Kalpana Besar

Highly sensitive NH3 detection based on organic field-effect transistors with tris(pentafluorophenyl)borane as receptor.

We have increased organic field-effect transistor (OFET) NH(3) response using tris(pentafluorophenyl)borane (TPFB) as a receptor. OFETs with this additive could detect concentrations of 450 ppb v/v, with a limit of detection of 350 ppb, the highest sensitivity reported to date for semiconductor films; in comparison, when triphenylmethane (TPM) or triphenylborane (TFB) was used as an additive, no obvious improvement in the sensitivity was observed. These OFETs also showed considerable selectivity with respect to common organic vapors and stability toward storage. Furthermore, excellent memory of exposure was achieved by keeping the exposed devices in a sealed container stored at -30 °C, the first such capability demonstrated with OFETs.

Label-free brain injury biomarker detection based on highly sensitive large area organic thin film transistor with hybrid coupling layer

We describe a sensitive, large-area thin film transistor (TFT) sensor platform for real time detection of low-concentration protein analytes in solution. The sensing area is 7 mm by 7 mm. p-channel (pentacene) and n-channel (a naphthalenetetracarboxylic diimide, NTCDI) organic molecules were each used as semiconductors in conjunction with a newly designed receptor–antibody-functionalized top dielectric layer. This layer, incorporating both a fluorinated polymer and vapor-deposited hydrocarbon, provided maximum capacitive coupling and minimal interference from the aqueous analyte solution, and allowed convenient solvent processing of the antibody coupling layer. Additionally, a new antibody immobilization method was introduced, which led to high immobilization yield and surface coverage. Using glial fibrillary acidic protein (GFAP) as a model protein analyte, this sensor platform demonstrated significant selectivity and recognition of target protein even in much more concentrated non-target protein backgrounds. The dose–response relationship yielded a Langmuir isotherm from which a reasonable affinity constant was calculated for the protein and antibody. A zeta potential measurement provided further evidence of the surface potential change being detected by the TFTs. We explicitly verified for the first time that the response is in fact predominantly from perturbations of TFT channel current. To the best of our knowledge, this is the most sensitive organic TFT (OTFT) protein sensor yet reported, and also the first demonstration of the expected opposite current responses by p- and n-channel semiconductors to the same protein.

Demonstration of Hole Transport and Voltage Equilibration in Self-Assembled π-Conjugated Peptide Nanostructures Using Field-Effect Transistor Architectures

π-Conjugated peptide materials are attractive for bioelectronics due to their unique photophysical characteristics, biofunctional interfaces, and processability under aqueous conditions. In order to be relevant for electrical applications, these types of materials must be able to support the passage of current and the transmission of applied voltages. Presented herein is an investigation of both the current and voltage transmission activities of one-dimensional π-conjugated peptide nanostructures. Observations of the nanostructures as both semiconducting and gate layers in organic field-effect transistors (OFETs) were made, and the effect of systematic changes in amino acid composition on the semiconducting/conducting functionality of the nanostructures was investigated. These molecular variations directly impacted the hole mobility values observed for the nanomaterial active layers over 3 orders of magnitude (∼0.02 to 5 × 10(-5) cm(2) V(-1) s(-1)) when the nanostructures had quaterthiophene cores and the assembled peptide materials spanned source and drain electrodes. Peptides without the quaterthiophene core were used as controls and did not show field-effect currents, verifying that the transport properties of the nanostructures rely on the semiconducting behavior of the π-electron core and not just ionic rearrangements. We also showed that the nanomaterials could act as gate electrodes and assessed the effect of varying the gate dielectric layer thickness in devices where the conventional organic semiconductor pentacene spanned the source and drain electrodes in a top-contact OFET, showing an optimum performance with 35-40 nm dielectric thickness. This study shows that these peptides that self-assemble in aqueous environments can be used successfully to transmit electronic signals over biologically relevant distances.

Sequence-dependent mechanical, photophysical and electrical properties of pi-conjugated peptide hydrogelators

The ability to modulate intermolecular interactions in such a way as to impact nano-, micro- and even macroscale properties is an attractive aspect of self-assembling systems. We present an investigation of sequence-dependent rheological, photophysical and electrical properties of semiconducting peptide hydrogelators. Five different π-conjugated peptides containing a quaterthiophene core were studied, wherein the relative size and hydrophobicity of the amino acid residues adjacent to the π-electron core were varied in order to assess the impact of molecular variation on nanoscale and bulk material properties. Steady-state spectroscopic measurements of the peptides once assembled into 1D-nanostructures show distinct spectral characters as the relative size of the amino acid side chain adjacent to the π-electron core increases. Those peptides that formed hydrogels differed in network topography and rheological properties, with storage modulus (G′) values ranging from ∼3 to 20 kPa. The electrical properties of the peptide nanostructures were characterized by measuring the sheet resistance of dried peptide films on glass substrates. This study provides insights on the effects of amino acid sequence on the nanoscale to the macroscale electrical transport and mechanical properties of nanostructure-forming π-conjugated peptides.

