Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

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Nanoscale c1nr11039j

PAPER

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Self-assembly of thiolated cyanine aggregates on Au(111)

and Au nanoparticle surfaces

Guillermo O. Men�endez, Emiliano Cort�es, Doris Grumelli,*

Lucila P. M�endez De Leo, Federico J. Williams,Nicol�as G. Tognalli, Alejandro Fainstein, Mar�ıa Elena Vela,Elizabeth A. Jares-Erijman and Roberto C. Salvarezza

J-Aggregates of CNN molecules in solution are preferentiallyadsorbed on the Au(111) surface but adsorbed CNN monomerscannot organize into aggregates on the substrate surface: S–Auinteraction is not able to disorganize the large J-aggregatesstabilized by p–p stacking but it is strong enough to hinder the

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p–p stacking when CNNs are chemisorbed as monomers.

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Dynamic Article LinksC<Nanoscale

Cite this: DOI: 10.1039/c1nr11039j

www.rsc.org/nanoscale PAPER

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Self-assembly of thiolated cyanine aggregates on Au(111) and Aunanoparticle surfaces†

Guillermo O.Men�endez,a Emiliano Cort�es,bDoris Grumelli,*b Lucila P. M�endez De Leo,c Federico J. Williams,c

Nicol�as G. Tognalli,d Alejandro Fainstein,d Mar�ıa Elena Vela,b Elizabeth A. Jares-Erijman‡a

and Roberto C. Salvarezzab

Received 6th August 2011, Accepted 26th October 2011

DOI: 10.1039/c1nr11039j
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Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive

candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on

planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In

this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs

from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-

IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on

the Au surfaces by forming thiolate–Au bonds. We found that the J-aggregates are preferentially

adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot

organize into aggregates on the substrate surface. These results indicate that the CNN–Au interaction is

not able to disorganize the large J-aggregates stabilized by p–p stacking to optimize the S–Au binding

site but it is strong enough to hinder the p–p stacking when CNNs are chemisorbed as monomers. The

optical properties of the J-aggregates remain active after adsorption. The possibility of covalently

bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au

distance opens a new path regarding their improved stability and the wide range of biological

applications of both CNN and AuNP biocompatible systems.

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1. Introduction

Cyanine (CNN) is a family of cationic dyes typically consisting of

two nitrogen containing heterocyclic ring systems, which include

a chain of conjugated double bonds. These dyes have found

widespread use as sensitizers for color photography,1 as fluo-

rescent labels for biomolecules (proteins and nucleic acids) either

through covalent or noncovalent bonding,2 photoinitiators in

photopolymerization,3 DNA sequencing,4 determination of

nucleic acids5 and proteins,6 flow cytometry,7 near laser dyes,8

aCHIDECAR, Departamento de Qu�ımica Org�anica, FCEN, UBA, CiudadUniversitaria, Pabell�on II, Buenos Aires, 1428, ArgentinabInstituto de Investigaciones Fisicoqu�ımicas Te�oricas y Aplicadas(INIFTA), Universidad Nacional de La Plata-CONICET, Sucursal 4Casilla de Correo 16 La Plata, 1900 Buenos Aires, Argentina. E-mail:[email protected], CONICET, Departamento de Qu�ımica Inorg�anica,Anal�ıtica y Qu�ımica F�ısica FCEN, UBA, Ciudad Universitaria, Pabell�onII, Buenos Aires, 1428, ArgentinadCentro At�omico Bariloche and Instituto Balseiro, Comisi�on Nacional deEnerg�ıa At�omica and Universidad Nacional de Cuyo, San Carlos deBariloche, 8400, R�ıo Negro, Argentina

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1nr11039j

‡ In memory of Prof. Elizabeth A. Jares-Erijman.

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This journal is ª The Royal Society of Chemistry 2011

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optical recording and storage media,1,9 as well as ultra-trace

determinations.10 CNNs are environmentally sensitive probes for

reporting on local properties such as viscosity, polarity, and pH-

sensitive fluorescent probes for intracellular studies11 due to the

fact that near infrared dyes absorption is poorly adsorbed by

biomolecules, so it can penetrate deeply into tissues avoiding cell

autofluorescence. These dyes exhibit very high extinction coeffi-

cients so it is possible to conceive their detection at extremely low

levels such as in monolayers on metal surfaces using absorption

spectroscopy.

One peculiar and interesting property of cyanines is the ability

to form spontaneously aggregates in solution, and at liquid/solid

and liquid/air interfaces.12 The self-assembly of these amphiphilic

dyes into aggregates is determined by the sensitive balance

between different intermolecular forces: the dispersion force of

the cyanine backbone, the entropic forces of the alkyl chains, the

electrostatic forces and H-bonding of the ionic groups, and

the general van der Waals forces.13,14 It has been estimated that

the free energy of aggregation for one dimer is high, in the order

of 20 kcal mol�1.15 The aggregation behavior of cyanine dyes is

important because it markedly influences the photophysical and

photochemical properties. In fact, the absorption maxima are

shifted to shorter wavelengths (H-aggregation) or sharply shifted

to longer wavelengths (J-aggregation). It is commonly accepted

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Nanoscale, 2011, xx, 1–11 | 1

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Fig. 1 Chemical structure of the novel thiolated heptamethinecyanine

dye used in this work.

