âAggregation-to-Deaggregationâ Colorimetric Signal Amplification

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An “Aggregation-to-Deaggregation” Colorimetric Signal Amplification Strategy for Ag+ Detection at A Femtomolar Level with Dark- Field Microscope Observation Jingjing Li, Hongyan Xi, Caiyun Kong, Qing-Yun Liu, and Zhengbo Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03739 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Analytical Chemistry

An “Aggregation-to-Deaggregation” Colorimetric Signal Amplification Strategy for Ag+ Detection at A Femtomolar Level with DarkField Microscope Observation Jingjing Li,1 Hongyan Xi,1 Caiyun Kong,1 Qingyun Liu,2 Zhengbo Chen1* 1 2

Department of Chemistry, Capital Normal University, Beijing, 100048, China

College of Chemical and Environmental Engineering, Shandong University of Science and Technology,

Qingdao, 266590, China * Corresponding author. Tel.: 010-68903047 E-mail: [email protected] ABSTRACT: Robust but ultrasensitive aptasensors with an ability of detecting lower concentration of heavy metal ions enable detection of serious environmental and health issues. We herein develop a label-free aptasensor for ultrasensitive detection of silver ion (Ag+) utilizing gold nanoparticle (AuNP) intensity measurement methodology by dark-field microscopy, which is based on target Ag+ and exonuclease Ш (Exo Ш)-dependent DNA cleavage recycling amplification. In the presence of target Ag+, thymine (T) bases at two termini of hairpin DNA bind with Ag+ through C-Ag+-C coordination to form DNA duplex, Exo Ш can recognize the blunt 3’ end of DNA duplex and digest it from the 3’ end to the 5’ direction. The released target Ag+ then binds with another hairpin DNA via C-Ag+-C pairs. After many cycles of the digestion of the DNA duplex by Exo III, numerous remaining singlestranded DNA (ssDNA) is generated. These ssDNA is absorbed on the surface of AuNPs, enhancing the repulsion force between AuNPs, which further promotes the dispersion of AuNPs, leading to a significantly decreased intensity of yellow and red dots (aggregated AuNPs) under dark-field microscopy observation in contrast to that of the blank solution (without target Ag+). On this basis, the detection limits of 41 fM and 39 fM were achieved for Ag+ in Tris-HCl buffer and river water, respectively.

struments, skilled operators, and complex protocols.13-15 Such requirements greatly preclude practical applications of these methods in resource-limited areas. Alternatively, colorimetric approaches are widely recognized as a kind of simple but powerful detection technique for Ag+ because they can be readily manipulated by less-trained personnel with an inexpensive spectroscopic instrument or even the naked eye.16-27 However, the bottleneck for these colorimetric methods showed the lower detection sensitivity, which confined to the nM level, compared with that of the aforementioned typical sensitive methods. For this purpose, gold nanoparticles (AuNPs)-based darkfield microscope affords an alternative solution to this issue (low sensitivity). The dark-field microscope, as a simple, costeffective, and particularly ultrasensitive technique, can offer a powerful tool to the direct image at the single nanoparticle level.28,29 The mechanism of the proposed colorimetric aptasensor for Ag+ detection is illustrated in Scheme 1. The system is composed of hairpin-like DNA and AuNPs. In the absence of target Ag+, the anionic AuNPs are initially well dispersed in the solution containing negatively charged hairpin-

INTRODUCTION Silver ion (Ag+), which was assigned to the highest toxicity class,1 has long received considerable attention. Chronic exposure to Ag+ can cause undesirable consequences on human heath, such as organ failure, cytotoxicity, brain damage, reduction in mitochondrial function, and disruption of the immune system.2-5 According to the U.S. Environmental Protection Agency, a concentration higher than 1.6 nM is toxic to fish and microorganisms, and the maximum permissible Ag+ concentration in drinking water was 930 nM enacted by the Secondary Drinking Water Standards of the U.S. Environmental Protection Agency (EPA). Hence, the determination of the trace amount of Ag+ is of paramount essentiality for public health, water quality control, and environmental monitoring. Typical sensitive methods for detection of Ag+ include atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), voltammetry and fluorescence spectroscopy.6-12 Nevertheless, these approaches are time consuming and require sophisticated in-

