Surface-enhanced raman scattering substrate for multiplex detection

Abstract

Various embodiments provide a surface-enhanced Raman scattering (SERS) substrate, a biosensor system, and a method for performing multiplex detection of different analytes. The SERS substrate includes: a distributed Bragg reflector (DBR) including alternating layers of chalcogenide phase change materials and dielectric materials, terminated by an end layer of chalcogenide phase change material, wherein the end layer is thicker than each alternating layer of chalcogenide phase change materials; and a conductor layer disposed on the DBR. The chalcogenide phase change materials have a refractive index higher than that of the dielectric materials. The DBR has a thickness dimensioned to simultaneously excite multiple Tamm modes at different Raman excitation wavelengths and to continuously tune the multiple Tamm modes within photonic bandgaps of the DBR to achieve an enhancement factor. The conductor layer provides a surface roughness for generating Raman signals with the enhancement factor.

Description

DESCRIPTIONTITLE OF THE INVENTION: SURFACE-ENHANCED RAMAN SCATTERING SUBSTRATE FOR MULTIPLEX DETECTIONFIELD OF THE INVENTION

[0001] The invention generally relates to the field of analytical spectroscopy. More particularly, various embodiments relate to a surface-enhanced Raman scattering (SERS) substrate for multiplex detection of different analytes, a method for fabricating the SERS substrate, and a method for performing multiplex detection of different analytes.BACKGROUND OF THE INVENTION

[0002] Detecting a single biomarker using conventional techniques may not accurately provide information about the disease and its treatment efficacy. In the past, if multiple biomarkers were present, the sample had to be divided into appropriate fractions to analyze it individually. However, this approach is time- and sample-consuming, not cost-effective, and unsuitable for the increasing demand for high throughput detection. Therefore, the development of multiplexed biosensor platforms with high throughput is in high demand for the management of various diseases by simultaneously analyzing the multiple biomarkers. The multiplexed detection platform can detect and analyze the analytes in a single run qualitatively and quantitatively and saves sample volume, assay time, and cost, which is more efficient in clinical applications. At the same time, developing reusable or cost-effective non-invasive multiplexed biosensors is essential for point-of-care testing (POCT) such as early detection of diseases and its monitoring. In this direction, optical biosensors have shown great progress in basic biomedical research due to their label-free and multiplexed detection capability, high sensitivity, low cost, reusability, low noise, and immunity to electromagnetic interference.

[0003] Among optical techniques that may potentially be used for multiplex detection of biomarkers are fluorescence and surface plasmon resonance (SPR). However, fluorescence is not ideal for multiplex detection as spectral bands are broad and difficult to decouple the signal for multiplex detection both in label-free and labelled detection modes. Furthermore, a bulk prism configuration is required for SPR, and the sensitivity is limited.

[0004] There is thus a need for a sensor platform that addresses at least the problems mentioned above. Furthermore, other desirable features and characteristics will becomeapparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.SUMMARY

[0005] In one aspect, there is provided a surface-enhanced Raman scattering (SERS) substrate for multiplex detection of different analytes, the SERS substrate including: a distributed Bragg reflector (DBR) including alternating layers of chalcogenide phase change materials and dielectric materials, terminated by an end layer of chalcogenide phase change material, wherein the end layer is thicker than each alternating layer of chalcogenide phase change materials; and a conductor layer disposed on the DBR, wherein the chalcogenide phase change materials have a refractive index higher than that of the dielectric materials, the DBR has a thickness dimensioned in a manner such that the DBR is operable to substantially simultaneously excite multiple Tamm modes at different Raman excitation wavelengths and to continuously tune the multiple Tamm modes within photonic bandgaps of the DBR to match a resonance wavelength of each Tamm mode to an absorption peak wavelength of each of the different analytes to achieve an enhancement factor, and the conductor layer provides a surface roughness for generating Raman signals with the enhancement factor.

[0006] In one embodiment, the chalcogenide phase change materials may include SbzSa.

[0007] In one embodiment, the chalcogenide phase change materials may include SbzSea or GezSbzSe^ei.

[0008] In one embodiment, the end layer may have a thickness ranging from 250 nm to 400 nm.

[0009] In one embodiment, each alternating layer of chalcogenide phase change materials may have a thickness ranging from 160 nm to 200 nm.

[0010] In one embodiment, the dielectric materials may be selected from the group consisting of SiO2, MgF2, CaF2, and BaF2.

[0011] In one embodiment, each alternating layer of the dielectric materials may have a thickness ranging from 70 nm to 120 nm.

[0012] In one embodiment, the DBR may include at least three pairs of alternating layers of chalcogenide phase change materials and dielectric materials.

[0013] In one embodiment, the conductor layer may include a gold film having a thickness ranging from 10 nm to 20 nm.

[0014] In one embodiment, the conductor layer may include a first conductor layer disposed on the DBR overlying the end layer, the first conductor layer arranged to be in contact with a sample comprising the different analytes.

[0015] In one embodiment, the SERS substrate as described herein may further include a dielectric film disposed in between the DBR and the first conductor layer.

