Sanghee Lee

Colorimetric detection of pathogenic bacteria using platinum-coated magnetic nanoparticle clusters and magnetophoretic chromatography

A colorimetric method that uses platinum-coated magnetic nanoparticle clusters (Pt/MNCs) and magnetophoretic chromatography is developed to detect pathogenic bacteria. Half-fragments of monoclonal Escherichia coli O157:H7 (EC) antibodies were functionalized to Pt/MNCs and used to capture E. coli bacteria in milk. After magnetic separation of free Pt/MNCs and Pt/MNC-EC complexes from the milk, a precision pipette was used to imbibe the E. coli-containing solution, then a viscous polyethylene glycol solution. Due to difference in viscosities, the solutions separate into two liquid layers inside the pipette tip. The Pt/MNC-EC complexes were separated from the free Pt/MNCs by applying an external magnetic field, then added to a tetramethylbenzidine (TMB) solution. Catalytic oxidation of TMB by Pt produced color changes of the solution, which enabled identification of the presence of 10 cfu mL(-1) E. coli bacteria with the naked eye. The total assay time including separation, binding and detection was 30 min.

Facile and Sensitive Method for Detecting Cardiac Markers using Ubiquitous pH Meters

A sensitive and easy method was developed for the detection of the cardiac marker troponin I using magnetic immunoassay and ubiquitous pH meters. Monoclonal antibody-functionalized Fe3O4 magnetic nanoparticle clusters (MNCs) were synthesized to capture troponin in human serum, and MNC-troponin complexes were magnetically isolated using a permanent magnet. These complexes were subsequently conjugated to polyclonal antibody-functionalized acetylcholinesterase (AchE) and dispersed in acetylcholine (Ach) solution. As the Ach was hydrolyzed to choline and acetic acid, the pH of the solution decreased, and the resulting pH change was measured in real time using a pH meter. The sensitivity of detection of this assay was found to be 10 pg/mL of troponin in human serum after 10 min of the hydrolysis reaction. Further, the pH change could be determined with the naked eye from the color change of a pH indicator strip.

Magnetorheological Elastomer Films with Tunable Wetting and Adhesion Properties

We fabricated magnetorheological elastomer (MRE) films consisting of polydimethylsiloxane and various concentrations of fluorinated carbonyl iron particles. The application of a magnetic field to the MRE film induced changes in the surface morphology due to the alignment of the iron particles along the magnetic field lines. At low concentrations of iron particles and low magnetic field intensities, needle-like microstructures predominated. These structures formed more mountain-like microstructures as the concentration of iron particles or the magnetic field intensity increased. The surface roughness increased the water contact angle from 100° to 160° and decreased the sliding angle from 180° to 10°. The wettability and adhesion properties changed substantially within a few seconds simply upon application of a magnetic field. Cyclical measurements revealed that the transition was completely reversible.

Facile detection of Troponin I using dendritic platinum nanoparticles and capillary tube indicators

A facile method was developed for the detection of Troponin I (Tnl) using dendritic platinum nanoparticles and capillary tube indicators. Dendritic platinum nanoparticles were functionalized with TnI antibodies, which were used to capture TnI in human serum. The captured TnI was conjugated to the inner surface of a glass vial, to which a hydrogen peroxide (H2O2) solution was added. After sealing the glass vial with a screw cap containing a silicon septum, a capillary tube containing a drop of ink was inserted through the septum. The catalytic dissociation of H2O2 to water and oxygen increased the pressure inside the glass vial and raised the ink level in the capillary tube. The ink level increased with the platinum nanoparticle concentration, which is proportional to the TnI concentration. The sensitivity of this assay for TnI in human serum after a 5-min dissociation reaction, detected with the naked eye, was 0.1 ng/mL, which was better than the sensitivity of the conventional colorimetric method using the TMB oxidation reaction under the same experimental conditions. A control experiment using alpha-fetoprotein, interleukin-5, and C-reactive protein revealed that the developed method was highly selective for the detection of.

Detection of Salmonella bacteria in milk using gold-coated magnetic nanoparticle clusters and lateral flow filters

A novel method was developed for the detection of Salmonella bacteria using gold-coated magnetic nanoparticle clusters (Au/MNCs) and lateral flow filters. Unlike a conventional lateral flow immunoassay (LFA) membrane, where antibodies are immobilized on test and control lines to observe a signal, the test line in the new method is formed simply by pressing a nitrocellulose membrane to decrease its thickness. Half-antibody fragment-functionalized Au/MNCs were used to capture Salmonella in milk and then dispersed in a small volume of buffer solution in which one end of the lateral flow filter was immersed. The free Au/MNCs rose to the pressed test line, while the Salmonella–Au/MNC complexes remained in the solution because they were too large to pass through the pores inside the lateral flow filter. The flow of free Au/MNCs was blocked at the test line because of the reduced pore size, and their accumulation caused the test line to darken. The color of the test line was inversely proportional to the Salmonella concentration, and the limit of detection for Salmonella in milk was determined to be 103 CFU mL−1 after image processing. A control test using Escherichia coli, Staphylococcus aureus, and Vibrio parahaemolyticus revealed that the new lateral flow filtration method with Au/MNCs is highly selective for the detection of Salmonella.

