Fluorometric determination of Au3+ via hydrolysis of thiocarbonate with bromide as a masking agent

Article abstract of DOI:10.1016/j.dyepig.2019.01.001

Gold ions are very important in chemistry and materials science owing to their various uses in chemical reactions as well as industrial and biological applications. In this study, we developed a novel reaction-based probe (GP), which is a thiocarbonate derivative of the 2-(2-hydroxyphenyl)benzothiazole dye, for the selective and sensitive determination of Au³⁺. The probe GP showed turn-on fluorescence signaling behavior toward Au³⁺ ions triggered by the metal-induced hydrolysis of the thiocarbonate group. Interference from other thiophilic ions, Ag⁺ and Hg²⁺, was effectively eliminated by using bromide ions as a masking agent. Prominent off–on type fluorescent sensing of Au³⁺ over other coexisting metal ions was possible with a detection limit of 4.8 × 10^?8 M (9.5 ppb). Fluorescence sensing of Au³⁺ by GP was fast and applicable over a wide pH range between 4.0 and 10.5. Application of the probe to the analysis of Au³⁺ in synthetic wastewater was conducted using an easily accessible smartphone.

Full text of DOI:10.1016/j.dyepig.2019.01.001

Accepted Manuscript
Fluorometric determination of Au3+ via hydrolysis of thiocarbonate with bromide as a
masking agent
So Young Park, Myung Gil Choi, Dae Sung Lim, Suk-Kyu Chang
PII:
S0143-7208(18)32321-0
https://doi.org/10.1016/j.dyepig.2019.01.001
DYPI 7274
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 October 2018
Revised Date: 2 January 2019
Accepted Date: 3 January 2019
Please cite this article as: Park SY, Choi MG, Lim DS, Chang S-K, Fluorometric determination of Au3+
via hydrolysis of thiocarbonate with bromide as a masking agent, Dyes and Pigments (2019), doi: https://
doi.org/10.1016/j.dyepig.2019.01.001.
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Graphical Abstract

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3
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Fluorometric determination of Au via hydrolysis of
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thiocarbonate with bromide as a masking agent
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So Young Park, Myung Gil Choi, Dae Sung Lim, 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)
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Abstract:
Gold ions are very important in chemistry and materials sciences owing to their
various uses in chemical reactions as well as industrial and biological applications. In
this study, we developed a novel reaction-based probe (GP), which is a thiocarbonate
derivative of the 2-(2-hydroxyphenyl)benzothiazole dye, for the selective and
3
+
sensitive determination of Au . The probe GP showed turn-on fluorescence
3
+
signaling behavior toward Au ions triggered by the metal-induced hydrolysis of the
+
2+
thiocarbonate group. Interference from other thiophilic ions, Ag and Hg , was
effectively eliminated by using bromide ions as a masking agent. Prominent off–on
1

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3
+
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type fluorescent sensing of Au over other coexisting metal ions was possible with a
–
8
3+
detection limit of 4.8 × 10 M (9.5 ppb). Fluorescence sensing of Au by GP was
fast and applicable over a wide pH range between 4.0 and 10.5. Application of the
3
+
probe to the analysis of Au in synthetic wastewater was conducted using an easily
accessible smartphone.
3
+
Keywords: Au
sensing; 2-(2-Hydroxyphenyl)benzothiazole; Thiocarbonate
hydrolysis; Fluorescent signaling; Masking agent; Smartphone.
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1. Introduction
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Gold is an essential metal species in various industrial applications owing to its
unique properties such as high malleability, ductility, electrical conductivity, and
resistance to corrosion and most other chemical reactions [1]. In addition, gold
possesses interesting biological characteristics such as anti-inflammatory properties
and biocompatibility, and is used in pharmaceutical preparations for the treatment of
diseases including arthritis, tuberculosis, and cancer [2,3]. Despite its diverse
biological and chemical properties, gold ions can cause cell toxicity in living
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organisms [3] by binding tightly to biomolecules such as enzymes and DNA [4].
Moreover, if they accumulate in organisms at certain concentrations [3], gold ions
can cause damage to the liver, the kidneys, and the peripheral nervous system [5].
