Plasmon Shift Research Articles

Optimizing geometry and metal-dependent performance of Si-based Schottky plasmonic photodetectors

In this study, different Schottky photodetectors with a plasmonic assist which receives Near-infrared (NIR) light at room temperature were demonstrated. We achieved remarkable detection over a wide range of wavelengths through a comparative study of Schottky plasmonic photodetectors employing Au, Ti, Mo, and Cr as a Schottky metal, a diffusion barrier, an adhesion promoter, and to suppress the formation of intermetallic compounds and plasmonic resonance shifts. The device structure is determined by extensive experimental and modeling results, ensuring a wavelength-specific plasmonic photodetection and a near one-to-one correspondence between incident angles and wavelengths within the NIR region. The combination of different metals in the plasmonic structure efficiently broadened the SPR response, due to the variation of junction barrier height and enhanced interface properties. It exhibited exceptional photocurrent response under room temperature and zero biasing conditions, characterized by high linearity and sensitivity. Light acquisition for longer NIR wavelengths became possible, and applicable in gas sensing, particularly within the vibrational absorption bands of alkane groups. These findings provide valuable design guidelines for optimizing device structures and insights into their application for various scenarios.

A selective LSPR biosensor for molecular-level glycated albumin detection

A biosensor specifically engineered to detect glycated albumin (GA), a critical biomarker for diabetes monitoring, is presented. Unlike conventional GA monitoring methods, the biosensor herein uniquely employs localised surface plasmon resonance (LSPR) for signal transduction, leveraging a novel fabrication process where gold nanoparticles are deposited on a quartz substrate using flame spray pyrolysis. This enables the biosensor to provide mean glucose levels over a three-week period, correlating with the glycation status of diabetes patients. The sensor's DNA aptamer conjugation selectively binds GA, inducing a plasmonic wavelength shift; resulting in a detection limit of 0.1 μM, well within the human GA range of 20–240 μM. Selectivity experiments with diverse molecules and an exploration of sensor reusability were carried out with positive results. The novelty of the biosensor presented includes specificity, sensitivity and practical applicability; which is promising for enhanced diabetes diagnosis using a rapid and inexpensive process.

Single-Particle Plasmon Sensor to Monitor Proteolytic Activity in Real Time.

We have established a label-free plasmonic platform that monitors proteolytic activity in real time. The sensor consists of a random array of gold nanorods that are functionalized with a design peptide that is specifically cleaved by thrombin, resulting in a blueshift of the longitudinal plasmon. By monitoring the plasmon of many individual nanorods, we determined thrombin's proteolytic activity in real time and inferred relevant kinetic parameters. Furthermore, a comparison to a kinetic model revealed that the plasmon shift is dictated by a competition between peptide cleavage and thrombin binding, which have opposing effects on the measured plasmon shift. The dynamic range of the sensor is greater than two orders of magnitude, and it is capable of detecting physiologically relevant levels of active thrombin down to 3 nM in buffered conditions. We expect these plasmon-mediated label-free sensors to open the window to a range of applications stretching from the diagnostic and characterization of bleeding disorders to fundamental proteolytic and pharmacological studies.

Double plasmonic peak shift sensitivity: an analysis of a highly sensitive LSPR-PCF sensor for a diverse range of analyte detection

Double plasmonic peak shift sensitivity: an analysis of a highly sensitive LSPR-PCF sensor for a diverse range of analyte detection

Machine Learning Color Feature Analysis of a High Throughput Nanoparticle Conjugate Sensing Assay.

