WO2019182997A1 - Thin film-based dynamic structural color system for sensing, camouflage, and adaptive optics - Google Patents

Abstract

A thin-film interferometer having a featureless hydrogel layer covalently bonded to a substrate is provided as a simple and universal adaptive color platform. The thin hydrogel exhibits at least one color arising from interference of light waves. External stimuli causes a thickness change in the hydrogel layer, and the thickness change then produces a color change in the hydrogel. Colorimetric analyses may be performed by computer-based or smartphone-based programs.

Description

THIN FILM-BASED DYNAMIC STRUCTURAL COLOR SYSTEM FOR SENSING,

CAMOUFLAGE, AND ADAPTIVE OPTICS

FIELD OF THE INVENTION

[0001] The present invention relates to ultra-sensitive sensors with small-scale and fast-response through simple design and low-cost fabrication, and more particularly to a hydrogel-based interferometer sensor system.

BACKGROUND OF THE INVENTION

[0002] Many animals can change their colors for acclimatization, camouflage, and communication, by tuning the volume or thickness of the soft layer of cells or proteins in response to environmental cues. Applying soft materials to artificial adaptive color systems could promote the development of various areas such as sensing, displays, optical filters, anti-counterfeiting, and stealth technologies.

[0003] Hydrogels are a class of soft materials that are capable of changing their volume by several folds in response to certain conditions. They have been used as stimuli-responsive components in structural color systems commonly in the form of photonic crystals. However, photonic crystals rely on highly ordered structures and periodically arranged refractive indices, requiring careful co-assembling of multiple materials and micro/nano building blocks to produce uniform color in large scale, limiting their rapid and convenient fabrication. The multilayer structures also prolong the time needed for swelling and hinder real-time monitoring applications, further demonstrating the need for improved system designs.

SUMMARY OF THE DISCLOSURE

[0004] Many embodiments of the present disclosure relate to ultra-sensitive sensors with small-scale and fast-response through simple design and low-cost fabrication.

[0005] Various such embodiments are directed to thin-film interferometers including:

• a reflective substrate; • a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the reflective substrate, wherein the thin-film layer has at least one defined thickness, wherein the boundary between the reflective substrate and the thin-film layer defines a thin-film/substrate interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

• wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin-film interface and the thin-film layer/substrate interface;

• wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer; and

• wherein the change in the thickness of the thin-film layer produces a change in the color in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the reflective substrate and the thickness of the thin-film layer.

[0006] In various other embodiments, the highly porous cross-linked polymer network is formed of a hydrogel.

[0007] In still various other embodiments, the hydrogel forms a scaffold onto which is bonded a plurality of ligands specific to a target analyte. In some such embodiments, the mass ratio of ligand to hydrogel polymer is 1 to 5. In some such embodiments, the binding of the target analyte with the ligand concentrates the analyte 10⁹ fold.

[0008] In yet various other embodiments, the thin-film layer has a thickness of from 0.1 to 1 pm.

[0009] In still yet various other embodiments, the thin-film layer has a thickness comparable to the wavelength of visible light.

[0010] In still yet various other embodiments, the volume change of the think-film layer on exposure to the external stimulus is at least 100 fold.

[0011] In still yet various other embodiments, the stimulus is selected from the group consisting of humidity, temperature, light, mechanical stress, magnetic or electrical field, and specific chemical and biological molecules, including pH, metal ions, and anions, in both gaseous and liquidus forms.

[0012] In still yet various other embodiments, the thin-film layer is formed by mixing a plurality of at least one material selected form the group of pre-polymers, monomers, and functional molecules.

[0013] In still yet various other embodiments, the reflective substrate is formed of a Si wafer.

[0014] In still yet various other embodiments, the thin-film layer is homogeneous.

[0015] In still yet various other embodiments, the thin-film interferometer further comprises a colorimetric analyzer. In some such embodiments, the colorimetric analyzer determines the concentration of a target analyte by reference to a correlation between a measured reflection spectra, the thin-film thickness and analyte concentration. In some such embodiments the colorimetric analyzer is configured to compare a sample image from a thin-film interferometer against a database of comparable interferometer images to determine analyte concentration. In some such embodiments the analyzer outputs a four-dimensional matrix image wherein each dimension consists of data for each color channel and data on an image transparency value.

[0016] In still yet various other embodiments, the reflective substrate is formed of a flexible material.

[0017] In still yet various other embodiments, the sensor further includes a layer of reflective material disposed atop the thin-film layer. In some such embodiments the reflective material is Au and has a nanometer-scale thickness.

[0018] In still yet various other embodiments, the thin-film layer is formed of a hydrogel, wherein the ligand is an imidazole, and wherein the analyte is Cu ions. In some such embodiments the hydrogel is formed of a poly(acrylamide-co-acrylic acid -co-N- allylacrylamide) (poly(AAm-co-AAc-co-AAene)). In some such embodiments the sensor further includes a nanometer-scale thick Au layer disposed at the thin-film layer/substrate interface.

[0019] In still yet various other embodiments, the volume of analyte is less than 10 pL.

[0020] In still yet various other embodiments, the time scale of diffusion is 0.01 s. [0021] In still yet various other embodiments, the sensor has a picomole sensitivity to the analyte.

[0022] In still yet various other embodiments, the analyte is a glycoprotein, and wherein the sensor has a sensitivity between 1.0 ^c10 ¹⁰ mg/mL to 1.0 ^c10⁶ mg/mL. In some such embodiments the ligand is a phenylboronic acid.

[0023] In still yet various other embodiments, the sensor comprises an array of separate interferometer elements. In some such embodiments the separate interferometer elements are configured to detect different target analytes. In some such embodiments the target analytes are selected from the group of metal ions, volatile organic compounds and biological compounds.

[0024] Many embodiments are directed to transmissive thin-film interferometers including:

• a substrate assembly comprising a first transparent substrate and a reflective thin film, where the reflective thin film has a thickness configured to preserve the transparency of the substrate assembly and has a refractive index higher than 1 ;

• a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the substrate assembly, wherein the thin-film layer has at least one defined thickness, wherein the boundary between the substrate assembly and the thin-film layer defines a thin-film layer/substrate assembly interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

• wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin-film interface and the thin-film/substrate assembly interface

• wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer;

• wherein the change in the thickness of the thin-film layer produces a change in the color in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the substrate assembly and the thickness of the thin-film layer; and

• wherein the sensor is configured such that incoming light may be directed from one of either the thin-film or substrate sides of the sensor, and wherein the detection may occur from one of either the thin-film or substrate sides of the sensor.

[0025] In many other embodiments, the reflective thin-film is disposed between one of either the substrate and the thin-film layer, or on the surface of the substrate opposite the thin-film layer.

[0026] In still many other embodiments, sensors further include a second reflective thin-film disposed atop the thin-film layer.

[0027] In yet many other embodiments, the highly porous cross-linked polymer network is formed of a hydrogel.

[0028] In still yet many other embodiments, the hydrogel forms a scaffold onto which is bonded a plurality of ligands specific to a target analyte.

[0029] In still yet many other embodiments, the first transparent substrate is formed from glass or plastic.

[0030] In still yet many other embodiments, the first transparent substrate is formed from a flexible material selected from PET or PDMS.

[0031] Several embodiments are directed to encryption devices including:

• a substrate;

• a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the reflective substrate, wherein the thin-film layer has at least two defined thicknesses, wherein the boundary between the reflective substrate and the thin-film layer defines a thin-film/substrate interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

• wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin-film interface and the thin-film layer/substrate interface; • wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer; and

• wherein the change in the thickness of the thin-film layer produces a differential change in the color of portions of the sensor in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the reflective substrate and the thicknesses of the thin-film layer such that an encryption key is reversibly displayed.

[0032] Several embodiments are directed to methods of forming a thin-film interferometer including:

• providing a substrate;

• spin-coating a highly porous cross-linked polymer network precursor in a layer having a fixed thickness atop the substrate;

• polymerizing the highly porous cross-linked polymer network precursor in- situ atop the substrate such that it covalently binds thereto.

[0033] In several other embodiments, the highly porous cross-linked polymer network is a hydrogel.

[0034] In still several other embodiments, the methods further include patterning the highly porous cross-linked polymer network atop the substrate by spatially controlled polymerization to form areas of the highly porous cross-linked polymer network having different thicknesses.

[0035] In yet several other embodiments, the methods further include disposing a nano-scale reflective layer in one or both atop the highly porous cross-linked polymer network and between the highly porous cross-linked polymer network and the substrate.

[0036] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure. SUMMARY OF THE FIGURES

[0037] The summary of the invention will be more fully understood with reference to the included figures. The included figures represent exemplary embodiments of the invention and should construed as a complete recitation of the scope of the invention, wherein:

[0038] FIG. 1 illustrates a sensing mechanism of the hydrogel interferometer platform, including the complete chemical-mechanical-optical signal transduction process in accordance with embodiments.

[0039] FIGs. 2a to 2d illustrate: (2a) the bioinspired soft hydrogel layer based adaptive color platform according to embodiments of the invention; (2b) a schematic of interference of two reflected light waves from the air-hydrogel and hydrogel-substrate interfaces according to exemplary embodiments; (2c) a color palette of hydrogel films with tunable thickness; and (2d) corresponding reflection spectra at an incident angle of 0° according to exemplary embodiments.

[0040] FIG. 3 illustrates a schematic of a patterning and smartphone-based analysis process that may be utilized with a hydrogel interferometer in accordance with exemplary embodiments.

[0041] FIGs. 4a and 4b illustrate two transmission mode designs in accordance with exemplary embodiments.

[0042] FIG. 5 illustrates a configuration of a sensor on transparent substrates in accordance with exemplary embodiments.

[0043] FIGS. 6a to 6d illustrate reflective spectra of a hydrogel interferometer on a glass substrate to sense Cu²⁺ ions from different detection directions in accordance with exemplary embodiments.

[0044] FIG. 7 illustrates a method for manufacturing sensor platforms in accordance with exemplary embodiments.

[0045] FIG. 8a illustrates a synthetic route for poly(AAm-co-AAc) in accordance with exemplary embodiments. [0046] FIG. 8b illustrates a synthetic route for poly(AAm-co-AAc-co-AAene) in accordance with exemplary embodiments.

[0047] FIG. 8c illustrates a ¹³C NMR spectrum of poly(AAm-co-AAc) in accordance with exemplary embodiments.

[0048] FIG. 8d illustrates a ¹FI NMR spectrum of poly(AAm-co-AAc-co-AAene) in accordance with exemplary embodiments.

[0049] FIG. 9 illustrates a fabrication process of a Cu²⁺ ions sensor in accordance with exemplary embodiments.

[0050] FIGs. 10a and 10b illustrate configurations of hydrogel interferometers: (10a) without Au layers; and (10b) with Au layers when sensing liquidus analytes in aqueous solution in accordance with exemplary embodiments.

