Halochromic Properties Research Articles

Cellulose-based halochromic sensor for real-time surveillance of spoilage of packed fish

The development of a biopolymer-based color-changing visual sensor for monitoring the spoilage of fishery products is described in this study. At first, a cellulose-based composite film containing mixed-indicator dyes was fabricated. The film was characterized using UV–visible and FT-IR spectroscopy, scanning electron microscopy, thermogravimetric analysis, lab, and RGB measurements. The halochromic properties of the film were evaluated through ammonia test and colorimetry. The developed film sensor is tested for monitoring the spoilage of packed seer fish fillets at room temperature for up to 48 h and at 4 °C for up to 10 days. The sensor displayed distinct color changes correlating with the spoilage of the packed fish. Analysis of total volatile basic nitrogen and pH levels of the samples revealed a correlation between the halochromism of the sensor and the spoilage of the fish. The color-changing pattern of the sensor was consistent with the results of microbial analysis. The present study reveals a promising material combination for fabricating simple visual sensors from biomaterials, which can be used to monitor the real-time deterioration of packaged fishery products.

Halochromic Polyaniline as smart detector packaging for fresh food: a review

Abstract Early spoilage is a challenge that is often encountered in fresh food. One prevention that can be done is by using packaging technology. There has been an increasing interest in using smart detector packaging for fresh products. Smart detector has high sensitivity and is responsive to visual changes when exposed to pH, temperature, water activity, or composition. The advantage of using a smart detector is that it can detect and give information on which products are safe to consume and which are not. Overall, the concept of a smart detector is not much different from the function of smart packaging. Smart packaging that is sensitive to pH is called halochromic properties. The mechanism of halochromic depends on the compound or the indicator used. Polyaniline (PANI) is one of the candidates for halochromic materials, which have high stability and sensitivity to pH changes. PANI also has a high sensitivity to volatile compounds such as ammonia, indicated by a change in colour from green to blue. This review will overview the use of PANI as a smart detector halochromic packaging and its potential application in the food industry. It has been found that the application of PANI has opportunities to advance the development of modern smart packaging and better food quality monitoring systems.

Thiopyran-Fused Polycyclic Aromatic Compounds Synthesized via Pt(II)-Catalyzed One-Pot Ring-Expansion and 6-endo Cyclization Reactions.

A series of thiopyran-fused polycyclic aromatic hydrocarbons (PAHs) have been straightforwardly synthesized from 2,5-di(1-en-3-ynyl)thiophene-containing precursors via one-pot ring-expansion and 6-endo cyclization reactions. The reaction monitoring and the density function theoretical calculation suggest that the ring-expansion reaction occurs prior to 6-endo cyclization. Moreover, the absorption profiles of the thiopyran-fused PAHs suggest that the π-conjugation extension on the side of the cyclopentadiene ring in the cyclopenta[b]thiochromene core is predominant in prolonging the effective conjugation length, while the effect from extension on the other side is negligible. Furthermore, all of the thiopyran-fused PAHs exhibit halochromic properties. Upon the addition of trifluoromethanesulfonic acid, fluorescence "off-on" switches can be found for these thiopyran-fused PAHs. Therefore, this work not only provides a new synthetic approach for one-pot ring-expansion and 6-endo cyclization reactions but also expands the diversity of thiopyran-fused PAHs.

Chromic properties of dibenzo[j,l]fluoranthenes exhibiting different resonance contributions.

Chromic molecules change colour in response to external stimuli and are utilized in applications such as food additive detection, light dimmers, and biological probes. One of the common design strategies for organic chromic molecules is based on changes in the π-conjugation. We have hypothesized that non-alternant polyaromatic hydrocarbon (PAH) skeletons can be used as backbones for chromic molecules. Herein, we synthesized hydroxy-substituted dibenzo[j,l]fluoranthenes, a class of non-alternant PAHs, as novel chromic compounds and evaluated their halochromic properties by UV-vis and fluorescence spectroscopy. Under basic conditions, the 1-hydroxy derivatives show a hyperchromic shift, whereas the 9-hydroxy derivatives show a bathochromic shift and fluorescence although the skeleton of the chromophore is the same. Density functional theory calculations indicated that the different chromic properties are attributed to the differences in their resonance structures.

