Exploiting the Higher Specificity of Silver Amalgamation: Selective

Aug 13, 2013 - In our proposed approach, Hg2+ detection is achieved by reducing the ... signal increase relative to the fluorescence without Hg2+ ions...
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Exploiting the Higher Specificity of Silver Amalgamation: Selective Detection of Mercury(II) by Forming Ag/Hg Amalgam Li Deng,† Xiangyuan Ouyang,† Jianyu Jin,† Cheng Ma,† Ying Jiang,† Jing Zheng,† Jishan Li,† Yinhui Li,† Weihong Tan,†,‡ and Ronghua Yang*,† †

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, China ‡ Center for Research at the Bio/Nano Interface, Department of Chemistry, and Department of Physiology and Functional Genomics, Shands Cancer Center and UF Genetics Institute, University of Florida, Gainesville, Florida, 32611-7200, United States S Supporting Information *

ABSTRACT: Heavy metal ion pollution poses severe risks in human health and the environment. Driven by the need to detect trace amounts of mercury, this article demonstrates, for the first time, that silver/mercury amalgamation, combining with DNA-protected silver nanoparticles (AgNPs), can be used for rapid, easy and reliable screening of Hg2+ ions with high sensitivity and selectivity over competing analytes. In our proposed approach, Hg2+ detection is achieved by reducing the mercury species to elemental mercury, silver atoms were chosen as the mercury atoms’ acceptors by forming Ag/Hg amalgam. To signal fluorescently this silver amalgamation event, a FAMlabeled ssDNA was employed as the signal reporter. AgNPs were grown on the DNA strand that resulted in greatly quenching the FAM fluorescence. Formation of Ag/Hg amalgam suppresses AgNPs growth on the DNA, leading to fluorescence signal increase relative to the fluorescence without Hg2+ ions, as well as marked by fluorescence quenching. This FAM fluorescence enhancement can be used for detection of Hg2+ at the a few nanomolar level. Moreover, due to excellent specificity of silver amalgamation with mercury, the sensing system is highly selective for Hg2+ and does not respond to other metal ions with up to millimolar concentration levels. This sensor is successfully applied to determination of Hg2+ in tap water, spring water and river water samples. The results shown herein have important implications in the development of new fluorescent sensors for the fast, easy, and selective detection and quantification of Hg2+ in environmental and biological samples.

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teins,20 and genetically engineered bacteria,21,22 have been actively pursued for providing a real-time Hg2+ determination. Despite significant contributions have been made to the Hg2+ assay, it should be noted that the molecular recognition step of these approaches is always based on interactions between the S-, N-, or O-functional groups of the acceptor molecules and Hg2+ through covalent chemical reactions or metal coordinations. They are either limited with respect to sensitivity and selectivity, long reaction time, or incompatible with aqueous environments. As a unique feature, mercury atoms can complex with certain metal atoms to form amalgams.23−25 Among them, Ag/Hg amalgam is of special interest with respect to the electronic and optical properties of the bimetallic structure.26−28 In the past decades, silver amalgam electrodes are intensively sought to substitute the convenient mercury electrode material for electrochemical detections of various targets from proteins and DNAs, to small molecules and metal ions.23−25,29,30 In addition, it has been reported that adsorption of Hg to silver or gold particles surfaces shifted the surface plasmon resonance

ontamination of the environment with heavy metal ions has been an important concern throughout the world for decades. The water-soluble mercuric ion (Hg2+) is an example of highly toxic and widespread pollutants, its damage to the brain, nervous system, endocrine system and even the kidneys is well-known.1,2 Therefore, these environmental and health problems of Hg2+ have prompted researchers to develop efficient methods for selective and sensitive assay of the metal to understand its distribution and pollution potential. Toward this goal, instrumental analysis methods, such as cold vapor technique with atomic absorption spectrometry,3,4 inductively coupled plasma atomic emission spectrometry,5 inductively coupled plasma mass spectrometry,6,7 surface-enhanced Raman scaterring,8,9 and impedance spectrometry,10,11 etc., have been established. Although these methods encompass high sensitivity, the bulky instrumentation and long-term sample pretreatment processes limits their use for on-site analysis, bringing the shortcomings of being complicated, costly, and time-consuming. To alleviate these problems and simplify sample preparation and instrumentation costs, development of new approaches for the handiness and quickness detection and quantification of Hg2+ ions is highly desirable. In the past decades, a variety of Hg2+ sensing systems, based upon organic molecules,12,13 oligonucleotides,14−16 DNAzymes,17−19 pro© XXXX American Chemical Society

