Search

Titin, a giant protein containing multiple tandem domains, is essential in maintaining the superior mechanical performance of muscle. The consecutive and reversible unfolding and refolding of the domains are crucial for titin to serve as a modular spring. Since the discovery of the mechanical features of a single titin molecule, the exploration of biomimetic materials with titin-emulating modular structures has been an active field. However, it remains a challenge to prepare these modular polymers on a large scale due to the complex synthesis process. In this study, we propose modular DNA with multiple hairpins (MH-DNA) as the fundamental block for the bottom-up design of advanced materials. By analyzing the unfolding and refolding dynamics of modular hairpins by atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS), we find that MH-DNA shows comparable stability to those of polyproteins like titin. The unique low hysteresis of modular hairpin makes it an ideal molecular spring with remarkable mechanical efficiency. On the basis of the well-established DNA synthesis techniques, we anticipate that MH-DNA can be used as a promising building block for advanced materials with a combination of superior structural stability, considerable extensibility, and high mechanical efficiency.

The synthesis of crystalline polymer with a well-defined orientated state and a two-dimensional crystalline size beyond a micrometer will be essential to achieve the highest physical feature of polymer material but remain challenging. Herein, we show the synthesis of the crystalline unipolymer monolayer with an unusual ultrahigh modulus that is higher than the ITO substrate and high conductance by simultaneous electrosynthesis and manipulation. We find that the polymer monolayer has fully extended in the vertical and unidirectional orientation, which is proposed to approach their theoretically highest density, modulus, and conductivity among all aggregation formations of the current polymer. The modulus and current density can reach 40 and 1000 times higher than their amorphous counterpart. It is also found that these monolayers exhibit the bias- and length-dependent multiple charge states and asymmetrically negative differential resistance (NDR) effect, indicating that this unique molecular tailoring and ordering design is promising for multilevel resistive memory devices. Our work demonstrates the creation of a crystalline polymer monolayer for approaching the physical limit of polymer electronic materials and also provides an opportunity to challenge the synthetically iterative limit of an isolated ultra-long polymer.

As a weak non-covalent interaction, hydrogen bond (H-bond) is highly susceptible to the environmental interference. However, the direct quantification of a single H-bond under an interference-free condition is still a challenge. Herein, the intramolecular H-bond in a model system, poly(N-isopropylacrylamide), is studied in high vacuum by single-molecule atomic force microscopy and steered molecular dynamics simulations, which allows the precise quantification of H-bond strength in an interference-free state. Control experiments show that the H-bond is significantly weakened in nonpolar solvent, even if the dielectric constant is very close to vacuum. If a polar solvent is used as the environment, the H-bond will be further weaker or even broken. These results imply that for experiments in any liquid environment, the H-bond strength (ΔG) will be only ∼ 50% or even less of that measured in vacuum. Further analysis shows that in liquid environments, ΔG decays in a quasi-linear way with the increase of the dielectric constant (e). For H-bond studies in future, the result measured in vacuum can be set as the standard value, namely, the inherent strength. This approach will provide fundamental insights into the H-bond participated nano-structures and materials in different environments.

High elasticity is one of the most important characteristics of polymers. The entropy-driven elastic behavior dominates the low force regime. The entropic elasticity of single polymer chains can be...

It is generally accepted that water is deeply involved in the structures and functions of DNA and proteins. For polysaccharides, however, the role of water remains poorly understood. Amylose and natural cellulose (NC) are two polysaccharides with similar molecular structures but different linkages (α or β) between the pyranose rings. In this study, the effects of H-bonds on the single-molecule mechanics and affinity for water of amylose and NC are explored by single-molecule atomic force microscopy (AFM) and molecular dynamics (MD) simulations, respectively. The experimental results show that the single-molecule mechanics of both amylose and NC are dependent on the solvent polarity. Accordingly, the status of H-bonds of each polysaccharide can be inferred. We find that the two polysaccharides present the same status of H-bonds in a given organic solvent: the intrachain H-bonds can be formed in a nonpolar solvent (nonane), while they are completely prohibited in a highly polar solvent (dimethyl sulfoxide, ...

