Wavelength, concentration, and distance dependence of nonradiative energy transfer to a plane of gold nanoparticles

A plasmonic metasurface reveals differential motility of breast cancer cell lines at initial phase of adhesion

A plasmonic metasurface composed of a self-assembled monolayer of gold nanoparticles allows for fluorescence imaging with high spatial resolution, owing to the collective excitation of localized surface plasmon resonance. Taking advantage of fluorescence imaging confined to the nano-interface, we examined actin organization in breast cancer cell lines with different metastatic potentials during cell adhesion. Live-cell fluorescence imaging confined within tens of nanometers from the substrate shows a high actin density spanning < 1 μm from the cell edge. Live-cell imaging revealed that the breast cancer cell lines exhibited different actin patterns during the initial phase of cell adhesion (∼ 1 h). Non-tumorous MCF10A cells exhibited symmetric actin localization at the cell edge, whereas highly metastatic MDA-MB-231 cells showed asymmetric actin localization, demonstrating rapid polarization of MDA-MB-231 cells upon adhesion. The rapid actin organization observed by our plasmonic metasurface-based fluorescence imaging provides information on how quickly cancer cells sense the underlying substrate.

Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Single-nucleobase resolution of a surface energy transfer nanoruler for in situ measurement of aptamer binding at the receptor subunit level in living cells

In situ identification of aptamer-binding targets on living cell membrane surfaces is of considerable interest, but a major challenge, specifically, when advancing recognition to the level of membrane receptor subunits. Here we propose a novel nanometal surface energy transfer (NSET) based nanoruler with a single-nucleobase resolution (SN-nanoruler), in which FAM-labeled aptamers and single-sized gold nanoparticle (GNP) antibody conjugates act as a donor and an acceptor. A single nucleobase resolution of the SN-nanoruler was experimentally illustrated by molecular size, orientation, quenching nature, and other dye-GNP pairs. The SN-nanoruler provides high reproducibility and precision for measuring molecule distance on living cell membranes at the nanometer level owing to only the use of single-sized antibody-capped GNPs. In situ identification of the aptamer binding site was advanced to the protein subunit level on the living cell membrane for the utilization of this SN-nanoruler. The results suggest that the proposed strategy is a solid step towards the wider application of optical-based rulers to observe the molecular structural configuration and dynamic transitions on the membrane surface of living cells.

Enhanced resonance energy transfer in gold nanoparticles bifunctionalized by tryptophan and riboflavin and its application in fluorescence bioimaging

Gold nanoparticles were functionalized by amino acid tryptophan and vitamin riboflavin - a resonance energy transfer (RET) pair of biomolecules. The presence of the gold nanoparticles resulted in 65% increase in RET efficiency. Because of enhanced RET efficiency, the photobleaching dynamics of the fluorescent molecules at the surface of the nanoparticles is different from that of molecules in solution. The observed effect was used for detection of the functionalized nanoparticles within biological material rich with autofluorescent species. Synchrotron radiation deep-ultraviolet fluorescence microscopy is used to study the photobleaching dynamics of the fluorescence centers within human hepatocellular carcinoma Huh7.5.1 cells incubated with the nanoparticles. The fluorescent centers were classified according to their photobleaching dynamics, which enabled the discrimination of the cell areas where the accumulation of the nanoparticles takes place, even though the particles were smaller than the spatial resolution of the images.

Förster and nanometal surface-energy transfer in CsPbCl3/Yb3+ quantum-cutting multilayer structures

Recently, in nanophotonics, thin metal films owing to the plasmon modes they support and their perovskite nanostructures exhibit novel optical properties, which have attracted considerable interest. Both the Förster resonant energy transfer (FRET) of the dopant-induced right-angled Yb³⁺-V_Pb-Yb³⁺ defect state and a pair of Yb³⁺ ions in all-inorganic perovskite nanocrystal (PeNC) CsPbCl₃:Yb³⁺ quantum-cutting (QC) materials and the nanometal surface-energy transfer (NSET) of the excitons of PeNC-Ag nanoparticles (NPs) were investigated experimentally in CsPbCl₃:Yb³⁺/PMMA/Ag/Si (CYA_ii = 1, 2, 3, 4, 5, 6), CsPbCl₃:Yb³⁺/PMMA/Si (CY_i), and CsPbCl₃/PMMA/Ag/Si (CA_i), representing three species of multilayer structures. It was found that due to the mediation of the Ag film and an increase in the interaction volume of donors-acceptors, FRET efficiencies increased from 26% to 66% as the spacer (or wave-guiding layer) thicknesses decreased from 63.7 to 17.8 nm. The energy-transfer efficiencies of CA_i in the NSET in the surface-surface scheme followed a d^-1.6-distance dependence. This distance dependence approached the d^-2-distance dependence expected of a point-to-surface or 0D-2D energy transfer (ET). The ET in quantum cutting (QC) modulated by plasmons undoubtedly paves a way for improving the FRET and NSET performances of materials.