[0051] FIG. 1 1 illustrates a calculated reflective spectrum with different film thickness, where the parameters are set as: from 400 nm to 800 nm, the medium is air, the layer is a material with Rl 1 .4, and the substrate is Si in accordance with exemplary embodiments.

[0052] FIG. 12 illustrates FTIR spectra for hydrogels with and without imidazole ligands (where the insets are larger scale from 1500 cm ¹ to 1300 cm^-1) in accordance with exemplary embodiments.

[0053] FIG. 13 illustrates XPS data for hydrogels with and without imidazole ligands in accordance with exemplary embodiments.

[0054] FIG. 14 illustrates reflection peak shifts of the Cu²⁺ sensor at different wavelengths with the Cu²⁺ ions concentrations from 10.0 fM to 10.0 pM (where the dash line represents the resolution of the spectrometer) in accordance with exemplary embodiments.

[0055] FIG. 15 illustrates reflection peak shifts of the sensor without the ligands inside at different wavelengths with the Cu²⁺ ions concentrations from 10.0 pM to 1.0 mM (where the dash line represents the resolution of the spectrometer) in accordance with exemplary embodiments.

[0056] FIG. 16 illustrates reflection peak shifts of the sensor in different pH solutions in accordance with exemplary embodiments. [0057] FIG. 17 illustrates reflection peak shifts of the sensor with the NaCI concentrations from 1 .0 pM to 1 .0 mM in accordance with exemplary embodiments.

[0058] FIGs. 18a and 18b illustrate SEM images of hydrogel before (a) and after (b) adding Cu2+ ions (where the scale bar is 1 pm).

[0059] FIGs. 19a to 19d illustrate a Cu²⁺ sensor in accordance with exemplary embodiments, where: (19a) illustrates a reflective spectra of the Cu²⁺ sensor with and without the sputtered gold film on the surface; (19b) illustrates the complete reflective spectra of the sensor with different concentrations of Cu²⁺ (where the curves for blank solution and 10.0 fM overlap due to the small peak shift); (19c) illustrates the reflective peak shift and the swelling ratio measured as a function of the concentration of Cu²⁺ at the wavelength of 639.8 nm; and (19d) illustrates reflective peak shifts at 458.7 nm induced by Cu²⁺ of 10 ¹¹ M and 14 different metal ions of 10^-9 M, as well as a mixture of them (where the dash lines represent the resolution of the spectrometer (1 .5 nm), and all error bars indicate the standard deviation of three parallel experiments).

[0060] FIGs. 20a to 20d illustrate the localized binding of Cu²⁺ in a sensor in accordance with exemplary embodiments, where: (20a) illustrates the experimental setup for verifying the localized binding (where the distance between Spot 1 and Spot 2 is about 5.0 mm); (21 b & 21 c) illustrate the reflective spectra before and after applying 10 pL of Cu²⁺ with 10 ¹¹ M is recorded as in (21 b) spot 1 and (21 c) spot 2; and (21 d) illustrates the estimated diameter of the effective area of binding and the local concentration of Cu²⁺ within the effective area.

[0061] FIGs. 21 a to 21 d illustrate the generality of a sensing platform in accordance with exemplary embodiments, where: (21 a) illustrates the detection mechanism of the glycoprotein-specific sensor; (21 b) illustrates the reflective peak shift at the wavelength of 601 .0 nm as a function of the concentration of FIRP (from 10 ¹¹ mg/mL to 10 ⁶ mg/mL); (21 c) illustrates reflection peak shifts at the wavelength of 452.0 nm of 10 ¹° mg/mL HRP compared to 10 ⁸ mg/mL of seven other different proteins (where the dash line represents the resolution of the spectrometer (1 .5 nm)); and (21 d) illustrates the peak shift at 1 % and 10% crosslinking densities. [0062] FIGs. 22a to 22c illustrate a hydrogel sensor on transparent substrates in accordance with embodiments, where: (22a) illustrates the configuration of the hydrogel sensor on transparent substrates; (22b) illustrates a photo of the hydrogel sensor on glass substrate; and (22c) illustrates reflective peak shifts of the hydrogel sensor on glass substrate induced by Cu²⁺ from different projecting-detecting directions including top-top, bottom -bottom, bottom-top, and top-bottom.

[0063] FIGs. 23a to 23i illustrate a hydrogel sensor on flexible substrates in accordance with embodiments, where: (23a) illustrates reflective spectra of the hydrogel sensor on PET substrate before and after cycles of bending; (23b) illustrates reflective spectra of the hydrogel sensor on PDMS substrate before and after cycles of stretching (where all error bars indicate the standard deviation of three parallel experiments); (23c & 23d) illustrate microscope images of the hydrogel interferometer on PET substrate (23c) before and (23d) after 20 times bending (where the scale bar is 50 pm); (23e) illustrates reflective spectra of the hydrogel interferometer on PET substrate to sense Cu²⁺ ions after bending; (23f & 23g) illustrate microscope images of the hydrogel interferometer on PDMS substrate (23 f) before and (23g) after 20 times stretching (where the scale bar is 50 pm); (23h) illustrates reflective spectra of the hydrogel interferometer on PDMS substrate to sense Cu²⁺ ions after bending; and (23i) illustrates reflection spectra of the hydrogel interferometer on PDMS substrates with and without the 150% strain.

[0064] FIGs. 24a to 24c illustrate data from an ethanol vapor sensor based on a poly(FIEMA-co-AAc) hydrogel interferometer according to an embodiment of the invention, where: (24a) provides data for real-time monitoring of ethanol indicated by the reflectance change of the second-order destructive interference centered at 472 nm for an embodiment with a scale bar of 500 pm); (24b) illustrates experimental and theoretical hydrogel film thickness as a function of the ethanol partial pressure; and (24c) provides a quantitative analysis of ethanol vapor concentration based on RBG values in accordance with an embodiment of the invention.

[0065] FIGs. 24d to 24f illustrate pattern-based recognition of multiple volatile organic compounds (VOCs) with single hydrogel material based sensor array in accordance with embodiments of the invention, where: (24d) illustrates a colorimetric response of the sensor array to eight VOCs; (24e) shows RGB value patterns of the sensor array to each VOC for this embodiment; and (24f) illustrates a 2D score plot of linear discriminant analysis (LDA) on the data for this embodiment.

[0066] FIGs. 25a to 25c illustrate an exemplary information encryption device in accordance with embodiments of the invention, where: (25a) provides an image of such a device (with a scale bar of 2 mm); FIG. 25b illustrates thickness changes of the image (a water droplet pattern) and surrounding areas with increased relative humidity for another exemplary embodiment; and FIG. 25c illustrates a humidity indicator based on a multicolor image pattern in accordance with still other embodiments (with a scale bar of 2 mm).

[0067] FIGs. 26a to 26c illustrate images for Cu²⁺-responsive sensors in accordance with exemplary embodiments, where: (26a) illustrates before; (26b) illustrates after adding 20.0 mM Cu²⁺; and (26c) illustrates the correlated spectrum for the images.

[0068] FIGs. 27a to 27c illustrate images for glucose-responsive sensors in accordance with exemplary embodiments, where: (27a) illustrates before; (27b) illustrates after adding 20.0 mM glucose; and (27c) illustrates the correlated spectrum for the images.

[0069] FIGs. 28a to 28c illustrate images for glucose-responsive sensors made by Method B in accordance with exemplary embodiments, where (28a) illustrates before; (28b) illustrates after adding 20.0 mM glucose; and (28c) illustrates the correlated spectrum for the images.

DETAILED DISCLOSURE

[0070] Turning to the data and figures, ultra-sensitive sensors with small-scale and fast response, and methods of their manufacture, are provided. In various embodiments, the universal sensing platform is based on a hydrogel interferometer with femtomol-level sensitivity. Many embodiments of hydrogel sensing platforms utilize a chemically engineered, stimuli-responsive hydrogel covalently bonded to a substrate. In various such embodiments, the soft hydrogel layer rapidly changes a physical parameter, such as, for example, thickness in response to external stimuli, resulting in a color change. In some such embodiments, interference colors provide a visual and quantifiable means of revealing rich environmental metrics.

[0071] Embodiments of the general sensing platform based on hydrogel interferometer exhibit remarkable high performance by taking advantage of coupling two attributes: optical interference and responsive hydrogels. Responsive hydrogels in accordance with embodiments are capable of large deformation triggered by external stimuli such as solvent, pH, temperature and humidity. The dehydrated hydrogel with ligands incorporated according to embodiments can facilitate the absorption of a large amount of analyte in aqueous solution within very small volume, which spontaneously localizes and concentrates the analyte concentration. Embodiments incorporating the local concentrating effect, multiplied by the signal amplification effect, enables the ultra- high sensitivity of this hydrogel interferometer-based sensor. In addition, hydrogel sensor platforms according to embodiments may be functionalized to become responsive to different environmental cues, by linking specific functional groups or monomers to its polymer chains, which provides an analyte-specific matrix. Many embodiments allow for the fabrication of sub-micrometer hydrogels and testing various analytes is low, as they require only small quantities of materials. Various embodiments of the hydrogel-based sensor platforms may be bio-compatible with good stretchability, making them suitable material for wearable sensor.

[0072] Many embodiments of the sensor platform, couple such operational principles, with broad chemistries, simple physics, and modular design allow for high performance in detecting a wide variety of analytes, including metal ions and proteins. In some exemplary embodiments, the sensitivity can be 10 ¹⁴ M for copper ions and 1 .0 ^c 10 ¹¹ mg/mL for glycoprotein with an enhancement of 2 to 4 orders-of-magnitude. Many embodiments allow for the scale of the sensing platform to be of a pm-size. Several embodiments utilize a soft gel allowing the sensing platform to be transparent, flexible, stretchable, and compatible with a variety of substrates, allowing for high sensing stability and robustness after 200 cycles of bending or stretching.

[0073] With an ever-higher demand for real-time health and environment monitoring and prevalent smart technologies, high-precision molecule detection with micro-sized devices is playing a vital role with many exciting progresses recently. (See, e.g., T. M. Swager, Angew. Chem. Int. Ed. 2018, 57, 4248, the disclosure of which is incorporated herein by reference.) Ultra-sensitivity is critical for detecting trace-amounts of analyte, which is vital for healthcare, ecology, and industries. (See, e.g., S. Bai, et al. , Small 2015, 1 1 , 5807; S. Yang, et al., Sensors and Actuators B: Chemical 2016, 226, 478; and T. Wang, et al., Small 2016, 12, 3748, the disclosures of which are incorporated herein by reference.)

