Photoacoustic imaging a promising new diagnostic medical imaging modality can provide high contrast images of molecular features by introducing highly-absorbing plasmonic nanoparticles. Avasimibe (CI-1011) greater than dye alternatives . The optical response of plasmonic NPs can be tuned by modifying their size and shape  or by modifying their surface characteristics . Their small size allows systemic distribution in vivo  making NPs particularly useful contrast brokers for medical imaging applications such as optical coherence tomography  vital reflectance confocal microscopy  diffuse optical tomography  surface plasmon resonance imaging  and photoacoustic imaging [14 38 When light is usually assimilated by plasmonic NPs during optical imaging thermal energy results from electron-phonon interactions . Photoacoustic imaging uses this thermal energy to Avasimibe (CI-1011) generate images of NPs within tissue. Photoacoustic imaging which uses pulsed incident light to locally and transiently heating an absorber detects the pressure wave resulting from the thermoelastic expansion of the heated absorber [10 25 This pressure wave propagates through tissue and can be received at tissue surfaces using an ultrasound transducer [37 45 48 Plasmonic NPs are highly efficient photoacoustic imaging contrast agents due to their high optical absorption cross-sections and efficient thermal relaxation mechanisms [11 32 Additionally since NPs can be conjugated to molecular targeting moieties plasmonic NPs can be used to obtain an image of the molecular composition of a tissue region [3 12 34 Taking full advantage of the optical wavelength tunability of plasmonic NPs combined with tunable lasers for photoacoustic imaging we can implement photoacoustic molecular imaging in a multiplex format capable of distinguishing between multiple molecular receptors within a single tissue region [8 30 Besides providing imaging contrast NPs can also be used for therapeutics. During photothermal therapy (PTT) laser heating of plasmonic NPs results in increased cell death in targeted regions containing gold NPs [19 20 33 Applying an extended duration continuous wave laser (typically lasting minutes) as used for most PTT applications leads to increased regional heating as a result of the NP photothermal processes; this increased heating likely Avasimibe (CI-1011) causes protein denaturation leading to the observed cellular damage [13 18 However when using a nanosecond pulsed laser to excite plasmonic NPs it is not expected Rabbit polyclonal to Hsp70. that bulk heating will be sufficient to lead to cell death. Accepted models of nanosecond laser-induced NPs heating and the subsequent conduction of heat to the surrounding environment predict a large temperature increase within the NPs during the laser pulse [28 29 but this heat is usually dissipated through the environment during the time between pulses. Since nanosecond lasers used for photoacoustic imaging typically have a pulse interval greater than milliseconds this rapid dissipation results in a negligible bulk temperature increase. Despite this several groups have demonstrated the ability to cause cell death by exciting Avasimibe (CI-1011) Avasimibe (CI-1011) plasmonic NPs with nanosecond laser pulses [35 46 53 The mechanisms of cell death are likely to be a combination of photothermal pressure and photochemical effects . At this point however a comprehensive study of the impact of the nanosecond pulsed laser used during photoacoustic imaging of cells made up of endocytosed NPs is usually lacking. We sought to establish guidelines for “safe” imaging versus therapeutic applications of nanosecond pulsed laser irradiation of NP-labeled cells. Typically researchers can rely upon the American National Standards Institute (ANSI) safety guidelines to determine safe laser fluences for imaging ; however our experiments show that these Avasimibe (CI-1011) limits are insufficient for estimating the damage threshold for plasmonic nanoparticle-loaded cells exposed to nanosecond laser pulses. 2 Materials and methods 2.1 Gold nanosphere synthesis Gold nanospheres were synthesized using seed-mediated growth as previously described . A 71 mL volume of 0.27 mM HAuCl4 in nanopure water was brought to a boil under reflux. A 3.75 mL volume of 34 mM sodium citrate was added to the solution under vigorous stirring and allowed to stir for several minutes. Then the solution temperature was allowed to return to room temperature. To make larger nanospheres 7.5 mL of 25 mM HAuCl4 was mixed with 15.61 mL of 0.2 M NH2OH and 750 mL of nanopure water. To that.