Research

The Department is engaged in four major areas of research. These areas together with their associated investigators are as follows:

A selection of publications from these research areas is listed below.

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2018

Electrowetting on dielectric (EWOD) of sessile microdroplets containing gold nanoparticles

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By Crismar Patacsil, Erwin P. Enriquez, and Raphael A. Guerrero 

Our electrowetting set-up consists of a bottom copper electrode coated with a thin insulating layer of dielectric (uncured polydimethylsiloxane) and a platinum wire upper electrode in contact with the sessile gold nanofluid droplet sitting on the dielectric layer. EWOD experiments are conducted with the following concentrations of gold nanofluid (in μM): 0.5, 0.33, 0.25, 0.05, 0.01, 0.005 and control fluid. Results show enhanced electrowetting response for microdroplets containing higher concentrations of gold nanoparticles. The specific capacitance is calculated for each concentration and is found to increase with increasing concentration, in agreement with an electromechanical model for the electrowetting phenomena.

DOI: 10.1016/j.matpr.2018.01.034
 

Controlled Microwave-Hydrolyzed Starch as a Stabilizer for Green Formulation of Aqueous Gold Nanoparticle Ink for Flexible Printed Electronics

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 By Nikita P. Bacalzo, Jr., Lance P. Go, Christine Joy Querebillo, Peter Hildebrandt, F. T. Limpoco, and Erwin P. Enriquez
 
Controlled Microwave-Hydrolyzed Starch as a Stabilizer for Green Formulation of Aqueous Gold Nanoparticle Ink for Flexible Printed Electronics. Nikita P. Bacalzo, Jr., Lance P. Go, Christine Joy Querebillo, Peter Hildebrandt, F. T. Limpoco, and Erwin P. Enriquez. ACS Applied Nano Materials 2018 1 (3), 1247-1256 Gold electrodes are important in some devices and certain applications where an inert, highly conductive feature is required. An aqueous gold nanoparticle (AuNP) ink suitable for inkjet printing was synthesized and formulated using starch and microwave-assisted heating. By varying the hydrolysis conditions of starch, the size, yield, and stability of the AuNP suspension can be controlled and optimized to achieve a jettable ink. The optimized formulation has a very low starch loading of only 1.75 wt % relative to gold, forming a highly stable AuNP ink, which upon drying already forms a very conductive film and sinters at low temperature. The overall synthesis protocol thus provides a greener and cheaper alternative to other AuNP synthesis methods. The sintering behavior of the film was monitored, wherein, upon heating, starch is degraded, crystallite growth increased, and the morphology changed from individual nanoparticles to a network of fused particles. The film sheet resistance decreased concomitant with these physical changes. By heating the film to at least 200 °C, a sheet resistance of <1.0 Ω/□ is achieved. This aqueous-formulated ink, therefore, offers an alternative to the usual organic solvent ink formulations used in printed electronics. It proved printing of conductive films on various substrates for possible applications in flexible electronic devices.

DOI: 10.1021/acsanm.7b00379

Smartphone-based image analysis and chemometric pattern recognition of the thin-layer chromatographic fingerprints of herbal materials

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 By Sarah May Sibug-Torres, Isagani D. Padolina, Philip Cruz, Felan Carlo Garcia, Mark Joseph Garrovillas, Maria Regina Yabillo and Erwin P. Enriquez

Thin-layer chromatography (TLC) is commonly used as a screening method to verify the identity and quality of dried herbal medicinal plant material. While TLC is relatively simple, the method still requires technical experience and relies on the subjective classification of sample TLC profiles as “within-specifications” or “off-specifications.” In this work, we report the development of an objective TLC-based system for the identification and quality assessment of herbal medicinal materials. Our proposed system is a miniaturized Pharmacopeia-based TLC method coupled with a smartphone app that allows for an objective interpretation of TLC profiles via multivariate image analysis and chemometric fingerprinting. An image of the TLC profile is captured using a smartphone camera interfaced with a 3D-printed photo-box, and the analysis is automated using a framework of pre-uploaded algorithms hosted on a cloud server. The TLC profile image is converted to an unfolded red, green, and blue (RGB) channel intensity profile, and classified as “within-specifications” or “off-specifications” using aggregated Soft Independent Modeling of Class Analogy (SIMCA) models. We present the application of our system to two herbal medicinal plants, Blumea balsamifera and Vitex negundo. The proposed system demonstrates 90.2% sensitivity and 86.2% specificity for B. balsamifera classification, and 81.4% sensitivity and 92.0% specificity for V. negundo classification when compared to the respective laboratory-based Pharmacopeia TLC protocols for the ability to distinguish authentic samples from non-authentic and degraded samples. The system developed in this work is a cost-effective, rapid method that can serve as a herbal material quality assessment tool in resource-limited settings.

