An Acid-Inert Fluorescent Probe for the Detection of Nitrite

Article abstract of DOI:10.1007/s10895-017-2071-9

The fluorescent detection of nitrite ions has been challenging owing to the poor acid-stability of typical fluorescent probes. Herein, an acid-inert rhodamine-based fluorescent probe has been developed for the fast detection of trace amount of nitrite ion

Full text of DOI:10.1007/s10895-017-2071-9

J Fluoresc
DOI 10.1007/s10895-017-2071-9
ORIGINAL ARTICLE
An Acid-Inert Fluorescent Probe for the Detection of Nitrite
1
,2
2
1
2
Meiyi Cai & Xiaoyun Chai & Xuedong Wang & Ting Wang
Received: 12 December 2016 /Accepted: 2 March 2017
#
Springer Science+Business Media New York 2017
Abstract The fluorescent detection of nitrite ions has been
challenging owing to the poor acid-stability of typical fluores-
cent probes. Herein, an acid-inert rhodamine-based fluores-
cent probe has been developed for the fast detection of trace
amount of nitrite ions with turn-on mode. The detection limit
of nitrite ions was determined to be 9.4 nM (S/N = 3) and the
linear range with high linear correlation was observed be-
tween 0.025 to 2.5 μM. A test paper method was developed
for rapid visual detection of nitrite ions. Quantitative analysis
of two real water samples also confirmed the reliability and
sensitivity of the developed fluorescent method.
diseases, such as intrauterine growth restriction, spontaneous
abortions, infant methemoglobinemia and birth defects in cen-
tral nervous system [5–7]. Additionally, in acidic environ-
−
ment, NO2 play the role as a reaction substrate for the gen-
eration of highly carcinogenic N-nitrosamine compounds [8].
−
Considering the potential toxicity and hazards of NO , a
2
−
simple and sensitive detection of NO2 level in water and food
is highly demanded. Ion chromatography, capillary electro-
phoresis, colorimetry and electrochemistry are conventionally
−
used for the detection of NO2 [3, 9–13]. However, these
aforementioned methods are often limited by complicated
procedures or low selectivity. In comparison, design of fluo-
−
.
.
.
Keywords Nitrite Silicon Rhodamine Fluorescence
rescent probes for NO2 has already attracted great attention
enhancement
owing to the high sensitivity and simplicity. So far, several
fluorescent probes have been developed for the detection of
nitrite [4, 14–21]. These probes are typically based on the
diazotization of amine, which requires strong acidic condition.
Introduction
−
In particular, rhodamine-based fluorescent probes for NO2
−
Nitrite (NO ) ion, commonly detected in water and food, has
are especially preferred because of their remarkable chromo-
2
−
proven to be a great threat to human health [1–4]. Excessive
genic and fluorogenic responses [14, 15]. Triggered by NO ,
2
intake of NO2− with water or food can lead to a variety of
diazotization of amine induces the ring-opening of
spirolactam in the probe, thereby restoring the absorption
and emission. However, most of the developed fluorescent
+
Electronic supplementary material The online version of this article
probes are sensitive to H , and the rhodamine-based probe is
(doi:10.1007/s10895-017-2071-9) contains supplementary material,
not an exception. In acidic condition, the rhodamine based
which is available to authorized users.
+
probes go through a H -induced ring-opening process, giving
undesired background signals, which leads to the loss of sen-
*
*
Xuedong Wang
xdwangwfmc@163.com
−
sitivity and false positive response to NO . To eliminate these
2
+
−
H -induced side effects, the detections of NO are usually
2
Ting Wang
restricted to low temperature or extra addition of base [14, 15].
Recently, we have found that the spiroring in Si-rhodamine
can tolerate a relatively strong acidic condition, existing as the
non-fluorescent spirocyclic form over a wide pH range
[22–25]. The stability of the spirolactam suppresses the back-
ground signal, thereby providing a promising platform for
wangting1983927@gmail.com
1
2
College of Pharmacy, Weifang Medical University, Weifang 261053,
China
Department of Organic Chemistry, College of Pharmacy, Second
Military Medical University, Shanghai 200433, China

