1

The Deposition and Aggregation of Aspirin Molecules on a

Phospholipid Bilayer Pattern

Guangzhao Mao,* Dongzhong Chen,§ Hitesh Handa

Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA

Wenfei Dong, Dirk G. Kurth,† Helmuth Möhwald

Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany

GZMAO@ENG.WAYNE.EDU

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required

according to the journal that you are submitting your paper to)

TITLE RUNNING HEAD: ASPIRIN DEPOSITION ON BILAYER PATTERN

* Corresponding author.

§ Current address: Key Laboratory of Mesoscopic Chemistry of MOE and Department of Polymer Science & Engineering, College of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.

† National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.

2

ABSTRACT

Aspirin and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) are deposited from their alcoholic

mixed solution onto highly oriented pyrolytic graphite (HOPG) by spin coating. The film structure and

morphology are characterized by atomic force microscopy (AFM). The barely soluble DMPE forms a highly

oriented stripe phase due to its 1-D epitaxy with the HOPG lattice. The bilayer stripe pattern exposes the cross

section of the lipid bilayer lamellae, and enables the direct visualization of the molecular interactions of drug or

biological molecules with either the hydrophobic or hydrophilic part of the phospholipid bilayer. The bilayer

pattern affects the aspirin molecular deposition and aggregation. AFM shows that the aspirin molecules prefer

to deposit and aggregate along the aliphatic interior part of the bilayer pattern, giving rise to parallel dimer rods

in registry with the underlying pattern. The nonpolar interactions between aspirin and the phospholipid bilayer

are consistent with the lipophilic nature of aspirin. The bilayer pattern not only stabilizes the rod-like aggregate

structure of aspirin at low aspirin concentration, but also inhibits crystallization of aspirin at high aspirin

concentration. Molecular models show that the width of the DMPE aliphatic chain interior can accommodate no

more than 2 aspirin dimers. The bilayer confinement may prevent aspirin from reaching its critical nucleus size.

This study illustrates a general method to induce a metastable or amorphous form of an active pharmaceutical

ingredient (API) by chemical confinement under high undercooling conditions. Metastable and amorphous solids

often display better solubility and bioavailability than the stable crystalline form of the API.

KEYWORDS: Phospholipid bilayer, aggregation, crystallization, patterning, and AFM.

3

INTRODUCTION

The ability to manipulate and characterize materials at the nanoscale has led to explosive research activities in

the molecular thin films, crystals, and devices. In supramolecular pharmaceutics, the same active pharmaceutical

ingredient (API) molecules are manipulated by various non-covalent interactions (hydrogen bonding, van der

Waals, π-π stacking, and electrostatic interactions) into different solid-state forms ranging from amorphous to

crystalline states.1 The solid-state form of the API affects its compressibility, solubility, dissolution rate, chemical

stability, and bioavailability. Some new drug discovery methods involve screening different crystallization

conditions in small volume2 and on engineered surfaces.3 Micro-patterns of self-assembled monolayers (SAMs)

have been used to control crystallization by relying on guest molecules to recognize different surface functional

groups and by micro-confinement.4 The integration of bottom-up to top-down approaches in the developing

technologies, such as the high-throughput screening of drugs, requires the knowledge of placing molecules on

ever diminishing patterns.

Alkanes and alkane derivatives are known to self-assemble into a long-range ordered stripe phase on highly

oriented pyrolytic graphite (HOPG) due to the one-dimensional (1-D) epitaxial match between the 1,3-

methylene group distance (= 0.251 nm) and the distance of the next nearest neighbor of the HOPG lattice (=

0.246 nm).5,6 Recently, this molecular pattern has been used to align small organic molecules7,8 as well as

macromolecules.9,10,11,12 The amphiphilic pattern at the solid and liquid interface has been reproduced in thin

solid films by spin coating, and has been used to synthesize sulfide molecular rod arrays on the copper

arachidate template.13 Both the arachidate pattern and the subsequent inorganic rod arrays have been

characterized by atomic force microscopy (AFM). AFM is capable of resolving the location and shape of guest

molecular aggregates in relation to a specific type of functional groups on the host pattern. The amphiphilic

pattern of phospholipids serves as a model for the study of the molecular interactions of drug or biological

compounds with biomembranes and cells. The phospholipid stripe phase is structurally similar to the plane

4

perpendicular to the bilayer lamellae. The bilayer pattern exposes the hydrophobic and hydrophilic parts

simultaneously for guest molecular recognition and also for AFM imaging. This enables detailed structural

analysis of drug or biological molecular interactions with specific functional groups of the phospholipid bilayer.

