Detection of label-free H2O2 based on sensitive Au nanorods as sensor

Colloids and Surfaces B: Biointerfaces 102 (2013) 327–330

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Detection of label-free H2 O2 based on sensitive Au nanorods as sensor Guiye Shan ∗ , Shujing Zheng, Shaopeng Chen, Yanwei Chen, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key laboratory for UV light-emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, PR China

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Article history: Received 28 February 2012 Received in revised form 18 July 2012 Accepted 20 July 2012 Available online 14 August 2012 Keywords: Au Nanorod H2 O2 LSPR Sensor

a b s t r a c t A rapid, reproducible, cost-effective approaches for the detection of hydrogen peroxide has been developed based on the change of localized surface plasmon resonance (LSPR) peak of Au nanorods (NRs). Au NRs were prepared by silver ion-assisted seed-mediated method, which are characterized by UV-vis spectroscopy and transmission electron microscopy. The longitudinal plasmon band of Au nanorods is highly sensitive to their aspect ratios so that LSPR peak of Au NRs was shift with change of their aspect ratios. Hydrogen peroxide (H2 O2 ) with high oxidation potential can decompose Au NRs. As a result, Au NRs can be shortened through an oxidation reaction by H2 O2 . After shortening Au NRs, the LSPR peaks show blue shift. The LSPR peak of Au NRs displays the dependence of spectral shift on concentration of H2 O2 . It provides a more simple and sensitive method for detecting H2 O2 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction H2 O2 has been involved in many chemical, biological, pharmaceutical, clinical, environmental, and food processes. The analytical determination of H2 O2 is an important topic that has relevance pertaining to environmental, pharmaceutical, clinical and industrial research [1–7]. Many analytical techniques of H2 O2 based on different mechanisms have been developed including chemiluminescence, fluorometry, colorimetry, electrochemical approaches [8]. These methods have showed sensitivity, rapidity for the detection of H2 O2 . However, expensive reagents, timeconsuming and complex operating procedures limit their use. Therefore, it is still a challenge to develop a novel approach with high sensitivity, low cost, and simple operation to detect the H2 O2 . Au NRs are promising materials for biomedical sensor due to their tunable optical properties. Au NRs exhibit two different plasmonic modes including transverse and longitudinal modes [9–14]. For the longitudinal mode, plasmon oscillation is parallel to the major axis of Au NRs so that the longitudinal resonance wavelength increases with increasing aspect ratio. Au NRs show potential applications for chemical and biological sensing by the change of aspect ratio. H2 O2 is a powerful oxidizing agent. The standard potential is dependent on the pH value of the solution. Mallouk [15] reported that electrokinetics in PtAu nanorod was explored based on the decomposition of H2 O2 . H2 O2 show high electrochemical potential than Au NRs [15,16]. Therefore, H2 O2

∗ Corresponding author. Tel.: +86 431 85608803; fax: +86 431 85608803. E-mail address: [email protected] (G. Shan). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.07.041

should affect the LSPR peaks of Au NRs by change their aspect ratio, which can be applied as an alternative method for detecting H2 O2 . In the present work, we present a new approach to detect H2 O2 based on Au NRs. The change of aspect ratio in Au NRs determined the change of LSPR peaks. The variation of LSPR peaks of Au NRs was quantitatively correlated with different concentration of H2 O2 . It provides a simple method to detect the concentration of H2 O2 .

2. Experimental 2.1. The preparation of Au NRs Au NRs were prepared by silver ion-assisted seed-mediated method. Typically, the seed solution was prepared by the addition of HAuCl4 (0.01 M, 0.25 mL) into cetyltrimethylammonium bromide (CTAB, 0.1 M, 10 mL) in a 15-mL plastic tube with gentle mixing. A freshly prepared, ice-cold NaBH4 solution (0.01 M, 0.6 mL) was then injected quickly into the mixture solution, followed by rapid inversion for 2 min. The seed solution was kept at room temperature for at least 2 h before use. To grow Au NRs, HAuCl4 (0.01 M, 2.0 mL) and AgNO3 (0.01 M, 0.4 mL) were mixed with CTAB (0.1 M, 40 mL) in a 50-mL plastic tube. HCl (1.0 M, 0.8 mL) was then added to adjust the pH of the solution to 1–2, followed by the addition of ascorbic acid (0.1 M, 0.32 mL). Finally, the seed solution (0.096 mL) was injected into the growth solution. The solution was gently mixed for 10 s and left undisturbed at room temperature for at least 6 h before use.

