A robust tissue laser platform for analysis of formalin-fixed paraffin-embedded biopsies

Properties and Applications of Random Lasers as Emerging Light Sources and Optical Sensors: A Review

In a random laser (RL), optical feedback arises from multiple scattering instead of conventional mirrors. RLs generate a laser-like emission, and meanwhile take advantage of a simpler and more flexible laser configuration. The applicability of RLs as light sources and optical sensors has been proved. These applications have been extended to the biological field, with tissues as natural scattering materials. Herein, the current state of the RL properties and applications was reviewed.

Monitoring Amyloidogenesis with a 3D Deep-Learning-Guided Biolaser Imaging Array

Amyloidogenesis is a critical hallmark for many neurodegenerative diseases and drug screening; however, identifying intermediate states of protein aggregates at an earlier stage remains challenging. Herein, we developed a peptide-encapsulated droplet microlaser to monitor the amyloidogenesis process and evaluate the efficacy of anti-amyloid drugs. The lasing wavelength changes accordingly with the amyloid peptide folding behaviors and nanostructure conformations in the droplet resonator. A 3D deep-learning strategy was developed to directly image minute spectral shifts through a far-field camera. By extracting 1D color information and 2D features from the laser images, the progression of the amyloidogenesis process could be monitored using arrays of laser images from microdroplets. The training set, validation set, and test set of the multimodal learning model achieved outstanding classification accuracies of over 95%. This study shows the great potential of deep-learning-empowered peptide microlaser yields for protein misfolding studies and paves the way for new possibilities for high-throughput imaging of cavity biosensing.

Cellular lasers for cell imaging and biosensing

The possibility to produce laser action involving biomaterials, in particular (single) biological cells, has fostered the development of cellular lasers as a novel approach in biophotonics. In this respect, cells that are engineered to carry gain medium (e.g., fluorescent dyes or proteins) are placed inside an optical cavity (i.e., typically a sandwich of highly reflective mirrors), allowing the generation of stimulated emission upon sufficient optical pumping. In another scenario, micron-sized optical resonators supporting whispering-gallery mode (WGM) or semiconductor-based laser probes can be internalized by the cells and support light amplification. This review summarizes the recent advances in the fields of biolasers and cellular lasers, and most importantly, highlights their potential applications in the fields of in vitro and in vivo cell imaging and analysis. They include biosensing (e.g., in vitro detection of sodium chloride (NaCl) concentration), cancer cell imaging, laser-emission-based microscope, cell tracking, cell distinction study, and tissue contraction monitoring in zebrafish. Lastly, several fundamental issues in developing cellular lasers including laser probe fabrication, biocompatibility of the system, and alteration of local refractive index of optical cavities due to protein absorption or probe aggregation are described. Cellular lasers are foreseen as a promising tool to study numerous biological and biophysical phenomena. STATEMENT OF SIGNIFICANCE: Biolasers are generation of laser involving biological materials. Biomaterials, including single cells, can be engineered to incorporate laser probes or fluorescent proteins or fluorophores, and the resulting light emission can be coupled to optical resonator, allowing generation of cellular laser emission upon optical pumping. Unlike fluorescence, this stimulated emission is very sensitive and is capable of detecting small alterations in the optical property of the cells and their environment. In this review, recent development and applications of cellular lasers in the fields of in vitro and in vivo cell imaging, cell tracking, biosensing, and cell/tissue analysis are highlighted. Several challenges in developing cellular lasers including probe fabrication and biocompatibility as well as alteration of cellular environment are explained.

Identification of Thioflavin T Binding Modes to DNA: A Structure-Specific Molecular Probe for Lasing Applications

The binding mechanism of thioflavin T (ThT) to DNA was studied using polarized light spectroscopy and fluorescence-based techniques in solutions and in solid films. Linear dichroism measurements showed that ThT binds to DNA duplex by intercalation. Time-resolved fluorescence studies revealed a second binding mode which is the external binding to the DNA phosphate groups. Both binding modes represent the nonspecific type of interactions. The studies were complemented with the analysis of short oligonucleotides having DNA cavities. The results indicate that the interplay between three binding modes-intercalation, external binding, and binding inside DNA cavities-determines the effective fluorescence quantum yield of the dye in the DNA structures. External binding was found to be responsible for fluorescence quenching because of energy transfer between intercalated and externally bound molecules. Finally, amplified spontaneous emission (ASE) was successfully generated in the ThT-stained films and used for detecting different DNA structures. ASE measurements show that ThT-stained DNA structures can be used for designing bioderived microlasers.

Biophotonic probes for bio-detection and imaging

The rapid development of biophotonics and biomedical sciences makes a high demand on photonic structures to be interfaced with biological systems that are capable of manipulating light at small scales for sensitive detection of biological signals and precise imaging of cellular structures. However, conventional photonic structures based on artificial materials (either inorganic or toxic organic) inevitably show incompatibility and invasiveness when interfacing with biological systems. The design of biophotonic probes from the abundant natural materials, particularly biological entities such as virus, cells and tissues, with the capability of multifunctional light manipulation at target sites greatly increases the biocompatibility and minimizes the invasiveness to biological microenvironment. In this review, advances in biophotonic probes for bio-detection and imaging are reviewed. We emphatically and systematically describe biological entities-based photonic probes that offer appropriate optical properties, biocompatibility, and biodegradability with different optical functions from light generation, to light transportation and light modulation. Three representative biophotonic probes, i.e., biological lasers, cell-based biophotonic waveguides and bio-microlenses, are reviewed with applications for bio-detection and imaging. Finally, perspectives on future opportunities and potential improvements of biophotonic probes are also provided.

