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Ultrasensitive colorimetric detection of Cu2+ using gold nanorods Xiaofang Niu,a Dong Xu,b Yunhui Yang*a and Yan He*b
Received 22nd November 2013 Accepted 1st March 2014 DOI: 10.1039/c3an02164e www.rsc.org/analyst
We utilized Cu2+ induced gold nanorod shortening in the presence of Na2S2O3 and NH3 for colorimetric sensing of copper ions. Compared with conventional methods, this method has good reproducibility, fast response time and very high sensitivity to Cu2+. The sensor has a large dynamic range for Cu2+ covering 5 nM to 500 mM with a detection limit of 1.6 nM, which is lower than previously reported for the colorimetric detection of copper ions.
Copper (Cu) is an essential trace element for the health of all living things and the environment. In the human body, copper is necessary for the proper functioning of organs and metabolic processes.1,2 It was reported in 1928 that Cu deciency results in anaemia.3 According to the World Health Organization (W. H. O.), 0.9 mg per day of copper is recommended for adults, but the intake should not exceed 10 mg per day.4 Excessive amounts of Cu2+ in the body can also pose health risks,5 such as liver disease, severe neurological decits and even lead to death. Therefore, development of a highly sensitive method for the detection of Cu2+ is particularly critical.6 Many methods have been proposed to detect Cu2+ in the eld of environmental and biomedical science, for example, ame atomic absorption spectrometry (FAAS),7 electrochemical devices8,9 and uorescence sensors.10,11 However, these methods have a number of shortcomings when used for Cu2+ detection, for instance, they are time-consuming, require expensive instruments, are complicated in operation, and their detection limits are oen higher than the standard of the Environmental Protection Agency (EPA). In recent years, accompanied with the fast development of nanotechnology, different kinds of metal nanoparticles have been prepared.12 Among these nanomaterials, gold nanorods
(GNRs) have attracted much interest due to their size and shapedependent optical and physical properties, and they have been applied to catalysis and optical sensing. GNRs with controlled aspect ratios can be readily prepared in high yield.13 Importantly, GNRs give rise to longitudinal or transverse localized surface plasma resonance (LSPR) absorption and scattering under the illumination of visible and near-infrared light of certain wavelengths, when surface electrons of the GNRs oscillate either parallel or perpendicular to their length direction. The longitudinal LSPR maximum is dependent on the aspect ratio, surface charge and refractive index of the surrounding medium. So far GNRs have been utilized to detect various biologically important species and environmental toxins.14 Some research groups have utilized GNRs for the detection of antibiotics,15 cancer cells16 and heavy metal ions.17 Recently, Placido et al. demonstrated a method for GNR synthesis and the subsequent detection of mercury ions (Hg2+).18 In their work, GNRs functionalized with a pyrazole-derived amino ligand (PyL) were prepared rst. Before adding Hg2+ solution, the functionalized GNRs exist in the solution in the form of stable monodisperse particles. The addition of Hg2+ can induce the GNRs into end-toend assembly. Compared with the UV-vis absorption spectrum before, the resulting absorption spectrum is red shied and its absorbance decreased greatly. Herein, we present a GNR-based colorimetric sensor for the detection of Cu2+. First of all, a new approach for the seed-mediated synthesis of GNRs was demonstrated. Secondly, a sensitive sensor was developed to detect Cu2+ using GNRs as the probe. The procedure is shown in
a
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming, China 650500. E-mail: [email protected]; Fax: +86 871 5941086; Tel: +86 871 5941087
b
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, China 410082. E-mail: [email protected]; Fax: +86 731 88821818; Tel: +86 731 88821818
This journal is © The Royal Society of Chemistry 2014
Scheme 1
Schematic diagram of the detection of Cu2+ based on
GNRs.
