Colorimetric detection of iron ions (III) based on the highly sensitive plasmonic response of the N-acetyl-L-cysteine-stabilized silver nanoparticles.

G Model ACA 233842 No. of Pages 8

Analytica Chimica Acta xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Colorimetric detection of iron ions (III) based on the highly sensitive plasmonic response of the N-acetyl-L-cysteine-stabilized silver nanoparticles Xiaohui Gao a,b , Yizhong Lu a,b , Shuijian He a,b , Xiaokun Li a , Wei Chen a, * a b

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China University of Chinese Academy of Sciences, Beijing 100039, China

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

N-acetyl-L-cysteine-stabilized Ag nanoparticles are synthesized by a chemical reduction method. The surface plasmon resonance intensity of the silver nanoparticles decreases with Fe3+ concentration. The silver nanoparticles can be used for sensitive and selective detection of Fe3+ ions in water. A new detection mechanism of oxidation–reduction reaction between Ag NPs and Fe3+ ions is proposed.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 February 2015 Received in revised form 29 March 2015 Accepted 1 April 2015 Available online xxx

We report here a facile colorimetric sensor based on the N-acetyl-L-cysteine (NALC)-stabilized Ag nanoparticles (NALC–Ag NPs) for detection of Fe3+ ions in aqueous solution. The Ag NPs with an average diameter of 6.55 1.0 nm are successfully synthesized through a simple method using sodium borohydride as reducing agent and N-acetyl-L-cysteine as protecting ligand. The synthesized silver nanoparticles show a strong surface plasmon resonance (SPR) around 400 nm and the SPR intensity decreases with the increasing of Fe3+ concentration in aqueous solution. Based on the linear relationship between SPR intensity and concentration of Fe3+ ions, the as-synthesized water-soluble silver nanoparticles can be used for the sensitive and selective detection of Fe3+ ions in water with a linear range from 80 nM to 80 mM and a detection limit of 80 nM. On the basis of the experimental results, a new detection mechanism of oxidation–reduction reaction between Ag NPs and Fe3+ ions is proposed, which is different from previously reported mechanisms. Moreover, the NALC–Ag NPs could be applied to the detection of Fe3+ ions in real environmental water samples. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Colorimetric sensing N-acetyl-L-cysteine Silver nanoparticles Iron ions Surface plasmon resonance Chemical sensor

* Corresponding author. Tel.: +86 431 85262061. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.aca.2015.04.002 0003-2670/ ã 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: X. Gao, et al., Colorimetric detection of iron ions (III) based on the highly sensitive plasmonic response of the N-acetyl-L-cysteine-stabilized silver nanoparticles, Anal. Chim. Acta (2015), http://dx.doi.org/10.1016/j.aca.2015.04.002


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X. Gao et al. / Analytica Chimica Acta xxx (2015) xxx–xxx

