe2O3/GCE

(pseudo

shuttle-like

Fe2O3

modified

GCE),

Ph-Fe2O3/GCE

(polyhedron-like Fe2O3 modified GCE) and D-Fe2O3/GCE (drum-like Fe2O3 modified GCE) were examined using cyclic voltammetric (CV) method in 0.2 mM DA in PBS solution (Figure 3a). As shown in Figure 3a, an obvious pair of redox peaks can be observed at 0.16 and 0.11 V after the addition of 0.2 mM DA, these peaks appeared relatively sharp compared with that of the bare electrode, which can be ascribed to the oxidation and reduction of DA, respectively, while no redox peaks were found in the blank PBS solution. Additionally, the S-Fe2O3 NPs modified GCE shows the best electrocatalytic activity. By contrast, the peak current intensity for the electrode made from the Ps-, Ph-, and D-Fe2O3 NPs was only about one-third of the value measured from the S-Fe2O3 NP modified electrode. This means that the S-Fe2O3 NPs, which is surrounded by high intensity of atomic steps, possess a more sensitive performance toward DA sensing. In other words, the DA molecules prefer to attach on the surface of α-Fe2O3 NPs with high surface energy, or to be more specific, the DA sensing process is strongly affected by the facet type of the α-Fe2O3 NPs. The onset potential of the DA formation in the S-, Ps-, Ph-, and D-Fe2O3 NPs modified electrode are 0.055, 0.071, 0.078 and 0.080 V, respectively. A Sudden increase in current intensity can be observed after the onset potential for the S-Fe2O3/GCE. Additionally, the reduction potential due to the reduction of adsorbed DA layer on the S-Fe2O3 NPs shows a negative shift compared with that of the other samples, indicating a stronger interaction between the adsorbed oxygen or hydroxyl and Fe2O3 surface. It has been proved that the high-index facets and kink/step sites could have a much stronger adsorption toward oxygen or hydroxyl species, which generally induce a negative shift of the onset potential.46-52 Combining with the above TEM studies, it indicates that there are a large amount of


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low-coordinated atoms existing on the S-Fe2O3 surfaces, and that is to say, the catalytic performance of the Fe2O3 surface was determined by the atomic structure on their surface. That should be the reason why the cyclic voltammetry is quite sensitive to the surface structures of the nanocrystals. Additionally, in order to test the effects of Nafion film on the catalytic performance of the electrode, a blank electrode was prepared by coating 10 µL of Nafion solution on the bare GCE electrode. As shown in Figure S5, almost no enhancement in the electrocatalytic performance of the Nafion/GCE electrode was observed when the electrode was only covered with Nafion compared with that of bare GCE, indicating the catalytic contributions from the Nafion film was negligible. Figure 3b show the current response with increasing the concentration of DA measured on the S-Fe2O3/GCE electrode. Obviously, the peak current of the redox couple increased with the concentration of DA increasing from 0.02 mM to 2 mM, and the liner regression equations are Ipa= 28.307C + 5.764 (R2 = 0.981) and Ipc= -18.045C - 3.116 (R2 = 0.996) for positive and negative scanning, respectively. The slope of the calibration plot for DA obtained from the S-Fe2O3/GCE electrode is 28.31 for the positive scanning (Figure 3b, inset), which is much higher than those obtained from Ps-Fe2O3/GCE (slope: 25.50, Figure S7a, insert), Ph-Fe2O3/GCE (slope: 24.95, Figure S8a, insert) and D-Fe2O3/GCE (slope: 23.49, Figure S9a, insert) modified GCE. These results further indicate that the electrode modified by S-Fe2O3 NPs is more sensitive than that of the others.


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Figure 3. (a) Typical cyclic voltammograms (CVs) measured with and without 0.2 mM DA use S-Fe2O3/GCE, Ps-Fe2O3/GCE, Ph-Fe2O3/GCE and D-Fe2O3/GCE. (b) Cyclic voltammograms of S-Fe2O3/GCE in the presence of different concentrations of DA, inset, the relationship of anodic and cathodic peak. The onset potentials are marked by arrows. Test environment: PBS solution (pH 7.0). The scan rate was kept at 50 mVs-1. Effect of Scan Rate To investigate the reaction kinetics, the influences of potential sweep rates on the peak potentials and the peak currents of DA were also investigated. Figure 4a shows the CVs of the S-Fe2O3/GCE sensor at the potential scanning increasing from 5 to 500 mVs-1 in PBS solution


