Review of biosensing with whispering-gallery mode lasers

Recent advances in dynamic single-molecule analysis platforms for diagnostics: Advantages over bulk assays and miniaturization approaches

Single-molecule science is a unique technique for unraveling molecular biophysical processes. Sensitivity to single molecules provides the capacity for the early diagnosis of low biomarker amounts. Furthermore, the miniaturization of instruments for portable diagnostic tools toward point-of-care testing (POCT) is a crucial development in this field. Herein, we discuss recent developments in single-molecule sensing platforms and their advantages for diagnostics over bulk measurements including molecular size measurements, interaction dynamics, and fast biomarker sensing and sequencing at low concentrations. We highlight the capabilities of dynamic optical and electrical sensing platforms for single-biomolecule and single-vesicle monitoring associated with neurodegenerative disorders, viral diseases, cancers, and more. Current approaches to instrument miniaturization have brought technology closer to portable diagnostics settings via smartphone-based devices, multifunctional portable microscopes, handheld electrical circuit devices, and remote single-molecule assays. Finally, we provide an overview of the clinical applications of single-molecule sensors in POCT assays. Altogether, single-molecule analyses platforms exhibit significant potential for the development of novel portable healthcare devices.

Single-molecule manipulation and detection by WGM-coupled photonic nanojets

Optical manipulation and detection of single molecules, such as biomolecules and bacterial viruses, are crucial in single-molecule mechanics and biosensing. The interaction between light and individual molecules is weak due to the size of biomolecules (≤ 10 nm) being significantly smaller than the wavelength (λ) of light. This limitation results in a reduced optical gradient force and diminished detection sensitivity of light on biomolecules. To overcome this challenge, we propose a single-molecule trapping and sensing method that utilizes whisper-gallery mode (WGM) coupled photonic nanojets (PNJs). Our theoretical analysis demonstrates that a focused beam with a waist radius of λ/6 can be generated by WGM-coupled PNJs. By harnessing this sub-diffraction-limit focused beam, we create a stable nano-optical potential well for DNA molecules. The stiffness of the potential well is measured at 0.04 pN/nm/W, which is four orders of magnitude greater than that of conventional optical tweezers. Furthermore, the molecular configuration and refractive index of an individual DNA molecule can be detected by analyzing the shift in the WGM resonance peak and the intensity variation of the backscattering signal. This work provides theoretical guidance for the trapping and sensing of single molecules in the fields of chemistry, biology, and materials science.

Perspectives on Devices for Integrated Phononic Circuits

The phonon wavelength, being much shorter than that of photons at the same frequency, offers phononic devices a unique niche in radio frequency (RF) applications. However, the current limitations of these devices, particularly their restricted functionality, hinder their broader integration and application. Currently, many functions are achieved using alternative signal forms like electric and photonic signals, requiring bulky converters to transform between phonon signals and other forms. The development of functional phononic devices paves the way for integrated phononic circuits, which aim to minimize the need for signal conversion while accomplishing all necessary functions. In this perspective, a brief overview of several types of functional phononic devices is provided that hold promise for integration, such as phononic modulators, amplifiers, lasers, nonreciprocal devices, and those inspired by topological physics. It is envisioned that through continued developments in materials, fabrication techniques, and designs, it's possible to realize integrated phononic circuits which will be applied in miniaturized communication devices with reduced size, weight, power consumption, and cost (SWaP-C), as well as in other fields including quantum information science, sensing, biomedical engineering, and beyond.

Exploring Carbon Dots for Biological Lasers

Biological lasers, representing innovative miniaturized laser technology, hold immense potential in the fields of biological imaging, detection, sensing, and medical treatment. However, the reported gain media for biological lasers encounter several challenges complex preparation procedures, high cost, toxicity concerns, limited biocompatibility, and stability issues along with poor processability and tunability. These drawbacks have impeded the sustainable development of biological lasers. Carbon dots (CDs), as a novel solution-processable gain materials characterized by facile preparation, low cost, low toxicity, excellent biocompatibility, high stability, easy modification, and luminescence tuning capabilities along with outstanding luminescence performance. Consequently, they find extensive applications in diverse fields such as biology, sensing, photoelectricity, and lasers. Henceforth, they are particularly suitable for constructing biological lasers. This paper provides a comprehensive review on the classification and application of existing biological lasers while emphasizing the advantages of CDs compared to other gain media. Furthermore, it presents the latest progress made by utilizing CDs as gain media and forecasts both promising prospects and potential challenges for biological lasers based on CDs. This study aims to enhance understanding of CD lasers and foster advancements in the field of biological lasers.

Highly sensitive microdisk laser sensor for refractive index sensing via periodic meta-hole patterning

Microdisk lasers have emerged as compact on-chip optical sensors due to their small size, simple structure, and efficient lasing capabilities. However, conventional microdisk laser sensors face challenges in enhancing interactions with external analytes, as their energy remains predominantly confined within the laser material. In this study, we present a novel microdisk laser sensor incorporating periodic meta-hole patterning, designed to enhance external interaction while maintaining the integrity of the whispering gallery mode (WGM). Numerical simulations show that in an InGaAsP microdisk laser (5 μm diameter, 250 nm thickness), the WGM remains stable with periodic meta-holes (period a = 340 nm, diameter d < 0.4a), achieving a resonant wavelength near 1,500 nm. The inclusion of meta-holes led to a substantial improvement in sensitivity, reaching up to 100.8 nm/RIU - a 2.26-fold increase over nonpatterned microdisks. Experimental validation confirmed lasing in structures with a d/a ratio of 0.32, achieving a maximum sensitivity of 74.5 nm/RIU, which represents a 2.02-fold enhancement compared to nonpatterned designs. This advancement in microdisk laser design not only opens new possibilities for high-performance, miniaturized optical sensors but also holds significant potential for integration into next-generation on-chip sensing technologies.

