Functional Polymeric Membranes with Antioxidant Properties for the Colorimetric Detection of Amines

Herein, the ability of highly porous colorimetric indicators to sense volatile and biogenic amine vapors in real time is presented. Curcumin-loaded polycaprolactone porous fiber mats are exposed to various concentrations of off-flavor compounds such as the volatile amine trimethylamine, and the biogenic amines cadaverine, putrescine, spermidine, and histamine, in order to investigate their colorimetric response. CIELAB color space analysis demonstrates that the porous fiber mats can detect the amine vapors, showing a distinct color change in the presence of down to 2.1 ppm of trimethylamine and ca. 11.0 ppm of biogenic amines, surpassing the limit of visual perception in just a few seconds. Moreover, the color changes are reversible either spontaneously, in the case of the volatile amines, or in an assisted way, through interactions with an acidic environment, in the case of the biogenic amines, enabling the use of the same indicator several times. Finally, yet importantly, the strong antioxidant activity of the curcumin-loaded fibers is successfully demonstrated through DPPH● and ABTS● radical scavenging assays. Through such a detailed study, we prove that the developed porous mats can be successfully established as a reusable smart system in applications where the rapid detection of alkaline vapors and/or the antioxidant activity are essential, such as food packaging, biomedicine, and environmental protection.

Development of a pH-sensing indicator for shrimp freshness monitoring: Curcumin and anthocyanin-loaded gelatin films

An intelligent pH-sensing indicator containing Roselle (Hibiseus sabdariffa L.) (RS) anthocyanin and curcumin (CR) was developed and characterized as on-package indicator tags to check the freshness of shrimp during the storage at 4°C. FE-SEM and FT-IR analysis showed that RS and CR were successfully immobilized in the gelatin-glycerol film-forming substrate. The addition of various natural dyes increased the thickness and antioxidant action of the colorimetric film. To assess the response to changes in the pH, the colorimetric film was immersed in different buffers. Based on volatile amines secreted by shrimp, a test application of a colorimetric film containing natural dyes at a ratio of CR:RS = 1:4 (v/v) was conducted in shrimp at 4°C. The total volatile basic nitrogen (TVB-N) and the pH of shrimp were monitored during refrigerated storage for 10 days, and the color changes of the indicator were recorded simultaneously. The results indicated that the designed colorimetric film could produce various colors, which are thought to be indicative of the freshness and spoilage of packaged shrimp. Therefore, the target film can be utilized as a promising smart packaging material for monitoring the freshness of shrimp and aquatic products in real time.

Development and characterization of a new active and intelligent packaging system based on soluble soybean polysaccharide-Malva sylvestris extract

This work aimed to develop an active and intelligent film based on soluble soybean polysaccharide (SSPS)/Malva sylvestris extract (MSE) to extend the shelf life of foodstuff and detect indirectly the food spoilage. The influence of MSE content on physico-mechanical properties, biological activity, and pH sensitivity of the SSPS-based films was examined. When the MSE concentration increased from 0 to 6% (w/w), water solubility and water vapor permeability of the films decreased (p ˂ 0.05). Clear antioxidant and antibacterial capacities were observed for SSPS films incorporated with different concentrations of MSE. SSPS/MSE films could sense the pH variation in a pH range of 7 to 8. SSPS/MSE film was used to detect the spoilage of shrimp and showed a pH-sensitive highly distinctive color variation from grey to dark green as the shrimp's quality altered. Overall, SSPS/MSE film can be introduced as a promising candidate for application as active and intelligent packaging.

