Color Changes In Response Research Articles

Cellulose-based colorimetric sensor with N, S sites for Ag+ detection

A novel cellulose-based colorimetric sensor (DAC-Tu) with N, S sites for Ag+ was prepared. The DAC-Tu exhibited excellent selectivity and sensitivity for Ag+ with a naked eyes color change from white to black in mixed metal ions aqueous solutions. The naked-eye in-situ detection limit of DAC-Tu for Ag+ was 10−6 mol/L within 10 min. The cellulose matrix and the grafted functional groups (CS, NH2) of DAC-Tu all contributed to the low naked-eye in-situ detection limit. The mechanism of the visual recognition of DAC-Tu sensor for Ag+ was proposed, the N and S atoms of DAC-Tu preferentially coordination chelated with Ag+ to form NAg, SAg, and Ag2S, and enriched on the cellulose matrix, thus the DAC-Tu presented different color change in response to different concentrations of Ag+. The as-prepared DAC-Tu colorimetric sensor showed a great application prospect for in-situ naked eyes detection of Ag+.

Privet golden leaves adapt unexpectedly well to light changes

Golden-leaf privet (Ligustrum × vicaryi) is widely used as a horticultural shrub because of its upper golden leaves, but its lower leaves are green. However, the putative mechanisms of its upper golden leaves and the leaf color changes in response to light shifts have not been well studied so far. Here, chlorophylls (Chl), carotenoids, and Chl precursors from both golden and green leaves grown in full sunlight (approximately 1200 μmol photons m−2 s−1 at noon) or low-light conditions (180 μmol m−2 s−1) were determined spectrophotometrically. In addition, their gas exchange parameters and Chl fluorescence were measured in situ. Metabolic flux analysis of chlorophyll intermediates indicated that the conversion of prochlorophyllide to chlorophyllide was significantly blocked in golden leaves when compared with green leaves. Green leaves showed higher photosynthetic capacity in low light than golden leaves, but golden leaves presented unexpectedly stronger photosynthetic capacity and lower reactive oxygen species accumulation under the high-light condition. Furthermore, golden leaves showed a higher level of nonphotochemical quenching (NPQ) after the light-to-dark shift and presented a stronger adaptive ability to a broad range of light environments. Higher NPQ values and less oxidative damage in golden leaves may be correlated with their higher carotenoid levels. The results imply that lower chlorophyll levels and higher carotenoid levels in canopy leaves may help privet plants acclimate better to illumination changes. This study demonstrates the key role of irradiance in generating the two types of Ligustrum × vicaryi leaves and sheds a light on cultivation of other ornamental foliage plants.

Plastic sexual ornaments: Assessing temperature effects on color metrics in a color-changing reptile.

Conspicuous coloration is an important subject in social communication and animal behavior, and it can provide valuable insight into the role of visual signals in social selection. However, animal coloration can be plastic and affected by abiotic factors such as temperature, making its quantification problematic. In such cases, careful consideration is required so that metric choices are consistent across environments and least sensitive to abiotic factors. A detailed assessment of plastic trait in response to environmental conditions could help identify more robust methods for quantifying color. Temperature affects sexual ornamentation of eastern fence lizards, Sceloporus undulatus, with ventral coloration shifting from green to blue hues as temperatures rise, making the calculation of saturation (color purity) difficult under conditions where temperatures vary. We aimed to characterize how abiotic factors influence phenotypic expression and to identify a metric for quantifying animal color that is either independent from temperature (ideally) or best conserves individual's ranks. We compared the rates of change in saturation across two temperature treatments using seven metrics: three that are based on fixed spectral ranges (with two of them designed by us specifically for this system) and three that track the expressed hue (with one of them designed by us to circumvent spurious results in unornamented individuals). We also applied a lizard visual sensitivity model to understand how temperature-induced color changes may be perceived by conspecifics. We show that the rate of change in saturation between two temperatures is inconsistent across individuals, increasing at a higher rate in individuals with higher baseline saturation at lower temperatures. In addition, the relative color rank of individuals in a population varies with the temperature standardized by the investigator, but more so for some metrics than others. While we were unable to completely eliminate the effect of temperature, current tools for quantifying color allowed us to use spectral data to estimate saturation in a variety of ways and to largely preserve saturation ranks of individuals across temperatures while avoiding erroneous color scores. We describe our approaches and suggest best-practices for quantifying and interpreting color, particularly in cases where color changes in response to environmental factors.

