P. Ravi Selvaganapathy


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Tuning the Chemical and Mechanical Properties of Conductive MoS<sub>2</sub> Thin Films by Surface Modification with Aryl Diazonium Salts
Dipankar Saha, Shayan Angizi, Maryam Darestani-Farahani, Johnson Dalmieda, P. Ravi Selvaganapathy, Peter Kruse
Langmuir, Volume 38, Issue 12

Molybdenum disulfide (MoS2) is a promising material for applications in sensors, energy storage, energy conversion devices, solar cells, and fuel cells. Because many of those applications require conductive materials, we recently developed a method for preparing a conductive form of MoS2 (c-MoS2) using dilute aqueous hydrogen peroxide in a simple and safe way. Here, we investigate modulating the chemical and mechanical surface properties of c-MoS2 thin films using diazonium chemistry. In addition to a direct passivation strategy of c-MoS2 with diazonium salts for electron-withdrawing groups, we also propose a novel in situ synthetic pathway for modification with electron-donating groups. The obtained results are examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The degree of surface passivation of pristine and functionalized c-MoS2 films was tested by exposing them to aqueous solutions of different metal cations (Fe2+, Zn2+, Cu2+, and Co2+) and detecting the chemiresistive response. While pristine films were found to interact with several of the cations, modified films did not. We propose that a surface charge transfer mechanism is responsible for the chemiresistive response of the pristine films, while both modification routes succeeded at complete surface passivation. Functionalization was also found to lower the coefficient of friction for semiconducting 2H-MoS2, while all conductive materials (modified or not) also had lower coefficients of friction. This opens up a pathway to a palette of dry lubricant materials with improved chemical stability and tunable conductivity. Thus, both in situ and direct diazonium chemistries are powerful tools for tuning chemical and mechanical properties of conductive MoS2 for new devices and lubricants based on conductive MoS2.

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Facile fabrication of conductive MoS<sub>2</sub> thin films by sonication in hot water and evaluation of their electrocatalytic performance in the hydrogen evolution reaction
Dipankar Saha, Vinay Patel, P. Ravi Selvaganapathy, Peter Kruse
Nanoscale Advances, Volume 4, Issue 1

Molybdenum disulfide (MoS 2 ) has long been used in catalysis and is a promising material for energy conversion devices.

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Colorimetric Detection of Heavy Metal Ions Using Superabsorptive Hydrogels and Evaporative Concentration for Water Quality Monitoring
Mahmoud F. Fathalla, P. Ravi Selvaganapathy
ACS ES&T Water, Volume 2, Issue 4

Prevalence of high levels of metal ions in natural and drinking water is a growing problem to both ecosystems and human health. Several methods are broadly used for heavy metal monitoring in water resources, but most of them are laboratory-based. Here, we describe a method that simplifies the measurement process by enabling passive aliquoting and preconcentration of heavy metals. We use superabsorbent polymer beads that can take up hundreds of times their volume to aliquot the sample and preconcentrate the ionic species present in them by 2 orders of magnitude. We then use commercially available colorimetric dyes that are sensitive only at high concentrations to reveal a visible range change in the bead color that can be measured optically using a camera. Using this approach, we have detected the concentration of copper(II) ions in water as low as 5.4 ppb. We demonstrate that this method can also be used for drinking water and tap water samples to assess concentrations of copper and iron. This solid-state method significantly simplifies the analytical procedure and provides extremely low detection levels of heavy metals, eliminating the need for expensive equipment and hence could be useful in remote settings.

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Reagent-Free Hydrogen Peroxide Sensing Using Carbon Nanotube Chemiresistors with Electropolymerized Crystal Violet
Vinay Patel, Dipankar Saha, Peter Kruse, P. Ravi Selvaganapathy
ACS Applied Nano Materials, Volume 5, Issue 3

Hydrogen peroxide (H2O2) is an intermediate molecule generated in numerous peroxidase assays used to measure concentrations of biomolecules such as glucose, galactose, and lactate. Here, we develop a solid-state reagent-free chemiresistive H2O2 sensor, which can measure H2O2 over a wide measuring range of 0.5–1000 ppm (0.015–29.4 mM). The sensor was fabricated using a network of functionalized single-walled carbon nanotubes (SWCNTs) as a sensitive layer and a xurographically patterned gold leaf as a contact electrode. The SWCNTs were functionalized with crystal violet to impart selective detection of H2O2. The crystal violet was self-assembled on the SWCNT film and subsequently polymerized via cyclic voltammetry to improve its retention on the sensing layer. The functionalized sensor exhibited good selectivity against common interferents such as uric acid, urea, glucose, and galactose. In addition, the sensor was used to measure in situ H2O2 generated during peroxidase assays performed using enzymes like glucose oxidase. The sensor was tested in standard buffer solutions for both enzymes. The glucose oxidase assay was also demonstrated in spiked pooled human plasma samples. The glucose oxidase-coated sensor exhibited a glucose detection range of 2–20 mM in standard buffer and blood plasma solutions, with a good recovery rate (∼95–107%) for glucose measurements in blood plasma.

