Victor M. Kariuki

Greener synthesis and characterization, antimicrobial and cytotoxicity studies of gold nanoparticles of novel shapes and sizes

We hereby present a novel approach for the synthesis of gold nanoparticles (AuNPs) using water soluble, naturally-derived flavonoids. Quercetin pentaphosphate (QPP), quercetin sulfonic acid (QSA) and apigenin triphosphate (ATRP) were utilized as reducing agents and stabilizers for the gold nanoparticle synthesis. Synthesis was achieved at room temperature using water as a solvent and it requires no capping agents. The AuNPs were characterized using UV-vis, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive absorption spectroscopy (EDS), high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED). The resulting AuNPs were spherical, triangular, cubicle, hexagonal and rectangular in shape. The average particle sizes of 4.85 nm, 9.56 nm and 13.54 nm were obtained for the nanoparticles derived from QPP, ATRP and QSA respectively. The surface plasmon resonance peak of the AuNPs derived from QSA, ATRP and QPP was observed at 541 nm, 544 nm and 547 nm respectively. The AuNPs exhibited excellent antibacterial activities of 99.9%, 100% and 99.9% growth inhibition for Escherichia coli ATCC® 25922™, Staphylococcus epidermidis ATCC® 12228™ and Citrobacter freundii ATCC® 8090 at 104 cfu inoculations. The AuNPs were observed to retain stability after 150 days compared to those reported using conventional approaches of 30 days. This work also provides insights into the mechanism of flavonoid-based nanoparticle synthesis while eliminating the use of hazardous and toxic organic solvents and adopting the use of water as a solvent.

Synthesis and antibacterial characterization of sustainable nanosilver using naturally-derived macromolecules.

Greener nanosynthesis utilizes fewer amounts of materials, water, and energy; while reducing or replacing the need for organic solvents. A novel approach is presented using naturally-derived flavonoids including Quercetin pentaphosphate (QPP), Quercetin sulfonic acid (QSA) and Apigenin Triphosphate (ATRP). These water soluble, phosphorylated flavonoids were utilized both as reducing agent and stabilizer. The synthesis was achieved at room temperature using water as a solvent and it requires no capping agents. The efficiency of the resulting silver nanoparticle synthesis was compared with naturally-occurring flavonoid such as Quercetin (QCR). Results show that QCR reduced Ag(+) faster followed by QPP, QSA and ATRP respectively. This is the first evidence of direct utilization of QCR for synthesis of silver nanoparticles (AgNPs) in water. The percentage conversion of Ag(+) to Ag(0) was determined to be 96% after 35min. The synthesized nanoparticles were characterized using Transmission electron microscopy (TEM), Energy dispersive absorption spectroscopy (EDS), UV-vis spectroscopy, High resolution TEM (HR-TEM) with selected area electron diffraction (SAED). The particle sizes ranged from 2 to 80nm with an average size of 22nm and in the case of ATRP, the nanoparticle shapes varied from spherical to hexagonal with dispersed particle size ranging from 2 to 30nm. Crystallinity was confirmed by XRD and the SAED of (111), (200), and the fringes observed in HRTEM images. Results were in agreement with the UV resonance peaks of 369-440nm. The particles also exhibit excellent antibacterial activity against Staphylococcus epidermidis, Escherichia coli and Citrobacter freundii in water.

Capture, isolation and electrochemical detection of industrially-relevant engineered aerosol nanoparticles using poly (amic) acid, phase-inverted, nano-membranes

Workplace exposure to engineered nanoparticles (ENPs) is a potential health and environmental hazard. This paper reports a novel approach for tracking hazardous airborne ENPs by applying online poly (amic) acid membranes (PAA) with offline electrochemical detection. Test aerosol (Fe2O3, TiO2 and ZnO) nanoparticles were produced using the Harvard (Versatile Engineered Generation System) VENGES system. The particle morphology, size and elemental composition were determined using SEM, XRD and EDS. The PAA membrane electrodes used to capture the airborne ENPs were either stand-alone or with electron-beam gold-coated paper substrates. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to conceptually illustrate that exposure levels of industry-relevant classes of airborne nanoparticles could be captured and electrochemically detected at PAA membranes filter electrodes. CV parameters showed that PAA catalyzed the reduction of Fe2O3 to Fe(2+) with a size-dependent shift in reduction potential (E(0)). Using the proportionality of peak current to concentration, the amount of Fe2O3 was found to be 4.15×10(-17)mol/cm(3) PAA electrodes. Using EIS, the maximum phase angle (Φmax) and the interfacial charge transfer resistance (Rct) increased significantly using 100μg and 1000μg of TiO2 and ZnO respectively. The observed increase in Φmax and Rct at increasing concentration is consistent with the addition of an insulating layer of material on the electrode surface. The integrated VENGES/PAA filter sensor system has the potential to be used as a portable monitoring system.