This paper reports the effect of the phospholipid bilayer pattern on the deposition and aggregation behavior of

aspirin. Aspirin, also called acetylsalicylic acid or 2-(acetyloxy)-benzoic acid, was first synthesized by Bayer in

1897. Aspirin is a nonsteroidal anti-inflammatory drug (NSAID), widely used to treat human inflammatory

disorders, such as blood coagulation, thrombosis, and atherosclerosis, by inhibiting platelet aggregation.14

Recently, aspirin has also been found to reduce the risk of heart attack and to be effective against colorectal

cancer. The primary mechanism of aspirin drug action is its interference with the biosynthesis of inflammatory

prostaglandins (PGs).15 Another known effect of NSAIDs is their capability to perturb the phospholipid

ordering in the biomembranes, and thus affecting the normal functions of the membrane proteins.16 Aspirin is a

weak acid and lipophilic.17 Aspirin is found to increase the fluidity of liposomes by inserting itself in between the

hydrocarbon chains of the phospholipid molecules. Aspirin has only one crystal form,18 which makes its

structural analysis less complicated. The aspirin crystal consists of hydrogen-bonded dimers. The hydrogen-

bonded dimers are the most common supramolecular synthon for monocarboxylic acid crystals.19 The

supramolecular synthon is the smallest molecular building block, whose symmetry and connectivity predispose

the symmetry and packing in the final crystal structure. This paper presents a first study on the adsorption of

supramolecular aggregates in a predefined way on the graphite-based templates. The study of the aggregation

behavior of the aspirin dimers may shed light on nanoscale confinement means to engineer crystals with self-

replicating building blocks.

5

EXPERIMENTAL

Materials. Aspirin (Aldrich, +99.5%), DMPE (Sigma, 99%), ethanol (Pharmco, 100%), and methanol

(Mallinckrodt Chemicals, 100%) are used as received. ZYB grade HOPG (Mikromasch) is hand-cleaved with

an adhesive tape just before film preparation until a smooth surface is obtained.

Film preparation. Aspirin and DMPE are dissolved in methanol or ethanol in different molar ratios. The

solubility of aspirin in alcohol is 1.11 M,17 while DMPE dissolves sparingly and slowly in alcohol.

Approximately 10-5 M DMPE is dissolved after shaking the solution for 10 minutes. DMPE concentration is

maintained at 10-5 M, while the molar ratio of aspirin to DMPE is varied from 1 to 5000. 100 µl of a freshly

prepared solution, filtered with 0.22 µm PTFE filter (Millipore), is placed on a HOPG substrate rotating at

3000 rpm (PM101DT-R485 Photo Resist Spinner, Headway Research) at room temperature for 1 minute.

Characterization. The optical images are captured by an Olympus BX60 microscope with a SONY DXC-

970MD camera in transmitted light. AFM images are obtained with either an E scanner (Nanoscope III,

VEECO) or a Dimension Scan Head (Dimension 3100, VEECO). Height, amplitude, and phase images are

obtained in Tapping Mode in ambient air with silicon tips (TESP, VEECO). Only height images are shown here

unless specified. Scan rate is 1 Hz. Integral and proportional gains are approximately 0.4 and 0.7 respectively.

The crystal structures and structural analysis are made with the Materials Studio software programs from

Accelrys.

6

RESULTS AND DISCUSSION

Aspirin crystals recrystallized in alcohol. Aspirin recrystallized from methanol and ethanol by slow

solvent evaporation forms rectangular plates as shown in Figure 1. The largest aspirin crystal face from alcohol

is the (001) face, followed by two other low-energy faces (100) and (011).20 Aspirin recrystallized from hexane

forms needles. Aspirin crystals with the needle habit dissolve faster in water than the crystals with the plate

habit.21,22 Therefore commercial aspirin tablets contain a significant amount of needle crystals.

