Fenton-like Reaction-Mediated Etching of Gold Nanorods for Visual

Dec 8, 2014 - Because of the high redox potential of Au(I)/Au(0), the targets should ... method of GNRs and apply it to the sensitive visual detection...
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Fenton-like Reaction-Mediated Etching of Gold Nanorods for Visual Detection of Co2+ Zhiyang Zhang,†,‡ Zhaopeng Chen,*,† Dawei Pan,† and Lingxin Chen*,† †

Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS); Shandong Provincial Key Laboratory of Coastal Environmental Processes, YICCAS, Yantai Shandong 264003, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: We have proposed a Fenton-like reactionmediated etching of gold nanorods and applied it to the sensitive visual detection of Co2+ ions. With the presence of bicarbonate (HCO3−) and hydrogen peroxide(H2O2), Co2+ ions trigger a Fenton-like reaction, resulting in the generation of superoxide radical (O2•−). As a result, the gold nanorods are gradually etched by O 2 •− in the presence of SCN−, accompanied by an obvious color change from green to red. The gold nanorods etching process preferentially occurs along the longitudinal direction, which is observed by transmission electron microscope. The etching mechanism is carefully proved by investigating the effects of different radical scavengers (e.g., dimethyl sulfoxide). The auto-oxidation of hydroxylamine assay further confirms the mechanism. Then, the main factors, including reactants concentrations, temperature, and incubation time, are specifically investigated. Under optimized conditions, we get an excellent sensing performance for Co2+ with a lower detection limit of 1.0 nM via a spectrophotometer and a visual detection limit of 40 nM. In addition, this principle may provide a new concept of “intermediate-mediated etching of nanoparticles” for sensing.

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INTRODUCTION In the past two decades, much attention has been focused on the gold nanoparticles-based visual detection methods due to their simplicity, sensitivity as well as the potential application to on-site detection.1−3 Most of these methods are based on the principal that targets directly or indirectly trigger the nanoparticles aggregation, which leads to a shift of the localized surface plasmon resonance absorption (extinction), accompanied by a color change from red to blue. However, these methods generally require the incorporation of analyte recognition reagents onto the nanoparticle surface, suffering from the tedious procedure for the modification of nanoparticles. In addition, all the nanoparticle-aggregation-based sensors suffer from the autoaggregation of nanoparticles in complex samples, which often causes high backgrounds or false positive results, and thus the application of these methods is limited to a certain extent. To overcome the drawbacks mentioned above, a promising platform based on etching of gold nanoparticles has been developed for visual sensing of some targets,4−16 including H2O2, Cu2+, Pb2+, Fe3+, Cr(VI), CN−, I−, Cl−, and NO2−. This kind of sensor often does not need the modification of nanoparticles and can also avoid false positive signal owing to the autoaggregation of nanoparticles. Unfortunately, this platform can be only applied to monitoring limited targets. © 2014 American Chemical Society

Because of the high redox potential of Au(I)/Au(0), the targets should either have higher redox potential than that of Au(I)/ Au(0) at certain conditions or can reduce the redox potential of Au(I)/Au(0) by formation a stable complex or metal-Au alloys. Recently, to speed up the etching of gold nanorods (GNRs) by H2O2, Nie and coworkers utilize the higher oxidizing property of hydroxyl radical (•OH) to etch GNRs and realize the blood glucose detection.17 As well-known, superoxide radical (O2•−) is one of the active oxygen radicals produced in human body. Because of its high oxidizing property, O2•− is greatly adverse to human body. Inspired by this, herein, we propose a new etching method of GNRs and apply it to the sensitive visual detection of Co2+ ions. Co2+ triggers a Fenton-like reaction, generating a large amount of superoxide radical (O2•−). Then, GNRs are quickly oxidized to Au(I) ions by O2•− in the presence of SCN−. As a result, the color of GNRs changed from green to red. The color change can be applied to quantifying the concentrations of Co2+ via the naked eye. Cobalt is one of the biological essential trace elements and plays many important roles in biological systems. However, cobalt and some of its compounds are also suggested to be Received: October 28, 2014 Revised: November 30, 2014 Published: December 8, 2014 643

