Magnetic Levitation in Chemistry, Materials Science, and Biochemistry.

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).

Diagnosis of iron deficiency anemia using density-based fractionation of red blood cells.

Iron deficiency anemia (IDA) is a nutritional disorder that impacts over one billion people worldwide; it may cause permanent cognitive impairment in children, fatigue in adults, and suboptimal outcomes in pregnancy. IDA can be diagnosed by detection of red blood cells (RBCs) that are characteristically small (microcytic) and deficient in hemoglobin (hypochromic), typically by examining the results of a complete blood count performed by a hematology analyzer. These instruments are expensive, not portable, and require trained personnel; they are, therefore, unavailable in many low-resource settings. This paper describes a low-cost and rapid method to diagnose IDA using aqueous multiphase systems (AMPS)-thermodynamically stable mixtures of biocompatible polymers and salt that spontaneously form discrete layers having sharp steps in density. AMPS are preloaded into a microhematocrit tube and used with a drop of blood from a fingerstick. After only two minutes in a low-cost centrifuge, the tests (n = 152) were read by eye with a sensitivity of 84% (72-93%) and a specificity of 78% (68-86%), corresponding to an area under the curve (AUC) of 0.89. The AMPS test outperforms diagnosis by hemoglobin alone (AUC = 0.73) and is comparable to methods used in clinics like reticulocyte hemoglobin concentration (AUC = 0.91). Standard machine learning tools were used to analyze images of the resulting tests captured by a standard desktop scanner to 1) slightly improve diagnosis of IDA-sensitivity of 90% (83-96%) and a specificity of 77% (64-87%), and 2) predict several important red blood cell parameters, such as mean corpuscular hemoglobin concentration. These results suggest that the use of AMPS combined with machine learning provides an approach to developing point-of-care hematology.

A Paper-Based "Pop-up" Electrochemical Device for Analysis of Beta-Hydroxybutyrate.

This paper describes the design and fabrication of a "pop-up" electrochemical paper-based analytical device (pop-up-EPAD) to measure beta-hydroxybutyrate (BHB)-a biomarker for diabetic ketoacidosis-using a commercial combination BHB/glucometer. Pop-up-EPADs are inspired by pop-up greeting cards and children's books. They are made from a single sheet of paper folded into a three-dimensional (3D) device that changes shape, and fluidic and electrical connectivity, by simply folding and unfolding the structure. The reconfigurable 3D structure makes it possible to change the fluidic path and to control timing; it also provides mechanical support for the folded and unfolded structures that enables good registration and repeatability on folding. A pop-up-EPAD designed to detect BHB shows performance comparable to commercially available plastic test strips over the clinically relevant range of BHB in blood when used with a commercial glucometer that integrates the ability to measure glucose and BHB (combination BHB/glucometer). With simple modifications of the electrode and the design of the fluidic path, the pop-up-EPAD also detects BHB in buffer using a simple glucometer-a device that is more available than the combination BHB/glucometer. Strategies that use a "3D pop-up"-that is, large-scale changes in 3D structure and fluidic paths-by folding/unfolding add functionality to EPADs (e.g., controlled timing, fluidic handling and path programming, control over complex sequences of steps, and alterations in electrical connectivity) and should enable the development of new classes of paper-based diagnostic devices.

Fractionating Polymer Microspheres as Highly Accurate Density Standards.

This paper describes a method of isolating small, highly accurate density-standard beads and characterizing their densities using accurate and experimentally traceable techniques. Density standards have a variety of applications, including the characterization of density gradients, which are used to separate objects in a variety of fields. Glass density-standard beads can be very accurate (±0.0001 g cm(-3)) but are too large (3-7 mm in diameter) for many applications. When smaller density standards are needed, commercial polymer microspheres are often used. These microspheres have standard deviations in density ranging from 0.006 to 0.021 g cm(-3); these distributions in density make these microspheres impractical for applications demanding small steps in density. In this paper, commercial microspheres are fractionated using aqueous multiphase systems (AMPS), aqueous mixture of polymers and salts that spontaneously separate into phases having molecularly sharp steps in density, to isolate microspheres having much narrower distributions in density (standard deviations from 0.0003 to 0.0008 g cm(-3)) than the original microspheres. By reducing the heterogeneity in densities, this method reduces the uncertainty in the density of any specific bead and, therefore, improves the accuracy within the limits of the calibration standards used to characterize the distributions in density.