Anthocyanin-based sensors derived from food waste as an active use-by date indicator for milk

Active use-by date (AUBD) or freshness indicators hold great potential to reduce food waste. Herein, we develop an anthocyanin AUBD indicator that is capable of discriminating between fresh, spoiling, and spoiled milk. The sensor undergoes a visible blue to purple to pink color change in response to lactic acid, which is an indicator of microbial spoilage in milk. Anthocyanin is cast into a range of materials and the composite’s suitability to monitor pH changes (pH 6.8 fresh milk vs pH 4.0 spoiled milk) is assessed. Of the materials studied, an anthocyanin-agarose film is nominated as the optimum materials with the best colorimetric performance. We introduce a new method to quantify anthocyanin color change by measuring red chromatic shift by digital analysis. The anthocyanin sensors will provide a real-time indication of actual milk quality, surpassing the function of traditional date marking tools that provide an indication of the expected shelf life.

Phenotypic flexibility in background-mediated color change in sticklebacks.

Phenotypic flexibility may incur a selective advantage in changing and heterogeneous environments, and is increasingly recognized as an integral aspect of organismal adaptation. Despite the widespread occurrence and potential importance of rapid and reversible background-mediated color change for predator avoidance, knowledge gaps remain regarding its adaptive value, repeatability within individuals, phenotypic correlates, and whether its expression is context dependent. We used manipulative experiments to investigate these issues in two fish species, the three-spined stickleback (Gasterosteus aculeatus) and nine-spined stickleback (Pungitius pungitius). We sequentially exposed individuals to dark and light visual background treatments, quantified color change from video recordings, and examined associations of color change with phenotypic dimensions that can influence the outcome of predator-prey interactions. G. aculeatus expressed a greater degree of color change compared to P. pungitius. In G. aculeatus, the color change response was repeatable within individuals. Moreover, the color change response was independent of body size but affected by sex and boldness, with males and bolder individuals changing less. Infection by the parasite Schistocephalus solidus did not affect the degree of color change, but it did modulate its association with sex and boldness. G. aculeatus adjusted the expression of color change in response to predation risk, with enhanced color change expression in individuals exposed to either simulated attacks, or olfactory cues from a natural predator. These results provide novel evidence on repeatability, correlated traits, and context dependence in the color change response and highlight how a suite of factors can contribute to individual variation in phenotypic flexibility.

Reversible Nontoxic Thermochromic Microcapsules

Thermochromic materials exhibit a color change in response to a change in temperature. Creating nontoxic microcapsules containing thermochromic materials for applications in ink and film materials is historically challenging. In this study, we develop a nontoxic chlorophenol red (CPR)-water thermochromic system and its microcapsules with silicone shells via a reaction between water and octadecyltrichlorosilane (OTS) at the interface of a w/o emulsion. The obtained microcapsules exhibit a clear color change with full reversibility and are successfully used as inks by screen printing and as additives in films. Nontoxicity of both microcapsules and films is demonstrated through cell cytotoxicity assays. These features make these novel materials applicable to the next generation of intelligent sensors, coating, and food packaging materials.

