Surface plasmons on isolated gold dimers can initiate intermolecular reactions of adsorbed p-nitrothiophenol. At the single molecule level when dimerization is not possible an intramolecular reaction can be observed. Experimental evidence indicates that plasmon-induced hot electrons provide the required activation energy

Deckert, Volker

Deckert-Gaudig, Tanja

Singh, Pushkar

Zhang, Zhenglong

English

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

This journal is©The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51, 3069--3072 | 3069Cite this:Chem. Commun., 2015,51, 3069Single molecule level plasmonic catalysis – adilution study of p-nitrothiophenol on golddimers†Zhenglong Zhang,a Tanja Deckert-Gaudig,a Pushkar Singha and Volker Deckert*abSurface plasmons on isolated gold dimers can initiate intermolecularreactions of adsorbed p-nitrothiophenol. At the single molecule levelwhen dimerization is not possible an intramolecular reaction can beobserved. Experimental evidence indicates that plasmon-inducedhot electrons provide the required activation energy.In the presence of surface plasmons molecular chemical reactionscan be investigated at the nanoscale, as it has been demonstratedfor a variety of catalyzed systems.1–19 Recently, plasmon catalyzeddimerization of p-nitrothiophenol (pNTP) to 4,4-dimercaptoazo-benzene (DMAB) has been realized using surface-enhancedRaman spectroscopy (SERS) and tip-enhanced Raman spectro-scopy (TERS).2–6 For any dimerization the distance between thereactants is decisive. If the distance between pNTP moleculesgets too large a dimerization reaction cannot occur. In this casethe question arises, whether surface plasmons can initiate afurther intramolecular reaction like dissociation or rearrangement.To address this issue, a detection sensitivity of signals from a few orultimately even a single educt molecule is required. In our experi-ments we employed SERS, which is widely used for noninvasivedetection, biological and chemical sensing.20–25 In SERS laserirradiation of silver or gold nanoparticles induces localized surfaceplasmon resonances at the nanostructured surface generatingenhanced electromagnetic fields.25 If molecules are brought inclosest vicinity to such nanoparticles (e.g. by adsorption) a largeenhancement of the Raman scattering signal can be observed.Using the junctions or gaps of nanoparticles, so-called ‘‘hot spots’’,very high enhancement factors (B108) can be achieved, providingsufficient signal enhancement for single molecule detection. There-fore such junctions are ideally suited for the intended reactivitydetection.23–27In previous studies, it was found that the plasmon catalyzedintermolecular reaction of pNTP (adsorbed on gold or silvernanoparticles) to DMAB can be controlled by laser intensity.3,4A higher laser intensity generates a stronger plasmon resonancewith higher kinetic energy yielding an increased reaction rate. Ifthe concentration, however, is low enough to prevent clusteringand self-assembly, the distance of pNTP molecules adsorbed onthe nanoparticles should become too large for an intermolecularreaction and no reaction should occur even at high laser powers.Then, any detected SERS signal can be related to single or only afew isolated and non-interacting separated molecules.In this communication, single gold nanoparticle dimers wereemployed for SERS detection of pNTP molecules. In the presentedexperiments a so far unknown behavior of pNTP could bemonitored, which strongly differs from previous reports asseparated pNTP molecules exclusively reacted to thiophenol(TP), and is a new finding in surface catalyzed experiments. Inaddition, a step-like signal intensity change during the processstrongly indicates that the dissociation reaction of pNTP to TPoccurs at or close to a single molecule level.As shown in Fig. 1, SERS active gold nanoparticle dimers (GDs)were prepared using a wet chemistry method.28 Further experi-mental details can be found in the ESI.† In order to achieve SERS ofseparated molecules, three different concentrations of aqueouspNTP solutions (5  107, 108 and 109 M) were mixed withFig. 1 (a) SEM image of gold nanoparticle dimers spin-coated onto asilicon wafer, (b) a single gold nanoparticle dimer on glass as used in theSERS measurement (inset: optical image of the selected nanoparticledimer, which is placed in the center of the laser focus).a Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena,Germany. E-mail: volker.deckert@ipht-jena.deb Institute of Physical Chemistry and Abbe Center of Photonics,Friedrich-Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany† Electronic supplementary information (ESI) available: Experimental details andFig. S1–S3. See DOI: 10.1039/c4cc09008jReceived 11th November 2014,Accepted 6th January 2015DOI: 10.1039/c4cc09008jwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 07 January 2015. Downloaded on 6/23/2022 12:51:43 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issue3070 | Chem. Commun., 2015, 51, 3069--3072 This journal is©The Royal Society of Chemistry 2015a gold nanoparticle dimer colloidal solution. The dimerizationof pNTP to DMAB is characterized by the disappearance of theRaman band at 1332 cm1 (nNO2), and the appearance of newRaman bands around 1140 (bC–H), 1387 (nNN + nCC + nC–N) and1432 (nNN + nCC + bC–H) cm1, which are related to the –NQN–unit of DMAB.2First, SERS experiments of the 107 M pNTP–GD mixturewere made with increasing laser intensity. Starting with 3 mW(see Fig. 2(a)) only the three main peaks of pNTP were detectedin the SERS spectrum. Increasing the laser power to 15 mW and30 mW, respectively, dimerization of pNTP to DMAB was verifiedby the afore-mentioned bands at 1140, 1387 and 1432 cm1(Fig. 2(b)). Fig. 2(b) and (c) indicate also that an increased laserpower yields a higher reaction rate. After continuous irradiationfor 5 min almost all pNTP molecules in the laser spot havereacted to DMAB (see dotted red curve in Fig. 2(c)).In the next experiment the 108 M pNTP–GD mixture wereinvestigated, all other experimental parameters except the laserpower as stated below remained unchanged. The correspondingSERS spectra are shown in Fig. 2(d)–(f). Similar to the previousexperiment, a dimerization of pNTP to DMAB occurred, however,higher laser intensities (150 mW) were needed to initiate thereaction. The results of both experiments demonstrate that with107 M and 108 M pNTP solutions the gold surface is stilldensely covered with pNTP molecules enabling a dimerization.For the final experiment a 109 M pNTP solution was used.The resulting SERS spectra are shown in Fig. 3(a)–(c). Duringthese measurements no dimerization of pNTP to DMAB wasobserved. Even at highest laser power (3 mW) for one hour (seeFig. 3(d)–(h) and for the full trace ESI,† Fig. S1) no DMAB signalscould be detected. It was concluded that the distance betweenpNTP molecules in the plasmonic hot-spot of the GD was indeedtoo large to allow intermolecular reactions under these condi-tions. That the reaction of pNTP to DMAB could not be initiatedeven after 1 hour of continuous irradiation, indicates that SERSsignals from only a few separated molecules in the hot-spot wereacquired. This finding is in accordance with the estimatedmolecular coverage n of pNTP on a single gold dimer that wasestimated as follows:n ¼ nMnGD¼ cMcGD¼ cMmAumGD NA  V(1)where cM and cGD are the molarity of pNTP and GD nanoparticles,mAu and mGD refer to the mass of Au and one single GD nano-particle, respectively. NA is Avogadro’s constant; V is the volume ofGDs solution in the preparation process. According to the SEMimages the diameter of gold nanoparticles is about 40 nm, and themolarity of the GDs solution is about 3.7 1010 M. Consequently,there are approximately 1000, 100 and 10 molecules absorbed on asingle nanoparticle dimer for the respective 107, 108 and 109 Msolutions. Since not all molecules can be absorbed on GDs in theexperiment, the actual number of molecules on average will beeven lower in the experiments. From the absence of dimerization inthe low concentration case we can also conclude that the moleculesdo not cluster and are statistically absorbed on the GD surface. Theaverage distance d between two molecules can be estimated asd ¼ Rffiffiffiffiffiffiffiffiffiffi8p=np: (2)R is the radius of the gold nanoparticle and n is the number ofmolecules absorbed on single nanoparticle dimer, which can becalculated by eqn (1). The calculated distances between two mole-cules of 107 and 108 M are B3 nm and 10 nm, respectively,which is either close enough for two pNTP molecules to dimerize toDMAB or, more likely, at such distances collisions happen oftenenough for a dimerization. But for the 109 M solution, the averagedistance between two molecules is B32 nm, which is obviouslypreventing the dimerization of two pNTP molecules. Thus, SERSspectra of separated pNTP and the absence of DMAB molecules atthe lowest concentration used here can be explained.Probably the most striking observation in the SERS experimentsof pNTP at 109 M was an intensity decrease of the dominating bandat 1332 cm1 (nNO2) over time. This peak disappeared after 10 min,simultaneously a new peak at 1017 cm1 was detected after 5 min,as shown in Fig. 3(d)–(h). Since the NO2 mode of pNTP disappeared,we hypothesize that the nitro group was cleaved from the benzeneFig. 2 (a)–(c) SERS spectra of 107 M pNTP excited at 3, 15 and 30 mWlaser power, respectively. (d)–(f) SERS spectra of 108 M pNTP excited at0.015, 0.15 and 1.5 mW laser power, respectively. The dotted red curves in(c) and (f) refer to spectra after continuous irradiation for 5 minutes.Fig. 3 (a)–(c) Laser intensity dependent SERS spectra excited at 0.03, 0.15and 3 mW laser power, respectively. (d)–(h) Time dependent SERS spectraat 3 mW laser at the sample.Communication ChemCommOpen Access Article. Published on 07 January 2015. Downloaded on 6/23/2022 12:51:43 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51, 3069--3072 | 3071ring in the hot-spot of the gold dimer resulting in the formationof thiophenol (TP). Comparing SERS spectra of neat TP (Fig. 4(b))with ‘‘pNTP on GDs’’ spectra from late stages (see Fig. 4(a) andalso Fig. 3(h)) it is obvious that the new bands at 995 (bCCC) and1017 (bCH) cm1 as well as the bands at 420 and 685 cm1 inthe low wavenumber region match those of TP strikinglywell (see also ESI,† Fig. S3). The necessary hydrogen atom forthe formation of TP was most probably abstracted from thesurrounding water layer that was ubiquitously present sincepNTP was