A near-infrared optical nanosensor for measuring aerobic respiration in microbial systems

Oxygen-Sensitive Optical Nanosensors: Current Advances and Future Perspectives

Oxygen sensing is essential across a wide range of fields, from understanding cellular metabolism and disease mechanisms to optimizing industrial and environmental processes. In this Perspective, we highlight key developments in optical architectures (at the nanometer to sub-micrometer scale), including their transduction methods and applications to in vitro, in vivo/in situ, and nonbiological systems. We also discuss future directions for the field in the domain of expanding extra/intracellular and nonbiological sensing. We address improving accessibility for nonexpert users through the need for standardized protocols and scalable production methods. Furthermore, we advocate for fostering interdisciplinary collaborations through academic incubators, conference networking, and strategic citation practices to bridge gaps between fundamental research and applied science to expand the impact of these tools to researchers outside the sensing field. Addressing these challenges will help drive the development of more versatile and widely accessible oxygen sensors, thus advancing innovation across diverse disciplines.

Smart Luminescent Materials for Emerging Sensors: Fundamentals and Advances

Smart luminescent materials have drawn a significant attention owing to their unique optical properties and versatility in sensor applications. These materials, encompassing a broad spectrum of organic, inorganic, and hybrid systems including quantum dots, organic dyes, and metal-organic frameworks (MOFs), offer tunable emission characteristics that can be engineered at the molecular or nanoscale level to respond to specific stimuli, such as temperature, pH, and chemical presence. This adaptability makes them crucial in developing advanced sensor technologies for environmental monitoring, biomedical diagnostics, and industrial applications with the help of the luminescence mechanisms, such as fluorescence, phosphorescence, and upconversion. Recent advancements have been driven by the integration of nanotechnology, which enhances the sensitivity and selectivity of luminescent materials in sensor platforms. The development of photoluminescent and electrochemiluminescent sensors, for instance, has enabled real-time detection and quantification of target analytes with high accuracy. Additionally, the incorporation of these materials into portable, user-friendly devices, such as smartphone-based sensors, broadens their applicability and accessibility. Despite their potential, challenges remain in optimizing the stability, efficiency, and biocompatibility of these materials under different conditions. This review provides a comprehensive overview of the fundamental principles of smart luminescent materials, discusses recent innovations in their use for sensor applications, and explores future directions aimed at overcoming current limitations and expanding their capabilities in meeting the growing demand for rapid and cost-effective sensing solutions.

A scalable and autoclavable oxygen nanosensor platform for metabolic monitoring of Saccharomyces cerevisiae in a bioreactor and other in situ systems

Polymer-encapsulated dye nanoparticle sensors are a valuable approach to achieving in situ analyte measurements with luminescence; however, typical emulsion-based nanosensors are poorly suited for large-scale biological samples due to limitations of synthesis scalability and stability. Branched polyethylenimine (PEI) is a versatile polymer scaffold ideal for constructing nanoparticles with various covalently conjugated moieties due to their high density of reactive primary amines, high water solubility, and biological stability. In this work, we used branched polyethylenimine as a scaffold-based approach for making a stable and scalable ratiometric oxygen sensor. Pt (II) tetracarboxyporphine was used as an oxygen-sensing dye and coumarin 343 as a reference dye, all covalently linked to the PEI scaffold producing a product that could withstand sterilization procedures and easily be scaled. To minimize toxicity from the PEI scaffold, we conjugated it with 2000 MW PEG. The applicability of the sensors was demonstrated in a 200 mL Saccharomyces cerevisiae yeast culture, using orthogonal luminescent and electrochemical oxygen measurements to validate sensor response and measure the metabolic activity of the yeast in our culture. This approach was able to match the sensitivity of our electrochemical measurements while improving upon drawbacks of other luminescent methods of oxygen detection, demonstrating effective monitoring for at least 20 h. Our scaffold-based approach is a modular and easily translatable technology that could be useful in various biotechnological applications.

The impact of zwitterionic surfactants on optode-based nanosensors via different fabrication approaches and sensing mechanisms

In this work, we explored the impact of zwitterionic surfactants, sulfobetaine 16 (SB-16) and a PEG-phospholipid conjugate (DSPE-PEG), on nanosensor performance. We fabricated four sensors (for Na+, K+, Al3+, and O2) and examined how these surfactants influenced various aspects, including fabrication methods, sensing mechanisms, and the incorporation of nanomaterials. Our results highlighted SB-16's role in enhancing selectivity in ion-exchange sensors (Na+ and K+) while maintaining sensitivity akin to its PEG counterpart. The liquid-liquid extraction based sensors (Al3+) were unaffected by surfactant choice, while the O2 sensors that operate via collisional quenching exhibited reduced sensitivity with SB-16 when compared to its PEG-based counterpart. Additionally, the SB-16 sensors proved adaptable to different fabrication approaches (SESE - single emulsion solvent evaporation and FNP - flash nanoprecipitation), showcased good cell viability and maintained a functional lifetime of at least five days. Furthermore, via the use of quantum dots, we showed that it is possible to incorporate other nanomaterials into the sensing phase of SB-16 sensors. Future investigations could target enhancing the pH stability of zwitterionic surfactants to further advance their applicability in sensor technologies.

