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Depth-profiling X-ray photoelectron spectroscopy (XPS) analysis of interlayer diffusion in polyelectrolyte multilayers

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Functional organic thin films often demand precise control over the nanometer-level structure. Interlayer diffusion of materials may destroy this precise structure; therefore, a better understanding of when interlayer diffusion occurs and how to control it is needed. X-ray photoelectron spectroscopy paired with C60(+) cluster ion sputtering enables high-resolution analysis of the atomic composition and chemical state of organic thin films with depth. Using this technique, we explore issues common to the polyelectrolyte multilayer field, such as the competition between hydrogen bonding and electrostatic interactions in multilayers, blocking interlayer diffusion of polymers, the exchange of film components with a surrounding solution, and the extent and kinetics of interlayer diffusion. The diffusion coefficient of chitosan (M = ∼100 kDa) in swollen hydrogen-bonded poly(ethylene oxide)/poly(acrylic acid) multilayer films was examined and determined to be 1.4*10(-12) cm(2)/s. Using the high-resolution data, we show that upon chitosan diffusion into the hydrogen-bonded region, poly(ethylene oxide) is displaced from the film. Under the conditions tested, a single layer of poly(allylamine hydrochloride) completely stops chitosan diffusion. We expect our results to enhance the understanding of how to control polyelectrolyte multilayer structure, what chemical compositional changes occur with diffusion, and under what conditions polymers in the film exchange with the solution.
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Depth-proling X-ray photoelectron spectroscopy
(XPS) analysis of interlayer diffusion in
polyelectrolyte multilayers
Jonathan B. Gilbert
a
, Michael F. Rubner
b,1
, and Robert E. Cohen
a,1
Departments of
a
Chemical Engineering and
b
Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
Edited by Michael L. Klein, Temple University, Philadelphia, PA, and approved March 12, 2013 (received for review December 20, 2012)
Functional organic thin lms often demand precise control over the
nanometer-level structure. Interlayer diffusion of materials may
destroy this precise structure; therefore, a better understanding of
when interlayer diffusion occurs and how to control it is needed. X-
ray photoelectron spectroscopy paired with C
60
+
cluster ion sputter-
ing enables high-resolution analysis of the atomic composition and
chemical state of organic thin lms with depth. Using this technique,
we explore issues common to the polyelectrolyte multilayer eld,
such as the competition between hydrogen bonding and electro-
static interactions in multilayers, blocking interlayer diffusion of
polymers, the exchange of lm components with a surrounding so-
lution, and the extent and kinetics of interlayer diffusion. The diffu-
sion coefcient of chitosan (M =100 kDa) in swollen hydrogen-
bonded poly(ethylene oxide)/poly(acrylic acid) multilayer lms was
examined and determined to be 1.4*10
12
cm
2
/s. Using the high-
resolution data, we show that upon chitosan diffusion into the hy-
drogen-bonded region, poly(ethylene oxide) is displaced from the
lm. Under the conditions tested, a single layer of poly(allylamine
hydrochloride) completely stops chitosan diffusion. We expect our
results to enhance the understanding of how to control polyelectro-
lyte multilayer structure, what chemical compositional changes occur
with diffusion, and under what conditions polymers in the lm
exchange with the solution.
XPS depth proling
|
layer-by-layer lms
|
interdiffusion
Layer-by-layer assembly of polyelectrolyte multilayers (PEMs)
allows for the precise deposition of ultrathin organic lms that
can conformally coat features of any shape and size. These lms can
incorporate a variety of species, leading to a wide range of appli-
cations, including antifogging (1), antireection (2), drug delivery
(37), fuel cells (8), and responsive materials (9). Because the
multilayer lms are assembled through a sequential self-limiting
adsorption process onto a substrate (10), a major advantage of the
technique is the ability to constrain the location of certain materials
within the lm at the nanoscale simply by controlling the order of
material deposition.Such nanoscale spatial control has allowed the
creation of complex periodic heterostructures not easily realized by
other deposition techniques (11). In some cases, however, diffusion
of the constituent macromolecular species in and out of the lm
may occur, changing the desired lm stratication, composition,
growth prole, and properties (12). This type of diffusion, known as
interlayer diffusion, often is detrimental to the desired properties
but also might be an opportunity to impart new functions. However,
interlayer diffusion has proven difcult to fully characterize and
control and a more thorough understanding is needed.
Interlayer diffusion in PEM lms may be detrimental if the de-
sired stratied heterostructure is lost during the assembly process,
during a postassembly treatment, or in use. For example, in the
cases of the sequential release of therapeutics (7), structural color
(13), organic light-emitting diode devices (14), solar cells (15), and
on-demand release of PEM lms (5, 16), the loss of stratication
due to interlayer diffusion results in loss of the desired function.
