Direct writing of optical microresonators in a lab-on-a-chip for label-free biosensing

Direct laser writing-enabled 3D printing strategies for microfluidic applications

Over the past decade, additive manufacturing-or "three-dimensional (3D) printing"-has attracted increasing attention in the Lab on a Chip community as a pathway to achieve sophisticated system architectures that are difficult or infeasible to fabricate via conventional means. One particularly promising 3D manufacturing technology is "direct laser writing (DLW)", which leverages two-photon (or multi-photon) polymerization (2PP) phenomena to enable high geometric versatility, print speeds, and precision at length scales down to the 100 nm range. Although researchers have demonstrated the potential of using DLW for microfluidic applications ranging from organ on a chip and drug delivery to micro/nanoparticle processing and soft microrobotics, such scenarios present unique challenges for DLW. Specifically, microfluidic systems typically require macro-to-micro fluidic interfaces (e.g., inlet and outlet ports) to facilitate fluidic loading, control, and retrieval operations; however, DLW-based 3D printing relies on a micron-to-submicron-sized 2PP volume element (i.e., "voxel") that is poorly suited for manufacturing these larger-scale fluidic interfaces. In this Tutorial Review, we highlight and discuss the four most prominent strategies that researchers have developed to circumvent this trade-off and realize macro-to-micro interfaces for DLW-enabled microfluidic components and systems. In addition, we consider the possibility that-with the advent of next-generation commercial DLW printers equipped with new dynamic voxel tuning, print field, and laser power capabilities-the overall utility of DLW strategies for Lab on a Chip fields may soon expand dramatically.

Development of lab-on-chip biosensor for the detection of toxic heavy metals: A review

Recently, a decrease in water availability and quality has been raised due to rapid industrialization, unsustainable agricultural activities and anthropogenic activities. Heavy metals are considered significant pollutants in the water environment, cause environmental hazards and health effects to humans. For monitoring water contaminants utilized different conventional techniques. Still, they have some drawbacks, such as cost expensive, ecological issues, and processing time, requiring technicians and researchers to operate them effectively. Biosensors have become reasonable devices for screening and identifying environmental contaminants because of their different benefits contrasted with other detecting techniques. This review summarizes the toxic effect of heavy metal and their source, occurrence. A detailed discussion is provided on the heavy metal recognition materials for detecting heavy metals in wastewater. Lab on chip (LOC) is an emerging micro-electrical mechanical system (MEMS) device that intakes liquid and makes it move through the micro-channels, to accomplish fast, cost-effective and profoundly sensitive analysis with significant yield. LOC also provided a discussion on numerous laboratory functions on a single platform. This article attempts to discuss the detection of heavy metals using lab on a chip by suitable recognition materials. Further, the design and fabrication mechanism and their recognition abilities of LOC were also reviewed. The review mainly focuses on the application of LOC biosensors, pros, and cons, and suggests a roadmap towards future development to enhance the practical use in pollutant monitoring.

Two-photon polymerization simulation and fabrication of 3D microprinted suspended waveguides for on-chip optical interconnects

Quantum and neuromorphic computational platforms in integrated photonic circuits require next-generation optical functionalities. Often, increasingly complex on-chip light-routing that allow superpositions not attainable by planar technologies are paramount e.g. for artificial neural networks. Versatile 3D waveguides are achievable via two-photon polymerization (TPP)-based microprinting. Here, a 3D morphology prediction tool which considers experimental TPP parameters, is presented, enabling on-chip 3D waveguide performance simulations. The simulations allow reducing the cost-intensive systematic experimental optimization process. Fabricated 3D waveguides show optical transmission properties in agreement with simulations, demonstrating that the developed morphology prediction methodology is beneficial for the development of versatile on-chip and potentially inter-chip photonic interconnect technology.

