Maung Kyaw Khaing Oo

Rapid, sensitive DNT vapor detection with UV-assisted photo-chemically synthesized gold nanoparticle SERS substrates

We report rapid, sensitive, and direct detection of 2,4-dinitrotoluene (DNT) vapor using tailored gold nanoparticles (Au-NPs) as the SERS substrate. The Au-NPs were synthesized using the UV-assisted photo-chemical reduction method and subsequently formed a monolayer on the glass slide through polymer-mediated self-assembly. The SERS substrate such prepared has high SERS enhancement, high affinity towards DNT vapor, and rapid response to the DNT adsorption/desorption. We systematically studied the effect of the Au-NP size and surface density on the SERS performance such as enhancement factor and response time. With the optimized SERS substrate, an enhancement factor over 5.6 × 10(6) was achieved. Furthermore, real-time detection of DNT vapor with only 0.35 second data acquisition time was demonstrated using a 12 mW laser. Compared to previously reported results, we achieved two orders of magnitude reduction in detection time and more than one order of magnitude reduction in excitation laser power. The detection limit is estimated to be 0.4 attogram, which corresponds to a sub-ppb DNT concentration in air. This work will lead to the development of ultra-fast and ultra-sensitive SERS devices for explosive identification and monitoring.

Optofluidic laser for dual-mode sensitive biomolecular detection with a large dynamic range

Enzyme-linked immunosorbent assay (ELISA) is a powerful method for biomolecular analysis. The traditional ELISA employing light intensity as the sensing signal often encounters large background arising from non-specific bindings, material autofluorescence and leakage of excitation light, which deteriorates its detection limit and dynamic range. Here we develop the optofluidic laser-based ELISA, where ELISA occurs inside a laser cavity. The laser onset time is used as the sensing signal, which is inversely proportional to the enzyme concentration and hence the analyte concentration inside the cavity. We first elucidate the principle of the optofluidic laser-based ELISA, and then characterize the optofluidic laser performance. Finally, we present the dual-mode detection of interleukin-6 using commercial ELISA kits, where the sensing signals are simultaneously obtained by the traditional and the optofluidic laser-based ELISA, showing a detection limit of 1 fg ml(-1) (38 aM) and a dynamic range of 6 orders of magnitude.

Robust silica-coated quantum dot-molecular beacon for highly sensitive DNA detection

We synthesized and characterized small yet highly robust silica-coated quantum dots (QDs) and then used them to develop highly sensitive molecular beacon (MB) for DNA detection. As compared to the previously reported methods, our silica coating approach enabled simple and rapid synthesis of silica-coated QDs in large quantities and high concentrations with a well-controlled silica layer. The QDs such made were stable and had a high quantum yield in a wide range of pH values (1-14) and high salt concentrations (up to 2 M). They were less than 10 nm in diameter, much smaller than current silica-coated QDs, thus allowing for efficient energy transfer. The MB sensor based on these silica-coated QDs was capable of rapidly detecting the target DNA at 0.1 nM concentration within 15 min. It could also differentiate the target DNA from the single base mismatched DNA. The QD-MB developed in this work can be used for highly sensitive and selective detection of DNA and other biomolecules in homogeneous solution and inside a cell, as well as in harsh environment.

Ultrasensitive vapor detection with surface-enhanced Raman scattering-active gold nanoparticle immobilized flow-through multihole capillaries.

We developed novel flow-through surface-enhanced Raman scattering (SERS) platforms using gold nanoparticle (Au-NP) immobilized multihole capillaries for rapid and sensitive vapor detection. The multihole capillaries consisting of thousands of micrometer-sized flow-through channels provide many unique characteristics for vapor detection. Most importantly, its three-dimensional SERS-active micro-/nanostructures make available multilayered assembly of Au-NPs, which greatly increase SERS-active surface area within a focal volume of excitation and collection, thus improving the detection sensitivity. In addition, the multihole capillarys inherent longitudinal channels offer rapid and convenient vapor delivery, yet its micrometer-sized holes increase the interaction between vapor molecules and SERS-active substrate. Experimentally, rapid pyridine vapor detection (within 1 s of exposure) and ultrasensitive 4-nitrophenol vapor detection (at a sub-ppb level) were successfully achieved in open air at room temperature. Such an ultrasensitive SERS platform enabled, for the first time, the investigation of both pyridine and 4-nitrophenol vapor adsorption isotherms at very low concentrations. Type I and type V behaviors of the International Union of Pure and Applied Chemistry isotherm were well observed, respectively.

