Colorimetric pH Sensor Research Articles

Biopolymeric sensor based on natural deep eutectic solvents for monitoring meat spoilage

Ensuring food safety by monitoring freshness is crucial, particularly for high-protein meat products susceptible to spoilage. Smart packaging with pH-sensitive indicators, using natural pigments such as anthocyanin, provides a cost-effective and non-invasive solution. In this work, a novel colorimetric pH sensor based on biopolymer and natural deep eutectic solvents (NADES) is proposed. The NADES composed by Betaine: Glycerol: Water (BGH) was used for the anthocyanin extraction from Brassica oleracea (Purple Cabbage, PC), using the ultrasonic “one-step” method and BGH extraction efficiency was compared with water. Then, the BGH extract was mixed with different concentrations of xanthan gum (XG) 5, 10, 15 and 20%. The biopolymeric materials were characterized in terms of structure, mechanical properties, stability and pH response. The combination containing 10 % of xanthan gum (PC-BGH-XG10) presented suitable features to be used as a sensor. Thus, PC-BGH-XG10 was applied for real-time monitoring of meat spoilage. Interestingly colour changes were visible to the naked eye and they were successfully correlated with microbiological assays. To the best of our knowledge, this is the first integration of NADES and xanthan gum in the development of a colorimetric pH sensor. The proposed approach represents a green strategy for in-situ control of packaged foods intended for final consumption.

A New Soluble Copper Phthalocyanine Derivative as a Smart Material

Copper phthalocyanine (CuPc) and its derivatives are considered as candidate materials in many applications. Particularly, easy and sensitive film-forming ability, commercial availability, chemical stability, and ease in tailoring its molecular structure make CuPc a versatile material. On the other hand, main challenge that Pcs often exhibit is their poor solubility in organic solvents. In this sense, this work involves designing of new CuPc derivatives by introducing suitable substitutions to improve the solubility in organic solvents. Specifically, [2,9,16,23-tetra{(4,5-Diphenyl-1H-imidazole)-2-yl-thio}phthalocyaninato-copper(II)] (1) and [2,9,16,23-tetra{(4,5-Diphenyl-1-methyl-1H-imidazole)-2-yl-thio}phthalocyaninato-copper(II)] (2) were prepared. The results show that compound 1 is soluble in tetrahydrofuran (THF), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), and compound 2 is soluble in chloroform, acetone, methanol, THF, DMF, and DMSO. Optical and spectroscopic properties of the synthesized compounds were also investigated, and it was determined that the energy bang gaps of compounds 1 and 2 are 1.70 eV and 1.56 eV, respectively. Strikingly, we demonstrate that compound 1 is exhibiting a rapid and reversible color change behavior upon altering pH in the entire pH spectrum. As it is known, materials that respond reversibly to chemical and/or physical stimuli in a controllable fashion are regarded as smart materials. Hence, we report that compound 1 is actually a smart material that can be used as a simple yet efficient pH sensor.

Synthesis, characterizations and colorimetric pH sensors of freestanding Janus conjugated Covalent Organic Framework (COF) films

Ultrathin, freestanding and optically active conjugated COF films based on 2,4,6-trihydroxybenzene-1,3,5-trialdehyde (TFP) and 4,4’,4’’-triaminotriphenylamine (TPA) have been synthesized and studied. A new mechanism of interfacial polymerization-interfacial film formation, based on the direct observation of the aqueous droplet volume changes in the organic phase, is proposed. Depth profiling using X-ray photoelectron spectroscopy (XPS) revealed the changes in the oxygen element composition after annealing, with a corresponding increase in hydrophilic groups exposed near the film's surface. Capacitance tests demonstrated a slight decrease in capacitance after annealing, attributed to a lower percentage of oxygen element. SEM analysis revealed a slight increase in particle size after annealing. Additionally, the COF films treated with hydrochloric acid (HCl) exhibited stronger Electron Spin Resonance (ESR) signals. It was also discovered that the as-prepared conjugated COF films undergo pH-dependent color changes, rendering them suitable for optical sensors. A colorimetric optical pH sensor based on the conjugated COF films was fabricated and evaluated.