In-fibre micro-channel: its potential for in-fibre detection

Micro-channels (μ-channels) in microstructure fibres can be regarded as natural in-fibre flow channels. Thus, the advent of microstructure fibres with μ-channels makes it possible to realize in-fibre integrated microfluidic devices, and microstructure fibres with μ-channels provide the possibility of creating new online monitoring systems and define new concepts. Herein, we developed a novel compact in-fibre detection platform, which is combined with a μ-channel in a new-type microstructure fibre, side-hole fibre, for in-fibre detection. The optical component of this proposed in-fibre detection platform is made of a simple cross-axial open-cavity Fabry-Perot interferometer. This miniaturized cross-axial open-cavity Fabry-Perot interferometer is formed by a 45°-angled side-hole fibre, which is fabricated by a simple end-face polishing process. For a 45°-angled fibre, the incident optical axis can be steered based on total internal reflection at the oblique fibre-air interface, and the reflected light will enter the side-hole μ-channel, and the front and rear inner μ-channel walls form the cavity; the natural in-fibre μ-channel functions as a (liquid/gas) flow channel. Experimentally, this proposed in-fibre detection platform was fabricated, and its spectral characteristics were investigated. Its relative humidity characteristics and potential application in breath sensing were calibrated by measuring the evolution of the reflection spectrum. As a whole, the proposed detection platform demonstrates the advantages of simple structure, easy fabrication without additional sensitive materials, and potential application in breath sensing or lab-in-fibre technology.

Superhydrophilic-Superhydrophobic Multifunctional Janus Foam Fabrication Using a Spatially Shaped Femtosecond Laser for Fog Collection and Detection

Fog collection is an effective method for addressing water shortages in arid areas. By constructing a Janus structure with asymmetric wettability on its two sides, flexible and efficient fog capture can be achieved. However, in situ detection and fog collection on a Janus surface are still challenging tasks. Herein, a novel method for producing a superhydrophilic-superhydrophobic Janus fog collector is proposed; the method utilizes a combined process in which a spatially shaped femtosecond laser treatment (superhydrophilic) is applied to one side of a copper foam and a chemical replacement reaction (superhydrophobic) is applied to the other side of the copper foam. Two configurations of the Janus structure were designed to study different water transport behaviors. Furthermore, the Au micro-nanoparticle prepared adhered to the Janus structure, indicating the effectiveness of surface-enhanced Raman spectroscopy detection. The Janus foam shows excellent sensitivity and stability on testing the fog mixed with rhodamine 6G. This surface allows for the simultaneous collection and detection of fog, which can provide insights into the preparation of Janus multifunction structures and how such structures can play a key role in the subsequent purification and usage of water resources.

In-Fiber Polymer Microdisk Resonator and Its Sensing Applications of Temperature and Humidity

We proposed and realized an all-in-fiber polymer microdisk whispering-gallery mode (WGM) resonator, which is composed of a nanoscale polymer waveguide in conjunction with a polymer microdisk. The resonator is manufactured by femtosecond laser-induced two-photon polymerization inside a single-mode optical fiber, and its transmission spectrum has been investigated theoretically and experimentally. The WGM resonance was excited successfully, exhibiting a high Q factor of 2.3 × 10³ at a resonant wavelength of 1416.6 nm. The temperature and humidity responses of the resonator were tested as examples of possible application. Temperature sensitivity of -96 pm/°C when the temperature increased from 25 to 60 °C and humidity sensitivity of 54 pm/%RH when the relative humidity increased from 30 to 90% were obtained. The proposed in-fiber microdisk resonator is highly suitable for detection of microorganisms, bacteria, and single molecules.

Fiber-tip polymer clamped-beam probe for high-sensitivity nanoforce measurements

Micromanipulation and biological, material science, and medical applications often require to control or measure the forces asserted on small objects. Here, we demonstrate for the first time the microprinting of a novel fiber-tip-polymer clamped-beam probe micro-force sensor for the examination of biological samples. The proposed sensor consists of two bases, a clamped beam, and a force-sensing probe, which were developed using a femtosecond-laser-induced two-photon polymerization (TPP) technique. Based on the finite element method (FEM), the static performance of the structure was simulated to provide the basis for the structural design. A miniature all-fiber micro-force sensor of this type exhibited an ultrahigh force sensitivity of 1.51 nm μN-1, a detection limit of 54.9 nN, and an unambiguous sensor measurement range of ~2.9 mN. The Young's modulus of polydimethylsiloxane, a butterfly feeler, and human hair were successfully measured with the proposed sensor. To the best of our knowledge, this fiber sensor has the smallest force-detection limit in direct contact mode reported to date, comparable to that of an atomic force microscope (AFM). This approach opens new avenues towards the realization of small-footprint AFMs that could be easily adapted for use in outside specialized laboratories. As such, we believe that this device will be beneficial for high-precision biomedical and material science examination, and the proposed fabrication method provides a new route for the next generation of research on complex fiber-integrated polymer devices.