[0074] For example, free copper ions in the ocean should be critically maintained between picoM~femtoM (10^-12 - 10^-15 M) to be micronutrients for organisms, which would become toxic at a higher concentration. (See, e.g., S. Takano, et al., Nat. Commun. 2014, 5, 5663; A. L. Campbell, et al., Environ. Sci. Technol. 2014, 48, 9745; J. M. Lee, et al., Anal. Chim. Acta 201 1 , 686, 93; and C. K. H., B. K. W., Limnol. Oceanogr. 1988, 33, 1084, the disclosures of which are incorporated herein by reference.) Hence, accurately mapping and real-time monitoring copper distribution are important for marine biological recycling and scavenging. Likewise, Wilson’s disease, a hepatic and neurological disorder, is diagnosed by the 24-h urinary copper excretion (>100 pg/24 h) of a patient. (See, e.g., A. Ala, et al., Lancet 2007, 369, 397, the disclosure of which is incorporated herein by reference.) On-site fast detection of the trace-amount copper ions with personal metabolism monitoring devices could effectively prevent or reverse many manifestations of this disorder. (See, e.g., C. Hazra, et al., J. Mater. Chem. C 2018, 6, 153, the disclosure of which is incorporated herein by reference.) However, current methods of detecting such low amounts of metal ions still rely on complex and costly analyte pre-concentrating or presorting, as well as large instruments operated by well-trained laboratory personnel, such as inductively coupled plasma mass spectroscopy (ICP-MS) and atomic absorption/emission spectroscopy (AAS/AES). (see, e.g., W. Zheng, et al., Small 2018, 14, 1703857, the disclosure of which is incorporated herein by reference.) Thus, real-time monitoring of seawater cannot be achieved using these methods. Similarly, detecting specific proteins typically requires the use of western blot or enzyme-linked immunosorbent assay (ELISA), both of which involve lengthy multi-step processes of proteins enrichment, staining, and detection. These detection methods can reach desired low limit of detection (LOD), but their large equipment size and high cost prevent them from facilitating personal healthcare or point-of-care technologies. (See, e.g., F. Watzinger, K. Ebner, T. Lion, Mol. Aspects Med. 2006, 27, 254; and A. Ymeti, et al. , Nano Lett. 2007, 7, 394, the disclosures of which are incorporated herein by reference.)

[0075] There have been a number of efforts directed to advancing microscale sensors for detecting metal ions and biomolecules (e.g., protein and enzyme); however, the sensitivity of state-of-the-art methods, for example, copper ion detection is only 10 ⁴ - 10 ¹⁰ M, and for horse radish protease detection sensitivity is only 10 ⁴ - 10 ⁷ M. Accordingly, these techniques have not reached the aforementioned desired sensitivity levels. (See, e.g., J. Wang, et al., Chem. Eur. J. 2018, 24, 3499; S. Wang, et al., ACS Sens. 2017, 2, 364; T. Liu, et al., Sens. Actuator B-Chem. 2016, 235, 568; H. Ouyang, et al., Biosens. Bioelectron. 2016, 85, 157; R. Hu, et al., Adv. Funct. Mater. 2017, 27, 1702232; A. K. Yetisen, et al., Anal. Chem. 2015, 87, 5101 ; J. Ye, et al., Angew. Chem. Int. Ed. 2014, 53, 10386; Y. J. Liu, et al., ACS Appl. Mater. Interfaces 2017, 9, 25559; H. Y. Peng, et al., J. Anal. At. Spectrom. 2014, 29, 1 1 12; C. H. Lu, et al., Biosens. Bioelectron. 2012, 31 , 439, the disclosures of which are incorporated herein by reference.)

[0076] In summary, creating small-scale synthetic chemical sensors with high sensitivity and high selectivity has proven a daunting challenge, due to the time- consuming and still expensive synthesis, purification, and manufacturing processes required by conventional methods. (See, e.g., J. A. Cotruvo, Jr., et al., Chem. Soc. Rev. 2015, 44, 4400; K. P. Carter, et al., Chem. Rev. 2014, 1 14, 4564; T. Hirayama, et al., Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 2228; L. Xi, et al., Adv. Mater. 2018, 30, 1800177, the disclosures of which are incorporated herein by reference.) To address these issues, embodiments are directed to a low-cost, general-purpose platform for chemical detection utilizing a sensing platform incorporating hydrogel interferometry. Hydrogel interferometry according to embodiments can achieve chemical detection of exceptionally high sensitivity and selectivity through the synergy of chemistry, mechanics, and optics. Embodiments of Sensor Platforms

[0077] Many embodiments of a sensor platform comprise a highly porous cross-linked polymer network thin-film interferometer consisting of a reflective substrate coated with a single thin film of a highly porous cross-linked polymer network, as shown in FIG. 1 a. In many embodiments, the highly porous cross-linked polymer network comprises a hydrogel. Hydrogels are chosen according to many embodiments for their large volume change ratio (up to ~10 times), facile chemical functionalization for broad stimuli sensitivities, and mechanical flexibility. (See, e.g., L. Ionov, Mater. Today 2014, 17, 494; X. He, et al., Nature 2012, 487, 214; A. Shastri, et al., Nat. Chem. 2015, 7, 447; Z. Zhao, et al., Environ. Sci. Technol. 2016, 50, 12401 , the disclosures of which are incorporated herein by reference.) These advantages together make the responsive hydrogel an ideal material for fabricating high-performance sensing systems, and being integrated with optical interference.

[0078] In many such embodiments, the hydrogel layer is covalently bonded to the reflective substrate, forming a hydrogel-substrate interface. In such embodiments, the polymer network of the hydrogel provides a scaffold to carry a large number of ligands specific to an analyte of interest (e.g., ligand-to-polymer mass ratio = 1 :5). The time scale of response for embodiments of the sensor platform may in some cases be governed by diffusion, which decreases quadratically with the feature size of the hydrogel. Accordingly, in many embodiments the thickness of hydrogel layers according to embodiments is maintained at an 0.1 to 1 pm scale enabling 10 to 100 second-scale fast responses for real-time sensing. In many embodiments, the thickness of the hydrogel is comparable to the wavelength of visible light (~300 nm at dry state and ~1000 nm at hydrated state). In some such embodiments, the thickness of the hydrogel film is at the scale of ~100 nm.

[0079] In accordance with embodiments of the invention, and as illustrated in FIG. 1 and 2a, external stimuli can produce changes in thickness of the hydrogel layer, resulting in a change in the color exhibited by the hydrogel. In such embodiments, stimuli- responsive hydrogels may change their volume significantly in response to small alterations of certain environmental parameters; the volume changes can be more than hundred fold via the absorption or release of molecules. In addition, many embodiments may comprise hydrogels that can be chemically adjusted to provide a large assortment of sensitivities such as humidity, temperature, light, mechanical stress, magnetic or electrical field, and specific chemical and biological molecules, including pH, metal ions, and anions, in both gaseous and liquidus forms. In such embodiments, the chemical composition of the hydrogel may be flexibly tuned by mixing different pre-polymers or monomers and functional molecules as may be known in the art. Accordingly, embodiments of the current invention may be used in diverse applications such as a volatile-vapor sensor, a colorimetric sensor array for multi-analyte recognition, breath- controlled information encryption, and a colorimetric humidity indicator.

[0080] Turning to the structure of the substrate, in many embodiments the substrate is formed of a reflective material suitable for bonding the hydrogel layer. Some exemplary embodiments of such a substrate comprise a silicon (Si) wafer modified with reactive functional groups to allow covalent bonding with the hydrogel, or other transparent and/or flexible substrate materials, such as glass, PET, PDMS, or almost any materials, coated with an ultrathin reflective metal layer.

[0081] During operation of a sensor platform in accordance with embodiments, when a drop of analyte-containing solution is applied on the surface of the hydrogel, the analyte diffuses into the hydrogel, causing a cascade of signal transduction, involving chemical reaction, mechanical deformation, and optical detection (C M O), as shown schematically in FIG. 1 . According to embodiment, the ligands capture and localize the analyte by forming complexes producing a strong and local ligand-analyte binding which effectively concentrates the analyte within an extremely small volume of the gel (e.g., as high as 10⁹ fold, concentrating, for example, an analyte droplet of 10 pL or 1x10¹⁰ pm³ into a 1 .7 pm³ volume). Compared to other high-performance sensors based on rigid porous carrier such as metal-organic framework, the unique large volumetric shrinkage (~10%) of the soft gel network further enhances the magnitude of this local concentrating effect. (See, e.g., X. Lin, et al., Chem. Commun. 2015, 51 , 16996, the disclosure of which is incorporated herein by reference.)

[0082] As shown schematically in FIGs. 1 and 2a, in embodiments of the sensor platform the ligand-analyte binding results in the gel locally contracting or swelling, depending on the ligand-to-analyte ratio, which leads to local thickness change. A thickness change as small as a few nanometers can be detected through optical interference, known as a significant signal amplifier. The optical interference can be: (1 ) recognized by optical spectrometer and analyzed by quantifying the reflectance or transmittance spectrum shifts before and after exposure to the analytes, (2) recognized by taking images by camera and analyzing the color change before and after exposure to the analytes, and/or (3) directly by naked eye and analyzed with the aid of color chart or standard curves of colors. Overall, the two key merits, the analyte concentrating effect (in C—M) and the subsequent signal amplification (in M 0) jointly lead to the remarkably low limit of detection (LOD) at an unprecedented level while possessing great selectivity.

[0083] In exemplary embodiments of the invention, at least one portion of the hydrogel layer exhibits at least one color, where the color arises from interference of light waves reflected from the air-hydrogel interface and the hydrogel-substrate interface, as depicted schematically in FIG. 1 . The adaptive coloration of such a simple structure having only one homogenous hydrogel layer on a reflective substrate obeys a simple governing equation of thin-film interference physics, as illustrated in FIG. 2b. In some such embodiments, ni, ri2, and ri3 are refractive indices of air (1 ), hydrogel (1 .52), and silicon (3.8), with d as the thickness of the hydrogel, and Q as the incident angle. If ni < n₂ < ri3, the condition for constructive interference is given by:

2n₂dcosd = mA, (EQ. 1 )

and the condition for destructive interference is in turn given by:

2n₂dcosd = (m - 0.5)l, (EQ. 2)

where l is the wavelength of interference light, and m is an integer. As can be derived from these relationships, at normal incidence (cos0 = 1 ), the color is mainly determined by the thickness of hydrogel film, where d is comparable to the wavelength of visible light. Accordingly, although hydrogel films may take many forms, as previously described, in various embodiments, the hydrogel layer has at least one defined thickness, is featureless, and has an air-hydrogel interface.