Preparation of green colorimetric pH sensor using Humulus lupulus L. (common hop) biomolecular extract for sweat monitoring.

Novel smart cotton diagnostic assay was developed toward onsite sensing of sweat pH variations for possible medical applications such as drug test and healthcare purposes. Humulus lupulus L. extract was obtained according to previously reported procedure. As reported by high-performance liquid chromatography (HPLC), the extract demonstrated the presence of hop acids, prenylchalcones, and prenylflavanones, which is responsible for the colorimetric changes. The extract was applied to cellulose fibers employing potassium aluminum sulfate as mordant. This was observed by the formation of mordant/xanthohumol nanoparticles onto cotton surface. The absorption spectra and CIE (Commission Internationale de l'Eclairage) Lab screening of the prepared cotton assay showed colorimetric changes in association with hypsochromic shift from 600nm to 433 nm upon exposure to sweat simulant fluid (pH < 7). The biochromic activity of the xanthohumol-finished cotton depends mainly on the halochromic performance of the xanthohumol chromophore to show a colorimetric switch from yellow to white owing to intramolecular charge transfer in the xanthohumol molecule. No substantial defects were detected in gas-permeability and stiffness of the treated fabrics. Satisfactory fastness was approved for the xanthohumol-dyed diagnostic cotton assay.

A Novel Indole Derivative with Superior Photophysical Performance for Fluorescent Probe, pH-Sensing, and Logic Gates.

An indole-related molecules have been considered as the potential fluorescent probes for biological and electrochemical sensing. However, most of the indole probes have been usually used in a single detection mode. Indolium probes that enable accurate detection in complex environments are rarely reported. Here, four novel indole derivatives including the phenyl group substituted with different functional moieties were designed on the basis of the donor-π-acceptor (D-π-A) concept. These derivatives exhibit positive solvatochromism owing to their varied molecular conformations upon contacting to various solvents and the different HOMO-LUMO gaps caused by the difference in electronic push-pull capability of the substituents. Their solid-state fluorescence emissions and multiple chromisms are observed due to the inherent twisted geometries and aggregation modes. In addition, these derivatives show dramatic color and fluorescence responses due to the protonation of the nitrogen and oxygen containing groups, and thus novel colorimetric pH sensors, fluorescent papers and logic gates have been designed.

A wearable colorimetric sweat pH sensor-based smart textile for health state diagnosis.

Sweat pH is an important indicator for diagnosing disease states, such as cystic fibrosis. However, conventional pH sensors are composed of large brittle mechanical parts and need additional instruments to read signals. These pH sensors have limitations for practical wearable applications. In this study, we propose wearable colorimetric sweat pH sensors based on curcumin and thermoplastic-polyurethane (C-TPU) electrospun-fibers to diagnose disease states by sweat pH monitoring. This sensor aids in pH monitoring by changing color in response to chemical structure variation from enol to di-keto form via H-atom separation. Its chemical structure variation changes the visible color due to light absorbance and reflectance changes. Furthermore, it can rapidly and sensitively detect sweat pH due to its superior permeability and wettability. By O2 plasma activation and thermal pressing, this colorimetric pH sensor can be easily attached to various fabric substrates such as swaddling and patient clothing via surface modification and mechanical interlocking of C-TPU. Furthermore, the diagnosable clothing is durable and reusable enough to neutral washing conditions due to the reversible pH colorimetric sensing performance by restoring the enol form of curcumin. This study contributes to the development of smart diagnostic clothing for cystic fibrosis patients who require continuous sweat pH monitoring.

Fast Responsive, Reversible Colorimetric Nanoparticle-Hydrogel Complexes for pH Monitoring.

