l-Cysteine-Modified Gold Nanostars for SERS-Based Copper Ions

Article pubs.acs.org/Langmuir

L‑Cysteine-Modified

Gold Nanostars for SERS-Based Copper Ions Detection in Aqueous Media

Pancras Ndokoye, Jun Ke, Jie Liu, Qidong Zhao, and Xinyong Li* Key Laboratory of Industrial Ecology and Environmental Engineering, Key Laboratory of Fine Chemicals, School of Environmental Sciences and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: Gold nanostars coated with cysteine (Cys-AuNSs) were successfully synthesized and used in SERS-based copper ions (Cu2+) detection in aqueous media. The strong coordination ability of cysteine (Cys) with Cu2+ and the resulting Cys-AuNSs-Cu complex formation led to AuNSs aggregation and the drastic change in intensity and strength of COOH band spectra. The aggregation of AuNSs yielded distinct SERS signals, which exhibited remarkable sensitivity and selectivity for Cu2+ over other metal ions. Using this SERS-based sensing method, we have achieved a practical detection limit of 10 μM. Such AuNSs-based detection could provide promising alternative choices for future SERS-active AuNSs application.

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INTRODUCTION Gold nanoparticles (AuNPs) own large optical field that arises from the resonant oscillation of their free electrons known as localized surface plasmon resonance (LSPR).1,2 They exhibit a high sensitivity originating from the nature, size, and shape of particles, interparticle distance, and the surrounding media.3,4 In recent years, AuNPs in particular with their well-defined nanostructures have been widely used in many applications because of their low dimensionality, high extinction coefficients, and strong distance-dependent optical properties.5,6 Consequently, various anisotropic gold nanostructures such as gold nanorods (AuNRs)7−13 and gold nanostars (AuNSs)14 have attracted researchers in many ways, especially AuNSs15−17 due to their high sensitivity to local environment18,19 and large surface areas from nanoscaled branches.20−22 Hence, they have been typically involved in some important applications such as medicine,4 optics,23,24 catalysis,25 and heavy metal detection.26 For instance, chitosan-coated AuNSs have shown higher colloidal stability and been used as suitable mediators in cell photothermolysis.27 Mercaptopropyltrimethoxysilane-coated AuNSs exhibited an efficient photothermal response upon laser excitation by inducing local hyperthermia and efficient killing of Staphylococcus aureus biofilms.28 Owing to their photophysical properties, they are potential candidates for in vitro and in vivo cancer therapy29,30 and drug delivery.30 The application of antibody-AuNSs in immune-surface-enhanced Raman spectroscopy (Immune-SERS) has demonstrated the abundance of the tumor suppressor in the basal epithelium of benign prostate tissue.31 More investigations on the performance of AuNSs-based biosensing have proven that detection of molecular and breast cancer cells is successfully achieved via SERS amplification.28,32 © 2014 American Chemical Society

SERS-based techniques rely on the spectral positions of LSPRs excited via the change in dielectric environment around the tips.33 It is not only in biomedical domain, SERS signals have been utilized in heavy metals detection. For instance, by analyzing SERS signal intensities, cadmium ions (Cd2+) were sensitively and selectively detected via the process of Cd2+induced nanoparticle aggregation.34 A self-assembled nanostar dimer through Hg2+-mediated T−T base pair of ssDNA has been also developed as a highly sensitive SERS sensor for Hg2+ detection in water. Formation of dimer structures has demonstrated the electromagnetic field enhancement which highly facilitates SERS intensity.26 To date, SERS-based copper ions (Cu2+) detection is not reported yet. In this work, we investigate the SERS-based selective detection of Cu2+ with L-cysteine-modified AuNSs (CysAuNSs). Meanwhile, the mechanism on the selectivity of Cu2+ detection is carefully analyzed, and the role of the interaction between Cu2+ ion and L-cysteine (Cys) is clarified as well.

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EXPERIMENTAL SECTION

Materials and Instruments. Hydrogen tetrachloroaurate (HAuCl4·3H2O, 99.99%), cetyltrimethylammonium bromide (CTAB, 99%), benzyldimethylammonium chloride hydrate (BDAC, 98%), ascorbic acid (AA, 99.7%), sodium borohydride (NaBH4, 98%), silver nitrate (AgNO3, 99.8%), hydrochloride (HCl, 36%−38%), and sodium hydroxide (NaOH,99%) were purchased from Sinopharm Chemical (Shanghai, China). KBr, NaCl, KCl, Cr(NO3)3·6H2O, Co(NO3)2· 6H2O, MgCl2·6H2O, NiCl2·6H2O, FeCl2·4H2O, ZnCl2, CdCl2·5H2O, Received: September 5, 2014 Revised: October 18, 2014 Published: October 22, 2014 13491

