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Highly infectious viral diseases are a serious threat to mankind as they can spread rapidly among the community, possibly even leading to the loss of many lives. Early diagnosis of a viral disease not only increases the chance of quick recovery, but also helps prevent the spread of infections. There is thus an urgent need for accurate, ultrasensitive, rapid, and affordable diagnostic techniques to test large volumes of the population to track and thereby control the spread of viral diseases, as evidenced during the COVID-19 and other viral pandemics. This review paper critically and comprehensively reviews various emerging nanophotonic biosensor mechanisms and biosensor technologies for virus detection, with a particular focus on detection of the SARS-CoV-2 (COVID-19) virus. The photonic biosensing mechanisms and technologies that we have focused on include: (a) plasmonic field enhancement via localized surface plasmon resonances, (b) surface enhanced Raman scattering, (c) nano-Fourier transform infrared (nano-FTIR) near-field spectroscopy, (d) fiber Bragg gratings, and (e) microresonators (whispering gallery modes), with a particular emphasis on the emerging impact of nanomaterials and two-dimensional materials in these photonic sensing technologies. This review also discusses several quantitative issues related to optical sensing with these biosensing and transduction techniques, notably quantitative factors that affect the limit of detection (LoD), sensitivity, specificity, and response times of the above optical biosensing diagnostic technologies for virus detection. We also review and analyze future prospects of cost-effective, lab-on-a-chip virus sensing solutions that promise ultrahigh sensitivities, rapid detection speeds, and mass manufacturability.

MicroRNA exhibits differential expression levels in cancer and can affect cellular transformation, carcinogenesis and metastasis. Although fluorescence techniques using dye molecule labels have been studied, label-free molecular-level quantification of miRNA is extremely challenging. We developed a surface plasmon resonance sensor based on two-dimensional nanomaterial of antimonene for the specific label-free detection of clinically relevant biomarkers such as miRNA-21 and miRNA-155. First-principles energetic calculations reveal that antimonene has substantially stronger interaction with ssDNA than the graphene that has been previously used in DNA molecule sensing, due to thanking for more delocalized 5 s /5 p orbitals in antimonene. The detection limit can reach 10 aM, which is 2.3–10,000 times higher than those of existing miRNA sensors. The combination of not-attempted-before exotic sensing material and SPR architecture represents an approach to unlocking the ultrasensitive detection of miRNA and DNA and provides a promising avenue for the early diagnosis, staging, and monitoring of cancer. Label-free molecular-level quantification of MicroRNA (miRNA) remains challenging. Here, the authors develop a new surface plasmon resonance sensor based on two-dimensional nanomaterial of antimonene for the specific label-free detection of clinically relevant biomarkers such as miRNA-21 and miRNA-155.

Polyelectrolyte multilayer (PEM) thin films, fabricated at nano-scale by Layer-by-Layer (LbL) self-assembly techniques, have diverse nanotechnology applications. Precise thickness measurement during layer buildup is crucial in controlling the thickness. In this work, we present a novel optical measurement technique for in-situ analysis of the PEM film build-up by utilizing Etched Fiber Bragg Gratings (EFBG)-based sensors to quantify the deposited thickness. PEM films were deposited over EFBG by alternative deposition of weak polyelectrolytes Poly(allylamine hydrochloride)(PAH) and Poly(acrylic acid)(PAA) with quantitative analysis of adsorption and desorption steps at varying pH conditions. Further, the desorption process was observed in detail at the sub-nanometer scale, with PAA exhibiting a linear desorption while PAH follows an exponential desorption. This validates the inter-diffusive behavior of the low molecular weight polyelectrolytes during the PEM buildup, not only during the adsorption process but also during the desorption process. Thus, EFBG could be utilized as a precision tool to extract fundamental information during individual PEM layer build-up, thereby fine-tuning the nano-scale architecture of multilayers. Polyelectrolyte multilayer thin films have diverse applications in nanotechnology but controlling the thickness of each layer during their layer-by-layer assembly remains challenging. Here, an etched fiber Bragg grating technique is introduced to quantify the thickness of the deposited layers during film build-up, which enables real-time monitoring of the adsorption and desorption processes at sub-nanometer scale.