Spin-coated DMPE films. Spin coating of 10-5 M DMPE methanol solution produces the stripe phase on

HOPG with close to a monolayer coverage. Figure 2a-c display the spin-coated film structures as captured by

AFM. Similar film structures are obtained also from ethanol. The stripe phase domain is rectangular in shape

with an average domain size of 200 nm. The domain size is consistent with the single crystalline domain size of

ZYB grade HOPG. Two sides of the rectangle are terminated clearly at the stripe edges, while the other sides

are less obvious because of higher boundary energy. The domain orientation displays the 3-fold symmetry of

the HOPG lattice, as expected from the 1-D epitaxy (Figure 2a). The domain height is 0.7 nm by the sectional

height analysis (Figure 2b). At full monolayer coverage, the film appears to be identical to HOPG at the

micrometer scale, showing the typical folded layer ledges of HOPG. The AFM tip is capable of sweeping the

DMPE molecules away when scanning at a force = 100 nN repeatedly in Contact Mode. The AFM tip digs a

square trench with the same size of the previous scans as a result. Sectional height analysis shows that the

unperturbed film portion is 0.7 nm above the square trench. It is found that the tip does not damage the HOPG

surface at the same force level. The measured thickness is much smaller than the DMPE chain length, which

excludes the normal monolayer structure with vertically oriented hydrocarbon chains. The interchain spacing in

triclinic alkane crystals is 0.42 nm and 0.48 nm in the direction perpendicular and parallel to the zigzag carbon

chain plane respectively.23 The film thickness of 0.7 nm is consistent with the DMPE double chains lying parallel

to the HOPG basal plane but in a tilted configuration in order to maximize the chain packing. The center-to-

7

center distance between neighboring stripes is measured to be 5.2 nm by the 2-D Fourier transform analysis

(Figure 2c inset). This periodicity, of which 3.3 nm belongs to the hydrophobic tails,24 agrees well with the

DMPE bilayer thickness as determined by x-ray scattering.25 Energetically the lipid bilayer should be continuous

across its hydrophobic interior in a tail-to-tail configuration with a small separation existing between the

opposing headgroups from neighboring bilayers. The same energy consideration also requires the stripe phase

terminating at the hydrophilic headgroups. Figure 2c shows that the domain edge matches the edge and not the

center of a bright stripe. It is evident that the center of the bright stripe should correspond to the hydrophobic

center of the lipid bilayer, while the dark gap should correspond to the hydrophilic headgroup region.

Spin-coated aspirin films. No aspirin molecules are observed on HOPG when the aspirin concentration

is below 10-3 M. At 10-3 M, the HOPG surface is sparsely covered by the molecular aggregates presumably

belonging to aspirin. The aggregates are irregular in shape and do not bind strongly to HOPG (Figure 3a). A

minimum force in Tapping Mode can still sweep the aggregates away. The average height of the aggregates is

0.5 nm (Figure 3b). Above 2×10-3 M, localized crystalline layers of aspirin of at least 10 nm in thickness appear

on HOPG. Figure 3c shows the aspirin film spin coated at 10-2 M in ethanol. The layers have orthogonal

boundaries with a minimum differential thickness of 1.1 nm (Figure 3d). This is consistent with the texture

orientation of the aspirin crystal (001) face. Some 60° angles also exist, which indicates likely azimuthal

correlation between the aspirin (001) face and the HOPG basal plane.

Spin-coated aspirin and DMPE films. The molar ratio of aspirin to DMPE is varied from 1 to 5000 with

the DMPE concentration fixed at 10-5 M in methanol or ethanol. When the aspirin to DMPE molar ratio is less

than 5, only the DMPE stripe phase is observed on HOPG. On the other hand, when the aspirin to DMPE

molar ratio is greater than 50, the film morphology is largely that of the aspirin crystalline layers. When the molar

ratio is between 5 and 50, the film shows both the molecular aggregates of aspirin on top of the DMPE stripe

domains and the plate-like aspirin crystals on bare HOPG areas in between the DMPE domains, as illustrated

by the scheme in Figure 4a. The spin-coated films from binary solutions generally show phase separation. If the

8

two components are similar to each other in solubility and surface affinity, lateral phase separation occurs.

