Plasmonics

, Volume 5, Issue 3, pp 273–286

α,ω-Aliphatic Diamines as Molecular Linkers for Engineering Ag Nanoparticle Clusters: Tuning of the Interparticle Distance and Sensing Application

Authors

    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Irene Izquierdo-Lorenzo
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Rogelio Rodriguez-Oliveros
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Jose A. Sanchez-Gil
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Santiago Sanchez-Cortes
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Jose Vicente Garcia-Ramos
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
  • Concepcion Domingo
    • Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas
Article

DOI: 10.1007/s11468-010-9143-x

Cite this article as:
Guerrini, L., Izquierdo-Lorenzo, I., Rodriguez-Oliveros, R. et al. Plasmonics (2010) 5: 273. doi:10.1007/s11468-010-9143-x

Abstract

The controlled tuning of interparticle distance at the nanoscale level is a major challenge for nanofabrication of surface-enhanced Raman scattering (SERS) active clusters and their application to molecular sensing. In fact, the geometrical properties of the narrow gaps between nanoparticles play a key role in determining the local field enhancement (and therefore, the SERS enhancement factor) and the spatial enhancement distribution in the gap region. Besides, very short interparticle distances may block the access of the analyte to the hot zone. In this paper, we report the synthesis of silver colloid NP clusters with interparticle distances fine tuned in the ≤2 nm range, by exploiting the chemical properties of linear α,ω-aliphatic diamines as molecular linkers with varying chain length. The bifunctional diamines also form intermolecular cavities within their self-assembled monolayers, suitable to host molecular analytes for nanosensing applications, as evidenced by SERS detection of organochlorine insecticides at the trace level. In this regard, the extension of the aliphatic chain played a crucial role in determining the SAM conformation and thus the final sensitivity of the functionalized SERS substrate.

Keywords

SERSPlasmonic nanoparticlesInterparticle distanceSensing applicationOrganochlorine insecticides

Introduction

Surface-enhanced Raman scattering (SERS) has attracted a renewed interest in recent years due in part to single molecule detection and the fabrication of new metal nanostructured substrates [1, 2]. The SERS effect has been attributed to localized surface plasmon resonances (LSPR) occurring in nanostructured metal surfaces [3]. The optical properties associated with the excitation of plasmon resonances on metal nanoparticles (NPs) are known to be crucial in providing large local electromagnetic fields [4, 5], as required not only in SERS spectroscopy [2, 6], but also in a variety of nanostructure-enhanced emission processes, such as two-photon luminescence [7, 8], fluorescence [9], or quantum-dot emission [10]. A controlled fabrication of NP substrates with optimized configurations yielding enhanced local fields would no doubt pave the way to a wealth of applications in nano-photonics and (bio)molecular sensing [6, 1014]. In this regard, whatever the metal NP configuration with related plasmon resonances, a key geometrical factor in order to obtain huge electromagnetic field enhancements is the existence of narrow gaps that allow for strong optical coupling between NPs. Basically, this is accomplished either by lithographic techniques, with well-defined gaps in between coupled metal NPs with given shapes, often referred to as dimer nanoantennas [7, 8, 10, 15], or by colloidal suspensions of spherical NPs, which exhibit complex aggregates with interstitial gaps [1]. Major experimental drawbacks are, respectively, the limitation to gaps >10 nm for nanolithography and the large (number, shape, and geometry) nonhomogeneities for NP aggregates. Other attempts rely on attaching a metal NP to a scanning probe that is coupled in turn to a single NP on a substrate [16], but is impractical for obtaining samples with a large number of dimers. Recently, many efforts have been devoted to the controlled aggregation of NP clusters. In particular, the use of bifunctional molecules as NP cross-linkers showed to be a promising strategy [1721]. Moskovits et al. demonstrated that dithiol-containing adsorbates [17, 18], 4,4′-diaminoazobenzene [21], and small proteins [22] can act as linker molecules, leading to the chemically driven production of SERS active systems consisting of small assemblies of strongly interacting NPs. Recently, 1,6-diaminohexane has been used by Braun et al. [19] as molecular linker to mediate the controlled assembly of colloidal NPs to dimers and small clusters. The subsequent polymer-encapsulation step structurally stabilized these nanoaggregates resulting in active SERS substrates with high uniformity. By contrast to dithiol linkers, the diamine compound can be displaced at a later stage by a target molecule with higher affinity for the metal surface without compromising the integrity of the pre-linked dimer or small aggregate

Our group has shown that, in addition to promoting the formation of nanoscale interparticle junctions, bifunctional compounds as viologen dications [20, 23] modify the surface chemical properties of the metal so that the detection of pollutants without SERS activity can be achieved. This NP assembly strategy leads to the approaching of the analyte near to the so-formed hot spot (HS) providing an extremely sensitive and selective LSPR based nanosensor, which allowed the detection of pyrene down to few molecules [23]. In our previous works [20, 2328], we emphasized the importance of the host molecule conformation on the functionalized NP surfaces for the SERS sensing of pollutants showing poor affinity toward the metal. In fact, a critical factor for the detection of these analytes is the presence of inter- or intramolecular cavities within the self-assembled monolayer (SAM) of the host molecules, which is strictly related to the special organization adopted by the receptor upon the adsorption on the metal surface. Besides, by controlling the surface chemistry carefully, it is possible to achieve high level of detection specificity in addition to the intrinsic sensitivity of the SERS technique.