1. Introduction In modern society, more and more people pay attention to their living standard, including usual daily food, drink, and surrounding environment. Metal elements are essential components for living body. The metals in the form of ions can coordinate with protein, nucleic acid, vitamin and hormone to generate metalloprotein and metalloenzyme etc. which play critical roles in regulating the important biochemical and physiological processes of life [1,2]. However, any metal ion with too higher and/or lower level would destroy the balance of metabolism. Thus quantitative determination of metal ions concentration is the key to ensure the metal ions on healthy level. Iron is one of essential trace element and it is a necessity for many physiological processes in human and animal bodies [3,4]. Iron ions in body are usually responsible for the transfer and transport of oxygen and block materials. The lack of iron ions will result in a lot of physiological and pathological diseases, such as iron-deficiency anemia, methemoglobinemia, liver and kidney damage, diabetes and heart diseases, and so on. Thus, qualitative and quantitative detection of Fe3+ is required to ensure body health [3–5]. Nowadays, many analytical techniques, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry and inductively coupled plasma emission spectrometry, have been applied to the detection of metal ions. However, these methods usually need sophisticated equipments and tedious sample preparation steps. Recently, chemsensors fabricated from fluorescent nanoparticles, such as carbon dots, graphene quantum dots, and noble metal nanoparticles or nanoclusters for sensing of metal ions have attracted wide attention due to their good selectivity, high sensitivity and easy operation [6–8]. Colorimetric assay has also been extensively utilized to detect heavy metal ions in aqueous solution because of the cost-efficient and less time-consuming procedures compared to other methods [9–12]. For example, Yin et al. developed a facile colorimetric sensor for ultrasensitive determination of Cu2+ based on catalytic oxidation of L-cysteine [13]. Meanwhile, metal nanoparticles and nanorods have recently received increasing interest in colorimetric assay because of the pronounced surface plasmon resonance (SPR), highly stable dispersion, good biocompatibility, and tunable physical and chemical properties dependent on size and shape [14–17]. Among the coinage metal nanoparticles (Cu, Ag, Au), it is well-known that silver has the strongest surface plasma resonance, even with the size down to 2 nm. Therefore, most of SPR-related fundamental investigations and sensing applications have focused on silver nanoparticles. For instance, Duan et al. realized the colorimetric detection of Hg2+ based on silver nanoparticles [18]. Annadhasan et al. reported the simultaneous detection of Hg2+/Mn2+ by using green-synthesized silver nanoparticles [19]. In another report, silver nanoparticles were also used to detect copper ions with paper-based devices [20]. It should be pointed out that although such method has achieved much development, to the best of our knowledge, the sensitive detection of Fe3+ based on plasmonic silver nanoparticles has not yet been reported. Herein, we aim to synthesize appropriate silver nanoparticles for the colorimetric sensing of Fe3+ with high sensitivity and selectivity. For preparing metal nanoparticles, it is common to use organic protecting ligands to improve the stability and dispersity of nanoparticle. As an example, oleylamine-stabilized Au–Ag alloy nanoparticles with tunable size and surface plasmon resonance frequency have been reported [21]. In another work, cysteamine was also utilized as efficient protecting ligand to prepare Ag nanoparticles in aqueous medium [22]. Among the used stabilizing agents, amino acid (AA) containing both amino-group and carboxyl group could show either hydrophilic or hydrophobic property. Due to the unique structure and properties, AA with either acidic or basic characteristic has been widely used to stabilize metal

