Studies of the Properties of Designed Nanoparticles Using Atomic Force Microscopy

The purpose of the research in this dissertation was to elucidate the intrinsic properties of how nanoparticles are different from bulk materials. This was done by mechanical and electronic studies of the properties of designed nanoparticles using advanced modes of atomic force microscopy. Information relating to the work functions, contact potential difference, Young’s Moduli, elasticity, and viscoelasticity can be investigated using state-of-the-art atomic force microscope (AFM) experiments. Subsurface imaging of polystyrene encapsulated cobalt nanoparticles was achieved for the first time using Force Modulation Microscopy (FMM) in conjunction with contact mode AFM. Previously prepared sample of polystyrene coated cobalt nanoparticles were studied. Tapping-mode AFM was used to evaluate the size of coated nanoparticles. Force modulation microscopy was used to visualize details of the outer polystyrene coating. Differences between the softer polystyrene outer coating and the harder cobalt nanoparticle core was visualized based upon the elastic and viscoelastic properties. Variances in sample elasticity were monitored via the amplitude channel that monitors the oscillation amplitude of the cantilever while scanning. Viscoelastic differences were mapped by the phase channel which provides information of the phase lag of the probe. The identification of designed nanoparticles based upon electrochemical properties was evaluated using the Kelvin Probe Force Microscopy (KPFM) mode of AFM. The contact potential difference between the tip and the sample is measured using an AC bias that is offset with a compensating DC bias while operating in either tapping-mode or non-contact mode AFM. The contact potential difference is more commonly referred to as the difference in work function between the tip and the sample. The work function of a material can be calculated using a reference material with a known work function. Cobalt nanoparticles and gold nanoparticles were imaged using KPFM and baseline experimental contact potential difference values were obtained. Thus far, co-deposition of a mixed nanoparticle solution led to inconclusive results as the experimental and theoretical contact potential difference values were calculated. However, future studies relating to this experiment are planned

Risk of COVID-19 after natural infection or vaccinationResearch in context

Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health