Immunoassays on thiol-ene synthetic paper generate a superior fluorescence signal

Methods to spontaneously generate three dimensionally arrayed microdroplets triggered by capillarity for bioassays and bioengineering

Herein, I review our recent work toward developing methods for generating three-dimensional (3D) droplet arrays driven by capillarity. Microdroplet array-based systems are useful for bioassays and bioengineering because they require only small amounts of samples and reagents and provide the high throughput. Various methods have been developed for preparing droplet arrays, among which methods based on capillarity have attracted considerable attention owing to their simplicity. I and collaborators have developed such methods based on capillary flow, including a method for preparing droplet arrays via oil-water replacement. We recently proposed our own concept of "fluid-fluid interfacial energy driven 3D structure emergence in a micropillar scaffold (FLUID3EAMS)" and its application. FLUID3EAMS allows a 3D droplet (or hydrogel bead) array to be generated in a micropillar scaffold by passing a fluid-fluid interface through the scaffold. This approach is useful for applications requiring ordered or arrayed microdroplets in biosensors, biophysics, biology, and tissue engineering. This review is an extended version of the article "FLUID3EAMS: Fluid-Fluid Interfacial Energy Driven 3D Structure Emergence in a Micropillar Scaffold and Development in Bioengineering" published in Seibutsu Butsuri (vol. 62, p. 110-113, 2022).

FLUIDOT: A Modular Microfluidic Platform for Single-Cell Study and Retrieval, with Applications in Drug Tolerance Screening and Antibody Mining

Advancements in lab-on-a-chip technologies have revolutionized the single-cell analysis field. However, an accessible platform for in-depth screening and specific retrieval of single cells, which moreover enables studying diverse cell types and performing various downstream analyses, is still lacking. As a solution, FLUIDOT is introduced, a versatile microfluidic platform incorporating customizable microwells, optical tweezers and an interchangeable cell-retrieval system. Thanks to its smart microfluidic design, FLUIDOT is straightforward to fabricate and operate, rendering the technology widely accessible. The performance of FLUIDOT is validated and its versatility is subsequently demonstrated in two applications. First, drug tolerance in yeast cells is studied, resulting in the discovery of two treatment-tolerant populations. Second, B cells from convalescent COVID-19 patients are screened, leading to the discovery of highly affine, in vitro neutralizing monoclonal antibodies against SARS-CoV-2. Owing to its performance, flexibility, and accessibility, it is foreseen that FLUIDOT will enable phenotypic and genotypic analysis of diverse cell samples and thus elucidate unexplored biological questions.

Use of Polymer Micropillar Arrays as Templates for Solid-Phase Immunoassays

We investigate the use of periodic micropillar arrays produced by high-fidelity microfabrication with cyclic olefin polymers for solid-phase immunoassays. These three-dimensional (3D) templates offer higher surface-to-volume ratios than two-dimensional substrates, making it possible to attach more antibodies and so increase the signal obtained by the assay. Micropillar arrays also provide the capacity to induce wicking, which is used to distribute and confine antibodies on the surface with spatial control. Micropillar array substrates are modified by using oxygen plasma treatment, followed by grafting of (3-aminopropyl)triethoxysilane for binding proteins covalently using glutaraldehyde as a cross-linker. The relationship between microstructure and fluorescence signal was investigated through variation of pitch (10-50 μm), pillar diameter (5-40 μm), and pillar height (5-57 μm). Our findings suggest that signal intensity scales proportionally with the 3D surface area available for performing solid-phase immunoassays. A linear relationship between fluorescence intensity and microscale structure can be maintained even when the aspect ratio and pillar density both become very high, opening the possibility of tuning assay response by design such that desired signal intensity is obtained over a wide dynamic range compatible with different assays, analyte concentrations, and readout instruments. We demonstrate the versatility of the approach by performing the most common immunoassay formats-direct, indirect, and sandwich-in a qualitative fashion by using colorimetric and fluorescence-based detection for a number of clinically relevant protein markers, such as tumor necrosis factor alpha, interferon gamma (IFN-γ), and spike protein of severe acute respiratory syndrome coronavirus 2. We also show quantitative detection of IFN-γ in serum using a fluorescence-based sandwich immunoassay and calibrated samples with spike-in concentrations ranging from 50 pg/mL to 5 μg/mL, yielding an estimated limit of detection of ∼1 pg/mL for arrays with high micropillar density (11561 per mm2) and aspect ratio (1:11.35).

Off-Stoichiometry Thiol–Enes Polymers Containing Silane Groups for Advanced Packaging Technologies

New modified off-stoichiometry thiol–enes polymers, called OSTE-MS polymers, were developed by introducing mercaptosilane into the polymer mixture. This modification made it possible to introduce silane groups into the polymer frame, due to which the polymer gained the ability to bond with silicon wafers without modification of the wafer surface by any adhesive. The optimal composition for creating 3D polymer structures on a chip was selected, which consists of a volume ratio of 6:6:1 of allyl monomer, mercapto monomer, and mercaptosilane, respectively. The hardness, shift force, tensile strength, Young’s modulus, optical transparency, glass transition temperature, thermal stability, and chemical resistance of the OSTE-MS polymer, and the viscosity for the prepolymer mixture were studied. On the basis of the OSTE-MS polymer, 3D polymer structures of the well type and microfluidic system on the silicon chips were obtained.