Development of two devices for high-throughput screening of ethanol-producing microorganisms by real-time CO2 production monitoring

Bioethanol's importance as a renewable energy carrier led to the development of new devices for the high-throughput screening (HTS) of ethanol-producing microorganisms, monitoring ethanol production, and process optimization. This study developed two devices based on measuring CO₂ evolution (an equimolar byproduct of microbial ethanol fermentation) to allow for a fast and robust HTS of ethanol-producing microorganisms for industrial purposes. First, a pH-based system for identifying ethanol producers (Ethanol-HTS) was established in a 96-well plate format where CO₂ emission is captured by a 3D-printed silicone lid and transferred from the fermentation well to a reagent containing bromothymol blue as a pH indicator. Second, a self-made CO₂ flow meter (CFM) was developed as a lab-scale tool for real-time quantification of ethanol production. This CFM contains four chambers to simultaneously apply different fermentation treatments while LCD and serial ports allow fast and easy data transfer. Applying ethanol-HTS with various yeast concentrations and yeast strains displayed different colors, from dark blue to dark and light green, based on the amount of carbonic acid formed. The results of the CFM device revealed a fermentation profile. The curve of CO₂ production flow among six replications showed the same pattern in all batches. The comparison of final ethanol concentrations calculated based on CO₂ flow by the CFM device with the GC analysis showed 3% difference which is not significant. Data validation of both devices demonstrated their applicability for screening novel bioethanol-producer strains, determining carbohydrate fermentation profiles, and monitoring ethanol production in real time.

Halochromic Textiles for Real-Time Sensing of Hazardous Chemicals and Personal Protection

Chemical protective clothing (CPC) has become mandatory when performing various tasks to ensure user protection and prevent chemicals from contacting the skin and causing severe injuries. In addition to protection, there is a need to develop a simple mechanism that can be attached to CPC and be capable of detecting and alerting the user to the presence of harmful chemical agents. In this study, a double-sensor strategy was investigated, using six different pH indicators stamped on cotton and polyester knits to detect acidic and alkaline substances, both liquid and gaseous. Functionalized knits underwent microscopic characterization, air permeability and contact angle evaluation. All samples exhibited hydrophobic behavior (contact angle > 90°) and air permeability values above 2400 L/min/cm²/bar, with the best condition demonstrating a contact angle of 123° and an air permeability of 2412.5 L/min/cm²/bar when the sensor methyl orange and bromocresol purple (MO:BP) was stamped on polyester. The performed tests proved the functionality of the sensors and showed a visible response of all knits when contacting with different chemicals (acids and bases). Polyester functionalized with MO:BP showed the greatest potential, due to its preeminent color change. Herein, the fiber coating process was optimized, enabling the industrial application of the sensors via a stamping method, an alternative to other time- and resource-consuming techniques.

Functional Filaments: Creating and Degrading pH-Indicating PLA Filaments for 3D Printing

With the rapid pace of advancements in additive manufacturing and techniques such as fused filament fabrication (FFF), the feedstocks used in these techniques should advance as well. While available filaments can be used to print highly customizable parts, the creation of the end part is often the only function of a given feedstock. In this study, novel FFF filaments with inherent environmental sensing functionalities were created by melt-blending poly(lactic acid) (PLA), poly(ethylene glycol) (PEG), and pH indicator powders (bromothymol blue, phenolphthalein, and thymol blue). The new PLA-PEG-indicator filaments were universally more crystalline than the PLA-only filaments (33-41% vs. 19% crystallinity), but changes in thermal stability and mechanical characteristics depended upon the indicator used; filaments containing bromothymol blue and thymol blue were more thermally stable, had higher tensile strength, and were less ductile than PLA-only filaments, while filaments containing phenolphthalein were less thermally stable, had lower tensile strength, and were more ductile. When the indicator-filled filaments were exposed to acidic, neutral, and basic solutions, all filaments functioned as effective pH sensors, though the bromothymol blue-containing filament was only successful as a base indicator. The biodegradability of the new filaments was evaluated by characterizing filament samples after aging in soil and soil slurry mixtures; the amount of physical deterioration and changes in filament crystallinity suggested that the bromothymol blue filament degraded faster than PLA-only filaments, while the phenolphthalein and thymol blue filaments saw decreases in degradation rates.

