Article pubs.acs.org/Langmuir
In Situ Non-DLVO Stabilization of Surfactant-Free, Plasmonic Gold Nanoparticles: Effect of Hofmeister’s Anions Vivian Merk,†,# Christoph Rehbock,†,# Felix Becker,‡ Ulrich Hagemann,‡ Hermann Nienhaus,‡ and Stephan Barcikowski*,† †
Technical Chemistry I, University of Duisburg-Essen and Center for NanoIntegration Duisburg-Essen CENIDE, Universtitaetsstrasse 5, 45141 Essen, Germany ‡ Faculty of Physics, University of Duisburg-Essen and Center for NanoIntegration Duisburg-Essen CENIDE, Lotharstr. 1-21, 47048 Duisburg, Germany S Supporting Information *
ABSTRACT: Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water−metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV−vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br−, SCN−, or I−) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F− or SO42−) seem to destabilize the gold colloid, whereas Cl− and NO3− give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.
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INTRODUCTION By the end of the 19th century Franz Hofmeister published a series of landmark papers about ion-specific effects in such different fields as protein precipitation, colloidal ferric oxide, or sodium oleate1,2 Since then, so-called Hofmeister effects have been observed in a broad range of scientific contexts as in physical properties of simple aqueous electrolytes3−5 in biological systems like proteins or enzymes3−6 and colloidal particles or macromolecules.3−5 Continuum electrostatic models, however, exclude shortrange phenomena like the ion-specific water affinity and other ion-specific effects as ions are generally treated as equivalent point-charges.7 The theoretical description of the diffuse double layer based on the Poisson−Boltzmann equation neglects the nonelectrostatic interactions between counterions, co-ions, and the particle surface and disregards gradients of the dielectric permittivity.8 Even though the origin of ion-specific effects based upon experimental data and model calculations are still under vivid debate, it has been proposed that based on a modified Gouy− © 2014 American Chemical Society
Chapman−Stern model these effects occur due to ion adsorption in the fixed Stern layer close to the surface. In order to determine local ion concentration e.g. of chlorides around the gold nanoparticle, the application of Cl− sensitive fluorophores has been proven to be a suitable tool involving complex organic ligands.9 In contrast specific ion effects on ligand-free nanoparticles are mainly influenced by three parameters, ion−surface, ion− water, as well as water−surface interactions, namely hydrophilicity and hydrophobicity10,11 while water−water interactions around the ions have proven to be less significant as interactions are predominantly limited to the first water layer.12 As to ion−surface effects, Pearson’s rule states that binding interactions are favored in the case of matching polarizability of ion and surface, categorizing ions with low polarizability as “hard ions” and ions with high polarizability as “soft ions”. Received: November 26, 2013 Revised: March 12, 2014 Published: March 13, 2014 4213
dx.doi.org/10.1021/la404556a | Langmuir 2014, 30, 4213−4222
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Figure 1. (a) Flow-through setup for preparation of gold colloids in electrolytes in the absence of organic stabilizers via laser ablation process. Mechanism of particle formation (b), absorption of laser pulse (c), formation of plasma plume, and (d) particle formation by coalescence and growth during and after cavitation bubble formation and collapse in the micromolar electrolyte.
effects or monolayers potentially blocking the ions’ access to the nanoparticle surface. Additionally, the Hückel regime of nanoscaled particles29 dispersed in highly dilute electrolytes has, except for e.g. Hofmeister effects on α-alumina surfaces,30 and at the air-water interface31 hardly been addressed in the literature so far since specific ion effects are supposed to occur primarily at relatively high concentrations (above 100 mM) when long-range electrostatic interactions are screened.2−4 Pulsed laser ablation of solid metal targets in liquids is a suitable method to fabricate ligand-free spherical gold nanoparticles often with relatively broad size distributions32−34 which are negatively charged. This charge is caused by a partial surface oxidation to Au+/Au3+ followed by addition of oxygen species on the nanoparticle surface leading to a pH-dependent equilibrium between Au−OH/AuO− and AuCO3− groups.35,36 Since this preparation method avoids the use of any chemical precursors or further stabilization agents,32,33,40 it allows to gain insight into specific ion effects at pure curved gold/water interfaces. This high purity of the colloids is the determining parameter which makes laser ablation in liquid superior to other physical synthesis methods like e.g. electric arc generation (Bredig method), which predominantly yields colloids with contaminations from electrolytes and oxidized material.37 Furthermore, laser ablation in liquids offers the opportunity to study ion−nanoparticle interactions during the nanoparticle formation process (in situ), which has been reported to occur via coalescence and growth following the cooling of the plasma plume resulting from the laser ablation process,32,38 while crystalline nanoparticle species were observed inside the formed cavitation bubble.39 The concept of particle formation by pulsed laser ablation in liquid is illustrated in Figure 1. Prior investigations of laser ablation in liquids in the presence of electrolytes already reported differing colloidal stabilities of gold nanoparticles depending on the used salt35 though a systematic
