NIR-Active Plasmonic Gold Nanocapsules Synthesized Using

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NIR active Plasmonic Gold Nanocapsules Synthesized using Thermally Induced Seed Twinning for Surface Enhanced Raman Scattering Applications Prem Singh, Tobias A.F. Koenig, and Amit Jaiswal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14445 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Applied Materials & Interfaces

NIR active Plasmonic Gold Nanocapsules Synthesized using Thermally Induced Seed Twinning for Surface Enhanced Raman Scattering Applications

Prem Singh1, Tobias A. F. König 2, 3, Amit Jaiswal 1 * 1

School of Basic Sciences, Indian Institute of Technology Mandi, Kamand, Mandi-175005, Himachal Pradesh, India

2

Leibniz-Institut für Polymerforschung Dresden e.V., Institute of Physical Chemistry and Polymer Physics, Hohe Str. 6, D-01069 Dresden, Germany

3

Cluster of Excellence Centre for Advancing Electronics Dresden (CFAED), Technische Universität Dresden, D-01062 Dresden, Germany *Corresponding Author E-mail: [email protected]

Abstract Hollow and porous core shell nanostructures with defined interior nanogaps are of great significance in the field of surface enhanced Raman scattering (SERS) applications because of the presence of intrinsic electromagnetic (EM) hotspots, multi-polar resonances and multiple facets. Further, nanomaterials having extinction in the NIR region are particularly important for SERS and biomedical applications and thus, it is highly desirable to synthesize NIR active plasmonic

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nanostructures. Herein, we report the synthesis of gold nanocapsules having a solid Au bead as core and a thin porous rod-shaped shell with extinction in both NIR-I and NIR-II region. Thermally induced twinned seeds were used for the silver free synthesis of pentatwinned Au bead which served as the foundation for the directed growth of Ag nanorods which was finally converted to Au nanocapsules following Galvanic replacement reaction (GRR). Detailed investigation was carried out to understand the effect of thermal treatment duration in the seed morphology and its subsequent growth to anisotropic Au beads. Ag overgrowth on Au beads yielded uniform Au bead@Ag nanorods whose size can be tuned by varying the Ag precursor. Five different sized Au bead@Ag nanorods was studied and it was converted to Au nanocapsules following GRR. We explored the size dependent SERS activity of the prepared Au nanocapsules along with its comparison with solid pentatwinned Au beads and found that the smallest sized Au nanocapsules were the best SERS performer. FDTD simulation revealed the presence of intense EM hotspots in the smallest sized Au nanocapsule and corroborated the experimental SERS data. Finally, we fabricated a simple flexible cellulose-based SERS substrate by using the smallest sized Au nanocapsules and investigated its SERS sensing ability for the detection of 2-napthalenethiol (2NT), as a model analyte, and were able to achieve its detection down to 1fM concentration.

Keywords: Gold nanocapsules, SERS, galvanic replacement reaction, NIR responsive, nanorattles, sensing

INTRODUCTION Colloidal plasmonic gold nanoparticles have emerged as protagonists in various fields of science and technology setting up a perfect example of ‘one man, many roles’. Due to its inertness, ease


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of synthesis, tuneable optical, electronic, and catalytic properties, these nanomaterials find applications in plasmonics, catalysis, sensing, bioimaging, photothermal therapy, surface enhanced Raman scattering, etc

1-11.

Further, the gold nanostructure allows the growth of other

metals like silver, palladium etc. on its surface due to the lattice match between their crystal structure leading to the synthesis of bimetallic core-shell type nanostructures 3,12-14. The growth of another metal onto the surface of gold leads to creation of different dipolar and multipolar surface plasmon resonance

15,16.

Directed overgrowth of Ag on Au can yield nanostructures whose SPR

can be tuned from the visible to NIR region

1,4,16.

In addition to the presence of surface capping

agents specific towards a crystal plane, or halide ions, the growth direction is highly dependent on the core Au geometry 17-19. For example, growth of Ag on single crystal Au nanorods leads to the formation of Au@Ag nanocuboids where the Ag deposition mainly occurs in the transverse direction of the underlying nanorods

4,9,20,21.

Interestingly, using a pentatwined gold nanorod as

core leads to the directional growth of Ag towards the longitudinal axis of the core nanorods 1,3,12,16,22-26.

