We report a phosphorescent chemosensor based on a trinuclear Au(I) pyrazolate complex or [Au(3-CH,5-COOH)Pz] (aka AuPz) stabilized in aqueous chitosan (CS) polymer media. AuPz is synthesized in situ within aqueous CS media at pH ∼ 6.5 and room temperature (RT). AuPz exhibits strong red emission (λ ∼ 690 nm) in such solutions. On addition of silver salt to AuPz/CS aqueous media, a bright-green emissive adduct (AuPz/Ag) with a peak maximum within 475-515 nm is developed. The silver adduct exhibits a 4-fold increase in quantum yield (0.19 ± 0.02) compared to AuPz alone (0.05 ± 0.01), along with a corresponding increase in phosphorescence lifetime. With almost zero interference from 15 other metal ions tested, AuPz exhibits extreme selectivity for Ag with nM/ppb detection limits (6.4-72 ppb, depending on %CS and on the sensitivity basis being a signal-to-noise ratio (S/N) = 3 or a baseline-corrected signal change = 10%). AuPz exhibits sensitivity to higher concentrations (>1 mM) of other metal ions (Tl/Pb/Gd). The sensing methodology is simple, fast, convenient, and can even be detected by the naked eye. On addition of ethylenediaminetetraacetic acid (EDTA), the red AuPz emission can be restored. AuPz and its silver adduct retain their characteristic photophysical properties in thin film forms. Remarkable photostability with <7% photobleaching after 4 h of UV irradiation is attained for AuPz solutions or thin films.
Chitosan (CS) is a natural polymer derived from chitin that has found its usage both in research and commercial applications due to its unique solubility and chemical and biological attributes. The biocompatibility and biodegradability of CS have helped researchers identify its utility in the delivery of therapeutic agents, tissue engineering, wound healing, and more. Industrial applications include cosmetic and personal care products, wastewater treatment, and corrosion protection, to name a few. Many researchers have published numerous reviews outlining the physical and chemical properties of CS, as well as its use for many of the above-mentioned applications. Recently, the cationic polyelectrolyte nature of CS was found to be advantageous for stabilizing fascinating photonic materials including plasmonic nanoparticles (e.g., gold and silver), semiconductor nanoparticles (e.g., zinc oxide, cadmium sulfide), fluorescent organic dyes (e.g., fluorescein isothiocyanate (FITC)), luminescent transitional and lanthanide complexes (e.g., Au(I) and Ru(II), and Eu(III)). These photonic systems have been extensively investigated for their usage in antimicrobial, wound healing, diagnostics, sensing, and imaging applications. Highlighted in this review are the different works involving some of the above-mentioned molecular-nano systems that are prepared or stabilized using the CS polymer. The advantages and the role of the CS for synthesizing and stabilizing the above-mentioned optically active materials have been illustrated.
Stimuli-responsive phosphorescent hydrogel microspheres have been synthesized by incorporating a water-soluble phosphorescent Au(I) complex, Na(8)[Au(TPPTS)(3)], TPPTS = tris(3,3',3''-trisulfonatophenyl)phosphine, into the polymer network of poly(N-isopropylacrylamide) (PNIPAM). Remarkable sensitization of the Au-centered emission takes place in the resulting phosphorescent hydrogels (by up to 2 orders of magnitude!) compared to that of the gold complex alone in pure water. Results of pH- and temperature-dependent luminescence titrations show that the sensitization is further magnified at physiological conditions, which is desirable for biomedical applications that will include bioimaging and drug delivery. The physical properties of PNIPAM microgels are not negatively impacted by the presence of the gold luminophore, as the colloidal crystallinity and phase transition properties remain intact. Phosphorescent microspheres have been further cross-linked by covalently bonding to neighboring particles, leading to brightly phosphorescent/high-water-content crystalline hydrogel networks with more stable crystallinity vs microgel soft crystals. These gel networks exhibit the same green phosphorescence seen in the hydrogel microspheres and pure Na(8)[Au(TPPTS)(3)] aqueous solutions with a broad unstructured profile and peak maximum at ∼525 nm. Dehydration leads to further emission sensitization and gradual blue shifts that can be fine-tuned to ultimately reach a turquoise emission at ∼490 nm in the freeze-dried form of the gel, corresponding to the emission of single crystals of Na(8)[Au(TPPTS)(3)], in agreement with the photoinduced Jahn-Teller distorted excited state model we reported earlier. Remarkable sensitivity to temperature and pH takes place in the emission enhancement with particularly favorable results at physiological conditions. The work herein represents a unique example of a stimulus-responsive phosphorescent hydrogel from a transition metal-based as opposed to lanthanide-based phosphor in an aqueous medium.
A novel Zinc oxide (ZnO)-Hydrogel fluorescent colloidal semiconductor nanomaterials system is presented for potential bio-medical applications such as cell and tissue imaging. ZnO nanoparticles (NPs) synthesized using arc discharge technique has been conjugated to bio-compatible Poly N-isopropylacrylamide (PNIPAM) based hydrogel polymer matrix. The stability and fluorescence of ZnO nanoparticles are significantly enhanced using hydrogel colloidal dispersion. Photoluminescence spectroscopy indicates approximately 10 times enhancement in fluorescence in ZnO-Hydrogel colloidal system compared to ZnO-Water system, confirming the surface modification of ZnO nanoparticles by hydrogel polymer matrix. Femtosecond time resolved fluorescence measurement demonstrates that the fluorescence is due to the enhancement in absorption by the ZnO nanoparticles due to scattering by PNIPAM nanospheres.
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