Liquid Core Waveguide Research Articles

Roboticized AI-assisted microfluidic photocatalytic synthesis and screening up to 10,000 reactions per day

The current throughput of conventional organic chemical synthesis is usually a few experiments for each operator per day. We develop a robotic system for ultra-high-throughput chemical synthesis, online characterization, and large-scale condition screening of photocatalytic reactions, based on the liquid-core waveguide, microfluidic liquid-handling, and artificial intelligence techniques. The system is capable of performing automated reactant mixture preparation, changing, introduction, ultra-fast photocatalytic reactions in seconds, online spectroscopic detection of the reaction product, and screening of different reaction conditions. We apply the system in large-scale screening of 12,000 reaction conditions of a photocatalytic [2 + 2] cycloaddition reaction including multiple continuous and discrete variables, reaching an ultra-high throughput up to 10,000 reaction conditions per day. Based on the data, AI-assisted cross-substrate/photocatalyst prediction is conducted.

Boundary Feedback Fiber Random Microcavity Laser Based on Disordered Cladding Structures

The cavity form of complex microcavity lasers predominantly relies on disordered structures, whether found in nature or artificially prepared. These structures, characterized by disorder, facilitate random lasing through the feedback effect of the cavity boundary and the internal scattering medium via various mechanisms. In this paper, we report on a random fiber laser employing a disordered scattering cladding medium affixed to the inner cladding of a hollow-core fiber. The internal flowing liquid gain establishes a stable liquid-core waveguide environment, enabling long-term directional coupling output for random laser emission. Through theoretical analysis and experimental validation, we demonstrate that controlling the disorder at the cavity boundary allows liquid-core fiber random microcavities to exhibit random lasing output with different mechanisms. This provides a broad platform for in-depth research into the generation and control of complex microcavity lasers, as well as the detection of scattered matter within micro- and nanostructures.

Waveguide‐based Raman enhancement strategies

AbstractWaveguide‐enhanced Raman scattering (WERS) is a powerful branch of enhanced Raman technologies that has gained significant progress in recent years because of its advantages, such as reproducibility and robustness. As a complementary tool to surface‐enhanced Raman spectroscopy (SERS), WERS provides a powerful solution for reproducible quantification of analytes. According to different Raman enhancement mechanisms, five major WERS implementation strategies, namely, (1) single‐mode dielectric waveguide, (2) liquid core waveguide, (3) metal cladding waveguide, (4) resonance mirror waveguide, and (5) double metal cladding waveguide, are classified and described in detail in this review. The flexibility of WERS structures makes them easy to be integrated with 2D devices to obtain a complete on‐chip detection scheme, allowing the WERS chip to combine excitation, detection, and data analysis in integrated chips, providing a powerful prospect for real‐time and on‐site analysis of target samples. This article highlights the principles, implementations, and application scenarios of WERS techniques and evaluates their advantages and limitations, respectively. Finally, the strengths and weaknesses of WERS techniques are summarized, and promising future applications are proposed. This review provides a panoramic view for researchers interested in waveguide‐enhanced Raman technology.

Online Coupling of Size Exclusion Chromatography to Capillary Enhanced Raman Spectroscopy for the Analysis of Proteins and Biopharmaceutical Drug Products.

The online coupling of size exclusion chromatography (SEC) to capillary enhanced Raman spectroscopy (CERS) based on a liquid core waveguide (LCW) flow cell was applied for the first time to assess the higher-order structure of different proteins. This setup allows recording of Raman spectra of the monomeric protein within complex mixtures, since SEC enables the separation of the monomeric protein from matrix components such as excipients of a biopharmaceutical product and higher molecular weight species (e.g., aggregates). The acquired Raman spectra were used for structural elucidation of well characterized proteins such as bovine serum albumin, hen egg white lysozyme, and β-lactoglobulin and of the monoclonal antibody rituximab in a medicinal product. Additionally, the CERS detection of the disaccharide sucrose, which is used as a stabilizing excipient, was quantified to achieve a limit of detection (LOD) of 120 μg and a limit of quantification (LOQ) of 363 μg injected on the column.

