Insights into photo-electrochemical sensing of herbicides driven by Chlamydomonas reinhardtii cells

The unicellular green alga Chlamydomonas reinhardtii was exploited as bio-sensing element for the construction of an electrochemical whole-cell based biosensor to detect herbicides in water samples. To preserve the algal photosynthetic functionality, C. reinhardtii cells were entrapped in an alginate gel directly onto the surface of commercial screen-printed electrodes. Cyclic voltammetry (CV) measurements indicated the higher performance of screen-printed carbon nanotube electrodes compared to graphite and gold as working materials. Moreover, it provided insights into the electrochemical reactions occurring at the electrode/algal cells interface, and indicated the optimum buffer pH (pH 7.0) and potential values-(0.7 ± 0.03)V vs Ag/AgCl reference electrode to maximize current signals in chronoamperometry (CA) experiments. Electrodes with different number of cells and related chlorophyll content were tested by CA to optimize the electrochemical signal in terms of peak current intensity and signal to noise ratio. The oxygen reduction signal originated from the algal activity in response to red LEDs light exposure was monitored by amperometry, and the bio-sensing element response was expressed as a ratio between the current intensities registered in the absence and in the presence of herbicides. As competitive inhibitors of the plastoquinone (QB) binding to the reaction center D1 protein, triazine and urea-type herbicides block the photosynthetic electron transport leading to a reduction of the biosensor output currents in a concentration-dependent manner. For linuron and simazine the limits of detection were 6 × 10-9 and 9 × 10-8 M, respectively, while the inhibition constant values (I50) were (1.2 ± 0.1) × 10-7 and (2.3 ± 0.2) × 10-6 M. The operational half-life of the bio-recognition element lasted approximately 9 hours, while room temperature storage stability tests indicated a 2 and 24% signal loss after 3 and 20 days, respectively. The inhibition of the biosensor photosynthetic activity was irreversible at low herbicide concentrations, and highly reversible at medium-high doses. These results were discussed considering the presence in the Photosystem II pigment-protein complex of two herbicide binding niches with different binding affinities. Beyond introducing a promising prototype for commercial applications, this research shed light on current functional issues related to the not yet fully explored plastoquinone/herbicide binding site.