Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 132–137
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric detection of manganese(II) ions using gold/dopa nanoparticles Kannan Badri Narayanan, Hyun Ho Park ⇑ Department of Biochemistry, School of Biotechnology, Yeungnam University, Gyeongsan 712 749, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Facile colorimetric detection of Mn(II)
ions by dopa-capped gold nanoparticles. Synthesis of AuNPs is cost-effective and eco-friendly without toxic chemicals. Ratio of absorbance (A700/A550) was linear against the concentration of [Mn2+] ions.
a r t i c l e
i n f o
Article history: Received 11 January 2014 Received in revised form 5 April 2014 Accepted 17 April 2014 Available online 26 April 2014 Keywords: Gold nanoparticles Metal ions Dopa Manganese(II) Sensor Eco-friendly
a b s t r a c t We report here a one-pot, greener, eco-friendly strategy for the synthesis of gold nanoparticles using L-dopa. The as-prepared dopa-functionalized gold nanoparticles (AuNPs/dopa) can detect low concentrations of manganese(II) metal ions in aqueous solution. The binding forces between dopa and Mn2+ ions cause dopa-functionalized gold nanoparticles to come closer together, decreasing the interparticle distance and aggregating it with a change in color of colloidal solution from red to purplish-blue. Dynamic light scattering (DLS) analysis showed a decreased surface charge on the surface of gold nanoparticles when exposed to Mn2+ ions, which caused cross-linking aggregation. Transmission electron microscopic (TEM) images also revealed the aggregation of gold nanoparticles with the addition of Mn2+ ions. The extinction ratio of absorbance at 700–550 nm (A700/A550) was linear against the concentration of [Mn2+] ions. Thus, the optical absorption spectra of gold colloidal solution before and after the addition of Mn2+ ions reveal the concentration of Mn2+ ions in solution. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Nanomaterials are nanodimensioned materials with a size of 3.5 nm diameter) induces interparticle surface plasmon coupling, resulting in a visible color change from red to blue at nanomolar concentrations. This color change aids in the utilization of plausible absorption-based colorimetric sensing of any target analyte that induces AuNPs aggregation [10,11]. Thus, strategies based on nanoparticle aggregation have attracted a great deal of attention. Since the surface of AuNPs can be easily functionalized with various ligands, it is possible to modulate the interaction of external ions with stabilizing functional groups for development of diverse metal sensors. Gold nanoparticles exhibit higher extinction coefficients, as well as size or distance-dependent optical properties. The aggregation/dispersion states of AuNPs show changes in color when observed by the naked eye, enabling easy detection of DNA [12,13], protein [14,15], and metal ions [16,17]. Upon aggregation, spherical AuNPs undergo red-shifts of the SPR band due to increased size, which are used to sense analytes based on colorimetric assays. Several cross linking and non-cross-linking aggregations of AuNPs have been reported to have sensing ability. Several biocompatible molecules such as proteins, amino acids, and sugars are used as reducing and capping agents in the synthesis of metal nanoparticles [18,19]. The use of L-dopa in protein engineering has recently been increasing in protein engineering due to its structural similarity with the natural amino acid tyrosine. Dopa has multifunctional groups including amino, carboxylic and hydroxyl groups, which have the ability to reduce metal ions such as Au3+ and Ag+ to metal nanoparticles. Manganese (Mn) is an essential trace element for both plants and animals that is most stable in the 2+ valence state. Mn2+ activates many enzymatic reactions in carbohydrate metabolism and in the metabolism of organic acids, nitrogen and phosphorous. Although it is required for humans, Mn deficiency is associated with lipid and carbohydrate metabolism, growth retardation and reproductive failure in adults and ataxia and skeletal abnormalities in neonatals [20]. Additionally, higher concentrations can cause adverse neurological effects, and high levels of manganese (>14 mg/l) in drinking water
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can cause manganism, which is characterized by a Parkinson-like syndrome [21]. Mn is also a Toxic Release Inventory (TRI) chemical, and some of its compounds are also listed as air contaminants by the Occupational Safety and Health Administration (OSHA). In 1973, the Food and Nutrition Board of the National Research Council (NRC) determined the safe and adequate daily dietary intake of Mn to be 2–5 mg/day for adults [22]. Herein, we describe the synthesis of gold nanoparticles using L-dopa (L-3,4-dihydroxyphenylalanine) and the subsequent utilization of gold/dopa nanoparticles (AuNPs/dopa) to detect and quantify manganese (Mn2+) ions. Experimental Chemicals and reagents L-3,4-dihydroxyphenylalanine (L-dopa) and manganese(II) chloride (MnCl24H2O) were purchased from Sigma–Aldrich (USA). Chloroauric acid (HAuCl4) was purchased from Alfa Aesar Chemicals (South Korea). All experiments were carried out in deionized Milli-Q (18.2 O) water. Preparation of dopa-capped gold nanoparticles A total of 50 ml aqueous L-dopa solution (100 lM) at pH 6.5 was stirred at 80 °C for 30 min with the addition of 1.5 ml of HAuCl4 (6 