sufficient robustness, our supramolecular system, having a fibrous 3D network with
nanoscopic pores, should be able to separate particles having nanoscopic sizes.
Notably, all commercially available filtration membranes are composed of high
molecular weight polymers or ceramics [62–65]. The technological application of
membrane-based techniques includes various large-scale industrial processes,
including food processing, biomedical applications, and water purification
[62, 66], while most separation processes deal with aqueous solutions. Membranes
for pressure-driven filtration applications pose a dilemma in material science: on the
one hand, high material robustness is required (the membrane must be stable and
must retain its nanoscopic structure under the pressure and flux of solvent and
solutes); on the other hand, adaptive properties are highly desirable (e.g., membranes
with a dynamically controllable pore size, self-healing, and recyclability).
The membranes were fabricated in one step by filtering a solution of 8 in water
over a commercial cellulose acetate support, thus forming a layer with a 3D fibrous
nanostructure (Fig. 13a–e) [61]. Other commercial supports are suitable as well, and
cellulose acetate was selected due to its availability, good wettability, and low costs.
The supramolecular membranes were stable under a pressure-driven (up to 0.8 bar of
overpressure) flux of water for several hours and could be used immediately after
preparation to separate various nanoparticles according to size. The nature of the
particles and their capping layers did not influence the membrane’s performance.
The membrane thickness was readily adjusted by changing the amount of supramolecular solution that was used for its preparation. A thin (~12 μm) membrane was
used for filtering various gold nanoparticles, and it consistently showed a 5-nm
cutoff size (an example is shown in Fig. 13f–k). The membrane’s permeance
(pressure normalized flux) of 1.1 Â 10
2 L h
À1 m
À2 bar
À1 is comparable to commercially available membranes with similar rejection properties.
1
To investigate the mechanism underlying the observed size-selective separation,
we performed cryo-SEM imaging of a membrane sample that was prepared following
Au nanoparticle deposition, which revealed that the particles permeate through the
membrane, with the depth of permeation depending on the nanoparticle size (Fig. 14a)
(depth filtration mechanism [62]). Evidently, even a very thin membrane layer
exhibited a nice particle distribution, suggesting that size separation of particles
smaller than 5 nm in the regime of chromatography should be feasible. This can be
important for semiconductor nanoparticles (quantum dots, QD), whose photonic
properties strongly depend on the size, with several classes of QDs exhibiting a
substantial variation of photonic characteristics in the 1–5 nm range. Addressing this
possibility, we used a 45-μm-thick membrane to fractionate a mixture of 2.5 and 4.0nm CdTe quantum dots. The large particles penetrated the supramolecular network
slower than the small ones, resulting in chromatographic separation according to size
(Fig. 14b).
1 See for example technical specifications of Koch Membrane Systems HFM-100/180, HFK-131,
or GE Osmonics KN1CP04700.
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
381
nanoscopic pores, should be able to separate particles having nanoscopic sizes.
Notably, all commercially available filtration membranes are composed of high
molecular weight polymers or ceramics [62–65]. The technological application of
membrane-based techniques includes various large-scale industrial processes,
including food processing, biomedical applications, and water purification
[62, 66], while most separation processes deal with aqueous solutions. Membranes
for pressure-driven filtration applications pose a dilemma in material science: on the
one hand, high material robustness is required (the membrane must be stable and
must retain its nanoscopic structure under the pressure and flux of solvent and
solutes); on the other hand, adaptive properties are highly desirable (e.g., membranes
with a dynamically controllable pore size, self-healing, and recyclability).
The membranes were fabricated in one step by filtering a solution of 8 in water
over a commercial cellulose acetate support, thus forming a layer with a 3D fibrous
nanostructure (Fig. 13a–e) [61]. Other commercial supports are suitable as well, and
cellulose acetate was selected due to its availability, good wettability, and low costs.
The supramolecular membranes were stable under a pressure-driven (up to 0.8 bar of
overpressure) flux of water for several hours and could be used immediately after
preparation to separate various nanoparticles according to size. The nature of the
particles and their capping layers did not influence the membrane’s performance.
The membrane thickness was readily adjusted by changing the amount of supramolecular solution that was used for its preparation. A thin (~12 μm) membrane was
used for filtering various gold nanoparticles, and it consistently showed a 5-nm
cutoff size (an example is shown in Fig. 13f–k). The membrane’s permeance
(pressure normalized flux) of 1.1 Â 10
2 L h
À1 m
À2 bar
À1 is comparable to commercially available membranes with similar rejection properties.
1
To investigate the mechanism underlying the observed size-selective separation,
we performed cryo-SEM imaging of a membrane sample that was prepared following
Au nanoparticle deposition, which revealed that the particles permeate through the
membrane, with the depth of permeation depending on the nanoparticle size (Fig. 14a)
(depth filtration mechanism [62]). Evidently, even a very thin membrane layer
exhibited a nice particle distribution, suggesting that size separation of particles
smaller than 5 nm in the regime of chromatography should be feasible. This can be
important for semiconductor nanoparticles (quantum dots, QD), whose photonic
properties strongly depend on the size, with several classes of QDs exhibiting a
substantial variation of photonic characteristics in the 1–5 nm range. Addressing this
possibility, we used a 45-μm-thick membrane to fractionate a mixture of 2.5 and 4.0nm CdTe quantum dots. The large particles penetrated the supramolecular network
slower than the small ones, resulting in chromatographic separation according to size
(Fig. 14b).
1 See for example technical specifications of Koch Membrane Systems HFM-100/180, HFK-131,
or GE Osmonics KN1CP04700.
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
381