Oxochlorin frameworks confining a β-hydroxyketone moiety.

Nominally, meso-hydroxyoxochlorins, like known 5-hydroxy-7-oxo-octaethylchlorin (9), its nickel complex [5-hydroxy-7-oxo-octaethylchlorinato]nickel(II) (9Ni), or the novel 5-hydroxy-7,17-dioxo-octaethylbacteriochlorin (10), incorporate an acetylacetonate (acac)-moiety in the enol form in their chromophore structures. X-Ray diffraction studies of the compounds show the presence of a strong H-bond between the enol and flanking β-ketone. Like acac, the functionality can be deprotonated. However, unlike regular acac-like moieties, we did not find any indication that this functionality is competent in chelating any of the 3d or 4d transition metal ions tested. Evidently, the conjugation that contributes to the stability of acac as a ligand cannot be expressed in the meso-hydroxyoxochlorins since it would perturb the aromaticity of the porphyrinic chromophores; in other words, the metal binding energies do not offset the loss in aromaticity. The halochromic properties of the molecules provide some more insight into the location of the protonation/deprotonation sites. The interpretation of the findings is supported by computations.

PVA Nanofibers as an Insoluble pH Sensor.

Turmeric has been widely studied as a color indicator for pH variations due to its halochromic properties. It has been tested in solution or included in some polymeric matrices. Some studies have demonstrated that its change in color is due to the tautomeric species of curcumin, and this property can be observed even if turmeric is assimilated in a film or nanofiber. Chitosan/polyethylene oxide (PEO) polymers have been tested in previous studies. Polyvinyl alcohol (PVA) nanofibers are used as potential carriers of drugs once they are insolubilized. The aim of this work is to cross-link PVA with citric acid (CA) to insolubilize the nanofibers and determine the effect on turmeric's halochromic properties. The nanofibers were treated with a sodium hydroxide (NaOH) solution, and a chromatic study was undertaken to determine color change. The change in color was assessed by eye (subjective) and by spectroscopy (objective). The nanofibers were characterized, in addition to the colorimetric study, by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) as well. The results demonstrate how thermal treatment induces cross-linking between the nanofibers, allowing them to keep their shape once the NaOH solution is applied to them. The opposite effect (solubilization) can be observed for non-cross-linked (NCL) samples. Although the final color varied, the cross-linked (CL) nanofibers' halochromic behavior was maintained. It was demonstrated that during cross-linking, ester groups are formed from the free carboxyl group in the cross-linked CA and the ketones present in the curcumin under acid conditions. So, CA acts as an acid catalyst to bond turmeric to the cross-linked PVA nanofibers.

Chromogenesis of a black fluoran dye: Theoretical and spectral studies

Fluoran dyes are widely used as chromogenic reagents in thermochromic and photochromic materials due to their intrinsic halochromic properties. However, theoretical and spectral reports on fluoran-based chromic materials are still rare. This paper investigates the chromogenic mechanism of a black fluoran dye 2-(2,4-Dimethylphenylamino)-3-methyl-6-diethylaminofluoran (B-15) through spectral experiments and theoretical calculations. Computational chemistry was employed to optimize the geometry of B-15 and validate a chromogenic mechanism. The proposed chromogenic structure of B-15 was further verified by infrared spectra. The chromogenic performances of B-15 by four kinds of color developers were quantitatively analyzed using spectrometric methods. The study comprehensively examines the chromogenic mechanism of fluoran dyes and provides a valuable reference for practical chromogenic systems based on fluoran dyes.

