Fe Foil-Guided Fabrication of Uniform Ag@AgX Nanowires for

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Biological and Medical Applications of Materials and Interfaces

Fe Foil-Guided Fabrication of Uniform Ag@AgX Nanowires for Sensitive Detection of Leukemia DNA Cuiting Liu, Baojuan Wang, Ting Han, Dongmin Shi, and Guangfeng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18700 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Fe Foil-Guided Fabrication of Uniform Ag@AgX Nanowires for Sensitive Detection of Leukemia DNA Cuiting Liu†, Baojuan Wang‡, Ting Han†, Dongmin, Shi†, and Guangfeng Wang†* † Anhui Province Key Laboratory of Chem-Biosensing, Anhui Province Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, No. 1, Beijing East Road, 241000, Wuhu, Anhui, China ‡ Institute of Molecular Biology and Biotechnology, Anhui Provincial Key Laboratory of the Conservation and Exploitation of Biological Resources, College of Life Sciences, Anhui Normal University, No. 1, Beijing East Road, 241000, Wuhu, Anhui, China KEYWORDS: Ag@AgX (X = Cl, Br) nanowires; in-situ; etch; Leukemia DNA; photoelectrochemical detection; Fe foil ABSTRACT: Herein, we report a novel Fe foil guided, in-situ etching strategy for the preparation of highly uniform Ag@AgX (X = Cl, Br) nanowires (NWs) and applied the photoelectric-responsive materials for sensitive photoelectrochemical (PEC) detection of Leukemia DNA. The Ag@AgX NWs formation process was discussed from the redox potential and Ksp value. The fabricated PEC platform for sensing Leukemia DNA showed good assay performance with a wide linear range (0.1 pM to 50 nM) and low detection limit of 0.033 pM. We envision that our Fe foil-guided synthetic method could be applied to synthesize more photoactive materials for sensitive PEC detections. 1. INTRODUCTION ACS Paragon Plus Environment

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Photoelectrochemistry has been dynamically developing in various fields including photovoltaics, photocatalysis, photosynthesis, photoelectrochemical (PEC) detection, etc.1-4 Among these fields, PEC detection, a newly developed approach, has drawn a lot of interests and been widely applied in sensing DNA, enzymes, proteins and other small molecules because of its distinguished features including facile and convenient operation, high sensitivity, low background signal, and good reproducibility.5-7 Organic/inorganic semiconductors such as porphyrin, phthalocyanine, TiO2, CdS and CdSe quantum dots (QDs), C3N4, PbS nanomaterials are conventional photoactive materials for PEC detection.8-15 Since photoelectric conversion efficiency is the key factor of the photoactive material to improve its PEC detection performance,16 a lot of efforts have been put to design and fabricate novel hybrid materials with better PEC detection performance. As we know, coupling semiconductors with noble metals can improve their efficiencies of not only light utilization but also charge separation, leading to obviously enhanced photoelectric conversion efficiency.17 Among the noble metals, silver is significantly superior for this purpose because of its distinguished and tunable

surface

plasmon

resonance

(SPR)

absorbance

in

the

desirable

wavelength,

optimal

electroconductivity (6.3 × 107 S m-1) and cheapest price.18 For example, due to the efficient charge transfer between Ag and AgX through the Schottky junction, silver-based AgX heterogeneous nanowires (named Ag@AgX NWs, X=Cl, Br) have shown distinguished photocatalytic performance as irradiated by visiblelight.. Nevertheless, to the best of knowledge, using Ag@AgX NWs for PEC detection has not been reported. To fabricate Ag@AgX NWs, in situ etching and oxidation of the prefabricated Ag NWs is a good strategy due to its methodological simplicity and ability to maintain the wire shape. For examples, Ag@AgCl and Ag@AgBr NWs have been reported to be prepared by in-situ oxidizing the Ag NWs with FeCl3 and CuBr2, respectively.19,20 However, due to fast oxidation of Ag by the cation Fe3+ or Cu2+, the as-synthesized AgX

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nanoparticles depositing on the Ag NWs were not only too large in size but also non-uniform in growth, which may be disadvantageous for the photoelectric conversion efficiency of the Ag@AgX NWs.

