Preparation and photo-oxidative functions of poly(ethylene-co-methacrylic acid) (PE-co-MAA) nanofibrous membrane supported porphyrins

Article abstract of DOI:10.1039/c2jm16703d

Surface functionalized poly(ethylene-co-methacrylic acid) (PE-co-MAA) nanofibrous membranes were successfully prepared and applied as solid supports to immobilize photosensitizers. Several activation agents were used to facilitate the surface functionalization of PE-co-MAA nanofibrous membranes, and PCl₅ was found to be the most efficient one to activate surface carboxylic acid groups. Three spacers with variable lengths were introduced to the membrane surfaces prior to immobilizations of protoporphyrin IX (PPIX), a photosensitizer. The results revealed that the membranes with longer spacer chain incorporated more photoactive macrocyclic compounds, and the supported PPIX provided a more powerful catalytic effect on the photo-oxidation of 1,5-dihydroxynaphthalene. After five times of repeated catalytic reactions, the catalytic activity of the solid supported PPIX was maintained at 80% of the original power, and the membrane morphology was intact. The nanofibrous membrane supported photosensitizer provided high catalytic activity, easy handling, good durability and reusability. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16703d

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PAPER
Preparation and photo-oxidative functions of poly(ethylene-co-methacrylic
acid) (PE-co-MAA) nanofibrous membrane supported porphyrins
*
Jing Zhu and Gang Sun
Received 20th December 2011, Accepted 14th March 2012
DOI: 10.1039/c2jm16703d
Surface functionalized poly(ethylene-co-methacrylic acid) (PE-co-MAA) nanofibrous membranes were
successfully prepared and applied as solid supports to immobilize photosensitizers. Several activation
agents were used to facilitate the surface functionalization of PE-co-MAA nanofibrous membranes,
and PCl5 was found to be the most efficient one to activate surface carboxylic acid groups. Three
spacers with variable lengths were introduced to the membrane surfaces prior to immobilizations of
protoporphyrin IX (PPIX), a photosensitizer. The results revealed that the membranes with longer
spacer chain incorporated more photoactive macrocyclic compounds, and the supported PPIX
provided a more powerful catalytic effect on the photo-oxidation of 1,5-dihydroxynaphthalene. After
five times of repeated catalytic reactions, the catalytic activity of the solid supported PPIX was
maintained at 80% of the original power, and the membrane morphology was intact. The nanofibrous
membrane supported photosensitizer provided high catalytic activity, easy handling, good durability
and reusability.
(mesoporous materials, nanoparticles, nanotubes, nanosheets
Introduction
and nanofibrous membranes) as support media for organic
photosensitizers.12–16 In comparison with conventional support
media, the enormous high specific surface areas of nanoscaled
materials provide more sites and enhanced accessibility of
immobilized photoactive molecules, thus enabling higher
quantum yield of singlet oxygen for improved activities. Despite
these superiorities, a major issue related to mesoporous media is
low oxygen diffusion rates into and out of the solid matrices,
which is especially important for oxygen involved reactions. In
addition, solvent dependent dispersion properties and difficult
isolation processes of nanoparticles and nanotubes also limited
the practical applications of these nanosized media. In the
absence of the abovementioned drawbacks, nanofibrous
membranes serve as an attractive alternative to achieve high
amounts of immobilized photosensitizers as well as well-main-
tained catalytic activity. Composite nanofibrous membranes
with porphyrin moieties have been fabricated by an electrospun
method from solutions of mixed photosensitizers and target
polymers. However, leaching of coordinatively bound porphy-
rins from the mixture media limited the applications.14,15
Furthermore, Xu et al. reported the synthesis of copolymers of
vinyl porphyrins with acrylonitrile and the subsequent electro-
spinning of the resulting copolymer into porphyrin containing
nanofibers. The polymer matrices may block light exposure of
the embedded photosensitizers, which may have caused
a reduced photocatalytic activity,16 whereas covalently immobi-
lized porphyrins on the surface of nanofibrous membranes as
catalysts for photo-oxidation reactions have not received much
attention yet.
Singlet oxygen induced photo-oxidation reactions in the presence
of visible light, oxygen and photosensitizers have attracted much
attention due to the effective, economical and environmentally
friendly reaction pathway.1–3 By efficiently capturing and trans-
ferring radiant energies from light to molecular oxygen, non-
metallated porphyrins as organic photosensitizers are capable of
generating high quantum yields of singlet oxygen. As a result,
a variety of photo-oxidation reactions mediated by porphyrin
derivatives were extensively studied.4 However, the key challenge
for using these homogeneous catalysts is the reusability of the
porphyrin compounds owing to the difficult recovering processes
from reaction media.
