SERS-active linear barcodes by microfluidic-assisted patterning.

Simple, low-cost, robust, and scalable fabrication of microscopic linear barcodes with high levels of complexity and multiple authentication layers is critical for emerging applications in information security and anti-counterfeiting. This manuscript presents a novel approach for fabrication of microscopic linear barcodes that can be visualized under Raman microscopy. Microfluidic channels are used as molds to generate linear patterns of end-grafted polymers on a substrate. These patterns serve as templates for area-selective binding of colloidal gold nanoparticles resulting in plasmonic arrays. The deposition of multiple taggant molecules on the plasmonic arrays via a second microfluidic mold results in a linear barcode with unique Raman fingerprints that are enhanced by the underlying plasmonic nanoparticles. The width of the bars is as small as 10 µm, with a total barcode length on the order of 100 µm. The simultaneous use of geometric and chemical security layers provides a high level of complexity challenging the counterfeiting of the barcodes. The additive, scalable, and inexpensive nature of the presented approach can be easily adapted to different colloidal nanomaterials and applications.

In vitro analysis of multiple blood flow determinants using red blood cell dynamics under oscillatory flow.

The flow behavior of blood is determined mainly by red blood cell (RBC) deformation and aggregation as well as blood viscoelasticity. These intricately interdependent parameters should be monitored by healthcare providers to understand all aspects of circulatory flow dynamics under numerous cases including cardiovascular and infectious diseases. Current medical instruments and microfluidic systems lack the ability to quantify these parameters all at once and in physiologically relevant flow conditions. This work presents a handheld platform and a measurement method for quantitative analysis of multiple of these parameters from 50 µl undiluted blood inside a miniaturized channel. The assay is based on an optical transmission analysis of collective RBC deformation and aggregation under near-infrared illumination during a 1 s damped oscillatory flow and at stasis, respectively. Measurements with blood of different hemo-rheological properties demonstrate that the presented approach holds a potential for initiating simultaneous and routine on-chip blood flow analysis even in resource-poor settings.

Electro-Viscoelastic Migration under Simultaneously Applied Microfluidic Pressure-Driven Flow and Electric Field.

Under the simultaneous use of pressure-driven flow and DC electric field, migration of particles inside microfluidic channels exhibits intricate focusing dynamics. Available experimental and analytical studies fall short in giving a thorough explanation to particle equilibrium states. Also, the understanding is so far limited to the results based on Newtonian and neutral viscoelastic carrier fluids. Hence, a holistic approach is taken in this study to elaborate the interplay of governing electrophoretic and slip-induced/elastic/shear gradient lift forces. First, we carried out experimental studies on particle migration in Newtonian, neutral viscoelastic, and polyelectrolyte viscoelastic media to provide a comprehensive understanding of particle migration. The experiments with the viscoelastic media led to contradictory results with the existing explanations. Then, we introduced the Electro-Viscoelastic Migration (EVM) theory to give a unifying explanation to particle migration in Newtonian and viscoelastic solutions. Confocal imaging with fluorescent-labeled polymer solutions was used to explore the underlying migration behavior. A surprising outcome of our results is the formation of cross-sectionally nonuniform viscoelasticity that may have unique applications in microfluidic particle focusing.

Real-time impedimetric droplet measurement (iDM).

Droplet-based microfluidic systems require a precise control of droplet physical properties; hence, measuring the morphological properties of droplets is critical to obtain high sensitivity analysis. The ability to perform such measurements in real-time is another demand which has not been addressed yet. In this study, we used coplanar electrodes configured in the differential measurement mode for impedimetric measurement of size and velocity. To obtain the size of the droplets, detailed 3D finite element simulations of the system were performed. The interaction of the non-uniform electric field and the droplet was investigated. Electrode geometry optimization steps were described and design guideline rules were laid out. User-friendly software was developed for real-time observation of droplet length and velocity together with in situ statistical analysis results. A comparison between impedimetric and optical measurement tools is given. Finally, to illustrate the benefit of having real-time analysis, iDM was used to synthesize particles with a predefined monodispersity limit and to study the response times of syringe pump and pressure pump driven droplet generation devices. This analysis allows one to evaluate the 'warm-up' time for a droplet generator system, after which droplets reach the desired steady-state size required by the application of interest.

Reversible decryption of covert nanometer-thick patterns in modular metamaterials.

Continuous development of security features is mandatory for the fight against forgery of valuable documents and products, the most notable example being banknotes. Such features demonstrate specific properties under certain stimuli such as fluorescent patterns glowing under ultraviolet light. These security features should also be hard to copy by unlicensed people and be interrogated by anyone using easily accessible tools. To this end, this Letter demonstrates the development of an ideal security feature enabled by the realization of modular metamaterials based on metal-dielectric-metal cavities that consist of two separate parts: metal nanoparticles on an elastomeric substrate and a bottom mirror coated with a thin dielectric. Patterns generated by creating nanometer-thick changes in the dielectric layer are invisible (encrypted) and can only be detected (decrypted) by sticking the elastomeric patch on. The observed optical effects such as visibility and colors can only be produced with the correct combination of materials and film thicknesses, making the proposed structures a strong alternative to compromised security features.

Impedance-based viscoelastic flow cytometry.

Elastic nature of the viscoelastic fluids induces lateral migration of particles into a single streamline and can be used by microfluidic based flow cytometry devices. In this study, we investigated focusing efficiency of polyethylene oxide based viscoelastic solutions at varying ionic concentration to demonstrate their use in impedimetric particle characterization systems. Rheological properties of the viscoelastic fluid and particle focusing performance are not affected by ionic concentration. We investigated the viscoelastic focusing dynamics using polystyrene (PS) beads and human red blood cells (RBCs) suspended in the viscoelastic fluid. Elasto-inertial focusing of PS beads was achieved with the combination of inertial and viscoelastic effects. RBCs were aligned along the channel centerline in parachute shape which yielded consistent impedimetric signals. We compared our impedance-based microfluidic flow cytometry results for RBCs and PS beads by analyzing particle transit time and peak amplitude at varying viscoelastic focusing conditions obtained at different flow rates. We showed that single orientation, single train focusing of nonspherical RBCs can be achieved with polyethylene oxide based viscoelastic solution that has been shown to be a good candidate as a carrier fluid for impedance cytometry.

Sheathless Microflow Cytometry Using Viscoelastic Fluids.

Microflow cytometry is a powerful technique for characterization of particles suspended in a solution. In this work, we present a microflow cytometer based on viscoelastic focusing. 3D single-line focusing of microparticles was achieved in a straight capillary using viscoelastic focusing which alleviated the need for sheath flow or any other actuation mechanism. Optical detection was performed by fiber coupled light source and photodetectors. Using this system, we present the detection of microparticles suspended in three different viscoelastic solutions. The rheological properties of the solutions were measured and used to assess the focusing performance both analytically and numerically. The results were verified experimentally, and it has been shown that polyethlyene oxide (PEO) and hyaluronic acid (HA) based sheathless microflow cytometer demonstrates similar performance to state-of-the art flow cytometers. The sheathless microflow cytometer was shown to present 780 particles/s throughput and 5.8% CV for the forward scatter signal for HA-based focusing. The presented system is composed of a single capillary to accommodate the fluid and optical fibers to couple the light to the fluid of interest. Thanks to its simplicity, the system has the potential to widen the applicability of microflow cytometers.

Perfectly absorbing ultra thin interference coatings for hydrogen sensing.

Here we numerically demonstrate a straightforward method for optical detection of hydrogen gas by means of absorption reduction and colorimetric indication. A perfectly absorbing metal-insulator-metal (MIM) thin film interference structure is constructed using a silver metal back reflector, silicon dioxide insulator, and palladium as the upper metal layer and hydrogen catalyst. The thickness of silicon dioxide allows the maximizing of the electric field intensity at the Air/SiO2 interface at the quarter wavelengths and enabling perfect absorption with the help of highly absorptive palladium thin film (∼7 nm). While the exposure of the MIM structure to H2 moderately increases reflection, the relative intensity contrast due to formation of metal hydride is extensive. By modifying the insulator film thickness and hence the spectral absorption, the color is tuned and eye-visible results are obtained.