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• W2892556516 abstract "•An artificially synthesized molecular actuator for nanoparticles•Reversible manipulation of the nanoscale gap distance between two nanoparticles•Optical signal output of artificial molecular machines in action Artificial molecular muscles are arising as a species of man-made molecular materials with muscle-like contraction/extension capability under external stimuli. Some efforts have been made in macroscopic molecular muscles by integrating the actuation motions of molecular collections. However, to undertake nanoscale actuation tasks by artificial molecular muscles remains rarely explored, although its realization is intriguing and vital for the evolution of artificial molecular machines toward nanoactuators and even nanorobots in the future. In this work, we designed and artificially synthesized a muscle-like molecular actuator based on [c2]daisy chain rotaxane, which could be used as a linear actuator to reversibly manipulate the nanoscale distance between two gold nanoparticles. Our study offers new perspectives in the underexploited functions of artificial molecular machines, especially those working at the single- to few-molecule level. Muscle tissue performs crucial contraction/extension motions that generate mechanical force and work by consuming chemical energy. Inspired by this naturally created biomolecular machine, artificial molecular muscles are designed and synthesized to undertake linear actuation functions. However, most of these muscle-like actuators are performed at large ensembles, while to realize the nanoscale actuation at the single- to few-molecule level remains challenging. Herein, we developed an artificial muscle-like molecular actuator that can reversibly control the proximity of the attached nano-objects, gold nanoparticles, within the single-molecule length level by its stimuli-responsive muscle-like linear contraction/extension motion. The molecular actuation motion is accompanied by an optical signal output resulting from the plasmonic resonance properties of gold nanoparticles. Meanwhile, the thermal noise of the muscle-like molecular actuator can be overcome by integrating the optical signal over a sufficiently long period. Muscle tissue performs crucial contraction/extension motions that generate mechanical force and work by consuming chemical energy. Inspired by this naturally created biomolecular machine, artificial molecular muscles are designed and synthesized to undertake linear actuation functions. However, most of these muscle-like actuators are performed at large ensembles, while to realize the nanoscale actuation at the single- to few-molecule level remains challenging. Herein, we developed an artificial muscle-like molecular actuator that can reversibly control the proximity of the attached nano-objects, gold nanoparticles, within the single-molecule length level by its stimuli-responsive muscle-like linear contraction/extension motion. The molecular actuation motion is accompanied by an optical signal output resulting from the plasmonic resonance properties of gold nanoparticles. Meanwhile, the thermal noise of the muscle-like molecular actuator can be overcome by integrating the optical signal over a sufficiently long period. Artificial molecular machines have become an emerging theme of major interest over the past decades.1Sauvage J.P. From chemical topology to molecular machines (Nobel lecture).Angew. Chem. Int. Ed. 2017; 56: 11080-11093Crossref PubMed Scopus (459) Google Scholar, 2Stoddart J.F. Mechanically interlocked molecules (MIMs)—molecular shuttles, switches, and machines (Nobel lecture).Angew. Chem. Int. Ed. 2017; 56: 11094-11125Crossref PubMed Scopus (525) Google Scholar, 3Feringa B.L. The art of building small: from molecular switches to motors (Nobel lecture).Angew. Chem. Int. Ed. 2017; 56: 11060-11073Crossref PubMed Scopus (425) Google Scholar, 4Peplow M. March of the machines.Nature. 