Au Nanohole Research Articles

A cavity induced mode hybridization plasmonic sensor for portable detection of exosomes

Exosomes have been considered as promising biomarkers for cancer diagnosis due to their abundant information from originating cells. However, sensitive and reliable detection of exosomes is still facing technically challenges due to the lack of a sensing platform with high sensitivity and reproducibility. To address the challenges, here we propose a portable surface plasmon resonance (SPR) sensing of exosomes with a three-layer Au mirror/SiO2 spacer/Au nanohole sensor, fabricated by an economical polystyrene nanosphere self-assembly method. The SiO2 spacer can act as an optical cavity and induce mode hybridization, leading to excellent optimization of both sensitivity and full width at half maximum compared with normal single layer Au nanohole sensors. When modified with CD63 or EpCAM aptamers, a detection of limit (LOD) of as low as 600 particles/μL was achieved. The sensors showed good capability to distinguish between non-tumor derived L02 exosomes and tumor derived HepG2 exosomes. Additionally, high reproducibility was also achieved in detection of artificial serum samples with RSD as low as 2%, making it feasible for clinical applications. This mode hybridization plasmonic sensor provides an effective approach to optimize the detection sensitivity of exosomes, pushing SPR sensing one step further towards cancer diagnosis.

Tip-based Lithography with a Sacrificial Layer.

The fabrication of a highly controlled gold (Au) nanohole (NH) array via tip-based lithography is improved by incorporating a sacrificial layer-a tip-crash buffer layer. This inclusion mitigates scratches during the nano-indentation process by employing a 300nm thick poly(methyl methacrylate) layer as a sacrificial layer on top of the Au film. Such a precaution ensures minimal scratches on the Au film, facilitating the creation of sub-50nm Au NHs with a 15nm gap between the Au NHs. The precision of this method exceeds that of fabricating Au NHs without a sacrificial layer. Demonstrating its versatility, this Au NH array is utilized in two distinct applications: as a dry etching mask to form a molybdenum disulfide hole array and as a catalyst in metal-assisted chemical etching, resulting in conical-shaped silicon nanostructures. Additionally, a significant electric field is generated when Au nanoparticles (NPs) are placed within the Au NHs. This effect arises from coupling electromagnetic waves, concentrated by the Au NHs and amplified by the Au NPs. A notable result of this configuration is the enhancement factor of surface-enhanced Raman scattering, which is an order of magnitude greater than that observed with just Au NHs and Au NPs alone.

The Sensitivity of a Hexagonal Au Nanohole Array under Different Incident Angles

Surface plasmon resonance sensors have been widely used in various fields for label-free and real-time detection of biochemical species due to their high sensitivity to the refractive index change of the surrounding environment. The common practices to achieve the improvement of sensitivity are to adjust the size and morphology of the sensor structure. This strategy is tedious and, to some extent, limits the applications of surface plasmon resonance sensors. Instead, the effect of the incident angle of excited light on the sensitivity of a hexagonal Au nanohole array sensor with a period of 630 nm and a hole diameter of 320 nm is theoretically investigated in this work. By exploring the peak shift of reflectance spectra of the sensor when facing a refractive index change in (1) the bulk environment and (2) the surface environment adjacent to the sensor, we can obtain the bulk sensitivity and surface sensitivity. The results show that the bulk sensitivity and surface sensitivity of the Au nanohole array sensor can be improved by 80% and 150%, respectively, by simply increasing the incident angle from 0° to 40°. The two sensitivities both remain nearly unchanged when the incident angle further changes from 40° to 50°. This work provides new understanding of the performance improvement and advanced sensing applications of surface plasmon resonance sensors.

