Impact of Chemical and Physical Properties of Organic Solvents on the Gas and Hydrogen Formation during Laser Synthesis of Gold Nanoparticles

Laser ablation in liquids has been established as a scalable preparation method of nanoparticles for various applications. Particularly for materials prone to oxidation, it is established to suppress oxidation by using organic solvents as a liquid medium. While this often functionalizes the nanoparticles with a carbon shell, the related chemical processes that result from laser-induced decomposition reactions of the organic solvents remain uncertain. Using a systematic series of C₆ solvents complemented by n-pentane and n-heptane during the nanosecond laser ablation of gold, the present study focuses on the solvent-dependent influence on gas formation rates, nanoparticle productivity, and gas composition. Both the permanent gas and hydrogen formation was found to be linearly correlated with ablation rate, ΔH_vap , and pyrolysis activation energy. Based on this, a decomposition pathway linked to pyrolysis is proposed allowing the deduction of first selection rules for solvents that influence the formation of carbon or permanent gases.

Color-Tunable Multifunctional Excited-State Intramolecular Proton Transfer Emitter: Stimulated Emission of a Single Dye

The excited-state intramolecular proton transfer chromophores were regarded as good materials for laser action generation due to their inherent four-level photocycle. The excitation-dependent properties of these compounds enable light amplification from two distinct forms: both enol and keto, making it possible to obtain dual fluorescence emission. Herein, we report that a third option is possible for the first time stimulated emission was realized with a deprotonated ESIPT molecule based on a novel rigidified 2-(2'-hydroxyphenyl)benzothiazole derivative, triggering the possibility to fabricate real-time tunable active material. Through the rational engineering of the ratio of each emissive species, a red-green-blue device was fabricated with the possibility of white light generation. The degenerated two-wave mixing setup was applied to construct a continuously tunable distributed feedback laser.

Laser Induced Thermal Effect on the Polymerization Behavior in Upconversion Particle Assisted Near-Infrared Photopolymerization

Laser induced thermal effect is inevitable in upconversion particle assisted near-infrared polymerization (UCAP). Herein, the influence of thermal effects on the polymerization behavior are investigated. The effects of up-conversion particles content and NIR laser intensity on the polymerization rate and surface oxygen inhibition were systematically investigated, and the temperature evolution and complex viscosity changes in the polymerization system during the polymerization process were also monitored. In addition, polymerization experiments conducted on a controlled temperature platform were used to study the effect of NIR heating on the polymerization behavior. The results show that the near-infrared thermal effect promotes the polymerization reaction, but also causes severe oxygen inhibition which has an adverse effect on polymerization. Finally, NIR curing materials with enhanced mechanical properties than those of conventional UV curing materials were obtained.

From Nanosecond Photochemistry to Optical Force Chemistry: My Journey

Laser was invented in 1960 and soon introduced to chemistry research. We started time-resolved spectroscopy and photochemistry and initial trial was focused to nanosecond and then picosecond electronic absorption spectroscopy for studying molecular electronic excited states, charge separation in molecular complexes, and intermolecular electron transfer in solution. We considered that not only time-resolved but also space-resolved chemistry would be important for future laser-based chemistry and combined pulsed lasers with optical microscopes. Spectroscopy, photochemistry, ablation, and spatial arrangement of single microparticles and microdroplets in solution were carried out. Further we shifted from micro to nano and opened a new field covering spectroscopy, ablation, phase transition, crystallization, patterning, and fabrication. The progress is summarized and discussed as time-resolved nano spectroscopy, ablation nano dynamics, and optical force chemistry.