Identification of natural dyes with halochromic properties and their potential use as food spoilage indicators

PurposeThe purpose of this research is to identify and investigate some natural dyes with halochromic properties for potential use as food spoilage indicators to reduce waste and curve the negative effects of food borne diseases.Design/methodology/approachExactly 10 potential dye-yielding plants were selected based on their colour (mostly purple, red, maroon and pink). Solvent extraction was used to extract the dyes and pH differential method was used to determine the concentrations of anthocyanin in the extracted dyes. Different concentrations of hydrochloric acid and sodium hydroxide (0.1 M, 1 M and 2 M) in drops and in excess as acidic and basic solution, respectively, were used to test the halochromicity of the extracted dyes. Methyl red (a synthetic dye) was used as a reference standard/control. The pH of the dyes was recorded before and after addition of both NaOH and HCl solutions.FindingsFive out of the 10 dyes extracted (labelled as dye A–E for Ti plant (green Cordyline fruticosa), coleus (Coleus blumei), paper flower (Bougainvillea glabra), painted nettle (Palisandra coleus) and purple heart (Setcresea purpurea), respectively, were found to be halochromic (even at low doses) by changing its colour when exposed to both acidic and basic solutions. While other dyes labelled F–J for red acalypha (Acalypha wilkesiana), golden shower (Cassia fistula), golden dew drop (Duranta repens), wild sage (Lantana camara var Aculeata) and pink oleander (Apocynaceae Nerium oleander), respectively, were either completely insensitive to the solutions in drops, slightly sensitive at high doses or the colour change is insignificant. Although some dyes were found to be more sensitive than others but in most cases, the colour changes in halochromic dyes were more stable in acidic conditions than in basic making it more sensitive to the basic than the acidic solution with the exception of dye A and E (to some extent) which was sensitive to both acidic and basic solution. The anthocyanin contents of dye A–J were found to be between the range of 2.28–10.35 mg/l with dye E having the lowest and dye J with the highest anthocyanin concentration, respectively. The initial pH of all the dyes falls within the range of 4.8–7.3 with most found within the acidic range.Originality/valueHalochromic dye research studies are still at the infancy stage in developing world despite the vast available and abundant potential natural halochromic dye-yielding plants. The study explored this area of research and gives an opportunity for the development of smart packaging for pH-sensitive foods using natural dyes as an alternative to conventional synthetic dyes to reduce cost and also curve the negative effect of synthetic dyes as well as food borne diseases.

Synthesis of monoazo disperse dyes derived from N-(1-phthalimidyl)-naphthalimides and their dyeing properties on polyester fabrics

PurposeThis paper aims to synthesize a novel series of monoazo disperse dyes based on N-(1-phthalimidyl)-naphthalimides by coupling with substitute anilines, naphthylamines and naphthol derivatives.Design/methodology/approachThe purification of the intermediates and the dyes was carried out by recrystallization. The structures of the synthesized intermediates and the dyes were elucidated by spectroscopic techniques. The absorption maxima, molar extinction coefficient and halochromic properties of the dyes were determined spectrophotometrically using solvents of different polarity.FindingsThe dyes were applied on polyester using a high-temperature high-pressure dyeing machine, and the dyeing performance parameters such as colour build-up on fabrics, wash fastness, perspiration fastness and light fastness were evaluated. The colour build-up was found to be very good and the wash fastness (4–5) and perspiration fastness (4–5) were excellent, whereas the light fastness was found to vary from moderate to very good (3–6).Research limitations/implicationsIt is not possible to investigate the structure of the synthesized dyes by nuclear magnetic resonance spectroscopic analysis due to the low solubility of dyes in deuterated solvents.Originality/valueA novel method for the synthesis of a new category of monoazo disperse dyes based on N-(1-phthalimidyl)-naphthalimides was developed. These dyestuffs could be used in textile printing of polyester fabrics.

Halochromic properties of new nitrophenol-based azochromophores

A number of new azo dyes were synthesized using azo-coupling of nitrohydroxyphenols (2-nitroresorcinol and nitrophloroglicinol) with ortho-aminophenol and its nitro- and sulfonic-substituted derivatives. The dye molecules contain multiple hydroxy groups which can be easily deprotonated, and their ionization affects the conjugated π-electron system of the dye molecule leading to the significant spectral changes. Spectrophotometric titration of the obtained dyes with an acid and an alkali revealed their halochromic behaviour, i.e. an ability to alter color upon pH change. All the dyes absorb light with wavelength shorter 500 nm in acidic media, and with wavelength longer 500 nm in alkaline media. Variation of substituents in the dye molecules allows to tune the pH range of the color change. The obtained dye series covers the pH range from 0 to 10. DFT calculations of the dye molecules revealed their structures and preferential isomeric forms. TD-DFT calculations of the excited states allowed to clarify the acid-base equilibrium based on the spectrophotometric titration, and to determine the nature of the observed absorption bands. The dyes were adsorbed on polyamide fabrics, and the dyed fabrics reversibly changed their color upon treating with acids and alkalis. These fabrics with applied dyes showed good resistance to aggressive environments and mechanical influences. Results of tests of the dyed fabrics showed that the obtained materials can be used as flexible textile pH sensors.

