Gradient Guided-Mode Resonance Biosensor with Smartphone Readout.

Integrating biosensors with smartphones is becoming an increasingly popular method for detecting various biomolecules and could replace expensive laboratory-based instruments. In this work, we demonstrate a novel smartphone-based biosensor system with a gradient grating period guided-mode resonance (GGP-GMR) sensor. The sensor comprises numerous gratings which each correspond to and block the light of a specific resonant wavelength. This results in a dark band, which is observed using a CCD underneath the GGP-GMR sensor. By monitoring the shift in the dark band, the concentration of a molecule in a sample can be determined. The sensor is illuminated by a light-emitting diode, and the light transmitted through the GGP-GMR sensor is directly captured by a smartphone, which then displays the results. Experiments were performed to validate the proposed smartphone biosensor and a limit of detection (LOD) of 1.50 × 10-3 RIU was achieved for sucrose solutions. Additionally, multiplexed detection was demonstrated for albumin and creatinine solutions at concentrations of 0-500 and 0-1 mg/mL, respectively; the corresponding LODs were 1.18 and 20.56 µg/mL.

Handheld Biosensor System Based on a Gradient Grating Period Guided-Mode Resonance Device.

Handheld biosensors have attracted substantial attention for numerous applications, including disease diagnosis, drug dosage monitoring, and environmental sensing. This study presents a novel handheld biosensor based on a gradient grating period guided-mode resonance (GGP-GMR) sensor. Unlike conventional GMR sensors, the proposed sensor's grating period varies along the device length; hence, the resonant wavelength varies linearly along the device length. If a GGP-GMR sensor is illuminated with a narrow band of light at normal incidence, the light resonates and reflects at a specific period but transmits at other periods; this can be observed as a dark band by using a complementary metal oxide semiconductor (CMOS) underneath the sensor. The concentration of a target analyte can be determined by monitoring the shift of this dark band. We designed and fabricated a handheld device incorporating a light-emitting diode (LED) light source, the necessary optics, an optofluidic chip with an embedded GGP-GMR sensor, and a CMOS. LEDs with different beam angles and bandpass filters with different full width at half maximum values were investigated to optimize the dark band quality and improve the accuracy of the subsequent image analysis. Substrate materials with different refractive indices and waveguide thicknesses were also investigated to maximize the GGP-GMR sensor's figure of merit. Experiments were performed to validate the proposed handheld biosensor, which achieved a limit of detection (LOD) of 1.09 × 10-3 RIU for bulk solution measurement. The sensor's performance in the multiplexed detection of albumin and creatinine solutions at concentrations of 0-500 µg/mL and 0-10 mg/mL, respectively, was investigated; the corresponding LODs were 0.66 and 0.61 µg/mL.

A Compact Detection Platform Based on Gradient Guided-Mode Resonance for Colorimetric and Fluorescence Liquid Assay Detection.

This paper presents a compact spectral detection system for common fluorescent and colorimetric assays. This system includes a gradient grating period guided-mode resonance (GGP-GMR) filter and charge-coupled device. In its current form, the GGP-GMR filter, which has a size of less than 2.5 mm, can achieve a spectral detection range of 500-700 nm. Through the direct measurement of the fluorescence emission, the proposed system was demonstrated to detect both the peak wavelength and its corresponding intensity. One fluorescent assay (albumin) and two colorimetric assays (albumin and creatinine) were performed to demonstrate the practical application of the proposed system for quantifying common liquid assays. The results of our system exhibited suitable agreement with those of a commercial spectrometer in terms of the assay sensitivity and limit of detection (LOD). With the proposed system, the fluorescent albumin, colorimetric albumin, and colorimetric creatinine assays achieved LODs of 40.99 and 398 and 25.49 mg/L, respectively. For a wide selection of biomolecules in point-of-care applications, the spectral detection range achieved by the GGP-GMR filter can be further extended and the simple and compact optical path configuration can be integrated with a lab-on-a-chip system.

Biosensor based on two-dimensional gradient guided-mode resonance filter.

