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Non-aggregation based label free colorimetric sensor for the detection of Cr (VI) based on selective etching of gold nanorods
Sensors and Actuators B 155 (2011) 817–822
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Non-aggregation based label free colorimetric sensor for the detection of Cr (VI) based on selective etching of gold nanorods Fei-Ming Li a , Jia-Ming Liu a,∗ , Xin-Xing Wang a , Li-Ping Lin a , Wen-Lian Cai a , Xuan Lin a , Yi-Na Zeng a , Zhi-Ming Li b , Shao-Qin Lin c a b c
Department of Chemistry and Environmental Science, Zhangzhou Normal College, Zhangzhou 363000, PR China Department of Food and Biological Engineering, Zhangzhou Institute of Technology, Zhangzhou 363000, PR China Department of Biochemistry, Fujian Education College, Fuzhou 350001, China
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Article history: Received 30 November 2010 Received in revised form 13 January 2011 Accepted 27 January 2011 Available online 2 February 2011 Keywords: Gold nanorod Etching Non-aggregation based label free colorimetric sensor Cr (VI)
a b s t r a c t Gold nanorods (GNRs) exhibit strong longitudinal surface plasmon resonance absorption (LPA), which is highly dependent on its aspect ratio (length/width). The strong oxidization of Cr (VI) enables it to etch GNRs selectively at tips. The redox etching causes the aspect ratio of GNRs to decrease, resulting in the LPA blue shifts and the color of GNRs distinctly changes. Besides, the blue shift is linear to the concentration of Cr (VI) in the range of 0.1–20 M. Thus, a non-aggregation based label free colorimetric sensor for the detection of Cr (VI) has been developed based on the selective etching of GNRs. The proposed colorimetric sensor is responsive, simple, sensitive (detection limit is 8.8 × 10−8 M) and selective, and it has been successfully applied to the detection of Cr (VI) in drinking water and sea water. Moreover, the mechanism of colorimetric sensor for the detection of Cr (VI) was also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In natural environments, chromium is generally in two most stable states of Cr (III) and Cr (VI), and their toxicities and mobilities differ significantly. Cr (III) is an essential trace element that improves the efficiency of insulin in individuals with impaired glucose tolerance, and generally precipitates onto mineral surfaces and so it is immobile in aquatic environments. But, Cr (VI) is known to be highly soluble and toxic with carcinogenic effect, which is hazardous to human beings [1]. Thus, the US Environmental Protection Agency recommends that the concentration of Cr (VI) in drinking water should be less than 0.1 g mL−1 . Nowadays, chromium has been increasingly used in a number of industrial processes including chrome planting, dye and pigment fabrication, leather tanning, and wood preserving. Due to the increasing threat of Cr (VI) exposure in the environment, there has been a growing interest in the development of highly sensitive and selective assays for the determination of Cr (VI) over the past few years. Various sensor systems have been reported [2–10]. Most of these systems, however, have either limitations with aspect to sensitivity, simplicity or the need of surface modification for substantial selectivity. Therefore, it should be highly desirable to develop a sensor that is not
∗ Corresponding author. Tel.: +86 596 2591352. E-mail address: [email protected] (J.-M. Liu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.054
only sensitive, selective and reliable but also simple, practical and economical in its operation. Colorimetric methods are extremely attractive, in particular, for the field detection, because they can be easily read out with the naked eye, offering advantages of simplicity and rapidity, along with the additional benefits of cost-effectiveness and no requirement of any sophisticated instrumentation. Gold nanoparticles (AuNPs) exhibit strong surface plasmon resonance (SPR) absorption with extremely high extinction coefficients (108 –1010 M−1 cm−1 ) in the visible wavelength range [11]. SPR is highly dependent on the composition, size, and shape of the nanoparticles and sensitive to the dielectric constant of the surrounding medium. Taking advantage of these characteristics, lots of colorimetric sensors have been developed for the detection of ions [12–16], biomolecules [17–21], organic molecules [22–25], most of which can be distinguished as aggregation sensors based on cross-linking and electrostatic absorption. However, most of these analyte-induced aggregation colorimetric sensors display drawbacks in terms of actual applicability. For cross-linking colorimetric sensors, crosslinkers that possess at least two binding tags have to be synthesized in order to connect individual AuNPs. The complex synthesis procedures involved limit the applicability of the sensors [25]. Moreover, AuNPs aggregation induced by inter-particle crosslinking sometimes takes several hours to observe the color changes [19]. In comparison with cross-linking colorimetric sensors, aggregation sensors based on electrostatic interaction and physical adsorp-
