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Abstract
We report on the synthesis of a new metallic nanoarchitecture, namely, hairy gold nanorods that were carefully designed and engineered the seed-mediated growth of gold nanowires on the sub-nanometer scale gold nanorod substrate. The thickness of the gold nanowires grown could be tuned from 5 to 9 nm by controlling the ratio of HAuCl4 to 4-Mercaptobenzoic acid (MBA) from 2.5 to 25 while the length of gold nanowires could be controlled between 47 nm to 15 µm by varying the concentration of silica coated gold nanorod in the gold solution. The high-aspect-ratio hairy gold nanowires tethered to concentric gold nanorod could be used for fabrication of soft flexible high performance resistive strain sensors and soft surface-enhanced Raman scattering substrate.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metallic nanoparticles have exhibited intriguing optical, [1–4] electronic, [5–7] catalytical [8–10] and magnetic properties [11–13] which are vastly different from their corresponding bulk form. Driven by these exciting size- and shape-dependent properties, a number of synthetic strategies have been developed over the past about 30 years, leading to the formulation of so-called ‘nanoparticle periodic table’. [14] Despite of these encouraging progresses in metallic nanoparticles synthesis, [15–19] researchers are still far from capable of constructing any arbitrary nanostructures in a well-controlled manner. Besides, metallic nanoparticles in its single form also has limited functionality, performance and application. Combination of nanostructures of different types and unique properties will offer additional functionalities broadening the scope of applications. [20,21]
Over the past decade, researchers have demonstrated assembly of nanomaterials which lead to the use in design and development of stretchable and wearable electronics. [22–25] Most of these reported materials were one dimensional (1D) nanowires due to their high aspect ratio in conjunction with excellent mechanical and electrical property, enabling construction of percolation conductive network with low materials consumption. [25–28]
Controlled seed-mediated growth of 1D gold nanowires have been achieved on macroscopic surfaces such as silicone-based substrates [29,30] and trumpet shells. [31] This approach has also been extended to sub-micrometer sized silica fibres [32] and magnetic beads. [20] On the other hand, gold nanowires have demonstrated novel applications of the gold nanowires in wearable energy devices, [33] wearable electronics, [30,34,35] surface enhanced Raman scattering [36] and wearable electrochemical sensors. [37–39] In this context, it would be beneficial to precisely grow 1D gold nanowires in a highly site-specific manner. Such specifically designed 1D nanostructures may possess multifunctionalities, broadening the scope of potential applications.
In the seed-mediated growth, the gold nanoseeds define where gold nanowires will form. Previously, our group had reported on self-assembly of gold nanoparticles onto caged-gold nanorod (CGNR) surface via amine-functionalization of the silica coated CGNR. [40] Using this technique, herein, we extended the similar gold seed-anchoring chemistry to plasmonic gold nanorods, which were then used in a seed-defined gold nanowire growth, leading to formation of hairy gold nanorods (HGNRs). The HGNRs were thoroughly characterized by electron microscopes and the growth process was optimised by tuning concentrations of seed particles, gold precursors and growth solutions. The localized surface plasmon resonance of gold nanorods could be used to monitor nanowire formation process via UV-visible spectrophotometry. The optimized HGNRs were then used to fabricate soft resistive strain sensors and stretchable SERS substrates. Our results indicated HGNRs are multifunctional nanomaterials with dual sensing capabilities of electrical and optical signal transduction.
