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Single gold nanoparticle-based colorimetric detection of picomolar mercury ion with dark-field microscopy Xiaojun Liu, Zhangjian Wu, Qingquan Zhang, Wenfeng Zhao, Chenghua Zong, and Hongwei Gai Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03653 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016
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Single gold nanoparticle-based colorimetric detection of picomolar mercury ion with dark-field microscopy
Xiaojun Liu, Zhangjian Wu, Qingquan Zhang, Wenfeng Zhao, Chenghua Zong, Hongwei Gai* Jiangsu Key Laboratory of Green Synthesis for Functional Materials, School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, Jiangsu, 221116, China.
*Corresponding author. Email: [email protected] Fax: 86-516-83536972
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Abstract Mercury severely damages the environment and human health, particularly when it accumulates in the food chain. Methods using colorimetric detection of Hg2+ have been increasingly developed over the past decade because of the progress in nanotechnology. However, the limit of detections (LODs) of these methods are mostly either comparable to or higher than the allowable maximum level (10 nM) in drinking water set by the US Environmental Protection Agency. In this study, we report a single Au nanoparticle (AuNP)-based colorimetric assay for Hg2+ detection in a solution. AuNPs modified with oligonucleotides were fixed on the slide. The fixed AuNPs bound with free AuNPs in the solution in the presence of Hg2+ because of oligonucleotide hybridization. This process was accompanied with the color changing from green to yellow as observed under an optical microscope. The ratio of changed color spots corresponded with Hg2+ concentration. The LOD was determined as 1.4 pM, which may help guard against mercury accumulation. The proposed approach was applied to environmental samples with recoveries of 98.3% ± 7.7% and 110.0% ± 8.8% for Yuquan River and industrial waste water, respectively.
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Mercury pollution has drawn global concern because mercury can severely damage the environment and human health even at low concentration. 1,2 Therefore, monitoring trace amount of mercury is highly important. Various techniques have been exploited to quantify Hg2+. The analysis techniques are atomic fluorescence spectrophotometry, 3 atomic absorption spectrometry, 4 circular dichroism, 5 inductively coupled plasma mass spectrometry, 6 atomic force microscope, 7 surface plasmon resonance, 8 quartz crystal microbalance, 9 magnetic resonance imaging, 10 surface-enhanced Raman scattering, 11 and electrochemical-based analytical techniques. 12,13 These techniques require complex and expensive hardware, well-trained operators, and/or dedicated sample treatments which are usually costly, labor-intensive, and time-consuming. Fluorescent and colorimetric sensors for Hg2+ have recently drawn increasing attention because of their simplicity, novelty, and no requirement of advanced instruments. These sensors are similar in many aspects. 14–16 Both sensors are composed of two necessary components, namely, recognition and signal transducer moiety. The core strategy is that recognition interaction induces detectable change in spectrum of transducer moiety. The transducer is basically either organic fluorescent compound or nanomaterials, in particular, Au nanoparticle (AuNP). AuNP has advantages over organic fluorescent compound because of its convenient synthesis in aqueous solution, high solubility in water, high molar extinction coefficient, strong photostability, tunable optical properties, etc. These characteristics have been summarized in several reviews. 14–17 Thus, AuNP-based colorimetric detection for Hg2+ is more attractive. AuNP-based Hg2+ recognition reaction either generally directly bridges polymeric network-like aggregation or induces disaggregation of AuNPs. Aggregation or disaggregation will remarkably alter localized surface plasmon resonance (LSPR) spectrum of AuNPs caused by the electric dipole–dipole interaction between the neighboring nanoparticle plasmons, accompanied with color change. An example of this phenomenon is Hg2+ bond with poly(c-glutamic acid) that was adsorbed onto the gold colloids. The bindings cross-linked AuNPs, the color of which turned to purple blue from red. 18 Another example, Hg2+ broke the AuNP aggregation induced by 4-mercaptobutanol (4-MB) because Hg2+ had higher affinity to 4-MB and displaced AuNP. 19 Table 1 summarizes recent developments in determining Hg2+ by AuNPs. The types of functional ligands on the AuNPs were used to classify the recognition moieties into three groups, namely, oligonucleotides, 20–25 oligopeptides, 26–28 and the rest. 18, 29–36 Oligonucleotides are complementary DNA strands, except T–T mismatches. These molecules can form a DNA duplex via T-Hg2+-T coordination. Oligonucleotides are designed flexibly and constructed as specific probes toward Hg2+.
