Shin-Ichiro Sato

Fullerene attracts much interest due to its potential in optoelectronic and biomedical applications. However, its insolubility in water and in many common organic solvents hinders further development. Herein, cyclized Pluronic L64 and 17R4 and cyclized poly(ethylene glycol) (PEG), namely, c-L64, c-17R4, and c-PEG, respectively, were used to disperse C60 and C70 in water as well as in ethanol and acetone, and the dispersion was shown to be governed by the polymer topology. Through simple ultrasonication in the presence of the cyclized polymers followed by centrifugation, C60 and C70 were dispersed without covalent modification of the fullerene structure. Under identical dispersion conditions, the cyclized polymers consistently outperformed their linear counterparts bearing either hydroxy or methoxy end groups: 1 g of c-L64 dispersed 0.53 g of C60, whereas HO–L64–OH and MeO–L64–OMe dispersed only 0.14 and 0.05 g, respectively, corresponding to a roughly 4-fold enhancement upon cyclization. The same topology-dependent trend was observed for the Pluronic 17R4 and PEG series and for C70, and it was accompanied by a smaller cluster size for every cyclized dispersant, as determined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Notably, this is the first dispersion of fullerene by a cyclized PEG homopolymer without any chemical modification, achieved solely by the topological transformation. Moreover, the complexes of fullerene and the cyclized polymers were redispersed in ethanol and acetone. Finally, because the fullerene structure remained intact, the dispersions exhibited a superior •OH radical scavenging ability compared with water-soluble polyhydroxylated fullerene, in which a substantial proportion of the π-conjugation was lost. This research proposes a facile dispersion method for intact fullerene based on the polymer topology effect, with potential biological applications such as photodynamic therapy and drug delivery systems.

Properties of polymers with diverse topologies are highly intriguing. The synthesis of 8-shape poly(ethylene glycol) (PEG) and its unique multicyclic topology effects including enhancement of the dispersion stabilization of gold nanoparticles (AuNPs) are studied. By employing liquid chromatography at critical conditions (LCCC), high-purity 8-shape and tadpole-shape PEGs were isolated, overcoming traditional difficulties in separating diverse molecular topologies with the same chemical structure in the repeating units. Through small-angle X-ray scattering (SAXS), it was demonstrated that increasing the number of rings significantly reduces the polymer’s radius of gyration (Rg) in solution. When applied to AuNPs, 8-shape PEG facilitated even more robust physisorption than previously reported monocyclic PEG, generating a thick, protective layer without chemical bonding to the nanoparticle surface, as characterized by dynamic light scattering (DLS), ζ-potential measurements, ultraviolet–visible (UV–Vis) spectroscopy, and small-angle neutron scattering (SANS). This topological design of polymeric materials provides superior dispersion stability under environmental conditions, offering a highly efficient, noninvasive alternative to traditional chemical modification methods of nanoparticles in various applications.

Dispersion stabilization of nanoparticles for catalytic reactions is an important issue. However, dispersing agents should be carefully selected not to hinder catalytic performance. In the present study, physisorption of cyclic poly(ethylene glycol) (c-PEG) onto platinum nanoparticles (PtNPs) was investigated in comparison with unmodified PtNPs (PtNPs/No PEG), PtNPs mixed with linear PEG (PtNPs/HO-PEG-OH), and PtNPs chemisorbed with HS-PEG-OMe (PtNPs/HS-PEG-OMe). DLS showed a significant increase in the particle size for PtNPs/c-PEG and PtNPs/HS-PEG-OMe compared to PtNPs/No PEG and PtNPs/HO-PEG-OH. ζ-potential measurements revealed values around −30 mV for PtNPs/No PEG and PtNPs/HO-PEG-OH, whereas PtNPs/c-PEG and PtNPs/HS-PEG-OMe approached 0 mV, which indicated that c-PEG and HS-PEG-OMe adsorb onto PtNPs to form a shielding layer. Moreover, PtNPs/c-PEG and PtNPs/HS-PEG-OMe were stable in a phosphate-buffered saline (PBS) solution, but PtNPs/No PEG and PtNPs/HO-PEG-OH immediately aggregated. This suggests that high dispersion stability by c-PEG is comparable to ordinary surface modification using HS-PEG-OMe. Furthermore, the catalytic ability of PtNPs/c-PEG and PtNPs/HS-PEG-OMe was compared in various reactions. As a result, physisorbed PtNPs/c-PEG showed suitable catalytic activities, whereas chemisorbed PtNPs/HS-PEG-OMe was significantly hampered by the blocking of the catalytic sites with thiol in some reactions. Thus, physisorption of c-PEG endows PtNPs with dispersion stability and maintains the catalytic ability, leading to an alternative way of modifying metal nanoparticles.

