- Email: [email protected]
Functionalized magnetic core–shell Fe@SiO2 nanoparticles as recoverable colorimetric sensor for Co2+ ion
Chemical Engineering Journal 281 (2015) 428–433
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Functionalized magnetic core–shell Fe@SiO2 nanoparticles as recoverable colorimetric sensor for Co2+ ion Uiseok Jeong, Hyeon Ho Shin, Younghun Kim ⇑ Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of 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
Recoverable colorimetric sensor using
core–shell Fe@SiO2 nanoparticle. Rapid, naked-eye detection of trace
Co2+ ions at concentrations as low as 0.08 mM. Distinguished SPR bands revealed by metal–amine complex and self-seed generated Co NPs.
a r t i c l e
i n f o
Article history: Received 27 April 2015 Received in revised form 16 June 2015 Accepted 2 July 2015 Available online 6 July 2015 Keywords: Colorimetric sensor Core–shell Cobalt ion Surface plasmon resonance Fe@SiO2
a b s t r a c t This study presents a sensitive colorimetric probe for sensing Co2+ ions based on surface amine-modified iron oxide–silica core–shell nanoparticles (APTES–Fe@SiO2 NPs) and the appearance of an additional plasmonic peak in the UV–vis spectra due to NP aggregation. This colorimetric strategy based on the component dependence of core–shell NPs offered a highly sensitive and selective detection method for Co2+ ions. This cost-effective colorimetric probe afforded rapid, naked-eye detection of trace Co2+ ions at concentrations as low as 0.08 mM. The amine moiety of APTES–Fe@SiO2 NPs readily formed the metal–amine configuration complex (k980 peak) and induced the formation of Co NPs by self-seed generation (k520, k590, and k630 peaks). Preliminary study data supported the promising practical applicability of the current method for the colorimetric detection of Co2+ ions in aqueous phase. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Colorimetric sensors capable of being observed by the naked eye are appropriate for the on-site real-time detection of target heavy metal ions due to their simple configuration and on-site portability [1]. To date, various metal nanoparticles (NPs) have been used as colorimetric detectors for heavy metal ions in the aqueous phase. In particular, gold (Au) and silver NPs (AgNPs) offer excellent localized surface plasmon resonance (LSPR) properties, exhibit a strong and well-defined color and enable easy ⇑ Corresponding author. Tel.: +82 2 940 5769; fax: +82 2 941 5769. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.cej.2015.07.006 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
visualization of color change [2,3]. Visual detection has been based on the well-known metal–ligand coordination, where the metal and ligand act as electronic acceptor and donor, respectively. A simple colorimetric assay for the detection of Co2+ ions, using mercaptopyridine–glutathione (GSH)-modified, AgNPs (30 nm), was previously reported based on the metal ions-GSH coordination compounds inducing the aggregation of AuNPs [2]. The core–shell NPs could be used as candidate materials for colorimetric sensors. The SPR of Au@Ag core–shell nanostructures is highly dependent on the dimensional shell-to-core ratio, and thus its slight variation often leads to an obvious spectral and color change [4]. Especially, when iron NPs (FeNPs) were used as the core material, magnetic core–shell NPs could be used as a
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
429
recoverable sensor/catalyst [5,6] or magnetic resonance imaging contrast agent [7]. Magnetic core–shell Fe@SiO2 NPs have aroused great interest in current research, due to their biocompatibility, easily renewability, and stability against degradation [8]. The inner FeNP core with a silica outer shell not only stabilizes Fe@SiO2 NPs in solution but also provides sites for surface modification with various chemical and biomedical ligands. In addition, an inert silica coating on the surface of magnetic NPs prevents their aggregation in solution and provides better protection against toxicity [9]. The present study describes a simple strategy for preparing amine functionalized FeNPs based on a multistep process in which the FeNPs are modified with silica. An amine functional group can be easily attached to the silica shell via a sol–gel reaction. The (3-aminopropyl)triethoxysilane (APTES) is selected as the amine functional material because it is a widely used grafting agent for the specific detection of heavy metal ions [10]. APTES is conjugated with the magnetic core–shell APTES–Fe@SiO2 NPs and used as a colorimetric sensor for heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+).
2. Experimental 2.1. Preparation of FeNPs Magnetic 20-nm-diameter NPs were synthesized based on a slight modification of a published one-pot chemical coprecipitation method [8]. Firstly, FeCl3H2O (2.18 mg), FeCl2H2O (0.78 mg), and tetramethylammonium (TMA, 3 mL) were added to deionized (DI) water, and the solution was purged with nitrogen gas with vigorous stirring. Then, ammonium hydroxide (5.6 mL, 28%) was added slowly as the solution color slowly turned from black to red-brown. The precipitate was collected by centrifugation (SUPRA22, Hanil) at 14,000 rpm, washed with DI water and ethanol five times, and then suspended in DI water. 2.2. Preparation of core–shell Fe@SiO2 NPs FeNP-coated SiO2 NPs were prepared by simple sol–gel reaction. 1 mL of FeNP solution was suspended in 9 mL of ethanol with a probe-type ultrasonicator (Ulh700S, ULSSO HI-Tech). Since aqueous silane at high pH formed a particulate sol by Ostwald ripening mechanism, 0.3 mL of tetraethyl orthosilane (TEOS) was added to the resulting solution after the pH was adjusted to 10 by addition of ammonium hydroxide. After 24 h, Fe@SiO2 NPs were isolated via centrifugation, washed with ethanol and DI water five times to remove unreacted chemicals, and then suspended in ethanol. The morphology of FeNPs and Fe@SiO2 NPs was analyzed using transmission electron microscopy (TEM, JEM-1010, JEOL). 2.3. Colorimetric sensing of heavy metal ions To detect the heavy metal ions, and amine group was functionalized on the Fe@SiO2 NPs using amino silane (APTES). As shown in Fig. 1, the amine group of APTES on the Fe@SiO2 NPs readily formed the metal–amine configuration complex and thus induced the aggregation of core–shell NPs. After 0.3 mL of APTES was drop-wise added to 10 mL of Fe@SiO2 NP solution and stirred at 70 °C at 500 rpm, the resulting APTES–Fe@SiO2 NP solution showed a dark-brown color. APTES–Fe@SiO2 NPs were isolated via centrifugation, washed with ethanol and DI water five times to remove unreacted chemicals, and then suspended in DI. Stock solutions (100 ppm) of heavy metal ions were prepared by CoCl2, CuCl2, FeCl2, and HgCl2. After addition of heavy metal ions into the APTES–Fe@SiO2 NP solutions, the color change (i.e., absorbance
Fig. 1. Schematic diagram for colorimetric detection via aggregation mechanism of core–shell nanoparticles.