Received: April 16, 2013 Accepted: August 13, 2013

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Scheme 1. Schematic Illustration of Fluorescent Detection of Hg2+ Ions by Forming Ag/Hg Amalgam that Suppresses AgNPs Growth on a Dye-Labeled DNA Scaffold (FAM-DNA)

spectroscopy of the nanoparticles to shorter wavelengths.27,28 On the basis of this feature, ionic or elemental mercury species was determined by adsorption of mercury vapor to gold and silver surfaces.27,31,32 Therefore, this unique property of silver atom would provide us a potential acceptor for mercury atoms by forming Ag/Hg amalgam. On the other hand, nucleic acids which present N- and O-functional groups to coordinate with heavy metal ions offer an advanced approach to prepare silver nanostructures through a DNA-protected process.33−35 DNAtemplated silver nanostructures are a recently developed class of fluorophore that are particularly responsive to optical modulation of their emission.36,37 Specifically, if a dye-labeled DNA is used to form AgNPs, emission from the fluorescent tag should be quenched by the nanoparticles,38 making the DNAfunctionalized silver nanostructures attractive materials for biosensors design.36−38 Inspired by the findings mentioned above, and as a continuation of our studies on Hg2+ fluorescent sensors designs based on oligonucleotides,16,39,40 we would present herein a new methodology, by integrating the silver amalgamation and the formation of DNA-protected AgNPs, to realize facile and rapid fluorescence detection of Hg2+ ions with high selectivity and sensitivity. In our proposed approach, the Hg2+ detection is achieved by forming Ag/Hg amalgam upon reduction of the mercury species to elemental mercury, along with the silver atoms as the mercury atoms’ acceptors. To signal fluorescently this silver amalgamation event, a dye-labeled ssDNA was employed as the signal reporter. Scheme 1 shows the schematic illustration of our design, which is based on (1) the ability of AgNPs that are grown on dye-labeled ssDNA to effectively quench the emission of fluorescent tag and (2) competitive complex of mercury atoms and silver atoms with the ssDNA suppressing AgNPs formation on the DNA strand, thus resulting in fluorescence signal increments relative to the fluorescence without Hg2+ ions. The strong quenching efficiency of DNA-templated AgNPs, combined with the high specificity of silver amalgamation and the simplicity of detection, provides a new and efficient approach for developing a rapid and handy alternative for to the monitoring of Hg2+ in environmental or biological samples.

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CTAA-3′ and FAM-P2, 5′-FAM-CCTTTAACCTTTAACCTTTAACCTTTAA-3′) were synthesized by Shanghai Sangon Biological Technology & Services Co., Ltd., China. All sequences were dissolved in highly pure water (sterile Millipore water, 18.3 MΩ) as stock solutions. The concentrations of the DNA stock solutions were estimated by UV absorption using published sequence-dependent absorption coefficients.41 All metal ion stock solutions were prepared from nitrate salts. All other reagents were of analytical reagent grade and were used without further purification or treatment. All work solutions were prepared with 0.1 mM Tris-HNO3 buffer solution (pH 7.2) UV−vis absorption spectra were obtained on a Hitachi U4100 UV−vis spectrophotometer (Kyoto, Japan). Fluorescence measurements were recorded on a Hitachi F-7000 fluorescence spectrofluorometer. Transmission electron microscopy (TEM) was performed on a JEOL JEM-3010. Energy-dispersive X-ray (EDX) spectra were obtained using the TEM microscope. pH was measured by a model 868 pH meter (Orion). ICP-MS experiments were performed on X series II. The samples for TEM analysis were prepared by pipetting 10 μL of the colloidal DNA/AgNPs conjugates or Ag/Hg amalgam solutions onto standard holey carbon-coated copper grids, and the grids were dried in air for about 12 h. Then the grids were loaded into the vacuum chamber of the electron microscope for detection. These samples were not subjected to heavy metal staining or other treatments. Synthesis of DNA-Templated AgNPs. The DNAtemplated AgNPs were prepared using a literature procedure.37 Aliquot of stock solution of AgNO3 was transferred into a 1.0 mL volumetric pipe, a certain volume of FAM-labeled DNA (FAM-P1) stock solution was added. The mixed solution was diluted to 1.0 mL with the Tris-HNO3 buffer. The final concentrations of FAM-P1 and AgNO3 in the buffer were 5.0 μM and 800 μM, respectively. The solution was incubated at 0 °C for 15 min to form the DNA/Ag+ complex, and was stored at 4 °C for further usage. To synthesize DNA-templated AgNPs, aliquot of 10 μL of the DNA/Ag+ mixture solution was added to a 500 μL volumetric pipe with Tris-HNO3 buffer. Then 10 μL NaBH4 (1 mM) was added to the solution with slightly shaking. After incubation for 20 min at room temperature, the DNA-templated AgNPs were formed. The size and composition of nanoparticles were characterized by TEM and EDX. Fluorescent Detection of Hg2+. For detection of Hg2+ in buffer or real water samples, 5 μL of the as-prepared DNA/Ag+

