On the adhesion between fine particles and nanocontacts: an atomic force microscope study

In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.     

Manipulation of gold nanoparticles: influence of surface chemistry, temperature, and environment (vacuum versus ambient atmosphere)

We have manipulated raw and functionalized gold nanoparticles (with a mean diameter of 25 nm) on silicon substrates with dynamic atomic force microscopy (AFM). Under ambient conditions, the particles stick to silicon until a critical amplitude is reached by the oscillations of the probing tip. Beyond that threshold, the particles start to follow different directions, depending on their geometry and adhesion to the substrate. Higher and lower mobility were observed when the gold particles were coated with methyl- and hydroxyl-terminated thiol groups, respectively, which suggests that the adhesion of the particles to the substrate is strongly reduced by the presence of hydrophobic interfaces. Under ultrahigh vacuum conditions, where the water layer is absent, the particles did not move, even when operating the atomic force microscope in contact mode. We have also investigated the influence of the temperature (up to 150 degrees C) and of the geometrical arrangement of the particles on the manipulation process. Whereas thermal activation has an important effect in enhancing the mobility of the particles, we did not find differences when manipulating ordered versus random distributions of particles.     

Probing colloidal forces between a Si3N4 AFM tip and single nanoparticles of silica and alumina

The atomic force microscope (AFM) has been used to measure surface forces between silicon nitride AFM tips and individual nanoparticles deposited on substrates in 10(-4) and 10(-2) M KCl solutions. Silica nanoparticles (10 nm diameter) were deposited on an alumina substrate and alumina particles (5 to 80 nm diameter) were deposited on a mica substrate using aqueous suspensions. Ionic concentrations and pH were used to manage attractive substrate-particle electrostatic forces. The AFM tip was located on deposited nanoparticles using an operator controlled offset to achieve stepwise tip movements. Nanoparticles were found to have a negligible effect on long-range tip-substrate interactions, however, the forces between the tip and nanoparticle were detectable at small separations. Exponentially increasing short-range repulsive forces, attributed to the hydration forces, were observed for silica nanoparticles. The effective range of hydration forces was found to be 2-3 nm with the decay length of 0.8-1.3 nm. These parameters are in a good agreement with the results reported for macroscopic surfaces of silica obtained using the surface force apparatus suggesting that hydration forces for the silica nanoparticles are similar to those for flat silica surfaces. Hydration forces were not observed for either alumina substrates or alumina nanoparticles in both 10(-4) M KCl solution at pH 6.5 and 10(-2) M KCl at pH 10.2. Instead, strong attractive forces between the silicon nitride tip and the alumina (nanoparticles and substrate) were observed.     

Solvent-free synthesis of Janus colloidal particles

Taking advantage of the quick and efficient access of vapor to surfaces, a simple, solvent-free method is demonstrated to synthesize Janus colloidal particles in large quantity and with high efficiency. First, at the liquid-liquid interface of emulsified molten wax and water, untreated silica particles adsorb and are frozen in place when the wax solidifies. The exposed surfaces of the immobilized particles are modified chemically by exposure to silane vapor and, in principle, subsequent dissolution of the wax opens up the inner particle surface for further chemical modification. Applying this scheme, this paper describes the production of amphiphilic Janus particles (hydrophobic on one side, hydrophilic on the other) and dipolar Janus particles (positively charged on one side, negatively charged on the other). Janus geometry is confirmed by fluorescence microscopy and flow cytometry. Amphiphilic Janus particles are found to adsorb strongly to the water-oil interface, whereas dipolar particles assemble into chains in the aqueous phase.     

