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In this study, we developed a liquid crystal (LC) droplet-based sensing platform for the detection of carboxylesterase (CES) and its inhibitors. The LC droplet patterns in contact with myristoylcholine chloride (Myr) exhibited dark cross appearances, corresponding to homeotropic anchoring of the LCs at the aqueous/LC interface. However, in the presence of CES, Myr was hydrolyzed; therefore, the optical images of the LC patterns changed to bright fan-shaped textures, corresponding to a planar orientation of LCs at the interface. In contrast, the presence of CES inhibitors, such as benzil, inhibits the hydrolysis of Myr; as a result, the LC patterns exhibit dark cross textures. This principle led to the development of an LC droplet-based sensing method with a detection limit of 2.8 U/L and 10 μM, for CES detection and its inhibitor, respectively. The developed biosensor not only enables simple and label-free detection of CES but also shows high promise for the detection of CES inhibitors.

A simple, novel concept for the one-step fabrication of a low-cost, easy-to-use droplet-based electrochemical (EC) sensor is described, in which the EC reagents are contained in a droplet and the droplet assay is operated on a simple planar surface instead of in a complicated closed channel/chamber. In combination with an elegant carbon electrode configuration, screen-printed on a widely available polyethylene terephthalate (PET) substrate, the developed sensor exhibits a stable solution-restriction capacity and acceptable EC response, and thus can be used directly for the detection of different analytes (including ascorbic acid (AA), copper ions (Cu2+), 2′-deoxyguanosine 5′-triphosphate (dGTP) and ferulic acid (FA)), without any pretreatment. The obtained, acceptable linear ranges/detection limits for AA, Cu2+, dGTP and FA are 0.5–10/0.415 mM, (0.0157–0.1574 and 0.1574–1.5736)/0.011 mM, 0.01–0.1/0.008 mM and 0.0257–0.515/0.024 mM, respectively. Finally, the utility of the droplet-based EC sensor was demonstrated for the determination of AA in two commercial beverages, and of Cu2+ in two water samples, with reliable recovery and good stability. The applicability of the droplet-based sensor demonstrates that the proposed EC strategy is potentially a cost-effective solution for a series of biochemical sensing applications in public health, environmental monitoring, and the developing world.

Herein, we report the development of a simple liquid crystal (LC)-based droplet sensor for the specific detection of mercury ions (Hg 2+ ) in an aqueous medium, using trimethyl octadecyl ammonium bromide (OTAB) and an Hg 2+ -binding aptamer. In terms of the mechanism, the system senses the homeotropic orientation of the LCs at the LC–aqueous interface that is induced by the self-assembled monolayer of positively charged OTAB molecules. In the absence of Hg 2+ , electrostatic interactions between the positively charged OTAB and negatively charged aptamer molecules disturb the organization of the OTAB monolayer, causing the LCs to adopt a planar orientation. By contrast, an aptamer hairpin structure formed in the presence of Hg 2+ weakens the interactions between the OTAB and aptamer molecules, causing the OTAB monolayer to self-reassemble and the LCs to consequently adopt a homeotropic orientation. These LC orientational transitions are observable under a polarized optical microscope. This sensor had a low limit of detection of 100 and 250 pM for buffer and tap water samples, respectively. Therefore, our LC-based droplet sensor is a suitable platform for the simple, specific, and convenient detection of Hg 2+ in an aqueous medium.

Droplet‐based lab‐on‐a‐chip systems offer vast possibilities in the manipulation, guidance, tracking, and labeling of individual droplet‐based bioreactors. One of the targeted application scenarios is in drug discovery where millions of unique codes are required, which is out of reach for current technologies. Here, a concept for the realization of multiparametric codes, where information is stored in distinct physical and chemical parameters, is proposed and validated. Exemplarily, the focus is on the use of impedance and magnetic sensing by monitoring ionic concentration as well as magnetic content per droplet and droplet volume. Codes based on aqueous ferrofluid droplets are prepared using a tubing‐based millifluidic setup and consist of up to six droplets of different combinations of volumes and magnetic concentration. It is demonstrated that a droplet chain of three single droplets of different volumes with nine different magnetic nanoparticle concentrations accompanied with four different ionic concentrations per droplet offers up to 3 million unique codes. The developed fluidic platform can be readily extended to other types of sensors including optical ones to boost the coding capacity even further.

This paper presents a comprehensive study focusing on the detection and characterization of droplets with volumes in the nanoliter range. Leveraging the precise control of minute liquid volumes, we introduced a novel spectroscopic on-chip microsensor equipped with integrated microfluidic channels for droplet generation, characterization, and sensing simultaneously. The microsensor, designed with interdigitated ring-shaped electrodes (IRSE) and seamlessly integrated with microfluidic channels, offers enhanced capacitance and impedance signal amplitudes, reproducibility, and reliability in droplet analysis. We were able to make analyses of droplet length in the range of 1.0–6.0 mm, velocity of 0.66–2.51 mm/s, and volume of 1.07 nL–113.46 nL. Experimental results demonstrated that the microsensor’s performance is great in terms of droplet size, velocity, and length, with a significant signal amplitude of capacitance and impedance and real-time detection capabilities, thereby highlighting its potential for facilitating microcapsule reactions and enabling on-site real-time detection for chemical and biosensor analyses on-chip. This droplet-based microfluidics platform has great potential to be directly employed to promote advances in biomedical research, pharmaceuticals, drug discovery, food engineering, flow chemistry, and cosmetics.

