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Colorimetric detection of Hg2+ ions in aqueous media using CA–Au NPs
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 737–740
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric detection of Hg2+ ions in aqueous media using CA–Au NPs Zening Liu a, Jiao Hu a, Sijia Tong a, Qihua Cao b, Hong Yuan a,⇑ a b
State Key Laboratory of Agricultural Microbiology, College of Science, Huazhong Agricultural University, Wuhan 430070, PR China School of Resource and Environmental Science, Wuhan University, Wuhan 430072, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" We developed a colorimetric method
"
"
"
"
for detecting Hg2+ ions using Au NPs and CA. The experiment validated the selective interaction between CA and Hg2+ ions. The experiment validated the antiaggregation of CA–Au NPs induced by NaCl. We also evaluated the sensitivity and selectivity of the sensor of Hg2+ ions. The use of a highly stable and commercially available CA as Hg2+ acceptor avoids any other labeling or modification steps.
a r t i c l e
i n f o
Article history: Received 24 May 2012 Accepted 15 June 2012 Available online 10 July 2012 Keywords: Colorimetric method Hg2+ ions Gold nanoparticles Cyanuric acid
a b s t r a c t Based on the selective interaction between Hg2+ ions and cyanuric acid (CA) and the anti-aggregation of CA stabilized gold nanoparticles (CA–Au NPs), a simple colorimetric method was developed for detecting Hg2+ ions. In a medium of pH 7.4 tris–HCl buffer containing 8 10 3 M NaCl, the CA–Au NPs solution was red, which was due to CA adsorbed onto the surface of Au NPs, stabilizing Au NPs against aggregation. When CA–HgII–CA complex was formed in the presence of Hg2+, the stability of CA–Au NPs reduced, and then aggregation of Au NPs occurred. Consequently, the color of the solution changed from red to blue and could easily be measured with a common spectrophotometer. The aggregation of Au NPs was also validated using transmission electron microscopy (TEM). The controlled experiment showed that other ions including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, and Ni2+ ions did not induce any distinct spectral changes, which constituted a Hg2+-selective sensor. A dynamic range of 1.6–16 10 6 M Hg2+ ions was observed at the optimized reaction condition. This method provides a potentially useful tool for Hg2+ detection. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Heavy-metal pollution (e.g. mercury) is an important environmental concern because it can cause serious human health problems [1,2]. It has been reported that mercury could cause permanent harmful effects in living organisms even at relatively low dose, such as memory loss, neuronal, hepatic, and nephritic ⇑ Corresponding author. Tel.: +86 27 87284018; fax: +86 27 87282133. E-mail address: [email protected] (H. Yuan). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.06.051
damage, decrease in the rate of fertility, as well as birth defects in offspring [2]. Thus, there is a pressing need to develop specific ion sensors for the rapid detection of mercury ions (Hg2+). Many methods have been developed for the determination of Hg2+ ions in different biological, industrial and food samples. Traditional quantitative approaches to HgII analysis (e.g. atomic absorption spectroscopy, cold vapor atomic fluorescence spectrometry, and gas chromatography) require complicated, multistep sample preparation and/or sophisticated instrumentation [3–7]. Compared with conventional techniques, chemical sensors based
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on optical signal measurement are considered as the advanced techniques because of its simplicity, reasonable selectivity, improved sensitivity, and fieldwork applicability. Despite the above strongpoint, the use of chemical sensors suffers from several drawbacks, such as the complicated organic synthesis procedure involved [8]. As such, there has still been a growing need or desire for constructing optical chemical sensors for fast and economical monitoring of Hg2+ ions. The highly selective determination of Hg2+ ions is usually accomplished by spectroscopic methods upon binding to the specific receptors. It has recently been found that Hg2+ ions can bind with two thymine (T) residues of DNA to form the T–HgII–T complex [9,10]. The stability of this T–HgII–T base pair is higher than that of a T–A Watson–Crick pair, and this interaction is highly specific (only Hg2+ ions can stabilize the T–T base pair) [10]. It provides a rationale for applying T-containing oligonucleotide sequences for specifically sensing aqueous Hg2+ in diverse ways [11–15]. Moreover, Au NPs-based sensing methods have attracted more and