Ultrasonic Scission Research Articles

Folate-Conjugated Halloysite Nanotubes, an Efficient Drug Carrier, Deliver Doxorubicin for Targeted Therapy of Breast Cancer

To carry doxorubicin (DOX) on breast cancer site effectively, halloysite nanotubes conjugated with poly(ethylene glycol) and folate (HNTs-PEG-FA) is designed as a targeted drug delivery system. Halloysite nanotubes (HNTs) are shortened to ∼200 nm by ultrasonic scission and functionalized with amide groups to conjugate with N-hydroxylsuccinimide-polyethylene glycol carboxylic acid (NHS-PEG-COOH) and folate (FA). DOX@HNTs-PEG-FA is prepared by loading DOX on HNTs-PEG-FA via physical adsorption. The sustained and controlled release of DOX from DOX@HNTs-PEG-FA is up to 35 h in an acidic environment (pH 5.3). DOX@HNTs-PEG-FA, performed as a new nanodelivery system, shows significant inhibition of proliferation and induction of death in MCF-7 cells with positive FA receptor but not in L02 cells with negative FA receptor. Results of acridine orange/ethidium bromide and flow cytometric assay indicate that DOX@HNTs-PEG-FA induces cell death through apoptosis. Compared to the same dose of DOX, DOX@HNTs-PEG-FA gener...

Facile preparation of homogeneous and length controllable halloysite nanotubes by ultrasonic scission and uniform viscosity centrifugation

Halloysite nanotubes (HNTs) are natural “green” nanomaterials with tubular structures and have exciting potential applications in many fields. Preparation of homogeneous HNTs remains significant challenges in their fundamental studies and applications. Herein, we provide a method for preparing homogeneous and length controllable HNTs by the combination of ultrasonic treatment and two-step uniform viscosity centrifugation. Raw halloysite bundles have been successfully dispersed and cut by ultrasonic scission. Length controllable HNTs (140–240nm) can be prepared simply by changing ultrasonic treatment time or ultrasonic power. Short HNTs have been successfully separated from the sonicated suspension of raw HNTs by two-step uniform viscosity centrifugation. Transmission electron microscopy, X-ray diffraction and sequential X-ray fluorescence spectrometer measurements demonstrate that the purified HNTs are homogeneous with much less impurities. This simple, rapid, and effective method can be scaled up for industrial preparation of homogeneous and length controllable HNTs. The purified HNTs can be used as high value-added materials in many industrial fields and used as a carrier for a wide range of active agents, such as drugs, cosmetics, and catalysts. The separation method can be used to separate chemically synthesized nanotubes. In addition, a simple method for quick determination of the concentration of HNTs by UV–Vis spectrophotometer has been developed.

Influence of the CO2Antisolvent Effect on Ultrasound-Induced Polymer Scission Kinetics

Ultrasound-induced polymer scission is a nonrandom process which alters the molecular weight distribution of polymers. However, transient cavitation, and consequently polymer scission, is not possible in concentrated polymer solutions due to the high liquid viscosity. The addition of an antisolvent can be used to circumvent this problem because the antisolvent decreases the gyration radius of polymer chains, which induces a reduction in liquid viscosity. To determine the influence of carbon dioxide (CO2) as an antisolvent on the ultrasound-induced scission rate, ultrasonic scission experiments of poly(methyl methacrylate) have been performed in bulk methyl methacrylate (MMA) as well as in CO2-expanded MMA. Modeling the experimental time-dependent molecular weight distributions (MWD) has revealed the scission kinetics at different polymer concentrations and CO2 fractions. At low polymer concentrations, the scission rate is decreased upon an increased CO2 content. This is a result of the higher vapor pressu...

Kinetic Equations for Ultrasonic Scission of Polymers

When a long chain of polymer is sonicated it is known that it preferentially breaks at the middle and long chain radicals result. The termination step is especially important to control the degree of polymerisation Pn. In this paper, in the absence of the radical traps the nature of the phenomenological kinetic equations derived in [4] are studied. It has been found that for all possible values of the parameters, the only equilibrium point is linearly stable. But because of the nonlinear effects, stability of the equilibrium point depends on the parameter space. For some set of parameters and initial conditions, it has been observed that the solutions are asymptotically periodic. But large time behaviour of all these solutions are not realistic in the sense that functions representing nonnegative physical quantities assume negative values. A modified version of the kinetic equations whose solutions remain nonnegative in the time course is also proposed.

