iron oxide magnetic nanoparticles used as probing agents to study the nanostructure of mixed...

Iron oxide magnetic nanoparticles used as probing agents to study thenanostructure of mixed self-assembled monolayers

Benoit P. Pichon,*a Gregory Barbillon,b Pascal Marie,c Matthias Paulya and Sylvie Begin-Colina

Received 5th July 2011, Accepted 25th August 2011

DOI: 10.1039/c1nr10729a

Self-assembled monolayers (SAMs) of organic molecules are of exceptional technological importance

since they represent a convenient, flexible, and simple system for tuning the chemical and physical

properties of surfaces. The fine control of surface properties is directly dependent on the structure of

mixed SAMs which is difficult to characterize at the nanoscale with usual techniques such as scanning

probe microscopies. In this study, we report on a general method to investigate at the nanoscale the

structure of molecular patterns which consist in SAMs of two components. Iron oxide nanoparticles

(NPs) have been used as probing agents to study indirectly the structure of mixed SAMs. Mixed SAMs

were prepared by the replacement of mercaptododecane (MDD) adsorbed by mercaptoundecanoic

acid (MUA) molecules on gold substrates. Therefore, the SAM surface displays both chelating

carboxylic terminal groups and non-chelating methylene terminal groups. As NPs have been previously

demonstrated to specifically interact with carboxylic acid groups, the increasing density in NPs was

correlated with the evolution of the COOH/CH3 terminal groups ratio. Therefore the structure of

mixed SAMs was studied indirectly as well as the kinetic of the replacement reaction and its mechanism.

With this aim, we took advantage of the SPR properties of the gold substrate and of the high refractive

index of iron oxide nanoparticles to follow their assembling on mixed SAMs as a time resolved study.

The high sensitivity and tuning of the SPR signal over a wide range of wavelengths are correlated with

the NP density. Furthermore, SEM combined with image analysis has allowed studying the

replacement rate of MDD by MUA in SAMs. We took also advantages of the magnetic properties of

NPs to evaluate qualitatively the replacement of thiol molecules.

Introduction

Self-assembled monolayers (SAMs) of organic molecules are of

exceptional technological importance for a wide range of appli-

cations depending on surface modifications.1,2 They represent

a convenient, flexible, and simple system for processing and

modifying the chemical and physical properties of surfaces

(hydrophilic/phobic, self-cleaning, antibacterial, etc.). SAMs are

usually prepared by the spontaneous adsorption of alkanethiols

on gold substrates. A wide range of molecules has been used to

generate well-defined organic surfaces which display functional

groups. An interesting achievement consists in mixed SAMs of

multiple components which offer a high tunability of the surface

activity as well as patterning down to the nanoscale.3 Mixed

SAMs are usually prepared by the co-adsorption of two different

molecules which is difficult to control since the molar ratio of

both components adsorbed on surfaces usually differs from that

of the solution.1 Indeed, the relative solubility of the molecules,

their relative interaction with solvent and the substrate and the

intrinsic properties of the head and tail groups will induce pref-

erential molecular interactions between adsorbates of the same

type.4,5 Therefore the composition and the structure of such

SAMs often result in phase segregation.4,6,7 Nanodomains have

been shown several times in mixed SAMs by STM7–10 and

AFM.7,11 While phase segregation results in the patterning of

surfaces, its formation process is still unclear and usually

proceeds to intermediate and partial phase separation which is

difficult to control. Therefore, the SAM structure is difficult to

predict since many parameters have to be taken into account.

Another method to pattern surfaces by using mixed SAMs

consists in taking advantage of the reversible absorption of thiol

molecules on gold surfaces.12–15 Their preparation consists in the

gradual replacement of self-assembled molecules in the mono-

layer by others and is reported to occur preferentially at

boundaries and defect sites. In contrast to the co-adsorption

aInstitut de Physique et de Chimie des Mat�eriaux de Strasbourg (UMRCNRS-UdS 7504), 23 rue du Loess – BP43, 67034 Strasbourg Cedex 2,France. E-mail: [email protected]; Fax: +33 (0)3 88 10 72 47;Tel: +33 (0)3 88 10 71 33bLaboratoire Charles Fabry de l’Institut d’Optique (CNRS UMR 8501),Universit�e Paris-Sud, Campus Polytechnique, RD128, 2 avenue AugustinFresnel, 91127 Palaiseau Cedex, FrancecInstitut Charles Sadron, Universit�e de Strasbourg, CNRS UPR 22, 23 ruedu Loess – BP 84047, 67034 Strasbourg Cedex 2, France

4696 | Nanoscale, 2011, 3, 4696–4705 This journal is ª The Royal Society of Chemistry 2011

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method which is ruled by the competitive adsorption of mole-

cules, the replacement method enables to direct the surface

patterning more precisely by the spatial arrangement of defect

sites and the replacement time. The replacement of hex-

adecanethiol (HDT) monolayers with 12-mercaptododecanoic

acid (MDDA) has been shown to proceed domain wise and to

result in phase separation in the SAM into sizable domains.14

Nevertheless, the structure of mixed SAMs is very difficult to

investigate precisely. Unless analysis techniques such as XPS

(composition) and scanning probe microscopies (spatial

arrangement) have greatly enhanced the understanding and the

optimization of the patterning of SAMs at the nanoscale, they

still remain difficult to process.10 STM or AFMoperates at a very

local scale and molecules with similar lengths are difficult to

discriminate when absorbed on a surface. An alternative is the

use of probing agents to study indirectly the structure of mixed

SAMs. Recently, iron oxide nanoparticles were found to be

useful to study the patterning of surfaces by mixed SAMs

prepared by the absorption method.16 The spatial arrangement

of chelating carboxylic acid and non-coordinating methylene

terminal groups at the SAMs surface was investigated as

a function of the NPs assembling. Indeed, nanoparticles were

observed to assemble only on specific regions of SAMs decorated

with carboxylic acid.

