Improving sensitivity of a new Hg2+-Selective fluorescent sensor by silver nanoparticles via plasmonic enhancement

Article abstract of DOI:10.1016/j.jphotochem.2020.113064

A new Hg²⁺-selective fluorescent sensor which consisted of tripodal ligand connected to three 7-nitrobenzo-2-oxa-1,3-diazolyl (NBD) moieties (TNBD) was successfully synthesized. This sensor exhibited high Hg²⁺-selectivity over other interfering ions with the detection limit of 12.5 ppb. Furthermore, plasmonic enhancement of the synthesized purple silver nanoparticles (P-AgNPs) was utilized to enhance sensitivity of TNBD. Remarkably, the sensor containing P-AgNPs (TNBD + P-AgNPs) preserved high selectivity toward Hg²⁺with better detection limit (3.8 ppb) compared to the bare sensor. Moreover, Hg²⁺ detection in real samples (drinking water) by TNBD + P-AgNPs was accomplished which indicated that the sensor could operate in the presence of aqueous-consisted matrix. Therefore, the developed sensor was practical and could be applied for Hg²⁺-detection in biological samples.

Full text of DOI:10.1016/j.jphotochem.2020.113064

Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Improving sensitivity of a new Hg2+-Selective fluorescent sensor by silver
nanoparticles via plasmonic enhancement
Sasiwimon Kraithong, Nirumon Chailek, Jitnapa Sirirak, Kullatat Suwatpipat,
Nantanit Wanichacheva, Pattanawit Swanglap*
Department of Chemistry, Faculty of Science, Silpakorn University, Nakhon Pathom, 73000, Thailand
A R T I C L E I N F O
A B S T R A C T
A new Hg2+-selective fluorescent sensor which consisted of tripodal ligand connected to three 7-nitrobenzo-2-
oxa-1,3-diazolyl (NBD) moieties (TNBD) was successfully synthesized. This sensor exhibited high Hg2+-selec-
tivity over other interfering ions with the detection limit of 12.5 ppb. Furthermore, plasmonic enhancement of
the synthesized purple silver nanoparticles (P-AgNPs) was utilized to enhance sensitivity of TNBD. Remarkably,
the sensor containing P-AgNPs (TNBD þ P-AgNPs) preserved high selectivity toward Hg2+with better detection
limit (3.8 ppb) compared to the bare sensor. Moreover, Hg2+ detection in real samples (drinking water) by TNBD
þ P-AgNPs was accomplished which indicated that the sensor could operate in the presence of aqueous-
consisted matrix. Therefore, the developed sensor was practical and could be applied for Hg2+-detection in
biological samples.
Keywords:
Silver nanoparticles
Plasmonic enhancement
Hg2+sensor
Fluorescent sensor
Colorimetric sensor
1. Introduction
fluorescent sensor in the presence of Hg2+, which allows easy visuali-
zation, is also desirable.
Mercury ion (Hg2+) is a toxic contaminant which can cause serious
health problems [1–5]. Mercury emissions from human activity such as
powerplant, mine, and petroleum industries can lead to mercury
contamination in aqueous environment such as ocean and lake [2,6].
The bacteria in these aqueous environments can change the contami-
nated mercury to the inorganic methylmercury (CH3Hg+) [1,6], which
can accumulate in human body via biological food chains and cause
severe damage to DNA, brain, and central nervous systems including
Minamata’s disease [1–5]. Up to 2.1 ppm of mercury contamination was
found in bluefin tuna in the Atlantic ocean [7] and up to 11 ppm of
mercury contamination was found in the sediment close to petroleum
industry site [8]. To protect human health safety, the maximum
contaminated limit of Hg2+ in drinking water regulated by World Health
Organization (WHO) is 6 ppb [9]. Due to high toxicity of Hg2+, a variety
of techniques including fluorescence spectroscopy through fluorescent
sensor have been developed for Hg2+ detection [10–16]. Hg2+ fluores-
cent sensors can provide high sensitivity and selectivity as well as
real-time monitoring possibility with relatively quick and easy operation
compare to conventional techniques such as inductively coupled plasma
atomic emission spectroscopy (ICP-OES) and atomic absorption spec-
troscopy (AAS). In addition, accompanied colorimetric change of the
Among the available fluorescent dye, 7-nitrobenzo-2-oxa-1,3-diaz-
olyl (NBD) and its derivatives possess excellent photophysical proper-
ties in terms of strong fluorescent emission, excitation and emission in
visible wavelength, and large stroke shift. NBD-based fluorescent sen-
sors have been utilized to detect several metal ions via intramolecular
charge transfer (ICT) mechanism [17–20]. Particularly, fluorescence of
NBD-based fluorescent sensors is drastically quenched by Hg2+ due to
paramagnetic properties of Hg2+ [21–25]. Therefore, complexation of
the NBD-based fluorescent sensor and Hg2+, which led to reduction of
electron density of NBD, and consequently distinct changes of fluores-
cence and color of the sensor. Therefore, NBD derivative was chosen to
serve as a fluorophore for Hg2+ sensor in our work. Additionally, an
acyclic tripodal ligand containing N and S atoms, which provide selec-
tive binding of Hg2+ and sensor, was linked to the sensor to achieve
self-assembly of Hg2+ and the sensor through host-guess chemistry [26].