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Scheme 1. Initially the GNRs were coated with surface absorbed sodium thiosulfate and ammonia. Aer Cu2+ was added, an oxidation–reduction reaction occurred in the presence of Cu2+, leading to etching of the GNRs and a reduction in the size and aspect ratio. Such a morphological change decreases the absorbance peak of the GNR solution. In this process, Cu2+ was added to the ammonia and a complex of Cu(NH3)42+ was produced. As the GNRs were dispersed in the solution, the Cu(NH3)42+ could quickly combine the thiosulfate and gold atoms. The primary chemical reactions for the Cu2+–Cu+ redox system were19–21 Cu2+ + 4NH3 ¼ Cu(NH3)42+ Cu(NH3)42+ + 3S2O32 + e ¼ Cu(S2O3)35 + 4NH3 Banthia and Samanta22 have proved that the reduction of Cu complexes to Cu+ is more stable and easier. Moreover, aer sodium thiosulfate was added into the gold nanorod solution, the copper ions and ammonia showed a catalytic performance. The reason for this is that Cu2+ and NH3 form Cu (NH3)4+, which acts as an oxidant in the system. NH3 and S2O32 undergo the following reaction in the presence of Cu2+ in the solution. 2+
Au + Cu(NH3)42+ + 4S2O32 ¼ Au(S2O3)23 + Cu(S2O3)23 + 4NH3
Experimental apparatus and conditions include the following: cetyltrimethylammonium bromide (CTAB), tetrachloroauric acid (HAuCl4), sodium borohydride, silver nitrate, ascorbic acid, sodium thiosulfate, MgCl2, ZnCl2, TiCl, NaCl, KCl, and CuCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The UV-1800 spectrophotometer was from Shimadzu (Japan). An Ocean Optics USB2000 miniature ber-optic spectrometer (Ocean Optics, Dunedin, FL) was used to produce the absorption spectra instantaneously. To prepare the GNRs, a seed mediated method was adapted according to previous reports. 40.5 mL HAuCl4 (24.28 mM) was added to 4.0 mL of cetyltrimethylammonium bromide (CTAB) (0.1 M) and the solution was shaken in order to mix thoroughly. Subsequently, 24 mL ice-cold NaBH4 (0.1 M) was injected into the solution and allowed to stabilize for 2 h at 28 C (the color of the solution changed from yellow to brown). During this process, Au3+ was reduced to Au atoms.23,24 The seeds were then acquired from the aqueous solution. 206 mL of chloroauric acid (0.1 M), 10 mL of AgNO3 (0.1 M) and 70.5 mL of ascorbic acid (0.1 M) were added to 10.0 mL of CTAB (0.1 M), respectively, shaking it for 10 s until colorless. 30 mL of seeds were added to the solution, which was kept stationary for 3 h at 28 C. Finally, gold nanorods 40–50 nm in length and 10–15 nm in diameter were obtained.25,26 In order to remove CTAB from the GNR solution, a centrifugal method was used to wash it twice with ultrapure water. A relatively clean GNR solution containing a small quantity of CTAB was then acquired. The GNR solution was subsequently diluted to 10%. 20 mL of ammonia (1.4 M) was
2692 | Analyst, 2014, 139, 2691–2694
then added to 900 mL of the GNR solution and mixed for 5 minutes. 70 mL of Na2S2O3 (0.1 M) was then added to the solution, which was mixed, and remained undisturbed for about 30 minutes to ensure sufficient reaction occurred. The absorption intensity was measured using a UV-vis spectrophotometer. Following this, 10 mL of the test sample containing Cu2+ was added to the solution, which was shaken and then kept motionless to let Cu2+ etch the GNRs for 5 minutes. The sample solution was then detected using a UV-vis spectrophotometer. Fig. 1 shows a typical TEM image (500 K magnication) of GNRs obtained before and aer different concentrations of Cu2+ were added. The distribution of length and width was also provided. These GNRs are straight and very uniform in dimension with an average length of 59.87 11.40 nm and an average diameter of 17.70 5.20 nm without Cu2+ (100 GNRs were counted for the nanorod