Au Nanospheres@Ag Nanorods for Wide Linear Range Colorimetric

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Au Nanospheres@Ag Nanorods for Wide Linear Range Colorimetric Determination of Hypochlorite Xinxin Li, Xiang Lin, Shuang Lin, Xichen Sun, Di Gao, Benkang Liu, Haiyan Zhao, Jing Zhang, Shu-Lin Cong, and Li Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00475 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Nano Materials

Au Nanospheres@Ag Nanorods for Wide Linear Range Colorimetric Determination of Hypochlorite Xinxin Li,†,‡ Xiang Lin,*,† Shuang Lin,§ Xichen Sun,† Di Gao,† Benkang Liu,† Haiyan Zhao,† Jing Zhang,† Shulin Cong,‡ and Li Wang*,† †School

of Physics and Materials Engineering, Dalian Minzu University, Dalian 116600, P. R. China of Physics, Dalian University of Technology, Dalian 116024, P. R. China §National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin, 150080, P. R. China *Corresponding author: Xiang Lin, [email protected]; Li Wang, [email protected] ‡School

ABSTRACT: In this study, we prepared the Au nanospheres (NSs)@Ag nanorods (NRs) for colorimetric monitoring of hypochlorite (ClO-). The sensing strategy is based on the redox reaction between Ag and ClO-, which give rise to the decrease in aspect ratio and a more than 180 nm blue-shift of the longitudinal localized surface plasmon resonance (LSPR) peak of Au NSs@Ag NRs. Meanwhile, a visible color change from yellow to green, blue, purple and red was observed, which could be utilized for the qualitative determination of ClO- by the naked eyes. Moreover, the longitudinal LSPR peak showed a linear correlation with the length of Au NSs@Ag NRs, which agreed with the simulation results conducted by the FDTD method. Additionally, a good linear relationship between the blue-shift of the longitudinal LSPR peak and the concentration of ClO- over the range of 0.5-30 μM indicated that the Au NSs@Ag NRs could be used for quantitative detection of ClO-. And the limit of detection was calculated to be 0.24 μM. Most notably, this ClO- sensor exhibited good selectivity and excellent sensitivity. Finally, the proposed method was employed to detect ClO- in tap water and obtained satisfying results. Therefore, the proposed colorimetric assay provided a promising platform for determination of ClO- in real water samples.

KEYWORDS: Colorimetric sensing, Au nanospheres@Ag nanorods (Au NSs@Ag NRs), Hypochlorite, Etching, Localized surface plasmon resonance (LSPR)

INTRODUCTION Free chlorine has been widely employed in our daily life as both bleaching agent and oxidant, such as household bleach and disinfection of drinking water and swimming pool water. The sum of dissolved chlorine gas (Cl2), hypochlorous acid (HClO) and hypochlorite (ClO-) in water are referred to as free chlorine. In water treatment, the concentration of residual chlorine must be strictly controlled in the range of millimole to micromole,1,2 because low concentration of free chlorine cannot kill bacteria and viruses in water.3 However, a high level of residual chlorine is harmful to the health of humans and animals. Excessive intake of ClO- in organism can cause a number of medical issues, such as