nanoparticles. For example, water-soluble gold nanoparticles have been successfully synthesized by using tryptophan and L-cysteine as protecting ligands [23,24]. Sastry and his co-workers successfully synthesized Ag and Au nanoparticles by using cysteine and lysine as capping ligands, respectively [25,26]. As far as we know, there is scarce report of the usage of N-acetyl-L-cysteine as protecting ligand for the preparation of noble metal nanoparticles, except for its participation in the synthesis of small Au and Ag nanoclusters [27]. Recently, N-acetyl-L-cysteine-protected silver nanoparticles have been reported as optical sensors and colorimetric assay for Ni2+ ions detection [28]. Here, by using the strong interaction between cysteine ( SH) and silver, we present a simple method to prepare N-acetyl-Lcysteine-stabilized Ag nanoparticles (denoted as NALC–Ag NPs) without tedious post-synthesis step. The prepared Ag nanoparticles are water-soluble and show strong surface plasmon resonance around 400 nm. It was found that the SPR intensity of the as-synthesized Ag nanoparticles decreases when Fe3+ ions were gradually introduced into the solution. Moreover, the color of the Ag nanoparticle solution showed a change upon addition of different concentrations of Fe3+. On the basis of the SPR intensity of the silver nanoparticles, the sensitive detection of Fe3+ has been successfully realized. The sensitivity and limit of detection were calculated to be 0.00578 mM 1 and 80 nM, respectively. Meanwhile, the color change of the Ag colloidal solution can be distinguished by naked eyes, which can also be used to directly check whether the presence of Fe3+ or not in the solution. More importantly, different from the previously reported detection mechanisms based on ligand-induced aggregation of Ag nanoparticles and formation of nanoalloys, the present study demonstrates a new sensing mechanism based on Fe3+-induced decomposition of Ag nanoparticles, resulting in highly sensitive plasmonic response. 2. Experimental 2.1. Materials Silver nitrate (AgNO3), potassium hydroxide (KOH), and ethanol (CH3CH2OH) were purchased from Beijing Chemical Reagent. Nacetyl-L-cysteine (C5H9O3N3S) and L-cysteine were obtained from Alfa Aesar. Sodium borohydride (NaBH4) was purchased from ACROS. Standard stock (10 mM) solution of metal ions (Hg2+, Ca2+, Fe3+, Na+, Cu2+, K+, Ni2+, Co2+, Pb2+, Ag+, Mn2+, Mg2+, Zn2+, Cd2+, MnO4 and Cr2O72 ) were prepared with ultrapure water from the respective metal salts (NaCl, KCl, CuCl2H2O, ZnCl2, CaCl2, FeCl36H2O, AgNO3, MgSO4, Hg(NO3)21/2H2O, MnCl2, NiCl26H2O, Pb(NO3)2, CoCl26H2O, Cd(CH COO)2H2O, KMnO4 and K2Cr2O7). All chemicals were used as received without any further purification. Water was supplied by a Nanopure water system (18.3 MV cm). 2.2. Instruments UV–vis spectra were recorded on UV-3000PC Spectrophotometer (Shanghai Mapada Instruments Co., Ltd.). X-ray photoelectron spectroscopy (XPS) measurements were performed by using AVG Thermo ESCALAB250 spectrometer (VG Scientific) operated at 120 W. Fourier-transformed infrared spectroscopy (FTIR) study was conducted on a Vertex 70 FTIR with a KBr wafer technique, i.e., the solid mixture of KBr and Ag NP (NALC) were ground into fine powder and then pressed into transparent sheet by sheeting-out mill. The size and morphology of the as-synthesized Ag NPs were examined by using a Hitachi H-600 transmission electron microscope (TEM) operated at 100 kV. High resolution transmission electron microscope (HRTEM) images were obtained from a JEM-2010 (HR) microscope operated at 200 kV. Dynamic light




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complicated post-treatment. In the preparation, the precursors, including AgNO3,N-acetyl-L-cysteine, KOH and NaBH4, were dissolved in ethanol, while the generated product was insoluble in ethanol. Thus the prepared Ag NPs can be spontaneously precipitated from reaction solution and it is easy to purify the produced Ag NPs. Before the addition of NaBH4, silver cations can react with the N-acetyl-L-cysteine ligands, forming NALC–Ag(I) complex. When NaBH4, a strong reducing agent, was added into the solution, the pre-formed NALC–Ag(I) was reduced to Ag(0), then experienced nucleation and aggregation and finally grew into NALC–Ag NPs. The stabilizing ligand, N-acetyl-L-cysteine, was readily dissolved in water, which renders the N-acetyl-L-cysteinecapped Ag NPs shows good water-solubility and potential application for detection of metal ions. The successful preparation of Ag NPs was first proved by the UV–vis absorption spectrum. As shown in Fig. 1(a), the characteristic SPR peak of silver nanoparticles can be observed at around 400 nm. At the same time, the pronounced absorption with sharp peak indicates the monodispersity of the formed Ag NPs. Moreover, compared with the previous report [19], the absence of shoulder peak at longer wavelength suggests neither large silver spherical particles nor Ag anisotropy ellipsoids or rod was produced with the present synthetic process. The composition of the Ag NPs was then studied by X-ray photoelectron spectrum (XPS) measurements using C 1s as reference. The survey XPS spectrum in Fig. 1(b) clearly shows the binding energy peaks from Ag 3d, S 2p, C 1s, N 1s, O 1s, indicating the presence of Ag, S, C, N, O elements in the product [29,30]. In Fig. 1(c), the binding energies of Ag 3d5/2 and Ag 3d3/2 at 367.6 and 373.6 eV are the typical characteristics of metallic silver. Combined with UV–vis absorption result, the generation of Ag NPs could be undoubtedly confirmed. It should be noted that there is only small binding energy difference (0.5 eV) between Ag(0) and Ag(I). According to the previous study [31], there may exist Ag(I) on the particle surface due to the charge transfer in the Ag–NALC bonds. The XPS spectrum of S 2p3/2 presented in Fig. 1(d) can be deconvoluted into two different components at 161.7 and 163.0 eV, which may correspond to the Ag(0)–S and Ag(I)–S, respectively.

