Journal of Colloid and Interface Science 439 (2015) 7–11
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Polyamine-capped gold nanorod as a localized surface Plasmon resonance probe for rapid and sensitive copper(II) ion detection Yingshuai Liu ⇑, Yanan Zhao, Yuchen Wang, Chang Ming Li Institute for Clean Energy & Advanced Materials, Faculty of Materials & Energy, Southwest University, Chongqing 400715, China Chongqing Engineering Research Center for Rapid Diagnosis of Dread Disease, Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Southwest University, Chongqing 400715, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Accepted 16 October 2014 Available online 23 October 2014 Keywords: Gold nanorods Localized surface Plasmon resonance Cu2+ detection Polyethylenimine
a b s t r a c t Polyamine-capped gold nanorods (AuNRs) were developed as nanoprobes for localized surface Plasmon resonance (LSPR)-based simple, selective, and sensitive detection of Cu2+ ions. Poly(sodium-4-styrenesulfonate) (PSS) and polyethylenimine (PEI) was successively adsorbed on the positively charged AuNRs via electrostatic adsorption, resulting in polyamine-capped AuNRs (called ‘‘PEI–PSS–AuNRs’’ thereafter), in which PEI offered bifunctions of providing sufficient positive charges and static hindrance to ensure stability of the AuNRs and serving as a Cu2+ ion recognition molecule via specific chelation. The as-prepared PEI–PSS–AuNRs were characterized by UV–vis spectroscopy, zeta potential analyzer, and transmission electron microscopy (TEM). Experimental results show that the polyelectrolytes PSS and PEI have been successfully adsorbed on AuNRs. The PEI–PSS–AuNRs were then employed as nanoprobes for Cu2+ ion detection. A linear range from 1 lM to 5 mM and a detection limit (3r/k) of 0.24 lM were achieved in PBS. The concentration dependent shifts of longitudinal extinction peak of PEI–PSS–AuNRs notably results from the specific PEI–Cu2+ chelation-induced changes of dielectric property of polyelectrolyte film attached on nanoprobes. The negligible interference from other metal ions demonstrates good selectivity of the PEI–PSS–AuNRs for Cu2+ sensing. Moreover, the developed probes were successfully used to detect Cu2+ in river water, demonstrating their feasibility for analysis of surface water sample. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Copper, an essential trace element, plays an important role in various biological processes [1]. However, exposure to excess copper in drinking water or other environmental sources is highly toxic to organisms. It has been reported that many serious diseases such as Alzheimer’s and Wilson’s disease are related to the toxicity of copper. Cu2+ ion has been listed as a major pollutant by the US Environmental Protection Agency (EPA) [2]. Recognition and detection of Cu2+ ion is of very importance in both environmental surveillance and public healthcare monitor. Gold nanorods (AuNRs) have attracted much attention due to their unique optical properties, in particular the localized surface Plasmon resonance (LSPR) characteristics. It is well known that AuNRs exhibit two Plasmon absorption bands corresponding to the transverse band and the longitudinal band, which can be tuned from visible to near infrared (NIR) region (650–900 nm) by simple adjusting their aspect ratio [3,4]. Compared to LSPR of spherical ⇑ Corresponding author. Fax: +86 23 68254969. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jcis.2014.10.023 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
gold nanoparticles, the longitudinal extinction peak of AuNRs is much more sensitive to dielectric changes of local environments including solvents, adsorbates, and the interparticle distance [5–7]. In the past years, AuNRs have been widely employed for various applications such as biosensing, bioimaging, and cancer therapy [8,9]. Among them the most popular application is label-free detection of biomacromolecules such as proteins and antibodies [10,11], which can cause significant dielectric changes of the environment locally around the AuNRs, thereby resulting in detectable LSPR wavelength-shift. AuNRs have also been applied for chemosensing of environmental toxins [12], cysteine [13], and Hg2+ ions [14], which cause LSPR changes by inducing assembly or aggregation of AuNRs. Cysteine (Cys)-modified AuNRs were developed for colorimetric detection of Cu2+ ions, in which a stable Cys–Cu–Cys complex was generated due to the coordination of Cu2+ with Cys, resulting in aggregation of colloidal nanorods along with a color change from blue–green to dark gray [15]. However, Cys-modified AuNRs are also sensitive to pH, ionic concentration, and other metal ions (Pb2+, Hg2+) [13,16,17], leading to low specificity and reliability. More recently, AuNRs were employed as probes for Cu2+ detection
8
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
based on the copper-mediated leaching along the longitudinal direction [18]. Although a low detection limit was achieved by this scheme, the etching process is dependent on the dissolved oxygen, which concentration may change dramatically in different samples. In addition, the sensing system has to be heated to 75 °C to accelerate the copper-mediated etching of AuNRs. Thus, a simple, reliable, and sensitive sensor is highly demanded for Cu2+ detection in environmental matrix and biological fluidics. In this work, polyethylenimine (PEI), a well-known Cu2+-chelating agent, capped AuNRs were prepared for LSPR-based detection of Cu2+ ions for the first time. Polyanionic poly(sodium-4-styrenesulfonate) (PSS) and polycationic PEI were electrostatically adsorbed on positive charged cetyltrimethylammonium bromide (CTAB)coated AuNRs through a simple layer-by-layer self-assembly method, resulting in PEI–PSS–AuNRs, where PEI not only served as a specific recognition molecule, but also worked as a protection agent to ensure their stability in various samples. High sensitivity and selectivity were achieved for Cu2+ detection in lab buffer and spiked river water owing to the sensitivity of longitudinal LSPR of AuNRs and the good coordination specificity of PEI with Cu2+ ion. Scheme 1 shows the principle for LSPR-based Cu2+ detection using PEI-PSS-AuNRs as sensing probes. 