2. Results and discussion
In essence, the synthesis of HGNRs went through two main wet-chemistry based seed-mediated growth processes. The first one was the known process for synthesis of monodispersed gold nanorods using gold nanoseed solution in the presence of cetyltrimethylammonium bromide (CTAB) ligand. Briefly, cetyltrimethylammonium bromide (CTAB) stabilized gold nanorod were first synthesized following the well-known seed-mediated growth method. [41,42] CTAB was then partially replaced by thiolated polyethylene glycol (SH-PEG) via ligand exchange, which could promote selective coating of uniform silica shell by sol-gel reaction with tetraethyl orthosilicate (TEOS) the precursor. The second main process was the seed-mediated growth of gold nanowires on gold nanorod with the strong binding 4-mercaptobenzoic acid (4-MBA) ligand. The silica shell was functionalized with amino groups by (3-aminopropyl) trimethoxysilane (APTMS) to enable the attachment of gold seeds by electrostatic forces. Finally, the seeds-tethered to gold nanorods were used to catalyze the growth of ultrathin gold nanowires using 4-mercaptobenzoic acid as binding ligands and HAuCl4/ascorbic acid as growth solution. The overall synthesis process for the HGNR particle is illustrated in Fig. 1.
The growth of gold nanowires on the silica-PEGylated gold nanorod was characterized stepwisely by transmission electron microscope (TEM) as shown in Fig. 2. The monodispersed gold nanorods with an average length of 53 ± 2.1 nm and width of 16 ± 0.4 nm, corresponding to an aspect ratio of 3.2 ± 0.2, were obtained (Fig. 2(a)). Further sol-gel reactions led to a uniform silica coating of 42 ± 3.8 nm as shown in Fig. 2(b). After surface modification with amine moieties by APTMS, citrate-stabilized gold nanoparticle seeds could be tethered to silica shell surfaces (Fig. 2(c)). These immobilized seeds could catalyze the growth of long gold nanowires (Fig. 2(d)).
Fig. 2. TEM characterization of (a) gold nanorod, (b) gold nanorod coated with SiO2, (c) gold nanorod with ∼2 nm gold nanoparticle seed and (d) gold nanowires grown on the silica coated gold nanorod, also known as hairy gold nanorod.
The evolution of HGNRs led to evident changes of plasmonic spectra (Fig. 3(a)). The as-synthesized gold nanorods displayed two distinct localized surface plasmon resonance (LSPR) bands situated at 513 and 684 nm, corresponding to the transversal and longitudinal LSPR modes, respectively. To ensure uniform silica coating, modification of gold nanorods by polyethylene glycol was crucial. There are two important roles performed by the PEG ligand: 1) To provide sufficient stability for them to be transferred into ethanol; 2) To increase the affinity of silica to the gold surface. [43] The conformal coating of silica led to further red-shift of the longitudinal modes to 701 nm due to the increase of effective refractive index. The attachment of gold seeds caused further red shifting of the plasmonic peak to 717 nm, due to the strong surface plasmonic coupling between seeds and nanorods. [40,44] The time-series UV-Vis spectra of gold nanowires growth process was recorded and plotted in Fig. 3(b).
The blue line in Fig. 3(b) is the AuNP/SiO2/NR seed before nanowires growth, which show two UV-Vis spectrum peaks at 518 nm and 717 nm. In the nanowire growth process, the longitudinal LSPR modes (717 nm) disappeared while the transversal LSPR modes (518 nm) become stronger, which may be attributed to the transverse mode of gold nanowires. We can also see that the gold nanowires growth process was rapid, within 10 seconds the longitudinal LSPR modes disappeared and transversal LSPR modes becomes the main peak. The hairy morphologies of HGNR were confirmed by scanning electron microscope (SEM). Figure 8(a) shows a typical SEM image of HGNR, clearly demonstrating that long nanowires of around 1.5 µm long and about 7.3 nm wide could be firmly tethered to gold nanorod surfaces. The firm attachment (Fig. 8(b)) indicates the strong chemical binding interactions between nanowires and silica surfaces. The length of gold nanowires could reach up to ∼15 µm long (Fig. 9) and their number density could reach as high as 140 ± 17 nanowires per nanorod as analyzed closely from Fig. 4(b).
Fig. 4. TEM images of (a) whole HGNR (b) NR core. (c) HRTEM image of silica shell and nanowire root interface region. Inset is a selected area electron diffraction pattern of HGNR, showing the (111), (200), (220) and (311) reflections of gold.
