The behavior of nanocarriers, even though they are well-defined at equilibrium conditions, is unpredictable in living system. Using the complexes formed by plasmid DNA (pDNA) and K20 (K: lysine), protamine, or polylysine (PLL) as models, we studied the dynamic behavior of gene carriers in the presence of fetal bovine serum (FBS) and under different shear rates, a condition mimicking the internal physical environment of blood vessels. Without shear, all the positively charged complexes bind to the negatively charged proteins in FBS, leading to the formation of large aggregates and even precipitates. The behaviors are quite different under shear. The shear generates two effects: a mechanical force to break down the complex into smaller size particles above a critical shear rate and a stirring effect leading to secondary aggregation of complexes below the critical shear rate. In the studied shear rate from 100 to 3000 s–1, the mechanical force plays a key role in K20/pDNA and protamine/pDNA, while the stirring effect is dominant in PLL/pDNA. A model study shows that the interfacial tension, the chain density, and the elasticity of the complexes determine their responsiveness to shear force. This study is helpful to understand the fate of drug/gene carriers under physiological conditions. It also gains insight in designing drug/gene carriers with desirable properties for in vivo applications.

Physically coating liposomes with peptides of desirable functions is an economic, versatile, and less time-consuming approach to prepare drug delivery vehicles. In this work, we designed three peptides—Ac-WWKKKGGNNN-NH2 (W2K3), Ac-WWRRRGGNNN-NH2(W2R3), Ac-WWGGGGGNNN-NH2(W2G3)—and studied their coating ability on negatively charged liposomes. It was found that the coating was mainly driven by the electrostatic interaction between the peptides’ cationic side groups and the acidic lipids, which also mediated the “anchoring ” of Trp residuals in the interfacial region of lipid bilayers. At the same conditions, the amount of the coated W2R3 was more than that of W2K3, but the stability of the liposome coated with W2R3 was deteriorated. This was caused by the delocalized charge of the guanidinium group of arginine. The coating of the peptide rendered the liposome pH-responsive behavior but did not prominently change the phase transition temperature. The liposome coated with peptides displayed appropriate pH/temperature dual responsive characteristics and was able to release the content in a controlled manner.

The mutual interaction between enzymes and their environments plays a key role in various life processes. In this study, using the complexes formed by salmon DNA and a de novo designed peptide, Ac-RRRRRRRRRGALGLPGKGGGLQRLTALDGR-NH2 (abbreviated as RR-30), as a model, we studied the activity of collagenase encapsulated inside the complex. Collagenase is able to cleave RR-30 at a LG/LP site, generating two shorter length peptides, which decreases the stability of the complex. Results show that the complex dissociates with time in the presence of collagenase. The dissociation rate is linearly proportional to the collagenase concentration. On the other hand, the collagenase activity is severely deteriorated inside the complex, where only 1/3 of the enzyme is active. We attribute it to the electrostatic interaction and hydrophobic interaction between collagenase and the components of the complex. Therefore, the mutual interaction determines the structure and kinetics of the DNA/peptide complex.

Human serum albumin (HSA) is the most abundant plasma protein and has an inherent ability to target tumor cells. It is an excellent candidate for drug delivery. However, HSA cannot form complex with DNA or RNA, because it is negatively charged under physiological conditions. In this work, we reported a simple method to prepare HSA/RNA nanoparticles mainly by physical interaction. Firstly, the solution pH is adjusted to 4.0, under which condition HSA is positively charged. It forms complex with RNA via electrostatic interaction. The solution is then heated at 75 oC for 15 min to stabilize the structure and the size of the formed complex. The HSA/RNA nanoparticle prepared by this method has a diameter about 110 nm and a narrow distribution. It is also stable for days under physiological conditions. Cellular essays demonstrate that these particles exhibit a high cellular uptake efficiency and non-toxicity to HeLa cells.

Using two series of de novo designed antimicrobial peptides, we studied the effects of peptide length and hydrophobicity/charge (H/C) ratio on the antimicrobial activities. For these peptides, a correlation was established between their antimicrobial efficacy and the leakage, aggregation, and fusion activities on artificial membrane. The results showed that peptides with an H/C ratio of 1.3, and a length of about half of the membrane thickness caused most potent membrane leakage, and negligible membrane aggregation and fusion. In addition, such peptide also exhibited the highest antimicrobial activity. Analysis of the hydrophobic and electrostatic interactions showed that the strength, the order and the position of these interactions determined the activities of the peptides on the artificial membrane and thus the antimicrobial efficacy. Further analyses on the tilt angle of the peptides on the membrane surface indicated that the peptides distorted the membrane in a dynamic mode, instead of being fixed in the membrane at a constant angle.

The effect of chain rigidity on the mechanic properties of DNA hydrogels was studied. Counterintuitively, the hydrogel formed by mainly flexible chains exhibited better stability, stretchability, and much mechanical properties than the hydrogel containing only rigid chains. Calculations showed that the crosslinking ratio in the hydrogel formed by flexible chains was about twice that of the hydrogel formed by rigid chains under the same conditions. We attributed this to the ease of conformational adjustment of flexible chains. Incorporation of 25% rigid chains further improved the performance of DNA hydrogel by shrinking the pore size and tuning its distribution.