Nanoparticles may be used as probing agents which enable to

study indirectly the structure of mixed SAMs on gold substrates.

Furthermore, the use of gold substrate may enable to study the

kinetic of the NPs assembling by the Surface Plasmon Resonance

(SPR) technique. In recent decades, the SPR technique became

very attractive to probe and characterize chemical modifications

on noble metal surfaces such as gold.17 The SPR is very sensitive

to surrounding medium and, in particular, to small local changes

of the refractive index. Therefore, SPR was used to characterize

surface modifications resulting from the adsorption of organic

molecules.18 Gold nanoparticles, also featured by localized

surface plasmon (LSPR), were naturally used to improve the

sensitivity of this technique by extending the specific surface of

the film.19–26 They were assembled onto gold substrates func-

tionalized by SAMs of thiol molecules. However, the use of non-

metallic nanoparticles to study the changes of the SPR signal has

been poorly reported.22,27 Among them, iron oxide nanoparticles

have been used to detect very low concentration of biological

materials by the SPR technique with a high sensitivity and

selectivity,28–32 thanks to their high refractive index (2.42 for

magnetite).33

In this study, we report on a general method to investigate the

structure of molecular patterns at the nanoscale. Iron oxide

nanoparticles are used as probing agents to perform indirect and

time resolved studies of the structure and of the formation

mechanism of mixed SAMs. Mixed SAMs were prepared by the

gradual replacement of molecules containing non-chelating

methylene terminal groups by molecules displaying chelating

carboxylic terminal groups at their surface. The composition of

the SAMs and the spatial arrangement of methylene and

carboxylic acid terminal groups are investigated as a function of

the replacement time. As nanoparticles interact specifically with

carboxylic acid groups, their assembling on mixed SAMs enables

to determine the patterning of the surface as a function of the

replacement time. The assembling of NPs was studied by using

the SPR technique which was combined with SEM image anal-

ysis. The use of high refractive index iron oxide nanoparticles

results in the enhancement of the variation of the SPR properties

of gold substrates to determine quantitatively the NPs density.

We took also advantages of the magnetic properties of NPs to

evaluate qualitatively the replacement of organosulfur

molecules.

Experimental section

Chemicals and materials

11-Mercaptoundecanoic acid (MUA) and mercaptododecane

(MDD) were purchased from Aldrich. Absolute ethanol and

tetrahydrofurane were used as received.

Gold film preparation

Glass substrates are cleaned in a freshly prepared piranha solu-

tion (3 : 1 H2SO4 (98%), H2O2 (30%)) for 30 min. Once cooled,

the glass substrates are rinsed abundantly with deionized water

and dried with N2 gas. The last step is to evaporate a gold thin

layer (48 nm) in order to realize the metallic film. Previously, an

adhesion layer (Cr) for gold is evaporated (2 nm).

Preparation of self-assembled monolayers (SAMs)

Freshly cleaned ion sputtered gold substrates under O2/Ar

plasma (2 min) were used for all preparation. Mixed SAMs were

prepared by the replacement method which occurs in two

steps.14,34,35 First, MDD-SAMs were prepared by soaking gold

substrates at room temperature for 24 hours in 10 mmol etha-

nolic solutions of mercaptododecane (MDD). Second, freshly

washed and dried MDD-SAMs were then immersed in

a 10 mmol ethanolic solution of 11-mercaptoundecanoic acid

(MUA) at room temperature for different times (30 minutes, 2, 4,

7, 24, 96, 170 and 288 hours). SAM substrates were then washed

extensively with absolute ethanol to remove unbound thiol

molecules and dried under N2 stream. The corresponding SAMs

were respectively named SAM-0.5H, SAM-2H, SAM-4H, SAM-

7H, SAM-24H, SAM-96H, SAM-170H and SAM-288H. Both

molecules have similar chain lengths to maximize the reciprocal

influence of the chelating and non-chelating abilities of carbox-

ylic and non-polar methylene terminal groups towards the mean

surface activity and mixture.

Preparation of iron oxide nanoparticles (NPs)

Iron oxide NP were produced by the thermal decomposition

method and following the procedure that is detailed in ref. 36–38.

It consists in the preparation of an iron(III)/oleate complex

(Fe(oleate)3), which is thermally decomposed in a high boiling

solvent in the presence of oleic acid. Fe(oleate)3 was prepared

from FeCl3$H2O (10.8 g, 40 mmol, 97%, Aldrich) which was

dissolved in 60 mL H2O (Milli-Q) and 80 mL ethanol. This

solution was mixed with a solution of sodium oleate (36.5 g,

120 mmol, 82%, Riedel-de Ha€en) dissolved in hexane (140 mL)

and refluxed at 70 �C for 4 h. The organic phase containing the

iron oleate complex was separated, washed three times with

distilled water (30 mL) to extract salts, dried using MgSO4, and

This journal is ª The Royal Society of Chemistry 2011 Nanoscale, 2011, 3, 4696–4705 | 4697

finally hexane was evaporated. The resulting iron oleate complex

was a reddish-brown viscous solution and stored at 4 �C. A

combination of Fe(oleate)3 (2 g, 2.2 � 10�3 mol), oleic acid

(1.24 g, 3.3 � 10�3 mol) and octyl ether (20 mL) was stirred for

1 h to dissolve the reactants. The temperature was carefully

raised to reflux with a heating rate of 5 �C min�1 without stirring

for 120 min under air. After cooling down to room temperature,

the black suspension of nanocrystals was washed 3 times by

addition of ethanol and centrifugation (8000 rpm, 10 min). The

obtained nanocrystals could be easily suspended in various

organic solvents to raise a highly stable suspension which can be

stored for several months. The size monodispersity of NPs was

improved by applying a size selection precipitation process.39 The

nanoparticles were suspended in hexane at a concentration of

1 mg mL�1 and precipitated by adding the same volume of

acetone followed by centrifugation. The precipitate was redis-

persed in tetrahydrofurane (THF) to prepare a highly stable

suspension of coated nanoparticles with a specific concentration

of 3.7 mg mL�1.

