Physical Chemistry Chemical Physics

This paper is published as part of a PCCP Themed Issue on:

New Frontiers in Surface-Enhanced Raman Scattering

Guest Editor: Pablo Etchegoin

Editorial

Quo vadis surface-enhanced Raman scattering? Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b913171j

Perspective

Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists canvas Kristin L. Wustholz, Christa L. Brosseau, Francesca Casadio and Richard P. Van Duyne, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904733f

Communication

Tip-enhanced Raman scattering (TERS) of oxidised glutathione on an ultraflat gold nanoplate Tanja Deckert-Gaudig, Elena Bailo and Volker Deckert, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904735b

Papers

Self-assembly of , -aliphatic diamines on Ag nanoparticles as an effective localized surface plasmon nanosensor based in interparticle hot spots Luca Guerrini, Irene Izquierdo-Lorenzo, José V. Garcia-Ramos, Concepción Domingo and Santiago Sanchez-Cortes, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904631c

Single-molecule vibrational pumping in SERS C. M. Galloway, E. C. Le Ru and P. G. Etchegoin, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904638k

Silver nanoparticles self assembly as SERS substrates with near single molecule detection limit Meikun Fan and Alexandre G. Brolo, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904744a

Gated electron transfer of cytochrome c6 at biomimetic interfaces: a time-resolved SERR study Anja Kranich, Hendrik Naumann, Fernando P. Molina-Heredia, H. Justin Moore, T. Randall Lee, Sophie Lecomte, Miguel A. de la Rosa, Peter Hildebrandt and Daniel H. Murgida, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904434e

Investigation of particle shape and size effects in SERS using T-matrix calculations Rufus Boyack and Eric C. Le Ru, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905645a

Plasmon-dispersion corrections and constraints for surface selection rules of single molecule SERS spectra S. Buchanan, E. C. Le Ru and P. G. Etchegoin, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905846j

Redox molecule based SERS sensors Nicolás G. Tognalli, Pablo Scodeller, Victoria Flexer, Rafael Szamocki, Alejandra Ricci, Mario Tagliazucchi, Ernesto J. Calvo and Alejandro Fainstein, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905600a

Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch Seth Michael Morton, Ebo Ewusi-Annan and Lasse Jensen, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904745j

Interfacial redox processes of cytochrome b562 P. Zuo, T. Albrecht, P. D. Barker, D. H. Murgida and P. Hildebrandt, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904926f

Surface-enhanced Raman scattering of 5-fluorouracil adsorbed on silver nanostructures Mariana Sardo, Cristina Ruano, José Luis Castro, Isabel López-Tocón, Juan Soto, Paulo Ribeiro-Claro and Juan Carlos Otero, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b903823j

SERS imaging of HER2-overexpressed MCF7 cells using antibody-conjugated gold nanorods Hyejin Park, Sangyeop Lee, Lingxin Chen, Eun Kyu Lee, Soon Young Shin, Young Han Lee, Sang Wook Son, Chil Hwan Oh, Joon Myong Song, Seong Ho Kang and Jaebum Choo, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904592a

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View Article Online / Journal Homepage / Table of Contents for this issue

Nanospheres of silver nanoparticles: agglomeration, surface morphology control and application as SERS substrates Xiao Shuang Shen, Guan Zhong Wang, Xun Hong and Wei Zhu, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904712c

SERS enhancement by aggregated Au colloids: effect of particle size Steven E. J. Bell and Maighread R. McCourt, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b906049a

Towards a metrological determination of the performance of SERS media R. C. Maher, T. Zhang, L. F. Cohen, J. C. Gallop, F. M. Liu and M. Green, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904621f

Ag-modified Au nanocavity SERS substrates Emiliano Cortés, Nicolás G. Tognalli, Alejandro Fainstein, María E. Vela and Roberto C. Salvarezza, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904685m

Surface-enhanced Raman scattering studies of rhodanines: evidence for substrate surface-induced dimerization Saima Jabeen, Trevor J. Dines, Robert Withnall, Stephen A. Leharne and Babur Z. Chowdhry, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905008f

Characteristics of surface-enhanced Raman scattering and surface-enhanced fluorescence using a single and a double layer gold nanostructure Mohammad Kamal Hossain, Genin Gary Huang, Tadaaki Kaneko and Yukihiro Ozaki, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b903819c

High performance gold nanorods and silver nanocubes in surface-enhanced Raman spectroscopy of pesticides Jean Claudio Santos Costa, Rômulo Augusto Ando, Antonio Carlos Sant Ana, Liane Marcia Rossi, Paulo Sérgio Santos, Márcia Laudelina Arruda Temperini and Paola Corio, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904734d

Water soluble SERS labels comprising a SAM with dual spacers for controlled bioconjugation C. Jehn, B. Küstner, P. Adam, A. Marx, P. Ströbel, C. Schmuck and S. Schlücker, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905092b

