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    Aluminum as a competitive plasmonic material for the entire electromagnetic spectrum: A review
    (Elsevier, 2024-11) Arora, Pankaj
    With plasmonics taking the lead in most sensing applications, research has geared towards alternative, cost-effective materials that can strive for large-scale production along with CMOS compatibility. Aluminum (Al) is among those competitive plasmonic metal films that have seen unprecedented research in recent years. The ability to exhibit appreciable plasmonic response in the entire electromagnetic spectrum has been reported along with improved performance sensing parameters. This review article covers different aspects of Al-based nanostructures, nano-films, and nano-particles in different wavelength regimes, displaying efficient plasmonic sensing for myriad purposes. A comprehensive review is conducted to explore the diverse and exciting possibilities emerging from Al-based tunable plasmons at the metal-dielectric interface. Al has already entered many applications, from on-chip plasmonic integration to point-of-care diagnosis. Thus, the application of Al in wide applications (heath, fluorescence, image-filtering techniques, and many more) is discussed here, along with the corresponding limitations and future scope associated with them.
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    Fluorescence Brightness, Photostability, and Energy Transfer Enhancement of Immobilized Single Molecules in Zero-Mode Waveguide Nanoapertures
    (ACS, 2022-05) Patra, Satyajit
    Zero-mode waveguide (ZMW) nanoapertures are widely used to monitor single molecules beyond the range accessible to normal microscopes. However, several aspects of the ZMW influence on the photophysics of fluorophores remain inadequately documented and sometimes controversial. Here, we thoroughly investigate the ZMW influence on the fluorescence of single immobilized Cy3B and Alexa 647 molecules, detailing the interplays between brightness, lifetime, photobleaching time, the total number of emitted photons, and Förster resonance energy transfer (FRET). Despite the plasmonic-enhanced excitation intensity in the ZMW, we find that the photostability is preserved with similar photobleaching times as on the glass reference. Both the fluorescence brightness and the total number of photons detected before photobleaching are increased, with an impressive gain of nearly five times that found for Alexa 647 dyes. Finally, the single-molecule data importantly allow a loophole-free characterization of the ZMW influence on the FRET process. We show that the FRET rate constant is enhanced by 50%, demonstrating that nanophotonics can mediate the energy transfer. These results deepen our understanding of the fluorescence enhancement in ZMWs and are of immediate relevance for single-molecule biophysical applications.
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    Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides
    (ACS, 2019-09) Patra, Satyajit
    Single molecule detection provides detailed information about molecular structures and functions but it generally requires the presence of a fluorescent marker which can interfere with the activity of the target molecule or complicate the sample production. Detecting a single protein with its natural UV autofluorescence is an attractive approach to avoid all the issues related to fluorescence labeling. However, the UV autofluorescence signal from a single protein is generally extremely weak. Here, we use aluminum plasmonics to enhance the tryptophan autofluorescence emission of single proteins in the UV range. Zero-mode waveguide nanoapertures enable the observation of the UV fluorescence of single label-free β-galactosidase proteins with increased brightness, microsecond transit times, and operation at micromolar concentrations. We demonstrate quantitative measurements of the local concentration, diffusion coefficient, and hydrodynamic radius of the label-free protein over a broad range of zero-mode waveguide diameters. Although the plasmonic fluorescence enhancement has generated a tremendous interest in the visible and near-infrared parts of the spectrum, this work pushes further the limits of plasmonic-enhanced single molecule detection into the UV range and constitutes a major step forward in our ability to interrogate single proteins in their native state at physiological concentrations.
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    Extending Single-Molecule Förster Resonance Energy Transfer (FRET) Range beyond 10 Nanometers in Zero-Mode Waveguides
    (ACS, 2019-07) Patra, Satyajit
    Single-molecule Förster resonance energy transfer (smFRET) is widely used to monitor conformations and interaction dynamics at the molecular level. However, conventional smFRET measurements are ineffective at donor–acceptor distances exceeding 10 nm, impeding the studies on biomolecules of larger size. Here, we show that zero-mode waveguide (ZMW) apertures can be used to overcome the 10 nm barrier in smFRET. Using an optimized ZMW structure, we demonstrate smFRET between standard commercial fluorophores up to 13.6 nm distance with a significantly improved FRET efficiency. To further break into the classical FRET range limit, ZMWs are combined with molecular constructs featuring multiple acceptor dyes to achieve high FRET efficiencies together with high fluorescence count rates. As we discuss general guidelines for quantitative smFRET measurements inside ZMWs, the technique can be readily applied for monitoring conformations and interactions on large molecular complexes with enhanced brightness.
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    On-chip label-free plasmonic based imaging microscopy for microfluidics
    (IOP, 2018-08) Arora, Pankaj
    In this work, we demonstrated an on-chip label-free imaging microscopy using real and Fourier Plane (FP) dark field images of surface plasmons, by integrating engineered plasmonic substrates with different shapes of microfluidic channels. After successful integration of fabricated plasmonic nanostructures with SU-8 based microfluidic channels, on-chip label-free index monitoring of analytes with different refractive indices was demonstrated and an index resolution of 1.63 × 10−4 RIU was achieved by quantifying CMYK components of captured images. Label-free imaging for interface of colorless miscible and immiscible analytes flowing on plasmonic nanostructures in the microfluidic channels was performed using color-selective filtering nature of plasmonic nanostructures. Hydrodynamic focusing where the width of the focused stream of one liquid was controlled by the relative flow rates of the three liquids was demonstrated and utilized to capture the flow of air bubbles on plasmonic nanostructures with real and FP images. Since, the imaging is realized on a chip and does not need any complicated and bulky arrangement, it will benefit the development of flat optical components for sensing applications and will be well suited for on-chip point of care diagnostics
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    Design and comparative analysis of aluminum-MoS2 based plasmonic devices with enhanced sensitivity and Figure of Merit for biosensing applications in the near-infrared region
    (Elsevier, 2021-02) Arora, Pankaj
    Aluminum (Al)-Molybdenum Disulfide (MoS2) based plasmonic structures act as excellent biosensors when exploited in the near-infrared region. While Al is economical as well as compatible with the optoelectronic devices, MoS2 is an emerging 2D nanomaterial with the promise of initiating better plasmonic activity. Based on Kretschmann's arrangement, we have explored angular interrogation over four different combinations of heterostructures with Al as the plasmonic metal layer, at a wavelength of 1550 nm. After studying the effect of Al thickness on the conventional configuration, the intermediate layers between the metal layer and the analyte were optimized. Inclusion of graphene along with MoS2 results in better interaction with the sensing medium. The effect of including silicon is also studied for sensitivity enhancement. In addition, a comparative analysis of sensor performances of the proposed devices is presented taking into account the two important parameters i.e. sensitivity as well as the Figure of Merit (FOM). Among the optimized multi-layered MoS2 based configurations, a maximum sensitivity of about 141°/RIU is obtained along with FOM of about 335.13 RIU−1. Finally, the single-stranded DNA sensing on the proposed devices shows that the structures can be used as a highly sensitive refractive index biosensor for bio-medical applications.