MoS2/WS2/BN-Silver Thin-Film Hybrid Architectures Displaying Enhanced Fluorescence via Surface Plasmon Coupled Emission for Sensing Applications

Smartphone-Based Attomolar Cyanide Ion Sensing Using Au-Graphene Oxide Cryosoret Nanoassembly and Benzoxazolium-Based Fluorophore in a Surface Plasmon-Coupled Enhanced Fluorescence Interface

Photoplasmonic platforms are being demonstrated as excellent means for bridging nanochemistry and biosensing approaches at advanced interfaces, thereby augmenting the sensitivity and quantification of the desired analytes. Although resonantly coupled electromagnetic waves at the surface plasmon-coupled emission (SPCE) interface are investigated with myriad nanomaterials in order to boost the detection limits, rhodamine moieties are ubiquitously used as SPCE reporter molecules in spite of their well-known limitations. In order to overcome this constraint, in this work, a benzoxazolium-based fluorescent molecule, (E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]oxazol-3-ium iodide (DSBO), was synthesized to selectively detect the cyanide (CN^-) ions in water samples. To this end, the sensitivity of the fabricated SPCE substrates is tested in spacer, cavity, and extended cavity nanointerfaces to rationalize the configurational robustness. The performance of the sensor is further improved with the careful engineering of gold (Au)-graphene oxide (GO) cryosoret nanoassemblies fabricated via an adiabatic cooling technology. The unique dequenching (turn-on) of the quenched (turn-off) fluorescent signal is demonstrated with the hybridized metal-π plasmon synergistic coupling in the nanovoids and nanocavities assisting delocalized Bragg and localized Mie plasmons. The spectro-plasmonic analysis yielded highly directional, polarized (>95%), and enhanced emission attributes with an attomolar limit of detection of 10 aM of CN^- ions with high linearity (R² = 0.996) and excellent reliability, in addition to an exceptional correlation with the theoretically obtained TFclac simulations. The CN^- ion sensing is experimentally validated with the smartphone-based cost-effective SPCE detection technology to render the device amenable to resource-limited settings. We believe that the unique fluorophore-cryosoret nanoassemblage presented here encourages development of frugal, unconventional, and highly desirable strategies for the selective quantitation of environmentally and physiologically relevant analytes at trace concentrations for use in point-of-care diagnostics.

Titanium nitride as an alternative and reusable plasmonic substrate for fluorescence coupling

The development of alternative plasmonic materials that can replace gold and silver is of long-standing interest in materials research. In this study, we have prepared and characterized thin films of TiN, an emerging plasmonic material, and examined its effectiveness for fluorescence coupling in metal-dielectric structures having TiN as the plasmonically active component. We have used a combination of experiment and reflectivity calculations to determine the nature and dispersion of the optical modes sustained by the metal-dielectric structures, which furthermore are adjustable by varying the thickness of the dielectric layer. Our results reveal that fluorophores placed on the TiN substrates can couple with the surface-plasmon mode and/or the waveguide modes supported by these structures, to provide polarized and directional emission over narrow angular ranges. The performance of TiN substrates for surface plasmon-coupled emission (SPCE) and waveguide-coupled emission (WGCE) is found to be comparable with conventional Au substrates. Importantly, the TiN thin films are reusable, which is certainly advantageous for their use in SPCE or WGCE-based fluorescence sensing applications.

Recent progress in sensing application of metal nanoarchitecture-enhanced fluorescence

Fluorescence analytical methods, as real time and in situ analytical approaches to target analytes, can offer advantages of high sensitivity/selectivity, great versatility, non-invasive measurement and easy transmission over long distances. However, the conventional fluorescence assay still suffers from low specificity, insufficient sensitivity, poor reliability and false-positive responses. By exploiting various metal nanoarchitectures to manipulate fluorescence, both increased fluorescence quantum yield and improved photostability can be realized. This metal nanoarchitecture-enhanced fluorescence (MEF) phenomenon has been extensively studied and used in various sensors over the past years, which greatly improved their sensing performance. Thus in this review, we primarily give a general overview of MEF based sensors from mechanisms to state-of-the-art applications in environmental assays, biological/medical analysis and diagnosis areas. Finally, their pros and cons as well as further development directions are also discussed.