cardiovascular disease, kidney disease, liver injury and cancer.4-6 Therefore, detection of ClO- in water is completely necessary. In recent years, many methods have developed for determination of ClO-, including electrochemistry,7 fluorescence8-10 and liquid chromatography.11 These methods are sensitive and selective, but suffer from some drawbacks such as the need of sophisticated instruments, time-consuming and tedious sample preparation procedures, which certainly limit their applications for on-site detection. Compared with the above approaches, colorimetric assay has attracted considerable attention owing to the advantages of high sensitivity, selectivity, cheapness, simplicity, fast response and can be read out by naked eyes. It is well known that noble metal nanomaterial (Au and Ag) are more suitable for ultrasensitive plasmon colorimetric sensors, owing to the localized surface plasmon resonance (LSPR) absorption from the visible to near-infrared region.12-20 Furthermore, the LSPR absorption is highly dependent on the size, shape, composition, dielectric environment and inter-particle distance.21-25 Compared with Au nanomaterial, Ag nanomaterial is more attractive in plasmon colorimetric sensing, owing to the high refractive index sensitivity in the visible and near-infrared regions and low-cost.26-28 For example, Kolekar et al. proposed a sensing strategy for direct measuring the concentration of ClO- based on the aggregation of Ag nanoparticles (NPs).29 After addition of ClO-, the nearby Ag NPs tended to coupling accompanied with the absorbance decreased and an obvious color change from yellowish brown to colorless. However, this method could also be caused by many factors, including PH, temperature, solvent, salt and charged molecules,30 resulting in a lower accuracy in terms of quantitative sensing of ClO-. As a typical example, Malaichamy et al. developed a simple and sensitive colorimetric assay for determination of ClO- based on the morphological transformation from triangular Ag nanoprisms to spherical particles.31 Unfortunately, their sharp edges were often rounded automatically when stored in aqueous solutions,32,33 which limited their practical application. Hence, the preparation of stable Ag nanomaterial for colorimetric monitoring of ClO- is of great significance. Compared with single Ag nanomaterial, bimetallic Au@Ag core-shell structures are more attractive in colorimetric sensing field, owing to their properties of narrow plasmon linewidths, chemical stability and multicolor change in the visible region.34-38 For instance, Chen’ group reported a colorimetric method for

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determination of Cu2+ based on the etching of Au@Ag nanorods (NRs) in the presence of S2O32-.39 By the addition of Cu2+, the thickness of Ag shell decreased, resulting in the longitudinal LSPR peak red-shift and the shift of wavelength were linearly related to the concentration of Cu2+ over the 3-1000 nM. Typically, Guo et al. produced the Au@Ag NRs for determination of benzoyl peroxide with the limit of detection (LOD) as low as 0.75 μM. The etching of Ag shell only caused 50 nm red-shift of the longitudinal LSPR peak of the Au@Ag NRs.40 More recently, Zhao et al. employed ClO- as the oxidation for etching Ag shell of Au nanobipyramids (NBs)@Ag NRs and achieved an ultralow detection limit of 22 nM with a linear range of 0.1-20 μM.41 These previous reports indicated that the anisotropic Au@Ag NRs have made great achievements in colorimetric sensing field. However, the maximum wavelength shift only reached 100 nm and the linear range was also narrow. On the other hand, firstly blue-shift and then red-shift of the longitudinal LSPR peak was occurred during the etching process. Herein, we reported the selective oxidation etching of the one-dimensional (1D) Au nanospheres (NSs)@Ag NRs by ClO- at room temperature. The length of Au NSs@Ag NRs decreased with the increasing concentration of ClO- due to the formation of AgCl between Ag and ClO-, resulting in blue-shift of the longitudinal LSPR peak (Δλmax) and a gradual decrease of the absorption intensity. Meanwhile, a multicolor change from yellow, to green, blue, purple and red was easily recognized by naked eyes. More importantly, the linear relationship between the longitudinal LSPR peak and the length of Au NSs@Ag NRs was further verified by FDTD simulations. Moreover, the linear relationship between the Δλmax and the concentration of ClO- (c) can be expressed as Δλmax=6.012c-2.078. The Δλmax was used for quantitative detection of ClO- with a linear range from 0.5 μM to 30 μM, which was broader than previously reported colorimetric method. Furthermore, the present approach for determination of ClO- in tap water had achieved satisfying results. The results indicated that the Au NSs@Ag NRs could be utilized as a good alternative for quantitative measuring ClO- in real samples.

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Figure 1 Schematic illustration of sensing mechanism for the detection of ClO- based on the etching of Au NSs@Ag NRs.

