Digital monitoring of the microchannel filling flow dynamics using a non-contactless smartphone-based nano-liter precision flow velocity meter

Microfluidic systems find widespread applications in diagnostics, biological research, chemistry, and engineering studies. Among their many critical parameters, flow rate plays a pivotal role in maintaining the functionality of microfluidic systems, including droplet-based microfluidic devices and those used in cell culture. It also significantly influences microfluidic mixing processes. Although various flow rate measurement devices have been developed, the challenge remains in accurately measuring flow rates within customized channels. This paper presents the development of a 3D-printed smartphone-based flow velocity meter. The 3D-printed platform is angled at 30° to achieve transparent flow visualization, and it doesn't require any external optical components such as external lenses and filters. Two LED modules integrated into the platform create a uniform illumination environment for video capture, powered directly by the smartphone. The performance of our platform, combined with a customized video processing algorithm, was assessed in three different channel types: uniform straight channels, straight channels with varying widths, and vessel-like channel patterns to demonstrate its versatility. Our device effectively measured flow velocities from 5.43 mm/s to 24.47 mm/s, with video quality at 1080p resolution and 60 frames per second, for which the measurement range can be extended by adjusting the frame rate. This flow velocity meter can be a useful analytical tool to evaluate and enhance microfluidic channel designs of various lab-on-a-chip applications.

Investigation of the molecular switching process between spin crossover states of triazole complexes as basis for optical sensing applications

With the advent of the first laser sources and suitable detectors, optical sensor applications immediately also came into focus. During the last decades, a huge variety of optical sensor concepts were developed, yet the forecast for the future application potential appears even larger. In this context, the development of new sensor probes at different scales down to the atomic or molecular level open new avenues for research and development. We investigated an iron based triazole molecular spin-crossover complex changing its absorption characteristics significantly by varying environmental parameters such as humidity, temperature, magnetic or electric field, respectively, with respect to its suitability for a new class of versatile molecular sensor probes. Hereby, besides the investigation of synthesized pure bulk material using different analyzing methods, we also studied amorphous micro particles which were applied in or onto optical waveguide structures. We found that significant changes of the reflection spectra can also be obtained after combining the particles with different types of optical waveguides.The obtained results demonstrate the suitability of the material complex for a broad field of future sensor applications.

SERS-based pH-Dependent detection of sulfites in wine by hydrogel nanocomposites

Sulfur dioxide (SO2) and sulfites are well-known additives in winemaking due to their preservative properties. Although they can prevent oxidation and inhibit microbial growth, they pose health risks and require limitations on their use. Consequently, the total level of SO2 is regulated and several quantification strategies have been proposed. The approved detection methods require the extraction of SO2 by heating and/or acid treatment. Then, iodine or acid/base titrations are conducted for the detection of liberated SO2. Although these methods can provide sensitive detection of SO2, they are complex, time-consuming, and require sample preparation steps and skilled operators. Thus, to overcome these disadvantages, an easy-to-use method, involving simple sample preparation steps, and offering high sensitivity and selectivity, is desirable. Herein, we introduce a SERS-based strategy for SO2 detection in liquids using hydrogel nanocomposites. The hydrogels are prepared by poly(ethylene glycol) diacrylate (PEGDA) in the presence of gold nanoparticles (AuNPs), acting as the SERS substrate. The use of hydrogels ensures a homogenous signal distribution and an efficient collection of SO2, and drying the hydrogels enhances and stabilizes the obtained SO2 signal. The detection strategy is based on the pH-dependent dissociation of SO2. By adjusting the pH value of wine to 10 through simple dilutions, SO2 can be directly detected in wine, down to 0.4 ppm, well below the regulatory limits. The proposed method allows for sensitive, direct, cost-effective detection of SO2 by eliminating the loss of the gaseous form of the sample and avoids titration-based detection methods.

Blue-red emission color change from a heavily-doped Eu@MOF composite: Synthesis, characterization and application for 2,4,6-trinitrophenol sensing

2,4,6-trinitrophenol (TPA) natural degradation is nearly impossible and its accumulation threatens ecosystem. Optical sensing is an attractive detection method for TPA with low demand of equipment and data processing, but still needs to be improved. This work was dedicated to increasing probe-loading content so as to improve sensing sensitivity. Three probes derived from Eu(III)-benzimidazole were designed, with their active H atoms replaced by alkyl groups to eliminate the hydrogen bond with supporting host and thus to improve probe-loading content. Their molecular structure, absorption, emission, and excitation spectra were discussed to confirm their sensing potential to TPA. Then these three probes were loaded into host (bio-MOF-1) via ionic exchange method, which was confirmed by XRD, N2 adsorption/desorption, ICP, and SEM. The loading content and sensing performance of these three probes in bio-MOF-1 were compared. It was found that the elimination of active H atoms indeed increased probe loading content from 44% to 78%, with sensing coefficient increased from 0.010 μM-1 to 0.029 μM-1. A ratiometric sensing towards TPA was observed, with blue emission from bio-MOF-1 host increased and red emission from Eu(III) probe decreased, which was detectable by naked eyes. Linear working equations were fitted with high selectivity.

A multispectroscopic approach for ultra-trace sensing of prostate specific antigen (PSA) by iron nanocomposite fabricated on graphene nanoplatelet

Herein we report an easy, rapid and cost-effective method for spectroscopic sensing of a prostate cancer biomarker prostate specific antigen (PSA) using a novel nanocomposite. The material is a synthetic quinoxaline derivative-based iron nanocomposite fabricated on graphene nanoplatelet surface (1d-Fe-Gr). Presence of graphene enhanced the efficacy of synthesized 1d-Fe-Gr to sense PSA in serum medium with an impressive limit of detection (LOD) value of 0.878 pg/mL compared to 1d-Fe alone (LOD 17.619 pg/mL) using UV-visible absorption spectroscopy. LOD of PSA by 1d-Fe-Gr using Raman spectroscopy is even more impressive (0.410 pg/mL). Moreover, presence of interfering biomolecules like glucose, cholesterol, bilirubin and insulin in serum improves the detection threshold significantly in presence of 1d-Fe-Gr which otherwise cause LOD values of PSA to elevate in control sets. In presence of these biomolecules, the LOD values improve significantly as compared to healthy conditions in the range 0.623-3.499 pg/mL. Thus, this proposed detection method could also be applied efficiently to the patients suffering from different pathophysiological disorders. These biomolecules may also be added externally during analyses to improve the sensing ability. Fluorescence, Raman and circular dichroism spectroscopy were used to study the underlying mechanism of PSA sensing by 1d-Fe-Gr. Molecular docking studies confirm the selective interaction of 1d-Fe-Gr with PSA over other cancer biomarkers.