Scheme 1. Schematic illustration of (a) Fe guided X- etching Ag NWs to form Ag@AgX core-shell NWs and (b) applied for DNA assay Herein, we proposed a general method of in-situ preparing highly uniform Ag@AgX NWs and applied the photoelectric-responsive materials for PEC detection of Leukemia DNA. For Ag@AgX preparations, Fe foil was used to guide the uniform growth of AgX nanoparticles with several nanometers in diameter on the Ag NW surfaces (Scheme 1a). It was investigated that Ag played important roles: improving the conductivity and inhibiting the photoreduction of AgX, beneficial to be as the photoelectrochemical active materials. Electrodes modified with as-prepared Ag@AgX NWs first showed fast response, high sensitivity and selectivity towards gallic acid (GA). Finally, in the presence of GA as electron donor, the Ag@AgCl NWmodified electrode was explored for the highly sensitive and selective detection of leukemia DNA, as shown in Scheme 1b. 2. EXPERIMENTAL SECTION 2.1. Preparation of Ag@AgX (X=Cl, Br) NWs The apparatus and chemical reagent were described in detail in the supporting information. For the prepartation of Ag@AgX (X=Cl, Br) NWs, first, the Ag NWs were synthesized through a modified polyol reduction process. Briefly, the mixture of 15 mL of ethylene glycol with 0.1 g polyvinylpyrrolidone (PVP) in


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three-neck round flask was reacted in an oil bath at 160 °C for 5 min. Subsequently, 2 mL ethylene glycol containing 0.074 g AgNO3 and 0.3 mL 0.05 M NaCl was added into the above mixture solution and reacted for 15 min at 160 °C to obtain the Ag NWs. Magnetic stirring was applied throughout the entire synthesis process. Finally, the as-obtained Ag NWs were washed with acetone removing the ethylene glycol and surfactant PVP, and deionized water for three times, successively. After that, Ag@AgX (X=Cl, Br) NWs were prepared by anion ion etching Ag NWs. In the synthesis, Ag NWs were dissolved in 2 mL 0.1 M NaCl, or NaBr solution, and then a slice of clean Fe foil (1cm×2cm) was immersed into the solution. The reaction solution was placed at room temperature for a period of time (60 or 72 h) and subsequently the Fe foil was taken out. Each of the products was washed with ultrapure water and ethanol for several times to obtain Ag@AgX (X=Cl, Br) NWs. 2.2. Preparation of Ag@AgX (X=Cl, Br) NWs modified electrodes Firstly, a glassy carbon electrode (GCE) was polished with an emery paper and furthre slurries of alumina (0.3 and 0.05 μm), respectively. After that, the electrode was washed by sonicating in ultrapure water and isopropyl alcohol for 1 min each and dried under nitrogen atomosphere. Meanwhile, 5 mL of the obtained Ag@AgX NWs products were dissolved in N, N-dimethyl formamide solution (5 mL) with ultrasonic dispersion for 10 min. 20 μL of the mixed solution was drop casted onto the GCE and the solvent allowed to evaporate at ambient temperature for 12 h. Finally, the Ag@AgX NWs modified GCE (Ag@AgX/GCE) was obtained. As a control, AgX NWs modified GCE (AgX/GCE) was prepared also by dipping AgX NWs solution on the GCE as the above procedure. 2.3. PEC Measurement Photoelectrochemical signal was measured by the current-time curve experimental technique under visible light irradiation with the applied potential of 0.0 V. Electrolyte buffer of phosphate solution (PBS) (0.1 M, pH 7.4) was deaerated with nitrogen for 10 min prior to the photoelectrochemcial measurement. At the 100th ACS Paragon Plus Environment

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seconds, the analyte was added and the light was turned on. After that, the PEC current signal was captured for 20 seconds and then the light was turned off for the next cycle. 2.4. DNA Assay All DNA sequences were in Tables S1. First, 20 μL of 100 pM hairpin like DNA (HP1) was pretreated by 10 mM tris(2-carboxyethyl)-phosphine (TCEP) for 1 h to destroy the bond of disulfide and was then dropped onto the Ag@AgCl/GCE for assembly at 4 °C for 12 h. Furthermore, using the washing buffer (Tris-HCl buffer pH = 7.4, 10 mM), the electrode was rinsed. After that, the HP1 modified electrode was incubated with 20 μL 1 mM mercaptoethanol (MCH) for 2 h to avoid non-specific adsorbing effects and block the rest active sites. Subsequently, the HP1 modified Ag@AgCl/GCE was washed and incubated with 50 μL of 0.01 nM or different concentrations of target DNA (tDNA) for 2h at ambient temperature. Subsequently, the electrode was then immediately soaked in 200 μL 20 nM Zn2+ solution containing 1.0 nM 8–17 DNAzyme for 90 min at 37 °C. In the end, the resulting electrode was transferred for PEC detection after rinsed with washing buffer. 3. RESULTS AND DISCUSSION The morphologies of the produced nanomaterials were characterized by scanning electron microscopy (SEM). Compared with Ag NWs (Fig. 1a), the Ag@AgX NWs (Fig. 1b and 1c) showed a rougher surface. The transmission electron microscope (TEM) images also clearly demonstrated that the as-obtained Ag@AgX NWs (Fig. 1e and 1f) possessed unique core-shell structure with uniform nanoparticles dispersed on the surface of Ag NWs. From the high resolution TEM (HRTEM) images of the Ag and Ag@AgX NWs, lattice fringes can be clearly observed. Silver nanocrystal has distinct fringes with an interval of 0.235 nm or so and it can be indexed to the (111) lattice plane (Fig. 1g). Meanwhile, the obvious lattice spacings of 0.28 nm is assigned to the (200) lattice plane of AgCl (Fig. 1h) while that at 0.289 nm is associated with the (200)