change in the range of 350–1100 nm) was analyzed with UV–vis spectra (UV-18,000, Shimadzu). 3. Results and discussion 3.1. Characterization of Fe@SiO2 NPs These core–shell NPs are usually used to confine the charges and protect the sensitive particles using an inert material shell. Some metal–silica core–shell nanocomposites are suitable for highly sensitive biosensing, imaging, and chemical sensing. For example, silica–Au core–shells show nanogaps between them that provide room to accommodate metal ions as the hot spots, thereby improving the sensitivity [11]. Hot spots are attributed to enhanced electromagnetic field, and thus SPR has emerged as a sustainable bio-friendly analytical technique for the non-invasive detection of chemical, environmental, and biological materials [12]. Herein, for colorimetric detection of heavy metal ions through the change of LSPR properties of core–shell NPs, amine-functionalized magnetic core–shell NPs were prepared by FeNP-core and SiO2-shell. Au@Ag core–shell NPs were also used as colorimetric probes for various chemicals. The etching process of Au@Ag NPs with the addition of cyanide [4] or chromium [13] ions changes the SPR absorption and then the solution color. Namely, Au@Ag or Ag@Au core–shell NPs used for colorimetric detection were destructive. Therefore, a non-destructive core–shell that could be used in the colorimetric detection of heavy metal ions was prepared based on the magnetic silica NPs. As shown in Fig. 2a, the iron oxide (Fe3O4) NPs exhibited a spherical shape with a diameter of ca. 20 nm and a red-brown solution color. In the UV–vis spectrum of FeNPs dispersed in DI water (Fig. 2b), no characteristic peak was found in the range of 350–1100 nm. After silica coating on the FeNP-core, clear core– shell architectures were observed (Fig. 3a). The Fe@SiO2 NPs were sized 100–150 nm with a 40- to 60-nm-thick silica shell. The silica layer is required to avoid aggregation and degradation of the
430
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
Fig. 2. (a) TEM image and (b) UV–vis spectrum of FeNPs dispersed in the aqueous phase. Inset is a picture of FeNP solution contained in a UV cuvette.
Fig. 3. (a) TEM image and (b) UV–vis spectrum of Fe@SiO2 dispersed in the aqueous phase. Inset is a picture of Fe@SiO2 solution contained in a UV cuvette.
FeNP-core. Therefore, the Stöber process for the synthesis of silica NPs does not change the core–shell structure of the FeNP-coated silica NPs. As shown in Fig. 3b, the UV–vis spectrum was similar to that of FeNPs, indicating that the silica shell did not influence the SPR change of FeNPs. Amine ligands bind heavy metal ions by forming the metal– amine complex via amine–Mn+–amine couplings with two to four ligands (Fig. 1). When a monolayer of APTES is formed on the Fe@SiO2 NPs, the maximum grafting density (Dg) could be estimated by the following equation [14]
Dg ¼
S SSi
Mw Na
where Ssi is the occupied surface area of a silane ligand, Mw the molecular weight of APTES (221 g/mol), and Na the Avogadro constant (6.02 1023/mol). The specific surface area (S) of the bare SiO2 NP (m2/g) was calculated to be 22.8 m2/g according to the following equation: S = 6/qd (q = 2.1 g/cm3, d = 125 nm) [15]. Assuming that Ssi is 0.2 nm2 [16], the grafting density of APTES on Fe@SiO2 NPs was estimated as ca. 41.8 mg/g. Therefore, since the immobilization of APTES on Fe@SiO2 NPs is very low, the UV–vis spectrum and solution color of APTES–Fe@SiO2 NPs were unchanged (Fig. 4). 3.2. Colorimetric sensing of heavy metal ions Generally, colorimetric detection using metal NPs relies on the aggregation of NPs due to the formation of a chelating complex
between the metal ions and functional groups, namely, ion-templated chelation. The color change induced by the formation of the metal–amine complex was evaluated for various heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+) at 100 ppm. As shown in Fig. 4b, the solution color depended on the kinds of heavy metal ion. The exposure of Co2+, Cu2+, Fe2+, and Hg2+ to APTES–Fe@SiO2 NP solution resulted in a solution color of indigo, sky blue, yellow, and light brown, respectively. In the UV–vis spectrum (Fig. 4a), only a solution of Co2+ ions bonded with APTES–Fe@SiO2 NPs showed a dramatic increment of the absorbance intensity at 500–700 nm. The APTES–Fe@SiO2 NPs underwent aggregation due to the formation of a strong chelating complex via an amine group. The presence of Co2+ ions led to the emergence of new peaks at ca. 520, 590, and 650 nm in the UV–vis spectra. These three plasmon peaks suggest the possible formation of Co NPs by metal– amine complex. As reported by Moon et al. [17], amines act as a complexing agent by forming a suitable metal complex for self-seed generation, and the amines play an additional role as a capping agent by aggressive adsorption on the metal NPs. Namely, a metal–amine complex could be formed spontaneously the metal NPs by self-seed generation with silanization of APTES and acted as plasmonic NPs. For example, spherical 25-nm-diameter Co NPs exhibited the maximum SPR peak at 510 nm [18]. The three distinctive peaks were assigned to the out-of-plane quadrupole resonance (the first peak) and the in-plane quadrupole resonance (the two additional peaks) [19]. Therefore, the first peak at 520 nm in Fig. 4a was attributed to the Co NPs. To check the CoNP self-seed generation in solution,
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
431
Fig. 4. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions after addition of 100 ppm of Co2+, Cu2+, Fe2+, and Hg2+ ions. (c) TEM image of Co NPs formed by metal–amine complex through silanization.