Adaptive Two-Dimensional Microgas Chromatography

We proposed and investigated a novel adaptive two-dimensional (2-D) microgas chromatography system, which consists of one 1st-dimensional column, multiple parallel 2nd-dimensional columns, and a decision-making module. The decision-making module, installed between the 1st- and 2nd-dimensional columns, normally comprises an on-column nondestructive vapor detector, a flow routing system, and a computer that monitors the detection signal from the detector and sends out the trigger signal to the flow routing system. During the operation, effluents from the 1st-dimensional column are first detected by the detector and, then, depending on the signal generated by the detector, routed to one of the 2nd-dimensional columns sequentially for further separation. As compared to conventional 2-D GC systems, the proposed adaptive GC scheme has a number of unique and advantageous features. First and foremost, the multiple parallel columns are independent of each other. Therefore, their length, stationary phase, flow rate, and temperature can be optimized for best separation and maximal versatility. In addition, the adaptive GC significantly lowers the thermal modulator modulation frequency and hence power consumption. Finally, it greatly simplifies the postdata analysis process required to reconstruct the 2-D chromatogram. In this paper, the underlying working principle and data analysis of the adaptive GC was first discussed. Then, separation of a mixture of 20 analytes with various volatilities and polarities was demonstrated using an adaptive GC system with a single 2nd-dimensional column. Finally, an adaptive GC system with dual 2nd-dimensional columns was employed, in conjunction with temperature ramping, in a practical application to separate a mixture of plant emitted volatile organic compounds with significantly shortened analysis time.

Integrated Separation Columns and Fabry-Perot Sensors for Microgas Chromatography Systems

We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation column and on-column, nondestructive Fabry-Pérot (FP) vapor sensors on a single silicon chip. The device was fabricated using deep reactive ion etching of silicon to create fluidic channels and polymers were deposited on the same silicon chip to act as a stationary phase or an FP sensor, thus avoiding dead volumes caused by the interconnects between the column and sensor traditionally used in μGC. Two integration designs were studied. In the first design, a 25-cm long μGC column was coated with a layer of polymer that served as both the stationary phase and the FP sensor, which has the greatest level of integration. This design was capable of sub-second response times and detection limits under 10 ng. In the second design, an FP sensor array spray coated with different vapor sensing polymers was integrated with a 30-cm long μGC column, which significantly improves the system flexibility and detection sensitivity. With this design, we show that the FP sensors have a detection limit on the order of tens of picograms or ~500 ppb with a sub-second response time. Furthermore, the FP sensor array are shown to respond to a mixture of analytes separated by the integrated separation channel, allowing for the construction of response patterns, which, along with retention time, can be used as a basis of analyte identification.

Sensitive sulfide ion detection by optofluidic catalytic laser using horseradish peroxidase (HRP) enzyme

We report an optofluidic catalytic laser for sensitive sulfide ion detection. In the catalytic reaction, horseradish peroxidase (HRP) enzyme is used for catalyzing the non-fluorescent substrate, 10-Acetyl-3,7-dihydroxyphenox-azine (ADHP), to produce highly fluorescent resorufin, which was used as gain medium for lasing. Using sulfide ions as inhibitors, the catalytic reaction slows down, resulting in a delay in the lasing onset time, which is used as the sensing signal. The sensing mechanism of the catalytic laser is theoretically analyzed and the performance is experimentally characterized. Sulfide anion is chosen as a model ion because of its broad adverse impacts on both environment and human health. Due to the optical feedback provided by the laser, the small difference in the sulfide ion concentration can be amplified. Consequently, a detection limit of 10nM is achieved with a dynamic range as large as three orders of magnitude, representing significant improvement over the traditional fluorescence and colorimetric methods. This work will open a door to a new catalytic-laser-based chemical sensing platform for detecting a wide range of species that could inhibit the catalytic reaction.