Modified and Optimized Glass Electrode for pH Measurements in Hydrated Ethanol Fuel

One of the quality control parameters of ethanol fuel is pH, established by the Brazilian standard ABNT NBR 10891, whose scope is specific for hydrated ethanol fuel, and by the American standard ASTM D 6423, which focuses on anhydrous ethanol fuel. This study presented a modified and optimized structure using a single solvent, both for the glass electrode and the external reference electrode, to minimize the presence of the liquid junction potential for measuring the pH of hydrated ethanol fuel. The Box-Behnken design enabled us to determine the optimal condition expected for the new measurement system, which was compared with the systems proposed by the standard references and the turning range of acid-base indicators using parametric and nonparametric tests. The results revealed that the pH values obtained by the different systems are statistically different, and that only the values obtained by this proposal are suitable for the pH range found by the indicators. The optimized electrode presented an adequate response sensitivity to the Nernst equation, having an operational behavior adequate for the modified and optimized glass electrode for pH measurements in hydrated ethanol fuel.

Colorimetric Indicators Based on Anthocyanin Polymer Composites: A Review

This review explores the colorimetric indicators based on anthocyanin polymer composites fabricated in the last decade, in order to provide a comprehensive overview of their morphological and compositional characteristics and their efficacy in their various application fields. Notably, the structural properties of the developed materials and the effect on their performance will be thoroughly and critically discussed in order to highlight their important role. Finally, yet importantly, the current challenges and the future perspectives of the use of anthocyanins as components of colorimetric indicator platforms will be highlighted, in order to stimulate the exploration of new anthocyanin sources and the in-depth investigation of all the possibilities that they can offer. This can pave the way for the development of high-end materials and the expansion of their use to new application fields.

Preparation and Testing of Food Freshness Indicators: an Application-Oriented Learning Module Integrating Basic Concepts of Microbiology and Chemistry Laboratory

We describe a novel 4-day food microbiology laboratory learning module for a first-year, introductory undergraduate course. In the module, the students test the suitability of four different pH indicator dyes as freshness indicators for dairy products. The concepts of serial dilutions, microbial growth, microbial metabolism, pH as well as pKa, and basic microbial laboratory practices are a part of the designed activity. It is a relatively inexpensive module and can be executed with little infrastructural support. It can be delivered as a stand-alone structured inquiry. The associated variables and applications indicate that the activity can perhaps be developed into a more elaborate course-based undergraduate research experience, or CURE.

Conformal Coating of Orthopedic Plates with X-ray Scintillators and pH Indicators for X-ray Excited Luminescence Chemical Imaging through Tissue

We describe a pH-indicating material that can be directly implanted or coated on orthopedic implant surfaces to provide high-spatial-resolution pH mapping through tissue by X-ray excited luminescence chemical imaging (XELCI). This is especially useful for detecting local pH changes during treatment of implant-associated infections. The material has two layers: an X-ray scintillator layer with Gd₂O₂S:Eu in epoxy, which emits 620 and 700 nm light when irradiated with X-rays, and a pH indicator dye layer, which absorbs some of the 620 nm light in a pH-dependent fashion. To acquire each pixel in the image, a focused X-ray beam irradiates a small region of scintillators and the ratio of 620 to 700 nm light is acquired through the tissue. Scanning the X-ray beam across the implant surface generates high-spatial-resolution chemical measurements. Two associated challenges are (1) to make robust sensors that can be implanted in tissue to measure local chemical concentrations specifically for metal orthopedic implants and (2) to conformally coat the implant surface with scintillators and pH indicator dyes in order to make measurements over a large area. Previously, we have physically pressed or glued a pH-sensitive hydrogel sensor onto the surface of an implant, but this is impractical for imaging over large irregular device areas such as an orthopedic plate with holes and edges. Herein, we describe a chemically sensitive and biocompatible XELCI sensor material that can conformally coat the implant surface. A two-part commercial-grade epoxy resin was mixed with Gd₂O₂S:Eu and adhered to the titanium surface. Sugar and salt particles were added to the surface of the epoxy as it cured to create a roughened surface and increase the surface area. On this roughened surface, a secondary layer of diacrylated polyethylene glycol (PEG) hydrogel, containing a pH sensitive dye, was polymerized. This combination of epoxy-PEG layers was found to adhere well to the metal implant unlike other previously tested polymer surfaces, which delaminated when exposed to water or humidity. The focused X-ray beam enabled 0.5 mm spatial resolution through 1 cm-thick tissue. The pH sensor-coated orthopedic plate was imaged with XELCI, through tissue, with different pH levels to acquire a calibration curve. The plates were also imaged through tissue, with a low pH region on one section due to growth of a Staphylococcus aureus biofilm. A pH sensor-coated stainless-steel rod with two distinct pH regions was inserted in a rabbit tibia specimen, and the pH was imaged through both bone and soft tissue. These studies demonstrate the use of pH sensor-coated orthopedic plates and rods for mapping the local pH through tissue during biofilm formation by XELCI.

