Colorimetric pH Sensor Research Articles

Poly( N-isopropylacrylamide) Cross-Linked Gels as Intrinsic Amphiphilic Materials: Swelling Properties Used to Build Novel Interphases.

The hydrophilic nature of hydrogels allows their swelling in aqueous solutions. In that way, any substance loaded inside the gel is exposed to the aqueous media and could be released if it is soluble in water. However, only substances that are soluble in water can be loaded inside a gel, which can be swelled only in water. In this work, we studied the swelling of poly( N-isopropylacrylamide) (PNIPAM) gels in nonaqueous solvents and their solutions with water. PNIPAM gels swell strongly in highly polar solvents, but they do not swell in slightly polar solvents (e.g., toluene). However, it is possible to swell the gel in mixtures containing toluene. The observed properties of PNIPAM gels allow describing them both as solvogels or amphigels. When the loaded substance is soluble in one solvent (e.g., water) and not in another (e.g., chloroform), the substance is not released but exposed to the new media. As a proof of concept, a colorimetric pH sensor active in CHCl3 and a Cu1+ sensor in water were built. Moreover, using a ternary solution containing toluene linear polystyrene can be loaded inside the gel, making a semi-interpenetrated network. Because PNIPAM swells in water and solvents immiscible in water, a liquid/liquid interphase can be set inside a gel. A near-infrared absorbing dye (soluble in CHCl3) is loaded in only half of a thermoresponsive PNIPAM gel. Upon near-infrared irradiation, only the region where the dye is loaded heats up driving the phase transition of PNIPAM.

Covalent Organic Framework Functionalized with 8-Hydroxyquinoline as a Dual-Mode Fluorescent and Colorimetric pH Sensor.

Real-time and accurate detection of pH in aqueous solution is of great significance in chemical, environmental, and engineering-related fields. We report here the use of 8-hydroxyquinoline-functionalized covalent organic framework (COF-HQ) for dual-mode pH sensing. In the fluorescent mode, the emission intensity of COF-HQ weakened as the pH decreased, and also displayed a good linear relationship against pH in the range from 1 to 5. In addition, COF-HQ showed discernible color changes from yellow to black as the acidity increased and can be therefore used as a colorimetric pH sensor. All these changes are reversible and COF-HQ can be recycled for multiple detection runs owing to its high hydrolytical stability. It can be further assembled into a mixed matrix membrane for practical applications.

Simple on package indicator label for monitoring of grape ripening process using colorimetric pH sensor

A simple on package indicator label for grapes (Vitis vinifera L.) ripeness has been developed based on chlorophenol red (CPR). The CPR was immobilized onto the filter paper via absorption method to make the CPR membrane. The membrane as a colorimetric label for grape ripeness works based on pH increase as the volatile organic acids decreased gradually in the package headspace due to the formation of sugars during berry ripening process. Subsequently, the color of the indicator label will change from white to beige then finally to yellow for over-ripe indication, which is easily detected by naked eye. The results show that the indicator label could be used to monitor the ripeness of grapes since the color changes of indicator label toward the ripeness of the grapes are in similar tendency and have a good linear correlation. Thus, the indicator label can be used as an effective tool for on-line ripeness monitoring of grapes packaging. Finally, the indicator label was successfully used as on-package indicator label for on-line ripeness monitoring of grapes in ambient and chilled conditions.

Robust and low cytotoxic betaine-based colorimetric pH sensors suitable for cancer cell discrimination

The local pH level in tumor microenvironment usually turns to an acidic range of 5.5-7.0 from the healthy level of 7.4. For this pH change, robust sensors with high sensitivity, water solubility and biocompatibility are highly desirable. Herein, novel zwitterionic betaines were designed and synthesized for the first time as sensitively colorimetric pH sensors which featured low cytotoxicity, excellent water solubility and reversibility. When the solution pH was changed from 5.0 to 7.2, new absorption bands appeared with gradually increased absorbance and were accompanied by naked-eye-visible color changes from yellow to green, indicating a new approach to detect pH changes by a simple ratiometric method. Moreover, these zwitterionic betaine sensors showed promising applications for biological and pathological samples due to their low cytotoxicity. In the pathological samples of prostate cancer cell extract solutions, they displayed sensitively colorimetric responses and strong anti-interference ability to pH changes. Therefore, it is promising for them to be as colorimetric indicators to pH changes in pathological samples for diagnostic and targeted therapeutic purposes.

