Analyte chemisorption and sensing on n- and p-channel copper phthalocyanine thin-film transistors.

Chemical sensing properties of phthalocyanine thin-film transistors have been investigated using nearly identical n- and p-channel devices. P-type copper phthalocyanine (CuPc) has been modified with fluorine groups to convert the charge carriers from holes to electrons. The sensor responses to the tight binding analyte dimethyl methylphosphonate (DMMP) and weak binding analyte methanol (MeOH) were compared in air and N(2). The results suggest that the sensor response involves counterdoping of pre-adsorbed oxygen (O(2)). A linear dependence of chemical response to DMMP concentration was observed in both n- and p- type devices. For DMMP, there is a factor of 2.5 difference in the chemical sensitivity between n- and p-channel CuPc thin-film transistors, even though it has similar binding strength to n- and p-type CuPc molecules as indicated by the desorption times. The effect is attributed to the difference in the analyte perturbation of electron and hole trap energies in n- and p-type materials.

Electrode independent chemoresistive response for cobalt phthalocyanine in the space charge limited conductivity regime.

The electrical properties of 50 nm thick metallophthalocyanine films, prepared by organic molecular beam epitaxy (OMBE) on interdigitated electrodes, were studied with DC current-voltage measurements and impedance spectroscopy. The transition from Ohmic behavior at low voltages to space-charge-limited conductivity (SCLC) at higher voltages depends on the metal electrode (Pt, Pd, and Au), but does not correlate with the work function of the electrode. Impedance spectroscopy studies show the coexistence of low- and high-frequency traps in the thin film devices, and the contribution of low-frequency traps associated with Ohmic behavior diminishes at higher bias. Although device resistances are strongly influenced by the electrode material, and vary by a factor of over 300, the relative chemical sensor responses on exposure to dimethyl methylphosphonate (DMMP), methanol, water, or toluene vapors are similar for CoPc on Pt, Pd, and Au electrodes when these devices are operated in the SCLC regime at room temperature. When the devices are operated at voltages where the low-frequency interfacial traps are filled, the sensor response to analyte becomes uniform and reliable regardless of the specific interfacial electrode contact.

The Vroman effect: a molecular level description of fibrinogen displacement.

The molecular level details of the displacement of surface adsorbed fibrinogen from silica substrates were studied by atomic force microscopy, immunochemical assays, fluorescence microscopy, and vibrational sum frequency spectroscopy. The results showed that human plasma fibrinogen (HPF) can be readily displaced from the interface by other plasma proteins near neutral pH because the positively charged alpha C domains on HPF sit between the rest of the macromolecule and the underlying surface. The alpha C domains make weak electrostatic contact with the substrate, which is manifest by a high degree of alignment of Lys and Arg residues. Upon cycling through acidic pH, however, the alpha C domains are irreversibly removed from this position and the rest of the macromolecule is free to engage in stronger hydrogen bonding, van der Waals, and hydrophobic interactions with the surface. This results in a 170-fold decrease in the rate at which HPF can be displaced from the interface by other proteins in human plasma.

Thermodynamics of phase transitions in langmuir monolayers observed by vibrational sum frequency spectroscopy.

Vibrational sum-frequency spectroscopy (VSFS) was used to study gauche defects in octadecylamine (ODA) monolayers at the air/water interface. The VSFS spectra provide unique insights into phase transitions that occur as a result of changes in the structure of the monolayer's hydrophobic region. These changes can be attributed to the increased presence of gauche conformers in the ODA alkyl chains during the monolayer's transition from the solid to liquid phase. Temperature-dependent spectra from monolayers at several different pressures were used to assign the phase transition temperature based on the observed changes in microscopic structure. Through application of a two-dimensional form of the Clapeyron equation, the first in situ measurements of the entropy and enthalpy changes associated with gauche conformers in a monolayer were made.