Triple Signal Readout for Colorimetric Sensing of Mercury Ions by

Jan 27, 2016 - ... Sensing of Mercury Ions by Plasmonic Cu2-xSe Nanoparticles .... Bin Ren ( Associate Editor, Analytical Chemistry ) , Michael J. Sai...
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Ratiometry, Wavelength, and Intensity: Triple Signal Readout for Colorimetric Sensing of Mercury Ions by Plasmonic Cu2‑xSe Nanoparticles Hui Zhang and Yunsheng Xia* Key Laboratory of Functional Molecular Solids, Ministry of Education; College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China S Supporting Information *

ABSTRACT: In this study, we have reported a new colorimetric platform for sensitive and selective sensing of Hg2+ using nearinfrared (NIR) plasmonic Cu2‑xSe nanoparticles (NPs) as reporters. Because of ultrahigh affinity between Hg2+ and Se2−, the added Hg2+ can react with Cu2‑xSe NPs and exchange their Cu+/Cu2+, yielding a HgSe layer around the host NPs. Accordingly, the absorption profiles of the Cu2‑xSe NPs are modulated substantially: The absorbance at 400−600 nm is increased, and the NIR localized surface plasmon resonance dramatically decreases with a more than 150 nm bathochromic shift. Thus, the system possesses triple signal responses, namely, ratiometry, wavelength, and intensity, to the analytes simultaneously. Such uniquely multiple signal output not only provides more choices for the quantification, but also enhances the reliability in the analyte detection. By rationally choosing poly(allylamine hydrochloride) as the NP template, Hg2+ ions can be determined as ranging from 0−800 nM. The detection limit is as low as 2.7 nM, which is nearly 4 times lower than the limit value (10 nM) defined by the U.S. Environmental Protection Agency for drinking water. Other heavy/transition metal ions, such as Cu2+, Ag+, Pb2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Cr3+, Fe2+, and FeF63−, do not interfere with the sensing. Especially, Hg2+ contents can be quantified, even if their concentrations are as low 10 nM in tap water and common environmental water samples. Due to favorable analytical performance, the proposed Cu2‑xSe NPs based system has potential applications in monitoring trace Hg2+ in various real samples, even in drinking water. KEYWORDS: Cu2‑xSe nanoparticles, Hg2+ assay, colorimetric chemosenosor, localized surface plasmon resonance

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dispersion/aggregation modulation. This strategy is simple and has a distinct color change (wine ↔ purple). However, it is not highly specific, and its moderate sensitivity is not ideal in real sample sensing,23 especially for monitoring trace amounts of Hg2+. Then, in terms of surface modification, the multistep centrifugation purification processes not only are costly and tedious, but often lead to irreversible agglomeration of the NPs.24 So, it is desirable to develop specific, highly sensitive, cost-effective, and robust tools for Hg2+ assay. In recent years, copper chalcogenide NPs (CuS, CuSe, CuTe) have attracted growing interest ranging from controllable synthesis to optical properties to various applications.25 Especially, self-doped copper-deficient Cu2‑xX (X = S, Se, or Te) NPs possess localized surface plasmon resonances (LSPR) at near-infrared (NIR) wavelengths originating from the high density of holes in the valence band.26,27 Furthermore, the physicochemical property and LSPR modulation of Cu2‑xX NPs are different from those of traditional noble metal NPs.28−31 So, plasmonic semiconductor NPs are promising as a new kind of

