C.J. Rha, et al.
Inorganic Chemistry Communications 121 (2020) 108244
Scheme 1. Synthesis of APBN.
group is a great chromogenic moiety due to the presence of the long
conjugation which can utilize as a colorimetric probe [25,26]. Mean-
while, the C]N bond of Schiff base can be cleft by a hypochlorite [27].
With these points in mind, we expected that a long-conjugated Schiff
base having azo dye and naphthol derivatives would operate as a col-
orimetric probe for sensing hypochlorite.
In this work, we demonstrate a novel and effective azo-naphthol
based colorimetric sensor APBN which displayed a greatly selective
−
detection to ClO by color change of yellow to colorless in aqueous
media. The apparent color change could be visualized directly through
the naked eye and show the practical test-kit application. In addition,
−
APBN demonstrated good sensitivity to ClO with low detection limit
−
(
0.47 μM) and fast reaction. The sensing process of APBN for ClO was
1
understood by the UV–vis, ESI-mass, H NMR titration, and calcula-
tions.
Compound APBN was synthesized by combination of sodium 4-
aminoazobenzene-4′- sulfonate and 2-hydroxy-1-naphthaldehyde in
1
13
−5
−
MeOH (72%, Scheme 1). It was verified by H NMR, C NMR, ESI-mass
Fig. 2. UV absorption change of APBN (2 × 10
M) at various ClO con-
centrations (from 0 to 7.0 equiv). Inset: Plot of the absorption at 470 nm as a
function of ClO− concentration.
(
Figs. S1–S3) and elemental analysis.
To confirm the sensing ability of APBN, diverse analytes including
−
−
−
–
2−
–
−
−
−
−
ClO , I , tBuOOH, CN , S , N , OAc , Br , AcOOH, H
−
3
2
PO
4
, BzO ,
−
–
SCN , Cl , NO
2
, and H
2
O
2
, F were tested in PBS buffer solution
and DMF, but the sensing rate was slower than in water due to the low
−
−
(
pH = 7.4). When 6.5 equiv of analytes was added to APBN, only ClO
mixability of ClO with the other organic solvents. The results in-
−
accompanied remarkable spectral (Fig. 1a) and significant color
dicated that APBN may operate as a colorimetric probe for ClO
.
−
changes of orange to colorless (Fig. 1b). APBN also showed the same
To study the reacting properties of APBN toward ClO , the UV–vis
−
−
sensing ability for ClO in other solvents such as MeOH, MeCN, DMSO,
titration was executed (Fig. 2). As the amount of ClO increased, the
absorption peak at 470 nm was steadily decreased and blue-shifted to
2
70 nm with an isosbestic point at 301 nm. The appearance of the
isosbestic point meant that a new product was formed. To prove the
−
quantitative sensing capability of APBN for ClO , a calibration curve
was made in the range of 0.0 μM to 36.0 μM and reasonable linearity
2
could be obtained (R = 0.9922) (Fig. 3). Based on the curve, detection
−
limit for ClO was given as 0.47 μM by 3σ/slope, which was the second
lowest among those previously reported for the colorimetric hypo-
chlorite probes in a near-perfect aqueous media (Table S1).
1
The H NMR titration was executed to comprehend the reaction
−
−
mechanism of APBN toward ClO (Fig. S4). As the amount of ClO
increased, the proton H of the C]N bond disappeared with broadness
5
of the rest proton peaks. The results led us to presume that APBN would
−
undergo the cleavage of the C]N bond by ClO . For further eluci-
dating of the cleavage sensing mechanism, the ESI-mass test for the
−
reaction of APBN with ClO was conducted (Fig. S5). The positive-ion
mass spectrum confirmed the formation of the oxidative product ([P-1
+
+
(
2-hydroxy-1-naphthoic acid) + Na + 2H
2
O] [calcd. 247.06]) of
APBN at m/z = 247.21. Moreover, the P-1 molecule was again con-
firmed on thin layer chromatography (TLC) compared to the authentic
P-1. Based on the results of NMR titration, ESI-mass, TLC test and
previously reported literatures [27–30], the plausible sensing process of
−
APBN for ClO was proposed (Scheme 2).
To check the possible influences caused by other analytes, the
Fig. 1. (a) Absorption variation of APBN (2 × 10−5 M) after adding (6.5 equiv)
various anions and oxidants in PBS buffer. (b) The photograph of APBN
−
competition test for ClO was conducted (Fig. 4). When other anions
−
and oxidants coexisted with the same concentration of ClO , most of
−
5
the analytes showed no inhibition to ClO−, except for NO
–
, AcOOH,
(
2 × 10
M) after adding (6.5 equiv) various anions and oxidants.
2
2
Kim, Cheal
