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Nanocolloidal gels (NCGs) are an emerging class of soft matter, in which nanoparticles act as building blocks of the colloidal network. Chemical or physical crosslinking enables NCG synthesis and assembly from a broad range of nanoparticles, polymers, and low-molecular weight molecules. The synergistic properties of NCGs are governed by nanoparticle composition, dimensions and shape, the mechanism of nanoparticle bonding, and the NCG architecture, as well as the nature of molecular crosslinkers. Nanocolloidal gels find applications in soft robotics, bioengineering, optically active coatings and sensors, optoelectronic devices, and absorbents. This review summarizes currently scattered aspects of NCG formation, properties, characterization, and applications. We describe the diversity of NCG building blocks, discuss the mechanisms of NCG formation, review characterization techniques, outline NCG fabrication and processing methods, and highlight most common NCG applications. The review is concluded with the discussion of perspectives in the design and development of NCGs. [ABSTRACT FROM AUTHOR]

Organotypic micrometre-size 3D aggregates of skin cells (multicellular spheroids) have emerged as a promising in vitro model that can be utilized as an alternative of animal models to test active ingredients (AIs) of skincare products; however, a reliable dermal spheroid-based microfluidic (MF) model with a goal of in vitro AI screening is yet to be developed. Here, we report a MF platform for the growth of massive arrays of dermal fibroblast spheroids (DFSs) in a biomimetic hydrogel under close-to-physiological flow conditions and with the capability of screening AIs for skincare products. The DFSs formed after two days of on-chip culture and, in a case study, were used in a time-efficient manner for screening the effect of vitamin C on the synthesis of collagen type I and fibronectin. The computational simulation showed that the uptake of vitamin C was dominated by the advection flux. The results of screening the benchmark AI, vitamin C, proved that DFSs can serve as a reliable in vitro dermal model. The proposed DFS-based MF platform offers a high screening capacity for AIs of skincare products, as well as drug discovery and development in dermatology. [ABSTRACT FROM AUTHOR]

Colloidal stability of nanoparticles (NPs) strongly influences their synthesis, processing, and applications. For gold NPs stabilized with cetyl trimethylammonium bromide (CTAB) and polymer ligands we show that gradual increase in polarity of the water/aprotic solvent mixture leads to stabilization–aggregation–stabilization–aggregation transitions. We propose that these transitions are mediated by structural rearrangements of the CTAB layer on the NP surface. [ABSTRACT FROM AUTHOR]

Direct formic acid fuel cells hold great potential for utilizing formic acid as an energy source via formic acid oxidation (FAO). We report a new anodic material composed of branched Pd nanoparticles (BNPs) with enhanced performance for the electrocatalytic FAO reaction. The results of computational studies indicate that the surface morphology of the nanoparticles favours the binding of FAO intermediates while allowing for field-induced reagent concentration (FIRC) at sharp tips leading to amplified catalytic activity and improved stability. Our findings highlight the importance of morphological control of high-energy surfaces for effective fuel cell anodes. [ABSTRACT FROM AUTHOR]

Linear assemblies of nanoparticles show promising applications due to their collective electronic, optical and magnetic properties. Rational design and controllable organization of nanoparticles in one-dimensional structures can strongly benefit from the marked similarity between conventional step-growth polymerization reactions and directional step-wise assembly of nanoparticles in linear chains. Here we show different aspects of the “polymerization” approach to the solution-based self-assembly of polymer-functionalized metal nanoparticles with different chemical compositions, shapes and dimensions. The self-assembly was triggered by inducing solvophobic attraction between polymer ligands, due to the change in solvent quality. We show that both anisotropic (patchy) nanoparticles and nanoparticles uniformly capped with polymer molecules can self-assemble in linear chains. We explore the control of chain length, morphology, and composition, discuss the ability to form isotropic and hierarchical structures and show the properties and potential applications of linear assemblies of plasmonic nanoparticles. [ABSTRACT FROM AUTHOR]