ercury is one of the most toxic heavy metal elements to human health and environmental safety.1,2 It can lead to severe damage to many organs such as the kidney and brain, as well as deleterious effects on the human body in the forms of tremors, vegetative nerve functional disturbance, and so forth.3,4 The World Health Organization (WHO) defines the maximum allowable level of inorganic mercury in drinking water as no more than 30 nM,5 which is further decreased to 10 nM by the U.S. Environmental Protection Agency (EPA).6 Thus, great efforts have been devoted to developing various methods and strategies for monitoring ionic mercury (Hg2+),1,3,7−11 the most stable and widespread inorganic form in the environment and organisms.12 Among these, colorimetric sensing systems have attracted considerable interest because of their simplicity and practicality. In parallel, gold nanoparticles (AuNPs) have been widely employed as colorimetric reporters due to their higher extinction coefficient, tunable plasmonic band, and readout distinguishable to the naked eye.13−16 By a combination of versatile surface modification and various recognition principles, large amounts of AuNP based colorimetric systems have been designed and applied to Hg2+ sensing.17−22 Despite these substantial achievements, there are a few potential problems in practical applications. First of all, most of the developed platforms are based on AuNP © XXXX American Chemical Society

Received: December 8, 2015 Accepted: January 27, 2016

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Figure 1. SEM (A), TEM (B), and HRTEM (C) images, and absorption spectrum (D) of the PAH-templated Cu2‑xSe NPs. The inset in (C) is the SAED pattern, and the inset in (D) is the NP containing solution in room light. NiCl2·6H2O, BaCl2·2H2O, Zn(CH3COO)2·2H2O, CoCl2·6H2O, FeCl3·6H2O, Bi(NO3)3·5H2O, CdCl2·2.5H2O, Sr(NO3)2, Cr(NO3)3· 9H2O, Al(NO3)3·9H2O, Ti(SO4)2, FeSO4·7H2O, SnSO4, HAuCl4· 3H2O, CaCl2, NaCl, Mg(NO3)2·6H2O, KCl, Na2SO4, NaNO3, NaHCO3, and HgCl2 were acquired from Shanghai Chemical Reagent Co. All solutions were prepared with double deionized water. Instruments and Characterization. Characterization of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) mapping was carried out on Hitachi S-4800 under the accelerating voltage of 5 kV. Characterization of transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) was carried out on Tecnai G2 F20 UTwin (FEI) under the accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed by an ESCALAB 250 Xi XPS system of Thermo Scientific, where the analysis chamber was 1.5 × 10−9 mbar and the X-ray spot was 500 μm. A U-4100 spectrometer was used to record the absorption spectra. The dynamic light scattering (DLS) was measured by Zetasizer Nano ZS series, Malvern Instruments with a 633 nm laser wavelength and a measurement angle of 173° (backscatter detection) at 25 °C. The crystal phases in Cu2‑xSe NPs were determined using powder XRD (Bruker D8 Advance X-ray diffractometer with Cu Kα X-ray source). Synthesis of Cu2‑xSe NPs. Cu2‑xSe NPs were prepared in aqueous solution by the modified method reported previously.37 In a typical procedure, PAH (50 mg/mL, 480 μL) was dissolved in 6.62 mL of water and then to it was added SeO2 (0.2 M, 0.1 mL) and Vc (0.4 M, 0.3 mL), successively. After 10 min, a mixed solution of CuSO4·5H2O (0.4 M, 0.1 mL) and Vc (0.4 M, 0.4 mL) was added under vigorous stirring at 30 °C until a green solution was obtained in 3 h. After cooling to room temperature, the Cu2‑xSe NP containing solution was centrifuged at 4000 rpm for 10 min twice. The purified Cu2‑xSe NPs were preserved at 4 °C for further use. The procedures for the fabrication of other surfactants templated by Cu2‑xSe NPs were similar, but PAH was replaced by CTAB (30 mM), SDS (10 mg/mL), and PSS (10 mg/mL), respectively. Procedures for Hg2+ Sensing. PBS (0.01 M, 100 μL) buffer solution (pH = 7.0) and 50 μL of purified Cu2‑xSe NPs (1.2 × 10−10 M) were placed in a series of 5 mL colorimetric tubes. The concentration of the Cu2‑xSe NPs was estimated according to the method reported previously.38 Then, different amounts of Hg2+ were added. The mixtures were diluted to 1.7 mL with water and mixed thoroughly. Ten minutes later, their absorption spectra were recorded at ambient conditions. Procedures for Hg2+ Sensing in Real Samples. Tap water (from the lab), pond water (from Jinghu Lake, Wuhu), and river water (from Yangtze River) samples were measured after three times filtration using 0.22 μm filters. For the sensing, 850 μL of pure or Hg2+- (10 nM, 30 nM) spiked water samples were first added to NaF (100 μM, 150 μL); the resulting solutions were then introduced to the systems containing the Cu2‑xSe NPs (1.2 × 10−10 M, 50 μL) and PBS (0.01 M, 100 μL), respectively. The obtained mixtures were diluted to 1.7 mL and incubated for 10 min before measuring their absorption spectra. Note: Due to their lower energy, the absorption spectra of the Cu2‑xSe NPs were a little rough in the NIR region. To improve quantification, several original data points (Figures 3A−D and 7) were

