Teaching Old Polymers New Tricks: Improved Synthesis and Anomalous Crystallinity for a Lost Semi-Fluorinated Polyaryl Ether via Interfacial Polymerization of Hexafluoroacetone Hydrate and Diphenyl Ether.

A practical and direct electrophilic polymerization of hexafluoroacetone hydrate with diphenyl ether toward the preparation of semi-fluorinated polyaryl ethers (PAE) is reported. Electrophilic aromatic substitution (EAS) polymerization under interfacial conditions with phase transfer catalyst (Aliquat 336) proceeds in trifluoromethanesulfonic anhydride by generation of trifluoromethanesulfonic acid and the protonated hexafluoroacetone (HFA) in situ affording 1,1,1,3,3,3-hexafluoroisopropylidene (6F) PAE with high regioselectivity (4,4'-DPE) and high molecular weight (≈60 kDa). Although first reported in a 1966 US Patent by DuPont using harsh conditions, improved synthetic methods or modern characterization has not been disclosed until now. Despite the presence of the 6F group, known to impart disordered morphology, this simple semi-fluorinated PAE exhibits anomalous crystallinity with polymorphic melting points (Tm ) ranging from 230-309 °C, high solubility in common organic solvents, a glass transition (Tg ) of 163 °C, and thermo-oxidative stability above 500 °C. Tough optically clear films prepared from solution give transmittance higher than 90% throughout the visible region. Synthesis, mechanistic aspects, and characterization including surface and dielectric properties are discussed.

Scattering and absorption differ drastically in their inner filter effects on fluorescence, resonance synchronous, and polarized resonance synchronous spectroscopic measurements.

The sample inner filter effect (IFE) induces spectral distortion and affects the linearity between intensity and analyte concentration in fluorescence, Raman, surface enhanced Raman, and Rayleigh light scattering measurements. Existing spectrofluorometric-based measurements treat light scattering and absorption identically in their sample IFEs. Reported herein is the finding that photon scattering and absorption differ drastically in inducing the sample IFE in Stokes-shifted fluorescence (SSF) spectrum, resonance synchronous spectrum (RS2), and the polarized resonance synchronous spectrum (PRS2) measurements. Absorption with an absorption extinction as small as 0.05 imposes significant IFE on SSF, RS2, and PRS2 measurements. However, no significant IFE occurs even when the scattering extinction is as high as 0.9. For samples that both absorb and scatter light, one should decompose their UV-vis extinction spectra into absorption and scattering extinction component spectra before correcting the sample IFE. An iteration PRS2 method was introduced for the experimental decoupling of the photon absorption and scattering contribution. The methodology presented in this work can be easily implemented by researchers with access to one conventional UV-vis spectrophotometer and one spectrofluorometer equipped with a pair of excitation and detection polarizers. This work should be of broad significance in chemical research given the popularity of fluorescence spectroscopy in material characterization applications.

Determining the Liquid Light Scattering Cross Section and Depolarization Spectra Using Polarized Resonance Synchronous Spectroscopy.

Rayleigh scattering is a universal material property because all materials have nonzero polarizability. Reliable quantification of the material light scattering cross section in the liquid phase and its depolarization spectra is, however, challenging due to a host of sample and instrument issues. Using the recently developed polarized resonance synchronous spectroscopic method, we reported the light scattering cross section and depolarization spectra measured for a total of 29 liquids including water, methanol, ethanol, 1-propanol, 1-butanol, dimethylformamide, carbon disulfide, dimethyl sulfoxide, hexane and two hexane isomers (3-methylpentane and 2,3-dimethylbutane), tetrahydrofuran, cyclohexane, acetonitrile, pyridine, chloromethanes including di-, tri, tetrachloromethane, acetone, benzene and eight benzene derivatives (toluene, fluorobenzene, 1,2-, 1,3-, and 1,4-difluorobenzene, chlorobenzene, 1,2- and 1,3-dichlorobenzene, and nitrobenzene). The solvent light scattering depolarization is wavelength-independent for the model solvents, and it varies from 0.023 ± 0.011 for CCl4 to 0.619 ± 0.022 for nitrobenzene. The light scattering cross-section spectra can be approximated with the function of σ(λ) = αλ-4 with the α value varying from 7.2 ± 0.2 × 10-45 cm6 for water to a maximum of 8.5 ± 0.6 × 10-43 cm6 for nitrobenzene. Structural isomerization has no significant effect on either the depolarization or the scattering cross sections for both hexanes and difluorobenzene isomers. This work represents the most comprehensive experimental study on liquid light scattering features. The insight from this work should be important for understanding the correlation between the material structure and optical properties. The described method can be readily implemented by researchers with access to conventional spectrofluorometers equipped with excitation and detection polarizers.

Kinetic spectroscopic quantification using two-step chromogenic and fluorogenic reactions: From theoretical modeling to experimental quantification of biomarkers in practical samples.