PCR-free paper-based nanobiosensing platform for visual detection of telomerase activity via gold enhancement

Telomerase activity has been demonstrated in a wide variety of most solid tumors and considered as a well-known cancer biomarker. The commonly utilized method for its detection is polymerase chain reaction (PCR)-based telomeric repeat amplification protocol (TRAP). However, the TRAP technique suffers from false-negative results caused by the failure of PCR step. Moreover, it requires advanced equipment with a tedious and time-consuming procedure. Herein, we presented a portable nitrocellulose paper-based nanobiosensing platform for ultrafast and equipment-free detection of telomerase activity based on a simple colorimetric assay that enabled naked-eye visualization of the color change in response to enzyme activity. In this platform, hybridization was initially performed between telomere complementary oligonucleotide immobilized on gold nanoparticles (GNPs) and telomerase elongated biotinylated probe. Thereafter, the assembly was attached on activated paper strip via avidin-biotin interaction. The signal amplification was carried out by enlargement of the attached GNPs on the paper strip, forming tightly compact rod-shaped submicron structures of gold representing a visual color formation. Thanks to significant sensitivity enhancement, the color change was occurred for down to 6 cells, which can be easily observed by the naked eye. Due to the desired aspects of the developed assay including PCR-free, low cost, simple, and high sensitivity, it can be used for evaluation of telomerase activity in cell extracts for future clinical applications. Furthermore, this design has the ability to be easily integrated into lab-on-chip devices for point-of-care telomerase sensing.

Effective methods for the synthesis of hydrazones, quinazolines, and Schiff bases: reaction monitoring using a chemometric approach.

Synthesis of hydrazones (1a–4a and 1b–4b), quinazolines (3c·MeOH and 3d·MeOH), and hydrazone-Schiff bases (4c and 4d) is achieved by combining suitable aldehydes (2,3- or 2,4-dihydroxybenzaldehyde) with four hydrazides (isonicotinic, nicotinic, and 2- or 4-aminobenzoic acid hydrazide). A suite of approaches for their preparation is described: solution-based synthesis, mechanosynthesis, and solid-state melt reactions. The mechanochemical approach is generally a better choice for the quinazolines, while the solid-state melt reaction is more efficient for derivatives of (iso)nicotinic based hydrazones. Crystalline amine-functionalised hydrazones 4a and 4b undergo post-synthetic modifications in reactions with 3- or 4-pyridinecarbaldehyde vapours to form hydrazone-Schiff bases 4a-3py, 4b-3py, 4a-4py, and 4b-4py. Mechanochemical and vapour-mediated reactions are followed by ex situ powder X-ray diffraction and IR-ATR methods, respectively. The chemometric analysis of these data using principal component analysis provided an insight into the reaction profiles and reaction times. Azines (5a and 5b), achieved from aldehydes and hydrazine, reversibly change colour in response to temperature changes. The structures of all products are ascertained by a combined use of spectroscopic and X-ray diffraction methods. The cytotoxic and antimicrobial activities of all compounds against selected human cancer cell lines and bacterial strains are evaluated.

Vapochromic luminescence of a spin-coated copper(I) complex thin film by the direct coordination of vapour molecules.

A homogeneous thin film of a simple and highly luminescent Cu(i) complex, [CuI(PPh3)2(py)] (PPh3 = triphenylphosphine, py = pyridine) (Cu-py), was fabricated via spin coating using polyvinylpyrrolidone (PVP) and pyridine without destroying the complex. The thin film (Cu-py@PVP), with a thickness of 1 μm, exhibited efficient response to vapour, exhibiting reversible luminescence changes between blue-green and yellow upon exposure to vapours of N-heteroaromatic compounds such as py and 2-methylpyrazine (Mepyz). Vapochromic luminescence colour change resulting from ligand substitution was also observed in the crystal state, but the response of the thin film was remarkably faster than that of the crystalline samples. The vapour-induced ligand exchange on the thin film was fully characterised by comparing the luminescence properties of the Cu-py crystal with the newly prepared Cu(i) complex, [CuI(Mepyz)(PPh3)2] (Cu-Mepyz).

Temperature-dependent changes in the molecular orientation and visible color of phthalocyanine films.