2015; 525: 18-21Crossref PubMed Scopus (59) Google Scholar, 5Erbas-Cakmak S. Leigh D.A. Mcternan C.T. Nussbaumer A.L. Artificial molecular machines.Chem. Rev. 2015; 115: 10081-10206Crossref PubMed Scopus (1237) Google Scholar, 6Abendroth J.M. Bushuyev O.S. Weiss P.S. Barret C.J. Controlling motion at the nanoscale: rise of the molecular machines.ACS Nano. 2015; 9: 7746-7768Crossref PubMed Scopus (325) Google Scholar, 7Balzani V. Venturi M. Credi A. Molecular Devices and Machines: Concepts and Perspectives for the Nanoworld. John Wiley & Sons, 2008Crossref Scopus (637) Google Scholar, 8Ma X. Tian H. Bright functional rotaxanes.Chem. Soc. Rev. 2010; 39: 70-80Crossref PubMed Google Scholar The inspiration mostly comes from the ubiquitous biomolecular machines existing in nature. In living systems, these biomolecular machines transduce chemical energy derived from adenosine triphosphate fuels to perform cellular functions.9Hirokawa N. Kinesin and dynein superfamily proteins and the mechanism of organelle transport.Science. 1998; 279: 519-526Crossref PubMed Scopus (1369) Google Scholar, 10Alberts B. The cell as a collection of protein machines: preparing the next generation of molecular biologists.Cell. 1998; 92: 291-294Abstract Full Text Full Text PDF PubMed Scopus (1007) Google Scholar A representative example involves the muscle tissue, a hierarchical organization of myosin and actin to undertake the vital mechanical motions in living organisms. Artificial molecular muscles, which can generate mechanical forces and then perform mechanical work by varying the distance between the two points under an external input of energy, have been designed and constructed to mimic this ability.11Jiménez M.C. Dietrich-Buchecker C. Sauvage J.P. Towards synthetic molecular muscles: contraction and stretching of a linear rotaxane dimer.Angew. Chem. Int. Ed. 2000; 39: 3284-3287Crossref PubMed Scopus (503) Google Scholar, 12Badjić J.D. Balzani V. Credi A. Silvi S. Stoddart J.F. A molecular elevator.Science. 2004; 303: 1845-1849Crossref PubMed Scopus (873) Google Scholar, 13Liu Y. Flood A.H. Bonvallet P.A. Vignon S.A. Northrop B.H. Tseng H.R. Jeppesen J.O. Huang T.J. Brough B. Baller M. et al.Linear artificial molecular muscles.J. Am. Chem. Soc. 2005; 127: 9745-9759Crossref PubMed Scopus (611) Google Scholar, 14Wu J. Leung K.C.F. Benítez D. Han J.Y. Cantrill S.J. Fang L. Stoddart J.F. An acid-base-controllable [c2]daisy chain.Angew. Chem. Int. Ed. 2008; 47: 7470-7474Crossref PubMed Scopus (174) Google Scholar, 15Juluri B.K. Kumar A.S. Liu Y. Ye T. Yang Y.W. Flood A.H. Fang L. Stoddart J.F. Weiss P.S. Huang T.J. A mechanical actuator driven electrochemically by artificial molecular muscles.ACS Nano. 2009; 3: 291-300Crossref PubMed Scopus (210) Google Scholar, 16Bruns C.J. Li J. Frasconi M. Schneebeli S.T. Iehl J. Jacquot de Rouville H.P. Stupp S.I. Voth G.A. Stoddart J.F. An electrochemically and thermally switchable donor-acceptor [c2]daisy chain rotaxane.Angew. Chem. Int. Ed. 2014; 53: 1953-1958Crossref PubMed Scopus (56) Google Scholar, 17Bruns C.J. Stoddart J.F. Rotaxane-based molecular muscles.Acc. Chem. Res. 2014; 47: 2186-2199Crossref PubMed Scopus (400) Google Scholar, 18Coutrot F. Romuald C. Busseron E.A. New pH-switchable dimannosyl [c2] daisy chain molecular machine.Org. Lett. 2008; 10: 3741-3744Crossref PubMed Scopus (177) Google Scholar, 19Chuang C.J. Li W.S. Lai C.C. Liu Y.H. Peng S.M. Chao I. Chiu S.H. A molecular cage-based [2]rotaxane that behaves as a molecular muscle.Org. Lett. 2009; 11: 385-388Crossref PubMed Scopus (87) Google Scholar, 20Chang J.C. Tseng S.H. Lai C.C. Liu Y.H. Peng S.M. Chiu S.H. Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles.Nat. Chem. 2017; 9: 128-134Crossref Google Scholar Daisy chain rotaxanes, a species of mechanically interlocked molecules, are considered as excellent candidates for functional artificial molecular muscles because of their unique ability of generating muscle-like contraction/extension motion at the expense of external stimuli, such as light,21Dawson R.E. Lincoln S.F. Easton C.J. The foundation of a light driven molecular muscle based on stilbene and α-cyclodextrin.Chem. Commun. 