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1. SOFI for Plasmonics: Extracting Near-field Intensity in the Far-Field at High Density

   Boutelle, R.; Yi, X.; Neuhauser, D.; Weiss, S. (October 2017, arXiv.org)

   Unlike normal fluorescent methods that use the intensity as a direct measurement of the localized enhanced field, we use blinking statistics of quantum dots (QDs). We have already shown that blinking gives a more accurate characterization of the near-field. When an emitter is situated close to a metallic surface, non-radiative pathways are opened up, leading to quenching of the exciton. Blinking statistics, however, is only minimally affected by quenching, and therefore can be used to probe emitters in close proximity to metallic surfaces. We have expanded our method (COFIBINS) to high densities using superresolution technique SOFI. A proof of principle for SOFI-COFIBINS is demonstrated with a defocused point spread function. The method is then applied to surface plasmon polaritons. SOFI-COFIBINS shows excellent agreement with the average fluorescence intensity. 

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2. Plasmonic-Photonic Hybrid System for Enhanced Fluorescent-based Digital Resolution Biosensing

   Xiong, Yanyu; Barya, Priyash; Shepherd, Skye; Gupta, Rohit; Akin, Lucas; Tibbs, Joseph; Lee, Han Keun; Liu, Shengyan; Singamaneni, Srikanth; Cunningham, Brian T (May 2023, International Conference on Surface Plasmon Photonics (SPP10))

   Plasmonic and photonic technologies have attracted strong interest in the past few decades toward several interdisciplinary applications stemming from unique light-matter interactions fostered by materials at the nanoscale. The versatility of plasmonic and photonic sensors for ultrasensitive, rapid, analyte sensing without extensive sample pre-treatment steps or sophisticated optics have resulted in their strong foothold in the broad arena of biosensing. Fluorescence-based bioanalytical techniques are widely used in liquid-biopsy diagnostics applications, but require many labeled target molecules to combine their emission output to achieve a practically useful signal-to-noise ratio. Approaches capable of amplifying fluorescence signals can provide signal-to-noise sufficient for digitally counting single emitters for ultrasensitive assays that are detected with simple and inexpensive instruments. [1]. Plasmonic and nano-photonics can function in synergy to amplify fluorescence signals. By concentrating optical energy well below the diffraction limit, plasmonic nanoantenna provide spatial control over excitation light, but their quality factor (Q) is modulated by radiative and dissipative losses. Photonic crystals (PC) as dielectric microcavities have a diffraction-limited optical mode volume despite being able to generate a high Q-factor. Here, we demonstrate a plasmonic-photonic hybrid system to produce a much stronger fluorescent enhancement for digital resolution biosensing. With an optimized dielectric spacer layer, around 200 Alexa-647 fluorophores have been coated over heterometallic Ag@Au core-shell plasmonic nanostructures with minimized Ohmic losses and quenching effects [2]. The target-specific molecule capture events enabled this plasmonic fluor to attach to the PC surface, forming a Plasmonic-Photonic hybrid mode. With much stronger local field enhancement, far-field directional emission, large Purcell enhancement, and high quantum efficiency, we report a two-orders signal enhancement from PC-enhanced plasmonic-fluor (104-fold brighter than a single fluorophore). This improved signal-to-noise ratio enabled us to perform single molecule imaging even with a 10x (NA=0.2) objective lens while offering 3 orders of magnitude boost in the limit of detection of Interleukine-6 (common biomarker for cancer, inflammation, sepsis, and autoimmune disease) compared with standard immunoassays in human plasma 

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3. Ultrasmall HgTe Quantum Dots with Near-Unity Photoluminescent Quantum Yields in the Near and Shortwave Infrared

   https://doi.org/10.1021/acs.chemmater.4c01619  

   Coffey, Belle; Skytte, Elise; Ahmed, Tasnim; Vasileiadou, Eugenia S; Lin, Eric Yu; Sueh_Hua, Ash; Cook, Elijah; Tenney, Stephanie M; Sletten, Ellen M; Caram, Justin Ryan (August 2024, Chemistry of Materials)

   We demonstrate a low-temperature synthesis of ultrasmall (<2 nm) HgTe quantum dots (QDs) with superlative optical properties in the near and shortwave infrared. The tunable cold-injection synthesis produces HgTe QDs ranging from 1.7 to 2.3 nm in diameter, with photoluminescence maxima ranging from 900 to 1180 nm and a full-width at half-maximum of ∼100 nm (∼130 meV). The synthesized quantum dots display high photoluminescence quantum yields (PLQY) ranging from 80 to 95% based on both relative and absolute methods. Furthermore, samples retain their high PLQY (∼60%) in the solid state, allowing for first-of-their-kind photoluminescence imaging and blinking studies of HgTe QDs. The facile synthesis allows for the isolation of small, photostable HgTe quantum dots, which can provide valuable insight into the extremes of quantum confinement. 

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4. Gas-phase grafting for the multifunctional surface modification of silicon quantum dots

   https://doi.org/10.1039/D2NR04902C  

   Schwan, Joseph; Wang, Kefu; Tang, Ming Lee; Mangolini, Lorenzo (December 2022, Nanoscale)

   Photon upconversion in systems incorporating inorganic quantum dots (QDs) is of great interest for applications in solar energy conversion, bioimaging, and photodynamic therapy. Achieving high up-conversion efficiency requires not only high-quality inorganic nanoparticles, but also precise control of their surface functional groups. Gas-phase surface functionalization provides a new pathway towards controlling the surface of small inorganic nanoparticles. In this contribution, we utilize a one-step low-temperature plasma technique for the synthesis and in-flight partial functionalization of silicon QDs with alkyl chains. The partially functionalized surface is then modified further with 9-vinylanthracene via thermal hydrosilylation resulting in the grafting of 9-ethylanthracene (9EA) groups. We have found that the minimum alkyl ligand density necessary for quantum dot solubility also gives the maximum upconversion quantum yield, reaching 17% for silicon QDs with Si-dodecyl chains and an average of 3 9EA molecules per particle. 

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5. Blinking-Based Multiplexing (BBM): Harnessing Molecular Photophysics for Single-Emitter Classification

   Wustholz, K.; Hoy, G.; DeSalvo, G; Kogan, I.; Haile, S.; Smith, E. (April 2023, Bulletin of the American Physical Society)

   Single-molecule fluorescence approaches have revolutionized biological and materials microscopy. However, many questions can only be addressed by multicolor imaging of multiple targets, a capability that is limited by the small subset of available, well-performing, and spectrally-distinct fluorescent probes. We recently introduced an alternative single-molecule multiplexing approach termed blinking-based multiplexing (BBM), wherein individual molecules are classified on the basis of their intrinsic blinking dynamics. We demonstrate accurate (>93.5%) binary classification of spectrally-overlapped rhodamine and quantum dot emitters using BBM, even when substantial blinking heterogeneity is observed. Classification can be accomplished using change point detection (CPD) analysis of blinking dynamics or a deep learning (DL) algorithm, the latter of which provides up to 96.6% accuracy. Here, we use CPD and DL algorithms to probe the excitation power, environmental, and molecular dependence of BBM. In addition to providing new opportunities in single-molecule spectroscopy and imaging, BBM represents a new take on single-molecule research, where blinking dynamics can be harnessed for more than just traditional localization or nanoreporting. 

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