A simple phthalocyanine zinc complex exhibits a visible color change in response to weak external stimuli, i.e., changes in solvent and temperature. Its chromism was attributed to its controlled aggregation via weak interactions between the central metal and peripheral oxygen atoms. In solution, intense absorption and fluorescence bands appeared in both the longer-wavelength and NIR region in non-coordinating solvents, while a simple sharp Q band was observed in coordinating solvents. Variable-temperature absorption spectra and fluorescence lifetime measurements were used to characterize the aggregation-induced absorption and emission in non-coordinating solvents. A selective aggregation–disaggregation process was also observed in thin films of this phthalocyanine zinc complex, and the optical properties of the film depend on the annealing temperature. The changes in the NIR region lead to a sizable visible color change that is recognizable by the naked human eye. The temperature-dependent control of the aggregation process in the thin film was confirmed using operando spectroscopy techniques.

Reversible Redox Switching of Concurrent Luminescence and Visual Color Change Based on Lanthanide Metallogel.

The development of reversible redox supramolecular gels capable of concurrent luminescence switch and visible color change with the facile redox process has always been an intriguing challenge. A redox-responsive supramolecular lanthanide metallogel with strong luminescence and yellow color is obtained via coordination interaction between 3,5-dinitrosalicylic acid (DNSA) and europium (Eu3+). Upon the addition of TiO2 to the prepared gel (DNSA/Eu3+ gel), the oxidation process of the gel (DNSA/Eu3+/TiO2 gel) can be easily achieved by UV irradiation. The DNSA/Eu3+/TiO2 gel exhibits a concurrent reversible "on-off" luminescence and color change in response to redox stimuli. The DNSA/Eu3+/TiO2 gel shows a concurrent quench of luminescence and a color change from yellow to red when the gel was stimulated by the reductant. Upon UV irradiation, the luminescence and color of the reduced DNSA/Eu3+/TiO2 gel restored to its initial state due to the strong oxidation ability of hydroxyl radicals derived from photocatalytic oxidation of TiO2. The results of UV-vis and mass spectroscopy indicated that the reversible redox responsiveness of DNSA/Eu3+/TiO2 gel depends on the reversible oxidation-reduction reactions of DNSA. Moreover, DNSA/Eu3+/TiO2 gel remains stable because the morphology of the gel had no change during the redox process. Exemplarily, the application of DNSA/Eu3+/TiO2 gels to achieve luminescent patterning was investigated. The results demonstrated that the prepared metallogel has potential applications in the fields of writable materials, anticounterfeiting, sensors, and others.

Creating Chameleon-like Smart Actuators

Inspired by how chameleons change their skin color, Du and co-workers have developed a general biomimetic strategy for preparing smart photonic crystal actuators with the fast and reversible colorimetric response and programmable shape transformation.

Color-spectrum-broadened ductile cellulose films for vapor-pH-responsive colorimetric sensors

We introduce a smart colorimetric sensor platform, in which iridescent chiral nematic films composed of oxidized cellulose nanocrystals (OxCNCs) exhibit pH-responsive color changes. To provide dried OxCNC films with the controlled reflection of circularly polarized lights at specific wavelengths, we manipulated the half-pitch distance from 145 to 270nm by adjusting the surface charges of OxCNCs during 2,2,6,6-tetramethylpiperidine-1-oxyl-radical-mediated oxidation. Utilizing the controlled electrostatic repulsion generated by the difference in the surface charge of OxCNCs, the reflection wavelengths of the films could be tuned across a broad color spectrum. In addition, we improved flexibility or ductility of OxCNC composite films by coassembly with a hygroscopic polymer, polyethylene glycol, while maintaining their iridescent colors. We finally demonstrated that the OxCNC composite films could show reversible color changes in response to vapors of aqueous solutions with different pH values, thus enabling the development of a vapor-pH-responsive colorimetric sensor technology.