Laser Access to Quercetin Radicals and Their Repair by Co-antioxidants

We have demonstrated the feasibility and ease of producing quercetin radicals by photoionization with a pulsed 355 nm laser. A conversion efficiency into radicals of 0.4 is routinely achieved throughout the pH range investigated (pH 2-9), and the radical generation is completed within a few ns. No precursor other than the parent compound is needed, and the ionization by-products do not interfere with the further fate of the radicals. With this generation method, we have characterized the quercetin radicals and studied the kinetics of their repairs by co-antioxidants such as ascorbate and 4-aminophenol. Bell-shaped pH dependences of the observed rate constants reflect opposite trends in the availability of the reacting protonation forms of radical and co-antioxidant and even at their maxima mask the much higher true rate constants. Kinetic isotope effects identify the repairs as proton-coupled electron transfers. An examination of which co-antioxidants are capable of repairing the quercetin radicals and which are not confines the bond dissociation energies of quercetin and its monoanion experimentally to 75-77 kcal mol-1 and 72-75 kcal mol-1 , a much narrower interval in the case of the former than previously estimated by theoretical calculations.

Formic Acid, a Ubiquitous but Overlooked Component of the Early Earth Atmosphere

Terrestrial volcanism has been one of the dominant geological forces shaping our planet since its earliest existence. Its associated phenomena, like atmospheric lightning and hydrothermal activity, provide a rich energy reservoir for chemical syntheses. Based on our laboratory simulations, we propose that on the early Earth volcanic activity inevitably led to a remarkable production of formic acid through various independent reaction channels. Large-scale availability of atmospheric formic acid supports the idea of the high-temperature accumulation of formamide in this primordial environment.

Laser-Induced Wurtz-Type Syntheses with a Metal-Free Photoredox Catalytic Source of Hydrated Electrons

Upon irradiation with ns laser pulses at 355 nm, 2-aminoanthracene in SDS micelles readily produces hydrated electrons. These "super-reductants" rapidly attack substrates such as chloro-organics and convert them into carbon-centred radicals through dissociative electron transfer. For a catalytic cycle, the aminoanthracene needs to be restored from its photoionization by-product, the radical cation, by a sacrificial donor. The ascorbate monoanion can only achieve this across the micelle-water interface, but the monoanion of ascorbyl palmitate results in a fully micelle-contained regenerative electron source. The shielding by the micelle in the latter case not only increases the life of the catalyst but also strongly suppresses the interception of the carbon-centred radicals by the hydrogen-donating ascorbate moiety; and in conjunction with the high local concentrations effected by the pulsed laser, termination by radical dimerization thus dominates. We have obtained a complete and consistent picture through monitoring the individual steps and the assembled system by flash photolysis on fast and slow timescales, from microseconds to minutes; and in preparative studies on a variety of substrates, we have achieved up to quantitative dimerization with a turnover on the order of 1 mmol per hour.

Synthesis of Black Phosphorus Quantum Dots with High Quantum Yield by Pulsed Laser Ablation for Cell Bioimaging

Black phosphorus quantum dots (BPQDs), with an average diameter of about 6 nm and a height of about 1.1 nm, are successfully synthesized by means of a pulsed laser ablation (PLA) method in isopropyl ether (IPE) solvent. The photoluminescence PL quantum yield of the as-prepared sample is as high as 20.7 %, which is 3 times that of BPQDs prepared by means of probe ultrasonic exfoliation (approximately 7.2 %). The stable and blue-violet PL emission of the BPQDs is observed. It can be elucidated that electrons transit from the LUMO energy level to the HOMO energy level, as well as energy levels below the HOMO (H1 and H2). In addition, BPQDs are also utilized in bioimaging in HeLa cells, showing an intense and stable PL signal and excellent biocompatibility. Hence, this work indicates that the obtained BPQDs with high quantum yield and stable PL emission have great potential for biomedical applications, including biolabeling, bioimaging, and drug delivery.