Assembling of iron oxide nanoparticles on SAMs

Replaced SAMs were immersed directly after preparation in the

suspension of oleic acid coated nanoparticles in THF at room

temperature for only 10 minutes. Each substrate was then

extensively rinsed with THF and placed in an ultrasonic bath in

THF for 10 seconds to remove any physisorbed nanoparticles

and finally dried under a stream of nitrogen.

Characterization of the samples

Scanning electron microscopy was performed using a JEOL 6700

microscope equipped with a field emission gun (SEM-FEG)

operating at an accelerating voltage of 3 kV. Atomic Force

Microscopy (AFM) was performed using a Digital Instrument

3100 microscope coupled to a Nanoscope IIIa recorder.

Measurements were done in the tapping mode onto substrates

before and after exposition to the suspension of nanoparticles.

Collected data were analyzed with WSXM software.40

Image analysis

The digitized SEM images were analyzed with the Visilog�

software (Noesis, France) following an edge detection procedure

to determine the density of nanoparticles and their position.

Several techniques were combined such as filtering (background

correction and Gaussian smoothing for removing detail and

noise), edge detection methods (Laplacian of Gaussian and zero-

crossing operators), and adaptive threshold to locate the

boundary (or edge) of each particle. The nanoparticles were

analyzed from SEM pictures as we reported previoulsy16 so as to

extract their number and positions within the image (X and Y

coordinates of the centre of mass). Densities in NPs were

calculated on each mixed SAMs from the number of nano-

particles on a precise area. The surface of the SAMs covered by

NPs was deduced according to the size, the spherical shape and

the density in NPs. NPs can be assimilated to circles with

a maximum compacity of 0.9 if they are ideally organized in a 2D

hexagonal closed packed (hcp) lattice. Therefore the effective

surface covered by NPs is compared to the maximum value of

90%.

Surface Plasmon Resonance measurements

The used setup is the Kretschmann configuration with

a TM-polarized white light source and a fixed angle of illumi-

nation (q ¼ 57�) (Scheme 1). The wavelength range used in our

experiments is 600–900 nm. An equilateral SF10 glass prism has

been used, and for glass substrates, the refractive index is similar

to the glass prism. The plane face of the prism was coupled to the

SF10 glass substrates via index matching oil (Cargille Labora-

tories Inc.). The water is used as reference, because the spectral

shifts due to the adsorption of magnetic nanoparticles will

remain in the 600–900 nm range, which is our work range. The

spectral measurements were chosen in order to use a simple

model described by Campbell et al. (see the Results and discus-

sion section) to determine the effective thickness of the adsorbed

layer from the spectral shifts and thus to calculate the paving

density of magnetic nanoparticles. With the value of effective

thickness d, we determine the paving density by dividing this

value d by the magnetic nanoparticles size (diameter), which is

12 nm. Then, this value is divided by 0.9 corresponding to the

compacity of the hexagonal 2D lattice, and we thus obtain the

paving density value expressed as a percentage of the maximum

value of paving density (here 0.9).

Magnetic measurements

The magnetic properties of nanoparticles assembled on SAMs

were investigated by using a superconducting quantum inter-

ference device (SQUID) magnetometer (Quantum Design model

MPMS-XL). The magnetization was recorded at 5 K as a func-

tion of the applied magnetic field. The substrates containing the

assemblies of nanoparticles were placed parallel to the applied

magnetic field. The magnetization values measured for nano-

particles assembled on SAMs were normalized according to the

weight of each substrate.

Results and discussion

The assembling of NPs in 2D arrays can be controlled by the

surface patterning generated by SAMs displaying different

functional head groups.16,41 Nanoparticles have been demon-

strated to assemble specifically on SAM surfaces decorated with

carboxylic acid head groups and not with methylene head

Scheme 1 Simplified scheme of principle of the surface plasmon reso-

nance measurements.


groups. Therefore mixed SAMs of 11-mercaptoundecanoic acid

(MUA) and mercaptododecane (MDD) were prepared by the

solution phase technique.1 Both molecules have similar chain

lengths to maximize the reciprocal influence of the chelating and

the non-coordinating abilities of carboxylic acid and methylene

groups, respectively on the mean surface activity and mixture. In

this study, the fine tuning of the chemical nature of surfaces was

performed by preparing mixed SAMs according to the replace-

ment method which consists in a stepwise deposition method

(Scheme 2).14,34,35 First, MDD-SAMs were prepared by the

immersion of gold substrates for 24 hours in an ethanolic solu-

tion of MDD molecules (10 mM). Second, the MDD molecules

were progressively exchanged by dipping the MDD-SAMs in

a solution of MUA molecules (10 mM) for times from 0.5 to

288 hours. SAMs were named as a function of the replacement

time, respectively SAM-0.5H, SAM-2H, SAM-4H, SAM-7H,

SAM-24H, SAM-96H, SAM-170H and SAM-288H. The modi-

fication of the surface chemistry was confirmed by contact angle

measurements with water (Table 1). The contact angle measured

for the SAM-MDD fits perfectly with a highly hydrophobic

surface.42 This value decreases with the replacement time to

a value which is similar to SAM-MUA43 and demonstrates the

gradual replacement of the methylene groups by the carboxylic

acid groups at the SAM surface.43 The use of methylene and

carboxylic acid groups as terminal functional groups induces

a large difference in the wetting properties of the corresponding

mixed SAMs. Nevertheless, contact angle measurements were

performed onto relatively large surfaces, and this technique is not

suited to distinguish the arrangement of the functional head

groups at the nanoscale. Therefore, iron oxide NPs have been

used as probing agents to study the formation of mixed SAMs

and the spatial arrangement of carboxylic acid groups at their

surface.

