Plasmon-enhanced Fluorescence Research Articles

Correction to “Surface Plasmon-Enhanced Fluorescence Spectroscopy on Silver Based SPR Substrates”

Correction to “Surface Plasmon-Enhanced Fluorescence Spectroscopy on Silver Based SPR Substrates”

Single-Molecule Fluorescence Probes Interactions between Photoactive Protein-Silver Nanowire Conjugate and Monolayer Graphene.

In this work, we apply single-molecule fluorescence microscopy and spectroscopy to probe plasmon-enhanced fluorescence and Förster resonance energy transfer in a nanoscale assemblies. The structure where the interplay between these two processes was present consists of photoactive proteins conjugated with silver nanowires and deposited on a monolayer graphene. By comparing the results of continuous-wave and time-resolved fluorescence microscopy acquired for this structure with those obtained for the reference samples, where proteins were coupled with either a graphene monolayer or silver nanowires, we find clear indications of the interplay between plasmonic enhancement and the energy transfer to graphene. Namely, fluorescence intensities calculated for the structure, where proteins were coupled to graphene only, are less than for the structure playing the central role in this study, containing both silver nanowires and graphene. Conversely, decay times extracted for the latter are shorter compared to a protein-silver nanowire conjugate, pointing towards emergence of the energy transfer. Overall, the results show that monitoring the optical properties of single emitters in a precisely designed hybrid nanostructure provides an elegant way to probe even complex combination of interactions at the nanoscale.

Dual-functional nano-photosensitizers: Eosin-Y decorated gold nanorods for plasmon-enhanced fluorescence and singlet oxygen generation.

Photosensitizer (PS) with enhanced fluorescence is attractive for image-guided photodynamic therapy (PDT) due to its dual functional role in Singlet Oxygen Generation (SOG) and producing high fluorescence signals. Here, Eosin-Y (Ey) decorated polymer coated gold nanorods (GNRs) of different aspect ratios are synthesized and introduced as novel plasmon-enhanced nano-photosensitizers for this purpose. We show, upon excitation at 519 nm, simultaneous enhancement in fluorescence and SOG was achieved for the hybrid nanostructure. The best enhancement factors of 110 and 18 for metal-enhanced fluorescence and metal-enhanced SOG, respectively, are obtained with GNRs of length 133 nm and width 45 nm, where Ey is positioned at 12.6 nm from the metal core using layer-by-layer assembly of oppositely charged polymers. The observed plasmonic effect is critically analysed by comparing the near field damping rate along with decay length, far field scattering and nonradiative energy transfer of the nanohybrids.

Sandwich Immuno-RCA Assay with Single Molecule Counting Readout: The Importance of Biointerface Design.

The analysis of low-abundance protein molecules in human serum is reported based on counting of the individual affinity-captured analyte on a solid sensor surface, yielding a readout format similar to digital assays. In this approach, a sandwich immunoassay with rolling circle amplification (RCA) is used for single molecule detection (SMD) through associating the target analyte with spatially distinct bright spots observed by fluorescence microscopy. The unspecific interaction of the target analyte and other immunoassay constituents with the sensor surface is of particular interest in this work, as it ultimately limits the performance of this assay. It is minimized by the design of the respective biointerface and thiol self-assembled monolayer with oligoethylene (OEG) head groups, and a poly[oligo(ethylene glycol) methacrylate] (pHOEGMA) antifouling polymer brush was used for the immobilization of the capture antibody (cAb) on the sensor surface. The assay relying on fluorescent postlabeling of long single-stranded DNA that are grafted from the detection antibody (dAb) by RCA was established with the help of combined surface plasmon resonance and surface plasmon-enhanced fluorescence monitoring of reaction kinetics. These techniques were employed for in situ measurements of conjugating of cAb to the sensor surface, tagging of short single-stranded DNA to dAb, affinity capture of the target analyte from the analyzed liquid sample, and the fluorescence readout of the RCA product. Through mitigation of adsorption of nontarget molecules on the sensor surface by tailoring of the antifouling biointerface, optimizing conjugation chemistry, and by implementing weak Coulombic repelling between dAb and the sensor surface, the limit of detection (LOD) of the assay was substantially improved. For the chosen interleukin-6 biomarker, SMD assay with LOD at a concentration of 4.3 fM was achieved for model (spiked) samples, and validation of the ability of detection of standard human serum samples is demonstrated.

Plasmonic Gold Nanostar-Based Probes with Distance-Dependent Plasmon-Enhanced Fluorescence for Ultrasensitive DNA Methyltransferase Assay.

