We demonstrate controlled transitions between competing radiative and nonradiative decay channels in the up-conversion luminescence of NaYF4:Er3+/Yb3+ nanocrystals placed in proximity to metallic nanowires. The nanocrystal-nanowire separation is used as a key control parameter governing the optical response. An essential aspect of our approach is the reduction of inhomogeneous residual polymer layers from the nanowires, thereby reducing uncertainties associated with distance control and improving reproducibility in emitter-metal hybrid nanostructures. Replacing them with a well-defined polymer spacer yields controlled access to three qualitatively distinct interaction regimes: luminescence quenching, plasmonic enhancement, and effective decoupling. Transitions between these regimes are shown to reflect changes in the dominant energy relaxation pathways: from nonradiative losses in direct contact with the metal, through modification of the local photonic density of states and coupling to plasmon-mediated modes, to behavior characteristic of quasi-isolated emitters. The plasmonic origin of the emission enhancement in the intermediate regime is evidenced by an increase in luminescence intensity accompanied by shortened decay times, as revealed by fluorescence lifetime imaging microscopy of up-conversion nanocrystals. All experiments were performed on single nanostructures, thereby excluding artefacts arising from aggregation effects and strengthening the interpretation of the observed phenomena. The presented approach provides controlled access to plasmon-modified decay channels and offers a basis for the rational design of functional nanophotonic and sensing structures with tailored optical properties.

In the quest to construct photonic circuits, the key issue is the coupling of light into subwavelength devices. While plasmonic waveguides can be used as building blocks in novel nanodevices, for efficient energy transmission, the distance between the plasmonic nanostructures must not exceed a few nanometers due to the rapidly decreasing coupling. Here, we study a system of two silver nanowires positioned precisely in order to control the separation between their ends. In such a structure, any measurable energy propagation between the nanowires is possible upon direct contact, as when the ends of the nanowires are away, they are also isolated optically from each other. We show that by combining precise positioning of the nanowires with the deposition of aqueous microdroplets, the coupling between two silver nanowires can be restored using colloidal quantum dots (QDs) as an interfacing medium. This effect is observed even if the QD-interfaced nanowires are separated by over a micrometer. The energy transfer between QDs in the microdroplet, as evidenced by shortening of the decay time of the QD fluorescence, is efficient enough to consider extending this method to facilitate and control the coupling between any type of nanostructure in nanophotonic devices.

Objective: Endovascular surgery requires accurate measurement of parameters such as pressure, temperature, and biomarkers within vessels for real-time tissue response monitoring and ensuring targeted therapeutic interventions. However, the availability of small tip-based sensors capable of precise application, for example, navigating an aneurysm's lumen, is limited. With their capabilities for real-time analysis, flexibility, and biocompatibility, optical fiber sensors (OFS) hold promise in addressing this need. This proof-of-concept study investigates the feasibility of OFS in endovascular surgery scenarios. Methods: The sensor is based on a single-mode silica fiber with an interferometric forward-facing thin-film tip. The thin-film materials may be tailored for detecting various physical parameters and, when functionalized, also specific analytes. Materials applied in this sensor are thin metal oxides deposited using magnetron sputtering. A full-scale 3D-printed vascular model was employed to simulate endovascular setup. Results: The experiments showed the high mechanical robustness of the approach, i.e., the sensor maintained functionality while being maneuvered through the endovascular model. The forward-facing tip remained intact and worked adequately, ensuring consistent and stable readouts. Moreover, the fiber showed sufficient flexibility, with no significant bending loss observed during simulations. Finally, the performance of the OFS in bovine serum samples was assessed. The sensor performed well in serum, and the results suggest that low-concentration serum may be used to reduce nonspecific surface interactions. Conclusion: Overall, this OFS system offers a promising solution for endovascular surgery and other biomedical applications, allowing for precise and on-the-spot analysis. Significance: Our study pioneers the feasibility of thin-film interferometric label-free OFS with a forward-facing sensitive area for sensing during endovascular procedures.

Expression of concern for 'An impedimetric immunosensor based on diamond nanowires decorated with nickel nanoparticles' by Palaniappan Subramanian et al., Analyst, 2014, 139, 1726-1731, https://doi.org/10.1039/C3AN02045B.

Expression of concern for ‘Preparation of reduced graphene oxide–Ni(OH)2 composites by electrophoretic deposition: application for non-enzymatic glucose sensing’ by Palaniappan Subramanian et al., J. Mater. Chem. A, 2014, 2, 5525–5533, https://doi.org/10.1039/C4TA00123K.