Chromic materials for responsive surface-enhanced resonance Raman scattering systems: a nanometric pH sensor Rômulo A. Ando, Nicholas P. W. Pieczonka, Paulo S. Santos and Ricardo F. Aroca, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b904747f

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Self-assembly of a,x-aliphatic diamines on Ag nanoparticles as an

effective localized surface plasmon nanosensor based in interparticle

hot spots

Luca Guerrini, Irene Izquierdo-Lorenzo, Jose V. Garcia-Ramos,

Concepcion Domingo and Santiago Sanchez-Cortes*

Received 6th March 2009, Accepted 11th June 2009

First published as an Advance Article on the web 28th July 2009

DOI: 10.1039/b904631c

The adsorption and self-assembly of a,o-aliphatic diamines on silver nanoparticles is studied in

this work by surface-enhanced Raman scattering (SERS) spectroscopy and plasmon resonance.

These bifunctional diamines can act as linkers of metal nanoparticles (NPs) inducing the

formation of hot spots (HS), i.e. interparticle junctions or gaps between metal NPs, which are

points where a huge intensification of the electromagnetic field occurs. In addition, the dicationic

nature of these diamines leads to the formation of cavities just at the induced hot spots which can

be applied to molecular recognition of analytes. The influence of the surface coverage and the

aliphatic chain length in diamines on their self-assembly was tested by the vibrational spectra and

correlated to the different plasmon resonances of the dimers detected in the extinction spectra.

These factors can be used for tuning the plasmon resonance of dimers formed by two metal

nanoparticles where interparticle hot spots are formed. Finally, the analytical potential of these

functionalized Ag nanoparticles is demonstrated for the trace detection of the pesticide aldrin.

Introduction

The single molecule detection (SMD) by surface-enhanced

Raman scattering (SERS) has been widely investigated in the

last decade.1–4 SERS is possible thanks to localized surface

plasmon resonance (LSPR) occurring in nanostructured metal

surfaces inducing a large electromagnetic field intensification

in certain points or areas of the surface. The most effective

locations for extremely high enhancements are interstitial sites

of nanometre-scale between nanoparticles (NPs), named hot

spots (HS).5 Therefore, the controlled fabrication of nano-

structured junctions in SERS substrates is now a critical target

in nanotechnology.4 One of these applications is the fabrica-

tion of extremely sensitive and selective LSPR based

nanosensors.6 An ideal situation for building interparticle HS

is the use of bifunctional molecules which act as NPs linkers.

Moskovits et al. demonstrated7,8 that dithiol-containing

adsorbates can act as linker molecules, leading to the chemically

driven production of SERS active systems consisting of

assemblies of strongly interacting nanoparticles (NPs).

However, thiol linkers form very tight self-assembled monolayers

(SAM) due to the strong intermolecular interactions between

aliphatic chains.9 The functionalization of silver NPs by

bifunctional viologen dications gives rise to the formation of

highly sensitive interparticle junctions by simultaneously

creating intermolecular cavities (IC) where the approaching

of pollutants is possible, with low affinity toward the metal

surface, near to these so-formed HS.10,11 This system allowed

the detection of pyrene down to a few molecules.11 In previous

works we demonstrated the importance of inter- or intra-

molecular cavities within the SAM on the functionalized

surface as a critical factor for the detection of pollutants

showing poor affinity toward the metal substrate.10,12–17 The

conformation of the organic molecules employed in the func-

tionalization has a key role on the possible analytical applica-

tions of these systems. This conformation is a consequence of

the special organization adopted by these assemblers upon

their self-assembly on the surface.

Another group of bifunctional molecules with promising

properties for the creation of HS + IC sites are a,o-aliphaticdiamines (AD). The influence of these diamines on the NPs

architecture could be interesting as these compounds can also

act as linkers of NPs. In fact, these diamines are completely

protonated in the colloidal suspension, presenting two

positively charged nitrogen atoms at the side ends of the alkyl

chains which are able to form ion pairs with the halide anions

adsorbed on the metal surface. Besides, the coulombic repul-

sions between the amino head groups may result in the

formation of cavities within the formed SAM. The adsorption

and self-assembly of linear AD on metal surfaces is a key issue

to understand the sensing properties of AD-functionalized

metal NPs, although it has been poorly studied and limited

to the diaminoethane.18 However, the use of these compounds

as NP linkers and host molecules in the design of LSPR

nanosensors could be advantageous because of the relative

weakness of their Raman features and their structural simplicity.

The former property is essential to avoid interferences in the

SERS spectra of the analytes, while the latter is crucial for

making an appropriate interpretation of their adsorption onto

the metal surface. In fact, the structure of AD is characterized

by the existence of an aliphatic chain between two aminoInstituto de Estructura de la Materia, CSIC. Serrano, 121,28006-Madrid, Spain. E-mail: imts158@iem.cfmac.csic.es

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 7363–7371 | 7363

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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groups, the length of which will determine the NP–NP inter-

particle distance.

