Lasing in Live Mitotic and Non-Phagocytic Cells by Efficient Delivery of Microresonators

Hyperspectral confocal imaging for high-throughput readout and analysis of bio-integrated microlasers

Integrating micro- and nanolasers into live cells, tissue cultures and small animals is an emerging and rapidly evolving technique that offers noninvasive interrogation and labeling with unprecedented information density. The bright and distinct spectra of such lasers make this approach particularly attractive for high-throughput applications requiring single-cell specificity, such as multiplexed cell tracking and intracellular biosensing. The implementation of these applications requires high-resolution, high-speed spectral readout and advanced analysis routines, which leads to unique technical challenges. Here, we present a modular approach consisting of two separate procedures. The first procedure instructs users on how to efficiently integrate different types of lasers into living cells, and the second procedure presents a workflow for obtaining intracellular lasing spectra with high spectral resolution and up to 125-kHz readout rate and starts from the construction of a custom hyperspectral confocal microscope. We provide guidance on running hyperspectral imaging routines for various experimental designs and recommend specific workflows for processing the resulting large data sets along with an open-source Python library of functions covering the analysis pipeline. We illustrate three applications including the rapid, large-volume mapping of absolute refractive index by using polystyrene microbead lasers, the intracellular sensing of cardiac contractility with polystyrene microbead lasers and long-term cell tracking by using semiconductor nanodisk lasers. Our sample preparation and imaging procedures require 2 days, and setting up the hyperspectral confocal microscope for microlaser characterization requires <2 weeks to complete for users with limited experience in optical and software engineering.

Intracellular biocompatible hexagonal boron nitride quantum emitters as single-photon sources and barcodes

Color centers in hexagonal boron nitride (hBN) have been emerging as a multifunctional platform for various optical applications including quantum information processing, quantum computing and imaging. Simultaneously, due to its biocompatibility and biodegradability hBN is a promising material for biomedical applications. In this work, we demonstrate single-photon emission from hBN color centers embedded inside live cells and their application to cellular barcoding. The generation and internalization of multiple color centers into cells was performed via simple and scalable procedure while keeping the cells unharmed. The emission from live cells was observed as multiple diffraction-limited spots, which exhibited excellent single-photon characteristics with high single-photon purity of 0.1 and superb emission stability without photobleaching or spectral shifts over several hours. Due to different emission wavelengths and peak widths of the color centers, they were employed as barcodes. We term them Quantum Photonic Barcodes (QPBs). Each QPB can exist in one out of 470 possible distinguishable states and a combination of a few QPBs per cell can be used to uniquely tag virtually an unlimited number of cells. The barcodes developed here offer some excellent properties, including ease of production by a single-step procedure, biocompatibility and biodegradability, emission stability, no photobleaching, small size and a huge number of unique barcodes. This work provides a basis for the use of hBN color centers for robust barcoding of cells and due to the single photon emission, presented concepts could in future be extended to quantum-limited sensing and super-resolution imaging.

Microcavity- and Microlaser-Based Optical Barcoding: A Review of Encoding Techniques and Applications

Optical microbarcodes have recently received a great deal of interest because of their suitability for a wide range of applications, such as multiplexed assays, cell tagging and tracking, anticounterfeiting, and product labeling. Spectral barcodes are especially promising because they are robust and have a simple readout. In addition, microcavity- and microlaser-based barcodes have very narrow spectra and therefore have the potential to generate millions of unique barcodes. This review begins with a discussion of the different types of barcodes and then focuses specifically on microcavity-based barcodes. While almost any kind of optical microcavity can be used for barcoding, currently whispering-gallery microcavities (in the form of spheres and disks), nanowire lasers, Fabry-Pérot lasers, random lasers, and distributed feedback lasers are the most frequently employed for this purpose. In microcavity-based barcodes, the information is encoded in various ways in the properties of the emitted light, most frequently in the spectrum. The barcode is dependent on the properties of the microcavity, such as the size, shape, and the gain materials. Various applications of these barcodes, including cell tracking, anticounterfeiting, and product labeling are described. Finally, the future prospects for microcavity- and microlaser-based barcodes are discussed.

