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Selective cancer therapy using IR-laser-excited gold nanorods

Phototherapy (either chemical or thermal) has been used to treat cancers (tumors) employing various sensitizing dyes to absorb visible light from lasers and LEDs. The application of nanoparticles in medicine includes bioimaging and biosensing,1, 2 drug delivery,3, 4 cancer cell diagnostics, and therapeutics.5–12 By changing the shape of nanoparticles from spheres to nanorods, the absorption or scattering wavelength changes from the visible to the near-IR (NIR) region and offers the advantages of larger absorption and scattering cross-sections and much deeper penetration in tissues.7, 11, 13 Recent studies have shown that gold nanorods (GNRs) attached to antibodies6 and viral vectors9 could be used for selective and efficient photothermal therapy. The theory behind these nanomedical applications is characterized by the selectivity and sensitivity of the associated surface plasmon resonance (SPR) in response to incident light, i.e., changes in transmission or reflection. The selectivity of SPR is given by its absorption peak, which is redshifted when the sizes of nanospheres increase or when aspect ratio (R) increases in the the case of nanorods. Figure 1 shows the resonant absorption wavelength of various lasers in the visible spectrum (for R D 1) and NIR (for R > 1).13 NIR lasers for cancer therapy offer much deeper penetration depths than do visible lasers. However, they are limited to normal tissues. At resonant laser wavelengths, the absorption coefficients of cancer tissues with GNR components increase significantly to about 2.0–5.0cm 1, depending on the concentration of GNRs. Consequently, penetration depth in cancer tissues is still limited to about 0.2–0.5cm. In addition, the distribution of the aspect ratios of GNRs and their concentrations inside tumors are difficult to control for perfect matching to the laser peak absorption. Overheating of the surface area of targeted tissues is another important issue in photothermal therapy. To overcome these intrinsic difficulties, I previously proposed novel Figure 1. The resonant wavelength is redshifted from the visible (for spherical nanoparticles, with R D 1) to near-IR (for nanorods, with R > 1). R: Aspect ratio. NIR: Near-IR.