- Open Access
Increased sensitivity of Au-Pd nanolayer on tapered optical fiber sensor for detecting aqueous Ethanol
© The Author(s) 2017
Received: 30 May 2017
Accepted: 5 October 2017
Published: 16 October 2017
Optical fiber sensors gains popularity as an alternative and a better solution compared to its electronics counterpart. Especially in detecting hazardous materials, optical fiber sensors prove to have many advantages such as miniaturization, remote yet real-time sensing and immune to electromagnetics interference.
In this paper, we used a 40um tapered standard multi-mode fiber coated with gold (Au), palladium (Pd) and mixtures of Au-Pd nano-composites to detect hazardous ethanol in its various concentration in aqueous environment. Interrogation were made possible using UV-VIS light source, and changes in absorbance and intensity were recorded via spectrometer.
Results and Conclusion
Ethanol ranging from 20% till 100% were investigated. It was found that the composition of nanomaterial coating on the developed sensors strongly affected the sensing performance. From the experiments carried out, the sensor with a gold palladium nanocomposite layer with a 2: 1 ratio of gold to palladium produced the highest sensitivity, which is 0.74/vol% concentration of Ethanol. The fabricated fiber sensor also exhibits fast response and recovery time of 13 s and 6 s respectively.
Recently, optical fiber sensors have been in the limelight since they are being utilized in various fields such as environmental monitoring, medical imaging and diagnostics, forensics, aviation and even in the military [1, 2]. Optical fiber sensors earn this accolade due to their many advantages such as miniaturization, flexibility, fast response and capability for real time and in-situ sensing deployment as well as capability of remote sensing [3, 4]. This increases the desirability of optical fiber sensors as compared to electrical based transducers [5, 6], especially when it comes to chemical detection [7, 8], humidity [9, 10], and also vibration sensing [11, 12].
Tapered fiber optic sensors are particularly popular due to their high sensitivity towards their surroundings [13, 14]. It is not only the enhancement of sensitivity of the tapered fibers sensors that makes them a preferable choice for optical fiber based sensors, their ease of fabrication makes them even more desirable as they are easily reproducible and reliable [15, 16]. The tapering causes a huge reduction in the size of the core and that leads to penetration of light signal into the cladding which in turn enables the interaction of the light with the surrounding medium which allow the tapered fiber to be employed as a sensor [17, 18].
Ethanol is a commonly utilized chemical in various industries such as medical, food processing, beverages and even as a replacement fuel in automotives [19, 20]. Ethanol has a high volatility and flammability that can be dangerous for humans and living organisms, thus making the sensing of Ethanol crucial and important . Mostly, commercial Ethanol sensors available are electrical based transducers which rely on resistance and voltage readings for detection [22, 23]. Safety features are required when electrical based sensors are used due to Ethanol’s flammability that might cause it to ignite when sparks are formed or heat is transferred. This makes the monitoring of Ethanol concentrations risky because of the fire hazards associated with Ethanol.
Researchers are now looking to optical fiber sensors as an alternative for detecting Ethanol. According to previous research conducted, a fiber optic surface Plasmon resonance (SPR) sensor coated with a layer of gold was used to detect Ethanol in an aqueous solution . This sensing was based on refractive index changes of the solution as the ethanol concentration increased caused changes to the SPR wavelength. Other research include tapered fiber optic tip coated with graphene oxide layer and used as a reflectance based sensor to detect Ethanol concentrations . A sensitivity of 0.0086/vol% units of Ethanol in water was obtained. In this paper we propose a tapered multimode fiber optic coated with Au and Pd nanocomposite layer in order to obtain a highly sensitive response towards ethanol detection.
Au:Pd (2: 0.7)
Au:Pd (2: 1)
Results and discussion
The sensor’s dynamic response is obtained by integrating the absorbance levels of the sensor from 600 to 700 nm for each concentration of ethanol. This is performed using the time response feature of the spectrasuite software. First 20 ml of Ethanol solution is inserted into the chamber using syringe. Once the response stabilized, the Ethanol is removed using syringe again. The sensor is allowed to recover to a stable base line. Following the recovery of the sensor, the next concentration of the Ethanol is inserted. These steps are repeated until all the concentrations are completed.
The dynamic response of the sensor was recorded at the wavelength range from 600 nm to 700 nm at room temperature. This wavelength was chosen because the sensor exhibited the best response at this region. An average of 18 s was the time taken by the sensor to respond towards Ethanol and 9 s was the approximated time taken for the sensor to recover to a stable baseline when Ethanol is removed. The dynamic response of the Au coated tapered MMF fiber optic is shown in Fig. 5c.
A corresponding decrease is intensity levels can be observed in the intensity spectrums as expected. The dynamic response of the 40 μm diameter MMF coated with Pd is shown in Fig. 6c. The sensor shows fast response and recovery time of approximately 17 and 12 s respectively for all of the five concentrations of Ethanol.
These results are due to the properties of gold and palladium when they are exposed to Ethanol. Generally, the optical properties of metallic complexes change when VOCs are present. These changes normally happen in refractive index and color. In return, these changes cause a corresponding change to the absorbance spectrum of the tapered fiber sensor. This is similar with the other reported work on fiber optic based Ethanol sensors [20, 26].
Performance comparison of developed sensors
Fibre optic coating
Gold: Palladium 2:0.7
Gold: Palladium 2:1
Where: Sn= sensitivity, A = absorbance and C = concentration.
All the four sensors developed in this research show strong responses towards Ethanol concentrations. The response and recovery time for all 4 sensors are below 20 s. The fastest response is observed with the sensor coated with a gold palladium ratio of 2 to 1, which are 13 s for response and 6 s for recovery. This sensor also demonstrated the highest sensitivity towards ethanol concentrations which is 0.074/vol%.
This due to the fact that the Au, Pd combination causes a unique interaction between the light signal to the sensor surface. The gold nanoparticle enhanced the optical signal in the fiber causing a strong evanescent wave to be present to the surface of the sensor. This will allow the sensor interaction between the light and the surrounding media. The palladium nanoparticles show a strong affinity towards ethanol molecules . Therefore, the combination of the gold and palladium gives a strong response toward Ethanol detection.
A tapered multimode fiber sensor coated with gold, palladium and gold palladium nanocomposite with ratios 1:1 and 2:1 has been successfully developed to detect ethanol concentration in water. The sensor was interrogated in the visible region using a broadband white light source and a spectrometer to detect the output. The developed sensors demonstrated high sensitivity towards ethanol concentrations, with the best results obtained from the sensor with the gold palladium coating with a 2:1 ratio, where a sensitivity of 0.074/vol% is reported. This sensor also gave the fastest dynamic response towards ethanol. This study presents improvement to the sensitivity of fiber optic based ethanol sensor using a larger taper waist diameter. This will contribute towards tapered fiber optic sensors that are more robust and easier to handle.
The work reported in this paper has partly supported by the Universiti Putra Malaysia’s Research University Grant Schemes (RUGS) (Ref: 05–01-12-1626RU and 05–02-12-2015RU) and Ministry of Higher Education, Malaysia’s Fundamental Research Grant Scheme (FRGS) (Ref: 03–04-10-795FR). Both RUGS were used to partly fund the student scholarship throughout his study, whereas FRGS were used to buy all the components and consumable items used in the experiments.
AT: Sensing experiments and Material characterization. MR: Material Characterization. PT, NT and AM: Data analysis. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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