Liquid–liquid extraction is an important separation and purification method; however, it faces a challenge in reducing the energy consumption and the environmental impact of solvent (extractant) recovery. The reversible chemical reactions of switchable solvents (nitrogenous bases) with carbon dioxide (CO2) can be implemented in reactive liquid–liquid extraction to significantly reduce the cost and energy requirements of solvent recovery. The development of new effective switchable solvents reacting with CO2 and the optimization of extraction conditions rely on the ability to evaluate and screen the performance of switchable solvents in extraction processes. We report a microfluidic strategy for time- and labour-efficient studies of CO2-mediated solvent extraction. The platform utilizes a liquid segment containing an aqueous extractant droplet and a droplet of a solution of a switchable solvent in a non-polar liquid, with gaseous CO2 supplied to the segment from both sides. Following the reaction of the switchable solvent with CO2, the solvent becomes hydrophilic and transfers from the non-polar solvent to the aqueous droplet. By monitoring the time-dependent variation in droplet volumes, we determined the efficiency and extraction time for the CO2-mediated extraction of different nitrogenous bases in a broad experimental parameter space. The platform enables a significant reduction in the amount of switchable solvents used in these studies, provides accurate temporal characterization of the liquid–liquid extraction process, and offers the capability of high-throughput screening of switchable solvents. [ABSTRACT FROM AUTHOR]

Adsorption of polymers from solutions moving past solid or liquid surfaces controls a broad range of phenomena in science, technology, and medicine. In the present work, a microfluidic methodology was developed to study polymer adsorption in flow under well-defined conditions by integrating an attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectrometer with a microfluidic device. Polymer adsorption in flow using exemplary polyelectrolytes such as polystyrene sulfonate and polyacrylic acid was studied under varying flow rates, polymer concentrations, pH values, and ionic strengths of the solution. Furthermore, the microfluidic platform was utilized to study layer-by-layer adsorption of alternating anionic and cationic polyelectrolytes such as polyacrylic acid and polyallylamine hydrochloride. The proposed methodology paves the way for studies of in-flow adsorption of biologically relevant molecules, which would mimic processes occurring in the cardiovascular microcirculation system. [ABSTRACT FROM AUTHOR]

We introduce oscillatory segmented flow as a compact microfluidic format that accommodates slow chemical reactions for the solution-phase processing of colloidal nanomaterials. The strategy allows the reaction progress to be monitored at a dynamic range of up to 80 decibels (i.e., residence times of up to one day, equivalent to 720–14 400 times the mixing time) from only one sensing location. A train of alternating gas bubbles and liquid reaction compartments (segmented flow) was initially formed, stopped and then subjected to a consistent back-and-forth motion. The oscillatory segmented flow was obtained by periodically manipulating the pressures at the device inlet and outlet via square wave signals generated by non-wetted solenoid valves. The readily implementable format significantly reduced the device footprint as compared with continuous segmented flow. We investigated mixing enhancement for varying liquid segment lengths, oscillation amplitudes and oscillation frequencies. The etching of gold nanorods served as a case study to illustrate the utility of the approach for dynamic characterization and precise control of colloidal nanomaterial size and shape for 5 h. Oscillatory segmented flows will be beneficial for a broad range of lab-on-a-chip applications that require long processing times. [ABSTRACT FROM AUTHOR]

Self-assembly of plasmonic nanoparticles offers a labour- and cost-efficient strategy for the expansion of the library of plasmonic nanostructures with highly tunable, coupled optical properties. This review covers recent advances in solution-based self-assembly of plasmonic nanoparticles, modelling of the self-assembly process and of the optical properties of the resulting nanostructures, and potential applications of self-assembled plasmonic nanostructures. [ABSTRACT FROM AUTHOR]

Microfluidics (MFs) offers a promising method for the preparation of polymer microgels with exquisite control over their dimensions, shapes and morphologies. A challenging task in this process is the generation of droplets (precursors for microgels) from highly viscous polymer solutions. Spatial separation of MF emulsification and gelation of the precursor droplets on chip can address this challenge. In the present work, we explored the application of the “direct injection” method for the preparation of microgels by adding a highly concentrated polymer solution or a gelling agent directly into the precursor droplets. In the first system, primary droplets were generated from a dilute aqueous solution of agarose, followed by the injection of the concentrated agarose solution directly in the primary droplets. The secondary droplets served as precursors for microgels. In the second system, primary droplets were generated from the low-viscous solution of methyl-β-cyclodextrin and poly(ethylene glycol) end-terminated with octadecyl hydrophobic groups. Addition of surfactant directly into the primary droplets led to the binding of methyl-β-cyclodextrin to the surfactant, thereby releasing hydrophobized poly(ethylene glycol) to form polymer microgels. Our results show that, when optimized, the direct injection method can be used for microgel preparation from highly viscous liquids and thus this method expands the range of polymers used for MF generation of microgels. [ABSTRACT FROM AUTHOR]

