Selection of Fluorescent Nanodiamonds Based on Optical Properties by Resonant Optical Force

Fluorescent nanodiamonds (F-ND) have significant potential in bio-sensing and quantum information processing because of their high photostability and low toxicity. Their small particle size and chemical stability make them suitable for applications such as fluorescent markers and single-photon emitters.<br/>Our group has successfully synthesized NDs containing a few SiV or GeV centers using a detonation process (SiV-DND/GeV-DND)[1,2]. This method enables rapid large-scale synthesis. However, this process also produces DNDs without a color center (Undoped-DND), necessitating efficient post-process optical property-based selection[3,4]. However, because the optical properties of F-ND are attributed to color center, existing sorting methods such as chromatography, centrifugation, and mass spectrometry are not applicable to F-ND.<br/>Recent advances in optical force-based selection have shown promise. When a material refracts, absorbs, or scatters light, optical force is exerted. Recently, selective manipulation using resonant optical response has been demonstrated. Since a large force acts only on substances that resonantly absorb light, selective transport is possible. While effective for 1D nanomaterials such as carbon nanotubes[5], its application to 0D nanomaterials has been limited by weak optical forces and large Brownian motion. Previous studies have required complicated setups to reduce the effects of non-resonant optical force and Brownian motion, which act besides resonant optical force, and only a few particles could be sorted under room temperature conditions [6].<br/>This study aims to develop a practical, large-scale optical sorting system for SiV-DND based on resonant optical force. The small volume of SiV-DND significantly reduces non-resonant optical force compared to resonant optical force. By loosely focusing a high-intensity laser beam, it is possible to overcome three-dimensional Brownian motion without restricting the dimensions of movement, allowing for effective sorting.<br/>A water dispersion of SiV-DND/GeV-DND was prepared and sealed in a glass capillary. For selective manipulation, the capillary was irradiated with a CW laser (1.68 eV: SiV resonant light, 1.58 eV: SiV/GeV non-resonant light) along its axis. In addition, the capillary was irradiated with a CW laser (2.34 eV) from a direction perpendicular to the axis of the capillary, and the time evolution of the fluorescence intensity of SiV-DND and GeV-DND was monitored. The fluorescence intensity of SiV-DNDs increased under resonant light irradiation and decreased under non-resonant light irradiation. GeV-DND showed minimal changes, indicating that the SiV-DND separation by resonant optical force was successful.<br/>Furthermore, we quantified the optical forces exerted on the SiV-DND/GeV-DND and calculated the transport distances induced by these forces. Theoretical calculations confirm that each DND has a very small non-resonant optical force due to the very small particle size. On the other hand, for SiV-DNDs, the resonant optical force due to the large dipole moment was confirmed to be dominant. Finally, the high durability of SiV-/GeV-DND enables long-term selection with high-power laser beams. [3,4].<br/>This new and scalable sorting system for 0D nanomaterials has achieved large-scale practical separations at room temperature. It enables the purification of fluorescent DND with homogeneous fluorescence properties, paving the way for large-scale manufacturing and application in biotechnology and quantum technologies. The versatility of this method suggests the potential for broad application in the production of other fluorescent nanoparticles.<br/><br/>[1] Y. Makino, et al., Diam. Relat. Mat. 2021.<br/>[2] Y. Makino, et al., Diam. Relat. Mat. 2022.<br/>[3] Y. Makino, et al., Phys. Status Solidi a 2022.<br/>[4] Y. Makino, et al., Jpn. J. Appl. Phys. 63, 2024.<br/>[5] S. E. S. Spesyvtseva, et al., Phys. Rev. Appl. 2015.<br/>[6] H. Fujiwara, et al., Sci. Adv. 7, 2021.