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The probes exhibit LODs of 0.025 nM to 250 nM. Meanwhile, for oligopeptides, certain block units, such as Lys, has affinity to Hg2+. LODs of this group vary between 26 nM and 20 µM. The third group includes alkanethiols and derivatives of mercaptoaliphatic acid and urea with LODs of 0.5 nM to 100 nM. All aforementioned studies were performed at ensemble level and cannot achieve the ultimate detection sensitivity of chemical assays, that is, to count the number of single molecules without any further chemical amplification or enriched steps. We used our previous studies on wide-field single-particle imaging 37–39 to develop further the single-particle counting technique to quantify trace amount of mercury ion in water. In our studies, the two oligonucleotides with three T–T mismatched bases hybridized in the presence of Hg2+, which resulted in AuNP aggregation. The color changed from green to yellow under dark-field microscopy observation. The calculated aggregation occurrence is related to Hg2+ concentration as shown by the counted number of yellow AuNPs. LOD was measured to be 1.4 pM. The proposed approach was applied successfully to determine Hg2+ in environmental samples. The recoveries were 98.3% ± 7.7% and 110.0% ± 8.8% for river water and industrial waste water, respectively. EXPERIMENTAL SECTION Chemical and material AuNPs of 70 nm in diameter were obtained from Nano Partz Inc. (Loveland, CO). The nanoparticle was protected with citrate groups. The concentration of AuNPs was 1.6 × 1010/mL. DNA oligonucleotides were synthesized by Sangon Biotechnology Inc. (Shanghai, China). Their base sequences were designed as follows: oligonucleotide 1 [5′SH(CH2)6TCAGTTTGGC3′] and its T–T mismatched base oligonucleotide 2 [5′SH(CH2)6GCCTTTCTGA3′]. The oligonucleotides were dissolved in 10 mM Tris buffer (pH 7.5), and the concentration of the stock solutions was 100 µM. 3-Mercaptopropyl triethoxysilance was obtained from Sigma (St. Louis, MO). Cover slip was purchased from Fisher Scientific (Hanover Park, IL). Other chemicals were obtained from local vendor and used without further purification. Fabrication of Hg2+ detection sensors The whole fabrication of Hg2+ detection sensors was divided into two steps. Figure 1 shows that the AuNPs were firstly immobilized onto the glass slide, and the citrate groups of AuNPs were replaced with oligonucleotide 1. In the first step, the glass slides were modified with thiol groups according to the reported method. 37 Then, the slides were immersed into 15 mL of AuNP solution in a custom-designed beaker. The glass slide stands vertically in the beaker because the size of the beaker is slightly bigger than the width of the glass slide. The same kind of beaker was used in the following experiment. This immobilization was allowed to continue for 30 min followed by extensively washing the glass slide with distilled water. In the second
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step, the glass slide was immersed into an aliquot (15 mL) of 10 nM oligonucleotide 1 solution. The ligand exchange lasted for 30 min. The sensor was washed with 10 mM Tris buffer (pH 7.4), dried at room temperature, and stored at 4 °C until use. Detection of Hg2+ at single-particle level Hg2+ was captured via T-Hg2+-T binding prior to Hg2+ determination at single-particle level (Figure 1). The stored sensor slide was placed into 15 mL of standard Hg2+ solution or real water sample containing 1 nM oligonucleotide 2 in 20 °C water bath. Hg2+ could be adsorbed onto the AuNPs because of the hybridization of oligonucleotides 1 and 2. The reaction proceeded for 30 min. The glass slide was then consecutively washed with 5× SSC and 0.1× SSC. The remaining solution on the surface was sucked away with a water pump, and 2.5 µL of AuNP solution at concentration of 1.6×109/mL in 2.5 mM Tris buffer (pH 7.4) was added. The glass slide was immediately covered with a cover slip. The sensor was ready for observation under dark-field microscopy. The detection was performed using an Olympus IX71 microscope. The numerical aperture of 100× oil immersion objective was adjusted to 0.6. The microscope was equipped with two CCDs, namely, an EMCCD (Evolve 512; Photometrics, Tucson, USA) and a colorful CCD (QIClickTM; QImaging, Surrey, BC, Canada). The temperature, gain, and exposure time of the EMCCD camera were maintained at −80 °C, 16, and 0.3 s, respectively. A transmission grating with 70 lines/mm was purchased from Edmund Scientific (Barrington, NJ). Image J software was used to process data. Hg2+ induced AuNP aggregation in bulk solution Oligonucleotide 1 (90 nM), oligonucleotide 2 (90 nM), and Hg2+ standard solution (90 nM) or distilled water were mixed at a volume ratio of 1:1:1. The mixture was then stored at 20 °C water bath for 30 min. The AuNPs, with a diameter of 80 nm, were added into this solution at a volume ratio of 1:1. Afterward, 10 µL of the solution was carefully placed onto a grid for transmission electron microscopy (TEM). After 10 min, the solution was drained with a filter paper. The deposition of AuNPs onto the grid was repeated thrice. The TEM was conducted using a FEI Tecnai G2 T12 (USA). Environmental sample preparation Yuquan River water and industry waste water were used as model environmental samples. The environmental samples were spiked with standard Hg2+ solution and then mixed with the stock solution of the oligonucleotide 2 after they were filtered with 0.22 µm membrane. The final concentration of the oligonucleotide 2 was 1 nM. Hg2+ was quantitated following the above mentioned procedure. RESULTS AND DISCUSSION Sensing mechanism
