Detection of mercury(II) ions using colorimetric gold nanoparticles on paper-based analytical devices.

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Detection of mercury (II) ions using colorimetric gold nanoparticles on paper-based analytical devices Guan-Hua Chen, Wei-Yu Chen, Yu-Chun Yen, Chia-Wei Wang, Huan-Tsung Chang, and Chien-Fu Chen Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2014 Downloaded from http://pubs.acs.org on June 21, 2014

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Analytical Chemistry

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A schematic illustration of the proposed on-site Hg2+ sensing strategy. Water samples extracted from suspected polluted sites are first added to the ssDNA-attached AuNP colloids and colorimetric sensing is triggered with the addition of a sodium chloride solution. After color development, the mixture is placed on hydrophobic-wax-confined chromatography paper for color enhancement, and the detection results are then recorded and transmitted to a server to instantly receive quantitative data via cloud computing. 92x52mm (300 x 300 DPI)


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UV–Vis absorbance spectra of colorimetric AuNPs and Hg2+-ssDNA complex-based Hg2+ sensing. Based on the Ext 715/520 plot, the detection limit of 50 nM was obtained (n=5). A linear correlation is observed over the range of 25 nM – 750 nM (y = 0.0007x + 0.1272 and R2 = 0.98). 102x124mm (300 x 300 DPI)



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The colorimetric results for mercury ion detection (a) in cuvettes and (b) on cellulose paper. Panels (c) and (d) show the analytical blue/red color values of the spots. This paper-based colorimetric sensing platform has a detection limit of 50 nM, with a turnaround time of 40 min (n=5). The length of the inset scale bar is 3 mm. 127x193mm (300 x 300 DPI)


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SEM images of spots of detected AuNP mixtures (a) with and (b) without Hg2+ on cellulose paper. The bright spots in the images are AuNPs. 33x13mm (300 x 300 DPI)



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The specificity test results obtained for samples (a) in cuvettes, (b) samples on cellulose paper and (c) the analytical blue/red color values of the spots obtained using the proposed sensing platform. Although the concentrations of the other metal ions were 10-fold higher than the concentration of Hg2+, the blue/red values remain < 1 (n=5). The length of the inset scale bar is 3 mm. 72x63mm (300 x 300 DPI)


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The colorimetric results of mercury ion detection from Hg2+-spiked (a) pond and (b) river water. The detection limit for both the Hg2+-spiked pond and river water remained 50 nM, which demonstrates the consistency of the sensing platform (n=5). A linear correlation for both cases are over the range of 25 nM – 100 nM (y = 0.0002x + 0.9923 and y = 0.0002x + 0.9634, respectively). 115x159mm (300 x 300 DPI)



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Detection of Mercury (II) Ions Using Colorimetric Gold Nanoparticles on Paper-Based Analytical Devices Guan-Hua Chen,† Wei-Yu Chen,‡ Yu-Chun Yen,† Chia-Wei Wang,‡ Huan-Tsung Chang‡ and Chien-Fu Chen†* †

Graduate Institute of Biomedical Engineering, National Chung Hsing University, Taichung 402, Taiwan ‡Department

of Chemistry, National Taiwan University, Taipei 106, Taiwan

ABSTRACT: An on-field colorimetric sensing strategy employing gold nanoparticles and a paper-based analytical platform was investigated for mercury ion (Hg2+) detection at water sources. By utilizing thymine-Hg2+-thymine (T-Hg2+-T) coordination chemistry, label-free detection oligonucleotide sequences were attached to unmodified gold nanoparticles (AuNPs) to provide rapid mercury ion sensing without complicated and time-consuming thiolated or other costly labeled probe preparation processes. This strategy’s sensing mechanism is not only specific toward Hg2+, rather than other metal ions, but the conformational change in the detection oligonucleotide sequences also introduces different degrees of AuNP aggregation that causes the color of AuNPs to exhibit mixture variance. To eliminate the use of sophisticated equipment and


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minimize the power requirement for data analysis and transmission, the color variance of multiple detection results were transferred and concentrated on cellulose-based paper analytical devices, and the data were subsequently transmitted for the readout and storage of results using cloud computing via a smartphone. As a result, a detection limit of 50 nM for Hg2+ spiked pond and river water could be achieved. Furthermore, multiple tests could be performed simultaneously with a 40-min turnaround time. These results suggest that the proposed platform possesses the capability for sensitive and high-throughput on-site mercury pollution monitoring in resource-constrained settings.

