Real-time detection of DNA interactions with long-period fiber-grating-based biosensor

Rapid and quantitative detection of DNA hybridization using a simplified Fabry-Perot interferometric biosensor

This study introduces a miniaturized fiber-optic Fabry-Perot (FP) interferometric biosensor, distinctively engineered for cost-effective, rapid, and quantitative DNA sequence detection. By leveraging the interference patterns generated within a Fabry-Perot microcavity, our sensor precisely monitors DNA hybridization events in real-time. We have verified the sensor's biofunctionalization via fluorescent labeling and have extensively validated its performance through numerous hybridization and regeneration cycles with 1 μM single-stranded DNA (ssDNA) solutions. Demonstrating remarkable repeatability and reusability, the sensor effectively discerns ssDNA sequences exhibiting varying degrees of mismatches. Its ability to accurately distinguish between sequences with 2 and 7 mismatches underscores its potential as a valuable asset for swift DNA analysis. Characterized by its rapid response time-typically yielding results within 6 minutes-and its adeptness at mismatch identification, our biosensor stands as a potent tool for facilitating accelerated DNA diagnostics and research.

Distributed optical fiber biosensor based on optical frequency domain reflectometry

In situ acquisition of spatial distribution of biochemical substances is important in cell analysis, cancer detection and other fields. Optical fiber biosensors can achieve label-free, fast and accurate measurements. However, current optical fiber biosensors only acquire single-point of biochemical substance content. In this paper, we present a distributed optical fiber biosensor based on tapered fiber in optical frequency domain reflectometry (OFDR) for the first time. To enhance evanescent field at a relative long sensing range, we fabricate a tapered fiber with a taper waist diameter of 6 μm and a total stretching length of 140 mm. Then the human IgG layer is coated on the entire tapered region by polydopamine (PDA) -assisted immobilization as the sensing element to achieve to sense anti-human IgG. We measure shifts of the local Rayleigh backscattering spectra (RBS) caused by the refractive index (RI) change of an external medium surrounding a tapered fiber after immunoaffinity interactions by using OFDR. The measurable concentration of anti-human IgG and RBS shift has an excellent linearity in a range from 0 ng/ml to 14 ng/ml with an effective sensing range of 50 mm. The concentration measurement limit of the proposed distributed biosensor is 2 ng/ml for anti-human IgG. Distributed biosensing based on OFDR can locate a concentration change of anti-human IgG with an ultra-high sensing spatial resolution of 680 μm. The proposed sensor has a potential to realize a micron-level localization of biochemical substances such as cancer cells, which will open a door to transform single-point biosensor to distributed biosensor.

Long-Period Fiber Grating Sensors for Chemical and Biomedical Applications

Optical fiber biosensors (OFBS) are being increasingly proposed due to their intrinsic advantages over conventional sensors, including their compactness, potential remote control and immunity to electromagnetic interference. This review systematically introduces the advances of OFBS based on long-period fiber gratings (LPFGs) for chemical and biomedical applications from the perspective of design and functionalization. The sensitivity of such a sensor can be enhanced by designing the device working at or near the dispersion turning point, or working around the mode transition, or their combination. In addition, several common functionalization methods are summarized in detail, such as the covalent immobilization of 3-aminopropyltriethoxysilane (APTES) silanization and graphene oxide (GO) functionalization, and the noncovalent immobilization of the layer-by-layer assembly method. Moreover, reflective LPFG-based sensors with different configurations have also been introduced. This work aims to provide a comprehensive understanding of LPFG-based biosensors and to suggest some future directions for exploration.

Relative humidity sensor based on a silver nano-grating made from DVD discs

A relative humidity sensor based on a silver nano-grating was proposed. By stripping and cleaning commercially available CD and DVD discs, polycarbonate plates with different grating periods are obtained. These plates as templates are coated with a layer of sputtered silver film to form silver nano-gratings, which exhibit refractive index sensing sensitivities of 517 nm/RIU and 742.9 nm/RIU, respectively. The finite-difference time-domain simulation results conform the excited surface plasmon polariton modes and localized surface plasmon modes on the nano-grating. By spin coating a layer of humidity-sensitive porous silica with optimized thickness, the silver nano-grating shows a relative humidity detection sensitivity of 0.23 nm/%RH.

Micromachined Optical Fiber Sensors for Biomedical Applications

Optical fibers revolutionized the rate of information reception and transmission in telecommunications. The revolution has now extended to the field of physicochemical sensing. Optical fiber sensors (OFSs) have found a multitude of applications, spanning from structural health monitoring to biomedical and clinical measurements due to their unique physical and functional advantages, such as small dimensions, light weight, immunity to electromagnetic interference, high sensitivity and resolution, multiplexing, and remote operation. OFSs generally rely on the detection of measurand-induced changes in the optical properties of the light propagating in the fiber, where the OFS essentially functions as the conduit and physical link between the probing light waves and the physicochemical parameters under investigation. Several advanced micromachining techniques have been developed to optimize the structure of OFSs, thus improving their sensing performance. These techniques include fusion splicing, tapering, polishing, and more complicated femtosecond laser micromachining methods. This chapter discusses and reviews the most recent developments in micromachined OFSs specifically for biomedical applications. Step-by-step procedures for several optical fiber micromachining techniques are detailed.