We further employed high-resolution transmission electron microscopy (HRTEM) to characterize a particular HGNR (Fig. 4). HRTEM image in Fig. 4(c) shows zoomed-in interface between silica shell and roots of nanowires, giving clear the crystal lattices with d-spacing of 0.235 nm and 0.202 nm corresponding to the (111) and (200) planes of face-centered cubic (FCC) gold nanowires. The selected area electron diffraction pattern (SAED) of gold nanowires (inset of Fig. 4(c)) with four bright rings corresponding to the (111), (200), (220) and (311) planes indicated that the entire nanowires were polycrystalline. The close inspection of a single gold nanowire (Fig. 10) demonstrates their highly crystalline structures.
The length of nanowires could be controlled by adjusting the mole ratio between HAuCl4 and nanorods. Four ratios at 1.15×109, 5.75×108, 2.30×108, and 1.15×108 were chosen to grow hairy nanorod of different length, while all other growth conditions are kept the same (HAuCl4 to MBA ratio at 12.5, see experiment section for details). From the SEM images (Fig. 5), we can see that the length of nanowires increased as the mole ratio increase. The longest hairy nanorod of 1623 ± 163 nm is obtained with HAuCl4: nanorod mole ratio at 1.15×109, which is regarded as long hairy nanorod (L-HGNR). Similarly, medium hairy nanorod (M-HGNR), short hairy nanorod (S-HGNR), and extra short hairy nanorod (XS-HGNR) were obtained by decreasing the HAuCl4: nanorod mole ratio as shown in Fig. 5. It is also possible to tune the number density of nanowires by changing the gold nanoparticle seed concentration, which was reported in our previous publication [34]. Interestingly, although the lengths of these nanowires were different, their widths were similar.
Fig. 5. The length of nanowire tuned by changing the mole ratio between HAuCl4 and nanorod. Insets are the corresponding SEM images of various length nanowires.
For the growth of hairy gold nanowires, the HAuCl4 precursors and MBA capping ligands were in a dynamic competition for gold deposition and ligand binding. [31] More MBA usually gave thinner nanowires due to the fast ligand binding; in contrast, more HAuCl4 usually resulted in faster gold deposition speed, leading to the formation of thicker nanowires.
An optimum mole ratio of 3 for HAuCl4 versus MBA led to nanowires with a width of 6 nm was reported earlier. [31] In this study, we investigated the correlation of HAuCl4/MBA mole ratio and nanowire growth while keeping the gold nanorod concentration at 0.0033 nM. The gold nanowires were able to grow with the mole ratio of HAuCl4 and MBA at the range of 2.5 to 25 (SEM images in Fig. 11(b) and (c)), however, they were unable to grow under or beyond the range, for instance mole ratio of 1.25 and 75 as shown in Fig. 11(a) and (d). We speculate that the reason behind this is mainly due to the dynamic competition between the precursor and capping ligands. Table 1 summarized concentration of gold precursor and ligand and their mole ratio in the growth solution, and the ratios where nanowires can form were highlighted in red. Our study also confirmed that the mole ratio determines the width of nanowire. With ratio increased from 2.5 to 3, 12.5 and 25, nanowire width increased from 5 nm, 5.5 nm, 7.3 nm and 9 nm respectively.
In addition to the mole ratio of HAuCl4 and MBA, their respective concentration also played an important role in the growth of HGNRs. In our study, we found that even HAuCl4 and MBA was in the ratio range of 2.5 to 25, nanowires still wouldn’t form when their concentions were too low or too high. As the table shows, the gold nanowires can grow at HAuCl4 concentration ranged from 0.43 to 1.7 mM, and MBA concentration between 35 to 170 µM. Out of these concentration ranges, no nanowires were observed even the ratio was within the range. In the report by He et. al., the concentration of HAuCl4 and MBA were at 1.7 mM and 550 µM respectively, [31] where the concentration is much higher than the concentration reported in this work. This could be because of our use of nanorods as nanoscale nanoscale colloidal template rather than macroscopic planar surface.
