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2018
Gold nanoparticle decorated Au-Ag
alloy tubes: A bifunctional substrate for label-free and in situ surface-enhanced
Raman scattering based reaction monitoring
Ankudze, Bright
Elsevier BV
Tieteelliset aikakauslehtiartikkelit
© Authors
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.apsusc.2018.05.041
https://erepo.uef.fi/handle/123456789/6712
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Full Length Article
Gold nanoparticle decorated Au-Ag alloy tubes: A bifunctional substrate for label-free and in situ surface-enhanced Raman scattering based reaction monitoring
Bright Ankudze, Tuula T. Pakkanen
⇑Department of Chemistry, University of Eastern Finland, P.O Box 111, FI-80101 Joensuu, Finland
a r t i c l e i n f o
Article history:
Received 1 February 2018 Revised 16 April 2018 Accepted 7 May 2018 Available online 9 May 2018
Keywords:
Gold nanoparticles Au-Ag alloy
Surface-enhanced Raman scattering Reaction monitoring
a b s t r a c t
A quantitative monitoring of a heterogeneously catalyzed reaction based on surface-enhanced Raman scattering (SERS) requires fabrication of a bifunctional substrate with exposed SERS and catalytic sites.
Fabrication of such dual-functional substrates is challenging, owing to the different size limits of metal nanoparticles set by both SERS and catalysis. Larger-sized nanoparticles are suitable for SERS, whereas catalysis requires smaller particles, hence, the integration of both features into a single nanostructured material can be demanding. In this study, we report access to simple fabrication of a bifunctional nanos- tructure based on gold nanoparticle decorated Au-Ag tubes (Au@Au-AgTs) having both SERS and catalytic features. A facile approach using polymer linkages facilitated the immobilization of catalytically active gold nanoparticles on Au-AgTs. The decoration process involves a simple mixing of polyethylenimine capped gold nanoparticles with Au-Ag alloy tubes, and requires no harsh conditions or complex synthetic procedures. The synergistic effect of gold and silver metals enabled a sensitive SERS performance of Au@Au-AgTs, and the small gold nanoparticles functioned as active catalytic sites. Using the catalytic conversion of 4-nitrothiophenol to 4-aminothiophenol as a model reaction, the Au@Au-AgTs exhibited the ability to function as an active catalytic and SERS-responsive platform in quantifying the reaction kinetics.
Ó2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
A quantitative monitoring of molecular transformations occur- ring on the surface of a metal catalyst can provide an insight to the reaction kinetics and mechanism[1,2]. Conventionally, use of analytical tools such as Fourier transform infrared spectroscopy, gas chromatography and UV–vis absorption spectroscopy are lim- ited by a low sensitivity, a poor surface selectivity or chemical specificity (inability to provide adequate information about the molecules) [3]. Surface-enhanced Raman spectroscopy, on the other hand, combines the high sensitivity (down to a single mole- cule detection) and the chemical specificity, and can thus function as a suitable technique to quantify and monitor reaction kinetics [4]. In principle, roughened metal nanostructures, preferably of gold or silver, are mostly used as SERS platforms, due to their wide range of light absorption in the visible region of the electromag- netic spectrum [5]. The interaction of light with noble metal nanoparticles leads to a coherent oscillation of conduction
electrons and a subsequent excitation of a localized surface plas- mon resonance (LSPR)[6]. The excited LSPRs induce strong local electromagnetic fields at the surface of the nanoparticles, thus amplifying the weak Raman modes of the molecules[6].
Anin situreaction monitoring by surface-enhanced Raman scat- tering requires fabrication of an active bifunctional nanostructure having high SERS and catalytic capabilities. However, integrating both SERS and catalytic features into a single nanostructured mate- rial remains a challenge, because while SERS is favored by nanoparticles with sizes ranging from 30 to 100 nm, catalysis requires smaller particles, preferably less than 10 nm[7,8]. To date, much efforts have been focused on core-shell and bimetallic nanocrystals, which have exposed catalytic and SERS active sites.