Halochromic Silk Fabric as a Reversible pH-Sensor Based on a Novel 2-Aminoimidazole Azo Dye

Textiles are important components for the development of lightweight and flexible displays useful in smart materials. In particular, halochromic textiles are fibrous materials with a color-changing ability triggered by pH variations mainly based on pH-sensitive dye molecules. Recently, a novel class of 2-aminoimidazole azo dyes was developed with distinct substituent patterns. In this work, silk fabric was functionalized through exhaustion for the first time with one of these dyes (AzoIz.Pip). The halochromic properties of the dye were assessed in an aqueous solution and after silk functionalization. The solutions and the fabrics were thoroughly analyzed by ultraviolet-visible (UV-vis) spectra, color strength (K/S), color difference (∆E), CIE L*a*b* coordinates, and the ultraviolet protection factor (UPF). The dyeing process was optimized, and the halochromic performance (and reversibility) was assessed in universal Britton-Robinson buffers (ranging from pH 3 to 12) and artificial body fluids (acid and alkaline perspiration, and wound exudate). AzoIz.Pip showed vibrant colors and attractive halochromic properties with a hypsochromic shift from blue (557 nm) to magenta (536 nm) in aqueous buffered solutions. Similarly, the functionalized silk showed a shift in wavelength of the maximum K/S value from 590 nm to 560 nm when pH increases. The silk fabric showed a high affinity to AzoIz.Pip, and promoted additional color stabilization of the dye, avoiding color loss as observed when the dye is in solution at alkaline pH after 24 h. The color reversibility was effective up to the fourth cycle and the fastness tests denoted suitable results, except washing fastness. The cytotoxicity of the silk fabric extracts was assessed, depicting reduced viability of HaCaT cells to <70% only when the dye concentration in the fabric is higher or equal to 64 μg·mL-1. Nevertheless, lower concentrations were also very effective for the halochromic performance in silk. These materials can thus be a helpful tool for developing sensors in several sectors such as biomedicine, packaging, filtration, agriculture, protective apparel, sports, camouflage, architecture, and design.

Synthesis, Characterization, and Antiproliferative Properties of New Bio-Inspired Xanthylium Derivatives.

Xanthylium derivatives are curcumin analogs showing photochromic properties. Similarly, to anthocyanins, they follow the same multistate network of chemical species that are reversibly interconverted by external stimuli. In the present work, two new asymmetric monocarbonyl analogues of curcumin, 4-(4-hydroxy-3-metoxybenzylidene)-1,2,3,4-tetrahydroxanthylium chloride (compound 3) and 4-(4-hydroxybenzylidene)-6-methoxy-1,2,3,4-tetrahydroxanthylium chloride (compound 4) were synthesized, and their photochromic and biological properties were investigated. The UV-Vis spectroscopy and the direct and reverse pH-jumps studies confirmed the halochromic properties and the existence of different molecular species. A network of chemical reactions of these species was proposed. Furthermore, the antiproliferative properties of both compounds were evaluated using P19 murine embryocarcinoma cells and compared with each other. The results demonstrate that both new xanthylium derivatives modify the progression through the cell cycle of P19 cells, which translates into a significant antiproliferative effect. The effect of the methoxy group position is discussed and several checkpoint proteins are advanced as putative targets.

Red-shifted and pH-responsive imidazole-based azo dyes with potent antimicrobial activity.

A novel route is described to obtain 2-aminoimidazole azo dyes with a unique substituent pattern in the heteroaryl unit that provides halochromic properties, exhibiting vibrant colours that change from magenta to deep blue. Potent antimicrobial properties against infectious yeasts were demonstrated. No cytotoxicity was detected for concentrations lower than 16 μg mL-1.

