1 O 2 Generation DOI: 10.1002/anie.201105236 Metal Nanoparticles Sensitize the Formation of Singlet Oxygen** Raviraj Vankayala, Arunachalam Sagadevan, Priya Vijayaraghavan, Chien-Lin Kuo, and Kuo Chu Hwang* Singlet oxygen ( 1 O 2 ) is known to play an indispensible role in the photodynamic therapy (PDT) treatment of cancer, [1–5] and is an important oxidant for hydroperoxidation of olefins in organic synthesis. [6, 7] Singlet O 2 is conventionally formed by sensitization by organic photosensitizers, such as Rose Bengal, silicon phthalocyanine, etc. [1–7] These organic or organometallic dyes are, however, prone to photoinduced degradation and enzymatic degradation, which becomes problematic in PDT treatments, and reduces the efficiency of the generation of singlet O 2 . [5, 8] It is, therefore, important to search for photosensitizers with highly efficient singlet O 2 generation and large absorption coefficients that are photo- chemically more stable and less prone to enzymatic degrada- tion. Previously, it was reported that the yield of singlet oxygen production by a photosensitizer, namely, Rose Bengal, was enhanced by a silver island film through the metal-enhanced absorption of photosensitizer. [9, 10] It was also reported that a gold nanodisk could enhance the phosphorescence decay rate of singlet oxygen, leading to a larger characteristic phosphor- escence emission band of singlet oxygen at 1270 nm. [11] In another two studies, it was observed that the quantum yield of singlet O 2 formation generated by phthalocyanine photo- sensitizers can be enhanced by the presence of gold nano- particles. [12, 13] Herein we report an unprecedented observa- tion that singlet oxygen can be formed through direct sensitization by metal nanoparticles (M NPs, M = Ag, Pt, and Au) without the presence of any organic photosensitizers. Unambiguous experimental evidence includes direct obser- vation of singlet oxygen emission at roughly 1268 nm, hydro- peroxidation of cyclohexene, green fluorescence from a selective singlet oxygen fluorescent sensor, namely, Singlet Oxygen Sensor Green (SOSG, Molecular Probe), and quenching of singlet oxygen phosphorescence by sodium azide. As shown in Figure 1, photoexcitation of M NPs at the surface plasmon resonance absorption bands of Ag (d = 55, 42 nm), Pt (10 nm), and Au (22 nm) in D 2 O results in characteristic singlet oxygen emission at 1264 and 1268 nm, respectively. Control experiments show that in the absence of metal nanoparticles, photoexcitation of poly(vinyl pyrroli- done (PVP) in D 2 O using either 254 or 508 nm light did not result in any detectable singlet O 2 emission signal (see the Figure 1. a) Phosphorescence emission spectra of singlet oxygen sensi- tized by Au, Ag or Pt nanoparticles in D 2 O, and a control experiment with PVP in D 2 O in the absence of metal nanoparticles (purple line, l ex = 254 nm and blue line, l ex = 508 nm). b) The extinction spectra (solid lines) of Ag, Pt, and Au NPs, and the excitation spectra (dashed lines) of singlet O 2 phosphorescence at 1270 nm in the presence of Ag, Pt, and Au NPs, respectively. A longpass filter of 850 nm was put between the sample and the detector for all experiments (unless otherwise mentioned) to filter away any stray light and the second harmonic of the excitation light with wavelengths shorter than 850 nm. The excitation light profiles of 508 nm are also shown (the gray blue line was recorded without the LP850 filter). [*] R. Vankayala, A. Sagadevan, P. Vijayaraghavan, C.-L. Kuo, Prof. K. C. Hwang Department of Chemistry, National Tsing Hua University Hsinchu, Taiwan (R.O.C.) E-mail: kchwang@mx.nthu.edu.tw [**] We thank the National Science Council, Taiwan, for financial support. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201105236. Communications 10640  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 10640 –10644