3
+
Therefore, the development of simple and convenient methods to detect Au ions
has attracted continuous research efforts.
For the analysis of gold ions, analytical methods involving complex instruments,
such as inductively coupled plasma optical emission spectroscopy (ICP-OES), ICP
atomic emission spectroscopy, ICP mass spectroscopy, and anodic stripping
voltammetry, have been routinely used [6]. However, compared to these instrument-
based techniques, the optical methods based on colorimetry and fluorometry are
advantageous owing to their sensitivity, easily obtainable data, and simple
experimental procedures. Thus, the development of fluorescent or colorimetric
sensors and probes for gold and other precious metal ions, such as silver and
platinum, has attracted intense research interest [7]. In particular, for optical
3
+
detection of Au , many sophisticated probes have been reported that operate via
selective complex formation or reaction-based mechanisms taking advantage of the
3
+
unique alkynophilic behavior and reactivity of Au ions toward a variety of
functional group transformations (Table S1, Supplementary data). For instance,
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signaling systems using Au -promoted cyclization reactions such as transformation
of propargylamide to oxazolecarbaldehyde [8] and thioamide-substituted alkyne to 5-
ketothiazole [9], intramolecular hydroamination of 2’-ethynyl biphenylamine to the
phenanthridine moiety [10], and endo-dig or exo-dig cyclization of alkynes [11]. In
3
+
addition, Au -catalyzed hydroarylation [12], oxidation of propargylimide to ketone
[13], and irreversible hydrolysis of the hydrazone C=N bond [14–17] and the
propargyl C≡C bond [18] have been successfully utilized. Moreover, probes based
on nano-structured platforms, such as graphene quantum dots [19,20], fluorescent
graphene oxide-poly(vinyl alcohol) hybrid [21], and dansyl-hydrazide-tagged silver
nanoparticles, have been successfully developed [22].
Thiocarbonates have been used in organic synthesis for the deoxygenation of
alcohols [23] and conversion of vicinal diols to alkenes [24] and as dehydrating
agents for the condensation of carboxylic acids and amines [25,26]. In polymer
chemistry, thiocarbonates have been employed as monomers for the preparation of
sulfur-containing polymers such as polythioethers and polythioesters by ring-opening
polymerization [27]. However, this useful functional group has rarely been used for
the construction of reaction-based probes [28].
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The design of a probe that is selective for a single analyte is a challenging task
because many sensors or probes also give undesirable responses induced by closely
related interfering species. To circumvent this problem, a number of masking agents
and additives have been ingeniously employed for the effective sequestering of the
interfering
species
[29].
For
instance,
efficient
chelating
agent
ethylenediaminetetraacetic acid (EDTA) is used as a masking agent and the selective
+
2+
2+
2+
sensing of Ag over Hg and Hg over Cu has been accomplished with enhanced
selectivity [30–32]. On the other hand, by utilizing another valuable masking agent
2
+
of iodide ions, Hg interference was readily removed through the formation of stable
3
+
iodide complexes [33]. Meanwhile, the most prominent drawback of many Au
+
sensing systems is cross-affinity with other alkynophilic metal species such as Ag ,
2
+
2+
2+
Cu , Hg , and Pd . In particular, because of their similar coordination properties, it
is especially difficult to construct a molecular sensor or probe that can differentiate
3
+
2+
between Au and Hg species [13,34,35]. We attempted to overcome this problem
by using bromide ions as a masking agent for the notable interfering ionic species of
2
+
+
Hg and Ag . Actually, bromide ions are not very widely used as a masking agent
2
+
although they form stable complexes with a number of metal ions, such as Hg (pK
sp
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= 18.9) and Ag (pK = 12.30) [36]. Some examples have been reported on the use
sp
3
+
2+
of bromide ions for the masking of Tl [37] and Hg [38–40].