Although, there have been many reports of Hg2+ fluorescent sensors,
some of them exhibited moderate sensitivity due to the low fluorescent
quantum yield which limited their practical use [27–30]. Thus, devel-
oping an approach to improve sensitivity of Hg2+ fluorescent sensor is
an attractive challenge. One of a promising approach is utilizing plas-
monic enhancement of noble metal nanoparticles (NPs), which have
* Corresponding author.
E-mail address: swanglap_p@silpakorn.edu (P. Swanglap).
https://doi.org/10.1016/j.jphotochem.2020.113064
Received 30 September 2020; Received in revised form 27 November 2020; Accepted 30 November 2020
Available online 2 December 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
been widely used to increase the emissive quantum yield of the
photo-active molecules via plasmonic enhancement [31,32]. Many ap-
plications that taking advantage of the plasmonic enhancement such as
energy transfer along nanostructure [33], Surface-Enhanced Raman
Spectroscopy (SERS) [34–37], and plasmon-enhanced fluorescence
spectroscopy have been achieved [38,39]. These plasmonic enhance-
ments is originated from an increasing of electric field in proximal of the
surface of NPs, arises from Surface Plasmon Resonance (SPR) of NPs,
which are coherent oscillations of conductive electrons upon electro-
magnetic wave excitation [40]. And we have recently proved that
non-radiative energy transfer from SPR of plasmonic nanoparticles,
which led to plasmonic enhancement could increase sensitivity of the
turn on-Hg2+ fluorescent sensor. [41]
from the reported procedure. [29] The first step was synthesizing of tris
(2-chloroethyl)amine (Tris-Cl). In 250 mL round bottom flask, SOCl2
(52 mL, 700 mmol) was dissolved and stirred in CHCl3 (80 mL). Next,
triethanolamine (29.8 g, 200 mmol) in CHCl3 (50 mL) was slowly
dropped into SOCl2 solution and the solution mixture was stirred until
the gas disappeared. Then the solution mixture was refluxed for 4 h.
After that, the solution mixture was cooled to room temperature and
then was filtered under vacuum. The white residue was washed 3 times
by cold CHCl3 (30 mL) to acquire the white solid of Tris-Cl with quan-
titative yield of 41.0 g (200 mmol). 1H NMR (300 MHz, CDCl3): δ 1.46
(br-s, 4NH2), 1.71 (quint, 4H, J =3.0 Hz), 2.53 (t, 4H, J =6.0 Hz), 2.75 (t,
4H, J =6.0 Hz), 2.88 (t, 4H, J =6.0 Hz) ppm; 13C NMR (75 MHz, CDCl3) δ
28.77 (2CH2), 31.41 (2CH2), 36.37 (2CH2), 41.18 (2CH2) ppm. HR-ESI
MS calcd for C8H21N2S2 (M+H)+ 209.1146 m/z, found 209.1073 m/z.
In the second step, tris(ethylisothiouronium)amine (Tris-SN) was
synthesized. Tris-Cl (10.0 g, 49 mmol) was disssolved and stirred in
EtOH (30 mL) in a 100 mL round bottom flask. Then thiourea (9.4 g. 123
mmol) was quickly added to the solution mixture then the solution
mixture was refluxed at ambient condition for 24 h. After that, the so-
lution mixture was left stirring until the temperature reached the room
temperarture and then the soltion mixture was filtered under vacuum.
The white residue was washed 3 times by cold EtOH. Then the white
solid of Tris-SN was collected (14.2 g, 44 mmol, 90 %yield). 1H NMR
(300 MHz, D2O): δ 3.26 (t, 6H, J =6.6 Hz), 3.45 (t, 6H, J =6.3 Hz) ppm;
13C NMR (75 MHz, D2O): δ 27.4 (3CH2), 51.7 (3CH2) ppm; HR-ESI MS
calcd for C9H22 N7S+3 (M+H)+ 324.1093 m/z, found 324.1123 m/z.
In the final step, Tris-SN (14.2 g, 44 mmol) was dissolved in water
(30 mL) and then was stirred in a 100 mL round bottom flask with
addition of NaOH (11.3 g, 283 mmol). Next, the solution mixture was
heated at 100 ◦C for 5 min. After that, the solution mixture was rapidly
Herein, we propose a new Hg2+-selective colorimetric and fluores-
cent sensor based on tripodal ligand containing three subunits of NBD
fluorophore. In this study, a new strategy to increase sensitivity of turn
off- Hg2+ fluorescent sensor TNBD through non-radiative energy
transfer from SPR of plasmonic nanoparticles, which led to plasmonic
enhancement, by using of the synthesized purple silver nanoparticles (P-
AgNPs) has been demonstrated without compromising the selectivity of
the newly developed sensor. The calculated detection limit of sensor
containing P-AgNPs (TNBD þ P-AgNPs) was adequate to detect the
maximum-permitted Hg2+ contamination in drinking water of WHO (6
ppb). The accompanied colorimetric change of our new fabricated
sensor will allow practical, quick, and easy monitoring of the Hg2+
contamination in the aqueous environmental samples with reliable
detection. In addition, we have demonstrated that plasmonic nano-
particles could be used to increase the sensitivity of fluorescent sensor
for both turn on [41] and turn off fluorescent sensors via
plasmonic-fluorescence enhancement mechanisms [42,43]. The under-
standing and approach of the plasmonic enhancement to improve the
sensitivity of the fluorescent sensor from our work will benefit devel-
oping of the ultra-sensitive fluorescent sensors for immediate sensing,
on-site screening, and easy detection of various toxic contamination in
the aqueous environmental samples.
