Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development We believe this review will promote communications among biology, chemistry, physics, and materials science

Emerging Droplet Microfluidics

Fogging occurs when moisture condensation takes the form of accumulated droplets with diameters larger than 190 nm or half of the shortest wavelength (380 nm) of visible light. This problem may be effectively addressed by changing the affinity of a material’s surface for water, which can be accomplished via two approaches: i) the superhydrophilic approach, with a water contact angle (CA) less than 5°, and ii) the superhydrophobic approach, with a water CA greater than 150°, and extremely low CA hysteresis. To date, all techniques reported belong to the former category, as they are intended for applications in optical transparent coatings. A well-known example is the use of photocatalytic TiO2 nanoparticle coatings that become superhydrophilic under UV irradiation. Very recently, a capillary effect was skillfully adopted to achieve superhydrophilic properties by constructing 3D nanoporous structures from layer-by-layer assembled nanoparticles. The key to these two “wet”-style antifogging strategies is for micrometer-sized fog drops to rapidly spread into a uniform thin film, which can prevent light scattering and reflection from nucleated droplets. Optical transparency is not an intrinsic property of antifogging coatings even though recently developed antifogging coatings are almost transparent, and the transparency could be achieved by further tuning the nanoparticle size and film thickness. To our knowledge, the antifogging coatings may also be applied to many fields that do not require optical transparency, including, for example, paints for inhibiting swelling and peeling issues and metal surfaces for preventing corrosion. These types of issues, which are caused by adsorption of moisture, are hard to solve by the superhydrophilic approach because of its inherently “wet” nature. Thus, a “dry”-style antifogging strategy, which consists of a novel superhydrophobic technique that can prevent moisture or microscale fog drops from nucleating on a surface, is desired. Recent bionic researches have revealed that the self-cleaning ability of lotus leaves and the striking ability of a water-strider’s legs to walk on water can be attributed to the ideal superhydrophobicity of their surfaces, induced by special microand nanostructures. To date, the biomimetic fabrication of superhydrophobic microand/or nanostructures has attracted considerable interest, and these types of materials can be used for such applications as self-cleaning coatings and stain-resistant textiles. Although a superhydrophobic technique inspired by lotus leaves is expected to be able to solve such fogging problems because the water droplets can not remain on the surface, there are no reports of such antifogging coatings. Very recently, researchers from General Motors have reported that the surfaces of lotus leaves become wet with moisture because the size of the fog drops are at the microscale—so small that they can be easily trapped in the interspaces among micropapillae. Thus, lotuslike surface microstructures are unsuitable for superhydrophobic antifogging coatings, and a new inspiration from nature is desired for solving this problem. In this communication, we report a novel, biological, superhydrophobic antifogging strategy. It was found that the compound eyes of the mosquito C. pipiens possess ideal superhydrophobic properties that provide an effective protective mechanism for maintaining clear vision in a humid habitat. Our research indicates that this unique property is attributed to the smart design of elaborate microand nanostructures: hexagonally non-close-packed (ncp) nipples at the nanoscale prevent microscale fog drops from condensing on the ommatidia surface, and hexagonally close-packed (hcp) ommatidia at the microscale could efficiently prevent fog drops from being trapped in the voids between the ommatidia. We also fabricated artificial compound eyes by using soft lithography and investigated the effects of microand nanostructures on the surface hydrophobicity. These findings could be used to develop novel superhydrophobic antifogging coatings in the near future. It is known that mosquitoes possess excellent vision, which they exploit to locate various resources such as mates, hosts, and resting sites in a watery and dim habitat. To better understand such remarkable abilities, we first investigated the interaction between moisture and the eye surface. An ultrasonic humidifier was used to regulate the relative humidity of the atmosphere and mimic a mist composed of numerous tiny water droplets with diameters less than 10 lm. As the fog was C O M M U N IC A IO N

The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography: a superhydrophobic state with high adhesive force

This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead-in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors.

Photonic Crystals for Chemical Sensing and Biosensing

This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions

Advances in colloidal assembly: the design of structure and hierarchy in two and three dimensions.