Chlamydia trachomatis Cell-to-Cell Spread through Tunneling Nanotubes

Tunneling nanotubes (TNTs) are transient cellular connections that consist of dynamic membrane protrusions. They play an important role in cell-to-cell communication and mediate the intercellular exchanges of molecules and organelles. TNTs can form between different cell types and may contribute to the spread of pathogens by serving as cytoplasmic corridors. We demonstrate that Chlamydia (C.) trachomatis-infected human embryonic kidney (HEK) 293 cells and other cells form TNT-like structures through which reticulate bodies (RBs) pass into uninfected cells. Observed TNTs have a life span of 1 to 5 h and contain microtubules, which are essential for chlamydial transfer. They can bridge distances of up to 50 μm between connecting neighboring cells. Consistent with the biological role for TNTs, we show that C. trachomatis spread also occurs under conditions in which the extracellular route of chlamydial entry into host cells is blocked. Based on our findings, we propose that TNTs play a critical role in the direct, cell-to-cell transmission of chlamydia. IMPORTANCE Intracellular bacterial pathogens often undergo a life cycle in which they parasitize infected host cells in membranous vacuoles. Two pathways have been described by which chlamydia can exit infected host cells: lytic cell destruction or exit via extrusion formation. Whether direct, cell-to-cell contact may also play a role in the spread of infection is unknown. Tunneling nanotubes (TNTs) interconnect the cytoplasm of adjacent cells to mediate efficient communication and the exchange of material between them. We used Chlamydia trachomatis and immortalized cells to analyze whether TNTs mediate bacterial transmission from an infected donor to uninfected acceptor cells. We show that chlamydia-infected cells build TNTs through which the intracellular reticulate bodies (RBs) of the chlamydia can pass into uninfected neighboring cells. Our study contributes to the understanding of the function of TNTs in the cell-to-cell transmission of intracellular pathogens and provides new insights into the strategies by which chlamydia spreads among multicellular tissues.

Paradigm shift in future biophotonics for imaging and therapy: Miniature living lasers to cellular scale optoelectronics

Advancements in light technology, devices and its applications have tremendously changed the facets of biomedical science and engineering to provide powerful diagnostic and therapeutic capabilities ranging from basic research to clinics. Recent novel innovations and concepts in the field of material science, biomedical optics, processing technology and nanotechnology have enabled increasingly sophisticated technologies such as cellular scale, wireless, remotely controlled micro device for in vivo integrations. This review deals with such futuristic applications of biophotonics like miniature living lasers, wireless remotely controlled implantable and cellular optoelectronics for novel imaging, diagnostic and therapeutic applications. We begin with an overview of the competency and progress in biophotonics as one of the most active frontiers in advanced analytical, diagnostic and therapeutic modalities. This is further followed by comprehensive discussion on recent advances, importance and applications, towards miniaturization size of laser to integrate into live cells as biological lasers, and wearable and implantable optoelectronic devices. Such applications form a novel biocompatible platform for intracellular sensing, cytometry and imaging devices. Further, the opportunities and possible challenges for future research directions to transform this basic research to clinical applications are also discussed.

Laser particle activated cell sorting in microfluidics

Laser particles providing bright, spectrally narrowband emission renders them suitable for use as cellular barcodes. Here, we demonstrate a microfluidic platform integrated with a high-speed spectrometer, capable of reading the emission from laser particles in fluidic channels and routing cells based on their optical barcodes. The sub-nanometer spectral emission of each laser particle enables us to distinguish individual cells labeled with hundreds of different laser colors in the near infrared. Furthermore, cells tagged with laser particles are sorted based on their spectral barcodes at a kilohertz rate by using a real-time field programmable gate array and 2-way electric field switch. We demonstrate several different flavors of sorting, including isolation of barcoded cells, and cells tagged with a specific laser color. We term this novel sorting technique laser particle activated cell sorting (LACS). This flow reading and sorting technology adds to the arsenal of single-cell analysis tools using laser particles.

