1. Modification of Ultrafiltration Membranes
for the Improved Purification of
Nanoparticle Dispersions
Tyler J. Myers, Nkem Alele M. Sc., Prof. Dr. Mathias Ulbricht
Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen, 45117 Essen, Germany
1. Background
The need to fractionate and purify solutions, particularly nanoparticle dispersions, poses a challenging task
in multiple areas of research, including biochemical and medicinal. Many former techniques in this area,
such as size exclusion chromatography and gel electrophoresis, are quite time-extensive and low-volume.
Since a variety of nanoparticle properties depend on size and uniformity, the exploitation of ultrafiltration
(UF) membranes provide a fast, efficient, and easily-scalable method for the purification of broad
nanoparticle distributions1. The sieving properties of such membranes, coupled with pressure driven
processes, provide for a precise fractionation with little to no dilution or contamination of target products.
However, the short lifespan of ultrafiltration membranes in terms of pore blockage or surface fouling, proves
a difficult obstacle in exploiting the full efficiency of the material.
aaaajkjkhjj
2. Objectives
Bb
5. Results - Characterization
aa
6. Conclusions
This study primarily focuses on the proposition and evaluation of a
surface modification method that aims to reduce ultrafiltration
membrane fouling and improve separation processes with respect to
the fractionation of proteins (e.g. Bovine Serum Albumin) and
nanoparticles, specifically silica-type. Polyethersulfone membranes
with a molecular weight cut off of 300 kDa will be modified in
reference to membranes with a cut off of 100 kDa in order to give the
PES300 membranes the sieving properties of the PES100
membranes while maintaining an anti-fouling layer.
Membrane Contact Angle
[°]
PES100
PES300
PES300_Mod
140.46 ± 12.91
127.97 ± 17.70
33.54 ±1.84
Preliminary results of this project shows the ultraviolet grafting of polyethersulfone membranes
as a quick, efficient, and successful procedure for their modification. The entire modification
process can be completed within a day and results in membranes that no longer foul or lose
performance upon use. Although many of the experiments demonstrate incomplete grafting,
where not the entire membrane surface has been coated with the anti-fouling layer, many of the
experiments show the presence of said polymer layer. With this method in particular, the large
difference in silica nanoparticle and BSA rejection poses a high potential for the fractionation of
nanoparticle dispersions that contain particles with diameters of around 50 nm. The modification
method is still being improved with work being put into a solution for the low permeability after
long grafting times, as well developing a modified membrane with even smaller pores for full
rejection of silica nanoparticles. Also in the future, research may be completed to find other
types of monomer solution that may yield an even better performance in order to cut costs and
provide a greater supply of anti-fouling membranes for this particular area of research.
aa
7. References & Acknowledgments
Organische Chemie
%Transmittance
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
3927
3897
3686
3408
3105
3077
2973
2901
2761
2719
2641
2524
2415
2351
2323
2289
2195
2164
2114
2050
1980
1696
1579
1484
1406
13041304
1228
1144
1108
1074
1057
1015
863
830
704
PES300_Mod2(Down)_BL_ATR
3900 3800 3700 3600 3500 3400 3300 3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100
Wavenumber
N-H Stretch
(Secondary)
C-H Stretch
(Alkane)
C-H Stretch
(Alkene)
C=O Stretch
(Ketone)
Figure 6. FT-IR spectrum of a ultrafiltration following the modification procedure. The labeled peaks
correspond to the monomer, 2-Acrylamido-2-methylpropane sulfonic acid (AMPS). AMPS contains
very similar peaks to the additional monomer, DMAPAA.
t
Membrane Preparation
Polyethersulfone (PES) membranes with molecular weight cut-off (MWCO) of 300 kiloDaltons
and 100 kiloDaltons were checked for discrepancies with a Kaiser Pro Lite Basic 2 High Frequency
Light Box. The marked membranes were cut into small disks with a diameter of 25 millimeters using a
rubber hammer and metal hollow-punches. The cut membranes were then immersed in a solution of
ethanol and water in a 1:1 ratio and placed on a mechanical shaker for one hour at 100 rpm. After an
hour of shaking, the membranes were rinsed three times with pure Milli-Q water and shook for 30
minutes two times in pure Milli-Q water. The membranes were then washed overnight in Milli-Q water
and stored in fresh water the next morning.
Rejection Procedure
Using a dead-end filtration system equipped with a nitrogen gas line for applying pressure, the
initial water flux was measured at a pressure of 0.05 bar. A 100 milliter solution of 200 mg/L BSA in pH
8 buffer was prepared for the rejection. 15 milliliters of the solution were added to the filtration cell and
7.5 milliliters were filtered through the membrane and stored as the permeate. The remaining 7.5
milliliters in the cell were stored as the retentate. The time required for filtration was taken for flux
measurements. Immediately after, the water flux was measured again and then the membrane was
flipped over so the rough side faced up. The membrane was then rinsed with Milli-Q water for 5
minutes at a pressure of 1 bar to dislodge any particles that may remain in the pores of the membrane.
