Raman Spectroscopy on Brain Disorders: Transition from Fundamental Research to Clinical Applications

Brain disorders such as brain tumors and neurodegenerative diseases (NDs) are accompanied by chemical alterations in the tissues. Early diagnosis of these diseases will provide key benefits for patients and opportunities for preventive treatments. To detect these sophisticated diseases, various imaging modalities have been developed such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET). However, they provide inadequate molecule-specific information. In comparison, Raman spectroscopy (RS) is an analytical tool that provides rich information about molecular fingerprints. It is also inexpensive and rapid compared to CT, MRI, and PET. While intrinsic RS suffers from low yield, in recent years, through the adoption of Raman enhancement technologies and advanced data analysis approaches, RS has undergone significant advancements in its ability to probe biological tissues, including the brain. This review discusses recent clinical and biomedical applications of RS and related techniques applicable to brain tumors and NDs

Growth Dynamics of Colloidal Silver-Gold Core-Shell Nanoparticles Studied by Second Harmonic Generation and Extinction Spectroscopy

The growth dynamics of colloidal silver-gold core-shell (Ag@Au CS) nanoparticles (NPs) in water are monitored in a stepwise synthesis approach using time-dependent second harmonic generation (SHG) and extinction spectroscopy. Three sequential additions of chloroauric acid, sodium citrate, and hydroquinone are added to the silver nanoparticle solution to grow a gold shell around a silver core. The first addition produces a stable urchin-like surface morphology, while the second and third additions continue to grow the gold shell thickness as the surface becomes more smooth and uniform, as determined using transmission electron microscopy. The extinction spectra after each addition are compared to finite-difference time-domain (FDTD) calculations, showing large deviations for the first and second additions due to the bumpy surface morphology and plasmonic hotspots while showing general agreement after the third addition reaches equilibrium. The SHG signal is dominated by the NP surface, providing complementary information on the growth time scales due to changes to the surface morphology. This combined approach of synthesis and characterization of Ag@Au CS nanoparticles with SHG spectroscopy, extinction spectroscopy, and FDTD calculations provides a detailed foundation for investigating complex colloidal nanoparticle growth mechanisms and dynamics in developing enhanced plasmonic nanomaterial technologies

Efficient photoinduced energy transfer in porphyrin-based nanomaterials

© 2020 American Chemical Society. Synthesis, characterization, and ultrafast dynamics of porphyrin- and zinc porphyrin-based nanomaterials are reported. Spherical nanoparticles composed of a group of uniform materials based on organic salts (nanoGUMBOS) are prepared from either porphyrin or zinc porphyrin with trihexyl(tetradecyl)phosphonium in aqueous colloidal suspension with sizes of approximately 50 nm in diameter. Ultrafast excited-state dynamics of porphyrin and zinc porphyrin nanoGUMBOS in water are measured using transient absorption spectroscopy with 400 nm excitation. Results are compared to corresponding measurements of the porphyrin molecular dye parent compounds in water. Porphyrin and zinc porphyrin have long-lived excited states arising from intersystem crossing of the first-excited singlet S1 state to the triplet T1 state. These excited-state lifetimes are significantly faster in porphyrin-based nanoGUMBOS as compared to the corresponding porphyrin molecules due to intermolecular energy transfer, electronic delocalization, and altered chemical environments of the nanomaterials. Additionally, these results demonstrate that porphyrin-based nanoGUMBOS are promising nanomaterials for light harvesting in solar cells and optoelectronics

Monitoring the growth dynamics of colloidal gold-silver core-shell nanoparticles using in situ

© 2019 Author(s). The growth dynamics of gold-silver core-shell (Au@Ag) nanoparticles are studied using in situ time-dependent second harmonic generation (SHG) and extinction spectroscopy to investigate the nanoparticle shell formation. The silver shell is grown by reduction of silver cations onto a 14 nm gold core using ascorbic acid in colloidal aqueous solution under varying reaction concentrations producing Au@Ag nanoparticles of final sizes ranging from 51 to 78 nm in diameter. The in situ extinction spectra show a rapid increase in intensity on the timescale of 5-6 s with blue shifting and narrowing of the plasmonic peak during the silver shell formation. The in situ SHG signals show an abrupt rise at early times of the reaction, followed by a time-dependent biexponential decrease, where the faster SHG lifetime corresponds to the timescale of the shell growth, and where the slower SHG lifetime is attributed to changes in the nanoparticle surface charge density. A large enhancement in the SHG signal at early stages of the reaction is caused by plasmonic hot spots due to the nanoparticle surface morphology, which becomes smoother as the reaction proceeds. The final extinction spectra are compared to finite-difference time-domain (FDTD) calculations, showing general agreement with experiment, where the plasmon peak red shifts and increases in spectral width as the silver shell thickness increases. These in situ SHG and extinction spectroscopy results, combined with FDTD calculations, help characterize the complicated processes involved in colloidal nanoparticle shell formation in real time for developing potential plasmon-enhanced nanomaterial applications

Monitoring the Seed-Mediated Growth of Gold Nanoparticles Using in Situ Second Harmonic Generation and Extinction Spectroscopy

© 2018 American Chemical Society. In situ second harmonic generation (SHG) coupled with extinction spectroscopy is used for real-time monitoring of seed-mediated growth dynamics of colloidal citrate-stabilized gold nanoparticles in water. The time-dependent in situ SHG results capture an early stage of the growth process where a large enhancement in the SHG signal is observed, which is attributed to the formation of plasmonic hot spots from a rough and uneven nanoparticle surface. The temporal peak in the SHG signal is followed by a decay that is fit to an exponential function to characterize the size-dependent nanoparticle growth lifetime, which varies from 0.45 to 1.7 min for final nanoparticle sizes of 66 and 94 nm, respectively. This early growth stage also corresponds to a broadening of the plasmon spectra, as monitored using time-dependent in situ extinction spectroscopy. Over the course of the seed-mediated growth reaction, the nanoparticle becomes more thermodynamically stable through surface reconstruction resulting in a smoother, more uniform surface, corresponding to lower, stable SHG signals and narrower plasmon spectra. With real-time monitoring of nanoparticle formation, in situ SHG spectroscopy combined with in situ extinction spectroscopy provides an important insight for controlling nanoparticle synthesis and surface morphology for potential nanoscale engineering of different colloidal nanomaterials