Breathing Laser-Spectral Mapping of Cavity-Enhanced Redox Reactions with Subcellular Resolution

Precise and dynamic observation of redox reactions in living organisms holds significant importance for the study of physiological processes and pathological mechanisms. However, the current technologies still make it challenging to monitor this process in a nondestructive and highly sensitive manner. Herein, we introduced a bioactive laser approach for ultrasensitive and real-time monitoring of intracellular redox reactions. Resazurin, as a popular cell viability assay reagent, has lasing behaviors and photostability, which makes it suitable for the development of bioactive lasers. Due to the strong interactions of light and matter within the laser cavity, subtle changes in resazurin concentration during the redox reaction can be translated into detectable wavelength shifts in the lasing spectrum. With narrow laser peaks, the sensing resolution can reach down to 30 pM per 10 pm wavelength shift. Combined with a scanning platform, we mapped the intracellular and intercellular heterogeneities in metabolism. Further applications in cell identification, oxidative stress assessment, and drug evaluation revealed the universal applicability of this method in cell assays and biomedical analysis, providing insights into disease diagnosis and drug screening.

DNA Sensing with Whispering Gallery Mode Microlasers

Nucleic acid sensing is crucial for advancing diagnostics, therapeutic monitoring, and molecular biology research by enabling the precise identification of DNA and RNA interactions. Here, we present an innovative sensing platform based on DNA-functionalized whispering gallery mode (WGM) microlasers. By correlating spectral shifts in laser emission to changes in the refractive index, we demonstrate real-time detection of DNA hybridization and structural changes. The addition of gold nanoparticles to the DNA strands significantly enhances sensitivity, and exclusively labeling the sensing strand or a hairpin strand eliminates the need for secondary labeling of the target strand. We further show that ionic strength influences DNA compactness, and we introduce a hairpin-based system as a dual-purpose sensor and controlled release mechanism for drug delivery. This versatile WGM-based platform offers promise for sequence-specific nucleic acid sensing, multiplexed detection, and in vivo applications in diagnostics and cellular research.

Electric Field Polarity Controls Distribution of Viral Bioreceptors within Near-Field Electrospun Biohybrid Microfiber Optical Biosensors

Microorganisms (e.g., bacteria, fungi, and viruses) add indispensable functionality to a range of electrospun polymer materials and devices. The optimal distribution of bioactive agents on either the interior or exterior of the fiber is application-specific. Current microbe surface immobilization strategies and core-confinement techniques continue to pose a number of challenges. Here, we explore a simple strategy, utilizing electrostatic forces, to control the migration and surface concentration of the M13 bacteriophage within near-field electrospun poly(vinyl alcohol) (PVA) microfibers. Both the surface charge of the electrospun virus and the applied electric field polarity altered microbe placement. When doped with Rhodamine 6G (R6G), the circular microfiber cross sections formed active whispering gallery mode (WGM) resonators. These relatively high-quality (Q) optical cavities enabled us to sensitively probe the virus content of their outer layer, while functioning as label-free optical biosensors with phage-based streptavidin biorecognition elements. Coulomb forces displayed significant control over M13 surface coverage during near-field electrospinning, increasing biosensor response by nearly a factor of 4 to 1310 nM streptavidin. These findings are an important demonstration of electrostatic forces as a simple, yet adaptable method to enhance biohybrid fiber functionality and performance by tailoring microbe distribution.

Improvement of the optical cavity-based biosensor's limit of detection using optimal 3-aminopropyltriethoxysilane process

Optical resonator-based biosensors are important for advancing medical diagnostics and environmental monitoring due to their high sensitivity and label-free detection capabilities. In this study, we present a systematic comparison of three 3-aminopropyltriethoxysilane (APTES) functionalization methods - ethanol-based, methanol-based, and vapor-phase - on an Optical Cavity-based Biosensor (OCB) designed to detect streptavidin. The APTES process is an important first step for surface functionalization to form a linker to immobilize receptor molecules on the sensor surface. Our aim was to identify the deposition conditions that yield a uniform APTES layer, with an enhanced bioreceptor immobilization and improved sensor performance. By using a differential detection approach using two laser diodes at 808 nm and 880 nm, we achieved real-time intensity measurements in the OCB that enabled sensitive detection of target analyte. Among the three APTES methods tested, the methanol-based protocol (0.095% APTES) led to a significantly improved limit of detection (LOD) of 27 ng/mL, a threefold improvement over our previous results. Detailed atomic force microscopy (AFM), contact angle, and dose-response analyses confirmed the high quality of the monolayer formed under optimal conditions, emphasizing the importance of solvent choice and controlled deposition parameters for obtaining stable functional layers. These findings emphasize how the improved APTES functionalization directly enhances the sensitivity and reliability of our OCB system, offering a robust and adaptable approach for real-time, label-free detection in diverse biosensing applications.

Efficient multilayer near-infrared micro-bottle laser pumped by a 532 nm nanosecond laser

This paper explores the development and optimization of organic near-infrared micro-cavity lasers for biophotonic applications. Four micro-bottle laser configurations inclouding single-layer, two-layer, and three-layer structures were designed and fabricated using Nile-Blue (NB) and Rhodamine B (RhB) laser dyes doped in SU-8 polymer as laser-active materials. While NB achieves lasing near 750 nm, its absorption of common pump sources such as Nd: YAG lasers at 532 nm is limited. Therefore, Forster resonance energy transfer (FRET) between RhB and NB was employed to enhance NB's lasing efficiency under 532 nm excitation. Experimental and simulation results demonstrate that multilayer designs, particularly the three-layer configuration, outperform others, achieving higher emission intensity, improved stability, and reduced lasing thresholds. The inclusion of RhB optimizes pump absorption and enables efficient energy transfer, facilitating stable Near-IR lasing at 720-750 nm. These findings highlight the potential of multilayer micro-cavity lasers for compact, efficient, and stable organic laser systems in biophotonic and sensing applications.

Editorial for the Glassy Materials and Micro/Nano Devices Section

Glass is an amorphous solid, renowned for its transparency and versatility, and has been widely used for centuries in both scientific instruments and daily life [...].

