The story of chemically powered nanomotors: Where did it all begin?

The history of scientific article citations occurs in real time, at the behest of a highly dynamic and fickle community. In surging fields, some papers maintain large amounts of citations, widely known as canonical articles (the first few DNA papers from Watson and Crick come to mind). Other papers may spur a field but become lost as the volume of publications increases. Here, Professor Ozin recalls the early contributions to nanomotors. The history of scientific article citations occurs in real time, at the behest of a highly dynamic and fickle community. In surging fields, some papers maintain large amounts of citations, widely known as canonical articles (the first few DNA papers from Watson and Crick come to mind). Other papers may spur a field but become lost as the volume of publications increases. Here, Professor Ozin recalls the early contributions to nanomotors. Following an exciting research breakthrough in science, one that undergoes a huge expansion, sometimes references to pioneering papers get overlooked or submerged in the fog of a massive publication surge. It is interesting to observe how literature citations become distorted or obscured over time with a sudden loss of memory and early work ceases to be regularly cited. Such seems to be the case with the discovery of "chemically powered nanomotors," characterized as self-propelled nanoscale objects whose mobility is derived from the onboard catalytic conversion of an energy-rich fuel in their surroundings. One early practitioner in this exciting endeavor feels obligated to set the record straight on behalf of the talented students and colleagues who initiated the field in the first place. They deserve to be fittingly recognized for their innovative, impactful, timely, and technologically relevant contributions to a field with promise for untold societal benefits in medicine and environmental sustainability. Recently, the Royal Society of Chemistry Celebratory 60th Anniversary Board Member Collection of Chemical Communications highlighted pioneering papers published over the six-decade history of the journal, https://blogs.rsc.org/cc/2024/03/18/chemcomm-60th-anniversary-board-member-collection/?doing_wp_cron=1711199647.6802639961242675781250. One of the selected articles in the historic collection of papers from North America was a paper entitled "Synthetic Self-Propelled Nanorotors." It first appeared online in Chemical Communications on the 29th of November 2004.1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar The article described the preparation of self-propelled nanorotors made from barcoded gold–nickel nanorods. On the 21st of September 2004, a similar paper entitled "Catalytic Nanomotors: Autonomous Movement of Striped Nanorods", with gold-platinum barcodes, appeared online in The Journal of the American Chemical Society2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar (Figure 1). Note that the dates these breakthroughs were made public knowledge online both occurred in 2004, while the time it took to publish them as hard copy in the respective journals was 2005 and 2004, respectively. This is how they appear in later citations yet the research of the two groups was done independently and showcased to the world in 2004, a situation to ponder when giving credit and precedence to an important discovery (if only a preprint server such as ChemRxiv existed 20 years ago!). The 2-month difference in the online appearance of these two papers is emphasized in the context of this opinion piece as it appears to have passed unnoticed in accreditation of the pioneering papers in the field of chemically powered nanomotors. Fast-forward two decades with thousands of peer reviewed published papers on the topic, it is now abundantly clear that these two-curiosity-inspired and independently implemented studies that became public knowledge within 2 months of each other in the same year should be recognized as the groundbreaking research that sparked the birth and inspired the rapid growth of a veritable "autonomous nanomotor industry" (see Figure 1). Were these discoveries the long-awaited prototypes of the 1966 science fiction adventure film Fantastic Voyage, about the miniaturization of a submarine crew to microscopic dimensions who undertook an adventurous journey through the body of a scientist to restore function of an injured brain? These real-world "Fantastic Voyages" were both founded in a chemistry laboratory, with the discovery that