Persistent Luminescence Nanosensors: A Generalized Optode-Based Platform for Autofluorescence-Free Sensing in Biological Systems

Fluorescent nanosensors have revolutionized diagnostics and our ability to monitor cellular dynamics. Yet, distinguishing sensor signals from autofluorescence remains a challenge. Here, we merged optode-based sensing with near-infrared-emitting ZnGa2O4:Cr3+ persistent luminescence nanoparticles (PLNPs) to create nanocomposites for autofluorescence-free "glow-in-the-dark" sensing. Hydrophobic modification and incorporation of the persistent luminescence nanoparticles into an optode-based nanoparticle core yielded persistent luminescence nanosensors (PLNs) for five analytes (K+, Na+, Ca2+, pH, and O2) via two distinct mechanisms. We demonstrated the viability of the PLNs by quantifying K+ in fetal bovine serum, calibrating the pH PLNs in the same, and ratiometrically monitoring O2 metabolism in cultures of Saccharomyces cerevisiae, all the while overcoming their respective autofluorescence signatures. This highly modular platform allows for facile tuning of the sensing functionality, optical properties, and surface chemistry and promises high signal-to-noise ratios in complex optical environments.

Organosilica Nanosensors for Monitoring Spatiotemporal Changes in Oxygen Levels in Bacterial Cultures

Oxygen plays a central role in aerobic metabolism, and while many approaches have been developed to measure oxygen concentration in biological environments over time, monitoring spatiotemporal changes in dissolved oxygen levels remains challenging. To address this, we developed a ratiometric core-shell organosilica nanosensor for continuous, real-time optical monitoring of oxygen levels in biological environments. The nanosensors demonstrate good steady state characteristics (KpSV = 0.40 L/mg, R2 = 0.95) and respond reversibly to changes in oxygen concentration in buffered solutions and report similar oxygen level changes in response to bacterial cell growth (Escherichia coli) in comparison to a commercial bulk optode-based sensing film. We further demonstrated that the oxygen nanosensors could be distributed within a growing culture of E. coli and used to record oxygen levels over time and in different locations within a static culture, opening the possibility of spatiotemporal monitoring in complex biological systems.

Increasing aggregate size reduces single-cell organic carbon incorporation by hydrogel-embedded wetland microbes

Microbial degradation of organic carbon in sediments is impacted by the availability of oxygen and substrates for growth. To better understand how particle size and redox zonation impact microbial organic carbon incorporation, techniques that maintain spatial information are necessary to quantify elemental cycling at the microscale. In this study, we produced hydrogel microspheres of various diameters (100, 250, and 500 μm) and inoculated them with an aerobic heterotrophic bacterium isolated from a freshwater wetland (Flavobacterium sp.), and in a second experiment with a microbial community from an urban lacustrine wetland. The hydrogel-embedded microbial populations were incubated with 13C-labeled substrates to quantify organic carbon incorporation into biomass via nanoSIMS. Additionally, luminescent nanosensors enabled spatially explicit measurements of oxygen concentrations inside the microspheres. The experimental data were then incorporated into a reactive-transport model to project long-term steady-state conditions. Smaller (100 μm) particles exhibited the highest microbial cell-specific growth per volume, but also showed higher absolute activity near the surface compared to the larger particles (250 and 500 μm). The experimental results and computational models demonstrate that organic carbon availability was not high enough to allow steep oxygen gradients and as a result, all particle sizes remained well-oxygenated. Our study provides a foundational framework for future studies investigating spatially dependent microbial activity in aggregates using isotopically labeled substrates to quantify growth.

Optical sensors (optodes) for multiparameter chemical imaging: classification, challenges, and prospects

Chemical gradients and uneven distribution of analytes are common in natural and artificial systems. As a result, the ability to visualize chemical distributions in two or more dimensions has gained significant importance in recent years. This has led to the integration of chemical imaging techniques into all domains of analytical chemistry. In this review, we focus on the use of optical sensors, so-called optodes, to obtain real-time and multidimensional images of two or more parameters simultaneously. It is important to emphasize that multiparameter imaging in this context is not confined solely to multiple chemical parameters (analytes) but also encompasses physical (e.g., temperature or flow) or biological (e.g., metabolic activity) parameters. First, we discuss the technological milestones that have paved the way for chemical imaging using optodes. Later, we delve into various strategies that can be taken to enable multiparameter imaging. The latter spans from developing novel receptors that enable the recognition of multiple parameters to chemometrics and machine learning-based techniques for data analysis. We also explore ongoing trends, challenges, and prospects for future developments in this field. Optode-based multiparameter imaging is a rapidly expanding field that is being fueled by cutting-edge technologies. Chemical imaging possesses the potential to provide novel insights into complex samples, bridging not only across various scientific disciplines but also between research and society.