scattering and zeta potential were measured using Malvern Nano ZS90. X-ray diffraction measurements were performed on D8 ADVANCE (Germany) using Cu Ka radiation with Ni filter (l = 0.154059 nm at 30 kV and 15 mA). 2.3. Synthesis of NALC–Ag NPs Ag nanoparticles were synthesized through a facile and rapid chemical reduction method. In a typical run, 48 mg AgNO3 was added to 12 mL of ethanol solution containing 170 mg KOH under vigorous magnetic stirring. After AgNO3 dissolving, 163 mg of NALC was introduced into the mixture. Then 66 mg of NaBH4 solid powder was directly poured into the reaction vessel after half an hour. The whole reaction was conducted under vigorous stirring at room temperature. The brown precipitate was collected and purified through centrifugation, and finally dispersed in water for further use. The synthesized Ag NPs showed strong characteristic SPR around 400 nm. 2.4. Colorimetric detection of Fe3+ The procedure for colorimetric detection of Fe3+ is as follows. 3 mL of diluted silver nanoparticle aqueous solution containing 200 mM L-cysteine was used as primary testing sample. Then different concentrations of Fe3+ stock solution was introduced into the above solution and the corresponding UV–vis absorption spectra were recorded. The detection selectivity was examined just by replacing Fe3+ with other metal ions, i.e., Hg2+, Ca2+, Fe3+, Na+, Cu2+, K+, Ni2+, Co2+, Pb2+, Ag+, Mn2+, Mg2+, Zn2+, and Cd2+ and two strong oxidizing anions MnO4 and Cr2O72 . 3. Results and discussion 3.1. Synthesis and characterization of NALC–Ag nanoparticles As described in Section 2, the synthesis of silver nanoparticles was successfully achieved through a one-step process without 1.2 Newly prepared Ag NPs Six months later

Absorbance

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Fig. 1. (a) UV–vis absorption spectra of the newly synthesized NALC–Ag NPs and the NPs stored for six months. XPS spectra of the NALC–Ag NPs; (b) survey spectrum; (c) Ag 3d and (d) S 2p.



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Such result also indicates that the prepared Ag NPs are capped by N-acetyl-L-cysteine protecting ligands. The stability of the NALC– Ag NPs was examined by monitoring the UV–vis absorption change with time. The change of UV–vis absorption spectrum was tracked in six months. By comparing the UV–vis spectra of the freshly prepared Ag NPs and the particles stored for six months (Fig. 1(a)), negligible change could be observed, indicating the high stability of the synthesized Ag NPs. The morphology and size of the Ag NPs were characterized by transmission electron microscope (TEM). Fig. 2(a) shows the typical TEM image of the product, from which spherical silver nanoparticles with good dispersion can be observed. From the nanoparticle size histogram shown in Fig. 2(a) inset, the Ag NPs have a narrow size distribution ranging from 5 to 8 nm and the average size calculated from one hundred particles is 6.55 1.0 nm, which agrees with the UV–vis absorption. The morphology of the as-synthesized Ag nanoparticles was also characterized by HRTEM. It can be seen from Fig. S1 that the Ag nanoparticles exhibit spherical shape with uniform particle size. Fig. S1 inset shows the magnified HRTEM image of a Ag nanoparticle. The lattice fringe space was measured to be 0.23 nm which corresponds to Ag (111). To further analyze the crystal structure of the obtained Ag nanoparticles, XRD measurement of the Ag NPs was also performed. As shown in Fig. S2, the diffraction peaks in the XRD pattern of Ag NPs can be ascribed to the (111), (2 0 0), (2 2 0) and (3 11) planes of silver, indicating the face-centered cubic (fcc) structure of the synthesized silver nanoparticles. On the other hand, the dynamic light scattering (DLS) measurement (not shown