The former process leads to the blue shift in the SPR spectrum of the particle while the

later allows to tune the SPR towards NIR by varying the amount of silver precursor used during the growth process 1,12,13,16,27. The importance of the presence of SPR in the NIR region is that it can suitably be used for SERS sensing and biomedical applications 5. The advantage of using NIR for SERS sensing is the absence of background fluorescence which is a very common source of SERS signal loss

28.

Further, the higher penetration depth of NIR light enables deep tissue

penetration and can suitably be used for photothermal therapy or bioimaging applications 8,20,28-30. However, Ag is known to be toxic 31. Thus, an ideal system should be nanostructure free from or having the least amount of silver atoms. This can be achieved by replacing the Ag atoms from the Ag shell present in case of Au@Ag nanoparticles into Au nanorattles through Galvanic


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replacement reaction (GRR). This process can lead to the creation of structure with solid Au core and thin-porous Au shell 32-34. Recently, Singamaneni et al. have reported the synthesis of cuboidal gold nanorattles having a nanorod core 7. The structure was prepared by growing silver shell on single crystal gold nanorods where the growth takes place preferentially along the transverse axis of the solid core nanorod. This structure was later converted to cuboidal rattles following GRR. For all the prepared structures, the growth and replacement were mainly along the transverse axis of the core and the SPR was present only in the NIR I region. They further evaluated the structure for its SERS activity. Till date, most of the synthesized Au nanoshell, nanocage or nanorattle structure with potential biomedical applications have SPR band in the NIR I region and only few reports are available where nanomaterials have demonstrated the presence of SPR in the NIR II region or both. The second biological window (1000 – 1700 nm) is more preferred for imaging and other biomedical applications like photothermal therapy because of centimetre scale penetration depth with micron scale resolution of anantomic features (upto 3 mm), which is not resolvable with the traditional first biological window (750 – 900 nm) 29,30. Thus, it is highly desirable to synthesize nanostructures which are active in NIR II region and can be exploited for SERS as well as other biomedical applications. Herein, we report the synthesis of Au nanocapsules with a pentatwined Au solid bead as core and thin porous rod-shaped Au shell having SPR peak both in the first and the second biological window. Due to capsular morphology of the structure having an encapsulated solid Au bead, we label them as Au nanocapsule. We carried out detailed investigation of the synthesis process by modulating the different parameters which led to the creation of different sized nanorattles with tuneable extinction. Further, due to the rattle like structure, it has intrinsic


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electromagnetic hot spots which demonstrates excellent SERS performance. We studied the size dependent SERS activity of the prepared nanocapsules and realized that the smallest sized Au nanocapsules were the best SERS performer due to the presence of intense EM hotspots within the nanosystem. The experimental data was further corroborated by FDTD simulations. Finally, we fabricated a flexible cellulose-based SERS substrate by depositing the Au nanocapsules onto it and investigated the SERS sensing ability of the prepared substrate towards the detection of trace level analytes using 2-NT as a model analyte molecule. Distinguishable Raman band was observed down to a concentration of 1 fM (S/N ratio > 4) enabling a highly sensitive detection of analyte molecules even at trace levels.

EXPERIMENTAL SECTION Materials. Chloroauric acid (HAuCl4), hexadecyltrimethylammonium chloride (CTAC), benzyldimethylhexadecylammonium chloride (BDAC), citric acid, sodium borohydride (NaBH4), L-ascorbic, 2- napthalenethiol (2-NT), hydrogen peroxide (H2O2, 30%), sulphuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%) and nitric acid (HNO3, 98%). CTAC was purchased from TCI. Hydrogen peroxide, sulphuric acid, hydrochloric acid (HCl, 37%) and nitric acid were purchased from Merck, and the rest of the chemicals were purchased from Sigma-Aldrich. Nanopure water (resistivity 18.2 MΩ·cm) and aqua regia cleaned glasswares were used in all experiments.

Instrumentation: The extinction spectra of synthesized nanoparticles were recorded using Lambda 750 UV/VIS/NIR spectrophotometer (Perkin Elmer) and UV-1800 spectrophotometer (Shimadzu, Japan). Magnetic stirrer IKA (C-MAG HS7) was used for stirring. The morphological


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and topographical study of the synthesized nanostructures was performed using transmission electron microscopy (TEM, FP 5022/22-Tecnai G2 20 STWIN, FEI) and scanning electron microscopy (SEM, Nova Nano SEM-450, FEI). The samples for TEM characterization were prepared by drop-casting around 5 μL of the samples on a carbon-coated copper grid. For SEM characterization, 10 μL of the samples were dropcasted on precleaned silicon wafers.