Two-Photon Polymerization: Functionalized Microstructures, Micro-Resonators, and Bio-Scaffolds

The direct laser writing technique based on two-photon polymerization (TPP) has evolved considerably over the past two decades. Its remarkable characteristics, such as 3D capability, sub-diffraction resolution, material flexibility, and gentle processing conditions, have made it suitable for several applications in photonics and biosciences. In this review, we present an overview of the progress of TPP towards the fabrication of functionalized microstructures, whispering gallery mode (WGM) microresonators, and microenvironments for culturing microorganisms. We also describe the key physical-chemical fundamentals underlying the technique, the typical experimental setups, and the different materials employed for TPP.

Femtosecond laser micromachining of an optofluidics-based monolithic whispering-gallery mode resonator coupled to a suspended waveguide

A monolithic lab-on-a-chip fabricated by femtosecond laser micromachining capable of label-free biosensing is reported. The device is entirely made of fused silica, and consists of a microdisk resonator integrated inside a microfluidic channel. Whispering gallery modes are excited by the evanescent field of a circular suspended waveguide, also incorporated within the channel. Thermal annealing is performed to decrease the surface roughness of the microstructures to a nanometric scale, thereby reducing intrinsic losses and maximizing the Q-factor. Further, thermally-induced morphing is used to position, with submicrometric precision, the suspended waveguide tangent to the microresonator to enhance the spatial overlap between the evanescent field of both optical modes. With this fabrication method and geometry, the alignment between the waveguide and the resonator is robust and guaranteed at all instances. A maximum sensitivity of 121.5 nm/RIU was obtained at a refractive index of 1.363, whereas near the refractive index range of water-based solutions the sensitivity is 40 nm/RIU. A high Q-factor of 105 is kept throughout the entire measurement range.

Fabrication of Functional Microdevices in SU-8 by Multi-Photon Lithography

This review surveys advances in the fabrication of functional microdevices by multi-photon lithography (MPL) using the SU-8 material system. Microdevices created by MPL in SU-8 have been key to progress in the fields of micro-fluidics, micro-electromechanical systems (MEMS), micro-robotics, and photonics. The review discusses components, properties, and processing of SU-8 within the context of MPL. Emphasis is focused on advances within the last five years, but the discussion also includes relevant developments outside this period in MPL and the processing of SU-8. Novel methods for improving resolution of MPL using SU-8 and discussed, along with methods for functionalizing structures after fabrication.

In-Fiber BaTiO3 Microsphere Resonator for High-Sensitivity Temperature Measurement

A fiber optic whispering gallery mode (WGM) resonator was proposed and realized by integrating an inline polymer waveguide with a microsphere mounted on it. The polymer waveguide with a diameter of 1 μm was printed with femtosecond laser-assisted multiphoton polymerization in a section of a grooved hollow-core fiber, which was sandwiched between two single-mode fibers. Two WGW resonators assembled with microspheres of different sizes were prepared. The transmission spectra of those stimulated WGMs were investigated both in simulation and experimentally. The temperature response of the resonators was particularly studied, and a linear sensitivity of −593 pm/°C was achieved from 20 °C to 100 °C.

