As global demand for food rises and agricultural systems face unprecedented stress from environmental challenges, understanding the role of ions (i.e., key nutrient components) in crop productivity has never been more critical. Unfortunately, current tools for ion analysis in plants rely on destructive sap collection that fails to capture the dynamic changes in ionic concentrations. On the other hand, noninvasive optical methods lack practicality for field applications due to their reliance on expensive equipment and complex operational procedures. Recent advancements in microneedle (MN) sensing technology have demonstrated significant potential for real-time monitoring of plants' health by enabling the direct detection of various important biomarkers, including but not limited to ions. By offering a minimally invasive approach, MN sensors allow continuous in-planta monitoring with precise penetration into plant tissues, ensuring natural growth remains undisturbed. However, the application of MN sensors, especially for in vivo ion measurement, is still in its very early stage. Herein, we delve into the technological potential and application avenues of plant MN sensors, with a focus on tailoring sensor designs to meet the specific requirements of various plant growth environments and analytical performances for ion detection. This perspective paper also introduces the essential relevance of ion levels in plants, provides a comprehensive assessment of existing ion detection methods, and identifies key challenges associated with achieving effective in planta monitoring. Notably, we highlight the potential of MN sensors as a transformative approach for unveiling plant stress responses, optimizing crop yields, and fulfilling diverse roles that bridge the fields of precision agriculture and plant science research.

Accurate temperature monitoring plays a crucial role in understanding the physiological status of patients and the early diagnosis of diseases commonly associated with local and global infections. Intradermal temperature measurement is, in principle, more precise than skin surface detection, as it prevents interference from environmental temperature changes and skin secretions. However, to date, precise and reliable intradermal temperature monitoring in a real-time and continuous manner remains a challenge. We propose herein high-resolution 3D printing to fabricate a mechanically robust and biocompatible hollow microneedle, filled with a temperature-responsive conducting polymer (poly(3,4-ethylenedioxythiophene): polystyrenesulfonate, PEDOT:PSS) to develop a microneedle temperature sensor (T-MN). The significance is 2-fold: rational design of robust MNs with high resolution in the micrometer domain and the implementation of a conducting polymer in a MN format for temperature sensing. The analytical performance of the developed T-MN is in vitro evaluated under mimicked intradermal conditions, demonstrating good sensitivity (−0.74%° C^-1), resolution (0.2 °C), repeatability (RSD = 2%), reproducibility (RSD = 2%), reversibility, and medium-term stability. On-body temperature monitoring is performed on six euthanized rats for 80 min. The results presented good agreement with those obtained using a commercial optical temperature probe, which was intradermally inserted into the rat skin. The reliability of utilizing the T-MN for precise and continuous intradermal temperature monitoring was successfully demonstrated, noting its potential use for patient monitoring in the near future but also temperature compensation for MN (bio)sensors that may need it.

Monitoring of carbon dioxide (CO2) body levels is crucial under several clinical conditions (e.g., human intensive care and acid–base disorders). To date, painful and risky arterial blood punctures have been performed to obtain discrete CO2 measurements needed in clinical setups. Although noninvasive alternatives have been proposed to assess CO2, these are currently limited to benchtop devices, requiring trained personnel, being tedious, and providing punctual information, among other disadvantages. To the best of our knowledge, the literature and market lack a wearable device for real-time, on-body monitoring of CO2. Accordingly, we have developed a microneedle (MN)-based sensor array, labeled as CO2–MN, comprising a combination of potentiometric pH- and carbonate (CO32–)-selective electrodes together with the reference electrode. The CO2–MN is built on an epidermal patch that allows it to reach the stratum corneum of the skin, measuring pH and CO32– concentrations directly into the interstitial fluid (ISF). The levels for the pH–CO32– tandem are then used to estimate the PCO2 in the ISF. Assessing the response of each individual MN, we found adequate response time (t95 < 5s), sensitivity (50.4 and −24.6 mV dec–1 for pH and CO32–, respectively), and stability (1.6 mV h–1 for pH and 2.1 mV h–1 for CO32–). We validated the intradermal measurements of CO2 at the ex vivo level, using pieces of rat skin, and then, with in vivo assays in anesthetized rats, showing the suitability of the CO2–MN wearable device for on-body measurements. A good correlation between ISF and blood CO2 concentrations was observed, demonstrating the high potential of the developed MN sensing technology as an alternative to blood-based analysis in the near future. Moreover, these results open new horizons in the noninvasive, real-time monitoring of CO2 as well as other clinically relevant gases. 