Stable suspensions of nanoparticles with narrow size distri-

bution and no aggregation have to be prepared with the aim to

correlate the evolution of the SAM structure with the changes of

the SPR signal and with the NP density. 12 nm sized iron oxide

nanoparticles which crystallize in the spinel structure (Fe3–dO4)

have been synthesized following the thermal decomposition

method.37,44,45 The coating of nanoparticles with oleic acid leads

to highly stable suspensions in many organic solvents and avoids

their agglomeration. The assembling process was performed by

dipping each mixed SAM for 10 minutes in the suspension of

nanoparticles in THF.16,41 Specific interactions between carbox-

ylic acid terminal groups at the SAM surfaces and the NP surface

occurred through a ligand exchange process. Then the SAMs

were extensively rinsed with THF to remove the physisorbed NPs

which may remain attached onto the first NPs layer.

The assembling of iron oxide NPs on each mixed SAMs has

been studied by the SPR technique. The increasing density in

NPs immobilized onto mixed SAMs was expected to strongly

influence the optical properties of the gold film. Therefore the

quantitative study of SPR changes is correlated with the amount

of non-plasmonic nanoparticles assembled on SAMs. The

reflectivity is measured as a function of the wavelength of the

light source (i.e. at a fixed angle of incidence), which is directly

Table 1 Contact angle values (degrees) measured with water on mixed SAMs as a function of the replacement time

SAM-MDD SAM-0.5H SAM-2H SAM-4H SAM-24H SAM-96H SAM-288H SAM-MUA

103 � 1.2 82 � 1.9 77 � 1.6 75 � 1.6 73 � 2.4 69 � 2.3 54 � 0.8 49 � 1.3

Scheme 2 Preparation of mixed SAMs by the replacement method and assembling of nanoparticles on the SAMs.


related to the modification of the metal surface. At the minimum

of reflectivity, the wavelength is directly related to the refractive

index and to the thickness of the adsorbed layer. On the basis of

these considerations, the high refractive index of iron oxide

nanoparticles (2.42 for magnetite)33 is expected to enhance the

shift of the wavelength.

Fig. 1 shows the evolution of the SPR by monitoring the

minimum of the reflectivity as a function of the wavelength. The

reflectivity curve with a minimum centred at the lowest SPR

wavelength (lAu ¼ 679 nm) represents the SPR peak of the gold

film without functionalization. The second curve displays

a minimum which is shifted to a higher wavelength lAu+MDD of

683 nm. This is consistent with the adsorption of MDD mole-

cules on the gold film.46 The next reflectivity curves represent the

peaks of surface plasmon resonance after each replaced SAM

was immersed in the NP suspension. All curves display

a minimum in reflectivity which is shifted to higher wavelengths

when the replacement time increases. Shifts increase from 19 to

181 nm with respect to the wavelength corresponding to the

SAM-MDD (Table 2). This observation demonstrates that the

SPR reflectivity is modified and depends on the density of NPs

immobilized on SAMs. Moreover, a pronounced and regular

increase of the minimum reflectivity was observed with the SAM

replacement time. This phenomenon has already been assigned

to the 2D assembling of gold NPs on a gold substrate by Tamada

et al.23,24 These experiments show the large tuning of the SPR

signal over a range of almost 200 nm. The spectral shift is far

larger in comparison to studies that relate on the adsorption of

molecules onto gold surfaces. We ascribe such a variation to the

high refractive index of magnetite, which agrees with the simple

immobilization of nanoparticles onto gold surfaces functional-

ized by mixed SAMs.

The wavelength shiftDl of the minimum in reflectivity for each

mixed SAM covered by NPs has been determined from the

minimum of the MDD-SAM which is uncovered by NPs

(Table 2). From these values, we calculated the effective thick-

ness d of the deposited layer of NPs according to the simple

model described by Campbell et al.:47

Dl ¼ mDn [1 � exp (�2d/ld)]

where Dl is the wavelength shift, m is the sensitivity of our gold

film to the local refractive index, Dn is the change in refractive

index induced by an adsorbate (Dn ¼ nadsorbate � nwater), d is the

effective adsorbate layer thickness and ld is the characteristic

evanescent electric field decay length. The values ofm and ld were

calculated by the Finite Difference Time Domain (FDTD).24,25

We found the following values for our gold film: m ¼ 3330 nm

per refractive index unit (RIU) and ld ¼ 253 nm. The change in

the refractive index Dn is determined from a reference, which is

the refractive index of the medium in which the experiment is

performed, i.e. water (n ¼ 1.33). Iron oxide magnetic nano-

particles are coated with oleic acid, which represent a thickness of

about 1.5 nm. Therefore, the refractive index of both compo-

nents (nFe3O4¼ 2.42, nOA ¼ 1.45) has to be taken into account to

calculate the refractive index of these hybrid nanoparticles. It is

reasonable to consider that the refractive indices of these hybrid

nanoparticles (nNP ¼ 2.12) result from the combination of the

refractive index of each component in the proportion of their

volume fraction, respectively about 0.7 and 0.3. The effective

thickness values d corresponding to each spectral shift increase

regularly to 8.9 nm with the replacement time (Table 2). It should

be noticed that after 288 hours of replacement time, this value

remains lower than the diameter of 12 nm sized NPs. Therefore,

the corresponding paving density was determined from these

values of d by taking into account the hexagonal compact

packing of 12 nm sized NPs with a spherical shape and coated by

oleic acid. We observe the paving density to increase regularly

from 8.4% on SAM-0.5H to 82.0% on SAM-288H. The latter is

in good agreement with earlier results41 showing that NPs cover

Fig. 1 SPR measurements on replaced SAMs after exposure to the NP suspension.