Fiber-Tip Polymer Microcantilever for Fast and Highly Sensitive Hydrogen Measurement

Hydrogen as an antioxidant gas has been widely used in the medical and biological fields for preventing cancer or treating inflammation. However, controlling the hydrogen concentration is crucial for practical use due to its explosive property when its volume concentration in air reaches the explosive limit (4%). In this work, a polymer-based microcantilever (μ-cantilever) hydrogen sensor located at the end of a fiber tip is proposed to detect the hydrogen concentration in medical and biological applications. The proposed sensor was developed using femtosecond laser-induced two-photon polymerization (TPP) to print the polymer μ-cantilever and magnetron sputtering to coat a palladium (Pd) film on the upper surface of the μ-cantilever. Such a device exhibits a high sensitivity, roughly -2 nm %^-1 when the hydrogen concentration rises from 0% to 4.5% (v/v) and a short response time, around 13.5 s at 4% (v/v), making it suitable for medical and environmental applications. In addition to providing an ultracompact optical solution for fast and highly sensitive hydrogen measurement, the polymer μ-cantilever fiber sensor can be used for diverse medical and biological sensing applications by replacing Pd with other functional materials.

Plasmonic Metasensors Based on 2D Hybrid Atomically Thin Perovskite Nanomaterials

In this work, we have designed highly sensitive plasmonic metasensors based on atomically thin perovskite nanomaterials with a detection limit up to 10⁻¹⁰ refractive index units (RIU) for the target sample solutions. More importantly, we have improved phase singularity detection with the Goos–Hänchen (GH) effect. The GH shift is known to be closely related to optical phase signal changes; it is much more sensitive and sharp than the phase signal in the plasmonic condition, while the experimental measurement setup is much more compact than that of the commonly used interferometer scheme to exact the phase signals. Here, we have demonstrated that plasmonic sensitivity can reach a record-high value of 1.2862 × 10⁹ µm/RIU with the optimum configurations for the plasmonic metasensors. The phase singularity-induced GH shift is more than three orders of magnitude larger than those achievable in other metamaterial schemes, including Ag/TiO₂ hyperbolic multilayer metamaterials (HMMs), metal–insulator–metal (MIM) multilayer waveguides with plasmon-induced transparency (PIT), and metasurface devices with a large phase gradient. GH sensitivity has been improved by more than 10⁶ times with the atomically thin perovskite metasurfaces (1.2862 × 10⁹ µm/RIU) than those without (918.9167 µm/RIU). The atomically thin perovskite nanomaterials with high absorption rates enable precise tuning of the depth of the plasmonic resonance dip. As such, one can optimize the structure to reach near zero-reflection at the resonance angle and the associated sharp phase singularity, which leads to a strongly enhanced GH lateral shift at the sensor interface. By integrating the 2D perovskite nanolayer into a metasurface structure, a strong localized electric field enhancement can be realized and GH sensitivity was further improved to 1.5458 × 10⁹ µm/RIU. We believe that this enhanced electric field together with the significantly improved GH shift would enable single molecular or even submolecular detection for hard-to-identify chemical and biological markers, including single nucleotide mismatch in the DNA sequence, toxic heavy metal ions, and tumor necrosis factor-α (TNFα).

Reusable, Non-Invasive, and Ultrafast Radio Frequency Biosensor Based on Optimized Integrated Passive Device Fabrication Process for Quantitative Detection of Glucose Levels

The increase in the number of people suffering diabetes has been the driving force behind the development of glucose sensors to overcome the current testing shortcomings. In this work, a reusable, non-invasive and ultrafast radio frequency biosensor based on optimized integrated passive device fabrication process for quantitative detection of glucose level was developed. With the aid of the novel biosensor design with hammer-shaped capacitors for carrying out detection, both the resonance frequency and magnitude of reflection coefficient can be applied to map the different glucose levels. Meanwhile, the corresponding fabrication process was developed, providing an approach for achieving quantitative detection and a structure without metal-insulator-metal type capacitor that realizes low cost and high reliability. To enhance the sensitivity of biosensor, a 3-min dry etching treatment based on chlorine/argon-based plasma was implemented for realizing hydrophilicity of capacitor surface to ensure that the biosensor can be touched rapidly with glucose. Based on above implementation, a non-invasive biosensor having an ultrafast response time of superior to 0.85 s, ultralow LOD of 8.01 mg/dL and excellent reusability verified through five sets of measurements are realized. The proposed approaches are not limited the development of a stable and accurate platform for the detection of glucose levels but also presents a scheme toward the detection of glucose levels in human serum.

