
The point defect in diamond known as the NV center—a nitrogen atom substituted for a carbon atom and adjacent lattice vacancy—has become a promising ingredient in recent efforts to develop atomic-scale sensors. When optically excited, the defect exhibits stable fluorescence, even in a crystal as small as 5 nm. And its ground state is magnetically sensitive—the spin 0 level is separated from degenerate spin ±1 levels by a microwave transition of 2.9 GHz. That sensitivity allows one to detect weak magnetic fields by observing the quantum spin state, which can be manipulated by microwave pulses and then read out optically by monitoring the fluorescence intensity. (The intensity depends on which of the three spin states is populated.) Researchers led by the University of Melbourne’s Lloyd Hollenberg have now performed such magnetic resonance experiments on individual nanodiamonds placed inside human cells. The quantum spin levels of the defects acted both as local magnetometers and as fingerprints to spectrally distinguish each nanodiamond in the complicated cell environment. Using a confocal microscope, the researchers were able to identify and track at nanometer precision individual NV centers in the cells from the optical emission (red). They were also able to measure the coherence time of the spin states; that work sets the stage for sensing the cells’ local magnetic fluctuations in response to, for example, the transport of charge through cell membrane ion channels, which are important drug targets. (L. P. McGuinness et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2011.64.)—R. Mark Wilson














Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by
Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now,
Lightness, strength, and moldability are among the most desired material properties for aircraft, sporting equipment, and many structural applications. Those sometimes opposing properties converge in bulk metallic glasses—supercooled amorphous metal alloys that can be cast into complex shapes and are resilient under large elastic strains. However, their toughness is suspect: Under repeated stress, BMGs fatigue and develop fatal cracks much more quickly than crystalline metal alloys do. To control crack propagation,
Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaflike sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how. 
In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from
Overuse of antibiotics has spawned strains of bacteria whose cell walls are impervious to the crippling blows once delivered by penicillin and its derivatives. One such so-called superbug, methicillin-resistant staphylococcus aureus, although found primarily in prisons and hospitals, has now spread beyond those confines. Despite the controlled use of the drug vancomycin, a last line of defense against MRSA, the latest threat are vancomycin-resistant bacteria, which mutate by deleting a key hydrogen bond that allows the drug to bind and inhibit cell wall growth, thereby mechanically weakening the bacteria . Rachel McKendry at
In scanning microscopy, images are put together by sweeping a single narrow beam back and forth over a sample. If you had a wide array of multiple beams, one scan would suffice. And if the sample moved over the array, you wouldn't need to scan at all. That idea is behind a new optofluidic imaging scheme developed for biological applications by Caltech's