Toward multiplexed quantification of biomolecules on surfaces using time-of-flight secondary ion mass spectrometry

Accurate detection and quantification of individual molecules is important for the development of improved diagnostic methods as well as biochemical characterization of disease progression and treatments. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a surface analysis technique capable of imaging the distribution of specific molecules on surfaces with a high spatial resolution (<1 μm) and high sensitivity. ToF-SIMS is particularly suitable for detection of molecules up to ∼2 kDa, including lipids, whereas larger molecules, such as peptides and proteins, are fragmented during analysis, which makes them difficult to identify. In this study, an approach for extending the molecular detection capability of ToF-SIMS is presented, based on the specific binding of functionalized liposomes to molecular targets on the sample surface and subsequent detection of the liposomes by ToF-SIMS. Furthermore, by using different recognition elements conjugated to liposomes with different lipid compositions, simultaneous detection of different targets was accomplished. This multiplexing capability was investigated for two types of recognition elements (antibodies and cholera toxin) and for target molecules immobilized on surfaces using two frequently applied surface functionalization strategies: a supported lipid bilayer aimed to mimic a cell membrane and a polyethylene glycol modified surface, commonly employed in bioanalytical sensor applications. The efficacy of the conjugation protocols and the specificity of the recognition mechanism were confirmed using quartz crystal microbalance with dissipation monitoring, while fluorescence microscopy was used to validate the ToF-SIMS data and the reliability of the freeze-drying step required for ToF-SIMS analysis. The results demonstrated specific binding of the two types of liposomes to each target and showed a concentration-dependent binding to the targets on the different model surfaces. In particular, the possibility to use the contrasts in the mass spectra of SIMS to identify the concentration dependent coverage of different liposomes opens up new opportunities for multiplexed detection and quantification of molecules at biotechnology relevant interfaces.