Microstructured Optical Fiber-based Biosensors: Reversible and Nanoliter-Scale Measurement of Zinc Ions

Peptides in the detection of metal ions

By means of their specific interactions with different metal ions, naturally occurring proteins control structures and functions of many biological processes and functions in organisms. In view of natural metallopeptides, scientists have proposed artificial peptides which coordinate with metal ions through their functional groups either for introducing a special reactivity or for constructing various sensors. However, the design of new peptide ligands requires a deep understanding of the structures, assembly properties, and dynamic behaviors of such peptides. This review briefly describes detection strategies of metal ions via coordination to the binding sites in peptides. The principles and functions of sensing systems are described as well. We also highlight some examples of a metal-induced peptide self-assembly with relevance to biotechnology applications.

Single-fiber probes for combined sensing and imaging in biological tissue: recent developments and prospects

Single-fiber-based sensing and imaging probes enable the co-located and simultaneous observation and measurement (i.e., 'sense' and 'see') of intricate biological processes within deep anatomical structures. This innovation opens new opportunities for investigating complex physiological phenomena and potentially allows more accurate diagnosis and monitoring of disease. This prospective review starts with presenting recent studies of single-fiber-based probes for concurrent and co-located fluorescence-based sensing and imaging. Notwithstanding the successful initial demonstration of integrated sensing and imaging within single-fiber-based miniaturized devices, the realization of these devices with enhanced sensing sensitivity and imaging resolution poses notable challenges. These challenges, in turn, present opportunities for future research, including the design and fabrication of complex lens systems and fiber architectures, the integration of novel materials and other sensing and imaging techniques.

Microfluidic analysis of 3T3 cellular transport in a photonic crystal fiber: part I

This microfluidic-optical fiber sensor is an experimental system designed to transport and monitor 3D cell cultures, facilitating medical research and technology. This system includes a photonic crystal fiber with a hollow core diameter of 22 µm, which functions as a bridge between two microfluidic devices. The purpose of this system was to transport 3T3 cells (of diameters from 15 µm to 23 µm) between the two devices. At low Reynold's and capillary numbers, spectroscopic analysis confirmed the presence of cellular aggregation at the interface of the fiber and microfluidic device. The transcapillary conductance, T C, is a separate analysis that models the behavior of a cellular aggregate through the hollow channel of a photonic crystal fiber. For the experimental system, conventional fluid mechanics theory is limited and requires special treatment of conditions at the microscale, such that transcapillary conductance treatment was employed. The transcapillary conductance, T C, was empirically derived to model cellular transport at the microfluidic scale and is useful for comparing transport events. For example, for a pressure differential of Δ p=1.5⋅103 c m H 2 O, the transcapillary conductance values were determined to be 10-12 Collapse

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A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

Optical Fibre-Enabled Photoswitching for Localised Activation of an Anti-Cancer Therapeutic Drug

Local activation of an anti-cancer drug when and where needed can improve selectivity and reduce undesirable side effects. Photoswitchable drugs can be selectively switched between active and inactive states by illumination with light; however, the clinical development of these drugs has been restricted by the difficulty in delivering light deep into tissue where needed. Optical fibres have great potential for light delivery in vivo, but their use in facilitating photoswitching in anti-cancer compounds has not yet been explored. In this paper, a photoswitchable chemotherapeutic is switched using an optical fibre, and the cytotoxicity of each state is measured against HCT-116 colorectal cancer cells. The performance of optical-fibre-enabled photoswitching is characterised through its dose response. The UV–Vis spectra confirm light delivered by an optical fibre effectively enables photoswitching. The activated drug is shown to be twice as effective as the inactive drug in causing cancer cell death, characterised using an MTT assay and fluorescent microscopy. This is the first study in which a photoswitchable anti-cancer compound is switched using an optical fibre and demonstrates the feasibility of using optical fibres to activate photoswitchable drugs for potential future clinical applications.

Binding Analysis of Functionalized Multimode Optical-Fiber Sandwich-like Structure with Organic Polymer and Its Sensing Application for Humidity and Breath Monitoring

In recent years, the chemical modification of optical fibers (OFs) has facilitated the manufacture of sensors because OFs can identify several analytes present in aqueous solutions or gas phases. Nevertheless, it is imperative better to understand the chemical interactions in this molecular system to generate low-cost and efficient sensors. This work presents a theoretical and experimental study of organic polymeric functionalized OF structures and proposes a cost-effective alternative to monitor breathing and humidity. The device is based on silicon optical fibers functionalized with (3-Aminopropyl) triethoxysilane (APTES) and alginate. The theoretical analysis is carried out to validate the activation of the silicon dioxide fiber surface; moreover, the APTES–alginate layer is discussed. The computational simulation suggests that water can be absorbed by alginate, specifically by the calcium atom linked to the carboxylic acid group of the alginate. The analysis also demonstrates a higher electrostatic interaction between the water and the OF–APTES–alginate system; this interaction alters the optical fiber activated surface’s refractive index, resulting in transmission power variation. The humidity analysis shows a sensitivity of 3.1288 mV/RH, a time response close to 25 s, and a recovery time around 8 s. These results were achieved in the range of 50 to 95% RH. Moreover, the recovery and response time allow the human breath to be studied. The proposed mechanism or device is competitive with prior works, and the components involved made this sensor a cost-effective alternative for medical applications.