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crystal plane of AgBr (Fig. 1i), respectively. The energy dispersive X-ray spectrometer (EDX) was used to analyze the ingredient of the composites, also illustrating the heterogeneous NWs were composed of Ag and X elements (Fig. S1).

Fig. 1 SEM (a-c), TEM (d-f) and HRTEM (g-i) of Ag NWs (a, d and g), Ag@AgCl NWs (b, e and h); Ag@AgBr NWs (c, f and j), respectively. The X-ray diffraction (XRD) patterns of the Ag and Ag@AgX NWs depicted in Fig. 2a and 2d, revealed that all the products were composed of two types of distinct crystal structures. Apart from the diffraction peaks attributed to Ag (JCPDS no. 04-0783), all the recorded peaks can be indexed to the AgCl (JCPDS no.311238) (Fig. 2a) and AgBr (JCPDS Card no.6-438) (Fig. 2d), respectively. Besides, the composition of the nanocomposites was further characterized by using X-ray photoelectron spectra (XPS) analysis. Fig. 2b and 2c display the survey spectra of the Ag@AgCl NWs. In the XPS of Ag 3d, the peaks located at 368.04 and 374.09 eV were assigned to the Ag 3d5/2 and Ag 3d3/2 of the metallic Ag0. While the peaks of 367.7 and 373.7 eV should be assigned to the binding energies of Ag 3d5/2 and Ag 3d3/2 from Ag+ ions in AgCl nanoparticles (Fig. 2b). This confirmed the existence of Ag0 and Ag+ in Ag@AgCl NWs, consistent with the XRD results. The chlorin anions in the lattice of AgCl contributed to the peaks of Cl 2p3/2 and Cl 2p1/2 at approximately 198.11 and 199.83 eV(Fig. 2c), correspondingly. The XPS spectra of Ag@AgBr were showed


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in Fig. 2e and 2f. The electron binding energies of Ag 3d5/2 and Ag 3d3/2 (Fig. 2e) were similar to that in the Ag@AgCl. Differently, there were two Br 2p3/2 and 2p1/2 peaks nearly at 67.95 and 68.97 eV corresponding to Br of AgBr (Fig. 2f). To probe into the growth process of Ag@AgX NWs, time-dependent reactions were further investigated.

Fig. 2 XRD patterns of Ag@AgCl (a) and Ag@AgBr (d) NWs, respectively; XPS images: Ag 3d (b) and Cl 2p (c) of Ag@AgCl NWs, Ag 3d (e) and Br 3d (f) of Ag@AgBr NWs. Taking the Ag@AgCl NWs to illustrate, when Ag NWs were immersed with Fe foil in Cl- solution, at the initial stage of the reaction (30 h), the Ag NWs became rough and some AgCl nanoparticles were deposited on their surface (Fig. S2b). With reaction time prolonged to 60 h, uniform AgCl nanoparticles grew up gradually and formed a continuous layer on the surface of the Ag NWs (Fig. S2c). The growth process of Ag@AgBr NWs (Fig. S2d-f) was similar to that of Ag@AgCl NWs (Fig. S2a-c) but with a longer reaction time of 72 h. According to the redox potentials of E0 (AgCl/Ag) = +0.22 V vs Standard Hydrogen Electrode (SHE), and E0 (AgBr/Ag) = +0.07 V vs SHE, which were both smaller than that of E0 (Fe3+/Fe2+) = +0.77 V ACS Paragon Plus Environment

