Karthik Reddy - Academia.edu (original) (raw)

Papers by Karthik Reddy

AIP Advances, 2020

The microfluidic chip can be used to separate particles via the acoustic radiation force and can ... more The microfluidic chip can be used to separate particles via the acoustic radiation force and can be applied to the separation of impurity particles from lubricating oil. A device that separates particles by controlling the acoustic radiation force via standing surface acoustic waves (SSAWs) was proposed. The concentration and separation of suspended particles were simulated by using the COMSOL Multiphysics modeling software. The force exerted on suspended particles and their subsequent motion in the microfluidic channel were analyzed, and then the concentration of particles exposed to SSAWs was verified. We also investigate how the frequency of the SSAW affects the particle concentration and discuss the advantage of using SSAWs to concentrate and separate particles. The separating feasibility was verified by suspended particles in lubricating oil experiments according to simulation results.

The need to diminish the worldwide anthropogenic carbon di oxide has energized specialists to sca... more The need to diminish the worldwide anthropogenic carbon di oxide has energized specialists to scan for manageable building materials. Bond, the second most devoured item on the planet, contributes about 7% of worldwide carbon di oxide discharge. Geo polymer concrete (GPC) is fabricated utilizing mechanical waste like fly cinder, GGBS is considered as a more eco neighborly other option to common Portland bond (OPC) base cement. In GPC no bond is utilized, rather fly fiery remains, GGBS which are rich in Silica (Si) and Aluminum (Al) are initiated by antacid fluids, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate Na2O, SiO2) or potassium silicate to make the folio important to produce elite eco amicable cement. Fly Fiery remains and GGBS based polymer concrete has magnificent compressive quality, endures exceptionally low drying shrinkage, low crawl, magnificent impervious to sulfate assault and great corrosiveresistance. The exploratory program w...

Sensor Letters, 2018

The accumulation of suspended particles for lubricating oil used for dynamic systems causes disas... more The accumulation of suspended particles for lubricating oil used for dynamic systems causes disasters. Removal of these particles from the lubricating oil is vital to improve the quality of oil products and ensure the safety of the equipment. The status of current particle separation techniques is reviewed and the disadvantages of the current these techniques are reported in this article. By using ultrasonic standing-wave separation technique, the key issues in particle separation techniques are discussed. Which provide a reference for the research of theory and technology of particle separation.

The Astrophysical Journal Supplement Series, 2021

Observations of positional offsets between the location of X-ray and radio features in many resol... more Observations of positional offsets between the location of X-ray and radio features in many resolved, extragalactic jets indicates that the emitting regions are not cospatial, an important piece of evidence in the debate over the origin of the X-ray emission on kiloparsec scales. The existing literature is nearly exclusively focused on jets with sufficiently deep Chandra observations to yield accurate positions for X-ray features, but most of the known X-ray jets are detected with tens of counts or fewer, making detailed morphological comparisons difficult. Here we report the detection of X-ray-to-radio positional offsets in 15 extragalactic jets from an analysis of 22 sources with low-count Chandra observations, where we utilized the Low-count Image Reconstruction Algorithm. This algorithm has allowed us to account for effects such as Poisson background fluctuations and nearby point sources which have previously made the detection of offsets difficult in shallow observations. Using...

The Astrophysical Journal, 2019

Over the past two decades, the most commonly adopted explanation for high and hard X-ray emission... more Over the past two decades, the most commonly adopted explanation for high and hard X-ray emission in resolved quasar jets has been inverse Compton upscattering of the Cosmic Microwave Background (IC/CMB), which requires jets which remain highly relativistic on 10-1000 kpc scales. In more recent years various lines of observational evidence, including gamma-ray upper limits, have disfavored this explanation in favor of a synchrotron origin. While the IC/CMB model generally predicts a high level of gamma-ray emission, it has never been detected. Here we report the detection of a lowstate Fermi/LAT gamma-ray spectrum associated with two jetted AGN which is consistent with the predictions of the IC/CMB model for their X-ray emission. We have used archival multiwavelength observations to make precise predictions for the expected minimum flux in the GeV band, assuming that the X-ray emission from the kpc-scale jet is entirely due to the IC/CMB process. In both sources-OJ 287 and PKS 1510-089-the minimum-detected gamma-ray flux level agrees with predictions. Both sources exhibit extreme superluminal proper motions relative to their jet power, which argues for the well-aligned jets required by the IC/CMB model. In the case of PKS 1510-089, it cannot be ruled out that the minimum gamma-ray flux level is due to a low state of the variable core which only matches the IC/CMB prediction by chance. Continued long-term monitoring with the Fermi/LAT could settle this issue by detecting a plateau signature in the recombined light-curve which would clearly signal the presence of a non-variable emission component.

