Organic Semiconductor-based Chemical Sensors

The field known as organic or plastic electronics is centered on field effect transistor (FET)-based circuits mounted on large-area and/or flexible substrates. When the semiconductor is organic, the device is referred to as an organic field-effect transistor (OFET). Work on OFET has been extensively reviewed, most notably and comprehensively in Chemistry of Materials and Journal of Materials Research special issues, to which the one of us has contributed three articles [1-3]. 

Several new approaches to designing semiconducting materials for enhanced chemical sensitivity have been reported very recently [4], In some cases, these materials are intended to enhance the capability of an existing technique. In other cases, the materials are the basis of an entirely new sensing platform. The materials are employed as the current-carrying media in transistors or resistors whose 

The conductivity of a generic semiconductor film is likely to be modulated by absorption of a polar species on the film surface. For example, a group at the Weiz-mann Institute has examined GaAs surfaces and found that porphyrin receptors linked to it will attract nitrogen oxide NO, and the binding of NO caused a change in resistance [5]. Other embodiments of GaAs sensors were found to be sensitive to ions in solution [25]. 

Though not strictly functioning as resistors/conductors, carbon nanotubes have just been reported in an extraordinarily vapor-sensitive capacitive device [19]. The electric field lines emanating from the nanotubes are responsible for a localized dielectric response that can be modulated by minute quantities of adsorbate on the nanotube surface. A layer of hydrogen-bonding polymer, or even a mono-layer terminated in mildly acidic groups, increased sensitivity to parts per billion levels. Response strength was correlated with the dipole moment of the analytes. 

A very recent paper details the biomolecular functionalization of a polypyrrole nanowire. ZnSe/CdSe quantum dots were functionalized with avidin and these quantum dots were incorporated into a polypyrrole nanowire during its electrochemical formation between two electrodes 100 nm apart [32], Biotin with a DNA strand attached was shown to modulate the conductivity of the avidin nanowire, but not an unfunctionalized polypyrrole nanowire fabricated similarly. Presumably, the DNA-biotin-avidin complex would also be sensitive to changes in the hybridization state of the DNA. 

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Huang J, Miragliotta J, Becknell A, Katz HE (2007) Hydroxy-terminated organic semiconductor-based field-effect transistors for phosphonate vapor detection. J Am Chem Soc 129 9366-9376 Janata J, Josowicz M (1998) Chemical modulation of work function as a transduction mechanism for chemiceil sensors. Acc Chem Res 31 241-248... 

Potje-Kamloth K (2002) Chemical gas sensors based on organic semiconductor polypyrrole. Crit Rev Anal Chem 32 121-140... 

To test this novel architecture as a tool for classification, a simulated experiment was performed. The case of chemo-resistive sensors was considered because of the simple involved electronics. This class of sensors is rather wide and can include sensors based either on inorganic (e.g. metal-oxide semiconductors) or organic (e.g. conducting polymers) sensitive materials. The concepts here illustrated can be extended, with a proper modification of the AORN architecture, to different kinds of chemical sensors. Actually, the features of the olfactive epithelium define the following structure of the AORN. 

Wohltjen H, Barger WR, Snow AW, Jarvis NL (1985) A vapor-sensitive chemiresistor fabricated with planar microelectrodes and a Langmuir-Blodgett organic semiconductor film. IEEE Trans Electron Dev ED-32 1170-1174 Wolfbeis OS (1991) Fiber optic chemical sensors and biosensors, vol 1. CRC, Boca Raton, FL Wolfbeis OS (1992) Fiber optic chemical sensors and biosensors, vol 2. CRC, Boca Raton, FL Wolfbeis OS (2005) Materials for fluorescence-based optical chemical sensors. J Mater Chem 15 2657-2669 Wollenstein J, Plaza JA, Can6 C, Min Y, Bottner H, Tuller HE (2003) A novel single chip thin film metal oxide array. Sens Actuators B 93 350-355... 

Solid-state potentiometric sensors that are based on the chemical modulation of the work function of organic semiconductors were reviewed. These include the chemically sensitive field-effect transistor in which the conventional gate of... 

In principle, the choice of materials used to develop both the electrodes and the insulator of an OFET-based sensor should be basically the same as for OFETs developed for other applications. Similarly, the design of the device should follow the same rules. However, there are some unique aspects that are intrinsic to sensors and deserve special attention. For instance, the sensing area of the device this is the area where the external stimulus (chemical or physical) must be applied to the device without affecting its integrity and/or robustness. The first example of an ISOFET reported in the literature [13] was fabricated on silicon and the only organic component of it was the semiconductor employed as the active layer. In order to develop this structure, it is necessary to etch the highly doped silicon from the back side of the device... 

Results of numerous items of research have shown that, in theory, any material can be used in the design of a gas sensor, regardless of its physical, chemical, structural, or electrical properties (Korotcenkov 2010,2011). Prototypes of gas sensors based on covalent semiconductors, semiconducting metal oxides, solid electrolytes, polymers, ionic membranes, organic saniconductors, and ionic salts have already been tested (Sadaoka 1992 Gopel 1996 Haugen and Kvaal 1998 Monkman 2000 Talazac et al. 2001 Eranna et al. 2004 Adhikari and Majumdar 2004). As shown in Table 1.23, these materials may be used... 

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