3
+
In this study, we report an easily prepared Au -selective fluorescent probe that
operates via the hydrolysis of the thiocarbonate of the 2-(2-
3
+
hydroxyphenyl)benzothiazole dye. The Au -assisted deprotection of the
thiocarbonate-protecting group resulted in the recovery of the strong fluorescence of
the parent dye. The interference from other thiophilic metal ions was circumvented
by using bromide ions as a masking agent. The application of this probe to the
3
+
analysis of Au ions in synthetic wastewater was demonstrated using a smartphone
as a convenient field-oriented stand-alone tool.
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2. Materials and Methods
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2.1 General.
O-Phenyl chlorothionoformate, triethylamine, and 2-(2-hydroxyphenyl)benzothiazole
were purchased from Aldrich Chemical Co. All other chemicals and spectroscopic
1
13
grade solvents were obtained from commercial sources. H (600 MHz) and C (150
MHz) NMR spectra were measured on a Varian VNS NMR spectrometer. UV–vis
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spectra were acquired using a Scinco S-3100 spectrophotometer equipped with a
Peltier temperature controller. Fluorescence spectra were obtained using a Scinco
FluoroMate FS-2 spectrofluorometer. Mass spectra were obtained on a Micromass
AutoSpec mass spectrometer. Syringe filtration was achieved using a Millex syringe
filter unit (polyvinylidene fluoride membrane, 0.45 µm). Column chromatography
was performed using silica gel (Merck, 240 mesh).
2.2 Preparation of thiocarbonate-based probe GP.
O-Phenyl chlorothionoformate (0.15 mL, 1.1 mmol) was added to a mixture of 2-(2-
hydroxyphenyl)benzothiazole (0.23 g, 1.0 mmol) and triethylamine (0.204 mL, 1.5
mmol) in dichloromethane (30 mL). The mixture was stirred for 1 h at room
temperature. After confirmation of the completion of the reaction by TLC monitoring,
the reaction mixture was washed with distilled water several times and the organic
layers were collected. The volatiles were removed under reduced pressure and the
residue was purified by column chromatography (silica gel, eluent: ethyl
acetate:hexane = 1:3, v/v) to yield GP (0.15 g, 43%). White powder. mp: 231.7 °C.
1
H NMR (600 MHz, CDCl ) δ 8.36 (d, J = 7.6 Hz, 1H), 8.14 (d, J = 8.1 Hz, 1H),
3
7.95 (d, J = 8.0 Hz, 1H), 7.56–7.49 (m, 2H), 7.45 (t, J = 7.6 Hz, 1H), 7.43–7.38 (m,
1
3
3H), 7.35 (d, J = 8.1 Hz, 1H), 7.28 (t, J = 7.4 Hz, 1H), 7.20 (d, J = 8.2 Hz, 2H). C
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NMR (150 MHz, CDCl ) δ 193.8, 162.1, 153.7, 153.1, 150.7, 135.7, 131.6, 130.6,
3
129.7, 127.4, 126.9, 126.46, 126.50, 125.6, 123.8, 123.6, 121.8, 121.5; HRMS (FAB),
+
+
found: [M+H] 364.0461, molecular formula C H NO S requires [M+H] :
2
0
14
2 2
364.0460.
2.3 Preparation of stock solutions.
−
4
A stock solution of probe GP (5.0 × 10 M) was prepared using spectroscopic grade
+
+
+
2+
2+
2+
3+
2+
DMSO. Stock solutions of metal ions (Li , Na , K , Mg , Ca , Ba , Al , Mn ,
3
+
2+
2+
2+
2+
+
2+
2+
2+
3+
Fe , Co , Ni , Cu , Zn , Ag , Cd , Hg , and Pb as perchlorate salts and Au
−
2
as a chloride salt, 1.0 × 10 M) were prepared in distilled water. Synthetic
wastewater was formulated according to a reference procedure [41] (Table S2,
Supplementary data).
2.4 Measurement of signaling behavior.
3
+
The fluorescent signaling behavior of probe GP toward Au was measured in a
mixture of acetate buffer (pH 4.76) and DMSO (1:1, v/v). The measurement solutions
were prepared by adding buffer (300 µL, 0.1 M), metal stock solution (15 µL, 1.0 ×
−
2
−4
10 M), and the probe (60 µL, 5.0 × 10 M) to a 10 mL vial. The solution was
diluted using distilled water and DMSO to give a final volume of 3.0 mL and a
composition of 50% aqueous DMSO solution (H O:DMSO = 1:1, v/v). The final
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concentrations of the probe, acetate buffer, and metal ions in the solution were 1.0 ×
−
5
−2
−5
10 M, 1.0 × 10 M, and 5.0 × 10 M, respectively. All measurements were
performed 30 min after sample preparation.