Robust low threshold full-color upconversion lasing in rare-earth activated nanocrystal-in-glass microcavity

Visible light microlasers are essential building blocks for integrated photonics. However, achieving low-threshold (μW), continuous-wave (CW) visible light lasing at room temperature (RT) has been a challenge because of the formidable requirement of population inversion at short wavelengths. Rare-earth (RE)-activated microcavities, featuring high-quality factor (Q) and small mode volume of whispering gallery modes, offer a great opportunity for achieving infrared-to-visible upconversion (UC) lasing. Here, we report that batch-produced nano-glass composite (GC) microspheres incorporating RE-doped fluoride nanocrystals show efficient UC emissions. These multi-phase composite microspheres exhibit a high Q value (≥105), comparable to that of conventional multi-component glass microspheres. The UC lasing with pure red, green, and blue (RGB) emissions are demonstrated based on a highly efficient tapered fiber-microsphere system. More importantly, the GC microspheres manifest reduced (by 45%) lasing threshold and enhanced (more than four times) slope efficiency. These characteristics, together with excellent long-term stability, suggest a promising solution to achieving highly robust, stand-alone, low-threshold, and versatile UC microlasers.

Whispering-gallery mode sensor based on coupling of tapered two-mode fiber and glass capillary

A novel whispering-gallery mode (WGM) sensor is fabricated by coupling a tapered two-mode fiber and a glass capillary. By utilizing the relatively large orifice of glass capillaries, polydimethylsiloxane (PDMS) and magnetic fluid are directly injected into two WGM structured glass capillaries, respectively, allowing these materials to substantially interact with the light field of the WGM, thereby achieving temperature, pressure, and magnetic field measurements. λ1 and λ2 are the two resonant peak wavelengths of the WGM after injecting PDMS into a glass capillary. λ3 is the resonant peak wavelength of the WGM after injecting the magnetic fluid into the glass capillary. The experiments found that λ1 and λ2 exhibit high sensitivity to temperature and air pressure, and λ3 is sensitive to magnetic field. The temperature and air pressure sensitivities of λ1 are -624 pm/°C and -7.04 nm/MPa, respectively. The temperature and air pressure sensitivities of λ2 are -964 pm/°C and -15.08 nm/MPa, respectively. The magnetic field sensitivity of λ3 is 107 pm/mT in the range of 9.45-62.91 mT. The proposed sensors have the advantages of low cost, simple fabrication, and high sensitivity, and they can be applied to temperature, gas pressure, and magnetic field measurements.

pH-responsive hydrogels embedded in hollow-core optical resonators

Whispering-gallery-mode (WGM) microresonators are typically studied for surface (bio)chemical sensing, mainly relying on small refractive index changes occurring within a nanometer range from their walls surface. This high sensitivity, reaching up to 10-5 refractive index unit (RIU, ∼2.5 nm/RIU and measured at a femtometer resolution) leads to broad ranges of applications, especially for biosensing purposes through the monitoring of molecular binding events. In this article, we investigate the gelling of thin layers of poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hydrogels inside a WGM microbubble resonator (MBR), fabricated from a silica capillary. The formation of such layers is achieved by withdrawing a liquid solution of 25% PVA/PAA in pure water into the MBR and locally heating the microbubble region, leading to hydrogel formation only in the WGM cavity. The capillary is then rinsed and tested under varying pH solutions. The swelling ability of these hydrogels is proportional to the pH of samples brought into contact with the cavity, leading to physical deformations of the layers consequently leading to changes in the WGM resonance condition. We show the preliminary results obtained for the gelling and characterization of these thin layers in microbubble resonators and present the related wavelength shifts observed for several pH values. We discuss the kinetics and practical uses, such as reversibility and tunable detection of small pH changes.

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.

Open whispering gallery mode resonators

There are some issues with traditional whispering gallery mode (WGM) resonators such as poor light extraction and a dense mode spectrum. In this paper, we introduce a solution to these limitations by proposing open WGM (OWGM) resonators that effectively reduce the mode density and enable directional radiation through a connected waveguide at the expense of some lowering in Q-factor. Numerical simulations of two-dimensional metallic and dielectric disk resonators with holes reveal a significant increase in intermode distance. The study also extends to three-dimensional dielectric OWGM resonators, demonstrating the formation of sparse spectra suitable for photonic applications. Additionally, the design of a cylindrical Bragg microresonator connected to a single-mode fiber via an optimized topology-based connector achieves near-unity transmission and efficient coupling. This approach enhances the development of new photonic devices, addressing the limitations of traditional high Q-factor WGM resonators and offering potential advancements in laser technology and optical communications.

Microfluidics and Nanofluidics in Strong Light-Matter Coupling Systems

The combination of micro- or nanofluidics and strong light-matter coupling has gained much interest in the past decade, which has led to the development of advanced systems and devices with numerous potential applications in different fields, such as chemistry, biosensing, and material science. Strong light-matter coupling is achieved by placing a dipole (e.g., an atom or a molecule) into a confined electromagnetic field, with molecular transitions being in resonance with the field and the coupling strength exceeding the average dissipation rate. Despite intense research and encouraging results in this field, some challenges still need to be overcome, related to the fabrication of nano- and microscale optical cavities, stability, scaling up and production, sensitivity, signal-to-noise ratio, and real-time control and monitoring. The goal of this paper is to summarize recent developments in micro- and nanofluidic systems employing strong light-matter coupling. An overview of various methods and techniques used to achieve strong light-matter coupling in micro- or nanofluidic systems is presented, preceded by a brief outline of the fundamentals of strong light-matter coupling and optofluidics operating in the strong coupling regime. The potential applications of these integrated systems in sensing, optofluidics, and quantum technologies are explored. The challenges and prospects in this rapidly developing field are discussed.

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.

Single Molecule Thermodynamic Penalties Applied to Enzymes by Whispering Gallery Mode Biosensors

Optical microcavities, particularly whispering gallery mode (WGM) microcavities enhanced by plasmonic nanorods, are emerging as powerful platforms for single-molecule sensing. However, the impact of optical forces from the plasmonic near field on analyte molecules is inadequately understood. Using a standard optoplasmonic WGM single-molecule sensor to monitor two enzymes, both of which undergo an open-to-closed-to-open conformational transition, the work done on an enzyme by the WGM sensor as atoms of the enzyme move through the electric field gradient of the plasmonic hotspot during conformational change has been quantified. As the work done by the sensor on analyte enzymes can be modulated by varying WGM intensity, the WGM microcavity system can be used to apply free energy penalties to regulate enzyme activity at the single-molecule level. The findings advance the understanding of optical forces in WGM single-molecule sensing, potentially leading to the capability to precisely manipulate enzyme activity at the single-molecule level through tailored optical modulation.