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The core of the proposed approach was to set up evaluation criteria under dark-field microscopy to evaluate whether AuNPs aggregated. Figure 1 shows that the oligonucleotide 1 (blue) attached on the AuNPs sitting onto the glass slide hybridizes with the partial complementary oligonucleotide 2 (grey) with the aid of T-Hg2+-T bindings. The double-strand DNA segment behaves as a rigid rod, 40 which caused the thiol group of the oligonucleotide 2 to stick out. Therefore, the subsequently added AuNP could be captured by the fixed AuNP through S-Au binding. When 1 µM Hg2+ solution was used, 100% of AuNPs were yellow, whereas 98.3% of AuNPs were green without Hg2+, indicating that the color of the aggregated AuNPs changes from green to yellow under dark-field microscopy. We further used transmission grating-based spectral imaging dark-field microscopy to characterize LSPR spectra of AuNP, which has been detailed in our previous studies. 37–39 A transmission grating before EMCCD splitted and diffracted light scattered from an object into two beams travelling in different directions as shown in Figure 2. The dot-shaped light corresponding to direct transmission is referred to as the zeroth order dot, and the streak-shaped light deviating from direct transmission is referred to as the first-order streak. The scattering wavelength of the object (λ) is calculated as follows: λ = L × d/S where S, d, and L represent the distance between the grating and EMCCD chip, grating constant, and distance between the zeroth order dot and the first-order streak, respectively. For a given optical setup, the spectrum was obtained by determining the distance between the zeroth order dot and the first-order streak belonging to an individual nanoparticle, which was started from matching the zeroth order dot with its corresponding first-order streak. Figure 3 compares the images of the same location of the Hg2+ sensor incubated with 1.0 nM Hg2+ captured by colorful CCD and EMCCD. In Figure 3A, the zeroth order dot and the first-order streak split from an individual AuNP or follow-up aggregation are labeled with triangles in the same color. The others are unpaired because the corresponding zeroth order dots/the first order streaks are out of range of the EMCCD. The colorful image of the same area is shown in Figure 3B to determine whether the AuNPs in the black and white image are aggregated. Using color as criterion, the AuNPs numbered 0, 2, and 6 are aggregated. Figure 3C shows the typical scattering spectra of the aggregated and non-aggregated AuNPs. The LSPR peak wavelengths, which were obtained by averaging 20 spectra, were 579.1 ± 1.2 and 564.0 ± 2.1 nm for aggregated and non-aggregated AuNPs, respectively. These values fall into the range of yellow and green, respectively, which is consistent with our deduction that the color of the aggregated AuNPs changes from green to yellow. AuNP aggregation induced by Hg2+ was conducted in bulk solution and analyzed by TEM. Figure 4 illustrates the typical transmission electron
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micrograph in the absence and presence of Hg2+. Thus, statistical results from 100 particles showed that 30 nM Hg2+ resulted in 99.0% aggregation of AuNPs, in contrast to 2.22% aggregation of AuNPs without Hg2+. The aggregated AuNPs appeared brighter and larger in all the dark-field microscopy images, which is attributed to the increase in intensity caused by LSPR coupling of the aggregated AuNPs. Sensing performance for Hg2+ detection Standard Hg2+ solutions at different concentrations were analyzed to evaluate the performance of our method. The number of yellow AuNPs continuously increased in Figures 5 A–D, suggesting an increase in AuNP aggregation with increasing concentration of Hg2+. Quantitative results were obtained by plotting the aggregation occurrence percentage of the AuNPs against the logarithm of Hg2+ concentration in Figure 5 E. A linear dependence of the aggregation occurrence percentage on the Hg2+ quantity was found in the range of 0.005 nM to 25.0 nM. Aggregation occurrence percentage reached saturation (>98%) when Hg2+ concentration was above 25 nM, indicating that AuNPs aggregated almost completely. Meanwhile, the aggregation percentage was 3.0% ± 0.8% at 0.005 nM Hg2+, which was higher than the blanket value of 1.7% ± 0.8%. Therefore, the maximum lowest detectable concentration of Hg2+ is 0.005 nM. The LOD of our approach is estimated to be 1.4 pM at a signal-to-noise of three σ. Tables 1 and S1 summarize the recent development of Hg2+ detection by Au nanosphere-based colorimetric assays and by a wide array of techniques. The LOD of our method is at least 1–5 and 1–2 orders of magnitudes lower than those of the reported colorimetric methods using Au nanosphere 20–36 and those of the common methods involving ICP/MS, atomic fluorescence spectrophotometry, and atomic absorption spectrometry. 3,4, 41–43 Selectivity and stability The selectivity of our method was investigated by testing the sensor response to other environmentally relevant metallic ions, such as K+, Ag+, Ca2+, Zn2+, Cd2+, Co2+, Fe2+, Ni2+, Pb2+, Mn2+, and Al3+. Figure 6 shows that 99.3% ± 1.5% of AuNPs were aggregated in the presence of 25 nM Hg2+, whereas less than 7.7% of AuNPs were aggregated in the presence of the other metallic ions (25 nM). This result indicates that significant interference was not obtained from the other metal ions. The stability of our sensors was assessed by monitoring the performance of the sensors stored at 4 °C. The aggregation occurrence percentage of the sensors did not fluctuate within 18 days in Figure 7, indicating the stability of the fabricated sensors. We did not test the sensors stored longer than 18 days. Environmental sample detection River and waste water were collected from Yuquan River in our campus and Xuzhou