Fig. 5. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions with decreasing Co2+ ion concentration.
Co2+ ion was exposed into APTES solution without Fe@SiO2. Namely, in order to prevent be confused with Fe@SiO2 NPs, this nanoparticles are excluded. A volume of 0.1 mL of 4.3 M APTES was added to 3 mL of 100 ppm Co2+ ion. After APTES combined with Co2+ ions, the particles that were present in the solution were analyzed with TEM. As shown in Fig. 4c, the small Co NPs were found and showed irregular sphere. As well as these three peaks, an additional peak appeared at 980 nm in the UV–vis spectra (Fig. 4a) and was assigned to the in-plane dipole plasmon resonance [19]. This plasmon peak was assigned to the change of local refractive index on the Fe@SiO2 NPs surface caused by the specific binding of APTES–Fe@SiO2 NPs with metal ions and the interparticle interaction resulting from the NP assembly. Therefore, the aggregation of NPs in the presence of metal ions induces both the four new peaks in the plasmon band energy and the color change.
Based on the two interpretation, the amine moiety of APTES– Fe@SiO2 NPs readily formed the metal–amine configuration complex (k980 peak) and induced the formation of Co NPs by self-seed generation (k520, k590, and k630 peaks), as illustrated in Fig. 1. At the low concentration of 1 ppm, even though the metal–amine complex was formed, the k980 peak appeared, but the quantity of Co-amine complex was considered insufficient for the formation of NPs for self-seed generation. Therefore, the characteristic peaks (k520, k590, and k630 peaks) of Co NPs were decreased in the UV–vis spectra at low Co2+ ion concentration. Quantitative analysis was performed by adding different concentrations of Co2+ ions into the APTES–Fe@SiO2 NP solution and monitoring the absorption peak in the UV–vis spectra (Fig. 5). With decreasing metal ion concentration, the absorbance intensity of the three peaks was decreased (Fig. 5a), accompanying the solution color change from indigo to sky blue (Fig. 5b). The UV–vis
432
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
Fig. 6. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions with decreasing Cu2+ ion concentration.
Fig. 7. (a) Regeneration of Co2+–APTES–Fe@SiO2 NPs complex and (b) recovery efficiency with cycle change.
absorbance (k650) increased linearly with increasing (0.08– 1.69 mM) Co2+ ion concentration. The determination coefficient (R2) was high, at 0.99. A linear correlation between the absorbance ratio and the Co2+ ion concentration makes it suitable for the quantitative determination of target metal ions in aqueous solutions. The objective of this Fe@SiO2 NP colorimetric sensor is to detect the presence of metal ions in aqueous phase by naked-eye observation. Therefore, this cost-effective colorimetric probe afforded the rapid, naked-eye detection of trace Co2+ ions at concentrations as low as 0.08 mM. Moreover, lower concentrations could be detected based on the measurements of the UV–vis spectra in aqueous solutions at room temperature. The UV–vis absorbance of Cu2+ ions, as shown in Fig. 4, was increased and the shoulder peak was found at ca. 630 nm. In addition, similarly to the Co2+ case, the peak for metal–amine complex (Cu2+–APTES Fe@SiO2 NPs) was found at 970 nm in the UV–vis spectra. As shown in Fig. 6, the UV–vis absorbance (k630) increased linearly with increasing (0.08–1.57 mM) Cu2+ ion concentration, but that of k970 was not linearly dependent. For colorimetric sensing of Hg2+ and Fe2+ ions, UV–vis absorbance was increased without a characteristic peak, but a single peak for metal–amine complex (M2+–APTES Fe@SiO2 NPs) was found at 970 nm in the UV–vis spectra. Consequently, the appearance of a peak at 970 or 980 nm in the UV–vis spectra revealed the aggregation of Fe@SiO2 NPs via metal–amine configuration complex, and thus the solution color change to reddish color. The experimental values of the standard molar Gibbs energy of hydration (DhydG°) for Cu2+, Co2+, Hg2+, and Fe2+ were 2010, 1915, 1760, and 1840 kJ/mol, respectively [20]. The negative DG values
indicate spontaneous adsorption of heavy metal ions through the metal–amine complex. Increasingly negative DG values indicate stronger formation of metal–amine complex and thus a preference for amine–Cu2+–amine coordinative coupling over amine–Hg2+– amine complex. Therefore, the APTES Fe@SiO2 NPs are suitable for the colorimetric detection of Co2+ and Cu2+ ions, compared to Hg2+ and Fe2+ ions. An important aspect of these magnetic core–shell NPs is their ability to be magnetically separated for use in subsequent water treatment, thereby reducing their environmental impact in aqueous phase. An important consideration is the ability to completely remove the NPs from the treated water to prevent them from acting as contaminants themselves. The magnetic NPs used in the colorimetric detection of heavy metal ions were successfully separated in a few minutes by magnet. The magnetically separated M2+–APTES Fe@SiO2 NPs complex could be regenerated by adjustment of pH using 0.1 M NaOH or HCl. Since silica-shell could be partially dissolved by NaOH treatment, HCl treatment is more stable re-generation method. In regeneration step (Fig. 7a), solution color of metal–NP complex was changed from blue to yellow, and then re-generated NPs were recovered by magnet. Finally, recovered NPs was concentrated and re-dispersed in DW to reuse. Recovery efficiency was decreased with regeneration cycles, and reached to 80% after 5 cycles (Fig. 7b). 4. Conclusions An efficient colorimetric sensor for the detection of heavy metal ions was prepared using magnetic core–shell NPs. The amine
