Single-cell photonic nanocavity probes

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.

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.

Biomimetic design of photonic materials for biomedical applications

Photonic crystal (PC) materials with bio-inspired structure colors have drawn increasing attention as their potentials have been rapidly progressed in the field of biomedicine. After elaborate integration with smart materials or preparations through advanced techniques, PC materials have shown significant advantages in biosensing, bio-probing, bio-screening, tissue engineering, and so forth. In this review, we first introduced the fundamentals of PC materials as well as their fabrication strategies with different dimensional outputs. Based on these diversified PC materials, their biomedical potentials as biosensing elements, cell carriers, drug delivery systems, screening methods, cell scaffolds for tissue engineering, cell imaging probes, as well as the monitoring means for biological processes were then highlighted. In addition to these, we finally listed and discussed some emerging applications of PCs integrated with functional materials and newly developed material engineering technologies. In short, this review will provide a panoramic view of PCs-based biomedicines, and moreover, the progressive discussions from fundamentals to advanced applications in this review may also encourage researchers to innovate PC materials or devices for broader biomedical applications.

Nanoparticle enhanced optical biosensing technologies for Prostate Specific Antigen biomarker detection

Prostate Cancer (PCa) is one of the deadliest forms of Cancer among men. Early screening process for PCa is primarily conducted with the help of a FDA approved biomarker known as Prostate Specific Antigen (PSA). The PSA-based screening is challenged with the inability to differentiate between the cancerous PSA and Benign Prostatic Hyperplasia (BPH), resulting in high rates of false-positives. Optical techniques such as optical absorbance, scattering, surface plasmon resonance (SPR), and fluorescence have been extensively employed for Cancer diagnostic applications. One of the most important diagnostic applications involves utilization of nanoparticles (NPs) for highly specific, sensitive, rapid, multiplexed, and high performance Cancer detection and quantification. The incorporation of NPs with these optical biosensing techniques allow realization of low cost, point-of-care, highly sensitive, and specific early cancer detection technologies, especially for PCa. In this work, the current state-of-the-art, challenges, and efforts made by the researchers for realization of low cost, point-of-care (POC), highly sensitive, and specific NP enhanced optical biosensing technologies for PCa detection using PSA biomarker are discussed and analyzed.

Efficient stochastic simulation of rate equations and photon statistics of nanolasers

Based on a rate equation model for single-mode two-level lasers, two algorithms for stochastically simulating the dynamics and steady-state behaviour of micro- and nanolasers are described in detail. Both methods lead to steady-state photon numbers and statistics characteristic of lasers, but one of the algorithms is shown to be significantly more efficient. This algorithm, known as Gillespie's first reaction method (FRM), gives up to a thousandfold reduction in computation time compared to earlier algorithms, while also circumventing numerical issues regarding time-increment size and ordering of events. The FRM is used to examine intra-cavity photon distributions, and it is found that the numerical results follow the analytics exactly. Finally, the FRM is applied to a set of slightly altered rate equations, and it is shown that both the analytical and numerical results exhibit features that are typically associated with the presence of strong inter-emitter correlations in nanolasers.

Enhanced Optical Absorption and Slowed Light of Reduced-Dimensional CsPbBr3 Nanowire Crystal by Exciton-Polariton

Metallic halide perovskites are promising for low-cost, low-consumption, flexible optoelectronic devices. However, research is lacking on light propagation and dielectric behaviors as fundamental properties for optoelectronic perovskite applications, particularly the mechanism supporting a strong light-matter interaction and the different properties of low-dimensional structures from their bulk counterparts. We use spatially resolved photoluminescence (SRPL) spectroscopy to explore light propagation and measure the refractive index of CsPbBr₃ nanowires (NWs). Owing to strong exciton-photon interactions, light is guided as an exciton-polariton inside the NWs at room temperature. Remarkable spatial dispersion is confirmed, in which both the real and imaginary parts of the refractive index increase dramatically approaching exciton resonance, thus slowing light and enhancing absorption, respectively. Reducing the NWs dimension increases exciton-photon coupling and the exciton fraction, increasing the light absorption coefficient and group index 5- and 3-fold, respectively, relative to those of bulk films and slowing the light group velocity by ∼74%. Furthermore, dispersive absorption induces an energy redshift to the propagating PL at 4.1-5.5 meV μm^-1 until the bottleneck region. These findings clarify light-matter interaction in confined perovskite structures to improve their optoelectronic device performance.