here) indicates that the prepared Ag NPs have an average hydrodynamic size of 12.0 4.0 nm. The larger hydrodynamic size than that obtained from TEM image suggests the strong hydrophilic property of the Ag NPs arising from the hydrophilic protecting ligands (N-acetyl-L-cysteine). In addition, the test of zeta potential ( 19.7 mV) implies that the surface of the assynthesized Ag NPs was negative-charged. Such result suggests that the OH groups of NALC molecules bonded on silver particle surface have been ionized, which can be also further confirmed from the following FT-IR characterization. The surface chemistry of the Ag NPs was studied by FT-IR. Fig. 2(b) presents the FT-IR spectra of pure NALC (red line) and NALC–Ag nanoparticles (black line). By comparing the two FT-IR spectra, the peaks in the range of 1000– 1700 cm 1 from the stretching modes of C H, N H, COO or COOH are basically same for the two samples, suggesting the binding of N-acetyl-L-cysteine protecting ligands onto the Ag particle surface [32]. However, there are also some significant differences between the two spectra. First, it is obvious that the characteristic band of S H at 2548 cm 1 from NALC disappeared in the spectrum of NALC–Ag NPs [33]. This observation strongly suggests the cleavage of S H bonds and the formation of Ag S bonds during the formation of Ag–NALC nanoparticles. Second, the peak from the OH and NH groups (3376 cm 1) in the amino acid become wider for the NALC–Ag NPs, which may be due to the deprotonation of the functional groups in the alkali synthesis condition or the existence of water molecules in the Ag NPs solid sample because of the strong hydrophilic property of N-acetyl-L-cysteine. Moreover, due to the

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0 Fig. 2. (a) TEM image of the as-synthesized NALC–Ag NPs. The inset shows the nanoparticle size histogram. (b) FT-IR spectra of pure N-acetyl-L-cysteine (NALC, red line) and the NALC-capped Ag NPs (NALC–Ag NPs, black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1E-4 1E-3 1E-2 1E-1 1 Concentration of NaCl (mol/L)

Fig. 3. Dependence of the absorption intensity of NALC–Ag NPs at 400 nm on pH (a) and salting strength (b) of solution. Inset in (a) shows the UV–vis absorption spectra of the NALC–Ag NPs in different pH solutions.




deprotonation of COOH into COO , the peaks at 1722 and 1917 cm 1 shift to lower wavenumber for the NALC–Ag NPs. The FTIR characterizations again indicate the formation of NALCcapped silver surface and the result is consistent with those from the above XPS, UV–vis and TEM measurements. It should be pointed out that as usual, the size of NALC-capped silver nanoparticles will increase when decreasing the NALC/Ag ratio. Meanwhile, without the presence of KOH, the product shows deeper yellow color and the nanoparticles are unstable and easy to be degraded. 3.2. Colorimetric detection of Fe3+ based on the highly sensitive plasmonic response of NALC–Ag NPs In order to examine the stability of our synthesized Ag NPs in different conditions, we firstly investigated the influences of some chemically environmental factors, such as pH, salting strength of the solution, on the surface plasmon resonance of the Ag NPs. Fig. 3(a) shows the dependence of SPR absorption intensity at 400 nm on the pH of solution and Fig. 3(a) inset displays the UV–vis absorption spectra of the Ag NPs in different solutions. It can be seen obviously that the influence of pH on the surface plasmon resonance intensity of Ag NPs could be ignored. Especially, when pH changes from 4 to 13, the absorption value basically keeps steady. Fig. 3(b) presents the influence of salting strength on the absorption intensity of the Ag NPs, from which the influence of