Actively Tunable Spectral Filter for Compact Hyperspectral Camera Using Angle‐Sensitive Plasmonic Structures

AbstractHyperspectral imaging provides enhanced classification and identification of veiled features for diverse biomedical applications such as label‐free cancer detection or non‐invasive vascular disease diagnostics. However, hyperspectral cameras still have technical limitations in miniaturization due to the inherently complex and bulky configurations of conventional tunable filters. Herein, a compact hyperspectral camera using an active plasmonic tunable filter (APTF) with electrothermally driven spectral modulation for feature‐augmented imaging is reported. APTF consists of angle‐sensitive plasmonic structures (APS) over an electrothermal MEMS (Microelectromechanical systems) actuator, fabricated by combining nanoimprint lithography, and MEMS fabrication ona 6‐inch wafer. APS have a complementary configuration of Au nanohole and nanodisk arrays supported on asilicon nitride membrane. APTF shows a large angular motion at operational voltages of 5–9 VDC for continuous spectral modulation between 820 and 1000 nm (45 nm/V). The compact hyperspectral camera was fully packaged with a linear polarizer, APTF and amonochromatic camera, exhibiting asize of 16 mm (ϕ) × 9.5 mm (h). Feature‐augmented images of subcutaneous vein and a fresh fruit have been successfully demonstrated after the hyperspectral reconstruction and spectral feature extraction. This functional camera provides a new compact platform for point‐of‐care or in vivo hyperspectral imaging in biomedical applications.

Plasmonic crystals fabricated by nanosphere lithography for advanced biosensing.

Plasmonic nanostructures have attracted wide attention in the past few years for their promising applications such as surface-enhanced spectroscopies, chemical or biosensing, and so on. However, the fabrication of plasmonic nanostructures relies on traditional photolithography methods such as electron beam lithography and focused ion beam lithography, which have inherent shortcomings, such as high fabrication cost and being time-consuming. Here, using the nanosphere lithography approach, we fabricate large-area long-range ordered periodic Au nanohole arrays on an opaque Au substrate. The structure supports spectral-isolation and well-defined plasmonic resonances favorable to spectral monitoring at normal incidence of light. The bulk sensitivity of up to 403 nm/RIU is measured for the plasmon modes. Furthermore, we assess the surface-sensing performance of the system and obtain a near-field decay length of about 240 nm, meaning that it is desirable to detect the biological protein molecules. The suggested plasmonic-sensing platform has broad application prospects in the development of low-cost and high-throughput biosensor chips.

Eccentricity improved plasmon refractive index sensing and SERS performances of Au nano-donuts

The extraordinary electric-field enhancement within the inherent gap of Au nano-donuts promises their strong capability in surface enhanced Raman scattering (SERS) and plasmon refractive-index sensing. It is crucial to unveil the eccentricity effect on their localized surface plasmon resonance (LSPR) properties to access their optimized sensing and SERS performances. By finite element method, we demonstrate that elliptic-nano-donut monomers are able to provide improved refractive-index sensitivity (S), figure of merit (FOM), and SERS enhancement than that of concentric ones. The corresponding optimized values are revealed to reach 444.5 nm/RIU, 10.3RIU−1 and 6.25×106, respectively, which are superior than that of similar sized Au nanospheres and nanoholes. The effect of the electromagnetic coupling in between dimeric components on the improvement of S, FOM and SERS enhancement is also discussed. The obtained eccentricity dependent LSPR properties are not only fundamentally important, but also provide new clues for future developing efficient SERS and plasmon sensing substrates.

In Situ Chemical Patterning Technique

AbstractPlasmonic nanochemistry is employed to develop a chemical patterning technique for multiscale distributed plasmonic nanoparticle (NP) arrays by a spatial confined in situ growth process. Ag NPs grow exclusively following the maximum plasmonic field areas of Au nanohole arrays under light illumination, and Ag NP‐assembly arrays (NAAs) with controllable morphology from nanoring to nanodisk are realized precisely. Combining with conventional photolithography, letters, and patterns ranging from macroscopic to sub‐microscale are accurately obtained. The region‐specific generated Ag NAAs acted as an excellent surface‐enhanced Raman scattering substrate to realize quantitative detection and Raman imaging, which are further applied to multilevel encryption applications. Moreover, the lithography strategy shows good universality where it can be utilized to fabricate Au NAAs and polypyrrole nanodisks, and applied on flexible polydimethylsiloxane and curved glass substrates. Overall, this lithography strategy has good general applicability to obtain a multiscale engineered large‐area array of colloidal metallic NPs and organic polymers without the presence of either heat or specific adhesive layers, thus offers an approach to plasmonic engineering for a myriad of applications ranging from plasmonic anti‐counterfeit labels to electrical devices.