However, in other cases, such as surface planarization for the cre-
ation of higher-efciency dye-sensitized solar cells (15) or ordering
of the internal or surface arrangement of PEM lms(17,18),in-
terlayer diffusion may be harnessed to provide functional benets.
Interlayer diffusion also may be used to tune material properties
such as the critical dissolution pH in hydrogen-bonding systems
through the addition of small amounts of electrostatic cross-links
(16) or viscoelasticity through diffusion of stiffer polymer compo-
nents (19). In all these cases, interlayer diffusion must be un-
derstood and controlled.
Interlayer diffusion also affects the mechanism of PEM lm
growth. Some polymer systems show linear growth where the bi-
layer thickness is invariant with deposition cycles, whereas other
systems show exponential growth with progressively increasing bi-
layer thicknesses (20). It is widely believed that exponential growth
arises from rapid interlayer diffusion of polymers throughout the
lm during the fabrication steps (20, 21); however, some disagree
with this conclusion (22). Improved analytic techniques that pro-
vide spatial information about the location of specicmolecules
within a multilayer thin lm therefore clearly are needed.
Because of the importance of understanding interlayer diffusion,
a variety of techniques have been used to analyze it, with varying
degrees of success. These techniques include confocal microscopy
(2325), FRET (26, 27), FTIR (28), neutron reectivity (2931),
and X-ray reectometry (32). Confocal microscopy is limited in
spatial sensitivity, as lms much thicker than the typical PEM thick-
ness (500 nm) are required because of a relatively low z-resolution
(2325). FRET is more sensitive but relies on uorescent modi-
cation of polymers for indirect measurements of diffusion (26, 27).
FTIR may provide valuable information on the exchange of poly-
mers in solution with PEM lm components but commonly probes
the full thickness of the lm, limiting the ability to spatially resolve
the effects of diffusion (28). Neutron reectivity and X-ray re-
ectometry (2932) require nuclear contrast and electron density
contrast, respectively, in the lm and commonly require the use of
special deuterated polymers. In comparison, X-ray photoelectron
spectroscopy (XPS) is a highly sensitive surface analysis method
that probes the top 10 nm of a lm. When combined with sputtering
or etching sources to remove material slowly between analysis
cycles without damaging underlying material, depth-proling XPS
enables high-resolution chemical analysis of polymer lms. The
information provided by this technique might expand the un-
derstanding of how to control PEM structure, what compositional/
structural changes occur with interlayer diffusion, and when poly-
mers in the lm exchange with deposition/postassembly solutions.
Development of less destructive sputtering or etching sources has
been the enabling step in advancing polymer depth-proling capa-
bilities. Many depth-proling techniques use single-atom sputtering
sources such as argon, applicable to inorganic materials but severely
damaging to polymers (3335). Only recently have cluster ion
sources such as C
60
+
been used in conjunction with XPS for analysis
of polymer lms with depth (33, 36, 37). Cluster ion C
60
+
sputtering
Author contributions: J.B.G., M.F.R., and R.E.C. designed research; J.B.G. performed re-
search; J.B.G., M.F.R., and R.E.C. analyzed data; and J.B.G., M.F.R., and R.E.C. wrote
the paper.
The authors declare no conict of interest.
This article is a PNAS Direct Submission.
1
To whom correspondence may be addressed. E-mail: recohen@mit.edu or rubner@mit.
edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1222325110/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1222325110 PNAS
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CHEMISTRY
is much less damaging because the energy transfer from the ion to
the material occurs primarily at the surface, minimizing the chem-
ical damage deep into the lm(38).Therefore,mostofthedamaged
material is removed from the surface, minimizing its interference
with the proper analysis of the exposed surface (39).
One strategy in the fabrication of functional PEM lms is the use
of blocking layers to minimize interlayer diffusion. Earlier studies
(7, 24, 4042) showed that the properties of a successful blocking
layer depend on the diffusing species under consideration and the
conditions of diffusion. Some have found that covalent cross-link-
ing is the only way to stop interlayer diffusion of polymers (7, 43),
whereas others have noted that electrostatic interactions may be
used to stop interlayer diffusion (24, 44, 45). Of interest to this study
is how blocking layers enable the controlled production of free-
oating PEM lms by maintaining the desired dissolution proper-
ties of a sacricial region that anchors a pH-stable PEM lm to
a substrate surface. Once released, these free-oating assemblies
have been used for tissue engineering (46) and drug delivery (5, 6).
