ABSTRACT
The sample inner filter effect (IFE) induces spectral distortion and affects the linearity between intensity and analyte concentration in fluorescence, Raman, surface enhanced Raman, and Rayleigh light scattering measurements. Existing spectrofluorometric-based measurements treat light scattering and absorption identically in their sample IFEs. Reported herein is the finding that photon scattering and absorption differ drastically in inducing the sample IFE in Stokes-shifted fluorescence (SSF) spectrum, resonance synchronous spectrum (RS2), and the polarized resonance synchronous spectrum (PRS2) measurements. Absorption with an absorption extinction as small as 0.05 imposes significant IFE on SSF, RS2, and PRS2 measurements. However, no significant IFE occurs even when the scattering extinction is as high as 0.9. For samples that both absorb and scatter light, one should decompose their UV-vis extinction spectra into absorption and scattering extinction component spectra before correcting the sample IFE. An iteration PRS2 method was introduced for the experimental decoupling of the photon absorption and scattering contribution. The methodology presented in this work can be easily implemented by researchers with access to one conventional UV-vis spectrophotometer and one spectrofluorometer equipped with a pair of excitation and detection polarizers. This work should be of broad significance in chemical research given the popularity of fluorescence spectroscopy in material characterization applications.
ABSTRACT
The stability of citrate-residues on gold nanoparticles (AuNPs) against ligand displacement has been controversial. Using AuNPs synthesized with deuterated citrate in combination with in-situ surface-enhanced Raman spectroscopic (SERS) analysis, we report that both citrate-residues and solution impurities can be simultaneously adsorbed onto citrate-reduced AuNPs in solution. The citrate-residues can be readily displaced from AuNPs by organosulfur such as organothiols (RS-H), organodisuflide (R-S-S-R), and non-specific ligands including halides and adenine. Control experiments conducted on high-purity gold films sputter-coated onto silicone substrates indicate that air-borne and solvent-borne impurities rapidly contaminate the gold surfaces. Head-to-head comparison of ligand-functionalized AuNPs by in-situ SERS measurements verses those from the ex-situ X-ray photoelectron spectroscopic (XPS) measurements reveal that the impurity deposition can compromise the reliability of ex-situ XPS identification of surface adsorbates on AuNPs in solution. These insights are of general significance to nanoscience research given the broad interest in nanoparticle surface chemistry and popularity of XPS for nanomaterial characterizations.
ABSTRACT
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.