Generation of nanostructures containing from several to thousands of inorganic nanorods (NRs) organized in a highly ordered manner paves the way for applications that exploit directional properties of NR arrays. Self-assembly of NRs provides a simple and cost-efficient strategy for producing NR ensembles. This tutorial review highlights recent advances in the field of NR synthesis, summarizes the types of ligands used for NR synthesis and stabilization, reviews experimental and theoretical work on NRself-assembly that is driven by interactions between the ligands and describes current properties and applications of self-assembled NR structures. [ABSTRACT FROM AUTHOR]

Generation of nanostructures containing from several to thousands of inorganic nanorods (NRs) organized in a highly ordered manner paves the way for applications that exploit directional properties of NR arrays. Self-assembly of NRs provides a simple and cost-efficient strategy for producing NR ensembles. This tutorial reviewhighlights recent advances in the field of NR synthesis, summarizes the types of ligands used for NR synthesis and stabilization, reviews experimental and theoretical work on NR self-assembly that is driven by interactions between the ligands and describes current properties and applications of self-assembled NR structures. [ABSTRACT FROM AUTHOR]

We report an efficient method for the fabrication of microfluidic electrochemical reactors. The technique relies on soft lithography and micromolding in capillaries. Intrinsic to our method of fabrication is the capability of controlling the inter-electrode gap between 40 to 200 µm, and the ability to produce microchannels with complex geometries. The material selected to fabricate the reactor is resistant to most organic solvents, whereas its relative softness eliminates the need for additional sealants required by other methods for the fabrication of microfluidic electrochemical reactors. [ABSTRACT FROM AUTHOR]

Herein we demonstrate the photothermally-triggered selfassembly of poly(N-isopropylacrylamide)-functionalized gold nanorods in one-dimensional structures. [ABSTRACT FROM AUTHOR]

A microfluidic strategy has been used for the time- and labour-efficient evaluation of the relative efficiency and thermodynamic parameters of CO 2 binding by three Lewis acid/base combinations, where efficiency is based on the amount of CO 2 taken up per binding unit in solution. Neither t Bu 3 P nor B(C 6 F 5 ) 3 were independently effective at CO 2 capture, and the combination of the imidazolin-2-ylidenamino-substituted phosphine (NI i Pr) 3 P and B(C 6 F 5 ) 3 was equally ineffective. Nonetheless, an archetypal frustrated Lewis pair (FLP) comprised of t Bu 3 P and B(C 6 F 5 ) 3 was shown to bind CO 2 more efficiently than either the FLP derived from tetramethylpiperidine (TMP) and B(C 6 F 5 ) 3 or the highly basic phosphine (NI i Pr) 3 P. Moreover, the proposed microfluidic platform was used to elucidate the thermodynamic parameters for these reactions.

Capitalizing on the benefits of microscale segmented flows, e.g., enhanced mixing and reduced sample dispersion, so far requires specialist training and accommodating a few experimental inconveniences. For instance, microscale gas-liquid flows in many current setups take at least 10 min to stabilize and iterative manual adjustments are needed to achieve or maintain desired mixing or residence times. Here, we report a cruise control strategy that overcomes these limitations and allows microscale gas-liquid (bubble) and liquid-liquid (droplet) flow conditions to be rapidly "adjusted" and maintained. Using this strategy we consistently establish bubble and droplet flows with dispersed phase (plug) velocities of 5-300 mm s(-1), plug lengths of 0.6-5 mm and continuous phase (slug) lengths of 0.5-3 mm. The mixing times (1-5 s), mass transfer times (33-250 ms) and residence times (3-300 s) can therefore be directly imposed by dynamically controlling the supply of the dispersed and the continuous liquids either from external pumps or from local pressurized reservoirs. In the latter case, no chip-external pumps, liquid-perfused tubes or valves are necessary while unwanted dead volumes are significantly reduced.

