A Microscopic View of Macromolecule Transfer in the Vacuum Using Gas and Bismuth Clusters

Abstract

Transferring large nonvolatile molecules in the vacuum is key to both their characterization by mass spectrometry (ion mobility, etc.) and their use as nanofabrication building blocks in soft-landing-type experiments. Recently, our group performed the successful transfer and redeposition of intact and bioactive lysozyme (14 kDa) with large gas cluster ion beams, in the absence of a solvent or matrix, demonstrating that such beams could serve as tools for macromolecule manipulation. A number of fundamental questions arose in relation with this experimental proof-of-concept, concerning for instance the maximum molecular size for successful transfer, the effect of the cluster projectile incidence angle, the dependence of molecular dissociation on the projectile energy and size, the internal energy uptake ultimately leading to metastable decay reactions, or the influence of the surface interactions on desorption. To address these questions, a series of molecular dynamics simulations were conducted involving cluster impacts on an adsorbed globular polystyrene (PS) macromolecule of 60 kDa, with a mass and shape comparable to those of a protein such as bovine serum albumin. Conditions of intact desorption of this PS molecule by Ar clusters are identified, in terms of energy per atom and incidence angle, and the results are compared to 30 keV Bi5 impacts, commonly used for molecular analysis and 2D imaging in our time-of-flight secondary ion mass spectrometry experiments. When Ar cluster impact conditions are appropriate for intact desorption, the take-off angle of the macromolecule is larger than the projectile incidence. Bombardment by a massive methane projectile (6.6 × 104 methane molecules) does not induce qualitatively different results, and in general, the energy per atom or energy per unit mass remains the deciding factor concerning the survival or dissociation of the bombarded molecule. In contrast with large gas clusters, Bi5 projectiles largely fail at desorbing the intact macromolecules, because of the detrimental effect of the collision cascade and the insufficient momentum transferred by the crater expansion for molecular lift off. Additionally, 10 keV Ar5000 projectiles with 45−75° incidence with respect to the surface normal directly transfer momentum to the macromolecule via their backscattered Ar atoms and small clusters, inducing molecular desorption with center-of-mass velocities of the order of 1 km/s. The implications for future experiments are discussed