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
moiety was immobilized on the Fe@SiO2 NPs to form a metal– amine complex and induce a color change by aggregation between NPs. Four different heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+) were selected to demonstrate the colorimetric detection. After the addition of metal ions in APTES–Fe@SiO2 NP solutions, the appearance of the same peak at 970 or 980 nm in the UV–vis spectra of all samples revealed the aggregation of Fe@SiO2 NPs via metal–amine configuration complex. In the colorimetric detection of Co2+ ions, the observation of three addition peaks at 520, 590, and 630 nm in the UV–vis spectra indicated the formation of Co NPs by self-seed generation. Consequently, Co2+ ions could be detected colorimetrically via the aggregation of metal–APTES– Fe@SiO2 NPs, accompanied by the formation of Co NPs. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF-2010-0007050), the Korea Environmental Industry & Technology Institute (201400140002), and Kwangwoon University (2015). References [1] Y. Cho, S.S. Lee, J.H. Jung, Recyclable fluorimetric and colorimetric mercuryspecific sensor using porphyrin-functionalized Au@SiO2 core/shell nanoparticles, Analyst 135 (2010) 1551–1555. [2] H.K. Sung, S.Y. Oh, C. Park, Y. Kim, Colorimetric detection of Co2+ ion using silver nanoparticles with sphere, plate, and rod shapes, Langmuir 29 (2013) 8978–8982. [3] D. Vilela, M.C. González, A. Escarpa, Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: chemical creativity behind the assay. A review, Anal. Chim. Acta 751 (2012) 24–43. [4] J. Zeng, Y. Cao, J. Chen, X. Wang, J. Yu, B. Yu, Z. Yan, X. Chen, Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing, Nanoscale 6 (2014) 9939–9943. [5] X. Tian, Z. Dong, R. Wang, J. Ma, A quinoline group modified Fe3O4@SiO2 nanoparticles for sequential detection of Zn2+ and hydrogen sulfide in aqueous solution and its logic behavior, Sens. Actuators B 183 (2013) 446–453.
433
[6] C. Liu, J. Li, J. Qi, J. Wang, R. Luo, J. Shen, X. Sun, W. Han, L. Wnag, Yolk-shell Fe0@SiO2 nanoparticles as nanoreactors for fenton-like catalytic reaction, ACS Appl. Mater. Interfaces 6 (2014) 13167–13173. [7] L. Babes, B. Denizot, G. Tanguy, J.J.L. Jenune, P. Jallet, Synthesis of iron oxide nanoparticles used as MRI contrast agents: a parametric study, J. Colloid Interface Sci. 212 (1999) 474–482. [8] X. Peng, Y. Wang, X. Tang, W. Liu, Functionalized magnetic core–shell Fe3O4@SiO2 nanoparticles as selectivity-enhanced chemosensor for Hg(II), Dyes Pigm. 91 (2011) 26–32. [9] E.J. Park, H.N. Umh, Y. Kim, D.H. Choi, S.W. Kim, M.H. Cho, J.H. Kim, W. Choi, Magnetic iron oxide nanoparticles induce autophagy preceding apoptosis through mitochondrial damage and ER stress in RAW264.7 cells, Toxicol. In Vitro 28 (2014) 1402–1412. [10] B. Lee, Y. Kim, H. Lee, J. Yi, Synthesis of functionalized porous silicas via templating method as heavy metal ion adsorbents: the introduction of surface hydrophilicity onto the surface of adsrobents, Microporous Mesoporous Mater. 50 (2001) 77–90. [11] D.K. Lim, K.S. Jeon, J.H. Hwang, H. Kim, S. Kwon, Y.D. Suh, J.M. Nam, Highly uniform and reproducible surface-enhanced Raman scattering from DNAtailorable nanoparticles with 1-nm interior gap, Nat. Nanotechnol. 6 (2011) 452–460. [12] M. Li, S.K. Cushing, N. Wu, Plasmon-enhanced optical sensors: a review, Analyst 140 (2015) 386–406. [13] J. Xin, F. Zhang, Y. Gao, Y. Feng, S. Chen, A. Wu, A rapid colorimetric detection method of trace Cr(VI) based on the redox etching of Agcore–Aushell nanoparticles at room temperature, Talanta 101 (2012) 122–127. [14] Y. Liu, Y. Ki, X.M. Li, T. He, Kinetics of (3-aminopropyl)triethoxylsilane (APTES) silanizatioin of superparamagnetic iron oxide nanoparticles, Langmuir 29 (2013) 15275–15282. [15] H. Duan, X. Lin, G. Liu, L. Xu, F. Li, Synthesis of Co nanoparticles and their catalytic effect on the decomposition of ammonium perchlorate, Chin. J. Chem. Eng. 16 (2008) 325–328. [16] K. Kojio, A. Takahara, K. Omote, T. Kajiyama, Molecular aggregation state of noctadecyltrichlorosilane monolayers prepared by the Langmuir and chemisorption methods, Langmuir 16 (2000) 3932–3936. [17] D.V.R. Kumar, I. Kim, Z. Zhong, K. Kim, D. Lee, J. Moon, Cu(II)-alkyl amine complex mediated hydrothermal synthesis of Cu nanowires: exploring the dual role of alkyl amines, Phys. Chem. Chem. Phys. 16 (2014) 22107–22115. [18] J. Zhang, C.Q. Lan, Nickel and cobalt nanoparticles produced by laser ablation of solids in organic solution, Mater. Lett. 62 (2008) 1521–1524. [19] R. Jin, Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Photoinduced conversion of silver nanopheres to nanoprisms, Science 94 (2001) 1901–1903. [20] Y. Marcus, Thermodynamics of salvation of ions, J. Chem. Soc. Faraday Trans. 87 (1991) 2995–2999.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Functionalized magnetic core–shell Fe@SiO2 nanoparticles as recoverable colorimetric sensor for Co2+ ion Uiseok Jeong, Hyeon Ho Shin, Younghun Kim ⇑ Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of 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
Recoverable colorimetric sensor using
core–shell Fe@SiO2 nanoparticle. Rapid, naked-eye detection of trace
Co2+ ions at concentrations as low as 0.08 mM. Distinguished SPR bands revealed by metal–amine complex and self-seed generated Co NPs.