Although π-π interactions have been studied for several decades, the quantification of the strength of π-π interactions in a macromolecule remains a big challenge. Herein, we utilize single-molecule atomic force microscopy and steered molecular dynamics simulations to study the π-π interactions in polystyrene (PS). It is found that in high vacuum, the single-chain mechanics of PS differs largely from that of polyethylene (PE). Accordingly, the strength of intrachain π-π interactions in PS is estimated to be 0.7 kcal/(mol stack), which is much lower than that in a small-molecule system (benzene dimer, 2-3 kcal/(mol stack)). Further study shows that in high vacuum, there are two types of π-π stacking in the single PS chain, i.e., the every-other-moiety (E) type and the adjacent-moiety (A) type. Upon force stretching, a transition from E-type to A-type π-π stacking can be observed.

Since the discovery of amorphous red phosphorus (a-red P) in 1847, many possible structures have been proposed. However, the exact molecular structure has not yet been determined because of its amorphous nature. Herein several methods are used to investigate basic properties of a-red P. Data from scanning tunneling microscopy (STM) and gel permeation chromatography (GPC) confirm that a-red P is a linear inorganic polymer with a broad molecular weight distribution. The theoretical single-molecule elasticities of the possible a-red P structures are obtained by quantum mechanical (QM) calculations. The experimental single-molecule elasticity of a-red P measured by single-molecule AFM matches with the theoretical result of the zig-zag ladder structure, indicating that a-red P may adopt this structure. Although this conclusion needs further validation, this fundamental study represents progress towards solving the structure of a-red P. It is expected that the strategy utilized in this work can be applied to study other inorganic polymers.

"The Tao begets the One. One begets all things of the world." This quote from Tao Te Ching is still inspiring for scientists in chemistry and materials science: The "One" can refer to a single molecule. A macroscopic material is composed of numerous molecules. Although the relationship between the properties of the single molecule and macroscopic material is not well understood yet, it is expected that a deeper understanding of the single-chain mechanics of macromolecules will certainly facilitate the development of materials science. Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) has been exploited extensively as a powerful tool to study the single-chain behaviors of macromolecules. In this review, we summarize the recent advances in the emerging field of environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by means of AFM-SMFS. First, the single-chain inherent elasticities of several typical linear macromolecules are introduced, which are also confirmed by one of three polymer models with theoretical elasticities of the corresponding macromolecules obtained from quantum mechanical (QM) calculations. Then, the effects of the external environments on the single-chain mechanics of synthetic polymers and biomacromolecules are reviewed. Finally, the impacts of single-chain mechanics of macromolecules on the development of polymer science especially polymer materials are illustrated.

Ubiquitous van der Waals (vdW) forces are very important for nanostructures. Although the vdW forces between two surfaces (or two layers) have been measured for several decades, a direct detection at the single-molecule level is still difficult. Herein, we report a novel method to solve this problem in high vacuum by means of AFM-based single-molecule force spectroscopy (SMFS). Solvent molecules and surface adsorbed water are removed thoroughly under high vacuum so that the situation is greatly simplified. A constant force plateau can be observed when a polymer chain is peeled off from a substrate in high vacuum. Accordingly, the vdW forces between one polymer repeating unit and the substrates can be obtained. The experimental results show that the vdW forces (typical range: 21–54 pN) are dependent on the species of substrates and the size of polymer repeating unit, which is in good accordance with the theoretical results. It is expected that this novel method can be applied to detect other non-covalent interactions (such as hydrogen bond and π-π stacking) at the single-molecule level in the future.

It is generally recognized that water is deeply involved in the structures and functions of DNA and proteins. For polysaccharides, however, the role of water remains unclear. Due to the force-induced conformational transition of the sugar rings, a fingerprint plateau can be observed in the single-chain force-extension (F-E) curves of amylose and some other polysaccharides in aqueous solutions. In this study, the effects of water content of the mixed solvents on the fingerprint plateau of amylose are explored by single-molecule AFM. The experimental results obtained in a series of water/alcohol mixed solvents clearly show that both the appearance and the fingerprint plateau height in the F-E curves of amylose are dependent on the water content. Since water is a good solvent for amylose but alcohols are not, the higher water content of a mixed solvent corresponds to a better solvent quality. Thus, the observed results can be associated with the solvent quality to amylose. The present study implies that water is not only a solvent but also an active constituent in the amylose solution.