Monitoring Various Bioactivities at the Molecular, Cellular, Tissue, and Organism Levels via Biological Lasers

The laser is considered one of the greatest inventions of the 20th century. Biolasers employ high signal-to-noise ratio lasing emission rather than regular fluorescence as the sensing signal, directional out-coupling of lasing and excellent biocompatibility. Meanwhile, biolasers can also be micro-sized or smaller lasers with embedded/integrated biological materials. This article presents the progress in biolasers, focusing on the work done over the past years, including the molecular, cellular, tissue, and organism levels. Furthermore, biolasers have been utilized and explored for broad applications in biosensing, labeling, tracking, bioimaging, and biomedical development due to a number of unique advantages. Finally, we provide the possible directions of biolasers and their applications in the future.

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.

Red-Shifted Excitation and Two-Photon Pumping of Biointegrated GaInP/AlGaInP Quantum Well Microlasers

Biointegrated intracellular microlasers have emerged as an attractive and versatile tool in biophotonics. Different inorganic semiconductor materials have been used for the fabrication of such biocompatible microlasers but often operate at visible wavelengths ill-suited for imaging through tissue. Here, we report on whispering gallery mode microdisk lasers made from a range of GaInP/AlGaInP multi-quantum well structures with compositions tailored to red-shifted excitation and emission. The selected semiconductor alloys show minimal toxicity and allow the fabrication of lasers with stable single-mode emission in the NIR (675-720 nm) and sub-pJ thresholds. The microlasers operate in the first therapeutic window under direct excitation by a conventional diode laser and can also be pumped in the second therapeutic window using two-photon excitation at pulse energies compatible with standard multiphoton microscopy. Stable performance is observed under cell culturing conditions for 5 days without any device encapsulation. With their bio-optimized spectral characteristics, low lasing threshold, and compatibility with two-photon pumping, AlGaInP-based microlasers are ideally suited for novel cell tagging and in vivo sensing applications.

Deep tissue localization and sensing using optical microcavity probes

Optical microcavities and microlasers were recently introduced as probes inside living cells and tissues. Their main advantages are spectrally narrow emission lines and high sensitivity to the environment. Despite numerous novel methods for optical imaging in strongly scattering biological tissues, imaging at single-cell resolution beyond the ballistic light transport regime remains very challenging. Here, we show that optical microcavity probes embedded inside cells enable three-dimensional localization and tracking of individual cells over extended time periods, as well as sensing of their environment, at depths well beyond the light transport length. This is achieved by utilizing unique spectral features of the whispering-gallery modes, which are unaffected by tissue scattering, absorption, and autofluorescence. In addition, microcavities can be functionalized for simultaneous sensing of various parameters, such as temperature or pH value, which extends their versatility beyond the capabilities of standard fluorescent labels.

Cellular Features Revealed by Transverse Laser Modes in Frequency Domain

Biological lasers which utilize Fabry-Pérot (FP) cavities have attracted tremendous interest due to their potential in amplifying subtle biological changes. Transverse laser modes generated from cells serve as distinct fingerprints of individual cells; however, most lasing signals lack the ability to provide key information about the cell due to high complexity of transverse modes. The missing key, therefore, hinders it from practical applications in biomedicine. This study reveals the key mechanism governing the frequency distributions of transverse modes in cellular lasers. Spatial information of cells including curvature can be interpreted through spectral information of transverse modes by means of hyperspectral imaging. Theoretical studies are conducted to explore the correlation between the cross-sectional morphology of a cell and lasing frequencies of transverse modes. Experimentally, the spectral characteristics of transverse modes are investigated in live and fixed cells with different morphological features. By extracting laser modes in frequency domain, the proposed concept is applied for studying cell adhesion process and cell classification from rat cortices. This study expands a new analytical dimension of cell lasers, opening an avenue for subcellular analysis in biophotonic applications.

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.

Whispering-gallery microlasers for cell tagging and barcoding: the prospects for in vivo biosensing

Researchers in the field of whispering-gallery-mode (WGM) microresonators have proposed biointegrated low-threshold WGM lasers, to enable large-scale parallel single-cell tracking and barcoding. Although the reported devices have so far been primarily investigated in model applications, most recent results represent important steps towards the development of in vivo tags and sensors that utilize the unique and narrow spectral features of miniature WGM lasers.