mM) and then cooled to room temperature to prepare dopa-functionalized gold nanoparticles (AuNPs/dopa). The assynthesized gold colloidal solution was further characterized and also directly exposed to various concentrations of metal ions for colorimetric assays. Characterization of L-dopa-capped gold nanoparticles The reduction of gold ions to gold nanoparticles was monitored using UV–Vis spectroscopy (Beckman Coulter DU-730) and visual observation. Additionally, the morphology and size of as-synthesized AuNPs/dopa were characterized by TEM analysis of samples that had been placed on carbon-coated copper grids and dried prior to observation using a Hitachi (Model: H-7600) microscope at an accelerating voltage of 120 kV. Furthermore, the dynamic light scattering (DLS) method was employed to measure the size and surface charge of nanoparticles and the interparticle interactions. The hydrodynamic diameter and zeta potential of AuNPs/ dopa were characterized using a Zetasizer Nano series (Malvern Instruments) equipped with a He–Ne laser (k = 633 nm) and operated at a scattering angle of 173°. Powder X-ray diffraction (XRD) was conducted to determine the crystallinity of gold nanoparticles. The powdered nanoparticles were measured in transmission mode on a PANalytical X’pert PRO X-ray diffractometer (Netherlands) at a voltage of 40 kV and a current of 30 mA with Cu Ka1 radiation (1.541 Å). Samples were scanned in the region of 2h from 30° to 85°. The nanocrystallite size was calculated using Scherrer equation [23]. Fourier transform infrared spectroscopy (FTIR) measurements were performed to identify the functional groups involved in the formation and stabilization of dopa-functionalized AuNPs. The AuNPs/dopa in the solution were separated using high-speed centrifugation (25,000g) for 20 min and then washed twice using sterile deionized water. Finally, the dried pellets were ground with KBr and analyzed on a Jasco FTIR 5300 spectrophotometer in transmittance mode over 4000–400 cm1. Colorimetric detection of manganese(II) ions using AuNPs/dopa sensor To demonstrate the detection of metal ions using AuNPs/dopa, a stock solution of 10 mM manganese(II) chloride was prepared.
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K.B. Narayanan, H.H. Park / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 132–137
Fig. 1. (A) UV–Vis spectrum of dopa-functionalized gold nanoparticles indicating SPR peak, (B) XRD pattern of dopa-capped gold nanoparticles. The principal Bragg reflections are identified.
900 ll of synthesized gold/dopa nanoparticles were directly exposed to 100 ll of various concentrations of Mn2+ ions and incubated at room temperature for 10 min, after which the color change was visually observed and the optical measurements were carried out using a UV–Vis spectrophotometer (Beckman Coulter DU-730) at 300–900 nm. Results and discussions Characterization of gold nanoparticles Colloidal gold nanoparticles interact with visible light to produce colors, which has applications in sensory probes, catalysis and therapeutic agents [11]. These optical and electronic properties of gold nanoparticles are tunable by changing the size, morphology, surface chemistry or dispersity state [24]. Several low-molecular weight biomolecules such as amino acids are used for the greener synthesis of metal nanoparticles [25]. When HAuCl4 was added to oxidized L-dopa solution the color of the solution turned from yellowish to reddish with a sharp surface plasmon resonance (SPR) peak at 548 nm (Fig. 1A). The characteristic reddish color of AuNPs confirms the formation of monodispersed AuNPs [26]. The dihydroxyl groups at the ortho position of dopa were involved in the reduction of metal ions. During the preparation of AuNPs, the dihydroxyl groups of the added dopa molecules lose two electrons to form a quinone molecule, which further forms cyclodopa that is adsorbed onto the surface of AuNPs to form
Fig. 2. FTIR spectra of (A) dopa and (B) dopa-capped gold nanoparticles.
monodispersed AuNPs [27]. The FTIR spectra of the dopa and dopacapped AuNPs were then measured (Fig. 2). The spectrum of dopa showed many small peaks, which is a common feature of small molecules, whereas AuNPs/dopa showed peaks around 1615 cm1 and 3420 cm1 that were attributed to the stretching vibration of the aromatic rings and catechol –OH groups, respectively [28]. The use of dopa to functionalize gold nanoparticles can be used to immobilize nanoparticles onto polymer surfaces for several applications [29]. TEM images showed the formation of monodispersed spherical nanoparticles ranging from 3.5 to 17.3 nm in size with an average diameter of around 13.7 nm. XRD measurements revealed the formation of face-centered cubic (fcc) gold nanoparticles with intense peaks corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) Bragg’s reflection (Fig. 1B). The average crystallite size of AuNPs/ dopa was calculated to be 7.7 ± 0.87 nm using Scherrer’s formula by taking the average of all the five peaks, which matches the
Fig. 3. (A) Photographs of dopa-functionalized AuNPs colloid with different concentrations of Mn2+. From left to right: (1) 0 lM; (2) 5 lM; (3) 10 lM; (4) 20 lM; (5) 50 lM; (6) 100 lM. (B) UV–Vis spectrum of dopa-functionalized AuNPs colloid containing different concentrations of Mn2+. From upper to lower: (1) 0 lM; (2) 5 lM; (3) 10 lM; (4) 20 lM; (5) 50 lM; (6) 100 lM. Inset graph describes the plot of A700/A550 against CMn for manganese analysis.