On the strength of the above findings, we have recently employed different α,ω-aliphatic diamines as molecular linkers in the functionalization of Ag NPs [29]. These diamines are completely protonated in the colloidal suspension, presenting two positively charged nitrogen atoms at the side ends of the aliphatic chains, which are able to form ion pairs with the chloride anions adsorbed on the metal surface. Furthermore, by contrast to what happens to the adsorption of thiols at gold surfaces, which produces densely packed films in a crystalline arrangement [30, 31], the coulombic repulsions between the amino head groups avoid a thick molecular packing on the surface, thus leading to the formation of intermolecular spaces, which are actual cavities able to host molecular analytes [29]. An additional important feature for these compounds to be applied as receptors is their small Raman cross-section, which avoids extended band overlapping in the final host/guest SERS spectrum.

The interparticle distance in an NP–NP nanoscale junction plays a key role in determining the characteristics of the gap-plasmon resonance [32], including the local field enhancement (and therefore the SERS enhancement factor) and the spatial enhancement distribution in the gap region [32]. Besides, very short interparticle distances may block the access of the analyte to the hot zone. Thus, the controlled tuning of interparticle distance at the nanoscale level is a major challenge for nanofabrication of SERS active clusters and their application to molecular sensing.

In a previous article [29], we described in detail the structural properties of the self-assembled monolayers of linear α,ω-aliphatic diamines, ADn (where n is the number of methylene groups in the chain, n = 2, 6, 8, 10, and 12; Fig. 1) on Ag NP surfaces by SERS spectroscopy. In this work, we focus the attention on the effects of alkyl chain length and surface coverage on (1) the interparticle spatial properties of chemically driven assembled NP clusters and (2) the hosting properties of the diamine SAMs on the metal surface. A fine tuning of the interparticle distance in the ≤2 nm range is achieved via careful control of these two variables. This is revealed by redshifted dimer resonances in the absorption spectra for closing distances, in agreement with calculations based on the hybridization model [33]. The adsorbed diamines on Ag NPs may also act as molecular hosts to target trace detection of persistent organic pollutants, as organochlorine insecticides. Organochlorine compounds are non-selective insecticides, which develop their toxicity mainly on the nerve cell membranes, by inhibiting the transport of cations as Ca2+, K+, Na+, and Mg2+ [3436]. The use of ADn as receptor was motivated by the fact that their SAMs formed upon the adsorption on Ag NPs can mimic the main features of the biological membranes promoting the interaction with the insecticides under investigation. AD8 functionalization of metal NPs allowed the detection of α-endosulfan (α-ES), β-endosulfan (β-ES) and aldrin (ALD) insecticides down to about 10−8 M.
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Fig. 1

Structures of linear α,ω-aliphatic diamines, ADn (where n is the number of methylene groups in the chain, n = 2, 6, 8, 10, and 12) and the respective distances between the positively charged amino head groups for extended chain with all-trans configurations

Experimental

Materials

1,2-Diaminoethane (AD2) and 1,12-diaminododecane (AD12) were purchased from Aldrich with a purity of >98% w/w. 1,6-Diaminohexane (AD6), 1,8-diaminooctane (AD8), and 1,10-diaminodecane (AD10) were purchased from Fluka with a purity of >98% w/w. Aqueous stock solutions of AD2, AD6, and AD8 were prepared in Milli-Q water to a final concentration of 10−2 M. Absolute ethanol stock solutions of AD10 and AD12 were dissolved in absolute ethanol to a final concentration of 10−3 M. α-Endosulfan (α-ES), β-Endosulfan (β-ES) and Aldrin (ALD) were purchased from Riedel-de-Haën with a purity of >99%. Stock solutions of these compounds in absolute ethanol were prepared to a final concentration of 10−3 M. All the employed reagents were of analytical grade.

Synthesis of Ag nanoparticles and SERS measurements

Silver NPs were prepared by reduction in silver nitrate with hydroxylamine hydrochloride at room temperature [37]. The average NP diameter is about 35–40 nm [29]. The preparation of ADn-functionalized NPs was carried out as follows: 40 µL of 0.5 M NaCl aqueous solution was added to 1 mL of colloid then an aliquot of the ADn solution was added up to the desired concentration. The aggregation induced by the salt leads to a slight change of the color and a slight redshift of the plasmon absorption, thus indicating that the aggregation extent is limited. Moreover, the chloride anion promotes the adsorption of diamines on the metal surface via ionic interaction. The SERS spectra of the diamine/insecticide complexes were acquired by adding appropriate aliquots of the insecticide ethanolic solution to the aforementioned ADn-NPs suspensions.

