MICROFLUIDIC GIANT MAGNETORESISTANCE DETECTION OF MAGNETIC PARTICLES IN FLOW
Julian Sheats1, Lorena P. Maldonado-Camargo2, Carlos Rinaldi2, Srinand Sreevatsan1, Maria A. Torija3 and Kevin D. Dorfman1* 1 University of Minnesota, USA 2 University of Florida, USA 3 NVE Corporation, USA ABSTRACT Magnetic particles are ubiquitous in biological research due to their facile handling and functionalization. We developed a new method for detecting such particles in flow by integrating microfluidics with commercially available, giant magnetoresistance (GMR) sensors. Our method paves the way to the general use of GMR sensors in lab-on-a-chip devices in a wide range of potential applications. KEYWORDS: Microfluidic, Giant Magnetoresistance, Nanoparticles INTRODUCTION We are particularly interested in GMR detection as a component of a new flow-based sensor for pathogen detection in a bead-based, aptamer modality. Pathogen detection is a vitally important tool for the prevention of diseases within the food and medical care industries, especially in the developing world. There remains a need for rapid and inexpensive detection technology despite a plethora of existing sensors [1,2]. In typical magnetic bead-based sensors, such as immunoassays, the functionalized beads adhere to a particular target and are then immobilized in an external magnetic field for detection. The advantages of flow-based methods are the rapid passage of the analytes and reusability of the device. Magnetic detection is also desirable relative to optical methods, since it works in turbid solutions, which are common for biological fluids. Thus, a magnetic, flow-based assay is ideal for point-of-care and field use. The critical issue in GMR detection is the need to bring the particles close to the detector without adsorption onto the sensor itself. The GMR sensor [3] is a commercially available stack of different metal layers on a silicon substrate. The design of this device effectively isolates the sensor from the fluid, simplifying usage and reusability. In particular, separating the fluid from the sensor with a non-porous insulating layer allows the electronics (on the front-side of the wafer) to be decoupled from the fluidics (on the backside). This membrane, used in conjunction with magnetic nanoparticles whose surface has been appropriately functionalized, aids in eliminating issues due to absorption and facilitates cleaning. Additionally, the thin membrane supporting the sensor allows the particles to come in close proximity to the sensor to increase sensitivity. To demonstrate the ability of our device to detect a relevant analyte in flow, magnetic nanoparticles [4] approximately 20 nm in diameter were pegylated, in preparation for future aptamer conjugation, and detected in an aqueous medium. EXPERIMENTAL The GMR sensor was created by physical deposition via sputtering. Low-stress LPCVD silicon nitride-coated wafers were used to first lay down the sensors, similar to that of [5], following which the microfluidic channel was created by standard anisotropic potassium hydroxide (KOH) etch. This produced a nitride membrane supporting the sensor and preventing direct fluid contact. The etch temperature was carefully controlled so as to not fracture the membrane, and the sensors were protected during the wet etch using an etch resistant fixture (AMMT, GmbH). An example of an etched channel is shown in an SEM image in Figure 1A. The thin nature of the membrane, ~200 nm, makes it visible 978-0-9798064-7-6/µTAS 2014/$20©14CBMS-0001 1295
18th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 26-30, 2014, San Antonio, Texas, USA
optically, as shown in a sensor-side view in Figure 1B. A PDMS slab was used to connect inlet and outlet ports with ~1 mm holes punched. Due to the metal layers on the sensor, an oxygen plasma clean, typical for adhering such a device to PDMS, was prohibitive. For this reason, a PDMS glue was used to adhere the SiN bottom layer to the PDMS slab [6]. Mixed and uncured PDMS was spun on a slide at 6000 rpm for 15 min, after which the PDMS slab was pressed into this layer, removed, aligned and pressed onto the etched channel and allowed to cure. Cleaving the sensor dies from the wafer was used in place of a dicing saw due to fracture caused by the saw’s water jets; a protective photoresist layer did not prevent fracture. Cleaving membranes 2 mm in length was possible with only a scribe. However, cut lines were added to the etch mask for membranes greater than 2 mm in length to aid the scribe usage. Nevertheless, most membranes 300 µm wide and 30 mm long fractured upon cleavage due to strain upon bending. A two-step etch where the final few microns of silicon are dry etched after the device had been cleaved may aid future largemembrane die removal. The sensor die was formed with redundant features: narrow (~40 µm-wide) or wide (208 µm-wide) sensors, as well as both those using a Wheatstone above the channel and two layers are away from the channel, shown in Figure 1B, bridge setup or non-bridged sensors. The Wheatstone-bridged wide sensor, where two of the GMR layers are located was found to be most capable of reducing noise and detecting the particles. Additionally, in order to detect the superparamagnetic particles used here, we used pulses of magnetic field. If particles are present, the oscillation in the field detected by the active legs of the bridge is different than when the particles are absent.