From the bench to the field in low-cost diagnostics: two case studies.

Despite the growth of research in universities on point-of-care (POC) diagnostics for global health, most devices never leave the laboratory. The processes that move diagnostic technology from the laboratory to the field--the processes intended to evaluate operation and performance under realistic conditions--are more complicated than they might seem. Two case studies illustrate this process: the development of a paper-based device to measure liver function, and the development of a device to identify sickle cell disease based on aqueous multiphase systems (AMPS) and differences in the densities of normal and sickled cells. Details of developing these devices provide strategies for forming partnerships, prototyping devices, designing studies, and evaluating POC diagnostics. Technical and procedural lessons drawn from these experiences may be useful to those designing diagnostic tests for developing countries, and more generally, technologies for use in resource-limited environments.

Using magnetic levitation for non-destructive quality control of plastic parts.

Magnetic levitation (MagLev) enables rapid and non-destructive quality control of plastic parts. The feasibility of MagLev as a method to: i) rapidly assess injection-molded plastic parts for defects during process optimization, ii) monitor the degradation of plastics after exposure to harsh environmental conditions, and iii) detect counterfeit polymers by density is demonstrated.

Evaluation of a density-based rapid diagnostic test for sickle cell disease in a clinical setting in Zambia.

Although simple and low-cost interventions for sickle cell disease (SCD) exist in many developing countries, child mortality associated with SCD remains high, in part, because of the lack of access to diagnostic tests for SCD. A density-based test using aqueous multiphase systems (SCD-AMPS) is a candidate for a low-cost, point-of-care diagnostic for SCD. In this paper, the field evaluation of SCD-AMPS in a large (n = 505) case-control study in Zambia is described. Of the two variations of the SCD-AMPS used, the best system (SCD-AMPS-2) demonstrated a sensitivity of 86% (82-90%) and a specificity of 60% (53-67%). Subsequent analysis identified potential sources of false positives that include clotting, variation between batches of SCD-AMPS, and shipping conditions. Importantly, SCD-AMPS-2 was 84% (62-94%) sensitive in detecting SCD in children between 6 months and 1 year old. In addition to an evaluation of performance, an assessment of end-user operability was done with health workers in rural clinics in Zambia. These health workers rated the SCD-AMPS tests to be as simple to use as lateral flow tests for malaria and HIV.

Density-based separation in multiphase systems provides a simple method to identify sickle cell disease.

Although effective low-cost interventions exist, child mortality attributable to sickle cell disease (SCD) remains high in low-resource areas due, in large part, to the lack of accessible diagnostic methods. The presence of dense (ρ > 1.120 g/cm(3)) cells is characteristic of SCD. The fluid, self-assembling step-gradients in density created by aqueous multiphase systems (AMPSs) identifies SCD by detecting dense cells. AMPSs separate different forms of red blood cells by density in a microhematocrit centrifuge and provide a visual means to distinguish individuals with SCD from those with normal hemoglobin or with nondisease, sickle-cell trait in under 12 min. Visual evaluation of a simple two-phase system identified the two main subclasses of SCD [homozygous (Hb SS) and heterozygous (Hb SC)] with a sensitivity of 90% (73-98%) and a specificity of 97% (86-100%). A three-phase system identified these two types of SCD with a sensitivity of 91% (78-98%) and a specificity of 88% (74-98%). This system could also distinguish between Hb SS and Hb SC. To the authors' knowledge, this test demonstrates the first separation of cells by density with AMPSs, and the usefulness of AMPSs in point-of-care diagnostic hematology.

Universal mobile electrochemical detector designed for use in resource-limited applications.