RESULT AND DISCUSSION A simple illustration of the sensing mechanism based on the Au NSs@Ag NRs is shown in Figure 1. In this work, we utilize the 1D Au NSs@Ag NRs to measure the concentration of ClO-. The experimental strategy involves four main steps: (1) synthesis of Au NBs by seed-mediated growth method, (2) preparation of Au NSs through the oxidative etching of Au NBs, (3) synthesis of Au NSs@Ag NRs with Ag overgrowth and (4) colorimetric detection of ClO- through the etching of Au NSs@Ag NRs. The redox reaction taking place between Au NBs and AuCl4- is based on the following equation (1) and the detection mechanism is based on the redox reaction between Ag part and ClO- via the equation (2).42,43 AuCl4- + 2Au0 + 2Cl - ↔3AuCl20

NaClO + 2Ag + 2Cl

―

+

(1)

+ 2H →2AgCl↓ + NaCl + H2O

(2)

Synthesis processes of Au NSs@Ag NRs In the present work, we have successfully prepared the Au NSs@Ag NRs via a synthetic pathway combined the seed-mediated growth method and oxidative etching technique.44-46 Initially, the Au NBs were prepared according to the seed-mediated growth method proposed by Liz-Marzán’s group.44 More notably, the color of the solution was dark brown, indicating the high purity of Au NBs formed. Therefore, no size selection or any other purification process was required. As shown in Figure 2(a) and Figure S1(a) (Supporting Information), the monodispersed Au NBs with excellent size and shape

Figure 2 TEM images of (a) Au NBs, (b) pentatwined Au NSs, (c) Au NSs@Ag NRs. (d) Extinction spectra and the photograph of Au NBs, pentatwined Au NSs and Au NSs@Ag NRs. uniformities have been obtained. Figure S2(a) and S2(b) showed that the Au NBs had an average length of 73.9±2.2 nm and diameter of 25.9±1.4 nm. Subsequently, HAuCl4 acted as etching agent and converted Au NBs into Au NSs before Ag overgrowth, which could increase the adjustable range of the extinction spectrum. It should be note that the prepared Au NSs had narrower size distributions, indicating the excellent uniformity of our samples (Figure S2(c) of Supporting Information). Finally, the pentatwined Au NSs were used as core to prepared 1D Au NSs@Ag NRs. In detail, the preparation of Au NSs@Ag NRs was realized through the reduction of AgNO3 by ascorbic acid in the presence of CTAC as the capping agent and then the Ag atom deposited on the surface of Au NSs. More interestingly, we found the directional overgrowth of Ag preferentially along the one end longitudinal axis of the core Au NSs, as shown in Figure 2(c). The asymmetric growth of Au NSs@Ag NRs may be controlled by the kinetics.47-49 The Au NSs@Ag NRs obtained through this approach were exhibited high uniformity in terms of both size and shape (length 100.8±2.5 nm and width 31.7±1.7 nm, see Figure S2(d) and S2(e) of the Supporting Information). The high uniformity morphology and narrow size distributions of Au


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Figure 3 TEM images of Au NSs@Ag NRs incubated with different concentrations of ClO- (a) 0 μM, (b) 10 μM, (c) 20 μM, (d) 30 μM, (e) 40 μM and (f) 50 μM, respectively. (g) Extinction spectra and photograph of Au NSs@Ag NRs incubated with various concentrations of ClO- (0 μM, 10 μM, 20 μM, 30 μM, 40 μM and 50 μM). (h) The shift of wavelength vs the concentration of ClO-. (i) The calibration curve between experimental longitudinal LSPR peak position and the length of Au NSs@Ag NRs. NSs@Ag NRs allowed us to evaluate the performance for sensing applications. In addition, the optical properties of Au NBs, pentatwined Au NSs and Au NSs@Ag NRs were examined by the UV/Vis-NIR absorption spectra, as shown in Figure 2(d). Figure 2(d) presented that the longitudinal LSPR peak of the bare Au NBs was located at 761 nm together with a transverse LSPR peak at 515 nm. After etching into Au NSs, only one plasmon resonance peak at 522 nm was observed and the intensity decreased significantly due to Au metal dissolution from Au NBs. After the deposition of Ag, the predominant plasmon band was red-shift to 885 nm, while a transverse plasmon band at 510 nm was also appeared. Simultaneously, the spectral evolution of the samples gave rise to vivid color changes from brown, orange and finally to yellow, as shown in the insets of Figure 2(d).

