Real-Time Monitoring of Critical Care Analytes in the Bloodstream with Chemical Sensors: Progress and Challenges

Biosensors for Public Health and Environmental Monitoring: The Case for Sustainable Biosensing

Climate change is a profound crisis that affects every aspect of life, including public health. Changes in environmental conditions can promote the spread of pathogens and the development of new mutants and strains. Early detection is essential in managing and controlling this spread and improving overall health outcomes. This perspective article introduces basic biosensing concepts and various biosensors, including electrochemical, optical, mass-based, nano biosensors, and single-molecule biosensors, as important sustainability and public health preventive tools. The discussion also includes how the sustainability of a biosensor is crucial to minimizing environmental impacts and ensuring the long-term availability of vital technologies and resources for healthcare, environmental monitoring, and beyond. One promising avenue for pathogen screening could be the electrical detection of biomolecules at the single-molecule level, and some recent developments based on single-molecule bioelectronics using the Scanning Tunneling Microscopy-assisted break junctions (STM-BJ) technique are shown here. Using this technique, biomolecules can be detected with high sensitivity, eliminating the need for amplification and cell culture steps, thereby enhancing speed and efficiency. Furthermore, the STM-BJ technique demonstrates exceptional specificity, accurately detects single-base mismatches, and exhibits a detection limit essentially at the level of individual biomolecules. Finally, a case is made here for sustainable biosensors, how they can help, the paradigm shift needed to achieve them, and some potential applications.

Smart Hydrogel Swelling State Detection Based on a Power-Transfer Transduction Principle

Stimulus-responsive (smart) hydrogels are a promising sensing material for biomedical contexts due to their reversible swelling change in response to target analytes. The design of application-specific sensors that utilize this behavior requires the development of suitable transduction concepts. The presented study investigates a power-transfer-based readout approach that is sensitive to small volumetric changes of the smart hydrogel. The concept employs two thin film polyimide substrates with embedded conductive strip lines, which are shielded from each other except at the tip region, where the smart hydrogel is sandwiched in between. The hydrogel's volume change in response to a target analyte alters the distance and orientation of the thin films, affecting the amount of transferred power between the two transducer parts and, consequently, the measured sensor output voltage. With proper calibration, the output signal can be used to determine the swelling change of the hydrogel and, consequently, to quantify the stimulus. In proof-of-principle experiments with glucose- and pH-sensitive smart hydrogels, high sensitivity to small analyte concentration changes was found along with very good reproducibility and stability. The concept was tested with two exemplary hydrogels, but the transduction principle in general is independent of the specific hydrogel material, as long as it exhibits a stimulus-dependent volume change. The application vision of the presented research is to integrate in situ blood analyte monitoring capabilities into standard (micro)catheters. The developed sensor is designed to fit into a catheter without obstructing its normal use and, therefore, offers great potential for providing a universally applicable transducer platform for smart catheter-based sensing.

An Isoniazid Based Schiff Base Sensor for Selective Detection of Pd2+ Ions

Here, we developed a novel isoniazid based fluorescent probe (E)-N'-(thiophen-2-ylmethylene)isonicotinohydrazide (TINH) through simple condensation reaction and employed for selective detection of Pd2+ ions with a low detection limit of 4.102 × 10-11 M. Among the many existing cations, TINH bound Pd2+ ions with an association affinity of 9.794 × 105 M-1. Adding Pd2+ ions to ligand solution increased the absorption intensity in UV-Visible and quenched the emission intensity in fluorescence spectroscopic experiments. More importantly, this TINH complexed to Pd2+ ions in 1:1 stoichiometric ratio. To evaluate the stability of complexed system, pH experiments has been performed. The binding insights among the ligand and Pd2+ ions has been confirmed by IR spectroscopic and MASS spectrometric methods. Additionally, TINH also applied to real water samples for the identification and measurement of Pd2+ ions. Hence, this system could be highly applicable for detection of Pd2+ ions in environmental and industrial samples with in low detection range.