diameter) (Fig. 1A). Aer 500 nM Cu2+ was added, the GNRs became smaller with an average length of 46.60 10.30 nm and an average diameter of 16.60 6.70 nm (60 GNRs were counted for the nanorod diameter) (Fig. 1B). Moreover, aer 500 mM Cu2+ was added, the length of the GNRs decreased to 37.57 10.80 and the width decreased to 15.80 8.60 nm (Fig. 1C). In addition, when 5 mM Cu2+ was added, the average length of the GNRs became 29.48 12.89 nm with a width of 14.6 7.1 nm (Fig. 1D). Fig. 1E shows the changes in aspect ratio of GNRs with different Cu2+ concentrations. It clearly suggests that Cu2+ etched GNRs in the solution, and that the ratio of length-to-diameter became smaller with the increasing concentration of Cu2+ added. This is because the GNRs have a high curvature radius and both sides
Contrast TEM image of the GNRs sensing Cu2+. (A) Before Cu2+ was added to the GNR solution. From (B) to (D), the concentration of Cu2+ added was 500 nM, 500 mM and 5 mM of Cu2+, respectively. (E) The aspect ratio of GNRs in different Cu2+ concentrations. Fig. 1
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have much CTAB protection.27 CTAB contains ammonia, which could prevent Cu2+ from combining with the GNRs. In order to identify the etching time of GNRs by Cu2+, further testing was conducted in the detection process. The relationship between the Cu2+ etching time and the absorption intensity has been researched using an Ocean Optics USB2000 miniature ber-optic spectrometer. A verication test was required and is shown in Fig. 2. From the image, it can be clearly seen that the UV-vis absorption peak of the longitudinal SPR band was almost constant before Cu2+ (100 mM) was added to the system. However, once Cu2+ was added to the solution, the UV absorption intensity of the longitudinal band (at a wavelength of about 880 nm) decreased quickly and reached a stable platform aer 4 minutes and almost remained unchanged. Meanwhile, because GNRs have been etched to be small and aggregated together, the transverse plasma absorption (at a wavelength of about 522 nm) has a tendency to increase.28 As the UV absorption almost remained stable aer Cu2+ was added aer 4 to 7 minutes (Fig. 2B), 5 min was selected as the optimal reaction time. To demonstrate the specicity of the colorimetric sensor, several potentially interfering metal ions such as Ca2+, Cd2+, Fe3+, Mg2+, Mn2+, Ni2+, Zn2+, Ag1+, Au3+ (50 mM) and Cu2+ (1 mM) were separately added to the solution of GNRs under the same conditions and were detected using the UV-vis spectrophotometer. It was found that the absorption peak had almost no noticeable changes (as shown in Fig. 3), suggesting that these metal ions do not etch GNRs and that the sensor has high specicity for Cu2+. To study the sensitivity of the system, different concentrations of Cu2+ were added to the GNR solution to ascertain the linear range. The absorbance curve of different concentrations of Cu2+ was investigated and is shown in Fig. 4A. The GNR solution showed a longitudinal absorption peak at 860 nm. With increasing Cu2+ concentration, the intensity of the UV-vis absorption decreased. Moreover, as the concentration of Cu2+ increased from 5 nM to 500 mM, the longitudinal absorption peaks blue shied (860 nm to 830 nm), interestingly when the concentration of Cu2+ increased from 500 mM to 500 mM, the absorption peaks red shied (860 nm to 880 nm). According to a previous report, the amount of CTAB capped on both sides was more than on the ends of the GNRs which could protect them
Fig. 2 The influence of Cu2+ etching time for GNRs on UV-vis spectra. (A) The absorption spectra at different etching times of Cu2+. (B) The relationship between UV-vis absorbance at 860 nm with different Cu2+ etching times. Cu2+ was added to the solution for 7 minutes.
