We demonstrate efficient coupling to the optical whispering gallery modes (WGMs) of nematic liquid crystal (NLC) microdroplets immersed in an immiscible aqueous environment. An individual NLC microdroplet, confined at the tip of a microcapillary, was coupled via a tapered optical fiber waveguide positioned correctly within its vicinity. Critical coupling of the taper-microdroplet system was facilitated by adjusting the gap between the taper and the microdroplet to change the overlap of the evanescent electromagnetic fields; efficient and controlled power transfer from the taper waveguide to the NLC microdroplet is indeed possible via the proposed technique. We also found that NLC microdroplets can function as highly sensitive thermal sensors: A maximum temperature sensitivity of 267.6 pm/°C and resolution of 7.5 × 10⁻² °C were achieved in a 78-μm-diameter NLC microdroplet.

We demonstrate a detection method for heavy metal (HM) ions based on whispering gallery mode (WGM) lasing in a liquid crystal (LC) microdroplet biosensor. By doping with stearic acid, nematic LC 4-cyano-4’-pentylbiphenyl (5CB) microdroplets are biochemically functionalized and used as both optical microresonators and sensing elements. Typical WGM lasing emission is observed in stearic acid-doped 5CB microdroplets under a pulse laser pump. Our results show that quantitative spectral shift of WGMs can serve as a real-time indicator of the adsorption of HM ions on the microdroplet surface. The detection limit of our sensor is as low as 40 pM for Cu(II) ions, six orders of magnitude better than the exposure threshold defined by the World Health Organization. Furthermore, this sensing system has an ability to discriminate between heavy and light metal ions. We believe that this novel biosensor has great application potential for environmental monitoring and drinking water quality testing.

Spores of Aspergillus niger (denoted as A. niger) were used as a novel biosorbent to remove hexavalent chromium from aqueous solution. The effects of biosorbent dosage, pH, contact time, temperature and initial concentration of Cr(VI) on its adsorption removal were examined in batch mode. The Cr(VI) uptake capacity increased with an increase in Cr(VI) concentration until saturation, which was found to be about 97.1 mg g⁻¹ at pH 2.0, temperature of 40 °C, adsorbent dose of 2.0 g L⁻¹ and initial concentration of 300 mg L⁻¹. Scanning electron microscopy, energy dispersive X-ray spectroscopy, field-emission transmission electron microscopy (FETEM), XPS and Fourier-transform infrared spectroscopy were applied to study the microstructure, composition and chemical bonding states of the biomass adsorbent before and after spore adsorption. The mechanisms of chromate anion removal from aqueous solution by the spores of A. niger were proposed, which included adsorption of Cr(VI) onto the spores followed by its reduction to Cr(III). The reduced Cr(III) was rebound to the biomass mainly through complexation mechanisms, redox reaction and electrostatic attraction. The removal of Cr(VI) by spores of A. niger followed pseudo-second-order adsorption kinetics. Monolayer adsorption of Cr(VI) was revealed by the better fitting of the Langmuir model isotherm rather than multilayer adsorption for the Freundlich model. The results indicated that A. niger spores can be used as a highly efficient biosorbent to remove Cr(VI) from contaminated water.

Real-time monitoring of the internal temperature of a battery with an optical fiber sensor based on β-NaYF₄:Er³⁺/Yb³⁺@NaYF₄ nanoparticles.

A compact, humidity-insensitive, fiber optic temperature sensor based on a quartz capillary, antiresonant reflecting optical waveguide was proposed and experimentally demonstrated. The transmission spectral responses of the proposed sensor were experimentally investigated regarding temperature variation and environmental humidity. Resonant dips exhibited temperature sensitivity as large as 201 pm/°C from −30 to 45 °C in a humid environment. By coating a sufficiently thick gold film onto the sensor surface, the humidity cross-sensitivity issue was effectively resolved. This proposed sensor was anticipated to find potential applications in humid environments, and moreover, immunity to humidity-sensitivity ensures its applicability in marine environments.