Angular-Dependent Metal-Enhanced Fluorescence from Silver Island Films

In this letter we report the observation of angular-dependent Metal Enhanced Fluorescence (MEF) from fluorophores deposited onto silver island films (SiFs). When illuminated with laser light (473 nm) at angles of 45 and 90 degrees from the surface, SiFs scattered light at wide observation angles biased by the direction of the incident light. We observed angular-dependent MEF (10-fold) from FITC-HSA immobilized onto the SiFs, again slightly biased with respect to the direction of the incident light. We also measured the photostability of FITC from the back of the glass substrate at angles of 225 and 340 degrees.     

Metal-Enhanced S(2) Fluorescence from Azulene

In this paper, we report the first observation of metal-enhanced S(2) emission at room and low temperature (77K). The S(2) emission intensity of Azulene is enhanced by close proximity to Silver island films (SiFs). In this regard, a ≈ 2-fold higher S(2) fluorescence intensity of Azulene was observed from SiFs as compared to a glass control sample. This suggests that S(2) excited states can couple to surface plasmons and enhance S(2) fluorescence yields, a helpful observation in our understanding the interactions between plasmons and lumophores, and our continued efforts to develop a unified plasmon-lumophore/fluorophore theory.     

Electrochemical Synthesis of Plasmonic Nanostructures

Thanks to their tunable and strong interaction with light, plasmonic nanostructures have been investigated for a wide range of applications. In most cases, controlling the electric field enhancement at the metal surface is crucial. This can be achieved by controlling the metal nanostructure size, shape, and location in three dimensions, which is synthetically challenging. Electrochemical methods can provide a reliable, simple, and cost-effective approach to nanostructure metals with a high degree of geometrical freedom. Herein, we review the use of electrochemistry to synthesize metal nanostructures in the context of plasmonics. Both template-free and templated electrochemical syntheses are presented, along with their strengths and limitations. While template-free techniques can be used for the mass production of low-cost but efficient plasmonic substrates, templated approaches offer an unprecedented synthetic control. Thus, a special emphasis is given to templated electrochemical lithographies, which can be used to synthesize complex metal architectures with defined dimensions and compositions in one, two and three dimensions. These techniques provide a spatial resolution down to the sub-10 nanometer range and are particularly successful at synthesizing well-defined metal nanoscale gaps that provide very large electric field enhancements, which are relevant for both fundamental and applied research in plasmonics.     

溶剂效应制备核壳纳米银及荧光素金属增强荧光

直链或支链高分子可用来制备和稳定纳米材料,具有丰富羟基的高分子通过分子间和分子内氢键作用形成分子级别的"胶囊",用作生长纳米颗粒的模板[1].可溶性淀粉主要是直链淀粉,是由多个葡萄糖单元构成的含有丰富羟基的高分子,同时具有疏水性和亲水性[2].     

DNA-Barcoded Plasmonic Nanostructures for Activity-Based Protease Sensing

We report the development of a new class of protease activity sensors called DNA-barcoded plasmonic nanostructures. These probes are comprised of gold nanoparticles functionalized with peptide-DNA conjugates (GPDs), where the peptide is a substrate of the protease of interest. The DNA acts as a barcode identifying the peptide and facilitates signal amplification. Protease-mediated peptide cleavage frees the DNA from the nanoparticle surface, which is subsequently measured via a CRISPR/Cas12a-based assay as a proxy for protease activity. As proof-of-concept, we show activity-based, multiplexed detection of the SARS-CoV-2-associated protease, 3CL, and the apoptosis marker, caspase 3, with high sensitivity and selectivity. GPDs yield >25-fold turn-on signals, 100-fold improved response compared to commercial probes, and detection limits as low as 58 pM at room temperature. Moreover, nanomolar concentrations of proteases can be detected visually by leveraging the aggregation-dependent color change of the gold nanoparticles. We showcase the clinical potential of GPDs by detecting a colorectal cancer-associated protease, cathepsin B, in three different patient-derived cell lines. Taken together, GPDs detect physiologically relevant concentrations of active proteases in challenging biological samples, require minimal sample processing, and offer unmatched multiplexing capabilities (mediated by DNA), making them powerful chemical tools for biosensing and disease diagnostics.     

