Small-angle X-ray scattering in droplet-based microfluidics

Micro- and milli-fluidic sample environments for in situ X-ray analysis in the chemical and materials sciences

X-ray-based methods are powerful tools for structural and chemical studies of materials and processes, particularly for performing time-resolved measurements. In this critical review, we highlight progress in the development of X-ray compatible microfluidic and millifluidic platforms that enable high temporal and spatial resolution X-ray analysis across the chemical and materials sciences. With a focus on liquid samples and suspensions, we first present the origins of microfluidic sample environments for X-ray analysis by discussing some alternative liquid sample holder and manipulator technologies. The bulk of the review is then dedicated to micro- and milli-fluidic devices designed for use in the three main areas of X-ray analysis: (1) scattering/diffraction, (2) spectroscopy, and (3) imaging. While most research to date has been performed at synchrotron radiation facilities, the recent progress made using commercial and laboratory-based X-ray instruments is then reviewed here for the first time. This final section presents the exciting possibility of performing in situ and operando X-ray analysis in the 'home' laboratory and transforming microfluidic and millifluidic X-ray analysis into a routine method in physical chemistry and materials research.

Physicochemical Characterization of Asphaltenes Using Microfluidic Analysis

Crude oils are complex mixtures of organic molecules, of which asphaltenes are the heaviest component. Asphaltene precipitation and deposition have been recognized to be a significant problem in oil production, transmission, and processing facilities. These macromolecular aromatics are challenging to characterize due to their heterogeneity and complex molecular structure. Microfluidic devices are able to capture key characteristics of reservoir rocks and provide new insights into the transport, reactions, and chemical interactions governing fluids used in the oil and gas industry. Understanding the microscale phenomena has led to better design of macroscale processes used by the industry. One area that has seen significant growth is in the area of chemical analysis under flowing conditions. Microfluidics and microscale analysis have advanced the understanding of complex mixtures by providing in situ imaging that can be combined with other chemical characterization methods to give details of how oil, water, and added chemicals interface with pore-scale detail. This review article aims to showcase how microfluidic devices offer new physical, chemical, and dynamic information on the behavior of asphaltenes. Specifically, asphaltene deposition and related flow assurance problems, interfacial properties and rheology, and evaluation of remediation strategies studied in microchannels and microfluidic porous media are presented. Examples of successful applications that address key asphaltene-related problems highlight the advances of microscale systems as a tool for advancing the physicochemical characterization of complex fluids for the oil and gas industry.

Flow Chemistry: A Sustainable Voyage Through the Chemical Universe en Route to Smart Manufacturing

Microfluidic devices and systems have entered many areas of chemical engineering, and the rate of their adoption is only increasing. As we approach and adapt to the critical global challenges we face in the near future, it is important to consider the capabilities of flow chemistry and its applications in next-generation technologies for sustainability, energy production, and tailor-made specialty chemicals. We present the introduction of microfluidics into the fundamental unit operations of chemical engineering. We discuss the traits and advantages of microfluidic approaches to different reactive systems, both well-established and emerging, with a focus on the integration of modular microfluidic devices into high-efficiency experimental platforms for accelerated process optimization and intensified continuous manufacturing. Finally, we discuss the current state and new horizons in self-driven experimentation in flow chemistry for both intelligent exploration through the chemical universe and distributed manufacturing. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

An innovative data processing method for studying nanoparticle formation in droplet microfluidics using X-rays scattering