The ultrasensitive DNA methyltransferase (Dam MTase) assay is of high significance for biomedical research and clinical diagnosis because of its profound effect on gene regulation. However, detection sensitivity is still limited by shortcomings, including photobleaching and weak signal intensities of conventional fluorophores at low concentrations. Plasmonic nanostructures with ultrastrong electromagnetic fields and fluorescence enhancement capability that can overcome these intrinsic defects hold great potential for ultrasensitive bioanalysis. Herein, a silica-coated gold nanostars (Au NSTs@SiO2)-based plasmon-enhanced fluorescence (PEF) probe with 20 "hot spots" was developed for ultrasensitive detection of Dam MTase. Here, the Dam Mtase assay was achieved by detecting the byproduct PPi of the rolling circle amplification reaction. It is worth noting that, benefiting from the excellent fluorescence enhancement capability of Au NSTs originating from their 20 "hot spots", the detection limit of Dam Mtase was reduced by nearly 105 times. Moreover, the proposed Au NST-based PEF probe enabled versatile evaluation of Dam MTase inhibitors as well as endogenous Dam MTase detection in GW5100 and JM110 Escherichia coli cell lysates, demonstrating its potential in biomedical analysis.

Composite nanostructure comprising silver nanopyramids and silver nanoparticles for plasmon-enhanced fluorescence

In this study, a composite silver nanostructure is presented to serve as the platform for highly efficient plasmon-enhanced fluorescence (PEF). The composite nanostructure is realized by first employing nanosphere lithography (NPL) combined with reactive ion etching (RIE) technology to create a silver nanopyramid array, which is then further modified with silver nanoparticles. Utilizing DCJTB as the fluorophore, this composite nanostructure is found to outperform the use of either silver nanoparticles or silver nanopyramid array. Notably, it not only enhances the photoluminescence (PL) but also reduces the carrier lifetime. The influence of the particle size of silver nanoparticles and the height of the silver nanopyramids is also investigated. It is found that larger silver nanoparticles and higher silver nanopyramids lead to stronger local surface plasmon resonance (LSPR), thereby enhancing the PEF phenomenon. Compared with the performance of using bare glass substrate, the PL intensity of using the composite nanostructure is enhanced to 19.4 times and the carrier lifetime is shortened by 61%.

Continuous Monitoring Biosensing Mediated by Single-Molecule Plasmon-Enhanced Fluorescence in Complex Matrices.

Continuous detection of critical markers directly at the point of interest and in undiluted biological fluids represents the next fundamental step in biosensing. The goal of realizing such a platform is utterly challenging because it requires a reversible biosensor that enables the tracking of pico- to nanomolar molecular concentrations over long time spans in a compact device. Here we describe a sensing method based on plasmon-enhanced fluorescence capable of single-molecule detection of unlabeled analyte by employing biofunctionalized gold nanoparticles. The very strong plasmon-enhanced fluorescence signals allow for single-molecule sensing in unaltered biological media, while the use of low-affinity interactions ensures the continuous tracking of increasing and decreasing analyte concentrations with picomolar sensitivity. We demonstrate the use of a sandwich assay for a DNA cancer marker with a limit of detection of picomolar and a time response of 10 min. The enhanced single-molecule signals will allow for miniaturization into a small and cheap platform with multiplexing capability for application in point-of-care diagnostics, monitoring of industrial processes, and safe keeping of the environment.

Light and endogenous enzyme triggered plasmonic antennas for accurate subcellular molecular imaging with enhanced spatial resolution.

Developing accurate tumor-specific molecular imaging approaches holds great potential for evaluating cancer progression. However, traditional molecular imaging approaches still suffer from restricted tumor specificity due to the "off-tumor" signal leakage. In this work, we proposed light and endogenous APE1-triggered plasmonic antennas for accurate tumor-specific subcellular molecular imaging with enhanced spatial resolution. Light activation ensures subcellular molecular imaging and endogenous enzyme activation ensures tumor-specific molecular imaging. In addition, combined with the introduction of plasmon enhanced fluorescence (PEF), off-tumor signal leakage at the subcellular level was effectively reduced, resulting in the significantly enhanced discrimination ratio of tumor/normal cells (∼11.57-fold) which is better than in previous reports, demonstrating great prospects of these plasmonic antennas triggered by light and endogenous enzymes for tumor-specific molecular imaging at the subcellular level.

Plasmonic silver and gold nanoparticles: shape- and structure-modulated plasmonic functionality for point-of-caring sensing, bio-imaging and medical therapy.