Fluorescence Based on Surface Plasmon Coupled Emission for Ultrahigh Sensitivity Immunoassay of Cardiac Troponin I

This work demonstrates the quantitative assay of cardiac Troponin I (cTnI), one of the key biomarkers for acute cardiovascular diseases (the leading cause of death worldwide) using the fluorescence-based sandwich immune reaction. Surface plasmon coupled emission (SPCE) produced by non-radiative coupling of dye molecules with surface plasmons being excitable via the reverse Kretschmann format is exploited for fluorescence-based sandwich immunoassay for quantitative detection of cTnI. The SPCE fluorescence chip utilizes the gold (2 nm)-silver (50 nm) bimetallic thin film, with which molecules of the dye Alexa 488 (conjugated with detection antibodies) make a near field coupling with the plasmonic film for SPCE. The experimental results find that the SPCE greatly improves the sensitivity via enhancing the fluorescence signal (up to 50-fold) while suppressing the photo-bleaching, permitting markedly enhanced signal-to-noise ratio. The limit of detection of 21.2 ag mL-1 (atto-gram mL-1) is obtained, the lowest ever reported to date amid those achieved by optical technologies such as luminescence and label-free optical sensing techniques. The features discovered such as ultrahigh sensitivity may prompt the presented technologies to be applied for early diagnosis of cTnI in blood, particularly for emergency medical centers overloaded with patients with acute myocardial infarction who would suffer from time-delayed diagnosis due to insufficient assay device sensitivity.

Engineering of Exciton-Plasmon Coupling Using 2D-WS2 Nanosheets for 1000-Fold Fluorescence Enhancement in Surface Plasmon-Coupled Emission Platforms

Enhancement of fluorescence emission from single-photon quantum emitters on plasmonic nanomaterials using surface plasmon-coupled emission (SPCE) platforms has seen significant advancements. In parallel, there has also been an exponential rise in applications involving two-dimensional (2D) transition-metal dichalcogenides (TMDs) that exhibit unique exciton-plasmon interactions. Although both these Frontier research areas have impacted the development of sensor and sensing technologies, no study coalesces these two arenas for translational applications. In this work, we use thin WS₂ nanosheets for realizing 1000-fold fluorescence enhancement on the SPCE platform. Structure-dependent fluorescence enhancement exhibited by WS₂ provides new insight into the use of TMDs and exciton-plasmon coupling in SPCE substrates. Cellphone-based detection of the emitting dipole is another unique aspect of this work that presents a low-cost alternative in comparison with high-end detectors.

Chemical sensing with Au and Ag nanoparticles

Noble metal nanoparticles (NPs) are ideal scaffolds for the fabrication of sensing devices because of their high surface-to-volume ratio combined with their unique optical and electrical properties which are extremely sensitive to changes in the environment. Such characteristics guarantee high sensitivity in sensing processes. Metal NPs can be decorated with ad hoc molecular building blocks which can act as receptors of specific analytes. By pursuing this strategy, and by taking full advantage of the specificity of supramolecular recognition events, highly selective sensing devices can be fabricated. Besides, noble metal NPs can also be a pivotal element for the fabrication of chemical nose/tongue sensors to target complex mixtures of analytes. This review highlights the most enlightening strategies developed during the last decade, towards the fabrication of chemical sensors with either optical or electrical readout combining high sensitivity and selectivity, along with fast response and full reversibility, with special attention to approaches that enable efficient environmental and health monitoring.

Atomically Thin Boron Nitride as an Ideal Spacer for Metal-Enhanced Fluorescence

Metal-enhanced fluorescence (MEF) considerably enhances the luminescence for various applications, but its performance largely depends on the dielectric spacer between the fluorophore and plasmonic system. It is still challenging to produce a defect-free spacer having an optimized thickness with a sub-nanometer accuracy that enables reusability without affecting the enhancement. In this study, we demonstrate the use of atomically thin hexagonal boron nitride (BN) as an ideal MEF spacer owing to its multifold advantages over the traditional dielectric thin films. With rhodamine 6G as a representative fluorophore, it largely improves the enhancement factor (up to ∼95 ± 5), sensitivity (10^-8 M), reproducibility, and reusability (∼90% of the plasmonic activity is retained after 30 cycles of heating at 350 °C in air) of MEF. This can be attributed to its two-dimensional structure, thickness control at the atomic level, defect-free quality, high affinities to aromatic fluorophores, good thermal stability, and excellent impermeability. The atomically thin BN spacers could increase the use of MEF in different fields and industries.

Electron-plasmon interaction on lithium niobate with gold nanolayer and its field distribution dependent modulation

Surface plasmon resonance (SPR) enables strong field confinement, opening thereby new avenues for device miniaturization and reducing energy consumption. Plasmonic devices with electrical tunability attract tremendous interest for various applications. Most of the current researches achieved SPR modulation with relatively large driving voltages, or by other relatively low-speed tuning approaches, such as thermo-optic, magneto-optic, acousto-optic etc. In this paper, we propose and demonstrate an efficiently electrical SPR modulation based on lithium niobate (LN) with gold nanolayer (~81 nm) via electron-plasmon interaction. Efficient intensity modulation and wavelength shift (in visible band) of ~5.7 dB/V and ~36.3 nm/V are respectively obtained with low DC current. More importantly, modulation phenomenon of field distribution dependent is also observed and experimentally unveiled. Further performance is analyzed in terms of AC modulation and polarization characteristics. This key achievement opens up opportunities for applications such as optical interconnection, electric field sensing, electrically plasmonic modulation, etc.