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A xurography based rapid prototyping method to fabricate low-cost and high quality metal thin film micropatterns using metal leaves
Vinay Patel, Peter Kruse, P. Ravi Selvaganapathy
Materials Today Communications, Volume 30

Metal leaves are commercially available for decoration purposes and offers a low-cost alternative to sputtering thin metal films. Although thin metal leaves have been sparingly used in physical and chemical sensing and solar cells, their application has been limited primarily due to lack of a simple patterning methods and to form microscale features with them. Here, a low-cost, rapid and simple xurography based cutting method has been developed for direct pattering of metal leaves. The method was able to pattern features with line width of < 100 µm and it was also able to cut patterns with a pitch of < 100 µm. Conductive lines < 250 µm were also achieved which is a sufficient resolution for application in sensors and most biomedical devices. The versatile capability of this method to cut various geometric shapes like circle, rectangle, triangles and hexagons was also demonstrated. The method is robust and can be applied to pattern leaves made of several materials or which gold, silver, palladium, aluminum and copper were demonstrated. This patterning method was used to fabricate contact electrodes for chemiresistive sensors with low and high surface roughness. These sensors were evaluated using the resistance and noise characteristics. The peak-to-peak noise for gold contact electrodes (11.5 nA) for chemiresistive sensors was significantly lower than the copper tape contact electrodes (18.2 nA). The process was also used to fabricate gold interdigitated electrodes for biamperometric glucose sensing at low potential (~10 mV). Finally, the method was used to indirectly pattern gold leaf on a shrink film to fabricate high surface 3D electrodes costing around one-fifth (~20%) of a sputtered gold electrode.


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Enhancing the sensitivity of cobalt based solid-state phosphate sensor using electrical pretreatment
Vinay Patel, P. Ravi Selvaganapathy
Sensors and Actuators B: Chemical, Volume 349

Phosphate is an important analyte to monitor in various water bodies. Cobalt based sensors are attractive for this application as they are solid-state, have a quick response time, are easy to fabricate and can perform reagent-less measurements. However, these sensors have lower sensitivity, limited dynamic range and require a chemical conditioning in a standard solution before measurement. In this study, an in situ anodic current pretreatment method in sample solution itself is used to enhance the sensitivity of the sensor and alleviate the need of chemical conditioning before measurement. With electrical pretreatment, the sensor exhibited a linear range from 10 −6 M to 10 −3 M with a sensitivity of −91.4 mV/decade of change in dihydrogen phosphate concentration. No significant interference was detected with common interfering anions that are typically present in field water samples such as nitrate, sulfate and chloride. Finally, the sensor was also responsive when tested real water samples such as tap water, lake water and creek water spiked with phosphate. • A new in situ electrical pretreatment method is used to enhance the sensitivity of cobalt based phosphate sensors. • The in situ electrical pretreatment method eliminates the need of the tedious chemical pretreatment in standard solution. • The rapid pretreatment protocol can even extend the range of measurements to much lower concentrations (10-8 M). • Use of electrical pretreatment makes this a practical format for field use as standard solutions are not needed.

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Chemiresistive detection of silver ions in aqueous media
Johnson Dalmieda, Ana Zubiarrain-Laserna, Devanjith Ganepola, P. Ravi Selvaganapathy, Peter Kruse
Sensors and Actuators B: Chemical, Volume 328

• Chemiresistive sensors can be fabricated from percolation networks of few-layer graphene (FLG) flakes. • Functionalization with suitable ligands can achieve selective sensor response to Ag + ions in the 3 ppb to 1 ppm range. • Sensors are robust and reusable, can be reset at pH3 due to a shift in the complexation equilibrium. • The sensor response was tested in an environmental sample (river water) and found to correlate well with ICP-MS data. Silver is used as a water disinfectant in hospital settings as well as in purifiers for potable water. Although there are no strict regulations on the concentration of silver in water, adverse effects such as argyria and respiratory tract irritation have been correlated to excess silver consumption. Based on this, the levels of silver in water are recommended to be maintained below 100 ppb to ensure safety for human consumption. In this work, we present a silver sensor for use in aqueous media that utilizes bathocuproine, a silver selective chromophore, adsorbed onto few-layer graphene (FLG) flake networks for the chemiresistive detection of silver. Complexation of silver to bathocuproine modulates the conductivity of the FLG film, which can be probed by applying a small voltage bias. The decrease in resistance of the film correlates with the concentration of silver in solution between 3 ppb and 1 ppm. Exposing the sensor to a lower pH resets the sensor, allowing it to be reused and reset multiple times. This sensor demonstrates a new pathway to chemiresistive cation sensing using known selective complexing agents adsorbed onto graphitic thin films. This concept can be expanded to the detection of other relevant analytes in domestic, industrial and environmental water sources.