colorimetric platform for sensing. However, in comparison to their applications in photothermal therapy, imaging, and plasmonic solar cells,32−34 little attention has been paid to the functions of colorimetric reporters. Probably, one of the major reasons is that the employment of metal (Au, Ag) NPs for colorimetric assay is too deep-rooted. In these regards, it is urgent to explore the sensing mechanisms, analytical performance, as well as potential applications of plasmonic semiconductor NPs. Herein, we present a novel colorimetric system using NIR plasmonic Cu2‑xSe NPs as reporters for sensitive and selective sensing of Hg2+ in real water samples. As we known, the pKsp of HgSe, namely, the solubility product constant, reaches 64.5, whereas those of Cu2+, Cu+, Pb2+, Ag+, Co2+, Zn2+, Cd2+, Ni2+, Fe2+, Sn2+, and Mn2+ are ca. 49.0, 60.8, 38.0, 53.7, 31.2, 31.0, 35.2, 32.7, 26.0, 38.4, and 26.0 respectively.35,36 We envision the ultrahigh affinity between Hg2+ and Se2− as an available character for the corresponding assay applications. As Hg2+ ions are introduced into Cu2‑xSe NP solution, they can replace the Cu+/Cu2+ ions of the NPs and form a HgSe layer around the host particles because of lower solubility of HgSe material. As a result, the Cu2‑xSe NPs’ absorbance at 400−600 nm is increased; and the NIR LSPR band dramatically decreases with a more than 150 nm bathochromic shift. Thus, the system possesses triple signal responses, namely, ratiometry, wavelength, and intensity, to the analytes simultaneously. Such uniquely multiple signal output not only provides more choices for the quantification, but also enhances the reliability in the analyte detection. By rational choice of poly(allylamine hydrochloride) (PAH) as the particle templates, Hg2+ can be determined ranging from 0 to 800 nM. The detection limit is as low as 2.7 nM, which is nearly 4 times lower than the limit value (10 nM) defined by the U.S. Environmental Protection Agency (EPA) for drinking water. In addition to high sensitivity, the system possesses excellent selectivity. Various heavy/transition metal ions do not interfere with the sensing (the slight interference of Fe3+ can be eliminated by the masking effect of F− ions). Especially, both tap water and common environmental water samples can be assayed only by a double dilution. Due to favorable analytical performance, the proposed Cu2‑xSe NPs based system has potential applications in monitoring trace Hg2+ in various real samples, even for drinking water.

