Miniaturized pH Holographic Sensors for the Monitoring of Lactobacillus casei Shirota Growth in a Microfluidic Chip

Electrochemical monitoring of hydrogen peroxide by a signal-amplified microfluidic chip coupled with colloidal VO2 nanostructures as a peroxidase enzyme mimic

We present a novel electrochemical microfluidic device for the sensitive and selective detection of hydrogen peroxide (H₂O₂) through a VO₂ nanostructure enzyme mimic. The low-cost ($0.50) microfluidic chip was fabricated using a simple and rapid prototyping technique via three syringe needles. Each needle played the role of an electrode (working, reference, and counter), and was connected by micro-hoses to a construction of the electrochemical microfluidic chip. The colloidal VO₂ nanoflakes with peroxidase-like activity could be easily transferred on to the electrodes by a syringe, for development of a novel electrochemical platform to enable the detection of H₂O₂. The unique microfluidic electrochemical sensor delivered a wide linear dynamic range from 0.5 to 300 μM, with a limit of detection of 0.14 μM. The facile, rapid, sensitive, and selective as-fabricated H₂O₂ sensors were proven to be appropriate for the real-time monitoring of H₂O₂ released from PC12 cells. The integration of a microfluidic sensor with an enzyme mimic nanostructure is essentially a promising strategy for the low-cost and accurate monitoring of physiological processes.

Theoretical design of an absorption hologram-based sensor for dose quantification in daylight photodynamic therapy

Daylight photodynamic therapy (D-PDT) is an effective and almost painless treatment for many skin conditions, where successful treatment relies on daylight activation of a topical photosensitizer. Optimization of D-PDT requires accurate assessment of light dose received. There is a requirement for a small-area sensor that can be placed adjacent to the treatment site to facilitate accurate dose quantification. Here, a novel, to the best of our knowledge, configuration for a D-PDT dose sensor, consisting of a holographic absorption grating fabricated in a photosensitive film, is presented. Theoretical modeling of the sensor's response (i.e., change in grating diffraction efficiency due to change in grating absorption modulation, α₁, on exposure to daylight) was conducted using Kogelnik's coupled-wave theory. The influence of the different grating parameters (initial film absorption, thickness, spatial frequency, and reconstruction wavelength) on the sensor response was examined and revealed that the initial absorption and grating thickness values have a large impact on both the magnitude and rate of the D-PDT sensor response. The optimum design for an absorption grating-based D-PDT sensor is described.

Hydrogel-based holographic sensors and biosensors: past, present, and future

Hydrogel-based holographic sensors consist of a holographic pattern in a responsive hydrogel that diffracts light at different wavelengths depending on the dimensions and refractive index changes in the material. The material composition of hydrogels can be designed to be specifically responsive to different stimuli, and thus the diffraction pattern can correlate with the amount of analyte. According to this general principle, different approaches have been implemented to achieve label-free optical sensors and biosensors, with advantages such as easy fabrication or naked-eye detection. A review on the different approaches, sensing materials, measurement principles, and detection setups, and future perspectives is offered.

Fabrication, Potentiometric Characterization, and Application of Screen-Printed RuO2 pH Electrodes for Water Quality Testing

Screen-printed sensing electrodes attract much attention for water pollution monitoring due to their small size, physical and chemical durability, and low cost. This paper presents the fabrication and broad potentiometric characterization of RuO₂ pH sensing electrodes deposited by screen printing on alumina substrates and sintered in the 800–900 °C temperature range. All the fabricated electrodes showed close to Nernstian sensitivity, good linearity, fast response, small drift, low hysteresis, and low cross-sensitivity toward various interfering cations and anions. Furthermore, decreasing the sintering temperature led to better adhesion of the RuO₂ layer and a negligible response to interfering ions. The measurements in real-life samples from different water sources showed that the fabricated electrodes are on par with conventional glass electrodes with a maximum deviation of 0.11 pH units, thus indicating their potential for application in water quality monitoring.

Plasmonic Elastic Capsules as Colorimetric Reversible pH-Microsensors

There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists of exploiting the distance-dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH-sensitive sensors remains a challenge. Here, it is shown how to combine optically active plasmonic gold nanoparticles and pH-responsive thin shells into "plasmocapsules." Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron-size sensors can thus be easily fabricated. They are nonintrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, their potential use for the detection of bacterial growth is demonstrated, thus proving that plasmocapsules are a new class of sensing materials.

Quantifying Acute Fuel and Respiration Dependent pH Homeostasis in Live Cells Using the mCherryTYG Mutant as a Fluorescence Lifetime Sensor

Intracellular pH plays a key role in physiology, and its measurement in living specimens remains a crucial task in biology. Fluorescent protein-based pH sensors have gained widespread use, but there is limited spectral diversity for multicolor detection, and it remains a challenge to measure absolute pH values. Here we demonstrate that mCherryTYG is an excellent fluorescence lifetime pH sensor that significantly expands the modalities available for pH quantification in live cells. We first report the 1.09 Å X-ray crystal structure of mCherryTYG, exhibiting a fully matured chromophore. We next determine that it has an extraordinarily large dynamic range with a 2 ns lifetime change from pH 5.5 to 9.0. Critically, we find that the sensor maintains a p Ka of 6.8 independent of environment, whether as the purified protein in solution or expressed in live cells. Furthermore, the lifetime measurements are robustly independent of total fluorescence intensity and scatter. We demonstrate that mCherryTYG is a highly effective sensor using time-resolved fluorescence spectroscopy on live-cell suspensions, which has been previously overlooked as an easily accessible approach for quantifying intracellular pH. As a red fluorescent sensor, we also demonstrate that mCherryTYG is spectrally compatible with the ATeam sensor and EGFP for simultaneous dual-color measurements of intracellular pH, ATP, and extracellular pH. In a proof-of-concept, we quantify acute respiration-dependent pH homeostasis that exhibits a stoichiometric relationship with the ATP-generating capacity of the carbon fuel choice in E. coli. Broadly speaking, our work presents a previously unemployed methodology that will greatly facilitate continuous pH quantification.