INTRODUCTION The economies of developing countries have been booming in recent years. However, the excessive number of plants constructed in these countries produce serious air pollution, and discharged toxic heavy metal ions, such as mercury ions, may ultimately accumulate in human bodies through the food chain.1,

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According to World Health Organization (WHO) reports,

mercury poses a serious threat to human health, particularly to fetal development in utero and infants.3 In addition, it also affects the nervous, immune and digestive systems and causes damage to the brain as well as kidneys and lungs of human beings.4-7 The WHO and the U.S. Environmental Protection Agency (EPA) has set the maximum allowable levels of Hg in drinking water at 30 nM and 10 nM, respectively. Due to the location of a majority of water resources and suspicious monitoring sites in suburban areas, where power is limited, the transportation of collected samples is time-consuming and the risk of sample loss and contamination is high, a simple, rapid, light-weight, highly sensitive and selective detection


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platform for mercury ion detection in resource-constrained settings is required to achieve better pollution control and environmental monitoring. The most widely used technique for mercury ion detection is inductively coupled plasma mass spectrometry (ICP-MS), which allows for the accurate, sensitive and parallel analysis of a broad range of metal ions in a rapid manner.8-10 In addition, various detection methods utilizing organic fluorophores,11-13 chromophores,14 polymers,15,

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oligonucleotides,17,

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DNAzymes,19,

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proteins21, 22,cells23 and gold nanoelectrodes24 have been developed, adopting optical, chemical and biological mechanisms for mercury sensing and quantification. Although the detection limit of Hg2+ using ICP-MS and electrochemical detection platform can reach 0.1 nM,10,

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the

requirement of sophisticated analytical instrumentation, well-trained personnel or complex processes make such techniques difficult to implement for field detection. Over the past decades, with the advancement of various nanomaterial synthesis procedures and biochemical analytical platforms, versatile biochemical sensors have been developed based on the high surface-to-volume of nanomaterials to supply large areas for high-density biochemical interaction and on their extraordinary opto-physical and chemical characteristics for more sensitive, specific and rapid analysis.25-31 Among these attractive nanomaterials, AuNPs have been widely adopted as colorimetric sensors to provide alternative schemes to conventional detection methods for mercury detection based on the ease with which AuNPs of various sizes and shapes can be synthesized and surface-modified with different functionalities and the particles’ excellent biocompatibility and unique optoelectronic and chemical properties.17, 32-35 When the surface charge of dispersed AuNPs approaches neutralization or specific conjugation or binding takes place with surrounding molecules to reduce the distance between AuNPs, surface plasmon coupling occurs, resulting in a red shift in the absorbance wavelength of


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colloidal AuNPs and a color change from red to blue; it is the degree to which this color changes that can be a quantitative indicator of mercury concentrations.33 In addition, AuNPs exhibit stronger molar absorptivities than organic dye molecules due to their high extinction coefficients;36, 37 thus, several fluorescent “turn on” and “turn off” mechanisms through which the distance between fluorescent labels and AuNPs is modified by the presence of mercury have been proposed.17, 32, 34 In addition to the above-mentioned detection mechanisms, the surface chemistry underlying the binding of deoxyribonucleic acid (DNA) to AuNP surfaces has been combined with the formation of T-Hg2+-T complexes to create thymine-containing DNA that specifically recognizes Hg2+ rather than forming an A-T base pair.17,

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Although the results of this AuNP-based

colorimetric assay can be observed by the naked eye, most of the above-mentioned methods still depend on instruments to readout results; therefore, the feasibility of applying these tests for onsite measurement is limited. This paper describes a colorimetric sensing strategy combining unmodified AuNPs and microfluidic paper-based analytical devices (µPADs) for mercury ion detection in resourcelimited settings. The Hg2+ sensing mechanism developed in this study is based on the surface plasmon resonance (SPR)-induced color variance of AuNPs colloid, which is correlated to the distance between adjacent AuNPs.38-41 The surface charge of AuNPs synthesized using the citrate reduction method is negative; therefore, the Coulombic force-induced repulsion makes AuNPs disperse in aqueous solution. At this moment, the peak absorbance wavelength is centered at 520 nm and the visible color of the colloid is red.39, 40 Once a salt solution is added, the dissociative ions shelter the surface charge on the AuNPs, causing the color of the AuNP colloid to change from red to purple and then to blue according to the degree of AuNP


