Fluorescence sensing of Ag+ ions by desulfurization of an acetylthiourea derivative of 2-(2-hydroxyphenyl)benzothiazole

Article abstract of DOI:10.1016/j.dyepig.2017.08.041

A novel Ag⁺-selective reaction-based probe based on an acetylthiourea derivative of 2-(2-hydroxyphenyl)benzothiazole dye was investigated. The designed probe showed pronounced off-on type fluorescence signaling behavior towards Ag⁺ i

Full text of DOI:10.1016/j.dyepig.2017.08.041

Accepted Manuscript
+
Fluorescence sensing of Ag ions by desulfurization of an acetylthiourea derivative of
2-(2-hydroxyphenyl)benzothiazole
Keum Saem Hwang, Ka Young Park, Da Bin Kim, Suk-Kyu Chang
PII:
S0143-7208(17)31424-9
10.1016/j.dyepig.2017.08.041
DYPI 6203
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 June 2017
Revised Date: 23 August 2017
Accepted Date: 23 August 2017
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Please cite this article as: Hwang KS, Park KY, Kim DB, Chang S-K, Fluorescence sensing of Ag
ions by desulfurization of an acetylthiourea derivative of 2-(2-hydroxyphenyl)benzothiazole, Dyes and
Pigments (2017), doi: 10.1016/j.dyepig.2017.08.041.
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Graphical Abstract

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Fluorescence sensing of Ag+ ions by desulfurization of an
acetylthiourea derivative of 2-(2-hydroxyphenyl)benzothiazole
Keum Saem Hwang, Ka Young Park, Da Bin Kim, and Suk-Kyu Chang*
Department of Chemistry, Chung-Ang University, Seoul 06974, Republic of Korea
*Corresponding author. Tel.: +82 2 820 5199; Fax: +82 2 825 4736.
E-mail address: skchang@cau.ac.kr (S.-K. Chang)
Abstract:
A novel Ag+-selective reaction-based probe based on an acetylthiourea derivative of
2-(2-hydroxyphenyl)benzothiazole dye was investigated. The designed probe showed
pronounced off-on type fluorescence signaling behavior in the presence of Ag+ ions
via Ag+-induced desulfurization of acetylthiourea to acetylurea. The signaling was
superior to the analogous thioamide-to-amide conversion in terms of both speed and
signaling contrast. Interference from thiophilic Hg2+ ions was effectively suppressed
using the chelating agent EDTA as a masking agent. The practical application of the
system to the detection of Ag+ in simulated wastewater with a detection limit of 0.76
ꢀM using a smartphone as an easy-to-use data capturing and processing device was
successfully demonstrated.
Keywords: Ag+ sensing; Fluorescence; Desulfurization; Acetylthiourea; 2-(2-
hydroxyphenyl)benzothiazole; Smartphone.
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1. Introduction
Silver and silver-containing compounds are important resources that are used in
the electronics, photographic, imaging, and pharmaceutical industries [1–4]. Ag+
exhibits rich biological chemistry, and acts as a widely used antibacterial agent [5]
and a transcriptional initiator in plants [6] and mammals [7,8]. However, when the
concentration of Ag+ ions becomes sufficiently high, silver-related technologies can
lead to potentially hazardous side effects in the environment [9–12]. It is known that
silver ions can deactivate sulfhydryl enzymes and are also capable of combining with
the imidazole, amine, and carboxyl groups of various metabolites, which can lead to
harmful effects in human beings [3,4,13]. Repeated exposure to Ag+ can produce
anemia, cardiac enlargement, growth retardation, and degenerative changes in
animals [14]. The United States Environmental Protection Agency (USEPA)
recommends a maximum concentration of silver ions in drinking water of 100 ꢀg/L
[15]. Therefore, the development of rapid, selective, and sensitive methods of
detecting Ag+ ions is essential for both environmental protection and human health
[16].
Hitherto, various instrumental techniques such as atomic absorption spectrometry,
inductively coupled plasma mass spectrometry, and electrochemical methods have
been used to detect silver ions; however, all of these require expensive instruments
and/or time-consuming procedures [17–23]. Optical methods such as colorimetry and
fluorescence spectroscopy are more convenient than these traditional techniques due
to their high sensitivities, noninvasive natures, and suitability for real-time detection
[24,25].