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that a J-aggregate is a polymolecular state in which planar dye

molecules are packed together in a plane-to-plane fashion

stabilized by p–p stacking. The shift angle (a) in a slipped deck

of molecules determines the spectral absorption of the aggregate.

Calculations demonstrated that a ¼ 60� corresponds to H-

aggregates, whereas a ¼ 30� and a ¼ 19� correspond to J-

aggregates. The J-aggregation process is most typical of poly-

methine dyes. J-Aggregates display sharp, intense fluorescence

emission. Spectral properties of J- and H-aggregates make them

attractive candidates for developing a variety of chem-bio-

sensing applications.

The possibility of immobilizing cyanine species at solid

surfaces has been explored on different substrates.16,17 In

particular, adsorption of cyanine on gold has attracted consid-

erable attention because of the remarkable stability of Au to

oxidation and its well-defined optical and electronic properties.

In this way it has been reported that cyanine dyes can organize as

J-aggregates on a self-assembled monolayer of 11-mercaptoun-

decanoic acid (HS–(CH2)10–COOH)18 and cysteamine19 on Au

surfaces. In this case 1.3 � 1014 molecules cm�2 dye molecules are

individually anchored to the surface amino groups in the form of

ammonium sulfonates. Formation of J-aggregates on gold

nanoparticle assemblies has also been reported during self-

assembly of cyanine on negatively charged Au nanoparticles

(AuNPs).20 From the optical point of view, J-aggregates assem-

bled on plasmonic substrates have recently become the focus of

strong attention due to the interaction of the aggregate localized

excitations with the intense near field of surface plasmons. This

results in the formation of mixed states, as a consequence of the

coupling between excitons and plasmons (plexcitons). These

artificial-new states have important implications in optoelec-

tronics and nonlinear optical devices, functional metamaterials

and surface-enhanced molecular sensing, among others.21

However, in the examples discussed above the J-aggregates are

only weakly bonded by van der Waals forces to the thiol SAM-

covered Au surface and charged nanoparticles. In this context

a thiolated CNN molecule should provide a more effective

anchor to the Au surface. However, the question is if the S–Au

bond, that dictates optimal 0.5 nm nearest neighbor distances in

thiols, is strong enough to effectively compete with the p–p

stacking of the CNN molecules in the J-aggregates (0.33 nm)

leading to aggregate disorganization. Also the thiolated CNN–

Au interaction is expected to be stronger on the surface of

AuNPs.22

Therefore, the survival or not of the aggregates on these Au

surfaces is an interesting point that deserves special attention.

Besides, the structure of self-assembled chromophores on Au

surfaces is not well understood because the chromophore

component is usually much larger than alkyl chains, thus

avoiding long-range order in the SAMs. In fact, it is well known

that the surface order in SAMs of aromatic thiols is more difficult

than aliphatic thiols.23 In many cases diluted phases coexist with

aggregates whose structure and origin are under debate.24 It has

been reported that SAMs of large aromatic thiols can form

disordered aggregates, ordered SAMs with molecules at

distances compatible with p–p stacking, or 5 � O3 surface

structures depending on the molecule.25

In this work we study the self-assembly of a new water-soluble

thiolated heptamethinecyanine (Fig. 1) on Au(111) and Au

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nanoparticle (AuNP) surfaces. The new cyanine has very

attractive photophysical properties such as near IR fluorescence,

high absorption coefficients, relatively high fluorescence

quantum yields, stability against photodegradation, and envi-

ronment sensitivity. In contrast to other works on CNN immo-

bilization on Au by weak interactions26 our CNN has been

designed to self-assemble on the Au surface by a thiol group on

one side of the molecule. In the opposite side, specific sites for

functionalization comprising two hydrocarbon chains ending in

carboxylate groups have been inserted, in order to allow further

conjugation to biotin, nitriloacetic acid, or binding to metallic

cations.

STM images, Raman spectra, XPS and surface coverage

derived from the reductive desorption curves indicate that the

CNN SAMs on Au(111) consist mainly of J-aggregates with the

CNN molecules chemisorbed on the Au surfaces by forming

thiolate–Au bonds. The optical properties of the thiolated CNN

remain active after adsorption on the Au surface. The fact that J-

aggregates survive on the Au(111) surface indicates that the

interaction forces resulting from p–p stacking between large

numbers of CNN molecules introduce an energy barrier that

cannot be overcome to optimize the S–Au bond distance (0.5 nm)

on the substrate, i.e. they are too stable to be disorganized by the

interaction with the Au surface. Similar results are obtained for

J-aggregate adsorption on citrate-capped AuNPs 12 nm in size

despite the fact that stronger CNN–Au interactions are expected

in this type of defective surfaces.27 The possibility to covalently

bond CNN J-aggregates to Au surfaces opens new paths

regarding their improved stability and the wide range of appli-

cations combining both abilities of physiological friendly

compatible CNN dyes and the low toxicity of AuNPs compared

with other nanosystems.