This paper describes an inexpensive, handheld device that couples the most common forms of electrochemical analysis directly to "the cloud" using any mobile phone, for use in resource-limited settings. The device is designed to operate with a wide range of electrode formats, performs on-board mixing of samples by vibration, and transmits data over voice using audio--an approach that guarantees broad compatibility with any available mobile phone (from low-end phones to smartphones) or cellular network (second, third, and fourth generation). The electrochemical methods that we demonstrate enable quantitative, broadly applicable, and inexpensive sensing with flexibility based on a wide variety of important electroanalytical techniques (chronoamperometry, cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and potentiometry), each with different uses. Four applications demonstrate the analytical performance of the device: these involve the detection of (i) glucose in the blood for personal health, (ii) trace heavy metals (lead, cadmium, and zinc) in water for in-field environmental monitoring, (iii) sodium in urine for clinical analysis, and (iv) a malarial antigen (Plasmodium falciparum histidine-rich protein 2) for clinical research. The combination of these electrochemical capabilities in an affordable, handheld format that is compatible with any mobile phone or network worldwide guarantees that sophisticated diagnostic testing can be performed by users with a broad spectrum of needs, resources, and levels of technical expertise.

Printed indium gallium zinc oxide transistors. Self-assembled nanodielectric effects on low-temperature combustion growth and carrier mobility.

Solution-processed amorphous oxide semiconductors (AOSs) are emerging as important electronic materials for displays and transparent electronics. We report here on the fabrication, microstructure, and performance characteristics of inkjet-printed, low-temperature combustion-processed, amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) grown on solution-processed hafnia self-assembled nanodielectrics (Hf-SANDs). TFT performance for devices processed below 300 °C includes >4× enhancement in electron mobility (µFE) on Hf-SAND versus SiO2 or ALD-HfO2 gate dielectrics, while other metrics such as subthreshold swing (SS), current on:off ratio (ION:IOFF), threshold voltage (Vth), and gate leakage current (Ig) are unchanged or enhanced. Thus, low voltage IGZO/SAND TFT operation (<2 V) is possible with ION:IOFF = 10(7), SS = 125 mV/dec, near-zero Vth, and large electron mobility, µFE(avg) = 20.6 ± 4.3 cm(2) V(-1) s(-1), µFE(max) = 50 cm(2) V(-1) s(-1). Furthermore, X-ray diffraction analysis indicates that the 300 °C IGZO combustion processing leaves the underlying Hf-SAND microstructure and capacitance intact. This work establishes the compatibility and advantages of all-solution, low-temperature fabrication of inkjet-printed, combustion-derived high-mobility IGZO TFTs integrated with self-assembled hybrid organic-inorganic nanodielectrics.

Oxygen "getter" effects on microstructure and carrier transport in low temperature combustion-processed a-InXZnO (X = Ga, Sc, Y, La) transistors.

In oxide semiconductors, such as those based on indium zinc oxide (IXZO), a strong oxygen binding metal ion ("oxygen getter"), X, functions to control O vacancies and enhance lattice formation, hence tune carrier concentration and transport properties. Here we systematically study, in the IXZO series, the role of X = Ga(3+) versus the progression X = Sc(3+) → Y(3+) → La(3+), having similar chemical characteristics but increasing ionic radii. IXZO films are prepared from solution over broad composition ranges for the first time via low-temperature combustion synthesis. The films are characterized via thermal analysis of the precursor solutions, grazing incidence angle X-ray diffraction (GIAXRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging. Excellent thin-film transistor (TFT) performance is achieved for all X, with optimal compositions after 300 °C processing exhibiting electron mobilities of 5.4, 2.6, 2.4, and 1.8 cm(2) V(-1) s(-1) for Ga(3+), Sc(3+), Y(3+), and La(3+), respectively, and with I(on)/I(off) = 10(7)-10(8). Analysis of the IXZO TFT positive bias stress response shows X = Ga(3+) to be superior with mobilities (µ) retaining >95% of the prestress values and threshold voltage shifts (ΔV(T)) of <1.6 V, versus <85% µ retention and ΔV(T) ≈ 20 V for the other trivalent ions. Detailed microstructural analysis indicates that Ga(3+) most effectively promotes oxide lattice formation. We conclude that the metal oxide lattice formation enthalpy (ΔH(L)) and metal ionic radius are the best predictors of IXZO oxygen getter efficacy.

