Guanjia
Zhao
a,
Samuel
Sanchez
b,
Oliver G.
Schmidt
b and
Martin
Pumera
*a
aDivision of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore. E-mail: pumera@ntu.edu.sg; Fax: +65 6791-1961
bInstitute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany
First published on 7th February 2013
Self-propelled catalytic microjets have attracted considerable attention in recent years and these devices have exhibited the ability to move in complex media. The mechanism of propulsion is via the Pt catalysed decomposition of H2O2 and it is understood that the Pt surface is highly susceptible to poisoning by sulphur-containing molecules. Here, we show that important extracellular thiols as well as basic organic molecules can significantly hamper the motion of catalytic microjet engines. This is due to two different mechanisms: (i) molecules such as dimethyl sulfoxide can quench the hydroxyl radicals produced at Pt surfaces and reduce the amount of oxygen gas generated and (ii) molecules containing –SH, –SSR, and –SCH3 moieties can poison the catalytically active platinum surface, inhibiting the motion of the jet engines. It is essential that the presence of such molecules in the environment be taken into consideration for future design and operation of catalytic microjet engines. We show this effect on catalytic micromotors prepared by both rolled-up and electrodeposition approaches, demonstrating that such poisoning is universal for Pt catalyzed micromotors. We believe that our findings will contribute significantly to this field to develop alternative systems or catalysts for self-propulsion when practical applications in the real environment are considered.
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Scheme 1 Poisoning of Pt-catalyst microjets with small molecules containing sulphur. (A) In the absence of the Pt-poisoning molecules, robust decomposition of H2O2 takes place in the tubular cavity, which is catalysed by the Pt surface of the cavity; (B) in the presence of Pt-poisoning molecules, the decomposition reaction is quenched or inhibited to a certain degree, fewer or no oxygen bubbles could be generated, leading to a lower or zero speed of the microjets. |
However, when real-world applications are considered, one should keep in mind that the components of those environments may have a negative impact on the motion of the microjet engine. Pt as a catalyst is prone to poisoning mostly by sulphur-containing compounds. These compounds such as sulphur containing peptides and amino acids are present in blood and in cells. As for other sulphur containing compounds like sulphoxides, they may be present in environmental waters dedicated for remediation. To date, there is a lack of studies on the effect of environmental molecules on the Pt-catalyst micromotors. Most of the current research is focused on fabrication of more powerful nano/microengines2,14,15 or on demonstration of the proof of principle of the applications;8–10,12,13 however, the issue of negative influence of components of the environment on the movement of microjets is rarely taken into account.19
In this paper, we demonstrate that at a constant hydrogen peroxide concentration (9 wt%), the motion of the microjets can be significantly inhibited by chemicals present in the environment. Since the motion of the microjets depends on the rate at which oxygen bubbles are generated,20 the presence of certain molecules would have a significant effect on the motion if they can affect the reaction of hydrogen peroxide. Some organic and/or biological molecules are capable of quenching or inhibiting the H2O2 decomposition reactions catalysed by Pt. In this study, dimethyl sulfoxide (DMSO) was used to chemically quench the ˙OH radical generated in the decomposition of H2O2. In addition, several sulphur containing peptides and amino acids were also chosen to study the poisoning effect they have on the microjets since sulphur containing species (H2S, RSH, RSSR…) are known poisons for all catalytic processes using Pt-catalysts.21 With the presence of the –SH or –SCH3 moiety, the motion of the microjets was reduced in terms of speed and the number of microjets running (Scheme 1). It is also shown that a higher concentration would be required for the –SCH3 containing methionine molecules to affect the motion of the microjets, indicating that the Pt catalyst is much more sensitive to –SH moieties. Our results reveal that organic/biological molecules present in the natural/biological environment can dramatically inhibit the motion of microjet engines based on the Pt catalyst. This must be carefully considered when employing the microjet for real-world tasks.
Since the generation of O2 gas proceeds through the radical mechanism, the motion of the jets can be very sensitive to molecules that are capable of quenching the radicals. To run the jets in a chemically complex medium – which is the case if we use them in a natural sample for certain applications, it may be difficult to avoid the presence of such ˙OH radical-quenching molecules. In this study, DMSO was used as a model “˙OH radical quenching molecule” as it is a well-known quencher for the ˙OH radicals.24
In an aqueous solution of 9 wt% of H2O2, we studied the influence of DMSO concentration on the motion of the microjets (if not mentioned otherwise, we discuss in the following text microjets prepared by rolled-up nanotech).19Fig. 1 summarizes the findings. In the presence of 20 mM of DMSO, more than half (56.25%; n = 32, where n is the total number of jets counted) of the microjets showed absolutely no motion; they were completely deactivated by the quenching effect, see Fig. 1A. DMSO molecules are able to quench the radicals that were produced in situ, even at a high concentration of hydrogen peroxide (9 wt%; this corresponds to a ratio of ∼145 H2O2 molecules vs. 1 DMSO molecule). As the DMSO molecules are homogenously distributed in the solution; they are taken in by the microjet inlet together with the H2O2 fuel, quenching the radicals as soon as they are formed. When the concentration of the DMSO was increased to 80 mM, all the microjets were deactivated – in other words, no microjets showed any motion.