Water droplet evaporation from sticky superhydrophobic surfaces

The evaporation dynamics of water from sticky superhydrophobic surfaces was investigated using a quartz crystal microresonator and an optical microscope. Anodic aluminum oxide (AAO) layers with different pore sizes were directly fabricated onto quartz crystal substrates and hydrophobized via chemical modification. The resulting AAO layers exhibited hydrophobic or superhydrophobic characteristics with strong adhesion to water due to the presence of sealed air pockets inside the nanopores. After placing a water droplet on the AAO membranes, variations in the resonance frequency and Q-factor were measured throughout the evaporation process, which were related to changes in mass and viscous damping, respectively. It was found that droplet evaporation from a sticky superhydrophobic surface followed a constant contact radius (CCR) mode in the early stage of evaporation and a combination of CCR and constant contact angle modes without a Cassie–Wenzel transition in the final stage. Furthermore, AAO membranes with...

One-step immobilization of antibodies on ZIF-8/Fe3O4 hybrid nanoparticles for the immunoassay of Staphylococcus aureus

Zeolitic imidazolate framework-8 (ZIF-8)-coated Fe3O4 magnetic nanoparticle clusters (MNCs) were synthesized and used to detect pathogenic bacteria in milk. Hydrothermally synthesized MNCs were encapsulated with ZIF-8 via sonochemical reactions. Half fragments of monoclonal Staphylococcus antibodies were conjugated to the low-coordinated Zn sites located on the outer layer of ZIF-8 via Zn–S bonding, which allows one-step immobilization of antibodies with favorable orientations on ZIF-8. Furthermore, ZIF-8 encapsulation improved the stability of MNCs in water by suppressing the Fe3O4 oxidation. After the capture and magnetic separation of Staphylococcus in milk using hybrid nanoparticles, bacteria concentration was determined with a portable ATP luminometer and the detection limit was found to be 300 cfu mL−1.

Immunomagnetic separation and size-based detection of Escherichia coli O157 at the meniscus of a membrane strip

We developed a facile method for the detection of pathogenic bacteria using gold-coated magnetic nanoparticle clusters (Au@MNCs) and porous nitrocellulose strips. Au@MNCs were synthesized and functionalized with half-fragments of Escherichia coli O157 antibodies. After the nanoparticles were used to capture E. coli O157 in milk and dispersed in a buffer solution, one end of a test strip was dipped into the solution. Due to the size difference between the E. coli–Au@MNC complexes (approximately 1 μm) and free Au@MNCs (approximately 180 nm), only E. coli–Au@MNC complexes accumulated at the meniscus of the test strip and induced a color change. The color intensity of the meniscus was proportional to the E. coli concentration, and the detection limit for E. coli in milk was 103 CFU mL−1 by the naked eye. The presence of E. coli–Au@MNC complexes at the meniscus was confirmed using a real-time PCR assay. The developed method was highly selective for E. coli when compared with Salmonella typhimurium, Listeria monocytogenes, and Staphylococcus aureus.

Direct synthesis of platinum nanodots in ZIF-8/Fe3O4 core–shell hybrid nanoparticles

A novel method was developed for synthesizing platinum nanodots inside zeolitic imidazolate framework nanostructures without using additional reducing agents. Fe3O4 magnetic nanoparticle clusters (MNCs) were synthesized and coated with ZIF-8 (ZIF) shells via a hydrothermal reaction. Upon addition of ZIF/MNC hybrid nanoparticles into a platinum precursor (K2PtCl4) solution, platinum ions were reduced to metallic platinum nanodots by the 2-methyl imidazolate groups. The resulting platinum nanodots were ∼2 nm in diameter and uniformly distributed in the pores of the ZIF layer. The catalytic activity of the platinum nanodots was examined by using Pt/ZIF/MNCs for the reduction of 4-nitrophenol. The resulting high catalytic activity was attributed to the high surface area of the platinum nanodots and the absence of capping layers. Furthermore, the hybrid nanoparticles were recovered using a permanent magnet and were found to maintain their catalytic activity after multiple cycles.