Stretchy polymer lights up under stress

Materials that light up or change color in response to stress could enable responsive clothing or status reports on electronic skin. But many force-sensitive materials produce colors by breaking chemical bonds, making the changes hard to reverse and limiting reusability. Now, an international team of researchers has made polyurethanes that glow when stretched and then instantly switch off when relaxed, thanks to mechanically interlocked molecular structures called rotaxanes (ACS Cent. Sci. 2019, DOI: 10.1021/acscentsci.9b00173). The team made blue-, green-, and orange-glowing polymers and combined them to produce a white one. Learn more about the chemistry behind the polymers at cenm.ag/stretch.—Kerri Jansen Music: “Loopster” by Kevin MacLeod is licensed under CC BY 3.0.

Automatic characterisation of dye decolourisation in fungal strains using expert, traditional, and deep features

Fungi have diverse biotechnological applications in, among others, agriculture, bioenergy generation, or remediation of polluted soil and water. In this context, culture media based on colour change in response to degradation of dyes are particularly relevant, but measuring dye decolourisation of fungal strains mainly relies on a visual and semiquantitative classification of colour intensity changes. Such a classification is a subjective, time-consuming, and difficult to reproduce process. In order to deal with these problems, we have performed a systematic evaluation of different image-classification approaches considering ad hoc expert features, traditional computer vision features, and transfer-learning features obtained from deep neural networks. Our results favour the transfer learning approach reaching an accuracy of 96.5% in the evaluated dataset. In this paper, we provide the first, at least up to the best of our knowledge, method to automatically characterise dye decolourisation level of fungal strains from images of inoculated plates.

Paper Analytic Device to Detect the Presence of Four Chemotherapy Drugs

A paper analytic device, the chemoPAD, was developed and validated to visually detect methotrexate, doxorubicin, cisplatin, and oxaliplatin at concentrations commonly found in injectable dosage forms. By testing residual solution after patient treatment, the chemoPAD can be used to monitor drug quality without restriction of patient access to medication. The chemoPAD is produced by wax printing on Ahlstrom paper to create separate reaction areas and deposits small amounts of chemicals to create color changes in response to different active pharmaceutical ingredients (APIs). This creates a unique color bar code to identify each medication. Internal validation studies show that the chemoPAD has excellent sensitivity and specificity to differentiate between samples of 100% and 0% API, which is the distinction relevant to the majority of reported falsified chemotherapy cases. The platinum-containing drugs, cisplatin and oxaliplatin, can be detected semiquantitatively. The cards can be read either visually by comparison with a standard image or by using computer image analysis. Dosage forms were collected from the Ethiopian health care system and analyzed with the chemoPAD followed by high-performance liquid chromatography. A substandard sample was discovered and reported to the Ethiopian Food Medicine and Healthcare Administration and Control Authority.

Surface modification of TEMPO-oxidized cellulose nanofibrils for composites to give color change in response to pH level

This study aimed to modify the surface of cellulose nanofibrils (CNFs) prepared by TEMPO-mediated oxidation via two approaches: esterification and amination for further reaction with activated bromocresol green (BCG) to prepare CNF-BCG composites capable of changing color to pH level. Both the esterified and aminated CNFs were reacted with the activated BCG to obtain the composites of CNF-BCG1 and CNF-BCG2, respectively. Then, these composites were exposed to different pH levels to observe any color changes. Fourier transform infrared (FTIR) spectroscopy, thermogravimetry analysis (TGA), and UV–Vis spectroscopy were employed to characterize the modified CNFs and CNF-BCG composites. FTIR spectra showed that CNFs have been esterified or aminated, and the modified CNFs had been successfully reacted with the activated BCG to form CNF-BCG1 and CNF-BCG2, respectively. TGA results also supported this result with a greater thermal stability of both CNF-BCG composites than those of the CNFs. The CNF-BCG1 became yellow below pH 2 while it became green above pH 5. Although these CNF-BCG composites showed color changes to relevant pH levels, the CNF-BCG1 had more distinctive color changes than that of the CNF-BCG2. These results suggested that the esterification of CNFs was a better route to the reaction with the activated BCG for the color change, showing a possibility of using the CNF-BCG1 composites for colorimetric application.