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Industry District. Hg2+ was not detected in both samples. Recovery experiments were performed using Hg2+-spiked Yuquan River and waste water. The final concentration of spiked Hg2+ was 0.3 nM. The recoveries of spiked Hg2+ were 98.3% ± 7.7% and 110.0% ± 8.8% for the Yuquan River and industry wastewater, respectively. The good recovery reveals that our method was reliable in determining trace amount of Hg2+ in environmental water samples. Conclusion We developed a single nanoparticle-based colorimetric approach to determine Hg2+ under dark-field microscopy. The LOD and dynamic range were 1.4 pM and between 0.005 and 25.0 nM, respectively. The analysis of the real environmental samples showed the suitability and reliability of our approach in environment control. Acknowledgements The authors are grateful to the Natural Science Foundation of China (NSFC 21405064, 21575053), the Jiangsu Province Natural Science Foundation of China (BK20140233), and Priority Academic Program Development of Jiangsu Higher Education Institutions. References (1) Bonzongo, J. C. J.; Heim, K. J.; Chen, Y. A.; Lyons, W. B.; Warwick, J. J.; Miller, G. C.; Lechler, P. J. Environ. Toxicol. Chem. 1996, 15, 677-683. (2) Wolfe, M. F.; Schwarzbach, S.; Sulaiman, R. A. Environ. Toxicol. Chem. 1998, 17, 146-160. (3) Edwards, S. C.; Macleod, C. L.; Corns, W. T.; Williams, T. P.; Lester, J. N. Intern. J. Environ. Anal. Chem. 1996, 63, 187-193. (4) Lopez-Garcia, I.; Rivas, R. E.; Hernandez-Cordoba, M. Anal. Chim. Acta. 2012, 743, 69-74. (5) Yan, W. J.; Wang, Y. J.; Zhuang, H.; Zhang, J. H. Biosens. Bioelectron. 2015, 68, 516-520. (6) Rodriguez-Reino, M. P.; Rodriguez-Fernandez, R.; Pena-Vazquez, E.; Dominguez-Gonzalez, R.; Bermejo-Barrera, P.; Moreda-Pineiro, A. J. Chromatogr. A 2015, 1391, 9-17. (7) Zhang, T.; Cheng, Z. G.; Wang, Y. B.; Li, Z. G.; Wang, C. X.; Li, Y. B.; Fang, Y. Nano Lett. 2010, 10, 4738-4741. (8) Zhang, H. Y.; Yang, L. Q.; Zhou, B. J.; Liu, W. M.; Ge, J. E. C.; Wu, J. S.; Wang, Y.; Wang, P. F. Biosens. Bioelectron. 2013, 47, 391-395. (9) Wang, M. H.; Liu, S. L.; Zhang, Y. C.; Yang, Y. Q.; Shi, Y.; He, L. H.; Fang, S. M.; Zhang, Z. H. Sensor. Actuat. B-Chem. 2014, 203, 497-503. (10) Liang, G. H.; Zhang, P.; Li, H. X.; Zhang, Z. Y.; Chen, H.; Zhang, S.; Kong, J. L. Anal. Chim. Acta. 2012, 726, 73-78.
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Captions of figures and table 2+
Figure 1. Scheme of Hg sensor at single-particle level with dark-field microscopy Figure 2. Scheme of transmission grating-based dark-field spectral imaging microscopy Figure 3. Comparison of colorful and spectral imaging at the presence of 1 nM Hg2+. (A) Transmission grating-based spectral imaging by EMCCD. The zeroth order dot and first-order streak split from an individual Au nanoparticle(AuNP)/aggregation are pointed by same color triangles; (B) Corresponding dark-field microscopy image by colorful CCD; (C) Representative spectra of aggregated and non-aggregated AuNPs in red and blue, respectively. Figure 4. Typical transmission electron micrographs of the sensors without Hg2+ (A) and 30 nM Hg2+ (B) Figure 5. Images of AuNPs at different concentrations of Hg2+. From (A) to (D) Hg2+ concentrations are 0.01, 0.1, 1.0, and 50 nM, respectively. Scale bar is 6 µm in length. (E) Plots of AuNP aggregation percentage against the logarithm of Hg2+ concentration. Figure 6. Selectivity of the sensor for Hg2+ over other metal ions. The concentration is 25 nM for all metal ions. Figure 7. Stability of the sensor for 1.0 nM Hg2+ detection. Table 1 Performance summary of representative metal nanoparticle-based colorimetric detection of Hg2+
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Figure 1
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Figure 7
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Table 1 Probes
LOD (nM)
Selectivity
Mechanism
Real Sample
References
DNA-AuNPs
30
Hg2+
Aggregation
Xiang River water
20
DNA-AuNPs
17.3
Hg2+
Aggregation
Tap water, river, 21 and lake water
DNA-AuNPs
250
Hg2+
Aggregation
No
22
DNA-AuNPs
0.025
Hg2+
Aggregation
No
23
DNA/AuNPs
0.6
Hg2+
Aggregation
Tap and lake water
24
2+
DNA/AuNPs
250
Hg
Aggregation
No
25
Peptide-AuNPs
20,000
Hg2+
Aggregation
No
26
Peptide-AuNPs
26
Aggregation
No
27
Acid protien-AuNPs
1,000
Hg2+
Aggregation
No
28
Thiocyanuric acid-AuNPs
0.5
Hg2+
Anti-aggregation
Tap and lake water
29
Diethyldithiocarbamate-AuNPs
10
Hg2+
Aggregation
Drinking water
33
2+
Hg2+, Co2+, Pd4+,
Pt2+
Pd2+,
Citrate-AuNPs
2.9
Hg
Aggregation
Tap water
32
2,2'-Bipyridyl-AuNPs
38
Hg2+
Anti-aggregation
Jialing River water
30
Poly (γ-glutamic acid)-AuNPs
1.9
Hg2+
Aggregation
Tap and mineral 18 water
Thymine derivative-AuNPs
0.8
Hg2+
Aggregation
No
34
Quaternary ammonium-AuNPs
30
Hg2+
Aggregation
Drinking water
35
Cys-AuNPs
100
Hg2+
Aggregation
No
31
Thiourea-AuNPs
193
2+
Hg , Ag
Aggregation
No
36
DNA-AuNPs
0.0014
Hg2+
Aggregation
+
River and industry In our study waste water
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Single gold nanoparticle-based colorimetric detection of picomolar mercury ion with dark-field microscopy Xiaojun Liu, Zhangjian Wu, Qingquan Zhang, Wenfeng Zhao, Chenghua Zong, and Hongwei Gai Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03653 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016
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Single gold nanoparticle-based colorimetric detection of picomolar mercury ion with dark-field microscopy
Xiaojun Liu, Zhangjian Wu, Qingquan Zhang, Wenfeng Zhao, Chenghua Zong, Hongwei Gai* Jiangsu Key Laboratory of Green Synthesis for Functional Materials, School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, Jiangsu, 221116, China.