2. Experimental 2.1. Chemical and reagents Chloroauric acid hydrate (HAuCl43H2O), cetyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), ascorbic acid (AA), silver nitrate (AgNO3), poly(sodium-4-styrenesulfonate) (PSS), (MW = 70,000), polyethylenimine (PEI) (50%, wv%, MW = 11,235), copper(II) chloride (CuCl2) were purchased from Sigma– Aldrich (Shanghai, China). All other reagents are of analytical grade and used as received without further purification. Glassware used in this work was cleaned by aqua regia and intensively rinsed with deionized (DI, 18 MX) water before use. DI water was produced by a water purification system (Q-GradÒ1, Millipore). Phosphate-buffered saline (PBS) solutions with different pH values were prepared by titrating 0.01 M phosphoric acid with a concentrated sodium hydroxide solution (1 M). 2.2. Synthesis of gold nanorods AuNRs were synthesized by the well-documented seed-mediated growth method with a minor modification [19]. Spherical gold seeds were prepared by reducing HAuCl4 with NaBH4. Specifically, 104 lL of 0.024 M HAuCl4 was mixed with 10 mL of 0.1 M colorless CTAB solution under gentle stirring. 0.6 mL of fresh-prepared icecold 0.01 M NaBH4 solution was then rapidly added to the stirred
solution. Under continuous stirring for 7 min, brownish yellow seed solution was obtained. It was stored at 25 °C and used for synthesis of AuNRs. The growth solution was prepared by sequentially adding 250 lL of 4 mM silver nitrate and 208 lL of 0.024 M HAuCl4 to a 10 mL of 0.1 M CTAB solution under continuous stirring, into which 55 lL of 0.0788 M ascorbic acid solution was then injected. Subsequently, 12 lL of the seed solution was added to the growth solution under gentle stirring to initiate growth of AuNRs. After a thorough mixing, the mixture was kept undisturbed at 30 °C for overnight. Solution color gradually changed from colorless through bluish violet to purplish red during the growth, indicating the successful formation of AuNRs. 2.3. Preparation of polyelectrolytes coated AuNRs The as-prepared colloidal AuNRs solution was centrifuged twice at 9000 rpm for 15 min each to remove excess CTAB. After carefully remove the supernatant, pellets formed at the bottom of centrifuge tubes were redispersed in DI water. The resulting AuNRs suspension was dropwise added to a solution containing 2 mg mL 1 PSS and 6 mM NaCl under vigorous stirring. After 2 h incubation, anionic polyelectrolyte PSS was electrostatically adsorbed on positive charged CTAB–AuNRs to form PSS-capped AuNRs. Excess PSS in the supernatant fraction was removed by centrifugation at 9000 rpm for 15 min, and the pellet was redispersed with DI water. Positive charged PEI was coated on PSS-capped AuNRs through repetition of the above procedures, resulting in PEI–PSS–AuNRs. 2.4. Characterization Ultraviolet–visible (UV–vis) absorption spectra were recorded by Shimadzu UV-2550 spectrophotometer with a 10 mm pathlength quartz cuvette. Transmission electronmicroscopy (TEM) measurements were performed on a JEOL, JEM-2100 with an accelerating voltage of 200 kV. Zeta potentials of pristine and modified AuNRs were measured using a Nano-ZS Zetasizer ZEN 3600 (Malvern Instruments Ltd., UK) in DI water. 2.5. Metal ion titration For LSPR-based Cu2+ detection, 5 lL of prepared Cu2+ standard solutions was mixed with a 495 lL of PEI–PSS–AuNRs (2.74 nM) PBS suspensions to prepare a series of mixtures with a final Cu2+concentration of 1 lM, 10 lM, 100 lM, 1 mM, 5 mM. Before UV–vis measurement, the mixture was kept at room temperature (25 °C) for 20 min to allow the coordination between PEI and Cu2+ ion. Concentration of Cu2+ ions was quantified by peak-shift (Dk) (Dk = k0 k, where k0 and k are the longitudinal SPR peak
Scheme 1. Schematic illustration for LSPR-based detection of Cu2+ ion using PEI–PSS–AuNRs.
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
wavelength of PEI–PSS–AuNRs in the absence and presence of Cu2+ ions, respectively). The LSPR-based Cu2+ detection totally takes less than 25 min with consideration of UV–Vis measurements. 3. Results and discussion 3.1. Characterization of pristine and polyelectrolytes coated AuNRs The as-prepared CTAB-capped AuNRs and PEI–PSS–AuNRs were first characterized by TEM. For preparation of TEM specimens, excess CTAB and polyelectrolytes were first removed from the AuNR solutions by twice centrifugation at 9000 rpm for 15 min each. Then a drop of the nanoparticle dispersion was placed on a carbon-coated copper grid and drying in a vacuum oven at room temperature. Fig. 1 shows that the obtained AuNRs have reasonably uniform size and shape. Based on statistic analysis, the length and width of the AuNRs are 46.9 ± 7.9 nm and 15 ± 2.5 nm, respectively. Thus aspect ratio is determined to be 3.1. After PSS and PEI coating, AuNRs still keep monodisperse instead of aggregation, indicating that the nanorods were individually coated by the polyelectrolytes [20], although the coating layer is not visible in TEM image. The adsorbed polyelectrolytes work as a protection layer to stabilize the nanorods through charge repulsion and steric hindrance. One of the prerequisites for sequential deposition of oppositely charged polyelectrolyte onto surfaces is charge reversal after each coating step. Zeta potential measurement, the most efficient way to determine the changes of surface charge, was employed to monitor each layer deposition. The as-prepared AuNRs are coated by a double-layer CTAB, resulting in strong positive charged surface, which is indicated by the positive zeta potential at around +35 mV (Fig. 2A). Charge reversal upon sequential adsorption of polyanionic PSS and polycationic PEI is approved by the negative and positive zeta potentials as shown in Fig. 2A, indicating the success of PSS and PEI coating on colloidal AuNR surface. It is worth noting that the zeta potentials of pristine and modified AuNRs are greater than 20 mV (positive or negative), resulting in strong interparticle repulsion to maintain good colloidal stability. UV–vis spectra were measured to further confirm the polyelectrolyte depositions and evaluate the LSPR changes upon each coating. As shown in Fig. 2B two characteristic absorption bands at 520 nm for transverse LSPR peak and 753 nm for longitudinal LSPR peak are recorded from solution of the as-prepared AuNRs (black line). A 12 nm blue shift in the longitudinal peak (741 nm) is observed from PSS capped AuNRs compared with pristine AuNRs, indicating the successful deposition of PSS, which induces the change of local dielectric properties surrounding the nanoparticles [21]. Similarly, the adsorption of PEI results in an additional 7 nm blue shift in the longitudinal LSPR peak. The UV–vis results further confirm that PSS and PEI have been adsorbed on AuNRs through the simple layer-by-layer approach. Furthermore, two typical LSPR