X-ray scattering techniques provide a powerful means of characterizing the formation of nanoparticles in solution. Coupling these techniques to segmented-flow microfluidic devices that offer well-defined environments gives access to in situ time-resolved analysis, excellent reproducibility, and eliminates potential radiation damage. However, analysis of the resulting datasets can be extremely time-consuming, where these comprise frames corresponding to the droplets alone, the continuous phase alone, and to both at their interface. We here describe a robust, low-cost, and versatile droplet microfluidics device and use it to study the formation of magnetite nanoparticles with simultaneous synchrotron SAXS and WAXS. Lateral outlet capillaries facilitate the X-ray analysis and reaction times of between a few seconds and minutes can be accommodated. A two-step data processing method is then described that exploits the unique WAXS signatures of the droplets, continuous phase, and interfacial region to identify the frames corresponding to the droplets. These are then sorted, and the background scattering is subtracted using an automated frame-by-frame approach, allowing the signal from the nanoparticles to be isolated from the raw data. Modeling these data gives quantitative information about the evolution of the sizes and structures of the nanoparticles, in agreement with TEM observations. This versatile platform can be readily employed to study a wide range of dynamic processes in heterogeneous systems.

In Situ X-ray Absorption Spectroscopy and Droplet-Based Microfluidics: An Analysis of Calcium Carbonate Precipitation

Droplet-based microfluidic systems are ideally suited for the investigation of nucleation and crystallization processes. To best leverage the features of such platforms (including exquisite time resolution and high-throughput operation), sensitive and in situ detection schemes are needed to extract real-time chemical information about all species of interest. In this regard, the extension of conventional (UV, visible, and infrared) optical detection schemes to the X-ray region of the electromagnetic spectrum is of high current interest, as techniques such as X-ray absorption spectroscopy (XAS) provide for the element-specific investigation of the local chemical environment. Accordingly, herein, we report for the first time the integration of millisecond droplet-based microfluidics with XAS. Such a platform allows for the sensitive acquisition of X-ray absorption data from picoliter-volume droplets moving at high linear velocities. Significantly, the high-temporal resolution of the droplet-based microfluidic platform enables unprecedented access to the early stages of the reaction. Using such an approach, we demonstrate in situ monitoring of calcium carbonate precipitation by extracting XAS spectra at the early time points of the reaction with a dead time as low as 10 ms. We obtain insights into the kinetics of the formation of amorphous calcium carbonate (ACC) as a first species during the crystallization process by monitoring the proportion of calcium ions converted into ACC. Within the confined and homogeneous environment of picoliter-volume droplets, the ACC content reaches 60% over the first 130 ms. More generally, the presented method offers new opportunities for the real-time monitoring of fast chemical and biological processes.

Evaluation of microflow configurations for scale inhibition and serial X-ray diffraction analysis of crystallization processes

The clean and reproducible conditions provided by microfluidic devices are ideal sample environments for in situ analyses of chemical and biochemical reactions and assembly processes. However, the small size of microchannels makes investigating the crystallization of poorly soluble materials on-chip challenging due to crystal nucleation and growth that result in channel fouling and blockage. Here, we demonstrate a reusable insert-based microfluidic platform for serial X-ray diffraction analysis and examine scale formation in response to continuous and segmented flow configurations across a range of temperatures. Under continuous flow, scale formation on the reactor walls begins almost immediately on mixing of the crystallizing species, which over time results in occlusion of the channel. Depletion of ions at the start of the channel results in reduced crystallization towards the end of the channel. Conversely, segmented flow can control crystallization, so it occurs entirely within the droplet. Consequently, the spatial location within the channel represents a temporal point in the crystallization process. Whilst each method can provide useful crystallographic information, time-resolved information is lost when reactor fouling occurs and changes the solution conditions with time. The flow within a single device can be manipulated to give a broad range of information addressing surface interaction or solution crystallization.

