Mercaptopyridine-Functionalized Au Nanoparticles for Fiber-Optic

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Mercaptopyridine-Functionalized Au Nanoparticles for Fiber-Optic Surface Plasmon Resonance Hg2+ Sensing huizhen yuan, Wei Ji, shuwen chu, qiang liu, siyu qian, jianye guang, jiabin wang, Xiuyou Han, Jean-François Masson, and Wei Peng ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01558 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Mercaptopyridine-Functionalized Au Nanoparticles for Fiber-Optic Surface Plasmon Resonance Hg2+ Sensing

Huizhen Yuan,† Wei Ji*,‡ Shuwen Chu,† Qiang Liu,† Siyu Qian,† Jianye Guang,† Jiabin Wang,† Xiuyou Han,† Jean-Francois Masson,§ Wei Peng,*,† College of Physics and Optoelectronics Engineering and ‡ School of Chemistry, Dalian University of Technology, Dalian,

†

116024, China Department of Chemistry, Université de Montréal, Montréal, Québec H3C 3J7, Canada

§

*Corresponding author: E-mail: [email protected], [email protected]

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ABSTRACT: As a highly toxic heavy metal ion, divalent mercuric ion (Hg2+) is one of the most widely diffused and hazardous environmental pollutants. In this paper, a simple, portable, and inexpensive fiberoptic sensor based on surface plasmon resonance (SPR) effect was developed for Hg2+ detection, which takes advantage of 4-mercaptopyridine (4-MPY) functionalized Au nanoparticles (Au NPs/4-MPY) as a signal amplification tag. Based on the coordination between Hg2+ and nitrogen in the pyridine moiety, we developed the sensor by self-assembling 4-MPY on Au film surfaces to capture Hg2+, and then introducing Au NPs/4-MPY to generate a plasmonic coupling structure with the configuration of nanoparticle-on-mirror. The coupling between localized SPR increased changes in SPR wavelength, which allowed highly sensitive Hg2+ sensing in aqueous solution. The sensor exhibited superior selectivity for Hg2+ detection compared with other common metal ions in water. The sensor’s Hg2+ detection limit is 8 nM under optimal conditions. Furthermore, we validated the sensor’s practicality for Hg2+ detection in tap water samples, and demonstrated its potential application for environmental water on-site monitoring.

KEYWORDS ： surface plasmon resonance, fiber-optic SPR, mercury, 4-mercaptopyridine, Au nanoparticles.

Heavy metals are one of the most important causes of environmental pollution.1 Excessive levels of heavy metals in the human body can lead to various diseases, which pose a great threat to human health.2 As a source of heavy metal pollution, mercury is a great threat for ecological systems due to its bioaccumulative effect, which can be enriched in the human body through the food chain.3,4 The most prevalent form of mercury contamination is water-soluble divalent mercuric ion (Hg2+). It is highly toxic even at low concentration, because Hg2+ permanently destroys liver, kidney, heart, and nervous system.5-7 So far, the methods for Hg2+ detection are mainly based on colorimetric,8 electrochemiluminescent immunoassay,9 and surface plasmon resonance biochemical analysis.10 The promising results achieved by these methods, but great efforts are still needed to develop novel approaches with low-cost, high sensitivity, and selectivity. Surface plasmon resonance (SPR) technique can effectively detect the changes of the refractive index (RI) of surrounding the medium near a thin metallic film. Thus, SPR-based sensors monitor the variation