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EXPERIMENTAL SECTION

Materials. Selenious dioxide (SeO2) was pursed from Aladdin. PAH (MW: 15 000) was obtained from Sigma-Aldrich. Ascorbic acid (Vc), CuSO4·5H2O, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polystyrenesulfonate (PSS, MW: 70 000), Li2CO3, AgNO3, Pb(NO3)2, CuSO4·5H2O, MnCl2·4H2O, B

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Figure 2. (A) Absorption spectra of the PAH-templated Cu2‑xSe NPs in the absence and presence of 10 μM Hg2+. (B) DLS data of the Cu2‑xSe in the absence (top) and presence (bottom) of 10 μM Hg2+. (C) HRTEM image of the Cu2‑xSe NPs in the presence of 10 μM Hg2+. (D) SAED pattern recorded from a single particle after addition of 10 μM Hg2+. fitted by the polynomial fitting procedure.39,40 The corresponding fitted data are shown in Figures S7 and S8 (Supporting Information).

element is detected in the Hg2+ added Cu2‑xSe NPs. This result indicates that the analyte should react with the NPs and form certain mercury-containing products. Then, we tried to employ the HRTEM technique to further investigate the interactions of Hg2+ and the Cu2‑xSe NPs. As shown in Figure 2C, in the presence of 10 μM Hg2+, a new kind of crystal lattice with 0.351 nm is observed around the host NPs, whose lattice plane spacing just corresponds to the (111) facet of cubic HgSe. Furthermore, there is an angle of approximately 160° between the new-formed crystal lattice and the original one, which probably results from the stress effect of the two materials. The HRTEM result indicates that a hybrid Cu2‑xSe@HgSe core@ shell structure is generated in the presence of Hg2+ ions. The formation of HgSe is further confirmed by SAED. As described in Figure 2D, two sets of SAED pattern are distinctly observed in the products, which demonstrates the formation of the hybrid core@shell structure. Herein, the formation of a HgSe layer is attributed to the cation exchange effect.42 It is known that the solubility of HgSe is much lower than that of CuSe and Cu2Se. Due to the higher affinity, Hg2+ ions replace the sublattice Cu+/Cu2+ ions and form more stable HgSe. The absorption spectrum changes of the Cu2‑xSe NPs (Figure 2A) result from the formation of an additional HgSe layer by the cation exchange reaction. First, the absorbance at 400−600 nm is obviously enhanced because of the band edge absorption of the newly formed HgSe.43 The band gap of bulk HgSe material is 0.35 eV,44 which substantially blue shifts due to a quantum confinement effect. Then, the decrease in the LSPR intensity results from the interactions of the charge carriers within the host Cu2‑xSe particles and the newly formed HgSe layer, respectively. As a kind of n-type semiconductor material,45 HgSe possesses affluent free electrons, which annihilate the holes of the Cu2‑xSe NPs by electron−hole combinations. Finally, the HgSe layer formed around the Cu2‑xSe NPs cause the red shift of the LSPR band because its refractive index is larger than that of water (2.72 vs 1.33).46 Factors Affecting the Colorimetric Sensing of Hg2+. The present Hg2+-induced absorption band changes are promising for corresponding sensing applications. For better analytical performance, the experimental conditions were then optimized. We first studied the effects of organic templates on Hg2+ sensing. Three other polymers/surfactants, namely, CTAB, SDS, and PSS, were employed for the synthesis of Cu2‑xSe NPs. As shown in Figure S5 (Supporting Information), the three resulting kinds of NPs are also quasi-spherical and monodisperse, and their diameters are 16, 33, and 40 nm, respectively. Although the absorption spectra of the four Cu2‑xSe NPs can be changed by Hg2+ ions, the degrees of response are rather different, especially in the presence of trace