a r t i c l e
i n f o
Article history: Received 27 April 2015 Received in revised form 16 June 2015 Accepted 2 July 2015 Available online 6 July 2015 Keywords: Colorimetric sensor Core–shell Cobalt ion Surface plasmon resonance Fe@SiO2
a b s t r a c t This study presents a sensitive colorimetric probe for sensing Co2+ ions based on surface amine-modified iron oxide–silica core–shell nanoparticles (APTES–Fe@SiO2 NPs) and the appearance of an additional plasmonic peak in the UV–vis spectra due to NP aggregation. This colorimetric strategy based on the component dependence of core–shell NPs offered a highly sensitive and selective detection method for Co2+ ions. This cost-effective colorimetric probe afforded rapid, naked-eye detection of trace Co2+ ions at concentrations as low as 0.08 mM. The amine moiety of APTES–Fe@SiO2 NPs readily formed the metal–amine configuration complex (k980 peak) and induced the formation of Co NPs by self-seed generation (k520, k590, and k630 peaks). Preliminary study data supported the promising practical applicability of the current method for the colorimetric detection of Co2+ ions in aqueous phase. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Colorimetric sensors capable of being observed by the naked eye are appropriate for the on-site real-time detection of target heavy metal ions due to their simple configuration and on-site portability [1]. To date, various metal nanoparticles (NPs) have been used as colorimetric detectors for heavy metal ions in the aqueous phase. In particular, gold (Au) and silver NPs (AgNPs) offer excellent localized surface plasmon resonance (LSPR) properties, exhibit a strong and well-defined color and enable easy ⇑ Corresponding author. Tel.: +82 2 940 5769; fax: +82 2 941 5769. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.cej.2015.07.006 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.
visualization of color change [2,3]. Visual detection has been based on the well-known metal–ligand coordination, where the metal and ligand act as electronic acceptor and donor, respectively. A simple colorimetric assay for the detection of Co2+ ions, using mercaptopyridine–glutathione (GSH)-modified, AgNPs (30 nm), was previously reported based on the metal ions-GSH coordination compounds inducing the aggregation of AuNPs [2]. The core–shell NPs could be used as candidate materials for colorimetric sensors. The SPR of Au@Ag core–shell nanostructures is highly dependent on the dimensional shell-to-core ratio, and thus its slight variation often leads to an obvious spectral and color change [4]. Especially, when iron NPs (FeNPs) were used as the core material, magnetic core–shell NPs could be used as a
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
429
recoverable sensor/catalyst [5,6] or magnetic resonance imaging contrast agent [7]. Magnetic core–shell Fe@SiO2 NPs have aroused great interest in current research, due to their biocompatibility, easily renewability, and stability against degradation [8]. The inner FeNP core with a silica outer shell not only stabilizes Fe@SiO2 NPs in solution but also provides sites for surface modification with various chemical and biomedical ligands. In addition, an inert silica coating on the surface of magnetic NPs prevents their aggregation in solution and provides better protection against toxicity [9]. The present study describes a simple strategy for preparing amine functionalized FeNPs based on a multistep process in which the FeNPs are modified with silica. An amine functional group can be easily attached to the silica shell via a sol–gel reaction. The (3-aminopropyl)triethoxysilane (APTES) is selected as the amine functional material because it is a widely used grafting agent for the specific detection of heavy metal ions [10]. APTES is conjugated with the magnetic core–shell APTES–Fe@SiO2 NPs and used as a colorimetric sensor for heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+).
2. Experimental 2.1. Preparation of FeNPs Magnetic 20-nm-diameter NPs were synthesized based on a slight modification of a published one-pot chemical coprecipitation method [8]. Firstly, FeCl3H2O (2.18 mg), FeCl2H2O (0.78 mg), and tetramethylammonium (TMA, 3 mL) were added to deionized (DI) water, and the solution was purged with nitrogen gas with vigorous stirring. Then, ammonium hydroxide (5.6 mL, 28%) was added slowly as the solution color slowly turned from black to red-brown. The precipitate was collected by centrifugation (SUPRA22, Hanil) at 14,000 rpm, washed with DI water and ethanol five times, and then suspended in DI water. 2.2. Preparation of core–shell Fe@SiO2 NPs FeNP-coated SiO2 NPs were prepared by simple sol–gel reaction. 1 mL of FeNP solution was suspended in 9 mL of ethanol with a probe-type ultrasonicator (Ulh700S, ULSSO HI-Tech). Since aqueous silane at high pH formed a particulate sol by Ostwald ripening mechanism, 0.3 mL of tetraethyl orthosilane (TEOS) was added to the resulting solution after the pH was adjusted to 10 by addition of ammonium hydroxide. After 24 h, Fe@SiO2 NPs were isolated via centrifugation, washed with ethanol and DI water five times to remove unreacted chemicals, and then suspended in ethanol. The morphology of FeNPs and Fe@SiO2 NPs was analyzed using transmission electron microscopy (TEM, JEM-1010, JEOL). 2.3. Colorimetric sensing of heavy metal ions To detect the heavy metal ions, and amine group was functionalized on the Fe@SiO2 NPs using amino silane (APTES). As shown in Fig. 1, the amine group of APTES on the Fe@SiO2 NPs readily formed the metal–amine configuration complex and thus induced the aggregation of core–shell NPs. After 0.3 mL of APTES was drop-wise added to 10 mL of Fe@SiO2 NP solution and stirred at 70 °C at 500 rpm, the resulting APTES–Fe@SiO2 NP solution showed a dark-brown color. APTES–Fe@SiO2 NPs were isolated via centrifugation, washed with ethanol and DI water five times to remove unreacted chemicals, and then suspended in DI. Stock solutions (100 ppm) of heavy metal ions were prepared by CoCl2, CuCl2, FeCl2, and HgCl2. After addition of heavy metal ions into the APTES–Fe@SiO2 NP solutions, the color change (i.e., absorbance
Fig. 1. Schematic diagram for colorimetric detection via aggregation mechanism of core–shell nanoparticles.