Design and analysis of refractive index sensors based on slotted photonic crystal nanobeam cavities with sidewall gratings

We propose and numerically investigate a refractive index sensor based on a one-dimensional slotted photonic crystal nanobeam cavity with sidewall gratings for refractive index sensing in a gaseous environment. By using the three-dimensional finite-difference time-domain method, we demonstrate that our proposed sensor simultaneously possesses a high quality factor of $ 3.71 \times {10^6} $3.71×10⁶ and a high sensitivity of 508 nm/RIU (refractive index unit) at the resonant wavelength near 1583 nm, yielding a detection limit as low as $ 1.97 \times {10^{ - 6}} $1.97×10^-6 RIU. Moreover, the mode volume of the cavity's fundamental resonant mode is found to be as small as $ 0.022(\lambda /n)^3 $0.022(λ/n)³, resulting in a very compact effective sensing area. We finally study and assess the effect of fabrication disorder on the performances of our proposed sensor. We believe our proposed sensor will be a promising candidate for applications not only in multiplexed biochemical sensing and multielement mixture detection, but also in optical trapping of single biomolecules or nanoparticles.

Ultra-small photonic crystal (PhC)-based test tool for gas permeability of polymers

We present an ultra-small photonic crystal-based test tool for gas permeability of polymers. It features a fully-etched photonic crystal (PhC) structure occupying an area of 20 µm × 800 µm on silicon-on-insulator wafer. The light-matter interaction in the PhC cavity with deformed Polydimethylsiloxane (PDMS) under pressure difference was investigated with finite element method and finite-difference time-domain method numerically. Next, three PDMS membranes of different mixing ratios were utilized for the characterization of gas permeation flux. The feasibility and effectiveness of the proposed working mechanism are verified through clearly distinguishing the gas permeability of these three testing samples. Compared with conventional test tools, this proposed test tool has fast response while it consumes less testing gas volume in a testing system with reduced footprint. Potentially, it can be integrated into lab-on-a-chip devices to measure gas permeation in nano scale.

Lasing action in low-resistance nanolasers based on tunnel junctions

We experimentally demonstrate the lasing action of a new nanolaser design with a tunnel junction. By using a heavily doped tunnel junction for hole injection, we can replace the p-type contact material of a conventional nanolaser diode with a low-resistance n-type contact layer. This leads to a significant reduction of the device resistance and lowers the threshold voltage from 5 V to around 0.95 V at 77 K. The lasing behavior is verified by the light output versus the injection current (L-I) characterization and second-order coherence function measurements. Because of less Joule heating during current injection, the nanolaser can be operated at temperatures as high as 180 K under CW pumping. The incorporation of heavily doped tunnel junctions may pave the way for other nanoscale cavity design for improved heat management.

Single-cell biomagnifier for optical nanoscopes and nanotweezers

Optical microscopes and optical tweezers, which were invented to image and manipulate microscale objects, have revolutionized cellular and molecular biology. However, the optical resolution is hampered by the diffraction limit; thus, optical microscopes and optical tweezers cannot be directly used to image and manipulate nano-objects. The emerging plasmonic/photonic nanoscopes and nanotweezers can achieve nanometer resolution, but the high-index material structures will easily cause mechanical and photothermal damage to biospecimens. Here, we demonstrate subdiffraction-limit imaging and manipulation of nano-objects by a noninvasive device that was constructed by trapping a cell on a fiber tip. The trapped cell, acting as a biomagnifier, could magnify nanostructures with a resolution of 100 nm (λ/5.5) under white-light microscopy. The focus of the biomagnifier formed a nano-optical trap that allowed precise manipulation of an individual nanoparticle with a radius of 50 nm. This biomagnifier provides a high-precision tool for optical imaging, sensing, and assembly of bionanomaterials.