2. Results and discussion

2.1. CNN on Au(111) characterization

2.1.1. STM. STM images taken after Au(111) immersion in

the CNN containing solution show Au(111) terraces (Fig. 2a)

covered by a large number of randomly distributed nanometre

sized spots that correspond to adsorbed CNN species. In some

places the spots seem to follow some preferred orientation

forming parallel rows (center of the large terrace in Fig. 2a) as

already described for other aromatic compounds on Au(111).24b

Short-range order is visible in Fig. 2b where an ordered array of

spots (2–3 nm in size) are separated by 3.5 nm distances on the

Au(111) surface. However, other spots are placed completely at

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Fig. 2 (a) 200 � 200 nm2 top view STM image (raw data) of the CNN

covered Au substrate. Randomly distributed CNN species (bright spots)

are observed on the Au surface. (b) 75 � 200 nm2 STM image showing

short-range ordered domains of CNN adsorbates. (c) 50 � 50 nm2 3D

STM image of randomly distributed CNN adsorbates. (d) Power spectral

density showing a broad size distribution of species. The green line

indicates the average size of the CNN adsorbates (2.6 nm).

Fig. 3 High resolution XPS spectra of the CNN SAMs: (a) S 2p and (b)

N 1s.

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random as seen in Fig. 2c. These spots, which cover almost all the

Au surface, exhibit different contrast, some are bright while

others appear darker, more diffuse in the background. Bright

spots following step edges are shown in Fig. 2b at the right-hand

lower corner. The power spectral density analysis of the

randomly distributed spots shows a broad size distribution with

an average mean size of 2.6 nm. This figure is slightly higher than

the longer axis dimension of the CNNmolecules (2.1 nm, Fig. 1).

The broad size distribution (Fig. 2d) with a long tail shifted to

larger wavelength values suggests the presence of a fraction of

CNN aggregates, as already described for cyanine molecules in

solution and surfaces.14 Disordered assemblies of molecular

aggregates have also been observed in 1-(11-mercaptoundecyl)

pyrene SAMs on Au(111) surfaces.25 The structure of the CNN

SAM differs completely from that observed in spin coating

heptamethinecyanine films that exhibit 50 nm grains.28 Cross-

section analysis of the bright spots shows values between 0.2 nm

and 0.37 nm that are far from the height of the molecule (1.61

nm, Fig. 1). This is reasonable taking into account that the spots

are so close together that the cross-section does not represent the

real height of the CNN aggregates. Also, the STM images

contain electronic and topographic components. The scatter in

the cross-section values (z-direction) could reflect the lack of

preferential orientation in the aggregates with respect to the

substrate normal and/or disorder in the long outmost hydro-

carbon tails (Fig. 1).

At this stage we cannot decide if the 2.6 nm spots correspond

to a single CNN molecule or to an aggregate. In fact, if an iso-

lated molecule rotates freely it could bring about a spot similar to

those shown in the images. On the other hand, in a 2.6 nm spot

we can assemble �7 CNN elongated molecules stabilized by

robust p–p stacking. The nature of the 2.6 nm elements will be

clarified by using combined electrochemical techniques and

Raman spectroscopy as discussed in detail in Section 2.1.5.

Despite this drawback the power spectral density indicates the

presence of large elements that clearly correspond to aggregates.

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However, there are also many other questions related to the

adsorbed CNN molecules that can be answered. (i) How are the

molecules bound to the Au(111) surface: chemisorption or

physisorption? (ii) Are the CNN molecules intact after adsorp-

tion? (iii) What is the surface coverage of thiolate bonds (that

gives a direct measure of the number of chemisorbed CNN

molecules)? In the next section we will address all these points by

using XPS, PM-IRRAS, electrochemistry and Raman

spectroscopy.

2.1.2. XPS. The XPS S 2p signal of the CNN SAM (Fig. 3a)

can be fitted by considering two main components corresponding

to chemisorbed thiols (162 eV) and unbounded thiols (163 eV)29

with a 162 eV/163 eV ratio 3 : 1. The main component at 162 eV

confirms that a significant amount of the CNN molecules are

bonded to the Au substrate by the S atom (Fig. 1) forming

a thiolate bond. The presence of the adsorbed CNN molecules is

also consistent with the N 1s spectra shown in Fig. 3b. The main

peak at 399 eV has been assigned to the N in the CNN rings30 and

amine groups.31 The analysis of the XPS data yields a S 2p 162

eV/N 1S ratio z3, i.e. a value expected for the CNN molecule

(Fig. 1). We also obtain from the XPS data a quantitative rela-

tionship C : O : N : S (162 eV component) ¼ 42 : 11 : 3 : 1 very

close to 44 : 4 : 3 : 1 expected for the molecules shown in Fig. 1,

with the exception of O, which is often present as physisorbed

water in hydrophilic SAMs.31

As mentioned above, the XPS data indicate that most of the

CNN molecules are chemisorbed on the Au(111) surface by

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Table 1 IR assignment of the CNN on Au(111)

Assignment Wavelength/cm�1

KBr pelletMonolayeron Au(111)

naCH22918 2926

nsCH22850 2855

naC]O 1735 1730nC]C (coupled) and/orcollected ring modes

1566 1585

nC]N and/orcollected ring modes

1464 1456

DC–H 1384 1393nC–C 1134 (broad) 1206 (broad)In-plane CH bending + ringvibration � orthodi-substituted benzene

1067

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a thiolate bond (S 2p 162 eV component). However, the XPS

data also show that about 1/4 of the CNN molecules are phys-

isorbed as the SH group remains intact (S 2p 163 eV component).