◦
cooled to 20 C by an ice bath and then was neutralized by 5 M HCl.
Then the mixture solution was extracted by CHCl3 (3 × 20 mL). The
organic phase was dried by anh.Na2SO4 and solvent was removed under
vacuum. Finally, the pale pink oil of Tris-SH was obtained (3.6 g, 180
mmol, 41 %yield). 1H NMR (300 MHz, CDCl3): δ 1.77 (d, 3H, J =1.8 Hz),
2.62–2.81 (m, 12 H) ppm; 13C NMR (75 MHz, CDCl3): δ 23.2 (3CH2),
57.1 (3CH2) ppm; HR-ESI MS calcd for C6H16NS+3 (M+H)+ 198.0439 m/
z, found 198.0439 m/z.
2. Experiments
2.1. Chemicals and materials
2.4. Synthesis of NBDBr
All solvents and reagents for experiment were obtained from Fluka
Chemical Corporation and all chemicals were used as received. All of the
cationic salts in this study were obtained from Strem chemicals, Inc.
In a 25 mL round bottom flask, 2-bromoethylamine (0.1 g, 0.49
mmol) was stirred with Et3N (0.15 mL, 1.1 mmol) in dry CH2Cl2 (5 mL)
for 30 min. Next, 4-chloro-7-nitrobenzofurazan (0.097 g, 0.49 mmol)
was added and the reaction was kept at ambient temperature for 2 h.
Then the solution mixture was extracted by CH2Cl2 3 times (3 × 20 mL).
After that, the organic phase was dry by anh. Na2SO4 and removed under
vacuum. The obtaining crude product was then purified by column
chromatography using 5:3 hexane:EtOAc as eluent (Rf = 0.85) to
aqquire orange-brown solid of NBDBr (0.059 g, 2.1 mmol, 40 %yield).
1H NMR (300 MHz, CDCl3): δ 1.76 (quint, 4H, J =4.2 Hz), 2.62 (t, 4H, J
=6.6 Hz), 2.96 (t, 4H, J =6.6 Hz), 3.71 (t, 4H, J =6.0 Hz), 6.23 (d, 2H, J
=8.7 Hz), 8.53 (d, 2H, J =8.7 Hz) ppm; 13C NMR (75 MHz, CDCl3): δ
28.11 (4CH2), 30.62 (4CH2), 100.78 (2CH), 102.82 (2C), 106.74 (2C),
119.03 (4C), 137.95 (2CH) ppm; HR-ESI MS calcd for C20H22N8NaO6S2+
(M + Na)+ 557.1000 m/z, found 557.0913 m/z.
2.2. Characterization techniques
The Bruker Avance spectrometer were used to collect 1H and 13C
NMR spectra of all compounds with 300 MHz and 75 MHz operating
frequency, respectively. The mass spectra were determined by an elec-
trospray ionization ion trap mass spectrometer (ThermoElectron LCQ-
DECA-XP). A single beam spectrophotometer (Hewlett Packard 8453)
was used to collect all absorption spectra. Fluorescence spectra were
recorded by a Luminescence spectrometer (Perkin Elmer LS 55) with
both emission and excitation slit widths of 5.0 nm. The transmission
electron microscopy was used to characterized the morphology of the
synthesized P-AgNPs (TEM, JEM-2100 with 160 kV operating voltage).
The scanning electron microscope with energy dispersive x-ray spec-
troscopy (SEM-EDS, TESCAN-MIRA3 with 20.0 kV operating voltage)
was used to characterized the morphology and elemental analyze of the
P-AgaNPs. The particle size analyzer (Malvern, Nano-ZS Zetasizer) was
used to characterized the size distribution (hydrodynamic radius) and
zeta potential of the synthesized P-AgaNPs.
2.5. Synthesis of TNBD
Tris-SH (0.030 g, 0.15 mmol) was stirred with K2CO3 (0.15 g, 1.1
mmol) in dry acetone (4 mL) for half an hour. Then NBDBr (0.3 g, 1.0
mmol) was instantaneous added to the mixed solution and the reaction
mixture was then refluxed at ambient condition for 24 h. Next, the re-
action was then cooled to room temperature and was filtered. The sol-
vent was removed under vacuum and the obtianing crude product was
finally purified by column chromatography using 3% v/v MeOH/CH2Cl2
2.3. Synthesis of tris(2-mercaptoethyl)amine (Tris-SH)
The synthesis procedure of tris(2-mercaptoethyl)amine was modified
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S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
Fig. 1. a) Synthesis outline of TNBD. And optimized geometries in MeOH of b) TNBD and c) TNBD:Hg2+ complex by the Gaussian 09 program.