[0084] Utilizing such embodiments, allows for the use of a wide variety of optical interferometry techniques. For example, as shown in FIG. 2c, an increase in hydrogel layer thickness from 89 nm to 370 nm, results in a broad color spectrum from violet to yellow-green. It will be understood that these color changes are merely exemplary. The specific colors for any embodiment will correspond to the peaks (constructive interference) and valleys (destructive interference) in the reflection spectra. The hydrogel thin films in some embodiments will show colors determined by reflection peaks (m = 1 , 2) or complementary colors ( m = 0.5, 1 .5, 2.5) determined by reflection valleys as represented in FIG. 2d. In some embodiments, (e.g., when the hydrogel film thickness is in the range of 66 nm - 132 nm) only reflection valley (first-order destructive interference) can be observed, and the hydrogel film will show complementary color to the valley. In other embodiments, (e.g., when the hydrogel film is thicker than 132 nm) the color will correspond to the reflection peak. Such an adaptive color platform in accordance with embodiments of the current invention provides a means of unveiling chemical signals by visual and quantifiable color change.

[0085] In various embodiments, such color changes may be used in conjunction with colorimetric analysis (e.g., computer-based analysis programs such as MATLAB). In such embodiments, a relationship of the correlation of reflection spectra, hydrogel film thickness, and analyte concentrations, may be provided allowing a user to read the analyte concentration from the images of the hydrogel film before and after analyte exposure. In other embodiments, colorimetric analysis may be used for a portable and easy-to-use sensing system, where qualitative and quantitative analysis of targeting compounds can be performed by matching RBG color values with a corresponding database stored on the device. An exemplary embodiment is shown in FIG. 3. In one such embodiment a displayed application may match sample photos with the closest records in the stored database and give a detection result based on this comparison. In these embodiments, a sample image may be converted to a four-dimensional matrix, with each dimension consisting of data for each color channel as well as data on the image’s transparency. Algorithms may also be used to precisely crop the image for analysis. Analysis for yet other embodiments may be performed with a color scale bar or table, a gradient pattern design, or a bar code or QR code. [0086] Although embodiments have been described in relation to a reflection interferometry mode, it should be understood that the sensor platform according to embodiments may also be used in a transmission mode. The transmission mode embodiments, like the reflection mode embodiments, also utilize a hydrogel layer having: at least one defined thickness, a featureless surface, and an air-hydrogel interface; however, according to transmission mode embodiments of the invention, the hydrogel layer is bonded to a substrate assembly comprising a first transparent substrate and a reflective thin film, as shown schematically in FIGs. 4a and 4b. In exemplary embodiments, the first transparent substrate may be, for example, glass or plastic. According to other embodiments of a transmission mode sensor platform, the substrate may include a reflective thin film configured to enhance the reflection of the first transparent substrate while remaining thin enough to maintain the transparency of the whole platform. In such embodiments, the reflective thin film may be any material with a refractive index higher than 1 , such as a gold thin film or other metal thin film. In some embodiments, the first transparent substrate is disposed between the reflective thin film and the hydrogel film, as shown in FIG. 4a. In other embodiments, the reflective thin film is disposed between the first transparent substrate and the hydrogel film, as shown in FIG. 4b.

[0087] As in reflective mode embodiments of sensor platforms, in transmission mode embodiments, at least a portion of the hydrogel layer exhibits at least one color, and said color arises from interference of transmitted light waves passing through the hydrogel layer and the substrate assembly. Similar to the discussion above with respect to the reflective mode, the hydrogel thickness in embodiments of the sensor platform in transmission mode also changes in response to at least one external stimulus and the thickness change produces a similar change in the hydrogel color. One distinctive feature of these transmissive embodiments is that, observing the color in transmission mode from the substrate side, reveals the color with absorption spectra complementary to that in reflection mode. Accordingly, sensor platform embodiments incorporating transmission modes allows for the observation of the color or detection of the optical readout signal from the hydrogel films from the back of the substrate, as illustrated in FIGs. 4a and 4b, while the environmentally-sensitive component of the adaptive color platform, the hydrogel film, is exposed to an enclosed environment. Such embodiments may have special application as sensors for testing and monitoring the environment inside various closed packages, such as food packaging materials.

[0088] Still other embodiments of the hydrogel interferometer may be made with different transparent substrates that provide flexibility of color detection from different sides with respect to the analyte. Exemplary embodiments may use glass, PET, or PDMS as transparent substrate material. FIG. 5 provides a configuration of a sensor in accordance with such embodiments - in this example, a first thin layer of gold, a hydrogel thin film, and a second thin gold layer are disposed on a transparent substrate. Additionally, FIGs. 6a to 6d provide reflective spectra of a hydrogel interferometer on a glass substrate to sense Cu²⁺ ions in accordance with an exemplary embodiment. As shown, utilizing such a configuration allows for different detection methods to be employed. Specifically, light from top and detector also from top (FIG. 6a), light from bottom and detector also from bottom (FIG. 6b), light from bottom and detector from top (FIG. 6c), and light from top and detector from bottom (FIG. 6d). Accordingly, the use of a transparent substrate in combination with a reflective layer provided multiple options for detection and analysis, increasing the flexibility of the sensor platform.

Embodiments of Methods for Forming Sensor Platforms

[0089] Although embodiments directed to sensor platforms have been discussed thus far, embodiments are also directed to methods of forming such sensor platforms. As shown in FIG. 7, in various embodiments, the hydrogel interferometer is prepared by a one-step method, with spin-coating deposition and in-situ polymerization of hydrogel precursor solutions onto a reflective, non-transparent substrate. Although these embodiments have been described in relation to hydrogel layers having a single fixed thickness, in still other embodiments, sensing elements with different hydrogel layer thicknesses on a single interferometer may be realized through on-demand patterning by spatially controlled polymerization, e.g., by use of a mask an multiple exposure. Embodiments Overview

[0090] A universal hydrogel interferometry sensor has been described that can effectively enhance the chemical detection sensitivity for several orders of magnitude (e.g., 10²-10⁴), by remarkable local concentrating effect (10⁹ times) and large signal amplification in a chemo-mechano-optical signal transduction. Specifically, strong analyte binding by way of large concentrations of ligands carried on a shrinkable porous gel matrix produces unique local concentrating effect at factors of 10⁹ times, allowing for the detection of extremely low concentrations of molecules and the elimination of pre- concentration or complex multi-step processing. Meanwhile, the specific chemical reaction between the selected ligand and the target analyte gives the platform high selectivity. As will be described in greater detail in the Exemplary Embodiments, embodiments of the sensor platform demonstrate picoM-femtoM high sensitivity (10 ¹³- 10 ¹⁵ M) in metal ion detection against 14 interfering ions, as well as protein detection. These examples demonstrate the promising potential in on-site analysis and even real- time monitoring of seawater or wastewater and in new point-of-care or health monitoring technologies, without the needs of complex procedures and costly large equipment required by current methods.

[0091] Embodiments of sensor platforms are extremely manufacturable. Sensor platforms may be formed having pm-scale effective sensing areas, optical transparency, and mechanical flexibility (robustness over cycles of bending or stretching), and are compatible with different substrates showing great potential as micro-scale wearable sensors and easy integration. Overall, this simple and general design principle is applicable to almost any porous soft materials, providing a practical solution to enhancing the performance of many sensors of different sensing mechanisms with different responses to not only various chemicals but also temperature, mechanical and other physical environmental cues. In short, this platform will open the avenues to new development of sensors with high performance, low cost, and easy fabrication for health and environmental real-time monitoring. EXEMPLARY EMBODIMENTS

[0092] As will be discussed in greater detail in the following Exemplary Embodiments section, below. Exemplary embodiments of the invention display many advantages such as remarkable color uniformity, fast response, high robustness, and easy fabrication. In some embodiments, the sensors are configured to detect biological and chemical species. In many embodiments, the sensors are configured to provide a local concentrating effect (up to 10⁹ fold) in the hydrogel induced by strong analyte binding and large numbers of ligands. In some embodiments, the local concentrating effect combines with a signal amplification effect resulting from optical interference to endow embodiments of the sensing platform with an ultra-high sensitivity. In various embodiments, the specificity of the chemical reaction between selected ligands and target analytes provides high selectivity in detecting complex fluids.

[0093] In the following examples, sensor platforms capable of providing LODs of 10 ¹⁴ M in Cu²⁺ detection and 10 ¹¹ mg/mL in glycoprotein detection against multiple interfering species in mixture analyte fluids have been demonstrated, which present a 2-4 order-of- magnitude enhancement in sensitivity over the state-of-the-art methods in analyzing real- life complex fluids. Moreover, achieving such high sensitivity and selectivity is achieved on a pm-sized hydrogel-based interferometer sensor platform. In addition, such hydrogel interferometer can be constructed into transparent and flexible sensors with various substrates. This simple but general platform with a single layer of hydrogel can accommodate broad choices of ligands and substrates. Therefore, it can be readily customized for detecting many chemical and biological species in the fields from healthcare to environment safety and integrated to wearable electronics. Overall, this unique hydrogel interferometer-based sensing platform adopts a highly efficient chemo- mechano-optical signal transduction that enables fM-level sensitivity on a sub-pm³ sensing active region. In combination with the great selectivity, optical transparency, and mechanical flexibility, it presents promise for the next-generation high-performance micro-sensors. [0094] Although specific techniques and embodiments are discussed below, these techniques and embodiments are meant to be exemplary in nature, and are not to be construed as limiting the scope of this disclosure

Experimental Methods

General Methods and Materials

[0095] 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA), acrylic acid (AAc), acrylic amide (AAm), sodium hydroxide ammonium persulfate, N-(3-dimethylaminopropyl)-N'- ethylcarbodiimide hydrochloride (EDC HCI), 2-hydroxy-4'-(2-hydroxyethoxy)-2- methylpropiophenone (Irgacure 2959), allylamine, benzoin methyl ether, cystamine, N- hydroxysuccinimide (NHS), 1 -vinylimidazole, barium chloride anhydrous, calcium chloride, cobalt chloride, chromium chloride, copper chloride, iron(lll) chloride, magnesium chloride hexahydrate, manganese chloride tetrahydrate, nickel chloride hexahydrate, potassium chloride, sodium chloride, lead chloride, zinc chloride anhydrous, peroxidase from horseradish, ribonuclease A, b-lactoglobulin A from bovine milk, myoglobin from equine heart, bovine serum albumin, hemoglobin from bovine blood, albumin from chicken egg white, b-casein, cytochrome c from equine heart, sodium phosphate dibasic, potassium phosphate monobasic, HEPES, acetic acid, ethanol, isopropanol, polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) were purchased from Fisher Scientific or Sigma-Aldrich. 3-Methacrylamidophenylboronic acid was purchased from Combi-Blocks. Silicon wafers were purchased from Waferpro. All chemicals were commercially available unless noted otherwise.

NMR

[0096] The 1 H NMR and 13C NMR spectra were recorded in solution of deuterium oxide on a Varian 400 MHz spectrometer.

