Photothermal imaging of nanoparticles and cells

This review summarizes the findings of recent applications of time-domain far-field photothermal (PT) technique to the detection and imaging of nanoscale absorbing particles. This two-beam (pump-probe) technique is based on time-resolved PT visualization of laser-induced thermal effects around nanoparticles. Imaging is accomplished, after an adjustable time delay after the pump laser pulse, with a second probe beam that senses the nanotarget. Using a tunable optical parametric oscillator laser (wavelength, 420 to 570 nm; energy, 0.1-300 μJ; pulse width, 8 ns) as the pump laser and a Raman shifter (639 nm, 10 nJ, 13 ns) as the probe laser, with a tunable delay of 0 to 5 000 ns of the probe pulse relative to the pump pulse, this approach has demonstrated the capability to visualize nanoscale gold particles (2 to 250 nm) alone and in cells, liposomes (30 to 90 nm), neutral red-stained particles (30 to 500 nm), and polystyrene beads. Different applications of the time-resolved PT technique are discussed, including imaging of absorbing cellular nanostructures and optimization of selective killing of cancer cells and bacteria.