A nanoplasmonic biosensor

Abstract

The proposed invention is an optical sensor with new plasmonic architecture which enables the simplification of the current complex diagnosis instruments.

Description

[0001] A NANOPLASMONIC BIOSENSOR

[0002] Technical Field

[0003] The proposed invention is an optical sensor with new plasmonic architecture which enables the simplification of the current complex diagnosis instruments.

[0004] Prior Art

[0005] Biosensors are the core components of biodetection and medical diagnosis technologies and they emerge from improvement of the concurrent technologies or invention of new and innovative technologies to achieve lower cost and higher accessibility.

[0006] The global biosensors market was valued at 19.6 billion USD in 2019 and is expected to have a staggering annual growth rate of 7.9%, reaching 36 billion USD in 2027. The point-of-care (PoC) biosensors constitute nearly half of the biosensor market [1], The healthcare industry’s increasing demand concentrates on PoC clinical screenings, real-time diagnosis of diseases, and personalized treatments. Biosensor technology focuses on low-cost, fast, and portable automated systems with sensitive and real-time detection capabilities to supply this demand. Even though the plethora of existing successful detection methodologies, there is always room for improvement to eliminate the drawbacks of each method. Among many, optical sensors stand out with their rapid, non-destructive, and cost-effective sensing potential for multiplexing and miniaturization.

[0007] To meet the growing demands for predictive diagnoses of various diseases, a number of biosensor technologies have been developed to detect a wide range of biomarkers. Recent technical advances, for example, have enhanced the sensitivity and / or chip availability for detection and quantification of protein biomarkers in biological samples via binding to antibodies or aptamers. Biosensors detecting and quantifying binding events via optical, colorimetric, electrical, electrochemical, acoustic and / or magnetic means have been developed. However, these biosensors have not translated to point-of-care (PoC) technologies capable of rapid, label-free, sensitive, high throughput detection of various biomarkers in a whole blood sample. Therefore, current disadvantages persist regarding facile and rapid analysis of biological samples for disease diagnosis. The nanopl asmonics field focuses on the phenomenon of light-induced excitations of collective free electron oscillations between dielectrics and materials with a high number of free or free- like electrons. The collective oscillation of electrons is called surface plasmon resonance (SPR) in general. The SPR can enable the manipulation of light below the diffraction limit at a nanometer regime. The SPR resonance can be engineered with both geometric parameters and dispersion properties of the constituent materials. With the rapid progress in nanophotonics, the SPR principle can be employed in optoelectronics, coinciding with the current miniaturization effort in the sensing field. The first application of surface plasmons in biosensors was to utilize noble metal thin films as active surfaces. In such configurations, incoming photons excite propagating surface plasmons in a metal-dielectric interface, having a rapidly decaying evanescent field in the dielectric layer, which is in return highly susceptible to the changes in the refractive index of the surrounding environment. The proximity-based configuration enables high sensitivity and low limits of detection. SPRs are confined modes, propagating in flat metal / dielectric interfaces. The coupling of photons from the dielectric to surface-confined mode requires a phase-matching condition (momentum matching) that can be surpassed using a high-RI prism or periodic gratings on the metal-dielectric interface.

[0008] SPR is widely used to investigate the binding kinetics of biological macromolecules like proteins and nucleic acids. However, detection of small molecule analytes and analytes at low concentrations remains to be challenging. The difficulty resides in the tiny RI variations close to the metal-dielectric interface. While being widely accepted and commercialized, SPR still has significant drawbacks limiting its use in point-of-care diagnostics: For instance, excitation of SPR requires light coupling setups to match the wavevectors of photons and plasmons. Often angular detection setups are needed to achieve desired sensitivity levels. These conditions severely limit the application of SPR in miniaturized portable devices. SPR shows high sensitivity to bulk refractive index changes in solutions. Subsequent injects of analyte dispersions, buffer changes, and convective thermal fluctuations can detrimentally affect the measurements, which requires highly controlled measurement conditions. In SPR, the evanescent field decay length can reach 100-500 nm away from the metal-dielectric interface, leading to the detection of weakly-bound non-specific interactions and convective thermal fluxes with less orientation / conformation-based sensitivity. An alternative to SPR, metallic nanostructures smaller than the incident light’s wavelength can support locally confined non-propagating collective harmonic oscillations of free-electrons in a metal-dielectric interface. The phenomenon is called localized surface plasmon resonance (LSPR), illustrated in Figure IB. LSPR can provide highly confined and enhanced electric fields in the proximity of the interface compared to SPR with a more rapid evanescent field decay hence the high confinement. LSPR does not require coupling elements like prisms or angular detection; strong scattering and absorption coefficients of LSPR-supporting nanostructures eliminate the need for optical coupling elements or polarization restrictions. LSPR spectral shifts can be tens of nanometers thus can be detected even by the naked eye. Compared to SPR, LSPR can provide a higher surface sensitivity due to a lower field decay length of 10-30 nm while remaining less sensitive to bulk RI changes and convective temperature fluctuations. Thus, LSPR-based biosensing systems can track analytes near their surfaces without interfering with faraway molecules in the analyte solution. The LSPR can be excited by unpolarized light and does not demand highly complex optics. Significantly short evanescent field decay into the surrounding dielectric media offer a higher potential for miniaturization and multiplex detection. However, inherent absorptive losses in plasmonic metal at visible range and heating of plasmonic nanostructures remain among the field problems. A higher number of parallel microfluidic channels can be utilized and allows realtime monitoring of reading sites independent of each other. These properties make LSPR-based platforms more advantageous for low-cost portable PoC devices with excellent miniaturization and multiplexing potential with the incorporation of microfluidic distribution systems.

