High-frequency electromagnetic dynamics properties of THP1 cells using scanning microwave microscopy
Highlights
► Experimental results of scanning microwave microscopy applied to biological cells. ► Electromagnetic dynamic characteristics in THP1 cell by SMM. ► Sub-micron spatial topographical image of the cellular surface. ► Information from underneath the cell surface collected in microwave images. ► Variations in the dielectric properties of cells at different resonant frequencies.
Introduction
A variety of near-field scanning microscopic methods has been developed for probing local variations of electromagnetic properties and analyzing material structures. This effort is associated with the concept of near field interactions between a source and a sample for which near-zone fields or evanescent waves with high spatial frequency are created. The impedance measurement of the electromagnetic response of materials at microwave frequencies is perhaps the most important approach for both fundamental and practical reasons. The major advantage of microwave fields over IR and optical waves is their penetrability into certain media that render possible imaging subsurface features in poorly conducting materials, due to the greater penetration depth in insulators. Hence characterization by microwaves is not limited to conducting or semiconducting media, making it a very versatile tool for a wide range of materials [1], [2].
More recently, near field microscopy techniques have allowed localizing electro-dynamic properties of materials and quantitative measurements with spatial resolutions much less than the wavelength [3]. Scanning microwave microscopy (SMM) provides unique measurement and imaging capabilities that are not afforded with other existing scanning probe microscopy tools. Easily and generally applicable, it does not require a bottom electrode or high quality oxide on the specimen. For example, SMM was used to simultaneously map variations in resistivity, permittivity or permeability of materials over a wide range of frequencies [4]. Combined microwave and atomic force microscopy (AFM) scans can be performed in contact mode, non-contact mode and other scanning modes. The AFM probe in close proximity to a sample locally enhances radiation generated by a focused far-field beam. Due to the enhanced field strength, detection through the local microscopic probe is facilitated. Performing microwave scans at different frequencies, and detecting both the magnitude and phase of the signal, multi-frequency and multi-modal imaging was accomplished so as to obtain additional information regarding the embedded structures, material properties, and non-uniformity [5], [6].
Moreover, microwave techniques are attractive for biological applications because of their sensitivity to water and dielectric contrast. Since water has a high dielectric constant and exhibits losses in the microwave frequency regime, SMM signals from many biological samples are dominated by the water contents of the samples. In bio-materials and cells, the resistivity is directly determined by the water and ionic content, while permittivity increases with density. In cases where resonant absorption of a molecule falls in the operational frequency range of a microwave, SMM is capable of imaging a variety of non-uniformities in biological materials quantifying tissue conductivity, permittivity, and density variations [7]. Moisture content, ionic species such as Na+, K+, [2] free radicals, and iron content of blood affect the conductivity of biological materials [7], [8]. In addition, density variations, which influence the dielectric constant can be mapped and quantified [9], [10]. Overall, SMM has the ability to promote characterization of biomaterials in a nondestructive manner.
In the present work, SMM was utilized to characterize changes of reflected amplitude and phase signals of biological cells on metal and glass substrates at micro- to nano-meter resolution by applying different resonance microwave frequencies.
Section snippets
Sample preparation
Cells were adhered to poly-l-lysine covered gold substrates or glass cover slips. Human monocytic leukemia cells (THP1, American type culture collection, ATCC) were transduced with a lentiviral vector encoding for YFP-labeled human CD1d and were grown in RPMI 1640, 10% FBS, and 2 mM l-glutamine. 1 mM sodium pyruvate, penicillin (100 units ml−1), and streptomycin (100 μg ml−1) were added to the medium to inhibit bacterial contamination. Cells were maintained between 2×105 and 9×105 cells ml−1 and grown
Results and discussion
S11 reflection amplitude curves were acquired in dependence on the frequency after positioning the cantilever close to a THP1 cell surface (Fig. 1). In Fig. 1(a), a plot of the reflected impedance signal vs. frequency at different distances between the tip and the sample surface is shown. The maximum of the reflected impedance decreases with increasing tip-surface distance. However, only a small variation is evident at close distances, but a strong reflection decrease appears between 100 and
Conclusion
We have presented first experimental results of scanning microwave microscopy applied to biological cells. The most powerful feature of the SMM technique is mapping the distribution of dielectric properties of materials at high resolution. Here, we have reached a sub-micron spatial topographical image of the cellular surface, simultaneously with information from underneath the cell surface collected in microwave images. The resonance frequency changed in a complex manner according to the
Acknowledgments
This study was supported by the Christian Doppler Society.
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