Nanofiber formation of hydroxylpropylcellulose (HPC)

The aggregative behaviors of hydroxypropylcellulose (HPC) molecules in aqueous solution and on substrates have been observed by employing laser light scattering (LLS) and, after deposition on a mica surface, atomic force microscopy (AFM). LLS studies showed that the HPC molecules formed large aggregates through self-association when the concentration of the solution was above the critical concentration c(t). AFM measurements revealed that when a dilute aqueous solution of HPC molecules was deposited onto a mica substrate at a temperature below its lower critical solution temperature (LCST) thin nanofibers were formed with a height of 0.9 nm, whereas thick nanofibers were formed when an aqueous solution of HPC molecules was deposited onto a substrate above its LCST. Furthermore, the growth of nanofibers led to the formation of fan structures.     

Short-range forces between glass surfaces in aqueous solutions

We found that the force between glass surfaces measured with an atomic force microscope (AFM) has universal character in the short range, less than approximately 1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion size, charge and concentration and pH. Our results suggest that the excess DLVO force, obtained by subtracting the DLVO theory with a charge regulation model from the AFM force data, essentially does not change with the electrolytes Na, Ca, and Al, in the range of concentration from 10(-6) to 10(-2) M and the range of pH from 3.1 to 7.9. Single force curves for a glass-silica system in a 10-4 M aqueous NaCl solution at pH approximately 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule. We postulate that the excess force between glass surfaces arises from a surface-induced solvent effect, from the creation of a hydrogen-bonding network at the surface level, rather than from a solvent-induced surface steric hindrance.     

Transient violations of the second law of thermodynamics in protein unfolding examined using synthetic atomic force microscopy and the fluctuation theorem

The synthetic atomic force microscopy (AFM) method is developed to simulate a periodically replicated atomistic system subject to force and length fluctuations characteristic of an AFM experiment. This new method is used to examine the forced-extension and subsequent rupture of the alpha-helical linker connecting periodic images of a spectrin protein repeat unit. A two-dimensional potential of mean force (PMF) along the length and a reaction coordinate describing the state of the linker was calculated. This PMF reveals that the basic material properties of the spectrin repeat unit are sensitive to the state of linker, an important feature that cannot be accounted for in a one-dimensional PMF. Furthermore, nonequilibrium simulations were generated to examine the rupture event in the context of the fluctuation theorem. These atomistic simulations demonstrate that trajectories which are in apparent violation of the second law can overcome unfolding barriers at significantly reduced rupture forces.     

Nanoscale repulsive forces between mica and silica surfaces in aqueous solutions

Nanoscale repulsive forces between mineral surfaces in aqueous solutions were measured for the asymmetric mica-silica system. The force measured with an atomic force microscope (AFM) has universal character in the short range, less than ～1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion (Na, Ca, Al), concentration (10(-6)-10(-2)M), and pH (3.9-8.2). Notably, the force is essentially the same as for the glass-silica system. Single force curves for a mica-silica system in a 10(-4)M aqueous NaCl solution at pH ～ 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule, a consequence of a layer-by-layer dehydration of the surfaces when pushed together. This result provides additional support to the idea that nanoscale repulsive forces between mineral surfaces in aqueous solutions arise from a surface-induced water effect; the water between two mineral plates that are pushed together becomes structured and increasingly anchored to the surface of the plates by the creation of a hydrogen-bonding network that prevents dehydration of the surfaces.     

Chemistry on surface-confined molecules: an approach to anchor isolated functional units to surfaces.

The synthesis of surface-confined, nanometer-sized dendrimers and Au nanoparticles was performed starting from single Pd(II) pincer adsorbate molecules (10) embedded as isolated species into 11-mercapto-1-undecanol and decanethiol self-assembled monolayers (SAMs) on gold. The coordination of monolayer-protected Au nanoclusters (MPCs) bearing phosphine moieties at the periphery (13), or dendritic wedges (8) having a phosphine group at the focal point, to SAMs containing individual Pd(II) pincer molecules was monitored by tapping mode atomic force microscopy (TM AFM). The individual Pd(II) pincer molecules embedded in the decanethiol SAM were visualized by their coordination to phosphine MPCs 13; isolated objects with a height of 3.5 +/- 0.7 nm were observed by TM AFM. Reaction of these embedded Pd(II) pincer molecules with the dendritic wedge 8 yielded individual molecules with a height of 4.3 +/- 0.2 nm.     

