Nanoporous GaN-Ag composite materials prepared by metal-assisted electroless etching for direct laser desorption-ionization mass spectrometry.

Three-dimensional nanoporous gallium nitride(PGaN) produced by metal-assisted electroless etching is chemically embedded with silver nanoparticles via electroless deposition, forming a metallized semiconductor membrane with large surface area and nanoscale metal features. A new application utilizing the unique chemical and morphological features of these composite nanostructures is described here, laser induced desorption-ionization(LDI) of biomolecules(e.g., cholesterol and nucleotides) for direct mass analysis, without use of additional organic matrix. Although PGaN itself is a poor matrix for direct LDI mass spectrometry, the combination of Ag and PGaN greatly improves ion signals relative to PGaN or Ag nanostructure surfaces alone. This behavior is attributed to the combination of strong UV absorption, enhanced surface area, and favorable thermal properties of PGaN. Importantly, Ag-PGaN is shown to facilitate the formation of Ag adduct ions in some cases, for example adenine, where adducts are not observed from either porous anodic aluminum membranes or surfaces presenting Ag nanoparticles in isolation. Nanopore-embedded Ag nanostructures serve a dual role: as cationization agents and to assist thermal desorption under UV laser irradiation. The results reported here suggest that the combination of Ag nanostructures embedded in PGaN has the capacity for high quality matrix-free LDI mass analysis.