Engineered porosity for tissue-integrating, bioresorbable lifetime-based phosphorescent oxygen sensors

Versatile silk protein-based material formats were studied to demonstrate bioresorbable, implantable optical oxygen sensors that can integrate with the surrounding tissues. The ability to continuously monitor tissue oxygenation in vivo is desired for a range of medical applications. Silk was chosen as the matrix material due to its excellent biocompatibility, its unique chemistry that facilitates interactions with chromophores, and the potential to tune degradation time without altering chemical composition. A phosphorescent Pd (II) benzoporphyrin chromophore was incorporated to impart oxygen sensitivity. Organic solvent-based processing methods using 1,1,1,3,3,3-hexafluoro-2-propanol were used to fabricate: 1) silk-chromophore films with varied thickness and 2) silk-chromophore sponges with interconnected porosity. All compositions were biocompatible and exhibited photophysical properties with oxygen sensitivities (i.e., Stern-Volmer quenching rate constants of 2.7-3.2 × 104 M-1) useful for monitoring physiological tissue oxygen levels and for detecting deviations from normal behavior (e.g., hyperoxia). The potential to tune degradation time without significantly impacting photophysical properties was successfully demonstrated. Furthermore, the ability to consistently monitor tissue oxygenation in vivo was established via a multi-week rodent study. Histological assessments indicated successful tissue integration for the sponges, and this material format responded more quickly to various oxygen challenges than the film samples.

Optical detection of total cholesterol based on a dye-displacement method

In the present study, the dye methylene blue (MB) was entrapped in an agarose gel and used as a sensing probe for the detection of total cholesterol. When methylene blue-entrapped agarose cubes were added to the cholesterol solution, methylene blue was displaced by cholesterol and released into the solution. A calibration curve was prepared by plotting the rate of release of methylene blue at 664 nm against varying cholesterol concentrations. A linear response was observed in the concentration range of 1 to 5 mM (40 mg/dL to 200 mg/dL) which covers normal and elevated cholesterol levels in humans. Optical detection of cholesterol using this dye-replacement method is simple, economical, and non-toxic. Characterisation of the system was carried out by FT-IR spectroscopy and cyclic voltammetry. The optical method was validated to determine total cholesterol in serum samples with reasonable accuracy.

Simple colorimetric copper(II) sensor - Spectral characterization and possible applications

New o-hydroxyazocompound L bearing pyrrole residue was obtained in the simple synthetic protocol. The structure of L was confirmed and analyzed by X-ray diffraction. It was found that new chemosensor can be successfully used as copper(II) selective spectrophotometric regent in solution and can be also applied for the preparation of sensing materials generating selective color signal upon interaction with copper(II). Selective colorimetric response towards copper(II) is manifested by a distinct color change from yellow to pink. Proposed systems were effectively used for copper(II) determination at concentration level 10^-8 M in model and real samples of water.

Optode-based chemical imaging of laboratory burned soil reveals millimeter-scale heterogeneous biogeochemical responses

Soil spatial responses to fire are unclear. Using optical chemical sensing with planar 'optodes', pH and dissolved O₂ concentration were tracked spatially with a resolution of 360 μm per pixel for 72 h after burning soil in the laboratory with a butane torch (∼1300 °C) and then sprinkling water to simulate a postfire moisture event. Imaging data from planar optodes correlated with microbial activity (quantified via RNA transcripts). Post-fire and post-wetting, soil pH increased throughout the entire ∼13 cm × 17 cm × 20 cm rectangular cuboid of sandy loam soil. Dissolved O₂ concentrations were not impacted until the application of water postfire. pH and dissolved O₂ both negatively correlated (p < 0.05) with relative transcript expression for galactose metabolism, the degradation of aromatic compounds, sulfur metabolism, and narH. Additionally, dissolved O₂ negatively correlated (p < 0.05) with the relative activity of carbon fixation pathways in Bacteria and Archaea, amoA/amoB, narG, nirK, and nosZ. nifH was not detected in any samples. Only amoB and amoC correlated with depth in soil (p < 0.05). Results demonstrate that postfire soils are spatially complex on a mm scale and that using optode-based chemical imaging as a chemical navigator for RNA transcript sampling is effective.

Yolk-Shell Nanostars@Metal Organic Frameworks as Molecular Sieves for Optical Sensing and Catalysis

Hybrid composites between nanoparticles and metal organic frameworks (MOFs) have been described as optimal materials for a wide range of applications in optical sensing, drug delivery, pollutant removal or catalysis. These materials are usually core shell single- or multi-nanoparticles, restricting the inorganic surface available for reaction. Here, we develop a method for the preparation of yolk-shells consisting in a plasmonic gold nanostar coated with MOF. This configuration shows more colloidal stability, can sieve different molecules based on their size or charge, seems to show some interesting synergy with gold for their application in photocatalysis and present strong optical activity to be used as SERS sensors.

Optical and quantitative detection of cobalt ion using graphitic carbon nitride-based chemosensor for hydrometallurgy of waste lithium-ion batteries

A hydrometallurgy is one of the most important techniques for recycling waste LIBs, where identifying the exact composition of the metal-leached solution is critical in controlling the metal extraction efficiency and the stoichiometry of the regenerated product. In this study, we report a simple and selective optical detection of high-concentrated Co²⁺ using a graphitic carbon nitride (g-CN)-based fluorescent chemosensor. g-CN is prepared by molten salt synthesis using dicyandiamide (DCDA) and LiI/KI. The mass ratio of LiI/KI to DCDA modifies the resulting g-CN (CNI) in terms of in-plane molecular distances of base sites including cyano functional groups (─CN) and fluorescent emission wavelength via nucleophilic substitution. The fluorescent sensing performance of CNIs is evaluated through photoluminescence (PL) emission spectroscopy in a broad Co²⁺ concentration range (10^-4-10⁰ M). The correlation between the surface exposure of hidden nitrogen pots (base sites) and PL intensity change is achieved where the linear relationship between the PL quenching and the logarithm of Co²⁺ concentration in the analyte solution is well established with the regression of 0.9959. This study will provide the design principle of the chemosensor suitable for the fast and accurate optical detection of Co²⁺ present in a broad concentration range for hydrometallurgy for the recycling of waste LIBs.

