Biocompatible nanoscale probes for sensitive detection of paramagnetic species and molecules associated with their (bio)chemical transformations would provide a desirable tool for a better understanding of cellular redox processes. Here, we describe an analytical tool based on quantum sensing techniques. We magnetically coupled negatively charged nitrogen-vacancy (NV) centers in nanodiamonds (NDs) with nitroxide radicals present in a bioinert polymer coating of the NDs. We demonstrated that the T1 spin relaxation time of the NV centers is very sensitive to the number of nitroxide radicals, with a resolution down to ∼10 spins per ND (detection of approximately 10-23 mol in a localized volume). The detection is based on T1 shortening upon the radical attachment, and we propose a theoretical model describing this phenomenon. We further show that this colloidally stable, water-soluble system can be used dynamically for spatiotemporal readout of a redox chemical process (oxidation of ascorbic acid) occurring near the ND surface in an aqueous environment under ambient conditions.

Department of Biomedical Technology Faculty of Biomedical Engineering Czech Technical University Prague Sitna sq 3105 27201 Kladno Czechia

Department of Inorganic Chemistry Faculty of Science Charles University Hlavova 2030 128 40 Prague 2 Czechia

IMOMEC Division IMEC Wetenschapspark 1 B 3590 Diepenbeek Belgium

Institute for Advanced Biosciences UMR 5309 Allée des Alpes 38700 la Tronche France

Institute for Materials Research Hasselt University Wetenschapspark 1 B 3590 Diepenbeek Belgium

Institute for Quantum Optics and IQST Ulm University Albert Einstein Allee 11 D 89081 Ulm Germany

Institute of Biology and Medical Genetics 1st Faculty of Medicine Charles University Albertov 4 128 00 Prague Czechia

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Flemingovo namesti 2 166 10 Prague Czechia

Institute of Theoretical Physics and IQST Ulm University Albert Einstein Allee 11 D 89081 Ulm Germany

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