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RESULTS AND DISCUSSION Characterization of the PAH-Templated Cu2‑xSe NPs. The large-scale SEM and TEM images of the as-prepared Cu2‑xSe NPs are shown in Figure 1A and B, respectively, which demonstrates that the obtained NPs are quasi-spherical, welldispersed, and monodisperse. Their diameter is 80 nm, and the size distribution is only 6%. The HRTEM image in Figure 1C shows that the lattice spacing distance is 0.333 nm, corresponding to that of the (111) facet of cubic Cu2‑xSe. The highly consistent lattice fringe indicates that the products probably possess monocrystalline structure. The SAED image (inset of Figure 1C) has regular, symmetric, and well-defined diffraction spots, which further demonstrates the monocrystal nature of the products. Furthermore, the observed spots can be indexed to the reflections of (111), (200), and (220) of cubic Cu2‑xSe facets, respectively (inset of Figure 1C). The XRD pattern (Figure S1 in Supporting Information) shows several clear peaks at 26.75°, 31.03°, 44.60°, 52.91°, 64.98°, and 71.59°, which match the (111), (200), (220), (311), (400), and (331) reflections of cubic Cu2‑xSe (JCPDS no. 06−0680).41 As shown in Figure 1D, the Cu2‑xSe NPs exhibit two distinct absorption bands located at 400−600 and 600−1300 nm, respectively. The first one has no absorption peak but only a shoulder at 450 nm, which is ascribed to direct band gap absorption. In contrast, the second one is broad and has a definite peak at 920 nm. According to the literature,25−27 this NIR band is the LSPR of the NPs arising from the copper deficiency. The as-prepared Cu2‑xSe NPs are well dispersed in water and form homogeneous green solution (inset of Figure 1D), and it can stabilize for more than 2 weeks in ambient conditions at 4 °C. Interactions of the PAH-Templated Cu2‑xSe NPs with Hg2+. As Hg2+ ions are introduced into the Cu2‑xSe solution, the absorbance at 400−600 nm is increased; at the same time, the NIR band exhibits an intensity decrease with an obvious bathochromic shift. Such a wavelength shift can reach 158 nm as 10 μM Hg2+ is added (Figure 2A). The absorption changes indicate that the interactions of Hg2+ ions and the Cu2‑xSe NPs alter one or more parameters of the particles (size, shape, composition, dispersed states, etc.).25−31 To demonstrate this point, several characterizations have been conducted. As shown in TEM images (Figure S2 in Supporting Information), the size and shape of the Cu2‑xSe NPs remain almost invariable in the presence of Hg2+. Then, the in situ DLS test demonstrates that Hg2+ ions do not cause aggregation of the NPs, which have dispersed well all along (Figure 2B). Based on EDS and XPS (Figures S3 and S4 in Supporting Information), mercury C

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Figure 3. Absorption spectra of the CTAB- (A), SDS- (B), PSS- (C), and PAH- (D) templated Cu2‑xSe NPs in the presence of various concentrations of Hg2+. (E) LSPR absorbance of the four Cu2‑xSe NPs templated by different surfactants in the presence of lower concentrations of Hg2+. (F) Schematic illustration of the proposed interaction procedures of the PAH-templated Cu2‑xSe NPs and Hg2+ ions.

amounts of Hg2+. As shown in Figure 3A−D, as the concentrations of the added Hg2+ ions range from 0 to 100 nM, only the PAH-templated NPs exhibit apparent responses (Figure 3E). As shown in Figure 1A and Figure S5 (Supporting Information), the PAH-templated Cu2‑xSe NPs are 80 nm in diameter, which are larger than the other three ones. So, the more remarkable response does not originate from their higher surface-to-volume ratio. In terms of the other three NPs, their interactions with Hg2+ ions can be considered as follows: First, Hg2+ ions diffuse from homogeneous bulk solution onto the particle surface; then, these Hg2+ ions react with the NPs and exchange their Cu+/Cu2+ ions. As the concentrations of Hg2+ are lower, the diffusion effect cannot make enough Hg2+ ions approach/attach to particle surface. So, the exchange reaction does not happen and the signal responses are correspondingly not observed. However for PAH-templated NPs, the situations are different. In addition to the diffusion effect, large amounts of amino groups around the particles can coordinate with Hg2+ ions,47,48 which forcibly “pull” Hg2+ ions from bulk solution onto the particle surface (Figure 3F). This “enriching” effect makes Hg2+ ions more efficiently react with Cu2‑xSe NPs and results in higher sensitivity. The effects of pH value were then investigated. As shown in Figure 4A, the signal responses are weaker at acidic or basic conditions, while neutral medium (pH = 7.0) is most suitable for the sensing. At pH < 7.0, −NH2 groups protonize and form −NH3+. So, their coordination with Hg2+ ions is correspondingly weaker and leads to the decrease of the “enriching” effect. In contrast, at higher pH conditions, the increased OH− anions probably interact with Cu2+/Cu+ ions around the particle surface, which prevents the Hg2+ exchange reaction. So, pH 7.0 is chosen for the sensing. Finally, the response time was studied. As shown in Figure 4B, the interaction of the Cu2‑xSe NPs and Hg2+ is rather fast, which can reach a balance within 15 min. This result demonstrates that the present method is time-saving. Analytical Performances for Hg2+ Sensing. Under the optimized conditions discussed above, the linear response range of the sensing system was measured. As shown in Figure 3D, with the increase of the added Hg2+ ions, the intensity of the absorption band at 400−600 nm is gradually enhanced, while