In this paper, we designed a model system that enables the study
of common attributes of interlayer diffusion found in many PEM
systems, including blocking-layer effectiveness. The sacricial com-
ponent of this model PEM system is a hydrogen-bonded region
[poly(acrylic acid)/poly(ethylene oxide)] (PAA/PEO) that is in-
soluble at low pH but becomes soluble at a critical higher pH. A
pH-stablePEM system based on chitosan and hyaluronic acid (HA)
is assembled on top of this sacricial region with the goal of creating
an on-demandpH-triggered release of the chitosan /HA multilayer
lms. Previously, we used a related approach to create cellular
backpacks that attach to immune system cells via specic inter-
actions between HA and CD-44 receptors on the cell surface (5).
From these earlier studies, it became apparent that the assembly of
chitosan /HA onto a hydrogen-bonded region rendered the entire
multilayer system insoluble under pH conditions that should dis-
solve the sacricial region. Thus, this work seeks to determine
whether interlayer diffusion of chitosan/HA causes the changes in
solubility and how suitable blocking layers can prevent these
changes. From a fundamental perspective, this model system allows
the exploration of elements such as the competition between hy-
drogen bonding and electrostatic interactions in multilayers, the
design of an effective blocking layer, the exchange of lm compo-
nents with a surrounding solution, and the extent and kinetics of
interlayer diffusion. Using XPS depth-proling data acquired with
C
60
+
cluster ion sputtering, we nd that chitosan diffuses effectively
into the hydrogen-bonded region of the multilayer lm and dis-
places the hydrogen-bonded component PEO. In addition, weshow
that this interlayer diffusion process may be blocked completely
with only a single adsorbed layer of a polycation.
Results and Discussion
The model PEM lms examined in this work were assembled on
glassslidescoatedwithan80-nmpoly(diallyldimethylammonium
chloride) (PDAC) and poly(styrene sulfonate) (SPS) adhesion-
promoting multilayer. Above this adhesion layer, was a hydrogen-
bonded lm composed of PAA and PEO. The solubility of this hy-
drogen-bonded PEM system is pH sensitive, and above a pH of 3.6, it
will dissolve in water (47). Fig. 1 shows a cartoon representation of
the multilayer heterostructures examined in this work, along with
the experimentally determined thicknesses of the various regions of
the multilayer. In Fig. 1A, the experiments involved immersing
a hydrogen-bonded multilayer lm in a chitosan solution for vary-
ing amounts of time. In Fig. 1B, blocking layers containing varying
numbers of poly(allylamine hydrochloride) (PAH) and SPS layers
were deposited on top of the hydrogen-bonded region followed by
the assembly of a multilayer of HA and chitosan.
These stratied lms were dried then analyzed using depth-pro-
lingXPSpairedwithC
60
+
sputteringtocollectC1s,O1s,N1s,and
Si2p high-resolution spectra. It is important to note that prolonged
X-ray exposure and C
60
+
sputtering may alter the chemical com-
position of PEMs and decrease the interface resolution (36, 48). As
described in Fig. S1, the choice of XPS data acquisition parameters
and sputtering conditions is very important because long periods of
X-ray exposure reduced the O-to-C ratio, particularly the signal of
the carboxyl peak at 289 eV (49). As a result, we chose acquisition
parameters and C
60
+
sputtering conditions to minimize the total X-
ray exposure time while still obtaining an acceptable resolution and
signal-to-noise ratio at each point in the depth prole.
Diffusion of Chitosan in Hydrogen-Bonded Multilayers. To explore
the question of whether the adsorbed chitosan diffuses into the
swollen hydrogen-bonded region and by how much, the nitrogen
signal from the amine on chitosan was analyzed as a function of
depth for hydrogen-bonded multilayers exposed to a 0.1% (wt/vol)
CHI solution at pH 3 for a specied amount of time. The chitosan
solution acted as an innite supply for diffusion of chitosan into the
hydrogen-bonded region. All samples were rinsed with water for 4
min and dried with nitrogen gas before analysis. The compiled
spectra for chitosan exposure times of 1, 3, 10, and 60 min (CHI1,
CHI3, CHI10, and CHI60) are plotted in Fig. 2 AD. Color was
added to highlight the approximate locations of the distinct regions
of the PEM lm, using the same color scheme shown in Fig. 1A.
The depth of the (red) chitosan region was determined by ana-
lyzing the intensity of the N1s signal with depth. When the N1s
signal dropped to background levels, the spectrum was colored
yellow to denote the hydrogen-bonded region. Finally, the (black)
adhesion layer starts when the N1s signal increases at the base of
the lm as a result of the presence of nitrogen-containing PDAC.
