On-Chip Photonic Detection Techniques for Non-Invasive In Situ Characterizations at the Microfluidic Scale.

Microfluidics has emerged as a robust technology for diverse applications, ranging from bio-medical diagnostics to chemical analysis. Among the different characterization techniques that can be used to analyze samples at the microfluidic scale, the coupling of photonic detection techniques and on-chip configurations is particularly advantageous due to its non-invasive nature, which permits sensitive, real-time, high throughput, and rapid analyses, taking advantage of the microfluidic special environments and reduced sample volumes. Putting a special emphasis on integrated detection schemes, this review article explores the most relevant advances in the on-chip implementation of UV-vis, near-infrared, terahertz, and X-ray-based techniques for different characterizations, ranging from punctual spectroscopic or scattering-based measurements to different types of mapping/imaging. The principles of the techniques and their interest are discussed through their application to different systems.

Microfluidic lab-on-a-chip characterization of nano- to microparticles suspensions by light extinction spectrometry.

The analysis of nano- and microparticle suspensions with micro systems affords improved space-time yields, selectivity, reaction residence times and conversions capabilities. These capabilities are of primary importance in various fields of research and industry. The few microfluidic lab-on-a-chip approaches that have been developed are essentially designed to analyse fluid phases or involve the use of benchtop particle sizing instruments. We report a novel microscale approach to characterize the particle size distribution and absolute concentration of colloidal suspensions. The method is based on a photonic lab-on-a-chip with three scale-specific detection channels to record simultaneous light extinction spectra. Experiments carried out on particle standards with sizes ranging from 30 nm to 0.5 µm and volume concentrations of 1 to 1000ppm, clearly demonstrate the value and potential of the proposed method.

Microfluidic In-Situ Spectrophotometric Approaches to Tackle Actinides Analysis in Multiple Oxidation States.

The study and development of present and future processes for the treatment/recycling of spent nuclear fuels require many steps, from design in the laboratory to setting up on an industrial scale. In all of these steps, analysis and instrumentation are key points. For scientific reasons (small-scale studies, control of phenomena, etc.) but also with regard to minimizing costs, risks, and waste, such developments are increasingly carried out on milli- or microfluidic devices. The logic is the same for the chemical analyses associated with their follow-up and interpretation. Due to this, over the last few years, opto-microfluidic analysis devices adapted to the monitoring of different processes (dissolution, liquid-liquid extraction, precipitation, etc.) have been increasingly designed and developed. In this work, we prove that photonic lab-on-a-chip (PhLoC) technology is fully suitable for all actinides concentration monitoring along the plutonium uranium refining extraction (plutonium, uranium, reduction, extraction, or Purex) process. Several PhLoC microfluidic platforms were specifically designed and used in different nuclear research and development (R&D) laboratories, to tackle actinides analysis in multiple oxidation states even in mixtures. The detection limits reached (tens of µmol·L-1) are fully compliant with on-line process monitoring, whereas a range of analyzable concentrations of three orders of magnitude can be covered with less than 150 µL of analyte. Finally, this work confirms the possibility and the potential of coupling Raman and ultraviolet-visible (UV-Vis) spectroscopies at the microfluidic scale, opening the perspective of measuring very complex mixtures.

Enabling Microscale Processing: Combined Raman and Absorbance Spectroscopy for Microfluidic On-Line Monitoring.

Microfluidics have many potential applications including characterization of chemical processes on a reduced scale, spanning the study of reaction kinetics using on-chip liquid-liquid extractions, sample pretreatment to simplify off-chip analysis, and for portable spectroscopic analyses. The use of in situ characterization of process streams from laboratory-scale and microscale experiments on the same chemical system can provide comprehensive understanding and in-depth analysis of any similarities or differences between process conditions at different scales. A well-characterized extraction of Nd(NO3)3 from an aqueous phase of varying NO3- (aq) concentration with tributyl phosphate (TBP) in dodecane was the focus of this microscale study and was compared to an earlier laboratory-scale study utilizing counter current extraction equipment. Here, we verify that this same extraction process can be followed on the microscale using spectroscopic methods adapted for microfluidic measurement. Concentration of Nd (based on UV-vis) and nitrate (based on Raman) was chemometrically measured during the flow experiment, and resulting data were used to determine the distribution ratio for Nd. Extraction distributions measured on the microscale were compared favorably with those determined on the laboratory scale in the earlier study. Both micro-Raman and micro-UV-vis spectroscopy can be used to determine fundamental parameters with significantly reduced sample size as compared to traditional laboratory-scale approaches. This leads naturally to time, cost, and waste reductions.

