Raman Microspectroscopic Study on Polymerization and Degradation Processes of a Diacetylene Derivative at Surface Enhanced Raman Scattering Active Substrates. 2. Confocal Raman Microscopic Observation of Polydiacetylene Adsorbed on Active Sites

Topochemical polymerizations for the solid-state synthesis of organic polymers

Topochemical polymerizations are solid-state reactions driven by the alignment of monomers in the crystalline state. The molecular confinement in the monomer crystal lattice offers precise control over the tacticity, packing and crystallinity of the polymer formed in the topochemical reaction. As topochemical reactions occur under solvent- and catalyst-free conditions, giving products in high yield and selectivity/specificity that do not require tedious chromatographic purification, topochemical polymerizations are highly attractive over traditional solution-phase polymer synthesis. By this method, polymers having sophisticated structures and desired topologies can be availed. Often, such ordered packing confers attractive properties to the topochemically-synthesized polymers. Diverse categories of topochemical polymerizations are known, such as polymerizations via [2+2], [4+4], [4+2], and [3+2] cycloadditions, and polymerization of diynes, triynes, dienes, trienes, and quinodimethanes, each of which proceed under suitable stimuli like heat, light or pressure. Each class of these reactions requires a unique packing arrangement of the corresponding monomers for the smooth reaction and produces polymers with distinct properties. This review is penned with the intent of bringing all the types of topochemical polymerizations into a single platform and communicating the versatility of these lattice-controlled polymerizations. We present a brief history of the development of each category and comprehensively review the topochemical synthesis of fully-organic polymers reported in the last twenty years, particularly in crystals. We mainly focus on the various molecular designs and crystal engineering strategies adopted to align monomers in a suitable orientation for polymerization. Finally, we analyze the current challenges and future perspectives in this research field.

A Simple Way to Achieve Legible and Local Controllable Patterning for Polymers Based on a Near-Infrared Pulsed Laser

This study developed a simple way to achieve legible and local controllable patterning for polymers based on a near-infrared (NIR) pulsed laser. The polycarbonate-coated nano antimony-doped tin oxide (nano ATO) was designed as a core-shell structure that was tailored to be responsive to a 1064 nm NIR laser. The globular morphology of polycarbonate-coated nano ATO with a diameter of around 2-3 μm was observed by scanning electron microscopy and transmission electron microscopy. This core-shell structure combined the excellent photothermal conversion efficiency of nano ATO and the high char (carbon) residue of polycarbonate. The X-ray photoelectron spectroscopy results of a polymer-patterning plate after laser irradiation demonstrated that, through local controlled photochromism, the well-defined legible patterns can be fabricated on the polymer surfaces contribute to the synergistic effect consisting of polycarbonate carbonization and nano ATO photothermal conversion. Furthermore, polymers doped with a minimal content of polycarbonate-coated nano ATO can achieve a remarkable patterning effect. This novel laser-patterning approach will have wide promising applications in the field of polymer NIR pulsed-laser patterning.

Laser-Marking Mechanism of Thermoplastic Polyurethane/Bi2O3 Composites

Using bismuth oxide (Bi2O3) as a laser-marking additive and thermoplastic polyurethane (TPU) as the matrix, TPU/Bi2O3 composite materials were prepared by melt blending in a torque rheometer. The sheet samples prepared from the TPU/Bi2O3 composites were treated in air by scanning with a neodymium-doped yttrium aluminum garnet (Nd: YAG) pulsed laser beam at a wavelength of 1064 nm. Compared with the pure TPU sample, the laser-marked composite samples exhibited differences in marking contrast as the Bi2O3 content increased from 0.1% to 1.0% based on stereomicroscope analysis. Scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, thermogravimetry analysis, and X-ray diffraction were used to characterize the laser-marked surface material of the composite samples. Furthermore, a mechanism for the laser-effected darkening of the TPU/Bi2O3 composites was proposed. The results herein indicated that the addition of the Bi2O3 laser-sensitive additive to TPU resulted in laser darkening of the TPU/Bi2O3 composites. The marking contrast and visual appearance of the surface of the TPU/Bi2O3 composites after laser irradiation was due to a synergistic effect consisting of carbonization via TPU pyrolysis and reduction of Bi2O3 to black bismuth metal.

Vibrational and theoretical study of diacetylenic acids

Three selected diacetylenic acids (DAs) with side-chains of various length (CH3-(CH2)m-C≡C-C≡C(CH2)n-COOH, where m=7, 9, 11, and n=3, 8) were analyzed using vibrational spectroscopy and quantum-chemical calculations. The conformational analysis was followed by potential energy distribution (PED) calculations to gain deeper insight into their FT-Raman and FT-IR spectra. The analysis was focused on spectral features of the diacetylene system sensitive to the substitution. In particular, the electron donor-acceptor properties of the substituent and the influence of side-chain length were studied. FT-IR spectra were measured by using two techniques, i.e. transmission (with KBr substrate) and Attenuated Total Reflection (ATR), and the latter seems to be less adequate for DAs measurements because the bands in the fingerprint region as well as the ν(C≡C)as mode are relatively of low intensity. Additionally, polymerization process of DAs was recognized using FT-Raman spectroscopy and strong and well-separated bands of diacetylenic polymers. Temperature and exposure to the sunlight are the factors of an important influence on the polymerization process of DAs. Since the investigated DAs are carboxylic acids, the interpretation of experimental spectra was performed on the basis of monomer and dimer calculations.

