DOI: 10.1126/science.277.5330.1232
, 1232 (1997);
277 Science Gero Decher
Fuzzy Nanoasmblies: Toward Layered Polymeric Multicomposites
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is a Science 1997 by the American Association for the Advancement of Science; all rights rerved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n D e c e m b e r 4, 2012
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9.The scheme requires consideration of shape and
packing at the atomic and molecular level as well as at higher levels of scale leading to the three dimen-sional structure of the overall aggregate.Bringing the t of components together into the requisite array in
a single process step(for example,by cooling the
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10.A.H.Simmons,C.A.Michal,L.W.Jelinski,Science
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32.We have benefited from interactions with many stu-
dents and co-workers and would like to thank former
members of our rearch groups.In particular,
C.K.O.thanks G.-P.Mao and J.-G.Wang and E.L.T.
thanks J.T.Chen.Funding from NSF(DMR-970527)
(to M.M.,C.K.O.,and E.L.T.)and both NSF(DMR-
9201845)and the Air Force Office of Sponsored
Rearch(MURI PC218975)(to C.K.O.and E.L.T.)
is gratefully acknowledged.C.K.O.thanks the Office
of Naval Rearch for support of the fluoropolymer
work.
Fuzzy Nanoasmblies:
Toward Layered Polymeric
Multicomposites
Gero Decher
Multilayer films of organic compounds on solid surfaces have been studied for more than 60years becau they allow fabrication of multicomposite molecular asmblies of tailored architecture.However,both the Langmuir-Blodgett technique and chemisorp-tion from solution can be
ud only with certain class of molecules.An alternative approach—fabrication of multilayers by concutive adsorption of polyanions and poly-cations—is far more general and has been extended to other materials such as proteins or colloids.Becau polymers are typically flexible molecules,the resulting superlattice architectures are somewhat fuzzy structures,but the abnce of crystallinity in the films is expected to be beneficial for many potential applications.
I n the last two to three decades,materials
science has developed into an interdiscipli-
nary field that encompass organic,poly-
meric,and even biological components in
addition to the classic metals and inorgan-
ics.Although carbon-bad molecules offer
an enormous structural diversity and tun-
ability in terms of potential properties or
processability,they also typically suffer from
a lack of stability when expod to heat,
oxidizing agents,electromagnetic radiation,
or(as in the ca of complex biomolecules)
dehydration.Multicomposites make it pos-
sible to combine two or more desirable prop-
erties,as in the classic reinforced plastics,or
to provide additional stability for otherwi
highly labile functional biomolecules or bio-
molecular asmblies.Even higher device
functionality will ari from a combination
of physical and chemical process(such as
electron or energy transfer)and chemical
transformations found in nature(such as
photochemical energy conversion).Such
devices require control of molecular orien-
tation and organization on the nanoscale,as
their function strongly depends on the local
chemical environment.It is therefore highly
desirable to develop methods for the con-
trolled asmbly of multicomponent nano-
structures,although it is also clear that struc-
tures as complex as tho found in the bio-
logical world,such as the flagellar motor,
cannot yet be fabricated.
It is,however,possible to concutively
deposit single molecular layers onto planar
solid supports and to form multilayers in
which nanoscale arrangements of organic
molecules can be controlled at least in one
dimension(along the layer normal).This
approach also fulfills another prerequisite
for functional macroscopic devices:a fixed
relation between nanoscopic order and
macroscopic orientation.To fully exploit an
asmbled structure,it is necessary to know
the location or orientation(or both)of
every molecule,not only with respect to
each other(as in ordered or pha-parated
bulk systems at the nanometer scale,such as
liquid crystals,copolymers,or zeolites),but
also with respect to a macroscopic coordi-
nate.Only materials that have such struc-
tural hierarchy(1)allow molecular proper-
ties to be fully exploited in macroscopic
devices,as,for example,in organic
waveguides for cond-order nonlinear op-
tics or in bionsors.