Generalized indicator-based determination of solution pH

pH indicators can be used both fast responsive as well as long-term stable sensors. They have been extensively used for monitoring pH changes in fast kinetic reactions as well as slowly changing pH in oceanic waters. If the pH range that needs to be covered is narrow it is possible to use only one indicator of appropriate protonation constant; otherwise, mixtures of two or more indicators are used for monitoring pH values covering a broad range of pH. In this paper we presented a new methodology for determining pH of solutions using mixtures of pH indicators. The pH calculation is based on the strict application of the basic laws of mass action and mass conservation. The proposed method was evaluated by the successful determination of the pH values of solutions containing three indicators (neutral red, phenol red (two different protonation constants), and methyl orange) covering a wide range of pH values from 0.5 to 9. The method was also applied for rapid monitoring of pH changes in stopped-flow measurements, investigating the reactions of CO₂ in aqueous amine solutions.

pH-responsive allochroic nanoparticles for the multicolor detection of breast cancer biomarkers

Estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2) are the three crucial biomarkers for the clinical diagnosis of breast cancer. Sensitive and precise detection of ER, PR, and HER2 is of great significance for the diagnosis of breast cancer. Herein, through a simple solvent-induced self-assembly process, the self-carried allochroic nanoparticles were prepared by using some hydrophobic pH indicator molecules for allochroic NPs-linked immunosorbent assay (named ALISA) of ER, PR, and HER2, respectively. Meanwhile, the introduction of bovine serum albumin (BSA) and antibody (Ab) enhanced the dispersity of the self-assembled nanoparticles as signal tags. Since the ultra-high loading and high-efficiency release of pH indicators, the ALISA exhibitssatisfactory selectivity and sensitivity, which demonstrated the great potential in the early diagnosis and postoperative monitoring of breast cancers. Furthermore, the smartphone was introduced to combine with the ALISA for point-of-care testing, indicating the high feasibility in practical applications.

Illuminating Brain Activities with Fluorescent Protein-Based Biosensors

Fluorescent protein-based biosensors are indispensable molecular tools for life science research. The invention and development of high-fidelity biosensors for a particular molecule or molecular event often catalyze important scientific breakthroughs. Understanding the structural and functional organization of brain activities remain a subject for which optical sensors are in desperate need and of growing interest. Here, we review genetically encoded fluorescent sensors for imaging neuronal activities with a focus on the design principles and optimizations of various sensors. New bioluminescent sensors useful for deep-tissue imaging are also discussed. By highlighting the protein engineering efforts and experimental applications of these sensors, we can consequently analyze factors influencing their performance. Finally, we remark on how future developments can fill technological gaps and lead to new discoveries.

