Methods for studying protein aggregation are crucial to understanding the associated disease pathologies and for functional biomaterial synthesis in nature and in the laboratory. The ideal measurement platform is low-volume, label-free, and noncontact, as well as easily integrated into continuous-flow microfluidic experiments to provide scalability. Current approaches realize only a subset of these requirements. Here, we demonstrate a new technique for studying protein aggregates and in situ aggregation within hollow-core photonic crystal fibers. These optofluidic waveguides allow us to perform continuous-flow microfluidic label-free analysis of silk fibroin protein in the form of preformed nanofibrillar aggregates and on the native protein as it undergoes aggregation in situ in the optofluidic waveguide. We demonstrate label-free ultraviolet absorbance measurements on both calibration-standard nanospheres and silk fibroin aggregates as well as monitoring the aggregation of native silk fibroin protein solution via simultaneous ultraviolet absorbance and intrinsic fluorescence measurements in situ. This technique forms a platform for the study of protein aggregation that is low volume, label-free, and optical, thereby providing a valuable optofluidic tool for a range of protein biophysics.

We introduce a flexible microscale all-fiber-optic Raman probe which can be embedded into devices to enable operando in situ spectroscopy. The facile-constructed probe is composed of a nested antiresonant nodeless hollow-core fiber combined with an integrated high refractive index barium titanate microlens. Pump laser 785 nm excitation and near-infrared collection are independently characterized, demonstrating an excitation spot of full-width-half-maximum 1.1 μm. Since this is much smaller than the effective collection area, it has the greatest influence on the collected Raman scattering. Our characterization scheme provides a suitable protocol for testing the efficacy of these fiber probes using various combinations of fiber types and microspheres. Raman measurements on a surface-enhanced Raman spectroscopy sample and a copper battery electrode demonstrate the viability of the fiber probe as an alternative to bulk optic Raman microscopes, giving comparable collection to a 10 objective, thus paving the way for operando Raman studies in applications such as lithium battery monitoring.

Light transmission through a multimode fiber (MMF) has gained major importance for imaging and manipulation. The majority of phase retrieval algorithms used for a MMF implicitly assume light propagation to be described by a unitary operation, yet the transmission matrix of a multimode fiber is inherently non-unitary. It is demonstrated that this erroneous assumption can impede the performance of many commonly used MMF phase retrieval algorithms and demonstrate that the weighted Yang-Gu algorithm outperforms other phase retrieval algorithms in this scenario. Once accounted for, the non-unitary property of the transmission matrix can be leveraged to generate intricate intensity and phase patterns at the output of the fiber, and shape specific output fields. This is experimentally demonstrated by generating Laguerre-Gaussian beams that carry orbital angular momentum, and by forming images in planes offset from the distal end of the fiber facet.

Carbon nanodots are nanosized light-harvesters emerging as sustainable next-generation photosensitizers in photocatalytic reactions. Despite their ever-increasing potential applications, the intricate details underlying their photoexcited charge carrier dynamics are yet to be elucidated. In this study, nitrogen-doped graphitic carbon nanodots (NgCNDs) are selectively excited in the presence of methyl viologen (MV2+, redox mediator) and different electron donors, namely ascorbic acid (AA) and ethylenediaminetetraacetic acid (EDTA). The consequent formation of the methyl viologen radical cation (MV·+) is investigated, and the excited charge carrier dynamics of the photocatalytic system are understood on a 0.1 ps to 1 ms time range, providing spectroscopic evidence of oxidative or reductive quenching mechanisms experienced by optically excited NgCNDs (NgCNDs*) depending on the electron donor implemented. In the presence of AA, NgCNDs* undergo oxidative quenching by MV2+ to form MV·+, but is short-lived due to dehydroascorbic acid, a product of photoinduced hole quenching of oxidized NgCNDs. The EDTA-mediated reductive quenching of NgCNDs* is observed to be at least two orders of magnitude slower due to screening by EDTA-MV2+ complexes, but the MV·+ population is stable due to the irreversibly oxidized EDTA preventing a back reaction. In general, our methodology provides a distinct solution with which to study charge transfer dynamics in photocatalytic systems on an extended time range spanning 10 orders of magnitude. This approach generates a mechanistic understanding to select and develop suitable electron donors to promote photocatalytic reactions.