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(pH 7.0), in presence of 0.2 mM DA. The redox peak current increased consistently with the scan rate. At the same time, the anodic and cathodic peaks show a slight shift to more positive and negative potentials at higher scan rates, respectively, which can be attributed to the presence of ohmic drop and relatively high energy is required for anodic striiping.11,53 The Figure S6 indicates the dependency of anodic and cathodic peak currents to the scanning rate. It can be seen that, on one hand, a well linear dependence of redox peak currents on the scan rate below 60 mVs-1 can be observed, indicating the electron transfer on the S-Fe2O3/GCE is a surface adsorption controlled redox process (Figure 4b). On the other hand, the peak current intensity also presents a linear increase with the increase of scanning rate, and their linear regression equations can be expressed as: Ipa = 0.2259x + 4.4651, R2 = 0.9952; Ipc = - 0.2129x -0.2999, R2 = 0.9987, respectively. According to the equation (1):

I =

[1]

Where R is 8.314 J mol-1 K-1, n is the number of transferred electron, F is Faraday constant, Q is the quantity of charge (C), v is the scanning rate, T is surrounding temperature (298.15 K). The integration area under the reduction peaks educed the constant charge (Q) values, which is independent with the scanning rate below 60 mVs-1, revealing that the redox reaction DA molecular on the surface of S-Fe2O3/GCE sensor is a quasi-reversible surface-adsorption controlled process. Moreover, the peak currents intensity become proportional to the square root of v in the range of 70 - 500 mVs-1 (Figure 4c) when further increases the scanning rate. It reveals that the electrochemical redox reaction of DA is a diffusion-controlled process,54,55 implying that the as-prepared sensor meets the requirement of the quantitative detection for practical applications. At higher scanning rates, it can be seen that the separation in the redox


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peak position increased obviously, as shown in Figure 4d, which indicates that the charge transfer kinetics partly limits the occurance of redox reactions, leading to a increased separation in the redox peak position. Accordingly, by measuring the change of peak potential with scanning rate, the electron transfer rate constant (ks) and charge transfer coefficient (α) can be calculated using Laviron equations (2), (3) and (4), respectively.11,54, 56, 57 .

E = E + () logv [2]

E = E −

.

logv [3]

logk = α log(1 − α) + (1 − α)logα − log −

()∆$% .

[4]

Where ∆E is the peak separation of the S-Fe2O3/GCE redox couple. According to the above equations, the value of n is calculated to be 0.9649，implying a one-electron and one-proton transformation process at the sensor interface.58 The transfer coefficient (α) and the electron transfer rate constant (ks) of the S-Fe2O3/GCE is calculated to be 0.6346 and 0.2000s-1, respectively, indicating relative fast electron transfer kinetics compared with the reported polymer-modified carbon ceramic electrode.59 And the electrochemical analysis for Ps-Fe2O3/GCE, Ph-Fe2O3/GCE and D-Fe2O3/GCE were shown in Figure S7-S9, respectively. The surface coverage concentration of the electroactive species, i.e. DA in current case, of the sensor electrode (Γ) can be calculated from the CVs through the equation (5):

Γ = '

[5]

The value of calculated Γ is 1.81 × 10-5 mol/cm2, which is seven times higher than the value 2.86 × 10-12 mol/cm2 measured from the bare GCE, indicating a much enhanced adsorption


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capacity of DA molecular at the surface of S-Fe2O3/GCE sensor. And on the other hand, as listed in Table S2, the Γ value calculated from the S-Fe2O3 modified electrode is obviously higher than those obtained from the Ps-Fe2O3 (1.01 × 10-5 mol/cm2), Ph-Fe2O3 (0.74 × 10-5 mol/cm2) and D-Fe2O3 (0.68 × 10-5 mol/cm2), suggesting that the S-Fe2O3 NPs possess a much improved chemisorption activity toward DA. Furthermore, the slope of electrical response equations is found to follow the order of S-Fe2O3 > Ps-Fe2O3 > Ph-Fe2O3 > D-Fe2O3, similar trends are also found in the electron transfer coefficient(α).