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A number of elaborately designed optical sensors and probes have been developed
for the determination or visualization of chemically, biologically, and
environmentally important chemical species [24,26,27]. Recently, reaction-based
probes have attracted much research interest because they provide a number of
advantages that enable the design of selective and efficient signaling systems for the
detection of various important metal ions, anions, and oxidants [28]. Especially,
fluorescence probes that utilize reaction-based transformations for the detection of
intrinsically quenching ions of Hg2+, Cu2+, and Ag+ are particularly attractive
because they afford off-on type signaling responses, which are more desirable than
on-off type sensing systems.
Desulfurization reactions have been one of the most successful strategies of the
many sophisticated approaches to the design of reaction-based probes for thiophilic
metal ions such as Hg2+, Cu2+, and Ag+. Czarnik and co-workers reported the first
fluorescent Hg2+-selective probe based on the sulfur to oxygen exchange reaction of
anthracene-thioamide, which is promoted by Hg2+ and Ag+ ions [29]. After this, a
number of probes using desulfurization processes have been developed for the
thiophilic metal ions. However, when compared with probes for Hg2+ and Cu2+ ions,
Ag+-selective probes have been less widely investigated. Representative examples of
reaction-based probes for the determination of Ag+ ions are developed using
desulfurization of the thioamide group of 9-[(methylamino)thiocarbonyl]anthracene
[29] and the deprotection of 1,3-dithiane-modified BODIPY [30]. Desulfurization of
a coumarin derivative containing an N′-acetylthiourea group [31], deselenation of
rhodamine B selenolactone [32], and irreversible tandem ring-opening and formation
of oxazoline from rhodamine spirolactam are also noteworthy [33].
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In reality, many Ag+-selective sensors and probes frequently suffered from
significant interference from other commonly encountered transition metal ions, such
as Hg2+ and Cu2+ [34]. A valuable tactic for circumventing this interference is the use
of a chelating agent that can discriminate between the target and the interfering ions.
In the design of Ag+-selective sensors, EDTA and thiosulfate have been successfully
used to mask responses from Hg2+ and Cu2+ ions [35]. However, we could not find
any literature on using such chelating agents for the realization of Ag+-selective
reaction-based probes.
In this paper, a new Ag+-selective fluorescence signaling probe based on the Ag+-
induced
desulfurization
of
an
acetylthiourea
derivative
of
2-(2-
hydroxyphenyl)benzothiazole dye is described. The designed probe employs the
acetylthiourea group as a switch to turn-on the Ag+-induced fluorescence revival of
2-(2-hydroxyphenyl)benzothiazole. 2-(2-Hydroxyphenyl)benzazoles are versatile
fluorescent compounds that exhibit unique excited-state intramolecular proton
transfer (ESIPT) phenomena [36]. Moreover, the 4-substituted amine analogue 2-(5-
amino-2-hydroxyphenyl)benzothiazole and its benzoxazole analogue have been
utilized as platforms for the design of ESIPT-type signaling probes for a number of
important species. These are based on the generation of an ESIPT-capable
fluorophore by deprotection of O-protected functional groups on the phenol moiety,
such as deallylation for detection of Pd2+ [37], vinyl ether hydrolysis for detection of
Hg2+ [38], desilylation for detection of fluoride [39,40], and phenylboronate
hydrolysis for detection of H2O2 [41]. The present investigation shows that an
acetylthiourea-based probe exhibited selective signaling behavior towards thiophilic
Ag+ ions via a smooth desulfurization process. Using EDTA as a masking agent for
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interfering Hg2+ ions, we could realize an exclusive Ag+-selective signaling probe
system. To demonstrate a practical application of the system, the determination of
Ag+ in simulated wastewater using a readily available smartphone was conducted.
