Dependence of Adenine Raman Spectrum on Excitation Laser Wavelength: Comparison between Experiment and Theoretical Simulations

New insights into the surface Enhanced Raman Scattering (SERS) response of adenine using chemometrics

The SERS response of adenine is one of the most studied, due to its outstanding exaltation. However, the spectra obtained strongly differ according to the experimental conditions and still remain not well understood. To be able to search for the presence of this molecule in complex environments, it is essential to better understand the SERS response of adenine alone. After a brief presentation of the literature on the subject, we present results suggesting that the experimental spectra would result from the overlap of different spectroscopic signatures, that may probably be due to different non-covalent interactions or different electromagnetic fields experienced by adenine molecules. An independent component analysis is reported. Our results underline the difficulty to precisely analyze the experimental data, the need to continue this research and to constitute data banks that would allow comparing the spectra obtained in different laboratories according to the experimental conditions.

DNA Antiadhesive Layer for Reusable Plasmonic Sensors: Nanostructure Pitch Effect

A long-term reusable sensor that provides the opportunity to easily regenerate the active surface and minimize the occurrence of undesired absorption events is an appealing solution that helps to cut down the costs and improve the device performances. Impressive advances have been made in the past years concerning the development of novel cutting-edge sensors, but the reusability can currently represent a challenge. Direct shielding of the sensor surface is not always applicable, because it can impact the device performance. This study reports an antiadhesive layer (AAL) made of 90 mg/mL DNA sodium salt from salmon testes (ssstDNA) for passivating gold plasmonic sensor surfaces. Our gold two-dimensional (2D) nanostructured plasmonic metasurfaces modified with AAL were used for DNA quantification. AAL is thin enough that the plasmonic sensor remains sensitive to subsequent deposition of DNA, which serves as an analyte. AAL protects the gold surface from unwanted nonspecific adsorption by enabling wash-off of the deposited analyte after analysis and thus recovery of the LSPR peak position (rLSPR). The calibration curve obtained on a single nanostructure (Achiral Octupolar, 100 nm pitch) gave an LOD = 105 ng/mL and an extraordinary dynamic range, performances comparable or superior to those of commercial UV-vis spectrometers for acid nucleic dosage. Two different analytes were tested: ssstDNA (∼2000 bp) in deionized water and double-strand DNA (dsDNA) of 546-1614 bp in 100 mM Tris buffer and 10 mM MgCl₂. The two nanostructures (Achiral Octupolar 25 and 100) were found to have the same sensitivity to DNA in deionized water but different sensitivity to DNA in a salt/buffer solution, opening a potential for solute discrimination. To the best of our knowledge, this is the first report on the use of AAL made of several kilobase-pairs-long dsDNA to produce a reusable plasmonic sensor. The working principle and limitations are drawn based on the LSPR and SERS study.

Surface-Enhanced Raman Spectroscopy for Environmental Monitoring of Aerosols

Surface-enhanced Raman spectroscopy (SERS) is conducted from single aerosol particles held in a linear electrodynamic quadrupole trap. SERS measurements from two representative types of ambient aerosol particles, semi-liquid and solid aerosols, are demonstrated; aerosol composed of adenine where the metallic nanoparticles (MNPs) are volume distributed throughout the particle and aerosol composed of polystyrene latex (PSL) beads where the MNPs are surface coated. An enhancement factor > 10⁶ is demonstrated from 5 μm aerosols containing trace amounts of adenine (0.1% by mass), with a detection limit of 10^-8 M corresponding to 5 × 10⁵ molecules (equivalent to 100 ag in mass or a 50 nm diameter sphere), and a ratio of 100 adenine molecules per Ag NP. SERS signal intensities are linear with particle adenine concentration up to a saturation point. Both the linearity and enhancement factor were confirmed by SERS measurements of adenine as bulk suspensions. The SERS spectra of adenine as bulk suspensions were explored as a function of excitation wavelength ranging from 400 to 800 nm. The two main Raman peaks of adenine at 738 and 1336 cm^-1 exhibit SERS maxima for excitation in the 450-500 nm range for commercially available 40 nm spherical Ag nanoparticles (NPs) used in this study, which shifts to longer wavelengths with the addition of NaCl. Shifts in SERS and spontaneous Raman shifts were observed between aqueous and dry adenine, in agreement with the literature, demonstrating the utility of SERS to possibly study water uptake of aerosols. SERS is measured from MNP surface-coated PSL beads with an enhancement factor of 30 for 5 μm PSLs. Theoretical extrapolation demonstrates that the enhancement factor will increase for decreasing particle size with an estimated enhancement factor of 140 for 1 μm PSLs.