Water-stable organic transistors and their application in chemical and biological sensors

Abstract

The development of low-cost, reliable sensors will rely on devices capable of converting an analyte binding event to an easily read electrical signal. Organic thin-film transistors (OTFTs) are ideal for inexpensive, single-use chemical or biological sensors because of their compatibility with flexible, large-area substrates, simple processing, and highly tunable active layer materials. We have fabricated low-operating voltage OTFTs with a cross-linked polymer gate dielectric, which display stable operation under aqueous conditions over >10(4) electrical cycles using the p-channel semiconductor 5,5'-bis-(7-dodecyl-9H-fluoren-2-yl)-2,2'-bithiophene (DDFTTF). OTFT sensors were demonstrated in aqueous solutions with concentrations as low as parts per billion for trinitrobenzene, methylphosphonic acid, cysteine, and glucose. This work demonstrates of reliable OTFT operation in aqueous media, hence opening new possibilities of chemical and biological sensing with OTFTs.

Fig. 1.

Cross-linked polymer gate insulator and its corresponding electrical properties. (A) Structure of the top-contact OTFT sensor. (B) Chemical structure of cross-linked PVP with HDA. (C) Capacitance vs. frequency. (D) Leakage current vs. voltage for various PVP-HDA films.

Fig. 2.

Electrical characteristics of p-channel OTFTs with a PVP-HDA insulator layer and a source-drain electrode geometry of W/L = 20. Output and transfer (Inset) characteristics of OTFTs with 40-nm thermally evaporated films of pentacene on OTS (A) and DDFTTF (B). The gate current is shown in dashed lines.

Fig. 3.

DDFTTF OTFT operation in aqueous media. (A) Optical micrograph of an OTFT with DDFTTF and 22-nm PVP-HDA under aqueous conditions. (B) Transfer characteristics (V_DS = −0.6 V) in ambient (black) and water (red). The right axis shows the semilog plot of I_DS vs. V_G and the left axis shows I_DS^1/2 vs. V_G. (C) Transfer characteristics recorded sequentially after the addition of water. (D) Output characteristics (I_DS vs. V_DS) in ambient. (E) Output characteristics under water. (F) I_DS vs. V_G (0.3 to −1 V) at a V_DS = −0.6 V measured over 10⁴ cycles (over a period of 12 h) with the initial 20 cycles expanded.

Fig. 4.

Chemical detection in aqueous systems based on OTFTs (V_G = −1 V, V_DS = −0.6 V). (A) Schematic showing an OTFT with DDFTTF in flow cell for aqueous-phase sensing. (B) Drain current, I_DS, response of a DDFTTF OTFT to pH. I_DS response curves for the pH 5 solution were measured before (solid) and after (dashed) the exposure to other pH solutions. (C–F) I_DS response to different analytes: trinitrobenzene (TNB) (C), glucose (D), cysteine (E), and methylphosphonic acid (MPA) (F). (E Inset) I_DS response for 10-ppm cysteine as a function of V_G. The magnitude of the change in current, referenced to the drain current at time 0 s (the baseline current varies with V_G), increases as V_G is changed from 0.2 to 0 to −0.2 V.

Fig. 5.

OTFT drain current sensitivity to 1 part per thousand (ppth) solutions of MeOH (dark gray), EtOH (gray), i-PrOH (light gray), and water (black) for V_G switching on and off (1 V), etc., for “on” V_G of 0.4, 0.2, 0, −0.2, and −0.4 V. (Inset) Expansion of OTFT drain current from around V_G = 0 V.