UV–vis Spectroscopic Properties of <i>n</i>C₆₀ Produced via Extended Mixing

Colloidally stable C₆₀ suspensions produced via extended mixing in water (aq/<i>n</i>C₆₀) are highly heterogeneous with respect to particle size and morphology. Ultraviolet–visible (UV–vis) absorption spectra of aq/<i>n</i>C₆₀ are often used as a supplemental tool to dynamic light scattering (DLS), transmission electron microscopy (TEM), and other analytical methods to characterize aq/<i>n</i>C₆₀. In the present study, the UV–vis spectra provide information about the average particle size and the interactions between C₆₀ and water. We report that the decrease in relative absorption in the 240–290 nm range is a function of magnetic stirring time, that the average size (<i>Z</i>_ave) of an aq/<i>n</i>C₆₀ suspension determines the position of absorbance maximum of its 360 nm band, and that the methods used to prepare and fractionate <i>n</i>C₆₀ affect the extent of the blue shift in this band that occurs due to a decrease in <i>Z</i>_ave. We also confirm that the broad absorption band in the 400–600 nm region is a result of C₆₀ aggregation

MGITC Facilitated Formation of AuNP Multimers

Malachite green isothiocyanate (MGITC) is frequently used as a surface bound Raman reporter for metal nanoparticle-enabled surface enhanced Raman scattering (SERS). To date, however, no study has focused on the application of MGITC for the formation of stable “hot-spot” aggregates for Raman imaging applications. Herein we report a method to produce a series of suspensions of MGITC functionalized gold nanoparticles (MGITC-AuNPs) that at one extreme consist primarily of monomers and at the other extreme as mixtures of multimers and monomers. Monomer and multimer morphologies were characterized by scanning electron microscopy and atomic force microscopy using a reliable spin-coating deposition sampling method. The multimers generally include 2, 3, or 4 individual AuNPs with an average number of 3 ± 1. The number of multimers produced in a given suspension was found to be dependent on the volume and concentration of MGITC initially applied. The surface binding of MGITC to both monomeric and multimeric MGITC-AuNPs was investigated by Raman and SERS, and the degree of aggregation in the multimer suspension was evaluated based upon the measured variation of the MGITC SERS intensity of the AuNPs. Using an estimated extinction coefficient of 1.22 ± 0.41 × 10¹¹ M^–1 cm^–1 at ≈850 nm for the localized surface plasmon resonance (LSPR) band of the MGITC-AuNP multimers, the multimer concentrations were calculated by Beer’s Law

Nanoclustered Gold Honeycombs for Surface-Enhanced Raman Scattering

A honeycomb-shaped gold substrate was developed for surface-enhanced Raman imaging (SERI). The honeycombs are composed of clusters of 50–70 nm gold nanoparticles and exhibit high Raman enhancement efficiency. An average surface enhancement factor (ASEF) of 1.7 × 10⁶ was estimated for a monolayer of l-cysteine molecules adsorbed to gold via a thiol linkage. The presence of a linear relationship in the low concentration region was observed in SERI detection of malachite green isothiocyanate (MGITC). These results together with the high reproducibility and simple and cost-effective fabrication of this substrate suggest that it has utility for applications of surface-enhanced Raman scattering (SERS) in quantitative diagnoses and analyte detection

Uncontrolled Variability in the Extinction Spectra of C₆₀ Nanoparticle Suspensions

To properly investigate the environmental transport, fate, and impact of fullerene C₆₀ nanoparticles (<i>n</i>C₆₀), it is necessary to reproducibly obtain <i>n</i>C₆₀ suspensions and to accurately determine their concentration ([C₆₀]). The results in the present study, however, clearly illustrate that the production of <i>n</i>C₆₀ via extended mixing and via sonication are highly stochastic top-down processes subject to widely divergent end points. <i>n</i>C₆₀ suspensions exhibit variable characteristics (e.g., [C₆₀], average particle size, size distribution, etc.) that make it challenging, if not impossible, to acquire reproducible UV–vis extinction spectra. The mass extinction coefficient, which is the absorptivity of a suspension with [C₆₀] = 1 mM obtained by normalizing UV–vis spectra by the mass concentration of C₆₀ in the suspension, decreases with a given suspension’s hydrodynamic diameter, whereas the particle extinction coefficient, which is the absorptivity of a suspension containing one mole of <i>n</i>C₆₀ nanoparticles with the same size distribution as the target suspension and calculated based upon the suspension nanoparticle size distribution, increases with its number weighted average diameter. Other spectroscopic properties of <i>n</i>C₆₀ (e.g., absorbance bandwidth, position of absorption maximum, and relative extinction intensity) also change with average particle size. As a result of the extant variability between samples, when UV–vis spectra are employed to calculate or represent [C₆₀] for fullerene nanoparticle suspensions, extreme care must be taken and other colloidal properties of this suspension must be measured to obtain an accurate result

Controlled Evaluation of Silver Nanoparticle Dissolution Using Atomic Force Microscopy

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7.0, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, the in-plane radius decreased by 5–11 nm, and the height increased by 6–12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10–550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of unaggregated AgNP dissolution

Porous Media-Induced Aggregation of Protein-Stabilized Gold Nanoparticles

Gold-nanoparticles (AuNPs) are employed for cancer treatment, drug delivery, chemical analyses, and many other uses. As AuNP manufacture increases, it is imperative that we understand the environmental fate of these nanomaterials. We investigated the transport and stability of AuNPs under simulated groundwater conditions. Batch experiments indicated that 16 nm AuNPs stabilized with bovine serum albumin (BSA-cit-AuNPs) was slightly more stable under high ionic strength conditions than citrate-functionalized AuNPs (cit-AuNPs) of the same core size. Both types of AuNPs were injected into glass bead-packed columns and subjected to transport with varying NaCl and CaCl₂ concentrations. BSA-cit-AuNPs deposited less than cit-AuNPs in the presence of increasing concentrations of CaCl₂, but the opposite trend was observed in the presence of increasing concentrations of NaCl. This finding differed from the results obtained in the batch studies. Calculated attachment efficiencies (α) failed to reflect the observed experimental column data, with α at maximum only approaching 0.1 even though a majority of the AuNPs were retained in the column. Colloid filtration theory fails to predict and explain this discrepancy. We conclude that media induced nanoparticle aggregation is responsible for the inconsistency

Dissolution and Persistence of Copper-Based Nanomaterials in Undersaturated Solutions with Respect to Cupric Solid Phases

Dissolution of copper-based nanoparticles (NPs) can control their environmental persistence and toxicity. Previous research has generally reported limited dissolution of Cu-based NPs at circumneutral pH, but the environmentally important case of dissolution in solutions that are undersaturated with respect to copper mineral phases has not been investigated thoroughly. In this study, immobilized Cu-based NPs were fabricated on solid supports. Metallic copper (Cu), cupric oxide/hydroxide (Cu_ox), and copper sulfide (Cu_<i>x</i>S) NPs were investigated. Dissolution rate constants were measured <i>in situ</i> by an atomic force microscope equipped with a flow-through cell. A mass-balance model indicated that the flowing solution was consistently undersaturated with respect to cupric solid phases. Based on the measured rate constants, Cu_ox NPs are expected to dissolve completely in these undersaturated conditions within a matter of hours, even at neutral to basic pH. The expected persistence of metallic Cu NPs ranges from a few hours to days, whereas Cu_<i>x</i>S NPs showed no significant dissolution over the time scales studied. Field deployment of Cu-based NP samples in a freshwater stream confirmed these conclusions for a natural aquatic system. These results suggest that Cu and Cu_ox NPs will be short-lived in the environment unless dissolution is hindered by a competing process, such as sulfidation

Surface-Enhanced Raman Spectroscopy (SERS) Cellular Imaging of Intracellulary Biosynthesized Gold Nanoparticles