In simple multilayer systems,this de-
mand is reduced to the quence of layers
and to the orientation of molecules with
respect to the layer normal.For about60
years,the molecularly controlled fabrica-Universite´Louis Pasteur and CNRS,Institut Charles Sad-
ron,6,rue Boussingault,F-67083Strasbourg Cedex,
France.E-mail:decher@ics-crm.u-strasbg.fr
SCIENCE⅐VOL.277⅐29AUGUST1997⅐www.sciencemag 1232o n D e c e m b e r 4 , 2 0 1 2 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o m
tion of nanostructured films has been dom-inated by the so-called Langmuir-Blodgett (LB)technique,in which monolayers are formed on a water surface and then trans-ferred onto a solid support(2,3).Indeed, the pioneering work on synthetic nanoscale heterostructures of organic molecules was carried out by Kuhn and co-workers in the late1960s using the LB technique(4). Their experiments with donor and acceptor dyes in different layers of LB films provided direct proof of distance-dependent Fo¨rster energy transfer on the nanoscale.The experiments were also the first true nano-manipulations,as they allowed for the me-chanical handling of individual molecular layers(such as paration and contact for-mation)with angstrom precision(5).
The LB technique requires special equipment and has vere limitations with respect to substrate size and topology as well as film quality and stability.Since the early 1980s,lf-asmbly techniques bad main-
ly on silane-SiO
2(6)and metal phospho-
nate chemistry(7)were developed as an alternative to LB films.However,lf-as-mbled films bad on covalent or coordi-nation chemistry are restricted to certain class of organics,and high-quality multi-layer films cannot be reliably obtained. The problems are most likely caud by the high steric demand of covalent chem-istry and the verely limited number of reactions with exactly100%yield,which is a prerequisite for the prervation of func-tional group density in each layer.
It was therefore desirable to develop a simple approach that would yield nanoar-chitecture films with good positioning of individual layers,but who fabrication would be largely independent on the nature, size,and topology of the substrate.The elec-trostatic attraction between oppositely charged molecules emed to be a good can-didate as a driving force for multilayer build-up,becau it has the least steric demand of all chemical bonds.Since the early1990s, our group has developed a technique for the construction of multicomposite films of rod-like molecules equipped with ionic groups at both ends(8),polyelectrolytes(9),or other charged materials through layer-by-layer ad-sorption from aqueous solution(10,11). The process,which is extremely simple,is depicted in Fig.1for the ca of polyanion-polycation deposition on a positively charged surface.Strong electrostatic attrac-tion occurs between a charged surface and an oppositely charged molecule in solution; this p
henomenon has long been known to be a factor in the adsorption of small organ-ics and polyelectrolytes(12),but it has rare-ly been studied with respect to the molecular details of layer formation(13).In principle, the adsorption of molecules carrying more than one equal charge allows for charge
reversal on the surface,which has two im-
portant conquences:(i)repulsion of equal-
ly charged molecules and thus lf-regula-
tion of the adsorption and restriction to a
single layer,and(ii)the ability of an oppo-
sitely charged molecule to be adsorbed in a
cond step on top of the first one.Cyclic
repetition of both adsorption steps leads to
the formation of multilayer structures.
The crucial factor of charge reversal of a
surface upon adsorption of an oppositely
charged polyelectrolyte has long been
known for the ca of polyion adsorption on
colloids,but has also been obrved on mac-
roscopic surfaces(14,15).The concutive
adsorption of cationic colloids compod of
a heparin-hexadecylamine complex and of
pure heparin on polyethylene surfaces that
were oxidized or sulfated(or both)leads to
films with interesting nonthrombogenic
properties(16).However,the films were
reported to be homogeneous monolayers
that ari from submonolayer coverage after
the first deposition cycle and subquent
completion of surface coverage in cycles2
to5;additional deposition cycles lead to
surface flocculation and destruction of layer
uniformity.This report was rather discour-
aging given the early and promising exper-
iments of Iler on the fabrication of multi-
layers of charged inorganic colloids by con-
cutive adsorption(17),which were never
proven to be layered structures or the pro-
tein-polyelectrolyte multilayers propod by
Fromherz in1980(18).
Sequential cationic-anionic polyelectro-
lyte addition has important conquences
for flocculation and is therefore of interest
in large-scale industrial process such as
wage dewatering or paper making,and
the two-step treatment of colloids or cel-
lulosic fibers with polycations and polya-
nions has been studied for many years(19,
20).However,the process is considered
difficult and the resulting structures are
not well understood;therefore,existing
industrial applications may benefit from a
琅琊榜电视剧下载
better understanding of polyelectrolyte
multilayer films as model systems,as the
can be well characterized by a wide variety
of physical techniques.
