Citation: Padamati, S.K.; Vedelaar,
T.A.; Perona Martínez, F.; Nusantara,
A.C.; Schirhagl, R. Insight into a
Fenton-like Reaction Using
Nanodiamond Based Relaxometry.
Nanomaterials 2022, 12, 2422. https://
doi.org/10.3390/nano12142422
Academic Editors: Deepak Kukkar
and Ki-Hyun Kim
Received: 25 April 2022
Accepted: 7 July 2022
Published: 15 July 2022
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Article
Insight into a Fenton-like Reaction Using Nanodiamond
Based Relaxometry
Sandeep Kumar Padamati
†
, Thea Annie Vedelaar
†
, Felipe Perona Martínez, Anggrek Citra Nusantara
and Romana Schirhagl *
University Medical Center Groningen, University of Groningen, Antonius Deusinglaan 1,
9713AW Groningen, The Netherlands; sandeepimschcu@gmail.com (S.K.P.); solar_sola@hotmail.com (T.A.V.);
felipeperona@gmail.com (F.P.M.); anggrek.citra.n@gmail.com (A.C.N.)
* Correspondence: romana.schirhagl@gmail.com
† These authors contributed equally to this work.
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 H
2
O
2
, a by-product of
oxygen metabolism. In this study, the reactions of copper with H
2
O
2
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.
Keywords: NV-centers; nanodiamonds; copper; Fenton-like reactions; diamonds
1. Introduction
Copper is one of the vital elements present in cells. It is especially important in
enzymes, which catalyze a variety of biological processes including oxidation, photosyn-
thesis or cell wall metabolism [
1
]. However, copper in its free hydrated form, i.e., Cu
2+
,
can be toxic to both plants and animals by altering membrane permeability and affect-
ing chromatin structure, protein synthesis, and various enzyme activities [
2
]. In humans,
several neurodegenerative diseases including Alzheimer’s and Parkinson’s disease are
characterized by modified copper homeostasis [
3
]. Changes in copper metabolism in the
brain either directly or indirectly increase oxidative stress, which is an important factor in
neuronal toxicity. Copper-based materials are regarded as efficient catalysts in Fenton-like
reactions at neutral pH and have thus been considered excellent candidates for developing
new cancer treatments [
4
]. Notably, copper is used in chemodynamic therapy treatment
for cancers by triggering ROS production. For this application, copper(I)-nanoparticles
selectively kill tumor cells [
5
]. Another important process that involves copper in biology
is the generation of hydroxyl radicals (HO
•
) and/or other reactive oxygen species (ROS).
These are generated as a result of the reaction of copper with hydrogen peroxide (H
2
O
2
)
in cells as a by-product of oxygen metabolism [
6
]. These reactions are studied by quanti-
fying hydroxyl radicals using coumarin dyes [
7
] or other hydroxyl-specific dyes such as
disodium terephthalic acid [
8
]. While these dyes have the advantage to be radical specific,
they are limited by photo-bleaching and do not provide real-time measurements. Standard
electron paramagnetic resonance (EPR) measurements can be used to detect copper(II).
However, aqueous solutions are affected by the absorption of microwaves by water, which
complicates measurements in biological samples [
9
]. In order to investigate Fenton-like
reactions, in this case a reaction of copper with H
2
O
2
, we use T1-relaxometry in this article.
Nanomaterials 2022, 12, 2422. https://doi.org/10.3390/nano12142422 https://www.mdpi.com/journal/nanomaterials