Rapid and sensitive detection of Staphylococcus aureus by using a long-period fiber grating immunosensor coated with egg yolk antibody

The rapid and reliable detection of bacteria plays an important role in clinical and veterinary practice. A stable, label free, compact, and sensitive long-period fiber grating (LPFG) sensor based on egg yolk antibody (IgY) was proposed for the detection of Staphylococcus aureus (S. aureus). LPFG was fabricated with laser writing technology, and specific IgY was further immobilized on the grating region and then combined with the corresponding bacteria. S. aureus was detected by tracking the change of resonance wavelength in the LPFG transmission spectrum caused by bacteria-antibody interaction induced by the increase of biological cover thickness and density after the immune reaction. The testing results showed that the proposed sensor was selective and sensitive to S. aureus measurement, and the detection limit is approximately 33 CFU/ml. The proposed antibody immobilization method is very simple, and the optical fiber can be manufactured in batch to reduce the cost. The detection time of the sensor is around 20 min, which is fast and suitable for detection. The assay was successfully applied for the quantitative analysis of S. aureus in natural waters and met the needs of on-site screening trace pathogenic bacteria in food safety control.

Au-NPs signal amplification ultra-sensitivity optical microfiber interferometric biosensor

An optical microfiber interferometric biosensor for the low concentration detection of sequence-specific deoxyribonucleic acid (DNA) based on signal amplification technology via oligonucleotides linked to gold nanoparticles (Au-NPs) is proposed and experimentally analyzed. The sensor uses a "sandwich" detection strategy, in which capture probe DNA (DNA-c) is immobilized on the surface of the optical microfiber interferometer, the reporter probe DNA (DNA-r) is immobilized on the surface of Au-NPs, and the DNA-c and DNA-r are hybridized to the target probe DNA (DNA-t) in a sandwich arrangement. The dynamic detection of the DNA-t was found to range from 1.0×10^-15 M to 1.0×10^-8 M, and the limit of detection (LOD) concentration was 1.32 fM. This sensor exhibited not only a low LOD but also excellent selectivity against mismatched DNA-t, and it can be further developed for application in various sensing platforms.

A Lamping U-Shaped Fiber Biosensor Detector for MicroRNA

This study presents a U-shaped optical fiber developed for a facile application of microRNA detection. It is fabricated by the lamping process and packaged in a quartz tube to eliminate human negligence. In addition, silanization and electrostatic self-assembly are employed to bind gold nanoparticles and miRNA-133a probe onto the silicon dioxide of the fiber surface. For Mahlavu of hepatocellular carcinoma (HCC), detection is determined by the wavelength shift and transmission loss of a U-shaped optical fiber biosensor. The spectral sensitivity of wavelength and their coefficient of determination are found at −218.319 nm/ ng/mL and 0.839, respectively. Concurrently, the sensitivity of transmission loss and their coefficient of determination are found at 162.394 dB/ ng/mL and 0.984, respectively. A method for estimating the limit of detection of Mahlavu is at 0.0133 ng/mL. The results show that the proposed U-shaped biosensor is highly specific to miRNA-133a and possesses good sensitivity to variations in specimen concentration. As such, it could be of substantial value in microRNA detection.

Immunosensor Based on Long-Period Fiber Gratings for Detection of Viruses Causing Gastroenteritis

Since the norovirus is the main cause of acute gastroenteritis all over the world, its fast detection is crucial in medical diagnostics. In this work, a rapid, sensitive, and selective optical fiber biosensor for the detection of norovirus virus-like particles (VLPs) is reported. The sensor is based on highly sensitive long-period fiber gratings (LPFGs) coated with antibodies against the main coat protein of the norovirus. Several modification methods were verified to obtain reliable immobilization of protein receptors on the LPFG surface. We were able to detect 1 ng/mL norovirus VLPs in a 40-min assay in a label-free manner. Thanks to the application of an optical fiber as the sensor, there is a possibility to increase the user’s safety by separating the measurement point from the signal processing setup. Moreover, our sensor is small and light, and the proposed assay is straightforward. The designed LPFG-based biosensor could be applied in both fast norovirus detection and in vaccine testing.

In-situ DNA hybridization detection based on a reflective microfiber probe

A label-free biosensor based on a reflective microfiber probe for in-situ real-time DNA hybridization detection is proposed and experimentally demonstrated. The microfiber probe is simply fabricated by snapping a non-adiabatic biconical microfiber through closing the oxyhydrogen flame during fiber stretching. Assisted with the Fresnel reflection at the end of microfiber, a reflective microfiber modal interferometer is realized. The in-situ DNA hybridization relies on the surface functionalization of a monolayer of Poly-L-lysine (PLL) and synthetic DNA sequences that bind to a given target with high specificity. The detection processes of DNA hybridization in various concentration of target DNA solutions are monitored in real-time and the experimental results present a minimum detectable concentration of 10pM with good repeatability. Additionally, the detection specificity is also investigated by immersing the microfiber probe into the non-complementary ssDNA solutions and observing the spectral variation. The proposed biosensor has advantages of high sensitivity, compact size, ease of use and simple fabrication, which makes it has great potential to be applied in a lot of fields such as disease diagnosis, medicine, and environmental science.