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in shift or intensity of the resonant peak in response to external stimuli.11-13 SPR-based sensors has been received increasing attention due to its high sensitivity and non-destructive analysis.14,15 Some technological solutions have been proposed for Hg2+ detection. Lai’s group reported a polypyrrolemodified SPR device for Hg2+ sensing.16 Mahnaz M. Abdi et al. reported a SPR sensor to detect trace amounts of Hg2+ relied on a specific binding of chitosan with Hg2+.17 All these SPR sensors can be used for the Hg2+ detection, but most of them are proposed by using the commercial SPR devices, which limited their practical on-site applications. Indeed, the traditional commercial SPR device mostly adopts a prism-based structure, which is usually bulky and expensive. It is thus attractive to use a fiber-optic SPR sensor in an attempt to miniaturize the instrument. Compared with traditional SPR sensor, fiber-optic SPR sensor has some advantages, such as facility of integration, ease of manipulation, and excellent flexibility.18-20 Fiber-optic SPR sensor has been used to analyze solution concentration and refractive index (RI).21-23 The sensitivity of SPR sensor is proportional to the mass of the analyte that binds to the surface, and thus the analyte with small molecular weight lead to small, often undetectable change in SPR signal. Obviously, direct SPR detection of small molecules with low concentration is still challenging, not to mention the metal ions.24,25 LSPR is a result of the interactions between incident light and surface conduction electrons in metallic nanoparticles or nanostructures. When the incident photon frequency conforms to the oscillation frequency of conductive electrons, the resonance phenomenon occurs. And the localized electromagnetic field is extremely enhanced. Its frequency can be tuned by changing the size, geometry, material, and surrounding dielectrics of the nanoparticles.26-31 Au NPs with advantages of good electrical conductivity, large surface area, high chemical stability, and favorable biocompatibility, have been widely used for biological studies after surface modification.32 Recently, functionalized Au NPs were also used in SPR sensor for enhancing the sensitivity owing to their high RI and strong plasmon absorption.33-35 For example, anti-testosterone-modified and boronic acid-modified Au NPs have been demonstrated for the detection of testosterone36 and RNA,37 respectively. Besides, the nanoparticle-on-mirror structure can be constructed to detect small molecules, such as TNT38 and glucose.39 In these well-designed sensing structures, Au NPs can couple with the surface plasmon wave (SPW) associated with the Au film, leading to an enhancement in SPR response. In this way, the SPR signal can be enhanced more than 20 times due to the electromagnetic resonance coupling effect.40


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Herein, we propose a fiber-optical SPR sensor based on the 4-mercaptopyridine (4-MPY) modified Au NPs as the signal amplification tags. Figure 1 shows the sensing principle for Hg2+ detection. 4-MPY was self-assembled on the Au-coated sensors surface through the forming of Au-S bond, thus exposing the pyridinic nitrogen to solution. Notably, nitrogen of the pyridine moiety could coordinate with Hg2+ via multidentate N-bonding to form Hg(pyridine)2 complex.41 Accordingly, Hg2+ first coordinate with 4MPY assembled on Au film, and then Au NPs/4-MPY was captured by Hg2+, forming (Au film/4-MPY)Hg2+-(Au NPs/4-MPY) sandwich structure. The surface concentration of Au NPs/4-MPY attached on fiber-optic SPR sensor sensing region is dependent on the concentration of Hg2+. The coupling effect between Au film and Au NPs generates strong localized SPR, which shows a great influence on SPR wavelength. Based on this sensing principle, the sensitivity and selectivity of the present fiber-optic SPR sensor were carefully investigates. Furthermore, we also validated the practicality of this SPR sensor through Hg2+ detection in tap water samples.

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EXPERIMENTAL SECTION

Materials and reagents. Gold (III) chloride solution (HAuCl4) and 4-MPY were purchased from Sigma Aldrich (Shanghai, China). Sodium Citrate (99%) was purchased from Energy Chemical (Shanghai, China). Metal salts including HgCl2, FeCl2·4H2O, CuCl2·2H2O, KCl, NaCl, PbCl2, NiCl2·6H2O, BaCl2, CaCl2, FeCl3·6H2O, and ZnCl2 were purchased from the Sinopharm Chemical Reagent Co. (Shanghai, China). All of the reagents were of analytical grade and used without further purification. Apparatus Fabrication of Gold-Coated Optical Fibers. The RI and numerical aperture of optical fibers are 1.49 and 0.50, respectively. About 5 mm length of cladding was removed and rinsed by deionized water, ethanol, and acetone to remove any residual cladding. The fiber-optic sensing region was then coated with 2 nm thick chromium and 60 nm thick gold by ion beam sputtering deposition. The fiber-optic SPR equipment consists of halogen light source (Ocean Optics HL-2000), optical platform, fiber optic connectors, and a spectrophotometer (Ocean Optics HR4000). The optical resolution and detecting region of HR4000 spectrophotometer are 0.25 nm and 300-1200 nm, respectively. Hg2+ samples were injected by peristaltic pump at 0.13 mL/min flow rate. The SPR signal is processed through an analysis program written in LabVIEW.


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Figure 1. Operation principle of fiber-optic SPR sensor for Hg2+ detection. (a) Capture 4-MPY modification on the sensing surface. (b) Hg2+ on the sensing surface. (c) 4-MPY/Au NPs selectively bind with Hg2+ to amplify the signal.

