Preparative Electrospray

Electrospray is not limited to applications involving chemical analysis, but is in fact used in a preparative sense quite frequently. The small size of droplets produced by electrospray makes them ideal for the production of micro- and nanoparticles from solutions. Particle formation is accomplished through solvent evaporation from electrosprayed droplets, which are subjected to charge neutralization (by interaction with a plasma) to prevent Coulombic fission events, and thus the formation of much smaller droplets. In cases where uniform particle size is desired, an AC frequency may be superimposed on the DC voltage applied to the emitter, creating controlled breakup of droplets of uniform size [33]. Thin films can also be readily produced by electrospray deposition (ESD) onto surfaces [34]. ESD for thin film production is advantageous in that droplets do not coalesce and are efficiently deposited on the target surface, factors governed by the highly charged nature of the droplets [35].

In recent years, electrospray has also been demonstrated as a means of performing chemical reactions at accelerated rates, relative to their solution-phase counterparts. The acceleration is likely due to several effects in evaporating microdroplets including a rapid concentration increase, an increase in relative surface area, and the large changes in pH [36, 37]. This effect occurring in elec- trosprayed droplets has been demonstrated through online derivatization of ketos- teroids for analysis by desorption electrospray ionization (DESI) [36], a base-catalyzed Claisen-Schmidt condensation [38], the cross-linking of peptides [39], and in the Hantzsch synthesis of 1,4-dihydropyridines [40]. The accelerated reaction rates occurring in small droplets and upon the impact of electrosprayed droplets with surfaces provides a unique method of exploring reactions with small quantities of reagents and solvent that is easily coupled with MS for analysis of reaction products.

In addition to organic synthesis, nanoESI has been used to synthesize metallic nanoparticles (NPs) on surfaces. In these experiments, a wire composed of the metal of interest is used to apply a high potential to a solution of acetonitrile (ACN) in a nanospray emitter. Metal ions are generated in solution by electrolysis of the wire, and are carried by the resulting charged droplets to a grounded surface. At the surface, the metal ions are reduced and aggregate to form NPs. Particle size is dictated by the number of ions deposited, a function of both electrospray current and time. NPs generated by this method have been used as catalysts for organic reactions as well as Raman active substrates for surface enhanced Raman spectroscopy (SERS) [41, 42]. Because the particle size is easily controlled, SERS active substrates can be tuned for different excitation wavelengths. Moreover, ions can be directed to distinct regions through the use of atmospheric pressure ion optics and electrostatic masks.

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