89% of the surface of a MUA-SAM displaying only COOH

terminal groups at the surface.

To confirm the efficiency of SPR analysis as a quantitative

technique, NP assemblies onto replaced SAMs were also studied

by scanning electron microscopy (SEM) (Fig. 2). SAM-MDD

was used as a reference and as previously observed,41 no NPs

were immobilized after exposure to the NP suspension, only gold

grains were observed. In contrast, assemblies of NPs were clearly

observed on every replaced SAMs. The density in NPs clearly

increases as long as the initial MDD-SAM has been previously

immersed in the solution of MUA molecules. On the basis that

the carboxylic acid terminal groups at the SAM surface are

responsible for the immobilization of iron oxide nanoparticles,

we can conclude that the amount of COOH terminal groups

increases with the replacement times from 0.5 to 288 hours.

Moreover, the NP density remains unchanged after an increase

of the immersion time from 10 minutes to one hour. Taking into

consideration the rather fast kinetic of adsorption (below

10 minutes) of iron oxide NPs on a SAM fully decorated with

carboxylic acid group,41 the increasing density in nanoparticles is

thus correlated with the increasing amount of these groups on the

SAM surface.

The density in NPs immobilized on SAMs was calculated by

using the Visiolog� software (see the Experimental section). The

values are plotted as a function of the replacement time of MDD

Table 2 Effective thicknesses d and paving density of magnetic nano-particle layers calculated from the spectral shifts Dl of the minimum inreflectivity of SPR curves. Values are given as a function of the replace-ment time

Replacement time/hSpectralshift Dl/nm

Effectivethickness d/nm

Pavingdensity (%)

0.5 19 0.91 8.42 33 1.6 154 53 2.54 247 75 3.61 3324 99 4.79 4496 133 6.5 60170 170 8.34 77288 181 8.9 82

Fig. 2 SEM images of NPs assembled on replaced SAMs. (a) SAM-MDD, (b) SAM-0.5H, (c) SAM-2H, (d) SAM-4H, (e) SAM-7H, (f) SAM-24H, (g)

SAM-96H, (h) SAM-170H and (i) SAM-288H.


by MUA in Fig. 3a. The density in NPs continuously increases

with the replacement time and thus with the amount of chelating

carboxylic acid groups. Gold grains which are clearly observed

on SAM-MDD tend to be hidden as the density of NPs assem-

bled on mixed SAMs increases. More precisely, it increases

linearly and very fastly from 650 � 115 NP mm�2 on the SAM-

0.5H to 1611 � 79 NP mm�2 on the SAM-7H. Taking into

account the surface of 12 nm sized NPs shelled by a 1.5 nm thick

layer of oleic acid38 which corresponds to an occupied area of

176 nm2, these densities correspond to 13� 1.4% to 33� 1.6% of

the surface coverage, respectively. In contrast, the NP density on

SAM-24H to SAM-288H tends to level off from 3355 � 150 NP

mm�2 to 4023 � 101 NP mm�2. On SAM-288H, the surface

covered by NPs reaches a maximum of 80.8 � 2.0% which

remains slightly lower than in the case of NPs assembled onto

a SAM-MUA which is fully decorated with chelating carboxylic

acid terminal groups.41 It shows that the replacement of MDD

molecules by MUA molecules tends to reach the equilibrium

despite the excess of MUA molecules in the solution during the

preparation of mixed SAMs. In addition, if NPs would have been

assembled into the ideal 2D-hexagonal compact packing (hcp)

with a compacity of 0.9, NPs would cover almost 90%. This

difference is due to the random deposition of NPs which results

in some uncovered areas. Fig. 3b demonstrates that the density

values measured from SEM images are perfectly correlated with

the SPR shift (Dl) at the whole scale of each sample. Therefore,

the SPR technique is a very efficient way to quantify the amount

of NPs assembled on a gold surface at the sample scale.

SEM pictures also show that NPs initially assembled at the

gold grain junction. By increasing the replacement time, they

form domains which increase in size. The spatial distribution of

NPs assembled on SAM surfaces was evaluated by plotting the

pair correlation function (PCF) for each sample. This function

determines the average distance between each NP and its nearest

neighbours. It can be interpreted as an averaged probability of

finding the centre of a particle at a given distance, r, from the

centre of another particle normalized to the uniform probability

at large distances.48,49 Fig. 4 shows the PCFs for NPs assembled

on replaced SAMs. We note that for all SAMs, the PCFs are very

similar and reveal the typical features of NPs assembled with

short-range topological ordering. The g(r) function exhibits, in

all cases, a well pronounced primary maximum at the distance r

between the particles in the range between 12.0 and 13.6 nm.

These values are in good agreement with 12 nm sized NPs shelled

by a 1.5 nm thick layer of oleic acid which are tightly packed in

the assemblies. Moreover, the PCF is almost uniform for the

distribution of particles. Indeed, g(r) approaches unity for r > 25

nm which is significant of particles randomly located (jammed

disordered packing), they have essentially no structure. Never-

theless very weak peaks are observed at about 22 and 33 nm after

96 h of replacement time which may correspond to a short range

ordering of NPs. However, this accounts much more from the

high density in NPs on SAM-96H and SAM-288H than from

a real organization of NPs. These results indicate that the

structure of the NPs assemblies is not changing appreciably

although the chemical nature of the SAMs and the NPs density

change.