Physics of Fluids, 2015

The current work puts forth an implementation of a dynamic procedure to locally compute the value... more The current work puts forth an implementation of a dynamic procedure to locally compute the value of the model constant CDES , as used in the eddy simulation branch of Delayed Detached Eddy Simulation (DDES). Former DDES formulations [P. R. Spalart et al., "A new version of detached-eddy simulation, resistant to ambiguous grid densities," Theor. Comput. Fluid Dyn. 20, 181 (2006); M. S. Gritskevich et al., "Development of DDES and IDDES formulations for the k-ω shear stress transport model," Flow, Turbul. Combust. 88, 431 (2012)] are not conducive to the implementation of a dynamic procedure due to uncertainty as to what form the eddy viscosity expression takes in the eddy simulation branch. However, a recent, alternate formulation [K. R. Reddy et al., "A DDES model with a Smagorinsky-type eddy viscosity formulation and log-layer mismatch correction," Int. J. Heat Fluid Flow 50, 103 (2014)] casts the eddy viscosity in a form that is similar to the Smagorinsky, LES (Large Eddy Simulation) sub-grid viscosity. The resemblance to the Smagorinsky model allows the implementation of a dynamic procedure similar to that of Lilly [D. K. Lilly, "A proposed modification of the Germano subgrid-scale closure method," Phys. Fluids A 4, 633 (1992)]. A limiting function is proposed which constrains the computed value of CDES , depending on the fineness of the grid and on the computed solution.

Submitted for the DFD14 Meeting of The American Physical Society Dynamic DDES On DES Type Grid ZI... more Submitted for the DFD14 Meeting of The American Physical Society Dynamic DDES On DES Type Grid ZIFEI YIN, PAUL DURBIN, None-A dynamic procedure allows a DES formulation that we developed to adjust C DES for different flow configurations. Similarly to the dynamic Smagorinsky model, the grid is required to be fine enough to resolve a significant portion of the inertial range. In some cases, that requirement conflicts with the goal of DES to cut down computing cost. The current effort is therefore to determine a proper C DES value by approximately recovering some unresolved small scales from primary, filtered solution. Repeated test filtering is adopted here to compute the approximation of the unfiltered solution. The formulation is based on the dynamic l 2 w DDES model and different geometries with varies grid resolution are tested to determine the applicability of proposed formultion on DES type grids.

Frontiers in Biological Detection: From Nanosensors to Systems III, 2011

Optical fiber provides a unique and versatile platform for developing point-of-care optical sensi... more Optical fiber provides a unique and versatile platform for developing point-of-care optical sensing systems. Here we first propose a Fabry-Pérot (FP) based flow-through optofluidic biosensor, and then construct an all-fiber system which fully utilizes optical fibers to achieve rapid, sensitive, label-free biomolecular detection. This sensor consists of two single mode fibers (SMFs) with reflecting surfaces and a photonic crystal fiber (PCF) vertically sandwiched by them. Firstly, the SMFs act as waveguides for delivering light into and out of an optofluidic device (like PCF); secondly, instead of using the optical properties of the PCF, we take advantage of its inherent multiple fluidic channels and large sensing surface; thirdly, the two reflective surfaces and the PCF form a Fabry-Pérot microresonator and its resonance mode is sensitive to the change in the fluidic channels, which can be used to detect the substances flowing through the fluidic channels or adsorbing on the channel surface. In this paper, we explore the operating principle of the FP-based optofluidic biosensor, theoretically and experimentally investigate its feasibility and capability. The results show that the all-fiber optofluidic sensor is a promising technology platform for rapid, sensitive and high-throughput biological and chemical sensing.

