1-D and 2-D photonic crystals as optical methods for amplifying biomolecular recognition

Biosensing platforms based on silicon nanostructures: A critical review

Biosensors are revolutionizing the health-care systems worldwide, permitting to survey several diseases, even at their early stage, by using different biomolecules such as proteins, DNA, and other biomarkers. However, these sensing approaches are still scarcely diffused outside the specialized medical and research facilities. Silicon is the undiscussed leader of the whole microelectronics industry, and novel sensors based on this material may completely change the health-care scenario. In this review, we will show how novel sensing platforms based on Si nanostructures may have a disruptive impact on applications with a real commercial transfer. A critical study for the main Si-based biosensors is herein presented with a comparison of their advantages and drawbacks. The most appealing sensing devices are discussed, starting from electronic transducers, with Si nanowires field-effect transistor (FET) and porous Si, to their optical alternatives, such as effective optical thickness porous silicon, photonic crystals, luminescent Si quantum dots, and finally luminescent Si NWs. All these sensors are investigated in terms of working principle, sensitivity, and selectivity with a specific focus on the possibility of their industrial transfer, and which ones may be preferred for a medical device.

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.

Emission Enhancement of Fluorescent Molecules by Antireflective Arrays

Traditional fluorescence enhancement based on a match of the maximum excitation or emission of fluorescence molecule with the spectra of the nanostructure can hardly enhance blue and red fluorescent molecules. Here, an enhanced method which is a new strategy based on the antireflective array has been developed to enhance the emission of blue and red fluorescent molecules. The fluorescence emission is enhanced by increasing the absorption at excitation wavelengths of the fluorescent molecules and reducing the fluorescent energy dissipation with an antireflective array. By introducing the antireflective arrays, the emission enhancement of blue and red fluorescent molecules is, respectively, up to 14 and 18 fold. It is a universal and effective strategy for enhancing fluorescence emission, which could be applied to enhance the intensity of organic LED and imaging.

Numerical study of sensitivity enhancement in a photonic crystal microcavity biosensor due to optical forces

Photonic crystal microcavity biosensors can detect single biomolecules, but reliance on diffusion from microfluidic flow for particle delivery limits the minimum detectable particle concentration. Here the particle equation of motion is solved to find the sensitivity enhancement due to optical forces. The enhancement is examined for a range of parameters, including input optical power, fluid flow rate, device quality factor, and particle size.

A label-free, multiplex competitive assay for small molecule pollutants

Understanding the amount of exposure individuals have had to common chemical pollutants critically requires the ability to detect those compounds in a simple, sensitive, and specific manner. Doing so using label-free biosensor technology has proven challenging, however, given the small molecular weight of many pollutants of interest. To address this issue, we report the development of a pollutant microarray based on the label-free arrayed imaging reflectometry (AIR) detection platform. The sensor is able to detect three common environmental contaminants (benzo[a]pyrene, bisphenol A, and acrolein) in human serum via a competitive binding scheme.

Label-Free Biosensor Imaging on Photonic Crystal Surfaces

We review the development and application of nanostructured photonic crystal surfaces and a hyperspectral reflectance imaging detection instrument which, when used together, represent a new form of optical microscopy that enables label-free, quantitative, and kinetic monitoring of biomaterial interaction with substrate surfaces. Photonic Crystal Enhanced Microscopy (PCEM) has been used to detect broad classes of materials which include dielectric nanoparticles, metal plasmonic nanoparticles, biomolecular layers, and live cells. Because PCEM does not require cytotoxic stains or photobleachable fluorescent dyes, it is especially useful for monitoring the long-term interactions of cells with extracellular matrix surfaces. PCEM is only sensitive to the attachment of cell components within ~200 nm of the photonic crystal surface, which may correspond to the region of most interest for adhesion processes that involve stem cell differentiation, chemotaxis, and metastasis. PCEM has also demonstrated sufficient sensitivity for sensing nanoparticle contrast agents that are roughly the same size as protein molecules, which may enable applications in "digital" diagnostics with single molecule sensing resolution. We will review PCEM's development history, operating principles, nanostructure design, and imaging modalities that enable tracking of optical scatterers, emitters, absorbers, and centers of dielectric permittivity.