OSTE+ for in situ SAXS analysis with droplet microfluidic devices

In recent years, microfluidic-based sample preparation techniques have emerged as a powerful tool for measurements at large scale X-ray facilities. Most often the microfluidic device was a form of hybrid system, i.e. an assembly of different materials, because a simple, versatile and inexpensive microfabrication method, on the one hand, and X-ray compatibility, on the other hand, cannot generally be achieved by the same material. The arrival of a new polymer family based on off-stoichiometric thiol-ene-epoxy (OSTE+) has recently redistributed the cards. In this context, we studied the relevance and the compatibility of OSTE+ for small-angle X-ray scattering (SAXS) studies. The material was characterized regarding its X-ray properties (transmission coefficient, attenuation coefficient, scattering pattern and polymer aging under X-ray light) and their comparison with those of the usual polymers used in microfluidics and/or for synchrotron radiation experiments. We show that OSTE+ has a better SAXS signal than polyimide, the polymer of reference in the SAXS community. Then a detailed protocol to manufacture a suitably thin full OSTE+ chip (total thickness <500 μm) is described and the potency of full OSTE+ devices for in situ SAXS studies is highlighted in two case-studies: the characterization of gold nanoparticles and the precipitation of cerium oxalate particles, both in moving droplets. Additionally, a method to analyze the scattering signals from droplet and carrier phase in a segmented flow is proposed.

Dynamic Crystallization Pathways of Polymorphic Pharmaceuticals Revealed in Segmented Flow with Inline Powder X-ray Diffraction

Understanding the transitions between polymorphs is essential in the development of strategies for manufacturing and maximizing the efficiency of pharmaceuticals. However, this can be extremely challenging: crystallization can be influenced by subtle changes in environment, such as temperature and mixing intensity or even imperfections in the crystallizer walls. Here, we highlight the importance of in situ measurements in understanding crystallization mechanisms, where a segmented flow crystallizer was used to study the crystallization of the pharmaceuticals urea: barbituric acid (UBA) and carbamazepine (CBZ). The reactor provides highly reproducible reaction conditions, while in situ synchrotron powder X-ray diffraction (PXRD) enables us to monitor the evolution of this system. UBA has two polymorphs of almost equivalent free-energy and so is typically obtained as a polymorphic mixture. In situ PXRD analysis uncovered a progression of polymorphs from UBA III to the thermodynamic polymorph UBA I, where different positions along the length of the tubular flow crystallizer correspond to different reaction times. Addition of UBA I seed crystals modified this pathway such that only UBA I was observed throughout, while transformation from UBA III into UBA I still occurred in the presence of UBA III seeds. Information regarding the mixing-dependent kinetics of the CBZ form II to III transformation was also uncovered in a series of seeded and unseeded flow crystallization runs, despite atypical habit expression. These results illustrate the importance of coupling controlled reaction environments with in situ XRD to study the phase relationships in polymorphic materials.

3D-MiXD: 3D-printed X-ray-compatible microfluidic devices for rapid, low-consumption serial synchrotron crystallography data collection in flow

Serial crystallography has enabled the study of complex biological questions through the determination of biomolecular structures at room temperature using low X-ray doses. Furthermore, it has enabled the study of protein dynamics by the capture of atomically resolved and time-resolved molecular movies. However, the study of many biologically relevant targets is still severely hindered by high sample consumption and lengthy data-collection times. By combining serial synchrotron crystallography (SSX) with 3D printing, a new experimental platform has been created that tackles these challenges. An affordable 3D-printed, X-ray-compatible microfluidic device (3D-MiXD) is reported that allows data to be collected from protein microcrystals in a 3D flow with very high hit and indexing rates, while keeping the sample consumption low. The miniaturized 3D-MiXD can be rapidly installed into virtually any synchrotron beamline with only minimal adjustments. This efficient collection scheme in combination with its mixing geometry paves the way for recording molecular movies at synchrotrons by mixing-triggered millisecond time-resolved SSX.

Using the singular value decomposition to extract 2D correlation functions from scattering patterns

The truncated singular value decomposition (TSVD) is applied to extract the underlying 2D correlation functions from small-angle scattering patterns. The approach is tested by transforming the simulated data of ellipsoidal particles and it is shown that also in the case of anisotropic patterns (i.e. aligned ellipsoids) the derived correlation functions correspond to the theoretically predicted profiles. Furthermore, the TSVD is used to analyze the small-angle X-ray scattering patterns of colloidal dispersions of hematite spindles and magnetotactic bacteria in the presence of magnetic fields, to verify that this approach can be applied to extract model-free the scattering profiles of anisotropic scatterers from noisy data.