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vs SHE, it was believed Fe3+ ions could proceed the oxidation of the Ag atom on the surface of Ag NWs into AgX at room temperature. Therefore, the growth mechanism of the reaction was speculated to carry out as the equations (eq. 1-4). Firstly, as the addition of Fe foil in the Ag NWs solution, the Fe foil was etched and oxidized in aqueous solution, gradually releasing Fe3+ (Fig. S3). The released Fe3+ could oxidize Ag0 into Ag+ which then combined with Cl-, or Br- to form the homogenous product AgCl, or AgBr on the surface of Ag NWs. It was suggested that in the present system, due to the oxidization of Fe foil in the aqueous solution was a weak and homogenous process, the oxidized Fe3+ could be released gradually in small amount to etch Ag NWs more evenly (as Fig. S2b, c, e, f showed). In addition, because the solubility product constant Ksp value of AgX (X=Cl, Br) (Ksp AgCl=1.8×10-10, Ksp AgBr=5×10−13) is very low, the deposition is so fast that the produced AgX has no enough time to disperse into the water solution but deposited on the surface of Ag NWs. With the reaction occured, these deposited AgX gradually covered the surface of Ag nanowires (Fig. S2), which inhibited further oxidation of Ag NWs, resulting in the limited production of AgX on Ag NWs and the Ag NWs substrate exsited. Notably, different reaction time of 60 and 72 h was required for the deposition of AgCl and AgBr on Ag NWs, respectively. We suggested that the increased reaction time is ascribed to the larger Ksp value of AgBr. Besides, due to E0 (AgI/Ag) = −0.15 V vs SHE is also lower than E0 (Fe3+/Fe2+) = +0.77 V vs SHE, Ag@AgI NWs were also desired to be prepared using this method. However, according to the result of SEM, TEM and EDX (Fig. S4), we can find out that Ag@AgI NWs were difficult to form. We assumed the reason may be that I- possessed strong reducibility and it would react with Fe3+ resulting in the decrease of the free I- in solution (eq. 5), which impeded the production of uniform AgI on Ag NWs.” Fe + O2 + H2O → Fe3+ + 3OH-

(eq1)

Fe3+ + Ag → Ag+ + Fe2+

(eq2)

Ag+ + Cl- → AgCl

(eq3)

Ag+ + Br- → AgBr

(eq4)


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2Fe3+ + 2I- → 2Fe2+ + I2


(eq5)

The role of Ag NWs was further investigated. On one hand, the electrochemical impedance spectroscopy (EIS) result showed that the electron transfer resistance (Ret) of Ag@AgX NWs was smaller than that of the corresponding AgX particles (Fig. S5), implying that Ag NWs improved the conductivity of the complex. On the other hand, the reducibility of silver may improve the stability of the AgX upon photoreduction. From the result of XPS, it was obvious to see that the ratio of Ag0 content in Ag@AgX before and after irradiation changed less than that in AgX (Table S2), indicating that the presence of Ag inhibited the AgX photoreduction, beneficial to be as effective photoactive materials in PEC. The photocurrent responses of Ag@AgX NWs modified on glassy carbon electrode were further monitored (Fig. 3). Upon visible light photoexcitation, Ag@AgCl NWs showed a relatively high photocurrent of 17.03 nA (Fig. 3a, curve a0). A low photocurrent of 2.35 nA was presented on the Ag@AgBr/GCE, ascribing to its poor conductivity (Fig. 3d, curve a0), in accordance with the EIS result (Fig. S5). We suggest it may be due to the anion radius of Cl- is smaller than that of Br-, resulting in different conductivity of AgX on the electrode. Furthermore, PEC detection of GA on the presented Ag@AgX/GCE was investigated. With the addition of 100 μM GA, the photocurrent of Ag@AgCl/GCE enlarged obviously (Fig. 3a, curve a1). Similarly, the photocurrent of Ag@AgBr/GCE increased (Fig. 3d, curve a1). These results generally showed that GA had an obvious photoelectrochemical response on the Ag@AgX/GCE, which was attributed to the oxidization of GA by the AgX holes, blocking the recombination of electrons and holes (Scheme S1). Furthermore, at the suitable applied potential 0 V, the photoelectric assay performance of Ag@AgX/GCE was evaluated towards different concentrations of GA (Fig. 3b and 3e). It was observed that the photocurrent response displayed a linear increase as the GA concentration increased from 20 to 1000 μM (Fig. 3c and 3f). Ag@AgX (X=Cl, Br) NWs also exhibited an excellent specificity towards GA against other oxidants (Fig. S6). With a wide response range, low detection limit, rapid response (

Fe Foil-Guided Fabrication of Uniform Ag@AgX Nanowires for

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