Immobilization of the photosensitizers onto solid support
materials provides a simple and effective way to circumvent the
limitation mentioned above. A variety of support media
including beads, gels and membranes have been developed for
this purpose.5–10 The resulting products of support media and
organic photosensitizers as heterogeneous catalysts exhibited
synergetic benefits of easy recovery, ready reusability and
improved flexibility for various working solvents. However, these
advantages are usually accompanied by undesired weakening of
catalytic activity, as a direct result of decreased quantum yield of
singlet oxygen.11 Inspired by their intrinsic properties, consider-
able efforts have been devoted to utilize nanoscaled materials
Fiber and Polymer Science, University of California, Davis, CA 95616,
USA. E-mail: gysun@ucdavis.edu; Fax: +1 (530) 752-7584; Tel: +1
(530) 752-0840
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Recently, a high throughout procedure to fabricate thermo-
plastic nanofibers was developed by melt extrusion of an
immiscible polymer blend of the target polymer with a sacrificial
matrix, in which the fiber size was controllable via varied polymer
blend ratios.17,18 Poly(ethylene-co-methacrylic acid) (PE-co-
MAA) nanofiber was successfully prepared by using this
method.19 The resulting PE-co-MAA nanofibers possess good
thermal and chemical stability due to the polyethylene back-
bones. The carboxylic acid groups from the methacrylic acid
units are available for chemical attachment of various functional
groups. Most importantly, the inherent high specific surface area
from the open porous structure and ultrafine fiber size of nano-
fibrous membranes are beneficial for possessing high density of
surface functional groups as well as being easily accessible to the
incorporation of the macrocyclic porphyrins. Considering these
advantages, PE-co-MAA nanofibrous membranes are envisioned
as ideal solid media for supporting the photosensitizers.
substrate/catalyst ratio, solvent media and oxygen concentration
in this catalytic system were also investigated.
Experimental
1. Materials
Poly(ethylene-co-methacrylic acid) (PE-co-MAA; methacrylic
acid 15 wt%), phosphorus pentachloride (PCl5), thionyl chloride,
oxalyl chloride, N-hydroxybenzotriazole (HOBt), N,N-diisopro-
pylethylamine (DIPEA), 1,3-diaminopropane, 1,8-diamine-3,6-
dioxaoctane, 4,9-dioxa-1,12-dodecanediamine, protoporphyrin
IX (PPIX), 1,5-dihydroxynaphthalene, O-(benzotriazol-1-yl)-
N,N,N0,N0-tetramethyluronium hexafluorophosphate (HBTU),
N-(9-fluorenylmethoxycarbonyl)-glycine (Fmoc-Gly-OH) and
methyl piperidine were purchased from Sigma-Aldrich (Milwau-
kee, WI, USA). Cellulose acetate butyrate (CAB; butyryl content
35–39%), carbonyldiimidazole (CDI), diisopropylcarbodiimide
(DIC), acetone, acetonitrile (MeCN), dichloromethane (DCM),
N,N-dimethylformamide (DMF), chloroform and tetrahydro-
furan (THF) were obtained from Acros Chemical (Pittsburgh,
PA, USA). All chemicals were used as received.
In this study, PE-co-MAA nanofibrous membranes were
prepared and surface functionalized to examine the feasibility as
solid support media. Protoporphyrin IX (PPIX), a widely studied
photosensitizer to generate singlet oxygen, was selected as
a model photosensitizer and covalently immobilized onto the
activated PE-co-MAA nanofibrous membranes (Scheme 1). Due
to the close adjacency to solid surfaces, the performance of
immobilized photoactive molecules could be directly influenced
by the nature of the support material.20 Therefore, three diamine
spacers with different lengths were employed to anchor PPIX to
PE-co-MAA polymer backbones. The catalytic activity of the
supported PPIX was evaluated through a photo-oxidation
reaction of 1,5-dihydroxynaphthalene in organic solutions
with irradiation of visible light. And the effects of spacers,
2. Fabrication of PE-co-MAA nanofibrous membranes
PE-co-MAA nanofibers were prepared according to a previously
published procedure.19 Briefly, CAB as a sacrificial matrix was
mixed with PE-co-MAA in a blend ratio of CAB/PE-co-MAA ¼
85/15, which was gravimetrically fed into a Leistritz co-rotating
twin-screw (18 mm) extruder (Model MIC 18/GL 30D, Nurn-
berg, Germany) at a feed rate of 12 g minꢀ1. The blends
were extruded into composite fibers through a two strand
Scheme 1 Surface functionalization and PPIX immobilization on PE-co-MAA nanofibrous membranes.