Sensing mechanism of ClO- induced the etching of Au NSs@Ag NRs

A simple and high sensitivity colorimetric assay for determination of ClO- was developed using the Au NSs@Ag NRs. The sensing mechanism is that ClO- can etch the Ag part to generate AgCl at room temperature, inducing the length of Au NSs@Ag NRs decreased and in turn the longitudinal LSPR peak blue-shift. Specially, the products needed to wash with high concentration of CTAC (100 mM) to remove the by-product of AgCl.50 To confirm the etching mechanism, the morphology and optical properties of Au NSs@Ag NRs incubated with different concentrations of ClO- were characterized by TEM and UV/Vis-NIR absorption spectra, as displayed in Figure 3. In a typical etching process, the as-prepared Au NSs@Ag NRs solution (10 mL) was added into ultrapure water (10 mL), followed by the addition of ClO- solution (20 mL) and stirring for 30 min at room temperature. The concentration of ClO- employed in our experiments in a range of 0-50 μM. The size of the samples was measured on the TEM images and the corresponding size distributions were illustrated in Figure S4 (Supporting Information). The average length/width of the Au NSs@Ag NRs



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were 100.8±2.5 nm/31.7±1.7 nm, 90.5±2.6 nm/31.6±1.8 nm, 81.2±2.6 nm/31.6±1.8 nm, 73.8±2.7 nm/31.5±1.9 nm, 63.4±2.8 nm/31.5±1.8 nm and 51.3±3.1 nm/31.5±1.8 nm, which were incubated with 0, 10, 20, 30, 40 and 50 μM of ClO- solution, respectively. As presented in Figure 3(a)-(f), the Au NSs@Ag NRs were gradually sculptured with the increasing concentration of ClO-, leading the length decreased and the width remain basically unchanged. It is worth mentioned that all the samples possessed highly uniform size and shape. To better understand the relationship between the Δλmax and the concentration of ClO-, the optical properties of the samples were performed by the UV/Vis-NIR spectroscopy. Figure 3(g) showed the extinction spectra of Au NSs@Ag NRs incubated with various concentrations of ClO- under same conditions. As the increasing concentration of ClO-, the longitudinal LSPR peak of Au NSs@Ag NRs blue-shifted from 740 nm to 697 nm, 652 nm, 622 nm, 578 nm and 539 nm. At the same time, the color of the solution changed from yellow to green, blue, purple and red, as shown in the insets of Figure 3(g). The narrow plasmon linewidths and excellent size uniformity of Au NSs@Ag NRs allowed us to investigate the relationship between Δλmax and the concentration of ClO- (c). The linear regression equation could be described as Δλmax= 3.98c+3.24 with a correlation coefficient (R2) of 0.997. In addition, the longitudinal LSPR peak showed a linear correlation with the length of Au NSs@Ag NRs could be regressed as λmax= 4.112d+322.2 (R2=0.996), where λmax and d were the longitudinal LSPR peak and the average length obtained in TEM image, respectively. Therefore, the etching effect of ClO-

only caused the decreased length of Au NSs@Ag NRs, which improved the accuracy for measuring the concentration of ClO-.

Figure 4 (a) Extinction spectra of Au NSs@Ag NRs simulated by FDTD. (b) Theoretically calculated longitudinal LSPR peak position vs the length of Au NSs@Ag NRs.

Figure 5 (a) Extinction spectra change of the Au NSs@Ag NRs with every 5 min after addition of ClO- (10 μM). (b) The change of longitudinal LSPR peak with increasing time (each 5 min).

