M M U N I Nonblinking and Nonbleaching Upconverting Nanoparticles as an Optical Imaging Nanoprobe and T1 Magnetic Resonance Imaging Contrast Agent C A T IO N By Yong Il Park, Jeong Hyun Kim, Kang Taek Lee, Ki-Seok Jeon, Hyon Bin Na, Jung Ho Yu, Hyung Min Kim, Nohyun Lee, Seung Hong Choi, Sung-Il Baik, Hyoungsu Kim, Seung Pyo Park, Beom-Jin Park, Young Woon Kim, Sung Ho Lee, Soo-Young Yoon, In Chan Song, Woo Kyung Moon, Yung Doug Suh,* and Taeghwan Hyeon*