Optical Whispering-Gallery-Mode Microbubble Sensors

Optimized helical intermedium-period fiber grating for breathing monitoring

The investigation into the spectral properties and refractive index (RI) sensitivities at low RI region of helical intermedium-period fiber gratings (HIPFGs) with varied periods ranging from 10-48 μm is presented in detail for the first time. The structure of HIPFG is optimized for RI sensing in the RI range of 1.3-1.33 by comparing the optical properties of HIPFGs with different grating periods. The HIPFG with optimized structure is demonstrated to have a high average sensitivity of 302.5 nm/RIU in the RI ranging from 1.3 to 1.33, which is two orders more elevated than the traditional long-period fiber gratings. The improved HIPFG is also experimentally applied to breath monitoring in different states. Normal breath, slow breath, fast breath, and unhealthy breath are distinguished based on breathing rate, intensity, and time of exhalation and inhalation. The fastest response time is determined to be 10 ms. The results demonstrate that the optical fiber's sensitivity in the low RI region can be increased by shortening its period, offering a special strategy for improving detection performance of HIPFGs. By verifying its performance in breathing monitoring, it is proved that the optimized HIPFG sensor has the great potential to expand medical applications.

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.

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.

Machine learning-assisted high-accuracy and large dynamic range thermometer in high-Q microbubble resonators

Whispering gallery mode (WGM) resonators provide an important platform for fine measurement thanks to their small size, high sensitivity, and fast response time. Nevertheless, traditional methods focus on tracking single-mode changes for measurement, and a great deal of information from other resonances is ignored and wasted. Here, we demonstrate that the proposed multimode sensing contains more Fisher information than single mode tracking and has great potential to achieve better performance. Based on a microbubble resonator, a temperature detection system has been built to systematically investigate the proposed multimode sensing method. After the multimode spectral signals are collected by the automated experimental setup, a machine learning algorithm is used to predict the unknown temperature by taking full advantage of multiple resonances. The results show the average error of 3.8 × 10^-3°C within the range from 25.00°C to 40.00°C by employing a generalized regression neural network (GRNN). In addition, we have also discussed the influence of the consumed data resource on its predicted performance, such as the amount of training data and the case of different temperate ranges between the training and test data. With high accuracy and large dynamic range, this work paves the way for WGM resonator-based intelligent optical sensing.

Ultrasensitive optofluidic coupled Fabry-Perot capillary sensors

Refractive index (RI) measurements are pertinent in concentration and biomolecular detection. Accordingly, an ultrasensitive optofluidic coupled Fabry-Perot (FP) capillary sensor based on the Vernier effect for RI sensing is proposed. Square capillaries integrated with the coupled FP microcavity provide multiple microfluidic channels while reducing the complexity of the fabrication process. The incoherent light source and spectrometer used during measurement facilitate the development of a low-cost sensing system. An ultrahigh RI sensitivity of 51709.0 nm/RIU and detection limit of 2.84 × 10^-5 RIU are experimentally demonstrated, indicating acceptable RI sensing performance. The proposed sensor has significant potential for practical and low-cost applications such as RI, concentration, or biomolecular sensing.

Editorial for the Special Issue on Microfluidics and Lab-on-a-Chip Applications for Biosensing

Microfluidics refers the use of interdisciplinary science and engineering concepts that can control and manipulate small fluidic volumes [...].

Label-free detection of cardiac troponin-I with packaged thin-walled microbubble resonators

The measurement of cardiac troponin-I (cTnI) is widely used for diagnosing acute myocardial infarction (AMI) diseases because of its myocardial specificity. Packaged microbubble resonators with thin wall are utilized for label-free and specific detection of cTnI based on whispering gallery mode (WGM). This packaged structure can provide a good protection for the biosensor, improve the anti-interference ability of the sensor and reduce the system noise. The theoretical detection limit of the biosensors for cTnI in phosphate buffer saline (PBS) is 0.4 ag mL^-1 (0.02 a_M ). Furthermore, we demonstrated that the biosensors can be used to detect cTnI molecules in simulated serum and the theoretical detection limit is also 0.4 ag mL^-1 (0.02 a_M ). These results are much far below the clinical cut-off value and show a huge application potential for the detection of cardiac biomarkers of AMI.