Discrimination of "specific" and "nonspecific" binding in two-dimensional photonic crystals

In this paper, we propose a design for a 2D slab photonic crystal (PhC) virus sensor and an associated signal analysis methodology that together enable single-virus detection while rejecting false positives that occur due to non-specific interactions of serum proteins and small molecules with the sensor surface. The slab-PhC design takes advantage of both the optical and geometrical properties of its incorporated structures by physically limiting virus infiltration to only the most sensitive region of the PhC sensor, while allowing simultaneous measurement of both site-selective virus infiltration and non-specific small molecule accumulation across the device. Notably, the proposed sensor transducer is compatible with both standard semiconductor fabrication procedures and lab-on-a-chip style microfluidic delivery systems. 3D finite-difference time-domain electromagnetic field computation results are presented, the outcomes of which indicate that both specific (target) virus capture and non-specific (non-target) binding can be simultaneously measured and discerned from one another. This type of capacity for background-corrected, single-pathogen target detection would provide a new and novel advancement toward sensitive, label-free virus diagnostics.

Two-dimensional photonic crystals for sensitive microscale chemical and biochemical sensing

Photonic crystals - optical devices able to respond to changes in the refractive index of a small volume of space - are an emerging class of label-free chemical- and bio-sensors. This review focuses on one class of photonic crystal, in which light is confined to a patterned planar material layer of sub-wavelength thickness. These devices are small (on the order of tens to hundreds of microns square), suitable for incorporation into lab-on-a-chip systems, and in theory can provide exceptional sensitivity. We introduce the defining characteristics and basic operation of two-dimensional photonic crystal sensors, describe variations of their basic design geometry, and summarize reported detection results from chemical and biological sensing experiments.

A photonic crystal based sensing scheme for acetylcholine and acetylcholinesterase inhibitors

A 2-layer approach towards optical sensing of acetylcholine and detection of an acetylcholinesterase inhibitor using photonic crystal sensor technology.

Detection of single nano-defects in photonic crystals between crossed polarizers

We investigate, by simulations and experiments, the light scattering of small particles trapped in photonic crystal membranes supporting guided resonance modes. Our results show that, due to amplified Rayleigh small particle scattering, such membranes can be utilized to make a sensor that can detect single nano-particles. We have designed a biomolecule sensor that uses cross-polarized excitation and detection for increased sensitivity. Estimated using Rayleigh scattering theory and simulation results, the current fabricated sensor has a detection limit of 26 nm, corresponding to the size of a single virus. The sensor can potentially be made both cheap and compact, to facilitate use at point-of-care.

Silica colloidal crystals as emerging materials for high-throughput protein electrophoresis

Silica colloidal crystals are a new type of media for protein electrophoresis, and they are assessed for their promise in rapidly measuring aggregation of monoclonal antibodies. The nature of silica colloidal crystals is described in the context of the need for a high-throughput separation tool for optimizing the formulations of protein drugs for minimal aggregation. The fundamental relations between molecular weight and mobility in electrophoresis are used to make a theoretical comparison of selectivity between gels and colloidal crystals. The results show that the selectivity is similar for these media, but slightly higher, 10%, for gels, and the velocity is inherently lower than that for gels due to the smaller free volume fraction. These factors are more than compensated for by lower broadening in colloidal crystals. These new media give plate heights of only 0.15 μm for the antibody monomer and 0.42 μm for the antibody dimer. The monoclonal antibody is separated from its dimer in 72 s over a distance of only 6.5 mm. This is five times faster than size-exclusion chromatography, with more than tenfold miniaturization, and amenable to parallel separations, all of which are promising for the design of high-throughput devices for optimizing protein drug formulations.

Selective virus detection in complex sample matrices with photonic crystal optical cavities

Rapid, sensitive, and selective detection of viruses is critical for applications in medical diagnostics, biosecurity, and environmental safety. In this article, we report the application of a point-defect-coupled W1 photonic crystal (PhC) waveguide biosensor to label-free optical detection of viruses. Fabricated on a silicon-on-insulator (SOI) substrate using electron-beam (e-beam) lithography and reactive-ion-etching, the PhC sensing platform allows optical detection based on resonant mode shifts in response to ambient refractive index changes produced by infiltration of target biomaterial within the holes of the PhC structure. Finite difference time domain (FDTD) calculations were performed to assist with design of the sensor, and to serve as a theoretical benchmark against which experimental results could be compared. Using Human Papillomavirus virus-like particles (VLPs) spiked in 10% fetal bovine serum as a model system, we observed a limit of detection of 1.5 nM in simple (buffer only) or complex (10% serum) sample matrices. The use of anti-VLP antibodies specific for intact VLPs with the PhC sensors provided highly selective VLP detection.