Microfluidic Integrated Organic Electrochemical Transistor with a Nanoporous Membrane for Amyloid-β Detection

Alzheimer’s disease (AD) is a neurodegenerative disorder associated with a severe loss in thinking, learning, and memory functions of the brain. To date, no specific treatment has been proven to cure AD, with the early diagnosis being vital for mitigating symptoms. A common pathological change found in AD-affected brains is the accumulation of a protein named amyloid-β (Aβ) into plaques. In this work, we developed a micron-scale organic electrochemical transistor (OECT) integrated with a microfluidic platform for the label-free detection of Aβ aggregates in human serum. The OECT channel–electrolyte interface was covered with a nanoporous membrane functionalized with Congo red (CR) molecules showing a strong affinity for Aβ aggregates. Each aggregate binding to the CR-membrane modulated the vertical ion flow toward the channel, changing the transistor characteristics. Thus, the device performance was not limited by the solution ionic strength nor did it rely on Faradaic reactions or conformational changes of bioreceptors. The high transconductance of the OECT, the precise porosity of the membrane, and the compactness endowed by the microfluidic enabled the Aβ aggregate detection over eight orders of magnitude wide concentration range (femtomolar–nanomolar) in 1 μL of human serum samples. We expanded the operation modes of our transistors using different channel materials and found that the accumulation-mode OECTs displayed the lowest power consumption and highest sensitivities. Ultimately, these robust, low-power, sensitive, and miniaturized microfluidic sensors helped to develop point-of-care tools for the early diagnosis of AD.

. FESEM image of Aβ aggregates captured on the surface of a CR-modified nanoporous membrane.            In c), a log scale was used to better visualize the response at the low concentration range.

Capacitance characteristics of µf-OECTs
To gain physical insight into the capacitance changes of the OECT interfaces, we analyzed the transfer characteristics of PEDOT:PSS based devices exposed to various Aβ concentrations using  Figure S17.

Numerical Simulations
The coupled Poisson-Nernst-Planck (PNP) equations incorporate ion transport under the electric field gradients for a dilute, completely dissociated ionic solution. The equations were solved using COMSOL Multiphysics 5.5. Since the microfluidic channel's width is much larger than its height, we considered ion accumulation at the OECT channel surface in a rectangular two-dimensional system. A finer mesh was distributed near the OECT channel surface to resolve ion concentration and capture the large electric field gradients. For all cases, the initial cation and anion concentrations were set to bulk concentration, and initial potential was set to zero such that the system was at rest at t=0. From t=0 onward, a time-stepping routine combined with the directsolving method (MUMPS) was used to compute the time-dependent ion accumulation behavior at an applied voltage for each channel material. Note that our numerical model does not consider ions penetrating the channel. We only simulated the fluidic part where the electric field and transport equations were solved to predict the system behavior. The governing equations for the electrode/electrolyte system are dictated by Gauss' law (S1a and S1c) and the charge conservation equation (S1b): where is the space charge density, is the permittivity of the fluid, is the electric field, and is the electrical conductivity of the medium. We neglected the contribution of the convective term and magnetic field effect by assuming a quasi-electrostatic field.
The simulation domain is depicted in Figure S20a. The concentration of ions inside the medium (Na + and Cl -) are described by the Nernst-Planck equation: where is the diffusion coefficient of species and is the electric potential. The electric field in the media is obtained by solving Poisson equation: where, is the electric field ( ).
The simulation parameters and the boundary conditions are as follows. The operating temperature was 300 K and diffusion coefficients of Na + and Clions were fixed at 1.33 10 -9 m 2 /s and 2.30 10 -9 m 2 /s, respectively. The relative permittivity of medium was considered to be 78. All the boundaries were fixed as insulating boundary condition, except the gate and the channel which were excited at a constant voltage ( Figure S20a). Once a nanopore was clogged by the Aβ aggregate, an extra floating potential was introduced, which enhanced or hindered ion motion towards the OECT channel. The surface potential of the Aβ aggregate was measured as 15 mV.
We thus added an external constant potential layer right above the channel.

Isoporous membrane fabrication
The membranes were prepared from a 18% of PS-b-P4VP block copolymer solution in a ternary solvent mixture with 24% dimethylformamide, 42% 1,4 dioxane and 16% acetone (all in weight percent). We stirred this solution at room temperature for 24h and then cast it on a glass plate using a doctor blade with a gap of 200 µm. Following 10 s solvent partial evaporation, we immersed the cast solution layer in a water precipitation bath. Subsequently to the solvent-water exchange, we stored the membrane for 24 in deionized water to remove any solvent traces. We obtained an asymmetric membrane with approximatively 40 nm regular pores on the top layer formed by block-copolymer self-assembly in mixed solvents and phase separation in water. The larger pores in the membrane bulk are formed by spinodal decomposition promoted by the solvent-non solvent exchange after immersion in the water bath. Figure S23. Schematic representation the of PS-b-P4VP membrane fabrication.

Organic electrochemical transistor (OECT) fabrication
The OECT fabrication starts with sputtering to deposit chromium (10 nm)/gold (100 nm) on glass wafers. We patterned the metal via a lift-off process using a bilayer resist structure (S1813 photoresist, Microchemicals GmbH; LOR 5B MicroChem Corp. Westborough, MA). We performed lift-off in appropriate solvents followed by the encapsulation of the wafer in Parylene C via vaporization of the dimer (PDS 2010 Labcoater 2, Specialty Coating Systems, Indianapolis, IN). We spin-coated an anti-adhesion layer on the encapsulated wafer for the deposition of a sacrificial Parylene C layer, which allows for the patterning of the polymer in the channel. The metal contacts and channels were exposed using reactive ion etching with O2 (Plasma lab 100 -ICP 380, Oxford Instruments). The polymers were spin-coated on the substrates and parylene-C was peeled off to complete the device.