All matter has density. The recorded uses of density to characterize matter date back to as early as ca. 250 BC, when Archimedes was believed to have solved “The Puzzle of The King's Crown” using density.[1] Today, measurements of density are used to separate and characterize a range of materials (including cells and organisms), and their chemical and/or physical changes in time and space. This Review describes a density‐based technique—magnetic levitation (which we call “MagLev” for simplicity)—developed and used to solve problems in the fields of chemistry, materials science, and biochemistry. MagLev has two principal characteristics—simplicity, and applicability to a wide range of materials—that make it useful for a number of applications (for example, characterization of materials, quality control of manufactured plastic parts, self‐assembly of objects in 3D, separation of different types of biological cells, and bioanalyses). Its simplicity and breadth of applications also enable its use in low‐resource settings (for example—in economically developing regions—in evaluating water/food quality, and in diagnosing disease).

This work describes the development of magnetic levitation (MagLev) using ring magnets and a configuration (which we call “axial MagLev”) to remove the physical barriers to physical sampling in the magnetic field present in “standard MagLev” and to simplify the procedures used to carry out density-based analyses, separations, and manipulations. The optimized, linear magnetic field generated between the two ring magnets (coaxially aligned and like-poles facing) enables the levitation of diamagnetic (and weakly paramagnetic, e.g., aluminum) materials in a paramagnetic suspending medium and makes density measurements more straightforward. This “axial” configuration enables (i) simple procedures to add samples and paramagnetic medium from an open end and to retrieve samples while levitating in the magnetic field (e.g., a subpopulation of a cluster of small particles); (ii) simple accesses and the abilities to view the samples 360° around the sample container and from the top and bottom; and (iii) convenient density measurements of small quantities (as small as a single submillimeter particle as demonstrated) of samples. The compact design, portability, affordability, and simplicity in use of the “axial MagLev” device will broaden the uses of magnetic methods in analyzing, separating, and manipulating different types of samples (solids, liquids, powders, pastes, gels, and also biological entities) in areas such as materials sciences, chemistry, and biochemistry.

This work describes the development of an integrated analytical system that enables high-throughput density measurements of diamagnetic particles (including cells) using magnetic levitation (MagLev), 96-well plates, and a flatbed scanner. MagLev is a simple and useful technique with which to carry out density-based analysis and separation of a broad range of diamagnetic materials with different physical forms (e.g., liquids, solids, gels, pastes, gums, etc.); one major limitation, however, is the capacity to perform high-throughput density measurements. This work addresses this limitation by (i) re-engineering the shape of the magnetic fields so that the MagLev system is compatible with 96-well plates, and (ii) integrating a flatbed scanner (and simple optical components) to carry out imaging of the samples that levitate in the system. The resulting system is compatible with both biological samples (human erythrocytes) and nonbiological samples (simple liquids and solids, such as 3-chlorotoluene, cholesterol crystals, glass beads, copper powder, and polymer beads). The high-throughput capacity of this integrated MagLev system will enable new applications in chemistry (e.g., analysis and separation of materials) and biochemistry (e.g., cellular responses under environmental stresses) in a simple and label-free format on the basis of a universal property of all matter, i.e., density.

This work describes the development of magnetic levitation (MagLev) to characterize the kinetics of free-radical polymerization of water-insoluble, low-molecular-weight monomers that show a large change in density upon polymerization. Maglev measures density, and certain classes of monomers show a large change in density when monomers covalently join in polymer chains. MagLev characterized both the thermal polymerization of methacrylate-based monomers and the photopolymerization of methyl methacrylate and made it possible to determine the orders of reaction and the Arrhenius activation energy of polymerization. MagLev also made it possible to monitor polymerization in the presence of solids (aramid fibers, and carbon fibers, and glass fibers). MagLev offers a new analytical technique to materials and polymer scientists that complements other methods (even those based on density, such as dilatometry), and will be useful in investigating polymerizations, evaluating inhibition of polymerizations, and studying polymerization in the presence of included solid materials (e.g., for composite materials).