The presence of silica was critical for the formation of gold nanowires, as per the claim in the previous report. [31] As shown in Fig. 12, under same condition as Fig. 8 only 30 nm nanoparticles with worm shape obtained without gold nanowires formed. In the absence of silica, APTMS is unable to modify the surface of gold nanorods, consequently no seed particle attachment. Upon addition of growth solution, gold nanorods would seed their growth into the worm-like particles.
The unique soft and elastic nanowires around gold nanorod cores allowed us to use HGNRs as a new conductive building block for stretchable sensors, which require the integration of electrical conductivity with elastic mechanics. To prove this, we drop-casted HGNRs solution on a thin nitrile rubber sheet substrate to form a dark gold nanopatch. Typically, the nanopatch had a sheet resistance of 59.5 ± 14.4 kΩ sq−1 after repeatedly drop-casted for 10 times.
To assess the ability of HGNRs as an active strain sensing material, the tests were carried out at a voltage of 1V by applying different levels of strains. Figure 6(b) shows strain-induced relative resistance responses over three cycles of the applied strain from 1%, 5% to 10%. The overall electrical resistance increased with increasing strains. HGNR-based sensors also exhibited high durability. Figure 6(c) is the plot of durability test under a cyclic strain of 0%-5%-0% at a frequency of 0.5 Hz. The resistance change curves were recorded for 1,000 cycles, and 50 cycles of data was presented in each 250 cycles recording. The stable and reproducible responds to the cyclic demonstrated the high durability of HGNR sensors (Fig. 6(c)). The morphology changes of the strain sensor during stretching was also investigated as shown in Fig. 13. With the increase of strain, cracks were observed, leading to the increasement in the resistance of the film. Interestingly, the cracks were self-repaired after the full strain recovery, which explained the high durability of our strain sensors [23].
Fig. 6. (a) Possible mechanism of HGNR film before and under strain. (b) Resistance-time characteristics of the sample’s stretchability test for three applied strain (1%, 5% and 10%). (Input voltage: 1V; frequency: 0.5 Hz) (c) The durability test under a strain of 5% at a frequency of 0.5 Hz. The resistance change curves were recorded after each 500 cycles and 50 cycles of data was presented in each recording.
The conductivity and sensitivity of the strain sensors fabricated with HGNR with different nanomorphologies were also compared. Specifically, L-HGNR, M-HGNR, S-HGNR, XS-HGNR, and bare gold nanorod solution were used to drop-casted a thin film on the thin nitrile rubber. The nanorod amount (50 µl at nanorod concentration of 0.68 nM) and size of thin film (5 × 25 mm2) were kept the same. As shown in Fig. 14(a), the sheet resistance of HGNR increased with the increase of nanowire length, while the thin films based on XS-HGNR and bare gold nanorod were not conductive. This may be due to the much higher aspect ratio of long nanowire compared to short ones, offering more conductive pathways at the same nanorod concentration. On the other hand, the sensitivity of the strain sensor is decreased with the increase of nanowire length (Fig. 14(b)). Here the sensitivity of the sensor is quantified by the gauge factor, which is defined as GF=(R-Roff)/ɛ Roff, where ɛ is the strain, Roff is electrical resistance under no strain. It is worth noting that the HGNR thin film with longer nanowire showed much higher stretchability compares to that with shorter nanowire. This is consistent with the literature that high-aspect-ratio building blocks are good candidates of stretchable strain-insensitive conductors. [25]
The excellent performance indicated high elasticity of HGNR film and their strong adhesion to nitrile rubber support. This property is attributed to unique structures of HGNR, where the nanorod core in middle radical were multi-directionally wrapped by nanowires with high aspect ratio of over 200. Such unique structural features made HGNR films to work as mesh conducting film, exhibiting high elasticity. Furthermore, the stability of the senor was tested in 30 days. The sensor film was put in a petri dish in ambient condition without any further sealing process, but the electrical resistance did not show any evident fluctuations. The highly stable nanowire is attributed by the inert nature of noble gold material which are against oxidation under ambient conditions [45] and also the MBA capping molecules which form a compact layer on the metal surface during nanowire growth preventing facile excess of oxygen.
