Force-Detected Magnetic Resonance Imaging of Influenza Viruses in the Overcoupled Sensor Regime

Abstract

Long and thin scanning force cantilevers are sensitive to small forces, but also vulnerable to detrimental noncontact interactions. Here we present an experiment with a cantilever whose spring constant and static deflection are dominated by the interaction between the tip and the surface, a regime that we refer to as "overcoupled." The interactions are an obstacle for ultrasensitive measurements such as nanoscale magnetic resonance imaging (nano-MRI). We discuss several strategies to overcome the challenges presented by overcoupling, and demonstrate proton nano-MRI measurements of individual influenza virus particles.

Nanoscale Magnets Embedded in a Microstrip

“…We test the performance of our magnet-in-microstrip device in a force-detected NanoMRI setup and find that the field gradient in x -direction, the direction of the cantilever oscillation, is boosted by a factor of ∼4 relative to magnet-on-microstrip devices . As a consequence, we detect nuclear spin ensembles within an averaging time of 10 s, roughly 1 order of magnitude faster than in previous work . We further demonstrate a best-effort spatial resolution below 1 nm in the direction of oscillation.…”

“…In Figure a,b, we show a two-dimensional spatial scan measured with an integration time of only 10 s per pixel, compared to typical integration times between 60 and 300 s in earlier measurements. ,, The SNR we obtain reaches values above 8 over a wide range of the scan. The signal strength and SNR clearly surpass the results obtained for the same sample with a magnet-on-microstrip device, where a 6-fold longer integration time and smaller tip–magnet distance were used (see Figure d,e). This speedup, enabled by the large G x values, will be valuable when performing large three-dimensional scans of extended nanoscale samples.…”

“…To test the functionality of the embedded nanomagnets, we mount them in a magnetic resonance force microscopy (MRFM) setup. ,, Our setup is suspended in a vacuum chamber inside a liquid helium bath cryostat, with a base temperature of T = 4.7 K and a pressure of p = 1 × 10 –7 mbar. The force sensor is an in-house-fabricated silicon cantilever (see Figure a). ,,, The cantilever has dimensions of 150 μm × 4 μm × 130 nm, a free resonance frequency f 0 = 3500 Hz, a mass m = 100 pg, and a quality factor Q = 25000 at cryogenic temperatures around 4 K. These properties result in an intrinsic spring constant of k 0 = 50 μN/m and a damping coefficient of γ 0 = 8.8 × 10 –14 kg/s. Sensor readout is achieved using a fiber-optic interferometer. , By analyzing the static and oscillating components of the reflected light, we extract the cantilever frequency and static deflection, respectively.…”

“…The laser temperature is stabilized to within 10 mK to reduce signal drift . Samples are attached to the end of the cantilever via a silicon nanorod , and then brought into close proximity with the nanomagnet on the chip. A scanning electron microscopy (SEM) micrograph of the magnet-in-microstrip device, which forms the main advance of this work, is shown in Figure b together with a traditional magnet-on-microstrip device, shown in Figure c.…”

“…Direct comparison of magnet-in-microstrip (a–c) and magnet-on-microstrip (d–f) devices. Both scans image an identical virus sample . (a) Two-dimensional MRFM measurement recorded with the magnet-in-microstrip device with an integration time of 10 s and h = 100 nm.…”