as eluent (Rf = 0.33) to obtain brown solid of TNBD (0.080 g, 0.10
mmol, 65 %yield, mp 119ꢀ 120 ◦C). 1H NMR (300 MHz, CDCl3): δ 2.72
(t, 6H, J =6 Hz), 2.91 (t, 6H, J =6 Hz), 2.99 (t, 6H, J =6 Hz), 3.27 (t, 6H,
J =6 Hz), 6.21(d, 3H, J =6 Hz), 8.46 (d, 3H, J =6 Hz) ppm; 13C NMR (75
MHz, CDCl3): δ 28.44 (3CH2), 28.52 (3CH2), 37.34 (3CH2), 52.93
(3CH2), 96.56 (3CH), 100.29 (3C), 121.40 (3C), 134.13 (3CH), 141.60
(3C), 141.91 (3C) ppm; HR-ESI MS calcd for C20H22N8NaO6S+2 (M +
Na)+ 557.1000 m/z, found 557.0913 m/z.
2.7. Sensitivity and selectivity studies of TNBD and TNBD + P-AgNPs
The sensitivity and selectivity of sensor TNBD and TNBD þ P-AgNPs
were studied in 5% v/v water/MeOH by fluorescence measurement. The
stock solutions of 1.0 × 10ꢀ 2 M acetate salts were dissolved in the
deionized water. And the fluorescent titration of TNBD (2.3 μM) and
TNBD þ P-AgNPs (2.0
μM, 100 nM) solution were obtained with
addition of gradient concentration of metal ions using fluorescence
spectrophotometer with excitation wavelength (λex) of 465 nm and
emission wavelength from 470 nm to 650 nm.
2.6. Synthesis of the purple silver nanoparticles (P-AgNPs)
The synthesis procedure of P-AgNPs was slightly modified from the
2.8. Computational study of TNBD
reported study [44]. First of all, 50 μL of 0.05 M AgNO3 and 0.50 mL of
75 mM trisodium citrate solution TSC were added into 24.1 mL of
deionized water in a 100 mL beaker then the mixture was vigorously
stirred at ambient temperature for 5 min. Next, NaBH4 (100 mM, 0.5
mL) was rapidly added into the mixture to obtain light yellow solution
indicating formation of the Ag seeds, which resulted from reduction of
TNBD and binding of TNBD and Hg2+ (TNBD-Hg2+ complex) were
optimized by density functional theory calculations (DFT) utilizing the
Gaussian 09 program [45]. And the geometrical optimizations of both
TNBD and TNBD-Hg2+ complex were achieved by LanL2DZ for Hg2+
and B3LYP level using 6ꢀ 31 G for main group element with MeOH as
the solvents. The optimized molecular figures were produced by the
VMD program [46].
Ag+ to Ag◦. After that, 90
μL of 30 % H2O2 was added and the mixture
was stirred for 30 min to obtain deep purple solution indicating for-
mation of P-AgNPs. Finally, poly(vinylpyrrolidone) (PVP, Mw = 40,000
g/mol, 17.5 mM, 0.10 mL) was added to the reaction mixture to prevent
aggregation of the synthesized P-AgNPs. The absorption spectrum of
P-AgNPs were collected by UV–vis absorption spectrophotometer and
morphologies of the P-AgNPs were determined by transmission electron
microscopy (TEM, JEM-2100 with 160 kV operating voltage).
3. Result and discussion
3.1. Molecular designing and synthesizing of TNBD
Here in fluorescent sensor TNBD was synthesized via a conventional
five-step synthesis, as shown in the synthetic outline of Fig. 1a). Tris(2-
mercaptoethyl)amine (Tris-SH) was synthesized with a high yield by
three steps via substitution reaction of triethanolamine with SOCl2 to
provide Tris-Cl and then substitution of Tris-Cl with thiourea followed
by hydrolysis using NaOH. On the other hand, NBDBr was obtained with
a good yield by electrophilic aromatic substitution of 4-chloro-7-nitro-
benzofurazan and 2-bromoethylamine. After that, substitution reaction
of Tris-SH with NBDBr using K2CO3 as base provided the sensor TNBD.
The selective fluorescent sensor TNBD was inspired by the host-guess
interaction of the supramolecular chemistries. Chelating of Hg2+ to
fluorescent sensor TNBD via a cyclic tripodal ligand was designed based
on the host-guess interaction. The symmetrical tripodal ligand that
contained three sulfur atoms as well as four nitrogen atoms could
improve selectivity of sensor TNBD and Hg2+ via high affinity of “soft
acid” (Hg2+) and “soft ligand” (cyclic tripodal ligand containing sulfur
atoms). Furthermore, the additional fluorophore moieties could
enhance a sensitivity of the system. In this work, we have shown that
TNBD consisted of tripodal ligand and three subunits of NBD provided
high selectivity and high sensitivity toward Hg2+ through favorable
electrostatic interactions.
Fig. 2. Fluorescent emissions of sensor TNBD (10 μM) at 556 nm (excitation
wavelength of 465 nm) of the bare sensor and the sensor after addition of Hg2+
In order to get a geometry details of the coordination behavior be-
(5.3 μM) as the function of water percentages in MeOH.
tween TNBD and Hg2+, the geometry optimization of TNBD:Hg2+
3

S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
Fig. 5. Competitive-selectivity studies of TNBD (2.3
μ
M) in 5% aqueous MeOH
with Hg2+ (67
also presence.