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like DNA probes due to strong electrostatic repulsion. However, upon addition of Exo Ш and NaCl, Exo III can not digest the hairpin-like ssDNA, and the electrostatic repulsion among AuNPs is shielded, leading to AuNP aggregation. The color of the aggregated AuNPs shows yellow and red under dark-field microscopy observation. In the presence of target Ag+, the protrudent C bases at two termini of the hairpin-like DNA can be folded to form a rigid DNA duplex through the formation of a Ag+-mediated base pair (C-Ag+-C). In this case, Exo III can digest DNA duplex from the blunt 3’-end to 5’, and Ag+ is released simultaneously. Subsequently, Ag+ binds with the remaining hairpin-like DNA and initates another DNA cleavage cycle with the assistance of Exo Ш. Such Ag+ and Exo Шdependent DNA cleavage recycling amplification leads to generation of more ssDNA molecules, the released ssDNA can be absorbed on the surface of AuNPs and effectively prevents AuNPs from aggregation in high salt media. The color changed from yellow and red (aggregated AuNPs) to green (disperse AuNPs) under dark-field microscopy observation. A further insight into the mechanism of aggregation-todispersion emergence can be obtained by transmission electron microscopy (TEM). Fig. 1a shows the AuNPs were aggregated in aqueous solution in the absence of target Ag+. Whereas the presence of target Ag+ made the AuNPs deaggregation (Fig. 1b).

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EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate(III) hydrate (HAuCl4·3H2O, >99.0%) and trisodium citrate were purchased from Sigma-Aldrich Co. Ltd.. The DNA sequence was purchased from Sangon Biotech Co., Ltd. (Shanghai). The sequence of oligonucleotides is listed as follows: DNA: 5'- CCC CCC CGT GGG TAG GGC GGG TTG GAC CCT ACC CAC CCC CCC G-3' The DNA samples were prepared by dissolving in Tris-HCl buffer solution (20 mM, pH=7.4), and the concentration of the DNA solution was 100 µM. The DNA in Tris-HCl buffer solution was heated to 85 °C for 5 min and then allowed to cool to room temperature prior to use. Ultrapure water with a resistivity of 18.2 MΩ cm was produced using a Milli-Q apparatus (Millipore) and used as a solvent in all experiments. Instrumentation. The detection was performed using a microscope (DS-Fi2, Nikon). Image-Pro Plus 6.0 software was used to process the dark-field images of AuNPs. The morphology and size of AuNPs were characterized by Transmission electron microscope (TEM H-7650, Hitachi, Japan). Preparation of 50 nm AuNPs. All glass containers and the magnetic stir-bars were immersed in aqua regia for 1 hour and washed thoroughly with deionized water prior to the preparation of AuNPs. The 50 nm AuNPs were synthesized through two-step gold growth method30 as follows: Uniform 13 nm AuNPs as the seeds were prepared by reduction of HAuCl4 with citrate in deionized water. Briefly, to a 250 mL of boiling aqueous solution of HAuCl4 (1 mM) was added 25 mL of trisodium citrate (38.8 mM) rapidly, then the solution was heated with vigorous stirring for 15 min. The resulting solution changed to wine red in color, indicating the formation of 13 nm AuNPs. Then, 2 mL of aqueous hydroxylamine hydrochloride (0.2 M) and 1 mL of the above AuNPs seeds were added into 125 mL of deionized water under magnetic stirring. Subsequently, 0.8 mL of aqueous chloroauric acid (100 mM) was added dropwise and kept at vigorous stirring for 30 min. Detection Procedure for Ag+. A 40 µL aliquot of Ag+ standards with various concentrations and 10 µL of 1 U/µL Exo III were initially injected into a 0.5 mL centrifuge tube containing 30 µL of 42.9 nM hairpin DNA in 20 mM Tris-HCl buffer (pH 7.4). The mixture was incubated at 37 °C for 30 min and heated to 85 °C for 10 min to inactivate Exo Ш. Then, 510 µL of AuNP solution (17 pM) and 60 µL of NaCl (8.57 mM) were added to the above mixture for 10 min of incubation at room temperature. Finally, we drew up 20 µL of the solution and slightly dripped it on the glass slide. The glass slide was immediately covered with a coverslip. The samples were ready for observation under the dark-field microscopy. The numerical aperture of an 100×oil immersion objective was adjusted to 0.6.

