Anal Bioanal Chem (2011) 401:1241–1249 DOI 10.1007/s00216-011-5177-y
ORIGINAL PAPER
Sensoric potential of gold–silver core–shell nanoparticles Andrea Steinbrück & Ondrej Stranik & Andrea Csaki & Wolfgang Fritzsche
Received: 3 May 2011 / Revised: 1 June 2011 / Accepted: 13 June 2011 / Published online: 9 July 2011 # Springer-Verlag 2011
Abstract The sensitivities of five different core–shell nanostructures were investigated towards changes in the refractive index of the surrounding medium. The shift of the localized surface plasmon resonance (LSPR) maximum served as a measure of the (respective) sensitivity. Thus, gold–silver core–shell nanoparticles (NPs) were prepared with different shell thicknesses in a two-step chemical process without the use of any (possibly disturbing) surfactants. The measurements were supported by ultramicroscopic images in order to size the resulting core–shell structures. When compared to sensitivities of nanostructures reported in the literature with those of the (roughly spherical) gold–silver core–shell NPs, the latter showed comparable (or even higher) sensitivities than gold nanorods. The experimental finding is supported by theoretical calculation of optical properties of such core–shell NP. Extinction spectra of ideal spherical and deformed core–shell NPs with various core/shell sizes were calculated, and the presence of an optimal silver shell thickness with increased sensitivity
Electronic supplementary material The online version of this article (doi:10.1007/s00216-011-5177-y) contains supplementary material, which is available to authorized users. A. Steinbrück : O. Stranik : A. Csaki : W. Fritzsche (*) Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany e-mail:
[email protected] O. Stranik School of Physical Science, Dublin City University, Dublin 9, Ireland Present Address: A. Steinbrück Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
was confirmed. This effect is explained by the existence of two overlapping plasmon bands in the NP, which change their relative intensity upon change of refractive index. Results of this research show a possibility of improving LSPR sensor by adding an extra metallic layer of certain thickness. Keywords Core–shell nanoparticles . Gold–silver nanoparticles . Localized surface plasmon resonance . Biosensor . Sensing
Introduction Noble metal nanoparticles (NPs) exhibit strong absorption of light at visible range of spectrum due to the localized surface plasmon resonance (LSPR) effect [1]. This LSPR is associated with charge density oscillation within a NP and creating a strong electromagnetic field confinement around the NP at specific resonance wavelength of the incident light [2]. The character (position, intensity, and type) of LSPR depends on the geometry of NPs, material of NPs, and the local refractive index around NPs. Many sensing platforms are based on the latter effect, in which the change of LSPR is monitored. In general, the monitored event of interest, such as temperature increase, change of pH, NP aggregation, and adsorption of molecules on a NP, causes change of refractive index around the NP, which is then detected by a shift of LSPR [3, 4]. A specially promising area for LSPR sensors is in biosensing [5–8]. A major advantage of LSPR sensing is restriction to nanometerrange detection volumes, which enables a detection of just a few molecules. The sensitivity of such sensors is proportional to the induced shift of LSPR in the metal nanostructures by a
1242
change of refractive index around the nanostructures. A lot of research is focused on investigating sensitivity of metal nanostructures of different shapes [9–11]. Generally, sensitivity is enhanced by diverging from a spherical shape to non-spherical one, such as rods, cubes, and triangles. This is due to increased electromagnetic field at the sharp corners of the non-spherical structure. Regarding the optimization of the material of NP, mostly gold (Au) and silver (Ag; partially aluminum) are used because they have lower damping, and therefore, NPs made of such metals exhibit sharper LSPR [1]. Regarding the synthesis, the first scientifically based chemical synthesis of spherical Au NPs was utilized by Faraday in 1857 [12]. Today, citrate is commonly used as reducing agent in Au NP synthesis [13, 14]. Soon, the syntheses were extended to other materials (e.g., Ag) [15] and shapes such as nanorods [16], triangular structures [17], cubes [18], cages [19], and nanoshells [20]. Moreover, bimetallic NPs gained interest because of the possibility of tuning the optical properties of the nanostructures [21]. Bimetallic