We review how optofluidic hollow-core photonic crystal fibres (HC-PCF) can be integrated with in-line microfluidic circuits to achieve sensitive, label-free detection of proteins at low concentrations.[1] Proteins are the key building blocks of life, and sensing proteins is a challenging problem encountered across disciplines, from the study of fundamental interactions in biophysics to clinical applications in medicine. As nanometer-sized objects of low refractive index contrast, they are difficult to detect but have to be quantified to high precision. Because of their biological origin, they are also resource-intensive to synthesise or extract, requiring sensors to be capable of detecting small sample volumes at low concentrations. A key development in studying proteins came with the development of bright fluorescent markers known as labels. They revolutionised the field by adding fluorophores to the proteins either through genetic engineering or by chemical staining. However, increasing evidence shows that these labels alter protein behaviour, necessitating new sensing technologies. Hollow-core fibre-based sensors confine light into sub-microliter volumes while achieving high light-matter interaction over long path lengths. This enables them to achieve higher sensitivity compared to conventional non-waveguiding geometries. Here, we discuss our recent advances in realising this system experimentally and highlight future directions.

Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the Co-I and Co-II states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady-state signal, with the former being assigned to the Co-I state and the latter being assigned to the Co-II state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure-activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

We use optofluidic hollow-core fibres (HCFs), connected to microfluidic coupling cells, to monitor photocatalytic reactions in systems that combine molecular catalysts with particulate light absorbers. Our recent work focuses on molecular catalysts combined with carbon nanodots. Carbon nanodots (CNDs) are one of the most promising light-absorber materials due to their unique scalability, biocompatibility, water solubility, and stable optical properties. Key to improving their performance in solar catalysis are charge-transfer processes. This talk will review how these processes can be investigated through fibre-enhanced absorption-, fluorescence-, and Raman spectroscopy.

Improved analytical tools are urgently required to identify degradation and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochemical processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chemistry of a carbonate-based liquid electrolyte during electrochemical cycling in Li-ion batteries with a graphite anode and a LiNi 0.8 Mn 0.1 Co 0.1 O 2 cathode. By embedding a hollow-core optical fibre probe inside a lab-scale pouch cell, we demonstrate the effective evolution of the liquid electrolyte species by background-free Raman spectroscopy. The analysis of the spectroscopy measurements reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage and show the potential to track the lithium-ion solvation dynamics. The proposed operando methodology contributes to understanding better the current Li-ion battery limitations and paves the way for studies of the degradation mechanisms in different electrochemical energy storage systems.

A complex-valued transmission matrix describing a scattering medium can be constructed from a sequence of many interferometric measurements. A major challenge in such experiments is to correct for rapid phase drift of the optical system during the data acquisition process, especially when the phase drifts significantly between consecutive measurements. Therefore, a new method is presented where the exact phase drift between two measurements is characterized and corrected using a single additional measurement. This approach removes the need to continuously track the phase and significantly relaxes the phase stability requirements of the interferometer, allowing transmission matrices to be constructed in the presence of fast and erratic phase drift.

We report the use of optofluidic hollow-core photonic crystal fibres as microreactors for Stern-Volmer (SV) luminescence quenching analysis of visible-light photocatalytic reactions. This technology enables measurements on nanolitre volumes and paves the way for automated SV analyses in continuous flow that minimise catalyst and reagent usage. The method is showcased using a recently developed photoredox-catalysed α-C-H alkylation reaction of unprotected primary alkylamines.

Multimode devices and components have attracted considerable attention in the last years, and different research topics and themes have emerged very recently. The multimodality can be seen as an additional degree of freedom in designing devices, thus allowing for the development of more complex and sophisticated components. The propagation of different modes can be used to increase the fiber optic capacity, but also to introduce novel intermodal interactions, as well as allowing for complex manipulation of optical modes for a variety of applications. In this roadmap we would like to give to the readers a comprehensive overview of the most recent developments in the field, presenting contributions coming from different research topics, including optical fiber technologies, integrated optics, basic physics and telecommunications.

The effects of time-varying measurement noise on transmission matrix acquisition processes are considered for the first time, to our knowledge. Dominant noise sources are discussed, and the noise properties of a typical interferometer system used for characterizing a multimode fiber transmission matrix are quantified. It is demonstrated that an appropriate choice of measurement basis allows a more accurate transmission matrix to be more quickly obtained in the presence of measurement noise. Finally, it is shown that characterizing the noise figure of the experimental system allows the inverse transmission matrix to be constructed with an ideal amount of regularization, which can in turn be used for optimal image acquisition.