A biophotonic approach to measure pH in small volumes in vitro: Quantifiable differences in metabolic flux around the cumulus-oocyte-complex (COC)

Unfertilised eggs (oocytes) release chemical biomarkers into the medium surrounding them. This provides an opportunity to monitor cell health and development during assisted reproductive processes if detected in a non-invasive manner. Here we report the measurement of pH using an optical fibre probe, OFP1, in 5 μL drops of culture medium containing single mouse cumulus oocyte complexes (COCs). This allowed for the detection of statistically significant differences in pH between COCs in culture medium with no additives and those incubated with either a chemical (cobalt chloride) or hormonal treatment (follicle stimulating hormone); both of which serve to induce the release of lactic acid into the medium immediately surrounding the COC. Importantly, OFP1 was shown to be cell-safe with no inherent cell toxicity or light-induced phototoxicity indicated by negative DNA damage staining. Pre-measurement photobleaching of the probe reduced fluorescence signal variability, providing improved measurement precision (0.01-0.05 pH units) compared to previous studies. This optical technology presents a promising platform for the measurement of pH and the detection of other extracellular biomarkers to assess cell health during assisted reproduction.

Functionalized Microstructured Optical Fibers: Materials, Methods, Applications

Microstructured optical fiber-based sensors (MOF) have been widely developed finding numerous applications in various fields of photonics, biotechnology, and medicine. High sensitivity to the refractive index variation, arising from the strong interaction between a guided mode and an analyte in the test, makes MOF-based sensors ideal candidates for chemical and biochemical analysis of solutions with small volume and low concentration. Here, we review the modern techniques used for the modification of the fiber's structure, which leads to an enhanced detection sensitivity, as well as the surface functionalization processes used for selective adsorption of target molecules. Novel functionalized MOF-based devices possessing these unique properties, emphasize the potential applications for fiber optics in the field of modern biophotonics, such as remote sensing, thermography, refractometric measurements of biological liquids, detection of cancer proteins, and concentration analysis. In this work, we discuss the approaches used for the functionalization of MOFs, with a focus on potential applications of the produced structures.

Microstructured Optical Waveguide-Based Endoscopic Probe Coated with Silica Submicron Particles

Microstructured optical waveguides (MOW) are of great interest for chemical and biological sensing. Due to the high overlap between a guiding light mode and an analyte filling of one or several fiber capillaries, such systems are able to provide strong sensitivity with respect to variations in the refractive index and the thickness of filling materials. Here, we introduce a novel type of functionalized MOWs whose capillaries are coated by a layer-by-layer (LBL) approach, enabling the alternate deposition of silica particles (SiO₂) at different diameters—300 nm, 420 nm, and 900 nm—and layers of poly(diallyldimethylammonium chloride) (PDDA). We demonstrate up to three covering bilayers consisting of 300-nm silica particles. Modifications in the MOW transmission spectrum induced by coating are measured and analyzed. The proposed technique of MOW functionalization allows one to reach novel sensing capabilities, including an increase in the effective sensing area and the provision of a convenient scaffold for the attachment of long molecules such as proteins.

Surface Functionalization of Exposed Core Glass Optical Fiber for Metal Ion Sensing

One of the biggest challenges associated with exposed core glass optical fiber-based sensing is the availability of techniques that can be used to generate reproducible, homogeneous and stable surface coating. We report a one step, solvent free method for surface functionalization of exposed core glass optical fiber that allows achieving binding of fluorophore of choice for metal ion sensing. The plasma polymerization-based method yielded a homogeneous, reproducible and stable coating, enabling high sensitivity aluminium ion sensing. The sensing platform reported in this manuscript is versatile and can be used to bind different sensing molecules opening new avenues for optical fiber-based sensing.