more attention due to their intrinsically high sensitivity and easy colorimetric read-out [16–18]. Hence, a variety of colorimetric sensors based on T-containing DNA/Au NPs have been developed for the selective detection of Hg2+ ions [19–25]. However, the design and synthesis of various sophisticated DNA oligomer probes is tedious and expensive, and the enzymatic DNA degradation is unavoidable when detecting Hg2+ from environmental water samples. The structure of CA with high stability was similar as that of thymine (diimide groups), so it is possible to replace T or DNA by using CA as the specific receptors for determining Hg2+ ions. Herein, we demonstrated that Hg2+ ions could be recognized selectively in aqueous solution by the colorimetry based on cyanuric acid stabilized Au NPs (CA–Au NPs). In our experiment, the addition of Hg2+ ions reduced the stability of CA–Au NPs, and then aggregation of Au NPs occurred after adding NaCl, which induced visible colorimetric response of CA–Au NPs solution from red to purple. The sensitivity and selectivity of the colorimetric assay were also investigated. 2. Experimental 2.1 Reagents and apparatus HAuCl44H2O, Na3(C6H5O7)2H2O, Hg(NO3)2, NaCl, concentrated HCl, and concentrated HNO3 were purchased from Sinopharm Group, Shanghai, China. All chemicals were of analytical-reagent grade. Deionized water produced by a Milli-Q system (Millipore, USA) was used for preparing solutions. All glassware used in these preparations was thoroughly cleaned in aqua regia (3 parts HCl, 1 part HNO3), rinsed in triply distilled H2O, and oven-dried prior to use. Unless otherwise noted, the experiments were carried out at room temperature (20–25 °C). UV/vis absorption spectra were recorded with an Evolution 300 UV/vis spectrophotometer (Thermo Scientific, USA). TEM images were acquired by using Hitachi-7650 (HITACHI, Japan) transmission electron microscopes operated at 120 kV.
solution of colloidal Au NPs was characterized by absorption spectroscopy and transmission electron microscopy (TEM). The spherical Au NPs obtained were characterized by a maximum absorption at 520 nm and had uniform size 14 nm (Figure S1). 2.3 Procedure of detecting Hg2+ ions In a 10 mL test tube, 300 lL Au NPs was mixed with 5.0 mL of Tris–HCl buffer. 25 lL of 10 3 mol L 1 of CA and 4 mL of distilled H2O was added, and equilibrated for 10 min at room temperature. Different concentrations of Hg2+ solution were added into this mixture, and then, 100 lL of NaCl (0.4 mol L 1) was added to the resulted solution before being incubated for another 5 min. The absorbance of solution was measured in the range of 400– 1000 nm. All reported concentrations of reagents or samples were the initial values in reaction mixtures.
3. Results and discussion 3.1 The sensing mechanism and feasibility for detecting Hg2+ ions Our basic idea of the present work is on the basis of the adsorption of CA on the surface of Au NPs, and the addition of Hg2+ ions, which could react with CA through CA–HgII–CA coordination, reduces the stability of CA–Au NPs so that NaCl could readily induce the aggregation of Au NPs, resulting in shifting surface plasmon resonance (SPR) signals owing to the distance changes between Au NPs [17,18]. The detailed sensing mechanism of CA–Au NPs sensor for Hg2+ ions is shown in Scheme 1. In order to study the feasibility of our approach, two below experiments were conducted: the anti-aggregation of CA–Au NPs, and the interaction between CA and Hg2+. In Turkevich method, citrate ions act as both a reducing agent and a capping agent, which sticks to Au NP surface to prevent them from aggregating [16,17]. However, the interaction force between citrate ions and Au NPs is weak, so it is easy to replace citrate ions by other molecules with heterocyclic N and SH groups [16–18,23,27,28]. It is shown from Figure S2 that adding 0.3 ml NaCl to colloidal Au NPs led to their aggregation, which increased the relative absorption at longer wavelength region. However, in the presence of CA, the addition of 0.3 ml NaCl did not change the relative absorption of Au NPs solution. It is said that the anti-aggregation ability of Au NPs was enhanced because CA was absorbed on the surface of Au NPs via its heterocyclic N [16,17]. Then, we explored whether CA could reacted with Hg2+ for the structure of CA was similar as that of thymine (diimide groups) [9–15]. After different amount of Hg2+ ions was added to the solution of 10 4 mol L CA, the absorption increased in the UV range (Figure S3). The above change resulted from the interaction between CA and Hg2+ ions, so CA could be used as functional group for detecting Hg2+. From the above experiments, it is possible to develop the sensor for Hg2+ by CA and Au NPs.