3
+
2.5 Preparation of Au signaling product 1.
Probe GP (34 mg, 0.09 mmol) was dissolved in 10 mL of methanol, and gold
trichloride (61 mg, 0.18 mmol) in 0.1 mL of distilled water was added to the solution.
The generated gold sulfide and impurities were removed by chromatography on a
short silica column. Signaling product 1 was obtained after evaporation of the
1
remaining organic solvent (13 mg, 57%). H NMR (600 MHz, CDCl ) δ 12.50 (s,
3
1H), 7.98 (d, J = 8.2 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.69 (dt, J = 7.8, 1.5 Hz, 1H),
7.50 (tt, J = 8.4, 1.5 Hz, 1H), 7.42–7.36 (m, 2H), 7.10 (dt, J = 8.2, 1.3 Hz, 1H), 6.97–
1
3
6.93 (m, 1H). C NMR (150 MHz, CDCl ) δ 169.3, 157.9, 151.8, 132.7, 132.6,
3
128.4, 126.6, 125.5, 122.1, 121.5, 119.5, 117.8, 116.8. MS (EI), found: 227.0,
+
molecular formula C H NOS [1] requires: 227.0.
1
3
9
3
+
2.6 Determination of the detection limit for Au .
3
+
As per IUPAC recommendations, the detection limit of Au was calculated using the
expression 3s /m, where s is the standard deviation of the blank measurements
bl
bl
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(number of measurements = 10) and m is the calibration sensitivity estimated from
the slope of the calibration plot [42].
3
+
2.7 Determination of Au ions using a smartphone.
3
+
Determination of the Au concentration in synthetic wastewater using a smartphone
as a stand-alone signal measurement and processing tool was performed following
slightly modified literature procedure [43].
2.7.1 Preparation of sample solutions.
All measurements were carried out using the optimized conditions of a 1:1 (v/v)
mixture of acetate-buffered synthetic wastewater (pH 4.76) and DMSO. Sample
3
+
solutions were prepared by adding buffer (300 µL, 0.10 M), Au stock solution (30
−
3
−4
µL, 1.0 × 10 M), and the probe (60 µL, 5.0 × 10 M) to a 10 mL vial. The solution
was diluted using synthetic wastewater and DMSO to give a final volume of 3.0 mL
and a composition of 50% aqueous DMSO solution. The final concentrations of the
3
+
−5
−2
probe, acetate buffer, and Au ions in the solution were 1.0 × 10 M, 1.0 × 10 M,
−
5
and 1.0 × 10 M, respectively.
3
+
2.7.2 Determination of Au ions using a smartphone.
First, to prevent interference from environmental light, a cube-shaped box with a
square hole in the lid, for the illumination of the samples by a hand-held UV lamp
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(VILBER, VL-4LC), was prepared. The samples were kept in the box, and images of
the solution under UV illumination were taken using a smartphone (Galaxy S7,
Samsung Electronics) without any special settings. The red, green, and blue channel
levels of the acquired fluorescence images were analyzed using a smartphone
application (RGB Grabber, Shunamicode). The calibration curve was obtained by
plotting the changes in the blue channel value, which showed the most variation
3
+
among the three, as a function of [Au ] using a smartphone-embedded spreadsheet
program (Excel, Microsoft Corporation).
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3. Results and discussion
3
+
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The designed Au -signaling probe GP was prepared by the simple reaction of 2-(2-
hydroxyphenyl)benzothiazole with phenyl thionochloroformate (triethylamine,
dichloromethane, 43% yield) (Scheme 1). The optimized signaling conditions of
3
+
acetate-buffered DMSO solution for Au sensing by GP were identified by varying
the pH and composition of the solution. Using these optimized conditions, screening
experiments for the metal selectivity of GP were performed using fluorescence
measurements, because the fluorescence responses of the probe were much better
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than its UV–vis behaviors (Fig. S1, Supplementary data). Probe GP showed very
weak emission above 400 nm when compared to its parent dye due to the blocked
phenolic OH moiety. Preliminary experiments revealed that GP exhibited prominent
2
+
+
fluorescence enhancements in the presence of the thiophilic metal ions Hg and Ag ,
3
+
as well as Au .
