We report an approach for the detection of magnetic resonance imaging without superconducting magnets and cryogenics: optical atomic magnetometry. This technique possesses a high sensitivity independent of the strength of the static magnetic field, extending the applicability of magnetic resonance imaging to low magnetic fields and eliminating imaging artifacts associated with high fields. By coupling with a remote-detection scheme, thereby improving the filling factor of the sample, we obtained time-resolved flow images of water with a temporal resolution of 0.1 s and spatial resolutions of 1.6 mm perpendicular to the flow and 4.5 mm along the flow. Potentially inexpensive, compact, and mobile, our technique provides a viable alternative for MRI detection with substantially enhanced sensitivity and time resolution for various situations where traditional MRI is not optimal.low field ͉ remote detection

We demonstrate remote detection of nuclear magnetic resonance (NMR) with a microchip sensor consisting of a microfluidic channel and a microfabricated vapor cell (the heart of an atomic magnetometer). Detection occurs at zero magnetic field, which allows operation of the magnetometer in the spin-exchange relaxationfree (SERF) regime and increases the proximity of sensor and sample by eliminating the need for a solenoid to create a leading field. We achieve pulsed NMR linewidths of 26 Hz, limited, we believe, by the residence time and flow dispersion in the encoding region. In a fully optimized system, we estimate that for 1 s of integration, 7 ؋ 10 13 protons in a volume of 1 mm 3 , prepolarized in a 10-kG field, can be detected with a signal-to-noise ratio of Ϸ3. This level of sensitivity is competitive with that demonstrated by microcoils in 100-kG magnetic fields, without requiring superconducting magnets.microfluidics ͉ signal-to-noise ratio ͉ mass-limited sample R emote detection of nuclear magnetic resonance (NMR) (1), in which polarization, encoding or evolution, and detection are spatially separated, has recently attracted considerable attention in the context of magnetic resonance imaging (2), microfluidic flow profiling (3, 4), and spin-labeling (5). Detection can be performed with superconducting quantum interference devices (SQUIDs), inductively at high field as in refs. 3-5 or with atomic magnetometers as in ref. 2. To most efficiently detect the flux from the nuclear sample, it is typically necessary to match the physical dimensions of the sensor and the sample. Thus, small, sensitive detectors of magnetic flux reduce the detection volume, thereby reducing the quantity of analyte. Microfabricated atomic magnetometers (6) with sensor dimensions on the order of 1 mm operating in the spin-exchange relaxation-free (SERF) regime (8) have recently demonstrated sensitivities of Ϸ0.7 nG/ ͌ Hz (7), with projected theoretical sensitivities several orders of magnitude higher. (In this article, we use Gaussian units; 1 nG ϭ 100 fT.)In this work, we demonstrate remote detection of pulsed and continuous-wave (CW) NMR with a compact sensor assembly consisting of an alkali vapor cell and microfluidic channel, fabricated with lithographic patterning and etching of silicon. We realize pulsed NMR linewidths of Ϸ26 Hz, limited, we believe, by residence time and flow dispersion in the encoding region. Estimates of the fundamental sensitivity limit for an optimized system, assuming a modest 10-kG prepolarizing field, indicate detection limits competitive with those demonstrated by microcoils in superconducting magnets (9-14). Hence, the technique described here offers a promising solution to NMR of mass-limited samples-for example, in the screening of new drugs-without requiring superconducting magnets.The atomic magnetometer operates in the SERF regime (achieved when the Larmor precession frequency is small compared with the spin exchange rate), currently the most sensitive technique in atomic magnetometry. Optical pum...

Magnetite (Fe3O4) nanoparticles (NPs) are attractive nanomaterials in the field of material science, chemistry, and physics because of their valuable properties, such as soft ferromagnetism, half-metallicity, and biocompatibility. Various structures of Fe3O4 NPs with different sizes, geometries, and nanoarchitectures have been synthesized, and the related properties have been studied with targets in multiple fields of applications, including biomedical devices, electronic devices, environmental solutions, and energy applications. Tailoring the sizes, geometries, magnetic properties, and functionalities is an important task that determines the performance of Fe3O4 NPs in many applications. Therefore, this review focuses on the crucial aspects of Fe3O4 NPs, including structures, synthesis, magnetic properties, and strategies for functionalization, which jointly determine the application performance of various Fe3O4 NP-based systems. We first summarize the recent advances in the synthesis of magnetite NPs with different sizes, morphologies, and magnetic properties. We also highlight the importance of synthetic factors in controlling the structures and properties of NPs, such as the uniformity of sizes, morphology, surfaces, and magnetic properties. Moreover, emerging applications using Fe3O4 NPs and their functionalized nanostructures are also highlighted with a focus on applications in biomedical technologies, biosensing, environmental remedies for water treatment, and energy storage and conversion devices.