EXPERIMENTAL SECTION

Chemicals and Apparatus. AgNO3(99.99%), Hg(NO3)2 (99.999%), and NaBH4 (99.99%) were purchased from Sigma Aldrich and used as received. The dye-labeled oligonucleotides (FAM-P1, 5′-FAM-CCCCTAACCCCTAACCCCTAACCCB

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mixture were added to a 500 μL volumetric pipe containing 465 μL Tris-HNO3 buffer/water sample. Then 10 μL Hg(NO3)2 solution of various concentrations was added. After incubating for 10 min with gentle shaking, 20 μL NaBH4 (1 mM) were added to the mixture. The samples were incubated for 10 min at room temperature before fluorescence measurement. The spring water were taken from Yuelu mountain in Changsha, the river water were obtained from Xiang river in Changsha. Gel Electrophoresis. The gel was prepared containing 3% agarose and 1 × TBE, pH 8.0. Twenty microliters of different reaction products ([FAM-P1] = 1.0 μM) containing 15% glycerol were added to each lane. The gel was run at 100 V for 60 min. The running buffer also contained 1 × TBE. The photograph was taken by BIO-RAD Molecular Imager (ChemiDoc XRS + imaging system) under UV-Trans model after exposure for 2.0 s.

Figure 2. UV−vis absorption (A) and fluorescence emission (B) spectra of FAM-P1/Ag+ induced by NaBH4. The spectra were collected in the Tris-HNO3 buffer solution containing: (a) FAM-P1 + AgNO3, (b) a + NaBH4, and (c) a + Hg(NO3)2 + NaBH4. Inset: Photographs of the corresponding absorption and fluorescent species. [FAM-P1] = 50 nM, [AgNO3] = [Hg(NO3)2] = 8.0 μM, [NaBH4] = 20 μM. λex = 480 nm.

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RESULTS AND DISCUSSION To demonstrate the feasibility of our design, a 28-mer Ag+specific ssDNA (FAM-P1) was employed as the Ag+ scaffold for forming DNA-templated AgNPs.33 To produce a fluorescence signal, 6-carboxyfluorescein (FAM) was labeled on the 5′-end of the DNA strand. To ensure saturation of all possible DNA binding sites, a defined stoichiometry of AgNO3 and FAM-P1 (160: 1) in the Tris-HNO3 buffer was mixed that causes slight quenching of the FAM-P1 fluorescence. After reducing the FAM-P1/Ag+ complex with NaBH4, AgNPs were formed on FAM-P1 (FAM-P1/AgNPs). As characterized by TEM, the FAM-P1/AgNPs were shown to be uniform and monodispersed nanoparticles with a size distribution from 6.0 to 12.0 nm and an average size of 10.0 nm (Figure 1A). EDX

