El. Florin et al., PHOTONIC FORCE MICROSCOPE BASED ON OPTICAL TWEEZERS AND 2-PHOTON EXCITATION FOR BIOLOGICAL APPLICATIONS, Journal of structural biology, 119(2), 1997, pp. 202-211
A new scanning probe microscope, the photonic force microscope (PFM),
based on optical tweezers and two photon absorption processes for biol
ogical applications is described. Optical tweezers are used to trap a
fluorescent latex bead with a diameter of 200 nm in an aqueous solutio
n in all three dimensions. The fluorescent dye is chosen to fulfill th
e two-photon absorption criterion for the 1064-nm line of a Nd:YVO4 la
ser. The intensity of the fluorescence emission is utilized as a very
sensitive position sensor along the optical axis. Two-dimensional imag
es are formed by laterally scanning the trapped latex bead across biol
ogical samples while recording the two-photon-induced fluorescence int
ensity. A scanning probe image of the outer surface of a small neurite
from a cultured rat hippocampal neuron is shown, which is hardly visi
ble under differential interference contrast microscopy. The lateral r
esolution is given by the bead diameter; the axial resolution is 40 nm
. Under the experimental conditions the maximal imaging force applied
by the probe is below 5 pN. (C) 1997 Academic Press.