Scheme 1 Schematic Illustration of Detecting Ag+ Through A Specific C-Ag+-C Coordination With Dark-Field Microscopy.

RESULTS AND DISCUSSION Optimization of Experimental Conditions. To achieve better sensing performance, several important experimental parameters including AuNP concentration, NaCl concentration, Exo III concentration, and DNA concentration were optimized. As depicted in Fig. 2, the intensities of the blank AuNPs (without Ag+) and AuNPs (with Ag+) were increased with the increase of AuNP concentration from 13 to 51 pM. When Ag+

Figure 1. TEM images of AuNPs in the absence (a) and presence (b) of 57 nM Ag+ in the presence of 8.57 mM NaCl, 15 U Exo III, and 42.9 nM DNA.


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Analytical Chemistry concentration was 17 pM, the intensity change (I0/I) reached the maximum. Thus, 17 pM AuNPs was employed for the following experiment. The effect of NaCl concentration on intensity response was also explored, as shown in Fig. 3, the intensity of aggregated AuNPs increased dramatically with the increasing NaCl concentration from 0 to 8.57 mM and decreased sharply beyond 8.57 mM. Therefore, 8.57 mM was selected as the optimum concentration of NaCl. Exo III concentration played a crucial role in the sensitivity. As shown in Fig. 4, the intensity was decreased within the range of 0 to 15 U and then enhanced beyond 15 U. Thus, the optimal concentration of Exo III was 15 U. The impact of DNA concentration on the intensity of AuNPs was investigated (Fig. 5). The intensity was diminished with the DNA concentration up to 42.9 nM and began to decrease with extra addition of the DNA. Thus, 42.9 nM DNA was used for the following work.

Figure 4. Dark-field images of AuNPs in the absence (a) and presence (b-e) of different concentrations of Exo III. (g) Effect of different Exo III concentrations on the AuNP intensities. Error bars represent the standard deviation of triplicates.

Figure 5. Dark-field images of AuNPs in the absence (a) and presence (b-e) of different concentrations of DNA. (g) Effect of different DNA concentrations on the AuNP intensities. Error bars represent the standard deviation of triplicates. The sensitivity. To evaluate the sensitivity of the colorimetric aptasensor, the dark-field images of the assay were measured after addition of different concentrations of Ag+ (0, 57 fM, 0.57 pM, 5.7 pM, 57 pM, 570 pM, 5.7 nM, and 57 nM) (Fig. 6(a-h)). From the dark-field images, the number of yellow and red dots increased with increasing concentration of target Ag+, indicative of the increase in aggregated AuNPs. The quantitative analysis was achieved by measuring the intensity change of the aggregated AuNPs under dark-field microscope via the Image-Pro Plus 6.0 software. As shown in Fig. 6i, the intensity of AuNPs was very sensitive to the change in the Ag+ concentration and decreased as an increase in the Ag+ concentration. A linear correlation of the intensity of aggregated AuNPs with the logarithm of Ag+ concentration was observed in the Ag+ concentration range from 57 fM to 57 nM (with R2 = 0.98) (Fig. 6j). An ultralow limit of detection (LOD) of the assay was estimated to be 41 fM (mean+3×standard deviation of intensity of AuNPs in blank) (inset of Fig. 6i), which was far lower than the standard of silver (∼930 nM) in drinking water permitted by the United States EPA. The high sensitivity can be attributed to the target Ag+ and Exo III-triggered DNA recycle-induced colorimetric signal amplification. The LOD of the developed Ag+ sensor was much lower in comparison with those of the previously

