Kangtaek Lee

Solution of the population balance equation using constant-number Monte Carlo

Abstract We formulate a Monte Carlo simulation of the mean-field population balance equation by tracking a sample of the population whose size (number of particles in the sample) is kept constant throughout the simulation. This method amounts to expanding or contracting the physical volume represented by the simulation so as to continuously maintain a reaction volume that contains constant number of particles. We call this method constant-number Monte Carlo to distinguish it from the more common constant-volume method. In this work, we expand the formulation to include any mechanism of interest to population balances, whether the total mass of the system is conserved or not. The main problem is to establish connection between the sample of particles in the simulation box and the volume of the physical system it represents. Once this connection is established all concentrations of interest can be determined. We present two methods to accomplish this, one by requiring that the mass concentration remain unaffected by any volume changes, the second by applying the same requirement to the number concentration. We find that the method based on the mass concentration is superior. These ideas are demonstrated with simulations of coagulation in the presence of either breakup or nucleation.

Simultaneous coagulation and break-up using constant-N Monte Carlo

We present a new Monte Carlo method for solving the population balance with multiple growth processes. The method samples a constant number of particles regardless of whether the actual growth process results in increase or decrease of the particle concentration. By decoupling the size of the simulated sample from the concentration of the actual system we achieve constant accuracy throughout the simulation. We apply this method to coagulation with simultaneous binary break-up. We examine the results for three cases with analytical solutions and show that the constant-N method yields accurate results.

The effects of sub-lethal concentrations of silver nanoparticles on inflammatory and stress genes in human macrophages using cDNA microarray analysis.

Because of the limited information on size-dependent particle-mediated effects, the present study was conducted to determine if the changes in induced protein expression between 5 nm silver nanoparticles and 100 nm particles after exposure to sub-lethal concentrations. A total of 28,000 cDNA profiles were screened using 5 nm silver nanoparticles and 100 nm silver nanoparticles in a macrophage cell line. Based on results obtained from cDNA microarray we also assessed protein levels of hemeoxygenase-1 (HO-1), heat shock protein-70 (HSP-70) and interleukin-8 (IL-8), which were shown to significantly increase. Together with results obtained using N-acetylcystein (NAC), we were able to clearly show that low level and early stage exposure to 5 nm silver nanoparticles, but not 100 nm, induces expression of IL-8 as well as stress genes against reactive oxygen species (ROS). Therefore, we provide important data to understand and identify the early effects of silver nanoparticles on the immune system.

Surface-enhanced plasmon resonance detection of nanoparticle-conjugated DNA hybridization.

We have investigated surface-enhanced plasmon resonance detection of DNA hybridization. Surface enhancement was based on the excitation of localized surface plasmon using subwavelength nanogratings, at a 300 nm period, coated with 24-mer ssDNA oligonucleotide, while optical signatures of DNA were amplified at the same time by gold nanoparticles conjugated with complementary ssDNA strands. When using nanoparticles of different sizes, maximum sensitivity enhancement, of more than 18 times, was obtained with nanoparticles of 20 nm diameter. This enhancement is mainly due to nanoparticle-associated signal amplification. Additional surface enhancement boosted the detection sensitivity by 57%. We have also confirmed the sensitivity enhancement to be linearly related to nanoparticle volume.

Grating-based surface plasmon resonance detection of core-shell nanoparticle mediated DNA hybridization

In this report, we have investigated enhanced surface plasmon resonance (SPR) detection of DNA hybridization using gold core - silica shell nanoparticles in localized plasmonic fields. The plasmonic fields were localized by periodic linear gratings. Experimental results measured for hybridization of 24-mer single-stranded DNA oligomers suggest that core-shell nanoparticles (CSNPs) on gratings of 400 nm period provide enhanced optical signatures by 36 times over conventional thin film-based SPR detection. CSNP-mediated DNA hybridization produced 3 times larger angular shift compared to gold nanoparticles of the same core size. We have also analyzed the effect of structural variation. The enhancement using CSNPs was associated with increased surface area and index contrast that is combined by improved plasmon coupling with localized fields on gratings. The combined approach for conjugated measurement of a biomolecular interaction on grating structures is expected to lower the limit of detection to the order of a few tens of fg/mm(2).

Vascular tube formation and angiogenesis induced by polyvinylpyrrolidone-coated silver nanoparticles.