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tion is a simple process that reducing complexity of crosslinkers preparation. However, the binding is not strong enough to yield stable surfaces capable of standing the necessary washing steps and incubation conditions in biological studies on subsequent reaction [18,19]. For example, long time incubation with buffer solution which contains attacking molecules like dithiothreitol (a small, uncharged molecule with two thiol groups, used to protect proteins from oxidation) and salt of high concentration (generally used in DNA hybridization experiment) result in a strong non-specific interaction between the AuNPs probes and analytes, which leads to decrease detection selectivity [17,19]. GNRs exhibit strong LPA due to coherent electronic oscillation along the long axis. The extinction coefficient of GNRs is larger than AuNPs and it is four orders of magnitude than those of traditional organic chromophores [26]. Furthermore, LPA is dependent on the rod length of GNRs and has sensitive response to the rod length change [27]. We have observed that Cr (VI) could shorten the length of GNRs due to the redox etching of GNRs at tips. Moreover, the etching effect enhances as the concentration of Cr (VI) increases with a significant blue shift of LPA and color change of the GNRs. Motivated by the sensitive response of GNRs to Cr (VI), a nonaggregation based label free colorimetric sensor for the detection of Cr (VI) was achieved with simple visual inspection in less than 30 min. Besides, the proposed sensor obviates the surface modification, and has high selectivity and sensitivity, making it a promising technique in Cr (VI) analysis. 2. Experimental 2.1. Instrumentation Absorbance measurements were carried out using a Shimadzu UV-2550 spectrophotometer with one pair of 10-mm quartz cell. Inductively coupled plasma mass spectroscopy (ICP-MS) was performed with ICP-MS instrument (Agilent 7500, USA). A cole Parmer microfiltration apparatus with microfiltration systems (MFS) membrane filter (0.45 m pore size) was used for the filtration of water samples. 2.2. Chemicals HAuCl4 ·3H2 O (>99%), AgNO3 (>99%), NaBH4 (99%), l-ascorbic acid (>99%), hexadecyltrimethylammoniumbromide (CTAB, 99%), and all other fundamental reagents (analytical-reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. Nanopure deionized and distilled water (18.2 M) was used for all experiments. 2.3. Synthesis of GNRs GNRs were synthesized according to silver ion-assisted seedmediated method previously reported by El-Sayed [28]. Briefly, 5.00 mL of 0.2 M CTAB was mixed with 5.00 mL of 0.02 mM HAuCl4 and stirred. Thereafter, 0.60 mL of ice-cold 0.01 M NaBH4 was added, and the color of the solution changed from dark yellow to brownish yellow under vigorous stirring, indicating the formation of the seed solution. The seed solution was kept in a water bath at 25 ◦ C for at least 2 h before use. To prepare GNRs growth solution, 75.00 mL of 0.2 M CTAB was mixed with 1.25 mL of 4 mM AgNO3 aqueous solution and 75.00 mL of 1 mM HAuCl4 . After gently mixing the solution, 1.05 mL 0.10 M l-ascorbic acid was added and the growth solution was obtained. With continuously stirring growth solution, 180 L seed solution was taken out and added to the growth solution at 25 ◦ C. The color of the solution gradually changed to dark red in the first 15 min until finally stabilized. The solution was aged for 24 h to ensure full growth of GNRs. The
Fig. 1. UV–Vis absorption spectra of GNRs sensor responses to Cr (VI) of different concentrations after reacting at 50 ◦ C for 30 min under the condition of pH 1.0 and 0.02 M CTAB.
obtained GNRs solution was centrifugated at 8000 rpm for 15 min to remove some small spherical particles since they retained in the supernatant. The GNRs were then resuspended in 0.02 M CTAB for use. 2.4. GNRs based Sensor for Cr (VI) detection For Cr (VI) sensing, 1.00 mL GNRs were placed in a 5-mL test tube, and 1.00 mL of 0.5 M HCl was added to keep the system under acidic condition. Then 1.00 mL Cr (VI) of different concentrations (0.1–20 M) was added. After quick mixing, the solution was incubated at 50 ◦ C for 30 min and then put on ice for 2 min to cool down the solution. The reagent blank experiment was also carried out. Absorption spectra of the test solution and reagent blank were recorded and the blue shift ( = 1 − 2 , 1 and 2 are the LPA wavelength of reagent blank and test solution) was calculated. 2.5. Analysis of Cr (VI) in sea water and drinking water Acidic digestion of real samples was performed according to Ref. [4]. Briefly, 1000.00 mL water sample was treated with 5.00 mL HNO3 (65%) and boiled for 20 min to remove coexisting organic substances and convert Fe2+ to Fe3+ . The pH of the residual solution was then adjusted to 9.0 with ammonia and heated again for 20 min to precipitate Fe3+ , Cr3+ and Cu2+ . Then the treated water samples was filtered and adjusted to the initial volume and pH value was adjusted to 1.0 with HCl before use. Finally, the Cr (VI) content of test solution was separately determined by the developed sensing technique and ICP-MS. 3. Results and discussion 3.1. Sensing strategy To understand the role that Cr (VI) plays in etching of GNRs, SPR absorption of GNRs was monitored (Fig. 1). The initial GNRs exhibit two SPR absorption peaks located at 531 and 713 nm, corresponding to transverse surface plasmon resonance adsorption (TPA) and LPA, respectively. As the concentration of Cr (VI) increases, the
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Table 1 Tolerance ratio of co-existing ions on the determination of 5.0 M Cr (VI) using the proposed sensor (conditions: 5.0 × 10−4 M of GNRs, 30 min reaction time at 50 ◦ C, pH 1.0 and 0.02 M CTAB). Co-existing ions
Tolerance ratio (mol/mol)
Li+ , K+ , Na+ , Mg2+ , Co2+ , Zn2+ , Fe2+ , Ni2+ , Mn2+ , Ca2+ , Ba2+ , Cd2+ , Cr3+ , Cl− , NO3 − , SO4 2− , CO3 2− Pb2+ , Hg2+ , PO3− , Br− 4 I− Cu2+ Fe3+
>1000 100 50 16 12