Resonant energy transfer between rare earth atomic layers in nanolaminate films

Förster resonant energy transfer between atoms separated at a distance of a few nanometers has strong relevance to different properties of matter. In this work, the resonant energy transfer rate is derived from the electric potential in a system with one dipole interacting with a separated 2D plane of dipoles. It shows an R-² (R: distance between dipole and 2D plane of dipoles) dependency on the distance of dipole layers, which is different from previous theoretical evaluations with an R-⁴ dependency. The electroluminescence (EL) properties are studied in different rare earth (Re: Tm, Tb, Ho, Yb, Er) distributed single atomic layer doped Al₂O₃ nanolaminates prepared by atomic layer deposition, in which the distance between single atomic layers of Re³⁺ is modulated at the atomic scale. Our theoretical results are consistent with the changes of EL intensity and decay time with the distance between the single atomic rare earth doping layers. This result is crucial for increasing the accuracy in biosensing and design of photonic materials.

Understanding Förster Resonance Energy Transfer in the Sheet Regime with DNA Brick-Based Dye Networks

Controlling excitonic energy transfer at the molecular level is a key requirement for transitioning nanophotonics research to viable devices with the main inspiration coming from biological light-harvesting antennas that collect and direct light energy with near-unity efficiency using Förster resonance energy transfer (FRET). Among putative FRET processes, point-to-plane FRET between donors and acceptors arrayed in two-dimensional sheets is predicted to be particularly efficient with a theoretical 1/r⁴ energy transfer distance (r) dependency versus the 1/r⁶ dependency seen for a single donor-acceptor interaction. However, quantitative validation has been confounded by a lack of robust experimental approaches that can rigidly place dyes in the required nanoscale arrangements. To create such assemblies, we utilize a DNA brick scaffold, referred to as a DNA block, which incorporates up to five two-dimensional planes with each displaying from 1 to 12 copies of five different donor, acceptor, or intermediary relay dyes. Nanostructure characterization along with steady-state and time-resolved spectroscopic data were combined with molecular dynamics modeling and detailed numerical simulations to compare the energy transfer efficiencies observed in the experimental DNA block assemblies to theoretical expectations. Overall, we demonstrate clear signatures of sheet regime FRET, and from this we provide a better understanding of what is needed to realize the benefits of such energy transfer in artificial dye networks along with FRET-based sensing and imaging.

Surface energy transfer in hybrid halide perovskite/plasmonic Au nanoparticle composites

The incorporation of plasmonic metal nanoparticles (NPs) into the multilayered architecture of perovskite solar cells (PSCs) has been a recurrent strategy to enhance the performance of photovoltaic devices from the early development of this technology. However, the specific photophysical interactions between the metal NPs and the hybrid halide perovskites are still not completely understood. Herein, we investigate the influence of Au NPs on the photoluminescence (PL) signal of a thin layer of the CH₃NH₃PbI₃ hybrid perovskite. Core-shell Au@SiO₂ NPs with a tunable thickness of the SiO₂ shell were used to adjust the interaction distance between the plasmonic NPs and the perovskite layer. Complete quenching of the PL signal in the presence of the Au NPs is measured together with the gradual recovery of the PL intensity at a thicker thickness of the SiO₂ shell. A nanometal surface energy transfer (NSET) model is employed to reasonably fit the experimental quenching efficiency. Thus, the energy transfer deactivation is revealed as a detrimental process occurring in the PSCs since it funnels the photon energy into the non-active excited state of the Au NPs. This work indicates that tuning the distance between the plasmonic NPs and the perovskite materials by a silica shell may be a simple and straightforward strategy for further improving the efficiency of PSCs.

Investigating the effect of 6-mercaptohexanol on the performance of a biosensor based on nanosurface energy transfer between gold nanoparticles and quantum dots

Nanosurface energy transfer (NSET)-based sensors have been widely developed using various pairs of nanomaterials including gold nanoparticles (AuNPs) and quantum dots (QDs). However, a low signal to background ratio is one of the most important problems that researchers are continually trying to solve. Herein, we present a 6-mercaptohexanol (MCH) modified MCH/DNA-Au-QD sensor for the detection of nucleic acids and MUC1. Interestingly, an unexpected effect of MCH was found in enhancing the fluorescence recovery ratio, therefore yielding a higher signal to background ratio. Through further investigation, we perceive the enhancement as a result of lowering of the NSET efficiency between free DNA-AuNPs and free DNA-QDs, which arises from the stretching of adsorbed DNA on the surface of AuNPs. The employment of MCH endowed the sensor with a wider linear range from 5 nM to 120 nM and a relatively lower LOD of 1.19 nM in nucleic acid detection, outperforming the original DNA-Au-QD sensor. Furthermore, the application of the sensor can be further extended to MUC1 detection. This study offers a better understanding of the NSET process between QDs and AuNPs and also initiates a new approach for the performance optimization of analogous NSET-based sensors.

Modulated Luminescence of Lanthanide Materials by Local Surface Plasmon Resonance Effect

Lanthanide materials have great applications in optical communication, biological fluorescence imaging, laser, and so on, due to their narrow emission bandwidths, large Stokes' shifts, long emission lifetimes, and excellent photo-stability. However, the photon absorption cross-section of lanthanide ions is generally small, and the luminescence efficiency is relatively low. The effective improvement of the lanthanide-doped materials has been a challenge in the implementation of many applications. The local surface plasmon resonance (LSPR) effect of plasmonic nanoparticles (NPs) can improve the luminescence in different aspects: excitation enhancement induced by enhanced local field, emission enhancement induced by increased radiative decay, and quenching induced by increased non-radiative decay. In addition, plasmonic NPs can also regulate the energy transfer between two close lanthanide ions. In this review, the properties of the nanocomposite systems of lanthanide material and plasmonic NPs are presented, respectively. The mechanism of lanthanide materials regulated by plasmonic NPs and the scientific and technological discoveries of the luminescence technology are elaborated. Due to the large gap between the reported enhancement and the theoretical enhancement, some new strategies applied in lanthanide materials and related development in the plasmonic enhancing luminescence are presented.

