Nanoparticle Size Influences the Self-Assembly of Gold Nanorods Using Flexible Streptavidin–Biotin Linkages

The self-assembly of colloidal nanocrystals remains of robust interest due to its potential in creating hierarchical nanomaterials that have advanced function. For gold nanocrystals, junctions between nanoparticles yield large enhancements in local electric fields under resonant illumination, which is suitable for surface-enhanced spectroscopies for molecular sensors. Gold nanorods can provide such plasmonic fields at near-infrared wavelengths of light for longitudinal excitation. Through the use of careful concentration and stoichiometric control, a method is reported herein for selective biotinylation of the ends of gold nanorods for simple, consistent, and high-yielding self-assembly upon addition of the biotin-binding protein streptavidin. This method was applied to four different sized nanorods of similar aspect ratio and analyzed through UV–vis spectroscopy for qualitative confirmation of self-assembly and transmission electron microscopy to determine the degree of self-assembly in end-linked nanorods. The yield of end-linked assemblies approaches 90% for the largest nanorods and approaches 0% for the smallest nanorods. The number of nanorods linked in one chain also increases with an increased nanoparticle size. The results support the notion that the lower ligand density at the ends of the larger nanorods yields preferential substitution reactions at those ends and hence preferential end-to-end assembly, while the smallest nanorods have a relatively uniform ligand density across their surfaces, leading to spatially random substitution reactions

A Demonstration of Le Chatelier’s Principle on the Nanoscale

Photothermal desorption of molecules from plasmonic nanoparticles is an example of a light-triggered molecular release due to heating of the system. However, this phenomenon ought to work only if the molecule–nanoparticle interaction is exothermic in nature. In this study, we compare protein adsorption behavior onto gold nanoparticles for both endothermic and exothermic complexation reactions, and demonstrate that Le Chatelier’s principle can be applied to predict protein adsorption or desorption on nanomaterial surfaces. Polyelectrolyte-wrapped gold nanorods were used as adsorption platforms for two different proteins, which we were able to adsorb/desorb from the nanorod surface depending on the thermodynamics of their interactions. Furthermore, we show that the behaviors hold up under more complex biological environments such as fetal bovine serum

Gold Nanorods Indirectly Promote Migration of Metastatic Human Breast Cancer Cells in Three-Dimensional Cultures

Gold nanomaterials are intensively studied for applications in disease detection, diagnosis and therapeutics, and this has motivated considerable research to determine their interaction with biomolecules, cells and cell behaviors. However, few studies look at how nanomaterials alter the extracellular matrix (ECM) and cell–ECM interactions. Nanomaterials in the body would interact with the entire cellular environment, and it is imperative to account for this when studying the impact of nanomaterials on living systems. Furthermore, recent evidence finds that migration rates of cells in 2D can be affected by nanomaterials, and uptake of the nanomaterials is not necessary to exert an effect. In this study, three-dimensional nested type I collagen matrices were utilized as a model ECM to study how gold nanorods affect the migration of MDA-MB-231 human breast cancer cells. Spontaneous cell migration through collagen containing gold nanorods was found to increase with increasing concentrations of gold nanorods, independent of intracellular uptake of the nanorods. Gold nanorods in the collagen matrix were found to alter collagen mechanical properties and structure, molecular diffusion, cellular adhesion, cell morphology, mode of migration and protease expression. Correlation between decreased cellular adhesion and rounded cell morphology and locomotion in nanorod-containing collagen suggests the induction of an amoeboid-like migratory phenotype

Evidence for Patchy Lipid Layers on Gold Nanoparticle Surfaces

Gold nanoparticles bearing multiple surface ligands are becoming favored candidates as multifunctional targeting, imaging, and therapeutic vehicles for biomedicine. The question of spatial location of different ligands on nanoparticle surfaces, especially with those of diameters less than 100 nm, is an important one that is difficult to quantitatively address. Here we functionalize the surface of 20, 50, and 90 nm gold nanoparticles with two different lipids, both single and mixed, using two different surface chemical procedures. Mass spectrometry supports the presence of both lipids in the mixed-lipid systems on nanoparticles, while electron microscopy evidence shows domain sizes for one lipid apparently a quarter to a half the projected diameter for 50 and 90 nm particles; but for 20 nm particles, there is no evidence for the existence of patches of the two lipids. Larger gold nanoparticles (90 nm) can be decorated with an array of 12 nm gold nanoparticles by use of a third lipid and antibody–antigen connectors; the display of the 12 nm particles about the 90 nm particles can be controlled to some extent by the initial surface chemistry and is quantified via a new angle analysis procedure

Polyelectrolyte Wrapping Layers Control Rates of Photothermal Molecular Release from Gold Nanorods

Gold nanorods show great promise as light-controlled molecular release systems. Dye molecules were loaded within a variable number of polyelectrolyte multilayers wrapped around gold nanorods. The dye photoinduced release rate depended on the quantity and type of polyelectrolyte trapping layers and could be tuned by a factor of 100. Only two molecular capping layers were sufficient to turn off release. Comparison of the phototriggered molecular release rate to a pure thermal experiment provides an estimate of the effective temperature of the nanorod solution upon irradiation

Nanovacuums: Nanoparticle Uptake and Differential Cellular Migration on a Carpet of Nanoparticles