*Corresponding author. Email: [email protected] Fax: 86-516-83536972
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Abstract Mercury severely damages the environment and human health, particularly when it accumulates in the food chain. Methods using colorimetric detection of Hg2+ have been increasingly developed over the past decade because of the progress in nanotechnology. However, the limit of detections (LODs) of these methods are mostly either comparable to or higher than the allowable maximum level (10 nM) in drinking water set by the US Environmental Protection Agency. In this study, we report a single Au nanoparticle (AuNP)-based colorimetric assay for Hg2+ detection in a solution. AuNPs modified with oligonucleotides were fixed on the slide. The fixed AuNPs bound with free AuNPs in the solution in the presence of Hg2+ because of oligonucleotide hybridization. This process was accompanied with the color changing from green to yellow as observed under an optical microscope. The ratio of changed color spots corresponded with Hg2+ concentration. The LOD was determined as 1.4 pM, which may help guard against mercury accumulation. The proposed approach was applied to environmental samples with recoveries of 98.3% ± 7.7% and 110.0% ± 8.8% for Yuquan River and industrial waste water, respectively.
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Mercury pollution has drawn global concern because mercury can severely damage the environment and human health even at low concentration. 1,2 Therefore, monitoring trace amount of mercury is highly important. Various techniques have been exploited to quantify Hg2+. The analysis techniques are atomic fluorescence spectrophotometry, 3 atomic absorption spectrometry, 4 circular dichroism, 5 inductively coupled plasma mass spectrometry, 6 atomic force microscope, 7 surface plasmon resonance, 8 quartz crystal microbalance, 9 magnetic resonance imaging, 10 surface-enhanced Raman scattering, 11 and electrochemical-based analytical techniques. 12,13 These techniques require complex and expensive hardware, well-trained operators, and/or dedicated sample treatments which are usually costly, labor-intensive, and time-consuming. Fluorescent and colorimetric sensors for Hg2+ have recently drawn increasing attention because of their simplicity, novelty, and no requirement of advanced instruments. These sensors are similar in many aspects. 14–16 Both sensors are composed of two necessary components, namely, recognition and signal transducer moiety. The core strategy is that recognition interaction induces detectable change in spectrum of transducer moiety. The transducer is basically either organic fluorescent compound or nanomaterials, in particular, Au nanoparticle (AuNP). AuNP has advantages over organic fluorescent compound because of its convenient synthesis in aqueous solution, high solubility in water, high molar extinction coefficient, strong photostability, tunable optical properties, etc. These characteristics have been summarized in several reviews. 14–17 Thus, AuNP-based colorimetric detection for Hg2+ is more attractive. AuNP-based Hg2+ recognition reaction either generally directly bridges polymeric network-like aggregation or induces disaggregation of AuNPs. Aggregation or disaggregation will remarkably alter localized surface plasmon resonance (LSPR) spectrum of AuNPs caused by the electric dipole–dipole interaction between the neighboring nanoparticle plasmons, accompanied with color change. An example of this phenomenon is Hg2+ bond with poly(c-glutamic acid) that was adsorbed onto the gold colloids. The bindings cross-linked AuNPs, the color of which turned to purple blue from red. 18 Another example, Hg2+ broke the AuNP aggregation induced by 4-mercaptobutanol (4-MB) because Hg2+ had higher affinity to 4-MB and displaced AuNP. 19 Table 1 summarizes recent developments in determining Hg2+ by AuNPs. The types of functional ligands on the AuNPs were used to classify the recognition moieties into three groups, namely, oligonucleotides, 20–25 oligopeptides, 26–28 and the rest. 18, 29–36 Oligonucleotides are complementary DNA strands, except T–T mismatches. These molecules can form a DNA duplex via T-Hg2+-T coordination. Oligonucleotides are designed flexibly and constructed as specific probes toward Hg2+.