Silver and gold nanoparticles have found extensive biomedical applications due to their strong localized surface plasmon resonance (LSPR) and intriguing plasmonic properties. This review article focuses on the correlation among particle geometry, plasmon properties and biomedical applications. It discusses how particle shape and size are tailored via controllable synthetic approaches, and how plasmonic properties are tuned by particle shape and size, which are embodied by nanospheres, nanorods, nanocubes, nanocages, nanostars and core-shell composites. This article summarizes the design strategies for the use of silver and gold nanoparticles in plasmon-enhanced fluorescence, surface-enhanced Raman scattering (SERS), electroluminescence, and photoelectrochemistry. It especially discusses how to use plasmonic nanoparticles to construct optical probes including colorimetric, SERS and plasmonic fluorescence probes (labels/reporters). It also demonstrates the employment of Ag and Au nanoparticles in polymer- and paper-based microfluidic devices for point-of-care testing (POCT). In addition, this article highlights how to utilize plasmonic nanoparticles for in vitro and in vivo bio-imaging based on SERS, fluorescence, photoacoustic and dark-field models. Finally, this article shows perspectives in plasmon-enhanced photothermal and photodynamic therapy.

Plasmon-enhanced photoluminescence spectroscopy of a single molecule in the subnanometer cavity

We investigate the influence of quantum tunneling on the plasmon-enhanced electric field and photoluminescence spectroscopy in the subnanometer nanogap between two adjacent Ag nanoparticles using the quantum-corrected method. We analyze the contribution of nonlocal dielectric response at different separation distances to plasmon-enhanced fluorescence using the D-parameters method. Our results demonstrate the high sensitivity of the quantum-induced effect and nonlocal dielectric response to the separation distance. In cases of shorter separation distances, the localized fields start to decrease due to tunneling current short circuit, resulting in a substantial reduction in Raman enhancement. Additionally, the lower quantum yield caused by loss of electron-hole pairs further quenches the fluorescence intensity in the narrowing gaps. We achieve calculated Raman and fluorescence enhancement factors of 13 and 6 orders of magnitude. Under excitation wavelengths of 660 nm, we achieve spatial resolutions of 1.5 nm and 2.2 nm for Raman and fluorescence spectroscopy.

Enhancement and quenching of plasmon-enhanced spectroscopy of single molecule confined in metallic nanoparticle dimers

We present a theoretical analysis of plasmon-enhanced fluorescence (PEF) and Raman scattering (PERS) spectroscopy of a single molecule confined in the laser-irradiated metallic nanoparticles (NPs) dimer, focusing on the origin of the spectral enhancement and quenching effects. The theoretical method of D-parameters has been used to calculate the dimer distance-dependent nonlocal dielectric effect in Ag and Au NPs. Meanwhile, other damping rates and electric field enhancements are quantitatively computed by finite element method. Moreover, PEF and PERS spectra of rhodamine 6G are obtained within the density-functional theory. Our calculated results show that the PERS mainly depend on the excitation and emission field enhancements, and thus it occurs at the narrower dimer gap due to the stronger localized plasmon coupling. The PEF is related to fluorescence rate caused by the competition between excitation electric field and quantum efficiency, and the increase of former may enhance the fluorescence intensity while the lower latter lead to reduce the intensity as decreasing the dimer distance. The contribution of nonlocal dielectric effect can significantly reduce the quantum efficiency at smaller distance so that it overcomes the excitation field enhancement, leading to the fluorescence quenching for Au NPs dimer. Furthermore, by optimizing the dimer distance and NPs size, the maximum PERS and PEF cross sections reach 10−14 and 10−15 under 2.45 eV laser excitation for Ag NPs dimer, and 10−18 for Au NPs. Our study finely explains the experiment results showed either fluorescence enhancement or quenching with the change of molecule-NPs distance, and better guidance for optimizing the experiments.

Plasmon-enhanced fluorescence of gold nanoparticle/graphene quantum dots for detection of Cr3+ ions

Graphene quantum dots (GQDs), fascinating semiconductors with stable photoluminescence (PL), have important potential applications in the fields of biology, medicine, and new semiconductor devices. However, it is still challenging to overcome the weak PL intensity. Here, we report a strategy for selective resonance enhancement of GQD fluorescence using gold nanoparticles (AuNPs) as plasmas. Interestingly, the addition of low concentration AuNP makes AuNP/GQDs exhibit significant fluorescence enhancement of 2.67 times in the visible range. The addition of high concentration AuNP leads to the formation of an excitation peak at 421 nm and selectively enhances certain radiation modes. We concluded that the main reason for the selective enhancement of PL intensity in high concentration AuNP is the transfer of generous hot electrons at high energy states from AuNP to GQD and relaxation to the ground state. The electron resonance of low concentration AuNP transfers to GQD and relaxes to lower energy levels, exhibiting an overall enhancement of PL intensity. We apply it for detection of the heavy metal ion Cr3+, and verify that it has a correlation coefficient of 97.36%. We believe AuNP/GQDs can be considered excellent candidates for heavy metal detection and high fluorescence bio-imaging.