Figure 4. Effects of pH values (A) and incubation time (B) on the sensing of Hg2+. The concentration of the added Hg2+ was 2 μM.

the NIR LSPR absorbance decreases step by step with a regular red shift. So, the present sensing system exhibits triple signal readout, namely, ratiometry, wavelength, and intensity, which are further expressed by Figure 5A,B,C, respectively. Furthermore, all three signals, namely, A920, A450/A920 and (Δλ) can be employed for the quantification of Hg2+ from 0 to 800 nM. Because A920 vs the concentrations of Hg2+ possesses the best linear relation (R = −0.995), we choose it for the analyte quantification. On the other hand, wavelength shift and/or ratiometric signal outputs cause more striking and identified changes in the profiles of the absorption spectra, which are especially significant for qualitative analysis. To assess the selectivity of the present method for Hg2+ ions, the effects of common metal ions, including Cu2+, Ag+, Pb2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Cr3+, Fe2+, Ti4+, Au3+, Ba2+, Sn2+, Al3+, Bi3+, Sr2+, and Fe3+, on the sensing were first examined. As described above, the pKsp values of CuSe and Cu2Se reach 49.0 and 60.8, respectively, which leads to the high stability of the Cu2‑xSe NPs. So, these cations have very little effect on the NPs absorption except for Fe3+ (Figure 6). As shown in Figure 6C D

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Figure 5. (A) Plot of A920 (A), A450/A920 (B), and Δλ (C) vs the concentrations of Hg2+ by the Cu2‑xSe NP based system. Each inset is the corresponding linear plot as a function of Hg2+ concentration (0−800 nM); R = −0.995 (A), R = 0.992 (B), R = 0.991 (C).

