Optical beam deflection near a heated surface was recently introduced as a method of photothermal spectroscopy. Photothermal spectroscopy (PTS) is closely related to photoacoustic spectroscopy (PAS) in its ability to measure optical properties of opaque solids and liquids. This paper develops a theory of this effect and compares this theory with extensive experimental observations. Both are in excellent agreement. The relationship between this form of PTS and PAS is explicitly developed. Applications to the measurement of thermal diffusivity of gases is described.
The effect of a nontoxic, nonionic block co-polymeric surface active agent, poloxamer 188, on electroporation of artificial lipid membranes made of azolectin, was investigated. Two different experimental protocols were used in our study: charge pulse and voltage clamp. For the charge pulse protocol, membranes were pulsed with a 10-micronsecond rectangular voltage waveform, after which membrane voltage decay was observed through an external 1-M omega resistance. For the voltage clamp protocol the membranes were pulsed with a waveform that consisted of an initial 10-microsecond rectangular phase, followed by a negative sloped ramp that decayed to zero in the subsequent 500 microseconds. Several parameters characterizing the electroporation process were measured and compared for the control membranes and membranes treated with 1.0 mM poloxamer 188. For both the charge pulse and voltage clamp experiments, the threshold voltage (amplitude of initial rectangular phase) and latency time (time elapsed between the end of rectangular phase and the onset of membrane electroporation) were measured. Membrane conductance (measured 200 microseconds after the initial rectangular phase) and rise time (tr; the time required for the porated membrane to reach a certain conductance value) were also determined for the voltage clamp experiments, and postelectroporation time constant (PE tau; the time constant for transmembrane voltage decay after onset of electroporation) for the charge pulse experiments. The charge pulse experiments were performed on 23 membranes with 10 control and 13 poloxamer-treated membranes, and voltage pulse experiments on 49 membranes with 26 control and 23 poloxamer-treated membranes. For both charge pulse and voltage clamp experiments, poloxamer 188-treated membranes exhibited a statistically higher threshold voltage (p = 0.1 and p = 0.06, respectively), and longer latency time (p = 0.04 and p = 0.05, respectively). Also, poloxamer 188-treated membranes were found to have a relatively lower conductance (p = 0.001), longer time required for the porated membrane to reach a certain conductance value (p = 0.05), and longer postelectroporation time constant (p = 0.005). Furthermore, addition of poloxamer 188 was found to reduce the membrane capacitance by approximately 4-8% in 5 min. These findings suggest that poloxamer 188 adsorbs into the lipid bilayers, thereby decreasing their susceptibility to electroporation.
Optical-beam deflection (OBO) photothermal imaging uses spatially localized excitation to observe spatial variations in the sample surface temperature. This paper analyzes OBO signals produced by localized excitation in terms of three-dimensional thermal diffusion in the sample and in the fluid region in contact with the sample. The dependence of the signals on the local optical absorption coefficient, on gas/sample thermal properties, on modulation frequency, and on the probe/excitation beam radii are discussed with special attention being given to determining the spatial resolution possible for OBO imaging. A criterion for photothermal "saturation" appropriate to localized optical absorption is developed. Finally, a new variant of the OBO technique is introduced, which is especially adapted to studying optical and thermal boundaries in the plane of the sample. Some comparisons between theory and experiment are provided which illustrate transverse thermal diffusion.
The effect of thermal parameters on photothermal spectroscopy and photothermal imaging for certain types of spatially heterogeneous samples has been studied. Results have been obtained for both photoacoustic and photothermal deflection modes of detection. Specific attention has been paid to optical saturation in samples whose properties exhibit gradual spatial changes and to the dependence of the photothermal signal on the individual thermal parameters. Most photothermal spectroscopy has been done using thermally homogeneous samples, and most photothermal imaging using optically saturated samples. In this paper, these limitations are relaxed, and the photothermal signal is assumed to depend simultaneously on gradual spatial variations in the optical absorption coefficient, thermal conductivity, and thermal capacity of the sample.
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