Significant difference in solubility or surface affinity between the two components results in vertical phase

separation and the formation of a wetting layer.26,27,28 This AFM study reveals a largely vertical phase

separation between aspirin and DMPE, with DMPE forming the wetting layer. This is due to the large difference

in alcohol solubility between aspirin and DMPE. For example, when the DMPE concentration is 10-5 M and the

aspirin to DMPE molar ratio is 50, the solution is saturated with DMPE but highly under-saturated with aspirin

(aspirin concentration/solubility = 0.00045). DMPE precipitates upon the slight solvent evaporation. Its epitaxial

interaction with HOPG warrants the formation of the wetting monolayer as well as its self-organization into the

stripe pattern. Aspirin precipitates only when its solubility limit is reached with further solvent evaporation, either

on the DMPE layer, or, if the surface is not fully covered, on HOPG. AFM images show that aspirin molecules

recognize the amphiphilic pattern inherent in the DMPE stripe phase. Aspirin molecules adsorb only on the

hydrophobic interior of the DMPE bilayer stripe while avoiding the hydrophilic boundaries. This molecular

recognition should be due to the strong lipophilicity of aspirin. The bilayer pattern should promote the 1-D

aggregation of aspirin dimers, while limiting the 2-D and 3-D crystallization. Inhibition of the growth of the (001)

face is desirable as this face is less water-soluble than the (100) face. Here the nonpolar interactions are

sufficient to stabilize an otherwise highly unstable molecular rod structure. Meta-stable and amorphous forms of

APIs are often more desirable than the stable crystalline form because they tend to show higher bioavailability.

Figure 4b-d shows the film made from a mixed solution with the aspirin to DMPE molar ratio of 10, as

imaged at 800-nm, 300-nm, and 150-nm scan size respectively. The individual domains display the 3-fold

azimuthal orientation as defined by the stripe edges. The domain has a rough appearance, which is different

from the parallel stripe pattern of the DMPE layer. A closer look (Figure 4c+d) shows an additional layer

partially covering the DMPE stripes. The second layer is 0.7 nm above the first stripe layer (the stripe layer

thickness is still 0.7 nm) as measured in Figure 4e. Pure DMPE spin coated at 10-5 M, on the other hand, forms

a sub-monolayer structure. Presumably the bottom layer belongs to the DMPE wetting layer and the second

9

layer is made of aspirin aggregates. Figure 4d shows that the height undulation of the rod-like aspirin aggregates

is in phase with that of the underlying DMPE stripes. This means that the aspirin aggregates are situated directly

on top of the hydrophobic center of the DMPE bilayer stripe. This is consistent with the lipophilic nature of

aspirin, which interacts through nonpolar interactions with the hydrophobic interior of biomembranes and the

hydrophobic pockets of the protein receptors. When the aspirin to DMPE molar ratio is increased to 20,

elongated particles with ill-defined shapes are distributed randomly but evenly on the surface, as shown in

Figure 5a. The orientation of these particles is analyzed by measuring the orientation angles of those particles

whose long axes are clearly defined. The orientation histogram is presented in Figure 5b. The mutual orientation

angle of the majority particles maintains the 3-fold symmetry of the HOPG basal plane. Figure 5c+e are two

characteristic features at higher magnifications. Figure 5c shows that aspirin forms a continuous layer on the

DMPE stripe phase. The layer is 0.7 nm above the DMPE stripes (Figure 5d). The long sides of the aspirin

layer are parallel to the DMPE stripe and thus better defined. The other sides display no clear boundaries

against the DMPE background. Figure 5e shows rectangular aspirin crystals located in between the DMPE

stripe domains. The height of the crystals, 1.1 nm, is one unit length along the [001] axis. It is clear that the

DMPE bilayer pattern inhibits the crystallization of aspirin by stabilizing the rod-like molecular aggregates.

Structural analysis and modeling. It is possible to understand the AFM images further by studying the

molecular packing structures of aspirin and DMPE in the solid state. The structural analysis provides a likely

model for the aspirin molecular aggregation on the DMPE bilayer pattern. The DMPE crystal structure is

unavailable, and therefore the crystal structure of 1,2-dilauroyl-sn-glycero-3-phosphoethanolamine acetic acid

(DLPE) is used instead. DLPE has the same molecular structure as DMPE with the exception that it is shorter

by two carbons in each aliphatic chain. DLPE forms a monoclinic crystal structure P21/c with the following unit

cell parameters: a = 47.700 Å, b = 7.770 Å, c = 9.950 Å, and β = 92.000º.29,30 One reaches a similar

hydrophobic layer thickness of 32.920 Å from the crystal structure as the value by x-ray scattering24 by adding

an ethylene unit (= 4.992 Å) to each of the DLPE chains. The hydrophilic headgroup thickness of the DMPE