The structure and stability of polyelectrolyte complex are controlled not only by electrostatic interaction but also by hydrogen bonding and hydrophobic interaction if they are present. The complexes formed by such multiple interactions should exhibit different responses to the environmental changes, such as ionic strength and pH. In this work, we designed a positively charged peptide S5R4, which can interact with poly(ethylene glycol)-block-poly(glutamate sodium) (PEG114-PGlu64) via electrostatic interaction, hydrogen bonding, and hydrophobic interaction. In deionized water at pH 7.1, the complexes formed by PEG114-PGlu64 and S5R4 assemble into wormlike micelles, spheres, and even hierarchical “wool balls”, depending on mixing ratio. However, a distinct dissociation–reassembly process is observed when 30 mM NaCl is added to screen the electrostatic interaction. The spheres transform into loose clusters after reassembly. This process is caused by the switch of driving force from electrostatic interaction to hydrogen bonding. Similarly, when the driving force is switched from electrostatic interaction to hydrophobic interaction by increasing solution pH to above 8.7, the original structure quickly dissociates and reassembles into dense aggregates. The rich structures formed by polyelectrolyte complexes and their drastic and sensitive responses to environmental changes are helpful to understand the working mechanism of biomolecules regulated by pH or ion strength.

The kinetics of DNA assembly is determined not only by temperature but also by the flexibility of the DNA tiles. In this work, the flexibility effect was studied with a model system of Y-junctions, which contain single-stranded thymine (T) loops in the center. It was demonstrated that the incorporation of a loop with only one thymine prominently improved the assembly rate and tuned the final structure of the assembly, whereas the incorporation of a loop of two thymines exhibited the opposite effect. These observations could be explained by the conformation adjustment rate and the intermotif binding strength. Increasing DNA concentration hindered the conformational adjustment rate of DNA strands, leading to the formation of hydrogels in which the network was connected by ribbons. Therefore, the gel can be treated as a metastable state during the phase transition.

Under an electric field, the complexes formed by DNA and polylysine exhibit novel features, such as selective merging of particles, ejecting of daughter vehicles, and differentiation of particles of varying mobility. The mobility of the complex could be three times faster than that of free DNA.

Quantitatively describing macromolecular confinement is still a challenge. Using the assembly of DNA tiles in a polyacrylamide network as a model, we studied the effect of macromolecular confinement on the growth of the filament by scaling theory. The results show that the confinement regulates the morphology, the initial growth rate v, and the eventual length of the filament Nm. The initial growth rate is dependent on the medium viscosity η as ν ∝ η−0.94, and the filament adjusts its length in the given confined space as Nm ∝ (ξ/Rg)1.8, with ξ being the mesh size of the polyacrylamide solution and Rg being the radius of gyration of polyacrylamide.

The mechanism of DNA assembly is revealed by analyzing the energy barriers during nucleation and growth. The assembly is controlled by two competing parameters: the conformation adjustment rate of DNA strands and the spreading rate of new strands on the nuclei surface, both of which are temperature dependent and can be tuned by sequence design.

How polyamines such as spermidine cooperate with histone to condense and de-condense DNA during transcription has not been clarified. In this work, using the complex of DNA and poly(L-lysine) (PLL) at +/− ratio of 0.5 as a model of nucleosome, we monitored the de-condensation of DNA in the presence of spermidine. As revealed by the results from atomic force microscopy and time-resolved laser light scattering, spermidine was able to transform the spherical complex into a core–shelled structure, with the hard core being the DNA–PLL complex and the soft shell being DNA and spermidine. The soft shell evolved into a coiled DNA conformation with time. Such a local de-condensation process should be helpful in understanding the DNA transcription and cell division process in vivo.

Polyelectrolyte complexes (PECs) are of great importance in drug delivery and gene therapy. The density and the distribution of the charges are key parameters of a polyelectrolyte, determining the structure of the complex and the kinetics of the complexation. Using peptides of precisely-controlled charge density as model molecules, we showed that the presence of weakly-charged peptides, (KGGG)5 or (KGKG)5, did not affect the complexation of highly-charged peptides (KKKK)5 with 21 bp oligonucleotides. However, peptide containing blocks of different charge densities, such as (KKKK)5-b-(KGGG)5 or (KKKK)5-b-(KGKG)5, exhibited superior performance during complexation. With a relatively uniform small size, the complex was also stable in serum. More importantly, the cellular uptake of the complex was greatly enhanced by a ratio of 40–60%, compared to that of the complex formed by uniformly-charged peptides. We attributed the improvement to the structure of the complex, in which the highly-charged blocks form the core with the oligonucleotide whilst the weakly-charged blocks dangle outside, preventing the complexes from further aggregation.

Kanamycin A, an amino modified sugar, can interact with poly(ethylene glycol)-block-poly(glutamate sodium) (PEG114–PGlu64) via electrostatic interactions (with PGlu) and hydrogen bonding (with PEG). The interplay of these two forces determines the assembly process and the resulting structure. In deionized water, kanamycin A and PEG114–PGlu64 form a spherical structure at [+]/[−] = 3.5. This structure dissociates instantly and completely in the presence of 30 mM NaCl. However, a new structure is reassembled in about 2 hours. A similar phenomenon is observed when the buffer pH is increased from 7.8 to 8.3. We attribute the distinct dissociation/reassembly process to the reestablishment of the balance between electrostatic interactions and hydrogen bonding. The dissociation/reassembly process in response to environmental changes offers a novel approach to release the loaded cargo in a controlled manner.

The endomembrane system, including the endoplasmic reticulum, Golgi apparatus, lysosomes, and endosomes, is located in the crowded intracellular environment. An understanding of the cellular structure and functions requires knowledge of how macromolecular crowding and confinement affect the activity of membrane and its proteins. Using negatively charged liposome and the peptide K3L8K3 as a model system, we studied the aggregation behavior of liposome in a matrix of polyacrylamide and hyaluronic acid. Without matrix, the liposomes form spherical aggregates in the presence of K3L8K3. However, they orient in one dimension and fuse into a tube up to 40 μm long in the matrix. The growth of the tube is via end-to-end connection. This anisotropic growth is mainly due to the macromolecular confinement provided by the polymer network. The study of the interactions between liposome and peptide in the crowded environment helps to reveal the mechanism of membrane-related processes in vivo.

