Photocatalysis-Based Nanoprobes Using Noble MetalâSemiconductor

Article pubs.acs.org/ac

Photocatalysis-Based Nanoprobes Using Noble Metal− Semiconductor Heterostructure for Visible Light-Driven in Vivo Detection of Mercury Lihua Zhi,†,§ Xiaofan Zeng,‡ Hao Wang,† Jun Hai,† Xiangliang Yang,‡ Baodui Wang,*,† and Yanhong Zhu*,‡ †

State Key Laboratory of Applied Organic Chemistry and Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou, Gansu 730000, People’s Republic of China ‡ College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China § College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China S Supporting Information *

ABSTRACT: The development of sensitive and reliable methods to monitor the presence of mercuric ions in cells and organisms is of great importance to biological research and biomedical applications. In this work, we propose a strategy to construct a solar-driven nanoprobe using a 3D Au@MoS2 heterostructure as a photocatalyst and rhodamine B (RB) as a fluorescent and color change reporter molecule for monitoring Hg2+ in living cells and animals. The sensing mechanism is based on the photoinduced electron formation of gold amalgam in the 3D Au@MoS2 heterostructure under visible light illumination. This formation is able to remarkably inhibit the photocatalytic activity of the heterostructure toward RB decomposition. As a result, “OFF−ON” fluorescence and color change are produced. Such characteristics enable this new sensing platform to sensitively and selectively detect Hg2+ in water by fluorescence and colorimetric methods. The detection limits of the fluorescence assay and colorimetric assay are 0.22 and 0.038 nM for Hg2+, respectively; these values are well below the acceptable limits in drinking water standards (10 nM). For the first time, such photocatalysis-based sensing platform is successfully used to monitor Hg2+ in live cells and mice. Our work therefore opens a promising photocatalysis-based analysis methodology for highly sensitive and selective in vivo Hg2+ bioimaging studies.

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catalysis-based analysis methodology may possess some advantages over traditional methods such as fluorescencebased ones in terms of detection stability and targeting abilities against interference from complicated media (i.e., blood). Unfortunately, to the best of our knowledge, no study has reported the use of catalysis-based analysis methodology to detect Hg2+ in living cells and animals. Recently, metal−semiconductor heterostructures have emerged as ideal photocatalytic materials, with the semiconductor generating charge carriers and the metal acting as a sink for trapping electrons. For the case of metal Au, electron transfer is facilitated as the surface plasmon of Au couples with the semiconductor exciton and enhances the excited-state lifetime of the photoelectron.12−17 Among semiconductor counterparts, monolayer MoS2, with a band gap of 1.8 eV,18 is generally considered a feasible candidate for use in photocatalysts19 due to the semiconducting property with band gaps ranging from the visible to the near-infrared and the

s well-known environmental pollutants, mercury ions (Hg2+) exert serious medical effects because mercury accumulation in the human body through food chains and drinking water causes dysfunction of cells and results in a wide variety of diseases in the brain, kidney, central nervous system, etc.1−4 As a result of these deleterious effects of Hg2+ on humans, highly sensitive and on-site detection of Hg2+ ions in environmental and biological settings is important.5,6 Recently, different approaches based on electrochemical and optical sensors, organic chromophores or fluorophores, conjugated polymers, oligonucleotides, DNAase and proteins, as well as inorganic nanostructures have been developed for the sensitive and selective detection of Hg2+ ions.7−10 However, these methods might suffer from either low detection sensitivity and throughput or poor analysis stability and abilities against background interferences. With the development of nanoscience and nanotechnology, noble metal nanomaterial-based nanozymes have been utilized to probe heavy metal ions such as Hg2+ ions that could inhibit and stimulate their catalytic activity through a specific Au+− Hg2+ interaction and the change in the surface properties of Au nanoparticles (NPs) by forming Ag−Hg alloys.11 Such a © XXXX American Chemical Society

Received: May 1, 2017 Accepted: June 14, 2017 Published: June 14, 2017 A

DOI: 10.1021/acs.analchem.7b01602 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Figure 1. Illustration of the preparation of 3D Au@MoS2 heterostructure (A). Different magnification SEM images of MoS2 NSs (B) and 3D Au@ MoS2 heterostructure (C).