Self-assembly of graphene oxide at interfaces

Due to its amphiphilic property, graphene oxide (GO) can achieve a variety of nanostructures with different morphologies (for example membranes, hydrogel, crumpled particles, hollow spheres, sack-cargo particles, Pickering emulsions, and so on) by self-assembly. The self-assembly is mostly derived from the self-concentration of GO sheets at various interfaces, including liquid-air, liquid-liquid and liquid-solid interfaces. This paper gives a comprehensive review of these assembly phenomena of GO at the three types of interfaces, the derived interfacial self-assembly techniques, and the as-obtained assembled materials and their properties. The interfacial self-assembly of GO, enabled by its fantastic features including the amphiphilicity, the negatively charged nature, abundant oxygen-containing groups and two-dimensional flexibility, is highlighted as an easy and well-controlled strategy for the design and preparation of functionalized carbon materials, and the use of self-assembly for uniform hybridization is addressed for preparing hybrid carbon materials with various functions. A number of new exciting and potential applications are also presented for the assembled GO-based materials. This contribution concludes with some personal perspectives on future challenges before interfacial self-assembly may become a major strategy for the application-targeted design and preparation of functionalized carbon materials.

Facile preparation of polydiacetylene-based uniform porous fluorescent microspheres for potential immunoassay applications

Fluorescent microspheres are prepared by loading PDA onto the substrate microspheresviaa self-assembled vesicle precursor pathway.

Improving correlated SERS measurements with scanning electron microscopy: an assessment of the problem arising from the deposition of amorphous carbon

For surface-enhanced Raman scattering (SERS) substrates with nonspherical symmetry, it is critical to correlate spectroscopy measurements with imaging by scanning electron microscopy (SEM). However, the deposition of carbon resulting from e-beam exposure during SEM imaging contaminates the surface of nanoparticles, potentially preventing their further functionalization with Raman probe molecules. In addition, the deposited carbon leads to unwanted background SERS signals. In this study, we systematically investigated the deposition of carbon during SEM imaging and examined how it affects the functionalization of nanoparticles with probe molecules and impacts the detection of SERS signals. Significantly, we found that the carbon could be removed or replaced from the surface of Ag nanoparticles through chemical or physical means, rendering the nanoparticles the capability for correlated SEM/SERS studies.

A strategy to prevent signal losses, analyte decomposition, and fluctuating carbon contamination bands in surface-enhanced Raman spectroscopy

Signal losses and fluctuating carbon contamination bands are "bottlenecks" in the application of surface-enhanced Raman spectroscopy (SERS) for reliable chemical analysis. They originate mainly from prolonged laser irradiation of the sample during data collection, which causes analyte decomposition and/or loss of the enhancing capabilities of the adsorption site. In this work, a laser illumination/signal collection technique, the "multiple points collection" (MPC) method is introduced to circumvent these problems. The MPC method is based on the use of a pair of galvanic mirrors to scan the laser beam rapidly and steadily across the sample surface. Each position is irradiated for <10 mus, at a rate of approximately 0.5 Hz. The SER spectrum is obtained by summing the signals collected from a large array of non-overlapping sample points. The MPC is compared with the conventional "single point collection" method, in which the laser beam is statically focused onto a particular spot and the scattered signals acquired. The MPC has the following advantages: (1) illumination and collection efficiencies are not compromised, (2) signal losses originating from analyte decomposition and/or alteration of the enhancing capabilities of the adsorption site are avoided, (3) high-quality SER spectra for analytes such as biomolecules and dipicolinic acid (a common marker for bacteria spores) can be easily obtained, and (4) the occurrence of broad amorphous carbon bands and the commonly observed temporal fluctuations in SERS are prevented. The success of the MPC is attributed to the reduction of local sample heating, as the time interval between the laser irradiations of a spot is much longer than the actual irradiation time itself.

Re-examining the origins of spectral blinking in single-molecule and single-nanoparticle SERS

Single metal nanoparticles and nanoaggregates are known to emit intense bursts of surface-enhanced Raman scattering (SERS) in an intermittent on and off fashion. The characteristic "blinking" timescales range from milliseconds to seconds. Here we report detailed temperature dependence (both heating and cooling) and light-intensity studies to further examine the origins of this intriguing phenomenon. The results indicate that blinking SERS contains both a thermo-activated component and a light-induced component. Several lines of evidence suggest that the observed fluctuations are caused by thermally activated diffusion of individual molecules on the particle surface, coupled with photo-induced electron transfer and structural relaxation of surface active sites or atomic-scale roughness features.

Role of O2 in Inducing Intensive Fluctuations of Surface-Enhanced Raman Scattering Spectra

Confocal Raman microscopic measurements were performed on silver electrodes covered with hydrogenated amorphous carbon (a-C:H). When short accumulation time was used, the subsequently measured surface-enhanced Raman scattering (SERS) spectra exhibited fluctuations. As previously reported for other systems, the intensity of fluctuations of SERS spectra significantly decreases if O2 was removed from the ambient medium. In this contribution we show that intensive SERS fluctuations can be also observed for a-C:H/Ag samples immersed in the deoxygenated electrolyte after applying a negative potential pulse to the silver electrode. It means that the O2-mediated Burstein mechanism of SERS fluctuations, which has been previously proposed to explain the SERS O2 effect, is not adequate for these results. We suggest that oxygen chemisorbed on the silver surface decreases the average strength of the interaction between a-C:H clusters and the metal surface (and hence the speed of movement of a-C:H clusters across the metal surface) and that the SERS O2 effect should be rather explained using the "classical" model of SERS fluctuations, in which fluctuations are interpreted as a result of the thermally activated diffusion of carbon segments in and out of the SERS "hot spots". A numerical algorithm for modeling of the fluctuations of SERS intensity has been proposed, and some example simulations of SERS fluctuations have been carried out. For the first time, strongly fluctuating bands due to the stretching vibrations of significantly weakened C-H bonds have been identified.