A novel cleaning method of gold-coated atomic force microscope tips for their chemical modification

Rapid improvements in charge carrier mobility at ionic liquid/pentacene single crystal interfaces by self-cleaning

We report the rapid improvement in the carrier mobility of the electric double layer field-effect transistor based on the ionic liquid (IL)/pentacene single crystal interface. Generally, the surface oxidation of the pentacene single crystal is unavoidable, and the considerable degradation restricts the performance of the field-effect transistor. However, the formation of the IL/pentacene single crystal interface resolves this problem by increasing the carrier mobility by approximately twice the initial value within a few hours. Furthermore, frequency-modulation atomic force microscopy revealed that the aforementioned rapid improvement is attributed to the appearance of a clean and flat surface of the pentacene single crystal via the defect-induced spontaneous dissolution of pentacene molecules into the IL.

Rapid transport of germ-mimetic nanoparticles with dual conformational polyethylene glycol chains in biological tissues

Polyethylene glycols (PEGs) can improve the diffusivity of nanoparticles (NPs) in biological hydrogels, while extended PEG chains severely impede cellular uptake of NPs. Inspired by invasive germs with flagellum-driven mucus-penetrating and fimbriae-mediated epithelium-adhering abilities, we developed germ-mimetic NPs (GMNPs) to overcome multiple barriers in mucosal and tumor tissues. In vitro studies and computational simulations revealed that the tip-specific extended PEG chains on GMNP functioned similarly to flagella, facilitating GMNP diffusion (up to 83.0-fold faster than their counterparts). Meanwhile, the packed PEG chains on the bodies of GMNP mediated strong adhesive interactions with cells similarly to the fimbriae, preserving cellular uptake efficiency. The in vivo results proved the superior tumor permeability and improved oral bioavailability provided by the GMNP (21.9-fold over administration of crystalline drugs). These findings offer useful guidelines for the rational design of NPs by manipulating surface polymer conformation to realize multiple functions and to enhance delivery efficacy.

In situ characterization of nanoscale contaminations adsorbed in air using atomic force microscopy

Under ambient conditions, surfaces are rapidly modified and contaminated by absorbance of molecules and a variety of nanoparticles that drastically change their chemical and physical properties. The atomic force microscope tip-sample system can be considered a model system for investigating a variety of nanoscale phenomena. In the present work we use atomic force microscopy to directly image nanoscale contamination on surfaces, and to characterize this contamination by using multidimensional spectroscopy techniques. By acquisition of spectroscopy data as a function of tip-sample voltage and tip-sample distance, we are able to determine the contact potential, the Hamaker constant and the effective thickness of the dielectric layer within the tip-sample system. All these properties depend strongly on the contamination within the tip-sample system. We propose to access the state of contamination of real surfaces under ambient conditions using advanced atomic force microscopy techniques.

Molecularly clean ionic liquid/rubrene single-crystal interfaces revealed by frequency modulation atomic force microscopy

Frequency modulation atomic force microscopy was employed to show a molecularly clean interface between an ionic liquid and a rubrene single crystal for possible applications to electric double-layer field-effect transistors.

Significant improvements in stability and reproducibility of atomic-scale atomic force microscopy in liquid

Recent advancement of dynamic-mode atomic force microscopy (AFM) for liquid-environment applications enabled atomic-scale studies on various interfacial phenomena. However, instabilities and poor reproducibility of the measurements often prevent systematic studies. To solve this problem, we have investigated the effect of various tip treatment methods for atomic-scale imaging and force measurements in liquid. The tested methods include Si coating, Ar plasma, Ar sputtering and UV/O₃ cleaning. We found that all the methods provide significant improvements in both the imaging and force measurements in spite of the tip transfer through the air. Among the methods, we found that the Si coating provides the best stability and reproducibility in the measurements. To understand the origin of the fouling resistance of the cleaned tip surface and the difference between the cleaning methods, we have investigated the tip surface properties by x-ray photoelectron spectroscopy and contact angle measurements. The results show that the contaminations adsorbed on the tip during the tip transfer through the air should desorb from the surface when it is immersed in aqueous solution due to the enhanced hydrophilicity by the tip treatments. The tip surface prepared by the Si coating is oxidized when it is immersed in aqueous solution. This creates local spots where stable hydration structures are formed. For the other methods, there is no active mechanism to create such local hydration sites. Thus, the hydration structure formed under the tip apex is not necessarily stable. These results reveal the desirable tip properties for atomic-scale AFM measurements in liquid, which should serve as a guideline for further improvements of the tip treatment methods.

Direct observation of layered structures at ionic liquid/solid interfaces by using frequency-modulation atomic force microscopy

Presence of inhomogeneous layered structures of ionic liquid (IL) molecules at IL/HOPG and IL/mica interfaces was directly detected and imaged by using frequency-modulation atomic force microscopy. High stability of the layered structures may disturb their interface applications to catalysis and electrochemistry.