S
S
O
HO
O
Cl
O
S
N
S
N
TEA / DCM, rt
2
-(2-hydroxyphenyl)benzothiazole
GP
1
99
00
3
+
2
Scheme 1. Preparation of the Au -selective signaling probe GP.
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To obtain an optimum selectivity toward a single target ion, we tested several
masking agents: EDTA, N,N,N’,N’-tetrakis(2-pyridylmethyl)ethanediamine (TPEN),
and halide ions (bromide and iodide) (Fig. S2, Supplementary data). Nearly perfect
3
+
Au -selective signaling behavior was obtained when using bromide ions as the
2
+
+
masking agent (Fig. 1). We also confirmed that the interferences of Hg and Ag on
3
+
Au -selective signaling by probe GP were successfully suppressed in the presence
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3+
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of minimum 5.0 × 10 M of bromide as a masking agent without affecting the Au
signaling of probe GP (Fig. S3, Supplementary data). In the presence of TPEN or
+
2+
iodide ions, the probe’s responses triggered by the interfering Ag and Hg ions
3
+
were completely suppressed; however, the signal induced by the target Au ions also
totally disappeared. With EDTA as the masking agent, the most pronounced
2
+
3+
+
signaling response was observed with Hg , followed by Au and Ag . Therefore, all
the subsequent signaling experiments were carried out using bromide ions as a
+
2+
masking agent for Ag and Hg . Probe GP showed no measurable responses toward
the rest of the surveyed metal ions: the fluorescence intensity ratio in the presence
and absence of metal ions I/I at 472 nm was nearly constant and varied between 1.1
0
+
2+
(for Na ) and 2.4 (for Hg ).
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1
1
0
5
0
5
0
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60
40
20
0
3
+
GP + Au
GP only
n+
GP + M
3
50
400
450
500
550
600
Wavelength (nm)
2
20
21
3
+
2
Fig. 1. Au -selective signaling of probe GP as expressed by the fluorescence intensity
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226
227
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231
ratio (I/I ) at 472 nm. Inset: fluorescence spectra of GP in the presence of the metal ions
0
3
+
shown in the main graph. Inset photograph: cuvettes containing GP and GP with Au .
–
5
n+
–5
–
–3
[GP] = 1.0 × 10 M, [M ] = 5.0 × 10 M, [Br ] = 1.0 × 10 M in a 1:1 (v/v) mixture of
an acetate buffer solution (pH = 4.76, 20 mM) and DMSO. λex = 330 nm.
Under the competitive conditions using bromide as a masking agent, the presence
+
2+
of background metal ions such as Ag and Hg resulted in slightly turbid solutions
–
13
due to the formation of precipitates (K for AgBr and HgBr = 5.0 × 10 and 1.3 ×
sp
2
–
19
10 ) [36] and significantly affecting the fluorescence signaling as shown in Fig. S4
(Supplementary data). This effect could be easily eliminated by simply passing the
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resulting solutions through a syringe filter (Fig. 2). Time-dependent fluorescence
3
+
measurements revealed that the signaling of Au by GP reached maximum intensity
within 30 min of sample preparation (Fig. S5, Supplementary data). Thus, all
measurements were performed 30 min after sample preparation. Under the signaling
conditions, the stability of probe GP itself was found to be satisfactory showing
sustained fluorescence intensity for greater than 1 h. Because the fluorescence
3
+
intensity of the resulting signaling solution of probe GP was dependent on the [Au ]
3
+
(Fig. S5, Supplementary data), we concluded that the Au signaling of probe GP is
based on the irreversible reaction by consuming the analyte to form very stable
3
+
Au S , not by the Au -catalyzed hydrolysis of thiocarbonate of GP.