The spectra from Fig. 2 ADwere analyzed to determine the
atomic percentage of nitrogen with depth, as seen in Fig. 2E.Alm
not exposed to chitosan is shown in Fig. S2. The concentration of
nitrogen in the multilayer lm increased systematically with time of
exposure to the chitosan solution. Also, the maximum depth at
which an appreciable nitrogen signal was observed increased with
time. Separate experiments showed that the PDAC from the ad-
hesion layers does not enter the hydrogen-bonded region during the
assembly process, even after many hours at pH 3 (Fig. S2). Thus, the
only source of nitrogen in the lm, above the 80-nm adhesion layer,
is from the chitosan that diffused from the top of the hydrogen-
bonded region. For the samples CHI1, CHI3, and CHI10, the lo-
cation of the diffusion front (where N1s concentration is 50% of the
maximum value) advanced 181, 238, and 299 nm, respectively, as
measured in dry lms. Because the nal dry thickness remained
relatively constant, independent of chitosan diffusion depth, the
diffusion of chitosan does not expand or collapse the lm greatly. As
a result, the dry diffusion distance of chitosan directly correlates with
the thickness of the portion of the (PAA3/PEO3) lm that was al-
tered by chitosan diffusion. Therefore, to estimate the diffusion
coefcient, the chitosan penetration distances were multiplied by
a factor of 2.5 to account for the 250% swelling of the (PAA3/
PEO3) lm in pH 3 water (Table 1 and Table S1). After 60 min in
the chitosan solution (CHI60), the chitosan diffused through the
entire hydrogen-bonded region, as seen by the uniformly high ni-
trogen content throughout the lm in Fig. 2E. These results clearly
show that the adsorbed chitosan diffuses into the hydrogen-bonded
region. Chitosan is known to be highly diffusive because of a charge
density lower than that of typical polyamines, such as PAH, and the
presence of multiple hydrogen-bonding acceptors (25, 26).
To estimate the diffusion coefcient of chitosan into the hy-
drogen-bonded region, we used the data in Table 1 and the
Fig. 1. Schematic of systems used to test (A) chitosan (CHI) diffusion into the
hydrogen-bonded region and (B) electrostatic blocking-layer effectiveness.
The number after the polymer abbreviation is the deposition solution pH.
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www.pnas.org/cgi/doi/10.1073/pnas.1222325110 Gilbert et al.
characteristic diffusion length, L=
ffiffiffiffiffiffiffi
4Dt
p(50). As seen in Table 1,
the calculated diffusion coefcient is consistent for the three time
points sampled and is 1.4*10
12
cm
2
/s. Recent reports on in-
terlayer diffusion coefcients in polyelectrolyte multilayers range
from 10
20
cm
2
/s for SPS in linearly growing (PAH/SPS) lms (29)
to 10
7
cm
2
/s for poly(L-lysine) in exponentially growing poly(L-
lysine)/HA lms (20). This wide range of reported interlayer dif-
fusion coefcients is the result of a fundamental difference in the
lm growth mechanism between linearly and exponentially grow-
ing lms. In linear growth conditions, the deposited polymers
generally interact only with the top surface and thus generally have
interlayer diffusion coefcients below 10
17
cm
2
/s (2931). In
comparison, exponentially growing systems require some amount
of interlayer diffusion to occur during the dipping cycle (20) and, as
a result, have higher reported interlayer diffusion coefcients, in
the range of 10
16
to 10
7
cm
2
/s depending on the conditions and
polyelectrolytes used (20, 26). A recent paper by Lundin et al. (26)
used FRET and showed that the interlayer diffusion of chitosan in
exponentially growing lms made of chitosan and heparin was
10
15
cm
2
/s for 150-kDa chitosan. Our reported interlayer dif-
fusion coefcient of 10
12
cm
2
/s for chitosan of roughly the same
molecular weight is larger but well within the range of other ex-
ponentially growing polymer systems previously studied. In addi-
tion, the diffusion coefcient we report would be higher than the
diffusion coefcient of chitosan in a pure lm of chitosan/heparin,
as Xu et al. (30) showed that weaker matrix interactions enable
a higher diffusion coefcient. Given that in our study chitosan
diffused in a hydrogen-bonded (PAA3/PEO3) matrix with a low
interaction strength, as measured by dissolution pH, and did not
have an internal structure, as measured by neutron reectivity
(51), it thus would allow for a diffusion coefcient higher than that
of the more strongly interacting electrostatic matrix of chitosan
and heparin.