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Figure 1. (a) UV−vis−NIR absorption spectra of solutions in C and (b, c) TEM and (d) SEM image from samples in bottle C. HgSO4, Pb(NO3)2, and CuSO4·5H2O were obtained from Aladdin Reagent Company Ltd. (Shanghai, China). A DXD Raman microscope was used for SERS measurements. The signals from the samples were collected upon excitation with 633 nm laser lines. FTIR spectroscopy was performed using a Bruker Vertex 70-FTIR spectrometer. Transmission electron microscopy (TEM) was performed on a Tencai F 20 instrument and operated at 200 kV. UV−vis absorption spectra were recorded using a Lambda 950 UV−vis spectrophotometer from PerkinElmer. All chemicals were used without further purification. All glasswares were cleaned with aqua regia (volume ratio HCl:HNO3 = 3:1) and thoroughly rinsed with Millipore water (18.0 M·cm). Deionized water (18 MΩ) was used in all the experiments. Synthesis of Metal Seeds. Gold seeds were prepared as previously reported.35 Briefly, a 20 mL aqueous solution containing 2.5 ×10−4 M HAuCl4 and 2.5 ×10−4 M trisodium citrate was prepared in a conical flask. Next, 0.6 mL of ice cold 0.1 M NaBH4 solution was added into the solution all at once under stirring. The solution turned pink immediately after adding NaBH4, indicating particle formation. The particles in this solution were used as seeds within 2−5 h after preparation. Synthesis of Gold Nanostars. To prepare AuNSs, we used seeds prepared in accordance with the Nikoobakht method,35 and the seeding was done into three steps for the purpose of investigating the effect of seeds on AuNSs growth. Typically, three small bottles of 9 mL (labeled as A, B, and C), each containing of 2.5 × 10−4 M HAuCl4 and 2.5 g of 0.1 M CTAB mixed with 1.2 g of 0.20 M BDAC, were mixed with 0.05 mL of 0.1 M AgNO3 and then 0.1 M ascorbic acid as reducing agent. Because of the reduction of Au3+ to Au+, solutions in all bottles became colorless. Next, 1.0 mL of seed solution was added into and mixed with solution A, then 1 mL was picked from solution A and transferred to bottle B 15 s later, and 1 mL of solution B was transferred to bottle C at 30 s after adding seed solution into A. After 4 h AuNSs were successfully grown in bottle C. Particles contained in bottle C (Figure 1b−d) were typically used in copper detection. Separation of these particles was performed by means of centrifugation

at 1200 rpm in 25 min. This reaction of AuNSs synthesis was performed at 25 °C, and pH was maintained at 7.6. Preparation of Cys-AuNSs. Before detecting various metal ions, AuNSs was modified with Cys in order to recognize metal ions. Cys can anchor to Au surface via S−Au covalent bonding, and thus COO− and NH3+ groups of Cys-AuNs could bind to positive ions through coordination bonding.36,37 Thereby, Cys was selected as the functional material to recognize the target metal ion. 12 mL of solution was picked from the solution of purely centrifuged AuNSs and mixed with 6 mL of 2 × 10−4 M L-cysteine solution under stirring. The solution was stirred for 1.5 h at 40 °C. The Cys-AuNSs solution (Figure 2a−c) was isolated by centrifugation (400 rpm/5 min) and redispersed in pure water for further use. Surface-Enhanced Raman Scattering (SERS). The SERS measurements were carried out with in DXD Raman microscope. The signals from samples were collected upon the excitation with 633 nm laser lines. 300 μL of different solutions containing the complex of Cys-AuNSs with metals was dropped into a silicon cuvette, and the measurements were performed at different positions on each sample. Characterization and Samples Preparation. The samples were characterized by using ultraviolet−visible−near-infrared (UV−vis− NIR) spectroscopy (Hitachi U-2800 spectrometer), scanning electron microscopy (SEM, Hitachi S4800, 10 kV), transmission electron microscopy (TEM JEOL-2010, 200 kV), and Fourier transforminfrared (FTIR). Prior to measurement, 1 mM stock solution of metals (Hg2+, Cu2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+) was diluted with deionized water up to 8.5 μM. On the other hand, Cu2+ was diluted and adjusted to the following concentrations: 8.5, 9, 10, 15, 20, 25, 35, and 40 μM. The solutions containing 25 μM of different metal together with Cys-AuNSs were prepared for proper SERS measurement. To obtain enough powder for FTIR measurement, all samples were dried and each residue was carefully collected and mixed with KBr to get enough powder for FTIR measurement. Cys-Cu2+ was prepared in the same procedure for FTIR measurement. 13492

