The photothermal effect of metal nanoparticles generates extreme, localized heat. This heat can be used to destroy cancer cells, enhance photocatalytic efficiency, and also quickly and cleanly drive bulk chemical transformations — without raising the bulk temperature. Our previous work has shown that laser irradiation of metal nanoparticles substantially increases the reaction rate of urethane polymerization, even as the bulk system remains close to room temperature. In order to further characterize the effects of photothermal heating on the reaction kinetics, we investigated the photothermal heating of a hexyl isocyanate–1-butanol model system: a small-molecule combination reaction that forms one urethane bond, but does not polymerize. We conducted the reaction at room temperature both with and without pulsed laser irradiation at 532 nm, and with and without the presence of ~2 nm hexanethiol-protected gold nanoparticles. The extent of reaction rate enhancement was quantified by infrared spectroscopy. Reaction progress was tracked by measurement of the intensities of reactant (isocyanate) and product (urethane) bands over time. With photothermal heating, the overall reaction proceeds 15% faster than without photothermal heating. Considering that only a fraction (12%) of the sample is exposed to the laser light, the rate enhancement within that volume is actually a factor of 9.97. In addition, the rate is only enhanced during the brief time nanoparticles are hot, which is when they are exposed to the laser light. Taking into account the total irradiation time, given the laser pulse duration (8 ns) and frequency (10 Hz), reveals an additional enhancement factor of 1.25 × 107, for a total rate enhancement of 1.25 × 108.

Kinetic studies of molecular scale heating using the photothermal effect

255th National Meeting (2018)

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