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The probes exhibit LODs of 0.025 nM to 250 nM. Meanwhile, for oligopeptides, certain block units, such as Lys, has affinity to Hg2+. LODs of this group vary between 26 nM and 20 µM. The third group includes alkanethiols and derivatives of mercaptoaliphatic acid and urea with LODs of 0.5 nM to 100 nM. All aforementioned studies were performed at ensemble level and cannot achieve the ultimate detection sensitivity of chemical assays, that is, to count the number of single molecules without any further chemical amplification or enriched steps. We used our previous studies on wide-field single-particle imaging 37–39 to develop further the single-particle counting technique to quantify trace amount of mercury ion in water. In our studies, the two oligonucleotides with three T–T mismatched bases hybridized in the presence of Hg2+, which resulted in AuNP aggregation. The color changed from green to yellow under dark-field microscopy observation. The calculated aggregation occurrence is related to Hg2+ concentration as shown by the counted number of yellow AuNPs. LOD was measured to be 1.4 pM. The proposed approach was applied successfully to determine Hg2+ in environmental samples. The recoveries were 98.3% ± 7.7% and 110.0% ± 8.8% for river water and industrial waste water, respectively. EXPERIMENTAL SECTION Chemical and material AuNPs of 70 nm in diameter were obtained from Nano Partz Inc. (Loveland, CO). The nanoparticle was protected with citrate groups. The concentration of AuNPs was 1.6 × 1010/mL. DNA oligonucleotides were synthesized by Sangon Biotechnology Inc. (Shanghai, China). Their base sequences were designed as follows: oligonucleotide 1 [5′SH(CH2)6TCAGTTTGGC3′] and its T–T mismatched base oligonucleotide 2 [5′SH(CH2)6GCCTTTCTGA3′]. The oligonucleotides were dissolved in 10 mM Tris buffer (pH 7.5), and the concentration of the stock solutions was 100 µM. 3-Mercaptopropyl triethoxysilance was obtained from Sigma (St. Louis, MO). Cover slip was purchased from Fisher Scientific (Hanover Park, IL). Other chemicals were obtained from local vendor and used without further purification. Fabrication of Hg2+ detection sensors The whole fabrication of Hg2+ detection sensors was divided into two steps. Figure 1 shows that the AuNPs were firstly immobilized onto the glass slide, and the citrate groups of AuNPs were replaced with oligonucleotide 1. In the first step, the glass slides were modified with thiol groups according to the reported method. 37 Then, the slides were immersed into 15 mL of AuNP solution in a custom-designed beaker. The glass slide stands vertically in the beaker because the size of the beaker is slightly bigger than the width of the glass slide. The same kind of beaker was used in the following experiment. This immobilization was allowed to continue for 30 min followed by extensively washing the glass slide with distilled water. In the second
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step, the glass slide was immersed into an aliquot (15 mL) of 10 nM oligonucleotide 1 solution. The ligand exchange lasted for 30 min. The sensor was washed with 10 mM Tris buffer (pH 7.4), dried at room temperature, and stored at 4 °C until use. Detection of Hg2+ at single-particle level Hg2+ was captured via T-Hg2+-T binding prior to Hg2+ determination at single-particle level (Figure 1). The stored sensor slide was placed into 15 mL of standard Hg2+ solution or real water sample containing 1 nM oligonucleotide 2 in 20 °C water bath. Hg2+ could be adsorbed onto the AuNPs because of the hybridization of oligonucleotides 1 and 2. The reaction proceeded for 30 min. The glass slide was then consecutively washed with 5× SSC and 0.1× SSC. The remaining solution on the surface was sucked away with a water pump, and 2.5 µL of AuNP solution at concentration of 1.6×109/mL in 2.5 mM Tris buffer (pH 7.4) was added. The glass slide was immediately covered with a cover slip. The sensor was ready for observation under dark-field microscopy. The detection was performed using an Olympus IX71 microscope. The numerical aperture of 100× oil immersion objective was adjusted to 0.6. The microscope was equipped with two CCDs, namely, an EMCCD (Evolve 512; Photometrics, Tucson, USA) and a colorful CCD (QIClickTM; QImaging, Surrey, BC, Canada). The temperature, gain, and exposure time of the EMCCD camera were maintained at −80 °C, 16, and 0.3 s, respectively. A transmission grating with 70 lines/mm was purchased from Edmund Scientific (Barrington, NJ). Image J software was used to process data. Hg2+ induced AuNP aggregation in bulk solution Oligonucleotide 1 (90 nM), oligonucleotide 2 (90 nM), and Hg2+ standard solution (90 nM) or distilled water were mixed at a volume ratio of 1:1:1. The mixture was then stored at 20 °C water bath for 30 min. The AuNPs, with a diameter of 80 nm, were added into this solution at a volume ratio of 1:1. Afterward, 10 µL of the solution was carefully placed onto a grid for transmission electron microscopy (TEM). After 10 min, the solution was drained with a filter paper. The deposition of AuNPs onto the grid was repeated thrice. The TEM was conducted using a FEI Tecnai G2 T12 (USA). Environmental sample preparation Yuquan River water and industry waste water were used as model environmental samples. The environmental samples were spiked with standard Hg2+ solution and then mixed with the stock solution of the oligonucleotide 2 after they were filtered with 0.22 µm membrane. The final concentration of the oligonucleotide 2 was 1 nM. Hg2+ was quantitated following the above mentioned procedure. RESULTS AND DISCUSSION Sensing mechanism