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À
Á
(2 mm in diameter) rod die, hot-drawn by a take-up device with
a drawing ratio of 25 (the area of cross-section of the die to that
of the extrudates) and air cooled to room temperature. And then,
PE-co-MAA nanofibers were obtained via Soxhlet extraction of
acetone to remove CAB from the CAB/PE-co-MAA composite
fiber. The prepared nanofibers were subsequently made into
stable suspensions and deposited onto a polyester monofilament
fabric as a releasing surface. After evaporation of solvents, the
PE-co-MAA nanofibrous membrane was formed. The quantifi-
cation of loading capacity of surface carboxylic acid groups on
the prepared membrane was evaluated by an acid/base titration
method described before.21
1000 ꢁ A
M ꢁ 7800 ꢁ D
Primary amine loading mmol gꢀ1
¼
(1)
where A is the UV absorbance, M is the weight of membranes in
the unit of mg and D is the dilution factor which is determined as
0.00125 in this study. Meanwhile, the original PE-co-MAA
nanofibrous membrane treated under the same procedure was
tested as a control.
4. Immobilization of PPIX onto aminated PE-co-MAA
nanofibrous membranes
To incorporate photosensitizers onto membrane surfaces,
0.15 mmol of PPIX was first mixed with an equal amount of CDI
in 5 mL DMF. The mixture was heated to 60 ꢂC and vigorously
shaken for 4 h. Then, 20 mg of the aminated membranes was
added, and the reaction continued overnight. After that, the
PPIX immobilized membranes were extensively washed with
DMF (20 mL ꢁ 3) and MeCN (20 mL ꢁ 3) to remove physically
absorbed PPIX and any other unreacted chemical. Element
analysis was also used to determine the total nitrogen content of
the PPIX immobilized membranes and the loading capacity of
immobilized PPIX was calculated from the difference of nitrogen
content before and after PPIX immobilization.
3. Surface functionalization of PE-co-MAA nanofibrous
membranes
Variable activation agents, PCl5, thionyl chloride, oxalyl chlo-
ride, CDI, DIC/HOBt and HBTU, were first used to activate the
surface carboxylic acid groups on PE-co-MAA nanofibrous
membranes, respectively. Typically, 0.2 mmol of PCl5 was dis-
solved in 20 mL DCM, and then 40 mg of PE-co-MAA nano-
fibrous membranes was immersed in this solution. This mixture
was gently shaken for 4 h at room temperature, and then the
resulting membranes were washed with DCM (20 mL ꢁ 3). A
similar reaction condition was used for other activation agents
except the reaction time for CDI was 12 h.
5. Characterization
To attach spacers onto membrane surfaces, 40 mg of the
activated membranes were immersed into 20 mL DCM, in which
1 mmol of a diamine spacer was previously dissolved. For the
membranes activated by PCl5, thionyl chloride and oxalyl chlo-
ride, 1.5 mmol of DIPEA was also added into the reaction
solution. After shaking overnight at room temperature, ami-
nated membranes were washed with DCM (20 mL ꢁ 2), MeCN
(20 mL ꢁ 2) and distilled water (20 mL ꢁ 2), and then dried in
a vacuum. The prepared membranes from 1,3-diaminopropane,
1,8-diamine-3,6-dioxaoctane and 4,9-dioxa-1,12-dodecanedi-
amine were assigned to PE-co-MAA1, PE-co-MAA2 and
PE-co-MAA3, respectively.