Plasmonic nanoparticles are finding applications within the single molecule sensing field in a "dimer" format, where interaction of the target with hairpin DNA causes a decrease in the interparticle distance, leading to a localized surface plasmon resonance shift. While this shift may be detected using spectroscopy, achieving statistical relevance requires the measurement of thousands of nanoparticle dimers and the timescales required for spectroscopic analysis are incompatible with point-of-care devices. However, using dark-field imaging of the dimer structures, simultaneous digital analysis of the plasmonic resonance shift after target interaction of thousands of dimer structures may be achieved in minutes. The main challenge of this digital analysis on the single-molecule scale was the occurrence of false signals caused by non-specifically bound clusters of nanoparticles. This effect may be reduced by digitally separating dimers from other nanoconjugate types. Variation in image intensity was observed to have a discernible impact on the color analysis of the nanoconjugate constructs and thus the accuracy of the digital separation. Color spaces wherein intensity may be uncoupled from the color information (hue, saturation, and value (HSV) and luminance, a* vector, and b* vector (LAB) were contrasted to a color space which cannot uncouple intensity (RGB) to train a classifier algorithm. Each classifier algorithm was validated to determine which color space produced the most accurate digital separation of the nanoconjugate types. The LAB-based learning classifier demonstrated the highest accuracy for digitally separating nanoparticles. Using this classifier, nanoparticle conjugates were monitored for their plasmonic color shift after interaction with a synthetic RNA target, resulting in a platform with a highly accurate yes/no response with a true positive rate of 88% and a true negative rate of 100%. The sensor response of tested single stranded RNA (ssRNA) samples was well above control responses for target concentrations in the range of 10 aM-1 pM.

Sodium Lauryl Sulfate-Conjugated Cationic Gemini-Surfactant-Capped Gold Nanoparticles as Model System for Biomolecule Recognition

Surfactant-based nanostructures are promising materials for designing novel colorimetric biosensors based on aggregation/disaggregation phenomena. In this work, a colorimetric sensor based on the plasmonic shift of surfactant-capped gold nanoparticles via the disaggregation mechanism was developed. To perform this, the optimum SDS concentration was firstly determined in order to form Au@16-s-16/SDS complex aggregates with a well-defined SPR band in the blue region. Once the optimal SDS concentration for Au@16-s-16 aggregation was established, the sensing method depended on the nature of the electrostatic charge of the biopolymer studied where both the strength of the biopolymer/SDS and biopolymer/Au@16-s-16 interactions and the cationic gold nanoparticles play a key role in the disaggregation processes. As a result, an instantaneous color change from blue to red was gradually observed with increasing biopolymer concentrations. The response of the sensor was immediate, avoiding problems derived from time lapse, and highly dependent on the order of addition of the reagents, with a detection limit in the nanomolar and picomolar range for DNA and Lysozyme sensing, respectively. This behavior can be correlated with the formation of different highly stabilized Au@16-s-16/biopolymer/SDS complexes, in which the particular biopolymer conformation enhances the distance between Au@16-s-16 nanoparticles among the complexes.

Quantitative Insights into a Plasmonic Ruler Equation from the Perspective of Enhanced Near Field.

The plasmonic shift of resonance wavelength induced by near-field coupling enables one to measure nanoscale distances optically. Empirically, the well-known ruler equation correlating plasmon shift with interparticle spacing was proposed. Though it has been widely used in analyzing simulation and experimental outcomes, little is known about the underlying physical mechanism of the characteristic exponential form of the plasmon ruler equation and the universal decay constant therein. In this work, we attempt to decrypt these from the perspective of plasmon near-field enhancement. Based on an analytical quasi-normal mode formula for plasmon shifts, we proved that the exponential decaying electric field is the critical reason that results in the exponential form of the plasmon ruler equation and quantitatively, we found that the universal decay constant in the plasmon ruler equation actually reflects the range of the enhanced near field. This work hopefully helps to deepen the understanding of the mechanism of light-matter interaction in corresponding plasmonic processes.

Gold Nanoparticle Enabled Localized Surface Plasmon Resonance on Unique Gold Nanomushroom Structures for On-Chip CRISPR-Cas13a Sensing.

