Spin‐Dependent Photoluminescence in Carbon‐Based Quantum Dots

The ability to modulate the photoluminescence (PL) of nanomaterials via spin-related effects is vital for many emerging quantum technologies, with nanoscale quantum sensing and imaging being particular areas of focus. Carbon-based quantum dots (CQDs) are among the most common forms of luminescent nanomaterials, appealing due to their ease of synthesis, tunability through organic chemistry, high brightness, and natural biocompatibility. However, the observation of room-temperature, spin-dependent PL has remained elusive. Here, we report on the observation of PL modulation of CQDs by magnetic fields ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>∼</mml:mo> <mml:mn>10</mml:mn></mml:mrow> <mml:annotation>$\sim 10$</mml:annotation></mml:semantics> </mml:math> mT) under ambient conditions. Using pyrolysis, we synthesize a series of CQDs using 19 different amino acids as the starting material. These provide samples with a range of PL emission spectra. Surprisingly, the vast majority of them exhibit a clear magneto-PL effect (up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mo>%</mml:mo></mml:mrow> <mml:annotation>$\sim 1\%$</mml:annotation></mml:semantics> </mml:math> change) in dry form, which generally persists in solution. Furthermore, an electron spin resonance is detected in the PL with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>g</mml:mi> <mml:annotation>$g$</mml:annotation></mml:semantics> </mml:math> -factor of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>g</mml:mi> <mml:mo>≈</mml:mo> <mml:mn>2</mml:mn></mml:mrow> <mml:annotation>$g\approx 2$</mml:annotation></mml:semantics> </mml:math> , suggesting a process similar to the radical pair mechanism is responsible for the spin-dependent PL. Finally, we show that the magneto-PL contrast decreases in the presence of paramagnetic species, which we attribute to an increase in magnetic noise-induced spin relaxation in the CQDs. Our work brings new functionalities to these commonly used and biocompatible luminescent nanoparticles, opening new opportunities for in situ quantum sensing and imaging of biological samples.