Supported lipid bilayers (SLBs) are popular models of cell membranes with potential biotechnological applications, yet the mechanism of SLB formation is only partially understood. In this study, the adsorption and subsequent conformational changes of sonicated unilamellar vesicles on silica supports were investigated by quartz crystal microbalance with dissipation monitoring and atomic force microscopy, using mixtures of zwitterionic, negatively charged, and positively charged lipids, both in the presence and in the absence of Ca 2 + ions. Four different pathways of vesicle deposition could be distinguished. Depending on their charge, vesicles i), did not adsorb; ii), formed a stable vesicular layer; or iii), decomposed into an SLB after adsorption at high critical coverage or iv), at low coverage. Calcium was shown to enhance the tendency of SLB formation for negatively charged and zwitterionic vesicles. The role of vesicle-support, interbilayer, and intrabilayer interactions in the formation of SLBs is discussed.

Pathways of lipid vesicle deposition on solid surfaces: A combined QCM-D and AFM study

Graphene as a transducer material has produced some of the best-performing sensing approaches to date opening the door toward integrated miniaturized all-carbon point-of-care devices. Addressing this opportunity, laser-scribed graphene (LSG) electrodes are demonstrated here as highly sensitive and reliable biosensor transducers in blood serum analysis. These flexible electrodes with large electrochemical surface areas were fabricated using a direct-write laser process on polyimide foils. A universal immobilization approach is established by anchoring 1-pyrenebutyric acid to the graphene and subsequently covalently attaching an aptamer against the coagulation factor thrombin as an exemplary bioreceptor to the carboxyl groups. The resulting biosensor displays extremely low detection limits of 1 pM in buffer and 5 pM in the complex matrix of serum.

Laser-Scribed Graphene Electrodes for Aptamer-Based Biosensing

Monodisperse, highly negatively charged, cross-linked polystyrene nanoparticles with diameters between 80 and 120 nm have been incorporated into a polyacrylamide hydrogel, where they display an iridescent color that conventionally is attributed to the so-called photonic crystal effect. The film is of red color if placed in plain water but turns to green in the presence of a 1 mM solution of an electrolyte such as sodium chloride and to purple in 100 mM solutions of electrolytes. Quantitative reflection spectroscopy was performed at various wavelengths and resulted in plots of reflected light wavelength versus ionic strength (IS) that are almost linear in the logarithmic concentration range from 5 × 10–5 to 10–2 mol·L–1. We show that such films are capable of monitoring the IS of aqueous solutions in the pH range from 5 to 9. We also show that, in addition to visual and instrumental readout, the sensor films can be analyzed with a digital camera at fixed angle. The digital images were separated into their ...

Optical sensing of the ionic strength using photonic crystals in a hydrogel matrix.

(Bio)analytical applications often require efficient signal enhancement strategies that are directly integrated into the biorecognition process. Nanomaterials are increasingly used for this purpose, as their small size provides numerous unique properties. This review identifies recent trends in (bio)analytical sensing and discusses the use of nanomaterials as signal enhancement tags and as carriers in enzymatic amplification enabling significantly higher performance compared to traditional assays. Gold nanoparticles are one of the most established amplification tools and still play a major role in the development of highly sensitive (bio)assays. These are based on signal enhancement for detection approaches that range from colorimetric over plasmonic and electrochemical to mass spectrometric systems. We also discuss the opportunities of other nanomaterials such as nanocomposites including carbon nanotubes and graphene, magnetite nanoparticles, polymeric nanomaterials, liposomes and quantum dots that are increasingly used in this field. Based on the observed trends, future perspectives in (bio)assay development are presented.

Nanomaterials as versatile tools for signal amplification in (bio)analytical applications

Monodisperse polystyrene nanoparticles with a diameter of 173 nm were incorporated into a polydimethylsiloxane matrix where they display an iridescent color that can be attributed to the photonic crystal effect. The film is of violet color if placed in plain water, but turns to red in the presence of the non-polar solvent n-hexane. Several solvents were studied in some detail. We show that such films are capable of monitoring the water content of ethanol/water mixtures, where only 1% (v/v) of water leads to a shift of the peak wavelength of reflected light by 5 nm. The method also can be applied to determine, both visually and instrumentally, the fraction of methanol in ethanol/methanol mixtures. Here, a fraction of 1% of methanol (v/v) results in a wavelength shift of 2 nm. The reflected wavelength is not influenced by temperature changes nor impeded by photobleaching. The signal changes are fully reversible and response times are <1 s.

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Christoph Fenzl

Papers

Photonic Crystals for Chemical Sensing and Biosensing

Laser-Scribed Graphene Electrodes for Aptamer-Based Biosensing

Optical sensing of the ionic strength using photonic crystals in a hydrogel matrix.

Nanomaterials as versatile tools for signal amplification in (bio)analytical applications

Photonic Crystal Based Sensor for Organic Solvents and for Solvent-Water Mixtures