Visual Cu2+ Detection of Gold-Nanoparticle Probes and its Employment for Cu2+ Tracing in Circuit System

Highly sensitive, simple and reliable colorimetric probe for Cu²⁺-ion detection was visualized with the L-cysteine functionalized gold nanoparticle (LS-AuNP) probes. The pronounced sensing of Cu²⁺ with high selectivity was rapidly featured with obvious colour change that enabled to visually sense Cu²⁺ ions by naked eyes. By employing systemic investigations on crystallinities, elemental compositions, microstructures, surface features, light absorbance, zeta potentials and chemical states of LS-AuNP probes, the oxidation-triggered aggregation effect of LS-AuNP probes was envisioned. The results indicated that the mediation of Cu²⁺ oxidation coordinately caused the formation of disulfide cystine, rendering the removal of thiol group at AuNPs surfaces. These features reflected the visual colour change for the employment of tracing Cu²⁺ ions in a quantitative way.

Ionic modification on COF with rare earth ions for the selective optical sensing and removal of picronitric acid

In this work, we reported a modified COF material for trinitrophenol (TPA) ratiometric sensing and removal. Here a cationic covalent organic framework (C-COF) was prepared as host, while two Tb(III)-based ions were doped into C-COF as probe by ionic exchange reaction with probe loading level of ∼15%. In the absence of TPA, weak Tb(III) emission (489 nm, 545 nm, 585 nm) and bright red COF emission were observed (633 nm). The addition of TPA increased Tb(III) emission and decreased COF emission, following linear response within TPA concentration region of 0-9 μM. Their limit of detection values were determined as 0.9 μM and 4.5 μM, respectively. Corresponding working equations were fitted as I/I₀ = 1.225 + 6.914 × 10⁵ M^-1[TPA], R² = 0.997 for TbCF₃-COF and I/I₀ = 1.063 + 9.222 × 10⁴ M^-1 [TPA], R² = 0.993 for TbDBM-COF. TbCF₃-COF showed better sensing performance than TbDBM-COF, due to its suitable ligand triplet energy level. Their sensing mechanism was revealed as dopant "replacement", where dopant molecules loaded in COF micropore were replaced by TPA molecules, accompanied with energy competing on Tb(III) ⁵D₄ level, showing ratiometric signals. Good selectivity and removal capacity (∼7.4 wt%) for TPA were achieved.

Experimental data manipulations to assess performance of hyperspectral classification models of crop seeds and other objects

BACKGROUND

Optical sensing solutions are being developed and adopted to classify a wide range of biological objects, including crop seeds. Performance assessment of optical classification models remains both a priority and a challenge.

METHODS

As training data, we acquired hyperspectral imaging data from 3646 individual tomato seeds (germination yes/no) from two tomato varieties. We performed three experimental data manipulations: (1) Object assignment error: effect of individual object in the training data being assigned to the wrong class. (2) Spectral repeatability: effect of introducing known ranges (0-10%) of stochastic noise to individual reflectance values. (3) Size of training data set: effect of reducing numbers of observations in training data. Effects of each of these experimental data manipulations were characterized and quantified based on classifications with two functions [linear discriminant analysis (LDA) and support vector machine (SVM)].

RESULTS

For both classification functions, accuracy decreased linearly in response to introduction of object assignment error and to experimental reduction of spectral repeatability. We also demonstrated that experimental reduction of training data by 20% had negligible effect on classification accuracy. LDA and SVM classification algorithms were applied to independent validation seed samples. LDA-based classifications predicted seed germination with RMSE = 10.56 (variety 1) and 26.15 (variety 2), and SVM-based classifications predicted seed germination with RMSE = 10.44 (variety 1) and 12.58 (variety 2).

CONCLUSION

We believe this study represents the first, in which optical seed classification included both a thorough performance evaluation of two separate classification functions based on experimental data manipulations, and application of classification models to validation seed samples not included in training data. Proposed experimental data manipulations are discussed in broader contexts and general relevance, and they are suggested as methods for in-depth performance assessments of optical classification models.

Ratiometric intracellular pH sensors based on nitrogen-doped graphene oxide quantum dots

Intracellular pH (pHi) is very essential for the function of cells and organisms. Thus, it is of great scientific and technical significance to develop nanosensors for probing pHi. In this work, nitrogen-doped graphene oxide quantum dots (N-GOQDs) with fluorescent efficiency of 54% are prepared. The fluorescent spectrum excited at 340 nm contains two remarkable bands at 430 and 520 nm. Interestingly, when pH value increases from 3.6 to 10.5, the blue band at 430 nm slightly changes, while the green band at 520 nm significantly quenches. The change of fluorescent intensities also can be reflected by the variation of fluorescent color. The dual-emissive N-GOQDs are developed as ratiometric fluorescent probes for pHi, which can avoid the influence of several deviations, such as probe concentration, optical path length, and detector efficiency. As a proof of concept, pHi of Hela cells is monitored successfully. This work demonstrates the construction of nano-biosensors based on N-GOQDs with bright fluorescence, high-stability, and good biocompatibility.