Fabrication of Polyelectrolyte and
Related Multilayers
Multilayer structures compod of polyions
or other charged molecular or colloidal ob-
jects(or both)are fabricated as schemati-
cally outlined in Fig.1.Becau the process
only involves adsorption from solution,
there are in principle no restrictions with
respect to substrate size and topology;mul-
tilayers have been prepared on colloids and
on objects with dimensions of veral tens
of centimeters.Film deposition on a glass
slide from ordinary beakers can be carried
out either manually or by an automated
device(21)(Fig.1A).A reprentation of
the buildup of a multilayer film at the mo-
lecular level(Fig.1B)shows a positively
charged substrate adsorbing a polyanion
and a polycation concutively;in this ex-
ample,the counterions have been omitted
1. Polyanion
S
u
b
s
t
r
a
戕害t
e
2. Wash
3. Polycation
4. Wash
SO3– Na+
NH3+ CI–
A
C
Fig.1.(A)Schematic of the
film deposition process us-
ing slides and beakers.
Steps1and3reprent the
adsorption of a polyanion
and polycation,respectively,
and steps2and4are wash-
ing steps.The four steps are
the basic buildup quence
for the simplest film archi-
tecture,(A/B)
n
.The con-
struction of more complex
film architectures requires
only additional beakers and
a different deposition -
quence.(B)Simplified mo-
lecular picture of the first two
adsorption steps,depicting
film deposition
starting with a
positively charged substrate.
locoCounterions are omitted for
鹬蚌相争翻译clarity.The polyion confor-
mation and layer interpene-
tration are an idealization of
the surface charge reversal
with each adsorption step.
(C)Chemical structures of
two typical polyions,the so-
单程票英语dium salt of poly(styrene sulfonate)and poly(allylamine hydrochloride).
www.sciencemag⅐SCIENCE⅐VOL.277⅐29AUGUST19971233o n D e c e m b e r 4 , 2 0 1 2 w w w . s c i e n c e m a g . o r g d f r o m
and the stoichiometry of charged groups between polyions and between the substrate and polyanion is arbitrary (e below).Two typical polyelectrolytes,sodium poly(sty -rene sulfonate)and poly(allylamine hydro -chloride),are shown in Fig.1C.
The u of polyelectrolytes rather than small molecules is advantageous mainly be -cau good adhesion of a layer to the un -derlying substrate or film requires a certain number of ionic bonds.Therefore,the over -compensation of the surface charge by the incoming layer is more a prop
erty of the polymer than a property of the surface.This is becau polymers can simply bridge over underlying defects;their conformation at the surface (and thus also the newly created film surface)is mostly dependent on the chon polyelectrolytes and adsorption con -ditions and much less dependent on the substrate or the substrate charge density (10,22).The linear increa of film thick -ness with the number of deposited layers is often similar even if different substrates are ud,which makes the film properties rath -er independent of the substrate.In cas where substrate charge densities are very small,the first layer binds to the surface with only a few groups and expos a larger number of oppositely charged groups to the solution.This effective “multiplication of surface functionality”often continues over a few layers before a linear deposition re -gime is reached (22–26).
Similar to this lf -regulation of thick -ness increments per layer,there is a tenden -cy toward a certain value of the interfacial overlap between a polyanion layer and a polycation layer and a certain roughness at the film -air interface;the attributes are probably a property of the polyanion -poly -cation pair rather than a property of the substrate.We have obrved that polyelec -trolyte multilayers have similar surface roughness,regardless of the roughness of the underlying substrates.One possible expla -nation is that the surface roughness of rough polyelectrolyte films ca
n be “an -nealed”to smaller values by concutively immersing films in solutions of salt and pure water (27).Presumably,in this post -prepa -ration treatment of the films,the salt breaks some of the anion -cation bonds,and its removal by washing in pure water leads to their reformation in a more equilibrated conformation of the polymer chains.
Films are typically deposited from adsor -bate concentrations of veral milligrams per milliliter.The concentrations are much greater than that required to reach the plateau of the adsorption isotherm,but this excess ensures that the solutions do not become depleted during the fabrication of films compod of veral hundred layers.One or more washing steps are usually ud after the adsorption of each layer to avoid contamination of the next adsorption solu -tion by liquid adhering to the substrate from the previous adsorption step.The washing step also helps to stabilize weakly adsorbed polymer layers (24).Typical adsorption times per layer range from minutes in the ca of polyelectrolytes (24,25,28)to hours in the ca of gold colloids (29,30),depending on molar mass,concentra -tions,and agitation of the solutions.
