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Controlled growth of gold nanocrystals on biogenicAs–S nanotubes by galvanic displacementTo cite this article: Fang Liu et al 2018 Nanotechnology 29 055604

 

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Controlled growth of gold nanocrystals onbiogenic As–S nanotubes by galvanicdisplacement

Fang Liu1,3,4 , Wilfred Chen1 and Nosang V Myung2,3

1Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716,United States of America2Department of Chemical and Environmental Engineering, University of California-Riverside, Riverside,CA 92521, United States of America

E-mail: liufang@udel.edu, fliu006@gmail.com and myung@engr.ucr.edu

Received 11 September 2017, revised 30 November 2017Accepted for publication 8 December 2017Published 5 January 2018

AbstractTraditional methods for fabricating nanoscale arrays are usually based on lithographic techniqueswhile alternative new approaches rely on the use of nanoscale templates made of synthetic orbiological materials. Here, gold (Au) nanocrystals were grown on the surface of themicrobiologically formed As–S nanotubes through the process of galvanic displacement. Thesize and organization of the synthesized Au nanocrystals were affected by the pH dependentspeciation of HAuCl4 precursors as well as the initial ratio of As–S/HAuCl4. We found that aspH increased, the Au nanocrystals grown on As–S nanotubes had smaller sizes but were morelikely to assemble in one-dimension along the nanotubes. At a proper initial ratio ofAs–S/HAuCl4, Au nanotubes were formed at pH 6.0. The mechanism of Au nanostructuresformation and the synthesis process at different pHs were proposed. The resulting Aunanoparticle/As–S nanotube and Au nanotube/As–S nanotube hetero-structures may provideimportant properties to be used for novel nano-electronic devices.

Keywords: Au nanostructures, As–S nanotubes, galvanic displacement, hetero-structures

(Some figures may appear in colour only in the online journal)

1. Introduction

Au nanostructures have shown to be one of the most pro-mising nanomaterials and have a wide range of potentialapplications in electronics, biomedicine, biological imaging,optics, catalysis and sensing [1–5]. The unique optical andelectronic properties of Au nanostructures are related to theirsize and shape [6–9] and therefore methods to control theirsize and shape are of special interest.

Since the wider application of traditional top-down fab-rication of nanomaterials is impeded by its high cost and sizelimitation of the synthesized nanostructures, the bottom-up

approach has become complimentary to the top-down meth-ods by achieving cost-effective nanofabrication with a highspatial resolution [10]. The fabrication of Au nanostructureshave been achieved by aqueous solution self-assembly [11],seeded growth procedures [12, 13], templating techniquesusing natural materials such as viruses [14, 15], proteins[16–18], peptides [19, 20] as well as other predesignednanostructures. These nanoscale templates can not only directthe deposition, but also assemble Au nanostructures into one-,two-, or even three-dimensional functional architectures,which provides an important way to tune the optical andelectronic properties of Au nanomaterials. Au nanomaterialsexhibit distinctive surface plasmon resonance (SPR) proper-ties [21] that are highly sensitive to their size, shape, andorientation. The SPR properties of Au nanomaterials makethem attractive for applications such as cancer treatment,

Nanotechnology

Nanotechnology 29 (2018) 055604 (9pp) https://doi.org/10.1088/1361-6528/aaa061

3 Authors to whom any correspondence should be addressed.4 Current address: Department of Biomass Science & Conversion Technol-ogies, Sandia National Laboratories, Livermore, CA 94550, United States ofAmerica.

0957-4484/18/055604+09$33.00 © 2018 IOP Publishing Ltd Printed in the UK1

sensors, and optoelectronics [21, 22]. One dimensional Aunanostructures, in particular, have attracted great attentionbecause they have been developed as building blocks forsensors, catalysts, optics, and nanoelectronic devices [23–26].In addition, Au nanomaterial based hetero-nanostructuresenabled them with even new optical and electronic properties.The assembly of Au nanoparticles on one-dimensional inor-ganic nanostructures have been templated by carbon nano-tubes [27], silica nanofibers [28], and CdSe nanotubes [29] toform the hybrid nanostructures with applications in a varietyof areas. For examples, Au-carbon nanotube nanocompositeshave demonstrated the applications in biosensors, gas sensorsand drug delivery [30].