The structural and dynamic properties of linear chain

systems, such as n-alkanes or lipid bilayers, are determined

by the relative importance of both intra- and inter-chain

interactions. Raman spectral parameters, such as band

frequencies, intensities, band shapes and splitting of the

vibration bands, are very sensitive to the conformational

arrangement of lipid molecules in the supramolecular assem-

blies of the lipid bilayer or the micelle.19 The Raman-active

bands responding to motion and conformation changes of

alkanes and lipids have been thoroughly studied by several

authors.19–33 This information can be used to understand the

adsorption of AD on the metal surface and to further control

the NP–NP interparticle distance. Specifically, the alkylic

linear diamines have been extensively studied by vibrational

spectroscopy and theoretical methods by Batista de Carvalho

et al.34–37

In this work a systematic analysis of the self-assembly

of linear a,o-aliphatic diamines with different lengths (ADn

where n = 2, 6, 8, 10 and 12; Fig. 1) on Ag metal surface was

performed. This study was done as a previous step to under-

standing the sensing activity of AD/NPs systems in the detec-

tion of pollutants. The structural changes occurring in ADn

was controlled by checking the structural marker bands of the

linear aliphatic chain and those of the charged amino heads.

The effect of the length and surface coverage on the structure

and the self-assembly of diamines was investigated. The

relationship between ADn structure and packing with the

NP–NP interparticle distance was studied by plasmon

resonance. Finally, an example of the application of function-

alized Ag NPs in the detection of the pesticide aldrin is

presented. A more systematic study on the application of

these SERS substrates is currently being done for a list of

polychlorinated pesticides.

Experimental

1,2-Diaminoethane (AD2) and 1,12-diaminododecane (AD12)

were purchased from Aldrich with a purity of 498% w/w.

1,6-Diaminohexane (AD6), 1,8-diaminooctane (AD8) and

1,10-diaminodecane (AD10) were purchased from Fluka with

a purity of 498% w/w. Aqueous stock solutions of AD2,

AD6 and AD8 were prepared in Milli-Q water to a final con-

centration of 10�2 M. AD10 and AD12 were dissolved in

absolute ethanol to a final concentration of 10�3 M.

Silver colloids were prepared by reduction of silver nitrate

with hydroxylamine hydrochloride at room temperature.38

The resulting nanoparticles are predominantly spherical

showing an average diameter of 35 nm.38 SERS measurements

on Ag colloidal suspensions were performed by adding an

aliquot of aqueous chloride solution to a final concentration of

2 � 10�2 M and, subsequently, by adding aliquots of ADn

solutions up to the desired concentration. The effect of the

Cl� is double: it induces the aggregation of Ag colloid

and promotes the adsorption of the diamines via ion pair

interactions.

The SERS spectra were measured with a Renishaw Raman

Microscope System RM2000 equipped with argon laser at

514.5 nm, a Leica microscope, and an electrically refrigerated

CCD camera. The laser power in the sample was 2.0 mW. The

spectral resolution was set at 2 cm�1.

Samples for UV-visible absorption spectroscopy were

prepared in the same way as those for the corresponding

SERS spectra and were recorded in a Helios l spectrometer.

The colloidal suspensions were diluted to 50% in water and

placed in 1 cm optical path quartz cuvette.

Samples for transmission electron microscopy (TEM) and

scanning electron microscopy (SEM) were prepared by adding

an aliquot of AD8 up to a low final concentration (10�5 M) to

reduce the extent of the aggregation. TEM micrographs were

recorded in a JEOL 2000TX (200 kV). SEM micrographs were

obtained in an environmental scanning electron microscope,

(ESEM) PHILIPS XL30 with tungsten filament operating

under high vacuum mode (25 kV). For secondary electrons,

the standard Everhard–Thornley detector was used. Samples

coating was accomplished using a high resolution magnetron

sputter coater Polaron SC 7640. All samples were coated

at room temperature with a 3–4 nm of gold–palladium

(Au/Pd) layer using a high resolution magnetron sputter

coater operated at 800 V and 5mA plasma current to get a

coating rate of 0.5 nm min�1.

Results and discussion

SERS of a,x-aliphatic diamines adsorbed on the metal surface

Fig. 2–6 show the Raman and SERS spectra of AD2, AD6,

AD8, AD10 and AD12, respectively. Two main spectral regions

are found to be the most sensitive, gathering the most useful

information about the structural and dynamic properties of

these systems. These are the regions corresponding to C–C and

C–H stretching vibrations.