The complexes formed by DNA or siRNA interacting with polycations showed great potential as nonviral vectors for gene delivery. The physicochemical properties of the DNA/siRNA complexes, which could be tuned by adjusting the characteristics of polycations, were directly related to their performance in gene delivery. Using 21 bp double-stranded oligonucleotide (ds-oligo) and two icosapeptides (with the repeating units being KKGG and KGKG, respectively) of the same charge density as model molecules, we investigated the effect of charge distribution on the kinetics of complexation and the structure of the final complexes. Even though the distribution of the charged groups in peptides was only adjusted by one position, the complexes formed by (KKGG)5 and ds-oligo were larger in size and easier to precipitate than those formed by (KGKG)5. Counterintuitively, it was not the charged groups but the hydrophilic neutral spacers that determined the kinetics and the structure of the complex. We attributed such an effect to the water-mediated disproportionation process. The hydrophilic spacers next to each other were better than that in the separated pattern in holding water molecules after forming the complex. The water-rich domains in the complex functioned as a lubricant and facilitated the relaxation of the polyelectrolyte, resulting in a fast complexation process. The resulting complex was thus larger in size and lower in surface energy.

Aminoglycosides are capable of expelling water molecules when forming a complex with DNA via electrostatic interaction. The “water-proof” nature of the complex leads to the formation of capsules, which possess hierarchical shell structures with a smooth and rigid outer layer and a viscoelastic inner layer.

The phase behaviors of the complex formed by didodecyldimethylammonium bromide (DDAB) and cetyltrimethylammonium bromide (CTAB) interacting with three different types of DNAs, salmon testes DNA (∼2000 bp), 21-bp double-stranded oligonucleotides (oligo-dsDNA), and 21-nt single-stranded oligonucleotides (oligo-ssDNA) were studied by synchrotron small-angle X-ray scattering. It was found that the DNA length and flexibility, together with the positive/negative charge ratio, determined the final structure. At higher charge ratios, the DNA length exhibited negligible effect. Both oligo-dsDNA and salmon DNA formed inverted hexagonal packing of cylinders with CTAB, as well as bilayered lamella with DDAB. However, at lower charge ratios, oligo-dsDNA formed a distorted hexagonal phase with CTAB and a new structure with DDAB, which was different from the behaviors of salmon DNA. The flexible oligo-ssDNA formed rich structures that were subject to environmental disturbance. Kinetic study also indicated that the structures of the complex formed by oligo-ssDNA took much longer to mature than the structures formed by oligo-dsDNA. We attributed this result to the conformational adjustment of oligo-ssDNA in the complex.

DNA complex has been widely used as non-viral vectors for the delivery of genes or siRNA. Owing to the strong and long-ranged electrostatic interaction, the structure and property of the DNA complex should evolve with time in a long-term. To test this hypothesis, we choose 2000 bp double-stranded DNA and 21 bp oligonucleotide as the model molecules, and comparatively studied their complexation with narrowly-distributed poly-l-lysine (PLL150) by time-resolved laser light scattering. In the time range of about one week, the complexation of both DNA samples underwent three stages: formation of preliminary complex, further aggregation, followed by precipitation. The aggregation and precipitation rate of the complex formed by oligonucleotide was much faster than that of the complex formed by 2000 bp dsDNA. After precipitation, the amount of the longer chain polyelectrolyte, as determined by UV and fluorescence, was about twice that of the short chain polyelectrolyte in the sediment. The precipitates were far from being fully neutralized. Moreover, the component ratio in the sediment was independent of the mixing charge molar ratio. A rational complex mechanism was proposed on the basis of these findings. During complexation, the relaxation of polyelectrolyte inside the complex and the exchange of polyelectrolyte between complex determined the aggregation and precipitation rate. The competition of the two kinetic processes governed the structure and property of the complex in the solution and in the sediment.

Using three designed peptides with precisely-controlled charge density and three types of DNAs with different length and flexibility, the effect of charge density on the formation of PEC was studied. Highly charged (KKKK)5 interacts strongly with 21 bp dsDNA to form large complex, followed by precipitation; while the medium charged (KGKG)5 only form complex with 21 bp dsDNA at proper +/− charge ratios; and no prominent complex between weakly charged (KGGG)5 and 21 bp dsDNA is observed at the same conditions. Similar trend is observed when the peptides form complex with 2000 bp DNA or 21nt ssDNA. It is also found that the complex formed by adding peptide to DNA is in random coil conformation, but the complex prepared by the inverse order is in molten globule state. Re-dissolution of the complex occurs only when DNA is added to peptides with similar or shorter length.

Physically coating liposomes with peptides of desirable functions is an economic, versatile, and less time-consuming approach to prepare drug delivery vehicles. In this work, we designed three peptides—Ac-WWKKKGGNNN-NH2 (W2K3), Ac-WWRRRGGNNN-NH2(W2R3), Ac-WWGGGGGNNN-NH2(W2G3)—and studied their coating ability on negatively charged liposomes. It was found that the coating was mainly driven by the electrostatic interaction between the peptides’ cationic side groups and the acidic lipids, which also mediated the “anchoring ” of Trp residuals in the interfacial region of lipid bilayers. At the same conditions, the amount of the coated W2R3 was more than that of W2K3, but the stability of the liposome coated with W2R3 was deteriorated. This was caused by the delocalized charge of the guanidinium group of arginine. The coating of the peptide rendered the liposome pH-responsive behavior but did not prominently change the phase transition temperature. The liposome coated with peptides displayed appropriate pH/temperature dual responsive characteristics and was able to release the content in a controlled manner.