Dialkoxybithiazole: a new building block for head-to-head polymer semiconductors.

Polymer semiconductors have received great attention for organic electronics due to the low fabrication cost offered by solution-based printing techniques. To enable the desired solubility/processability and carrier mobility, polymers are functionalized with hydrocarbon chains by strategically manipulating the alkylation patterns. Note that head-to-head (HH) linkages have traditionally been avoided because the induced backbone torsion leads to poor π-π overlap and amorphous film microstructures, and hence to low carrier mobilities. We report here the synthesis of a new building block for HH linkages, 4,4'-dialkoxy-5,5'-bithiazole (BTzOR), and its incorporation into polymers for high performance organic thin-film transistors. The small oxygen van der Waals radius and intramolecular S(thiazolyl)···O(alkoxy) attraction promote HH macromolecular architectures with extensive π-conjugation, low bandgaps (1.40-1.63 eV), and high crystallinity. In comparison to previously reported 3,3'-dialkoxy-2,2'-bithiophene (BTOR), BTzOR is a promising building block in view of thiazole geometric and electronic properties: (a) replacing (thiophene)C-H with (thiazole)N reduces steric encumbrance in -BTzOR-Ar- dyads by eliminating repulsive C-H···H-C interactions with neighboring arene units, thereby enhancing π-π overlap and film crystallinity; and (b) thiazole electron-deficiency compensates alkoxy electron-donating characteristics, thereby lowering the BTzOR polymer HOMO versus that of the BTOR analogues. Thus, the new BTzOR polymers show substantial hole mobilities (0.06-0.25 cm(2)/(V s)) in organic thin-film transistors, as well as enhanced I(on):I(off) ratios and greater ambient stability than the BTOR analogues. These geometric and electronic properties make BTzOR a promising building block for new classes of polymer semiconductors, and the synthetic route to BTzOR reported here should be adaptable to many other bithiazole-based building blocks.

Spectral aspects of cavity tuned absorption in organic photovoltaic films.

Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Using optical transfer matrix formalism, we model the absorption of organic photovoltaic films as a function of active layer thickness and incident wavelength. In our simulations we consider the absorption in the optical cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode. We find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We also observe distinct spectral effects due to frequency pulling which results in enhanced long-wavelength absorption. Spectral sculpting can be carried out by cavity design without affecting the open circuit voltage as the spectral shifts are purely optical effects. We have experimentally verified aspects of our modeling and suggest methods to improve device design.

Bithiopheneimide-dithienosilole/dithienogermole copolymers for efficient solar cells: information from structure-property-device performance correlations and comparison to thieno[3,4-c]pyrrole-4,6-dione analogues.

Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor-acceptor copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOs--essential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R(2)Si→R(2)Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π-π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure-property-device performance correlations are provided by a thieno[3,4-c]pyrrole-4,6-dione (TPD)-dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit-subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π-π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC(71)BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V(oc) > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V(oc) with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure-property relationships in TPD, BTI, and related polymer semiconductors.

A "zig-zag" naphthodithiophene core for increased efficiency in solution-processed small molecule solar cells.

A solution-processed small molecule utilizing a novel 5,10-bis((2-ethylhexyl)oxy)-naphtho[2,3-b:6,7-b0]dithiophene [corrected] "zig-zag" core (zNDT) exhibits high hole mobility, upshifted frontier MO energies, and enhanced photovoltaic cell short-circuit currents, fill-factors, and power conversion efficiencies (4.7%) versus the linear NDT isomer.

In-situ probe of gate dielectric-semiconductor interfacial order in organic transistors: origin and control of large performance sensitivities.

