Optical Biosensing Platform Research Articles

Embedded Plasmonic Nanomenhirs as Location-Specific Biosensors

We introduce a novel optical biosensing platform that exploits the asymmetry of nanostructures embedded in nanocavities, termed nanomenhirs. Upon oblique illumination using plane polarized white light, two plasmonic resonances attributable to the bases and the axes of the nanomenhirs emerge; these are used for location-specific sensing of membrane-binding events. Numerical simulations of the near field distributions confirmed the experimental results. As a proof-of-concept, we present a model biosensing experiment that exploits the dual-sensing capability, the size selectivity offered by the sensor geometry, and the possibility to separately biochemically modify the nanomenhirs and the nanocavities for the specific binding of lipid membrane structures to the nanomenhirs.

Planar lens integrated capillary action microfluidic immunoassay device for the optical detection of troponin I

Optical based analysis in microfluidic and lab-on-a-chip systems are currently considered the gold standard methodology for the determination of end point reactions for various chemical and biological reaction processes. Typically, assays are performed using bulky ancillary apparatus such as microscopes and complex optical excitation and detection systems. Such instrumentation negates many of the advantages offered by device miniaturisation, particularly with respect to overall portability. In this article, we present a CO2 laser ablation technique for rapidly prototyping on-chip planar lenses, in conjunction with capillary action based autonomous microfluidics, to create a miniaturised and fully integrated optical biosensing platform. The presented self-aligned on-chip optical components offer an efficient means to direct excitation light within microfluidics and to directly couple light from a LED source. The device has been used in conjunction with a miniaturised and bespoke fluorescence detection platform to create a complete, palm sized system (≈60 × 80 × 60 mm) capable of performing fluoro-immunoassays. The system has been applied to the detection of cardiac Troponin I, one of the gold standard biomarkers for the diagnosis of acute myocardial infarction, achieving a lower detection limit of 0.08 ng/ml, which is at the threshold of clinically applicable concentrations. The portable nature of the complete system and the biomarker detection capabilities demonstrate the potential of the devised instrumentation for use as a medical diagnostics device at the point of care.

Graphene-based nanoprobes and a prototype optical biosensing platform

Biochemical and biomedical applications of graphene are critically dependent on the interaction between biomolecules and the nanomaterial. In this work, we developed a graphene-based signal-amplification nanoprobe by combining anti-immunoglobulin G (anti-IgG) and horseradish peroxidase (HRP) with graphene oxide (GO). The structure and function of HRP in the nano-interface of GO were firstly investigated, which demonstrated that the enzyme retained 78% of its native activity and 77% of its native α-helix content. HRP and anti-IgG were then co-adsorbed onto GO to form bifunctional nanoprobes. The nanoprobes provide both improved binding ability and signal-amplification ability. Comparing with conventional bioconjugates such as enzyme-linked antibody, co-adsorption could avoid chemical conjugation between biomolecules, keeping their bioactivity well. As an example for their application, the nanoprobes were used to obtain amplified signals in a sandwich-type immunoassay for cancer marker, instead of conventional colorimetric conjugates. This approach provided a detection limit of 10pg/mL alpha-fetoprotein (AFP), which was much more sensitive than conventional enzyme-linked immunosorbent assay (ELISA) methods. The easily fabricated GO-based nanoprobes have the potential to become universal probes for molecular diagnostics.

Control of density and LSPR of Au nanoparticles on graphene

In this study, we introduce the tunable density and localized surface plasmon resonance (LSPR) of plasmonic gold (Au) nanoparticles which were formed on monolayer graphene at room temperature, based on the difference of the reduction potential between graphene and the Au3+ precursor. The size of the Au nanoparticles was ∼40 nm, which is very desirable to provide an optical enhancement effect by LSPR in the full visible range. It is demonstrated that the density of the Au nanoparticles was modulated by the surface energy of the graphene on the substrate as well as the concentration of the Au3+ precursor. Furthermore, the cycle number of the reduction process strongly affected the distribution of the nanoparticle size and their optical properties. The LSPR of the plasmonic Au nanoparticles was red-shifted from 560 to 620 nm and its full width at half maximum broadened as the Au3+ precursor concentration was increased and the cyclic reduction process progressed. Based on the optical enhancement of the plasmonic Au nanoparticles and the extraordinary physical characteristics of graphene, the Au/graphene assembly may offer a promising optoelectronic platform for next-generation flexible optical electronics or biosensors.