Spiropyran-Based Nanocarrier: A New Zn2+ -Responsive Delivery System with Real-Time Intracellular Sensing Capabilities

A new spiropyran-based stimuli-responsive delivery system is fabricated. It encapsulates and then releases an extraneous compound in response to elevated levels of Zn²⁺ , a critical factor in cell apoptosis. A C₁₂ -alkyl substituent on the spiropyran promotes self-assembly into a micelle-like nanocarrier in aqueous media, with nanoprecipitation and encapsulation of added payload. Zn²⁺ binding occurs to an appended bis(2-pyridylmethyl)amine group at biologically relevant micromolar concentration. This leads to switching of the spiropyran (SP) isomer to the strongly fluorescent ring opened merocyanine-Zn²⁺ (MC-Zn²⁺ ) complex, with associated expansion of the nanocarriers to release the encapsulated payload. Payload release is demonstrated in solution and in HEK293 cells by encapsulation of a blue fluorophore, 7-hydroxycoumarin, and monitoring its release using fluorescence spectroscopy and microscopy. Furthermore, the use of the nanocarriers to deliver a caspase inhibitor, Azure B, into apoptotic cells in response to an elevated Zn²⁺ concentration is demonstrated. This then inhibits intracellular caspase activity, as evidenced by confocal microscopy and in real-time by time-lapsed microscopy. Finally, the nanocarriers are shown to release an encapsulated proteasome inhibitor (5) in Zn²⁺ -treated breast carcinoma cell line models. This then inhibits intracellular proteasome and induces cytotoxicity to the carcinoma cells.

A Liposomal Platform for Sensing of Extracellular Analytes Near Cells

Cell-permeable fluorescent chemosensors (calcein, monochlorobimane, and a recently reported spiropyran-based sensor SP2) have been incorporated into yeast total lipid extract-based liposomes to suppress inherent cell permeability to allow the detection of extracellular Ca²⁺, GSH, and Zn²⁺, respectively. The repurposed sensors have enhanced aqueous solubility and the ability to quantitatively measure biologically relevant concentrations of Ca²⁺ (0.25 mM–1 mM), Zn²⁺ (6.25 µM–50 µM), and GSH (0.25 mM–1 mM) by fluorescence in aqueous media. In addition, the liposomal sensors are nontoxic to HEK293 cells and have the ability to detect exogenously added Zn²⁺ (1 mM), Ca²⁺ (1 mM), or GSH (1 mM) near cells without internalisation. This new sensing platform provides a means to repurpose a range of intracellular fluorescent sensors to specifically detect extracellular analytes, while also improving biocompatibility for overall enhanced use in a wide range of biomedical applications.

Light-responsive polymers for microfluidic applications

While the microfluidic device itself may be small, often the equipment required to control fluidics in the chip unit is large e.g. pumps, valves and mixing units, which can severely limit practical use and functional scalability. In addition, components associated with fluidic control of the device, more specifically the valves and pumps, contribute significantly to the overall unit cost. Here we sketch the problem of a gap between high end accurate, but expensive sensor platforms, versus less accurate, but widely employable hand-held low-cost devices. Recent research has shown that the integration of light-responsive materials within microfluidic devices can provide the function of expensive fluidic components, and potentially enable sophisticated measurements to be made using much less expensive equipment. An overview of the most recent developments will be presented for valves, mixers, transport and sample handling inside microfluidic devices.

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.

Rationally Designed Probe for Reversible Sensing of Zinc and Application in Cells

Biologically compatible fluorescent ion sensors, particularly those that are reversible, represent a key tool for answering a range of fundamental biological questions. We report a rationally designed probe with a 6'-fluoro spiropyran scaffold (5) for the reversible sensing of zinc (Zn²⁺) in cells. The 6'-fluoro substituent overcomes several limitations normally associated with spiropyran-based sensors to provide an improved signal-to-background ratio and faster photoswitching times in aqueous solution. In vitro studies were performed with 5 and the 6'-nitro analogues (6) in HEK 293 and endothelial cells. The new spiropyran (5) can detect exogenous Zn²⁺ inside both cell types and without affecting the proliferation of endothelial cells. Studies were also performed on dying HEK 293 cells, with results demonstrating the ability of the key compound to detect endogenous Zn²⁺ efflux from cells undergoing apoptosis. Biocompatibility and photoswitching of 5 were demonstrated within endothelial cells but not with 6, suggesting the future applicability of sensor 5 to study intracellular Zn²⁺ efflux in these systems.

A Rationally Designed Reversible 'Turn-Off' Sensor for Glutathione

γ-Glutamyl-cysteinyl-glycine (GSH) plays a critical role in maintaining redox homeostasis in biological systems and a decrease in its cellular levels is associated with diseases. Existing fluorescence-based chemosensors for GSH acts as irreversible reaction-based probes that exhibit a maximum fluorescence (‘turn-on’) once the reaction is complete, regardless of the actual concentration of GSH. A reversible, reaction-based ‘turn-off’ probe (1) is reported here to sense the decreasing levels of GSH, a situation known to occur at the onset of various diseases. The more fluorescent merocyanine (MC) isomer of 1 exists in aqueous solution and this reacts with GSH to induce formation of the ring-closed spiropyran (SP) isomer, with a measurable decrease in absorbance and fluorescence (‘turn-off’). Sensor 1 has good aqueous solubility and shows an excellent selectivity for GSH over other biologically relevant metal ions and aminothiol analytes. The sensor permeates HEK 293 cells and an increase in fluorescence is observed on adding buthionine sulfoximine, an inhibitor of GSH synthesis.