was collected upon addition of AgNPs. As shown in Figure S3A (SI), addition of increasing amounts of colloidal AgNPs to the solution of FAM-P1 also resulted in fluorescence quenching, but the maximal quenching efficiency is around 35% as shown in Figure S3B (SI), which is significantly lower than the DNAtemplated AgNPs under the same condition. Next, we speculated that specific interaction between Ag atoms and Hg atoms would weaken the strength of the interaction between Ag+ and DNA and thus suppress the formation of DNA-templated AgNPs. To support this hypothesis, we carried out NaBH4 reduction of FAM-P1/Ag+ in the presence of Hg2+ ions. The TEM images reveal that the presence of Hg2+ during the reducing process leads to the formation of irregular particles with wide size distributions from 5.0 to 22.0 nm (Figure 1B). The plasmon absorption band of aggregates shifts to shorter wavelengths, while the area of the absorption band becomes greater related to plasmon absorption band of DNA-templated AgNPs (Figure 2A,c), indicating the formation of bimetallic particles of silver and mercury.27,28 Figure 2A,c also shows only one absorption band is present, implying that no monometallic particles were present. The structural composition of the particles by estimating the atom contents from EDX spectra revealed the coexistence of Ag and Hg, no N, O, and P elements appeared (Figure S1B, SI), suggesting that the Ag/Hg particles were formed out of the DNA strand. In addition, elemental analysis indicates the Ag/ Hg particles contained mercury and silver in the ratio about Hg/Ag = 2: 1 (Table S2, SI), suggesting the particle structure is not “core-shell” type but alloys of silver and mercury.28 Figure 2B,c shows the fluorescence emission spectrum of FAM-P1/Ag+ when Ag+ ions and Hg2+ ions were reduced simultaneously by NaBH 4 . As expected, the FAM-P1 fluorescence emission is retained by the presence of Hg2+. In the presence of 8.0 μM Hg2+, the FAM-P1 fluorescence intensity at 518 nm is 31-fold higher than that without Hg2+. To test whether this fluorescence enhancement is due to the interaction of Hg2+ ion with FAM-P1/AgNPs, we first obtained FAM-P1/AgNPs and then added increasing concentrations of Hg2+ ions to the colloid solution of FAM-P1/AgNPs (Figure S4, SI). Although Hg2+ ions induced FAM-P1/AgNPs fluorescence increase, the enhancement factor is greatly smaller than that of Figure 2B,c. In the presence of 10.0 μM Hg2+, the fluorescence intensity of FAM-P1/AgNPs at 518 nm is 5.7-fold higher than that without of Hg2+ ions. These results demonstrate it is the interaction of silver and mercury to

Figure 1. TEM images of FAM-P1/AgNPs (A) and Ag/Hg amalgams (B). The nanoparticles were obtained by NaBH4 reducing FAM-P1/ Ag+ in the absence and the presence of Hg(NO3)2.

spectroscopy analysis shows the coexistence of Ag, O, N and P elements (Figure S1A and Table S1, see Supporting Information (SI)). The color of FAM-P1/AgNPs exhibits yellow color with a maximal absorption band centered at ∼400 nm (Figure 2A), which is attributable to the surface plasmon resonance of small AgNPs.36,42 The fluorescence spectrum of FAM-P1/Ag+ in Tris-HNO3 buffer shows strong fluorescence emission upon excitation at 480 nm, owing to the presence of the FAM-based dye. However, the FAM fluorescence was almost quenched when NaBH4 was added to the solution FAM-P1/Ag+. Figure 2B reveals that approximate 96% decrease of the FAM-P1 emission at the 518-nm peak. Higher quenching efficiency can be achieved by further increase of the concentrations Ag+ and NaBH4 (Figure S2, SI). To find out whether the fluorescence quenching is due to formation of DNA-templated AgNPs, the FAM-P1 fluorescence spectrum C

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form Ag/Hg amalgam that weakens the interaction between Ag+ and FAM-P1, which, in turn, suppresses the AgNPs growth on the DNA strand. We used agarose gel electrophoresis to confirm this hypothesis. As shown in Figure 3, FAM-P1/Ag+ appears as a