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salinity is also almost negligible. Such high stability of the Ag NPs may be due to the used N-acetyl-L-cysteine ligand containing both NH2 and COOH groups, which can be protonated or deprotonated with the change of environment. Based on these results, it can be assumed that the Ag NPs synthesized in this work can be applied to a variety of sensing systems owing to its high resistance to acid and base, and high salt-tolerance. The sensing performance of the Ag NPs to different metal ions was then investigated. In the present work, the sensing properties of the NALC-stabilized Ag NPs for different metal ions were first investigated separately, including Hg2+, Ca2+, Fe3+, Na+, Cu2+, K+, Ni2 + , Co2+, Pb2+, Ag+, Mn2+, Mg2+, Zn2+ and Cd2+ at the same concentration of 80 mM. Upon introduction of various metal ions into the Ag NPs aqueous solution, the changes of UV–vis absorption curves are shown in Fig. 4(a). It can be seen that the addition of most metal ions did not cause the UV–vis absorption change of the Ag nanoparticle solution. However, the introduction of Fe3+ and Hg2+ can lead to the significant decrease of the absorption intensity, with more efficient quenching from Fe3+. Here, in order to improve the sensing selectivity of Ag NPs to Fe3+, we added amine acid, L-cysteine, to the solution, which exhibited excellent sheltering effect for the interference Hg2+. Fig. 4(b) gives the absorption difference at 400 nm upon the addition of different metal ions with absence or presence of L-cysteine in the solution. Clearly, with the presence of 200 mM cysteine, the interference from Hg2+ can be largely reduced. Meanwhile, the possible interference from dichromate ion (Cr2O72 ) and permanganate ions (MnO42 ) were also investigated. As shown in Fig. S3, without the presence of masking agent cysteine, both ions could change the intensity of surface plasmon resonance. The SPR intensity of Ag NP decreases upon the addition of permanganate ions and increases with the introduction of dichromate ions. However, in the presence of cysteine, the interfering effects from Cr2O72 and MnO42 were largely reduced. Based on above results, the prepared NALC–Ag NPs can be used as sensing materials for selective detection of Fe3+ by using cysteine as an effective chelating agent. The effect of cysteine

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Fig. 4. (a) UV–vis absorption spectra and (b) the absorption peak intensity change of the NALC–Ag NPs solution (58 mg mL 1) with addition of different kinds of metal ions (80 mM), including Ca2+, Cd2+, Co2+, Cu2+, Mn2+, Ag+, K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+, Fe3+, and Hg2+. In panel (b), the black and red bars represent the absorption difference of the Ag NPs solution with and without addition of 200 mM cysteine. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. (a) UV–vis absorption spectra of NALC–Ag NPs (58 mg mL 1) with different concentrations of Fe3+. Inset shows the linear relationship between the intensity difference of UV–vis absorption and concentration of Fe3+. (b) Change of the UV–vis absorption spectra of NALC–Ag NPs (76 mg mL 1) with adding different concentrations of Fe3+ into the lake water system. Inset shows the linear relationship between the SPR absorption difference and concentration of Fe3+ in the lake water system. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)



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Table 1 Brief summary of sensing performances of different sensors for Fe3+ detection. Detection probe

Detection mechanism

Linear range

Detection limit

Ref.

Gold nanorods Gold nanoparticles Fluorophores Carbon dots GQDsa N-GQDs N-GQDs S-GQDs MOF (MIL-53)b Silver nanoparticles

Aggregation Aggregation Coordination Coordination Coordination Coordination Coordination Coordination Ions exchange Reduction

– 10–60 mM 0–100 mM 2 nM–1 mM 0–400 mM 1–1945 mM 1–70 mM 0.01–0.70 mM 3–200 mM 0.08–80 mM

100 ppb 5.6 mM – 2 nM 7.22 mM 90 nM 80 nM 4.2 nM 9 mM 80 nM

[40] [41] [42] [43] [44] [8] [45] [46] [47] This work

a b

GQDs: graphene quantum dots. MOF: metal organic framework.

