Aquamarine blue emitting silver nanoparticles as fluorescent sensor for melamine detection

https://doi.org/10.1016/j.saa.2019.03.051Get rights and content

Highlights

  • Water-soluble silver nanoparticles (Ag NPs) are prepared by chemical reduction methods.

  • Synthesis of Ag NPs by using polyethyleneimine (PEI) as a stabilizing agent and citric acid (CA) as the reducing agent

  • A rapid, sensitive and selective fluorescence sensing system is developed for the detection of melamine.

  • The detection limit is 132 nM.

Abstract

Designing an exceptional probe to detect minute quantities of melamine is of huge importance for the safety and health of the human race. In this study, an aquamarine blue and rapid fluorescent sensor was devised by chemical reduction synthesis method for highly selective and sensitive detection of melamine in milk using polyethyleneimine protected silver nanoparticles (PEI-AgNPs). The optical properties of PEI-AgNPs were studied as a function of reactive material concentration, incubation time, temperature and pH. The formation of PEI-AgNPs was verified by using transmission electron microscopy (TEM) and found that the PEI-AgNPs were spherical in shape with an average diameter of 15 nm. The prepared PEI-AgNPs were aquamarine blue luminescent under UV light and show a maximum emission at 470 nm, when excited at 348 nm. The fluorescence of the PEI-AgNPs can be effectively quenched by the addition of melamine due to its ability to interact with citric acid (CA) through hydrogen bonding and form silver-melamine complexes with Ag+ via coordination bonds, while the addition of other nitro compounds induced feeble quenching of PEI-AgNPs, suggesting the high selectivity of PEI-AgNPs. The present sensor system exhibits a good linear response to melamine ranging from 0.16 to 56 μM, and the limit of detection was found to be 132 nM (Sb/K = 3). Significantly, the fluorescence sensor exhibited good selectivity and acted as a potential probe for detection of melamine in milk samples.

Introduction

Melamine (1, 3, 5-triazine-2, 4, 6-triamine, C3H6N3), also known as protein essence, is a triazine nitrogen-containing heterocyclic ring organic compound. Owing to its high production-volume in industry, it has been widely used in plastics, coatings, synthetic fire-retardant laminate adhesives, water repellent fixatives or hardeners, heat-resistant decorative sheets, moisture-proof paper and gray leather suede, as well as anti-folding and anti-shrinking treatment agents for textiles [1]. Although its industrial use is extensive, it is not allowed to be added to food. However, some unscrupulous people always blend melamine into animal feed as well as in pet and human foods to increase the apparent content of protein due to its high nitrogen content (66% by mass) [2]. For instance, Sanlu Group Co., Ltd. added melamine into infant formula causing over 300,000 children in China to develop kidney stones in 2008 [3]. Apart from this, long-term intake of melamine can cause damage to the reproductive and urinary systems, bladder stones, and can further induce bladder cancer [4,5]. Hence, a simple, accurate satisfactory and feasible method of detection of melamine in regularly consumed dairy products is urgently needed.

At present, a large variety of confirmation and screening methods have been reported to detect melamine, such as electrochemical biosensor [6], enzyme-linked immune sorbent assay (ELISA) [7], colorimetric method [8], gas chromatography–mass spectrometry method (GC–MS) [9], liquid chromatography-mass spectrometry (LC-MS) [10], waveguide fluorescence immune sensor [11], Raman spectroscopy [12] and so on. In spite of the high sensitivity of these conventional methods for melamine analysis, these approaches are generally not popular due to the exorbitant instrumentation and usually occupy too much space.

Compared with the aforementioned approaches, the fluorescence methods have emerged as powerful supporting tool for the detection analysis in recent decades owing to its high spatial and temporal resolution, good reliability, highly selectivity, simple instrumentation and ease of operation [[13], [14], [15], [16], [17]]. Although much progress had been made in the development of fluorescence probes for detection of melamine [[18], [19], [20], [21]], there are still have some drawbacks, such as high detection limit, participation of organic solvents, the detection of melamine with high sensitivity, selectivity and low ratio of organic solvent is a persistent challenge.