Ppb-level gas detection using on-beam quartz-enhanced photoacoustic spectroscopy based on a 28 kHz tuning fork

In this paper, an on-beam quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a custom quartz tuning fork (QTF) acting as a photoacoustic transducer, was realized and tested. The QTF is characterized by a resonance frequency of 28 kHz, ~15% lower than that of a commercially available 32.7 kHz standard QTF. One-dimensional acoustic micro resonator (AmR) was designed and optimized by using stainless-steel capillaries. The 28 kHz QTF and AmRs are assembled in on-beam QEPAS configuration. The AmR geometrical parameters have been optimized in terms of length and internal diameter. The laser beam focus position and the AmR coupling distance were also adjusted to maximize the coupling efficiency. For comparison, QEPAS on-beam configurations based on a standard QTF and on the 28 kHz QTF were compared in terms of H2O and CO2 detection sensitivity. In order to better characterize the performance of the system, H2O, C2H2 and CO2 were detected for a long time and the long-term stability was analyzed by an Allan variance analysis. With the integration time of 1 s, the detection limits for H2O, C2H2 and CO2 are 1.2 ppm, 28.8 ppb and 2.4 ppm, respectively. The detection limits for H2O, C2H2 and CO2 can be further improved to 325 ppb, 10.3 ppb and 318 ppb by increasing the integration time to 521 s, 183 s and 116 s.

Optical Sensing in Cell-Free Expression

Light can be used as a control switch for gene expression with potential advantages, avoiding the defects induced by chemical substances. By transplanting components capable of emitting light at a specific wavelength from cells into a cell-free synthesis system, controlled gene expression can be achieved in vitro. Here, we describe an effective method to achieve optical sensing in cell-free protein synthesis (CFPS) based on Escherichia coli crude extract containing the two-component system (TCSs) YF1/FixJ, which was able to respond to blue light. Plasmids capable of interacting with the photosensitive components were constructed, and the fluorescent protein mCherry was used as a reporter. This protocol provides a detailed procedure guiding how to construct the blue-light sensing system in CFPS.

Advances in electrochemical and optical sensing techniques for vitamins detection: a review

Vitamins are essential nutrients that aid in metabolism, cell growth, and the appropriate functioning of other biomolecules. They are required for the proper functioning of various systems in human body. Both vitamin shortage and excess can pave the way for a variety of illnesses. They enter the body via food and supplements eaten, making it critical to measure the vitamin concentrations in food, medicines, and biological fluids. The concentrations of these vitamins are determined using a variety of techniques. The performance measure of the techniques like selectivity, sensitivity, and limit of detection is crucial in their utilization. Among the many techniques of determination, electrochemical sensing and optical sensing have garnered widespread interest because of their potential to improve performance. Additionally, the introduction of innovative materials has added a lot of benefits to sensing. The aim of this article is to summarize significant work toward recent improvements in electrochemical and optical methods for detecting different vitamins. Additionally, it attempts to assess the gaps in vitamin sensing in order to encourage researchers to fill such gaps that will benefit the community.

Assessing citric acid-derived luminescent probes for pH and ammonia sensing: A comprehensive experimental and theoretical study

The present work reports on the assessment of luminescent probes derived from citric acid (CA) and β-aminothiols (namely, l-cysteine (Cys) and cysteamine) for instrumental and smartphone-based fluorimetric sensing purposes. Remarkably, the evaluated luminescent probes derived from natural compounds showed pH-dependent dual excitation/dual emission features. Both fluorophores hold promise for the ratiometric fluorimetric sensing of pH, being especially convenient for the smartphone-based sensing of pH via ratiometric analysis by proper selection of B and G color channels. Time dependent density functional theory (TDDFT) calculations allowed to substantiate the pH dependent structure-property relationship and to unveil the critical role of the CA derived carboxyl group, these findings contributing to the fundamental knowledge on these systems for the rational design of new fluorophores and in establishing fluorescence sensing mechanisms of CA-derived systems. Besides, paper-based devices modified with CA-Cys were implemented in a three-phase separation approach for sensitive and selective ammonia sensing, yielding a remarkable enrichment factor of 389 and a limit of detection of 37 μM under optimal conditions. The proposed approach was successfully applied to the determination of ammonia nitrogen and extractable ammonium in water samples and marine sediments, respectively.

A shining proposal for the detection of dissolved O2 in aqueous medium: Self-calibrated optical sensing via a covalent hybrid structure of carbon-dots&Ru

O₂ is a life-supporting gas and has been widely recognized as an important analyte in life science, medical care and environmental science. Optical sensing for gaseous oxygen has been widely reported owing to the simple, cost-effective and easy-to-go procedure. On the other hand, optical sensors for dissolved oxygen in aqueous media have been rarely reported, since most of them are incompatible with water, leading to poor sensitivity and linearity. In this effort, we tried the combination of Ru(II)-bpy complex and carbon dots (CDs) via covalent bonds, where bpy = bipyridine. A hybrid structure, named as Ru@CD, was constructed for the detection of dissolved oxygen, using Ru(II)-bpy as sensing probe and CDs as water-compatible supporting matrix. Ru@CD was carefully characterized to confirm its hybrid structure. Detailed analysis suggested that its emission showed self-calibrated sensing signals for dissolved oxygen. A good linearity of 99.1% was realized. Its sensitivity (3.18) was higher than most literature values for dissolved oxygen detection. Its working equation was confirmed as a corrected Stern-Volmer equation (Lehrer mode). Good selectivity and signal stability were observed.

Advancing sensing technology with CRISPR: From the detection of nucleic acids to a broad range of analytes - A review

The CRISPR/Cas technology, derived from an adaptive immune system in bacteria, has been awarded the Nobel Prize in Chemistry in 2020 for its success in gene editing. Increasing reports reveal that CRISPR/Cas technology has a wide scope of applications and it could be incorporated into biosensors for detecting critical analytes. CRISPR-powered biosensors have attracted significant research interest due to their advantages including high accuracy, good specificity, rapid response, and superior integrity. Now the CRISPR technology is not only admirable in nucleic acid monitoring, but also promising for other kinds of biomarkers' detection, including metal ions, small molecules, peptides, and proteins. Therefore, it is of great worth to explore the prospect, and summarize the strategies in applying CRISPR technology for the recognition of a broad range of targets. In this review, we summarized the strategies of CRISPR biosensing for non-nucleic-acid analytes, the latest development of nucleic acid detection, and proposed the challenges and outlook of CRISPR-powered biosensors.

Open-source lab hardware: Driver and temperature controller for high compliance voltage, fiber-coupled butterfly lasers

This article describes the development of a compact, relatively low-cost, high compliance voltage laser driver that can provide the constant optical laser output required for a range of applications. The system contains an integrated, high-precision temperature controller that can be implemented with butterfly-style lasers containing an internal thermoelectric cooler. The laser parameters can be controlled manually or via an onboard microcontroller. Additionally, an adjustable over-current protection circuit safeguards the laser diode from potential damage.