Fluorescence Coupling to Internal Modes of 1D Photonic Crystals Characterized by Back Focal Plane Imaging

The coupling of fluorescence with surface electromagnetic modes, such as surface plasmons on thin metal films or Bloch surface waves (BSW) on truncated one-dimensional photonic crystals (1DPC), are presently utilized for many fluorescence-based applications. In addition to the surface wave, 1DPCs also support other electromagnetic modes that are confined within the 1DPC structure. These internal modes (IMs) have not received much attention for fluorescence coupling due to lack of spatial overlap of their electric fields with the surface bound fluorophores. However, our recent studies have indicated that the fluorescence coupling with IMs occurs quite efficiently. This observed internal mode-coupled emission (IMCE) is (similar to BSW-coupled emission) indeed wavelength dependent, directional and S-polarized. In this paper, we have carried out back-focal plane (BFP) imaging to reveal that the IMs of 1DPCs can couple with surface bound excited dye molecules, with or without a BSW mode presence. Depending on the emission wavelength, the coupling is observed with BSW and IMs or only IMs of the 1DPC structure. The experimental results are well matching with numerical simulations. The occurrence of IMCE regardless of the availability of BSWs removes the dependence on just the surface mode for obtaining coupled emission from 1DPCs. The observation of IMCE is expected to widen the scope of 1DPCs for surface-based fluorescence sensing and assays.

Förster Resonance Energy Transfer and the Local Optical Density of States in Plasmonic Nanogaps

Förster resonance energy transfer (FRET) is a fundamental phenomenon in photosynthesis and is of increasing importance for the development and enhancement of a wide range of optoelectronic devices, including color-tuning LEDs and lasers, light harvesting, sensing systems, and quantum computing. Despite its importance, fundamental questions remain unanswered on the FRET rate dependency on the local density of optical states (LDOS). In this work, we investigate this directly, both theoretically and experimentally, using 30 nm plasmonic nanogaps formed between a silver nanoparticle and an extended silver film, in which the LDOS can be controlled using the size of the silver nanoparticle. Experimentally, uranin-rhodamine 6G donor-acceptor pairs coupled to such nanogaps yielded FRET rate enhancements of 3.6 times. This, combined with a 5-fold enhancement in the emission rate of the acceptor, resulted in an overall 14-fold enhancement in the acceptor's emission intensity. By tuning the nanoparticle size, we also show that the FRET rate in those systems is linearly dependent on the LDOS, a result which is directly supported by our finite difference time domain (FDTD) calculations. Our results provide a simple but powerful method to control FRET rate via a direct LDOS modification.

Fluorophore Coupling to Internal Modes of Bragg Gratings

Multilayer structures with two dielectrics having different optical constants and no structural features in the x-y plane can display photonic band gaps (PBGs) and are called one-dimensional photonic crystals (1DPCs). If the top layer thickness is carefully selected, the electromagnetic energy can be trapped at the top surface. These highly enhanced fields are called Bloch surface waves (BSWs). The BSW resonance angles are sensitive to the dielectric constant above the top dielectric layer. As a result, BSW structures have been used for surface plasmon resonance-like measurements without the use of a metal film. However, the emphasis on surface-localized BSWs has resulted in limited interest in fluorophore interactions with other optical modes of 1DPCs or Bragg gratings without the different thickness top layer. Herein, three different fluorescent probes were used to cover the short, center, and long wavelengths of the PBG. We demonstrate efficient coupling of fluorophores to both the BSW and internal modes (IMs) of a 1DPC. Coupling to the IM is expected to be low because of the micron-scale distances between the fluorophores and IM, which exists inside the Bragg gratings. At different wavelengths or observation angles, the IM-coupled emission (IMCE) can occur with the first three modes of the multilayer. This coupling is not dependent on a BSW mode. IMCE was also observed for a monolayer of fluorophore-labeled protein. IMCE enables sensitive detection of surface-bound fluorophores. Applications are anticipated in high sensitivity detection and super-resolution imaging.

Plasmonic Enhanced Gold Nanoclusters-Based Photoelectrochemical Biosensor for Sensitive Alkaline Phosphatase Activity Analysis

Low-toxicity gold nanoclusters-decorated Ag@SiO₂ (Au NCs-Ag@SiO₂) nanocomposites modified plasmonic photoelectrodes were first fabricated to improve the photoelectric properties of Au NCs and practical application in biological detection. Through adjusting distance between Au NCs and plasmonic silver nanoparticles (Ag NPs), the photocurrent intensity of Au NCs enhanced by 3.8 times attributed to strong competition between enhancement functions of hot electron transfer, local electric field, light scattering effects, and quenching functions of nonradiative energy transfer. Further comparison between experimental results and theoretical simulations were conducted to gain a deeper understanding toward the photoelectric enhancement mechanism. Moreover, Au NCs-Ag@SiO₂ nanocomposites was successfully applied to the construction of photoelectrochemical (PEC) biosensors for sensitively detecting alkaline phosphatase activity. This proposed PEC biosensor showed a wide linear range from 0.04 to 400 U·L^-1, and a low detection limit of 0.022 U·L^-1.