Label-Free Optical Single-Molecule Micro- and Nanosensors

Label-free optical sensor systems have emerged that exhibit extraordinary sensitivity for detecting physical, chemical, and biological entities at the micro/nanoscale. Particularly exciting is the detection and analysis of molecules, on miniature optical devices that have many possible applications in health, environment, and security. These micro- and nanosensors have now reached a sensitivity level that allows for the detection and analysis of even single molecules. Their small size enables an exceedingly high sensitivity, and the application of quantum optical measurement techniques can allow the classical limits of detection to be approached or surpassed. The new class of label-free micro- and nanosensors allows dynamic processes at the single-molecule level to be observed directly with light. By virtue of their small interaction length, these micro- and nanosensors probe light-matter interactions over a dynamic range often inaccessible by other optical techniques. For researchers entering this rapidly advancing field of single-molecule micro- and nanosensors, there is an urgent need for a timely review that covers the most recent developments and that identifies the most exciting opportunities. The focus here is to provide a summary of the recent techniques that have either demonstrated label-free single-molecule detection or claim single-molecule sensitivity.

Living Nanospear for Near-Field Optical Probing

Optical nanoprobes, designed to emit or collect light in the close proximity of a sample, have been extensively used to sense and image at nanometer resolution. However, the available nanoprobes, constructed from artificial materials, are incompatible and invasive when interfacing with biological systems. In this work, we report a fully biocompatible nanoprobe for subwavelength probing of localized fluorescence from leukemia single-cells in human blood. The bioprobe is built on a tapered fiber tip apex by optical trapping of a yeast cell (1.4 μm radius) and a chain of Lactobacillus acidophilus cells (2 μm length and 200 nm radius), which act as a high-aspect-ratio nanospear. Light propagating along the bionanospear can be focused into a spot with a full width at half-maximum (fwhm) of 190 nm on the surface of single cells. Fluorescence signals are detected in real time at subwavelength spatial resolution. These noninvasive and biocompatible optical probes will find applications in imaging and manipulation of biospecimens.

Photonic crystal elliptical-hole tapered low-index-mode nanobeam cavities for sensing

A one-dimensional photonic crystal elliptical-hole tapered low-index-mode nanobeam cavity sensor fully encapsulated in a water environment is proposed. In the proposed structure, to confine the light in the low-index region and enhance the light-matter interaction, a tapered major axis of the elliptical hole away from the nanobeam cavities center is optimized. Through a three-dimensional finite-difference time-domain simulation, the results show that the low-index-mode of the middle geometry cell is confined in the photonic bandgap of two-sided cells. The highest quality factor of 6.04×10⁵ is achieved when 13 tapered segments and 5 mirror segments are placed at both sides of the host waveguide. The proposed nanobeam structure theoretically possesses a sensitivity of 244.7 nm/RIU (refractive index unit) in a water environment. Moreover, an ultra-compact footprint of 6.4  μm×0.85  μm is achieved, which is only half of the size compared to the best value reported for the nanobeam structure. The results indicate that it is a promising sensor for excellent on-chip sensing with respect to the very small footprint.

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.

Applications of Photonic Crystal Nanobeam Cavities for Sensing

In recent years, there has been growing interest in optical sensors based on microcavities due to their advantages of size reduction and enhanced sensing capability. In this paper, we aim to give a comprehensive review of the field of photonic crystal nanobeam cavity-based sensors. The sensing principles and development of applications, such as refractive index sensing, nanoparticle sensing, optomechanical sensing, and temperature sensing, are summarized and highlighted. From the studies reported, it is demonstrated that photonic crystal nanobeam cavities, which provide excellent light confinement capability, ultra-small size, flexible on-chip design, and easy integration, offer promising platforms for a range of sensing applications.

