Whispering Gallery Mode Biosensors Research Articles

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 movethrough 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.

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.

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.

Design of a high-sensitivity graphene-silicon hybrid micro-disk in a square cavity whispering gallery mode biosensor

Design of a high-sensitivity graphene-silicon hybrid micro-disk in a square cavity whispering gallery mode biosensor

Near-field electrospinning of polymer/phage whispering gallery mode microfiber resonators for label-free biosensing

Whispering gallery mode (WGM) resonators have attracted attention as optical biosensors due to their capacity for rapid label-free detection. Separately, the M13 bacteriophage has been investigated as an alternative biorecognition element. In this work, a novel polymer WGM biosensor functionalized with filamentous virus biocapture agents was fabricated using near-field electrospinning (NFES) in a single step. Polyvinyl alcohol (PVA) was mixed with streptavidin-binding M13 bacteriophage and rhodamine 6 G in aqueous solution for blend electrospinning of active cavity microfiber-based resonators. Confocal fluorescence microscopy revealed alignment of M13 bioreceptors along the fiber axis, while x-ray photoelectron spectroscopy (XPS) and streptavidin-conjugated nanoparticle binding studies indicated that bioactive M13 bacteriophage were on the fiber surface. Dye-doped, crosslinked PVA/M13 fibers supported moderate quality (Q) factor first order WGM resonances within their cross-sections that exhibited a spectral shift in response to streptavidin binding. Composite PVA/M13 microfibers demonstrated specific label-free biosensing of streptavidin with a theoretical limit of detection of 3 nM and a maximum surface coverage of 21 ± 5%. These results demonstrate the potential of NFES to fabricate WGM resonators from PVA/M13 microfibers and use them as a platform for rapid, sensitive label-free sensing.

Design and analysis of a multi-core whispering gallery mode bio-sensor for detecting cancer cells and diabetes tear cells

Whispering gallery modes (WGM) have revolutionized the field of optical sensors. This paper presents a design and simulation of a novel structure called a “multi-core whispering gallery mode (WGM)” based on multiple evanescent waves coupling for detecting cancer cells and diabetes tear cells. This work is totally simulation based, and the simulation is done by a finite element method based simulation tool. From the simulation, it is expected that the proposed sensor exhibits a sensitivity of 650 nm/RIU, 666.67 nm/RIU, and 642.285 nm/RIU, respectively, for the detection of cancerous Basal, HeLa, and MDB-MB-231 cells. In addition, it is also capable of detecting affected diabetes tear cells from healthy tear cells with a sensitivity of 650 nm/RIU. To the best of our knowledge, the resultant sensitivity of the proposed sensor is probably the highest compared to other WGM based bio-sensors till now.

Hyperboloid-Drum Microdisk Laser Biosensors for Ultrasensitive Detection of Human IgG.

Whispering gallery mode (WGM) microresonators have been used as optical sensors in fundamental research and practical applications. The majority of WGM sensors are passive resonators that require complex systems, thereby limiting their practicality. Active resonators enable the remote excitation and collection of WGM-modulated fluorescence spectra, without requiring complex systems, and can be used as alternatives to passive microresonators. This paper demonstrates an active microresonator, which is a microdisk laser in a hyperboloid-drum (HD) shape. The HD microdisk lasers are a combination of a rhodamine B-doped photoresist and a silica microdisk. These HD microdisk lasers can be utilized for the detection of label-free biomolecules. The biomolecule concentration can be as low as 1 ag mL-1 , whereas the theoretical detection limit of the biosensor for human IgG in phosphate buffer saline is 9 ag mL-1 (0.06 aM ). Additionally, the biosensors are able to detect biomolecules in an artificial serum, with a theoretical detection limit of 9 ag mL-1 (0.06 aM ). These results are approximately four orders of magnitude more sensitive than those for the typical active WGM biosensors. The proposed HD microdisk laser biosensors show enormous detection potential for biomarkers in protein secretions or body fluids.

Towards next-generation label-free biosensors: recent advances in whispering gallery mode sensors.

Whispering gallery mode biosensors have been widely exploited over the past decade to study molecular interactions by virtue of their high sensitivity and applicability in real-time kinetic analysis without the requirement to label. There have been immense research efforts made for advancing the instrumentation as well as the design of detection assays, with the common goal of progressing towards real-world sensing applications. We therefore review a set of recent developments made in this field and discuss the requirements that whispering gallery mode label-free sensors need to fulfill for making a real world impact outside of the laboratory. These requirements are directly related to the challenges that these sensors face, and the methods proposed to overcome them are discussed. Moving forward, we provide the future prospects and the potential impact of this technology.