Hybridization of solitons in realistic non-instantaneous fibers

Non-local nonlinearities are an integral part of numerous physical systems. Yet, their dissipative nature makes it hard to identify and verify stable eigenstates rigorously. This work sheds new light onto the existence of temporally non-local solitons – a class of dissipative self-maintaining states in time theoretically predicted to emerge in media with a time-delayed (i.e., non-instantaneous) nonlinear response. Revisiting the non-instantaneous nonlinear Schrödinger equation introduced by Conti et al., I introduce a semi-analytical approach to reveal such non-instantaneous solitons in liquid-type media which exhibit realistic non-exponential response functions featuring both rise and decay times. I investigate the susceptibility of such states to typical perturbations, such as third-order dispersion, instantaneous Kerr nonlinearity, and causality of the material response. Empirical numeric experiments illustrate how non-instantaneous solitons may compete against and even fuse with instantaneous (Kerr-type) solitons to an inseparable hybrid solitary waves. While finding a numerical solution to define the nature of this mixed state remains a challenge, I introduce a phase balance condition and other benchmarks which will allow to identify the occurrence of such states in realistic liquid-core fiber systems in future work. The findings of this study give reason to hypothetize a new type of dissipative solitary state and provide new means to understanding information transport and system dynamics of highly non-instantaneous nonlinear systems, particularly liquid-core optical fibers and waveguides.

Gas-permeable liquid-core waveguide coupled to LC-MS for studying the influence of oxygen on photodegradation processes

Light-induced degradation (LID) strongly depends on experimental conditions, among which the presence of oxygen is a major parameter. Elucidating LID processes is of high concern to many areas of interest, e.g., for the conservation of cultural heritage, safety and shelf life of food, and UV-disinfection methods for water purification. Recently, we presented a new, fully automated tool to study molecular photodegradation in solution. The tool employs a gas-permeable liquid-core waveguide (LCW) as a light-exposure cell with in-situ absorption spectroscopy for real-time monitoring, coupled on-line to liquid chromatography (LC) with diode-array (DAD) and mass spectrometry (MS) for characterization of the photodegradation products. The current work reports on the assessment of the potential of the LCW in a tube-in-tube geometry for studying the role of oxygen in photodegradation processes, using Riboflavin and Eosin Y as model compounds. The LID results obtained for Riboflavin and Eosin Y using the LCW set-up were in line with reported data obtained with conventional approaches. On-line LC-MS analysis allowed semi-quantitative monitoring of LID differences under oxic and anoxic circumstances. An increase in degradation by 9% and 30% for Riboflavin and Eosin Y, respectively, was observed after 10 min under anoxic conditions. Moreover, for the first time, the fully debrominated species of Eosin Y (i.e. fluorescein), was identified as a photodegradation product in solution. The presented set-up can be highly useful for the investigation of photodegradation mechanisms and kinetics in solution, including the role of oxygen while increasing analytical efficiency and reducing time spent in the lab.

Optical fiber inclinometer with dynamically controllable excitation length of quantum dots liquid-core waveguide based on a photo-controlled bubble.

An ultracompact fiber inclinometer based on a bubble controlled by Marangoni force is proposed in this Letter. By coupling a 980-nm laser, the bubble can suspend in a quantum dots (QDs) liquid-core waveguide (LCW) due to the Marangoni effect. Under the excitation of a 405-nm laser, QDs LCW exhibit green emissions centered at 523 nm. When the tilt angle changes, the position of the bubble changes as well, which causes the variation of the 523-nm fluorescence intensity. The experimental results show that the sensitivity based on the peak intensity ratio (PIR) reaches 0.22/° with a linearity of 0.979 from 0° to 35°. Furthermore, the sensor has excellent stability and repeatability.

Use of liquid-core waveguides as photochemical reactors and/or for chemical analysis – An overview

The study of photochemical reactions is of great importance in many fields including the pharmaceutical, food, and paint industry. Most of these photochemical processes are being studied to better understand how to apply them for a specific purpose or how unwanted effects can be prevented. Advances are still being made in photoreactor design, where in-situ detection of the involved reagents and products is an important development. Liquid-core waveguides (LCWs) allow simultaneous illumination and optical assessment of liquid samples and, therefore, constitute one way of combining photoreactor design with on-line or in-situ analytical detection methods. LCWs possess several interesting characteristics, such as low light loss, increased optical path length, and possibilities for coupling with spectroscopic techniques. The current review discusses the state-of-the-art of LCWs applied as photoreactors, for analytical detection, and their combinations. We discuss the differences between several total internal reflection (TIR)-based LCWs, including polymer and polymer-coated capillaries, and silica aerogels, and interference-based waveguides, including Bragg fibers, holey fibers, Kagomé fibers and anti-resonance reflecting optical waveguides (ARROWs). Assessed characteristics include the (freedom of) design, the degree of light attenuation, the range of transmittable wavelengths, gas permeability, compatibility with analytical techniques, current challenges, and applications.