2008; : 3980-3982Crossref PubMed Scopus (131) Google Scholar acid/base,14Wu J. Leung K.C.F. Benítez D. Han J.Y. Cantrill S.J. Fang L. Stoddart J.F. An acid-base-controllable [c2]daisy chain.Angew. Chem. Int. Ed. 2008; 47: 7470-7474Crossref PubMed Scopus (174) Google Scholar, 18Coutrot F. Romuald C. Busseron E.A. New pH-switchable dimannosyl [c2] daisy chain molecular machine.Org. Lett. 2008; 10: 3741-3744Crossref PubMed Scopus (177) Google Scholar redox,13Liu Y. Flood A.H. Bonvallet P.A. Vignon S.A. Northrop B.H. Tseng H.R. Jeppesen J.O. Huang T.J. Brough B. Baller M. et al.Linear artificial molecular muscles.J. Am. Chem. Soc. 2005; 127: 9745-9759Crossref PubMed Scopus (611) Google Scholar or metal ion.11Jiménez M.C. Dietrich-Buchecker C. Sauvage J.P. Towards synthetic molecular muscles: contraction and stretching of a linear rotaxane dimer.Angew. Chem. Int. Ed. 2000; 39: 3284-3287Crossref PubMed Scopus (503) Google Scholar, 20Chang J.C. Tseng S.H. Lai C.C. Liu Y.H. Peng S.M. Chiu S.H. Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles.Nat. Chem. 2017; 9: 128-134Crossref Google Scholar Sauvage et al. created the pioneering [c2]daisy chain rotaxane as a molecular “muscle” that can undertake a ∼2-nm contractile length variation driven by metal ion exchange.11Jiménez M.C. Dietrich-Buchecker C. Sauvage J.P. Towards synthetic molecular muscles: contraction and stretching of a linear rotaxane dimer.Angew. Chem. Int. Ed. 2000; 39: 3284-3287Crossref PubMed Scopus (503) Google Scholar Stoddart et al. and Grubbs et al. independently reported the covalent oligopolymerization of [c2]daisy chain rotaxanes and their pH-responsive operations.22Fang L. Hmadeh M. Wu J. Olson M.A. Spruell J.M. Trabolsi A. Yang Y.W. Elhabiri M. Albrecht-Gary A.-M. Stoddart J.F. Acid-base actuation of [c2]daisy chains.J. Am. Chem. Soc. 2009; 131: 7126-7134Crossref PubMed Scopus (169) Google Scholar, 23Clark P.G. Day M.W. Grubbs R.H. Switching and extension of a [c2]daisy chain dimer polymer.J. Am. Chem. Soc. 2009; 131: 13631-13633Crossref PubMed Scopus (116) Google Scholar Giuseppone et al. developed a supramolecular polymerization strategy to realize the integrated operation of thousands of [c2]daisy chain rotaxanes as muscle-like bundles,24Du G. Moulin E. Jouault N. Buhler E. Giuseppone N. Muscle-like supramolecular polymers: integrated motion from thousands of molecular machines.Angew. Chem. Int. Ed. 2012; 51: 12504-12508Crossref PubMed Scopus (190) Google Scholar, 25Goujon A. Du G. Moulin E. Fuks G. Maaloum M. Buhler E. Giuseppone N. Hierarchical self-assembly of supramolecular muscle-like fibers.Angew. Chem. Int. Ed. 2016; 55: 703-707Crossref PubMed Scopus (77) Google Scholar and also the actuation of the sol-gel transition of supramolecular polymers.26Goujon A. Mariani G. Lang T. Moulin E. Rawiso M. Buhler E. Giuseppone N. Controlled sol-gel transitions by actuating molecular machine based supramolecular polymers.J. Am. Chem. Soc. 2017; 139: 4923-4928Crossref PubMed Scopus (93) Google Scholar Harada et al. incorporated light-responsive [c2]daisy chain rotaxanes into a gel network that allowed the gel to perform light-powered mechanical actuation work.27Iwaso K. Takashima Y. Harada A. Fast response dry-type artificial molecular muscles with [c2]daisy chains.Nat. Chem. 2016; 8: 625-632Crossref PubMed Scopus (273) Google Scholar These efforts successfully evolved artificial molecular muscles from microscopic molecular scale to macroscopic polymer scale, showing the promising applications as smart polymeric materials. On the other hand, it is also a worthwhile challenge to undertake the nanoscale actuation tasks by artificial molecular machines28Muraoka T. Kinbara K. Aida T. Mechanical twisting of a guest by a photoresponsive host.Nature. 2006; 440: 512-515Crossref PubMed Scopus (585) Google Scholar, 29Zhao D. van Leeuwen T. Cheng J. Feringa B.L. Dynamic control of chirality and self-assembly of double-stranded helicates with light.Nat. Chem. 2017; 9: 250-256Crossref PubMed Scopus (142) Google Scholar owing to their unique capability to generate controlled mechanical motion on the molecular scale. It has been an extremely attractive but challenging hypothesis to manipulate nanostructures by muscle-like molecular actuator, such as operating as addressable linkages between coupled nanoparticles (NPs) at nanoscale.30Coskun A. Banaszak M. Astumian R.D. Stoddart J.F. Grzybowski B.A. Great expectations: can artificial molecular machines deliver on their promise?.Chem. Soc. Rev. 2012; 41: 19-30Crossref PubMed Scopus (693) Google Scholar However, to the best of our knowledge, this hypothesis remains unrealized, although its realization would be a major promotion for the bright functionalization and application