Fluorescence Lifetime Measurements. Fluorescence lifetimes were measured using a custom-built far-field epifluorescence microscope coupled to a spectrometer and time-gated intensified charge-coupled device (CCD) camera (Imager QE, LaVision GmbH, Goettingen, Germany). The microscope setup permitted use of very small excitation powers (~30 flW) while maintaining a strong photoemission signaL The excitation light source was coupled to the microscope collection axis using a 400 nm dichroic filter and a 0.3 NA lOx objective (Nikon, Tokyo, Japan) placed after the dichroic filter was used to focus the excitation light onto the sample and collect the emitted fluorescence signal. An additional long pass filter inserted after the dichroic mirror was used to eliminate residual signal from the laser line and transmit only relevant (PL) signals onto the CCD detector. A pulsed GaN diode laser (414 nm, 5 MHz, 90 ps full width at half-maximum, model LDH400, PicoQuant GmbH, Berlin, Germany) was used to excite the sample. The overall instrument response time was limited to 500 ps by the CCD image intensifier microchannel plate. Data were collected at room temperature using a 10 mm optical path fluorescence cell filled with 3 mL of QD conjugates in buffer solution