Microcontact printed diaphorase monolayer on glass characterized by atomic force microscopy and scanning electrochemical microscopy

Micropatterns of diaphorase (Dp) were fabricated on glass substrates by the microcontact printing (μCP) method and characterized with atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM). AFM images of the printed samples revealed that the mean height of the Dp patterns was 3–5 nm, indicating the formation of a monolayer pattern. The Dp molecules on the surface organized themselves into two-dimensional arrays. We used two kinds of inking solutions: Dp–phosphate buffer solution (PBS) (pH 7.0) and Dp–PBS (pH 7.0) with glutaraldehyde (GA, 1% v/v) as a cross-linking reagent. Although the AFM imaging showed high-quality Dp monolayer patterns in both cases, SECM measurements indicated that the enzymatic activity of Dp was almost lost when Dp–PBS with GA was used as the inking solution, whereas clear enzymatic activity was found when Dp–PBS was used.     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

ATRP synthesis and association properties of temperature responsive dextran copolymers grafted with poly(N-isopropylacrylamide)

Temperature responsive copolymers of dextran grafted with poly(N-isopropylacrylamide) (Dex-g-PNIPAAM) were prepared by atom transfer radical polymerization (ATRP) in homogeneous mild conditions without using protecting group chemistry. Dextran macroinitiator was synthesized by reaction of dextran with 2-chloropropionyl chloride at room temperature in DMF containing 2% LiCl. ATRP was carried out in DMF:water 50:50 (v/v) mixtures at room temperature with CuBr/Tris(2-dimethylaminoethyl)amine (Me₆TREN) as catalyst. Several grafted copolymers with well defined number and length of low polydispersity grafted chains were prepared. Temperature induced association properties in aqueous solution were studied as a function of temperature and polymer concentration by dynamic light scattering, fluorescence spectroscopy and atomic force microscopy (AFM). LCST, ranging from 35 to 41 °C, was significantly affected by number and length of grafted chains. The fine tuning of LCST around body temperature is an important characteristic not obtainable by conventional radical grafting of PNIPAAM. Well defined spherical nanoparticles were formed above the LCST of PNIPAAM. Hydrodynamic diameter was in the range 73-98 nm.     

Comparative observation of the recombinant adeno-associated virus 2 using transmission electron microscopy and atomic force microscopy.

Adeno-associated virus (AAV) is a defective, nonpathogenic human parvovirus, which coinfects with a helper adenovirus or herpes virus. AAV's unique characteristics have made it an appealing vector system for gene delivery. AAV or recombinant AAV (rAAV) has been widely detected using negative stain transmission electron microscopy (TEM) but little has been detected using atomic force microscopy (AFM). In this article, we used AFM and TEM to observe the recombinant AAV-2 (rAAV-2) virus particles and applied statistical analysis to the AFM and TEM images. The results indicated that the rAAV-2 particle was a slightly elliptic particle close to round when it was detected by TEM (the mean length of major and minor axes of rAAV-2 particles was 24.77 +/- 1.78 nm and 21.84 +/- 1.57 nm, respectively), whereas when detected by AFM, the rAAV-2 particle was almost round. Even though the dimensions of the rAAV-2 particle exhibited a polymorphous distribution via off-line particle analysis of AFM, most of the rAAV-2 particles had a mean diameter of approximate 22.04 nm, which was similar to the results obtained by TEM. The results above suggested that AFM was important for accurately determining the average dimensions and distributions of virus particles.     

Annealing behavior of the novel morphology of poly(butylene succinate) lath‐shaped single crystals

Lamellar single crystals of poly(butylene succinate) (PBS) with novel morphologies were prepared from a chloroform/methanol solution by self‐seeding methods. Crystal structures and morphologies were investigated by means of atomic force microscopy (AFM). Lath‐shaped crystal and hexagonal‐shaped crystals coexist in one PBS single crystal and this has a lamellar thickness of around 5–6 nm as determined by AFM. The thickening of lamellae from 5–6 to 7–9 nm occurred during heating from 41 to 84 °C. In situ temperature‐controlled AFM observations demonstrated that the lath‐shaped crystal sections melted first and then the hexagonal sections while the edge of the single crystals remained regular during annealing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1492–1496, 2009     

Self-localization of polyacrylic acid molecules on polar ZnO(0001)-Zn surfaces

The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.     