detection of 0.22 and 0.038 nM, respectively. The detection system could also monitor Hg2+ in cells and living bodies. Our strategy is the first to provide a new type of photocatalysisbased nanoprobe with high selectivity and sensitivity and is therefore suitable for cell and living animal imaging.

catalytic activity of its edge sites. However, heterostructures involving Au and MoS2 have been studied mainly as photocatalysts20−22 and not as a sensor platform for the sensitive recognition of target species in biological bodies. Moreover, the synthesis of these materials, which could facilitate metal/semiconductor Schottky contact and favorable electron transfer through this interface for photocatalytic reactions, remains a challenge. Furthermore, previous studies suggest that 3D architectures based on noble metal NPs and semiconductor materials are ideal photocatalysts because they could offer long optical paths for efficient light absorption and rapid electron−hole separation.23 Nonetheless, 3D structures assembled from MoS2 nanosheets (NSs) and noble metal NPs with a Schottky junction are neither reported nor explored for the detection of Hg2+ ions or other analytes in living cells and animals. The present study is the first to present a novel photocatalysis-based 3D Au@MoS2 heterostructure nanoprobe for sensitive Hg2+ detection. In this work, a 3D Au@MoS2 heterostructure was prepared by photoirradiation of a DMF solution containing HAuCl4 and MoS2 NSs under visible light illumination. The formed Au NPs directly deposited on the MoS2 NSs to form a 3D frame structure (Figure 1A), in which the effective Au/MoS2 Schottky contact was produced. In this assembly process, no further ligand exchange steps were required. The obtained 3D Au@MoS2 heterostructure showed remarkable absorption from the ultraviolet to the near-infrared region and exhibited superior photocatalytic activity toward rhodamine B (RB) decomposition. These effects resulted in the turning OFF of the fluorescence and color of RB (Figure 1). However, in the presence of Hg2+, Hg2+ wrapped around the Au NPs of the 3D Au@MoS2 heterostructure because of the strong affinity between Hg(II) and Au NPs.24 The Hg2+ on the surface of the Au NPs were reduced to Hg0 in the form of gold amalgamation by the photoinduced electrons. Thus, the photocatalytic activity of the 3D Au@MoS2 heterostructure toward RB decomposition was suppressed, resulting in the turning ON of the fluorescence and color of RB (Figure 1). Such characteristics enabled the 3D Au@MoS2 heterostructure to sensitively and selectively detect Hg2+ in water by both fluorescence and colorimetric methods, achieving limits of

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EXPERIMENTAL SECTION Chemicals. HAuCl 4 ·4H 2 O, molybdenum(IV) sulfide (MoS2, 99%), 1.6 M n-butyllithium solution, Rhodamine B (RB), and metallic salts (NaNO3, KNO3, AgNO3, CaCl2, MgSO4, Cd(NO3)2, Cu(NO3)2·3H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, FeSO4, Al(NO3)·9H2O, Fe(NO3)3·9H2O, and Hg(NO3)2) were purchased from major suppliers such as Sigma-Aldrich and Alfa Aesar and used as received. MoS2 nanosheet solution was prepared by chemical exfoliation method according to the published method.25 Instrumentation. The morphology of the samples was investigated by field-emission scanning electron microscopy (FE-SEM, FEI, Sirion 200). TEM images were taken on a Tecna i-G2-F30 (FEI) transmission electron microscope at an acceleration voltage of 300 kV. X-ray powder diffraction (XRD) patterns of the nanomaterials were recorded on a Bruker AXS D8-Advanced diffractometer with Cu Kα radiation (λ = 1.5418 Å). N2 adsorption−desorption isotherms were measured at 77 K after heating the samples at 150 °C for 8 h to remove any moisture and solvent molecules presented in the pores with a Micromeritics TriStar II 3020 analyzer. The Brunauer− Emmett−Teller (BET) method was utilized to calculate the specific surface areas (SBET). By using the Barrett−Joyner− Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI-5702 multifunctional spectrometer using Al Kα radiation. Raman spectra were collected using a confocal microprobe Raman system (Renishaw, RM2000). FT-IR spectra were recorded on a Nicolet FT-170SX spectrometer. Au contents are determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Varian VISTA-MPX). UV−visible adsorption spectra (UV−vis) were carried out on a UV 1750 spectrometer. B