2. Experimental
General: 5-Aminosalicylic acid, 2-aminothiophenol, 2,4-bis(4-methoxyphenyl)-2,4-
dithioxo-1,3,2,4-dithiadiphosphetane (Lawesson’s reagent), acetyl chloride,
potassium thiocyanate, and all spectroscopic grade solvents were purchased from
Aldrich Chemical Co. Polyphosphoric acid was obtained from Alfa Aesar. Column
chromatography was performed using silica gel (240 mesh). 1H NMR (300 MHz) and
13C NMR (150 MHz) spectra were measured using Varian Gemini 2000 and Varian
VNS spectrometers, and were referenced to the residual solvent signals. UV−vis
spectra were recorded using a Scinco S-3100 spectrophotometer. Fluorescence
spectra were measured using a Scinco FS-2 fluorescence spectrophotometer. High
resolution mass spectra (HRMS) using fast atom bombardment (FAB) and
electrospray ionization (ESI) were recorded on JEOL JMS-700 and Bruker Compact
mass spectrometer, respectively. FT-IR measurements were carried out using a
Thermo Scientific Nicolet 6700 spectrophotometer using KBr pellets.
Preparation of probe 2. A mixture of acetyl chloride (887 mg, 11.3 mmol) and
potassium thiocyanate (2.19 g, 22.5 mmol) in acetonitrile (25 mL) was stirred under
reflux for 1 h. After the solution was cooled to room temperature, 2-(5-amino-2-
hydroxyphenyl)benzothiazole 1 (500 mg, 2.06 mmol) was added, and the reaction
mixture was stirred at room temperature for 1.5 h. The resulting precipitate was
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filtered, washed with acetonitrile several times, and dried to obtain 608 mg (86%) of
l
probe 2 as a grey powder. mp: 240 ºC (decomposed). H NMR (300 MHz, DMSO-d6):
δ 12.36 (s, 1H), 11.73 (s, 1H), 11.49 (s, 1H), 8.42 (d, J = 2.7 Hz, 1H), 8.15 (d, J =
7.8 Hz, 1H), 8.06 (d, J = 8.0 Hz, 1H), 7.56−7.54 (m, 2H), 7.45 (t, J = 7.5 Hz, 1H)
7.15 (d, J = 8.7 Hz, 1H), 2.17 (s, 3H); 13C NMR (150 MHz, DMSO-d6) δ 179.4,
172.8, 163.9, 154.6, 151.5, 134.8, 130.0, 129.2, 126.6, 125.2, 124.5, 122.3, 122.1,
118.6, 117.1, 24.0. IR (KBr): νmax/cm–1 3425 (br, O–H stretch), 3220 (m, N–H
stretch), 3149 (m, N–H stretch), 3060 (w, Ar C–H stretch), 1702 (s, C=O stretch),
1547 (m, N–H bend), 1538 (m, N–H bend), 1547 (s, C=S stretch). HRMS: (ESI+);
calcd for [2+Na]+ (C16H13N3NaO2S2), m/z = 366.0341, observed: 366.0341.
Preparation of probe 4. Acetamide derivative 3 was prepared by the reaction of 1
with acetyl chloride following a literature method [42]. Thioamide 4 was prepared by
the reaction of acetamide 3 with Lawesson’s reagent. A mixture of acetamide 3 (284
mg, 1.0 mmol) and Lawesson’s reagent (459 mg, 1.1 mmol) in toluene (20 mL) was
refluxed for 18 h. The resulting reaction mixture was cooled, evaporated, and
partitioned between dichloromethane and water. The organic phase was separated
and evaporated to obtain probe 4 as a brown powder. Purification was carried out by
column chromatography (CH2Cl2:MeOH, 20:1 v/v). Yield: 52%. mp: 250 ºC
l
(decomposed). H NMR (300 MHz, DMSO-d6): δ 11.63 (s, 2H), 8.63 (d, J = 2.6 Hz,
1H), 8.21–8.12 (m, 1H), 8.06 (d, J = 8.0 Hz, 1H), 7.86 (dd, J = 8.8, 2.7 Hz, 1H),
7.63–7.51 (m, 1H), 7.50–7.40 (m, 1H), 7.10 (d, J = 8.8 Hz, 1H), 2.62 (s, 2H); 13C
NMR (150 MHz, DMSO-d6) δ 198.7, 164.2, 154.0, 151.4, 134.5, 131.9, 127.7, 126.5,
125.1, 123.1, 122.2, 122.1, 118.1, 116.7, 34.8. IR (KBr): νmax/cm–1 3450 (br w, O–H
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stretch), 3164 (m, N–H stretch), 3060 (w, Ar C–H stretch), 2991 (w, Ar C–H stretch),
2939 (w, Ar C–H stretch), 1548 (m, N–H bend), 1498 (m, C–N stretch), 1153 (s,
C=S stretch). HRMS: (FAB+); calcd for [4]+ (C15H13N2OS2), m/z = 301.0400,
observed: 301.0407.
