α,ω-Aliphatic Diamines as Molecular Linkers for Engineering Ag Nanoparticle Clusters: Tuning of the Interparticle Distance and Sensing Application
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- Guerrini, L., Izquierdo-Lorenzo, I., Rodriguez-Oliveros, R. et al. Plasmonics (2010) 5: 273. doi:10.1007/s11468-010-9143-x
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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.
KeywordsSERSPlasmonic nanoparticlesInterparticle distanceSensing applicationOrganochlorine insecticides
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 . 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 , or quantum-dot emission . 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, 10–14]. 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 . 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 , 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 [17–21]. Moskovits et al. demonstrated that dithiol-containing adsorbates [17, 18], 4,4′-diaminoazobenzene , and small proteins  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.  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 . In our previous works [20, 23–28], 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 . 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 . 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 , including 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. 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.
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 . The average NP diameter is about 35–40 nm . 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.
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.
The resonance energies strongly depend on the distance between the NPs of the dimer . 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 = 2πc/ω1 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 .
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 . 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 . 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 . 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 . 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 . This common trend fails for n = 2 because of the different chemical behavior resulting from the shortness of the alkyl chain . At the other end of the dimensional scale, as the diamine molecules become too large (n = 12), the chain flexibility drastically increases , 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 . 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 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.
The resonance plasmon spectra shown in Fig. 2 (), were recorded at diamine concentrations above C2, at which the most linear conformation of ADn is attained. In Fig. 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 (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) . 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 , 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 .
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 . 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 . 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 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, ), 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, ). 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 . In fact, AD2 interacts through both amino groups with the same NP surface . 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.
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.
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.