Silver nanoparticles (AgNPs) are one of the most commonly used nanomaterials due to their antibacterial properties. In this study, we examined the effects of polyvinylpyrrolidone (PVP)-coated AgNPs (average size 2.3nm) on angiogenesis in both an in vivo model and an in vitro endothelial cell line, SVEC4-10. Increased angiogenesis was detected around the injection site of AgNP-containing Matrigel in vivo. AgNPs also increased the infiltration of endothelial cells and the hemoglobin (Hb) content in AgNP-Matrigel plugs implanted into mice. AgNPs induced endothelial cell tube formation on growth factor-reduced Matrigel, production of reactive oxygen species (ROS), and production of angiogenic factors, such as vascular endothelial growth factor (VEGF) and nitric oxide (NO), in SVEC4-10 cells. In addition, AgNPs promoted the activation of FAK, Akt, ERK1/2, and p38, which are all involved in VEGF receptor (VEGFR)-mediated signaling. Finally, AgNP-treated tumors caused angiogenesis around tumors in B16F10 melanomas after they were injected into mice, and the Hb concentration in the tumors increased in a concentration-dependent manner with AgNP treatment. Thus, our study suggests that exposure to AgNPs can cause angiogenesis through the production of angiogenic factors.

Comparison Study of Mixing Effect on Batch Cooling Crystallization of 3-Nitro-1,2,4-triazol-5-one (NTO) Using Mechanical Stirrer and Ultrasound Irradiation

The insensitive explosive 3-nitro-1,2,4-triazol-5-one (NTO) has been recrystallized from water in an effort to prepare crystals with smaller size and narrower distribution in a batch cooling crystallizer. Two mixing devices, i.e., a mechanically stirred system with and without ultrasound in aqueous media were employed to compare the mixing effect on the crystallization. Under ultrasound irradiation, the metastable zone width was significantly reduced by more than 2 fold and the crystal size was shifted from 140∼160 μm to 50∼70 μm with a narrower CSD compared to the mechanically stirred system. However in the mechanical stirrer, the mixing effect on NTO crystallization was negligible if the impeller speed was sufficient to suspend all crystals in the crystallizer. It was found that the crystal growth was not influenced by mixing. We suggest that the NTO crystals were formed by primary heterogeneous nucleation that is common in batch cooling system. Finally, the population balance model (PBM), with the empirical nucleation and growth kinetic expressions, was solved numerically to predict the crystal size and the CSD with batch time, and the results were in good agreement with the experimental data.

Colorimetric genotyping of single nucleotide polymorphism based on selective aggregation of unmodified gold nanoparticles

We have developed a colorimetric method for genotyping of single nucleotide polymorphism (rs2131877) in 15 human DNA samples using selective aggregation of unmodified gold nanoparticles. First, we designed two different types of oligonucleotide probes with either thymine or cytosine at the end that are perfect complementary to the target allele sequence. After hybridization of the probe and target DNAs, the double-stranded DNA was added to the unmodified gold nanoparticle suspensions. By adjusting the salt concentration, we could induce aggregation of gold nanoparticles exclusively for the samples with the perfectly matched double-stranded DNAs, which resulted in a distinct color change of the suspension. This enabled us to discern samples with three different genotypes of the target sequences by naked eye: (i) the genotype with only T (thymine) alleles; (ii) that with only C (cytosine) alleles; (iii) that with both T and C alleles. We also confirmed these results by an independent direct sequencing method. These results suggest that the selective aggregation of unmodified gold nanoparticles can successfully be used to discern genotypes of single nucleotide polymorphisms.

Fluorescence-based detection of point mutation in DNA sequences by CdS quantum dot aggregation.

We present a novel method for the detection of single base mismatch based on fluorescence quenching that unmodified CdS quantum dots exhibit upon aggregation. Target DNA sequences of interest are breast cancer 2 (BRCA2) and signal-induced proliferation-associated gene 1 (Sipa1) sequences. We monitor aggregation of CdS quantum dots upon addition of double-stranded DNAs at different salt concentration using quasi-elastic light scattering (QELS), transmission electron microscopy (TEM), photoluminescence spectroscopy, and zeta potential measurement. Our results indicate that the double-stranded DNA with a perfectly matched sequence can easily be discerned by naked eye from the single base mismatched one due to the fluorescence quenching phenomenon caused by selective aggregation of the CdS quantum dots.

Electrodeposition of Manganese-Nickel Oxide Films on a Graphite Sheet for Electrochemical Capacitor Applications

Manganese-nickel (Mn-Ni) oxide films were electrodeposited on a graphite sheet in a bath consisting of manganese acetate and nickel chloride, and the structural, morphological, and electrochemical properties of these films were investigated. The electrodeposited Mn-Ni oxide films had porous structures covered with nanofibers. The X-ray diffractometer pattern revealed the presence of separate manganese oxide (γ-MnO2) and nickel oxide (NiO) in the films. The electrodeposited Mn-Ni oxide electrode exhibited a specific capacitance of 424 F/g in Na2SO4 electrolyte. This electrode maintained 86% of its initial specific capacitance over 2000 cycles of the charge-discharge operation, showing good cycling stability.