Filamentous, arsenic–sulfide (As–S) nanotubes that are20–100 nm in diameter and 30 μm in length can be massivelyproduced by the dissimilatory metal-reducing bacteriumShewanella sp. via the reduction of As(V) and -S O2 3

2 [31].These biogenic nanotubes provide powerful templates forbottom-up fabrication of Au nanostructures. Galvanic dis-placement, a electroless deposition process that occurs effi-ciently under room temperature, has applications in manyareas, particularly in the interfacing of metals (e.g. Ni, Pd,Au) with semiconductor (e.g. InP, GaAs, Si, Ge) surfaces atthe nanoscale level [32–34]. Here, we demonstrated the in situformation of Au nanocrystals on the surface of As–S nano-tubes via galvanic displacement. Biogenic As–S nanotubesserved as both the reducing agent and the nanoscale templatesduring the process.

Aqueous HAuCl4 is the most commonly used Au pre-cursor in templated synthesis of Au nanocrystals. HAuCl4solution is easily hydrolyzed in water and the predominantspecies is depending on the pH of the solution and [Cl−]concentration [35]. Although studies of the pH effect on Aunanocrystals synthesis have been reported, the pH dependentspeciation of HAuCl4 has seldom been considered as a factorthat will influence the synthesis process and Au nanos-tructures. Reduction of HAuCl4 by H2O2 at pH ranging from2.9 to 12 produced Au NPs with different morphologies,which was due to the pH-dependent reduction ability of H2O2

[36]. The size-controllable synthesis of glutathione capped AuNPs by varying the pHs was attributed to the formation ofAu(I)glutathione polymer precursors, which changed theirsize and density depending on pH [20]. Only in a recent

report, Wang et al showed that aqueous HAuCl4 of differentpHs resulted in Au colloids of different particle sizes [37]. Inthis study, we consider the aqueous pH as an importantparameter that would affect the growth of Au nanocrystals onAs–S nanotubes. We found that by systematically varying thepH and ratio of As–S/HAuCl4, the size and organization ofAu nanocrystals could be altered.

2. Experimental section

2.1. Bacterial growth conditions and synthesis of As–Snanotubes

Basal media for dissimilatory iron-reducing bacteria wereprepared as described previously (Lee et al 2007). Themedium was adjusted to pH 6.9 with 10 N NaOH. Facultativebacterium, Shewanella sp. HN-41, from laboratory culturecollection was grown on Luria–Bertani agar in the dark at30 °C for 24 h. Cells were resuspended in sterile Hepes buffer(30 mM, pH 7.0) to achieve optical density of 2.0 at 600 nm.Subsequently cell suspensions were inoculated to a serumbottle containing the basal medium supplemented with10 mM lactate (as sodium DL-lactate), 5 mM thiosulfate(Na2S2O3 · 5H2O) and 5 mM arsenate (Na2HAsO4 · 7H2O) toa final optical density of 0.02. Cultures, in sealed serumbottles, were incubated in the dark environment at 30 °C for 7days. After the As–S nanotubes were formed, the serumbottles were opened and the samples were cleaned withdeionized (DI) water and resuspended in DI water.

2.2. Synthesis of Au nanostructures

After cleaned As–S nanotubes were prepared, 500 μl of As–Snanotube solution and 500 μl of HAuCl4 (containing 0.01MNaCl) with different concentrations or pHs were added to thecentrifuge tube and mixed well. The amount of As(III), totalaqueous As and Au ions in the mixture during the reactionwere analyzed by the atomic absorption spectrometer(AAnalyst 800, PerkinElmer). As(V) concentration was cal-culated by subtracting As(III) concentration from the total Asconcentration.

Figure 1. Solution color after 6 h reaction of As–S nanotubes (f) with 1 mM HAuCl4 (containing 0.01 M NaCl) at pH 2.9 (a), pH 4.2 (b),pH 5.3 (c), pH 6.0 (d), pH 7.0 (e).

