Optical Label-free Biosensing Research Articles

Smartphone-based dynamic measurements of electro-optically modulated lossy-mode resonance and its biosensing applications

In this work, we discuss an application of a smartphone-based spectrometer for dynamic analysis of optical response of an electro-optically modulated lossy-mode resonance (LMR) optical fiber sensor. The LMR was achieved when a section of a multimode optical fiber core was coated with a fluorine-doped tin oxide (FTO) thin film. The FTO, thanks to its electrical properties, has been simultaneously applied as an electrochemical electrode. It has been shown that when an electrical potential is applied to the obtained dual-domain sensor, the distribution of charge carriers in the FTO changes and it can be dynamically measured optically using the smartphone-based spectrometer with a time resolution below 0.5 s. Moreover, when charge carriers' accumulation is induced at the FTO surface, the changes on the sensor surface, such as biomaterial binding, can be identified with the device. The optical label-free biosensing performance of the system has been verified using streptavidin and green fluorescent protein (GFP) as receptor and target, respectively. The outcome of optical and electrical measurements, acquired simultaneously, were compared. It has been found that the system can detect concentration of GFP as low as 1 ng/mL. It must be noted that the smartphone-based dynamic optical measurements were possible with only the device, a low-cost detachable adapter, and a custom-made software application. Electrical measurements using a smartphone with such low-cost modifications are hardly possible. The setup can be further simplified using just a battery for setting the potential difference, thus establishing electro-optical modulation of the sensor. Such a system may be considered as suitable for a broad spectrum of sensors and their label-free biosensing applications.

3D structured biochip for label free determinations at the point-of-need

A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors in their application of on-site food analysis. The proposed methodology (GRADual thin film IntErferometry, GRADE) is based on light interferometry employing an illumination source of a few nm spectral width and a 3D micro-patterned sensor surface consisting of multilevel staircase structures, fabricated through grayscale electron beam lithography and appropriate pattern transfer on thick dielectric layer. The formation of the biomolecular adlayer on the 3D structured surface sensor, is monitored with a 2D camera, without the need for broadband illumination sources and spectral analysis instrumentation. Consequently, this novel sensing approach has the potential to enable the cost-efficient monitoring of bioreactions employing a simple optical setup, a low-cost photodetector and light source, thus being an ideal platform for use at the Point-of-Need. In the present study the system is demonstrated for the detection of aflatoxin B 1 (AFB1), a very potent carcinogen detected in various food categories. • A novel optical label-free biosensing approach is introduced aiming to remove the current limitations of optical biosensors. • Based on a sensing area with a gradually-varying-thickness dielectric, fabricated through grayscale e-beam lithography. • Cost-efficient monitoring of bioreactions, ideal platform for use at the Point-of-Need.

Ultrasensitive Optical Label-Free Biosensing and Fluorescent Measurements using UV Surface Plasmons in Gold Nanolayers Launched with the Support of a Specially Designed Photonic Crystal

Recently, we used Photonic Crystal (PC) - supported surface plasmons (SP) propagating along thin Pd, Co (in red spectral range) and Au (in blue, 405 nm) nanolayers for ultrasensitive gas sensing (Pd, Au) and magnetoplasmonic researches (Co). In all these cases, without the exploitation of a specially designed PC, the propagation length of the plasmons is just of the order of the wavelength hence there would be no reason to speak about SP at all. Here we experimentally show that the same concept is applicable even in UV spectral range at 375 nm despite the extremely non-favorable conditions of SP excitation at bulk gold - dielectric interface: the real part of the dielectric permittivity of gold at this wavelength is minus 1.5, and its imaginary part 5.6. Nevertheless, the use of a specially designed PC, containing seven pairs of transparent Ta2O5/SiO2 layers plus an additional Ta2O5 and 8 nm-thick gold layer, enabled us to attain the angular width of the surface plasmon excitation around 0.250 and thus the UV SP propagation length of tens of microns. This same structure can be used in quite usual fashion as an ultrasensitive optical label-free biosensor. Besides, the short UV wavelength exploited enables also to use it for excitation of fluorescence for very broad class of studied compounds. The perspectives of the application of this approach to deeper UV, at 280 nm, will be discussed. Such structures will enable direct label-free excitation and measurement of fluorescence from many proteins and DNA/RNA molecules/complexes.