μ
M) when the other competitive metal ions (670 M) were
μ
3.2. Influence of water content on the fluorescent emissions
The influences of water contents on fluorescent emissions of TNBD
were investigated by varying ratio of water in MeOH to assess the use of
sensor for aqueous and environmental samples. The influences of water
contents (percentages) on the fluorescences of TNBD were tested in the
absence and presence of Hg2+
.
As shown in Fig. 2, it was found that the fluorescent intensity of
TNBD depended on the content of water. The fluorescence of the bare
sensor was gradually decreased when the water percentages was
Fig. 3. a) UV–vis absorption titrations of TNBD (12
μ
M) in 5% aqueous MeOH
M, b was 1.6 M, c was 4.9
M, d was 8.3 μM, e was 11.6 μM, f was 14.9 μM, g was 20.9 μM, and h was 24.3
solution after Hg2+(aq) addition. [Hg2+] of a was 0
μ
μ
increased. Addition of Hg2+ (5.3
μ
M) led to distinguished decreasing of
μ
μ
M. b) Fluorescent titrations of 1 (2.3 μM) at 556 nm in 5% aqueous MeOH
fluorescent emission when the content of water was lower or equal to 20
%. In contrast, only small decreasing of fluorescent emission was
observed when the content of water was higher than 20 %. Therefore,
TNBD in 5% aqueous MeOH solution was chosen for study of fluores-
cence sensing of the sensor in the presence of different metal cations.
solution after Hg2+(aq) addition. [Hg2+] of a was 0
μ
M, b was 1.5
μ
M, c was 2.5
M, i
μ
M, d was 4.1
μ
M, e was 5.5
μ
M, f was 7.8
μ
M, g was 9.8
μ
M, h was 14.5 μ
was 22.3
μ
M, and j was 27.5 μM.
complex in MeOH (Fig. 1b)) were performed by the Gaussian 09 using
DFT-B3LYP as LanL2DZ basis set for Hg2+ and 6ꢀ 31 G basis set for all of
the main group elements utilizing the integral equation formalism
polarizable continuum model
3.3. Sensitivity studies of TNBD
The absorption and fluorescent titration of TNBD as a function of
Hg2+ concentrations were studied in 5% aqueous MeOH solution. The
absorption maxima of the sensor was displayed at 335 nm and 465 nm
(IEEPCM). As shown in Fig. 1c), the optimized TNBD:Hg2+ complex
displayed a nearly planar geometry. chelation of Hg2+ and three sulfur
atoms of tripodal ligand was observed with the length of 2.90, 2.96 and
3.11 Å via ion‒dipole interaction. These results indicated that TNBD
was selective to Hg2+ through self-assembly of its tripodal ligand and
(Fig. 3a)), corresponding to π-π transition and ICT from amine, electron-
donating group to nitro, electron-withdrawing group [47]. The ab-
sorption titration of TNBD as a function of Hg2+ concentrations as
depicted in Fig. 3a), the absorption peaks at 335 nm and 465 nm were
slowly decreased when Hg2+ concentration was increased with notice-
ably three isosbestic points at 333 nm, 394 nm, and 474 nm. The lower
Hg2+
.
Fig. 4. a) Normalized fluorescence intensities of TNBD in 5% aqueous MeOH when the concentrations of various metal cations change. The cations were Cu2+, Ag+,
Ni2+, Zn2+, Cd2+, Fe3+, Pb2+, Ba2+, Ca2+, Na+, Mg2+, K+, Mn2+, and Co2+ b) Fluorescence spectra of TNBD (2.3
μM) after addition of various metal ions (18.6 μM).
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S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
Fig. 6. a) Colorimetric changes and b) Fluorescent changes of TNBD (15
μ
M) in 5% aqueous MeOH solutions after addition of different metal cations. The tested
cations were Cu2+, Ag+, Ni2+, Zn2+, Cd2+, Fe3+, Pb2+, Ba2+, Ca2+, Na+, Mg2+, K+, Mn2+ and Co2+
.
of the absorption peaks at 335 nm and 465 nm were accompanied by the
new emerging absorption peaks at 345 nm and 485 nm. The observed
bathochromic shifts (10 and 20 nm), which led to the colorimetric
change of TNBD solution from yellow to red, was possibly caused by the
reduction of ICT from amine, electron-donating, group to nitro,
Fig. 7. TEM image of the synthesized P-AgNPs. The morphologies of the P-
AgNPs were truncated triangular nanoplates with various side lengths in the
range of 10 to 50 nm. The magnification of the image was 135,000 times. And
optical image of the P-AgNPs (inset, top left).
Fig. 9. a) Fluorescent spectra of TNBD (2.0
μ
M) and TNBDþP-AgNPs (2.0
μM,
100 nM) with and without Hg2+ addition (49
μ
M). The excitation wavelength
was 465 nm and 5% aqueous MeOH were used as solvents. b) Fluorescent
titration of TNBD þ P-AgNPs (2.0
of a was 0 M, b was 1.5 M, f was
14.0 M, g was 22.0 M, h was 27.0 μM, i was 39.0 μM, and j was 49.0 μM. The
μ
M, 100 nM) with addition of Hg2+. [Hg2+
]
μ
μM, c was 3.0
μ
M, d was 5.0
μ
M, e was 9.0
μ
Fig. 8. Absorption spectra of sensor TNBD without P-AgNPs, bare P-AgNPs,
Tris-SH ionophore, P-AgNPs with Tris-SH addition. All spectra
were normalized.