2
3
1
0
.8
.6
.4
0
0
0.2
0
3
+
+
Au
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+
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Fig. 2. Au -selective signaling of GP in the presence of background metal ions as
–
5
3+
n+
expressed by the ratio I(Metal+Au(III))/IAu(III). [GP] = 1.0 × 10 M, [Au ] = [M ] = 5.0
–
5
–
–3
× 10 M, [Br ] = 1.0 × 10 M in a 1:1 (v/v) mixture of an acetate buffer solution
(pH = 4.76, 20 mM) and DMSO. λex = 330 nm. The results for turbid solution
+
2+
containing Ag and Hg ions were obtained after removing the precipitates in the
measurement solution with a syringe filter.
3
+
3+
The sensing of Au ions by probe GP is due to the Au -induced hydrolytic
deprotection of the thiocarbonate functionality to form the highly fluorescent parent
dye 1 and insoluble precipitate Au S (Scheme 2) [44]. The hydrolysis is facilitated
2
3
3
+
by the coordination of thiophilic Au ions to the thiocarbonyl moiety of the probe
(Scheme S1, Supplementary data). The signaling product was obtained after
purification of the resulting signaling solution by passing it through a short silica pad.
1
The H NMR spectrum of the purified signaling product was identical to that of the
reference compound, 1 (Fig. 3). Most of the aromatic proton resonances were
upfield-shifted compared to those for GP, and the appearance of a singlet at 12.50
ppm (indicated with a red asterisk) for the hydroxyl proton of 1 was particularly
noticeable. Measurement of the mass spectrum (electron impact ionization) also
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confirmed the formation of the signaling product through the presence of a
+
diagnostic peak at m/z = 227.0 (calcd. for C H NOS = 227.0) (Fig. S6,
1
3
9
Supplementary data).
2
66
67
3
+
2
Scheme 2. Au signaling by hydrolysis of thiocarbonate-based probe GP.
268
2
69
70
1
3+
3+
2
Fig. 3. Partial H NMR spectra of GP, GP upon treatment with Au (GP + Au ),
–
3
271
and reference compound 1. [GP] = [1] = 5.0 × 10 M in CDCl . The spectrum of GP
3
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+
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+ Au was obtained after purification of the signaling product by passing it through
a short silica pad.
3
+
The effect of pH on the Au sensing of probe GP was assessed by adjusting the
final pH of the acetate buffer solution by adding various amounts of HCl or NaOH.
As shown in Fig. S7 (Supplementary data), the signaling contrast of the probe with
3
+
and without Au was nearly constant over the wide pH range between 4.0 and 10.5.
This observation implies that GP could be used as a signaling tool in a wide pH
range including the physiological region near pH 7.0.
3
+
Titration of probe GP with Au was carried out to determine quantitative
analytical characteristics of the probe. The fluorescence intensity of the probe at 472
3
+
nm increased steadily with the increase in [Au ] (Fig. 4). The low concentration
3
+
region of the plot has a somewhat sigmoidal form due to Au also forming
–
complexes with the added bromide ions (logKassoc for AuBr = 32.3) [45]. The plot
4
3
+
showed that up to 20 µM Au could be determined using GP by the fluorescence
–
6
measurements, and the dynamic linear range was found to be from 2.5 × 10 M to
–
5
2
1.2 × 10 M with a correlation coefficient (R ) of 0.986 (Fig. S8, Supplementary
data). The detection limit was estimated from this plot following IUPAC
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recommendations (3s /m) and found to be 4.8 × 10 M (9.5 ppb) [42]. In addition,
bl
to confirm its practical utility in real-world applications, we applied GP to the
3
+
determination of Au ions in a sample of synthetic wastewater, which was prepared
following the reported guidelines (Table S2, Supplementary data) [41]. From the
3
+
concentration-dependent fluorescence profile, the detection limit for Au
−
8
determination in synthetic wastewater was calculated to be 5.7 × 10 M (11.2 ppb)
(Fig. S9, Supplementary data). As a matter of fact, the obtained detection limit of
3
+
3+
probe GP for Au ion is not superior to the previously reported Au signaling
sensors and probes (Table S1, Supplementary data) [46–50]. As can be seen from the
3
+
Table S1 (Supplementary data), majority of the reported Au signaling systems are
3
+
focused on the determination of Au ion in biological samples. However, probe GP
3
+
is easily obtainable and could conveniently determine the Au ions in environmental
and industrial samples with high selectivity using a smartphone as a field-oriented
device as described below.
