1. Berkeley Sensor and Actuator Center, Department of
Bioengineering, §Department of Electrical
Engineering and Computer Science, Biophysics Graduate Program, University of California at Berkeley, Berkeley, California 94720, United States
2. Department of Chemistry, School of Physics and Chemistry, ⊥School of Materials
Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea