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

Whispering Gallery Mode Micro/Nanolasers for Intracellular Probing at Single Cell Resolution

Intracellular probing at single cell resolution is key to revealing the heterogeneity of cells, learning new cell subtypes and functions, understanding the pathophysiology of disease, and ensuring precise diagnosis and treatment. Despite the best efforts, an enormous challenge remains due to the very small size, extremely low content, and dynamic microenvironment of a single cell. Whispering gallery mode (WGM) micro/nanolasers (active WGM) offer unique advantages of small mode volume, high quality factors, bright and low threshold laser emission, and narrow line width, particularly suitable for integration within a single cell. In this review, we provide a focused overview of WGM micro/nanolasers for intracellular probing. We deliver information on WGM micro/nanolaser concepts, sensing mechanism, and biocompatibility, as well as recent progress in intracellular probing applications mainly covering cellular-level sensing, molecular-level detection, and feasibility for cellular imaging. At the end, challenges and prospects of WGM micro/nanolasers for intracellular applications are discussed.

Bacterial Hybrid Light-Emitting Diodes

Photon down-converting filters with fluorescent proteins (FPs) are a new frontier in the quest for rare-earth-free and non-toxic color filters for white light-emitting diodes. There are, however, concerns related to the FP purification costs and lack of FP recyclability/reuse. Here, the direct use of bacteria in photon down-converting filters can be of utmost relevance, eliminating purification and allowing in situ production of new FPs. However, their high background autofluorescence/scattering and low stability in polymer coatings have traditionally hampered the application of Engineering Living Materials (ELMs) for photon manipulation. Indeed, there are no examples of ELMs in lighting systems. This work discloses the first protocol to prepare living spheroplasts with > 90% scattering reduction, high FP expression fairly keeping their photoluminescence figures-of-merit, and excellent resilience in polymer films over 1 year under ambient storage. This unlocked the preparation of the first bacteria hybrid light-emitting diodes integrating ELMs for photon conversion. These devices feature similar stabilities to those using purified FPs, while enabling a cost-effective strategy and active FP recycling by the simple recultivation of spheroplasts. Overall, this work introduces a successful case toward bacteria-polymer photon manipulation, in general, and a new living lighting concept, in particular.

Microfluidics-Driven Dripping Technique for Fabricating Polymer Microspheres Doped with AgInS2/ZnS Quantum Dots

Fluorescent microspheres are at the forefront of biosensing technologies. They can be used for a wide range of biomedical applications. They consist of organic dyes and polymers, which are relatively immune to photobleaching and other environmental factors. However, recently developed AgInS2/ZnS quantum dots are a water-soluble, low-toxicity class of semiconductor nanocrystals with enhanced stability as fluorescent materials. Here, we propose a simple way for making microspheres: a microfluidic dripping technique for acrylamide polymer spheres doped with quantum dots. Analyses of their spectra show that the emission of quantum dots, dispersed in water, is saturated with an increasing pump intensity, while quantum dots embedded into polymer microspheres exhibit a more sustained emission. Moreover, our study unveils a remarkable reduction in the luminescence lifetime of quantum dots embedded in microspheres: the mean value of the decay time for quantum dots in solutions was 91 and 3.5 ns for similar quantum dots incorporated into polymer microspheres.

Enhanced performance of amplified spontaneous emission in Dion-Jacobson phase quasi-2D perovskite by facilitating carrier co-radiation

Dion-Jacobson (DJ) structured quasi-2D perovskites are promising candidates for new generation gain medium due to their excellent photoelectric performance, super environmental, and structure stability. The isolated carrier recombination with inhomogeneous mixed phase is detrimental in enhancing amplified spontaneous emission (ASE) of optically pumped DJ phase quasi-2D perovskites lasers. Here, in 1.3-propanediamine (PDA)-based DJ perovskites, the carrier dynamic behavior from the pristine sample to the Cremophor EL (Cre EL) treated sample is unraveled. Remarkably, the Cre EL treated sample displays a well-proportioned large n domain distribution, resulting in an increased radiation-state density and hence enhancing collaboration emitting between carriers. The improved collaboration emitting promotes carriers' fast relay radiation, resulting in a higher ASE performance with a threshold reduced from 11.7 to 4.8μJ/cm2, optical gain coefficient increased from 775 to 1559 cm-1 and degree-of-polarization (DOP) improved from 0.59 to 0.98. Our findings suggest that the development of DJ structured quasi-2D perovskite laser gain medium should target facilitating fast carrier co-radiation recombination.

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.

Quantifying local stiffness and forces in soft biological tissues using droplet optical microcavities

Mechanical properties of biological tissues fundamentally underlie various biological processes and noncontact, local, and microscopic methods can provide fundamental insights. Here, we present an approach for quantifying the local mechanical properties of biological materials at the microscale, based on measuring the spectral shifts of the optical resonances in droplet microcavities. Specifically, the developed method allows for measurements of deformations in dye-doped oil droplets embedded in soft materials or biological tissues with an error of only 1 nm, which in turn enables measurements of anisotropic stress inside tissues as small as a few pN/μm2. Furthermore, by applying an external strain, Young's modulus can be measured in the range from 1 Pa to 35 kPa, which covers most human soft tissues. Using multiple droplet microcavities, our approach could enable mapping of stiffness and forces in inhomogeneous soft tissues and could also be applied to in vivo and single-cell experiments. The developed method can potentially lead to insights into the mechanics of biological tissues.