change in the range of 350–1100 nm) was analyzed with UV–vis spectra (UV-18,000, Shimadzu). 3. Results and discussion 3.1. Characterization of Fe@SiO2 NPs These core–shell NPs are usually used to confine the charges and protect the sensitive particles using an inert material shell. Some metal–silica core–shell nanocomposites are suitable for highly sensitive biosensing, imaging, and chemical sensing. For example, silica–Au core–shells show nanogaps between them that provide room to accommodate metal ions as the hot spots, thereby improving the sensitivity [11]. Hot spots are attributed to enhanced electromagnetic field, and thus SPR has emerged as a sustainable bio-friendly analytical technique for the non-invasive detection of chemical, environmental, and biological materials [12]. Herein, for colorimetric detection of heavy metal ions through the change of LSPR properties of core–shell NPs, amine-functionalized magnetic core–shell NPs were prepared by FeNP-core and SiO2-shell. Au@Ag core–shell NPs were also used as colorimetric probes for various chemicals. The etching process of Au@Ag NPs with the addition of cyanide [4] or chromium [13] ions changes the SPR absorption and then the solution color. Namely, Au@Ag or Ag@Au core–shell NPs used for colorimetric detection were destructive. Therefore, a non-destructive core–shell that could be used in the colorimetric detection of heavy metal ions was prepared based on the magnetic silica NPs. As shown in Fig. 2a, the iron oxide (Fe3O4) NPs exhibited a spherical shape with a diameter of ca. 20 nm and a red-brown solution color. In the UV–vis spectrum of FeNPs dispersed in DI water (Fig. 2b), no characteristic peak was found in the range of 350–1100 nm. After silica coating on the FeNP-core, clear core– shell architectures were observed (Fig. 3a). The Fe@SiO2 NPs were sized 100–150 nm with a 40- to 60-nm-thick silica shell. The silica layer is required to avoid aggregation and degradation of the
430
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
Fig. 2. (a) TEM image and (b) UV–vis spectrum of FeNPs dispersed in the aqueous phase. Inset is a picture of FeNP solution contained in a UV cuvette.
Fig. 3. (a) TEM image and (b) UV–vis spectrum of Fe@SiO2 dispersed in the aqueous phase. Inset is a picture of Fe@SiO2 solution contained in a UV cuvette.
FeNP-core. Therefore, the Stöber process for the synthesis of silica NPs does not change the core–shell structure of the FeNP-coated silica NPs. As shown in Fig. 3b, the UV–vis spectrum was similar to that of FeNPs, indicating that the silica shell did not influence the SPR change of FeNPs. Amine ligands bind heavy metal ions by forming the metal– amine complex via amine–Mn+–amine couplings with two to four ligands (Fig. 1). When a monolayer of APTES is formed on the Fe@SiO2 NPs, the maximum grafting density (Dg) could be estimated by the following equation [14]
Dg ¼
S SSi
Mw Na
where Ssi is the occupied surface area of a silane ligand, Mw the molecular weight of APTES (221 g/mol), and Na the Avogadro constant (6.02 1023/mol). The specific surface area (S) of the bare SiO2 NP (m2/g) was calculated to be 22.8 m2/g according to the following equation: S = 6/qd (q = 2.1 g/cm3, d = 125 nm) [15]. Assuming that Ssi is 0.2 nm2 [16], the grafting density of APTES on Fe@SiO2 NPs was estimated as ca. 41.8 mg/g. Therefore, since the immobilization of APTES on Fe@SiO2 NPs is very low, the UV–vis spectrum and solution color of APTES–Fe@SiO2 NPs were unchanged (Fig. 4). 3.2. Colorimetric sensing of heavy metal ions Generally, colorimetric detection using metal NPs relies on the aggregation of NPs due to the formation of a chelating complex
between the metal ions and functional groups, namely, ion-templated chelation. The color change induced by the formation of the metal–amine complex was evaluated for various heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+) at 100 ppm. As shown in Fig. 4b, the solution color depended on the kinds of heavy metal ion. The exposure of Co2+, Cu2+, Fe2+, and Hg2+ to APTES–Fe@SiO2 NP solution resulted in a solution color of indigo, sky blue, yellow, and light brown, respectively. In the UV–vis spectrum (Fig. 4a), only a solution of Co2+ ions bonded with APTES–Fe@SiO2 NPs showed a dramatic increment of the absorbance intensity at 500–700 nm. The APTES–Fe@SiO2 NPs underwent aggregation due to the formation of a strong chelating complex via an amine group. The presence of Co2+ ions led to the emergence of new peaks at ca. 520, 590, and 650 nm in the UV–vis spectra. These three plasmon peaks suggest the possible formation of Co NPs by metal– amine complex. As reported by Moon et al. [17], amines act as a complexing agent by forming a suitable metal complex for self-seed generation, and the amines play an additional role as a capping agent by aggressive adsorption on the metal NPs. Namely, a metal–amine complex could be formed spontaneously the metal NPs by self-seed generation with silanization of APTES and acted as plasmonic NPs. For example, spherical 25-nm-diameter Co NPs exhibited the maximum SPR peak at 510 nm [18]. The three distinctive peaks were assigned to the out-of-plane quadrupole resonance (the first peak) and the in-plane quadrupole resonance (the two additional peaks) [19]. Therefore, the first peak at 520 nm in Fig. 4a was attributed to the Co NPs. To check the CoNP self-seed generation in solution,
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
431
Fig. 4. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions after addition of 100 ppm of Co2+, Cu2+, Fe2+, and Hg2+ ions. (c) TEM image of Co NPs formed by metal–amine complex through silanization.