Microfluidic devices for small-angle neutron scattering

A comparative examination is presented of materials and approaches for the fabrication of microfluidic devices for small-angle neutron scattering (SANS). Representative inorganic glasses, metals, and polymer materials and devices are evaluated under typical SANS configurations. Performance criteria include neutron absorption, scattering background and activation, as well as spatial resolution, chemical compatibility and pressure resistance, and also cost, durability and manufacturability. Closed-face polymer photolithography between boron-free glass (or quartz) plates emerges as an attractive approach for rapidly prototyped microfluidic SANS devices, with transmissions up to ∼98% and background similar to a standard liquid cell (I ≃ 10-3 cm-1). For applications requiring higher durability and/or chemical, thermal and pressure resistance, sintered or etched boron-free glass and silicon devices offer superior performance, at the expense of various fabrication requirements, and are increasingly available commercially.

Label-Free Sensing in Microdroplet-Based Microfluidic Systems

Droplet microfluidic systems have evolved as fluidic platforms that use much less sample volume and provide high throughput for biochemical analysis compared to conventional microfluidic devices. The variety of droplet fluidic applications triggered several detection techniques to be applied for analysis of droplets. In this review, we focus on label-free droplet detection techniques that were adapted to various droplet microfluidic platforms. We provide a classification of most commonly used droplet platform technologies. Then we discuss the examples of various label-free droplet detection schemes implemented for these platforms. While providing the research landscape for label-free droplet detection methods, we aim to highlight the strengths and shortcomings of each droplet platform so that a more targeted approach can be taken by researchers when selecting a droplet platform and a detection scheme for any given application.

Droplet microfluidic SANS

The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H₂O/D₂O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (∼1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.

SAXS on a chip: from dynamics of phase transitions to alignment phenomena at interfaces studied with microfluidic devices

The field of microfluidics offers attractive possibilities to perform novel experiments that are difficult (or even impossible) to perform using conventional bulk and surface-based methods. Such attractiveness comes from several important aspects inherent to these miniaturized devices. First, the flow of fluids under submillimeter confinement typically leads to a drop of inertial forces, meaning that turbulence is practically suppressed. This leads to predictable and controllable flow profiles, along with well-defined chemical gradients and stress fields that can be used for controlled mixing and actuation on the micro and nanoscale. Secondly, intricate microfluidic device designs can be fabricated using cleanroom standard procedures. Such intricate geometries can take diverse forms, designed by researchers to perform complex tasks, that require exquisite control of flow of several components and gradients, or to mimic real world examples, facilitating the establishment of more realistic models. Thirdly, microfluidic devices are usually compatible with in situ or integrated characterization methods that allow constant real-time monitoring of the processes occurring inside the microchannels. This is very different from typical bulk-based methods, where usually one can only observe the final result, or otherwise, take quick snapshots of the evolving process or take aliquots to be analyzed separately. Altogether, these characteristics inherent to microfluidic devices provide researchers with a set of tools that allow not only exquisite control and manipulation of materials at the micro and nanoscale, but also observation of these effects. In this review, we will focus on the use and prospects of combining microfluidic devices with in situ small-angle X-ray scattering (and related techniques such as small-angle neutron scattering and X-ray photon correlation spectroscopy), and their enormous potential for physical-chemical research, mainly in self-assembly and phase-transitions, and surface characterization.

Cyclic olefin copolymer as an X-ray compatible material for microfluidic devices

The combination of microfluidics and X-ray methods attracts a lot of attention from researchers as it brings together the high controllability of microfluidic sample environments and the small length scales probed by X-rays. In particular, the fields of biophysics and biology have benefited enormously from such approaches. We introduce a straightforward fabrication method for X-ray compatible microfluidic devices made solely from cyclic olefin copolymers. We benchmark the performance of the devices against other devices including more commonly used Kapton windows and obtain data of equal quality using small angle X-ray scattering. An advantage of the devices presented here is that no gluing between interfaces is necessary, rendering the production very reliable. As a biophysical application, we investigate the early time points of the assembly of vimentin intermediate filament proteins into higher-order structures. This weakly scattering protein system leads to high quality data in the new devices, thus opening up the way for numerous future applications.