Direct fabrication and characterization of gold nanohole arrays.

We demonstrate a one-step fabrication method to realize desired gold (Au) nanoholes arrays by using a one-photon absorption based direct laser writing technique. Thanks to the optically induced thermal effect of Au material at 532 nm excitation wavelength, the local temperature at the laser focus area can reach as high as 600°C, which induces an evaporation of the Au thin film resulting in a metallic nanohole. By controlling the laser spot movement and exposure time, different two-dimensional Au nanoholes structures with periodicity as small as 500 nm have been demonstrated. This allows obtaining plasmonic nanostructures in a single step without needing the preparation of polymeric template and lift-off process. By this direct fabrication technique, the nanoholes do not have circular shape as the laser focusing spot, due to the non-uniform heat transfer in a no-perfect flat Au film. However, the FDTD simulation results and the experimental measurement of the transmission spectra show that the properties of fabricated plasmonic nanoholes arrays are very close to those of ideal plasmonic nanostructures. Actually, the plasmonic resonance depends strongly on the periodicity of the metallic structures while the heterogeneous form of the holes simply enlarges the resonant peak. Furthermore, it is theoretically demonstrated that the non-perfect circular shape of the Au hole allows amplifying the electromagnetic field of the resonant peak by several times as compared to the case of perfect circular shape. This could be an advantage for application of this fabricated structure in laser and nonlinear optics domains.

Monitoring Solid-Phase Reactions in Self-Assembled Monolayers by Surface-Enhanced Raman Spectroscopy.

Nanopatterned surfaces enhance incident electromagnetic radiation and thereby enable the detection and characterization of self‐assembled monolayers (SAMs), for instance in surface‐enhanced Raman spectroscopy (SERS). Herein, Au nanohole arrays, developed and characterized as SERS substrates, are exemplarily used for monitoring a solid‐phase deprotection and a subsequent copper(I)‐catalyzed azide–alkyne cycloaddition “click” reaction, performed directly on the corresponding SAMs. The SERS substrate was found to be highly reliable in terms of signal reproducibility and chemical stability. Furthermore, the intermediates and the product of the solid‐phase synthesis were identified by SERS. The spectra of the immobilized compounds showed minor differences compared to spectra of the microcrystalline solids. With its uniform SERS signals and the high chemical stability, the platform paves the way for monitoring molecular manipulations in surface functionalization applications.

Improve the Local Electric Field Enhancement of Au Cylindrical Nanohole by Pt Coating

Local electric field enhancement in a long cylindrical Au nanohole with Pt coating has been theoretically studied based on quasi-static model. Calculation results show that both the peak wavelength and local field factor in the nanohole are greatly dependent on the Pt coating thickness. Because of the Pt coating, a new local surface plasmon resonance (LSPR) band takes place in the ultraviolet wavelength below 400 nm, which also results in intense local electric field enhancement in the nanohole. As the Pt coating thickness is increased, the local field factor peak corresponding to the Pt shell red shifts and gets intense rapidly, which is much greater than that of Au shell. However, the increasing Pt thickness can also leads to the decrease of the local field factor when the dielectric constant of inner hole is much greater than that of outer environment. The local field factor in Pt shell is also sensitive to the Pt thickness and local dielectric environment. At the inner surface of the Pt shell, the increasing Pt thickness results in non-monotonic change of the local field factor when the dielectric constant of inner hole is equal or smaller than that of outer environment. Whereas at the outer surface of the Pt shell, the increasing Pt thickness always results in monotonic decrease of the local field factor, which is independent on the difference of the dielectric constant between inner hole and outer surrounding. This Pt coating-controlled local field enhancement in cylindrical Au@Pt nanohole presents design criteria and synthetic strategies toward local field effect-induced surface-enhanced Raman scattering (SERS), surface-enhanced fluorescence, and biosensing applications.