of artificial molecular machines in nanotechnology and molecular devices. The challenges of this topic originate from the following fundamental questions: (1) how to attach these muscle-like molecular actuators between the coupled objects at nanoscale; (2) how to overcome the thermal noise effect from the individual molecular actuator; and (3) whether or not the reversibility of the linked molecular actuator could be retained at the interface. Herein, we report the successful construction of a muscle-like molecular actuator to reversibly modulate the gap distance between two linked gold (Au) NPs (Figure 1). This molecular muscle actuator enables the equipped Au NPs with stimuli-responsive capability by the acid/base-powered molecular transformation. As a result, the distance-dependent plasmonic resonance optical signal of the Au NPs can be used to reflect the nanoscale molecular actuation motion of few-molecule scale and overcome the thermal noise of the bistable molecular actuator by integrating the optical signal in a sufficiently long period. We believe this research not only provides a reliable approach to addressable nano-objects but also represents a significant step in the functional output of artificial molecular machines at the single- to few-molecule level. A carefully designed acid/base-driven muscle-like molecular actuator, 1·2H, with tunable molecular lengths, is applied in this research (Figure 2). It consists of an interpenetrated dibenzo-24-crown-8 (DB24C8)-based [c2]daisy chain structure in the middle of the linear backbone, while two stoppers are positioned at each end to form a mechanical bond. Both stoppers bear three pendant thiol groups for the covalent anchor to the surface of Au NPs by the widely used Au-S bonding.31Klajn R. Stoddart J.F. Grzybowski B.A. Nanoparticles functionalised with reversible molecular and supramolecular switches.Chem. Soc. Rev. 2010; 39: 2203-2237Crossref PubMed Scopus (451) Google Scholar It should be noted that two types of recognition sites for macrocycle DB24C8 are introduced in the [c2]daisy chain rotaxane 1·2H: benzylalkylammonium (BAA) sites as the primary recognition site and N-methyltriazolium (MTA) sites as the secondary recognition site, enabling the bistability of the [c2]daisy chain rotaxanes controlled by external acid/base stimuli. To synthesize this elaborate molecule, we prepared compound A4 (the synthetic route can be found in Supplemental Information) as the monomer to form interpenetrated [c2]daisy chain pseudo[2]rotaxane structure in apolar solvent. The formed [c2]daisy chain pseudo[c2]rotaxane and compound B6 were used to synthesize [c2]daisy chain 2·2H by a typical threading-followed-by-stopping strategy32Fu X. Zhang Q. Rao S.J. Qu D.H. Tian H. One-pot synthesis of a [c2]daisy-chain-containing hetero[4]rotaxane via a self-sorting strategy.Chem. Sci. 2016; 7: 1696-1701Crossref PubMed Google Scholar with a yield of 82.4% (Figure 2). The terminal olefinic bonds on the two stoppers of [c2]daisy chain 2·2H provided handles for reaction with thiol groups. Hence, a photoinitiated thiol-ene click reaction between [c2]daisy chain 2·2H and excess pentaerythritol tetrakis(3-mercaptopropionate) afforded the target [c2]daisy chain 1·2H with a yield of 79.2%. The molecular structure of [c2]daisy chain 1·2H was confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray ionization mass spectroscopy (HR-ESI-MS) (detailed synthetic procedures and characterization are shown in Supplemental Experimental Procedures and Supplemental Information). The HR-ESI-MS of [c2]daisy chain 1·2H showed a peak at m/z 1137.8558, corresponding to the species having lost three PF6− counterions ([M − 3PF6−]2+), and a peak at m/z 817.1067, corresponding to the species having lost four counterions ([M − 4PF6−]2+). Meanwhile, we have designed and synthesized a reference compound, [c2]daisy chain 3 (Figure 2; The synthesis route and structural characterization can be found in Supplemental Information), which bears similar [c2]daisy chain structure with the muscle-like actuator 1·2H but no responsiveness to acid/base stimuli because of the acetylated BAA sites. The contraction/extension behavior of the [c2]daisy chain 1·2H was further investigated in solution. As previously reported in other [c2]daisy chain rotaxanes, the stronger noncovalent interaction between electron-poor BAA sites and electron-rich DB24C8 made the [c2]daisy chain 1·2H in an extended state with a calculated length of 11.3 nm (see Figure S1 for the optimized geometry). Upon addition of base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), deprotonation of BAA sites occurred and the DB24C8 macrocycles moved to the secondary MTA recognition sites. This process resulted in the contraction of 1·2H to the shorter state as [c2]daisy chain 1, bearing a calculated length of 8.5 nm (see Figure S1 for the optimized geometry). 