7Single Particle Detection and Tracking

Single‐molecule spectroscopy is a technique that investigates the photophysical properties of individual molecules [1]. It overcomes the limitations of ensemble‐averaged measurements to allow the examination of chemical and biological systems with unprecedented details. The first single‐molecule measurement by fluorescence detection in the condensed phase was conducted in 1990 by Orrit and Bernard [2] for pentacene in a p‐terphenyl crystal at temperatures lower than 4 K. Seven years later, Wrächtrup and coworkers [3] took advantage of the excellent photostability and the high fluorescence brightness of NV centers to study the individual defects in bulk diamond. Moreover, the team went a step further and obtained the first spin resonance spectra of single NV centers at room temperature [3]. The studies have aroused considerable interest of using these single spins as room‐temperature quantum sensors for both physics and biology since then [4].

As described in the previous chapter, red fluorescent nanodiamonds (FNDs) are nanoscale diamonds containing an ensemble of NV centers as built‐in fluorophores. More than 10 NV centers can be hosted in a FND particle of 30 nm in diameter or larger. Therefore, each of them can be readily detected by confocal fluorescence microscopy, a standard tool used in biological research [5]. This outstanding feature, together with the capability of emitting far‐red emission, makes it possible to use red FNDs ...

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