The samples examined by electron microscopies were prepared by adding aliquots of AD8 or MA8 stock solutions to a freshly made silver hydroxylamine colloid, up to a concentration of 1.5 × 10−5 M. After 10 min, samples were diluted three times with water, in order to quench the aggregation process. Samples examined by SEM were dried on a glass slide at room temperature, while samples for TEM were directly added dropwise on the copper grid.

Instrumentation

The SERS spectra were measured with a Renishaw Raman Microscope System RM2000 equipped with argon laser at 514.5 nm and a diode laser at 785 nm, a Leica microscope, and an electrically refrigerated charge-coupled device camera. The laser power in the sample was 2.0 mW. The spectral resolution was set at 2 cm−1. The microspectra shown here were obtained using a ×100 objective. 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 λ spectrometer. The colloidal suspensions were diluted to 50% in water and placed in 1-cm optical path quartz cuvettes.

Theoretical calculations

The numerical results have been obtained with a Fortran-90 program, which has been programmed by the authors in order to solve the eigenvalue problem appearing in the hybridization method. The results of our program have been checked against the results in the original articles of the method in the bibliography. In order to calculate the plasmon resonances in our dimer configurations with emphasis on the spectral shifts stemming from (fine) tuning of the interparticle distance, we make use of the hybridization method [29]. This method is based upon describing the sea of electrons on the surface of the NPs as an incompressible fluid [33]. This fluid can be expressed in terms of a set of vibration modes, with given resonance energies associated. The resonance energies of the dimers are thus obtained by the hybridization of the non-interacting modes of the single spheres, which is described by the Mie theory formula:
$$ {{\omega_l} = {\omega_B}\sqrt {{\frac{l}{{{\hbox{2l}} + {1}}}}} } $$

The resonance energies strongly depend on the distance between the NPs of the dimer [38]. More precisely, in order to solve this problem, we built the Lagrangian of the system, containing the kinetic and the interaction terms of the plasmon modes. The resulting Euler–Lagrange equations lead to an eigenvalue problem of a symmetric matrix, where the diagonal terms are the single particle energies, and the off-diagonal terms account for the interaction between modes: Finally, the calculated eigenvalues yield the plasmon resonance energies of the NP dimer. The radius of the two identical NPs forming the dimer is 20 nm, and the calculation has been performed up to 50 coupling modes. We have fixed the bulk plasmon resonance (needed in the theoretical calculations) in order to match the wavelength of the single particle resonances λ1 = 2πc1 to that experimentally obtained from the absorption spectra (λ1 = 355 nm). For the sake of comparison, two other values of λ1 have been considered in the calculations that fit within the half-width of the single NP plasmon band. We only consider the contribution to the dimer resonance of the lower (longitudinal) symmetric mode [39].

Results and discussion

SERS characterization of ADn adsorption on Ag NPs

The previous analysis of the SERS spectra of α,ω-aliphatic diamines provided new insight into the orientation and interaction mechanism of these linear molecules when adsorbed on a metal surface [29]. In particular, the chain packing on the surface of ADn–NP systems mainly influences two intramolecular and intermolecular disordering processes: the trans/gauche isomerization along each chain and the interchain lateral interactions [40]. The ADn Raman bands are sensitive to these processes, and thus, SERS spectra can be readily used to monitor the structural and dynamic properties of the diamine layer over the metal surface [40]. Specifically, the peak height ratio of the bands at about 1,130 and 1,080 cm−1 (H1,130/H1,080), assigned to symmetric and anti-symmetric C–C stretching vibrations respectively, provides quantitative information about the relative amounts of trans/gauche conformers, which is indeed related to the order/disorder along the linear chain. On the other hand, the spectral features in the C–H stretching region (3,000–2,800 cm−1 range) are deeply affected by the lateral packing interactions due to the re-orientational fluctuations of the aliphatic chains [41]. The spectral parameter sensitive to these intermolecular interactions is the peak height ratio of the bands at about 2,905–2,910 cm-1 and about 2,845 cm−1 (H2905/H2845), assigned to symmetric and anti-symmetric C–H stretching vibration, respectively, which increases as the lateral packing tightens.

The above spectral marker bands, related to the structure of these linear molecules, are especially sensitive to the surface covering and the aliphatic chain length. In the series n = 6, 8, and 10, the increasing of the diamine concentration and the enlarging of the CH2 chain promotes the organization in higher ordered self-assembled monolayers, where the diamine units preferably adopt a perpendicular orientation with respect to the surface [29]. This common trend fails for n = 2 because of the different chemical behavior resulting from the shortness of the alkyl chain [29]. At the other end of the dimensional scale, as the diamine molecules become too large (n = 12), the chain flexibility drastically increases [29], allowing the formation of highly packed chain layer even at low surface coverage. The identification of the structural marker bands of diamines was crucial to interpret the results obtained in the analysis of the insecticides detection.

