Hydrodynamic fractionation of finite size gold nanoparticle clusters

We demonstrate a high-resolution in situ experimental method for performing simultaneous size classification and characterization of functional gold nanoparticle clusters (GNCs) based on asymmetric-flow field flow fractionation (AFFF). Field emission scanning electron microscopy, atomic force microscopy, multi-angle light scattering (MALS), and in situ ultraviolet-visible optical spectroscopy provide complementary data and imagery confirming the cluster state (e.g., dimer, trimer, tetramer), packing structure, and purity of fractionated populations. An orthogonal analysis of GNC size distributions is obtained using electrospray-differential mobility analysis (ES-DMA). We find a linear correlation between the normalized MALS intensity (measured during AFFF elution) and the corresponding number concentration (measured by ES-DMA), establishing the capacity for AFFF to quantify the absolute number concentration of GNCs. The results and corresponding methodology summarized here provide the proof of concept for general applications involving the formation, isolation, and in situ analysis of both functional and adventitious nanoparticle clusters of finite size.     

Quantitative Scanning Probe Microscopy for Nanomechanical Forensics

Atomic force microscopy (AFM) was used to assess the indentation modulus M _s and pull-off force F _po in four case studies of distinct evidence types, namely hair, questioned documents, fingerprints, and explosive particle-surface interactions. In the hair study, M _s decreased and F _po increased after adding conditioner and bleach to the hair. For the questioned documents, M _s and F _po of two inks were markedly different; ballpoint pen ink exhibited smaller variations relative to the mean value than printer ink. The fingerprint case study revealed that both maximum height and F _po decreased over a three-day period. Finally, the study on explosive particle-surface interactions illustrated that two fabrics exhibited similar M _s, but different F _po. Overall, it was found that AFM addresses needs in forensic science as defined by several federal agencies, in particular an improved ability to expand the information extracted from evidence and to quantify its evidentiary value.     

Recent Advances in Micro,Nano, and Cell Mechanics

Experimental Mechanics -     

Tracking Defects and Microstructural Heterogeneities in Meso-Scale Tensile Specimens Excised from Additively Manufactured Parts

Experimental Mechanics - The commercialization of additive manufacturing (AM) is underway in the aerospace and biomedical device industries [1, 2]. However, most metal parts produced by AM are...     

Material Flaw Populations and Component Strength Distributions in the Context of the Weibull Function

Experimental Mechanics - A clear relationship between the population of brittle-fracture controlling flaws generated in a manufactured material and the distribution of strengths in a group of...     

Quantitative determination of competitive molecular adsorption on gold nanoparticles using attenuated total reflectance-Fourier transform infrared spectroscopy

Surface-sensitive quantitative studies of competitive molecular adsorption on nanoparticles were conducted using a modified attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy method. Adsorption isotherms for thiolated poly(ethylene glycol) (SH-PEG) on gold nanoparticles (AuNPs) as a function of molecular mass (1, 5, and 20 kDa) were characterized. We find that surface density of SH-PEG on AuNPs is inversely proportional to the molecular mass (M(m)). Equilibrium binding constants for SH-PEG, obtained using the Langmuir adsorption model, show the binding affinity for SH-PEG is proportional to M(m). Simultaneous competitive adsorption between mercaptopropionic acid (MPA) and 5 kDa SH-PEG (SH-PEG5K) was investigated, and we find that MPA concentration is the dominant factor influencing the surface density of both SH-PEG5K and MPA, whereas the concentration of SH-PEG5K affects only SH-PEG5K surface density. Electrospray differential mobility analysis (ES-DMA) was employed as an orthogonal characterization technique. ES-DMA results are consistent with the results obtained by ATR-FTIR, confirming our conclusions about the adsorption process in this system. Ligand displacement competitive adsorption, where the displacing molecular species is added after completion of the ligand surface binding, was also interrogated by ATR-FTIR. Results indicate that for SH-PEG increasing M(m) yields greater stability on AuNPs when measured against displacement by bovine serum albumin (BSA) as a model serum protein. In addition, the binding affinity of BSA to AuNPs is inhibited for SH-PEG conjugated AuNPs, an effect that is enhanced at higher SH-PEG M(m) values.     

Effect of formation temperature and roughness on surface potential of octadecyltrichlorosilane self-assembled monolayer on silicon surfaces

Kelvin probe force microscopy (KPFM) and atomic force microscopy (AFM) are employed to probe the surface potential and topography of octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3] self-assembled monolayers (SAMs) on oxidized Si(100) and polycrystalline silicon surfaces as a function of deposition temperature and substrate roughness with particular attention paid to the monitoring of SAM adsorption on highly rough surfaces. In these studies, it is found that the surface potential magnitude of the adsorbed layer is larger for monolayers formed in the liquid-condensed (LC) phase than for those formed in the liquid-expanded (LE) phase. Experiments on individual islands in the LC phase show that surface potential and monolayer thickness increase with increasing island size; islands larger than about 1.5 microm reach maximum potential and height values of 48+/-4 mV and 2.7+/-0.1 nm, with respect to the underlying oxidized surface. It is also shown that KPFM is suitable for the study of monolayer adsorption on polycrystalline surfaces, for which preexisting surface texture makes the use of traditional scanning probe techniques for molecular recognition difficult. In these scenarios it is shown that OTS growth occurs preferentially along grain boundaries in fingerlike patterns having a molecular arrangement comparable to that of LC phase islands on atomically smooth silicon. These findings indicate that surface potential measurements provide a highly accurate, local means of probing monolayer morphology on rough surfaces encountered in many applications.     

Effect of Tempering on the Ductile-to-Brittle Transitional Behavior of Ni-Cr-Mo Low-Alloy Steel

Experimental Mechanics - About 10 years ago, super-high energy Charpy specimens at the National Institute of Standards and Technology were removed from inventory due to unacceptable...     

Quantifying dithiothreitol displacement of functional ligands from gold nanoparticles

Dithiothreitol (DTT)-based displacement is widely utilized for separating ligands from their gold nanoparticle (AuNP) conjugates, a critical step for differentiating and quantifying surface-bound functional ligands and therefore the effective surface density of these species on nanoparticle-based therapeutics and other functional constructs. The underlying assumption is that DTT is smaller and much more reactive toward gold compared with most ligands of interest, and as a result will reactively displace the ligands from surface sites thereby enabling their quantification. In this study, we use complementary dimensional and spectroscopic methods to characterize the efficiency of DTT displacement. Thiolated methoxypolyethylene glycol (SH-PEG) and bovine serum albumin (BSA) were chosen as representative ligands. Results clearly show that (1) DTT does not completely displace bound SH-PEG or BSA from AuNPs, and (2) the displacement efficiency is dependent on the binding affinity between the ligands and the AuNP surface. Additionally, the displacement efficiency for conjugated SH-PEG is moderately dependent on the molecular mass (yielding efficiencies ranging from 60 to 80?% measured by ATR-FTIR and ≈90?% by ES-DMA), indicating that the displacement efficiency for SH-PEG is predominantly determined by the S–Au bond. BSA is particularly difficult to displace with DTT (i.e., the displacement efficiency is nearly zero) when it is in the so-called normal form. The displacement efficiency for BSA improves to 80?% when it undergoes a conformational change to the expanded form through a process of pH change or treatment with a surfactant. An analysis of the three-component system (SH-PEG?+?BSA?+?AuNP) indicates that the presence of SH-PEG decreases the displacement efficiency for BSA, whereas the displacement efficiency for SH-PEG is less impacted by the presence of BSA.