Plasmonic spectral determination of Hg(II) based on surface etching of Au-Ag core-shell triangular nanoplates: From spectrum peak to dip

Highlights

• Au-Ag core-shell triangular nanoplates could be etched by Hg²⁺.

• The Hg²⁺ induced etching and spectrum change is affected by the initial thickness of Ag shell.

• This spectral change of “peak-to-dip” greatly enlarges the detection range of mercury ions.

• The linear range spans a large scope from 10 to 1000 μM.

Abstract

In this work, we develop a simple and selective sensing method for the detection of mercury ions based on surface plasmon resonance (SPR) spectrum change of Au-Ag core-shell triangular nanoplates. When the concentration of mercury is increased, the etching-induced change of particle size and shape also leads to the decrease of the absorption peak at the fixed wavelength, until a spectrum dip takes place. This spectral change of “peak-to-dip” greatly enlarges the detection range of mercury ions, which could be fine tuned by changing the initial thickness of the Ag coating. Under optimal conditions, the decrease of the logarithmic absorption intensity has a good linear response with the concentration of mercury ions increasing from 10 to 1000 μM, and the limit of detection (LOD) is 0.88 μM. Interference studies and real samples test indicate that, this new sensing method has a good selection for mercury ions and can be practically used in lake water. This work shows the surface etching-induced SPR shift can also leads to the intensity change with “peak-to-dip” fashion, which greatly enlarge the concentration range of the detection and could be widely applied in the spectroscopy sensing based on SPR.

Introduction

Mercury as a heavy metal is highly toxic, which has serious danger to human health. As we know, mercury can not only cause neurological damages, but also increase the risk of myocardial infarction [1]. Therefore, detection of mercury in water and food is essential. In recent years, more and more methods have been developed to detect mercury [2]. Traditional methods of mercury detection include instrumental method such as atomic absorption spectroscopy, plasma mass spectrometry, atomic fluorescence spectrometry and atomic emission spectrometry [3]. Although these methods can precisely detect mercury, they all need expensive equipment and complicated procedures. Mercury ions could also be detected by using other methods based on chemical sensor and optical sensor such as organic molecules, biomaterial and polymeric materials [[4], [5], [6], [7]]. However, these methods need a complex synthesis process or molecules modification process. Therefore, a rapid, simple, high selective method is needed to detect mercury ions.

Recently, noble metal nanostructures have been studied deeply for their superior properties in detecting chemical or biological molecule. Owning to their high surface to volume ratio and superior optical properties, these metal nanostructures are good to serve as spectroscopic and colorimetric sensors [8]. The optical properties of these nanostructures are dependent on its morphological, structure and aggregation, which can be sensitive to some toxic metal ions. Thus, noble metal nanoparticles have been widely used to detect toxic metal ions [[9], [10], [11], [12], [13]]. For example, gold and silver nanoparticle probes both have been used to detect Hg²⁺ through recognition units [14,15]. The detection mechanism of heavy metal ions is to trigger etching or aggregation of silver and gold nanoparticles through recognition units including protein molecules [16], fluorophores [17], polymers [18], oligonucleotides [19], small thiolate ligand [20]. Because of the lower cost and higher molar extinction coefficient, silver nanoparticles are more sensitive than gold. So, silver nanoparticles are more popular to be used as nanosensors and probes [21,22]. In the report of Duan et al., 6-Thioguanine can lead to the aggregation of Ag nanoparticles, while Hg²⁺ can prevent the aggregation [23]. In this way, the concentration of Hg²⁺ has been detected. Liu et al. reported a colorimetric detection of Hg²⁺ using Au nanoparticles through variation of three colors. In the method, original Au nanoparticles become Au-Hg alloy due to the addition of Hg²⁺, then aggregate. During these processes, three different colors appear, which have been used for the detection of Hg²⁺ [24]. Based on the redox between Hg²⁺ and Ag, Lokhande et al. reported a sensitive method to detect Hg²⁺. In their method, silver thin film on the plastics substrate can detect Hg²⁺ in solution by color changing and the time to change. When the concentration of Hg²⁺ is high, the color of silver thin film changed from brownish to cloudy white quickly, which can show the density of Hg²⁺ in the liquor [25].

Compared with single metal nanoparticles, core-shell type bimetallic nanoparticles have two metallic elements, which are more attractive. Because the core size, shell thickness and chemical elements can be controlled independently, it is easier to control the plasmonic resonance wavelength and strength. Besides, the interaction between different metals and different metal surfaces can enhance the plasmonic resonance coupling and create more plasmon modes [26]. In the study of Guha et al., Au-Ag core-shell nanoparticles with controlled shell thickness have been used for Hg²⁺ detection by fluorescent. When the shell thickness of core-shell nanoparticles varies, the optical property also varies [27]. Au-Ag core-shell nanoparticles can also be used to detect glucose and cholesterol. According to Zhang et al., under the catalytic action of glucose or cholesterol oxidase, H₂O₂ can be produced and etched the Ag shell, which causes the decrease of SPR peak [28]. Zhu et al. reported a spectral sensing method for detecting of Hg²⁺ by Au-Ag core-shell nanorods. In their method, gold nanorods modified with cysteine were etched by Hg²⁺. Thus the absorption peak decreases because of the Hg²⁺-induced aggregation [29]. Hg²⁺ can also be detected by silver nanoparticles through anti-aggregation. By using fungal extracted aqueous method, Pd-Ag bimetallic nanoparticles were synthesized by Mallikarjuna et al. [30]. The electrode modified by these Pd-Ag nanoparticles displayed the highest electrocatalytic activity for the detection of uric acid. Reddy et al. also reported a one-step procedure for the fabrication of Pd-Ag bimetallic nanoparticles on the graphene oxide(rGO) support [31]. This Pd–Ag/rGO based electrochemical sensor was proved to be ultrasensitive in the detection of acetaminophen in the presence of etilefrine. Recently, a nanocomposite of SiO₂@Fe₃O₄ was also synthesized by Reddy et al. [32]. By the electropolymerization of dipicolinic acid on the surface of SiO₂@Fe₃O₄ nanocomposite immobilizing the carbon paste electrode, the poly(dipicolinic acid)/SiO₂@Fe₃O₄ based electrochemical based sensor was fabricated.

Compared to spherical nanoparticles, triangular metal nanoplates have intense electromagnetic field in the corners [33]. What's more, silver or gold atoms at the tips possess high energy, which are easier to be etched by oxidation [14]. Therefore, we aim to develop a rapid and selective spectral determination of Hg²⁺ using Au-Ag core-shell triangular nanoplates. In our study, Au-Ag core-shell triangular nanoplates have been prepared by seed growth and silver coating. Because of the Hg²⁺–induced etching of Au-Ag nanoplates, the SPR absorption peak decreases rapidly and depends on the concentration of Hg²⁺. By tuning the initial thickness of the Ag coating, the spectral change with “peak-to-dip” fashion could be obtained and greatly enlarges the detection range of mercury ions, which could be widely applied in the spectroscopy sensing based on metal nanoparticles and SPR.