K.B. Narayanan, H.H. Park / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 132–137
TEM results. The physical and optical stability of nanoparticles have also been evaluated. It was found that there was an indication of agglomeration of AuNPs/dopa with slight change in optical spectrum along with some change in physical appearance after three months of storage at room temperature (Supplementary Fig. 1).
Fig. 4. (A) Photograph and (D) TEM image of dopa-functionalized AuNPs without addition of Mn2+; (B) photograph of dopa-functionalized AuNPs with EDTA and Mn2+; (C) photograph and (E) TEM image of dopa-functionalized AuNPs with the addition of Mn2+.
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Effect of manganese (Mn2+) ions on AuNPs/dopa The synthesis of size-controlled monodispersed gold nanoparticles is advantageous in colorimetric sensing of metal ions. The functionalized groups stabilizing these nanoparticles have different affinities towards different metal ions. To synthesize gold nanparticles to sense a specific metal ion, the knowledge on the surface functional groups, which has a selective affinity towards a particular metal ion is prerequisite. Using L-dopa as a stabilizing agent in the synthesis of dopa/gold nanoparticles exhibited a strong affinity towards Mn2+ ions. Fig. 3 shows the UV–Vis spectrum of reaction solution with different concentrations (5, 10, 20, 50, 100 lM) of Mn2+. In the absence of Mn2+, the gold nanoparticles were red in color and exhibited a sole sharp peak at 548 nm, indicating the monodispersed state. However, as the concentration of Mn2+ increased, the red color of AuNPs gradually changed to dark purplish-blue and showed decreased absorbance at 548 nm with a concomitant increase in another peak at around 700 nm due to aggregation. The extinction ratio of absorbance at 700–550 nm (A700/A550) is linear against the concentration of Mn2+ in the range of (5–100 lM), and the calibration curve is A700/A550 = 0.09561 + 0.00751 CMn (R = 0.99). In the presence of Mn2+ ions, reddish AuNPs become purplishblue in 10 min due to the aggregation of monodispersed spherical nanoparticles (Fig. 4). TEM images also revealed the aggregation of nanoparticles with the addition of Mn2+ ions. The Mn2+ ions interact with dopa-functionalized AuNPs through Mn–O interactions [30]. Due to the binding forces between dopa and Mn2+ ions, dopa-capped AuNPs tend to come closer, decreasing the interparticle distance and gets aggregated (Fig. 5). This results in a change of optical properties of AuNPs, causing purplish-blue color. However, when chelating agent (EDTA) was added with divalent cation Mn2+, the color of the solution did not change from red to purplish-blue, indicating that Mn2+ was chelated by EDTA, preventing interaction with AuNPs/dopa (Fig. 4). DLS analysis also revealed increased
Fig. 5. Schematic diagram of the mechanism of colorimetric detection of Mn2+ with AuNPs/dopa.
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aggregation of nanoparticles with increased levels of Mn2+ ions in the solution. When the interparticle distance is greater than the average nanoparticle diameter AuNPs appear red, while a decrease in interparticle distance to less than the average particle diameter results in the AuNPs becoming purplish-blue [31]. Particle charge is also an important factor to determine the physical stability of colloidal suspensions. Particle charge is quantified as zeta potential (f), which is a measure of the electrical force (approximation of surface charge) that exists between nanoparticles in a solution. This aggregation was influenced by the surface charge (f potential) of nanoparticles. As the concentration of Mn2+ ions in solution increased, there was a gradual decrease in zeta potential and a progressive increase in agglomeration of nanoparticles with increased hydrodynamic size. Additionally, the surface charge changed from positive to negative with the addition of Mn2+ ions, while further increases in the concentration of Mn2+ led to a gradual decrease in negative charge (repulsive forces), resulting in a tendency to aggregate (Fig. 6). For potential reuse of gold/dopa nanoparticles, EDTA was added to the solution which chelates Mn2+ ions. This solution was further centrifuged at 16,000 rpm for 30 min to sediment the gold nanoparticles from the solution, which can be washed with deionized water and resuspended in a homogenous aqueous solution by mild sonication. Several cross linking and non-cross-linking aggregations of AuNPs have been reported for its sensing ability. Specific hybridization of DNA–DNA, antibody–antigen interactions or metal–ligand coordination is normally involved in aggregation. Mirkin et al. [32] used DNA–AuNPs for the first time to detect target DNA by hybridization with complementary DNA based on aggregation of AuNPs. DNA functionalized AuNPs with thiolatedDNA sequences (50 -thiol-C10A10TA10 and 50 -thiol-C10T10TT10), which are complementary except for a single thymine–thymine (T–T) mismatch that detects metal Hg2+ ions by coordinating to a T–T pair [33,34]. Thymine (T)–AuNPs sensitively and selectively detected Hg2+ ions by inducing T–Hg2+–T coordination [14]. Sugunan et al. [35] used chitosan-capped AuNPs to sense Zn2+ and Cu2+ ions. Lactose-functionalized AuNPs (16 nm) detects Ca2+ ions colorimetrically [36]. Additionally, Liu et al. [37] devised a simple colorimetric sensor for detection of Hg2+ in aqueous solution using gold nanoparticles modified with quaternary ammonium group-terminated thiols at room temperature. Among several metal ions investigated for the selectivity of colorimetric sensor, metal ions such as Li+, Cd2+, Ca2+, Mg2+, Hg2+, Ni2+, Zn2+, K+, Na+, and Rb+ did not show any color change, whereas Pb2+, Ba2+, Cu2+ and Mn2+ ions caused AuNPs to turn from red to purple-blue color. Among the positive ions, Mn2+ exhibited a profound
Fig. 6. Hydrodynamic size and zeta potential (f) of dopa-functionalized AuNPs with different concentrations of Mn2+ ions.