Among various fluorescence probes, the fluorescent metal (e.g., gold, silver and copper) nanoparticles have been studied extensively and emerged as a novel type of fluorescent nanoprobe in biochemical analysis, such as the detection of metal ions, small molecules, and proteins, due to their excellent physical properties and luminescence [[22], [23], [24]]. For example, Liu reported that Triton X-100 modified citrate-capped AuNPs could highly efficiently and selectively detect melamine through the ligand exchange interaction between melamine and citrate ions on the surface of AuNPs [25]. Jiang developed core-shell Ag@SiO2NPs for highly sensitive and selective detection of Cu2+ [26]. Up to now, the frequently used adopted protective agent in Au/AgNPs synthesis is thiolate due to the strong interaction between thiol and gold/silver, and numerous well-defined thiolate-protected Au/AgNPs have been reported for various applications [[27], [28], [29]]. However, the synthetic route exploration is quite complicated, thus limiting their simplicity and rapidity in practical application. Moreover, as far as we know, there are some researches about melamine detection with silver nanoparticles [30,31], however, silver nanoparticle based fluorescence method is rare reported.

Herein, a sensitive method is proposed for the determination of melamine in milk using polyethyleneimine protected silver nanoparticles (PEI-AgNPs) as a fluorescence probe (Scheme 1). The fluorescence intensity of PEI-AgNPs gets quenched during the addition of melamine and quenching was proportional to the concentration of melamine. The quenching is the result of the hydrogen bond formation between amino group in melamine and carboxyl group on the surface of AgNPs. Interference study performed with different constituents present in milk showed negligible interference. The real sample analysis with satisfactory results proved the system to be appropriate for the analysis of melamine in milk.

Section snippets

Reagents

All the chemicals used in the experiments were of analytical grade and commercially available. Silver nitrate (AgNO3) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Citric acid (CA) was bought from Sailboat Chemical Reagent Technology Co., Ltd. (Tianjin, China). Polyethyleneimine (PEI) was purchased from Chengdu Grecia Chemical Technology Co., Ltd. (Chengdu, China). Melamine was acquired from Dingshengxin Chemical Co., Ltd. (Tianjin, China). Ancient city milk was

Optimization synthesis condition of PEI-AgNPs

In order to successfully acquire AgNPs with optimum fluorescence performance, herein, an easy and green approach combined with microwave and sonochemical was employed to rapidly afford intensely aquamarine blue fluorescent emission AgNPs using polyethyleneimine (PEI) as a stabilizing agent and citric acid (CA) as the reducing agent. The influencing factors, including the concentration of PEI and CA, the medium pH, the incubation time and the reaction temperature, could determine the synthesis

Conclusions

In summary, a rapid, simple and green melamine sensing assay was fabricated using aquamarine blue fluorescent AgNPs probe with polyethyleneimine (PEI) and citric acid (CA) as the stabilizing agent and reducing agent, respectively. The advantage of the AgNPs probe is the easy synthesis and application as fluorescent sensor at room temperature for sensitive detection of melamine. The melamine detection is based on fluorescence quenching of AgNPs on complex compound between interaction of melamine

Acknowledgements

This work is supported by the Natural Science Foundation of Shanxi Province of China (Grant No. 201601D02024) and sponsored by the Fund for Shanxi "1331 Project" Key Subjects Construction of Shanxi Province of China (Grant No. 2017122).

References (33)

Cited by (8)

  • Fluorescent probes in public health and public safety

    2021, Coordination Chemistry Reviews
    Citation Excerpt :

    One is that melamine can interact with multiple –OH groups of citric acid through hydrogen bonds, and the other is that Ag+ ions can interact with amines and melamine through acceptor–donor interactions. PEI-AgNPs have been successfully applied to the determination of melamine in milk samples, and interference studies on other different components in milk have shown that the interference was negligible (Fig. 9B) [63]. Zou et al. established a sensitive Au@CQDs-based fluorescence method for the visual and accurate detection of melamine in milk.

View all citing articles on Scopus
View full text