2.2 Colloidal gold preparation [26] Colloidal gold were prepared following the described method. 50 milliliters of 1 mM HAuCl4 was brought to a round-bottomed flask under vigorous stirring. 5 millilters of 38.8 mM trisodium citrate was rapidly added to the vortex of the boiling solution, resulting in a color change from pale yellow to burgundy. Boiling was prolonged for 10 min. The heating mantle was then removed, while the stirring was continued for an additional 15 min. The resulting
: CA
: Au NPs
: Hg2+
Scheme 1. Detection of Hg2+ ions using Au NPs coated with CA.
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A 0.18
1.35
-6
x10 M Hg 0 1.6 2.0 4.0 8.0 12.0 16.0
0.15
Absorbance
A660 nm/A520 nm
1.30
1.25
0.12 0.09
2+
0.06
1.20
0.03 1.15 0.00 400 7.0
7.5
8.0
8.5
500
600
9.0
700
800
900
1000
Wavelength / nm
pH Fig. 1. Effect of pH on the colorimetric response of CA–Au NPs. The concentration of Hg2+ ions was 4 10 5 mol L 1.
B
1.0
The effects of experimental conditions on the detection of mercury ions were investigated. From the studies on sensing Hg2+ ions via thymine, it is known that the effect of pH on the formation of T–HgII–T complex is very obvious, and the most favorable experimental environment is weak alkaline [10,23]. We investigated the effect of pH on colorimetric response of Hg2+ sensor in Tris– HCl buffer. It can be seen in Fig. 1 that the colorimetric response was pH-dependent. The A660nm/A520nm value increased with the increase of pH value and reached to the maximum at pH = 7.4. It can be seen that, in a range of pH from 7.4 to 8.5, pH change does not affect the determination of Hg2+ with the proposed method. The results were parallel with the previous report that Hg2+ coordinated favorably and strongly with the diimide group of thymine at appropriate pH [10,23]. This is because the diimide group is easily deprotonated in basic solutions which is valuable for the subsequent formation of CA–HgII–CA. The results supported the hypothesis that the colorimetric response was resulted from the formation of CA–HgII–CA, which decreased the stability of Au NPs. Therefore, pH 7.4 Tris–HCl buffer solution was selected as an ideal experimental condition. Next, the reaction time was investigated. The relative absorption intensity was flatted after 5 min. The change of relative absorption was very small as the incubation time was extended. Therefore, the incubation time of 5 min selected. 3.2 Analytical performance of the sensor Based on these studies, we evaluated the analytical performance of the sensor for quantitative assays of Hg2+. Firstly, the analytical performance of the optical sensor was investigated by
A660 nm/A520 nm
0.8
0.6
0.4
0.2
0.0 0
2
4
6
8
10
12
14
16
18
2+
C (Hg ) / µM Fig. 2. (A) UV–Vis absorption spectra of CA–Au NPs with different concentrations of Hg2+ ions after the addition of NaCl. (B) A plot of A660nm/A520nm versus the concentration of Hg2+ ions.
