Single-Walled Carbon Nanotube–Poly(porphyrin) Hybrid for Volatile Organic Compounds Detection

Porphyrins due to their unique and interesting physicochemical properties have been widely investigated as functional materials for chemical sensor fabrication. However, their poor conductivity is a major limitation toward the realization of porphyrin-based field-effect transistor/chemiresistor sensor. The issue of conductivity can be overcome by exploiting the excellent electrical property of single-walled carbon nanotubes (SWNTs) to make a SWNTs-based hybrid device in which SWNTs would act as a transducer and porphyrin as a sensory layer. The present attempt was to fabricate a SWNTs–poly­(tetraphenylporphyrin) hybrid through electrochemical route and to evaluate its potential as a low-power chemiresistor sensor for sensing acetone vapor as a model for volatile organic compounds. Functionalization of SWNTs with porphyrin polymer by the electrochemical method resulted in a fuller coverage of SWNTs surface compared to a partial coverage by adsorption and thereby higher sensitivity. SWNTs were coated with poly­(tetraphenylporphyrin) of different thickness by applying different charge density to optimize sensing performance. Differences in sensing performance were noticed for hybrids fabricated at varying charge densities, and the optimum sensing response was found at 19.65 mC/cm². The hybrid exhibited a wide dynamic range for acetone vapor sensing from 50 to ∼230 000 ppm with a limit of detection of 9 ppm. The field-effect transistor studies showed a negative threshold voltage shift and almost constant transconductance when exposed to air/analyte, indicating electrostatic gating dominated sensing mechanism. Further, the results confirmed a good stability of the device over a period of 180 days. The long-term device stability along with the sensing capability at low analyte concentration with a wide dynamic range and easily scalable fabrication technique signify the potential of SWNT–poly­(porphyrin) hybrid for volatile organic compound sensing applications

Porphyrin-Functionalized Single-Walled Carbon Nanotube Chemiresistive Sensor Arrays for VOCs

Single-walled carbon nanotubes (SWNTs) have been used extensively for sensor fabrication due to their high surface-to-volume ratio, nanosized structure, and interesting electronic property. Lack of selectivity is a major limitation for SWNT-based sensors. However, surface modification of SWNTs with a suitable molecular recognition system can enhance the sensitivity. On the other hand, porphyrins have been widely investigated as functional materials for chemical sensor fabrication due to their several unique and interesting physicochemical properties. Structural differences between free-base and metal-substituted porphyrins make them suitable for improving the selectivity of sensors. However, their poor conductivity is an impediment in the fabrication of prophyrin-based chemiresistor sensors. The present attempt is to resolve these issues by combining free-base and metallo-porphyrins with SWNTs to fabricate SWNT–porphyrin hybrid chemiresistor sensor arrays for monitoring volatile organic compounds in the air. Differences in sensing performance were noticed for porphyrins with different functional groups and with different central metal atoms. The mechanistic study for acetone sensing was done using field-effect transistor measurements and revealed that the sensing mechanism of the ruthenium octaethyl porphyrin hybrid device was governed by the electrostatic gating effect, whereas the iron tetraphenyl porphyrin hybrid device was governed by electrostatic gating and Schottky barrier modulation in combination. Further, the recorded electronic responses for all hybrid sensors were analyzed using a pattern-recognition analysis tool. The pattern-recognition analysis confirmed a definite pattern in response for different hybrid materials and could efficiently differentiate analytes from one another. This discriminating capability of the hybrid nanosensor devices opens up the possibilities for further development of highly dense nanosensor arrays with suitable porphyrins for E-nose applications