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(0.075 M), 25 μL NaHCO3 (0.1 M), and 20 μL H2O2 (0.1 M) were added into the borate buffer solution. Second, the prepared GNRs (200 μL, 1.5 nM) solution was added to the mixed solution as the colorimetric indictor. Finally, the mixture solution was incubated at 95 °C for 7 min and then subjected to record the extinction spectra. Detection of Co2+ in Drinking Water. Different concentrations of Co2+ were spiked in drinking water, and then the pH of the solutions was adjusted to 9.0 by using the high concentration borate buffer solution. Then a similar detection procedure to that described above was conducted.

potential etiological toxins and likely present carcinogenic effects.18 Thus, many analytical methods, including atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICPMS), electrochemistry, and colorimetry, have been established for detection of Co2+.19−39 Among these methods, colorimetric methods are more suitable for on-site detection because they can avoid tedious sample pretreatment and expensive instrumentation, and the detection signal can be observed by the naked eye. However, conventional colorimetric Co2+ sensors based organic dye molecules often face poor sensitivity with detection limit at ∼10−6 M levels.23,24 To improve the sensitivity, many nanoparticlesaggregation-based sensors for Co2+ have been developed.36−39 However, most of them cannot meet the visual sensitivity requirements and all of them suffer from the drawbacks of the type of sensors mentioned above, such as modification process and false positive result. Therefore, it is essential to develop a more sensitive and reliable colorimetric method for the on-site detection of Co2+ ions. The proposed method exhibits better on-site detection performance compared to existing methods: (1) excellent visual sensitivity with a naked-eyes-detectable limit of 40 nM, avoiding apparatus; (2) no false positive result owing to autoaggregation; (3) no modification of good nanorods; and (4) short detection time (7 min). Additionally, from the perspective of principle, compared with the existing methods based on target-induced etching of nanoparticles, this method is entirely different. Co2+ neither has the higher potential to etch gold nanoparticles directly, nor can it reduce the redox potential of Au(I)/Au(0). Co2+ just acts as a catalyst to trigger the generation of an intermediate (O2•−) which can etch gold nanoparticles. Thus, this principle may propose a new concept of “intermediate-mediated etching of nanoparticles” for other analyte detection.

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RESULTS AND DISCUSSION Principle for the Etching of GNRs. The extinction spectrum of GNRs exhibits two plasmon peaks, which result from the transversal and longitudinal surface plasmon resonance (LSPR), respectively.41,42 The LSPR peak of GNRs reflects the aspect ratio of GNRs. Normally, with the increase in the aspect ratio, the LSPR peak shifts to longer wavelength.43 Scheme 1 illustrates the principle of the etchingScheme 1. Schematic Illustration for Visual Detection of Co2+ Based on Fenton-like Reaction-Mediated Etching of GNRs

EXPERIMENTAL SECTION

based visual detection of Co2+. The GNRs with an average aspect ratio of 2.0:1 (Figure 1A and Figure 2a) exhibited a LSPR peak at 660 nm (curve a in Figure 1D). In the absence of Co2+, H2O2 etched GNRs slowly. The LSPR peak blue-shifted to 620 nm (curve b in Figure 1D) and the color of GNRs solution changed slightly (inset in Figure 1A and B). The corner angles on the GNRs surface disappeared (Figure 1B), and the average aspect ratio of GNRs turned to 1.9:1 (Figure 1B and Figure 2b). The results were also consistent with the previous study.44,45 With the addition of 0.1 μM Co2+, the Fenton-like reaction happened in the presence of hydrocarbonate. Co2+ catalyzed the decomposition of H2O2 to produce a large amount of superoxide anion radical (O2•−). Then, O2•− etched GNRs quickly along the longitudinal direction in the presence of SCN−. The average aspect ratio of GNRs decrease to 1.4:1 (Figure 1C and Figure 2c) accompanied by an obvious color change from green to red (inset in Figure 1C). The preferential etching along the longitudinal direction results from the less surface passivation and/or the higher reaction activities at the tips of GNRs. The Fenton-like reaction-mediated anisotropic etching of GNRs also led to a great blue-shift of the LSPR (from 620 to 560 nm, curve c in Figure 1D). This phenomenon can be used for the visual detection of Co2+ ions. In the presence of hydrocarbonate, the Fenton-like reaction produced a few kinds of oxidants, such as some free radicals and singlet oxygen as shown in the following reactions:46−48

