Label-free imaging of fibroblast membrane interfaces and protein signatures with vibrational infrared photothermal and phase signals

Increasing contrast in water-embedded particles via time-gated mid-infrared photothermal microscopy

The transient dynamics of photothermal signals provide interesting insights into material properties and heat diffusion. In a mid-infrared (mid-IR) photothermal microscope, the imaging contrast in a standard amplitude imaging can decrease due to thermal diffusion effects. It is shown that contrast varies for poly-methyl 2-methylpropenoate (PMMA) particles of different sizes when embedded in an absorbing medium of water (H2O) based on levels of heat exchange under the water absorption resonance. Using time-resolved boxcar (BC) detection, analysis of the transient thermal dynamics at the bead-water interface is presented, and the time decay parameters for 500 nm and 100 nm beads are determined. Enhanced (negative) imaging contrast is observed for less heat exchange between the water and bead, as in the case for the 100 nm bead. For the 500 nm bead, boxcar imaging before heat exchange starts occurring, leads to an increase of the imaging contrast up to a factor of 1.6.

Dual phase-detected infrared photothermal microscopy

Infrared photothermal microscopy (IPM) has recently gained considerable attention as a versatile analytical platform capable of providing spatially resolved molecular insights across diverse research fields. This technique has led to numerous breakthroughs in the study of compositional variations in functional materials and cellular dynamics in living cells. However, its application to investigate multiple components of temporally dynamic systems, such as living cells and operational devices, has been hampered by the limited information content of the IP signal, which only covers a narrow spectral window (< 1 cm-1). Here, we present a straightforward approach for measuring two distinct IPM images utilizing the orthogonality between the in-phase and quadrature outputs of a lock-in amplifier, called dual-phase IR photothermal (DP-IP) detection. We demonstrate the feasibility of DP-IP detection for IPM in distinguishing two different micro-sized polymer beads.

Monitoring the synthesis of neutral lipids in lipid droplets of living human cancer cells using two-color infrared photothermal microscopy

There has been growing interest in the functions of lipid droplets (LDs) due to recent discoveries regarding their diverse roles. These functions encompass lipid metabolism, regulation of lipotoxicity, and signaling pathways that extend beyond their traditional role in energy storage. Consequently, there is a need to examine the molecular dynamics of LDs at the subcellular level. Two-color infrared photothermal microscopy (2C-IPM) has proven to be a valuable tool for elucidating the molecular dynamics occurring in LDs with sub-micrometer spatial resolution and molecular specificity. In this study, we employed the 2C-IPM to investigate the molecular dynamics of LDs in both fixed and living human cancer cells (U2OS cells) using the isotope labeling method. We investigated the synthesis of neutral lipids occurring in individual LDs over time after exposing the cells to excess saturated fatty acids while simultaneously comparing inherent lipid contents in LDs. We anticipate that these research findings will reveal new opportunities for studying lesser-known biological processes within LDs and other subcellular organelles.

Time-Resolved Mid-Infrared Photothermal Microscopy for Imaging Water-Embedded Axon Bundles

Few experimental tools exist for performing label-free imaging of biological samples in a water-rich environment due to the high infrared absorption of water, overlapping with major protein and lipid bands. A novel imaging modality based on time-resolved mid-infrared photothermal microscopy is introduced and applied to imaging axon bundles in a saline bath environment. Photothermally induced spatial gradients at the axon bundle membrane interfaces with saline and surrounding biological tissue are observed and temporally characterized by a high-speed boxcar detection system. Localized time profiles with an enhanced signal-to-noise, hyper-temporal image stacks, and two-dimensional mapping of the time decay profiles are acquired without the need for complex post image processing. Axon bundles are found to have a larger distribution of time decay profiles compared to the water background, allowing background differentiation based on these transient dynamics. The quantitative analysis of the signal evolution over time allows characterizing the level of thermal confinement at different regions. When axon bundles are surrounded by complex heterogeneous tissue, which contains smaller features, a stronger thermal confinement is observed compared to a water environment, thus shedding light on the heat transfer dynamics across aqueous biological interfaces.

Single virus fingerprinting by widefield interferometric defocus-enhanced mid-infrared photothermal microscopy

Clinical identification and fundamental study of viruses rely on the detection of viral proteins or viral nucleic acids. Yet, amplification-based and antigen-based methods are not able to provide precise compositional information of individual virions due to small particle size and low-abundance chemical contents (e.g., ~ 5000 proteins in a vesicular stomatitis virus). Here, we report a widefield interferometric defocus-enhanced mid-infrared photothermal (WIDE-MIP) microscope for high-throughput fingerprinting of single viruses. With the identification of feature absorption peaks, WIDE-MIP reveals the contents of viral proteins and nucleic acids in single DNA vaccinia viruses and RNA vesicular stomatitis viruses. Different nucleic acid signatures of thymine and uracil residue vibrations are obtained to differentiate DNA and RNA viruses. WIDE-MIP imaging further reveals an enriched β sheet components in DNA varicella-zoster virus proteins. Together, these advances open a new avenue for compositional analysis of viral vectors and elucidating protein function in an assembled virion.