LPA gradually blue shifts and absorbance decreases, resulting in that the LPA becomes a shoulder of the TPA and finally disappears (not shown). Besides, the color of the GNRs distinctly changes (Fig. 2). Taking accounting to these observations and with regard to the investigations of Yang and co-workers [27] and Stucky and co-workers [29], it is realized that the blue-shift is due to the decreasing of aspect ratio (length/width) of GNRs and a preferential shortening along the axial direction. Based on the blue shift of LPA and color changes of the GNRs, Scheme 1 outlines the sensing strategy employed in this study. The standard electron potential of Au (I)/Au (0) is 1.691 eV. The electron potential of Au (I)/Au (0) decreases when the Br− ion of CTAB and Cl− of HCl act as ligands of gold (AuBr2 − + e → Au + 2Br− , E = 0.959 eV [29], AuCl2 − + e → Au + 2Cl− , E = 1.15 eV [27]). The standard electron potential of Cr (VI)/Cr (III) (1.33 eV) is higher than that of Au (I)/Au (0) in the presence of Br− and Cl− ion, which enables Cr (VI) to oxidize GNRs. The redox etching induced by Cr (VI) causes a significant decrease of GNRs in length but little change in diameter, which attributes to less surface passivation and higher chemical reactivity of the tips of the GNRs [27,29]. Upon further oxidation, GNRs are shortened, converted into gold nanospheres and eventually completely oxidized, exhibiting distinct color changes varying with LPA. Besides, the blue shift is linear to the concentration of Cr (VI), allowing determination of Cr (VI) using GNRs as non-aggregation colorimetric sensor. 3.2. Optimization of the conditions for the Cr (VI) measurement In order to optimize the conditions for the determination of Cr (VI), a number of parameters that influence on the sensitivity, selectivity, accuracy and stability were investigated in a univariate approach. 3.2.1. Effect of the concentration of GNRs The effect of the concentration of GNRs on the system was examined in order to obtain a wide linear range and high sensitivity. When lowering the concentration of GNRs, the etching of GNRs by Cr (VI) increases and blue shift accordingly enhances, with a favorable sensitivity but narrow linear range comparing with high concentration of GNRs. In contrast, higher concentration expands the linear range but limits sensitivity. In consideration of both lin-
ear range and sensitivity, 5.0 × 10−4 M of GNRs was chosen for the future studies. 3.2.2. Effect of pH on the performance of determination of Cr (VI) When pH is higher than 7.0, the OH− can form ion pairs with CTA+ through electrostatic force, leading the amount of CTA+ packed on the {1 1 0} face of GNRs to decrease [30]. In this case, GNRs will agglomerate irreversibly even in very low ionic strength solution as pH increases and the stability of GNRs decreases. In order to improve the stability of GNRs, the effect of pH value on sensor response to 10 M Cr (VI) was investigated only under the acid condition (Fig. 3). As is observed in Fig. 3, the etching of GNRs enhances as the pH value decreases and reaches maximum when pH is 1.0, which ascribes to the electron potential and oxidation ability of Cr (VI) increase when pH value decreases. To support this point, simple calculations are conducted to roughly estimate the electron potentials of Cr (VI) at different pH values. Because there is little Cr (III) at the beginning, it is reasonable to assume that only 0.01% of Cr (VI) turns into the reductive species Cr (III) for the calculation of the initial electron potential. Then, it is found that the electron potentials of Cr (VI)/Cr (III) are 1.271 and 0.995 eV when the pH values are 1.0 and 3.0, respectively. Besides, the increasing Cl− in the solution could further reduce the electron potential of the gold species (Au (I)/Au (0) is 1.268 and 1.032 when Cl− is 0.001 M and 0.1 M, assuming that only 0.01% of Au turns into the oxidative species Au (I) for the calculation of the initial electron potentials) and facilitate the etching of GNRs. In an attempt to identify the best acid agent, such as HCl, HNO3 and H2 SO4 were investigated. The detection of Cr (VI) was interfered when HNO3 and H2 SO4 were used because they could also etch GNRs when pH was 1.0. In consideration of the stability of GNRs and sensor response to Cr (VI), HCl was chosen for further investigation. 3.2.3. Effect of the concentration of CTAB CTAB could effectively absorb on the surface of the GNRs [31], which is supposed to prohibit chemical etching. However, Fig. 4
60
Δλ
40
20
0 1
2
3
4
5
6
7
pH Fig. 2. Color responses when GNRs react with Cr (VI) of various concentrations after incubation at 50 ◦ C for 30 min under the condition of pH 1.0 and 0.02 M CTAB.
Fig. 3. Effect of pH value on the of GNRs (conditions: 5.0 × 10−4 M of GNRs, 10 M of Cr (VI), 0.02 M of CTAB, 30 min reaction time at 50 ◦ C).
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Scheme 1. Schematic illustration on the etching mechanism of the GNRs induced by Cr (VI).
50 40 30
Δλ
illustrates the etching process of the GNRs speeds up with the increasing concentration of CTAB, attributing to the considerable amount of Br− introduced from CTAB reduces the electron potential of the gold species (AuBr2 − + e → Au + 2Br− , E = 0.959 V) [27,29]. Hence, the increasing concentration sensitizes the sensor response to Cr (VI). Nevertheless, the great electron potential decrease of the gold species induced by Br− (Au (I)/Au (0) is 0.924 and 0.824 when CTAB is 0.02 M and 0.14 M, assuming that only 0.01% of Au turns into the oxidative species Au (I) for the calculation of the initial electron potentials) leads to increasing interference from other oxidative cations, lowering the selectivity of proposed sensor. Giving an attention to selectivity and sensitivity, 0.02 M CTAB was employed in the present study.
20 10 0
3.2.4. Effect of reaction temperature and time El-Sayed et al. found that the aspect ratio of GNRs decreases with increasing temperature [30]. In order to avoid thermal reshaping of GNRs, the temperature effect on the etching reaction was investigated only in the range of 25–50 ◦ C. As shown in Figs. 5 and 6, the selective etching () is dependent on the both reaction temperature and time. At a fixed reaction time of 20 min, the selective etching enhances as the temperature rises (Fig. 5). By adjustment of the reaction temperature at 50 ◦ C, the selective etching by 10 M Cr (VI) completes in 30 min (Fig. 6) and the system remains stable for 6 h (data not shown). Thus, in the following studies, the reaction is carried out by incubating the system at 50 ◦ C for 30 min.