Effect of ADn Adsorption on NPs Aggregation

The bifunctional nature of linear α,ω-aliphatic diamines leads to the chemically driven formation of NP clusters. In principle, the interparticle distance is thus governed by the different alkyl chain lengths of the diamine linkers. However, when adsorbed on silver nanoparticles, ADn compounds cannot be treated as rigid rods, since they can undergo significant structural reorganizations depending on factors such as the surface coverage [29]. This indeed provides an additional method to further tune the interparticle distance by changing the boundary conditions, since the end-to-end distance depends on the internal structure of the molecule linker.

The formation of NP clusters by the action of diamines should yield corresponding plasmon resonances with associated interparticle hot spots, which manifest through the absorption spectra of the Ag NP suspension [29], as similarly performed in a previous work [20] where viologen dications were employed as hot spots builders. In order to extract the contribution of the diamines to the final colloidal aggregation, the absorption spectrum of Ag NPs acquired after the addition of Cl was subtracted from the spectrum in presence of diamine, too, in order to eliminate the contribution of the ionic aggregation [29]. All the ADn difference spectra (where n = 6, 8, 10, and 12) display three main bands appearing in the 340–370, 400–500, and 700–1,000 nm regions (Fig. 2, [1]). The position of the first band does not practically change and can be assigned to non-aggregated NPs that are too small as to be aggregated (TEM micrographs show the presence of a large number of isolated 10 nm NPs [29]). Conversely, the position and width of the two latter bands remarkably change depending on the structure and concentration of the adsorbed diamine. In particular, the band appearing at 400–500 nm may correspond to the longitudinal dimer plasmon modes according to various calculation [4247]. The broadband at higher wavelengths may be due to several contributions from larger aggregates or multimers, which position is also dependent on the 450–500 nm band as the formation of dimers is likely the previous step in the appearance of multimers. By contrast, AD2 shows a different behavior: We do not observe the splitting between the dimer and the multimers bands but an intense feature centered at about 505 nm extends over a wider spectral range (Fig. 2, [1], n = 2). In fact, in the singular case of AD2, several different geometries are imposed by the diamine on the NP aggregation, rather than a dominating one as for the other diamines [29]. The experimental control, the mono-amine molecule MA8, poorly promotes the dimer formation as indicated by the weak dimer band in its absorption spectra (Fig. 2, [1], MA8) and as seen in the microscopy (Fig. 3). We suggest monitoring the effect of ADn linkers on the architecture of NP–NP junction by following the dimer plasmon band, which spectral position is directly related to the interparticle distance within the gap.
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Fig. 2

[1] UV-Vis absorbance difference spectra of MA8 5 × 10−4 M and ADn, where (n = 2) AD2 1 × 10−3 M, (n = 6) AD6 1 × 10−3 M, (n = 8) AD8 5 × 10−4 M, (n = 10) AD10 1 × 10−4 M and (n = 12) AD12 3 × 10−5 M. Insert Schematic representations of MA8 and ADn molecules adsorbed on the NP surface. [2]aBlue Experimental shift of the dimer band position when changing n (number of CH2 groups in the chain). The quadratic standard deviation (shorter error bar) as well as the half width at the half height (larger error bar) is displayed. Red Calculated plasmon resonances of NP dimers as a function of interparticle distance for three different values of the single particle, lowest resonance wavelength: λ1 = 346, 355, and 364 nm. b Scheme illustrating the chemically driven formation of dimers by ADn and some proposed configurations of AD2 molecules adsorbed on the metal surface

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Fig. 3

Examples of SEM (a) and TEM (b) images of silver colloids aggregated by AD8 addition. (c) Average rating of lose nanoparticles, dimers, small aggregates, and large aggregates for unaggregated Ag NPs and MA8 or AD8 functionalized NPs systems

The average rate of nanoparticle dimers obtained in the presence of the bifunctional diamines was determined by direct counting of the diverse structures present in several samples analyzed through transmission and scanning electron microscopies (Fig. 3a, b). Figure 3c shows how the addition of AD8 promotes the linking of nanoparticles and the achieving of a high concentration of dimers in the colloidal system. In fact, in the absence of the diamine linkers, it was observed that the colloid consisted mainly of lose nanoparticles, along with very small proportions of aggregates. By contrast, the NP functionalization with n-octylamine (MA8), employed as experimental control due to its lack of bifunctional property, leads to a wider dispersion of particles as well as a decreasing in the average number and size of the aggregates, indicating that MA8 acts as a stabilizing agent.