Cleaning AFM colloidal probes by mechanically scrubbing with supersharp "brushes"

Contamination control of atomic force microscope (AFM) tips (including standard but supersharp imaging tips and particle/colloidal probes) is very important for reliable AFM imaging and surface/interface force measurements. Traditional cleaning methods such as plasma, UV-ozone and solvent treatments have their shortcomings. Here, we demonstrate that calibration gratings with supersharp spikes can be employed to scrub away contaminants accumulated on a colloidal sphere probe by scanning the probe against the spikes at high load at constant-force mode. The present method is superior to traditional cleaning methods in several aspects. First, accumulated lump-like organic/inorganic material can be removed; second, removal is non-destructive and highly efficient based on a "targeted removal" strategy; third, removal and probe shape/morphology study can be completed in a single step (we report, to our best knowledge, the first evidence of the wear of the colloidal sphere during force measurements); and fourth, both colloidal/particle probes and standard but supersharp AFM imaging tips can be treated.

Internal friction of single polypeptide chains at high stretch

Experiments that measure the viscoelasticity of single molecules from the Brownian fluctuations of an atomic force microscope (AFM) have provided a new window onto their internal dynamics in an underlying conformational landscape. Here we develop and apply these methods to examine the internal friction of unfolded polypeptide chains at high stretch. The results reveal a power law dependence of internal friction with tension (exponent 1.3 +/- 0.5) and a relaxation time approximately independent of force. To explain these results we develop a frictional worm-like chain (FWLC) model based on the Rayleigh dissipation function of a stiff chain with dynamical resistance to local bending. We analyse the dissipation rate integrated over the chain length by its Fourier components to calculate an effective tension-dependent friction constant for the end-to-end vector of the chain. The result is an internal friction that increases as a power law with tension with an exponent 3/2, consistent with experiment. Extracting the intrinsic bending friction constant of the chain it is found to be approximately 7 orders of magnitude greater than expected from solvent friction alone; a possible explanation we offer is that the underlying energy landscape for bending amino acids and/or peptide bond is rough, consistent with recent results on both proteins and polysaccharides.

Peeling single-stranded DNA from graphite surface to determine oligonucleotide binding energy by force spectroscopy

We measured the force required to peel single-stranded DNA molecules from single-crystal graphite using chemical force microscopy. Force traces during retraction of a tip chemically modified with oligonucleotides displayed characteristic plateaus with abrupt force jumps, which we interpreted as a steady state peeling process punctuated by complete detachment of one or more molecules. We were able to differentiate between bases in pyrimidine homopolymers; peeling forces were 85.3 - 4.7 pN for polythymine and 60.8 +/- 5.5 pN for polycytosine, substantially independent of salt concentration and the rate of detachment. We developed a model for peeling a freely jointed chain from the graphite surface and estimated the average binding energy per monomer to be 11.5 +/- 0.6 k(B)T and 8.3 +/- 0.7 k(B)T in the cases of thymine and cytosine nucleotides, respectively. The equilibrium free-energy profile simulated using molecular dynamics had a potential well of 18.9 k(B)T for thymidine, showing that nonelectrostatic interactions dominate the binding. The discrepancy between the experiment and theory indicates that not all bases are adsorbed on the surface or that there is a population of conformations in which they adsorb. Force spectroscopy using oligonucleotides covalently linked to AFM tips provides a flexible and unambiguous means to quantify the strength of interactions between DNA and a number of substrates, potentially including nanomaterials such as carbon nanotubes.

A Model System To Study the Insertion of Cholesterol into a Phospholipid Monolayer

Colloidal probe atomic force microscopy (AFM) was used to study the interaction between a surface bearing tethered cholesterol groups and an egg phosphatidylcholine (egg-PC) monolayer. The cholesterol bearing surface was comprised of a mixed self-assembled monolayer comprised of O-cholesteryl N-(8'-mecapto-3',6'-dioxaoctyl)carbamate (CPEO3) molecules and beta-mercaptoethanol formed on a 20 mum diameter gold-coated silica particle. The egg-PC monolayer was adsorbed onto an octadecylthiol monolayer formed on template-stripped gold. The force between the surfaces, as a function of separation, was measured for surface concentrations of CPEO3 from 0 to 100 mol %. At all concentrations there was a long-range repulsive double-layer force due to weak surface charges. At surface concentrations of CPEO3 from 1 to 29 mol % the interaction on the approach of the surfaces showed a maximum in the repulsive force, followed by a small (2-5 nm) jump into a force minimum corresponding to adhesion of the surfaces. On separation, a normalized pull-off force of 1.0-1.6 mN m(-1) was measured. Over the same concentration range, the calculated interaction energy per CPEO3 molecule decreased from 1.1 +/- 0.2 kT to 0.04 kT. At surface concentrations of 35 mol % and above there was no reproducible adhesion between the cholesterol-bearing surface and the phospholipid monolayer. We attribute the occurrence of short-range attraction and adhesion in the 1-29 mol % regime to the insertion of (some) cholesterol groups into the phospholipid monolayer. At higher surface concentrations the efficiency of insertion is reduced due to steric effects. We discuss the experimental results in the light of the energetics of the insertion of a cholesterol molecule into a lipid bilayer.