FTIR Spectra

[0097] Fourier transform infrared spectroscopy (FTIR) spectra were performed on a Jasco 420 FTIR spectrophotometer with samples prepared on silicon substrates. XPS Measurements

[0098] X-ray photoelectron spectroscopy (XPS) measurements were performed on an Omicron Nanotechnology system with a base pressure of 2 ^c 10-10 Torr. An MgKa radiation source was used for the XPS measurement.

Optical Measurements

[0099] The sensor was first dried on hot plate for the hydrogel film to reach the same thickness for each measurement and then 10 pL solution was added on the surface of the sensor (for glycoprotein detection, the solutions with different concentrations of proteins are prepared in PBS buffer (pH 7.4, 150 mM) and the hydrogel sensor is incubated for 30 min for complete diffusion and binding reaction.). After using a piece of cover glass to cover the sensor, the sensor was transferred and placed on the stage of the microscope (Leica DM5000) with light illuminating vertically from the top or the bottom. The reflected light was collected by a UV-Vis spectrometer (Ocean Optics USB2000+). Treatment of the Substrates

[00100] Si Substrates. The Si substrates were washed by isopropanol and deionized water twice. After drying, the substrates were treated with oxygen plasma (Plasma Cleaner PDC-001 , Harrisk Plasma) and then immersed in the silane solution (200 ml_ ethanol, 6 ml_ of 10% acetic acid and 2m L TMSPMA) for 4 h. After the incubation, the substrates were washed with ethanol and completely dried.

[00101] Glass. PET and PDMS substrates. A thin layer of Au was first sputtered on the surface of the glass, PET film or PDMS substrates. Then, according to the technique disclosed by O. A. Raitman, et al. , Anal. Chim. Acta 2004, 504, 101 , the disclosure of which is incorporated herein by reference, the substrates were merged into cystamine solution (50 mM solution in phosphate buffer, pH = 7.3) for 3 h. After that, the substrates were rinsed with distilled water, and reacted with a solution containing 0.1 mM AAc in HEPES buffer, pH = 7.4, in the presence of EDC HCI (10 mM) for 2 h. After these modifications, the substrates were washed with distilled water and flushed by a flow of nitrogen. Synthesis of the Polymer poly(AAm-co-AAc-co-AAene)

[00102] As summarized in FIG. 8a, in the first step, AAc (514.2 pl_), AAm (3020.9 mg) and ammonium persulfate (570.5 mg) were dissolved in 50.0 ml_ water and this mixture was tuned to pH 10 using NaOH solution. Then, the mixture was purged with N2 for 30 min to remove the oxygen. After that, the solution was heated and maintained at 50 °C for 2 h. After the reaction, the copolymer poly (AAm -co-AAc) was obtained by dialysis (MWCO: 12000).

[00103] As shown in FIG. 8b, Poly(AAm-co-AAc) (500.0 mg), EDC HCI (500.0 mg), NHS (300.0 mg) and 20 ml_ water were first stirred for 1 h. Then 200.0 mI_ allylamine was added to the mixture and the mixture was kept to stir for another 24 h at 50 °C. The final product ene-function copolymer poly(AAm-co-AAc-co-AAene) was obtained by using dialysis tubing (MWCO: 12000).

[00104] The polymer chemical structures were obtained by NMR. FIG. 8c provides 13C NMR spectrum of poly (AAm -co-AAc). The peak at 182.48 ppm was assigned to -CH- COOH and the peak at 179.35 ppm was assigned to -CH-CO-NH2. The ratio of these two parts (m:n) was 1 : 1 1 . Further, poly(AAm-co-AAc-co-AAene) was characterized by 1 H NMR, as shown in FIG. 8d. -CH=CH2 on AAene part was shown a peak at 5.69 ppm. The peaks of -CH2-CH- on the polymer chain was from 1 .20 ppm to 2.32 ppm. According to m:n = 1 1 : 1 and the integrations in FIG. 8d, the polymer poly(AAmm-co-AAcn-x-co- AAenex) configuration was determined with m:(n-x):x = 1 1 :0.32:0.68.

Fabrication of the Sensor Platform

[00105] As shown in FIG. 9, in various exemplary embodiments the precursor solution contained 3% wt poly(AAm-co-AAc-co-AAene), 2% wt ligands (1 -vinylimidazole (for Cu2+ sensing) or 3-methacrylamidophenylboronic acid (for glycoprotein sensing), 0.4% initiator in 40% acetic acid-water solution. The hydrogel precursor solution was spin-coated onto the different substrates with UV exposure (Spectroline Pencil UV Lamps) to initiate in situ polymerization. Each time 15 pL precursor solution was added on the substrate, followed by spin-coating at 12000 rpm for 60 s. After the polymerization, the sensor was washed in deionized water to remove unreacted monomers. At last, a thin Au was layer sputtered on the top surface of the sensor using a mask. The thickness of the Au layer is about 2.0 nm.

[00106] Au Lavers. As shown in FIG. 10a, when incident angles are vertical, optical intensity (energy) reflection ratio at an interface is given by:

* = & <^EQ- ³>

The optical intensity reflection ratio at the interface between the air and the water (interface 1 ) is Ri = 0.02; and thus the transmission ratio (interface 1 ) is Ti = 0.98. As the refractive index of water and hydrogel film are almost the same, optical intensity reflection ratio at their interface (interface 2) is R « 0 and Ϊ2 « 1 . And the optical intensity reflection ratio at the interface between hydrogel film and Si wafer layer (interface 3) is R3 = 0.357; and the transmission ratio (interface 3) is T3 = 0.653. The intensity recorded by the spectrometer is the interference result of optical complex field reflected from all interfaces is given by:

A_r 1 = A_x exp(icr) + A₂ exp(i/?) + A₃ exp(iy) (EQ. 4)

Assuming that the incident light intensity is /, so:

A_r 1 = A_x exp(icr)

(icr) + 0 +

_\/0.342 V7 exp(iy) (EQ. 5)

Thus,

0.5277.

[00107] As shown in FIG. 10b, when the Au film is about 2.0 nm, it is half transmission, R_b2 = 0.5 and T_b2 = 0.5. So:

p(icr) + V0.4802 V exp(i/5) + V0.086 7 exp(ty) (EQ. 6)

Thus, I_2min = 0, l_2max = (V0O2 + 0 802 + V0O86)² / * 1.2727.

[00108] Based on the above set of calculation, when there is an Au layer on the top surface of the hydrogel, the peak value of intensity curve increases from 0.527I without Au to 1 .2721 with Au. The peak recorded by the spectrometer will thus turn stronger. This sharp peak is good for sensing due to the high signal to noise ratio. [00109] Hydrogel Film Thickness.

[00110] The effect of film thickness on the sensing performance was studied in the following way. By controlling the spin coating conditions, the hydrogel thickness was tuned to be ~1000 nm at its swollen state as the desirable range, so that the reflective spectrum has 3 or 4 peaks in the visible range, as shown in FIG. 1 1 . Measuring the multiple peak shifts provides us an accurate value of hydrogel film thickness change, with the experimental error minimized while the short response time maintained. By contrast, smaller thickness, e.g., less than 500 nm, generates fewer peaks in the visible range of the spectrum, reducing the sensing accuracy and reliability. Larger thickness (>1000 nm), on the other hand, generates more peaks, but the responsive time increases quadratically with the thickness, due to the increased diffusion length of analyte into the gel.

[00111] With the help of a reflectance calculator, the reflective spectrum of the hydrogel sensor with different thicknesses, from 100 nm to 2000 nm, was simulated. The results are shown in FIG. 1 1 . The thickness here is referred to the thickness that the hydrogel is swollen in the solution. When the thickness is 100 nm, there is only half peak in the wavelength 400 nm - 800 nm. This will bring huge difficulty when the peak moves because it is hard to find the peak or the valley in this spectrum. When the thickness increases to 500 nm, two peaks could be found in the spectrum which is better than that for 100 nm. But two peaks aren’t enough for multi-peak analysis. If the hydrogel thickness is too large and reaches 2000 nm, although it has many peaks, the responsive time will be increased due the larger diffusion distance. So, the spectrum for the thicknesses is optimum between these extremes, e.g., between 1000 nm and 1200 nm.

[00112] The thickness of the hydrogel sensor may also be calculated from the UV-Vis reflective spectrum. Each peak wavelength l from the reflective spectrum was obtained. The interference order m of adjacent peaks must be consecutive. The value m of 1 st, 2nd, ... , peak must be equal to a, a+1 , ... , where a is a positive integer. Each peak wavelength A and its corresponding m value is a constant 2nd, according to the Bragg’s Law: 2nd = mA, and assuming that the refractive index of the hydrogel n is a constant for all the wavelengths. The standard deviation of all m_/A value was defined as o, and o will have the minimum value when a proper a was achieved. By calculating each o for each positive integer a starting from 1 , the desired a value could be fitted and obtained during this process. After a was fixed, the corresponding d for the thickness of the hydrogel could be known from the Bragg’s Law: 2nd = mA.

Sensor Performance Characterization

[00113] FTIR was used to characterize the samples. The up curve was the FTIR spectrum for the sensor with imidazole ligands and the low curve is that for the sensor without the ligands. As shown in FIG. 12, the peak at 1420 cm ¹ in the up curve, which was assigned to the symmetric C=N-C=C stretching vibration, increased a little compared to the same position in the low curve, because imidazole molecules were in the hydrogel scaffolds. Also, the characteristic peaks at 641 cm^-1 was ascribed to the vibration of imidazole ring.

[00114] Also, the imidazole amount in the hydrogel was calculated by XPS, as shown in FIG. 13. According to percentages of the elements C, N and O, we estimated that the ratio of the poly(AAm-co-AAc-co-AAene) and imidazole ligand was about 5:1 . It was assumed that the hydrogel density in a dry status is the same as the density of the acrylamide monomer, which is 1 .32 g/cm³. The volume of the hydrogel sensor in a dry status is about 1 .3 ^c 10 ¹¹ m³. Thus it was possible to know that the mass of imidazole in the hydrogel sensor is about 9.5 ^c 10 ⁶ g, which equals that in this sensor the molar amount of the imidazole is about 10⁸ mol. The above measurements confirmed that the imidazole ligands were successfully introduced in the hydrogel matrices.

[00115] A larger wavelength will give more peak shifts due to the Bragg’s law. When 2nM is constant, the peak at high wavelength will have a small m, and thus will give a larger peak shift. When 1 .0 ^c 10 ¹⁴ M Cu²⁺ was added on the sensor, only the peak at 639.8 nm gives a significant shift about 2.3 nm. The peaks at 458.7 nm and 533.6 nm changed very little (0.4 nm and 0.4 nm) and were lower than the resolution of the instruments. When the concentration of Cu²⁺ is over 1.0 ^c 10 ¹² M, all the peaks shifted more than 1.5 nm, as shown in FIG. 14. This again verifies the ultra-sensitive detecting limit of the current sensor. The analysis of multiple peaks in one sensor behaves as a sensor array, and leads to more reliable sensing results. [00116] A sample sensor without imidazole ligand in the hydrogel was also made to verify the function of the ligand. As shown in FIG. 15, there were no obvious peak shift when the Cu²⁺ concentration was under 1 .0 ^c 10 ⁹ mol/L, compared to the shift with ligands inside (about 7.5nm). This confirmed that the binding between the ligand and metal ions cause the shrink of the hydrogel.