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Peroxide-Induced Tuning of the Conductivity of Nanometer-Thick MoS<sub>2</sub> Films for Solid-State Sensors
Dipankar Saha, P. Ravi Selvaganapathy, Peter Kruse
ACS Applied Nano Materials, Volume 3, Issue 11

Applications of molybdenum disulfide (MoS2) in energy storage devices, solar cells, electrocatalysts, and sensors require good electrical conductivity. However, neither of the current ways to prepa...

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Solid State Sensors for Hydrogen Peroxide Detection
Vinay Patel, Peter Kruse, P. Ravi Selvaganapathy
Biosensors, Volume 11, Issue 1

Hydrogen peroxide (H2O2) is a key molecule in numerous physiological, industrial, and environmental processes. H2O2 is monitored using various methods like colorimetry, luminescence, fluorescence, and electrochemical methods. Here, we aim to provide a comprehensive review of solid state sensors to monitor H2O2. The review covers three categories of sensors: chemiresistive, conductometric, and field effect transistors. A brief description of the sensing mechanisms of these sensors has been provided. All three sensor types are evaluated based on the sensing parameters like sensitivity, limit of detection, measuring range and response time. We highlight those sensors which have advanced the field by using innovative materials or sensor fabrication techniques. Finally, we discuss the limitations of current solid state sensors and the future directions for research and development in this exciting area.

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Influence of wastewater microbial community on the performance of miniaturized microbial fuel cell biosensor
Nan Xiao, P. Ravi Selvaganapathy, Rong Wu, Jinhui Jeanne Huang‬‬‬‬
Bioresource Technology, Volume 302

Microbial fuel cells (MFCs) based sensors had been studied in measuring biochemical oxygen demand (BOD) or the equivalent chemical oxygen demand (COD) recently. Limited attention has been paid to the effect of the microbial communities in wastewater on the responses of these sensors. This study systematically evaluated, for the first time, the effect of wastewater samples from a variety of sources on the electrical response of a micro-fabricated double-chamber MFC device. It was found that the response of the MFC is positively correlated with the bacterial composition, in particular electroactive bacteria. The presence of aerobic bacteria in the sample reduces the current generation. These findings indicated that the bacterial content of the water sample could be a significant interference source and must be considered in the use of µMFC-based sensors. Filtering samples may be effective in improving the reliability of these microsensors.

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Anode surface modification regulates biofilm community population and the performance of micro-MFC based biochemical oxygen demand sensor
Nan Xiao, Rong Wu, Jinhui Jeanne Huang‬‬‬‬, P. Ravi Selvaganapathy
Chemical Engineering Science, Volume 221

• Plasma increased hydrophilicity, encouraging bacterial growth and diversity. • CNT changed anode surface morphology, encouraging electroactive bacteria growth. • Both plasma and CNT treatment do not increase the sensitivity of the biosensor. • The conditions optimal for power generation may not be optimal for MFC sensors. The anode surface is known to play an important role in the microbial growth and in mediating electron transfer between electroactive bacteria and the electrodes in power generating microbial fuel cells (MFCs). However, the effect of the anode surface and its modification on MFC-based biosensor performance has not been studied previously. In this study, our results show that the surface modification influences certain aspect of the biosensor performance. Plasma treatment makes the carbon cloth electrode hydrophilic with contact angle of 82 ± 5° from that of 139 ± 3° without treatment which consequently increases the amount of biofilm and produces higher current generation. Carbon nanotube (CNT) treatment doesn’t increase the amount of biofilm but significantly changes its electroactive microorganism composition from 2.3% to 17.3% that improves current generation. Interestingly, the sensitivity of the MFC sensor was not improved by either of these treatments. These findings would be important for the optimized design and manufacturing of biosensing MFCs.