FDTD simulation To further determine the linear correlation between the longitudinal LSPR peak position and the length of Au NSs@Ag NRs, the theoretical extinction spectra were simulated by using the FDTD method. The simulated model was shown in Figure S5 (Supporting Information) and the size of the Au NSs@Ag NRs was based on the statistical results measured from the TEM images. As depicted in Figure 4(a), the longitudinal LSPR band displayed a blue-shift from 743 nm to 541 nm with the length of Au NSs@Ag NRs decreased from 100.8 nm to 51.3 nm. It was clearly observed that the simulated extinction spectra agreed well with the experimental results. However, the experimental and theoretical results still had some differences due to the inevitable factors, such as surrounding medium, structural model, and size distributions.51 A plot of the longitudinal LSPR peak versus the length of the Au NSs@Ag NRs gave a straight line and the fitting curve can be expressed as λmax=4.116d+320.9, where λmax and d were the longitudinal LSPR peak and the average length obtained in TEM image, as shown in Figure 4(b). It is also consistent with the experiment results.

Analytical performance for colorimetric detection of ClOBefore analytical performance for ClO- detection, the UV/VisNIR absorption spectra were carried out to monitor the etching processes of Au NSs@Ag NRs over time. In detail, the as-prepared Au NSs@Ag NRs solution (0.5 mL) was added into


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Figure 6 (a) Extinction spectra of Au NSs@Ag NRs incubated with various concentrations of ClO-. (b) Plot of wavelength shift as a function of ClO- concentration. (c) The photograph of Au NSs@Ag NRs incubated with various concentrations of ClO-. The concentrations of ClO- employed in experiments in the range of 0.5-30 μM. ultrapure water (1 mL), followed by the addition of ClO- solution (1.5 mL) and thoroughly mixed by a vortex for 20 s at room temperature. Figure 5 showed the time-dependent extinction spectra and the longitudinal LSPR peak position of Au NSs@Ag NRs after addition of ClO- (10 μM). It is found that the longitudinal LSPR peak blue-shift and the intensity of extinction spectra decreased greatly within 10 min. The Δλ reached the maximum after 30 min. Similarly, the time-dependent extinction spectra of Au NSs@Ag NRs incubated with ClO- (20 μM) was displayed in Figure S5 (Supporting Information). The reaction also basically completed within 30 min. Therefore, 30 min was chosen as the optimal reaction time in the following experiments. In addition, we also studied that the aqueous medium pH-dependent correlation of the wavelength shift of Au NSs @Ag NRs with a fixed concentration of ClO- at 10 μM. As a result, the pH of the aqueous medium does have an effect on the wavelength shift of Au NSs@Ag NRs, as shown in Figure S7. Considering the stock ClO- solution stored in an alkaline solution, we first pretreated the aqueous medium with NaOH solution. Then the stock ClO- solution was diluted with the pretreated aqueous medium to prepare the spiked samples with pH of 13.15. This would ensure the same pH of the different concentrations of ClOsolution. To determine the sensitivity for colorimetric detection of ClO-, the extinction spectra of Au NSs@Ag NRs after introducing various concentrations of ClO- for 30 min were recorded, as demonstrated in Figure 6(a). The reaction condition had been detailed in the Supporting Information. As shown in Figure 6(a), the longitudinal LSPR peak of Au NSs@Ag NRs blue-shifted from 740 nm to 560 nm and the intensity decreased gradually when the concentration of ClO- increased from 0.5 μM to 30 μM. As depicted in Figure 6(b), the Δλmax was plotted against the ClOconcentration and the linear relationship was obtained with ClOconcentration in a range of 0.5-30 μM. Moreover, the linear relationship between the Δλmax and the concentration of ClO- (c) could be expressed as Δλmax=6.012c-2.078 (R2=0.999). The LOD