Morphologies of nanofibrous membranes were observed by
using a scanning electron microscope (SEM) (XL 30-SFEG, FEI/
Philips, USA) at 5 kV accelerating voltage on gold sputter coated
samples. Attenuated total reflectance-Fourier transform infrared
(ATR-FTIR) spectra were measured by a Nicolet 6700 spec-
trometer (Thermo Fisher Scientific, USA). A laser Raman
microscope (Renishaw RM1000) with excitation at 514 nm was
used to obtain Raman spectra. A PDZ Europa ANCA-GSL
elemental analyzer interfaced to a PDZ Europa 20-20 isotope
ratio mass spectrometer (Sercon Ltd., Cheshire, UK) was
employed for element analysis. 1H NMR was measured using an
Avance DRX-500 MHz NMR spectrometer (Bruker BioSpin,
Billerica, MA) with XWINNMR software (version 3.5). Mass
spectrometry analysis was performed with a Waters Micromass
ZQ 2000 (ESI-MS) mass spectrometer.
The total amount of amine groups on the aminated
PE-co-MAA nanofibrous membrane was determined by element
analysis of total nitrogen content with the original PE-co-MAA
membrane as a control. The quantification of accessible primary
amine loading on the membrane surfaces was tested by Fmoc
analysis.21,22 Specifically, 5 mmol Fmoc-Gly-OH in 20 mL DMF
was activated by equal amounts of DIC and HOBt for 30 min at
room temperature, followed by adding 50 mg of aminated
PE-co-MAA nanofibrous membrane. The mixture was shaken
overnight to couple all accessible primary amine on membrane
surfaces with N-terminal protected amino acid. At the end of this
coupling reaction, the resulting membrane was washed with
DMF (20 mL ꢁ 2) and MeCN (20 mL ꢁ 2). After being dried in
a vacuum to a constant weight, 50 mg of Fmoc-protected glycine
attached dry PE-co-MAA membranes were treated by 10 mL
methyl piperidine/DMF solution (volume ratio 1 : 4) for 4 h,
from which 50 mL of the reaction solution was collected and
diluted to 4 mL. UV absorbance (300 nm) of the diluted solution
was measured with a UV spectrometer (Evolution 600, Thermo,
USA). And the primary amine loading was calculated from
eqn (1):
6. Photo-oxidation reactions catalyzed by supported
protoporphyrin IX (PPIX)
1 mmol of 1,5-dihydroxynaphthalene was dissolved in 20 mL of
MeCN in which oxygen gas was purged for 10 min. According to
the desired substrate/catalyst ratio, a certain amount of free or
supported PPIX was added into the mixture, respectively, to
initiate the photo-oxidation reaction. And the reaction media
were irradiated by three 50 W lamps which were filtered by
a 410 nm cut filter to shield UV light. The reaction rate was
monitored by a UV-vis spectroscope at 420 nm. After the reac-
tion was completed, the supported PPIX was removed and
washed thoroughly with MeCN until no substrate in the washing
solutions was detected by the measurement of UV-vis absor-
bance. At the end, the reaction solution and all washing solutions
were collected and evaporated, the subsequent residue was
separated by flash chromatography on a Combi-flash column
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(Isco, Inc., Nebraska, USA) with chloroform/MeCN mixture
(volume ratio 9 : 1) as an eluent. The resulting oxidative product
was characterized via 1H NMR, mass spectra, and FTIR,
respectively, and identified as 5-hydroxy-1,4-naphthalenedione.
This reaction was also carried out in chloroform or THF to study
the solvent effect. Moreover, air gas or nitrogen gas was purged
into the reaction solution to study the effect of oxygen concen-
tration. To achieve a reliable result, each test was repeated three
times at least and all of the results reported were the average
value of the replicated tests.
membrane morphology remained intact after photosensitizer
immobilization and catalytic photo-oxidation reactions,
distinctly shown in Fig. 1(b) and (c), suggesting that PE-co-MAA
nanofibrous membranes were chemically stable for surface
functionalizations and as support media for catalytic
applications.
The amount of carboxylic acid groups from PE-co-MAA
polymers was calculated as 1.74 mmol gꢀ1 based on the weight
percentage of methacrylic acid segments. However, the actual
surface density of these carboxylic acid groups on PE-co-MAA
nanofibrous membranes was only around 0.72 mmol gꢀ1
,
5-Hydroxy-1,4-naphthalenedione. m/z ¼ 174 [M + H]+; 1H
NMR (500 MHz, CDCl3)/ppm: 7.59 (1H, d, H-naph.), 7.61 (1H,
d, H-naph.), 7.92–7.97 (3H, m, H-naph.), 12.22 (1H, s, OH);
FTIR (KBr): 1601 cmꢀ1 (aromatic bond), 1664 and 1640 cmꢀ1
(aromatic ketone).
measured by an acid/base titration method. The reduced value
was probably due to a result that certain carboxylic acid groups
were embedded inside the polymer after the formation of the
nanofibrous membranes, leading to only 41.38% of the existing
carboxylic acid groups accessible. Also, the hydrophobic nature
of PE-co-MAA polymer provided unfavorable contact between
the membrane surfaces and aqueous solution, enabling some
reactive sites inside the membranes undetectable by the titration
method mentioned above.
