Label-Free Biosensing with High Selectivity in Complex Media using Microtoroidal Optical Resonators

High-Q WGM microcavity-based optofluidic sensor technologies for biological analysis

High-quality-factor (Q) optical microcavities have attracted extensive interest due to their unique ability to confine light for resonant circulation at the micrometer scale. Particular attention has been paid to optical whispering-gallery mode (WGM) microcavities to harness their strong light-matter interactions for biological applications. Remarkably, the combination of high-Q optical WGM microcavities with microfluidic technologies can achieve a synergistic effect in the development of high-sensitivity optofluidic sensors for many emerging biological analysis applications, such as the detection of proteins, nucleic acids, viruses, and exosomes. They can also be utilized to investigate the behavior of living cells in human organisms, which may provide new technical solutions for studies in cell biology and biophysics. In this paper, we briefly review recent progress in high-Q microcavity-based optofluidic sensor technologies and their applications in biological analysis.

Advancements and Perspectives in Optical Biosensors

Optical biosensors exhibit immense potential, offering extraordinary possibilities for biosensing due to their high sensitivity, reusability, and ultrafast sensing capabilities. This review provides a concise overview of optical biosensors, encompassing various platforms, operational mechanisms, and underlying physics, and it summarizes recent advancements in the field. Special attention is given to plasmonic biosensors and metasurface-based biosensors, emphasizing their significant performance in bioassays and, thus, their increasing attraction in biosensing research, positioning them as excellent candidates for lab-on-chip and point-of-care devices. For plasmonic biosensors, we emphasize surface plasmon resonance (SPR) and its subcategories, along with localized surface plasmon resonance (LSPR) devices and surface enhance Raman spectroscopy (SERS), highlighting their ability to perform diverse bioassays. Additionally, we discuss recently emerged metasurface-based biosensors. Toward the conclusion of this review, we address current challenges, opportunities, and prospects in optical biosensing. Considering the advancements and advantages presented by optical biosensors, it is foreseeable that they will become a robust and widespread platform for early disease diagnostics.

Roadmap on Universal Photonic Biosensors for Real-Time Detection of Emerging Pathogens

The COVID-19 pandemic has made it abundantly clear that the state-of-the-art biosensors may not be adequate for providing a tool for rapid mass testing and population screening in response to newly emerging pathogens. The main limitations of the conventional techniques are their dependency on virus-specific receptors and reagents that need to be custom-developed for each recently-emerged pathogen, the time required for this development as well as for sample preparation and detection, the need for biological amplification, which can increase false positive outcomes, and the cost and size of the necessary equipment. Thus, new platform technologies that can be readily modified as soon as new pathogens are detected, sequenced, and characterized are needed to enable rapid deployment and mass distribution of biosensors. This need can be addressed by the development of adaptive, multiplexed, and affordable sensing technologies that can avoid the conventional biological amplification step, make use of the optical and/or electrical signal amplification, and shorten both the preliminary development and the point-of-care testing time frames. We provide a comparative review of the existing and emergent photonic biosensing techniques by matching them to the above criteria and capabilities of preventing the spread of the next global pandemic.

On-demand inkjet-printed microdisk laser with air cladding by liquid flow microetching

We have novelly, to the best of our knowledge, developed the liquid flow microetching method that can treat a single microdisk in a microregion with precise position control for inkjet-printed microdisk lasers. The injection-drain wet etching setup consisted of two microneedles that successfully performed a formation of a fine undercut structure of an inkjet-printed microdisk on a pre-pedestal layer through the individual wet etching process. Then measurement of the undercut structure using scanning electron microscopy and lasing characteristics with whispering gallery modes were carried out to demonstrate performance of the etched microdisks. The measured lasing threshold decreased by half compared with that of the unetched microdisk directly printed on a fluorine-type film. A point to note is that this etching method exhibits an excellent undercut and lasing characteristics even when using a clad pre-pedestal layer having a refractive index higher than that of core microdisks. This technique, combined with inkjet printing, offers a powerful tool for individually designing a microdisk and can help develop novel devices that comprise several inkjet-printed microdisks being evanescently coupled.

Permanent tuning of optical resonant modes of chalcogenide-coated microresonators

Chalcogenide materials are promising for optical resonant mode tuning of whispering gallery mode (WGM) microresonators due to their high nonlinearity. In this study, this phenomenon was demonstrated for Ge₂Sb₂Te₅-coated toroidal microresonators using an optical postprocess, which utilizes the intrinsically photosensitive property of the Ge₂Sb₂Te₅ coating. A signal laser was used to illuminate the resonator for permanent tuning of the WGMs in a sensitive manner. 0.01 nm and 0.02 nm permanent tuning of the WGMs was recorded for 5 nm and 10 nm coated resonators, respectively. This technique enables resonance matching of coupled optical resonators, which could pave the way for optoelectronic circuitries employing multiple optical microresonators.

Functional lasing microcapillaries for surface-specific sensing

Lasing-based sensors have several advantages over fluorescent devices, specifically related to the high light intensity and narrow mode linewidth that can improve the speed and accuracy of the sensor performance. In this work, a microcapillary-based lasing sensor is demonstrated, in which the lasing wavelengths are sensitive to the surface binding of specific materials. In order to achieve this, we utilized lasing into the "star" and "triangle" modes of a conventional microcapillary and tracked the mode positions after the deposition of a polyelectrolyte tri-layer and the subsequent amide binding of carboxy-functionalized polystyrene microspheres. While the lasing mode spectrum becomes increasingly complicated by the addition of the surface layers, careful mode selection can be used to monitor the layer-by-layer surface binding in a mechanically and optically robust device. For polystyrene microspheres, the detection limits were 9.75 nM based upon the lasing mode shift, which compares favorably with fluorescence-based devices. The methods presented in this work could readily be extended to other surface binding schemes and lasing wavelengths, showing that capillary microlasers could be used for many potential applications that capitalize on stable lasing-based detection methods.