Chemicals and Apparatus. Hydrogen tetrachloroaurate(III) dehydrate, cetyltrimethylammonium bromide (CTAB), ascorbic acid, hydroxylammonium chloride, mannitol, dimethyl sulfoxide, sodium azide, H2O2, NaBH4, AgNO3, NaHCO3, KSCN, and CoCl2 were obtained from Sinopharm Chemical Reagent (China). Superoxide dismutase (SOD) was purchased from Sigma-Aldrich. All other chemicals were analytical reagent grade or better. Solutions were prepared with deionized water (18.2 MΩ, Pall Cascada). Transmission electron microscopy (TEM) analyses were performed on a JEM-1230 electron microscope (Japan) operating at 100 kV. Absorption (extinction) spectra were measured on a Thermo Scientific NanoDrop 2000/2000C spectrophotometer. Preparation of GNRs. The GNRs were synthesized using a modified method by changing the amount of AgNO3.40 (1) Seed Solution: 0.25 mL of HAuCl4·3H2O (0.01 M) was added to 7.5 mL of CTAB (0.10 M) solution. Afterward, 0.60 mL of ice-cold NaBH4 (0.01 M) was mixed with the solution. The final solution was kept in a 26 °C water bath for 2 h. (2) Growth Solution: 1.2 mL of an aqueous 0.05 M HAuCl4·3H2O solution was mixed with 100 mL of CTAB (0.10 M) in a beaker under stirring. Then, 0.3 mL AgNO3 (0.01 M) and 0.96 mL ascorbic acid (0.1 M) were added to the solution in sequence. Finally, 0.2 mL seed solution prepared in step (1) was added at room temperature. The color of the solution gradually changed to purple within 10−20 min. The flask was then left undisturbed for 2 days. The obtained GNRs were centrifuged twice at 8000 rpm for 15 min to remove excess CTAB. The obtained soft sediment was resuspended in deionized water and kept at room temperature. The colloid was found to be stable for at least 6 months. Procedure for Detection of Co2+. The measurement was carried out in 50 mM borate buffer solution (800 μL, pH 9.0) containing 2.0 mM CTAB and different concentrations of Co2+. First, 10 μL KSCN

Co2 + + H 2O2 → Co3 + + OH− + •OH 644

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At pH 9.0, all the practical redox potentials of these radicals and singlet oxygen are higher than that of Au(SCN)2−/Au(0.69 V, vs NHE).49 That means all of them have the ability to oxidize Au. To investigate which one plays the leading role in the etching of GNRs, several control experiments were implemented in the following steps. Ascorbic acid is a well-known common free radical scavenger.46 Figure 3A shows the effect of ascorbic acid on the Fenton-like reaction-mediated etching of GNRs. It was observed that, with the removal of free radical by the addition of ascorbic acid, the etching of GNRs was slowed with the less significant blue-shift of the LSPR peak. The result confirmed the radical reaction mechanism of the proposed method. The generation of free radicals appeared to be the critical controlling factors in the etching of GNRs. DMSO and mannitol are effective scavengers for hydroxyl radical (•OH).50,51 The inhibition effects of DMSO and mannitol on the etching of GNRs were also carried out as shown in Figure 3B (curves c and d). The results indicated that the etching was almost not affected by the addition of DMSO or mannitol. Therefore, we could conclude that •OH was not the etching chemical, or the generated •OH was transformed into HO2• quickly in the system. NaN3, a scavenger for singlet oxygen (1O2), was also added to the systems. No inhibition effect was observed (curve c in Figure 3C), suggesting that no 1O2 was involved in the etching reaction.46 It is well-known that SOD can catalyze the dismutation reaction of O2•− to give ground state molecular oxygen and H2O2.52 Figure 3D shows the inhibition effect of SOD on the etching of GNRs (curve c). The LSPR absorption peak shifted much less than that without addition of SOD, indicating the etching was inhibited considerably. The result demonstrated that GNRs were possibly etched by O2•− in the presence of SCN−.

Figure 1. TEM images and extinction spectra of GNRs before (A, a) and after incubation with 0 (B, b) and 0.1 μM Co2+ (C, c) for 7 min in borate buffer solution containing 2.5 mM of HCO3−, 0.75 mM of SCN− and 2.0 mM of H2O2. •

OH + H 2O2 → HO2• + H 2O

Co3 + + H 2O2 → Co2 + + HO2• + H+

HO2• → O2•− + H+ O2•− +• OH → 1O2 + OH−

Figure 2. Length (I), width (II), and aspect ratio (III) distributions of CTAB-stabilized GNRs before (a) and after incubation with 0 (b) and 0.1 μM Co2+ (c) for 7 min in borate buffer solution containing 2.5 mM of HCO3−, 0.75 mM of SCN− and 2.0 mM of H2O2. 645

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Figure 3. Extinction spectra of GNRs in borate buffer solution containing different chemicals after incubation at 95 °C for 7 min. The “blank” in A, B, C, and D represent borate buffer solution containing 2.5 mM of HCO3−, 0.75 mM of SCN−, and 2.0 mM of H2O2.

