How to deposit a durable layer of sensing molecules in a PVC matrix on the tip of an optical fiber?
We have no idea how to do this, since we dont have any step/by/step instruction, reaction time, environment we need to maintain for this.


Let's redefine the general task:


How to deposit a layer of 2,4,5-triphenylimidazole (lophine) on the tip of an optical fibre?


Requirements:


The layer must be permanent/durable and does not wear or wash off during measurements of the nitrate concentration in drinking water.


The workflow therefore includes:


How to prepare the fibre?
How to deposit one (or several) layers of lophine?
How to cure the layers to make sure that they do not wear/wash off?


As far as the preparation of the fibre is concerned, your primary paper (Optics and Lasers in Engineering, 2014, 60, 38-43) gives some hints on page 39:


  To deposit the sensitive layer on
  the optical fiber, the coating was removed in a 1 cm section, and
  the exposed cladding was sterilized. Isopropyl Alcohol Level was
  used for cleaning the optical fiber in special cleaning cloths and
  immediately the LI–PVC was introduced [...].


Apparently, lophine is deposited on the fibre in a PVC matrix and you have cited the composition of the mixture used. 

How is this actually done?

An earlier publication by the same authors gives some directions! It's reference 19 in the paper that you cited! 

In Sensors, 2008, 8, 847-859, the describe authors describe the dipping method in chapter 2.2. and provide further references.

In summary, the prepared fibre is 


dipped into the solution (lophine, PVC, TBP, THF) and then 
cured by placing the fibre in an oven for 15 minutes at 80 °C.


These two steps are repeated as often as necessary to obtain the required total layer thickness.