Layer-By-Layer Nanoassembly Of Polyelectrolites Engineered With Bioreceptors For High-Sensitivity Optical Label-Free Biosensing With Nanostructured Porous Silicon

Here we report the layer-by-layer (LbL) nanoassembly of charged polyelectrolytes engineered with bioreceptors as an effective and robust alternative to covalent biofunctionalization (e.g., organosilanization and hydrosilylation) for affinity biosensing with porous silicon (PSi) interferometers [1]. As a proof of concept demonstration, a bi-layer of positively-charged poly(allylamine hydrochloride) (PAH) and negatively-charged biotinylated poly(methacrylic acid) (b-PMAA) is assembled onto the surface of oxidized PSi interferometers for the affinity detection of streptavidin in buffer and raw saliva. The LbL nanoassembly allows a homogenous coating of the inner PSi surface, ensuring a robust anchoring of the bioreceptors and, in turn, high sensitive and selective detection of streptavidin, also in raw saliva, down to a theoretical detection limit of 600 fM. This pushes PSi based biosensors at detection limit comparable to that of state-of-the-art nanostructured photonic and plasmonic platforms for biosensing. Further, the development and employment of polymers engineered with several bioreceptors or with stronger dissociation properties (e.g. PSS) could broaden the applications LbL nano-assembly in biosensing and in biomedical applications.[1] S. Mariani et al., Nature Communications. 2018, 9, 5256, 1-13.

Biofunctionalization Effectiveness of Titanium Oxide Thin Films Obtained with Physical and Chemical Vapour Deposition Methods for Optical Label-free Biosensing Applications

An influence of titanium oxide (TiOx) deposition method, i.e., chemical (atomic layer deposition, ALD) and physical (reactive magnetron sputtering, RMS) vapor deposition, on the optical properties and surface chemical modification were investigated. TheTiOx film was used as a functional coating for enhancing sensing properties of two types of label-free optical sensors, i.e., localized surface plasmon resonance (LSPR) sensors based on gold nanoparticles and induced in optical fibre long-period gratings (LPGs) working in visible and infrared spectral range, respectively. We have found that independently on deposition method the films increase the sensitivity of LPG and enhance biofunctionalization capabilities.

Diatom-based label-free optical biosensor for biomolecules.

Diatoms are unicellular algae, which fabricates ornate biosilica shells called frustules that possess a surface rich in reactive silanol (Si-OH) groups. The intrinsic patterned porous structure of diatom frustules at nanoscale can be exploited in the effective detection of biomolecules. In this study, the frustules of a specific diatom Amphora sp. has been functionalized to detect bovine serum albumin (BSA). The functionalization of the diatom frustule substrate is achieved by using 3-aminopropyltriethoxysilane (APES). The field emission scanning electron microscopy (FESEM) results showed an ornately patterned surface of the frustule valve ordered at nanoscale. The Fourier transform infrared (FTIR) spectra confirmed the N-H bending and stretching of the amine group after amine functionalization. The emission peaks in the photoluminescence (PL) spectra of the amine-functionalized diatom biosilica selectively enhanced the intensity by a factor of ten when compared to that of a bare diatom biosilica. The result showed a significant quenching of PL intensity of BSA at around 445nm due to the interaction of amine-functionalized diatom-BSA protein complex. The detection limit was found to be 3 × 10(-5)M of BSA protein. Hence, the study proves that the functionalized frustule of Amphora sp. is an effective quantitative analytical tool for optical label-free biosensing applications.

Description of an Advantageous Optical Label-Free Biosensing Interferometric Read-Out Method to Measure Biological Species

In this article we report a new, simple, and reliable optical read-out detection method able to assess Rotavirus present in human sera as well as in the viral pollution sources. It is based on the interference of two interferometers used as biophotonic transducers. The method significantly improves the optical label-free biosensing response measuring both, the concentration of the AgR and its corresponding size. Two different immunoassays were carried out: Bovine Serum Albumin (BSA), and the recognition by its antibody (anti-BSA); and Rotavirus (AgR) and the recognition by its antibody (anti-AgR). In the cases studied, and using as model interferometer a simple Fabry-Perot transducer, we demonstrate a biosensing enhancement of two orders of magnitude in the Limit of Detection (LoD). In fact, this read-out optical method may have significant implications to enhance other optical label-free photonic transducers reported in the scientific literature.