[0016] In one embodiment, the dielectric film may include SiOz.

[0017] In one embodiment, the SERS substrate as described herein may further include an underlying substrate arranged adjacent to the DBR and opposite to the first conductor layer, wherein the DBR may be disposed between the first conductor layer and the underlying substrate.

[0018] In one embodiment, the underlying substrate may be selected from the group consisting of quartz, silicone, glass, semiconductor and polymers.

[0019] In one embodiment, the conductor layer may further include a second conductor layer disposed on the DBR opposite to the end layer.

[0020] In another aspect, there is provided a biosensor system for multiplex detection of different analytes, the biosensor system including: a SERS substrate as described herein configured to receive the different analytes; and an excitation source configured to emit a plurality of excitation radiations of different wavelengths onto the different analytes and the SERS substrate, wherein the SERS substrate is configured to generate the Raman signals of each of the different analytes with the enhancement factor.

[0021] In one embodiment, the biosensor system as described herein may further include a spectrometer configured to receive the Raman signals with the enhancement factor.

[0022] In another aspect, there is provided a method for performing multiplex detection of different analytes, the method comprising: (a) providing a SERS substrate as described herein; (b) contacting a sample comprising the different analytes with the conductor layer of the SERS substrate, wherein the conductor layer is disposed on the DBR overlying the end layer; (c) applying a plurality of excitation radiations to the sample and the SERS substrate to substantially simultaneously excite multiple Tamm modes, wherein each excitation radiation has a predetermined excitation wavelength that is close to or equal to a resonance wavelength of each of the multiple Tamm modes; and (d) generating Raman signals for multiplex detection.

[0023] In one embodiment, for the plurality of excitation radiations, each of the predetermined excitation wavelengths may be close to or equal to an absorption peak wavelength of each of the different analytes, and wherein generating the Raman signals forthe multiplex detection may include generating the Raman signals with an enhancement factor.

[0024] In one embodiment, applying the plurality of excitation radiations may cause continuous tuning of the DBR of the SERS substrate by modulating a structural phase of the chalcogenide phase change materials of the DBR to match the resonance wavelength of each of the multiple Tamm modes to the absorption peak wavelength of each of the different analytes.

[0025] In the context of various embodiments, the phrase “modulating a structural phase of the chalcogenide phase change materials of the DBR (e.g. each SbzSa layer) from a lower crystalline state to a higher crystalline state” is meant to refer to increasing a degree of crystallization of the chalcogenide phase change materials of the DBR (e.g. SbzSa layers) between an amorphous phase and a crystalline phase. For example, the chalcogenide phase change materials are initially in the fully amorphous phase. The degree of crystallization increases with annealing, and the chalcogenide phase change materials become fully crystalline. It would be generally appreciated that the chalcogenide phase change materials of the DBR (SbzSa layers) may be fully amorphous, fully crystalline, or any degree in between (i.e. partially crystalline). In one embodiment, the structural phase of all SbzSa layers may be modulated together and to the same degree. In a preferred embodiment, the modulation of the structural phase of the chalcogenide phase change materials may be achieved by thermal annealing.

[0026] In one embodiment, the modulation of the structural phase of the chalcogenide phase change materials may be achieved by thermal annealing, or laser-induced heating, or electric current-induced heating.

[0027] In one embodiment, the enhancement factor may correspond to an improved sensitivity of up to 2 orders.

[0028] In one embodiment, the plurality of excitation radiations may include light having a wavelength ranging from a visible region of between 400 nm and 780 nm to a near-infrared region between 780 nm and 1400 nm.

[0029] In one embodiment, the plurality of excitation radiations may include light having a wavelength in an infrared region of between 780 nm to 8000 nm.

[0030] In another aspect, there is provided a method of fabricating the SERS substrate as described herein, the method comprising: (a) forming the DBR by thin film deposition of each of the alternating layers of chalcogenide phase change materials and dielectric materials, and the end layer of chalcogenide phase change material; and (b) depositing the conductor layer on the DBR using thermal evaporation.BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0032] Fig. 1 shows an exemplary spectrum illustrating the concept of multiplex detection using SERRS.

[0033] Fig. 2A shows a schematic representation of the proposed multimode tunable Tamm cavity, according to an embodiment.

[0034] Fig. 2B shows a spectral reflectance curve denoting calculated three TPP modes with different spectral tunability for three modes, according to various embodiments.

[0035] Fig. 2C shows a spectral reflectance curve illustrating the multistate non-volatile tuning capability of the cavity with different degrees of crystallization for TPP1.

[0036] Fig. 2D shows a spectral reflectance curve illustrating the multistate non-volatile tuning capability of the cavity with different degrees of crystallization for TPP2.

[0037] Fig. 2E shows a spectral reflectance curve illustrating the multistate non-volatile tuning capability of the cavity with different degrees of crystallization for TPP3.

[0038] Fig. 3A shows a simulated cross-sectional intensity field distribution of the amorphous cavity at the resonance wavelength of 605 nm for TPP1 , according to an example.