Reconfigurable Plasmonic Nanostructures Controlled by DNA Origami

Precise surface functionalization and reconfigurable capability of nanomaterials are essential to construct complex nanostructures with specific functions.Here we show tire assembly of a reconfigurable plasmonic nanostructure,which executes both conformational and plasmonic changes in response to DNA strands.In this work,different sized gold nanoparticles(AuNPs)were arranged site-specifically on the surface of a DNA origami clamp nanostructure.The opening and closing of the DNA origami clamp could be precisely controlled by a series of strand emplacement reactions.Therefore,the patterns of these AuNPs could be switched between two different configura-tions.The observed plasmon band shift indicates the change of the plasmonic interactions among the assembled AuNPs.Our study achieves the construction of reconfigurable nanomaterials with tunable plasmonic interactions,and will enrich the toolbox of DNA-based functional nanomachinery.     

Ag@SiO2纳米复合物金属增强荧光释放法检测葡萄糖

制备了Ag@SiO2纳米复合物,罗丹明B通过物理掺杂结合在SiO2壳层。由于金属增强荧光效应,罗 丹明B的荧光增强到4.7倍。Ag核易被H2O2氧化,Ag核氧化后产生荧光增强释放效应。基于金属增强荧光 释放建立了一种新型葡萄糖检测方法,采用交联法在罗丹明B掺杂的Ag@SO2纳米复合物的SiO2壳层固定 葡萄糖氧化酶。检测浓度范围为0.2～6.8 mmol/L,检测限可达0.06 mmol/L。由于H2O2氧化Ag核反应迅 速,检测体系对葡萄糖的响应快速。     

Distance-Dependent Metal-Enhanced Intrinsic Fluorescence of Proteins Using Polyelectrolyte Layer-by-Layer Assembly and Aluminum Nanoparticles

Previously reported studies indicate that aluminum nanostructured substrates can potentially find widespread use in metal-enhanced fluorescence (MEF) applications particularly in the UV or near-UV spectral region toward label-free detection of biomolecules. MEF largely depends on several factors, such as chemical nature, size, shape of the nanostructure and its distance from the fluorophore. A detailed understanding of the MEF and its distance-dependence are important for its potential application in biomedical sensing. Our goal is to utilize intrinsic protein fluorescence for label-free binding assays. This is made possible by the use of metallic nanostructures which provide localized excitation and enhanced fluorescence of UV fluorophores and will also provide a way to separate the surface-bound proteins from the bulk samples. We evaluated varied probe distances from plasmonic nanostructures by the well-established layer-by-layer (LbL) technique. The investigated proteins were adsorbed on different numbers of alternate layers of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). Bovine serum albumin (BSA) was electrostatically attached to the positively charged PAH layer, and goat and rabbit IgG were attached to negatively charged PSS layer. We obtained a maximum of a ~ 9 fold increase in fluorescence intensity from BSA at a distance of ~9 nm from the Al nanostructured surface. Approximately 6- and 7- fold increases were observed from goat and rabbit IgG at a distance of ~8 nm, respectively. The minimum lifetimes were about 3-fold shorter than those on bare control quartz slides for all three proteins. The time-resolved intensity decays were analyzed with a lifetime distribution model to understand the distance effect on the metal-fluorophore interaction in detail. The present study indicates the distance dependence nature of metal-enhanced intrinsic fluorescence of proteins and potential of LbL assembly to control the metal-to-fluorophore distance in the UV wavelength region.     

Recent Advances in Plasmonic Nanostructures Applied for Label-free Single-cell Analysis

Cellular heterogeneity presents a major challenge in understanding the relationship between cells of particular genotype and response in disease. In order to characterize the cell-to-cell differences during the biochemical processes, single-cell analysis is necessary. Profiting from the unique localized surface plasmon resonance (LSPR) and Mie scattering, plasmonic nanostructures have revealed stable and adjustable scattering signals, avoiding photobleaching, blinking and autofluorescence phenomenon. These characterizations are propitious to the dynamic trace and biological image of single living cells. In this review, we discuss the recent advances in plasmonic nanostructures applied for label-free detection and monitoring of target cells at single-cell level by using three different techniques, surface-enhanced Raman scattering (SERS), surface-enhanced Infrared absorption spectroscopy (SEIRAS), and dark-field microscopy. Various avenues to design plasmonic probes combining spectra and imaging for single-cell analysis are demonstrated as well. We hope this review can highlight the superiority of plasmonic nanostructures in single cellular analysis, and further motivate the development of label-free cell analysis technique to elucidate cellular diversity and heterogeneity.     

