Optical pH Measurement Research Articles

(Second Place Poster Award) Development of Optical Nanosensors for pH Measurements in Model Biofluids

Changes in pH in biofluids such as urine, sweat, and blood can reveal pathophysiological conditions like tumor metastasis, microbial infection, acidosis, cystic fibrosis, wound healing, etc. Thus, monitoring pH could provide insights on those patho-physiological changes. Among others, sweat analysis provides convenient and non-invasive way to track personal health. Sweat pH changes can reveal information about atopic dermatitis, fungus infections, and cystic fibrosis. Thus, sweat pH measurements could aid in accurately identifying certain health issues. Although solution pH can be measured by various methods including electrochemical, colorimetric and fluorescence measurements, optical pH measurements have gained popularity in recent decades due to versatile material choices and new imaging technologies. Semiconducting single-walled carbon nanotubes (SWCNTs) are emerging molecular optical probes and sensors due to their outstanding photophysical properties, which include photo-stable near-infrared (nIR) fluorescence and sensitive molecular recognition. Here, we report that SWCNT-based sensors can reliably detect pH changes in a model biofluid - artificial sweat. The sensor exhibited significant responses within biologically relevant pH ranges within minutes through changes in the nIR fluorescence. The nanotubes' optical response to pH changes provides new opportunities to develop optical pH sensors for healthcare, bioengineering, environmental sciences, and chemistry.

Optimized formulation of polyaniline-pectin optical film sensor for pH measurement

Novel polyaniline-pectin (PAN-PEC) composite sensing films were prepared for optical pH measurement. Box–Behnken experiment design and the response surface method were used to optimize the PAN-PEC synthesis to produce films with a maximum color change response to pH change in the pH range of 2 to 12. The pH sensing PAN-PEC nanoparticles for incorporation in the sensing film were made by oxidative polymerization of aniline using ammonium persulfate as the oxidant and pectin as the stabilizer. The experimental factors were the quantity of aniline, the concentration of hydrochloric acid, and the mass of ammonium persulfate in the reaction mixture, for a fixed quantity of pectin. A relationship between spectrophotometric absorption at 420 nm and the pH was developed for estimating the pH of the samples.

Preparation and Application of Highly Fluorescent Purple-emissive Carbon Dots for Optical pH Measurement

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Amorphous and crystalline CaCO3 phase transformation at high solid/liquid ratio – Insight to a novel binder system

The transformation reactions in aqueous calcium carbonate systems are relevant for biomineralization processes, functionalised materials design and recently for the potential development of inorganic binders. The so-called ‘calcium carbonate cements’ are obtained by mixing amorphous (ACC) and crystalline CaCO3 (herein the anhydrous minerals calcite, aragonite and vaterite) with low amounts of water. The mechanisms behind the setting and hardening of these binders are still not clear. In the present study, the dissolution and precipitation reactions in low water ACC systems with and without crystalline CaCO3 have been investigated by in-situ techniques, Raman spectroscopy and optical pH measurements, coupled with reaction time resolved mineralogical and chemical analyses of solid and liquid samples. The results clearly indicate the presence of initial crystalline CaCO3 polymorphs to control the reactions kinetics and the resulting individual microstructure. Vaterite, with a considerably higher surface area than calcite and aragonite, produces more bonded and connected material. In the ACC-vaterite system the transformation of vaterite to calcite takes places in two dissolution-precipitation stages, accompanied by typical pH in- and decreases. Time-resolved analyses and hydrochemical modelling clearly reveal the solid transformation behaviour to be governed by the presence/absence of Mg2+ in the solution and the impurities, such as Na+, liberated from the educts to the reactive solution. The control and adjustment of the identified influencing parameters could allow for promising future development of CaCO3-based binders.

Development of an optical pH measurement system based on colorimetric effect

<p>This paper proposed an optical pH measurement system developed to measure the changes of pH levels based on colorimetric reactions with phenol red reagent. The optical sensing was achieved through the implementation of a pair of light emitting diode (LED) and photodiode. The detection mechanism was based on different absorbance of light intensity at pH values of 2.36, 8.32, 9.08 and 12.83. The data processing method was carried out using LabVIEW software and interfaced with NI USB DAQ 6008. The measured voltage showed a good correlation in relation to the pH level with R2 equals to 0.9624. This relationship was used as a calibration curve in the final system testing. The final measurement of pH showed a good agreement between the actual and measured values with an error of less than 5%, thus indicating the reliability of the proposed system.</p>

Fluorescent sensor array for high-precision pH classification with machine learning-supported mobile devices