Localization and Orientation of Dye Molecules at the Surface of a Levitated Microdroplet in Air Revealed by Whispering Gallery Mode Resonances

Microdroplets offer unique environments that accelerate chemical reactions; however, the mechanisms behind these processes remain debated. The localization and orientation of solute molecules near the droplet surface have been proposed as factors for this acceleration. Since significant reaction acceleration has been observed for electrospray- and sonic-spray-generated aerosol droplets, the analysis of microdroplets in air has become essential. Here, we utilized whispering gallery mode (WGM) resonances to investigate the localization and orientation of dissolved rhodamine B (RhB) in a levitated microdroplet (∼3 μm in diameter) in air. Fluorescence enhancement upon resonance with the WGMs revealed the localization and orientation of RhB near the droplet surface. Numerical modeling using Mie theory quantified the RhB orientation at 68° to the surface normal, with a small fraction randomly oriented inside the droplet. Additionally, low RhB concentrations increased surface localization. These results support the significance of surface reactions in the acceleration of microdroplet reactions.

Rare-Earth-Metal-Free Solid-State Fluorescent Carbonized-Polymer Microspheres for Unclonable Anti-Counterfeit Whispering-Gallery Emissions from Red to Near-Infrared Wavelengths

Colloidal carbon dots (CDs) have garnered much attention as metal-free photoluminescent nanomaterials, yet creation of solid-state fluorescent (SSF) materials emitting in the deep red (DR) to near-infrared (NIR) range poses a significant challenge with practical implications. To address this challenge and to engineer photonic functionalities, a micro-resonator architecture is developed using carbonized polymer microspheres (CPMs), evolved from conventional colloidal nanodots. Gram-scale production of CPMs utilizes controlled microscopic phase separation facilitated by natural peptide cross-linking during hydrothermal processing. The resulting microstructure effectively suppresses aggregation-induced quenching (AIQ), enabling strong solid-state light emission. Both experimental and theoretical analysis support a role for extended π-conjugated polycyclic aromatic hydrocarbons (PAHs) trapped within these microstructures, which exhibit a progressive red shift in light absorption/emission toward the NIR range. Moreover, the highly spherical shape of CPMs endows them with innate photonic functionalities in combination with their intrinsic CD-based attributes. Harnessing their excitation wavelength-dependent photoluminescent (PL) property, a single CPM exhibits whispering-gallery modes (WGMs) that are emission-tunable from the DR to the NIR. This type of newly developed microresonator can serve as, for example, unclonable anti-counterfeiting labels. This innovative cross-cutting approach, combining photonics and chemistry, offers robust, bottom-up, built-in photonic functionality with diverse NIR applications.

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.

Avalanche-assisted transient optical phenomenon in aggregated toluidine blue dye

Pulse-modulated CW laser heat deposition modulates the darkness or the transparency of an aggregated medium in the high signal optical regimen. A recently reported work found that transient optical responses of molecular aggregates can be different depending on whether the sample is excited with a laser wavelength tuned within the absorption band of the monomer or within the absorption band of the aggregates. The different transient responses were attributed to different dynamic processes during the laser-induced disassembling of the molecular aggregates and may have implications in the field of organic electronics and optical devices, such as optical logical gates, optical power limiters and all-optical switching. In this paper laser beams with wavelengths of 663 nm and 532 nm were used to produce sudden changes in the thermodynamic equilibrium of the aggregation states of the ortho-toluidine blue dye, which allowed to observe the occurrence of the avalanche - mediated transient phenomenon in the laser-induced disassembling of ortho-toluidine blue (TBO) aggregates. A double exponential model was adjusted to the registered transient data. The obtained values for the fast components of the transient time responses of ortho-toluidine blue dye, for the studied concentrations, ranged from ∼ 6.5 to 9.5 ms at 532 nm, and from ∼ 43 to 48 ms at 663 nm. A single beam experiment was employed to evaluate the performance of the ortho-toluidine blue dye in a beam power-damping device, driven by the simultaneous and cooperative actions of the laser induced disassembling of aggregated dye units and the thermal lensing effect. It was found that the phenomenon of laser-induced dye disassembling of TBO, acting cooperatively with the thermal lensing effect, damps the laser beam power faster than the thermal lensing phenomenon alone. In addition, the results showed that the speed of the laser beam power-damping is dye dependent.

Synergizing Nanomaterials and Artificial Intelligence in Advanced Optical Biosensors for Precision Antimicrobial Resistance Diagnosis

Antimicrobial resistance (AMR) poses a critical global One Health concern, ensuing from unintentional and continuous exposure to antibiotics, as well as challenges in accurate contagion diagnostics. Addressing AMR requires a strategic approach that emphasizes early stage prevention through screening in clinical, environmental, farming, and livestock settings to identify nonvulnerable antimicrobial agents and the associated genes. Conventional AMR diagnostics, like antibiotic susceptibility testing, possess drawbacks, including high costs, time-consuming processes, and significant manpower requirements, underscoring the need for intelligent, prompt, and on-site diagnostic techniques. Nanoenabled artificial intelligence (AI)-supported smart optical biosensors present a potential solution by facilitating rapid point-of-care AMR detection with real-time, sensitive, and portable capabilities. This Review comprehensively explores various types of optical nanobiosensors, such as surface plasmon resonance sensors, whispering-gallery mode sensors, optical coherence tomography, interference reflection imaging sensors, surface-enhanced Raman spectroscopy, fluorescence spectroscopy, microring resonance sensors, and optical tweezer biosensors, for AMR diagnostics. By harnessing the unique advantages of these nanoenabled smart biosensors, a revolutionary paradigm shift in AMR diagnostics can be achieved, characterized by rapid results, high sensitivity, portability, and integration with Internet-of-Things (IoT) technologies. Moreover, nanoenabled optical biosensors enable personalized monitoring and on-site detection, significantly reducing turnaround time and eliminating the human resources needed for sample preservation and transportation. Their potential for holistic environmental surveillance further enhances monitoring capabilities in diverse settings, leading to improved modern-age healthcare practices and more effective management of antimicrobial treatments. Embracing these advanced diagnostic tools promises to bolster global healthcare capacity to combat AMR and safeguard One Health.