Surface structured silver-copper bimetallic nanoparticles by irradiation of excimer laser pulses to bilayer thin films

Optical chips based on plasmonic bimetal nanoparticles are of particular interest for various applications, including optical sensors, as well as plasmon-enhanced fluorescence and plasmon-enhanced Raman scattering due to their extraordinary optical properties. In the paper, the formation of the bimetallic nanoparticles on the BK7 substrate and tuning of the LSPR wavelengths by irradiation of excimer laser pulses (193 nm output wavelength and 15 ns temporal pulse linewidth) to silver-copper thin films deposited by the physical vapor deposition (PVD) method is studied. Applying four and five ArF laser pulses with the fluence of 140 mJ cm−2 leads to the formation of spherical Ag-Cu nanoparticles and clusters, respectively, with several LSPR peaks in the UV and visible ranges. Also, sharp and intense LSPR peaks in the 560 to 700 nm spectral range are observed in the absorption spectra of the produced samples with four laser pulses which indicates the ability of the method for tuning the LSPR wavelength. Furthermore, the highest enhancement in the Raman scattering of R6G solution is provided using this sample as a substrate for Raman spectroscopy. Therefore, the laser-produced spherical Ag-Cu NPs by these parameters of laser irradiation can be a good candidate for applications based on LSPR, and Raman or fluorescence enhancement.

Tailoring Geometry of Large-Area Hexagonal Close-Packed Plasmonic Nano-Array Structures for Biosensing

Generally, a large area of plasmonic nanoarray patterns are employed for biosensing. Electromagnetic (EM) field and Quality-factor (Q-factor, the ratio of the resonance wavelength over its bandwidth) of plasmonic patterns, are important to the sensitivity of plasmon-enhanced biosensors, for example, fluorescence and surface enhanced Rama spectroscopy (SERS) biosensors. Hexagonal close-packed plasmonic nanostructures arranged in nano-disk, nano-ring, nano-disk/ring patterns are prevailing configurations using batch processes (e.g., anodic aluminum oxide (AAO)). Different patterns due to size/geometry difference can have different Q-factors. However, the geometry effect on Q-factor in such configurations is not clear. Therefore, we adopted Finite-difference time-domain (FDTD) simulation to numerically study how EM field and Q-factor are modulated by geometric design of large-area hexagonal close-packed gold disk/ring structures. The simulation results provide guidelines to increase Q-factor to improve the sensitivity of plasmon-enhanced biosensors.

Gold nanohole arrays with ring-shaped silver nanoparticles for highly efficient plasmon-enhanced fluorescence

To enhance the interaction between electromagnetic fields and reduce photoluminescence (PL) lifetime in plasmon-enhanced fluorescence, this study presents a novel approach involving the fabrication of gold (Au) nanohole arrays (ANA) decorated with ring-shaped silver nanoparticles (AgNPs) on a silicon-dioxide (SiO2)/silver (Ag) substrate. The surface plasmon resonance coupling in terms of the PL reactions of ANA substrates with and without ring-shaped AgNPs is investigated via experiments and numerical simulations. The remarkable enhancement of PL intensity of the proposed substrate is attributed to increased absorption, which enables the tuning of surface-enhanced electromagnetic fields. Specifically, the Raman signal of rhodamine 6G (R6G) dye and the PL intensity of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) molecules are significantly enhanced 3.7 and 2.2 times, respectively, compared to those of the bare ANA substrate. Meanwhile, the PL lifetime is reduced by 46.15%. These results confirm that ANAs decorated with ring-shaped AgNPs can significantly improve plasmon-enhanced fluorescence. The findings presented herein demonstrate the potential to revolutionize biosensing, imaging, and photonics.

Use of Single-Molecule Plasmon-Enhanced Fluorescence to Investigate Ligand Binding to G-Quadruplex DNA.

