Localized surface plasmon resonance based tapered fiber optic ethanol sensor

Ethanol is mostly found in drinks, food, beverages and in the human body fluids, e.g. urine, serum, sweat, saliva, and blood etc. A high concentration of ethanol in body fluids causes various metabolic disorders like diabetes, cirrhosis and hepatitis [1]. Therefore, the detection of ethanol is essential for clinical and forensic investigation. In this study, we report the fabrication and characterization of a localized surface plasmon resonance (LSPR) based tapered fiber optic ethanol sensor. Nowadays LSPR is a very useful technique for biosensing applications. For the LSPR phenomenon, the size of the nanoparticles should be smaller than the wavelength of the light. When an electromagnetic wave is incident on metal nanoparticles, the collective oscillation of the free electrons inside the metal nanoparticles occurs. The maximum amplitude occurs when the frequency of oscillation matches with the frequency of the incident light. Due to the frequency matching a strong field enhancement is observed around the nanoparticles. In the phenomenon called as LSPR, the peak absorbance wavelength depends on the dielectric constant of the medium surrounding the nanoparticles [2]. To fabricate the LSPR based fiber optic probe for ethanol sensing, we have synthesized gold nanoparticles (AuNP) using Turkevich method [3]. The TEM image of AuNPs is shown in fig. 1 (a). The schematic of the LSPR probe is shown in fig 1 (b). The probe is consists of AuNPs over the core of the tapered optical fiber followed by the immobilized layer of enzyme alcohol dehydrogenase (ADH) and coenzyme nicotinamide adenine dinucleotide (NAD). For the attachment of gold nanoparticles the core was cleaned by pirhana solution and kept in amino silane solution. Dip coating was used to coat the AuNPs over the silanized core. After this, the probe was incubated in 1 mM aqueous solution of cystamine dihydrochloride for 1 h for attachment of -NH₂ groups over the AuNPs-coated fiber core. Finally, the probe was incubated for 12 h into the ADH and NAD solution. This completed the probe fabrication step.