Designing a main-chain visible-light-labile polymer is still a challenging task in related fields. In this work, a picolinium-caged main-chain photolabile polymer (MCPP) is firstly reported, where the C–O bonds of picolinium groups are efficiently cleaved (>95%) upon visible light irradiation (452 nm). The degradation rate of the MCPP is easily tuned by adjusting the concentration of the catalyst, irradiation intensity and time. Importantly, after photodegradation, the molecular weight of degradation products is decreased to less than 650 g mol−1. It is found that, unlike forming a helix structure of pure polyethylene glycol in water, the picolinium-caged MCPP is in a linear conformation. Moreover, nanopore-based single-molecule analysis indicates that both the MCPP and degradation products can easily pass through a biomimetic lipid membrane, and the passing speed can be tuned by varying the applied bias voltage. Cell cytotoxicity and proliferation experiments demonstrate that both the MCPP and degradation products are nontoxic, but only the MCPP is favorable for cell proliferation. On the basis of these results, it is anticipated that the MCPP is an exceptional candidate of photodegradable materials in chemistry, pharmacy, materials science and medicine.

The single-chain elasticity of a completely unfolded protein ((I27)8, modules of human cardiac titin) is studied in different liquid environments by the atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS). The experimental results show that there is a clear deviation between the force curves obtained in the aqueous and nonaqueous environments. Such a deviation can be attributed to the additional energy consumed by the rearrangement of the bound water molecules around the chain of the completely unfolded (I27)8 chain upon stretching in aqueous solution, which is very similar to the partial dehydration process from a denatured/unfolded to a native/folded protein. Through the analysis of the free energy changes involved in protein folding, we conclude that it is due to the weak disturbance of water molecules and the special backbone structures of proteins that the self-assembly of proteins can be achieved in physiological conditions. We speculate that water is likely to be an important criterion for the selection of self-assembling macromolecules in the prebiotic chemical evolution.

A novel environment-friendly system is proposed to fabricate polymer brush, which has the advantages including non-toxic and inexpensive initiator (eosin Y), visible-light exposure (λ = 515 nm), water medium and ambient environment. The experimental results from UV-Vis spectroscopy, AFM-based single molecule force spectroscopy (SMFS) and other measurements indicate that a polymer brush with a living nature is fabricated via free radical polymerization. This polymer brush may find applications in coatings, bio-interfaces and so forth.

The interactions between monovalent counterions and polyelectrolyte are important in chemical and biological systems. The condensation and screening effect of counterions complicate the polyelectrolyte solutions. By means of single-molecule AFM, the single-chain mechanics of a strong polyelectrolyte, poly(sodium styrenesulfonate) (PSSNa), in KCl aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KCl solutions with a parameter of single-chain modulus (K0). Along with the increase of the concentration of KCl from zero to almost the saturation concentration, a reentrant variation of K0 of single PSSNa chain can be observed. When [K+] is between 0.01 to 3 M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K0 has a positive correlation with the net charge of the polymer chain, the increased K0 at very high KCl concentrations...

Sample preparation is crucial to the studies of polymers on surfaces and interfaces. For studies using single-molecule force spectroscopy (SMFS), sample preparation is the key to obtain high-quality data. In each of the previous SMFS studies, either physisorption or chemisorption was applied in the sample preparation. However, a direct comparison on the same polymer species with both of the two strategies has not been reported yet. With two methods (physisorption, “PS”, or chemisorption, “CS”) and two surfaces (AFM tip or quartz substrate), four types of samples of poly(ethylene glycol) (PEG) can be prepared from the polymer solution. The performance of these samples with the same species of PEG are directly compared by SMFS. It is found that among these samples, two of them, i.e., tip-CS and substrate-PS, are suitable for SMFS. The advantage of substrate-PS is the simple preparation. In contrast, the advantage of tip-CS is the higher rupture force and the lower sample consumption. The former feature will be time saving if a high rupture force is needed in the analysis. The latter feature will be economic when an expensive sample is used. The other two types of samples, i.e., tip-PS and substrate-CS show lower data yield and lower rupture force. In summary, the tip-CS and substrate-PS are recommended for sample preparation in SMFS. The tip-CS is the most promising protocol, if a functionalized polymer sample is available.