Results and discussion
1. Preparation of PE-co-MAA nanofibrous membrane
supported protoporphyrin IX
1.2. Activation of nanofibrous membranes. The surface
density of reactive sites, their reactivity and accessibility on
nanofibrous membranes have major impacts on the efficiency of
surface functionalization. To enhance the reactivity of carboxylic
acid groups on the surfaces of PE-co-MAA nanofibrous
membranes, six activation agents were initially employed to
activate the carboxylic acid groups which further reacted with
1,3-diaminopropane. And the activation efficiencies on the acid
groups were evaluated based on the amount of resulting primary
amines determined by Fmoc analysis. As Fig. 2 shows, PCl5 was
found to be the most effective activation agent, which can effi-
ciently and rapidly convert carboxylic acid into acid chloride, the
most reactive acid derivative with amine groups. Thionyl chlo-
ride and oxalyl chloride, as agents that can form acid chlorides,
also afforded a relatively high amount of primary amine loadings
on the membranes in comparison to three other common
coupling agents, which proceeded by forming intermediates with
the catalysts and then having nucleophilic substitution by amino
1.1. Fabrication of nanofibrous membranes. Poly(ethylene-co-
methacrylic acid) (PE-co-MAA), a random copolymer consisting
of ethylene and methacrylic acid segments with a weight blend
ratio of 85/15, was fabricated into nanofibers by using a recently
developed high throughput production process.17 The resultant
nanofibers were dispersed into stable suspensions and then
deposited onto releasable substrates to form PE-co-MAA
nanofibrous membranes. The morphologies of the membrane
surfaces were observed via SEM, and the results are presented in
Fig. 1. The original PE-co-MAA nanofibrous membranes
possessed an open porous non-woven like structure with uniform
and continuous nanofibers. The average diameter of these
nanofibers was measured as 210.76 nm. In addition, the unique
Fig. 1 SEM images of original PE-co-MAA nanofibrous membranes
(a), PPIX immobilized PE-co-MAA nanofibrous membranes (b), PPIX
immobilized PE-co-MAA nanofibrous membranes after 5 repeated cycles
of photo-oxidation reactions (c) and the distribution of fiber size of
original PE-co-MAA nanofibrous membranes (d).
Fig. 2 Primary amine loadings of PE-co-MAA nanofibrous membranes
activated by different activation agents.
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groups. Having determined the superior performance of PCl5,
this activation method was used in the following studies.
The direct adjacency of carboxylic acid groups to their poly-
mer backbones leads to steric hindrance from the polymer chain,
limiting the accessibility of these reactive sites to macrocyclic
porphyrins. The introduction of spacers was a promising strategy
to circumvent this drawback.23 In this study, three diamine
spacers with various molecular lengths were used to investigate
their influence on the surface functionalization and subsequent
catalytic performance (Table 1). The chemical structure changes
of PE-co-MAA nanofibrous membranes during the activation
with PCl5 and amination with spacers were observed via ATR-
FTIR (Fig. 3). The characteristic peak of carboxylic acid groups
at 1701 cmꢀ1 from the original membrane disappeared after the
activation reaction, instead, the specific peak for acid chloride
groups was found around 1798 cmꢀ1. For all three aminated
membranes with different spacer chains, two strong peaks
appeared at 1641 and 1528 cmꢀ1, assigned to C]O stretching
(Amide I) and N–H bending (Amide II), respectively, which
indicated the covalent amide linkage between the membrane
surfaces and diamine spacers.24 In addition, the new peaks
around 1120 cmꢀ1 were assigned to C–O stretching from alkyl-
oxide groups of longer spacer chains. The ATR-FTIR spectra
demonstrated the presence of diamine spacers covalently bonded
to the membrane surfaces.
Fig.