The major advantages of layer -by -layer adsorption from solution are that many dif -ferent materials can be incorporated in in -dividual multilayer films and that the film architectures are completely determined
by
Fig.2.(A )Curves XR-1,XR-2,and XR-3are specular x-ray reflectivity scans of multilayer films compod of poly(styrene sulfonate)and poly(al-lylamine hydrochloride).Only partial curves for XR-1and XR-2are shown,to keep the same range of the scat-tering vector q for all scans and for better comparison with (B).The film thickness increa from 16.5nm to 120.5nm becau of different num-bers of layers and deposition from so-lution containing different amounts of salt.All curves can be modeled quan-titatively with a film of homogeneous thickness and no internal structure.Scan XR-3was taken from the same film specimen as curve NR-6in (B);however,its internal layer structure is only detected by neutron reflectiv-ity.XR-4is a reflectivity scan of a multilayer with the architecture ((A /B)3A /G)4,where G consists of negatively charged gold colloids of 13.5nm diameter (30).The superlat-tice formed by the layers of the gold colloids is clearly en;the high in-tensity of the (001)Bragg peak aris-es from the large electron density difference between the polymers and the gold.Scans are shifted in the y direction to prerve clarity.(B )Curves NR-1to NR-6are specular neutron reflectivity scans of multilayer films;all except NR-5were obtained from films compod of sodium poly(styrene sulfonate)and poly(allylamine).NR-1and NR-2are scans of films in which every cond layer was deuterated [(A/B d )n film architecture].NR-3has an (A /B/A /B d )n architecture and was measured from the same film specimen as NR-4after 11months of storage in a regular laboratory,demonstrating the good stability of multilayer films over time.Scan NR-6has ((A /B)2A /B d )
n architecture and was measured from the same specimen that does not show Bragg peaks in x-ray reflectivity [(A),scan XR-3].Although differ-ent in architecture,the positions of the deuterated layers in samples NR-2and NR-3were approximately matched by depositing NR-2from 3M NaCl and NR-3from 2M NaCl;therefore,Bragg peaks,if prent,should appear at similar q values.This result rules out the hypothesis that low count rates at high Q values are responsible for the abnce of Bragg peaks in NR-1and NR-2and shows that the deuteron concentration along the layer normal in (A/B d )n films is constant with respect to experimental resolution.NR-5is a reflectivity scan of a multilayer originally fabricated from poly(sty-rene sulfonate)and the tetrahydrothiophenium precursor polyelectrolyte of PPV;every fourth poly(styrene sulfonate)layer is perdeuterated.After 32min of elimination at 120°C,the conversion to PPV is almost complete but the layer structure remains intact,as evidenced from the Bragg peak.Scans are shifted in the y direction to prerve clarity.
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⅐VOL.277⅐29AUGUST 1997⅐www.sciencemag
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w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m
the deposition quence.The most remark-able current examples of multicomposite films include proteins(31–33),clay plate-lets(31,34–37),virus particles(38),and gold colloids(30,39).Nanostructured sur-face-confined films do,of cour,have bulk analogs;there are similarities between poly-electrolyte multilayers and their bulk coun-terparts(40).Polyelectrolyte-clay multilay-ers and bulk organoclay nanocomposites (41–46),which have interesting materials properties themlves,may be structurally even clor.However,no straightforward strategy exists to prepare bulk multicompos-ites with more than two components in which the distance of all constituents or their orientation(or both)can be molecu-larly controlled.Template approaches such as layer-by-layer asmbly are much more promising in this respect.Another differ-ence between bulk systems and surface-con-fined multilayers is that bulk nanocompos-ites are often turbid materials,whereas lay-er-by-layer asmbled films can be applied as wavelength-thick transparent coatings on,for example,optical devices.