High-quality quasi-bound state in the continuum enabled single-nanoparticle virus detection

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality(Q-) factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by introducing a quasi-BIC (qBIC) supported by an elaborated all-dielectric dimer grating. Thanks to the excellent field confinement within the air gap of grating enabled by such a high-Q qBIC, the figure of merit (FOM) of a biosensor is up to 18,908.7 RIU-1. Furthermore, we demonstrated that such a high-Q grating can help push the limit of optical biosensing to the single-particle level. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentration.

Free-space laser emission from Nd:YAG elliptical microdisks

The utilization of deformed microcavities, such as elliptical microdisks, has been widely acknowledged as an effective solution for achieving free-space emission in microcavity lasers. However, the deformations introduced in the microcavity structure tend to decrease the quality factor (Q factor), resulting in weakened output intensity. To address this issue, one potential approach is to employ highly efficient laser gain media that can compensate for the negative impact of the structure on the output intensity. In this study, we employed the exceptional laser crystal material Nd:YAG as the laser gain medium and successfully fabricated an elliptical microdisk laser with a major semiaxis of 15 µm and an eccentricity ratio of 0.15. By utilizing an 808 nm laser for pumping, we were able to achieve free-space laser emission with a slope efficiency of 1.7% and a remarkable maximum output power of 58 µW. This work contributes toward the advancement of the application of deformation microcavity lasers.

A Wearable Thin-Film Hydrogel Laser for Functional Sensing on Skin

Flexible photonics offers the possibility of realizing wearable sensors by bridging the advantages of flexible materials and photonic sensing elements. Recently, optical resonators have emerged as a tool to improve their oversensitivity by integrating with flexible photonic sensors. However, direct monitoring of multiple psychological information on human skin remains challenging due to the subtle biological signals and complex tissue interface. To tackle the current challenges, here, we developed a functional thin film laser formed by encapsulating liquid crystal droplet lasers in a flexible hydrogel for monitoring metabolites in human sweat (lactate, glucose, and urea). The three-dimensional cross-linked hydrophilic polymer serves as the adhesive layer to allow small molecules to penetrate from human tissue to generate strong light--matter interactions on the interface of whispering gallery modes resonators. Both the hydrogel and cholesteric liquid crystal microdroplets were modified specifically to achieve high sensitivity and selectivity. As a proof of concept, wavelength-multiplexed sensing and a prototype were demonstrated on human skin to detect human metabolites from perspiration. These results present a significant advance in the fabrication and potential guidance for wearable and functional microlasers in healthcare.

Highly Sensitive Force Sensor Based on High-Q Asymmetric V-Shaped CaF2 Resonator

Whispering gallery mode (WGM) resonators have high-quality factors and can be used in high-sensitivity sensors due to the narrow line width that allows for the detection of small external changes. In this paper, a force-sensing system based on a high-Q asymmetric V-shaped CaF2 resonator is proposed. Based on the dispersion coupling mechanism, the deformation of the resonator is achieved by loading force, and the resonant frequency is changed to determine the measurement. By adjusting the structural parameters of the asymmetric V-shaped resonator, the deformation of the resonator under force loading is improved. The experimental results show that the sensitivity of the V-shaped tip is 18.84 V/N, which determines the force-sensing resolution of 8.49 μN. This work provides a solution for force-sensing measurements based on a WGM resonator.

Tailoring whispering-gallery fields in optical black hole cavities

The ability to confine light has great significance in both fundamental science and practical applications. Optical black hole (OBH) cavities show intriguing zero radiation loss and strong field confinement. In this work, we systematically explore the whispering gallery mode (WGM) in a group of generalized OBH cavities, featuring bound states and strong field confinement. The field confinement in generalized OBH cavities is revealed to be enhanced with the increase of index-modulation factors, resulting from the increase of a potential barrier. Furthermore, we reveal the anomalous external resonant modes, exhibiting fascinating field enhancement in the low-index region far beyond the cavity boundary. These anomalous WGMs are attributed to the potential bending effect and above-barrier resonance. Our work may shed light on tailoring WGM fields in gradient-index cavities and find potential applications in light coupling and optical sensing.

Self-injection-locked thin-film regenerative laser amplifier

Organic lasers based on distributed feedback (DFB) microcavities have been extensively investigated. However, the application of these lasers is limited by their low output power and large beam divergence. Therefore, laser amplifiers are needed to achieve practically applicable laser intensity and controllable lasing modes for far-field applications. In this work, we report self-injection-locked laser amplifiers using the combination of a DFB microcavity and a Bragg reflector, where a high-reflection mirror acts as the Bragg reflector and its feedback supplies the external-cavity injection. The coherent coupling between the DFB microcavity and the Bragg amplifier is crucial for achieving high conversion efficiency and high-contrast transverse modes. An amplification factor larger than 20 and a single output laser spot with high contrast that has been achieved. Such an integration design of the self-injected DFB microcavity amplifier can be directly utilized in the realization of high-performance thin-film laser sources for practical applications.

Self-Sustaining Water Microdroplet Resonators Using 3D-Printed Microfluidics

Microdroplet resonators provide an excellent tool for optical studies of water, but water microdroplets are difficult to maintain outside a carefully controlled environment. We present a method for maintaining a water microdroplet resonator on a 3D-printed hydrophobic surface in an ambient environment. The droplet is maintained through a passive microfluidic system that supplies water to the droplet through a vertical channel at a rate equivalent to its evaporation. In this manner, we are able to create and passively maintain water microdroplet resonators with quality factors as high as 3×108.