interaction causing a dark purplish-blue color change. In addition, there was a linear relationship between the extinction ratio of absorbance at 700–550 nm (A700/A550) and the concentration of [Mn2+]. Conclusions The method described herein enables the facile colorimetric detection of manganese(II) ions based on dopa-functionalized AuNPs. The synthesis of biocompatible AuNPs is cost-effective, environmentally-friendly, and does not involve toxic chemicals. The ratio of absorbance at 700–550 nm of gold/dopa nanoparticles against the known concentration of Mn(II) provides the concentration of Mn(II) ions in desired solution. Acknowledgement This study was supported by the 2012 Yeungnam University Research Grant. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.04.081. References [1] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078–1081. [2] H. Li, L.J. Rothberg, J. Am. Chem. Soc. 126 (2004) 10958–10961. [3] J. Li, X. Chu, Y. Liu, J.H. Jiang, Z. He, Z. Zhang, G. Shen, R.Q. Yu, Nucleic Acids Res. 33 (2005) e168. [4] X. Xu, M.S. Han, C.A. Mirkin, Angew. Chem. Int. Ed. 46 (2007) 3468–3470. [5] Y.F. Huang, Y.W. Lin, Z.H. Lin, H.T. Chang, J. Nanopart. Res. 11 (2009) 775–783. [6] M. Faraday, Philos. Trans. R. Soc. Lond. 147 (1857) 145–181. [7] J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55–75. [8] G. Mie, Ann. Phys. (Leipzig) 25 (1908) 377–445. [9] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. [10] S. Srivastava, B.L. Frankamp, V.M. Rotello, Chem. Mater. 17 (2005) 487–490. [11] K. Saha, S.S. Agasti, C. Kim, X. Li, V.M. Rotello, Chem. Rev. 112 (2012) 2739– 2779. [12] A.P. Xin, Q.P. Dong, C. Xiong, L.S. Ling, Chem. Commun. (2009) 1658–1660. [13] H.L. Yan, C. Xiong, H. Yuan, Z.X. Zeng, L.S. Ling, J. Phys. Chem. C 113 (2009) 17326–17331. [14] C.W. Liu, Y.T. Hsieh, C.C. Huang, Z.H. Lin, H.T. Chang, Chem. Commun. (2008) 2242–2244. [15] Y. Zhuo, Y.Q. Chai, R. Yuan, L. Mao, Y.L. Yuan, J. Han, Biosens. Bioelectron. 26 (2011) 3838–3844. [16] T. Li, L.L. Shi, E.K. Wang, S.J. Dong, Chem. Eur. J. 15 (2009) 3347–3350. [17] K.W. Huang, C.J. Yua, W.L. Tseng, Biosens. Bioelectron. 25 (2010) 984–989. [18] K.B. Narayanan, N. Sakthivel, Colloids Surf. A: Physicochem. Eng. Aspects 380 (2011) 156–161. [19] K.B. Narayanan, N. Sakthivel, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 116 (2013) 485–490. [20] J.H. Freeland-Graves, C. Llanes, Models to Study Manganese Deficiency, CRC Press, Boca Raton, FL, 1994, pp. 59–86. [21] D. Mergler, G. Huel, R. Bowler, A. Iregren, S. Belanger, M. Baldwin, R. Tardif, A. Smargiassi, L. Martin, Environ. Res. 64 (1994) 151–180. [22] USEPA, Manganese, United States Environmental Protection Agency, Integrated Risk Information System (IRIS), Washington, DC, 1997. [23] A. Monshi, M.R. Foroughi, M.R. Monshi, World J. Nano Sci. Eng. 2 (2012) 154– 160. [24] K.B. Narayanan, N. Sakthivel, World J. Microbiol. Biotechnol. 29 (2013) 2207– 2211. [25] K.M. Siskova, J. Straska, M. Krizek, J. Tucek, L. Machala, R. Zboril, Proc. Environ. Sci. 18 (2013) 809–817. [26] J.K. Young, N.A. Lewinski, R.J. Langsner, L.C. Kennedy, A. Satyanarayan, V. Nammalvar, A.Y. Lin, R.A. Drezek, Nanoscale Res. Lett. 6 (2011) 428. [27] T.E. Young, J.R. Griswold, M.H. Hulbert, J. Org. Chem. 39 (1974) 1980–1982. [28] M. Zhang, X. Zhang, X. He, L. Chen, Y. Zhang, Chem. Lett. 39 (2010) 552–553. [29] Y. Liao, Y. Wang, X. Feng, W. Wang, F. Xu, L. Zhang, Mater. Chem. Phys. 121 (2010) 534–540. [30] P. Li, X. Duan, Z. Chen, Y. Liu, T. Xie, L. Fang, X. Li, M. Yin, B. Tang, Chem. Commun. 47 (2011) 7755–7757. [31] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678–700. [32] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607–609.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric detection of manganese(II) ions using gold/dopa nanoparticles Kannan Badri Narayanan, Hyun Ho Park ⇑ Department of Biochemistry, School of Biotechnology, Yeungnam University, Gyeongsan 712 749, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Facile colorimetric detection of Mn(II)
ions by dopa-capped gold nanoparticles. Synthesis of AuNPs is cost-effective and eco-friendly without toxic chemicals. Ratio of absorbance (A700/A550) was linear against the concentration of [Mn2+] ions.
