## 2 Materials and Methods
### Synthesis of RhB-Conjugated AOA (AOA-RhB)  
Exactly 10.0 mg of Rhodamine B isothiyanate and 2.9 mg 5-aminoorotic acid were dissolved in 1.0 mL of \(N\),\(N\)-dimethylformamide (DMF). The solution was stirred at room temperature for 24 h and then dialyzed against water to remove the excess of Rhodamine B and finally obtain AOA-RhB.  
Detection of Melamine in Water Using [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(\bullet\bullet\bullet\bullet\bullet\)Melamine\(\bullet\bullet\bullet\bullet\)AOA-RhB] as a Matrix  
A melamine stock solution was prepared by accurately dissolving 1.0 mg of melamine in 10.0 mL of water. The gold magnetic nanoparticle (Fe\({}_{3}\)O\({}_{4}\)/Au-AOA) and the RhB-conjugated AOA (AOA-RhB) solution were prepared at concentrations of 2.5 and 0.1 mg/mL, respectively. Aliquots of the melamine stock solution above were added to the mixture of 0.1 mL of Fe\({}_{3}\)O\({}_{4}\)/Au-AOA and 0.1 mL of AOA-RhB solution to make a series of solutions, respectively containing 1.0 \(\upmu\)g/mL, 2.5 \(\upmu\)g/mL, 5.0 \(\upmu\)g/mL, 10.0 \(\upmu\)g/mL, and 15.0 \(\upmu\)g/mL of melamine. These solutions were oscillated at room temperature for 30 min and then transferred to concave glass slides with pipettes. A strong NdFeB cylinder magnet (10 mm in diameter, 3 mm in thickness, Jiangdong Magnetic Industry of Ningbo Co. Ltd., Ningbo, China) was placed under each slide to separate and concentrate the gold magnetic nanoparticles to a pellet point. The laser irradiation of the pellets produced Raman spectra of RhB and its characteristic peak at 1510 cm\({}^{-1}\) was analyzed to build the calibration curve for melamine since the peaks between 1200 and 1400 cm\({}^{-1}\) were prone to interference with the glass background of slides [47]. Each recordedspectrum is an average of five independent readings taken at an integration time of 1 s. The total analysis time can be less than 1 h.  
Detection of Melamine in Milk Using [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(\bullet\)\(\bullet\)\(\bullet\)\(\bullet\)Melamine\(\bullet\)\(\bullet\)\(\bullet\)\(\bullet\)AOA-RhB] as a Matrix  
The gold magnetic nanoparticle (Fe\({}_{3}\)O\({}_{4}\)/Au-AOA) and the RhB-conjugated AOA (AOA-RhB) solution were prepared at a concentration of 4.0 and 0.5 mg/mL, respectively. A melamine stock solution was prepared by accurately dissolving 10.0 mg of melamine in 10.0 mL of ethanol/H\({}_{2}\)O 50:50 (\(v\)/\(v\)). Aliquots of the melamine stock solution above were added into 0.2 mL of milk. Then, the milk solution was mixed with 0.1 mL of Fe\({}_{3}\)O\({}_{4}\)/Au-AOA and 0.1 mL of AOA-RhB solution to make a series of solutions, respectively containing 2.0, 2.5, 5.0, 10.0, and 15.0 mg/mL of melamine. These solutions were oscillated at room temperature for 30 min and then transferred to concave glass slides with pipettes. A strong NdFeB cylinder magnet (10 mm in diameter, 3 mm in thickness, Jiangdong Magnetic Industry of Ningbo Co. Ltd, Ningbo, China) was placed under each slide to separate and concentrate the gold magnetic nanoparticles into a pellet point. The supernatant was removed by a filter paper. The laser irradiation of the pellets produced Raman spectra of RhB, and its characteristic peak at 1510 cm\({}^{-1}\) was analyzed to build the calibration curve of melamine. Each recorded spectrum is an average of five independent readings taken at an integration time of 1 s. The total analysis time can be less than 1 h.