As already mentioned, the linker concentration plays a crucial role in the plasmon absorption and thus in the formation of NP dimers. When increasing the diamine concentration, the absorption spectra put in evidence two main steps in the chemically driven aggregation process (Fig. 4a), which are likely related to phase transitions in the self-assembled diamine systems on the metal surface. In the first step occurring at the concentration C1, we observed a sudden and drastic aggregation of the NPs in suspension, with the appearance of a broad absorption band in the 400–800 nm range. The second step was observed for higher diamine surface covering (concentration C2) with the appearance of intense dimer absorption (Fig. 4a). The former step is possibly related to the change of the adsorption configuration of the diamine molecules from a curved geometry with both positively charged amino head groups interacting with the same NP surface, only possible at low surface coverage, to a perpendicular orientation within diamine patches over the metal surface, as deduced from the SERS spectra [29]. In fact, whereas the perpendicular arrangement highly favors the NP linking, the curved orientation exposes the alkyl chain to the bulk, thus reducing the extension of the dimer formation. At ADn concentration higher than C1, the orientation of diamine is perpendicular, and the formation of dimers starts, although the surface coverage is still low. Thus, the diamine assemblies present high conformational disorder due to the reorientational fluctuations of the alkyl chains leading to a great variability of the interparticle distances in dimers. This accounts for the broadness of the plasmon absorption band (Fig. 4a, center). A similar phase transition was observed by Chernyshova et al. [48] when studying the dodecylamine salts adsorption of on silicates. When this monoamine is adsorbed below the concentration of 2 × 10−5 M, the alkyl chains are chaotically oriented and the distances between the amine ions on the surface are large. Whereas above this critical value, the chains adopt a perpendicular orientation forming well-packed patches on the silicate [48]. Therefore, as the concentration is raised above C2, a full coverage of NPs is reached, and both the interchain interactions and the extension of each chain increase, leading finally to a higher and well-defined packing order. This entails a specific interparticle geometry between the linked NPs, as a narrow dimer absorption band at lower wavelength is seen (Fig. 4a, right). Again, AD2 represents an exception, and no transitions in the aggregation process were noticed when increasing the surface coverage.
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Fig. 4

a Outline of the progression of dimer formation when increasing the AD8 concentration. Two main transitions are observed in the aggregation process at the diamine concentration C1 and C2. bC1 and C2 values (molarity) for different diamines

The resonance plasmon spectra shown in Fig. 2 ([1]), were recorded at diamine concentrations above C2, at which the most linear conformation of ADn is attained. In Fig. 2 ([2]), the corresponding dimer plasmon absorption maxima are plotted against the number of CH2 in the alkyl chain. We observe a blueshift of the dimer plasmon absorption band position when enlarging the alkyl chain from n = 2–10, due to an increase of the interparticle distance, which leads to a weaker electromagnetic coupling in the NP dimers. As can be seen, AD12 functionalized NP systems depart from this common behaviour, exhibiting a weak redshift. We are of the opinion that because of the dominant interchain interactions observed for this highly flexible diamine, relevant torsions of the adsorbed AD12 molecules are likely causing the shortening of the interparticle distance with respect to the hypothetical full-extended chain configuration (thus consistent with a plasmon resonance blueshift), as sketched in Fig. 2 ([2]b). This agrees with the experimental values C1 and C2 found for each ADn (Fig. 4b). As can be seen, both C1 and C2 concentrations decrease as the alkyl chain length increases up to n = 10. This is due both to the increase in interchain interaction and to the lower repulsion between NPs. For AD12, a change in these trends is observed. Again, this can be explained in terms of increasing chain flexibility and decreasing the electrostatic repulsions between the charged NPs forming the dimer. The theoretical calculations reproduce fairly well the trend of the experimental measurements, thus providing further evidence that colloid NP dimers with chemically tuned interparticle distances are being synthesized.

Sensing of organochlorine insecticides

In addition to promoting the formation of interparticle junctions, the self-assembly of diamines on NPs drastically modifies the metal surface chemical properties and, therefore, its affinity toward various analytes in the medium. SERS data indicate that, at a sufficiently high surface coverage, 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) but showing reduced lateral interactions (i.e., liquid-like structure) [29]. Therefore, hydrophobic intermolecular cavities are present within the SAM of diamines. At the same time, positively charged amino head groups point away from the surface, providing available sites for ionic interaction with negatively charged species. As a result, the surface functionalization with diamines transforms molecules as organochlorine insecticides into SERS active probes [29], by inducing their approach to the metal substrate. In fact, without diamines, no SERS spectrum from these pollutants can be obtained due to their low affinity toward the metal surface [29].

Organochlorine insecticides are persistent organic pollutants, which develop their toxicity mainly on the nerve cell membranes, by inhibiting the transport of cations as Ca2+, K+, Na+ and Mg2+ [3436]. The strong interaction with nerve cells likely takes place because of their lipophilic character [49], and it has been suggested that a primary target of these compounds is the membrane lipid matrix [50, 51]. Owing to the non-selectivity of these insecticides, other organisms, including humans, may suffer their toxic action [5254]; therefore, the trace detection of these pollutants is of great interest. Figure 5 shows the structure of the insecticides studied in this paper: α-ES, β-ES, and ALD. It is possible to distinguish two main moieties within their structures: (1) the chlorinated part, which constitutes the active wedge of the molecule bringing the large excess of charge responsible for the interaction with electron receptors [55], and (2) the unchlorinated part, which acts as an anchor supporting the altering action of the insecticide [56] on the cell membranes. Therefore, even the unchlorinated moiety strongly affects the relative toxicity of the insecticide, and as a proof of that, it is reported that the α-endosulfan displays much higher insecticidal property than the β-isomer [57].
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Fig. 5

