Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer

Abstract

Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50–1.5 ml min−1) sheath stream with a low-flow-rate (5–90 µl min−1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in 3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection–diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

Stream lamination and rapid mixing in a microfluidic jet for X-ray spectroscopy studies

“…The lower Q sa condition shows the sample stream fluorescence is nearly completely quenched at x = 0.6 mm. We quantified the degree of quenching by computing the quenched intensity ratio, 𝛽, as a function of spatial location along the jet (Huyke et al, 2021). Briefly, we integrated the quenched and unquenched pixel intensities along the spanwise z-direction, obtained the ratio of these (streamwise) distributions and plotted this ratio as a function of the streamwise coordinate, x.…”

“…protein folding) (Pollack et al., 1999). In large part because of the line-of-sight integrating nature of X-ray methods, the mixing function of these microfluidic devices has been achieved using three-dimensional (3-D) hydrodynamic focusing (Huyke et al., 2020, 2021; Plumridge et al., 2018). Furthermore, the sample injection task is increasingly achieved using microfluidic devices that produce free jets to accommodate the highly powerful radiation of next-generation X-ray sources (Crissman et al., 2022; Ishigami et al., 2019; Knoška et al., 2020; Wang, Weierstall, Pollack, & Spence, 2014).…”

“…3d transition metal complexes. The latter can be used to achieve ~3 times higher resolution spectra as compared with the Fe K-edge XAS data described earlier for a hydrodynamic focusing mixer (Huyke et al., 2021). The main advantage of this mixer configuration is therefore to enable direct access by the X-ray beam to the reactants and products carried by the liquid stream within approximately 100 nm of the free surface of the jet.…”

Iron Kβ X-ray Emission Spectroscopy: The Origin of Spectral Features from Atomic to Molecular Systems Using Multi-configurational Calculations

“…The sensitivity of the Kβ XES spectra to the spin on the metal center , has been widely used to determine spin and oxidation states, e.g., to track excited-state intermediates in time-resolved experiments or reactive intermediates of chemical and biological reactions. − Beyond the probe of local spin, some studies indicated that the Kβ XES can also be used to probe the orbital covalency. − The energy difference between Kβ 1,3 and Kβ′ was found to be proportional to the nominal spin state that can be further modulated by the orbital covalency, or the effective 3d spin, , as the energy splitting is connected to the effective spin on the metal center and hence the magnitude of exchange interactions. When going to lower-spin states, the Kβ 1,3 –Kβ′ energy splitting becomes more and more challenging to determine due to the reduced intensity of the Kβ′ feature and the strong intensity of the main line.…”

X-ray compatible microfluidics for in situ studies of chemical state, transport and reaction of light elements in an aqueous environment using synchrotron radiation

“…Huyke et al employ a flow focusing geometry with a polyimide capillary providing X-ray access, however the relatively long path length through the sheath fluid renders this device unsuitable for tender X-ray studies. 50 Chan et al combined etched-silicon flow-focusing channels with SiN x windows to study the kinetics of a nanocrystal cation exchange. 51 The high aspect ratio of the channel employed is not designed for investigating diffusion reactions, and the addition of a SU8 layer in the beam path and deep channel would also lead to high attenuation of tender X-rays.…”