Silica Spacer Research Articles

Signal-amplified detection of amyloid-β aggregates based on plasmon-enhanced fluorescence

Protein aggregation and misfolding are the main causes of neurodegenerative diseases. The development of a sensitive detection method and early diagnosis is significant for the prevention, diagnosis and treatment of these diseases. Herein, boron dipyrromethene (BODIPY)-modified probe based on plasmon-enhanced fluorescence (PEF) was developed for the detection of amyloid-β protein (Aβ) aggregates. Gold nanoparticles (Au NPs) and BODIPY were as the plasma, fluorescent substance, respectively. Meanwhile, the distance between the Au NPs and the BODIPY was controlled by silica spacer. Compared with the original BODIPY, the fluorescence signal of BODIPY-decorated Au NPs (Au NPs@SiO2-BODIPY) exhibited 13-fold fluorescence enhancement. Three dimensions finite difference time domain (3D-FDTD) result demonstrated the signal-amplified was ascribed to the increase of electric field intensity (3.78 V/m). The linear range and detection limit for Aβ42 detection were lowered to 5–60 μM and 0.85 μM. The developed PEF probe was applied to selective and sensitive recognize Aβ aggregates in mouse colorectal carcinoma cells (CT26) and mouse pancreatic cancer cells (PAN02), which also showed a higher fluorescent signal than the original BODIPY. This method has a broad application prospect in the prediction and detection of neurodegenerative diseases.

Ultra-narrow dual-band perfect absorber based on double-slotted silicon nanodisk arrays

Due to its unique advantage of optical properties, nanophotonic metamaterials have gained extensive applications in perfect absorbers. However, achieving both dual-band and ultra-narrow linewidth in absorbers simultaneously remains a challenge for typical metal-dielectric based metamaterials. In this work, a dual-band ultra-narrow perfect absorber consisting of a double slotted silicon nanodisk array that located on a silver film with a silica spacer layer is proposed theoretically. By combining the hybrid mode excited by the coupling of diffraction wave mode and magnetic dipole mode with the anapole–anapole interaction, two absorption peaks can be induced in the near-infrared regime, achieving nearly perfect absorbance of 99.31% and 99.61%, with ultra-narrow linewidths of 1.92 nm and 1.25 nm respectively. In addition, the dual-band absorption characteristics can be regulated by changing the structural parameters of the as-proposed metamaterials. The as-designed metamaterials can be employed as efficient two-channel refractive index sensors, with sensitivity and figure of merit (FOM) of 288 nm RIU−1 and 150 RIU−1 for the first band, and sensitivity and FOM of up to 204 nm RIU−1 and 163.2 RIU−1 for the second band. This work not only opens up a new design idea for the realization of dual-band perfect absorber synchronously with ultra-narrow linewidth, but also provides potential attractive candidates for developing dual-frequency channel sensors.

Toward Complete Optical Coupling to Confined Surface Polaritons.

Optical coupling between propagating light and confined surface polaritons plays a pivotal role in the practical design of nanophotonic devices. However, the coupling efficiency decreases dramatically with the degree of mode confinement due to the mismatch that exists between the light and polariton wavelengths, and despite the intense efforts made to explore different mechanisms proposed to circumvent this problem, the realization of a flexible scheme to efficiently couple light to polaritons remains a challenge. Here, we experimentally demonstrate an efficient coupling of light to surface-plasmon polaritons assisted by engineered dipolar scatterers placed at an optimum distance from the surface. Specifically, we fabricate gold disks separated by a silica spacer from a planar gold surface and seek to achieve perfect coupling conditions by tuning the spacer thickness for a given scatterer geometry that resonates at a designated optical frequency. We measure a maximum light-to-plasmon coupling cross section of the order of the square of the light wavelength at an optimum distance that results from the interplay between a large particle-surface interaction and a small degree of surface-driven particle-dipole quenching, both of which are favored at small separations. Our experiments, in agreement with both analytical theory and electromagnetic simulations, support the use of optimally placed engineered scatterers as a disruptive approach to solving the long-standing problem of in/out-coupling in nanophotonics.