Triplet-Triplet Annihilation Upconversion-Based Oxygen Sensors to Overcome the Limitation of Autofluorescence

Autofluorescence is one of the many challenges in bioimaging as it can mask the emission from fluorescent probes or markers, a limitation that can be overcome via upconversion. Herein, we have developed a nanosensor that uses triplet-triplet annihilation upconversion to optically report changes in the dissolved oxygen concentration. Using a sensitizer-annihilator dye pairing of platinum(II) octaethylporphyrin and 9,10-diphenylanthracene, we monitored the oxygen consumption (as a proxy for metabolic activity) over time in a biological system─Saccharomyces cerevisiae (brewing yeast). The nanosensor demonstrated good reversibility over multiple cycles and showed good signal and colloidal stability when tested over the course of 7 days, and it was sensitive to dissolved oxygen from 0.00 to 3.17 mg/L O2. Additionally, there was no signal overlap between the nanosensor emission and S. cerevisiae autofluorescence, thus underscoring the utility of upconversion as a facile and economical means of overcoming autofluorescence.

An In Silico Molecular Modelling-Based Prediction of Potential Keap1 Inhibitors from Hemidesmus indicus (L.) R.Br. against Oxidative-Stress-Induced Diseases

The present study investigated the antioxidant potential of aqueous methanolic extracts of Hemidesmus indicus (L.) R.Br., followed by a pharmacoinformatics-based screening of novel Keap1 protein inhibitors. Initially, the antioxidant potential of this plant extract was assessed via antioxidant assays (DPPH, ABTS radical scavenging, and FRAP). Furthermore, 69 phytocompounds in total were derived from this plant using the IMPPAT database, and their three-dimensional structures were obtained from the PubChem database. The chosen 69 phytocompounds were docked against the Kelch-Neh2 complex protein (PDB entry ID: 2flu, resolution 1.50 Å) along with the standard drug (CPUY192018). H. indicus (L.) R.Br. extract (100 µg × mL-1) showed 85 ± 2.917%, 78.783 ± 0.24% of DPPH, ABTS radicals scavenging activity, and 161 ± 4 μg × mol (Fe (II)) g-1 ferric ion reducing power. The three top-scored hits, namely Hemidescine (-11.30 Kcal × mol-1), Beta-Amyrin (-10.00 Kcal × mol-1), and Quercetin (-9.80 Kcal × mol-1), were selected based on their binding affinities. MD simulation studies showed that all the protein-ligand complexes (Keap1-HEM, Keap1-BET, and Keap1-QUE) were highly stable during the entire simulation period, compared with the standard CPUY192018-Keap1 complex. Based on these findings, the three top-scored phytocompounds may be used as significant and safe Keap1 inhibitors, and could potentially be used for the treatment of oxidative-stress-induced health complications.

On the Use of Polymer-Based Composites for the Creation of Optical Sensors: A Review

Polymers are widely used in many areas, but often their individual properties are not sufficient for use in certain applications. One of the solutions is the creation of polymer-based composites and nanocomposites. In such materials, in order to improve their properties, nanoscale particles (at least in one dimension) are dispersed in the polymer matrix. These properties include increased mechanical strength and durability, the ability to create a developed inner surface, adjustable thermal and electrical conductivity, and many others. The materials created can have a wide range of applications, such as biomimetic materials and technologies, smart materials, renewable energy sources, packaging, etc. This article reviews the usage of composites as a matrix for the optical sensors and biosensors. It highlights several methods that have been used to enhance performance and properties by optimizing the filler. It shows the main methods of combining indicator dyes with the material of the sensor matrix. Furthermore, the role of co-fillers or a hybrid filler in a polymer composite system is discussed, revealing the great potential and prospect of such matrixes in the field of fine properties tuning for advanced applications.

Sensors in a Flash! Oxygen Nanosensors for Microbial Metabolic Monitoring Synthesized by Flash Nanoprecipitation

Flash nanoprecipitation (FNP) is an efficient and scalable nanoparticle synthesis method that has not previously been applied to nanosensor fabrication. Current nanosensor fabrication methods have traditionally exhibited poor replicability and consistency resulting in high batch-to-batch variability, highlighting the need for a more tunable and efficient method such as FNP. We used FNP to fabricate nanosensors to sense oxygen based on an oxygen-sensitive dye and a reference dye, as a tool for measuring microbial metabolism. We used fluorescence spectroscopy to optimize nanosensor formulations, calibrate the nanosensors for oxygen concentration determination, and measure oxygen concentrations through oxygen-sensitive dye luminescence. FNP provides an effective platform for making sensors capable of responding to oxygen concentration in gas-bubbled solutions as well as in microbial environments. The environments we tested the sensors in arePseudomonas aeruginosa biofilms andSaccharomyces cerevisiae liquid cultures─both settings where oxygen concentration is highly dependent on microbial activity. With FNP now applied to nanosensor fabrication, future nanosensor applications can take advantage of improved product quality through better replicability and consistency while maintaining the original function of the nanosensor.

Development of optical chemosensors based on photochromic polymer nanocarriers

Spiropyran-containing photochromic polymer nanoparticles with hydroxyl or amine functional groups and particle size of below 100 nm were used to design chemosensors for sensing pH of aqueous media.