A novel localized surface plasmon resonance (LSPR) system based on the coupling of gold nanomushrooms (AuNMs) and gold nanoparticles (AuNPs) is developed to enable a significant plasmonic resonant shift. The AuNP size, surface chemistry, and concentration are characterized to maximize the LSPR effect. A 31 nm redshift is achieved when the AuNMs are saturated by the AuNPs. This giant redshift also increases the full width of the spectrum and is explained by the 3D finite-difference time-domain (FDTD) calculation. In addition, this LSPR substrate is packaged in a microfluidic cell and integrated with a CRISPR-Cas13a RNA detection assay for the detection of the SARS-CoV-2 RNA targets. Once activated by the target, the AuNPs are cleaved from linker probes and randomly deposited on the AuNM substrate, demonstrating a large redshift. The novel LSPR chip using AuNP as an indicator is simple, specific, isothermal, and label-free; and thus, provides a new opportunity to achieve the next generation multiplexing and sensitive molecular diagnostic system.

Nature of the Anomalous Size Dependence of Resonance Red Shifts in Ultrafine Plasmonic Nanoparticles

Plasmonic red shifts of nanoparticles are commonly used in imaging technologies to probe the character of local environments, and the understanding of their dependence on size, shape, and surrounding media has therefore become an important target for research. The red shift of plasmon resonances changes character at about 8–10 nm of size for spherical gold nanoparticles─above this value, the red shift progresses linearly with particle size, while below this size, the red shift changes nonlinearly and more strongly with size. Using an atomistic discrete interaction model, we have studied the special properties of the nanoparticle surface layers and discovered its importance for ultrafine plasmonic nanoparticles and their red shifts. We find that the physical origin for the specific properties inherent to the surface layer of atoms near the nanoparticle boundary is related to the anisotropy of the local environment of atoms in this layer by other atoms. The anisotropy changes the conditions for light-induced nonlocal interactions of neighboring atoms with each other and with the incident radiation compared to the atoms located in the particle core with isotropic nearest surroundings by other atoms. The local anisotropy of the nanoparticle crystal lattice is a geometric factor that increases toward its boundary and that is the most fundamental factor underlying the physical differences between the nanoparticle surface layer and the core material. It is shown that the inflexion point at 8–10 nm is due to a change in the dominant physical origin of the red shift─from chaotization of atomically light-induced dipoles within the surface layer in the case of ultrafine nanoparticles to retardation effects for large nanoparticles in which the relative volume of the surface layer decreases rapidly to a negligible value with increasing nanoparticle size. The patterns revealed are the basis for predicting the manifestation of surface layer effects in ultrafine plasmonic nanoparticles of different shapes and composed of different plasmonic materials.

Ag NPs/PMMA nanocomposite as an efficient platform for fluorescence regulation of riboflavin.

The fluorescence detection platform has broad application in many fields. In this paper, we report a simple and efficient fluorescence detection platform based on the synergistic effects of Ag nanoparticles (Ag NPs) and polymethylmethacrylate (PMMA). Ag NPs were introduced to realize the plasmon enhancement fluorescence and a thin PMMA layer was used to adjust the distance between Ag NPs and riboflavin. The thin PMMA layer not only enhances the fluorescence by enhancing adhesion of substrate, but also optimizes the plasmon enhancement fluorescence effect by serving as the spacer. The fluorescence enhancement factor based on this platform shows a trend of increasing with the decrease of the concentration of riboflavin, and the detection of riboflavin is realized based on this feature, the lowest detectable concentration is as low as 0.27 µM. In addition to the detection based on plasmon enhancement fluorescence, the detection of riboflavin at low concentrations can also be realized by the shift and broadening of the fluorescence peak due to the Ag NPs. The combination of the two ways of plasmon enhancement fluorescence and shift of the fluorescence spectra is used for the detection of riboflavin. These results show that the platform has great potential applications in the field of detection and sensing.

Do We Really Need Quantum Mechanics to Describe Plasmonic Properties of Metal Nanostructures?

