Microfluidic ion-sensing devices

A Highly Sensitive and Selective Optical Sensor for the On-Line Detection of Cesium in Water

In this study, we have undertaken the development of two fluorescent sensors based on calixarene compounds for the purpose of detecting cesium in water. By introducing the sulfonate functional groups, we have considerably improved the water solubility of sensors, enabling complete dissolution of products in aqueous media and direct analysis of polluted water samples. Through rigorous experiments, we have demonstrated that the complexation of Cs+ ions with sensors 1 and 2 in water leads to a remarkable enhancement of fluorescence. This fluorescence enhancement serves as a reliable indication of cesium presence and allows for sensitive detection. To further advance the practical application of our sensors, we have successfully integrated calixarene sensors 1 and 2 into a microfluidic sensor chip. This integration has enabled real-time, on-line measurements and has resulted in the development of a portable detection device capable of detecting cesium ions in water samples at parts per billion (ppb) levels. This device holds great promise for environmental monitoring and assessment, providing a convenient and efficient solution for cesium detection. Our work represents a significant advancement in the field of cesium detection, displaying the efficacy of calixarene-based fluorescent sensors and their integration into microfluidic systems. The enhanced water solubility, fluorescence response, and portability of our detection device offers tremendous potential for applications in environmental monitoring, water quality assessment, and emergency response scenarios where rapid and accurate cesium detection is crucial.

Microfluidic assisted recognition of miRNAs towards point-of-care diagnosis: Technical and analytical overview towards biosensing of short stranded single non-coding oligonucleotides

MiRNAs are short stranded single non-coding oligonucleotides that play an important role in regulating gene expression. MiRNAs are stable in RNase enriched environments such as human body fluids and their dysregulation or abnormal abundance in human body fluids as a diagnostic biomarker has been associated with several diseases. Due to the low concentration of miRNAs, it is difficult to detect using interactive methods (ideal detection limit is femtomolar range). However, clinicians lack sensitive and reliable methods for quantifying miRNA. Microfluidic devices integrated with electrochemical, optical (fluorometric, SERs, FRET, colorimetric), electrochemiluminescence and photoelectrochemical signal readout led to development innovative diagnostic device test, can probably overcome the limitations of the traditional methods. In the present review, microfluid methods for the sensitive and selective recognition of miRNA in various biological matrices are surveyed. Also, advantages and limitation of recognition methods on the performance and efficiency of microfluidic based biosensing of miRNAs are critically investigated. Finally, the future perspectives on the diagnosis of disease based on microfluidic analysis of miRNAs are provided.

On-chip high ion sensitivity electrochromic nanophotonic light modulator

Since the discovery of electrochromism, the prospect of employing various electrochromic materials for smart window glass, variable reflectivity mirrors, and large-area displays has been the main drive for such an intriguing phenomenon. However, with advances in nanofabrication and the emergence of improved electrochromic materials offering reversible large changes in dielectric properties upon electrically induced redox reactions, the application strategies are starting to encompass the field of nanophotonics and nanoplasmonics. Herein, a novel strategy is proposed and demonstrated for offering both ultrahigh light modulation depth and high sensitivity ion detection in a single nanophotonic waveguiding platform. By using WO₃ to ionically-drive dynamic light control via modulating the refractive index and the losses within the waveguide at ±1.5 V, ultrahigh optical modulation depth of 10⁶, rapid response speed of <0.56 s, long cyclic life, and very sensitive Na⁺ ion detection ability in 1 mM-1 M concentration, are achieved within a volume of a few μm³. It is envisioned that our introduction of such a multifunctional electrochromic nanophotonic waveguide platform will stimulate and promote further efforts toward fundamental research on technologically promising on-chip integrated next-generation nanophotonic and nanoplasmonic devices for various niche applications.

A Minimally Invasive Microsensor Specially Designed for Simultaneous Dissolved Oxygen and pH Biofilm Profiling

A novel sensing device for simultaneous dissolved oxygen (DO) and pH monitoring specially designed for biofilm profiling is presented in this work. This device enabled the recording of instantaneous DO and pH dynamic profiles within biofilms, improving the tools available for the study and the characterization of biological systems. The microsensor consisted of two parallel arrays of microelectrodes. Microelectrodes used for DO sensing were bare gold electrodes, while microelectrodes used for pH sensing were platinum-based electrodes modified using electrodeposited iridium oxide. The device was fabricated with a polyimide (Kapton^®) film of 127 µm as a substrate for minimizing the damage caused on the biofilm structure during its insertion. The electrodes were covered with a Nafion^® layer to increase sensor stability and repeatability and to avoid electrode surface fouling. DO microelectrodes showed a linear response in the range 0–8 mg L⁻¹, a detection limit of 0.05 mg L⁻¹, and a sensitivity of 2.06 nA L mg⁻¹. pH electrodes showed a linear super-Nernstian response (74.2 ± 0.7 mV/pH unit) in a wide pH range (pH 4−9). The multi-analyte sensor array was validated in a flat plate bioreactor where simultaneous and instantaneous pH and DO profiles within a sulfide oxidizing biofilm were recorded. The electrodes spatial resolution, the monitoring sensitivity, and the minimally invasive features exhibited by the proposed microsensor improved biofilm monitoring performance, enabling the quantification of mass transfer resistances and the assessment of biological activity.

