In recent years, the design and synthesis of fluorescent sensors have drawn considerable attention for the specific and sensitive detection of biologically and environmentally important metal ions in living cells and aqueous solutions. Herein, we reported the photophysical properties of a Schiff base fluorescent sensor N′-(2-hydroxy-3-methoxybenzylidene)-3-methoxybenzohydrazide (RN5) that possesses a high affinity for the Ni2+ at a nanomolar concentration. Through fluorescence titration analysis at 600 nm, we confirmed that the sensor RN5 exhibited a colorimetric (from colorless to yellow) and fluorometric turn-on response (fluorescence enhancement) in CH3CN:H2O (2/8, v/v) medium in the presence of Ni2+, whereas no noticeable emission was observed in the case of other examined metals ion. The complexation reaction of Ni2+ with RN5 has been investigated by the colorimetric, absorption spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The 1:1 binding mode was confirmed between the sensor RN5 and Ni2+ by job plot evaluation and theoretically confirmed by DFT studies. The detection limit for Ni2+ was found to be 12.8 nM estimated by the titration method, which was lower than the guideline for drinking water, 1.2 μM by the Environmental Protection Agency (EPA), USA. The sensor RN5 exhibited the excellent fluorescence sensing behavior in a wide pH range of 5–12. The turn-on fluorescence behavior of the Ni2+ with the sensor RN5 was fast, so that it could be employed for quantitative and qualitative determination. Test kits, containing RN5 were fabricated which could perform as the suitable and complete assay “in-the-field” detection of Ni2+. Finally, the sensor RN5 was applied successfully for the detection of Ni2+ in the real water samples.
The supplementary files can be downloaded from the webpage of this article in Photonic Sensors (https://link.springer.com/journal/13320).
Supplementary material is available for this article at https://doi.org/10.1007/s13320-025-0763-3 and is accessible for authorized users.
This study presented the enhancement of a plasmonic optical sensor utilizing the surface plasmon resonance (SPR) to detect potassium ions (K+). The sensor performance was improved by integrating a nanocomposite carbon-based material, i.e., multiwalled carbon nanotubes-nanocrystalline cellulose (MWCNT-NCC), as the sensing layer. The SPR curve analysis was carried out by the evaluation of critical parameters, including the detection range, binding affinity, sensitivity, full width at half maximum (FWHM), data accuracy (DA), and signal-to-noise ratio (SNR). The results showed that the sensor detection range was between 0.08 ppm and 0.6 ppm before reaching saturation. The sensor also had a good sensitivity value of 0.595 6 °·ppm−1. The Langmuir and Sips isotherm models were used for the binding affinity, and the calculated binding affinity constants were 1.586 6×105 M−1 and 1.644 1×105 M−1, respectively, much higher than the previously reported binding affinity constant for metal ion detection. Based on the Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis, it was demonstrated that the MWCNT-NCC thin film contained COOH functional groups that might bind with K+ through the electrostatic interaction, leading to improved sensing capabilities of the sensor.
Eigenmode expansion (EME) is a widely used method for modeling the electromagnetic wave propagation in multimode waveguides, where it breaks down signals into local eigenmodes and calculates them independently. Nevertheless, this methodology may challenge the causality mandated by the theory of special relativity, thus potentially disrupting the cause-and-effect relationship. This study experimentally explored light transmission in the multimode coreless fiber and found discrepancies between the EME method and measurement. To reconcile these inconsistencies, we introduced a light cone model, providing an alternative interpretation guided by the principles of special relativity. Remarkably, this innovative model did not merely resolve the observed discrepancies between the theory and experiments, but also presented a pioneering technique for designing microbend sensors. Through experimentation, we achieved the remarkable sensitivity of 500 dB/m−1 at a bending curvature of 0 m−1. Our research advances the understanding of multimode systems and paves the way for innovative sensing and communications applications in compact devices.
The photoacoustic spectroscopy apparatus based on an external cavity quantum cascade laser was employed to simultaneously detect four SF6 decomposition by-products, i.e., SO2F2, SOF2, CF4, and SO2 for the application in diagnostics of the SF6 insulation equipment widely used in high-voltage transmission lines. The influence of background gases SF6 and N2 with the He addition in different ratios on the SOF2 photoacoustic signal was investigated, with the pure SF6 showing the most pronounced enhancement effect on the photoacoustic signal of the target analyte. The limit of detection of SOF2 in pure SF6 was estimated by comparing the limits of detection of SOF2, CF4, and SO2 in the 95% SF6/5% He combination and pure SF6. The estimated detection limits for SO2F2, SOF2, CF4, and SO2 were 635.4 ppb, 24.0 ppb, 10.3 ppb, and 97.8 ppb, respectively. Finally, the concentration of each decomposition component in the gas mixture of SO2F2, SOF2, CF4, and SO2 in the 95% SF6/5% He background gas was quantified by multiple linear regression.
