Listening to Colloidal Silica Samples: Simultaneous Measurement of Absorbed and Scattered Light Using Pulsed-Laser Photoacoustics

Abstract

Laser photoacoustic spectroscopy (LPAS) has been used to simultaneously measure scattered light and absorbed light with the use of a single piezoelectric transducer detector. Samples of Ludox™ colloidal silica, with and without added potassium chromate, were illuminated with pulses of 308 or 355 nm light. Signals were measured with a 1 MHz ultrasonic transducer clamped to the side of the cuvette. The resulting oscilloscope tracing shows a pattern of signals originating from light scatter and light absorption that is consistent with experimental geometry and the speeds of light and sound, and linear with incident laser pulse energy. The absorbed light signals are independent of the distance of the laser beam from the transducer, while the scattered light signals are strongly distance-dependent. The absorbed light signals are more indicative of true sample absorbance than are readings from a standard spectrophotometer. A spectrophotometer sums contributions from scattering and absorbance to give an optical density reading, while LPAS separates scattering from absorbance to give quantitative information on each function. Scattered light and fluorescent light behave very similarly in the LPAS technique. Our experiments were done with well-characterized colloidal silica samples, but the technique is readily extended to larger, more heterogeneous, and photochemically complex biological and environmental samples.

Far- and near-field properties of gold nanoshells studied by photoacoustic and surface-enhanced Raman spectroscopies

“…The reason for exploiting this method is that the PAS signal, being generated by a photothermal process involving absorption and heat release, is intrinsically insensitive to light scattering. 38 Therefore, scattering contributions will be the cause of an amplitude loss when the PAS signal of a nanoparticle sample is compared with that of a calorimetric reference. This is a substance which immediately releases all the energy absorbed from the excitation light as heat, 39 and negligibly scatters.…”

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

“…This signal, along with the 'normal' acoustic wave coming from absorbance in the solution, has been used to quantitate absorbance and scattering from a colloidal silica solution. 54 Comparison with spectrophotometric data has shown that the determination of absorbance using this method is potentially superior.…”

Triplet quantum yields in light-scattering powder samples measured by laser-induced optoacoustic spectroscopy (LIOAS)

“…18 However, in spite of the intense work dedicated by several research groups to the determination of quantum yields in particulate light scattering systems, the determination of triplet quantum yields, Φ T , remains elusive and eventually based on uncertain assumptions, such as neglecting internal conversion. 30 In addition to its well known application to solution systems, [31][32][33] laser-induced optoacoustic spectroscopy (LIOAS) has been successfully applied to the study of non-radiative processes in turbid suspensions, [34][35][36] thin films, 37,38 intact leaves, 39,40 and polymeric matrices, 41 among others. Regarding dyed powders, application was restricted to the measurement of absorption spectra due to the difficulty in achieving reproducible acoustic contact.…”