Figure 4. (a) Typical CVs of S-Fe2O3/GCE at different potential scanning rates increasing from 5 to 500 mVs-1 in a PBS solution (pH 7.0, containing 0.2 mM DA); The proportionality of anodic and cathodic peak currents intensity to the linear relation (c) and square root of potential scanning rate (c). (d) The change relationship of peak potential vs. Logarithm of scan rates. Effect of Amperometric Response Figure 5a shows the amperometric responses of S-Fe2O3/GCE electrode with increasing the DA concentrations from 0.2 µM to 0.39 mM at the potential of 0.16 V. The results indicates that


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the S-Fe2O3/GCE electrode exhibits a fast and linear response to the variation of DA concentrations increasing from 0.2 µM to 0.107 mM (Figure S10a, magnify part in Figure 5a). The related current response equation is I (µA) = 0.1315x + 0.2214, R2 = 0.9959. On the basis of the linear region of the calibration curve, the sensitivity and detection limit of the S-Fe2O3/GCE electrode are estimated to be 671.02 µA mM-1 cm-2 and 31.25 nM (signal to noise ratio, S/N = 3), respectively. The corresponding calibration curve of α-Fe2O3 NPs were showed in Figure S9 and the detailed amperometric response parameters are listed in Table S3. Under the same applied potential, α-Fe2O3 NPs has much wider linear range, fast and step-like response on DA. Especially, the sensitivity of the S-Fe2O3/GCE was 0.1315 µA µM-1, being 2.31, 1.87, and 1.51 times as high as that of Ps-Fe2O3, Ph-Fe2O3, and D-Fe2O3, respectively. The main challenge in the electrochemical detection of DA is the coexisted interference. For instance, ascorbic acid (AA), potassium chloride (KCl), uric acid (UA), urea and L-dopa that were usually co-exist with DA and probably interfering the sensor sensitive. The concentration range of AA and DA is 10-7-10-3 M and 10-8- 10-6 M, respectively. Therefore, one of the most desirable properties of S-Fe2O3/GCE electrode is the ability to specifically bond to the target molecules, which was investigated by adding a series of DA solutions containing various interfering substance with a concentration of 2 µM at the potential of 0.16 V. Figure 5c clearly shows that the response signals toward the interferents are negligible and the S-Fe2O3/GCE electrode still exhibits a much better linear relation (R2= 0.9903) (Figure S11). Although the ascorbic acid has a similar redox potential to that of DA, and meanwhile, L-dopa is known to have a similar molecular structure to DA. These results demonstrate that the S-Fe2O3/GCE has an excellent specificity and selectivity for DA detection in the presence of the interferents and is suitable for the complex testing environment. In addition, the response current of S-Fe2O3/GCE


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electrode displays a step-like profile even when 31.25 nM DA was successively injected into the test system and they also present short response time by less than 2 s, as shown in Figure 5d. Moreover, As shown in Table S4, compared with the other reported sensor materials, such as graphene, and some of the noble metal/metal oxide based DA biosensor electrodes, the α-Fe2O3 electrode exhibits a much better sensing performances in the sensitivity, linear range, detection limit and response time.

Figure 5. (a) Amperometric response of the α-Fe2O3/GCE electrode with successive addition of DA at regular intervals. S-, Ps-, Ph- and D-Fe2O3/GCE (top to bottom). The applied potential was +0.16 V vs. SCE, stirring speed: 300 rpm. (b) The histogram of the parameters of amperometric response of α-Fe2O3/GCE. (c) Amperometric response S-Fe2O3/GCE to the stepwise injection of AA, KCl, L-dopa, urea and UA (2 µM), followed by the successive addition of DA. (d) Amperometric response of S-Fe2O3/GCE electrode with the successive addition of 31.25 nM DA into the solution. Test environment: PBS solution (pH 7.0). As was mentioned above, the higher sensitivity of S-Fe2O3/GCE electrode toward the DA was due to their special surface, which is more inclined to adsorb the DA molecules. To further


