A Solid‐State Reference Electrode Based on a Self‐Referencing Pulstrode

A smartphone-based sensor for detection of iron and potassium in food and beverage samples

A novel approach for simultaneous detection of iron and potassium via a smartphone-based potentiometric method is proposed in this study. The screen printed electrodes were modified with carbon black nanomaterial and ion selective membrane including zinc (II) phtalocyanine as the ionophore. The developed Fe3+-selective electrode and K+-selective electrode exhibited detection limits of 1.0 × 10-6 M and 1.0 × 10-5 M for Fe3+ and K+ ions, respectively. The electrodes were used to simultaneously detect Fe3+ and K+ ions in apple juice, skim milk, soybean and coconut water samples with recovery values between 90%-100.5%, and validated against inductively coupled plasma-optical emission spectrometry. Due to the advantageous characteristics of the sensors and the portability of Near Field Communication potentiometer supported with a smartphone application, the proposed method offers sensitive and selective detection of iron and potassium ions in food and beverage samples at the point of need.

Thermoelectric Nanofluidics Probing Thermal Heterogeneity inside Single Cells

Accurate temperature measurement in one living cell is of great significance for understanding biological functions and regulation. Here, a nanopipet electric thermometer (NET) is established for real-time intracellular temperature measurement. Based on the temperature-controlled ion migration, the temperature change in solution results in altered ion mobilities and ion distributions, which can be converted to the thermoelectric responses of NET in a galvanostatic configuration. The exponential relationship between the voltage and the temperature promises highly sensitive thermoelectric responses up to 11.1 mV K-1, which is over an order of magnitude higher than previous thermoelectric thermometry. Moreover, the NET exhibits superior thermal resolution of 25 mK and spatiotemporal resolution of 100 nm and 0.9 ms as well as excellent stability and reproducibility. Benefiting from these unique features, both thermal fluctuations in steady-state cells and heat generation and dissipation upon drug administration can be successfully monitored, which are hardly achieved by current methods. By using NET, thermal heterogeneities of single cancer cells during immunotherapy were reported first in this work, in which the increased intracellular temperature was demonstrated to be associated with the survival benefit and resistance of cancer cells in immunotherapy. This work not only provides a reliable method for microscopic temperature monitoring but also gains new insights to elucidate the mechanism of immune evasion and therapeutic resistance.

Spontaneous Mesoporosity-Driven Sequestration of Ionic Liquids from Silicone-Based Reference Electrode Membranes

Nanopore-driven sequestration of ionic liquids from a silicone membrane is presented, a phenomenon that has not been reported previously. Reference electrodes with ionic liquid doped polydimethylsiloxane (PDMS) reference membranes and colloid-imprinted mesoporous carbon (CIM) as solid contact are not functional unless special attention is paid to the porosity of the solid contact. In the fabrication of such reference electrodes, a solution of a hydroxyl-terminated silicone oligomer, ionic liquid, cross-linking reagent, and polymerization catalyst is deposited on top of the carbon layer, rapidly filling the pores of the CIM carbon. The catalyzed polymerization curing of the silicone quickly results in cross-linking of the hydroxyl-terminated polydimethylsiloxane oligomers, forming structures that are too large to penetrate the CIM carbon pores. Therefore, as solvent evaporation from the top of freshly prepared membranes drives the diffusional transport of solvent toward that membrane surface, the solvent molecules that leave the CIM carbon pores can only be replaced by the ionic liquid. This depletes the ionic liquid in the reference membrane that overlies the CIM carbon solid contact and increases the membrane resistance by up to 3 orders of magnitude, rendering the devices dysfunctional. This problem can be avoided by presaturating the CIM carbon with ionic liquid prior to the deposition of the solution that contains the silicone oligomers and ionic liquid. Alternatively, a high amount of ionic liquid can be added into the membrane solution to account for the size-selective sequestration of ionic liquid into the carbon pores. Either way, a wide variety of ionic liquids can be used to prepare PDMS-based reference electrodes with CIM carbon as a solid contact. A similar depletion of the K⁺ ionophore BME-44 from ion-selective silicone membranes was observed too, highlighting that the depletion of active ingredients from polymeric ion-selective and reference membranes due to interactions with high surface area solid contacts may be a more common phenomenon that so far has been overlooked.

