(a) Schematic of nanoneedle biosensor three-dimensional and side view of horizontal nanoneedles (Not to Scale). (b) Optical micrograph of bird's eye view of aluminum-polysiliconhybridnanoneedle biosensor. (c) SEM image of the tip of a nanoneedles biosensor; 1 & 3 are the electrodes; 2 is the oxide in between the electrodes
Presence of single stranded DNA modulates the measured impedance. (a) Various concentrations of DNA were injected onto our sensor surface sequentially. Between every step that DNA was added we dried out the measurement well. As the concentration of DNA in the solution decreases, the measured impedance increases getting closer and closer to the baseline value. (b) Impedance change plotted with error bars over three measurements per point.
Presence of non-adsorbed streptavidin protein modulates the measured impedance. (a) Various concentrations of streptavidin were injected onto our sensor surface sequentially: (1) water, (2) 250 ng/ml streptavidin, (3) 25 ug/ml streptavidin, (4) 25 mg/ml streptavidin. Between every step that protein was added we dried out the measurement well. As the concentration of protein in the solution decreases, the measured impedance increases getting closer and closer to the baseline value. (b) Impedance response of polystyrene beads injected onto sensor resulting in increase in impedance. (c) Impedance change plotted with error bars over three measurements per point.
Circuit model of the nanoneedle sensor-electrolyte interface. (a) Full model where CW represents the fringing capacitance at the sensor interface. Rw represents the resistance across the double layer (on top of the insulator) between the electrodes. Cdl represents the double layer capacitance on each electrode surface. Rf represents the tunneling resistance or the electron transfer resistance from the electrode into the bulk solution. Rb represents the bulk resistance of the electrolyte. Rtr represents the trace resistance of the electrode leading up to the bonding pads. Cb represents the body capacitance between the electrodes along the body of the sensor. (b) Simplified model which is valid at f = 15 kHz.
Plot of the electrical impedance spectrum across the device from f = 1 Hz all the way to f = 1 MHz. The magnitude of the impedance is shown for both water and streptavidin solution.
(a) Optical image of streptavidin beads bound to nanoneedle sensor coated with biotinilated BSA. (b) Representative results of real-time measurement of impedance as (1) sensor is covered with water (2) biotinilated BSA (250 mg/ml) is physically adsorbed on the surface of the nanoneedle sensor, which results in a drop in impedance. (3) The sensor surface is washed with water resulting in increase in impedance. (4) Streptavidin (5 mg/ml) was injected in a decrease in impedance, (5) followed by a wash step afterwards, resulting in an increase in impedance. The final impedance levels after both wash steps (after biotinilated BSA adsorption and after streptavidin binding) are compared with each other to quantify the amount of protein bound to the sensor surface. (c) Impedance change plotted with error bars over three measurements per point. ΔZ1 represents the difference in impedance of steps (5) and (3) for the experiments where biotinalated BSA was immobilized on surface. ΔZ2 represents the difference in impedance of steps (5) and (3) for the experiments where unconjugated BSA (control experiment) was immobilized on surface. (d). Control experiments where representative results of real-time measurement of impedance where (1) sensor is covered with water then (2) unconjugated BSA (250 mg/ml) is adsorbed to the surface resulting in a partial drop in impedance. (3) The sensor surface was washed with water to wash out the unbound BSA molecules resulting in an increase in impedance. (4) Streptavidin (5 mg/ml) was injected onto the sensor surface resulting in a decrease in impedance similar to the positive experiment. (5) The sensor surface is washed. The difference in electrical response of the control experiment and positive experiment lies in the final wash step which results in increasing the impedance level all the way back up to the original baseline level where only BSA was immobilized.
Average real and imaginary impedance values for experiments in Figure 2 for steps with DI water and 1 μM Oligo.
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