Single-Nanoparticle Plasmonic Electro-optic Modulator Based on MoS2 Monolayers

The manipulation of light in an integrated circuit is crucial for the development of high-speed electro-optic devices. Recently, molybdenum disulfide (MoS₂) monolayers generated broad interest for the optoelectronics because of their huge exciton binding energy, tunable optical emission, direct electronic band-gap structure, etc. Miniaturization and multifunctionality of electro-optic devices further require the manipulation of light-matter interaction at the single-nanoparticle level. The strong exciton-plasmon interaction that is generated between the MoS₂ monolayers and metallic nanostructures may be a possible solution for compact electro-optic devices at the nanoscale. Here, we demonstrate a nanoplasmonic modulator in the visible spectral region by combining the MoS₂ monolayers with a single Au nanodisk. The narrow MoS₂ excitons coupled with broad Au plasmons result in a deep Fano resonance, which can be switched on and off by applying different gate voltages on the MoS₂ monolayers. A reversible display device that is based on this single-nanoparticle modulator is demonstrated with a heptamer pattern that is actively controlled by the external gates. Our work provides a potential application for electro-optic modulation on the nanoscale and promotes the development of gate-tunable nanoplasmonic devices in the future.

Plasmonics of 2D Nanomaterials: Properties and Applications

Plasmonics has developed for decades in the field of condensed matter physics and optics. Based on the classical Maxwell theory, collective excitations exhibit profound light-matter interaction properties beyond classical physics in lots of material systems. With the development of nanofabrication and characterization technology, ultra-thin two-dimensional (2D) nanomaterials attract tremendous interest and show exceptional plasmonic properties. Here, we elaborate the advanced optical properties of 2D materials especially graphene and monolayer molybdenum disulfide (MoS2), review the plasmonic properties of graphene, and discuss the coupling effect in hybrid 2D nanomaterials. Then, the plasmonic tuning methods of 2D nanomaterials are presented from theoretical models to experimental investigations. Furthermore, we reveal the potential applications in photocatalysis, photovoltaics and photodetections, based on the development of 2D nanomaterials, we make a prospect for the future theoretical physics and practical applications.

Giant Enhancement of Defect-Bound Exciton Luminescence and Suppression of Band-Edge Luminescence in Monolayer WSe2-Ag Plasmonic Hybrid Structures

We have investigated how the photoluminescence (PL) of WSe₂ is modified when coupled to Ag plasmonic structures at low temperature. Chemical vapor deposition (CVD) grown monolayer WSe₂ flakes were transferred onto a Ag film and a Ag nanotriangle array that had a 1.5 nm Al₂O₃ capping layer. Using low-temperature (7.5 K) micro-PL mapping, we simultaneously observed enhancement of the defect-bound exciton emission and quenching of the band edge exciton emission when the WSe₂ was on a plasmonic structure. The enhancement of the defect-bound exciton emission was significant with enhancement factors of up to ∼200 for WSe₂ on the nanotriangle array when compared to WSe₂ on a 1.5 nm Al₂O₃ capped Si substrate with a 300 nm SiO₂ layer. The giant enhancement of the luminescence from the defect-bound excitons is understood in terms of the Purcell effect and increased light absorption. In contrast, the surprising result of luminescence quenching of the bright exciton state on the same plasmonic nanostructure is due to a rather unique electronic structure of WSe₂: the existence of a dark state below the bright exciton state.

Label-Free Fluorescence Assay of S1 Nuclease and Hydroxyl Radicals Based on Water-Soluble Conjugated Polymers and WS₂ Nanosheets

We developed a new method for detecting S1 nuclease and hydroxyl radicals based on the use of water-soluble conjugated poly[9,9-bis(6,6-(N,N,N-trimethylammonium)-fluorene)-2,7-ylenevinylene-co-alt-2,5-dicyano-1,4-phenylene)] (PFVCN) and tungsten disulfide (WS₂) nanosheets. Cationic PFVCN is used as a signal reporter, and single-layer WS₂ is used as a quencher with a negatively charged surface. The ssDNA forms complexes with PFVCN due to much stronger electrostatic interactions between cationic PFVCN and anionic ssDNA, whereas PFVCN emits yellow fluorescence. When ssDNA is hydrolyzed by S1 nuclease or hydroxyl radicals into small fragments, the interactions between the fragmented DNA and PFVCN become weaker, resulting in PFVCN being adsorbed on the surface of WS₂ and the fluorescence being quenched through fluorescence resonance energy transfer. The new method based on PFVCN and WS₂ can sense S1 nuclease with a low detection limit of 5 × 10(-6) U/mL. Additionally, this method is cost-effective by using affordable WS₂ as an energy acceptor without the need for dye-labeled ssDNA. Furthermore, the method provides a new platform for the nuclease assay and reactive oxygen species, and provides promising applications for drug screening.

Synthesis and porous h-BN 3D architectures for effective humidity and gas sensors

3D (three dimensional) architectures synthesised using an easily scalable solid state method which results in an interconnected network of porous h-BN sheets with boron trioxide are reported in this study.