Synthesis and application of an indenoquinoline dione conjugate as a dual fluorescent and colorimetric pH sensor

An indenoquinoline dione conjugate has been synthesized by a convenient and environmentally benign methodology using task specific ionic liquid [NMP]H2PO4 and the structure has been characterized by X-ray crystallography. UV–visible and fluorescence studies of 1 have been investigated in different solvents. pH dependent absorption and emission properties of this probe were also investigated. The fluorescence spectra of 1 were revealed a bathochromic shift and the fluorescence intensity was gradually increased along with the increase of pH value. Possible mechanism based on intramolecular charge transfer (ICT) for this fluorescent pH sensor has been proposed. Theoretical calculations based on density functional theory (DFT) have been done to understand the ICT in presence of acid and base of this particular sensor. The color change of this sensor could be directly detected by naked eyes which confirmed that it is also a promising candidate for colorimetric pH sensor.

A route to new colorimetric pH sensors

Abstract The detection of target molecules via luminescence changes of appropriate sensors is a powerful tool to determine the presence of analytes qualitatively and quantitatively. Therefore, the exploration of synthetic routes to new sensors is important to facilitate the recognition of a large range of analytes. In this report we describe such a new route to sensors and discuss the behavior of the synthesized sensors in the presence of different analytes. The prepared anthracene derivatives belong to two classes of substances. On the one hand imines were synthesized wherein the isomerization of their inherent carbon nitrogen double bond provides an effective non-radiative relaxation pathway. On the other hand amines were received by reduction of the mentioned imines. Their intramolecular charge transfer (ICT) based sensing mechanism allows the detection of protons in a reversible colorimetric fashion. Furthermore, the addition of zinc cations in the aprotic solvent DCM provides the first example showing that the detection of these colorimetric pH sensors is not limited to protons.

Highly Stable Mixed‐Lanthanide Metal–Organic Frameworks for Self‐Referencing and Colorimetric Luminescent pH Sensing

AbstractA strategy utilizing mixed‐lanthanide metal–organic frameworks (MOFs) for self‐referencing and colorimetric luminescent pH sensing has been developed. The mixed‐lanthanide MOFs EuxTb1−xHdpda (x=0.097, 0.205, 0.303, H4dpda=4‐(3,5‐dicarboxyphenyl)pyridine‐2,6‐dicarboxylic acid) exhibit excellent stability in phosphate buffer solution with pH values varying from 3.90 to 7.50. Density functional theory (DFT) calculations indicate that the deprotonation of uncoordinated carboxyl groups in ligands results in the elevation of the triplet excited state (T1) energy level of H4dpda, thus increasing the energy transfer probability from the T1 to 5D4 level of Tb3+. As a result, the emission intensity of Tb3+ in EuxTb1−xHdpda increases gradually while that of Eu3+ is almost not changed with the pH varying from 3.90 to 7.50. The intensity ratios of Eu3+ to Tb3+ can be correlated to the pH value, suggesting they are promising self‐referencing luminescent pH sensors. Furthermore, the significant pH‐dependent emission color change enables them to act as a luminescent colorimetric pH sensor for in situ and real‐time pH imaging. Because the energy transfer between the ligand and lanthanide ion can be easily tuned by selecting an appropriate ligand to further optimize the pH response range and sensitivity, such a strategy opens a new perspective for the development of self‐referencing luminescent pH sensors.

Synthesis and application of novel tricyanofuran hydrazone dyes as sensors for detection of microbes

Acknowledging the need to develop rapid and sensitive bacterial recognition approaches, we functionalised the tricyanofuran hydrazone molecular switch. Of significant interest in relation to the synthesised hydrazones is the formation of two different conjugated structures upon exposure to different pH values. Many bacteria release ammonia gas, which alkalises environments. Herein we report the synthesis of a tricyanofuran hydrazone having the function of a colorimetric pH sensor. The UV‐visible absorption and fluorescence spectra exhibit reversible colour changes of the tricyanofuran hydrazone solution in acetonitrile under acid–base conditions. Our results indicate that the tricyanofuran hydrazone probe can identify the bacterial targets quickly with high sensitivity. The infected samples exhibit a significant colour change from orange to blue and in the mean time there is a decrease in fluorescence emission as a function of ammonia and volatile amines released from bacterial metabolites. This tricyanofuran hydrazone chromophore is proposed for use in food packaging with a pH‐sensing capability.

Dipicolylamine styryldiazine derivatives: Synthesis and photophysical studies

Different D–π–A push–pull molecules in which dipicolylamine (DPA) is used as the donor group, different diazines as the acceptor groups, and styryl as the π-conjugated spacer have been synthesized in a straightforward manner by aldol condensation of 4-(di-2-picolylamino)benzaldehyde and the appropriate methyldiazine. All of the compounds showed π–π* transitions in the UV or visible region and the emission of yellow-green light upon irradiation. Significant red shifts were observed in the fluorescence emission maxima of these compounds on increasing the solvent polarity, a finding that suggests the formation of an intramolecular charge-separated emitting state that is also supported by semi-empirical calculations. In some cases, protonation led to marked color changes, thus showing the ability of these molecules to function as colorimetric pH sensors. The DPA and diazine units can act as coordination sites for metal cations such as Zn2+, Cd2+ or Hg2+, leading to a blue or red shift in the fluorescence spectra due to the change in the intramolecular charge transfer properties. This phenomenon could become a powerful tool for the creation of multiple emission colors with a single molecule after suitable design.

