Enhanced plasmonic resonance energy transfer in mesoporous silica-encased gold nanorod for two-photon-activated photodynamic therapy

Abstract

The unique optical properties of gold nanorods (GNRs) have recently drawn considerable interest from those working in in vivo biomolecular sensing and bioimaging. Especially appealing in these applications is the plasmon-enhanced photoluminescence of GNRs induced by two-photon excitation at infrared wavelengths, owing to the significant penetration depth of infrared light in tissue. Unfortunately, many studies have also shown that often the intensity of pulsed coherent irradiation of GNRs needed results in irreversible deformation of GNRs, greatly reducing their two-photon luminescence (TPL) emission intensity. In this work we report the design, synthesis, and evaluation of mesoporous silica-encased gold nanorods (MS-GNRs) that incorporate photosensitizers (PSs) for two-photon-activated photodynamic therapy (TPA-PDT). The PSs, doped into the nano-channels of the mesoporous silica shell, can be efficiently excited via intra-particle plasmonic resonance energy transfer from the encased two-photon excited gold nanorod and further generates cytotoxic singlet oxygen for cancer eradication. In addition, due to the mechanical support provided by encapsulating mesoporous silica matrix against thermal deformation, the two-photon luminescence stability of GNRs was significantly improved; after 100 seconds of 800 nm repetitive laser pulse with the 30 times higher than average power for imaging acquisition, MS-GNR luminescence intensity exhibited ~260% better resistance to deformation than that of the uncoated gold nanorods. These results strongly suggest that MS-GNRs with embedded PSs might provide a promising photodynamic therapy for the treatment of deeply situated cancers via plasmonic resonance energy transfer.

Keywords: Gold nanorods; photodynamic therapy; plasmonic resonance energy transfer; surface plasmon resonance; two-photon luminescence..

Scheme 1

Schematic illustration of two-photon-activated photodynamic therapy (TPA-PDT) using mesoporous silica-encased gold nanorods. Pd-porphyrins (Green sphere), conjugated onto the walls of nanochannels within the mesoporous silica shell, are activated via intra-particle plasmonic energy transfer (ET) from two-photon-excited gold nanorods, to generate cytotoxic singlet oxygen.

Figure 1

TEM image and photophysical properties of MS-GNRs. (a) TEM image of MS-GNRs, with insert showing an enlarged view of its mesoporous structure. (b) Photoluminescence spectrum of MS-GNRs in an aqueous solution irradiated with femtosecond laser at 800 nm. (c) Dependence of the integrated photoluminescence intensity of (b), versus the excitation power; slope, obtained from the regression fitting, is ~2 indicating that the measured photoluminescence is indeed TPL. (d) Steady-state absorption of MS-GNRs (solid line) and the TPL excitation spectrum (open triangles).

Figure 2

Enhancement of photostability of GNRs via mesoporous envelopment. (a) The TPL intensities of GNRs (■) and MS-GNRs (red ●) monitored as a function of time, under the exposure of 120 mW repetitive laser pulses. TEM images of MS-GNRs (b) and GNRs (c) following 100 seconds of laser irradiation.

Figure 3

(a) TEM imaging of MS-GNR-PdTPPs; scale bar: 0.10 µm (b) Energy transfer in MS-GNR-PdTPPs, showing the TPLs of MS-GNRs (black) and MS-GNR-PdTPPs (red). (c) Confocal microscopy imaging of the endocytosis of MS-GNR-PdTPPs, with the nuclei stained by Hochest 33342 (blue signal); white foci are the TPLs of MS-GNR-PdTPPs under 800 nm laser excitation; scale bar: 10 µm. (d) Decay of optical absorption of ADPA at 378 nm, caused by the generation of singlet oxygen from GNRs (○), PDTPPs (▲) and MS-GNR-PdTPPs (■) as a function of laser irradiation exposure time.

Figure 4

Confocal fluorescence microscopy images showing the TPA-PDT performance (¹O₂ cytotoxicity) of MS-GNR-PdTPPs (a)-(c) and MS-GNRs (d)-(f) in treated breast cancer cells under 3 mW repetitive laser irradiation of 3 durations: 0, 108, and 194 seconds. YOPRO-1 (green) and propidium iodide signals (red) denote leakage of the cell and nuclear membranes, respectively. Scale bar: 10 µm. Fluorescence intensity of YOPRO-1 and propidium iodide as a function of irradiation period for cells treated with MS-GNR-PdTPPs (g) and MS-GNRs (h).

Figure 5

TEM images of a 70 nm-thick section of tumor harvested from an anesthetized mouse 24 hours after administration of MS-GNR-PdTPPs and TPA-PDT; scale bar: 500 nm. Insert shows an enlarged image of localized MS-GNR-PdTPPs within the tumor; scale bar: 100 nm.

Figure 6

In vivo studies illustrating TPA-PDT therapeutic benefits derived from intra-tumor injection of MS-GNR-PdTPPs. Histologic analyses of harvest tumor sections 24 hours post-irradiation were made with hematoxylin and eosin (first row, scale car: 100 µm), TUNEL (second row, green coloring), caspase-3 immuno-histogram (third row, red coloring) and DAPI (blue coloring) staining. Scale bar: 25 µm. Represented tumor images (n=3 for each group) acquired 14 days after treatments were shown for quantitative analysis of tumor growth (fourth row). Scale bar: 5 mm.