10

bilayer is 14 Å according to the molecular model. The periodicity in the AFM images is slightly larger the

expected bilayer thickness of DMPE, which is also its crystal unit cell length along the [100] direction. The

hydrocarbon chains in the DLPE crystal lie along the [100] axis perpendicular to the bilayer plane. The height of

the DMPE layer measured by AFM matches the unit cell length along the [010] direction, about one and half

the hydrocarbon chain diameter. The plane containing the two hydrocarbon chains of a DMPE molecule should

be tilted against the HOPG basal plane for maximum packing. The zwitterionic phosphoethanolammonium

headgroup dipoles are parallel to the bilayer surface and along the [010] axis in the DLPE crystal. The

ammonium groups form short bonds (bond length = 2.7 Å) with neighboring phosphate groups with partial

hydrogen and the rest ionic bonding character.31 This rigid bonding structure among headgroups may enhance

the epitaxial ordering of DMPE on HOPG. It can be concluded that DMPE exhibits an azimuthal orientation on

the HOPG basal plane with the hydrocarbon chains along the interhexagonal direction of the HOPG lattice.

Aspirin recrystallizes from ethanol into a monoclinic crystal structure P21/c with the following unit cell

parameters: a = 11.430 Å, b = 6.591 Å, c = 11.395 Å, and β = 95.68º.18,32 Aspirin forms the inversion-

symmetric, hydrogen-bonded carboxylic acid dimer as illustrated in Figure 6a. The (001) and (100) faces are

displayed in Figure 6b as viewed along the [010] direction. The (001) face contains the methyl and phenyl

groups, while the (100) face contains the ester groups. Aspirin dimers most likely prefer to face the DMPE

bilayer with the (001) face in order to maximize the nonpolar-nonpolar interactions. A previous AFM study by

Danesh et al. shows that the methyl-terminated AFM tip interacts more favorably with the more hydrophobic

(001) face, while the carboxyl-terminated AFM tip is attracted more to the (100) face.33 Aspirin dimers are

free to aggregate through π-π stacking along the [010] axis along the DMPE stripe direction; however, the

aggregation in the direction perpendicular to the stripe is inhibited by the hydrophilic bilayer boundaries. The

DMPE hydrocarbon region is 3.3 nm wide, which can fit up to 2 unit cells along the aspirin [100] axis. Aspirin

aggregates may not be able to reach the required critical nucleus size in order for crystallization to occur on the

DMPE pattern. Therefore aspirin forms only molecular aggregates on DMPE at the same concentration where

11

crystals are observed on the bare HOPG substrate. The AFM-determined aggregate height, 0.7 nm, less than

the unit cell length along [001], also indicates that crystallization has not occurred. The height is close to the size

of aspirin, 7.433 Å, as measured lengthwise from the methyl to the phenyl group from its crystal structure.

Figure 6c displays the possible model of the aspirin aggregation on the DLPE bilayer, which is consistent with

the AFM study.

12

CONCLUSIONS

This paper describes the molecular aggregation behavior of aspirin molecules on the bilayer stripe pattern of

DMPE. The pattern is formed spontaneously by spin coating the DMPE alcoholic solution on HOPG. When

aspirin is added to the DMPE solution, it deposits either on top of the DMPE stripe pattern or on uncovered

HOPG surface during spin coating. Aspirin molecules do not deposit randomly, but prefer to aggregate along

the hydrophobic center of the DMPE bilayer. It shows that the aspirin molecules can bind to a specific region of

the lipid bilayer through the nonpolar van der Waals interactions. While the bilayer pattern promotes the aspirin

dimer aggregation via π−π stacking along the length of the stripe, the narrow width of the stripe inhibits the

crystallization of aspirin, probably by limiting the association among aspirin dimers in the width direction of the

bilayer stripe. The experimental approach described here may offer a simple strategy to study the effect of

surfactant mesophases and liposomes on the crystal engineering and encapsulation of the APIs. It is especially

relevant to the solid-state preparation methods that require small volume and high undercooling. One may utilize

the various non-covalent interactions between the API molecule and surfactant or polymer additive molecule to

control the degree of crystallinity and even the solid form in order to achieve different aggregate structures from

the same API molecule.

13

ACKNOWLEDGMENT

This work is partially supported by the National Science Foundation (CTS-0221586 and CTS-0216109).

G. Mao acknowledges the financial support from the German-American Fulbright Commission.

14

FIGURE CAPTIONS

Figure 1. Optical micrograph of aspirin recrystallized from ethanol by slow solvent evaporation.