The Au/Pt/Au raspberry [9] with high Pt surface area and plas- monic properties, Au-Pd nanoparticles grown at the ends of single crystal Au nanorods [10], SiO2@Au@Pd-islands [11] and Au@Pt@Au-islands [12] represent a few of the numerous dual- functional catalytic and SERS-responsive materials. However, due to the complicated and multistep processes involved in the fabrica- tion of the bifunctional core shell and bimetallic nanostructures, a more convenient approach based on a self-assembly method has
https://doi.org/10.1016/j.apsusc.2018.05.041
0169-4332/Ó2018 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑ Corresponding author.
E-mail address:Tuula.Pakkanen@uef.fi(T.T. Pakkanen).
Contents lists available atScienceDirect
Applied Surface Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c
been introduced. Zhang et al. [13] successfully assembled small and big nanoparticle hierarchies by an interfacial self-assembly method, and used the obtained nanostructure as an effective SERS-based reaction monitoring platform. Li et al.[14]also used the self-assembly method to assemble small gold nanoparticles on silver nanowires by utilizing an electrostatic interaction and demonstrated their use in the SERS-based reaction monitoring.
The majority of catalytic and SERS-responsive platforms have been bulk nanostructures, but tubular nanostructures will offer a larger surface area with highly exposed catalytic and SERS active sites for the reaction monitoring[15]. Au-Ag nanotubes represent one of the hollow nanostructures with high SERS properties. The bimetallic Au-Ag system has unique electronic and optical proper- ties compared to the individual metals. These properties can make them efficient SERS materials compared to the monometallic coun- terparts [16,17]. The Au-Ag nanotubes have recently been exploited as a potential platform for investigating the progress of reaction using SERS. However, it is only the porous form that has so far been used[18].
Despite the high sensitivity, nanoporous Au-Ag nanotubes obtained by the conventional galvanic replacement reaction (GRR) route involve some structural challenges. First, the galvanic replacement reaction may lead to the formation of nanoparticle islands on the surface of the Au-Ag nanotubes[19,20]. Second, cre- ating a uniform nanosized holes on the Au-Ag nanotubes with the GRR method has not yet been obtained. Since nanoparticle islands and nanosized holes on nanoporous Au-Ag nanotubes are the main active sites for SERS and catalysis, their inhomogeneous distribu- tion will pose a practical challenge. In the case of SERS, the inho- mogeneity of the active sites (hot spots) will lead to a poor signal reproducibility. Therefore, a more controlled approach to integrate both SERS and catalytic properties into highly sensitive Au-Ag nanotubes is needed.
Here, we demonstrate a novel catalytic and SERS-responsive bifunctional tubular nanostructure based on gold nanoparticles decorated Au-Ag tubes (Au@Au-AgTs). The free amino groups of branched polyethylenimine (PEI) was utilized in binding PEI- capped gold nanoparticles to the Au-AgTs (Schemes 1 and 2). We deliberately chose gold nanoparticles of sub-ten nanometers because smaller nanoparticles are known to exhibit high catalytic activities due to a high number of surface atom and empty coordi- nation sites [7]. The fabrication of the bifunctional substrate required simple mixing of PEI-capped gold nanoparticles and Au- AgTs. The method is facile and requires no precise temperature control and harsh conditions. The Au-catalyzed conversion of 4- nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4
was selected as the model reaction. In the present study, the con- version of 4-NTP to 4-ATP by Au@Au-AgTs proceed without the influence of photocatalysis.
2. Experimental
2.1. Materials
Silver nitrate (AgNO3, Merck, Reg. Ph), polyvinylpyrrolidone (PVP, Fluka, Mr360,000), and ethylene glycol (EG, Merck 99%) were used to synthesize silver wires. Branched polyethylenimine (PEI, Aldrich, Mw25,000), sodium borohydride (NaBH4, Aldrich 99%) and gold(III) chloride trihydrate (HAuCl43H2O, Alfa Aesar, 99%) were used to synthesize gold nanoparticles. HAuCl43H2O was also used in a galvanic replacement reaction. A saturated sodium chloride solution in H2O (NaCl, VWR, Prolabo99.8%) was used to remove AgCl from Au-AgTs. The SERS and catalytic properties of Au@Au-AgTs was studied with 4-nitrothiophenol.
