On-demand fabrication of surface-enhanced Raman scattering arrays by pen writing, and their application to the determination of melamine in milk

An on-demand, disposable and portable surface-enhanced Raman scattering (SERS) array has been prepared by modifying the surface of cellulose paper and patterning it with silver nanoparticles (AgNPs) via pen-writing technique. The paper is initially modified with hexadecenyl succinic anhydride to generate a hydrophobic top layer that is capable of concentrating melamine (and other hydrophobic analytes) by preventing the mainly aqueous sample from spreading on the paper. The AgNPs are subsequently written onto the hydrophobic paper with a high degree of control to form large-scale SERS sensing arrays. The resulting arrays demonstrate outstanding Raman scattering enhancement as demonstrated for the fluorescent dye Rhodamine 6G which can be detected by SERS with an 80 pM limit of detection (LOD) without interference by fluorescence. The reproducibility in the preparation of spots has a relative standard deviation of < 15% (for n = 128 spectra). The arrays were applied to the determination of the illicit milk additive melamine. The plot of SERS intensity at Delta gamma = 681 cm(-1) versus the logarithm of melamine concentration is linear in the concentration range from 0.3 to 20 mg.L-1, and the LOD is 0.27 mg.L-1. In our perception, the disposable arrays presented here integrate the advantages of easy production and reliable reproducibility, and therefore are an ideal choice for use in SERS detection.