Optical fiber plasmonic sensor for the ultrasensitive detection of copper (II) ion based on trimetallic Au@AgPt core-shell nanospheres
Introduction
Copper is an essential trace element and a component of many important enzymes in living organisms, which can promote the absorption and utilization of iron and maintain the functions of the central nervous system. When the amount of copper ions (Cu2+) ingested in the human body is excessive, it is easy to over-burden the liver, gallbladder and other organs in the body, which makes the human body metabolism disordered, resulting in necrotic hepatitis and hemolytic anemia [1,2]. Therefore, the pollution-free, highly sensitivity and cost-effective designs for the recognition and trace analysis of Cu2+ are extremely important in environmental science and medical monitoring. Classic analytical methods for Cu2+ detection mainly include fluorescence spectrometry [[3], [4], [5]], colorimetry [[6], [7], [8]], electrochemistry [9,10], photothermal effect [11] and localized surface plasmon resonance [12,13]. Among all the above, localized surface plasmon resonance (LSPR) has been broadly widely recognized as one of the most cost-effective and convenient tools for rapid and ultra-sensitive Cu2+ detection.
LSPR is derived from the resonance of collective oscillation interaction between the incident electromagnetic wave and free electrons in the conduction band of metallic nanoparticles (NPs) at resonant frequency, which in turn leads to local electromagnetic (EM) fields amplification and an intense optical extinction band [14]. This enhanced electric field decays sharply with the increase of the distance away from the surface of the particle [15]. This phenomenon supplies an effective platform for measuring the changes of the refractive index (RI) near the surfaces of noble metal NPs. The RI change of the medium around the plasmonic NPs leads to a significant variation of the plasmon extinction band [16]. Generally, the wavelength change or the dip in transmission is approximately in proportional to the amount of analytes present in the surrounding environment [17].
LSPR sensor system can be designed for measuring a wide range of parameters including refractive index [18], hydrogen [19,20], DNA [21], protein [22] and heavy metal ions [23]. Liu et al. reported a LSPR sensor of Cu2+ detection based on the polyethylenimine (PEI)-capped gold nanorods, and the detection limit was determined as 2.4 × 10−7 mol/L [24]. Hu et al. developed a copper (II) ion sensor based on the amino-functionalized graphitic carbon encapsulated gold nanoparticles, achieving a detection limit of 10-8 mol/L [13]. However, the LSPR sensors described above are mostly conducted in aqueous solution, which are highly susceptible to the interference of the complex ion components from the surrounding environment, and lead to inaccurate analysis of experimental data and low detection sensitivity. In order to improve the specific sensitivity of the Cu2+, LSPR sensors mediated with novel plasmonic NPs based on optical fiber structure could be an ideal solution. By exploiting the enhanced light-matter interactions between plasmonic NPs and the evanescent field, LSPR optical fiber sensor is emerging as a vital tool for biochemical sensing [[25], [26], [27], [28], [29], [30]]. To achieve and enhance the evanescent field for higher sensitivity, optical fiber structure, such as cladding-removed fiber [25], side-polished/D-shaped fiber [26,27], tapered optical fiber [28], U-shaped fiber [29], and fiber Bragg grating [30] are usually further fabricated. Since these optical fibers inevitably require the complete removal of the cladding region or at least making it thinner by mechanical polishing or chemical dissolution, it greatly gives rise to complicated fabrication processes and inferior mechanical robustness. To get rid of this, hetero-core spliced optical fibers have been developed with promising characteristics for SPR sensing, such as strong evanescent field, miniature design, excellent biocompatibility and high stability [31,32]. The multimode-singlemode-multimode fiber (MMF-SMF-MMF) structure was firstly proposed by Iga et al., where the cladding layer of singlemode fiber (SMF) worked as a sensing surface, and a layer of silver film was coated on surface of the SMF for refractive index detection [31]. Especially, MMF-SMF-MMF structure spliced with two types of fibers with different core diameter has been proposed to deliberately leak the transmitted light wave into the cladding layer at the interfaces of hetero-core splicing. The localized surface plasmon wave is thereby excited due to the existence of an evanescent wave which in turn interact with deposited gold/silver NPs on the cladding surface.