All aqueous based reactions were conducted using 18.4 MXcm1 ultrapure water. A Millipore Milli-Q filtration and deionization sys- tem was used to obtain ultrapure water.
2.2. Fabrication of gold nanoparticles decorated Au-Ag alloy tube (Au@Au-AgTs)
2.2.1. Synthesis of gold nanoparticles
In the preparation of gold nanoparticles, 0.45 ml of 10 mM NaBH4solution was added to 10 ml of 0.5 mM HAuCl43H2O and 0.005 ml of 0.6 mM PEI solution. The resulting solution was stirred for 15 min under ambient conditions[21].
2.2.2. Synthesis of silver wires (AgWs)
Silver wires were synthesized based on a slightly modified polyol process [22]. In a typical reaction, 0.08 M AgNO3 and 0.375 M PVP solutions were first prepared in ethylene glycol (EG). Then, 5 ml of EG was heated to 180C under reflux. After an hour of heating, the prepared solutions of AgNO3 and PVP (3 ml Scheme 1.Schematic representation for the fabrication of Au@Au-AgTs. Addition of
HAuCl43H2O to silver wires lead to the formation of Au-Ag tubes. Polyethylenimine capped gold nanoparticles are added to the Au-AgTs by simple mixing to form Au@Au-AgTs.
Scheme 2.A proposed structure of the Au@Au-AgTs. (a), a structure of branched polyethylenimine (PEI), (b) a linkage of gold nanoparticle to Au-AgTs by amino groups of PEI.
342 B. Ankudze, T.T. Pakkanen / Applied Surface Science 453 (2018) 341–349
each) were added dropwise simultaneously. The addition lasted for about 6–9 min under a rigorous stirring. The resulting solution was kept at 180C for additional one hour, leading to formation of silver wires. The silver wires were collected by centrifugation, washed three times with 18.4 MXcm1 ultrapure water, and dispersed in 10 ml water for further use.
2.2.3. Synthesis of Au-Ag tubes (Au-AgTs)
In the galvanic replacement reaction, 1 ml of an as-prepared suspension of silver wires was first diluted with water to a volume of 10 ml. The resulting silver wire suspension was heated to about 100°C under reflux. After 5 min of heating, 2 ml of 0.5 mM HAuCl4∙3H3O aqueous solution was added to the suspension in drops under a rigorous stirring. After additional 5 min of stirring, Au-Ag Ts were obtained. The tubes were collected by centrifuga- tion, washed first with saturated sodium chloride, and three times with 18.4 MXcm1ultrapure water. The purified Au-Ag Ts were dispersed in 1 ml of water for further use.
2.2.4. Immobilization of gold nanoparticles on Au-Ag tubes
In the immobilization of gold nanoparticles on Au-Ag tubes, various volumes, ranging from 0.2 ml to 0.6 ml of a PEI-capped gold nanoparticle solution were added to 0.5 ml of Au-AgTs sus- pension. The resulting solution was sonicated in an ultrasonic bath and allowed to stand overnight.
2.3. Characterization
The structures and sizes of the silver wires, Au-Ag tubes (Ts), gold nanoparticles and Au@Au-AgTs were determined with a Hita- chi S-4800 FE-SEM (field emission scanning electron microscope).
The plasmonic absorptions of the silver wires, the Au-Ag tubes, and the gold nanoparticles were determined with a Perkin Elmer Lambda 900 UV/Vis/NIR spectrophotometer. A Bruker D8 X-ray diffractometer was used to examine the crystallinity of silver wires, Au-AgTs, and Au@Au-AgTs.
Elemental identification of the Au-AgTs sample was acquired with an energy dispersive X-ray spectrometer (EDS) fitted with a Noran system six (NSS) software. The gold content of Au-AgTs was determined with a Varian atomic absorption spectroscopy (AAS) 220 spectrometer with flame atomization. The AAS was first calibrated with three gold standards and further validated with a gold validation standard before the gold content was determined.