[00117] The swelling behavior of the hydrogel sensor was also measured in different pH solutions. Since the hydrogel contains the weak base group imidazole and the acid group COOH, which is from the residue of the synthesis (FIGs. 8b and 8d), the swelling behavior is similar to the polyampholyte hydrogels. As shown in FIG. 16, in the case of low pH (< 3.5) (region I), the dominant charges in the gel are the protonated imidazole group (NH⁺) and the gel swells. While at pH 3.5-5.0 (region II) around the isoelectric point (IEP) of the gel, the numbers of NH⁺ and COO groups are nearly equal, the surrounding osmotic pressure causes the gel to shrink. At region III with a high pH (5.0-8.0), the gel did not change the volume so much.

[00118] The effect of salt concentration was also investigated by monitoring the peak shift under different concentration of NaCI, from 1 .0 pM to 1.0 mM. As shown in FIG. 17, with the increase of the NaCI concentration, the peak gradually shifted and the hydrogel shrank.

Mechanism of Binding

[00119] The binding constant between the ligand and Cu2+ (LogKa = 12.6) is much higher than those between the ligand and other metal ions, as shown in the Table 1 , below.

Effective Sensing Area

[00120] The effective area of the sensor was estimated with a computer simulation method by adopting a swelling theory of hydrogels under large deformation. Here it is assumed that after a drop of Cu²⁺ sensing solution is applied on the hydrogel film, all the Cu²⁺ ions are concentrated and homogeneously distributed throughout the hydrogel thickness, but within an effective area of diameter a in the planar direction. This effective area indicates the minimum area of the hydrogel film to sense the Cu²⁺ in the current volume of solution (10 pL). a corresponds to the diameter in the dry gel state and depends on the concentration of Cu²⁺ in the sensing solution.

[00121] The concentration of Cu²⁺ in the sensing solution is denoted as c, the volume of the sensing solution as V, and the thickness of the dry gel film as t. Based on the above assumption, the number of Cu²⁺ ions divided by the volume of the effective area of gel in the dry state, denoted as C, is calculated by:

(EQ. 7)

[00122] Because C is much smaller than the concentration of ligands in the gel, it is assumed that all the Cu²⁺ ions bind the ligands. Each Cu²⁺ ion is linked to 4 ligands, thus generating 4 additional polymer chains. As a result, the additional number of polymer chains per unit volume of the dry gel is given by:

[00123] The effective area of hydrogel film is assumed to swell homogeneously from thickness t to At, where l is the swelling ratio in the vertical direction. It is further assumed that the dry gel is constrained on the substrate without any pre-stretch. As a result, the volumetric swelling ratio is J = A From the theory of large deformation and swelling of hydrogels, the swelling ratio l is determined by the following equation:

where N is the total number of polymer chains per unit volume of the dry gel, W is the volume of water molecule, and c is the Flory parameter. N is the sum of the initial concentration of polymer chains No without any Cu²⁺ and the additional concentration of polymer chains dN generated by Cu²⁺ according to:

N = No + 5N (EQ. 10)

[00124] The swelling ratio A is measured in experiments as a function of the Cu²⁺ concentration c. The initial concentration No is then calculated from the swelling ratio of the gel in pure water without any Cu²⁺. Combining EQ. 8 and EQ. 10, the following is obtained:

where N is calculated from EQ. 9 with the experimentally measured swelling ratio A.

[00125] From experiments, it is possible to take V= 10 pL, t = 322 nm, No = 1 .42 ^c 1026 m-3, and W = 3 ^c 10-29 m³ Further it is possible to take = 0.49 as the Flory parameter of polyacrylamide. All together, the diameter of the effective area in the dry gel state is plotted, specifically 1 pm ~ 1 mm, as a function of the Cu²⁺ concentration in the sensing solution from 10 ¹³ to 10 ⁵ M, in FIG. 20b.

Example 1 : Embodiments Implementing Cu Ion Sensors

[00126] Various embodiments of sensor platforms are configured as metal ion sensors with high accuracy for quantitative estimation. In an exemplary embodiment a high- sensitivity Cu²⁺ sensor was formed. It is well known that imidazole can bind with Cu²⁺ ions specifically to form complexes in aqueous solutions. (See, e.g., R. J. Sundberg, R. B. Martin, Chem. Rev. 1974, 74, 471 , the disclosure of which is incorporated herein by reference.) By grafting imidazole ligands on the polymer chains of a hydrogel in accordance with embodiments, the hydrogel becomes Cu²⁺-sensitive. When a droplet of Cu²⁺ analyte aqueous solution is applied onto such an imidazole-rich hydrogel, the Cu²⁺ ions diffuse in and bind with the imidazole ligands on the polymer chains. Because the binding constant between Cu²⁺ and imidazole is fairly high, and the concentration of imidazole is much higher than that of Cu²⁺, these favor for the right-shifting of the coordination reaction, facilitating the formation of the Cu²⁺-imidazole complexes. In the case of detecting Cu²⁺ at low concentrations with the large numbers if ligands in the hydrogel, the Cu²⁺-imidazole complexes form at a 1 :4 ratio. Flence, each Cu²⁺ ion brings multiple surrounding polymer chains together, the complexes serve as additional crosslinks of the hydrogel, and as a result, the hydrogel swelling ratio decreases with the concentration of Cu²⁺ in the analyte solution. As the SEM images in FIGs. 18a and 18b show, after adding Cu²⁺ into the hydrogel, the hydrogel pore size significantly reduced and the gel network became much denser. Such a Cu²⁺ ion-induced thickness change can be readily captured by the reflective spectrum with a spectrometer.

[00127] In the exemplary embodiment a poly(acrylamide-co-acrylic acid -co-N- allylacrylamide) (poly(AAm-co-AAc-co-AAene)) was used as the hydrogel network with covalently linked imidazole ligands. A thin film of the hydrogel was fabricated via in-situ photo-polymerization during spin-coating on a Si wafer as the reflective substrate (as discussed above in relation to FIG. 9). Considering a hydrated hydrogel has almost identical refractive index to the aqueous solution of analyte, a nanometer-thin layer of gold (Au) was subsequently sputtered on the hydrogel surface, for the purpose of enhancing the reflectivity at the aqueous solution-gel interface for optimal interference, while ensuring good optical transparency and liquid permeability. When a droplet of Cu²⁺ aqueous solution is applied on the surface of the hydrogel film with gold coating, clear and sharp peaks in the spectrum appears, indicating the effective improvement of interference compared to the hydrogel film without gold coating (as shown in FIG. 19a). Such simple design with gold modification is proven to effectively produce a high signal- to-noise ratio in the optical sensing, allowing for the detection of liquidus analytes (as discussed in relation to FIG. 9). Importantly, this successfully expands the use cases of the platform to accommodate all forms of analytes.

[00128] As the hydrogel film is covalently bonded to the Si wafer substrate during the in-situ polymerization, the swelling of the hydrogel induces the thickness increase of the film. The film is washed, dried and swelled to a same initial thickness before applying different analyte solutions for multiple cycles of sensing tests. The thicknesses of the film before and after applying the analyte were measured by the reflective spectrum with an optical spectrometer under an illumination normal to the hydrogel surface. The thickness of the swollen hydrogel d was calculated using the Bragg’s Law 2nd = mA, where n is the refractive index of the hydrogel, m is a known integer, and l is the wavelength of the incident wave (detailed information, see Supporting Information).

[00129] FIG. 19b shows the complete reflective spectra for the Cu²⁺-specific hydrogel sensing a droplet of 10 pL with different concentrations of Cu²⁺. Each curve exhibits three peaks around 458.7 nm, 533.6 nm and 639.8 nm. These peaks shift towards shorter wavelength as the concentration of Cu²⁺ increases. Taking the 639.8-nm peak as an example (FIG. 2c), the peak shifts from 639.8 nm to 589.1 nm with different concentrations of Cu²⁺. Considering the resolution of the optical spectrometer used here is 1 .5 nm, only a peak shift above this resolution is considered as an effective sensing signal. Based on this, we achieved a wide detecting range of Cu²⁺ concentration from 10 ¹⁴ M to 10 ⁴ M. When a drop of 10 ¹⁴ M Cu²⁺ was applied on the film, the peak blue-shifted by 2.3 nm, still above the resolution of 1 .5 nm. According to the curve in FIG. 2c, the lowest concentration we can detect is about 1 .3 ^c 10 ¹⁵ M Cu²⁺, mainly limited by the resolution of the spectrometer used in this study (1 .5 nm). Without such instrument limit, theoretically such a sensing principle could allow the lowest LOD to reach 2.2* 10 ¹⁷ M on such a simple hydrogel thin-film system. The low limits of detection (LOD) of our sensor is thus far below the maximum tolerable concentration of Cu²⁺ in the standard drinking water (2 ^c 10 ⁵ M) established by the U.S. Environmental Protection Agency (EPA). The thickness of the swollen hydrogel film under each Cu²⁺ concentration was then calculated accordingly from the peak shift. The swelling ratio of the film was characterized as the ratio between the thickness of the swollen hydrogel and that of the dry gel (FIG. 2c). The swelling ratio decreases from 3.31 to 3.07 as the concentration changes from 10 ¹⁴ M to 10 ⁴ M, which confirms the effective additional crosslinks of hydrogel generated by the Cu²⁺-ligand complex.

[00130] The reliability of the detection was further enhanced by reading the shifts of all the three peaks. Four different concentrations of Cu²⁺ were chosen and all the three corresponding peak shifts (as discussed in relation to FIG. 14). The peak shifts are all higher than the resolution (1 .5 nm) for Cu²⁺ concentration as low as 1.0 ^c 10 ¹² M. The ultra-low detecting limit is again confirmed for the sensor platform. With rationally selected hydrogel thickness, multiple peaks can be obtained in one sensor at once, serving as a built-in sensor array and leading to more reliable sensing results.

[00131] Next, the specificity of the Cu²⁺ sensor was verified against 14 other interfering metal ions, including Ag⁺, Ba²⁺, Ca²⁺, Co²⁺, Cr³⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺ and Zn²⁺. These were tested by applying a mixture solution of all the 15 ions, which contains 1 .0 ^c10 ¹¹ M Cu²⁺ and other ions of 1 .0 ^c10 ⁹ M (two orders of magnitude higher than Cu²⁺ concentration). By high contrast, the peak shifts induced by all the other ions are significantly below the detection limit of the spectrometer and much smaller than the peak shift induced by Cu²⁺ (4.7 nm) (FIG. 19c & 19d). This suggests a high selectivity of embodiments of the sensor platform in identifying Cu²⁺, with great potential for real-life applications with various water or biofluid sources. The sensor can be recovered by being rinsed with or immersed in an acid solution.