Slotted Photonic Crystal Sensors

Optical biosensors are increasingly being considered for lab-on-a-chip applications due to their benefits such as small size, biocompatibility, passive behaviour and lack of the need for fluorescent labels. The light guiding mechanisms used by many of them results in poor overlap of the optical field with the target molecules, reducing the maximum sensitivity achievable. This review article presents a new platform for optical biosensors, namely slotted photonic crystals, which provide higher sensitivities due to their ability to confine, spatially and temporally, the optical mode peak within the analyte itself. Loss measurements showed values comparable to standard photonic crystals, confirming their ability to be used in real devices. A novel resonant coupler was designed, simulated, and experimentally tested, and was found to perform better than other solutions within the literature. Combining with cavities, microfluidics and biological functionalization allowed proof-of-principle demonstrations of protein binding to be carried out. Higher sensitivities were observed in smaller structures than possible with most competing devices reported in the literature. This body of work presents slotted photonic crystals as a realistic platform for complete on-chip biosensing; addressing key design, performance and application issues, whilst also opening up exciting new ideas for future study.

Organic Silicone Sol–Gel Polymer as a Noncovalent Carrier of Receptor Proteins for Label-Free Optical Biosensor Application

Optical biosensing techniques have become of key importance for label-free monitoring of biomolecular interactions in the current proteomics era. Together with an increasing emphasis on high-throughput applications in functional proteomics and drug discovery, there has been demand for facile and generally applicable methods for the immobilization of a wide range of receptor proteins. Here, we developed a polymer platform for microring resonator biosensors, which allows the immobilization of receptor proteins on the surface of waveguide directly without any additional modification. A sol-gel process based on a mixture of three precursors was employed to prepare a liquid hybrid polysiloxane, which was photopatternable for the photocuring process and UV imprint. Waveguide films were prepared on silicon substrates by spin coating and characterized by atomic force microscopy for roughness, and protein adsorption. The results showed that the surface of the polymer film was smooth (rms = 0.658 nm), and exhibited a moderate hydrophobicity with the water contact angle of 97°. Such a hydrophobic extent could provide a necessary binding strength for stable immobilization of proteins on the material surface in various sensing conditions. Biological activity of the immobilized Staphylococcal protein A and its corresponding biosensing performance were demonstrated by its specific recognition of human Immunoglobulin G. This study showed the potential of preparing dense, homogeneous, specific, and stable biosensing surfaces by immobilizing receptor proteins on polymer-based optical devices through the direct physical adsorption method. We expect that such polymer waveguide could be of special interest in developing low-cost and robust optical biosensing platform for multidimensional arrays.

Single laboratory validation of an optical biosensor method for the determination of folate in foods

The scope of a recently developed optical biosensor platform for application to milk products was extended in this study to the determination of folate in supplemented and non-supplemented foods. The technique utilises folate-binding protein within an automated high throughput inhibition assay format and was subjected to single-laboratory validation. Critical performance factors were evaluated, including establishing four-parameter logistic calibration parameters, confirmation of equivalent molar cross-reactivity as a function of poly-γ-glutamate chain length, demonstration of analytically insignificant non-specific binding, and verification of the absence of food-derived matrix interferences. A panel of certified reference food materials was used to estimate assay precision (HorRatr: 0.3–0.8), confirm recovery within accepted guidelines (72–112%) and establish method accuracy based on data compliance with assigned folate values. The optimised method was applied to a range of food items (milk, cereal, flour, broccoli, egg, fishmeal, liver) and the influence of assay variables including preheat and trienzyme treatments was demonstrated to be minor. The described assay is an expedient alternative to current methods used for the determination of the folate content in foods.

Graphene Oxide: Graphene Oxide as an Optical Biosensing Platform (Adv. Mater. 25/2012)

As an oxygenated lattice of donor/acceptor molecules exposed in a planar surface, graphene oxide enables unprecedented optical biosensing strategies to detect DNA, cancer biomarkers, viruses, and more. It has excellent capabilities for direct wiring with biomolecules, heterogeneous chemical and electronic structures, the ability to be solution-processed, and the ability to be tuned as either an insulator, semiconductor or semi-metal. On page 3298, E. Morales-Narváez and A. Merkoçi discuss the rationale behind the use of graphene oxide in biosensing applications and review current approaches.