This component could be assigned to a fraction of CNN mole-

cules intercalated in the SAM or in a second layer that cannot

interact directly with the Au surface but remain trapped by

intermolecular forces.32

2.1.3. PM-IRRAS. PM-IRRAS was also used to confirm the

presence of the intact CNNmolecule and further characterize the

orientation and degree of order of these molecules on the Au

(111) surface. An infrared transmission spectrum of a KBr pellet

of the CNN was taken and compared to the spectrum of the

monolayer on the surface. Fig. 4 shows these results: the presence

of the stretching mode associated with the carboxylic acid group

in both systems, naC]O at 1735–1730 cm�1. The bands corre-

sponding to collective ring modes are also observed at �1566 to

1585 cm�1 and �1464 to 1456 cm�1.33 These two bands can be

also assigned to resonant C]C and C]N groups34 and are

probably the sum of both contributions. The band observed at

1384–1393 cm�1 can be assigned to the C–H deformation mode

(dC–H) and the broad band below 1200 cm�1 can be assigned both

to alkyl chain C–C stretching modes and to ortho di-substituted

aromatic ring vibrations. In the high frequencies region, the

peaks corresponding to naCH2and nsCH2

are observed and can be

attributed to the carboxylated methylene chain.26a,b In the KBr

pellet, these peaks appear at 2918 and 2850 cm�1 respectively,

indicating that the alkyl tails are fully extended with all-trans

conformation while in the monolayer on the Au(111) surface

there is a shift to higher wavenumbers (2926 and 2855 cm�1)

indicating that these chains are curled. Table 1 summarizes these

assignments.

In general terms, Fig. 4 shows the same bands at the same

position and with the same relative intensities for the KBr

pellet and the CNN SAM, implying that there is no preferential

orientation of the molecule on the surface. The shift of the

naCH2and nsCH2

peaks implies that on the monolayer the alkyl

tails (Fig. 1) are more disordered than in the pellet. By these

results we can follow the derivatization process and confirm

that the CNN is intact on the surface. Also these results

confirm the lack of preferential orientation of the aggregates

and disorder in the long outmost hydrocarbon tails as sug-

gested by the STM data.

Fig. 4 (Black line) PMIRRAS spectra of the CNN SAM on Au(111). Res

Transmission FTIR spectra of CNN in KBr pellets. Resolution was set to 4

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2.1.4. Electrochemical data. It is well-known that reductive

desorption of thiol-containing monolayers gives voltammetric

cathodic peaks in aqueous solutions due to the cleavage of S–Au

thiolate bonds. We have used the reductive desorption curves to

estimate the coverage and the electrochemical stability of the

CNN SAM. In fact, the peak potential reflects the strength of the

S–Au bond while the charge density of the peak allows an esti-

mation of the thiolate coverage.

In Fig. 5 we compare the reductive desorption of our CNN

with the well-known desorption behavior of hexanethiol (HT)

SAMs used here as a reference. The reductive desorption for

SAMs of HT and CNN shows defined cathodic current peaks

preceding the hydrogen evolution (HER) reaction. Results

shown in this figure are typical of a series of 10 reductive

desorption measurements. The curves reveal that CNN SAMs

are desorbed from the Au substrate at a peak potential of

Ep ¼ �0.92 V while HT SAMs are desorbed at Ep ¼ �0.99 V, in

agreement with previous results.26c The HT and CNN SAMs

exhibit a difference in peak potentials of 0.07 eV (assuming one

electron per thiolate bond for the reductive desorption). Thus,

HT seems to be a good dilution agent for the CNN molecules in

order to form mixed SAMs.

The charge density involved in the HT desorption peak is q ¼75 � 7 mC cm�2, i.e. a figure that leads to a thiolate surface

coverage 1/3 in agreement with the c(4 � 2) structure usually

found in the STM images of this SAM.35 In contrast, the

olution was set to 4 cm�1 and 1500 scans were accumulated. (Red line)

cm�1 and 200 scans were accumulated.

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Fig. 5 Reductive desorption curve of CNN (red line), and HT (blue line)

SAMs on Au(111) recorded at 0.05 V s�1 in 0.1 M NaOH. Inset: similar

desorption curves recorded for SAMs of CNN monomers (green line)

and CNN J-aggregates + monomers (red).

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desorption peak of CNN SAMs involves q ¼ 22 � 5 mC cm�2,

a figure consistent with the larger size of the CNN molecule.

On the other hand, the slight decrease in the CNN monolayer

electrochemical stability in relation to the HT SAM shown in

Fig. 5 could be assigned to the greater solubility of the CNN

in aqueous electrolyte that could assist desorption. In fact, in

aqueous solution hydrophobic forces contribute to stabilize the

HT molecule in the Au surface.