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2.2. Fabrication of H2 O2 sensors

relation with dielectric constant of metal (εm ) and aspect ratio (b/a) as equation:

The detection of H2 O2 was carried out by adding an appropriate volume of H2 O2 into the starting Au NRs solution for two hours. For monitoring of LSPR optical characteristics, H2 O2 solutions (0.5 mM) diluted with 50 mM phosphate buffer (pH 7.2) were introduced into Au NRs solution and its optical characteristics were evaluated by UV–vis absorption spectrum. 2.3. 3.Instrumentation The initial micrographs were taken with Hitachi H-8100IV transmission electron microscope operating at 200 kV (Japan). Samples for TEM were prepared by casting one drop of the cluster ˚ formvar film solution onto a standard carbon-coated (200–300 A) on copper grid (230 meshes). The UV–vis–NIR spectra were taken with a Lambda 950 spectrophotometer (Perkin-Elmer, Waltham, MA). 3. Results and discussion 3.1. The effect of H2 O2 on LSPR optical characteristics of Au NRs Starting Au NRs possess two plasmon bands including a shorter wavelength band at 520 nm originating from the transverse plasmon oscillation and a longer wavelength band at 760 nm originating from the longitudinal plasmon oscillation. After addition of 1.8 mM H2 O2 into Au NRs solution, the pink color from original Au NRs was drastically changed to yellow color. The color change from pink to yellow should be attributed to the effect of H2 O2 on the LSPR optical absorption. To detect the effect of H2 O2 on the LSPR peak position of Au NRs, different concentration of H2 O2 including 4.5 ␮M, 9 ␮M, 0.45 mM, 0.9 mM and 1.8 mM were added into the same Au NRs solution for two hours. The extinction spectra of solution were obtained as shown in Fig. 1. The LSPR and transverse plasmon peak of the starting Au NRs were located at 750 nm and 520 nm, respectively. After the addition of H2 O2 , the LSPR peaks occur to blue-shifted. The results reveal that the shift of LSPR for mixtures is large upon increasing the concentration of H2 O2 . During this process, the corresponding LSPR peaks were decreased in intensity, but transverse surface plasmon resonance (TSPR) peaks shift little. With increasing the amount of H2 O2 , the LSPR peaks merge together with TSPR peaks. The blue-shifts of LSPR peaks resulted from the change in the frequency of longitudinal plasmon oscillation. The LSPR absorption peak shows the following 0.40 b 0.35

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Wavelength(nm) Fig. 1. Extinction spectra of Au NRs before and after treatment with different amount of H2 O2 including 4.5 ␮M, 9 ␮M, 0.45 mM, 0.9 mM and 1.8 mM.

max = 33.34εm

b − 46.31εm + 472.31 a

The variation of aspect ratio (b/a) of Au NRs resulted in the change in the frequency of longitudinal plasmon oscillation. With decreasing aspect ratio, the corresponding frequency of longitudinal plasmon oscillation increased and the exaction spectra show blue-shift. Therefore, The shift of LSPR peak and merging of the two plasmon peaks indicate that the aspect ratio of Au NRs was decreased, which resulted in the transformation of nanorods into nanospheres. Furthermore, we used the transmission electron microscopy (TEM) measurements to characterize the morphology properties of Au NRs in the absence and presence of H2 O2 as shown in Fig. 2. TEM imaging shows that original Au NRs have a good curve on the two side of long axis. The width of Au NRs corresponding to the LSPR peak at 760 nm is about 20 nm as shown in Fig. 2(a). After addition of H2 O2 , the long axis was shortened and the tip of Au NRs becomes sharpen to form bipyramid, which lead to the shift of LSPR peak to 660 nm. But the short axis still keeps about 20 nm as that of original Au NRs in Fig. 2(b). When the concentration of H2 O2 was increased to 1.8 mM, Au NRs were changed to Au nanospheres and show one exaction peaks at 580 nm as shown in Fig. 2(c). The results demonstrated that Au NRs had been reacted with H2 O2 along long axis direction, but not along short axis direction. For Au NRs, CTAB acts as a capping agent as it can selectively bind to {1 0 0} facets of Au NRs and the edges may be less protected with the surfactant compared to the lateral face. Thus, CTAB effectively block the oxidation along the lateral direction and only allow oxidation along the vertical axis, which led to the selective oxidation of Au NRs in the longitudinal direction. The results demonstrate that the short axis has not been oxidized for the coverage of surface ligand of CTAB but long axis has been oxidized due to high surface activation. So, Au NRs undergo a shape change from rods to nanosphere. During this process, Au NRs decrease in length and the diameter keeps constant. The TEM results also indicate that oxidation starts at the ends of the Au NRs. It is corresponding to the results from absorption spectra that the LSPR of Au NRs shifts toward to blue with addition of more H2 O2 . 3.2. The effect of pH on etching Au NRs with H2 O2 The oxidation rate of Au NRs is controlled by the acid concentration. As known, the electrochemical potential of H2 O2 is dependent on the pH of the solution so that the acid concentration may control the oxidation rate of Au NRs. To demonstrate the effect of acid concentration on oxidation rate, we investigated the effect of pH on the LSPR peak of Au NRs in this reaction. The concentration of chosen H2 O2 is 0.9 mM. The pH-dependent LSPR change in this work was shown in Fig. 3. The effect of pH on the LSPR change was studied for two reactions with different pH 3(A) and pH 7(B). Both reactions took place in a similar time scale (1 h). With lower pH at 3, LSPR peaks of Au NRs show quickly blue-shift from 760 nm to 610 nm. When the pH is 7 in solution, LSPR peaks of Au NRs exhibit little shift as Fig. 3(B). The results indicated pH in solution has great effect on oxidation rate of H2 O2 . It is known that the oxidation potential of H2 O2 can be tuned by changing the pH, which affects the reactivity of Au NRs. To quantify the oxidation rate of H2 O2 at different pH value, the LSPR wavelength as a function of time was plotted at pH 3, pH 7 and pH 9.5 as shown in Fig. 4. Three curves (a, b, c) were obtained at the same time scale. The resulting slope was used to characterize the oxidation rate of Au NRs under different pH. The results show that the oxidation rate increased on the condition of deceasing pH in the solution. The faster oxidation of Au NRs obtained is due to the enhancement of the oxidizing capability