Lipid-Stabilized Water-Oil Interfaces Studied by Microfocusing Small-Angle X-ray Scattering

Water-in-oil (w/o) simple emulsions are dispersed microconfined systems that find applications in many areas of advanced materials and biotechnology, such as the food industry, drug delivery, and cosmetics, to name but a few. In these systems, the structural and chemical properties of the boundary layer at the w/o interface are of paramount importance in determining functionality and stability. Recently, microfluidic methods have emerged as a valuable tool for fabricating monodisperse emulsion droplets. Because of the intrinsic flexibility of microfluidics, different interfaces can be obtained, and general principles governing their stability are needed to guide the experimental approach. Herein, we investigate the structural characteristics of the region encompassing the liquid/liquid (L/L) interface of w/o emulsions generated by a microfluidic device in the presence of phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and other intercalating amphiphiles (dopants) using microfocused small-angle X-rays scattering (μ-SAXS). We show that phospholipids provide a stable and versatile boundary film of ∼100 μm whose basic units are swollen and uncorrelated DMPC bilayers. The internal arrangement of this interfacial film can be tuned by adding molecules with a different packing parameter, such as cholesterol, which is able to increase the stiffness of the lipid membranes and trigger interbilayer correlation.

Chemical and Biological Dynamics Using Droplet-Based Microfluidics

Recent years have witnessed an increased use of droplet-based microfluidic techniques in a wide variety of chemical and biological assays. Nevertheless, obtaining dynamic data from these platforms has remained challenging, as this often requires reading the same droplets (possibly thousands of them) multiple times over a wide range of intervals (from milliseconds to hours). In this review, we introduce the elemental techniques for the formation and manipulation of microfluidic droplets, together with the most recent developments in these areas. We then discuss a wide range of analytical methods that have been successfully adapted for analyte detection in droplets. Finally, we highlight a diversity of studies where droplet-based microfluidic strategies have enabled the characterization of dynamic systems that would otherwise have remained unexplorable.

Innovative High-Throughput SAXS Methodologies Based on Photonic Lab-on-a-Chip Sensors: Application to Macromolecular Studies

The relevance of coupling droplet-based Photonic Lab-on-a-Chip (PhLoC) platforms and Small-Angle X-Ray Scattering (SAXS) technique is here highlighted for the performance of high throughput investigations, related to the study of protein macromolecular interactions. With this configuration, minute amounts of sample are required to obtain reliable statistical data. The PhLoC platforms presented in this work are designed to allow and control an effective mixing of precise amounts of proteins, crystallization reagents and buffer in nanoliter volumes, and the subsequent generation of nanodroplets by means of a two-phase flow. Spectrophotometric sensing permits a fine control on droplet generation frequency and stability as well as on concentration conditions, and finally the droplet flow is synchronized to perform synchrotron radiation SAXS measurements in individual droplets (each one acting as an isolated microreactor) to probe protein interactions. With this configuration, droplet physic-chemical conditions can be reproducibly and finely tuned, and monitored without cross-contamination, allowing for the screening of a substantial number of saturation conditions with a small amount of biological material. The setup was tested and validated using lysozyme as a model of study. By means of SAXS experiments, the proteins gyration radius and structure envelope were calculated as a function of protein concentration. The obtained values were found to be in good agreement with previously reported data, but with a dramatic reduction of sample volume requirements compared to studies reported in the literature.

