Dynamics of Ion Assembly in Solution: 2DIR Spectroscopy Study of LiNCS in Benzonitrile

The solute−solvent interaction dictates the equilibrium structure of ionic assemblies in solution, ranging from free ions, ion pairs, ion-pair dimers, and ion-pair tetramers to nanoparticles and ionic crystals. We use two-dimensional infrared (2DIR) spectroscopic measurements of the antisymmetric CN stretch of thiocyanate (NCS⁻) to study the equilibrium chemical exchange between the spectrally distinct ion pair and the ion-pair dimer in benzonitrile. We observe this chemical exchange to occur with a 36 ± 4 ps time constant. Our measurement indicates that 2DIR will provide a useful tool for investigating the dynamics of ion assembly in solution

Dynamics of Solvent-Mediated Electron Localization in Electronically Excited Hexacyanoferrate(III)

We have used polarization-resolved UV pump–mid-IR probe spectroscopy to investigate the dynamics of electron hole localization for excited-state ligand-to-metal charge-transfer (LMCT) excitation in Fe­(CN)₆^3–. The initially generated LMCT excited state has a single CN-stretch absorption band with no anisotropy. This provides strong evidence that this initial excited state preserves the octahedral symmetry of the electronic ground state by delocalizing the ligand hole in the LMCT excited state on all six cyanide ligands. This delocalized LMCT excited state decays to a second excited state with two CN-stretch absorption bands. We attribute both peaks to a single excited state because the formation time for both peaks matches the decay time for the delocalized LMCT excited state. The presence of two CN-stretch absorption bands demonstrates that this secondary excited state has lower symmetry. This observation, in conjunction with the solvent-dependent time constant for the formation of the secondary excited state, leads us to conclude that the secondary excited state corresponds to a LMCT state with a localized ligand hole

Imaging Laser-Triggered Drug Release from Gold Nanocages with Transient Absorption Lifetime Microscopy

Nanoparticles have shown promise in loading and delivering drugs for targeted therapy. Many progresses have been made in the design, synthesis, and modification of nanoparticles to fulfill such goals. However, realizing targeted intracellular delivery and controlled release of drugs remains challenging, partly because of the lack of reliable tools to detect the drug-releasing process. In this paper, we applied femtosecond laser pulses to trigger the explosion of gold nanocages (AuNCs) and control the intracellular release of loaded aluminum phthalocyanine (AlPcS) molecules for photodynamic therapy (PDT). AuNCs were found to enhance the encapsulation efficiency and suppress the PDT effect of AlPcS molecules until they were released. More importantly, we discovered that the excited-state lifetimes of the AlPcS–AuNC conjugate (∼3 ps) and free AlPcS (∼11 ps) differ significantly, which was utilized to image the released drug molecules using transient absorption lifetime microscopy with the same laser source. This technique extracts information similar to fluorescence lifetime imaging microscopy but is superior in imaging the molecules that hardly fluoresce or are prone to photobleaching. We further combined a dual-phase lock-in detection technique to show the potential of real-time imaging based on the change in transient optical behaviors. Our method may provide a new tool for investigating nanoparticle-assisted drug delivery and release

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump–probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron–phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump–probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron–phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material

Aqueous Mg²⁺ and Ca²⁺ Ligand Exchange Mechanisms Identified with 2DIR Spectroscopy

Biological systems must discriminate between calcium and magnesium for these ions to perform their distinct biological functions, but the mechanism for distinguishing aqueous ions has yet to be determined. Ionic recognition depends upon the rate and mechanism by which ligands enter and leave the first solvation shell surrounding these cations. We present a time-resolved vibrational spectroscopy study of these ligand exchange dynamics in aqueous solution. The sensitivity of the CN-stretch frequency of NCS^– to ion pair formation has been utilized to investigate the mechanism and dynamics of ligand exchange into and out of the first solvation shell of aqueous magnesium and calcium ions with multidimensional vibrational (2DIR) spectroscopy. We have determined that anion exchange follows a dissociative mechanism for Mg²⁺ and an associative mechanism for Ca²⁺

Site-Specific Measurement of Water Dynamics in the Substrate Pocket of Ketosteroid Isomerase Using Time-Resolved Vibrational Spectroscopy