Preparation of Au NPs/4-MPY and Sensing Surface. In this work, the Au NPs with a diameter of 17 nm was synthesized as previously reported.42 In a typical preparation process, a total of 10 μL of 4-MPY (1 mM) was added into 10 mL of Au NPs solution under vigorous stirring for 15 min. Then, the mixture was reacted without agitation at 4 °C for overnight. The sensing surface of fiber-optic SPR was prepared by self-assembly method. Freshly Au-coated optical fibers were immersed in 4-MPY (1 mM) ethanolic solution for 12 h to form a 4-MPY monolayer on the sensing region, and then rinsed successively with deionized water, ethanol and acetone. SPR Assay. A stock solution of HgCl2 solution (50 μM) was used to prepare standard solutions in the Hg2+ concentration range of 0.008–30 μM. The 4-MPY modified SPR sensing surface was exposed to metal ion solution for 15 min and then incubated with the Au NPs/4-MPY colloidal solution for another 30 min. To investigate the selectivity, different metal ions (concentrations were 100 μM), including Fe2+, Cu2+, K+, Na+, Pb2+, Ni2+, Ba2+, Ca2+, Zn2+, and Fe3+ were added into the sensing solution individually and the differences in the SPR wavelength shift were recorded.

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RESULTS AND DISCUSSION


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Figure 2. FEA (Finite Element Analysis) of the SPR sensor. (a) 4-MPY-Hg2+-4-MPY on the SPR sensors surface; (b) The electric field one Au NPs (Spatial electric field (|EX|) distribution for Au NPs on SPR), SPR angle = 80° (c) The SPR signal at the Au film with Au NPs.

Sensing Principle. In order to understand the sensing mechanism, finite element analysis (FEA) simulation was performed to demonstrate the SPR wavelength induced by the presence of Au NPs on sensor surface. The field distribution and SPR was calculated by COMSOL Multiphysics. It was considered that stronger SPR on the sandwich substrates were derived from both the electromagnetic coupling of the localized surface plasmons (LSPs) of the Au NPs and surface plasmon polaritons (SPPs) supported by the underlying metal film, and the lateral plasmon coupling between metal nanoparticles. The time-varying dipole induced in the junction at the Au NPs /mirror interface can launch SPPs that can propagate outward as circular waves along the interface and contribute to the electromagnetic fields resident in the NP-mirror hot spot of neighboring NPs when it crosses those points.43-45 The SPP phenomenon has been widely used in the design of various biosensing devices to detect refractive index changes in the vicinity of metal surfaces due to the presence of biomolecules. Au NPs with 17 nm and the space between Au NPs and SPR sensing film is fixed as 1.36 nm, which is corresponding to the size


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of 4-MPY-Hg2+-4-MPY (Figure S1). The sensing film with a fixed pitch (1000 nm) was performed to investigate the simulation of SPR response. The different numbers of Au NPs were added into fixed pitch to simulate of different Hg2+ concentrations. The simulation results for different number of Au NPs fixed on Au film sensor region were shown in Figure S2 and Figure 2(b). As a metal ion receptor, we select 4mercaptopyridine (MPY), which is known to strongly bind gold surfaces via its mercapto group and coordinate mercuric species via the nitrogen of the pyridine Hg2+ in water to form Hg(pyridine)2 complex. And Hg2+ first coordinate with 4-MPY assembled on Au film, and then Au NPs/4-MPY was captured by Hg2+, forming (Au film/4-MPY)-Hg2+-(Au NPs/4-MPY) sandwich structure. When Au NPs was introduced, the electric field intensity is increased by about 6 times than that of the pure sensing gold film structure. The electromagnetic field is localized around the nanoparticles. The change of the refractive index of the medium makes the SPR more sensitive, and the LSPR phenomenon occurs.46,47 The Au NPs on the sensing surface was increases, the local electromagnetic field also increasesed. The electric field is mostly confined in the gap between Au NPs and metal film. The field enhancements by two and three Au NPs are slightly increased as compared with that of one Au NPs. Increasing the amounts of Au NPs increases the SPR wavelength and peak depth due to the strong localized plasmonic coupling (Figure 2).48 These results demonstrated the feasibility of our proposed fiber-optic SPR principle for Hg2+ detection in theory.

