pH-sensitive Fibers Research Articles

Controlled preparation and chemical actuation of polyacrylonitrile fibers

Pure polyacrylonitrile (PAN) fibers with diameter size at micrometric scale were obtained and collected radially using an immersion-jet wet spinning .system. This technique is a fast and easy approach to fabricate micrometric PAN fibers. The diameter of the fiber can be easily controlled by adjusting the size of the spinneret. Uniform, smooth and continuous PAN microfibers were suitable modified by thermal stabilization and alkaline saponification to obtain pH-sensitive fibers. The effect of diameter size fiber on the chemical actuation behavior was investigated in terms of length change characteristics under the influence of pH solution. The microfibers showed expanding/contracting behavior and force generation stimulated by changes in the environment pH. The fibers structural and chemical properties were characterized using the FT-IR spectroscopy and SEM microscopy techniques.

A semen cassia gum-based film with visual-olfactory function for indicating the freshness change of animal protein-rich food.

Bromothymol blue (BTB) was fixed on the cationic cellulose fibers (CCFs) to prepare pH sensitive fibers (pH-SFs). The pH-SFs as intelligent indicator were added into the semen cassiae gum (SCG) as a weakly acidic matrix to prepare a visual-olfactory film. The 13C NMR results show that the CCFs were successfully obtained by introducing hydroxypropyltriethylamine groups which showed strong affinity to BTB molecules. Rheology results demonstrated that all of the film-forming solutions were shear-thinned fluids with non-Newtonian behavior. Scanning electronic microscopy showed that the addition of pH-SFs makes the film surface rougher and rougher. The addition of pH-SFs < 3% improved the tensile strength of the film. The visual-olfactory film was sensitive to ammonia with a highly visible color change from pale yellow to blue-green. The SCG-3SFs film pre-treated in NaOH solution changed from blue to pale yellow while the raw milk was close to spoilage. The light yellow SCG-3SFs film changed to blue-green as the freshwater shrimp changed from fresh to spoilage. The results indicate that the visual-olfactory film can be used for perceiving the freshness of milk and freshwater shrimp.

In-Situ pH-Sensitive Fibers via the Anchoring of Bromothymol Blue on Cellulose Grafted with Hydroxypropyltriethylamine Groups via Adsorption.

In-situ pH-sensitive cellulose fibers (IS-pH-SCF) were prepared by anchoring bromothymol blue (BTB) onto cellulose fibers (CF) modified with hydroxypropyltriethylamine (HPTTL) groups. Fourier transform infrared and X-ray photoelectron spectrum analyses demonstrated that the HPTTL groups were grafted onto the CF. X-ray diffraction proved that cellulose I in the CF transformed into cellulose II after quaternization. Scanning electron microscopy suggested that the quaternized CF (QCF) surface was clean and uniformly ridged. The adsorption of BTB onto QCF was carried out via batch adsorption experiments. A kinetic study illustrated that the adsorption was a spontaneous process and described well by pseudo-second-order, Freundlich and Temkin isotherms. The activation energy for the BTB adsorption onto QCF was 52.89 kJ/mol, which proved that the BTB adsorption onto QCFs was chemically controlled. The pH response demonstrated that the IS-pH-SCF was highly sensitive to pH, with an obvious color change for pH 4 to 8. The release tests showed that BTB was anchored on QCFs and that no BTB was released. IS-pH-SCF has a potential use for indicating pH changes in food.

Flexible pH‐Sensing Hydrogel Fibers for Epidermal Applications

Epidermal pH is an indication of the skin's physiological condition. For example, pH of wound can be correlated to angiogenesis, protease activity, bacterial infection, etc. Chronic nonhealing wounds are known to have an elevated alkaline environment, while healing process occurs more readily in an acidic environment. Thus, dermal patches capable of continuous pH measurement can be used as point-of-care systems for monitoring skin disorder and the wound healing process. Here, pH-responsive hydrogel fibers are presented that can be used for long-term monitoring of epidermal wound condition. pH-responsive dyes are loaded into mesoporous microparticles and incorporated into hydrogel fibers using a microfluidic spinning system. The fabricated pH-responsive microfibers are flexible and can create conformal contact with skin. The response of pH-sensitive fibers with different compositions and thicknesses are characterized. The suggested technique is scalable and can be used to fabricate hydrogel-based wound dressings with clinically relevant dimensions. Images of the pH-sensing fibers during real-time pH measurement can be captured with a smart phone camera for convenient readout on-site. Through image processing, a quantitative pH map of the hydrogel fibers and the underlying tissue can be extracted. The developed skin dressing can act as a point-of-care device for monitoring the wound healing process.

