Self-Assembly of Synthetic and Biological Polymeric Systems of ...

typical AFM is shown. Cantilevers commonly are either V-shaped or a rectangular “diving
board” shaped. The cantilever posses a sharp tip at its free end, which acts as the interaction
probe. This probe has most commonly the form of a square-based pyramid or cylindrical cone.
The probe is brought into and out of contact with the sample surface by the use of a piezo-
crystal where the cantilever or the surface itself is mounted, depending on the particular
equipment used. The movement in this direction is conventionally referred to as the Z-axis. A
laser light beam is reflected from the reverse side of the cantilever onto a position-sensitive
photodetector. The configuration of the photodetector consisted of a quadrant photodiode
divide into four parts with a horizontal and vertical dividing line. If each section of the detector
is labelled from A to D, as shown in Figure 2.12, then the deflection signal is calculated by the
signal difference detected in each part of the detector (38).
2.6.- Fluorescence spectroscopy
Once a molecule is excited by the absorption of a photon, this can return its ground state
through either fluorescence emission, internal energy conversion (i.e. direct return to the
ground state without fluorescence emission for example, by heat radiation), intersystem
crossing (possibly followed by phosphorescence emission), intramolecular charge transfer
and/or conformational change. All these processes can be visualized in the Perrin-Jablonski
diagram (Figure 2.13). The singlet electronic states are denoted as S0 (fundamental electronic
state), S1, S2, ... and the triplet states as, T1, T2, ..., with different vibrational levels associated
with each electronic state. It is important to note that energy absorption is very fast (≈10 -15 s)
with respect to all other processes (so that there is no concomitant displacement of the nuclei
according to the Franck-Codon principle) (39)(40). The vertical arrows corresponding to the
energy absorption process start from the 0 vibrational energy level, S0, since most of molecules
are in this state at room temperature. Absorption of a photon, hence, can bring a molecule to
one of the vibrational levels of S1, S2, ...
Emission of photons accompanying the S1 S0 relaxation is called fluorescence. It should be
emphasized that, apart from a few exception, fluorescence emission occurs from S1 and,
therefore, its characteristics (except polarization) do not depend on the excitation wavelength
(of course only one species exists in the ground state). The transition between the ground
state and the excited stated (0-transition) is usually the same for absorption and fluorescence.
However, the fluorescence spectrum is located at higher wavelengths than the absorption one
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