A Miniature Fibre-Optic Raman Probe Fabricated by Ultrafast Laser-Assisted Etching

Optical biopsy describes a range of medical procedures in which light is used to investigate disease in the body, often in hard-to-reach regions via optical fibres. Optical biopsies can reveal a multitude of diagnostic information to aid therapeutic diagnosis and treatment with higher specificity and shorter delay than traditional surgical techniques. One specific type of optical biopsy relies on Raman spectroscopy to differentiate tissue types at the molecular level and has been used successfully to stage cancer. However, complex micro-optical systems are usually needed at the distal end to optimise the signal-to-noise properties of the Raman signal collected. Manufacturing these devices, particularly in a way suitable for large scale adoption, remains a critical challenge. In this paper, we describe a novel fibre-fed micro-optic system designed for efficient signal delivery and collection during a Raman spectroscopy-based optical biopsy. Crucially, we fabricate the device using a direct-laser-writing technique known as ultrafast laser-assisted etching which is scalable and allows components to be aligned passively. The Raman probe has a sub-millimetre diameter and offers confocal signal collection with 71.3% ± 1.5% collection efficiency over a 0.8 numerical aperture. Proof of concept spectral measurements were performed on mouse intestinal tissue and compared with results obtained using a commercial Raman microscope.

Tunable microfluidic device fabricated by femtosecond structured light for particle and cell manipulation

Smart devices made of stimuli-responsive (SR) hydrogel can realize accurate shape control with high repeatability attributed to their fast swelling and shrinking upon the change of external stimuli. Integrating these devices into microfluidic chips and utilizing their controllable deformation capability are highly promising approaches to enrich the functions of microfluidic devices and reduce their external apparatuses. Herein we propose and demonstrate a tunable microfluidic device (TMFD) by integrating a pH-sensitive hydrogel microring array into a microchannel. Instantaneous and reversible deformation of the microrings can be finished in less than 200 ms. The array gaps of the microrings are reversibly switched to realize the capture or release of microobjects. In addition, a femtosecond laser holographic processing method is firstly used to pattern and integrate the pH-sensitive hydrogel microrings into a microchannel, and the pH-responsive properties of the hydrogel affected by laser processing dosages are theoretically and experimentally investigated. With this method, the height, diameter (6-16 μm), swelling ratio (35-65%), and diameter change (2-5 μm) can be precisely controlled. As a proof of concept, the filtering of polystyrene particles with multiple sizes and complete trapping of PS particles and cells are demonstrated by these TMFDs. The developed TMFD can be easily integrated by the femtosecond laser holographic processing method, and operates robustly without the need for external precision apparatuses, which hold great promise in the applications of microobject manipulation and biomedical analysis.

Optofluidic in-fiber integrated surface-enhanced Raman spectroscopy detection based on a hollow optical fiber with a suspended core

In this Letter, we propose, to the best of our knowledge, the first in-fiber optofluidic Raman surface-enhanced Raman spectroscopy (SERS) sensor based on a microstructured hollow fiber (MHF) with a suspended core. Taking advantage of the unique internal structure, we immobilize silver nanoparticles with an SERS effect in the MHF by chemical bonding. The Raman signal of the microfluidic sample is excited by the excitation light in the suspended core through an evanescent field. Then the online SERS signal can be coupled back into the core and detected. To demonstrate the feasibility of the device, rhodamine 6G is chosen as the analyte, and high-quality SERS spectra are detected with the limit of detection of 1×10^-14  M. Furthermore, an online optofluidic test is conducted on ceftriaxone (C₁₈H₁₈N₈O₇S₃) to examine its capabilities in antibiotic sensing. The results show that the LOD of the samples is 10^-6  M. Significantly, this Letter provides an integrated optofluidic in-fiber SERS sensor with a microchannel that can be integrated with chip devices without spatial optical coupling, which has a broad application in medicine and food safety, as well as various aspects of biological in-fiber sensing.