Silver-Gold Nanocomposite Substrates for Metal-Enhanced Fluorescence: Ensemble and Single-Molecule Spectroscopic Studies

In recent years, there has been a growing interest in the studies involving the interactions of fluorophores with plasmonic nanostructures or nanoparticles. These interactions lead to several favorable effects such as increase in the fluorescence intensities, increased photostabilities, and reduced excited-state lifetimes that can be exploited to improve the capabilities of present fluorescence methodologies. In this regard, we report the use of newly developed silver-gold nanocomposite (Ag-Au-NC) structures as substrates for metal-enhanced fluorescence (MEF). The Ag-Au-NC substrates have been prepared by a one-step galvanic replacement reaction from thin silver films coated on glass slides. This approach is simple and suitable for the fabrication of MEF substrates with large area. We have observed about 15-fold enhancement in the fluorescence intensity of ATTO655 from ensemble fluorescence measurements using these substrates. The fluorescence enhancement on the Ag-Au-NC substrates is also accompanied by a reduction in the fluorescence lifetime of ATTO655, which is consistent with the fluorophore-plasmon coupling mechanism. Single-molecule fluorescence measurements have been performed to gain more insight into the metal-fluorophore interactions and to unravel the heterogeneity in the interaction of individual fluorophores with the fabricated substrates. The single-molecule studies are in good agreement with the ensemble measurements and show maximum enhancements of ~50-fold for molecules located in proximity to the "hotspots" on the substrates. In essence, the Ag-Au-NC substrates have a very good potential for various MEF applications.     

Enhanced Photocurrent Generation in Self-Assembled Monolayers Formed at Plasmonic Gold Nanostructures

The effect of localized surface plasmon resonance (SPR) appearing at the surfaces of gold nanoparticles (AuPs) was successfully applied for the enhancement of photocurrents from porphyrin ( Po ) immobilized on AuPs. The electrode consisting of AuPs with different sizes (∼15 or ∼35 nm) was prepared using the AuP film formed at the liquid/liquid interface, and then Po was self-assembled on the gold surface. The SPR effect for the AuP film was verified from the attenuated total reflection SPR and absorption measurements. Photocurrents from the modified electrodes were compared with the corresponding planar electrode. Appreciable enhancement of photocurrents was observed.     

Feasibility of Using Bimetallic Plasmonic Nanostructures to Enhance the Intrinsic Emission of Biomolecules

Detection of the intrinsic fluorescence from proteins is important in bio-assays because it can potentially eliminate the labeling of external fluorophores to proteins. This is advantageous because using external fluorescent labels to tag biomolecules requires chemical modification and additional incubation and washing steps which can potentially perturb the native functionality of the biomolecules. Hence the external labeling steps add expense and complexity to bio-assays. In this paper, we investigate for the first time the feasibility of using bimetallic nanostructures made of silver (Ag) and aluminum (Al) to implement the metal enhanced fluorescence (MEF) phenomenon for enhancing the intrinsic emission of biomolecules in the ultra-violet (UV) spectral region. Fluorescence intensities and lifetimes of a tryptophan analogue N-acetyl-L-tryptophanamide (NATA) and a tyrosine analogue N-acetyl-L-tyrosinamide (NATA-tyr) were measured. Increase in fluorescence intensities of upto 10-fold and concurrent decrease in lifetimes for the amino acids were recorded in the presence of the bimetallic nanostructures when compared to quartz controls. We performed a model protein assay involving biotinylated bovine serum albumin (bt-BSA) and streptavidin on the bimetallic nanostructured substrate to investigate the distance dependent effects on the extent of MEF from the bimetallic nanostructures and found a maximum enhancement of over 15-fold for two layers of bt-BSA and streptavidin. We also used finite difference time domain (FDTD) calculations to explore how bimetallic nanostructures interact with plane waves and excited state fluorophores in the UV region and demonstrate that the bimetallic substrates are an effective platform for enhancing the intrinsic emission of proteins and other biomolecules.     