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aggregation.41 The colorimetric mechanism has been combined with label-free oligonucleotide sequences that can be attached to the AuNP surfaces via hydrophobic interaction and expose the negatively charged phosphate backbone to enhance resistance to the ion sheltering effect.38 However, when the surface-attached single-stranded deoxyribonucleic acid (ssDNA) sequences possess T–T mismatches, the addition of Hg2+ ions changes the structure of ssDNA into a hairpin structure based on T–Hg2+–T coordination chemistry.42, 43 As a result, the zeta potential on the AuNP surfaces and the electrostatic repulsion between adjacent AuNPs were decreased; therefore, AuNPs aggregate in a salt solution. To obtain quantitative results, the blue/red ratio gained from the red-green-blue (RGB) values of each spot was calculated based on the color change of the mixture, which was correlated with the different degrees of AuNPs aggregation and an increase in the concentration of Hg2+. To avoid using sophisticated equipment and minimize the required resources, µPAD and cloud computing were adopted for data storage and result readout. µPADs have been developed for biomedical sensing and analysis in resource-limited settings based on their advantages of cost effectiveness, disposability, simple fabrication, low sample volume required, wettability (which helps eliminate external flow control systems) and their ease of storage and delivery without damage.44-46 Furthermore, with other paper-based analytical devices such as dipstick and lateral flow assays, it is difficult to obtain multiplex and qualitative results. In contrast, µPADs can simultaneously detect multiple analytes from extracted samples and receive qualitative results via a scanner or smartphone.47 In this study, a smartphone was chosen based on its portability, light weight and on-site image recording and data transmission capabilities, which are suitable for resource-constrained settings.


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As a result, the specificity provided by single thymine mismatched ssDNA toward Hg2+ is exclusive; moreover, 50 nM spiked Hg2+ in pond and river water can be detected via the color variance of AuNP spots on paper, which allows the turnaround time following sample extraction from suspicious sites to be limited to 40 min. EXPERIMENTAL Chemicals. Methanol, 2-propanol, ethanol, sodium chloride, boric acid, trisodium citrate, phosphate tris-borate-EDTA, tris(hydroxymethyl)aminomethane and all metallic salts were purchased from Sigma-Aldrich (St. Louis, MO). Hydrogen tetrachloroaurate(III) trihydrate was ordered from Acros Organics (Geel, Belgium). Oligonucleotide sequences were synthesized and purified by Bio Basic Inc. (Bio Basic, Ontario, Canada). Ultrapure water (18.2 mΩ‧cm) was filtered through a Milli-Q system (Millipore, Milford, MA). Whatman Grade 3-mm chr cellulose chromatography paper was obtained from GE Healthcare (Little Chalfont, UK). Instrumentation and Characterization. The distribution of colorimetric AuNPs on chromatography paper was inspected using a scanning electron microscope (SEM; JSM-6700F, JEOL, Tokyo, Japan). A transmission electron microscope (TEM; H7100, Hitachi HighTechnologies, Tokyo, Japan) was used to verify the size and morphology of the synthesized AuNPs. The concentration and interparticle distance of AuNPs was characterized by a UV–Vis spectrometer (Cintra 10e, GBC, Victoria, Australia). The RGB values of the colorimetric detection results of metal ions on chromatography paper were recorded and transmitted for cloud computing using a smartphone (butterfly, HTC, Taiwan). All captured images were simultaneously uploaded to a cloud storage space (Dropbox, San Francisco, CA) via the internet, followed by the analysis of RGB values using the free software program ImageJ (National Institutes of Health, US).