Synthesis of pentatwinned gold nano-beads (Au-bead): Seed-mediated method was used for the preparation of gold beads. Hexadecyltrimethylammonium chloride (CTAC) stabilized seeds were prepared by adding 0.25 mL of ice-cold sodium borohydride solution (25 mM) into 10 mL of solution containing CTAC (50 mM), HAuCl4 (0.18 mM) and citric acid (5mM) under vigorous magnetic stirring at room temperature. After 5 minutes of stirring, the vial was closed and placed in an oil bath at 90 ºC under mild stirring. After 400 minutes of the thermal treatment, the seed solution was removed from the oil bath and stored at room temperature. For the further growth of gold beads, 0.10 mL of gold seeds was added under vigorous stirring to a growth solution containing benzyldimethylhexadecylammonium chloride (BDAC) (10 mL, 93 mM), HAuCl4 (0.1 mL, 35.6 mM) and ascorbic acid (0.075 mL, 100 mM) at 30 ºC. The resulting mixture was left undisturbed at 30 °C for 30 minutes. Prior to use, the as-synthesized pentatwinned gold beads were centrifuged at 5000 rpm for 15 min and then re-dispersed in equal volume of CTAC (80 mM) followed by the sonication of 15 min. After sonication, solution was again centrifuged at 5000 rpm for 15 min and pellet was redispersed in equal volume of CTAC solution (80 mM).

Synthesis of pentatwinned Ag/Au bimetallic rod (Au-beads@Ag nanorod): For the subsequent growth of Au-beads@Ag rod with different sizes, 2 mL of processed Au-beads and 2.5 mL of


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CTAC (90 mM) were mixed at 65 °C under magnetic stirring for 30 min. For the synthesis of different size of rods, we used the different volume of AgNO3 (2 mM) i.e. 0.8, 1.2, 1.4, 1.6 and 1.8 mL and keeping half the volume of ascorbic acid (0.1M) with respect to AgNO3 volume. After 4 hours of stirring, the as-synthesized Au-beads@Ag rods solution was cooled and stored at room temperature.

Synthesis of Au nanocapsules with Au-bead as core (Au nanocapsules): Au nanocapsules were synthesized by using a GRR where Ag shell of Au-beads@Ag nanorods were transformed into hollow and porous shell of Au. The synthesized Au-beads@Ag rods were centrifuged and dispersed in CTAC solution (50 mM), followed by heating at 120 °C. Under stirring condition, the aqueous solution of HAuCl4 (0.5mM) was added at a rate of 0.25 mL/min until the color of the solution turned to blue.

SERS measurements: As-synthesized Au nanocapsules of different sizes were centrifuged twice at 4000 rpm for 10 min to remove the excess stabilizer present in the solution. Samples for SERS measurements were prepared by loading 10 μL of 2-napthalenethiol (10 mM in ethanol) to 990 μL of washed Au nanocapsules followed by the overnight mixing in a dancing shaker. After overnight mixing, samples were removed from the shaker and washed with nanopure water at once to remove unloaded 2- NT and then deposited on piranha cleaned silicon substrate. SERS spectra were collected using a Jobin Yvon LabRam HR evolution Raman spectrometer. Spectra were collected using the 785 nm laser, which was focused on the sample using a 20× objective with 30 s exposure time. The laser power was measured to be approximately 0.9 mW. For sensing experiment, the asobtained Au nanocapsules of smallest size were washed twice and finally resuspended in nanopure


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water. The cellulose fibre membrane was then immersed in the tenfold concentrated colloidal solution of the nanocapsules for overnight at room temperature and was washed thoroughly with ethanol and water to remove unattached Au nanocapsule from its surface. This fabricated substrate then was used for sensing experiment by dipping it in 2-NT solution (from 10-3 M to 10-15 M) for 6 hrs. Finally, SERS spectra from the plasmonic substrate were collected using the 785 nm laser, which was focused on the sample using a 20× objective with 30 s exposure time. For statistical analysis, five different spectra were collected from different spots across each substrate.

FDTD simulation: A commercial-grade simulator based on the finite-difference time-domain method was used to perform the calculations (Lumerical FDTD, version 8.16)

35.