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Figure 2. (A) TEM images of MoS2. (B,C) Different magnification TEM images of 3D Au@MoS2 heterostructure. Inset: Nanoparticle size distribution of Au in 3D Au@MoS2 heterostructure. (D) HRTEM image of 3D Au@MoS2 heterostructure. Inset: Selected area electron diffraction of 3D Au@MoS2 heterostructure. (E) Dark field of STEM images. (F−I) Element mapping images of the 3D Au@MoS2 heterostructure.

added to the aqueous solution of rhodamine B (4 mL, 100 μM). Hg2+ with different concentration then was added to the above solution. Subsequently, the mixture was stirred under visible light irradiation for 25 min. Finally, the supernatant was analyzed using fluorescence spectra and UV−visible adsorption spectra. Selectivity Analyses of 3D Au@MoS2 Heterostructure for Various Metal Ions. To measure the selectivity of metal ions, Na+, K+, Ag+, Ca2+, Mg2+, Cd2+, Cu2+, Co2+, Ni2+, Zn2+, Fe2+, Al3+, Fe3+, and Hg2+ were solved in Milli-Q water at higher stock concentrations, and we repeated the method of Hg2+ detecting experiments mentioned above. To check if other metal ions have an effect on Hg2+ detection, all of the metal salts (100 μM) were added before Hg2+ addition (50 μM). The fluorescence and absorbance intensity were checked as above. Cell Cytotoxicity. 3T3 cells were seeded at a density of 6 × 104 cells/well in 96-well plates, cultured at 37 °C and 5% CO2 for 24 h. Different concentrations of 3D Au@MoS 2 heterostructure (0, 100, 200, 300, 400, 500, and 600 μg/mL, diluted in DMEM) were then added to the wells. The cells were subsequently incubated for 24 or 48 h at 37 °C under 5% CO2. The numbers of viable and total cells were quantified using the methyl thiazolyl tetrazolium (MTT) assay. The

Photocalysis was performed using a xenon lamp (HSX-F300) equipped with a cutoff glass filter transmitting λ ≥ 420 nm surrounded by a water-cooling quartz jacket to cool the lamp. Synthesis of 3D Au@MoS2 Heterostructure. Twenty milligrams of MoS2 nanosheets was dissolved in 20 mL of DMF and 20 mL of deionized water to form a homogeneous solution. After being sonicated for 30 min, 2 mL of HAuCl4 (2 mM) solution was added to the above solution. The resulting mixture was stirred under visible light irradiation (λ ≥ 420 nm, distance 20 cm) for 1 h. After the reaction finished, the products were collected by centrifuging and washed thoroughly with EtOH and water. At last, the 3D Au@MoS2 heterostructure was dried by freeze-drying for 12 h at −50 °C. Photocatalytic Test. A 60 mL solution (H2O:EtOH = 9:1) containing rhodamine B (100 μM) and the photocatalyst (15 mg) was stirred in the dark for 20 min to reach adsorption/ desorption equilibrium. The mixture then was stirred under light irradiation. During the stirring period, the supernatant was taken out at regular intervals and then centrifuged to remove the catalyst. Finally, the supernatant was analyzed by fluorescence spectra and UV−visible adsorption spectra. Detection of Hg2+ Using 3D Au@MoS2 Heterostructure. One milligram of 3D Au@MoS2 heterostructure was C


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Figure 3. (A) XRD pattern of the 3D Au@MoS2 heterostructure (a) and MoS2 NSs (b). (B) Nitrogen adsorption/desorption isotherms of the 3D Au@MoS2 heterostructure. Inset: Pore size distribution of the 3D Au@MoS2 heterostructure. (C) UV−vis absorption of 3D Au@MoS2 heterostructure, MoS2 NSs, and Au NPs. (D) Plots of (ahν)0.5 versus photon energy for calculation of bandgap energies of 3D Au@MoS2 heterostructure and MoS2 NSs.