varying the Hg2+ concentration. The CA–Au NPs probes were prepared by mixing Au NPs with CA, after which the CA–Au NPs solution still maintained wine red for its strong SPR at 520 nm. When Hg2+ was added to the solution of CA–Au NPs, their stability decrease, and the aggregation of CA–Au NPs induced by NaCl was completed within a few minutes, in which the dramatic color change was from wine red to violet blue. Spectroscopically, significant changes in UV/Vis absorption were identified 5 min after mixing Hg2+ with CA–Au NPs. Both a decreased absorbance of the plasmon band at 520 nm and an increased absorbance at about 660 nm were observed (Fig. 2). The absorbance values of CA–Au NPs solutions at 520 and 660 nm are related to the quantities of dispersed and aggregated Au NPs, respectively. The absorption ra-
Fig. 3. TEM images of CA–Au NPs in the absence (A) and presence (B) of 40 10
6
mol L
1
Hg2+ solution.
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0.15
along with the concentration of Hg2+ ions could be accordingly tuned. This method offers below advantages: (1) the assay is rather simple: all it takes is recording the absorption spectrum or observing the color change of the sensing system. (2) The use of a highly stable and commercially available CA as Hg2+ acceptor avoids any other labeling or modification steps. Therefore, we expect that this strategy may offer a new approach for developing low-cost and rapid sensor for detecting of Hg2+ ions, and will be highly useful in a wide range of applications.
0.10
Acknowledgements
0.05
This work was supported by the National Natural Science Foundation of China (20907015), the Program for Chenguang Young Scientist for Wuhan (201050231071), and the Fundamental Research Funds for the Central Universities (2011PY131).
0.35 0.30
A660 nm/A520 nm
0.25 0.20
0.00 2+
Cd
2+
Cu
2+
Mg
2+
Mn
2+
Ni
2+
Zn
2+
Co
2+
Ca
2+
Ba
2+
black Hg
Fig. 4. The A660nm/A520nm value of solution in the presence of 4 lM Hg2+ and 40 lM each of other metal ions, including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, Ni2+.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.051.
tion at 520 nm and 660 nm (A660nm/A520nm) was used to reflect the ratio of dispersed and aggregated Au NPs. It was known from Fig. 2 that the absorption ration increased along with an increase in the Hg2+ concentration in the range of 1.6–16 10 6 mol L 1, and the linear correlation was obtained (R = 0.97). Transmission electron microscopy (TEM) further revealed the aggregation of CA–Au NPs (Fig. 3b), comparing to initial dispersion in the absence of Hg2+ (Fig. 3a). Moreover, with the decrease of CA concentration, the linear range was narrowed because CA–Au NPs solution has aggregated so sharply that sedimentation takes place quickly. It is obvious that CA solution with the higher concentration is less prone to aggregate at a fixed Au NPs concentration. This phenomenon may be due to the lack of CA per particle to stabilize. Thus, it is possible to use CA solution of given concentration to analyze Hg2+ ions as need [29]. Secondly, the selectivity of metal ion sensors is of great importance to explore the application of real sample detection. To realize the selectivity of our system, some common metal ions were chosen for the investigation, including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, Ni2+ (Fig. 4). It was found from experimental data that every tested metal ion could not induce the aggregation of CA–Au NPs, and no absorption change was observed within 30 min. The color of CA–Au NPs immediately changed within 5 min only in presence of Hg2+. The highly selective interaction between CA and Hg2+ resulted to decreasing the stability of CA–Au NPs, which was due to the structure similarity of CA and thymine. Other metal ions could not react with CA to change the stability of CA–Au NPs, with the consequence of aggregation of CA–Au NPs. 4. Conclusions In summary, a simple colorimetric detection method for Hg2+ ions is developed based on the anti-aggregation of CA–Au NPs, and the selective interaction between CA and Hg2+. CA could increase the stability of colloidal Au NPs, whereas, the addition of Hg2+ ions decreased the anti-aggregation of CA–Au NPs. The aggregation of Au NPs induced by NaCl occurred. The color of the solution changed from red to blue within 5 min and could easily be measured with a common spectrophotometer. The color displayed
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Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric detection of Hg2+ ions in aqueous media using CA–Au NPs Zening Liu a, Jiao Hu a, Sijia Tong a, Qihua Cao b, Hong Yuan a,⇑ a b
State Key Laboratory of Agricultural Microbiology, College of Science, Huazhong Agricultural University, Wuhan 430070, PR China School of Resource and Environmental Science, Wuhan University, Wuhan 430072, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" We developed a colorimetric method
"
"
"
"
for detecting Hg2+ ions using Au NPs and CA. The experiment validated the selective interaction between CA and Hg2+ ions. The experiment validated the antiaggregation of CA–Au NPs induced by NaCl. We also evaluated the sensitivity and selectivity of the sensor of Hg2+ ions. The use of a highly stable and commercially available CA as Hg2+ acceptor avoids any other labeling or modification steps.