Dipeptide nanoparticle and aptamer-based hybrid fluorescence platform for enrofloxacin determination

A novel fluorescence platform was fabricated for enrofloxacin determination by using cDNA-modified dipeptide fluorescence nanoparticles (FDNP-cDNA) and aptamer-modified magnetic Fe₃O₄ nanoparticles (Fe₃O₄-Apt). The FDNP were prepared via tryptophan-phenylalanine self-assembling. When magnetic Fe₃O₄-Apt incubated with standard solution or sample extracts, the target enrofloxacin was selectively captured by the aptamer on the surface of the Fe₃O₄ nanoparticles. After removing interference by washing with phosphate-buffered saline, the FDNP-cDNA was added, which can bind to the aptamer on the surface of the Fe₃O₄ nanoparticles not occupied by the analyte. The higher the concentration of the target enrofloxacin in the standard or sample solution is, the less the FDNP-cDNA can be bound with the Fe₃O₄ nanoparticles, and the more the FDNP-cDNA can be observed in the supernatant. Fluorescence intensity (E_x/E_m = 310/380 nm) increased linearly in the enrofloxacin concentration range 0.70 to 10.0 ng/mL with a detection limit of 0.26 ng/mL (S/N = 3). Good recoveries (88.17-99.30%) were obtained in spiked lake water, chicken, and eel samples with relative standard deviation of 2.7-6.2% (n = 3).

High-Performance Passive Plasma Separation on OSTE Pillar Forest

Plasma separation is of high interest for lateral flow tests using whole blood as sample liquids. Here, we built a passive microfluidic device for plasma separation with high performance. This device was made by blood filtration membrane and off-stoichiometry thiol-ene (OSTE) pillar forest. OSTE pillar forest was fabricated by double replica moldings of a laser-cut polymethylmethacrylate (PMMA) mold, which has a uniform microstructure. This device utilized a filtration membrane to separate plasma from whole blood samples and used hydrophilic OSTE pillar forest as the capillary pump to propel the plasma. The device can be used to separate blood plasma with high purity for later use in lateral flow tests. The device can process 45 μL of whole blood in 72 s and achieves a plasma separation yield as high as 60.0%. The protein recovery rate of separated plasma is 85.5%, which is on par with state-of-the-art technologies. This device can be further developed into lateral flow tests for biomarker detection in whole blood.

Addressing the Theoretical and Experimental Aspects of Low-Dimensional-Materials-Based FET Immunosensors: A Review

Electrochemical immunosensors (EI) have been widely investigated in the last several years. Among them, immunosensors based on low-dimensional materials (LDM) stand out, as they could provide a substantial gain in fabricating point-of-care devices, paving the way for fast, precise, and sensitive diagnosis of numerous severe illnesses. The high surface area available in LDMs makes it possible to immobilize a high density of bioreceptors, improving the sensitivity in biorecognition events between antibodies and antigens. If on the one hand, many works present promising results in using LDMs as a sensing material in EIs, on the other hand, very few of them discuss the fundamental interactions involved at the interfaces. Understanding the fundamental Chemistry and Physics of the interactions between the surface of LDMs and the bioreceptors, and how the operating conditions and biorecognition events affect those interactions, is vital when proposing new devices. Here, we present a review of recent works on EIs, focusing on devices that use LDMs (1D and 2D) as the sensing substrate. To do so, we highlight both experimental and theoretical aspects, bringing to light the fundamental aspects of the main interactions occurring at the interfaces and the operating mechanisms in which the detections are based.

Controlling Capillary Flow Rate on Lateral Flow Test Substrates by Tape

Controlling capillary flow rate of sample liquid is of high interest for lateral flow tests, since the flow rate can affect the dissolution and mixing of the immunoreagents and the efficiency of immunoreactions. Here we develop a facile method to adjust the capillary flow rate on lateral flow test substrates by using tape to cover the surface of substrates. We test this method on the traditional lateral flow test substrate—nitrocellulose and a novel lateral flow test substrate—synthetic paper, which is a porous media made by interlocked off-stoichiometry thiol-ene (OSTE) micropillars. We found that after the surface was covered by tape, the average flow rate decreased to 61% of the original flow rate on nitrocellulose, while the average flow rate increased to at least 320% of the original flow rate on synthetic paper. More interesting, besides the increase of flow rate, the volume capacity of synthetic paper also increases after covered by tape. Furthermore, we investigated the influence of length and position of tape on the capillary flow rate for nitrocellulose. A longer tape will lead to a smaller flow rate. The influence of tape of same length on the flow rate is bigger when the tape is placed closer to the loading pad. These results can help in the flow rate control on lateral flow test substrates, and potentially improve the performance of lateral flow tests.