Hydrogels containing redox-sensitive colorimetric nanoparticles (NPs) have been used to sense ambient pH in many fields owing to their simple and fast visualization capabilities. However, real-time pH monitoring still has limitations due to its poor response rate and irreversibility. Herein, we developed a fast responsive colorimetric hydrogel called ferrocene adsorption colorimetric hydrogel (FACH). Ferrocene, an organometallic compound, plays a vital role as an electron transfer mediator (i.e., redox catalyst) within the hydrogel network. FACH shows fast color change performance with high reactivity and penetrability to ambient pH changes. In detail, FACH shows distinct color change within 2 min under various pH conditions from four to eight, with good reliability. The speed for color change of FACH is approximately six times faster than that of previously developed colorimetric hydrogels, suggesting the fastest hydrogel-based colorimetric pH sensor. Furthermore, FACH shows reversibility and repeatability of the redox process, indicating scalable utility as a sustainable pH monitoring platform.

Ultra-fast pH determination with a new colorimetric pH-sensing hydrogel for biomedical and environmental applications

In this study that demonstrates many firsts in the literature, the acrylate derivative (BTBm) of bromothymol blue, retaining its indicator feature, was synthesized as a monomer via using an organic indicator bromothymol blue (BTB). After the structural characterization of BTBm synthesized by ourselves for the first time, copolymeric hydrogel systems were formed with three different commercial monomers in cationic (3-Acrylamidopropyl-trimethylammonium chloride / APTMACl), anionic (3-Sulfopropyl acrylate potassium salt / SPA) and neutral (acrylamide / AA) structures. Structural characterization of BTBm-based hydrogels, which was successfully synthesized by using a 0.5% methylene bisacrylamide (MBA) crosslinker, was elucidated by FT-IR spectroscopy. The morphological structures and elemental compositions of the hydrogels were examined by SEM and EDX elemental mapping analyses. The color changes of BTBm-based hydrogels, which are intended to be used as colorimetric pH sensors, were examined in different pH environments both with naked-eye and by using a UV–Vis spectrophotometer. It has been observed that APTMACl-BTBm hydrogel can be a more beneficial colorimetric pH sensor material than other BTBm-based hydrogels in terms of its ability to respond to pH changes in a wide color range, short response time, and mechanical strength performance.

The potential use of colorimetric pH sensor from Clitoria ternatea flower for indicating bacterial infection in wound dressing application

Treatment of chronic wounds requires good monitoring for the wound infection. One of the indicators for the infected wound is the change of pH values around the wound area. In this study, the visual pH sensor from Clitoria ternatea (CT) flower was used to indicate the pH value and evaluate the wound status. The CT extract contains anthocyanins which are pigments that appear in different colors depending on the pH values. The color of CT extract at pH values ranges from 2.0 to 12.0 was investigated by the UV–vis spectrophotometer and the naked eye. The antioxidant activity, antibacterial activity, and cytotoxicity of CT extract were evaluated for their biological activities. The concentration of CT extract that can scavenge 50% of DPPH free radicals was 0.80 mg/mL. Besides, the CT extract showed non-toxic to NCTC clone 929 cells. However, the CT extract demonstrated low inhibition of S. aureus, E. coli, P. aeruginosa, and S. epidermidis bacteria. Also, the CT extract was loaded on the paper to observe the color changes in bacterial solutions. The color changed from blue-purple to blue-green color in bacterial solutions indicating the wound infection. Therefore, the natural dyes extracted from the CT flower had the potential to apply for colorimetric pH sensor in wound dressing application.