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The selectivity of this method. (A) The UV-vis absorption curve of the GNRs after adding different metal ions: Ca2+ (50 mM), Cd2+ (50 mM), Fe3+ (50 mM), Mg2+ (50 mM), Mn2+ (50 mM), Ni2+ (50 mM), Zn2+ (50 mM), Ag1+ (50 mM), Au3+ (100 mM) and Cu2+ (1 mM) ions. (B) The decrease in UV-vis absorbance intensity at 860 nm of GNRs in different metal ion solutions. Fig. 3
from Cu2+ etching.29 When the concentration of Cu2+ was low, the ends of the GNRs adsorbed Cu2+ rst and the etching process started from opposite ends of the GNRs. When a high concentration of Cu2+ was added, all the surfaces of the GNRs were capped with Cu2+ and the opposite ends and both sides were etched together. With increasing concentration of Cu2+, more and more Cu2+ capped on both sides of the GNRs and resulted in both sides being etched quickly and a red shi occurred. The relationship curve between the longitudinal absorption intensity and the logarithm of concentration of Cu2+ is presented in Fig. 4B. It shows that the sensor has a large dynamic range for Cu2+ covering 5 nM to 500 mM with a detection limit of 1.6 109 M, which is better than the values reported by Jingmin Liu and Shasha Wang.30,31 In conclusion, with the seed mediated growth method, we have successfully synthesized stable and functional GNRs. A novel strategy for the colorimetric sensing of Cu2+ based on GNRs has been developed. The colorimetric sensor showed high sensitivity and selectivity for the detection of Cu2+ in aqueous solutions, which can be used in biomedical and environmental analysis. Compared with recently reported colorimetric Cu2+ sensing, using complex and expensive experiment apparatus, the procedure for the detection of Cu2+ in this work is simple and of low cost. Moreover, the strategy suggests that the
Fig. 4 The relationship between different concentrations of Cu2+ and UV-vis absorbance. (A) The absorbance curves for concentrations of 0 nM, 5 nM, 25 nM, 50 nM, 500 nM, 5 mM, 50 mM, 500 mM, 5 mM, 50 mM, 250 mM and 500 mM of Cu2+. (B) The relationship between decreasing UV-vis absorbance and the logarithm of different concentrations of Cu2+.
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detection range can be extended and the detection limit can be pushed down. The rst two authors contributed equally to this work. This work was supported by the NSFC 21165023, NSFC 20865006 and PCSIRT.
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Notes and references 1 E. Gaggelli, H. Kozlowski, D. Valensin and G. Valensin, Chem. Rev., 2006, 106, 1995–2044. 2 J. Y. Uriu-Adams and C. L. Keen, Mol. Aspects Med., 2005, 26, 268–298. 3 E. B. Hart, H. Steenbock, J. Waddell and C. A. Elvehjem, J. Biol. Chem., 1928, 77, 797–833. 4 F. P. M. Araya, M. Olivares, M. Arredondo, M. Gonz´ alez and M. M´ endez, Biol. Res., 2006, 39, 183–187. 5 G. Williams and C. D'Silva, Analyst, 1994, 119, 187–190. 6 S. Lee, I. Choi, S. Hong, Y. In Yang, J. Lee, T. Kang and J. Yi, Chem. Commun., 2009, 45, 6171–6173. 7 S. X. Li, F. Y. Zheng, Y. C. Li, T. S. Cai and J. Z. Zheng, J. Agric. Food Chem., 2012, 60, 11691–11695. 8 W. Yantasee, Y. Lin, T. S. Zemanian and G. E. Fryxell, Analyst, 2003, 128, 467–472. 9 S. M. Marzouk, W. A. Al-Ariqui and S. M. Hassan, Anal. Bioanal. Chem., 2003, 375, 1186–1192. 10 S. Sarkar, S. Roy, A. Sikdar, R. N. Saha and S. S. Panja, Analyst, 2013, 138, 7119–7126. 11 S. C. Dodani, S. C. Leary, P. A. Cobine, D. R. Winge and C. J. Chang, J. Am. Chem. Soc., 2011, 133, 8606–8616. 12 K. Critchley, B. P. Khanal, M. Ł. G´ orzny, L. Vigderman, S. D. Evans, E. R. Zubarev and N. A. Kotov, Adv. Mater., 2010, 22, 2338–2342. 13 B. Nikoobakht and M. A. El-Sayed, Chem. Mater., 2003, 15, 1957–1962. 14 B. I. Kharisov, H. V. R. Dias, O. V. Kharissova, V. M. JimenezPerez, B. O. Perez and B. M. Flores, RSC Adv., 2012, 2, 9325– 9358.