For 5.0 M Cr (VI), the selectivity of the sensor was evaluated by testing the response of the sensor to other environmentally relevant ions under optimum conditions. Each ion was considered as interfering agent, when the relative error was more than ±5%. As shown in Table 1, most ions could be allowed at high concentrations (>100-fold). Fe3+ and Cu2+ had mild interference and their concentrations should be kept at no more than 12-fold and 16-fold of Cr (VI), respectively. At present, pretreatment would be neces-
35
40
45
50
T/ºC Fig. 5. Effect of reaction temperature on the of GNRs (conditions: 5.0 × 10−4 M GNRs, 10 M of Cr (VI), 20 min reaction time, pH 1.0).
sary. Fe3+ and Cu2+ can be easily obviated by adding ammonia to precipitate. The results indicate that the sensor has a good selectivity.
The calibration curves for the determination of Cr (VI) were constructed under the optimum conditions. The measurement linear range for the Cr (VI) is from 0.10 M to 20.0 M. The linear regression equation is y = 4.538x (M) + 12.47 with a correlation coefficient of 0.9913 (n = 6). The R.S.D. obtained for 1.0 × 10−6 M of Cr (VI) is 2.3% (n = 6) and the 3 limits of detection for Cr (VI) is 8.8 × 10−8 M (here represents the standard deviation of 11 blank measurements).
76
64
72
60
Δλ
68 64
56 52
60 0.00
30
3.4. Analytical figures of merit
3.3. Interference effect
Δλ
25
48 0.04
0.08
0.12
0.16
CCTAB / M Fig. 4. Effect of CTAB concentration on the of GNRs (conditions: 5.0 × 10−4 M of GNRs, 10 M of Cr (VI), 30 min reaction time at 50 ◦ C, pH 1.0).
10
20
30
40
50
60
t/min Fig. 6. Effect of reaction time on the of GNRs (conditions: 5.0 × 10−4 M GNRs, 10 M of Cr (VI), 50 ◦ C reaction temperature, pH 1.0).
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Table 2 Determination of Cr (VI) in water samples. Sample
Average found (g L−1 ) (n = 6)
Cr (VI) added (g L−1 )
Cr (VI) found (g L−1 )
Recovery (%)
R.S.D. (%) (n = 6)
Cr (VI) found by ICP-MS (g L−1 )
Drinking water 1 Drinking water 2 Sea water 1 Sea water 2
– – – –
20.00 25.00 30.00 35.00
20.70 25.60 31.34 36.70
103.5 102.4 104.5 104.8
3.7 4.4 2.9 3.9
20.90 25.82 30.98 36.13
3.5. Application of the sensor The determination of Cr (VI) in drinking water and sea water was carried out according to the procedure described in Section 2.5. Besides, the standard recovery experiment was also carried out. The comparisons between the proposed sensor and ICP-MS were summarized in Table 2. The detection results of proposed method agree well with those of ICP-MS, indicating this sensor has high accuracy.
[10]
[11] [12]
[13]
4. Conclusion A highly sensitive and selective non-aggregation based colorimetric sensor for the detection of Cr (VI) has been developed based on the etching of GNRs induced by Cr (VI). In comparison with aggregation based colorimetric sensor based on cross-linking or electrostatic absorption, the proposed non-aggregation colorimetric sensor shows selective and sensitive response towards Cr (VI) without surface modification. Additionally, the sensor for the detection of Cr (VI) was achieved with simple visual inspection in less than 30 min without special instrument, making it a promising sensor to on-line detect Cr (VI) in water sample. Furthermore, the proposed method can also provide an alternative way to obtain the GNRs with desirable aspect ratios and specific optical properties.
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgements
[20]
This project supported by the Fujian Province Natural Science Foundation (Nos. 2010J01053, JK2010035, 2009J1017 and 2008J0313), Fujian Province Education Committee (JA08252, JB08262, JA10203 and JA10277) and Scientific Research Program of Zhangzhou Institute of Technology Foundation (Nos. ZZY 1007, ZZY1009 and ZZY1014). At the same time, we are very grateful to precious advices raised by the reviewers.
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Biographies Fei-Ming Li is currently a graduate student in the Zhangzhou Normal College, under the supervision of Prof. Jia-ming Liu. Li’s research interests include the development of new colorimetric sensing materials and approach for colorimetric sensors. Jia-Ming Liu is a secondary professor and postgraduate tutor in Zhangzhou Normal College, and a winner of the State Council special government allowances from 2001. His research interests include research of photoluminescence anal-
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ysis and analysis study of catalytic kinetics and the design of colorimetric sensing.
Xuan Lin now is an associate professor in Zhangzhou Normal College and engages in photoluminescence analysis study.
Xin-Xing Wang received her bachelor degree in 2009, is a postgraduate of 2009 in Zhangzhou Normal College and engages in photoluminescence analysis study.
Yi-Na Zeng is a student in Zhangzhou Normal College and engages in photoluminescence analysis study.
Li-Ping Lin received her bachelor degree in 2009, now is a postgraduate of 2009 in Zhangzhou Normal College and engages in photoluminescence analysis study. Wen-Lian Cai is a band four professor in Zhangzhou Normal College and engages in photoluminescence analysis study.