Tracking micro-optical resonances for identifying and sensing novel procaspase-3 protein marker released from cell cultures in response to toxins

The response of cells to toxins is commonly investigated by detecting intracellular markers for cell death, such as caspase proteins. This requires the introduction of labels by the permeabilization or complete lysis of cells. Here we introduce a non-invasive tool for monitoring a caspase protein in the extracellular medium. The tool is based on highly sensitive optical micro-devices, referred to as whispering-gallery mode biosensors (WGMBs). WGMBs are functionalized with antibodies for the specific and label-free detection of procaspase-3 released from human embryonic kidney HEK293 and neuroglioma H4 cells after introducing staurosporine and rotenone toxins, respectively. Additional tests show that the extracellular accumulation of procaspase-3 is concomitant with a decrease in cell viability. The hitherto unknown release of procaspase-3 from cells in response to toxins and its accumulation in the medium is further investigated by Western blot, showing that the extracellular detection of procaspase-3 is interrelated with cytotoxicity of alpha-synuclein protein (aSyn) overexpressed in H4 cells. These studies provide evidence for procaspase-3 as a novel extracellular biomarker for cell death, with applications in cytotoxicity tests. Such WGMBs could be applied to further identify as-yet unknown extracellular biomarkers using established antibodies against intracellular antigens.

Yield enhancement in whispering gallery mode biosensors: microfluidics and optical forces

We analyze the transport of 25 nm polystyrene nanoparticles through a microfluidic geometry housing a microtoroidal whispering gallery mode (WGM) sensor, taking into account diffusion, convection and optical forces. In particular, we look at the effects of tightly confining the flow of nanoparticles near the microtoroid’s equatorial rim, and calculate the average nanoparticle detection time as well as the total yield for passage of a 1 L sample. Compared to purely diffusive transport, flow rates on the order of 2 nL min give yield enhancements of up to 15 , with nanoparticles being detected every 12 s. Even shorter detection times (1.2 s) can be achieved, but at the expense of lower yield. The sensor’s transport properties are further improved due to gradient optical forces exerted on the nanoparticles.

Periodic plasmonic enhancing epitopes on a whispering gallery mode biosensor

We propose the attachment of a periodic array of gold nanoparticles (epitopes) to the equator of a Whispering Gallery Mode Biosensor for the purpose of plasmonically enhancing nanoparticle sensing in a self-referencing manner while increasing the capture rate of analyte to antibodies attached to these plasmonic epitopes. Our approach can be applied to a variety of whispering gallery mode resonators from silicon/silica rings and disks to capillaries. The interpretation of the signals is particularly simple since the optical phase difference between the epitopes is designed to be an integer multiple of ?, allowing the wavelength shift from each binding event to add independently.

Flow-enhanced transient response in whispering gallery mode biosensors

Whispering gallery mode (WGM) optical resonator sensors are an extremely sensitive label-free technology for detecting the binding of biomolecules in solution. To better understand the fast transient response observed with these devices, we model mass transfer to spherical and toroidal WGM sensors of identical outer radius. Finite element simulations predict a 3-10 fold higher binding frequency for toroidal sensors. These results agree to within an order of magnitude with experimental data from the literature and suggest a design strategy to improve the transient response of a sensor by making the device small only in the dimension that governs boundary layer development.

Whispering gallery mode biosensor quantification of fibronectin adsorption kinetics onto alkylsilane monolayers and interpretation of resultant cellular response

A Whispering Gallery Mode (WGM) biosensor was constructed to measure the adsorption of protein onto alkysilane self-assembled monolayers (SAMs) at solution concentrations unattainable with other techniques. The high sensitivity was provided by a WGM resonance excited in a silica microsphere that was functionalized with alkylsilane SAMs and integrated in a microfluidic flow cell under laminar flow conditions. It was found that FN adsorbed at biologically relevant surface densities, however, the adsorption kinetics and concentration dependent saturation values varied significantly from work published utilizing alkanethiol SAMs. Mathematical models were applied to the experimental results to interpret the observed kinetics of FN adsorption. Embryonic hippocampal neurons and skeletal myoblasts were cultured on the modified surfaces, a live–dead assay was used to determine the viability of the FN surfaces for cell culture, and major differences were noted in the biological response to the different SAMs. The high sensitivity and simplicity of the WGM biosensor, combined with its ability to quantify the adsorption of any dilute protein in a label-free assay, establishes the importance of this technology for the study of surface accretion and its effect on cellular function, which can affect biomaterials for both in vivo and in vitro applications.

Ultrasensitive nanoparticle detection using a portable whispering gallery mode biosensor driven by a periodically poled lithium-niobate frequency doubled distributed feedback laser

We demonstrate a significant reduction in the limit of label-free detection of individual viral-sized nanoparticles in aqueous solution through the use of a frequency doubled telecom laser constructed from a distributed feedback-periodically poled lithium-niobate (DFB-PPLN) union. By driving a whispering gallery mode biosensor near a wavelength of 650 nm with this device we have detected real-time adsorption steps for particles 36 nm in radius with a signal to noise ratio of 8. The noise equivalent detection limit is ∼20 ag (17 nm radius). This new lower limit is attributed to the ultralow resonance wavelength noise [(Δλ(r))(rms)/λ(r)<10(-9)] associated with the use of the DFB-PPLN device.

