Coronavirus disease 2019 (COVID-19) is a newly emerging human infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, previously called 2019-nCoV). Based on the rapid increase in the rate of human infection, the World Health Organization (WHO) has classified the COVID-19 outbreak as a pandemic. Because no specific drugs or vaccines for COVID-19 are yet available, early diagnosis and management are crucial for containing the outbreak. Here, we report a field-effect transistor (FET)-based biosensing device for detecting SARS-CoV-2 in clinical samples. The sensor was produced by coating graphene sheets of the FET with a specific antibody against SARS-CoV-2 spike protein.The performance of the sensor was determined using antigen protein, cultured virus, and nasopharyngeal swab specimens from COVID-19 patients. Our FET device could detect the SARS-CoV-2 spike protein at concentrations of 1 fg/mL in phosphate-buffered saline and 100 fg/mL clinical transport medium. In addition, the FET sensor successfully detected SARS-CoV-2 in culture medium (limit of detection [LOD]: 1.6 × 10 1 pfu/mL) and clinical samples (LOD: 2.42 × 10 2 copies/mL). Thus, we have successfully fabricated a promising FET biosensor for SARS-CoV-2; our device is a highly sensitive immunological diagnostic method for COVID-19 that requires no sample pretreatment or labeling.

In this work, we experimentally and theoretically explore voltage controlled oscillations occurring in microbeams of vanadium dioxide. These oscillations are a result of the reversible insulator to metal phase transition in vanadium dioxide. Examining the structure of the observed oscillations in detail, we propose a modified percolative-avalanche model which allows for voltage-triggering. This model captures the periodicity and waveshape of the oscillations as well as several other key features. Importantly, our modeling shows that while temperature plays a critical role in the vanadium dioxide phase transition, electrically induced heating cannot act as the primary instigator of the oscillations in this configuration. This realization leads us to identify electric field as the most likely candidate for driving the phase transition.

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We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial scales on the order of one wavelength at 1 THz. Thus we show the viability of elements for use in future devices with potential applications in beamforming and communications.Metamaterials have progressed from academic curiosities 1,2 to candidates for real-world applications. 3,4 Emerging metamaterial applications range from radio frequency (RF) 5 communications to millimeter radar. 6 One key technique which promises to further the applicability of metamaterials is tunability. Operational frequency tuning has been presented as one solution to the narrow bandwidth often present in metamaterial devices. 7,8 Frequency-agile metamaterial designs have been demonstrated across a wide spectrum from microwave 9 to near-visible frequencies. 10 To date, tuning has generally been homogeneously implemented across the entire device as a whole. Developing techniques for controllable spatially variable tuning will present the possibility of devices with a reconfigurable Gradient Index of Refraction (GRIN). GRIN devices have already proven an attractive area for metamaterials, 11 as the metamaterial design process naturally allows for the control needed to fabricate GRIN structures. Additionally, use of spatially nonuniform tuning can leverage the narrow bandwidth of metamaterials in a unique way. For narrow-band operation, minor adjustments in the resonance frequency of a metamaterial can translate to large changes in the index of refraction at that frequency. Overall, spatial control of resonance tuning allows for post-fabrication modification of the index of refraction and therefore the creation of a reconfigurable gradient.In this work, we demonstrate a spatially reconfigurable THz hybrid metamaterial with vanadium dioxide (VO 2 ) and split ring resonators (SRRs) as constituent elements. The SRR has been the "fruit fly" of metamaterials research, allowing for convenient implementation of optical characteristics which are unattainable without a) mgoldfla@physics.ucsd.edu metamaterials. 1,12,13 Our device is composed of an array of 100 nm thick gold SRRs (dimensions shown in figure 2) lithographically fabricated on 90 nm thick VO 2 grown on a sapphire substrate. VO 2 undergoes an insulator to metal transition 14 which can be triggered thermally electrically 15 or optically. 16 The phase transition is hysteretic, and therefore, changes in the conductivity of VO 2 generally persist, provided the device temperature is maintained. Hybrid metamaterial-VO 2 devices benefit from this memory, 17 and from the large tuning dynamic range achievable with VO 2 . Persistent tuning uses this memory to eliminate the need for continu...

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In order to investigate bistable switching characteristics of planar junction devices based on vanadium dioxide (VO2) thin films, we have measured the optical power dependence of the threshold voltage of the device, at which a current jump, regarded as the Mott metal-insulator transition (MIT), happened, by using an infrared laser with a wavelength of ∼1.55 μm, illuminated onto the VO2 film. In a test closed loop circuit connecting a DC voltage source, a standard resistor, and a VO2 thin film device in series, the bistability of the voltage across the device (VD) was examined with respect to a variety of illumination powers (PLs). By triggering the forward or reverse phase transition (Mott MIT) of the VO2 film with “SET” or “RESET” optical pulse, respectively, the photo-assisted bistable switching of VD in the test circuit properly DC biased could be realized at an intermediate PL chosen between optical powers of “SET” and “RESET” pulses. In particular, the transient response of VD showed not only bistable states of VD but also their switching speed.

With COVID-19 widespread worldwide, people are still struggling to develop faster and more accurate diagnostic methods. Here we demonstrated the label-free detection of SARS-CoV-2 spike protein by employing a SARS-CoV-2 spike antibody-conjugated phase-shifted long-period fiber grating (PS-LPFG) inscribed with a CO 2 laser. At a specific cladding mode, the wavelength separation ( λ D ) between the two split dips of a PS-LPFG varies with the external refractive index, although it is virtually insensitive to ambient temperature variations. To detect SARS-CoV-2 spike protein, SARS-CoV-2 spike antibodies were immobilized on the fiber surface of the fabricated PS-LPFG functionalized through chemical modification. When exposed to SARS-CoV-2 spike protein with different concentrations, the antibody-immobilized PS-LPFG exhibited the variation of λ D according to the protein concentration, which was caused by bioaffinity binding-induced local changes in the refractive index at its surface. In particular, we also confirmed the potential of our sensor for clinical application by detecting SARS-CoV-2 spike protein in virus transport medium. Moreover, our sensor could distinguish SARS-CoV-2 spike protein from those of MERS-CoV and offer efficient properties such as reusability and storage stability. Hence, we have successfully fabricated a promising optical transducer for the detection of SARS-CoV-2 spike protein, which can be unperturbed by external temperature disturbances.