Anisotropic Plasmonic Metasurfaces with Linear Polarization‐Dependent Focusings

AbstractAs a burgeoning approach to modulate electromagnetic waves, metasurfaces fascinate many researchers. However, it is a persistent problem to realize independent wavefront tailoring in different polarization states. In this work, anisotropic plasmonic metasurfaces are presented with linear polarization‐dependent focusings. The designed plasmonic meta‐atoms consist of cross‐shaped gold, silica spacer, and gold substrate. Phase modulation in different polarization channels is independently achieved by adjusting the dimensions of cross‐shaped gold in x‐ and y‐directions. Three metasurfaces are presented to verify linear polarization‐dependent focusings at 200 THz. First, a focusing metasurface is designed with focal lengths of 5.2 µm under x‐polarized incidence and 6.5 µm under y‐polarized incidence. Second, by applying convolution operation, the second metasurface exhibits different focusings with different deflection angles at opposite directions under orthogonally polarized incidences. Finally, a multi‐focal metasurface is demonstrated, which switches between dual‐ and quad‐focal points depending on polarization state. The work may provide a novel platform for near‐infrared integrated photonics.

Enhanced dual-band absorption of graphene mediated by an aluminum metastructure

Graphene absorption from the visible to infrared spectrum has great potential and broad applications in miniature of modern optoelectronic biosensors and photodetectors. However, graphene has zero bandgap energy, which limits its absorption to 2.3% in the visible and infrared spectrums. Here, we propose a metastructure to optimize graphene absorption in the visible to near-infrared frequency regions. The metastructure, comprising an array of aluminum square blocks (Al-SBs) on a graphene layer, a silica spacer, and an Al reflector, is investigated for absorption enhancement. This work deciphers the effect of the periodicity of decorated Al-SBs on the evolution of dual-band absorption in single-layer graphene under normal incidence. The electromagnetic signatures of two excited modes indicate that surface plasmons and magnetic dipole plasmons are mediators of absorption. The investigation into the impact of geometrical parameters illustrates that the coexisting phenomena of a relative broad peak and a relative sharp peak have been achieved simultaneously with high efficiency. The dynamic manipulation of surface plasmons and magnetic dipole plasmons presents great potential for a diverse range of applications, such as sensing and imaging. By controlling the periodicity of Al-SBs, it is possible to achieve active control of surface plasmon resonance, and a detection range of 300 nm is observed. Dynamic control of the magnetic dipole plasmon is successfully achieved by modifying the electrical environment of the graphene layer, which is realized by altering the underlying spacer material. Collectively, the findings of this study demonstrate the significant potential of the suggested metastructure for its prospective applications in optoelectronic devices, including biosensors, photovoltaics, and photodetectors that rely on the dynamic control of surface and magnetic plasmon resonances.

Colloidal self-assembly based ultrathin metasurface for perfect absorption across the entire visible spectrum

Abstract Perfect absorption over the entire visible spectrum can create a dark background for acquiring images with high contrast and improved resolution, which is crucial for various applications such as medical imaging, biological detection, and industrial non-destructive testing. The broadband absorption is desired to be achieved in an ultrathin structure for low noise as well as high integration. Here, we experimentally demonstrate a metasurface broadband perfect absorber with an ultrathin thickness of 148 nm and a large area of ∼10 cm2. Such a metasurface, with more than 97% absorption in the wavelength range from 400 to 800 nm, is composed of chromium nanodisk hexagonal array deposited on a chromium substrate with a silica spacer. A self-assembly based colloidal lithography nanofabrication method is developed for the scalable fabrication of the proposed nanostructure. We attribute the broadband absorption to the spectrally overlapped Fabry–Perot resonance, surface plasmon polariton, and localized surface plasmon resonances. Our results offer a novel approach to wafer-scale and low-cost manufacturing of absorption-based devices for applications such as high-contrast imaging and optical modulation.

Terahertz state switching of holograms enabled by vanadium dioxide-based metasurfaces.