Optical bio-chemical sensors based on whispering gallery mode resonators

Whispering gallery mode (WGM) resonators have attracted extensive attention and their unique characteristics have led to some remarkable achievements. In particular, when combined with optical sensing technology, the WGM reonator-based sensor offers the advantages of small size, high sensitivity and a real-time dynamic response. At present, this type of sensor is widely applied in the bio-chemical sensing field. In this paper, we briefly review the sensing principle, the structures and the sensing applications of optical bio-chemical sensors based on the WGM resonator, with particular focuses on their sensing properties and their advantages and disadvantages. In addition, the existing problems and future development trends of WGM resonator-based optical bio-chemical sensors are discussed.

Organic-inorganic hybrid microdisk laser with dye and silica mixed doping prepared by ink-jet printing method

We developed an ink-jet printing method for fabricating inorganic microdisks at room temperature, which is much lower than the melting point of solid-state inorganic oxide, and have fabricated an organic-inorganic hybrid microdisk laser. Silica was used as the inorganic disk material, and microdisk-shaped aggregates were formed by the ink-jet printing method using a solution in which nanosilica particles were dispersed in propylene glycol monomethylether (PGME) solvent. Then, a microdisk capable of laser oscillation was also prepared by preliminarily adding the laser dye rhodamine 6G to the ink to form a mixed organic material. The structural evaluation of the printed microdisk was first conducted using an optical microscope, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The results of laser oscillation evaluation by optical excitation showed that the printed microdisk sufficiently functions as an optical resonator with a low optical loss. In these evaluations, excellent values such as a surface roughness of 5.83 nm from root mean square (R. M. S.) which is one forth smaller than the particle diameter, and a laser oscillation threshold of 4.76 µJ/mm² at a wavelength of 601.4 nm were obtained. To the best of our knowledge, this is the first time that an inorganic microdisk has been fabricated at room temperature to realize an organic-inorganic hybrid microdisk laser.

Label-Free Optical Biodetection of Pathogen Virulence Factors in Complex Media Using Microtoroids with Multifunctional Surface Functionality

Early detection of pathogens or their virulence factors in complex media has a key role in early diagnosis and treatment of many diseases. Nanomolar and selective detection of Exotoxin A, which is a virulence factor secreted from Pseudomonas aeruginosa in the sputum of Cystic Fibrosis (CF) patients, can pave the way for early diagnosis of P. aeruginosa infections. In this study, we conducted a preliminary study to demonstrate the feasibility of optical biodetection of P. aeruginosa Exotoxin A in a diluted artificial sputum mimicking the CF respiratory environment. Our surface engineering approach provides an effective biointerface enabling highly selective detection of the Exotoxin A molecules in the complex media using monoclonal anti-Exotoxin A functionalized microtoroids. The highly resilient microtoroid surface toward other constituents of the sputum provides Exotoxin A detection ability in the complex media by reproducible measurements. In this study, the limit-of-detection of Exotoxin A in the complex media is calculated as 2.45 nM.

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.

Wafer-scale arrays of high-Q silica optical microcavities

On-chip high-Q microcavities possess significant potential in terms of integration of optical microresonators into functional optoelectronic devices that could be used in various applications, including biosensors, photonic-integrated circuits, or quantum optics experiments. Yet, despite the convenience of fabricating wafer-scale integrated microresonators with moderate Q values using standard microfabrication techniques, surface-tension-induced microcavities (STIMs), which have atomic-level surface roughness enabling the observation of Q values larger than 10⁶, could only be produced using individual thermal treatment of every single microresonator within the devised area. Here, we demonstrate a facile method for large-scale fabrication of silica STIMs of various morphologies. Q values exceeding 10⁶ are readily obtained using this technique. This study represents a significant advancement toward fabrication of wafer-scale optoelectronic circuitries.

Optical Microbubble Resonators with High Refractive Index Inner Coating for Bio-Sensing Applications: An Analytical Approach

The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an analytical method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse electric (TE) and transverse magnetic (TM) polarization. The obtained results do provide physical insights for the development of OMBR-based biosensor.

Oligonucleotide-based label-free detection with optical microresonators: strategies and challenges

This review targets diversified oligonucleotide-based biodetection techniques, focusing on the use of microresonators of whispering gallery mode (WGM) type as optical biosensors mostly integrated with lab-on-a-chip systems. On-chip and microfluidics combined devices along with optical microresonators provide rapid, robust, reproducible and multiplexed biodetection abilities in considerably small volumes. We present a detailed overview of the studies conducted so far, including biodetection of various oligonucleotide biomarkers as well as deoxyribonucleic acids (DNAs), ribonucleic acids (RNAs) and proteins. We particularly advert to chemical surface modifications for specific and selective biosensing.

Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging

Whispering-gallery-mode microresonators enable materials for single-molecule label-free detection and imaging because of their high sensitivity to their micro-environment. However, fabrication and materials challenges prevent scalability and limit functionality. All-glass on-chip microresonators significantly reduce these difficulties. Construction of all-glass toroidal microresonators with high quality factor and low mode volume is reported and these are used as platforms for label-free single-particle imaging.