A novel biosensor based on a two-dimensional gradient (TDG) guided-mode resonance (GMR) filter was introduced in this study. The TDG-GMR is demarcated in terms of the gradient grating period (GGP) in one dimension and gradient waveguide thickness (GWT) in the other dimension. A single compact sensor can combine these two features to simultaneously provide a broad detection range through GGP and high resolution through GWT. A detection range of 0.109 RIU (0%-60% sucrose content) with a limit of detection of 5.62 × 10-4 was demonstrated in this study by using a TDG-GMR with a size of 140.8 × 125.4 µm2. This value cannot be achieved using one dimensional gradient GMR sensor. Label-free (LF) biomolecule detection through TDG-GMR was also experimentally demonstrated in a model assay of albumin. The result confirms that the GWT-GMR provides a better resolution, whereas the GGP-GMR provides a broader detection range. A device for multiplex measurement could be easily implemented with a compact sensor chip and a simple readout directly from a charge-coupled device. This system would require a narrow-band source such as a light emitting diode or a laser diode, in addition to a limited number of other components such as a polarizer and a collimator. The proposed TDG-GMR could easily be integrated with smartphones and portable devices.

Gradient Waveguide Thickness Guided-Mode Resonance Biosensor.

Portable systems for detecting biomolecules have attracted considerable attention, owing to the demand for point-of-care testing applications. This has led to the development of lab-on-a-chip (LOC) devices. However, most LOCs are developed with a focus on automation and preprocessing of samples; fluorescence measurement, which requires additional off-chip detection instruments, remains the main detection method in conventional assays. By incorporating optical biosensors into LOCs, the biosensing system can be simplified and miniaturized. However, many optical sensors require an additional coupling device, such as a grating or prism, which complicates the optical path design of the system. In this study, we propose a new type of biosensor based on gradient waveguide thickness guided-mode resonance (GWT-GMR), which allows for the conversion of spectral information into spatial information such that the output signal can be recorded on a charge-coupled device for further analysis without any additional dispersive elements. A two-channel microfluidic chip with embedded GWT-GMRs was developed to detect two model assays in a buffer solution: albumin and creatinine. The results indicated that the limit of detection for albumin was 2.92 µg/mL for the concentration range of 0.8-500 µg/mL investigated in this study, and that for creatinine it was 12.05 µg/mL for the concentration range of 1-10,000 µg/mL. These results indicated that the proposed GWT-GMR sensor is suitable for use in clinical applications. Owing to its simple readout and optical path design, the GWT-GMR is considered ideal for integration with smartphones or as miniaturized displays in handheld devices, which could prove beneficial for future point-of-care applications.

A Cylindrical Ion Sensor Tip with a Diameter of 1.5 mm for Potentially Invasive Medical Application.

A fine cylindrical chemical sensor tip is developed with optical fiber in the core, surrounded by a transparent cylinder of photopolymer Norland Optical Adhesive 61 (NOA 61), and covered by a polymer hydrogel mixed with sensing molecules. The overall diameter is as small as 1.5 mm. pH response is demonstrated using two approaches of sensing materials: (i) absorbing probe Phenol Red mixed with Rhodamine 6G fluorescent dye and (ii) 8-hydroxypyrene-1,3,6-trisulfonic acid fluorescent probe. Both the optical excitation and fluorescence signal collection are through the optical fibers. A time resolution of 10 s is achieved for pH variations. Good linearity is observed in the physiological range from pH 7.0 to pH 8.6 with reversible and reproducible outcomes. For in vitro urea measurement, the sensor tip can distinguish 1, 3, and 5 mM urea solution, which is a crucial range in saliva urea concentration. The miniaturized tip with such simple cylindrical symmetry is designed to detect vital signs during minimally invasive surgeries and can be potentially accompanied with endoscopes to enter human bodies.

Hydrogel-based zinc ion sensor on optical fiber with high resolution and application to neural cells.