Manufacture of Three-Dimensional Optofluidic Spot-Size Converters in Fused Silica Using Hybrid Laser Microfabrication

We propose a hybrid laser microfabrication approach for the manufacture of three-dimensional (3D) optofluidic spot-size converters in fused silica glass by a combination of femtosecond (fs) laser microfabrication and carbon dioxide laser irradiation. Spatially shaped fs laser-assisted chemical etching was first performed to form 3D hollow microchannels in glass, which were composed of embedded straight channels, tapered channels, and vertical channels connected to the glass surface. Then, carbon dioxide laser-induced thermal reflow was carried out for the internal polishing of the whole microchannels and sealing parts of the vertical channels. Finally, 3D optofluidic spot-size converters (SSC) were formed by filling a liquid-core waveguide solution into laser-polished microchannels. With a fabricated SSC structure, the mode spot size of the optofluidic waveguide was expanded from ~8 μm to ~23 μm with a conversion efficiency of ~84.1%. Further measurement of the waveguide-to-waveguide coupling devices in the glass showed that the total insertion loss of two symmetric SSC structures through two ~50 μm-diameter coupling ports was ~6.73 dB at 1310 nm, which was only about half that of non-SSC structures with diameters of ~9 μm at the same coupling distance. The proposed approach holds great potential for developing novel 3D fluid-based photonic devices for mode conversion, optical manipulation, and lab-on-a-chip sensing.

Combining Photodegradation in a Liquid-Core-Waveguide Cell with Multiple-Heart-Cut Two-Dimensional Liquid Chromatography

Photodegradation greatly affects everyday life. It poses challenges when food deteriorates or when objects of cultural heritage fade, but it can also create opportunities applied in advanced oxidation processes in water purification. Studying photodegradation, however, can be difficult because of the time needed for degradation, the inaccessibility of pure compounds, and the need to handle samples manually. A novel light-exposure cell, based on liquid-core-waveguide (LCW) technology, was embedded in a multiple-heart-cut two-dimensional liquid chromatography system by coupling the LCW cell to the multiple-heart-cut valve. The sample was flushed from the heart-cut loops into the cell by an isocratic pump. Samples were then irradiated using different time intervals and subsequently transferred by the same isocratic pump to a second-dimension sample loop. The mixture containing the transformation products was then subjected to the second-dimension separation. In the current setup, about 30–40% of the selected fraction was transferred. Multiple degradation products could be monitored. Degradation was found to be faster when a smaller sample amount was introduced (0.3 μg as compared to 1.5 μg). The system was tested with three applications, that is, fuchsin, a 19th-century synthetic organic colorant, annatto, a lipophilic food dye, and vitamin B complex.

Planar Optofluidic Integration of Ring Resonator and Microfluidic Channels.

We report an optofluidic hybrid silicon-polymer planar ring resonator with integrated microfluidic channels for efficient liquid delivery. The device features a planar architecture of intersecting liquid-core waveguides and microfluidic channels. A low-loss integration of microfluidic channels is accomplished by exploiting the interference pattern created by the self-imaging effect in the multimode interference-based coupler waveguides. Numerical simulations have been performed in order to minimize the propagation losses along the ring loop caused by the integration of microfluidic channels. The device has been fabricated and optically characterized by measuring the quality factor, obtaining a value of 4 × 103. This result is comparable with the quality factor of an optofluidic ring with the same optical layout but without integrated microfluidic channels, thus, confirming the suitability of the proposed approach for microfluidics integration in planar optofluidic design.

Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation

In many areas, studying photostability or the mechanism of photodegradation is of high importance. Conventional methods to do so can be rather time-consuming, laborious, and prone to experimental errors. In this paper we evaluate an integrated and fully automated system for the study of light-induced degradation, comprising a liquid handler, an irradiation source and exposure cell with dedicated optics and spectrograph, and a liquid chromatography (LC) system. A liquid core waveguide (LCW) was used as an exposure cell, allowing efficient illumination of the sample over a 12 cm path length. This cell was coupled to a spectrograph, allowing in situ absorbance monitoring of the exposed sample during irradiation. The LCW is gas-permeable, permitting diffusion of air into the cell during light exposure. This unit was coupled online to LC with diode array detection for immediate and automated analysis of the composition of the light-exposed samples. The analytical performance of the new system was established by assessing linearity, limit of detection, and repeatability of the in-cell detection, sample recovery and carryover, and overall repeatability of light-induced degradation monitoring, using riboflavin as the test compound. The applicability of the system was demonstrated by recording a photodegradation time profile of riboflavin.

A Microfluid Fiber Device for Trace Detection of Aggregation Induced Emission Molecules

An ultra-sensitive microfluid device for detecting aggregation of induced emission molecules is proposed and demonstrated. The minimum detectable concentration is greatly reduced by use of a micromachined fiber tip and a liquid core waveguide. The fiber tip is a tapered-microlens structure with optimized shape parameters. The liquid waveguide is formed with low refractive index Teflon capillary. The dependence of photoluminescence intensity on the concentration change is tested with the device. The limit of detection is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.316 \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> , the volume of the sample solution used is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.649 \boldsymbol {\mu }\text{L}$ </tex-math></inline-formula> and the minimum detectable concentration is down to 100 nM. It is a valuable result obtained with fiber spectrometer (a CCD-type machine), better than that obtained with traditional fluorospectro photometer (a photomultiplier tubes-type one). The reversibility and stability are also testified through experimental method. Together with the compact size, small sample volume and high sensitivity, the durable microfluidic sensing device has great potential for biosensing applications.

Embedding a Sensitive Liquid-Core Waveguide UV Detector into an HPLC-UV System for Simultaneous Quantification of Differently Dosed Active Ingredients during Drug Release.

Individual dosing of pharmaceutics and personalized medicine have become important with regard to therapeutic safety. Dose adjustments, biorelevant drug release and combination of multiple active substances in one dosage form for the reduction in polymedication are essential aspects that increase the safety and acceptance of the patient’s pharmacotherapy. Therefore, not only innovative drug products but also new analytical methods are needed during the drug development phase and for quality control that can simultaneously determine different active ingredients and cover wide concentration ranges. We investigated a liquid-core waveguide UV absorbance flow cell detector coupled to an existing HPLC-UV system. A Teflon AF 2400 capillary tubing of 20 cm length was connected in series to the HPLC flow line and enabled a lower limit of quantification of 1 ng/mL pramipexole (increase in sensitivity by 20 compared to common 0.9 cm flow cells). This allowed the low-concentration of pramipexole and the higher concentrations of levodopa and benserazide occurring during drug release to be determined in a single chromatographic run within 22.5 min.

Comparing different light-degradation approaches for the degradation of crystal violet and eosin Y

Organic colourants have important applications in many fields. Their photostability is an important characteristic. Several methods to study photodegradation were compared in this work. Eosin Y (C.I. Generic name: Acid Red 87, EY) and crystal violet (C.I. Generic Name: Basic Violet 3, CV) were used as test compounds, both in solution and dyed on silk. Commonly applied methods were included, viz. Xenotest, Microfading-Tester, and light-box (Spectrolinker) experiments. A novel method was based on a liquid-core-waveguide (LCW) cell. After photodegradation on textile, extraction was performed using dimethyl sulfoxide (DMSO). The degraded solutions and extracts were analysed with liquid chromatography combined with diode-array detection and mass spectrometry. The degradation products were compared between techniques. Degradation in the LCW cell progressed much faster than in standard tests (Xenotest and Spectrolinker) and could be performed online, without a need for extraction or sample transfer. The degradation of CV in the LCW was comparable to its degradation in standard tests. For EY, there was a clear difference in degradation mechanisms between in-solution and on-textile samples. This could be due to the matrix or to incomplete extraction. Because the light sources used in the different experiments differed in energy and spectral emission, the results could not be quantitatively compared. However, the degradation products formed were shown to be independent of the light source. Therefore, the LCW is an attractive method for rapid and efficient studies into the chemistry of photodegradation.