Preparation of stock solutions. Stock solutions of 2 and 4 (5.0 × 10−4 M) were
prepared in spectroscopic grade DMSO. Stock solutions (1.0 × 10−2 M) of metal ions
were prepared in deionized water or simulated wastewater. Simulated wastewater
was prepared in deionized water following the literature [43,44].
Measurement of signaling behavior. The fluorescence signaling behaviors of
probes 2 and 4 toward Ag+ were measured in mixed solutions of acetate buffer (pH
4.0) and DMSO (1:1, v/v). Solutions were prepared for measurement by adding the
probe stock solution (30 ꢀL, 5.0 × 10−4 M), acetate buffer (150 ꢀL, 2.0 × 10−1 M),
EDTA (15 ꢀL, 1.0 × 10−1 M), and metal ion stock solutions (15 ꢀL, 1.0 × 10−2 M) to
a 10 mL vial. The solutions were diluted with distilled water and DMSO to give a
final volume of 3.0 mL (buffer solution : DMSO = 1:1, v/v for 2 and 1:9, v/v for 4).
The final concentrations of the probe, acetate buffer, EDTA, and metal ions in the
measuring solutions were 5.0 × 10−6 M, 1.0 × 10−2 M, 5.0 × 10−4 M, and 5.0 × 10−5 M,
respectively.
Detection limit of Ag+ ions. Following IUPAC recommendation, the detection limit
of Ag+ was assessed by the equation 3sbl/m, where sbl is the standard deviation of the
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blank measurements (number of measurements = 15) and m is the calibration
sensitivity obtained from the slope of the calibration plot.
Smartphone-assisted detection of Ag+ ions in simulated wastewater. Images of the
signaling solutions prepared from standard solutions with varying Ag+ concentrations
were captured using a smartphone in a dark room and used to plot the corresponding
calibration curve. Calculated amounts of simulated wastewater stock solutions
containing Ag+, acetate buffer, EDTA, and probe 2 were sequentially added to a vial, and
the resulting solutions were diluted to 3.0 mL using simulated wastewater and DMSO.
The final concentrations of 2, EDTA, and Ag+ in the test solutions were 5.0 × 10−6 M,
5.0 × 10−4 M, and 0−5.0 × 10−6 M, respectively, in a mixture of acetate buffered (pH 4.0,
final concentration = 1.0 × 10−2 M) simulated wastewater and DMSO (1:1, v/v). Images
of the solutions under a UV light (365 nm) were obtained in a dark room using a
smartphone (Samsung Electronics Co., Ltd., Galaxy S7). The green channel levels of the
resulting images were estimated using a smartphone-embedded RGB Grabber
application. Using a plot of this value vs. [Ag+] as a working calibration curve, the
concentration of Ag+ in simulated wastewater was estimated.
3. Results and discussion
The designed acetylthiourea-based probe 2 was prepared by the reaction of
benzothiazolyl-aminophenol 1 with acetyl isothiocyanate, as outlined in Scheme 1.
Benzothiazolyl-aminophenol 1 was prepared by the reaction of 2-aminothiophenol with
5-aminosalicylic acid in polyphosphoric acid following a reported procedure (79% yield)
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[37]. The reaction of 1 with acetyl isothiocyanate, which was prepared by the treatment
of potassium thiocyanate with acetyl chloride, in acetonitrile afforded desired
acetylthiourea 2 in a good yield (86%). Thioamide 4 was prepared as a control
compound by the thionation of acetamide 3, which was obtained by the reaction of 1
with acetyl chloride (DMF, 91% yield), with Lawesson’s reagent (toluene, 52% yield).