As we have mentioned before, CNN-containing solutions are

usually formed by CNNmonomers and CNN aggregates. In this

way, two mechanisms could contribute to the presence of the

aggregates on the Au(111) surface: (i) aggregate formation in

the methanolic solution followed by preferential adsorption of

the aggregates on the Au(111) surface and (ii) monomer

adsorption from solution followed by aggregation on the

substrate surface. Also both mechanisms could operate simul-

taneously. In the next section we will elucidate the self-assembly

of the thiolate CNN dye on Au(111) by Raman spectroscopy, an

efficient characterization technique to study the presence of J-

aggregates in metal surfaces.26e,36

2.1.5. UV-vis in solution and Raman scattering spectra of

CNN SAMs on Au(111). The UV-vis spectrum recorded in 50

mM CNN methanolic solutions after 24 h of preparation, the

time used for the CNN self-assembly, exhibits a large maximum

Fig. 6 (a) UV-visible spectra of 50 mM thiolated-CNN methanolic solutions.

(111) at 647.1 nm excitation line and (c) at 775 nm excitation line. Black (red

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at 650 nm corresponding to individual solvated molecules

(monomeric form), which do not interact with each other; a red-

shifted small maximum at 800 nm that corresponds to J-aggre-

gates; and an additional minor blue-shifted (400–500 nm)

shoulder that can be ascribed to H-aggregates (Fig. 6a, black

curve).

The Raman measurements of CNN SAM on the Au(111)

surface taken with two specific laser lines: 647.1 nm, where the

monomers have their maximum absorption in the UV-visible

spectra and 775 nm where J-aggregates absorb, are shown in

Fig. 6b and c. Important to note for our study is that the Raman

spectra show the low-frequency bands (450–800 cm�1) typical of

J-aggregates physisorbed on metal surfaces.37 These Raman

modes have been detected and theoretically predicted when the

aggregation process takes place (i.e. they are not present or

nearly visible in CNN monomers)38 and have previously been

assigned to a vibration out of plane in similar CNN dyes.39

Notably, we also observe an intensity increase and split of the 558

cm�1 band (at 647.1 nm) to 523 cm�1 and 558 cm�1 (at 775 nm),

see Fig. 6c. The fact that the low-frequency bands are enhanced

when the incident laser line is 775 nm could be related to

a resonant Raman process related to the laser excitation of the J-

aggregate electronic states, where they have an intense absorp-

tion, as shown in the UV-visible spectrum.26e

Therefore, the presence of the low-frequency bands and their

modification with the incident-energy laser line is clear evidence

of the presence of J-aggregates on Au(111) as suggested from the

analysis of the previously reported data.

Raman spectroscopy can also help us to determine which

mechanism takes place at the liquid–solid interface and the origin

of the J-aggregates at the Au(111) surface. In order to answer the

question if the aggregation process occurs after monomer

adsorption or if selective adsorption of aggregates from solution

takes place, we have prepared a new solution containing only

thiolate CNN monomers. This has been possible just by heating

the solution in order to disaggregate the molecules, as it has been

already reported.14 The UV-visible spectra of the monomeric

thiolated CNN dye are also shown in Fig. 6a (red curve).

Raman spectra of both samples show a band centered at 310

cm�1 that has been previously assigned to the Au–S bond,40

indicating that the thiolated-CNNs have been self-assembled by

forming thiolate bonds. By comparing the low-frequency bands

(b) Low frequency bands in the Raman spectra of thiolated-CNN on Au

) lines correspond to CNN with (without) J-aggregates.

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Fig. 7 Low frequency bands in the SERS spectra of thiolated-CNN on

AuNPs in solution at 647.1 nm excitation line.

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in the Raman spectra between both samples after adsorption in

Au(111) it is possible to confirm that no aggregation takes place

when only monomers are present in the former solution, as can

be clearly observed in Fig. 6b and c. The fact that J-aggregates of

our thiolated CNN cannot be formed from monomers on the Au

(111) surface contrasts with previous results showing that

aggregation is promoted on liquid/solid interfaces.12

In the next section we discuss the differences between the self-

assembly of J-aggregates and CNN monomers on the Au(111)

surface. The integration of the 310 cm�1 band intensity corre-

sponding to the Au–S bond (Fig. 6b) shows that the amount of

thiolates is twice when the J-aggregates are present on the Au

(111) surface. This relationship was also confirmed by the charge

densities involved in the desorption curves (Fig. 5 and the inset)

recorded for the SAMs formed from the mixed J-aggregates +

monomers (q ¼ 22 � 5 mC cm�2) and from the plain monomer

solutions (q ¼ 11 � 2 mC cm�2) (Fig. 5, inset). Considering that

the CNN SAM consists of a close packed array of monomers

with size 2.1 nm separated by 0.5 nm (the nearest neighbor

distance found for alkanethiolates35 and small aromatic thiols25

on Au(111)), it is possible to estimate that the q value should be

�16 mC cm�2. The experimental charge density q ¼ 11 � 2 mC

cm�2 is not far from this value. It suggests that the CNN

monomers form SAMs with the nearest neighbor CNN distance

z0.5 nm. However it is important to note that q ¼ 16 mC cm�2 is

the maximum charge value expected for thiolated CNN sepa-

rated by 0.5 nm. Therefore, the value q ¼ 22 � 5 mC cm�2 found

for the SAM containing J-aggregates implies that the nearest

neighbor CNN distance should be smaller, i.e. the 0.33 nm

expected for these aggregates in solution.26d In fact, an ideal close

packed array of CNNmolecules of 2.1 nm in length separated by

0.33 nm should lead to qz 23 mC cm�2. This number is very close

to the experimental q value (22 mC cm�2). Note that 0.33 nm

nearest neighbor distances have also been observed in SAMs of

large conjugated aromatic thiols on Au(111) by STM imaging.25

The fact that SAMs of J-aggregates have a larger number of

thiolate bonds per unit area also explains why the J-aggregates

win over the monomer adsorption, i.e. they are preferentially

adsorbed on the Au(111) surface. In other words the main

thermodynamic factor for SAM stabilization is the number of

thiolate bonds that can be formed per unit area of substrate.