μ
μ
excitation wavelength was 465 nm and 5% aqueous MeOH were used
as solvents.
5

S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
Scheme 1. The Hg2+ sensing mechanisms of a) TNBD þ P-AgNPs with plasmonic enhancement and b) TNBD.
electron-withdrawing, group through Hg2+ chelation and increasing of
polarity of the system [19]. The noticeably color change after addition of
Hg2+ demonstrated that TNBD could serve as Hg2+-colorimetric sensor
with clear and easy visualization.
function of Hg2+ concentration. And the fluorescent quenching up to 44
% was observed with the calculated detection limit (calculated by 3SD/
slope) [48,49] of 12.5 ppb (6.2 × 10ꢀ 8 M). The fluorescent quenching
could be attributed to the reducing of ICT from the amine,
electron-donating, group to the nitro, electron-withdrawing, group due
to coordination of Hg2+ similar to the previous NBD-derivative fluo-
rescent dyes [17,19,47]. The lower fluorescence quantum yield of
TNBD:Hg2+ complex (0.00) compared to sensor TNBD (0.10) in MeOH
(relative to coumarin 6 in EtOH, φf = 0.78), confirmed that the
Fluorescence sensing of TNBD was also studied. The fluorescence
behavior of TNBD in 5% aqueous MeOH exhibited ON-OFF switching
after addition of Hg2+ (Fig. 3b)). The bare sensor exhibited strong
fluorescence with maximum emission at 556 nm. In contrary, adding of
Hg2+ resulted in gradually decreasing of its fluorescent emission as a
Table 1
The comparison of sensitivities and sensing performances of TNBD and TNBD þ P-AgNPs and recent reported Hg2+ fluorescent sensors.
Hg2+ sensor
Working system
Detection limit (M)
λex/ λem (nm)
Application
Reference
A
CH3CN
1.6 × 10ꢀ 8
3.5 × 10ꢀ 7
1.08 × 10ꢀ 6
2.7 × 10ꢀ 7
3.0 × 10ꢀ 6
6.2 × 10ꢀ 8
1.9 × 10ꢀ 8
520/585
500/540
396/475
355/417
530/612
lake and tap water
HepG-2 cells
[14]
[11]
[13]
[12]
[16]
B
DMSO/H2O (5/1, v/v)
CH3CN/H2O (3/7, v/v)
CH3CN/H2O (1/1, v/v)
acetone
C
HeLa cells
D
drinking and tap water
test strips
E
TNBD
5% aqueous MeOH
465/556
drinking water
This work
TNBD þ P-AgNPs
6

S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
real environmental samples.
Moreover, the chromogenic change of TNBD from yellow to orange
was only observed with addition of Hg2+ as displayed in Fig. 6. The
result indicated that TNBD could also serve as Hg2+ colorimetric sensor
with high Hg2+-selectivity.
3.5. Increasing sensitivity of TNBD by the plasmonic silver nanoparticles
For this work, plasmonic enhancement were utilized to increase
sensitivity of TNBD. The purple silver nanoparticles (P-AgNPs) with
maximum absorption at 500 nm and broad absorption from 375 nm to
700 nm were synthesized, and were incorporated to TNBD solution to
obtain TNBD þ P-AgNPs sensor. The distribution and morphologies of
the synthesized P-AgNPs were characterized by TEM as shown in TEM
image (Fig. 7), and the appearance of the P-AgNPs in solution was
shown (inset Fig. 7, top left). The average size (hydrodynamic radius) of
the P-AgNPs which was analyzed by particle analyzer was 61.5 nm and
their zeta potential were -37.8 mV, which indicated that P-AgNPs were
stable in the colloidal solution (Figure S1). The SEM image also showed
that the particles shape was truncated triangular nanoplates and the EDS
analysis showed that element of the P-AgNPs was silver (Figure S2). And
the absorption of TNBD, P-AgNPs, Tris-SH ionophore, and P-
AgNPs+Tris-SH were shown in Fig. 8. Addition of Tris-SH (ionophore)
to P-AgNPs led to red-shifting of the SPR with the new maximum ab-
sorption at approximately 600 nm as shown in Fig. 8. This observation
indicated that the Tris-SH ionophores were indeed in close proximity of
P-AgNPs via sulfhydryl bond similar to the previous studies. [41,50]
Therefore, the plasmonic enhancement that originated from
non-radiative energy transfer from SPR of P-AgNPs to TNBD could be
achieved, which resulted in increasing of fluorescent intensity [42,43].