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Fig. 1 The presence of the DMSO in the running solution can significantly reduce the ability of catalytic microjets to move. (A) Influence of the DMSO concentration on the population of the microjet engines found to be exhibiting motion. Note that more than half of the jets stopped running at a low concentration of DMSO at 20 mM (running percentage in solutions without DMSO was defined as 100%), and no motion could be observed at all when the concentration of DMSO increased to 80 mM; (B) influence of the concentration of DMSO on the velocities of the microjets which exhibited non-zero velocity motion. Note that at 80 mM of DMSO, there was no motion observed for any microjet. Tracking data were obtained for a timescale of 10 seconds from 5 independent running experiments in order to get the average speed. |
Other than the complete deactivation of a large percentage of the jets (or all of them at concentrations of ≥80 mM), the presence of DMSO also resulted in a significant decrease of the velocities of the microjets, among those which were still able to acquire motion, as shown in Fig. 1B. A significant reduction of the velocities was observed even with the presence of only 20 mM of DMSO. The average speed decreased from ∼180 to ∼99 μm s−1. This is due to the quenching of the ˙OH radicals which led to the decreased production of oxygen bubbles, and this in turn resulted in a weaker thrust in the motion of the jets.
Several extracellular thiols, such as cysteine, methionine and glutathione, are of important physiological significance. They exist in extracellular as well as in intracellular liquids, acting as signal transducers and antioxidants. The levels of these compounds present in biological fluids, such as plasma and urine are important biomarkers in various clinical situations and they are often present in concentrations of 1–10 mM.27–30 These thiols contain either an R–SH or R–S–S–R moiety. Hence it is of paramount importance to determine whether the presence of these extracellular thiols can poison the Pt interior of a microjet engine and hinder the catalytic conversion of H2O2 to oxygen bubbles, which would disable the motion of the microjets. We first investigated one of the simplest of these thiols, cysteine. We also investigated the reason behind the poisoning to determine if it is indeed the –SH group that is responsible, by using serine as a control additive to the solution. Serine has the same structure as cysteine with the exception of the side chain moiety, which is –SH for cysteine and –OH for serine. The cysteine molecules are readily oxidized by hydrogen peroxide to form a disulfide bond in the presence of hydrogen peroxide in the running solution.31 The presence of a disulfide bond and a possible residual –SH group in the running solution can function as poisoning agents for the catalytically active Pt metal surface,21 which is evident from Fig. 2. Two sets of experiments under the same conditions were carried out for cysteine-containing and serine-containing solutions. As shown in Fig. 2A, even in the presence of 10 μM (10−5 M) of cysteine, about 15% of the microjets were disabled, accompanied with a significant reduction of the velocity of remaining microjets. This is due to the coverage of the inner Pt surface of these microjets with an active R–SH group, which suppressed the disproportionation of H2O2. At 1 mM concentration of cysteine-containing solutions, only less than half of the microjets were found moving; and when the concentration was increased to 10 mM, none of the microjets were found running. In contrast, almost 100% of the microjets were found to be running for the serine-containing solution at 10 mM concentration. Furthermore, not only do the cysteine molecules “disable” the jets but they also reduce the power for the moving ones, in a similar manner to that of the DMSO molecules. The average speed in the first 10 seconds decreased from ∼180 μm s−1 to ∼86.1 μm s−1 at 0.1 mM of cysteine concentrations, and continued decreasing to 16.7 μm s−1 for 1 mM and eventually to 0 μm s−1 at 10 mM concentration of cysteine. In addition, the velocities of the microjets remained the same whether it is in the absence or presence of 10 mM of serine (∼180 μm s−1). As seen for the cysteine-containing solutions, partial deactivation of the active Pt catalytic sites led to a lower production of oxygen bubbles, resulting in a weaker thrust pushing the jets to move; hence a reduction in the velocities was observed. In order to prove that such decrease of microjet performance is indeed due to the –SH group present in cysteine, we used an analogue of cysteine, as a control experiment. Serine differs from cysteine only by the substitution of the –SH group by the –OH group. In comparison, such reduction of velocities was not observed for the serine-containing solutions. Since the difference between the structures of a cysteine and a serine molecule lies only in the –SH and –OH side-chain moiety, it is thus clear that it is indeed the –SH groups that react in the solution, and are responsible for the inhibition of the motion of the microjets.