Figure 4. Real-time fluorescence recordings of the Ag+ complexes of FAM-P1 (A) and FAM-P2 (B) upon addition of NaBH4 without (red trace) or with 8.0 μM Hg2+ ions (blue trace). For each spectrum, we distinguished three steps: (1) the cuvette was filled with 500 μL TrisHNO3 buffer containing 50 nM of the fluorescent DNA and 8.0 μM AgNO3; (2) 8.0 μM Hg(NO3)2 was introduced in the cuvette; and (3) 20.0 μM NaBH4 was added. The transition between each regime is marked with an arrow. Excitation was at 480 nm, and emission was monitored at 518 nm. Figure 3. Gel electrophoresis of the reaction products of FAM-P1with Ag+ and NaBH4. From left to right: Lane 1, FAM-P1 + AgNO3; lane 2, FAM-P1 + AgNPs; lane 3, FAM-P1 + AgNO3 + NaBH4; lane 4, FAMP1 + AgNO3 + Hg(NO3)2 + NaBH4. Lane 5 is the reaction product of FAM-P2 + AgNO3 + Hg(NO3)2 + NaBH4. For performance of the gel electrophoresis, aliquots of 10 μL of each sample containing 15% glycerol were loaded onto a 3% agarose gel and electrophoresed at 100 V for 60 min at room temperature.

thymine to Hg2+ promotes the formation of Ag/Hg amalgams on the DNA template. This hypothesis is further confirmed by the agarose gel electrophoresis, as shown in Figure 3. Finally, to evaluate the validity of this approach for Hg2+ sensing, the FAM-P1/Ag+ complex was titrated with varying concentrations of Hg2+ ions and, subsequently, certain amount of NaBH4, and the kinetics of FAM fluorescence change at 518 nm was then monitored (Figure 5). In the presence of Hg2+,

sharp, fast-moving band along with strong fluorescence under UV excitation (lane 1). The mixture of FAM-P1 with AgNPs behaves in a manner similar to FAM-P1 when run on a gel (lane 2). However, when FAM-P1/Ag+ was reduced by NaBH4, the conjugates move significantly slower than FAM-P1 and appear as continuous smears with very weak visible fluorescence (lane 3). On the other hand, the reaction products of FAM-P1, Ag+, Hg2+, and NaBH4 exhibit a fast-moving band with strong fluorescence (lane 4). This result is similar to that of FAM-P1 + AgNPs when run on a gel. Taken together, this evidence suggests that the formation of DNA-templated AgNPs could lower the mobility of the DNA strands, whereas the formation of an Ag/Hg amalgam causes the interaction between the DNA and nanoparticles to weaken. Since Hg2+ can selectively bind in between two DNA thymine (T) bases and promote these T−T mismatches to form stable T−Hg2+−T base pairs,43−45 we reasoned the response behavior of the approach to Hg2+ ions would be DNA sequences dependent. To test this hypothesis, we incorporated an Hg2+-specific binding element into FAM-P1 by replacing the cytosine bases with thymine bases (FAM-P2). Real-time kinetics of interactions of the FAM−P1/Ag+ and FAM−P2/ Ag+ with NaBH4 in the absence and the presence of Hg2+ ions were then measured by recording the FAM fluorescence emission as a function of time. Figure 4 shows that both the fluorescence emissions of FAM-P1 and FAM-P2 are almost completely quenched by NaBH4 within a few seconds, and the quenching efficiency is suppressed by the presence of Hg2+. It is worth noting that, for a given Ag+ and Hg2+ concentration, the measured fluorescence enhancement strongly depends on the DNA sequences. Thus, in the presence of 8.0 μM Hg2+, around 86% of the FAM-P1 fluorescence is retained; however, only ∼17% of the FAM-P2 fluorescence is retained under the same conditions. The lower fluorescence enhancement of FAM-P2 relative to FAM-P1 indicates the specific binding ability of

Figure 5. Real-time fluorescence records of FAM-P1/Ag+ in the TrisHNO3 solution upon addition of different concentrations of Hg2+, separately, and subsequently NaBH4. For each spectrum, we distinguished three steps: (1) the cuvette was filled with 500 μL Tris-HNO3 buffer solution containing 50 nM FAM-P1 and 8.0 μM AgNO3, (2) Hg2+ ions were introduced in the cuvette, and (3) 20.0 μM NaBH4 was added. The transition between each regime is marked with an arrow. Excitation was at 480 nm, and emission was monitored at 518 nm.