A Solid-Contact Reference Electrode Based on Silver/Silver Organic Insoluble Salt for Potentiometric Ion Sensing

Solid-contact ion-selective electrodes are a type of ion measurement devices that have been focused in wearable biotechnology based on the features of miniaturization and integration. However, the solid-contact reference electrodes (SC-REs) remain relatively less focused compared with numerous working (or indicator) electrodes. Most SC-REs in wearable sensors rely on Ag/AgCl reference electrodes with solid electrolytes, for example, the hydrophilic electrolyte salts in polymer matrix, but face the risk of electrolyte leakage. Herein, we report a type of SC-REs based on the silver/silver tetraphenylborate (Ag/AgTPB) organic insoluble electrode. The SC-RE consists of a Ag substrate, a solid contact (AgTPB), and a plasticized poly(vinyl chloride) (PVC) membrane containing the hydrophobic organic salt of tetrabutylammonium tetraphenylborate (TBATPB). The potentiometric measurements demonstrated that the SC-RE of Ag/AgTPB/PVC-TBATPB showed a reproducible standard potential in various electrolytes and disclosed high long-term stability. This SC-RE was further fabricated on a flexible substrate and integrated into all-solid-state wearable potentiometric ion sensor for sweat Cl^- monitoring.

Microfluidic-based ion-selective thermoplastic electrode array for point-of-care detection of potassium and sodium ions

A microfluidic paper-based thermoplastic electrode (TPE) array has been developed for point-of-care detection of Na⁺ and K⁺ ions using a custom-made portable potentiometer. TPEs were fabricated using polystyrene as the binder and two different types of graphite to compare the electrode performance. The newly designed TPE array embedded in a polymethyl methacrylate chip consists of two working electrodes modified with carbon black nanomaterial and an ion-selective membrane, and an all-solid-state reference electrode modified with Ag/AgCl ink and poly(butyl methacrylate-co-methyl methacrylate) membrane via drop-casting. Ion-selective membrane compositions and conditioning steps were optimized. Under optimized conditions, ion-selective TPEs demonstrated fast response time (4 s) and good stability. The TPE array demonstrated a Nernstian behavior for K⁺ with a sensitivity of 59.2 ± 0.2 mV decade^-1 and near-Nernstian response for Na⁺ with a sensitivity of 54.0 ± 1.1 mV decade^-1 in the range 10^-1 - 10^-4 M and 1 -  10^-3 M, respectively. The detection limits were 1 × 10^-5 M and 1 × 10^-4 M for K⁺ and Na⁺, respectively. In addition, a K⁺ and Na⁺ selective microfluidic paper-based analytical device (µPAD) was applied to artificial serum analysis and found in good agreement with average recoveries of 101.3% and 99.7%, respectively, suggesting that the developed ISE array is suitable for detection of sodium and potassium in complex matrix.

Graphene Biodevices for Early Disease Diagnosis Based on Biomarker Detection

The early diagnosis of diseases plays a vital role in healthcare and the extension of human life. Graphene-based biosensors have boosted the early diagnosis of diseases by detecting and monitoring related biomarkers, providing a better understanding of various physiological and pathological processes. They have generated tremendous interest, made significant advances, and offered promising application prospects. In this paper, we discuss the background of graphene and biosensors, including the properties and functionalization of graphene and biosensors. Second, the significant technologies adopted by biosensors are discussed, such as field-effect transistors and electrochemical and optical methods. Subsequently, we highlight biosensors for detecting various biomarkers, including ions, small molecules, macromolecules, viruses, bacteria, and living human cells. Finally, the opportunities and challenges of graphene-based biosensors and related broad research interests are discussed.