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confirm this inference, the XPS characterizations for all samples after adsorption of DA were performed to investigate their adsorptivity toward DA. The representative high-resolution spectra of the N 1s regions are shown in Figure S12-S13. The N 1s region is consistent with three peaks, which can be assigned to primary (R-NH2, amine groups, 401.9 ± 0.3 eV), secondary (R-NH-R, substituted amines, 399.9 ± 0.1 eV), and tertiary/aromatic (=N-R, imino groups, 398.6 ± 0.1 eV) amine functionalities.60-62 Based on the proposed chemical structures of polydopamine, the primary, secondary and tertiary amine are related to dopamine, polydopamine, and the tautomeric species of the intermediates, such as 5,6-dihydroxyindole and 5,6-indolequinone, respectively.63 As shown in Figure 6a, compared with the XPS spectra obtaining from the pure Fe2O3 NPs, a new peak of N 1s and much enhanced peak intensity of O 1s can be observed from the sample after adsorbing DA molecules, implying the adsorption ability of Fe2O3 NPs can be well investigated through XPS. Figure 6b shows the magnified peak region of N 1s at ca. 401-403 eV for different Fe2O3 NPs, from which one can see that the N 1s signal measured from the S-Fe2O3 NPs is obviously higher than that of the others, indicating that the S-Fe2O3 NPs more easily adsorb the DA molecules compared to other samples. In addition, as shown in Figure S13, the N 1s signal resulting from the polydopamine (R-NH-R) indicates that the autoxidation of dopamine occurred when the DA molecules are adsorbed on the surface of Fe2O3 NPs.60


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Figure 6. XPS spectra of S-Fe2O3, Ps-Fe2O3, Ph-Fe2O3, D-Fe2O3 NPs. (a) survey spectra of S-Fe2O3 with and without absorbed DA, (b) N 1s spectra. The spectra of Fe2O3-DA were obtained after the treatment of DA at room temperature. The adsorption ability, so as to the sensor ability of the faceted Fe2O3 NPs is closely related to their ability of adsorbing oxygen or hydroxyl species, which greatly influence the thickness of the depletion layer near the surface of sensing materials. The XPS analysis was further carried out to test the status of oxygen in the adsorbed species on the as prepared α-Fe2O3 samples. Figure 7a shows the XPS spectra of the Fe 2p region of the four samples, the spectral line shape is identical and characteristic of Fe3+ in Fe2O3. Figure 7b-e shows the O 1s XPS peaks of each samples, from which one can see that the oxygen states measured from the surface of different


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Fe2O3 NPs shows remarkable differences. The O 1s XPS peak can be decomposed into three fitted Gaussian components centered at ca. 530 eV (OL), 531 eV (OV) and 532 eV (OC), respectively. The OL component can be ascribed to the O2- ions in the Fe2O3 lattice. The OV component, which located at the medium binding energy, is related to the O2- ions around the oxygen vacancy regions within the matrix of Fe2O3. The OC component is due to the chemisorbed and dissociated oxygen species or OH.32,42 Therefore one can estimate the oxygen-chemisorption ability of different Fe2O3 NPs in accordance with the intensity of OC component in the O 1s XPS peak. As shown in Table S5 and Figure 7f (blue), the percentages of the OC component follow the order of S-Fe2O3 > Ps-Fe2O3 > Ph-Fe2O3 > D-Fe2O3. These results reveal that the S-Fe2O3 NPs prefer to absorb the ionized oxygen species, in other words, the surface of S-Fe2O3 NPs is more electrophilic than other samples, which can be further verified by measuring their zeta potential. As shown in Figure 7f, the red histograms present the zeta potential values of four samples. Similarly with the percentages of the OC component (blue histograms), the value of zeta potential of S-Fe2O3 NPs is -15.5 mV, more negative than that of the other samples (Table S6), further implying that the S-Fe2O3 NPs prefer to absorb more negative charged analytes, i.e. DA and dissociated oxygen species in current case, thus inducing the formation of a more negative diffuse layer, which contributes greatly to their electrochemically sensing performance toward DA.. The result is quite accordant with that of XPS analysis.


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Figure 7. (a) Fe 2p XPS spectra and O 1s XPS spectra of the samples: (b) S-Fe2O3, (c) Ps-Fe2O3, (d) Ph-Fe2O3, (e) D-Fe2O3. (f) The histogram of the percentage of chemisorbed oxygen (blue) and the values of zeta potential (red). ■ CONCLUSIONS In summary, uniform hematite α-Fe2O3 NPs with shuttle, pseudo shuttle, polyhedron and drum-like morphologies were successfully synthesized with a surfactant-free hydrothermal method. The detailed structural characterization indicated that the S-Fe2O3NPs was enclosed by the facet with much highdensity of atomic steps, which endowed them with enhanced surface activity. Thus when the as-prepared α-Fe2O3 NPs were used in electrochemical detection of DA, the highest sensitivity of 0.1315 µAµM-1 was achieved using S-Fe2O3 NPs, being 2.31, 1.87 and 1.51 times as high as that of Ps-Fe2O3, Ph-Fe2O3, and D-Fe2O3, respectively. Additionally, S-Fe2O3 NPs also exhibited an admirable selectivity and quick response ability toward DA in the presence of AA, KCl, L-dopa, urea and UA. Further XPS and zeta potential analysis confirmed


