A new method is developed to perform local measurements of fluorophore excited state lifetimes in turbid media without collecting the fluorescence emission. The method is based on a pump-probe approach where a first laser pulse excites the dye and then a second laser pulse is used for photoacoustic probing of the transient absorption. The photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Repeating the measurement for increasing pump-probe time delays yields a series of photoacoustic signals that are used to extract the lifetime of the excited state. The method is validated by measuring the lifetime of an oxygen sensitive dye solution at different concentrations of dissolved oxygen. The dye is pumped with a 532-nm pulsed laser and the transient absorption at 740 nm is probed using a second pulsed laser system. The photoacoustic-based results are in close agreement with those obtained from time-dependent fluorescent measurements. The method can be extended to photoacoustic lifetime imaging by using a receiver array instead of a single receiver. Potential applications of this method include tissue oxygen imaging for cancer diagnostics and mapping molecular events such as resonant energy transfer and ion collisions in a biological environment.
We have developed a new method to perform local measurements of fluorophore excited state lifetimes in turbid
media without collecting the fluorescence emission. The method is based on a double pulse illumination where a first
laser pulse excites the dye and then a second laser is used for photoacoustic probing of the transient absorption. The
photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Varying the time delay
between excitation and probing allows for tracking the relaxation dynamics of the excited state. The method was
validated by measuring the lifetime of an oxygen sensitive dye (Pt(II) octaethylporphine) solution at different
concentrations of dissolved oxygen. The dye was excited with a 532 nm pulsed laser and the transient absorption at 740
nm was probed using a second pulsed laser system. The photoacoustics based results coincide with those obtained from
simultaneous time-resolved fluorescent measurements. The method can be extended to photoacoustic lifetime imaging
by using a receiver array instead of a single receiver. This opens unique possibilities for non-invasive, clinical functional
imaging. For example, combined with oxygen sensitive dye, 3D imaging of tissue oxygenation could be developed for
accurate diagnosis of cancer tumors, better planning of radiation therapy, and monitoring efficacy of treatment. Other
potential applications include: in-vivo mapping of ion (e.g. Ca) concentration and dynamics and imaging of enzymes
activity and metabolic functions, as well as environmental studies in turbid media.
Metallic nanoparticles are known to experience enhanced optical trap strengths compared to dielectric particles due to the increased optical volume, or polarizability. In our experience, larger metallic particles (~micron) are not easily trapped because momentum effects due to reflection become significant. Hybrid particles comprised of both metal and dielectric materials can circumvent this limitation while still utilizing a larger polarizability. Heterogeneous nanosystems were fabricated by embedding/coating silica nanoparticles with gold or silver in varying amounts and distributions. These compound particles were manipulated via optical tweezers, and their trapping characteristics quantitatively and qualitatively compared to homogeneous particles of comparable size. The parameters explored include the dependence of the trapping force on the percentage of loading of gold, the size of the embedded colloids, and the distribution of metal within the surrounding matrix or on its surface.
In this study we show that steady-state fluorescence anisotorpy within PEBBLEs can be used for the optochemical sensing of analytes such as Zn2+, O2, and Ca2+. Steady-state fluorescence anisotropy is a non- time resolved method that measures a combination of rotational and fluorescence lifetimes. This eliminates the need for reference dyes and ratiometic techniques to obtain quantitative results, even when using intensity-based sensor dyes. An advantage to working with PEBBLE nanosensors is that the encapsulated dye is localized in a constant rotational environment. This is in contrast to the use of free dyes, which can be affected by interferents such as protein binding.
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