[0039] Fig. 3B shows a simulated cross-sectional intensity field distribution of the amorphous cavity at the resonance wavelength of 730 nm for TPP2, according to an example.

[0040] Fig. 3C shows a simulated cross-sectional intensity field distribution of the amorphous cavity at the resonance wavelength of 1190 nm for TPP3, according to an example.

[0041] Fig. 4A shows an SEM image of the topology of a cavity surface, according to an example.

[0042] Fig. 4B shows a spectral reflectance curve (the measured normal incidence reflectance spectrum) of an annealed TPP cavity, showing two TPP modes.

[0043] Fig. 5A shows Raman spectra of R6G with 633 and 785 nm excitation wavelength.

[0044] Fig. 5B shows Raman spectra of Cy7 with 633 and 785 nm excitation wavelength.

[0045] Fig. 5C shows a Raman spectrum of a mixed solution of R6G and Cy7 for the excitation wavelength of 633 nm.

[0046] Fig. 5D shows a Raman spectrum of a mixed solution of R6G and Cy7 for the excitation wavelength of 785 nm.

[0047] Fig. 6 shows a spectral reflectance curve depicting the calculated reflectance spectra of tunable Tamm cavity in the infrared wavelength, according to an example.DETAILED DESCRIPTION OF THE INVENTION

[0048] In one aspect, there is provided a surface-enhanced Raman scattering (SERS) substrate for multiplex detection of different analytes, the SERS substrate including: a distributed Bragg reflector (DBR) comprising alternating layers of chalcogenide phase change materials and dielectric materials, terminated by an end layer of chalcogenide phase change material, wherein the end layer is thicker than each alternating layer of chalcogenide phase change materials; and a conductor layer disposed on the DBR, wherein the chalcogenide phase change materials have a refractive index higher than that of the dielectric materials, the DBR has a thickness dimensioned in a manner such that the DBR is operable to substantially simultaneously excite multiple Tamm modes at different Raman excitation wavelengths and to continuously tune the multiple Tamm modes within photonic bandgaps of the DBR to match a resonance wavelength of each Tamm mode to an absorption peak wavelength of each of the different analytes to achieve an enhancement factor, and the conductor layer provides a surface roughness for generating Raman signals with the enhancement factor.

[0049] The term “tunable” in the context of “tunable substrate” or “tunable thin film cavity” is meant to refer to the substrate / cavity being capable of exciting Tamm modes where the Tamm modes can be tuned by the DBR of the substrate / cavity. In particular, the phrase “tuning the Tamm mode” is meant to refer to tuning the resonance wavelength of the Tamm mode.

[0050] The thickness of DBR (i.e. each pair of alternating layers of chalcogenide phase change materials and dielectric materials) may be determined using the formula of nd=AI4, where d is the thickness of each layer, is the wavelength of light (optimized DBR reflectance wavelength) and n is the refractive index at the specific selected wavelength. In one embodiment, the thickness of the SbzSa termination layer may determine the multiple Tamm modes for Raman excitation wavelengths.

[0051] Advantageously, the thicker end layer (e.g. SbzSa termination layer) of the present disclosure reduces the number of alternating layers required for the DBR to achieve the 100% reflection in the photonic bandgap and to excite three Tamm modes at the conventional Raman laser wavelengths of 633 nm, 785 nm, and 1064 nm. Moreover, the surface roughness of the conducting layer (e.g. metal-coated sensing layer) further increases with thicker SbzSa film due to SbzSa being a soft material. It should be noted that high surface roughness is required for the SERS application because the cavity is not nanopatterned. By decreasing thenumber of layers required in the DBR, the cost of fabrication can be reduced. Furthermore, the substrate of the present disclosure may be optimized such that its multiple plasmon bands match with standard lasers (e.g. having wavelengths of 633 nm, 785 nm and 1064 nm) used for Raman spectroscopy, thereby offering easier integration for multiplex detection.

[0052] In a preferred embodiment, the chalcogenide phase change material may include a high refractive index material such as SbzSa. It should be appreciated that other lossless PCM such as SbzSea and GezSbzSe^ei may also be used. In contrast, the dielectric material may include a comparably low refractive index material such as SiOz.

[0053] In one embodiment, the chalcogenide phase change materials may include SbzSa.

[0054] In one embodiment, the chalcogenide phase change materials may include SbzSea or GezSbzSe^ei.

[0055] In one embodiment, the end layer may have a thickness ranging from 250 nm to 400 nm. In a preferred embodiment, the thickness of the end layer may be about 290 nm when the DBR is used to excite multiple Tamm modes at the Raman laser wavelengths of 633 nm, 785 nm and 1064 nm.

[0056] In one embodiment, each alternating layer of chalcogenide phase change materials may have a thickness ranging from 160 nm to 200 nm. In a preferred embodiment, each of the alternating layers of chalcogenide phase change materials may have a thickness of about 160 nm.

[0057] In one embodiment, the dielectric materials may be selected from the group consisting of SiO2, MgF2, CaF2, and BaF2.