J Fluoresc
fabricating acid-inert probes with high sensitivity. Herein, we
developed a Si-rhodamine-based probe SiRB-Nitrite for the
detection of NO2− (Scheme 1). Under acidic condition with
nitrite ions, probe readily undergoes diazotization, resulting in
a ring-opening process with remarkable chromogenic and
fluorogenic responses. The acid-stability has been further con-
firmed by comparing the optical properties with its rhodamine
analogous RB-Nitrite (Scheme 1). Owing to the selectivity
and sensitivity, probe SiRB-Nitrite has been applied in mon-
itoring NO2− level in water.
Fluorescence spectroscopic studies were performed on a
Hitachi F-7000.
Absorption and Fluorescence Analysis
Absorption spectra were obtained with 1.0-cm glass cells.
Fluorescence emission spectra were obtained with a Xenon
lamp and 1.0-cm quartz cells. The fluorescence intensity was
measured at 685 nm for probes SiRB-Nitrite. The slit width
was 5.0 nm for both excitation and emission of probe. The
photomultiplier voltage was 600 V. Phosphate buffer solution
at different pH was respectively added into DMSO (1%, v/v)
solutions containing probe SiRB-Nitrite (250 μM).
Materials and Methods
Materials
Preparation of Colorimetric Test Paper
Colorimetric test paper was prepared by immersing strips of
white filter paper (10 mm × 30 mm) into a methanol solution
of SiRB-Nitrite (250 μM) for 30 min and then dried gently.
The colorimetric test paper was stored in a vacuum desiccator.
Before the usage, the colorimetric test paper was exposed to
hydrogen chloride gas for 10 s. Then the paper was dipped
into the aqueous solution of nitrite ions for 1 s and dried in air.
Blue color appears in the dried colorimetric test paper.
All reagents and solvents were of the best grade available.
Methanol, phosphorus oxychloride, 1,2-dichloroethane, acetoni-
trile, ethyl acetate, petroleum ether and triethylamine were sup-
plied by Shanghai Chemical Reagent Co. o-Diaminobenzene,
rhodamine B, sodium nitrite and all salts used in interference
experiment were supplied by Adamas-beta. Unless otherwise
stated, all commercial reagents were used without additional pu-
rification. Solvents were dried according to standard procedures
prior to use.
Real Sample Pre-Treatment and Analysis
Instrument
Water samples were taken from the tape water collected from
lab and pond water from the university in Shanghai city. River
All reactions were monitored by thin-layer chromatography
water and tap water were analyzed after filtration and acidifi-
−
(
TLC) on gel F254 plates. Flash chromatography was carried
cation (pH was adjusted to 2.0), which were spiked with NO
at different concentration for the measurement of recovery.
2
out on silica gel (300–400 mesh). NMR spectra were recorded
1
on a Bruker AC-300P spectrometer (300 MHz for H NMR
1
3
and at 75 MHz for C) or on a BrukerAC-600P
Synthesis of Probes SiRB-Nitrite and RB-Nitrite
1
spectrometer(600 MHz for H NMR and at 150 MHz for
1
3
C NMR). Spectral data are reported in ppm relative to
SiRB-Nitrite To a solution of compound SiRB (726 mg,
1.5 mmol) in dry 1,2-dichloroethane (10 mL) at room temper-
ature, phosphorus oxychloride (690 mg, 4.5 mmol) was added
dropwise over a period of 5 min. After being refluxed for 4 h,
the reaction mixture was cooled and concentrated under vac-
uum to give a blue solid. This blue solid was further dissolved
in dry acetonitrile (10.0 mL). The solution was slowly added
to a solution of o-diaminobenzene (810 mg, 7.5 mmol) in dry
tetramethylsilane (TMS) as internal standard. High-
resolution mass spectra (HRMS) were recorded on an
Aglilent Technologies 6538 UHD Accurate-Mass Q-TOF
MS spectrometer using ESI. All pH measurements were per-
formed with a pH-3c digital pH-meter with a combined glass-
calomel electrode. UV-visible spectra were obtained on a
Analytikjena Specord 210 PLUS UV-vis spectrophotometer.
Scheme 1 Chemical structure of
probe SiRB-Nitrite and its
analogous RB-Nitrite