Long-Term Quantitatively Imaging Intracellular Chloride Concentration Using a Core-/Shell-Structured Nanosensor and Time-Domain Dual-Lifetime Referencing Method

Luminescence lifetime-based nanosensors for chloride ions were designed by incorporating a luminescent ruthenium dye [Ru(1,10-phenanthroline)₃] inside silica nanoparticles and chemically labelling their outer surface with chloride ion-sensitive fluorescent dyes (N,N'-bis(carboxypropyl)-9,9'-biacridine). The nanosensor surface was further functionalized with positively charged amino groups to facilitate intracellular uptake via endocytosis and target lysosomes. The nanosensors have an average diameter of 52 nm and are monodispersed in aqueous solutions. Because of the long lifetime of the reference ruthenium dye, the sensor response can be analyzed using the time-domain dual-lifetime referencing (td-DLR) approach. The use of pulsed excitation in td-DLR rather than intense continuous illumination in ratiometric measurements greatly prevents the dye from photobleaching which significantly improves its measurement stability and reproducibility for long-term monitoring. At optimum conditions, the sensor can measure chloride concentration in the range of 0-200 mM with a large ratiometric signal change from 140.9 to 40.2. Combined with our custom-built microscopic td-DLR system, variations of intracellular chloride concentration in lysosomes were imaged quantitatively with a high spatial resolution and accuracy.

An Integrated Multi-Function Heterogeneous Biochemical Circuit for High-Resolution Electrochemistry-Based Genetic Analysis

Modular construction of an autonomous and programmable multi-functional heterogeneous biochemical circuit that can identify, transform, translate, and amplify biological signals into physicochemical signals based on logic design principles can be a powerful means for the development of a variety of biotechnologies. To explore the conceptual validity, we design a CRISPR-array-mediated primer-exchange-reaction-based biochemical circuit cascade, which probes a specific biomolecular input, transform the input into a structurally accessible form for circuit wiring, translate the input information into an arbitrary sequence, and finally amplify the prescribed sequence through autonomous formation of a signaling concatemer. This upstream biochemical circuit is further wired with a downstream electrochemical interface, delivering an integrated bioanalytical platform. We program this platform to directly analyze the genome of SARS-CoV-2 in human cell lysate, demonstrating the capability and the utility of this unique integrated system.

Solid-Contact Ion-Selective Electrodes: Response Mechanisms, Transducer Materials and Wearable Sensors

Wearable sensors based on solid-contact ion-selective electrodes (SC-ISEs) are currently attracting intensive attention in monitoring human health conditions through real-time and non-invasive analysis of ions in biological fluids. SC-ISEs have gone through a revolution with improvements in potential stability and reproducibility. The introduction of new transducing materials, the understanding of theoretical potentiometric responses, and wearable applications greatly facilitate SC-ISEs. We review recent advances in SC-ISEs including the response mechanism (redox capacitance and electric-double-layer capacitance mechanisms) and crucial solid transducer materials (conducting polymers, carbon and other nanomaterials) and applications in wearable sensors. At the end of the review we illustrate the existing challenges and prospects for future SC-ISEs. We expect this review to provide readers with a general picture of SC-ISEs and appeal to further establishing protocols for evaluating SC-ISEs and accelerating commercial wearable sensors for clinical diagnosis and family practice.

Direct Potentiometric Sensing of Anion Concentration (Not Activity)

Potentiometric probes used in direct potentiometry are attractive sensing tools. They give information on ion activities, which is often uniquely useful. If, instead, concentrations are desired as sensor output, the ionic strength of the sample must be precisely known, which is often not possible. Here, for the first time, direct potentiometry can be made to report concentrations, rather than activities. It is demonstrated for the detection of monovalent anionic species by using a self-referencing Ag/AgI pulstrode as the reference element instead of a traditional reference electrode. This reference pulstrode releases a discrete quantity of iodide ions from the electrode and the resulting reference potential varies with the activity coefficient of iodide. The effects of activity coefficient on the indicator and reference electrode are therefore compensated and the observed cell potential may now be described in a Nernstian manner against anion concentration, rather than activity. Theoretical simulations and experimental results support the validity of this approach. For most monovalent anions of practical relevance, the potential difference between this approach and from a traditional activity coefficient calculation is less than 0.5 mV. The concept is validated with an all-solid-state nitrate sensor as well as a commercial fluoride-selective electrode, giving Nernstian responses in different ionic strength backgrounds against concentration without the need for correcting activity coefficients or liquid junction potentials.