A Proposal for a Novel Surface-Stress Based BioMEMS Sensor Using an Optical Sensing System for Highly Sensitive Diagnoses of Bio-particles

In this paper, a BioMEMS sensor by using a surface-stress sensing approach, connected to a highly sensitive optical sensing system, is proposed to diagnose various types of biomolecules. The MEMS transducer is composed of a fixed-fixed beam with immobilized receptors on the surface which is connected to a Ring Resonator (RR) filter. The interaction between the target biomolecules and the receptors induces surface stresses on the beam. This stress results in the beam deformation which leads to changes in the coupling coefficient of the RR. Consequently, the transmission spectrum of the RR experiences changes, measured by using an optical photo-detector. Therefore, by analyzing the response of the photo-detector output, one can detect the number of target biomolecules in the sample and assign a level of contamination, infection or bioparticles, caused by the specific disease. Furthermore, the MEMS functional characteristics and the optical properties of the proposed biosensor are designed and analyzed respectively by using the finite element method (FEM) and the finite difference time domain (FDTD) approach. The obtained functional characteristics of the proposed device show that the present optical BioMEMS sensor can be used for highly sensitive diagnoses of various types of diseases and their progress level.

How 3D printing can boost advances in analytical and bioanalytical chemistry

3D printing fabrication methods have received lately an enormous attention by the scientific community. Laboratories and research groups working on analytical chemistry applications, among others, have advantageously adopted 3D printing to fabricate a wide range of tools, from common laboratory hardware to fluidic systems, sample treatment platforms, sensing structures, and complete fully functional analytical devices. This technology is becoming more affordable over time and therefore preferred over the commonly used fabrication processes like hot embossing, soft lithography, injection molding and micromilling. However, to better exploit 3D printing fabrication methods, it is important to fully understand their benefits and limitations which are also directly associated to the properties of the materials used for printing. Costs, printing resolution, chemical and biological compatibility of the materials, design complexity, robustness of the printed object, and integration with commercially available systems represent important aspects to be weighted in relation to the intended task. In this review, a useful introductory summary of the most commonly used 3D printing systems and mechanisms is provided before the description of the most recent trends of the use of 3D printing for analytical and bioanalytical chemistry. Concluding remarks will be also given together with a brief discussion of possible future directions.

The synthesis of a series of fluorescent emitters and their application for dye lasing and cation sensing

The following paper reported four carbazole-modified fluorescent emitters. Their molecular structure and electronic transition were analyzed via their single crystal and theoretical calculation. Their photophysical parameters, including absorption, emission and quantum yield, were determined and discussed. It was found that the emission performance of benzo-thiazole-based dyes was better than that of benzo-imidazole-based dyes, owing to the electron-donating effect from the S atom. Upon the presence of metal cations, these photophysical parameters were re-measured. Benzo-thiazole-based dyes were found insensitive towards most metal cations, while benzo-imidazole-based dyes showed obvious photophysical variation upon these metal cations, including absorption red shift and emission quenching. Detailed sensing performance of a representative dye was discussed. A linear working curve with good selectivity was finally observed. Its sensing mechanism was confirmed as the coordination between metal cation and deprotonated benzo-imidazole group. Benzo-thiazole-based dyes showed amplified spontaneous emission (ASE) behavior, with threshold energy of ~220 μJ. Given the optimal condition, a highest ASE efficiency of ~2% was observed, with FWHM value of 6 nm and emission peak of 435 nm. The major novelty and advancement of these fluorescent dyes shall be the stable ASE output (dye 4) under UV excitation and the linear sensing curve with good selectivity (dye 3), which was a hard task for emission turn off sensing probes. We attributed its causation to the valent-recognizing sensing mechanism.

Controlled Assembly of Luminescent Lanthanide-Organic Frameworks via Post-Treatment of 3D-Printed Objects

Complex multiscale assemblies of metal-organic frameworks are essential in the construction of large-scale optical platforms but often restricted by their bulk nature and conventional techniques. The integration of nanomaterials and 3D printing technologies allows the fabrication of multiscale functional architectures. Our study reports a unique method of controlled 3D assembly purely relying on the post-printing treatment of printed constructs. By immersing a 3D-printed patterned construct consisting of organic ligand in a solution of lanthanide ions, in situ growth of lanthanide metal-organic frameworks (LnMOFs) can rapidly occur, resulting in macroscopic assemblies and tunable fluorescence properties. This phenomenon, caused by coordination and chelation of lanthanide ions, also renders a sub-millimeter resolution and high shape fidelity. As a proof of concept, a type of 3D assembled LnMOFs-based optical sensing platform has demonstrated the feasibility in response to small molecules such as acetone. It is anticipated that the facile printing and design approach developed in this work can be applied to fabricate bespoke multiscale architectures of functional materials with controlled assembly, bringing a realistic and economic prospect.

On the application of multi-parametric optical phenotyping of bacterial colonies for multipurpose microbiological diagnostics

The development of new diagnostics techniques and modalities is critical for early detection of microbial contamination. In this study, the novel integrated system for multi-parametric optical phenotyping and characterization of bacterial colonies, is presented. The system combines Mach-Zehnder interferometer with a spectral imaging system for capturing multispectral diffraction patterns and multispectral two-dimensional transmission maps of bacterial colonies, along with the simultaneous interferometric profilometry. The herein presented investigation was carried out on five representative bacteria species and nearly 3000 registered multispectral optical signatures. The interferograms were analyzed by four-step phase shift algorithm to reconstruct the colony profile to enable the obtaining of the comparable optical signatures. The dedicated image processing algorithms were used for extraction of quantitative features of these signatures. The random forest algorithm was applied for selection of the most predictive set of features, which were used in classification model based on Support-Vector Machine. Obtained results have shown that the use of multiple multispectral optical signatures provide a multi-parametric bacteria identification at an exceptionally high accuracy (99.4-100%), significantly better than in case of classification based on each of these signatures (multispectral diffraction patterns, two-dimensional transmission coefficient maps), separately. Obtained results revealed that analysis of multispectral signatures can also be applied for characterisation of physical, physicochemical and chemical properties of the bacterial colonies in the presence of the antimicrobial factors. Therefore, the proposed label-free, non-destructive optical technique has perspectives to be exploited in the multipurpose diagnostics and it can be used as a pre-screening tool in microbiological laboratories.