Displacement of PEO from Film by Chitosan. One possible effect of
interlayer diffusion in PEMs is the displacement of materials from
the lm into solution (28, 40, 52). In our model system, interlayer
diffusion of chitosan into the hydrogen-bonded region changes the
dominant interaction from hydrogen bonding between PAA and
PEO to electrostatic interactions between PAA and chitosan. FTIR
conrmed these new electrostatic interactions. In lms with large
amounts of chitosan diffusion, the PAA ionization level increased
as a result of the titration of carboxylic acid groups to carboxylate
groups by cationic chitosan (Fig. S3) (53). Therefore, upon chitosan
diffusion, electrostatic interactions between chitosan and PAA
displace the weaker hydrogen-bonding interactions between PEO
and PAA. As a result, the hydrogen-bonding acceptor PEO no
longer is associated with the lm and may diffuse out.
Enabled by the high sensitivity of XPS, PEO displacement can
be explored directly. Fig. 3 shows the high-resolution C1s data
from the CHI1 and CHI3 samples using the color scheme from
Fig. 1A(CHI10 and CHI60 data in Fig. S4). The red region is
inltrated with chitosan, and its location in this region was de-
termined by analyzing the intensity of the N1s signal with depth as
in Fig. 2. In the remaining depth of the lm, the N1s signal is very
low (<0.5% atomic conc. N), signifying that in this yellow region,
the lm contains little to no chitosan. To analyze the displacement
of PEO from the lm, the C1s spectra from the (red) chitosan-
inltrated regions were compared with the spectra in the (yellow)
hydrogen-bonded regions of the lm. In particular, we focused on
the change in C1s signal intensity at 286.5 eV. Both PEO and
chitosan have a signal at this point, but because the extent of
chitosan diffusion can be determined independently by the nitro-
gen signal, the changes in C1s spectra may be used to analyze the
displacement of PEO from the lm. The C1s spectra of pure PEO,
chitosan, and PAA may be seen in Fig. S5.
As shown in Fig. 3 Aand B, the red regions, where chitosan
has diffused into the lm, have a markedly lower signal at 286.5
eV than the yellow hydrogen-bonded region. The change in
signal intensity at 286.5 eV is highlighted in Fig. 3C, which
compares the chitosan-inltrated regions from Fig. 3 Aand B
with the (PAA3/PEO3) hydrogen-bonded region. The spectra of
all chitosan-exposed samples were obtained from 450 nm above
the glass surface to minimize differences due to X-ray exposure
time or C
60
+
sputtering time. The decrease in signal intensity at
286.5 eV is a result of chitosan diffusion displacing PEO and
Fig. 2. Diffusion of chitosan into hydrogen-bonded lms. Spectra of hydrogen-bonded (PAA3/PEO3) lms exposed to chitosan solution for different amounts
of time: (A) 1-min exposure, (B) 3-min exposure, (C) 10-min exposure, and (D) 60-min exposure to chitosan. The color scheme is the same as that of Fig. 1A. Red
spectra represent chitosan-infused areas, yellow spectra represent the hydrogen-bonded (PAA3/PEO3) area, and black spectra represent the (PDAC4/SPS4)
adhesion layer. (E) Quantication of ADto determine the atomic concentration of nitrogen with depth in the lm. Data points are individual dots, and the
lines show the result of a SavitzkyGolay ve-point quadratic algorithm.
Table 1. Diffusion of chitosan in a swollen hydrogen-bonded
lm
Sample t,s
Dry lm
distance, nm
Swollen lm
distance, nm D, cm
2
/s
CHI1 360 181 452 1.42E-12
CHI3 720 238 594 1.23E-12
CHI10 900 299 748 1.56E-12
Dry distance multiplied by 2.5 to account for 250% lm swelling at pH
3. D, distance; t, time.
Gilbert et al. PNAS
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CHEMISTRY
allowing it to diffuse out of the lm. Because chitosan also has
a peak at 286.5 eV, if PEO was not diffusing out of the lm, this
signal would increase. Fig. 3Calso shows that the decrease in the
signal at 286.5 eV correlates with the holding time in chitosan
solution, which is consistent with the diffusion of PEO out of
the lm.
Electrostatic Blocking Layer Stops Chitosan Diffusion. In many cases,
it is desirable to stop interlayer diffusion to maintain distinct
functional regions of a multilayer heterostructure. The effect of
electrostatic blocking layers on the diffusion of chitosan into the
hydrogen-bonded region was investigated using the lm archi-
tecture shown in Fig. 1B. Unlike the previous study, above the
hydrogen-bonded region there is an electrostatic blocking layer
that varies from a single layer of PAH (<1 nm) to 9.5 bilayers of
PAH3/SPS3 (10 nm). On top of the blocking region, the nal
region is a 20-nm HA3/CHI3 multilayer lm. To determine the
location of the distinct regions of the PEM lm and apply the
color scheme shown in Fig. 1B, information from the C1s and N1s
spectra was combined. For example, in Fig. 4A, the transition
from the red HA/CHI region at the surface to the green (PAH3/
SPS3) blocking region was determined by the change in shape of
the C1s spectrum. The shape change is a result of more carbon
carbon bonds at 285 eV and fewer carbonoxygen bonds at 286.5
eV in the green region compared with the red region. Because
XPS analyzes approximately the top 10 nm and the blocking layers
are less than 10 nm, the green spectrum representing the blocking
layers likely contains signal from an adjacent region as well.
