Double-Layered Silica-Engineered Fluorescent Nanodiamonds for Catalytic Generation and Quantum Sensing of Active Radicals
Abstract
Fluorescent nanodiamonds (FNDs) hosting nitrogen-vacancy (NV) centers have attracted considerable attention for quantum sensing applications, particularly owing to notable advancements achieved in the field of weak magnetic signal detection in recent years. Here, we report a practical quantum-sensing platform for the controlled production and real-time monitoring of ultra-short-lived reactive free radicals using a double-layered silica modification strategy. An inner dense silica layer preserves the intrinsic properties of NV centers, while an outer porous silica layer facilitates efficient adsorption and stabilization of hydroxyl radicals and their precursor reactants. By doping this mesoporous shell with gadolinium (III) catalysts, we achieve sustained, light-free generation of hydroxyl radicals via catalytic water splitting, eliminating reliance on external precursors. The mechanism underlying this efficient radical generation is discussed in detail. The radical production is monitored in real time and in situ through spin-dependent T1 relaxometry of the NV centers, demonstrating stable and tunable radical fluxes, with concentration tunable across a continuous range from approximately 100 mM to molar levels by adjusting the catalyst condition. This study extends the technical application of nanodiamonds from relaxation sensing to the controlled synthesis of reactive free radicals, thereby providing robust experimental evidence to support the advancement of quantum sensing systems in intelligent manufacturing.
Summary
This paper presents a novel quantum sensing platform for the controlled generation and real-time monitoring of hydroxyl radicals using double-layered silica-engineered fluorescent nanodiamonds (FNDs). The core innovation lies in the design of a nanostructure where an inner dense silica layer protects the nitrogen-vacancy (NV) centers within the FNDs from environmental noise, while an outer porous silica layer facilitates the adsorption and stabilization of hydroxyl radicals and their precursor reactants. The authors further enhance the system by doping the outer porous shell with gadolinium (III) catalysts, enabling sustained, light-free generation of hydroxyl radicals via catalytic water splitting. This eliminates the need for external precursors or photo-activation, offering a more stable and controllable method for radical production. The researchers used a combination of experimental techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS), electron paramagnetic resonance (EPR) spectroscopy, and spin-dependent T1 relaxometry of the NV centers, along with density functional theory (DFT) calculations. The key findings demonstrate the successful synthesis and characterization of the double-layered silica-engineered FNDs, the efficient light-free generation of hydroxyl radicals, and the real-time, in-situ monitoring of radical production via NV center relaxometry. The radical concentration was tunable across a range from approximately 100 mM to molar levels by adjusting the catalyst concentration. This work is significant because it extends the application of nanodiamonds from passive sensing to active catalytic sensing, providing a robust platform for studying transient radical dynamics in various chemical and biological systems.
Key Insights
- •Novel double-layered silica architecture on FNDs: An inner dense silica layer protects NV centers, while an outer porous layer facilitates radical stabilization and catalyst doping.
- •Light-free catalytic water splitting: Gadolinium (III) doping in the porous silica layer enables continuous hydroxyl radical generation without external stimuli like light or ultrasound. DFT calculations show that Gd3+ catalysis lowers the water dissociation energy barrier, making the process more efficient.
- •Real-time, in-situ monitoring of radical production: Spin-dependent T1 relaxometry of NV centers allows for quantitative tracking of radical generation kinetics, with radical concentration tunable from 100 mM to molar levels.
- •Homogeneous catalytic activity: Single-particle analysis of 177 MS-silica-FND particles demonstrates uniform radical generation across the nanoparticle ensemble, even at low gadolinium (III) doping concentrations (100 aM Gd(DTPA) 2-).
- •Demonstrated the stability of the silica-FND with respect to their altered relaxation rates by showing that surface modification with silica effectively passivates the FND surface and increases the separation between the NV centers and the external interface, thereby preserving stable relaxation signals even in the presence of micromolar concentrations of gadolinium ions.
- •Conventional EPR measurements have further confirmed that hydroxyl radicals are generated via water splitting on the surface of gadolinium-doped porous silica under light-free conditions.
- •The rate of water dissociation on gadolinium-doped porous silica is 2.2×10^9 times faster than on pure porous silica, according to transition state theory calculations.
Practical Implications
- •Real-time monitoring of redox biology: The platform can be used to study cellular oxidative stress responses and metabolic activities in real-time and in-situ.
- •Catalytic reaction dynamics: The system enables investigation of reaction kinetics and mechanisms involving highly reactive intermediates, providing insights into catalytic processes.
- •Targeted radical-based therapies: The controlled generation and delivery of hydroxyl radicals can be utilized for targeted cancer therapy and antimicrobial applications, as shown by the enhanced germicidal activity against Escherichia coli even in dark conditions.
- •Intelligent chemical manufacturing: The platform opens avenues for precise, in-situ control over reactive intermediates in chemical synthesis, enabling selective control over reaction pathways and product yields.
- •Future research: Explore other metal ion catalysts, optimize the core-shell structure for specific applications, and develop closed-loop control systems for dynamic regulation of radical production based on real-time sensing signals.