The tube sample was first dissolved in 3 ml of aqua regia and fur- ther diluted to 50 ml in a volumetric flask. The content of carbon (C), hydrogen (H) and nitrogen (N) in the Au-AgT was determined with an Elementar Vario Micro Cube instrument at a 1150°C com- bustion temperature.
A Renishaw invia Raman spectrometer using a wire 3.4TMsoft- ware was used to record and monitor the catalytic conversion of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) using SERS technique. A laser power of about 1.5 mW, an excitation laser wavelength of 785 nm, a 20objective lens, and an exposure time (the time the Raman signals are exposed to the detector) of 10 s were used in the spectral acquisition.
2.4. Estimation of the analytical enhancement factor (AEF)
The analytical enhancement factor of the Au@Au-AgTs was esti- mated with 4-nitrothiophenol using Eq.(1) [23],
AEF¼ ISERS
IRamanCRaman
CSERS ð1Þ
where ISERS is the intensity of the probe molecule collected on Au@Au-AgTs,CSERSis the concentration of the probe molecule giving rise toISERS,IRamanis the intensity of the probe molecule collected on
a pure glass plate, and CRamanis the concentration of the probe molecule giving rise toIR.TheAEFof the Au@Au-AgTs was 2.5 105for 4-NTP.
2.5. In situ reaction monitoring by SERS
A small amount of Au@Au-AgTs was drop dried on a glass plate treated with piranha solution. The Au@Au-AgTs film was then incubated in 4-NTP solution (1104M) for two hours. NaBH4
soution (0.01 M) was added to the chemisorbed 4-NTP to initiate the conversion to 4-ATP on Au@Au-AgTs. The conversion reaction was simultaneously monitored by SERS.
3. Results and discussion
3.1. Morphology of gold nanoparticles
Branched polyethylenimine (PEI), a cationic polyelectrolyte, was used as a capping agent in the synthesis of gold nanoparticles.
The size of as-prepared nanoparticles was 9 ± 2 nm (Fig. 1(a)), and the concentration of the solution, if the reduction of Au3+ is assumed to be complete, was calculated to be 0.0499 mg/ml[24–
26]. The plasmonic absorptions of the 9 ± 2 nm colloidal gold nanoparticle solutions was observed around 519 nm in the visible region of the electromagnetic spectrum.
3.2. Morphology of silver wires (AgWs)
Fig. 1(b) illustrates a SEM image of silver wires grown by the polyol method[22]. The polyol method works under completely anhydrous conditions and at high temperatures[27]. The polyol (ethylene glycol (EG)) in this case serves as solvent, and can also reduce the silver ions at high temperatures[28]. Upon reduction, the silver atoms form, through a homogeneous nucleation, small silver nanoparticles, which can subsequently grow by Ostwald ripening[29]. The facets of some silver nanoparticles are passi- vated into multiple twinned structures when PVP is introduced into the reaction system[27]. In this study, a PVP: AgNO3 molar ratio of 5:1 was found to be optimal to passivate selectively the (1 0 0) planes of the silver nanoparticles leaving the (1 1 1) planes to grow anisotropically in the [1 1 0] direction. The anisotropic growth process results in the formation of silver wires[27].
The as-prepared silver wires had a pentagonal cross-section (Scheme 1(a)) with an average diameter of 167 ± 6 nm and a length of about 40
l
m. InFig. 2(a), the plasmonic absorption of the silver wire suspension was observed around 402 nm in the visible region of the electromagnetic spectrum. The x-ray diffraction signal at the 2hangles of 38°, 44°, 64°, 77°and 81°inFig. 2(b) can be assigned to the reflections from the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of a face-centered cubic (FCC) silver nanoparticle, respectively.3.3. Morphology of Au-Ag alloy tubes (Au-AgTs)
Fig. 1(c) shows the SEM image of as-synthesized Au-AgTs hav- ing a pentagonal cross-section. The Au-AgTs were fabricated by the galvanic replacement reaction of silver wires with chloroauric acid (Scheme 1(b)). The driving force for the galvanic replacement reaction is the difference in the standard redox potentials of Ag+/Ag pair (0.80 V, versus the standard hydrogen electrode, (SHE)), and AuCl4/Au pair (0.99 V, versus the SHE)[30]. Because of the elec- trode potential difference, silver wire is oxidized to silver ions when brought into contact with chloroauric acid. Upon oxidation of silver, gold ions deposit as gold atoms via the following galvanic reaction[30]
3AgðsÞ+ AuCl4ðaqÞ!3AgþðaqÞ+ AuðsÞ+ 4ClðaqÞ ð2Þ The deposited solid gold grows epitaxially, and mimics the structure of the silver wire template due to the good lattice match between silver and gold[30]. The average external diameter of the Au-AgTs was 215 ± 29 nm, which corresponds to an increase of about 50 nm with respect to silver wires. InFig. 2(a), the plasmonic absorption maximum of Au-AgTs was observed around 608 nm. An EDS mapping of the Au-AgT inFig. 3(a) and (b) confirms the pres- ence of gold and silver in the structure.
