Functionalized Ionic Microgel Sensor Array for Colorimetric Detection and Discrimination of Metal Ions

A functional ionic microgel sensor array was developed by using 1-(2-pyridinylazo)-2-naphthaleno (PAN)- and bromothymol blue (BTB)-functionalized ionic microgels, which were designed and synthesized by quaternization reaction and anion-exchange reaction, respectively. The PAN microgels (PAN-MG) and BTB microgels (BTB-MG) were spherical in shape with a narrow size distribution and exhibited characteristic colors in aqueous solution in the presence of various trace-metal ions, which could be visually distinguished by the naked eye. Such microgels could be used for the colorimetric detection of various metal ions in aqueous solution at submicromolar levels, which were lower than the U.S. Environmental Protection Agency standard for the safety limit of metal ions in drinking water. A total of 10 species of metal ions in aqueous solution, Ba²⁺, Cr³⁺, Mn²⁺, Pb²⁺, Fe³⁺, Co²⁺, Zn²⁺, Ni²⁺, Cu²⁺, and Al³⁺, were successfully discriminated by the as-constructed microgel sensor array combined with discriminant analysis, agglomerative hierarchical clustering, and leave-one-out cross-validation analysis

4‑(2-Pyridylazo)-resorcinol Functionalized Thermosensitive Ionic Microgels for Optical Detection of Heavy Metal Ions at Nanomolar Level

4-(2-Pyridylazo)-resorcinol (PAR) functionalized thermosensitive ionic microgels (PAR-MG) were synthesized by a one-pot quaternization method. The PAR-MG microgels were spherical in shape with radius of ca. 166.0 nm and narrow size distribution and exhibited thermo-sensitivity in aqueous solution. The PAR-MG microgels could optically detect trace heavy metal ions, such as Cu²⁺, Mn²⁺, Pb²⁺, Zn²⁺, and Ni²⁺, in aqueous solutions with high selectivity and sensitivity. The PAR-MG microgel suspensions exhibited characteristic color with the presence of various trace heavy metal ions, which could be visually distinguished by naked eyes. The limit of colorimetric detection (<i>D</i>_L) was determined to be 38 nM for Cu²⁺ at pH 3, 12 nM for Cu²⁺ at pH 7, and 14, 79, 20, and 21 nM for Mn²⁺, Pb²⁺, Zn²⁺, and Ni²⁺, respectively, at pH 11, which was lower than (or close to) the United States Environmental Protection Agency standard for the safety limit of these heavy metal ions in drinking water. The mechanism of detection was attributed to the chelation between the nitrogen atoms and <i>o</i>-hydroxyl groups of PAR within the microgels and heavy metal ions

Thermosensitive Ionic Microgels with pH Tunable Degradation via <i>in Situ</i> Quaternization Cross-Linking

Degradable thermosensitive ionic microgels were synthesized via surfactant-free emulsion polymerization (SFEP) of <i>N</i>-isopropylacrylamide (NIPAm) and 1-vinylimidazole (VIM) at 70 °C with degradable 1,4-phenylene bis­(4-bromobutanoate) or 1,6-hexanediol bis­(2-bromopropionate) as quaternized cross-linkers. VIM could be quaternized by 1,4-phenylene bis­(4-bromobutanoate) or 1,6-hexanediol bis­(2-bromopropionate), leading to the formation of degradable cross-linking network and ionic microgels. Combined techniques of transmission electron microscopy (TEM), dynamic light scattering (DLS), electrophoretic light scattering (ELS), UV–vis spectroscopy, FT-IR spectra, and gel permeation chromatography (GPC) were employed to systematically investigate the sizes, morphologies, and properties of the obtained microgels before and after degradation as well as the degradation mechanism. The obtained microgels were spherical in shape with narrow size distribution and exhibited thermosensitive behavior and controllable degradation. The disintegration of the microgels was confirmed to be resulted from the hydrolysis of ester bonds of the cross-linkers. The degradation rate of the obtained microgels could be regulated by tuning the pH value of microgel suspensions. The PNI-Ph series of microgels fabricated with 1,4-phenylene bis­(4-bromobutanoate) as the cross-linking agent could gradually degrade even in neutral solution with lifetimes of 44–53 h, depending on the quaternization ratio. The degradation of PNI-Ph series of microgels experienced two reaction processes, that is, the hydrolysis of ester bonds of the cross-linkers and the oxidation of generated hydroquinone to form benzoquinone. It was also demonstrated that different silica nanostructures could be fabricated by using such degradable thermosensitive ionic microgels as the template at various temperatures

Thermosensitive Ionic Microgels via Surfactant-Free Emulsion Copolymerization and in Situ Quaternization Cross-Linking