Ultrasensitive tantalum oxide nano-coated long-period gratings for detection of various biological targets

In this work we discussed a label-free biosensing application of long-period gratings (LPGs) optimized in refractive index (RI) sensitivity by deposition of thin tantalum oxide (TaO_x) overlays. Comparing to other thin film and materials already applied for maximizing the RI sensitivity, TaO_x offers good chemical and mechanical stability during its surface functionalization and other biosensing experiments. It was shown theoretically and experimentally that when RI of the overlay is as high as 2 in IR spectral range, for obtaining LPGs ultrasensitive to RI, the overlay's thickness must be determined with subnanometer precision. In this experiment the TaO_x overlays were deposited using Atomic Layer Deposition method that allowed for achieving overlays with exceptionally well-defined thickness and optical properties. The TaO_x nano-coated LPGs show RI sensitivity determined for a single resonance exceeding 11,500 nm/RIU in RI range n_D= 1.335-1.345 RIU, as expected for label-free biosensing applications. Capability for detection of various in size biological targets, i.e., proteins (avidin) and bacteria (Escherichia coli), with TaO_x-coated LPGs was verified using biotin and bacteriophage adhesin as recognition elements, respectively. It has been shown that functionalization process, as well as type of recognition elements and target analyte must be taken into consideration when the LPG sensitivity is optimized. In this work optimized approach made possible detection of small in size biological targets such as proteins with sensitivity reaching 10.21 nm/log(ng/ml).

Immunosensing platform with large detection range using an excessively tilted fiber grating coated with graphene oxide

We report an immunosensing platform with a large detection range using an excessively tilted fiber grating (ExTFG) coated with graphene oxide (GO). ExTFG was inscribed in standard single-mode fiber; GO film was coated on the fiber surface through hydrogen bond. The effectiveness and uniformity of GO deposited on the ExTFG surface were investigated by field emission scanning electron microscopy and energy spectrum method. Bovine serum albumin (BSA) monoclonal antibodies (MAbs) were used as biometric units to link the GO film through a covalent bond for the specific detection of BSA, so as to evaluate the performances of the proposed biosensor. The whole dynamic immobilization process of BSA MAbs and BSA detection were observed by the spectral evolution of the sensor. Experimental results show that the fabricated GO-coated ExTFG biosensor has a large detection range from 1.5 nM-75 nM and fast response for BSA antigen; the limit of detection is ∼0.88  nM by using an optical spectrum analyzer with a resolution of 0.03 nm, and the dissociation constant K_D and the affinity constant K_A are calculated to be ∼6.66×10^-9  M and ∼1.5×10⁸  M^-1, respectively. The proposed GO-coated ExTFG immunosensing platform could lay a foundation for the specific detection of other biomolecules.

Selective and sensitive Escherichia coli detection based on a T4 bacteriophage-immobilized multimode microfiber

Escherichia coli bacteria have been found to be responsible for various health outbreaks caused by contaminated food and water. Accurate and rapid test of E. coli is thus crucial for protecting the public health. A fast-response, label-free bacteriophage-based detection of E. coli using multimode microfiber probe is proposed and demonstrated in this article. Due to the abrupt taper and subwavelength diameter, different modes are excited and guided in the microfiber as evanescent field that can interact with surrounding E. coli directly. The change of E. coli concentration and corresponding binding of E. coli bacteria on microfiber surface will lead to the shift of optical spectrum, which can be exploited for the application of biosensing. The proposed method is capable of reliable detection of E. coli concentration as low as 10³ cfu/mL within the range of 10³ to 10⁷  cfu/mL. Owing to the advantages of high sensitivity and fast response, the microfiber probe has great potential application in the fields of environment monitoring and food safety.

Fabrication of Long Period Gratings by Periodically Removing the Coating of Cladding-Etched Single Mode Optical Fiber Towards Optical Fiber Sensor Development

Here, we present a novel method to fabricate long period gratings using standard single mode optical fibers (SMF). These optical devices were fabricated in a three-step process, which consisted of etching the SMF, then coating it with a thin-film and, the final step, which involved removing sections of the coating periodically by laser ablation. Tin dioxide was chosen as the material for this study and it was sputtered using a pulsed DC sputtering system. Theoretical simulations were performed in order to select the appropriate parameters for the experiments. The responses of two different devices to different external refractive indices was studied, and the maximum sensitivity obtained was 6430 nm/RIU for external refractive indices ranging from 1.37 to 1.39.

Study on spectral and refractive index sensing characteristics of etched excessively tilted fiber gratings

We investigated the spectral and refractive index (RI) sensing characteristics of the excessively tilted fiber grating (Ex-TFG) with different cladding diameters. The Ex-TFG is inscribed in standard single-mode fiber, and the cladding reduces from 125 μm to around 15 μm by the chemical etching method. Experimental results show that the number of cladding modes decreases, and the spacing of adjacent resonance peaks becomes larger and larger with the reduction of the cladding diameter in the observed wavelength range of 1250-1650 nm. The average RI sensitivity in the index region of 1.33-1.38, the one near 1.33, and the one at around 1.38 of the etched Ex-TFG with a diameter of 15 μm is ∼6.3, ∼5.3, and ∼6.67 fold compared to those of the no-etched Ex-TFG, respectively. Also, the RI sensing performances of the etched Ex-TFG with a diameter smaller than 30 μm are better than those of the Ex-TFG inscribed in SM1500 (4.2 μm/80 μm) fiber in the index region of 1.33. The proposed micronano Ex-TFG has higher RI sensitivity and a more compact structure in biosensing applications, compared to the standard Ex-TFGs and Ex-TFGs inscribed in SM1500 fiber.