Resveratrol Radical Repair by Vitamin C at the Micelle-Water Interface: Unexpected Reaction Rates Explained by Ion-Dipole Interactions

Repair reactions of lipophilic phenoxy radicals by hydrophilic co-antioxidants at model membranes are important for understanding the factors that govern the interactions between radical scavengers in biological systems. By using near-UV photoionization, we have selectively generated the phenoxy radical of the famous antioxidant resveratrol inside anionic (SDS), cationic (DTAC), or neutral (TX-100) micelles, as well as in homogeneous aqueous solution, and have compared its repairs in these media by the water-soluble co-antioxidants ascorbic acid and ascorbate monoanion. With all surfactants, these reactions are dynamic processes at the micelle-water interface. Whereas for the combinations ascorbate monoanion/ ionic micelle the repair rates can be rationalized by the Coulombic interactions, unexpected effects were observed with the neutral ascorbic acid and the charged micelles: for the anionic micelles, this repair is three times faster than in homogeneous solution, and two orders of magnitude faster than for the cationic micelles. Given that the repair by a concerted proton-electron transfer demands a coplanar arrangement of the resveratrol phenoxy centre sticking out into the Stern layer and the co-antioxidant hydroxy moiety approaching from the aqueous bulk, we explain these results by ion-dipole interactions: only at a negatively charged micellar surface does the direction of the large dipole moment of ascorbic acid lead to an orientation favourable for the repair.

Selection of a Single Isotope of Multiply Charged Xenon (A Xez+ , A=128-136, z=1-6) by Using a Bradbury-Nielsen Ion Gate

The inclusion of an ion gate in a tandem mass spectrometer allows a specific precursor ion to be selected, and the fragment ions are then used for structure analysis and to investigate chemical reactions. However, the performance of an ion gate has been judged simply by whether or not the target ion was selected. In this study, we designed, manufactured, constructed, and characterized a Bradbury-Nielsen ion gate (BNG). The actual ion selection ability, i.e. the gate function, of the BNG was measured for isotopes of Xe^z+ (z=1-6). The gate function of the BNG was 36.5±0.5 ns in width and 3-13 ns in rise and fall times. The BNG provides a simple way to select multiply charged molecular cations of small organic molecules as well as large molecules such as proteins and peptides.

Cavitation-Free Continuous-Wave Laser Ablation from a Solid Target to Synthesize Low-Size-Dispersed Gold Nanoparticles

Continuous wave (CW) radiation from a Yb-fiber laser (central wavelength 1064 nm, power 1-200 W) was used to initiate ablation of a gold target in deionized water and to synthesize bare (unprotected) gold nanoparticles. We show that the formed nanoparticles present a single low-size-dispersed population with a mean size of the order of 10 nm, which contrasts with previously reported data on dual populations of nanoparticles formed during pulsed laser ablation in liquids. The lack of a second population of nanoparticles is explained by the absence of cavitation-related mechanism of material ablation, which typically takes place under pulsed laser action on a solid target in liquid ambience, and this supposition is confirmed by plume visualization tests. We also observe a gradual growth of mean nanoparticle size from 8-10 nm to 20-25 nm under the increase of laser power for 532 nm pumping wavelength, whereas for 1064 nm pumping wavelength the mean size 8-10 nm is independent of radiation power. The growth of the nanoparticles observed for 532 nm wavelength is attributed to the enhanced target melting and splashing followed by additional heating due to an efficient excitation of plasmons over gold nanoparticles. Bare, low-size-dispersed gold nanoparticles are of importance for a variety of applications, including biomedicine, catalysis, and photovoltaics. The use of CW radiation for nanomaterial production promises to improve the cost efficiency of this technology.

