Biosensors are essential tools which have been traditionally used to monitor environmental pollution and detect the presence of toxic elements and biohazardous bacteria or virus in organic matter and biomolecules for clinical diagnostics. In the last couple of decades, the scientific community has witnessed their widespread application in the fields of military, health care, industrial process control, environmental monitoring, food-quality control, and microbiology. Biosensor technology has greatly evolved from in vitro studies based on the biosensing ability of organic beings to the highly sophisticated world of nanofabrication-enabled miniaturized biosensors. The incorporation of nanotechnology in the vast field of biosensing has led to the development of novel sensors and sensing mechanisms, as well as an increase in the sensitivity and performance of the existing biosensors. Additionally, the nanoscale dimension further assists the development of sensors for rapid and simple detection in vivo as well as the ability to probe single biomolecules and obtain critical information for their detection and analysis. However, the major drawbacks of this include, but are not limited to, potential toxicities associated with the unavoidable release of nanoparticles into the environment, miniaturization-induced unreliability, lack of automation, and difficulty of integrating the nanostructured-based biosensors, as well as unreliable transduction signals from these devices. Although the field of biosensors is vast, we intend to explore various nanotechnology-enabled biosensors as part of this review article and provide a brief description of their fundamental working principles and potential applications. The article aims to provide the reader a holistic overview of different nanostructures which have been used for biosensing purposes along with some specific applications in the field of cancer detection and the Internet of things (IoT), as well as a brief overview of machine-learning-based biosensing.

We present the design, fabrication, and response of a polymer-based Laterally Amplified Chemo-Mechanical (LACM) humidity sensor based on mechanical leveraging and parametric amplification. The device consists of a sense cantilever asymmetrically patterned with a polymer and flanked by two stationary electrodes on the sides. When exposed to a humidity change, the polymer swells after absorbing the analyte and causes the central cantilever to bend laterally towards one side, causing a change in the measured capacitance. The device features an intrinsic gain due to parametric amplification resulting in an enhanced signal-to-noise ratio (SNR). Eleven-fold magnification in sensor response was observed via voltage biasing of the side electrodes without the use of conventional electronic amplifiers. The sensor showed a repeatable and recoverable capacitance change of 11% when exposed to a change in relative humidity from 25–85%. The dynamic characterization of the device also revealed a response time of ~1 s and demonstrated a competitive response with respect to a commercially available reference chip.

This paper details the design, fabrication, and characterization of highly uniform batch-fabricated sidewall etched vertical nanogap tunneling junctions for bio-sensing applications. The device consists of two vertically stacked gold electrodes separated by a partially etched sacrificial spacer layer of sputtered α-Si and Atomic Layer Deposited (ALD) SiO₂. A ~10 nm wide air-gap is formed along the sidewall by a controlled dry etch of the spacer. The thickness of the spacer layer can be tuned by adjusting the number of ALD cycles. The rigorous statistical characterization of the ultra-thin spacer films has also been performed. We fabricated nanogap electrodes under two design layouts with different overlap areas and spacer gaps, from ~4.0 nm to ~9.0 nm. Optical measurements reported an average non-uniformity of 0.46 nm (~8%) and 0.56 nm (~30%) in SiO₂ and α-Si film thickness respectively. Direct tunneling and Fowler–Nordheim tunneling measurements were done and the barrier potential of the spacer stack was determined to be ~3.5 eV. I–V measurements showed a maximum resistance of 46 × 10³ GΩ and the average dielectric breakdown field of the spacer stack was experimentally determined to be ~11 MV/cm.

We present the design, fabrication and response of a humidity sensor based on electrical tunneling through temperature-stabilized nanometer gaps. The sensor consists of two stacked metal electrodes separated by ~2.5 nm of vertical air gap. Upper and lower electrodes rest on separate 1.5 μm thick polyimide patches with nearly identical thermal expansion but different gas absorption characteristics. When exposed to a humidity change, the patch under the bottom electrode swells but the patch under the top electrode does not, as it is covered with a water-vapor diffusion barrier ~8 nm of Al₂O₃. The air gap thus decreases leading to increase in the tunneling current across the junction. The gap however is independent of temperature fluctuations as both patches expand or contract by near equal amounts. Humidity sensor action demonstrates an unassisted reversible resistance reduction R_max/R_min ~10⁵ when the device is exposed to 20–90 RH% at a standby DC power consumption of ~0.4 pW. The observed resistance change when subject to a temperature sweep of 25–60 ° C @24% RH was ~0.0025% of the full device output range.

Objective: Presbyopia, an age-related ocular disorder, is characterized by the loss in the accommodative abilities of the human eye. Conventional methods of correcting presbyopia divide the field of view, thereby resulting in significant vision impairment. We demonstrate the design, assembly and evaluation of autofocusing eyeglasses for restoration of accommodation without dividing the field of view. Methods: The adaptive optics eyeglasses comprise of two variable-focus liquid lenses, a time-of-flight range sensor and low-power, dual microprocessor control electronics, housed within an ergonomic frame. Subject-specific accommodation deficiency models were utilized to demonstrate high-fidelity accommodative correction. The abilities of this system to reduce accommodation deficiency, its power consumption, response time, optical performance and MTF were evaluated. Results: Average corrected accommodation deficiencies for 5 subjects ranged from -0.021 D to 0.016 D. Each accommodation correction calculation was performed in ∼67 ms which consumed 4.86 mJ of energy. The optical resolution of the system was 10.5 cycles/degree, and featured a restorative accommodative range of 4.3 D. This system was capable of running for up to 19 hours between charge cycles and weighed ∼132 g. Conclusion: The design, assembly and performance of an autofocusing eyeglasses system to restore accommodation in presbyopes has been demonstrated. Significance: The new autofocusing eyeglasses system presented in this article has the potential to restore pre-presbyopic levels of accommodation in subjects diagnosed with presbyopia.

We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 V_RMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.