A Self-Assembled Plasmonic Substrate for Enhanced Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer (FRET) has found widespread uses in biosensing, molecular imaging, and light harvesting. Plasmonic metal nanostructures offer the possibility of engineering photonic environment of specific fluorophores to enhance the FRET efficiency. However, the potential of plasmonic nanostructures to enable tailored FRET enhancement on planar substrates remains largely unrealized, which are of considerable interest for high-performance on-surface bioassays and photovoltaics. The main challenge lies in the necessitated concurrent control over the spectral properties of plasmonic substrates to match that of fluorophores and the fluorophore-substrate spacing. Here, a self-assembled plasmonic substrate based on polydopamine (PDA)-coated plasmonic nanocrystals is developed to effectively address this challenge. The PDA coating not only drives interfacial self-assembly of the nanocrystals to form closely packed arrays with customized optical properties, but also can serve as a tailored nanoscale spacer between the fluorophores and plasmonic nanocrystals, which collectively lead to optimized fluorescence enhancement. The biocompatible plasmonic substrate that allows convenient bioconjugation imparted by PDA has afforded improved FRET efficiency in DNA microarray assay and FRET imaging of live cells. It is envisioned that the self-assembled plasmonic substrates can be readily integrated into fluorescence-based platforms for diverse biomedical and photoconversion applications.

Exploring Energy Transfer in a Metal/Perovskite Nanocrystal Antenna to Drive Photocatalysis

The use of all-inorganic perovskite nanocrystals (PeNCs) in photocatalytic systems has been limited because of their instability in polar solvents. Encapsulation of PeNCs in inorganic or polymeric matrices has been shown to be effective in overcoming such instability issues, yet studies on charge and energy extraction from these composite systems are still rare. Herein, we explore the capacity of CsPbBr₃ PeNC/AlO_x composite films to drive chemical reactions by coupling them to plasmonic AgNCs. AlO_x is used both as a stabilizing layer and as a spacer to study distance-dependent excitation energy transfer, which reveals a migration of energy from the PeNCs toward the AgNCs. We then utilize this pooled energy for a plasmon-mediated methylene blue desorption where we demonstrate enhancement effects of spectral and spatial absorption on the reaction outcome due to the coupling to PeNCs.

Plasmon-mediated nonradiative energy transfer from a conjugated polymer to a plane of graphene-nanodot-supported silver nanoparticles: an insight into characteristic distance

Hybrid nanostructures comprising conjugated polymers (CPs) and plasmonic metals show excellent performance in light harvesting. However, the energy transfer mechanism of the CP film to nearby metal nanoparticles, especially knowledge of the characteristic distance, is still unclear. Here, quenching of the emission of a CP film in proximity to a monolayer of graphene-nanodot-supported silver nanoparticles (GND-Ag NPs) is investigated. Uniform Ag NPs with D = 3.2 nm were grown on GNDs in situ under mild light irradiation, and a series of bilayer structures of GND-Ag NPs/CPs were constructed by spin-coating blue, green and red light-emitting poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV),respectively, on top of the GND-Ag NP plane. The spacer distance was controlled by the layers of assembled polyelectrolytes. Both steady and transient photoluminescence (PL) spectra showed emission quenching of the bilayer structures, providing the maximum efficiency of 99% for the F8BT films. The surface density of GND-Ag NPs and the spacer distance-dependent PL quenching data were analyzed within the plasmonic resonant energy transfer model, and the extracted characteristic distances are 6 nm, 3 nm and 10 nm for the PFO, F8BT and MEH-PPV systems, respectively. Current-sensing atomic force microscopy shows that the GND-Ag NPs/F8BT film exhibits enhanced electrical conductivity. These results are believed to be important for the development of plasmonic enhanced polymer photovoltaics and photocatalysis.

Plasmon-Enhanced Fluorescence Resonance Energy Transfer

In this review, we firstly introduce physical mechanism of fluorescence resonance energy transfer (FRET), the methods to measure FRET efficiency, and the applications of FRET. Secondly, we introduce the principle and applications of plasmon-enhanced fluorescence (PEF). Thirdly, we focused on the principle and applications of plasmon-enhanced FRET. This review can promote further understanding of FRET and PE-FRET.

Excitation Wavelength-Dependent Photoluminescence Decay of Hybrid Gold/Quantum Dot Nanostructures

Hybrid nanostructures comprised of metal nanoparticles (MNPs) and quantum dots (QDs) have been found to exhibit unique, new optical properties due to the interaction that occurs between the MNPs and QDs. The aim of this work is to understand how the exciton-plasmon interaction in these systems is dependent on the excitation wavelength. The nanoassemblies consisted of gold (Au) NPs coated in a silica (SiO₂) shell of a controlled thickness and core/shell CdSe/CdS QDs adsorbed onto the SiO₂ shells. Our findings show that the photoluminescence lifetimes of the hybrid constructs are dependent on the excitation wavelength relative to the localized surface plasmon resonance (LSPR) of the Au NPs. When the excitation wavelength is closer to the LSPR, the photoluminescence decay of the hybrid structures is faster. We demonstrate that by tuning the excitation wavelength close to the resonance, there is an enhancement in the exciton-plasmon coupling between the Au NPs and QDs resulting in a shortening in the QD photoluminescence lifetime. We then propose a possible mechanism to explain this excitation wavelength-dependent phenomenon.

