Facet-Inspired CoreâShell Gold Nanoislands on Metal Oxide

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Biological and Medical Applications of Materials and Interfaces

Facet Inspired Core-Shell Gold Nanoislands on Metal Oxides Octadecahedral Heterostructures: High Sensing Performance towards Sulfide in Biotic Fluids Muhammad Asif, Ayesha Aziz, Ghazala Ashraf, Zhengyun Wang, Junlei Wang, Muhammad Azeem, Xuedong Chen, Fei Xiao, and Hongfang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12186 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Facet Inspired Core-Shell Gold Nanoislands on Metal Oxides Octadecahedral Heterostructures: High Sensing Performance towards Sulfide in Biotic Fluids Muhammad Asif,†,‡ Ayesha Aziz,† Ghazala Ashraf,† Zhengyun Wang,† Junlei Wang,† Muhammad Azeem,† Xuedong Chen,‡ Fei Xiao†,* and Hongfang Liu†,* †

Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Hubei

Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China. ‡

State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong

University of Science and Technology, Wuhan 430074, P. R. China ABSTRACT: The development of structurally modified metal oxide heteroarchitectures with higher energy facets exposed has been of extensive research interests because of their unique construction and synergy effect of multi-functioning characteristics. In this study, we reported for the first time the development of a distinct type of gold nanoislands (AuNIs) on metal oxides (i.e., Cu2O-CuO) octadecahedral (ODH) heterostructures through galvanic exchange reaction, where Cu2O not only acts as stabilizer but also functions as reductant. The electrocatalytic performance of the resultant core-shell Cu2O-CuO@AuNI ODH based electrochemcial sensing platform has been evlauted in ultrasensitive detection of sulfide as early diseases diagnostics and bacterial marker. Owing to the synergistic collaboration of enhanced surface active sites, exposed {110} crystallographic facets, mixed valances of copper that encourage redox reactions at electrode material/analyte interface, and the polarization effect provide by AuNIs decorated onto Cu2O surface, Cu2O-CuO@AuNI ODH modified electrode has demonstrated striking electrochemical sensing performance towards sulfide oxidation in terms of broad linear range, real detection limit down to 1 nM (S/N=3), incredible durability and reproducibility. In virtue of marvelous efficiency, the proposed electrochemical sensor based on Cu2O-CuO@AuNI ODH has been employed in in situ senstive detection of ubiquitous amount of sulfide engendered by sulfate reducing bacteria and real-time tracking of sulfide efflux from live cells as early diagnostic strategies. . 1 ACS Paragon Plus Environment

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KEYWORDS:

Core-shell

structure;

Gold

nanoislands;

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Metal

oxides

octadecahedral

heterostructures; Electrochemical sensor; Sulfide detection 1. INTRODUCTION: The sensitive and reliable detection of sulfide compounds (i.e., H2S, HS-, and S2-), with swiftly growing research themes, has aroused immense attention of researchers because of their recently established various pathophysiological capabilities.1 It has been well-documented that H2S is one of the most crucial neuromodulators in central nervous system and third gasotransmitter following nitric oxide and carbon monoxide.2 The ubiquitous H2S, engendered by numerous enzymatic processes in living organisms, has been considered to be contributed in many physiological and/or pathological functions such as protection of neurons and glia, reduction in blood pressure, anti-oxidation, anti-inflammation, and apoptosis.3,4 The studies have profiled the adequate H2S level in central nerve system and blood plasma which should be in physiologic ranges of 50~160 µM and 10~100 µM respectively.5 It is worth mentioning that abnormalities in H2S concentrations can lead to some chronic diseases including Alzheimer’s disease Parkinson’s disease, ischemic stroke, diabetes, traumatic brain injury6,7 and cancer diseases.8 Moreover, sulfate-reducing bacteria (SRB), extensively scattered in anoxic environments and causing severe contaminations, acquire their energy by unique enzymatic reduction of sulfate to sulfide.9 Sulfides being highly corrosive, reactive and toxic, are tremendously detrimental for industry and mammals as well.10 Consequently, sulfide concentration in environmental samples can be used as marker of SRB detection. Thus, monitoring of exact levels of endogenous sulfide with high temporal resolution under biological conditions is mandatory in order to ensure reliable diagnostics of lethal diseases, significance of clinical treatment and to circumvent industrial issues. Until now, various analytical techniques have been employed for the detection of sulfide e.g., spectrophotometry,11

fluorimetry,12

chemiluminescence,

chromatography,13

atomic

emission/absorption spectroscopy14 and biosensors.15,16,17 Electrochemical biosensors have attracted wealth of interest owing to their distinct advantages of superb selectivity, sensitivity, ease while operating, quick response and further offering trustworthy real-time analysis,18 which are superior to those of aforementioned analytical methodologies. For the electrochemical detection of sulfide, enzymatic and microbial biosensors have been well-established. Nevertheless, the enzymatic biosensrors possess some inherent drawbacks of enzymes such as 2 ACS Paragon Plus Environment

