Fig. 3
Confirmation of AuNP anchoring to the outer cell surface by TEM. Images were collected from MKN28 cells. The cells were fixed after 1 h from the completion of AuNP attachment, in order to confirm also the lack of endocytosis during SERS experiments. The cells embedded into resin were trimmed through their thickness using an ultramicrotome to obtain 90 nm thin sections. The images in a and b were taken from the same section, but at different magnification (7 and 17 k, respectively). Similarly, images in c–e belong to another section of the same cell at magnification 17, 50, and 50 k, respectively. In b the pH-sensitive 4-MBA SAM of each AuNP illuminated by the laser beam is depicted with green lines. The thickness of the lines does not represent the real thickness of the SAM, which can be assumed as 0.78 nm31. Red arrows in d indicate sections of microvilli. The 90 nm thickness of the slices for TEM analysis is equal to the average size of the AuNP, namely some of the AuNP were trimmed by the diamond knife. For this reason, in the nanosensors indicated by the green arrow in e the point of contact with the microvillus may have been removed during trimming. The blue arrow in e points out an aggregation of AuNP lying on outer cell membrane, whose maximum thickness can be estimated as about 100 nm. Scale bars: 1 µm in a, 500 nm in c, 200 nm in d and e. In b the laser beam waist is 700 nm (length between the two white arrows)