Synthesis of pH indicators for Cerenkov imaging by electrophilic substitution of bromine by fluorine in an aromatic system

Direct electrophilic fluorination using molecular fluorine gas is used in organic synthesis to create novel fluorine-containing compounds with potential beneficial activity that could not be obtained by nucleophilic substitution. In this paper, we report a novel electrophilic substitution of bromine by fluorine in an aromatic system. The mechanism of this type of fluorination was explored using the reaction between bromothymolsulfonphthalein (Bromothymol Blue) and dilute fluorine gas under acidic conditions. Substitution occurs in the bromine atoms located in the ortho-position relative to the hydroxyl group. A similar electrophilic fluorination of thymolsulfonphthalein (Thymol Blue) leads to a substitution of hydrogen atoms in the same position (ortho to hydroxyl). NMR spectroscopy was used to confirm the fluorination sites. NMR spectra of thymolsulfonphthalein and its derivatives under basic conditions can be explained by considering the absence of resonance between the two phenolic rings. Both dibromothymol blue and fluorobromothymol blue revealed intermolecular attenuate Cerenkov radiation selectively near their maximum absorbance in a pH dependent manner.

Mass Spectra of Sulfonephthalein pH Indicator Dyes and Their Impurities

Sulfonephthalein pH indicator dyes have historically been noted for impurities originating from the synthesis process, which lead to batch to batch differences in pH measurements. Uncertainties up to 0.1 pH units have been attributed to impurities in these reagents. In an effort to understand the extent of impurities in these dyes and the potential impact on the expanded uncertainty of spectrophotometric pH measurements, we have examined seven commercially available sulfonephthalein pH indicators using liquid chromatography-mass spectrometry (LC-MS): bromocresol green (BCG), bromocresol purple (BCP), bromothymol blue (BTB), cresol red (CR), meta-cresol purple (MCP), phenol red (PR), and thymol blue (TB). Peaks eluting from the LC were subjected to fragmentation by mass spectrometry. The resulting dataset will aid in efforts to improve the synthesis or purification of these dyes.

X-ray excited luminescent chemical imaging (XELCI) for non-invasive imaging of implant infections

X-ray excited luminescent chemical imaging (XELCI) uses a combination of X-ray excitation to provide high resolution and optical detection to provide chemical sensing. A key application is to detect and study implant-associated infection. The implant is coated with a layer of X-ray scintillators which generate visible near infrared light when irradiated with an X-ray beam. This light first passes through a pH indicator dye-loaded film placed over the scintillator film in order to modulate the luminescence spectrum according to pH. The light then passes through tissue is collected and the spectral ratio measured to determine pH. A focused X-ray beam irradiates a point in the scintillator film, and a pH image is formed point-by-point by scanning the beam across the sample. The sensor and scanning system are described along with preliminary results showing images in rabbit models.

A smartphone-based colorimetric reader coupled with a remote server for rapid on-site catechols analysis

The search of a practical method to analyze cis-diol-containing compounds outside laboratory settings remains a substantial scientific challenge. Herein, a smartphone-based colorimetric reader was coupled with a remote server for rapid on-site analysis of catechols. A smallest-scale 2×2 colorimetric sensor array composed of pH indicators and phenylboronic acid was configured. The array was able to distinguish 13 catechols at 6 serial concentrations, through simultaneous treatment via principal component analysis, hierarchical cluster analysis, and linear discriminant analysis. After both the discriminatory power of the array and the prediction ability of the partial least squares quantitative models were proved to be predominant, the smartphone was coupled to the remote server. All the ΔRGB data were uploaded to the remote server wherein linear discriminant analysis and partial least squares processing modules were established to provide qualitative discrimination and quantitative calculation, respectively, of the analytes in real time. The applicability of this novel method to a real-life scenario was confirmed by the on-site analysis of various catechols from a water sample of the Yangtze River; the feedback result in the smartphone showed the method was able to identify the catechols with 100% accuracy and predict the concentrations to within 0.706-2.240 standard deviation.