Optical detection enhancement in porous volumetric microfluidic capture elements using refractive index matching fluids

Biosensors Based on Inorganic Composite Fluorescent Hydrogels

Fluorescent hydrogels are promising candidate materials for portable biosensors to be used in point-of-care diagnosis because (1) they have a greater capacity for binding organic molecules than immunochromatographic test systems, determined by the immobilization of affinity labels within the three-dimensional hydrogel structure; (2) fluorescent detection is more sensitive than the colorimetric detection of gold nanoparticles or stained latex microparticles; (3) the properties of the gel matrix can be finely tuned for better compatibility and detection of different analytes; and (4) hydrogel biosensors can be made to be reusable and suitable for studying dynamic processes in real time. Water-soluble fluorescent nanocrystals are widely used for in vitro and in vivo biological imaging due to their unique optical properties, and hydrogels based on these allow the preservation of these properties in bulk composite macrostructures. Here we review the techniques for obtaining analyte-sensitive fluorescent hydrogels based on nanocrystals, the main methods used for detecting the fluorescent signal changes, and the approaches to the formation of inorganic fluorescent hydrogels via sol-gel phase transition using surface ligands of the nanocrystals.

Phase wavefront perturbation calculation model for spectroscopic refractive index matching of hybrid materials

A low-cost flexible spectroscopic refractive index matching (SRIM) material with bandpass filtering properties without incidence angle and polarization dependence by randomly dispersing inorganic C a F ₂ particles in organic polydimethylsiloxane (PDMS) materials was proposed in our previous study. Since the micron size of the dispersed particles is much larger than the visible wavelength, the calculation based on the commonly used finite-difference time-domain (FDTD) method to simulate light propagation through the SRIM material is too bulky; however, on the other hand, the light tracing method based on Monte Carlo theory in our previous study cannot adequately explain the process. Therefore, a novel approximate calculation model, to the best of our knowledge, based on phase wavefront perturbation is proposed that can well explain the propagation of light through this SRIM sample material and can also be used to approximate the soft scattering of light through composite materials with small refractive index differences, such as translucent ceramics. The model simplifies the complex superposition of wavefront phase disturbances and the calculation of scattered light propagation in space. The scattered and nonscattered light ratios; the light intensity distribution after transmission through the spectroscopic material; and the influence of absorption attenuation of the PDMS organic material on the spectroscopic performance are also considered. The simulation results based on the model are in great agreement with the experimental results. This work is important to further improve the performance of SRIM materials.

AI-based analysis of 3D position and orientation of red blood cells using a digital in-line holographic microscopy

The morphological and mechanical characteristics of red blood cells (RBCs) largely vary depending on the occurrence of hematologic disorders. Variations in the rheological properties of RBCs affect the dynamic motions of RBCs, especially their rotational behavior. However, conventional techniques for measuring the orientation of biconcave-shaped RBCs still have some technical limitations, including complicated optical setups, complex post data processing, and low throughput. In this study, we propose a novel image-based technique for measuring 3D position and orientation of normal RBCs using digital in-line holographic microscopy (DIHM) and artificial intelligence (AI). Formaldehyde-fixed RBCs are immobilized in coagulated polydimethylsiloxane (PDMS). Holographic images of RBCs positioned at various out-of-plane angles are acquired by precisely manipulating the PDMS-trapped RBC sample attached to a 4-axis optical stage. With the aid of deep learning algorithms for data augmentation and regression analysis, the out-of-plane angle of RBCs is directly predicted from the captured holographic images. The 3D position and in-plane angle of RBCs are acquired by employing numerical reconstruction and ellipse detection methods. Combining these digital image processing techniques, the 3D positional and orientational information of each RBC recorded in a single holographic image is measured within 23.5 and 3.07 s, respectively. The proposed AI-based DIHM technique that can extract the 3D position, orientation, and morphology of individual RBCs would be utilized to analyze the dynamic translational and rotational motions of abnormal RBCs with hematologic disorders in shear flows through further research.

Covalent immobilisation of antibodies in Teflon-FEP microfluidic devices for the sensitive quantification of clinically relevant protein biomarkers

This study reports for the first time the sensitive colorimetric and fluorescence detection of clinically relevant protein biomarkers by sandwich immunoassays using the covalent immobilisation of antibodies onto the fluoropolymer surface inside Teflon®-FEP microfluidic devices. Teflon®-FEP has outstanding optical transparency ideal for high-sensitivity colorimetric and fluorescence bioassays, however this thermoplastic is regarded as chemically inert and very hydrophobic. Covalent immobilisation can offer benefits over passive adsorption to plastic surfaces by allowing better control over antibody density, orientation and analyte binding capacity, and so we tested a range of different and novel covalent immobilisation strategies. We first functionalised the inner surface of a 10-bore, 200 μm internal diameter FEP microcapillary film with high-molecular weight polyvinyl alcohol (PVOH) without changing the outstanding optical transparency of the device delivered by the matched refractive index of FEP and water. Glutaraldehyde immobilisation was compared with the use of photoactivated linkers and NHS-ester crosslinkers for covalently immobilising capture antibodies onto PVOH. Three clinically relevant sandwich ELISAs were tested against the cytokine IL-1β, the myocardial infarct marker cardiac troponin I (cTnI), and the chronic heart failure marker brain natriuretic peptide (BNP). Overall, glutaraldehyde immobilisation was effective for BNP assays, but yielded unacceptable background for IL-1β and cTnI assays caused by direct binding of the biotinylated detection antibody to the modified PVOH surface. We found NHS-ester groups reacted with APTES-treated PVOH coated fluoropolymers. This facilitated a novel method for capture antibody immobilisation onto fluoropolymer devices using a bifunctional NHS-maleimide crosslinker. The density of covalently immobilised capture antibodies achieved using PVOH/APTES/NHS/maleimide approached levels seen with passive adsorption, and sensitive and quantitative assay performance was achieved using this method. Overall, the PVOH coating provided an excellent surface for controlled covalent antibody immobilisation onto Teflon®-FEP for performing high-sensitivity immunoassays.

