M. Basak et al.
Dyes and Pigments 196 (2021) 109779
Scheme 2. Schematic representation of the proposed binding model for self-assembled Probe 1 and PA.
interaction between PA and Probe 1 (
ε
= 98697.4 Mꢀ 1 cmꢀ 1) (Fig. 2a).
time-resolved fluorescence spectra of Probe 1 (5.0 μM, λex = 400 nm) in
Further, the visual appearance of Probe 1 in solution changed imme-
diately from colorless to mustard yellow, possibly due to the formation
of the charge-transfer complex.
an aqueous medium was recorded before and after the addition of PA.
Both the time-resolved decays were fitted to bi-exponential decay along
with a significantly decreased lifetime from 3.85 ns to 1.52 ns upon
addition of PA (Table S1, Supporting Information). The decrease in
lifetime with the gradual addition of PA signified a dynamic quenching
mechanism, i.e., diffusion-controlled collision [43] between the excited
sensor and the quencher. The limit of detection [44] for PA was calcu-
lated to be ~56 ppb (Fig. S9, Supporting Information) using an earlier
reported procedure in the literature [45]. The Ksv and LOD obtained for
the probe for PA detection in the current study compares well with
previous reports (Table 1). The limit of quantification (LOQ) for PA was
estimated to be 186 ppb (Fig. S9, Supporting Information). The inter-
ference and selectivity studies with other NACs, common cations and
anions (2,4-DNP; 4-NP, 2-NP, 3-NP, 4-NT, NB, 3,4-DNT, 3,6-DNT,
The fluorescence spectra of self-assembled Probe 1 (5.0 μM, Quan-
tum yield = 0.91) manifested an intense emission maxima at 484 nm
when excited at 400 nm in an aqueous solution. Surprisingly, after the
addition of various NACs and non-nitro compounds, prominent
quenching was observed in the case of nitrophenol solutions, while the
others revealed minimal changes. A significant quenching of emission
(~95%) was observed upon addition of 6 Equiv. of PA, while for 2,4-
DNP and 4-NP, quenching efficiency was only 51% and 38%, respec-
tively (Fig. 2b). As the number of nitro groups increased, electron
deficiency enhanced in NACs, and electron drifts were elevated from the
electron-rich sensor to NACs followed by strong intermolecular in-
teractions, leading to a higher extent of fluorescence quenching.
Consequently, with the addition of PA, the quenching efficiency was
maximum (Quantum yield = 0.087) as compared to other nitro-
aromatics, suggesting the selectivity of self-aggregated Probe 1 toward
PA. Since nitrophenols displayed substantial quenching among all other
NACs, fluorescence titration experiments with only PA (Fig. 2c), ,4-DNP
and 4-NP were pursued. For these three nitrophenols, the emission in-
tensity of Probe 1 decreased with the incremental addition of the ana-
lyte. The quenching mechanism was also evaluated by Stern-Volmer
(SV) equation. The SV plot was found to be linear at a lower concen-
tration of PA with a Stern-Volmer constant (Ksv) of 22.6 × 106 Mꢀ 1
(Fig. 2d, inset), which is more significant than earlier reported values in
the literature [41], while at a higher concentration of PA, the plot was
apparently exponential (Fig. 2d). The linear response at lower concen-
tration of PA and a high Ksv value may be ascribed to static quenching,
while the non-linear nature at higher concentration of PA signified the
involvement of dynamic quenching in presence of an additional energy
transfer process [42]. Interestingly, Ksv values for 2,4-DNP and 4-NP
were lower than the Ksv value for PA (22.6 × 106 Mꢀ 1) and was esti-
mated to be 4.26 × 106 and 1.68 × 106 Mꢀ 1, respectively with a linear
response plot (Figs. S7 and S8, Supporting Information). Hence, the Ksv
values could render discrimination amongst PA, DNP, and NP. To
illustrate the mechanistic aspect of fluorescence quenching,
phenol, Fe3+, Al3+, Cr3+, Hg2+, Cd2+, Pb2+, Cu2+, Co2+, Ni2+, Zn2+
,
,
Ca2+, Mg2+, PO43ꢀ , Clꢀ , Iꢀ , H2PO4ꢀ , HSO4ꢀ , NO3ꢀ , OHꢀ , HSO3
ꢀ
HCO3ꢀ , SHꢀ , oxalate2ꢀ ) were also examined. Their effect on sensitivity
and selectivity of Probe 1 for PA was very negligible (Fig. S10, Sup-
porting Information).
FESEM analysis and fluorescence microscopic studies were con-
ducted to ascertain the variation in surface morphology of self-
assembled Probe 1 in the absence and presence of PA. Self-assembled
Probe 1 exhibited leaf-like morphology, which became smaller and
displayed round-shaped morphology upon interaction with PA (Fig. 3a
and b). In fluorescence microscope analysis, the intense fluorescence
emitted by the self-assembled probe was completely dissipated upon
interaction with PA (Fig. 3c–f). Conceivably, interaction with PA
resulted in disaggregation of the self-assembled Probe 1. This tenet was
corroborated by DLS studies, wherein the self-assembled Probe 1 dis-
played a higher hydrodynamic radius (939.8 nm) as compared to Probe
1-PA complex (353.7 nm) (Fig. S11, Supporting Information), strongly
suggesting dissociation of the self-assembled Probe 1. In order to gain
further insight into the sensing phenomenon (Scheme 2), Job’s plot was
generated, which indicated that Probe 1-PA formed a 1:1 stoichiometric
complex (Fig. S12, Supporting Information). HRMS studies further
verified the complexation between Probe 1 and PA. (Fig. S13, Sup-
porting Information). Additionally, 1H NMR study (Fig. S14, Supporting
5