Whispering-Gallery Mode Optoplasmonic Microcavities: From Advanced Single-Molecule Sensors and Microlasers to Applications in Synthetic Biology

Optical microcavities, specifically, whispering-gallery mode (WGM) microcavities, with their remarkable sensitivity to environmental changes, have been extensively employed as biosensors, enabling the detection of a wide range of biomolecules and nanoparticles. To push the limits of detection down to the most sensitive single-molecule level, plasmonic nanorods are strategically introduced to enhance the evanescent fields of WGM microcavities. This advancement of optoplasmonic WGM sensors allows for the detection of single molecules of a protein, conformational changes, and even atomic ions, marking significant contributions in single-molecule sensing. This Perspective discusses the exciting research prospects in optoplasmonic WGM sensing of single molecules, including the study of enzyme thermodynamics and kinetics, the emergence of thermo-optoplasmonic sensing, the ultrasensitive single-molecule sensing on WGM microlasers, and applications in synthetic biology.

Hybrid quantum-classical polarizability model for single molecule biosensing

Optical whispering gallery mode biosensors are able to detect single molecules through effects of their polarizability. We address the factors that affect the polarizability of amino acids, which are the building blocks of life, via electronic structure theory. Amino acids are detected in aqueous environments, where their polarizability is different compared to the gasphase due to solvent effects. Solvent effects include structural changes, protonation and the local field enhancement through the solvent (water). We analyse the impact of these effects and find that all contribute to an increased effective polarizability in the solvent. We also address the excess polarizability relative to the displaced water cavity and develop a hybrid quantum-classical model that is in good agreement with self-consistent calculations. We apply our model to calculate the excess polarizability of 20 proteinogenic amino acids and determine the minimum resolution required to distinguish the different molecules and their ionised conformers based on their polarizability.

Microsphere-Enabled Micropillar Array for Whispering Gallery Mode Virus Detection

Rapid detection of pathogens and analytes at the point of care offers an opportunity for prompt patient management and public health control. This paper reports an open microfluidic platform coupled with active whispering gallery mode (WGM) microsphere resonators for the rapid detection of influenza viruses. The WGM microsphere resonators, precoated with influenza A polyclonal antibodies, are mechanically trapped in the open micropillar array, where the evaporation-driven flow continuously transports a small volume (∼μL) of sample to the resonators without auxiliaries. Selective chemical modification of the pillar array changes surface wettability and flow pattern, which enhances the detection sensitivity of the WGM resonator-based virus sensor. The optofluidic sensing platform is able to specifically detect influenza A viruses within 15 min using a few microliters of sample and displays a linear response to different virus concentrations.

Optical Method for Detection and Classification of Heavy Metal Contaminants in Water Using Iso-pathlength Point Characterization

Water pollution caused by hazardous substances, particularly heavy metal (HM) ions, poses a threat to human health and the environment. Traditional methods for measuring HM in water are expensive and time-consuming and require extensive sample preparation. Therefore, developing robust, simple, and sensitive techniques for the detection and classification of HM is needed. We propose an optical approach that exploits the full scattering profile, meaning the angular intensity distribution, and utilizes the iso-pathlength (IPL) point. This point appears where the intensity is constant for different scattering coefficients, while the absorption coefficient is set. The absorption does not affect the IPL point position, it only reduces its intensity. In this paper, we explore the wavelength influence on the IPL point both in Monte Carlo simulations and experimentally. Next, we present the characterization of ferric chloride (FeCl2) by this phenomenon. Eventually, we exhibit the detection of FeCl2 and intralipid mixed in concentrations of 50-100 and 20-30 ppm, respectively. These findings endorse the idea that the IPL point is an intrinsic parameter of a system serving as an absolute calibration point. The method provides an efficient way of differentiating contamination in water. Its characterization technique is easy, precise, and versatile making it preferable for water monitoring.

Determination of Enantiomeric Excess by Optofluidic Microlaser near Exceptional Point

Enantiomeric excess (ee) is an essential indicator of chiral drug purification in the pharmaceutical industry. However, to date the ee determination of unknown concentration enantiomers generally involves two separate techniques for chirality and concentration measurement. Here, a whispering-gallery mode (WGM) based optofluidic microlaser near exceptional point to achieve the ee determination under unknown concentration with a single technique is proposed. Exceptional point induces the unidirectional WGM lasing, providing the optofluidic microlaser with the novel capability to measure chirality by polarization, in addition to wavelength-based concentration detection. The dual-parameters detection of optofluidic microlaser empowers it to achieve ee determination of various unknown enantiomers without additional concentration measurements, a feat that is challenging to accomplish with other methods. Featuring the sensitivity enhancement and miniature structure of the WGM sensors, the obtained chiroptical response of the present approach is ≈30-fold higher than that of the conventional optical rotation-based polarimeter, and the reagent consumption is reduced by three orders of magnitude.

Functionalization of Phosphate and Tellurite Glasses and Spherical Whispering Gallery Mode Microresonators

Active whispering gallery mode resonators made as spherical microspheres doped with quantum dots or rare earth ions achieve high quality factors and are excellent candidates for biosensors capable of detecting biomolecules at low concentrations. However, to produce quantum dot-doped microspheres, new low melting temperature glasses are sought, which require surface functionalization and antibody immobilization for biosensor development. Here, we demonstrate the successful functionalization of three low melting point glasses and microspheres made of them. The glasses were made from sodium borophosphate, sodium aluminophosphate, and tellurite, and then, they were functionalized using (3-glycidyloxypropyl)trimethoxysilane in ethanol- and toluene-based protocols. Proper silanization was confirmed by energy-dispersive X-ray spectroscopy and fluorescence microscopy of an amino-modified luminescent oligonucleotide probe. Fluorescence imaging showed successful silanization for all tested samples and no degradation for aluminophosphate and tellurite glasses. The strongest signal was registered for tellurite glass samples functionalized using the toluene-based silanization protocol. This conclusion implies that this functionalization method is the most efficient and is highly recommended for future antibody immobilization and biosensing application.