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Preparation of AuNPs. Citrate-capped AuNPs with an average diameter at 13 nm were synthesized by the citrate-mediated reduction of HAuCl4.48 Briefly, a 25-mL solution containing 38.8 mM trisodium citrate was added to 250 mL of a boiling 1 mM HAuCl4 solution and consistently stirred and heated for an additional 15 min until the color of the mixture turned from light yellow to dark red. After cooling to room temperature, the concentration of the AuNP colloid was estimated according to the Beer–Lambert law to analyze the absorbance spectra.49 Mercury Ion Detection via Combination of Unmodified AuNPs and ssDNA Sequences. Detection oligonucleotide sequences were dissolved in tris-borate buffer (100 mM, pH 9.0) and mixed with a AuNP colloid. The final concentrations of AuNPs and ssDNA sequences were adjusted using ultrapure water to 1 nM and 10 nM, respectively. The mixture was then heated to 900C for 10 min and stored at 40C before testing. Before the Hg2+ detection tests, the ssDNA-attached AuNP solution was brought to ambient conditions without further temperature control. Different concentrations of Hg2+ ranging from 25 nM to 2 µM were added separately, followed by the addition of NaCl (30 mM) to each solution and 30-min incubation for color development. For UV-Vis spectra and paper-based analytical device measurements, the required sample volumes are 1 mL and 50 µL, respectively. Colorimetric Sensing Results Obtained Through µPADs. The Hg2+ detection results were obtained using µPADs and a smartphone. µPADs were formed on cellulose paper using the solid-ink printing method. The sizes of the detection spots were all 3 mm in diameter. They were controlled by the circular hydrophobic barriers formed by using a printer (Xerox 8570, Fuji, Japan) to print solid wax on the surface of the paper and then heating to 160°C for 2 min to allow the wax to penetrate the paper. In addition, reference color bars were simultaneously formed on the µPADs to adjust for lighting conditions.


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After color development, 50 µL of the mixture was spotted at the center of the printed circles. To achieve rapid drying, a wiper was fixed beneath the paper devices using double sided tape. When the mixture spots completely filtered through paper and dried, data were recorded using a smartphone under artificial white light without flash. Images were simultaneously transmitted for cloud computing via a free app (Dropbox, San Francisco, CA). Specificity Tests for Metal Ions. To investigate the specificity of the proposed platform, aqueous solutions spiked with 10 different kinds of metal ions were separately tested. 1 µM Hg2+ and 10 µM Co2+, Mn2+, Pb2+, Ca2+, Cd2+, Cu2+, Ni2+, Zn2+ and Cr3+ were used here as the target analytes. Analysis of Pond Water and River Samples. Water samples from a pond on campus and a nearby river were used for environmental sample testing. Sample solutions were collected and then filtered membrane 3 times through a filter with a pore size of 0.2 µm. Hg2+ at concentrations over the range of 25 nM to 2 µM were spiked into the pond and river water samples. The detection processes are identical to those mentioned above utilizing coordination chemistry between AuNPs and T-T mismatched ssDNA sequences, followed by spotting colorimetric results on µPADs and transferring and receiving data via a smartphone. RESULTS AND DISCUSSION In this study, AuNPs and oligonucleotide sequences were combined via T-Hg2+-T coordination chemistry to create a salt-induced colorimetric sensing platform that provides a rapid, specific, simple and cost-effective means for Hg2+ detection. Unmodified ssDNA sequences are adopted as cost-effective detection probes rather than thiolated or fluorescent-labeled ssDNA; moreover, the simplicity of the preparation of the AuNP probes further reduces the total assay time. In addition, to further achieve favorable outcomes in resource-limited settings, a µPAD was


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adopted for data recording and a smartphone was used for image transmission to obtain quasiquantitative results. A schematic illustration of the on-site mercury detection system is shown in Scheme 1. Unknown extracted water samples from suspicious sites are first added to the pre-prepared T-T mismatched ssDNA-attached AuNP mixture, followed by salt-induced colorimetric sensing via the addition of a sodium chloride solution. When the extracted water samples consist of Hg2+, some of the T-T mismatched oligonucleotide sequences form a hairpin structure dependent on the relative concentration of Hg2+; thus, the remaining ssDNA cannot prevent the aggregation of AuNPs when the salt is present. Different degrees of aggregation cause the color of the AuNP colloid to vary from red to purple to blue. After color development, the mixture is spotted and concentrated within a circular area confined by a hydrophobic wax barrier for color enhancement. The color of cellulose paper, hydrophobic barriers and reference color bars is utilized to adjust lighting conditions. The detection results are then calculated using cloud computing and stored in a cloud space via a smartphone for data transmission and reception. Colorimetric Sensing of Hg2+ via T-Hg2+-T Coordination. A 24-mer oligonucleotide sequence, 5’-TTT-GTT-TGT-TGG-GGT-TCT-TTC-TTT-3’, was selected as the detection ssDNA sequence in this study. To maximize the variance of the color change, 20 mM sodium chloride was selected for the following tests based on the maximum amount of salt that can be added without inducing any serious aggregation of ssDNA-attached AuNPs. The proposed colorimetric sensing mechanism was first verified via UV-Vis absorbance spectra (Figure 1). The TEM image shows that the average diameter of the synthesized AuNPs used in this study was 13 nm ± 1 nm (Figure 1a). As the concentration of added Hg2+ increased, the absorption wavelength of the AuNPs mixture was centered at 520 nm, and then gradually red shifted to 715