We further investigated Surface-Enhanced Raman Scattering (SERS) performance of the HGNR particle under stretched states. To do this, HGNR particle was drop casted and dried on nitrile rubber sheet, followed by swiping 1mM on benzocaine on it. The substrate was then placed under Raman microscope and Raman signal for benzocaine analyte was collected. This was to determine the Raman signal enhancement from HGNR substrate and compare to bare gold nanorod particle substrate. Besides, the ability of HGNR in enhancing the Raman signal under 20% stretched state was also studied and shown in Fig. 7(a). With excitation of 514 nm laser, the characteristic peaks for benzocaine located at peaks located at 860, 1173, 1280, 1604, and 1682 cm−1 were identified. The strongest peak at 1604 cm-1, which corresponds to the C-C stretching mode were used to characterize the SERS enhancement by comparing the intensities obtained from HGNR and GNR substrates. The results clearly show that the HGNR particle exhibited stronger SERS signal than GNR nanoparticles, with ∼3.1 fold higher SERS enhancement as shown in Fig. 7(b). Interestingly, the HGNR substrate is also able to enhance the Raman signal in its stretched state. The gold nanowires on the GNR creates strong coupling effects, leading to strong local electromagnetic fields for physical enhancement of Raman signals for benzocaine analyte. When being stretched, the coupling strength decreased and reduced the electromagnetic field, hence, leading to decreased SERS enhancement factor. Nevertheless, the ability of identifying chemicals under mechanically stretched state makes our HGNR a promising soft, flexible and stretchable SERS substrate suitable for continuous monitoring chemicals sitting on topologically complex real-world soft surfaces.
Fig. 7. (a) SERS spectra of benzocaine on HGNR and GNR substrate in their unstretched and stretched state (b) SERS signal intensity of benzocaine analyte at 1604 cm-1.
3. Conclusions
In summary, a new type of metal nanoarchitecture – hairy gold nanorods were successfully obtained by the two-step seed-mediated growth processes. We could obtain hairy gold nanowires with a ratio of HAuCl4 to MBA from 2.5 to 25, which is much wider region than literature report. What is more, width of nanowires could be controlled in the range of 5 to 9 nm by tuning the ratio of the growth solution, and width changed from 5 nm to 9 nm in this ratio range. We could control the length of nanowires from 47 nm to 15 µm, simply by adjusting amount of nanorod. The unique hairy high-aspect-ratio gold nanowires tethered to concentric nanorod allowed for constructing flexible and stretchable percolation conductivity network. This conductive network could be used as high performance piezoresistive strain sensors with high stretchability and durability.
4. Experiment section
4.1 Materials
Gold (III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%), cetyltrimethyl-ammoniumbromide (CTAB), sodium borohydride (NaBH4), silver nitrate (AgNO3), L-ascorbic acid (AA), tetraethyl orthosilicate (TEOS), (3-aminopropyl) trimethoxysilane (APTMS), 4-Mercaptobenzoic acid (MBA) and benzocaine were purchased from Sigma Aldrich. Thiol-functionalized polystyrene (Mn = 50 000 g/mol, Mw/Mn = 1.09) was purchased from Polymer Source Inc. Ethanol was obtained from Merck KGaA. Ammonia hydroxide solution (28.0-30%) was purchased from Fisher Scientific). All chemicals were used as received unless otherwise indicated. Deionized water was used in all aqueous solutions, which were further purified with a Milli-Q system (Millipore). All glassware used in the following procedures was cleaned in a bath of freshly prepared aqua regia and rinsed thoroughly in MilliQ water prior to use. Nitrile rubber was brought from MEDIflex industries. Silver paste was purchased from Sigma Aldrich. Stainless thin conductive thread was purchased from Adafruit Industries.