Review of biosensing with whispering-gallery mode lasers

Lasers are the pillars of modern optics and sensing. Microlasers based on whispering-gallery modes (WGMs) are miniature in size and have excellent lasing characteristics suitable for biosensing. WGM lasers have been used for label-free detection of single virus particles, detection of molecular electrostatic changes at biointerfaces, and barcode-type live-cell tagging and tracking. The most recent advances in biosensing with WGM microlasers are described in this review. We cover the basic concepts of WGM resonators, the integration of gain media into various active WGM sensors and devices, and the cutting-edge advances in photonic devices for micro- and nanoprobing of biological samples that can be integrated with WGM lasers.

Bioresponsive microlasers with tunable lasing wavelength

Lasing particles are emerging tools for amplifying light-matter interactions at the biointerface by exploiting its strong intensity and miniaturized size. Recent advances in implementing laser particles into living cells and tissues have opened a new frontier in biological imaging, monitoring, and tracking. Despite remarkable progress in micro- and nanolasers, lasing particles with surface functionality remain challenging due to the low mode-volume while maintaining a high Q-factor. Herein, we report the novel concept of bioresponsive microlasers by exploiting interfacial energy transfer based on whispering-gallery-mode (WGM) microdroplet cavities. Lasing wavelengths were manipulated by energy transfer-induced changes of a gain spectrum resulting from the binding molecular concentrations at the cavity surface. Both protein-based and enzymatic-based interactions were demonstrated, shedding light on the development of functional microlasers. Finally, tunable lasing wavelengths over a broad spectral range were achieved by selecting different donor/acceptor pairs. This study not only opens new avenues for biodetection, but also provides deep insights into how molecules modulate laser light at the biointerface, laying the foundation for the development of smart bio-photonic devices at the molecular level.

Advances in materials for cellular applications (Review)

The goal of this review is to highlight materials that show exciting promise for either entirely new cellular-level applications or new approaches to long-standing biological challenges. The authors start with two more established materials, graphene and carbon nanotubes, and then progress to conducting polymers, followed by an overview of the microresonators, nanowires, and spasers used as intracellular lasers. These materials provide new approaches to gene and drug delivery, cellular regeneration, mechanical sensing, imaging, and the modulation and recording of cellular activity. Of specific interest is the comparison of these materials with existing technologies, the method of cellular delivery, and the all-encompassing challenge of biocompatibility. Concluding remarks examine the extension of these materials from cellular-level experiments to in vivo applications, including the method of activation: light, electricity, and ultrasound. Overall, these materials and their associated applications illustrate the most recent advances in material-cell interactions.

Non-obstructive intracellular nanolasers

Molecular dyes, plasmonic nanoparticles and colloidal quantum dots are widely used in biomedical optics. Their operation is usually governed by spontaneous processes, which results in broad spectral features and limited signal-to-noise ratio, thus restricting opportunities for spectral multiplexing and sensing. Lasers provide the ultimate spectral definition and background suppression, and their integration with cells has recently been demonstrated. However, laser size and threshold remain problematic. Here, we report on the design, high-throughput fabrication and intracellular integration of semiconductor nanodisk lasers. By exploiting the large optical gain and high refractive index of GaInP/AlGaInP quantum wells, we obtain lasers with volumes 1000-fold smaller than the eukaryotic nucleus (V_laser < 0.1 µm³), lasing thresholds 500-fold below the pulse energies typically used in two-photon microscopy (E_th ≈ 0.13 pJ), and excellent spectral stability (<50 pm wavelength shift). Multiplexed labeling with these lasers allows cell-tracking through micro-pores, thus providing a powerful tool to study cell migration and cancer invasion.

Microsphere-based interferometric optical probe

Fluorescent optical probes have rapidly transformed our understanding of complex biological systems by providing specific information on biological targets in the natural living state. However, their utility is often limited by insufficient brightness, photostability, and multiplexing capacity. Here, we report a conceptually new optical probe, termed ‘reflectophore’, which is based on the spectral interference from a dielectric microsphere. Reflectophores are orders-of-magnitudes brighter than conventional fluorophores and are free from photobleaching, enabling practically unlimited readout at high fidelity. They also offer high-degree multiplexing, encoded in their optical size, which can be readily decoded through interferometric detection with nanoscale accuracy, even in turbid biological media. Furthermore, we showcase their biological applications in cellular barcoding and microenvironmental sensing of a target protein and local electric field.