Novel fluorescent and colorimetric pH sensors derived from benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid

Fluorescent sensors N-[2-(dimethylamino)ethyl]benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylamide (L1) and its corresponding quaternary ammonium salt L2 were prepared. Probe L1 exhibited fluorescence ‘turn-on’ response toward H+ in acetonitrile/H2O (9:1) solution based on photoinduced electron transfer (PET) process. The fluorescence intensity of L1 gradually increased about 45-fold from pH 9.2 to 4.2 with a pKa value of 6.67. L2 exhibited colorimetric response toward OH− in water. The solution color of L2 changed from yellow to colorless under visible light and changed from yellow to blue under UV light from pH 8.9 to 11.5. 1H NMR spectroscopy studies confirmed hydration of the carbonyl group (CO) in fluorophore of L2 occurred under basic condition. The reason of fluorescence enhancement of L1 toward H+ and color changes of L2 toward OH− were demonstrated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations with 6-31G(d) basis sets using a suite of Gaussian 03 programs.

Synthesis and photophysical studies of a series of quinazoline chromophores.

The synthesis of a series of push-pull arylvinyl (styryl), aryl, and arylethynyl quinazoline derivatives by means of different straightforward protocols is reported. The photophysical properties of the compounds are described. The preparation of arylvinylquinazolines was performed by aldol condensation of the appropriate methylquinazoline and functionalized benzaldehyde. Suzuki and Sonogashira cross-coupling reactions were used to prepare the aryl and arylethynyl compounds, respectively, starting from chloroquinazolines. Optical studies revealed that all of the compounds reported here behave in a way similar to that of their pyrimidine counterparts, with absorption bands in the UV or visible region and the emission of green light upon irradiation. Large red shifts were observed in the fluorescence emission maxima upon increasing the solvent polarity. This strong emission solvatochromism suggests the formation of an intramolecular charge-separated emitting state. The materials can be easily and reversibly protonated at the nitrogen atoms of the heterocyclic ring, and this causes dramatic color changes. This phenomenon opens up the possibility of developing colorimetric pH sensors that can be efficiently modified a posteriori for specific applications.

Efficient Colorimetric pH Sensor Based on Responsive Polymer–Quantum Dot Integrated Graphene Oxide

In this paper, we report the development of a versatile platform for a highly efficient and stable graphene oxide (GO)-based optical sensor that exhibits distinctive ratiometric color responses. To demonstrate the applicability of the platform, we fabricated a colorimetric, GO-based pH sensor that responds to a wide range of pH changes. Our sensing system is based on responsive polymer and quantum dot (QD) hybrids integrated on a single GO sheet (MQD-GO), with the GO providing an excellent signal-to-noise ratio and high dispersion stability in water. The photoluminescence emissions of the blue and orange color-emitting QDs (BQDs and OQDs) in MQD-GO can be controlled independently by different pH-responsive linkers of poly(acrylic acid) (PAA) (pKa=4.5) and poly(2-vinylpyridine) (P2VP) (pKa=3.0) that can tune the efficiencies of Förster resonance energy transfer from the BQDs to the GO and from the OQDs to the GO, respectively. As a result, the color of MQD-GO changes from orange to near-white to blue over a wide range of pH values. The detailed mechanism of the pH-dependent response of the MQD-GO sensor was elucidated by measurements of time-resolved fluorescence and dynamic light scattering. Furthermore, the MQD-GO sensor showed excellent reversibility and high dispersion stability in pure water, indicating that our system is an ideal platform for biological and environmental applications. Our colorimetric GO-based optical sensor can be expanded easily to various other multifunctional, GO-based sensors by using alternate stimuli-responsive polymers.

Rational design of a colorimetric pH sensor from a soluble retinoic acid chaperone.

Reengineering of cellular retinoic acid binding protein II (CRABPII) to be capable of binding retinal as a protonated Schiff base is described. Through rational alterations of the binding pocket, electrostatic perturbations of the embedded retinylidene chromophore that favor delocalization of the iminium charge lead to exquisite control in the regulation of chromophoric absorption properties, spanning the visible spectrum (474-640 nm). The pKa of the retinylidene protonated Schiff base was modulated from 2.4 to 8.1, giving rise to a set of proteins of varying colors and pH sensitivities. These proteins were used to demonstrate a concentration-independent, ratiometric pH sensor.

