Single mSiNC tracking showed that, unlike QDs, mSiNCs neither blink nor photobleach over the total observation period of 300 min. (A) A typical TIRF microscope image of single HPLC-purified mSiNCs, immobilized on a polylysine-coated cover glass and excited by the 405-nm line of a solid-state laser. (B) Distribution of fluorescence intensities of 530 × 530–nm areas enclosing single mSiNC spots (top, mSiNC) and single Qdot655 (bottom, Qdot655). The fluorescence intensity is after background subtraction. The intensity distribution of mSiNC could be well fitted by a single log-normal function (red curve), and the possible dimer fraction was less than a few percentages, as determined by the two-component fitting, whereas the intensity distribution of Qdot655 was extremely broad. (C) Typical time-dependent changes of the fluorescence intensities of an mSiNC spot (top) and of a Qdot655 spot (bottom; typical among 30 mSiNC and 30 Qdot655 particles, examined here. Other typical examples are shown in Fig. S3). Because of the background subtraction, the plot includes occasional negative signal intensities (0 is shown by red broken lines). The mean signal intensity is shown by solid red horizontal lines. Both fluorophores were excited by the same laser (440 nm; 0.34 µW/µm² at the sample plane). Images were recorded at video rate (33 ms/frame); see Video 1 and Fig. S3 A. The Qdot655 blinked frequently and photobleached gradually, whereas the mSiNCs never blinked during the video rate observations. Over 80% of the single Qdot655s observed in this study stopped emitting measurable fluorescence after the 10-min observation under these conditions. a.u., arbitrary unit.