Electrospray Ion Beam Deposition of Clusters and Biomolecules

An ion beam source using electrospray ionization is presented for nondestructive vacuum deposition of mass‐selected large organic molecules and inorganic clusters. Electrospray ionization is used to create an ion beam from a solution containing the nanoparticles or molecules to be deposited. To form and guide the ion beam, radio frequency and electrostatic ion optics are utilized. The kinetic energy distribution of the particles is measured to control the beam formation and the landing process. The particle mass‐to‐charge ratio is analyzed by in situ time‐of‐flight mass spectrometry. To demonstrate the performance of the setup, deposition experiments with gold nanoclusters and bovine serum albumin proteins on graphite surfaces were performed and analyzed by ex situ atomic force microscopy. The small gold clusters are found to form three‐dimensional agglomerations at the surface, preferentially decorating the step edges. In contrast, bovine serum albumin creates two‐dimensional fractal nanostructures on the substrate terraces due to strong intermolecular interactions. Protein deposition: A general vacuum deposition apparatus for large molecules and clusters has been constructed by combining electrospray ionization and differential pumping. The energy and composition of the particle beam can be controlled by ion optics and mass spectrometry. The deposition technique was demonstrated with albumin proteins, which form fractal agglomerations on a substrate surface (see AFM image), and gold colloids, which are organized into 3D clusters.