Fig. 5. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions with decreasing Co2+ ion concentration.
Co2+ ion was exposed into APTES solution without Fe@SiO2. Namely, in order to prevent be confused with Fe@SiO2 NPs, this nanoparticles are excluded. A volume of 0.1 mL of 4.3 M APTES was added to 3 mL of 100 ppm Co2+ ion. After APTES combined with Co2+ ions, the particles that were present in the solution were analyzed with TEM. As shown in Fig. 4c, the small Co NPs were found and showed irregular sphere. As well as these three peaks, an additional peak appeared at 980 nm in the UV–vis spectra (Fig. 4a) and was assigned to the in-plane dipole plasmon resonance [19]. This plasmon peak was assigned to the change of local refractive index on the Fe@SiO2 NPs surface caused by the specific binding of APTES–Fe@SiO2 NPs with metal ions and the interparticle interaction resulting from the NP assembly. Therefore, the aggregation of NPs in the presence of metal ions induces both the four new peaks in the plasmon band energy and the color change.
Based on the two interpretation, the amine moiety of APTES– Fe@SiO2 NPs readily formed the metal–amine configuration complex (k980 peak) and induced the formation of Co NPs by self-seed generation (k520, k590, and k630 peaks), as illustrated in Fig. 1. At the low concentration of 1 ppm, even though the metal–amine complex was formed, the k980 peak appeared, but the quantity of Co-amine complex was considered insufficient for the formation of NPs for self-seed generation. Therefore, the characteristic peaks (k520, k590, and k630 peaks) of Co NPs were decreased in the UV–vis spectra at low Co2+ ion concentration. Quantitative analysis was performed by adding different concentrations of Co2+ ions into the APTES–Fe@SiO2 NP solution and monitoring the absorption peak in the UV–vis spectra (Fig. 5). With decreasing metal ion concentration, the absorbance intensity of the three peaks was decreased (Fig. 5a), accompanying the solution color change from indigo to sky blue (Fig. 5b). The UV–vis
432
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
Fig. 6. (a) UV–vis spectra and (b) color change of APTES–Fe@SiO2 solutions with decreasing Cu2+ ion concentration.
Fig. 7. (a) Regeneration of Co2+–APTES–Fe@SiO2 NPs complex and (b) recovery efficiency with cycle change.
absorbance (k650) increased linearly with increasing (0.08– 1.69 mM) Co2+ ion concentration. The determination coefficient (R2) was high, at 0.99. A linear correlation between the absorbance ratio and the Co2+ ion concentration makes it suitable for the quantitative determination of target metal ions in aqueous solutions. The objective of this Fe@SiO2 NP colorimetric sensor is to detect the presence of metal ions in aqueous phase by naked-eye observation. Therefore, this cost-effective colorimetric probe afforded the rapid, naked-eye detection of trace Co2+ ions at concentrations as low as 0.08 mM. Moreover, lower concentrations could be detected based on the measurements of the UV–vis spectra in aqueous solutions at room temperature. The UV–vis absorbance of Cu2+ ions, as shown in Fig. 4, was increased and the shoulder peak was found at ca. 630 nm. In addition, similarly to the Co2+ case, the peak for metal–amine complex (Cu2+–APTES Fe@SiO2 NPs) was found at 970 nm in the UV–vis spectra. As shown in Fig. 6, the UV–vis absorbance (k630) increased linearly with increasing (0.08–1.57 mM) Cu2+ ion concentration, but that of k970 was not linearly dependent. For colorimetric sensing of Hg2+ and Fe2+ ions, UV–vis absorbance was increased without a characteristic peak, but a single peak for metal–amine complex (M2+–APTES Fe@SiO2 NPs) was found at 970 nm in the UV–vis spectra. Consequently, the appearance of a peak at 970 or 980 nm in the UV–vis spectra revealed the aggregation of Fe@SiO2 NPs via metal–amine configuration complex, and thus the solution color change to reddish color. The experimental values of the standard molar Gibbs energy of hydration (DhydG°) for Cu2+, Co2+, Hg2+, and Fe2+ were 2010, 1915, 1760, and 1840 kJ/mol, respectively [20]. The negative DG values
indicate spontaneous adsorption of heavy metal ions through the metal–amine complex. Increasingly negative DG values indicate stronger formation of metal–amine complex and thus a preference for amine–Cu2+–amine coordinative coupling over amine–Hg2+– amine complex. Therefore, the APTES Fe@SiO2 NPs are suitable for the colorimetric detection of Co2+ and Cu2+ ions, compared to Hg2+ and Fe2+ ions. An important aspect of these magnetic core–shell NPs is their ability to be magnetically separated for use in subsequent water treatment, thereby reducing their environmental impact in aqueous phase. An important consideration is the ability to completely remove the NPs from the treated water to prevent them from acting as contaminants themselves. The magnetic NPs used in the colorimetric detection of heavy metal ions were successfully separated in a few minutes by magnet. The magnetically separated M2+–APTES Fe@SiO2 NPs complex could be regenerated by adjustment of pH using 0.1 M NaOH or HCl. Since silica-shell could be partially dissolved by NaOH treatment, HCl treatment is more stable re-generation method. In regeneration step (Fig. 7a), solution color of metal–NP complex was changed from blue to yellow, and then re-generated NPs were recovered by magnet. Finally, recovered NPs was concentrated and re-dispersed in DW to reuse. Recovery efficiency was decreased with regeneration cycles, and reached to 80% after 5 cycles (Fig. 7b). 4. Conclusions An efficient colorimetric sensor for the detection of heavy metal ions was prepared using magnetic core–shell NPs. The amine