Highly sensitive, modification-free, and dynamic real-time stereo-optical immuno-sensor

Modification-free biosensing with high specificity and sensitivity is essential for miniaturized, online, integrated, and rapid, or even real-time molecular analyses. However, most optical biosensors are based on surface pre-modification or fluorescent labeling, and have either low sensitivity or low quality factor (Q). To address these difficulties, in this study, an optical sensor prototype was developed with a microbubble optofluidic channel integrated inside a Fabry-Pérot cavity to three-dimensionally tailor the intra-cavity light field via the intra-cavity lensing (microbubble) configuration. A high Q-factor (∼105), small mode volume, and high light energy density were experimentally achieved with this "stereo-sensor" while maintaining an ultrahigh refractive index (RI) sensitivity (679 nm/RIU) and ultra-small RI resolution (∼10-7 RIU at 950 nm). Moreover, specific detection of very low concentration of biomolecules (5 fg/mL for human IgG and 0.5 pg/mL for human serum albumin (HSA)) and wide range of protein concentrations (e.g., fg/mL-ng/mL for human IgG and pg/mL-ng/mL for HSA) without probe pre-modification were achieved owing to the RI change specifically associated with the probe-target binding and the corresponding bio-macromolecular conformation change. This modification-free stereosensing scenario is applicable to continuous, real-time, and multiplexed operations, thus showing potential for online, integrated, dynamic, biomolecular analyses in vitro or in vivo, such as the dynamic metabolic analysis of single cells or organoids and point-of-care tests.

Distributed optical fiber biosensor based on optical frequency domain reflectometry

In situ acquisition of spatial distribution of biochemical substances is important in cell analysis, cancer detection and other fields. Optical fiber biosensors can achieve label-free, fast and accurate measurements. However, current optical fiber biosensors only acquire single-point of biochemical substance content. In this paper, we present a distributed optical fiber biosensor based on tapered fiber in optical frequency domain reflectometry (OFDR) for the first time. To enhance evanescent field at a relative long sensing range, we fabricate a tapered fiber with a taper waist diameter of 6 μm and a total stretching length of 140 mm. Then the human IgG layer is coated on the entire tapered region by polydopamine (PDA) -assisted immobilization as the sensing element to achieve to sense anti-human IgG. We measure shifts of the local Rayleigh backscattering spectra (RBS) caused by the refractive index (RI) change of an external medium surrounding a tapered fiber after immunoaffinity interactions by using OFDR. The measurable concentration of anti-human IgG and RBS shift has an excellent linearity in a range from 0 ng/ml to 14 ng/ml with an effective sensing range of 50 mm. The concentration measurement limit of the proposed distributed biosensor is 2 ng/ml for anti-human IgG. Distributed biosensing based on OFDR can locate a concentration change of anti-human IgG with an ultra-high sensing spatial resolution of 680 μm. The proposed sensor has a potential to realize a micron-level localization of biochemical substances such as cancer cells, which will open a door to transform single-point biosensor to distributed biosensor.

Autonomous Microlasers for Profiling Extracellular Vesicles from Cancer Spheroids

Self-propelled micro/nanomotors are emergent intelligent sensors for analyzing extracellular biomarkers in circulating biological fluids. Conventional luminescent motors are often masked by a highly dynamic and scattered environment, creating challenges to characterize biomarkers or subtle binding dynamics. Here we introduce a strategy to amplify subtle signals by coupling strong light-matter interactions on micromotors. A smart whispering-gallery-mode microlaser that can self-propel and analyze extracellular biomarkers is demonstrated through a liquid crystal microdroplet. Lasing spectral responses induced by cavity energy transfer were employed to reflect the abundance of protein biomarkers, generating exclusive molecular labels for cellular profiling of exosomes derived from 3D multicellular cancer spheroids. Finally, a microfluidic biosystem with different tumor-derived exosomes was employed to elaborate its sensing capability in complex environments. The proposed autonomous microlaser exhibits a promising method for both fundamental biological science and applications in drug screening, phenotyping, and organ-on-chip applications.

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.

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.

Droplet Lasers for Smart Photonic Labels

Microscopic lasers represent a promising tool for the development of cutting-edge photonic devices thanks to their ability to enhance light-matter interaction at the microscale. In this work, we realize liquid microlasers with tunable emission by exploiting the self-formation of three-dimensional liquid droplets into a polymeric matrix driven by viscoelastic dewetting. We design a flexible device to be used as a smart photonic label which is detachable and reusable on various types of substrates such as paper or fabric. The innovative lasing emission mechanism proposed here is based on whispering gallery mode emission coupled to random lasing, the latter prompted by the inclusion of dielectric compounds into the active gain medium. The wide possibility of modulating the emission wavelength of the microlasers by acting on different parameters, such as the cavity size, type and volume fraction of the dielectrics, and gain medium, offers a multitude of spectroscopic encoding schemes for the realization of photonic barcodes and labels to be employed in anticounterfeiting applications and multiplexed bioassays.