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that the higher content of iron atom or oxygen vacancy exposed on the surface of S-Fe2O3 NPs inducing their possessing a more positive charged crystal surface. The high-resolution XPS analys is confirmed that the adsorption of DA molecules was much enhanced on the surface of S-Fe2O3 NPs and accordingly improved their electrocatalytic activity towards DA sensing. This work not only opens up new possibilities for designing efficient electrode materials for DA detection, but also provides advanced insights into enhancing their electrochemical sensor properties by means of shape modification. ■ASSOCIATED CONTENT Supporting Information XRD patterns, EDAX spectra, UV/Vis spectra, Magnetic hysteresis loops, Typical cyclic voltammograms, Summary of electrochemistry parameters, XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86-29-82663034. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The SEM (or TEM) work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi’an, China; the authors also thank Ms. Yanzhu Dai and Mr.


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Chuansheng Ma for the help in using SEM and TEM. This work is supported by National Natural Science Foundation of China (No. 51271135), the Fundamental Research Funds for the Central Universities, and the Natural Science Foundation and the project of Innovative Team of Shanxi Province (No. 2015JM5166 and 2013KCT-05). ■NEFERENCES

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Palladium Nanoparticles Decorated Graphene Modified Glassy Carbon Electrode. J. Electrochem. Soc. 2015, 162, 651-660. (56) Chen, A.; Ding, Y.; Yang, Z.; Yang, S. Constructing Heterostructure on Highly Roughened Caterpillar-like Gold Nanotubes with Cuprous Oxide Grains for Ultrasensitive and Stable Nonenzymatic Glucose Sensor. Biosens. Bioelectron. 2015, 74, 967-973. (57) Laviron, E. General expression of the Liner Potential Sweep Voltammogram in the Case of Diffusionless Electrochemical Systems. J. Electroanal. Chem. 1979, 101, 19-28. (58) Wang, C.; Du, J.; Wang, H.; Zou, C.; Jiang, F.; Yang, P.; Du, Y. A Facile Electrochemical Sensor Based on Reduced Graphene Oxide and Au Nanoplates Modified Glassy Carbon Electrode for Simultaneous Detection of Ascorbic Acid, Dopamine and Uric Acid. Sensors Actuators, B Chem. 2014, 204, 302-309. (59) Bhim, B. P.; Darshika. J.; Mahavir, P. T.; A dual-template imprinted polymer-modified carbon ceramic electrode for ultra trace simultaneous analysis of ascorbic acid and dopamine. Biosens. Bioelectron. 2013, 50, 19-27 (60) Clark, M. B.; Gardella, J. A.; Schultz, T. M.; Patil, D. G.; Salvati, L. Solid-State Analysis of Eumelanin Biopolymers by Electron Spectroscopy for Chemical Analysis. Anal. Chem. 1990, 62, 949-956. (61) Zangmeister, R. A.; Morris, T. A.; Tarlov, M. J. Characterization of Polydopamine Thin Films Deposited at Short Times by Autoxidation of Dopamine. Langmuir 2013, 29, 8619-8628.


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(62) Bernsmann, F.; Ponche, A.; Ringwald, C.; Hemmerle, J.; Raya, J.; Bechinger, B.; Voegel, J. C.; Schaaf, P.; Ball, V. Characterization of Dopamine-Melanin Growth on Silicon Oxide. J. Phys. Chem. C 2009, 113, 8234-8242. (63) Lepoittevin, B.; Bedel, S.; Dragoé, D.; Bruzaud, J.; Barthés-Labrousse, M. G.; Mazerat, S.; Herry, J. M.; Bellon-Fontaine, M. N.; Roger, P. Antibacterial Surfaces Obtained through Dopamine and Fluorination Functionalizations. Prog. Org. Coat. 2015, 82, 17-25.


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CS Applied Materials Page & 34Interfac of 34

ACS 1 Paragon Plus Environmen 2 3

Improving Surface Adsorption via Shape Control of Hematite Î±-Fe2O3

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