Plasmonic Aluminum Thin Films as Substrate Materials for Label-Free Optical Detection and Surface-Enhanced MALDI Mass Spectrometry

In this work, we report the plasmonic properties of aluminum films as substrate materials for multiple analytical platforms, including surface plasmon resonance (SPR) and MALDI-MS. The intrinsic optical sensitivity was characterized with ionic polymer coatings, lipid vesicles, and medically relevant biomarkers. In SPR imaging mode, the aluminum film allowed for the sensitive quantification of kinetic differences of binding interactions between the ionic polymer and biomarker peptides of CXCL8 and CXCL10. The binding was found to be correlated to the charge densities of the biomarkers and the polymer coating, and the use of an artificial urine matrix could alter the association behavior. The e-beam fabricated Al film was also shown to be effective for enriching phosphorylated peptides from milk proteins for mass spectrometric profiling. The surface-assisted ionization process was further investigated by comparing MALDI spectra of biomarkers obtained on conventional stainless steel plates, Au films, and Al films. Results indicate that aluminum films have m/z intensity values significantly higher than those on a steel plate and Au film, suggesting the electronic and plasmonic properties of aluminum thin films, especially those under UV conditions, may lead to an improved performance in MALDI signals. We believe that Al thin films have great potential as substrates for developing bioanalytical methods and can have vast benefits for the future study of biophysical interactions.