Scheme 1. Preparation of Ag+-selective acetylthiourea (2) and thioamide (4) probes.
The signaling behaviors of probes 2 and 4 towards common metal ions were
investigated by UV−vis and fluorescence spectroscopy. Acetylthiourea 2 showed weak
emission at 415 nm and broad emission at 490 nm in an acetate buffer (pH 4.0) solution
containing 50% DMSO. Upon treatment with metal ions, prominent fluorescence
responses to Hg2+ and Ag+ ions were observed (Figure S1, Supplementary data). That is
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due to the metal-induced desulfurization of acetylthiourea 2 to produce acetylurea 5
(vide infra). In contrast to this, no considerable changes in the UV−vis spectrum of 2
were observed on exposure to the surveyed metal ions (Figure S2, Supplementary data).
For most analytical purposes, selectivity for a single target metal ion is far more
desirable than multiple responses from closely related metal ions. We attempted to
improve the metal ion selectivity of 2 by fine-tuning the signaling conditions by altering
the solvent composition and pH, as well as the use of extra masking agents that could
interact differently with Hg2+ and Ag+ ions. After a systematic survey, we found that
pronounced peak selectivity for Ag+ ions could be realized by using EDTA as a masking
agent. Interference from Hg2+ ions was effectively suppressed by complex formation,
allowing exclusive selectivity of 2 towards Ag+ ions to be obtained. For instance, in an
acetate-buffered solution containing EDTA (10 equiv) and DMSO (1:1, v/v), probe 2
showed marked fluorescence enhancement only with Ag+ ions (Figure 1 and Figure S3,
Supplementary data). The color of the solution under illumination with a UV lamp (λex =
365 nm) changed from a faint dark blue to a bright greenish blue on exposure to Ag+,
and the fluorescence intensity enhancement of 2 at 490 nm (I/I0) was 53-fold. Other
metal ions showed negligible responses (I/I0 = 0.83−1.32).
It is worth noting that we initially aimed to develop the analogous thioamide 4 as an
Ag+-selective probe. However, thioamide 4 showed less pronounced Ag+-selectivity
(Figure S4, Supplementary data) as well as much slower signaling behavior than
acetylthiourea 2 (Figure S5, Supplementary data). The large increase in signaling speed
of 2 as compared with thioamide 4 is due to the formation of intramolecular hydrogen
bonding between the acetylthiourea moiety, and is in good agreement with the results
obtained by Tsukamoto et al., for a 7-hydroxycoumarin-based acetylthiourea compound
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[30].
The highly Ag+-selective signaling of 2 in the present system can be rationalized by
referring to the formation constants ꢀlog ꢁ ꢃ of the EDTA complexes of Ag+ and Hg2+,
ꢂ
which are 7.32 and 21.7, respectively [45]. However, below pH 10.24, EDTA (H4Y) does
not fully dissociate into its complex formation form, Y4-, and therefore the conditional
formation constant ꢀꢁꢄ = ꢅꢆ ꢁ ꢃ should be considered, where ꢅ
denotes the
ꢇꢈ
ꢇꢈ
ꢂ
ꢂ
ꢆ
fraction of EDTA in its fully deprotonated form (Y4−). At pH 4.0, ꢅꢆ for EDTA is 3.8
ꢇꢈ
× 10−9 [45]. Therefore, the conditional formation constants ꢀlog ꢁ ′ꢃ of Ag+ and Hg2+ at
ꢂ
pH 4.0 are −1.10 and 13.3, respectively. The value of the Ag+-EDTA complex at pH 4.0
(log ꢁ ′ = −1.10) enables us to postulate that the Ag+ ions weakly interact with EDTA,
ꢂ
thereby a reaction between Ag+ and acetylthiourea should be possible. On the other hand,
Hg2+ is tightly complexed to EDTA at pH 4 (log ꢁ ′ = 13.3), and so cannot participate
ꢂ
in the desulfurization signaling process. Meanwhile, the selective sensing of Ag+ ions
using probe 2 in the presence of competing Hg2+ ions is believed to be infeasible under
physiological conditions, because the conditional formation constants ꢀlog ꢁ ′ꢃ of
ꢂ
both Ag+ and Hg2+ ions at physiologically relevant pH 7.4 are high (4.33 and 18.7,
respectively).