Long-period fiber grating: a specific design for biosensing applications

In this paper, a detailed investigation on the modeling of long-period fiber grating (LPFG) sensors is discussed with the aim of providing a more realistic solution for their use in biosensing. Add-layer sensitivity, i.e., sensitivity of the sensor to an additional layer adhered onto the fiber surface, is quantified and a clear and complete analysis about the influence of the average thickness of the deposited biological sensing layers, as well as the change in refractive index of these layers, on the resonant wavelength of the cladding modes of an LPFG is provided. Add-layer sensitivity of LPFG sensors close to mode transition (MT) and also at turn-around point (TAP) are taken into account. Adsorbed layer thicknesses, as estimated from measured wavelength shifts of the LPFG, are found to have a good match with the values obtained through other measurement techniques.

Immobilized optical fiber microprobe for selective and sensitive Escherichia coli detection

Escherichia coli (E. coli) bacteria have been found to be responsible for various health outbreaks caused by contaminated food and water. Accurate and rapid test of E. coli is thus crucial for protecting the public health. A fast-response, label-free bacteriophage-based detection of E. coli using multimode microfiber probe is proposed and demonstrated in this paper. Due to the abrupt taper and subwavelength diameter, different modes are excited and guided in the microfiber as evanescent field that can interact with surrounding E. coli directly. The change of E. coli concentration and corresponding binding of E. coli bacteria on microfiber surface will lead to the shift of optical spectrum, which can be exploited for the application of biosensing. The proposed method is capable of reliable detection of E. coli concentration as low as 103 cfu/mL within the range of 103 to 107  cfu/mL. Owing to the advantages of high sensitivity and fast response, the microfiber probe has great potential application in the fields of environment monitoring and food safety.

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors

The measurement of chemical and biomedical parameters can take advantage of the features exclusively offered by optical fibre: passive nature, electromagnetic immunity and chemical stability are some of the most relevant ones. The small dimensions of the fibre generally require that the sensing material be loaded into a supporting matrix whose morphology is adjusted at a nanometric scale. Thanks to the advances in nanotechnology new deposition methods have been developed: they allow reagents from different chemical nature to be embedded into films with a thickness always below a few microns that also show a relevant aspect ratio to ensure a high transduction interface. This review reveals some of the main techniques that are currently been employed to develop this kind of sensors, describing in detail both the resulting supporting matrices as well as the sensing materials used. The main objective is to offer a general view of the state of the art to expose the main challenges and chances that this technology is facing currently.

Towards a Uniform Metrological Assessment of Grating-Based Optical Fiber Sensors: From Refractometers to Biosensors

A metrological assessment of grating-based optical fiber sensors is proposed with the aim of providing an objective evaluation of the performance of this sensor category. Attention was focused on the most common parameters, used to describe the performance of both optical refractometers and biosensors, which encompassed sensitivity, with a distinction between volume or bulk sensitivity and surface sensitivity, resolution, response time, limit of detection, specificity (or selectivity), reusability (or regenerability) and some other parameters of generic interest, such as measurement uncertainty, accuracy, precision, stability, drift, repeatability and reproducibility. Clearly, the concepts discussed here can also be applied to any resonance-based sensor, thus providing the basis for an easier and direct performance comparison of a great number of sensors published in the literature up to now. In addition, common mistakes present in the literature made for the evaluation of sensor performance are highlighted, and lastly a uniform performance assessment is discussed and provided. Finally, some design strategies will be proposed to develop a grating-based optical fiber sensing scheme with improved performance.

Enzyme-functionalized thin-cladding long-period fiber grating in transition mode at dispersion turning point for sugar-level and glucose detection

Enzyme-functionalized dual-peak long-period fiber grating (LPFG) inscribed in 80 - ? m -cladding B/Ge codoped single-mode fiber is presented for sugar-level and specific glucose detection. Before enzyme functionalization, the dual-peak LPFG was employed for refractive index sensing and sugar-level detection and high sensitivities of ? 4298.20 ?? nm / RIU and 4.6696 ?? nm / % were obtained, respectively. Glucose detection probe was attained by surface functionalization of the dual-peak LPFG via covalent binding with aminopropyl triethoxysilane used as a binding site. Optical micrographs confirmed the presence of enzyme. The surface-functionalized dual-peak LPFG was tested with D-(+)-glucose solution of different concentrations. While the peak 2 at the longer wavelength was suitable only to measure lower glucose concentration (0.1 to 1.6 ?? mg / ml ) recording a high sensitivity of 12.21 ± 0.19 ?? nm / ( mg / ml ) , the peak 1 at the shorter wavelength was able to measure a wider range of glucose concentrations (0.1 to 3.2 ?? mg / ml ) exhibiting a maximum resonance wavelength shift of 7.12 ± 0.12 ?? nm / mg / ml . The enzyme-functionalized dual-peak LPFG has the advantage of direct inscription of highly sensitive grating structures in thin-cladding fibre without etching, and most significantly, its sensitivity improvement of approximately one order of magnitude higher than previously reported LPFG and excessively tilted fibre grating (Ex-TFG) for glucose detection.