Zhi-Ming Li now is a professor in Zhangzhou Institute of Technology and engages in photoluminescence analysis study. Shao-Qin Lin is a band four professor in Fujian Institute of Education and engages in photoluminescence analysis study.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Non-aggregation based label free colorimetric sensor for the detection of Cr (VI) based on selective etching of gold nanorods Fei-Ming Li a , Jia-Ming Liu a,∗ , Xin-Xing Wang a , Li-Ping Lin a , Wen-Lian Cai a , Xuan Lin a , Yi-Na Zeng a , Zhi-Ming Li b , Shao-Qin Lin c a b c
Department of Chemistry and Environmental Science, Zhangzhou Normal College, Zhangzhou 363000, PR China Department of Food and Biological Engineering, Zhangzhou Institute of Technology, Zhangzhou 363000, PR China Department of Biochemistry, Fujian Education College, Fuzhou 350001, China
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Article history: Received 30 November 2010 Received in revised form 13 January 2011 Accepted 27 January 2011 Available online 2 February 2011 Keywords: Gold nanorod Etching Non-aggregation based label free colorimetric sensor Cr (VI)
a b s t r a c t Gold nanorods (GNRs) exhibit strong longitudinal surface plasmon resonance absorption (LPA), which is highly dependent on its aspect ratio (length/width). The strong oxidization of Cr (VI) enables it to etch GNRs selectively at tips. The redox etching causes the aspect ratio of GNRs to decrease, resulting in the LPA blue shifts and the color of GNRs distinctly changes. Besides, the blue shift is linear to the concentration of Cr (VI) in the range of 0.1–20 M. Thus, a non-aggregation based label free colorimetric sensor for the detection of Cr (VI) has been developed based on the selective etching of GNRs. The proposed colorimetric sensor is responsive, simple, sensitive (detection limit is 8.8 × 10−8 M) and selective, and it has been successfully applied to the detection of Cr (VI) in drinking water and sea water. Moreover, the mechanism of colorimetric sensor for the detection of Cr (VI) was also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In natural environments, chromium is generally in two most stable states of Cr (III) and Cr (VI), and their toxicities and mobilities differ significantly. Cr (III) is an essential trace element that improves the efficiency of insulin in individuals with impaired glucose tolerance, and generally precipitates onto mineral surfaces and so it is immobile in aquatic environments. But, Cr (VI) is known to be highly soluble and toxic with carcinogenic effect, which is hazardous to human beings [1]. Thus, the US Environmental Protection Agency recommends that the concentration of Cr (VI) in drinking water should be less than 0.1 g mL−1 . Nowadays, chromium has been increasingly used in a number of industrial processes including chrome planting, dye and pigment fabrication, leather tanning, and wood preserving. Due to the increasing threat of Cr (VI) exposure in the environment, there has been a growing interest in the development of highly sensitive and selective assays for the determination of Cr (VI) over the past few years. Various sensor systems have been reported [2–10]. Most of these systems, however, have either limitations with aspect to sensitivity, simplicity or the need of surface modification for substantial selectivity. Therefore, it should be highly desirable to develop a sensor that is not
∗ Corresponding author. Tel.: +86 596 2591352. E-mail address: [email protected] (J.-M. Liu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.054
only sensitive, selective and reliable but also simple, practical and economical in its operation. Colorimetric methods are extremely attractive, in particular, for the field detection, because they can be easily read out with the naked eye, offering advantages of simplicity and rapidity, along with the additional benefits of cost-effectiveness and no requirement of any sophisticated instrumentation. Gold nanoparticles (AuNPs) exhibit strong surface plasmon resonance (SPR) absorption with extremely high extinction coefficients (108 –1010 M−1 cm−1 ) in the visible wavelength range [11]. SPR is highly dependent on the composition, size, and shape of the nanoparticles and sensitive to the dielectric constant of the surrounding medium. Taking advantage of these characteristics, lots of colorimetric sensors have been developed for the detection of ions [12–16], biomolecules [17–21], organic molecules [22–25], most of which can be distinguished as aggregation sensors based on cross-linking and electrostatic absorption. However, most of these analyte-induced aggregation colorimetric sensors display drawbacks in terms of actual applicability. For cross-linking colorimetric sensors, crosslinkers that possess at least two binding tags have to be synthesized in order to connect individual AuNPs. The complex synthesis procedures involved limit the applicability of the sensors [25]. Moreover, AuNPs aggregation induced by inter-particle crosslinking sometimes takes several hours to observe the color changes [19]. In comparison with cross-linking colorimetric sensors, aggregation sensors based on electrostatic interaction and physical adsorp-