Defect-free cyclic poly(ethylene glycol) (c-PEG) has been previously reported to be physisorbed onto gold nanoparticles (AuNPs) to drastically enhance the dispersion stability. The present study investigates the distinct behaviors of the physisorption of c-PEG and the chemisorption of linear thiol-terminated PEG (HS-PEG-OMe) onto gold substrates and AuNPs. Thermogravimetric analysis (TGA) and quartz crystal microbalance (QCM) were used to quantify the adsorption and characterize the PEG layers, finding that c-PEG exhibits lower adsorption density and the adsorption mass and energy dissipation of the layer are mostly independent of the molecular weight, unlike HS-PEG-OMe. Furthermore, dynamic light scattering (DLS) and small-angle neutron scattering (SANS) assessed the complexes under varying temperatures, demonstrating that AuNPs modified with c-PEG maintain a consistent structure, whereas AuNPs with HS-PEG-OMe experience significant structural shrinkage as the temperature increases. The results suggest that c-PEG interacts with the gold surface at multiple points, leading to superior structural stability compared to that of the single-point attachment of HS-PEG-OMe, which is more prone to dehydration.

Fullerenes attract much interest due to the potentials to various applications. However, their insolubility in water and some organic solvents often hinders development. Herein, poly(ethylene glycol) (PEG), polyvinylpyrrolidone (PVP), poly(vinyl alcohol) (PVA), polyacrylic acid (PAA), and Pluronic L64 and 17R4, are attempted for dispersing fullerene in water via ultrasonication, and some of these are found to disperse an exceedingly large amount. The maximum concentration of C ₆₀ is achieved with Pluronic L64, being 9.8 g/L (13.6 m m ) without re‐aggregation for a long time. Electron paramagnetic resonance (EPR) spectroscopy shows a significant amount of radical species existing in the fullerene–polymer complexes, which are stable for several weeks. DLS and TEM exhibit the formation of nanoparticles, and NMR, FT‐IR, and MALDI‐TOF MS are used to characterize the fullerene nanoparticles–polymer complexes. The aqueous dispersions of the complexes can be dried and redispersed in water and polar organic solvents. Column chromatographic separation is performed to give unreacted fullerene and fullerene–polymer complexes, the latter of which shows a strong EPR signal. Density functional simulations reveal partial electron transfer from the PEG segment to fullerene, which causes charge separation in the complexes, resulting in the excellent dispersibility in water, while the poly(propylene glycol) (PPG) segment likely assists the complexation by hydrophobic interactions with fullerene.

Conjugated polymer nanoparticles (CP NPs) attract attention as nanoscale materials used for a variety of applications. In relation to this, the internal structure of CP NPs is an important factor for their properties, and numerous investigations have been carried out to control their nanomorphology. Here, we report the formation of CP NPs from defect-free cyclic poly(3-hexylthiophene) (c-P3HT) using a microfluidic device, and the effect of polymer topology on their structural and solvatochromic properties was investigated. CP NPs from c-P3HT exhibited reduced particle sizes and hypsochromic shifts in the absorption spectrum when compared to CP NPs obtained from corresponding linear P3HT (l-P3HT). Furthermore, steady responses in the solvatochromism of CP NPs from c-P3HT were observed, while those from l-P3HT displayed molecular weight dependency. These topology effects were caused by the change in the conjugation length, solubility, and crystallinity upon cyclization. Grazing incidence X-ray scattering (GIXS) studies of spin-coated P3HT films further showed a reduced interchain order and a larger proportion of face-on molecular orientation on a substrate for c-P3HTs. The various distinct structures observed for c-P3HT indicate the use of polymer topology as a means of nanostructure regulation.

Surface functionalization of gold nanoparticles (AuNPs) to impart colloidal stability is imperative for their applications, especially in biomedical fields. A drastic dispersion stabilizing effect of defect-free cyclic poly(ethylene glycol) (c-PEG) through their physisorption onto the surface of AuNPs was previously discovered, which was even superior to the de facto standard, thiolated PEG (HS-PEG-OMe). Here, we report a comprehensive characterization of the unique structure of the AuNPs/c-PEG complex using nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS). Furthermore, molecular dynamics (MD) simulations were employed to elucidate the distinct mechanism of physisorption. Quantification of the adsorbed c-PEG molecules by NMR revealed that the adsorption density (σ) is approximately 1 order of magnitude smaller than that of HS-PEG-OMe. Model-independent and -dependent analyses of the SANS data were used to further identify the relatively thin and sparse PEG layer as a characteristic of c-PEG adsorption. MD simulations of the physisorption events revealed the absolute value of free energy of adsorption for c-PEG to be greater than that for linear HO-PEG-OH. The favorable adsorption of c-PEG is suggested to arise from its less hydrated state and the larger number of water molecules that are released from the gold surface upon PEG adsorption.