Nature Communications, 2014

Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding... more Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding changes the charge density of the sensor and leads to sensing signal. However, intrinsically slow dynamics of interface-trapped charges and defect-mediated charge-transfer processes significantly limit those sensors' response to tens to hundreds of seconds, which has long been known as a bottleneck for studying the dynamics of molecule-nanomaterial interaction and for many applications requiring rapid and sensitive response. Here we report a fundamentally different sensing mechanism based on molecular dipole detection enabled by a pioneering graphene nanoelectronic heterodyne sensor. The dipole detection mechanism is confirmed by a plethora of experiments with vapour molecules of various dipole moments, particularly, with cis-and trans-isomers that have different polarities. Rapid (down to B0.1 s) and sensitive (down to B1 ppb) detection of a wide range of vapour analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronics sensors.

Optics Express, 2010

We theoretically analyze the ability of 3-dimensionally confined optofluidic ring resonators (OFR... more We theoretically analyze the ability of 3-dimensionally confined optofluidic ring resonators (OFRRs) for detection of a single nanoparticle in water and in air. The OFRR is based on a glass capillary, on which bottleshaped and bubble-shaped ring resonators can form. The spectral position of the whispering gallery mode in the OFRR shifts when a nanoparticle is attached to the OFRR inner surface. For both ring resonator structures, the electric field at the inner surface can be optimized by choosing the right wall thickness. Meanwhile, different electric field confinement along the capillary longitudinal axis can be achieved with different curvatures. Both effects significantly increase the sensitivity of the ring resonator for single nanoparticle detection. It is found that the sensitivity is enhanced about 10 times, as compared to that of a solid microsphere biosensor recently reported, and that the smallest detectable nanoparticle is estimated to be less than 20 nm in radius for a Δλ/λ resolution of 10 8. The high sensitivity and the naturally integrated capillary based microfluidics make the OFRR a very promising sensing platform for detection of various nano-sized bio/chemical species in liquid as well as in air.

Lab on a Chip, 2012

We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for m... more We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for microgas chromatography (mGC) applications. The sensor is based on the Fabry-P erot (FP) interferometer formed by a micrometre-thin vapor-sensitive polymer layer coated on a silicon wafer. The thickness and the refractive index of the polymer vary in response to the vapor analyte, resulting in a change in the reflected intensity of the laser impinged on the sensor. In our study, four different polymers were coated on four wells pre-etched on a silicon wafer to form a spatially separated sensor array. A CMOS imager was employed to simultaneously monitor the polymers' response, thus enabling multiplexed detection of a vapor analyte passing through the GC column. A sub-second detection time was demonstrated. In addition, a sub-picogram detection limit was achieved, representing orders of magnitude improvement over the on-chip vapor sensors previously reported.

Journal of Microelectromechanical Systems, 2013

We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation co... more We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation column and oncolumn, 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.

Applied Physics Letters, 2011

Applied Physics Letters, 2011

Single mode lasing from the polydimethylsiloxane based on-chip coupled optofluidic ring resonator... more Single mode lasing from the polydimethylsiloxane based on-chip coupled optofluidic ring resonator ͑OFRR͒ with the lasing threshold of a few J / mm 2 is demonstrated using the Vernier effect. The single mode operation is highly stable even at high pump energy densities. The effect of the OFRR size and coupling strength on the single mode emission is investigated, showing that the excessive coupling results in incomplete side mode suppression. Tuning of the lasing wavelength is achieved by modifying the dye solution.

Applied Physics Letters, 2011

An optofluidic Fabry-Pérot cavity label-free biosensor with integrated flow-through micro-/ nanoc... more An optofluidic Fabry-Pérot cavity label-free biosensor with integrated flow-through micro-/ nanochannels is proposed and demonstrated, which takes advantages of the large surface-to-volume ratio for analyte concentration and high detection sensitivity and built-in fluidic channels for rapid analyte delivery. The operating principle is first discussed, followed by assembly of a robust sensing system. Real-time measurements are performed to test its sensing feasibility and capability including bulk solvent change and removal/binding of molecules from/onto the internal surface of fluidic channels. The results show that this sensor provides a very promising platform for rapid, sensitive, and high-throughput biological and chemical sensing.

Analytical Chemistry, 2012

We proposed and investigated a novel adaptive two-dimensional (2-D) microgas chromatography syste... more 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 oncolumn 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.

Analytical Chemistry, 2012

We developed novel flow-through surface-enhanced Raman scattering (SERS) platforms using gold nan... more 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 capillary's 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.