Conjugation Extension and Halochromic Behaviors of S-Fused Polycyclic Aromatic Hydrocarbons Bearing Cyclopenta[b]thiopyran Moieties.

Three S-fused polycyclic aromatic hydrocarbons (PAHs) bearing cyclopenta[b]thiopyran moieties have been designed and successfully synthesized. With the conjugation extension, the absorption onset of the longest PAH reaches 1110 nm. All the three S-fused PAHs exhibit significant halochromic properties in both solution and solid states. Upon protonation, the proton is incorporated on the cyclopentadiene ring while the positive charge is localized on the thiopyrylium ring. Moreover, no significant difference can be found for the two shorter PAHs upon the protonation by different organic acids, such as trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH), while the longest PAH can be only mono-protonated by TFA but di-protonated by stronger TfOH. Furthermore, after protonation, the non-emissive S-fused PAHs exhibit strong fluorescence and can be regenerated by simply neutralization with triethylamine. The enhanced emission of mono-protonated products stem from S2 →S0 transitions, which disobey the Kasha's rule.

Fabrication of halochromic polylactic acid filament for 3D Printing

With increasing concerns regarding plastic waste and pollution, researchers have been looking to develop advanced materials from biodegradable plastics. This study features the additive manufacturing of halochromic polylactic acid (PLA) filaments and the testing of halochromic properties of the specimens. PLA/polyethylene glycol (PEG)/bromocresol purple (BCP) compound was extruded and wound to produce a 3D printing filament. The halochromic responsiveness and colour reversibility were tested by exposing the samples to liquid and vaporized pH solutions for 30 min and then placing them back into ambient conditions for 48 h. The compatibility of PLA, PEG and BCP were found to be good, and the halochromic compound formed was homogenous. Dumbbell-shaped and rectangular specimens were successfully fabricated from the compound through 3D printing. Overall, the 3D printed halochromic PLA specimens show promising results and have proven their functionality as pH sensors which offers wide applications in many industries such as medical, environmental and packaging.

A Multi-Stimuli responsive organic luminogen with aggregation induced emission for the selective detection of Zn2+ ions in solution and solid state

Organic luminogens capable of excited state intramolecular electron transfer (ESIPT) have drawn prodigious attraction due to their enhanced emission in solid-state. A novel Schiff base molecule, 3,5-dibromo-2-hydroxybenzylidenenicotinohydrazide (DHN) exhibited stimuli-induced reversible fluorescence switching and selective binding propensity towards zinc in aqueous media, and the concentration-dependent studies showed a limit of detection of 9.135 nM. DHN was found to be weakly fluorescent in polar solvents with a quantum yield ranging between 0.0365 and 0.0789 but exhibited a very strong fluorescence in solid state (λexc = 370 nm) due to aggregation induced emission (AIE). The ESIPT fluorophore renders significant reversible halochromic properties in solution and solid-state. In addition, utilizing the solid-state fluorescence, we have prepared PVA-probe green-emitting composite films, which can be used for the on-site detection of Zn2+ in aqueous media. The practical applicability of DHN was proven by detecting Zn2+ in real drug samples. Finally, the ESIPT fluorophore was used for fluorescent imaging of intracellular zinc in the cells acquired from the nervous tissue of rats (N2a). The investigations carried out highlight the versatility of ESIPT Schiff bases used for the development of multi-responsive fluorescent materials for selective sensing of metal ions in both solid and solution states.

Shikonin: Extraction, properties and applications in active and intelligent packaging

AbstractShikonin is a naphthoquinone compound, generally extracted from the roots of the Oriental medicinal herb Lithospermum erythrorhizon or the Indian medicinal herb Arnebia euchroma. It has a rich history of being utilized as a pharmacological compound to treat various diseases such as inflammation, wounds and microbial infections. Besides, it also possesses anticancer, antiviral and antioxidant properties, which interest current medical researchers. Food packaging researchers have recently utilized shikonin's antimicrobial and antioxidant properties for active food packaging. Its halochromic properties and ability to change colour with pH variation have attracted tremendous attention for intelligent packaging applications. This review article covers the important extraction processes for shikonin. Further, the important properties of these functional compounds have been discussed. Finally, the advancements in applying shikonin as a multifunctional material in biopolymer‐based films for smart packaging applications have been reviewed for the first time.