Figure 3. Au NPs for surface analysis: (a) the UV–vis of Au NPs and Au NPs/4-MPY (pH 6.05); (b) SERS spectroscopy of Au NPs/4-MPY.

Au NPs for surface analysis. The UV–vis NIR adsorption spectra of the as-prepared Au NPs and Au NPs/4-MPY are given in Figure 3(a). Compared with Au NPs, a relatively weak absorption band at 650 nm was observed for Au NPs/4-MPY. It may be due to that the 4-MPY modified Au NPs shows a slightly


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aggregation, but such aggregation is no obvious interference in the following SPR sensing experiments (see Figure S3 for details). In order to simplify the preparation process of signal amplification tag, only 4-MPY molecule was used both as stabilizing and recognizing reagent. We consider that pyridine moiety of 4-MPY could provide sufficient stability to support Hg2+ sensing. We have tested the reproducibility of our present SPR sensor by using seven batches of Au NPs/4-MPY. The relative standard deviation of Hg2+ detection using seven batches of Au NPs/4-MPY is calculated to be 8.33% (Figure S5a), indicating that the slight aggregation of Au/4-MPY do not show obviously impact on the reproducibility of our present sensor. Surface-enhanced Raman scattering (SERS) spectroscopy was used to further demonstrate the absorption of 4-MPY on Au NPs surface. We use SERS and Raman spectra (SERS-ID, Real-Time Analyzers, Inc., USA ； the excitation wavelength of 785 nm (30 mW)) to test SERS spectroscopy. As shown in Figure 3(b), the bands below 900 cm-1 were assigned to the deformation and aromatic C-H out-of-plane bending modes. The bands at 1671, 1600, 1478, 1451, and 1395 are due to the C-C stretching mode. The strongly enhanced band at 1100 cm-1 ascribed to the C-S stretching mode, indicating that 4-MPY was adsorbed on the Au NPs surface through the formation of Au-S bond. These vibrational bands are coincided well with the previous reports,49,50 which further confirmed that 4-MPY was successfully modified on the surface of Au NPs.

Figure 4. TEM and SEM characterization of the Au NPs: (a) TEM micrograph of Au NPs/4-MPY, (b) SEM of Au film on fiberoptic SPR sensor surface, the fiber-optic SPR sensor surface exposing to Au NPs/4-MPY in the (c) absence and (d) presence of Hg2+.


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Optical fiber surface analysis. The morphology of the Au NPs/4-MPY was firstly characterized by high-magnification transmission electron microscopy (TEM). As shown in Figure 4(a) and Figure S4, Au NPs are spheres with the average diameters of 17 nm. Figure 4(b)-(d) show the field-emission scanning electron microscopy (SEM) images of Au film surface in fiber-optical SPR sensing region before and after exposure to Hg2+ or/and Au NPs/4-MPY. Figure 4(b) shows the SEM image of ion beam sputtered Au film on the sensing region. The modification of 4-MPY on Au film do not change the surface structure and there is no Au NPs/4-MPY was attached on the fiber surface in the absence of Hg2+ (Figure 4(c)). However, it was found that Au NPs/4-MPY tags randomly distributes on the 4-MPY modified Au film surface in the present of Hg2+ (Figure 4(d)). Therefore, the SEM results clearly reveal that the Hg2+-dependent nanoparticles-on-mirror structure can be achieved.

Figure 5. SPR spectra of 4-MPY -functionalized gold-coated fiber sensing region which incubated with different Hg2+ concentrations; (a) The SPR wavelength shift of different Hg2+ concentrations; (b) Calibration curve of the Hg2+ sensors.

Hg2+ Assay of SPR Sensor. To test the sensitivity of the present sensor for Hg2+ detection, the SPR wavelength were monitored upon addition of increasing Hg2+concentrations and addition of Au NPs /4MPY. As observed in Figure 5(a), the SPR wavelength increased with increasing the concentration of Hg2+. Figure 5(b) shows the SPR wavelength as a function of Hg2+ concentrations in the range of 0.008 to 30 μM. A good linear correlation with the Hg2+ concentration was exhibited in the concentration range from 8 to 100 nM. The limit of detection (LOD) of the present sensor for Hg2+ detection is calculated to be 3.34 nM at a signal-to-noise ratio of 3, indicating that our present sensor is sensitive enough to the assessment of pollution levels. The comparison between our present sensor and other published Hg2+ detection methods was summarized in Table S1. It can be seen that our present SPR sensor is comparable to the limits obtained by other optical or electrochemical sensor. The corresponding regression coefficient of the linear part was 0.98. The lowest concentration for quantification of Hg2+ is as low as 8


