## 1 Introduction  
Melamine is a common industrial chemical and has been widely used to manufacture thermoset plastics, coating materials, flame retardants, wood adhesives, glues, and many other applications. The chemical became a global concern in 2008 because a high amount of melamine had been illegally added to pet food and even infant milk formula in China [1; 2]. The long-term ingestion of melamine may cause severe damages like kidney stones [3], bladder stones [4; 5], and even bladder cancer [6; 7]. Under the effect of gastric juice, part of melamine may transfer into cyanuric acid and react with the remaining melamine to produce insoluble crystals of melamine cyanurate, which induce renal malfunction, metabolic problems of the urinary system [8], or even death in infants. For both health and social reasons, there remains an urgent need for a rapid, on-site, sensitive, and cost-effective analysis of melamine. The current detection methods for melamine analysis primarily include high performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), enzyme-linked immunosorbent assay (ELISA), infrared spectrometry (IR), and surface enhanced Raman spectroscopy (SERS) [9; 10; 11]. Though HPLC is a powerful method of high sensitivity and selectivity to determine melamine, its complicated and time-consuming pretreatment of samples brings much inconvenience for on-site rapid detection [12]. LC-MS or GC-MS requires expensive equipment, complicated operations, professional skills, and high test costs, and thus is suitable for laboratory testing only [13]. IR has a fast detection speed, less sample consumption, and high operability, but it is less sensitive and less accurate in quantitative analysis. ELISA is easy to carry out and its short analysis time makes it possible for mass screening, but ELISA often gives false positive results and thus is difficult to achieve a rapid onsite determination of melamine [14]. The phenomenon of SERS was first observed by [PERSON] and coworkers in 1974 [15]. [PERSON] and [PERSON] [16] later found that the Raman scattering signal of pyridine on the rough surface of silver electrodes was about \(10^{6}\) times higher than the same amount of pyridine in solution. Under ideal conditions, SERS is able to detect a single molecule of the detected analytes [17]. Rough gold or silver surfaces as well as gold or silver nanoparticles are generally used as Raman active substrates due to their high surface enhanced Raman scattering effect [18]. With the help of a portable spectrometer, SERS can provide simple, rapid, and on-site analysis due to its inherent simplicity and high sensitivity and thus has been developed as a fast detection platform for biothreat investigation [19], DNA and RNA identification [20; 21; 22], cancer-diagnostics [23], protein immunoassay [24; 25; 26; 27; 28], and food safety analysis [29; 30; 31; 32; 33; 34; 35; 36; 37]. For example, SERS has been explored to detect melamine in milk using hollow gold chips or other SERS active substrates fabricated on glass or silicon wafers [32; 33; 34] and using melamine-induced aggregation of gold nanoparticles as an enhanced substrate [35]. Printed SERS substrates combined with paper chromatograph have been successfully applied in the direct detection of melamine in proteinaceous samples [38]. Melamine has been directly detected in infant formula with a limit of detection of 100 ppb using commercial gold nanofinger SERS sensor chips [10] and has also been detected in pure water with a limit of detection of 1 ppb using ultrasensitive Ag nanoparticles over Au film coated on PS colloid monolayer as a substrate [39]. However, these direct detections of melamine by SERS may not be applicable to other complex samples in practice such as blood, urine, pet foods, and fertilizers.