A New Model for the Evolution of the Molecular Weight Distribution of Polymers During Ultrasonic Chain Scission

A new model for ultrasonic scission of polymer chains has been developed. The model takes the coiled nature of the polymer chains into consideration. The time evolution of the molecular weight distribution for several values of the solvent quality parameter is obtained using the new model. It is seen that the model leads to higher scission rates and lower final molecular weights for better solvents in accord with the experiments. Simulated gel permeation chromatography results are also in line with the experiments.

Block copolymer synthesis and chain extension from TEMPO-terminated chains formed by ultrasonic chain scission

Long poly(ethyl methacrylate) (Mn = 2,300,000) and polystyrene (Mn = 1,200,000) chains were subjected to ultrasonic scission in the presence of a radical scavenger, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). This procedure yielded polymers with lower molecular weights and TEMPO terminal units. Application of these polymers in stable radical mediated polymerization of styrene resulted in chain extension and block copolymers, depending on the precursor polymer. Block copolymer formation was evidenced by NMR measurement, and chain extension was shown by GPC analysis. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1950–1953, 2000

Termination mechanism of polymethyl methacrylate and polystyrene studied by ultrasonic degradation technique

A new method for determining the dominant termination mechanism in radical polymerization based on ultrasonic scission of long chains is used to study the termination of polystyrene and polymethyl methacrylate. The method is extended to obtain the disproportionation/combination ratio. Long dead polymer chains in solution were broken by ultrasound. The chain radicals thus formed were then allowed to terminate in the presence and absence of a chain terminating agent (radical trap). The resulting molecular weights are compared to find the dominant termination mechanism. It is found that the dominant mechanism in polystyrene is combination and that in polymethyl methacrylate is disproportionation. These results are in accordance with those quoted in the literature and obtained by other methods.The effectiveness of the radical trap used (2-chloroethyl benzene) was tested by NMR and it was found that when the trapping agent is present it terminates 100% of the polymethyl methacrylate chains. The time evolution of the degree of polymerization was compared to simulations based on Schmid's model. The disproportionation/combination ratios were found for polymethyl methacrylate as 2 and for polystyrene as 1/7 respectively through simulation studies.

Molecular Weight Distribution and Intrinsic Viscosity of Sonicated and Successively Fractionated Double-Stranded Deoxyribonucleic Acid and Polyribonucleotides

The ultrasonic scission was studied for deoxyribonucleic acid (DNA) and five polyribonucleotide duplexes; three antiparallel-stranded helices (Poly(G)·Poly(C), Poly(A)·Poly(U), Poly(I)·Poly(C)) and two parallel-stranded helices (Poly(C)·Poly(C+) and Poly(A)·Poly(A+)). The sonication was carried out in 0.1—0.2 M NaCl solutions at 0°C and 200 watts for totally 20 minutes under helium gas atmosphere. The sonicated sample was fractionated by successive precipitation with acetone as the precipitant. For each duplex, several fractions were obtained in large quantities. The weight-average molecular weight Mw, the degree of polydispersity Mw/Mn, and the molecular weight distribution were determined for unsonicated, sonicated, and fractionated samples by the gel-permeation-chromatography/low-angle-laser-light-scattering method. The Mw values were reduced to a limit of 105 and the Mw/Mn values were narrowed to a 2—1.5 range by sonication. These values were further refined by fractionation. The molecular weight distribution profiles of well-defined fractions could be fitted by the logarithmic-normal function. The relationship between intrinsic viscosity and Mw was evaluated for each duplex. The result showed that the rigidity or flexibility of polymer chain depends on the molecular weight of fractionated samples and the kind of duplex examined.