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RESULTS AND DISCUSSION Synthesis and Characterization. The monodisperse monolayer MoS2 NSs were prepared through a chemical exfoliation method.25 A controlled amount of MoS2 NSs was ultrasonicated in DMF/water to form a homogeneous dispersed solution. HAuCl4·4H2O was added to the above dispersed solution. In this aqueous dispersion, the MoS2 NSs interacted with AuCl4− to form MoS2−AuCl4−. Under visible light irradiation, electron and hole pairs were generated within MoS2. The photoexcited electrons further facilitated the reduction of AuCl4− ions to Au0 for the formation of Au NPs, hence the formation of a 3D Au@MoS2 heterostructure (Figure 1A).26 At the same time, holes were scavenged by the DMF solvent. The phenomenon of the photochemical synthesis of metal NPs using a photoreducing agent is well supported in the literature.26 However, the use of HAuCl4· 4H2O alone without MoS2 NSs in DMF/water did not generate Au NPs under visible light irradiation (Figure S1). The surface morphology and structure of the products were examined via scanning electron microscopy (SEM), which revealed that the obtained samples possessed a 3D honeycomblike structure with numerous pores (Figure 1C). These pores cross-linked with one another to form a 3D macroporous network, the walls of which were composed of single- or fewlayer MoS2 NSs with a clear curved profile, unlike pure 2D MoS2 NSs (Figure 1B). The structure of the 3D Au@MoS2 heterostructure was further studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). As described in Figure 2A, the bare MoS2 clearly exhibited thin and transparent sheets. However, after formation of 3D Au@ MoS2 heterostructure, the clearly observed Au NPs were attached on the nanosheets of MoS2 with an average size of 22 nm (Figure 2B,C). The HRTEM image in Figure 2D shows that the 3D Au@MoS2 heterostructure displayed a sandwichlike structure and that the lattices of Au and MoS2 were in close

following formula was used to calculate the inhibition of cell growth: cell viability (%) = (mean Abs value of treatment group /mean Abs value of control) × 100%

In Vitro Image Studies. 3T3 cells were washed with PBS buffer three times, and then the cells were incubated with 3D Au@MoS2 heterostructure (30 μL, 2 mg/mL), rhodamine B (20 μL, 0.1 mM) in a mixture of PBS buffer (pH 7.4, 10 mM) and DMSO (4/1, v/v), and the Hg2+ (100 μL, 1 mM in a mixture of water) for 1 h at 37 °C. After irradiation for 25 min under visible light irradiation, the fluorescence imaging of intracelluar Hg2+ was then carried out after washing the cells with PBS. The 3T3 cells without the pretreatment of Hg2+ under the dark and with the pretreatment of Hg2+ under visible light irradiation were set as a control. All confocal images were performed with a Leica TCS SP8 inverted epifluorescence/ reflectance laser scanning confocal microscope. Excitation was at 550 ± 10 nm, and emission was at 580 ± 10 nm. In Vivo Image Studies. Kunming mice (20−25 g) were anesthetized by intraperitoneal of xylazine (10 mg/kg) and ketamine (80 mg/kg). The mice then were given a subcutaneous of the mixtured 3D Au@MoS2 heterostructure (50 μL, 3 mg/mL), rhodamine B (20 μL, 0.1 mM) in a mixture of PBS buffer (pH 7.4, 10 mM) and DMSO (4/1, v/v) and the Hg2+ (50 μL, 1 mM in a mixture of water) after irradiating for 1 h under the visible light. The mice were then imaged by using an IVIS Lumina XR in vivo imaging system. The mice that did not inject Hg2+ under the dark and injected Hg2+ under visible light irradiation were set as a control. D


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Figure 4. Absorption spectroscopy (A) and fluorescence spectroscopy of 3D Au@MoS2 heterostructure on the degradation of RB under visible light irrdiation. Photocatalytic degradation efficiencies of different photocatalysts toward RB measured by absorption spectroscopy (B) and fluorescence spectroscopy (D).

