We present a simple device that measures the optical transmission of a suspension of magneto-tactic bacteria under application of a magnetic field. The system consists of a measurement head with a RGB LED, a photodetector and three sets of orthogonal coils that generate a field up to 3.5 mT. In addition we realized a module based on an ESP32 microprocessor that sets the coil currents, performs data collection, and features a touchscreen and Wifi access. Source files such as source code for the software as well as links to the designs for hardware (3D prints, circuit boards) are available online at https://github.com/LeonAbelmann/MagOD, so that anyone with average electronic skills (for instance EE students) can construct such a magnetic Optical Density meter (MagOD) for under 1000 Eu. We demonstrate to application of the MagOD system on two types of magneto-tactic bacteria. Magnetospirillum bacteria (MSR-1) are elongated and have very different scattering properties depending on the direction of the incident light. Figure 1 shows that there is a clear change in transmitted light when the magnetic field is rotated from parallel to perpendicular to the light path. The signal difference (red lines) is a measure for the fraction of magnetic bacteria in the culture. Since the MagOD meter measures at data acquisition rates up to 800 Hz, we can also time-resolve the rotation of the bacteria (blue lines). The time constant for rotation is a measure for the ratio between bacteria length and their magnetic moment. Unlike MSR-1, which swim along the magnetic field lines but reverse frequently in direction, Magnetococcus bacteria (MC-1) swim in one direction only. This allows us to perform marathon experiments, as shown in figure 2. First, the bacteria are collected at the bottom of the container by a vertical magnetic field. After reversal of the field, the entire groups swim up and passes the photodetector after some time. From the reduction in transmission, we can obtain the distribution in swimming velocity. These are just two examples of the wealth of information one can obtain with such an inexpensive system. Not all possibilities have been explored yet. Next to magneto-tactic bacteria, also suspensions of magnetic nanoparticles can be analyzed with respect to sedimentation and clustering. Since the LED used for the light beam has three different wavelengths, which can be individually tuned in intensity, simple spectral analysis methods can be applied. Moreover next to transmitted light also the scattered light can be detected. Since the housing is 3D printed, other geometries can be easily realized. By making the source code available, we hope that researchers in other fields will make use of and expand on our efforts.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/34803d5b19fbdfb0bfe99155bb581fe1.jpg)
Fig 1. The scattering properties of magnetospirillum bacteria is dependent on their orientation with respect to the incident light beam. By rotating the magnetic field, we can obtain information on the amount of magnetic bacteria in the suspension from the amplitude of the signal (Cmag) and information on the magnetic moment from the time response (gamma).

![](https://assets.underline.io/uploads/markdown_image/1/image/7ceb7ab967a7f5ec83236c2905f337fb.jpg)
Fig. 2. Magnetococcus bacteria can be sent to the bottom of the container by a vertical magnetic field. When the field is reversed, the entire group swims up (like in a marathon). From the resulting dip in transmission, we can determine the velocity distribution of the bacteria.

Analysis of magneto-tactic bacteria using an open-source optical density meter

We demonstrate the realisation of nanoparticle concentration gradients by means of Halbach arrays positioned below channels dedicated for cell culture studies. The nanoparticles can be magnetic as well as non-magnetic if a biocompatible chelated Gadolinium carrier fluid is used. These gradients are extremely helpful for the dose-response analysis of nano-particle toxicity. Conventionally, concentration ranges are prepared in individual containers (wells). Even though precise, this method is labour intensive both in preparation and analysis and requires considerable space inside the incubator. The use of gradients allows for rapid scanning of the concentration range of interest at small form factor. The Halbach array was realized using 48 commercially available 1.0x1.5x5.0 mm NdFeB magnets. Analytical solutions are given for the forces on magnetic nanoparticles and non-magnetic spheres in a paramagnetic solution. We demonstrate this method on human liver cells inside Ibidi micro-Slide channels of 400 um height using Fe2O3 and silver nanoparticles.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/d7061caed5097b142d18229f01bee10d.jpg)
Fig. 1. Top: Concentration gradient of oxide nanoparticles of 5 nm diameter inside a channel slide dedicated for cell culture analysis. Bottom: Using a paramagnetic fluid, also concentration gradients of non-magnetic particles can be realized.

 Concentration gradients of magnetic nanoparticles in non-magnetic fluids and non-magnetic particles in magnetic fluids for rapid toxicity screening

A major fraction of airborne particulate matter is magnetic. The origin of these magnetic particles is anthropogenic (industry, traffic), and often accompanied by traces of very toxic heavy metal pollutants. The magnetic fraction of airborne particulate matter is therefore a good indicator for the amount of air pollution, and can serve as a first indicator for toxicity. We collected airborne magnetic particulate matter directly onto silicon substrates by means of a stack of permanent NdFeB magnets (Figure 1). Collection was performed in open air along a campus road for a period of 2-4 weeks. For comparison, a control substrate was mounted in a similar fashion, nearby but not on a magnet. The silicon substrates can be readily used in a variety of surface analysis techniques. The morphology of the captured particles was analyzed by optical and electron microscopy, as well as by AFM. The substrate that was on the magnet stack showed an increased amount of particles. The particle size varied from 50 micrometer down to diameters below 100 nm. Many of the particles are perfectly round spheres with a diameter of a few up to tens of micrometers. SEM-EDX analysis showed that particles that are captured magnetically mainly consist of iron-oxide. VSM analysis showed that the substrate that was on the magnet has a magnetic moment that is six times higher than the substrate that was not in the magnetic field gradient (Figure 2). The average collection of magnetic particulate matter on the 5mm×5mm area is approximately 4 nAm2/day, which translates to 46 ng/day if the particles are made entirely of iron-oxide. These experiments show that one can collect magnetic particulate matter directly from the air, without using filters and complex post-treatment procedures that change the particle composition and cause clustering. Magnetic particulate matter sampling is therefore a promising complementary technique that can be applied in for instance air quality monitoring stations.

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/11c6c4c908f151e15109ac69ec88de7b.jpg)
Fig 1. A stack of NdFeB magnets ranging in diameter from 70 down to 4 mm is used to attract magnetic particulate matter onto a 5 mm x 5 mm silicon substrates. The substrate is shielded from rain by a tube, and the magnets are protected by a PVC enclosure.

![](https://assets.underline.io/uploads/markdown_image/1/image/7752478b701390e1c101a1464837f6ef.jpg)
Fig. 2. The magnetic moment of the substrate that was outside on the magnet stack for a month (red curve) is at least six times higher than that of a substrate that was outside for the same period, but not on a magnet (blue curve). The reference (black curve) is the signal of a substrate that was kept in a box.

 Capture of magnetic particulate matter directly from air on silicon substrates

*Session Co-Chair: Jose Mardegan, DESY*

*Session Co-Chair: Marcin Sikora, AGH University of Science and Technology*

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AC: Instrumentation and Measurement Techniques

technical paper

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JG: Bio-applications of Magnetism I

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Leon Abelmann

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Analysis of magneto-tactic bacteria using an open-source optical density meter

Concentration gradients of magnetic nanoparticles in non-magnetic fluids and non-magnetic particles in magnetic fluids for rapid toxicity screening

Capture of magnetic particulate matter directly from air on silicon substrates

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