Large‐Area Plasmonic Metamaterial with Thickness‐Dependent Absorption

AbstractPlasmonic metamaterials are receiving increasing attentions due to their great ability to confine light energy at the nanoscale for perfect absorption and various applications in the near infrared and visible region. However, the common practices to achieve the tunable absorption are to adjust the morphology of plasmonic metamaterials. This strategy has a low fabrication efficiency, which limits the applications of plasmonic metamaterials. Here, the absorption efficiency and peak position of the plasmonic metamaterial consisting of Au nanohole array/SiO2 spacer/Au film are tuned by simply controlling the thicknesses of the hexagonal Au nanohole array and the spacer layer, respectively. An ultrahigh absorption of ≈99% and a high surface‐enhanced Raman scattering (SERS) effect (with an average surface enhancement factor of 106) are achieved at the desired laser wavelength. The substrate can be as large as 10 mm in diameter and shows a high uniformity for SERS detection. This work provides a simple way to flexibly tune the optical properties of plasmonic metamaterials for various promising applications such as plasmon‐mediated chemical reactions and biosensing.

Opposite Polarity Surface Photovoltage of MoS2 Monolayers on Au Nanodot versus Nanohole Arrays.

We prepared MoS2 monolayers on Au nanodot (ND) and nanohole (NH) arrays. Both these sample arrays exhibited enhanced photoluminescence intensity compared with that of a bare SiO2/Si substrate. The reflectance spectra of MoS2/ND and MoS2/NH had clear features originating from excitation of localized surface plasmon and propagating surface plasmon polaritons. Notably, the surface photovoltages (SPV) of these hybrid plasmonic nanostructures had opposite polarities, indicating negative and positive charging at MoS2/ND and MoS2/NH, respectively. Surface potential maps, obtained by Kelvin probe force microscopy, suggested that the potential gradient led to a distinct spatial distribution of photo-generated charges in these two samples under illumination. Furthermore, the local density of photo-generated excitons, as predicted from optical simulations, explained the SPV spectra of MoS2/ND and MoS2/NH. We show that the geometric configuration of the plasmonic nanostructures modified the polarity of photo-generated excess charges in MoS2. These findings point to a useful means of optimizing optoelectronic characteristics and improving the performance of MoS2-based plasmonic devices.

Au Nanohole–Nanodisk Hybrid Array for Plasmonic Sensing

The phenomenon of extraordinary light transmission through sub‐wavelength‐sized holes in noble metal films has attracted significant attention due to its potential applications in optical filters and plasmonic sensors. Herein, an Au nanohole–nanodisk hybrid array, which allows controllable extraordinary light transmission, is presented to detect the target analytes. A 3D electromagnetic simulation proves that the proposed plasmonic nanostructure array with its constant geometry can vary the light transmittance by altering the refractive index of the surrounding medium. As the refractive‐index‐dependent plasmon coupling between the Au nanohole and nanodisk can drastically enhance the light transmission, the Au nanohole–nanodisk hybrid array can be utilized as a sensitive and feasible plasmonic sensor.

Facile synthesis of size- and shape-controlled freestanding Au nanohole arrays by sputter deposition using anodic porous alumina templates

To fabricate a freestanding through-hole Au membrane using an anodic porous alumina template, Au was deposited on the outermost surface of an anodic film followed by the removal of the template. Alumina templates with different dimensions (e.g. diameters and number of pores) were prepared by two-step anodization in the range of 40–80 V and pore-widening. The Au thin films were deposited onto alumina templates with well-controlled surface morphologies by sputter deposition using a commercially available ion sputter coater. After the removal of the alumina template, a variety of Au membranes with nanoholes, nanotubes, or branched pores were obtained, which reflect the morphology of the alumina template. When the sputtered Au penetrates the pores of the alumina film, Au nanotube arrays with an aspect ratio of ∼3 can be fabricated. The present method is much simpler than the traditional template process involving multi-step replication because there is no need to separate the alumina template from the aluminum substrate before Au deposition.

Sub-10 nm Nanolaminated Al2O3/HfO2 Coatings for Long-Term Stability of Cu Plasmonic Nanodisks in Physiological Environments.

