Carbon nanotubes-based chemiresistive immunosensor for small molecules: detection of nitroaromatic explosives

Abstract

In recent years, there has been a growing focus on use of one-dimensional (1-D) nanostructures, such as carbon nanotubes and nanowires, as transducer elements for label-free chemiresistive/field-effect transistor biosensors as they provide label-free and high sensitivity detection. While research to-date has elucidated the power of carbon nanotubes- and other 1-D nanostructure-based field effect transistors immunosensors for large charged macromolecules such as proteins and viruses, their application to small uncharged or charged molecules has not been demonstrated. In this paper we report a single-walled carbon nanotubes (SWNTs)-based chemiresistive immunosensor for label-free, rapid, sensitive and selective detection of 2,4,6-trinitrotoluene (TNT), a small molecule. The newly developed immunosensor employed a displacement mode/format in which SWNTs network forming conduction channel of the sensor was first modified with trinitrophenyl (TNP), an analog of TNT, and then ligated with the anti-TNP single chain antibody. Upon exposure to TNT or its derivatives the bound antibodies were displaced producing a large change, several folds higher than the noise, in the resistance/conductance of SWNTs giving excellent limit of detection, sensitivity and selectivity. The sensor detected between 0.5 ppb and 5000 ppb TNT with good selectivity to other nitroaromatic explosives and demonstrated good accuracy for monitoring TNT in untreated environmental water matrix. We believe this new displacement format can be easily generalized to other one-dimensional nanostructure-based chemiresistive immuno/affinity-sensors for detecting small and/or uncharged molecules of interest in environmental monitoring and health care.

Figure 1

(A) Schematic diagram of the SWNT immunosensor to detect TNT. Anti-TNP scAb is leaving from the sensor platform due to displacement by TNT. (B) SEM image of aligned SWNTs between two gold electrodes. (C) Sequencial modification of the sensor.

Figure 2

Sequential responses of the sensor during the fabrication and the sensing. When TNP-OVA was immobilized on the SWNTs, the slope of the I-V plot decreased due to accumulation of negative charge of the protein. Antibody binding to TNP on the SWNTs also led to a decrease of the slope. After adding TNT, the slope was increased back; Bare SWNTs (◆), after modification with OVA-TNP (■), incubation with anti-TNT scAb (▲), and treatment with 5000 ng/mL of TNT (●).

Figure 3

Sensor response to different concentrations of TNT; (A) I-V plot according to the each concentration of TNT; anti-TNP scAb was bound to functionalized SWNTs (▲), 0.5 ng/mL (ж), 5 ng/mL (◆), 50 ng/mL (●), 500 ng/mL (■) and 5000 ng/mL (—) of TNT were added. (B) Calibration curve of the sensor for TNT in buffer. Inset graph represents a function of the logarithm of concentration. Each data is an average of three measurements, and error bar stands for the standard deviation.

Figure 3

Sensor response to different concentrations of TNT; (A) I-V plot according to the each concentration of TNT; anti-TNP scAb was bound to functionalized SWNTs (▲), 0.5 ng/mL (ж), 5 ng/mL (◆), 50 ng/mL (●), 500 ng/mL (■) and 5000 ng/mL (—) of TNT were added. (B) Calibration curve of the sensor for TNT in buffer. Inset graph represents a function of the logarithm of concentration. Each data is an average of three measurements, and error bar stands for the standard deviation.

Figure 4

Calibration curves of the sensor for different derivatives of TNT; TNB (●), 2A-4,6-DNT (▼), 2,4-DNT (◆) and Toluene (■). Each data is an average of five measurements, and error bars stand for the standard deviation.