The behavior of prostate carcinoma (PC3) cells and human dermal fibroblast (HDF) cells when incubated with sedimented Au NPs in vitro is studied. Darkfield microscopy demonstrates that both PC3 and HDF cells can “vacuum” Au NPs from the surface. Mean square displacement and mean cumulative square distance of cells shows that PC3 migration decreases in the presence of Au NPs while for HDF, migration is dependent on the surface charge and shape of Au NPs

Identification of Nanoparticles with a Colorimetric Sensor Array

A simple colorimetric sensor array technique was developed for the detection of various different nanoparticles (NPs) in aqueous solutions. The sensor array consists of five different cross-reactive chemoresponsive dyes, whose visible absorbances change in response to their interactions with NPs. Although no single dye is specific for any one NP, the pattern of color changes for all dyes provides a unique molecular fingerprint for each type of NP studied. Based on the responses of various dyes, a semiquantitative determination of concentration of each type of NP could also be accomplished with excellent sensitivity (<100 ng/mL). A variety of chemically distinct NPs were unambiguously identified using a standard chemometric approaches, including gold nanospheres (2 through 40 nm diameter), gold nanorods (2.4 and 3.5 aspect ratios), and multifunctional carbon nanospheres without errors in 112 trials. This colorimetric approach may pave the way for a fast, reliable, and inexpensive method to detect nanopollution and to characterize the physiochemical properties of NPs

Distance and Plasmon Wavelength Dependent Fluorescence of Molecules Bound to Silica-Coated Gold Nanorods

Plasmonic nanoparticles can strongly interact with adjacent fluoro­phores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. This dipolar coupling is dependent upon nanoparticle composition, distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore’s absorbance/emission and the surface plasmon band of the nanoparticles. In this work, we examine the distance and plasmon wavelength dependent fluorescence of an infrared dye (“IRDye”) bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11–26 nm thick. IRDye is covalently attached to the nanoparticle surface <i>via</i> a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed with 17 nm shells when the nanorod plasmon maximum is resonant with IRDye absorption. Time-resolved photo­luminescence reveals multi­exponential decay and a sharp reduction in fluorescence lifetime with decreasing silica shell thickness and when the plasmon maximum is closer to IRDye absorption/emission. Control experiments are carried out to confirm that the observed changes in fluorescence are due to plasmonic interactions, is simply surface attachment. There is no change in fluorescence intensity or lifetime when IRDye is bound to mesoporous silica nanoparticles. In addition, IRDye loading is limited to maintain a distance between dye molecules on the surface to more than 9 nm, well above the Förster radius. This assures minimal dye–dye interactions on the surface of the nanoparticles

Nanovacuums: Nanoparticle Uptake and Differential Cellular Migration on a Carpet of Nanoparticles

The behavior of prostate carcinoma (PC3) cells and human dermal fibroblast (HDF) cells when incubated with sedimented Au NPs in vitro is studied. Darkfield microscopy demonstrates that both PC3 and HDF cells can “vacuum” Au NPs from the surface. Mean square displacement and mean cumulative square distance of cells shows that PC3 migration decreases in the presence of Au NPs while for HDF, migration is dependent on the surface charge and shape of Au NPs

Understanding the Seed-Mediated Growth of Gold Nanorods through a Fractional Factorial Design of Experiments

Since the development of simple, aqueous protocols for the synthesis of anisotropic metal nanoparticles, research into many promising, valuable applications of gold nanorods has grown considerably, but a number of challenges remain, including gold-particle yield, robustness to minor impurities, and precise control of gold nanorod surface chemistry. Herein we present the results of a composite fractional factorial series of experiments designed to screen seven additional potential avenues of control and to understand the seed-mediated silver-assisted synthesis of gold nanorods. These synthesis variables are the amount of sodium borohydride used and the rate of stirring when producing seed nanoparticles, the age of and the amount of seeds added, the reaction temperature, the amounts of silver nitrate and ascorbic acid added, and the age of the reduced growth solution before seed nanoparticles are added to initiate rod formation. This statistical experimental design and analysis method, besides determining which experimental variables are important and which are not when synthesizing gold nanorods (and quantifying their effects), gives further insight into the mechanism of growth by measuring the degree to which variables interact with each other by mapping out their mechanistic connections. This work demonstrates that when forming gold nanorods by the reduction of auric ions by ascorbic acid onto seed nanoparticles, ascorbic acid determines how much gold is reduced, and the amount of seeds determine how it is divided, yet both influence the intrinsic growth rates, in both width and length, of the forming nanorods. Furthermore, this work shows that the reduction of gold proceeds via direct reduction on the surface of seeds and not through a disproportionation reaction. Further control over the length of gold nanorods can be achieved by tuning the amount of silver nitrate or the reaction temperature. This work shows that silver does not directly influence rod length or width, and a new primary role for silver is proposed as a catalyst promoting the reduction of gold on the ends of forming nanorods. Furthermore, this silver catalyst is removed from the reaction by adsorption onto the surface of the growing nanorod. This work also demonstrates the importance of freshly prepared silver nitrate and ascorbic acid solutions, free from even a few hours of photodegradation, in preparing gold nanorods with high shape purity and gold yield