Holography is an important topic in optical research. Metasurface holography has attracted increasing attention in recent years. However, it is still challenging to achieve dynamic tuning of holograms in the terahertz band. As an excellent phase change material, vanadium dioxide (VO2) is widely employed to dynamically manipulate electromagnetic waves. Here, VO2 meta-atoms are designed to manipulate phase and amplitude by changing the state of VO2 at 3.0 THz. These meta-atoms are composed of a VO2 block, silica spacer, and gold substrate. As the metallic VO2 is involved, 360° phase coverage is achieved by changing the dimension of VO2. The phase difference between the VO2 meta-atoms is approximately equal to 90°. Holograms are generated by aligning these meta-atoms. By combining convolution operations, holograms are deflected and reproduced. As the insulating VO2 is involved, the phase difference between the VO2 meta-atoms vanishes and the reflection amplitudes of the meta-atoms almost reach 100%. Using the phase transition of VO2, three types of metasurfaces are designed to manipulate holograms and they realize state switching of the hologram generator, state switching of hologram deflection, and state switching of the multi-beam hologram. Our work may find applications in optical holography and information privacy.

An optical keypad lock with high resettability based on a quantum dot-porphyrin FRET nanodevice.

Due to their appealing properties, nanomaterials have become ideal candidates for the implementation of computing systems. Herein, an optical keypad lock based on a Förster resonance energy transfer (FRET) nanodevice is developed. The nanodevice is composed of a green-emission quantum dot with a thick silica shell (gQD@SiO2) and peripheric blue-emission quantum dots with ultrathin silica spacer (bQD@SiO2), on which 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TSPP) is covalently linked. The nanodevice outputs dual emission-based ratiometric fluorescence, depending on the FRET efficiency of bQD-porphyrin pairs, which is highly sensitive to the metalation of TSPP: values are 59.7%, 44.8%, and 10.1% for bQD-Zn(ii)TSPP, bQD-TSPP, and bQD-Fe(iii)TSPP pairs, respectively. As such, by using the competitive chelation-induced transmetalation of TSPP, the nanodevice is capable of implementing a 3-input keypad lock that is unlocked only by the correct input order of Zn(ii) chelator, iron ions, and UV light. Interestingly, the reversible transmetalation of TSPP permits the reset (lock) operation of the keypad lock with the correct input order of ascorbic acid, Zn(ii), and UV light. Application of the nanodevice is exemplified by the construction of paper and cellular keypad locks, respectively, both of which feature signal readability and/or high resettability, showing high potential for personal information identification and bio-encryption applications.

Tunable High-Q Factor Substrate for Selectively Enhanced Raman Scattering

Most Surface-enhanced Raman scattering (SERS) substrates enhance all the Raman signals in a relative broad spectral range. The substrates enhance both the interested and background signals together. To improve the identification of target molecules from numerous background ones, substrates with multi high-quality (Q) factor resonance wavelengths can be designed to achieve the selective enhancement of specific Raman transitions. When the resonance frequencies are modulated to match the excitation and Raman scattering frequencies, the detection of the target molecule can be more effective. In this paper, we design a tunable high-Q SERS substrate with periodic silver bowtie nanoholes on silica spacer and silver film. The substrate possessed three high-Q and high electric field resonance modes, which resulted from the interaction of the localized surface plasmon resonance (LSPR) of the bowtie nanoholes, the surface plasmon polariton (SPP) of the period bowtie nanoholes and the Fabry–Perot (FP) resonance between the bowtie and silver film bottom. The interaction between these resonance modes resulted in not only a higher quality (Q) factor, but also a higher electric field, which can be employed to realize a potential substrate in high-sensitivity and selective-detection fields.