Amyloid formation is now considered a universal and intrinsic property of all proteins, irrespective of their sequences. Therefore, it is interesting to see whether random copolymers of amino acids can also form amyloid aggregates. Here we use a copolymer of 4 amino acids, mimicking the clinically used drug Glatiramer, and demonstrate that it does form amyloid-like fibrils in the aqueous solution despite its random sequence structure. The fibrillar aggregates show an alanine-rich β-sheet secondary structure, proving the high tolerance of amyloid aggregates to the sequence irregularity in poly(amino acid)s, and suggesting the potential application of random copolymers as amyloid materials.

Biological membranes are heterogeneous systems. Their functions are closely related to the lipid lateral segregation in the presence of membrane proteins. In this work, we designed two peptides, amphiphilic cationic peptides K3L8K3 and nonamphiphilic peptides K20, and studied their interactions with binary liposomes in different phases (Lα, Lβ′, and Lα/Lβ′). As mimics of membrane proteins, both K3L8K3 and K20 can cause the liposomes to aggregate, fuse, or leak. These processes were closely related to the phases of liposomes. For the liposomes in Lα phase, heavy aggregation, fusion, and leakage were observed in the presence of either K20 or K3L8K3. For the liposomes in Lβ′ phase, neither K3L8K3 nor K20 can induce fusion or leakage. For the liposomes in Lα/Lβ′ phase, K3L8K3 caused the liposomes to aggregate, fuse, and leak, while K20 only led to aggregation. The kinetics of aggregation, fusion, and leakage in each phase were recorded, and they were related to the lipid demixing in the presence of the peptide. Our work not only gained insight into the effect of the lipid demixing on the interactions between peptide and membrane, but also helped in developing drug delivery vehicles with liposomes as the platform.

Polycations and cationic lipids have been widely employed as non-viral vectors for siRNA delivery. However, the stability of siRNA–polycation complexes is still rarely reported. Using highly charged dendrimer poly(amidoamine) (PAMAM) as an example, we studied the complexation of siRNA with a polycation by laser light scattering and atomic force microscopy. PAMAM forms a complex of more than 100 nm in diameter with siRNA right after mixing. However, the complex is not stable. It experiences an interesting kinetic process of aggregation, followed by phase separation. The rate of phase separation is related to the chemical composition of the polycations. Gene expressions on A549 cells and Hela cells demonstrate that the transfection efficiency decreases with the kinetics of the complexes. Cross-linking of the primary amino group of the PAMAM stabilizes the siRNA complex and causes the transfection results to be steady with time.

We describe an efficient approach to modulate the hydrolysis of the ortho ester-containing polymers by incorporating different amount of tertiary amino (TA) groups. The block copolymers (NE0–NE3) contain a hydrophilic poly(ethylene glycol) (PEG) segment and hydrophobic chains constituted by random methacrylate copolymers with pendent acid-labile cyclic ortho ester and TA groups and were synthesized by a two-step approach. First, atom transfer radical copolymerization of 2-hydroxyethyl methacrylate and 2-(diethylamino)ethyl methacrylate using mPEG45-Br as a macroinitiator afforded the block copolymer precursors. Then, reaction of the precursors with 2-ethylidene-4-methyl-1,3-dioxolane transformed the pendent hydroxyl groups into cyclic ortho ester groups. The hydrophobic chains are similar in degree of polymerization, but the percent molar content of TA increases from 0% for NE0 to ∼15% for NE3. In phosphate buffer at pH 8.4, all the four amphiphilic block copolymers can self-assemble into stable nanoparticles with a monomodal distribution, which have a similar hydrophilic/hydrophobic balance as revealed by using pyrene and Nile red (NR) as fluorescent probes. Kinetic measurements of the ortho ester hydrolysis in the copolymer nanoparticles were studied at different pH values by 1H NMR spectroscopy, NR fluorescence probe, and light scattering. The results indicated that all the copolymer nanoparticles exhibit the pH-dependent hydrolysis behaviors with the half-life times ranging from hundreds of minutes at pH 5.4 to several days at neutral pH. More importantly, we found that the TA units have an amphoteric effect on the hydrolysis kinetics of the surrounding pendent ortho esters in acidic media. When compared with copolymer nanoparticles of NE0 with no TA unit, the NE1 nanoparticles with a small amount of TA unit hydrolyzed much slower, whereas a faster hydrolysis was observed for the NE3 nanoparticles containing a higher amount of TA unit.

The polyelectrolyte complexes formed by DNA and polycations have been widely used as non-viral vectors for gene delivery. The DNA complex prepared in aqueous solutions is usually controlled by kinetics. In this work, we demonstrated, by using salmon testes DNA (2000 bp), poly-(L-lysine) (PLL35), and poly(ethylene oxide)-b-Poly-(L-lysine) (PEO45-b-PLL35), that the DNA complexes formed in dimethylformamide (DMF) was milder than those formed in aqueous solutions. In DMF, no precipitation of DNA complex is observed even at the stoichiometric charge ratio. Taking advantage of the weak complexation and better solubility of complex in DMF, we premixed DNA and PLL at higher concentration in DMF, followed by quenching the mixture into a large amount of aqueous solution. This method alleviates the kinetic control to a large extent, and the complex obtained is smaller in size and lower in molar mass than those prepared solely in aqueous solution. The morphology of the complex, as studied by AFM, is also different.