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15 Y. Zhu, C. Qu, H. Kuang, L. Xu, L. Liu, Y. Hua, L. Wang and C. Xu, Biosens. Bioelectron., 2011, 26, 4387–4392. 16 B. Yoza, M. Matsumoto and T. Matsunaga, J. Biotechnol., 2002, 94, 217–224. 17 Y. Zhu, L. Xu, W. Ma, Z. Xu, H. Kuang, L. Wang and C. Xu, Chem. Commun., 2012, 48, 11889–11891. 18 T. Placido, G. Aragay, J. Pons, R. Comparelli, M. L. Curri and A. Merkoçi, ACS Appl. Mater. Interfaces, 2013, 5, 1084–1092. 19 C. Abbruzzese, P. Fornari, R. Massidda, F. Vegli` o and S. Ubaldini, Hydrometallurgy, 1995, 39, 265–276. 20 M. G. Aylmore and D. M. Muir, Miner. Eng., 2001, 14, 135– 174. 21 M. Orban and I. R. Epstein, J. Am. Chem. Soc., 1989, 111, 2891–2896. 22 S. Banthia and A. Samanta, New J. Chem., 2005, 29, 1007– 1010. 23 X. Ye, L. Jin, H. Caglayan, J. Chen, G. Xing, C. Zheng, V. Doan-Nguyen, Y. Kang, N. Engheta, C. R. Kagan and C. B. Murray, ACS Nano, 2012, 6, 2804–2817. 24 J. Boleininger, A. Kurz, V. Reuss and C. Sonnichsen, Phys. Chem. Chem. Phys., 2006, 8, 3824–3827. 25 N. R. Jana, L. Gearheart and C. J. Murphy, Adv. Mater., 2001, 13, 1389–1393. 26 L. Vigderman, B. P. Khanal and E. R. Zubarev, Adv. Mater., 2012, 24, 4811–4841. 27 J. Rodr´ıguez-Fern´ andez, J. P´ erez-Juste, P. Mulvaney and L. M. Liz-Marz´ an, J. Phys. Chem. B, 2005, 109, 14257–14261. 28 L. Wang, Y. Zhu, L. Xu, W. Chen, H. Kuang, L. Liu, A. Agarwal, C. Xu and N. A. Kotov, Angew. Chem., Int. Ed., 2010, 49, 5472–5475. 29 P. L. Gai and M. A. Harmer, Nano Lett., 2002, 2, 771–774. 30 J. M. Liu, H. F. Wang and X. P. Yan, Analyst, 2011, 136, 3904– 3910. 31 S. Wang, Z. Chen, L. Chen and R. Liu, Analyst, 2013, 138, 2080–2084.
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Ultrasensitive colorimetric detection of Cu2+ using gold nanorods Xiaofang Niu,a Dong Xu,b Yunhui Yang*a and Yan He*b
Received 22nd November 2013 Accepted 1st March 2014 DOI: 10.1039/c3an02164e www.rsc.org/analyst
We utilized Cu2+ induced gold nanorod shortening in the presence of Na2S2O3 and NH3 for colorimetric sensing of copper ions. Compared with conventional methods, this method has good reproducibility, fast response time and very high sensitivity to Cu2+. The sensor has a large dynamic range for Cu2+ covering 5 nM to 500 mM with a detection limit of 1.6 nM, which is lower than previously reported for the colorimetric detection of copper ions.