[0009] On the other hand, extraordinary optical transmission (EOT) is another phenomenon that originates from incident photons’ interaction with periodic arrays of nanoholes (Figure 1C). EOT combines both the propagating SPR and localized SPs in single geometry. A periodic array acts as a grating and matches the incoming light’s momentum to the surface plasmons. Nanoholes act as inverse hollow nanostructures that confine the incoming EM field in their perimeter since metal nanostructures in a dielectric medium are interchangeable with hollow or dielectric nanostructures in metal media in electrostatic approximation. This way, EOT does not require complex optics and can be excited with unpolarized light. LSPR-like confinement of EM field around the nanohole edges can be effective sensing sites for exploiting the fast decay lengths, even leading to flow-through sensing. The wavelength-specific transmission through these nanohole arrays offers a major resonant frequency shift within their evanescent fields. The technique eliminates the need for moving parts like angle-dependent measurement and enables portable and compact sensing platforms. LSPR / EOT-based nanoplasmonic biosensors can be realized with a broadband light source and portable spectrometers without complex optical setups. However, the amount of the transmitted light over the incident light, in other words, signal intensity is very low in these systems, which is the main drawback of these systems.

[0010] Aims of the Invention and Brief Description

[0011] This invention relates to a nanoplasmonic biosensor. The periodic nanohole array on the top metal layer enables the formation of electric field enhancement and facilitates the highly sensitive biosensing. The presence of insulator layer ensures the sharpness of the plasmonic resonance and formation of waveguide modes within MIM structure. The bottom metal layer acts as a back reflector and facilitates the formation of waveguide modes and enhancement of electric field intensity on the patterned top metal layer. The high enhancement of electric field intensity enables refractometric and biosensing through improve light-analyte interactions resulting in high surface sensitivity and a lower limit of detection.

[0012] The aim of this invention is to remedy the disadvantages listed below for label-free nanoplasmonic biosensing for multiplexing and miniaturization;

[0013] • In contrast to the low signal response of LSPR and EOT-based systems, MIM based nanoplasmonic biosensor can provide high signal quality due to its back reflector based MIM waveguide.

[0014] • MIM based nanoplasmonic biosensor has more structural tolerance to nanofabrication errors than other methods

[0015] • Unlike LSPR and EOT, MIM nanoplasmonic sensors do not exhibit dominant substrate-metal surface plasmon polaritons in their spectral response.

[0016] Advantages of the invention;

[0017] • The invention provides reflection-based refractometric sensing of analytes in the vicinity of the metal -di electric interface. The local refractive index changes provide a considerable shift in the resonance wavelength of MIM-based nanohole array. • The invention provides higher signal-to-noise ratio and signal quality (Q factor) than LSPR- based systems. Since the invention uses reflection-based spectral response, the change in the spectral light intensity provides a higher signal-to-noise ratio. The spectral response of the invention shows narrow full-width half-maximum peaks thus enhancing the signal quality.

[0018] • The invention provides higher signal-to-noise and signal strength (Q factor) than EOT-based systems

[0019] • The invention provides metal-medium plasmonic resonance response which do not get affected by the effective refractive of the substrate, unlike EOT and LSPR-based system.

[0020] Definition of the Figures

[0021] Figure 1. Schematic representation of the working principle of (A) SPR, (B) LSPR, (C) EOT, and (D) MIM based biosensors

[0022] Figure 2. Geometric parameters for MIM nanohole arrays.