Figure 2. Dark-field images of AuNPs before (a0-d0) and after (a-d) incubation of 3.6 nM Ag+ in the presence of different concentrations of AuNPs. (e) AuNP intensity as a function of AuNP concentration in the absence and presence of 3.6 nM Ag+. (f) Effect of AuNP concentration on the intensity change (I0/I). I0 stands for the intensity of the AuNPs in the absence of Ag+, and I was intensity of the AuNPs after incubation with 3.6 nM Ag+, respectively. Error bars represent the standard deviation of triplicates.

Figure 3. Dark-field images of AuNPs in the absence (a) and presence (b-f) of different concentrations of NaCl. (g) Effect of different Ag+ concentrations on the AuNP intensities. Error bars represent the standard deviation of triplicates.



reported optical sensor (Table 1), although some of these sensors had the similar molecular recognition element, i.e., the aptamer containing the C-Ag+-C coordination.

obvious change in intensity of the aggregated AuNPs (Fig. 7h) were observed compared with those of the blank solution. While 2 nM target Ag+ exhibited a significant change in the color change (Fig. 7g) and aggregated AuNP intensity (Fig. 7h), implying the excellent selectivity of the aptasensor that arose from highly specific coordination of the C-Ag+-C pairs.

Figure 6. Dark-field images of AuNPs before and after incubation of Ag+ with varying concentrations in Tris-HCl buffer (pH=7.4). Ag+ concentration: (a) 0, (b) 57 fM, (c) 0.57 pM, (d) 5.7 pM, (e) 57 pM, (f) 570 pM, (g) 5.7 nM, and (h) 57 nM. (i) The intensities of aggregated AuNPs as a function of different concentrations of Ag+ (0-57 nM) in Tris-HCl buffer (pH=7.4). Inset: the calculation of the LOD according to the 3σ rule. (j) The intensities of aggregated AuNPs versus the logarithm of Ag+ concentrations (57 fM-57 nM). The error bars represent the standard deviation about the mean of three independent data sets. Table 1 Comparison of different optical Ag+ sensors. method

mateiral

linear range

detection limit

ref.

