arXiv subjects

Rushna Quddus

Publications and source records attributed to Rushna Quddus.

5 recordsLinked to original sources

arXiv2026

Self-Assembly of Lipid-Biopolymer Periodic Nanostructures on Photonic Length Scales

The self-assembly of photonic nanostructures in insects involves chitin, proteins, and lipids. While synthetic photonic systems have been extensively studied, current lipid-based self-assembly systems are limited in periodicity to $68\,\text{nm}$ compared to photonic length scales ($\approx 450\,\text{nm}$) observed in biological organisms. We hypothesise that lipids facilitate how structural colour arises in vivo by acting as templates for the self-assembly of biopolymers via lipidic lyotropic liquid crystal mesophases. Here, we aim to understand and identify how structural colour is produced in insects by the co-assembly of lipids and biopolymers. We study the effect of biopolymers, pH, temperature, surface charge, and stability on lipid vesicles using dynamic light scattering, X-ray scattering, and zeta potential analysis. Using cryo-electron microscopy, we demonstrate that these vesicles interact with the biopolymers and generate periodic nanostructures with periodicities ranging from $700\,\text{nm}$ to $1.2\,\mu\text{m}$ (more than ten times larger than for purely lipidic systems) and dimensionalities ranging from 1D to 3D. Our results establish that lipid mesophases and biopolymers can induce reorganisation into ordered nanostructures, overcoming key limitations of periodicities achieved by lipid-only systems, and providing a methodology for recreating the physicochemical mechanisms underlying biophotonic structural colour.

cond-mat.soft↗

arXiv2025

A Quantitative Holographic Agglutination Assay for Immunoglobulin A

This study introduces a Holographic Agglutination Assay for quantifying levels of the immunoglobulin protein IgA in biological samples. This is the first example of a label-free and bead-free assay that quantifies protein agglutinates by direct detection using Total Holographic Characterization. A proof-of-concept assay for human serum immunoglobulins is demonstrated using Jacalin, the galactose-specific plant lectin, to induce selective agglutination. By analyzing the size, refractive index, and number of particles in an assay sample, we obtain a reproducible and quantitative measurement of galactosylated immunoglobulins in a given sample. The assay is calibrated for a physiologically relevant reference interval of IgA concentrations in a 10x diluted emulated biological sample from low (80 mg/dL, 5 {\mu}M) to high (320 mg/dL, 20 {\mu}M) levels. The assay clearly distinguishes samples containing IgA from samples containing IgG. More broadly, this study introduces a platform for creating lectin-mediated Holographic Agglutination Assays to monitor levels of immunoglobulins in biological samples. The ability to quantify immunoglobulin levels efficiently in clinical samples is likely to be valuable for diagnostics and will provide a basis for assaying other proteins that can be induced to agglutinate.

physics.optics↗

arXiv2025

Multiplexed holographic molecular binding assays with internal calibration standards

Holographic molecular binding assays detect macromolecules binding to colloidal probe beads by monitoring nanometer-scale changes in the beads' diameters with holographic microscopy. Measured changes are interpreted with Maxwell Garnett effective-medium theory to infer the surface coverage of analyte molecules and therefore to measure the analyte concentration in solution. The precision and accuracy of those measurements can be degraded by run-to-run instrumental variations, which introduce systematic errors in the holographic characterization measurements. We detect and mitigate these errors by introducing a class of inert reference beads whose polymer brush coating resists macromolecular binding. The holographically measured diameter and refractive index of those beads serve as internal standards for THC measurements. To characterize the reference beads, we introduce a general all-optical method to measure the grafting density of the polymer brush that combines holographic characterization of the bead diameter with a refractometry measurement of the polymer's specific volume. The latter technique shows the specific volume of poly(ethylene oxide) to be 1.308(4) cubic nanometers per kilodalton. We use this suite of techniques to demonstrate a multiplexed immunoassay for immunoglobulin G (IgG) whose success validates the effective-medium analysis of holographic characterization measurements. Internal negative controls provided by the reference beads are validated by negative control measurements on alcohol dehydrogenase (ADH), which has a similar molecular weight to IgG but does not bind to the probe beads' binding sites.

cond-mat.soft↗

arXiv2020

Holographic characterization and tracking of colloidal dimers in the effective-sphere approximation

An in-line hologram of a colloidal sphere can be analyzed with the Lorenz-Mie theory of light scattering to measure the sphere's three-dimensional position with nanometer-scale precision while also measuring its diameter and refractive index with part-per-thousand precision. Applying the same technique to aspherical or inhomogeneous particles yields the position, diameter and refractive index of an effective sphere that represents an average over the particle's geometry and composition. This effective-sphere interpretation has been applied successfully to porous, dimpled and coated spheres, as well as to fractal clusters of nanoparticles, all of whose inhomogeneities appear on length scales smaller than the wavelength of light. Here, we combine numerical and experimental studies to investigate effective-sphere characterization of symmetric dimers of micrometer-scale spheres, a class of aspherical objects that appear commonly in real-world dispersions. Our studies demonstrate that the effective-sphere interpretation usefully identifies dimers in holographic characterization studies of monodisperse colloidal spheres. The effective-sphere estimate for a dimer's axial position closely follows the ground truth for its center of mass. Trends in the effective-sphere diameter and refractive index, furthermore, can be used to measure a dimer's three-dimensional orientation. When applied to colloidal dimers transported in a Poiseuille flow, the estimated orientation distribution is consistent with expectations for Brownian particles undergoing Jeffery orbits.

cond-mat.soft↗

arXiv2020

Holographic immunoassays

The size of a probe bead reported by holographic particle characterization depends on the proportion of the surface area covered by bound target molecules and so can be used as an assay for molecular binding. We validate this technique by measuring the kinetics of irreversible binding for the antibodies immunoglobulin G (IgG) and immunoglobulin M (IgM) as they attach to micrometer-diameter colloidal beads coated with protein A. These measurements yield the antibodies' binding rates and can be inverted to obtain the concentration of antibodies in solution. Holographic molecular binding assays therefore can be used to perform fast quantitative immunoassays that are complementary to conventional serological tests.

cond-mat.soft↗