Tagging and tracking cells with multiplexed labels can help study complex cellular behaviors in living systems. Here, Jo et al. propose and demonstrate the use of Fabry-Perot-like resonances in dielectric microspheres as such a label and call these reflectophores.

Intracellular near-Infrared Microlaser Probes Based on Organic Microsphere-SiO2 Core-Shell Structures for Cell Tagging and Tracking

Conventional near-infrared (NIR) luminescent probes, such as DsRed and Cy5, utilize spontaneous emission (SE) signals, which are broad (fwhm >50 nm) and often have low quantum yield. Herein, we developed smart NIR intracellular whispering-gallery mode (WGM) microlaser probes made by organic microspheres of (E)-3-(4-(diptolylamino)phenyl)-1-(1-hydroxynaphthalen-2-yl)prop-2-en-1-one (DPHP) coated with a silica shell. The overall small diameter ( D, adjustable between 2 and 10 μm) and the biocompatible silica shell ensure our core-shell microspheres (CSmSPs) to be engulfed in cells as a microlaser operating around 720 nm with a low threshold of 0.78 μJ/cm². Considering that WGM mode spacing depending strongly on its size, it will be possible to distinguish millions of individual macrophages through well-defined WGM lasing peaks (fwhm ≤2 nm) of CSmSPs of different sizes. Furthermore, we monitored the transformation of normal macrophages to foamy ones by encoding them with our NIR CSmSPs microlaser probes, which deliver constant WGM lasing signals with a spectral fluctuation <0.02 nm and excellent stability.

Photonic integration platform with pump free microfluidics

Chip based particle sensing using 3D capillary fill microfluidics integrated with monolithically integrated lasers and photodetectors is used to demonstrate the feasibility of true chip scale photonic measurements of fluids. The approach is scalable and manufactured using industry standard compound semiconductor fabrication tools. The need for fluid speed regulation via external pumps is removed by measuring local particle velocity at the point of interrogation and particle position within the fluid flow is derived from multiple time resolved forward scattered light signals. Particle size discrimination of 10 and 15 μm polystyrene microbeads is used as an example.

An integrated microwell array platform for cell lasing analysis

Biological cell lasers are emerging as a novel technology in biological studies and biomedical engineering. The heterogeneity of cells, however, can result in various lasing behaviors from cell to cell. Thus, the capability to track individual cells during laser investigation is highly desired. In this work, a microwell array was integrated with high-quality Fabry-Pérot cavities for addressable and automated cell laser studies. Cells were captured in the microwells and the corresponding cell lasing was achieved and analyzed using SYTO9-stained Sf9 cells as a model system. It is found that the presence of the microwells does not affect the lasing performance, but the cell lasers exhibit strong heterogeneity due to different cell sizes, cycle stages and polyploidy. Time series laser measurements were also performed automatically with the integrated microarray, which not only enables the tracking and multiplexed detection of individual cells, but also helps identify "abnormal" cells that deviate from a large normal cell population in their lasing performance. The microarrayed cell laser platform developed here could provide a powerful tool in single cell analysis using lasing emission that complements conventional fluorescence-based cell analysis.

Spectral reading of optical resonance-encoded cells in microfluidics

The ability to label individual cells is useful for single-cell-level studies of complex cellular interactions and heterogeneity. Optically readable cell labeling is attractive as it can be investigated non-invasively and repeatedly at high speeds. Here, we demonstrate the feasibility of large-scale cell barcoding and identification using fluorescent polystyrene microbeads loaded into cells. Intracellular beads with different diameters in a range of 5 to 12 μm generate spectrally distinguished features or barcodes. A microfluidic chip was used to measure fluorescence resonance peaks emitted from individual cells. An algorithm comparing the peak wavelengths to a reference barcode library allowed barcode identification with high accuracy. This work provides a guideline to increase the number of unique identifiers and reduce various false-positive and false-negative errors.