Photothermal release of DNA from gold nanoparticles either by thermolysis of the Au-S bonds used to anchor the oligonucleotides to the nanoparticle or by thermal denaturation has great therapeutic potential, however, both processes have limitations (a decreased particle stability for the former process and a prohibitively slow rate of release for the latter). Here we show that these two mechanisms are not mutually exclusive and can be controlled by adjusting laser power and ionic strength. We show this using two different double-stranded (ds)DNA-nanoparticle conjugates, in which either the anchored sense strand or the complementary antisense strand was labeled with a fluorescent marker. The amounts of release due to the two mechanisms were evaluated using fluorescence spectroscopy and capillary electrophoresis, which showed that irradiation of the decorated particles in 200 mM NaOAc containing 10 mM Mg(OAc)(2) with a pulsed 532 nm laser operating at 100 mW favors denaturation over Au-S cleavage to an extent of more than six-to-one. Due to the use of a pulsed laser, the process occurs on the order of minutes rather than hours, which is typical for continuous wave lasers. These findings encourage continued research toward developing photothermal gene therapeutics.

Please release me: The heat generated when metal nanoparticles absorb light results in a significant increase in the temperature of the environment around the particles and is used to selectively break bonds within a molecular system anchored to the nanoparticle surface (see picture). This process represents an advantageous and more universal method to deliver chemicals locally, while avoiding excessive tissue damage.

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New methods for the spatially and temporally controlled delivery of chemical and/or biochemical species using lowenergy visible light offers greater opportunities in photodynamic therapy, [1] chemical synthesis, [2] and photolithography. [3] The fact that designer metal nanoparticles absorb light of specific frequencies (at their plasmon resonance) and efficiently convert it into heat (through the photothermal effect) is a particularly appealing phenomenon that has been effectively used to trigger cell death [4] and melt ice [5] within the vicinity of the nanoparticles. However, to date, the heat generated when visible light is absorbed by such structures has not been used to initiate delivery events. This is surprising, as this technique would provide a convenient and general method to break chemical bonds and release biologically relevant payloads from the nanoparticle surfaces. Both detrimental (poisonous) and beneficial (therapeutic) agents could be released on command by using such a universal method.We recently set out to develop a new method to harness the local heat that dissipates from the nanoparticles when they are stimulated with visible light, in order to trigger thermally activated bond-breaking reactions close to the nanoparticle surfaces without significantly increasing the temperature of the surrounding environment. This approach has a benefit over others in that specific agents can be delivered to a cell without damaging it. Herein, we describe how we can trigger the release of a fluorescent dye from coreshell nanoparticles as the first illustration of a generalized approach to photothermal release.We chose the retro-Diels-Alder reaction to demonstrate our new release method because the temperature at which it is activated can be easily tuned by modifying the structure of the two chemical components (the diene and dienophile). [6] This cycloaddition reaction is particularly well-suited for surface-localized chemistry and has been used to selectively decorate [7] and control nanoparticle aggregation [8] by directly applying heat to suspensions of nanoparticles. The photothermal effect has yet to be taken advantage of in this context. We identified a specific retro-Diels-Alder reaction (shown in the substructure in Figure 1 a). Our choice was based on the fact that the bond-breaking reaction of 7-oxa-bicyclo-[2.2.1]hept-5-ene-2,3-dicarboxylic imide to release a furan and a maleimide component barely occurs at room temperature but proceeds above 60 8C. Model studies on both the endo and exo isomers of the depicted bicyclic system suggest that, as expected, the use of the former isomer is advantageous [9][10][11] since is cleanly converts into its two components at Figure 1. a) Release of the fluorescein dye from the surface of a silicagold core-shell nanoparticle by using the photothermal effect to induce the retro-Diels-Alder reaction of an anchored 7-oxa-bicyclo-[2.2.1]hept-5-ene-2,3-dicarboxylic imide derivative. UV/Vis absorption spectra of aqueous dispersions with TEM images of each type of pa...

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