A fluorescent colorimetric pH sensor and the influences of matrices on sensing performances

A fluorescent colorimetric pH sensor was developed by a polymerization of a monomeric fluorescein based green emitter (SM1) with a monomeric 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran derived red emitter (SM2) in poly(2-hydroxyethyl methacrylate)-co-polyacrylamide (PHEMA-co-PAM) matrices. Polymerized SM1 (PSM1) in the polymer matrices showed bright emissions at basic conditions and weak emissions at acidic conditions. Polymerized SM2 (PSM2) in the polymer matrices exhibited a vastly different response when compared to PSM1. The emissions of PSM2 are stronger under acidic conditions than those under basic conditions. When SM1 and SM2 were polymerized in the same polymer matrix, a dual emission sensor acting as a ratiometric pH sensor (PSM1,2) was successfully developed. Because the PSM1 and PSM2 exhibited different pH responses and separated emission windows, the changes in the emission colors were clearly observed in their dual color sensor of PSM1,2, which changed emission colors dramatically from green at pH 7 to red at pH 4, which was detected visually and/or by using a color camera under an excitation of 488nm. In addition to the development of the dual color ratiometric pH sensor, we also studied the effects of different matrix compositions, crosslinkers, and charges on the reporting capabilities of the sensors (sensitivity and pKa).

Development of a colorimetric microfluidic pH sensor for autonomous seawater measurements

High quality carbonate chemistry measurements are required in order to fully understand the dynamics of the oceanic carbonate system. Seawater pH data with good spatial and temporal coverage are particularly critical to apprehend ocean acidification phenomena and their consequences. There is a growing need for autonomous in situ instruments that measure pH on remote platforms. Our aim is to develop an accurate and precise autonomous in situ pH sensor for long term deployment on remote platforms. The widely used spectrophotometric pH technique is capable of the required high-quality measurements. We report a key step towards the miniaturization of a colorimetric pH sensor with the successful implementation of a simple microfluidic design with low reagent consumption. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n=20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO2, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The measurements were made close to in situ temperature (+0.2̊C) in a sampling chamber which had a continuous flow of the ship’s underway seawater supply. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. The use of a three wavelength LED with detection that integrated power across the whole of each LED output spectrum indicated that low wavelength resolution detectors can be used instead of the current USB mini spectrophotometer. Artefacts due to the polychromatic light source and inhomogeneity in the absorption cell are shown to have a negligible impact on the data quality. The next step in the miniaturization of the sensor will be the incorporation of a photodiode as detector to replace the spectrophotometer.

Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on Polyethylenimine-Capped Silver Nanoclusters

Silver nanoclusters capped by hyperbranched polyethylenimine (PEI) have been developed as a highly sensitive fluorescent and colorimetric pH sensor. The probe responds rapidly to pH fluctuations and has such absorption characteristics that the color changes from the colorless or a nearly colorless state to a colored state with increasing acidity, so PEI-capped Ag nanoclusters could be used as a color indicator for colorimetric pH detection. Quantitatively, the fluorescence intensity of PEI-capped Ag nanoclusters exhibits a linear fashion over the pH range of 5.02-7.96 and increases by around 10-fold approximately with greater fluorescence at higher pH values. The repulsion development and conformational change of PEI with decreasing pH induce the aggregation of Ag nanoclusters, leading to an obvious color change and fluorescence quenching of Ag nanoclusters at low pH values. As expected, the pH probe is also sensitive to the different buffer solutions, except for those containing some anions that could react with Ag nanoclusters. Besides, the ionic strength of the buffers has a little influence on the pH-responsive behavior. Our pH sensor with nanoscaled physical dimensions would be a promising candidate in the applications in biological, medical, and pharmaceutical fields.

Tunable, colorimetric DNA-based pH sensors mediated by A-motif formation

A colorimetric pH sensor has been developed based on GNP aggregation mediated by A-motif formation under acidic conditions. The pH response of the sensor can be tuned in the range of pH 2-5.5 by changing the length and the sequence of the A-motif forming poly dA tracts.

Quantification of indicator content in silica-based pH solid sensors by diffuse reflectance spectroscopy

The amount of alizarin red, brilliant yellow or acridine encapsulated within silica matrixes as colorimetric pH sensors, produced by three sol–gel routes (acid catalyzed, base catalyzed and non-hydrolytic routes), was investigated using diffuse reflectance spectroscopy in the UV-Vis region. Univariate calibration curves using silica (commercial or sol–gel synthesized) as a blank were linear (R2 > 0.99) but did not allow coherent quantification of the pH indicator, likely due to the matrix effect. The standard addition method using the silica blank (produced by the corresponding sol–gel route) to correct background afforded coherent results with standard deviation between 0.017 and 0.59, depending on the sol–gel route and indicator.

Intracellular pH.

Intracellular pH.