J Fluoresc
Scheme 2 Synthesis of probe
SiRB-Nitrite and its analogous
RB-Nitrite
acetonitrile (10 mL) containing triethylamine (5.0 mL). After
stirring at room temperature overnight, the mixture was con-
centrated in vacuo and the crude product was purified by
column chromatography on silica gel (ethyl acetate: petro-
acetonitrile (10.0 mL). The solution was slowly added to a solu-
tion of o-diaminobenzene (810 mg, 7.5 mmol) in dry acetonitrile
(10 mL) containing triethylamine (5.0 mL). After stirring at room
temperature overnight, the mixture was concentrated in vacuo
and the crude product was purified by column chromatography
on silica gel (ethyl acetate: petroleum ether = 1: 4) to give SiRB-
leum ether = 1: 4) to give SiRB-Nitrite as a white solid
1
(
458 mg, 53% yield). H NMR (600 MHz, CDCl ): δ 8.03
3
1
(
dd, J = 5.6, 2.9 Hz, 1H), 7.54 (m, 2H), 7.17 (d, J = 8.1 Hz,
Nitrite as a light red solid (447 mg, 56% yield). H NMR
1
H), 6.92–6.86 (m, 1H), 6.51–6.58 (m,7H), 6.32 (t,
(300 MHz, CDCl ): δ 8.06 (d, J = 6.9 Hz, 1H), 7.63–7.54 (m,
3
J = 10.0 Hz, 1H), 5.80 (d, J = 7.7 Hz, 1H), 3.35 (d,
J = 1.0 Hz, 8H), 1.16 (t, J = 7.0 Hz, 12H), 0.42 (s,
2H), 7.27 (s, 1H), 6.98 (t, J = 7.6 Hz, 1H), 6.69 (d, J = 8.5 Hz,
2H), 6.58 (d, J = 7.9 Hz, 1H), 6.53–6.18 (m, 5H), 6.12 (d,
J = 7.8 Hz, 1H), 3.36 (d, J = 6.7 Hz, 8H), 1.18 (t, J = 6.7 Hz,
1
3
3
1
1
1
1
H), −0.32 (s, 3H); C NMR (151 MHz, CDCl ) δ
3
13
67.34, 154.95, 146.17, 145.50, 137.26, 132.69,
32.30,130.29, 128.87, 127.91, 124.24, 123.53, 122.02,
18.05, 116.76, 114.88, 114.01, 75.04, 53.42, 44.23,
2.45, −0.57, −1.70.
12H). C NMR (75 MHz, CDCl ) δ 166.34, 153.84, 152.22,
3
148.73, 144.42, 132.60, 131.85, 128.63,128.71, 128.34,
124.21, 123.40, 122.02, 118.14, 116.93, 107.97, 97.96,
77.18,67.92, 44.42, 12.38.
RB-Nitrite To a solution of compound rhodamine B (664 mg,
.5 mmol) in dry 1,2-dichloroethane (10 mL) at room temper-
1
Results and Discussion
ature, phosphorus oxychloride (690 mg, 4.5 mmol) was added
dropwise over a period of 5 min. After being refluxed for 4 h, the
reaction mixture was cooled and concentrated under vacuum to
give a red solid. This blue solid was further dissolved in dry
Probe SiRB-Nitrite and its rhodamine analogous RB-Nitrite
were synthesized according to reported procedures
(Scheme 2) [25, 26]. Since the diazotization process requires
Fig. 1 a Effect of pH on the
probe SiRB-Nitrite (2.5μM,
black circle) and its fluorescent
–
2
responses to 1.0 equiv. of NO
(red circle). b The relative
fluorescence intensity changes of
probes RB-Nitrite (2.5μM) and
SiRB-Nitrite (2.5μM) in the
absence (F
.0 equiv. of NO
and pH = 1.0, respectively
0
) or presence (F) of
−
1
2
at pH = 2.0

J Fluoresc
+
that the spirolactam in probe RB-Nitrite is sensitive to H ,
resulting in relatively high background fluorescence. The fluo-
−
rescent response of probe SiRB-Nitrite to NO2 was carefully
examined at a series of pH values. Upon the addition of 1.0
−
equiv. of NO , probe SiRB-Nitrite presented remarkable fluo-
2
rescence enhancement with the increasing acidity of solution,
−
and exhibited the best response performance to NO2 at
pH = 2.0. The fluorescence enhancement of the probe started
to decrease with further increasing of acidity.
The reference probe RB-Nitrite shared similar responses to
−
1
.0 equiv. of NO2 at pH = 2.0 (Fig. S1, Supporting
Information). However, the fluorescence enhancements in
probe RB-Nitrite (pH = 2.0 and pH = 1.0) are both lower than
that in probe SiRB-Nitrite (pH = 2.0) due to the high back-
ground fluorescence in acidic environment (Fig. 1b).
Comparing with probe RB-Nitrite, it is more feasible to use
Fig. 2 Time-dependent fluorescence intensity changes of probe SiRB-
−
–
probe SiRB-Nitrite to sensitively monitor NO2 without extra
Nitrite (2.5μM) upon addition of various concentrations of NO
2
at
pH = 2.0
procedures.
At room temperature, probe SiRB-Nitrite can respond to
−
acidic condition, the effect of pH on the probe SiRB-Nitrite and
its fluorescent response to NO2 were evaluated in the pH range
NO
2
in relatively short time. As shown in Fig. 2, after adding
−
−
a certain amount of NO at pH = 2.0, the fluorescence emis-
2
from 1.0 to 8.0. As illustrated in Fig. 1a, SiRB-Nitrite displays
constant low background fluorescence within a pH range from
sion at 685 nm remarkably increases within the first 10 min
and reaches a plateau in about 15 min.
8
.0 to 1.0, demonstrating that the probe can tolerate a relatively
broad range of pH, even under strong acidic condition
pH = 1.0). In sharp contrast, the reference probe RB-Nitrite
Under the optimal conditions, the absorption and fluores-
cence responses of probe SiRB-Nitrite with the titration of
−
(
NO
2
were examined. Free probe SiRB-Nitrite formed a col-
shows pH-dependent fluorescence enhancement under acidic
conditions (Fig. S1and S2, Supporting Information), suggesting
orless and non-fluorescent solution, indicating the predomi-
nant existence of the spirocyclic form. With the addition of
Fig. 3 a Absorption change of
probe SiRB-Nitrite (2.5μM) with
−
the increase of NO
2
(0.0–
2
5.0μM). b Emission change of
probe SiRB-Nitrite (2.5 μM)
−
with the increase of NO
5.0μM). c The relative
fluorescence intensity (F/F
changes with the increase of
2
(0.0–
2
0
)
−
NO
2
(0.0–25.0μM). d
−
Calibration curve for NO
2
detection (0.025–2.5 μM). The
excitation and emission
wavelengths are 630 and 685 nm.
Error bars indicate standard
deviation for three replicates