Fiber optic lifetime pH sensing based on ruthenium(II) complexes with dicarboxybipyridine

The complexes of ruthenium(II) with phenanthroline (Phen), diphenylphenanthroline (Ph 2Phen) and with 4,4′-dicarboxy-2,2′-bipyridine acid (Dcbpy) ([Ru(Phen) 2Dcbpy]Cl 2 and [Ru(Ph 2Phen) 2Dcbpy]Cl 2) were synthesized and the variation of the correspondent fluorescence intensity and lifetime with the pH characterized. Luminescence intensity, emission wavelength and excited state lifetime all show a typical sigmoid variation with pH in the range 3–9, demonstrating the suitability of this complex for luminescence sensing applications. In aqueous solutions (28% ethanol) the complexes [Ru(Phen) 2Dcbpy]Cl 2 and [Ru(Ph 2Phen) 2Dcbpy]Cl 2 show, respectively, the following properties: apparent p K a of 3.6 ± 0.4 and 3.7 ± 0.4; lifetimes of the protonated species 0.46 ± 0.01 μs and 0.38 ± 0.02 μs and ionised species 0.598 ± 0.001 μs and 0.61 ± 0.08 μs. The [Ru(Phen) 2Dcbpy]Cl 2 complex was immobilised in the tip of optical fibers using a hybrid sol–gel procedure based on tetraethoxysilan and phenyltriethoxysilan enabling pH sensitive fiber probes. The immobilised complex shows the following lifetimes: protonated species 1.05 ± 0.04 μs and ionised species 1.16 ± 0.04 μs. These characteristics show that these ruthenium(II) complexes are good indicators for pH sensing, either in aqueous solution or immobilised in sol–gel, and are well suited for intensity and/or frequency domain interrogation.

Effect of copolymer architecture on the response of pH sensitive fibers based on acrylonitrile and acrylic acid

Copolymers of acrylonitrile and acrylic acid with high acrylic acid feed ratio of 43 mol% were synthesized using free radical polymerization. The architecture of copolymers was modified by regulating the dosing of more reactive comonomer-acrylic acid. 13C NMR analysis confirmed that two copolymers – one (A) containing enriched blocks of individual monomer-residues (architecture close to a block copolymer) and the second (B) having nearly random distribution of the comonomers, could be successfully synthesized. The resultant acrylic acid content was determined to be nearly 50 mol% for both the copolymers. These copolymers were converted to fine fibers by solution spinning in DMF-water system, drawn in coagulation bath, and annealed at 120 °C for 2 h. The fibers were evaluated for pH response behavior, mechanical stability, and retracting stresses. The fiber A was found to have significantly higher swelling percentage (3300–3700%), faster response, and higher stability to repeated cycling compared to fiber B. Also, Fiber A showed lower thermal shrinkage, better mechanical properties during swelling and higher retracting forces during deswelling. These results indicate that the copolymer with enriched block architecture could possibly form segregated domain structure with acrylic acid domains facilitating enhanced pH response while acrylonitrile domains providing physical crosslinks for stronger mechanical strength. The study suggests that above approach may be more useful than chemically cross-linked gel rods in producing artificial muscles with faster response and good mechanical properties.

Influence of the copolymer architecture and composition on the response and mechanical properties of pH-sensitive fibers

A series of copolymers based on acrylonitrile (AN) and acrylic acid (AA) with varying architecture and composition were synthesized using free radical polymerization. The distribution of monomers in the copolymer chains could be successfully controlled by regulating the addition of more reactive monomer (AA). Copolymers having nearly random distribution of comonomer moieties to block type distribution with different composition (10–50 mol % AA) were synthesized to investigate the effect of polymer architecture and composition on pH response and mechanical properties of resultant structures. These copolymers were solution spun from dimethylformamide-water system, drawn in coagulation bath, and annealed at 120°C for 2 h to make pH-sensitive fibers which were structurally stable without the need of chemical crosslinking. The fibers from block copolymers showed significantly better tensile strength (34.3 MPa), higher retractive forces (0.26 MPa), and enhanced pH response (swelling 3890%) in comparison with fibers from random copolymer (13.55 MPa, 0.058 MPa, and 1723%, respectively). The tensile strength and retractive forces could be further improved to a value of 72 MPa and 0.36 MPa, respectively, by changing the composition of the block copolymer while retaining the swelling percentage similar to the random copolymer mentioned above. It is proposed that on processing to fibers, the block copolymers could form a segregated domain structure with separate domains of AA and AN, where AN domains were responsible for high structural integrity by providing connectivity among polymer chains, while AA domains showed improved response to changing pH of the environment. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007