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Raman spectroscopy has become a popular tool for analyzing biological samples such as DNA. It allows us to access to the vibrational levels of molecules and thus to identify the chemical composition and to observe the structure of molecular systems.[1] In this work, we recorded the SERS spectra of DNA strands with different sequences and grafted at the surface of gold nanoparticles. Our objective is to study the correlation between the DNA sequences and its base composition and the intensity of Raman bands observed in SERS. To reach a high density of DNA on the surface of the gold nanoparticles, the DNA strands include a thiol group (SH) at their 5' end and are conjugated to gold nanoparticles using the freeze-thaw cycle method. SERS measurements are performed on dried drops deposited on a glass slide. We recorded 5 spectra for each sample using a 785nm excitation wavelength. We observed several bands that can be assigned to the different bases and to the phosphate backbone. For instance, the bands of ring breathing mode of adenine and the carbonic skeleton of the DNA are observable at 733cm-1 and 1029cm-1 respectively.[2] We then compare the relative intensity of the different bands such as the 733cm-1 or 1029cm-1 ones. We found that the relative intensity of the 733cm-1 band is correlated with the amount of adenine in the DNA sequence. This study provides a new approach for reliable quantification and analysis of genetic information associated with DNA molecule sequencing.

International audience; Optimum amplification in surface enhanced Raman scattering (SERS) from individual nanoantennas is expected when the excitation is slightly blue-shifted with respect to the localized surface plasmon resonance (LSPR), so that the LSPR peak falls in the middle between the laser and the Stokes Raman emission. Recent experiments have shown when moving the excitation from the visible to the near-infrared that this rule of thumb is no more valid. The excitation has to be red-shifted with respect to the LSPR peak, up to 80 nm, to obtain highest SERS. Such discrepancy is usually attributed to a near-field (NF) to far-field (FF) spectral shift. Here we critically discuss this hypothesis for the case of gold nanocylinders. By combining multiwavelength-excitation SERS experiments with numerical calculations, we show that the red-shift of the excitation energy does not originate from a spectral shift between the extinction (FF) and the near-field distribution (NF), which is found to be not larger than 10 nm. Rather, it can be accounted for by looking at the peculiar spectral dependence of the near-field intensity on the cylinders diameter, characterized by an initial increase, up to 180 nm diameter, followed by a decrease and a pronounced skewness.