Single drop microextraction using tetraalkylammonium bromide coated silver nanoparticles (SDME-AgNPs) prepared in toluene has been successfully applied as electrostatic affinity probes to preconcentrate peptide mixtures in biological samples prior to atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry (AP-MALDI-MS) analysis. This approach is based on the isoelectric point (pI) of peptides and surface charge of AgNPs. Using the SDME-AgNPs technique, from a peptide mixture, Met- and Leu-enkephalins (Met-enk and Leu-enk) were extracted into a droplet of toluene containing AgNPs, but not the neutral peptides (gramicidins). The best peptide extraction efficiency for SDME-AgNPs was observed with the optimized parameters: extraction time 2 min, sample agitation rate 240 rpm, and sample pH 7. The limits of detection (LODs) of the SDME-AgNPs/AP-MALDI-MS technique for Met-enk and Leu-enk peptides were 160 and 210 nM, respectively. Furthermore, the application of the technique has been shown for the analysis of peptides from a sample containing high matrix interferences such as 1% Triton X-100 and 6 M urea. Finally, this approach has been compared with the SDME-AuNPs technique and the results have clearly revealed that the SDME-AgNP affinity probe exhibits higher affinity to extract the sulfur-bearing peptide (Met-enk). We also compared this electrostatic affinity probe of AgNPs with the previously demonstrated hydrophobic affinity probe of AgNPs and found that the electrostatic probe can greatly reduce the extraction time from 1.5 h to 2 min. This is due to the fact that electrostatic attraction forces are much stronger than the hydrophobic attraction forces. Therefore, we concluded that the electrostatic affinity probe based on SDME-AgNPs coupled with AP-MALDI-MS is a high-throughput technique for the analysis of low-abundance peptides from biological samples containing complex matrices.
Rapid Communications in Mass Spectrometry 22(19), pp.3076-3086