Influence of substrate hydrophobicity on the adsorption of an amphiphilic diblock copolymer

The adsorption of poly(tert-butylmethacrylate)-block-poly(2-(dimethylamino-ethyl) methacrylate) (PtBUMA-b-PDMAEMA) was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis performed on dried samples. The copolymer was dissolved in toluene at concentrations below (0.01 wt%) and above (0.05 and 1 wt%) the CMC; silicon (SiOH) and CH(3)-grafted silicon (SiCH(3)) were used as substrates. Whatever the concentration and the substrate, a layer of individual copolymer molecules, 1.5-3 nm thick, formed rapidly. The adsorbed amount was slightly higher and the resistance to AFM tip scraping was stronger on SiOH than on SiCH(3). This is attributed to hydrogen bonding between the PDMAEMA block and the OH groups of the silicon surface, leading to polarization of the adsorbed layer. Above the CMC, on SiOH, randomly scattered dot-like features (about 5 nm high) observed by AFM were attributed to individual micelles, which were not displaced by drying. On SiCH(3), the particles found on the top of the adsorbed layer were micelle aggregates, about 50 nm thick, the lateral size of which was strongly influenced by the rate of drying. This further difference between SiCH(3) and SiOH is tentatively attributed to the exposure of PDMAEMA by the adsorbed layer formed on SiCH(3), while only PtBUMA would be exposed by the layer adsorbed on SiOH. The red blood cell shape and the size of the micelles observed in single layers indicate that the PtBUMA corona was not made compact as a result of drying.     

Organization of polyhydroxyalkanoate synthase for in vitro polymerization as revealed by atomic force microscopy

Individual polyhydroxyalkanoate synthase molecules from Ralstonia eutropha (PhaCRe) were directly visualized on highly oriented pyrolytic graphite (HOPG) by atomic force microscopy (AFM). PhaCRe molecule was observed as a spherical particle of 2.9 +/- 0.4 nm in height and 28 +/- 4 nm in width. In vitro polymerization reaction on HOPG was carried out for 5 min by reacting the PhaCRe molecules with (R)-3-hydroxybutyryl-CoA monomers. The reaction product was then observed after the removal of water solution. Several PhaCRe molecules associated with each other to form an assembly, which was attached to a fibrillar structure of ca. 0.2-0.3 nm in height. The fibrillar structure that elongated from the PhaCRe assembly was interpreted as the poly[(R)-3-hydroxybutyrate] polymer chain. High resolution AFM suggested that the PhaCRe assembly was composed of 3-4 subunits of PhaCRe molecules. This was further supported by SDS-PAGE analysis of the cross-linked PhaCRe enzyme. These results suggest that more than two subunits of PhaCRe are necessary for the in vitro polymerization of PHB molecular chains.     

Self-assembly of acridine orange dye at a mica/solution interface: formation of nanostripe supramolecular architectures

Optical waveguide spectroscopy and atomic force microscopy (AFM) have been used to characterize the supramolecular architectures of acridine orange (AO) dye self-assembled at a mica/aqueous solution interface. Under the saturated adsorption conditions, optical waveguide spectroscopy revealed that the dye formed H-type aggregates at the interface. In situ AFM visualized interesting morphology of the dye aggregates showing nanosized meandering stripes with the width of approximately 1.5 nm (or brightness periodicity of approximately 3 nm). Electrostatic adsorption of the dye cations onto a mica surface as well as the intermolecular pi-pi stacking brought about the ordered nanostructures. We propose an interfacial aggregation model that shows a meandering staircase structure with the intermolecular slip angle of 60 degrees. According to the model, the AO molecule occupies a surface area of about 1.0 nm2.     

衬底温度对全氟取代酞菁锌固体薄膜微结构的影响

用UV-Vis光谱、X射线衍射(XRD)和原子力显微镜(AFM)对全氟取代酞菁锌(F₁₆PcZn)在不同温度下的石英衬底上的分子堆砌方式进行了研究,F₁₆PcZn分子沉积按3个阶段进行.首先通过F^-原子与石英衬底的强相互作用形成“card-packing”奠基层,进而形成“amorphousaccumulation”过渡层,最后形成有序的“brick-stacking”结晶层.三种分子堆砌方式对应的吸收光谱谱峰分别出现在640,670和810nm附近.810nm附近吸收峰形状的变化说明衬底温度升高有利于“brick-stacking”堆砌方式的实现,AFM观察的结果证实衬底温度升高使结晶微畴增大.常温下结晶层中分子以单斜点阵平行排列,点阵常数a为1.494nm,在250～300℃下分子发生位型微调,F₁₆PcZn分子采取更致密的堆积方式(a=1.428nm).     