Structures of α-endosulfan (α-ES), β-endosulfan (β-ES), and Aldrin (ALD)

In the absence of diamine, no SERS spectrum from these pollutants can be obtained due to their low affinity to the metal surface. By contrast, the proper functionalization of the metal surface with ADn allowed the detection of the pollutant due to the formation of host/guest complexes (Fig. 6b, in the case of AD8/ALD complex). In addition to the detection of the analytes, the sensing methods based on vibrational spectroscopies provide useful tools for studying the host–guest interaction mechanism and its effects on both the host and guest structures. In order to emphasize the chemical sensing of the insecticides by the designed ADn–NPs systems, SERS difference spectra were extracted (Fig. 6c; in this case, the example of AD8 and ALD is shown) by subtracting the diamine SERS spectrum (Fig. 6a) from the spectrum of the diamine/pollutant complex (Fig. 6b).
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Fig. 6

SERS spectra of a AD8 (10−3 M) and b AD8/ALD (10−3 M/10−4 M) complex. c Difference spectrum (ba). The spectra were recorded at λexc = 785 nm

The diamine/insecticide interaction likely occurs through an ionic pair between the positively charged amino head groups of the host and the Cl atoms of the Cl–C = C–Cl fragment in the insecticide structure (the presence of the double bond promotes the electron donor ability of chloride). The stability of the so-formed complex is completed by the hydrophobic interaction between the unchlorinated moiety of the insecticide and the aliphatic chain environment provided by the self-assembled diamines on the metal.

We investigated in details the effect of alkyl chain length and diamine surface coverage on the SERS detection of organochlorine insecticides. In particular, AD2, AD8, and AD12 were selected as host receptors because of their different self-assembling behavior when adsorb on the metal surface [29]. In this way, we intended to verify the different host abilities of the SAMs of the linear diamines over a wide range of chain length, resulting from the disordered aggregation of AD2 up to the tightly packed layer formed by AD12 and going through the intermediate behavior of AD8 [29]. We repeated the same study at different excitation wavelengths, using the laser lines at 785 and 514.5 nm. In fact our previous works [24, 28, 58], which dealt with the functionalization of metal surfaces for the trace detection of pollutants, pointed out that the excitation wavelength is an important experimental parameter to optimize in these studies.

The Raman spectra of the insecticides present the most intense bands in the region below 800 cm−1, which is instead free of ADn features (Fig. 7). These bands are mainly assigned to C–Cl vibrations [56, 59] and are sensitive to the molecular conformation of the pollutant [60]. In particular, the intense feature at about 400 cm−1 can be attributed to the C–Cl stretching of the Cl–C = C–Cl fragment (i.e., the most electron-donor group of the insecticide molecule), whereas the C–Cl modes appearing at lower frequencies can be assigned to vibrations involving C atoms of less polar portions of the structure [60]. The comparison between the SERS difference spectra of ALD and α-ES, obtained by employing AD2, AD8, and AD12 as hosts, and the Raman spectra of the solid pollutants shows an overall similarity (Fig. 7), indicating that the insecticides substantially retain their conformation after the complexation with the diamines. However, some significant changes can be observed. Specifically, the spectra of AD2/insecticide complexes show marked variation in the ν(C–Cl) band of the Cl–C = C–Cl fragment, with the appearance of a new feature at 415–411 cm−1. In contrast, the complexation with AD8 and AD12 induces only slight modifications in the C–Cl bands of less polar groups (i.e., the band at 328 cm−1 in ALD and that at 345 cm−1 in α-ES). The influence of the shortest diamine on the most polar fragment of the insecticides is attributed to the higher polarity of AD2 in relation to AD8 and AD12, as also reported by Batista de Carvalho et al. [61]. This possibly leads to a stronger interaction between the positively charged amino head groups of AD2 and the Cl–C = C–Cl wedge of the analyte. For the sake of brevity, the data obtained for the special case of β-ES will not be shown throughout the discussion of the SERS spectra, since the results are very similar to what observed for the α isomer.
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Fig. 7

Detail of the ν(C–Cl) spectral region. [1] SERS difference spectra obtained by subtracting the SERS spectrum of ADn to the SERS of ADn/ALD complexes: a AD12/ALD (3 × 10−5 M/10−4 M), b AD8/ALD (1 × 10−3 M/10−4 M) and c AD2/ALD (5 × 10−4 M/10−4 M). d Raman spectrum of the solid ALD. [2] SERS difference spectra obtained by subtracting the SERS spectrum of ADn to the SERS of ADn/α-ES complexes: a AD12/α-ES (3 × 10−5 M/10−4 M), b AD8/α-ES (1 × 10−3 M/10−4 M) and c AD2/α-ES (5 × 10−4 M/10−4 M). d Raman spectrum of the solid α-ES. The spectra were obtained at λexc = 785 nm