on the sensing system was also studied. As shown in Fig. S4, the addition of Cys could not influence the absorption intensity of silver nanoparticles and we found that 200 mM Cys is enough to shield Hg2+. Therefore, 200 mM Cys was used in the following detections. The sensitivity for Fe3+ detection was investigated by adding different concentrations of Fe3+ ions into the Ag NPs solution. The changes of UV–vis absorption curves and the corresponding SPR band intensities were monitored by UV–vis spectroscopy. As shown in Fig. 5(a), the SPR band intensity gradually decreases with increasing Fe3+ concentration in the Ag NPs solution. Fig. 5(a) inset depicts the linear relationship between the SPR peak intensity and the concentration of Fe3+ within the range from 80 nM to 80 mM. The fitting line can be expressed as: DA = 0.00578 [Fe3+] 0.00877 (DA = A0 A, A0 and A represent the SPR peak intensities of Ag NPs without and with the addition of Fe3+, respectively, [Fe3+] refers to the concentration of Fe3+) with a linear regression coefficient (R2) of 0.997. Under the current experimental conditions, the limit of detection (LOD) was calculated to be 80 nM on the basis of a signalto-noise of 3 (LOD = 3s /s). Table 1 summarizes the reported sensing probes for Fe3+ detection. It can be seen that the present NALC–Ag NPs sensing material exhibits lower LOD than most of the reported systems. On the other hand, the qualitative determination of Fe3+ could be realized by the color change of the solution, as demonstrated in Fig. 6. By comparing Fig. 6(a and b), upon addition of different metal ions into the Ag NPs solution, only Fe3+ can result

in a colorless solution, further indicating the high selectivity of the NALC–Ag NPs sensing material for Fe3+ detection. Moreover, it can be seen from Fig. 6(c) that the solution color is sensitive to the added amount of Fe3+. Therefore, after further optimization of detection conditions, the semi-quantitative detection of Fe3+ could be realized by naked eyes using the present NALC–Ag NPs material. To examine the practical sensing capability of the NALC–Ag NPs system for detection of Fe3+ ions, we investigated the sensing performance in real water sample that was obtained from the South Lake of Changchun, Jilin province, China. Before the detection experiments, the lake water samples were filtered through a 0.2 mM membrane and centrifuged at 10,000 rpm for 15 min. Then different volumes of Fe3+ stock solution were introduced into the treated water samples and analyzed with the standard addition method. From Fig. 5(b), we can see that the SPR band intensity of the Ag NPs testing solution shows decrease with increasing the concentration of Fe3+. Although there may be many impurities, such as microorganisms and mineral substances in the lake water, the sensor based on Ag NPs still works for the sensitive detection of Fe3+ in the real water system. Fig. 5(b) inset shows the linear relationship between the absorption peak intensity and the concentration of Fe3+. The good linearity again indicates the high sensing performance of NALC–Ag NPs for Fe3+ detection. It should be pointed out that according to the obtained calibration curve, the direct addition of real lake water can not lead to the obvious change of SPR band intensity of the NALC–Ag NPs,

Fig. 6. Photographs of the NALC–Ag NPs solution (58 mg mL 1) containing 200 mM cysteine before (a) and after (b) addition of different metal ions (80 mM). (c) Photographs of the NALC–Ag NPs solution (high concentration, 505 mg mL 1) upon addition of different concentrations of Fe3+ (from left to right, the concentrations are 0, 0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM, 0.5 mM, 0.6 mM, 0.7 mM, 0.8 mM, 0.9 mM, respectively).




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Scheme 1. The processes of formation of the NALC-protected Ag nanoparticles and the sensing mechanism for Fe3+ ions.