Pushing Optical Virus Detection to a Single Particle through a High-Q Quasi-bound State in the Continuum in an All-dielectric Metasurface

Bound states in the continuum (BICs) have emerged as a powerful platform for boosting light-matter interactions because they provide an alternative way of realizing optical resonances with ultrahigh quality factors, accompanied by extreme field confinement. In this work, we realized an optical biosensor by harnessing a quasi-BIC (qBIC) supported by an all-dielectric metasurface with broken symmetry, whose unit cell is composed of a silicon cuboid with two asymmetric air holes. Thanks to the excellent field confinement within the air gap of a metasurface enabled by such a high-Q qBIC, the figure of merit (FOM) of the biosensor is up to 2136.35 RIU-1. Futhermore, we demonstrated that such a high-Q metasurface can push the detection limit to a few virus particles. Our results may find exciting applications in extreme biochemical sensing like COVID-19 with ultralow concentrations.

Poly(lactic acid) stereocomplex microspheres as thermally tolerant optical resonators

Thermally tolerant polymer optical resonators are fabricated from a stereocomplex of poly(L-lactic acid) and poly(D-lactic acid) through the oil-in-water miniemulsion method. The thermal stability of the microspheres of the stereocomplex poly(lactic acid) (SC-PLA) is superior to that of the homochiral poly(lactic acid) (HC-PLA). As a result of the high thermal stability, the optical resonator properties of the SC-PLA microspheres are preserved at an elevated temperature of up to 230 °C, which is 70 °C higher than that of microspheres formed from HC-PLA.

Submonolayer biolasers for ultrasensitive biomarker detection

Biomarker detection is key to identifying health risks. However, designing sensitive and single-use biosensors for early diagnosis remains a major challenge. Here, we report submonolayer lasers on optical fibers as ultrasensitive and disposable biosensors. Telecom optical fibers serve as distributed optical microcavities with high Q-factor, great repeatability, and ultralow cost, which enables whispering-gallery laser emission to detect biomarkers. It is found that the sensing performance strongly depends on the number of gain molecules. The submonolayer lasers obtained a six-order-of-magnitude improvement in the lower limit of detection (LOD) when compared to saturated monolayer lasers. We further achieve an ultrasensitive immunoassay for a Parkinson's disease biomarker, alpha-synuclein (α-syn), with a lower LOD of 0.32 pM in serum, which is three orders of magnitude lower than the α-syn concentration in the serum of Parkinson's disease patients. Our demonstration of submonolayer biolaser offers great potentials in high-throughput clinical diagnosis with ultimate sensitivity.

A whispering-gallery scanning microprobe for Raman spectroscopy and imaging

Optical whispering-gallery-mode microsensors are a promising platform for many applications, such as biomedical monitoring, magnetic sensing, and vibration detection. However, like many other micro/nanosensors, they cannot simultaneously have two critical properties - ultrahigh sensitivity and large detection area, which are desired for most sensing applications. Here, we report a novel scanning whispering-gallery-mode microprobe optimized for both features and demonstrate enhanced Raman spectroscopy, providing high-specificity information on molecular fingerprints that are important for numerous sensing applications. Combining the superiorities of whispering-gallery modes and nanoplasmonics, the microprobe exhibits a two-orders-of-magnitude sensitivity improvement over traditional plasmonics-only enhancement; this leads to molecular detection demonstrated with stronger target signals but less optical power required than surface-enhanced-Raman-spectroscopy substrates. Furthermore, the scanning microprobe greatly expands the effective detection area and realizes two-dimensional micron-resolution Raman imaging of molecular distribution. The versatile and ultrasensitive scanning microprobe configuration will thus benefit material characterization, chemical imaging, and quantum-enhanced sensing.

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.

Snapshot hyperspectral imaging of intracellular lasers

Intracellular lasers are emerging as powerful biosensors for multiplexed tracking and precision sensing of cells and their microenvironment. This sensing capacity is enabled by quantifying their narrow-linewidth emission spectra, which is presently challenging to do at high speeds. In this work, we demonstrate rapid snapshot hyperspectral imaging of intracellular lasers. Using integral field mapping with a microlens array and a diffraction grating, we obtain images of the spatial and spectral intensity distribution from a single camera acquisition. We demonstrate widefield hyperspectral imaging over a 3 × 3 mm2 field of view and volumetric imaging over 250 × 250 × 800 µm3 (XYZ) volumes with a lateral (XY) resolution of 5 µm, axial (Z) resolution of 10 µm, and a spectral resolution of less than 0.8 nm. We evaluate the performance and outline the challenges and strengths of snapshot methods in the context of characterizing the emission from intracellular lasers. This method offers new opportunities for a diverse range of applications, including high-throughput and long-term biosensing with intracellular lasers.

Rapid and high-precision displacement sensing based on the multiple mode dip areas in a SNAP microresonator

Whispering gallery mode (WGM) microresonators offer significant potential for precise displacement measurement owing to their compact size, ultrahigh sensitivity, and rapid response. However, conventional WGM displacement sensors are prone to noise interference, resulting in accuracy loss, while the demodulation process for displacement often exhibits prolonged duration. To address these limitations, this study proposes a rapid and high-precision displacement sensing method based on the dip areas of multiple resonant modes in a surface nanoscale axial photonics microresonator. By employing a neural network to fit the nonlinear relationship between displacement and the areas of multiple resonant dips, we achieve displacement prediction with an accuracy better than 0.03 µm over a range of 200 µm. In comparison to alternative sensing approaches, this method exhibits resilience to temperature variations, and its sensing performance remains comparable to that in a noise-free environment as long as the signal-to-noise ratio is greater than 25 dB. Furthermore, the extraction of the dip area enables significantly enhanced speed in displacement measurement, providing an effective solution for achieving rapid and highly accurate displacement sensing.

