Steven R. Saunders

Size-selective fractionation of nanoparticles at an application scale using CO2 gas-expanded liquids

Size-based fractionation of nanoparticles remains a non-trivial task for the preparation of well-defined nanomaterials for certain applications and fundamental studies. Typical fractionation techniques prove to be inefficient for large nanoparticle quantities due to several factors including the expense of equipment, throughput constraints, and the amount of organic solvent waste produced. Through the use of the pressure-tunable physico-chemical properties of CO2-expanded liquids, a rapid, precise, and environmentally sustainable size-selective fractionation of ligand-stabilized nanoparticles is possible through simple variations in applied CO2 pressure. An apparatus capable of fractionating large quantities of nanoparticles into distinct fractions with the ability to control mean diameters and size distributions has been developed. This apparatus consists of three vertically mounted pressure vessels connected in series with needle valves. This process, at current design scales, operated at room temperature, and CO2 pressures between 0 and 50 bar, results in a batch size-selective fractionation of a concentrated nanoparticle dispersion. This paper presents this new apparatus and the separation results of various single pass fractionations as well as recursive fractionations.

Production of gold nanoparticles by electrode-respiring Geobacter sulfurreducens biofilms.

The goal of this work was to synthesize gold nanoparticles (AuNPs) using electrode-respiring Geobacter sulfurreducens biofilms. We found that AuNPs are generated in the extracellular matrix of Geobacter biofilms and have an average particle size of 20nm. The formation of AuNPs was verified using TEM, FTIR and EDX. We also found that the extracellular substances extracted from electrode-respiring G. sulfurreducens biofilms reduce Au3+ to AuNPs. From FTIR spectra, it appears that reduced sugars were involved in the bioreduction and synthesis of AuNPs and that amine groups acted as the major biomolecules involved in binding.

Biological synthesis of nanoparticles in biofilms

The biological synthesis of nanoparticles (NPs) by bacteria and biofilms via extracellular redox reactions has received attention because of the minimization of harmful chemicals, low cost, and ease of culturing and downstream processing. Bioreduction mechanisms vary across bacteria and growth conditions, which leads to various sizes and shapes of biosynthesized NPs. NP synthesis in biofilms offers additional advantages, such as higher biomass concentrations and larger surface areas, which can lead to more efficient and scalable biosynthesis. Although biofilms have been used to produce NPs, the mechanistic details of NP formation are not well understood. In this review, we identify three critical areas of research and development needed to advance our understanding of NP production by biofilms: 1) synthesis, 2) mechanism and 3) stabilization. Advancement in these areas could result in the biosynthesis of NPs that are suitable for practical applications, especially in drug delivery and biocatalysis. Specifically, the current status of methods and mechanisms of nanoparticle synthesis and surface stabilization using planktonic bacteria and biofilms is discussed. We conclude that the use of biofilms to synthesize and stabilize NPs is underappreciated and could provide a new direction in biofilm-based NP production.

Switchable Surfactants for the Preparation of Monodisperse, Supported Nanoparticle Catalysts

Synthesis methods for the preparation of monodisperse, supported nanoparticles remain problematic. Traditional synthesis methods require calcination following nanoparticle deposition to remove bound ligands and expose catalytic active sites. Calcination leads to significant and unpredictable growth of the nanoparticles resulting in polydisperse size populations. This undesired increase in nanoparticle size leads to a decrease in catalytic activity due to a loss of total surface area. In this work, we present the use of silylamines, a class of switchable solvents, for the preparation of monodisperse, supported nanoparticles. Silylamines are switchable molecules that convert between molecular and ionic forms by reaction with CO2. Upon addition of an alkane, the switchable solvent behaves as a switchable surfactant (SwiS). The SwiS is used to template nanoparticles to aid in synthesis and subsequently used to release nanoparticles for deposition onto a support material. The use of SwiS allowed for the preservation of nanoparticle diameter throughout the deposition process. Finally, it is demonstrated that supported gold nanoparticle catalysts prepared using SwiS are up to 300% more active in the hydrogenation of 4-nitrophenol than their traditionally prepared analogues.

Manipulating ligand–nanoparticle interactions and catalytic activity through organic-aqueous tunable solvent recovery

Dispersed gold nanoparticles demonstrate much higher catalytic activities compared to their supported counterparts. Current methods for nanoparticle recovery are difficult to scale up, can alter the morphology of the nanoparticles, require large amounts of energy or solvents, and/or need highly specialized syntheses. Herein, we propose a facile system for the recovery of nanoparticles from reaction mixtures using Organic-Aqueous Tunable Solvents (OATS). OATS are homogeneous mixtures of an organic solvent and water which have the inherent property to phase separate into a heterogeneous mixture under moderate CO2 pressure. A 60 vol% acetonitrile and 40 vol% water mixture was chosen to disperse gold nanoparticles for these experiments. Poly(vinylpyrrolidone) (PVP) was selected because it is water soluble and is believed to allow access to the entire metal surface for reactive compounds. We synthesized gold nanoparticles of average size 9.4 ± 1.4 nm and performed four different thermal treatments (no thermal treatment, 40, 50, 60 °C). The phase separation of the OATS mixture was determined to occur between 9.6 and 11.3 bar of absolute CO2 pressure. Complete recovery of gold nanoparticles was achieved for all thermal treatments based on UV-vis absorbance of the localized surface plasmons collected from each phase. Catalytic activity of the nanoparticles was benchmarked using the hydrogenation of 4-nitrophenol. Reduction of catalytic activity occurred after the nanoparticles underwent thermal treatments or were subjected to pressure separation in OATS but still remained highly active. The decrease in catalytic activity can be attributed to additional PVP functional groups having passivated the surface of the nanoparticles and blocking active sites as the system minimized free energy indicating that PVP does not allow access to the entire nanoparticle surface. The additional surface-bound ligands also prevented precipitation and growth in OATS, but slowed dynamic surface restructuring, ultimately decreasing the apparent catalytic activity.