colorimetric

AuNPs and DNA

57 fM to 57 nM

41 fM

this work

colorimetric

test strip

0 to 50 µM

1.69 µM

31

colorimetric

Pt nanocubes

0.01to10000 nM

80 pM

32

colorimetric

AgNPs and DNA

0.1 to 75 µM

58 nM

33

colorimetric

AuNPs

1.09 to 109 nM

1.09 nM

34

colorimetric

palladium nanozyme

0 to 100 nM

1.2 nM

35

fluorescent

MoS2-RhoBS nanocomplex

10 nM to 10 µM

10 nM

36

fluorescent

pyridyl

0 to 80 µM

2.5 µM

37

fluorescent

graphene quantum dots

0 to 115.2 µM

250 nM

38

Figure 7. Dark-field images of AuNPs in the absence and presence of different metal ions: (a) blank, (b) Co2+, (c) Fe3+, (d) Mn2+, (e) Ni2+, (f) Pb2+, and (g) Ag+. (h) Effects of different metal ions on the intensity of AuNPs. Ag+ concentration: 2 nM, and concentration of the other interfering metal ions: 20 nM. The Real Sample Detection. Encouraged by the high sensitivity and selectivity of the aptasensor toward Ag+, we further extended the determination ability of the aptasensor toward Ag+ in river water. The river water samples were collected from Beijing moat. The river water was filtered through 0.22 µM nitrocellulose membranes to dispose of physical impurities prior to measurement. No Ag+ in the river water samples was detected using ICP-MS (Agilent 7500ce). Ag+ was spiked into the river water at different concentrations (0, 57 fM, 0.57 pM, 5.7 pM, 57 pM, 570 pM, 5.7 nM, and 57 nM) and measured by the colorimetric aptasensor. (Fig. 8). Fig. 8a manifests the resulted intensities of aggregated AuNPs after incubation with various concentrations of Ag+. As shown, the intensity of aggregated AuNPs was proportional to logarithm of Ag+ concentrations in a linear range from 57 fM to 0.57 nM (R2=0.97) (Fig. 8b), with a LOD of 39 fM (inset of Fig. 8a), estimated by the 3σ rule, as also verified by the corresponding dark-field images (Fig. 8 (c-j)), the number of yellow and red dots (AuNP aggregates) continuously decreased as Ag+ concentrations increased. The obtained detection results of Ag+ in TrisHCl buffer exhibited a good agreement with those values in river water samples, which indicates the developed dark-field image-based colorimetric aptasensor possessed excellent applicability for real environmental sample analysis.

The Selectivity. The selectivity of the assay for Ag+ was examined by monitoring the color changes observed from dark- field images and the intensities of aggregated AuNPs. The aptasensor was incubated in sample solutions containing different interfering substances including Co2+, Fe3+, Mn2+, Ni2+, Pb2+, and Ag+ under the same experimental conditions. As shown in Fig. 7, for 20 nM interfering metal ions, neither the color changes in the dark-field images (Fig. 7 (a-f)) nor an


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Analytical Chemistry This study was supported by Scientific Research Project of Beijing Educational Committee (Grant No. KM201710028009), Youth Innovative Research Team of Capital Normal University, and Capacity Building for Sci-Tech Innovation-Fundamental Scientific Research Funds (025185305000/195).

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Figure 8. (a) The intensities of aggregated AuNPs as a function of Ag+ concentration (0-57 nM) separately spiked in river water samples. Inset: the calculation of the LOD according to the 3σ rule. (b) The intensities of aggregated AuNPs versus the logarithm of Ag+ concentrations (57 fM-0.57 nM). Error bars are the standard deviation of three repetitive experiments. Dark-field images of AuNPs in the absence and presence of target Ag+ in river water with different concentrations: (c) 0, (d) 57 fM, (e) 0.57 pM, (f) 5.7 pM, (g) 57 pM, (h) 570 pM, (i) 5.7 nM, and (j) 57 nM.

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CONCLUSIONS In summary, we have successfully demonstrated an “aggregation-to-deaggregation” signal amplification strategy for ultarsensitive detection of Ag+ at a low femtomolar level, which utilizes the intensity of AuNPs as signal readout with the dark-field microscope plus Image-Pro Plus 6.0 software. The assay combines the ultrasensitive AuNP intensity measurement technique with target Ag+ and Exo Ш-triggered DNA digestion recycling to realize signal amplification. LOD of 41 fM for Ag+ was achieved, which is far below the cutoff values the standard of silver (∼930 nM) in drinking water permitted by the United States EPA. Additionally, to demonstrate the potential of this aptasensor in environmental samples, Ag+ in the river water samples was detected with the LOD of 39 fM. Of note, no expensive detection instruments and consumables or complex operating procedures are involved. It is very evident that our sensor is a promising prospect in environmental monitoring and medical diagnosis where Ag+ concentration is extremely low.

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AUTHOR INFORMATION

(25) Y. Chang, Z. Zhang, J. Hao, W. Yang, J. Tang, Sens. Actuators, B 2016, 232, 692-697.

Corresponding Author

(26) Y. Sung, S. Wu, Sens. Actuators, B 2014, 197, 172-176.

* Phone: +86-010-68903047. E-mail: [email protected]

Notes

(27) B. Li, Y. Du, S. Dong, Anal. Chim. Acta 2009, 644, 78-82.

The authors declare no competing financial interest.

(28) P. K. Jain, X. H. Huang, I. H. El-Sayed, M. A. El-Sayed, Acc. Chem. Res. 2008, 41, 1578-1586.

ACKNOWLEDGMENTS



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