The main chain C–C skeletal stretching region from

1050 cm�1 to 1150 cm�1 is sensitive to the intra-molecular

conformation along each chain.19,20 For an extended chain

with all-trans structures, two intense bands appear at

ca. 1050 cm�1 and ca. 1130 cm�1, assigned to symmetric and

anti-symmetric C–C stretching vibrations, respectively. When

Fig. 1 Structures of 1,2-diaminoethane (AD2), 1,6-diaminohexane

(AD6), 1,8-diaminooctane (AD8), 1,10-diaminodecane (AD10) and

1,12-diaminododecane (AD12), and the respective distances between

the two head nitrogen atoms for extended chain with all-trans

structures. The amino head groups are positively charged at the pH

of the suspended Ag NPs (5.5).

7364 | Phys. Chem. Chem. Phys., 2009, 11, 7363–7371 This journal is �c the Owner Societies 2009

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the numbers of trans C–C bonds in the chain decrease in favor

of gauche conformers, the intensity of these two bands lowers

and a strong broad band at about 1080 cm�1 rises. The

peak height ratio of the bands at 1130 cm�1 and 1080 cm�1

(H1130/H1080) provides quantitative information about the

relative amounts of trans/gauche conformers, which is indeed

related to the order/disorder in the aliphatic chain.

The spectral features in the C–H stretching region ranging

from 2800 cm�1 to 3000 cm�1 are found to be deeply affected

by the lateral packing interactions due to the re-orientational

fluctuations of the alkyl chains.21 The spectral para-

meter sensitive to these inter-molecular interactions is the

peak height ratio of the bands at ca. 2890 cm�1 and

ca. 2845 cm�1 (H2890/H2845), assigned to symmetric and anti-

symmetric C–H stretching vibration, respectively, which

increases as the lateral packing is stronger. AD2 presents a

remarkably different behavior regarding the other linear

diamines.36 In fact, the shortness of its alkyl chain does not

allow an efficient electronic charge delocalization through the

Fig. 2 (a) Raman spectrum of neat AD2 in liquid state; and (b) SERS

spectra of AD2 (10�3 M). Excitation at lexc = 514.5 nm.

Fig. 3 Raman spectra of AD6 in (a) aqueous solution (0.1 M); and

(b) in solid state. (c) SERS spectrum of AD6 (10�3 M). Excitation at

lexc = 514.5 nm.

Fig. 4 Raman spectra of AD8 in (a) aqueous solution (1.0 M); and

(b) in solid state. (c) SERS spectrum of AD8 (10�3 M). Excitation at

lexc = 514.5 nm.

Fig. 5 (a) Raman spectra of AD10 in the solid state. SERS spectra of

AD10 at (b) 10�4 M and (c) 10�5M. Excitation at lexc = 514.5 nm.

Fig. 6 (a) Raman spectra of AD12 in the solid state. SERS spectra of

AD12 at (b) 10�4 M, (c) 5 � 10�5 M and (d) 3 � 10�6 M. Excitation at

lexc = 514.5 nm.

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C skeleton when intermolecular H2N–H interactions simul-

taneously form in both ends of the molecule. Therefore, the

dimeric species is the most favourable in the condensed phase.

By contrast, as the alkyl chain lengthens, the relative weight of

the amine hydrogens in the molecule becomes smaller and the

oligomer forms are likely to occur in the solid. These dis-

similarities are reflected in the C–H stretching region of the

spectra, where the AD2 presents significant differences in the

number, relative intensities and frequencies of the observed

bands compared to the aliphatic diamines with more than two

methylene groups (Fig. 2).

As can be seen in Fig. 3b, 4b, 5a, and 6a, the trans

conformation is predominant in the C–C bonds of ADn in

the solid state. This can be deduced by the strong symmetric

and anti-symmetric C–C skeletal vibration bands at ca. 1120

and 1060 cm�1. Conversely, in the liquid state (Fig. 2a) or in

aqueous solution (Fig. 3a and 4a), a broad band in the

1095–1070 cm�1 region corresponding to gauche C–C

conformers dominates this spectral region, thus indicating a

high disorder in the liquid phase.

The SERS spectra of ADn (Fig. 2b, 3c, 4c, 5b, 6b and c)

indicates that the adsorption of these compounds on Ag NPs

induces significant changes in the diamine structure, which

depends on the length of the alkylic chain and the diamine

concentration, i.e. the surface coverage.

According to the SERS spectra, two different adsorption

patterns can be identified on varying the concentration. The

AD2, AD6 and AD8 diamines show SERS spectra with an

invariant profile, the intensity of which decreases with the

lowering the diamine concentration. This indicates that the

adsorption mechanism is maintained no matter what the con-

centration. On the contrary, AD10 and AD12 (Fig. 5 and 6)

display two different spectra profiles: at high concentration the

spectra are similar to those of the shorter diamines, but at very

low surface covering, the AD10 and AD12 spectra (Fig. 5c and

6d, respectively) are dominated by the symmetric and anti-

symmetric d(N–H) modes of the amino head groups at

ca. 1510 cm�1 and the doublet at ca. 1580 and 1610 cm�1,39

while the C–C skeletal vibration disappears and both the CH2

wagging vibration at 1365–1360 cm�1 and the CH2 bending

modes at ca. 1455–1440 cm�1 undergo a strong weakening.