a r t i c l e
i n f o
Article history: Received 11 January 2014 Received in revised form 5 April 2014 Accepted 17 April 2014 Available online 26 April 2014 Keywords: Gold nanoparticles Metal ions Dopa Manganese(II) Sensor Eco-friendly
a b s t r a c t We report here a one-pot, greener, eco-friendly strategy for the synthesis of gold nanoparticles using L-dopa. The as-prepared dopa-functionalized gold nanoparticles (AuNPs/dopa) can detect low concentrations of manganese(II) metal ions in aqueous solution. The binding forces between dopa and Mn2+ ions cause dopa-functionalized gold nanoparticles to come closer together, decreasing the interparticle distance and aggregating it with a change in color of colloidal solution from red to purplish-blue. Dynamic light scattering (DLS) analysis showed a decreased surface charge on the surface of gold nanoparticles when exposed to Mn2+ ions, which caused cross-linking aggregation. Transmission electron microscopic (TEM) images also revealed the aggregation of gold nanoparticles with the addition of Mn2+ ions. The extinction ratio of absorbance at 700–550 nm (A700/A550) was linear against the concentration of [Mn2+] ions. Thus, the optical absorption spectra of gold colloidal solution before and after the addition of Mn2+ ions reveal the concentration of Mn2+ ions in solution. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Nanomaterials are nanodimensioned materials with a size of 3.5 nm diameter) induces interparticle surface plasmon coupling, resulting in a visible color change from red to blue at nanomolar concentrations. This color change aids in the utilization of plausible absorption-based colorimetric sensing of any target analyte that induces AuNPs aggregation [10,11]. Thus, strategies based on nanoparticle aggregation have attracted a great deal of attention. Since the surface of AuNPs can be easily functionalized with various ligands, it is possible to modulate the interaction of external ions with stabilizing functional groups for development of diverse metal sensors. Gold nanoparticles exhibit higher extinction coefficients, as well as size or distance-dependent optical properties. The aggregation/dispersion states of AuNPs show changes in color when observed by the naked eye, enabling easy detection of DNA [12,13], protein [14,15], and metal ions [16,17]. Upon aggregation, spherical AuNPs undergo red-shifts of the SPR band due to increased size, which are used to sense analytes based on colorimetric assays. Several cross linking and non-cross-linking aggregations of AuNPs have been reported to have sensing ability. Several biocompatible molecules such as proteins, amino acids, and sugars are used as reducing and capping agents in the synthesis of metal nanoparticles [18,19]. The use of L-dopa in protein engineering has recently been increasing in protein engineering due to its structural similarity with the natural amino acid tyrosine. Dopa has multifunctional groups including amino, carboxylic and hydroxyl groups, which have the ability to reduce metal ions such as Au3+ and Ag+ to metal nanoparticles. Manganese (Mn) is an essential trace element for both plants and animals that is most stable in the 2+ valence state. Mn2+ activates many enzymatic reactions in carbohydrate metabolism and in the metabolism of organic acids, nitrogen and phosphorous. Although it is required for humans, Mn deficiency is associated with lipid and carbohydrate metabolism, growth retardation and reproductive failure in adults and ataxia and skeletal abnormalities in neonatals [20]. Additionally, higher concentrations can cause adverse neurological effects, and high levels of manganese (>14 mg/l) in drinking water
133
can cause manganism, which is characterized by a Parkinson-like syndrome [21]. Mn is also a Toxic Release Inventory (TRI) chemical, and some of its compounds are also listed as air contaminants by the Occupational Safety and Health Administration (OSHA). In 1973, the Food and Nutrition Board of the National Research Council (NRC) determined the safe and adequate daily dietary intake of Mn to be 2–5 mg/day for adults [22]. Herein, we describe the synthesis of gold nanoparticles using L-dopa (L-3,4-dihydroxyphenylalanine) and the subsequent utilization of gold/dopa nanoparticles (AuNPs/dopa) to detect and quantify manganese (Mn2+) ions. Experimental Chemicals and reagents L-3,4-dihydroxyphenylalanine (L-dopa) and manganese(II) chloride (MnCl24H2O) were purchased from Sigma–Aldrich (USA). Chloroauric acid (HAuCl4) was purchased from Alfa Aesar Chemicals (South Korea). All experiments were carried out in deionized Milli-Q (18.2 O) water. Preparation of dopa-capped gold nanoparticles A total of 50 ml aqueous L-dopa solution (100 lM) at pH 6.5 was stirred at 80 °C for 30 min with the addition of 1.5 ml of HAuCl4 (6 mM) and then cooled to room temperature to prepare dopa-functionalized gold nanoparticles (AuNPs/dopa). The assynthesized gold colloidal solution was further characterized and also directly exposed to various concentrations of metal ions for colorimetric assays. Characterization of