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nm when the concentration of Hg2+ reached 2 µM. The detection limit of 50 nM was obtained from the plot of the 715 nm/520 nm absorption ratio, which demonstrates the degree of aggregation of the AuNP mixture, versus the concentration of added Hg2+. The limit of detection is defined as the concentration of Hg2+ that produces a signal that is three times greater than the baseline noise signal of the control. However, if the colorimetric sensing results are interpreted using the naked eye without instrumentation, the color variance is limited relative to the color of the reference (Figure 2a), and it can easily be misjudged under different environmental conditions and self-perception. µPAD with Unmodified AuNPs Colorimetric Assay for Hg2+. The colorimetric sensing results of Hg2+ obtained using a µPAD are shown in Figure 2. Compared with to the low color variance of Hg2+-added mixtures in the cuvettes, the colorimetric results obtained by the µPADs are more concentrated and more easily differentiable especially when the Hg2+ concentration is ≤ 100 nM (Figure S1).. The SEM images of spots from different color mixtures further demonstrate that the drying process on paper did not cause any aggregation that may have led to a false-positive result (Figure 3). No aggregation could be observed on the cellulose paper after drying when the AuNPs were dispersed in the mixture and the color was red. On the other hand, seriously aggregated AuNPs clusters could be observed for the spot of a blue color mixture. In addition, the amount of each test mixture spotted on each hydrophobic-wax-confined circle was 50 µL, which makes the heavy metal sensing platform safer, produces a more rapid response and less expensive compared with a single UV-Vis analysis, which usually requires a sample volume of 1 mL. In addition, the small sample volume requirement also results in a shorter diffusion length


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during the color development, so the total assay time can be reduced from hours to 40 min (Figure S2). To obtain quantitative results, the blue/red ratio gained from the RGB values of each spot was calculated based on the color change of the mixture, which was correlated with the increase in the concentration of Hg2+. The Hg2+ concentration was acquired using an interpolation method according to the calibration curves for controls containing 0 – 2 µM Hg2+ that were prepared in advance and input into the database. This colorimetric platform has a detection limit of 50 nM at a signal-to-noise ratio of 3. Tens to hundreds of spots from different extracted sites can be analyzed at once to further reduce the detection time of large-scale screening. Compared with the detection results obtained via UV-Vis spectroscopy, not only can the same detection limit can be achieved but parallel analysis can also effectively reduce assay times without instrumentation. Selectivity of the Colorimetric Assay for Hg2+. The specificity of the proposed analytical platform was investigated by comparing the degrees of color variance of mixture spots containing Hg2+ and other metal ions (Figure 4). In these tests, the concentrations of Hg2+ and other metal ions added to mixtures of AuNPs were 1 µM and 10 µM (1:10), respectively. A 10fold concentration difference between Hg2+ and other metal ions was adopted to determine whether the colorimetric sensing mechanism would be affected by other metal ions in a highpollutant solution. The result shows that the T-Hg2+-T-coordination-based sensing mechanism possesses good specificity toward Hg2+. Even at high concentrations, the color was red for all other metal ion solutions in cuvettes, and the mean blue/red ratios were < 1 (0.96~0.98) for all of the spots on the cellulose paper. In contrast, the color of the AuNP mixture containing 1 µM Hg2+ was blue, and the blue/red ratio of the spot was > 1 (~1.03). The results also support the