[0058] In one embodiment, each alternating layer of the dielectric materials may have a thickness ranging from 70 nm to 120 nm. In a preferred embodiment, each of the alternating layers of the dielectric materials may have a thickness of about 80 nm.

[0059] In one embodiment, the DBR may include at least three pairs of alternating layers of chalcogenide phase change materials and dielectric materials.

[0060] In one embodiment, the conductor layer may include a gold film having a thickness ranging from 10 nm to 20 nm.

[0061] Minimizing the thickness of the conductor layer (such as to between 10 nm and 20 nm) allows higher surface roughness to be obtained, which may be desirable when used in a SERS or SERRS method. When used in a SEIRA method, the thickness of the conductor layer may be increased to allow sharper resonances. The thickness of the conductor layer may be designed in a manner to optimize surface roughness and resonance. In various embodiments, the conductor layer may be disposed on the DBR in a manner to allow the conductor layer to be in contact with one of the layers of chalcogenide phase change materials(e.g. SbzSa) as described herein. In one embodiment, a first conductor layer may be in contact with the end layer of SbzSa. Where a second conductor layer is present, the second conductor layer may be in between the bottom SbzSa layer of the DBR and the underlying substrate. For example, when using the SERS substrate for SERS application, having the first conductor layer overlying the end layer may suffice. Preferably, when using the SERS substrate for SERS and SEIRA simultaneously, a first thinner conductive layer (e.g. 10 nm) may be deposited overlying the end layer and a second thicker conductive layer (e.g. 20 nm) may be deposited at the bottom of the DBR (opposite to the end layer and above the underlying substrate). Placing the second conductive layer at the bottom of the DBR may allow multiple Tamm modes to be excited in the mid-infrared wavelengths for SEIRA.

[0062] In one embodiment, the conductor layer may include a first conductor layer disposed on the DBR overlying the end layer, the first conductor layer arranged to be in contact with a sample including the different analytes.

[0063] In one embodiment, the SERS substrate as described herein may further include a dielectric film disposed in between the DBR and the first conductor layer.

[0064] In one embodiment, the dielectric film may include SiOz.

[0065] Optionally, for SERS measurement, a dielectric film may be disposed in between the SbzSa end layer and the first conductor layer to avoid damage of SbzSa due to laser heating because SbzSa is a soft material. In one example, the dielectric film may include an ultrathin (<5 nm) SiOz layer. The thickness of this dielectric film is negligible when compared to the thicker layers of the DBR. It should be appreciated that other dielectric materials such as Si3N4, TiOz, BTO or AI2O3 may be used for the dielectric film. In a different embodiment, the dielectric film may not be required for SEIRA because SEIRA measurement is typically performed using a FTIR system or broadband infrared source (i.e. not using laser). In another embodiment, the dielectric film may not be required for SERS if a low power laser is used.

[0066] In one embodiment, the SERS substrate as described herein may further include an underlying substrate arranged adjacent to the DBR and opposite to the first conductor layer, wherein the DBR is disposed between the first conductor layer and the underlying substrate. In other words, the SERS substrate may have a stacked structure of the underlying substrate followed by the alternating layers of the DBR, the end layer, and the (first) conductor layer (in this order), all being substantially in parallel to the underlying substrate.

[0067] In one embodiment, the underlying substrate may be selected from the group consisting of quartz, silicone, glass, semiconductor and polymers. In a preferred embodiment, the underlying substrate may include quartz or silicone. It would generally be appreciated that any substrate that is not damaged by annealing (up to 280°C) may be used. In addition toquartz and silicone, such substrates may include glass and semiconductor substrates, as well as flexible polymers such as polyamide (kapton).

[0068] In one embodiment, the conductor layer may further include a second conductor layer disposed on the DBR opposite to the end layer.

[0069] In another aspect, there is provided a biosensor system for multiplex detection of different analytes, the biosensor system including: a SERS substrate as described herein configured to receive the different analytes; and an excitation source configured to emit a plurality of excitation radiations of different wavelengths onto the different analytes and the SERS substrate, wherein the SERS substrate is configured to generate the Raman signals of each of the different analytes with the enhancement factor.

[0070] In the biosensor system for multiplex detection of different analytes, the SERS substrate that is configured to receive the different analytes may be a functionalised SERS substrate. An example of a functionalised SERS substrate may be a SERS substrate that is modified with functional groups such as specific chemical groups or molecules, thereby allowing it to selectively bind and enhance the Raman signal of a target analyte(s). The excitation source may be configured to emit excitation radiation of different wavelengths onto the different analytes and the functionalised SERS substrate. The multiple excitation wavelengths (i.e. the excitation radiation of different wavelengths) used for detecting the multiple analytes allow individual biomolecules to be distinguished accurately, and allows for multiplex detection to be performed without labelling.

[0071] In one embodiment, the biosensor system as described herein may further include a spectrometer configured to receive the Raman signals with the enhancement factor.