AIP Advances, 2020

The microfluidic chip can be used to separate particles via the acoustic radiation force and can ... more The microfluidic chip can be used to separate particles via the acoustic radiation force and can be applied to the separation of impurity particles from lubricating oil. A device that separates particles by controlling the acoustic radiation force via standing surface acoustic waves (SSAWs) was proposed. The concentration and separation of suspended particles were simulated by using the COMSOL Multiphysics modeling software. The force exerted on suspended particles and their subsequent motion in the microfluidic channel were analyzed, and then the concentration of particles exposed to SSAWs was verified. We also investigate how the frequency of the SSAW affects the particle concentration and discuss the advantage of using SSAWs to concentrate and separate particles. The separating feasibility was verified by suspended particles in lubricating oil experiments according to simulation results.

The need to diminish the worldwide anthropogenic carbon di oxide has energized specialists to sca... more The need to diminish the worldwide anthropogenic carbon di oxide has energized specialists to scan for manageable building materials. Bond, the second most devoured item on the planet, contributes about 7% of worldwide carbon di oxide discharge. Geo polymer concrete (GPC) is fabricated utilizing mechanical waste like fly cinder, GGBS is considered as a more eco neighborly other option to common Portland bond (OPC) base cement. In GPC no bond is utilized, rather fly fiery remains, GGBS which are rich in Silica (Si) and Aluminum (Al) are initiated by antacid fluids, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate Na2O, SiO2) or potassium silicate to make the folio important to produce elite eco amicable cement. Fly Fiery remains and GGBS based polymer concrete has magnificent compressive quality, endures exceptionally low drying shrinkage, low crawl, magnificent impervious to sulfate assault and great corrosiveresistance. The exploratory program w...

Sensor Letters, 2018

The accumulation of suspended particles for lubricating oil used for dynamic systems causes disas... more The accumulation of suspended particles for lubricating oil used for dynamic systems causes disasters. Removal of these particles from the lubricating oil is vital to improve the quality of oil products and ensure the safety of the equipment. The status of current particle separation techniques is reviewed and the disadvantages of the current these techniques are reported in this article. By using ultrasonic standing-wave separation technique, the key issues in particle separation techniques are discussed. Which provide a reference for the research of theory and technology of particle separation.

The Astrophysical Journal Supplement Series, 2021

Observations of positional offsets between the location of X-ray and radio features in many resol... more Observations of positional offsets between the location of X-ray and radio features in many resolved, extragalactic jets indicates that the emitting regions are not cospatial, an important piece of evidence in the debate over the origin of the X-ray emission on kiloparsec scales. The existing literature is nearly exclusively focused on jets with sufficiently deep Chandra observations to yield accurate positions for X-ray features, but most of the known X-ray jets are detected with tens of counts or fewer, making detailed morphological comparisons difficult. Here we report the detection of X-ray-to-radio positional offsets in 15 extragalactic jets from an analysis of 22 sources with low-count Chandra observations, where we utilized the Low-count Image Reconstruction Algorithm. This algorithm has allowed us to account for effects such as Poisson background fluctuations and nearby point sources which have previously made the detection of offsets difficult in shallow observations. Using...

The Astrophysical Journal, 2019

Over the past two decades, the most commonly adopted explanation for high and hard X-ray emission... more Over the past two decades, the most commonly adopted explanation for high and hard X-ray emission in resolved quasar jets has been inverse Compton upscattering of the Cosmic Microwave Background (IC/CMB), which requires jets which remain highly relativistic on 10-1000 kpc scales. In more recent years various lines of observational evidence, including gamma-ray upper limits, have disfavored this explanation in favor of a synchrotron origin. While the IC/CMB model generally predicts a high level of gamma-ray emission, it has never been detected. Here we report the detection of a lowstate Fermi/LAT gamma-ray spectrum associated with two jetted AGN which is consistent with the predictions of the IC/CMB model for their X-ray emission. We have used archival multiwavelength observations to make precise predictions for the expected minimum flux in the GeV band, assuming that the X-ray emission from the kpc-scale jet is entirely due to the IC/CMB process. In both sources-OJ 287 and PKS 1510-089-the minimum-detected gamma-ray flux level agrees with predictions. Both sources exhibit extreme superluminal proper motions relative to their jet power, which argues for the well-aligned jets required by the IC/CMB model. In the case of PKS 1510-089, it cannot be ruled out that the minimum gamma-ray flux level is due to a low state of the variable core which only matches the IC/CMB prediction by chance. Continued long-term monitoring with the Fermi/LAT could settle this issue by detecting a plateau signature in the recombined light-curve which would clearly signal the presence of a non-variable emission component.