A wide-range pH indicator based on colorimetric patterns of gold@silver nanorods

The potential of hydrogen (pH) is a basic and critical parameter representing the function of numerous chemicals/biomolecules. Due to the widespread applications of pH in diverse fields, the development of rapid and simple yet reliable probes for the determination of pH has attracted significant interest. In this paper, by using AuNRs, silver ions, and ascorbic acid as colorimetric pH sensor, a multicolorimetric nanosensor is described for pH measurement. The reduction of silver ions by ascorbic acid which is strongly influenced by pH, results in silver nanoshell deposition on the surface of AuNRs. Consequently, the formation of Au@Ag core-shell NRs causes a series of blue shifts in the longitudinal peaks of AuNRs and elegant color changes according to pH values ranging from 2.0 to 12.0. As a result of spectral shifting, distinct color variations were observed from brownish pink/light green color (acidic media) to dark green/blue color (neutral media) and eventually purple/brown color (basic media). Furthermore, a paper-based analytical device (PAD) was introduced for on-site pH screening. The pH-sensitive PAD was simply fabricated by immobilizing AuNRs on the surface of filter paper. Finally, the developed plasmonic probe was validated to be capable of accurate pH monitoring in urine, spoiled meat, drinking water, and seawater samples.

Colorimetric Paper-Based Dual Indicator Label for Real-Time Monitoring of Fish Freshness.

Fish freshness and quality monitoring are of high importance for consumers, retailers and fishing industry. Therefore, developing novel approaches that are simple, fast, non-destructive and inexpensive to monitor fish freshness in real time is of great value. One alternative is using Intelligent or smart packaging to monitor the freshness or conditions of packaged fish. On-package dual indicator label based on paper-based pH sensors was developed for real-time monitoring of the milkfish (Chanos chanos) freshness. The paper-based pH sensor was prepared using bromocresol purple (BCP) and bromothymol blue (BTB) that were immobilized onto a filter paper by dip coating. Herein, the fish degradation could be monitored visually by the dual indicator label, where the BCP changes from yellow to pink, then finally to purple, while the BTP changes from orange to green-yellow, and finally to green-blue to indicate fresh, medium fresh or spoiled product, respectively. The label responds to the pH change caused by the fish degradation and the colour of dual indicator changes to show the fish freshness at room temperature and chiller conditions. This pH change was followed by changes in the other parameters related to fish freshness, such as total volatile basic nitrogen (TVBN), total viable count (TVC), texture and odour. The threshold of fish spoilage at room temperature was observed at 8 h and under chiller conditions at 7 days when the deterioration time point was indicated by the colour changes. Thus, it can be concluded that the dual indicator label can be applied as a simple and low-cost on-package active label for fish freshness monitoring. Increasing consumer concerns about quality and safe food worldwide has boosted the search for a novel approach to food monitoring. In this work, a simple and practical on-package dual indicator label for real-time monitoring of fish freshness was developed. The colorimetric pH sensor was obtained simply by dip-coating of filter paper, yet it enables easy and accurate detection of fish spoilage with the naked eye. Similarly, the dual indicator label changes colour for other freshness parameters, such as TVBN, TVC, texture and odour.

Development of a PCL-PEO double network colorimetric pH sensor using electrospun fibers containing Hibiscus rosa sinensis extract and silver nanoparticles for food monitoring

Major anthocyanin, cyanidin-3-sophoroside (318.1 mg/mL), and other minor copigments were identified in the ethanol extract of Hibiscus rosa sinensis. The extracts can be coelectrospun with polycaprolactone and polyethylene oxide into fiber mats and were sensitive to pH changes from 1 to 13 with a unique color code (ΔE > 5). The pH sensor was used to monitor shrimp quality under isothermal conditions to obtain the respective activation energy (Ea in kJ/mol) of the sensors’ color-change response (20.2), measured pH (20.6), and trimethylamine nitrogen (24.6), indole (27.1), and total microbial counts (30.8). Together with the Pearson correlation coefficient, the results showed high correlations between the sensors’ color change and other quality parameters (p < 0.001). The regression equation developed by conducting the kinetic analysis was also suitable for predicting shrimp quality at refrigeration temperatures (4–10 °C) and can be used as a marker to monitor shrimp quality by visually inspecting the item condition.