Recent advances in X-ray compatible microfluidics for applications in soft materials and life sciences

The increasingly narrow and brilliant beams at X-ray facilities reduce the requirements for both sample volume and data acquisition time. This creates new possibilities for the types and number of sample conditions that can be examined but simultaneously increases the demands in terms of sample preparation. Microfluidic-based sample preparation techniques have emerged as elegant alternatives that can be integrated directly into the experimental X-ray setup remedying several shortcomings of more traditional methods. We review the use of microfluidic devices in conjunction with X-ray measurements at synchrotron facilities in the context of 1) mapping large parameter spaces, 2) performing time resolved studies of mixing-induced kinetics, and 3) manipulating/processing samples in ways which are more demanding or not accessible on the macroscale. The review covers the past 15 years and focuses on applications where synchrotron data collection is performed in situ, i.e. directly on the microfluidic platform or on a sample jet from the microfluidic device. Considerations such as the choice of materials and microfluidic designs are addressed. The combination of microfluidic devices and measurements at large scale X-ray facilities is still emerging and far from mature, but it definitely offers an exciting array of new possibilities.

Scanning-SAXS of microfluidic flows: nanostructural mapping of soft matter

The determination of in situ structural information of soft matter under flow is challenging, as it depends on many factors, such as temperature, concentration, confinement, channel geometry, and type of imposed flow. Here, we combine microfluidics and scanning small-angle X-ray scattering (scanning-SAXS) to create a two-dimensional spatially resolved map, which represents quantitatively the variation of molecular properties under flow. As application examples, mappings of confined amyloid fibrils and wormlike micelles under flow into various channel geometries are compared. A simple process to fabricate X-rays resistant chips, based on polyimide and UV-curing resin, is discussed. During experiments, these chips remained in high-energy synchrotron radiation for more than 24 hours, causing constant low background scattering. Thus, sufficient statistics were obtained from sample scattering at exposure times as low as 0.1 s, even with the small scattering volumes in microfluidic channels. Scanning-SAXS of microfluidic flows has many potential applications from biology to fundamental soft matter physics. In general, any fluid which has enough contrast for X-ray scattering can be measured to obtain the dependence of molecular shape, conformation, alignment and size on the flow field. Besides, dynamic processes of soft matter caused by flow, temperature, concentration gradient, and confinement, for example self-assembling, aggregation, mixing, diffusion, and disintegration of macromolecules, can be quantified and visualized on a single image by this mapping technique.

LabDisk for SAXS: a centrifugal microfluidic sample preparation platform for small-angle X-ray scattering

We present a centrifugal microfluidic LabDisk for protein structure analysis via small-angle X-ray scattering (SAXS) on synchrotron beamlines. One LabDisk prepares 120 different measurement conditions, grouped into six dilution matrices. Each dilution matrix: (1) features automatic generation of 20 different measurement conditions from three input liquids and (2) requires only 2.5 μl of protein solution, which corresponds to a tenfold reduction in sample volume in comparison to the state of the art. Total hands on time for preparation of 120 different measurement conditions is less than 5 min. Read-out is performed on disk within the synchrotron beamline P12 at EMBL Hamburg (PETRA III, DESY). We demonstrate: (1) aliquoting of 40 nl aliquots for five different liquids typically used in SAXS and (2) confirm fluidic performance of aliquoting, merging, mixing and read-out from SAXS experiments (2.7-4.4% CV of protein concentration). We apply the LabDisk for SAXS for basic analysis methods, such as measurement of the radius of gyration, and advanced analysis methods, such as the ab initio calculation of 3D models. The suitability of the LabDisk for SAXS for protein structure analysis under different environmental conditions is demonstrated for glucose isomerase under varying protein and NaCl concentrations. We show that the apparent radius of gyration of the negatively charged glucose isomerase decreases with increasing protein concentration at low salt concentration. At high salt concentration the radius of gyration (Rg) does not change with protein concentrations. Such experiments can be performed by a non-expert, since the LabDisk for SAXS does not require attachment of tubings or pumps and can be filled with regular pipettes. The new platform has the potential to introduce routine high-throughput SAXS screening of protein structures with minimal input volumes to the regular operation of synchrotron beamlines.

Small Angle X-ray Scattering for Nanoparticle Research

X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle's size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research where X-ray scattering has played a pivotal role, including in situ nanoparticle synthesis, nanoparticle assembly, and operando studies of catalysts and energy storage materials. Throughout this review we highlight the unique capabilities of X-ray scattering for structural characterization of materials in their native environment.