There is growing research interest from many scientific, healthcare, and industrial applications toward the development of high-precision optical pH sensors that cover a broad pH range. Despite enthusiastic endeavors, however, it remains challenging to develop cost-effective, high-precision, and broadband working paper-strip-type optical pH measurement systems, particularly for on-site or in-the-field pH sensing applications. We develop a fluorescent array based on a KIz system for accurate pH level classification. Based on the indolizine fluorescent core skeleton, a library of 30 different pH-responsive fluorescent probes is rationally designed and efficiently synthesized. Spotting the compounds in a checkered pattern (5 × 6) allows for the development of a disposable compound array on wax-printed cellulose paper. Compounds sharing a single chemical core skeleton result in the interrogation of all the components of a system with a single excitation light, resulting in a simple system design for pH classification. Furthermore, we design a 3D-printed enclosure to capture the fluorescence pattern changes of the array by using an intelligent, smartphone-based, handheld pH detection system. Specifically, by exploiting a random forest-based machine learning algorithm on a smartphone, we can effectively analyze the fluorescence pattern changes. Our results suggest that our proposed system can classify pH levels in fine-grain (0.2 pH) units.

Optical pH measurement system using a single fluorescent dye for assessing susceptibility to dental caries (Erratum).

.The erratum corrects a grant number listed in Acknowledgments section of the original article.

Optical pH measurement system using a single fluorescent dye for assessing susceptibility to dental caries.

.Sugar-rich diets and poor dental hygiene promote the formation of a biofilm (plaque) that strongly adheres to the dental enamel surface and fosters the evolution of aciduric bacteria. The acid contributes to demineralization of the exterior tooth enamel, which accelerates after the pH drops below a critical value () for extended time periods resulting in the need for restorative procedures. Preventative techniques to alert the dentist and caries-susceptible patients regarding vulnerability to dental decay require a clinical measure of plaque activity. Therefore, there is a need to evaluate the acid production capability of plaque deposits in the pits and fissures of occlusal and interproximal regions. A ratiometric fluorescence pH-sensing device has been developed using an FDA-approved dye and LED excitation. Fluorescein spectral profiles were collected using a spectrometer and analyzed with a spectral unmixing algorithm for calibration over the pH range of 4.5 to 7. An in vivo pilot study on human subjects was performed using a sucrose rinse to accelerate bacterial metabolism and to measure the time-dependent drop in pH. The optical system is relatively immune to confounding factors such as photobleaching, dye concentration, and variation in excitation intensity associated with earlier dye-based pH measurement techniques.

A 3-Bit Pseudo Flash ADC Based Low-Power CMOS Interface Circuit Design for Optical Sensor

The paper presents a CMOS interface circuit desig nf or optical pH measurement that can produce an 8-bit digital output representing the color information (i.e., wavelength, l). In this work we are focusing at reducing the component count by design and hence reducing the cost and silicon area. While it could be further optimized for lower power consumption, the proposed design has been implemented using standard cells provided by the foundry (i.e., AMS 0.35 ! mC MOS) for proof of concept. The biasing current and power consumption of the fabricated chip are measured at 11 ! A and 37 ! Wr espectively using 3.3 Vs upply voltage. Experimental results have further validated the proposed design concept. The number of detectable colors is eight and can be extended to a higher number without any major change in the architecture.

Real-time optical pH measurement in a standard microfluidic cell culture system

The rapid growth of microfluidic cell culturing in biological and biomedical research and industry calls for fast, non-invasive and reliable methods of evaluating conditions such as pH inside a microfluidic system. We show that by careful calibration it is possible to measure pH within microfluidic chambers with high accuracy and precision, using a direct single-pass measurement of light absorption in a commercially available phenol-red-containing cell culture medium. The measurement is carried out using a standard laboratory microscope and, contrary to previously reported methods, requires no modification of the microfluidic device design. We demonstrate the validity of this method by measuring absorption of light transmitted through 30-micrometer thick microfluidic chambers, using an inverted microscope fitted with a scientific-grade digital camera and two bandpass filters. In the pH range of 7-8, our measurements have a standard deviation and absolute error below 0.05 for a measurement volume smaller than 4 nL.

A new method of optical detection of yeast acidification power

We describe here a newly developed method for a contact-free optical pH measurement in yeast suspensions supplemented with glucose, and containing the pH sensitive triphenylmethane dye bromocresol green. It is suitable for performing the acidification power test (based on measuring the rate of pH drop of yeast suspension caused by active extrusion of acidity from cells after glucose addition) used for assessing yeast vitality in fermentation industries. Using this methodology we monitored the pH in yeast suspensions in the course of acidification in the pH range of 3.5-5.3. Optical pH measurement allows simultaneous testing of several samples, minimizes the sample volume, simplifies sample handling and reduces the hands-on time in sample processing.

Combination of On-line pH and Oxygen Transfer Rate Measurement in Shake Flasks by Fiber Optical Technique and Respiration Activity MOnitoring System (RAMOS).

