Insight into a Fenton-like Reaction Using Nanodiamond Based Relaxometry

At a Glance

Metadata	Details
Publication Date	2022-07-15
Journal	Nanomaterials
Authors	Sandeep K. Padamati, Thea Vedelaar, Felipe Perona Martínez, Anggrek Citra Nusantara, Romana Schirhagl
Institutions	University Medical Center Groningen, University of Groningen
Citations	8
Analysis	Full AI Review Included

Executive Summary

This research demonstrates the successful application of Nitrogen-Vacancy (NV) center T1 relaxometry using Fluorescent Nanodiamonds (FNDs) to monitor the kinetics of a copper-catalyzed Fenton-like reaction in real time.

Core Value Proposition: The method provides non-destructive, real-time measurement of paramagnetic species (Cu²⁺ and radicals) in aqueous solutions at nanomolar concentrations, overcoming the limitations of conventional Electron Paramagnetic Resonance (EPR) (microwave absorption by water) and traditional fluorescent probes (photo-bleaching).
Detection Performance: Nanomolar detection limits (100 nM) were achieved for copper(II) (Cu²⁺) in solution, confirming the high sensitivity of the NV relaxometry technique.
Kinetic Insight: T1 relaxation time measurements tracked the dynamic conversion of Cu²⁺ to non-paramagnetic species (Cu⁺ or Cu³⁺) upon addition of H₂O₂, followed by a subsequent increase in T1 as Cu²⁺ concentration decreased.
Multimodal Validation: Results were strongly correlated with complementary spectroscopic techniques, including UV-Vis absorption (Cu²⁺ decay), Raman spectroscopy (H₂O₂ consumption), and fluorescence spectroscopy (OH• radical formation).
Sensor Stability: NV centers offer an infinitely stable sensor platform, making them ideal for long-duration monitoring in complex biological or chemical environments.

Technical Specifications

Parameter	Value	Unit	Context
Sensor Material	Fluorescent Nanodiamonds (FNDs)	N/A	HPHT synthesis, oxygen terminated surface.
FND Average Size	~70	nm	Stock solution concentration: 1 mg/mL.
Sensing Defect	Ensemble Nitrogen-Vacancy (NV) Centers	N/A	Used for T1 relaxometry measurements.
Cu(II) Detection Limit (FND)	100	nM	Quantified concentration in solution.
T1 Measurement Time Resolution	~2.5	min	Achieved using rolling window analysis (10,000 repetitions).
Laser Polarization Wavelength	532	nm	Used for optical polarization of NV centers.
Laser Polarization Time	5	µs	Duration of laser pulse for polarization.
Detection Wavelength (Filter)	>550	nm	Long-pass filter used before Avalanche Photo Diode (APD).
Reaction Temperature	~22	°C	All experiments conducted at room temperature.
OH• Radical Concentration (Measured)	0.9	µM	Detected via HTA fluorescence (CuSO₄ 1 mM, H₂O₂ 10 mM).
H₂O₂ Consumption (Raman)	0.3	mM	Measured decrease over 20 min reaction time (Initial H₂O₂: 100 mM).
Cu(II) Concentration Range (T1)	100 nM to 10	mM	Range tested for calibration and reaction monitoring.
H₂O₂ Concentration Range (T1)	10 nM to 10	mM	Range tested in reaction mixtures.

Key Methodologies

The study utilized a combination of quantum sensing (T1 relaxometry) and conventional spectroscopy to characterize the copper(II)/H₂O₂ reaction.

1. Fluorescent Nanodiamond (FND) Preparation

Surface Treatment: Glass-bottom Petri dishes were pre-treated with air plasma for 15 minutes.
FND Deposition: FND stock (1 mg/mL) was diluted to 10 µg/mL. 20 µL of the diluted solution was added to the dish.
Drying: Solvent was evaporated for 1 hour in a fume hood to affix the FNDs to the glass surface.

