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.

In Part I of the three companion articles, we reported the effects of light scattering on experimental quantification of scattering extinction, intensity, and depolarization in solutions that contain only scatterers with no significant absorption and photoluminescence activities. The present work (Part II) studies the effects of light scattering and absorption on a series of optical spectroscopic measurements done on samples that contain both absorbers and scatterers, but not emitters. The experimental UV−vis spectrum is the sum of the sample absorption and scattering extinction spectra. However, the upper limit of the experimental Beer's-law-abiding extinction can be limited prematurely by the interference of forward scattered light. Light absorption reduces not only the sample scattering intensity but also the scattering depolarization. The impact of scattering on sample light absorption is complicated, depending on whether the absorption of scattered light is taken into consideration. Scattering reduces light absorption along the optical path length from the excitation source to the UV−vis detector. However, the absorption of the scattered light can be adequate to compensate the reduced light absorption along such optical path, making the impacts of light scattering on the sample total light absorption negligibly small (<10%). The latter finding constitutes a critical validation of the integrating-sphere-assisted resonance synchronous spectroscopic method for experimental quantification of absorption and scattering contribution to the sample UV−vis extinction spectra. The techniques and general guidelines provided in this work should help improve the reliability of optical spectroscopic characterization of nanoscale or larger materials, many of which are simultaneous absorbers and scatterers. The insights from this work are foundational for Part III of this series of work, which is on the cascading optical processes on spectroscopic measurements of fluorescent samples.

In the previous studies, Zhou et al. showed that the enhancement factor of the radiative decay rate of multiple emitters could be significantly smaller than that of one emitter near a silver nanoparticle surface at around its resonance wavelength. An asymmetric line shape was obtained in the enhancement factor spectrum when six or more emitters were included. Using a nanoparticle composed of silver, gold, glass, or materials with artificially defined real (n) and imaginary (k) parts of the index of refraction, we demonstrated that the enhancement factor of the radiative decay rate of six emitters near the nanoparticle surface will be smaller than that of one emitter when the localized surface plasmon resonance is excited. The asymmetric line shape of the enhancement factors for six emitters is affected by both the real and imaginary parts of the indices of refraction of the nanoparticle, and the asymmetric line shape will appear only when the real part (n) of the index of refraction of the nanoparticle is less than that of the environmental medium. The simulations show that a resonance peak in the enhancement spectrum of six emitters will be obtained when the induced electric field inside the particle is antiparallel to the incident polarization direction, whereas a resonance dip appears at wavelengths when the induced electric field inside the nanoparticle is parallel to the incident polarization direction. While the studies explained key parameters affecting the enhancement factor of the radiative decay rate of one and multiple emitters, developing analytical model is still necessary to reveal more physical insight behind the obtained numerical results.

Two novel urchinlike plasmonic nanostructures, hollow gold nanostars (HNSs) and silver-coated hollow gold nanostars (AgHNSs), were synthesized using hollow gold nanospheres (HGNs) as templates. The particle morphology was assessed by transmission electron microscopy (TEM), optical properties were characterized by UV–vis spectroscopy, and elemental composition was evaluated by both inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy-dispersive X-ray (EDX) analysis. Both the HNSs and AgHNSs exhibit tunable structural and optical properties. Their unique star-shaped structure provides the desired “hot spots” for enhancing the electromagnetic (EM) field for surface-enhanced Raman scattering (SERS) applications, while their near-infrared (NIR) absorption is ideal for biomedical applications. A comparative analysis of the relative SERS enhancement using rhodamine 6G (R6G) showed that HNSs reported herein have an order of magnitude higher relative enhancement over previously reported HNSs. Moreover, AgHNSs have a 4-fold increase in the SERS signal compared to HNSs. Discrete dipole approximation (DDA) simulations of electric field enhancement were used to corroborate the experimental findings. The comparison between the experimental and theoretical results suggests that the significant increase in SERS enhancement cannot be completely explained by the larger dielectric constant of silver. The branching of individual spikes, or the increased binding efficacy of the analyte to the nanoparticles, likely also plays an important role. To further explore the applicability of these novel structures, successful capping ligand exchange from citrate to pentanethiol was used to switch the surface charge from negative to positive to facilitate the SERS detection of dried bovine serum albumin (BSA). AgHNSs are stable and effective NIR-absorbing substrates for SERS detection of biological samples. Elemental analyses of AgHNSs, in conjunction with DDA simulations, provide insights into their enhancement mechanism.

Using the coupled dipole method, we examined the enhancement factor of the radiative decay rate of one or multiple emitters when they are placed near a silver nanoparticle with a 10 nm radius. The simulation results indicate that the induced dipole of the central nanoparticle plays a key role in the enhancement factor of the radiative decay rate of emitters. The results between one emitter and the central nanoparticle are consistent with the previous studies. The near-field coupling among multiple emitters is important and will determine the magnitude and phase of the induced dipole of the central nanoparticle. When the magnitude of the induced dipole is much larger than those of the emitting dipoles, a high enhancement factor will be obtained, and the phase of the induced dipole is less important. When the magnitude of the induced dipole is comparable to those of emitting dipoles, the relative phase between the induced dipole and those of emitting dipoles will lead to both far-field constructive and destructive interference and result in complex profiles in the enhancement factor spectra.