PCCP
Paper
to the surface of ZnS Qdot and thus, no free ligand is present;
Now, considering the two step reaction the surface equilibrium
(ii) surface coverage may be considered to be ‘1’ when ZnS Qdot can be expressed as
was treated with a high amount of HQ. Using PL spectroscopy,
0
0
ka(1 ꢃ y)[HQ] + ka (1 ꢃ y)[ZnQ2] = kdy + kd y
(4)
we monitored the reaction between HQ and ZnS Qdots in both
MeOH and DMSO media (Fig. 2A). Primarily, we investigated
whether binding of HQ on the surface of ZnS Qdot followed
Langmuir isotherm in both the solvents (i.e., MeOH and
DMSO). From Fig. 2B and C, it is evident that binding of HQ
on the surface of ZnS Qdot can be treated as an adsorption
phenomenon that follows Langmuir isotherm with better
agreement in case of MeOH compared to DMSO. The surface
complexation phenomenon can be speculated as adsorption of
HQ on the surface of ZnS Qdot following the formation of
surface zinc quinolato complex – more precisely, complexation
assisted adsorption. Upon formation of zinc quinolato complex
(possibly, ZnQ2) on the surface, desorption can possibly occur
either in the form of ZnQ2 or as the quinolato moiety itself.
However, the desorption of a particular species is dependent on
the solubility of that species in a solvent medium. Owing to the
higher solubility of ZnQ2 in DMSO than in MeOH, the extent of
desorption of the Z-type surface ZnQ2 complex would be much
higher in case of DMSO. The other possible path of desorption,
i.e., desorption of quinolato moiety by means of dissociation
of complex, may be considered as a part of complexation
equilibrium. Hence, in MeOH only dissociation-assisted
desorption of quinolato moiety was prevalent. However, in case
of DMSO, Z-type complex desorption as well as desorption of
quinolato moiety occurred, although Z-type desorption may
be the primary pathway in comparison to dissociation of a
chelating complex.
As free ZnQ2 is the intermediate species and HQ is added to the
mixture externally, we have [HQ] c [ZnQ2]. Thus, the equation
can be written as
0
ka(1 ꢃ y)[HQ] = (kd + kd )y
(5)
Hence,
Kca½HQꢄ
ka
y ¼
;
where Kca
¼
(6)
0
Kca½HQꢄ þ 1
kd þ kd
We term the new constant ‘Kca’ as complexation adsorption
constant. ‘ka/kd’ is similar to surface complexation constant.
Thus, ‘Kca’ herein represents the adsorption constant (K).
Importantly, ‘Kca’ takes care of the complexation-assisted
adsorption of HQ as well as solubility driven desorption of
ZnQ2, occurring simultaneously on the surface. Due to higher
solubility of ZnQ2 in DMSO than in MeOH, the extent of
desorption would be higher in case of DMSO. As a result,
desorption coefficient of ZnQ2 in DMSO would be much higher
0
0
than that of MeOH, [(kd )DMSO c (kd )MeOH] – which would
result in the lower value of ‘Kca’ in DMSO than in MeOH (i.e., for
0.177 mM ZnS Qdot, (Kca)MeOH = 2.61 ꢂ 108 Mꢃ1 and (Kca)DMSO
=
1.18 ꢂ 108 Mꢃ1, which were obtained from the value of ‘1/slope’
in Fig. 2C). The assumption that [HQ] c [ZnQ2] may not be
accurate in the case of treatment of ZnS Qdot with low
concentration of HQ in DMSO solvent. In this concentration
range, the extent of complex formation is high (based on Le
Chatelier’s principle) and desorption of Z-type ZnQ2 complex
may also occur. Thus, the deviation from simple Langmuir
isotherm in case of DMSO is possible. Therefore, the above
model projects the zinc quinolato complex (or, ZnQ2) as the
attached species and thus supports Z-type (1 : 2) binding of HQ
to the surface of the ZnS Qdots. However, this finding requires
further experimental substantiation.
Adsorption model of the surface complexation reaction
Based on the discussions, a model can be proposed for better
understanding of the ligand binding phenomenon. The reaction
can be visualized as two-step process:
(i) Adsorption of HQ followed by complexation on the
surface of ZnS Qdot (formation of 1 : 2 complex with surface
cation):
ka
Ð
Identification of the desorbed species by NMR spectroscopy
Surface ꢃ Zn2þ þ 2HQ
surface ꢃ ZnQ2
(1)
kd
The desorption of zinc-quinolato complex from the surface
of QDC in DMSO was substantiated by NMR spectroscopic
analysis. The supernatant obtained by centrifugation of QDC
dispersion in DMSO-d6 was used for 1H NMR analyses. Notably,
when the complexation reaction of HQ with Zn2+ ions in a
liquid medium (DMSO) was performed, a shift in the charac-
teristic peaks of HQ was observed (Fig. 3). For example, the
characteristic peak of HQ at 7.1 ppm (which was assigned to
proton number 6) shifted to 6.88 ppm following complexation
with Zn2+ ions (i.e., the formation of normal ZnQ2 complex;
Fig. 3). Interestingly, the supernatant of the HQ-treated ZnS
Qdot (QDC) exhibited characteristics peaks at 6.88 (for the
proton number 6 of HQ), which is akin to that of ZnQ2 complex
(ii) Adsorption equilibrium between adsorbed complex and
desorbed complex (Z-type ligand binding):
0
kd
Ð
Surface ꢃ ZnQ2
desorbed ꢃ ZnQ2
(2)
0
ka
Now, HQ is attached on the surface the Qdot via the formation
of a luminescent ZnQ2 complex. Thus, the surface coverage of
Qdot can be expressed as a function of PL intensity as follows:
I
y ¼
(3)
Imax
where y = average surface coverage, and Imax = maximum (Fig. 3). This clearly indicated the presence of ZnQ2 complex in
fluorescent intensity, which was determined using highly the supernatant of the HQ-treated ZnS Qdot (QDC). Moreover,
concentrated HQ.
the additional peaks present in the NMR spectrum of
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Phys. Chem. Chem. Phys., 2019, 21, 589--596 | 593