Label-free optical biosensors based on a planar optical waveguide

We review optical label-free biosensing platforms based on planar optical waveguides with their operation principles and performance characteristics. As the building blocks of refractometric optical transducers and plasmonic optical devices, optical planar waveguides are widely adopted to be core sensing platforms for label-free optical assay of biological and chemical interactions due to the high sensitivity with their invulnerability to external noise such as electromagnetic interference. The review scope includes the sensing schemes of conventional dielectric waveguides, the reverse symmetry waveguides, the anti-resonant reflecting optical waveguides (ARROW), and metal-clad leaky waveguides (MCLW). The configurations of the sensing systems with their operation features will be discussed together with the recent development and progress. In particular, we address in more detail the reverse symmetry waveguide and the MCLW based biosensors which have a capability of detecting micrometer-scale biological objects.

Mode-splitting-based optical label-free biosensing with a biorecognition-covered microcavity

A pair of counter-propagating high-Q whispering-gallery modes (WGMs) can couple to each other and produce two new modes with a significant resonance splitting when single or multi-Rayleigh scatterers are adsorbed on the microcavity surface. In this paper, we investigate the mode-splitting-based biosensing by using a biorecognition-covered WGM microcavity because the pre-covering is essentially necessary for label-free detection of specific biological targets. It is shown that the quantitative detection, e.g., nanoparticle sizing, is strongly affected by the biorecognition, and the minimum target size suitable for detection is subjected to the surface pre-covering ratio. To eliminate this degradation induced by biorecognition, we further propose to use the total linewidth broadening of the two new modes as the detection signal, which is immune to thermal fluctuation.

Label-free optical biosensor on a single chip

The healthcare and pharmaceutical sectors continuously demand more powerful analytical and diagnostic tools for the identification of disease, the development of new medicines, and better diagnostic tests. High-capability optical biosensing systems are emerging as a way to achieve these goals. Based on the combination of optical materials and photonic components, optical sensing offers many advantages. For instance, the technology is non-invasive, immune to electromagnetic interference, and the risk of electrical shocks or explosions is nonexistent. A key enabler for these systems is the development of costeffective miniaturization technology, which permits the efficient integration of mechanical, fluidic, and photonic components. In typical biosensors, detection of specific pathogens or proteins begins by immobilizing appropriate bioreceptors on the sensing areas of a chip. When analytes are introduced into those areas, only the target biomolecules will be bound to their corresponding biomolecular receptors. This binding process is usually monitored with commercial analytical techniques that require transduction labeling elements, such as fluorescent dyes or radioactive isotopes, to generate a physically readable signal from a recognition event. However, labeling chemistry is expensive and time-consuming. In contrast, recent progress in biosensing systems, in which biomolecules are unlabeled or unmodified (label-free biosensing), is showing promising results. The sensing transduction signals in optical label-free biosensing platforms are based on small changes in refractive index caused by the immobilization or binding reaction of biomolecules. Planar photonic biosensors have measured biomolecular binding interactions using well-known photonic structures, including surface plasmon resonance, 4 interferometers, resonators, 10 gratings, 12 and photonic crystals. 14 These photonic sensing biochips are attractive because they integrate multiple compact devices on a single chip. But, although they use low cost mature semiconductor technology, planar photonic biochips normally need complex optical coupling systems, such as inverted taper and grating couplers, which make the devices less cost effective. Our vision is to create a highly sensitive, label-free optical biosensing system that is fully compatible with semiconductor technology and can be fabricated and interrogated at wafer or disposable chip level. Our holistic approach combines biophotonic resonant or interferometric sensing cells and advanced micron and sub-micron spot size high-sensitivity optical interrogation technologies. This approach leads to a new generation of high-value biochips that are scalable at the wafer level without the need for complex coupling systems, thus offering an inexpensive solution for integration and packaging. Our optical sensing system uses a tightly focused beam (see Figure 1), which allows in situ measurement of micron and submicron size geometries. In our current work, we have already demonstrated the capability to measure extremely small volumes of analytes (on the order of 0.1fL per hole). 16 This system makes routine screening more cost effective and, because each wafer can integrate hundreds of sensing structures, it is also suitable for performing hundreds of measurements on one or multiple samples with high throughput and on-wafer monitoring. To determine the interactions with receptor biomolecules with more reliability and sensitivity, the system will be tremendously sensitive to refractive index variations: on the order of 10 or even 10 refractive index units (RIUs). This means that direct detection of picomolar or even lower concentrations is feasible. Both observation of the reflectivity profile and magnification of the biophotonic resonant or interferometric sensing cells will facilitate this sensitivity. Label-free optical biosensors are expected to grow most in importance in the healthcare, biomedical, and biopharmaceutical sectors, since they can provide new analytical tools with reduced size as well as facilitate large-scale screening of a very wide range of samples for many different parameters. Our optical biosensing system, which uses mixed technologies, such as op-