Bubbles, drops, and solid particles recognition from real or virtual photonic jets reconstructed by digital in-line holography.

The image of the photonic jet (also called caustic) formed by a large, transparent, and spherical particle, reconstructed by digital in-line holography, is shown to be similar to the Airy pattern observed at the focus of a diffraction-limited lens. The analysis of this image, real or virtual depending on whether the particle relative refractive index is above or below one, allows characterizing the particle composition via its refractive index. Experiments clearly demonstrate the value of this method for the simultaneous 3D characterization and differentiation of the dynamics, size, and composition of gas, liquid, and solid particles in multiphase flows.

Uranium(VI) On-Chip Microliter Concentration Measurements in a Highly Extended UV-Visible Absorbance Linearity Range.

The reduction of effluents deriving from analytical control is a serious concern in the nuclear industry, for both production and R&D units. In this work we report an alternative methodology for the standard UV-vis absorbance analyses for actinides concentration monitoring along the plutonium uranium refining extraction (PUREX) process. This methodology, based on photonic lab-on-a-chip (PhLoC) technology, enables drastic sampling reduction down to a few microliters and simultaneously allows to track concentrations over several orders of magnitude while maintaining a detection linearity range. A PhLoC microfluidic platform was specifically designed to allow online sample injection with zero dead volume connectivity and the on-chip spectrophotometric approach, based on a multiple optical path configuration, was tested for the determination of uranium(VI) concentrations from 0.1 to 200 g L-1, showing that linearity is maintained within high levels of confidence. These results provide the proof of concept for the transposition of current analytical methods for actinides, including plutonium, to microfluidic systems.

Droplet sizing and mixture fraction measurement in liquid-liquid flows with rainbow-angle diffractometry.

The capabilities and resolution of the rainbow technique were extended to estimate the size distribution and composition of droplets in liquid-liquid systems. For these droplets, essentially characterized by a low relative refractive index (m≈1.001-1.20), the first-order rainbow is localized in the near-forward to sideways region. It exhibits an unusually higher contrast in the parallel polarization due to the vicinity of the rainbow and the Brewster angles. A numerical study revealed that a few thousand to ten thousand droplets were necessary to obtain reliable estimations of the first moments of typical droplet size distributions when the diffractometer is operated as an ensemble averaging technique. The importance of the accuracy of the light scattering model and the inverse methods used are also documented. Experimental results performed on free-rising submillimeter to millimeter droplets of various compositions showed that a global resolution of 1% to 5% of their mean diameter and about 1.6×10-4 of the dispersion on their refractive index (i.e., 3% in the mixture fraction of oily droplets in water) could be achieved, which enhances the perspectives on mixing and extraction studies in liquid-liquid systems.

Broadcasting photonic lab on a chip concept through a low cost manufacturing approach.

A low cost fabrication process for photonic lab on a chip systems is here proposed. For the implementation of the masters suitable for cast molding fabrication, an inexpensive dry film photoresist, patternable using standard laboratory equipment, is benchmarked against standardized SU-8 masters obtained using UV lithography and systems manufacture in clean room facilities. Results show adequate system fabrication and a comparable performance of the photonic structures for absorbance/extinction measurements.

Photonic jet reconstruction for particle refractive index measurement by digital in-line holography.

A new and computationally efficient approach is proposed for determining the refractive index of spherical and transparent particles, in addition to their size and 3D position, using digital in-line holography. The method is based on the localization of the maximum intensity position of the photonic jet with respect to the particle center retrieved from the back propagation of recorded holograms. Rigorous electromagnetic calculations and experimental results demonstrate that for liquid-liquid systems and droplets with a radius > 30µm, a refractive index measurement with a resolution inferior to 4 × 10-3 is achievable, revealing a significant potential for the use of this method to investigate multiphase flows. The resolution for solid or liquid particles in gas is expected to be lower but sufficient for the recognition of particle material.