3
ATR-FTIR spectra of: original PE-co-MAA nanofibrous
membranes (a), PCl5 activated PE-co-MAA nanofibrous membrane (b),
1,3-diaminopropane aminated membranes (PE-co-MAA1) (c), 1,8-
diamine-3,6-dioxaoctane aminated membrane (PE-co-MAA2) (d), and
4,9-dioxa-1,12-dodecanediamine aminated membrane (PE-co-MAA3) (e).
To further investigate the effect of the spacers on surface
functionalization, the total amounts of amine groups on the
aminated PE-co-MAA nanofibrous membranes were assessed via
elemental analysis. The total amine loading values from Table 2
were found to be inversely related to the molecular lengths of the
used spacer chains, possibly due to the fact that the long spacer
chains might have lost reactivity because of potential inter- or
intramolecular entanglement. Basically, each activated
carboxylic acid group on the membrane surface should react with
one end-amine group from each diamine spacer to give one free
end-amine group for subsequent reactions. As a result, the
amount of the expected primary amine groups can be calculated
as half the amount of total amine groups that were reacted on the
membranes with values of 0.365, 0.216 and 0.186 mmol gꢀ1 for
PE-co-MAA1, PE-co-MAA2 and PE-co-MAA3, respectively.
However, the measured amounts of accessible primary amine, by
the Fmoc analysis, were found to be lower than the ones as
expected. Also shown in Table 2, the retention rates (measured
primary amine/calculated primary amine) of the primary amines
followed the order: PE-co-MAA1 (85.2%) > PE-co-MAA2
(72.6%) > PE-co-MAA3 (70.0%). This was ascribed to the fact of
crosslinking effect from the reactions between both two end-
amine groups and two surface acid chloride groups. And the
chance of this crosslinking reaction increased with the increment
of molecular length of the spacer chains.20
1.3. Covalent immobilization of protoporphyrin IX (PPIX).
The utilization of the prepared membranes as solid support
media for photosensitizers was demonstrated by using proto-
porphyrin IX (PPIX). According to Scheme 1, PPIX was cova-
lently attached to the aminated PE-co-MAA nanofibrous
membranes via nucleophilic substitution reaction between
carboxylic acid groups of PPIX and the primary amine groups on
the membrane surfaces. A coupling agent, N,N0-carbony-
ldiimidazole (CDI), was used to facilitate this reaction by acti-
vating the carboxylic acid groups to imidazole carboxylic ester
and then enabling the subsequent substitution reaction with the
primary amine groups. The resulting PE-co-MAA nanofibrous
membrane supported PPIXs with three different diamine spacer
lengths, 1,3-diaminopropane, 1,8-diamine-3,6-dioxaoctane and
4,9-dioxa-1,12-dodecanediamne, are referred to as PE-co-MAA-
PPIX1, PE-co-MAA-PPIX2 and PE-co-MAA-PPIX3, respec-
tively. Due to the symmetric molecular structure of PPIX,
Table 1 Various spacers used for surface activation of PE-co-MAA nanofibrous membranes
Spacers
Chemical structures
Length
Activated membranes
1,3-Diaminopropane
5 atoms
PE-co-MAA1
PE-co-MAA2
1,8-Diamine-3,6-dioxaoctane
10 atoms
4,9-Dioxa-1,12-
dodecanediamine
14 atoms
PE-co-MAA3
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Table 2 Surface functionalization of PE-co-MAA nanofibrous membranes with spacers and PPIX
Total amine loadinga Primary amine loadingb Retention ratec PPIX loading Yieldsd
Aminated membranes mmol gꢀ1
mmol gꢀ1
%
mmol gꢀ1
%
Supported photosensitizer
PE-co-MAA1
PE-co-MAA2
PE-co-MAA3
0.730 ꢃ 0.058
0.311 ꢃ 0.016
0.157 ꢃ 0.011
0.139 ꢃ 0.008
85.2
72.6
70.0
0.023 ꢃ 0.002 10.6
0.063 ꢃ 0.008 40.1
0.067 ꢃ 0.009 48.2
PE-co-MAA-PPIX1
PE-co-MAA-PPIX2
PE-co-MAA-PPIX3
0.432 ꢃ 0.041
0.372 ꢃ 0.034
Total amine loading was determined by elemental analysis. b Primary amine loading was determined by Fmoc analysis. c Retention rate ¼ [primary
a
amine loading/(total amine loading/2)] ꢁ 100%. d Yields ¼ (PPIX loading/primary amine loading) ꢁ 100%.