9
absorption bands are also observed from PSS–AuNRs and PEI–PSS– AuNRs, demonstrating their good dispersion after being coated by polyelectrolytes. It is very critical for LSPR-based sensor in solution. 3.2. Optimization of PEI–PSS–AuNR-based LSPR probe for Cu2+ detection In order to find optimal condition, the response of PEI–PSS– AuNRs to Cu2+ ion was investigated in PBS with different pH values. Since Cu2+ undergoes hydrolysis under alkaline conditions (pH > 7.0) [22], the optimization was carried out only in acidic media. Fig. 3A shows that Cu2+ does not induce observable LSPR shift at pH 2.0 and 3.0. Obviously, the amine groups on PEI are over-protonated under high acidity (pH 2.0 and 3.0), leading to loss of the coordination ability with Cu2+ ions to form cupric amine moieties. As shown in Fig. 3A, small blue shifts are observed at pH 5.0 and 6.0 in presence of 100 lM Cu2+ ion, while biggest blue shift is achieved at pH 4.0. Thus pH 4.0 is selected as an optimal condition and will be used in subsequent experiments. In addition, response of the pristine AuNRs and PSS–AuNRs to Cu2+ ions was investigated under the optimal condition. It is seen from Fig. 3B that no obvious response to 100 lM Cu2+ ions is observed from pristine AuNRs and PSS–AuNRs, while significant blue shift is obtained from PEI–PSS–AuNRs. The results prove that the response of PEI–PSS–AuNRs to Cu2+ ions results from the strong coordination of PEI with Cu2+, which induces the changes of dielectric properties of coating layers surrounding the nanoparticles [23]. 3.3. PEI–PSS–AuNR-based Cu2+ detection in lab buffer To evaluate the sensor performance, peak shift of the longitudinal band was investigated upon addition of various concentrations of Cu2+ ions. Fig. 4A shows that with an increase of Cu2+ concentration, the longitudinal peak of PEI–PSS–AuNRs gradually shifts to short wavelengths (blue shift). The dose-dependent curve with peak shifts versus Cu2+ concentrations is plotted as shown in Fig. 4B. A good linear relationship (R2 = 0.981) is achieved in the range from 1 lM to 5 mM (Fig. 4B). A detection limit (Detection of limit was calculated by the equation: Detection limit = 3r/k. Where r refers to the standard deviation of negative control, k is the slope of the dose-dependent curve.) is determined to be 243 nM, which is much lower than the maximum level 20 lM of Cu2+ in drinking water permitted by the U.S. Environmental Protection Agency (EPA) and 15 lM by Ministry of Environmental Protection of People’s Republic of China [15]. The high performance in terms of sensitivity and detection limit is mainly attributed to the strong chelation of Cu2+ with PEI [24] and the inherently high sensitivity of AuNRs to the changes of local dielectric environment [7,25,26]. Moreover, this work demonstrates many advantages over the existing LSPR-based Cu2+ sensor. First, the polyelectrolyte-coated
Fig. 1. TEM images of (A) as-prepared AuNRs; (B) PSS–PEI-coated AuNRs.
10
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
Fig. 2. Zeta potential (A) and UV–vis absorption spectra (B) of as-prepared AuNRs and polyelectrolytes coated AuNRs.
Fig. 3. (A) The peak shift of longitudinal band in the absence (solid line) and presence (dotted line) of Cu2+ ions at pH 2.0 (a, b), pH 3.0 (c, d), pH 4.0 (e, f), pH 5.0 (g, h), pH 6.0 (i, j). An inserted histogram clearly shows the pH effect on Cu2+ induced LSPR peak shift; (B) response of different AuNRs to Cu2+ ions in pH4.0 PBS solution. The concentration of Cu2+ ions is 100 lM.
Fig. 4. (A) UV–vis absorption spectra and (B) plot of peak shift of PEI–PSS–AuNRs against Cu2+ concentrations ranging from 1 lM to 5 mM in a pH 4 PBS.
AuNRs are highly dispersible and stable in diverse water samples, resulting in good reliability. Second, the detection is simply performed under room temperature without heating process. Third, the sensing effect is independent of other solute such as dissolved oxygen.
3.4. Selectivity of PEI–PSS–AuNR-based LSPR probe for Cu2+ ion detection Selectivity is another important parameter to evaluate performance of the developed LSPR sensor. Interferences from common
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
11
observed, demonstrating its good selectivity for Cu2+ detection in real sample. The result demonstrates that the developed novel PEI–PSS–AuNR LSPR probe has a potential usefulness for Cu2+ detection in various real samples. 4. Conclusion
Fig. 5. Selectivity of the PEI–PSS–AuNRs probe for Cu2+ detection over other metal ions. The concentration of all metal ions is 100 lM in PBS at pH 4.0.
metal ions including Ag+ Ba2+, Ca2+, Mn2+, Zn2+, Cd2+, Pb2+, Co2+, Al3+, and Mg2+ were studied under the condition identical to that for Cu2+ detection. As shown in Fig. 5, Mn2+, Cd2+, Co2+, Al3+, and Mg2+ at 100 lM do not induce any LSPR peak shift. Negligible responses are observed from the treatment of other metal ions, including Ag+, Ba2+, Ca2+, and Pb2+. However, red shift is monitored from PEI–PSS–AuNRs treated by Zn2+. The effect is completely opposite to that of Cu2+, indicating no interference to the developed LSPR sensor for Cu2+ detection. The good selectivity and specificity are ascribed to that amine groups on PEI has a stronger coordination affinity with Cu2+ compared with other metal ions [24,27].