Pulse-Width Dependence of the Cooling Effect on Sub-Micrometer ZnO Spherical Particle Formation by Pulsed-Laser Melting in a Liquid

Sub-micrometer spherical particles can be synthesized by irradiating particles in a liquid with a pulsed laser (pulse width: 10 ns). In this method, all of the laser energy is supposed to be spent on particle heating because nanosecond heating is far faster than particle cooling. To study the cooling effect, sub-micrometer spherical particles are fabricated by using a pulsed laser with longer pulse widths (50 and 70 ns). From the increase in the laser-fluence threshold for sub-micrometer spherical particle formation with increasing pulse width, it is concluded that the particles dissipate heat to the surrounding liquid, even during several tens of nanoseconds of heating. A particle heating-cooling model considering the cooling effect is developed to estimate the particle temperature during laser irradiation. This model suggests that the liquid surrounding the particles evaporates, and the generated vapor films suppress heat dissipation from the particles, resulting in efficient heating and melting of the particles in the liquid. In the case of small particle sizes and large pulse widths, the particles dissipate heat to the liquid without forming such vapor films.

Unraveling Cold Molecular Collisions: Stark Decelerators in Crossed-Beam Experiments

In the last two decades, enormous progress has been made in the manipulation of molecular beams. In particular, molecular decelerators have been developed with which advanced control over neutral molecules in a beam can be achieved. By using arrays of inhomogeneous and time-varying electric (or magnetic) fields, bunches of molecules can be produced with a tunable velocity, narrow velocity spreads, and almost perfect quantum-state purity. These monochromatic or "tamed" molecular beams are ideally suited to be used in crossed-molecular-beam scattering experiments. Here, we review the first generation of these "cold and controlled" scattering experiments that have been conducted in the last decade and discuss the prospects for this emerging field of research in the years to come.

Nanoporous Carbons Derived from Metal-Organic Frameworks as Novel Matrices for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) represents a powerful tool for the analysis of biomolecules, synthetic polymers, and even small organic compounds; its performances largely depend on the type of matrix materials utilized. Here, for the first time the employment of nanoporous carbons derived from metal-organic frameworks (MOFs) as novel matrices for SALDI-MS is demonstrated. The nanoporous carbons derived from MOFs not only circumvent the shortcomings of existing matrix materials but also demonstrate much higher efficiency of laser desorption/ionization for various compounds than any other nanoporous carbons reported so far. A new perspective for the development of matrix materials for SALDI-MS application is therefore provided.

In Situ Raman Monitoring of Silver(I)-Aided Laser-Driven Cleavage Reaction of Cyclobutane

The cyclobutane cleavage reaction is an important process and has received continuous interest. Herein, we demonstrate the visible laser-driven cleavage reaction of cyclobutane in crystal form by using in situ Raman spectroscopy. Silver(I) coordination-induced strain and thermal effects from the laser irradiation are the two main driving forces for the cleavage of cyclobutane crystals. This work may open up a new avenue for studying cyclobutane cleavage reactions, as compared to the conventional routes using ex situ techniques.

Tuning complex transition metal hydroxide nanostructures as active catalysts for water oxidation by a laser-chemical route

Diverse transition metal hydroxide nanostructures were synthesized by laser-induced hydrolysis in a liquid precursor solution for alkaline oxygen evolution reaction (OER). Several active OER catalysts with fine control of composition, structure, and valence state were obtained including (Lix)[Ni0.66Mn0.34(OH)2](NO3)(CO3) · mH2O, Lix[Ni0.67Co0.33(OH)2](NO3)0.25(ORO)0.35 · mH2O, etc. An operate overpotential less than 0.34 V at current density of 10 mA cm(-2) was achieved. Such a controllable laser-chemical route for assessing complex nanostructures in liquids opens many opportunities to design novel functional materials for advanced applications.

First highly efficient and photostable E and C derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) as dye lasers in the liquid phase, thin films, and solid-state rods