Fabrication and Characterization of Planar Plasmonic Substrates with High Fluorescence Enhancement

The use of plasmonic nanostructures for fluorescence signal amplification is currently a very active research field. The detection of submonolayers of proteins labeled with organic dyes is a widely used technique in surface-based immunoassays and DNA hybridization. There is a strong interest in the development of new optical and chemical methods to increase the signal from ultralow concentrations of dyes on the surface of sensor substrates. Herein, we have explored the possibility of using vacuum-deposited silver nanostructures on dielectric layers and silver mirrors as potential plasmonic substrates that effectively amplify fluorescence over a broad spectral range. By optimizing deposition parameters for dielectric layers and silver nanostructures and applying thermal annealing processes, we observed large fluorescence amplifications from three different dye-strept(avidin) conjugates: about 7-fold for a UV/blue dye AF350-Av, 49-fold for a blue-green dye AF488-SA, and up to 208-fold for red-emitting AF647-SA dye. The observed amplification factors for the ensemble of fluorophores are very promising for development of surface-based bioassays. These substrates can be prepared using simple vacuum deposition in which we circumvent using the expensive nanofabrication methods. In addition, unlike most nanofabrication methods, the present approach is appropriate for large scale fabrication of substrates with microscope slide surface area suitable for sensing applications.     

Dynamic and Quantitative Control of the DNA‐Mediated Growth of Gold Plasmonic Nanostructures

Reproducible and controllable growth of nanostructures with well‐defined physical and chemical properties is a longstanding problem in nanoscience. A key step to address this issue is to understand their underlying growth mechanism, which is often entangled in the complexity of growth environments and obscured by rapid reaction speeds. Herein, we demonstrate that the evolution of size, surface morphology, and the optical properties of gold plasmonic nanostructures could be quantitatively intercepted by dynamic and stoichiometric control of the DNA‐mediated growth. By combining synchrotron‐based small‐angle X‐ray scattering (SAXS) with transmission electron microscopy (TEM), we reliably obtained quantitative structural parameters for these fine nanostructures that correlate well with their optical properties as identified by UV/Vis absorption and dark‐field scattering spectroscopy. Through this comprehensive study, we report a growth mechanism for gold plasmonic nanostructures, and the first semiquantitative revelation of the remarkable interplay between their morphology and unique plasmonic properties.     

Yolk-Shell Nanocrystal@ZIF-8 Nanostructures for Gas-Phase Heterogeneous Catalysis with Selectivity Control

A general synthetic strategy for yolk-shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk-shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu(2)O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu(2)O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk-shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis.     

Sulforhodamine Adsorbed Langmuir-Blodgett Layers on Silver Island Films: Effect of Probe Distance on the Metal-Enhanced Fluorescence

Metal-Enhanced Fluorescence (MEF) has become an important method in biomedical sensing. In this paper, we present the distance-dependent MEF of sulforhodamine B (SRB) monolayer on silver island films (SIFs). SRB is electrostatically incorporated into the Langmuir-Blodgett (LB) layers of octadecylamine (ODA) deposited on glass and SIFs substrates. The distances between SRB and SIFs or glass surfaces are controlled by depositing a varied number of inert stearic acid (SA) spacer layers. SRB is incorporated into positively charged LB layers of ODA by immersing the ODA deposited substrates into aqueous solution of SRB. Dye incorporated ODA layers with 10 nm separation distance from the SIFs surface show maximum metal-enhanced fluorescence intensity; ~7-fold increase in intensity as compared to that from the glass surface. The corresponding enhancement factor is reduced with increasing or decreasing the probe distance from the SIFs surface. Additionally, SRB on SIF surfaces show reduced lifetimes. We observed the shortest lifetime from the SRB with 5 nm distance from the SIF surfaces and the lifetime increased consistently with increasing the distances between the fluorophore and the SIFs surface. These observed spectral changes, increase in fluorescence intensity and decreased fluorescence lifetimes, are in accordance with the expected effects due to near-field interactions between the silver nanoparticles and fluorophores. We have also analyzed the complex fluorescence heterogeneous decays on metallic nanostructured surfaces using continuous distributions of decay times. The decay-time distributions appear to be sensitive to the distance between the metal and fluorophore and represent the underlying heterogeneity of the samples. The present systematic study provides significant information on the effect of fluorophore distance on the metal-enhanced fluorescence phenomenon.