PEI–PSS–AuNRs were prepared via a simple layer by layer assembly approach and used as LSPR nanoprobes for detection of Cu2+ ions. A detection limit of 0.24 lM and a dynamic range from 1 lM to 5 mM have been achieved in PBS with the developed LSPR sensor. Good selectivity has also been demonstrated. The high performance is mainly attributed to the strong coordination of PEI with Cu2+ and the high sensitivity of longitudinal LSPR band to the changes of dielectric properties of coating layer/medium surrounding the AuNRs. Moreover, river water is successfully analyzed using the developed nanoprobe, demonstrating its great applicability for various real samples. In a word, this work provides a rapid, sensitive, selective, and low cost method for Cu2+ detection in many fields such as environment monitoring and food safety control. Acknowledgments This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies (Grant cstc2011pt), Chongqing Engineering Research Center for Rapid diagnosis of Dread Disease, the National Natural Science Foundation of China (Grant 31200604), Natural Science Foundation of Chongqing (Grant cstc2012jjA10152), Fundamental Research Funds for the Central Universities (Grant XDJK2012C083).
3.5. Application of PEI–PSS–AuNR-based LSPR probe in real sample References Applicability of the developed LSPR sensor is evaluated by performance of Cu2+ detection in real water sample obtained from Jia Ling River (Chongqing, China), where many unexpected substances are there. Before testing, the river water was first centrifuged at 12,000 rpm for 20 min and then filtered through a 0.22 lm membrane to remove particulate matters that may have influence on the analysis. River water samples spiked with standard solutions containing Cu2+ from 1 lM to 5 mM are mixed with PEI–PSS–AuNR solution at pH 4.0. A calibration curve was plotted with the values of peak shift versus the concentrations of Cu2+ over the range from 1 lM to 5 mM (Fig. 6). Despite numerous unexpected interferences in river water, a good linear response to Cu2+ (R2 = 0.975) is
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Fig. 6. Plot of longitudinal LSPR peak shift of PEI–PSS–AuNRs against Cu2+ concentrations ranging from 1 lM to 5 mM in river water.
[26] [27]
P.L. Fox, Biometals 16 (2003) 9–40. U.S. EPA, Fed. Reg. 56 (1991) 26460–26564. S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. L.F. Gou, C.J. Murphy, Chem. Mater. 17 (2005) 3668–3672. H.J. Chen, X.S. Kou, Z. Yang, W.H. Ni, J.F. Wang, Langmuir 24 (2008) 5233–5237. C.G. Wang, Y. Chen, T.T. Wang, Z.F. Ma, Z.M. Su, Chem. Mater. 19 (2007) 5809– 5811. M.M. Miller, A.A. Lazarides, J. Phys. Chem. B 109 (2005) 21556–21565. H.J. Parab, H.M. Chen, T.C. Lai, J.H. Huang, P.H. Chen, R.S. Liu, M. Hsiao, C.H. Chen, D.P. TSai, Y.K. Hwu, J. Phys. Chem. C 113 (2009) 7574–7578. X.H. Huang, I.H. El-Sayed, W. Qian, M.A. El-Sayed, J. Am. Chem. Soc. 128 (2006) 2115–2120. G.J. Nusz, S.M. Marinakos, A.C. Curry, A. Dahlin, F. Höök, A. Wax, A. Chilkoti, Anal. Chem. 80 (2008) 984–989. K.M. Mayer, S. Lee, H. Liao, B.C. Rostro, A. Fuentes, P.T. Scully, C.L. Nehl, J.H.A. Hafner, ACS Nano. 2 (2008) 687–692. L.B. Wang, Y.Y. Zhu, L.G. Xu, W. Chen, H. Kuang, L.Q. Liu, A. Agarwal, C.L. Xu, N.A. Kotov, Angew. Chem. Int. Ed. 49 (2010) 5472–5475. H.W. Huang, X.Y. Liu, T. Hu, P.K. Chu, Biosens. Bioelectron. 25 (2010) 2078– 2083. Y. Wang, Y.F. Li, J. Wang, Y. Sang, C.Z. Huang, Chem. Commun. 46 (2010) 1332– 1334. J.M. Liu, H.F. Wang, X.P. Yan, Analyst 136 (2011) 3904–3910. T. Placido, R. Comparelli, M. Striccoli, A. Agostiano, A. Merkoi, M.L. Curri, Sens. J. IEEE 13 (2013) 2834–2841. P.K. Sudeep, S.T.S. Joseph, K.G. Thomas, J. Am. Chem. Soc. 127 (2005) 6516– 6517. Z.Y. Zhang, Z.P. Chen, C.L. Qu, L.X. Chen, Langmuir 30 (2014) 3625–3630. B. Nikoobakht, M.A. El-Sayed, Chem. Mater. 15 (2003) 1957–1962. A. Gole, C.J. Murphy, Chem. Mater. 17 (2005) 1325–1330. A.D. McFarland, R.P. Van Duyne, Nano Lett. 3 (2003) 1057–1062. Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Anal. Chem. 84 (2012) 6220– 6224. A. Sugunan, C. Thanachayanont, J. Dutta, J.G. Hilborn, Sci. Technol. Adv. Mater. 6 (2005) 335–340. Y.L. Chen, B.C. Pan, H.Y. Li, W.M. Zhang, L. Lv, J. Wu, Environ. Sci. Technol. 44 (2010) 3508–3513. T.R. Jensen, M.L. Duval, K.L. Kelly, A.A. Lazarides, G.C. Schatz, R.P. Van Duyne, J. Phys. Chem. B 103 (1999) 9846–9853. K.A. Willets, R.P. Van Duyne, Annu. Rev. Phys. Chem. 58 (2007) 267–297. R. Molinari, S. Gallo, P. Argurio, Water Res. 38 (2004) 593–600.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Polyamine-capped gold nanorod as a localized surface Plasmon resonance probe for rapid and sensitive copper(II) ion detection Yingshuai Liu ⇑, Yanan Zhao, Yuchen Wang, Chang Ming Li Institute for Clean Energy & Advanced Materials, Faculty of Materials & Energy, Southwest University, Chongqing 400715, China Chongqing Engineering Research Center for Rapid Diagnosis of Dread Disease, Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Southwest University, Chongqing 400715, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Accepted 16 October 2014 Available online 23 October 2014 Keywords: Gold nanorods Localized surface Plasmon resonance Cu2+ detection Polyethylenimine