In a complementary study, the surface morphologies and

roughnesses of SAM-7H and SAM-96H after exposure to the NP

suspension were analyzed by AFM and compared to a SAM

before being exposed to the NP suspension (Fig. 5). The surface

coverage and the height of the covered areas were measured

assuming the length of both MDD and MUA to be identical.

NPs are clearly observed on both SAMs according to the cross-

section profiles which show average height differences of about

8.7 nm corresponding roughly to the NP size. These results are in

Fig. 3 (a) Wavelength shifts at the minimum of reflectivity (closed circles) measured by the SPR technique and NP density (opened circles) measured by

image analysis of SEM images on replaced SAMs after exposure to the NP suspension. (b) Wavelength shifts at the minimum of reflectivity plotted as

a function of the NP density.

Fig. 4 Pair correlation function of NPs assembled on replaced-SAMs.


good agreement with SEM images and confirm the higher NP

density on SAM-96H than on SAM-7H. In addition, the root

mean roughness in an area of 5 � 5 mm2 was determined to be

twice higher on SAM-96H (2.4 nm) than on SAM-7H (1.2 nm).

The increase of this value is consistent with the increasing density

of NPs when compared to SAMs before exposition to NPs which

are rather flat with low roughness (0.8 nm). As reported in

previous studies,41 gold substrates have a similar roughness

(0.9 nm) due to their topography which consists in gold grains.

The magnetic properties of iron oxide NPs were also used to

determine the amount of NP immobilized on mixed SAMs. The

magnetization was measured as a function of an external

magnetic field at 5 K (Fig. 6). TheM(H) curves of NPs assembled

on SAM-7H and SAM-96H are very similar and show hystereses

which correspond to a ferrimagnetic behaviour. These results

agree well with such 12 nm sized Fe3�dO4 nanoparticles prepared

by the decomposition technique.36,38,41,50 Both M(H) curves

exhibit a saturation magnetization (Ms) which corresponds to the

alignment of all the magnetic moments of the NPs in the direc-

tion of the applied field. Ms is directly and proportionally

dependent on the quantity of NPs. After dipping in the NP

suspension, SAM-96H exhibits a higherMs value than SAM-7H,

which is in good agreement with the higher NPs density on the

former as measured by the SPR technique and SEM images

analysis.

Discussion

The immobilization of nanoparticles on mixed SAMs leads to

interesting information on the nanostructure of mixed SAMs

and on the mechanism of the replacement reaction of organo-

thiol molecules adsorbed on a gold surface. On the basis of our

previous work,16,41 we know that NPs assemble on specific areas

of mixed SAMs containing carboxylic acid terminal groups.

Therefore the amount and the spatial arrangement of NPs

assembled on mixed SAMs are directly related to the amount and

the spatial arrangement of carboxylic acid terminal groups at the

SAM surface, i.e. the SAM structure.

The NP density was measured for each mixed SAMs by both

SPR technique and SEM image analysis. It increases as a func-

tion of the replacement time on mixed SAMs from 650 �

115 NP mm�2 up to 4023 � 101 NP mm�2 which is perfectly

correlated with the shift of the minimum in reflectivity from

683 nm to 864 nm. Such a wide shift accounts for an overall

tuning of more than 180 nm with respect to the curve of the

MDD-SAM and shows the high sensitivity of the SPR technique.

The SPR technique is also very suitable to measure the average

thickness of the NP layer. It reaches a maximum of 8.9 nm on

SAM-288H which is lower than the NPs diameter of 12 nm.

These different values can be explained by the spherical shape of

NPs which induces an inhomogeneous thickness. SEM andAFM

also show that the NP layer on SAM-288H is not fully covered by

the NPs (80.8 � 2.0%).

The assembling of NPs on mixed SAMs prepared with

different replacement times enabled us to perform a time resolved

study. Indeed, we followed the evolution of the NP density as

a function of the replacement of MDD by MUA molecules.

Therefore we investigated the kinetic of the replacement reaction

Fig. 5 AFM images of NPs assembled on SAM-7H (a, b, c and d) and SAM-96H (e, f, g and h). Topographic height images (a and e), phase images (b

and f), 3D images (c and g) and cross-section profiles (d and h) of panels observed at the horizontal line in topographic height images.

Fig. 6 Magnetization recorded as a function of an applied magnetic field

recorded at 5 K for SAM-7H and SAM-96H.


which clearly occurs very fast within the first 24 hours, while it

slows down for longer times as reported previously for

organothiols.51–53

The spatial arrangement of NPs on mixed SAMs also gives

some interesting information on the replacement mechanism.

Indeed, NPs assemble initially and preferentially in small

domains which correspond to the gold grain junctions. By

increasing the replacement time, NPs were immobilized in

domains with sizes increasing gradually from about 20 nm2

(SAM-0.5H) to 200–300 nm2 (SAM-24H). It shows that areas

occupied by carboxylic acid groups are initially located at the

gold grain junctions and extend at the surface of the SAMs.

Therefore, the replacement unequivocally takes place domain

wise and not in a random fashion. At longer replacement times,

some areas which correspond to gold terraces still remain

uncovered of NPs on SAM-288H and agree with a slower kinetic

of the replacement reaction in these areas. In addition, NPs tend

to be assembled individually on these areas, which agree with the

formation of small domains decorated by carboxylic acid groups

in larger methylene rich domains. This may account from some

rearrangement of MDD and MUA molecules in the SAM.

The phase segregation in mixed SAMs prepared by the co-

adsorption method is controlled by the affinity of organothiols

with solvents and the gold surface.16 In the case of the replace-

ment method, it is mainly controlled by the structural non-

homogeneity of the initial SAMs. Indeed the compactness of the

SAMs controls the kinetic of the exchange reaction. The different

miscibility of MUA and MDD molecules with regard to inter-

molecular interactions such as hydrogen bonding between

carboxylic acid groups against van der Waals interactions

between alkylene chains has to be taken into account.39,40 In

a first step, the replacement occurs preferentially in the domain

boundaries or at defect sites. Indeed gold grain junctions contain

steps, kinks, and other defects in contrast to gold flat terraces.