[0072] In another aspect, there is provided a method for performing multiplex detection of different analytes, the method including: (a) providing a SERS substrate as described herein; (b) contacting a sample comprising the different analytes with the conductor layer of the SERS substrate, wherein the conductor layer is disposed on the DBR overlying the end layer; (c) applying a plurality of excitation radiations to the sample and the SERS substrate to substantially simultaneously excite multiple Tamm modes, wherein each excitation radiation has a predetermined excitation wavelength that is close to or equal to a resonance wavelength of each of the multiple Tamm modes; and (d) generating Raman signals for multiplex detection.

[0073] In one embodiment, for the plurality of excitation radiations, each of the predetermined excitation wavelengths may be close to or equal to an absorption peak wavelength of each of the different analytes, and wherein generating the Raman signals forthe multiplex detection may include generating the Raman signals with an enhancement factor.

[0074] In one embodiment, applying the plurality of excitation radiations may cause continuous tuning of the DBR of the SERS substrate by modulating a structural phase of the chalcogenide phase change materials of the DBR to match the resonance wavelength of each of the multiple Tamm modes to the absorption peak wavelength of each of the different analytes. By doing so, high-quality factor multiple Tamm modes are excited without (i.e. in the absence of) any prism or grating coupling methods.

[0075] In one embodiment, the modulation of the structural phase of the chalcogenide phase change materials may be achieved by thermal annealing, or laser-induced heating, or electric current-induced heating.

[0076] In one embodiment, the modulation of the structural phase of the chalcogenide phase change materials may be achieved by thermal annealing, or laser-induced heating, or electric current-induced heating.

[0077] In one embodiment, the enhancement factor may correspond to an improved sensitivity of up to 2 orders.

[0078] In one embodiment, the plurality of excitation radiations may include light having a wavelength ranging from a visible region of between 400 nm and 780 nm to a near-infrared region between 780 nm and 1400 nm.

[0079] In one embodiment, the plurality of excitation radiations may include light having a wavelength in an infrared region of between 780 nm to 8000 nm.

[0080] In another aspect, there is provided a method of fabricating the SERS substrate as described herein, the method comprising: (a) forming the DBR by thin film deposition of each of the alternating layers of chalcogenide phase change materials and dielectric materials, and the end layer of chalcogenide phase change material; and (b) depositing the conductor layer on the DBR using thermal evaporation.

[0081] In one embodiment, a cost-effective scalable optical biosensor platform with high sensitivity, high accuracy, and high reproducibility may be provided for multiplexed POCT applications. Advantageously, the sensor platform as described herein may achieve simultaneous detection of biomarkers at high accuracy and reproducibility. These biomarkers may be interleukins (IL) IL-12, IL-17, and IL-23 for inflammatory skin conditions and cTnl, cTnT, and cTnC for cardiovascular diseases.

[0082] Various embodiments of the invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including","containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0083] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0084] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0085] EXAMPLES

[0086] Non-limiting examples of the invention and comparative examples will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

[0087] Example 1: Multiplex detection using SERRS

[0088] The technology as described herein relates to a surface-enhanced resonance Raman spectroscopy (SERRS)-based scalable optical sensing platform for multiplex detection. Even though the readout signal of multiplexed bioassays based on fluorescence and SERS has been widely investigated in basic biomedical research, the proposed approach is entirely different and promising for clinical applications. Existing SERS-based sensors require intense clean-room-based nanofabrication steps, which limit scalability for practically realizing these sensors for real-time point-of-care (POC) applications. In the present disclosure, a cost-effective scalable optical biosensor platform with high sensitivity, high accuracy, and high reproducibility for multiplexed POC is proposed. In particular, a lithography-free multilayered tunable thin film cavity was developed to realize surface-enhanced resonance Raman spectroscopy (SERRS) with improved sensitivity by 1-2 orders. Compared to existing SERS-based multiplexing systems, the substrate of the presentembodiments excites multiple plasmonic modes with tunable spectral features for multiplex detection. By using the nonvolatile phase change properties of the tunable film in the cavity, the plasmonic resonance may be exactly matched with the absorption peaks of various biomolecules and the Raman laser wavelengths (Fig.1). As a result, enhanced sensitivity is possible through SERRS effect with and without Raman reporter molecules. The conventional Raman reader with different excitation wavelengths is only required for the multiplexed detection. Advantageously, only a thin film deposition technique is required for the proposed sensor platform, thus wafer-scale fabrication at low cost is possible. The sensor of the present embodiments offers a highly sensitive and specific diagnostic tool that detects multiple biomarkers simultaneously and potentially translatable as an early and accurate disease diagnostic platform. In addition, the tunable thin film cavity of the present embodiments may be simultaneously used for SERS and surface-enhanced infrared absorption (SEIRA) spectroscopy because plasmonic modes may be excited from visible to infrared wavelengths.