However, it can be argued that in the case of J-aggregates the

thiolate bonds cannot be optimized because the distance is

shorter than the 0.5 nm distance preferred by thiols on Au. Even

though, the 0.33 nm distance is also commensurate with the Au

(111) surface, and the difference in adsorption energy for thiols

on the top, bridge, fcc and bridge–fcc sites of an unreconstructed

Au(111) surface is small.41 It has been also proposed that the S–

Au bond can occupy different adsorption sites on the Au(111)

surface.42 On the other hand, the larger CNN–CNN interactions

in the J-aggregates also contribute to SAM stabilization. It has

been reported that molecular aggregates of a large thiolated

aromatic molecule (1-(11-mercaptoundecyl)pyrene) can only be

eliminated after annealing to 343 K to form ordered SAMs with

2 � O3 lattices.25 Concerning the mechanism of adsorption it is

evident that the larger J-aggregates should be preferred over the

monomers in the initial physisorbed state (before S–H dissocia-

tion) due to their larger van der Waals interactions. This can

explain the selective adsorption of J-aggregates on the Au(111)

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6 | Nanoscale, 2011, xx, 1–11

surface even from a solution containing a large number of

monomers and a small number of J-aggregates (Fig. 6a). Selec-

tive adsorption of large J-aggregates in relation to smaller ones

has also been proposed on AgBr surfaces through a repetitive

adsorption–desorption process.43 Therefore, there are different

reasons for the formation of J-aggregate rich SAMs.

However, the following question should be answered: why are

the monomers unable to form the J-aggregates on the Au

surface? In the case of CNN monomers they arrive at the Au

surface and bind to the optimal sites (possibly the bridge fcc

positions) with the nearest neighbor distance of 0.5 nm. We

propose that the energy barrier to move the monomer from its

optimal site in order to form the aggregates (to reach the 0.33 nm

distance) on the surface is too large to be overcome, i.e. this

process is difficult to observe, at least in the scale of time of our

experiments.

2.2. AuNP–CNN

Finally, we have successfully self-assembled the J-aggregates on

AuNPs by displacement of the citrate capping after immersion in

the CNN solution (see ESI†). Note that this CNN solution is

composed of a large number of monomers and a small number of

J-aggregates as was already used for Au(111) surfaces (see UV-

visible spectra in Fig. 6a, black line). Fig. 7 shows the SERS

spectra recorded using the 647.1 nm excitation line. As in the

planar Au(111) surfaces (Fig. 6b and c) the spectra show

the band centered at 310 cm�1 consistent with the presence of the

thiolate bonds.40 The 558 cm�1 band indicates that J-aggregates

are also present on the AuNP surfaces.

2.3. Mixed HT and CNN SAMs

In order to control the amount of CNN immobilized on the

surface we perform the self-assembly process in the presence of

hexanethiol. In Fig. 8 we show STM images of a mixed SAM

prepared from immersion of the Au(111) substrate in a 1 : 10

CNN/HT ratio solution. Au terraces with the typical holes

induced by the HT adsorption and a few bright spots corre-

sponding to the CNN species are clearly observed (Fig. 8a). The

bright spots are embedded into HT domains where molecular

resolution is obtained (Fig. 8b). The HT domains exhibit the

same rows separated by 1 nm (Fig. 8c) typical of the c(4� 2) thiol

lattice on Au(111). The average size of the spots is similar to that

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reported for the self-assembly from the plain CNN solution also

suggesting the presence of aggregates. This finding is not

surprising as the UV-vis spectra of the self-assembly solution

show the presence of monomers and aggregates (Fig. 9). Even

more, the presence of HT seems to increase the J-aggregate/

monomer ratio, i.e. the surfactant induces the self-aggregation in

solution (Fig. 9a and b). In this case the cross-section analysis of

the isolated aggregates shows the height between 0.34 and 0.40

nm, i.e. the scatter in height is decreased with respect to the CNN

SAM. It could reflect that the near vertically arranged HT

molecules (tilt angle 30/50�) induce more order in the CNN

orientation on the surface. If one considers that the HT SAM in c

Fig. 8 STM images of mixed SAMs formed from 1 : 10 of cyanine

(CNN) : hexanethiol (HT) containing solution. (a) 200 � 200 nm2. Note

the small density of bright spots: (b) 50� 50 nm2 and (c) 30� 15 nm2 high

resolution images showing isolated CNN aggregates (bright spots)

embedded into the rows of the HT molecules.

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(4 � 2) configuration has �0.7 nm height from the substrate and

the CNN molecules of the aggregates emerge �0.4 nm from the

HT outer plane (Fig. 8b and c) the total height of the aggregates

should be �1.1 nm, still smaller than the height of the CNN

molecules (1.6 nm, Fig. 1). This can be explained considering the

disorder of the outmost hydrocarbon chains revealed by the

PMIRRAS results. This estimation is made under the assump-

tion that the STM contrast is dominated by topography although

it should also involve electronic effects.