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tion is a simple process that reducing complexity of crosslinkers preparation. However, the binding is not strong enough to yield stable surfaces capable of standing the necessary washing steps and incubation conditions in biological studies on subsequent reaction [18,19]. For example, long time incubation with buffer solution which contains attacking molecules like dithiothreitol (a small, uncharged molecule with two thiol groups, used to protect proteins from oxidation) and salt of high concentration (generally used in DNA hybridization experiment) result in a strong non-specific interaction between the AuNPs probes and analytes, which leads to decrease detection selectivity [17,19]. GNRs exhibit strong LPA due to coherent electronic oscillation along the long axis. The extinction coefficient of GNRs is larger than AuNPs and it is four orders of magnitude than those of traditional organic chromophores [26]. Furthermore, LPA is dependent on the rod length of GNRs and has sensitive response to the rod length change [27]. We have observed that Cr (VI) could shorten the length of GNRs due to the redox etching of GNRs at tips. Moreover, the etching effect enhances as the concentration of Cr (VI) increases with a significant blue shift of LPA and color change of the GNRs. Motivated by the sensitive response of GNRs to Cr (VI), a nonaggregation based label free colorimetric sensor for the detection of Cr (VI) was achieved with simple visual inspection in less than 30 min. Besides, the proposed sensor obviates the surface modification, and has high selectivity and sensitivity, making it a promising technique in Cr (VI) analysis. 2. Experimental 2.1. Instrumentation Absorbance measurements were carried out using a Shimadzu UV-2550 spectrophotometer with one pair of 10-mm quartz cell. Inductively coupled plasma mass spectroscopy (ICP-MS) was performed with ICP-MS instrument (Agilent 7500, USA). A cole Parmer microfiltration apparatus with microfiltration systems (MFS) membrane filter (0.45 m pore size) was used for the filtration of water samples. 2.2. Chemicals HAuCl4 ·3H2 O (>99%), AgNO3 (>99%), NaBH4 (99%), l-ascorbic acid (>99%), hexadecyltrimethylammoniumbromide (CTAB, 99%), and all other fundamental reagents (analytical-reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. Nanopure deionized and distilled water (18.2 M) was used for all experiments. 2.3. Synthesis of GNRs GNRs were synthesized according to silver ion-assisted seedmediated method previously reported by El-Sayed [28]. Briefly, 5.00 mL of 0.2 M CTAB was mixed with 5.00 mL of 0.02 mM HAuCl4 and stirred. Thereafter, 0.60 mL of ice-cold 0.01 M NaBH4 was added, and the color of the solution changed from dark yellow to brownish yellow under vigorous stirring, indicating the formation of the seed solution. The seed solution was kept in a water bath at 25 ◦ C for at least 2 h before use. To prepare GNRs growth solution, 75.00 mL of 0.2 M CTAB was mixed with 1.25 mL of 4 mM AgNO3 aqueous solution and 75.00 mL of 1 mM HAuCl4 . After gently mixing the solution, 1.05 mL 0.10 M l-ascorbic acid was added and the growth solution was obtained. With continuously stirring growth solution, 180 L seed solution was taken out and added to the growth solution at 25 ◦ C. The color of the solution gradually changed to dark red in the first 15 min until finally stabilized. The solution was aged for 24 h to ensure full growth of GNRs. The
Fig. 1. UV–Vis absorption spectra of GNRs sensor responses to Cr (VI) of different concentrations after reacting at 50 ◦ C for 30 min under the condition of pH 1.0 and 0.02 M CTAB.
obtained GNRs solution was centrifugated at 8000 rpm for 15 min to remove some small spherical particles since they retained in the supernatant. The GNRs were then resuspended in 0.02 M CTAB for use. 2.4. GNRs based Sensor for Cr (VI) detection For Cr (VI) sensing, 1.00 mL GNRs were placed in a 5-mL test tube, and 1.00 mL of 0.5 M HCl was added to keep the system under acidic condition. Then 1.00 mL Cr (VI) of different concentrations (0.1–20 M) was added. After quick mixing, the solution was incubated at 50 ◦ C for 30 min and then put on ice for 2 min to cool down the solution. The reagent blank experiment was also carried out. Absorption spectra of the test solution and reagent blank were recorded and the blue shift ( = 1 − 2 , 1 and 2 are the LPA wavelength of reagent blank and test solution) was calculated. 2.5. Analysis of Cr (VI) in sea water and drinking water Acidic digestion of real samples was performed according to Ref. [4]. Briefly, 1000.00 mL water sample was treated with 5.00 mL HNO3 (65%) and boiled for 20 min to remove coexisting organic substances and convert Fe2+ to Fe3+ . The pH of the residual solution was then adjusted to 9.0 with ammonia and heated again for 20 min to precipitate Fe3+ , Cr3+ and Cu2+ . Then the treated water samples was filtered and adjusted to the initial volume and pH value was adjusted to 1.0 with HCl before use. Finally, the Cr (VI) content of test solution was separately determined by the developed sensing technique and ICP-MS. 3. Results and discussion 3.1. Sensing strategy To understand the role that Cr (VI) plays in etching of GNRs, SPR absorption of GNRs was monitored (Fig. 1). The initial GNRs exhibit two SPR absorption peaks located at 531 and 713 nm, corresponding to transverse surface plasmon resonance adsorption (TPA) and LPA, respectively. As the concentration of Cr (VI) increases, the
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Table 1 Tolerance ratio of co-existing ions on the determination of 5.0 M Cr (VI) using the proposed sensor (conditions: 5.0 × 10−4 M of GNRs, 30 min reaction time at 50 ◦ C, pH 1.0 and 0.02 M CTAB). Co-existing ions
Tolerance ratio (mol/mol)
Li+ , K+ , Na+ , Mg2+ , Co2+ , Zn2+ , Fe2+ , Ni2+ , Mn2+ , Ca2+ , Ba2+ , Cd2+ , Cr3+ , Cl− , NO3 − , SO4 2− , CO3 2− Pb2+ , Hg2+ , PO3− , Br− 4 I− Cu2+ Fe3+
>1000 100 50 16 12