The X-ray diffraction signals at 2hangles around 38°, 44°, 64°, 77°and 81°recorded for Au-AgTs inFig. 2(b) can be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) diffraction planes of gold or silver nanoparticles. The absence of any characteristic peaks attributable to AgCl[27]shows that the obtained Au-AgTs are pure. The similarity of the diffraction angles of the Au-AgTs
to pure silver or gold nanoparticles can be due to the good match- ing of the crystal lattices of elemental gold and silver[31].
The composition of the Au-AgTs was analyzed with atomic absorption spectroscopy (AAS) to determine the gold content, which was found to be22 wt%. The determined percentage of gold indicates that the silver content in the Au-AgTs is higher.
Since the silver wires were synthesized in the presence of the PVP capping agent, the carbon (C), hydrogen (H), and nitrogen (N) of the Au@Au-AgTs were also determined. The analysis showed that the PVP content is around 1 wt%. The results from the elemen- tal analysis indicate that77 wt% of silver is present in the Au-AgT.
The Ag content in the Au-AgTs can be decreased by increasing the concentration of HAuCl4solution. However, introducing a higher amount of HAuCl4 can etch the surface of the AgWs leading to the formation of porous Au-AgTs[32].
Fig. 1.(a) a STEM image of 9 ± 2 PEI-capped gold nanoparticles, (b) a SEM image of AgWs with an average diameter of 167 ± 6 nm and a length of about 40lm, (c) a SEM image of Au-AgTs with an average external diameter of 215 ± 29 nm, (d) a SEM image of Au@Au-AgTs (0.4 ml of 9 nm gold nanoparticles on Au-AgTs).
Fig. 2.(a) UV–vis spectra of silver wires (AgWs) and Au-AgTs, (b), XRD profiles of AgWs, Au-AgTs, and Au@Au-AgTs.
344 B. Ankudze, T.T. Pakkanen / Applied Surface Science 453 (2018) 341–349
3.4. Morphology of gold nanoparticles decorating Au-Ag alloy tubes (Au@Au-AgTs)
Gold nanoparticles were immobilized on Au-AgTs to form Au@Au-AgTs (Scheme 1(c)). The immobilization was primarily due to the strong coordination property of the branched polyethylenimine (PEI). Branched PEI has numerous primary, sec- ondary and tertiary amino groups (Scheme 2(a)). By introducing PEI-capped gold nanoparticles into the Au-AgTs suspension, the free amino groups can anchor gold nanoparticles onto the Au- AgT surface through polymer linkages (Scheme 2(b)). The gold nanoparticles get immobilized on the Au-AgT by coordination interactions between the free amino groups on the gold nanoparti- cles and the metal atoms of Au-AgTs[33].
In the fabrication of Au@Au-AgTs, different volumes of the gold nanoparticle solution ranging from 0.2 ml to 0.6 ml were added to a fixed volume of the Au-AgTs suspension and the addition was found to yield Au@Au-AgTs with different surface densities of gold nanoparticles (Figs. S1 and S2). InFig. 4, the surface density of the gold nanoparticles on the Au-AgTs was found to increase with the increasing volume of the AuNP solution. The nanoparticles stood mostly isolated on the Au-AgTs, only when the volume of AuNP solution reached 0.5 ml were clusters of about 25 nm observed.