We present an automated microfluidic (MF) approach for the systematic and rapid investigation of carbon dioxide (CO(2)) mass transfer and solubility in physical solvents. Uniformly sized bubbles of CO(2) with lengths exceeding the width of the microchannel (plugs) were isothermally generated in a co-flowing physical solvent within a gas-impermeable, silicon-based MF platform that is compatible with a wide range of solvents, temperatures and pressures. We dynamically determined the volume reduction of the plugs from images that were accommodated within a single field of view, six different downstream locations of the microchannel at any given flow condition. Evaluating plug sizes in real time allowed our automated strategy to suitably select inlet pressures and solvent flow rates such that otherwise dynamically self-selecting parameters (e.g., the plug size, the solvent segment size, and the plug velocity) could be either kept constant or systematically altered. Specifically, if a constant slug length was imposed, the volumetric dissolution rate of CO(2) could be deduced from the measured rate of plug shrinkage. The solubility of CO(2) in the physical solvent was obtained from a comparison between the terminal and the initial plug sizes. Solubility data were acquired every 5 min and were within 2-5% accuracy as compared to literature data. A parameter space consisting of the plug length, solvent slug length and plug velocity at the microchannel inlet was established for different CO(2)-solvent pairs with high and low gas solubilities. In a case study, we selected the gas-liquid pair CO(2)-dimethyl carbonate (DMC) and volumetric mass transfer coefficients 4-30 s(-1) (translating into mass transfer times between 0.25 s and 0.03 s), and Henry's constants, within the range of 6-12 MPa.

We demonstrate continuous flow acid-base titration reactions as an educational microfluidic platform for undergraduate and graduate analytical chemistry courses. A series of equations were developed for controlling and predicting the results of acid-base neutralisation reactions conducted in a microfluidic format, including the combinations of (i) a strong base and a strong acid, (ii) a strong base and a weak acid, and (iii) a strong base and a multiprotic acid. Microfluidic titrations yielded excellent repeatability. The small experimental footprint is advantageous in crowded teaching laboratories, and it offers limited waste and exposure to potentially hazardous acids and bases. This platform will help promote the utilisation of microfluidics at an earlier stage of students' careers.

We report the development of a versatile microfluidic (MF) reactor with multiple analytical probes, which can be used for (i) quantitative characterisation of molecular vibrational signatures of reactants or products, (ii) the localised real-time monitoring of temperature and (iii) site-specific measurements of pH of the reaction system. The analytical probes utilised for in situ reaction analysis include an ATR-FTIR probe, a temperature probe, and a pH probe. We demonstrate the applications of the MF reactor with integrated probes for the parallel monitoring of multiple variables in acid/base neutralisation reaction, of changes in buffer pH, temperature, and vibrational absorption bands, and for monitoring the kinetics of the reaction between CO(2) and a buffer system with therapeutic applications.

This communication describes a novel strategy for the continuous microfluidic generation of highly monodispersed particle-coated microbubbles using temperature-dependent dissolution of carbon dioxide.

We report a microfluidic (MF) approach to studies of temperature mediated carbon dioxide (CO(2)) transfer between the gas and the liquid phases. Micrometre-diameter CO(2) bubbles with a narrow size distribution were generated in an aqueous or organic liquid and subsequently were subjected to temperature changes in the downstream channel. In response to the cooling-heating-cooling cycle the bubbles underwent corresponding contraction-expansion-contraction transitions, which we term 'bubble breathing'. We examined temperature-controlled dissolution of CO(2) in four exemplary liquid systems: deionized water, a 0.7 M aqueous solution of NaCl, ocean water extracted from Bermuda coastal waters, and dimethyl ether of poly(ethylene glycol), a solvent used in industry for absorption of CO(2). The MF approach can be extended to studies of other gases with a distinct, temperature-dependent solubility in liquids.

We report a cost-efficient Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) method for monitoring concentrations of solutes in solutions flowing through microfluidic channels. The method allows rapid acquisition of spectra and enables chemical characterisation and concentration measurements that are independent of the flow rate of liquids. The method enables independent measurement of concentrations of solutes with distinct spectral features in mixed solutions. For the polymer solutes studied in the present work, the method has a sensitivity of at least 10 microM (0.01 wt%). We also propose the applicability of the method for the differentiation between dissolved and adsorbed amphiphilic species.

Herein we demonstrate a new, straightforward method for the synthesis of macroporous hybrid polymers coated with gold nanorods, which are strongly and uniformly attached to the surface of pores.

We report a cost-efficient and easy to implement process for fabricating microfluidic reactors in thermoplastic materials. The method includes (i) the fabrication of an imprint template (master), which consists of a photoresist deposited on a metal plate; (ii) the thermoembossing of the reactor features into polymer sheets; (iii) the activation of the embossed and planar thermoplastic surfaces; and (iv) the low-temperature bonding of these surfaces. The generality of the method is established by fabricating microfluidic reactors with a complex geometry in a range of thermoplastic polymers, including cycloolefin, polycarbonate, and UV-transparent acrylic polymers and by the multiple, high-fidelity use of the master.