indicating the extremely low concentration of Fe3+ in the lake water. For comparison, the ICP-MS experiment can also not give the accurate concentration of Fe3+ in the lake water. These experimental results demonstrate that our prepared NALC–Ag NPs exhibit promising application for detection of Fe3+ in the surrounding environments. 3.3. The sensing mechanism of NALC–Ag NPs for Fe3+ detection In most cases, detection of transition metal ions, especially heavy metal ions, can be achieved by fluorescent Ag or Au nanoclusters based on the fluorescence quenching effect of metal ions [34–36]. The quenching effect was due to the strong interaction between fluorescent nanoclusters and metal ions. Meanwhile, non-fluorescent silver nanoparticles have also been used for the detection of Hg2+ ions on the basis of the dependence of SPR band intensity on the concentration of Hg2+ ions. In this case, anti-aggregation mechanism [18], or alloying process of Ag–Hg nanoparticles [19], were proposed. It should be noted that in the previous studies, the presence of Hg2+ ions can either cause the aggregation of silver nanoparticles or induce the formation of alloy nanoparticles, resulting in the intensity decrease and peak shifts of the SPR band. However, in the present study, it can be seen from Fig. 6 that the introduction of Fe3+ can lead to the colorlessness of particle solution. Meanwhile, when the NALC–Ag NPs system was incubated by all other metal ions with Fe3+, the mixed solution became colorless while retained the color without addition of Fe3+ (Fig. S5), indicating the sensitivity of the system to Fe3+. Also from Fig. 5(a and b), the SPR bands exhibited intensity decrease but no any blue- or red-shift. These results suggest that the intensity decrease of the prepared NALC–Ag NPs could be ascribed to the reduction in the amount of Ag NPs in solution but not to the size change. To further check whether there is Ag nanoparticle aggregation or not in the solution after the addition of Fe3+, TEM and UV–vis absorption of the bottom solution after centrifugation were measured. As shown in Fig. S6, after the addition of Fe3+, no large particle can be seen from the TEM image and the UV–vis spectrum of the bottom solution exhibited no characteristic absorption from large Ag nanoparticles. These measurements showed that no aggregated Ag nanoparticles were formed after Fe3+ ions were added. Based on these results, we propose here a new sensing mechanism of the as-synthesized NALC–Ag NPs for Fe3+ detection, as shown in Scheme 1. When Fe3+ ions were introduced in the NALC–Ag NPs solution, the Ag NPs can be decomposed by Fe3+ ions, resulting in the gradual decrease of SPR intensity dependent on Fe3+concentration. It was also found that the zeta potential of the NALC–Ag NPs ( 19.7 mV) is much

lower than that of the mixture of Ag NPs and Fe3+ ions solution (36.9 mV). On the other hand, the previous studies showed the possibility of oxidation–reduction reaction between Ag NPs and Fe3+ ions [37–39]. For example, in a recent report [37], Ag shell in the Ag@Au core–shell nanostructures can be etched by Fe3+ in the presence of CTAB (cetyltrimethylammonium bromide). These studies further indicate that our proposed mechanism is reasonable for the detection of Fe3+ ions, i.e., the possible oxidation– reduction reaction between Ag NPs and Fe3+ ions lead to the decompose of Ag NPs. Overall, optical sensor fabricated from the Nacetyl-L-cysteine-protected Ag NPs is promising for the detection of Fe3+ ions in environmental samples based on the oxidation– reduction mechanism. 4. Conclusion In summary, we successfully synthesized NALC-stabilized Ag NPs through a facile chemical reduction method without tedious posttreat steps. TEM and UV–vis absorption characterizations showed that the obtained Ag nanoparticles have an average diameter of 6.55 1.0 nm with a strong absorption around 400 nm. By taking advantage of the intensity change of SPR, the as-synthesized Ag NPs can be used to detect Fe3+ in water with high sensitivity and selectivity. The optical sensor based on the NALC–Ag NPs is reliable and can apply to the detection of Fe3+ in real water sample. Moreover, we proposed a new mechanism—oxidation–reduction process—for the sensing system based on the experimental results. Overall, our present work indicates that by changing the surface structure, Ag nanoparticles could be potential optical sensing materials for colorimetric sensing of metal ions. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21275136) and the Natural Science Foundation of Jilin province, China (No. 201215090). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2015.04.002. References [1] C.D. Lewis, U.K. Laemmli, Higher-order metaphase chromosome structure – evidence for metalloprotein interactions, Cell 29 (1982) 171–181. [2] D. Senthilnathan, A. Kalaiselvan, S.A. Vedha, P. Venuvanalingam, The metal delivery mechanism of transferrin and the role of bent metallocene metals



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