Fluorescence enhancement on silver nanoplates at the single- and sub-nanoparticle level

Triggered cagedSTORM microscopy

In standard SMLM methods, the photoswitching of single fluorescent molecules and the data acquisition processes are independent, which leads to the detection of single molecule blinking events on several consecutive frames. This mismatch results in several data points with reduced localization precision, and it also increases the possibilities of overlapping. Here we discuss how the synchronization of the fluorophores' ON state to the camera exposure time increases the average intensity of the captured point spread functions and hence improves the localization precision. Simulations and theoretical results show that such synchronization leads to fewer localizations with 15% higher sum signal on average, while reducing the probability of overlaps by 10%.

Metal-enhanced fluorescence of dyes with quadrupole surface plasmon resonance of silver nanoparticles

Silver colloidal films (SCFs) composed of homogeneous 60-220 nm silver nanoparticles were synthesized for optimal fluorescence enhancement of chromophores with the dipole and quadrupole surface plasmons. The fluorescence enhancements with the SCFs of three chromophores, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran, 4-dimethylamino-4'-nitrobiphenyl, and coumarin 343 whose emission spectra are centered distinctively in the 470-560 nm wavelength range were compared. Fluorescence enhancements and lifetime changes were investigated via time-resolved fluorescence spectroscopy. The spectral overlap between the chromophore's emission and the dipole or quadrupole surface plasmon resonance (SPR) bands determined the fluorescence enhancements with the SCFs. The dipole and quadrupole SPR bands both appeared to provide effective fluorescence enhancements of chromophores. This knowledge allows researchers to develop sensitive fluorescence sensors by combining nanoparticles with optimal dipole or quadrupole SPR bands in order to achieve fluorescence enhancement of a specific chromophore. The emission dynamics measurements with the SCFs were combined with the finite-difference time-domain simulation results for the local electric fields around the silver nanoparticles to enable discussion of metal-enhanced fluorescence mechanisms, including excitation and emission enhancements.

Label-Free LSPR-Vertical Microcavity Biosensor for On-Site SARS-CoV-2 Detection

Cost-effective, rapid, and sensitive detection of SARS-CoV-2, in high-throughput, is crucial in controlling the COVID-19 epidemic. In this study, we proposed a vertical microcavity and localized surface plasmon resonance hybrid biosensor for SARS-CoV-2 detection in artificial saliva and assessed its efficacy. The proposed biosensor monitors the valley shifts in the reflectance spectrum, as induced by changes in the refractive index within the proximity of the sensor surface. A low-cost and fast method was developed to form nanoporous gold (NPG) with different surface morphologies on the vertical microcavity wafer, followed by immobilization with the SARS-CoV-2 antibody for capturing the virus. Modeling and simulation were conducted to optimize the microcavity structure and the NPG parameters. Simulation results revealed that NPG-deposited sensors performed better in resonance quality and in sensitivity compared to gold-deposited and pure microcavity sensors. The experiment confirmed the effect of NPG surface morphology on the biosensor sensitivity as demonstrated by simulation. Pre-clinical validation revealed that 40% porosity led to the highest sensitivity for SARS-CoV-2 pseudovirus at 319 copies/mL in artificial saliva. The proposed automatic biosensing system delivered the results of 100 samples within 30 min, demonstrating its potential for on-site coronavirus detection with sufficient sensitivity.

Surface Plasmon Resonance (SPR)- and Localized SPR (LSPR)-Based Virus Sensing Systems: Optical Vibration of Nano- and Micro-Metallic Materials for the Development of Next-Generation Virus Detection Technology

The global damage that a widespread viral infection can cause is evident from the ongoing COVID-19 pandemic. The importance of virus detection to prevent the spread of viruses has been reaffirmed by the pandemic and the associated social and economic damage. Surface plasmon resonance (SPR) in microscale and localized SPR (LSPR) in nanoscale virus sensing systems are thought to be useful as next-generation detection methods. Many studies have been conducted on ultra-sensitive technologies, especially those based on signal amplification. In some cases, it has been reported that even a low viral load can be measured, indicating that the virus can be detected in patients even in the early stages of the viral infection. These findings corroborate that SPR and LSPR are effective in minimizing false-positives and false-negatives that are prevalent in the existing virus detection techniques. In this review, the methods and signal responses of SPR and LSPR-based virus detection technologies are summarized. Furthermore, this review surveys some of the recent developments reported and discusses the limitations of SPR and LSPR-based virus detection as the next-generation detection technologies.

