FIG. 1: a) Schematics of the experimental setup. BS, 50/50 beam splitter; DM, dichroic mirror; PS, piezoelectric scanner; PMT, photomultiplier tube; F, long pass filter; APD, avalanche photodiode; TCSPC, time correlated single photon counting card. b-d) Sample arrangements for diamond nanocrystals exposed to air, water, and TiO 2 particles. three-dimensional piezoelectric stage of a home-built microscope. Laser pulses at a wave- length of 532 nm, duration of 13 ps, and repetition rate of 3.8 MHz were used to excite the NV centers. This light was sent to a microscope objective (NA=1.4) via a 50/50 beam splitter. The scattering and fluorescence from the sample were collected through the same objective, separated via a dichroic mirror and two 540 nm long pass filters, and directed to a photomultiplier tube and an avalanche photodiode, respectively. Fluorescence lifetimes were determined with a time-correlated single-photon counter by recording the time delay between the excitation pulses and the detected photons. The intensity I measured on the photomul- tiplier tube resulted from the interference of the light reflected by the sample interface and that scattered by a given nano-object in the focus spot. The details of this interferometric scattering (iSCAT) detection method are described in our previous work [12, 13]. The added value of the iSCAT signal is to visualize nano-objects without the need for a fluorescence signal. In a first step, the DNCs were spin coated on a glass cover slide (see Fig. 1b) and simultaneously imaged in fluorescence and iSCAT by scanning the sample across the focus of the excitation laser beam. Figure 2a shows an example of a fluorescence map of DNCs while Fig. 2b displays the simultaneously recorded iSCAT image. The latter locates the same DNCs as well as a few other nonfluorescent nanoparticles which are most likely DNCs without color centers. The second step of the experiment involved the measurement of the excited state lifetime τ for each fluorescent DNC. For NV centers in bulk diamond, τ has been measured to be about 3