Swelling enhanced remanent magnetization of hydrogels cross-linked with magnetic nanoparticles.

Hydrogels that are pH-sensitive and partially cross-linked by cobalt ferrite nanoparticles exhibit remarkable remanent magnetization behavior. The magnetic fields measured outside our thin disks of ferrogel are weak, but in the steady state, the field dependence on the magnetic content of the gels and the measurement geometry is as expected from theory. In contrast, the time-dependent behavior is surprisingly complicated. During swelling, the remanent field first rapidly increases and then slowly decreases. We ascribe the swelling-induced field enhancement to a change in the average orientation of magnetic dipolar structures, while the subsequent field drop is due to the decreasing concentration of nanoparticles. During shrinking, the field exhibits a much weaker time dependence that does not mirror the values found during swelling. These observations provide original new evidence for the markedly different spatial profiles of the pH during swelling and shrinking of hydrogels.

Demagnetization treatment of remanent composite microspheres studied by alternating current susceptibility measurements.

The magnetic remanence of silica microspheres with a low concentration of embedded cobalt ferrite nanoparticles is studied after demagnetization and remagnetization treatments. When the microspheres are dispersed in a liquid, alternating current (AC) magnetic susceptibility spectra reveal a constant characteristic frequency, corresponding to the rotational diffusion of the microparticles; this depends only on particle size and liquid viscosity, making the particles suitable as a rheological probe and indicating that interactions between the microspheres are weak. On the macroscopic scale, a sample with the dry microparticles is magnetically remanent after treatment in a saturating field, and after a demagnetization treatment, the remanence goes down to zero. The AC susceptibility of a liquid dispersion, however, characterizes the remanence on the scale of the individual microparticles, which does not become zero after demagnetization. The reason is that an individual microparticle contains only a relatively small number of magnetic units, so that even if they can be reoriented magnetically at random, the average vector sum of the nanoparticle dipoles is not negligible on the scale of the microparticle. In contrast, on the macroscopic scale, the demagnetization procedure randomizes the orientations of a macroscopic number of magnetic units, resulting in a remanent magnetization that is negligible compared to the saturation magnetization of the entire sample.

Frequency-dependent magnetic susceptibility of magnetite and cobalt ferrite nanoparticles embedded in PAA hydrogel.

Chemically responsive hydrogels with embedded magnetic nanoparticles are of interest for biosensors that magnetically detect chemical changes. A crucial point is the irreversible linkage of nanoparticles to the hydrogel network, preventing loss of nanoparticles upon repeated swelling and shrinking of the gel. Here, acrylic acid monomers are adsorbed onto ferrite nanoparticles, which subsequently participate in polymerization during synthesis of poly(acrylic acid)-based hydrogels (PAA). To demonstrate the fixation of the nanoparticles to the polymer, our original approach is to measure low-field AC magnetic susceptibility spectra in the 0.1 Hz to 1 MHz range. In the hydrogel, the magnetization dynamics of small iron oxide nanoparticles are comparable to those of the particles dispersed in a liquid, due to fast Néel relaxation inside the particles; this renders the ferrogel useful for chemical sensing at frequencies of several kHz. However, ferrogels holding thermally blocked iron oxide or cobalt ferrite nanoparticles show significant decrease of the magnetic susceptibility resulting from a frozen magnetic structure. This confirms that the nanoparticles are unable to rotate thermally inside the hydrogel, in agreement with their irreversible fixation to the polymer network.

Highly luminescent (Zn,Cd)Te/CdSe colloidal heteronanowires with tunable electron-hole overlap.

We report the synthesis of ultranarrow (Zn,Cd)Te/CdSe colloidal heteronanowires, using ZnTe magic size clusters as seeds. The wire formation starts with a partial Zn for Cd cation exchange, followed by self-organization into segmented heteronanowires. Further growth occurs by inclusion of CdSe. The heteronanowires emit in the 530 to 760 nm range with high quantum yields. The electron-hole overlap decreases with increasing CdSe volume fraction, allowing the optical properties to be controlled by adjusting the heteronanowire composition.

Growth and stability of ZnTe magic-size nanocrystals.

A synthetic method for ZnTe magic-sized clusters (MSCs) is reported, and the stability and growth kinetics of these clusters are investigated. Four distinct MSC families, with lowest-energy absorption peaks at 330, 354, 378, and 392 nm, are observed. The stability and growth kinetics of the MSCs are strongly influenced by the reaction temperature, precursor concentration, and nature of the ligands used as the coordinating solvent. High precursor concentrations result in faster growth and MSC formation at lower temperatures. Higher temperatures accelerate the growth kinetics and lead to a gradual shift from the stepwise MSC growth regime to a continuous growth regime. For temperatures above 260 °C, only continuous growth of nanocrystals is observed. The nature of the ligands also influences the stability and growth of ZnTe MSCs, which are formed with primary alkylamines as ligands, but not when trioctylphosphine, trioctylphosphine oxide, or trioctylamine are used as the sole ligands. This demonstrates the crucial role of ligands in the formation of stable ZnTe MSCs using colloidal synthetic methods. Under optimal synthetic conditions (200 °C, hexadecylamine as ligand, and suitable precursor concentrations), the method presented here allows the synthesis and isolation of a single MSC family absorbing at 330 nm.