This article proposed for the first time that trimetallic nanoparticles modified multimode-singlemode-multimode fiber (MMF-SMF-MMF) structure for the accurate Cu2+ detection. The positively charged polyethylenimine (PEI) was successively adsorbed onto the surface of Au@AgPt NSs modified MMF-SMF-MMF structure through electrostatic interaction. The Cu2+ measurement was achieved by monitoring the transmission spectrum while satisfying the resonance condition. This method not only achieve specific and long-distance Cu2+ monitoring, but also greatly improves the lower detection limit of Cu2+ down to 10−16 mol/L, which demonstrates promising applications in environmental and biological fields.
Section snippets
Reagents and characterization
Chloroauric acid tetrahydrate (HAuCl4·4H2O, 99.99 %), silver nitrate (AgNO3, 99.8 %), hexachloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.8 %), trisodium citrate dehydrate, citric acid monohydrate, ethanol absolute, hydrogen peroxide (H2O2, 30 %) and glycerol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, 95 %–98 %) was bought from Xinyang Chemical Reagent Factory (Henan, China). 3-aminopropyl trimethoxysilane (APTES, 99 %) was obtained from
Characterization of Au@AgPt NSs
The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images of the selected area were performed to characterize the elemental composition of Au@AgPt NSs in Fig. 2. As shown in Fig. 2(a), there is a significant difference in the brightness between the inner and outer regions of the Au@AgPt NSs, which confirms that the nanospheres with core-shell structure have been successfully obtained. The displacement reaction between Ag and H2PtCl6 at room temperature can
Conclusions
In summary, this article describes for the first time that trimetallic nanoparticles-modified optical fiber for the accurate measurement of Cu2+. An efficient approach for synthesizing citrate-stabilized Au@AgPt core-shell nanospheres (NSs) with a discontinuous AgPt alloy shell has been demonstrated. The formation of which is owing to the synergistic effect of galvanic replacement between Ag and H2PtCl6·6H2O and reduction reaction between Na3Ct and H2PtCl6·6H2O under 100 °C. The Au@AgPt NSs
CRediT authorship contribution statement
Qing Huang: Writing - review & editing. Wenjie Zhu: Writing - review & editing. Yong Wang: Writing - review & editing. Zhao Deng: Writing - review & editing. Zhi Li: Writing - review & editing. Jiankun Peng: Writing - review & editing. Dajuan Lyu: Writing - review & editing. Elfed Lewis: Writing - review & editing. Minghong Yang: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China, NSFC (Project Number: 61575151). Prof. Elfed Lewis is supported by the Science Foundation Ireland under the MaREI Centre research programme (Grant SFI/12/RC/2302_P2). We thank Dr. Zhao Deng and Dr. Ting-ting Luo for the help with TEM/HRTEM and STEM EDS characterizations, we also thank Xiao-kun Wu for participating in the preparation of the fiber-optic sensing probe.
Minghong Yang received his PhD in physics from Huazhong University of Science and Technology, China, in 2003. From 2003 to 2005, he worked as a postdoctoral fellow at the Fraunhofer Institute for Applied Optics and Fine Mechanics, Jena, Germany. From 2006 to 2008, he worked as a research fellow at Berlin University of Technology, where he dealt with thin film sensors. He is now a research scientist in the National Engineering Laboratory for Optical Fiber Sensors at Wuhan University of
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Minghong Yang received his PhD in physics from Huazhong University of Science and Technology, China, in 2003. From 2003 to 2005, he worked as a postdoctoral fellow at the Fraunhofer Institute for Applied Optics and Fine Mechanics, Jena, Germany. From 2006 to 2008, he worked as a research fellow at Berlin University of Technology, where he dealt with thin film sensors. He is now a research scientist in the National Engineering Laboratory for Optical Fiber Sensors at Wuhan University of Technology, China. His research interest is thin film optical fiber sensors.