Analysis and modification of surfaces using molecular ions in the ambient environment

The analysis and modification of surfaces in their native conditions can be performed using new mass spectrometric methods. Ambient ionization sources, including desorption electrospray ionization (DESI), have been implemented for the rapid analysis of unmodified biological surfaces including whole plant material, tissue sections, algae, and bacterial colonies. Recent advances have shown promise for in vivo and high-throughput clinical analysis. Additionally, the recent development of ambient ion soft landing (SL) allows polyatomic ions to be deposited onto surfaces in open air. Ambient SL offers speed, control, and flexibility for surface reactions and modification.

Introduction

In the broad sweep of developments in surface analytical methodology the following trends are readily discerned: first, the emphasis, when vacuum technology allowed this in the 1960s, on the study of ideal surfaces, represented by single crystal metal surfaces dosed with known amounts of adsorbates, second, the interrogation of such surfaces with an increasingly wide array of probe beams (electrons [1, 2], ions [3], molecular beams [4], and laser beams [5]) of well-defined properties (mass, kinetic energy, wavelength) with information on the chemical nature of the surface being carried by the scattered or released particles [6]; third, the progression from the measurement of elemental to molecular properties and at the same time from static surface characterization to characterization of dynamic surfaces [7] as exemplified by reacting surfaces [8], binding constants with particular reagents, surface adsorbate mobility, etc.; fourth, the progression from small molecule adsorbates to larger molecule covalently bound adsorbates, the latter being especially useful in biomimetic studies; fifth, increased interest in surfaces at higher pressures driven by the wish to use realistic conditions to study reactions at surfaces, whether biological reactions, catalytic chemical reactions, or otherwise.

Another trend discernable in surface science is the increased attention being given to the manipulation as opposed to the analysis of surfaces. That is, surface science no longer approximates to surface analysis; surface preparation and chemical modification of surfaces have become topics of importance. While dosing with reactive neutral reagents is one aspect of these developments, ionic reagents are preferred for many reasons — ease of formation of ions with desired chemical structure, isotopic composition and (especially) requisite translational energy. Reactive collisions of ions with surfaces are easy to study because both the scattered product ion and/or the chemically modified surface can be characterized. There are many simple examples from the past two decades of functional group chemistry being performed using mass-selected ions colliding at surfaces bearing organic adsorbates [9, 10, 11, 12, 13, 14, 15, 16]. These cases include textbook examples of the simplest heterogeneous reactions [13, 14], based on single-collision processes. The energy exchange between projectile, surface, and released products (internal and translational) has been a subject of some interest, too [17]. A special method of modifying surfaces is to use low energies and soft land the ions onto the surface [18, 19]. Neutralization is not a foregone conclusion and soft landing (SL) with and without neutralization has been studied also. ‘Shaggy’ surfaces can protect the landed ion from reagents and so facilitate charge retention in the intact landed ion.

A most striking development in surface science in the past decade has been the extent to which biomimetic systems have become a major topic for analysis and preparation. Methods for generating surfaces bearing covalently modified groups which react selectively with particular biologically significant compounds have become common [19, 20]. The surfaces are often formed in vacuum but can be reacted in solution as well as in the gas phase. Model systems that bind to nucleic acids or to particular peptide residues are also well known.