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Nanotechnology 29 (2018) 055604 F Liu et al

2.3. Characterization of synthesized nanomaterials

The measurement of ultraviolet–visible (UV–vis) absorbancespectra during the reaction of As–S nanotubes with HAuCl4solution was performed on a Beckman UV–vis spectro-photometer. The As–S and HAuCl4 mixture was used directlyfor the UV–vis analysis.

After 6 h incubation of As–S nanotubes and HAuCl4solution, the product was collected by centrifugation. Theobtained product was rinsed three times with DI water andresuspended into DI water. The size and the shape of thematerial were observed with a scanning electron microscopy(SEM, XL30-FEG) and the chemical composition of theproduct was determined by energy-dispersive x-ray

Figure 2. UV–vis absorbance spectra of the mixture during the reaction of As–S nanotubes with 1 mM HAuCl4 solution at pH 2.9. Insertshows the UV–vis spectra of As–S nanotube solution, aqueous HAuCl4 solution and the mixture of As–S and HAuCl4 at 0 min. The 220 nmabsorption peak was not shown in the spectrum of HAuCl4 solution due to the out-of-limit reading in that range, but was shown in the spectraof after 35 min reaction.

Figure 3. Concentrations of As(V) and Au in the solution during the reactions of As–S nanotubes with 1 mM HAuCl4 at pH 2.9 (a), pH 4.2(b), pH 5.3 (c), pH 6.0 (d) and pH 7.0 (e).

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Nanotechnology 29 (2018) 055604 F Liu et al

spectroscopy. The sample was also analyzed using a trans-mission electron microscope (TEM, Philip CM 300) equippedwith selective area electron diffraction. The x-ray diffraction(XRD) analysis was performed using a Rigaku D/max dif-fractometer with Cu Kα irradiation (λ=0.154 18 nm) at ascan rate of 0.01° s−1 in the range of 10°–90°.

Au nanoparticles were randomly chosen from the SEMimages to measure their diameters. The nuclei density wasdetermined by dividing the number of particles by half of thesurface area of the As–S nanotube on which the number of Aunanoparticles was counted. The surface area of a single As–Snanotube was calculated as 4πdh, where the diameter d and

length h of the nanotube in the SEM image was measuredusing the Image J software.

2.4. Measurement of the electrical properties of thenanomaterials

The temperature-dependent electrical conduction propertiesof the formed nanostructures were investigated. To obtain theconductance from the I–V characteristics, a solution contain-ing the synthesized material was dispensed on prefabricatedgold electrodes, and the material precipitates bridging theelectrodes were air-dried. The I–V characteristics were mea-sured by using a semiconductor parameter analyzer, with therange of applied voltage from −0.5 to +0.5 V.

3. Results and discussion

3.1. The effect of pH and As–S/HAuCl4 ratio on Au nanocrystalgrowth

To study the effect of pH-dependent HAuCl4 speciation onthe growth of Au nanostructures using As–S nanotubes astemplates, we chose five different pH values, 2.9, 4.2, 5.3,6.0 and 7.0. According to the predominance diagram forgold hydroxide-chloride complexes [35], the major speciesis [AuCl4]

− at pH 2.9 and 4.2 when 0.01M [Cl−] ionsare present and the predominant species changes to[Au(OH)Cl3]

− at pH 5.3, [Au(OH)2Cl2]− at pH 6.0, and

[Au(OH)3Cl]− at pH 7.0 respectively.

The yellowish As–S nanotubes were synthesized after 7days of anaerobic incubation as described previously [38] and

Figure 4. SEM images of Au nanostructures formed on the surface of As–S nanotubes (a) with 1 mM HAuCl4 at pH 2.9 (b), pH 4.2 (c),pH 5.3 (d), pH 6.0 (e) and TEM image of the nanostructure synthesized at pH 7.0 (f). Inserts show SEM images with lower magnifications.