Structure of Polyelectrolyte
Multilayers
Reflectivity techniques,especially neutron and x-ray reflectometry,are well suited for the characterization of multilayer films,as they allow the determination of concentra-tion gradients along the layer normal.In many experiments on multilayer films com-pod of flexible,strong polyelectrolytes of approximately equal charge-to-charge dis-tances(polyanions and polycations with one charged group per monomer unit),x-ray reflectograms have only exhibited so-called Kiessig fringes that ari from the interference of x-ray beams reflected at the substrate-film and film-air interfaces(9, 47–49).Typical reflectivity curves are shown in Fig.2A(traces XR-1to XR-3).In the x-ray scans,different numbers of os-cillations ari from the different film thick-ness that were obtained either by chang-ing the total number of layers or by depos-iting from polyelectrolyte solutions of dif-ferent ionic strength(10,47).They show a large number of well-resolved Kiessig fring-es that were originally believed to be caud by the electron densities of two concutive layers being too clo to yield enough con-trast.However,neutron reflectograms of films in which all polyanion layers were
labeled with deuterium[(A/B
d )n film archi-
tecture,where A is a polycation,B is a polyanion,B d is a perdeuterated polyanion, and n is the number of deposition cycles] also showed Kiessig fringes as the only char-acteristic feature(Fig.2B,traces NR-1and NR-2).Only when we started to deuterate specific layer positions in a multilayer film
were Bragg peaks obrved by neutron re-
flectometry[((A/B)m A/B d)n film architec-
tures,mϭ1,2,...;Fig.2B,traces NR-3to
NR-6],which clearly demonstrated an in-
ternal layer structure(50,51).A single
Bragg peak was also obrved by x-ray re-
flectivity in films in which every fourth
layer was a polyanion containing side
groups of azo dyes(52).Thus,the abnce
of Bragg peaks in(A/B)n-type films does not
ari from small density differences between
different layers,but rather from large over-
laps between adjacent layers.
On the basis of this result,together with
our inability to detect significant amounts of
small counterions in the film,polyelectrolyte
multilayers should have a1:1stoichiometry of
anionic and cationic groups.In this ca,ev-
ery anionic group of a polyanion is bound to a
cationic group of a polycation,which is also
2014ama
the predominant ca in bulk polyion com-
plexes of flexible polyelectrolytes of high
charge density and similar molecular weights
(53).Note that for weak polyelectrolytes,not
all of the monomers need to be charged,so
that the overall stoichiometry may deviate
from1:1(54).This has the interesting con-
quence that within the resolution of the
charge-to-charge distance along the polyelec-
trolyte backbone(typically0.25to0.5nm),
the concentration of anionic and cationic
groups must be identical throughout the poly-
electrolyte multilayer film and constant along
the layer normal.At first,it would em that
such a homogenous distribution of charged
ionic groups in the film contradicts the notion
of defined individual layers of polyions within
such multilayer asmblies,but this apparent
discrepancy is easily explained.In a simplified
polyelectrolyte multilayer structure compod
of10layers(Fig.3),each layer is reprented
by an arbitrarily chon sinusoidal concentra-
tion profile.The50%overlap of layers of
equal charge has the conquence that at any
point inside the film(the substrate-film and
film-air interfaces are different),the sum of
the concentration of equal ionic groups is
unity in both the cationic and anionic ca,as
reprented by the lines compod of blue dots
(concentration profile of anionic groups)and
red dots(concentration profile of cationic
groups).The line compod of green dots
reprents the concentration profile for a label
applied to every fourth layer and shows that
chemical functional groups(or labels)can be
precily positioned at certain distances from
the substrate or with respect to each other.
Thus,Fig.3reprents a film model in which
the high overlap of layers of equal charge
allows for a1:1stoichiometry of anionic and
cationic groups within the film and provides
the ba for a true layer structure.
The deuterium concentration of such an
(A/B/A/B d)n film,which is the most impor-
tant contributor to the scattering length
density profile,is described by the line of
green dots in Fig.3.The profile is sinusoi-
dal,which agrees with the obrvation of a
single Bragg peak for this architecture (Fig.
1.0
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0.6
0.4
0.2
0.0
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12345678910
Layer number
Fig. 3.Schematic of a
polyelectrolyte multilayer
compod of10layers,
each reprented by an
arbitrarily chon sinu-
soidal concentration pro-
file(black lines).For a
positively charged sub-
strate,the five blue lay-
ers and five red layers
reprent polyanion and
polycation layers,re-
spectively.The spread of
each layer and the dis-
tance between them
were chon such that
every two layers of equal
charge start to overlap at
a relative concentration
of50%.The overlap of
blue and red layers(pur-
ple)has no physical
meaning.The lines com-
pod of blue dots(an-
ionic groups)and red
dots(cationic groups)
reprent the sum of
concentrations from all layers within the film.A positional shift of red layers with respect to blue layers caus changes in charge concentration only at the two interfaces,not in the center of the film.The line compod of green dots reprents the concentration profile for a label applied to every fourth layer
[(A/B/A/B
d
)
n
architectureϭdeuterium labels in layers3and7].
www.sciencemag⅐SCIENCE⅐VOL.277⅐29AUGUST19971235o n D e c e m b e r 4 , 2 0 1 2 w w w . s c i e n c e m a g . o r g D o w n l o a d e d f r o m