Droplet-based liquid microjunction surface sampling coupled with high-performance liquid chromatography (HPLC)-electrospray ionization (ESI)-tandem mass spectrometry (MS/MS) for spatially resolved analysis provides the possibility of effective analysis of complex matrix samples and can provide a greater degree of chemical information from a single spot sample than is typically possible with a direct analysis of an extract. Described here is the setup and enhanced capabilities of a discrete droplet liquid microjunction surface sampling system employing a commercially available CTC PAL autosampler. The system enhancements include incorporation of a laser distance sensor enabling unattended analysis of samples and sample locations of dramatically disparate height as well as reliably dispensing just 0.5 μL of extraction solvent to make the liquid junction to the surface, wherein the extraction spot size was confined to an area about 0.7 mm in diameter; software modifications improving the spatial resolution of sampling spot selection from 1.0 to 0.1 mm; use of an open bed tray system to accommodate samples as large as whole-body rat thin tissue sections; and custom sample/solvent holders that shorten sampling time to approximately 1 min per sample. The merit of these new features was demonstrated by spatially resolved sampling, HPLC separation, and mass spectral detection of pharmaceuticals and metabolites from whole-body rat thin tissue sections and razor blade (“crude”) cut mouse tissue. Graphical abstract Workflow of the droplet based liquid microjunction surface sampling process

Droplet-based microfluidics is a versatile tool to reveal the dose–response relationship of different effectors on the microbial proliferation. Traditional readout parameter is the temporal development of the cell density for different effector concentrations. To determine nonlinear or unconventional dose–response relationships, data with high temporal resolution and dense concentration graduation are essential. If microorganisms with slow microbial growth kinetics are investigated, a sterile and evaporation-free long-term incubation technique is required. Here, we present a modular droplet-based screening system which was developed to solve these issues. Beside relevant technical aspects of the developed modules, the procedural workflow, and exemplary dose–response data for 1D and 2D dose–response screenings are presented.

Droplet-based liquid microjunction surface sampling coupled with high-performance liquid chromatography (HPLC)-electrospray ionization (ESI)-tandem mass spectrometry (MS/MS) for spatially resolved analysis provides the possibility of effective analysis of complex matrix samples and can provide a greater degree of chemical information from a single spot sample than is typically possible with a direct analysis of an extract. Described here is the setup and enhanced capabilities of a discrete droplet liquid microjunction surface sampling system employing a commercially available CTC PAL autosampler. The system enhancements include incorporation of a laser distance sensor enabling unattended analysis of samples and sample locations of dramatically disparate height as well as reliably dispensing just 0.5 μL of extraction solvent to make the liquid junction to the surface, wherein the extraction spot size was confined to an area about 0.7 mm in diameter; software modifications improving the spatial resolution of sampling spot selection from 1.0 to 0.1 mm; use of an open bed tray system to accommodate samples as large as whole-body rat thin tissue sections; and custom sample/solvent holders that shorten sampling time to approximately 1 min per sample. Lastly, the merit of these new features was demonstrated by spatially resolved sampling, HPLC separation, and mass spectral detection of pharmaceuticals and metabolites from whole-body rat thin tissue sections and razor blade (“crude”) cut mouse tissue.

We are presenting a microfluidic droplet-based system for non-invasive, simultaneous optical monitoring of oxygen during bacterial cultivation in nL-sized droplets using ~350 nm nanobeads made from polystyrene and doped with the NIR-emitting oxygen probe platinum (II) 5, 10, 15, 20- meso -tetraphenyltetrabenzoporphyrin (PtTPTBP). Data were readout by a two-channel micro flow-through fluorimeter and a two-channel micro flow-through photometer. The time-resolved miniaturized optical multi endpoint detection was applied to simultaneously sense dissolved oxygen, cellular autofluorescence, and cell density in nL-sized segments. Two bacterial strains were studied that are resistant to heavy metal ions, viz. Streptomyces acidiscabies E13 and Psychrobacillus psychrodurans UrPLO1. The study has two main features in that it demonstrates (a) the possibility to monitor the changes in oxygen partial pressure during metabolic activity of different bacterial cultures inside droplets, and (b) the efficiency of droplet-based microfluidic techniques along with multi-parameter optical sensing for highly resolved microtoxicological screenings in aquatic systems. Graphical Abstract Microfluidic droplet-based system with multi-parameter optical sensing for bacterial cultivation and highly resolved microtoxicological screenings in nanoliter droplets.

Polymer microparticles containing an immobilized pH-sensitive dye are used for determination of pH inside microfluidic segments. The particles possess a hydrophilic surface in order to get a homogenous distribution inside the aqueous phase of microfluidic segments. The dye is coupled to the polymer matrix by a covalent bond. The pH can be determined by the read-out of fluorescence intensity. In contrast to dissolved indicator dyes, the chemical interference of the sensor particles with the surrounding solution is negligible. So, the particle-based sensing can easily be applied to the determination of pH changes during the cultivation of cells inside the microfluidic segments. The typical change of pH during cell cultivation can be used for monitoring the kinetic of cell cultivation inside single volumes in the nanoliter range, so that information about the metabolic activity of the organism can be obtained. An LED-based miniaturized flow-through fluorometer was developed to determine the fluorescence directly inside microtubes of an inner diameter of 0.5 mm. It allows measurement frequencies up to 60 Hz and is suited for characterization of fast moving large sequences of microfluidic segments.