Raman spectroscopy was used to observe the changes of chem-
ical structures during the PPIX immobilization (Fig. 4). In the
case of pure PPIX, two pairs of specific peaks around 1600 and
1350 cmꢀ1 were assigned to the vinyl groups and pyrrole rings
from the macrocyclic molecule,25,26 respectively, which were not
found from the aminated PE-co-MAA nanofibrous membranes.
However, the presence of these characteristic peaks from the
PPIX immobilized membranes illustrated the successful immo-
bilization of PPIX onto the nanofibers. Also, the quantitative
amounts of the immobilized PPIX on the membrane surfaces
were evaluated by elemental analysis of total nitrogen atoms
(Table 2). In comparison with other aminated membranes, a low
value of immobilized PPIX was found for PE-co-MAA1 with the
shortest spacer chain in spite of the high value of accessible
primary amine groups. This can be related to the possibility that,
due to the close proximity of primary amine groups to the
membrane surfaces, high steric hindrance blocked the approach
of the macrocyclic compound, PPIX, to these reactive sites,
leading to low reaction efficiency. In contrast, the amounts of
immobilized PPIX significantly enhanced by using longer
spacers, and the values were about 0.063 and 0.067 mmol gꢀ1 for
PE-co-MAA2 and PE-co-MAA3, respectively. Furthermore, the
reaction efficiency of the primary amines on the membrane
surfaces also improved proportionately to the increase of chain
length of the three used spacers. These results further revealed
that a longer spacer chain was preferred for the immobilizations
of the macrocyclic molecule onto the surfaces of the nanofibrous
membranes.
2. Photo-oxidation reactions catalyzed via the supported
protoporphyrin IX (PPIX)
2.1. Photo-oxidation of 1,5-dihydroxynaphthalene. It has
been reported that PPIX as an organic photosensitizer is capable
of catalyzing photo-oxidation reactions by efficiently generating
singlet oxygen under light exposure.27 In this study, the catalytic
performances of free PPIX and PE-co-MAA nanofibrous
membrane supported PPIX were investigated based on the
photo-oxidation of 1,5-dihydroxynaphthalene, which involves
1,4-cycloaddition of singlet oxygen onto a phenyl ring and
production of the corresponding 5-hydroxy-1,4-naph-
thalenedione8 (Scheme 2). The UV-Vis absorbance spectra of
1,5-dihydroxynaphthalene in the presence of the supported
photosensitizer, PE-co-MAA-PPIX3 (substrate/catalyst molar
ratio 100 : 1), under irradiation of a visible light source as
a function of exposure time are presented in Fig. 5. The inten-
sities of three major peaks from 1,5-dihydroxynaphthalene, 298,
316 and 330 nm, gradually decreased with correspondingly
increased intensities of new peaks around 249 and 428 nm, which
are the characteristics of the oxidized product. Therefore, the
completion of this reaction was determined to be when the peak
intensity at 428 nm ceased increasing. After 9 h of irradiation, the
three original peaks from 1,5-dihydroxynaphthalene totally dis-
appeared, and the color of the reaction solution changed from
light pink to yellow. And then, the oxidized product was isolated,
characterized and identified as 5-hydroxy-1,4-naphthalenedione,
Fig. 4 Raman spectra of pure PPIX (a), PPIX immobilized on 4,9-
dioxa-1,12-dodecanediamine aminated membrane (PE-co-MAA-PPIX3)
(b) and 4,9-dioxa-1,12-dodecanediamine aminated membrane
(PE-co-MAA3) (c).
Scheme 2 Photo-oxidation reaction of 1,5-dihydroxynaphthalene cata-
lyzed by photosensitizers in the presence of oxygen and light irradiation.
10586 | J. Mater. Chem., 2012, 22, 10581–10588
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Longer spacer chains enable the surface immobilized photosen-
sitizers to act with a higher freedom, which is more similar to
their free analogues in reaction solutions. Therefore, PE-co-
MAA-PPIX3 with the highest catalytic activity was employed for
further explorations.