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scarcity, complicated immobilization, easy to be destroyed and instability, which obstruct their frequent usability.19,20 While the microbial biosensors suffer from slow oxidation rate of sulfides which in turn gives feeble expression of enzyme involvement in sulfide biodegradation as well as complex culture conditions for sulfur-oxidizing bacteria.21 Consequently, there has always been an unequivocal investigation into nanomaterial based electrochemial sensors that possess excellent electrocatalytic activity, selectivity and good anti-interferrence ability for the detection of sulfide. Recently, metals and metal oxide nanoparticles (NPs) have become appealing candidates to effectively catalyze sulfide oxidation as enzyme mimicker in different environments.22,23,24 Further studies have provoked that the electrocatalytic properties of these transition metals can be improved by structurally integrating with noble metals to fabricate hydride electrode materials such as Au-Fe3O4 or PtPd-Fe3O4, Cu2O@CuO-AuPd NPs.25,26,27 The growth of noble metal NPs onto the surface of semiconducting nanomaterials can greatly influence their properties because of the strong coupling between electromagnetic waves and the charged species in metal semiconductor nanohybrids, which in turn re-distributes the interface charges. This redistribution in interfacial surface charges creates polarization, which may further improves the electrocatalytic capabilities.28 However, the construction of nonprecious (Fe, Co, Ni, Mn) metal based catalysts has been confined due to the highly insulating nature of these metal oxides/hydroxides.29 The development of metal oxides with high reduction potential as sacrificial template could be an effective stragety to achieve improved conductance and overall superior electrochemical performance. Additionally, the galvanic replacement protocol has also become one of the renowned technologies for the synthesis of diverse varieties of multifunctioning metal nanoarchitectures.30 A well-known economical metal oxide such as cuprous oxide (Cu2O) having low oxidation potential can be oxidized by numerous metallic cations like Au3+, Pd2+, Ag+, Pt+, Rh3+, and Ir3+.31 Cu2O is an exciting support material to evaluate catalytic behavior of NPs. Various researches have demanstrated that the cubic, octahedral, nanowires, nanospheres and dodecahedral nanoarchitectures can endow numerous effective interfaces to enhance catalytic abilities.32,33 Previous studies have corroborated the outstanding catalytic activity of polyhedral Cu2O than cubic because of the existence of {111} facets compared with {100} facets due to dangling bonds of {111} surfaces, in contrast there exist structured chemical bonds in {100} facets with the absence of dangling bonds. Besides, it is noteworthy that Cu2O octadecahedral 3 ACS Paragon Plus Environment


(ODH) with dominant {110} facets has higher catalytic performance compared with {111} and {100} planes.34 Thus, it is reasonable to convince that the creation of nanoscale hetrojunctions of Cu2O with partial coverage of Au NPs is favorable to harvest full advantages of facet inspired conductivity and interfacial interactions within heterojunction which in turn improve catalytic activity.

What's more, few research groups have reported the immobilization of noble metals