Tailoring terahertz wavefront with state switching in VO2 Pancharatnam–Berry metasurfaces

Metasurface provides a novel approach to control electromagnetic wave, and it attracts great attention. But, it is still challenging to realize wavefront tailoring with dynamically switchable functions. In this work, dynamically switchable metasurfaces are presented based on vanadium dioxide, and they realize state switching of reflected wavefront for circularly polarized wave in terahertz band. The designed meta-atom consists of vanadium dioxide strip, silica spacer, and gold substrate. Based on Pancharatnam-Berry phase, 360° phase modulation is achieved by axially rotating vanadium dioxide strip. Through phase transition of vanadium dioxide between metallic and insulating states, three applications are designed to demonstrate state switching of metasurfaces at 1.0 THz. First of all, a gradient metasurface is designed and anomalous reflection is formed. Anomalous reflection for metallic vanadium dioxide and specular reflection for insulating vanadium dioxide are dynamically switched. Secondly, vortex beam generators are implemented for metallic vanadium dioxide. Incident waves with different chirality achieve orbital angular momentums with different mode numbers, and it is switched to plane wave when vanadium dioxide is insulator. Finally, a reflective lens is presented with switchable focusing function. Our work may be applied in fields such as terahertz switching, wireless communication, and planar lens.

A FRET sensor based on quantum dots-porphyrin assembly for Fe(III) detection with ultra-sensitivity and accuracy.

Exploring sensors based on Förster resonance energy transfer (FRET) systems enables the continuous development of biological sensing technologies. Herein, we report the construction of a FRET sensor with dual-emissive quantum dots (QDs) and meso-tetra(4-sulfonatophenyl) porphine (TSPP). The sensor is composed of mesial green-emissive QDs with a thick silica shell (gQD@SiO2) and circumjacent blue-emissive QDs coated with ultra-thin silica spacer, on which is linked TSPP (bQD@SiO2-TSPP). The gQD@SiO2 endows the sensor with a fluorescent background. Due to the ultra-thin silica spacing, coupled with the superior resonance effect of bQD fluorescence and the Soret-band absorption of TSPP, the FRET efficiency is highly sensitive to the chelation state of TSPP. Relying on the absorbance transition of TSPP complexed with Fe(III), the FRET sensor is applied for ultra-sensitive Fe(III) detection. In aqueous solution, the sensor is demonstrated to linearly detect Fe(III) in the range of 0-1μM, with a limit of detection (LOD) of 40nM. More importantly, reliable Fe(III) detection can be achieved via the specific complexation of Fe(III) by TSPP and the ratiometric fluorescent response. As such, the inter-/intra-day precisions in standard samples, as well as the recovery rate in biological matrices, are fully validated. The excellent analytical performance, in combination with the excellent biocompatibility of the FRET sensor, allows semi-quantitative Fe(III) imaging in living cells.

Identifying high performance photosensitizer with simultaneous enhancement in fluorescence and singlet oxygen generation, from ‘(Ag/Au)-aggregation-induced emission-active fluorogen’ theranostic nanoparticles

Photodynamic therapy (PDT) is a non-invasive medical technique that combines photosensitizer (PS) molecules and light for cancer treatment. Driven by the need for more personalized medicine, shortened irradiation time and treatment efficiency of PDT are required. Herein, a library of novel theranostic nanoparticle systems for personalized medicine in PDT of cancer is reported. Ag and Au nanoparticles in a size range of 40–50 nm are prepared and used subsequently to synthesize core-shell plasmonic nanoparticles with various thicknesses, either with silica or polymer spacer. Metal-enhanced fluorescence and singlet oxygen generation (SOG) studies are performed for a naphthalene-linked pyridine-based neutral triarylethene as AIE-active PS molecule, by adsorbing on the Ag and Au core-shell nanoparticles. The factors that are varied during the study are the composition of the spacer, and the spacer length. Notably, the enhancement in fluorescence and SOG of the adsorbed AIEgen are observed at the same distance from the metallic surface, resulting from short-range near field plasmonic effect for all the theranostic PS prepared with Ag/Au. The best result, 7.9 fold enhancement in fluorescence and 10.4-fold enhancement in SOG, is observed on loading AIEgen on ∼42 nm Au nanoparticles with polymer spacer, showing promising advancement for image-guided PDT. The probable reasons behind the observed outcomes, are analyzed and discussed in detail, with the help of the Jablonski diagram of the PS molecule. The obtained results from the study has the potential to replace or support the conventional radiation therapy and chemotherapy with serious adverse effects.