These results suggest that at low surface covering the larger

diamines may interact, at least to a larger extent with respect to

the shorter diamines, through both amino groups with the same

NP surface, assuming a curved O configuration on the metal,

as schematically outlined in Fig. 7 (left panel). This curved

orientation exposes the alkyl chain to the bulk reducing the

extension of Ag NPs dimer formation. According to the

molecular modeling carried out by Weiger et al.,40 fully proto-

nated diamines with an alkyl chain length up to 8 methylene

groups were found to be completely surrounded by a water

shell, whereas for larger CH2-chains the surrounding water split

into two separated shells divided by a hydrophobic segment.

Moreover, their calculations pointed out the notably higher

flexibility of AD12 with respect to the other diamines.40 As a

result, the higher chain flexibility of AD10 and AD12 favors the

curved interaction with the metal surface at low concentration.

AD12 seems to display a more complex behavior, as a

significant enhancement of the d(N–H) bands in the

1500–1630 cm�1 region is seen when the diamine concentra-

tion is increased from 5 � 10�5 M (Fig. 6c) to 10�4 M

(Fig. 6b). This suggests a further alteration of the amino head

group conformation for highly packed monolayers.

At high diamine concentrations the bands corresponding to

C–C stretching motions undergo a general shift upwards

displaying a typical solid-like profile SERS spectra (Fig. 2b,

3c, 4c, 5b, 6b and c) indicating that the adsorption on the

metal surface leads to a prevailing trans geometry (Fig. 8). In

contrast, the spectral pattern in the C–H region still appears

more similar to the liquid state or the aqueous solutions

(Fig. 2a and b, 3a and c, 4a and c), suggesting that the lateral

packing in the adsorbed diamine layer is lower than in the

solid phase. The interesting mixture of solid and liquid charac-

teristics of the adsorbed ADn is a consequence of the special

self-assembly of these kind of compounds, and it is indeed

related to the reduced lateral packing and surface molecular

density induced by the positively charged amino head

groups.30 This is a key characteristic of the analytical applica-

tion of diamine SAM, since the low lateral packing favors the

formation of cavities where lipophilic analytes can be allocated

Fig. 7 Adsorption models of ADn at different surface coverages.

Fig. 8 Relation between the spectral marker bands and the self-

assembly of ADn on the metal surface: lateral packing interactions and

trans/gauche configuration of the alkyl backbone.

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(Fig. 8), just in interparticle junctions. In contrast, the SAMs

in alkyl thiols display a higher packing density and the

possibility of intermolecular cavities is much lower.41

The molecular orientation of ADn can be also deduced from

the relative intensity of bands in SERS spectra. For instance, a

relative enhancement is observed for: (a) the C–C skeletal

modes; (b) the band at ca. 1360 cm�1, assigned to the CH2

wagging vibration;30 and (c) the band at ca. 3260 cm�1,

assigned to the NH3+ stretching vibration. However, an

intensity decrease is seen for the CH2 twisting band at

ca. 1300 cm�1 and the bands at ca. 1455–1440 cm�1, attributed

to the CH2 bending modes.30 According to the selection rules

of the SERS effect,42 the selective enhancement of the Raman

modes possessing a strong perpendicular component with

regards to the metal surface, indicates a preferably perpendicular

orientation of the adsorbed diamines on the NPs, as outlined in

Fig. 9. This perpendicular arrangement implies the interaction of

both amino head groups with different Ag NPs (Fig. 7, right

panel), and this highly favours the NPs aggregation.

In what follows we present a detailed analysis of the

structural marker bands and the effect of the chain length

and the surface coverage on the diamine self-assembly.

C–H stretching region, 2800–3000 cm�1. In the C–H stretch-

ing region ranging from 2800 cm�1 to 3000 cm�1 (Fig. 10, left

panel), all the aliphatic diamines with more than two CH2

groups show a narrow band at ca. 2850 cm�1, corresponding

to symmetric n(C–H) mode, and a broad band which appears

as a unique band centred at 2914 cm�1 in AD6 and splits into

two components at 2920 and 2900 cm�1 as the length of ADn

increased. The latter band is assigned to a Fermi resonance

band resulting from the coupling of C–H asymmetric stretch-

ing mode and CH2 scissoring modes.43 As the length of the

chain is enlarged from AD6 (Fig. 10d, left panel) to AD12

(Fig. 10a, left panel), the former band results are hardly

affected in terms of width and frequency whereas the latter

feature broadens, indicating the presence of different types of

non-equivalent methylene groups in the structure.

The HB2910/H2850 ratio can be used as an indication of

intermolecular van der Waals interactions and display a clear

dependence with the diamine length and coverage (Fig. 10,

right panel). This ratio decreases as increasing the AD6, AD8

and AD10 concentration until reaching a constant value,

indicating an increase of the lateral packing density. However,

the AD12 peak ratio appears insensitive to the concentration,

suggesting a strong tendency of this diamine to form a highly

packed chain layer even at low surface covering.

