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A systematic analysis was used to understand electrical drift occurring in field-effect transistor (FET) dissolved-analyte sensors by investigating its dependence on electrode surface-solution combinations in a remote-gate (RG) FET configuration. Water at pH 7 and neat acetonitrile, having different dipoles and polarizabilities, were applied to the RG surface of indium tin oxide (ITO), SiO2, hexamethyldisilazane-modified SiO2, polystyrene, poly(styrene-co-acrylic acid), poly(3-hexylthiophene-2,5-diyl) (P3HT), and poly [3-(3-carboxypropyl)thiophene-2,5-diyl] (PT-COOH). We discovered that in some cases a slow reorientation of dipoles at the interface induced by gate electric fields caused severe drift and hysteresis because of induced interface potential changes. Conductive and charged P3HT and PT-COOH increased electrochemical stability by promoting fast surface equilibrations. We also demonstrated pH sensitivity of P3HT (17 mV/pH) as an indication of proton doping. PT-COOH showed further enhanced pH sensitivity (30 mV/pH). This combination of electrochemical stability and pH response in PT-COOH are proposed as advantageous for polymer-based biosensors.
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Low frequency electrochemical noise in AlGaN/GaN field effect transistor biosensors
Little has been studied on how the electrochemical noise impacts the limit of detection of field effect transistor (FET) biosensors. Herein, we investigate low frequency noise associated with phosphate-buffered saline (PBS) solutions at varying ionic strengths (Ni) under both weak and strong gate biases corresponding to saturation and sub-threshold regimes, respectively, in AlGaN/GaN heterojunction FET biosensors. We show that the electrochemical noise is strongly dependent on the ionic strength and gate biasing conditions. In the saturation regime (low bias), varying the ionic strength (a range of 10−6× PBS to PBS 1 × stock solutions used for testing) has little to no effect on the characteristic frequency exponent 𝛽(𝛽=1), indicating a predominately diffusion-based process. Conversely, under higher biases (sub-threshold regime), the β parameter varies from 1 to 2 with ionic strength exhibiting both diffusion and drift characteristics, with a “cut point” at approximately 10−5× PBS (𝑁𝑖≈9×1014/mL). Under a high bias, once the PBS concentration reaches 10−3×, the behavior is then drift dominant. This indicates that the higher bias likely triggers electrochemical reactions and by extension, faradaic effects at most physiologically relevant ionic strengths. The signal-to-noise ratio (SNR) of the device has an inverse linear relationship with the low frequency current noise. The device exhibits a higher SNR in the sub-threshold regime than in the saturation regime. Specifically, within the saturation regime, an inversely proportional relationship between the SNR and the ionic concentration is observed. The electrochemical noise induced from ionic activities is roughly proportional to 𝑁−1/2𝑖.
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- Award ID(s):
- 1809570
- PAR ID:
- 10314193
- Date Published:
- Journal Name:
- Applied physics letters
- Volume:
- 117
- ISSN:
- 1520-8842
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0014495