Rationally designed porous silicon as platform for optical biosensors

Optical porous silicon multilayer structures are able to work as sensitive chemical sensors or biosensors based in their optical response. An algorithm to simulate the optical response of these multilayers was developed, considering the optical properties of the individual layers. The algorithm allows designing and customizing the porous silicon structures according to a given application. The results obtained by the simulation were experimentally verified; for this purpose different photonic structures were prepared, such as Bragg reflectors and microcavities. Some of these structures have been derivatized by the introduction of aminosilane groups on the porous silicon surface. The algorithm also permits to simulate the effects produced by a non uniform derivatization of the multilayer.

Rapid detection of Mycobacterium tuberculosis biomarkers in a sandwich immunoassay format using a waveguide-based optical biosensor

Early diagnosis of active tuberculosis (TB) remains an elusive challenge, especially in individuals with disseminated TB and HIV co-infection. Recent studies have shown a promise for the direct detection of pathogen-specific biomarkers such as lipoarabinomannan (LAM) for the diagnosis of TB in HIV-positive individuals. Currently, traditional immunoassay platforms that suffer from poor sensitivity and high non-specific interactions are used for the detection of such biomarkers. In this manuscript, we demonstrate the development of sandwich immunoassays for the direct detection of three TB-specific biomarkers, namely LAM, early secretory antigenic target 6 (ESAT6) and antigen 85 complex (Ag85), using a waveguide-based optical biosensor platform. Combining detection within the evanescent field of a planar optical waveguide with functional surfaces that reduce non-specific interactions allows for the ultra-sensitive and quantitative detection of biomarkers (an order of magnitude enhanced sensitivity, as compared to plate-based ELISA) in complex patient samples (urine, serum) within a short time. We also demonstrate the detection of LAM in urine from a small sample of subjects being treated for TB using this approach with excellent sensitivity and 100% corroboration with disease status. These results suggest that pathogen-specific biomarkers can be applied for the rapid and effective diagnosis of disease. It is likely that detection of a combination of biomarkers offers greater reliability of diagnosis, rather than detection of any single pathogen biomarker. NCT00341601.

Graphene Oxide as an Optical Biosensing Platform

Since graphene exhibits innovative mechanical, electrical, thermal, and optical properties, this 2D material is increasingly attracting attention and is under active research. Among the various graphene forms with lattice-like nanostructure, graphene oxide (GO) displays advantageous characteristics as a biosensing platform due to its excellent capabilities for direct wiring with biomolecules, a heterogeneous chemical and electronic structure, the possibility to be processed in solution and the ability to be tuned as insulator, semiconductor or semi-metal. Moreover, GO photoluminescences with energy transfer donor/acceptor molecules exposed in a planar surface and is even proposed as a universal highly efficient long-range quencher, which is opening the way to several unprecedented biosensing strategies. Here, the rationale behind the use of GO in optical biosensing applications is discussed by describing different potentially exploitable properties of GO, and an overview of the current approaches are presented along with future perspectives and challenges.

Functional Nanostructured Porous Si/Hydrogel Hybrids: Synthesis, Characterization and Applications

ABSTRACTPorous Si/hydrogel thin films combine a porous Si optical nanostructure and a functional hydrogel. These hybrids show great promise as versatile platforms for the fabrication of miniaturized sensors with various transduction mechanisms, cell culture supports, autonomous drug delivery systems, and many other functional systems. The basic considerations in designing functional PSi/hydrogel hybrids, synthesis routes, and novel characterization methods are discussed. New exciting applications of these nanomaterials as label-free optical biosensing platforms for bacteria and organophosphates detection are described.