Near Infrared Metal-insulator-metal Surface Plasmon Resonances for Refractive Index Sensors

This work reports the optical properties of surface plasmon resonance (SPR) based on the metal-insulator-metal (MIM) structure towards a refractive index sensor. The MIM-SPR structure operating near infrared region consists of lateral periodicity of subwavelength gold patterns placed on a stack of thin silica spacer and silver film (acting as a reflector) on a silicon substrate. The reflection spectra and the electric field distributions of MIM-SPR structures can be tuned by modifying the geometrical properties and have been numerically investigated by using Lumerical’s finite-difference time-domain (FDTD) solutions. The square lattice configuration of 1200 nm to 1400 nm pitch of gold micro-disks of thickness from 80 nm to 120 nm have been conducted. The size of these considered gold patterns, i.e., the diameter of the micro-disks is in the range of 900 nm to 1000 nm. The proposed MIM-SPR structure possessing sensitivity of 370 nm per refractive index unit (RIU), can be applicable for a wide variety of plasmonic sensing, in particular for refractometric biosensors.

Visible to near-infrared nearly perfect absorption from alternate silica and chromium layers deposited by magnetron sputtering.

We present a novel, to the best of our knowledge, broadband and angle-insensitive nearly perfect absorber design composed of alternate silica and chromium layers. We show that by depositing a chromium nanofilm on a chromium substrate with a silica spacer, the absorption will significantly enhance from the visible to near-infrared. Then, another silica film is placed on the top of the layered structure as an antireflection coating, resulting in the broadband near-perfect absorption. We fabricate the proposed absorber by magnetron sputtering. The measured results show that our device has an average absorption over 97% in a wide range of wavelengths ranging from 350 to 1170 nm, and its absorption performance exhibits a good angular tolerance up to 50°. The presented absorber design offers a good prospect for large-scale and low-cost manufacturing of absorption-based optoelectronic devices.

Inflow nano-optics from the near-to the far-field detection based on Metal-Enhanced Fluorescence signaling

In this communication we evaluated the detection and tracking of Ultraluminescent gold core–shell nanoplatforms based on Metal-Enhanced Fluorescence (MEF) within colloidal dispersion by In-Flow Cytometry and Imaging Flow Cytometry (IFC). The enhanced properties were developed by the incorporation of Rhodamine B (RhB) as a laser dye reporter at variable controlled silica spacer lengths (Au@SiO2-RhB) from a gold core template. Thus, SiO2 was varied from 6 to 7 nm to 25 nm in order to place the laser dye sensitizer at different lengths of the higher electromagnetic fields surrounding the 45-nm gold core diameter. By Laser Fluorescence Microscopy, high intense emissions were shown at shorter SiO2 lengths, while lower emissions were found towards longer ones. These characteristics related to MEF were evaluated by in-flow cytometry with dual laser excitation and fluorescence detection, and by an IFC set-up. By In-flow cytometry, different scattered-light distributions were shown, depending on the nanoparticle concentrations in the colloidal dispersion and size of the Au@SiO2-RhB. Moreover, varied fluorescent event detections and counting were recorded according to the SiO2 lengths applied. A ratio of fluorescent event detection counting of core–shell/core-less of 40–42 was established for optimal MEF Enhancement Factors (MEFEF) of 41–43 determined by Laser Fluorescence Microscopy. Core-less nanoparticles showed a marked decrease of fluorescent event detection counting caused by higher photobleaching and absence of the MEF effect. Hence, an improved detection effect was shown within in-flow based on MEF from single nanoplatforms. Therefore, the Ultraluminescence emission of core–shell nanoarchitectures within in-flow techniques was shown and discussed. A non-classical light in real time within colloidal gold core–shell nanoparticles was generated by accurate control of their architectures.