We report a study of the continuous generation of polymer particles in parallel multiple modular microfluidic (M3) reactors. Each module consisted of sixteen parallel microfluidic reactors comprising emulsification and polymerization compartments. We identified and minimized the effects of the following factors that could result in the broadening of the distribution of sizes of the particles synthesized in the M3 reactors, in comparison with an individual microfluidic reactor: (i) the fidelity in the fabrication of multiple microfluidic droplet generators; (ii) the crosstalk between parallel droplet generators sharing liquid supply sources; and (iii) the coalescence of precursor droplets and/or partly polymerized polymer particles. Our results show that the M3 reactors can produce polymer microgel particles with polydispersity not exceeding 5% at a productivity of approximately 50 g/h.

We investigated the flow dynamics of biotin-conjugated microgel capsules in avidin-conjugated microchannel constrictions. Microgels were prepared using a microfluidic assembly approach. Biotinylated microgels passing through avidin-modified constrictions slowed relative to several control systems. This effect was observed below a critical velocity of the microgels in the channel-at-large. The reduction in microgel velocity in the constriction occurred for several different sizes of microgels and orifices. Soft compliant microgels showed a lower velocity in the constriction relative to rigid microgels with the same concentration of biotin on the surface, due to the ability of the softer microgels to deform in the orifice and maximize their surface area when in contact with the orifice wall.

This paper describes geometric coupling of the dynamics of break-up of liquid threads in parallel flow-focusing devices (FFD), which are integrated into a multiple quadruple-microfluidic droplet generator (QDG). We show weak parametric coupling between parallel FFDs with an identical design, which leads to the slight broadening of the distribution of sizes of droplets. Using parallel FFDs with distinct geometries we simultaneously generated several populations of droplets with different volumes, yet, each of these populations was characterized by a narrow size distribution. Simulation of the generation of droplets in the quadruple-microfluidic droplet generator based on hydraulic resistances to the flow of a single-phase fluid was in good agreement with the experimental results.

This article describes emulsification in a microfluidic double droplet generator (DDR) comprising two consecutive flow-focusing devices with locally modified surface chemistry. We generated W/O/W, O/O/W and O/W/O double emulsions with precisely controlled sizes and morphology of droplets. Secondly, by combining two mechanisms of droplet formation (the flow-focusing mechanism and the break up of liquid threads at T-junction) we produced multiple populations of droplets with varying size and/or composition. These droplets were used as the structural units for the formation of complex dynamic lattices.

We investigated the flow dynamics of microgel capsules in topographically patterned microfluidic devices. For microgels flowing through channel constrictions, or orifices, we observed three phenomena: (i) the effect of confinement, (ii) the role of interactions between the microgels and the channel surface, and (iii) the effect of the velocities of microgels prior to their passage through an orifice. We studied negatively charged alginate microgels and positively charged alginate microgels coated with N-(2-hydroxy)propyl-3-trimethylammonium chitosan chloride (HTCC). Aqueous dispersions of microgels were driven through poly(dimethyl siloxane) microchannels carrying a weak negative surface charge. The velocity of the continuous phase, and hence, the velocity of the microgels increased as they passed through topographically patterned orifices. Alginate microgels were observed to have a larger increase in velocity relative to HTCC-coated alginate microgels. This effect, which was attributed to electrostatic attraction or repulsion, was found to be strongest for orifices with dimensions close to the microgel diameter. For example, when 75 microm-diameter microgels flowed through a 76 microm orifice, alginate gels (negatively charged) experienced a 2x greater increase in velocity than HTCC-coated (positively charged) microgels. This effect was exaggerated at lower initial flow rates. For example, when 75 microm-diameter microgels flowed through an 80 microm orifice, a two-fold difference in the velocity changes of the two microgel types was observed when the initial flow rate was 275 microm s(-1), while a three-fold difference in velocity changes was observed when the initial flow rate was 130 microm s(-1). We speculate that these studies will be useful for modeling the flow of suspensions of cells or other biologically relevant particles for a wide range of applications.

The refractive index, molar refraction and Abbe number of polyferrocene derivatives are reported and the values indicate that these materials are very promising for a range of photonics applications.