Journal of Agricultural and Food Chemistry
Article
MATERIALS AND METHODS
578 nm was substantially elevated only by the subsequent
stimulation from SA, thus manifesting that the designed probe
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Instruments and Chemicals. All chemical reagents and
analytical reagent (AR) solvents were acquired from commercial
suppliers and used without further purification. Distilled water was
used throughout. Nuclear magnetic resonance (NMR) spectra were
obtained using a JEOL-ECX 500NMR spectrometer. High-resolution
mass spectroscopy (HRMS) spectra were performed on a Thermo
Scientific Q Exactive mass spectrometer. Fluorescence spectra were
performed by a Fluoromax-4 spectrofluorometer. Ultraviolet−visible
1
possesses a superior anti-interference characteristic for
(
(
UV−vis) spectra were recorded by a TU-1900 spectrophotometer
Beijing Purkinje General Instrument Co., China). Fluorescence
imaging was performed using a Nikon ECLIPSE Ti-S fluorescence
microscope.
The stock solution of probe 1 (1.0 × 10− M) was prepared in
methanol. Analyte stock solutions (1.0 × 10− M) of SA and its
distractors were prepared in methanol, respectively. The preparation
of samples for UV−vis and fluorescence detections follows the
operations: 0.1 mL of stock solution of probe 1 was added to a 10 mL
volumetric flask, and then certain amounts of detection objects were
added; the mixture was filled up to 10 mL with the associated
solvents; and finally, the related spectra were obtained after incubating
the mixture for 10 min.
3
2
Figure 3. Competition experiments for adding 600 μM SA into the
premixed solution consisting of probe 1 (10 μM) and various SA
analogues (600 μM): (1) blank, (2) ASA, (3) 2-MeOBA, (4) 4-
OHBA, (5) 2-MeBA, (6) catechol, (7) 2-NH BA, (8) MeSA, (9) BA,
2
(
10) phenol, (11) salicylaldehyde, and (12) 3-OHBA. Black bars,
RESULTS AND DISCUSSION
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fluorescence intensity for probe 1 with SA analogues at 578 nm; red
bars, after adding SA into the premixed solution containing probe 1
with SA analogues. Experimental conditions: λ , 554 nm; slits, 2/2
Rhodamine-based chemosensor 1 was prepared, as illustrated
in Figure 1a. The starting material Boc-glycine was reacted
with dimethylamine through typical condensation to provide
an intermediate a, which was then treated with trifluoroacetic
acid to remove the protective group (−Boc) to realize an
intermediate b. Finally, the target probe 1 was fabricated
through a simple cyclization reaction between the intermediate
b and rhodamine B chloride. The final molecular framework
ex
nm; and MeOH−H O, 9:1 (v/v).
2
plot experiment (Figure S10 of the Supporting Information),
3
−1
(Figure S11 of the Supporting Information). The concen-
tration-dependent titration assay demonstrated that the
fluorescent intensity at 578 nm increases gradually after adding
different dosages of SA. Consequently, a favorable linear
Information). Furthermore, the related detection limit was
obtained as 1.0 nM by testing the signal-to-noise ratio. Given
the above-mentioned results, a prospective probe that can
selectively monitor SA in vitro in a naked-eye detection manner
was discovered.
1
13
Supporting Information). Fluorescence and UV−vis spectra
were performed to evaluate the detection competence of probe
1
toward SA and its distractors (Figure 1c and panels a and b
of Figure 2). A methanol solvent and a methanol−water
solution (9:1, v/v) were rationally selected for monitoring SA
based on the fluorescence enhancement folds at 578 nm upon
and S8 of the Supporting Information). Probe 1 displayed
superior selectivity and sensitivity to those of other SA
analogues in detecting SA from the fluorescence spectrum
1H NMR spectra were performed to explain the possible
mechanism for monitoring SA in a concentration-dependent
manner (Figure 4). The protons (1′, 2′, 3′, and 4′) that belong
high fields because the molar ratio was 1:1 (Tables S2 and S3
of the Supporting Information). In contrast, further increasing
the amount of SA (2 or 3 equiv) resulted in a reduced variation
on the chemical shift, as illustrated by the corresponding
changes in protons 1′, 2′, 3′, and 4′: −0.03, −0.08, −0.05, and
−0.06 ppm for 1 equiv of SA and −0.02, −0.06, −0.04, and
−0.05 ppm for 3 equiv of SA. This phenomenon might
contribute to insufficient binding sites offered by probe 1 with
excess SA molecules directed by hydrogen bonding. In
contrast, the protons in probe 1 provided gradually increased
chemical shifts by elevating the abundance of SA. The protons
(11) of the methylene group showed significant low-field shifts
with the changed values of 0.08 ppm for 1 equiv of SA, 0.11 for
2 equiv of SA, and 0.13 ppm for 3 equiv of SA. This finding
might be due to the increased shielding effect from excess SA
molecules promoted by hydrogen bonding. This action
simultaneously led to rearranging the global electrons of
probe 1. The protons at the 5−10 positions presented
(
Figure 2a). These SA analogues included 4-hydroxybenzoic
acid, o-methylbenzoic acid, o-methoxybenzoic acid, salicylalde-
hyde, acetylsalicylic acid (ASA), 3-hydroxybenzoic acid,
catechol, methyl salicylate, benzoic acid, and anthranilic acid.
The significantly produced fluorescent intensity triggered by
adding SA revealed that SA can ring open the spirolactam
pattern probably promoted by hydrogen-bonding interactions.
This result was consistent with the newly generated UV
absorption peak depicted in Figure 2b, thereby confirming that
probe 1 can distinctively monitor SA. A pink color was only
observed after adding SA (Figure 2c), thus suggesting that a
colorimetric sensor was unexpectedly acquired for distinguish-
ing SA with a naked-eye detection feature. Definitively, probe 1
showed improved detection behavior compared to our
previously reported probe (Figure S9 of the Supporting
Information).
To validate the selectivity and anti-interference functions
further, competition assays should be conducted by sub-
sequently adding SA to the premixed solution containing probe
1
with diverse interfering ingredients. Fluorescent intensity at
C
J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Fu, Yi-Hong