A growing demand has emerged for new point-of-care (POC) platforms capable of delivering reliable clinical data in real-time through minimally invasive procedures. Currently, the majority of clinical data is derived from analyzing collected biological samples, primarily blood or plant sap. Unfortunately, these methods cost discomfort to patients, and are even destructive to plants. For example, conventional sap collection requires sacrifice the plants. The lack of portable tools for fast, on-site patient/plant monitoring has driven research into alternative strategies using biosensors.

The glucometer (i.e. blood glucose meter) stands out as one of the most successful examples of a POC device. It reflects the key features we strive for in such a biosensing platform: minimal limitations on who and where it can be used, combined with high reliability and affordability. Electrochemical readouts are advantageous in this case due to its fast response, wide detection range, and ease of integration into portable devices. This doctoral thesis introduces advancements of electrochemical biosensing platforms for detecting various analytes both in humans and plants. The key findings are summarized in the Results and Discussion section based on the four published research articles.

Briefly, the first type of electrochemical biosensor was developed for the determination of glycine in various human biofluids (e.g., blood, sweat, and urine). Considering the increasing importance of amino acid detection for clinical applications, we then created a new biosensing platform based on microneedles (MN) that aims to measure in dermal interstitial fluid. This minimally invasive strategy highlights the novelty of our second work. Third, we extended MN-based biosensors to another important analyte, lactate, which is previously widely analyzed in sweat. Finally, we demonstrated the first example of applying the MN sensors for continuous and real-time plant monitoring.

En växande efterfrågan har uppstått på nya point-of-care-plattformar (POC) som kan leverera tillförlitliga kliniska data i realtid genom minimalt invasiva procedurer. För närvarande härrör majoriteten av kliniska data från analys av insamlade biologiska prover, främst blod eller växtsaft. Tyvärr kostar dessa metoder obehag för patienter och är till och med destruktiva för växter, eftersom konventionell savinsamling kräver att växterna offras. Bristen på bärbara verktyg för snabb patient-/anläggningsövervakning på plats har drivit forskning på alternativa strategier som använder biosensorer.

Glukometern (dvs blodsockermätaren) framstår som ett av de mest framgångsrika exemplen på en POC-enhet. Det återspeglar nyckelfunktionerna vi strävar efter i en sådan biosensingplattform: minimala begränsningar för vem och var den kan användas, kombinerat med hög tillförlitlighet och prisvärdhet. Noterbart är den elektrokemiska avläsningen fördelaktig i detta fall på grund av dess snabba svar, breda detekteringsområde och enkla integration i bärbara enheter. I detta avseende introducerar denna doktorsavhandling framsteg inom elektrokemiska bioavkänningsplattformar för att detektera olika analyter både hos människor och växter. De viktigaste resultaten sammanfattas i avsnittet Resultat och diskussion baserat på de fyra publicerade artiklarna.

Först utvecklades den första typen av elektrokemisk biosensor för bestämning av glycin i olika humana biovätskor (t.ex. blod, svett och urin). Med tanke på den ökande betydelsen av aminosyradetektering för kliniska tillämpningar skapade vi sedan en ny biosensingplattform baserad på mikronålar (MN) som syftar till att mäta i interstitiell vätska (ISF). Denna minimalt invasiva strategi framhäver nyheten i vårt andra arbete. För det tredje utökar vi MN-baserade biosensorer till en annan viktig analyt, laktat, som tidigare analyserats flitigt i svett. Slutligen visade vi det första exemplet på att använda MN-sensorer för kontinuerlig och realtidsövervakning av anläggningar.