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.

The effect of permanent dipole moments of adsorbates upon I-V characteristics of a bilayer tunneling junction between self-assembled monolayers on an Au(111) substrate and a gold tip

It is important to understand an electronic property of an interface between an organic material and a metal electrode. In the present work, we measured current-voltage (I-V) curves of self-assembled monolayers (SAMs) on Au(111) using a conducting atomic force microscope (AFM) with chemically modified Au-coated AFM tips. This contact resulted in a bilayer junction between the Au(111) substrate covered with one SAM and the Au-coated tip with the other SAM. An alkanethiol (octanethiol) and benzenemethanethiols with various terminal groups (-H, -CH(3), -Cl, -CF(3)) were used as the adsorbates. The shapes of the I-V curves depended on the terminal groups. This phenomenon was attributed to the change in the work function of gold due to different permanent dipole moments of the terminal groups.

Imaging stretched single DNA molecules by pulsed-force-mode atomic force microscopy

The effect of a surface water layer on DNA strands deposited on a substrate was studied by atomic force microscopy (AFM). DNA molecules were deposited and stretched on chemically modified glass coverslips by a molecular combing method. Lambda bacteriophage DNA molecules were aligned on the organosilane-modified substrate surfaces by chemical and physical adsorption during the molecular combing. The combed DNA molecules were observed in humidity-controlled air and in aqueous solutions by pulsed-force-mode AFM (PFM-AFM). Chemical modification of cantilevers with an Au-coated tip by organothiol compounds was also applied to DNA observation. Mapping adhesive forces in aqueous media was useful to discriminate chemically the DNA strands from the substrate surface. The results suggest that PFM-AFM can be used widely to image the stretched DNA molecules on the silane-modified substrates.

A study of topographic effects on chemical force microscopy using adhesive force mapping

Origins of peak broadening in a histogram of measured adhesive forces were studied. The adhesive forces were measured in water by pulsed-force-mode atomic force microscopy. One sample was prepared by a microcontact printing method on a sputtered gold film with fine grains, on which CH(3)- and COOH-terminated regions were produced. Gold surfaces of other samples were chemically modified homogeneously by a self-assembling method in solution. Their surfaces were, however, topographically different, i.e. (i) an Au(111)-terrace-rich gold film prepared by vacuum vapor deposition at high temperature and (ii) sputtered gold films on cover glass with different grain sizes obtained by different deposition time. These sample surfaces and the probe tip surface were all CH(3)-terminated by self-assembled monolayers with CH(3)(CH(2))(19)SH. The main origin of peak broadening in the histogram was the topographic effect. Namely, the change in the grain sizes and the change in multiplicity of contacts between the tip and convexities of the grains resulted in the distribution of the observed adhesive forces.

Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation

Patterned self-assembled monolayers (SAMs) were formed on gold films and observed by friction force microscopy (FFM) and adhesive force mapping with pulsed-force mode atomic force microscopy (PFM-AFM). The substrate gold films were prepared by sputtering gold on flat surfaces of osmium-coated cover glass with surface roughness, Ra, of 0.3 nm. The patterned samples with the CH3 and COOH terminated regions were prepared using the Langmuir-Blodgett (LB) method, partial removal of the LB film by ultrasonication, and SAM formation. The CH3 and COOH terminated regions of the patterned SAMs in air and in water were observed by mapping friction and adhesive forces with FFM and PFM-AFM, respectively, using gold-coated AFM tips chemically modified with a thiol compound terminating in CH3 or COOH. The adhesive forces measured in air increased in the order of CH3/CH3, CH3/COOH (or COOH/CH3) and COOH/COOH, while those in water increased in reverse order. The enormous high adhesive force observed in water for CH3/CH3 was attributed to hydrophobic interaction between the CH3 tip and the CH3 terminated sample surface. With CH3 tip, the lower friction force was observed, however, in water on the CH3 terminated region than on the COOH terminated region. This experimental finding raises a question as to what is the effective normal load in friction measurements in water.

Study of microcontact printed patterns by chemical force microscopy

Patterned self-assembled monolayers (SAMs) on sputtered gold films prepared by microcontact printing (microCP) were studied by mapping adhesive forces with pulsed-force-mode atomic force microscopy. A stamp for microCP was fabricated by pouring polydimethylsiloxane (PDMS) over a photolithographically prepared master. The patterned SAMs were prepared by two methods. One is called the wet-inking method, in which inking was done by placing a thiol ethanol solution for 30 s on the stamp and then removing the excess ink solution under a stream of nitrogen. The other is called the contact-inking method, in which a pad made of PDMS was dipped overnight in a thiol ethanol solution and then the stamp was placed on the inker pad impregnated with the thiol ethanol solution. The second step for pattern formation was the same for both of the two different microCP methods. Namely, the gold surfaces stamped with alkanethiols were further reacted with a thiol terminating in COOH in ethanol. The resulting patterns with CH3- and COOH-terminated regions were analyzed by imaging the adhesive forces with the chemically modified gold coated AFM tips with a SAM of CH3 or COOH terminal functional groups.

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.