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Development of a xurographically fabricated miniaturized low-cost, high-performance microbial fuel cell and its application for sensing biological oxygen demand
Nan Xiao, Rong Wu, Jinhui Jeanne Huang‬‬‬‬, P. Ravi Selvaganapathy
Sensors and Actuators B: Chemical, Volume 304

Abstract The rapid quantification of biological oxygen demand (BOD) plays an important role in environmental management, for instance, wastewater treatment. This study used xurographic fabrication technology to rapidly fabricate a low cost miniaturized microbial fuel cell (MFC) and demonstrated its suitability to measure BOD. The miniaturized sensor could be fabricated in 10 min with low cost of $0.5 U.S. per device. The reaction volume was designed to be 1.8 μL to obtain faster response time. The sensor was tested using sodium acetate (NaAc) as a model BOD analyte. It could response to a wide range of BOD concentration between 20 and 490 mg/L which would cover the majority range of wastewater BOD concentration in a wastewater treatment plant. The response time of this microsensor was 1.1 min which was significantly shorter than other conventional methods for BOD measurements (5 days). This study demonstrated that the use of xurographic methods to fabricate MFCs could enable rapid fabrication of microsensors to measure BOD in a rapid manner. This study also identified the potential of the sensor for application in wastewater treatment plants to monitor BOD and provide guidance for controlling treatment processes.


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Fire-retardant, self-extinguishing triboelectric nanogenerators
Abdelsalam Ahmed, Maher F. El‐Kady, Islam Hassan, Ayman Negm, Amir Masoud Pourrahimi, Mit Muni, P. Ravi Selvaganapathy, Richard B. Kaner
Nano Energy, Volume 59

Abstract The development of highly sensitive sensors and power generators that could function efficiently in extreme temperatures and contact with fire can be lifesaving but challenging to accomplish. Herein, we report, for the first time, a fire-retardant and self-extinguishing triboelectric nanogenerator (FRTENG), which can be utilized as a motion sensor and/or power generator in occupations such as oil drilling, firefighting or working in extreme temperature environments with flammable and combustible materials. The device takes advantage of the excellent thermal properties of carbon derived from resorcinol-formaldehyde aerogel whose electrical, mechanical and triboelectric properties have been improved via the introduction of Polyacrylonitrile nanofibers and graphene oxide nanosheets. This FRTENG is not flammable even after 90 s of trying, whereas conventional triboelectric materials were entirely consumed by fire under the same conditions. The developed device shows exceptional charge transfer characteristics, leading to a potential difference up to 80 V and a current density up to 25 µA/m2. When integrated into firefighter's shoes, the FRTENG is able to discern the movements of a firefighter in hazardous situations, while providing the high thermal stability missing in conventional TENGs. The fire-retardant and self-extinguishing characteristics offered by the FRTENG makes it a path-breaking device for lifesaving wearable applications.

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Integrated Triboelectric Nanogenerators in the Era of the Internet of Things
Abdelsalam Ahmed, Islam Hassan, Maher F. El‐Kady, Ali Radhi, Chang Kyu Jeong, P. Ravi Selvaganapathy, Jean W. Zu, Shenqiang Ren, Qing Wang, Richard B. Kaner
Advanced Science, Volume 6, Issue 24

Since their debut in 2012, triboelectric nanogenerators (TENGs) have attained high performance in terms of both energy density and instantaneous conversion, reaching up to 500 W m-2 and 85%, respectively, synchronous with multiple energy sources and hybridized designs. Here, a comprehensive review of the design guidelines of TENGs, their performance, and their designs in the context of Internet of Things (IoT) applications is presented. The development stages of TENGs in large-scale self-powered systems and technological applications enabled by harvesting energy from water waves or wind energy sources are also reviewed. This self-powered capability is essential considering that IoT applications should be capable of operation anywhere and anytime, supported by a network of energy harvesting systems in arbitrary environments. In addition, this review paper investigates the development of self-charging power units (SCPUs), which can be realized by pairing TENGs with energy storage devices, such as batteries and capacitors. Consequently, different designs of power management circuits, supercapacitors, and batteries that can be integrated with TENG devices are also reviewed. Finally, the significant factors that need to be addressed when designing and optimizing TENG-based systems for energy harvesting and self-powered sensing applications are discussed.