In many structural components, a major interest lies on the monitoring of the exact condition of a component or the loads acting on it for which sensors are an important tool. The usage of fiber-Bragg-grating (FBG) sensors has many advantages considering sensor embedment and strain measurement in fiber reinforced composites. However, the direct calculation of applied loads based on the measured strain is impeded when FBG sensors are integrated conventionally, mainly due to parasitic effects from the cross coupling of axial and torsional strains. The presented work introduces an FBG patch produced in tailored fiber placement. A calculation approach is presented, which allows the calculation of superimposed loads from the FBG-strain while compensating for temperature and cross coupling effects. Experimental data from the use case of boring & trepanning association (BTA) deep hole drilling are presented to verify the calculation approach for the sensor patch and show vastly improved measurement accuracy compared to the conventional FBG integration.
Optical resonators are now essential in modern sensing applications, particularly in photoacoustic imaging technologies. Among these, three-dimensional photoacoustic computed tomography (3D-PACT) emerged as a significant area of research. This sophisticated technique involves two critical phases: first, the optical capture of acoustically scanned signals, and second, the optoelectrical demodulation of these acoustic responses. In this study, we present groundbreaking research on both facets and introduce a novel 3D-PACT system aimed at enhancing imaging performance. This system employs an array of 20 chalcogenide (Ge25Sb10S65) micro-ring resonators (MRRA) as the acoustic sensors, each micro-ring resonator featuring a radius of 20 µm and an average quality factor (Q-factor) of 5.5×105. Simultaneously, a digital optical frequency comb (DOFC) technique is introduced for parallel spectral detection and acoustic signal demodulation within the MRRA. By utilizing on-chip thermal electrodes to tune the resonance wavelengths of 20 micro-ring resonators, the DOFC method enables efficient parallel spectral demodulation of the MRRA, reducing the scanning time in the PACT by a factor of 20 compared to a single sensor. We demonstrate the performance of the 3D-PACT system using cross-sectional hair strands and leaf skeletons. The MRRA-based 3D-PACT system is a promising tool for structural, functional, and molecular imaging of deep biological tissues.
A biopsy needle is an essential surgical tool used to extract several pieces of tissue for diagnosing diseases such as prostate cancer. To facilitate cancer diagnosis, a real-time force-sensing biopsy needle comprising a stylet with an outer tubular sleeve (cannula) was developed in this study. For compatibility with magnetic resonance imaging (MRI)-guided prostate biopsy, fiber Bragg grating (FBG) sensors are embedded into the stylet. The performance of the individual stylet and the entire needle was experimentally evaluated, and the resolutions for force measurement were found to be 1.40 mN and 1.60 mN, respectively, with a root mean square error (RMSE) of 5.6 mN; these are sufficient for differentiating normal prostate tissue from cancer tissue. Since FBG sensors are affected by both strain and temperature, the effects of temperature due to the different sensor positions were analyzed to enable temperature compensation for the needle. The real-time force-sensing performance of the needle was evaluated at room temperature by inserting it into the heated chicken breast, which was used to replicate the typical human body temperature. The forces measured using the proposed needle were observed to be in close agreement with reference measurement from a force gauge; specifically, the measurement errors were found to be 0.021 2 N for the proposed biopsy needle.
An optically levitated rotating sphere is an ultrasensitive torque sensor. In this article, a method was presented to apply a specially manufactured defective hollow homogeneous sphere as a stable rotor. The numerical relationship between the external torque and attitude of a suspended rotating defective homogeneous sphere captured by a circularly polarized laser was determined. The trap stiffness and dynamic process of different particles were comparted to determine the feature of an ideal rotor. Particles with larger hollow radii and centrifugal distances had greater potential in torque detection. The simulation of the trail path and the stabilization process of particles showed that rotating motion could effectively cool particles and neutralize the optical force brought by the imbalance of the rotor. A defective nanoparticle was droved to rotate at 3 kHz and the cooling effect was successfully observed. The analytical formulae and simulation results analyzed the gyroscope effect and provided selection criteria for rotors in optical tweezers for precise torque detection.
Whispering gallery mode microresonators significantly enhance light-matter interactions, making them ideal platforms for a wide range of applications, including lasers, nonlinear converters, modulators, and sensors. Recently, the integration of sensitive materials such as graphene within optical microcavities has overcome the inert nature of the traditional optical microresonators, paving the way for highly sensitive biochemical detection. However, challenges such as Q factor deterioration, complex mode analysis, and demanding operation processes remain, resulting in intricate experimental setups, high excitation thresholds, and issues with device reliability and portability. Besides, the selectivity in the sensing process is also a challenge which relates to the material property. In this work, we present a gas sensor by combining functionalized graphene with a microrod resonator, addressing these challenges with the low threshold, simple structure, easy operation, high sensitivity, and switchable selectivity. By monitoring the shift of the resonant mode caused by the adsorption of gas molecules, we achieve the 1.1 ppb level detection of NH3 and CO2 in the P-doped graphene based microresonator and demonstrate 4 ppb level detection of NO2 with high selectivity by changing the doping state of graphene from P to N. Our approach showcases the advantages of low cost, high sensitivity, and switchable selectivity, providing a promising solution for flexible and high-performance chemical sensing systems.