The AFM observation of linear chain and crystalline conformations of ultrahigh molecular weight polyethylene molecules on mica and graphite

Atomic force microscopy (AFM) has been applied to visualize expanded linear chain and compact crystalline conformations of ultrahigh molecular weight polyethylene (PE) molecules deposited on mica and graphite from diluted solutions at elevated temperatures. Isolated PE chains are visualized on mica with the apparent negative AFM height and the contour length much shorter than the molecular length. The chain conformations have both the kinked random‐coil sites and the sites of the unexpectedly large two‐dimensional expansion. The crystalline conformations on mica are small single‐molecule rod‐like nanocrystallites and the isolated block‐type “edge‐on” nanolamellae comprising several PE molecules. Noticeable fluctuations of the fold length in the range of approximately 10–20 nm around the averaged value of about 15 nm are observed for nanocrystallites and on tips of some nanolamellae. The explanation of the experimentally observed features of chain surface conformations on mica is proposed. It implies the immobilization of PE molecules in the nm‐thickness salt layer formed on mica surface at ambient conditions after PE deposition and the presence along the chain of multiple expanded chain folds. Only isolated lamellae and lamellar domains of a monolayer height are observed on graphite samples. The substrate/polymer epitaxial incommensurability important for the observation of the PE linear chain surface conformations is discussed from the comparison of the results obtained for mica and graphite, the coil‐to‐crystal intramolecular transformation is assumed to be inhibited on mica surface. The slow disintegration of the original gel structure of PE stock‐solution used for the high‐temperature depositions was found to result in the characteristic large‐scale morphological heterogeneity of the samples. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 766–777, 2010     

Surface-confined single molecules: assembly and disassembly of nanosize coordination cages on gold (111)

A cavitand functionalized with four alkylthioether groups at the lower rim, and four tolylpyridine groups on the upper rim is able to bind to a gold surface by its thioether groups, and forms a coordination cage with [Pd(dppp)(CF(3)SO(3))(2)] by its pyridine groups. The cavitand or the cage complex can be inserted from solution into a self-assembled monolayer (SAM) of 11-mercaptoundecanol on gold. The inserted molecules can be individually detected as they protrude from the SAM by atomic force microscopy (AFM). The cages can be reversibly assembled and disassembled on the gold surface. AFM can distinguish between single cavitand and cage molecules of 2.5 nm and 5.8 nm height, respectively.     

Nanolithographic write, read, and erase via reversible nanotemplated nanostructure electrodeposition on alkanethiol-modified Au(111) in an aqueous solution

A write, read, and erase nanolithographic method, combining in situ electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal of HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape, and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of the deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.     

Penetration of living cell membranes with fortified carbon nanotube tips

We have fabricated robust nanosurgical needles suitable for single cell operations by modifying multiwalled carbon nanotube (MCNT)-terminated atomic force microscopy (AFM) tips. Extra-long MCNT AFM tips were prepared and fortified with molecular layers of carbon to overcome mechanical instabilities and then coated with an outer shell of gold to promote chemical versatility. The terminal diameters of the final fabricated tips were approximately 30-40 nm, and the MCNT probes were several micrometers in length. We illustrate the capability of these modified MCNT tips to carry nanoparticulate payloads and to penetrate the plasma membrane of living pleural mesothelial cells at the smallest indentation depths (100-200 nm) and lowest penetration forces (100-200 pN) currently reported for these procedures.     

Single molecule force spectroscopy on poly(vinyl alcohol) by atomic force microscopy

The elastic properties of poly(vinyl alcohol) (PVA) were investigated on the nanoscale using the new technique of single molecule force spectroscopy by atomic force microscopy (AFM). It was found that the elastic properties of PVA molecules scale linearly with their contour lengths. This finding corroborates that the deformation of individual PVA chains is measured. The force spectra of PVA show a kink at around 200 pN and cannot be fitted by an extended Langevin function. The deviation of the elastic behavior of PVA from a freely jointed chain model may indicate the presence of a suprastructure of PVA in NaCl solution.