ions is considerably weaker than that of Hg2+ ions, and the potential interference can be easily eliminated by the masking effect of F− ions (green curve in Figure 6C). We then investigated the interference of eight common ions in environmental water samples. As shown in Figure 6B, these ions have very little effect on the absorption of the NPs. These control experiments indicate that the proposed method has excellent selectivity for Hg2+ sensing. Sensitivity is one of key factors for a sensing system. Toward the Hg2+ assay, the detection limit is particularly significant because the target analyte is extremely toxic. As we know, the WHO and U.S. EPA define the maximum allowable level of inorganic mercury in drinking water to be no more than 30 and 10 nM, respectively. The detection limit of the present system is 2.7 nM (signal-to-noise ratio of 3), which is more sensitive than most gold NPs based sensing ones (Table S1 in Supporting Information).17−22,49−69 Obviously, the choice of PAH as the template is critical to the high sensitivity (Figure 3). It is noted that several Au NPs based systems have comparable and even higher sensitivity; however, considerable parts of them do not play a “real” role in practical applications. For example, in terms of refs 70 and 71, the concentrations of the assayed Hg2+ in real samples are 50−1000 nM and 10−45 nM, despite that their detection limits are only 2.8 and 0.6 nM, respectively.70,71 For several other systems, however (Table S2 in Supporting Information),72−74 the original water samples are diluted 6−20 times in the assay, leading to the lower significance of the assay results. In contrast, the present Cu2‑xSe NPs based platform is more “practical”. Hg2+ content can be quantified even when their concentrations are as low as 10 nM in original water samples, because the system can work well only if the water samples are simply treated with a double dilution (from 0.85 to 1.70 mL) (see below). Real Sample Assay. To evaluate the applicability of the colorimetric assay to real samples, various water samples, namely, drinking water (tap water) and environmental water samples (pond water and river water), were tested, respectively. As shown in Figure 7, the absorption spectra of the Cu2‑xSe NPs remains almost invariable in the presence of various water samples. As Hg2+ ions spiked, whether 10 or 30 nM, the NPs’ absorptivity at 400−600 nm is increased, and the LSPR band decreases with an observable bathochromic shift (green and blue curves in Figure 7). Such characteristic modulations of the absorption profiles indicate that the analyte is definitely detected. Furthermore, for all three water samples, the recovery rates are 90−97%, as 10 and 30 nM Hg2+ spiked, respectively (Table 1). Due to excellent analytical performance (high sensitivity and selectivity), the proposed system is competent for monitoring Hg2+ content in drinking water, even with regard to the standard of U.S. EPA.

Figure 6. Absorption responses of the Cu2‑xSe NPs to some transition/heavy metal ions (A) and eight common ions in environmental water samples (B). The concentrations of all the investigated ions are 1 μM. In (A), the “Mix” contains all the cations listed in the histogram except for Hg2+, and the concentration of each ion is also 1 μM. Effects of Fe3+ and FeF63− on the Cu2‑xSe NPs’ absorption spectra (C). The concentrations of Fe3+ and F− are 1.0 and 6.0 μM, respectively.

(red curve), the absorption profile of the Cu2‑xSe NPs shows a little change in the presence of Fe3+ (1 μM), indicating slight interference. In terms of the interactions of Fe3+ ions and the Cu2‑xSe NPs, two points should be noted. First, the LSPR intensity of the NPs is enhanced by Fe3+ ions, which is just opposite that of Hg2+ and indicates a probably different interaction mechanism. Then, the absorption response to Fe3+ E

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Figure 7. Sensing of Hg2+ in tap (A), pond (B), and river (C) water samples by the PAH-templated Cu2‑xSe NPs.

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Table 1. Hg2+ Assay in Real Water Samples sample

added Hg2+ (nM)

proposed method (nM)

recovery (%)

1

0 10 30 0 10 30 0 10 30

not found 9.4 29.0 not found 9.2 28.6 not found 9.0 28.2

/ 94 97 / 92 95 / 90 94

2

3

Corresponding Author

*E-mail: [email protected]. Fax: +86-553-3869303. Tel: +86-553-3869303. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 21275001 and 21422501).