High-Q, low-index-contrast photonic crystal nanofiber cavity for high sensitivity refractive index sensing

We present the design of simultaneous high-quality (Q)-factor and high-sensitivity (S) photonic crystal nanofiber cavities (PCNFCs) made of single silica nanofiber that have a low-index contrast (ratio=1.45). By using the three-dimensional finite-difference time-domain method, two different resonant modes, dielectric mode (DM) and air mode (AM), are designed and optimized to achieve an ultrahigh figure of merit (FOM), respectively. Numerical simulations are performed to study the Q-factors and sensitivities of the proposed PCNFCs. It shows that for both DM- and AM-based PCNFCs, respectively, the Q-factors and sensitivities of Q∼1.1×10⁷, S=563.6  nm/RIU and Q∼2.1×10⁵, S=736.8  nm/RIU can be estimated, resulting in FOMs as high as 4.31×10⁶ and 1.13×10⁵, respectively. To the best of our knowledge, this is the first silica nanofiber cavity geometry that simultaneously features high Q and high S for both DM and AM in PCNFCs. Compared with the state of the art of nanofiber-based cavities, the cavity Q-factor to mode volume (V) ratio (Q/V) in this work has been improved more than two orders of magnitude. The demonstration of a high Q/V cavity in low-index-contrast nanofibers can open up versatile applications using a broad range of functional and flexible fibers. Moreover, due to the extended evanescent field and small mode volumes, the proposed PCNFCs are ideal platforms for remote ultra-sensitive refractive-index-based gas sensing without the need for complicated coupling systems.

Nanowire lasers as intracellular probes

We investigate a cadmium sulfide (CdS) nanowire (NW) laser that is spontaneously internalized into a single cell to serve as a stand-alone intracellular probe. By pumping with nano-joule light pulses, green laser emission (500-520 nm) can be observed inside cells with a peak linewidth as narrow as 0.5 nm. Due to the sub-micron diameter (∼200 nm), the NW has an appreciable fraction of the evanescent field outside, facilitating a sensitive detection of cellular environmental changes. By monitoring the lasing peak wavelength shift in response to the intracellular refractive index change, our NW laser probe shows a sensitivity of 55 nm per RIU (refractive index units) and a figure of merit of approximately 98.

A Nanophotonic Structure Containing Living Photosynthetic Bacteria

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna-Matthews-Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton-photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.

Nanoneedle-Based Sensing in Biological Systems

Nanoneedles are high aspect ratio nanostructures with a unique biointerface. Thanks to their peculiar yet poorly understood interaction with cells, they very effectively sense intracellular conditions, typically with lower toxicity and perturbation than traditionally available probes. Through long-term, reversible interfacing with cells, nanoneedles can monitor biological functions over the course of several days. Their nanoscale dimension and the assembly into large-scale, ordered, dense arrays enable monitoring the functions of large cell populations, to provide functional maps with submicron spatial resolution. Intracellularly, they sense electrical activity of complex excitable networks, as well as concentration, function, and interaction of biomolecules in situ, while extracellularly they can measure the forces exerted by cells with piconewton detection limits, or efficiently sort rare cells based on their membrane receptors. Nanoneedles can investigate the function of many biological systems, ranging from cells, to biological fluids, to tissues and living organisms. This review examines the devices, strategies, and workflows developed to use nanoneedles for sensing in biological systems.

Degradation of silicon photonic biosensors in cell culture media: analysis and prevention

Silicon photonic biosensors are being widely researched as they combine high performance with the potential for low-cost mass-manufacturing. Sensing is typically performed in an aqueous environment and it is assumed that the sensor is chemically stable, as silicon is known to etch in strong alkaline solutions but not in liquids with a pH close to 7. Here, we show that silicon can be affected surprisingly strongly by typical cell culture media, and we observe etch rates of up to 2 nm/hour. We then demonstrate that a very thin (< 10 nm) layer of thermal oxide is sufficient to suppress the etching process and provide the long-term stability required for monitoring cells and related biological processes over extended periods of time. We also show that employing an additional pH buffering compound in the culture medium can significantly reduce the etch rate.