In addition, the broad LSPR of P-AgNPs, which spanned and covered the
absorption of TNBD ensured that the excitation wavelength at 465 nm
did not only excite TNBD but also excited the SPR of P-AgNPs which
resulted in energy transfer which was non-radiative process from SPR of
P-AgNPS to the fluorescent sensor. The optimum condition for the
highest plasmonic enhancement and fluorescent enhancement (2.8
times) was incorporation of 100 nM (calculated from AgNO3 precursor)
Fig. 10. Fluorescence spectra of TNBD þ P-AgNPs (2.0 μM, 100 nM) with
various other metal ions (49
μ
M) including Cu2+, Ag+, Ni2+, Zn2+, Cd2+, Fe3+
,
Pb2+, Ba2+, Ca2+, Na+, Mg2+, K+, Mn2+ and Co2+
.
fluorescence of TNBD was quenched by complexation with Hg2+ [19].
In addition, colorimetric change of TNBD solution from bright green to
dark orange with UV radiation was observed along with the fluorescent
quenching (inset Fig. 3b)).
3.4. Selectivity and competitive studies of sensor TNBD
The selective binding of TNBD to Hg2+ was tested by monitoring the
change of fluorescence of the sensor after addition of different metal ions
including Hg2+, Cu2+, Ag+, Ni2+, Zn2+, Cd2+, Fe3+, Pb2+, Ba2+, Ca2+
,
Na+, Mg2+, K+, Mn2+ and Co2+ as shown in Fig. 4. Only Hg2+ induced a
fluorescent quenching while the other metal ions did not affect the
fluorescent emission of sensor TNBD. In addition, the competitiveness of
TNBD was determined after addition of Hg2+ (67
μM) and 10 equiva-
lents of other metal cations, which included Cu2+, Ag+, Ni2+, Zn2+
,
Cd2+, Fe3+, Pb2+, Ba2+, Ca2+, Na+, Mg2+, K+, Mn2+ and Co2+as shown
in Fig. 5. The intensity ratios of the final fluorescence of TNBD with
Hg2+ in the presence of competition metal cations (IF) and initial fluo-
rescence (I0) of the sensor at emission wavelength of 556 nm were used
to determine tolerance of the sensor in the presence of the other metal
cations. The fluorescent intensity ratio (IF/I0) in which IF was fluorescent
intensity of TNBD with only Hg2+ addition was utilized as a reference
ratio. The result show that the changes of IF/I0 when the other
competitive cations were also presence, were negligible. This obsevation
indicated that TNBD was highly selective to Hg2+ and it could still serve
as Hg2+ sensor despite the existence of other metal cations. This
observation demonstrated the potential use of TNBD for Hg2+ sensing in
P-AgNPS and 2.0 μM TNBD in 5% aqueous MeOH (Fig. 9 a)). Incorpo-
ration of P-AgNPs led to non-radiative energy transfer from SPR of
plasmonic nanoparticles P-AgNPs to the TNBD, which served as Hg2+
recognition and fluorescent- responsive unit. This plasmonic enhance-
ment led to higher emissive quantum yield and stronger emission of
TNBD þ P-AgNPs, [42,43] the stronger fluorescent emission of TNBD þ
P-AgNPs indicated that smaller concentration of Hg2+ was required to
induce the observable fluorescent change. Therefore, addition of Hg2+
Fig. 11. Colorimetric changes (top) and fluorescent changes under UV irradiation (bottom) of sensor TNBD þ P-AgNPs (10.0
μ
M, 100 nM) in Hg2+‒spiked
drinking water.
7

S. Kraithong et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113064
fluorescent sensor despite the interference from the presence of other
metal cations. The result indicated that plasmonic enhancement and
improving sensitivity of TNBD þ P-AgNPs were achieved without
compromising of its selectivity.
Table 2
Results of Hg2+ concentrations, which were detected by our sensor and ICP-OES
method. a mean ± SD from triplicate analysis.
Sample
Mercury content (mol.Lꢀ 1
)
a
Our sensor
Reference method (ICP-OES)
3.7. Application of TNBD + P-AgNPs for Hg2+ sensing in a real sample
1
2
3
4
5
6
4.23×10ꢀ 6 ± 3.02×10-7
5.31×10ꢀ 6 ± 2.53×10-7
7.69×10ꢀ 6 ± 3.66×10-7
8.82×10ꢀ 6 ± 2.10×10-7
1.29×10ꢀ 5 ± 1.23×10-7
1.68×10ꢀ 5 ± 3.94×10-7
4.71×10ꢀ 6 ± 1.74×10-7
4.96×10ꢀ 6 ± 1.79×10-7
7.12×10ꢀ 6 ± 3.55×10-7
8.18×10ꢀ 6 ± 2.11×10-7
1.37×10ꢀ 6 ± 4.26×10-7
1.78×10ꢀ 6 ± 8.40×10-8
Sensor TNBD þ P-AgNPs exhibited high selectivity toward Hg2+ and
low detection limit in aqueous organic solvent which allowed the sensor
to detect Hg2+ in a real aqueous sample. In this study, drinking water
was chosen as an example of a real sample. As illustrated in Fig. 11, the
color of sensor gradually converted from yellow to red as concentration
of spiked Hg2+ (12.0–75.0
μM) in drinking water increased. The color
to TNBD þ P-AgNPs led to more pronounced change of fluorescent
intensity, which consequently lowered detection limit and led to higher
sensitivity of TNBD þ P-AgNPs compared to TNBD. The sensing
mechanisms of TNBD þ P-AgNPs with plasmonic enhancement and
TNBD were shown in scheme 1 a) and 1 b), respectively.
change was accompanied by the fluorescent quenching which led to
change of emission under ultraviolet light from bright green to dark
orange as shown in Fig. 11. The result indicated that sensing ability of
sensor was not affected by interferences in the sample media. In addi-
tion, Hg2+ detection of the sensor was compared to the result from ICP-
OES method as shown in Table 2. The statistic t-test showed that the
result from our sensor and ICP-OES was in good agreement with the
calculated tstatistic of 0.058 and the tcritical of 2.23 (95 % probability
level). The sensitivity, selectivity, and Hg2+-detection in a real sample
results clearly verified that sensor TNBD þ P-AgNPs is a promising
option for sensitive and selective Hg2+-sensing in both on-site and
environmental aqueous samples.