a r t i c l e
i n f o
Article history: Received 24 May 2012 Accepted 15 June 2012 Available online 10 July 2012 Keywords: Colorimetric method Hg2+ ions Gold nanoparticles Cyanuric acid
a b s t r a c t Based on the selective interaction between Hg2+ ions and cyanuric acid (CA) and the anti-aggregation of CA stabilized gold nanoparticles (CA–Au NPs), a simple colorimetric method was developed for detecting Hg2+ ions. In a medium of pH 7.4 tris–HCl buffer containing 8 10 3 M NaCl, the CA–Au NPs solution was red, which was due to CA adsorbed onto the surface of Au NPs, stabilizing Au NPs against aggregation. When CA–HgII–CA complex was formed in the presence of Hg2+, the stability of CA–Au NPs reduced, and then aggregation of Au NPs occurred. Consequently, the color of the solution changed from red to blue and could easily be measured with a common spectrophotometer. The aggregation of Au NPs was also validated using transmission electron microscopy (TEM). The controlled experiment showed that other ions including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, and Ni2+ ions did not induce any distinct spectral changes, which constituted a Hg2+-selective sensor. A dynamic range of 1.6–16 10 6 M Hg2+ ions was observed at the optimized reaction condition. This method provides a potentially useful tool for Hg2+ detection. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Heavy-metal pollution (e.g. mercury) is an important environmental concern because it can cause serious human health problems [1,2]. It has been reported that mercury could cause permanent harmful effects in living organisms even at relatively low dose, such as memory loss, neuronal, hepatic, and nephritic ⇑ Corresponding author. Tel.: +86 27 87284018; fax: +86 27 87282133. E-mail address: [email protected] (H. Yuan). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.06.051
damage, decrease in the rate of fertility, as well as birth defects in offspring [2]. Thus, there is a pressing need to develop specific ion sensors for the rapid detection of mercury ions (Hg2+). Many methods have been developed for the determination of Hg2+ ions in different biological, industrial and food samples. Traditional quantitative approaches to HgII analysis (e.g. atomic absorption spectroscopy, cold vapor atomic fluorescence spectrometry, and gas chromatography) require complicated, multistep sample preparation and/or sophisticated instrumentation [3–7]. Compared with conventional techniques, chemical sensors based
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on optical signal measurement are considered as the advanced techniques because of its simplicity, reasonable selectivity, improved sensitivity, and fieldwork applicability. Despite the above strongpoint, the use of chemical sensors suffers from several drawbacks, such as the complicated organic synthesis procedure involved [8]. As such, there has still been a growing need or desire for constructing optical chemical sensors for fast and economical monitoring of Hg2+ ions. The highly selective determination of Hg2+ ions is usually accomplished by spectroscopic methods upon binding to the specific receptors. It has recently been found that Hg2+ ions can bind with two thymine (T) residues of DNA to form the T–HgII–T complex [9,10]. The stability of this T–HgII–T base pair is higher than that of a T–A Watson–Crick pair, and this interaction is highly specific (only Hg2+ ions can stabilize the T–T base pair) [10]. It provides a rationale for applying T-containing oligonucleotide sequences for specifically sensing aqueous Hg2+ in diverse ways [11–15]. Moreover, Au NPs-based sensing methods have attracted more and more attention due to their intrinsically high sensitivity and easy colorimetric read-out [16–18]. Hence, a variety of colorimetric sensors based on T-containing DNA/Au NPs have been developed for the selective detection of Hg2+ ions [19–25]. However, the design and synthesis of various sophisticated DNA oligomer probes is tedious and expensive, and the enzymatic DNA degradation is unavoidable when detecting Hg2+ from environmental water samples. The structure of CA with high stability was similar as that of thymine (diimide groups), so it is possible to replace T or DNA by using CA as the specific receptors for determining Hg2+ ions. Herein, we demonstrated that Hg2+ ions could be recognized selectively in aqueous solution by the colorimetry based on cyanuric acid stabilized Au NPs (CA–Au NPs). In our experiment, the addition of Hg2+ ions reduced the stability of CA–Au NPs, and then aggregation of Au NPs occurred after adding NaCl, which induced visible colorimetric response of CA–Au NPs solution from red to purple. The sensitivity and selectivity of the colorimetric assay were also investigated. 