Enhanced stability of Janus nanoparticles by covalent cross-linking of surface ligands

A mercapto derivative of diacetylene was used as the hydrophilic ligand to prepare Janus nanoparticles by using hydrophobic hexanethiolate-protected gold (AuC6, diameter 5 nm) nanoparticles as the starting materials. The amphiphilic surface characters of the Janus nanoparticles were verified by contact angle measurements, as compared to those of the bulk-exchange counterparts where the two types of ligands were distributed rather homogeneously on the nanoparticle surface. Dynamic light scattering studies showed that the Janus nanoparticles formed stable superstructures in various solvent media that were significantly larger than those by the bulk-exchange counterparts. This was ascribed to the amphiphilic characters of the Janus nanoparticles that rendered the particles to behave analogously to conventional surfactant molecules. Notably, because of the close proximity of the diacetylene moieties on the Janus nanoparticle surface, exposure to UV irradiation led to effective covalent cross-linking between the diacetylene moieties of neighboring ligands, as manifested in UV-vis and fluorescence measurements where the emission characteristics of dimers and trimers of diacetylene were rather well-defined, in addition to the monomeric emission. In contrast, for bulk-exchange nanoparticles, no trimer emission could be identified, and the intensity of dimer emission was markedly lower (though the intensity increased with increasing diacetylene coverage on the particle surface) under the otherwise identical experimental conditions. This is largely because the diacetylene ligands were distributed on the entire particle surface, and it was difficult to find a large number of ligands situated closely so that the stringent topochemical principles for the polymerization of diacetylene derivatives could be met. Importantly, the cross-linked Janus nanoparticles were found to exhibit marked enhancement of the structural integrity, which was attributable to the impeded surface diffusion of the thiol ligands on the nanoparticle surface, as manifested in fluorescence measurements of aged nanoparticles.     

One-step fabrication of polymeric Janus nanoparticles for drug delivery

With its unique structure of two compartments, Janus particles can be used for many applications for which monomorphic particles are inadequate, including to be used as a drug delivery system to deliver multiple payloads with widely different solubility. Here we report on a fluidic nanoprecipitation system (FNPS), capable of fabricating biocompatible Janus polymeric nanoparticles comprised of the FDA-approved polymer poly(lactic-co-glycolic acid) (PLGA). The FNPS contains dual inlets, one for each half of the particle, that insert into the precipitation stream. The system provides a one-step approach for production of Janus polymeric particles with submicrometer diameters and is likely amenable to substantial scale-up. To the best of our knowledge, this is the first demonstration of biocompatible Janus nanoparticles that encapsulate a hydrophobic drug (paclitaxel) on one side and a hydrophilic drug (doxorubicin hydrochloride) on the other.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

LB技术制备FePt纳米粒子单层膜

FePt纳米粒子由于具有高的矫顽力而成为高密度垂直磁记录材料的研究热点之一,均匀有序排布的FePt纳米粒子薄膜将有利于提高磁记录介质的记录密度,因此,如何使FePt纳米颗粒均匀有序地排布是当前需要解决的关键问题之一,很多研究小组利用磁控溅射、自组装和电沉积等方法对FePt纳米颗粒的均匀分布进行了详细的研究,     

Role of eDNA on the adhesion forces between Streptococcus mutans and substratum surfaces: influence of ionic strength and substratum hydrophobicity

The aim of this study was to investigate the role of extracellular DNA (eDNA) on the adhesion strength of Streptococcus mutans LT11 on substrata with different hydrophobicities at high and low ionic strengths. AFM adhesion forces to a hydrophilic and hydrophobic substratum increased with increasing surface-delay times and ionic strength and were stronger on a hydrophobic than on a hydrophilic substratum. The presence of eDNA on the streptococcal cell surface enhanced its adhesion force to a hydrophobic substratum significantly more than to a hydrophilic substratum, especially after bond maturation. Bond maturation on a hydrophilic substratum was accompanied by an increasing number of minor adhesion peaks, indicating the involvement of acid-base interactions, whereas on the hydrophobic substratum surface the number of minor adhesion peaks remained low. More minor adhesion peaks developed on the hydrophilic substratum at low ionic strength than at high ionic strength. The final rupture distance in retraction force-distance curves was independent of ionic strength on a hydrophilic substratum and increased with increasing surface delay time. On the hydrophobic surface, the final rupture distance did not increase with surface delay time but was significantly smaller at low than at high ionic strength. Final rupture distances were different in presence and absence of eDNA, and the lower values of this difference coincided with the decrease in hydrodynamic radius of the streptococci upon increasing ionic strength, measured using dynamic light scattering. AFM also yielded higher values for the ionic strength induced difference in final rupture distance because in AFM rupture is forced, while in dynamic light scattering differences in radius are only induced by ionic strength differences.     