Electrochemical processes and mechanistic aspects of field-effect sensors for biomolecules

Electronic biosensing is a leading technology for determining concentrations of biomolecules. In some cases, the presence of an analyte molecule induces a measured change in current flow, while in other cases, a new potential difference is established. In the particular case of a field effect biosensor, the potential difference is monitored as a change in conductance elsewhere in the device, such as across a film of an underlying semiconductor. Often, the mechanisms that lead to these responses are not specifically determined. Because improved understanding of these mechanisms will lead to improved performance, it is important to highlight those studies where various mechanistic possibilities are investigated. This review explores a range of possible mechanistic contributions to field-effect biosensor signals. First, we define the field-effect biosensor and the chemical interactions that lead to the field effect, followed by a section on theoretical and mechanistic background. We then discuss materials used in field-effect biosensors and approaches to improving signals from field-effect biosensors. We specifically cover the biomolecule interactions that produce local electric fields, structures and processes at interfaces between bioanalyte solutions and electronic materials, semiconductors used in biochemical sensors, dielectric layers used in top-gated sensors, and mechanisms for converting the surface voltage change to higher signal/noise outputs in circuits.

Ethylene Detection Based on Organic Field-Effect Transistors With Porogen and Palladium Particle Receptor Enhancements

Ethylene sensing is a highly challenging problem for the horticulture industry because of the limited physiochemical reactivity of ethylene. Ethylene plays a very important role in the fruit life cycle and has a significant role in determining the shelf life of fruits. Limited ethylene monitoring capability results in huge losses to the horticulture industry as fruits may spoil before they reach the consumer, or they may not ripen properly. Herein we present a poly(3-hexylthiophene-2,5-diyl) (P3HT)-based organic field effect transistor as a sensing platform for ethylene with sensitivity of 25 ppm V/V. To achieve this response, we used N-(tert-Butoxy-carbonyloxy)-phthalimide and palladium particles as additives to the P3HT film. N-(tert-Butoxy-carbonyloxy)-phthalimide is used to increase the porosity of the P3HT, thereby increasing the overall sensor surface area, whereas the palladium (<1 μm diameter) particles are used as receptors for ethylene molecules in order to further enhance the sensitivity of the sensor platform. Both modifications give statistically significant sensitivity increases over pure P3HT. The sensor response is reversible and is also highly selective for ethylene compared to common solvent vapors.

Sensitive and Selective NO2 Sensing Based on Alkyl- and Alkylthio-Thiophene Polymer Conductance and Conductance Ratio Changes from Differential Chemical Doping

NO2-responsive polymer-based organic field-effect transistors (OFETs) are described, and room-temperature detection with high sensitivity entirely from the semiconductor was achieved. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecylthioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers to detect a concentration at least as low as 1 ppm of NO2. The proportional on-current change of OFETs using these polymers reached over 400% for PQTS12, which is among the highest sensitivities reported for a NO2-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and also decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO2 gas compared with PQT12. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, making this parameter a means of distinguishing responses to low concentrations for extended times from exposures to high concentrations from shorter times. The responses to nonoxidizing vapors were much lower, indicating good selectivity to NO2 of two polymers. This work demonstrates the capability of increasing selectivity and calibration of OFET sensors by modulating redox and aggregation properties of polymer semiconductors.

Inexpensive, Versatile, and Robust USB-Driven Sensor Platform

An ultralow cost vapor sensor platform is developed, incorporating a chemiresistor-based sensor that enables targeted sensing through surface functionalization. Humidity, ammonia, and acetone sensing are demonstrated by PEDOT-PSS, DPP-CN, and graphene surface modification, respectively. The integrated USB connector, use of a standard HMI interface, and microcontroller interface allows us installation-free data collection on any device with a USB port.

A flexible organic inverter made from printable materials for synergistic ammonia sensing

Ammonia is a nearly ubiquitous environmental pollutant, and occupational or random exposure to sufficiently high levels is known to result in a variety of lung ailments. In this work, we present a flexible architecture for an ammonia inverter sensor consisting of a conductive plastic substrate, a bi-layer polymer dielectric, and patterned gold electrodes. n- and p-Type semiconductors (2,2′-[(2,5-dihexadecyl-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diylidene)dithiene-5,2-diyli-dene]dimalononitrile (DPPCN) and poly(3-hexylthiophene-2,5-diyl) (P3HT), respectively) were deposited on the inverter in specific quantities in order to ensure complementary, reverse responses in the presence of ammonia gas. Our sensor shows a sensitivity down to 0.5 ppm ammonia, and has good stability in the presence of other vapors. The sturdy structure and easily understood transduction mechanism of this sensor make it potentially very useful for point-of-care sensing.

Design, synthesis, and static charge tuning of organic semiconductors for sensing applications

Organic and polymeric semiconductors are among the alternatives to silicon being considered for sensing devices and circuitry. Their synthesis is now well established, and some performance metrics such as charge carrier mobility and optoelectronic quantum yield exceed those of inorganic counterparts such as amorphous silicon. The best fit for organic semiconductors is in applications where inherent capabilities such as rational modification of carrier energy levels and covalent connection between charge channels and surface receptors are leveraged. This presentation will describe newly synthesized organic molecular solids and polymer films where these attributes are emphasized. For example, addition of a borane to a semiconductor enhances response to ammonia, and introduction of highly electron donating tetrathiafulvalenes into moderately electron-rich polymers enhances response to electron-poor analytes (for example, TNT), for the development of chemical sensors. Carrier energy levels are markedly and predictably altered by static charge embedded in polystyrene films adjacent to organic semiconductors, for multiple device activities to be obtained from a single device layout using one semiconductor, and also the avoidance of powering gate electrodes to set optimal sensor sensitivities during operation.