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The core of the proposed approach was to set up evaluation criteria under dark-field microscopy to evaluate whether AuNPs aggregated. Figure 1 shows that the oligonucleotide 1 (blue) attached on the AuNPs sitting onto the glass slide hybridizes with the partial complementary oligonucleotide 2 (grey) with the aid of T-Hg2+-T bindings. The double-strand DNA segment behaves as a rigid rod, 40 which caused the thiol group of the oligonucleotide 2 to stick out. Therefore, the subsequently added AuNP could be captured by the fixed AuNP through S-Au binding. When 1 µM Hg2+ solution was used, 100% of AuNPs were yellow, whereas 98.3% of AuNPs were green without Hg2+, indicating that the color of the aggregated AuNPs changes from green to yellow under dark-field microscopy. We further used transmission grating-based spectral imaging dark-field microscopy to characterize LSPR spectra of AuNP, which has been detailed in our previous studies. 37–39 A transmission grating before EMCCD splitted and diffracted light scattered from an object into two beams travelling in different directions as shown in Figure 2. The dot-shaped light corresponding to direct transmission is referred to as the zeroth order dot, and the streak-shaped light deviating from direct transmission is referred to as the first-order streak. The scattering wavelength of the object (λ) is calculated as follows: λ = L × d/S where S, d, and L represent the distance between the grating and EMCCD chip, grating constant, and distance between the zeroth order dot and the first-order streak, respectively. For a given optical setup, the spectrum was obtained by determining the distance between the zeroth order dot and the first-order streak belonging to an individual nanoparticle, which was started from matching the zeroth order dot with its corresponding first-order streak. Figure 3 compares the images of the same location of the Hg2+ sensor incubated with 1.0 nM Hg2+ captured by colorful CCD and EMCCD. In Figure 3A, the zeroth order dot and the first-order streak split from an individual AuNP or follow-up aggregation are labeled with triangles in the same color. The others are unpaired because the corresponding zeroth order dots/the first order streaks are out of range of the EMCCD. The colorful image of the same area is shown in Figure 3B to determine whether the AuNPs in the black and white image are aggregated. Using color as criterion, the AuNPs numbered 0, 2, and 6 are aggregated. Figure 3C shows the typical scattering spectra of the aggregated and non-aggregated AuNPs. The LSPR peak wavelengths, which were obtained by averaging 20 spectra, were 579.1 ± 1.2 and 564.0 ± 2.1 nm for aggregated and non-aggregated AuNPs, respectively. These values fall into the range of yellow and green, respectively, which is consistent with our deduction that the color of the aggregated AuNPs changes from green to yellow. AuNP aggregation induced by Hg2+ was conducted in bulk solution and analyzed by TEM. Figure 4 illustrates the typical transmission electron
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micrograph in the absence and presence of Hg2+. Thus, statistical results from 100 particles showed that 30 nM Hg2+ resulted in 99.0% aggregation of AuNPs, in contrast to 2.22% aggregation of AuNPs without Hg2+. The aggregated AuNPs appeared brighter and larger in all the dark-field microscopy images, which is attributed to the increase in intensity caused by LSPR coupling of the aggregated AuNPs. Sensing performance for Hg2+ detection Standard Hg2+ solutions at different concentrations were analyzed to evaluate the performance of our method. The number of yellow AuNPs continuously increased in Figures 5 A–D, suggesting an increase in AuNP aggregation with increasing concentration of Hg2+. Quantitative results were obtained by plotting the aggregation occurrence percentage of the AuNPs against the logarithm of Hg2+ concentration in Figure 5 E. A linear dependence of the aggregation occurrence percentage on the Hg2+ quantity was found in the range of 0.005 nM to 25.0 nM. Aggregation occurrence percentage reached saturation (>98%) when Hg2+ concentration was above 25 nM, indicating that AuNPs aggregated almost completely. Meanwhile, the aggregation percentage was 3.0% ± 0.8% at 0.005 nM Hg2+, which was higher than the blanket value of 1.7% ± 0.8%. Therefore, the maximum lowest detectable concentration of Hg2+ is 0.005 nM. The LOD of our approach is estimated to be 1.4 pM at a signal-to-noise of three σ. Tables 1 and S1 summarize the recent development of Hg2+ detection by Au nanosphere-based colorimetric assays and by a wide array of techniques. The LOD of our method is at least 1–5 and 1–2 orders of magnitudes lower than those of the reported colorimetric methods using Au nanosphere 20–36 and those of the common methods involving ICP/MS, atomic fluorescence spectrophotometry, and atomic absorption spectrometry. 3,4, 41–43 Selectivity and stability The selectivity of our method was investigated by testing the sensor response to other environmentally relevant metallic ions, such as K+, Ag+, Ca2+, Zn2+, Cd2+, Co2+, Fe2+, Ni2+, Pb2+, Mn2+, and Al3+. Figure 6 shows that 99.3% ± 1.5% of AuNPs were aggregated in the presence of 25 nM Hg2+, whereas less than 7.7% of AuNPs were aggregated in the presence of the other metallic ions (25 nM). This result indicates that significant interference was not obtained from the other metal ions. The stability of our sensors was assessed by monitoring the performance of the sensors stored at 4 °C. The aggregation occurrence percentage of the sensors did not fluctuate within 18 days in Figure 7, indicating the stability of the fabricated sensors. We did not test the sensors stored longer than 18 days. Environmental sample detection River and waste water were collected from Yuquan River in our campus and Xuzhou
