Photo-induced force microscopy was first introduced in Prof. Wickramasinghe’s lab in 2010 [1]. In this method, the very end of an AFM tip is illuminated by a modulated light. The AFM is operating in non-contact mode and its cantilever is vibrating at its first mechanical resonance mode. The modulation frequency of the light is the difference of the first and second mechanical resonance modes of the cantilever. Therefore, there will be a strong force at the second mechanical resonance mode of the AFM. In this way, the AFM can take topography image (at its first resonance) and PiFM image (at its second resonance) simultaneously. Fig. 1 shows topography image of a PMMA-PS block copolymer as well as its PiFM image at two different wavelengths corresponding to the absorption peaks of PS and PMMA [2]. The resolution is in nanometer scale. PiFM has also been used to map different components of the electric field [3-4] of the different illuminations.

Fig. 2: Longitudinal components of four different Gaussian beam excitations. [3] Photo-induced force microscopy was first introduced in Prof. Wickramasinghe’s lab in 2010 [1]. In this method, the very end of an AFM tip is illuminated by a modulated light. The AFM is operating in non-contact mode and its cantilever is vibrating at its first mechanical resonance mode. The modulation frequency of the light is the difference of the first and second mechanical resonance modes of the cantilever. Therefore, there will be a strong force at the second mechanical resonance mode of the AFM. In this way, the AFM can take topography image (at its first resonance) and PiFM image (at its second resonance) simultaneously. Fig. 1 shows topography image of a PMMA-PS block copolymer as well as its PiFM image at two different wavelengths corresponding to the absorption peaks of PS and PMMA [2]. The resolution is in nanometer scale. PiFM has also been used to map different components of the electric field [3-4] of the different illuminations. Fig. 2 shows spatial distribution of the longitudinal component of the E-field of (a) linearly polarized wave along the x-axis, (b) linearly polarized wave along the y-axis, (c) azimuthally polarized wave, and (d) radially polarized wave [3].
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[1] I. Rajapakse, K. Uenal, H. Kumar Wickramasinghe, “Image force microscopy of molecular resonance – a microscope principle”, Virtual Journal of Nanoscience and Technology, Vol. 22, issue 10, 2010.
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[2] D. Nowak, W. Morrison, H. K. Wickramasinghe, J. Jahng, E. Potma, L.Wan, R.Ruiz, T. R. Albrecht, K. Schmidt, J. Frommer, D. P. Sanders, S. Park, “ Nanoscale chemical imaging by photo-induced force microscopy” , Sci. Adv. 2016.
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[3] F. Huang, V.A Tamma, Z. Mamaghani, J. Burdett and H.K.Wickramasinghe “Imaging Nanoscale Electromagnetic Near-Field Distributions Using Optical Forces”, Scientific Reports, Nature Publishing, DOI 10.1038/srep10610, 2015.
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[4] F. Huang, V. K. Tamma, M. Rajaei, M. Almajhadi, and H. K. Wickramasinghe. “Measurement of laterally induced optical forces at the nanoscale.” Applied Physics Letters 110, no. 6 (2017): 063103.

This research project is supported by W. M. Keck Foundation and supervised by Prof. Filippo Capolino, Prof H. Kumar Wickramasinghe, Prof. Eric Potma, and Prof. V. Ara Apkarian, where the Wickramasinghe group as the major experimental group will lead the instrumentation development. The ultimate goal is to realize direct mapping of photo-induced magnetic force at nanoscale, by implementing a magnetic nanoprobe under special structured light illumination in the existing Photo-induced microscopy platform. In this aim, we proposed the magnetic nanoprobe in axis-aligned azimuthally polarized beam (APB) system that supports magnetic resonance at optical frequencies, to exclusively interact magnetic field light without the interference from the electric field counter-part [1]. The early experiment development includes using PiFM to characterize the electric near-field profile of the sharply focused APB as the ideal magnetic probing beam [3], and of a particular magnetic nanoprobe, Si nano-disk, under APB illumination as the core light-matter interaction system of the PiMFM [4]. These intermediate steps show promising results as indirect demonstration of our proposed system. We currently take endeavor into achieving direct magnetic force acquisition as the final step of this ambitious project.

[1] Caner Guclu, Mehdi Veysi, and Filippo Capolino. “Photoinduced magnetic nanoprobe excited by an azimuthally polarized vector beam.” ACS Photonics 3.11 (2016): 2049-2058.

[2] Caner Guclu, Venkata Ananth Tamma, H. Kumar Wickramasinghe, and Filippo Capolino. “Photoinduced magnetic force between nanostructures.” Physical Review B 92.23 (2015): 235111.

[3] Jinwei Zeng, Fei Huang, Caner Guclu, Mehdi Veysi, Mohammad Albooyeh, H. Kumar Wichramasinghe, and Filippo Capolino. “Sharply focused azimuthally polarized beams with magnetic dominance: near-field characterization at nanoscale by photoinduced force microscopy.” ACS Photonics (2017).

[4] Manuscript in preparation.

Dissecting the regulation of gene expression processes is fundamental to understand how cells function. Techniques used to obtain our current view of gene expressions rely on isolating mRNAs from large numbers of cells, which often associates with a loss or damage of the material, as well as the loss of spatial information. Also, individual cells within a population are unlikely to behave all in the same way, and current standard techniques are unable to detect cell-to-cell differences that can result from genetic variation, biological noise and different characteristics of genes within a population.

To solve these limitations, we developed a sensitive and non-destructive method and apparatus for tracking gene expressions on single cell level within a population of cells, either on a collagen cell culture gel or in a microfluidic environment. The developed bench-top instrument utilizes a modified AFM (Atomic Force Microscopy) probes for in situ extraction of mRNA molecules from single cells. The modified coaxial AFM probe serves as nanotweezer, and wherein the application of an alternating potential between the inner and outer electrodes of the co-axial cable creates a dielectrophoretic force for attracting target molecules toward the tip-end. For adherent cell lines, the samples are cultured on collagen gel for select-and-probe analysis. An integrated microfluidic/nanoprobe platform is also developed for gene expression analysis of suspension cells on single cell level.

This system directly targets and samples down to a few molecules within a single living cell, without the need for purification and averaging typically of conventional technologies. By providing reliable and exceptional sensitivity to identify differences between individual cells in a seemingly homogeneous population, the system creates possibilities for assaying the genomic analysis in living cells and tracking them in response to external stimuli. This technique has potential impacts on understanding the heterogeneity of transcriptional responses as well as its implications for cell function and disease. It is going to have broad application areas ranging from systems biology to cancer research.

[1] Yinglei Tao and H. Kumar Wickramasinghe, “Coaxial atomic force microscope probes for dielectrophoresis of DNA under different buffer conditions”, Applied Physics Letters 110, 073701 (2017)

[2] Xuan Li*, Yinglei Tao*, Do-Hyun Lee, H. Kumar Wickramasinghe and Abraham P. Lee, “In situ mRNA isolation from a microfluidic single-cell array using an external AFM nanoprobe”, Lab on a chip 17, 1635 (2017)