The relief induced by gold grain junctions is expected to dis-

favour the average coordination of gold atoms and to favour the

desorption of Au–organothiol complexes.52 It also favours the

disordering of alkyl chains of MDD molecules which results in

weaker interactions and in a higher solubility. In a second step,

the replacement reaction becomes much slower because defect

sites and domain boundaries decrease while crystalline domains

of tightly packed molecules are favoured by flat and large gold

terraces. The replacement reaction only occurs at the interface

between MDD and MUA domains. In these areas, the disorga-

nization of MDD molecules is favoured by the growing domains

ofMUAwhose formation is ruled by hydrogen-bonding between

carboxylic acid terminal groups.54 These interactions are

strengthened between the new adsorbates but disturb the van der

Waals interactions between the MDD molecules. As far as the

surface occupied by non-polar regions decreases, such an inter-

face between domains reduces and the replacement reaction is far

slower and tends to the equilibrium. This phenomenon is

concomitant to the reduction of the domain size and to the fact

that NPs tend to become independent after 170H of replacement

time. These observations may correspond to the rearrangement

of MUA molecules with the remaining MDD molecules within

the SAM. Nevertheless, due to the tight packing of the MDD

molecule and its very limited amount in the SAM, it cannot result

from the desorption/readsorption process. A surface diffusion

limited mechanism is much more possible and results in the

spatial rearrangement of the molecules to minimize the system

energy and reach the thermodynamic equilibrium.10

Conclusions

The assembling of iron oxide nanoparticles has been demon-

strated to be highly efficient to investigate the structure of mixed

SAMs at the nanoscale. The preparation of mixed SAMs by the

replacement method offers interesting possibilities to study the

structure of SAMs as a time resolved study. We took advantage

of both the SPR of gold substrate and the high refractive index of

iron oxide nanoparticles to determine precisely the density in

NPs at the scale of the samples by a simple and alternative

technique such as the surface plasmon resonance (SPR) spec-

troscopy. We showed the high sensitivity of the SPR of the gold

surface to the deposition of iron oxide (Fe3O4–d) NPs on mixed

SAMs. The minimum in reflectivity of the SPR signal has been

shifted on a large spectral range which is correlated quantita-

tively with the increasing density in NPs. These results were

confirmed by the density values calculated by SEM image anal-

ysis and by the qualitative evolution of the saturation magneti-

zation value which also depends on the amount of NPs

assembled on the mixed SAMs. These results were directly

correlated with the amount of carboxylic acid terminal groups at

the SAM surface. SEM and AFM also provided some informa-

tion on the spatial arrangement of NPs as a function of the

replacement time and thus on the mechanism of the replacement

of MDD byMUAmolecules. The replacement reaction of MDD

by MUA on the gold surface occurs in two distinctive steps: (i)

very fastly at defect and default sites in the SAM which are

favoured at the gold grain junctions and then level off at flat gold

terraces and (ii) by the diffusion of MUA in MDD rich domains

which remains on gold terraces. Finally, mixed SAMs prepared

by the replacement method result in a different patterning of the

surface in comparison to the co-adsorption method. The fine

adjustment of the surface activity of replaced SAMs by

controlling the immersion times of the initial MDD-SAM in the

solution of MUA molecules should be of primary interest to

study the magnetic properties of NP assemblies.

Acknowledgements

Financial supports were provided by the agence national pour la

recherche (ANR) and direction g�en�erale de l’armement (DGA).

Notes and references

1 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo andG. M. Whitesides, Chem. Rev., 2005, 105, 1103.

2 R. L. Grimm, N. M. Barrentine, C. J. H. Knox and J. C. Hemminger,J. Phys. Chem. C, 2008, 112, 890.

3 R. K. Smith, P. A. Lewis and P. S. Weiss, Prog. Surf. Sci., 2004, 75, 1.4 C. D. Bain, J. Evall and G. M. Whitesides, J. Am. Chem. Soc., 1989,111, 7155–7164.

5 J. P. Folkers, P. E. Laibinis and G. M. Whitesides, Langmuir, 1992, 8,1330–1341.

6 C. D. Bain and G.M.Whitesides, J. Am. Chem. Soc., 1989, 111, 7164.7 S. Chen, L. Li, C. L. Boozer and S. Jiang, Langmuir, 2000, 16, 9287.8 S. J. Stranick, A. N. Parikh, Y. T. Tao, D. L. Allara and P. S.Weiss, J.Phys. Chem., 2002, 98, 7636–7646.

9 I. Choi, Y. Kim, S. K. Kang, J. Lee and J. Yi, Langmuir, 2006, 22,4885–4889.


10 S. J. Stranick, A. N. Parikh, Y. T. Tao, D. L. Allara and P. S.Weiss, J.Phys. Chem., 1994, 98, 7636.

11 K. Tamada, M. Hara, H. Sasabe and W. Knoll, Langmuir, 1997, 13,1558.

12 P.-H. Lin and P. Guyot-Sionnest, Langmuir, 1999, 15, 6825.13 I. Takao, M. Wataru, T. Hiroshi, C. Nami, A. Uichi and

F. Masamichi, J. Vac. Sci. Technol. A, 2000, 18, 143714 T. Kakiuchi, K. Sato, M. Iida, D. Hobara, S.-i. Imabayashi and

K. Niki, Langmuir, 2000, 16, 7238.15 P. Gupta, A. Ulman, S. Fanfan, A. Korniakov and K. Loos, J. Am.

Chem. Soc., 2004, 127, 4.16 B. P. Pichon, M. Pauly, P. Marie, C. Leuvrey and S. Begin-Colin,

Langmuir, 2011, 27, 6235–6243.17 H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on

Gratings, Heidelberg, 1998.18 E. Ostuni, L. Yan andG.M.Whitesides,Colloids Surf., B, 1999, 15, 3.19 L.Aihua,A.GopalandM.Chuanbin,Adv.Mater., 2009,21, 1001–1005.20 G. Barbillon, J. L. Bijeon, J. Plain and P. Royer, Thin Solid Films,

2009, 517, 2997.21 E. Golub, G. Pelossof, R. Freeman, H. Zhang and I. Willner, Anal.