Copper (Cu) is an essential trace element for the health of all living things and the environment. In the human body, copper is necessary for the proper functioning of organs and metabolic processes.1,2 It was reported in 1928 that Cu deciency results in anaemia.3 According to the World Health Organization (W. H. O.), 0.9 mg per day of copper is recommended for adults, but the intake should not exceed 10 mg per day.4 Excessive amounts of Cu2+ in the body can also pose health risks,5 such as liver disease, severe neurological decits and even lead to death. Therefore, development of a highly sensitive method for the detection of Cu2+ is particularly critical.6 Many methods have been proposed to detect Cu2+ in the eld of environmental and biomedical science, for example, ame atomic absorption spectrometry (FAAS),7 electrochemical devices8,9 and uorescence sensors.10,11 However, these methods have a number of shortcomings when used for Cu2+ detection, for instance, they are time-consuming, require expensive instruments, are complicated in operation, and their detection limits are oen higher than the standard of the Environmental Protection Agency (EPA). In recent years, accompanied with the fast development of nanotechnology, different kinds of metal nanoparticles have been prepared.12 Among these nanomaterials, gold nanorods
(GNRs) have attracted much interest due to their size and shapedependent optical and physical properties, and they have been applied to catalysis and optical sensing. GNRs with controlled aspect ratios can be readily prepared in high yield.13 Importantly, GNRs give rise to longitudinal or transverse localized surface plasma resonance (LSPR) absorption and scattering under the illumination of visible and near-infrared light of certain wavelengths, when surface electrons of the GNRs oscillate either parallel or perpendicular to their length direction. The longitudinal LSPR maximum is dependent on the aspect ratio, surface charge and refractive index of the surrounding medium. So far GNRs have been utilized to detect various biologically important species and environmental toxins.14 Some research groups have utilized GNRs for the detection of antibiotics,15 cancer cells16 and heavy metal ions.17 Recently, Placido et al. demonstrated a method for GNR synthesis and the subsequent detection of mercury ions (Hg2+).18 In their work, GNRs functionalized with a pyrazole-derived amino ligand (PyL) were prepared rst. Before adding Hg2+ solution, the functionalized GNRs exist in the solution in the form of stable monodisperse particles. The addition of Hg2+ can induce the GNRs into end-toend assembly. Compared with the UV-vis absorption spectrum before, the resulting absorption spectrum is red shied and its absorbance decreased greatly. Herein, we present a GNR-based colorimetric sensor for the detection of Cu2+. First of all, a new approach for the seed-mediated synthesis of GNRs was demonstrated. Secondly, a sensitive sensor was developed to detect Cu2+ using GNRs as the probe. The procedure is shown in
a
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming, China 650500. E-mail: [email protected]; Fax: +86 871 5941086; Tel: +86 871 5941087
b
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, China 410082. E-mail: [email protected]; Fax: +86 731 88821818; Tel: +86 731 88821818
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Scheme 1
Schematic diagram of the detection of Cu2+ based on
GNRs.
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Scheme 1. Initially the GNRs were coated with surface absorbed sodium thiosulfate and ammonia. Aer Cu2+ was added, an oxidation–reduction reaction occurred in the presence of Cu2+, leading to etching of the GNRs and a reduction in the size and aspect ratio. Such a morphological change decreases the absorbance peak of the GNR solution. In this process, Cu2+ was added to the ammonia and a complex of Cu(NH3)42+ was produced. As the GNRs were dispersed in the solution, the Cu(NH3)42+ could quickly combine the thiosulfate and gold atoms. The primary chemical reactions for the Cu2+–Cu+ redox system were19–21 Cu2+ + 4NH3 ¼ Cu(NH3)42+ Cu(NH3)42+ + 3S2O32 + e ¼ Cu(S2O3)35 + 4NH3 Banthia and Samanta22 have proved that the reduction of Cu complexes to Cu+ is more stable and easier. Moreover, aer sodium thiosulfate was added into the gold nanorod solution, the copper ions and ammonia showed a catalytic performance. The reason for this is that Cu2+ and NH3 form Cu (NH3)4+, which acts as an oxidant in the system. NH3 and S2O32 undergo the following reaction in the presence of Cu2+ in the solution. 2+