was calculated to be 0.24 μM using 3σ/k, where σ was the standard deviation of blank measurements and k was the slope of working curve. The corresponding photograph of Au NSs@Ag NRs incubated with various concentrations of ClO- was represented in Figure 6(c). It is clearly observed that the color of the Au NSs@Ag NRs solution changed from yellow to green, blue, purple and red with the increasing concentration of ClO-. The distinct multicolor changes at the concentration of 10 μM could be easily read out via the naked eyes. The results indicated that Au NSs@Ag NRs could be employed to quantitatively detect ClO- with excellent sensitivity. A comparison with other reported approaches for detection of ClO- was listed in Table 1. Obviously, our proposed method by using Au NSs@Ag NRs for measuring ClO- is more attractive owing to the following advantages. First, this method was simple and convenient, which improved the practical application for on-site detection. Second, a multiple color changes could be used for the qualitative determination of ClO- concentration by the naked eyes. Third, the proposed method exhibited a wide linear range compared with other methods. Forth, Au NSs@Ag NRs possessed the narrower plasmon linewidths, which improved the sensing accuracy. To sum up, all these merits made Au NSs@Ag NRs an ideal colorimetric detection probe.

Selectivity study of the sensing method Excellent selectivity was also regard as the key factor for colorimetric sensing of ClO-. Therefore, we next investigated the extinction spectra of Au NSs@Ag NRs in the presence of various interfering ions under same conditions, including Cl-, Br-, CO32-, HCO3-, SO42-, NO2-, CH3COO- (AC-), NO3- and PO43-. As shown in Figure 7, only ClO- induced the longitudinal LSPR peak blue-shift significantly and the intensity decreased greatly, while other anions caused negligible changes. To further verify the selectivity for the practical detection of ClO-, we prepared a sample mixture containing all of the other anions employed in this study. It was easily seen that this method for the detection of ClO-



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Table 1. Performance Comparison of Various Detection Methods for the Determination of ClOMethod

Probe type

LOD

Detection linear range

Reference

Fluorescence method Fluorescence method Electrochemistry Colorimetry Colorimetry Colorimetry

ZnO quantum dots NIR fluorescent probe (CY-FPA) Grapheme-like carton Ag nanoprisms Ag nanoprisms Au NSs@Ag NRs

0.041 μM 0.13 μM Not mentioned 0.25 nM 0.07 μM 0.24 μM

0.05-0.7 μM Not mentioned 0.2-0.8 mg/L 0.2-1.2 μM 0.1-20 μM 0.5-30 μM

9 8 7 31 52

This work

Table2. Determination of ClO- in Tap Water (n=3) Samples 1 2 3 4

Added concentration (μM)

Found concentration (μM)

0.5 1 5 20

Recovery (%)

0.49 0.93 5.21 18.71

could still work in the presence of other anions. And the insets of Figure 7(b) exhibited that the other anions led to no color change. All the experiment results indicated that this proposed colorimetric assay had a good selectivity for ClO-.

Application for colorimetric detection of ClO- in tap water In order to verify the practicality in real sample, the developed

98.0 93.0 104.2 93.6

RSD (%) 2.51 4.72 3.78 4.83

method was further applied to measuring ClO- in tap water of our school. In the analysis, the tap water was spiked with various known concentration of ClO- and measured with standard curve method. In detail, we firstly prepared the NaOH solution (0.01 M) with the collected tap water and then filtered with 0.22 μm filter before detection the concentration of ClO-. Next, the pretreating tap water was spiked with different concentration of ClO- (0.5 μM, 1 μM, 5 μM and 20 μM) and the recovery of ClO- was measured. The results were summarized in Table 2 and the corresponding extinction spectra were shown in Figure S8 (Supporting Information). From Table 2, we clearly observed that the recovery was between 93.0% and 104.2% and the RSD range from 2.51%-4.83%, which demonstrated the accuracy and reliability of the proposed colorimetric assay. It was revealed that the Au NSs@Ag NRs could be applied to detecting ClO- in real samples.