P/P0 range of 0.45−1.0 signifies irreversible desorption, which is generally observed in specimens with relatively large mesopores and macropores.31 As indicated by the BJH analysis of the desorption branch of the isotherm, the pore size distribution fell in the ranges of 3.7−4.3 and 4.3−30 nm, further confirming the existence of small and large mesopores, as well as macropores. Here, the large mesopores originate from the pore formation between the MoS2 flakes after inserting the Au NPs.32−34 The BET specific surface area of the 3D Au@ MoS2 heterostructure was measured as 49.25 m2 g−1 (0.20 m3 g−1), which is 246 times that of the MoS2 nanosheets (0.19 m2 g−1) and is considerably greater than the value reported for the MoS2 aerogel (18 m2 g−1).35 The optical properties of the 2D MoS2 NSs and as-prepared 3D Au@MoS2 heterostructure were characterized through the UV−vis spectroscopy of their suspensions. The UV−vis spectrum of MoS2 exhibited clear characteristic peaks in the region from 350 to 500 nm, whereas the spectrum of the 3D Au@MoS2 heterostructure showed absorption bands similar to those of MoS2 and an additional absorption band at 625 nm (Figure 3C). The surface plasmon resonance (SPR) peak of spherical Au NPs with a diameter of 10−50 nm in diluted aqueous solutions is known to be centered at 517−533 nm.36 However, the growth of nanoparticles directly on disulfide nanostructure surfaces leads to a distortion of the spherical shape of Au NPs (Figure 2D). Such distortion can cause a red shift of the maximum of the SPR peak from the position typical for free spherical Au NPs. Thus, in this work, the SPR transversal band obviously red-shifted at 625 nm (Figure 3C). Combined with nonspherical morphology of the Au NPs and its SPR peak red-shift, we can conclude the electronic interactions between Au and MoS2.37 We determined the band gaps of the samples by fitting the optical transition at the absorption edges using the Tauc/David−Mott model.38 As shown in Figure 3D, the band gaps of MoS2 became increasingly narrow after the formation of the 3D Au@MoS2 heterostructure, which supports the observation of a red shift in the absorption band edge of 3D Au@MoS2 heterostructure as

contact. Each Au NP embedded in the 3D Au@MoS2 heterostructure showed clear lattice fringes with interplanar spacing of 0.234 nm, corresponding to the (111) interplane distance of Au (JCPDS no. 89-3697).27 Interlayer spacings of 0.27 and 0.635 nm, which correspond to the (100) and (002) crystal planes of MoS2, were also observed.27 In the HRTEM, the interface between MoS2 and the Au NPs revealed the close interfacial matching of the Au NPs and disulfide nanostructures, and the (111) planes of Au (0.234 nm) and the (100) planes of MoS2 (0.27 nm) exist with a considerably small mismatch of 13.1% (Figure 2D). In addition, such close examination of the interface revealed lattice fringe continuation, which may be the result of epitaxial growth of nanoparticle. The local composition of the formed 3D Au@MoS2 heterostructure was also studied using high-angle annular darkfield scanning TEM images, corresponding elemental mapping (Figures 2E− I), and energy-dispersive X-ray spectroscopy (Figure S2), all of which clearly showed the presence of gold in the grown NPs and its good dispersion on MoS2 NSs. The powder X-ray diffraction (XRD) pattern of the 3D Au@ MoS2 heterostructure is shown in Figure 3A. The evident diffraction peaks located at about 14.1°, 33.0°, 40.3°, 48.4°, and 58.6° for MoS2 matched the (002), (100), (103), (105), and (110) indexes of MoS2, as reported previously.28 After Au NP modification, new diffraction peaks emerged at 38.0°, 44.5°, 64.7°, and 77.8° for the 3D Au@MoS2 heterostructure samples; these peaks were ascribed to the (111), (200), (220), and (311) planes of Au (JCPDS no. 89-3697), respectively. According to the Raman spectra of the 3D Au@MoS2 heterostructure, the two prominent peaks located at 374.4 and 399.3 cm−1 corresponded to the E2g and A1g vibrational bands of MoS2,29,30 thus suggesting the existence of crystalline MoS2 in the nanohybrid framework (Figure S3). The XRD and Raman spectra offered preliminary proof of the successful fabrication of the 3D Au@MoS2 heterostructure. As shown in Figure 3B, the nitrogen adsorption−desorption curve of the 3D Au@MoS2 heterostructure is a type-IV adsorption isotherm. The apparent hysteresis loop of H2 in the E