However, the C1s spectra of the blocking layers remain distinct
from the red and yellow C1s spectra. The end of the green region is
determined by the drop in N1s signal, and thus the yellow hydro-
gen-bonded region begins. Finally, the black adhesion layer starts
when the N1s signal increases at the base of the lm because of the
presence of nitrogen-containing PDAC. These data reveal that
each of the various regions of the multilayer heterostructure il-
lustrated in Fig. 1Bcan be identied in XPS depth-prole spectra.
The N1s spectra from depth-proling samples with blocking
layers were analyzed to determine the atomic percentage of ni-
trogen with depth, as seen in Fig. 4B(N1s spectra from all samples
with blocking layers are seen in Fig. S6). From Fig. 4B, it is clear
that all three electrostatic blocking layers tested (PAH3/SPS3)
z
(z =0.5, 3.5, 9.5) with approximate thicknesses of <1 nm, 4 nm, and
Fig. 3. High-resolution C1s XPS depth proling of a hy-
drogen-bonded lm exposed to chitosan solution for (A)1
min and (B) 3 min. The color scheme is the same as that of
Fig. 1A. Red spectra represent chitosan-infused areas, yel-
low spectra represent the hydrogen-bonded (PAA3/PEO3)
areas, and black spectra represent the (PDAC4/SPS4) ad-
hesion layer. Comparing the red chitosan-diffused areas
with the yellow hydrogen-bonded areas, the chitosan-
diffused areas have a lower signal at 286.5 eV because PEO
has diffused out. (C) Comparison of C1s spectra with dif-
ferent chitosan-exposure times with the initial yellow hy-
drogen-bonded area. The longer the exposure to chitosan
solution, the more the PEO signal at 286.5 eV decreases. All
chitosan-exposed spectra were from 450 nm above the
glass surface.
Fig. 4. Effect of a blocking layer on interlayer diffusion
of chitosan. (A) C1s and N1s regions from depth-proling
XPS of a hydrogen-bonded sample with a (PAH3/SPS3)
3.5
blocking layer topped with (HA3/CHI3)
3.5
. The color scheme
is the same as that of Fig. 1B. Red spectra represent (HA3/
CHI3), green spectra represent the (PAH3/SPS3) electro-
static blocking layer, yellow spectra represent the (PAA3/
PEO3) hydrogen-bonded region, and black spectra repre-
sent the (PDAC4/SPS4) adhesion layer. (B) Quantication of
the nitrogen signal for different blocking-layer systems
tested. Data points are individual dots, and the line is the
result of a SavitzkyGolay ve-point smoothing algorithm.
All lms had (HA3/CHI3)
3.5
deposited on top of the block-
ing layer.
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10 nm, respectively, effectively stop the diffusion of chitosan into
the hydrogen-bonded region, as seen by the absence of any de-
tectable nitrogen signal in the bulk of the lm. Remarkably, even
though the lms spent over 30 min in chitosan solution during the
HA/chitosan lm fabrication, even a single adsorbed layer of PAH
was sufcient to block its diffusion into the hydrogen-bonded re-
gion. At pH 3, PAH is a fully charged polycation (pK
a
8) (53), so
it has a strong electrostatic interaction with the ionizable polyanion
PAA found at the top of the hydrogen-bonded region (54). Be-
cause of the strong electrostatic interaction, the PAH blocking
layer is kinetically trapped at the top of the lm (28) and effectively
stops the diffusion of chitosan into the hydrogen-bonded region
under the conditions used in this study. Recent literature (7, 24, 40,
43, 44) shows that different blocking layers work well for different
polymer systems and annealing conditions. In some cases, co-
valently cross-linked blocking layers are needed (7, 43), but in
other cases, electrostatic blocking layers may stop interlayer dif-
fusion as well (24, 44). In the conditions tested in this work, a single
electrostatic blocking layer is all that was needed to block the
interlayer diffusion of chitosan.