onto the surface of Cu2O nanocubes by galvanic replacement reactions to fabricate hybrid materials.35,36 As far as we know, there is no report on successful assembly of Au species on octadecahedral core-shell Cu2O-CuO heterostructures via galvanic replacement reaction. Benefitting from high reduction potential of Au (Eo=1.49V) compared with Cu (Eo=0.34V), in this work, we report the galvanic replacement process in ethanol solution at room temperature to synthesize core-shell gold nanoislands (AuNIs) on Cu2O surface (Cu2O-CuO@Au) octadecahedral (ODH) heterostructures, where Cu2O does not only act as stabilizer but also performs as reductant and sacrificial template as well. During the reaction, Au3+ ions on the surface are reduced to Au0 NPs by electron gain from Cu2O. Consequently, the surface Cu2O species are oxidized to CuO. This strategy is free of using any reducing agent and shortens the reaction time. The structural morphology, elemental composition and electrochemical properties have been characterized in detail. Under the optimized condition, the as-prepared core-shell Cu2O-CuO@Au ODH heterostructures has demonstrated superb catalytic activity for H2S oxidation in terms of a broad linear range and a detection limit down to 1 nM, owing to enlarged specific surface area, chemical stability, enhanced electron transfer kinetics, and more edges and corners in ODH composite with more active {110} facets. To the best of our knowledge, we have developed an electrochemical sensing platform for the first time for in vitro electrochemical detection of SRB using endogenous sulfide generated by characteristic metabolic process as the mark and sensing cellular sulfide secreted from live cells as well. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. CuCl2·H2O (99%), NaOH (96%), absolute ethanol (99.7%), NH2OH·HCl (99%), sodium dodecyl sulfate (SDS, 98.5%), Na2S·9H2O (98.5%), Dulbecco’s modified eagle medium (DMEM) and HAuCl4·4H2O (99.9%) were acquired from Sinopharm group chemical reagent Co, Ltd. (Shanghai, China) and used without any further purifications. Vascular endothelial growth factor (VEGF) was used to release H2S from live cells. The supporting electrolyte used was 0.1 4 ACS Paragon Plus Environment

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M phosphate buffer saline (PBS) solution (pH 7.4) and made up by mixing NaH2PO4 and Na2HPO4 in an appropriate proportion. The artificial oilfield produced water was used with chemical composition of 2900 mg L-1 NaHCO3, 37.5 mg L-1 MgSO4, 105 mg L-1 CaCl2, 7300 mg L-1 NaCl, 7 mg L-1 K2SO4 and 4880 mg L-1 Na2SO4. The desulfotomaculum nigrificans culture medium was used for SRB cultivation. This medium is comprised of 0.01 g L-1 K2HPO4, 0.2 g L-1 MgSO4·7H2O, 20.2 g L-1 (NH)2Fe(SO4), 10 g L-1 NaCl, 1.0 g L-1 yeast extract, 0.1 g L-1 vitamin C, in addition to 4.0 mL L-1 sodium lactate (pH 7.2). These entire chemicals were obtained from commercial suppliers and used as it is. 2.2. Fabrication of Cu2O ODH. As elaborated in Figure 1, the octadecahedral geometry of Cu2O was prepared with some alterations in previously reported method.32 In a typical procedure, 300 mL beaker was placed in water bath at 30 °C containing 100 mL deionized water. Then 11.25 mL of 0.1 M CuCl2 and 1.95 g of SDS was poured respectively into above solution under strong stirring. Once SDS was completely dissolved, 4 mL of 1 M NaOH was added rapidly into the above solution under vigorous stirring which turned the color of solution from colorless to light blue indicating the formation of Cu(OH)2 precipitates. Soon after, 52.5 mL of 0.1 M NH2OH·HCl was injected in rigorously stirring mixed solution and then the colored solution was kept at same temperature for 1 h more for crystal growth. The centrifugation of as obtained orange solution was done at 5000 RPM and washing was continued with mixture of water and ethanol till the removal of entire unreacted reagents and SDS surfactants. Finally, the orange precipitates of Cu2O were dried at 50 °C in vacuum oven. 2.3. Synthesis of Cu2O-CuO@AuNIs. Typically, first 50 mg of as-prepared Cu2O was mixed in 30 mL absolute ethanol with magnetic agitation followed by vigorous stirring. Secondly, 5 mg of HAuCl4 was injected into above colloidal suspension and under continued stirring for 24 h at room temperature. Then, the suspension was centrifuged and washed thoroughly with mixture of water and ethanol to get core-shell Cu2O-CuO@AuNIs ODH heterostructures. The material was dried at 50 °C in vacuum oven. The analogous reactions with 2.5 mg and 10 mg of HAuCl4 in 30 mL colloidal of 50 mg Cu2O were followed by workup similar to above mentioned protocol. 5 ACS Paragon Plus Environment