LPA gradually blue shifts and absorbance decreases, resulting in that the LPA becomes a shoulder of the TPA and finally disappears (not shown). Besides, the color of the GNRs distinctly changes (Fig. 2). Taking accounting to these observations and with regard to the investigations of Yang and co-workers [27] and Stucky and co-workers [29], it is realized that the blue-shift is due to the decreasing of aspect ratio (length/width) of GNRs and a preferential shortening along the axial direction. Based on the blue shift of LPA and color changes of the GNRs, Scheme 1 outlines the sensing strategy employed in this study. The standard electron potential of Au (I)/Au (0) is 1.691 eV. The electron potential of Au (I)/Au (0) decreases when the Br− ion of CTAB and Cl− of HCl act as ligands of gold (AuBr2 − + e → Au + 2Br− , E = 0.959 eV [29], AuCl2 − + e → Au + 2Cl− , E = 1.15 eV [27]). The standard electron potential of Cr (VI)/Cr (III) (1.33 eV) is higher than that of Au (I)/Au (0) in the presence of Br− and Cl− ion, which enables Cr (VI) to oxidize GNRs. The redox etching induced by Cr (VI) causes a significant decrease of GNRs in length but little change in diameter, which attributes to less surface passivation and higher chemical reactivity of the tips of the GNRs [27,29]. Upon further oxidation, GNRs are shortened, converted into gold nanospheres and eventually completely oxidized, exhibiting distinct color changes varying with LPA. Besides, the blue shift is linear to the concentration of Cr (VI), allowing determination of Cr (VI) using GNRs as non-aggregation colorimetric sensor. 3.2. Optimization of the conditions for the Cr (VI) measurement In order to optimize the conditions for the determination of Cr (VI), a number of parameters that influence on the sensitivity, selectivity, accuracy and stability were investigated in a univariate approach. 3.2.1. Effect of the concentration of GNRs The effect of the concentration of GNRs on the system was examined in order to obtain a wide linear range and high sensitivity. When lowering the concentration of GNRs, the etching of GNRs by Cr (VI) increases and blue shift accordingly enhances, with a favorable sensitivity but narrow linear range comparing with high concentration of GNRs. In contrast, higher concentration expands the linear range but limits sensitivity. In consideration of both lin-
ear range and sensitivity, 5.0 × 10−4 M of GNRs was chosen for the future studies. 3.2.2. Effect of pH on the performance of determination of Cr (VI) When pH is higher than 7.0, the OH− can form ion pairs with CTA+ through electrostatic force, leading the amount of CTA+ packed on the {1 1 0} face of GNRs to decrease [30]. In this case, GNRs will agglomerate irreversibly even in very low ionic strength solution as pH increases and the stability of GNRs decreases. In order to improve the stability of GNRs, the effect of pH value on sensor response to 10 M Cr (VI) was investigated only under the acid condition (Fig. 3). As is observed in Fig. 3, the etching of GNRs enhances as the pH value decreases and reaches maximum when pH is 1.0, which ascribes to the electron potential and oxidation ability of Cr (VI) increase when pH value decreases. To support this point, simple calculations are conducted to roughly estimate the electron potentials of Cr (VI) at different pH values. Because there is little Cr (III) at the beginning, it is reasonable to assume that only 0.01% of Cr (VI) turns into the reductive species Cr (III) for the calculation of the initial electron potential. Then, it is found that the electron potentials of Cr (VI)/Cr (III) are 1.271 and 0.995 eV when the pH values are 1.0 and 3.0, respectively. Besides, the increasing Cl− in the solution could further reduce the electron potential of the gold species (Au (I)/Au (0) is 1.268 and 1.032 when Cl− is 0.001 M and 0.1 M, assuming that only 0.01% of Au turns into the oxidative species Au (I) for the calculation of the initial electron potentials) and facilitate the etching of GNRs. In an attempt to identify the best acid agent, such as HCl, HNO3 and H2 SO4 were investigated. The detection of Cr (VI) was interfered when HNO3 and H2 SO4 were used because they could also etch GNRs when pH was 1.0. In consideration of the stability of GNRs and sensor response to Cr (VI), HCl was chosen for further investigation. 3.2.3. Effect of the concentration of CTAB CTAB could effectively absorb on the surface of the GNRs [31], which is supposed to prohibit chemical etching. However, Fig. 4
60
Δλ
40
20
0 1
2
3
4
5
6
7
pH Fig. 2. Color responses when GNRs react with Cr (VI) of various concentrations after incubation at 50 ◦ C for 30 min under the condition of pH 1.0 and 0.02 M CTAB.
Fig. 3. Effect of pH value on the of GNRs (conditions: 5.0 × 10−4 M of GNRs, 10 M of Cr (VI), 0.02 M of CTAB, 30 min reaction time at 50 ◦ C).
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Scheme 1. Schematic illustration on the etching mechanism of the GNRs induced by Cr (VI).
50 40 30
Δλ
illustrates the etching process of the GNRs speeds up with the increasing concentration of CTAB, attributing to the considerable amount of Br− introduced from CTAB reduces the electron potential of the gold species (AuBr2 − + e → Au + 2Br− , E = 0.959 V) [27,29]. Hence, the increasing concentration sensitizes the sensor response to Cr (VI). Nevertheless, the great electron potential decrease of the gold species induced by Br− (Au (I)/Au (0) is 0.924 and 0.824 when CTAB is 0.02 M and 0.14 M, assuming that only 0.01% of Au turns into the oxidative species Au (I) for the calculation of the initial electron potentials) leads to increasing interference from other oxidative cations, lowering the selectivity of proposed sensor. Giving an attention to selectivity and sensitivity, 0.02 M CTAB was employed in the present study.
20 10 0
3.2.4. Effect of reaction temperature and time El-Sayed et al. found that the aspect ratio of GNRs decreases with increasing temperature [30]. In order to avoid thermal reshaping of GNRs, the temperature effect on the etching reaction was investigated only in the range of 25–50 ◦ C. As shown in Figs. 5 and 6, the selective etching () is dependent on the both reaction temperature and time. At a fixed reaction time of 20 min, the selective etching enhances as the temperature rises (Fig. 5). By adjustment of the reaction temperature at 50 ◦ C, the selective etching by 10 M Cr (VI) completes in 30 min (Fig. 6) and the system remains stable for 6 h (data not shown). Thus, in the following studies, the reaction is carried out by incubating the system at 50 ◦ C for 30 min.
