Gold Nanoparticles as a Matrix for Visible-Wavelength Single-Particle Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Small Biomolecules

Abstract

Gold nanoparticles (GNPs) are used as the matrix for visible-wavelength matrix-assisted laser desorption/ionization (VIS-MALDI) of individual aerosol particles containing ∼50 attomole of a small peptide. A dual polarity time-of-flight mass spectrometer was used to obtain both positive and negative ion mass spectra simultaneously from individual particles using a tunable wavelength desorption/ionization laser. The wavelength of the laser was changed from λ = 440 to 680 nm to observe the wavelength dependence of analyte ion formation. Detection of the positive sodiated molecular ions and negative deprotonated molecular ion of a small peptide was only possible using 5-nm GNPs and not with larger sized (19- and 44-nm) GNPs. While the masses of gold within the sample particles were similar, surface areas were about 10 times more in the 5-nm GNPs, suggesting the total surface area of GNPs within the sample particles may play a role in the formation of molecular ions. At wavelengths near the peak plasmon resonance of the GNPs (λ = 500−540 nm), negative molecular ion signals from a small peptide was higher than with desorption/ionization at λ = 440 nm, with increased fragmentation observed at λ = 440 nm. At wavelengths longer than the peak plasmon absorption, the ability to generate a detectable ion signal decreased rapidly, which is consistent with the steep decrease in the absorbance of GNPs by surface plasmon resonance at these wavelengths. Silver nanoparticles, which also exhibit a surface plasmon resonance, were tested and under our conditions did not appear to work as well. The presented results demonstrate that noble metal nanoparticle matrices can be used for on-line VIS-MALDI analysis of small molecular weight species such as peptides or sugars.