1H NMR spectroscopy in Figures 3A and 3B confirmed this event by (1) the downfield shift (Δδ = +0.85 ppm) of the MTA proton signal Hn (1); (2) the upfield shift (Δδ = −0.59 ppm) of the MTA methyl proton signal Ho; (3) the broadening and downfield shifts (Δδ = +0.15 ppm) of proton signal Hp resting near by the MTA sites. Meanwhile, the inverse re-extension process was driven by the addition of two equivalents of trifluoroacetic acid (TFA) (Figure 3C), indicating the good reversibility of the [c2]daisy chain 1·2H working as a muscle-like molecular actuator in solution phase. We then focused on how to construct the proposed hybrid Au NP dimer linked with the synthesized molecular actuator 1·2H. We firstly tested the crosslinking capability of the [c2]daisy chain 1·2H for the homogeneous Au NP aqueous solution. 60 nm Au NP aqueous solution was selected with the protection of cetyltrimethyl ammonium bromide (CTAB) ligand. In a typical experiment, 20 μL of highly diluted acetonitrile solution (5 × 10−5 M) of molecular actuator 1·2H (extended) and 1 (contracted) was added into the quickly stirred aqueous solution of Au NPs. The ligand exchange reaction occurred immediately and caused the red solution to turn purple. The observed result by UV-visible (UV-vis) experiments (Figure S2) shows that the addition of both [c2]daisy chain 1·2H and 1 would lead to a new shoulder peak at around 650 nm, which is attributed to the coupled Au NPs connected with the [c2]daisy chain linkages. The larger red-shift of the peaks of coupled Au NPs is explained as the result of the shorter distance between the Au NPs linked by contracted 1.33Kundu P.K. Samanta D. Leizrowice R. Margulis B. Zhao H. Börner M. Udayabhaskararao T. Manna D. Klajn R. Light-controlled self-assembly of non-photoresponsive nanoparticles.Nat. Chem. 2015; 7: 646-652Crossref PubMed Scopus (347) Google Scholar The absorbance peaks at around 550 nm are attributed to the uncoupled Au NPs because of the presence of unlinked Au monomers. Hence, the crosslinking capability of the thiol-terminated molecular actuator at two different states, 1·2H and 1, is confirmed. However, the crosslinked Au NPs could not be successfully switched by external acid/base in solution because of the sensitivity of Au NPs aqueous solution to external acid/base. To overcome the sensitivity of Au NPs in solution, we transferred the Au NPs from solution to a surface. Indium tin oxide (ITO) glass substrate is well known to physically adsorb and fix monolayers of Au NPs on their surface and was selected for its optical transparency, chemical stability, and low surface roughness.34Li Y. Jing C. Zhang L. Long Y.T. Resonance scattering particles as biological nanosensors in vitro and in vivo.Chem. Soc. Rev. 2012; 41: 632-642Crossref PubMed Google Scholar The adsorption density of the Au NPs was positively correlated with the immersing time of the ITO glass substrate in the Au NP solution. Meanwhile, dark-field microscopy (DFM) equipped with a monochromator was used to collect the plasmonic resonance scattering spectroscopy (PRSS) spectra of the Au NPs on the surface of ITO glass substrate (Figure 4A). The PRSS spectrum of every single particle can be collected by the manual movement of the target particles entering the range of the fixed slit and the following integration of the scattering intensity of the particle in a given integration time (Figure S3). The PRSS of the detected Au NPs supported a significant parameter, λm, which represents the corresponding wavelength of the maximum scattering intensity of the peak in PRSS, for the plasmonic resonance circumstance. When two or multiple plasmonic NPs are adjoined at an appropriate distance, their plasmonic oscillations couple, leading to the highly sensitive distance-related variation in the λm of PRSS. The distinguishability corresponding to the λm is evaluated as ±0.4 nm (Figure S4), which enables this platform to be a reliable tool for evaluation of the interparticle distance.35Sönnichsen C. Reinhard B. Liphardt J. Alivisatos A.P. A molecular ruler based on plasmon coupling of single gold and silver nanoparticles.Nat. Biotechnol. 