Au + Cu(NH3)42+ + 4S2O32 ¼ Au(S2O3)23 + Cu(S2O3)23 + 4NH3
Experimental apparatus and conditions include the following: cetyltrimethylammonium bromide (CTAB), tetrachloroauric acid (HAuCl4), sodium borohydride, silver nitrate, ascorbic acid, sodium thiosulfate, MgCl2, ZnCl2, TiCl, NaCl, KCl, and CuCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The UV-1800 spectrophotometer was from Shimadzu (Japan). An Ocean Optics USB2000 miniature ber-optic spectrometer (Ocean Optics, Dunedin, FL) was used to produce the absorption spectra instantaneously. To prepare the GNRs, a seed mediated method was adapted according to previous reports. 40.5 mL HAuCl4 (24.28 mM) was added to 4.0 mL of cetyltrimethylammonium bromide (CTAB) (0.1 M) and the solution was shaken in order to mix thoroughly. Subsequently, 24 mL ice-cold NaBH4 (0.1 M) was injected into the solution and allowed to stabilize for 2 h at 28 C (the color of the solution changed from yellow to brown). During this process, Au3+ was reduced to Au atoms.23,24 The seeds were then acquired from the aqueous solution. 206 mL of chloroauric acid (0.1 M), 10 mL of AgNO3 (0.1 M) and 70.5 mL of ascorbic acid (0.1 M) were added to 10.0 mL of CTAB (0.1 M), respectively, shaking it for 10 s until colorless. 30 mL of seeds were added to the solution, which was kept stationary for 3 h at 28 C. Finally, gold nanorods 40–50 nm in length and 10–15 nm in diameter were obtained.25,26 In order to remove CTAB from the GNR solution, a centrifugal method was used to wash it twice with ultrapure water. A relatively clean GNR solution containing a small quantity of CTAB was then acquired. The GNR solution was subsequently diluted to 10%. 20 mL of ammonia (1.4 M) was
2692 | Analyst, 2014, 139, 2691–2694
then added to 900 mL of the GNR solution and mixed for 5 minutes. 70 mL of Na2S2O3 (0.1 M) was then added to the solution, which was mixed, and remained undisturbed for about 30 minutes to ensure sufficient reaction occurred. The absorption intensity was measured using a UV-vis spectrophotometer. Following this, 10 mL of the test sample containing Cu2+ was added to the solution, which was shaken and then kept motionless to let Cu2+ etch the GNRs for 5 minutes. The sample solution was then detected using a UV-vis spectrophotometer. Fig. 1 shows a typical TEM image (500 K magnication) of GNRs obtained before and aer different concentrations of Cu2+ were added. The distribution of length and width was also provided. These GNRs are straight and very uniform in dimension with an average length of 59.87 11.40 nm and an average diameter of 17.70 5.20 nm without Cu2+ (100 GNRs were counted for the nanorod diameter) (Fig. 1A). Aer 500 nM Cu2+ was added, the GNRs became smaller with an average length of 46.60 10.30 nm and an average diameter of 16.60 6.70 nm (60 GNRs were counted for the nanorod diameter) (Fig. 1B). Moreover, aer 500 mM Cu2+ was added, the length of the GNRs decreased to 37.57 10.80 and the width decreased to 15.80 8.60 nm (Fig. 1C). In addition, when 5 mM Cu2+ was added, the average length of the GNRs became 29.48 12.89 nm with a width of 14.6 7.1 nm (Fig. 1D). Fig. 1E shows the changes in aspect ratio of GNRs with different Cu2+ concentrations. It clearly suggests that Cu2+ etched GNRs in the solution, and that the ratio of length-to-diameter became smaller with the increasing concentration of Cu2+ added. This is because the GNRs have a high curvature radius and both sides
Contrast TEM image of the GNRs sensing Cu2+. (A) Before Cu2+ was added to the GNR solution. From (B) to (D), the concentration of Cu2+ added was 500 nM, 500 mM and 5 mM of Cu2+, respectively. (E) The aspect ratio of GNRs in different Cu2+ concentrations. Fig. 1
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have much CTAB protection.27 CTAB contains ammonia, which could prevent Cu2+ from combining with the GNRs. In order to identify the etching time of GNRs by Cu2+, further testing was conducted in the detection process. The relationship between the Cu2+ etching time and the absorption intensity has been researched using an Ocean Optics USB2000 miniature ber-optic spectrometer. A verication test was required and is shown in Fig. 2. From the image, it can be clearly seen that the UV-vis absorption peak of the longitudinal SPR band was almost constant before Cu2+ (100 mM) was added to the system. However, once Cu2+ was added to the solution, the UV absorption