Macromolecules
Article
immersing time lengthened. Although the APy molecule with
smaller molecular radius and hydrogen-bonded site (−NH2
functionality) should be able to enter into the nanochannels of
the polymeric materials, they were still rejected by the
nanochannels, which could be mainly ascribed to the
hydrogen-bonding and spatial match difference between APy
molecules and nanopores. For further proof of the LCP
material’s high-selective uptake of melamine, more adsorption
experiments using diverse organic molecules with different
charges and sizes, including anionic dye methyl orange (MO,
4.9 Å × 11.8 Å), cationic dye methylene blue (MB, 5.1 Å ×
13.8 Å), and neutral molecule meso-tetra(p-aminophenyl)-
porphine (TAPP, 14.2 Å × 14.2 Å), were studied (Figure S18).
As illustrated in Figure S20, all the absorbances of MO, MB,
and TAPP solutions were barely changed along with the
immersing time lengthened. On the one hand, MO and MB
molecules, although they could have accessibility to the
nanochannels, were significantly precluded due to the
uncharged nanopore surface, which revealed the exclusion of
anionic/cationic organic molecules. On the other hand, TAPP
with multiple −NH2 functionalities was barely absorbed due to
the size exclusion, which seemed to be essential for the
selective uptake mechanism since smaller molecules (i.e.,
melamine) showed higher uptake capability, in comparison to
those with larger molecular sizes.
The pH influence on the melamine uptake capacity of the
supramolecular LCP material was further investigated. A piece
of the nanoporous LCP material (0.55 mg, theoretically
maximum uptake of 23.62 μg melamine) was immersed in a
melamine solution (water, 1.00 mL, concentration = 100 μg
mL−1, 4.23 equiv to the maximum possible adsorption
quantity) under magnetic stirring at different pH values (pH
= 1, 4, 7, 10, and 13). In each experiment, 0.02 mL of the
original solution was extracted and diluted to 2.70 mL. The
concentration of the diluted solution and melamine uptake
quantity were measured by UV−vis spectroscopy and
calculated based on the UV absorbance vs melamine
concentration standard curve, which showed a good linearity
consistent with Lambert−Beer’s law (ε = 3.9186 × 10−4 L g−1
cm−1), as demonstrated in Figure S21. Here the adsorption
capacity (qt, eq 1) was defined as how many milligrams of
melamine was absorbed per gram of LCP material as a function
of uptake time.
uptake process. In detail, a small piece of the nanoporous LCP
material (0.38 mg) was immersed in a concentrated solution of
melamine (water, 1.00 mL, concentration = 70 μg mL−1, 4.29
equiv to the maximum possible adsorption quantity), 0.02 mL
of such a solution was extracted and diluted to 2.70 mL, which
was further investigated by UV−vis spectroscopy. The
concentration of the diluted solution and melamine uptake
quantity were calculated based on the standard curve (Figure
S21). Theoretically, a full refill of the Colh nanochannels of the
LCP material would take 16.32 μg of melamine. When the first
adsorption equilibrium reached, 9.52 μg of melamine was
absorbed, which accounted for 58.3% of occupied nanopores,
as shown in Figure 6c. After melamine removal, the second and
third adsorption capacities of the nanoporous material were
8.50 and 8.21 μg, and the occupation degrees were calculated
as 52.1% and 50.3%, respectively, which showed a good
recyclable adsorbing performance.
To better understand the uptake phenomena and process
dynamics, the melamine adsorption kinetics and mechanism of
the nanoporous LCP material were further studied. As can be
seen in Figure 6d and Figure S19b, melamine in the diluted
solution was adsorbed by the nanoporous LCP material as
indicated by UV absorbance decrease at 203 nm spectrophoto-
metrically along with the extended time. A fast absorption
process occurred in the first 96 h, possibly due to the presence
of a vast amount of hydrogen-bonded sites on the nanochannel
surfaces and a large concentration gradient between the
solution and the adsorbent at the beginning. The dynamics
platform with the maximum uptake capacity corresponding to
the adsorption equilibrium was eventually achieved after 240 h.
The quantitative and theoretical analysis of the melamine
uptake data was expressed by fitting the adsorption capacity
(qt) values to the Lagergren first-order (eq 2) and pseudo-
second-order (eq 3) kinetic models,63 whose equations are
presented as
qt = qe(1 − e−k
)
adt
(2)
t
qt
1
kadqe
1
qe
=
+
t
2
(3)
where qe is the maximum capacity of melamine adsorbed at
equilibrium, t is the uptake time, and kad is either the Lagergren
first-order (h−1) or pseudo-second-order (g mg−1 h−1) kinetic
constant.
The resulting first- and pseudo-second-order kinetic curves
for absorbing melamine were calculated by the linear
regression qt vs t and t/qt vs t, respectively, and illustrated in
Figure 6e,f. The correlation coefficient (R2 = 0.99939) for the
utilization of the pseudo-second-order model (qe = 14.61 mg
g−1, kad = 7.52 × 10−4 g mg−1 h−1) confirmed that this model
fitted the kinetic data more accurately than the first-order
kinetic model for the real-time detection of the adsorption
capacity of the nanoporous supramolecular LCP material.
(Ca − Cb)V
qt =
(1)
m
where Ca and Cb are the initial and final concentrations of
melamine (mg L−1), V is the volume (L) of the melamine
aqueous solution, and m is the mass of the nanoporous LCP
material (g). As shown in Figure 6b, the nanoporous LCP
material displayed a good melamine adsorption capacity even
in harsh environments; the q value reached 10.42, 7.77, and
8.64 mg g−1 at pH ∼ 7, 4, and 10, respectively. When the pH
value was set as either 1 or 13, the absorption peak at ca. 203
nm in the melamine solution could not be observed by UV−vis
spectroscopy because melamine was hydrolyzed under the
strongly acidic or basic conditions and converted into cyanuric
acid. Nonetheless, the nanoporous supramolecular LCP
material demonstrated a stable uptake capacity over a broad
range of pH values (pH 4−10).
CONCLUSION
■
In conclusion, we reported a nanoporous LCP material built
on melamine/thymine-derivative supramolecular structure,
which could specifically recognize and selectively absorb
melamine. Such a material presented a stable and recyclable
melamine adsorption capacity over a broad range of pH
environments (pH 4−10). To the best of our knowledge, this
work was the first example using the imidodicarbonyl
The recyclable uptake performance of the nanoporous LCP
material was elucidated by measuring the percentage of the
occupied nanopores (occupation degree) in each melamine
G
Macromolecules XXXX, XXX, XXX−XXX