Citation: Turino, M.;
Alvarez-Puebla, R.A.; Guerrini, L.
Plasmonic Azobenzene
Chemoreporter for Surface-Enhanced
Raman Scattering Detection of
Biothiols. Biosensors 2022, 12, 267.
https://doi.org/10.3390/
bios12050267
Received: 22 March 2022
Accepted: 21 April 2022
Published: 22 April 2022
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Article
Plasmonic Azobenzene Chemoreporter for Surface-Enhanced
Raman Scattering Detection of Biothiols
Mariacristina Turino
1
, Ramon A. Alvarez-Puebla
1,2,
* and Luca Guerrini
1,
*
1
Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Carrer de Marcel lí Domingo
s/n, 43007 Tarragona, Spain; mariacristina.turino@urv.cat
2
Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain
* Correspondence: ramon.alvarez@urv.cat (R.A.A.-P.); luca.guerrini@urv.cat (L.G.)
Abstract:
Low molecular weight thiols (biothiols) are highly active compounds extensively involved
in human physiology. Their abnormal levels have been associated with multiple diseases. In
recent years, major efforts have been devoted to developing new nanosensing methods for the low
cost and fast quantification of this class of analytes in minimally pre-treated samples. Herein, we
present a novel strategy for engineering a highly efficient surface-enhanced Raman scattering (SERS)
spectroscopy platform for the dynamic sensing of biothiols. Colloidally stable silver nanoparticles
clusters equipped with a specifically designed azobenzene derivative (AzoProbe) were generated as
highly SERS active substrates. In the presence of small biothiols (e.g., glutathione, GSH), breakage
of the AzoProbe diazo bond causes drastic spectral changes that can be quantitatively correlated
with the biothiol content with a limit of detection of ca. 5 nM for GSH. An identical response was
observed for other low molecular weight thiols, while larger macromolecules with free thiol groups
(e.g., bovine serum albumin) do not produce distinguishable spectral alterations. This indicates the
suitability of the SERS sensing platform for the selective quantification of small biothiols.
Keywords:
plasmonics; surface-enhanced Raman scattering; nanoparticles; biosensing; low molecular
weight thiols
1. Introduction
Low molecular weight thiols play a key role in human physiology, most notably in
the maintenance of cellular redox homeostasis [
1
]. Abnormal levels, such as those of the
most abundant biothiols (e.g., glutathione (GSH) and cysteine (Cys)), have been associated
with cancer [
2
], neurodegenerative disorders [
3
], and cardiovascular diseases [
4
,
5
], among
others. Moreover, the overall dysregulation of the dynamic thiol-disulfide homeostasis
has also been related to multiple diseases [
6
,
7
]. Due to the clinical relevance of biothiols
in human health, the development of methods for their rapid determination in biological
fluids is essential for early diagnosis and disease monitoring, as well as for acquiring a
better understanding of biothiol-related pathophysiological processes [
6
,
7
]. It is to note that
the concentrations of these biomolecules vary widely in different bodily fluids. For instance,
GSH typically exists in the 1–5
µ
M range in plasma or serum samples from healthy human
subjects, while in the whole blood, its content increases to millimolar levels [8].
Conventional approaches, such as those based on high-performance liquid chromatog-
raphy (HPLC), capillary electrophoresis, and mass spectrometry (MS), provide robust and
highly sensitive responses. However, these techniques are time-consuming, expensive, and
not suitable to be used in remote settings [
9
]. Notably, as biothiols are prone to oxidation,
methods that allow for fast quantification with minimal sample pre-treatment and no
storage are particularly needed to improve the reliability of the analysis [10].
In recent years, major efforts have been devoted to developing new analytical ap-
proaches that would overcome the intrinsic limitations of traditional techniques, in par-
ticular fluorescent methods [
9
,
11
,
12
]. Several other nanosensors have also been designed
Biosensors 2022, 12, 267. https://doi.org/10.3390/bios12050267 https://www.mdpi.com/journal/biosensors