Characteristics of strain-sensitive photonic crystal cavities in a flexible substrate

High-index semiconductor photonic crystal (PhC) cavities in a flexible substrate support strong and tunable optical resonances that can be used for highly sensitive and spatially localized detection of mechanical deformations in physical systems. Here, we report theoretical studies and fundamental understandings of resonant behavior of an optical mode excited in strain-sensitive rod-type PhC cavities consisting of high-index dielectric nanorods embedded in a low-index flexible polymer substrate. Using the three-dimensional finite-difference time-domain simulation method, we calculated two-dimensional transverse-electric-like photonic band diagrams and the three-dimensional dispersion surfaces near the first Γ-point band edge of unidirectionally strained PhCs. A broken rotational symmetry in the PhCs modifies the photonic band structures and results in the asymmetric distributions and different levels of changes in normalized frequencies near the first Γ-point band edge in the reciprocal space, which consequently reveals strain-dependent directional optical losses and selected emission patterns. The calculated electric fields, resonant wavelengths, and quality factors of the band-edge modes in the strained PhCs show an excellent agreement with the results of qualitative analysis of modified dispersion surfaces. Furthermore, polarization-resolved time-averaged Poynting vectors exhibit characteristic dipole-like emission patterns with preferentially selected linear polarizations, originating from the asymmetric band structures in the strained PhCs.

Subcellular and in-vivo Nano-Endoscopy

Analysis of individual cells at the subcellular level is important for understanding diseases and accelerating drug discovery. Nanoscale endoscopes allow minimally invasive probing of individual cell interiors. Several such instruments have been presented previously, but they are either too complex to fabricate or require sophisticated external detectors because of low signal collection efficiency. Here we present a nanoendoscope that can locally excite fluorescence in labelled cell organelles and collect the emitted signal for spectral analysis. Finite Difference Time Domain (FDTD) simulations have shown that with an optimized nanoendoscope taper profile, the light emission and collection was localized within ~100 nm. This allows signal detection to be used for nano-photonic sensing of the proximity of fluorophores. Upon insertion into the individual organelles of living cells, the nanoendoscope was fabricated and resultant fluorescent signals collected. This included the signal collection from the nucleus of Acridine orange labelled human fibroblast cells, the nucleus of Hoechst stained live liver cells and the mitochondria of MitoTracker Red labelled MDA-MB-231 cells. The endoscope was also inserted into a live organism, the yellow fluorescent protein producing nematode Caenorhabditis elegans, and a fluorescent signal was collected. To our knowledge this is the first demonstration of in vivo, local fluorescence signal collection on the sub-organelle level.

Direct Tracking of Amyloid and Tu Dynamics in Neuroblastoma Cells Using Nanoplasmonic Fiber Tip Probes

Amyloid plaques and neurofibrillary tangles are the pathological hallmarks of Alzheimer's disease. However, there has been a long-standing discussion on the dynamic relations between Aβ and tau proteins, partially due to the lack of a tool to track protein dynamics in individual live neurons at the early stage of Aβ generation and tau phosphorylation. Here, we developed nanoplasmonic fiber tip probe (nFTP) technology to simultaneously monitor Aβ42 generation and tau phosphorylation (at serine 262) in living, single neuroblastoma cells over 12 h. We observed that Aβ42 generation, under clinically relevant anesthetic treatment, preceded tau phosphorylation, which then facilitated Aβ42 generation. This observation is also supported by measuring proteins in cell lysates using the ultrasensitive label-free photonic crystal nanosensors. nFTP therefore provides an advanced method to investigate protein expression and post-translational modification in live cells and determine outcomes of intervention of Alzheimer's disease and other neurodegenerative disorders.