Enzyme engineering in biomimetic compartments

The success of a directed evolution approach to creating custom-made enzymes relies in no small part on screening as many clones as possible. The miniaturisation of assays into pico to femtoliter compartments (emulsion droplets, vesicles or gel-shell beads) makes directed evolution campaigns practically more straightforward than current large scale industrial screening that requires liquid handling equipment and much manpower. Several recent experimental formats have established protocols to screen more than 10 million compartments per day, representing unprecedented throughput at low cost. This review introduces the emerging approaches towards making biomimetic man-made compartments that are poised to be adapted by a wider circle of researchers. In addition to cost and time saving, control of selection pressures and conditions, the quantitative readout that reports on every library members and the ability to develop strategies based on these data will increase the degrees of freedom in designing and testing strategies for directed evolution experiments.

Implementation of in situ SAXS/WAXS characterization into silicon/glass microreactors

A successful implementation of in situ X-ray scattering analysis of synthetized particle materials in silicon/glass microreactors is reported. Calcium carbonate (CaCO3) as a model material was precipitated inside the microchannels through the counter-injection of two aqueous solutions, containing carbonate ions and calcium ions, respectively. The synthesized calcite particles were analyzed in situ in aqueous media by combining Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) techniques at the ESRF ID02 beam line. At high wavevector transfer, WAXS patterns clearly exhibit different scattering features: broad scattering signals originating from the solvent and the glass lid of the chip, and narrow diffraction peaks coming from CaCO3 particles precipitated rapidly inside the microchannel. At low wavevector transfer, SAXS reveals the rhombohedral morphology of the calcite particles together with their micrometer size without any strong background, neither from the chip nor from the water. This study demonstrates that silicon/glass chips are potentially powerful tools for in situ SAXS/WAXS analysis and are promising for studying the structure and morphology of materials in non-conventional conditions like geological materials under high pressure and high temperature.

An X-ray transparent microfluidic platform for screening of the phase behavior of lipidic mesophases

Lipidic mesophases are a class of highly ordered soft materials that form when certain lipids are mixed with water. Understanding the relationship between the composition and the microstructure of mesophases is necessary for fundamental studies of self-assembly in amphiphilic systems and for applications, such as the crystallization of membrane proteins. However, the laborious formulation protocol for highly viscous mesophases and the large amounts of material required for sample formulation are significant obstacles in such studies. Here we report a microfluidic platform that facilitates investigations of the phase behavior of mesophases by reducing sample consumption 300-fold, and automating and parallelizing sample formulation. The mesophases were formulated on-chip using less than 80 nL of material per sample and their microstructure was analyzed in situ using small-angle X-ray scattering (SAXS). The 220 μm-thick X-ray compatible platform was comprised of thin polydimethylsiloxane (PDMS) layers sandwiched between cyclic olefin copolymer (COC) sheets. Uniform mesophases were prepared using an active on-chip mixing strategy coupled with periodic cooling of the sample to reduce viscosity. We validated the platform by preparing and analyzing mesophases of the lipid monoolein (MO) mixed with aqueous solutions of different concentrations of β-octylglucoside (βOG), a detergent frequently used in membrane protein crystallization. Four samples were prepared in parallel on chip, by first metering and automatically diluting βOG to obtain detergent solutions of different concentration, then metering MO, and finally mixing by actuation of pneumatic valves. Integration of detergent dilution and subsequent mixing significantly reduced the number of manual steps needed for sample preparation. Three different types of mesophases typical for MO were successfully identified in SAXS data from on-chip samples. Microstructural parameters of identical samples formulated in different chips showed excellent agreement. Phase behavior of samples on-chip (~80 nL per sample) corresponded well with that of samples prepared via the traditional coupled-syringe method using at least two orders of magnitude more material ("off-chip", 35-40 μL per sample), further validating the applicability of the microfluidic platform for on-chip characterization of mesophase microstructure.