Multi-segmented CdS-Au nanorods for electrochemiluminescence bioanalysis

In this study, we have developed a programmable electrochemiluminescence (ECL) system based on multi-segmented CdS-Au nanorod arrays with a sequential and highly tunable structure. The nanorod arrays were synthesized by an electrodeposition method using anodic aluminum oxide (AAO) as the template in which the Au and CdS segments were alternately electrodeposited. Compared to pure CdS nanorod arrays, multi-segmented CdS-Au nanorod arrays have showed a better ECL performance, which can be attributed to two factors: the favorable electron transfer and the surface plasma resonance (SPR) effect of the Au segment. On the one hand, we demonstrated that the Au segment can increase the charge transfer rate of CdS, which is beneficial for the ECL process because the generation of the radical state needs to accept electrons and then generate the radical state. On the other hand, the SPR of Au plasmon-induced local electromagnetic field enhancement can increase the radiative decay rate of CdS which makes the ECL process more efficient and lead to a higher ECL intensity. And also, an ECL sensor with multi-segmented CdS-Au nanorod arrays was constructed to detect prostate protein antigen (PSA). This study provides some basis for designing high-performance ECL emission materials and the construction of biosensors.

Exciton-Plasmon Interaction between AuNPs/Graphene Nanohybrids and CdS Quantum Dots/TiO2 for Photoelectrochemical Aptasensing of Prostate-Specific Antigen

A competitive-displacement reaction strategy based on target-induced dissociation of gold nanoparticle coated graphene nanosheet (AuNPs/GN) from CdS quantum dot functionalized mesoporous titanium dioxide (CdS QDs/TiO₂) was designed for the sensitive photoelectrochemical (PEC) aptasensing of prostate-specific antigen (PSA) through the exciton-plasmon interaction (EPI) between CdS QDs and AuNPs. To construct such an aptasensing system, capture DNA was initially conjugated covalently onto CdS QDs/TiO₂-modified electrode, and then AuNPs/GN-labeled PSA aptamer was bound onto biofunctionalized CdS QDs/TiO₂ via hybridization chain reaction of partial bases with capture DNA. Introduction of AuNPs/GN efficiently quenched the photocurrent of CdS QDs/TiO₂ thanks to energy transfer. Upon addition of target PSA, the sandwiched aptamer between CdS QDs/TiO₂ and AuNPs/GN reacted with the analyte analyte, thus resulting in the dissociation of AuNPs/GN from the CdS QDs/TiO₂ to increase the photocurrent. Under optimum conditions, the aptasensing platform exhibited a high sensitivity for PSA detection within a dynamic linear range of 1.0 pg/mL to 8.0 ng/mL at a low limitat of detection of 0.52 pg/mL. The interparticle distance of exciton-plasmon interaction and contents of AuNPs corresponding to EPI effect in this system were also studied. Good selectivity and high reproducibility were obtained for the analysis of target PSA. Importantly, the accuracy and matrix effect of PEC aptasensor was evaluated for the determination of human serum specimens and newborn calf serum-diluted PSA standards, giving a well-matched result with the referenced PSA ELISA kit.

Localized surface plasmon resonance based biosensing

INTRODUCTION

Bioanalytical sensing based on the principle of localized surface plasmon resonance experiences is currently an extremely rapid development. Novel sensors with new kinds of plasmonic transducers and innovative concepts for the signal development as well as read-out principles were identified. This review will give an overview of the development of this field. Areas covered: The focus is primarily on types of transducers by preparation or dimension, factors for optimal sensing concepts and the critical view of the usability of these devices as innovative sensors for bioanalytical applications. Expert commentary: Plasmonic sensor devices offer a high potential for future biosensing given that limiting factors such as long-time stability of the transducers, the required high sensitivity and the cost-efficient production are addressed. For higher sensitivity, the design of the sensor in shape and material has to be combined with optimal enhancement strategies. Plasmonic nanoparticles from bottom-up synthesis with a post-synthetic processing show a high potential for cost-efficient sensor production. Regarding the measurement principle, LSPRi offers a large potential for multiplex sensors and can provide a high-throughput as well as highly paralleled sensing. The main trends are expected towards optimal LSPR concepts which represent cost-efficient and robust point-of-care solutions, and the use of multiplexed devices for clinical applications.

Environmentally responsive plasmonic nanoassemblies for biosensing

Assemblies of plasmonic nanoparticles enable new modalities for biosensing.

High-resolution imaging of a cell-attached nanointerface using a gold-nanoparticle two-dimensional sheet

This paper proposes a simple, effective, non-scanning method for the visualization of a cell-attached nanointerface. The method uses localized surface plasmon resonance (LSPR) excited homogeneously on a two-dimensional (2D) self-assembled gold-nanoparticle sheet. The LSPR of the gold-nanoparticle sheet provides high-contrast interfacial images due to the confined light within a region a few tens of nanometers from the particles and the enhancement of fluorescence. Test experiments on rat basophilic leukemia (RBL-2H3) cells with fluorescence-labeled actin filaments revealed high axial and lateral resolution even under a regular epifluorescence microscope, which produced higher quality images than those captured under a total internal reflection fluorescence (TIRF) microscope. This non-scanning-type, high-resolution imaging method will be an effective tool for monitoring interfacial phenomena that exhibit relatively rapid reaction kinetics in various cellular and molecular dynamics.