Polycations and cationic lipids have been widely used as non-viral vectors for the delivery of plasmid DNA, siRNA and anti-sense oligonucleotides. To demonstrate that one polycation can form a complex with several types of DNA, we conducted a comparative study on the complexation of poly(L-lysine) (PLL) with 2000 bp salmon testes DNA (dsDNA), 21 bp double-stranded oligonucleotides (ds-oligo), and 21 nt single-stranded oligonucleotides (ss-oligo) in PBS buffer. The complexes are prepared by a titration method and the process is monitored by laser light scattering. It was found that in most cases, ss-oligo and ds-oligo form complexes with higher molecular weights than the complex formed by dsDNA at the same +/− ratio immediately after mixing. More importantly, the complexes formed by oligonucleotides are not stable, the scattered intensity gradually decreases to the level of the solvent in weeks. Atomic force microscopy measurements also indicate that the freshly prepared complex is subject to environmental changes and could dissociate very quickly. The behaviour of oligonucleotides cannot be predicted by the classical polyelectrolyte theories.

Liposome-embedded hydrogels have been widely used for controlled drug release. In this work, by embedding egg phosphatidylcholine (EPC) liposome into a poly(N-isopropylacrylamide) (PNIPAm) hydrogel via chemical cross-linking, we systemically studied the physical interactions between the liposome and hydrogel matrix, as well as the release mechanism of the encapsulated content in the liposome at varying temperatures. It was found that the confinement of the network and the hydrophobic interactions between the liposome and PNIPAm determined the integrity of the liposome and, more importantly, the release profile of the encapsulated content, such as calcein. This liposome-embedded stimuli-responsive system is suitable for the delivery of mixed drugs with different release profiles, and easily achieves on-demand release.

C60(OH)8 is a fullerenol with all the hydroxyl groups located in the same hemisphere. It combines the fascinating assembly behaviour of a Janus particle and the unique properties of a fullerene. Since the range of interparticle hydrophobic attraction in water is much longer than the particle size, C60(OH)8 has a strong tendency to form aggregates. At concentrations above 1.0 × 10−6 M, the Rg of the aggregate is 110 nm and the aggregation number reaches 2.5 × 105. However, the conformation of the aggregate is not condensed, C60(OH)8 can still adjust its position by transportation or rotation. Therefore, when deposited on the surface, C60(OH)8 assembles into a spherical structure or a single-layered fiber on the hydrophilic surface, but it forms a toroidal structure with concentric rings on the hydrophobic surface. If illuminated with light during assembly, fibers composed of multi-braids are observed on the hydrophilic surface. Our work demonstrated that the assembly of Janus fullerenol offered a new approach to prepare novel carbon structures.

Block copolymers containg poly(ethylene oxide) (PEO) have been widely used in biomedical applications. The PEO block is generally treated as the hydrophilic moiety. In this work, we demonstrated that PEO could serve as the “hydrophobic” block under certain conditions by using PEO-b-poly(vinyl alcohol) (PEO-b-PVA) as the example. Water is a non-selective solvent for both of the blocks, however, the addition of NaCl decreases the solubility of PEO in aqueous solution while it shows no effect on the PVA block. With the solubility of PEO being deteriorated by adding NaCl, PEO-b-PVA exhibited amphiphilic feature in aqueous solution with PEO being the “hydrophobic” block. Therefore, by just adding salts, uniform gel-like particles were formed by PEO-b-PVA of proper block ratio. These solid particles formed by dual hydrophilic copolymers with excellent biocompability may have great potential in biomedical applications.

Using poly(vinyl alcohol)-b-poly(N-isopropylacrylamide) (PVA-b-PNIPAm) as an example, we demonstrate the concept of aggregation-induced gelation of PVA in dilute solution. PVA-b-PNIPAm forms aggregates in dilute solution at temperature above the transition point of PNIPAm, which significantly increases the local concentration of PVA chains. Therefore, PVA chains have a better chance to crystallize or to generate small crystalline nuclei, which function as cross-linkers in the later stages. When the solution is cooled to room temperature, PVA-b-PNIPAm gradually forms microgels at the time scale of days. We further investigated the effects of block ratio, heating rate, and urea concentration on the gelation process. It is found that larger PVA ratio, longer aging time and less urea content facilitated the gel formation. Rheological measurement at semidilute concentration also confirms gel formation after the heating/cooling treatment. The aggregation-induced gelation offers a new approach to prepare PVA gels without the introduction of toxic chemical cross-linkers.

The solubility and chain conformation of different types of homopolymers in low viscosity ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]) at 50 °C and 1-butyl-3-methylimidazolium formate ([BMIM][COOH]) at 25 °C, were studied by laser light scattering (LLS). For neutral polymers, such as polyvinyl alcohol and polysulfonamide, aggregation occurred in all the cases except for polyvinyl alcohol in [BMIM][COOH]. For negative polyelectrolytes, such as DNA and polystyrene sulfonate, single chain conformation was observed. However, the hydrodynamic radius of both polymers was much smaller than that in good solvents, suggesting that the chains were condensed. Cellulose was soluble in [AMIM][Cl], and non-diffusive mode was observed by dynamic light scattering. Zeta potential analysis indicated that cellulose exhibited the feature of polyelectrolyte. The solubility of homopolymers could be qualitatively explained by treating polymer/IL as a ternary system: polymer, cation, and anion. It was the mutual interactions determined the solubility and conformation of polymers in ILs.