4.2 Synthesis of gold nanorod
The gold nanorods were prepared according to reported method. A brownish-yellow seed solution was prepared by mixing CTAB (5.0 mL, 0.2 M) and HAuCl4 (5.0 mL, 0.5 mM) followed by adding ice-cold NaBH4 (0.6 mL, 0.01 M). The seed solution was aged at 30°C before seeding to growth solution. Then, CTAB (5 mL, 0.20 M) and HAuCl4 (5.0 mL, 1.0 mM) were added to (0.2 mL 4 mM) AgNO3 solution in sequence and with the addition of AA (0.08 mL, 0.08 M), the yellowish mixture became colorless and the growth solution was obtained. To grow nanorod, 12 µL of seed was added into the growth solution and aged at 30°C for two hours. The CTAB capped NRs were collected by centrifugation (7 000 rpm for 10 min) and washed with water twice, redispersed in 1 mL MilliQ water.
4.3 PEG functionalization
The stabilization agent, CTAB, on the surface of the gold nanorods was replaced by mPEG-thiol through ligand exchange. An mPEG (Mw = 5000) solution (1 mL, 20 mg/mL) was mixed with 1 mL nanorod solution to react 2 h. Then after washing with water 3 times to remove the non-reacting mPEG, the nanorods were redispersed in water (1.2 mL) for next silica coating.
4.4 Silica coating of PEG capped gold nanorods
A modified Stöber method [46,47] was used to grow a silica shell of controlled thickness on the PEGylated gold nanorods. Briefly, 1.2 mL of PEG capped gold nanorods was mixed with 8 mL ethanol, and sonicated for 1 min, then 67 µL ammonia solution was added to solution under vigorous stirring, followed by TEOS (8 µL) in ethanol (1 mL) dropwise added to the mixture. After stirring for 16 hours, the resulting samples were collected by centrifugation, washed with water 3 times and redispersed in 1 mL ethanol for amino functionalization. Functionalizing silica coated nanorod surface with APTMS. The amino-functionalized gold nanorod was prepared by mixing 1 mL silica coated gold nanorod with 4 mL 2% APTMS in ethanol for 1 hour, followed by washing with ethanol 3 times.
4.5 Synthesis gold seeds
0.2 mL of 25mM HAuCl4 and 0.147 mL 34 mM sodium citrate was added into conical flask with 20 mL H2O under vigorous stirring. After 1 min, 600 µL of ice-cold, fleshing prepared 0.1M NaBH4 solution was added with stirring. The solution turned brown immediately. The solution was stirred for 5 min and then stored at 4 °C until needed. The final concentration of the gold seed synthesized is ∼1 nM.
4.6 Adsorption of gold seeds onto APTMS site
Firstly, the amino functionalized nanorod sonicated for 30 seconds and added to 5 mL of 1nM AuNPs, then sit in room temperature for 6 hours for adsorption. The products were collected by centrifugation and then washed with water once, redispersed in MilliQ water as the nanorod seeds for nanowire growth.
4.7 Hairy gold nanorod growth
The reaction of ratio at 12.5 of HAuCl4 to MBA showed as following: 50 µL 6.1mM MBA water-ethanol solution (3:1, v/v) was firstly added under gently stirring. Then, 152 µL HAuCl4 (25 mM) and 1 mL nanorod seeds were added to the solution in turn. Subsequently, after 1min ingredient mixed completely, AA (22 µL, 0.41 M) was added. Keep the reaction under stirring for 1 minute and allowed to settle. The supernatant was removed, and sediments was kept for further characterizations.