Physics of Fluids, 2015

The current work puts forth an implementation of a dynamic procedure to locally compute the value... more The current work puts forth an implementation of a dynamic procedure to locally compute the value of the model constant CDES , as used in the eddy simulation branch of Delayed Detached Eddy Simulation (DDES). Former DDES formulations [P. R. Spalart et al., "A new version of detached-eddy simulation, resistant to ambiguous grid densities," Theor. Comput. Fluid Dyn. 20, 181 (2006); M. S. Gritskevich et al., "Development of DDES and IDDES formulations for the k-ω shear stress transport model," Flow, Turbul. Combust. 88, 431 (2012)] are not conducive to the implementation of a dynamic procedure due to uncertainty as to what form the eddy viscosity expression takes in the eddy simulation branch. However, a recent, alternate formulation [K. R. Reddy et al., "A DDES model with a Smagorinsky-type eddy viscosity formulation and log-layer mismatch correction," Int. J. Heat Fluid Flow 50, 103 (2014)] casts the eddy viscosity in a form that is similar to the Smagorinsky, LES (Large Eddy Simulation) sub-grid viscosity. The resemblance to the Smagorinsky model allows the implementation of a dynamic procedure similar to that of Lilly [D. K. Lilly, "A proposed modification of the Germano subgrid-scale closure method," Phys. Fluids A 4, 633 (1992)]. A limiting function is proposed which constrains the computed value of CDES , depending on the fineness of the grid and on the computed solution.

Submitted for the DFD14 Meeting of The American Physical Society Dynamic DDES On DES Type Grid ZI... more Submitted for the DFD14 Meeting of The American Physical Society Dynamic DDES On DES Type Grid ZIFEI YIN, PAUL DURBIN, None-A dynamic procedure allows a DES formulation that we developed to adjust C DES for different flow configurations. Similarly to the dynamic Smagorinsky model, the grid is required to be fine enough to resolve a significant portion of the inertial range. In some cases, that requirement conflicts with the goal of DES to cut down computing cost. The current effort is therefore to determine a proper C DES value by approximately recovering some unresolved small scales from primary, filtered solution. Repeated test filtering is adopted here to compute the approximation of the unfiltered solution. The formulation is based on the dynamic l 2 w DDES model and different geometries with varies grid resolution are tested to determine the applicability of proposed formultion on DES type grids.

Frontiers in Biological Detection: From Nanosensors to Systems III, 2011

Optical fiber provides a unique and versatile platform for developing point-of-care optical sensi... more Optical fiber provides a unique and versatile platform for developing point-of-care optical sensing systems. Here we first propose a Fabry-Pérot (FP) based flow-through optofluidic biosensor, and then construct an all-fiber system which fully utilizes optical fibers to achieve rapid, sensitive, label-free biomolecular detection. This sensor consists of two single mode fibers (SMFs) with reflecting surfaces and a photonic crystal fiber (PCF) vertically sandwiched by them. Firstly, the SMFs act as waveguides for delivering light into and out of an optofluidic device (like PCF); secondly, instead of using the optical properties of the PCF, we take advantage of its inherent multiple fluidic channels and large sensing surface; thirdly, the two reflective surfaces and the PCF form a Fabry-Pérot microresonator and its resonance mode is sensitive to the change in the fluidic channels, which can be used to detect the substances flowing through the fluidic channels or adsorbing on the channel surface. In this paper, we explore the operating principle of the FP-based optofluidic biosensor, theoretically and experimentally investigate its feasibility and capability. The results show that the all-fiber optofluidic sensor is a promising technology platform for rapid, sensitive and high-throughput biological and chemical sensing.