While optical fiber Bragg gratings (FBGs) have been exploited in the field of sensing, their potential for investigating quantum processes of photon-molecule interactions remains unexplored. Here, we experimentally demonstrated a method of probing photon-induced non-radiative thermal relaxation in fluorophores using the FBG technique. In response to various excitation wavelengths of photons, the FBG with fluorescent dye, Rhodamine B, present on its cladding, exhibits distinct Bragg wavelength shifts, reflecting the level of vibronic transitions and the absorption characteristics of the fluorophore based on non-radiative thermal release. The photoexcitation intensity-dependent response demonstrates that the FBG technique can probe localized photothermal relaxation at the micron-scale with LED intensity below 5 mW cm − 2 . Moreover, the modulation of the observed split in the Bragg wavelength spectrum provides further insights into photothermal localization in addition to yielding photothermal information. This approach of realizing photon-molecule interaction makes fiber Bragg grating-based quantum phenomena sensing accessible and can be extended for spectroscopy, biosensing, and quantum applications. Optical fiber Bragg gratings (FBGs) are widely applied in sensing, but their potential for investigating quantum processes of photon–molecule interactions remains unexplored. Here, the authors report a method for probing photon-induced non-radiative thermal relaxation in fluorophores with FBGs, using Rhodamine B as an example and observing distinct Bragg wavelength shifts based on its vibronic transitions and absorption properties.

In recent years, polaritons in two-dimensional (2D) materials have gained intensive research interests and significant progress due to their extraordinary properties of light-confinement, tunable carrier concentrations by gating and low loss absorption that leads to long polariton lifetimes. With additional advantages of biocompatibility, label-free, chemical identification of biomolecules through their vibrational fingerprints, graphene and related 2D materials can be adapted as excellent platforms for future polaritonic biosensor applications. Extreme spatial light confinement in 2D materials based polaritons supports atto-molar concentration or single molecule detection. In this article, we will review the state-of-the-art infrared polaritonic-based biosensors. We first discuss the concept of polaritons, then the biosensing properties of polaritons on various 2D materials, then lastly the impending applications and future opportunities of infrared polaritonic biosensors for medical and healthcare applications.

The reversible photostriction (photomechanical strain) in Ge 35 S 65 chalcogenide thin film deposited by a solvent casting method has been monitored using a fiber Bragg grating (FBG) sensor. The shift in Bragg wavelength is used as a probing parameter to quantitatively measure the photoinduced strain arising because of structural modifications in these films under illumination. Exposure to band gap light (405 nm) and above band gap light (302 and 254 nm) leads to a reversible photostriction effect of the order of 100 µε. The present study shows that FBG sensors can be used to effectively measure the optomechanical actuation in chalcogenide films caused by the reversible photostriction effect in the visible and ultraviolet wavelength region.

We used scattering-type scanning near-field optical microscopy (s-SNOM) to investigate the plasmonic properties of edges in well-defined graphene nanostructures, including sharp tapers, nanoribbons and nanogaps, which were all fabricated via the growth-etching chemical vapor deposition (GECVD) method. The obtained near-field images revealed the localized plasmon modes along the graphene nanoribbon; these modes strongly depended on the size of the graphene pattern, the angle of the tapered graphene and the infrared excitation wavelength. These interesting plasmon modes were verified by numerical simulations and explained by the reflection, and interference of electromagnetic waves at the graphene–SiO 2 edge. The constructive interference at the graphene nanogap caused by charge accumulation was demonstrated for the first time. Using the infrared nanoimaging technique, greater plasmon broadening was observed in the zigzag edge than in the armchair edge. Our study suggests that graphene edges should be separated by an effective working distance to avoid the overlapping of localized plasmon modes, which is very important for the design of graphene-based plasmonic circuits and devices. Graphene plasmonics: combining size and speed Scanning near-field optical microscopy has been used to explore plasmon modes on nanoribbons with well-defined edges. Since plasmons involve both electrons and light, plasmonics can combine the speed of optics with the tiny size of electronics. Qiaoliang Bao of Soochow University, China, and co-workers investigated the interaction of infrared light with various graphene nanostructures and found that the resulting plasmons strongly depend on the dimensions of the pattern and the wavelength of the infrared light. Based on their results, the researchers recommend separating graphene edges in real devices to prevent plasmons from overlapping. They anticipate that these findings may pave the way for the development of active sub-wavelength optics and a wide range of nano-optoelectronic devices, including graphene-based waveguides, quantum information devices and ultra-sensitive sensors.