MXenes: Are they emerging materials for analytical chemistry applications? - A review

MXenes are an emerging class of 2D materials that exhibit unique properties of high conductivity and hydrophilicity. They can be easily functionalized with other materials due to the abundance of surface terminated functionalities. The versatile chemistry of MXenes allows fine-tuning their properties for different analytical chemistry applications such as electrochemical and optical sensing. MXenes may also be useful adsorbents for analytical extractions due to their exceptional surface chemistry, high surface areas, and ease of functionalization as per the nature of the target compounds. The features of the MXenes that can make them excellent materials for analytical applications are listed and critically appraised. The emerging applications of MXenes in electrochemical and optical sensing are discussed with the pertinent examples. The potential of MXene-based sorbents for analytical extractions is highlighted based on the current literature that describes their applications in adsorptive removal and environmental remediation. In the end, limitations, challenges, and future opportunities are briefly presented.

Laser heterodyne spectroradiometer assisted by self-calibrated wavelength modulation spectroscopy for atmospheric CO2 column absorption measurements

We have developed a laser heterodyne spectroradiometer in combination with self-calibrated wavelength modulation spectroscopy based on a software-based lock-in amplifier to observe the atmospheric carbon dioxide (CO₂) column absorption near wavelength 1.57 μm in solar occultation mode. This combination facilitates miniaturization of laser heterodyne radiometer. Combined with our developed retrieval algorithm, the atmospheric carbon dioxide column concentration is measured to be 413.7 ± 1.9 ppm, in agreement with GOSAT satellite observation results. This system offers high spectral signal-to-noise ratio of ~333 for the zeroth harmonic (0f) normalized second harmonic (R2f) signal of CO₂ transition (R22e), with a measurement averaging time of 8 s, which can be further improved by increasing averaging time in accordance to the Allan deviation analysis for the noise fluctuation. This demonstrates the feasibility of the system for atmospheric investigation and the potential of ground-based, airborne and spaceborne observations for the variation of the global greenhouse gases.

Anion sensing properties of electrospun nanofibers incorporating a thiourea-based chromoionophore in methanol

To sense hydrophilic anions in protic solvents, we fabricated polymethylmethacrylate (PMMA) nanofibers incorporating 4-nitrophenyl azo thiourea polymer as a chromoionophore. When methanol solutions containing anions contacted the PMMA nanofiber, a bathochromic shift from 386 nm was observed in the absorption maximum of the chromoionophore. This spectral change is due to hydrogen bond formation between the urea moiety of the thiourea-based polymer and anions penetrating the nanofiber. This spectral change was not observed in PMMA film incorporating the same anion sensor, and the difference is attributed to the much larger specific surface area of the nanofiber compared to the film. As a result, many anions could react with the anion-sensing polymers in the nanofiber and induce a large spectral response.

A Luminescent Probe for Ratiometric Optical Detection of HgII and Turn-On Fluorescent Sensing of CuII

Luminescent detection of heavy metals continues to be of growing importance considering their increasing inseparable roles in modern day lifestyle. To this end, we report a simple yet interesting thiourea derivative that results in detection of Cu^II in a turn-on fashion via a chemodosimetric redox reaction leading to oxidative cyclization of the chemosensor molecule. Elaborate studies with regard to the sensing process have been performed along with the proposal of a plausible cyclization mechanism of the molecule. Furthermore, the probe optically detects Hg^II , yet another heavy metal of prime importance, discernable even to the naked eye. The occurrence of a coordination complex has been proved by UV/Visible spectroscopic experiments as well as ESI-mass spectrometry.

A turn-on graphene quantum dot and graphene oxide based fluorometric aptasensor for the determination of telomerase activity

A turn-on fluorometric assay is described for determination of the activity of enzyme telomerase. For this purpose, graphene quantum dots (GQDs) were first modified with the telomeric sequence (5'-amino-AATCCGTCGAGCAGAGTT-3') via a condensation reaction. Injection of graphene oxide causes instant quenching of the blue fluorescence of the GQDs. Addition of cell extract containing telomerase, triggers the extension of telomer via addition of specific sequence (TTAGGG)n to its 3' end. Fluorescence, best measured at excitation/emission wavelengths of 390/446 nm, is subsequently restored due to folding of the extended telomeric sequence into G-quadruplex structure. The method was applied to the determination of telomerase activity in crude cell extracts of as little as 10 HeLa cells. The linear dynamic range extends from 10 to 6500 cells. Graphical abstractIn this study, a new turn-on graphene quantum dotm and graphene oxide based fluorometric assay is developed for the determination of telomerase activity.

On the combination of luminescent rare earth MOF and rhodamine dopant with two sensing channels for picric acid

The present paper reported a hybrid structure for the optical recognition of PA (picric acid). This dye-MOF structure, named as R6h@EuBTC, consisted of a supporting matrix based on rare earth MOF and a sensing probe based on rhodamine dye, which was confirmed using XRD, IR, thermal and photophysical analysis. R6h@EuBTC's rhodamine absorption in visible region was enhanced by increasing PA concentrations, showing obvious color change and consequently colorimetric sensing. R6h@EuBTC's rhodamine emission component was increased by increasing PA concentrations, while its Eu emission component was slightly quenched by increasing PA concentrations, which offered self-calibrated sensing signals for ratiometric fluorescent sensing. Linear response and good selectivity were observed for both sensing channels with LOD of 3.9 μM. R6h@EuBTC's sensing mechanism towards PA was the combination of two procedures, which were the emission turn on effect of rhodamine component triggered by PA-released protons and the emission turn off effect of Eu component caused by its electron transfer procedure to PA, respectively. R6h@EuBTC's novelty was its two sensing channels and the practicability of naked eye detection.