Figure 1. Flow chart for the synthesis of core-shell Cu2O-CuO@AuNIs octadecahedral by galvanic replacement reaction 2.4. Material Characterization. The structure of as-fabricated material was investigated by X-ray diffraction (XRD) on a Rigaku D/max-r a diffractometer (Japan) using Cu Ka radiation (40 kV, 200 mA) with a Ni filter. The oxidation states and compositional analysis were evaluated with X-ray photoelectron spectroscopy (XPS) on an ESCALAB MKII spectrometer (VG Co., UK), using Mg Ka radiation (1253.6 eV) at a pressure of 2.0×10-10 mbar. Structural analysis was carried out with fieldemission scanning electron microscope (FSEM) on HITACHI X-650 FSEM (Hitachi Co., Japan) and transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) TECNAI G220 U-Twin instrument (Netherlands). For electrochemical measurements, CHI760 electrochemical workstation (CH Instrument Company, Shanghai, China) was employed with saturated calomel electrode, platinum wire and glassy carbon electrode as reference, counter and working electrode in typical three electrodes system through the experiment. All the assays were performed at room temperature. 2.5. SRB Cultivation and Amperometric Detection of Sulfide Generated. 6 ACS Paragon Plus Environment

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The oilfield produced water was seeded together with 10% (v/v) of desulfotomaculum nigrificans seed culture medium and placed in incubator at 37 °C for different days. Prior to seeding with culture medium, the oilfield produced water was autoclaved at 121 °C for 20 min, followed by the purging CO2 for 4 h to keep it deaerated. The estimated number of SRB cells were measured by using the most probable number (MPN) method with an MPN culture medium. After cultivation, the culture solutions were centrifuged at 12000 RPM to remove bacterial cells and remaining nutritional ingredients and supernatant was collected. To modify glassy carbon electrode (GCE), firstly its surface was polished thoroughly, rinsed in DI water and ethanol and then 5 µL of Cu2O-CuO@AuNIs suspension (3 mg mL-1) was dropped on the surface of GCE and dried at ambient temperature. The amount of sulfides produced by SRB was measured by inoculating different aliquots of above supernatant in 15 mL stirring PBS (pH 7.4). 2.6. Cell Culturing and Amperometric Monitoring of H2S Secreted. In addition, human melanoma cell line (A375) was received from the American Type Culture Collection (ATCC, Manassas, VA, USA). The live cells were seeded in 6-well plate with culture medium of Dulbecco’s Modified Eagle Medium (DMEM), and further 10% Fetal Bovine Serum (FBS), 100 units mL-1 penicillin and 100 mg mL-1 streptomycin was added in culture medium. Finally, the 6-well plate was incubated at 37°C under 5% CO2 and continued several passages. After appropriate culturing, cells in 6-well plate with 80% confluency were used for real-time monitoring of endogenous H2S efflux. Then GCE modified with Cu2O@CuO/AuNIs (Cu2O-CuO@AuNIs/GCE) was kept close to living cells in 6-well plate for the detection of extracellular H2S efflux in cells after being excreted by injecting vascular endothelial VEGF as stimulator. 3. RESULTS AND DISCUSSION 3.1. Characterization of Cu2O-CuO@AuNIs. The Brunauer-Emmett-Teller (BET) was firstly performed to evaluate the specific surface areas of Cu2O-CuO@AuNIs and other control samples. The surface area increases as we increase the Au deposition up to an intermediate level onto the surface of Cu2O ODH and reaches at highest degree of 45.85 m2g−1 (Supplementary Material, Table S1). The difference in surface areas of various nanostructures may possibly be owing to different stacking modes. The asprepared Cu2O ODH nanostructures via reduction of Cu2+ with NH2OH·HCl (Figure 1) were 7 ACS Paragon Plus Environment


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stable for few months. The typical XRD pattern of Cu2O and Cu2O-CuO@AuNIs has been presented in Figure 2A. The diffraction peaks at 2θ of 29.1º, 36.2º, 42.0º, 61.05º, and 73.6º correspond to (110), (111), (200), (220), (311) lattices in Cu2O phase (JCPDS card no. 05-0667). The reflections at 2θ of 32.4º, 36.2º, 38.2º, and 53.3º correspond to (110), (002), (111), (202), and (020) lattices in CuO moieties (JCPDS card no. 21-1272). The crystal peaks of (111), (220), and (311) at 2θ values of 39.7º, 64.6º, and 77.7º can be accredited to Au phase (JCPDS No.652870).37 As we have claimed about galvanic deposition of Au3+ onto the surface of Cu2O in ethanol solution, here is the unequivocal proof of our concept that the formation of CuO phase in composite material authenticates the donation of electrons by Cu2O required for reduction of Au3+ species and finally deposited onto its surface. Consequently, the surface Cu2O moieties transform into Cu(II) through galvanic replacement process construct a layer of CuO phase on the surface. In this process the redox reaction of the formation of Au NPs follows is as shown in the following [Equation (1)].36 2HAuCl4 + 3Cu2O + 3H2O

