Citation: Salih, A.E.; Elsherif, M.;
Alam, F.; Chiesa, M.; Butt, H. Rapid
Colorimetric pH-Responsive Gold
Nanocomposite Hydrogels for
Sensing Applications. Nanomaterials
2022, 12, 1486. https://doi.org/
10.3390/nano12091486
Academic Editors: Deepak Kukkar
and Ki-Hyun Kim
Received: 28 March 2022
Accepted: 22 April 2022
Published: 27 April 2022
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Article
Rapid Colorimetric pH-Responsive Gold Nanocomposite
Hydrogels for Sensing Applications
Ahmed E. Salih
1,
* , Mohamed Elsherif
1
, Fahad Alam
1
, Matteo Chiesa
1,2
and Haider Butt
1,
*
1
Department of Mechanical Engineering, Khalifa University, Abu Dhabi P.O. Box 127788,
United Arab Emirates; mohamed.elsherif@ku.ac.ae (M.E.); fahad.alam@ku.ac.ae (F.A.);
matteo.chiesa@ku.ac.ae (M.C.)
2
Department of Physics and Technology, UiT The Arctic University of Norway, 9010 Tromsø, Norway
* Correspondence: ahmed.salih@ku.ac.ae (A.E.S.); haider.butt@ku.ac.ae (H.B.)
Abstract:
Surface functionalization of metallic nanoparticles (NPs) with external groups can be
engineered to fabricate sensors that are responsive to various stimuli like temperature, pH, and
numerous ions. Herein, we report the synthesis of gold nanoparticles (GNPs) functionalized with
3-mercaptopropionic acid (GNPs-MPA) and the doping of these nanoparticles into hydrogel materials
using the breathing-in/breathing-out (BI-BO) method. MPA has a carboxyl group that becomes
protonated and, thus, ionized at a pH below its pK
a
(4.32); hence, the GNPs-MPA solutions and
gels were mostly pH-responsive in the range of 3–5. Optical properties were assessed through
ultraviolet-visible (UV-Vis) spectroscopy, namely: transmission and absorption, and the parameters
used to quantify the pH changes were the full width at half maximum (FWHM) and position of
surface plasmon resonance (SPR). The solutions and gels gradually changed their colors from red to
indigo with pH decrementation from 5 to 3, respectively. Furthermore, the solutions’ and doped gels’
highest FWHM sensitivities towards pH variations were 20 nm and 55 nm, respectively, while the
SPR’s position sensitivities were 18 nm and 10 nm, respectively. Also, transmission and scanning
electron microscopy showed synchronized dispersion and aggregation of NPs with pH change in
both solution and gel forms. The gel exhibited excellent repeatability and reversibility properties,
and its response time was instantaneous, which makes its deployment as a colorimetric pH-triggered
sensor practical. To the best of our knowledge, this is the first study that has incorporated GNPs into
hydrogels utilizing the BI-BO method and demonstrated the pH-dependent optical and colorimetric
properties of the developed nanocomposites.
Keywords: nanocomposites; Biosensors; pH sensors; colorimetric sensing; optical sensors
1. Introduction
Owing to their interesting optical, electronic, and chemical properties, gold nanoparti-
cles (GNPs) have been used extensively in biomedical and chemical related industries [
1
–
6
].
In a myriad of applications, GNPS undergo surface functionalization, which either enhances
the intrinsic properties of these nanoparticles (NPs) or introduces new ones [
7
–
10
]. Some of
the commonly used functional groups for gold nanoparticles include thiols, carboxylates,
and amines. Surface functionalization with such groups allows gold nanoparticles to exhibit
physiochemical properties that are responsive to external stimuli, such as pH, temperature,
and ions, making them attractive for biosensing and other biological applications [11,12].
Recently, pH-triggered colorimetric changes of functionalized gold colloidal NPs were
studied and utilized in numerous applications [
13
–
19
]. For example, Nam et al. functional-
ized GNPs with an amide group and used their pH-responsive behavior in photothermal
cancer therapy [
11
]. Thiols have widely been used as pH-triggered molecules and attached
to GNPs. For instance, Ansar et al. fabricated MUA-functionalized gold nanoparticles
and applied their pH-triggered aggregation and re-dispersion properties in catalysis [
7
].
Nanomaterials 2022, 12, 1486. https://doi.org/10.3390/nano12091486 https://www.mdpi.com/journal/nanomaterials