U. Jeong et al. / Chemical Engineering Journal 281 (2015) 428–433
moiety was immobilized on the Fe@SiO2 NPs to form a metal– amine complex and induce a color change by aggregation between NPs. Four different heavy metal ions (Co2+, Cu2+, Fe2+, and Hg2+) were selected to demonstrate the colorimetric detection. After the addition of metal ions in APTES–Fe@SiO2 NP solutions, the appearance of the same peak at 970 or 980 nm in the UV–vis spectra of all samples revealed the aggregation of Fe@SiO2 NPs via metal–amine configuration complex. In the colorimetric detection of Co2+ ions, the observation of three addition peaks at 520, 590, and 630 nm in the UV–vis spectra indicated the formation of Co NPs by self-seed generation. Consequently, Co2+ ions could be detected colorimetrically via the aggregation of metal–APTES– Fe@SiO2 NPs, accompanied by the formation of Co NPs. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF-2010-0007050), the Korea Environmental Industry & Technology Institute (201400140002), and Kwangwoon University (2015). References [1] Y. Cho, S.S. Lee, J.H. Jung, Recyclable fluorimetric and colorimetric mercuryspecific sensor using porphyrin-functionalized Au@SiO2 core/shell nanoparticles, Analyst 135 (2010) 1551–1555. [2] H.K. Sung, S.Y. Oh, C. Park, Y. Kim, Colorimetric detection of Co2+ ion using silver nanoparticles with sphere, plate, and rod shapes, Langmuir 29 (2013) 8978–8982. [3] D. Vilela, M.C. González, A. Escarpa, Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: chemical creativity behind the assay. A review, Anal. Chim. Acta 751 (2012) 24–43. [4] J. Zeng, Y. Cao, J. Chen, X. Wang, J. Yu, B. Yu, Z. Yan, X. Chen, Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing, Nanoscale 6 (2014) 9939–9943. [5] X. Tian, Z. Dong, R. Wang, J. Ma, A quinoline group modified Fe3O4@SiO2 nanoparticles for sequential detection of Zn2+ and hydrogen sulfide in aqueous solution and its logic behavior, Sens. Actuators B 183 (2013) 446–453.
433
[6] C. Liu, J. Li, J. Qi, J. Wang, R. Luo, J. Shen, X. Sun, W. Han, L. Wnag, Yolk-shell Fe0@SiO2 nanoparticles as nanoreactors for fenton-like catalytic reaction, ACS Appl. Mater. Interfaces 6 (2014) 13167–13173. [7] L. Babes, B. Denizot, G. Tanguy, J.J.L. Jenune, P. Jallet, Synthesis of iron oxide nanoparticles used as MRI contrast agents: a parametric study, J. Colloid Interface Sci. 212 (1999) 474–482. [8] X. Peng, Y. Wang, X. Tang, W. Liu, Functionalized magnetic core–shell Fe3O4@SiO2 nanoparticles as selectivity-enhanced chemosensor for Hg(II), Dyes Pigm. 91 (2011) 26–32. [9] E.J. Park, H.N. Umh, Y. Kim, D.H. Choi, S.W. Kim, M.H. Cho, J.H. Kim, W. Choi, Magnetic iron oxide nanoparticles induce autophagy preceding apoptosis through mitochondrial damage and ER stress in RAW264.7 cells, Toxicol. In Vitro 28 (2014) 1402–1412. [10] B. Lee, Y. Kim, H. Lee, J. Yi, Synthesis of functionalized porous silicas via templating method as heavy metal ion adsorbents: the introduction of surface hydrophilicity onto the surface of adsrobents, Microporous Mesoporous Mater. 50 (2001) 77–90. [11] D.K. Lim, K.S. Jeon, J.H. Hwang, H. Kim, S. Kwon, Y.D. Suh, J.M. Nam, Highly uniform and reproducible surface-enhanced Raman scattering from DNAtailorable nanoparticles with 1-nm interior gap, Nat. Nanotechnol. 6 (2011) 452–460. [12] M. Li, S.K. Cushing, N. Wu, Plasmon-enhanced optical sensors: a review, Analyst 140 (2015) 386–406. [13] J. Xin, F. Zhang, Y. Gao, Y. Feng, S. Chen, A. Wu, A rapid colorimetric detection method of trace Cr(VI) based on the redox etching of Agcore–Aushell nanoparticles at room temperature, Talanta 101 (2012) 122–127. [14] Y. Liu, Y. Ki, X.M. Li, T. He, Kinetics of (3-aminopropyl)triethoxylsilane (APTES) silanizatioin of superparamagnetic iron oxide nanoparticles, Langmuir 29 (2013) 15275–15282. [15] H. Duan, X. Lin, G. Liu, L. Xu, F. Li, Synthesis of Co nanoparticles and their catalytic effect on the decomposition of ammonium perchlorate, Chin. J. Chem. Eng. 16 (2008) 325–328. [16] K. Kojio, A. Takahara, K. Omote, T. Kajiyama, Molecular aggregation state of noctadecyltrichlorosilane monolayers prepared by the Langmuir and chemisorption methods, Langmuir 16 (2000) 3932–3936. [17] D.V.R. Kumar, I. Kim, Z. Zhong, K. Kim, D. Lee, J. Moon, Cu(II)-alkyl amine complex mediated hydrothermal synthesis of Cu nanowires: exploring the dual role of alkyl amines, Phys. Chem. Chem. Phys. 16 (2014) 22107–22115. [18] J. Zhang, C.Q. Lan, Nickel and cobalt nanoparticles produced by laser ablation of solids in organic solution, Mater. Lett. 62 (2008) 1521–1524. [19] R. Jin, Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Photoinduced conversion of silver nanopheres to nanoprisms, Science 94 (2001) 1901–1903. [20] Y. Marcus, Thermodynamics of salvation of ions, J. Chem. Soc. Faraday Trans. 87 (1991) 2995–2999.