Optical Nanosensor Architecture for Cell-Signaling Molecules Using DNA Aptamer-Coated Carbon Nanotubes

We report a novel optical biosensor platform using near-infrared fluorescent single-walled carbon nanotubes (SWNTs) functionalized with target-recognizing aptamer DNA for noninvasively detecting cell-signaling molecules in real time. Photoluminescence (PL) emission of aptamer-coated SWNTs is modulated upon selectively binding to target molecules, which is exploited to detect insulin using an insulin-binding aptamer (IBA) as a molecular recognition element. We find that nanotube PL quenches upon insulin recognition via a photoinduced charge transfer mechanism with a quenching rate of k(q) = 5.85 × 10(14) M(-1) s(-1) and a diffusion-reaction rate of k(r) = 0.129 s(-1). Circular dichroism spectra reveal for the first time that IBA strands retain a four-stranded, parallel guanine quadruplex conformation on the nanotubes, ensuring target selectivity. We demonstrate that these IBA-functionalized SWNT sensors incorporated in a collagen extracellular matrix (ECM) can be regenerated by removing bound analytes through enzymatic proteolysis. As proof-of-concept, we show that the SWNT sensors embedded in the ECM promptly detect insulin secreted by cultured pancreatic INS-1 cells stimulated by glucose influx and report a gradient contour of insulin secretion profile. This novel design enables new types of label-free assays and noninvasive, in situ, real-time detection schemes for cell-signaling molecules.

Engineering Nanostructured Porous SiO2 Surfaces for Bacteria Detection via “Direct Cell Capture”

An optical label-free biosensing platform for bacteria detection ( Escherichia coli K12 as a model system) based on nanostructured oxidized porous silicon (PSiO(2)) is introduced. The biosensor is designed to directly capture the target bacteria cells on its surface with no prior sample processing (such as cell lysis). The optical reflectivity spectrum of the PSiO(2) nanostructure displays Fabry-Pérot fringes characteristic of thin-film interference, enabling direct, real-time observation of bacteria attachment within minutes. The PSiO(2) optical nanostructure is synthesized and used as the optical transducer element. The porous surface is conjugated with specific monoclonal antibodies (immunoglobulin G's) to provide the active component of the biosensor. The immobilization of the antibodies onto the biosensor system is confirmed by attenuated total reflectance Fourier transform infrared spectroscopy, fluorescent labeling experiments, and refractive interferometric Fourier transform spectroscopy. We show that the immobilized antibodies maintain their immunoactivity and specificity when attached to the sensor surface. Exposure of these nanostructures to the target bacteria results in "direct cell capture" onto the biosensor surface. These specific binding events induce predictable changes in the thin-film optical interference spectrum of the biosensor. Our preliminary studies demonstrate the applicability of these biosensors for the detection of low bacterial concentrations. The current detection limit of E. coli K12 bacteria is 10(4) cells/mL within several minutes.

Label-Free Biosensing with Lipid-Functionalized Gold Nanorods

The fabrication of a label-free mass spectrometry and optical detection-based biosensor platform for the detection of low-abundance lipophilic analytes in complex mixtures is described. The biosensor consists of a lipid layer partially tethered to the surface of a gold nanorod. The effectiveness of the biosensor is demonstrated for the label-free detection of a lipophilic drug in aqueous solution and a lipopeptide in serum.

Total internal reflection ellipsometry as a label-free assessment method for optimization of the reactive surface of bioassay devices based on a functionalized cycloolefin polymer

We report a label-free optical detection technique, called total internal reflection ellipsometry (TIRE), which can be applied to study the interactions between biomolecules and a functionalized polymer surface. Zeonor (ZR), a cycloolefin polymer with low autofluorescence, high optical transmittance and excellent chemical resistance, is a highly suitable material for optical biosensor platforms owing to the ease of fabrication. It can also be modified with a range of reactive chemical groups for surface functionalization. We demonstrate the applications of TIRE in monitoring DNA hybridization assays and human chorionic gonadotrophin sandwich immunoassays on the ZR surface functionalized with carboxyl groups. The Ψ and Δ spectra obtained after the binding of each layer of analyte have been fitted to a four-layer ellipsometric model to quantitatively determine the amount of analytes bound specifically to the functionalized ZR surface. Our proposed TIRE technique with its very low analyte consumption and its microfluidic array format could be a useful tool for evaluating several crucial parameters in immunoassays, DNA interactions, adsorption of biomolecules to solid surfaces, or assessment of the reactivity of a functionalized polymer surface towards a specific analyte.