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result that the concentration effect exhibited by µPADs does not disturb the specificity but further enhances the color variance and thereby the sensing results. Environmental Sample Detection. Mercury is emitted into the air by human manufacturing, fuel, and from natural sources. It is then transported and deposited to the earth’s surface via circulation based on the location of the emission source, the form of mercury emitted, the surrounding atmosphere and the weather. Based on these effects, it is not easy to define the exact concentration of Hg2+ in the environment. In addition, because the Hg2+ concentration of natural water was non-detectable, Hg2+-spiked sample solutions were used to simulate the polluted samples. Environmental samples extracted from pond and river water were tested to verify the feasibility of the proposed nanomaterial/paper-based analytical platform in resource-limited settings (Figure 5). All Hg2+-spiked sample solutions were also used for ICP-MS tests to obtain the quantitative Hg2+ concentrations of the samples to verify the detection results. The sample preparation step consisted in pumping sample solutions through a plastic syringe with a 0.2-µm-pore filter by hand and then spiking the solutions with different concentrations of Hg2+, from 25 nM to 2 µM. Compared with the data obtained from a sample prepared using ultrapure water, the detection limit at a signal-to-noise ratio of 3 for Hg2+ found in pond and river samples remained the same at 50 nM. However, the standard deviation of each spot was relatively larger. This result can be attributed to the impurity concentration and simple sample preparation process of the environmental samples, especially for the pond samples extracted from a closed environment without external flow and therefore unable to completely remove existing salt. As a result, the blue/red values and variations are greater compared to those obtained by tests using ultrapure water. Furthermore, to overcome the differences in the water samples extracted from various environmental conditions and locations, the quantitative data


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were obtained through a comparison with previously examined calibration data stored in the cloud for adjustment and comparison. These extracted samples were also examined by UV-Vis absorption spectroscopy for comparison (Figure S3). The detection limits determined for pond and river water tests were 100 nM and 50 nM, respectively (S/N = 3). These results demonstrate that the colorimetric assay platform combining AuNPs, ssDNA, a µPAD and cloud computing for mercury ion detection can potentially be used in highly sensitive, specific and equipment-free environmental pollutionmonitoring applications. CONCLUSIONS With the rapid economic growth of developing countries, there has been an exponential increase in the number of industrial factories that inflict serious impacts such as metal ion pollution on our ecosystems. In this study, a colorimetric detection mechanism combining the photoelectric and chemical characteristics of AuNPs on µPADs with cloud computing was developed for on-field Hg2+ monitoring and water pollution control without the need for bulky analytical equipment and well-trained personnel. Unmodified AuNPs and ssDNA were adopted to eliminate complicated and time-consuming AuNPs probe preparation processes and the extra cost of thiolated or fluorescent labeled ssDNA. After the color variation results for AuNP mixtures containing different concentrations of Hg2+ were obtained, the colorimetric sensing results were further enhanced via µPADs and transmitted for cloud storage and computing for the parallel and quantitative analysis of multiple samples in remote settings. Based on a previous publication, the detection limit of Hg2+ utilizing unmodified AuNP and THg2+-T coordination chemistry was determined to be 250 nM by UV-Vis spectrometry.42 In this study, we have optimized all parameters and used different oligonucleotide sequences, so the


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detection limit using paper-based devices without using lab instruments has been improved to 50 nM, and the total turnaround time was 40 min. Although it has not yet reached the detection limit for an Hg2+ assay of WHO regulation, this work provides a portable and cost-effective toolset without using expensive materials or complicated preparation processes for Hg2+ detection in resource constrained settings. Even when the concentrations of other metal ions, including Co2+, Mn2+, Pb2+, Ca2+, Cd2+, Cr3+, Zn2+, Cu2+ and Ni2+, were 10-fold higher than the concentration of Hg2+ in aqueous solution, the T-Hg2+-T coordination chemistry provided high specificity toward Hg2+. The mixture volume required for each assay is only 50 µL, which not only makes the Hg2+-sensing platform safer but also reduces the cost of a single assay. For on-site tests, lighting varies with the environmental conditions such as weather, sunlight strength and angle, or even suspension particles. To overcome these factors, several images from single test were recorded under artificial white light and the color of cellulose paper, hydrophobic barriers and reference color bars was used for adjustment. Moreover, a sealed box with uniform artificial light sources and algorithms analysis for compensating measurement errors due to variability in ambient light are also being investigated.50, 51 In addition to other AuNP based colorimetric sensing mechanisms utilizing catalytic reactions or binding effects with a µPAD platform being investigating to achieve higher sensitivity and lower detection limits, different kinds of metal ions can be detected simultaneously in a single pot by incorporating other nanomaterial-based heavy metal detection methods for environmental pollution control and monitoring.


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Scheme 1. A schematic illustration of the proposed on-site Hg2+ sensing strategy. Water samples extracted from suspected polluted sites are first added to the ssDNA-attached AuNP colloids and colorimetric sensing is triggered with the addition of a sodium chloride solution. After color development, the mixture is placed on hydrophobic-wax-confined chromatography paper for color enhancement, and the detection results are then recorded and transmitted to a server to instantly receive quantitative data via cloud computing.