The effects of heating rate on the aggregate behavior of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions were investigated in detail by laser light scattering and TEM. By employing two separate heating protocols, step-by-step heating at < 5 K/step and one-step jump, to heat the sample from 15°C to the selected temperature, we found that the heating rate only showed significant effect on the aggregates above the cloud point. The aggregate formed by step-by-step heating exhibited a much larger size and a broader size distribution than those formed by one-step jump heating. Moreover, neither of the aggregates were ideal micellar structures as indicated by the size and the Rg/Rh values. On the contrary, at temperatures below the cloud point where the block copolymer formed core-shelled micelles, the heating rate showed negligible effect on the size and size distribution of the micelles. Since the system underwent a phase separation above the cloud point, the heating rate effect could be reasonably explained by the phase separation mechanisms: the nucleation-and-growth mechanism in the metastable region and the spinodal decomposition mechanism in the unstable region.

Using poly(acrylic acid) (PAA) and polyacrylamide (PAAm) as an example, we studied the mechanism of hydrogen-bonding complexation in dilute solution by both static and dynamic light scattering. In 20 mM phosphate buffer at pH = 3, the condition suitable for complexation of PAA and PAAm, neither PAA nor PAAm stayed as individual polymer chains at 8.0 mg/mL. Instead, most of PAA formed “multimacroion clusters” (or slow mode) due to the negative charges, and a small portion of PAAm formed associates mainly via hydrogen bonds. After PAA and PAAm solutions were mixed at stoichiometric ratio at room temperature, the slow mode, with PAA being dominant, persisted. The hydrogen-bonding complexation induced by cooling was evolved from the slow mode rather than from the single PAA or PAAm chains. Therefore, at the temperature where phase separation occurred, a fairly large amount of free PAAm chains still existed in the system. Our study demonstrated that the growth of the hydrogen-bonding complex, as well as the optimal ratio for complex, was highly dependent on the status of the component polymers before complexation.

Time-resolved laser light scattering, combined with transmission electron microscopy, was employed to study the kinetics of the sphere-to-rod transition. The brush−rod block copolymer based on poly[poly(ethylene glycol) monomethyl ether methacrylate] and poly{(+)-2,5-bis[4′-((S)-2-methylbutoxy)phenyl]styrene}, PEGMA37-b-MBPS141, formed spherical large compound micelles (LCM) in mixed solvent of THF and water. LCM underwent the transition to large compound rod (LCR) in the time scale of hours, and the transition was favored at higher polymer concentration and at intermediate water content. It was also found that a large amount of free polymer chains existed throughout the transition process. The depletion force generated by the free polymer chains was estimated to be in the order of 0.1kT. Therefore, besides the instability or defects of the LCM, the depletion effect also made a positive contribution to the sphere-to-rod transition.

The reentrant condensation of 21-bp oligonucleotide in the presence of spermidine was investigated by laser light scattering and capillary electrophoresis. 21-bp oligonucleotide showed a bimodal distribution in 1×TE buffer, with the slow mode being the characteristic diffusion of polyelectrolyte in solution without enough salt. At the fixed spermidine concentration, the reentry of oligonucleotide underwent aggregation, phase separation, and disassociation in sequence with time, and the kinetics was extremely slow. For example, it took more than 1200 h (50 days) for the reentry to complete at 21 mM spermidine. Higher spermidine concentration led to faster kinetics. After reentry, the slow mode disappeared, and the charges of oligonucleotide were at least partially neutralized. No prominent charge inversion was observed. The kinetics of oligonucleotide reentry in the presence of spermidine gained insight in the interactions of polyelectrolyte in aqueous solution.

The complexes formed by bovine serum albumin (BSA) with single-stranded oligonucleotide (ss-oligo) or double-stranded oligonucleotide (ds-oligo) were investigated by laser light scattering, zeta potential analysis, and atomic force microscopy. It was found that BSA was able to recognize ss-oligo and ds-oligo upon forming complexes in HCOOH-HCOONa buffer at pH 3.0. When oligonucleotide was added dropwise to BSA, BSA formed a complex with ss-oligo but not with ds-oligo in the studied charge ratio. When BSA was added to oligonucleotides, BSA formed complexes with both ss-oligo and ds-oligo but via different paths: the BSA/ds-oligo underwent two processes, heavy precipitation followed by reentry, with increasing BSA/oligo charge ratio, whereas BSA/ss-oligo underwent only aggregation process, but with a charge reversal occurred at BSA/oligo charge ratio about 0.1. Moreover, the complex formed by BSA and ds-oligo showed a kinetics much slower than that of BSA and ss-oligo. We attributed the big difference upon complexation to the physical nature of oligonucleotides as well as the conformational change of BSA under severe conditions. The differentiation of ss-oligo from ds-oligo by BSA via nonspecific interactions gained insight in the recognition of DNA or RNA by specific protein (enzyme) under physiological conditions.

