Paul Z. Chen

Branching and size of CTAB-coated gold nanostars control the colorimetric detection of bacteria

Rapid detection of pathogenic bacteria is challenging because conventional methods require long incubation times. Nanoparticles have the potential to detect pathogens before they can cause an infection. Gold nanostars have recently been used for colorimetric biosensors but they typically require surface modification with antibodies or aptamers for cellular detection. Here, CTAB-coated gold nanostars have been used to rapidly (<5 min) detect infective doses of a model Gram-positive pathogen Staphylococcus aureus by an instrument-free colorimetric method. Varying the amounts of gold nanoseed precursor and surfactant can tune the size and degree of branching of gold nanostars as studied here by transmission electron microscopy. The size and morphology of gold nanostars determine the degree and rate of color change in the presence of S. aureus. The optimal formulation achieved maximum color contrast in the presence of S. aureus and produced a selective response in comparison to polystyrene microparticles and liposomes. These gold nanostars were characterized using UV-Visible spectroscopy to monitor changes in their surface plasmon resonance peaks. The visual color change was also quantified over time by measuring the RGB components of the pixels in the digital images of gold nanostar solutions. CTAB-coated gold nanostars serve as a promising material for simple and rapid detection of pathogens.

Adhesion characteristics ofStaphylococcus aureusbacterial cells on funnel-shaped palladium–cobalt alloy nanostructures

ABSTRACT The adhesion properties of Staphylococcus aureus on palladium–cobalt (Pd–Co) alloy nanostructures with various cross-sectional geometries have been characterised. They include solid core, hollow, c-shaped, and x-shaped pillars. These pillars have unique funnel-shaped geometric features on the top surfaces with average included angles between ∼142o and ∼149o. The success rates of cell attachment on these pillar tops were quantified by using field emission scanning electron microscopy techniques. Results show the Staphylococcus aureus attachment rates of Pd–Co solid core, hollow, and x-shaped pillars are statistically indistinguishable with success rate up to 82%. X-shaped pillars have the lowest attachment rate among the four geometries of 46 ± 5%.

Magnetic flocculation for nanoparticle separation and catalyst recycling

Nanoparticles are heavily researched for environmental applications, such as photocatalytic water treatment, however practical separation of nanoparticles from colloidal dispersions remains a critical challenge. Here, we demonstrate a new approach to nanoparticle recovery, combining the advantages of flocculation with magnetic separation to enable simple collection of non-magnetic nanoparticles. Flocculant polymers were coated onto magnetic nanoparticles (Fe3O4@SiO2) to prepare reusable magnetic flocculants (MFs). When added to colloidal nanoparticle dispersions, MFs aggregate with the suspended nanoparticles to form magnetically responsive flocs, which upon separation can be reversibly deflocculated for nanoparticle release, and reused in a closed loop process. High separation efficiency was attained in a variety of nanoparticle suspensions, including Au, Ag, Pd, Pt, and TiO2, stabilized by different coatings and surface charge. The MFs were shown to be recyclable for photocatalytic treatment of naphthenic acids in oil sands process-affected water (OSPW) and selenate in flue gas desulfurization wastewater (FGDW). Magnetic flocculation thus represents a general platform and alternative paradigm for nanoparticle separation, with potential applications in water treatment and remediation of nanoparticle pollution. Furthermore, given that flocculant chemicals can be recovered and reused in this process, magnetic flocculation may also serve as an environmentally sustainable solution to conventional flocculation challenges.

Dual-band linear filter assisted envelope memory polynomial for linearizing multi-band power amplifiers

This paper proposes a dual-band linear filter assisted envelope memory polynomial model (EMP) devised for analog-RF predistortion (ARFPD) systems. The proposed model consists of two finite-impulse-response (FIR) filters preceding a newly formulated dual-band EMP block in forming the pruned 2D-FIR-EMP model. A linear estimation algorithm is devised to identify the coefficients of the proposed pruned 2D-FIR-EMP model. Furthermore, a proof of concept of the digitally-assisted dual-band ARFPD system based on two RF vector multipliers (RF-VM) built using off-the-shelf (OTS) components is presented. Measurement results obtained using the proposed pruned 2D-FIR-EMP proof-of-concept predistorter has demonstrated excellent linearization results in compensating for the distortions exhibited by a gallium nitride Doherty power amplifier driven by dual-band signals.

Trapping polystyrene and latex nanospheres inside hollow nanostructures using Staphylococcus aureus cells

A new methodology was developed to trap polystyrene or latex nanospheres in nanocrystalline nickel columnar nanostructures with various cross-sectional geometries. After filling the column interiors with polystyrene or latex nanospheres with diameters of ∼300 and 500 nm, respectively, the top openings of these small metallic nanopillars were capped with Staphylococcus aureus (S. aureus) bacterial cells. High resolution scanning electron microscopy inspection revealed that these bacterial cells were able to cover the top openings of C-shaped pillars within an hour of incubation at 37 °C. Results also demonstrated that capping S. aureus adhesive strengths to the nickel structures are greater than the cell wall cohesive strengths.

Low-fouling characteristics of ultrathin zwitterionic cysteine SAMs

Surface fouling remains an exigent issue for many biological implants. Unwanted solutes adsorb to reduce device efficiency and hasten degradation while increasing the risks of microbial colonization and adverse inflammatory response. To address unwanted fouling in modern implants in vivo, surface modification with antifouling polymers has become indispensable. Recently, zwitterionic self-assembled monolayers, which contain two or more charged functional groups but are electrostatically neutral and form highly hydrated surfaces, have been the focus of many antifouling coatings. Reports using various compositions of zwitterionic polymer brushes have demonstrated ultralow fouling in the ng/cm2 range. These coatings, however, are thick and can hinder the target application of biological devices. Here, we report an ultrathin (8.52 Å) antifouling self-assembled monolayer composed of cysteine that is amenable to facile fabrication. The antifouling characteristics of the zwitterionic surfaces were evaluated against bovine serum albumin, fibrinogen, and human blood in real time using quartz crystal microbalance and surface plasmon resonance imaging. Compared to untreated gold surfaces, the ultrathin cysteine coating reduced the adsorption of bovine serum albumin by 95% (43 ng/cm2 adsorbed) after 3 h and 90% reduction after 24 h. Similarly, the cysteine self-assembled monolayer reduced the adsorption of fibrinogen as well as human blood by >90%. The surfaces were further characterized using scanning electron microscopy: protein-enhanced adsorption and cellular adsorption in human blood was found on untreated surfaces but not on the cysteine SAM-protected surfaces. These findings suggest that surfaces can be functionalized with an ultrathin layer of cysteine to resist the adsorption of key proteins, with performance comparable to zwitterionic polymer brushes. As such, cysteine surface coatings are a promising methodology to improve the long-term utility of biological devices.