Reshaping of triangular silver nanoplates by a non-halide etchant and its application in melamine sensing

Although triangular silver (Ag) nanoplates are intrinsically unstable, this characteristic has been taken advantage of in the development of a novel sensing platform. However, most of these applications have relied on halide ions as etchants. In the current work, we used sodium 4-vinylbenzenesulfonate (Na-VBS) as a new powerful etchant of triangular silver (Ag) nanoplates. When aged with Na-VBS at room temperature, Na-VBS etched Ag nanoplates nearly as powerfully as halides did, and these nanoplates rapidly transformed into oblate nanospheroids. This shape evolution permitted tuning of the corresponding localized surface plasmon resonance (LSPR) features of the Ag nanostructures. Interestingly, passivation of the Ag nanoplate surface with melamine was shown to protect the nanoplates from Na-VBS-induced etching. The rate of change of the color and spectral features of the Ag nanoplate solution exposed to Na-VBS was found to be strongly correlated with the concentration of melamine in the solution. This association allowed us to apply this system to the development of a novel platform for sensing melamine.

Plasmon-Enhanced Electrochemiluminescence of Silver Nanoclusters for microRNA Detection

Surface plasmon-enhanced electrochemiluminescence (SPEECL) with excellent sensitivity and simplicity has attracted increasing attention. In this work, we reported a novel SPEECL with DNA templated silver nanoclusters (DNA-AgNCs) as ECL emitters and gold nanoparticles (AuNPs) as localized surface plasmon resonance (LSPR) source. The SPEECL with DNA-AgNCs as ECL luminophores possessed low toxicity and avoided the labeling process, which is favorable for its further sensing application. In addition, by investigation of the SPEECL under different distances between DNA-AgNCs and AuNPs, it was demonstrated that the SPEECL was distance dependent. Meanwhile, the SPEECL intensity changed with the sizes and interdistance of AuNPs under different electrodeposition time. Furthermore, by the combination of a cyclic amplification process with enzyme-free catalytic hairpin DNA, a sensitive SPEECL biosensor was proposed for the detection of microRNA (miRNA-21) successfully with a wide linear range from 1 aM to 10⁴ fM and a relatively low detection limit of 0.96 aM, which was applied in the detection of miRNA-21 in real samples with satisfying results. This novel, simple, sensitive, and selective SPEECL with label-free and low-toxic ECL emitters displayed a great potential for bioassay application.

Recent progress in the synthesis and applications of 2D metal nanosheets

The design and controlled synthesis of two-dimensional (2D) nanomaterials have been widely studied because the properties and functions of nanomaterials are highly dependent on their sizes, shapes, and dimensionalities. For instance, 2D metal nanosheets (2DMNSs) have attracted a significant amount of attention owing to their interesting properties, which are absent in corresponding bulk counterparts, and they have been confirmed to have potential applications in electrocatalysis, optics, and biomedicine. However, because of the close-packed structures of metals, the large-scale fabrication of 2DMNSs is challenging. In this review, we have outlined the research progress in the field of 2DMNSs, including the typical synthesis approaches and newly developed methods, as well as promising applications of the materials reported in recent years. Moreover, some preliminary and promising strategies to further improve the properties of 2DMNSs and some insights for the development of the field have been included.

Strongly enhanced molecular fluorescence with ultra-thin optical magnetic mirror metasurfaces

As a kind of two-dimensional metamaterial, metasurfaces can modify the amplitude, phase, and polarization of the transmitted or reflected electromagnetic wave, and thereby can be used for enhancing the light-matter interactions. Based on this notion, an optical magnetic mirror metasurface featuring periodic nanoscale grooves is designed to confine the strong electric field near the metal surface by magnetic responses. As a result, fluorescence from an ultra-thin layer of fluorescent polymer blend (∼15  nm) on the mirror surface can be strongly enhanced (by 45-fold in experiment). The fluorescence emission can be controlled by the polarization of excitation light since the responses of the magnetic mirror are polarization sensitive. This kind of magnetic mirror metasurface is potentially useful in biological monitors, optical sources, and chemical sensors.

