Picoliter-volume droplets within segmented flows can be probed in a rapid and efficient manner using optical detection methods. To date, however, most detection schemes for droplet content analysis have relied on the use of time-integrated fluorescence measurements. Despite its undoubted utility, the implementation of absorbance-based detectors is particularly challenging due to the reduced optical path lengths that are characteristic of microfluidic systems and deleterious scattering at droplet–oil interfaces. Unsurprisingly, efforts to develop sensitive absorbance-based detection schemes for the interrogation of rapidly moving droplets have primarily focused on ensuring adequate analytical sensitivity and, to date, have been exclusively limited to single-wavelength measurements. To address this limitation, and expand the information content associated with absorbance measurements on-chip, we herein describe a detection scheme for the extraction of broad-band absorbance spectra from pL-volume droplets with high sensitivity. The combination of a confocal optical system (that confines incident light to a reduced detection volume) and a postprocessing algorithm (that effectively removes the contribution of the carrier oil from the extracted spectra) engenders significant improvements in signal-to-noise ratios. Our system is initially calibrated by acquiring absorbance spectra from aqueous solutions of fluorescein isothiocyanate. These measurements confirm both excellent linearity over the studied range (from 0 to 100 μM) and a concentration limit of detection of 800 nM. The methodology is then used to monitor the salt-induced aggregation of gold nanoparticles with millisecond time resolution. This approach for small-volume absorbance spectroscopy allows for both high-throughput and high-information content measurements in subnanoliter volumes and will be highly desirable in a wide variety of bioanalytical applications where sensitivity and throughput are priorities.

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.

The potential of amyloid fibrils in artificial materials can be further enriched by their ability to form anisotropic assemblies. Like many other rod-like colloidal particles, aqueous suspensions of amyloid fibrils can self-assemble into phases with long-range orientational ordering, i.e., liquid crystals (LCs), driven by entropy. [11][12][13][14] In addition to the common nematic phase where there is no positional ordering, the inherent chirality of the fibrils also leads to cholesteric phases with helical twisting alignment of fibrils by controlling the fibrils length distribution and confinement. [15,16] These ordered states lead to anisotropy in the mechanical, rheological, and optical properties of the fibrils assemblies in meso-and macroscale, which, however, has yet to be taken full advantage of in the fabrication of functional materials. [7,8] An emerging research interest is the photonic properties of amyloid fibrils. First observed as UV-induced blue-green luminescence, [17] it has been extended to UV-vis-NIR range [18] and wavelength-dependent nonlinear absorption is also discovered in AFs. [19] Although the exact origin of the intrinsic fluorescence is still under debate, studies have demonstrated that it can be used to monitor the growth of the fibrils [20] or to image amyloid deposits in a noninvasive and contrast-agent-free fashion. [18,21] These findings may contribute to the diagnosis and treatment of human amyloidosis, as well as the development of novel bionanomaterials. However, the intrinsic fluorescence suffers from low quantum efficiency and weak signal to noise ratio, and therefore, hinders its potentials in practical applications. [18][19][20][21][22] In this work, we study the intrinsic fluorescence of amyloid fibrils in orientationally ordered phases and investigate their interaction with plasmonic nanoinclusions codispersed with the fibrils. We show that the fluorescence from the fibrils is dependent on the polarization of the excitation when they are in microdroplets with liquid crystalline ordering, i.e., tactoids. Then gold nanorods (GNRs) are introduced to the colloidal suspensions of AFs, forming hybrid tactoids with GNRs aligned by the LC orientation field of the fibrils. This alignment manifests through the selective activation of surface plasmon resonance (SPR) absorption bands of GNRs, resulting in color changes when illuminated with polarized white light. Furthermore, enhanced fluorescence from the hybrid material is observed due to the coupling between the GNRs' plasmonic effect and the fibrils' intrinsic fluorescence. The enhanced fluorescence is strongly dependent on the wavelength and polarization of the incident light, which is confirmed by numerical simulations of the electromagnetic field distribution in the vicinity of GNRs. We point out the unusual higher Despite their link to neurodegenerative diseases, amyloids of natural and synthetic sources can also serve as building blocks for functional materials, while possessing intrinsic photonic properties. Here, it is dem...