The application of metal nanoparticles as an efficient drug delivery system is one of the directions of cancer therapy development. However, this strategy requires precise information about how the drug interacts with the applied nanocarrier. In this study, atomic force microscopy combined with infrared spectroscopy (AFM-IR) was used for the first time to investigate the erlotinib adsorption structure on two different types of 15 nm metal nanoparticle mono-layers, namely, silver nanoparticle (AgNP) and gold nanoparticle (AuNP) mono-layers. Because the metal nanoparticles are loosely bound samples, only the tapping AFM-IR mode is suitable for the collection of IR maps and spectra for such a system. The obtained results indicated the relevance of the AFM-IR technique for characterizing drug interactions with a metal mono-layer surface. The investigated drug interacts with the AgNPs mainly through phenyl rings and methoxy moieties, while quinazoline, amino, and ethoxy moieties appear to be farther from the surface. For the AuNPs, the interaction occurs through both the phenyl ring and the quinazoline moiety. Additionally, the aliphatic groups of erlotinib directly participate in this interaction. The novelty of the present work is also related to the use of the tapping AFM-IR mode to study metal NP mono-layers with a drug adsorbed on them. The collected IR maps for the most enhanced erlotinib bands show specific areas with very high signal intensity. The connection between these areas and the "hot spots" typical for the surface plasmon resonance phenomenon of metals is considered.

A novel approach of combining conventional infrared spectroscopy (IR) and atomic force microscopy (AFM) is presented to better understand the behavior of a drug adsorbed on a metal substrate at the nanoscale level. Tip-enhanced infrared nanospectroscopy (TEIRA) was used for the first time to investigate Lu AA33810, a selective brain-penetrating Y5 receptor antagonist, after immobilization on gold nanoparticles (GNPs). Here, a gold coated AFM tip and gold substrate were used to obtain the near-field electromagnetic field trapping effect. Because of the huge signal enhancement, it was possible to obtain the spectral information regarding the self-assembled monolayer of the investigated molecule. The effect of two orthogonal polarizations (p- and s-polarization modulations) of the excitation laser beam on the spectral patterns is also discussed. The results show that there is a strong relationship between the state of polarization of the incident radiation and the relative infrared band intensities. Another factor affecting the observed spectral differences is the topology of the metal substrate, which may result in the induction of a cross-polarization effect. The performed analysis indicates that the C–C bond from the cyclohexyl group is oriented almost parallel to the metal surface. Conversely, the p- and s-polarized spectral variations suggest that the O=S=O angle is high enough to enable the simultaneous interaction of both oxygen atoms with the GNPs.