In this paper, the authors demonstrate how measurements of drug-loaded polymeric nanoparticles loosely adhered to a substrate can be taken using tapping AFM-IR spectroscopy and imaging. Infrared (IR) spectroscopy is well established as an important measurement capability for chemically characterizing the molecular structure of materials. New nanoscale chemical characterization capabilities, such as photothermal AFM-IR spectroscopy, have enabled the determination of nanoparticle structures, resulting in an improved understanding of important drug delivery mechanisms.
Until recently, however, AFM-IR could only be performed in contact mode, which can be problematic when samples such as polymeric drug-loaded nanoparticles are loosely adhered to a substrate. This is the theoretical basis of using the tapping AFM-IR approach for obtaining nanoscale IR spectra and chemical images described. The results obtained with this approach establish that the incorporated drug molecule pipemidic acid was preferentially accumulated in the outer layers of the 40-nm-diameter nanoparticles which were shown to consist of a poly(vinyl acetate) (PVA) core surrounded by a poly(lactic acid) (PLA) shell structure.