Freestanding Laser-Induced Graphene Ultrasensitive Resonative Viral Sensors.

Early and reliable detection of an infectious viral disease is critical to accurately monitor outbreaks and to provide individuals and health care professionals the opportunity to treat patients at the early stages of a disease. The accuracy of such information is essential to define appropriate actions to protect the population and to reduce the likelihood of a possible pandemic. Here, we show the fabrication of freestanding laser-induced graphene (FLIG) flakes that are highly sensitive sensors for high-fidelity viral detection. As a case study, we show the detection of SARS-CoV-2 spike proteins. FLIG flakes are nonembedded porous graphene foams ca. 30 µm thick that are generated using laser irradiation of polyimide and can be fabricated in seconds at a low cost. Larger pieces of FLIG were cut forming a cantilever, used as suspended resonators, and characterized for their electromechanics behavior. Thermomechanical analysis showed FLIG stiffness comparable to other porous materials such as boron nitride foam, and electrostatic excitation showed amplification of the vibrations at frequencies in the range of several kilo-hertz. We developed a protocol for aqueous biological sensing by characterizing the wetting dynamic response of the sensor in buffer solution and in water, and devices functionalized with COVID-19 antibodies specifically detected SARS-CoV-2 spike protein binding, while not detecting other viruses such as MS2. The FLIG sensors showed a clear mass-dependent frequency response shift of ∼1 Hz/pg, and low nanomolar concentrations could be detected. Ultimately, the sensors demonstrated an outstanding limit of detection of 2.63 pg, which is equivalent to as few as ∼5000 SARS-CoV-2 viruses. Thus, the FLIG platform technology can be utilized to develop portable and highly accurate sensors, including biological applications where the fast and reliable protein or infectious particle detection is critical.

Self-Sensing WS2 Nanotube Torsional Resonators.

We demonstrate self-sensing tungsten disulfide nanotube (WS2 NT) torsional resonators. These resonators exhibit all-electrical self-sensing operation with electrostatic excitation and piezoresistive motion detection. We show that the torsional motion of the WS2 NT resonators results in a change of the nanotube electrical resistance, with the most significant change around their mechanical resonance, where the amplitude of torsional vibrations is maximal. Atomic force microscopy analysis revealed the torsional and bending stiffness of the WS2 NTs, which we used for modeling the behavior of the WS2 NT devices. In addition, the solution of the electrostatic boundary value problem shows how the spatial potential and electrostatic field lines around the device impact its capacitance. The results uncover the coupling between the electrical and mechanical behaviors of WS2 and emphasize their potential to operate as key components in functional devices, such as nanosensors and radio frequency devices.

Height and morphology dependent heat dissipation of vertically aligned carbon nanotubes.

The continuous miniaturization of electronic devices substantially increases their power density, and consequently, requires effective cooling of these components. Vertically aligned carbon nanotubes (VA-CNTs) constitute one of the most promising materials for use as a high-end heat dissipation element due to their high thermal conductivity and large surface area. However, the lack of a clear understanding of the heat transfer mechanisms of VA-CNTs has so far impeded their large-scale use as cooling elements. Our infrared micro-thermography analysis revealed that the heat dissipation of VA-CNTs is determined mainly by their height, such that the heat dissipation behavior of tall samples was dominated by convection from the carbon nanotube (CNT) sidewalls. The mechanism of heat transfer in short VA-CNTs, in contrast, was determined by their morphology. Short VA-CNTs with highly organized CNT formations or with low thermal conductance exhibited convective heat dissipation similar to that of tall VA-CNTs, while other short VA-CNTs exhibited heat transfer dominated by conduction along the CNTs. This study provides important guidelines regarding the parameters that can be changed to optimize the performances of VA-CNTs in thermal applications. These applications include cooling elements in electronic devices, where convection is required, or thermal interface materials, where conduction is required.