Computational Modeling as a Tool to Drive the Development of a Novel, Chemical Device for Monitoring the Injured Brain and Body

Real-time measurement of dynamic changes, occurring in the brain and other parts of the body, is useful for the detection and tracked progression of disease and injury. Chemical monitoring of such phenomena exists but is not commonplace, due to the penetrative nature of devices, the lack of continuous measurement, and the inflammatory responses that require pharmacological treatment to alleviate. Soft, flexible devices that more closely match the moduli and shape of monitored tissue and allow for surface microdialysis provide a viable alternative. Here, we show that computational modeling can be used to aid the development of such devices and highlight the considerations when developing a chemical monitoring probe in this way. These models pave the way for the development of a new class of chemical monitoring devices for monitoring neurotrauma, organs, and skin.

The era of nano-bionic: 2D materials for wearable and implantable body sensors

Nano-bionics have the potential of revolutionizing modern medicine. Among nano-bionic devices, body sensors allow to monitor in real-time the health of patients, to achieve personalized medicine, and even to restore or enhance human functions. The advent of two-dimensional (2D) materials is facilitating the manufacturing of miniaturized and ultrathin bioelectronics, that can be easily integrated in the human body. Their unique electronic properties allow to efficiently transduce physical and chemical stimuli into electric current. Their flexibility and nanometric thickness facilitate the adaption and adhesion to human body. The low opacity permits to obtain transparent devices. The good cellular adhesion and reduced cytotoxicity are advantageous for the integration of the devices in vivo. Herein we review the latest and more significant examples of 2D material-based sensors for health monitoring, describing their architectures, sensing mechanisms, advantages and, as well, the challenges and drawbacks that hampers their translation into commercial clinical devices.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Paving the way towards continuous biosensing by implementing affinity-based nanoswitches on state-dependent readout platforms

Combining affinity-based nanoswitches with state-dependent readout platforms allows for continuous biosensing and acquisition of real-time information about biochemical processes occurring in the environment of interest.

Electrochemical methods for neural interface electrodes

Objective. Neural interfaces often rely on charge transfer processes between electrodes and the tissue or electrolyte. Electrochemical processes are at the core of electrode function and, therefore, the key to neural interface stability, electrode performance characterization, and utilization of electrodes as chemical sensors. Electrochemical techniques offer a variety of options to investigate the charge transfer and electrocatalytic properties of electrodes.Approach. In this tutorial, we present various experiments to illustrate the power of electrochemical methods, serve as a reference and guideline, and stimulate deeper understanding of the subject.Main results.As a basis for the following experiments, we discuss the platinum cyclic voltammogram and focus on understanding surface processes and roughness determination. We highlight the importance of appropriate instrumentation using potentiostats and how strongly it can influence results. We then discuss a number of potential-controlled and current-controlled methods for electrode characterization, including chronocoulometry, chronoamperometry, (active) potentiometry, and chronopotentiometry. They illustrate charge transfer caused by both electrode surface processes and the presence of redox-active species, such as dissolved oxygen and hydrogen, or hydrogen peroxide. We also discuss the electrode potential with respect to a reference electrode under various conditions and how it affects its electrochemical properties like surface state, catalytic properties and capability to transfer charge.Significance.Electrochemical methods are still underutilized in neural engineering, and valuable information is therefore often not accessed. Many studies on electrode characterization would benefit from a more consistent and target-oriented electrochemical methodology and instrumentation. That ranges from the investigation of new materials and processes, over electrode performance assessment to the development of more long-term stable and biocompatible neural interfaces. Ultimately, standardization, consistency and comparability will play a key role in the translation of microtechnology into biomedical and clinical applications.

Free-Base Porphyrins as Chemical Probes for Heavy Metal Ions Detection

Heavy metals are one of the major contaminants in water. They come from various human activities which include mining, smelting, and industrial discharge to name a few. Heavy metals pose a toxic danger to human beings even at their minute concentration. Current methods of detection suffer from numerous limitations such as complicated sample preparation, high cost of instruments, and the need for professional chemists. These challenges make them unsuitable for in situ and real-time monitoring of samples. Fluorescence spectroscopy has emerged as an alternative for sensing applications. They have high sensitivity, good selectivity, and only require small amounts of fluorescent probes. In this study, free base porphyrins were selected as a fluorescent chemical probe to detect the presence of commonly discharge heavy metals – lead (Pb2+) ions and nickel (Ni2+) ions. Solution-based assay and fluorescent measurements were used throughout the study. From the fluorescent analysis, porphyrins showed more selective and sensitivity towards Pb2+ ions. The study also investigated the photostability of porphyrins when porphyrins solutions were put under ambiance light conditions. This solution-based assay fluorescent measurement shows promising potential application of porphyrins to be used as chemical probes on another sensor medium such as on optical fibre and thin film study.