[00132] The experimentally observed high selectivity and sensitivity of the sensor platform embodiment can be explained by its chemical-mechanical-optical signal transducing process. The binding constant between the ligand and Cu²⁺ (Log a = 12.6) is much higher than those between the ligand and other metal ions (Table 1 ). The high selectivity of the sensor towards Cu²⁺ essentially originates from such a strong binding. Furthermore, it is hypothesized that the high sensitivity is due to the localized concentration in the hydrogel, described as following. The total amount of ligands in the hydrogel is 10 ⁸ mole, measured by X-ray photoelectron spectroscopy (XPS) (FIG. 13). Considering the planar length and width of the hydrogel film is 1 .0 cm, and the thickness is on the order of 1 pm, the average concentration of ligands in the hydrogel matrix is about 0.77 M in the fully swollen hydrogel. For comparison, the concentration of Cu²⁺ in the 10-pL analyte droplet ranges from 10 ¹⁴ to 10 ⁴ M. Because of the much higher ligand concentration together with the strong ion-ligand binding, as soon as the 10-pL (i.e., 10¹⁰ pm³) Cu²⁺ solution is applied on the hydrogel film, the Cu²⁺ will be locally confined within a much smaller volume of the hydrogel and thus be significantly concentrated. This concentrated Cu²⁺ then causes a detectable thickness change of the hydrogel using the spectrometer, even when the initial concentration in the sensing droplet is as small as 10 ¹⁴ M. [00133] To verify this hypothesis that the majority of Cu²⁺ ions were concentrated locally near the area where the drop was applied, we fabricated the same hydrogel sensor with two gold spots on its surface. As shown in FIG. 20a, the two spots with 5.0 mm apart from each other were located on the same hydrogel film. After adding 10 pL of Cu²⁺ at 10 ¹¹ M at Spot 1 , both the spectra were recorded at the two spots. An evident peak shift was observed at Spot 1 (FIG. 20b), but no shift at Spot 2 (FIG. 20c). This proves that almost all the Cu²⁺ ions have been effectively absorbed by the hydrogel at Spot 1 and formed complexes locally, without diffusing and reaching to the hydrogel at Spot 2. This experimentally revealed the diffusion length and the effective sensing area for 10 pl_ analyte of 10 ¹¹ M Cu²⁺ is no more than 5.0 mm, which is the minimum distance that we can set the two sensing spots apart in the experimental condition.

[00134] To further estimate the more accurate approximate size of the hydrogel area where Cu²⁺ ions are mostly concentrated, it is assumed that after diffusion and binding, the Cu²⁺ is homogeneously distributed throughout the thickness of the hydrogel, and within a circular area surrounding the location where the drop has been applied. This area is called the effective area of binding for the sensor. Outside the effective area, it is assumed that there is no Cu²⁺. The diameter of the estimated effective area is 1.44 pm for initially 10 ¹⁴ M of Cu²⁺, and 94.2 pm for initially 10 ⁹ M of Cu²⁺ (FIG. 4d). The local concentration of Cu²⁺ in the effective area in the hydrogel is further estimated to reach as high as nearly 10⁹ times of the original concentration in the droplet of Cu²⁺ solution (FIG. 4d). This large effect of localization enhances the signal transduction from the chemical binding to the optical spectrum, by increasing the change of the hydrogel swelling ratio at the local detecting spot.

[00135] The effect of binding localization also provides advantage of small detecting size of our sensor. The minimum size required for the hydrogel sensor to detect the 10 mI_ solution can be estimated by the effective binding area. For example, to detect the Cu²⁺ concentration of 10 ¹⁴ M, a sensor of only 1.44 pm diameter is needed. The current hydrogel sensor has the potential to be further fabricated with smaller size but same sensitivity and selectivity. [00136] In addition to the high selectivity and sensitivity, the sub-micrometer film thickness and the optical detection ensure the fast response of our sensor. The binding between the ligands and Cu²⁺ takes only a few minutes. The diffusivity of ions and water molecules in a hydrogel is approximately D ~ 10 ¹⁰ m²/s. Taking the thickness of the hydrogel as h ~ 800 nm, we estimate the time scale of the diffusion is h²ID ~ 0.01 s. As a result, the whole sensing process takes a few minutes, limited by the reaction of binding. The response of the sensor is at least as fast as other current sensing methods for Cu²⁺.

[00137] These embodiments of the invention present ultra-high sensitivity to copper ion up to the picomole level (one trillionth of a mole), which is one to three orders of magnitude higher than that of the sensitivity of other current copper ion detection methods. Such embodiments may be used for applications such as water monitoring in the environment for detecting and real-time monitoring of metal ions and chemical compounds, such as hazardous, noble, heavy, rare earth, or radiative metal ions.

Example 2: Embodiments Implementing Biological Sensors

[00138] Owing to the broad chemistry, robust physics, and modular design of the hydrogel interferometer, this universal chemical-mechanical-optical platform can be readily customized to sense a broad range of molecules. Here its capability of sensing biological macromolecules, such as proteins, is demonstrated by linking specific functional ligands to the polymer chains of the hydrogel. Specifically, the hydrogel functionalized with phenylboronic acid (PBA) as the ligand can sense glycoproteins peroxidase from horseradish (HRP) (FIG. 21 a). The PBA ligands can bind diols in the glycoprotein, forming a 1 :1 complex. The increase of anionic boronate species in the hydrogel upon binding with glycoproteins leads to hydrogel swelling, due to electrostatic force and osmotic pressure of ions. The gel film swelling redshifts the reflection wavelength. FIG. 21 b illustrates reflection peak shifts of the glycoprotein sensor at different wavelengths with protein concentrations ranging from 1 .0 x10 ¹¹ mg/mL to 1.0 *1 O ⁶ mg/mL, and FIG. 21 c provides reflection peak shifts of the sensor embodiment with 1 .0 x10 ⁸ mg/mL of seven different interfering proteins (HRP, Cyt C, RNase A, Myo, b-lac A, b-cas, Hemo, and BSA), with the dashed line representing the spectrometer resolution. Further, FIG. 21 d illustrates reflection peak shifts of a sensor embodiment with different crosslinking density inside, where the protein concentrations range from 1.0 *1 O ¹⁰ mg/mL to 1.0 x10 ⁶ mg/mL.

Example 3: Embodiments Implementing Transparent Substrates

The hydrogel sensor can also be fabricated on various substrates, such as glass, polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS). These optically transparent substrates provide versatility of projecting and detecting optical signals from arbitrary sides of the sensor. To achieve this, thin layers of gold were coated on top of the hydrogel film, as well as between the film and the substrate (FIG. 22a). With glass as the substrate, the sensor showed a slight golden color induced by interference, but still highly transparent (FIG. 22b). The transparent sensor was selected by choosing different combinations of the projecting-detecting optical signals, including directions of top-top, top-bottom, bottom-bottom and bottom-top. Evident peak shifts were observed in all cases (FIG. 22c), indicating effective sensing of the analyte. This omni-directional sensing broadens the flexibility in applying the hydrogel interferometer platform on various usage scenarios, such as detecting chemicals in a box or room from outside, without entering the enclosed environment, and wearable sweat sensor which biocompatible and soft like tissue hydrogels can adhere on the human skins directly to avoid the uncomfortable feelings caused by the hard electronic devices and show the sensing signals on the opposite directions after absorbing the sweats.

Example 4: Embodiments Implementing Flexible Substrates

[00139] Flexible and stretchable sensors for wearable devices, such as human- wearable sensors for health monitoring and feedback sensors in soft robots, have been actively developed. The transparent sensor demonstrated here can also be made flexible and stretchable using PET or PDMS as the substrates. All materials including hydrogels can be susceptible to fatigue under cyclic loads. To preliminarily test the resistance to fatigue of the stretchable sensor, we bent or stretched the hydrogel-PET/PDMS sensor for 200 cycles, and measured the reflective spectrum of the hydrogel film. After 200 cycles, the reflective spectrum remained nearly the same as what was measured before the cyclic bending or stretching (FIGs. 23a and 23b). No obvious crack or flaw was observed on the surface of the hydrogel (FIGs. 23c and 23d). With the same concentration of Cu²⁺ applied, the peak shift of the sensor kept the same before and after the cyclic test. As a stretchable sensor, the effect of mechanical stretch on the hydrogel film thickness change can be decoupled from the effect of analyte by individually measuring the peak shift during stretch without Cu²⁺. The reflective peak shifted by about 100 nm when the tensile strain was 50% (FIG. 23e). Being integrated in parallel as an array, the stretchable sensors are capable of sensing both metal ions and mechanical deformation. These exemplary embodiments demonstrates that flexible sensor platforms are capable of retaining sensing ability even after bending and stretching, and may even allow for application or integration with wearable or other existing devices.

Example 5: Embodiments Implementing Volatile Sensors

[00140] In another exemplary embodiment, as illustrated in FIGs. 24a to 24f, a poly(2- hyroxyethyl methacrylate-co-acrylic acid) (poly(FIEMA-co-AAc)) hydrogel interferometer may be used as a volatile-vapor sensor, with ethanol as a model vapor. In this embodiment, a poly(FIEMA-co-AAc) film having 233 nm thickness at ambient condition exhibits an orange color, with the second-order destructive interference centered at 472 nm. Flowever, when exposed to ethanol vapor, the hydrogel film swells and rapidly changes to blue, with red-shift of the valley from 472 nm to 577 nm and the occurrence of the second-order constructive interference centered at 441 nm. As shown in FIG. 24a, the response and recovery time are 140 ms and 210 ms, respectively, for this embodiment. In comparison, other dynamic structural color systems typically exhibit response time of several seconds and even longer recovery time. Additionally, said embodiment demonstrates the robustness of the sensor; the response-recovery cycles can be carried out repeatedly with highly consistent performance. FIG. 24b illustrate high correlation of the experimental and theoretical hydrogel film thickness as a function of the partial pressure of ethanol in an exemplary embodiment applied to sense various concentrations of ethanol vapor. FIG. 24c illustrates a quantitative analysis of ethanol vapor concentration based on RGB values using an embodiment of the invention, with two analyte samples used for validation. The arrow indicates increase in partial pressure of ethanol. [00141] With yet other embodiments, by integrating hydrogel interferometers into a sensor array, multi-analyte recognition may be realized using only one hydrogel material of different thicknesses. FIG. 24d illustrates pattern-based recognition of multiple volatile organic compounds (VOCs) with a sensor array composed of poly(HEMA-co-AAc) of three different film thicknesses (234 nm for S1 , 290 nm for S2, 362 nm for S3) in accordance with embodiments of the invention. As shown in FIGS. 24d to 24f, in some exemplary embodiments, VOCs may be selected from a group consisting of acetic acid, acetone, ethanol, ethyl acetate, hexane, isopropanol (IPA), methanol, or tetrahydrofuran (THF), saturated with N2. FIG. 24d shows that the sensor array in this embodiment shows a distinct color pattern for each VOC due to different swelling behaviors of the hydrogel in different vapors. Additionally, FIG. 24e illustrates the specific RGB-based response pattern of the sensor array for each VOC in this embodiment, even for chemically similar analytes such as alcohols. As shown in FIG. 24f, linear discriminant analysis (LDA) with acquired data for one such embodiment shows significant separation of different VOC clusters on the 2D LDA score plot, suggesting successful discrimination.