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Analytical Chemistry Scheme 1. Illustration of 3D Au@MoS2 Heterostructure for Sensing Hg2+

could not be completed with pure MoS2 NSs and Au NPs under the same conditions. From the above degradation results, we conclude that the formed 3D Au@MoS2 heterostructure exhibits much higher photocatalytic activities than the singlephase Au and MoS2 (Figure S5). The high photocatalytic activity observed for the 3D Au@MoS2 heterostructure may be attributed to Au effectively capturing photoinduced electrons. Such capture by Au inhibited the recombination of the photoinduced electrons with holes and improved photocatalytic activities. Moreover, recent studies have indicated that the SPR effect of a noble metal can accelerate charge carrier generation rate in the semiconductor and enhance solar energy conversion efficiency due to the transfer of the plasmon energy of the metal to the semiconductor through the HET or EMT process, or both.44−46 Thus, the high photocatalytic activity of 3D Au@ MoS2 heterostructure may be inseparable from its SPR effect. As is shown in Figure S6, when a trace amount of Hg2+ was added, the photocatalytic activity of the 3D Au@MoS2 heterostructure was dramatically decreased. To effectively understand the photocatalytic mechanism of the 3D Au@ MoS2 heterostructure before and after being treated with Hg2+ under visible light irradiation, we performed scavenger experiments by adding diisopropylethylamine as a hole scavenger, K2Cr2O7 as an electron scavenger, and benzoquinone as a radical scavenger during the photocatalytic degradation of RB. As shown in Figure S7, the photodegradation of RB was greatly inhibited by the addition of the electron scavenger. This result showed good agreement with the Hg2+ results. In sum, the results indicate the dominant role of electrons in the photocatalytic activities of nanocomposites. On the basis of our experimental results and some previously reported literature,47 the plausible Hg2+ detection mechanism of the 3D Au@MoS2 heterostructure under visible light irradiation is shown in Scheme 1. Under the excitation of light, the excited electrons of MoS2 can be transferred from the VB to the CB and then be directly injected into the Fermi level (Ef) of Au. When Hg2+ was added to the degradation system, Hg2+ wrapped around the Au NPs of the 3D Au@MoS2 heterostructure because of the strong affinity between Hg(II) and Au NPs.48 The Hg2+ on the surface of the Au NPs was reduced to Hg0 in the form of gold amalgamation by the photoinduced electrons. This process was demonstrated by XPS. As shown in Figure S8, the two binding-energy peaks at

compared to the pure MoS2 (Figure 3C). Generally, the formation of chemical bonding results in a red shift in the absorption band edge of the resulting material.39−41 In the 3D Au@MoS2 heterostructure, the band gap narrowing and enhanced visible light absorption of the nanocomposites were due to the formation of the chemical bonding between Au and the specific sites of MoS2 (Au−S bonding), which can be advantageous to enhancing the visible light photocatalytic activity of the nanocomposite. As presented in Figure S4A, the full survey X-ray photoelectron spectrometer (XPS) spectrum of the 3D Au@ MoS2 heterostructure revealed that the sample was composed of S, Mo, and Au elements. However, only S and Mo elements were found in the survey XPS spectrum of the MoS2 NSs. Figure S4B displays the high-resolution spectra of the Mo 3d at ∼231.48 and ∼228.27 eV, which correspond to the 3d5/2 and 3d3/2 doublets, as well as the peaks at ∼161.25 and ∼161.80 eV (Figure S4C), which are attributed to the 2p1/2 and 2p3/2 orbital of S. These binding energy values closely resemble the spectra of the binding energies of Mo4+ and S2− ions in MoS2 alone,42 with the important difference being the small decrease in the binding energy of 0.35 eV of the Mo 3d at ∼231.48 eV in comparison with the value for MoS2 alone (231.83 eV). The S 2p3/2 peak at ∼162.1 eV showed a similar downshift of 0.42 eV relative to the value of MoS2 alone (162.28). The shift in the Mo 3d5/2 peak and S 2p position to low binding energies is interesting because it reflects electron donation to the MoS2 sheets from gold.43 The peaks located at 87.5 and 83.9 eV corresponded to the 4f5/2 and 4f7/2 binding energies of the zero valence state of metallic Au (Figure S4D), respectively. This result further proved that Au NPs were successfully generated and introduced into the 3D Au@MoS2 heterostructure. Inhibition Effect of Hg2+ on the Photocatalytic Activity of 3D Au@MoS2 Heterostructure. As a result of its long-wavelength absorption and emission, high absorption coefficient and quantum efficiency, and good photo stability, RB was selected as the visible signaling reporter to evaluate the photocatalytic activity of the 3D Au@MoS2 heterostructure. As the control experiment, the photocatalytic activities of pure MoS2 NSs and Au NPs were also examined. In the presence of the 3D Au@MoS2 heterostructure (Figure 4), the degradation of RB was achieved completely within 25 min under visible light irradiation (λ ≥ 420 nm). However, the degradation of RB F


