Electron tunneling through thin ( _~ 1-50 nm), insulating materials can provide information regarding their band structure, density of states, surface and bulk defect densities and their energy levels and distributions. While electron tunneling has been used in thin inorganic insulating materials for nearly 7 decades, its applications in organic and biological materials, with exceptions of deoxyribonucleic acid and a few other materials, are not extensively explored. Here we report room temperature electron tunneling studies to differentiate between aptamers designed to attach to the capsid proteins on Zika viruses and the aptamer-Zika complexes. We show that the electron tunneling induced by the current versus voltage (I-V) measurements through these materials contain information that enable identification of the Zika virus without any need for DNA sequencing or additional molecular labels. The electron tunneling measurements were carried out using a conducting atomic force microscope, a home-made vertical tunneling junction and an electromigration-induced nanogap in a microfabricated 1 μm -wide gold line on silicon nitride bridges. The current through aptamers and aptamer-Zika complexes all have a threshold voltage just before the exponential increase in the junction current. Below this voltage, the leakage currents are different for the aptamer and aptamer-Zika samples. The tunnel nanogap device is suitable for detecting viruses that are nanometer-scale biomaterials.

Three different microbalances were coated with aptamers with thiol termination designed to attach to the capsid proteins of the whole Zika virus. The first microbalance was a 5 MHz commercial AT-cut (QSense) quartz microbalance that showed 1 Hz/ng sensitivity with minimum detectable signal (MDS) of 1 ng. The second sensor was a 10 MHz AT-cut quartz sensor that was extracted from a communication filter. It had 10 Hz/ng sensitivity and MDS of 0.1 ng. The third sensor was a 433 MHz Lithium Niobate (LiNbO ₃ ) sensor that was also extracted from a communication filter. It had 370 Hz/ng sensitivity and 2.7 pg MDS. These studies verify binding of the aptamers to the sensor surface and the Zika virus. They also clearly show the advantage of using the LiNbO ₃ high frequency microbalance.

Design and operation of a metal-oxide hydrogel field-effect transistor (MOHFET) with channel composed of a hydrogel embedded with gold nanoparticles (AuNP) is discussed. Hydrogels have low ionic and electronic conductivities that change as a function of their ionic contents. AuNP incorporated in the hydrogel increases its conductivity by ${>}2{\times}$ and also introduces large conduction non-linearity (field focusing by AuNP and smaller soft breakdown field) as a function of channel electric fields. The hydrogel provides a convenient medium with distributed springs connecting AuNPs to the MOHFET drain and source. The measured electrical conductivity in our pure hydrogel was 611 S/m and it increased to 1540 S/m in AuNP hydrogel. The hydrogel Young’s modulus was 14 ± 2 kPa that increased to 21 ± 2 kPa with Au-NP incorporation. The ${I} _{ \mathrm{\scriptscriptstyle ON}}/{I} _{ \mathrm{\scriptscriptstyle OFF}}$ ratio of MOHFET was ~10 ⁴ and its subthreshold swing was 148.72 mV/dec at ${V} _{\text {ds}}= 2$ V. Hydrogels’ porous structure and very flexible electrical and mechanical characteristics make them excellent candidate as binding media to incorporate suitable nanoparticles in sensors and electronic devices.

A unique field-effect transistor with an active channel composed of aptamers designed to bind to the Zika virus was fabricated. To immobilize the aptamers in the device channel, we used gold nanoparticles (AuNPs), a 20-40 nm thin film of sputtered gold (Au film), and a molecular sieve material zeolite. The aptamers were functionalized with a thiol end group that enabled them to bind with gold and most other materials. In the case of aptamer/AuNP, when Zika was added to the channel it reduced the channel conductivity from 4.4x102 S/cm to 3.7 S/cm. The gate field effect clearly showed that the aptamer/AuNP channel is p-type with a relatively large transconductance of 300 μS (VDS =2V) compared to the Zika/aptamer/AuNP channel (also p-type) but with lower transconductance of 2 μS (VDS=2V). There was large amount of charge trapping (~1015 cm2) in Zika/aptamer/AuNP channel. In other cases of aptamer/Au film and zeolite channels, there were large changes in the channel conductance when Zika was added. Aptamer/Au and aptamer/zeolite channels were both n-type and their conductivities increased when Zika was added. Combination of AuNP and zeolite biosensing FETs can be used to design complimentary bio-FET sensors. We show that the gate voltage in FET biosensors can be used to increase their sensitivity (24 times in Zika/aptamer/AuNP), selectivity and dynamic range (by 10x or more).

This paper presents two novel techniques for monitoring the response of smart hydrogels composed of synthetic organic materials that can be engineered to respond (swell or shrink, change conductivity and optical properties) to specific chemicals, biomolecules or external stimuli. The first technique uses microwaves both in contact and remote monitoring of the hydrogel as it responds to chemicals. This method is of great interest because it can be used to non-invasively monitor the response of subcutaneously implanted hydrogels to blood chemicals such as oxygen and glucose. The second technique uses a metal-oxide-hydrogel field-effect transistor (MOHFET) and its associated current-voltage characteristics to monitor the hydrogel’s response to different chemicals. MOHFET can be easily integrated with on-board telemetry electronics for applications in implantable biosensors or it can be used as a transistor in an oscillator circuit where the oscillation frequency of the circuit depends on the analyte concentration.