Real-time and wide-field mapping of cell-substrate adhesion gap and its evolution via surface plasmon resonance holographic microscopy

Enhanced plasmonic scattering imaging via deep learning-based super-resolution reconstruction for exosome imaging

Exosome analysis plays pivotal roles in various physiological and pathological processes. Plasmonic scattering microscopy (PSM) has proven to be an excellent label-free imaging platform for exosome detection. However, accurately detecting images scattered from exosomes remains a challenging task due to noise interference. Herein, we proposed an image processing strategy based on a new blind super-resolution deep learning neural network, named ESRGAN-SE, to improve the resolution of exosome PSI images. This model can obtain super-resolution reconstructed images without increasing experimental complexity. The trained model can directly generate high-resolution plasma scattering images from low-resolution images collected in experiments. The results of experiments involving the detection of light scattered by exosomes showed that the proposed super-resolution detection method has strong generalizability and robustness. Moreover, ESRGAN-SE achieved excellent results of 35.52036, 0.09081, and 8.13176 in terms of three reference-free image quality assessment metrics, respectively. These results show that the proposed network can effectively reduce image information loss, enhance mutual information between pixels, and decrease feature differentiation. And, the single-image SNR evaluation score of 3.93078 also showed that the distinction between the target and the background was significant. The suggested model lays the foundation for a potentially successful approach to imaging analysis. This approach has the potential to greatly improve the accuracy and efficiency of exosome analysis, leading to more accurate cancer diagnosis and potentially improving patient outcomes.

Analysis of tissue-substrate adhesion by hyperspectral surface plasmon resonance microscopy

The preparation of histology slides is a critical step in histopathology, and poor-quality histology slides with weak adhesion of tissue sections to the substrate often affect diagnostic accuracy and sometimes lead to diagnostic failure due to tissue section detachment. This issue has been of concern and some methods have been proposed to enhance tissue-substrate adhesion. Unfortunately, quantitative analysis of the adhesion between tissue sections and glass slides is still challenging. In this work, the adhesion of mouse brain tissue sections on gold-coated glass slides was analyzed using a laboratory-fabricated hyperspectral surface plasmon resonance microscopy (HSPRM) system that enabled single-pixel spectral SPR sensing and provided two-dimensional (2D) distribution of resonance wavelengths (RWs). The existence of the nanoscale water gap between the tissue section and the substrate was verified by fitting the RW measured in each pixel using the five-layer Fresnel reflection model. In addition, a 2D image of the tissue-substrate adhesion distance (AD) was obtained from the measured 2D distribution of RWs. The results showed that tissue-substrate AD was 20-35 nm in deionized water and 4-24 nm in saline solution. The HSPRM system used in this work has a wide wavelength range of 400-1000 nm and can perform highly sensitive and label-free detection over a large dynamic detection range with high spectral and spatial resolutions, showing significant potential applications in stain-free tissue imaging, quantitative analysis of tissue-substrate adhesion, accurate identification of tumor cells, and rapid histopathological diagnosis.

Guided wave resonance-based digital holographic microscopy for high-sensitivity monitoring of the refractive index

Surface plasmon resonance holographic microscopy (SPRHM) has been employed to measure the refractive index but whose performance is generally limited by the metallic intrinsic loss. Herein we first, to our knowledge, utilize guided wave resonance (GWR) with low loss to realize the monitoring of the refractive index by integrating with digital holographic microscopy (DHM). By depositing a dielectric layer on a silver film, we observe a typical GWR in the dielectric layer with stronger field enhancement and higher sensitivity to the surrounding refractive index compared to the silver film-supported SPR, which agrees well with calculations. The innovative combination of the GWR and DHM contributes to the highly sensitive dynamic monitoring of the surrounding refractive index variation. Through the measurement with DHM, we found that the GWR presents an excellent sensitivity, which is 2.6 times higher than that of the SPR on the silver film. The results will pave a new pathway for digital holographic interferometry and its applications in environmental and biological detections.