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20
16
12
8
20
+ Ag+
2
16
12
8
only
2
2
+ other metal ions
4
0
380
480
580
680
4
Wavelength (nm)
0
Figure 1. Ag+-selective signaling of 2 in the presence of a masking agent (EDTA). Inset:
changes in fluorescence spectrum. [2] = 5.0 × 10−6 M, [Mn+] = 5.0 × 10−5 M, and [EDTA]
= 5.0 × 10−4 M in a mixture of an acetate buffer (pH 4.0, final concentration = 10 mM)
and DMSO (1:1, v/v). λex = 366 nm.
Sensing of Ag+ is ascribable to the conversion of acetylthiourea 2 to acetylurea 5
(Scheme 2). The fluorescence of the 2-(2-hydroxyphenyl)benzothiazole framework,
which is suppressed in acetylthiourea due to the presence of the thiocarbonyl group, is
recovered after the conversion to acetylurea. The postulated conversion was confirmed
1
1
by H NMR and mass spectroscopy. Upon treatment of 2 with Ag+ ions, the H NMR
spectrum changed to that of acetylurea 5 (Figure 2). Particularly, the resonance of
acetylthiourea NH protons at 11.49 and 12.36 ppm were converted to acetylurea NH
protons at 10.46 and 10.65 ppm, respectively. In addition, in the mass measurement, an
intense molecular ion peak for 5 was observed at m/z = 350.0571 (calcd for
C16H13N3NaO3S = 350.0570) after conversion (Figure S6, Supplementary data).
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Scheme 2. Ag+ signaling by desulfurization of 2.
Figure 2. Partial 1H NMR spectra of 2 and 2 in the presence of Ag+ (2 + Ag+). [2] = 5.0
× 10−3 M and [AgClO4] = 1.0 × 10−2 M in DMSO-d6. The spectrum of (2 + Ag+) was
obtained after purification of the signaling product by passing it through a silica plug.
Competitive detection of a specific target analyte over other commonly coexisting
species is important for practical applications of probes. The Ag+-selective signaling of
probe 2 was not significantly affected by a background of other common metal ions
(Figure 3). The interference as expressed by the change in fluorescence intensity ratio at
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490 nm (I(Metal ion+Ag(I))/IAg(I)) varied narrowly between 0.96 for Li+ and 1.02 for Zn2+ ions.
This observation indicates that probe 2 could be used for the selective detection of Ag+
in analytes that are commonly encountered in chemical and environmental sciences.
20
+ Ag+
2
16
+ other metal ions + Ag+
2
12
8
only
2
4
0
380
430
480
530
580
630
680
Wavelength (nm)
1
0.8
0.6
0.4
0.2
0
+ Ag+
Figure 3. Changes in fluorescence spectrum of 2 when detecting Ag+ in the presence of
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various metal ions as background (upper). Fluorescence intensity ratio (I(Metal
ion+Ag(I))/IAg(I)) of 2 at 490 nm (lower). [Ag+] = [Mn+] = 5.0 × 10−5 M. Other conditions are
the same as those in Figure 1.
The pH-dependent signaling profile of probe 2 shows that most pronounced signaling
contrast between the two pertinent metal ions (Ag+ and Hg2+), as expressed by the
fluorescence intensity ratio (IAg(I)/IHg(II)) at 490 nm, was observed at pH 4.0 (Figure 4).
25
20
15
10
5
0
3
4
5
6
7
8
9
pH
Figure 4. Effect of pH on the selective fluorescence signaling of Ag+ by 2 as expressed
by the fluorescence intensity ratio IAg(I)/IHg(II) at 490 nm. [2] = 5.0 × 10−6 M, [Ag+] =
[Hg2+] = 5.0 × 10−5 M, and [EDTA] = 5.0 × 10−4 M in a mixture of an acetate buffer (pH
4–6, final concentration = 10 mM) or a phosphate buffer (pH 7–8, final concentration =
10 mM) and DMSO (1:1, v/v). λex = 366 nm.