Point-of-care testing (POCT) devices play a crucial role as tools for disease diagnostics, and the integration of biorecognition elements with electronic components into these devices widens their functionalities and facilitates the development of complex quantitative assays. Unfortunately, biosensors that exploit large conventional IgG antibodies to capture relevant biomarkers are often limited in terms of sensitivity, selectivity, and storage stability, considerably restricting the use of POCT in real-world applications. Therefore, we used nanobodies as they are more suitable for fabricating electrochemical biosensors with near-field communication (NFC) technology. Moreover, a flow-through microfluidic device was implemented in this system for the detection of C-reactive protein (CRP), an inflammation biomarker, and a model analyte. The resulting sensors not only have high sensitivity and portability but also retain automated sequential flow properties through capillary transport without the need for an external pump. We also compared the accuracy of CRP quantitative analyses between commercial PalmSens4 and NFC-based potentiostats. Furthermore, the sensor reliability was evaluated using three biological samples (artificial serum, plasma, and whole blood without any pretreatment). This platform will streamline the development of POCT devices by combining operational simplicity, low cost, fast analysis, and portability.

This work discusses label-free biosensing application of a double-layer optical fiber interferometer where the second layer tailors the reflection conditions at the external plain and supports changes in reflected optical spectrum when a bio-layer binds to it. The double-layer nanostructure consists of precisely tailored thin films, i.e., titanium (TiO2) and hafnium oxides (HfO2) deposited on single-mode fiber end-face by magnetron sputtering. It has been shown numerically and experimentally that the approach besides well spectrally defined interference pattern distinguishes refractive index (RI) changes taking place in a volume and on the sensor surface. These are of interest when label-free biosensing applications are considered. The case of myeloperoxidase (MPO) detection-a protein, which concentration rises during inflammation-is reported as an example of application. The response of the sensor to MPO in a concentration range of 1 x 10-11-5 x 10-6 g/mL was tested. An increase in the MPO concentration was followed by a redshift of the interference pattern and a decrease in reflected power. The negative control performed using ferritin proved specificity of the sensor. The results reported in this work indicate capability of the approach for diagnostic label-free biosensing, possibly also at in vivo conditions.

Precise deposition of materials on surfaces is one of the crucial steps in a broad range of applications and functional device fabrication at both the micro- and nanoscale. Semiconductor quantum dots (QDs), with their unique optical and physical properties, have frequently been a focus of attempts for micro- or nano-positioning. Here, we present a method for reproducible, repetitive, and precise deposition of QD-containing microdroplets using hydrophobic micropipettes without any need to apply an actuation voltage. We show that upon deposition, the droplets can be translated across the surface and placed in defined patterns. The incorporation of semiconductor QDs allows for the confirmation of the morphological integrity of the microdroplets after deposition and translation. Fluorescence blinking observed for droplets containing highly diluted QD solution proves that our approach is suitable for embedding individual emitters in such microdroplets.

Metallic structures play a pivotal role in the processes induced or enhanced by plasmon resonances. In this work, we describe the utilization of light energy converted to the surface plasmon polaritons (SPPs) for the initiation of photocatalytic reactions (semiconductor photocatalysis). Irradiation of silver nanowires coated with titanium dioxide results in the SPPs propagation and excitation of the photocatalytic system ca. 10 mu m away from the illumination spot. The reaction progress is revealed by transforming almost nonfluorescent 3 '-(p-aminophenyl) fluorescein to strongly emitting fluorescein. Remote plasmonic photocatalysis can be further explored in the fields of photocatalysis, photovoltaics, and sensorics.

Phage-derived affinity peptides have become widespread thanks to their easy selection via phage display. Interactions between a target protein and its specific peptide are similar to those between antibodies and antigens. The strength of these non-covalent complexes may be described by the dissociation constant (Kd). In this paper, protein-specific peptides are exposed on the pIII protein present in the M13 bacteriophage virion with up to five copies. Therefore, one phage particle can bind from one to five ligands. Here, we discuss the dependences between phage-displayed peptides and their ligands in solution using a model system based on troponin T (TnT) binding phages. Moreover, a method of calculating Kd values from ELISA experiments was developed and is presented. The determined Kd values are in the picomolar range.

Detection of biologically relevant substances is an important aspect of interdisciplinary research with aim to specifically, selectively and efficiently determine the presence of various analytes. In our research, we test the concept of applying patterned silver nanostructures featuring plasmon resonance for efficient fluorescence – based detection of photoactive proteins. We will describe the recent results about controlling spatial position of plasmonic paths made of silver islands, as well as of silver nanowires. Detection protocol is based on real-time imagining using wide-field fluorescence microscopy, which allows for observing emission of individual proteins. The approach is universal and applicable for variety of analytes.