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All printable snow-based triboelectric nanogenerator
Abdelsalam Ahmed, Islam Hassan, Islam M. Mosa, Esraa Elsanadidy, Gayatri Phadke, Maher F. El‐Kady, James F. Rusling, P. Ravi Selvaganapathy, Richard B. Kaner
Nano Energy, Volume 60

The development of power generators that can function in harsh snowy environments and in contact with snow can be beneficial but challenging to accomplish. Herein, we introduce the first snow-based triboelectric nanogenerator (snow-TENG) that can be used as an energy harvester and a multifunctional sensor based on the principle of snow-triboelectrification. In this work, we used a 3D printing technique for the precise design and deposition of the electrode and triboelectric layer, leading to flexible, stretchable and metal-free triboelectric generators. Based on the single electrode mode, the device can generate an instantaneous output power density as high as 0.2 mW/m2, an open circuit voltage up to 8 V, and a current density of 40 μA/m2. In addition, the snow-TENG can function as a miniaturized weather station to monitor the weather in real time to provide accurate information about the snowfall rate, snow accumulation depth, wind direction, and speed in snowy and/or icy environments. In addition, the snow-TENG can be used as a wearable power source and biomechanical sensor to detect human body motions, which may prove useful for snow-related sports. Unlike conventional sensor platforms, our design works without the need for batteries or image processing systems. We envision these devices could potentially be integrated into solar panels to ensure continuous power supply during snowy weather conditions.

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Sink electrical discharge machining of hydrophobic surfaces
Chuanyong Guo, Philip Koshy, Felipe Coelho, P. Ravi Selvaganapathy
CIRP Annals, Volume 68, Issue 1

Abstract The phenomenon of hydrophobicity observed in such surfaces as lotus leaves is typically manifest by hierarchical structures on low-energy surfaces. Sustained interest in fabricating hydrophobic surfaces has resulted in a myriad of processes, which are but limited by their largely referring to soft materials and/or involving multiple process steps. The present work explored the application of electrical discharge machining (EDM) for the single-step manufacture of durable, metallic hydrophobic surfaces. Simple sink EDM in a hydrocarbon dielectric, with no special process kinematic or tooling requirements, is demonstrated to rapidly generate surfaces that are intrinsically water repellent, with contact angles approaching 150°.

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Optofluidic Dissolved Oxygen Sensing With Sensitivity Enhancement Through Multiple Reflections
Eric Mahoney, Huan-Hsuan Hsu, Fei Du, Bo Xiong, P. Ravi Selvaganapathy, Qiyin Fang
IEEE Sensors Journal, Volume 19, Issue 22

The development of compact and low-cost dissolved oxygen (DO) sensors is essential for the continuous in situ monitoring of environmental water quality and wastewater treatment processes. The optical detection of dynamic and reversible quenching of fluorescent dyes by oxygen has been used for DO sensing. In this paper, we have optimized a multilayer optofluidic device based on the measurement of fluorescence quenching in a Ruthenium-based oxygen sensitive dye by employing total internal reflection (TIR) of the excitation light to achieve sensitivity enhancement for the detection of 0-20-ppm DO in water. The incident angles of light and sensitive layer thickness are optimized experimentally in order to increase the path length of light in the sensitive layer of the device through multiple reflections. A model is developed to demonstrate how light propagates through different layers of the device at varying angles of excitation and to describe the mechanism of fluorescence generation for each of the types of TIR observed. The design principles identified in this paper may be applied to the development and optimization of new multilayered optofluidic sensors by employing TIR for sensitivity enhancement.

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Suppression of Biofouling on a Permeable Membrane for Dissolved Oxygen Sensing Using a Lubricant-Infused Coating
Matthew Osborne, Aditya Aryasomayajula, Amid Shakeri, P. Ravi Selvaganapathy, Tohid F. Didar
ACS Sensors, Volume 4, Issue 3

Specific ranges of dissolved oxygen (DO) concentrations must be maintained in a waterbody for it to be hospitable for aquatic animals. DO sensor designs can employ selectively permeable membranes to isolate DO from untargeted compounds or organisms in waterbodies. Hence, the DO concentration can be monitored and the health of the water can be evaluated over time. However, the presence of bacteria in natural waterbodies can lead to the formation of biofilms that can block pores and prevent analyte from permeating the membrane, resulting in inaccurate readings. In this work, we demonstrate the implementation of a fluorosilane-based omniphobic lubricant-infused (OLI) coating on a selectively permeable membrane and investigate the rate of biofilm formation for a commercially available DO sensor. Coated and unmodified membranes were incubated in an environment undergoing accelerated bacterial growth, and the change in sensitivity was evaluated after 40, 100, 250, and 500 h. Our findings show that the OLI membranes attenuate biofouling by 70% and maintain sensitivity after 3 weeks of incubation, further demonstrating that oxygen transfer through the OLI coating is achievable. Meanwhile, unmodified membranes exhibit significant biofouling that results in a 3.35 higher rate of decay in oxygen measurement sensitivity and an over 70% decrease in static contact angle. These results show that the OLI coating can be applied on commercially available membranes to prevent biofouling. Therefore, OLI coatings are a suitable candidate to suppress biofilm formation in the widespread use of selectively permeable membranes for environmental, medical, and fluid separation applications.