Detection of Ag+ by 2 in the presence of EDTA was relatively fast, and a saturated
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signaling was observed after less than 1 min (Figure 5). In addition, probe 2 itself was
stable under the measurement conditions, and no measurable changes were observed 5 h
after sample preparation.
20
16
+ Ag+
2
12
8
only
2
4
0
0
5
10
15
Time (min)
20
25
30
Figure 5. Change in fluorescence intensity of Ag+ signaling by 2 at 490 nm over time. [2]
= 5.0 × 10−6 M, [Ag+] = 5.0 × 10−5 M. Other conditions are the same as those in Figure 1.
The quantitative signaling properties of 2 were assessed by fluorescence titration with
Ag+ ions. As [Ag+] increased, the fluorescence intensity of 2 at 490 nm increased
steadily (Figure 6). A plot of the change in fluorescence intensity as a function of [Ag+]
yielded a useful calibration plot for the determination of Ag+ ions up to 8.5 × 10−6 M.
The minor sigmoidal-type plot observed in the low concentration region ([Ag+] < 1 µM)
could be due to the involvement of Ag+ ions in additional complex formation equilibria
with the chelating agent EDTA. The detection limit for Ag+ ions was found to be 0.075
µM, as estimated using the IUPAC recommended procedure (3sb1/m), where sb1 and m
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denote the standard deviation of the blank and the slope of the calibration plot,
respectively [46].
20
20
15
16
10
5
12
[Ag+]
0
0
3
6
9
12
15
8
4
0
[Ag+] (µM)
380
480
580
680
Wavelength (nm)
Figure 6. Changes in the fluorescence spectrum of 2 as a function of [Ag+]. Inset:
changes in fluorescence intensity at 490 nm. [2] = 5.0 × 10−6 M, [Ag+] = 0−1.5 × 10−5 M.
Other conditions are the same as those in Figure 1.
Finally, a practical application of the probe to the detection of Ag+ in simulated
wastewater samples was demonstrated. Solutions containing varying amounts of Ag+
ions in simulated wastewater were prepared, and the fluorescence intensities of 2 in
these solutions were measured at 490 nm. Simulated wastewater was prepared following
literature examples [43,44], and the composition is shown in Table S1, Supplementary
data. The obtained calibration plots showed there was a linear relationship between
fluorescence intensity and [Ag+] that held up to 5 ꢀM in simulated wastewater samples
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(Figure 7). In parallel to this, images of the signaling solutions were captured using a
smartphone (Samsung, Galaxy S7). The change in color was analyzed using a
smartphone-embedded app (RGB Grabber). The plot between the green channel level
and [Ag+] revealed a useful correlation (Figure 8). From the concentration-dependent
change in green channel level, a detection limit of 0.76 ꢀM was estimated for Ag+
determination in wastewater samples using a smartphone as a stand-alone signal
capturing and processing device.
20
16
12
8
4
0
0
1
2
3
4
5
[Ag+] (µM)
Figure 7. Fluorescence signaling of Ag+ by 2 in simulated wastewater. [2] = 5.0 × 10−6
M, [Ag+] = 0−5.0 × 10−6 M. Other conditions are the same as those in Figure 1.
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Figure 8. Changes in green channel level as a function of [Ag+] in simulated
wastewater containing 2 obtained using a smartphone. Inset: fluorescence images
under illumination with a UV lamp (365 nm), which were analyzed using an app
(RGB Grabber). [2] = 5.0 × 10−6 M, [Ag+] = 0−5.0 × 10−6 M, and [EDTA] = 5.0 ×
10−4 M in a mixture of an acetate buffered solution (pH 4.0, final concentration = 10
mM) and DMSO (1:1, v/v).
4. Conclusions
We have developed a novel Ag+-selective reaction-based probe utilizing the Ag+-
assisted
acetylthiourea-acetylurea
transformation
of
2-(2-
hydroxyphenyl)benzothiazole-based dye. The acetylthiourea moiety satisfactorily
functioned as a selective Ag+-triggered switching-on handle for the recovery of 2-
(2-hydroxyphenyl)benzothiazole fluorescence. The designed probe showed
pronounced off-on type fluorescence signaling behavior in the presence of Ag+ ions.