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REFERENCES

(1) Nolan, E. M.; Lippard, S. J. Tools and tactics for the optical detection of mercuric ion. Chem. Rev. 2008, 108, 3443−3480. (2) Zahir, F.; Rizwi, S. J.; Haq, S. K.; Khan, R. H. Environ. Low dose mercury toxicity and human health. Environ. Toxicol. Pharmacol. 2005, 20, 351−360. (3) Onyido, I.; Norris, A. R.; Buncel, E. Biomolecule-mercury interactions: modalities of DNA base-mercury binding mechanisms. Remediation strategies. Chem. Rev. 2004, 104, 5911−5929. (4) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. Review: Environmental exposure to mercury and its toxicopathologic implications for public health. Environ. Toxicol. 2003, 18, 149−175. (5) Burin, G. J.; Becking, G. C. The World-Health-Organization (WHO) guidelines for drinking-water quality: a global perspective on trace contaminants of dinking-water. Trace Substances in Environmental Health  XXIV 1991, 207−219. (6) Aragay, G.; Pons, J.; Merkoçi, A. Recent trends in macro-, micro-, and nanomaterial-based tools and strategies for heavy-metal detection. Chem. Rev. 2011, 111, 3433−3458. (7) Wei, T.; Dong, T.; Wang, Z.; Bao, J.; Tu, W.; Dai, Z. Aggregation of individual sensing units for signal accumulation: conversion of liquid-phase colorimetric assay into enhanced surface-tethered electrochemical analysis. J. Am. Chem. Soc. 2015, 137, 8880−8883. (8) Qvarnström, J.; Lambertsson, L.; Havarinasab, S.; Hultman, P.; Frech, W. Determination of methylmercury, ethylmercury, and inorganic mercury in mouse tissues, following administration of thimerosal, by species-specific isotope dilution GC-inductively coupled plasma-MS. Anal. Chem. 2003, 75, 4120−4124. (9) Li, Q.; Zhou, X.; Xing, D. Rapid and highly sensitive detection of mercury ion (Hg2+) by magnetic beads-based electrochemiluminescence assay. Biosens. Bioelectron. 2010, 26, 859−862. (10) Zhu, X.; Zhou, X.; Xing, D. Ultrasensitive and selective detection of mercury(II) in aqueous solution by polymerase assisted fluorescence amplification. Biosens. Bioelectron. 2011, 26, 2666−2669. (11) Liu, X.; Tang, Y.; Wang, L.; Zhang, J.; Song, S.; Fan, C.; Wang, S. Optical detection of mercury(II) in aqueous solutions by using conjugated polymers and label-free oligonucleotides. Adv. Mater. 2007, 19, 1471−1474. (12) Lin, Y. H.; Tseng, W. L. Ultrasensitive sensing of Hg2+ and CH3Hg+ based on the fluorescence quenching of lysozyme type VIstabilized gold nanoclusters. Anal. Chem. 2010, 82, 9194−9200. (13) Wang, Z.; Ma, L. Gold nanoparticle probes. Coord. Chem. Rev. 2009, 253, 1607−1618.

CONCLUSIONS In summary, a new platform is presented for sensitive and selective colorimetric sensing of Hg2+ in drinking and environmental water samples based on NIR plasmonic Cu 2‑x Se NPs. Compared with conventional gold NP dispersion/aggregation based sensing systems, the proposed one is not only simpler, more cost-effective, and robust, but it also has multiple signal output. Due to high sensitivity and excellent anti-interference capacity, it can quantify Hg2+ content even when their concentrations are as low as 10 nM in various water samples. The present contribution, on one hand, provides a feasible method for Hg2+ assay and, on the other hand, demonstrates that semiconductor plasmonic Cu2‑xSe NPs are promising colorimetric reporters for sensing design and applications.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00275. Figure S1: Powder XRD pattern of the PAH-templated Cu2‑xSe NPs. Figures S2−S4: TEM, EDS, and XPS data of the PAH-templated Cu2‑xSe NPs before and after addition of 10 μM Hg2+. Figure S5: SEM images and size distributions of the CTAB-, SDS-, PSS-, and PAHtemplated Cu2‑xSe NPs. Absorption responses of the PAH-templated Cu2‑xSe NPs to Hg2+ ions at different pH. Tables S1−S2: Comparing the analytical performances of Hg2+ sensing with Au NP based colorimetric systems. Figure S7: The original, fitted and the overlapped absorption spectra of the CTAB-, SDS-, PSS-, and PAH-templated Cu2‑xSe NPs in the presence of different concentrations of Hg2+ ions. Figure S8: The original, fitted, and overlapped absorption spectra of the PAH-templated Cu2‑xSe NPs in real sample assay. (PDF) F

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