Fluorescence based fiber optic and planar waveguide biosensors. A review

The application of optical biosensors, specifically those that use optical fibers and planar waveguides, has escalated throughout the years in many fields, including environmental analysis, food safety and clinical diagnosis. Fluorescence is, without doubt, the most popular transducer signal used in these devices because of its higher selectivity and sensitivity, but most of all due to its wide versatility. This paper focuses on the working principles and configurations of fluorescence-based fiber optic and planar waveguide biosensors and will review biological recognition elements, sensing schemes, as well as some major and recent applications, published in the last ten years. The main goal is to provide the reader a general overview of a field that requires the joint collaboration of researchers of many different areas, including chemistry, physics, biology, engineering, and material science.

Partially reduced graphene oxide based FRET on fiber-optic interferometer for biochemical detection

Fluorescent resonance energy transfer (FRET) with naturally exceptional selectivity is a powerful technique and widely used in chemical and biomedical analysis. However, it is still challenging for conventional FRET to perform as a high sensitivity compact sensor. Here we propose a novel ‘FRET on Fiber’ concept, in which a partially reduced graphene oxide (prGO) film is deposited on a fiber-optic modal interferometer, acting as both the fluorescent quencher for the FRET and the sensitive cladding for optical phase measurement due to refractive index changes in biochemical detection. The target analytes induced fluorescence recovery with good selectivity and optical phase shift with high sensitivity are measured simultaneously. The functionalized prGO film coated on the fiber-optic interferometer shows high sensitivities for the detections of metal ion, dopamine and single-stranded DNA (ssDNA), with detection limits of 1.2 nM, 1.3 μM and 1 pM, respectively. Such a prGO based ‘FRET on fiber’ configuration, bridging the FRET and the fiber-optic sensing technology, may serve as a platform for the realization of series of integrated ‘FRET on Fiber’ sensors for on-line environmental, chemical, and biomedical detection, with excellent compactness, high sensitivity, good selectivity and fast response

Label-free Gram-negative bacteria detection using bacteriophage-adhesin-coated long-period gratings

This paper presents a novel application of a highly sensitive sensor based on long-period gratings (LPGs) coated with T4 bacteriophage adhesin for Gram-negative bacteria detection. We show here, that the sensor evidently recognizes Escherichia coli K-12 (PCM2560), whereas in the reference tests - ELISA and BIAcore - the results are questionable. For LPGs sensor the resonant wavelength shift observed for E. coli K-12 was approximately half of that measured for E.coli B (positive control). The BIAcore readings (RU) for E. coli K-12 were at 10% level of the signal obtained for E .coli B. These results confirm the improved sensitivity of the LPGs sensor. Moreover, we also show that application of adhesin may allow for efficient detection of E. coli O111 (PCM418), Klebsiella pneumoniae O1 (PCM1) and Yersinia enterocolitica O1 (PCM1879). The specificity of binding bacteria by the adhesin is discussed and it is determined by a distinct region of lipopolysaccharide receptors and/or by the presence of outer-membrane protein C in an outer membrane of Gram-negative bacteria.

Materiomics for Oral Disease Diagnostics and Personal Health Monitoring: Designer Biomaterials for the Next Generation Biomarkers

We live in exciting times for a new generation of biomarkers being enabled by advances in the design and use of biomaterials for medical and clinical applications, from nano- to macro-materials, and protein to tissue. Key challenges arise, however, due to both scientific complexity and compatibility of the interface of biology and engineered materials. The linking of mechanisms across scales by using a materials science approach to provide structure-process-property relations characterizes the emerging field of 'materiomics,' which offers enormous promise to provide the hitherto missing tools for biomaterial development for clinical diagnostics and the next generation biomarker applications towards personal health monitoring. Put in other words, the emerging field of materiomics represents an essentially systematic approach to the investigation of biological material systems, integrating natural functions and processes with traditional materials science perspectives. Here we outline how materiomics provides a game-changing technology platform for disruptive innovation in biomaterial science to enable the design of tailored and functional biomaterials--particularly, the design and screening of DNA aptamers for targeting biomarkers related to oral diseases and oral health monitoring. Rigorous and complementary computational modeling and experimental techniques will provide an efficient means to develop new clinical technologies in silico, greatly accelerating the translation of materiomics-driven oral health diagnostics from concept to practice in the clinic.

Biosensor based on excessively tilted fiber grating in thin-cladding optical fiber for sensitive and selective detection of low glucose concentration

We report a highly sensitive, high Q-factor, label free and selective glucose sensor by using excessively tilted fiber grating (Ex-TFG) inscribed in the thin-cladding optical fiber (TCOF). Glucose oxidase (GOD) was covalently immobilized on optical fiber surface and the effectiveness of GOD immobilization was investigated by the fluorescence microscopy and highly accurate spectral interrogation method. In contrast to the long period grating (LPG) and optical fiber (OF) surface Plasmon resonance (SPR) based glucose sensors, the Ex-TFG configuration has merits of nearly independent cross sensitivity of the environmental temperature, simple fabrication method (no noble metal deposition or cladding etching) and high detection accuracy (or Q-factor). Our experimental results have shown that Ex-TFG in TCOF based sensor has a reliable and fast detection for the glucose concentration as low as 0.1~2.5mg/ml and a high sensitivity of ~1.514 nm·(mg/ml)⁻¹, which the detection accuracy is ~0.2857 nm⁻¹ at pH 5.2, and the limit of detection (LOD) is 0.013~0.02 mg/ml at the pH range of 5.2~7.4 by using an optical spectrum analyzer with a resolution of 0.02 nm.