Little is known about the reorganization capacity of water molecules at the active sites of enzymes and how this couples to the catalytic reaction. Here, we study the dynamics of water molecules at the active site of a highly proficient enzyme, Δ⁵-3-ketosteroid isomerase (KSI), during a light-activated mimic of its catalytic cycle. Photoexcitation of a nitrile-containing photoacid, coumarin183 (C183), mimics the change in charge density that occurs at the active site of KSI during the first step of the catalytic reaction. The nitrile of C183 is exposed to water when bound to the KSI active site, and we used time-resolved vibrational spectroscopy as a site-specific probe to study the solvation dynamics of water molecules in the vicinity of the nitrile. We observed that water molecules at the active site of KSI are highly rigid, during the light-activated catalytic cycle, compared to the solvation dynamics observed in bulk water. On the basis of this result, we hypothesize that rigid water dipoles at the active site might help in the maintenance of the preorganized electrostatic environment required for efficient catalysis. The results also demonstrate the utility of nitrile probes in measuring the dynamics of local (H-bonded) water molecules in contrast to the commonly used fluorescence methods which measure the average behavior of primary and subsequent spheres of solvation

Optimizing Nonlinear Optical Visibility of Two-Dimensional Materials

Two-dimensional (2D) materials have attracted broad research interests across various nonlinear optical (NLO) studies, including nonlinear photoluminescence (NPL), second harmonic generation (SHG), transient absorption (TA), and so forth. These studies have unveiled important features and information of 2D materials, such as in grain boundaries, defects, and crystal orientations. However, as most research studies focused on the intrinsic NLO processes, little attention has been paid to the substrates underneath. Here, we discovered that the NLO signal depends significantly on the thickness of SiO₂ in SiO₂/Si substrates. A 40-fold enhancement of the NPL signal of graphene was observed when the SiO₂ thickness was varied from 270 to 125 nm under 800 nm excitation. We systematically studied the NPL intensity of graphene on three different SiO₂ thicknesses within a pump wavelength range of 800–1100 nm. The results agreed with a numerical model based on back reflection and interference. Furthermore, we have extended our measurements to include TA and SHG of graphene and MoS₂, confirming that SiO₂ thickness has similar effects on all of the three major types of NLO signals. Our results will serve as an important guidance for choosing the optimum substrates to conduct NLO research studies on 2D materials

Highly Efficient Destruction of Amyloid‑β Fibrils by Femtosecond Laser-Induced Nanoexplosion of Gold Nanorods

Alzheimer’s disease (AD) is associated with the aggregation of the amyloid-beta (Aβ) peptides into toxic aggregates. How to inhibit the aggregation of Aβ peptides has been extensively studied over recent decades. The investigation on eliminating preformed fibrils, however, has rarely been reported. In this paper, near-infrared femtosecond (fs) laser is applied for the destruction of preformed Aβ fibrils in conjunction with gold nanorods (AuNRs). Our results demonstrate that the 800 nm fs-laser irradiation can locally trigger the explosion of AuNRs due to the strong localized surface plasmon resonance effect. As a result, the majority of Aβ fibrils are efficiently destroyed into small fragments by the irradiation of fs-laser with a light dose less than 75 J·cm^–2. Meanwhile, significant reduction of β-sheet structures is observed by thioflavin T (ThT) fluorescence measurements. In contrast, the destruction effect by continuous wave (cw) laser irradiation is much weaker with equivalent power density and irradiation time. Furthermore, the laser-induced destruction of fibrils by Au nanoparticles (AuNPs) is also investigated, which reveals that most of the Aβ fibrils remain well under the surface explosion of spherical AuNPs. Overall, our results provide a novel design for the fast destruction of amyloid fibrils locally and biocompatibly, which may have remarkable potentials in the therapy of AD

Significantly Accelerated Hydroxyl Radical Generation by Fe(III)–Oxalate Photochemistry in Aerosol Droplets

Fe(III)–oxalate complexes are ubiquitous in atmospheric environments, which can release reactive oxygen species (ROS) such as H2O2, O•2–, and OH• under light irradiation. Although Fe(III)–oxalate photochemistry has been investigated extensively, the understanding of its involvement in authentic atmospheric environments such as aerosol droplets is far from enough, since the current available knowledge has mainly been obtained in bulk-phase studies. Here, we find that the production of OH• by Fe(III)–oxalate in aerosol microdroplets is about 10-fold greater than that of its bulk-phase counterpart. In addition, in the presence of Fe(III)–oxalate complexes, the rate of photo-oxidation from SO2 to sulfate in microdroplets was about 19-fold faster than that in the bulk phase. The availability of efficient reactants and mass transfer due to droplet effects made dominant contributions to the accelerated OH• and SO42– formation. This work highlights the necessary consideration of droplet effects in atmospheric laboratory studies and model simulations