The increase of AuNP solution also resulted in a decrease in the interparticle distance of the nanoparticles on the Au-AgTs. The interparticle distance between the nanoparticles on the Au-AgTs decrease from about 10 nm to few nanometer (about 2 nm). In Fig. 2(b), the X-ray diffraction signals at 2h angles around 38°, 44°, 64°, 77°and 81°recorded for Au@Au-AgTs can be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) diffraction planes of gold or silver nanoparticles.
3.5. SERS properties of the Au@Au-AgTs
In order to select an active SERS platform for a reaction moni- toring, SERS spectra of the prepared Au@Au-AgTs substrates were recorded. InFig. 5(a) and (b), the SERS sensitivity of Au@Au-AgTs towards 4-NTP was found to decrease with the increasing surface density of gold nanoparticles. A drastic decrease was observed when the volume of gold nanoparticles solution reached 0.5 ml.
It is worth noting that, since smaller nanoparticles (<20 nm) have a lower plasmon activity, the SERS properties of the Au@Au-AgTs can be attributed in a larger extent to the Au-AgTs[7]. The drastic decrease in SERS intensity inFig. 5may be attributed to the forma- tion of thick gold nanoparticle layer on the surface of the Au-AgT as the nanoparticle coverage increases. The nanoparticle layer blocks some of the incident light from reaching the SERS active Au-AgTs.
Thus, most of the incident light flux resides on the small gold nanoparticle compared to the Au-AgTs. This variation in the inci- dent light flux corresponds to plasmon distribution in the Au@Au-AgTs hierarchical structure. Therefore, as the surface den- sity of nanoparticles increases, the plasmon activity of the Au- AgTs decreases due to the limited light flux, hence a decrease in the SERS activity of the Au@Au-AgTs. A similar observation has been made by Oh et al.[34]. from nanosphere (NS) and rod (NR) hierarchies, where a greater part of light flux inside the NS-NR resides on the top, and the SERS activity of the multilayered struc- ture varies depending on the layer on top.
3.6. In situ reaction monitoring of the conversion 4-nitrothiophenol to 4-aminothiophenol
By monitoring the SERS behavior of the different Au@Au-AgTs substrates, an optimal structure was obtained with 0.4 ml of the gold nanoparticle solution. On this structure, the SERS intensity, using 4-NTP as the probe molecule, does not decrease much when compared to the SERS intensity of Au-AgTs (Fig. S3). Moreover, the structure had appreciable number of nanoparticles (Fig. 1(d)) immobilized across the entire outer surface of the Au-AgTs, hence, it could serve as an active substrate for heterogeneous catalysis.
For an experimental demonstration, we selected Au-catalyzed conversion reaction of 4-nitrothiophenol (4-NTP) to 4- aminothiophenol (4-ATP) under ambient conditions. It is widely Fig. 3.(a) STEM image and EDS mappings of as-synthesized Au-AgTs, (b) EDS
spectrum of Au-AgTs.
Fig. 4.Correlation between the surface density of gold nanoparticles on Au-AgTs and the volume of nanoparticle solution. An increasing volume of nanoparticle solution corresponds to an increase in the number of gold nanoparticles on Au- AgTs. The surface densities were determined by counting the nanoparticles from SEM images. In the determination, the Au-AgTs were considered cylindrical, and only the nanoparticles immobilized on the outer surface were considered.