2Au + 6CuO + 8HCl

(1)

XPS characterization was further employed to evaluate the chemical composition and metallic configuration of Cu2O-CuO@AuNIs. As shown in Figure. 2B, the wide scan spectrum illustrates the well distinctive peaks designated to Cu 2p, O 1s, C 1s and Au 4f in the sample. In core level spectrum of Cu 2p, the deconvoluted peaks of 254.6 eV and 234.8 eV can be assigned to CuO moieties in the sample respectively as depicted by Figure 2C.38 Moreover, the shake-uppeaks located at 962.7 eV, 944.2 eV and 942.6 eV are the fingerprint of CuO phase with d9 electronic configuration. Though, we cannot witness the peaks at 952.5 eV and 932.3 eV which are typical signals of Cu2O for Cu 2p1/2 and Cu 2p3/2 respectively which authenticate the effective galvanic replacement reaction where in turn Cu2O moieties are converted into CuO phases. However, these characteristic peaks of 952.5 eV and 932.3 eV for Cu2O have undoubtedly been observed prior to adding the HAuCl4 solution (Figure S1).39 Figure 2D displays the XPS core level spectrum of 4f5/2 and 4f7/2 for Au. The core fitting signals of 4f5/2 and 4f7/2 are positioned at 88.1 and 84.3 eV, respectively. These peaks are in good accordance with the standard peaks of Au(0), demonstrating the complete reduction of Au3+ species to Au(0).

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Figure 2. (A) XRD pattern of Cu2O and Cu2O-CuO@AuNIs. (B) XPS survey spectra, (C) Cu 2p core fitting XPS spectra, and (D) Au 4f core level XPS spectra of Cu2O-CuO@AuNIs. The SEM images at different magnifications as presented in Figure 3A and 3B exhibit the successful fabrication of Cu2O with controlled facets through modified method. The Cu2O particles with octadecahedral geometry possessing smoothed surfaces, an average size of 500 nm as well as hexagonal {110} and square {100} faces have been achieved. The as-prepared Cu2O crystals with more exposed {110} faces are more catalytically active. As presented by Figure 3C and 3D at different magnifications, the constructed hybrid material consists of almost octadecahedral structure with much rougher shape compared to pristine Cu2O particles. The rough surface of Cu2O-CuO@AuNIs confirms the adequate deposition of gold nanoparticles, accordingly the conversion of surface Cu2O to CuO phase.



Figure 3. (A) and (B) SEM images of Cu2O ODH nanostructures. (C) and (D) SEM images of core-shell Cu2O-CuO@AuNIs ODH at different magnifications. (E) SEM-EDX elemental mapping of Cu2O-CuO@AuNIs ODH. (F) magnified TEM image of Cu2O ODH. (G) and (H) TEM images of core-shell Cu2O-CuO@AuNIs ODH at different magnifications. (I) HRTEM image of Au nanoislands shell with lattice fringe of 0.23 nm. It is quite fascinating that the deposited Au NPs via this ratio of precursor salts are welldispersed and have partially covered the surface with slight agglomeration creating nanoislands. The deposition of Au NPs on surface using higher ratio of precursor salts causes agglomeration of Au NPs on the surface as well as away from the surface (Figure S2). Figure 3E depicts the SEM-EDX elemental mapping results of the constructed heterostructure corresponding to the signals of Cu, O and Au respectively, where the even distribution of elements has been witnessed on all the observable areas. These outcomes also substantiate the intimate heteroassembly of Cu2O-CuO@AuNIs hybrid. The morphology of as-fabricated heteroarchitectures has further been investigated by TEM. The low resolution TEM image in Figure 3F illustrates nearly monodispersed and smoothed 10 ACS Paragon Plus Environment