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16
12
8
4
0
2
1
1
0
5
0
5
0
3
+
[
Au ]
3
50 400 450 500 550 600
Wavelength (nm)
0
10
20
30
Au ] (µM)
40
50
3
+
[
3
05
06
3
+
–5
3+
3
Fig. 4. Fluorescence titration of GP with Au ions. [GP] = 1.0 × 10 M, [Au ] =
–
5
–
–3
307
308
309
310
311
312
313
314
315
316
0–5.0 × 10 M, [Br ] = 1.0 × 10 M in a 1:1 (v/v) mixture of acetate buffer solution
(pH = 4.76, 20 mM) and DMSO. λex = 330 nm.
Finally, practical application of probe GP to the field-oriented determination of
3
+
Au ions using a smartphone was carried out (Scheme 3). Smartphones have
recently attracted much interest and have been used as valuable devices for the
determination of chemically and environmentally relevant analytes in “in-the-field”
applications [51]. Acetate-buffered synthetic wastewater solutions were prepared
3
+
with various concentrations of Au ions. After a fluorescent signal from GP was
obtained, images of the fluorescent solutions were captured with a smartphone in a
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321
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325
dark chamber under illumination by a hand-held UV lamp. The captured images were
analyzed with an application on the smartphone by plotting the red, green, and blue
3
+
channel values as a function of [Au ] (Fig. S10, Supplementary data). Among the
3
+
three channels, the plot of the blue channel level vs. [Au ] afforded the most
pronounced result and provided a linear relationship for analyte concentrations of up
–
5
2
3+
to 1.0 × 10 M (R = 0.9944) (Fig. 5). The detection limit for Au ions when using
–
7
the smartphone was found to be 4.4 × 10 M (86.7 ppb) following IUPAC guidelines
(3s /m) [42].
bl
3
26
27
3
+
3
Scheme 3. Schematic view of the experimental setup for the determination of Au
ion in synthetic wastewater by probe GP using a smartphone.
328
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200
150
100
y = 20.535x + 16.991
2
R = 0.9944
Detection limit:
50
3
s /m = 3 x 3.008/20.535 = 0.44 µM
bl
0
0
2
4
6
8
10
3
+
[
Au ] (µM)
3
29
30
3
Fig. 5. Changes in the blue channel level of signaling images obtained by a
3
+
–5
331
332
333
334
335
336
smartphone as a function of [Au ] in synthetic wastewater. [GP] = 1.0 × 10 M,
3
+
–5
–
–3
[Au ] = 0–1.0 × 10 M, [Br ] = 1.0 × 10 M in a 1:1 (v/v) mixture of acetate-
buffered synthetic wastewater solution (pH = 4.76, 20 mM) and DMSO. Images were
captured under illumination by a hand-held UV lamp.
337
4. Conclusions
3
38
39
In this study, we developed a reaction-based probe with a simple structure for the
3
+
3
selective fluorometric determination of Au
ions via the hydrolysis of a
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+
340
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349
thiocarbonate functionalized 2-(2-hydroxyphenyl)benzothiazole dye. The Au -
selective signaling of the probe was not subjected to interference in the presence of
commonly encountered environmentally important metal ions. Interference from
2
+
+
other thiophilic ions (Hg and Ag ) was effectively circumvented by using bromide
ions as a masking agent. Signaling was fast and applicable over a wide pH range
from 4.0 to 10.5. Practical application of the probe in synthetic wastewater samples
was readily conducted using a smartphone as a convenient field-oriented device. The
3
+
probe could be used for the rapid and accurate determination of Au ions in
chemical and environmental applications.
350
Appendix. Supplementary data
3
51
52
Supplementary data associated with this article can be found in the online version at
doi:**.****.
3
3
53
54
3
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