processes
Article
Hg
2+
Detection with Rational Design of DNA-Templated
Fluorescent Silver Nanoclusters
Liam Yourston
1
, Polikron Dhoqina
2
, Nolan Marshall
1
, Rujani Mahmud
1
, Ethen Kuether
1
and Alexey Viktorovich Krasnoslobodtsev
1,
*
Citation: Yourston, L.; Dhoqina, P.;
Marshall, N.; Mahmud, R.;
Kuether, E.; Krasnoslobodtsev, A.V.
Hg
2+
Detection with Rational Design
of DNA-Templated Fluorescent Silver
Nanoclusters. Processes 2021, 9, 1699.
https://doi.org/10.3390/pr9101699
Academic Editor: Lukáš Richtera
Received: 16 August 2021
Accepted: 18 September 2021
Published: 23 September 2021
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4.0/).
1
Department of Physics, University of Nebraska at Omaha, Omaha, NE 68182, USA;
lyourston@unomaha.edu (L.Y.), nmarshall@unomaha.edu (N.M.), rujan.m@gmail.com (R.M.),
ekuether@unomaha.edu (E.K.)
2
Department of Physics, Faculty of Natural Sciences, University of Tirana, 1001 Tirana, Albania;
polikron.dhoqina@fshn.edu.al
* Correspondence: akrasnos@unomaha.edu
Abstract: Atomically precise silver nanoclusters (AgNCs) are small nanostructures consisting of only
a few atoms of silver. The combination of AgNCs with cytosine-rich single-stranded oligonucleotides
results in DNA-templated silver nanoclusters (DNA-AgNCs). DNA-AgNCs are highly luminescent
and can be engineered with reproducible and unique fluorescent properties. Furthermore, using
nucleic acids as templates for the synthesis of AgNCs provides additional practical benefits by
expanding optical activity beyond the visible spectral range and creating the possibility for color
tunability. In this study, we explore DNA oligonucleotides designed to fold into hairpin-loop (HL)
structures which modulate optical properties of AgNCs based on the size of the loop containing
different number of cytosines (HL-C
N
). Depending on the size of the loop, AgNCs can be manu-
factured to have either single or multiple emissive states. Such hairpin-loop structures provide an
additional stability for AgNCs and further control over the base composition of the loop, allowing
for the rational design of AgNCs’ optical properties. We demonstrate the potential of AgNCs in
detecting Hg
2+
by utilizing the HL-C
13
design and its variants HL-T
2
C
11
, HL-T
4
C
9
, and HL-T
6
C
7
.
The replacement of cytosines with thymines in the loop was intended to serve as an additional
sink for mercury ions extending the detectable range of Hg
2+
. While AgNC@HL-T
0
C
13
exhibits
an interpretable quenching curve, AgNC@HL-T
6
C
7
provides the largest detectable range of Hg
2+
.
The results presented herein suggest that it is possible to use a rational design of DNA-AgNCs based
on the composition of loop sequence in HL structures for creating biosensors to detect heavy metals,
particularly Hg
2+
.
Keywords: mercury; detection; biosensor; silver nanoclusters; fluorescence
1. Introduction
DNA-templated silver nanoclusters that are comprised of only a few silver atoms
display attractive optical properties. Measuring less than two nm in size, AgNCs are highly
luminescent [1,2]. The unique optical properties of AgNCs stem from their intermediate
nature between atomic and bulk metal [3]. Furthermore, atomic composition, shape, and
size of AgNCs control a unique optical behavior of these novel nanomaterials [4]. Single-
stranded (ss), cytosine-rich DNA oligonucleotides are the most suitable capping agents
for templating stable DNA-AgNCs due to high affinity of cytosine’s N3 heterocyclic atom
to silver ions, Ag
+
[5]. To date, various sequences have been reported to stabilize clusters
with unique optical properties, including bright emission bands in the visible part of
the spectrum [4]. Various colors have been previously reported based on the prevalent
emission wavelengths for a particular DNA-AgNC [6]. The use of nucleic acids (DNA
or RNA) as templates for synthesis of AgNCs provides additional practical benefits by
expanding optical activity beyond the visible spectral range [6,7].
Processes 2021, 9, 1699. https://doi.org/10.3390/pr9101699 https://www.mdpi.com/journal/processes