Plasmon-Coupled Resonance Energy Transfer

In this study, we overview resonance energy transfer between molecules in the presence of plasmonic structures and derive an explicit Förster-type expression for the rate of plasmon-coupled resonance energy transfer (PC-RET). The proposed theory is general for energy transfer in the presence of materials with any space-dependent, frequency-dependent, or complex dielectric functions. Furthermore, the theory allows us to develop the concept of a generalized spectral overlap (GSO) J̃ (the integral of the molecular absorption coefficient, normalized emission spectrum, and the plasmon coupling factor) for understanding the wavelength dependence of PC-RET and to estimate the rate of PC-RET W_ET. Indeed, W_ET = (8.785 × 10^-25 mol) ϕ_Dτ_D^-1J̃, where ϕ_D is donor fluorescence quantum yield and τ_D is the emission lifetime. Simulations of the GSO for PC-RET show that the most important spectral region for PC-RET is not necessarily near the maximum overlap of donor emission and acceptor absorption. Instead a significant plasmonic contribution can involve a different spectral region from the extinction maximum of the plasmonic structure. This study opens a promising direction for exploring exciton transport in plasmonic nanostructures, with possible applications in spectroscopy, photonics, biosensing, and energy devices.

Exciton enhancement and exciplex quenching by plasmonic effect of Aluminum nanoparticle arrays in a blue organic light emitting diode

We report the investigation of plasmonic effect of array of aluminum nanoparticles (Al-NPs) on blue micro-OLED subject to exciplex emission. N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) andcarbazol derivative 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) have been used as the emitting layer (EML) and hole transport layer (HTL), respectively. For the reference µ-OLED without Al-NPs, we observed two emission peaks attributed to CBP emission and exciplex emission formed at the NPB/CBP (EML/HTL) interface. By the incorporation of the Al-NPs array, obtained by e-beam lithography technique on the ITO anode, the exciplex emission has been widely depressed. Moreover, thanks to localized surface plasmon resonance (LSPR), an enhancement of the CBP emission has been achieved indicating an efficient energy coupling between the LSPR of the Al-NPs and the CBP excitons. Thus, an enhancement of about 20% of the efficiency of the µ-OLED with Al-NPs in comparison to the reference device has been obtained.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d ^-4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

Multiphoton Emission Enhancement from a Single Colloidal Quantum Dot Using SiO2-Coated Silver Nanoparticles

The enhancement of multiphoton emission from a single colloidal nanocrystal quantum dot (NQD) interacting with a plasmonic nanostructure was investigated using SiO₂-coated silver nanoparticles (Ag/SiO₂) as the plasmonic nanostructure. Using Ag/SiO₂ with five different SiO₂ shell thicknesses, we observed modification of the emission behavior depending on the distance between the NQD and silver nanoparticle (AgNP). The single-photon emission from a single NQD converted to multiphoton emission with a shortening of the emission lifetime as the NQD-AgNP distance decreased, whereas an increase and decrease in the emission intensity were observed. From the distance-dependent results, we concluded that the probability of multiphoton emission was increased by the quenching of the single-exciton state due to energy transfer from the NQD to the AgNP and that the emission intensity was modified by the enhancement of the excitation rate and quenching. These results indicate that the plasmonic nanostructure is very effective in controlling the emission photon statistics, that is, single- and multi-photon emission and the emission intensity from the single NQD, which is difficult to achieve in an NQD alone.

Influence of plasmonic array geometry on energy transfer from a quantum well to a quantum dot layer

A range of seven different Ag plasmonic arrays formed using nanostructures of varying shape, size and gap were fabricated using helium-ion lithography (HIL) on an InGaN/GaN quantum well (QW) substrate. The influence of the array geometry on plasmon-enhanced Förster resonance energy transfer (FRET) from a single InGaN QW to a ∼80 nm layer of CdSe/ZnS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix is investigated. It is shown that the energy transfer efficiency is strongly dependent on the array properties and an efficiency of ∼51% is observed for a nanoring array. There were no signatures of FRET in the absence of the arrays. The QD acceptor layer emission is highly sensitive to the array geometry. A model was developed to confirm that the increase in the QD emission on the QW substrate compared with a GaN substrate can be attributed solely to plasmon-enhanced FRET. The individual contributions of direct enhancement of the QD layer emission by the array and the plasmon-enhanced FRET are separated out, with the QD emission described by the product of an array emission factor and an energy transfer factor. It is shown that while the nanoring geometry results in an energy transfer factor of ∼1.7 the competing quenching by the array, with an array emission factor of ∼0.7, results in only an overall gain of ∼14% in the QD emission. The QD emission was enhanced by ∼71% for a nanobox array, resulting from the combination of a more modest energy transfer factor of 1.2 coupled with an array emission factor of ∼1.4.

Plasmonic Nanoantennas Enable Forbidden Förster Dipole-Dipole Energy Transfer and Enhance the FRET Efficiency

Förster resonance energy transfer (FRET) plays a key role in biochemistry, organic photovoltaics, and lighting sources. FRET is commonly used as a nanoruler for the short (nanometer) distance between donor and acceptor dyes, yet FRET is equally sensitive to the mutual dipole orientation. The orientation dependence complicates the FRET analysis in biological samples and may even lead to the absence of FRET for perpendicularly oriented donor and acceptor dipoles. Here, we exploit the strongly inhomogeneous and localized fields in plasmonic nanoantennas to open new energy transfer routes, overcoming the limitations from the mutual dipole orientation to ultimately enhance the FRET efficiency. We demonstrate that the simultaneous presence of perpendicular near-field components in the nanoantenna sets favorable energy transfer routes that increase the FRET efficiency up to 50% for nearly perpendicular donor and acceptor dipoles. This new facet of plasmonic nanoantennas enables dipole-dipole energy transfer that would otherwise be forbidden in a homogeneous environment. As such, our approach further increases the applicability of single-molecule FRET over diffraction-limited approaches, with the additional benefits of higher sensitivities and higher concentration ranges toward physiological levels.

