OptoReader as detection device for fiber optic sensor

The OptoReader is a highly sensitive detection device taking advantage of advanced photo-detection technologies. Its optical fiber based design and its capacity of both excitation and detection of fluorescence makes it the ideal device for working with fiber optic sensors.

Optoreader-fiber optic sensor

Fiber optic chemical sensors have important applications in challenging fields such as clinical diagnosis, pollution monitoring, oceanography, chemical process monitoring, …

In this application note, we demonstrate the ability of the OptoReader to detect the change in fluorescence of a tiny amount of molecules adsorbed at the tip of an optical fiber.

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Based on the Fluorescein isothiocyanate (FITC)’s fluorescence efficiency dependance on pH, a simple fiber optic sensor of pH can be thus fabricated by adsorbing FITC molecules at the extremity of an optical fiber.

Efficient adsorption of FITC molecules to the extremity of a glass optical fiber can be achieved using FITC-labeled poly(L-lysine). The polylysine is attractive to negatively charged surface. On the other hand, glass surfaces immersed in water is known to acquire a negative surface charge density, primarily through the dissociation of terminal silanol groups. Glass optical fiber with stable FITC coating can be obtained by dipping the fiber into a solution of poly(L-lysine)-FITC (PLL-FITC) in water. 

Materials and samples Fabrication of the fiber optic pH sensor

PLL-FITC solution

PLL-FITC is obtained from Sigma-Aldrich (P3069) and is deluted at the proportion of 50 µmol/l of into water.

Fiber optic sensor

The fiber optic sensor is made from a step-index optical fiber. The waveguiding effect is achieved by the difference of optical index between the core and the cladding. (See figure on the right for schematic structure of a step-index fiber). The core and cladding diameter used here are 400 µm and 425 µm, respectively and the numerical aperture is 0.5. 

The sensor is prepared as follows:

  • Remove the protection layers using cutting tools upto the plastique jacket protecting the glass optical fiber.
  • Remove the plastic jacket at the extremity of the fiber using an appropriate un-stripping tool.
  • Precharge negatively the fiber using corona discharge.
  • Dip the fiber into the PLL-FITC solution for about 30 seconds and rinse with deionized water.
FITC-labeled optical fiber

FITC-labeled step-index optical fiber. The adsorbed FITC molecules are illustrated by red dots. The glass core (1) and cladding (2) make up the wave guide channel by total internal reflection effect. They are usually protected by a plastique jacket (3) which is removed (un-stripped) at the extremity of the fiber.

 

The solutions of different pH are obtained by diluting acetic acid into water containing 10 mmol/l of Hepes. The latter is a pH buffer which stablizes the pH of the solution. The pH values are obtained using a commercial pH meter.

Un-strip the fiber using dedicated tool.

Un-strip the fiber using dedicated tool.

The fiber with the protection removed.

The fiber with the protection removed.

 

Precharge the fiber using corona discharge

Precharge the fiber using corona discharge.

 

FITC coating

FITC coating

 

OptoReader Duplex fiber based high sensitivity optical detection

We use the OptoReader to measure the change in the fluorescence efficiency of the FITC molecules attached to the fiber sensor. The OptoReader offers high sensitivity detection and is designed to only use one optical fiber for both illumination and detection. These features makes it ready for use with fiber optic sensor.

The OptoReader runs on the Elveflow® Smart Interface which offers smooth integration with other Elveflow® devices. SDK is also provided for simple interface with the popular programming languages (Matlab, Python, Labview, …).

  • Fast acquisition  
  • High sensitivity
  • Ambient light suppression by synchronous detection
  • Simultaneous measurement of fluorescence and reflection
  • Versatile integration

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Measurement

Connect the fiber optic sensor to OptoReader

Connect the fiber optic sensor to OptoReader

Fiber optic sensor

Optoreader fluorescence measurement

OptoReader is capable of simutaneously excite and measure the fluorescence.

The pH measurement of a solution is done by dipping the FITC-coated optical fiber therein. Between the successive measurements, the fiber is rinsed in water containing pH buffer (Hepes 10 mmol/l). The OptoReader performs both excitation and detection of fluorescence and records the fluorescence change due to the effect of pH.

It is known that low pH quench fluorescence of FITC fluorophore. Here, we use this property to measure pH by measuring the fluorescence at the tip of the FITC coated optical fiber. The following Figure shows the recorded fluorescence versus pH of the solution. 
Optical fluorescence and pH

Fluorescence intensity of FITC adsorbed at optical fiber tip as function of pH.

Conclusion

The OptoReader is a highly sensitive detection device taking advantage of latest photo-detection technologies and and fiber-based optics. Its capacity of simultaneous excitation and detection of fluorescence makes it a prominent candidate for working with fiber optic sensors.

Interested in fluorescent droplets monitoring? Check out our dedicated application note!

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