2005; 23: 741-745Crossref PubMed Scopus (1343) Google Scholar, 36Manna D. Udayabhaskararao T. Zhao H. Klajn R. Orthogonal light-induced self-assembly of nanoparticles using differently substituted azobenzenes.Angew. Chem. Int. Ed. 2015; 54: 12394-12397Crossref PubMed Scopus (111) Google Scholar, 37Klajn R. Wesson P.J. Bishop K.J.M. Grzybowski B.A. Writing self-erasing images using metastable nanoparticle “inks”.Angew. Chem. Int. Ed. 2009; 48: 7035-7039Crossref PubMed Scopus (298) Google Scholar, 38Zhao H. Sen S. Udayabhaskararao T. Sawczyk M. Kučanda K. Manna D. Kundu P.K. Lee J.W. Král P. Klajn R. Reversible trapping and reaction acceleration within dynamically self-assembling nanoflasks.Nat. Nanotechnol. 2016; 11: 82-88Crossref PubMed Scopus (242) Google Scholar In our system, two diameters of Au NPs protected with citrate, 60 nm and 15 nm, were selected and employed (the synthesis procedure of these Au NPs can be found in Supplemental Experimental Procedures). UV-vis spectra and transmission electron microscopy (TEM) images confirmed the good uniformity of the synthesized Au NPs (Figures S5–S8), which was important for the low noise and high sensitivity of the detected signal, the λm in PRSS. Notably, only 60-nm Au NPs could be observed in DFM images as well-dispersed bright-green dots because their scattering peaks were located in the visible region (Figures 4B and S9), while the 15-nm Au NPs were invisible in DFM images. This fact made the PRSS of the two diameters of Au NPs to be located individually, providing much convenience for the study of the coupled Au NPs by DFM and PRSS because it only requires the detection of λm in PRSS of the large NP, which was synchronously affected by the interparticle distance. The construction of the coupled Au NP dimer with [c2]daisy chain 1·2H includes a three-step route (as shown in Figure 4A): In step I, the precleaned ITO substrate was dipped in the highly diluted aqueous solution of 60-nm Au NPs for 10 min, when physical adsorption of the 60-nm Au NPs would occur spontaneously to afford an ITO substrate loaded with a well-dispersed 60-nm Au NP. The characteristic λm of 60-nm Au NPs on substrate could be detected as 553.3 nm (Figure 4C). Step II involved the careful dropwise addition of 200 μL of 1·2H solution (acetonitrile, 10−4 M) on the fixed ITO substrate loaded with 60-nm Au NPs. After standing for 10 min at room temperature, the droplet of 1·2H solution was carefully removed and the substrate surface was washed by acetonitrile three times to remove the nonimmobilized 1·2H molecules. The λm of molecular muscle 1·2H modified 60-nm Au NPs was then measured as 555.2 nm, showing a slight red-shift (Δλ = +1.9 nm) compared with that of step I (Figure 4C), which indicated the successful immobilization of 1·2H molecules on the 60-nm Au NPs by covalent Au-S bond. Step III involved the careful dropwise addition of 200 μL of aqueous solution of 15-nm Au NPs on the fixed substrate obtained in step II. The reaction time was set as 10 min and the solution of 15-nm Au NPs was removed. It should be noted that the reaction time makes a great difference in the formed proportion of expected Au NP dimers: a prolonged reaction time (over 10 min) would lead to further red-shift over 10 nm of the optical signal, which is attributed to the undesired oligomers. The as-obtained substrate was then detected under DFM and measured by PRSS. As a result, the λm was shifted to 563.3 nm (Δλ = +8.1 nm) (Figure 4C), which was attributed to the plasmonic scattering coupling effect after the successful connection of the Au NP dimers by [c2]daisy chain 1·2H. It should be noted that all of these manipulations were taken on the fixed substrate, meaning that the location of the Au NPs on the view of DFM images was unchanged, and hence the collected PRSS data were strictly in situ and at a