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Industry District. Hg2+ was not detected in both samples. Recovery experiments were performed using Hg2+-spiked Yuquan River and waste water. The final concentration of spiked Hg2+ was 0.3 nM. The recoveries of spiked Hg2+ were 98.3% ± 7.7% and 110.0% ± 8.8% for the Yuquan River and industry wastewater, respectively. The good recovery reveals that our method was reliable in determining trace amount of Hg2+ in environmental water samples. Conclusion We developed a single nanoparticle-based colorimetric approach to determine Hg2+ under dark-field microscopy. The LOD and dynamic range were 1.4 pM and between 0.005 and 25.0 nM, respectively. The analysis of the real environmental samples showed the suitability and reliability of our approach in environment control. Acknowledgements The authors are grateful to the Natural Science Foundation of China (NSFC 21405064, 21575053), the Jiangsu Province Natural Science Foundation of China (BK20140233), and Priority Academic Program Development of Jiangsu Higher Education Institutions. References (1) Bonzongo, J. C. J.; Heim, K. J.; Chen, Y. A.; Lyons, W. B.; Warwick, J. J.; Miller, G. C.; Lechler, P. J. Environ. Toxicol. Chem. 1996, 15, 677-683. (2) Wolfe, M. F.; Schwarzbach, S.; Sulaiman, R. A. Environ. Toxicol. Chem. 1998, 17, 146-160. (3) Edwards, S. C.; Macleod, C. L.; Corns, W. T.; Williams, T. P.; Lester, J. N. Intern. J. Environ. Anal. Chem. 1996, 63, 187-193. (4) Lopez-Garcia, I.; Rivas, R. E.; Hernandez-Cordoba, M. Anal. Chim. Acta. 2012, 743, 69-74. (5) Yan, W. J.; Wang, Y. J.; Zhuang, H.; Zhang, J. H. Biosens. Bioelectron. 2015, 68, 516-520. (6) Rodriguez-Reino, M. P.; Rodriguez-Fernandez, R.; Pena-Vazquez, E.; Dominguez-Gonzalez, R.; Bermejo-Barrera, P.; Moreda-Pineiro, A. J. Chromatogr. A 2015, 1391, 9-17. (7) Zhang, T.; Cheng, Z. G.; Wang, Y. B.; Li, Z. G.; Wang, C. X.; Li, Y. B.; Fang, Y. Nano Lett. 2010, 10, 4738-4741. (8) Zhang, H. Y.; Yang, L. Q.; Zhou, B. J.; Liu, W. M.; Ge, J. E. C.; Wu, J. S.; Wang, Y.; Wang, P. F. Biosens. Bioelectron. 2013, 47, 391-395. (9) Wang, M. H.; Liu, S. L.; Zhang, Y. C.; Yang, Y. Q.; Shi, Y.; He, L. H.; Fang, S. M.; Zhang, Z. H. Sensor. Actuat. B-Chem. 2014, 203, 497-503. (10) Liang, G. H.; Zhang, P.; Li, H. X.; Zhang, Z. Y.; Chen, H.; Zhang, S.; Kong, J. L. Anal. Chim. Acta. 2012, 726, 73-78.
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Captions of figures and table 2+
Figure 1. Scheme of Hg sensor at single-particle level with dark-field microscopy Figure 2. Scheme of transmission grating-based dark-field spectral imaging microscopy Figure 3. Comparison of colorful and spectral imaging at the presence of 1 nM Hg2+. (A) Transmission grating-based spectral imaging by EMCCD. The zeroth order dot and first-order streak split from an individual Au nanoparticle(AuNP)/aggregation are pointed by same color triangles; (B) Corresponding dark-field microscopy image by colorful CCD; (C) Representative spectra of aggregated and non-aggregated AuNPs in red and blue, respectively. Figure 4. Typical transmission electron micrographs of the sensors without Hg2+ (A) and 30 nM Hg2+ (B) Figure 5. Images of AuNPs at different concentrations of Hg2+. From (A) to (D) Hg2+ concentrations are 0.01, 0.1, 1.0, and 50 nM, respectively. Scale bar is 6 µm in length. (E) Plots of AuNP aggregation percentage against the logarithm of Hg2+ concentration. Figure 6. Selectivity of the sensor for Hg2+ over other metal ions. The concentration is 25 nM for all metal ions. Figure 7. Stability of the sensor for 1.0 nM Hg2+ detection. Table 1 Performance summary of representative metal nanoparticle-based colorimetric detection of Hg2+
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Figure 1
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Figure 2
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Figure 4
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Table 1 Probes
LOD (nM)
Selectivity
Mechanism
Real Sample
References
DNA-AuNPs
30
Hg2+
Aggregation
Xiang River water
20
DNA-AuNPs
17.3
Hg2+
Aggregation
Tap water, river, 21 and lake water
DNA-AuNPs
250
Hg2+
Aggregation
No
22
DNA-AuNPs
0.025
Hg2+
Aggregation
No
23
DNA/AuNPs
0.6
Hg2+
Aggregation
Tap and lake water
24
2+
DNA/AuNPs
250
Hg
Aggregation
No
25
Peptide-AuNPs
20,000
Hg2+
Aggregation
No
26
Peptide-AuNPs
26
Aggregation
No
27
Acid protien-AuNPs
1,000
Hg2+
Aggregation
No
28
Thiocyanuric acid-AuNPs
0.5
Hg2+
Anti-aggregation
Tap and lake water
29
Diethyldithiocarbamate-AuNPs
10
Hg2+
Aggregation
Drinking water
33
2+
Hg2+, Co2+, Pd4+,
Pt2+
Pd2+,
Citrate-AuNPs
2.9
Hg
Aggregation
Tap water
32
2,2'-Bipyridyl-AuNPs
38
Hg2+
Anti-aggregation
Jialing River water
30
Poly (γ-glutamic acid)-AuNPs
1.9
Hg2+
Aggregation
Tap and mineral 18 water
Thymine derivative-AuNPs
0.8
Hg2+
Aggregation
No
34
Quaternary ammonium-AuNPs
30
Hg2+
Aggregation
Drinking water
35
Cys-AuNPs
100
Hg2+
Aggregation
No
31
Thiourea-AuNPs
193
2+
Hg , Ag
Aggregation
No
36
DNA-AuNPs
0.0014
Hg2+
Aggregation
+
River and industry In our study waste water
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