Topological Encoded Vector Beams for Monitoring Amyloid-Lipid Interactions in Microcavity

Lasers are the pillars of modern photonics and sensing. Recent advances in microlasers have demonstrated its extraordinary lasing characteristics suitable for biosensing. However, most lasers utilized lasing spectrum as a detection signal, which can hardly detect or characterize nanoscale structural changes in microcavity. Here the concept of amplified structured light-molecule interactions is introduced to monitor tiny bio-structural changes in a microcavity. Biomimetic liquid crystal droplets with self-assembled lipid monolayers are sandwiched in a Fabry-Pérot cavity, where subtle protein-lipid membrane interactions trigger the topological transformation of output vector beams. By exploiting Amyloid β (Aβ)-lipid membrane interactions as a proof-of-concept, it is demonstrated that vector laser beams can be viewed as a topology of complex laser modes and polarization states. The concept of topological-encoded laser barcodes is therefore developed to reveal dynamic changes of laser modes and Aβ-lipid interactions with different Aβ assembly structures. The findings demonstrate that the topology of vector beams represents significant features of intracavity nano-structural dynamics resulted from structured light-molecule interactions.

Imaging-Based Optofluidic Biolaser Array Encapsulated with Dynamic Living Organisms

Optofluidic biolasers have emerged as promising tools for biomedical analysis due to their strong light-matter interactions and miniaturized size. Recent developments in optofluidic lasers have opened a new Frontier in monitoring biological processes. However, most biolasers require precise recording of the lasing spectrum at the single cavity level, which limits its application in high-throughput applications. Herein, a microdroplet laser array encapsulated with living Escherichia coli was printed on highly reflective mirrors, where laser emission images were employed to reflect the dynamic changes in living organisms. The concept of image-based lasing analysis was proposed by quantifying the integrated pixel intensity of the lasing image from whispering-gallery modes. Finally, dynamic interactions between E. coli and antibiotic drugs were compared under fluorescence and laser emission images. The amplification that occurred during laser generation enabled the quantification of tiny biological changes in the gain medium. Laser imaging presented a significant increase in integrated pixel intensity by 2 orders of magnitude. Our findings demonstrate that image-based lasing analysis is more sensitive to dynamic changes than fluorescence analysis, paving the way for high-throughput on-chip laser analysis of living organisms.

Fast and Reproducible ELISA Laser Platform for Ultrasensitive Protein Quantification

Optofluidic lasers are currently of high interest for sensitive intracavity biochemical analysis. In comparison with conventional methods such as fluorescence and colorimetric detection, optofluidic lasers provide a method for amplifying small concentration differences in the gain medium, thus achieving high sensitivity. Here, we report the development of an on-chip ELISA (enzyme-linked immunosorbent assay) laser platform that is able to complete an assay in a short amount of time with small sample/reagent volumes, large dynamic range, and high sensitivity. The arrayed microscale reaction wells in the ELISA lasers can be microfabricated directly on dielectric mirrors, thus significantly improving the quality of the reaction wells and detection reproducibility. The details of the fabrication and characterization of those reaction wells on the mirror are described and the ELISA laser assay protocols are developed. Finally, we applied the ELISA laser to detecting IL-6, showing that a detection limit of about 0.1 pg/mL can be achieved in 1.5 h with 15 μL of sample/reagents per well. This work pushes the ELISA laser a step closer to solving problems in real-world biochemical analysis.

Virus lasers for biological detection

The selective amplification of DNA in the polymerase chain reaction is used to exponentially increase the signal in molecular diagnostics for nucleic acids, but there are no analogous techniques for signal enhancement in clinical tests for proteins or cells. Instead, the signal from affinity-based measurements of these biomolecules depends linearly on the probe concentration. Substituting antibody-based probes tagged for fluorescent quantification with lasing detection probes would create a new platform for biomarker quantification based on optical rather than enzymatic amplification. Here, we construct a virus laser which bridges synthetic biology and laser physics, and demonstrate virus-lasing probes for biosensing. Our virus-lasing probes display an unprecedented > 10,000 times increase in signal from only a 50% increase in probe concentration, using fluorimeter-compatible optics, and can detect biomolecules at sub-100 fmol mL⁻¹ concentrations.

Many ligand-binding assays still rely on signals that scale linearly with probe concentration. The authors present lasing detection probes with a dye-labelled virus as the gain medium to optically amplify the signal, which could enable much higher signals than for fluorescent quantification.

Chromatin laser imaging reveals abnormal nuclear changes for early cancer detection

We developed and applied rapid scanning laser-emission microscopy (LEM) to detect abnormal changes in cell nuclei for early diagnosis of cancer and cancer precursors. Regulation of chromatins is essential for genetic development and normal cell functions, while abnormal nuclear changes may lead to many diseases, in particular, cancer. The capability to detect abnormal changes in "apparently normal" tissues at a stage earlier than tumor development is critical for cancer prevention. Here we report using LEM to analyze colonic tissues from mice at-risk for colon cancer (induced by a high-fat diet) by detecting pre-polyp nuclear abnormality. By imaging the lasing emissions from chromatins, we discovered that, despite the absence of observable lesions, polyps, or tumors under stereoscope, high-fat mice exhibited significantly lower lasing thresholds than low-fat mice. The low lasing threshold is, in fact, very similar to that of adenomas and is caused by abnormal cell proliferation and chromatin deregulation that can potentially lead to cancer. Our findings suggest that conventional detection methods, such as colonoscopy followed by histopathology, by itself, may be insufficient to reveal hidden or early tumors under development. We envision that this innovative work will provide new insights into LEM and support existing tools for early tumor detection in clinical diagnosis, and fundamental biological and biomedical research of chromatin changes at the biomolecular level of cancer development.