4.8 Strain sensor fabrication
The strain sensor was fabricated by the following procedure: nitrile rubber was first attached on a glass slide and patterned with polyimide masks (25 × 5 mm2 rectangular pattern size). Then high concentrated hairy gold nanorod solution (50 µL) was drop casted onto the soft substrates and dried in fume cupboard. After the solution was dried, polyimide masks and glass side were removed. Then silver paste was deposited onto both ends of the hairy nanorod strips connected with flexible conductive threads. After the silver paste was dried (100 °C for 10 min on hot plate), polyvinyl alcohol (PVA) glue was deposited on top of silver paste and dried in ambient condition (30 min), which permanently sealed the hairy gold nanorod film to conductive thread.
4.9 Fabrication of stretchable SERS substrate
The HGNR based stretchable SERS substrate was fabricated by drop casting 50 µL of highly concentrated HGNR sediments onto soft stretchable nitrile rubber sheet and allowed to dry in ambient condition. The dried substrate is then placed in the UV ozone chamber at an oxygen flow rate of 0.5L/min and plasma-treated for 4 minutes to remove the MBA ligand on the gold nanowires. After that, 1mM of benzocaine dissolved in ethanol was swiped onto the HGNR substrate surface using a cotton swab and air dried. A control substrate was fabricated by drop casting the highly concentrated gold nanorods (GNR) onto the soft stretchable nitrile rubber sheet. To obtain the highly concentrated GNR, the 1mL GNR solution above was centrifuged and supernatant was removed, then redispersed in 50 µL of MilliQ water. The dispersed solution was then dropped onto nitrile rubber sheet and allowed to dry. 1mM of benzocaine was again swiped onto the GNR substrate and dried before SERS measurement.
4.10 Characterization
UV-Vis absorption spectra were recorded using an Agilent 8453 UV–Vis spectrometer. The shape and morphology of synthesis process were characterized by Scan electron microscopy (SEM) imaging and transmission electron microscopy (FEI Tecnai T20 Twin TEM) imaging. For TEM imaging, a drop of gold nanorod suspension was placed on copper-formvar grids and dry with a filter paper. To test the strain-sensing characteristics, two ends of the samples were attached to motorized moving stages (THORLABS Model LTS150/M) then uniform stretching/releasing cycles were applied to the samples with a computer-based user interface (Thorlabs APT user), while the current differences and for the strain sensor were recorded by the Parstat 2273 electrochemical system (Princeton Applied Research) when a constant voltage is applied. The sheet resistances of the sample on nitrile substrate were carried out on a Jandel four-point conductivity probe by using a linear arrayed four-point head. SERS spectra were recorded by Renishaw RM2000 Confocal micro-Raman System with excitation laser wavelength of 514 nm (laser spot 1 µm, 10% laser power ∼ 0.1 mW, 50× microscope objective, data acquisition time 10 s). The data acquired are corrected by cubic spline baseline subtraction to exclude the fluorescence contribution. For all quantitative analysis, 10 SERS measurements were acquired on different locations and averaged.
Appendices
Fig. 10. (a, b, c, d) HRTEM images of four continuous parts along a typical nanowire started at silica shell. Inset is the overall view of the HGNR.
Fig. 11. SEM images of gold nanowires grew at different mole ratio of HAuCl4 to MBA. The molar ratio of HAuCl4 and MBA is at (a) 1.25, (b) 2.5, (c) 25, (d) 75 and (e) 3. The nanowire could be grew in the ratio range 2.5 to 25, however, they cannot grow when the ratio at 1.25 and 70.
Table 1. The concentration of gold precursor (HAuCl4) and ligand (MBA) and their mole ratio in the growth solution.
Fig. 12. SEM image of nanoparticles obtained from seed growth without silica substrate, under otherwise the same reaction conditions as for Fig. 8.
Fig. 13. (a)-(c) Optical images of the morphology of HGNRs thin film at 0% strain (a), at 10% strain (b), and back to 0% strain (c). (d) An enlarged image of (b) indicating the formation of cracks upon stretching.
Funding
Australian Research Council (DP180101715, LP160101192, LP60100521).
Acknowledgments
This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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