2. T1 Relaxometry Setup and Measurement

Instrumentation: Measurements were performed using a home-built magnetometer setup at room temperature (~22 °C).
Particle Selection: Diamonds were selected based on high intensity counts (10⁶ to 10⁷ counts per second). Outliers (T1 > 600 µs) were excluded.
Pulsing Sequence: NV centers were polarized using a 532 nm laser (5 µs pulse). The dark time between pulses varied (0.2 µs to 10 ms).
Signal Acquisition: The pulsing scheme was sent via a Pulseblaster to an Acousto-Optical Modulator (AOM). Fluorescence was detected by an APD through a 550 nm long-pass filter.
Data Processing: Each T1 curve was fitted using a bi-exponential model. Time resolution was increased to ~2.5 minutes using a rolling window method (combining 2500 repetitions per data point).

3. Chemical Reaction Monitoring

Calibration: T1 values were measured for CuSO₄ solutions ranging from 0.1 µM to 1 mM to establish a calibration curve for Cu²⁺ concentration.
Reaction Initiation: H₂O₂ (30 wt%) was added to the CuSO₄ solution (1 mM) to initiate the Fenton-like reaction, and T1 measurements were recorded in real time.

4. Spectroscopic Validation Techniques

UV-Vis Absorption: Used to monitor the decay of Cu²⁺ concentration by tracking the d-d band transition absorption at 800 nm over 20 minutes.
Raman Spectroscopy: Used to quantify H₂O₂ consumption by monitoring the O-O symmetric stretch band intensity at 876 cm^-1.
Emission Spectroscopy: Used to detect hydroxyl radicals (OH•) by employing disodium terephthalate (Na₂TH) as a chemical trap, measuring the resulting HTA fluorescence emission at 420 nm (Excitation: 330 nm).
Oxygen Sensor: Used to confirm the liberation of O₂ during the reaction, tracking dissolved oxygen concentration over time.

Commercial Applications

The NV-center relaxometry technique demonstrated here is highly relevant for several high-tech sectors due to its sensitivity, stability, and ability to operate in ambient conditions.

Industry/Sector	Application Area	Technical Relevance
Quantum Sensing & Metrology	Development of nanoscale magnetic sensors.	Enables room-temperature, high-sensitivity detection of paramagnetic species (spin noise) in liquid environments, crucial for miniaturized quantum devices.
Biomedical Diagnostics	Oxidative Stress and Metabolism Monitoring.	Real-time tracking of Reactive Oxygen Species (ROS) and free radicals (e.g., OH•) in biological samples, relevant for studying neurodegenerative diseases (Alzheimer’s, Parkinson’s).
Oncology & Drug Development	Chemodynamic Therapy (CDT) Efficacy.	Non-destructive monitoring of copper- or iron-catalyzed radical generation within tumor models, allowing optimization of CDT drug dosage and kinetics.
Analytical Chemistry	Heavy Metal and Catalyst Monitoring.	High-sensitivity, label-free quantification of transition metal ions (Cu²⁺, Fe²⁺) in complex solutions, surpassing the detection limits of standard UV-Vis or the aqueous limitations of EPR.
Nanomaterials Engineering	Surface Chemistry and Adsorption Studies.	Investigating the interaction kinetics and adsorption mechanisms of metal ions onto nanodiamond surfaces, critical for developing functionalized quantum probes.

View Original Abstract

Copper has several biological functions, but also some toxicity, as it can act as a catalyst for oxidative damage to tissues. This is especially relevant in the presence of H2O2, a by-product of oxygen metabolism. In this study, the reactions of copper with H2O2 have been investigated with spectroscopic techniques. These results were complemented by a new quantum sensing technique (relaxometry), which allows nanoscale magnetic resonance measurements at room temperature, and at nanomolar concentrations. For this purpose, we used fluorescent nanodiamonds (FNDs) containing ensembles of specific defects called nitrogen-vacancy (NV) centers. More specifically, we performed so-called T1 measurements. We use this method to provide real-time measurements of copper during a Fenton-like reaction. Unlike with other chemical fluorescent probes, we can determine both the increase and decrease in copper formed in real time.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12142422

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