Bio-Based pH Indicator Films for Intelligent Food Packaging Applications.

The widespread concerns about the environmental problems caused by conventional plastic food packaging and food waste led to a growing effort to develop active and intelligent systems produced from renewable biodegradable polymers for food packaging applications. Among intelligent systems, the most widely used are pH indicators, which are generally based on a pH-sensitive dye incorporated into a solid support. The objective of this study was to develop new intelligent systems based on renewable biodegradable polymers and a new bio-inspired pH-sensitive dye. The structure of the dye was elucidated through FT-IR and 1D and 2D NMR spectroscopic analyses. UV-VIS measurements of the dye solutions at various pH values proved their halochromic properties. Their toxicity was evaluated through theoretical calculations, and no toxicity risks were found. The new anthocyanidin was used for the development of biodegradable intelligent systems based on chitosan blends. The obtained polymeric films were characterized through UV-VIS and FT-IR spectroscopy. Their thermal properties were assessed through a thermogravimetric analysis, which showed a better stability of chitosan–PVA–dye and chitosan–starch–dye films compared to those of chitosan–cellulose–dye films and the dye itself. The films’ sensitivity to pH variations was evaluated through immersion in buffer solutions with pH values ranging from 2 to 12, and visible color changes were observed.

Halochromic Behavior and Anticancer Effect of New Synthetic Anthocyanidins Complexed with β-Cyclodextrin Derivatives.

Anthocyanidins, the aglycons of anthocyanins, are known, beyond their function in plants, also as compounds with a wide range of biological and pharmacological activities, including cytostatic effect against various cancer cells. The nature and position of the substituents in the flavylium cation is essential for such biological properties, as well as the equilibrium between the multistate of the different chemical species that are generated by the flavylium cation, including quinoidal base, hemiketal, and cis- and trans-chalcones. In this work, eight new flavylium derivatives were synthesized, characterized for confirmation of the structure by FT-IR and 2D-NMR, and investigated in vitro as possible cytostatic compounds against HCT116 and HepG2 cancer cells. The most active two compounds were explored for their halochromic properties that can influence the biological activity and subjected to molecular encapsulation in β-cyclodextrin derivatives in order to increase their solubility in water and bioavailability. The anticancer effect was influenced by the position (6-, 7-, or 8-) of the methoxy group in the β-ring of the methoxy-4′-hydroxy-3′-methoxyflavylium cation, while the study of the halochromic properties revealed the important role played by the chalcone species of the pH-dependent multistate in both the uncomplexed and inclusion complex forms of these anthocyanidins.

Sol-Gel Assisted Immobilization of Alizarin Red S on Polyester Fabrics for Developing Stimuli-Responsive Wearable Sensors

In the field of stimuli-responsive materials, introducing a pH-sensitive dyestuff onto textile fabrics is a promising approach for the development of wearable sensors. In this paper, the alizarin red S dyestuff bonded with a sol-gel precursor, namely trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane, was used to functionalize polyethylene terephthalate fabrics, a semi-crystalline thermoplastic polyester largely used in the healthcare sector mainly due to its advantages, including mechanical strength, biocompatibility and resistance against abrasion and chemicals. The obtained hybrid halochromic silane-based coating on polyester fabrics was investigated with several chemical characterization techniques. Fourier transform infrared spectroscopy and X-ray Photoelectron Spectroscopy confirmed the immobilization of the dyestuff-based silane matrix onto polyethylene terephthalate samples through self-condensation of hydrolyzed silanols under the curing process. The reversibility and repeatability of pH-sensing properties of treated polyester fabrics in the pH range 2.0–8.0 were confirmed with diffuse reflectance and CIELAB color space characterizations. Polyester fabric functionalized with halochromic silane-based coating shows the durability of halochromic properties conversely to fabric treated with plain alizarin red S, thus highlighting the potentiality of the sol-gel approach in developing durable halochromic coating on synthetic substrates. The developed wearable pH-meter device could find applications as a non-invasive pH sensor for wellness and healthcare fields.