[0089] Example 2: Tunable Tamm cavity-based sensor platform

[0090] The proposed tunable Tamm cavity-based sensor platform is shown in Fig. 2A, including a thin gold film-coated distributed Bragg reflector (DBR) made up of SbzSa phase change material (PCM) and SiOz. In the Tamm cavity, the DBR provides the required optical phase matching to excite the plasmonic resonance (Tamm plasmon polariton, TPP) within the photonic bandgap of DBR. It means that a planar thin film structure excites high-quality factor mode from normal incidence to a wide-angle range for both p-and s-polarizations of incident light. Since multiple photonic bandgaps may be designed for DBR, multiple TPPs may be excited. In order to tune the TPP resonance wavelength, a lossless (>550 nm) chalcogenide PCM such as SbzSa is used as the high refractive index dielectric layer of the DBR. PCMs offer non-volatile means of reversible tunability for a wide spectral band from ultraviolet to terahertz by switching the structural phase of PCM from amorphous to crystalline. In particular, the complex refractive index of PCM drastically changes with phase change. To generate Raman signal from the planar film, the surface roughness of the film is made to be significantly high by depositing a thin layer of Au on SbzSa termination layer. It is noted that the conventional SERS substrate mainly excites one plasmonic mode. In the present disclosure, by controlling the thickness of the DBR layers, multiple Tamm modes may be excited within the photonic bandgaps of the DBR. Interestingly, these Tamm modes may be continuously tuned by switching the structural phase of SbzSa layers from amorphous (Amp) (201 ) to crystalline (Cry) (203). As shown in Fig. 2B, three Tamm modes are excited, and the tunable spectral bandwidth (AA) of mode increases with increasing wavelength. The tunable spectral bandwidths of modes TPP1 , TPP2 and TPP3 are AAi = 115 nm, AA2 = 160 nm and AA3 = 235nm, respectively. Since the crystallization temperature of SbzSa is about 280°C, the degree crystallinity (a mixture of amorphous and crystalline molecules with different proportions) of SbzSa (e.g. amorphous 201, 20% crystalline 205, 40% crystalline 207, 60% crystalline 209, 80% crystalline 211, (100%) crystalline 203 as denoted in Figs. 2C, 2D and 2E) may be obtained by annealing it from 180°C to 280°C. As a result, each mode may be continuously tuned in the forward direction, which is shown in Figs. 2C to 2E. In the calculations of the present disclosure, experimentally obtained dielectric constants of SbzSa in both amorphous and crystalline phases are used and the effective dielectric constants of the partially crystallized SbzSa are obtained by an effective-medium theory using the Lorentz-Lorenz relation,are the dielectric constants of SbzSa in the intermediate, amorphous, and crystalline phases, respectively. The variable “p” ranges between 0 (for amorphous SbzSa) and 1 (for crystalline SbzSa), symbolizing the degree of crystallization.

[0091] Example 3: Intensity field distribution at resonance wavelength of three Tamm modes

[0092] The intensity field distribution of the amorphous cavity at resonance wavelength of three modes (605 nm, 730 nm and 1190 nm for TPP1, TPP2 and TPP3, respectively) was calculated. As shown in Figs. 3A to 3C, the intensity is tightly confined at the gold / distributed Bragg reflector (Au / DBR) interface and decays in both Au and DBR for three modes. This field confinement is important for SERS enhancement and hence for multiplex detection. These continuously tunable and high-quality factor Tamm modes may be utilized to realize multiplex detection, or these modes may be used as a probe for each biomolecule detection. To realize this, Tamm resonance may be excited at different Raman excitation wavelengths (three wavelengths for 3 analytes detection) and the resonance wavelength may be precisely tuned around these wavelengths to match the absorption peak wavelength of different biomolecules. That means that a tunable plasmonic platform is essential to realize an efficient SERRS effect. In particular, the multiplexing ability of the proposed sensor is based on the sensing capability of different Tamm resonances to the corresponding absorption peak wavelength of biomolecules. The detection may be performed in the presence and absence of the labels. Any analyte that may be conjugated to the Raman reporter molecule may be detected in labeled detection. This method may also be used to directly detect analytes (without label) due to the enhanced signal enhancement of the resonant SERS effect. In this case, analytes with absorption peaks lying within the Raman laser excitation wavelengths may be detected.

[0093] Example 4: Tamm cavity fabricated using thin film deposition technique

[0094] The designed Tamm cavity (Fig. 2A) was fabricated using the thin film deposition technique. Initially, the DBR structure with alternating thin layers of SbzSa and SiOz was deposited using RF magnetron sputtering. A thermal evaporator was used to deposit a thin Au layer of thickness 10 nm on top of the DBR to make it a Tamm cavity. An ultra-thin (5 nm) SiOz layer was also deposited between Au and SbzSa layers to avoid the possible laser-induced damage of SbzSa. Fig. 4A shows a SEM image of the annealed cavity surface. It is visible that the surface is significantly rough, and it is sufficient to generate detectable Raman signals. The normal incidence reflectance is measured using a microspectrometer (as shown in Fig. 4B). Since it is planned for the detection of two analytes simultaneously using 633 nm and 785 nm Raman laser wavelengths, the Tamm cavity is annealed at 210°C to obtain the TPP1 and TPP2 resonance around 633 nm and 785 nm, respectively (see Fig. 4B). It should be appreciated that the fabricated Tamm cavity excites TPP modes in the longer wavelengths (but not shown in the figures).