single-NP scale. We also investigated the statistical distribution of the modification proportion of steps II and III (Figure 4D). In step II, after immobilization of [c2]daisy chain 1·2H, the λm of the 60-nm Au NPs showed a red-shift at 2–3 nm (70% particles). The high modification proportion was attributed to the richness in thiol groups of [c2]daisy chain 1·2H. On the other hand, step III involved the connection between 15-nm Au NPs and the thiol groups on the other side of [c2]daisy chain 1·2H that had been anchored on the 60-nm Au NPs by Au-S bonding. Hence, considering the remarkably decreasing thiol amounts in the solution/interface phase, it was reasonable that a lower modification proportion was observed in step III: the major peak (Δλ = 6–8 nm) held a proportion of 40% in all the tested particles. Meanwhile, the large-area statistical data on the λm of the 60-nm Au NPs before and after modification of 15-nm Au NPs also showed the proportion of unbound 60-nm Au NPs to Au NP dimers to be about 1:1 (Figure S10), indicating the efficient modification yield by the step-by-step process in Figure 4A. Atomic force microscopy (AFM) images of the glass substrate showed the typical formed Au NP dimers after treatment by the three steps (Figures 4D [inset] and S11). Combining these data with the previous in situ experimental results in Figure 4C, it can be concluded that the observed Δλ at 6–8 nm was attributed to the formation of coupled Au NP dimers. Hence, we can obtain 40% particles as dimers by controlling the reaction time as 10 min at step III. Despite the presence of unreacted monomers and undesired oligomers, the dimers can be distinguished by the peak shifts of scattering spectra, and the distinguished dimers were used to undertake the following acid/base actuation in situ experiments at the single-particle level. After the successful fabrication and characterization of the hybrid Au NP dimers connected by molecular actuator 1·2H, we focused on the hypothesized actuation of the coupled Au NPs by the molecular actuator: the connected molecular actuator 1·2H was expected to undertake muscle-like contraction/extension motion between the nanogap of the two Au NPs under external base/acid stimuli. The predominant issue to be considered is the optical signal collection of the individual NP dimers, which has been well solved and demonstrated by the platforms of DFM and PRSS. On the other hand, another major challenge is how to overcome, rather than compete with, the natural thermal noise effect of individual molecular muscle. The thermal noise means that the molecular actuator between the two Au NPs was a dynamic state instead of an invariant state because of the natural thermal molecular motion. Hence, the contracted/extended molecular actuator would change its conformation repeatedly, resulting in the plasmonic resonance signal noise. In previously reported works,22Fang L. Hmadeh M. Wu J. Olson M.A. Spruell J.M. Trabolsi A. Yang Y.W. Elhabiri M. Albrecht-Gary A.-M. Stoddart J.F. Acid-base actuation of [c2]daisy chains.J. Am. Chem. Soc. 2009; 131: 7126-7134Crossref PubMed Scopus (169) Google Scholar, 23Clark P.G. Day M.W. Grubbs R.H. Switching and extension of a [c2]daisy chain dimer polymer.J. Am. Chem. Soc. 2009; 131: 13631-13633Crossref PubMed Scopus (116) Google Scholar, 24Du G. Moulin E. Jouault N. Buhler E. Giuseppone N. Muscle-like supramolecular polymers: integrated motion from thousands of molecular machines.Angew. Chem. Int. Ed. 2012; 51: 12504-12508Crossref PubMed Scopus (190) Google Scholar, 25Goujon A. Du G. Moulin E. Fuks G. Maaloum M. Buhler E. Giuseppone N. Hierarchical self-assembly of supramolecular muscle-like fibers.Angew. Chem. Int. Ed. 2016; 55: 703-707Crossref PubMed Scopus (77) Google Scholar, 26Goujon A. Mariani G. Lang T. Moulin E. Rawiso M. Buhler E. Giuseppone N. Controlled sol-gel transitions by actuating molecular machine based supramolecular polymers.J. Am. Chem. Soc. 2017; 139: 4923-4928Crossref PubMed Scopus (93) Go" @default.
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• W2892556516 date "2018-11-01" @default.
• W2892556516 modified "2023-10-18" @default.
• W2892556516 title "Muscle-like Artificial Molecular Actuators for Nanoparticles" @default.
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