arXiv subjects

Malavika Kayyil Veedu

Publications and source records attributed to Malavika Kayyil Veedu.

5 recordsLinked to original sources

arXiv2026

Reconfigurable Magnetic Nanopore Platform for Selective Trapping

Solid-state nanopores offer a powerful platform for nanoscale analysis of individual analytes, including biomolecules and functionalized nanoparticles, by confining them within a precisely defined sensing region. However, their inherently passive operation restricts practical applications, as they cannot precisely control particle position or dynamics inside the pore. Here, we introduce magnetic nanopore architectures that integrate a ferromagnetic layer into the nanopore system. Acting as a magnetic discontinuity within an otherwise uniformly magnetized film, the nanopore generates localized stray magnetic fields that enable magnetic tweezing of magnetic nanoparticles, which can be functionalized with fluorescent biomolecules. Importantly, the nanopore geometry is designed to reversibly switch between a nearly uniform magnetization state and a magnetic flux-closure state through the application of short magnetic field pulses of controlled amplitude. This capability allows the magnetic tweezing effect to be selectively activated or deactivated, enabling controlled capture and release of tagged biomolecules on demand. As a proof of concept, we demonstrate the selective magnetic trapping of fluorescent magnetic particles. These findings pave the way for reconfigurable, on-chip magnetic nanopore platforms capable of selective trapping and high-throughput single-particle detection. KEYWORDS: Nanopores, magnetic tweezers, fluorescence microscopy, vortex state, active control, magnetic nanoparticles

physics.optics↗

arXiv2026

DNA-Origami-Assembled Rhodium Nanoantennas for Deep-UV Label-Free Single-Protein Detection

Nanoparticles of plasmonic metals have significantly to the development of spectroscopic techniques, enabling strong confinement of electromagnetic fields at the nanoscale and corresponding signal amplification. However, to date, plasmonic applications have been limited mainly to the visible and near-infrared range, as materials supporting ultraviolet resonances typically exhibit poor chemical stability and lack robust surface functionalisation methods. In this work, we address these limitations by introducing a fully programmable approach to UV plasmonics based on rhodium nanocube dimers assembled using DNA origami templates. We have developed a reliable ligand exchange strategy that allows the functionalisation of rhodium nanocubes with DNA while maintaining their colloidal stability. These DNA-modified nanocubes act as modular building blocks that can be assembled into dimers with 69% efficiency and an average gap size of 10 nm. The DNA origami design also allows for the deterministic placement of a single streptavidin protein in the plasmonic gap, unlike previous methods based on stochastic diffusion. Experiments with single-molecule autofluorescence in UV, supported by numerical simulations, show an increase in brightness of up to 22, a reduction in fluorescence lifetime, and a more than tenfold increase in the total number of detected photons. By creating a robust and versatile platform for the production of UV-resonant plasmonic nanoantennas, this work extends the functionality of plasmonics to the deep UV spectrum and opens up new possibilities for labelling-free single-protein spectroscopy.

physics.optics↗

arXiv2025

Breaking the Low Concentration Barrier of Single-Molecule Fluorescence Quantification to the Sub-Picomolar Range

Single-molecule fluorescence techniques provide exceptional sensitivity to probe biomolecular interactions. However, their application to accurately quantify analytes at the picomolar concentrations relevant for biosensing remains challenged by a severe degradation in the signal-to-background ratio. This so-called 'low concentration barrier' is a major factor hindering the broad application of single-molecule fluorescence to biosensing. Here we break into the low concentration limit while keeping intact the confocal microscope architecture and without requiring complex microfluidics or preconcentration stages. Using fluorescence lifetime correlation spectroscopy (FLCS) and adding a diaphragm to the laser excitation beam, we achieve a limit of quantitation (LOQ) down to 0.1 pM, significantly below the state-of-the-art. We identify the physical parameters setting the LOQ and introduce a broadly applicable figure of merit (FoM) that determines the LOQ and allows for a clear comparison between experimental configurations. Our approach preserves the ability to monitor dynamic interactions, diffusion times, and distinguish species in complex mixtures. We illustrate this feature by measuring the biotin-streptavidin association rate constant which is highly challenging to assess quantitatively due to the strong affinity of the biotin-streptavidin interaction. These findings push the boundaries of single-molecule fluorescence detection for biosensing applications at sub-picomolar concentrations with high accuracy and simplified systems.

physics.optics↗

arXiv2024

Watching lanthanide nanoparticles one at a time: characterization of their photoluminescence dynamics at the single nanoparticle level

Lanthanide nanoparticles (LnNPs) feature sharp emission lines together with millisecond emission lifetimes which makes them promising luminescent probes for biosensing and bioimaging. Although LnNPs are gathering a large interest, their photoluminescence properties at the single nanoparticle level remain largely unexplored. Here, we employ fluorescence correlation spectroscopy (FCS) and photoluminescence burst analysis to investigate the photodynamics of Sm and Eu-based LnNPs with single nanoparticle sensitivity and microsecond resolution. By recording the photoluminescence intensity and the number of contributing LnNPs, we compute the photoluminescence brightness per individual nanoparticle, and estimate the actual number of emitting centers per nanoparticle. Our approach overcomes the challenges associated with ensemble-averaged techniques and provides insights into LnNP photodynamics. Moreover, we demonstrate our microscope capability to detect and analyze LnNPs at the single nanoparticle level, monitoring both photoluminescence brightness and burst duration. These findings expand our understanding of LnNPs and pave the way for their advanced biosensing applications at the single nanoparticle level.

physics.optics↗

arXiv2023

Unveiling the photoluminescence dynamics of gold nanoclusters with fluorescence correlation spectroscopy

Gold nanoclusters (AuNCs) have captured significant interest for their photoluminescent properties; however, their rapid photodynamics remain elusive while probed by ensemble-averaging spectroscopy techniques. To address this challenge, we use fluorescence correlation spectroscopy (FCS) to uncover the photoluminescence dynamics of colloidal Au18(SG)14 nanoclusters. Our FCS analysis reveals the photoluminescence (PL) brightness per nanocluster, elucidating the impact of photoexcitation saturation and ligand interactions. Unlike DNA-encapsulated silver nanoclusters, the gold counterparts notably exhibit minimal blinking, with moderate amplitudes and 0.2 ms characteristic times. Our data also clearly reveal the occurrence of photon antibunching in the PL emission, showcasing the quantum nature of the PL process, with each AuNC acting as an individual quantum source. Using zero-mode waveguide nanoapertures, we achieve a 16-fold enhancement of the PL brightness of individual AuNCs. This constitutes an important enabling proof-of-concept for tailoring emission properties through nanophotonics. Overall, our study bridges the gap between ensemble-averaged techniques and single-molecule spectroscopy, offering new insights into AuNC photodynamics for biosensing and imaging applications.

physics.optics↗