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polyhedral structure of the particles. Figure 3G shows the flower like morphology of Cu2OCuO@AuNIs architectures possessing typical core-shell assembly constructed by a compact hybrid core of Cu2O-CuO and shell of densely packed Au NPs in nanoislands fashion. It is clear from the image that the polyhedral structure has been successfully retained after thoroughly depositing Au NPs on the surface. The magnified portion in Figure 3H reveals the undoubted creation of Au nanoislands and further they have firmly anchored on the surface of Cu2O-CuO polyhedral. Further increase in Au loading results in high degree of agglomeration and accumulates away from Cu2O-CuO core in the form of irregular spheres (Figure S3 A&B). As depicted by high resolution TEM (HRTEM) image in Figure 3I, the lattice fringe with d-spacing of 0.23 nm has been observed that corresponds to (111) crystal plane of XRD for Au NPs.40 3.2. Electrochemical Sensing Performances of Cu2O@CuO/AuNIs Hybrid. The intrinsic interfacial properties of electrodes modified with Cu2O and Cu2OCuO@AuNIs in [Fe(CN)6]3-/4- redox probe have been explored by employing electron impedance spectroscopy (EIS) as shown in Figure 4A. The inset is the Randle equivalence circuit fitted with Rs, Rct, Cdl and W representing solution resistance, charge transfer resistance, double layer capacitance, and Warburg constant, respectively. The Cu2O ODH modified glass carbon electrode (GCE) Cu2O ODH/GCE exhibits a limited electron transfer process with Rct value of 2167 Ω, which is even smaller than bare GCE having Rct value of 3640 Ω because of the semiconducting aptitude of Cu2O particles. Interestingly, integrating Au NPs onto the surface of Cu2O by galvanic replacement reaction, Cu2O-CuO@AuNIs/GCE leads to further decrease of Rct to 492 Ω demonstrating efficient mass and charge transport capabilities of hybrid electrocatalyst. The electrochemical active surface areas of the electrodes have been calculated according to Randles–Sevcik equation (see supporting information). This can be accredited to uniform distribution and substantial loading of Au NIs on the surface of Cu2O nanostructures creating necessary conductive pathways for fast electron transportation on resultant electrode. The electrocatalytic performances of Cu2O-CuO@AuNIs/GCE have been evaluated using cyclic voltammetric (CV) measurements in the absence and presence of 1 mM sulfide in PBS (pH=7.4). As shown in Figure 4B, in the absence of sulfide, there is no any obvious peak except a pair of very small peaks, which can be assigned to redox couple of Cu(I) to Cu(II) species.41 While in the presence of 1 mM sulfide, Cu2O-CuO@AuNIs/GCE demonstrates a well-defined oxidation peak at potential of 0.30 V. With the injection of sulfide concentrations in PBS, the corresponding 11 ACS Paragon Plus Environment


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oxidation current densities are also increased linearly as shown in Figure 4C. To investigate the effect of pH, we have performed typical CV curves with Cu2O-CuO@AuNIs/GCE in the presence of 1 mM sulfide at various pH of 0.1 M PBS which is used as supporting electrolyte (Figure S6 A). The pH is ranging from 5.4 to 9.4. It has been concluded from Figure S6 that pH of PBS has noticeable influence on the electrocatalytic oxidation aptitude of as-fabricated material in sulfide sensing. The oxidation peak current density values increase in the range of pH 5.4 to 7.4 and the maximum values are observed at pH 7.4. When the PBS with pH greater than 7.4 is used, the peak current density values for sulfide oxidation drop gradually (Figure S6 B). Therefore, the optimal PBS supporting electrolyte pH 7.4 is used when detecting sulfide. The sensitivity of sulfide sensor depends upon pH of supporting electrolyte solution. Of note, hydrogen sulfide is a week acid with pKa1 and pKa2 values as given below in eq. 2. H2S ⇌ HS− pKa1 = 6.9,

HS− ⇌ S2- pKa2 = 14.15

(2)

More importantly, HS− is electrochemically detectable form of sulfides and the value of pKa can vary with salt content and temperature. In pH-dependent protolytic equilibria, a mixture with an approximate ratio of 20% H2S, 80% HS− and 0% S2− exists in phosphate buffer with pH 7.4, whereas at strong acidic pH and strong basic pH, H2S and S2- are the major forms of sulfide species respectively.42 To verify the most involved form of sulfide in catalytic reaction, buffers with different pH values have been used. The oxidation of sulfides at extreme acidic and basic pH is rather slow. Therefore, we use slightly basic phosphate buffer with physiological pH of 7.4 which consequently ensures the right pH environment for complete conversion of H2S to the electrochemically detectable sulfide (HS−) ion. Also aiming to implement the electrode in biological fluids, we use buffer under physiological conditions to evade the release of hydrogen sulfide from bound sulfane sulfur and the overestimation of free hydrogen sulfide in real tissues caused by extreme acidic or basic conditions.2 In addition, it is well documented that Cu(III) species serve as electron transfer mediator rather than Cu(I) and Cu(II). It is also believed that the presence of oxides and hydroxides are liable for electrooxidation of sulfides in an alkaline environment. Hence, the electrooxidation of sulfide on electrode modified with Cu-based hybrid material may have arisen through reaction with CuOOH to produce Cu(OH)2. Under the auspices of above assumption, we have proposed the possible mechanism for sulfide sensing on Cu2O-CuO@AuNIs modified GCE as given in 12 ACS Paragon Plus Environment