Note that the CNN aggregates seem to be preferentially

adsorbed at step edges or at the edges of the holes where HT

disorder exists as already reported for methylene blue molecules

intercalated on thiol SAMs.44 It is evident that the HT molecules

provide a favorable environment for the CNN aggregates at the

Au surfaces as it occurs in the methanolic HT/CNN solution

(Fig. 8b).

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3. Conclusions and outlooks

In the next section we discuss the implications of our results.

First, we have demonstrated from XPS and Raman data that

most of the thiolated CNN molecules are linked to the Au(111)

and AuNP surfaces by a thiolate bond as takes place for HT and

other thiols on Au(111). Secondly, we have shown from the STM

images, Raman spectra and coverage estimation from desorption

curves that the most of the CNN molecules exist on the Au(111)

surface as J-aggregates when they are present (even in a small

proportion) in the initial solution. In contrast, we have also

found that when only CNN monomers are present in the initial

solution, they are self-assembled as they cannot be organized into

J-aggregates on the substrate, i.e. when the thiolate–Au inter-

action is optimized by the monomer it becomes strong enough to

hinder p–p stacking between adsorbed monomers. From these

results one can conclude that the J-aggregates are preferentially

adsorbed on the Au(111) surface from the methanolic solutions

containing monomers and aggregates even when the aggregates

are only a minor component of the solution. The fact that J-

aggregates survive on the Au surface indicates that the interac-

tion forces resulting from p–p stacking between large numbers

of molecules introduces an energy barrier that cannot be over-

come to optimize the thiolate–Au bond on the substrate. Also, J-

aggregate adsorption results in double the amount of thiolates on

the surface (compared to CNN monomers) and this should be

also an important driving force in the final energy of the self-

assembly process. We have demonstrated that the self-assembly

of J-aggregates takes place not only on atomically smooth Au

surfaces but also on more defective surfaces such as AuNP

surfaces where the molecule–Au interaction should be high-

er.26c,27 In other words the J-aggregate is too stable to be disor-

ganized even for stronger CNN–Au interactions.

We also show that aliphatic thiols are efficient diluting agents

for J-aggregates so that their concentration on the Au(111)

surface can be controlled from the CNN/alkanethiol ratio in

solution allowing imaging of isolated species.

Another point to be mentioned is the versatile-synthetic

method used in this work. As we have presented, this heptame-

thinecyanine dye was further modified by a cysteamine thiol to

obtain this novel thiolate cyanine molecule. In addition (as

shown in the ESI†) this thiolating procedure does not modify the

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Nanoscale, 2011, xx, 1–11 | 7

Fig. 9 (a) UV-vis spectra of 50 mM CNN methanolic solutions after 24 h (black) and 1 : 10 CNN/HT methanolic solutions after 24 h (blue). (b)

Differential spectra (red) between [CNN/HT 1/10] � 10 mix and pure CNN dye.

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optical properties of the heptamethinecyanine dye in solution,

differing from what has been proposed for other cyanine mole-

cules.45 As we can see, also some fluorescence from the thiolate–

CNN after adsorption in Au(111) can be evidenced from the

intense background in the Raman measurements. However, the

thiolating procedure can be performed with other amine thiols in

order to improve particular qualities depending on specific

applications. For example, for fluorescence biomarkers longer

distances from the Au surface are desirable in order to get higher

fluorescence intensities avoiding quenching processes due to the

surface.46 11-Aminoundecanethiol or longer amine thiols could

be useful for this purpose.

30

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4. Experimental section

Synthesis of the thiolated cyanine

The thiolated cyanine was purified by HPLC and characterized

by spectroscopic methods such as Mass Spectroscopy and

Nuclear Magnetic Resonance. Details of the synthesis and

characterization can be found in the ESI†. UV-vis spectra of the

CNN solutions were acquired with a Lambda 35 double beam

spectrophotometer from Perkin Elmer (Waltham, MA, USA).

40

45

Gold substrates and chemicals

Gold evaporated on glass substrates were purchased from

Arrandee�, consisting of vapor deposited gold films (250� 50 nm

in thickness) on a thin layer of chromium supported on glass slides.

Substrates were prepared by annealing for 5 min in a butane–

propane flame until the film color turned to a dark red. After

annealing, these polycrystalline substrates exhibit large grainswith

atomically smooth (111) terraces separated by steps ofmonatomic

height. All of the chemicals used were of analytical reagent grade.
50
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SAMs preparation on the gold substrates

Self-assembled monolayers (SAMs) of hexanethiol (HT) and

thiolated cyanine (CNN) were prepared by immersing the Au

(111) substrates into freshly prepared 100 mM HT ethanolic

solution or 100 mM CNN methanolic solution for 24 h at room

temperature in the absence of light. A final and careful rinsing

was made with methanol before drying under N2. A 1 : 10

mixture of CNN and HT respectively was prepared by mixing

ART � C1NR

8 | Nanoscale, 2011, xx, 1–11

100 mMsolutions in the correct volume to reach the final relation.

In all cases the CNN solution containing a large number of

monomers and a small number of J-aggregates was used, except

when it is explicitly mentioned that another solution was used.

