Letter pubs.acs.org/JPCL
Identifying Enclosed Chemical Reaction and Dynamics at the Molecular Level Using Shell-Isolated Miniaturized Plasmonic Liquid Marble Xuemei Han,† Hiang Kwee Lee,†,‡ Yih Hong Lee,† Wei Hao,§ Yejing Liu,† In Yee Phang,‡ Shuzhou Li,§ and Xing Yi Ling*,† †
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ‡ Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634 § School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 S Supporting Information *
ABSTRACT: Current microscale tracking of chemical kinetics is limited to destructive ex situ methods. Here we utilize Ag nanocube-based plasmonic liquid marble (PLM) microreactor for in situ molecular-level identification of reaction dynamics. We exploit the ultrasensitive surface-enhanced Raman scattering (SERS) capability imparted by the plasmonic shell to unravel the mechanism and kinetics of aryl-diazonium surface grafting reaction in situ, using just a 2-μL reaction droplet. This reaction is a robust approach to generate covalently functionalized metallic surfaces, yet its kinetics remain unknown to date. Experiments and simulations jointly uncover a two-step sequential grafting process. An initial Langmuir chemisorption of sulfonicbenzene diazonium (dSB) salt onto Ag surfaces forms an intermediate sulfonicbenzene monolayer (Ag−SB), followed by subsequent autocatalytic multilayer growth of Ag−SB3. Kinetic rate constants reveal 19fold faster chemisorption than multilayer growth. Our ability to precisely decipher molecular-level reaction dynamics creates opportunities to develop more efficient processes in synthetic chemistry and nanotechnology.
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covalently functionalized metallic surfaces for important industrial applications,12−14 the dynamics of this highly reactive grafting currently remain unresolved.15−18 From our in situ SERS reaction tracking, we unveil a hitherto undocumented two-step grafting reaction and their respective kinetic rate constants. Experimental and simulation evidence reveal an initial Langmuir chemisorption of aryl-diazonium onto Ag to form a surface intermediate, followed by a subsequent autocatalytic multilayer growth. PLM functions as a confined microreactor for the in situ SERS monitoring of dynamic molecular events occurring during sulfonicbenzene diazonium (dSB) grafting of sulfonicbenzene species onto Ag surfaces. PLM is easily formed by using Ag nanocube building blocks to encapsulate a 2-μL aqueous reaction precursor droplet (nanocube edge length = 115 ± 6 nm; Figure 1a, b, Figure S1). The as-fabricated PLM comprises approximately four layers of Ag nanocubes and exhibits a clean featureless SERS background (Figure 1e, Supporting Information 2 and 3). Extensive 3D plasmon coupling among the Ag nanocubes gives rise to an analytical
iquid marbles are robust particle-encapsulated microreactors widely applied for the isolation of microlitersized (bio)chemical reactions arising from their ease of fabrication and manipulation.1−3 In particular, synergizing plasmonic liquid marbles (PLMs) assembled using metallic nanoparticles with ultrasensitive surface-enhanced Raman scattering (SERS) spectroscopy creates excellent microtestbeds to investigate the kinetics and pathways of confined reactions in their native environments without external interference, especially for hazardous or explosive reactions.4,5 Unraveling these molecular-level events, particularly for aqueous-based reactions, will enable us to rationally design and control synthetic approaches for the ultimate goal of improving reaction efficiency.6−8 In comparison with other reactiontracking platforms, PLM can potentially overcome current limitations that require large reaction volumes or sophisticated characterization tools without introducing platform instability.9−11 Here we combine PLM microreactor with vibrational spectroscopy-based SERS to elucidate molecular-level kinetic pathway of aryl-diazonium grafting reaction onto silver (Ag) surfaces using just 2 μL of reaction solution, and quantify its intrinsic kinetics without disturbing the reaction progress. While aryl-diazonium grafting is a powerful approach to form © XXXX American Chemical Society
Received: March 2, 2016 Accepted: April 6, 2016
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DOI: 10.1021/acs.jpclett.6b00501 J. Phys. Chem. Lett. 2016, 7, 1501−1506
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Assembling a PLM microreactor by using Ag nanocube building blocks to encapsulate a 2 μL aqueous reaction droplet. The reaction droplet contains sulfanilic acid (SA), NaNO2, and HCl for the generation of sulfonicbenzene diazonium (dSB) salt in situ. (b) Digital image of the as-fabricated PLM. (c) Schematic depicting in situ SERS monitoring of surface grafting reaction using PLM with 2.5 mM dSB. (d,e) Spectral evolution over the course of surface grafting. Time-resolved color-coded intensity map (d) and corresponding SERS spectra (e) show distinct appearance and diminishing of the 983 and 1555 cm−1 bands (blue, e) up to 2400 s. As these bands disappear, new bands at 1072 and 1572 cm−1 begin to appear after 360 s (orange, e).
SERS enhancement factor of >108 with high signal reproducibility (Supporting Information 2 and 4). These characteristics collectively enable us to utilize PLM as an efficient SERS platform to monitor the kinetics of short-lived chemical species formation and decomposition in our model reaction. Real-time and noninterfering SERS monitoring of the surface grafting reaction is performed directly in its native environment using 2.5 mM dSB salt as the reactant (Figure 1c). Owing to its high reactivity, dSB salt is generated freshly in situ within the PLM by premixing sulfanilic acid (SA), sodium nitrite (NaNO2), and hydrochloric acid (HCl) in the precursor droplet (Figure S5).19 A clear evolution of various vibrational fingerprints is evident in the time-dependent SERS spectra over the course of 1 h (Figure 1d,e). The initially featureless spectrum in the absence of dSB (background) gives rise to two distinct SERS bands at 983 and 1555 cm−1 at 150 s. Both SERS bands continue to increase in intensity until 360 s, followed by a steady signal reduction to below the detection limit of our system at 2400 s. The onset of signal reduction coincides with a concurrent emergence of two new distinct SERS bands at 1072 and 1572 cm−1 at t > 360 s. These new vibrational modes exhibit a gradual intensity increase before ultimately dominating the SERS spectrum at t > 2400 s. The SERS spectral changes are indicative of a multistep surface reaction. This is supported by the stable environment provided by our PLM with consistent SERS enhancement (40-fold enhanced SERS intensity with distinct SERS features (signalto-noise ratio >3; Figure 4c, Supporting Information 22). The strong and reproducible SERS signals are critical in enabling temporal investigation of rapid reactions or reaction species, especially small molecules with low SERS cross sections. We quantitatively uncover a two-step pathway for the covalent grafting of aryl-diazonium species onto metallic surfaces through the use of PLM coupled with ultrasensitive and real-time SERS reaction tracking. The spectral evolution can be accurately modeled using Langmuir chemisorption and logistic growth models for the first and second steps, respectively. By solving the respective intrinsic rate constants experimentally, we find a 19-fold faster Langmuir chemisorption than the subsequent multilayer growth. This kinetic
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X.Y.L., X.H., and Y.H.L. acknowledge the support from National Research Foundation, Singapore (NRF-NRFF201204), Nanyang Technological University’s start-up grant (M4080758). H.K.L. acknowledges A*STAR Graduate Scholarship support from A*STAR Singapore. S.L and W.H. acknowledge the support from MOE Tier 2 (ACR12/12).
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REFERENCES
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