Y. Jiang, H. Li, R. Chen et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 251 (2021) 119438
and 723 mg/kg, respectively [8]. The calcine produced by mercury
mining contains a large amount of inorganic mercury [9], which
extensively enriched in the surface soil causing severe pollution
[10,11]. The Loess Plateau of China is famous with soil erosion
and water shortage problems [12]. Droughts intercept more mer-
cury in soil, meanwhile, frequent windy days and lacking coverage
landscape increase long-range migration of mercury-carrying soil
particles. These causes extend the contaminated areas and aggra-
vate the mercury pollution, including soil, water, plants, etc., and
ultimately threaten human health [13–15]. Efficient and rapid
detection of Hg2+ content in loess is of great significance for under-
standing the impact of mercury pollution and protecting the eco-
logical environment of Loess Plateau. It can also contribute to
other loess regions in the world.
Atomic fluorescence spectrometry [16], atomic absorption spec-
trometry [17], atomic emission spectrometry [18], inductively cou-
pled plasma mass spectrometry [19] are common classic mercury
detection methods [20]. However, high technical requirements
for operators, advanced precision instruments, complicated prepa-
ration and high cost of these techniques hamper their applications
in field testing and daily monitoring of large area. Compared with
abovementioned methods, the fluorescent probe detection tech-
nology which closely combines molecular recognition and fluores-
cent technology has more advantages like sensitivity, specificity,
and convenience [21–23]. At present, many fluorescent probes
for Hg2+ have been reported, and the mainstream detection mech-
anisms include: intramolecular charge transfer (ICT) [24], photoin-
duced electron transfer (PET) [25,26], excited state intramolecular
proton transfer (ESIPT) [27], etc [28–30]. In addition to the general
advantages of fluorescent probes, PET fluorescent probes often
involve fluorescence quenching and turn-on, which is visible and
suitable for naked eye observation by direct ultraviolet lamp irra-
diation [31,32]. These probes validate the fluorescent probes have
remarkable ability and great potential in detecting the Hg2+ in
water samples and biological cells [33,34]. However, the detection
of Hg2+ in actual soil samples has been rarely reported, let alone
loess.
(NO3)3ꢁ9H2O, Co(NO3)2ꢁ6H2O, Mn(OAc)2, Na2CO3, NaOAcꢁ3H2O,
NaNO3, NaClO4ꢁH2O, NaClO, KClO3, Na2CrO4, Na2Cr2O7ꢁ2H2O, NaF.
Drugs and solvents are chemically pure or analytically pure, and
the salt solutions used in the experiment are all prepared with
ultrapure water.
2.2. Synthesis and characterization
Probe
DC-Hg:
Add
0.1405
g
(0.50
mmol)
3-
hydroxybiscoumarin, 10 mL dichloromethane, and 2 mL dimethyl-
formamide to a 50 mL round bottom flask containing magneton.
After dissolution, add 0.0945 g (0.55 mmol) phenyl thiochlorofor-
mate and 0.1293 g diisopropylethylamine, stirred at room temper-
ature for 4.5 h, vacuum distillation to remove the solvent. 10 mL
ethanol, 0.2 mL N, N-diethylethylamine and 10 mL water were
added to the reactant, and ultrasound treatment for 5 min, and
stand for 10 min. The yellow precipitation was collected by suction
filtration and washed with ethanol and water (1:1). The product
was dried to obtain a yellowish solid 0.1179 g with a yield of
56.40% (Scheme 1). According to C23H12O6S, the calculated molec-
ular weight of MS (ESI) is 439.0444 [M + Na], and the measured is
439.1238. 1H NMR (400 MHz, DMSO d6) d 8.59 (d, J = 9.6 Hz, 1H),
8.45 (d, J = 8.3 Hz, 1H), 7.88 (t, J = 7.8 Hz, 1H), 7.75 (d, J = 2.4 Hz,
1H), 7.59–7.51 (m,5H), 7.40 (dd, J = 7.6, 4.4 Hz, 3H). 13C NMR
(101 MHz, DMSO d6) d 193.37, 157.20, 155.86, 155.31, 155.08,
154.87, 153.71, 152.10, 135.56, 131.54, 130.52, 129.67, 127.65,
125.59, 122.23, 122.10, 119.50, 118.04, 115.61, 114.63, 111.95,
107.42, 100.00. (Figs. S1–S3)
2.3. Fluorescence property test
Metal solutions and probe solution used in the test: Metal solu-
tions (2 mM) were obtained in ultrapure water. And the DC-Hg
solution (2 mM) was prepared in DMSO. During the test, the
required metal solutions and probe solution were extracted by
microinjectors and injected into 2 mL PBS buffer, mixed evenly
and detected.