A) B) Figure 1: Etched microchannel. A) Microchannel interface to membrane (SEM image, scale bar=100 µm), B) Sensor side view of the membrane (optical image, scale bar= 200 µm). The membrane has been false-colored pink. RESULTS AND DISCUSSION Control of the etch rate was found to be critical in preventing fracture of the membrane. This was especially true for membranes of 30 mm in length. An etch rate corresponding to a bath temperature of 50-60oC was found to prevent fracture. Wrinkles appeared in many membranes. This appeared to be a function of membrane width and etch speed. Membranes etched at faster rates or ones that were of width greater than 10-20 µm had a greater degree of wrinkling. As a result, the Wheatstone bridge was not perfectly in balance when wrinkles were present. To increase sensitivity of the detection, the channel width was set to approximately 300 µm at the sensor side, which is 92 µm wider than the sensor. Using the Wheatstone-bridged sensor in pulsed-field mode, the 20 nm magnetic pegylated particles were detected upon reaching the sensor; a modest flow rate of approximately 3.4 µl/min was initially used to test membrane fracture. The amplitude increase in 1296
CONCLUSION We have demonstrated the performance of a microfluidic device for detection magnetic nanoparticles where the sensor is decoupled from the fluidic channel. Using a GMR sensor supported by a membrane above an analyte flow, a pulsedfield Wheatstone bridge mode detected superparamagnetic nanoparticles in a flow of 3.4 µl/min.
1.2 1.1
nanoparticles injected
1.0
Amplitude Increase (mV)
current in Figure 2 shows the detection of the particles. The relatively large signal, in the hundreds of µV range, demonstrates the performance of the sensor in overcoming noise issues. However, the spread of the pulse indicates significant plug spreading occurred. This was not unexpected due to the size of the inlet port (~1 mm) as well as the size and geometry of the channel, which is 250-500 µm deep with abrupt edges. Both an apparatus for controlling a small injection volume as well as refined channel geometry will reduce plug spread.
0
1
2
3
4
5
6
7
8
9
Time (min) Figure 2: Detection of pegylated magnetic nanoparticles in bridged GMR sensor channel under a flow of ~3.4 µl/min.
ACKNOWLEDGEMENTS This work was supported by the National Science Foundation (IIP-1321460). Portions of this work were performed in the Minnesota NanoCenter, which receives partial funding from the NSF through NNIN. REFERENCES [1] A.M. Foudeh, et al., “Microfluidic designs and techniques using lab-on-a-chip devices for pathogen detection for point-of-care diagnostics," Lab on a Chip, 12, 3249-3266, 2012. [2] J. Mairhofer, K. Roppert, and P. Ertl, “Microfluidic Systems for Pathogen Sensing: A Review,” Sensors, 9, 4804-4823, 2009. [3] NVE Corporation, website http://www.nve.com. [4] C. Barrera, et al., “Effect of poly(ethylene oxide)-silane graft molecular weight on the colloidal properties of iron oxide particles in biomedical applications,” Journal of Colloid and Interface Science, 377(1), 40-50, 2012. [5] N. Pekas, et al., “Giant magnetoresistance monitoring of magnetic picodroplets in an integrated microfluidic system,” Applied Physics Letters, 85(20), 4783-4785, 2004. [6] S. Satyanarayana, et al., “Stamp-and-stick room-temperature bonding technique for microdevices,” J. Microeletromech. Syst., 14(2), 392-399, 2005. CONTACT * K.D. Dorfman; phone: +1-612-624-5560;
[email protected]
1297