Figure 5. XRD pattern of the synthesized nanomaterials by As–Snanotubes and 1 mM HAuCl4 at pH 5.3.

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Nanotechnology 29 (2018) 055604 F Liu et al

were rinsed with DI water, after which the cleaned As–S wereresuspended into DI water with a final concentration of about3 mM. After 1 mM HAuCl4 solution at different pHs was addedto the As–S nanotube solution and incubated for 6 h, differentcolors for the reaction mixtures were observed-the color wasyellow-brown at pH 2.9 and 4.2, red-brown at pH 5.3, darkpurple at pH 6.0, and no obvious color change at pH 7.0(figure 1). UV–vis absorption study was performed during thereaction of As–S nanotubes with HAuCl4 solution of pH 2.9 atdifferent time points (figure 2). The spectrum of As–S nanotubesolution is featureless while HAuCl4 solution displayed theabsorption peaks at 220 and 290 nm, both of which were due tothe ligand-to-metal-charge-transfer (LMCT) bands of [AuCl4]

−

between gold and chloro ligands [39, 40]. During the reaction,the LMCT bands of the [AuCl4]

− decreased dramatically,suggesting that the Au3+ ions in HAuCl4 solution were reducedinto the metallic state [41]. The red shift of the 290 nm peak inthe spectrum indicated the increased overall size of the syn-thesized products during the reaction.

The time-dependent concentrations of Au(III) and As(V)in the solution were measured during the reaction to study thereaction kinetics. At all the pHs studied, the Au ion con-centration decreased during the reaction while the con-centration of As(V) increased (figure 3), indicating that Aswas displaced by Au from the As–S nanotubes and releasedinto the solution. The highest reaction rate occurred at pH 6.0,followed by pH 5.3, pH 2.9, and pH 4.2, while the slowestrate was observed at pH 7.0. These results are consistent with

the observed more significant color changes of the reactionmixtures with the higher reaction rates.

The morphology of the nanostructures was examinedunder SEM and TEM. At pH 2.9 and 4.2, large Au nano-particles with an average diameter of 261 nm were formedand discretely distributed on the surface of the As–S nano-tubes (figures 4(b) and (c)), while a small amount of Aunanoplates (triangle, hexagon, etc) were also observed. AtpH 5.3, smaller Au nanoparticles were grown along the As–Snanotubes in a higher density (figure 4(d)). The particle sizewas smaller with an average diameter of 96 nm (figure 6(b)).XRD analysis of the nanostructures showed powder patterns((111), (200), (220), (311), (222)) (figure 5), corresponding tothe randomly oriented polycrystalline Au crystallites. AtpH 6.0, the Au nanoparticles were uniformly deposited on theentire surface of As–S nanotubes (figure 4(e)) and the particlesize was even smaller with an average diameter of around31 nm (figure 6(c)). Moreover, the particle size distributionwas much narrower at pH 5.3 and pH 6.0 than at pH 2.9(figure 6), indicating the more uniform size of the synthesizedAu nanoparticles at pH 5.3 and pH 6.0. When thepH increased to 7.0, very few small nanoparticles around10 nm were found on the surface of the As–S nanotubes(figure 4(f)). Therefore, except at pH 7.0, the average size ofthe formed Au nanocrystals decreased while the density of theparticles deposited on As–S nanotubes increased with theincreasing pH of the reaction mixture. These results clearlydemonstrated the dependence of the morphology, size and

Figure 6. Histograms of diameter distribution of Au nanoparticles formed by As–S nanotubes and 1 mM HAuCl4 at pH2.9 (a), pH5.3 (b) andpH6.0 (c). (N: total number of particles that were included for the diameter measurement.)

Figure 7. SEM images of nanostructures synthesized by As–S nanotubes with 0.1 mM HAuCl4 (a), 0.5 mM HAuCl4 (b) and 1 mM HAuCl4(c) at pH5.3.

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Nanotechnology 29 (2018) 055604 F Liu et al

organization of the synthesized Au nanocrystals on the spe-ciation of aqueous HAuCl4 at different pHs.