synthetic chemically powered nanomotors, in which the catalytic conversion of hydrogen peroxide to water and oxygen, H2O2 → H2O + 1/2O2, served as the architype chemical fuel, can drive a bimetal nanorod device into either linear or rotational motion (Figure 2). This revelation represented the first small steps to energize, circumnavigate, and reconnoiter a nanoscale object in a wide range of fluid environments that were envisioned at the time to enable some kind of repair, replacement, augmentation, adjustment, remediation, or cargo pickup and delivery task.1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar Many follow-up studies materialized from the discovery of locomotion at the nanoscale powered by catalytic reactions of a chemical fuel on the surface of the nanomotor,1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar especially those tasked with elucidating the groundwork on how the properties of the nanomotor, fuel, and its environment determine the observed motion. These nanomotor characteristics included its elemental composition, size and shape, surface properties, and mechanical flexibility, as well as the fuel and fluid media. They all contributed to the velocity and trajectory of the nanomotor. Notably, chemical propulsion enabling nanomotor mobility3Khatri N. Kapral R. Clustering of chemically propelled nanomotors in chemically active environments.Chaos. 2024; 34033103Crossref Scopus (3) Google Scholar,4Robertson B. Huang M.J. Chen J.X. Kapral R. Synthetic nanomotors: working together through chemistry.Acc. Chem. Res. 2018; 51: 2355-2364Crossref Scopus (50) Google Scholar was contingent upon the generation of asymmetry of chemical activity, first theoretically simulated for a Janus nanomotor with generic opposite catalytic and non-catalytic hemispheres. The model was first reduced to practice with Janus platinum-silica nanospheres in aqueous hydrogen peroxide (Figure 3). These early studies sought to define the physicochemical origin of the nanomotor motion, which included bubble propulsion, self-electrophoresis, self-diffusiophoresis, and self-thermophoresis mechanisms. This remains an experimental and theoretical challenge, an endeavor that continues unabated today. It applies to the dynamics of single as well as more recently swarms of nanomotors, the collective motion of which originates from cooperative chemical and physical interactions between motor surfaces. Notably the relevant simulations and theory were pioneered by a University of Toronto departmental faculty colleague,3Khatri N. Kapral R. Clustering of chemically propelled nanomotors in chemically active environments.Chaos. 2024; 34033103Crossref Scopus (3) Google Scholar,4Robertson B. Huang M.J. Chen J.X. Kapral R. Synthetic nanomotors: working together through chemistry.Acc. Chem. Res. 2018; 51: 2355-2364Crossref Scopus (50) Google Scholar inspired by the experimental work.1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar The prototype nanomotors in the two original reports were powered chemically,1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar soon thereafter photo- and bio-chemically. Later work showed they can also be propelled by physical or mechanical means, based on acoustic, electrical, magnetic, and optical forces. Non-equilibrium, self-generated concentration and temperature gradients created by chemical reactions in the surrounding medium were proven responsible for directed nanomotor locomotion and clustering phenomenon.3Khatri N. Kapral R. Clustering of chemically propelled nanomotors in chemically active environments.Chaos. 2024; 34033103Crossref Scopus (3) Google Scholar,4Robertson B. Huang M.J. Chen J.X. Kapral R. Synthetic nanomotors: working together through chemistry.Acc. Chem. Res. 2018; 51: 2355-2364Crossref Scopus (50) Google Scholar,5Fusi A.D. Li Y. Llopis-Lorente A. Patiño T. van Hest J.C.M. Abdelmohsen L.K.E.A. Achieving Control in Micro-/Nanomotor Mobility.Angew. Chem. Int. Ed. Engl. 2023; 62: e202214754Crossref Scopus (33) Google Scholar,6Yang Z. Wang L. Gao Z. Hao X. Luo M. Yu Z. Guan J. Ultrasmall enzyme-powered Janus nanomotor working in blood circulation system.ACS Nano. 2023; 17: 6023-6035Crossref Scopus (27) Google Scholar,7Fraire J.C. Guix M. Hortelao A.C. Ruiz-González N. Bakenecker A.C. Ramezani P. Hinnekens C. Sauvage F. De Smedt S.C. Braeckmans K. Sánchez S. Light-Triggered Mechanical Disruption of Extracellular Barriers by Swarms of Enzyme-Powered Nanomotors for Enhanced Delivery.ACS Nano. 2023; 17: 7180-7193Crossref Scopus (15) Google Scholar,8Yuan X. Suárez-García S. De Corato M. Muñoz A.C. Pagonabarraga I. Ruiz-Molina D. Villa K. Self-degradable Photoactive Micromotors for Inactivation of Resistant Bacteria.Adv. Optical Mater. 