For 5.0 M Cr (VI), the selectivity of the sensor was evaluated by testing the response of the sensor to other environmentally relevant ions under optimum conditions. Each ion was considered as interfering agent, when the relative error was more than ±5%. As shown in Table 1, most ions could be allowed at high concentrations (>100-fold). Fe3+ and Cu2+ had mild interference and their concentrations should be kept at no more than 12-fold and 16-fold of Cr (VI), respectively. At present, pretreatment would be neces-
35
40
45
50
T/ºC Fig. 5. Effect of reaction temperature on the of GNRs (conditions: 5.0 × 10−4 M GNRs, 10 M of Cr (VI), 20 min reaction time, pH 1.0).
sary. Fe3+ and Cu2+ can be easily obviated by adding ammonia to precipitate. The results indicate that the sensor has a good selectivity.
The calibration curves for the determination of Cr (VI) were constructed under the optimum conditions. The measurement linear range for the Cr (VI) is from 0.10 M to 20.0 M. The linear regression equation is y = 4.538x (M) + 12.47 with a correlation coefficient of 0.9913 (n = 6). The R.S.D. obtained for 1.0 × 10−6 M of Cr (VI) is 2.3% (n = 6) and the 3 limits of detection for Cr (VI) is 8.8 × 10−8 M (here represents the standard deviation of 11 blank measurements).
76
64
72
60
Δλ
68 64
56 52
60 0.00
30
3.4. Analytical figures of merit
3.3. Interference effect
Δλ
25
48 0.04
0.08
0.12
0.16
CCTAB / M Fig. 4. Effect of CTAB concentration on the of GNRs (conditions: 5.0 × 10−4 M of GNRs, 10 M of Cr (VI), 30 min reaction time at 50 ◦ C, pH 1.0).
10
20
30
40
50
60
t/min Fig. 6. Effect of reaction time on the of GNRs (conditions: 5.0 × 10−4 M GNRs, 10 M of Cr (VI), 50 ◦ C reaction temperature, pH 1.0).
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Table 2 Determination of Cr (VI) in water samples. Sample
Average found (g L−1 ) (n = 6)
Cr (VI) added (g L−1 )
Cr (VI) found (g L−1 )
Recovery (%)
R.S.D. (%) (n = 6)
Cr (VI) found by ICP-MS (g L−1 )
Drinking water 1 Drinking water 2 Sea water 1 Sea water 2
– – – –
20.00 25.00 30.00 35.00
20.70 25.60 31.34 36.70
103.5 102.4 104.5 104.8
3.7 4.4 2.9 3.9
20.90 25.82 30.98 36.13
3.5. Application of the sensor The determination of Cr (VI) in drinking water and sea water was carried out according to the procedure described in Section 2.5. Besides, the standard recovery experiment was also carried out. The comparisons between the proposed sensor and ICP-MS were summarized in Table 2. The detection results of proposed method agree well with those of ICP-MS, indicating this sensor has high accuracy.
[10]
[11] [12]
[13]
4. Conclusion A highly sensitive and selective non-aggregation based colorimetric sensor for the detection of Cr (VI) has been developed based on the etching of GNRs induced by Cr (VI). In comparison with aggregation based colorimetric sensor based on cross-linking or electrostatic absorption, the proposed non-aggregation colorimetric sensor shows selective and sensitive response towards Cr (VI) without surface modification. Additionally, the sensor for the detection of Cr (VI) was achieved with simple visual inspection in less than 30 min without special instrument, making it a promising sensor to on-line detect Cr (VI) in water sample. Furthermore, the proposed method can also provide an alternative way to obtain the GNRs with desirable aspect ratios and specific optical properties.
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgements
[20]
This project supported by the Fujian Province Natural Science Foundation (Nos. 2010J01053, JK2010035, 2009J1017 and 2008J0313), Fujian Province Education Committee (JA08252, JB08262, JA10203 and JA10277) and Scientific Research Program of Zhangzhou Institute of Technology Foundation (Nos. ZZY 1007, ZZY1009 and ZZY1014). At the same time, we are very grateful to precious advices raised by the reviewers.
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Biographies Fei-Ming Li is currently a graduate student in the Zhangzhou Normal College, under the supervision of Prof. Jia-ming Liu. Li’s research interests include the development of new colorimetric sensing materials and approach for colorimetric sensors. Jia-Ming Liu is a secondary professor and postgraduate tutor in Zhangzhou Normal College, and a winner of the State Council special government allowances from 2001. His research interests include research of photoluminescence anal-
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ysis and analysis study of catalytic kinetics and the design of colorimetric sensing.
Xuan Lin now is an associate professor in Zhangzhou Normal College and engages in photoluminescence analysis study.
Xin-Xing Wang received her bachelor degree in 2009, is a postgraduate of 2009 in Zhangzhou Normal College and engages in photoluminescence analysis study.
Yi-Na Zeng is a student in Zhangzhou Normal College and engages in photoluminescence analysis study.
Li-Ping Lin received her bachelor degree in 2009, now is a postgraduate of 2009 in Zhangzhou Normal College and engages in photoluminescence analysis study. Wen-Lian Cai is a band four professor in Zhangzhou Normal College and engages in photoluminescence analysis study.
Zhi-Ming Li now is a professor in Zhangzhou Institute of Technology and engages in photoluminescence analysis study. Shao-Qin Lin is a band four professor in Fujian Institute of Education and engages in photoluminescence analysis study.