Low-cost label-free biosensors using photonic crystals embedded between crossed polarizers

There is a strong need for low-cost biosensors to enable rapid, on-site analysis of biological, biomedical, or chemical substances. We propose a platform for label-free optical biosensors based on applying the analyte onto a surface-functionalized photonic crystal slab and performing a transmission measurement with two crossed polarization filters. This dark-field approach allows for efficient background suppression as only the photonic crystal guided-mode resonances interacting with the functionalized surface experience significant polarization rotation. We present a compact biosensor demonstrator using a low-cost light emitting diode and a simple photodiode capable of detecting the binding kinetics of a 2.5 nM solution of the protein streptavidin on a biotin-functionalized photonic crystal surface.

A polyaniline-based optical biosensing platform using an entrapped oxidoreductase enzyme

A novel optical biosensing platform utilizing the unique solubility and chemochromic properties of polyaniline is presented. A facile, ion-induced reprecipitation method leads to the entrapment of a chosen oxidoreductase enzyme, which, in the presence of its associated substrate, catalyzes a reversible redox change in the host polymer. This change is monitored via the UV–vis absorption and subsequently analyzed to fit a Michaelis–Menten model. Here, in vitro prototype devices demonstrate selective sensing of glucose, choline, and uric acid, and the potential to be adapted for use as part of real-time in vivo monitoring systems is discussed.

An optical biosensing platform for proteinase activity using gold nanoparticles

The surface plasmon resonance (SPR) wavelength of colloidal gold nanoparticles (AuNPs) can vary when the AuNPs aggregate, have different sizes or shapes, or are modified with chemical molecules. In this study, an optical biosensing platform for a proteinase activity assay was established based on the SPR property of AuNPs. The 13-nm AuNPs were modified with gelatin (AuNPs-gelatin) as a proteinase substrate and subsequently modified with 6-mercaptohexan-1-ol (MCH) (AuNPs/MCH-gelatin). After proteinase (trypsin or gelatinase) digestion, the AuNPs lose shelter, and MCH increases the attractive force between the modified AuNPs. Therefore, the AuNPs gradually move closer to each other, resulting in AuNPs aggregation. The AuNPs aggregation can be monitored by the red shift of surface plasmon absorption and a visible color change of the AuNPs is from red to blue. Such a color change can be observed with the naked eye. For detection, the absorption ratio, A 625/ A 525, of the reacted AuNPs solution can be used to estimate quantitatively the proteinase activity. A linear correlation has been established with trypsin activity at concentrations from 1.25 × 10 −1 to 1.25 × 10 2 U and matrix metalloproteinase-2 activity at concentrations from 50 ng/mL to 600 ng/mL.

Self-assembled, photoluminescent peptide hydrogel as a versatile platform for enzyme-based optical biosensors

A self-assembled peptide hydrogel consisting of Fmoc-diphenylalanine has been employed as a biosensing platform through the encapsulation of enzyme bioreceptors (e.g., glucose oxidase or horseradish peroxidase) and fluorescent reporters (e.g., CdTe and CdSe quantum dots). Enzymes and quantum dots (QDs) were physically immobilized within the hydrogel matrix in situ in a single step by simply mixing aqueous solution containing QDs and enzymes with monomeric peptide (Fmoc-diphenylalanine) solution. By using atomic force microscopy and scanning transmission electron microscopy, we observed that the self-assembled peptide hydrogel had a three-dimensional network of nanofibers (with a diameter of approximately 70–90 nm) that physically hybridized with QDs and encapsulated enzyme bioreceptors with a minimal leakage. We successfully applied the peptide hydrogel to the detection of analytes such as glucose and toxic phenolic compounds by using a photoluminescence quenching of the hybridized QDs. The Michaelis–Menten constant ( K M) of the photoluminescent peptide hydrogel was found to be 3.12 mM (GOx for glucose) and 0.82 mM (HRP for hydroquinone), respectively, which were much lower than those of conventional gel materials. These results suggest that the peptide hydrogel is an alternative optical biosensing platform with practical advantages such as simple fabrication via self-assembly, efficient diffusion of target analytes, and high encapsulation efficiencies for fluorescent reporters and bioreceptors.