A type of thermosensitive ionic microgel was successfully prepared via the simultaneous quaternized cross-linking reaction during the surfactant-free emulsion copolymerization of <i>N</i>-isopropylacrylamide (NIPAm) as the main monomer and 1-vinylimidazole or 4-vinylpyridine as the comonomer. 1,4-Dibromobutane and 1,6-dibromohexane were used as the halogenated compounds to quaternize the tertiary amine in the comonomer, leading to the formation of a cross-linking network and thermosensitive ionic microgels. The sizes, morphologies, and properties of the obtained ionic microgels were systematically investigated by using transmission electron microscopy (TEM), dynamic and static light scattering (DLS and SLS), electrophoretic light scattering (ELS), thermogravimetric analyses (TGA), and UV–visible spectroscopy. The obtained ionic microgels were spherical in shape with narrow size distribution. These ionic microgels exhibited thermosensitive behavior and a unique feature of poly­(ionic liquid) in aqueous solutions, of which the counteranions of the microgels could be changed by anion exchange reaction with BF₄K or lithium trifluoromethyl sulfonate (PFM-Li). After the anion exchange reaction, the ionic microgels were stable in aqueous solution and could be well dispersed in the solvents with different polarities, depending on the type of counteranion. The sizes and thermosensitive behavior of the ionic microgels could be well tuned by controlling the quaternization extent, the type of comonomer, halogenated compounds, and counteranions. The ionic microgels showed superior swelling properties in aqueous solution. Furthermore, these ionic microgels also showed capabilities to encapsulate and release the anionic dyes, like methyl orange, in aqueous solutions

Segmental Relaxation Dynamics of the Core and Corona in a Single Dry Micelle

The segmental relaxation dynamics of the core and the corona in a single dry spherical micelle comprising a polystyrene (PS) or poly­(methyl methacrylate) (PMMA) core and a poly­(acrylic acid) (PAA) corona, along with the effects of the micelle density and the chain length of the core-forming and corona-forming polymers, were investigated by AFM. The results showed that the segmental relaxation temperature in the micelle core was close to its bulk <i>T</i>_g and independent of the micellar structure, such as micelle size, density, and corona thickness. By contrast, the segmental relaxation temperature in the micelle corona is dependent on the micellar structure. The depressed chain mobility in the corona was enhanced as the micelle density and the length of both the corona-forming and core-forming blocks increased. This phenomenon was attributed to conformational entropy loss in the micelle corona because of the chain adopting a conformation more perpendicular to the core as the micelle size and density increased

Segmental Relaxation Dynamics of the Core and Corona in a Single Dry Micelle

The segmental relaxation dynamics of the core and the corona in a single dry spherical micelle comprising a polystyrene (PS) or poly­(methyl methacrylate) (PMMA) core and a poly­(acrylic acid) (PAA) corona, along with the effects of the micelle density and the chain length of the core-forming and corona-forming polymers, were investigated by AFM. The results showed that the segmental relaxation temperature in the micelle core was close to its bulk <i>T</i>_g and independent of the micellar structure, such as micelle size, density, and corona thickness. By contrast, the segmental relaxation temperature in the micelle corona is dependent on the micellar structure. The depressed chain mobility in the corona was enhanced as the micelle density and the length of both the corona-forming and core-forming blocks increased. This phenomenon was attributed to conformational entropy loss in the micelle corona because of the chain adopting a conformation more perpendicular to the core as the micelle size and density increased

Highly Selective and Sensitive Detection of Pb²⁺ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin-Functionalized Thermosensitive Ionic Microgels

Tetra­(4-pyridyl)­porphyrin (TPyP)-functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have a hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically respond to trace Pb²⁺ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb³⁺, Gd³⁺, Ce³⁺, La³⁺, Bi³⁺, Ba²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cr³⁺, K⁺, Na⁺, Li⁺, Al³⁺, Cu²⁺, Ag⁺, Cd²⁺, and Fe³⁺) by using UV–visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb²⁺ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb²⁺ in aqueous solution at 50 °C is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher order derivative spectrophotometry and is much lower than the U. S. EPA standard for the safety limit of Pb²⁺ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have a potential application in the detection of Pb²⁺ in real world samples, which give consistent results with those obtained by elemental analysis

Critical Domain Sizes of Heterogeneous Nanopattern Surfaces with Optimal Protein Resistance

To investigate protein-resistant surfaces with heterogeneous nanopatterns, V-shaped polymer brushes composed of a hydrophilic methoxypoly­(ethylene glycol) (mPEG) arm and a hydrophobic polystyrene (PS) or fluorinated poly­(methyl methacrylate) (PMMA-<i>b</i>-PFMA) arm were prepared, in which the surface structure and phase-separation behavior were controlled by altering the relative lengths of the two arms. The protein resistance of these amphiphilic brushes was better than that of pure poly­(ethylene glycol) (PEG) brushes, and when the domain size of the phase-separated structures was about twice the size of the protein molecules, the surfaces exhibited optimal protein repellence. At the same time, the amount of protein adsorption was well related to both the adhesion and the relative friction coefficient of the protein on the brush surface. A heterogeneous surface with phase-separated domains twice the size of protein molecules may be beneficial for minimizing protein adsorption through the synergistic effect of hydrophobic and water-soluble domains. These results provide an important way for designing and preparing protein-resistant materials with heterogeneous surfaces