2. Experimental 2.1 Reagents and apparatus HAuCl44H2O, Na3(C6H5O7)2H2O, Hg(NO3)2, NaCl, concentrated HCl, and concentrated HNO3 were purchased from Sinopharm Group, Shanghai, China. All chemicals were of analytical-reagent grade. Deionized water produced by a Milli-Q system (Millipore, USA) was used for preparing solutions. All glassware used in these preparations was thoroughly cleaned in aqua regia (3 parts HCl, 1 part HNO3), rinsed in triply distilled H2O, and oven-dried prior to use. Unless otherwise noted, the experiments were carried out at room temperature (20–25 °C). UV/vis absorption spectra were recorded with an Evolution 300 UV/vis spectrophotometer (Thermo Scientific, USA). TEM images were acquired by using Hitachi-7650 (HITACHI, Japan) transmission electron microscopes operated at 120 kV.
solution of colloidal Au NPs was characterized by absorption spectroscopy and transmission electron microscopy (TEM). The spherical Au NPs obtained were characterized by a maximum absorption at 520 nm and had uniform size 14 nm (Figure S1). 2.3 Procedure of detecting Hg2+ ions In a 10 mL test tube, 300 lL Au NPs was mixed with 5.0 mL of Tris–HCl buffer. 25 lL of 10 3 mol L 1 of CA and 4 mL of distilled H2O was added, and equilibrated for 10 min at room temperature. Different concentrations of Hg2+ solution were added into this mixture, and then, 100 lL of NaCl (0.4 mol L 1) was added to the resulted solution before being incubated for another 5 min. The absorbance of solution was measured in the range of 400– 1000 nm. All reported concentrations of reagents or samples were the initial values in reaction mixtures.
3. Results and discussion 3.1 The sensing mechanism and feasibility for detecting Hg2+ ions Our basic idea of the present work is on the basis of the adsorption of CA on the surface of Au NPs, and the addition of Hg2+ ions, which could react with CA through CA–HgII–CA coordination, reduces the stability of CA–Au NPs so that NaCl could readily induce the aggregation of Au NPs, resulting in shifting surface plasmon resonance (SPR) signals owing to the distance changes between Au NPs [17,18]. The detailed sensing mechanism of CA–Au NPs sensor for Hg2+ ions is shown in Scheme 1. In order to study the feasibility of our approach, two below experiments were conducted: the anti-aggregation of CA–Au NPs, and the interaction between CA and Hg2+. In Turkevich method, citrate ions act as both a reducing agent and a capping agent, which sticks to Au NP surface to prevent them from aggregating [16,17]. However, the interaction force between citrate ions and Au NPs is weak, so it is easy to replace citrate ions by other molecules with heterocyclic N and SH groups [16–18,23,27,28]. It is shown from Figure S2 that adding 0.3 ml NaCl to colloidal Au NPs led to their aggregation, which increased the relative absorption at longer wavelength region. However, in the presence of CA, the addition of 0.3 ml NaCl did not change the relative absorption of Au NPs solution. It is said that the anti-aggregation ability of Au NPs was enhanced because CA was absorbed on the surface of Au NPs via its heterocyclic N [16,17]. Then, we explored whether CA could reacted with Hg2+ for the structure of CA was similar as that of thymine (diimide groups) [9–15]. After different amount of Hg2+ ions was added to the solution of 10 4 mol L CA, the absorption increased in the UV range (Figure S3). The above change resulted from the interaction between CA and Hg2+ ions, so CA could be used as functional group for detecting Hg2+. From the above experiments, it is possible to develop the sensor for Hg2+ by CA and Au NPs.