Refractive index biosensing using near-UV metal–dielectric–metal nanostructured arrays: fabrication and simulation

This work reports the design, simulation, and fabrication of a metal–dielectric–metal (MDM) nanohole array structure toward a realization of a refractive index biosensor. The MDM nanohole array structure operating near the UV range consists of a subwavelength hole array inscribed through the upper aluminum (Al) layer positioned on a stack of a thin silica ( S i O 2 ) spacer and an Al mirror on a silicon substrate. The quality factor of the resonance at ∼ 390 n m of the present nanostructure is ∼ 1.5 t i m e s higher than the previous result owing to the larger fill factor of Al material on the upper layer. This means at the UV range, the designed nanohole array structure closely resembles the dielectric gratings in the visible range. At resonance, we numerically observed a strong confinement of incident light localized inside the subwavelength apertures and the S i O 2 spacer, and thus high absorption and directive emission of up to 99% and ∼ 8.0 d e g , respectively. The MDM nanohole array structure has been fabricated by using plasma-enhanced chemical vapor deposition combining with the focused ion beam, and its reflection experiments are in good agreement with the finite-difference time-domain simulation results. The proposed MDM nanohole array structure can be applicable for a wide variety of plasmonic sensing, in particular for refractive index biosensors. Because of working in the near-UV range, this refractive index biosensor shows the figure of merit to have a selectivity that is ∼ 4.0 t i m e s higher than that of other established plasmonic MDM biosensors.

A smartphone-combined ratiometric fluorescence probe for specifically and visibly detecting cephalexin

A smartphone-combined dual-emission ratiometric fluorescence probe for specifically and visibly detecting cephalexin was first designed. In the probe, blue-emitting fluorescent carbon dots (CDs) was synthesized and covered with a layer of silica spacer. Red-emitting fluorescent CdTe QDs (r-QDs) was grafted onto the silica nanospheres as an analytical probe. Then, the cephalexin antibody was covalent grafted to the ratio sensor to increase the selectivity. The ratio of fluorescence intensity (FL) of r-QDs and CDs was quenched with the increasing concentration of cephalexin. The detection method has good linear response in the range of 1–500 μM and the detection limit was 0.7 μM. Then portable device based on smartphone detection was constructed according to the color change under UV lamp. The detection image was obtained through the smartphone camera, and the color picker APP installed in the smartphone captured the RGB value of the image. In addition, this method was also used to determine the amount of cephalexin in milk samples with recovery of 94.1%-102.2%. These results showed that it was a portable, simple and visible method to detect cephalexin in food analysis and environmental monitoring.

The ultraviolet absorption of graphene in the Tamm state

Ultraviolet (UV) light trapping by monolayer graphene is of great significance in optoelectronic devices. In this work, to enhance the UV absorption of monolayer graphene, a metallic thin film-type/graphene/Bragg mirror optical structure is developed using transfer matrix methods. Our results showed that the absolute UV absorption of monolayer graphene can reach approximately 83 % at the wavelength of 270 nm, which is a 9.2-fold enhancement compared to the intrinsic value of free-standing graphene (9%). This high absorption is due to the strong field confinement of the Tamm state in the silica spacer layer. The UV absorption of graphene can be tuned by adjusting the nanostructure parameters and incident angles. Furthermore, the proposed structure is less affected by machining errors. Considering the influence of processing precision, the light absorption of graphene in this structure is higher than that in the microcavity of a photonic crystal. The results obtained in this study may find promising applications in high-performance UV–graphene optoelectronic devices, such as modulators and photodetectors, as well as in photocatalysis.

Achieving broadband absorption and polarization conversion with a vanadium dioxide metasurface in the same terahertz frequencies.

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

Quasi-Periodic Dendritic Metasurface for Integral Operation in Visible Light.

A reflective metasurface model composed of silver dendritic units is designed in this study. The integral property of this metasurface, which consists of an upper layer of dendritic structures, a silica spacer, and a bottom silver substrate was demonstrated at visible wavelengths. The simulation results revealed that the metasurface can perform integral operation in the yellow and red bands; this can be easily generalized to the infrared and communication bands by scaling the transverse dimensions of this metasurface. A dendritic metasurface sample responding to red light was prepared via the bottom-up electrochemical deposition method. The integral operation property of the sample was verified experimentally. This dendritic metasurface, which can perform integral operation in visible light, can be used for big data processing technology, real-time signal processing, and beam shaping, and provides a new method for miniaturized and integrated all-optical signal processing systems.