Chem., 2009, 81, 9291.22 E. Hutter, J. H. Fendler and D. Roy, J. Phys. Chem. B, 2001, 105,

11159.23 M. Ito, F. Nakamura, A. Baba, K. Tamada, H. Ushijima,

K. H. A. Lau, A. Manna and W. Knoll, J. Phys. Chem. C, 2007,111, 11653.

24 X. Li, K. Tamada, A. Baba, W. Knoll andM.Hara, J. Phys. Chem. B,2006, 110, 15755.

25 M. Riskin, R. Tel-Vered, O. Lioubashevski and I. Willner, J. Am.Chem. Soc., 2009, 131, 7368.

26 Y. Yong, et al., Nanotechnology, 2006, 17, 2821.27 P. J. Cameron, X. Zhong and W. Knoll, J. Phys. Chem. C, 2007, 111,

10313.28 Y. Teramura, Y. Arima and H. Iwata,Anal. Biochem., 2006, 357, 208.29 J. Wang, A. Munir, Z. Zhu and H. S. Zhou, Anal. Chem., 2010, 82,

6782.30 Y. Sun, D. Song, Y. Bai, L.Wang, Y. Tian andH. Zhang,Anal. Chim.

Acta, 2008, 624, 294.31 K. S. Lee, M. Lee, K. M. Byun and I. S. Lee, J. Mater. Chem., 2011,

21, 5156.32 S. D. Soelberg, R. C. Stevens, A. P. Limaye and C. E. Furlong, Anal.

Chem., 2009, 81, 2357.33 D. Grigoriev, D. Gorin, G. B. Sukhorukov, A. Yashchenok,

E. Maltseva and H. M€ohwald, Langmuir, 2007, 23, 12388.

34 P.-H. Lin and P. Guyot-Sionnest, Langmuir, 1999, 15, 6825–6828.35 F. v. Wrochem, F. Scholz, A. Schreiber, H.-G. Nothofer, W. E. Ford,

P. Morf, T. Jung, A. Yasuda and J. M. Wessels, Langmuir, 2008, 24,6910.

36 A. Demorti�ere, B. Pichon, P. Panissod, B. Donnio, G. Pourroy,D. Guillon and S. B�egin-Colin, Nanoscale, 2011, 3, 225–232.

37 J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim,J.-H. Park, N.-M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891.

38 M. Pauly, B. P. Pichon, A. Demorti�ere, J. Delahaye, C. Leuvrey,G. Pourroy and S. B�egin-Colin, Superlattices Microstruct., 2009, 46,195.

39 W. L. Wilson, P. F. Szajowski and L. E. Brus, Science, 1993, 262,1242–1244.

40 I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero,J. G�omez-Herrero and A. M. Baro, Rev. Sci. Instrum., 2007, 78,013705.

41 B. P. Pichon, A. Demortiere, M. Pauly, K. Mougin, A. Derory andS. Begin-Colin, J. Phys. Chem. C, 2010, 114, 9041.

42 P. E. Laibinis and G. M. Whitesides, J. Am. Chem. Soc., 1992, 114,1990–1995.

43 N. Faucheux, R. Schweiss, K. L€utzow, C. Werner and T. Groth,Biomaterials, 2004, 25, 2721.

44 A. Demortiere, P. Panissod, B. P. Pichon, G. Pourroy, D. Guillon,B. Donnio and S. Begin-Colin, Nanoscale, 2011, 3, 225.

45 B. P. Pichon, O. Gerber, C. Lefevre, I. Florea, S. Fleutot, W. Baaziz,M. Pauly, M. Ohlmann, C. Ulhaq, O. Ersen, V. Pierron-Bohnes,P. Panissod, M. Drillon and S. Begin-Colin, Chem. Mater., 2011,23, 2886–2900.

46 C. Altavilla, E. Ciliberto, A. Aiello, C. Sangregorio and D. Gatteschi,Chem. Mater., 2007, 19, 5980.

47 L. S. Jung, C. T. Campbell, T. M. Chinowsky, M. N. Mar andS. S. Yee, Langmuir, 1998, 14, 5636.

48 Z. Adamczyk, Irreversible Adsorption of Particles, in AdsorptionTheory, Modeling and Analysis, ed. J. Toth, Marcel-Dekker, NewYork, 2002.

49 B. Senger, J. C. Voegel and P. Schaaf, Colloids Surf., A, 2000, 165,255.

50 B. P. Pichon, P. Louet, O. Felix, M. Drillon, S. Begin-Colin andG. Decher, Chem. Mater., 2011, 23, 3668–3675.

51 C. E. D. Chidsey, C. R. Bertozzi, T. M. Putvinski and A. M. Mujsce,J. Am. Chem. Soc., 1990, 112, 4301.

52 D. M. Collard and M. A. Fox, Langmuir, 1991, 7, 1192.53 J. B. Schlenoff, M. Li and H. Ly, J. Am. Chem. Soc., 1995, 117, 12528.54 E. Cooper and G. J. Leggett, Langmuir, 1999, 15, 1024–1032.


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