2.2 Colloidal gold preparation [26] Colloidal gold were prepared following the described method. 50 milliliters of 1 mM HAuCl4 was brought to a round-bottomed flask under vigorous stirring. 5 millilters of 38.8 mM trisodium citrate was rapidly added to the vortex of the boiling solution, resulting in a color change from pale yellow to burgundy. Boiling was prolonged for 10 min. The heating mantle was then removed, while the stirring was continued for an additional 15 min. The resulting
: CA
: Au NPs
: Hg2+
Scheme 1. Detection of Hg2+ ions using Au NPs coated with CA.
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Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 737–740
A 0.18
1.35
-6
x10 M Hg 0 1.6 2.0 4.0 8.0 12.0 16.0
0.15
Absorbance
A660 nm/A520 nm
1.30
1.25
0.12 0.09
2+
0.06
1.20
0.03 1.15 0.00 400 7.0
7.5
8.0
8.5
500
600
9.0
700
800
900
1000
Wavelength / nm
pH Fig. 1. Effect of pH on the colorimetric response of CA–Au NPs. The concentration of Hg2+ ions was 4 10 5 mol L 1.
B
1.0
The effects of experimental conditions on the detection of mercury ions were investigated. From the studies on sensing Hg2+ ions via thymine, it is known that the effect of pH on the formation of T–HgII–T complex is very obvious, and the most favorable experimental environment is weak alkaline [10,23]. We investigated the effect of pH on colorimetric response of Hg2+ sensor in Tris– HCl buffer. It can be seen in Fig. 1 that the colorimetric response was pH-dependent. The A660nm/A520nm value increased with the increase of pH value and reached to the maximum at pH = 7.4. It can be seen that, in a range of pH from 7.4 to 8.5, pH change does not affect the determination of Hg2+ with the proposed method. The results were parallel with the previous report that Hg2+ coordinated favorably and strongly with the diimide group of thymine at appropriate pH [10,23]. This is because the diimide group is easily deprotonated in basic solutions which is valuable for the subsequent formation of CA–HgII–CA. The results supported the hypothesis that the colorimetric response was resulted from the formation of CA–HgII–CA, which decreased the stability of Au NPs. Therefore, pH 7.4 Tris–HCl buffer solution was selected as an ideal experimental condition. Next, the reaction time was investigated. The relative absorption intensity was flatted after 5 min. The change of relative absorption was very small as the incubation time was extended. Therefore, the incubation time of 5 min selected. 3.2 Analytical performance of the sensor Based on these studies, we evaluated the analytical performance of the sensor for quantitative assays of Hg2+. Firstly, the analytical performance of the optical sensor was investigated by
A660 nm/A520 nm
0.8
0.6
0.4
0.2
0.0 0
2
4
6
8
10
12
14
16
18
2+
C (Hg ) / µM Fig. 2. (A) UV–Vis absorption spectra of CA–Au NPs with different concentrations of Hg2+ ions after the addition of NaCl. (B) A plot of A660nm/A520nm versus the concentration of Hg2+ ions.
