## 3 Results and Discussion
### Characterization of RbB-Conjugated AOA (AOA-RhB) by FTIR  
The infrared spectrum of Rhodamine B isothiocyanate (RhB) is shown in Figure 5a. RhB has a strong and broad IR adsorption of -OH stretching vibration between 3100 cm\({}^{-1}\) and 3600 cm\({}^{-1}\) and a characteristic adsorption of -N=C=S group at 2025 cm\({}^{-1}\). The peaks at 2977 cm\({}^{-1}\) and 2926 cm\({}^{-1}\) were ascribed to the anti-symmetric and symmetric stretching vibrations of -CH\({}_{3}\) of RhB. After the conjugation of RhB to AOA, the -N=C=S characteristic peak at 2025 cm\({}^{-1}\) completely disappeared while a strong peak at 1694 cm\({}^{-1}\) corresponding to the carboxyl group of AOA appeared (Figure 5b), indicating that the RhB had been connected to AOA through the reaction of -N=C=S with the amino group of AOA. Characterization of [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(  
Melamine had characteristic double peaks of amino group at 3466 cm\({}^{-1}\) and 3415 cm\({}^{-1}\) (Figure 6c), while Fe\({}_{3}\)O\({}_{4}\)/Au-AOA and AOA-RhB presented characteristic peak of carbonyl groups at 1696 cm\({}^{-1}\) and 1694 cm\({}^{-1}\), respectively (Figure 6a,b). The FTIR spectrum of the [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(\bullet\bullet\bullet\bullet\)melamine\(\bullet\bullet\bullet\)AOA-RhB] complex (Figure 6d) clearly showed that the double peaks of amino group of melamine disappeared, and the carbonyl group stretching vibration of AOA simultaneously shifted from 1696 cm\({}^{-1}\) to 1694 cm\({}^{-1}\) to a lower wavenumber at 1681 cm\({}^{-1}\), indicating the formation of a strong hydrogen bonding between melamine and AOA in the complex, as illustrated in Figure 1.  
Detection of Melamine in Water Using [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(\bullet\bullet\bullet\bullet\)Melamine\(\bullet\bullet\bullet\)AOA-RhB] as a Matrix  
The detection assay of melamine was assembled by the addition of aliquots of melamine solution 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 mixture solutions respectively containing 1.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, 2.5 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, 5.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, 10.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, and 15.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL of melamine. Figure 7 shows the difference in Raman spectra for the samples containing no melamine (Spectrum a) and 1.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL of melamine (Spectrum b). There are no apparent diagnostic signal peaks for RhB in Spectrum a because the RhB molecules were far from the surface of the gold magnetic nanoparticles due to the lack of strong interactions between Fe\({}_{3}\)O\({}_{4}\)/Au-AOA and AOA-RhB in the absence of melamine. In contrast, several characteristic peaks (between 1100 and 1700 cm\({}^{-1}\)) of RhB appeared in Spectrum b since the addition of melamine caused the formation of the supramolecular complex of [Fe\({}_{3}\)O\({}_{4}\)/Au-AOA\(\bullet\bullet\bullet\)melamine\(\bullet\bullet\)AOA-RhB] via strong multiple hydrogen bonding between melamine and AOA, and consequently brought the RhB molecules close enough to the surface of Fe\({}_{3}\)O\({}_{4}\)/Au nanoparticles for the generation of SERS signal of RhB. Figure 8 demonstrates a progressive increment in peak intensity at 1510 cm\({}^{-1}\) of RhB when melamine concentration was increased from 0 to 15.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL. A linear plot of the logarithm of the peak intensity at 1510 cm\({}^{-1}\) versus the melamine concentration indicates that the detection is linear within the concentration between 1.0 and 15.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL with the detection limit of melamine at 1.0 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in water. The linear equation is IgI = 2.07546 + 0.06641 \(\times\) C (IgI refers to the logarithmic peak intensity at 1510 cm\({}^{-1}\), C refers to melamine concentrations) and the linear regression coefficient was determined to be \(R^{2}\) = 0.9840.