Nucleobases in Supercritical Fluids: Solubility, Molecular Beam Expansion, and Surface Deposition

Supercritical fluids have found applications in a wide variety of fields, from solvents for reaction media to engineering of nanoparticles. They are also of interest in the analytical field for transferring non-volatile organic molecules into the gas phase for detection and deposition purposes. This work highlights the use of supercritical fluids as an analytical tool for the transfer of a group of non-volatile molecules, namely nucleobases, into the gas phase. The most commonly used supercritical fluid, carbon dioxide was found inefficient in dissolving the nucleobases even with the help of a cosolvent. Therefore, for this purpose a mixture of ethylene (pc = 50.6 bar and Tc = 9.35 ◦C) with a cosolvent was used as the supercritical solvent. A new bracketing method was developed for detecting the critical point of pure fluids and diluted mixtures of fluids. The shift in critical point of ethylene on addition of ethanol was determined and related to theoretical calculations by using the Soave Redlich Kwong equation of state. Comparing the experimental results to purely theoretical methods for calculating the critical point showed large deviations even at low concentrations. The critical temperature shifted by only 5.5 ◦C when the mole fraction of the cosolvent i.e. ethanol was 0.054. Five biologically relevant nucleobases i.e. adenine, guanine, cytosine, thymine and uracil were dissolved in supercritical ethylene using 3% of ethanol as cosolvent. The supersonic molecular beam composition of the expanded solution was analyzed quantitatively using a quadrupole mass spectrometer and the ratio of the nucleobases to ethylene in the beam was found to be of the order of 10−4 to 10−5. The solubility for different nucleobases increased in the following order: cytosine > uracil ∼ adenine > guanine > thymine. Surface deposition of the nucleobases through supercritical fluid solutions was carried out and the morphology was recorded using Atomic Force Microscopy. Remarkable differences were observed while comparing the morphology obtained after deposition using rapid expansion of supercritical solutions (RESS) and drop casting method. For example, in the case of cytosine, the drop casting method produced rather long (∼ μm range) curved rods with high aspect ratios, whereas spherical particles (ranging from nanometer to micrometer sizes) were observed in a thin particulate film when deposited using RESS. These differences are discussed in terms of diffusion, rate of evaporation of the solvent, degree of supersaturation, and the nucleation process.