varying the Hg2+ concentration. The CA–Au NPs probes were prepared by mixing Au NPs with CA, after which the CA–Au NPs solution still maintained wine red for its strong SPR at 520 nm. When Hg2+ was added to the solution of CA–Au NPs, their stability decrease, and the aggregation of CA–Au NPs induced by NaCl was completed within a few minutes, in which the dramatic color change was from wine red to violet blue. Spectroscopically, significant changes in UV/Vis absorption were identified 5 min after mixing Hg2+ with CA–Au NPs. Both a decreased absorbance of the plasmon band at 520 nm and an increased absorbance at about 660 nm were observed (Fig. 2). The absorbance values of CA–Au NPs solutions at 520 and 660 nm are related to the quantities of dispersed and aggregated Au NPs, respectively. The absorption ra-
Fig. 3. TEM images of CA–Au NPs in the absence (A) and presence (B) of 40 10
6
mol L
1
Hg2+ solution.
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Z. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 737–740
0.15
along with the concentration of Hg2+ ions could be accordingly tuned. This method offers below advantages: (1) the assay is rather simple: all it takes is recording the absorption spectrum or observing the color change of the sensing system. (2) The use of a highly stable and commercially available CA as Hg2+ acceptor avoids any other labeling or modification steps. Therefore, we expect that this strategy may offer a new approach for developing low-cost and rapid sensor for detecting of Hg2+ ions, and will be highly useful in a wide range of applications.
0.10
Acknowledgements
0.05
This work was supported by the National Natural Science Foundation of China (20907015), the Program for Chenguang Young Scientist for Wuhan (201050231071), and the Fundamental Research Funds for the Central Universities (2011PY131).
0.35 0.30
A660 nm/A520 nm
0.25 0.20
0.00 2+
Cd
2+
Cu
2+
Mg
2+
Mn
2+
Ni
2+
Zn
2+
Co
2+
Ca
2+
Ba
2+
black Hg
Fig. 4. The A660nm/A520nm value of solution in the presence of 4 lM Hg2+ and 40 lM each of other metal ions, including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, Ni2+.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.051.
tion at 520 nm and 660 nm (A660nm/A520nm) was used to reflect the ratio of dispersed and aggregated Au NPs. It was known from Fig. 2 that the absorption ration increased along with an increase in the Hg2+ concentration in the range of 1.6–16 10 6 mol L 1, and the linear correlation was obtained (R = 0.97). Transmission electron microscopy (TEM) further revealed the aggregation of CA–Au NPs (Fig. 3b), comparing to initial dispersion in the absence of Hg2+ (Fig. 3a). Moreover, with the decrease of CA concentration, the linear range was narrowed because CA–Au NPs solution has aggregated so sharply that sedimentation takes place quickly. It is obvious that CA solution with the higher concentration is less prone to aggregate at a fixed Au NPs concentration. This phenomenon may be due to the lack of CA per particle to stabilize. Thus, it is possible to use CA solution of given concentration to analyze Hg2+ ions as need [29]. Secondly, the selectivity of metal ion sensors is of great importance to explore the application of real sample detection. To realize the selectivity of our system, some common metal ions were chosen for the investigation, including Ba2+, Ca2+, Zn2+, Cd2+, Co2+, Mn2+, Cu2+, Mg2+, Ni2+ (Fig. 4). It was found from experimental data that every tested metal ion could not induce the aggregation of CA–Au NPs, and no absorption change was observed within 30 min. The color of CA–Au NPs immediately changed within 5 min only in presence of Hg2+. The highly selective interaction between CA and Hg2+ resulted to decreasing the stability of CA–Au NPs, which was due to the structure similarity of CA and thymine. Other metal ions could not react with CA to change the stability of CA–Au NPs, with the consequence of aggregation of CA–Au NPs. 4. Conclusions In summary, a simple colorimetric detection method for Hg2+ ions is developed based on the anti-aggregation of CA–Au NPs, and the selective interaction between CA and Hg2+. CA could increase the stability of colloidal Au NPs, whereas, the addition of Hg2+ ions decreased the anti-aggregation of CA–Au NPs. The aggregation of Au NPs induced by NaCl occurred. The color of the solution changed from red to blue within 5 min and could easily be measured with a common spectrophotometer. The color displayed
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