Nanoparticle- and microparticle-based luminescence imaging of chemical species and temperature in aquatic systems: a review

Most aquatic systems rely on a multitude of biogeochemical processes that are coupled with each other in a complex and dynamic manner. To understand such processes, minimally invasive analytical tools are required that allow continuous, real-time measurements of individual reactions in these complex systems. Optical chemical sensors can be used in the form of fiber-optic sensors, planar sensors, or as micro- and nanoparticles (MPs and NPs). All have their specific merits, but only the latter allow for visualization and quantification of chemical gradients over 3D structures. This review (with 147 references) summarizes recent developments mainly in the field of optical NP sensors relevant for chemical imaging in aquatic science. The review encompasses methods for signal read-out and imaging, preparation of NPs and MPs, and an overview of relevant MP/NP-based sensors. Additionally, examples of MP/NP-based sensors in aquatic systems such as corals, plant tissue, biofilms, sediments and water-sediment interfaces, marine snow and in 3D bioprinting are given. We also address current challenges and future perspectives of NP-based sensing in aquatic systems in a concluding section. Graphical abstract ᅟ.

Spectrophotometric, fluorimetric and electrochemical selective pyrophosphate/ATP sensing based on the dimethyltin(IV)-tiron system

Sensing of pyrophosphate anion (PPi) in the presence of nucleotide triphosphates allows the real time monitoring of the polymerase chain reaction. To get a deeper understanding of the factors involved in PPi/nucleotide triphosphate discrimination, a detailed study on the performance of a dimethyltin (IV)-catecholate complex capable of both separate fluorimetric or electrochemical detection of PPi in the presence of adenosine triphosphate (ATP) has been undertaken. Dimethyltin (IV) tightly binds PPi or ATP, and forms a stable 1:1 complex with tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) in water. The complexation equilibria with all components are characterized quantitatively by potentiometric and spectroscopic titrations. Pyrophosphate anion can be detected owing to its ability to release free tiron from the complex by measuring either a fluorimetric or an electrochemical signal. On the contrary, ATP does not displace tiron but causes an interference with PPi in the fluorimetric detection method due to the formation of a ternary Me₂Sn(IV)-tiron-ATP complex with optical properties intermediate between those of free and bound tiron. In the electrochemical (square wave voltammetry) method, the ternary ATP complex shows a separate peak which does not coincide with the peaks of neither free nor bound tiron, thus making possible the simultaneous detection of ATP in addition to PPi.

Ormosil-coated conjugated polymers for the detection of explosives in aqueous environments

A fluorescence-based sensor for detecting explosives, based on a conjugated polymer coated with an ormosil layer, has been developed for use in aqueous environments. The conjugated polymer Super Yellow was spin-coated onto glass substrates prior to a further spin-coating of an MTEOS/TFP-TMOS-based ormosil film, giving an inexpensive, solution-based barrier material for ruggedization of the polymer to an aqueous environment. The sensors showed good sensitivity to 2,4-DNT in the aqueous phase at micromolar and millimolar concentrations, and also showed good recovery of fluorescence when the explosive was removed.

Applications of vitamin B6 cofactor pyridoxal 5'-phosphate and pyridoxal 5'-phosphate crowned gold nanoparticles for optical sensing of metal ions

Vitamin B6 cofactor pyridoxal 5'-phosphate (PLP) and PLP crowned gold nanoparticles (PLP-AuNPs) was applied for the optical chemosensing of metal ions in aqueous medium. PLP showed a visually detectable colour change from colourless to yellow and 'turn-off' fluorescence in the presence of Fe³⁺. The fluorescence intensity of PLP at 433nm was also blue-shifted and enhanced at 395nm upon addition of Al³⁺. When the PLP was functionalized over AuNPs surface, the wine red colour of PLP-AuNPs was turned to purplish-blue and the SPR band at ~525nm was red-shifted upon addition of Al³⁺, Cd²⁺ and Pb²⁺ due to the complexation-induced aggregation of nanoparticles. The developed sensing systems exhibited good selectivity and specificity for the detected analytes (Fe³⁺, Al³⁺, Cd²⁺ and Pb²⁺).

Novel Technologies for the detection of Fusarium head blight disease and airborne inoculum

Many pathogens are dispersed by airborne spores, which can vary in space and time. We can use air sampling integrated with suitable diagnostic methods to give a rapid warning of inoculum presence to improve the timing of control options, such as fungicides. Air sampling can also be used to monitor changes in genetic traits of pathogen populations such as the race structure or frequency of fungicide resistance. Although some image-analysis methods are possible to identify spores, in many cases, species-specific identification can only be achieved by DNA-based methods such as qPCR and LAMP and in some cases by antibody-based methods (lateral flow devices) and biomarker-based methods ('electronic noses' and electro-chemical biosensors). Many of these methods also offer the prospect of rapid on-site detection to direct disease control decisions. Thresholds of spore concentrations that correspond to a disease risk depend on the sampler (spore-trap) location (whether just above the crop canopy, on a UAV or drone, or on a tall building) and also need to be considered with weather-based infection models. Where disease control by spore detection is not possible, some diseases can be detected at early stages using optical sensing methods, especially chlorophyll fluorescence. In the case of Fusarium infections on wheat, it is possible to map locations of severe infections, using optical sensing methods, to segregate harvesting of severely affected areas of fields to avoid toxins entering the food chain. This is most useful where variable crop growth or microclimates within fields generate spatially variable infection, i.e. parts of fields that develop disease, while other areas have escaped infection and do not develop any disease.

Current achievements of nanoparticle applications in developing optical sensing and imaging techniques

Metallic nanostructures have recently been demonstrated to improve the performance of optical sensing and imaging techniques due to their remarkable localization capability of electromagnetic fields. Particularly, the zero-dimensional nanostructure, commonly called a nanoparticle, is a promising component for optical measurement systems due to its attractive features, e.g., ease of fabrication, capability of surface modification and relatively high biocompatibility. This review summarizes the work to date on metallic nanoparticles for optical sensing and imaging applications, starting with the theoretical backgrounds of plasmonic effects in nanoparticles and moving through the applications in Raman spectroscopy and fluorescence biosensors. Various efforts for enhancing the sensitivity, selectivity and biocompatibility are summarized, and the future outlooks for this field are discussed. Convergent studies in optical sensing and imaging have been emerging field for the development of medical applications, including clinical diagnosis and therapeutic applications.