A new library of E- and C-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives has been synthesized through a straightforward protocol from commercially available BODIPY complexes, and a systematic study of the photophysical properties and laser behavior related to the electronic properties of the B-substituent group (alkynyl, cyano, vinyl, aryl, and alkyl) has been carried out. The replacement of fluorine atoms by electron-withdrawing groups enhances the fluorescence response of the dye, whereas electron-donor groups diminish the fluorescence efficiency. As a consequence, these compounds exhibit enhanced laser action with respect to their parent dyes, both in liquid solution and in the solid phase, with lasing efficiencies under transversal pumping up to 73 % in liquid solution and 53 % in a solid matrix. The new dyes also showed enhanced photostability. In a solid matrix, the derivative of commercial dye PM597 that incorporated cyano groups at the boron center exhibited a very high lasing stability, with the laser emission remaining at the initial level after 100 000 pump pulses in the same position of the sample at a 10 Hz repetition rate. Distributed feedback laser emission was demonstrated with organic films that incorporated parent dye PM597 and its cyano derivative. The films were deposited onto quartz substrates engraved with appropriate periodical structures. The C derivative exhibited a laser threshold lower than that of the parent dye as well as lasing intensities up to three orders of magnitude higher.

Photophysical and laser properties of cassettes based on a BODIPY and rhodamine pair

This work deals with the synthesis and the photophysical and laser properties of new BODIPY-rhodamine cassettes. These dyads differ in their rigid and conjugated spacer group (phenyl or acetylenephenyl) and in their linking positions (meta or para). The photophysical properties of these cassettes are controlled by the formation/opening of the spirolactone ring, which, in turn, switches off/on an energy-transfer process between the chromophores. Herein, we thoroughly describe the influence of the attached spacer group, as well as the distance and orientation between the donor-acceptor pair, on the excitation energy transfer. The observed fast dynamics and efficiency suggest that the process mainly takes place "through-bond", although the "through-space" mechanism also contributes to the whole process. As a result, efficient laser emission from the rhodamine is achieved upon excitation of the BODIPY, in particular for the cassette that contains an acetylenephenyl spacer group in a para disposition.

Optimizing the acquisition and analysis of confocal images for quantitative single-mobile-particle detection

Quantification of the fluorescence properties of diffusing particles in solution is an invaluable source of information for characterizing the interactions, stoichiometry, or conformation of molecules directly in their native environment. In the case of heterogeneous populations, single-particle detection should be the method of choice and it can, in principle, be achieved by using confocal imaging. However, the detection of single mobile particles in confocal images presents specific challenges. In particular, it requires an adapted set of imaging parameters for capturing the confocal images and an adapted event-detection scheme for analyzing the image. Herein, we report a theoretical framework that allows a prediction of the properties of a homogenous particle population. This model assumes that the particles have linear trajectories with reference to the confocal volume, which holds true for particles with moderate mobility. We compare the predictions of our model to the results as obtained by analyzing the confocal images of solutions of fluorescently labeled liposomes. Based on this comparison, we propose improvements to the simple line-by-line thresholding event-detection scheme, which is commonly used for single-mobile-particle detection. We show that an optimal combination of imaging and analysis parameters allows the reliable detection of fluorescent liposomes for concentrations between 1 and 100 pM. This result confirms the importance of confocal single-particle detection as a complementary technique to ensemble fluorescence-correlation techniques for the studies of mobile particle.

'Laser chemistry' synthesis, physicochemical properties, and chemical processing of nanostructured carbon foams

Laser ablation of selected coordination complexes can lead to the production of metal-carbon hybrid materials, whose composition and structure can be tailored by suitably choosing the chemical composition of the irradiated targets. This 'laser chemistry' approach, initially applied by our group to the synthesis of P-containing nanostructured carbon foams (NCFs) from triphenylphosphine-based Au and Cu compounds, is broadened in this study to the production of other metal-NCFs and P-free NCFs. Thus, our results show that P-free coordination compounds and commercial organic precursors can act as efficient carbon source for the growth of NCFs. Physicochemical characterization reveals that NCFs are low-density mesoporous materials with relatively low specific surface areas and thermally stable in air up to around 600°C. Moreover, NCFs disperse well in a variety of solvents and can be successfully chemically processed to enable their handling and provide NCF-containing biocomposite fibers by a wet-chemical spinning process. These promising results may open new and interesting avenues toward the use of NCFs for technological applications.