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Fig. 2. TEM images of Au NRs before (a) and after addition of different concentration of H2 O2 including 0.45 mM (b) and 1.8 mM (c) and their corresponding extinction spectra.

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Fig. 3. The LSPR dependence of Au NRs on different pH (A: pH 3, B: pH 7) with H2 O2 etching at different time intervals from 0 to 1 h.

of H2 O2 . The oxidation rate of Au NRs cannot only be changed by varying the amount of H2 O2 , but also pH of the solution. The comparison of pH-sensitive reaction curve indicated that pH in solution has great effect on the reaction ratio.

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3.3. The detection of H2 O2 based on Au NRs

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The aim of this study is to develop a simple, label-free, sensitive, and selective method for the detection of H2 O2 by Au NRs. The pH of the solution has a great effect on the LSPR of Au NRs in presence of H2 O2 . pH 3 was chosen for the detection of H2 O2 because lower pH in solution increase reaction ratio. Fig. 5 shows a sequence of extinction spectra taken as function of the reaction time right after the addition of H2 O2 treatment to determine the best reaction time for the assay on the condition of pH 3. As shown in Fig. 5, a sequence of absorption spectra was taken as a function of the reaction time after the addition of H2 O2 from a to f. The LSPR peaks of Au NRs exhibited different reactive rate under different amount of H2 O2 . The results indicated that small amount

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Time (hour) Fig. 4. The reaction ratio between Au NRs and H2 O2 at different pH including (a), pH 9.5 (n = 5), (b) pH 7 (n = 5), (c) pH 3 (n = 5). Error bars: ±SD.

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concentrations of H2 O2 . The detection limit curve has been plotted based on LSPR peak as a function of H2 O2 concentration range from 0.045 ␮M to 1.8 mM as shown in Fig. 6. The results indicated that the LSPR peaks changes linearly with the concentration of H2 O2 . In this experiment, a direct, quantitative detection of H2 O2 was provided. The LSPR-shift of Au NRs is highly sensitive to the concentration of H2 O2 . These results indicate that Au NRs exhibits a detection limit for H2 O2 as low as 0.045 ␮M. So they provide a quantitative measurement of H2 O2 concentration.

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Time (hour) Fig. 5. The dependence of time on the change of LSPR peaks with different concentration of H2 O2 including (a) 0.009 ␮M (n = 5), (b) 0.45 ␮M (n = 5), (c) 0.09 mM (n = 5), (d) 0.45 mM (n = 5), (e) 0.9 mM (n = 5), (f) 1.8 mM (n = 5). Error bars: ±SD.

We have developed a facile method for detecting the concentration of H2 O2 based on the change of LSPR of Au NRs. H2 O2 can etch Au NRs due to higher standard redox potential. By TEM analysis, the morphology of Au NRs shows obvious varition at long axis direction, but less varition at short axis direction. Furthermore, the absorption spectra shows that various ratio of LSPR peaks is proportional to the concentration of H2 O2 , which suggested that Au NRs can potentially serve as a new sensor for the detection of H2 O2 .

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Acknowledgments 700

This work was supported by the National Natural Science Foundation of China, (No. 11174047), and the Cultivation Fund of the Key Scientific, Technical Innovation Project, Ministry of Education of China (No. 70401F) and Science Foundation for Yong Teachers of Northeast Normal University.

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Concentration (mM) Fig. 6. The responses of Au NRs to various concentration of added H2 O2 (n = 5). Spectra were acquired in pH 3 at 25 ◦ C after incubation of the probe with H2 O2 for 30 min. Error bars: ±SD.

H2 O2 under 0.45 ␮M led to little change of Au NRs. However, the concentration of H2 O2 above 0.45 mM can significantly induced the change of Au NRs. After Au NRs solution was incubated in 1.8 mM H2 O2 for 10 min, the color of Au NRs solution was changed from pink to yellow and the LSPR peaks show blue shift. The reason for that is Au NRs was exposed to H2 O2 . H2 O2 may oxidize Au NRs and this property results in direct determination of the concentration of the analytes. By measuring the blue shift rate of absorption spectra, the detection of H2 O2 concentration may be accomplished. To evaluate whether Au NRs is capable of measuring H2 O2 concentration quantitatively, we performed LSPR-shift measurements at different

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