Dopamine (DA) and its polymer (PDA) have attracted broad interest in recent years. Due to the insolubility of PDA, however, it has been difficult to determine the exact structure and the polymerization mechanism. PDA is usually prepared under alkaline conditions (pH 8.5). In this case, the polymerization process may follow an equilibrium pathway to form indole-like repeating units, which lead to cross-linked structures. However, PDA can be prepared even under acidic conditions (pH 4.0) in the presence of an oxidant, ammonium persulfate (APS), which cannot be interpreted by previous mechanisms. Therefore, there should exist several pathways in parallel on the formation of PDA. Herein, a derivative of DA, 2-(4-methoxy-3-methylphenyl)ethylamine (MOE) that has fewer active sites, is used as a simplified model system to study the polymerization mechanism of DA. Experimental results from UV-Vis spectroscopy, single-molecule force spectroscopy and other measurements indicate that free radical polymerization of MOE occurs under both acidic and alkaline conditions in the presence of a polymerization initiator, APS. According to the mathematical principle of Set Theory and the fact that MOE has fewer active sites than DA, we speculate that free radical polymerization as a possible pathway should exist in parallel along with previously proposed pathways during the formation of PDA. These parallel pathways may be the main reason for the structural complexity of PDA.

Green fluorescent protein (GFP) is a widely used biomarker that demands systematical rational approaches to its structure function redesign. In this work, we mainly utilized atomistic molecular dynamics simulations to inspect and visualize internal fluctuation and coordination around chromophore inside GFP, from water to nonpolar octane solvent. We found that GFP not only maintains its β-barrel structure well into the octane, but also sustains internal residue and water coordination to position the chromophore stably while suppress dihedral fluctuations of the chromophore, so that functional robustness of GFP is achieved. Our accompanied fluorescence microscope measurements accordingly confirmed the GFP functioning into the octane. Furthermore, we identified that crucial water sites inside GFP along with permeable pores on the β-barrel of the protein are largely preserved from the water to the octane solvent, which allows sufficiently fast exchanges of internal water with the bulk or with the water layer kept on the surface of the protein. By additionally pulling GFP from bulk water to octane, we suggest that the GFP function can be well maintained into the nonpolar solvent as long as, first, the protein does not denature in the nonpolar solvent nor across the polar-nonpolar solvent interface; second, a minimal set of water molecules are in accompany with the protein; third, the nonpolar solvent molecules may need to be large enough to be nonpermeable via the water pores on the β-barrel.

Non-covalent interactions are very important for nanostructures. The detection of non-covalent interactions at the molecular scale is very difficult since they are too weak to detect in most cases. The disturbance from the liquid environment can be eliminated in high vacuum, which makes the detection of non-covalent interactions possible. This review introduces a method of high vacuum single molecule force spectroscopy (SMFS) and summarizes a series of recent studies on non-covalent interactions by using SMFS. These studies include the van der Waals forces between a polymer chain and the substrate as well as the intrachain π–π interactions of a polymer chain, which cast new light on the non-covalent interactions.

Polymers with a carbon–carbon (C–C) backbone are an important class of polymers, which can be regarded as the derivatives of polyethylene (PE). To investigate the effect of side chains on the single-chain enthalpic elasticity (SCEE) of polymers with a C–C backbone, several polymers with pendants or side chains of different lengths and shapes have been studied by single-molecule AFM. We find that both length and shape of the side chains count: only the side chains that are both long and bulky (i.e., bulky dendrons of second or higher generation as side chains) affect the SCEE. Thus, only rare polymers have special SCEE. For the vast majority of polymers, the SCEE is identical to that of PE, which means that the SCEE is determined by the nature of the C–C backbone. It is expected that this conclusion can also be popularized to all polymers with various backbones. This study is an important update to the understanding of polymers at the single-chain level.

