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

Rigid Single Carbon–Carbon Bond That Does Not Rotate in Water

Carbon–carbon bond is one of the most ubiquitous molecular building blocks for natural and man-made materials. Rotational isomerization is fundamentally important for understanding the structure and reactivity of chemical and biological molecules. Reported herein is the first demonstration that a single C–C bond does not rotate in water. The two distal C–S bonds in both 1,2-ethanedithiolate (⁻S–CH₂–CH₂–S^–, 1,2-EDT^2–) and 2,3-butanedithiolate (2,3-BuDT^2–) are exclusively in the <i>trans</i> conformer with reference to their respective center single C–C bond. In contrast, both <i>trans</i> and <i>gauche</i> conformers are observed in neutral 1,2-ethanedithiol (1,2-EDT) and 2, 3-butanedithiol (2,3-BuDT). The insight from this work should be important for understanding the charge effect on the molecular conformation in aqueous solutions

Quantitative Comparison of Raman Activities, SERS Activities, and SERS Enhancement Factors of Organothiols: Implication to Chemical Enhancement

Studying the correlation between the molecular structures of SERS-active analytes and their SERS enhancement factors is important to our fundamental understanding of SERS chemical enhancement. Using a common internal reference method, we quantitatively compared the Raman activities, SERS activities, and SERS enhancement factors for a series of organothiols that differ significantly in their structural characteristics and reported chemical enhancements. We find that while the tested molecules vary tremendously in their normal Raman and SERS activities (by more than 4 orders of magnitude), their SERS enhancement factors are very similar (the largest difference is less than 1 order of magnitude). This result strongly suggests that SERS chemical enhancement factors are not as diverse as initially believed. In addition to shedding critical insight on the SERS phenomena, the common internal reference method developed in this work provides a simple and reliable way for systematic investigation of the correlation between molecular structures and their normal Raman and SERS activities

Subnanogram Mass Measurements on Plasmonic Nanoparticles for Temperature-Programmed Thermal Analysis

Ultrasensitive thermogravimetric analysis of adsorbed organic molecules has been achieved on an ordered array of gold nanoparticles used as a novel plasmonic nanobalance. The extinction peaks of the resonating surface plasmon of nanoparticle arrays shift upon loading molecules and return to the original position after a linear temperature rise process. A good correlation exists between the film thickness and magnitude of peak shifts. The detection range of plasmonic nanobalance derived from our results can reach a subnanogram level (1.8 pg on an active area of 100 μm²), which is much lower than those of mechanical or electronic mass-measuring devices. Such high mass sensitivity, combined with the remote detection capability and high-temperature operation of plasmonic sensors, allows the in situ detections of the masses of loaded material and thermally desorbed molecules

Plasmonic Coupling in Single Silver Nanosphere Assemblies by Polarization-Dependent Dark-Field Scattering Spectroscopy

In this paper, we present an experimental and theoretical study of the plasmonic properties of single Ag nanospheres and the plasmon interactions in assemblies of Ag nanosphere dimers and trimers. High-quality Ag nanospheres with small size distribution are synthesized by etching prefabricated Ag nanocubes. We perform a 360° polarization-resolved scattering study on silver nanosphere dimers and trimers, and correlate the scattering anisotropy with nanoparticle structure through correlated dark-field spectroscopy and scanning electron microscopy (SEM) characterization. The polarization-resolved dimer scattering shows a dipolar pattern aligned with the long axis of the dimer. For single Ag nanosphere trimers assembled in an equilateral triangle geometry, we also observe the dipolar scattering pattern to a certain degree, although the dipolar pattern is not preferentially aligned with any sides of the triangle. Theoretical studies using the T-matrix method reveal that if the Ag nanospheres are perfectly spherical and are assembled in a trimer with <i>D</i>_3<i>h</i> symmetry, the scattering spectra should be polarization independent, in contrast to the observed experimental results. The same phenomena are demonstrated in Ag nanopshere assemblies in <i>D</i>_4<i>h</i>, <i>D</i>_5<i>h</i>, and <i>D</i>_6<i>h</i> symmetry as well. Using the discrete dipole approximation method, we find that slight elongation (5%) in one of the three axes of the Ag nanospheres can induce a significant anisotropy in the scattering pattern. We here have shown that even small variations in the nanoparticle geometry that are difficult to resolve with SEM can lead to significant effects in the plasmonic coupling, therefore affecting the scattering spectra of the assembled nanostructures

Fluorescence Quenching of Quantum Dots by Gold Nanoparticles: A Potential Long Range Spectroscopic Ruler

The dependence of quantum dot (QD) fluorescence emission on the proximity of 30 nm gold nanoparticles (AuNPs) was studied with controlled interparticle distances ranging from 15 to 70 nm. This was achieved by coassembling DNA-conjugated QDs and AuNPs in a 1:1 ratio at precise positions on a triangular-shaped DNA origami platform. A profound, long-range quenching of the photoluminescence intensity of the QDs was observed. A combination of static and time-resolved fluorescence measurements suggests that the quenching is due to an increase in the nonradiative decay rate of QD emission. Unlike FRET, the energy transfer is inversely proportional to the 2.7th power of the distance between nanoparticles with half quenching at ∼28 nm. This long-range quenching phenomena may be useful for developing extended spectroscopic rulers in the future