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Interference from Hg2+ ions was readily eliminated using a chelating agent, EDTA,
as an auxiliary masking agent. The practical application of the system to the
detection of Ag+ in simulated wastewater was successfully demonstrated using a
smartphone as a stand-alone data capturing and processing device suitable for field
applications.
Acknowledgements
This study was supported by Chung-Ang University Research Scholarship Grants in
2017 (G.Y.P.) and a fund from the National Research Foundation of Korea (NRF)
funded by the Korean Government (NRF-2015R1D1A1A01061470).
Appendix. Supplementary data
Supplementary data associated with this article can be found in the online version at
doi:**.****.
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Figure Captions
Scheme 1. Preparation of Ag+-selective acetylthiourea (2) and thioamide (4) probes.
Scheme 2. Ag+ signaling by desulfurization of 2.
Figure 1. Ag+-selective signaling of 2 in the presence of a masking agent (EDTA).
Inset: changes in fluorescence spectrum. [2] = 5.0 × 10−6 M, [Mn+] = 5.0 × 10−5 M,
and [EDTA] = 5.0 × 10−4 M in a mixture of an acetate buffer (pH 4.0, final
concentration = 10 mM) and DMSO (1:1, v/v). λex = 366 nm.
1
Figure 2. Partial H NMR spectra of 2 and 2 in the presence of Ag+ (2 + Ag+). [2] =
5.0 × 10−3 M, [AgClO4] = 1.0 × 10−2 M in DMSO-d6. The spectrum of (2 + Ag+) was
obtained after purification of the signaling product by passing it through a silica plug.
Figure 3. Changes in fluorescence spectrum of 2 when detecting Ag+ in the presence
of various metal ions as background (upper). Fluorescence intensity ratio (I(Metal
ion+Ag(I))/IAg(I)) of 2 at 490 nm (lower). [Ag+] = [Mn+] = 5.0 × 10−5 M. Other conditions
are the same as those in Figure 1.
Figure 4. Effect of pH on the selective fluorescence signaling of Ag+ by 2 expressed
by IAg(I)/IHg(II). [2] = 5.0 × 10−6 M, [Ag+] = [Hg2+] = 5.0 × 10−5 M, and [EDTA] = 5.0
× 10−4 M in a mixture of an acetate buffer (pH 4–6, final concentration = 10 mM) or
a phosphate buffer (pH 7–8, final concentration = 10 mM) and DMSO (1:1, v/v). λex
= 366 nm.
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Figure 5. Change in fluorescence intensity of Ag+ signaling by 2 at 490 nm over
time. [Ag+] = 5.0 × 10−5 M. Other conditions are the same as those in Figure 1.
Figure 6. Changes in the fluorescence spectrum of 2 as a function of [Ag+]. Inset:
changes in fluorescence intensity at 490 nm. [Ag+] = 0−1.5 × 10−5 M. Other
conditions are the same as those in Figure 1.
Figure 7. Fluorescence signaling of Ag+ by 2 in distilled water and simulated wastewater.
[Ag+] = 0−5.0 × 10−6 M. Other conditions are the same as those in Figure 1.
Figure 8. Changes in green channel level as a function of [Ag+] in simulated wastewater
containing 2 obtained using a smartphone. Inset: fluorescence images under illumination
with a UV lamp (365 nm), which were analyzed using an app (RGB Grabber). [2] = 5.0
× 10−6 M, [Ag+] = 0−5.0 × 10−6 M, and [EDTA] = 5.0 × 10−4 M in a mixture of an
acetate buffered solution (pH 4.0, final concentration = 10 mM) and DMSO (1:1, v/v).
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Highlights:
• A new reaction-based Ag+-selective fluorescence signaling probe has been explored.
• Signaling was based on Ag+-induced desulfurization of acetylthiourea to acetylurea.
• Interference from Hg2+ was effectively suppressed using EDTA as a masking agent.
• The probe could easily determine Ag+ level in simulated wastewater using a
smartphone.