Optical properties of metal nanostructures are the basis of several scientific and technological applications. When the nanostructure characteristic size is of the order of few nm or less, it is generally accepted that only a description that explicitly describes electrons by quantum mechanics can reproduce faithfully its optical response. For example, the plasmon resonance shift upon shrinking the nanostructure size (red-shift for simple metals, blue-shift for d-metals such as gold and silver) is universally accepted to originate from the quantum nature of the system. Here we show instead that an atomistic approach based on classical physics, ωFQFμ (frequency dependent fluctuating charges and fluctuating dipoles), is able to reproduce all the typical “quantum” size effects, such as the sign and the magnitude of the plasmon shift, the progressive loss of the plasmon resonance for gold, the atomistically detailed features in the induced electron density, and the non local effects in the nanoparticle response. To support our findings, we compare the ωFQFμ results for Ag and Au with literature time-dependent DFT simulations, showing the capability of fully classical physics to reproduce these TDDFT results. Only electron tunneling between nanostructures emerges as a genuine quantum mechanical effect, that we had to include in the model by an ad hoc term.

In Situ Monitoring of Scale Effects on Phase Selection and Plasmonic Shifts during the Growth of AgCu Alloy Nanostructures for Anticounterfeiting Applications

Tailoring of plasmon resonances is essential for applications in anticounterfeiting. This is readily achieved by tuning the composition of alloyed metal clusters; in the simplest case, binary alloys are used. Yet, one challenge is the correlation of cluster morphology and composition with the changing optoelectronic properties. Hitherto, the early stages of metal alloy nanocluster formation in immiscible binary systems such as silver and copper have been accessible by molecular dynamics (MD) simulations and transmission electron microscopy (TEM). Here, we investigate in real time the formation of supported silver, copper, and silver–copper-alloy nanoclusters during sputter deposition on poly(methyl methacrylate) by combining in situ surface-sensitive X-ray scattering with optical spectroscopy. While following the transient growth morphologies, we quantify the early stages of phase separation at the nanoscale, follow the shifts of surface plasmon resonances, and quantify the growth kinetics of the nanogranular layers at different thresholds. We are able to extract the influence of scaling effects on the nucleation and phase selection. The internal structure of the alloy cluster shows a copper-rich core/silver-rich shell structure because the copper core yields a lower mobility and higher crystallization tendency than the silver fraction. We compare our results to MD simulation and TEM data. This demonstrates a route to tailor accurately the plasmon resonances of nanosized, polymer-supported clusters which is a crucial prerequisite for anticounterfeiting.

Revealing the Photothermal Behavior of Plasmonic Gap Modes: Toward Thermostable Nanocavities

AbstractPlasmonic nanocavities based on nanoparticle‐on‐mirror geometries draw widespread interest due to their easy‐fabricated structures and highly confined plasmon modes, enabling light–matter interaction for developing state‐of‐the‐art photonic devices. To truly be ideal building blocks for practical nanophotonic devices, the nanoparticle‐on‐mirror nanocavities are expected to be simultaneously spectral tunable and thermostable under the focused laser irradiation. Here, the in‐situ optical tuning of the nanocavity mode is achieved in the nanocube‐on‐mirror nanocavities by continuous laser irradiation. Through the photothermal‐assisted morphology changes, the in‐situ shifting of the plasmon resonances to the lower and higher energies is accomplished by the deformation of the surfactant covering the silver nanocube and reshaping the rounding curvature of the nanocubes, respectively. Furthermore, it is found that the alumina‐encapsulated nanocube on mirrors constructed by surfactant‐eliminated nanocubes are capable of enduring more intense irradiation than the normal ones. On this basis, the factors affecting the plasmonic heating effect are also systematically investigated, and the internal relationship among light absorption, temperature increases, and plasmonic resonance shifts is established. In general, these results provide ideas for designing novel stable nanocavities, and the surfactant‐eliminated nanocube‐on‐mirror nanocavities presented here are a promising platform for future applications in boosting light–matter interaction.

Switching plasmonic nanogaps between classical and quantum regimes with supramolecular interactions.