Quantification of Gold Nanoparticle Ultraviolet–Visible Extinction, Absorption, and Scattering Cross-Section Spectra and Scattering Depolarization Spectra: The Effects of Nanoparticle Geometry, Solvent Composition, Ligand Functionalization, and Nanoparticle Aggregation

Using the recent polarized resonance synchronous spectroscopic (PRS2) technique, we reported the quantification of photon extinction, absorption, scattering cross-section spectra, and scattering depolarization spectra for AuNPs of different sizes and shapes. The effects of the solvent composition, ligand functionalization, and nanoparticle aggregation on the AuNP photon absorption and scattering have also been experimentally quantified. The light scattering depolarization is close to 0 for gold nanospheres (AuNSs) crossing the entire UV–vis region but is strongly wavelength-dependent for gold nanorods (AuNRs). Increasing the dielectric constant of the medium surrounding AuNPs either by solvents or ligand adsorption increases photon absorption and scattering but has no significant impact on the AuNP scattering depolarization. Nanoparticle aggregation increases AuNP photon scattering. However, even the extensively aggregated AuNPs remain predominantly photon absorbers with photon scattering-to-extinction ratios all less than 0.03 for the investigated AuNP aggregates at the AuNP peak extinction wavelength. The AuNP scattering depolarization initially increases with the AuNP aggregation but decreases when aggregation further progresses. The insights from this study are important for a wide range of AuNP applications that involve photon/matter interactions, while the provided methodology is directly applicable for experimental quantification of optical properties for nanomaterials that are commonly simultaneously photon absorbers and scatterers

Effects of Cascading Optical Processes: Part I: Impacts on Quantification of Sample Scattering Extinction, Intensity, and Depolarization

Light scattering is a universal matter property that is especially prominent in nanoscale or larger materials. However, the effects of scattering-based cascading optical processes on experimental quantification of sample absorption, scattering, and emission intensities, as well as scattering and emission depolarization, have not been adequately addressed. Using a series of polystyrene nanoparticles (PSNPs) of different sizes as model analytes, we present a computational and experimental study on the effects of cascading light scattering on experimental quantification of NP scattering activities (scattering cross-section or molar coefficient), intensity, and depolarization. Part II and Part III of this series of companion articles explore the effects of cascading optical processes on sample absorption and fluorescence measurements, respectively. A general theoretical model is developed on how forward scattered light complicates the general applicability of Beer’s law to the experimental UV–vis spectrum of scattering samples. The correlation between the scattering intensity and PSNP concentration is highly complicated with no robust linearity even when the scatterers’ concentration is very low. Such complexity arises from the combination of concentration-dependence of light scattering depolarization and the scattering inner filter effects (IFEs). Scattering depolarization increases with the PSNP scattering extinction (thereby, its concentration) but can never reach unity (isotropic) due to the polarization dependence of the scattering IFE. The insights from this study are important for understanding the strengths and limitations of various scattering-based techniques for material characterization including nanoparticle quantification. They are also foundational for quantitative mechanistic understanding on the effects of light scattering on sample absorption and fluorescence measurements

Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil

Nanoparticle self-assembly is fundamentally important for bottom-up functional device fabrication. Currently, most nanoparticle self-assembly has been achieved with gold nanoparticles (AuNPs) functionalized with surfactants, polymeric materials, or cross-linkers. Reported herein is a facile synthesis of gold and silver nanoparticle (AgNP) films assembled onto thiophene oil by simply vortex mixing neat thiophene with colloidal AuNPs or AgNPs for ∼1 min. The AuNP film can be made using every type of colloidal AuNPs we have explored, including sodium borohydride-reduced AuNPs with a diameter of ∼5 nm, tannic acid-reduced AuNPs of ∼10 nm diameter, and citrate-reduced AuNPs with particle sizes of ∼13 and ∼30 nm diameter. The AuNP film has excellent stability and it is extremely flexible. It can be stretched, shrunken, and deformed accordingly by changing the volume or shape of the enclosed thiophene oil. However, the AgNP film is unstable, and it can be rapidly discolored and disintegrated into small flakes that float on the thiophene surface. The AuNP and AgNP films prepared in the glass vials can be readily transferred to glass slides and metal substrates for surface-enhanced Raman spectral acquisition

Comparative Study of the Self-Assembly of Gold and Silver Nanoparticles onto Thiophene Oil

Nanoparticle self-assembly is fundamentally important for bottom-up functional device fabrication. Currently, most nanoparticle self-assembly has been achieved with gold nanoparticles (AuNPs) functionalized with surfactants, polymeric materials, or cross-linkers. Reported herein is a facile synthesis of gold and silver nanoparticle (AgNP) films assembled onto thiophene oil by simply vortex mixing neat thiophene with colloidal AuNPs or AgNPs for ∼1 min. The AuNP film can be made using every type of colloidal AuNPs we have explored, including sodium borohydride-reduced AuNPs with a diameter of ∼5 nm, tannic acid-reduced AuNPs of ∼10 nm diameter, and citrate-reduced AuNPs with particle sizes of ∼13 and ∼30 nm diameter. The AuNP film has excellent stability and it is extremely flexible. It can be stretched, shrunken, and deformed accordingly by changing the volume or shape of the enclosed thiophene oil. However, the AgNP film is unstable, and it can be rapidly discolored and disintegrated into small flakes that float on the thiophene surface. The AuNP and AgNP films prepared in the glass vials can be readily transferred to glass slides and metal substrates for surface-enhanced Raman spectral acquisition