Since GNRs were etched by O2•−, we concluded that other chemical reaction which could produce O2•− should also trigger the etching of GNRs. To confirm this speculation, hydroxylamine hydrochloride was added to the buffer solution containing GNRs in the absence of Co2+ and H2O2. The GNRs solution was incubated at 95 °C for 15 min. The LSPR peak shifted to short wavelength significantly (Figure 4). The

NH 2O• + O2 → NO− + O2•− + 2H+

Mn − 1 + O2 → O2•− + Mn +

On the basis of the above experimental results, we concluded that the generated O2•− in the course of Fenton-like reaction was responsible for the etching of GNRs. The practical redox potentials (φ, vs NHE) of Au(I)/Au(0) and O2•−/H2O2 at pH 9.0 and the possible etching reaction are described as follows:49,54 Au(SCN)2− + e− → Au + 2SCN−

φ = 0.69 V

O2•− + e− + 2H 2O → H 2O2 + 2OH−

φ = 0.77 V

Au + O2•− + 2SCN− + 2H 2O → Au(SCN)2− + H 2O2 + 2OH−

Sensitivity and Specificity of the Visual Detection for Co2+. Taking advantage of the outstanding catalytic property of Co2+ on the etching of GNRs, we attempted to use the property to develop a new visual method for sensing of Co2+. Figure 5 shows responses of the proposed sensors to different concentrations of Co2+ under optimal conditions (Figure S1− S7, Supporting Information). The LSPR peak of GNRs shifted to short wavelength gradually with the increase in Co2+ concentrations in the range of 1 to 200 nM (Figure 5A). A linear relationship between the peak-shift and Co2+ concentrations in the range of 5 to 100 nM was obtained (Figure 5B). When the concentration exceed 200 nM, the spectra of GNRs almost remained constant, indicating that the etching rate no longer increased. Although more O2•− is produced when the concentration of Co2+ exceeds 200 nM theoretically, we speculate that the etching reaction rate is just controlled by

Figure 4. Effect of hydroxylamine hydrochloride on the LSPR absorption spectrum after incubation in borate buffer solution containing SCN−.

peak shift is attributed to the etching of GNRs by O2•‑ generated in the process of auto-oxidation of hydroxylamine by dissolved oxygen, as shown in the following reactions, where the Mn+ may be Aun+ in the solution:53 NH 2OH + Mn + → NH 2O• + H+ + M(n − 1) + 646

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Figure 5. UV extinction spectra (A), LSPR peak shift (B), and color change (C) of GNRs after incubation with different concentrations of Co2+ at 95 °C for 7 min, respectively. Other conditions: H2O2 2.0 mM; SCN‑ 0.75 mM; carbonate 2.5 mM.

Table 1. A Comparison of the Performance of Different Analytical Methods for Detection of Co2+ method

technique or material

linear range

LOD

ref.

AAS ICPMS potentiometry chemiluminescence fluorescence fluorescence Fluorescence Fluorescence SERS colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry colorimetry

cloud point extraction using methyl-2-pyridylketone oxime combination with flow-injection online matrix separation p-(4-n-butylphenylazo)calix[4]arene contained polymer membranes electrode catlon-exchange liquld chromatography using lumlnol chemllumlnescence organic probes reaction-based fluorescent probe ZnO-based imine-linked coupled biocompatible chemosensor thioglycolicacid-capped CuInS2/ZnS quantum dots dithiocarbamate anchored terpyridine/Ag nanoparticles spiropyran−amide−dipicolylamine linkage coumarin-conjugated thiocarbanohydrazone organic chemosensor leaf-like poly(p-phenylenediamine) microcrystal carboxyl-functionalized CdS QDs bifunctionalized silver nanoparticles dopamine dithiocarbamate-functionalized silver nanoparticles thioglycolic acid-functionalized CTAB-modified gold nanoparticles hioglycolic acid-functionalized CTAB-modified gold nanoparticles Calix[4]arene-functionalized gold nanoparticles peptide-modified gold nanoparticles S2O32−-stabilized gold nanoparticles CTAB-stabilized gold nanorods