Decoration of Reduced Graphene Oxide Nanosheets with Aryldiazonium Salts and Gold Nanoparticles toward a Label-Free Amperometric Immunosensor for Detecting Cytokine Tumor Necrosis Factor-α in Live Cells

In this study, a label-free electrochemical immunosensor was developed for detection of cytokine tumor necrosis factor-alpha (TNF-α). First, AuNPs loaded reduced graphene oxides nanocomposites (RGO-ph-AuNP) were prepared, and then, a mixed layer of 4-carbxyphenyl and 4-aminophenyl phosphorylcholine (PPC) was modified to the surface of AuNPs for the subsequent modification of anti-TNF-α capture antibody (Ab₁) to form the capture surface (Au-RGO-ph-AuNP-ph-PPC(-ph-COOH)) for the analyte TNF-α with the antifouling property. For reporting the presence of analyte, the anti-TNF-α detection antibody (Ab₂) was modified to the graphene oxides which have been modified with the 4-ferrocenylaniline through diazonium chemistry to form Ab₂-GO-ph-Fc. Then, a sandwich assay was formed on gold surfaces for the quantitative detection of TNF-α based on the electrochemical signal of ferrocene. X-ray photoelectron spectra (XPS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis, and electrochemistry were used for characterization of the stepwise fabrications on the interface. The prepared electrochemical immunosensor was successfully used for the detection of TNF-α over the range of 0.1-150 pg mL^-1. The lowest detection limit of this immunosensor is 0.1 pg mL^-1 TNF-α in 50 mM phosphate buffer at pH 7.0. The fabricated immunosensor provided high selectivity and stability and can be used to detect TNF-α secreted by live BV-2 cells with comparable accuracy to enzyme-linked immunosorbent assay (ELISA) but with lower limit of detection.

Insight into a reversible energy transfer system

Resonance energy transfer (RET) processes have wide applications; these processes involve a unidirectional energy transfer from a particular donor to a particular acceptor. Here, we report a plasmonic resonance energy transfer (PRET), which occurs from the surface of gold nanoparticles to fluorescent organic dyes, and coexists with a nanometal surface energy transfer (NSET) that operates in the reverse direction. The coexistence of both PRET and NSET in opposite directions means that the roles of both donor and acceptor can be interchanged, which could be identified by using spectrofluorometric measurements and light scattering dark field microscopic imaging. The experimental data could be further theoretically supported using Persson and Lang's model, the quasi-static approximation and finite-difference time-domain simulation. Moreover, disruption of the PRET process by altering the energy transfer pairs suggests that interactions occur inside the reversible energy transfer system, which manifest by increasing the fluorescence quenching efficiency of the NSET process.

Plasmonic Effect on Exciton and Multiexciton Emission of Single Quantum Dots

Quantum dots are nanoscale quantum emitters with high quantum yield and size-dependent emission wavelength, holding promises in many optical and electronic applications. When quantum dots are situated close to noble metal nanoparticles, their emitting behavior can be conveniently tuned because of the interaction between the excitons of the quantum dots and the plasmons of the metal nanoparticles. This interaction at the single quantum dot level gives rise to reduced or suppressed photoluminescence blinking and enhanced multiexciton emission, which is difficult to achieve in isolated quantum dots. However, the mechanism of how plasmonic structures cause the changes in the quantum dot emission remains unclear. Because of the complexity of the system, the interfaces between metal, semiconductor, and ligands must be considered, in addition to factors such as geometry, interparticle distance, and spectral overlap. The challenges in the design and fabrication of the hybrid nanostructures as well as in understanding the exciton-plasmon coupling mechanism can be overcome by a cooperative effort in synthesis, optical spectroscopy, and theoretical modeling.

Dynamics of nanocubes embedding into polymer films investigated via spatially resolved plasmon modes

Integration of nanoparticles into thin films is essential for the development of functional materials, studies of fundamental interfacial processes, and exploitation of inherent properties from the particles themselves. In this work, we systematically investigate the process of incorporation of silver nanocubes into thin polystyrene films at temperatures just above the polymer glass transition. The process of nanocrystal incorporation can be precisely monitored via far-field spectroscopy to observe the response of spatially resolved hybrid plasmon modes. Each plasmon resonance has a distinct dynamic range and maximum sensitivity forming a complementary set of nanorulers that operates over a distance comparable to the edge length of the cubes. The approach explored in this work is a general robust method for the development of long-range polychromatic nanorulers.

Simple and Sensitive Colorimetric Detection of Dopamine Based on Assembly of Cyclodextrin-Modified Au Nanoparticles

A controlled assembly of natural beta-cyclodextrin modified Au NPs mediated by dopamine is demonstrated. Furthermore, a simple and sensitive colorimetric detection for dopamine is established by the concentration-dependent assembly.