Shake flasks are commonly used for process development in biotechnologyindustry. For this purpose a lot of information is required from the growth conditions duringthe fermentation experiments. Therefore, Anderlei et al. developed the RAMOS technology[1, 2], which proviedes on-line oxygen and carbondioxide transfer rates in shake flasks.Besides oxygen consumption, the pH in the medium also plays an important role for thesuccessful cultivation of micro-organisms and for process development. For online pHmeasurement fiber optical methods based on fluorophores are available. Here a combinationof the on-line Oxygen Transfer Rate (OTR) measurements in the RAMOS device with anon-line, fiber optical pH measurement is presented. To demonstrate the application of thecombined measurement techniques, Escherichia coli cultivations were performed and on-line pH measurements were compared with off-line samples. The combination of on-lineOTR and pH measurements gives a lot of information about the cultivation and, therefore, itis a powerful technique for monitoring shake flask experiments as well as for processdevelopment.

Optical pH Measurements with Water Dispersion of Polyaniline Nanoparticles and Their Redox Sensitivity

A new method for optical pH and redox measurements with a commercially available water dispersion of polyaniline (PANI) nanoparticles (mean particle size, 46 nm) is presented. The pH measurements are based on the acid-base equilibrium of PANI and were carried out either by combining both the automated sequential injection analysis (SIA) and UV-visible spectrophotometric techniques or with a fiber-optic light guide. In the former case, the detection was done in continuous mode at lambda = 800 nm by using the SIA technique for transporting the sample to a flow-through cell, which was placed in the light path of the photometer. With the fiber-optic light guide, the detection was done in batch mode at lambda = 400 and 580 nm. In both methods, fresh pH reagent (PANI) solution was used in each measurement, thus overcoming the problem with hysteresis (memory effect), which is usually observed with PANI films. The PANI nanoparticles were characterized with UV-visible spectroscopy in pH buffer solutions between pH 2-12 and a protonation constant of logK(0.5H,L)(H(0.5)L) = 4.4 was calculated from these data. Fast pH measurements can be done between pH 6 and 10.5 depending on the measuring technique. It is possible to determine pH with an accuracy of 0.1 pH unit between pH 8 and 10.5 (RSD, 0.5-2%). Redox transitions typical for PANI films were also observed for water solutions of PANI nanoparticles in the presence of the hexacyanoferrate(II/III) and the iron(II/III) oxalate redox couples. The absorbance at lambda = 875 nm is linearly dependent on the logarithm of the concentration ratio (0.1-10) of the iron oxalate redox couple.

The integration of an ultraviolet-visible spectrometer and a reaction calorimeter

A small ultraviolet-visible absorption spectrometer which uses fibre optic coupled immersion probes has been incorporated into a laboratory scale reaction calorimeter. The combined instrument has been tried out using the hydrolysis of acetic anhydride as a test reaction. With the calorimeter operating in the isoperibolic mode good agreement is found for the pseudo-first order reaction rate constant as determined from spectroscopic and calorimetric measurements. Experiments have been made in order to follow the reaction indirectly using optical pH measurements with acid-base indicators. The possibility of determining the temperature dependence of the rate constant in a single experiment has also been investigated.

Long-Lifetime Metal–Ligand pH Probe

We describe the synthesis and fluorescence spectral characterization of a pH-sensitive metal–ligand complex, [Ru(deabpy)(bpy)2]2+, where deabpy is 4,4′-diethylaminomethyl-2,2′-bipyridine. This metal–ligand complex (MLC) was found to display pH-dependent intensities, emission spectra, and decay times, with the changes centered near the physiological useful pH value of 7.5. The apparent pKavalues were not found to be dependent on ionic strength. The compound was found to be useful for lifetime-based sensing by phase-modulation fluorometry. Global analysis of the intensity decays over a range of pH values revealed two decay times of 235 and 380 ns, associated with the protonated and unprotonated forms, respectively. Because of its long decay time, optical pH measurements could be accomplished by phase-modulation fluorometry with a conveniently low modulation frequency of 700 kHz. The lifetime data were obtained with either a amplitude-modulated laser or with an amplitude-modulated blue-light-emitting diode. This pH-sensitive complex also displays a modest spectral shift with change in pH, allowing its use as a wavelengthratiometric MLC probe. One can imagine lifetime sensors for a variety of blood cations and point-ofcare assays based on long-lifetime metal–ligand complexes and simple solid-state light sources and detectors.

Experimental results on an optical pH measurement system for bioreactors

Properties of a pH-sensitive membrane have been tested with respect to its possible application in bioreactors. The influences of ionic strength, buffer type, buffer capacity, color and turbidity of the solution on the response of the membrane are studied. The developed membrane shows high reproducibility and long-term stability. The pH-sensing membrane can be sterilized chemically and by using heat. With the present opto-electronic set-up, an accuracy of ±0.1 pH between pH 6 and 10 can be achieved.