Solid-state zinc ion sensor is developed with high enough resolution and reproducibility for the potential application in brain injury monitoring. An optical diffuser is incorporated into the zinc ion sensor based on optical fiber and hydrogel doped with the fluorescent zinc ion probe molecule meso-2,6-Dichlorophenyltripyrrinone (TPN-Cl2). The diffuser transforms the high-peak-intensity excitation light near the fiber end into a broad light with moderate local intensity to reduce the degradation of the probe molecule. Reversible detection can be reached for 1, 2, and 5 µM (10-6 Molar), with slopes 0.3, 0.6, and 0.8 respectively. This is the pathophysiological concentration range after brain injury. The sensor is applied to neuron-glial cultures and macrophage under the stimulation of lipopolysaccharide (LPS), KCl and oxygen/glucose deprivation (OGD) that reflect inflammation, depolarization and ischemia respectively, mimicking events after brain injury. The zinc ion level is raised to 4-5 µM after LPS treatment, and then reduced to <3 µM after the co-treatment with the herbal drug silymarin. The results suggest the conditions of the neural cells under stress can be monitored.

Real-time CRP detection from whole blood using micropost-embedded microfluidic chip incorporated with label-free biosensor.

We demonstrate the detection of C-creative protein (CRP) from whole blood samples without sample pretreatment by using a lab-on-a-chip system consisting of a microfluidic chip and a label-free biosensor. The microfluidic chip includes an array of microposts for filtering blood cells and allows only plasma to flow through and reach the guided-mode resonance (GMR) biosensor for real-time monitoring. The developed GMR sensor can achieve a bulk sensitivity of 186 nm RIU-1, which supports a limit of detection of 3.2 ng mL-1 for recombinant CRP spiked in human serum. The results are comparable with those obtained using enzyme-linked immunosorbent assay. In addition, we demonstrate the efficacy of filtration of blood cells using microposts and simultaneous measurement of CRP concentration using a GMR sensor by using whole blood and plasma samples.

Compact spectrometer system based on a gradient grating period guided-mode resonance filter.

We demonstrate a compact spectrometer system by using a gradient grating period guided-mode resonance filter-mounted on a linear photodetector array-that exhibits spatially dependent resonance characteristics; a specific incident wavelength is reflected such that the underlying array pixels measure minimum intensity. A precalibrated transmission efficiency matrix is used to determine each pixel's transmission efficiency for specific wavelengths. Unknown spectral information can be calculated from the measured intensity. Grating periods of 250-388 nm in 2-nm increments are used in each 100-cycle period. Device length is 2.23 mm. Spectral range of 506-700 nm is measurable with 1-nm resolution.

Integration of a guided-mode resonance filter with microposts for in-cell protein detection.

We present an integrated microfluidic system consisting of a label-free biosensor of a guided-mode resonance filter (GMRF) and a microfluidic channel with a micropost filter. The GMRF was fabricated through replica molding using an ultraviolet-curable polymer and a plastic substrate. An array of microposts (a diameter and height of 26.5 and 56 µm, respectively, and a spacing between 7.5 and 9.5 µm), fabricated on a silicon substrate through photolithography, was used as the filter. A double-sided tape was used to laminate the GMRF and a microfluidic chip such that the integrated device provides two functions: filtration of the cell debris and quantification of the in-cell protein concentration. By measuring the changes in the resonant wavelength from the GMRF, the detection of ß-actin in an unprocessed lysed cell sample was demonstrated; the cell debris was separated using the micropost filter to prevent false measurement.

Photonic crystal micropost as a microarray platform.

This study demonstrates a photonic crystal micropost (PCMP) substrate for microarray applications. The substrate comprises an array of circular MPs with a PC on top of these MPs. This substrate enables biomolecule-containing droplets to form a composite contact upon deposition, thus allowing biomolecules to be attached on only the MPs, forming spots. When the device (PC) is excited on resonance, the electric field intensity is enhanced on only the top surface of the MPs. This enables the fluorescence intensities to be enhanced up to 5.50x; principally, this enhancement does not engender an increase in the background (intensity outside MP or spots) and noise intensities. The PCMP substrate enhances the spot intensity and minimizes the background intensity, enabling the detection of lower concentration analytes.

Photonic crystal-enhanced fluorescence through the extraction of dually polarized modes.