In addition to pH, it appears that the concentration ofaqueous HAuCl4 also affected the size of synthesized nano-particles. At pH 5.3 and a fixed concentration of As–Snanotubes, Au nanocrystals of increasing sizes were depos-ited onto the As–S nanotubes with increasing concentrationsof HAuCl4 from 0.1 to 1 mM. The average particle sizeincreased from 19 nm at 0.1 mM HAuCl4, 40 nm at 0.5 mMHAuCl4 to 90 nm at 1 mM HAuCl4 (figure 7).

3.2. Hypothesized mechanism of Au nanocrystals growth onAs–S nanotubes

Au nanocrystals were formed on the surface of As–S nano-tubes through a galvanic displacement process, in which free

Au(III) ions in the solution were reduced to Au(0) whileAs(III) was oxidized to As(V) and released into aqueoussolution from the nanotubes. Time-dependent SEM imageswere taken during the course of Au nanostructures synthesisin the mixture of As–S nanotubes and HAuCl4 solution ofpH 2.9 (figure 8(a)) to reveal the growth process of Aunanoparticles on the As–S nanotubes. The samples taken from0, 5 , 10 min and up to 8 h showed that the average particlesize of synthesized Au nanoparticles increased with time,from less than 50 to over 200 nm during the process(figure 8(b)). Base on this observation, we hypothesized thatduring the nanocrystals synthesis, Au atoms were con-centrated and supersaturated in the solution until Au nucleiwere formed along on As–S nanotubes. And the nuclei con-tinued to grow and formed nanoparticles by gathering thereduced metallic gold surrounding the nuclei (figure 9).

Figure 8. SEM images of Au nanoparticles synthesized by As–S nanotubes and 1 mM HAuCl4 at pH 2.9 with time course (a) and thecorresponding histograms of size distribution of synthesized nanoparticles at different reaction time (b) (N: total number of nanoparticleschosen for diameter measurement).

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Nanotechnology 29 (2018) 055604 F Liu et al

The surface chemistry of the As–S nanotubes enabledthem to serve as structured templates as well as reactionsubstrates for the site-specific nucleation of gold particles at arange of pHs. The galvanic displacement reaction happenedbetween As–S and Au ions at different pHs could be pre-sented as follows:

(1) e- = --( ) ( )eAs III 2 As V 0.67 V ;0NHE

e

e

e

e

+ + == =

+ + ++ = =

+ + ++ = =

+ + ++ = =

- - -

- + - -

- + - -

- + - -

( ) [ ] ( )

[ ( ) ] ( )

[ ( ) ]

[ ( ) ] ( )

e

e

e

e

2 AuCl 3 Au 0 4Cl 1.002 V ,at pH 2.9 and pH 4.2;

Au OH Cl H Au 0 3Cl

H O 2.33 V , at pH 5.3;

2 Au OH Cl 2H 2 Au O 4Cl

3H O 2.75 V , at pH 6.0;

Au OH Cl 3H 3 Au 0 Cl

3H O 1.439 V , at pH 7.0.

40

NHE

3

20

NHE

2 2 2

20

NHE

3

20

NHE

During the course of metal colloids formation, the colloidsize was determined by the nucleation process and the growthof nuclei [37]. At different pHs, the reactivity of the pre-dominant Au species was different. For example, [AuCl4]

−

and [Au(OH)Cl3]− were shown to be more easily reduced

than [Au(OH)3Cl]− and [Au(OH)4]

−. In another study,[Au(OH)Cl3]

− was found to be more chemically reactive than[AuCl4]

− [42]. In our case, based on the displacement reac-tions happened between As(III) and Au(III), the order of thereactivity of Au species at different pHs could be expre-ssed as [Au(OH)2Cl2]

−>[Au(OH)Cl3]−>[AuCl4]

−>Au[(OH)3Cl]

−, which was in agreement with the findings fromother studies. During the Au nanocrystals synthesis on As–Snanotubes, when [Au(OH)Cl3]