particles can be classified into two types by their ultrastructure: Alloy NPs with a homogeneous distribution of two metals are prepared by the simultaneous reduction of both metal salts in solution (alloy NPs are not considered in the work presented here). On the other hand, core–shell NPs are generated by the successive reduction of the two metal salts resulting in a heterogeneous arrangement of the two metals. Morriss and Collins pioneered the preparation of Au–Ag core–shell NPs [22]. Since that time, numerous publications reported the synthesis of Au–Ag core–shell NPs using various reducing agents such as citrate [23], ascorbic acid [24], and hydroquinone [21] for the electroless deposition of Ag onto Au NP cores. Even onion-like multilayer bimetallic NPs were generated by this strategy [25], and a comparison of Au–Ag with Ag–Au core–shell particles has been conducted [26]. Such particles have been utilized for DNA-particle constructs [27]. An ultrastructural study of this growth process at the single particle level revealed a linear growth in the case of Ag deposition [28]. As mentioned above, the localization of the maximum of the LSPR band depends on the chemical environment. Usually, an increase in the refractive index of the surrounding medium (by solvent or adsorbates) causes a red shift of the LSPR maximum [7]. In analytical applications, a high sensitivity is desired, reflected by a high value of the shift of the LSPR band per refractive index unit (RIU) of the respective NP system. Over the last years, numerous articles were published concerning refractive index sensing with metal NPs. In general, various kinds of NPs show sensoric potential. Spherical NPs exhibit the poorest sensitivities. Regarding the materials, Ag (120–160 nm/RIU) [29, 30] proved more sensitive than Au NPs (~70 nm/RIU) [7, 31]. Higher sensitivity could be realized using anisotropic
A. Steinbrück et al.
structures such as nanorods or triangular nanostructures. In the case of low aspect ratio Au nanorods, a sensitivity of ~150 nm/RIU was found [32], whereas the sensitivity of Ag nanorods reached 235 nm/RIU [33]. For triangular structures, values of 200–350 nm/RIU were reported [29, 30]. The investigation of more complex nanostructures such as hollow Au nanoshells yielded high sensitivities of ~400 nm/RIU, exceeded only by the sensitivity of Au nanostars (more than 600 nm/RIU possible) [34]. Here, we investigated theoretically as well as experimentally the sensoric properties of core–shell NPs for refractive index sensing in particle solutions. Thus, Au–Ag core–shell NPs were prepared with defined Ag shell thicknesses. The refractive indices of the solutions were varied from 1.33 to 1.37, a regime interesting for biological applications. In the theory part, the extinction spectra of core–shell NPs were calculated as well as the LSPR shift in the dependence on change of surrounding refractive index. The calculated data were compared with the measured ones, and the increased sensitivity was explained by analyzing the electromagnetic field distribution around NPs.
Experimental section Solutions of Au NPs were prepared according to the protocol of Turkevich [13] resulting in NPs of diameters of 13±2 nm (named Au1) and 15.6±1.4 nm (named Au2), respectively. Further, Au NPs (15.9±0.8 nm, named Au3) were purchased from British BioCell International (Cardiff, UK). The concentrations of prepared Au NPs solutions were estimated from measured optical density of the solutions, and then all Au NP solutions were adjusted to the concentration of 2× 1011 particles/ml by deionized water. To prepare core–shell NPs, Au NPs were mixed with the Ag enhancement kit SEKL15 of British BioCell International (Cardiff, UK) consisting of two components containing Ag salt and reducing agent, respectively. The Ag enhancement kit was used in a 1:10 dilution [26]. Varying volumes of the enhancement solution were added to Au NP solutions resulting in end concentrations of 0.9 to 13 vol%, respectively. After the Ag enhancement step, the concentration of Au and Au–Ag core–shell NP solutions was adjusted to the 6.7×109 particles/ml by dilution with deionized water (it was assumed that the number of NPs in solutions is not changed by the Ag enhancement step). All NP solutions and the time resolved monitoring of the shell growth of the core–shell NPs were measured by UV–Vis spectroscopy using a Jasco V530 and a Jasco V620 (Jasco, Groß-Umstadt, Germany) spectrophotometer. To determine the size and the ultrastructure of all NP samples, transmission electron microscopy (TEM) measurements were performed using a Zeiss DSM 960