CONCLUSIONS In summary, a facile and efficient approach towards colorimetric determination of ClO- with ultra-sensitivity and wide linear range has been developed based on the chemical etching of Au NSs@Ag NRs at room temperature. The etching process of Au NSs@Ag NRs preferentially occurred along the longitudinal direction. In the meantime, the color of the solution changed from yellow to green, blue, purple and red with the increasing concentration of ClO-. The calibration curve could be described as Δλmax=6.012c-2.078 and the LOD was determined to be 0.24 μM. Therefore, the concentration of ClO- could be quantitatively detected by extinction spectra or rapidly distinguished by naked eyes. What’s more, this proposed approach for determination of ClO- possessed a high selectivity and achieved ideal results in real sample measurement. We believe that the excellent properties of Au NSs@Ag NRs will enable it a promising candidate in the colorimetric sensing field.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Figure 7 Selective of the proposed method. Extinction spectra (a), wavelength shift and the photograph (b) of the Au NSs@Ag NRs incubated with ClO- and other inorganic anions. Mix stands for the reaction solution containing all the anions except for ClO- at 200 μM each.

The work was supported by the National Natural Science Foundation of China (Grant No. 61805033 and 21501021), the Open Fund of State Key Laboratory of Molecular Reaction Dynamics, DICP, CAS (SKLMRD-K201812) and the Dalian


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High-level Talent Innovation Support Program (Project No. 2017RQ067).

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ACS Paragon Plus Environment

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Table of Contents Graphic

Wide linear range colorimetric detection of hypochlorite based on the etching Au NSs@Ag NRs.


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Figure 1 Schematic illustration of sensing mechanism for the detection of ClO- based on the etching of Au NSs@Ag NRs. 85x60mm (300 x 300 DPI)


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Figure 2 TEM images of (a) Au NBs, (b) pentatwined Au NSs, (c) Au NSs@Ag NRs. (d) Extinction spectra and the photograph of Au NBs, pentatwined Au NSs and Au NSs@Ag NRs. 85x76mm (300 x 300 DPI)



Figure 3 TEM images of Au NSs@Ag NRs incubated with different concentrations of ClO- (a) 0 μM, (b) 10 μM, (c) 20 μM, (d) 30 μM, (e) 40 μM and (f) 50 μM, respectively. (g) Extinction spectra and photograph of Au NSs@Ag NRs incubated with various concentrations of ClO- (0 μM, 10 μM, 20 μM, 30 μM, 40 μM and 50 μM). (h) The shift of wavelength vs the concentration of ClO-. (i) The calibration curve between experimental longitudinal LSPR peak position and the length of Au NSs@Ag NRs. 175x154mm (300 x 300 DPI)


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Figure 4 (a) Extinction spectra of Au NSs@Ag NRs simulated by FDTD. (b) Theoretically calculated longitudinal LSPR peak position vs the length of Au NSs@Ag NRs. 85x127mm (300 x 300 DPI)



Figure 5 (a) Extinction spectra change of the Au NSs@Ag NRs with every 5 min after addition of ClO- (10 μM). (b) The change of longitudinal LSPR peak with increasing time (each 5 min). 85x127mm (300 x 300 DPI)


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Figure 6 (a) Extinction spectra of Au NSs@Ag NRs incubated with various concentrations of ClO-. (b) Plot of wavelength shift as a function of ClO- concentration. (c) The photograph of Au NSs@Ag NRs incubated with various concentrations of ClO-. The concentrations of ClO- employed in experiments in the range of 0.5-30 μM. 175x93mm (300 x 300 DPI)



Figure 7 Selective of the proposed method. Extinction spectra (a), wavelength shift and the photograph (b) of the Au NSs@Ag NRs incubated with ClO- and other inorganic anions. Mix stands for the reaction solution containing all the anions except for ClO- at 200 μM each. 85x120mm (300 x 300 DPI)


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Au Nanospheres@Ag Nanorods for Wide Linear Range Colorimetric

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