[0023] Figure 3. Schematic view of proposed Metal-Insulator-Metal (MIM) structure with sublayers

[0024] Figure 4. (A) Geometric parameters of nanofabricated MIM nanohole arrays, (B) Optical microscope image of MIM structure without nanohole array (top), MIM structure with nanohole array (bottom), (C) Scanning electron micrograph of MIM nanohole arrays with a period of 500 nm and hole diameter of 140 nm, (D) Scanning electron micrograph of MIM nanohole arrays with a period of 550 nm and hole diameter of 135 nm, (E) Scanning electron micrograph of MIM nanohole arrays with a period of 600 nm and hole diameter of 130 nm, (F) Scanning electron micrograph of MIM nanohole arrays with a period of 650 nm and hole diameter of 125 nm.

[0025] Figure 5. (A) Simulated reflection spectra of MIM nanohole arrays with periods of 500-650 nm and a hole diameter of 200 nm, (B) Simulated reflection spectra of MIM nanohole arrays with periods of 500-650 nm and a hole diameter of 130 nm, (C) Experimental reflection spectra of nanofabricated MIM nanohole arrays with periods of 500-650 nm.

[0026] Definition of the Elements in Figures

[0027] Definitions of elements in figure are listed below for better understanding of the invention.

[0028] 1 Substrate

[0029] 2 First Metal Layer 3 Insulator Layer

[0030] 4 Second metal layer with periodic nanohole array

[0031] Detailed Description

[0032] The novelty of the invention has been described with examples that shall not limit the scope of the invention and which have been intended to only clarify the subject matter of the invention.

[0033] The present invention has been described in detail below.

[0034] This invention relates to a nanoplasmonic biosensor which has metal-insulator-metal (MIM) structure with sublayers.

[0035] Figure 1. Surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR), extraordinary transmission based surface plasmon polaritons (EOT), and metal-insulator-metal (MIM) based plasmonic biosensing is represented comparatively with excitation modes, signal detection, spectral response of the sensing signal and electric field intensity distributions.

[0036] The commonly used methods such as surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR), and extraordinary transmission (EOT) based surface plasmon polaritons (EOT) were presented comparatively along with the proposed metal-insulator-metal (MIM) plasmonic resonator in Figure 1. Unlike the other plasmonic biosensing strategies, MIM-based plasmonic detection utilizes the plasmonic absorption of the waveguide modes to enhance the reflection minimum of the plasmonic response.

[0037] Figure 2. Geometric parameters for MIM nanohole arrays for nanosphere lithography (NSL) and electron beam lithography (EBL).

[0038] The geometric parameters for MIM nanohole arrays are as follow:

[0039] A, period of nanoholes, can be adjusted using polymer nanospheres with a range of diameters (A= 100-2000 nm)

[0040] D, the diameter of the nanoholes, can be adjusted with controlled etching of polymer nanospheres (D=50-200 nm) h, the thickness of the top metal layers (h=50-200 nm)

[0041] The material parameters for MIM nanohole arrays are as follows:

[0042] The optical properties of the substrate (n, k) (e.g., BK7, fused silica, soda lime glass) The optical properties of the bottom metal thin film (e.g., Au, Ag, Al)

[0043] The optical properties of the insulator layer (e.g., TiCh, ZnO, AI2O3, SisN4, SiCh)

[0044] The optical properties of the adhesion layers (e.g., Ti, Cr, TiN)

[0045] The simulation studies were performed for square, multiperiodic, and hexagonal MIM nanohole arrays varying the aforementioned geometric parameters using finite-difference time-domain (FDTD) method.

[0046] The first metal layer, which works as a back reflector, can be chosen from any metal with a high reflectivity since it has minimal interaction with surrounding media; therefore, chemical stability is not a crucial parameter for this layer. Top metal layer, on the other hand, interacts with the surroundings; thus, has to be a material with negative real permittivity like gold (Au) or silver (Ag). Insulator layer of the structure is a high refractive index ceramic material like silicon nitride (SisN4), silicon dioxide (SiCh) and aluminum oxide (AI2O3).

[0047] Figure 3, architecture of the sensor consists of 4 layers. The first layer is the substrate (item number (1) in figure 15) that provides foundation and robustness to the rest of the structure. Any flat and smooth material can be used as a substrate since the sensor itself works reflectionbased; hence, independent of the substrate. The second layer is the back reflector and the first metal layer of the metal-insulator-metal (MIM) structure, the third layer is the insulator layer and the final layer is the top metal layer with periodic nanohole array shown in figure 3, part number (2), (3) and (4) respectively.