Flow-through microfluidic immunosensors with refractive index-matched silica monoliths as volumetric optical detection elements

A sensitive and rapid absorbance based immunosensor that utilizes ex situ functionalized porous silica monoliths as volumetric optical detection elements is demonstrated in this study. The porous monolith structure facilitates high capture probe density and short diffusion length scales, enabling sensitive and rapid assays. Silica monoliths, synthesized and functionalized with immunocapture probes off-chip before integration into a sealed thermoplastic microfluidic device, serve to capture target antigens during perfusion through the porous structure. Gold nanoparticle immunoconjugates are combined with silver enhancement to create microscale silver clusters, followed by perfusion of an aqueous sucrose solution to limit light scattering and enhance optical signal. Using this approach, detection limits as low as 1 ng/mL are achieved for a sandwich assay, with a dynamic range of at least 4 logs. The results confirm that the combination of on-chip index matching with functionalized porous silica monoliths can enables simple and practical flow-through immunoassays for the sensitive and rapid detection of target antigens.

Novel functionalities of hybrid paper-polymer centrifugal devices for assay performance enhancement

The presented work demonstrates novel functionalities of hybrid paper-polymer centrifugal devices for assay performance enhancement that leverage the advantages of both paper-based and centrifugal microfluidic platforms. The fluid flow is manipulated by balancing the capillary force of paper inserts with the centrifugal force generated by disc rotation to enhance the signal of a colorimetric lateral flow immunoassay for pathogenic E. coli. Low-cost centrifugation for pre-concentration of bacteria was demonstrated by sample sedimentation at high rotational speeds before supernatant removal by a paper insert via capillary force after deceleration. The live bacteria capture efficiency of the device was similar to a commercial centrifuge. This pre-concentrated sample when combined with gold nanoparticle immunoconjugate probes resulted in a detection limit that is 10× lower than a non-concentrated sample for a lateral flow immunoassay. Signal enhancement was also demonstrated through rotational speed variation to prevent the flow for on-device incubation and to reduce the flow rate, thus increasing the sample residence time for the improved capture of gold nanoparticle-bacteria complexes in an integrated paper microfluidic assay. Finally, multiple sequential steps including sample pre-concentration, filtration, incubation, target capture by an integrated paper microfluidic assay, silver enhancement and quenching, and index matching were completed within a single device. The detection limit was 105 colony forming units per ml, a 100× improvement over a similar paper-based lateral flow assay. The techniques utilize the advantages of paper-based microfluidic devices, while facilitating additional functionalities with a centrifugal microfluidic platform for detection performance enhancement in a low-cost, automated platform amenable to point-of-care environments.

Quantitative Chemical Imaging of Nonplanar Microfluidics

Confocal and multiphoton optical imaging techniques have been powerful tools for evaluating the performance of and monitoring experiments within microfluidic devices, but this application suffers from two pitfalls. The first is that obtaining the necessary imaging contrast often requires the introduction of an optical label which can potentially change the behavior of the system. The emerging analytical technique stimulated Raman scattering (SRS) microscopy promises a solution, as it can rapidly measure 3D concentration maps based on vibrational spectra, label-free; however, when using any optical imaging technique, including SRS, there is an additional problem of optical aberration due to refractive index mismatch between the fluid and the device walls. New approaches such as 3D printing are extending the range of materials from which microfluidic devices can be fabricated; thus, the problem of aberration can be obviated simply by selecting a chip material that matches the refractive index of the desired fluid. To demonstrate complete chemical imaging of a geometrically complex device, we first use sacrificial molding of a freeform 3D printed template to create a round-channel, 3D helical micromixer in a low-refractive-index polymer. We then use SRS to image the mixing of aqueous glucose and salt solutions throughout the entire helix volume. This fabrication approach enables truly nonperturbative 3D chemical imaging with low aberration, and the concentration profiles measured within the device agree closely with numerical simulations.

Impedimetric Immunosensing in a Porous Volumetric Microfluidic Detector

A sensitive and rapid impedemetric immunosensor is demonstrated utilizing porous volumetric microfluidic detection elements and silver enhanced gold nanoparticle probes. The porous detection elements significantly increase capture probe density and decrease diffusion length scales compared to conventional planar sensors to improve target capture efficiency and enhance impedance signal. In this work, a packed bed of silica beads functionalized with antibody probes serves as a porous sensor element within a thermoplastic microchannel, with an interdigitated gold electrode microarray used to measure impedance changes caused by the concentration dependent formation of silver aggregates. The measured impedance change is independent of electrode spacing, enabling a device with low resolution electrodes to achieve a sandwich immunoassay detection limit between 1-10 ng/mL with a 4-log dynamic range, with a total assay time of 75 min.