EDITORIAL article Front. Phys., 18 November 2022Sec. Optics and Photonics https://doi.org/10.3389/fphy.2022.1052461

The fluorescent detection of proteins without labels or stains, which affect their behaviour and require additional genetic or chemical preparation, has broad applications to biological research. However, standard approaches require large sample volumes or analyse only a small fraction of the sample. Here we use optofluidic hollow-core photonic crystal fibres to detect and quantify sub-microlitre volumes of unmodified bovine serum albumin (BSA) protein down to 100 nM concentrations. The optofluidic fibre's waveguiding properties are optimised for guidance at the (auto)fluorescence emission wavelength, enabling fluorescence collection from a 10 cm long excitation region, increasing sensitivity. The observed spectra agree with spectra taken from a conventional cuvette-based fluorimeter, corrected for the guidance properties of the fibre. The BSA fluorescence depended linearly on BSA concentration, while only a small hysteresis effect was observed, suggesting limited biofouling of the fibre sensor. Finally, we briefly discuss how this method could be used to study aggregation kinetics. With small sample volumes, the ability to use unlabelled proteins, and continuous flow, the method will be of interest to a broad range of protein-related research.

Chemical recycling of synthetic polymers represents a promising strategy to deconstruct plastic waste and make valuable products. Inspired by small-molecule C-H bond activation, a visible-light-driven reaction is developed to deconstruct polystyrene (PS) into ∼40% benzoic acid as well as ∼20% other monomeric aromatic products at 50 °C and ambient pressure. The practicality of this strategy is demonstrated by deconstruction of real-world PS foam on a gram scale. The reaction is proposed to proceed via a C-H bond oxidation pathway, which is supported by theoretical calculations and experimental results. Fluorescence quenching experiments also support efficient electron transfer between the photocatalyst and the polymer substrate, providing further evidence for the proposed mechanism. This study introduces concepts from small-molecule catalysis to polymer deconstruction and provides a promising method to tackle the global crisis of plastic pollution.

A general method is presented for exciting discrete modes in waveguides of arbitrary geometry. Guided modes supported by the waveguide are first calculated using a finite difference frequency domain model. High efficiency holograms to excite these discrete modes are then generated using the Direct Search hologram generation algorithm. The Direct Search algorithm is optimised such that the inherent properties of waveguide modes are exploited to give faster execution times. A nodeless antiresonant photonic crystal fibre is considered as a test geometry, in which high-purity modes are experimentally excited and in-coupling efficiencies of up to 32.8% are obtained.

Nanomaterials capable of confining light are desirable for enhancing spectroscopies such as Raman scattering, infrared absorption, and nonlinear optical processes. Plasmonic superlattices have shown the ability to host collective resonances in the mid-infrared, but require stringent fabrication processes to create well-ordered structures. Here, we demonstrate how short-range-ordered Au nanoparticle multilayers on a mirror, self-assembled by a sub-nm molecular spacer, support collective plasmon-polariton resonances in the visible and infrared, continuously tunable beyond 11 µm by simply varying the nanoparticle size and number of layers. The resulting molecule-plasmon system approaches vibrational strong coupling, and displays giant Fano dip strengths, SEIRA enhancement factors ~ 10 6 , light-matter coupling strengths g ~ 100 cm −1 , Purcell factors ~ 10 6 , and mode volume compression factors ~ 10 8 . The collective plasmon-polariton mode is highly robust to nanoparticle vacancy disorder and is sustained by the consistent gap size defined by the molecular spacer. Structural disorder efficiently couples light into the gaps between the multilayers and mirror, enabling Raman and infrared sensing of sub-picolitre sample volumes.

We report analytical expressions for optical forces acting on particles inside waveguides. The analysis builds on our previously reported Fourier Transform method to obtain Beam Shape Coefficients for any beam. Here we develop analytical expressions for the Beam Shape Coefficients in cylindrical and rectangular metallic waveguides. The theory is valid for particle radius a ranging from the Rayleigh regime to large microparticles, such as aerosols like virus loaded droplets. The theory is used to investigate how optical forces within hollow waveguides can be used to sort particles in “optical chromatography” experiments in which particles are optically propelled along a hollow-core waveguide. For Rayleigh particles, the axial force is found to scale with a 6 , while the radial force, which prevents particles from crashing into the waveguide walls, scales with a 3 . For microparticles, narrow Mie resonances create a strong wavelength dependence of the optical force, enabling more selective sorting. Several beam parameters, such as power, wavelength, polarization state and waveguide modes can be tuned to optimize the sorting performance. The analysis focuses on cylindrical waveguides, where meter-long liquid waveguides in the form of hollow-core photonic crystal fibers are readily available. The modes of such fibers are well-approximated by the cylindrical waveguide modes considered in the theory.