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7.6. Figure 3 depicts the normal spectrum intensities of Cys and Cys-AuNSs (a) and SERS-based detection in aqueous media (b). According to the previous reports, the most prominent bands in the Raman spectra of Cys and other amino acids appear in the region of 3000 cm−1 and in the 500−1700 cm−1 range, with a gap from 1700 to 2850 cm−1 range,38 which is consistent with the spectrum shown in Figure 3a. By focusing on Cys-AuNSs, it is observed that after Cys is adsorbed on AuNSs the vibrational intensities relatively decrease, but it keeps the normal profile. Considering 1500−1250 cm−1 region on Cys-AuNSs (inset in Figure 3a), we find out that signals are apparently weak compared with other remarkable bands in the normal Raman spectra. However, SERS spectra of Cys adsorbed on colloidal Au surfaces have shown the enhanced symmetric and antisymmetric vibrations in the range of 1500−900 cm−1 in which symmetric vibration mode of COO¯ is observed relatively strong at 1400 cm−1. In addition, SERS spectra of the amino group (NH3+) vibrations modes also appear at 1123−1181, 1095, 1054−1074, and 846−856 cm−1.39,40 Herein, SERS response of Cys-AuNSs reacting with metal ions (Figure 3b) is clearly consistent with reports above, but here each metal responds differently due to their affinity toward Cys and the resulting effect on AuNSs as well. It is obvious that SERS signal intensities show dramatic increase when Cys-AuNSs forms complex with Cu2+ (Figure 3b). Previous reports mention that the coordination of Cys with Cu2+ forms a Cys-Cu-Cys complex through acidic (COOH) and basic (NH2) functional groups of cysteine,41,42 which leads to change of electrostatic repulsion inducing the aggregation phenomenon. As it has been reported, the aggregation of AuNSs results in very high SERS enhancements as a result of plasmon coupling effect.43−49 Apart from that advantageous phenomenon, AuNSs show a very high sensitivity toward local changes in the dielectric environment, which makes them SERS-active excellent tools.50,51 This could be the reason for sensitive SERS signals that exhibit high sensitivity on Cu2+ over other metal ions (Figure 4). Among the metal ions tested, Hg2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+ could coordinate with Cys but do not induce AuNSs aggregation. Therefore, the Cu2+-induced

Figure 2. (a) Absorption spectra of unmodified AuNSs (curve in dark green) and Cys-AuNSs (curve in gray) and (b, c) TEM images of CysAuNSs used for Cu2+ detection. Detection and Characterizations. The detection of aqueous Cu2+ was performed at room temperature. Briefly, 150 μL of 25 μM metals ions (Hg2+, Cu2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+) was respectively added to 200 μL of Cys-AuNSs. To investigate the specific concentration to be detected in aqueous media, 150 μL of various Cu2+ concentrations (8.5, 9, 10, 15, 20, 25, 30, 35, and 40 μM) were added to 200 μL of Cys-AuNSs. SERS intensities were measured with a DXD Raman microscope. The signals from the samples were collected upon excitation with 633 nm laser lines spectroscopy.

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RESULTS AND DISCUSSION Cys-AuNSs and Cu2+ Detection in Aqueous Media. Herein, 10 metal ions (Hg2+, Cu2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+) were used to measure the selectivity of Cys-AuNSs to Cu2+ ions in comparison with other metals, the solution of 150 μL (25 μM) for each metal was mixed with 200 μL of Cys-AuNSs at room temperature, and pH was adjusted to

Figure 3. (a) Normal Raman spectra of Cys (blue) and Cys-AuNSs (black) and (b) SERS signal intensities of Cys-AuNSs with different metal ions. The inset in (a) shows the normal spectra of Cys-AuNSs in 1500−500 cm−1 range with the emphasis on 1500−1250 cm−1 region. 13493