− and [Au(OH)2Cl2]− was the

predominant Au species at pH 5.3 and pH 6.0 respectively,Au nucleation proceeded fast via As(III) reduction, resulting

in the formation of ordered arrays of Au nanocrystals onAs–S nanotubes. The nuclei density was estimated by thedensity of nanoparticles formed on the surface of As–Snanotubes, assuming all the nuclei eventually grew to nano-particles. It was calculated by averaging the number ofnanoparticles formed on a single nanotube in the SEM imagedivided by half of the surface area of that nanotube. From thisestimation, the nuclei density at pH 5.3 was calculated to be112 nuclei μm−2 and 225 nuclei μm−2 at pH 6.0. However, atpH 2.9 and pH 4.2 where the dominant species was [AuCl4]

−,the nuclei density was much lower (5 nuclei μm−2 and 8nuclei μm−2 respectively) but the growth of Au nuclei wasstill relatively fast (figure 3), leading to the formation of largerand discrete Au nanostructures on As–S nanotubes. In thecase of pH 7.0, the reactivity of the dominant species[AuCl(OH)3]

− was so low that the nuclei formation andgrowth were both very slow, resulting in the slow formationof Au nanoparticles with small size on As–S nanotubes.Further validation of the hypothesized mechanism of Aunanocrystals formation on As–S nanotubes at different pHs,however, requires more detailed studies on the nucleationprocess and nuclei growth using the analytical techniquessuch as small-angle x-ray scattering and x-ray absorptionnear-edge spectroscopy [41].

3.3. Formation of Au nanotubes templated by As–S nanotubes

With the insights gained from the initial studies on the effectof pH and ratio of As–S/HAuCl4 on the formation of Aunanocrystals, we further investigated the feasibility of syn-thesizing Au nanotubes using As–S nanotubes as templatesby properly controlling these two parameters. In order to havecontinuous deposition of Au nanoparticles on As–S nano-tubes and anneal to form Au nanotubes, the Au nuclei densityshould be high as in the case of pH 5.3 and 6.0. Moreover, the

Figure 9. Proposed process of Au nanostructures forming on the surface of As–S nanotubes through galvanic displacement reaction at pH 2.9(a), pH 5.3 and pH 6.0 (b).

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Nanotechnology 29 (2018) 055604 F Liu et al

ratio of As–S/HAuCl4 should be optimal to avoid the for-mation of discrete nanoparticles. Based on these guidelines,deposition was conducted at pH 6.0 since the synthesized Aunanoparticles had the most continuous organization on As–Snanotubes, under a range of initial As–S/HAuCl4 ratios. Onlyat the ratio of As–S/HAuCl4 of 0.3:1 did we observe theformation of uniform and continuous Au nanotubes(figures 10(a) and (b)). The diffraction pattern showed aunique Au crystal plane which was consistent with the uni-formity of the synthesized nanostructures (figure 10(c)).

The temperature dependent I–V characterization of theresulting Au nanotubes formed at pH 6.0 showed anincreasing resistance of the nanomaterials with increasingtemperature (figure 11), which indicated a typical metallicbehavior. The difference of the I–V characteristics betweenthe nanostructure and gold bulk material was due to the smallboundary scattering effect.

4. Conclusions

In summary, we demonstrated the transformation of biogenicAs–S nanotubes to Au nanostructures by galvanic displace-ment and the size and organization of the synthesized Aunanocrystals was controllable by manipulation of chemicalparameters. The pH-dependent speciation of HAuCl4 pre-cursors affected the nucleation and growth of Au nanocrystals

and Au nanocrystals with smaller size were observed athigher reaction pHs. The size of the Au nanocrystals was alsotunable by varying HAuCl4 concentrations. Fairly uniformand continuous Au nanotubes were obtained using As–Snanotubes as templates at pH 6.0 using an initial ratio ofAs–S/HAuCl4 of 0.3:1. The possibility to form nanotube-nanoparticle and nanotube-nanotube hetero-structures offersthe potential to create materials with novel electronic, opticaland catalytic properties.

ORCID iDs

Fang Liu https://orcid.org/0000-0002-6770-3566

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