By supporting localized plasmon modes, metal-based plasmonic nanostructures can confine optical fields at deep-subwavelength scale in various applications, such as biological and chemical sensing, nanoscale light emission, and solar energy harvesting. While Cu is a low-cost complementary metal oxide semiconductor (CMOS) compatible material, its poor chemical stability limits the use of Cu plasmonic nanodevices in corrosive biochemical aqueous environments. In this paper, we demonstrate that sub-10 nm Al2O3/HfO2 nanolaminated coatings can significantly extend the lifetime of Cu nanodisk arrays from ∼5 h to ∼180 days in the physiological environment of 1× phosphate-buffered saline (PBS) at 37 °C. Cu nanodisk arrays are fabricated using freestanding Au nanohole array films as the physical vapor deposition masks and sub-10 nm nanolaminated coatings composed of alternating Al2O3 and HfO2 nanolayers are grown on Cu nanodisk arrays by atomic layer deposition (ALD). Time-dependent optical extinction measurements of Cu nanodisk arrays are conducted in 1× solutions at 37 °C to investigate the anticorrosion performance for different pure and nanolaminated ALD coatings. We observe a linear relationship between the lifetime of Cu nanodisk arrays in 1× PBS at 37 °C and the nanolaminated coating thickness, and ∼1.3 nm nanolaminated coatings of ∼10 ALD cycles can extend the lifetime of Cu plasmonics up to ∼20 days. Furthermore, we find that the anticorrosion performance of Al2O3/HfO2 nanolaminated ALD coatings strongly depends on the processing and the geometric parameters, such as the annealing temperature and the nanolaminated backbone unit size.

Ultrahigh-Sensitivity Sandwiched Plasmon Ruler for Label-Free Clinical Diagnosis.

Optical biosensors, especially those based on plasmonic structures, have emerged recently as a potential tool for disease diagnostics. Plasmonic biosensors have demonstrated impressive benefits for the label-free detection of trace biomarkers in human serum. However, widespread applications of these technologies are hindered because of their insufficient sensitivity, their relatively complex chemical immobilization processes, and the use of prism couplers. Accordingly, a sandwiched plasmon ruler (SW-PR) based on a Au nanohole array with ultrahigh sensitivity arising from the plasmonic coupling effect is developed. Highly confined surface charges caused by Bloch wave surface plasmon polarizations substantially increase the coupling efficiency. This platform exhibits thickness sensitivity as high as 61 nm nm-1 and can detect at least 200 000-fold lower analyte concentrations than a nanowell sensing platform with the same wavelength shift. Additionally, the sandwiched plasmonic biosensor allows precise and label-free testing of clinical biomarkers, namely C-reactive protein and procalcitonin, in patient serum samples without requiring a sophisticated prism coupler, extra antibodies, or a chemical immobilization technique. This study yields new insight into the structural design of plasmon rulers and will open exciting avenues for disease diagnosis and therapy follow-up at the point-of-care.

Large-scale highly ordered periodic Au nano-discs/graphene and graphene/Au nanoholes plasmonic substrates for surface-enhanced Raman scattering

In this paper, the study of using masks to directly generate large area, highly ordered and periodical nanostructure has been exhibited. Periodic Au nano-discs(NDs) arrays have been fabricated on top of graphene by using holey Si3N4 mask which is directly fixed on top of graphene and Au metal is deposited through the holes in mask by thermal evaporation method under vacuum condition. This fabrication method provides an easy, fast and cost efficiency way to generate periodical nanostructure. Also, Au nanoholes(NHs) structure has been studied by using holey Si3N4 as a template. The surface-enhanced Raman scattering (SERS) sensitivities of periodical Au NDs/graphene and graphene/Au NHs hybrid structures have been systematically studied. The internal mechanisms could be explained by chemical mechanism effect of graphene and electromagnetic mechanism effect of metallic nano-structures. The enhancement factors have been systematically investigated by varying the diameter and the thickness of Au discs and Au NHs. Raman mappings of Au NDs with 2.5 μm diameter illustrate that the larger SERS enhancements exist in the rim of NDs which has good agreement with the electric field simulation result. The SERE enhancement factors of fluorescein obtained from Au NDs/graphene substrates shows an improvement factor of 500% in comparison of graphene substrate. The calculated SERS enhancement factors of graphene/Au NHs achieve 1,200% in comparison of graphene/planar Au film substrate.