Similarly, the ν(C-C) and ν(C–H) spectral regions of the SERS spectra reveal changes related to modifications of the self-assembled AD8 and AD12 conformations. If the interaction of the insecticides with the AD2–NPs system does not induce significant alterations of the spectral profile (spectra not shown), the formation of the host–guest complex involving AD8 and AD12 does lead to an increase of the number of gauche conformers (positive band at about 1,120 cm−1 in the SERS difference spectra shown in Fig. 8), indicating a drop in the aliphatic chain order after the complexation. On the other hand, the spectral parameter sensitive to the inter-chain lateral packing (H2905/H2845) undergoes negligible variations but the intensity increase of the shoulder at about 2,920–2,925 cm−1 (Fig. 9), assigned to methylene scissoring fundamental [62], suggests that new inter-molecular forces perturb the aliphatic chains upon the analyte inclusion into the SAM.
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Fig. 8

Detail of the ν(C–C) spectral region. [1]a SERS spectrum of AD8 (10−3 M). SERS difference spectra obtained by subtracting the SERS spectrum of AD8 to the SERS of AD8/insecticide (10−3 M/10−4 M) complexes: b ALD and c α-ES. [2] SERS spectrum of a AD12 (3 × 10−5 M). SERS difference spectra obtained by subtracting the SERS spectrum of AD12 to the SERS of AD12/insecticide (3 × 10−5 M/10−4 M) complexes: b ALD and c α-ES. The spectra were obtained at λexc = 514.5 nm

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Fig. 9

Detail of the ν(C–H) spectral region. [1] SERS spectra of a AD8 (10−3 M), b AD8/ALD (10−3 M/10−4 M) and c AD8/α-ES (10−3 M/10−4 M). [2] SERS spectra of a AD12 (3 × 10−5 M), b AD12/ALD (3 × 10−5 M/10−4 M) and c AD12/α-ES (3 × 10−5 M/10−4 M). The spectra were recorded at λexc = 514.5 nm

In the case of AD12, further spectral changes are observed in the 1,600–1,300 cm−1 region. In the presence of the pollutant (Fig. 10b, c), the bands corresponding to the symmetric and anti-symmetric δ(N–H) modes of the amino head group, at 1,523 cm−1 and the doublet at 1,572 and 1,611 cm−1 respectively, decrease in intensity as compared to the CH2 bands (i.e., the wagging vibration at 1,359 cm−1, the CH2 bending modes at about 1,440–1,455 cm−1 and the C–C skeletal vibrations). In contrast, the symmetric ν(N–H) vibration at 3,262 cm−1 undergoes a relative intensity increase (result not shown). These changes are related to a reorientation of the amino head groups on the surface toward a more perpendicular configuration (Fig. 10, right), according to the SERS selection rules [6]. The peak height ratio of the bands at 3,262 and about 1,523 cm−1 (H3262/H1523) was chosen as spectral parameter sensitive to the NH3+ reorientation. As can be seen in the histogram of Fig. 10, the degree of perpendicular orientation of the amino head group increases after the complexation in the following order: ALD < α-ES < β-ES.
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Fig. 10

a SERS spectrum of AD12 (3 × 10−5 M). SERS difference spectra obtained by subtracting the SERS spectrum of AD12 to the SERS of AD12/insecticides (3 × 10−5 M/10−4 M) complexes: b ALD and c α-ES. The spectra were recorded at λexc = 514.5 nm. Histogram Changes of the H3262/H1523 ratio after the AD12/insecticides complexation. Right Schematic representation of the perpendicular and parallel orientations of the amino group on the metal surface

On the whole, the disturbing effects of the host–guest interaction on the diamine conformation are far more pronounced when enlarging the chain length from 8 to 12 methylene units (Figs. 8, 9, and 10). This can be explained in terms of higher flexibility of the longer chain, which makes it possible to overcome the electrostatic repulsions of the NH3+ groups with the formation of tighter chain packing [29]. As a consequence, an incidental reduction of the cavity size is likely to occur, making more difficult the analyte diffusion inside the diamine SAM. At these conditions, the inclusion of the insecticide will cause deeper changes in the conformation of the self-assembled AD12 molecules, as manifested in the corresponding SERS difference spectra. All these effects are schematically illustrated in Fig. 11.
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Fig. 11

Schematic effects of the insecticide inclusion on the SAMs of [1] AD8 and [2] AD12. [3] Reorientation of amino head groups of AD12 upon interaction with insecticides

The perturbing action of the insecticide inclusion on the diamine supramolecular assemblies emerges also from the analysis of the absorption spectra. Whereas the spectra of AD2–NPs and AD8–NPs suspensions are poorly affected by the interaction with the insecticides (spectra not shown), in the case of AD12 the dimer resonance at 462 nm undergoes a huge broadening and a significant upshift from 462 to about 510 nm (Fig. 12). This suggests a strong randomization of the interparticle distance at the NP–NP junctions, due to considerable structural alterations produced by the inclusion of the analyte in the SAM of the AD12 molecules.
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Fig. 12