a b s t r a c t Polyamine-capped gold nanorods (AuNRs) were developed as nanoprobes for localized surface Plasmon resonance (LSPR)-based simple, selective, and sensitive detection of Cu2+ ions. Poly(sodium-4-styrenesulfonate) (PSS) and polyethylenimine (PEI) was successively adsorbed on the positively charged AuNRs via electrostatic adsorption, resulting in polyamine-capped AuNRs (called ‘‘PEI–PSS–AuNRs’’ thereafter), in which PEI offered bifunctions of providing sufficient positive charges and static hindrance to ensure stability of the AuNRs and serving as a Cu2+ ion recognition molecule via specific chelation. The as-prepared PEI–PSS–AuNRs were characterized by UV–vis spectroscopy, zeta potential analyzer, and transmission electron microscopy (TEM). Experimental results show that the polyelectrolytes PSS and PEI have been successfully adsorbed on AuNRs. The PEI–PSS–AuNRs were then employed as nanoprobes for Cu2+ ion detection. A linear range from 1 lM to 5 mM and a detection limit (3r/k) of 0.24 lM were achieved in PBS. The concentration dependent shifts of longitudinal extinction peak of PEI–PSS–AuNRs notably results from the specific PEI–Cu2+ chelation-induced changes of dielectric property of polyelectrolyte film attached on nanoprobes. The negligible interference from other metal ions demonstrates good selectivity of the PEI–PSS–AuNRs for Cu2+ sensing. Moreover, the developed probes were successfully used to detect Cu2+ in river water, demonstrating their feasibility for analysis of surface water sample. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Copper, an essential trace element, plays an important role in various biological processes [1]. However, exposure to excess copper in drinking water or other environmental sources is highly toxic to organisms. It has been reported that many serious diseases such as Alzheimer’s and Wilson’s disease are related to the toxicity of copper. Cu2+ ion has been listed as a major pollutant by the US Environmental Protection Agency (EPA) [2]. Recognition and detection of Cu2+ ion is of very importance in both environmental surveillance and public healthcare monitor. Gold nanorods (AuNRs) have attracted much attention due to their unique optical properties, in particular the localized surface Plasmon resonance (LSPR) characteristics. It is well known that AuNRs exhibit two Plasmon absorption bands corresponding to the transverse band and the longitudinal band, which can be tuned from visible to near infrared (NIR) region (650–900 nm) by simple adjusting their aspect ratio [3,4]. Compared to LSPR of spherical ⇑ Corresponding author. Fax: +86 23 68254969. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jcis.2014.10.023 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
gold nanoparticles, the longitudinal extinction peak of AuNRs is much more sensitive to dielectric changes of local environments including solvents, adsorbates, and the interparticle distance [5–7]. In the past years, AuNRs have been widely employed for various applications such as biosensing, bioimaging, and cancer therapy [8,9]. Among them the most popular application is label-free detection of biomacromolecules such as proteins and antibodies [10,11], which can cause significant dielectric changes of the environment locally around the AuNRs, thereby resulting in detectable LSPR wavelength-shift. AuNRs have also been applied for chemosensing of environmental toxins [12], cysteine [13], and Hg2+ ions [14], which cause LSPR changes by inducing assembly or aggregation of AuNRs. Cysteine (Cys)-modified AuNRs were developed for colorimetric detection of Cu2+ ions, in which a stable Cys–Cu–Cys complex was generated due to the coordination of Cu2+ with Cys, resulting in aggregation of colloidal nanorods along with a color change from blue–green to dark gray [15]. However, Cys-modified AuNRs are also sensitive to pH, ionic concentration, and other metal ions (Pb2+, Hg2+) [13,16,17], leading to low specificity and reliability. More recently, AuNRs were employed as probes for Cu2+ detection
8
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
based on the copper-mediated leaching along the longitudinal direction [18]. Although a low detection limit was achieved by this scheme, the etching process is dependent on the dissolved oxygen, which concentration may change dramatically in different samples. In addition, the sensing system has to be heated to 75 °C to accelerate the copper-mediated etching of AuNRs. Thus, a simple, reliable, and sensitive sensor is highly demanded for Cu2+ detection in environmental matrix and biological fluidics. In this work, polyethylenimine (PEI), a well-known Cu2+-chelating agent, capped AuNRs were prepared for LSPR-based detection of Cu2+ ions for the first time. Polyanionic poly(sodium-4-styrenesulfonate) (PSS) and polycationic PEI were electrostatically adsorbed on positive charged cetyltrimethylammonium bromide (CTAB)coated AuNRs through a simple layer-by-layer self-assembly method, resulting in PEI–PSS–AuNRs, where PEI not only served as a specific recognition molecule, but also worked as a protection agent to ensure their stability in various samples. High sensitivity and selectivity were achieved for Cu2+ detection in lab buffer and spiked river water owing to the sensitivity of longitudinal LSPR of AuNRs and the good coordination specificity of PEI with Cu2+ ion. Scheme 1 shows the principle for LSPR-based Cu2+ detection using PEI-PSS-AuNRs as sensing probes. 2. Experimental 2.1. Chemical and reagents Chloroauric acid hydrate (HAuCl43H2O), cetyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), ascorbic acid (AA), silver nitrate (AgNO3), poly(sodium-4-styrenesulfonate) (PSS), (MW = 70,000), polyethylenimine (PEI) (50%, wv%, MW = 11,235), copper(II) chloride (CuCl2) were purchased from Sigma– Aldrich (Shanghai, China). All other reagents are of analytical grade and used as received without further purification. Glassware used in this work was cleaned by aqua regia and intensively rinsed with deionized (DI, 18 MX) water before use. DI water was produced by a water purification system (Q-GradÒ1, Millipore). Phosphate-buffered saline (PBS) solutions with different pH values were prepared by titrating 0.01 M phosphoric acid with a concentrated sodium hydroxide solution (1 M). 2.2. Synthesis of gold nanorods AuNRs were synthesized by the well-documented seed-mediated growth method with a minor modification [19]. Spherical gold seeds were prepared by reducing HAuCl4 with NaBH4. Specifically, 104 lL of 0.024 M HAuCl4 was mixed with 10 mL of 0.1 M colorless CTAB solution under gentle stirring. 0.6 mL of fresh-prepared icecold 0.01 M NaBH4 solution was then rapidly added to the stirred