Characterization of Pieces of Paper That Form Reagent Containers for Use as Portable Analytical Devices

Reagent-deposited pieces of paper were characterized by the use of a compact conductometer, a compact pH sensor, and a conventional spectrophotometer to assess their suitability for use as reagent containers. The pieces of paper were fabricated by wax printing to form a limited hydrophilic area to which a consistent volume of an aqueous reagent could be added. The pieces of paper without the reagent increased the conductivity of water gradually because of the release of sodium salts, whereas pH of NaOH decreased because of the acidity of the functional groups in the paper. Three reagents, sulfamic acid as an acid, Na₂CO₃ as a base, and BaCl₂ as a metal salt, were deposited on the pieces of paper to evaluate their ability to release from the pieces of paper. Sulfamic acid and Na₂CO₃ were released in quantities of 58 and 73% into water after 420 s, whereas 100% of BaCl₂ was released after 480 s. The conductometric titrations of NaOH, HCl, and Na₂SO₄, and the spectrophotometry of Fe²⁺ were examined using the pieces of paper that contained sulfamic acid, Na₂CO₃, BaCl₂, and 1,10-phenanthroline. Titrations using the pieces of paper suggested that the reagents were quantitatively released into the titrant, which resulted in a linear relationship between the endpoints and the equivalent points. In 120 s of soaking time, 60-70% of the reagents were released. The spectrophotometric measurements of Fe²⁺ indicated that when an excess amount of the reagents was deposited onto the pieces of paper, they nonetheless sufficiently fulfilled the role of a reagent container.

Monitoring ion activities in and around cells using ion-selective liquid-membrane microelectrodes

Determining the effective concentration (i.e., activity) of ions in and around living cells is important to our understanding of the contribution of those ions to cellular function. Moreover, monitoring changes in ion activities in and around cells is informative about the actions of the transporters and/or channels operating in the cell membrane. The activity of an ion can be measured using a glass microelectrode that includes in its tip a liquid-membrane doped with an ion-selective ionophore. Because these electrodes can be fabricated with tip diameters that are less than 1 μm, they can be used to impale single cells in order to monitor the activities of intracellular ions. This review summarizes the history, theory, and practice of ion-selective microelectrode use and brings together a number of classic and recent examples of their usefulness in the realm of physiological study.

Potentiometric and voltammetric polymer lab chip sensors for determination of nitrate, pH and Cd(II) in water

Due to their toxicity to humans and animals, heavy metals and nitrate in groundwater are of particular concern. The combination of high toxicity and widespread occurrence has created a pressing need for effective monitoring and measurement of nitrate and heavy metals in soil pore water and groundwater at shallow depths. In this work, a new electrochemical sensing platform with the self-assembly nanobeads-packed (nBP) hetero columns has been developed for the pH and nitrate measurements. In addition, for on-site determination of cadmium (Cd(II)), a bismuth (Bi(III)) based polymer lab chip sensor using the square-wave anodic stripping voltammetry (SWASV) sensing principle has been designed, fabricated and successfully characterized. Factors affecting sensitivity and precision of the sensor, including deposition potential and deposition time, were studied. Miniaturized electrochemical lab chip sensors could be very valuable in environmental monitoring area due to their many benefits, such as greatly reduced sensing cost, sensing system portability, and ease of use.

Fluorimetric lead detection in a microfluidic device

A microfabricated device has been developed for the selective detection of lead in water. It is based on the use of a selective and sensitive fluorescent molecular sensor for lead (Calix-DANS4) which contains a calix[4]arene bearing four dansyl groups. The microchip-based lead sensor contains a Y-shape microchannel equipped with a passive mixer and moulded on a glass substrate. An optimization of the microcircuit length has been performed in order to have a full complexation of the Calix-DANS4. The detection is performed by using a configuration in which the sensing molecules are excited by two optical fibres, each one connected to a 365 nm UV LED, and the light collection is made by another optical fibre with a photomultiplier. By using this configuration we have shown the possibility to detect lead with a detection limit of 5 ppb. The effect of interfering cations such as calcium has been evaluated. The obtained measurements have been validated by an alternative method (ASV).