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Figure 5. Selectivity studies. Fluorescence (A) and absorption (C) changes of RB + 3D Au@MoS2 heterostructure under visible light irradiation upon addition of 1 equiv of Hg2+ and 2 equiv of various metal ions. Black bar, RB + 3D Au@MoS2 heterostructure + metal ions; red bar, RB + 3D Au@MoS2 heterostructure + metal ions + Hg2+. The Photocatalytic activities of 3D Au@MoS2 heterostructure were inhibited by various metal ions. (B) The photographs were taken under a 365 nm UV lamp. (D) The photographs were taken under room light.

Figure 6. Sensitivity studies. (A) Fluorescence intensity changes caused by various concentrations of Hg2+ (5−50 μM). (B) Fluorescence response of the detection system versus Hg2+ concentration. (C) Absorption intensity changes caused by various concentrations of Hg2+ (5−50 μM). (D) Absorption response of the detection system versus Hg2+ concentration.

presence of visible light. As a result of the high specificity of the Hg−Au interaction, the 3D Au@MoS2 heterostructure showed excellent selectivity toward Hg2+ over other metal ions. As shown in Figure 5, the introduction of 2 equiv of other metal ions (Na+, K+, Ag+, Ca2+, Mg2+, Cd2+, Cu2+, Co2+, Ni2+, Zn2+, Fe2+, Al3+, and Fe3+) could hardly inhibit the photocatalytic activity of the 3D Au@MoS2 heterostructure. Correspondingly, the fluorescence of the test solution changed from yellow to

100.7 and 104.7 eV were ascribed to Hg 4f7/2 and Hg 4f5/2, respectively, which indicate that both Hg2+ and Hg0 existed on the surface of Au.27 Given that the photoinduced electrons were scavenged by Hg2+, the photocatalysis efficiency of the 3D Au@MoS2 heterostructure decreased. Hg2+ Sensing Studies. A colorimetric and fluorescent mercury sensor was established through the use of the 3D Au@ MoS2 heterostructure to catalyze degradation of the RB in the G


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Figure 7. Study on intracellular mercury ion imaging. Confocal images of 3T3 cells: (A−C) bright-field and (D−F) fluorescence image of 3T3 cells incubated with different materials under different conditions.

Figure 8. In vivo mercury ion imaging. In vivo fluorescence image of living mice pretreated with different materials under different conditions.

colorless (Figure 5B), and the color of the test solution changed from rose red to colorless (Figure 5D). However, only the introduction of 1 equiv of Hg2+ to the above system could inhibit the photocatalytic activity of the 3D Au@MoS2 heterostructure dramatically. Comparative experiments were carried out to evaluate the practicability of the 3D Au@MoS2 heterostructure. As depicted in Figure 5A and C, in the presence of various interference species, Hg2+ could still inhibit the photocatalytic activity of the 3D Au@MoS2 heterostructure. Taken together, these results indicate that the 3D Au@MoS2 heterostructure exhibits remarkably high selectivity toward Hg2+ over other metal ions. To evaluate the sensitivity of such Hg2+ sensor, we used the 3D Au@MoS2 heterostructure in catalyzing the photodegradation reaction of RB after reacting with different concentrations of Hg2+. As shown in Figure 6A and C, with the increase of Hg2+ concentration, the absorbance and fluorescence intensity of RB continuously increased. Moreover, the fluorescence and absorbance intensity against Hg2+