Adhesion of spherical polyelectrolyte brushes on mica: an in situ AFM investigation

We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.     

Substrate Morphology and Particle Adhesion in Reacting Systems

This paper describes an effort to measure and model changes in the adhesion of micron-scale particles to substrates in systems in which chemical reactions are occurring. Contact interactions between polystyrene latex spheres and silicon substrates (with surface oxide) immersed in aqueous KNO(3) solutions were studied. Two important results were obtained. First, it was shown that the AFM can be employed to monitor, in situ, changes in adhesive interactions induced by surface chemical reactions in this system. Second, the morphology of the interacting surfaces plays a controlling role in particle adhesion. In particular, for this system, changes in roughness of the substrate changed the interaction force by nearly 90%. Copyright 2000 Academic Press.     

The role of few-asperity contacts in adhesion

The surface roughness of a few asperities and their influence on the work of adhesion is of scientific interest. Macroscale and nanoscale adhesion data have seemingly given inconsistent results. Despite the importance of bridging the gap between the two regimes, little experimental work has been done, presumably due to the difficulty of the experiment needed to determine how small amounts of surface roughness might influence adhesion data lying in between the two scales. To investigate the role of few-asperity contacts in adhesion, the pull-off force was measured between different sized atomic-force microscope (AFM) tips (with different roughnesses) and sample surfaces that had well-controlled material properties. There were seventeen tips of four different types, with radii from 200 nm to 60 microm. The samples were unpatterned single crystal silicon with a chemical silicon dioxide surface resulting from a standard silicon wafer clean. Some of the samples were treated with a few angstroms of vapor deposited diphenylsiloxane. We observed that the uncorrected (for surface roughness) pull-off force was independent of the radius of the AFM tip, which was contrary to all continuum-mechanics model predictions. To explain this behavior, we assumed that the interactions between the AFM tip and sample were additive, material properties were constant, and that the AFM tip, asperities, and sample surfaces were of uniform density. Based on these assumptions, we calculated a simple correction due to the measured root mean square (RMS) surface roughness of the AFM tips. The simple correction for the RMS surface roughness resulted in the expected dependence of the pull-off force on radius, but the magnitudes were higher than expected. Commercial and heat-treated AFM tips have minimal surface roughness and result in magnitudes that are more reliable. The relative uncertainty for the pull-off force was estimated to be 10%. In this paper, we derive how the cantilever and tip parameters contribute to the measured pull-off force and show how the corrected results compare with theory. Although much work is still needed, the work presented here should advance the understanding of adhesion between the macroscale and nanoscale regimes.     

Au colloid monolayers as templates for nanostructure assembly

A new approach to nanostructure formation based on replication of an existing pattern of colloidal Au nanoparticles is described. The key steps in this strategy involve the coating of a well-defined Au colloid monolayer with an SiO_x sol-gel, HF etching until colloidal particles are exposed, and removal of colloidal Au with aqua regia. This process was carried out on a two-dimensional array of 12nm diameter particles on an In-doped SnO₂ (ITO) substrate, and monitored by UV-vis, electrochemistry, and atomic force microscopy (AFM). AFM data indicate the strategy is feasible, with clearly visible differences in surface morphology observed between bare ITO and colloidal Au-depleted, SiO_x-coated ITO.     

