Towards microfluidic reactors for in situ synchrotron infrared studies

Silicon microfabricated reactor for operando XAS/DRIFTS studies of heterogeneous catalytic reactions

A novel microreactor for operando XAS and DRIFTS studies of catalytic reactions is reported, exhibiting plug-flow, isothermal behaviour and absence of mass transfer resistances and dead volume, enabling time- and spatially-resolved experiments.

Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectromicroscopy Using Synchrotron Radiation and Micromachined Silicon Wafers for Microfluidic Applications

A custom-designed optical configuration compatible with the use of micromachined multigroove internal reflection elements (μ-groove IREs) for attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy and imaging applications in microfluidic devices is described. The μ-groove IREs consist of several face-angled grooves etched into a single, monolithic silicon chip. The optical configuration permits individual grooves to be addressed by focusing synchrotron sourced IR light through a 150 µm pinhole aperture, restricting the beam spot size to a dimension smaller than that of the groove walls. The effective beam spot diameter at the ATR sampling plane is determined through deconvolution of the measured detector response and found to be 70 µm. The μ-groove IREs are highly compatible with standard photolithographic techniques as demonstrated by printing a 400 µm wide channel in an SU-8 film spin-coated on the IRE surface. Attenuated total reflection FT-IR mapping as a function of sample position across the channel illustrates the potential application of this approach for rapid prototyping of microfluidic devices.

Synchrotron FTIR mapping of mineralization in a microfluidic device

Fourier transform infrared micro-spectroscopy provides an effective means of performing rapid, non-destructive, and label-free analysis of specimens according to their vibrational modes. However, as water absorbs very strongly in the infrared region, analysis of aqueous solutions in transmission mode can suffer from problems with signal saturation. We here describe the fabrication of a novel microfluidic device that overcomes this problem. Devices with channel depths of just 3 μm were constructed from calcium fluoride using photolithography and hot embossing bonding, where calcium fluoride was selected due to its transparency in the IR region. The utility of this device was then demonstrated by employing it to follow the precipitation pathways of calcium sulfate and calcium carbonate using synchrotron FTIR micro-spectroscopy. Importantly, due to the high brightness provided by synchrotron radiation, and the fact that the reacting ions (HCO₃^-, CO₃^2- and SO₄^2-) and the different mineral polymorphs all have finger print spectra in the measured IR range, this method can be used to acquire time-resolved, hyperspectral maps of the mineral particles formed within the sample cell, and then study the interaction and evolution of particles. The data provide new insight into the formation pathway of a population of crystals in confined volumes, and demonstrate that this in situ, real-time detection system provides a powerful tool for studying crystallization processes.