Example 6: Embodiments Implementing Encryption Devices

[00142] Several exemplary embodiments of patterned devices based on poly(acrylamide-co-acrylic acid) (poly(AAm-co-AAc)) hydrogel were formed in accordance with embodiments. In various embodiments the process of forming information encryption devices, where data and information can be easily encoded in the adaptive color platform, according to some embodiments - after spin coating and short- time ultraviolet (UV) exposure of the precursor solution, partially polymerized film is formed; continuous UV exposure is then conducted with a photomask to enable spatially controlled polymerization. The resulting hydrogel thin film is uniform, serving as an information encryption device, as provided in FIG. 25a. In this embodiment, decryption is triggered by moisture, and the information is hidden once moisture is removed. The reversible encryption or decryption process may thus be triggered by breath or humidity in some embodiments, providing potential for rapid information acquisition and protection without any additional equipment. FIG. 25b shows thickness changes of the image (water droplet pattern) and surrounding areas with increased humidity in an embodiment of the invention, and FIG. 25c illustrates a humidity indicator based on a multicolor image pattern according to yet another embodiment.

Example 7: Embodiments Implementing Colorimetric Analysis

[00143] Colorimetric sensing, which transduces environmental changes into visible color changes, provides a simple yet powerful detection mechanism that is well-suited to the development of low-cost and low-power sensors. Multilayer interference has an inherently simpler structure, which can be adopted to sense various chemical solvents and vapors when structured in multilayers. The color can be reversibly altered by introducing or removing an analyte into or out from hydrogel layers. As an extension of the precious hydrogel interferometry, this multilayers interference design can provide naked-eye-perceivable color change in the visible range, negating the need for extra detectors by making environmental changes visible to the unaided eye. Here two samples are provided for this multilayer interference design. As shown in FIGs. 26a to 26c, the sensor showed a yellow color before adding Cu ions. However, it changed to blue color. Also, the reflection peak shifted to the blue region. In addition, two glucose-recognition methods were explored using proteins and boronic acid respectively, producing promising color changes, as shown in FIGs. 27a to 27c and 28a to 28c. As shown in FIGs. 27a to 27c, the sensor has a peak shift to the left and the color of the sensor became blue after adding the glucose. As shown in FIGs. 28a to 28c, the sensor has a peak shift to the left a little.

[00144] Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention. Various other embodiments are possible within its scope. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

Claims

CLAIMS:

1. A thin-film interferometer comprising:

a reflective substrate;

a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the reflective substrate, wherein the thin-film layer has at least one defined thickness, wherein the boundary between the reflective substrate and the thin-film layer defines a thin-film/substrate interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin-film interface and the thin-film layer/substrate interface;

wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer; and wherein the change in the thickness of the thin-film layer produces a change in the color in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the reflective substrate and the thickness of the thin-film layer.

2. The thin-film interferometer of claim 1 , wherein the highly porous cross-linked polymer network is formed of a hydrogel.

3. The thin-film interferometer of claim 2, wherein the hydrogel forms a scaffold onto which is bonded a plurality of ligands specific to a target analyte.

4. The thin-film interferometer of claim 3, wherein the mass ratio of ligand to hydrogel polymer is 1 to 5.

5. The thin-film interferometer of claim 3, wherein the binding of the target analyte with the ligand concentrates the analyte 10⁹ fold.

6. The thin-film interferometer of claim 1 , wherein the thin-film layer has a thickness of from 0.1 to 1 pm.

7. The thin-film interferometer of claim 1 , wherein the thin-film layer has a thickness comparable to the wavelength of visible light.

8. The thin-film interferometer of claim 1 , wherein the volume change of the think-film layer on exposure to the external stimulus is at least 100 fold.

9. The thin-film interferometer of claim 1 , wherein the stimulus is selected from the group consisting of humidity, temperature, light, mechanical stress, magnetic or electrical field, and specific chemical and biological molecules, including pH, metal ions, and anions, in both gaseous and liquidus forms.

10. The thin-film interferometer of claim 1 , wherein the thin-film layer is formed by mixing a plurality of at least one material selected form the group of pre-polymers, monomers, and functional molecules.

11. The thin-film interferometer of claim 1 , wherein the reflective substrate is formed of a Si wafer.

12. The thin-film interferometer of claim 1 , wherein the thin-film layer is homogeneous.

13. The thin-film interferometer of claim 1 , wherein the thin-film interferometer further comprises a colorimetric analyzer.

14. The thin-film interferometer of claim 13, wherein the colorimetric analyzer determines the concentration of a target analyte by reference to a correlation between a measured reflection spectra, the thin-film thickness and analyte concentration.

15. The thin-film interferometer of claim 14, wherein the colorimetric analyzer is configured to compare a sample image from a thin-film interferometer against a database of comparable interferometer images to determine analyte concentration.

16. The thin-film interferometer of claim 13, wherein the colorimetric analyzer outputs a four-dimensional matrix image wherein each dimension consists of data for each color channel and data on an image transparency value.

17. The thin-film interferometer of claim 1 , wherein the reflective substrate is formed of a flexible material.

18. The thin-film interferometer of claim 1 , further comprising a layer of reflective material disposed atop the thin-film layer.

19. The thin-film interferometer of claim 18, wherein the reflective material is Au and has a nanometer-scale thickness.

20. The thin-film interferometer of claim 3, wherein the thin-film layer is formed of a hydrogel, wherein the ligand is an imidazole, and wherein the analyte is Cu ions.

21. The thin-film interferometer of claim 20, wherein the hydrogel is formed of a poly(acrylamide-co-acrylic acid-co-/V-allylacrylamide) (poly(AAm-co-AAc-co- AAene)).

22. The thin-film interferometer of claim 20, further comprising a nanometer-scale thick Au layer disposed at the thin-film layer/substrate interface.

23. The thin-film interferometer of claim 3, wherein the volume of analyte is less than 10 pL.

24. The thin-film interferometer of claim 3, wherein the time scale of diffusion is 0.01 s.

25. The thin-film interferometer of claim 3, wherein the sensor has a picomole sensitivity to the analyte.

26. The thin-film interferometer of claim 3, wherein the analyte is a glycoprotein, and wherein the sensor has a sensitivity between 1.0 *10 ¹⁰ mg/mL to 1.0 *1 O ⁶ mg/mL.

27. The thin-film interferometer of claim 26, wherein the ligand is a phenylboronic acid.

28. The thin-film interferometer of claim 1 , wherein the sensor comprises an array of separate interferometer elements.

29. The thin-film interferometer of claim 28, wherein the separate interferometer elements are configured to detect different target analytes.

30. The thin-film interferometer of claim 29, wherein the target analytes are selected from the group of metal ions, volatile organic compounds and biological compounds.

31. A transmissive thin-film interferometer comprising:

a substrate assembly comprising a first transparent substrate and a reflective thin film, where the reflective thin film has a thickness configured to preserve the transparency of the substrate assembly and has a refractive index higher than 1 ;

a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the substrate assembly, wherein the thin-film layer has at least one defined thickness, wherein the boundary between the substrate assembly and the thin-film layer defines a thin-film layer/substrate assembly interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin-film interface and the thin-film/substrate assembly interface;

wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer;

wherein the change in the thickness of the thin-film layer produces a change in the color in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the substrate assembly and the thickness of the thin-film layer; and wherein the sensor is configured such that incoming light may be directed from one of either the thin-film or substrate sides of the sensor, and wherein the detection may occur from one of either the thin-film or substrate sides of the sensor.

32. The transmissive thin-film interferometer of claim 31 , wherein the reflective thin- film is disposed between one of either the substrate and the thin-film layer, or on the surface of the substrate opposite the thin-film layer.

33. The transmissive thin-film interferometer of claim 32, further comprising a second reflective thin-film disposed atop the thin-film layer.

34. The thin-film interferometer of claim 31 , wherein the highly porous cross-linked polymer network is formed of a hydrogel.

35. The thin-film interferometer of claim 34, wherein the hydrogel forms a scaffold onto which is bonded a plurality of ligands specific to a target analyte.

36. The thin-film interferometer of claim 31 , wherein the first transparent substrate is formed from glass or plastic.

37. The thin-film interferometer of claim 31 , wherein the first transparent substrate is formed from a flexible material selected from PET or PDMS.

38. An encryption device comprising:

a substrate;

a thin-film layer of a highly porous cross-linked polymer network covalently bonded on a first side to the reflective substrate, wherein the thin-film layer has at least two defined thicknesses, wherein the boundary between the reflective substrate and the thin-film layer defines a thin-film/substrate interface, and wherein the second side of the thin-film layer defines an air/thin-film interface; and

wherein at least a portion of the thin-film layer exhibits at least one color, said color arising from interference of light waves reflected between the air/thin- film interface and the thin-film layer/substrate interface;

wherein the thin-film layer is configured such that at least one external stimulus produces a change in the thickness of the thin-film layer; and

wherein the change in the thickness of the thin-film layer produces a differential change in the color of portions of the sensor in accordance with destructive and constructive interference determined by the refractive indices of air, the polymer network and the reflective substrate and the thicknesses of the thin-film layer such that an encryption key is reversibly displayed.

39. A method of forming a thin-film interferometer comprising:

providing a substrate;

spin-coating a highly porous cross-linked polymer network precursor in a layer having a fixed thickness atop the substrate;

polymerizing the highly porous cross-linked polymer network precursor in-situ atop the substrate such that it covalently binds thereto.

40. The method of claim 39, wherein the highly porous cross-linked polymer network is a hydrogel.

41. The method of claim 39, further comprising patterning the highly porous cross- linked polymer network atop the substrate by spatially controlled polymerization to form areas of the highly porous cross-linked polymer network having different thicknesses.

42. The method of claim 39, further comprising disposing a nano-scale reflective layer in one or both atop the highly porous cross-linked polymer network and between the highly porous cross-linked polymer network and the substrate.