Enzyme catalyzed optofluidic biolaser for sensitive ion concentration detection

The enzyme horseradish peroxidase (HRP) has been extensively used in biochemistry for its ability to amplify a weak signal. By using HRP catalyzed substrate as the gain medium, we demonstrate sensitive ion concentration detection based on the optofluidic laser. The enzyme catalyzed reaction occurs in bulk solution inside a Fabry-Perot laser cavity, where the colorless, non-fluorescent 10-Acetyl-3,7-dihydroxyphenoxazine (ADHP) substrate is oxidized to produce highly fluorescent resorufin. Laser emission is achieved when pumped with the second harmonic wave of a Q-switched YAG laser. Further, we use sulfide anion (S2-) as an example to investigate the sensing performance of enzyme catalyzed optofluidic laser. The laser onset time difference between the sample to be tested and the reference is set to be the sensing output. Thanks to the amplification effects of both the enzymatic reaction and laser emission, we achieve a detection limit of 10 nM and a dynamic range of 3 orders of magnitude.

Ultra-fast and ultra-sensitive 2,4-dinitrotoluene vapor sensing using gold nanoparticle assembled SERS probes

Surface enhanced Raman scattering (SERS) amplifies the small Raman scattering cross section of molecules toward distinguishable signal. It has been advanced into an influential label-free nondestructive method to measure vibrational fingerprints of molecular structures directly. We report here the demonstration of vapor detection of 2,4-dinitrotoluene (2,4-DNT), a typical manufacturing impurity of trinitrotoluene (TNT) based explosives, using reproducible ultrasensitive SERS substrates, i.e., assembled gold nanoparticles (GNPs) synthesized by a UV photoreduction method. The estimated detection limit was achieved 0.4 attogram, which corresponds to a sub-ppb DNT concentration in air. The 2,4- DNT molecules was noticeable within a minute of exposure to the DNT vapor at room temperature. The detection time was as short as only 2 seconds with 12 mW 785 nm laser excitation at the SERS substrate. Our study shows that larger GNPs (~ 117 nm in diameter) with higher density, an enhancement factor of 5.6x106, exhibits the high sensitivity and the fast detection response, as compared to smaller and lower density GNPs. Dynamic depletion by laser heating indicates that our GNP based sensor is possible real time 2,4-DNT detection.

Rapid Mouse FSH Quantification and Estrus Cycle Analysis Using an Automated Microfluidic Chemiluminescent ELISA System

Follicle stimulating hormone (FSH) plays a critical role in female reproductive development and homeostasis. The blood/serum concentration of FSH is an important marker for reporting multiple endocrinal functions. The standardized method for mouse FSH (mFSH) quantification based on radioimmunoassay (RIA) suffers from long assay time (∼2 days), relatively low sensitivity, larger sample volume (60 μL), and small dynamic range (2-60 ng/mL); thus, it is insufficient for monitoring fast developing events with relatively small mFSH fluctuations (e.g., estrous cycles of mammals). Here, we developed an automated microfluidic chemiluminescent ELISA device along with the disposal sensor array and the corresponding detection protocol for rapid and quantitative analysis of mFSH from mouse tail serum samples. With this technology, highly sensitive quantification of mFSH can be accomplished within 30 min using only 8 μL of the serum sample. It is further shown that our technique is able to generate results comparable to RIA but has a significantly improved dynamic range that covers 0.5-250 ng/mL. The performance of this technology was evaluated with blood samples collected from ovariectomized animals and animals with reimplanted ovarian tissues, which restored ovarian endocrine function and correlated with estrus cycle analysis study.