In the realm of extreme nanophotonics, nanogap plasmons support reliable field enhancements up to 1000, which provide unique opportunities to access a single molecule for strong coupling and a single atom for quantum catalysis. The quantum plasmonics are intriguing but difficult to modulate largely because of the lack of proper spacers that can reversibly actuate the sub–1-nm gaps. Here, we demonstrate that supramolecular systems made of oligoamide sequences can reversibly switch the gap plasmons of Au nanoparticles on mirror between classical and quantum tunneling regimes via supramolecular interactions. The results reveal detailed plasmon shift near the quantum tunneling limit, which fits well with both classical- and quantum-corrected models. In the quantum tunneling regime, we demonstrate that plasmonic hot electron tunneling can further blue shift the quantum plasmons because of the increased conductance in the nanogaps, making it a promising prototype of optical tunable quantum plasmonic devices.

Nanoplasmonic Strip Test for Salivary Glucose Monitoring.

Nowadays, there is an increasing interest in Point-of-care (POC) devices for the noninvasive glucose assessment. Despite the recent progress in glucose self-monitoring, commercially available devices still use invasive samples such as blood or interstitial fluids, and they are not equipment-free and affordable for the whole population. Here, we report a fully integrated strip test for the semi-quantitative detection of glucose in whole saliva. The colorimetric mechanism consists of an enzyme-mediated reshaping of multibranched gold nanoparticles (MGNPs) into nanospheres with an associated plasmonic shift and consequent blue-to-red color change, clearly detectable in less than 10 min.

Tunable plasmon–polarizmon resonance and hotspots in metal–silicon core–shell nanostructures

Metal nanostructures create near-field super hotspots under light irradiation with a range limited to a few nanometers. The intense field in the spot affords enhanced nonlinear optical processes, such as Raman spectroscopy. The intense field, however, can cause heavy distortion and thermal damage to the molecular specimen as well as heavy convolution with the metal electronic structure. In this work, we simulate concentric layered silicon–metal core–shell (and its inverse) nanostructures that may alleviate the disadvantages of the pure metal environment. Our results using Mie and finite-difference time-domain scattering studies show that, in addition to the super hotspot at the gold–silicon interface, there emerges a super hotspot at the silicon–vacuum interface, whose intensities anti-correlate and are tuned by tuning the silicon thickness. Moreover, the plasmonic resonance red shifts with the thickness of the silicon shell, reaching a terminal wavelength of ∼840 nm. These features are understood in terms of induced polarization charge at the silicon–metal and silicon–vacuum interfaces, which for high κ materials (13.32) can be significant. The metal–silicon system creates plasmon–polarizmon hotspots tunable in strength and wavelength content that can be designed to alleviate high field damage, useful for Raman scattering and photovoltaic applications. The integrated metal–silicon system also promises field enhancement of visible luminescence of silicon nanoparticles, useful for imaging and tracking applications.

Near‐Infrared Multilayer MoS2 Photoconductivity‐Enabled Ultrasensitive Homogeneous Plasmonic Colorimetric Biosensing

AbstractThe ability to detect low‐abundance proteins in human body fluids plays a critical role in proteomic research to achieve a comprehensive understanding of protein functions and early‐stage disease diagnosis to reduce mortality rates. Ultrasensitive (sub‐fM), rapid, simple “mix‐and‐read” plasmonic colorimetric biosensing of large‐size (≈180 kDa) proteins in biofluids using an ultralow‐noise multilayer molybdenum disulfide (MoS2) photoconducting channel is reported here. With its out‐of‐plane structure optimized to minimize carrier scattering, the multilayer MoS2 channel operated under near‐infrared illumination enables the detection of a subtle plasmonic extinction shift caused by antigen‐induced nanoprobe aggregation. The demonstrated biosensing strategy allows quantifying carcinoembryonic antigen in unprocessed whole blood with a dynamic range of 106, a sample‐to‐answer time of 10 min, and a limit of detection of 0.1–3 pg mL−1, which is ≈100‐fold more sensitive than the clinical‐standard enzyme‐linked immunosorbent assays. The biosensing methodology can be broadly used to realize timely personalized diagnostics and physiological monitoring of diseases in point‐of‐care settings.