DOI: 10.1039/C8AY02698J

2014

Photoelectrochemical activity on Ga-polar and N-polar GaN surfaces for energy conversion

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Lin YG, Hsu YK, Basilio AM, Chen YT, Chen KH, Chen LC. 2014. Photoelectrochemical activity on Ga-polar and N-polar GaN surfaces for energy conversion. Optics Express. 22(S1): A21-A27.
 
Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical cells made of different facets of free-standing polar GaN system. To build the fundamental understanding at the differences of surface photochemistry afforded by the GaN {0001}and {000−1}polar surfaces, we correlated the relationship between the surface structure and photoelectrochemical performance on the different polar facets. The photoelectrochemical measurements clearly revealed that the Ga-polar surface had a more negative onset potential relative to the N-polar surface due to the much negative flat-band potential. At more positive applied voltages, however, the N-polar surface yielded much higher photocurrent with conversion efficiency of 0.61% compared to that of 0.55% by using the Ga-polar surface. The reason could be attributed to the variation in the band structure of the different polar facets via Mott-Schottky analyses. Based on this work, understanding the facet effect on photoelectrochemical activity can provide a blueprint for the design of materials in solar hydrogen applications.
 

Symmetry groups of single-wall nanotubes

By De Las Peñas, Ma. Louise Antonette N., Loyola, Mark L.

De Las Peñas MLAN, Loyola ML, Basilio AM, Santoso EB. 2014. Symmetry groups of single-wall nanotubes. Acta Cryst. A70: 12-23.
 
This work investigates the symmetry properties of single-wall carbon nanotubes and their structural analogs, which are nanotubes consisting of different kinds of atoms. The symmetry group of a nanotube is studied by looking at symmetries and color fixing symmetries associated with a coloring of the tiling by hexagons in the Euclidean plane which, when rolled, gives rise to a geometric model of the nanotube. The approach is also applied to nanotubes with non-hexagonal symmetry arising from other isogonal tilings of the plane.
 

Micellar electrokinetic chromatography of the constituents in Philippine lagundi (Vitex negundo) herbal products

By Guidote, Armando Victor, Jr. M.

Rabanes HR, Guidote AMJ, Quirino JP. 2014. Micellar electrokinetic chromatography of the constituents in Philippine lagundi (Vitex negundo) herbal products. Microchemical Journal. 112:153-158.
 
Vitex negundo or lagundi is an important herbal medicine in Asia and various commercial products of this herb's leaves have been flooding the market. Reports on the chemical constituents of such products were limited and directed to only one selected chemical marker. Here, micellar electrokinetic chromatography using a fused silica capillary was used for analysis of the methanolic extract of Philippine lagundi herbal products. The separation solution, injection, and separation voltage was 50 mM SDS with 50 mM phosphoric acid at pH 2.5, 6 s at 25 mbar, and 20 kV at negative polarity, respectively. With UV detection at 200 nm, the LODs (S/N = 3) were in the sub-μg/mL range which were sufficient for detection without the need for on-line or off-line sample concentration. The repeatability values were all below 3.9% for migration time and peak signals. When the method was used to real samples, a flavonoid (isoorientin) and two iridoid glycosides (agnuside and negundoside) were identified as the target chemical markers within a 12 min electrophoretic run. The accuracy was from 95% to 129%. The results also suggested that agnuside was the major component in Philippine lagundi. The method was simple, effective, low cost, and can be used for authenticity evaluation of lagundi products.
 

2013

Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation

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Hwang JS, Liu TY, Chattopadhyay S, Hsu GM, Basilio AM, Chen HW, Hsu YK, Tu WH, Lin YG, Chen KH, Li CC, Wang SB, Chen HY, Chen LC. 2013. Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation. Nanotechnology. 24(5): 055401.

Enhanced photoelectrochemical (PEC) performances of Ga2O3 and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga2O3 and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga2O3 NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga2O3. These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga2O3 NWs, or by incorporation of indium to form InGaN NWs.

DOI: 10.1088/0957-4484/24/5/055401