Hence, on hydrophobic surfaces like AgI or Hg adsorption of larger alkali cations is favored, following a direct lyotropic sequence Rb+ > Li+.13,14 However, on charged hydrophilic surfaces found in metal oxides like TiO215 or Al2O3,16 the presence of M−O− (M = metal) surface groups induces a strong attractive interaction with “hard cations”, causing an inverse lyotropic sequence Li+ > Cs+. As on hydrophilic metal oxide surfaces the presence of M−O− groups is pH-dependent; low pH values tend to favor direct lyotropic sequences, e.g., for MnO2 as the M−OH2+ groups found in that regime repulse “hard ions” more intensely than “soft ions”.17 Alternatively, the specific polarizability of the ions might account for diverging hydrophobic and dispersion forces exerted on particles.18,19 Concerning ion water interactions, Collins’ rule of matching water affinities says that inner-sphere ion pairs are only formed from ions with a comparable water affinity, correlating Hofmeister effects to the ability of ions to alter the water structure in their vicinity.7,11 Based on these Hofmeister or lyotropic effects, different ions should be expected to bind to nanoparticle surfaces with different affinity, significantly influencing the surface charge density on the nanoparticle and hence their stability based on electrostatic repulsion. Several recent studies compare the effectiveness of different ions in precipitating colloidal particles with different charges as well as hydrophilicity or hydrophobicity (water−surface interactions) of the surfaces.20−25 Up to now, Hofmeister effects have merely been investigated for gold nanoparticles fully covered with organic moieties such as citrate26 or oligo(ethylene glycol) thiols.27 In previous studies on colloidal stability, charged ligands residing at the gold nanoparticle surface provide electrostatic or steric stabilization but can cause ion scavenging.28 However, the synthesis of ligand-free particles allows to observe direct particle−ion interaction without disturbance by surfactant 4214
dx.doi.org/10.1021/la404556a | Langmuir 2014, 30, 4213−4222
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Figure 2. Colloidal stability (primary particle index) of ligand-free gold nanoparticles in the presence of (a) NaF, NaCl, NaBr, and NaI and (b) NaSCN, NaNO3, and Na2SO4 at varying ionic strengths. The lines are meant to guide the eye. The inset shows the low salinity regime (0−30 μM) where the colloidal stability scales linearly with the ionic strength. Transmission Electron Microscopy. A transmission electron microscope (Philips CM 12) was driven with an acceleration voltage of 120 kV (W cathode) and at a pressure of 10−6 mbar. Digital images were taken with a CCD camera (Hamamatsu). For the measurements a drop of the gold colloid was pipetted on a carbon-coated copper grid. The size determination of at least 500 gold nanoparticles was performed with the program ImageJ. Analytical Disk Centrifugation (ADC). Size determination with analytical disk centrifugation (ADC) was conducted with a DC 24000 from CPS instruments at 24 000 rpm for 20 min against a saccharose gradient and an external standard (PVC particles at 0.371 μm), using a sample volume of 0.1 mL. X-ray Photoelectron Spectroscopy. XPS spectra of selected samples were recorded using a hemispherical electron energy analyzer (Phoibos 100 by SPECS) with a pass energy of 20 eV and in the fixed analyzer transmission mode. A Mg anode, with the dominant Kα line at 1253.6 eV, served as a nonmonochromatic X-ray source. The background correction was Shirley type, and the fit of the data was done using a Gauss−Lorentz sum formula with the relative weights of 75% and 25%, respectively. The quality of the fit was also checked by achieving a residual value close to one. A small droplet of the investigated solution was pipetted onto a silicon wafer piece. After the fluid had evaporated it was placed into the load-lock of the vacuum system where it stayed for roughly 1 h. Finally, the samples were transferred into the XPS chamber for analysis.
investigation of these effects is still lacking. Additionally, a size reduction effect ionic strength has been reported,32 yielding monodisperse, monomodal surfactant-free gold colloids.40 In this paper we systematically investigate ion-specific effects on gold nanoparticles in highly dilute electrolytes with a focus on the Hofmeister’s anions, linking colloidal stability to ion properties. The laser-assisted synthesis of the particles provides a highly pure system comprising only colloidal gold, water, and the respective ions and allows studying the occurrence of ionspecific effects during the nanoparticle formation process. To this end, the stability of the colloids is assessed via UV−vis spectroscopy while further characterization of the nanoparticle’s properties is done by zeta potential measurements, transmission electron microscopy, and X-ray photoelectron spectroscopy.
