The increased availability of diamonds due to industrial production have extended their use beyond jewelry and abrasives. Their properties make possible their potential use as semiconductors suitable for building microchips, and as heat sinks in electronics. When made fluorescent, their applications extend into such realms as high tech sensors, and bit manipulation for quantum computing.
On the micro (1×10-6) and nano (1×10-9) scale, their unique properties make diamonds ideal for biomedical purposes. Diamonds are easily sterilized, are non-toxic, and their surfaces are easily functionalized for attachment of proteins. Their lubricity, which causes the diamond crystals to slide like ice against each other, can provide a durable, bio-compatible, non-wearing surface for artificial joints. And fluorescent nanodiamonds, when emitting in the near infrared region of the spectrum, can provide deep tissue imaging without background interference or photo-bleaching.
Pure diamond contains only carbon-carbon bonds, and is optically transparent between the ultraviolet, through the visible, to the microwave regimes. Non-carbon contaminants, (nitrogen, boron, nickel, etc.) and/or atomic defects (such as vacancies) within the diamond crystal produce visible colors, many of which are also fluorescent. Over 500 fluorescent diamond colors have been compiled and a number of these are of high interest as near-infrared (NIR) reagents for biomedical imaging. However, only a few types of fluorescent diamond have been studied in any great detail.
The best studied fluorescent diamond is the NV-color center, which consists of a nitrogen atom adjacent to a vacancy within the diamond lattice. NV centers are optically excited with a green laser and emit red fluorescence between 650 to 710 nm. The unique optical properties of NV-center diamonds include their intense fluorescence, and extreme photostability even under intense laser excitation (1GW/cm2). Finally, their unique photophysics makes them exquisite sensors for magnetic fields, electric fields, and thermal changes. The main features of NV-center diamonds are summarized below.
- Perfect photostability, even under intense laser excitation, offers long-term fluorescence for in vitro diagnostic applications as well as in vivo imaging.
- Fluorescent nanodiamonds are well suited to background-free imaging because their near-infrared emission is long relative to autofluorescence of endogenous cellular fluorophores.
- The fluorescence intensity of fluorescent nanodiamonds is comparable to quantum dots (QD), but the brightness can be increased as the number of NV centers increase within the nanodiamond. Our fluorescent nanodiamonds have at least 15 NV-centers per 50 nm diamond nanocrystal.
- Spin-state coupling of the fluorescence from NV-centers. The brightness emitted from an NV-center nanodiamond can be modulated by magnetic field strength. This magneto-optic property is unique among fluorescent crystals, which also include quantum dots and upconverting nanoparticles. Spin-coupled luminescence, or optically detected magnetic resonance (ODMR), forms the basis for using fluorescent nanodiamonds as ultrasensitive nanoscale field sensors (to monitor magnetic, or electric fields or temperature).
- Another advantage of fluorescent diamonds is their large Stokes shift. Stokes shift is measured as the difference (in nanometers) between the maximum wavelengths in the excitation and emission spectra. NV-center diamonds have a Stokes shift greater than 100 nm, as they can be excited at 532 nm and emit between 630 to 700 nm. In contrast, the Stokes shift for many organic fluorophores, such as Alexa Fluor 546, is very small (5 nm). This poses difficulties in fluorescence imaging, as it necessitates the use of expensive optical filters to effectively remove the excitation light from the fluorescence emission.