Both of temperature (in water) and composition (in the water/methanol mixed solvent) can induce the coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM). The atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension (F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic (monotonically increasing force) stretching but not plateau (constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.

Natural cellulose (NC) is the most abundant biomacromolecule. The fact that each pyranose ring of NC has three hydroxyl groups implies that NC should be highly hydrophilic. The paradoxical water insolubility of NC is usually explained by the strong tendency to form the hydrogen-bonding network by the high content of hydroxyl groups. In this study, we present the first experimental evidence that NC is rather hydrophobic, even if the chains are molecularly dispersed. By single-molecule force spectroscopy, the single-chain mechanics of NC has been studied in various liquid environments. In a common nonpolar solvent, octane, NC shows the elastic force–extension (F–E) curve. We find that this kind of F–E curve can be fitted well by the QM-FJC model, in which the single-chain elasticity obtained from quantum mechanical (QM) calculations is integrated into the freely jointed-chain (FJC) model. However, the result of NC obtained in water is different, which shows a long plateau in the F–E curve. Further study sho...

In this paper, we prepare a hydrogel in a green method via the redox reaction between phenol groups and Eosin Y by visible light irradiation. Eosin Y shows significant absorption at 515 nm while the reduced product does not. A series of control experiments show that almost 100% of the consumed Eosin Y reacted with phenol groups to form cross-link points. On the basis of the two results, the chemical cross-link density can be easily determined quantitatively by UV-vis spectroscopy. With this method, specimens from the state of the starting solution to solid gel can be measured in situ and in real time without any pretreatment. The result obtained by this new method is proved by traditional rheological measurements.

Most proteins are typically folded into predetermined three-dimensional structures in the aqueous cellular environment. However, proteins can be exposed to a nonpolar environment under certain conditions, such as inside the central cavity of chaperones and unfoldases during protein degradation. It remains unclear how folded proteins behave when moved from an aqueous solvent to a nonpolar one. Here, we employed single-molecule atomic force microscopy and molecular dynamics (MD) simulations to investigate the structural and mechanical variations of a polyprotein, I278, during the change from a polar to a nonpolar environment. We found that the polyprotein was unfolded into an unstructured polypeptide spontaneously when pulled into nonpolar solvents. This finding was corroborated by MD simulations where I27 was dragged from water into a nonpolar solvent, revealing details of the unfolding process at the water/nonpolar solvent interface. These results highlight the importance of water in maintaining folding stability, and provide insights into the response of folded proteins to local hydrophobic environments.

The material properties of polylactic acid (PLA) are largely determined by its stereo-regularity (tacticity). To find out the origin at the molecular level, the single-chain mechanics of poly-l-lactic acid (PLLA) and poly-d,l-lactide (PDLLA) were comparatively investigated by single-molecule atomic force microscopy (AFM). At a low concentration, PLLA adopted a random-coil conformation in a good solvent. At a high concentration, however, the PLLA chain can be induced into a helix, which consumed additional energy during unfolding by further stretching. Due to the random arrangement of l- and d-repeating units in the PDLLA chain, PDLLA adopts a random-coil conformation at all concentrations. The difference in single-chain mechanics of PLLA and PDLLA at high concentrations may be the cause of their different macroscopic properties. This is the first report to reveal the stereo-regularity-dependent mechanics of a polymer at the single-molecule level, which may help to bridge the gap between understanding single-molecule and materials properties.

Comparative studies of single molecule force spectroscopy and molecular dynamics simulations indicate that natural cellulose is more hydrophobic than amylose at the single-chain level, implying that the hydrophobicities of these polymeric isomers are regulated by only one parameter in the chains, the linkage between the sugar rings.