Resolution-improved in situ DNA hybridization detection based on microwave photonic interrogation

In situ bio-sensing system based on microwave photonics filter (MPF) interrogation method with improved resolution is proposed and experimentally demonstrated. A microfiber Bragg grating (mFBG) is used as sensing probe for DNA hybridization detection. Different from the traditional wavelength monitoring technique, we use the frequency interrogation scheme for resolution-improved bio-sensing detection. Experimental results show that the frequency shift of MPF notch presents a linear response to the surrounding refractive index (SRI) change over the range of 1.33 to 1.38, with a SRI resolution up to 2.6 × 10(-5) RIU, which has been increased for almost two orders of magnitude compared with the traditional fundamental mode monitoring technique (~3.6 × 10(-3) RIU). Due to the high Q value (about 27), the whole process of DNA hybridization can be in situ monitored. The proposed MPF-based bio-sensing system provides a new interrogation method over the frequency domain with improved sensing resolution and rapid interrogation rate for biochemical and environmental measurement.

Micro-capillary-based evanescent field biosensor for sensitive, label-free DNA detection

We proposed and demonstrated a micro-capillary-based, high-sensitivity evanescent field biosensor for the cost-effective, rapid, and sensitive analysis and detection of specific DNA sequences. By functionalizing the surface of the tubing wall with ssDNA probe sequences, label-free DNA detection is achieved. The wavelength shift response of the surface-functionalized biosensors to DNA hybridization is monitored in real time. Our experiments show that the biosensor can operate at room temperature and is capable of performing label-free hybridization detection, analyte concentration measurement and nucleotide mismatch detection through a single sensing device. The sensor has many advantages, such as a simple manufacturing process, standardized production control, reliable quality, low cost and an economic demodulator. The compact nature and miniature size of the biosensing detection system makes it a good candidate for the rapid and highly sensitive detection of low-concentration analytes in micro-samples for cost-effective, real-time, and on-site analysis in the fields of life science, pharmaceutical chemistry, medical science and criminal investigation.

Novel glucose sensor based on enzyme-immobilized 81° tilted fiber grating

We demonstrate a novel glucose sensor based on an optical fiber grating with an excessively tilted index fringe structure and its surface modified by glucose oxidase (GOD). The aminopropyltriethoxysilane (APTES) was utilized as binding site for the subsequent GOD immobilization. Confocal microscopy and fluorescence microscope were used to provide the assessment of the effectiveness in modifying the fiber surface. The resonance wavelength of the sensor exhibited red-shift after the binding of the APTES and GOD to the fiber surface and also in the glucose detection process. The red-shift of the resonance wavelength showed a good linear response to the glucose concentration with a sensitivity of 0.298 nm·(mg/ml)-1 in the very low concentration range of 0.0~3.0mg/ml. Compared to the previously reported glucose sensor based on the GOD-immobilized long period grating (LPG), the 81° tilted fiber grating (81°-TFG) based sensor has shown a lower thermal cross-talk effect, better linearity and higher Q-factor in sensing response. In addition, its sensitivity for glucose concentration can be further improved by increasing the grating length and/or choosing a higher-order cladding mode for detection. Potentially, the proposed techniques based on 81°-TFG can be developed as sensitive, label free and micro-structural sensors for applications in food safety, disease diagnosis, clinical analysis and environmental monitoring.

Microfluidic Flows and Heat Transfer and Their Influence on Optical Modes in Microstructure Fibers

A finite element analysis (FEA) model has been constructed to predict the thermo-fluidic and optical properties of a microstructure optical fiber (MOF) accounting for changes in external temperature, input water velocity and optical fiber geometry. Modeling a water laminar flow within a water channel has shown that the steady-state temperature is dependent on the water channel radius while independent of the input velocity. There is a critical channel radius below which the steady-state temperature of the water channel is constant, while above, the temperature decreases. However, the distance required to reach steady state within the water channel is dependent on both the input velocity and the channel radius. The MOF has been found capable of supporting multiple modes. Despite the large thermo-optic coefficient of water, the bound modes’ response to temperature was dominated by the thermo-optic coefficient of glass. This is attributed to the majority of the light being confined within the glass, which increased with increasing external temperature due to a larger difference in the refractive index between the glass core and the water channel.

Recognition of bacterial lipopolysaccharide using bacteriophage-adhesin-coated long-period gratings

In this paper we present a new type of highly sensitive label-free sensor based on long-period gratings (LPG) coated with T4 bacteriophage (phage) adhesin. The adhesin (gp37) binds Escherichia coli B (E. coli B) by recognizing its bacterial lipopolysaccharide (LPS). The LPG biofunctionalization methodology is based on coating LPG surface with nickel ions capable of gp37 histidine tag reversible binding. For the first time recombinant adhesive phage protein has been used as a receptor molecule in biosensing scheme. The specificity of LPS binding by adhesin has been tested with LPG-based device and confirmed using Western blot, Enzyme-Linked Immunosorbent Assay (ELISA) and BIACORE methods. The LPG-based sensor can measure bacterial contamination in real time and with a high accuracy. We show that T4 phage adhesin binds E. coli B LPS in its native or denatured form. The binding is highly specific and irreversible. The applied procedure allows for obtaining reusable biosensors.