Dual nanotransfer printing for complementary plasmonic biosensors

One of the main challenges in the widespread utilization of localized plasmon resonance-based biosensors is the fabrication of large-area and low-cost plasmonic nanostructures. In this work, we fabricated large-area and low-cost complementary plasmonic biosensors such as nanohole and nanodisk arrays using dual nanotransfer printing (NTP) with a single metal deposition and a single reusable mold. The suspended nanohole arrays and the suspended nanodisk arrays were fabricated using the subsequent dry etching process. We confirmed a maximum enhancement in bulk sensitivity in experiments and simulations by controlling the vertical and lateral etching depths of the dielectric layer underneath the gold (Au) nanohole and nanodisk arrays. Furthermore, we show that the surface sensitivity evaluated by atomic layer deposition of aluminum oxide increased because appropriate vertical and lateral etching depths allow the target analyte to access the additional near-field formed at the bottom of the Au nanostructure. The dual NTP method provides a practical solution for the realization of large-area and low-cost label-free plasmonic biosensing systems, with a reduction in complexity and cost of the fabrication process of complementary plasmonic structures and metasurfaces.

Gold nanoholes fabricated by colloidal lithography: novel insights into nanofabrication, short-range correlation and optical properties.

Colloidal lithography is widely used as a low cost and large-area deposition approach, alternative to the conventional small-area expensive lithographic techniques, for the fabrication of short-range ordered sub-wavelength metallic nanostructures. This paper contributes to the understanding of the impact of the fabrication protocol of a colloidal mask on the optical and sensing properties of short range-ordered nanohole (NH) distributions fabricated by colloidal lithography in optically thin (20 nm thick) gold films. We consider polystyrene nanospheres (PS-NSPs) with a nominal diameter of 80 nm, electrostatically adsorbed from a salt-free colloidal solution onto a polydiallyldimethylammonium (PDDA) countercharged monolayer. By avoiding the conventional polyelectrolyte multilayer and based on the interplay between the deposition times of both PDDA and PS-NSPs, we demonstrate effective simplification of the commonly applied deposition protocol and effective tuning of the NH-to-NH spacing (dNN) with negligible agglomeration. Comparison with NH samples prepared by salt-containing colloidal solutions points out the negative impact of salt addition on the optical properties. The effective tuning of dNN obtained by our protocol demonstrates highly correlated disorder under unsaturated adsorption and allows a discussion on the analogies of the optical response between long- and short- range ordered NH systems, which is a still debated topic. By Fast Fourier Transform of autocorrelation images of scanning electron microscopy micrographs we demonstrate quantitatively, rather than in principle, the correspondence between an inherent ordering length-scale and dNN. As optical transducers for detecting refractive index changes, our samples exhibit significant bulk sensitivity (∼309 nm RIU-1) in the framework of short range ordered NH systems.

Efficient capture and photothermal ablation of planktonic bacteria and biofilms using reduced graphene oxide-polyethyleneimine flexible nanoheaters.

Bacterial infections are one of the leading causes of disease worldwide. Conventional antibiotics are becoming less efficient, due to antibiotic-resistant bacterial strains. Therefore, the development of novel antibacterial materials and advanced treatment strategies are becoming increasingly important. In the present work, we developed a simple and efficient strategy for effective bacterial capture and their subsequent eradication through photothermal killing. The developed device consists of a flexible nanoheater, comprising a Kapton/Au nanoholes substrate, coated with reduced graphene oxide-polyethyleneimine (K/Au NH/rGO-PEI) thin films. The Au NH plasmonic structure was tailored to feature strong absorption in the near-infrared (NIR) region, where most biological matter has limited absorption, while PEI was investigated for its strong binding with bacteria through electrostatic interactions. The K/Au NH/rGO-PEI device was demonstrated to capture and eliminate effectively both planktonic Gram-positive Staphilococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria after 10 min of NIR (980 nm) irradiation and, to destroy and eradicate Staphilococcus epidermidis (S. epidermidis) biofilms after 30 min irradiation. The technique developed herein is simple and universal with potential applications for eradication of different micro-organisms.