The membrane was then flipped with the active side up and the water flux was measured once more
for flux recovery calculations. BSA was measured using a calibration curve generated by a UV-Visible
Spectrophotometer while the silica nanoparticle concentration was measured with a Cary Eclipse
Fluorescence Spectrophotometer.
Figure 1. A schematic depicting the path of UV irradiation during the surface modification process.
Petri
Dishes
Aqueous Monomer
Solution
Membrane
Optical
Filter
Green
Glass
Plates
Mercury Lamp
UV Radiation
Membrane Modification
Two milliliters of monomer solution was pipetted into each of three glass petri dishes and the
washed PES300 base membranes were placed active side down in the dish to soak for five minutes. The
membranes were then flipped active side up and covered with a slightly smaller glass petri dish to ensure
the entire membrane surface was covered with monomer solution. The three petri dishes were placed in
the UVACUBE 200 equipped with a high-pressure mercury lamp as the source of radiation emitting
wavelengths greater than 300 nm. Three green glass plates and a special glass optical plate filter the
radiation, bringing the intensity to between 5 and 7 mW/cm2, which was checked prior to and following
irradiation using a UV meter. All three petri dishes were placed into the chamber together and grafted for
a total of 5 minutes. After grafting, the modified membranes are rinsed in excess Milli-Q water for one
hour to remove excess monomer solution or any physically adsorbed polymer chains. The modified
membranes were then stored in Milli-Q water.
Figure 7. (A) SEM photograph of a modified ultrafiltration membrane. Some pore sizes have been
reduced but the ultimate effect comes from the new hydrophilicity of the membrane surface2,3. (B)
SEM photograph of an unmodified ultrafiltration membrane.
Table 1. Contact angles of
unmodified reference PES100,
unmodified PES300, and modified
PES300 membranes. Contact
angles less than 50 degrees
indicate hydrophilicity.
Measurements were performed
using “captive bubble” technique.
Bsccvcvvv
4. Results - Rejection
1 2 3 4 5
30
40
50
60
70
80
90
100
PercentRejection
Experiment
PES300_Mod_BSA
PES100_BSA
PES300_Mod_Silica
PES100_Silica
0 1 2 3 4 5 6
20
40
60
80
100
PercentRejection
Diavolumes
BSA
Silica
Figure 3. Percent rejections of Bovine Serum Albumin and silica nanoparticles
for reference PES100 membranes and modified PES300 membranes. The
modified membranes resulted in a 60% decrease in BSA rejection compared
to the references PES100 membranes. Effects of fouling have decreased
significantly in the modified membranes as they have let almost 70% of BSA
pass through while retaining 90-100% of the silica nanoparticles in solution.
Figure 5. Diavolumes for a mixture of BSA and silica nanoparticles
showing the concentration loss progression of BSA and silica in the
retentate. Although about 25% of the silica concentration is lost from
the original solution, it is lost at a much slower rate than BSA , where
only about 20% of the original BSA concentration was left in the
retentate after 6 diavolumes.
1 2
0
20
40
60
80
100
PercentRejection
Experiment
Silica
BSA
Figure 4. Percent rejections of BSA and silica nanoparticles when
tested in a solution together. The difference in rejection is not as large
as it was when the solutes were in individual solutions mostly due in
part to nanoparticles clogging the pores of the membrane.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
200
400
600
800
1000
1200
PES300_Mod_H2
O
PES300_Mod_BSA
PES100_H2
O
PES100_BSA
Flux(L/m2
*hr)
TMP (bar)
Figure 2. Flux vs TMP for water and BSA solutions through reference
PES100 membranes and modified PES300 membranes. Reference
PES100 membranes began to reach an isometric point where the flux
no longer increases, called the critical flux, around 0.6 bar. However,
the modified PES300 membranes continued to increase, even in the
BSA solution, indicating a significant decrease in fouling.
3. Experimental
1Baker, R. W. 2001. Membrane Technology. Encyclopedia Of Polymer Science and Technology. 3.
2Brans, G., Schroën, CGPH., van der Sman, RGM., Boom, RM. Journal of Membrane Science.
243, 1-2, 263-272.
3Yamagishi, H., Crivello, JV., Belfort, G. Journal of Membrane Science. 105, 3, 237-247.
Prof. Dr. Mathias Ulbricht & Nkem Alele
Universität Duisburg-Essen
German Academic Exchange Service (DAAD) & RISE