Magnetic particles are widely used as signal labels in a variety of biological sensing applications, such as molecular detection and related strategies that rely on ligand-receptor binding. In this review, we explore the fundamental concepts involved in designing magnetic particles for biosensing applications and the techniques used to detect them. First, we briefly describe the magnetic properties that are important for bio-sensing applications and highlight the associated key parameters (such as the starting materials, size, functionalization methods, and bio-conjugation strategies). Subsequently, we focus on magnetic sensing applications that utilize several types of magnetic detection techniques: spintronic sensors, nuclear magnetic resonance (NMR) sensors, superconducting quantum interference devices (SQUIDs), sensors based on the atomic magnetometer (AM), and others. From the studies reported, we note that the size of the MPs is one of the most important factors in choosing a sensing technique.

The specific binding between the two DNA strands in a double helix is one of the most fundamental and influential molecular interactions in biochemistry. Using force-induced remnant magnetization spectroscopy (FIRMS), we obtained well-defined binding forces of DNA oligomers, with a narrow force distribution of 1.8 pN. The narrow force distribution allows for directly resolving two DNA duplexes with a single base-pair difference in the same sample. Therefore, binding force can serve as a discriminating parameter for probing different DNA interactions. Furthermore, we observed that the binding forces depend on the position of the mismatching base pair. Our results show that FIRMS is capable of high-precision mechanical measurements of biochemical processes involving multiple DNA interactions and has the potential for characterizing the binding strength of materials based on DNA origami.

We report on the design, characterization, and applications of a sensitive atomic magnetic gradiometer. The device is based on nonlinear magneto-optical rotation in alkali-metal ͑ 87 Rb͒ vapor and uses frequency-modulated laser light. The magnetic field produced by a sample is detected by measuring the frequency of a resonance in optical rotation that arises when the modulation frequency equals twice the Larmor precession frequency of the Rb atoms. The gradiometer consists of two atomic magnetometers. The rotation of light polarization in each magnetometer is detected with a balanced polarimeter. The sensitivity of the gradiometer is 0.8 nG/ Hz 1/2 for near-dc ͑0.1 Hz͒ magnetic fields, with a base line of 2.5 cm. For applications in nuclear magnetic resonance ͑NMR͒ and magnetic resonance imaging ͑MRI͒, a long solenoid that pierces the magnetic shields provides an ϳ0.5 G leading field for the nuclear spins in the sample. Our apparatus is particularly suited for remote detection of NMR and MRI. We demonstrate a point-by-point free induction decay measurement and a spin echo reconstructed with a pulse sequence similar to the Carr-Purcell-Meiboom-Gill pulse. Additional applications and future improvements are also discussed.

The power stroke of a motor protein: The motor protein EF‐G generates a power stroke of 89 pN during ribosome translocation (see picture). This mechanical force is obtained by measuring the force‐induced dissociation of a series of DNA–mRNA duplexes. The dissociation is indicated by a decrease in magnetic signal.

This paper highlights the relation between the shape of iron oxide (Fe3O4) particles and their magnetic sensing ability. We synthesized Fe3O4 nanocubes and nanospheres having tunable sizes via solvothermal and thermal decomposition synthesis reactions, respectively, to obtain samples in which the volumes and body diagonals/diameters were equivalent. Vibrating sample magnetometry (VSM) data showed that the saturation magnetization (Ms) and coercivity of 100–225 nm cubic magnetic nanoparticles (MNPs) were, respectively, 1.4–3.0 and 1.1–8.4 times those of spherical MNPs on a same-volume and same-body diagonal/diameter basis. The Curie temperature for the cubic Fe3O4 MNPs for each size was also higher than that of the corresponding spherical MNPs; furthermore, the cubic Fe3O4 MNPs were more crystalline than the corresponding spherical MNPs. For applications relying on both higher contact area and enhanced magnetic properties, higher-Ms Fe3O4 nanocubes offer distinct advantages over Fe3O4 nanospheres of the same-volume or same-body diagonal/diameter. We evaluated the sensing potential of our synthesized MNPs using giant magnetoresistive (GMR) sensing and force-induced remnant magnetization spectroscopy (FIRMS). Preliminary data obtained by GMR sensing confirmed that the nanocubes exhibited a distinct sensitivity advantage over the nanospheres. Similarly, FIRMS data showed that when subjected to the same force at the same initial concentration, a greater number of nanocubes remained bound to the sensor surface because of higher surface contact area. Because greater binding and higher Ms translate to stronger signal and better analytical sensitivity, nanocubes are an attractive alternative to nanospheres in sensing applications.