Thickness measurement of bimetallic film using surface plasmon resonance holographic microscopy

Bimetallic film with high stability and sensitivity is often used to excite surface plasmon resonance (SPR). The thicknesses of the bimetallic film play an important role in quantitative retrieval of the sample's parameters, and a precise measurement method is not available until now. In this paper, we propose a method for measuring the thicknesses of bimetallic film using surface plasmon resonance holographic microscopy (SPRHM). Considering that the refractive index of the dielectric upon the bimetallic film sensitively modulates the SPR phase response, the two thickness parameters of bimetallic film can be calculated by two phase-contrast SPR images with two different liquid dielectrics. The capability of this method was verified with several Ag-Au film couples by using a compact SPRHM setup. Our work provides a precise characterization method for the parameters of SPR configuration and may find wide applications in the research fields of SPR sensing and imaging.

Site-selective heat boosting electrochemiluminescence for single cell imaging

In operando visualization of local electrochemical reactions provides mechanical insights into the dynamic transport of interfacial charge and reactant/product. Electrochemiluminescence is a crossover technique that quantitatively determines Faraday current and mass transport in a straightforward manner. However, the sensitivity is hindered by the low collision efficiency of radicals and side reactions at high voltage. Here, we report a site-selective heat boosting electrochemiluminescence microscopy. By generating a micron-scale heat point in situ at the electrode-solution interface, we achieved an enhancement of luminescence intensity up to 63 times, along with an advance of 0.2 V in applied voltage. Experimental results and finite element simulation demonstrate that the fundamental reasons are accelerated reaction rate and thermal convection via a photothermal effect. The concentrated electrochemiluminescence not only boosts the contrast of single cells by 20.54 times but also enables the site-selective cell-by-cell analysis of the heterogeneous membrane protein abundance. This electrochemical visualization method has great potential in the highly sensitive and selective analysis of local electron transfer events.

Label-free and dynamic monitoring of cell evolutions using wavelength-multiplexing surface plasmon resonance holographic microscopy

Dynamic characterizations of intracellular variations and cell-substrate interactions under different external environments are critical to study cell behaviors and exploring biological applications. However, techniques that are capable of dynamically and simultaneously measuring multiple parameters of living cells in a wide-field manner have rarely been reported. Here, we present a wavelength-multiplexing surface plasmon resonance holographic microscopy which allows wide-field, simultaneous, and dynamic measurements of cell parameters, including cell-substrate distance and cytoplasm refractive index (RI). We use two lasers of 632.8 nm and 690 nm as light sources. Two beam splitters are employed in the optical setup to separately adjust the incident angle of two light beams. Then, surface plasmon resonance (SPR) can be excited for each wavelength under SPR angles. We demonstrate the advances of the proposed apparatus by systematically studying the cell responses to osmotic pressure stimuli from the environmental medium at the cell-substrate interface. The SPR phase distributions of the cell are firstly mapped at two wavelengths, then the cell-substrate distance and cytoplasm RI are retrieved using a demodulation method. Based on phase response differences between two wavelengths and monotonic changes of SPR phase with cell parameters, cell-substrate distance, and cytoplasm RI can be determined simultaneously using an inverse algorithm. This work affords a new optical measurement technique to dynamically characterize cell evolutions and investigate cell properties in various cellular activities. It may become a useful tool in the bio-medical and bio-monitoring areas.

Recent advances in surface plasmon resonance imaging and biological applications

Surface Plasmon Resonance Imaging (SPRI) is a robust technique for visualizing refractive index changes, which enables researchers to observe interactions between nanoscale objects in an imaging manner. In the past period, scholars have been attracted by the Prism-Coupled and Non-prism Coupled configurations of SPRI and have published numerous experimental results. This review describes the principle of SPRI and discusses recent developments in Prism-Coupled and Non-prism Coupled SPRI techniques in detail, respectively. And then, major advances in biological applications of SPRI are reviewed, including four sub-fields (cells, viruses, bacteria, exosomes, and biomolecules). The purpose is to briefly summarize the recent advances of SPRI and provide an outlook on the development of SPRI in various fields.