Fundamental Properties of One-Dimensional Zinc Oxide Nanomaterials and Implementations in Various Detection Modes of Enhanced Biosensing

Recent bioapplications of one-dimensional (1D) zinc oxide (ZnO) nanomaterials, despite the short development period, have shown promising signs as new sensors and assay platforms offering exquisite biomolecular sensitivity and selectivity. The incorporation of 1D ZnO nanomaterials has proven beneficial to various modes of biodetection owing to their inherent properties. The more widely explored electrochemical and electrical approaches tend to capitalize on the reduced physical dimensionality, yielding a high surface-to-volume ratio, as well as on the electrical properties of ZnO. The newer development of the use of 1D ZnO nanomaterials in fluorescence-based biodetection exploits the innate optical property of their high anisotropy. This review considers stimulating research advances made to identify and understand fundamental properties of 1D ZnO nanomaterials, and examines various biosensing modes utilizing them, while focusing on the unique optical properties of individual and ensembles of 1D ZnO nanomaterials specifically pertaining to their bio-optical applications in simple and complex fluorescence assays.

Fundamental Properties of One-Dimensional Zinc Oxide Nanomaterials and Implementations in Various Detection Modes of Enhanced Biosensing

Recent bioapplications of one-dimensional (1D) zinc oxide (ZnO) nanomaterials, despite the short development period, have shown promising signs as new sensors and assay platforms offering exquisite biomolecular sensitivity and selectivity. The incorporation of 1D ZnO nanomaterials has proven beneficial to various modes of biodetection owing to their inherent properties. The more widely explored electrochemical and electrical approaches tend to capitalize on the reduced physical dimensionality, yielding a high surface-to-volume ratio, as well as on the electrical properties of ZnO. The newer development of the use of 1D ZnO nanomaterials in fluorescence-based biodetection exploits the innate optical property of their high anisotropy. This review considers stimulating research advances made to identify and understand fundamental properties of 1D ZnO nanomaterials, and examines various biosensing modes utilizing them, while focusing on the unique optical properties of individual and ensembles of 1D ZnO nanomaterials specifically pertaining to their bio-optical applications in simple and complex fluorescence assays.

On-chip integrated optofluidic complex refractive index sensing using silicon photonic crystal nanobeam cavities

Complex refractive index sensing is proposed and experimentally demonstrated in optofluidic sensors based on silicon photonic crystal nanobeam cavities. The sensitivities are 58 and 139 nm/RIU, respectively, for the real part (n) and the imaginary part (κ) of the complex refractive index, and the corresponding detection limits are 1.8×10(-5) RIU for n and 4.1×10(-6) RIU for κ. Moreover, the capability of the complex refractive index sensing method to detect the concentration composition of the ternary mixture is demonstrated without the surface immobilization of functional groups, which is impossible to realize with the conventional refractive index sensing scheme.

Nanoscale bio-platforms for living cell interrogation: current status and future perspectives

The living cell is a complex entity that dynamically responds to both intracellular and extracellular environments. Extensive efforts have been devoted to the understanding intracellular functions orchestrated with mRNAs and proteins in investigation of the fate of a single-cell, including proliferation, apoptosis, motility, differentiation and mutations. The rapid development of modern cellular analysis techniques (e.g. PCR, western blotting, immunochemistry, etc.) offers new opportunities in quantitative analysis of RNA/protein expression up to a single cell level. The recent entries of nanoscale platforms that include kinds of methodologies with high spatial and temporal resolution have been widely employed to probe the living cells. In this tutorial review paper, we give insight into background introduction and technical innovation of currently reported nanoscale platforms for living cell interrogation. These highlighted technologies are documented in details within four categories, including nano-biosensors for label-free detection of living cells, nanodevices for living cell probing by intracellular marker delivery, high-throughput platforms towards clinical current, and the progress of microscopic imaging platforms for cell/tissue tracking in vitro and in vivo. Perspectives for system improvement were also discussed to solve the limitations remains in current techniques, for the purpose of clinical use in future.