Sensoric potential of gold–silver core–shell nanoparticles
(Jena, Germany). TEM samples were prepared by placing a formvar/carbon-coated TEM grid into a sample solution for several hours and wiping away excess solution with blotting paper. For size determination and counting the NPs, the image analysis software ImageJ [35] was used. The sizes of NPs were determined, and histograms of size distributions of core and core–shell NPs were created. The average size of pure Au NP was directly obtained from the histogram, and the average thicknesses of Ag shell were obtained by cross-correlation of Au-core and Au–Ag core–shell histograms. The Au nanorods were provided by Carsten Sönnichsen (University Mainz). Their average size, determined by TEM, of short axis was 19.9±7.1 nm and of long axis was 42.9±10.1 nm, giving a ratio of 2.34. For measuring the sensitivity of the core–shell NPs against changes in the refractive index of the surrounding medium, several glucose solutions were prepared and mixed with equal volumes of the NP solutions resulting in glucose end concentrations of 0 to 30 wt.%. The experiments were repeated three times. All values presented here are averaged. The values of the refractive indices of the glucose solutions were measured with a refractometer (Pal-RI, Atago, Tokyo, Japan).
1243
simulation package. This method is based on generalization of the aforementioned method; the field is again expressed as superposition of M,N function with several origins along its boundary in each domain, and their expansions are obtained by numerically solving boundary conditions. Its advantage is low demand on computational power and accurate results for simpler geometric structures [40]. In the case of slightly deformed spherical (prolate and oblate ellipsoid) core–shell NPs, illuminated in the direction of their rotation axis, multipole and Bessel expansion at the origin for core and shell and a multipole (order up to 7, degree 1) for outer medium were used. In the case of an elliptical particle, illuminated perpendicular to the rotation axis and polarized along longer axis, four Bessel for inside and four multipole expansion (order up to 6, degree up to 3) for outside of NP distributed along the rotation axis were used. In all simulations, the relative error on the boundary was kept below 1%. The extinction spectra were calculated using optical theorem [37], where the field was calculated 1 mm away from NP in the direction of illumination. The optical constant for Au and Ag were taken from [41].
Results and discussions Simulation methods The interaction of electromagnetic field with a spherical core–shell NP can be solved analytically by a method described in Mie theory [36]. By assuming piecewise homogenous isotropic linear material, electromagnetic field in each domain (core, shell, and outer medium) is expressed as superposition of vector spherical harmonics functions M,N, which are solutions of Maxwell equations for a given domain. The unknown expansion coefficients of the M,N function are then obtained from a solution of boundary conditions at the domains' boundaries. Due to the spherical symmetry of the domains' M,N functions, the expansions coefficients have simple form and can be found for example in [37]. From the known field distribution, the extinction spectra are calculated and expressed in terms of the expansion coefficients [37]. These formulas were implemented into Matlab (MathWorks) and calculated for different core, shell sizes, and refractive index of the outer medium. The visualization of field distribution in the particles was obtained from OpenMax simulation package [38], which in the case of core–shell NP uses the identical above-mentioned method of solution. The interaction of electromagnetic field with nonspherical particles was solved by generalized multipole technique (GMT) [39], which is implemented in OpenMax
First, the Au NP solutions were adjusted to the particle concentration of 2×1011 particles/ml. Ag was electroless deposited on these NPs using a mixture of Ag salt and reducing agent solutions [42]. Here, the deposition of Ag was performed in five steps resulting in core–shell structures with varying thicknesses of the Ag shell in dependence on the volume of the Ag salt and the reducing agent added (Fig. 1). During the reaction, the color of the solutions changes from red to yellow/orange, indicating the growth of the Ag shell onto the Au core NPs
Fig. 1 UV–Vis spectra of gold core (sample A2) and core–shell nanoparticle solutions used in sensing experiments. The thickness of the silver shell of samples A, B, C, D, and E was