Lactate is an important diagnostic and prognostic biomarker of several human pathological conditions, such as sepsis, malaria, and dengue fever. Unfortunately, due to the lack of reliable analytical decentralized platforms, the determination of lactate yet relies on discrete blood-based assays, which are invasive and inefficient and may cause tension and pain in the patient. Herein, we demonstrate the potential of a fully integrated microneedle (MN) sensing system for the minimally invasive transdermal detection of lactate in an interstitial fluid (ISF). The originality of this analytical technology relies on: (i) a strategy to provide a uniform coating of a doped polymer-based membrane as a diffusion-limiting layer on the MN structure, optimized to perform full-range lactate detection in the ISF (linear range of response: 0.25–35 mM, 30 s assay time, 8 h operation), (ii) double validation of ex vivo and in vivo results based on ISF and blood measurements in rats, (iii) monitoring of lactate level fluctuations under the administration of anesthesia to mimic bedside clinical scenarios, and (iv) in-house design and fabrication of a fully integrated and portable sensing device in the form of a wearable patch including a custom application and user-friendly interface in a smartphone for the rapid, routine, continuous, and real-time lactate monitoring. The main analytical merits of the lactate MN sensor include appropriate selectivity, reversibility, stability, and durability by using a two-electrode amperometric readout. The ex-vivo testing of the MN patch of preconditioned rat skin pieces and euthanized rats successfully demonstrated the accuracy in measuring lactate levels. The in vivo measurements suggested the existence of a positive correlation between ISF and blood lactate when a lag time of 10 min is considered (Pearson’s coefficient = 0.85, mean difference = 0.08 mM). The developed MN-based platform offers distinct advantages over noncontinuous blood sampling in a wide range of contexts, especially where access to laboratory services is limited or blood sampling is not suitable. Implementation of the wearable patch in healthcare could envision personalized medicine in a variety of clinical settings.

Potassium and sodium ions (K+ and Na+) play crucial roles in influencing plant growth and health status. Unfortunately, current strategies to determine the concentrations of such ions are destructive for the plants because it is necessary to collect/extract the sap for further analysis and produce either scattered or delayed results. Here, we introduce a new potentiometric dual microneedle sensor for nondestructive, real-time, and continuous monitoring of K+ and Na+ concentrations in living plants. The developed sensors show a response time <5 s, close-to-Nernstian slope (∼55 mV dec–1), resiliency to five insertions on the stem, good repeatability (max. %RSD = 0.3%) and reversibility (max. %RSD = 3%), appropriate continuous operation for 24 h, and linear range of responses that cover expected plant physiological levels (5–50 mM for Na+ and 50–120 mM for K+). Moreover, the accuracy was successfully investigated by comparing the results provided by the microneedle sensors to those obtained by a standard reference method (e.g., ion chromatography). Finally, we demonstrate that the developed analytical device is capable of tracking K+ and Na+ transportation from the hydroponic solution to the stem within 5–10 min. This research will contribute to establishing a new generation of analytical platforms for smart agriculture offering real-time information.

Microneedle sensor technology offers exciting opportunities for decentralized clinical analyses. A novel issue puts forward herein is to demonstrate the uniqueness of membrane-based microneedles to accomplish real-time, on-body monitoring of multiple ions simultaneously. The use of multi-ion detection is clinically relevant since it is expected to provide a more complete and reliable assessment of the clinical status of a subject concerning electrolyte disorders and others. We present a microneedle system for transdermal multiplexed tracing of pH, Na+, K+, Ca2+, Li+, and Cl-. The device consists of an array of seven solid microneedles externally modified to provide six indicator electrodes, each selective for a different ion, and a common reference electrode, all integrated into a wearable patch read in a potentiometric mode. We show in vitro measurements at the expected clinical levels, resulting in a fast response time, excellent reversibility and repeatability, and adequate selectivity. Close-to-Nernstian sensitivity, sufficient stability and resiliency to skin penetration guarantee the sensor's success in transdermal measurements, which we demonstrate through ex vivo (with pieces of rat skin) and in vivo (on-body measurements in rats) tests. Accuracy is evaluated by comparison with gold standard techniques to characterize collected dermal fluid, blood, and serum. In the past, interstitial fluid (ISF) analysis has been challenging due to difficult sample collection and analysis. For ions, this has resulted in extrapolations from blood concentrations (invasive tests) rather than pure measurements in ISF. The developed microneedle patch is a relevant analytical tool to address this information gap.