the FAM-P1fluorescence intensity increased in a time-dependent manner, exhibiting a rapid increase in the first 5 min, followed by more gradual increase within 5−10 min. Higher concentrations of Hg2+ ions correspondingly produced higher rates of emission enhancement. Figure 6 reveals that the fluorescence emission is sensitive to the nanomolar concentration of Hg2+ ions and that a dramatic increase of fluorescence intensity is observed for the Hg2+ concentrations in the range of 10.0 nM to 8.0 μM. The limit of detection (LOD) was estimated to be 2.6 nM, as calculated with the following equation: LOD = 3σ/k, where k is the slope of the calibration curve, and σ is the standard deviation of the blank solution of 6 measurements. Such a LOD is lower than (at least comparable D

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S2− and other thiol compounds, could interact with Hg2+ or Ag+ ions to influence the formation of Ag/Hg amalgam, resulting in reduced fluorescence response toward Hg2+ ions, other anions, such as Cl− and I−, can influence the detection performance(reducing quenching efficiency of DNA-templated AgNPs) by forming precipitates with Ag+. However, this drawback could be lessen by adjusting the concentration of Ag+. With excellent selectivity and sensitivity in buffer solution, the proposed method was employed for Hg2+ detection with real environmental water samples to evaluate its practical application. Tap water, spring water and river water samples were chosen as the environmental water samples, for tap water, chlorine was eliminated by boiling the sample for 10 min, for river water, the sample collected was first filtered through a column (packed with an anionic−exchange resin) to remove oils and other organic impurities, and for spring water, no extra pretreatment. All the water samples were spiked with Hg2+ at concentrations of 0, 0.01, 0.5, and 4.0 μM, and then analyzed by the proposed method and compared with ICP-MS (inductively coupled plasma mass spectrometry), which served as referential standard. The mercury in river water samples were 5 nM measured by ICP-MS, while the Hg2+ in tap and spring water samples could not be detected. The results are showed in Table 2, which reveal high consistency in determination of Hg2+ in

Figure 6. Dependence of fluorescence enhancement, F/F0, of the sensing system on the increasing concentrations of selected metal ions. Where F and F0 are the fluorescence intensities of FAM-P1/Ag+ in the presence of NaBH4 with and without containing metal ions, respectively. The magnitude of the error bars was calculated from the uncertainty given by three independent measurements. Excitation was at 480 nm, and emission was monitored at 518 nm.

with) those of the reported methods (Table 1) and the wide dynamic range undoubtedly improves the sensing performance. Above all, this LOD meets the maximum contamination level of Hg2+ in drinking water (is 2.0 ppb or ∼10 nM)), defined by the U.S. Environmental Protection Agency (EPA).46 To characterize Hg2+ specificity, fluorescence responses of the approach to other metal ions were assayed (Figure S5, SI). As expected, only Hg2+ showed a significantly higher fluorescence enhancement, whereas even millimolar concentrations of typical competing metal ions, such as K+, Na+, Ca2+, Mg2+, Ba2+, Cu2+, Zn2+, Fe2+, Co2+, Ni2+, Mn2+, Cd2+, Pb2+, Fe3+, Cr3+, and Al3+ showed far weaker responses than Hg2+ at μM levels (Figure 6). Cu2+ exhibits small fluorescence enhancement under the same condition because of the strong complex of Cu2+ ions with the oligonucleotide through the N7 atoms of guanine bases and the N3 atoms of cytosine bases, which decreases the strength of the interactions of Ag+ ions with the oligonucleotide. Additionally, titrating Hg2+ in the presence of the interfering ions (Mg2+, Ba2+, Zn2+, Fe2+, Co2+, Ni2+, Mn2+, Cd2+, Pb2+, Fe3+, Cr3+, Al3+, and Cu2+ (10 μM)) gave a response nearly equal to that obtained in the presence of Hg2+ alone (SI Figure S6 and Figure 6). These results clearly indicate that our approach is not only insensitive to other metal ions but also selective toward Hg2+ ions when they are present. However, it should be noted that substrates, such as a high concentration of