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equations 3-6. While recording electrochemical responses, Cu(I) and Cu(II) would be converted into Cu(III) in alkaline medium which in turn catalyzes sulfide to generate sulfur. Recent studies have proved the enhanced electrocatalytic activity is warranted by synergistic effect of binary composition interfaces where quick electron transportation occurs between redox couples.43,44 The mixed Cu(I)/Cu(II) species can improve catalytic activity towards sulfide oxidation where Cu(II) as well as alkaline environment facilitate the transformation of Cu(III)/Cu(II) redox couple. Additionally, some neutral hydroxide radicals produced by the reaction of Cu(III) with hydroxide in basic medium, being active species in oxidation of sulfide may also be considered. The asprepared different control nanostructures with varied loading of Au NPs, Cu2O ODH and Cu2O NSs have also been examined for their electrocatalytic abilities in oxidizing sulfide as presented by Figure 4D and Figure S5. Benefitting from more edges, corners as well as highly active {111} and {110} facets, electrode modified by Cu2O ODH represents higher sulfide oxidation aptitude compared with Cu2O NSs but lower in comparison of Au NPs decorated Cu2O-CuO ODH. It is worth mentioning that after galvanic deposition of Au NPs at an intermediate level onto the surface of Cu2O ODH, oxidation peak current is further enhanced massively due to the epitaxial linkage of Cu2O ODH with Au NPs and slight electronic modulation in core-shell heterostructures. Here the Cu2O-CuO@AuNIs/GCE shows an oxidation potential of 0.3 V, which is even lower than that of Cu2O ODH alone and Cu2O-CuO@Au3 and it may owing to the maximum interfacial collaborations between two phases. Beyond the transitional deposition, further increasing the amount of Au NPs as shell results in gigantic agglomeration away from core surface creating least interfacial contact of Cu2O core and Au shell (Figure S2 and S3), which consequently decreases oxidation peak current with more positive overpotential. The superb electrocatalytic activity shown by Cu2O-CuO@AuNIs/GCE may refer to the combined effect of following factors, enlarged specific surface area which may not only be the reason for excellent performance: i) octadecahedral with exposed {110} crystallographic facets at their surface are more conductive and catalytically active;45 ii) CuO is more active because of Cu(II) to Cu(III) redox couple; iii) the existence of mixed valence Cu(I)/Cu(II) that encourages the redox reactions at electrode material/analyte interface;46 iv) Au NIs decorated onto the surface provide polarization effect at interface which boosts up the oxidation reactions.47 The possible catalytic mechanism towards sulfide oxidation can be described as follows: Cu2O + 2OH− + 3H2O Cu(OH)2 + OH−

2Cu(OH)2 + 2e− CuOOH + H2O + e− 13

ACS Paragon Plus Environment

(3) (4)


CuO + OH− CuOOH + HS−

CuOOH + e− S0 + Cu(OH)2

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(5) (6)

Figure 4. (A) Nyquist plots of Bare GCE, Cu2O ODH/GCE, and Cu2O-CuO@AuNIs/GCE in 0.1 M KCl containing 1.0 mM K3Fe(CN)6 and 1.0 mM K4Fe(CN)6, Frequency range: 0.1-105 Hz. Inset is the equivalent circuit, (B) Typical CV curves Cu2O-CuO@AuNIs/GCE in 0.1 M PBS in the absence (black line) and presence (red line) of 1 mM sulfide, (C) CV profiles of Cu2OCuO@AuNIs/GCE in 0.1 M PBS with different concentrations of sulfide. (D) CV responses of various modified electrodes in presence of 1 mM sulfide in 0.1 M PBS solution. To assess the sensitivity of Cu2O-CuO@AuNIs ODH, amperometric measurements have been applied upon successive inoculation of aliquots of sulfide concentrations at an applied voltage of 0.3 V in stirring 0.1 M PBS (pH 7.4) as elaborated by Figure 5A and B. The quick oxidation signal feedback is achieved upon the addition of different sulfide concentration with linear range from 10 nM to 11 mM, which reaches the steady-state current response of ∼95% 14 ACS Paragon Plus Environment