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results from the deprotonation of the thiol group upon binding readily and rapidly to the surface of the AuNSs; therefore, the band assigned to S−H almost disappears. On the other hand, Cys-AuNSs is marked with a shift (from 1300 to 1600 cm−1) for the position of COO̅ and NH3+ stretching, which probably originates from the change in their dipole moment when Cys binds on gold surface. Significantly, the Cys-AuNSs-Cu complex exhibits a particular change in stretching intensity and strength. It was reported that addition of Cu2+ into the system leads to the coordination of Cu2+ with the Cys in a Cys-Cu2+ molar ratio 2 (Cys-CuCys), and the coordination sites are acidic (COOH) and basic (NH2) functional groups of Cys.41,42 Therefore, a stronger effect on amine and carboxyl group is observed after AuNSsCys-Cu-Cys-AuNSs complex formation. This phenomenon resulted in the tremendous disappearance of S−H and drastic change of COO− spectra (Figure 6). This spectral response is consistent with aggregation occurring after AuNSs-Cys-Cu-Cys-AuNSs complex formation (Figure 7). The spectral variations probably indicate that the coordination of COOH and NH2 with Cu2+ leads to electrostatic repulsion change, which accelerates the aggregation phenomenon. Figure 8 depicts the band spectra of COOH for 10 metal ions. The evidence is that the Cu2+-induced aggregation of CysAuNSs entails the above-mentioned strong coordination between Cys and Cu2+. It is found out that the coordination ability of Cu2+ with Cys is so strong to the point that the intensity of COOH spectral band drastically changes (Figure 8). Even though COOH spectra change for the other metals ions, it seems that their coordinative bonding strength is not as remarkable as that on Cu2+.

Figure 4. Normalized SERS intensities showing the selectivity of CysAuNSs on Cu2+ compared with other heavy metal ions at 25 μM concentration for each metal ion.

aggregation may be due to the strong coordination of Cys toward Cu2+. On the other hand, we used the Cu2+ concentration range (8.5−40 μM) to figure out Cu2+ detection limit, and then, CysAuNSs complexes were mixed with various concentrations of Cu2+ ion. Figure 5a shows that 10 μM indicates the beginning of clear and detectable SERS signals. It is obvious that SERS signal intensities become stronger as Cu2+ concentration increases, which indicates that the higher the interparticle aggregation is, the stronger the SERS signal intensity becomes. With the increase of Cu2+ concentration, the SERS signal (Figure 5a) of AuNSs reaches the maximum at 40 μM Cu2+, indicating the complete aggregation of AuNSs (Figure 5b). Thus, SERS signals are activated as the aggregation takes place. In order to investigate the affinity of copper to Cys, we measured and analyzed FTIR spectra of Cys, Cys-AuNSs, and Cys-AuNSs-Cu (as shown in Figure 6). Focusing on Cys, the spectral bands at 1600 and 1390 cm−1 correspond to the asymmetric and symmetric stretching of CO (COOH). A band at 1532 cm−1 corresponds to N−H bending, and the very broad band of N−H and OH− stretch is observed in the 3000− 3500 cm−1 range. These results correspond with IR spectra agreement of a typical amino acid.52 In addition, a weak band near 2550 cm−1 virtually confirms the presence of S−H group in the Cys molecule.53−55 The Cys-AuNSs complex formation

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CONCLUSIONS We demonstrate SERS-based copper ions detection by using Cys-AuNSs. The presence of Cu2+ induces AuNSs aggregation through the excellent coordination of Cys toward Cu2+. AuNSs aggregation activates the SERS signals; hence, the unique sensitivity and selectivity for Cu2+ over other metal ions are obtained. The limit of detection (LOD) for Cu2+ is 10 μM, and the detection range varied from 8.5 to 40 μM. Cu2+ coordinates with COOH and NH2 to form the AuNSs-Cys-Cu-Cys-AuNSs complex, which leads to electrostatic repulsion change that accelerates the aggregation phenomenon. The aggregation

Figure 5. (a) SERS signal intensities of Cys-AuNSs with different concentration of Cu2+ and (b) aggregation of Cys-AuNSs-Cu. 13494

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Figure 6. (a, b) FTIR spectra of NH3+, SH, and COO¯ in Cys (green), Cys-AuNSs (blue), Cys-Cu (orange), and Cys-AuNSsCu (dark red).

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-411-8470-7733; Fax +86-411-8470-7733; e-mail [email protected] (X.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Nature Science Foundation of China (21377015, 21207015, and 51178076), the Major State Basic Research Development Program of China (973 Program) (No. 2011CB936002), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

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Figure 7. Schematic diagram for detection of Cu2+ using Cys-AuNSs in aqueous media.

Figure 8. FTIR spectra of COO− in Cys-AuNSs with different metal ions (Hg2+, Cu2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+).

occurs upon the formation of Cys-AuNSs-Cu complex, which causes the drastic change in spectral intensity of COOH band on FTIR spectrum as a result of strong coordination ability of Cys with Cu2+.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of Cys-Cu, Cys-AuNSs-Cu, and Cys-AuNSs; SERS signals of naked AuNPs and AuNSs as well as Cys-coated AuNPs and AuNSs Cys-AuNSs with Cu2+ ions. This material is available free of charge via the Internet at http://pubs.acs.org. 13495

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