Robust Sandwich-Structured Nanofluidic Diodes Modulating Ionic Transport for an Enhanced Electrochromic Performance.

Biomimetic solid‐state nanofluidic diodes have attracted extensive research interest due to the possible applications in various fields, such as biosensing, energy conversion, and nanofluidic circuits. However, contributions of exterior surface to the transmembrane ionic transport are often ignored, which can be a crucial factor for ion rectification behavior. Herein, a rational design of robust sandwich‐structured nanofluidic diode is shown by creating opposite charges on the exterior surfaces of a nanoporous membrane using inorganic oxides with distinct isoelectric points. Potential‐induced changes in ion concentration within the nanopores lead to a current rectification; the results are subsequently supported by a theoretical simulation. Except for providing surface charges, functional inorganic oxides used in this work are complementary electrochromic materials. Hence, the sandwich‐structured nanofluidic diode is further developed into an electrochromic membrane exhibiting a visual color change in response to redox potentials. The results show that the surface‐charge‐governed ionic transport and the nanoporous structure facilitate the migration of Li+ ions, which in turn enhance the electrochromic performance. It is envisioned that this work will create new avenues to design and optimize nanofluidic diodes and electrochromic devices.

A two-photon ratiometric probe for detection of hNQO1 enzyme activity in human colon tissue

Human colonic carcinomas are one of the most common cancers in all Western countries, but there is no reliable method for quantitative and rapid analysis of colon cancer. Herein, we report an hNQO1 enzyme-selective and two-photon emission ratiometric probe (SHC) for quantitative analysis of hNQO1 activity in human colon tissue. This probe shows a blue-to-yellow emission color change in response to hNQO1, high sensitivity and selectivity for hNQO1 over other bio-metabolites, and two-photon microscopy (TPM) imaging capability, thereby allowing precise monitoring of the activity of hNQO1 in situ. Ratiometric TPM imaging with SHC in human colon tissues revealed that hNQO1 activities gradually increased during cancer progression from normal to adenoma, and to adenocarcinoma colon tissues. These findings demonstrated that SHC labeling can detect the differential activities of hNQO1 in the distinct stages of colon neoplasm. Thus, this approach, enzyme-selective TP probe labeling, will likely provide a reliable and fast method for precise diagnosis of colon cancer.

Novel solvatochromic and halochromic sulfahydrazone molecular switch

Novel Sulfonamide-Tricyanofuran-Hydrazone (SULFATCFH) disperse dyes containing the sulfa drug nucleus and bearing a hydrazone recognition moiety were synthesized. The preparation involved azo-coupling between active methyl-containing tricyanofuran (TCF) heterocycle and the appropriately functionalized sulfonamide diazonium salt. The molecular structures of the produced sulfahydrazone compounds were confirmed using FTIR, 1H/13C NMR and (C, H, N) elemental analysis. Owing to the presence of an intramolecular charge transfer absorption in the visible region, these chromophores showed pronounced solvatochromic behavior in organic solvents of differing polarities. The pH molecular switch was accomplished by modulation of internal charge transfer via protonation and deprotonation of the sulfahydrazone in acetone solution. These systems exhibited reversible color changes in response to pH changes as a result of extensive charge delocalization in the hydrazono-form that led to a quinoid-type interaction. The UV–Vis absorption spectra demonstrated dramatic color change from yellow to blue due to solvatochromism and pH-responsiveness of the hydrazone recognition moiety. Mechanisms accounting for the solvatochromism and pH-responsiveness were proposed. The SULFATCFH compounds were applied to polyester fibers as disperse dyes and their fastness properties to wash, perspiration, rubbing and light were evaluated.