The HB2910/H2850 value at which a maximum packing density

is achieved, decreases with the increasing the number of CH2

groups in the chain, pointing out that the interaction strength

between aliphatic chains increases with the length at a constant

concentration. This is attributed to the higher flexibility in longer

chains, which makes it possible to overcome the electrostatic

repulsions of the positively charged amino head groups.

The SERS spectrum of AD2 in the C–H stretching region

(Fig. 10e, left panel), exhibits remarkable differences with

respect to the other diamines, as similarly observed in the

pure Raman spectra,36 suggesting that AD2 adopts a different

geometry onto the metal surface with respect to the other

diamines. In fact, an adsorption consisting in the interaction of

both amino groups with the same metal surface has been

proposed by Hu et al.18

C–C skeletal stretching region, 1050–1150 cm�1. Fig. 11, left

panel, illustrates the C–C skeletal stretching region of ADn

SERS spectra. The bands assigned to symmetric C–C stretch-

ing motions at ca. 1050 cm�1, related to the overall trans C–C

conformers fraction, and at ca. 1080 cm�1, corresponding to

gauche C–C conformers, undergo a notable upshift when

Fig. 9 Perpendicular adsorption scheme of ADn (n= 6, 8, 10 and 12)

on the metal surface and orientation of the corresponding vibrational

modes. The bands of modes perpendicular to the surface are enhanced

according to the SERS selection rules in relation to the parallel modes.

Fig. 10 [Left] Detail of the C–H stretching region from the SERS of

(a) AD12 (10�4 M); (b) AD10 (10�4 M); (c) AD8 (10�4 M); (d) AD6

(10�4 M) and (e) AD2 (10�3 M). [Right] Dependence of the

H2910/H2850 ratio with the diamine concentration.

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going from AD6 to AD12. On the contrary, the position of the

anti-symmetric C–C stretching vibration at ca. 1137 cm�1 is

almost insensitive to the chain length. We have measured the

order of the aliphatic chain by two methods: (a) by measuring

the intensity of the gauche band at ca. 1080 cm�1; and (b) by

measuring the relative intensity of the trans band at

ca. 1050 cm�1 with respect to the intensity of the n(Ag–Cl)

band at 243 cm�1 (Hns(C–C)/Hn(Ag–Cl)). The gauche band

undergoes a strengthening in going from longer to shorter

chains, indicating a decrease of the chain order as this length is

decreased. However, the overlapping of the stretching C–C

bands does not allow a reliable analysis of their relative

intensities. In contrast, the (Hns(C–C)/Hn(Ag–Cl)) ratio can be

better analysed. In Fig. 11, right panel, this ratio is plotted

against the diamine concentration. As can be seen, an increase

of the average number of trans C–C bonds, i.e. of the order

in the aliphatic chain, occurs when increasing the surface

coverage for each diamine and, for a given concentration,

when enlarging the chain length. This order increase is a

consequence of the higher molecular packing on increasing

the surface coverage. Thus, the conformational changes

occurring along the chain fully agree with the previously shown

variations in lateral packing order (Fig. 10, right panel).

Therefore, the SERS data point out that the diamine

molecule adsorbed onto the metal surface cannot be treated

as an independent and rigid cylindrical spacer but it has to be

considered as a part of the whole ensemble of molecules

forming the SAM, the properties of which (distribution of

the trans/gauche conformation and the inter-molecular inter-

actions) change when varying both the surface covering and

the length of the alkyl chain.

Plasmon adsorption spectra of a,x-aliphatic diamines

functionalized Ag NPs

Fig. 12 shows the initial plasmon resonance spectrum of the

Ag NPs colloid (Fig. 12a) as well as the difference plasmon

resonance spectra of Ag NPs functionalized with the diamines

AD8 (Fig. 12b) and AM8 (Fig. 12c) In general, for all the

investigated diamines three main bands appearing in the

340–370, 450–500 and 700–1200 nm regions can be seen in

the difference spectrum. The first band practically maintains

its position for all the investigated diamines and can be

assigned to non aggregated NPs that are too small as to

be aggregated. In fact Ag NPs of 10 nm in diameter can be

identified in the micrographs (Fig. 12d and f, upper micro-

graphs). However, the position and width of the two latter

Fig. 11 [Left] Detail of the C–C skeletal stretching region from the

SERS spectra of (a) AD12; (b) AD10; (c) AD8; (d) AD6 and (e) AD2

at the same concentrations that Fig. 10. [Right] Dependence of

Hn s(C�C)/Hn (Ag�Cl) ratio with the diamine concentration.