Bacterial cellulose-based re-swellable hydrogel: Facile preparation and its potential application as colorimetric sensor of sweat pH and glucose

Direct deposition of the negatively charged polyelectrolyte, carboxymethyl cellulose (CMC), into a bacterial cellulose (BC) matrix was used as a simple route to fabricate a re-swellable and biocompatible cellulose-based hydrogel. As a result of this non-destructive approach, the physical and mechanical property of the original BC were well-preserved within the resulting BC/CMC hydrogel. As a BC/CMC-based colorimetric pH sensor, it exhibited a rapid response with an easy color differentiation between each pH by the naked eye, and wide linear range of pH 4.0–9.0 with good linearity. For the detection of glucose in sweat, the BC/CMC-based colorimetric glucose sensor provided a low limit of detection (25 μM) with a wide linear detection range (0.0–0.5 mM) and high accuracy. These BC/CMC based sensors could potentially be applied as non-invasive semi-quantitative sensors for on-skin health monitoring.

New water-soluble colorimetric pH and metal ione sensor based on graphene quantum dot modified with alizarine red S

A new colorimetric sensor was designed for the screening pH changes in solutions, as well as, detection of some cations. The sensor preparation includes the chemical binding of alizarine red S (ARS) as a sensor of pH and cation to graphene quantum dots (GQD). Loading ARS on GQD led to the formation of water soluble sensor which finally responded to the colorimetric detection of some cations in water. Solubility and stability of the sensor in water indicate that the sensor is an ideal system for the biological and environmental applications. To demonstrate the applicability of the new sensor, the colorimetric responds of sensor were examined for some cations including Fe3+, Co2+, Ca2+, As3+, Cd2+, Hg2+, Pb2+, Sn2+, Al3+, and Cr3+. The colorimetric detections of all the ions were performable individually in a solution. In addition, GQD-ARS as a colorimetric sensor detected Co2+ at pH < 0.6 with limit of quantification 0.08 mM and Fe3+ at 0.6 < pH < 4.0 with limit of quantification 0.03 mM in the mixture of cations.

A Colorimetric pH Sensor Based on Clitoria sp and Brassica sp for Monitoring of Food Spoilage Using Chromametry.

A developed colorimetric pH sensor film based on edible materials for real-time monitoring of food freshness is described. The mixed natural dyes from edible plants Clitoria sp and Brassica sp were extracted and incorporated into ι-carrageenan film as a colorimetric pH sensor film for monitoring food spoilage and its freshness. The color changes of the developed colorimetric sensor film were measured with chromametry and UV-vis spectroscopy, respectively. Experimental results show that colorimetric pH sensor film demonstrated statistically significant differences (p < 0.05) between CIE-L*a*b* coordinates color system indicated that the developed colorimetric sensor film was able to give a gradual change in color over a wide pH range. The color of the colorimetric sensor film also changes discretely and linearly with factors that contribute to food spoilage using shrimp and durian samples. Moreover, the developed colorimetric pH sensor film has the potential to be used as a safe, non-destructive testing and also a flexibly visual method for direct assessment of food freshness indicator during storage.

Tuning the Photophysical Properties of Push-Pull Azaheterocyclic Chromophores by Protonation: A Brief Overview of a French-Spanish-Czech Project.

Conjugated push-pull molecules that incorporate nitrogen heterocycles as electron-withdrawing units are interesting materials because of their luminescence properties. These chromophores can be easily and reversibly protonated at the nitrogen atom of the heterocyclic ring and this can cause dramatic color changes. White and multi-color photoluminescence both in solution and in the solid state can be obtained by an accurate control of the amount of acid. Thus, with a suitable design these compounds have potential applications in the development of colorimetric pH sensors and the fabrication of OLEDs based on only one material. We provide here a brief overview of our collaborative efforts made in this area.