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Figure 1. UV–Vis absorbance spectra of colorimetric AuNPs and Hg2+-ssDNA complex-based Hg2+ sensing. Based on the Ext 715/520 plot, the detection limit of 50 nM was obtained (n=5). A linear correlation is observed over the range of 25 nM – 750 nM (y = 0.0007x + 0.1272 and R2 = 0.98).


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Figure 2. The colorimetric results for mercury ion detection (a) in cuvettes and (b) on cellulose paper. Panels (c) and (d) show the analytical blue/red color values of the spots. This paper-based colorimetric sensing platform has a detection limit of 50 nM, with a turnaround time of 40 min (n=5). The length of the inset scale bar is 3 mm.


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Figure 3. SEM images of spots of detected AuNP mixtures (a) with and (b) without Hg2+ on cellulose paper. The bright spots in the images are AuNPs.


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Figure 4. The specificity test results obtained for samples (a) in cuvettes, (b) samples on cellulose paper and (c) the analytical blue/red color values of the spots obtained using the proposed sensing platform. Although the concentrations of the other metal ions were 10-fold higher than the concentration of Hg2+, the blue/red values remain < 1 (n=5). The length of the inset scale bar is 3 mm.


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Figure 5. The colorimetric results of mercury ion detection from Hg2+-spiked (a) pond and (b) river water. The detection limit for both the Hg2+-spiked pond and river water remained 50 nM, which demonstrates the consistency of the sensing platform (n=5). A linear correlation for both cases are over the range of 25 nM – 100 nM (y = 0.0002x + 0.9923 and y = 0.0002x + 0.9634, respectively).


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

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Fax: +886 4 22852422. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This research was supported by the Ministry of Science and Technology, Taiwan (NSC 1012113-M-005-001-MY2 and NSC 102-2218-E-005-011) and the Biotechnology Center at National Chung Hsing University.

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Table of Contents Graphic


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dopa nanoparticles.

Colorimetric aptasensor using unmodified gold nanoparticles for homogeneous multiplex detection.

Colorimetric detection of mercury species based on functionalized gold nanoparticles.

Colorimetric biosensing of pathogens using gold nanoparticles.

Self-assembly of core-satellite gold nanoparticles for colorimetric detection of copper ions.

Selective colorimetric NO(g) detection based on the use of modified gold nanoparticles using click chemistry.

A sensitive and selective colorimetric method for detection of copper ions based on anti-aggregation of unmodified gold nanoparticles.

Detection of Copper(II) Ions Using Glycine on Hydrazine-Adsorbed Gold Nanoparticles via Raman Spectroscopy.

Highly sensitive colorimetric detection of lead using maleic acid functionalized gold nanoparticles.

Colorimetric detection of human chorionic gonadotropin using catalytic gold nanoparticles and a peptide aptamer.

Direct visualization of lead corona and its nanomolar colorimetric detection using anisotropic gold nanoparticles.

Colorimetric Detection of Plasmodium vivax in Urine Using MSP10 Oligonucleotides and Gold Nanoparticles.

Metal-induced aggregation of valine capped gold nanoparticles: An efficient and rapid approach for colorimetric detection of Pb2+ ions.

Ultrasensitive colorimetric detection of Cu2+ using gold nanorods.

Colorimetric detection of DNA hybridization based on a dual platform of gold nanoparticles and graphene oxide.

Colorimetric-based detection of TNT explosives using functionalized silica nanoparticles.

Gold nanozyme-based paper chip for colorimetric detection of mercury ions.

Colorimetric sensor array based on gold nanoparticles and amino acids for identification of toxic metal ions in water.

A portable lab-on-a-chip system for gold-nanoparticle-based colorimetric detection of metal ions in water.

Colorimetric detection of melamine in milk by citrate-stabilized gold nanoparticles.

Highly Sensitive Colorimetric Detection of Ochratoxin A by a Label-Free Aptamer and Gold Nanoparticles.

Phosphorylation-regulated crosslinking of gold nanoparticles: a new strategy for colorimetric detection of protein kinase activity.

Sensitive colorimetric detection of protein by gold nanoparticles and rolling circle amplification.

Lysine-promoted colorimetric response of gold nanoparticles: a simple assay for ultrasensitive mercury(II) detection.