Ultrasonic Scission of Deoxyribonucleic Acid in Aqueous Solution IV. Circular Dichroism of Sonicated and Unsonicated DNA in Water-Alcohol Mixed Solvents

The circular dichroic (CD) spectra of calf thymus deoxyribonucleic acid (hDNA) and the sonicated fragment (sDNA) with weight-average molecular weights of 580×104 and 14.2×104, respectively, were measured in the 220–320 nm wavelength region at 15°C in water-alcohol mixed solvents with an ionic strength of 0.4×10−3. The effect of ionic valence on CD spectra was examined with sodium or magnesium ions as the counterion and NaH2PO4/Na2HPO4 or Mg(CH3COO)2/CH3COOH as the buffer reagent. The CD spectra of both hDNA and sDNA with the Na+ counterion showed a B-form pattern in 0–70 vol% ethanol and transformed to an A-form profile in 72–90 vol%, the spectral change of hDNA being more cooperative than that of sDNA. Trifluoroethanol (TFE) induced changes in the CD spectra of sDNA as ethanol. In the case of the Mg2+ counterion, the CD spectra of h- and sDNA revealed the B-form pattern between 0 and 40% ethanol. A further addition of ethanol induced another spectral change, but fibrous precipitates appeared above 56% ethanol. From circular dichroic point of view, both hDNA and sDNA behave identically toward counterion, ethanol and TFE, their secondary structure probably remaining much the same.

Ultrasonic Scission of Deoxyribonucleic Acid in Aqueous Solution III. The Solution Properties of Sonicated Low Molecular Weight Samples as Revealed by Light Scattering and Viscosity

The light scattering and viscosity measurements were carried out at 25°C in 0.2 M NaCl solutions for 14 fractionated low molecular weight DNA (sDNA) samples, which were prepared by the precipitational fractionation of the sonicated calf thymus DNA fragments. The relationship between the intrinsic viscosity [η] and the weight-average molecular weight Mw could be expressed with an empirical formula, [η]=9.54×10−6Mw1.35 in cm3g−1 for 1.2×105≤Mw≤3×105, but a sigmoidal change followed with an inflection region (3×105≤Mw≤6×105). The z-averaged square radius of gyration ‹S2›z was also determined from the angular dependence of scattered light intensity for each sDNA sample. The dependence of ‹S2›z and [η] on Mw was calculated theoretically at various values of the persistence length for the A- and B-DNA conformations. It was found that the effect of the polydispersity on ‹S2›z and [η] is important, if a quantitative comparison is made between the observed and calculated quantities.

Ultrasonic Scission of Deoxyribonucleic Acid in Aqueous Solution I. Conditions for Sonication and Molecular Weights of Sonicated Samples

By irradiating ultrasonic wave (20 kHz).to calf thymus deoxyribonucleic acid (DNA) in aqueous solution (2 mg ml−1 containing 0.2 M NaCl) at 0°C, the molecular weight (Mw) was degraded from 8×106 to ca. 2×105 without any noticeable denaturation. The dependence of the molecular weight of sonicated DNA on the irradiation intensity was examined by varying ultrasonic energy levels. The molecular weights were remarkably decreased with the increase in ultrasonic power from 25 to 105 W, and reached a steady value (Mw=2.4×105) at 100—200 W.

Ultrasonic Scission of Deoxyribonucleic Acid in Aqueous Solution II. Precipitational Fractionation and Molecular Weights of Sonicated Samples

Four sonicated calf thymus deoxyribonucleic acid (DNA).samples irradiated at different energies, whose molecular weights Mw were 5.7×105, 3.4×105, 2.4×105, and 2.4×105, respectively, as described in the preceding paper, were fractionated in 0.2 M NaCl aqueous solutions by the successive precipitation method with acetone as the precipitant. Each sample was separated into four fractions, each of which was dissolved in 1 mM NaCl and freeze-dried for storage. The molecular weight and molecular weight distribution were determined by means of intrinsic viscosity (25°C), GPC (20°C), and gel electrophoresis (0°C), and compared with those of unfractionated samples. The molecular weight of each fractionated DNA sample was found to be in the range between 0.7×105 and 7×105, while the degree of polydispersity in terms of Mw/Mn was found to be between 1.06 and 1.77, any of which is narrower than the values of the corresponding unfractionated sample. The hyperchromicity of each fractionated sample was estimated from the melting curve. It was always larger than 34%, which indicates that the double-stranded helical structure was maintained in the process of fractionation.