Interferometric detection of mode splitting for whispering gallery mode biosensors

Sensors based on whispering gallery mode resonators can detect single nanoparticles and even single molecules. Particles attaching to the resonator induce a doublet in the transmission spectrum which provides a self-referenced detection signal. However, in practice this spectral feature is often obscured by the width of the resonance line which hides the doublet structure. This happens particularly in liquid environments that reduce the effective Q factor of the resonator. In this paper we demonstrate an interferometric setup that allows the direct detection of the hidden doublet and thus provides a pathway for developing practical sensor applications.

Whispering gallery mode bio-sensor for label-free detection of single molecules: thermo-optic vs reactive mechanism

Thermo-optic and reactive mechanisms for label-free sensing of bio-particles are compared theoretically for Whispering Gallery Mode (WGM) resonators (sphere, toroid) formed from silica and stimulated into a first order equatorial mode. Although it has been expected that a thermo-optic mechanism should "greatly enhance" wavelength shift signals [A.M. Armani et al, Science 317, 783-787 (2007)] accompanying protein binding on a silica WGM cavity having high Q (10(8)), for a combination of wavelength (680 nm), drive power (1 mW), and cavity size (43 microm radius), our calculations find no such enhancement. The possible reasons for this disparity are discussed.

In-vitro sensing of biomechanical forces in live cells by a whispering gallery mode biosensor

Direct measurement of the biomechanical stress induced by a live cell during endocytosis is reported. Fluorescent dye-doped polystyrene microspheres were used as microscopic remote optical sensors applying whispering gallery modes (WGMs) as transducer mechanism. Monitoring of the WGMs throughout the incorporation of the microsphere into the cell enabled the determination of the deformation experienced by the microsphere, characterized by both a broadening and a blue shift of the resonances, and consequently the stress induced by the cell. The results reveal an unexpectedly high stress with a magnitude of up to five times that of the passive cortical tension, which can be only explained by a so far undetermined active stress component induced by the cytoskeletal machinery during particle incorporation. The method is adaptable to the study of any other kind of phagocyte and thus provides a novel research tool of high interest for cell biology.

Whispering gallery mode biosensor operated in the stimulated emission regime

Whispering gallery modes (WGMs) are generated in fluorescent polymer microparticles in phosphate buffered saline (PBS) above the threshold for stimulated emission and compared to their characteristics below threshold. The WGM microresonators show an eightfold improvement of their signal-to-noise ratio and a threefold increase in their quality factor when operated above threshold. In an investigation on the benefits for biochemical sensing, a real-time adsorption kinetics of bovine serum albumin in PBS is monitored and compared with those kinetics acquired by means of a WGM microresonator operated below threshold as well as by surface plasmon resonance (SPR).

Whispering-gallery-mode biosensing: label-free detection down to single molecules

Optical label-free detectors, such as the venerable surface plasmon resonance (SPR) sensor, are generally favored for their ability to obtain quantitative data on intermolecular binding. However, before the recent introduction of resonant microcavities that use whispering gallery mode (WGM) recirculation, sensitivity to single binding events had not materialized. Here we describe the enhancement mechanisms responsible for the extreme sensitivity of the WGM biosensor, review its current implementations and applications, and discuss its future possibilities.

Whispering gallery mode biosensors in the low-Q limit

Whispering gallery modes (WGM) of dye-doped polystyrene beads with diameters from 20 down to 1.5 μm are studied with respect to their appearance and linewidth by excitation of the entire mode spectrum within the emission range of the dye. The lowest order (q=1) modes, which travel most closely to the inner particle surface, are assigned to their individual quantum numbers by means of a least-square-fit, resulting in a precise determination of particle radius and eccentricity. On this basis, the suitability of these microscopic cavities for applications in optical (bio-)sensing is explored. Due to the low quality (Q) factors of these small cavities, particles with diameters below 6 μm exhibit only q=1 modes, thereby causing a drastic simplification of the WGM spectrum. In such spectra, the shift in the WGM positions upon molecular adsorption can be easily monitored, as we demonstrate for the adsorption of bovine serum albumin as well as multiple layers of polyelectrolytes onto the surface of particles with 2 μm diameter. Mie simulations are used to confirm our findings. With a mass sensitivity limit of 3 fg, these microscopic sensors are highly competitive in the field of label-free detection techniques. Moreover, their small size and the simplified, dye-mediated excitation and detection scheme may pave the way to remote in-vitro biosensing in the future.