Detection of heavy metal ions using whispering gallery mode lasing in functionalized liquid crystal microdroplets

Abstract

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.

Applications of Microfluidics in Liquid Crystal-Based Biosensors

“…Compared with traditional biosensing platforms, which usually require complex instruments or fluorophores as reporters, the LC-based sensing platforms provide a simple, rapid, label-free, and cost-effective option for biosensing applications [4,9]. Various LC-based interfaces, such as LC-solid interface, LC-aqueous planar interface, and LC droplet-aqueous interface, have been developed for the detection of biological and chemical species, such as organophosphorus species [10][11][12], endotoxins [13][14][15], cells [16,17], metal ions [18][19][20], protein [21][22][23], nucleic acids [24][25][26], glucose [27][28][29], and bile acids [30][31][32][33]. The ordering of LCs is dictated by the modulation between the elastic energy stored in bulk LCs and the surface anchoring energy of LCs [6].…”

Label-Free Optical Resonator-Based Biosensors

“…WGM biosensors are highly sensitive to changes in the surface conditions and refractive index, and they have been developed for single-molecule detection [ 20 , 21 , 115 ], pico- to atto-molar scale detection [ 21 , 112 , 113 , 116 , 117 , 118 , 119 , 120 , 121 ], and monitoring biological processes [ 122 , 123 , 124 , 125 , 126 ] or enzymatic reactions [ 127 , 128 ]. In recent studies, researchers have also taken advantage of geometric modifications [ 129 , 130 ] and material doping [ 116 , 118 , 119 , 131 , 132 , 133 , 134 ] to improve the Q-factors and sensitivity of WGM-based resonators. Geometric differences in most resonators may depend on fabrication capabilities or desired optical qualities, such as mode confinement, increased binding surface area, simpler fabrication techniques, or material choice.…”

“…These are typically based on microbubbles, microspheres, or microdroplets. Some notable modifications were label-free surface functionalization [ 120 , 123 , 137 , 138 ], doping [ 118 , 119 ], and self-referencing [ 111 , 120 ].…”

“…A liquid crystal (LC) microdroplet WGM resonator was used to detect heavy metal (HM) ions in real-time [ 119 ]. The LC 5CB microdroplet solution was composed of 1% 4-cyano-4′-pentylbiphenyl (5CB) in n-heptane, with equal doping of 0.01% stearic acid and fluorescent dye (DCM).…”

Optically Transportable Optofluidic Microlasers with Liquid Crystal Cavities Tuned by the Electric Field

“…With a suitable choice of the constituent LC molecules, host liquid, and droplet-stabilizing surfactant, LC emulsions can be prepared in a number of phases ranging from nematic through smectic to cholesteric, with various boundary conditions imposed at the droplet surface. − Slight adjustments of these boundary conditions can then trigger a rearrangement of the director field within the droplet accompanied by corresponding changes of the refractive index. Due to this property, emulsion LC droplets have become attractive candidates for implementing ultra-sensitive environmental sensors , and optofluidic laser microcavities with largely tunable emission spectra. , …”