[0048] The upper metal layer of the MIM nanohole array is the layer used for biorecognition by trying to intensify the electric field. The intermediate insulator layer is involved in the sharpness of the resonance profile and the formation of waveguide modes for refractometric diagnosis. The lower layer acts as a back reflector and is used to increase both the generation of waveguide modes and the electric field strength of the periodic nanoholes in the upper metal layer. High electric field distribution increases the interaction of more electric fields with surface analytes for refractometric and biorecognition. In this way, it allows diagnosis with higher surface sensitivity and a lower detection limit.

[0049] The invention works under a light excitation. The working principle of the product is independent of the polarization and the incident angle of the light source. As the sensor interacts with light, the top metal layer with the nanohole array selectively transmits a narrow portion of the spectrum which depends on the refractive index, nanohole lattice, and the periodicity of the nanohole array; the rest of the spectrum was reflected back from the top layer. Transmitted light is reflected back from the bottom metal layer and gets channeled to the insulator layer where it gets damped. As a result, a sharp deep in reflection occurs. Since the wavelength of the transmitted light depends on the refractive index of the surrounding, in case of binding on the surface of the product, position of the deep in the reflection will shift accordingly. Hence, the invention can be used as a biosensing platform for a variety of analytes with different surface modifications.

[0050] Depending on the detailed information above, the invention is a nanoplasmonic biosensor which has a new plasmonic architecture, comprising metal-insulator-metal (MIM) structure with sublayers which are at least one substrate (1), at least one first metal layer (2), at least one insulator layer (3), at least one second metal layer with periodic nanohole array (3).

[0051] The sensor of the above information, wherein the

[0052] • At least one substrate (1), which is the first layer in sensor architecture at the bottom and provides foundation and robustness to the rest of the structure,

[0053] • At least one first metal layer (2), which is the second layer in sensor architecture from the bottom used for as a back reflector which is providing transmitted light reflects back and is used to increase both the generation of waveguide modes and the electric field strength of the periodic nanoholes in the up second metal layer with periodic nanohole array (4),

[0054] • At least one insulator layer (3), where the transmitted light back from first metal layer (2) gets damped and which is used to increase the sharpness of resonance profile and generation of waveguide modes for refractometric sensing, as well as to increase the electric field strength of around the nanoholes in metal,

[0055] • At least one second metal layer with periodic nanohole array (4) is the top metal layer which transmits some part of the spectrum (interested part of the visible and / or nearinfrared spectrum. It may vary according to the area of the working spectrum of the proposed biosensor. This change is adjusted by design of the nanohole array, which is the proposed second layer) depends on the refractive index, nanohole lattice structure and the periodicity of the nanohole array and is also used for biosensing by trying to concentrate local electric field respectively. Any metal with a high reflectivity in the visible light and near-infrared spectrum can be used in the first metal layer (2). The best example of ideal materials for first metal layer (2) is aluminum, gold, or silver.

[0056] Insulator layer (3) is a high refractive index ceramic material like silicon nitride (SisN4), silicon dioxide (SiCh), or aluminum oxide (AI2O3).

[0057] The second metal layer with periodic nanohole array (4) is gold (Au), silver (Ag) or aluminum (Al).

[0058] Substrate (1) is BK7, fused silica, or soda lime glass. BK7 is type of borosilicate glass with a constant refractive index.

Claims

CLAIMS A nanoplasmonic biosensor which has a new plasmonic architecture, comprising, metalinsulator-metal (MIM) structure with sublayers which are at least one substrate (1), at least one first metal layer (2), at least one insulator layer (3), at least one second metal layer with periodic nanohole array (3). The sensor of claim 1, further comprising,• At least one substrate (1), which is the first layer in sensor architecture at the bottom and provides foundation and robustness to the rest of the structure,• At least one first metal layer (2), which is the second layer in sensor architecture from the bottom used as a back reflector which provides reflection of the transmitted light and is used to increase both the generation of waveguide modes and the electric field strength of the periodic nanoholes in the up second metal layer with periodic nanohole array (4),• At least one insulator layer (3) where the transmitted light back from the first metal layer (2) gets damped and which used to increase the sharpness of resonance profile and generation of waveguide modes for refractometric sensing, as well as to increase the electric field strength of nanoholes in metal,• At least one second metal layer with periodic nanohole array (4) is the top metal layer which transmits interested part of the visible and / or near-infrared spectrum depend on the refractive index, lattice and the periodicity of the nanohole array and also used for biosensing by trying to concentrate electric field respectively. The sensor of claim 1, wherein said insulator layer (3) is a high refractive index ceramic material like silicon nitride (SisN4), silicon dioxide (SiCh) or aluminum oxide (AI2O3). The sensor of claim 1, wherein said second metal layer with periodic nanohole array (4) is gold (Au), silver (Ag), or aluminum (Al). The sensor of claim 1, wherein said substrate (1) is BK7, fused silica, or soda lime glass.

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