Absorption difference spectra of AD12–NPs ([AD12] = 3 × 10−5 M) in the 400–800 nm range in the presence of a no pollutant, b β-ES (10−4 M), c α-ES (10−4 M) and d ALD (10−4 M)

The entire architecture and the conformation of the self-assembled AD on the metal surface is a key factor determining the final host ability of the sensing system. For this reason, a complete study of the insecticide detection at different ADn surface coverage was performed by monitoring the height of the most intense SERS bands of ALD (351 cm−1), α-ES (400 cm−1), and β-ES (404 cm−1) over a wide range of diamine concentration. The pollutant concentrations were kept constant at 10−4 M. The optimal host concentrations turned out to be about 5 × 10−4 M for AD2, 1 × 10−3 M for AD8 and 3 × 10−5 M for AD12 for all the insecticides. The AD8–NPs systems provided the highest SERS intensities of the pollutant marker bands over a large range of host concentration (Fig. 13 shows the results for the special case of α-ES detection). The limits of detection were determined by keeping constant the diamine concentration at the above optimal values. The study was repeated at two excitation wavelengths (514.5 and 785 nm), in order to find the best conditions of the detection. The AD8−NPs system displayed the higher sensing efficiency for all the investigated insecticides, allowing their detection down to 10−8 M for 785 nm excitation wavelength.
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Fig. 13

SERS intensity of the α-ES marker band at 400 cm−1 versus the diamine concentration. All the measurements were carried out at λexc = 785 nm

The higher sensitivity of AD8–NPs with respect to AD12–NPs can be explained on the basis of the different SAM structure formed by these linear molecules once adsorbed on the metal. While AD8 molecules form more disordered SAMs, presenting intermolecular spaces or cavities available for the interaction with the pollutant (Fig. 11, [1]), the larger aliphatic chains of AD12 allow the formation of stronger intermolecular hydrophobic interactions which stabilize the SAM. As a result, the inclusion of the insecticide is less favored, since the creation of intermolecular cavities to host the pollutant is hindered by the high lateral packing of AD12 films (Fig. 11, [2]). On the other hand, when the alkyl chain is too short (AD2), the receptor does not provide a sufficient hydrophobic environment to anchor the unchlorinated moiety of the insecticide. However, the case of AD2 is more difficult to treat because of the different chemical behavior resulting from the shortness of the aliphatic chain [61]. In fact, AD2 interacts through both amino groups with the same NP surface [63]. Nonetheless, it is observed that higher AD2 concentrations are needed to obtain SERS signals from the pollutants, suggesting that the interaction mechanism followed by AD2 is different from AD8 and AD12 and involves a higher number of molecules to attract the insecticide.

Conclusions

In summary, silver colloid NP clusters are obtained with interparticle distances fine tuned in the ≤2 nm range. This is done by exploiting the chemical properties of linear α,ω-aliphatic diamines as molecular linkers with varying chain length, controlled by the number of methylene units in the structure, and surface coverage. This is revealed by redshifted plasmon resonances in the absorption spectra for closing distances, in agreement with theoretical calculations. At the same time, these molecular linkers form intermolecular cavities within their SAMs, suitable to host molecular analytes for nanosensing applications, as evidenced by SERS detection of organochlorine insecticides at the trace level. The mechanism of complexation and the molecular effects of the insecticide inclusion were monitored by SERS and plasmon absorption spectroscopy. In this regard, the extension of the aliphatic chain played a crucial role in determining the SAM conformation and thus the final sensitivity of the ADn–NPs system. AD8 provided the most effective SERS sensor, which allowed the detection of the insecticides down to 10−8 M. By contrast, the highly packed AD12 films hindered the inclusion of the analyte on the SAM because of the reduction of the intermolecular cavity size. On the other hand, the AD2 showed different chemical behavior due to the shortness of the aliphatic chain, which resulted less effective for insecticide detection.

These findings are expected to constitute the basis for employing linear α,ω-aliphatic diamines in the design of NP clusters with controlled interparticle distance at nanoscale level for sensing application. The self-assembled diamine layer on the metal surface may act itself as receptor for otherwise unactive SERS probes as organochlorine insecticides.

Acknowledgments

This work has been supported by the Spanish Ministerio de Ciencia e Innovación (grant no. FIS2007-63065, FIS2009-11264, and Consolider-Ingenio project EMET CSD2008-00066, and R.R.-O.’s Ph.D. scholarship), Comunidad de Madrid through the MICROSERES network (grant S-0505/TIC-0191), and CSIC (L.G.’s. and I.I.-L.’s Ph.D. scholarship). Fruitful discussions on the hybridization model with Pablo Albella are also acknowledged.

Copyright information

© Springer Science+Business Media, LLC 2010