Single-molecule measurements are crucial for studying the interactions between G-quadruplex (GQ) DNA and ligands, as they provide higher resolution and sensitivity compared to those of bulk measurements. In this study, we employed plasmon-enhanced fluorescence to investigate the real-time interaction between the cationic porphyrin ligand TmPyP4 and different topologies of telomeric GQ DNA at the single-molecule level. By analyzing the time traces of the fluorescence bursts, we extracted dwell times for the ligand. For parallel telomeric GQ DNA, the dwell time distribution followed a biexponential fit, yielding mean dwell times of 5.6 and 18.6 ms. For the antiparallel topology of human telomeric GQ DNA, plasmon-enhanced fluorescence of TmPyP4 was observed, with dwell time distributions following a single-exponential fit and a mean dwell time of 5.9 ms. Our approach allows the nuances of GQ-ligand interactions to be captured and holds promise for studying weakly emitting GQ ligands at the single-molecule level.

Plasmon-enhanced fluorescence for ellagic acid detection based on surface structure of gold nanoparticles.

Ellagic acid (EA), as a natural polyphenolic acid, is considered a naturally occurring inhibitor of carcinogenesis. Herein, we developed a plasmon-enhanced fluorescence (PEF) probe for EA detection based on silica-coated gold nanoparticles (Au NPs). A silica shell was designed to control the distance between silica quantum dots (Si QDs) and Au NPs. The experimental results indicated that an 8.8-fold fluorescence enhancement was obtained compared with the original Si QDs. Three-dimensional finite-difference time-domain (3D-FDTD) simulations further demonstrated that the local electric field enhancement around Au NPs led to the fluorescence enhancement. In addition, the fluorescent sensor was applied for the sensitive detection of EA with a detection limit of 0.14μM. It can be used to detect EA in pomegranate rind with a recovery rate of 100.26-107.93%. It can also be applied to the analysis of other substances by changing the identification substances. These experimental results indicated that the probe provides a good option for clinical analysis and food safety.

Self-assembled colloidal gold nanoparticles as substrates for plasmon enhanced fluorescence

Decades of intense research in the field of nanoscience have led to the ability to produce nanoparticles (NPs) with controlled composition, shape, and size. One of the next key challenges is the self-assembly of appropriate NP building blocks into larger systems to obtain micro- or macroscale materials. To achieve this, self-assembly protocols must not only produce high-quality structures, but also deliver the assemblies of interest to desired locations on a substrate. In this review, we discuss different self-assembly strategies, focusing on colloidal gold NPs and applications as plasmon-enhanced fluorescence (PEF) platforms. These plasmonic substrates have been used for biosensing and cell imaging, based on the enhancement of fluorescent emitters, and applied to improve the emission efficiency of luminescent NPs. It is important to note that higher fluorescence enhancement relies on precise control of the location of gold NPs and fluorescent emitters on the plasmonic substrate. Despite the diversity of available self-assembly strategies, many of them provide similar levels of structural control over the placement of gold NPs on the substrate. To highlight this, we have organized the discussion according to strategies that result in similar degrees of structural control over the placement of gold NPs and its associated PEF effect.

Plasmon-enhanced fluorescence from synergistic engineering of graphene oxide and sharp-edged silver nanorods mediated with castor protein for cellphone-based attomolar sensing

Environmentally benign plasmonic nanomaterials are increasingly used for sensing applications in nanophotonics and biomedical diagnostics. This study reports the utility of castor proteins as a unique biomaterial for plasmonic sensing application. We demonstrate an intriguing strategy to obtain rod-shaped silver nanoparticles (AgNPs) using biocompatible castor protein as both the reducing and capping agent in a frugal UV-light induced one-pot rapid synthesis. These AgNPs thus synthesized were juxtaposed with propagating surface plasmon polaritons of surface plasmon-coupled emission (SPCE) platform to generate the photo-plasmonic coupling phenomenon in the three nano-configurations of spacer, cavity, and extended cavity. Dramatically enhanced, sharply directional and distinctly polarized fluorescence enhancement was achieved using AgNPs on account of the sharp edges and pointed ends. The observed 1000-fold fluorescence enhancement is attributed to the hybrid coupling of the π-plasmons of graphene oxide (GO) with the AgNPs supporting lightning rod effect on account of the rod-shaped nano-morphology. Such realization of abundant plasmonic hotspots over the SPCE substrate assisted in attaining attomolar sensitivity. The performance of the subject platform was further validated using MATLAB simulations and user-friendly smartphone-based detector system for decentralizing the lab-confined biosensing architectures. We firmly believe that the eco-friendly method developed for nanosynthesis and the user-friendly technique demonstrated using smartphone technology would immensely benefit the biosensing approaches in resource-limited settings.

Large-scale controllable fabrication of aluminum nanobowls for surface plasmon-enhanced fluorescence

Large-scale controllable fabrication of aluminum nanobowls for surface plasmon-enhanced fluorescence