It is shown how the equilibrium pair correlation function between spin-bearing molecules in liquids may be incorporated as an effective force in the relative diffusion expressions, and how one may solve for the resulting time correlation functions and spectral densities needed for studies of spin relaxation by translational diffusion. The use of finite difference methods permits the solution no matter how complex the form of the pair correlation function (pcf) utilized. In particular, a Percus–Yevick pcf as well as one corrected from computer dynamics, both for hard spheres, are utilized. Good agreement with the experiments of Harmon and Muller on dipolar relaxation in liquid ethane is obtained from this analysis. Effects of ionic interactions in electrolyte solutions upon dipolar relaxation are also obtained in terms of Debye–Hückel theory for the pcf. Analytic solutions are given which are appropriate for the proper boundary-value problem for the relative diffusion of molecules (i.e., a distance of minimum approach) that has usually been neglected in the spin relaxation theories. Other molecular dynamics aspects of spin relaxation by translational diffusion in liquids are briefly discussed.

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We introduce a new category of nanoparticle-based T 1 MRI contrast agents (CAs) by encapsulating paramagnetic chelated gadolinium(III), i.e., Gd 3+ ·DOTA, through supramolecular assembly of molecular building blocks that carry complementary molecular recognition motifs, including adamantane (Ad) and β-cyclodextrin (CD). A small library of Gd 3+ ·DOTA-encapsulated supramolecular nanoparticles (Gd 3+ ·DOTA⊂SNPs) was produced by systematically altering the molecular building block mixing ratios. A broad spectrum of relaxation rates was correlated to the resulting Gd 3+ ·DOTA⊂SNP library. Consequently, an optimal synthetic formulation of Gd 3+ ·DOTA⊂SNPs with an r 1 of 17.3 s −1 mM −1 (ca. 4-fold higher than clinical Gd 3+ chelated complexes at high field strengths) was identified. T 1 -weighted imaging of Gd 3+ ·DOTA⊂SNPs exhibits an enhanced sensitivity with a contrast-to-noise ratio (C/N ratio) ca. 3.6 times greater than that observed for free Gd 3+ ·DTPA. A Gd 3+ ·DOTA⊂SNPs solution was injected into foot pads of mice, and MRI was employed to monitor dynamic lymphatic drainage of the Gd 3+ ·DOTA⊂SNPs-based CA. We observe an increase in signal intensity of the brachial lymph node in T 1 -weighted imaging after injecting Gd 3+ ·DOTA⊂SNPs but not after injecting Gd 3+ ·DTPA. The MRI results are supported by ICP-MS analysis ex vivo. These results show that Gd 3+ ·DOTA⊂SNPs not only exhibits enhanced relaxivity and high sensitivity but also can serve as a potential tool for diagnosis of cancer metastasis.

Direct evidence for a size effect in self-trapped exciton ͑STE͒ photoluminescence ͑PL͒ from silica-based nanoscale materials as compared with bulk type-III fused silica is obtained. Two kinds of mesostructures were tested: ͑1͒ silica nanoparticle composites with primary particle size of 7 and 15 nm, ͑2͒ ordered and disordered mesoporous silicas with pore size ranging from ϳ2 to ϳ6 nm and wall thickness ϳ1 nm. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity ϳ10 6 W cm Ϫ2 and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STE PL ͑STEPL͒ band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials. However, the use of this model leads to a contradiction showing that the model is completely unusable in the case of wide-band-gap nanoscale materials ͑the band-gap of bulk silica E g Х11 eV͒. In order to explain the experimental data, we propose a model that takes into account the laser heating of Frenkel-type free excitons ͑FE's͒. The heating effect is assumed to be due to the FE collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. According to the model, laser heating of FE's up to the temperature in excess of the activation energy required for the self-trapping give rise to the extremely hot STE's. Because the resulting temperature of the STE's is much higher than the lattice temperature, the cooling of STE's is dominated by the emission of lattice phonons. However, if the STE temperature comes into equilibrium with the lattice temperature, the absorption of lattice phonons becomes possible. As a result, the blue shift of the STEPL band is suggested to originate from the activation of hot ͑phonon-assisted͒ electronic transitions. Good agreement between experimental data and our FE laser heating model has been obtained.

Cancer theranostics is one of the most important approaches for detecting and treating patients at an early stage. To develop such a technique, accurate detection, specific targeting, and controlled delivery are the key components. Various kinds of nanoparticles have been proposed and demonstrated as potential nanovehicles for cancer theranostics. Among them, polymer-like dendrimers and copolymer-based core-shell nanoparticles could potentially be the best possible choices. At present, magnetic resonance imaging (MRI) is widely used for clinical purposes and is generally considered the most convenient and noninvasive imaging modality. Superparamagnetic iron oxide (SPIO) and gadolinium (Gd)-based dendrimers are the major nanostructures that are currently being investigated as nanovehicles for cancer theranostics using MRI. These structures are capable of specific targeting of tumors as well as controlled drug or gene delivery to tumor sites using pH, temperature, or alternating magnetic field (AMF)-controlled mechanisms. Recently, Gd-based pseudo-porous polymer-dendrimer supramolecular nanoparticles have shown 4-fold higher T1 relaxivity along with highly efficient AMF-guided drug release properties. Core-shell copolymer-based nanovehicles are an equally attractive alternative for designing contrast agents and for delivering anti-cancer drugs. Various copolymer materials could be used as core and shell components to provide biostability, modifiable surface properties, and even adjustable imaging contrast enhancement. Recent advances and challenges in MRI cancer theranostics using dendrimer- and copolymer-based nanovehicles have been summarized in this review article, along with new unpublished research results from our laboratories.

Photoluminescence (PL) from mesoporous silica (MS) with the pore size of ∼6 nm and the thickness of walls among pores of ∼1 nm has been studied at room temperature. The heat pretreatment of MS in air at different temperatures and the variation of the excitation wavelengths allow one to shift the PL peak through the whole visible spectral range. The PL is suggested to originate from nonbridging oxygens (red bands), hydrogen-related species (green bands), and water-carbonyl groups (blue bands). The spectroscopic properties of MS are found to be similar to those of surface-oxidized silicon nanocrystals and porous silicon.

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