Kinetic investigation into pH-dependent color of anthocyanin and its sensing performance

Dynamic changing process of pH-sensitive anthocyanin solutions was monitored by time-resolved ultraviolet–visible (UV–vis) absorption spectroscopy. Two-dimensional correlation spectroscopy (2DCOS) and principal component analysis (PCA) were further used to investigate the spectral data associated with chemical structure change of anthocyanin. The addition of acid resulted in the narrowing and blueshift of the absorption band of the anthocyanin solution around 528 nm. This may be caused by the formation of flavylium cation from carbinol pseudobase. The addition of alkali led to synchronous changes of the absorption bands around 528 and 607 nm, implying the conversion of flavylium cation to quinonoidal base. 2DCOS data suggested that the peak assigned to flavylium cation during the structural conversion could be located at 520 nm, which was rarely found in the time-resolved absorption spectra. Silk fabric was dyed with anthocyanin and used as a colorimetric pH sensor. The flexible pH sensing fabric showed visual sensitivity to pH changes, with great reusability and stability. The wearable pH fabric sensors fabricated from sustainable and renewable natural biomaterials have potential applications in environmental monitoring, medicine and smart clothing.

Facile synthesis of polyacrylate directed silver nanoparticles for pH sensing through naked eye

Poly(acrylic acid) (PAA) and its salt poly(acrylate) (PA) have been synthesized through modified free radical polymerization in environmentally begin aqueous medium under ultrasound to make the process robust. The synthesized polymer is well-characterized through conventional techniques. Its salt is employed to produce highly stable and stimuli sensitive colloidal silver (Ag-PA sol) without using any additional reducing chemical reagents like sodium borohydride, ascorbic acid, hydrazine, etc or UV/Gamma radiation. A detailed mechanistic path of the polymerization and reduction of Ag+ on polyacrylate chains has been evaluated. Finally Ag-PA sol is used for pH sensing through naked eye to eliminate the need for sophisticated instrument for data collection. The present work focuses mainly the development of a low cost pH sensing system based on colorimetric ‘smart polymer’ having high practical utility. The unique structural and photo-physical features of nano-scaled materials open new opportunities for the applications of colorimetric pH sensor. In the present work, Ag nano-clusters capped by PA− are employed as an effective colorimetric pH sensor for the first time, requiring no further functionalization.

Functionalized Quinazolines and Pyrimidines for Optoelectronic Materials

Background: Quinazolines, the important group of benzodiazines, are widely known in medicinal chemistry due to their broad spectrum of biological activities. Notably, extensive research on the synthesis and application of quinazoline derivatives for electronic devices, luminescent elements, photoelectric conversion elements, and image sensors has been published recently. Objective: This review reports luminescent small molecules and chelate compounds including a quinazoline or pyrimidine ring in their scaffold highlighting their applications related to photo- and electroluminescence. Conclusion: It is clear from the review of the topic that the incorporation of quinazoline and pyrimidine fragments into &amp;#960;-extended conjugated systems is of great value for the creation of novel optoelectronic materials. Polyhalogen derivatives represent the major starting materials for polysubstituted fluorescent quinazolines. Electroluminescent properties of aryl(hetaryl)substituted quinazolines with &amp;#960;-extended conjugated systems proved to be the most important. Incorporation of benzimidazole, carbazole, triphenylene or triphenylamine fragments into quinazoline scaffold allows fabricating materials for organic light-emitting diodes, including white OLEDs and highly efficient red phosphorescent organic light-emitting diodes. Moreover, arylvinylsubstituted quinazolines are of great interest as potential structures for nonlinear optical materials and for colorimetric pH sensors. Iridium complexes based on 2-aryl(thienyl) quinazoline or pyrimidine derivatives represent high-efficiency phosphorescent materials for OLEDs. Pyrimidine push-pull systems are of considerable importance as potential photosensitizers for dye-sensitized solar cells. Pyrimidine derivatives bearing phenylacridine or phenylphenoxazine fragments at the positions 4 and 6 are capable to function as thermally activated delayed fluorescence emitters.

Dual colorimetric and ratiometric fluorescent pH sensors for neutral and alkaline pH ranges

Dual colorimetric and ratiometric fluorescent pH sensors for neutral and alkaline pH ranges