Clearly, a relatively high fraction of aromatic C ar -H is prevalent in the nucleobases of ss-DNA, while the abundance is much less in natural amino acids and hence in the protein as well. Also, it is expected that ss-DNA binds the target protein and thus the appearance of the ν(C ar -H) band in the protein spectrum is not surprising. However, no C ar -H exists in the PEG molecule. The occurrence of the ν(C ar -H) band in the spectrum of the PEG rich phase is similar to the occurrence of the ν s (PO 2 ) -in this phase. One might attribute this to an intermixing of aptamer in the PEG, however, it must be remembered that the resolution of AFM-IR is limited to about 20 nm to 30 nm which is in the range of the lateral domain size observed here. This means that it cannot be unambiguously concluded whether the PEG-rich phase consists exclusively of PEG or has the aptamer intermixed. Taken together nano-IR spectroscopy in the higher wavenumber range supports the conclusion of a phase separated receptor layer and the observation of analyte binding on the single molecule level.