Molecular beacons immobilized within suspended core optical fiber for specific DNA detection

We propose and experimentally demonstrate a new class of sensor for specific DNA sequences based on molecular beacons (MB) immobilized on the internal surfaces of suspended core optical fibers (SCF). MBs, a type of hairpin structured DNA probe, are attached on the surface of the SCF core using a fuzzy nanoassembly process used in conjunction with a biotin-streptavidin-biotin surface attachment strategy. The proposed DNA sensor detects complementary DNA sequences (cDNA) while discriminating sequences differing from the target by just one base. This enables the detection of DNA in unprecedentedly small sample volumes (nL scale) and is, to the best of our knowledge, the first specific DNA detection using a DNA probe immobilized within a microstructured optical fiber.

Surface plasmon resonance based biosensor technique: a review

Optical Surface plasmon resonance (SPR) biosensors represent the most advanced and developed optical label-free biosensor technology. Optical SPR biosensors are a powerful detection and analysis tool that has vast applications in environmental protection, biotechnology, medical diagnostics, drug screening, food safety and security. This article reviews the recent development of SPR biosensor techniques, including bulk SPR and localized SPR (LSPR) biosensors, for detecting interactions between an analyte of interest in solution and a biomolecular recognition. The concepts of bulk and localized SPs and the working principles of both sensing techniques are introduced. Major sensing advances on biorecognition elements, measurement formats, and sensing platforms are presented. Finally, the discussions on both biosensor techniques as well as comparison of both SPR sensing techniques are made.

A protein-based biointerfacing route toward label-free immunoassays with long period gratings in transition mode

We present a fast and effective method for anchoring bioreceptors to optical waveguides exhibiting a poorly reactive polymer interface and that have to be minimally perturbed with respect to their design. The study originated from the need to biofunctionalize a fiber optic Long Period Grating (LPG) that is tuned in a highly sensitive working point, the so-called transition mode, through the deposition of a high refractive index overlay. In particular, a thin film of atactic polystyrene (PS) was dip-coated onto the LPG with a thickness suitable to optimize the LPG sensitivity to refractive index changes of the surrounding medium. Bovine serum albumin was selected as sacrificial layer for its well-known adhesion capabilities to PS surfaces, then glutaraldehyde was used to conjugate IgGs, serving as prototypical bioreceptor, on the device surface. The effectiveness of the immobilization method was assessed by studying the interaction between the immobilized IgG with a suitable anti-IgG. In a preliminary study performed by means of ELISA and surface plasmon resonance, optimal conditions for the biomolecular testing with the LPG were assessed. Four distinct interactions were thus monitored in real time following the shift of the LPG attenuation band. These experiments suggest a novel and interesting biofunctionalization approach of unreactive polymers with applications in immunosensing and basic life science research.

Optical fibre gratings as tools for chemical and biochemical sensing

Optical fibre gratings have recently been suggested as optical platforms for chemical and biochemical sensing. On the basis of the measurement of refractive index changes induced by a chemical and biochemical interaction in the transmission spectrum along the fibres, they are proposed as a possible alternative to the other label-free optical approaches, such as surface plasmon resonance and optical resonators. The combination of the use of optical fibres with the fact that the signal modulation is spectrally encoded offers multiplexing and remote measurement capabilities which the other technology platforms are not able to or can hardly offer. The fundamentals of the different types of optical fibre gratings are described and the performances of the chemical and biochemical sensors based on this approach are reviewed. Advantages and limitations of optical fibre gratings are considered, with a look at new perspectives for their utilization in the field.

Fiber loop ringdown DNA and bacteria sensors

We report a new type of refractive index-based biosensor using a fiber loop ringdown evanescent field (FLRD-EF) sensing scheme, in which the sensing signal is a time constant and detection sensitivity is enhanced by the multipass nature of the ringdown technique. Bulk index-based detections of three different single strand DNAs and one type of bacteria are demonstrated for the FLRD-EF sensors that utilize a partially-etched single mode fiber as the sensor head. Stepwise coating of the sensor head with poly-L-lysine and a probe DNA has enabled surface index-based label-free target DNA sensing. We expect an array of FLRD-EF biosensors to be created, which are superior to counterparts in terms of simplicity, low cost, and high sensitivity.

Detection of bacteria using bacteriophages as recognition elements immobilized on long-period fiber gratings

The paper presents for the first time a study of long-period gratings (LPGs) applied for label-free detection of specific bacteria using physically adsorbed bacteriophages. For the purposes of the experiment a number of highly sensitive LPGs working at the turning point of phase matching curve was fabricated in SMF28 fiber using UV exposure. We show that the device allows for real-time monitoring of phenomena taking place on the sensor's surface, including phage-bacteria interactions. For the applied conditions a resonance wavelength shift of ~1.3 nm induced by bacteria binding was observed.