Analysis of thermo-plasmonic lab-on-fiber probes in liquid environments

Lab-on-fiber (LOF) optrodes are recently emerging not only as valid platforms for biosensing, but also as promising light-controlled actuators in drug-delivery, optical trapping and thermo-ablation systems. In this regard, the thermo-plasmonic effect has been recognized as an intriguing tool for conferring to the optical fiber the capability of interacting with the external environment through the fine control of local overheating actuated by light in the range of few mW. However, the evaluation of the thermo-plasmonic overheating on small areas such as that of a standard single mode fiber tip is not trivial, especially in liquid solutions, where these probes typically operate. Here we demonstrate that by functionalizing the metallic nanostructure of LOF devices with a thermoresponsive smart materials, it is possible to measure the light-induced overheating on the fiber tip. Specifically, we monitored the plasmonic resonance wavelength shift induced by the temperature-dependent swelling dynamics of different microgel films deposited on the nanostructure. We find a local overheating of about 8 °C mW−1, i.e. also in line with our theoretical predictions based on numerical simulations. Our results demonstrate that the proposed approach is a valid methodology for the direct and continuous monitoring of the temperature changes in LOF devices induced by the input optical power in liquid environment. Our findings lay the basis for the analysis of thermo-plasmonic optical fiber probes exploitable in many applications, especially for the life science sector.

Design and Simulation of Terahertz Perfect Absorber with Tunable Absorption Characteristic Using Fractal-Shaped Graphene Layers

Graphene material, due to its unique conductivity and transparency properties, is utilized extensively in designing tunable terahertz perfect absorbers. This paper proposes a framework to design a tunable terahertz perfect absorber based on fractal triangle-shaped graphene layers embedded into dielectric substrates with the potential for spectral narrowing and widening of the absorption response without the need for geometric manipulation. In this way, the absorption cross-section spectra of the suggested configurations are achieved over the absorption band. First, the defection impact on the single-layer fractal triangle-shaped graphene structure inserted in insulators of the absorber is evaluated. Then, a flexible tunability of the absorbance’s peak is indicated by controlling the Fermi energy. By stacking fractal graphene sheets as a double graphene layer configuration in both the same and cross-states positioning, it is demonstrated that the absorption characteristics can be switched at 6–8 THz with a stronger amplitude, and 16–18 THz with a lower intensity. The impact of changing the Fermi potentials of embedded graphene layers is yielded, resulting in a plasmonic resonance shift and a significant broadening of the absorption bandwidth of up to five folds. Following, the absorption spectra related to the fractal triangle-shaped structures consist of a multi-stage architecture characterized by a spectral response experiencing a multiband absorbance rate and an absorption intensity of over 8 × 106 nm2 in a five-stage perfect absorber. Ultimately, the variations of the absorbance parameter and plasmonic mode under rotating the graphene sheet are explored for single and double fractal triangle-shaped perfect configurations on the absorption band. The presented mechanism demonstrates the tunability of the absorption spectrum in terms of narrowing or broadening and switching the plasmonic resonance by configuring multi-stage structures that can employ a broad range of applications for sensory devices.

Tunable and scalable fabrication of plasmonic dimer arrays with sub-10 nm nanogaps by area-selective atomic layer deposition

Nanogaps in metallic nanostructures produce local field enhancements with potential applications in surface enhanced spectroscopy, solar energy conversion, and photocatalysis. Atomic layer deposition is applied as a conformal coating to modify nanogap sizes and tune the optical properties of plasmonic dimer arrays with sub-10 nm nanogaps. Nanostructures are fabricated using layers of gold and palladium to combine features of plasmonics and area-selective atomic layer deposition, where copper metal is deposited on palladium-covered surfaces. Direct measurements of optical extinction for successive smaller nanogaps and thicker copper coatings show that spectral features become broadened at first due to heating-induced shape changes but subsequently sharpen as copper coatings form on palladium structures. Furthermore, longitudinal resonances of plasmonic dimers blue shift for thin coatings due to heating and decreasing aspect ratio, but thicker coatings lead to red shifts due to narrowing nanogaps. Together, these results show that area-selective atomic layer deposition is a promising tool for achieving large area arrays of plasmonic dimers with sub-10 nm nanogaps.