intensity of the longitudinal band (at a wavelength of about 880 nm) decreased quickly and reached a stable platform aer 4 minutes and almost remained unchanged. Meanwhile, because GNRs have been etched to be small and aggregated together, the transverse plasma absorption (at a wavelength of about 522 nm) has a tendency to increase.28 As the UV absorption almost remained stable aer Cu2+ was added aer 4 to 7 minutes (Fig. 2B), 5 min was selected as the optimal reaction time. To demonstrate the specicity of the colorimetric sensor, several potentially interfering metal ions such as Ca2+, Cd2+, Fe3+, Mg2+, Mn2+, Ni2+, Zn2+, Ag1+, Au3+ (50 mM) and Cu2+ (1 mM) were separately added to the solution of GNRs under the same conditions and were detected using the UV-vis spectrophotometer. It was found that the absorption peak had almost no noticeable changes (as shown in Fig. 3), suggesting that these metal ions do not etch GNRs and that the sensor has high specicity for Cu2+. To study the sensitivity of the system, different concentrations of Cu2+ were added to the GNR solution to ascertain the linear range. The absorbance curve of different concentrations of Cu2+ was investigated and is shown in Fig. 4A. The GNR solution showed a longitudinal absorption peak at 860 nm. With increasing Cu2+ concentration, the intensity of the UV-vis absorption decreased. Moreover, as the concentration of Cu2+ increased from 5 nM to 500 mM, the longitudinal absorption peaks blue shied (860 nm to 830 nm), interestingly when the concentration of Cu2+ increased from 500 mM to 500 mM, the absorption peaks red shied (860 nm to 880 nm). According to a previous report, the amount of CTAB capped on both sides was more than on the ends of the GNRs which could protect them
Fig. 2 The influence of Cu2+ etching time for GNRs on UV-vis spectra. (A) The absorption spectra at different etching times of Cu2+. (B) The relationship between UV-vis absorbance at 860 nm with different Cu2+ etching times. Cu2+ was added to the solution for 7 minutes.
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The selectivity of this method. (A) The UV-vis absorption curve of the GNRs after adding different metal ions: Ca2+ (50 mM), Cd2+ (50 mM), Fe3+ (50 mM), Mg2+ (50 mM), Mn2+ (50 mM), Ni2+ (50 mM), Zn2+ (50 mM), Ag1+ (50 mM), Au3+ (100 mM) and Cu2+ (1 mM) ions. (B) The decrease in UV-vis absorbance intensity at 860 nm of GNRs in different metal ion solutions. Fig. 3
from Cu2+ etching.29 When the concentration of Cu2+ was low, the ends of the GNRs adsorbed Cu2+ rst and the etching process started from opposite ends of the GNRs. When a high concentration of Cu2+ was added, all the surfaces of the GNRs were capped with Cu2+ and the opposite ends and both sides were etched together. With increasing concentration of Cu2+, more and more Cu2+ capped on both sides of the GNRs and resulted in both sides being etched quickly and a red shi occurred. The relationship curve between the longitudinal absorption intensity and the logarithm of concentration of Cu2+ is presented in Fig. 4B. It shows that the sensor has a large dynamic range for Cu2+ covering 5 nM to 500 mM with a detection limit of 1.6 109 M, which is better than the values reported by Jingmin Liu and Shasha Wang.30,31 In conclusion, with the seed mediated growth method, we have successfully synthesized stable and functional GNRs. A novel strategy for the colorimetric sensing of Cu2+ based on GNRs has been developed. The colorimetric sensor showed high sensitivity and selectivity for the detection of Cu2+ in aqueous solutions, which can be used in biomedical and environmental analysis. Compared with recently reported colorimetric Cu2+ sensing, using complex and expensive experiment apparatus, the procedure for the detection of Cu2+ in this work is simple and of low cost. Moreover, the strategy suggests that the
Fig. 4 The relationship between different concentrations of Cu2+ and UV-vis absorbance. (A) The absorbance curves for concentrations of 0 nM, 5 nM, 25 nM, 50 nM, 500 nM, 5 mM, 50 mM, 500 mM, 5 mM, 50 mM, 250 mM and 500 mM of Cu2+. (B) The relationship between decreasing UV-vis absorbance and the logarithm of different concentrations of Cu2+.
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detection range can be extended and the detection limit can be pushed down. The rst two authors contributed equally to this work. This work was supported by the NSFC 21165023, NSFC 20865006 and PCSIRT.
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