J Fluoresc
Fig. 4 a HRMS analysis of the
reaction between probe SiRB-
Nitrite and nitrite at pH = 2.0. b
The proposed reaction
mechanism of probe SiRB-
−
Nitrite with NO
2
NO2− solution, dramatic color change (colorless to sky blue,
Fig. S3, Supporting Information) was observed in the reaction
mixture, companied with the change in both absorption and
fluorescence emission. In absorption spectra, a sharp peak
centered at 675 nm increased rapidly and another gently peak
centered at 625 nm gradually increased (Fig. 3a).
Concomitantly, a gradual enhancement in fluorescence inten-
detection limit of probe SiRB-Nitrite to NO2− was deter-
mined to be 9.4 nM (S/N = 3), which is more sensitive than
the maximum allowable level of contamination of nitrite ions
in drinking water (1 ppm, 21.7 μM) defined by Environmental
Protection Agency [27].
The reaction mechanism of SiRB-Nitrite with NO2− was
revealed by high-resolution Mass Spectroscopy (HRMS)
analysis. In the reaction mixture at pH = 2.0, a unique peak
at m/z = 586.3006 was clearly found (Fig. 4a). The peak is
likely belongs to the highly fluorescent compound SiR-
benzotriazole (calc. 586.2997), which is the ring-opening
−
sity (λem = 685 nm) was noted with the increase of NO2
(
Fig. 3b). The fluorescence enhancement slowed down and
−
reached saturation with 5 equiv. of NO2 (Fig. 3c).
2
A good linear correlation (R = 0.996) between the relative
−
−
intensity of fluorescence enhancement and NO2 concentra-
product of the probe SiRB-Nitrite triggered by NO2 in acidic
tion from 0.025 to 2.5 μM was observed (Fig. 3d). The
solution (Fig. 4b). We also confirmed the high chemical sta-
bility of compound SiR-benzotriazole under acidic condition,
ensuring the reliability of the analytical application.
+
+
2+
2+
Many common cations, including Na , K , Ca , Cu ,
3
+
2+
2+
Fe , Mg and Zn , and some common anions in water, such
−
−
−
−
−
−
2−
2−
as AcO , NO , I , F , Cl , Br , CO
and C O
2 4
show
3
3
negligible interfere in the fluorescent emission even at 100-
fold higher concentrations of these ions when compared to
−
NO . In addition, the enhancement in fluorescence intensity
2
−
resulting from the addition of NO2 was not influenced by the
Fig. 5 Fluorescence responses of probe SiRB-Nitrite (2.5 μM) to
various ions. Gray bars represent the fluorescence response of SiRB-
Nitrite to the excess ion (250 μM) of interest. Black bars represent the
−
+
subsequent addition of 2.5 μM NO
2
to the solution. (1) probe; (2) Na ;
+
2+
2+
3+
2+
2+
−
(
3) K ; (4) Ca ; (5) Cu ; (6) Fe ; (7) Mg ; (8) Zn ; (9) AcO ; (10)
−
−
−
−
−
2−
2−
−
NO
3
; (11) I ; (12) F ; (13) Cl ; (14) Br ; (15) CO
3
; (16) C O
2 4
Fig. 6 Photographs of the test paper in the presence of NO
2

J Fluoresc
Table 1 Determination of nitrite ions in water with the proposed
method
Acknowledgements This work was supported by the National Natural
Science Foundation of China (No. 21205135 and 21602250). We are very
grateful for the help and suggestion of Dr. Xiaoyan Cui from East China
Normal University.
−
added (μM) NO −
c
Samples
NO
2
2
found (μM) Recovery (%)
Tap watera
0
0
0
0
0
0
-
.30
.60
0.31 ± 0.01
103.3
103.3
0.62 ± 0.03
0.21 ± 0.02
0.52 ± 0.01
0.79 ± 0.01
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Pond waterb
.30
.60
102.0
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