Smart Hydrogel Micromechanical Resonators with Ultrasound Readout for Biomedical Sensing

One of the main challenges for implantable biomedical sensing schemes is obtaining a reliable signal while maintaining biocompatibility. In this work, we demonstrate that a combination of medical ultrasound imaging and smart hydrogel micromechanical resonators can be employed for continuous monitoring of analyte concentrations. The sensing principle is based on the shift of the mechanical resonance frequencies of smart hydrogel structures induced by their volume-phase transition in response to changing analyte levels. This shift can then be measured as a contrast change in the ultrasound images due to resonance absorption of ultrasound waves. This concept eliminates the need for implanting complex electronics or employing transcutaneous connections for sensing biomedical analytes in vivo. Here, we present proof-of-principle experiments that monitor in vitro changes in ionic strength and glucose concentrations to demonstrate the capabilities and potential of this versatile sensing platform technology.

Why ammonium detection is particularly challenging but insightful with ionophore-based potentiometric sensors – an overview of the progress in the last 20 years

An overview of ionophore-based electrodes for ammonium sensing critically analyzing contributions in the last 20 years and with focus in analytical applications.

Quantitative determination of H2O2 for detection of alanine aminotransferase using thin film electrodes

The abnormal concentrations or absence of biomolecules (e.g., proteins) in blood can further be used in diagnosis of a particular pathology at an early stage. Current studies are intensely focusing on the analysis of interaction and detection of biomolecules via point-of-care systems (POCs), allowing miniaturized and parallelized reactions, simultaneously. Recent developments have shown that the collaboration of electrochemical sensing techniques and POCs to overcome challenging problems in health-care settings provides new approaches in diagnosis and treatment of diseases. The aim of this study was to adapt the alanine aminotransferase (ALT) enzyme to the platinum (Pt) thin film electrode system and quantitatively determine the enzyme levels via enzymatically generated H₂O₂ with differential pulse voltammetry (DPV). A simple potentiostat architecture with expanded sweep range utilizing dual LMP91000 devices was developed and adapted to the needs of the biosensor. In order to calibrate the system, known concentrations of H₂O₂ were also tested. Moreover, signals associated with the other electroactive species coming from the ALT reaction were eliminated. Resulted potential range has been achieved between +500 mV and +900 mV and the linear range was found to be 0.05 M-0.5 M for H₂O₂, whereas 5 UL^-1 to 120 UL^-1 for ALT enzyme.

Cytotoxicity Study of Ionophore-Based Membranes: Toward On-Body and in Vivo Ion Sensing

We present the most complete study to date comprising in vitro cytotoxicity tests of ion-selective membranes (ISMs) in terms of cell viability, proliferation, and adhesion assays with human dermal fibroblasts. ISMs were prepared with different types of plasticizers and ionophores to be tested in combination with assays that focus on the medium-term and long-term leaching of compounds. Furthermore, the ISMs were prepared in different configurations considering (i) inner-filling solution-type electrodes, (ii) all-solid-state electrodes based on a conventional drop-cast of the membrane, (iii) peeling after the preparation of a wearable sensor, and (iv) detachment from a microneedle-based sensor, thus covering a wide range of membrane shapes. One of the aims of this study, other than the demonstration of the biocompatibility of various ISMs and materials tested herein, is to create an awareness in the scientific community surrounding the need to perform biocompatibility assays during the very first steps of any sensor development with an intended biomedical application. This will foster meeting the requirements for subsequent on-body application of the sensor and avoiding further problems during massive validations toward the final in vivo use and commercialization of such devices.

