DNA Base Modifications Mediated by Femtosecond Laser-Induced Cold Low-Density Plasma in Aqueous Solutions.

Applications based on near-infrared femtosecond laser-induced plasma in biological materials involve numerous ionization events that inevitably mediate physicochemical effects. Here, the physical chemistry underlying the action of such plasma is characterized in a system of biological interest. We have implemented wavefront shaping techniques to control the generation of laser-induced low electron density plasma channels in DNA aqueous solutions, which minimize the unwanted thermo-mechanical effects associated with plasma of higher density. The number of DNA base modifications per unit of absolute energy deposited by such cold plasma is compared to those induced by either ultraviolet or standard ionizing radiation (γ-rays). Analyses of various photoinduced, oxidative, and reductive DNA base products show that the effects of laser-induced cold plasma are mainly mediated by reactive radical species produced upon the ionization of water, rather than by the direct interaction of the strong laser field with DNA. In the plasma environment, reactions among densely produced primary radicals result in a dramatic decrease in the yields of DNA damages relative to sparse ionizing radiation. This intense radical production also drives the local depletion of oxygen.

Tuning the size of gold nanoparticles produced by multiple filamentation of femtosecond laser pulses in aqueous solutions.

In the present study, we consider the self-regulated generation of spatially homogeneous low density plasma (LDP) micro-channels as a high intensity ionization source arising from the multi-filamentation of powerful femtosecond (fs) laser pulses in aqueous solutions. We investigate the modulation of the femtosecond laser multiple filamentation for tuning the size of gold nanoparticles (AuNPs) synthesized in an irradiated gold chloride solution. Previous studies on the radiation-induced synthesis of colloidal gold by more conventional ionization sources, such as high energy γ-rays and electron beams, highlighted the dependence of the size distribution of AuNPs on the density of energy deposited per unit of time, i.e. the dose rate. The present method of laser-induced production of AuNPs rests on a similar radiation-assisted process, i.e. the reduction of the solvated trivalent gold ions by the hydrated electrons produced upon ionization of water. We find that trivial optical manipulation varies the rate of deposited energy by laser irradiation, which can be considered equivalent to a variation of the dose rate. We investigate the influence of varying the density of energy deposited on the laser-induced gold cluster size distribution and made a comparison with the high energy radiation-induced synthesis of AuNPs. Here, our results highlight that the present method of laser irradiation, in the regime of LDP generation, mimics the radiolysis of water at an adjustable high dose rate. More generally, these spatially and temporally resolved plasmas could be developed as a tool for the unprecedented control of chemistry under ionizing radiation.

Dense ionization and subsequent non-homogeneous radical-mediated chemistry of femtosecond laser-induced low density plasma in aqueous solutions: synthesis of colloidal gold.

The "cold" low density plasma channels generated by the filamentation of powerful femtosecond laser pulses in aqueous solutions constitute a source of dense ionization. Here, we probed the radiation-assisted chemistry of water triggered by laser ionization via the radical-mediated synthesis of nanoparticles in gold chloride aqueous solutions. We showed that the formation of colloidal gold originates from the reduction of trivalent ionic gold initially present in solution by the reactive radicals (e.g. hydrated electrons) produced upon the photolysis of water. We analyzed both the reaction kinetics of the laser-induced hydrated electrons and the growth kinetics of the gold nanoparticles. Introduction of radical scavengers into the solutions and different initial concentrations of gold chloride provided information about the radical-mediated chemistry. The dense ionization results in the second order cross-recombination of the photolysis primary byproducts. Competition with recombination imposes the non-homogeneous interaction of reactive radicals with solute present in irradiated aqueous solutions. Such a laser-induced non-homogeneous chemistry suggests similarities with the radiation chemistry of water exposed to conventional densely ionizing radiation (high dose rate, high linear energy transfer).

Cancer radiotherapy based on femtosecond IR laser-beam filamentation yielding ultra-high dose rates and zero entrance dose.

Since the invention of cancer radiotherapy, its primary goal has been to maximize lethal radiation doses to the tumor volume while keeping the dose to surrounding healthy tissues at zero. Sadly, conventional radiation sources (γ or X rays, electrons) used for decades, including multiple or modulated beams, inevitably deposit the majority of their dose in front or behind the tumor, thus damaging healthy tissue and causing secondary cancers years after treatment. Even the most recent pioneering advances in costly proton or carbon ion therapies can not completely avoid dose buildup in front of the tumor volume. Here we show that this ultimate goal of radiotherapy is yet within our reach: Using intense ultra-short infrared laser pulses we can now deposit a very large energy dose at unprecedented microscopic dose rates (up to 10(11) Gy/s) deep inside an adjustable, well-controlled macroscopic volume, without any dose deposit in front or behind the target volume. Our infrared laser pulses produce high density avalanches of low energy electrons via laser filamentation, a phenomenon that results in a spatial energy density and temporal dose rate that both exceed by orders of magnitude any values previously reported even for the most intense clinical radiotherapy systems. Moreover, we show that (i) the type of final damage and its mechanisms in aqueous media, at the molecular and biomolecular level, is comparable to that of conventional ionizing radiation, and (ii) at the tumor tissue level in an animal cancer model, the laser irradiation method shows clear therapeutic benefits.