Exocytosis is a fundamental cellular process, pivotal in a wide range of cell types, used to deliver chemical messengers from one cell to another cell or tissue. While a tremendous amount of knowledge has been gained in the past several decades about the exocytotic machinery, recently, it has become clear that the role of membrane lipids is also crucial in this process. In particular, the critical role of the abundant and ubiquitous cholesterol molecules has not been well defined. Early insight has been gleaned from single cell amperometric studies on several commonly used secretory cell models, including chromaffin cells and PC12 cells; however, these secretory cell models are not ideal because manipulations of membrane cholesterol content may influence downstream cholesterol-dependent processes, making data interpretation difficult. Herein, blood platelets are employed as a simpler secretory cell model based on their anuclear nature and unique chemical messenger exocytosis behavior. Carbon-fiber microelectrochemistry was employed to measure real-time exocytosis from single platelets with depleted or enriched cholesterol either in the naturally occurring form or as the synthetic analog epicholesterol. The experimental results show that membrane cholesterol directly modulates the secretion efficiency of individual platelets as well as the kinetics of secretion events. Moreover, substitution of platelet membrane cholesterol with epicholesterol yields exocytotic behavior indistinguishable from normal platelets, arguing against the possibility of the cholesterol-specific interactions in regulating exocytosis. It is clear from this work that membrane cholesterol plays a critical biophysical, rather than biochemical, role in platelet exocytosis and likely in exocytosis in general.

Even though platelets are known to play a critical role in hemostasis, mediated in part by their uptake, storage, and release of serotonin, there are many unexplored aspects of this process. Herein, single-cell amperometry is employed to characterize the dynamic secretion of serotonin from platelet dense-body granules. On the basis of a three-dimensional random walk simulation that estimates detection efficiency with varied spacing between the carbon-fiber microelectrode and the platelet, it is clear that the detected charge likely represents complete oxidation of the released granule contents and, thus, is a good method to calculate the serotonin concentration in each granule. Using the measured charge and volume estimates based on transmission electron microscopy (TEM) data, the granular concentration of serotonin is approximately 0.5 M. The simulated spike widths are significantly narrower than most of the measured amperometric spikes, clearly indicating that the stored serotonin is highly associated with an aggregate rather than freely diffusible within the dense-body granule. Additionally, by varying extracellular buffer temperature and pH to adjust the driving forces for serotonin delivery from the dense-body granules to the extracellular space, it is clear that, although platelet chemical messenger storage and secretion is similar to that of other secretory cells, there are some important distinctions.

Magneto‐Archimedes levitation (MagLev) enables the separation of powdered mixtures of illicit drugs (cocaine, methamphetamine, heroin, fentanyl, and its analogues), adulterants, and diluents based on density, and allows the presumptive identification of individual components. Small samples (mass <50 mg), with low concentrations of illicit drugs, present a particular challenge to analysis for forensic chemists. The MagLev device, a cuvette containing a solution of paramagnetic gadolinium(III) chelate in a non‐polar solvent, placed between two like‐poles‐facing NdFeB magnets, allowed separation of seven relevant compounds simultaneously. In particular, initial separation with MagLev, followed by characterization by FTIR‐ATR, enabled identification of fentanyl in a sample of fentanyl‐laced heroin (1.3 wt % fentanyl, 2.6 wt % heroin, and 96.1 wt % lactose). MagLev allows identification of unknown powders in mixtures and enables confirmatory identification based on structure‐specific techniques.

Platelet aggregation in the blood stream is tightly associated with the secretory function of platelets based on several types of cytoplasmic secretory granules, each sequestering distinct chemical messenger species. Dense-body granules are one prominent type of secretory granules responsible for storing small molecule chemical messengers. Upon platelet activation, the timely and rapid release of these small molecules is critical in facilitating platelet aggregation. Therefore, techniques capable of measuring real-time granule content release are needed to understand the fundamental properties of platelet secretion and aggregation. Existing techniques lack adequate time resolution or require potentially toxic exogenous reagents for real-time measurement of granule content release. Herein, we demonstrate a label-free electrochemical method based on the endogenous electroactive chemical messenger serotonin (5-hydroxytryptamine or 5-HT) for the real-time measurement of dense-body granule secretion from platelet suspensions; fast-scan cyclic voltammetry (FSCV) using carbon-fiber microelectrodes was chosen based on its excellent temporal resolution, high sensitivity, and the ability to provide the electrochemical signature cyclic voltammograms for molecular identification. Real-time serotonin release from thrombinstimulated human platelet suspensions was successfully measured and the amount and time course of the bulk serotonin release were found to agree well with data obtained from single platelet measurements, thus confirming accurate characterization of granular secretion. Furthermore, this electrochemical method was applied to study the stimulation-secretion coupling in platelets, serotonin storage and release dynamics with applied pharmacological agents, and chemical messenger storage deficiency in Hermansky-Pudlak Syndrome (HPS) platelets, and has clearly demonstrated the potential of this method to reveal secretion behavior in both normal and diseased platelets.