Figure 2. AFM images of DMPE films on HOPG spin coated from 10-5 M methanol solution. (a) A 600-nm

scan showing sub-monolayer domains. The angle between domains as shown is 59.48º. (b) The cross sectional

height profile along the dotted line in (a). The domain height is 0.7 nm. (c) A 200-nm scan of the DMPE bilayer

stripes. The periodicity is 5.2 nm as determined from the 2-D Fourier transform pattern in the inset. The arrow

points to the direction of the interlinear spacing R.

Figure 3. AFM images of aspirin films on HOPG. (a) A 500-nm scan showing disordered molecular aggregates

spin coated from 10-3 M methanol solution. (b) The cross sectional height profile along the line in (a). The

aggregate height is 0.5 nm. (c) A 5-µm scan showing crystalline layers spin coated from 10-2 M ethanol

solution. The arrow points to a top crystalline layer with a step edge angle close to 90°. Some edges display a

60° angle as marked. (d) The cross sectional height profile along the dotted line in (c). The minimum step height

of the aspirin crystal is 1.1 nm.

Figure 4. AFM images of aspirin and DMPE mixed films on HOPG with aspirin to DMPE molar ratio = 10. (a)

Schematic graph of topographical features observed in the mixed film of aspirin and DMPE. (b) A 800-nm

scan. (c) A 300-nm scan. (d) A 150-nm scan. The arrows point out that the aspirin molecular aggregates sit

directly on top of the DMPE stripes, not in between. (e) The cross sectional height profile along the line in (c).

The height difference between the top and bottom layer is 0.7 nm.

15

Figure 5. AFM images of aspirin and DMPE mixed films on HOPG with aspirin to DMPE molar ratio = 20. (a)

A 2-µm scan showing elongated but otherwise ill-defined aspirin aggregates. (b) Particle orientation analysis of

the previous image. (c) A 300-nm scan showing an aspirin layer on top of the DMPE stripes. (d) The cross

sectional height analysis along the line in (c). The aspirin layer height is 0.7 nm. (e) A 250-nm scan showing

rectangular aspirin crystals deposited on HOPG in between the DMPE stripe domains. The phase image is

shown here because the corresponding height image does not show the aspirin crystal shape as clearly.

Figure 6. Aspirin structural analysis by the Materials Studio software program. (a) The basic building block of

aspirin crystal, the aspirin dimer. The hydrogen bonds are marked by the dotted lines. (b) The functional groups

on the (100) face and (001) face as viewed along the [010] direction. (c) A structural model of aspirin rod-like

molecular aggregates on the DLPE bilayer pattern on HOPG. The HOPG surface is not drawn to avoid

crowding. Only one dimer row is drawn for every bilayer stripe even though the stripe width can accommodate

up to 2 dimer rows. The DLPE layer is created by cleaving the crystal along its (010) face. A vacuum slab is

created above the DLPE supercell. 9 aspirin dimers along the [010] axis are selected from the aspirin crystal

structure and are docked above the DLPE supercell. A vacuum slab with DLPE (010) face at the bottom

enables the docking of aspirin dimers within the vacuum above so that the dimer does not see a periodic image

of the surface above it. The dimer row is arranged to parallel the [001] direction of the DLPE layer.

16

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nm32.3nm1265.089

122LH =×

+×= .

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18

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19

SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).

75 nm

Aspirin

DMPE

Aspirin Dimer Rod

DMPE Bilayer Pattern

Aspirin Dimer Rod

20

Figure 1

0.5mm

21

Figure 2a

Figure 2b

0.7 nm

22

Figure 2c

100 nm

R = 5.2 nm

23

Figure 3a

Figure 3b

0.5 nm

250 nm

24

Figure 3c

Figure 3d

1.1 nm

2.5 µm

25

Figure 4a

Figure 4b

400 nm

26

Figure 4c

Figure 4d

75 nm

Aspirin

DMPE

150 nm

27

Figure 4e

0.7 nm

28

Figure 5a

Figure 5b

1 µm

0

10

20

30

40

Angle

Fre

quen

cy

60 120 180

29

Figure 5c

Figure 5d

0.7 nm

150 nm

30

Figure 5e

125 nm

31

Figure 6a

Figure 6b

[100]

[001]

32

Figure 6c

Aspirin Dimer Rod

DLPE Bilayer Pattern

Aspirin Dimer Rod

1.1 nm

2.3 nm