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Digital archive of theses discussed at the University of Pisa


Thesis etd-02102009-123443

Thesis type
Tesi di dottorato di ricerca
Thesis title
Polymeric surfaces characterization by Atomic Force Microscopy
Academic discipline
Course of study
Relatore Prof. Solaro, Roberto
Relatore Prof. Cacialli, Franco
  • AFM
  • Biomaterials
  • SPM
  • Surfaces
Graduation session start date
Release date
The atomic force microscope (AFM) is a very promising and powerful tool for investigating a range of materials such as, oligopeptides, proteins, soft organic polymers as well hard materials (silicon and metals). The AFM consists of a microsized cantilever with a sharp probe at its end, with a radius of curvature of a few nanometers that is used to scan the specimen surface. When the probe is brought into proximity of the sample surface, forces between the probe and the sample lead to a relatively small deflection of the cantilever ruled by Hooke's law. Traditionally, the sample is mounted on a piezoelectric scanner that can move the object under examination in the z direction for maintaining a constant force, and in the x and y directions for scanning the sample.
The present thesis was focused on following main research lines: chemical force microscopy, phase imaging, mechanical characterization of materials, and high resolution imaging of biological materials (such as cells and proteins). The results obtained in these investigations are summarized by following.
In the first research line, chemical force microscopy was exploited to monitor the forces involved in protein swelling experiments. The basic idea was to ascertain the possibility of studying the binding of ligands attached to the AFM tip with surface-bound receptors by applying an increasing force to the complex until it dissociates at a measurable unbinding force. Accordingly, the behaviour of human serum albumin was tested by linking the protein to both the AFM tip and functionalized silicon surface. The loading-unloading curves presented two sequential steps that were attributed to albumin uncoiling and to the detachment of the protein from the surface, respectively.
The second line tackled the characterization of selected oligopeptides that self-aggregate to give different complex structures depending on the peptide structure, its concentration and the composition of the starting solution. The measurements were performed in the so-called “tapping mode” which is capable of acquiring both morphological and phase maps of the sample surface. The phase maps provides nanometre-scale information about surface structure and properties often not readily available by conventional techniques These measurements gave information on the dependence of oligopeptide aggregate size and shape from both the peptide structure and concentration. Basically, both the mean size and the dispersion of aggregates increased on increasing the oligopeptide concentration. A similar behaviour was found also for oligopeptides in buffer solution. However, in this case the size distribution was quite narrow.
Investigation of the mechanical properties of materials constituted the third goal of this thesis. A technique that can probe mechanical properties at the nanoscale has the potential to answer many important questions in the field of nanotribology and nanomechanics. Nanoindentation involves the application of a controlled load over the surface to induce local deformations. In the development of the present investigation, different materials including poly(methyl methacrylate), polystyrene, silicon, gold, aluminium, and Balinit, a very hard antiwear coating used in special engines, were tested. Nanoindentation measurements gave information on the hardness and the Young modulus at the nanoscale. Several theoretical models were considered and new ones were developed. Our model takes into account the actual shape of the probe, and provided a viable approach, as demonstrated by the substantial agreement between results and expectations and, more importantly, by the use of two different kinds of AFM probse, both correctly described in terms of shape and geometry.
The characterization of biologic materials by atomic force microscopy is the fourth topic of this thesis. In particular, AFM was used to determine the topological and structural features of rabbit spermatozoa. This study was addressed at investigating rabbit normal spermatozoa in order to set up a reference standard. Fresh ejaculated spermatozoa were adsorbed either passively on top a silicon slide or by motility from suspension on poly(L-lysine)-coated glass cover slips and then imaged in air and in buffer saline, respectively. AFM images clearly highlighted many details of spermatozoa head, neck, and tail. Distinct features were observed in the plasmatic membrane of spermatozoa. Altogether, the collected AFM images clearly defined a detailed map of spermatozoa outer morphology while giving some hints on the internal structure.
Finally, a method was investigated to induce phase separation in thin films of conjugated polymers blends used to build optoelectronic devices. Indeed, it is possible to tune the properties of optoelectronic devices by blending two different conjugated polymers in the same film. In particular, it is possible to improve the devices performance by a suitable choice of the blend components. To this end, the scanning thermal microscope (SThM) represents a new instrument for controlling the phase separation in such blends. The hot probe of the microscope, which is held in contact with the polymer film, heats the polymer locally and can be scanned to induce the phase separation, by reptation, in arbitrary patterns. The SThM was used to increase and/or to modify the phase separation in blends of an electron–transporting polymer poly(9,9’–dioctylfluorene–alt–benzothiazole) and a hole–transporting polymer poly(9,9’–dioctylfluorene–alt–bis–N,N’–(4–butylphenyl)–bis–N,N’–phenyl–1,4–phenylenediamine). Fluorescence, AFM, and micro-Raman analysis confirmed the phase separation, thus demonstrating that it is possible to induce a phase separation locally and with a well–determined architecture in such polymer blends.