L-dopa-capped gold nanoparticles The reduction of gold ions to gold nanoparticles was monitored using UV–Vis spectroscopy (Beckman Coulter DU-730) and visual observation. Additionally, the morphology and size of as-synthesized AuNPs/dopa were characterized by TEM analysis of samples that had been placed on carbon-coated copper grids and dried prior to observation using a Hitachi (Model: H-7600) microscope at an accelerating voltage of 120 kV. Furthermore, the dynamic light scattering (DLS) method was employed to measure the size and surface charge of nanoparticles and the interparticle interactions. The hydrodynamic diameter and zeta potential of AuNPs/ dopa were characterized using a Zetasizer Nano series (Malvern Instruments) equipped with a He–Ne laser (k = 633 nm) and operated at a scattering angle of 173°. Powder X-ray diffraction (XRD) was conducted to determine the crystallinity of gold nanoparticles. The powdered nanoparticles were measured in transmission mode on a PANalytical X’pert PRO X-ray diffractometer (Netherlands) at a voltage of 40 kV and a current of 30 mA with Cu Ka1 radiation (1.541 Å). Samples were scanned in the region of 2h from 30° to 85°. The nanocrystallite size was calculated using Scherrer equation [23]. Fourier transform infrared spectroscopy (FTIR) measurements were performed to identify the functional groups involved in the formation and stabilization of dopa-functionalized AuNPs. The AuNPs/dopa in the solution were separated using high-speed centrifugation (25,000g) for 20 min and then washed twice using sterile deionized water. Finally, the dried pellets were ground with KBr and analyzed on a Jasco FTIR 5300 spectrophotometer in transmittance mode over 4000–400 cm1. Colorimetric detection of manganese(II) ions using AuNPs/dopa sensor To demonstrate the detection of metal ions using AuNPs/dopa, a stock solution of 10 mM manganese(II) chloride was prepared.
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Fig. 1. (A) UV–Vis spectrum of dopa-functionalized gold nanoparticles indicating SPR peak, (B) XRD pattern of dopa-capped gold nanoparticles. The principal Bragg reflections are identified.
900 ll of synthesized gold/dopa nanoparticles were directly exposed to 100 ll of various concentrations of Mn2+ ions and incubated at room temperature for 10 min, after which the color change was visually observed and the optical measurements were carried out using a UV–Vis spectrophotometer (Beckman Coulter DU-730) at 300–900 nm. Results and discussions Characterization of gold nanoparticles Colloidal gold nanoparticles interact with visible light to produce colors, which has applications in sensory probes, catalysis and therapeutic agents [11]. These optical and electronic properties of gold nanoparticles are tunable by changing the size, morphology, surface chemistry or dispersity state [24]. Several low-molecular weight biomolecules such as amino acids are used for the greener synthesis of metal nanoparticles [25]. When HAuCl4 was added to oxidized L-dopa solution the color of the solution turned from yellowish to reddish with a sharp surface plasmon resonance (SPR) peak at 548 nm (Fig. 1A). The characteristic reddish color of AuNPs confirms the formation of monodispersed AuNPs [26]. The dihydroxyl groups at the ortho position of dopa were involved in the reduction of metal ions. During the preparation of AuNPs, the dihydroxyl groups of the added dopa molecules lose two electrons to form a quinone molecule, which further forms cyclodopa that is adsorbed onto the surface of AuNPs to form
Fig. 2. FTIR spectra of (A) dopa and (B) dopa-capped gold nanoparticles.
monodispersed AuNPs [27]. The FTIR spectra of the dopa and dopacapped AuNPs were then measured (Fig. 2). The spectrum of dopa showed many small peaks, which is a common feature of small molecules, whereas AuNPs/dopa showed peaks around 1615 cm1 and 3420 cm1 that were attributed to the stretching vibration of the aromatic rings and catechol –OH groups, respectively [28]. The use of dopa to functionalize gold nanoparticles can be used to immobilize nanoparticles onto polymer surfaces for several applications [29]. TEM images showed the formation of monodispersed spherical nanoparticles ranging from 3.5 to 17.3 nm in size with an average diameter of around 13.7 nm. XRD measurements revealed the formation of face-centered cubic (fcc) gold nanoparticles with intense peaks corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) Bragg’s reflection (Fig. 1B). The average crystallite size of AuNPs/ dopa was calculated to be 7.7 ± 0.87 nm using Scherrer’s formula by taking the average of all the five peaks, which matches the
Fig. 3. (A) Photographs of dopa-functionalized AuNPs colloid with different concentrations of Mn2+. From left to right: (1) 0 lM; (2) 5 lM; (3) 10 lM; (4) 20 lM; (5) 50 lM; (6) 100 lM. (B) UV–Vis spectrum of dopa-functionalized AuNPs colloid containing different concentrations of Mn2+. From upper to lower: (1) 0 lM; (2) 5 lM; (3) 10 lM; (4) 20 lM; (5) 50 lM; (6) 100 lM. Inset graph describes the plot of A700/A550 against CMn for manganese analysis.