Transition mode long period grating biosensor with functional multilayer coatings

We report our latest research results concerning the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. The main novelty of this work lies in a multilayer design that allows to decouple the problem of an efficient surface functionalization from that of the tuning in transition region of the cladding modes. An innovative solvent/nonsolvent strategy for the dip-coating technique was developed in order to deposit on the LPG multiple layers of transparent polymers. In particular, a primary coating of atactic polystyrene was used as high refractive index layer to tune the working point of the device in the so-called transition region. In this way, state-of-the-art-competitive sensitivity to surrounding medium refractive index changes was achieved. An extremely thin secondary functional layer of poly(methyl methacrylate-co-methacrylic acid) was deposited onto the primary coating by means of an original identification of selective solvents. This approach allowed to obtain desired functional groups (carboxyls) on the surface of the device for a stable covalent attachment of bioreceptors and minimal perturbation of the optical design. Standard 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide / N-hydrosuccinimide (EDC / NHS) coupling chemistry was used to link streptavidin on the surface of the coated LPG. Highly sensitive real-time monitoring of multiple affinity assays between streptavidin and biotinylated bovine serum albumin was performed by following the shift of the LPGs attenuation bands.

Fiber-optic Mach-Zehnder interferometer as a high-precision temperature sensor: effects of temperature fluctuations on surface biosensing

In this article, we report the use of a fiber-optic Mach-Zehnder interferometer as a high-precision temperature sensor, and we investigate the effects of small temperature fluctuations on the reliability of the device as a surface biosensor. We found that typical temperature fluctuations generally cause sensor responses large enough that they must be compensated for before reliable surface chemistry measurements can be undertaken. These findings are particularly relevant in light of the recent surge of interest in fiber-optic based biosensors of various designs.

Long period grating working in transition mode as promising technological platform for label-free biosensing

We present the development of a platform for label-free biosensing based on overlayered Long Period Gratings (LPGs) working in transition mode. Nano-scale layers of Polystyrene (PS) with different thicknesses were deposited onto the same LPG to test the performances of the device in different working points of its modified sensitivity characteristic. Adsorption dynamic of biotinylated bovine serum albumin (BBSA) onto the PS overlays was on-line monitored as well as a subsequent streptavidin (SA) binding dynamic on the biotinylated sites of the protein ad-layer. Experimental results show that overlayered LPGs are among the most sensitive refractive index transducers to be employed in label-free biochemical detection and that wide margins of further optimization exist.

Optical fibre-based detection of DNA hybridization

A dual-peak LPFG (long-period fibre grating), inscribed in an optical fibre, has been employed to sense DNA hybridization in real time, over a 1 h period. One strand of the DNA was immobilized on the fibre, while the other was free in solution. After hybridization, the fibre was stripped and repeated detection of hybridization was achieved, so demonstrating reusability of the device. Neither strand of DNA was fluorescently or otherwise labelled. The present paper will provide an overview of our early-stage experimental data and methodology, examine the potential of fibre gratings for use as biosensors to monitor both nucleic acid and other biomolecular interactions and then give a summary of the theory and fabrication of fibre gratings from a biological standpoint. Finally, the potential of improving signal strength and possible future directions of fibre grating biosensors will be addressed.

Advances in Nucleic Acid Detection and Quantification

The last decade has seen many changes in molecular biology at the bench, as we have moved away from a primary goal of cataloguing genes and mRNA towards techniques that detect and quantify nucleic acid molecules even within single cells. With the invention of the polymerase chain reaction (PCR), a nucleic acid sequence could now be amplified to generate a large number of identical copies, and this launched a new era in genetic research. PCR has developed in parallel to fluorescent hybridization probing to provide low-, medium- and high-throughput detection methods. However, PCR and hybridization detection have significant drawbacks as long-term solutions for routine research and diagnostics assays. Therefore many novel methods are being developed independently, but as yet no one technique has emerged as a clear replacement for PCR, microarrays or even sequencing. In order to examine the technological horizon in this area, around 90 delegates assembled at Hinxton Hall, Cambridge, U.K. on 28 and 29 October 2008 for a Biochemical Society/Wellcome Trust Focused Meeting sponsored by Thermo Fisher Scientific and the British Library. The title of the meeting was ‘Advances in Nucleic Acid Detection and Quantification’, and the primary aim was to bring together scientists from different disciplines who nevertheless are trying to develop reliable methods for the quantification or detection of RNA and DNA molecules. This meant that physical and organic chemists, microbial ecologists and clinicians appeared alongside molecular biologists. An introductory session on general nucleic acid detection technologies was initiated with a fascinating insight into single-molecule, singlecell hybridization from Professor Sir Edwin Southern. This served as an ideal base for sessions on single-cell molecular biology and an examination of current applications of emerging technology. This issue of Biochemical Society Transactions contains some of the papers prepared by speakers at the meeting, and highlights not only how PCR and microarrays are already being replaced, but also which methods are likely to replace them.

Sensitive DNA biosensor based on a long-period grating formed on the side-polished fiber surface

We demonstrate a sensitive DNA biosensor based on a long period grating (LPG) formed by a photolithograph process on the surface of a side-polished fiber. The biomolecules of the biosensor were immobilized on the silica surface between LPG patterns. The resonance wavelength was red-shifted after the binding of the poly-L-lysine, probe ssDNA and target ssDNA to the sensor surface. The overall wavelength shift after the successful DNA hybridization was 1.82 nm. The proposed LPG-based DNA biosensor is approximately 2.5 times more sensitive than the previously reported fiber grating-based DNA biosensors.