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.

Delayed ignition of autocatalytic combustion precursors: low-temperature nanomaterial binder approach to electronically functional oxide films.

Delayed ignition of combustion synthesis precursors can significantly lower metal oxide film formation temperatures. From bulk In(2)O(3) precursor analysis, it is shown here that ignition temperatures can be lowered by as much as 150 °C. Thus, heat generation from ~60 nm thick In(2)O(3) films is sufficient to form crystalline In(2)O(3) films at 150 °C. Furthermore, we show that the low processing temperatures of sufficiently thick combustion precursor films can be applied to the synthesis of metal oxide nanocomposite films from nanomaterials overcoated/impregnated with the appropriate combustion precursor. The resulting, electrically well-connected nanocomposites exhibit significant enhancements in charge-transport properties vs conventionally processed oxide films while maintaining desirable intrinsic electronic properties. For example, while ZnO nanorod-based thin-film transistors exhibit an electron mobility of 10(-3)-10(-2) cm(2) V(-1) s(-1), encasing these nanorods within a ZnO combustion precursor-derived matrix enhances the electron mobility to 0.2 cm(2) V(-1) s(-1). Using commercially available ITO nanoparticles, the intrinsically high carrier concentration is preserved during nanocomposite film synthesis, and an ITO nanocomposite film processed at 150 °C exhibits a conductivity of ~10 S cm(-1) without post-reductive processing.

Exploratory combustion synthesis: amorphous indium yttrium oxide for thin-film transistors.

We report the implementation of amorphous indium yttrium oxide (a-IYO) as a thin-film transistor (TFT) semiconductor. Amorphous and polycrystalline IYO films were grown via a low-temperature solution process utilizing exothermic "combustion" precursors. Precursor transformation and the IYO films were analyzed by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and optical transmission, which reveal efficient conversion to the metal oxide lattice and smooth, transparent films. a-IYO TFTs fabricated with a hybrid nanodielectric exhibit electron mobilities of 7.3 cm(2) V(-1) s(-1) (T(anneal) = 300 °C) and 5.0 cm(2) V(-1) s(-1) (T(anneal) = 250 °C) for 2 V operation.

Reduced contact resistance in inkjet printed high-performance amorphous indium gallium zinc oxide transistors.

Solution processing of amorphous metal oxide materials to fabricate thin-film transistors (TFTs) has received great recent interest. We demonstrate here an optimized "ink" and printing process for inkjet patterning of amorphous indium gallium zinc oxide (a-IGZO) TFTs and investigate the effects of device structure on derived electron mobility. Bottom-gate top-contact (BGTC) TFTs are fabricated and shown to exhibit electron mobilities comparable to a-Si:H. Furthermore, a record electron mobility of 2.5 cm(2) V(-1) s(-1) is demonstrated for bottom-gate bottom-contact (BGBC) TFTs. The mechanism underlying such impressive performance is investigated using transmission line techniques, and it is shown that the semiconductor-source/drain electrode interface contact resistance is nearly an order of magnitude lower for BGBC transistors versus BGTC devices.

Spectral aspects of cavity tuned absorption in organic photovoltaic films.

Concentration of light and infrared capture are two favored approaches for increasing the power conversion efficiency (PCE) of photovoltaic devices. Using optical transfer matrix formalism, we model the absorption of organic photovoltaic films as a function of active layer thickness and incident wavelength. In our simulations we consider the absorption in the optical cavity formed by the polymer bulk heterojunction active layer (AL) between the aluminum cathode and indium tin oxide (ITO) anode. We find that optical absorption can be finely tuned by adjusting the ITO thickness within a relatively narrow range, thus eliminating the need for a separate optical spacer. We also observe distinct spectral effects due to frequency pulling which results in enhanced long-wavelength absorption. Spectral sculpting can be carried out by cavity design without affecting the open circuit voltage as the spectral shifts are purely optical effects. We have experimentally verified aspects of our modeling and suggest methods to improve device design.