Optoacoustic conversion in diluted suspensions of medical and biological interest under the effect of laser pulses is
considered. The various conversion regimes with moderate values of the laser fluence are studied both theoretically and
experimentally.
The sensitivity of optoacoustic (OA) detection in diluted suspensions of gold nanoparticles under irradiation with nanosecond laser pulses was studied as a function of incident laser fluence. The range of moderate values of the laser fluence from 20 mJ/cm2 to 2 J/cm2 was studied theoretically and experimentally. Under these laser fluences, the usual thermoelastic mechanism of OA generation faces competition from laser-induced cavitation, a statistical process, which leads to considerable fluctuations of the acoustic response from one laser pulse to another. Analytical expressions for the statistical characteristics of the acoustic signal were obtained. A simulation of the statistical characteristics of the cavitation contribution to the signal was performed using the method of Monte Carlo. The experiment utilized the second harmonic pulses (532 nm) of an Nd:YAG laser to irradiate samples of water suspensions of spherical gold nanoparticles (NPs). A series of laser pulses each having from 100 to 2000 pulses were used to iradiate the samples. The statistical rank distributions of the magnitudes of optoacoustic signals recorded by a wide band ultrasonic transducer attached to the measurement cell were used as a tool for sensitive detection of a low concentration of the gold nanoparticles in water.
Regularities in some complex systems can sometimes be expressed in terms of simple laws. A peculiar regularity is identified concerning photoacoustic (optoacoustic) phenomena. In particular, the acoustic signals following phase transitions in liquid irradiated by laser pulses are distributed by magnitude according to the Zjif’s law. This power law distribution describes many man made and naturally occurring phenomena, including city sizes, incomes, word frequencies, and earthquake magnitudes. This law suggests connection with anomalous decay, i.e. it implies that small occurrences are extremely common, whereas large instances are extremely rare. We use this law for signal processing in the course of optoacoustic diagnostics of diluted suspensions. The irradiation of an inhomogeneous liquid sample with a long train of short laser pulses and subsequent recording of a histogram of the magnitudes of the acoustic responses can serve as a diagnostic tool for various applications. The absorption of an incident light by a suspended particle may cause a cavitation event. The random cavitation events also obey the Zipf’s law, this fact being used for detection of individual particles.
Pyrogens being introduced intravenously increase body temperature that leads to hazardous consequences and even to lethal outcome. One of the widespread pyrogen systems is presented by suspensions composed of bacterial endotoxins (or lypopolysaccharides, LPS). The aim of the work is to compare experimentally two methods for the determination of LPS at the submicrogram level and below. Both methods suppose that the LPS suspension is irradiated by a laser pulse. The thermal lens (TL) method (microsecond to millisecond irradiation cycle) detects LPS by a direct pick-up of the transient thermal field. The optoacoustic (OA) method (nanosecond laser pulses) has a potential to use non-thermal constitutents of the LPS response and to provide some selectivity of LPS detection with respect to optically uniform contaminants in the sample. In experiments, the selectivity was enhanced by means of analytical reagents, methylene blue and Stains All dyes. It was shown that both methods are mutually complementary. Then, their detectability potential increases and reaches 10 ppb if there occur ion pairs of LPS and cationic dye.
The results are presented which form the basis against which the technique of photoacoustic PA diagnostics of inhomogeneous liquid samples can be developed. Usually, a thin-layer small-volume inhomogeneous probe presents a considerable challenge for diagnostics. The paper shows that gas-microphone PA technique works well here, and moreover it presents an added bonus if the thickness of the layer is close to the dimensions of immersed inhomogeneities.
An optoacoustic device consisting of YAG laser and a measurement cell with an attached piezotransducer was used to detect atmospherical microparticles as well as artificial latex suspension. Monte Carlo simulation was used to predict the statistical parameters of the acoustical response.
The optoacoustic device consisting of a XeCl excimer laser and a measurement cell with an attached piezotransducer was used for detecting microparticles suspended in liquid probes. The potential of optoacoustic diagnostics of microinhomogeneous liquids was enhanced by applying information parameters of optoacoustic response. Probes of distilled water and Dow Chemical latex suspension were used in experiments. It was shown that the optimal laser energy deposit for individual particle detection corresponds to the coexistence of the thermal expansion mechanism of optoacoustic conversion and the cavitation mechanism.
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