The unique self-cleaning feature of the lotus-like superhydrophobic (SH) surface attracted worldwide interest in recent years. However, the mechanism of the self-cleaning phenomena remains unclear. Here, we attempt to provide a comprehensive understanding of why self-cleaning of the particles with a broad range of size can be realized on the lotus-like SH surfaces. After measurements and analysis of the force involved at the interface, we conclude that there are four main preconditions for self-cleaning: (1) contact angle (CA)90°, (2) low enough sliding angle, (3) low enough adhesion force, and (4) proper particle size. However, as far as the lotus-like SH surface and typical dust are concerned, all the preconditions will be satisfied automatically. We also observe that the particles with a broad range of size (from submicron level to the millimeter level) and density (virtually no limit) can be driven by a water droplet on the lotus-like SH surface. This interesting finding may be helpful for the design of novel engineering system at the micron-millimeter scale in the future.

At crystal or larger scales, natural cellulose was generally recognized to be a rigid material. Our single-chain mechanical measurements, however, reveal that the natural cellulose is as flexible as common synthetic polymers at the single-chain level, creating new opportunities in the designing of nano materials.

Superstretchable materials have many applications in advanced technological fields but are difficult to stretch to more than 1000× their original length. A superstretchable dynamic polymer network that can be stretched to 13 000× its original length is designed. It is revealed that superstretchability of the polymer network is derived from the synergistic effect of two different types of dynamic bonds, including a small number of strong dynamic imine bonds to maintain the network integrity during stretching and a large number of weak ionic hydrogen bonds to dissipate energy. This approach provides new insights into the design of superstretchable polymers.

Four double-stranded DNA films with different chain lengths were prepared on 3-aminopropyltriethoxysilane (APS)-modified mica surfaces in the NaCl solution with concentration ranging from 0.001 to 0.1 M. By using an atomic force microscope, the force-distance curves and friction behaviour of each DNA film were studied in the NaCl solution that was used in the sample preparation. When adsorbed on mica as films in salt solution, the conformation of DNA molecules would be a combination of loops and “train-like”. As the chain length increased from 50 to 20000 bp, the extension rate of DNA film increased from 7.1 to 11.5 in 0.001 mol/L NaCl solution, which suggested that the DNA molecule with long chain likely resulted in more extended conformation. In addition, under low normal load, low NaCl concentration could increase the friction of DNA film and the chain length revealed insignificant effect on the friction force of DNA film. Therefore, long chain DNA with low salt concentration is more conducive to the nanopore sequencing process, since extended conformation can make DNA molecules easier to reach into nanopore and the high friction can reduce the translocation speed. These results may benefit the development of the third-generation sequencing technique based on nanopore.

The single-chain mechanics of two similar thermosensitive polymers, poly(N,N-diethylacrylamide) (PDEAM) and poly(N-isopropylacrylamide) (PNIPAM), have been studied by atomic force microscopy-based single-molecule force spectroscopy (SMFS). In a typical nonpolar organic solvent, octane, both of the polymers show the same inherent elasticity, although they have different substitutional groups. However, the mechanics of the two polymers presents large differences in water. The energies needed for the rearrangement of the bound water during elongation at room temperature are estimated by the SMFS method at the single-chain level, which is ~1.13 ± 0.10 and ~5.19 ± 0.10 kJ/mol for PDEAM and PNIPAM, respectively. In addition, PNIPAM shows a temperature-dependent single-chain mechanics when the temperature is increased across the lower critical solution temperature (LCST), while PDEAM does not. These differences observed in aqueous solution originate from the different structures of the two polymers. With a hydrogen bond donor in the amide group, PNIPAM will be more hydrated when TLCST. When TLCST, PNIPAM will have larger changes in both conformation and hydration. These findings also suggest that PNIPAM is a good candidate for a thermo-driven single-molecule motor, while PDEAM is not.

Double-stranded DNA films (200 bp) were prepared on 3-aminopropyltriethoxy-silane (APS)-modified mica surfaces in the NaCl solution with concentration ranging from 0.001 to 0.1 M. By using an atomic force microscope, the force–distance curve and friction behaviour of each DNA film was studied in the NaCl solution that was used in the sample preparation. When the concentration of NaCl solution decreased from 0.1 to 0.001 M, the range of non-linear repulsion on DNA films increased from 8.6 to 29.1 nm, which suggested that lower salt concentration likely resulted in more extended DNA molecules. At the same time, the friction force on DNA films increased by 190% under a normal load of 2.5 nN, which was attributed to the conformation changes in the DNA molecules induced by the electrolyte ions. Therefore, low salt concentration is more conducive to the sequencing process, since it can not only make DNA molecules easier to reach into nanopore through extended conformation, but also reduce the passage rate by high friction. The results have important implications for the development of the third-generation sequencing technique based on nanopore.