[0095] Example 5: Simultaneous detection of two analytes using the SERRS effect

[0096] To demonstrate the simultaneous detection of two analytes using the SERRS effect, Rhodamine 6G (R6G) and Cyanin 7 (Cy7) dye were selected as the two model molecules. It should be noted that the absorption peak wavelengths of R6G and Cy7 are around 633 nm and 785 nm, respectively. An equal concentration of both model molecules (100pM for R6G and 1 mg / ml for Cy7) was prepared in distilled water. The samples were dipped in dye solutions for a few minutes and then performed the Raman measurements. A Renishaw InVia Raman upright microscope equipped with 633 nm and 785 nm lasers was used for Raman measurement. This instrument was coupled with a Leica microscope, and the laser light was focused onto the sample using an objective lens with a 50x magnification and 0.5 N.A. The scattered Raman signal was collected through the same lens, and prominent Rayleigh scattering was blocked using a notch filter. The beam spot size on the sample was carefully controlled to be approximately 2 pm to ensure high spatial resolution. To ensure statistical significance, over 10 different areas were analyzed on each sample, and 10 spectra were acquired from each area. To acquire each spectrum, the laser was focused on the sample for 10 seconds, and the Raman signal was integrated over the range of 600 cm-1to 1800 cm-1. To analyze the acquired spectra, WiRE™ v3.4 software was employed. The fluorescence background was subtracted using cubic spline interpolation, and the instrument was calibrated at 520 cm-1using standard silicon. The laser power (1.2 mM) is kept constant for 633 nm and 785 nm excitation. The results obtained using R6G and Cy7 are shown in Fig. 5A and Fig. 5B, respectively. As can be seen, a significant signal enhancement is obtained for R6G and Cy7using 633 nm (503) and 785 nm (501) excitation wavelengths, respectively. It confirms the SERRS effect by matching the absorption peak of R6G and Cy7 with 633 nm and 785 nm modes, respectively. A mixed solution of R6G and Cy7 with equal concentrations of both model molecules was then used. It is important to note that 633 nm excitation selectively enhances the R6G Raman peaks (Fig. 5C) and 785 nm excitation selectively enhances the Cy7 Raman peaks (Fig. 5D) from the mixed solution. It shows that selective simultaneous detection of R6G and Cy7 is possible using two excitation wavelengths.

[0097] In short, this approach may be used to detect at least three biomarkers simultaneously by matching the absorption peak wavelength of molecules with the Tamm resonances and using the matching wavelength as the excitation laser. Moreover, this tunable substrate may be simultaneously used for SERS and surface-enhanced infrared absorption (SEIRA) spectroscopy because it also excites tunable Tamm modes in the infrared wavelength. Fig. 6 shows a spectral reflectance curve depicting the calculated reflectance spectra of tunable Tamm cavity in the infrared wavelength, according to an example. In Fig.6, the structural phase of the chalcogenide phase change material layers amorphous switches from amorphous (Amp) (601) and crystalline (Cry) (603), and the tunable spectral bandwidths of modes are AAi, AA2, AA3, and AA4. The ability of the Tamm cavity to excite tunable narrowband modes from visible to infrared wavelengths may be used to develop a scalable dual function sensor for both SERS and SEIRA applications.

[0098] Advantageously, the SERS substrate of the present disclosure with multiple tunable plasmon resonances to match with laser excitation / absorption of analyte may achieve surface-enhanced resonance Raman scattering (SERRS) up to 2 orders of higher sensitivity. Various embodiments provide a scalable, cost-effective and mass-production capable lithography-free nanopatterning approach. Furthermore, the ability to tune the plasmon peaks that match with the absorption of analyte allows for label-free and labeled SERRS detection. The SERS-based sensing chip of the present disclosure may be used for the simultaneous detection of multiple biomarkers. Furthermore, the scalable SERS substrate of the present disclosure may be made up of multilayer thin films using a physical vapor deposition technique only. A non-volatile tunable spectral response may be achieved by using chalcogenide PCM as one of the layers of DBR. There is also the capability to excite high-quality factor multiple plasmonic modes without any prism or grating coupling methods. The thin film structure itself generates the SERS signal without any nanostructuring. In addition, the sensing chip may be further extended to have 2-in-1 sensing functions simultaneously for SERS and SEIRA by exciting plasmonic modes from visible to infrared wavelengths.

[0099] While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS1. A surface-enhanced Raman scattering (SERS) substrate for multiplex detection of different analytes, the SERS substrate comprising:a distributed Bragg reflector (DBR) comprising alternating layers of chalcogenide phase change materials and dielectric materials, terminated by an end layer of chalcogenide phase change material, wherein the end layer is thicker than each alternating layer of chalcogenide phase change materials; anda conductor layer disposed on the DBR,wherein the chalcogenide phase change materials have a refractive index higher than that of the dielectric materials,the DBR has a thickness dimensioned in a manner such that the DBR is operable to substantially simultaneously excite multiple Tamm modes at different Raman excitation wavelengths and to continuously tune the multiple Tamm modes within photonic bandgaps of the DBR to match a resonance wavelength of each Tamm mode to an absorption peak wavelength of each of the different analytes to achieve an enhancement factor, andthe conductor layer provides a surface roughness for generating Raman signals with the enhancement factor.

