Billion-fold rate enhancement of urethane polymerization via the photothermal effect of plasmonic gold nanoparticles

Recent Applications of Photothermal Conversion in Organic Synthesis

Photothermal conversion is a novel heating method that has emerged in recent years, wherein certain species can convert light to heat with great efficiency. These photothermal agents have shown immense promise for generating nanoscale thermal gradients under mild, visible light irradiation, providing a pathway for combining photochemistry with thermally driven reactivity. While this novel heating mechanism has been leveraged to great effect for applications such as photothermal therapeutics and steam water purification, it has seen limited use in organic synthesis. This outlook explores instances wherein the photothermal effect was used directly or as a synergistic component to drive organic reactions and postulates how it may be used moving forward.

Molecular Photothermal Conversion Catalyst Promotes Photocontrolled Atom Transfer Radical Polymerization

Photothermal conversion is a growing research area that promotes thermal transformations with visible light irradiation. However, few examples of dual photothermal conversion and catalysis limit the power of this phenomenon. Here, we take inspiration from nature's ability to use porphyrinic compounds for nonradiative relaxation to convert light into heat to facilitate thermal polymerization catalysis. We identify the photothermal conversion catalytic activity of a vitamin B12 derivative, heptamethyl ester cobyrinate (HME-Cob), to perform atom transfer radical polymerization (ATRP) under irradiation. Rapid polymerization are obtained under photothermal activation while maintaining good control over polymerization with the aid of a photoinitiator to enable light-induced catalyst regeneration. The catalyst exhibits exquisite temporal control in photocontrolled thermal polymerization. Ultimately, the activation of this complex is accessed across a broad range of wavelengths, including near-IR light, with excellent temporal control. This work showcases the potential of developing photothermal conversion catalysts.

Photothermal Conversion by Carbon Black Facilitates Aryl Migration by Photon-Promoted Temperature Gradients

The Newman Kwart Rearrangement (NKR) offers an efficient and high-yielding method for producing substituted thiophenols from phenols. While an industrially important protocol, it suffers from high activation energy barriers (35-43 kcal/mol), requiring the use of extreme temperatures (>200 °C) and specialty equipment. This report details a highly efficient and straightforward method for facilitating the NKR using photothermal conversion. This underused, unique reactivity pathway arises from the irradiation of nanomaterials that relax via a non-radiative decay pathway to generate intense thermal gradients. We show carbon black (CB) can be an inexpensive and abundant photothermal agent under visible light irradiation to achieve a facile NKR under mild conditions. The scope includes a wide array of stereo- and electronically diverse substrates with increasing difficulty of rearrangement, including BHT and BINOL as effective substrates. Furthermore, we demonstrate the unique application for temporal control in a thermal reaction and tunability of thermal gradients by modulating light intensity. Ultimately, photothermal conversion enables high-temperature reactions with simple, visible light irradiation.

Intrinsic Optical Properties and Emerging Applications of Gold Nanostructures

The collective oscillation of free electrons at the nanoscale surface of gold nanostructures is closely modulated by tuning the size, shape/morphology, phase, composition, hybridization, assembly, and nanopatterning, along with the surroundings of the plasmonic surface located at a dielectric interface with air, liquid, and solid. This review first introduces the physical origin of the intrinsic optical properties of gold nanostructures and further summarizes stimuli-responsive changes in optical properties, metal-field-enhanced optical signals, luminescence spectral shaping, chiroptical response, and photogenerated hot carriers. The current success in the landscape of nanoscience and nanotechnology mainly originates from the abundant optical properties of gold nanostructures in the thermodynamically stable face-centered cubic (fcc) phase. It has been further extended by crystal phase engineering to prepare thermodynamically unfavorable phases (e.g., kinetically stable) and heterophases to modulate their intriguing phase-dependent optical properties. A broad range of promising applications, including but not limited to full-color displays, solar energy harvesting, photochemical reactions, optical sensing, and microscopic/biomedical imaging, have fostered parallel research on the multitude of physical effects occurring in gold nanostructures.

Heterostructures of Cu2-xS/Cu2-xTe plasmonic semiconductors: disappearing and reappearing LSPR with anion exchange

Localized surface plasmon resonance (LSPR) of Cu_2-xS nanorods is quenched during the initial Cu_2-xS/Cu_2-xTe core/shell stage of anion exchange then returns as Cu_2-xTe progresses into the nanorod. Phase change within the core accounts for this behaviour illustrating the complexity emergent from anion exchange.

Direct sound printing

Photo- and thermo-activated reactions are dominant in Additive Manufacturing (AM) processes for polymerization or melting/deposition of polymers. However, ultrasound activated sonochemical reactions present a unique way to generate hotspots in cavitation bubbles with extraordinary high temperature and pressure along with high heating and cooling rates which are out of reach for the current AM technologies. Here, we demonstrate 3D printing of structures using acoustic cavitation produced directly by focused ultrasound which creates sonochemical reactions in highly localized cavitation regions. Complex geometries with zero to varying porosities and 280 μm feature size are printed by our method, Direct Sound Printing (DSP), in a heat curing thermoset, Poly(dimethylsiloxane) that cannot be printed directly so far by any method. Sonochemiluminescnce, high speed imaging and process characterization experiments of DSP and potential applications such as remote distance printing are presented. Our method establishes an alternative route in AM using ultrasound as the energy source.