Glycine (GLY) is gaining importance in medical diagnoses due to its relationship with multiple physiological functions. Today, GLY is exclusively analyzed using instrumentation centralized in clinical labs, and a tangible point-of-care tool that gathers real-time data from the patient for effective and fast evaluations is lacking. Relevant clinical advances are expected as soon as the rapid provision of both punctual and continuous measurements is possible. In that context, this work presents a microneedle (MN)-based biosensor for intradermal GLY detection in interstitial fluid (ISF). The MN tip is externally tailored to detect GLY levels through the hydrogen peroxide formed in its reaction with a quinoprotein-based GLY oxidase enzyme. The analytical performance of the MN biosensor indicates a fast response time (<7 s); acceptable reversibility, reproducibility, and stability; as well as a wide linear range of response (25-600 μM) that covers the physiological levels of GLY in ISF. The MN biosensor conveniently exhibits high selectivity for GLY over other compounds commonly found in ISF, and the response is not influenced by temperature, pH, or skin insertions. Validated intradermal measurements of GLY were obtained at the in vitro (with pieces of rat skin), ex vivo (on-body tests of euthanized rats) and in vivo (on-body tests of anesthetized rats) levels, demonstrating its ability to produce accurate physiological data. The developed GLY MN biosensor is skin-wearable and provides reliable, real-time intradermal GLY measurements in ISF by means of a minimally invasive approach.

We present herein the very first amperometric biosensor for the quantitative determination of glycine in diverse biological fluids. The biosensor is based on a novel quinoprotein that catalyzes the oxidation of glycine with high specificity. This process is coupled to the redox conversion of Prussian blue in the presence of hydrogen peroxide originating from the enzymatic reaction. The optimized tailoring of the biosensor design consists of the effective encapsulation of the quinoprotein in a chitosan matrix with the posterior addition of an outer Nafion layer, which is here demonstrated to suppress matrix interference. This is particularly important in the case of ascorbic acid, which is known to influence the redox behavior of the Prussian blue. The analytical performance of the biosensor demonstrates fast response time (<7 s), acceptable reversibility, reproducibility, and stability (<6% variation) as well as a wide linear range of response (25?500 ?M) that covers healthy (and even most unhealthy) physiological levels of glycine in blood/serum, urine and sweat. A total of 6 real samples from healthy patients and animals were analyzed: two serum, two urine and two sweat samples. The results were validated via commercially available fluorescence kit, displaying discrepancy of less than 9% in all the samples. The unique analytical features and effortless preparation of the new glycine biosensor position it at the forefront of current technologies towards decentralized clinical applications and sport performance monitoring.

Glycine monitoring is gaining importance as a biomarker in clinical analysis due to its involvement in multiple physiological functions, which results in glycine being one of the most analyzed biomolecules for diagnostics. This growing demand requires faster and more reliable, while affordable, analytical methods that can replace the current gold standard for glycine detection, which is based on sample extraction with subsequent use of liquid chromatography or fluorometric kits for its quantification in centralized laboratories. This work discusses electrochemical sensors and biosensors as an alternative option, focusing on their potential application for glycine determination in blood, urine, and cerebrospinal fluid, the three most widely used matrices for glycine analysis with clinical meaning. For electrochemical sensors, voltammetry/amperometry is the preferred readout (10 of the 13 papers collected in this review) and metal-based redox mediator modification is the predominant approach for electrode fabrication (11 of the 13 papers). However, none of the reported electrochemical sensors fulfill the requirements for direct analysis of biological fluids, most of them lacking appropriate selectivity, linear range of response, and/or capability of measuring at physiological conditions. Enhanced selectivity has been recently reported using biosensors (with an enzyme element in the electrode design), although this is still a very incipient approach. Currently, despite the benefits of electrochemistry, only optical biosensors have been successfully reported for glycine detection and, from all the inspected works, it is clear that bioengineering efforts will play a key role in the embellishment of selectivity and storage stability of the sensing element in the sensor.