Recent advances in label-free imaging of cell-matrix adhesions

Cell-matrix adhesions play an essential role in mediating and regulating many biological processes. The adhesion receptors, typically transmembrane integrins, provide dynamic correlations between intracellular environments and extracellular matrixes (ECMs) by bi-directional signaling. In-depth investigations of cell-matrix adhesion and integrin-mediated cell adhesive force are of great significance in biology and medicine. The emergence of advanced imaging techniques and principles has facilitated the understanding of the molecular composition and structure dynamics of cell-matrix adhesions, especially the label-free imaging methods that can be used to study living cell dynamics without immunofluorescence staining. This highlight article aims to give an overview of recent developments in imaging cell-matrix adhesions in a label-free manner. Electrochemiluminescence microscopy (ECLM) and surface plasmon resonance microscopy (SPRM) are briefly introduced and their applications in imaging analysis of cell-matrix adhesions are summarized. Then we highlight the advances in mapping cell-matrix adhesion force based on molecular tension probes and fluorescence microscopy (collectively termed as MTFM). The biomaterials including polyethylene glycol (PEG), peptides and DNA for constructing tension probes in MTFM are summarized. Finally, the outlook and perspectives on the further developments of cell-matrix adhesion imaging are presented.

Simultaneous measurement of near-water-film air temperature and humidity fields based on dual-wavelength digital holographic interferometry

We present a method to reconstruct the near-water-film air temperature and humidity distributions synchronously by measuring the phase delays based on dual-wavelength digital holographic interferometry. A falling water film device was used to create a water film evaporation environment and generate axially uniform temperature and humidity fields. The relationship between air temperature, humidity and phase delay is derived from the Edlen equations. With such relationship, the temperature and humidity distributions can be solved directly according to phase delays of two different wavelengths. An edge phase enhancement method and an error elimination method with PSO are presented to improve the measurement accuracy. The temperature and humidity fields in the falling water film model were experimentally reconstructed with temperature deviation of 0.06% and relative humidity deviation of 2.61%.

Dual-wavelength surface plasmon resonance holographic microscopy for simultaneous measurements of cell-substrate distance and cytoplasm refractive index

Studying the basic characteristics of living cells is of great significance in biological research. Bio-physical parameters, including cell-substrate distance and cytoplasm refractive index (RI), can be used to reveal cellular properties. In this Letter, we propose a dual-wavelength surface plasmon resonance holographic microscopy (SPRHM) to simultaneously measure the cell-substrate distance and cytoplasm RI of live cells in a wide-field and non-intrusive manner. Phase-contrast surface plasmon resonance (SPR) images of individual cells at wavelengths of 632.8 nm and 690 nm are obtained using an optical system. The two-dimensional distributions of cell-substrate distance and cytoplasm RI are then demodulated from the phase-contrast SPR images of the cells. MDA-MB-231 cells and IDG-SW3 cells are experimentally measured to verify the feasibility of this approach. Our method provides a useful tool in biological fields for dual-parameter detection and characterization of live cells.

Long-term cell culture and electrically in situ monitoring of living cells based on a polyaniline hydrogel sensor

Accurate, in situ and long-term electrically monitoring of cell development plays an important role in cell study, which brings in challenges in terms of biocompatibility, processability, and sensing capability of electrochemical sensors. Based on biocompatible conductive polyaniline (PAni) hydrogels, we constructed a flexible sensor with flexible carbon cloth for electrical analysis of living cells. The carbon fiber substrate modified with conductive PAni hydrogels was selected as the electrode to promote the current collection of the sensor. The three dimensional nanostructured mesoporous matrix of PAni hydrogels is favorable for in situ generation of catalytic Pt nanoparticles and cell growth. With these hierarchically nanostructured features, the hydrogel electrochemical sensor was endowed with high sensitivity and selectivity in the detection of H₂O₂ (with a low detection limit of 1.6 μM in 0.01 M PBS and a wide linear range from 10 μM to 10 mM), and good biocompatibility for cell growth as long as 5 days. The accurate detection of H₂O₂ released from cells enabled us to differentiate the physiological states of cells and imitate the different stimuli-responsive behavior, which can provide real-time information on cell biological events. With outstanding biocompatibility, operability and repeatability, this strategy can be expanded to the fields of other biosensor fabrication and cell-related biomarker monitoring, which exhibits a broad application potential in bioanalysis catering to new generation sensors.