Fig. 12 (a) Plasmon resonance spectrum of the hydroxylamine AgNPs

colloid, and plasmon resonance difference spectra of the Ag NPs colloid

functionalized with (b) AD8 (5 � 10�4 M) and (c) MA8 (5 � 10�4 M).

TEM (d) and SEM (e) micrographs of the AD8 functionalized Ag NPs

showing the presence of dimers, as well as multimers and unaggregated

NPs. A detail of the dimer formed by two polygonal Ag NPs in the

presence of AD8 is shown in (f) due to the linking of the diamine to two

different NPs (g). Plot of the plasmon resonance of NP–NP dimers

(band at 450–500 nm) against the number of CH2 groups in the chain

inducing a variable distance in interparticle junction by the adsorption

and self-assembly of ADn diamines is displayed in (h). The error bars

shown in this plot corresponds to the quadratic standard deviation from

three different measurements.

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bands remarkably changed depending on the structure and

concentration of the adsorbed diamine. In particular, the band

appearing at 450–500 nm can be assigned to the longitudinal

dimer plasmon modes. These dimers can be seen mainly in the

Ag NP colloid functionalized with diamines (Fig. 12d, e and f),

while are practically absent in the Ag NP colloid function-

alized with the monoamine. In a previous work, we reported

this plasmon resonance to appear at 500–600 nm, but in that

case the used Ag NPs had a mean diameter of 50–60 nm, as

corresponds to NPs prepared with citrate. This is the expected

result according to various calculations performed by other

authors.44–49 In the present case, the NPs are significantly

smaller (35 nm average diameter, Fig. 12 upper micrographs).

Furthermore, this resonance also agrees with that measured

for Ag NPs dimers recently reported by Li et al.,50 and are

close to the calculations carried out by us, in a work which will

be published in the near future, and other calculation by

Hao et al.47 for morphologically related NPs. Thus, from this

evidence we have assigned this band to the longitudinal

plasmon resonance of dimers, such as that shown in

Fig. 12f. The broad band at longer wavelengths includes

contributions from larger aggregates or multimers, the posi-

tion of which is also dependent on the lower 450–500 nm band.

Fig. 12 shows these bands for the special case of AD8 diamine

displaying three main bands centred at 363, 464 and 828 nm.

The mono-amine octylamine (MA8) was employed as the

experimental control. MA8 adsorption poorly promotes the

dimer formation as indicated by the weak dimer band in its

absorption spectra (Fig. 12c) and the obtained TEM and

SEM micrographs. This indeed indicating the importance of

bifunctionality in the formation of NP–NP dimers, as shown

in Fig. 12g.

The resonance plasmon spectra shown in Fig. 12 were

recorded at a diamine concentration at which AD8 is expected

to form the tighter SAM on the metal. Fig. 12h shows the

variation of the plasmon resonance corresponding to the

NP–NP dimer (450–500 nm band) against the diamine length,

i.e. the number of CH2 in the alkyl chain. As can be seen, a

blueshift of the dimer plasmon absorption band position is

observed when enlarging the alkyl chain from n= 2 to 10, due

to an increase of the interparticle distance which leads to a

weaker electromagnetic coupling in the NP dimers. In the case

of AD12 functionalized NPs this tendency is broken due to the

stronger interchain interactions which leads to a slight chain

shortening, in accordance to the SERS results.

The linker concentration has also a clear effect on the

plasmon absorption, and thus on the formation of NP dimers.

This is outlined in Fig. 13. When increasing the diamine

concentration, the absorption spectra put in evidence two

main steps in the diamine-driven aggregation process, dis-

played for AD8 in Fig. 13, left panel. This behaviour is likely

related to phase transitions occurring in the self-assembled

diamine systems on the metal surface. At very low concentra-

tion (usually under 10�5 M), no dimer band is observed

(Fig. 13a), and this is attributed to the adsorption of the

diamine on the same surface under the curved O configuration,

which prevent the metal NPs from aggregation. On increasing

the diamine concentration, a sudden and drastic aggregation

of the NPs in suspension occurs. This is related to the

appearance of a broad absorption band in the 400–800 nm

range which can be fitted to two components, each one

corresponding to NP dimers linked by diamines structurally

different with different order degree (Fig. 13b and c). Finally,

at high concentration (Fig. 13d and e) the dimer band is

enhanced and can be fitted to one single band due to NP–NP

dimers linked by highly ordered and packed diamines, as

indicated by the blue shift of the dimer band and the SERS

spectra at higher concentrations shown above.

Fig. 13, right panel, shows the evolution of the correspond-

ing dimer maxima at different surface coverages for different

diamines. The effect of concentration on these maxima reveals

a hairpin behaviour evolving from the red to the blue because

of the unification of two maxima observed at low concentra-

tions into a unique one. The concentration at which the bi-to-

monomodal transition occurs increases as the length of

diamines decreases, because the monomodal plasmon resonance

band is observed when the diamine SAM formed on the metal

surface is the tightest possible and the structure of ADn is highly

ordered. The appearance of this monomodal plasmon band

in shorter diamines is only possible if the concentration is

sufficiently increased, due to their higher tendency to disorder.