2. The SERS substrate according to claim 1, wherein the chalcogenide phase change materials comprise SbzSa.

3. The SERS substrate according to claim 1, wherein the chalcogenide phase change materials comprise SbzSea or Ge2Sb2Se4Tei.

4. The SERS substrate according to any one of claims 1 to 3, wherein the end layer has a thickness ranging from 250 nm to 400 nm.

5. The SERS substrate according to any one of claims 1 to 4, wherein each alternating layer of chalcogenide phase change materials has a thickness ranging from 160 nm to 200 nm.

6. The SERS substrate according to any one of claims 1 to 5, wherein the dielectric materials are selected from the group consisting of SiOz, MgFz, CaFz, and BaFz.

7. The SERS substrate according to claim 6, wherein each alternating layer of the dielectric materials has a thickness ranging from 70 nm to 120 nm.

8. The SERS substrate according to any one of claims 1 to 7, wherein the DBR comprises at least three pairs of alternating layers of chalcogenide phase change materials and dielectric materials.

9. The SERS substrate according to any one of claims 1 to 8, wherein the conductor layer comprises a gold film having a thickness ranging from 10 nm to 20 nm.

10. The SERS substrate according to any one of claims 1 to 9, wherein the conductor layer comprises a first conductor layer disposed on the DBR overlying the end layer, the first conductor layer arranged to be in contact with a sample comprising the different analytes.

11. The SERS substrate according to claim 10, further comprising a dielectric film disposed in between the DBR and the first conductor layer.

12. The SERS substrate according to claim 11 , wherein the dielectric film comprises SiOz.

13. The SERS substrate according to any one of claims 10 to 12, further comprising an underlying substrate arranged adjacent to the DBR and opposite to the first conductor layer, wherein the DBR is disposed between the first conductor layer and the underlying substrate.

14. The SERS substrate according to claim 13, wherein the underlying substrate is selected from the group consisting of quartz, silicone, glass, semiconductor and polymers.

15. The SERS substrate according to any one of claims 10 to 14, wherein the conductor layer further comprises a second conductor layer disposed on the DBR opposite to the end layer.

16. A biosensor system for multiplex detection of different analytes, the biosensor system comprising:a SERS substrate according to any one of claims 1 to 15 configured to receive the different analytes; andan excitation source configured to emit a plurality of excitation radiations of different wavelengths onto the different analytes and the SERS substrate, wherein the SERS substrate is configured to generate the Raman signals of each of the different analytes with the enhancement factor.

17. The biosensor system according to claim 16, further comprising a spectrometer configured to receive the Raman signals with the enhancement factor.

18. A method for performing multiplex detection of different analytes, the method comprising:(a) providing a SERS substrate according to any one of claims 1 to 15;(b) contacting a sample comprising the different analytes with the conductor layer of the SERS substrate, wherein the conductor layer is disposed on the DBR overlying the end layer;(c) applying a plurality of excitation radiations to the sample and the SERS substrate to substantially simultaneously excite multiple Tamm modes, wherein each excitation radiation has a predetermined excitation wavelength that is close to or equal to a resonance wavelength of each of the multiple Tamm modes; and(d) generating Raman signals for multiplex detection.

19. The method according to claim 18, wherein for the plurality of excitation radiations, each of the predetermined excitation wavelengths is close to or equal to an absorption peak wavelength of each of the different analytes, and wherein generating the Raman signals for the multiplex detection comprises generating the Raman signals with an enhancement factor.

20. The method according to claim 19, wherein applying the plurality of excitation radiations causes continuous tuning of the DBR of the SERS substrate by modulating a structural phase of the chalcogenide phase change materials of the DBR to match the resonance wavelength of each of the multiple Tamm modes to the absorption peak wavelength of each of the different analytes.

21. The method according to claim 20, wherein the modulation of the structural phase of the chalcogenide phase change materials is achieved by thermal annealing, or laser-induced heating, or electric current-induced heating.

22. The method according to claim 21, wherein the thermal annealing is carried out at a temperature ranging between 180°C and 280°C.

23. The method according to any one of claims 18 to 22, wherein the enhancement factor corresponds to an improved sensitivity of up to 2 orders.

24. The method according to any one of claims 18 to 23, wherein the plurality of excitation radiations comprises light having a wavelength ranging from a visible region of between 400 nm and 780 nm to a near-infrared region between 780 nm and 1400 nm.

25. The method according to any one of claims 18 to 23, wherein the plurality of excitation radiations comprises light having a wavelength in an infrared region of between 780 nm to 8000 nm.

26. A method of fabricating the SERS substrate according to one of claims 1 to 15, the method comprising:(a) forming the DBR by thin film deposition of each of the alternating layers of chalcogenide phase change materials and dielectric materials, and the end layer of chalcogenide phase change material; and(b) depositing the conductor layer on the DBR using thermal evaporation.

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