Photo- and thermo-activated polymerization and melting processes are dominant in Additive Manufacturing (AM) while ultrasound activated sonochemical reactions have not been explored for AM so far. Here, the authors demonstrate 3D printing of structures using acoustic cavitation produced directly by focused ultrasound which creates sonochemical reactions in highly localized cavitation regions.

Photothermal polymerization of benzoxazines

Visible irradiation of mixtures of benzoxazine monomers and metal salt catalysts leads to extensive photothermal polymerization, which allows the preparation of complex polybenzoxazine features via photolithography.

Exploiting the upconversion luminescence, Lewis acid catalytic and photothermal properties of lanthanide-based nanomaterials for chemical and polymerization reactions

Lanthanide-based nanocrystals possess three unique physical properties that make them attractive for facilitating photoreactions, namely photon upconversion, Lewis acid catalytic activity and photothermal effect. When co-doped with suitable sensitizer and...

Demonstration of the photothermal catalysis of the Sabatier reaction using nickel nanoparticles and solar spectrum light

The promise of light-to-chemical energy conversion has led researchers to explore photo-thermal processes for chemical reactivity of nanoparticles. Previous work has examined particles as an element of a supported catalyst, but not as an unsupported nanopowder. We present a preliminary examination of a photo-thermal Sabatier reaction performed on suspended Ni nanoparticles. This new system performs a catalytic Sabatier reaction at lower bulk temperatures than reported for a standard reaction, driven by light. This result occurs only when the particles are suspended freely in a gaseous mixture; implying that particle isolation enhances photo-thermal catalysis.

Ultrathin Near-Infrared Light Activated Nano-Hotplate Catalyst

A combined photothermal-catalytic system that contains a single active element, without using different entities for separate roles (catalytic vs photothermal), is designed here for efficient catalytic reactions. Herein, ultrathin (sub-6 nm) rectangular-like KNdF₄ nanoplates consisting of 3-4 unit cell layers are prepared where the Nd³⁺ ions act as a Lewis acid catalyst. In addition, the nanoplates undergo light-to-heat conversion when irradiated with NIR light due to cross-relaxation and nonradiative relaxation processes from excited Nd³⁺ . The cyanosilylation of a series of ketones is performed using the nano-hotplate catalysts to give near quantitative yields of the cyanohydrin trimethylsilyl ethers. This is because of the high surface area-to-volume ratio of the thin nanoplates that provides a large number of surface Nd³⁺ catalytic sites for reaction. The reaction kinetics are enhanced by the photothermal effect, leading to the observed > 10-fold increase in product yields.

Ultrafast relaxation dynamics in bimetallic plasmonic catalysts

Combining a plasmonic metal, such as gold, with other popular catalysts, such as Ni or Pt, can extend its benefits to many energy-extensive reactions catalyzed by those metals. The efficiency of a plasmon-enhanced catalytic reaction is mainly determined by the light absorption cross section and the photoexcited charge carrier relaxation dynamics of the nanoparticles. We have investigated the charge carrier relaxation dynamics of gold/nickel (Au/Ni) and gold/platinum (Au/Pt) bimetallic nanoparticles. We found that the addition of Ni or Pt to gold can reduce light absorption in gold nanoparticles. However, electron-phonon coupling rates of Au/Ni and Au/Pt nanoparticles are significantly faster than that of pure Au nanoparticles. This is due to the fact that both Ni and Pt possess significantly larger electron-phonon coupling constants and higher densities of states near the Fermi level in comparison with Au. Additionally, the phonon-phonon coupling rate of bimetallic Au/Pt and Au/Ni nanoparticles was significantly different from that of pure gold nanoparticles, due to the acoustic impedance mismatch at the nanoparticle/substrate interface. Our findings provide important insights into the rational design of bimetallic plasmonic catalysts.

Light-Induced Convective Segregation of Different Sized Microparticles

Using computational modeling, we design a facile method for sorting particles of different sizes in a fluid-filled microchamber. The microchamber is inclined at an angle with respect to the horizontal direction and contains suspended gold nanoparticles as well as the microparticles. With the application of ultraviolet light, the heat generated by illuminating the gold nanoparticles gives rise to thermal buoyancy effects, which drive the flow of the fluid in the chamber. This thermally driven, convective flow can be tailored by varying the intensity of the imposed light and the concentration of the gold nanoparticles in the solution. The competition between the drag force imposed by the fluid flows and the gravitational forces acting on the different sized particles produces the separation of the particles along the chamber's bottom, inclined wall. The separation distance between the particles can be increased by increasing the angle of inclination and the relative difference in the particle sizes. This system provides a label-free, membrane-less, and low-cost approach for sorting particles vital to a wide range of applications.

