NEW GENERATION OF SOL-GEL TRANSDUCERS FOR CHEMICAL OPTICAL SENSORS WORKING IN BIOLOGICAL MEDIA (BIOPTIC)

Context

The thesis makes the object of a collaboration between LMGP and the gymetrics company (Lausanne, Grenoble). The company develops single-use measurement systems operating in biological media. The primary application is cell culture monitoring by measuring physicochemical parameters such as temperature, pH, concentration of dissolved oxygen (DO) or CO₂…. To this end, the company designs, produces, and markets reliable and miniaturized sensors adapted to all types of cell culture. gymetrics has already developed electrochemical sensors meeting these requirements. The company now wishes to extend its business to optical transduction technology in order to develop a new generation of sensors. Researches focused on two types of optical sensors considered as priorities by gymetrics, i.e. DO and pH, are now in progress at LMGP. The operating principle of these sensors is based on changes in the luminescence signal (intensity or lifetime) when an organic luminescent dye, incorporated into a sol-gel thin film matrix permeable to gaseous or ionic species, is contacted with an analyte, e.g. the hydronium ion (H3O⁺) for a pH sensor or gaseous oxygen for a DO sensor. Ongoing researches are focused on the exploitation of thin film optical transducers, since a thin film configuration is suitably adapted for integration in miniaturized planar devices. However, this configuration suffers some limitations as far as the detection sensitivity is concerned:

·        The amount of active dyes embedded in the thin film matrix is limited by the small thickness of the transducer, which reduces the luminescence intensity and thus the detected signal.

·        Only a small area of the thin film transducer is able to conjointly receive the excitation signal produced by LEDs and to emit light toward the detection photodiode.

·        Since luminescence is isotropically emitted in the space, only a weak fraction of the light emitted by the dye is directed toward the photodiode.

Thesis objectives

While ongoing researches aim at short-term results, gymetrics also intends to explore new options opening up future routes for a second generation of more sensitive optical sensors. Exploratory researches conducted in the frame of the proposed thesis will aim at answering to this objective. We intend to combine two micro-structured architectures inspired by integrated optics and optoelectronic technologies.

1/ Planar waveguide configuration: using a light propagation mechanism, this configuration should significantly increase the amount of dye involved in the optical detection. The entire area covered by the dye in front of the LEDs will be excited and the emitted light will propagate toward the detection area where it will be collected by the photodiode.

2/ Diffraction gratings: with light diffraction effects, an improved optical coupling LED / dye and dye / photodiode should significantly increase signal detection. The diffraction gratings are expected to increase the fraction of excitation light injected in the waveguide and the fraction of emitted light extracted toward the photodiode.

These micro-structured architectures will rely on LMGP’s know-how in sol-gel chemistry. Sol-gel formulations have been developed to elaborate photoresists that can be patterned through a single insolation / washing lithographic step, avoiding costly and multistep traditional lithographic procedures. Hybrid organic-inorganic and all-inorganic (TiO₂-derived) sol-gel photoresists are based on a selective UVA insolation (e.g. through a chromium mask) and on the chemical stabilization of insolated areas, through photo-polymerization or photolytic decomposition mechanisms, while non-insolated ones can easily be removed by washing using various solvents (alcohols or else), which yields micrometric photo-patterns. These patterns have already been studied for planar waveguide or diffraction grating purposes.

Exploratory studies conducted during the thesis will aim at conjointly optimizing the experimental conditions (sol-gel formulations and protocols, imprinting conditions (photolithography in combination or not with nanoimprint, pre-bake and post-bake conditions) in order to provide opto-geometrical (width, thickness, refractive index, profile, and periodicity) of channel waveguides and diffraction gratings best suited to the aforementioned approaches. The studies will also focus on the best incorporation of luminescent dyes (nature, amount) in the micro-structured architectures in order to optimize the luminescence properties in connection with the tested analytes. The optically active dyes will be selected on the basis of ongoing researches performed at LMGP and dealing with pH and DO sensors.

The work will be performed at LMGP and the PhD student will also benefit from facilities available at gymetrics premises. The candidate should have strong knowledge in physical-chemistry of materials and be particularly interested in experimental research (elaboration, characterization). Knowledge in sol-gel chemistry and/or integrated optics are not prerequisites but would be appreciated.

Contact

Michel Langlet: Michel.Langlet@grenoble-inp.fr