solution. Under continuous stirring for 7 min, brownish yellow seed solution was obtained. It was stored at 25 °C and used for synthesis of AuNRs. The growth solution was prepared by sequentially adding 250 lL of 4 mM silver nitrate and 208 lL of 0.024 M HAuCl4 to a 10 mL of 0.1 M CTAB solution under continuous stirring, into which 55 lL of 0.0788 M ascorbic acid solution was then injected. Subsequently, 12 lL of the seed solution was added to the growth solution under gentle stirring to initiate growth of AuNRs. After a thorough mixing, the mixture was kept undisturbed at 30 °C for overnight. Solution color gradually changed from colorless through bluish violet to purplish red during the growth, indicating the successful formation of AuNRs. 2.3. Preparation of polyelectrolytes coated AuNRs The as-prepared colloidal AuNRs solution was centrifuged twice at 9000 rpm for 15 min each to remove excess CTAB. After carefully remove the supernatant, pellets formed at the bottom of centrifuge tubes were redispersed in DI water. The resulting AuNRs suspension was dropwise added to a solution containing 2 mg mL 1 PSS and 6 mM NaCl under vigorous stirring. After 2 h incubation, anionic polyelectrolyte PSS was electrostatically adsorbed on positive charged CTAB–AuNRs to form PSS-capped AuNRs. Excess PSS in the supernatant fraction was removed by centrifugation at 9000 rpm for 15 min, and the pellet was redispersed with DI water. Positive charged PEI was coated on PSS-capped AuNRs through repetition of the above procedures, resulting in PEI–PSS–AuNRs. 2.4. Characterization Ultraviolet–visible (UV–vis) absorption spectra were recorded by Shimadzu UV-2550 spectrophotometer with a 10 mm pathlength quartz cuvette. Transmission electronmicroscopy (TEM) measurements were performed on a JEOL, JEM-2100 with an accelerating voltage of 200 kV. Zeta potentials of pristine and modified AuNRs were measured using a Nano-ZS Zetasizer ZEN 3600 (Malvern Instruments Ltd., UK) in DI water. 2.5. Metal ion titration For LSPR-based Cu2+ detection, 5 lL of prepared Cu2+ standard solutions was mixed with a 495 lL of PEI–PSS–AuNRs (2.74 nM) PBS suspensions to prepare a series of mixtures with a final Cu2+concentration of 1 lM, 10 lM, 100 lM, 1 mM, 5 mM. Before UV–vis measurement, the mixture was kept at room temperature (25 °C) for 20 min to allow the coordination between PEI and Cu2+ ion. Concentration of Cu2+ ions was quantified by peak-shift (Dk) (Dk = k0 k, where k0 and k are the longitudinal SPR peak
Scheme 1. Schematic illustration for LSPR-based detection of Cu2+ ion using PEI–PSS–AuNRs.
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
wavelength of PEI–PSS–AuNRs in the absence and presence of Cu2+ ions, respectively). The LSPR-based Cu2+ detection totally takes less than 25 min with consideration of UV–Vis measurements. 3. Results and discussion 3.1. Characterization of pristine and polyelectrolytes coated AuNRs The as-prepared CTAB-capped AuNRs and PEI–PSS–AuNRs were first characterized by TEM. For preparation of TEM specimens, excess CTAB and polyelectrolytes were first removed from the AuNR solutions by twice centrifugation at 9000 rpm for 15 min each. Then a drop of the nanoparticle dispersion was placed on a carbon-coated copper grid and drying in a vacuum oven at room temperature. Fig. 1 shows that the obtained AuNRs have reasonably uniform size and shape. Based on statistic analysis, the length and width of the AuNRs are 46.9 ± 7.9 nm and 15 ± 2.5 nm, respectively. Thus aspect ratio is determined to be 3.1. After PSS and PEI coating, AuNRs still keep monodisperse instead of aggregation, indicating that the nanorods were individually coated by the polyelectrolytes [20], although the coating layer is not visible in TEM image. The adsorbed polyelectrolytes work as a protection layer to stabilize the nanorods through charge repulsion and steric hindrance. One of the prerequisites for sequential deposition of oppositely charged polyelectrolyte onto surfaces is charge reversal after each coating step. Zeta potential measurement, the most efficient way to determine the changes of surface charge, was employed to monitor each layer deposition. The as-prepared AuNRs are coated by a double-layer CTAB, resulting in strong positive charged surface, which is indicated by the positive zeta potential at around +35 mV (Fig. 2A). Charge reversal upon sequential adsorption of polyanionic PSS and polycationic PEI is approved by the negative and positive zeta potentials as shown in Fig. 2A, indicating the success of PSS and PEI coating on colloidal AuNR surface. It is worth noting that the zeta potentials of pristine and modified AuNRs are greater than 20 mV (positive or negative), resulting in strong interparticle repulsion to maintain good colloidal stability. UV–vis spectra were measured to further confirm the polyelectrolyte depositions and evaluate the LSPR changes upon each coating. As shown in Fig. 2B two characteristic absorption bands at 520 nm for transverse LSPR peak and 753 nm for longitudinal LSPR peak are recorded from solution of the as-prepared AuNRs (black line). A 12 nm blue shift in the longitudinal peak (741 nm) is observed from PSS capped AuNRs compared with pristine AuNRs, indicating the successful deposition of PSS, which induces the change of local dielectric properties surrounding the nanoparticles [21]. Similarly, the adsorption of PEI results in an additional 7 nm blue shift in the longitudinal LSPR peak. The UV–vis results further confirm that PSS and PEI have been adsorbed on AuNRs through the simple layer-by-layer approach. Furthermore, two typical LSPR
