The surface-enhanced Raman spectra of aflatoxins: spectral analysis, density functional theory calculation, detection and differentiation

High-quality surface-enhanced Raman scattering (SERS) spectra of aflatoxin (AF) B-1, B-2, G(1) and G(2) have been acquired using silver nanorod (AgNR) array substrates fabricated by oblique angle deposition method. Significant vibrational peaks are identified on the argon plasma-cleaned substrates, and those peaks agree very well with the Raman spectra calculated by density function theory (DFT). The concentration-dependent SERS detection is also explored. The relationship between the concentration (C) of different AFs and the SERS intensity (I) of the Raman peak at Delta v = 1592 cm(-1) is found to follow the general relationship I = AC(alpha), with alpha ranging from 0.32 to 0.46 for the four AFs. The limits of detection (LODs) reach 5 x 10(-5) mol L-1 for AFB(1), 1 x 10(-4) mol L-1 for AFB(2), and 5 x 10(-6) mol L-1 for both AFG(1) and AFG(2) in bulk solution, or 6.17 x 10(-16) mol/1.93 x 10(-4) ng of AFB(1), 1.23 x 10(-15) mol/3.88 x 10(-4) ng for AFB(2), 6.17 x 10(-17) mol/2.03 x 10(-5) ng for AFG(1), and 6.17 x 10(-17) mol/2.04 x 10(-5) ng for AFG(2) per laser spot. Principal component analysis (PCA) is used to successfully differentiate these four different kinds of AFs at different concentrations up to their detection limits. The LODs obtained from PCA agree with the LODs obtained by using peak fitting method. With such a low detection limit and outstanding differentiation ability, we prove the possibility of utilizing the SERS detection system as a platform for highly sensitive mycotoxin detection.