adsorbed from aqueous solution and the experimentwas performed under ambient conditions.The experimental data demonstrate that a dissociation ofpNTP to TP proceeds under our special SERS conditions at aconcentration of 109 M (corresponding to a surface coverageof B10 pNTP molecules per dimer). These results clearly differfrom the dimerization reaction of pNTP to DMAB. The lightinduced dissociation of nitro substituted aromatic molecules isnot new but usually requires drastic conditions. For example,the –NO2 group in nitrobenzene and nitrophenol can becleaved using UV excitation.29–31 The required dissociationenergy of the C–N bond is B3.03 eV,29 which is higher than theenergy provided by the incident photon of 632.8 nm (1.96 eV). Thepower dependent experiments in Fig. 3 show no nonlinear increaseof the products, so nonlinear optical effects including two/multi-photon absorption can be ruled out. Hence, the activation energy istoo large to be simply overcome by the excitation laser. However,localized surface plasmons excited on silver or gold nanoparticlesurfaces can decay non-radiatively into hot electrons with an energybetween the vacuum energy and Fermi level (B5 eV).32–34 Hotelectrons can scatter into an excited state of the absorbedmolecules, triggering a chemical reaction by reducing the activa-tion energy.34 Applied to the present reaction our hypothesis isthat plasmon-induced hot electrons initiate –NO2 cleavage fromthe phenyl ring. A scheme of the proposed mechanism is shownin Fig. 5. Initially, hot electrons generated from plasmon decayon the nanoparticle surface, soon lose coherence and form anonequilibrium Fermi–Dirac type distribution.8 The hot electronsin the high energy level have sufficient energy to transfer to theexcited state of pNTP molecule, creating a transient negative ion ofpNTP. This negative ion ‘‘travels’’ to the excited state of TP andtransfers the electron back to the GD surface, where it returns tothe ground state of TP and dissociates.It should be noted that the simultaneous detection of pNTPand TP in Fig. 4(a) – spectrum at 6 min – actually points to thepresence of at least two molecules in the hot spot. Although theexperiment was performed at a very low concentration it cannotbe regarded as ‘‘single molecule’’ SERS in the classical sense.According to the time dependent SERS spectra, the peak intensitychange vs. time is shown for the pNTP band at 1332 cm1, and theTP band at 1017 cm1 in Fig. 6, respectively. It was found that threeclear steps can be discerned: first, some molecules of pNTP weredetected in the first 5 min of the measurement; after 5 min, half ofthe pNTP molecules reacted to TP; the remaining pNTP moleculesreacted after 8 min. After that no significant change occurred in theremaining experiment (60 min). Consequently, it can be assumedthat two distant, non-interacting molecules were detected in thehotspot of a single GD because both pNTP and TP were detectedbetween 5–8 min. Apparently, the two molecules were too far apartfrom each other for an intermolecular reaction, and instead anintramolecular reaction was initiated. Because of the step-likereaction process, the other possibilities of having 4-2-0, 6-3-0, or8-4-0 molecules synchronously reacting seem very unlikely. Theinvolvement of even more molecules can be ruled out, as at 10 timeshigher concentration dimerization already starts.In summary, we present the detection of a new plasmoncatalyzed reaction of pNTP near the single molecule level.A dissociation of pNTP to TP was detected at low concentrationwhere the distance between pNTP molecules adsorbed onsingle isolated gold dimer became too large for a dimerizationreaction. In combination with the results with higher concen-trations this further confirms a dimerization reaction of pNTPto DMAB. In accordance with previous reports we assume thatplasmon-induced hot electrons provide the activation energynecessary for such a dissociation of pNTP. The tracked step-likeFig. 4 (a) Time dependent SERS spectra of reacting pNTP (c = 109 M) at3 mW laser at 2, 6 and 30 min. (b) and (c) normal SERS spectra of TP andpNTP, respectively.Fig. 5 Proposed mechanism of plasmon (hot electron) catalyzed dissociationof pNTP to TP.ChemComm CommunicationOpen Access Article. Published on 07 January 2015. Downloaded on 6/23/2022 12:51:43 PM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online3072 | Chem. Commun., 2015, 51, 3069--3072 This journal is©The Royal Society of Chemistry 2015process of the dissociation of pNTP to TP strongly indicates asingle molecule behavior. The results demonstrate that irradia-tion of an adsorbate in a gold nanoparticle gap can decrease theactivation energy barrier such, that a reaction usually requiringUV light can be triggered in the visible.We gratefully acknowledge support from the Deutsche For-schungsgemeinschaft (DEP4TERS, FR 1348/19-1) and the ThüringerAufbau Bank (No. 2011SE9048). Z. 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Single molecule level plasmonic catalysis – a dilution study of p-nitrothiophenol on gold dimers

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Single molecule level plasmonic catalysis – a dilution study of p-nitrothiophenol on gold dimers

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