In the absence of a blocking layer, chitosan from the (HA3/
CHI3) multilayer lm diffuses into the entire hydrogen-bonded
lm during fabrication, producing a high nitrogen signal through-
out, as seen in Fig. 4B. Although the lm has fewer total layers
deposited than the multilayer lms with blocking layers, the in-
terlayer diffusion of chitosan allows for a large increase in the nal
lm thickness, as shown by the leftmost data point in Fig. 4B.This
large increase in thickness is characteristic of exponential growth,
which is caused by the diffusion of polymers and polymer pairs in
and out of the lm during deposition (20). As a result, we expect
chitosan, not only from the rst bilayer deposition but also from the
subsequent depositions, to be present throughout the lm. Similar
chitosan diffusion into the hydrogen-bonded region also is observed
when the order of polymer deposition is switched from HA rst
(HA3/CHI3) to chitosan rst (CHI3/HA3), as revealed in Fig. S7.
As a result of macromolecules such as chitosan diffusing through-
out a lm, hydrogen-bonded lms might be used as scaffolds for
easy loading of drugs or other macromolecules of interest (3, 55).
An interesting question concerning the use of depth-proling
XPS with C
60
+
sputtering is: What level of vertical resolution is
possible, and is it sufcient to probe PEM heterostructure inter-
faces? The interface used to analyze this resolution was the sharp
boundary between the (PAH3/SPS3) blocking layers and the hy-
drogen-bonded region seen in Fig. 4B. The precipitous drop of
nitrogen signal from the blocking layer to the hydrogen-bonded
region occurs between two data points or 15 nm. Reducing the
sputtered thickness between successive XPS spectra would allow
the interface resolution to increase to 10 nm because this com-
monly is the depth of analysis during an XPS cycle. However, near
this limit of resolution, extended sputtering time may cause radi-
ation-induced diffusion and surface roughening, which must be
considered during experimental design (48).
pH Sensitivity of the Hydrogen-Bonded Region. The purpose of
a blocking layer is to maintain thedistinct propertiesof each region
of a multilayer lm. In our model system, the as-assembled (PAA3/
PEO3) hydrogen-bonded region dissolves above pH 3.6 (47). If the
lm is altered by sufcient chitosan interlayer diffusion, the multi-
layer becomes insoluble at neutral pH. Therefore, to test whether
the desired properties of the hydrogen-bonded region can be
maintained through the use of blocking layers, all lms character-
ized previously were exposed to a buffered salt solution of PBS, pH
7.4, for 30 min. After 30 min, the residual dry lm thickness was
compared with the initial dry lm thickness to determine whether
the hydrogen-bonded lm dissolved. As seen in Table 2, all multi-
layer lms with a blocking layer dissolved in PBS. Therefore, even a
single layer of PAH can effectively block chitosan diffusion and
maintain the pH-sensitive solubility of the hydrogen-bonded region.
In the absence of a blocking layer, the multilayer lms no longer
dissolved because pH-stable electrostatic cross-links formed be-
tween the diffused chitosan and PAA. In some cases, such as CHI1,
CHI3, and CHI10, this result was unexpected because chitosan did
not diffuse all the way through the lm (Fig. 2E)beforePBS
exposure. However, depth proling of CHI10 after PBS exposure
shows thatchitosan diffused throughout the lm and stabilized it to
pH changes (Fig. S8). Therefore, in the absence of a blocking layer,
chitosan interlayer diffusion was stopped by drying for analysis but
continued after exposure to PBS solutions. Because hydrogen-
bonded PEMs require a minute or two to dissolve (56), this brief
time allows for further chitosan diffusion, rendering the lm
insoluble in PBS.
Conclusions
XPS with C
60
+
cluster ion sputtering is a powerful technique for
analyzing the atomic composition and chemical state of organic
nanostructured lms. With the correct choice of conditions to
minimize sample damage, it can determine directly, to within 15
nm, the location of polymers through the thickness of a lm,
allowing analysis of interlayer diffusion as well as testing of the
efcacy of various blocking layers. Using our model system, we
have shown that chitosan is highly diffusive, with an interlayer
diffusion coefcient 1.4*10
12
cm
2
/s in hydrated hydrogen-bonded
(PAA3/PEO3) lms. Also, the high-resolution capabilities of XPS
show the displacement of hydrogen-bonded PEO in favor of elec-
trostatic interactions between chitosan and PAA. Finally, various
thicknesses of PAH-containing blocking layers were explored, in-
cludinga single layer of PAH that can stopthe diffusion of chitosan
into the hydrogen-bonded region.
We believe the ability to control and measure the interlayer
diffusion in PEMs will have applications in a variety of areas. For
example, exchanging hydrogen-bonding interactions for electro-
static ones may be important for the loading of biological mol-
ecules through postassembly modication of lms. Furthermore,
the design of lms capable of blocking macromolecule diffusion
is relevant to structured lms, sequential drug release, and pro-
duction of free-oating PEM lms.
