Photonic crystal fiber interferometric pH sensor based on polyvinyl alcohol/polyacrylic acid hydrogel coating.

We present a simple photonic crystal fiber interferometer (PCFI) that operates in reflection mode for pH measurement. The sensor is made by coating polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel onto the surface of the PCFI, constructed by splicing a stub of PCF at the distal end of a single-mode fiber with its free end airhole collapsed. The experimental results demonstrate a high average sensitivity of 0.9 nm/pH unit for the 11 wt.% PVA/PAA coated sensor in the pH range from 2.5 to 6.5. The sensor also displays high repeatability and stability and low cross-sensitivity to temperature. Fast, reversible rise and fall times of 12 s and 18 s, respectively, are achieved for the sensor time response.

Analysis of hollow-core photonic bandgap fibers for evanescent wave biosensing.

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 microM 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.

High-resolution photonic bandgap fiber-based biochemical sensor.

The use of photonic bandgap fibers (PBGF) for biomedical sensing has been demonstrated. The demonstrated PBGF has a blue wavelength shift of 280 nm in the falling photonic bandgap edge (PBE) when the ambient refractive indices inside the holey region change from 1.333 to 1.39, which agrees well with the analytical prediction. Combining this with the knowledge of immobilization techniques and biorecognition elements could open up a new class of PBGF-based label-free biosensors. A sensitivity on the order of 0.1 nmol/L could be achieved by consuming less than 1 microL of sample.

Double-pass Mach-Zehnder fiber interferometer pH sensor.

A biocompatible fiber-optic pH sensor based on a unique double-pass Mach-Zehnder interferometer is proposed. pH responsive poly(2-hydroxyethyl methacrylate-co-2-(dimethylamino)ethyl methacrylate) hydrogel coating on the fiber swells/deswells in response to local pH, leading to refractive index changes that manifest as shifting of interference dips in the optical spectrum. The pH sensor is tested in spiked phosphate buffer saline and demonstrates high sensitivity of 1.71 nm/pH, pH 0.004 limit of detection with good responsiveness, repeatability, and stability. The proposed sensor has been successfully applied in monitoring the media pH in cell culture experiments to investigate the relationship between pH and cancer cell growth.

Polyvinyl alcohol-coated hybrid fiber grating for relative humidity sensing.

A relative humidity (RH) sensor based on a hybrid fiber grating coated with polyvinyl alcohol is proposed and demonstrated experimentally. The hybrid fiber grating is formed by superimposing a normal fiber Bragg grating (FBG) and a tilted-FBG around the same position of a single-mode fiber so that it can work in the reflection mode. Optical power of the reflected signal changes with the refractive index of the moisture sensitive polyvinyl alcohol, and humidity measurement can be realized. Experimental results show that the measurement range is 30 to 95% with the maximum sensitivity of 0.737 nW∕% RH. The average response time is ∼2 s and the measurement is nearly insensitive to temperature. Compared with the wavelength detection method used in normal FBG-based relative humidity sensors, the intensity demodulation method in this report is simpler and more cost-efficient.