Clinical translation of microfluidic sensor devices: focus on calibration and analytical robustness

We present approaches to facilitate the use of microfluidics outside of the laboratory, in our case within a clinical setting and monitoring from human subjects, where the complexity of microfluidic devices requires high skill and expertise and would otherwise limit translation. Microfluidic devices show great potential for converting complex laboratory protocols into on-chip processes. We demonstrate a flexible microfluidic platform can be coupled to microfluidic biosensors and used in conjunction with clinical microdialysis. The versatility is demonstrated through a series of examples of increasing complexity including analytical processes relevant to a clinical environment such as automatic calibration, standard addition, and more general processes including system optimisation, reagent addition and homogenous enzyme reactions. The precision and control offered by this set-up enables the use of microfluidics by non-experts in clinical settings, increasing uptake and usage in real-world scenarios. We demonstrate how this type of system is helpful in guiding physicians in real-time clinical decision-making.

Measuring luteinising hormone pulsatility with a robotic aptamer-enabled electrochemical reader

Normal reproductive functioning is critically dependent on pulsatile secretion of luteinising hormone (LH). Assessment of LH pulsatility is important for the clinical diagnosis of reproductive disorders, but current methods are hampered by frequent blood sampling coupled to expensive serial immunochemical analysis. Here, we report the development and application of a Robotic APTamer-enabled Electrochemical Reader (RAPTER) electrochemical analysis system to determine LH pulsatility. Through selective evolution of ligands by exponential enrichment (SELEX), we identify DNA aptamers that bind specifically to LH and not to related hormones. The aptamers are integrated into electrochemical aptamer-based (E-AB) sensors on a robotic platform. E-AB enables rapid, sensitive and repeatable determination of LH concentration profiles. Bayesian Spectrum Analysis is applied to determine LH pulsatility in three distinct patient cohorts. This technology has the potential to transform the clinical care of patients with reproductive disorders and could be developed to allow real-time in vivo hormone monitoring.

Optical Sensor for Real-Time pH Monitoring in Human Tissue

This article reports on a fiber-based ratiometric optical pH sensor for use in real-time and continuous in vivo pH monitoring in human tissue. Stable hybrid sol-gel-based pH sensing material is deposited on a highly flexible plastic optical fiber tip and integrated with excitation and detection electronics. The sensor is extensively tested in a laboratory environment before it is applied in vivo in a human model. The pH sensor performance in the laboratory environment outperforms the state-of-the-art reported in the current literature. It exhibits the highest sensitivity in the physiological pH range, resolution of 0.0013 pH units, excellent sensor to sensor reproducibility, long-term stability, short response time of <2 min, and drift of 0.003 pH units per 22 h. The sensor also exhibits promising performance in in vitro whole blood samples. In addition, human evaluations conducted under this project demonstrate successful short-term deployment of this sensor in vivo.

Sensors for Fetal Hypoxia and Metabolic Acidosis: A Review

This article reviews existing clinical practices and sensor research undertaken to monitor fetal well-being during labour. Current clinical practices that include fetal heart rate monitoring and fetal scalp blood sampling are shown to be either inadequate or time-consuming. Monitoring of lactate in blood is identified as a potential alternative for intrapartum fetal monitoring due to its ability to distinguish between different types of acidosis. A literature review from a medical and technical perspective is presented to identify the current advancements in the field of lactate sensors for this application. It is concluded that a less invasive and a more continuous monitoring device is required to fulfill the clinical needs of intrapartum fetal monitoring. Potential specifications for such a system are also presented in this paper.

Electrochemical DNA-Based Immunoassay That Employs Steric Hindrance To Detect Small Molecules Directly in Whole Blood

The development of a universal sensing mechanism for the rapid and quantitative detection of small molecules directly in whole blood would drastically impact global health by enabling disease diagnostics, monitoring, and treatment at home. We have previously shown that hybridization between a free DNA strand and its complementary surface-bound strand can be sterically hindered when the former is bound to large antibodies. Here, we exploit this effect to design a competitive antibody-based electrochemical assay, called CeSHHA, that enables the quantitative detection of small molecules directly in complex matrices, such as whole blood or soil. We discuss the importance of this inexpensive assay for point-of-care diagnosis and for treatment monitoring applications.