Super-Resolution Imaging and Plasmonics

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Plasmon-enhanced fluorescence imaging with silicon-based silver chips for protein and nucleic acid assay

Metal-enhanced fluorescence shows great potential for improving the sensitivity of fluoroscopy, which has been widely used in protein and nucleic acid detection for biosensor and bioassay applications. In comparison with the traditional glass-supported metal nanoparticles (MNPs), the introduction of a silicon substrate has been shown to provide an increased surface-enhanced Raman scattering (SERS) effect due to the coupling between the MNPs and the semiconducting silicon substrate. In this work, we further study the fluorescence-enhanced effect of the silicon-supported silver-island (Ag@Si) plasmonic chips. In particular, we investigate their practical application of improving the traditional immunoassay such as the biotin-streptavidin-based protein assay and the protein-/nucleic acid-labeled cell and tissue samples. The protein assay shows a wavelength-dependent enhancement effect of the Ag@Si chip, with an enhancement factor ranging from 1.2 (at 532 nm) to 57.3 (at 800 nm). Moreover, for the protein- and nucleic acid-labeled cell and tissue samples, the Ag@Si chip provides a fluorescence enhancement factor of 3.0-4.1 (at 800 nm) and a significant improvement in the signal/background ratio for the microscopy images. Such a ready accommodation of the fluorescence-enhanced effect for the immunoassay samples with simple manipulations indicates broad potential for applications of the Ag@Si chip not only in biological studies but also in the clinical field.

Massively Screening the Temporal Spectra of Single Nanoparticles to Uncover the Mechanism of Nanosynthesis

Nanosynthesis is the basis of nanotechnology and its applications. It is necessary to understand the growth mechanism of nanoparticles and the functions of growth factors. An effective way to study the synthesis is at the single nanoparticle level. This study reports a single nanoparticle spectrometer, which is based on a commercial dark-field microscopy and a group of narrowband filters. This spectrometer has many advantages, such as high light transparency (35%-75%), low cost (<$1500), massive screening (≈200 nanoplates at a time), and a high time resolution (<5 s). By using this spectrometer, the galvanic replacement reaction (GRR) is studied on single Ag nanoplates in situ and in real time. The research reveals that GRR on single Ag nanoplates has three different types according to the change of peak wavelength during reaction. Such diverse reaction types can be attributed to the different relative reaction rates of GRR on the faces and edges of Ag nanoplate with different facets. Further research shows that the relative reaction rates of different facets vary a lot under different concentrations of tri-sodium citrate. This research successfully demonstrates that the new single nanoparticle spectrometer can study the growth of single nanoparticles and the effect of growth factors.

The artificial control of enhanced optical processes in fluorescent molecules on high-emittance metasurfaces

Plasmon-enhanced optical processes in molecules have been extensively but individually explored for Raman scattering, fluorescence, and infrared light absorption. In contrast to recent progress in the interfacial control of hot electrons in plasmon-semiconductor hybrid systems, plasmon-molecule hybrid systems have remained to be a conventional scheme, mainly assuming electric-field enhancement. This was because it was difficult to control the plasmon-molecule interface in a well-controlled manner. We here experimentally substantiate an obvious change in artificially enhanced optical processes of fluorescence/Raman scattering in fluorescent molecules on high-emittance plasmo-photonic metasurfaces with/without a self-assembled monolayer of sub-nm thickness. These results indicate that the enhanced optical processes were successfully selected under artificial configurations without any additional chemical treatment that modifies the molecules themselves. Although Raman-scattering efficiency is generally weak in high-fluorescence-yield molecules, it was found that Raman scattering becomes prominent around the molecular fingerprint range on the metasurfaces, being enhanced by more than 2000 fold at the maximum for reference signals. In addition, the highly and uniformly enhancing metasurfaces are able to serve as two-way functional, reproducible, and wavelength-tunable platforms to detect molecules at very low densities, being distinct from other platforms reported so far. The change in the enhanced signals suggests that energy diagrams in fluorescent molecules are changed in the configuration that includes the metal-molecule interface, meaning that plasmon-molecule hybrid systems are rich in the phenomena beyond the conventional scheme.

Distance mediated electrochemiluminescence enhancement of CdS thin films induced by the plasmon coupling of gold nanoparticle dimers

Theoretical and experimental studies of plasmon enhanced electrochemiluminescence of CdS QDs by gold nanoparticle monomers and dimers.