Materials and Methods
Materials. PAA (Aldrich; M =450 kDa), PAH (Aldrich; M =15 kDa), PEO
(Polysciences; M =20 kDa), PDAC (Aldrich; M =200350 kDa in 20% aqueous
solution), SPS (Aldrich; M =70 kDa), HA (from Streptococcus equi; Fluka;
M1,580 kDa), acetic acid (Sigma), and low molecular weight chitosan
(deacetylation 0.9; Sigma; M =50190 kDa) were used as received. The
nomenclature for PEMs follows (poly1X/poly2X)
z
, where X is the pH of the
polymer solutions and z is the number of bilayers deposited (one bilayer =
poly1 +poly2). A noninteger value of z indicates the assembly was termi-
nated with poly1.
Multilayer Film Deposition. Polymer solutions were made from Milli-Q 18.2-
MΩwater. Solutions of PAA, PDAC, and SPS were 0.01 M, and solutions of
PEO, HA, and chitosan were 0.1% (wt/vol). CHI solutions included 0.1 M of
acetic acid to aid dissolution. PDAC and SPS solutions for the adhesion layer
had 0.1 M NaCl at pH 4.0. All other solution pHs were adjusted to pH 3.0
with 1 M HCl and no added salt. Glass substrates were dipped sequentially in
the polymer solutions using an automated Zeiss programmable slide stainer
or nanoStrata dipping unit. Substrates were held in polymer solutions for
10 min and then rinsed for a total of 3 min in water with mild agitation. The
time in chitosan solution was altered for diffusion studies, but the rinse cycle
Table 2. pH sensitivity of the hydrogen-bonded region
Figure Top layer Blocking layer Dissolve in PBS, pH 7.4?
Fig. 4 (HA3/CHI3)
3.5
(PAH3) Yes
(HA3/CHI3)
3.5
(PAH3/SPS3)
3.5
Yes
(HA3/CHI3)
3.5
(PAH3/SPS3)
9.5
Yes
(HA3/CHI3)
3.5
None No
Fig. 2 CHI60 None No
CHI10 None No
CHI3 None No
CHI1 None No
After exposure to pH 7.4 PBS for 30 min, the remaining thickness was
compared with the initial thickness to determine whether the hydrogen-
bonded region dissolved.
Gilbert et al. PNAS
|
April 23, 2013
|
vol. 110
|
no. 17
|
6655
CHEMISTRY
used the same time proles. Fabrication details and polymer structures are
listed in SI Materials and Methods and Fig. S9, respectively. Dry lm thickness
was measured with a P-16 proler (KLA-Tencor Corp.).
XPS. Chemical composition ofthe surface was characterized using a PHI Versa-
Probe II X-ray photoelectronspectrometer with a scanning monochromated Al
source (1,486.6 eV; 50 W; spot size, 200 μm). The takeoff angle between the
sample surface and analyzer was 45°, and the X-ray beam collected C1s, N1s,
O1s, and Si2p elementalinformation while rastering over a 200 Χ700-μmarea.
Detailed XPS acquisitionparameters are found in Table S2. Depth prolingwas
accomplished using the instrumentsC
60
+
ion source operated at 10 kV, 10 nA,
and rastered over a 3 ×3-mm area at an angle of 70° to the surface normal.
Sputtering occurred in 1-min intervals while the sample was moved using con-
centric Zalar rotation at 1 rpm. Atomic composition was determined based on
photoelectron peak areas and the relative sensitivity factors provided in PHIs
MultiPak processing software. All data were background subtracted, smoothed
using a ve-point quadratic SavitzkyGolay algorithm, and charge corrected so
that the carboncarbon bond had a binding energy of 285.0 eV. The surface of
the glass was dened as the point at which the atomic concentration of silicon
reached 5% in the depth-proling data. The thickness as measured by prol-
ometry was compared with the number of sputter cycles that occurred before
reaching the surface of the glass. Data were plotted using Matlab.
pH Sensitivity of Hydrogen-Bonded Region. Films were tested by a 30-min
immersionin PBS (pH 7.4) on an orbital shaker at 100 rpm.After drying with N
2
,
the thickness was measured using prolometry and compared with the initial
thickness to determine whether the hydrogen-bonded region dissolved.
ACKNOWLEDGMENTS. We acknowledge support by the Materials Research
Science and Engineering Centers (MRSEC) Program of the National Science
Foundation (NSF) under Award DMR0819762, as well as the excellent as-
sistance of Libby Shaw of the MRSEC shared facilities. J.B.G. is supported
by a National Defense Science and Engineering Graduate (NDSEG) and
NSF fellowship.
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