The fluorescent titration of TNBD þ P-AgNPs with Hg2+ was per-
formed in 5% aqueous MeOH, as shown in Fig. 9 b). It was found that the
fluorescent intensity was progressively decreased with increasing of
Hg2+ concentration. In the presence of Hg2+, sensor TNBD þ P-AgNPs
exhibited fluorescent quenching up to approximately 59 % which was
significantly higher than the fluorescent quenching of the bare TNBD
(44 %). The greater fluorescent quenching after Hg2+ addition of sensor
TNBD þ P-AgNPs indicated that more pronounced change of fluores-
cent intensity could be observed, which consequently lowered detection
limit and led to higher sensitivity of the sensor. The detection limit of
sensor TNBD þ P-AgNPs, which was determined by 3SD/slope, [48,49]
was 1.9 × 10ꢀ 8 M or 3.8 ppb. This detection limit of sensor TNBD þ
P-AgNPs was approximately 3 times better than the detection limit of
the bare sensor TNBD in 5% aqueous MeOH (6.2 × 10ꢀ 8 M or 12.5 ppb).
Importantly, this improved detection limit of sensor TNBD þ P-AgNPs
was adequate to detect the maximum-allowed Hg2+ contamination in
drinking water of WHO (6 ppb). Therefore, sensor TNBD þ P-AgNPs
was applicable for Hg2+ sensing in aqueous samples from an environ-
ment. The comparison of sensitivities and sensing performances of
TNBD and TNBD þ P-AgNPs and recent reported Hg2+ fluorescent
sensors was shown in Table 1.
4. Conclusions
In summary, a new colorimetric and fluorescent sensor TNBD was
successfully designed and synthesized. The sensor showed high selec-
tivity and high sensitivity for Hg2+ determination in aqueous-organic
solvent. Additionally, utilization of TNBD together with P-AgNPs
could improve sensitivity of the sensor through non-radiative energy
transfer and plasmonic enhancement, which was adequate for Hg2+
-
sensing in drinking water as regulated by WHO. Importantly, improving
sensitivity of TNBD þ P-AgNPs were achieved without compromising of
its selectivity. Therefore, the strategy to utilize plasmonic enhancement
could benefit developing of the on-site and portable sensors for Hg2+
-
It could be noted that addition of P-AgNPs to TNBD, which yielded
TNBD þ P-AgNPs, improved sensing performance of the sensor. The
detection limit of TNBD þ P-AgNPs was comparable to the recent re-
ported Hg2+ fluorescent sensors and it could also detect Hg2+ in aqueous
sample. In addition, the sensor could still screen Hg2+ in the aqueous
sample after 1-month storage as shown in Figure S3.
determination. Furthermore, our study confirmed that non-radiative
energy transfer and plasmonic enhancement from the SPR of plas-
monic nanoparticle could also enhance a sensitivity of the turn off-Hg2+
fluorescent sensor.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Furthermore, the enhancing sensitivity of sensor TNBD was achieved
by using AgNPs with lower SPR maximum compared to absorption
maximum of the fluorescent molecule. This finding implied that over-
lapping of SPR of AgNPs and the absorption of sensor is adequate for
non-radiative energy transfer via plasmonic enhancement, which could
open the possibility of mixing and matching of different AgNPs and
fluorescent sensors to achieve plasmonic enhancement in our future
work.
Pattanawit Swanglap would like to thank financial support from
Research Fund for DPST Graduate with First Placement (Grant 018/
2558), The Institute for Promotion of Teaching Science and Technology
(IPST), Thailand. Nirumon Chailek also thanks the Royal Golden Jubilee
(RGJ) Ph.D. Program (grant No. PHD/0191/2561). And All authors
would like to thank support from Department of Chemistry, Faculty of
Science, Silpakorn University.
3.6. Hg2+-Selectivity study of sensor TNBD + P-AgNPs
The Hg2+-selectivity of sensor TNBD þ P-AgNPs was verified by
recording the fluorescence of the sensor after addition of different metal
cations, which included Hg2+, Cu2+, Ag+, Ni2+, Zn2+, Cd2+, Fe3+, Pb2+
,
Appendix A. Supplementary data
Ba2+, Ca2+, Na+, Mg2+, K+, Mn2+ and Co2+. The result showed that the
fluorescent quenching was generated by only Hg2+ whereas any other
metal ions barely affected the fluorescence of TNBD þ P-AgNPs
(Fig. 10). Hence, TNBD þ P-AgNPs could serve as Hg2+-selective
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
113064.
8

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