Two-Photon Absorbing Block Copolymer as a Nanocarrier for Porphyrin: Energy Transfer and Singlet Oxygen Generation in Micellar Aqueous Solution Ching-Yi Chen, Yanqing Tian, Yen-Ju Cheng, A. Cody Young, Jae-Won Ka, and Alex K.-Y. Jen* Department of Materials Science and Engineering, UniVersity of Washington, Seattle, Washington 98195-2120 Received February 14, 2007; E-mail: ajen@u.washington.edu Porphyrin-based derivatives are widely used as photosensitizers for singlet oxygen ( 1 O 2 ) generation, which is a key step for photodynamic therapy. 1 However, most porphyrins absorb light in the visible region and have relatively low two-photon absorption (2PA) cross-sections (δ: 1-10 GM; 1 GM ) 10 -50 cm 4 s photon -1 ). 2 These characteristics limit their effectiveness when excited with a near-infrared (NIR) light source (ranging from 700 to 1000 nm), which would allow deeper penetration and less damage to biological tissues. 3 Direct chemical modification of porphyrins by preparing porphyrin dimers and π-conjugated porphyrins 4 is a commonly used approach to solve these problems. Alternatively, the utilization of fluorescence resonance energy transfer (FRET) 5 in light harvesting dendrimers 6,7 and silica nanoparticles 8 is also quite effective. In these dendrimers, the peripheral donors possess a relatively large two-photon cross-section that can efficiently absorb NIR light and transfer the energy to the porphyrin core. As a result, these porphyrin-containing dendrimers could be used to efficiently generate 1 O 2 . 6 Herein, we report the use of a new amphiphilic 2PA-chro- mophore-containing block copolymer to enhance the efficiency of 1 O 2 generation. This is realized by using the block copolymer (P1, Figure 1) as a nanocarrier to encapsulate a hydrophobic porphyrin photosensitizer (1, Figure 1) inside its micelles. In aqueous solution, P1 forms micelles with the hydrophilic poly(ethylene glycol) (PEG) as the corona to ensure good water solubility. The 2PA-chro- mophore-containing hydrophobic core provides a good environment to accommodate the hydrophobic 1. Due to the size confinement in these micelles, efficient FRET from the 2PA chromophore to porphyrin occurs, which enhances the effective 2PA cross-sections of the porphyrin. This approach combines several advantages: (1) numerous photosensitizers can be easily incorporated into an aqueous system without encountering tedious chemical modifications; (2) biocom- patible 2PA block copolymers can be used as nanocarriers for bioapplications; and (3) the FRET through noncovalently bonded donors and acceptors within micelles enhances the efficiency of 1 O 2 generation under NIR light irradiation. The structures of two block copolymers synthesized (P1 and P2) are shown in Figure 1. In order to reduce chromophore aggregation within the micelles, the 2PA monomer (M) was copolymerized with styrene to produce P1. 9 The aryl amino-containing donors were connected with a fluorene bridge through triple bonds to enhance its photostability. 10 To differentiate the contribution of FRET to 1 O 2 generation, P2 without the 2PA chromophore was also synthesized for comparison. Micelles were prepared in aqueous solution using the typical dialysis method and were characterized by dynamic light scattering (DLS) and atomic force microscopy (AFM). The average diameters of the micelles are listed in Table 1. The fluorescence spectrum of the 2PA chromophore overlaps with both the Soret and Q-band absorptions of 1 (see Supporting Information for details), indicating that efficient FRET can occur from the 2PA chromophore to 1. Sensitizer 1 is insoluble and does not fluoresce in an aqueous solution. However, once it was encapsulated within a micelle, the aqueous solution began to fluoresce. For this study, the concentration of P1 was kept constant (0.075 mg/mL, which corresponds to a 2PA chromophore concen- tration of 7.5 μM). When the concentration of 1 in P1 (simplified as 1/P1) was increased, the fluorescence intensity of the 2PA chromophore decreases (Figure 2A). The energy transfer efficiency can be calculated to be as high as 91%. 11 This indicates that effective FRET indeed occurs within these micelles. 12 It also proves that the nanoconfinement provided by the 2PA block copolymer helps efficiently generate the porphyrin excited state through energy transfer. P1 in aqueous solution exhibits 2PA cross-sections of 288 GM at 750 nm and 120 GM at 800 nm when excited with a femtosecond mode lock Ti-sapphire laser. Although these values are moderate compared to other values reported in the literature, 13 they are still much higher than those from the porphyrins alone. The energy transfer efficiencies of 1/P1 excited at 800 nm are similar to those excited at 380 nm (Table 1). Furthermore, the emissions of 1 in 1/P1 through FRET are 2.7-6.5 14 times higher than the emissions Figure 1. The chemical structures of P1, P2, and 1. Table 1. Typical Physical Properties of P1 and P2 ET (%) Average sizes of the micelles systems ratio a 1PF b 2PF c AM d DLS f AFM g P1 0 0 59 (0.14) 53 ( 11 1/P1 0.1 47 50 e 66 (0.14) 50 ( 12 1/P1 0.2 65 71 2.7 67 (0.13) 48 ( 10 1/P1 0.6 86 89 6.5 70 (0.14) 52 ( 11 1/P1 1.0 91 96 4.4 73 (0.14) 52 ( 15 1/P2 1.0 77 (0.18) 54 ( 14 a Molar ratio of 1 to P1 or P2 determined by UV-vis spectra. b Energy transfer efficiency irradiated at 380 nm. c Energy transfer efficiency excited at 800 nm. d Amplification using P2 as counterpart excited at 800 nm. Integrated emission intensity of 1 was collected from 625 to 750 nm. e The signal is too weak to detect. f Determined by dynamic light scattering in solution at 25 °C. The data in the parentheses are the polydispersities. g Observed by AFM using dried sample on mica surface. Published on Web 05/17/2007 7220 9 J. AM. CHEM. SOC. 2007, 129, 7220-7221 10.1021/ja071057p CCC: $37.00 © 2007 American Chemical Society