Nonthermal Plasma Synthesis of Water-Dispersible and Photoluminescent Silicon Quantum Dots for Bioimaging with Near Infrared Emission

Silicon quantum dots (Si QDs) are considered as attractive probes for bioimaging applications because of their earth-abundance, and size-tunable optical properties, nontoxicity, biocompatibility and biodegradability. In this study, one-step synthesis of water-dispersible and red-emitting Si QDs is investigated using nonthermal plasma. Nonthermal plasma is popular to produce Si QDs with superior size and shape uniformities, high purity and crystallinity which are important to obtain decent and well-controlled optical properties. The one-step synthesis method is a simplified approach to produce surface-passivated Si QDs by combining QD synthesis and surface passivation processes. Therefore, it does not require additional treatments involving high temperatures and toxic chemicals; it can save energy as well as time and reduce hazardous chemicals. We prepared water-dispersible and red-emitting Si QDs using the one-step synthesis method for the first time. Si QDs were synthesized using silane as a precursor, and their surfaces were passivated by acrylic acid. In order to improve water dispersibility, oxygen gas or water vapor was added into the passivation stage. We estimated Si QDs’ size from X-ray diffraction (XRD) patterns, and the estimated diameter was 4 nm. The XRD patterns also showed the Si QDs are crystalline. The hydrodynamics diameters of QDs were measured by dynamic laser diffraction to investigate water-dispersibility. 10 to 11 nm of hydrodynamic diameters meant Si QDs can disperse well enough to be used as bioimaging probes; for instance, QDs smaller than 20 nm can cross the blood-brain barrier. The Si QDs excited by ultraviolet light (400 nm) emitted red light (peak position at ~830 nm) with 30% of photoluminescence quantum yield. Since skin, fat, tissue, and blood are relatively transparent in the red-to-NIR range, red-emitting Si QDs can offer deeper tissue penetration resulting in improved bioimaging quality. We conducted bioimaging using 3T3 cells cultured with the Si QDs and demonstrated the Si QDs existed inside the cells emitting red light. This result showed that our water-dispersible Si QDs enables bioimaging in NIR range. Thus, this study shows that the one-step Si QD preparation method is an energy- and time-saving as well as less hazardous route for fabricating water-dispersible and photoluminescent Si QDs which are promising materials as probes for bioimaging applications as well as diagnostic tools.