Coumarin is an outstanding fluorescent probe dye [35–38], in
which V-type-dicoumarin is a typical representative of structure
derivative and optimization of single coumarin. V-type-
dicoumarin is connected by two coumarins which expands the
conjugate plane containing many modification sites. As such, V-
type-dicoumarin is much likely to connect to other groups to form
fluorescent groups with excellent detection performance, and the
structure of the product can be controlled by adjusting the reaction
conditions [39]. Versus other dicoumarins, typically higher fluores-
cence quantum yields, acceptable Stokes shifts, and straightfor-
ward syntheses, V-type-dicoumarin holds great potential in
fluorescent applications [40]. Meanwhile, the specific reaction
between Hg2+ and S atom is one of the basic principles to improve
the detection performance of many Hg2+ detection probes [41–43].
Inspired by all these related researches, we boldly attempted to
use 3-hydroxy dicoumarin as the fluorophore matrix, which
reacted with phenyl thiochloroformate to introduce a carbon–sul-
fur double bonds as the Hg2+ detection group. A PET-type fluores-
cent probe was therefore made and then named as DC-Hg. Then,
the capacity of DC-Hg for detecting Hg2+ was reached, and the
application in loess was measured.
2.4. Loess Hg2+ detection
Loess samples collection and treatment: Take the random 0–
5 cm deep soil near the root of a tree (N 36°205100, E 103°5102900),
seal it and bring back to the laboratory. After picking up the resid-
ual branches and leaves and large pieces of gravel in the soil, they
were sifted with a 1 mm sieve in diameter, and were divided into
two parts after mixing evenly. One part was naturally dried to
determine the physical and chemical properties of the soil, and
the other was dried in an oven for 24 h (80 °C) for the preparation
of fluorescence detection samples. A specific amount of Hg2+ solu-
tion (2 mM, HgCl2) was accurately added to the dried soil, fully
ground to dry, and then passed through a 200-mesh sieve to pre-
pare mercury-containing soil samples with a concentration gradi-
ent of 50 mg/kg in the range of 0–400 mg/kg.
Loess samples detection: Weigh 0.4000 g ( 0.0003 g) mercury-
containing soil in 7 mL centrifuge tube, add 4 mL hydrochloric acid
solution (0.1 M, soil-liquid ratio is 1:10) to extract, then use 2 mL
needle to absorb soil solution filter head (0.45
solution. When performing fluorescence detection, transfer
100 L of soil solution in 1.9 mL of ultrapure water, and then
add 10 L of probe solution (2 mM) and stir to test.
lm) to get clear
l
l
2. Experiment
2.5. Detection rate
2.1. Materials
Dc
Ac
Dr ¼
ꢂ 100%
ð1Þ
Metal salts: CaCl2, ZnCl2, NiCl2ꢁ6H2O, Cd(NO3)2ꢁ4H2O, Cr(NO3)3-
ꢁ9H2O, Cu(NO3)2ꢁ3H2O, Fe(NO3)3ꢁ9H2O, Mg(NO3)2ꢁ6H2O, FeSO4, Al
2