Lithium phthalocyanine (LiPc) is a prototype of another generation of synthetic, metallic-organic, paramagnetic crystallites that appear very useful for in vitro and in vivo electron paramagnetic resonance oximetry. The peak-to-peak line width of the electron paramagnetic resonance spectrum of LiPc is a linear function ofthe partial pressure ofoxygen (PO2); this linear relation is independent of the medium surrounding the LiPc. It has an extremely exchange-narrowed spectrum (peak-to-peak line width = 14 mG in the absence of 02). Physicochemically LiPc is very stable; its response to pO2 does not change with conditions and environments (e.g., pH, temperature, redox conditions) likely to occur in viable biological systems. These characteristics provide the sensitivity, accuracy, and range to measure physiologically and pathologically pertinent 02 tensions (0.1-50 mmHg; 1 mmHg = 133 Pa). The application of LiPc in biological systems is demonstrated in measurements of PO2 in vivo in the heart, brain, and kidney of rats.The purpose of this article is to describe a technology based on electron paramagnetic resonance (EPR or equivalently, electron spin resonance, ESR) that can significantly improve the ability to measure the partial pressure of 02 (PO2) under biologically pertinent conditions in vitro, in vivo, and potentially in human subjects. This article focuses on a prototype ofa class of crystalline paramagnetic probes, lithium phthalocyanine (LiPc), and aims at providing sufficient detail to facilitate the use of these probes in viable biological systems. The critical capabilities of this technology are the ability to measure PO2 at the levels (usually <40 mmHg and can be as low as 0.1 mmHg; 1 mmHg = 133 Pa) and sites (e.g., in tissues in vivo and inside cells) needed to understand biological processes.

We report a solution NMR-based analysis of (16-mercaptohexadecyl)­trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nanospheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the headgroups are saturated on the ligand shell as the sizes of the nanoparticles increase beyond ∼10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from ∼3 molecules per nm2 for 25 nm particles to up to 5−6 molecules per nm2 in ∼10 nm and smaller particles for in situ measurements of bound ligands; after I2/I– treatment to etch away the gold cores, ligand density ranges from ∼2 molecules per nm2 for 25 nm particles to up to 4−5 molecules per nm2 in ∼10 nm and smaller particles. T 2 relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to ∼13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)­trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for ∼10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T 2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.

A longstanding challenge in nanoparticle characterization is to understand anisotropic distributions of organic ligands at the surface of inorganic nanoparticles. Here, we show that using electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope we can directly visualize and quantify ligand distributions on gold nanorods (AuNRs). These experiments analyze dozens of particles on graphene substrates, providing insight into how ligand binding densities vary within and between individual nanoparticles. We demonstrate that the distribution of cetyltrimethylammonium bromide (CTAB) on AuNRs is anisotropic, with a 30% decrease in ligand density at the poles of the nanoparticles. In contrast, the distribution of (16-mercaptohexadecyl)­trimethylammonium bromide (MTAB) is more uniform. These results are consistent with literature reported higher reactivity at the ends of CTAB-coated AuNRs. Our results demonstrate the impact of electron spectroscopy to probe molecular distributions at soft–hard interfaces and how they produce spatially heterogeneous properties in colloidal nanoparticles.

Understanding the origin and sensitivity of carbon dot emission will improve their utility in various applications.

Conspectus Plasmons, collective oscillations of conduction band electrons in nanoscale metals, are well-known phenomena in colloidal gold and silver nanocrystals that produce brilliant visible colors in these materials that depend on nanocrystal size and shape. Under illumination at or near the plasmon bands, gold and silver nanocrystals exhibit properties that enable fascinating biological applications: (i) the nanocrystals elastically scatter light, providing a straightforward way to image them in complex aqueous environments; (ii) the nanocrystals produce local electric fields that enable various surface-enhanced spectroscopies for sensing, molecular diagnostics, and boosting bound fluorophore performance; (iii) the nanocrystals produce heat, which can lead to chemical transformations at or near the nanocrystal surface, and can photothermally destroy nearby cells. While all the above-mentioned applications have already been well-demonstrated in the literature, this Account focuses on several other aspects of these nanomaterials, in particular gold nanorods that are approximately the size of viruses (diameters ~10 nm, lengths up to 100 nm). Absolute extinction, scattering, and absorption properties are compared for gold nanorods of various absolute dimensions, and references for how to synthesize gold nanorods of four different absolute dimensions are provided. Surface chemistry strategies are detailed that coat nanocrystals with smooth or rough shells; specific examples include mesoporous silica and metal-organic framework shells for porous (rough) coatings, and polyelectrolyte layer-by-layer wrapping for “smooth” shells. For self-assembled monolayer molecular coating ligands, the smoothest shells of all, a wide range of ligand densities have been reported from many experiments, yielding values from less than 1 to nearly 10 molecules/nm2 depending on nanocrystal size and ligand nature. Systematic studies of ligand density for one particular ligand with a bulky headgroup are highlighted, showing that the largest ligand density occurs for the smallest nanocrystals, even though these ligand headgroups are the most mobile as judged by NMR relaxation studies. Biomolecular coronas form around spherical and rod-shaped nanocrystals upon immersion into biological fluids; these proteins, and lipids, can be quantified, and their degree of adsorption depends on nanocrystal surface chemistry as well as biophysical characteristics of the adsorbing biomolecule. Photothermal adsorption and desorption of proteins on nanocrystals depend on the enthalpy of protein-nanocrystal surface interactions, leading to light-triggered alteration in protein concentrations near the nanocrystals. At the cellular scale, gold nanocrystals exert genetic changes at the mRNA level, with a variety of likely mechanisms that include alteration of local biomolecular concentration gradients, changing mechanical properties of the extracellular matrix, and physical interruption of key cellular processes - even without plasmonic effects. Microbiomes, both...

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The peak-to-peak line width (LW) of the first derivative electron spin resonance (EPR) spectrum of the coal maceral fusinite is reversibly broadened by O2. The extent of broadening per unit of partial pressure of oxygen (pO2) is unusually large, exceeding that of nitroxides by almost two orders of magnitude. This paramagnetic property of fusinite, combined with its very stable physicochemical properties and low toxicity, is shown to be of utility in the measurement of pO2 in vitro and in vivo. Fusinite particles are endocytosed by chinese hamster ovary (CHO) cells in vitro; this is useful for intracellular O2 measurements with commercially available EPR spectrometers operating at 9.1-9.3 GHz. For measurement of oxygen in vivo using low frequency EPR (1.1-1.3 GHz), fusinite provides a sensitive and persistent means to measure pO2 in tissues. Particles implanted into the gastrocnemius muscle of A/J mice remained interstitially in the same position for months with undiminished sensitivity to pO2 and no specific toxic effects.

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