Imaging approaches for monitoring three-dimensional cell and tissue culture systems

The past decade has seen an increasing demand for more complex, reproducible and physiologically relevant tissue cultures that can mimic the structural and biological features of living tissues. Monitoring the viability, development and responses of such tissues in real-time are challenging due to the complexities of cell culture physical characteristics and the environments in which these cultures need to be maintained in. Significant developments in optics, such as optical manipulation, improved detection and data analysis, have made optical imaging a preferred choice for many three-dimensional (3D) cell culture monitoring applications. The aim of this review is to discuss the challenges associated with imaging and monitoring 3D tissues and cell culture, and highlight topical label-free imaging tools that enable bioengineers and biophysicists to non-invasively characterise engineered living tissues.

Multicolor Photoacoustic Volumetric Imaging of Subcellular Structures

Photoacoustic imaging (PAI) has been widely used in multiscale and multicontrast imaging of biological structures and functions. Optical resolution photoacoustic microscopy (OR-PAM), an emerging submodality of PAI, features high lateral resolution and rich optical contrast, indicating great potential in visualizing cellular and subcellular structures. However, three-dimensional (3D) imaging of subcellular structures using OR-PAM has remained a challenge due to the limited axial resolution. In this study, we propose a multicolor 3D photoacoustic microscopy with high lateral/axial resolutions of 0.42/2 and 0.5/2.5 μm at 532 and 780 nm excitation, respectively. Owing to the significantly increased axial resolution, we could visualize the volumetric subcellular structures of melanoma cells using intrinsic contrast. In addition, we carried out multicolor imaging of labeled microtubules/clathrin-coated pits (CCP) and microtubules/mitochondria, respectively, with one scanning by using two different excitation wavelengths. The internal connections between different subcellular structures are revealed by quantitatively comparing the spatial distributions of microtubules/CCP and microtubules/mitochondria in a single cell. Current results suggest that the proposed OR-PAM may serve as an efficient tool for subcellular and cytophysiological studies.

Nanosecond-resolution photothermal dynamic imaging via MHZ digitization and match filtering

Photothermal microscopy has enabled highly sensitive label-free imaging of absorbers, from metallic nanoparticles to chemical bonds. Photothermal signals are conventionally detected via modulation of excitation beam and demodulation of probe beam using lock-in amplifier. While convenient, the wealth of thermal dynamics is not revealed. Here, we present a lock-in free, mid-infrared photothermal dynamic imaging (PDI) system by MHz digitization and match filtering at harmonics of modulation frequency. Thermal-dynamic information is acquired at nanosecond resolution within single pulse excitation. Our method not only increases the imaging speed by two orders of magnitude but also obtains four-fold enhancement of signal-to-noise ratio over lock-in counterpart, enabling high-throughput metabolism analysis at single-cell level. Moreover, by harnessing the thermal decay difference between water and biomolecules, water background is effectively separated in mid-infrared PDI of living cells. This ability to nondestructively probe chemically specific photothermal dynamics offers a valuable tool to characterize biological and material specimens.

Photothermal microscopy is limited for imaging of thermal dynamics. Here, the authors introduce a lock-in free, mid-infrared photothermal dynamic imaging system, which significantly increases SNR and imaging speed, and demonstrate metabolism analysis at single-cell level and background removal.

Erratum: Label-free imaging of fibroblast membrane interfaces and protein signatures with vibrational infrared photothermal and phase signals: publisher's note

[This corrects the article on p. 303 in vol. 12, PMID: 33520386.].

Bond-selective imaging by optically sensing the mid-infrared photothermal effect

Mid-infrared (IR) spectroscopic imaging using inherent vibrational contrast has been broadly used as a powerful analytical tool for sample identification and characterization. However, the low spatial resolution and large water absorption associated with the long IR wavelengths hinder its applications to study subcellular features in living systems. Recently developed mid-infrared photothermal (MIP) microscopy overcomes these limitations by probing the IR absorption-induced photothermal effect using a visible light. MIP microscopy yields submicrometer spatial resolution with high spectral fidelity and reduced water background. In this review, we categorize different photothermal contrast mechanisms and discuss instrumentations for scanning and widefield MIP microscope configurations. We highlight a broad range of applications from life science to materials. We further provide future perspective and potential venues in MIP microscopy field.