The effect of substrate/catalyst molar ratio was further studied
by adjusting the value at 500 : 1 and 1000 : 1 (entries 5 and 6,
Table 3). As expected, lower yields of the oxidized product and
longer time required to finish the photo-oxidation reaction were
found. Furthermore, the solvent systems could have an impact
on the photo-oxidation reactions, including varied oxygen
solubility and different lifetimes of generated singlet oxygen.29
Solvents could also swell the solid matrix and enhance contact
between the supported photosensitizers with reaction media,
leading to an improved catalytic performance.30 Three organic
solvents were used in this study in which both oxygen solubility
and singlet oxygen lifetimes followed the order: chloroform
> MeCN > THF. As shown in entries 4, 7 and 8 in Table 3, the
solvent with the most powerful catalytic effect was chloroform,
in which more than 99% of the reaction yield was achieved within
8 h. However, the solid supported photosensitizer in THF
exhibited significantly lower catalytic activity than the ones
observed in the other two solvents. Although chloroform and
THF are more favorable to the hydrophobic PE-co-MAA
nanofibrous membranes than MeCN, the remarkable difference
of catalytic performances of PE-co-MAA-PPIX3 in these three
solvent systems suggested that the oxygen solubility and singlet
oxygen lifetimes were the predominant factors influencing this
photo-oxidation reaction. Due to the oxygen involved reaction
mechanism, the effect of oxygen concentration was also studied
by purging pure oxygen, air and nitrogen to the reaction media,
respectively (entries 4, 9 and 10 in Table 3). The dramatically
reduced reaction rates and yields with the decrease of oxygen
concentration indicated the crucial importance of oxygen in this
Fig. 5 UV-vis spectra of 1,5-dihydroxynaphthalene in MeCN during the
photo-oxidation reaction catalyzed by PE-co-MAA-PPIX3. The inset
presents the color difference of the reaction media before and after the
photo-oxidation reaction.
which was consistent with the literature.8,28 Furthermore, no
product was found after 24 h of irradiation by using the original
PE-co-MAA nanofibrous membrane as a blank experiment.
2.2. Effect of spacers, substrate/catalyst ratio, solvent media
and oxygen concentration. The influence of spacers on catalytic
activities of the supported PPIX was investigated by comparing
the isolated yields of the oxidized product and the reaction time
under the same conditions, and the results are illustrated in
entries 1 to 4 in Table 3. Among all the supported photosensi-
tizers, PE-co-MAA-PPIX3 containing the longest spacer
exhibited the highest catalytic activity, with the performance
comparable to unbound PPIX. In addition, as the spacer chain
length decreases, reduced yield of the oxidized product,
5-hydroxy-1,4-naphthalenedione, and prolonged irradiation
time to complete the photo-oxidation reaction were observed.
For PE-co-MAA-PPIX1 with the shortest spacer arm, it took
almost 20 h to reach the final yield of around 75%. Such a low
activity indicated that the catalytic function of the supported
PPIX was mainly dependent on the distinct structures of spacers.
Table 4 Reusability of supported PPIX for photo-oxidation of 1,5-
dihydroxynaphthalenea
Running time
1st
2nd
3rd
4th
5th
Isolated yields (%)
Reaction time (h)
97
9
96
10
93
12
89
12
80
13
a
Photosensitizer: PE-co-MAA-PPIX3; substrate/photosensitizer ratio:
100/1; solvent: MeCN purged with oxygen.
Table 3 Photo-oxidation of 1,5-dihydroxynaphthalene catalyzed by free and supported PPIX
a
Entry
Photosensitizers
R ¼ ns/nc
Solvent
Isolated yields (%)
Reaction time (h)
1b
2b
3b
4b
5b
6b
7b
8b
9c
PPIX
100 : 1
100 : 1
100 : 1
100 : 1
500 : 1
1000 : 1
100 : 1
100 : 1
100 : 1
100 : 1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Chloroform
THF
99
75
93
97
82
61
97
36
46
17
5
20
11
9
11
18
6
24
20
24
PE-co-MAA-PPIX1
PE-co-MAA-PPIX2
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
PE-co-MAA-PPIX3
MeCN
MeCN
10d
a
Molar ratio of substrates and photosensitizers. b Reaction media were previously purged with oxygen. c Reaction media were previously purged with
air. d Reaction media were previously purged with nitrogen.
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photo-oxidation reaction, similar to the findings in the
literature.31
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Acknowledgements
This research was financially supported by Defense Threat
Reduction Agency (HDTRA1-08-1-0005). J. Zhu is grateful to
Jastro-Shields Graduate Student Research Fellowship Award at
the University of California, Davis.
10588 | J. Mater. Chem., 2012, 22, 10581–10588
This journal is ª The Royal Society of Chemistry 2012