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Fig. 2 Presence of cysteine molecules in the running solution can significantly reduce the motion of the rolled-up microjets. (A) More than half of the jets stopped running at 0.001 M of cysteine concentration, and no more motion could be observed when the concentration increased to 0.01 M. Nearly 100% of the jets were found running in serine-containing solution; (B) significant reduction in speed was observed for the jets, from ∼180 μm s−1 without cysteine to ∼16.7 μm s−1 at 0.001 M concentration of cysteine. The velocity remained at ∼180 μm s−1 for jets running in serine-containing solutions at 0.001 M concentration. Tracking data were obtained for a timescale of 10 seconds from 5 independent running experiments in order to obtain the average speed. |
Once we have proved that the –SH group is responsible for the poisoning the microjet engine, we proceeded to investigate more complex thiols. Glutathione is a very important tri-peptide which contains the –SH moiety (for the structure of glutathione, see Fig. 3), and it is responsible for its anti-oxidative function, present in ∼5 mM concentrations in the mammalian cells.27,28 The Pt catalytic microjets were found to be very susceptible to poisoning by glutathione at a concentration much lower than the physiological concentrations of this tri-peptide. At 0.1 mM (10−4 M) concentration, almost half of the microjets were disabled (exhibited no motion) and the velocity of the remaining moving jets was dramatically reduced from ∼180 to ∼70 μm s−1 (Fig. 3A). At the physiological concentrations of glutathione, which are in the 1–10 mM range, about 65–80% of jets were disabled and the remaining exhibited a crippled motion at 10–30 μm s−1 (compared to ∼180 μm s−1 in the absence of glutathione).
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Fig. 3 Comparison of the poisoning effect for glutathione and methionine molecules in the running solution (A) less than 40% of rolled-up tubes were running with only 0.001 M of glutathione present in the solution, and a significant reduction of speed was also observed for the jets, from ∼180 μm s−1 without glutathione to ∼30 μm s−1 at 0.001 M concentration of glutathione; (B) more than half of the jets stopped running at 0.15 M methionine concentration and no more motion could be observed when the concentration increased to 0.2 M, and a significant reduction of speed was also observed for the jets, from ∼180 μm s−1 without methionine to ∼60 μm s−1 at 0.15 M concentration of methionine; tracking data were obtained for a timescale of 10 seconds from 5 independent running experiments in order to obtain the average speed. |
Other than the R–SH and RS–SR′ moiety, the R–SR′ moiety is also able to poison the Pt catalyst. As shown in Fig. 3, nearly half of the jets were “disabled” at 150 mM of methionine. It is evident that the R–S–CH3 moiety of methionine is less poisoning than that of the R–SH moiety in cysteine (note that the R portion of a molecule of cysteine and that of methionine possess very close structures) and in glutathione, even though it is still strongly poisoning the Pt catalytic microjets at ∼100 mM concentration levels.
This is due to the fact that methionine (in contrast to cysteine) does not contain the –SH moiety but only the –SCH3 moiety, which exhibits a weaker bond to Pt (via chelation), in a similar manner as in methionine platinum dichloride.32
In order to show that the above-mentioned effects do not influence only the catalytic microjets prepared by rolled-up nanotech,25 we performed similar experiments on microjets of smaller dimensions prepared by electrochemical deposition. These microjets consisted of microtubes with a Pt interior, with a length of ∼10 μm and a diameter of ∼2 μm (compared to the size of rolled-up microtubes with a length of ∼50 μm and a diameter of ∼5 μm). Not surprisingly, these microjets behaved similarly to the ones prepared by rolled-up technology which we described in the previous sections. More specifically, microjets made from template-assisted electrodeposition were also found to be very susceptible to being poisoned by glutathione. At 50 mM (0.05 M) concentration of glutathione, more than 70% of the microjets were disabled (exhibited no motion) and the velocity of the remaining moving jets was dramatically reduced from ∼380 to ∼80 μm s−1 (Fig. 4). The mechanism of poisoning is the same as in the case of rolled-up microtubes.
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Fig. 4 The presence of glutathione in the running solution can reduce the ability of Cu/Pt catalytic microjets prepared by electrodeposition to move. (A) Influence of the glutathione concentration on the population of the microjet engines found to exhibit motion. No motion could be observed at all when the concentration of glutathione increased to 200 mM; (B) influence of the concentration of glutathione on the velocities of the microjets which exhibited non-zero velocity motion. Tracking data were obtained for a timescale of 10 seconds from 5 independent running experiments in order to obtain the average speed. |
This journal is © The Royal Society of Chemistry 2013 |