9
absorption bands are also observed from PSS–AuNRs and PEI–PSS– AuNRs, demonstrating their good dispersion after being coated by polyelectrolytes. It is very critical for LSPR-based sensor in solution. 3.2. Optimization of PEI–PSS–AuNR-based LSPR probe for Cu2+ detection In order to find optimal condition, the response of PEI–PSS– AuNRs to Cu2+ ion was investigated in PBS with different pH values. Since Cu2+ undergoes hydrolysis under alkaline conditions (pH > 7.0) [22], the optimization was carried out only in acidic media. Fig. 3A shows that Cu2+ does not induce observable LSPR shift at pH 2.0 and 3.0. Obviously, the amine groups on PEI are over-protonated under high acidity (pH 2.0 and 3.0), leading to loss of the coordination ability with Cu2+ ions to form cupric amine moieties. As shown in Fig. 3A, small blue shifts are observed at pH 5.0 and 6.0 in presence of 100 lM Cu2+ ion, while biggest blue shift is achieved at pH 4.0. Thus pH 4.0 is selected as an optimal condition and will be used in subsequent experiments. In addition, response of the pristine AuNRs and PSS–AuNRs to Cu2+ ions was investigated under the optimal condition. It is seen from Fig. 3B that no obvious response to 100 lM Cu2+ ions is observed from pristine AuNRs and PSS–AuNRs, while significant blue shift is obtained from PEI–PSS–AuNRs. The results prove that the response of PEI–PSS–AuNRs to Cu2+ ions results from the strong coordination of PEI with Cu2+, which induces the changes of dielectric properties of coating layers surrounding the nanoparticles [23]. 3.3. PEI–PSS–AuNR-based Cu2+ detection in lab buffer To evaluate the sensor performance, peak shift of the longitudinal band was investigated upon addition of various concentrations of Cu2+ ions. Fig. 4A shows that with an increase of Cu2+ concentration, the longitudinal peak of PEI–PSS–AuNRs gradually shifts to short wavelengths (blue shift). The dose-dependent curve with peak shifts versus Cu2+ concentrations is plotted as shown in Fig. 4B. A good linear relationship (R2 = 0.981) is achieved in the range from 1 lM to 5 mM (Fig. 4B). A detection limit (Detection of limit was calculated by the equation: Detection limit = 3r/k. Where r refers to the standard deviation of negative control, k is the slope of the dose-dependent curve.) is determined to be 243 nM, which is much lower than the maximum level 20 lM of Cu2+ in drinking water permitted by the U.S. Environmental Protection Agency (EPA) and 15 lM by Ministry of Environmental Protection of People’s Republic of China [15]. The high performance in terms of sensitivity and detection limit is mainly attributed to the strong chelation of Cu2+ with PEI [24] and the inherently high sensitivity of AuNRs to the changes of local dielectric environment [7,25,26]. Moreover, this work demonstrates many advantages over the existing LSPR-based Cu2+ sensor. First, the polyelectrolyte-coated
Fig. 1. TEM images of (A) as-prepared AuNRs; (B) PSS–PEI-coated AuNRs.
10
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
Fig. 2. Zeta potential (A) and UV–vis absorption spectra (B) of as-prepared AuNRs and polyelectrolytes coated AuNRs.
Fig. 3. (A) The peak shift of longitudinal band in the absence (solid line) and presence (dotted line) of Cu2+ ions at pH 2.0 (a, b), pH 3.0 (c, d), pH 4.0 (e, f), pH 5.0 (g, h), pH 6.0 (i, j). An inserted histogram clearly shows the pH effect on Cu2+ induced LSPR peak shift; (B) response of different AuNRs to Cu2+ ions in pH4.0 PBS solution. The concentration of Cu2+ ions is 100 lM.
Fig. 4. (A) UV–vis absorption spectra and (B) plot of peak shift of PEI–PSS–AuNRs against Cu2+ concentrations ranging from 1 lM to 5 mM in a pH 4 PBS.
AuNRs are highly dispersible and stable in diverse water samples, resulting in good reliability. Second, the detection is simply performed under room temperature without heating process. Third, the sensing effect is independent of other solute such as dissolved oxygen.
3.4. Selectivity of PEI–PSS–AuNR-based LSPR probe for Cu2+ ion detection Selectivity is another important parameter to evaluate performance of the developed LSPR sensor. Interferences from common
Y. Liu et al. / Journal of Colloid and Interface Science 439 (2015) 7–11
11
observed, demonstrating its good selectivity for Cu2+ detection in real sample. The result demonstrates that the developed novel PEI–PSS–AuNR LSPR probe has a potential usefulness for Cu2+ detection in various real samples. 4. Conclusion
Fig. 5. Selectivity of the PEI–PSS–AuNRs probe for Cu2+ detection over other metal ions. The concentration of all metal ions is 100 lM in PBS at pH 4.0.
metal ions including Ag+ Ba2+, Ca2+, Mn2+, Zn2+, Cd2+, Pb2+, Co2+, Al3+, and Mg2+ were studied under the condition identical to that for Cu2+ detection. As shown in Fig. 5, Mn2+, Cd2+, Co2+, Al3+, and Mg2+ at 100 lM do not induce any LSPR peak shift. Negligible responses are observed from the treatment of other metal ions, including Ag+, Ba2+, Ca2+, and Pb2+. However, red shift is monitored from PEI–PSS–AuNRs treated by Zn2+. The effect is completely opposite to that of Cu2+, indicating no interference to the developed LSPR sensor for Cu2+ detection. The good selectivity and specificity are ascribed to that amine groups on PEI has a stronger coordination affinity with Cu2+ compared with other metal ions [24,27].
