Sensitivity and colour intensity enhancement in lateral flow immunoassay tests by adjustment of test line position

SERS biosensors for liquid biopsy towards cancer diagnosis by detection of various circulating biomarkers: current progress and perspectives

Liquid biopsy has emerged as a promising non-invasive strategy for cancer diagnosis, enabling the detection of various circulating biomarkers, including circulating tumor cells (CTCs), circulating tumor nucleic acids (ctNAs), circulating tumor-derived small extracellular vesicles (sEVs), and circulating proteins. Surface-enhanced Raman scattering (SERS) biosensors have revolutionized liquid biopsy by offering sensitive and specific detection methodologies for these biomarkers. This review comprehensively examines the application of SERS-based biosensors for identification and analysis of various circulating biomarkers including CTCs, ctNAs, sEVs and proteins in liquid biopsy for cancer diagnosis. The discussion encompasses a diverse range of SERS biosensor platforms, including label-free SERS assay, magnetic bead-based SERS assay, microfluidic device-based SERS system, and paper-based SERS assay, each demonstrating unique capabilities in enhancing the sensitivity and specificity for detection of liquid biopsy cancer biomarkers. This review critically assesses the strengths, limitations, and future directions of SERS biosensors in liquid biopsy for cancer diagnosis.

No-cost ballpoint pen dispenser for lateral flow assays

We have developed a no-cost, lightweight, human-powered dispenser using an empty ballpoint pen. Used in lateral flow assays, this dispenser restricts antibody deposition to narrow zones, allowing freehand drawing of test and control lines. The lines can be drawn in widths ranging from 0.15 to 1.00 mm. Naphthol green B, a compatible stain, was used to label antibody solutions and certify handwriting traces. Using human chorionic gonadotropin (HCG) as a model antigen, we demonstrated that the pen dispenser can imprint antibodies on nitrocellulose membranes without affecting their microstructure and chromatographic function. A lateral flow assay using the pen dispenser detected HCG at 0.1 μg/mL, comparable to the sensitivity of standard tests using traditional benchtop dispensers.

Nitrite enhanced detection from saliva by simple geometrical modifications of paper-based micromixers

Dysregulation of nitric oxide (NO) and it's two relatively stable metabolites, nitrite, and nitrate, in SARS-CoV-2, are reported in infected populations, especially for nitrates levels > 68.4 μmol/L. In this paper, we measure the abnormal presence of nitrite in the saliva by developing a cheap μPAD for colorimetric detection through the modified Griess reaction. This includes a diazotization reaction between nitrite and Griess reagent, including Sulfanilamide and N-Naphthyl-ethylenediamine in an acidic medium, causing a pink Azo compound. The modifications are suggested by a numerical method model that couples the mass flux with the porosity medium equations (convection, diffusion and, dispersion) that improves the mixing process. The mixing index was quantified from the concentration deviation method via simulation of a homogeneous two-phase flow in a porous environment. Five μPAD designs were fabricated to verify the simulation results of mixing enhancement on the Griess reactants in saliva samples. The investigated geometries include straight, helical, zig-zag, square wave, and inclined jagged shapes fabricated by direct laser writing, suitable for low cost, mass fabrication. Inclined jagged micromixer exhibited the best performance with up to 40% improvement compared with the simple straight geometry. Deliberate geometrical modifications, exemplified here in a jagged micromixer on paper, cut the limit of detection (LOD) by at least half without impacting the linear detection range.

Effect of sample volume on the sensitivity of lateral flow assays through computational modeling

Lateral flow assays (LFAs) are extensively used in qualitative detection because of their convenience, low cost, fast results, and ease of operation. However, the sample volume used in a lateral flow assay is usually determined experimentally. We test and find that the flow velocity is influenced by sample volume, using fluorescent microspheres as label particles, when analyte concentration is fixed in a sandwich LFA. A model is developed based on mass-action kinetics and advection-diffusion-reaction equation, combing the conjugate pad and nitrocellulose membrane. The model shows predictions from 10 to 120 μL, and predicts accurately the experimental results from 50 to 120 μL where the fluid can flow to the test line. Over all, the model can provide predictions over a wide range of sample volumes for sensitivity analysis. On the basis of the model, the sensitivity of the LFA can be improved according to the sample volume added in the experiment.

Tutorial: design and fabrication of nanoparticle-based lateral-flow immunoassays

Lateral-flow assays (LFAs) are quick, simple and cheap assays to analyze various samples at the point of care or in the field, making them one of the most widespread biosensors currently available. They have been successfully employed for the detection of a myriad of different targets (ranging from atoms up to whole cells) in all type of samples (including water, blood, foodstuff and environmental samples). Their operation relies on the capillary flow of the sample throughout a series of sequential pads, each with different functionalities aiming to generate a signal to indicate the absence/presence (and, in some cases, the concentration) of the analyte of interest. To have a user-friendly operation, their development requires the optimization of multiple, interconnected parameters that may overwhelm new developers. In this tutorial, we provide the readers with: (i) the basic knowledge to understand the principles governing an LFA and to take informed decisions during lateral flow strip design and fabrication, (ii) a roadmap for optimal LFA development independent of the specific application, (iii) a step-by-step example procedure for the assembly and operation of an LF strip for the detection of human IgG and (iv) an extensive troubleshooting section addressing the most frequent issues in designing, assembling and using LFAs. By changing only the receptors, the provided example procedure can easily be adapted for cost-efficient detection of a broad variety of targets.

Micro-fabrication by wax spraying for rapid smartphone-based quantification of bio-markers

A cheap/rapid technique for the fabrication of μPADs is presented for point of care analysis. Hydrophobic channels were formed across the width of the paper by spraying hot pure wax. This biocompatible novel process yielded uniform 300 ± 5 μm hydrophilic microchannels on paper, without the use of the cleanroom, UV lamp, or organic solvents and was completed in a single step without the need for a hotplate. Hot wax is properly impregnated across paper thickness by spraying under optimized temperature and pressure. Our method is advantageous in the cost and ease of fabrication, process time (<1 min), the feasibility of mass-fabrication, readout, environmental considerations and multiplexing due to the embossed structure of remnant wax. The performance of the resulting μPAD was assessed on a multiplexed Uric acid and Nitride assay, bearing 95% of confidence level in the readout against standardized tests. A novel RGB processing app was developed for smart-phones to quantify colorimetric read-outs through a heuristic normalization equation that converts RGB to integer systems. This combinatorial sensor demonstrates a good linear range (up to 800 μM for Uric acid and 1250 μM for Nitride), low detection limit (100 μM for uric acid and 156 μM for nitride).