In this Letter, we demonstrate that fluorescence detection by using surface-bound fluorescent molecules can be enhanced by coupling the fluorescence emission with dually polarized resonant modes of a photonic crystal (PC) substrate. The PC was fabricated through nanoreplica molding by using a plastic substrate. It was designed such that the transverse magnetic (TM)- and transverse electric (TE)-polarized resonance coincided at a specific combination of the incident angle and the illumination wavelength. During excitation, the nonpolarized emission from cyanine-5 was simultaneously coupled with the TE and TM resonant modes and reradiated into the detection instrument, increasing the fluorescence collection efficiency and thus, enhancing the fluorescence detection.

SU8 inverted-rib waveguide Bragg grating filter.

A polymeric SU8 inverted-rib waveguide Bragg grating filter fabricated using reactive ion etching (RIE) and solvent assisted microcontact molding (SAMIM) is presented. SAMIM is one kind of soft lithography. The technique is unique in that a composite hard-polydimethysiloxane/polydimethysiloxane stamp is used to transfer the grating pattern onto an inverted SU8 rib waveguide system. The composite grating stamp can be used repeatedly several times without degradation. Using this stamp and inverter-rib waveguide structure, the Bragg grating filter fabrication can be significantly simplified. The experiment result shows an attenuation dip in the transmission spectra, with a value of -7 dBm at 1550 nm for a grating with a period of 0.492 µm on an inverted-rib waveguide with 6.6 µm width and 4 µm height.

Line-scanning detection instrument for photonic crystal enhanced fluorescence.

A laser line-scanning instrument was developed to optimize the near-field enhancement capability of a one-dimensional photonic crystal (PC) for excitation of surface-bound fluorophores. The excitation laser beam is shaped into an 8 µm × 1 mm line that is focused along the direction of the PC grating, while remaining collimated perpendicular to the grating. Such a beam configuration offers high excitation power density while simultaneously providing high resonant coupling efficiency from the laser to the PC surface. Using a panel of 21 immunofluorescence assays on the PC surface in a microarray format, the approach achieves an enhancement factor as high as 90-fold between on-resonance and off-resonance illumination. The instrument provides a capability for sensitive and inexpensive analysis of cancer biomarkers in clinical applications.

Multiplexed cancer biomarker detection using quartz-based photonic crystal surfaces.

A photonic crystal (PC) surface is demonstrated as a high-sensitivity platform for detection of a panel of 21 cancer biomarker antigens using a sandwich enzyme-linked immunosorbent assay (ELISA) microarray format. A quartz-based PC structure fabricated by nanoimprint lithography, selected for its low autofluorescence, supports two independent optical resonances that simultaneously enable enhancement of fluorescence detection of biomarkers and label-free quantification of the density of antibody capture spots. A detection instrument is demonstrated that supports fluorescence and label-free imaging modalities, with the ability to optimize the fluorescence enhancement factor on a pixel-by-pixel basis throughout the microarray using an angle-scanning approach for the excitation laser that automatically compensates for variability in surface chemistry density and capture spot density. Measurements show that the angle-scanning illumination approach reduces the coefficient of variation of replicate assays by 20-99% compared to ordinary fluorescence microscopy, thus supporting reduction in limits of detectable biomarker concentration. Using the PC resonance, biomarkers in mixed samples were detectable at the lowest concentrations tested (2.1-41 pg/mL), resulting in a three-log range of quantitative detection.

Spatially selective photonic crystal enhanced fluorescence and application to background reduction for biomolecule detection assays.

By combining photonic crystal label-free biosensor imaging with photonic crystal enhanced fluorescence, it is possible to selectively enhance the fluorescence emission from regions of the PC surface based upon the density of immobilized capture molecules. A label-free image of the capture molecules enables determination of optimal coupling conditions of the laser used for fluorescence imaging of the photonic crystal surface on a pixel-by-pixel basis, allowing maximization of fluorescence enhancement factor from regions incorporating a biomolecule capture spot and minimization of background autofluorescence from areas between capture spots. This capability significantly improves the contrast of enhanced fluorescent images, and when applied to an antibody protein microarray, provides a substantial advantage over conventional fluorescence microscopy. Using the new approach, we demonstrate detection limits as low as 0.97 pg/ml for a representative protein biomarker in buffer.