PEI–PSS–AuNRs were prepared via a simple layer by layer assembly approach and used as LSPR nanoprobes for detection of Cu2+ ions. A detection limit of 0.24 lM and a dynamic range from 1 lM to 5 mM have been achieved in PBS with the developed LSPR sensor. Good selectivity has also been demonstrated. The high performance is mainly attributed to the strong coordination of PEI with Cu2+ and the high sensitivity of longitudinal LSPR band to the changes of dielectric properties of coating layer/medium surrounding the AuNRs. Moreover, river water is successfully analyzed using the developed nanoprobe, demonstrating its great applicability for various real samples. In a word, this work provides a rapid, sensitive, selective, and low cost method for Cu2+ detection in many fields such as environment monitoring and food safety control. Acknowledgments This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies (Grant cstc2011pt), Chongqing Engineering Research Center for Rapid diagnosis of Dread Disease, the National Natural Science Foundation of China (Grant 31200604), Natural Science Foundation of Chongqing (Grant cstc2012jjA10152), Fundamental Research Funds for the Central Universities (Grant XDJK2012C083).
3.5. Application of PEI–PSS–AuNR-based LSPR probe in real sample References Applicability of the developed LSPR sensor is evaluated by performance of Cu2+ detection in real water sample obtained from Jia Ling River (Chongqing, China), where many unexpected substances are there. Before testing, the river water was first centrifuged at 12,000 rpm for 20 min and then filtered through a 0.22 lm membrane to remove particulate matters that may have influence on the analysis. River water samples spiked with standard solutions containing Cu2+ from 1 lM to 5 mM are mixed with PEI–PSS–AuNR solution at pH 4.0. A calibration curve was plotted with the values of peak shift versus the concentrations of Cu2+ over the range from 1 lM to 5 mM (Fig. 6). Despite numerous unexpected interferences in river water, a good linear response to Cu2+ (R2 = 0.975) is
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Fig. 6. Plot of longitudinal LSPR peak shift of PEI–PSS–AuNRs against Cu2+ concentrations ranging from 1 lM to 5 mM in river water.
[26] [27]
P.L. Fox, Biometals 16 (2003) 9–40. U.S. EPA, Fed. Reg. 56 (1991) 26460–26564. S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. L.F. Gou, C.J. Murphy, Chem. Mater. 17 (2005) 3668–3672. H.J. Chen, X.S. Kou, Z. Yang, W.H. Ni, J.F. Wang, Langmuir 24 (2008) 5233–5237. C.G. Wang, Y. Chen, T.T. Wang, Z.F. Ma, Z.M. Su, Chem. Mater. 19 (2007) 5809– 5811. M.M. Miller, A.A. Lazarides, J. Phys. Chem. B 109 (2005) 21556–21565. H.J. Parab, H.M. Chen, T.C. Lai, J.H. Huang, P.H. Chen, R.S. Liu, M. Hsiao, C.H. Chen, D.P. TSai, Y.K. Hwu, J. Phys. Chem. C 113 (2009) 7574–7578. X.H. Huang, I.H. El-Sayed, W. Qian, M.A. El-Sayed, J. Am. Chem. Soc. 128 (2006) 2115–2120. G.J. Nusz, S.M. Marinakos, A.C. Curry, A. Dahlin, F. Höök, A. Wax, A. Chilkoti, Anal. Chem. 80 (2008) 984–989. K.M. Mayer, S. Lee, H. Liao, B.C. Rostro, A. Fuentes, P.T. Scully, C.L. Nehl, J.H.A. Hafner, ACS Nano. 2 (2008) 687–692. L.B. Wang, Y.Y. Zhu, L.G. Xu, W. Chen, H. Kuang, L.Q. Liu, A. Agarwal, C.L. Xu, N.A. Kotov, Angew. Chem. Int. Ed. 49 (2010) 5472–5475. H.W. Huang, X.Y. Liu, T. Hu, P.K. Chu, Biosens. Bioelectron. 25 (2010) 2078– 2083. Y. Wang, Y.F. Li, J. Wang, Y. Sang, C.Z. Huang, Chem. Commun. 46 (2010) 1332– 1334. J.M. Liu, H.F. Wang, X.P. Yan, Analyst 136 (2011) 3904–3910. T. Placido, R. Comparelli, M. Striccoli, A. Agostiano, A. Merkoi, M.L. Curri, Sens. J. IEEE 13 (2013) 2834–2841. P.K. Sudeep, S.T.S. Joseph, K.G. Thomas, J. Am. Chem. Soc. 127 (2005) 6516– 6517. Z.Y. Zhang, Z.P. Chen, C.L. Qu, L.X. Chen, Langmuir 30 (2014) 3625–3630. B. Nikoobakht, M.A. El-Sayed, Chem. Mater. 15 (2003) 1957–1962. A. Gole, C.J. Murphy, Chem. Mater. 17 (2005) 1325–1330. A.D. McFarland, R.P. Van Duyne, Nano Lett. 3 (2003) 1057–1062. Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Anal. Chem. 84 (2012) 6220– 6224. A. Sugunan, C. Thanachayanont, J. Dutta, J.G. Hilborn, Sci. Technol. Adv. Mater. 6 (2005) 335–340. Y.L. Chen, B.C. Pan, H.Y. Li, W.M. Zhang, L. Lv, J. Wu, Environ. Sci. Technol. 44 (2010) 3508–3513. T.R. Jensen, M.L. Duval, K.L. Kelly, A.A. Lazarides, G.C. Schatz, R.P. Van Duyne, J. Phys. Chem. B 103 (1999) 9846–9853. K.A. Willets, R.P. Van Duyne, Annu. Rev. Phys. Chem. 58 (2007) 267–297. R. Molinari, S. Gallo, P. Argurio, Water Res. 38 (2004) 593–600.