2024; 2303137Crossref Google Scholar,9Chen Q. Liang K. Self-Propelled Nanoswimmers in Biomedical Sensing.Advanced Sensor Research. 2023; 22300056Crossref Google Scholar,10Gompper G. Winkler R.G. Speck T. Solon A. Nardini C. Peruani F. Löwen H. Golestanian R. Kaupp U.B. Alvarez L. et al.The 2020 motile active matter roadmap.J. Phys. Condens. Matter. 2020; 32193001Crossref Scopus (314) Google Scholar It is exciting to observe the emergence of a field entitled "active matter," the chemistry and physics concepts and principles of which can provide an understanding of the fundamental physicochemical phenomena that underpin nanomotor propulsion as well as motility-induced emergent collective behavior of large numbers of nanomotors. Theoretical and experimental research in active matter is being applied to processes inherent to living systems. The goal is to explore a unique form of non-equilibrium matter comprised of numerous interacting components that individually consume energy to cooperatively create movement or mechanical stress over nanoscopic to macroscopic scales.10Gompper G. Winkler R.G. Speck T. Solon A. Nardini C. Peruani F. Löwen H. Golestanian R. Kaupp U.B. Alvarez L. et al.The 2020 motile active matter roadmap.J. Phys. Condens. Matter. 2020; 32193001Crossref Scopus (314) Google Scholar 20 years later, experimental and theoretical research on nanomotors is a lively field. Myriad nanomotor architectures, exemplified by rods, tubes, cones, helices, and gears, shaped by bottom-up synthetic and top-down fabrication methods, made from inorganic, polymer, and biological materials, powered by chemical and biochemical fuels, are beginning to yield innovative technologies in which the nanomotor performs a specific task. These include drug delivery, diagnostics, antibacterial activity, cell surgery, biosensing, and environmental remediation,3Khatri N. Kapral R. Clustering of chemically propelled nanomotors in chemically active environments.Chaos. 2024; 34033103Crossref Scopus (3) Google Scholar,4Robertson B. Huang M.J. Chen J.X. Kapral R. Synthetic nanomotors: working together through chemistry.Acc. Chem. Res. 2018; 51: 2355-2364Crossref Scopus (50) Google Scholar,5Fusi A.D. Li Y. Llopis-Lorente A. Patiño T. van Hest J.C.M. Abdelmohsen L.K.E.A. Achieving Control in Micro-/Nanomotor Mobility.Angew. Chem. Int. Ed. Engl. 2023; 62: e202214754Crossref Scopus (33) Google Scholar,6Yang Z. Wang L. Gao Z. Hao X. Luo M. Yu Z. Guan J. Ultrasmall enzyme-powered Janus nanomotor working in blood circulation system.ACS Nano. 2023; 17: 6023-6035Crossref Scopus (27) Google Scholar,7Fraire J.C. Guix M. Hortelao A.C. Ruiz-González N. Bakenecker A.C. Ramezani P. Hinnekens C. Sauvage F. De Smedt S.C. Braeckmans K. Sánchez S. Light-Triggered Mechanical Disruption of Extracellular Barriers by Swarms of Enzyme-Powered Nanomotors for Enhanced Delivery.ACS Nano. 2023; 17: 7180-7193Crossref Scopus (15) Google Scholar,8Yuan X. Suárez-García S. De Corato M. Muñoz A.C. Pagonabarraga I. Ruiz-Molina D. Villa K. Self-degradable Photoactive Micromotors for Inactivation of Resistant Bacteria.Adv. Optical Mater. 2024; 2303137Crossref Google Scholar,9Chen Q. Liang K. Self-Propelled Nanoswimmers in Biomedical Sensing.Advanced Sensor Research. 2023; 22300056Crossref Google Scholar,10Gompper G. Winkler R.G. Speck T. Solon A. Nardini C. Peruani F. Löwen H. Golestanian R. Kaupp U.B. Alvarez L. et al.The 2020 motile active matter roadmap.J. Phys. Condens. Matter. 2020; 32193001Crossref Scopus (314) Google Scholar amazing inventions sparked by the original curiosity-driven research.1Fournier-Bidoz S. Arsenault A.C. Manners I. Ozin G.A. Synthetic self-propelled nanorotors.Chem. Commun. 2005; : 441-443Crossref Scopus (494) Google Scholar,2Paxton W.F. Kistler K.C. Olmeda C.C. Sen A. St. Angelo S.K. Cao Y. Mallouk T.E. Lammert P.E. Crespi V.H. Catalytic nanomotors: autonomous movement of striped nanorods.J. Am. Chem. Soc. 2004; 126: 13424-13431Crossref PubMed Scopus (1732) Google Scholar While the record is clear that a new field emerged in terms of number of publications, it should be recognized that many such fields spur from more than one critical or canonical "Kuhnian-like" paper. Indeed, many researchers embark on similar fantastic voyages of their own, driven by curiosity and intellectual adventure. The author declares no competing interests.