0.17−340 μM 9.2 μM to 0.1 M 0−25 nM 0.3012−90.36 μM 1.0−100 nM 1.0−10 μM

36 nM 20 nM 4.0 μM 5 pM ∼5.0 μM μM levels 0.4 nM 0.16 μM 1.0 nM 1.0 μM 1.0 μM 1.28 μM 0.35 μM 3.9 μM 7.0 μM 14 μM 0.3 μM 0.5 nM 1.0 nM 2.0 μM 40 nM 1.0 nM

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 This work

the GNRs’ surface at the moment, instead of O2•−. And as a result, only part of O2•− can react with GNRs within a limited time, and the extra O2•− transformed to other chemicals instantaneously, 1O2 for example. The detection limit was calculated to be 1.0 nM according to the S/N = 3 rule, which is lower than that of most visual sensors and is comparable with the results obtained by AAS, ICPMS, surface-enhanced Raman spectroscopy (SERS), fluorescence, etc. (Table 1).19−39 Although some of gold nanoparticle-aggregation-based visual sensors are more sensitive than the proposed method,36−39 they require tedious procedures for the modification of nanoparticles and easily suffer from false positive results due to the

0.5−100 μM 8.5−237.3 μM 5−100 μM 1.0-15 mM 0−5 nM 0.06−22 μM 2−10 μM 0.1−0.7 μM 5−100 nM

autoaggregation of nanoparticles. The digital photo (Figure 5C) shows that the color of GNRs’ solutions changed from blue to red with the increase in Co2+ concentration. The color change as induced by 40 nM Co2+ can be easily observed by the naked eye. To our best knowledge, the visual detection limit, 40 nM, is among the lowest reported for the detection of Co2+. The specificity of the sensor toward Co2+ was evaluated by examining the extinction spectra of GNRs in the presence of various other ions. The addition of 0.1 μM Co2+ caused significant changes in both the color and absorption spectrum, while no obvious change could be observed for other ions at different concentrations as shown in Figure 6. This result 647

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GNRs. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. Fax/phone: 086-535-2109133. *E-mail [email protected]. Fax/phone: 086-535-2109130. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financially supported by the Department of Science and Technology of Sh andong Province (BS2009DX006), NSFC (No. 21275158), the Project-sponsored by SRF for ROCS, and the 100 Talents Program of the CAS.

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Figure 6. LSPR peak shift of GNRs responding to different ions at concentration of 10 μM (1.0 μM for Mn2+, Ag+ and Ni2+, 0.5 μM for Hg2+ and Cu2+, and 0.1 μM Co2+).

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indicates the sensor displays excellent selectivity toward Co2+. Positive interference caused by more concentration of Hg2+ resulted from the formation of amalgam onto GNRs, which drove GNRs to form spherical nanoparticles.55 Negative interference coming from the high concentration of Cu2+ was attributed to the decomposition of H2O2 triggered by Cu2+.17,54 Detection of Co2+ in Drinking Water. The applicability of the proposed sensor for Co2+ was evaluated by comparing the detection results with the spiked concentration of Co2+ in drinking water. The detection results were consistent with the spiked concentrations (Table 2), indicating the sensor could be a practical tool for the rapid on-site monitoring of Co2+. Table 2. Recovery of Spiked Co2+ in Drinking Watera

a

sample

spiked/nM

detected/nM

RSD/%

recovery/%

1 2 3

5.0 20.0 50.0

5.1 19.2 47.3

16.0 8.3 6.7

102.0 96.0 94.6

The RSD of each sample was obtained by three measurements.

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CONCLUSION In conclusion, we have proposed a Fenton-like reactionmediated etching of GNRs for the sensitive visual sensing of Co2+. We conclude that other target analytes that can trigger the generation of high active oxidant (O2•− for example) at certain condition can also be detected by this strategy. As to this paper, the developed sensor displayed excellent on-site detection performance for Co2+, such as low visual detection limit (40 nM) and short response time (7 min). In addition, compared to nanoparticle-aggregation-based methods, this method also can avoid false positive results coming from the autoaggregation of nanoparticles. Possessing these merits, the sensor promises to be a practical tool for on-site monitoring of Co2+.

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dx.doi.org/10.1021/la504256c | Langmuir 2015, 31, 643−650