Photothermal Effectiveness of Magnetite Nanoparticles: Dependence upon Particle Size Probed by Experiment and Simulation

The photothermal effect of nanoparticles has proven efficient for driving diverse physical and chemical processes; however, we know of no study addressing the dependence of efficacy on nanoparticle size. Herein, we report on the photothermal effect of three different sizes (5.5 nm, 10 nm and 15 nm in diameter) of magnetite nanoparticles (MNP) driving the decomposition of poly(propylene carbonate) (PPC). We find that the chemical effectiveness of the photothermal effect is positively correlated with particle volume. Numerical simulations of the photothermal heating of PPC supports this observation, showing that larger particles are able to heat larger volumes of PPC for longer periods of time. The increased heating duration is likely due to increased heat capacity, which is why the volume of the particle functions as a ready guide for the photothermal efficacy.

Photothermal Nanoparticle Initiation Enables Radical Polymerization and Yields Unique, Uniform Microfibers with Broad Spectrum Light

Photothermal processes are utilized across a variety of fields, from separations to medicine, and are an area of active research. Herein, the action of a solar simulator upon carbon black nanoparticles is shown to result in photothermally initiated chain-growth polymerization of methyl acrylate, butyl acrylate, and methyl methacrylate initiated by benzoyl peroxide. With use of methyl acrylate as the model system, products from this reaction are shown to be apparently indistinguishable on the molecular level, but result in unique microstructures relative to the thermal controls. The relative contribution of bands of the UV/visible spectrum to the polymerization initiation show that red/infrared wavelengths are most important for the initiation to occur. Kinetic analysis of the initiator homolysis indicate that the apparent reaction rate is accelerated in the photothermal condition.

On-demand curing of polydimethylsiloxane (PDMS) using the photothermal effect of gold nanoparticles

The photothermal effect of gold nanoparticles (AuNPs) produces extremely localized heat that can be harnessed to drive large scale chemical reactions by simultaneously generating many individual reactions on the nanoscale. We use the photothermal effect to enhance the curing rate of polydimethylsiloxane (PDMS) by a factor of 4.9 × 10⁹. Photothermal curing occurs via crosslinking reactions between vinyl and Si-H groups of the pre-polymer, and the course of the reaction was followed by monitoring the disappearance of infrared bands associated with these functional groups. Using mass spectroscopy, we verify that the major polymer m/z peaks are identical for both traditionally and photothermally cured polymers, indicating that the photothermal effect of AuNPs is an effective way in which to supply on-demand curing of PDMS.

White-light-emitting magnetite nanoparticle-polymer composites: photonic reactions of magnetic multi-granule nanoclusters as photothermal agents

Magnetite nanoparticles combined with polymers produce white-light emission under multiphoton laser irradiation. Understanding the photonic reaction in magnetite-polymer composites is critical for application of magnetite NPs as photothermal agents. Laser irradiated magnetite nanoparticle-poly(methyl methacrylate) (PMMA) composites exhibit fluorescence due to the carbon double-bond formation resulting from the oxidation of the PMMA.

Oriented assembly of invisible probes: towards single mRNA imaging in living cells

Due to the complexity of biological systems and the ultralow concentration of analytes, improving the signal-to-noise ratio and lowering the limit of detection to allow highly sensitive detection is key to biomolecule analysis, especially intracellular analysis. Here, we present a method for highly sensitive imaging of mRNA in living cells by using novel invisible oriented probes to construct a turn-on signal generation mechanism from zero background. Two DNA probes (S1 and S2) are asymmetrically modified on two small gold nanoparticles (AuNPs) with a diameter of 20 nm. The hybridization of the two DNA probes with a single target mRNA leads to the formation of an AuNP dimer which shows a prominent plasmonic coupling effect. It generates a strong scattering signal from zero-background under a dark-field spectral analysis system. The unique design of the oriented assembly dimer has the ability to easily discriminate the target signal from the inherent cellular background noise in intracellular detection, thus making this approach a valuable technique for imaging single survivin mRNA and monitoring the distribution of survivin mRNA in tumor cells.

Solvent Effects on the Photothermal Regeneration of CO2 in Monoethanolamine Nanofluids

A potential approach to reduce energy costs associated with carbon capture is to use external and renewable energy sources. The photothermal release of CO2 from monoethanolamine mediated by nanoparticles is a unique solution to this problem. When combined with light-absorbing nanoparticles, vapor bubbles form inside the capture solution and release the CO2 without heating the bulk solvent. The mechanism by which CO2 is released remained unclear, and understanding this process would improve the efficiency of photothermal CO2 release. Here we report the use of different cosolvents to improve or reduce the photothermal regeneration of CO2 captured by monoethanolamine. We found that properties that reduce the residence time of the gas bubbles (viscosity, boiling point, and convection direction) can enhance the regeneration efficiencies. The reduction of bubble residence times minimizes the reabsorption of CO2 back into the capture solvent where bulk temperatures remain lower than the localized area surrounding the nanoparticle. These properties shed light on the mechanism of release and indicated methods for improving the efficiency of the process. We used this knowledge to develop an improved photothermal CO2 regeneration system in a continuously flowing setup. Using techniques to reduce residence time in the continuously flowing setup, such as alternative cosolvents and smaller fluid volumes, resulted in regeneration efficiency enhancements of over 200%.