Application of photonic crystal enhanced fluorescence to cancer biomarker microarrays.

We report on the use of photonic crystal surfaces as a high-sensitivity platform for detection of a panel of cancer biomarkers in a protein microarray format. The photonic crystal surface is designed to provide an optical resonance at the excitation wavelength of cyanine-5 (Cy5), thus providing an increase in fluorescent intensity for Cy5-labeled analytes measured with a confocal microarray scanner, compared to a glass surface. The sandwich enzyme-linked immunosorbent assay (ELISA) is undertaken on a microarray platform to undertake a simultaneous, multiplex analysis of 24 antigens on a single chip. Our results show that the resonant excitation effect increases the signal-to-noise ratio by 3.8- to 6.6-fold, resulting in a decrease in detection limits of 6-89%, with the exact enhancement dependent upon the antibody-antigen interaction. Dose-response characterization of the photonic crystal antibody microarrays shows the capability to detect common cancer biomarkers in the <2 pg/mL concentration range within a mixed sample.

Photobleaching on photonic crystal enhanced fluorescence surfaces.

The effect of resonant fluorescent enhancement from a photonic crystal surface upon the fluorescent photobleaching rate of Cyanine-5 labeled protein has been investigated. We show that the enhanced excitation mechanism for photonic crystal enhanced fluorescence, in which the device surface resonantly couples light from an excitation laser, accelerates photobleaching in proportion to the coupling efficiency of the laser to the photonic crystal. We also show that the enhanced extraction mechanism, in which the photonic crystal directs emitted photons approximately normal to the surface, does not play a role in the rate of photobleaching. We show that the photobleaching rate of dye molecules on the photonic crystal surface is accelerated by 30x compared to an ordinary glass surface, but substantial signal gain is still evident, even after extended periods of continuous illumination at the resonant condition.

Photonic crystal enhanced fluorescence using a quartz substrate to reduce limits of detection.

A Photonic Crystal (PC) surface fabricated upon a quartz substrate using nanoimprint lithography has been demonstrated to enhance light emission from fluorescent molecules in close proximity to the PC surface. Quartz was selected for its low autofluorescence characteristics compared to polymer-based PCs, improving the detection sensitivity and signal-to-noise ratio (SNR) of PC Enhanced Fluorescence (PCEF). Nanoimprint lithography enables economical fabrication of the subwavelength PCEF surface structure over entire 1x3 in2 quartz slides. The demonstrated PCEF surface supports a transverse magnetic (TM) resonant mode at a wavelength of λ = 632.8 nm and an incident angle of θ = 11°, which amplifies the electric field magnitude experienced by surface-bound fluorophores. Meanwhile, another TM mode at a wavelength of λ = 690 nm and incident angle of θ = 0° efficiently directs the fluorescent emission toward the detection optics. An enhancement factor as high as 7500 × was achieved for the detection of LD-700 dye spin-coated upon the PC, compared to detecting the same material on an unpatterned glass surface. The detection of spotted Alexa-647 labeled polypeptide on the PC exhibits a 330 × SNR improvement. Using dose-response characterization of deposited fluorophore-tagged protein spots, the PCEF surface demonstrated a 140 × lower limit of detection compared to a conventional glass substrate.

Large-core single-mode rib SU8 waveguide using solvent-assisted microcontact molding.

This paper describes a novel fabrication technique for constructing a polymer-based large-core single-mode rib waveguide. A negative tone SU8 photoresist with a high optical transmission over a large wavelength range and stable mechanical properties was used as a waveguide material. A waveguide was constructed by using a polydimethylsiloxane stamp combined with a solvent-assisted microcontact molding technique. The effects on the final pattern's geometry of four different process conditions were investigated. Optical simulations were performed using beam propagation method software. Single-mode beam propagation was observed at the output of the simulated waveguide as well as the actual waveguide through the microscope image.