known that this reaction proceeds only in the presence of metal catalyst [35–38]. Specifically, Au@Au-AgTs on a glass plate was incubated in 4-NTP solution for two hours, leading to a monolayer formation. Subsequent addition of NaBH4facilitated reduction of the chemisorbed 4-NTP to 4-ATP molecule.Fig. 6(a) and (b) shows the time dependent SERS spectra of a series of Au-catalyzed reduc- tions of 4-NTP to 4-ATP recorded on Au@Au-AgTs. The SERS spectra shows characteristic 4-NTP peaks at 1080 cm1, 1332 cm1, and 1571 cm1 that can be attributed to CAS stretching, OANAO stretching and phenyl ring mode, respectively[39]. After addition of NaBH4 (10 mM), two distinguishable spectral changes were observed, the OANAO stretching at 1332 cm1started to decrease while a new band at 1592 cm1, attributable to the phenyl ring mode of 4-ATP emerged, signifying a conversion of 4-NTP to 4-
ATP[40](Fig. 7(a)). The complete disappearance of the OANAO stretching mode occurred within 20 min under the current exper- imental conditions. To ascertain that the catalytic conversion is primarily from the immobilized gold nanoparticles, similar exper- imental conditions were setup, here using mere Au-AgTs. As shown inFig. 6(c) and (d), the OANAO stretching mode at 1332 cm1was still observable, and intense, after 20 min of consecutive spectra acquisition in a real time manner. This suggests that the catalytic property of the Au@Au-AgTs originates from the immobilized nanoparticles and not from as-prepared Au-AgTs.
It has been reported that the conversion of 4-NTP to 4-ATP pro- ceeds through an intermediates such as 4,40-dimercaptoazoben zene (DMAB) or hydroxylamine [3,41]. DMAB can be formed through surface plasmon resonance (SPR) induced photocatalytic Fig. 5.(a) SERS spectra of 4-NTP recorded on Au@Au-AgTs having different surface densities of gold nanoparticles, (b) a bar graphs showing the intensity variation as the volume of nanoparticle solution increases. The SERS intensity decreases with the increasing nanoparticle density.
Fig. 6.(a and b)In situSERS monitoring of the catalytic reduction of 4-NTP initiated by NaBH4on Au@Au-AgTs, and the corresponding image map, (c and d) SERS spectra of 4- NTP recorded on Au-AgTs after addition of NaBH4and the corresponding image map. The reactions were monitored for 20 min and it does not fully occur on Au-AgTs.
346 B. Ankudze, T.T. Pakkanen / Applied Surface Science 453 (2018) 341–349
dimerization of 4-NTP or 4-ATP on the surface of gold and silver. In Fig. 6(a) and (b), no band attributable to DMAB around 1140 cm1 and 1436 cm1[42–44]was observed during the progress of the Au-catalyzed reaction. In order to further investigate the presence or absence of DMAB, 20 consecutive spectra were acquired on the 4-NTP chemisorbed on Au@Au-AgTs in the absence of NaBH4. The spectra were recorded from the same spot on the 4-NTP function- alized Au@Au-AgTs using a 785 nm laser and a 1.5 mW power for 20 min (the same conditions used in the in situ reaction monitor- ing). The result shows that there are no spectral changes, neither is there any peak which can be attributed to DMAB (Fig. S4), there- fore, no DMAB molecule was observed in the reduction of 4-NTP catalyzed by Au@Au-AgTs. The absence of DMAB may be due to the low excitation laser power used in the spectral acquisition or because 4-NTP molecules are separated farther apart, so that chances of forming dimer with neighboring molecules is negligible, or DMAB reacted very fast and could not be detected.
The time evolution of the reduction of 4-NTP means that the reaction kinetics can be quantified. Since the concentration of sodium borohydride (10 mM) is far greater than that of 4-NTP (0.1 mM), it can be assumed that the concentration of NaBH4
remains constant throughout the reaction. In this respect, the cat- alytic reaction can be considered to follow a pseudo-first-order kinetics. Thus, the reaction rate can be quantified using Eq.(3) [8],
kt¼ ln Iðt=I0Þ ð3Þ
wheretis reaction time andIis the SERS intensity band at 1332 cm1.Fig. 7(b) shows a plot of ln(It/I0) versus time. The reaction rate constant (k) obtained from the slope of the linear regression of ln (It /I0) versus time was 3.4103s1with a standard deviation of 1 104s1.
To compare the catalytic properties of the 9 nm AuNP on Au- AgTs, PEI-capped gold nanoparticles having a diameter of 14 ± 4 nm (Fig. S5(a)) were prepared and used to decorate the Au-AgTs Fig. 7.(a) Time-dependent SERS spectra showing the disappearance of 4-NTP and evolution of 4-ATP, (b) a plot of ln(It/Io) against time, showing the rate constant (k) for the time-dependent disappearance of 4-NTP band at 1332 cm1recorded on Au@Au-AgTs.