In situ estimation of the chemical and mechanical contributions in local adhesion force measurement with AFM: the specific case of polymers

The atomic force microscope (AFM) was used to perform surface force measurements in contact mode to investigate surface properties of model systems at the nanoscale. Two types of model systems were considered. The first one was composed of a rigid substrate (silicon plates) which was chemically modified by molecular self-assembling (SAMs) to display different surface properties (hydroxyl, amine, methyl and ester functional groups). The second system consists of model polymer networks (cross-linked polydimethylsiloxane or PDMS) of variable mechanical properties, whose surfaces were chemically modified with the same groups as before with silicon substrates. The comparison of the force curves obtained from the two model systems shows that the viscoelastic or mechanical contribution dominates in the adhesion on polymer substrates. Finally, a relationship, which expresses the separation energy at a local scale as a function of the energy dissipated within the contact zone, on one hand and the surface properties of the polymer on the other, was proposed.     

Cleaning and hydrophilization of atomic force microscopy silicon probes

The silicon surface of commercial atomic force microscopy (AFM) probes loses its hydrophilicity by adsorption of airborne and package-released hydrophobic organic contaminants. Cleaning of the probes by acid piranha solution or discharge plasma removes the contaminants and renders very hydrophilic probe surfaces. Time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy investigations showed that the native silicon oxide films on the AFM probe surfaces are completely covered by organic contaminants for the as-received AFM probes, while the cleaning methods effectively remove much of the hydrocarbons and silicon oils to reveal the underlying oxidized silicon of the probes. Cleaning procedures drastically affect the results of adhesive force measurements in water and air. Thus, cleaning of silicon surfaces of the AFM probe and sample cancelled the adhesive force in deionized water. The significant adhesive force values observed before cleaning can be attributed to formation of a bridge of hydrophobic material at the AFM tip-sample contact in water. On the other hand, cleaning of the AFM tip and sample surfaces results in a significant increase of the adhesive force in air. The presence of water soluble contaminants at the tip-sample contact lowers the capillary pressure in the water bridge formed by capillary condensation at the AFM tip-sample contact, and this consequently lowers the adhesive force.     

Scanning of silicon wafers in contact with aqueous CTAB solutions below the CMC

Hydrophilic silicon wafers are studied against aqueous solutions of hexadecyl trimethyl ammonium bromide (CTAB) at concentrations between 0.05 mM up to 1 mM (CMC). AFM studies show that nanobubbles are formed at concentrations up to 0.4 mM. From 0.5 mM upward, no bubbles could be detected. This is interpreted as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic at about 0.5 mM. The high contact angle of the nanobubbles (140-150° through water) indicates that the nanobubbles are located on the surfactant domains. A combined imaging and colloidal probe AFM study serves to highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobubbles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The presence of the nanobubbles is correlated with force measurements between a silica probe and a silicon wafer surface. The study is a contribution to the better understanding of the short-range attraction between hydrophilic surfaces exposed to a surfactant solution.     

Langmuir-Blodgett films composed of amphiphilic double-decker shaped polyhedral oligomeric silsesquioxanes

New amphiphilic polyhedral oligomeric silsesquioxanes (POSSs) were synthesized, and their monolayer behavior on a water surface and Langmuir-Blodgett (LB) film formation were studied. Two kinds of amphiphilic POSS molecules, which have two or four di(ethylene glycol) units (2OH-DDSQ and 4OH-DDSQ, respectively), were synthesized by direct hydrosilylation of di(ethylene glycol) vinyl ether with double-decker shaped polyhedral oligomeric silsesquioxanes (DDSQs). Surface pressure (π)-area (A) isotherms and Brewster angle microscope (BAM) measurements indicated that both amphiphilic DDSQs form a stable monolayer at the air-water interface. In addition, 4OH-DDSQ can be deposited on a solid substrate by the LB technique. Atomic force microscope (AFM) images of a one-layer 4OH-DDSQ film showed a homogenous uniform surface on a hydrophilic silicon substrate, whereas nanometer scale dots were formed on a hydrophobic silicon substrate. Multilayer deposition on a hydrophobic substrate resulted in an increase of dot size with increasing deposition number of layers. Moreover, homogenous multilayer films with a few voids were obtained on a hydrophilic substrate. The results indicate that 4OH-DDSQ is a good candidate for preparing hybrid nanoassemblies.     