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within 3 s. This rapid response of modified electrode can be accredited to the facts that sulfides might easily be diffused and activated on large active sites produced by extremely rough surface of Au nanoislands on Cu2O faces as well as fast charge transport kinetics. The real-time lowest detection limit down to 1 nM has been achieved with signal-to-noise ratio of 3 (S/N=3) on asfabricated Cu2O-CuO@AuNIs/GCE as depicted by Figure 5C. The wide linear detection range between oxidation current densities and increasing sulfide concentrations is demonstrated in Figure 5D. These striking performances of our proposed biosensor are even comparable and far better than previously published articles on sulfide sensing over the last years (Table 1), which reflect its supremacy to be employed in practical applications.

Figure 5. (A) Amperometric response recorded with Cu2O-CuO@AuNIs/GCE upon successive additions of aliquots of varied sulfide concentrations in 0.1 M PBS at 0.3 V. Inset is amperometric records to several micromolars of sulfide, (B) The magnified response Cu2OCuO@AuNIs/GCE to micromolar level, (C) Amperometric records of real detection limit, (D) The linear calibration curve of current density vs. sulfide concentration, (E) Amperometric current responses for anti-interference observation upon 1 mM injection of each electroactive interfering species, (F) Variation of current responses to 1 mM sulfide, Inset is the current responses of six different modified electrodes to 1 mM sulfide.



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The selectivity is another fundamental parameter of any sensing platform, which has been tested for proposed biosensor based on Cu2O-CuO@AuNIs ODH against potential interfering substances by using amperometric measurements. Figure 5E indicates that the injection of 1 mM of various interfering species in stirring 0.1 M PBS does not cause any obvious interferrence at an applied detection potential of 0.3 V, while addition of 1 mM sulfide demonstrates its maximum oxidation current density vindicating the preferable anti-interferrence aptitude of Cu2OCuO@AuNIs ODH based biosensing platform for sulfide detection. Moreover, the long term stability and reproducibility of Cu2O-CuO@AuNIs/GCE stored at room temperature are investigated upon measuring the current density to the oxidation of 1 mM sulfide, which maintains 92% of its original current value upto 25 days of electrodes modification as illustrated in Figure 5F. A major problem with electrochemical sensing technique is electrode fouling. Fouling exists to some extent because in biological conditions the surface of electrode may unequivocally be blocked by the exiting biomolecules in the sample which in turn decrease the long term stability of electrode. In electrochemical fouling an insulating film is deposited on electrode surface because of the reactions occur for detection of analyte and this is typical of the oxidation of sulfide. During electrochemical oxidation, sulfide is converted into sulfur via twoelectron transfer and finally deposits on electrode that can considerably hinder the surface dependent redox properties of sulfide.42 In Figure 5E&F, the oxidation current density is going to decrease slightly with time in the presence of sulfide owing to the adsorption of oxidation products of sulfide on active cites of material. Moreover, six different modified electrodes represent excellent reproducible current density to 1mM sulfide oxidation having relative standard deviation (RSD) value less than 4.6%. Six successive assays with same electrode shows reproducible current density with RSD less than 3.8% (Figure 5F inset). All the aforementioned outcomes approve the remarkable sensitivity, selectivity and reproducibility of as-constructed biosensing system. Table 1 Comparison of sensing performances of several sulfide detection methods based on different nanomaterials reported previously Electrode materials

Linear range

Detection limit (nM)

Sensitivity (μA mM-1 cm-2)


Methods

Ref.

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Ligand L and nano-MOF PAC

-

16

-

Fluorescence

1

Polydimethylsil oxane chip

0.5~10 μM

170

-

Amperometry

2

Coated membrane

-

1400

-

Colorimetry

8

Magnetic CNT/Co/MoS2

5 µL ~60 µL

7.6

0.23

Amperometry

48

Alizarin– graphene

0.002 mM ~3.28 mM

1000

-

CV

49

E. coli/ NPG/GCE bioelectrode

50 μM~5 mM

2500

18.35

CV

50

Triple pulse amperometry

150 nM~15 μM

100

57.4

Amperometry

51

(N-Cdot)/TiO2 nanowire

-

10

-

PEC

52

Cu2OCuO@AuNIs ODH

10 nM~11 mM

Facet-Inspired CoreâShell Gold Nanoislands on Metal Oxide

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