The variation of the single-chain mechanics of poly(N-isopropyl-acrylamide) (PNIPAM) for the entire range of the methanol molar fraction (cmethanol) (0%e100%) in the water/methanol mixed solvent has been studied by AFM-based single-molecule force spectroscopy at room temperature (RT). In general, the single-chain mechanics of PNIPAM shows a reentrant variation upon the change of cmethanol from 0% to 100%. The possible mechanism for the reentrant variation of the single-chain mechanics is discussed. The current study may cast new light on the design of novel environmental responsive nano-devices in the future, when cmethanol changes from 16% to 0%.

Excluded-volume (EV) interaction, also known as the EV effect, can drive the collapse of polymer chains in a polymer solution and promote the crystallization of polymer chains. Herein we report, for the first time, the effect of EV interaction on the single-chain mechanics of a polymer, poly(ethylene glycol) (PEG). By using AFM-based single-molecule force spectroscopy, the single-chain mechanics of a PEG chain has been detected in various nonpolar organic solvents with different molecule sizes. It is observed that the nonpolar solvents can be classified into two categories. In the small-sized organic solvents (e.g., tetrachloroethane and n-nonane), PEG presents its inherent elasticity, which is consistent with the theoretical single-chain elasticity from quantum mechanical calculations. However, in the middle-sized solvents (e.g., n-dodecane and n-hexadecane), the single-chain entropic elasticity of PEG is influenced by EV interactions noticeably, which indicates that the PEG chain tends to adopt a compact conformation under these conditions. To stretch a PEG chain from a free state to a fully extended state, more energy (1.54 kBT per repeating unit) is needed in small-sized organic solvents than in middle-sized organic solvents. It is expected that a partially stretched PEG chain would shrink to some extent when the solvent is changed from a middle-sized organic solvent to a small-sized one. Accordingly, a novel design of a PEG-based single-molecule motor that works with solvent stimuli is proposed.

Radical polymerization from a single initiator molecule in a microenvironment is a nearly ideal system in which bimolecular termination, solution concentration, and viscosity changes could be neglected. In this study, we provide two facile methods of preparing polymers via atom-transfer radical polymerization (ATRP) under single-initiator conditions: tether initiators on planar substrates at superlow density through mixed self-assembled monolayers (SAMs) and encapsulated single initiators in microfluidic droplets. The molecular weight (MW) of the resultant polymers characterized by atomic force microscope-based single-molecule force spectroscopy (AFM-based SMFS) showed that the single-chain ATRP had an extraordinarily faster chain propagation rate (2 unit/s) on planar substrates and gave polymers with much higher MWs (10(5)-10(6) g/mol) than those obtained from traditional ATRP (10(3)-10(5) g/mol). The former method offered a general platform for single-chain polymer synthesis and investigation, and the latter could be amplified to obtain abundant single-chain polymers with ultrahigh molecular weight (UHMW) for commercial applications.

Poly(N-isopropyl-acrylamide) (PNIPAM) is a paradigm thermally sensitive polymer, which has a lower critical solution temperature (LCST) of ~32 °C in water. Herein by AFM-based single molecule force spectroscopy (SMFS), we measured the single chain elasticity of PNIPAM across the LCST in water. Below LCST, the force curves obtained at different temperatures have no remarkable difference; while above LCST, an unexpected temperature dependent elasticity is observed, mainly in the middle force regime. We found that 35 °C is a turning point of the variation: from 31 to 35 °C, the middle parts of the force curves drop gradually, whereas from 35 to 40 °C, the middle parts rise gradually. A possible mechanism for the unexpected temperature dependent mechanics is proposed. The single chain contraction against external force upon heating from 35 to 40 °C may cast new light on the design of molecular devices that convert thermal energy to mechanical work.