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TEM results. The physical and optical stability of nanoparticles have also been evaluated. It was found that there was an indication of agglomeration of AuNPs/dopa with slight change in optical spectrum along with some change in physical appearance after three months of storage at room temperature (Supplementary Fig. 1).
Fig. 4. (A) Photograph and (D) TEM image of dopa-functionalized AuNPs without addition of Mn2+; (B) photograph of dopa-functionalized AuNPs with EDTA and Mn2+; (C) photograph and (E) TEM image of dopa-functionalized AuNPs with the addition of Mn2+.
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Effect of manganese (Mn2+) ions on AuNPs/dopa The synthesis of size-controlled monodispersed gold nanoparticles is advantageous in colorimetric sensing of metal ions. The functionalized groups stabilizing these nanoparticles have different affinities towards different metal ions. To synthesize gold nanparticles to sense a specific metal ion, the knowledge on the surface functional groups, which has a selective affinity towards a particular metal ion is prerequisite. Using L-dopa as a stabilizing agent in the synthesis of dopa/gold nanoparticles exhibited a strong affinity towards Mn2+ ions. Fig. 3 shows the UV–Vis spectrum of reaction solution with different concentrations (5, 10, 20, 50, 100 lM) of Mn2+. In the absence of Mn2+, the gold nanoparticles were red in color and exhibited a sole sharp peak at 548 nm, indicating the monodispersed state. However, as the concentration of Mn2+ increased, the red color of AuNPs gradually changed to dark purplish-blue and showed decreased absorbance at 548 nm with a concomitant increase in another peak at around 700 nm due to aggregation. The extinction ratio of absorbance at 700–550 nm (A700/A550) is linear against the concentration of Mn2+ in the range of (5–100 lM), and the calibration curve is A700/A550 = 0.09561 + 0.00751 CMn (R = 0.99). In the presence of Mn2+ ions, reddish AuNPs become purplishblue in 10 min due to the aggregation of monodispersed spherical nanoparticles (Fig. 4). TEM images also revealed the aggregation of nanoparticles with the addition of Mn2+ ions. The Mn2+ ions interact with dopa-functionalized AuNPs through Mn–O interactions [30]. Due to the binding forces between dopa and Mn2+ ions, dopa-capped AuNPs tend to come closer, decreasing the interparticle distance and gets aggregated (Fig. 5). This results in a change of optical properties of AuNPs, causing purplish-blue color. However, when chelating agent (EDTA) was added with divalent cation Mn2+, the color of the solution did not change from red to purplish-blue, indicating that Mn2+ was chelated by EDTA, preventing interaction with AuNPs/dopa (Fig. 4). DLS analysis also revealed increased
Fig. 5. Schematic diagram of the mechanism of colorimetric detection of Mn2+ with AuNPs/dopa.
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aggregation of nanoparticles with increased levels of Mn2+ ions in the solution. When the interparticle distance is greater than the average nanoparticle diameter AuNPs appear red, while a decrease in interparticle distance to less than the average particle diameter results in the AuNPs becoming purplish-blue [31]. Particle charge is also an important factor to determine the physical stability of colloidal suspensions. Particle charge is quantified as zeta potential (f), which is a measure of the electrical force (approximation of surface charge) that exists between nanoparticles in a solution. This aggregation was influenced by the surface charge (f potential) of nanoparticles. As the concentration of Mn2+ ions in solution increased, there was a gradual decrease in zeta potential and a progressive increase in agglomeration of nanoparticles with increased hydrodynamic size. Additionally, the surface charge changed from positive to negative with the addition of Mn2+ ions, while further increases in the concentration of Mn2+ led to a gradual decrease in negative charge (repulsive forces), resulting in a tendency to aggregate (Fig. 6). For potential reuse of gold/dopa nanoparticles, EDTA was added to the solution which chelates Mn2+ ions. This solution was further centrifuged at 16,000 rpm for 30 min to sediment the gold nanoparticles from the solution, which can be washed with deionized water and resuspended in a homogenous aqueous solution by mild sonication. Several cross linking and non-cross-linking aggregations of AuNPs have been reported for its sensing ability. Specific hybridization of DNA–DNA, antibody–antigen interactions or metal–ligand coordination is normally involved in aggregation. Mirkin et al. [32] used DNA–AuNPs for the first time to detect target DNA by hybridization with complementary DNA based on aggregation of AuNPs. DNA functionalized AuNPs with thiolatedDNA sequences (50 -thiol-C10A10TA10 and 50 -thiol-C10T10TT10), which are complementary except for a single thymine–thymine (T–T) mismatch that detects metal Hg2+ ions by coordinating to a T–T pair [33,34]. Thymine (T)–AuNPs sensitively and selectively detected Hg2+ ions by inducing T–Hg2+–T coordination [14]. Sugunan et al. [35] used chitosan-capped AuNPs to sense Zn2+ and Cu2+ ions. Lactose-functionalized AuNPs (16 nm) detects Ca2+ ions colorimetrically [36]. Additionally, Liu et al. [37] devised a simple colorimetric sensor for detection of Hg2+ in aqueous solution using gold nanoparticles modified with quaternary ammonium group-terminated thiols at room temperature. Among several metal ions investigated for the selectivity of colorimetric sensor, metal ions such as Li+, Cd2+, Ca2+, Mg2+, Hg2+, Ni2+, Zn2+, K+, Na+, and Rb+ did not show any color change, whereas Pb2+, Ba2+, Cu2+ and Mn2+ ions caused AuNPs to turn from red to purple-blue color. Among the positive ions, Mn2+ exhibited a profound
Fig. 6. Hydrodynamic size and zeta potential (f) of dopa-functionalized AuNPs with different concentrations of Mn2+ ions.