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.

A bright carbon-dot-based fluorescent probe for selective and sensitive detection of mercury ions

In this work, we demonstrated a convenient and green strategy for the synthesis of bright and water-soluble carbon dots (CDs) by carbonizing sodium citrate and glutathione together in a hydrothermal method for the first time. Without post surface modification, the as-synthesized CDs display fluorescence quantum yield (QY) as high as 21.03% and show superior stability not only in concentrated salt solutions but also in neutral and alkaline media. The probe exhibits selective and sensitive recognition capability towards mercury ion (Hg²⁺) in aqueous solution. The fluorescence of CDs can be quenched by Hg²⁺ through an effective electron energy transfer process. It displays a linear quenching effect toward mercury ion in the concentration range of 0-15μM with a correlation coefficient (R²) of 0.99. The limit of detection is determined to be 25nM at the signal to noise ratio of 3. These attractive merits would enable the extensive applications of this probe in environmental science and analytical chemistry in the future.

Emerging trends in optical sensing of glycemic markers for diabetes monitoring

In the past decade, considerable attention has been focused on the measurement of glycemic markers, such as glycated hemoglobin and glycated albumin, that provide retrospective indices of average glucose levels in the bloodstream. While these biomarkers have been regularly used to monitor long-term glucose control in established diabetics, they have also gained traction in diabetic screening. Detection of such glycemic markers is challenging, especially in a point-of-care setting, due to the stringent requirements for sensitivity and robustness. A number of non-separation based measurement strategies were recently proposed, including photonic tools that are well suited to reagent-free marker quantitation. Here, we critically review these methods while focusing on vibrational spectroscopic methods, which offer highly specific molecular fingerprinting capability. We examine the underlying principles and the utility of these approaches as reagentless assays capable of multiplexed detection of glycemic markers and also the challenges in their eventual use in the clinic.

Fast and continuous microorganism detection using aptamer-conjugated fluorescent nanoparticles on an optofluidic platform

Fast and accurate pathogen detection in aquatic environments is challenging in many biomedical studies and microbial diagnostic applications. In this study, we developed a real-time, continuous, and non-destructive single cell detection method using target specific aptamer-conjugated fluorescent nanoparticles (A-FNPs) and an optofluidic particle-sensor platform. A-FNPs selectively bound to the surfaces of target bacteria (Escherichia coli) and labeled them with high affinity and selectivity so that target bacteria can be countable particles in an optofluidic particle-sensor. A-FNP-labeled target bacterial complexes were detected by the optofluidic particle-sensing system, which provides rapid and continuous single-cell detection. A-FNPs selectively bound to E. coli with a dissociation constant of 0.83 nM, but did not bind Enterobacter aerogenes or Citrobacter freundii strains, which lacked affinity for the aptamer used. We demonstrated that our optofluidic device achieves a detection throughput of ~100 particles per second with high accuracy (~85%) in detecting single bacterial cells conjugated with A-FNPs. This approach can be immediately extended to the real-time, high-throughput detection of other microorganisms such as viruses that are selectively conjugated with A-FNPs. Collectively, these data suggest that optofluidic systems are widely applicable for the fast and continuous detection of microbial cells.

Synthesis of fluorescent carbon dots via simple acid hydrolysis of bovine serum albumin and its potential as sensitive sensing probe for lead (II) ions

Carbon dots have great potential to be utilised as an optical sensing probe due to its unique photoluminescence and less toxic properties. This work reports a simple and novel synthesis method of carbon dots via direct acid hydrolysis of bovine serum albumin protein in a one-pot approach. Optimisation of the important synthetic parameters has been performed which consists of temperature effect, acid to protein ratio and kinetics of reaction. Higher temperature has promoted better yield with shorter reaction time. The carbon dots obtained shows a strong emission at the wavelength of 400 nm with an optimum excitation of 305 nm. The potential of the carbon dots as optical sensing probe has been investigated on with different cations that are of environmental and health concern. The fluorescence of the carbon dots was significantly quenched particularly by lead (II) ions in a selective manner. Further analytical study has been performed to leverage the performance of the carbon dots for lead (II) ions sensing using the standard Stern-Volmer relationship. The sensing probe has a dynamic linear range up to 6.0 mM with a Stern-Volmer constant of 605.99 M(-1) and a limit of detection (LOD) of 5.05 μM. The probe performance was highly repeatable with a standard deviation below 3.0%. The probe suggested in this study demonstrates the potential of a more economical and greener approach that uses protein based carbon dots for sensing of heavy metal ions.

Design and Fabrication of Fiber-Optic Nanoprobes for Optical Sensing

This paper describes the design and fabrication of fiber-optic nanoprobes developed for optical detection in single living cells. It is critical to fabricate probes with well-controlled nanoapertures for optimized spatial resolution and optical transmission. The detection sensitivity of fiber-optic nanoprobe depends mainly on the extremely small excitation volume that is determined by the aperture sizes and penetration depths. We investigate the angle dependence of the aperture in shadow evaporation of the metal coating onto the tip wall. It was found that nanoaperture diameters of approximately 50 nm can be achieved using a 25° tilt angle. On the other hand, the aperture size is sensitive to the subtle change of the metal evaporation angle and could be blocked by irregular metal grains. Through focused ion beam (FIB) milling, optical nanoprobes with well-defined aperture size as small as 200 nm can be obtained. Finally, we illustrate the use of the nanoprobes by detecting a fluorescent species, benzo[a]pyrene tetrol (BPT), in single living cells. A quantitative estimation of the numbers of BPT molecules detected using fiber-optic nanoprobes for BPT solutions shows that the limit of detection was approximately 100 molecules.

D-galactose/D-glucose-binding Protein from Escherichia coli as Probe for a Non-consuming Glucose Implantable Fluorescence Biosensor

D-Galactose/D-glucose-binding protein from E. coli (GGBP) is a monomer that binds glucose with high affinity. The protein structure of GGBP is organized in two principal domains linked by a hinge region that form the sugar-binding site. In this work we show that the mutant form of GGBP at the amino acid position 182 can be utilized as a probe for the development of a non-consuming analyte fluorescence biosensor to monitor the glucose level in diabetes health care.