Intracellular microlasers

Optical microresonators¹ which confine light within a small cavity are widely exploited for various applications ranging from the realization of lasers² and nonlinear devices^{3, 4, 5} to biochemical and optomechanical sensing^{6, 7, 8, 9, 10, 11}. Here we employ microresonators and suitable optical gain materials inside biological cells to demonstrate various optical functions in vitro including lasing. We explored two distinct types of microresonators: soft and hard, that support whispering-gallery modes (WGM). Soft droplets formed by injecting oil or using natural lipid droplets support intracellular laser action. The laser spectra from oil-droplet microlasers can chart cytoplasmic internal stress (~500 pN/μm²) and its dynamic fluctuations at a sensitivity of 20 pN/μm² (20 Pa). In a second form, WGMs within phagocytized polystyrene beads of different sizes enable individual tagging of thousands of cells easily and, in principle, a much larger number by multiplexing with different dyes.

Lasing within Live Cells Containing Intracellular Optical Microresonators for Barcode-Type Cell Tagging and Tracking

We report on a laser that is fully embedded within a single live cell. By harnessing natural endocytosis of the cell, we introduce a fluorescent whispering gallery mode (WGM) microresonator into the cell cytoplasm. On pumping with nanojoule light pulses, green laser emission is generated inside the cells. Our approach can be applied to different cell types, and cells with microresonators remain viable for weeks under standard conditions. The characteristics of the lasing spectrum provide each cell with a barcode-type label which enables uniquely identifying and tracking individual migrating cells. Self-sustained lasing from cells paves the way to new forms of cell tracking, intracellular sensing, and adaptive imaging.

Continuous wave blue lasing in III-nitride nanobeam cavity on silicon

III-V photonics on silicon is an active and promising research area. Here, we demonstrate room-temperature (RT) lasing in short-wavelength III-nitride photonic crystal nanobeam cavities grown on silicon featuring a single InGaN quantum well (QW). In the low-absorption QW region, high quality factors in excess of 10(4) are measured, while RT blue lasing under continuous-wave optical pumping is reported in the high-absorption wavelength range, hence the high QW gain region. Lasing characteristics are well accounted for by the large spontaneous emission coupling factor (β > 0.8) inherent to the nanobeam geometry and the large InGaN QW material gain. Our work illustrates the high potential of III-nitrides on silicon for the realization of low power nanophotonic devices with a reduced footprint that would be of prime interest for fundamental light-matter interaction studies and a variety of lab-on-a-chip applications including biophotonics.

Gold nanodome-patterned microchips for intracellular surface-enhanced Raman spectroscopy

Top-down patterned gold nanodome microchips are taken up by living cells and serve as a uniform and reproducible sensor for intracellular surface-enhanced Raman scattering.

Liquid sensor based on high-Q slot photonic crystal cavity in silicon-on-insulator configuration

We present the realization of an optical sensor based on an infiltrated high-Q slot photonic crystal cavity in a nonfreestanding membrane configuration. Successive infiltrations by liquids with refractive indices ranging from 1.345 to 1.545 yield a sensitivity S of 235 nm/RIU (refractive index unit), while the Q-factor is comprised between 8000 and 25,000, giving a sensor figure of merit up to 3700. This sensor has a detection limit of 1.25×10⁻⁵. The operation of this device on a silicon-on-insulator (SOI) substrate allows a straightforward integration in the silicon photonics platform, while providing a compliant mechanical stability.

Nanoscale label-free bioprobes to detect intracellular proteins in single living cells

Fluorescent labeling techniques have been widely used in live cell studies; however, the labeling processes can be laborious and challenging for use in non-transfectable cells, and labels can interfere with protein functions. While label-free biosensors have been realized by nanofabrication, a method to track intracellular protein dynamics in real-time, in situ and in living cells has not been found. Here we present the first demonstration of label-free detection of intracellular p53 protein dynamics through a nanoscale surface plasmon-polariton fiber-tip-probe (FTP).

Live-cell SERS endoscopy using plasmonic nanowire waveguides

Live-cell surface-enhanced Raman spectroscopy (SERS) endoscopy is developed by using plasmonic nanowire waveguides as endoscopic probes. It is demonstrated that the probe insertion does not stress the cell. Opposed to conventional SERS endoscopy, with excitation at the hotspot within the cell, the remote excitation method yields low-background SERS spectra from specific cell compartments with minimal associated photodamage.