A series of thermo- and pH-dual responsive dendronized polymers PSADG1−PSADG3 were successfully synthesized by attaching butylamide terminated poly(amidoamine) dendrons (DG1−DG3) to the alternating copolymers of styrene (St) and maleic anhydride (MAh). The structures and the molecular weights of the obtained polymers were characterized by 1H NMR and FTIR. The coverage degrees of DG1−DG3 dendrons were 83.5%, 64.9%, and 60.5%, respectively, indicating that the numbers of the attaching dendrons decreased in the order of G1 > G2 > G3. The turbidity measurements revealed that all the dendronized polymers exhibited reversible thermo-responsive property in deionized water, and the LCST values increased from PSADG1 to PSADG3 due to both the coverage degree and the structure of dendrons. Moreover, these dendronized polymers were sensitive to pH on account of the carboxylic and tertiary amino groups in the architecture. It has been found that, for example, the PSADG2 did not display thermal sensitivity in acidic environment (pH 2.4), whereas the phase transition occurred in near neutral (pH 6.0) and basic conditions (pH 10.6) with the LCST of 33.1 and 49.0 °C, respectively. It was also found that, at pH 6.0, the polymers formed larger aggregates with Rh,app of ca. 0.6 μm at elevated temperatures mainly because of the dehydration and the enhanced hydrophobic interactions. The morphology of the aggregates was monitored by optical microscopy, and uniform spherical aggregates with a diameter of 5−10 μm were observed above the LCST.

The behavior of poly(ethylene oxide)-block-poly(N,N-dimethylacrylamide) (PEO-b-PDMA) in aqueous solution at dilute concentrations was studied by laser light scattering. Even though water was a nonselective solvent for both PEO and PDMA blocks, PEO-b-PDMA formed a certain loose associate in aqueous solutions. Different from the micellization of block copolymers in selective solvents, the association of PEO-b-PDMA in aqueous solution showed weak concentration and temperature dependence as well as opposite salt effect. At 4.0 M NaCl, the association was completely suppressed and PEO-b-PDMA existed as individual polymer chains. This opposite “salting out” effect revealed the association mechanism of block copolymers in nonselective solvent: the solubility of PEO block in aqueous solution was unique, and it exhibited a better hydrophilicity and thick hydration shell than other water-soluble polymers. This inequality or weak incompatibility drove the block copolymer to associate into loose structures containing multidomains rich of either blocks. The dependence of the association on the block ratio also supported our conclusions.

The solvent-induced association and micellization processes of amphiphilic rod−coil diblock copolymers, poly(ethylene oxide)-b-poly{(+)-2,5-bis[4’-((S)-2-methylbutoxy)phenyl]styrene} (PEO104-b-PMBPSn, with n being 17, 30, 45, 53, and 106, respectively), were studied in situ by laser light scattering. In dioxane, a selective solvent for PMBPS block, the copolymer preserved the single-chain conformation. With the addition of water, a selective solvent for PEO block, the block copolymer underwent two difference processes: the association governed by PEO and the micellization governed by PMBPS. Because PMBPS would be dominant at elevated water content, three occurrences, the formation of associate, the formation of micelles, and the breakdown of associate, were observed in sequence with increasing water content. However, under the conditions where the rate of disassociation was much slower than that of micellization, the structure of the associate was “frozen” by the rod blocks, and mixed structures of associate and micelle were obtained. The associate formed at elevated water content was controlled by kinetics. Its formation was mainly determined by the PEO/PMBPS block ratio and the water addition rate.

An orthoester-containing monomer, trans-N-(2-ethoxy-1,3-dioxan-5-yl)acrylamide (tNEA), was synthesized. Atom transfer radical polymerization of tNEA using a poly(ethylene glycol) (PEG) macroinitiator afforded three acid-labile thermoresponsive block copolymers: PEG-b-PtNEA27, PEG-b-PtNEA56, and PEG-b-PtNEA73. These block copolymers are water-soluble at low temperatures (<13 °C). Thermally induced phase transition behaviors, including the critical aggregation temperatures (CATs), of these polymers were investigated by light scattering and 1H NMR. The results indicated that the longer the PtNEA chain length, the lower the CAT. Upon heating above the CATs, all the three polymers underwent a phase transition and formed polymeric micelles or micelle-like nanoparticles with PEG as the shell and PtNEA block as the core. Both the sizes and morphologies of the micelles were found to be affected by the heating rate and the salt concentration in the buffers. The micelles, formed through a fast heating procedure in the buffer with a relatively high salt concentration, have a smaller size and a more compacted structure. pH-dependent destabilization of the polymeric micelles prepared from PEG-b-PtNEA73 was studied by using light scattering and Nile Red fluorescence. The results demonstrated that hydrophobic Nile Red could be loaded in the micelles that were stable at pH 7.4, but destabilized in mildly acidic media. The dissociation of the micelles and the subsequent release of Nile Red were induced by the acid-triggered hydrolysis of the orthoester groups, which was proved by the 1H NMR spectra.

The well-defined block copolymer of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAm) was synthesized by the reversible addition fragmentation transfer (RAFT) polymerization, and its thermo-induced aggregation process in dilute aqueous solution was studied by laser light scattering. At temperatures below the cloud point of PNIPAm, the PEO-b-PNIPAm chains were associated together to form large and loose structures, which were in a fast equilibrium with the single chains. The association was weakened with increasing concentration, which was against the common ideas about aggregation. Because of such “abnormal” behavior, PEO-b-PNIPAm underwent three stages of transformation during the heating process at 0.1 mg/mL: the disassociation of loose associates at temperatures below 28 °C, the micellization above 42 °C, and the aggregation in between. At each stage, the size and Mw,app exhibited distinct features. On the basis of these observations, possible mechanisms of the association and aggregation were also proposed.

The polymerization-induced micellization of poly(ethylene oxide)-b-poly(styrene-alt-maleic anhydride) (PEO-b-P(S-alt-MAn)) in CHCl3 was monitored in situ by laser light scattering. Reversible addition−fragmentation chain transfer (RAFT) polymerization was used to grow the P(S-alt-MAn) block onto the PEO macromolecular chain transfer agents (macro-CTAs) at 55 °C. The whole process underwent three stages over time: the induction period, the formation of loose aggregates, and the formation of micelles. The polymerization process and the micellization process were affected mutually: the increase in the degree of polymerization induced the micellization of PEO-b-P(S-alt-MAn); meanwhile, the formation of core−shell micelles retarded the polymerization rate. At higher polymer concentrations, the mutual effect was even stronger, where smaller size micelles with higher density were obtained.