Label-Free Imaging of Nanoscale Displacements and Free-Energy Profiles of Focal Adhesions with Plasmonic Scattering Microscopy

Cell adhesion plays a critical role in cell communication, cell migration, cell proliferation, and integration of medical implants with tissues. Focal adhesions physically link the cell cytoskeleton to the extracellular matrix, but it remains challenging to image single focal adhesions directly. Here, we show that plasmonic scattering microscopy (PSM) can directly image the single focal adhesions in a label-free, real-time, and non-invasive manner with sub-micrometer spatial resolution. PSM is developed based on surface plasmon resonance (SPR) microscopy, and the evanescent illumination makes it immune to the interference of intracellular structures. Unlike the conventional SPR microscopy, PSM can provide a high signal-to-noise ratio and sub-micrometer spatial resolution for imaging the analytes with size down to a single-molecule level, thus allowing both the super-resolution lateral localization for measuring the nanoscale displacement and precise tracking of vertical distances between the analyte centroid and the sensor surface for analysis of free-energy profiles. PSM imaging of the RBL-2H3 cell with temporal resolution down to microseconds shows that the focal adhesions have random diffusion behaviors in addition to their directional movements during the antibody-mediated activation process. The free-energy mapping also shows a similar movement tendency, indicating that the cell may change its morphology upon varying the binding conditions of adhesive structures. PSM provides insights into the individual focal adhesion activities and can also serve as a promising tool for investigating the cell/surface interactions, such as cell capture and detection and tissue adhesive materials screening.

Dual-channel illumination surface plasmon resonance holographic microscopy for resolution improvement

Surface plasmon resonance holographic microscopy (SPRHM), combining digital holographic microscopy with surface plasmon resonance (SPR), can simultaneously obtain the amplitude and phase distributions of the reflected beam carrying specimen information in SPR. Due to the decaying length of the surface plasmon wave as large as tens of micrometers, the spatial resolution of SPRHM is lower than that of ordinary optical microscopes. In this work, we propose a scheme to improve the spatial resolution of SPRHM by applying dual-channel SPR excitations. Through the polarization multiplexing technique, two holograms carrying the information of SPR excited in orthogonal directions are simultaneously acquired. Via a numerical reconstruction and filtering algorithm for holograms, the lateral spatial resolution of SPRHM can be effectively enhanced to reach nearly 1 µm at a wavelength of 632.8 nm. This is comparable to the resolution of traditional optical microscopes, while possessing the advantages of wide-field imaging and high measurement sensitivity of SPR.

Response of NIH 3T3 Fibroblast Cells on Laser-Induced Periodic Surface Structures on a 15×(Ti/Zr)/Si Multilayer System

Ultrafast laser processing with the formation of periodic surface nanostructures on the 15×(Ti/Zr)/Si multilayers is studied in order to the improve cell response. A novel nanocomposite structure in the form of 15x(Ti/Zr)/Si multilayer thin films, with satisfying mechanical properties and moderate biocompatibility, was deposited by ion sputtering on an Si substrate. The multilayer 15×(Ti/Zr)/Si thin films were modified by femtosecond laser pulses in air to induce the following modifications: (i) mixing of components inside of the multilayer structures, (ii) the formation of an ultrathin oxide layer at the surfaces, and (iii) surface nano-texturing with the creation of laser-induced periodic surface structure (LIPSS). The focus of this study was an examination of the novel Ti/Zr multilayer thin films in order to create a surface texture with suitable composition and structure for cell integration. Using the SEM and confocal microscopies of the laser-modified Ti/Zr surfaces with seeded cell culture (NIH 3T3 fibroblasts), it was found that cell adhesion and growth depend on the surface composition and morphological patterns. These results indicated a good proliferation of cells after two and four days with some tendency of the cell orientation along the LIPSSs.