Surface plasmon resonance based fiber optic sensor for the detection of CrO₄²⁻ using Ag/ITO/hydrogel layers

Surface plasmon resonance based fiber optic sensor for the detection of CrO₄²⁻ in aqueous samples is reported. The probe is fabricated by coating layers of silver, indium-tin oxide (ITO) and hydrogel with (3-acrylamidopropyl)-trimethylammonium chloride (ATAC) over the unclad core of the optical fiber. The layers of silver and ITO over the unclad core of the fiber are deposited using a thermal evaporation technique, while the hydrogel with ATAC layer is coated using a dip-coating method. For the characterization of the sensor, samples of different concentrations of CrO₄²⁻ are prepared in aqueous solution of sodium chloride. The sensor is based on the principle of shrinkage/swelling of the hydrogel layer when the concentration of the CrO₄²⁻ sample around the hydrogel layer is increased/decreased. The shrinkage of the hydrogel layer occurs because of the polymerization of ATAC present in the hydrogel due to the formation of its ion pair with CrO₄²⁻ anion present in the aqueous solution. The sensor is based on the wavelength interrogation technique. The SPR spectra recorded for different concentrations of CrO₄²⁻ show that as the concentration of CrO₄²⁻ increases the resonance wavelength increases. To achieve the maximum shift in resonance wavelength and the maximum sensitivity of the sensor, the concentrations of ATAC in hydrogel, pH and concentration of sodium chloride in water and the thickness of the ITO layer are optimized. The selectivity of the sensor is investigated using samples of other anions and it is found that the present sensor is more selective for low concentrations of CrO₄²⁻. The sensor has a low limit of detection and has several advantages, such as immunity to electromagnetic interference, miniaturized probe, low cost, fast response, capability of online monitoring and remote sensing, due to the fabrication of the probe on an optical fiber.