Electrostatics plays a critical function in most biomolecules, therefore monitoring subtle biomolecular bindings and dynamics via the electrostatic changes of biomolecules at biointerfaces has been an attractive topic recently and has provided the basis in diagnosis and biomedical science. Here we present a bioelectrostatic responsive microlaser based on liquid crystal (LC) droplet and explored its application for ultrasensitive detection of negatively charged biomolecules. Whispering gallery mode (WGM) lasing from positively charged LC microdroplets was applied as the optical resonator, where the lasing wavelength shift was employed as a sensing parameter. With the dual impacts from whispering-gallery mode and liquid crystal, molecular binding signals will be amplified in such LC droplet sensors. It is found that molecular electrostatic changes at the biointerface of droplet triggered wavelength shift in lasing spectra. The total wavelength shift increased proportionally with the adhering target concentrations. Compared to a conventional polarized optical microscope, significant improvements in sensitivity and dynamic range by four orders of magnitude were achieved. Our work indicated that the surface-to-volume ratio plays a critical role in the detection sensitivity in WGM laser-based microsensors. Finally, bovine serum albumin and specific biosensing using streptavidin and biotin were exploited to demonstrate the potential applications of microlasers with a detection limit on the order of 1 pM. We anticipate this approach will open new possibilities for the ultrasensitive label-free detection of charged biomolecules and molecular interactions by providing a lower detection limit than conventional methods.
A novel magnetic field sensor by using magnetic fluid film in Sagnac loop is presented. The magnetic fluid introduces
controllable birefringence under external magnetic field inside the Sagnac loop to produce sinusoidal interference and
the corresponding transmission spectrum shifts with the change of external field strength. Sensitivity of 11.4 pm/Oe and
resolution of 0.88 Oe are achieved in the case of this proposed scheme.
A miniature modal interferometer based on a hollow-core photonic crystal fiber (HC-PCF) for refractive index measurement is demonstrated. The modal interferometer is fabricated by splicing the two ends of a 1.2-mm-long HC-PCF to a single-mode fiber (SMF). The air holes of the HC-PCF are fully collapsed by the discharge arc during the splicing procedure, and the length of each collapsed region is about 300 μm. The transmission spectra with different refractive indices outside the HC-PCF are measured. Measurement resolutions of an 8.1×10−4 refractive index unit (RIU) in the range of 1.35 to 1.39, and 4.3×10−4 RIU in the range of 1.39 to 1.43 are achieved, respectively. The temperature effect of the proposed refractometer is also analyzed.
The dispersion properties of LPCF were fully analyzed by using the beam propagation method (BPM).
It is shown that the zero dispersion wavelengh (ZDW) of LPCF can be altered to the designed value in
a certain range without the modification of the geometric structure. This property is useful for sensing
and nonlinear optics applications.
A new type fiber bending sensor based on a tilted fiber Bragg grating (TFBG) interacting with a multimode fiber (MMF)
is presented. The sensing head is formed by insertion of a small section of MMF between single-mode fiber (SMF) and
the TFBG. The reflection light from this sensor head includes two parts, i.e., the reflected Bragg mode and cladding
modes. The latter were first coupled from the core mode to counter-propagating cladding modes by the TFBG and then
coupled back into the core as a function of the MMF. The power of the cladding modes changes as the fiber is bent while
the Bragg one keeps unchanged. The average reflective power in the cladding modes decreased with the increase of
curvature. The measurement range of the curvature from 0m-1 to 2.5m-1 with a measurement sensitivity of -802.4nW/m-1
A novel temperature sensor by using photonic bandgap (PBG) effect is demonstrated. The solid core photonic bandgap
fiber (PBGF) with infiltrated high refractive index solution is used for temperature measurement. Two high index
solutions were infiltrated with refractive index of 1.58 and 1.64. It shows that the rising photonic bandgap edges of
PBGF have a blue shifting. The 112.40nm shifting was observed with 1.58 refractive index oil infiltrated when the
ambient temperature was changing from 24°C to about 65°C. The better sensitivity of the temperature measurement
2.74nm/°C was achieved.
In order to achieve photonic band-gap effect as sensing mechanism and improve biocompatibility of relatively lower-cost
silica-core phonic crystal fibers (SCPCF) and make use of photonic band-gap effect as sensing mechanism, polymer
with similar biocompatibility as PMMA coating in SCPCF air-holes is proposed in this paper. In order to evaluate the
polymer coating effect, a three-layer model of air hole is proposed. The wavelength shifts of photonic band-gap edges
(PBEs) were evaluated by plane wave expansion (PWE) method, assuming refractive index of silica ns, polymer np and
air na are 1.45, 1.50 and 1.00 respectively. Blue shifting of bands are observed in the simulation and the bandwidth of
each band-gap becomes narrower with the increasing of air ratio. The result shows that 1nm change of air hole is able to
obtain a wavelength shift of 0.43nm. Assuming the wavelength shift of 0.01nm can be detected, a small air hole variation
of 0.023nm can be measured.
Sol-gel entrapment technique is proposed for glucose oxidase immobilization in long period grating glucose sensor. The
glucose oxidase is encapsulated in transparent sol-gel matrix to detect the presence of D-glucose molecules. A sensitivity
of 39.8mM/nm was achieved for the fabricated glucose biosensors.
Hollow-core photonic bandgap fiber (HC-PBGF)–based evanescent wave biosensors are demonstrated and analyzed theoretically and experimentally. With 95% of the guided light power residing in the samples, the measured absorbance for a 30-cm-long fiber filled with a 0.2 µM Alexa Fluor 700–labeled DNA Oligo solution is 1.06. This is in good agreement with the theoretical prediction, which is evaluated by using the refractive index scaling law. The HC-PBGFs thus offer both efficiency and simplicity for the detection of biomolecules in ultra-small sample volumes.
Refractometric sensor utilizing spectral properties of antiresonant guiding photonic crystal fibers is proposed. The sensor
operation is based on the wavelength shift of the transmission spectrum with respect to the change of refractive index
inside the air holes of the photonic crystal fibers. Both numerical and experimental analyses are carried out to investigate
the spectral characteristics.