Nature Communications, 2014

Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding... more Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding changes the charge density of the sensor and leads to sensing signal. However, intrinsically slow dynamics of interface-trapped charges and defect-mediated charge-transfer processes significantly limit those sensors' response to tens to hundreds of seconds, which has long been known as a bottleneck for studying the dynamics of molecule-nanomaterial interaction and for many applications requiring rapid and sensitive response. Here we report a fundamentally different sensing mechanism based on molecular dipole detection enabled by a pioneering graphene nanoelectronic heterodyne sensor. The dipole detection mechanism is confirmed by a plethora of experiments with vapour molecules of various dipole moments, particularly, with cis-and trans-isomers that have different polarities. Rapid (down to B0.1 s) and sensitive (down to B1 ppb) detection of a wide range of vapour analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronics sensors.

Optics Express, 2010

We theoretically analyze the ability of 3-dimensionally confined optofluidic ring resonators (OFR... more We theoretically analyze the ability of 3-dimensionally confined optofluidic ring resonators (OFRRs) for detection of a single nanoparticle in water and in air. The OFRR is based on a glass capillary, on which bottleshaped and bubble-shaped ring resonators can form. The spectral position of the whispering gallery mode in the OFRR shifts when a nanoparticle is attached to the OFRR inner surface. For both ring resonator structures, the electric field at the inner surface can be optimized by choosing the right wall thickness. Meanwhile, different electric field confinement along the capillary longitudinal axis can be achieved with different curvatures. Both effects significantly increase the sensitivity of the ring resonator for single nanoparticle detection. It is found that the sensitivity is enhanced about 10 times, as compared to that of a solid microsphere biosensor recently reported, and that the smallest detectable nanoparticle is estimated to be less than 20 nm in radius for a Δλ/λ resolution of 10 8. The high sensitivity and the naturally integrated capillary based microfluidics make the OFRR a very promising sensing platform for detection of various nano-sized bio/chemical species in liquid as well as in air.

Lab on a Chip, 2012

We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for m... more We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for microgas chromatography (mGC) applications. The sensor is based on the Fabry-P erot (FP) interferometer formed by a micrometre-thin vapor-sensitive polymer layer coated on a silicon wafer. The thickness and the refractive index of the polymer vary in response to the vapor analyte, resulting in a change in the reflected intensity of the laser impinged on the sensor. In our study, four different polymers were coated on four wells pre-etched on a silicon wafer to form a spatially separated sensor array. A CMOS imager was employed to simultaneously monitor the polymers' response, thus enabling multiplexed detection of a vapor analyte passing through the GC column. A sub-second detection time was demonstrated. In addition, a sub-picogram detection limit was achieved, representing orders of magnitude improvement over the on-chip vapor sensors previously reported.

Journal of Microelectromechanical Systems, 2013

We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation co... more We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation column and oncolumn, 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.

Applied Physics Letters, 2011

Applied Physics Letters, 2011

Single mode lasing from the polydimethylsiloxane based on-chip coupled optofluidic ring resonator... more Single mode lasing from the polydimethylsiloxane based on-chip coupled optofluidic ring resonator ͑OFRR͒ with the lasing threshold of a few J / mm 2 is demonstrated using the Vernier effect. The single mode operation is highly stable even at high pump energy densities. The effect of the OFRR size and coupling strength on the single mode emission is investigated, showing that the excessive coupling results in incomplete side mode suppression. Tuning of the lasing wavelength is achieved by modifying the dye solution.

Applied Physics Letters, 2011

An optofluidic Fabry-Pérot cavity label-free biosensor with integrated flow-through micro-/ nanoc... more An optofluidic Fabry-Pérot cavity label-free biosensor with integrated flow-through micro-/ nanochannels is proposed and demonstrated, which takes advantages of the large surface-to-volume ratio for analyte concentration and high detection sensitivity and built-in fluidic channels for rapid analyte delivery. The operating principle is first discussed, followed by assembly of a robust sensing system. Real-time measurements are performed to test its sensing feasibility and capability including bulk solvent change and removal/binding of molecules from/onto the internal surface of fluidic channels. The results show that this sensor provides a very promising platform for rapid, sensitive, and high-throughput biological and chemical sensing.

Analytical Chemistry, 2012

We proposed and investigated a novel adaptive two-dimensional (2-D) microgas chromatography syste... more 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 oncolumn 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.

Analytical Chemistry, 2012

We developed novel flow-through surface-enhanced Raman scattering (SERS) platforms using gold nan... more 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 capillary's 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.