Point-of-care testing (POCT) devices are currently available for the detection of various important diseases owing to their easy-to-use, portable, and user-friendly nature. However, the multistep regent manipulation still impedes the performance of the device for end-users. To address this issue, we present a novel sequential flow-through microfluidic device that incorporates dual flow behaviors (fast-flow/delayed) into a single device for assessing the risk of cardiovascular disease (CVDs) using C-reactive protein (CRP) as the model analyte. The device fea-tures a fast-flow channel for automated washing of unbound CRP antigens and a delayed channel for flow of a redox reagent for electrochemical analysis. It requires only a single buffer loading, resulting in a total analysis time of 15 min. To overcome challenges related to storage stability and the affinity of antibodies in the manufacturing process, we integrated the P3-CRP peptide as a new bioreceptor for CRP detection. The perfor-mance of both the P3-CRP peptide and antibodies was assessed and examined using chronocoulometry. The proposed device exhibited a limit of detection at 47 pg mL-1 with a wide linear range (5 orders of magnitude). To validate its applicability, the device was tested with serum, plasma, and whole blood samples, yielding satis-factory results.

We developed a method of aligning silver nanowires in a microchannel and fixing them to glass substrates via appropriate functionalization. The attachment of nanowires to the substrate is robust with no variation of their angles over minutes. Specific conjugation with photoactive proteins is observed using wide-field fluorescence imaging in real-time for highly concentrated protein solution, both in a microchannel and in a chip geometry. In the latter case we can detect the presence of the proteins in the dropcasted solution down to single proteins. The results point towards possible implementation of aligned silver nanowires as geometrically defined plasmonic fluorescence sensing platforms.

We investigate the interactions between C-reactive protein (CRP) and new CRP-binding peptide materials using experimental (biological and physicochemical) methods with the support of theoretical simulations (computational modeling analysis). Three specific CRP-binding peptides (P2, P3, and P9) derived from an M13 bacteriophage have been identified using phage-display technology. The binding efficiency of the peptides exposed on phages toward the CRP protein was demonstrated via biological methods. Fibers of the selected phages/peptides interact differently due to different compositions of amino acid sequences on the exposed peptides, which was confirmed by transmission electron microscopy. Numerical and experimental studies consistently showed that the P3 peptide is the best CRP binder. A combination of theoretical and experimental methods demonstrates that identifying the best binder can be performed simply, cheaply, and fast. Such an approach has not been reported previously for peptide screening and demonstrates a new trend in science where calculations can replace or support laborious experimental techniques. Finally, the best CRP binder─the P3 peptide─was used for CRP recognition on silicate-modified indium tin oxide-coated glass electrodes. The obtained electrodes exhibit a wide range of operation (1.0-100 μg mL-1) with a detection limit (LOD = 3σ/S) of 0.34 μg mL-1. Moreover, the dissociation constant Kd of 4.2 ± 0.144 μg mL-1 (35 ± 1.2 nM) was evaluated from the change in the current. The selectivity of the obtained electrode was demonstrated in the presence of three interfering proteins. These results prove that the presented P3 peptide is a potential candidate as a receptor for CRP, which can replace specific antibodies.

This work describes an experimental study towards label-free sensing of C-reactive protein (CRP) – a protein recognized as a inflammation marker. A multimode optical fiber with a section of its core coated with indium tin oxide (ITO) thin film was used as a sensor. ITO film allows for guiding lossy modes and can simultaneously be used as a transparent electrode for electrochemical measurements. Therefore, optical and electrochemical detection based on a single sensor was possible. Such a dual-domain approach is practical, especially when the results in one of the domains are not accurate enough, which was the case in this work. A case of different functionalization methods of ITO surface was also pointed out. The proposed sensor allows for recognition as low as ng/mL.

Optical fiber probe based on fluorine-doped tin oxide thin film has been applied for electric potential measurements in electrochemical configuration. The sensitivity reaches up to 35 nm/V in potential range -0.6 V to 0.6 V.

In this work, we aimed to apply fluorescence microscopy to image protein conjugation to Ni-NTA modified silver nanowires in real time via the His-tag attachment. First, a set of experiments was designed and performed for the mixtures of proteins and silver nanowires in order to demonstrate plasmon enhancement of mCherry protein fluorescence as well as the ability to image fluorescence of single molecules. The results indicated strong enhancement of single-protein fluorescence emission upon coupling with silver nanowires. This conclusion was supported by a decrease in the fluorescence decay time of mCherry proteins. Real-time imaging was carried out for a structure created by dropping protein solution onto a glass substrate with functionalized silver nanowires. We observed specific attachment of mCherry proteins to the nanowires, with the recognition time being much longer than in the case of streptavidin–biotin conjugation. This result indicated that it is possible to design a universal and efficient real-time sensing platform with plasmonically active functionalized silver nanowires.