Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Aquamarine blue emitting silver nanoparticles as fluorescent sensor for melamine detection
Graphical abstract
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).
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