interaction causing a dark purplish-blue color change. In addition, there was a linear relationship between the extinction ratio of absorbance at 700–550 nm (A700/A550) and the concentration of [Mn2+]. Conclusions The method described herein enables the facile colorimetric detection of manganese(II) ions based on dopa-functionalized AuNPs. The synthesis of biocompatible AuNPs is cost-effective, environmentally-friendly, and does not involve toxic chemicals. The ratio of absorbance at 700–550 nm of gold/dopa nanoparticles against the known concentration of Mn(II) provides the concentration of Mn(II) ions in desired solution. Acknowledgement This study was supported by the 2012 Yeungnam University Research Grant. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.04.081. References [1] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078–1081. [2] H. Li, L.J. Rothberg, J. Am. Chem. Soc. 126 (2004) 10958–10961. [3] J. Li, X. Chu, Y. Liu, J.H. Jiang, Z. He, Z. Zhang, G. Shen, R.Q. Yu, Nucleic Acids Res. 33 (2005) e168. [4] X. Xu, M.S. Han, C.A. Mirkin, Angew. Chem. Int. Ed. 46 (2007) 3468–3470. [5] Y.F. Huang, Y.W. Lin, Z.H. Lin, H.T. Chang, J. Nanopart. Res. 11 (2009) 775–783. [6] M. Faraday, Philos. Trans. R. Soc. Lond. 147 (1857) 145–181. [7] J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55–75. [8] G. Mie, Ann. Phys. (Leipzig) 25 (1908) 377–445. [9] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. [10] S. Srivastava, B.L. Frankamp, V.M. Rotello, Chem. Mater. 17 (2005) 487–490. [11] K. Saha, S.S. Agasti, C. Kim, X. Li, V.M. Rotello, Chem. Rev. 112 (2012) 2739– 2779. [12] A.P. Xin, Q.P. Dong, C. Xiong, L.S. Ling, Chem. Commun. (2009) 1658–1660. [13] H.L. Yan, C. Xiong, H. Yuan, Z.X. Zeng, L.S. Ling, J. Phys. Chem. C 113 (2009) 17326–17331. [14] C.W. Liu, Y.T. Hsieh, C.C. Huang, Z.H. Lin, H.T. Chang, Chem. Commun. (2008) 2242–2244. [15] Y. Zhuo, Y.Q. Chai, R. Yuan, L. Mao, Y.L. Yuan, J. Han, Biosens. Bioelectron. 26 (2011) 3838–3844. [16] T. Li, L.L. Shi, E.K. Wang, S.J. Dong, Chem. Eur. J. 15 (2009) 3347–3350. [17] K.W. Huang, C.J. Yua, W.L. Tseng, Biosens. Bioelectron. 25 (2010) 984–989. [18] K.B. Narayanan, N. Sakthivel, Colloids Surf. A: Physicochem. Eng. Aspects 380 (2011) 156–161. [19] K.B. Narayanan, N. Sakthivel, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 116 (2013) 485–490. [20] J.H. Freeland-Graves, C. Llanes, Models to Study Manganese Deficiency, CRC Press, Boca Raton, FL, 1994, pp. 59–86. [21] D. Mergler, G. Huel, R. Bowler, A. Iregren, S. Belanger, M. Baldwin, R. Tardif, A. Smargiassi, L. Martin, Environ. Res. 64 (1994) 151–180. [22] USEPA, Manganese, United States Environmental Protection Agency, Integrated Risk Information System (IRIS), Washington, DC, 1997. [23] A. Monshi, M.R. Foroughi, M.R. Monshi, World J. Nano Sci. Eng. 2 (2012) 154– 160. [24] K.B. Narayanan, N. Sakthivel, World J. Microbiol. Biotechnol. 29 (2013) 2207– 2211. [25] K.M. Siskova, J. Straska, M. Krizek, J. Tucek, L. Machala, R. Zboril, Proc. Environ. Sci. 18 (2013) 809–817. [26] J.K. Young, N.A. Lewinski, R.J. Langsner, L.C. Kennedy, A. Satyanarayan, V. Nammalvar, A.Y. Lin, R.A. Drezek, Nanoscale Res. Lett. 6 (2011) 428. [27] T.E. Young, J.R. Griswold, M.H. Hulbert, J. Org. Chem. 39 (1974) 1980–1982. [28] M. Zhang, X. Zhang, X. He, L. Chen, Y. Zhang, Chem. Lett. 39 (2010) 552–553. [29] Y. Liao, Y. Wang, X. Feng, W. Wang, F. Xu, L. Zhang, Mater. Chem. Phys. 121 (2010) 534–540. [30] P. Li, X. Duan, Z. Chen, Y. Liu, T. Xie, L. Fang, X. Li, M. Yin, B. Tang, Chem. Commun. 47 (2011) 7755–7757. [31] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678–700. [32] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607–609.
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