Voltammetric Thin-Layer Ionophore-Based Films: Part 2. Semi-Empirical Treatment

This work reports on a semiempirical treatment that allows one to rationalize and predict experimental conditions for thin-layer ionophore-based films with cation-exchange capacity read out with cyclic voltammetry. The transition between diffusional mass transport and thin-layer regime is described with a parameter (α), which depends on membrane composition, diffusion coefficient, scan rate, and electrode rotating speed. Once the thin-layer regime is fulfilled (α = 1), the membrane behaves in some analogy to a potentiometric sensor with a second discrimination variable (the applied potential) that allows one to operate such electrodes in a multianalyte detection mode owing to the variable applied ion-transfer potentials. The limit of detection of this regime is defined with a second parameter (β = 2) and is chosen in analogy to the definition of the detection limit for potentiometric sensors provided by the IUPAC. The analytical equations were validated through the simulation of the respective cyclic voltammograms under the same experimental conditions. While simulations of high complexity and better accuracy satisfactorily reproduced the experimental voltammograms during the forward and backward potential sweeps (companion paper 1), the semiempirical treatment here, while less accurate, is of low complexity and allows one to quite easily predict relevant experimental conditions for this emergent methodology.

Controlled core-to-core photo-polymerisation – fabrication of an optical fibre-based pH sensor

The fabrication of fluorescence-based pH sensors, embedded into etched pits of an optical fibre via highly controllable and spatially selective photo-polymerisation is described and the sensors validated.

Rapid assessment of shock in a nonhuman primate model of uncontrolled hemorrhage: Association of traditional and nontraditional vital signs to mortality risk

BACKGROUND

Heart rate (HR), systolic blood pressure (SBP) and mean arterial pressure (MAP) are traditionally used to guide patient triage and resuscitation; however, they correlate poorly to shock severity. Therefore, improved acute diagnostic capabilities are needed. Here, we correlated acute alterations in tissue oxygen saturation (StO2) and end-tidal carbon dioxide (ETCO2) to mortality in a rhesus macaque model of uncontrolled hemorrhage.

METHODS

Uncontrolled hemorrhage was induced in anesthetized rhesus macaques by a laparoscopic 60% left-lobe hepatectomy (T = 0 minute). StO2, ETCO2, HR, as well as invasive SBP and MAP were continuously monitored through T = 480 minutes. At T = 120 minutes, bleeding was surgically controlled, and blood loss was quantified. Data analyses compared nonsurvivors (expired before T = 480 minutes, n = 5) with survivors (survived to T = 480 minutes, n = 11) using repeated-measures analysis of variance with Bonferroni correction. All p < 0.05 was considered statistically significant. Results were reported as mean ± SEM.

RESULTS

Baseline values were equivalent between groups for each parameter. In nonsurvivors versus survivors at T = 5 minutes, StO2 (55% ± 10% vs. 78% ± 3%, p = 0.02) and ETCO2 (15 ± 2 vs. 25 ± 2 mm Hg, p = 0.0005) were lower, while MAP (18 ± 1 vs. 23 ± 2 mm Hg, p = 0.2), SBP (26 ± 2 vs. 34 ± 3 mm Hg, p = 0.4), and HR (104 ± 13 vs. 105 ± 6 beats/min, p = 0.3) were similar. Association of values over T = 5-30 minutes to mortality demonstrated StO2 and ETCO2 equivalency with a significant group effect (p ≤ 0.009 for each parameter; R(2) = 0.92 and R(2) = 0.90, respectively). MAP and SBP associated with mortality later into the shock period (p < 0.04 for each parameter; R(2) = 0.91 and R(2) = 0.89, respectively), while HR yielded the lowest association (p = 0.8, R(2) = 0.83).

CONCLUSION

Acute alterations in StO2 and ETCO2 strongly associated with mortality and preceded those of traditional vital signs. The continuous, noninvasive aspects of Food and Drug Administration-approved StO2 and ETCO2 monitoring devices provide logistical benefits over other methodologies and thus warrant further investigation.