The above results reveal a good correlation between the

structural data provided by SERS spectra and those obtained

by plasmon resonance, and suggest an interesting possibility:

the plasmon resonance tuning in Ag NP dimers and multi-

mers by controlling the structure and conformation of the

molecular linker.

Application of diamine functionalized NPs for sensing pesticides

The aim of using diamines as NPs linkers was double: the

formation of interparticle HS with a controllable distance and

the formation of IC, i.e. recognizing cavities just at these HSs.

The first was demonstrated by the SERS and plasmon reso-

nance experiments. In particular, SERS spectra demonstrated

that the diamines interact with the metal surface retaining a

mainly trans conformation of the methylene groups with a

substantially extended alkyl chain (i.e. solid-like structure),

Fig. 13 [Left] UV-Vis absorbance difference spectra of AD8 at the

following diamine concentrations: (a) 1 � 10�5 M, (b) 3 � 10�5 M,

(c) 1 � 10�4 M, (d) 5 � 10�4 M and (e) 1 � 10�3 M. [Right] Hairpin

dependence of the plasmon corresponding to the dimer band on the

diamine concentration.

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but showing reduced lateral interactions (i.e. liquid-like

structure). This suggests the formation of intermolecular

cavities in between the dimer NPs (Fig. 14, inset), precisely

where the HS associated to the dimer plasmon resonance

occurs,44,46,48 which can be employed for analytical detections.

The sensing ability of ADn functionalized Ag NP was

illustrated by using the polychlorinated insecticide aldrin as

an example of a probe molecule. A more complete study on

the analytical applications of these systems regarding a group

of different polychlorinated pesticides is currently in progress

in our laboratory and will be published in the near future.

These pesticides interact with the plasmatic membrane of

neuronal cells modifying the nervous current toxicity.51 Since

this membrane is integrated by the alkylic chains of phospho-

lipids, the SAMs formed by ADn dications on metal NPs could

mimic the biological target of this pollutant. As a result, the

self-assembly of diamines on NPs modifies the chemical

properties of the surface in order to favour the approaching

of the polychlorinated insecticides.

In the absence of diamine, no SERS spectrum from aldrin

could be obtained due to its low affinity toward the metal

surface (spectrum not shown), whereas the proper functiona-

lization of the NPs surface with ADn (in this case AD8)

allowed its SERS detection due to the formation of the host/

guest complex (Fig. 14b). The characteristic bands of aldrin

can be seen in the low wavenumbers region which is comple-

tely free of diamine bands (Fig. 14b) and which can be better

seen in the difference spectrum (Fig. 14c). These bands are

mainly assigned to C–Cl stretching motions and constituted an

actual fingerprint of this molecule, which is completely different

from the vibrational spectrum of other related polychlorinated

molecules.52–54 The comparison with the Raman of solid

aldrin (Fig. 14d) reveals some differences due to the interaction

with AD8.

The adsorption of the aldrin near or at the interparticle

junctions formed by the diamine linkers led to the detection of

the pollutant, allowing its detection down to B5 ppb. The

resulting enhancement factor was estimated around 2 � 104.

Conclusions

In summary, silver colloid NP dimers are obtained with

interparticle distances finely tuned in the B1 nm range. This

was accomplished by exploiting the chemical properties of

a,o-aliphatic diamines as molecular linkers with varying

length controlled by the alkyl groups in the chain. SERS

spectroscopy was crucial to determine the structure of

adsorbed diamines on the metal surface thanks to the struc-

tural marker bands related to the order/disorder and mole-

cular packing. Both, the interaction strength between aliphatic

chains, i.e. the lateral packing, and chain conformational

order increases with the length and the diamine concentration.

The effects of diamine length and surface coverage on the

molecular structure of these linkers are also manifested on the

plasmon resonance of the dimers induced by the adsorption of

diamines. This effect is interesting as the structural modifica-

tion of these molecular linkers can be used to tune the plasmon

resonance of these dimers. Finally, we have shown that

ADn-functionalized Ag NP dimers can act as LSPR analyte

nanosensors, at the trace level, for SERS detection of poly-

chlorinated pesticides. This was possible thanks to inter-

molecular cavities formed by the diamines within dimer

gaps, where the SERS electromagnetic enhancement factors

are larger for the dimer plasmon resonances. Thus, the con-

trolled fabrication of NP dimers proposed here would be of

interest in a variety of applications in nano-photonics and

(bio) molecular sensing.

Acknowledgements

Authors acknowledge Ministerio de Educacion y Ciencia

project number FIS2007-63065 and Comunidad Autonoma de

Madrid project number S-0505/TIC/0191 MICROSERES for

financial support. L.G. and I.I.L. acknowledge CSIC for an

I3P and a JAE fellowship, respectively.

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