A Cost-effective Liquid Core Waveguide Based on a Concentrated Acid Medium for Colorimetric Determination of Sulfide.

A cost-effective long-path absorption liquid core waveguide (LCW) device was fabricated from a typical Teflon tube, and using a high refractive index (RI) medium of acid. It was incorporated in a flow injection-gas diffusion system for the sensitive determination of sulfide via methylene blue (MB) chemistry. The gas diffusion unit could prevent some interferences in samples. The limit of detection of 0.13 μmol L-1 was achieved without using a preconcentration procedure. The proposed device was successfully applied for sulfide determination in canal water samples with 88.7 - 103% recovery.

Optical trapping SiO2 nanoparticles based on liquid-core metal-cladding waveguide

Because of the demand for low-cost optical trapping small particles without directly touching in subwavelength volume, a liquid-core metal-cladding waveguide (LCMW) structure was put forward, and it retained all the merits of MCW, such as free space coupling, a large detection area, and polarization independence. LCMW was used for optical trapping of SiO2 colloid particles. The reflectivity of the guiding layer was monitored to investigate the factors influencing the self-assembly speed of the SiO2 particles (time for the reflectivity to reach a stable state). The results demonstrate that a smaller ultimate reflectivity of light, longer light-off time, or a higher sample refractive index increases the required optical trapping time for the SiO2 colloid particles to reach a stable state.

High-sensitivity microvolume UV absorption spectrometry for routine analysis of small-volume biological samples.

Substantial improvement of microvolume UV absorption spectrometry in sensitivity, robustness and ease of operation was achieved for routine biological applications. A unique microtubing-based absorption cell (208μm internal diameter) featuring enhanced light transmission with a liquid core waveguide technique provided dramatically enhanced absorption sensitivity, proportional to the extended path length (50mm, from the typical 1mm), whilerobust measurement performance was attained by implementation of preventive measures against bubble trapping along the light path. For pBR322 plasmid DNA, absorbance at 260nm wasreliably measurable down to 0.1ng/μl with repeatability typically 2-3% RSD. The detection limit was 0.03ng/μldsDNA, far lower than the current state-of-the-art∼1ng/μl. Sample consumption for each measurement was 2.4μl. Automated operation implemented for the first time in microvolume spectrophotometry facilitated the ease in handling with small-volume biological samples.

Greatly Enhanced Single Particle Fluorescence Detection Using High Refractive Index Liquid-Core Waveguides

High sensitivity and easy integration with microfabrication techniques has made silicon photonics one of the leading technologies used to build biosensors for diagnostic applications. Here we introduce a new silicon dioxide based optofluidic platform having a planar solid-core (SC) waveguide orthogonally intersecting a liquid-core (LC) waveguide with high refractive index ZnI2 salt solution as core. This enables both more uniform collection of particle fluorescence by the core mode and its propagation to an off-chip detector. This approach results in ultra-high sensitivity performance, demonstrated by achieving 8X enhancement in signal-to-noise ratio, a 45x increase in detection efficiency, and a 100x lower detection limit of 80 aM of fluorescent nanobeads. This represents a key step towards an ultrasensitive biosensor system for analyzing pathogens at clinical concentrations.

Application and validation of a coaxial liquid core waveguide fluorescence detector for the permeation analysis of desmopressin acetate

In recent years, the development of peptide drugs and alternative routes of administration, such as buccal and sublingual routes, has become increasingly important to the pharmaceutical industry. Performing experiments under physiologically relevant conditions is still a challenge that has not yet been fully mastered. The requirements associated with these alternative administration routes (e.g. permeation testing for buccal administration) push common analytical detection systems in pharmaceutical technology to their limits, especially with regard to large molecules and peptides. An HPLC-coupled coaxial liquid-core waveguide fluorescence detector has been developed and evaluated within this study to overcome these limits by achieving a more sensitive detection. Desmopressin acetate was selected as the peptide drug with the aim of investigating its permeation behavior during the clinically relevant application period of one hour. Based on the detector system, a complete validation according to the requirements of international guidelines was successfully performed. The results of the validation showed an increase in sensitivity resulting in a limit of detection of 4.7 ng/mL and a lower limit of quantification of 9.5 ng/mL. Moreover, it has been demonstrated that the permeation of desmopressin can be observed in clinically relevant dosages and time periods of up to one hour using this innovative detector system.