“Frequent hitter” phenomenon emerged in the high-throughput screening; one of the most common mechanisms behind artifactual inhibition is that some organic molecules formed large colloid-like aggregates which are able to sequester and thereby inhibit enzymes. To investigate the situation in Traditional Chinese Medicine (TCM), 60 medicinal herbs and 24 Chinese herbal formulae were detected by dynamic light scattering (DLS), and aggregates were observed in all the 84 solution mixtures. The aggregates of two Chinese herbal formulae, ‘Xue-Fu-Zhu-Yu Tang’ (XF) and ‘Jing-Guan Tang’ (JG), were not only able to survive in the gastro-intestinal environment, but also had the ability to pass through the monolayer of the Caco-2 cell. The activities of XF and JG against three cardiovascular targets were also aggregates-related. Based on these findings, a new possible mechanism of the action of Chinese medicine was proposed.

Aminoglycosides are capable of expelling water molecules when forming a complex with DNA via electrostatic interaction. The “water-proof” nature of the complex leads to the formation of capsules, which possess hierarchical shell structures with a smooth and rigid outer layer and a viscoelastic inner layer.

C60(OH)8 is a fullerenol with all the hydroxyl groups located in the same hemisphere. It combines the fascinating assembly behaviour of a Janus particle and the unique properties of a fullerene. Since the range of interparticle hydrophobic attraction in water is much longer than the particle size, C60(OH)8 has a strong tendency to form aggregates. At concentrations above 1.0 × 10−6 M, the Rg of the aggregate is 110 nm and the aggregation number reaches 2.5 × 105. However, the conformation of the aggregate is not condensed, C60(OH)8 can still adjust its position by transportation or rotation. Therefore, when deposited on the surface, C60(OH)8 assembles into a spherical structure or a single-layered fiber on the hydrophilic surface, but it forms a toroidal structure with concentric rings on the hydrophobic surface. If illuminated with light during assembly, fibers composed of multi-braids are observed on the hydrophilic surface. Our work demonstrated that the assembly of Janus fullerenol offered a new approach to prepare novel carbon structures.

Polyelectrolyte complexes (PECs) are of great importance in drug delivery and gene therapy. The density and the distribution of the charges are key parameters of a polyelectrolyte, determining the structure of the complex and the kinetics of the complexation. Using peptides of precisely-controlled charge density as model molecules, we showed that the presence of weakly-charged peptides, (KGGG)5 or (KGKG)5, did not affect the complexation of highly-charged peptides (KKKK)5 with 21 bp oligonucleotides. However, peptide containing blocks of different charge densities, such as (KKKK)5-b-(KGGG)5 or (KKKK)5-b-(KGKG)5, exhibited superior performance during complexation. With a relatively uniform small size, the complex was also stable in serum. More importantly, the cellular uptake of the complex was greatly enhanced by a ratio of 40–60%, compared to that of the complex formed by uniformly-charged peptides. We attributed the improvement to the structure of the complex, in which the highly-charged blocks form the core with the oligonucleotide whilst the weakly-charged blocks dangle outside, preventing the complexes from further aggregation.

Quantitatively describing macromolecular confinement is still a challenge. Using the assembly of DNA tiles in a polyacrylamide network as a model, we studied the effect of macromolecular confinement on the growth of the filament by scaling theory. The results show that the confinement regulates the morphology, the initial growth rate v, and the eventual length of the filament Nm. The initial growth rate is dependent on the medium viscosity η as ν ∝ η−0.94, and the filament adjusts its length in the given confined space as Nm ∝ (ξ/Rg)1.8, with ξ being the mesh size of the polyacrylamide solution and Rg being the radius of gyration of polyacrylamide.

Under an electric field, the complexes formed by DNA and polylysine exhibit novel features, such as selective merging of particles, ejecting of daughter vehicles, and differentiation of particles of varying mobility. The mobility of the complex could be three times faster than that of free DNA.

The mechanism of DNA assembly is revealed by analyzing the energy barriers during nucleation and growth. The assembly is controlled by two competing parameters: the conformation adjustment rate of DNA strands and the spreading rate of new strands on the nuclei surface, both of which are temperature dependent and can be tuned by sequence design.

Polycations and cationic lipids have been widely used as non-viral vectors for the delivery of plasmid DNA, siRNA and anti-sense oligonucleotides. To demonstrate that one polycation can form a complex with several types of DNA, we conducted a comparative study on the complexation of poly(L-lysine) (PLL) with 2000 bp salmon testes DNA (dsDNA), 21 bp double-stranded oligonucleotides (ds-oligo), and 21 nt single-stranded oligonucleotides (ss-oligo) in PBS buffer. The complexes are prepared by a titration method and the process is monitored by laser light scattering. It was found that in most cases, ss-oligo and ds-oligo form complexes with higher molecular weights than the complex formed by dsDNA at the same +/− ratio immediately after mixing. More importantly, the complexes formed by oligonucleotides are not stable, the scattered intensity gradually decreases to the level of the solvent in weeks. Atomic force microscopy measurements also indicate that the freshly prepared complex is subject to environmental changes and could dissociate very quickly. The behaviour of oligonucleotides cannot be predicted by the classical polyelectrolyte theories.