A STABLE MAGNETIC LOCOMOTIVE
5.4 Concluding remarks
In this Chapter, we demonstrated that Au NPs could be used for the catalytic decomposition of H2O2 producing O2 bubbles to propel a polymer spherical bead inside the liquid. The observations indicate that the bead containing the Au NPs moved upward with an acceleration that depended on the pH of the medium. The measured rate constants at different pH could be used in accounting for the vertical motion of the bead.
The average velocity profile of the bead as a function of pH (within the range 9.1-10.8) could be explained with the observed first order decomposition of H2O2 in alkaline medium. The present work not only opens a new avenue for using pH as a control in autonomous motion but also puts forward Au NPs as a potential material for catalytic decomposition of H2O2 in driving the motion. This would especially be useful when the functional group attached to the NP (such as an enzyme) would itself act as catalyst to drive the locomotive.
Chapter 6
INORGANIC pH TAXIS
In previous Chapters, we have discussed self-propelled motion of micronscale polymer resins and of smaller sized particles of CoFe2O4, incorporated with catalytic nanoparticles.
The particles when put in dilute H2O2 solution, decomposed the liquid catalytically inducing spontaneous propulsion in them. Such propulsion could be due to the increased buoyant force inside the O2bubbles formed as a result of the decomposition of the liquid or could be due to the recoil thrust of the detached bubbles – the exact mechanism being decided primarily by the dimension of the particles. To achieve control over such motions, both in terms of speed and directionality, the properties of the composite materials as well as that of the medium of movement were manipulated. So far, in all these studies, attempts were made to guide a submicroscopic object at every instant of its motion. In other words, for a particle guided towards a specific target, each point of its trajectory was decided instantaneously by tuning the values of one or more controlling parameters at the previous point. Such a process, although offers the flexibility of manipulating such motions at every instant of its motion, it involves difficulties in deciding the magnitudes of controlling parameters for each point of a predefined trajectory. Further, the act of such local guidance essentially demands a continuous monitoring of the system until the particles find their targets. Moreover, motion of sub micrometer scale objects can easily be purturbed with any small change in local environment of the particle – which adds uncertainly in its control. As an alternative, deterministic transport of a single or an ensemble of such inorganic particles could be realized with the minimum degree of randomness following the principle of taxis. To define in biological terms, taxis may is the responsive movement of a free-moving organism or cell towards or away from an external stimulus [101].There are different types of taxis that - identified and named with prefixes that specify the stimulus eliciting the response. Important among these are - aerotaxis (migration towards O2 concentration), phototaxis (response lowards light), thermotaxis
Inorganic pH Taxis
(movement along a temperature gradient) and chemotaxis (migration stimulated by a chemical).
Chemotaxis has been observed in a wide variety of microorganisms like Salmonella paratyphi [102] and Rhodospirillum rubrum [103]. The reason behind their motility in response to a chemical stimulus has been attributed to the creation of a gradient of oxygen which eventually helps these microorganisms to move preferentially in the direction of higher concentration of the chemical. Interestingly, in order to regulate the cytoplasmic pH, bacterial species like Escherichia coli are often seen to migrate away from acidic to alkaline sectors of a region characterized with a finite pH gradient [104].
Attentions have been paid to have a complete understanding of the mechanism behind bacterial pH taxis, which appears to be a model for designing small scale artificial carriers which could be directed towards specific targets. Unfortunately, complete understanding of bacterial pH taxis still remains a challenge with many fundamental points obscure. The situation therefore demands the development of artificial systems where the particle could be made to move collectively towards a target – much like bacterial migration in response to a chemical stimulus. Significant observations in this regard has been reported by Sen et al. realizing motion of Pt/Au bimetallic nanorods towards a region characterized with a higher peroxide concentration [105]. Very recently, controlled organization of Au microparticles in discrete regions within a liquid was attained with externally triggered electrolyte gradient by Wang and his co-workers [22].
In this Chapter, we discuss the first ever realization of inorganic pH taxis with Pd NP deposited micronscale polymer resins within 1% H2O2solution. Spontaneous migration of polymer-NP composite structures could be observed both over the surface and well within the liquid using bigger and crushed polymer resins respectively. The speed of these particles, both for two and three dimensional motion, was found to increase monotonically with the pH of medium. The motion of the bigger composite particles inside a liquid with a pH gradient could be explained considering the time dependent growth of O2 bubbles over these structures – as detailed in Chapter 5. To model the behavior of smaller catalytic particles in presence of a pH gradient, we recall the theory derived in Chapter 4, to explain the self-propulsion of Pd-CoFe2O4particles in H2O2. The theoretical trend in particle velocity matched quite well with the experimental observations establishing the validity of the model proposed. A schematic of inorganic pH taxis, as observed by us is given in Figure 6.1.
Chapter 6
Figure 6.1: Schematic of Inorganic pH taxis demonstrated by Pd NP deposited polymer resins.