Effect of roughness on particle adhesion in aqueous solutions: a study of Saccharomyces cerevisiae and a silica particle

An atomic force microscope (AFM) has been used to quantify the adhesion of living cells Saccharomyces cerevisiae on three different silica surfaces with defined roughness. The effects of support roughness on the adhesion forces of a smooth silica particle were studied in addition. A living single cell was immobilized at the apex of a tipless AFM cantilever using a key-lock mechanism. Adhesion was quantified from the force-distance data measured on a smooth silica substrate and two substrates coated with hydrophilic monodisperse silica particles with 110 and 240 nm in diameter to study the effect of roughness on particle adhesion. The AFM technique gives unique insight into the primary colonization event of biofilm formation. The new knowledge helps substantially to design surface coatings relevant for biotechnology, medicine and dentistry.     

Amphiphilic Janus Gold Nanoparticles Prepared by Interface‐Directed Self‐Assembly: Synthesis and Self‐Assembly

Materials with Janus structures are attractive for wide applications in materials science. Although extensive efforts in the synthesis of Janus particles have been reported, the synthesis of sub‐10 nm Janus nanoparticles is still challenging. Herein, the synthesis of Janus gold nanoparticles (AuNPs) based on interface‐directed self‐assembly is reported. Polystyrene (PS) colloidal particles with AuNPs on the surface were prepared by interface‐directed self‐assembly, and the colloidal particles were used as templates for the synthesis of Janus AuNPs. To prepare colloidal particles, thiol‐terminated polystyrene (PS‐SH) was dissolved in toluene and citrate‐stabilized AuNPs were dispersed in aqueous solution. Upon mixing the two solutions, PS‐SH chains were grafted to the surface of AuNPs and amphiphilic AuNPs were formed at the liquid–liquid interface. PS colloidal particles decorated with AuNPs on the surfaces were prepared by adding the emulsion to excess methanol. On the surface, AuNPs were partially embedded in the colloidal particles. The outer regions of the AuNPs were exposed to the solution and were functionalized through the grafting of atom‐transfer radical polymerization (ATRP) initiator. Poly[2‐(dimethamino)ethyl methacrylate] (PDMAEMA) on AuNPs were prepared by surface‐initiated ATRP. After centrifugation and dissolving the colloidal particles in tetrahydrofuran (THF), Janus AuNPs with PS and PDMAEMA on two hemispheres were obtained. In acidic pH, Janus AuNPs are amphiphilic and are able to emulsify oil droplets in water; in basic pH, the Janus AuNPs are hydrophobic. In mixtures of THF/methanol at a volume ratio of 1:5, the Janus AuNPs self‐assemble into bilayer structures with collapsed PS in the interiors and solvated PDMAEMA at the exteriors of the structures.     

Imaging of highly oriented pyrolytic graphite corrosion accelerated by Pt particles

Carbon corrosion that is presumed to occur at the proton exchange membrane fuel cell (PEMFC) cathode was visualized by atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) observations using a fundamental model electrode. Platinum nanoparticles were deposited on a highly oriented pyrolytic graphite (HOPG) substrate as a model cathode catalyst, and its stability in an acid solution at a fixed potential was investigated. The formation of blisters on the surface of the model electrode was observed by AFM after it was kept at 1.0 V vs. RHE, especially at and around the Pt particles. FE-SEM observations using a backscattered electron detector revealed that Pt particles remained unchanged at their original positions after the formation of blisters.