Table 2. Determination of Hg2+ in Water Samples Using the Proposed Method and ICP-MS Hg2+(μM) sample

added

tap water 1 tap water 2 tap water 3 tap water 4 river water 1 river water 2 river water 3 river water 4 spring water 1 spring water 2 spring water 3 spring water 4

0 0.010 0.500 4.000 0 0.010 0.500 4.000 0 0.010 0.500 4.000

proposed method meana ± SDb c 0.012 0.514 3.985 c 0.011 0.523 4.098 c 0.009 0.508 3.974

ICP-MS mean ± SD c 0.013 0.509 4.017 0.005 0.010 0.518 4.052 c 0.011 0.511 4.012

± 0.001 ± 0.040 ± 0.150 ± 0.002 ± 0.035 ± 0.176 ± 0.002 ± 0.047 ± 0.163

± ± ± ± ± ± ±

0.002 0.029 0.127 0.0008 0.002 0.040 0.187

± 0.001 ± 0.043 ± 0.154

a

Mean of three determinations. bSD, standard deviation. cNo Hg2+ concentration could be detected.

Table 1. Comparison of Assay Methods for Monitoring Mercury strategy AAS/sequential injection ICP-AES ICP-MS/cold vapor organic molecule/Hg2+-promoted cyclization of thiosemicarbazide T−Hg2+−T/FRET T−Hg2+−T/catalysis T−Hg2+−T/DNAzyme T−Hg2+−T/AuNPs T−Hg2+−T/AuNPs silver amalgamation a

target

dynamic range (nM)

detection limit (nM)

atomic absorption atomic emission mass spectrometry fluorescence, turn on

Hg2+ Hg2+ Hg2+ Hg2+

45−1575 3.5−1000 a 100−12000

45 3.5 4.0 100

3 5 6 13

fluorescence, turn off fluorescence, turn on ABTS absorption AuNPs absorption AuNPs absorption/fluorescence, turn on fluorescence, turn on

Hg2+ Hg2+ Hg2+ Hg2+ Hg2+

40−100 20−200 a 100−2000 96−6400

40 2.4 1.0 100 40

14 17 18 15 16

Hg2+

10−8000

2.6

detection signal

ref

this work

No reported results. E

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environmental water samples by present approach and conventional instrument, demonstrating the excellent performace of this sensor in practical application. In summary, the specific silver/mercury interaction to form Ag/Hg amalgam was first utilized for fluorescence detection of Hg2+ ions by employing dye-labeled ssDNA as the signal reporter. The present approach can be engineered in ways that offer unique advantages and capabilities that are not available from conventional Hg2+ sensing systems. First of all, the approach can detect a few nM Hg2+ ions, but it is silent to other metal ions with up to millimolar concentration levels. Its excellent selectivity results from the well-known silver amalgamation process that occurs specifically between silver and mercury atoms; while the outstanding sensitivity is contributed to the low background fluorescence of dye-labeled DNA/AgNPs conjugate. Second, this approach is simple and convenient, without specific designing of the Hg2+ recognition probe, requiring only the mixing of several solutions at room temperature to detect Hg2+ in just a few minutes. This simple, rapid and cost-effective process successfully achieves reliable monitoring of toxic mercury from environmental water samples, which is comparable to conventional Hg2+ ions sensing approach(ICP-MS). Finally, the strategy is versatile and it can be extended to develop a variety of sensor by employing not only different types of amalgamations processes but also different types of signal reporters. We envision that the designing principles presented here could open up new opportunities for the design and engineering of nanodevices for sensor applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-731-88822523. Fax: 86-731-88822523. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National Natural Science Foundation of China (21075032, 21005026, 21135001, and J1103312), ‘‘973’’National Key Basic Research Program (2011CB91100-0). We also sincerely appreciate Dr. Yi Lu and Dr. Hui Min Li for their kindly help with manuscript preparation and revision.

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REFERENCES

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dx.doi.org/10.1021/ac401408m | Anal. Chem. XXXX, XXX, XXX−XXX