Fig. 8.In situSERS monitoring of the catalytic reduction of 4-NTP, (a) plots of ln (It/Io) against time (s) for 14 nm and 9 nm gold nanoparticles on Au-AgTs. (b) Plots of pseudo- first-order rate constants (k) against Au@Au-AgTs with the different surface densities of 9 nm gold nanoparticles. (c) Plots of ln (It/Io) against time for different NaBH4
concentrations, showing the influence of NaBH4on the rate constant. (d) Linear dependence of the rate constant (k) on the NaBH4concentration.
(Fig. S5(b)). The prepared substrate was employed to monitor the reductive conversion of 4-NTP to 4-ATP under the same condition used for the 9 nm gold nanoparticles. A comparison made between the pseudo-first order rate constants (k) of the 9 nm and 14 nm gold nanoparticles decorated Au-AgTs inFig. 8(a) revealed a larger k for the 9 nm gold nanoparticles, indicating the size-dependent catalytic activity of small gold nanoparticles[7].
The reaction monitoring properties of the Au@Au-AgTs having different surface densities of 9 nm gold nanoparticles (Figs. 4and S1) were also examined in the conversion of 4-NTP to 4-ATP.
Fig. 8(b) shows a comparison of the rate constants of Au@Au- AgTs prepared with 0.3 ml, 0.4 ml, 0.5 ml and 0.6 ml of the gold nanoparticle solution. As illustrated inFig. 8(b), the pseudo-first- order rate constants (k) obtained for Au@Au-AgTs substrates pre- pared with 0.3 ml, 0.4 ml, 0.5 ml and 0.6 ml of the 9 nm AuNP solu- tion were 3.1103s1, 3.4103s1, 3.2103s1and 3.6 103 s1, respectively. Fig. 8(b) reveals no clear dependence of the rate constants on the amount of nanoparticles, as thekvalues obtained from the different Au@Au-AgTs catalysts are in the same order of magnitude. Nonetheless, the rate constant (k) showed a linear dependence on the NaBH4concentration. InFig. 8(c) a plot of ln (It/I0) against time obtained with 0.001, 0.004, 0.007 and 0.01 M NaBH4concentrations is presented.Fig. 8(d) shows a linear dependence of the pseudo-first-order rate constants on the NaBH4
concentration, indicating that the reaction rate of the reductive conversion of 4-NTP depends on the sodium borohydride concen- tration. The linear correlation betweenkand NaBH4allows us to obtain a normalized rate constant value (dividing the observed rate constant by the amount of NaBH4used[45]) of 0.34 s1M1, which is comparable to those reported for Au@Ag bimetallic nanocuboids [42]and nanorice[42], as well as for gold trisoctahedral[46]and quasi-spherical gold nanoparticles[46].
4. Conclusions
In this study, we have demonstrated a facile approach to the synthesis of a bifunctional nanostructure based on gold nanoparti- cles immobilized on Au-Ag alloy tubes for real-time and label-free SERS based reaction monitoring. Branched polyethylenimine (PEI) was used to bind gold nanoparticles onto Au-AgTs via polymer linkages. Immobilization of PEI-capped gold nanoparticles on Au- AgTs involves only a simple mixing, thus, requiring no multistep or complicated procedures. The Au-AgTs, due to the synergistic effect of gold and silver provides a strong electromagnetic field for SERS, whereas the small immobilized gold nanoparticles serve as active catalytic sites. Using the Au@Au-AgTs to catalyze and at the same time to monitor the reduction of 4-NTP to 4-ATP, we could quantify the reaction kinetics. The results from this study shows that the Au@Au-AgTs can serve as a robust platform for the investigation of molecular transformation occurring on the sur- face of a metal catalyst.
Acknowledgements
Scholarship from University of Eastern Finland is gratefully acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, athttps://doi.org/10.1016/j.apsusc.2018.05.041.
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