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EXPERIMENTAL METHODS
Materials. Sodium bromide, sodium chloride, sodium fluoride, sodium sulfate, lithium chloride, potassium chloride, and calcium chloride were purchased from Sigma-Aldrich. Sodium iodide and sodium thiocyanate were acquired from AppliChem and sodium nitrate from AnalaR. All salts were dissolved in Milli-Q deionized water (Millipore, ≤18.2 MΩ·cm). Preparation of Gold Colloids. Gold colloids were prepared via laser ablation in dilute electrolytes in a self-designed continuous flowthrough reactor (volume = 59 μL, flow rate = 1 mL/min) (Figure 1a) using a Nd:YAG nanosecond laser (Innolas SpitLight DPSS250-100) at λ = 1064 nm with a repetition rate of 100 Hz and a pulse energy of 45 mJ, as previously described in detail.40 A gold wire (d = 125 mm, Allgemeine Gold, Germany) was continuously fed via a motion control unit (Faulhaber GmbH, Germany, Model: 3242G024BX4 CS 32A 124:1) with an average speed of 147 ± 7 mg min−1 and ablated for 5 min, yielding a maximum NP concentration of 74 ± 4 μg mL−1. UV−vis Spectroscopy. UV−vis spectra were collected with a Thermo Scientific Evolution 201 spectrometer (Thermo) in a glass cuvette with 10 mm path length and a volume of 1.5 mL covering a spectral range from λ = 200−900 nm. Zeta-Potential Measurements. Zeta potentials were determined using a Malvern Zetasizer “Nano ZS” in a disposable capillary cell with a volume of 750 μL while fitting was performed using a Hückel model. Ultracentrifugation. Nanoparticles prepared in the presence of NaI were centrifuged with an OptimaTM MAX-XP ultracentrifuge (Beckman Coulter) at 30,000 g and 7 °C for 60 min to determine unbound iodide.
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RESULTS AND DISCUSSION
In this paper UV−vis spectroscopy is applied to shed light on the aggregation behavior of plasmonic gold nanoparticles prepared by pulsed laser ablation in diluted electrolytes at low salinities (1−2000 μM). The main goal here is to probe how ion type and ionic strength interfere with the colloidal stability. According to the classical Derjaguin−Landau−Verwey−Overbeek (DLVO) theory, an increment of the salinity is supposed to increase the aggregation speed independent of the type of ion. Herein, the primary particle index (PPI) derived from the UV−vis plasmon-resonance spectra is used to assess the aggregation tendencies of the particles. The PPI is a measure of colloidal stability defined as the ratio between the interband absorbance of gold at 380 nm and the scattering signal of aggregates, agglomerates, and bigger particles detected at 800 nm. An illustration of the PPI with representative UV−vis 4215
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subsequent reduction of the colloidal stability at even higher ionic strengths is most likely attributed to the non-ion-specific screening of surface charges frequently reported for colloids at higher ionic strengths. In order to link salt-induced effects on colloidal stability to specific properties of the used anions, the PPI at salinities F−) primarily form on anionic hydrophobic surfaces, while reversal (affinities I− < F−) is found when going to hydrophilic and cationic surface properties.56 In order to examine surface properties of the pristine gold nanoparticles, zeta potential measurements may be used to characterize the diffuse ion layer at the slipping plane, namely the electrokinetic surface charge. Concerning this, zetapotential measurements were conducted at varying ionic strengths in a regime from 3 to 1000 μM for NaI, NaBr, NaCl, and NaF (Figure S6). In the presence of F− zeta potentials of about 0 mV were found, while the standard deviations in all samples was extremely high, most likely due to particle aggregation processes. As zeta potential measurements are based on dynamic light scattering which is highly sensitive to larger particles, slight variations in aggregation tendencies may have very pronounced effects. These findings seem to point at a massive screening of the electrokinetic charge in nanoparticles laser-synthesized in the presence of kosmotropic F− ions, even at low salinities around 3 μM, which is in accordance with low PPI values found during UV−vis measurements. For colloids synthesized in the presence of the chaotropic anions Cl−, Br−, and I− negative electrokinetic surface charges of −30 to −40 mV were found. However, particularly in the presence of iodide reproducibility of these measurements was poor, probably due to time-dependent alterations of particle size and morphology in different samples caused by postsynthesis etching. Hence, similar to particles synthesized in the presence of fluoride, the uneven abundance of larger particles in different samples causes large deviations in the zeta potentials measured by light scattering instruments. This negative zeta potential is characteristic for laser-fabricated gold nanoparticles and is caused by the partial oxidation of the gold surface by oxygen species forming a pH-dependent equilibrium between AuOH/AuO− and AuCO3− surface groups. However, with all gold nanoparticles fabricated in the presence of chaotropic anions no significant change of the zetapotential with increasing ionic strength was found, even though in the same concentration regime stabilization increased as verified by UV−vis spectroscopy. This is due to the fact that zeta potential measurements may solely characterize electrokinetic potential in the diffuse Gouy−Chapman layer but are relatively insensitive to specific ion adsorption at the fixed stern layer and hence may not be directly used to characterize surface charge. However, previous examinations of laser-fabricated gold nanoparticles revealed that surface charge is indeed dominated
calibration can be found in the Supporting Information (Figure S4). From these data we can deduce that for ionic strengths