Silver Nanoshell Plasmonically Controlled Emission of Semiconductor Quantum Dots in the Strong Coupling Regime

Strong coupling between semiconductor excitons and localized surface plasmons (LSPs) giving rise to hybridized plexciton states in which energy is coherently and reversibly exchanged between the components is vital, especially in the area of quantum information processing from fundamental and practical points of view. Here, in photoluminescence spectra, rather than from common extinction or reflection measurements, we report on the direct observation of Rabi splitting of approximately 160 meV as an indication of strong coupling between excited states of CdSe/ZnS quantum dots (QDs) and LSP modes of silver nanoshells under nonresonant nanosecond pulsed laser excitation at room temperature. The strong coupling manifests itself as an anticrossing-like behavior of the two newly formed polaritons when tuning the silver nanoshell plasmon energies across the exciton line of the QDs. Further analysis substantiates the essentiality of high pump energy and collective strong coupling of many QDs with the radiative dipole mode of the metallic nanoparticles for the realization of strong coupling. Our finding opens up interesting directions for the investigation of strong coupling between LSPs and excitons from the perspective of radiative recombination under easily accessible experimental conditions.

Coupling Emitters and Silver Nanowires to Achieve Long-Range Plasmon-Mediated Fluorescence Energy Transfer

The development of quantum plasmonic circuitry requires efficient coupling between quantum emitters and plasmonic waveguides. A major experimental challenge is to simultaneously maximize the surface plasmon propagation length, the coupling efficiency into the plasmonic mode, and the Purcell factor. Addressing this challenge is also the key to enabling long-range energy transfer between quantum nanoemitters. Here, we use a dual-beam scanning confocal microscope to carefully investigate the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. By exciting the fluorescent nanoparticles via nanowire surface plasmons, we maximize the light-matter interactions and reach coupling efficiencies up to 44% together with 24× lifetime reduction and 4.1 μm propagation lengths. This improved optical performance enables the demonstration of long-range plasmon-mediated fluorescence energy transfer between two nanoparticles separated by micrometer distance. Our results provide guidelines toward practical realizations of efficient long-range fluorescence energy transfer for integrated plasmonics and quantum nano-optics.

Using carbon nanotubes-gold nanocomposites to quench energy from pinnate titanium dioxide nanorods array for signal-on photoelectrochemical aptasensing

On the basis of the absorption and emission spectra overlap, an enhanced resonance energy transfer caused by excition-plasmon resonance between carbon nanotubes-gold nanoparticles (CNTs-Au) and pinnate titanium dioxide nanorods array (P-TiO2 NA) was obtained. Three-dimensional single crystalline P-TiO2 were prepared successfully on fluorine-doped tin oxide conducting glass (FTO glass), and its optical absorption properties and photoelectrochemical (PEC) properties were investigated. With the synergy of CNTs-Au as energy acceptor, it resulted in the enhancement of energy transfer between excited P-TiO2 NA and CNTs-Au. Upon the novel sandwichlike structure formed via DNA hybridization, the exciton produced in P-TiO2 NA was annihilated and a damped photocurrent was obtained. With the use of carcinoembryonic antigen (CEA) as a model which bonded to its specific aptamer and destroyed the sandwichlike structure, the energy transfer efficiency was lowered, leading to PEC response augment. Thus a signal-on PEC aptasensor was constructed. Under the optimal conditions, the PEC aptasensor for CEA determination exhibited a linear range from 0.001 to 2.5ngmL(-1) with a detection limit of 0.39pgmL(-1) and was satisfactory for clinical sample detection. Furthermore, the proposed aptasensor shows satisfying performance, such as easy preparation, rapid detection and so on. Moreover, since different aptamer can specifically bind to different target molecules, the designed strategy has an expansive application for the construction of versatile PEC platforms.

Metal-Semiconductor Hybrid Aerogels: Evolution of Optoelectronic Properties in a Low-Dimensional CdSe/Ag Nanoparticle Assembly

Hybrid nanomaterials composed of metal-semiconductor components exhibit unique properties in comparison to their individual counterparts, making them of great interest for optoelectronic applications. Theoretical and experimental studies suggest that interfacial interactions of individual components are of paramount importance to produce hybrid electronic states. The direct cross-linking of nanoparticles (NPs) via controlled removal of the surfactant ligands provides a route to tune interfacial interactions in a manner that has not been thoroughly investigated. Herein, we report the synthesis of CdSe/Ag heteronanostructures (aerogels) via oxidation induced self-assembly of thiol-coated NPs and the evolution of optical properties as a function of composition. Three hybrid systems were investigated, where the first and second excitonic energies of CdSe were matched with plasmonic energy of Au or Ag NPs and Ag hollow NPs. Physical characterization of the aerogels suggests the presence of an interconnected network of hexagonal CdSe and cubic Ag NPs. The optical properties of hybrids were systematically examined through UV-vis, photoluminescence (PL), and time-resolved (TR) PL spectroscopic studies that indicate the generation of alternate radiative decay pathways. A new emission (640 nm) from CdSe/Ag aerogels emerged at Ag loading as low as 0.27%, whereas absorption band tailing and PL quenching effects were observed at higher Ag and Au loading, respectively. The TRPL decay time of the new emission (∼600 ns) is markedly different from those of the band-edge (1.83 ± 0.03 ns) and trap-state (1190 ± 120 ns) emission maxima of phase pure CdSe, supporting the existence of alternate radiative relaxation pathways in sol-gel derived CdSe/Ag hybrids.