Optically pumped Milliwatt Whispering-Gallery microcavity laser

Whispering-gallery-mode microcavity lasers possess remarkable characteristics such as high Q factors and compact geometries, making them an essential element in the evolution of microlasers. However, solid-state whispering-gallery-mode lasers have previously suffered from low output power and limited optical conversion efficiency, hindering their applications. Here, we present the achievement of milliwatt laser emissions at a wavelength of 1.06 µm from a solid-state whispering-gallery-mode laser. To accomplish this, we construct a whispering-gallery-mode microcavity (with a diameter of 30 µm) using a crystalline Nd: YAG thin film obtained through carbon-implantation enhanced etching of a Nd: YAG crystal. This microcavity laser demonstrates a maximum output power of 1.12 mW and an optical conversion efficiency of 12.4%. Moreover, our unique eccentric microcavity design enables efficient coupling of free-space pump light, facilitating integration with a waveguide. This integration allowed for single-wavelength laser emission from the waveguide, achieving an output power of 0.5 mW and an optical conversion efficiency of 6.18%. Our work opens up new possibilities for advancing solid-state whispering-gallery-mode lasers, providing a viable option for compact photonic sources.

Precise photoelectrochemical tuning of semiconductor microdisk lasers

Micro- and nano-disk lasers have emerged as promising optical sources and probes for on-chip and free-space applications. However, the randomness in disk diameter introduced by standard nanofabrication makes it challenging to obtain deterministic wavelengths. To address this, we developed a photoelectrochemical (PEC) etching-based technique that enables us to precisely tune the lasing wavelength with sub-nanometer accuracy. We examined the PEC mechanism and compound semiconductor etching rate in diluted sulfuric acid solution. Using this technique, we produced microlasers on a chip and isolated particles with distinct lasing wavelengths. Our results demonstrate that this scalable technique can be used to produce groups of lasers with precise emission wavelengths for various nanophotonic and biomedical applications.

Thermo-optomechanically induced optical frequency comb in a whispering-gallery-mode resonator

We present a theoretical study that combines thermal and optomechanical effects to investigate their influences on the formation of the optical frequency comb (OFC) in whispering-gallery-mode (WGM) microcavities. The results show that the cut-off order and center frequency of OFC affected by thermal effects exhibit an overall redshift by varying the power and detuning of the pump field, which provides the possibility of tuning the offset frequency of OFC. Our study demonstrates a method to characterize the effect on the generation of OFC and the tuning of its offset frequency in a WGM resonator with opto-thermo-mechanical properties and pave the way for the future development of OFC in thermo-optomechanical environments.

Highly sensitive and label-free detection of biotin using a liquid crystal-based optofluidic biosensor

A liquid crystal (LC)-based optofluidic whispering gallery mode (WGM) resonator has been applied as a biosensor to detect biotin. Immobilized streptavidin (SA) act as protein molecules and specifically bind to biotin through strong non-covalent interaction, which can interfere with the orientation of LCs by decreasing the vertical anchoring force of the alignment layer in which the WGM spectral wavelength shift is monitored as a sensing parameter. Due to the double magnification of the LC molecular orientation transition and the resonance of the WGM, the detection limit for SA can reach 1.25 fM (4.7 × 10^-13 g/ml). The measurable concentration of biotin and the wavelength shift of the WGM spectrum have an excellent linearity in the range of 0 to 0.1 pg/ml, which can achieve ultra-low detection limit (0.4 fM), i.e., seven orders of magnitude improvement over conventional polarized optical microscope (POM) method. The proposed optofluidic biosensor is highly reproducible and can be used as an ultrasensitive real-time monitoring biosensor, which will open the door for applications to other receptor and ligand models.

From Whispering Gallery Mode Resonators to Biochemical Sensors

Optical biosensors are frontrunners for the rapid and real-time detection of analytes, particularly for low concentrations. Among them, whispering gallery mode (WGM) resonators have recently attracted a growing focus due to their robust optomechanical features and high sensitivity, measuring down to single binding events in small volumes. In this review, we provide a broad overview of WGM sensors along with critical advice and additional "tips and tricks" to make them more accessible to both biochemical and optical communities. Their structures, fabrication methods, materials, and surface functionalization chemistries are discussed. We propose this reflection under a pedagogical approach to describe and explain these biochemical sensors with a particular focus on the most recent achievements in the field. In addition to highlighting the advantages of WGM sensors, we also discuss and suggest strategies to overcome their current limitations, leaving room for further development as practical tools in various applications. We aim to provide new insights and combine different knowledge and perspectives to advance the development of the next generation of WGM biosensors. With their unique advantages and compatibility with different sensing modalities, these biosensors have the potential to become major game changers for biomedical and environmental monitoring, among many other relevant target applications.

Glyphosate Separating and Sensing for Precision Agriculture and Environmental Protection in the Era of Smart Materials

The present article critically and comprehensively reviews the most recent reports on smart sensors for determining glyphosate (GLP), an active agent of GLP-based herbicides (GBHs) traditionally used in agriculture over the past decades. Commercialized in 1974, GBHs have now reached 350 million hectares of crops in over 140 countries with an annual turnover of 11 billion USD worldwide. However, rolling exploitation of GLP and GBHs in the last decades has led to environmental pollution, animal intoxication, bacterial resistance, and sustained occupational exposure of the herbicide of farm and companies' workers. Intoxication with these herbicides dysregulates the microbiome-gut-brain axis, cholinergic neurotransmission, and endocrine system, causing paralytic ileus, hyperkalemia, oliguria, pulmonary edema, and cardiogenic shock. Precision agriculture, i.e., an (information technology)-enhanced approach to crop management, including a site-specific determination of agrochemicals, derives from the benefits of smart materials (SMs), data science, and nanosensors. Those typically feature fluorescent molecularly imprinted polymers or immunochemical aptamer artificial receptors integrated with electrochemical transducers. Fabricated as portable or wearable lab-on-chips, smartphones, and soft robotics and connected with SM-based devices that provide machine learning algorithms and online databases, they integrate, process, analyze, and interpret massive amounts of spatiotemporal data in a user-friendly and decision-making manner. Exploited for the ultrasensitive determination of toxins, including GLP, they will become practical tools in farmlands and point-of-care testing. Expectedly, smart sensors can be used for personalized diagnostics, real-time water, food, soil, and air quality monitoring, site-specific herbicide management, and crop control.