Supplementary Information For

Transcription

1 Supplementary Information For Magnetoresistive performance and comparison of supermagnetic nanoparticles on giant magnetoresistive sensor-based detection system Wei Wang, Yi Wang, Liang Tu, Yinglong Feng, Todd Klein & Jian-Ping Wang * Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA Those authors contributed equally to this work. * Correspondence and requests for materials should be addressed to J.P.W. (jpwang@umn.edu)

2 Figure S1. Design layout design of UMN 64-sensor GMR biochip (16 mm 16 mm). The center is 8 8 sensor array (3 mm 3 mm). Each sensor had been numbered and accordingly connected to peripheral contact pads on the periphery of the chip via contact lines. These numbered pads serve as one electrode for each sensor, and the two bus pads connected to all sensors serve as another electrode. Even if one bus pad breaks, all sensors can still work via another bus pad. The two unnumbered pads are used as two reference points for automatic spotting rather than used for electronic purposes. Unlike microscope slide or micro-titer plate based diagnostic techniques, capture biomolecules should be accurately spotted and immobilized on sensor surface for GMR biosensor, and this is a prerequisite.

3 Figure S2. The transfer curve of our GMR is generated by measuring the resistance change of the sensor as applied external magnetic field changes by sweeping the field strength along the minor axis of the sensor. The sensor has a maximum resistance of 5623 Ω in the antiparallel state and a minimum resistance of 5479 Ω in the parallel state. The sensor has a magnetoresistance ratio (MR) of 2.6%.

4 Figure S3. (a) 8 8 sensor array in GMR chip. (b) Probe DNA solution or BSA solution was printed on individual sensor using sciflexarrayer S5 (Scienion, Germany). Each spot containing nanoliter solution is appropriate for our GMR chip. The printed solution can fully cover the sensor but not merge with adjacent spot. During the printing, relative humidity in the system chamber should exceed 90%. Engineering of surface hydrophilicity could be an approach to increase immobilized density of receptors on sensor surface and hence improve detection sensitivity.

5 Figure S4. The GMR biosensing detection system. It includes a power supply, a wheatstone bridge PCB, a connection stage with an electromagnet, data acquisition boards, and a laptop. Figure S5. The soft iron core of an electromagnet. A coil is wound on the core with 24-gauge and 600 turns copper wires. The maximal output of alternating in-plane field in the gap is over 200 Oe. During the signal measurement, the alternating in-plane field of 30 Oe at 50 Hz was applied along the minor axis of the sensor.

6 Figure S6. The probe DNA oligonucleotide (50 nmol/ml) was spotted and immobilized on 3- aminopropyltriethoxy silane and glutaraldehyde functionalized silicon wafer surface. Different concentrations of target DNA were hybridized to the surface, and streptavidin- AF555 (20 μg/ml) was added and bound. (a) Fluorescent images and (b) fluorescent intensity of the hybridization spots.

7 Table S1. Overview of physicochemical characteristics of streptavidin functionalized MNPs Company name Catalog number Diameter Concentration of stock MNPs ζ Potential (ph=7.4) Ocean NanoTech, USA Ocean NanoTech, USA Ocean NanoTech, USA Miltenyi Biotech, Germany Ademtech, France SHS-10 SHS-20 SHS-30 MACS ADM nm 20 nm 30 nm 50 nm 100 nm 1mg/mL (Fe) 1mg/mL (Fe) 1mg/mL (Fe) 2 mg/ml (Fe x O y ) -20 ~ -40 mv -20 ~ -40 mv -20 ~ -40 mv -11 mv -7.5 mv The three kinds of MNPs (10 30nm) have a higher ζ potenial at ph 7.4 than 50 nm and 100 nm MNPs, and high ζ potenial improves the stability of MNPs due to electrostatic repulsion of surface charge. Brownian motion can also help small MNPs fight against settling as large MNPs precipitate faster because of gravity. In the experiment, we found that the three kinds of MNPs (10 30nm) from Ocean NanoTech and 50 nm MNPs from Miltenyi Biotech can keep colloidally stable for at least one month, but some visible sediments emerge for the 100 nm MNPs solution after its 24 h static standing. The 100 nm MNPs solution can retain colloidally stable for several hours after briefly shaking. Thus, the 100 nm MNPs needs a shake to disperse before loading during signal measurement, and the other four kinds of MNPs can use directly.

8 Figure S7. Magnetic hysteresis loops for five kinds of MNPs in the range of to 3000 Oe. Figure S8. Magnetic hysteresis loops for five kinds of MNPs in the range of -100 to 100 Oe.

9 Table S2. Brownian and Néel relaxation times for different MNPs. Samples τ B (s, in water) τ N (s) SHS-10, 10 nm SHS-20, 20 nm SHS-30, 30 nm MACS, 50 nm Adembeads,100 nm 3.1* * * * * * * * * *10-8 Brownian relaxation time τ B = 3ηV/(kT). η is the viscosity of a fluid. It is 1.002*10-3 Pa s for water at 20 ºC ; V is the hydrodynamic volume of MNP; k is Boltzmann constant, 1.38*10-23 J K -1 ; T is thermodynamic temperature, here we use 293K; Assuming that all the MNP have 10nm polymer and protein coating. Néel relaxation time τ N = τ 0 *exp(kv /(kt)). V is the magnetic volume of MNP; k is Boltzmann constant, 1.38*10-23 J K -1 ; T is thermodynamic temperature, here we use 293K; K is the anisotropy constant, 1.6*10 4 J/m 3 ; τ 0 is 10-9 s. The 50nm and 100 nm MNPs are composed of 10 nm MNPs embedded in polymer matrix, so their magnetic volumes are still calculated in the form of 10 nm.

10 Normalized Amplitude nm 20nm 30nm MACS50 Bead Frequency (khz) Normalized Amplitude nm 20nm 30nm MACS50 Bead Frequency (Hz) Figure S9. The amplitude of MNPs magnetization under high and low frequency. As the applied field frequency is swept from 2 to 20 khz, the amplitude of 20 nm and 30 nm MNPs magnetization obviously decreases as the frequency increases. These two kinds of MNPs respond slower than the field with high frequency. However, the 10 nm, 50 nm and 100 nm MNPs show no obvious change along the frequency, because the flipping of their magnetization is dominated by Néel relaxation. The amplitude of all the MNPs magnetization shows no obvious decrease along frequency from 10 to 190 Hz. The maximal decrease is less than 2%.

11 Figure S10. SEM images of different MNPs bound on GMR sensor surface. (a) Control; (b) 20 nm MNPs; (c) 50 nm MNPs; (d) 100 nm MNPs. All the scale bars are 1 μm.