Fluorescence resonance energy transfer (FRET) between a quantum dot as donor and an organic fluorophore as acceptor has been widely used for detection of nucleic acids and proteins. In this paper, we developed a new method, characterized by 605-nm quantum dot (605QD) fluorescence intensity increase and corresponding Cy5 fluorescence intensity decrease, to detect adenosine triphosphate (ATP). The new method involved the use of three different oligonucleotides: 3'-biotin-modified DNA that binds to streptavidin-conjugated 605QD; 3'-Cy5-labelled DNA; and a capture DNA consisting of an ATP aptamer and a sequence which could hybridize with both 3'-biotin-modified DNA and 3'-Cy5-labelled DNA. In the absence of the target ATP, the capture DNA binds to 3'-biotin-modified DNA and 3'-Cy5-labelled DNA, bringing quantum dot and Cy5 into close proximity for greater FRET efficiency. When ATP is introduced, the release of the 3'-Cy5-labelled DNA from the hybridization complex took place, triggering 605QD fluorescence intensity increase and corresponding Cy5 fluorescence intensity decrease. Taken together, the virtue of FRET pair 605QD/Cy5 and the property of aptamer-specific conformation change caused by aptamer-ATP interaction, combined with the fluorescence intensity change of both 605QD and Cy5, provide prerequisites for simple and convenient ATP detection.

The development of simple and sensitive sensors with the capability of simultaneously detecting magnetic field intensity and its direction in three-dimensional (3D) space is a technical imperative for magnetic field detection. In this paper, we first demonstrated the mechanism of anisotropic manipulation of ferromagnetism nanoparticles (FMNPs) in a non-continuous magnetic fluid film with the aid of varying magnetic fields. Second, based on the anisotropic distribution of the FMNPs around an optical fiber, we fabricated a vector magnetometer with a direction error of ±1.9° and an intensity sensitivity of 222.0 pm/mT, respectively. The sensing mechanism relies on the magnetism-controllable effective refractive index modification of asymmetric cladding modes in an in-line fiber-optic Mach-Zehnder interferometer. Compared to the previously reported intensity-based magnetometers, the as-fabricated magnetometer also provides an avenue to monitor the vector direction of the magnetic field in 3D space.

We proposed and fabricated a tilted moiré FBG (TMFBG), whose grating section was made up of two consecutive scribed TFBGs. By adjusting the Bragg wavelengths and the tilt angles of the two TFBGs, the two cladding mode combs of the transmission spectrum are non-overlapped. When the TMFBG was used for refractive index detection, its resolution can reach 2 × 10 RIU, which is an order of magnitude higher than that of a single TFBG. And this result also has a good performance of temperature-insensitivity.

We proposed and demonstrated a flexible, endoscopic, and minimally invasive coherent anti-Raman Stokes scattering (CARS) measurement method for single-cell application, employing a tapered optical fiber probe. A few-mode fiber (FMF), whose generated four-wave mixing band is out of CARS signals, was selected to fabricate tapered optical fiber probes, deliver CARS excitation pulses, and collect CARS signals. The adiabatic tapered fiber probe with a diameter of 11.61 μm can focus CARS excitation lights without mismatch at the focal point. The measurements for proof-of-concept were made with methanol, ethanol, cyclohexane, and acetone injected into simulated cells. The experimental results show that the tapered optical fiber probe can detect carbon-hydrogen (C–H) bond-rich substances and their concentration. To our best knowledge, this optical fiber probe provides the minimum size among probes for detecting CARS signals. These results pave the way for minimally invasive live-cell detection in the future.

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