Green algae biosynthesize gold nanoparticles (AuNPs) in the presence of dissolved gold, but the precise biosynthesis mechanism remains unclear. Furthermore, few surface-enhanced Raman spectroscopy (SERS) spectra and even fewer SERS cellular images have been collected of intracellularly grown gold nanoparticles, despite the detailed information SERS can provide about nanosurface-associated molecules. In this effort, SERS imaging was used to detect intracellular and extracellular gold nanoparticles biosynthesized by the green algae Pseudokirchneriella subcapitata to identify surface-associated biomolecules and to evaluate the nanoparticle biosynthesis mechanism. Three-dimensional SERS spectral maps imaged AuNPs biosynthesized in the presence of 0.005–0.5 mM HAuCl₄ over a variety of pH conditions. Algal growth and AuNP biosynthesis were monitored over a 72 h exposure period using UV–vis spectroscopy, electron microscopy, and elemental analysis. Principle component analysis (PCA) and cluster analysis of SERS data demonstrate reproducible trends in the SERS spectral maps and simplify peak identification analyses. SERS cellular images contain peaks consistent with glutathione, β-carotene, chlorophyll <i>a</i>, hydroxyquinoline, NAD, and proteins such as a reductase enzyme. Each is a biomolecule previously thought to be involved in intracellular AuNP biosynthesis in bacteria and fungi. Little mechanistic study has been previously conducted with green algae. Identification of AuNP surface-associated biomolecules from SERS spectra requires prior knowledge of the system, but peaks not found in the SERS spectra can be used to narrow the list of potential AuNP surface-associated candidate molecules. Continued development of SERS spectral imaging will facilitate noble metal nanoparticle surface analyses to elucidate biosynthesis mechanisms relevant to green synthesis, to monitor nanomaterial function and stability in complex media, and to image AuNPs employed for drug delivery applications

Surface-Enhanced Raman Spectroscopy (SERS) Cellular Imaging of Intracellulary Biosynthesized Gold Nanoparticles

Green algae biosynthesize gold nanoparticles (AuNPs) in the presence of dissolved gold, but the precise biosynthesis mechanism remains unclear. Furthermore, few surface-enhanced Raman spectroscopy (SERS) spectra and even fewer SERS cellular images have been collected of intracellularly grown gold nanoparticles, despite the detailed information SERS can provide about nanosurface-associated molecules. In this effort, SERS imaging was used to detect intracellular and extracellular gold nanoparticles biosynthesized by the green algae Pseudokirchneriella subcapitata to identify surface-associated biomolecules and to evaluate the nanoparticle biosynthesis mechanism. Three-dimensional SERS spectral maps imaged AuNPs biosynthesized in the presence of 0.005–0.5 mM HAuCl₄ over a variety of pH conditions. Algal growth and AuNP biosynthesis were monitored over a 72 h exposure period using UV–vis spectroscopy, electron microscopy, and elemental analysis. Principle component analysis (PCA) and cluster analysis of SERS data demonstrate reproducible trends in the SERS spectral maps and simplify peak identification analyses. SERS cellular images contain peaks consistent with glutathione, β-carotene, chlorophyll <i>a</i>, hydroxyquinoline, NAD, and proteins such as a reductase enzyme. Each is a biomolecule previously thought to be involved in intracellular AuNP biosynthesis in bacteria and fungi. Little mechanistic study has been previously conducted with green algae. Identification of AuNP surface-associated biomolecules from SERS spectra requires prior knowledge of the system, but peaks not found in the SERS spectra can be used to narrow the list of potential AuNP surface-associated candidate molecules. Continued development of SERS spectral imaging will facilitate noble metal nanoparticle surface analyses to elucidate biosynthesis mechanisms relevant to green synthesis, to monitor nanomaterial function and stability in complex media, and to image AuNPs employed for drug delivery applications

Controlled Evaluation of the Impacts of Surface Coatings on Silver Nanoparticle Dissolution Rates

Silver nanoparticles (AgNPs) are increasingly being incorporated into a range of consumer products and as such there is significant potential for the environmental release of either the AgNPs themselves or Ag⁺ ions. When AgNPs are exposed to environmental systems, the engineered surface coating can potentially be displaced or covered by naturally abundant macromolecules. These capping agents, either engineered or incidental, potentially block reactants from surface sites and can alter nanoparticle transformation rates. We studied how surface functionalization affects the dissolution of uniform arrays of AgNPs fabricated by nanosphere lithography (NSL). Bovine serum albumin (BSA) and two molecular weights of thiolated polyethylene glycol (PEG; 1000 and 5000 Da) were tested as model capping agents. Dissolution experiments were conducted in air-saturated phosphate buffer containing 550 mM NaCl. Tapping-mode atomic force microscopy (AFM) was used to measure changes in AgNP height over time. The measured dissolution rate for unfunctionalized AgNPs was 1.69 ± 0.23 nm/d, while the dissolution rates for BSA, PEG1000, and PEG5000 functionalized samples were 0.39 ± 0.05, 0.20 ± 0.10, and 0.14 ± 0.07 nm/d, respectively. PEG provides a steric barrier restricting mass transfer of reactants to sites on the AgNP surface and thus diminishes the dissolution rate. The effects of BSA, however, are more complicated with BSA initially enhancing dissolution, but providing protection against dissolution over extended time