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nM, which is lower than the maximum allowable level (10-25 nM) of Hg2+ in drinking water permitted by the World Health Organization (WHO).51 Selectivity of the Hg2+. We further investigated the selectivity for Hg2+ detection in the presence of common metal ions in water, including Fe2+, Cu2+, K+, Na+, Pb2+, Ni2+, Ba2+, Ca2+, Zn2+, and Fe3+. The concentrations of interfering metal ions were 100 μM, which are five-folds more concentrated than the concentration of Hg2+ (20 μM). As shown in Figure 6, SPR wavelength remains unchanged after the addition of metal ions except Hg2+. The high selectivity of the proposed sensor is due to the specific coordination between Hg2+ and 4-MPY which forms stable 4-MPY–Hg2+– 4-MPY complexes.

Figure 6. SPR responses for dependent sandwich structure to different metal ions, relative to the signal of Hg2+. Hg2+ was at 20 μM and other interferents metal ions were tested using conditions at 100 μM.

Real sample analysis. In order to evaluate whether this SPR sensor is applicable to natural systems, the recoveries of Hg2+ in real water samples were investigated using our proposed SPR sensor. Water samples was taken from the tap in our laboratory. The tap water samples were spiked with Hg2+ in different concentrations (10, 50, and 100 nM). According to the SPR signal response, there is no detectable Hg2+ in the samples due to the low concentration of Hg2+ in tap water. However, Hg2+ was clearly detected in spiked water samples. As shown in Table 1, the recoveries of these samples were between 108% and 112%. These results indicate that this fiber-optic SPR sensor exhibits great potentials for practical Hg2+ detection in real water. Table 1. Hg2+ Recovery Tests in Tap Water by Fiber-Optic SPR.


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Recovery (%) Sample

Hg2+ added/nM

Hg2+ found/nM mean ± RSD, n = 3

1

10

11.1±0.9

111±9

2

50

53.8±3.9

108±8

3

100

112.1±6.6

112±7

*The standard deviation of each sample was obtained by three independent measurements.

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CONCLUSIONS

In summary, we have developed a sensitive and selective fiber-optic SPR sensor for Hg2+ detection by using Au/4-MPY as signal amplification tags. The signal amplification tag was synthesized by selfassembly of 4-MPY to Au NPs surface. In sensing process, a Hg2+ concentration-dependent particleson-mirror structure was constructed, the strong localized SPR existed between Au NPs and Au film produces an obvious change in SPR wavelength, which can be used for Hg2+ quantification detection. The Hg2+ sensor was calibrated with a series of aqueous solution with different mercury concentration. A wide linear concentration was observed from 8 to 100 nM. The lowest concentration detected for Hg2+ was found to be as low as 8 nM, which is lower than the maximum level in drinking water permitted by WHO. The results reveal that this chemical sensor has excellent selectivity for Hg2+ over other common metal ions in water. Furthermore, we applied the sensor for Hg2+ detection in spiked tap water samples with excellent recoveries (108-112%). This fiber-optic SPR sensor has the advantage of simplicity, high selectivity, high sensitivity, and cost-effective, which has great practical potentials for on-site Hg2+ detection in various environments.

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ASSOCIATED CONTENT

*S Supporting Information The molecular simulation for 4-MPY-Hg2+-4-MPY; The electric field with the different numbers of Au NPs; 4-MPY/Au NPs amplification signal stability; 4-MPY modified different batches of Au NPs for mercury ion detection. The particle size distribution histogram of Au NPs. SPR wavelength of Au sensor region modified with different concentration of 4-MPY. Au NPs modified with different volume of 4MPY. The limit of detection (LOD).


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AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (Wei Ji). *E-mail: [email protected] (Wei Peng). 

ORCID

Wei Ji: 0000-0001-6391-9768 Jean-François Masson: 0000-0002-0101-0468 Wei Peng: 0000-0002-0246-0698 Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (NSFCs. 61520106013,

61727016, 11474043, 21603021). We also acknowledge the support from the Fundamental Research Funds for the Central Universities (DUT18ZD215, DUT15RC(3)115).

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Mercaptopyridine-Functionalized Au Nanoparticles for Fiber-Optic

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