Gold nanoparticles: synthesis and characterization

Gold nanoparticle (AuNP) synthesis was done by the standard

method introduced by Turkevich et al.47 and refined by Frens.48

The AuNPs were characterized by UV-vis spectra and Trans-

mission ElectronMicroscopy (TEM) as it is described in the ESI†.

The AuNP–CNN functionalization was performed in the

following way: AuNPs stabilized in citrate buffer pH 7.5 and

a CNN solution in the same buffer were mixed in a 1 : 1000

molar ratio in order to replace the citrate capping by the CNN

molecules. The mix was left overnight at room temperature in the

dark. The AuNPs modified with the CNN SAM (AuNP–CNN)

were purified by repetitive centrifugation and characterized by

UV-vis, TEM (see ESI†) and Raman Spectroscopy.

SAMs characterization

The SAMs were characterized by XPS, PMIRRAS, STM,

Raman and electrochemical techniques. Measurements were

carried out at room temperature.

X-Ray Photoelectron Spectroscopy (XPS) measurements were

performed under UHV conditions (base pressure < 5 � 10�10

mbar) in a SPECS UHV spectrometer system equipped with

a 150 mm mean radius hemispherical electron energy analyzer

and a nine channeltron detector. XPS spectra were acquired at

a constant pass energy of 20 eV using an un-monochromated

MgKa (1253.6 eV) source operated at 12.5 kV and 20 mA and

a detection angle of 30 degrees with respect to the sample normal

on grounded conducting substrates. Quoted binding energies are

referred to the Au 4f 7/2 emission at 84 eV. Atomic ratios and

surface concentrations were calculated from the integrated

intensities of core levels after instrumental and photoionization

cross-section corrections.

Polarization Modulation InfraRed Reflection Absorption

Spectroscopy (PMIRRAS) experiments were made on a Thermo

Nicolet 8700 (Nicolet, Madison,WI) spectrometer equipped with

a custom made external tabletop optical mount, a MCT-A

detector (Nicolet), a photoelastic modulator, PEM (PM-90 with

II/Zs50 ZnSe 50 kHz optical head, Hinds Instrument, Hillsboro,

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OR), and Synchronous Sampling Demodulator, SSD (GWC

Instruments, Madison, WI). The IR spectra were acquired with

the PEM set for a half wave retardation at 2900 cm�1 for the CH

stretching region and at 1500 cm�1 for the CH bending and

aromatic region. The angle of incidence was set at 80�, whichgives the maximum of mean square electric field strength for the

air/Au(111) interface. The demodulation technique developed in

Corn’s laboratory was used in this work. The signal was cor-

rected by the PEM response using a method described by Frey

et al.49 Typically 8000 and 1500 scans were performed and the

resolution was set for 2 and 4 cm�1, respectively.

Transmission spectra for solid cyanineweremeasuredusingKBr

pellets.Resolutionwas set to 4 cm�1 and 200 scanswere performed.

The Raman scattering experiments on Au(111) surfaces and

the SERS (Surface Enhanced Raman Scattering) measurements

on AuNPs were performed on a Jobin-Yvon T64000 triple

spectrometer operating in subtractive mode and equipped with

a liquid N2 cooled charge coupled device (CCD). The excitation

was done using an Ar–Kr laser (specifically 647.1 nm), and a Ti-

Sapph laser continuously tunable between 690 and 1080 nm

(specifically 775 nm). Spectra were acquired 3 times (accumula-

tive) for 300 s for Au(111) and 2 times (accumulative) for 10 s for

AuNPs using 40 mW of laser power with an incidence angle of 25

degrees, and focused on a circular spot of 30 mm diameter. The

collection numerical aperture was 0.2, aligned normal to the

surface for Au(111) surfaces. AuNPs were measured in solution.

Scanning Tunneling Microscopy (STM) images were obtained

in air using a Nanoscope IIIa microscope from Veeco Instru-

ments (Santa Barbara, CA). Commercial Pt–Ir tips were used.

Typical tunneling currents and bias voltages for imaging cyanine

were 300 pA and 800–1000 mV, respectively.

The electrochemicalmeasurementswere performedwith aTEQ

potentiostat under computer control for data acquisition. The Au

(111) substrates were mounted in a conventional three-electrode

cell. Solutions were purged with nitrogen. All potentials were

measured and reported with respect to a SCE reference electrode.

Solutions of 0.1 M NaOH were prepared using deionized H2O

from aMilli-Q purification system (Millipore Products, Bedford).

Reductive electrodesorption of thiols from the Au substrates was

performed by scanning the potential from�0.2 to�1.40 V at 0.05

V s�1 in 0.1MNaOHaqueous solution, at room temperature. The

charge density and the peak potential involved in the reductive

desorption process were taken as an indication of the surface

coverage by the CNN and HT molecules.

Acknowledgements

We acknowledge financial support from ANPCyT (Argentina,

PICT-2010-2554, PICT 2008-0332, PICT-CNPQ 08-019, PICT

06-01061). EC, DG, NGT, AF and RCS are members of CONI-

CET.MEVis amemberof the research career ofCICBsAs.EAJ-E

acknowledges the Max Planck Society, University of Buenos

Aires, CONICET (Argentina) and ANPCyT (Argentina) for

financial support.

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Title: Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

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Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

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Transcript of Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces