Lighting Up Redox Propulsion with Luminol Electrogenerated Chemiluminescence

The development of systems at different scales that are remotely controlled or actuated in a wireless fashion is becoming a fast-developing area at the interface between chemistry and physics. In that framework, self-propelled macro-, microand nano-objects that are able to move in liquids when submitted to external physical fields or chemical fuels have recently been designed. In the latter case, the motion is induced by the conversion of locally available chemicals into mechanical energy via the production of gas bubbles. The mainstream strategy involves a metallic moiety which is able to catalyze the decomposition of hydrogen peroxide, resulting in the interfacial generation of oxygen bubbles. In contrast to this chemically-induced locomotion, physical methods involving electric fields have also been demonstrated. Bipolar electrochemistry (BPE) has recently encountered an important renewal thanks to the development of new applications in the fields of analytical chemistry and chemical sensing, surface science and generation of chemical gradients, catalysis and materials synthesis. The behaviour of a bipolar electrode (BE) is atypical with respect to conventional electrochemistry because it acts as an anode and a cathode at the same time. This asymmetric redox reactivity can also be advantageously used to set in motion conductive particles. The very first example of translational motion was proposed on the basis of an electrochemical self-regeneration with a zinc dendrite. Alternatively, water splitting at both reactive extremities of a BE is responsible for a spatially separated bubble production. This overall reaction generates twice more hydrogen bubbles at the cathodic pole when compared to oxygen at the anodic end. Here, the intrinsic stoichiometry controls the directionality of the resulting motion from the feeder anode to the feeder cathode. Enhancing the speed of motion can be achieved by introducing additive sacrificial redox compounds such as quinone or hydroquinone. In the latter case, the anodic counter reaction is not useful and simply allows to produce bubbles at only one pole of the BE. Interestingly, this recent approach allows as well the addition of new functionalities to dynamic systems like for example the emission of electrogenerated chemiluminescence (ECL). ECL is the light emission resulting from an initial electrochemical reaction. A model ECL system is based on the oxidation of luminol (5-Amino-2,3-dihydro-1,4-phthalazine-dione) in alkaline solution in the presence of hydrogen peroxide in order to promote blue light emission. Luminol and its derivatives are widely used in biochemical and clinical analysis such as enzymatic assays and immunoassays. For example, luminol ECL coupled to oxidasecatalysed reactions generating hydrogen peroxide constitutes a sensitive and versatile analytical strategy. ECL has not only been exploited in combination with BPE for analytical purposes, but also to design the first light emitting electrochemical swimmer involving [Ru(bpy)3] 2+ and tripropylamine (TPrA). The present contribution will show that this methodology can be generalized with another ECL system based on luminol, allowing to tune the wavelength of light emission. Moreover, all previously described BPE-driven motions were reported in neutral or acidic aqueous solutions with cathodic bubbles generation. In contrast, this first example in an alkaline solution proceeds through an alternative mechanism based on H2O2 oxidation and allowing the light emission at the same pole (anode) than the bubble production (Scheme 1). Simultaneous generation of gas bubbles and of ECL could be theoretically achieved when the interfacial difference of potential alongside a BE becomes sufficiently high. Preliminary