Kinetic chromogenic (CG) and fluorogenic (FG) quantification deduces analyte concentration based on the reaction rate between the CG/FG probe and its targeted molecule. Little progress has been made in the past half century in either the theory or the applications of the kinetic spectroscopic quantification methods. Current kinetic CG/FG quantification is limited only to a subset of CG/FG reactions that can be approximated as the single-step process, and more problematically, to research samples with no matrix interferences. Reported herein is a kinetic quantification model established for multistep CG/FG reactions and a proof-of-concept demonstration of direct kinetic FG quantification of biomarkers in practical samples. The kinetic spectral intensity of the CG/FG reactions with two rate-limiting steps comprises three temporal regions: an accelerating period where rate of signal change is increasingly rapid, a linear region where the rate of signal change is approximately constant, and a deceleration region where the rate of signal increase becomes progressively small. Kinetic quantification is performed through simple linear-curve-fitting of the kinetic signal in its linear time-course region. The theoretical model is validated with the dual CG/FG 2-thiobarbituric acid (TBA) and malondialdehyde (MDA) reaction. Proof-of-concept kinetic spectroscopic quantification of analytes in practical samples is demonstrated with the FG quantification of MDA in canned chicken. The only sample preparation is bench-top centrifugation followed by two sequential syringe filtrations. The total kinetic FG assay time is less than 10 min, more than 10 times more efficient than the current equilibrium-based MDA assay. The theoretical model and the measurement design strategies offered by this work should help transform the current kinetic spectroscopic quantification from a niche research tool to an indispensable technique for time-sensitive applications.

Linear Extrapolation of the Analyte-Specific Light Scattering and Fluorescence Depolarization in Turbid Samples.

Anisotropy and depolarization are two interconvertible parameters in fluorescence and light scattering spectroscopy that describe the polarization distribution of emitted and scattered photons generated with linearly polarized excitation light. Whereas anisotropy is more frequently used in fluorescence literature for studying association/dissociation of fluorophore-bearing reagents, depolarization is more popular in the light-scattering literature for investigating the effect of scatterers' geometries and chemical compositions. Presented herein is a combined computational and experimental study of the scattering and fluorescence depolarization enhancement induced by light scattering in turbid samples. The most important finding is that sample light scattering and fluorescence depolarization increases linearly with sample light-scattering extinction. Therefore, one can extrapolate the analyte-specific scattering and fluorescence depolarization through linear curve fitting of the sample light scattering and fluorescence depolarization as a function of the sample concentration or the path length of the sampling cuvettes. An example application of this linear extrapolation method is demonstrated for quantifying the fluorophore-specific fluorescence depolarization and consequently its anisotropy for an aggregation-induced-emission sample. This work should be important for a wide range of macromolecular, supramolecular, and nanoscale fluorescent materials that are often strong light scatterers due to their large sizes.

Surface Plasmon Resonance, Formation Mechanism, and Surface Enhanced Raman Spectroscopy of Ag+-Stained Gold Nanoparticles.

A series of recent works have demonstrated the spontaneous Ag+ adsorption onto gold surfaces. However, a mechanistic understanding of the Ag+ interactions with gold has been controversial. Reported herein is a systematic study of the Ag+ binding to AuNPs using several in-situ and ex-situ measurement techniques. The time-resolved UV-vis measurements of the AuNP surface plasmonic resonance revealed that the silver adsorption proceeds through two parallel pseudo-first order processes with a time constant of 16(±2) and 1,000(±35) s, respectively. About 95% of the Ag+ adsorption proceeds through the fast adsorption process. The in-situ zeta potential data indicated that this fast Ag+ adsorption is driven primarily by the long-range electrostatic forces that lead to AuNP charge neutralization, while the time-dependent pH data shows that the slow Ag+ binding process involves proton-releasing reactions that must be driven by near-range interactions. These experimental data, together with the ex-situ XPS measurement indicates that adsorbed silver remains cationic, but not as a charged-neutral silver atom proposed by the anti-galvanic reaction mechanism. The surface-enhanced Raman activities of the Ag+-stained AuNPs are slightly higher than that for AuNPs, but significantly lower than that for the silver nanoparticles (AgNPs). The SERS feature of the ligands on the Ag+-stained AuNPs can differ from that on both AuNPs and AgNPs. Besides the new insights to formation mechanism, properties, and applications of the Ag+-stained AuNPs, the experimental methodology presented in this work can also be important for studying nanoparticle interfacial interactions.

Facile displacement of citrate residues from gold nanoparticle surfaces.

The stability of citrate-residues on gold nanoparticles (AuNPs) against ligand displacement has been controversial. Using AuNPs synthesized with deuterated citrate in combination with in-situ surface-enhanced Raman spectroscopic (SERS) analysis, we report that both citrate-residues and solution impurities can be simultaneously adsorbed onto citrate-reduced AuNPs in solution. The citrate-residues can be readily displaced from AuNPs by organosulfur such as organothiols (RS-H), organodisuflide (R-S-S-R), and non-specific ligands including halides and adenine. Control experiments conducted on high-purity gold films sputter-coated onto silicone substrates indicate that air-borne and solvent-borne impurities rapidly contaminate the gold surfaces. Head-to-head comparison of ligand-functionalized AuNPs by in-situ SERS measurements verses those from the ex-situ X-ray photoelectron spectroscopic (XPS) measurements reveal that the impurity deposition can compromise the reliability of ex-situ XPS identification of surface adsorbates on AuNPs in solution. These insights are of general significance to nanoscience research given the broad interest in nanoparticle surface chemistry and popularity of XPS for nanomaterial characterizations.