exhibited a good linear correlation (Figures 6B and D). The detection limits (3σ/slope) of the fluorescence assay and colorimetric assay were 0.22 and 0.038 nM, respectively (Figure S9), which are below the threshold levels (10 nM) defined by the United States Environmental Protection Agency (EPA) for drinking water. In addition, appreciable color (from colorless to rose red) and fluorescence (from colorless to yellow) changes of the test solution could be distinguished by the naked eye (Figure 6D). Therefore, the 3D Au@MoS2 heterostructure could detect Hg2+ from water through fluorescence and colorimetric methods. Monitoring Hg2+ in Living Cells. Before the application of the 3D Au@MoS2 heterostructure in bioimaging, its possible cytotoxic activity was evaluated. As shown in Figure S10, the cell viability of the 3D Au@MoS2 heterostructure for 3T3 cells remained above 75% upon incubation at a concentration of 600 μg/mL for 48 h, thereby indicating the low cytotoxicity of the 3D Au@MoS2 heterostructure. To demonstrate the applicability of the RB system photocatalyzed with the 3D Au@MoS2 H



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heterostructure in monitoring intracellular Hg2+, we incubated 3T3 cells with the 3D Au@MoS2 heterostructure, RB, and Hg2+ successively in a serum-supplemented cell culture medium for 1 h at 37 °C. As shown in Figure 7A and D, the 3T3 cells incubated with the 3D Au@MoS2 heterostructure and RB for 1 h under dark conditions at 37 °C showed strong emission at 573 nm. By contrast, the 3T3 cells incubated with the 3D Au@ MoS2 heterostructure and RB for 1 h under visible light irradiation at 37 °C showed only a weak emission at 573 nm (Figure 7B and E). When the cells were supplemented with the 3D Au@MoS2 heterostructure, RB, and Hg2+ in the growth medium for 1 h at 37 °C under visible light irradiation, a strong enhancement in the red emission was observed in the intracellular region (Figure 7C and F). The confocal luminescence images demonstrated that the 3D Au@MoS2 heterostructure photocatalyzed RB system could be successfully applied to monitor intracellular Hg2+ in living cells via luminescence imaging. Hg2+ Imaging in Mice. We examined the ability of the 3D Au@MoS2 heterostructure photocatalyzed RB system to visualize Hg2+ in living animals. Kunming mice were selected as our model and were given a skin-pop injection of the 3D Au@MoS2 heterostructure, RB, and Hg2+ successively. The live mice injected with the 3D Au@MoS2 heterostructure and RB for 0.5 h under dark conditions showed strong emission at 550−650 nm (Figure 8A). When the living mice were injected with 3D Au@MoS2 heterostructure and RB under visible light irradiation, the emission significantly weakened (Figure 8B). When the living mice were injected with the 3D Au@MoS2 heterostructure, RB, and Hg2+ under visible light radiation, strong emission at 550−650 nm was observed (Figure 8C), thereby indicating that the 3D Au@MoS2 heterostructure photocatalyzed RB system can serve as a probe for mercury ion imaging in vivo.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b01602. EDX, Raman spectra, XPS, and cell cytotoxicity (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Baodui Wang: 0000-0003-1600-6557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (21671088, 21401091, 81573013, and 21501080).

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CONCLUSION

In summary, we proposed a new photocatalysis-based nanoprobe for mercury using a 3D Au@MoS2 heterostructure as a photocatalyst and rhodamine B (RB) as a fluorescent and color change reporter molecule for monitoring Hg2+ in living cells and animals. In such a detection system, Hg2+ was reduced by photoinduced electrons to form gold amalgam in the 3D Au@ MoS2 heterostructure under visible light illumination. This formation of gold amalgam suppressed the photocatalytic activity of the 3D Au@MoS2 heterostructure toward RB decomposition and led to the “OFF−ON” of the fluorescence and color change of RB. Such noteworthy signal change features made the 3D Au@MoS2 heterostructure photocatalyzed RB system an effective sensor platform for the selective and sensitive detection of Hg2+ through both fluorescence and colorimetric methods with detection limits of 0.22 and 0.038 nM, respectively. Such detection limit of Hg2+ is lower than the maximum level (10 nM) for drinking water set by the US EPA. The nanoprobe was also found to be capable of monitoring Hg2+ in cells and living bodies via luminescence bioimaging. Our successful photocatalysis-based nanoprobe for the sensing and bioimaging of Hg2+ provides a new design strategy for future novel probes for highly sensitive detection and bioimaging studies. I


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