Selective Sensing and Imaging of Penicillium italicum Spores and Hyphae Using Carbohydrate–Lectin Interactions

The blue-green mold Penicillium italicum is among the most problematic post-harvest plant infections limiting the integrity of citrus and many other crops during storage and transportation, but there is no sensor for its on-site or field detection. We hereby, for the first time, report the development of novel biomolecular sensor for assessing the presence of P. italicum spores and hyphae using carbohydrate-lectin recognitions. Two approaches were developed: (i) lateral tests using standalone poly(amic) acid (PAA) membranes and glass surfaces and (ii) quantitative tests on 96-well polystyrene plates and paper electrodes. In both cases, the surfaces were functionalized with novel derivatized sugar based ligands while staining was performed with gold nanoparticles. Both approaches provided strong signals for 104 spores/mL of P. italicum isolated from experimentally infected lemons as the lowest-reliable concentration. The 96-well plate-based gave the most sensitive detection with a 4 × 102 spores/mL limit of detection, a linear dynamic range between 2.9 × 103 and 6.02 × 104 spores/mL ( R2 = 0.9939) and standard deviation of less than 5% for five replicate measurements. The selectivity of the ligands was tested against Trichaptum biforme, Glomerulla cingulata ( Colletotrichum gloeosporioides), and Aspergillus nidulans fungi species. The highest selectivity was obtained using the sugar-based gold-nanoparticles toward both the spores and the hyphae of P. italicum. The advanced specificity was provided by the utilized sugar ligands employed in the synthesis of gold nanoparticles and was independent from size and shapes of the AuNPs. Accuracy of the sensor response showed dramatic dependence on the sample preparation. In the case of 5-10 min centrifugation at 600 rpm, the spores can be isolated free from hyphae and conidiophore, for which spiked recovery was up to 95% (std ±4). In contrast, for gravity-based precipitation of hyphae, the spiked recovery was 88% (std 11).

High performance paper-based microbial fuel cells using nanostructured polymers

In this work, we report paper-based microbial fuel cells (MFCs) that produce high power and current densities from one drop of bacteria-containing liquid. The devices feature (i) a simple and versatile fabrication technique by using paper as a substrate and (ii) an exceptional performance by incorporating novel nanostructured polymers, PAA-Poly (amic) acid) and PPDD-Poly(pyromellitic dianhydride-p-phenylene diamine), into the paper substrate. Four 3-D MFC configurations were designed by using different numbers of 2-D sheets of paper layers. Each device integrated four functional compartments (i.e. anode, reservoir, proton exchange membrane, and air-cathode) into one, two, three or four paper layers, respectively. The nanostructured polymers were engineered as a proton exchange membrane to enhance ion traveling efficiency or an oxygen mitigating layer to prevent diverting electrons away from the anode. Among the four MFC devices with different numbers of layers, two-layer paper-based MFC generated the highest current density of 47|UA/cm2 and power density of 4|UW/cm2, both of which are substantially greater than achieved by previous paper-based MFCs and even comparable to that of conventional micro-sale counterparts.

Erratum to: 3D π-Conjugated Poly(amic) Acid Polymer as Support Matrices for Ethanol Electro-Oxidation on Palladium and Platinum Catalysts

The search for new catalyst support matrices to replace the conventional carbon black support has been an important research field for decades. Aromatic conducting polymer (ACP)-based matrices have been found as prospective candidates. Poly(amic) acid (PAA) is one such polymer. We hereby present the catalytic activity and stability of electrodeposited palladium (PdNPs) and platinum (PtNPs) nanoparticles stabilized with PAA. We fabricated four electrodes: Pd/GCE, PAA/Pd/GCE, Pt/GCE, and PAA/Pt/GCE. Cyclic voltammetry and chronoamperometry were employed to assess the ethanol oxidation reaction using PAA as support matrix. PAA was used as support to enhance the stability of PdNPs and PtNPs in basic and acidic media, respectively. The synthesized PAA was characterized using NMR and FTIR. Scanning electron microscopy and X-ray diffraction (XRD) were used for nanoparticle characterization. Test results revealed that the presence of PAA layer on the catalysts leads to sluggish electron transfer kinetics as deduced from higher forward and reverse current densities (5 and 11 mA/cm2) for PdNPs on glassy carbon (Pd/GCE) electrode compared to PAA-stabilized PdNPs (PAA/Pd/GCE) (0.6 and 1.2 mA/cm2), respectively. A similar trend was reported for PtNPs. However, in presence of PAA, both PtNPs and PdNPs were observed to provide stability at up to 900 and 150 cycles, respectively. Chronoamperometric results reinforced the catalyst stabilization effect of the polymer, with findings revealing that the steady-state current density of PAA/Pd/GCE was ∼2.5 times higher than the bare Pd/GCE.