For the

simulation of the electromagnetic field, a total-field scattered-field source was used at 785 nm wavelength (long pulse excitation). Frequency-domain field monitors were used to obtain the electromagnetic field in transversal (electric field vector perpendicular to the geometric axis) and longitudinal excitations (electric field vector parallel to the geometric axis). The unpolarised electric field was determined by the square of the absolute value of the transversal and longitudinal mode. Zero-conformal-variant mesh refinement and an isotropic mesh overwrite region of 1 nm were used. All simulations reached the auto shut-off level of 10−5 before reaching 150 fs of simulation time. For the dielectric properties of gold, data from Johnson and Christy was fitted using six coefficients with a RMS error of 0.2 (fitting range between 300 and 1600 nm)36. For the dielectric core and the background index, we chose a fixed refractive index of 1.4 and 1.33, respectively. The mesh size was set to 1 nm. All simulations reached an auto-shutoff of at least 10-5 before reaching 300 fs simulation time. For the best simulation stability, the mesh area was chosen to be at least 100 nm larger than the existing structure in all three principal directions.


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Symmetric and anti-symmetric boundary conditions were used. For the simulation of the extinction efficiency, a plane wave source was used and the frequency points were set to be half the wavelength span. A 2D frequency – domain field and power monitors were used to obtain the optical responses

RESULTS AND DISCUSSION We start with the synthesis of Au nanobeads by following a seed mediated growth process. For this, we adapt the idea of inducing twin defect in seeds by thermal treatment as recently reported by Liz-Marzan et. al. 37. Next, we design our protocol for the synthesis of anisotropic penta-twined Au nanobeads using the thermally treated seeds and without using silver for the directional growth. The Au seed was prepared by reducing HAuCl4 using a strong reducing agent, NaBH4 in the presence of a cationic surfactant (CTAC) and citric acid. After NaBH4 addition, the colour of the solution immediately changes from yellow to brown indicating the formation of seeds. The resulting seed solution was thermally treated at 90 oC which is the key step for the formation of twinned particles. With time, we have observed change in colour of the seed solution from brown to red signalling the growth of the seed. Time- resolved UV visible spectroscopy revealed that prior to thermal treatment (0 min) there was no observable peak (Figure 1a). However, with increase in the duration of thermal treatment, signatures of LSPR peak of gold nanoparticles start evolving. The absence of any peak at zeroth minute is attributed to the small size of the seeds (~2 nm). The LSPR bands after thermal treatment of seeds at different time points i.e. 90 min, 240 min, 300 min and 400 min showed an increase in intensity accompanied with a slight red shift in its LSPR peak position (Figure 1a). After 90 min, the position of the LSPR band was at 511 nm which shifted to 525 nm after 400 min of thermal treatment (Figure 1 a). The reason behind this


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red shift in the extinction spectrum can be attributed to the increase in size of the seed during the thermal treatment process. HRTEM imaging was used to study the size and crystal habit of the thermally induced seeds (Figure 1 b) and we found that more than 90 % of the seeds were pentatwinned with an average diameter of 9.1 ± 1.3 nm (Figure S1). Polycrystalline nature of the seeds was also confirmed through SAED patterns (Figure S1b). The choice of surfactant and citrate ions play a crucial role in the formation of twin defects in the seeds. Presence of citrate ions in the reaction mixture blocks the oxidative etching on the gold surface and induce the formation of twin defects in the metal nanocrystal

14,37.

Further, chloride ions of the CTAC is known for its lower

affinity towards the gold surface which ultimately helps in maintaining high ratio of surfactant to metal precursor in the reaction mixture. Therefore, it is assumed that the presence of CTAC as a surfactant promotes the thermodynamic ripening under the thermal aging process which ultimately helps in converting the monocrystalline seeds into polycrystalline

16,18.

Thus, the thermal aging

process in presence of citrate ions and CTAC not only causes increase in the size of the seed but also induces twin defects.

Figure 1. (a) UV-vis spectra of thermally treated seeds for different time interval. (b) HRTEM image of seed after thermal treatment of 400 min. Black arrows indicates their twin planes. Scale bar of 1 (b) corresponds to 2 nm.


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Next, the thermally treated seeds were used for growth of nanoparticles. We use HAuCl4 as the gold precursor, ascorbic acid as the reducing agent and BDAC as stabilizer due to its higher micelle concentration than other surfactants. BDAC has higher flocculation rate at a very low surfactant concentration which helps to avoid some of the critical issues associated with higher surfactant concentration such as solubility, high viscosity and the unpredictable changeovers from spherical micelles to worm-like micelles 38. Thermally aged seeds of different time duration were used as seed to investigate if the thermal aging process plays a role in defining the structure of the finally grown nanoparticles. Using UV-vis spectroscopy, we monitored the LSPR bands of different growth solutions and found that using seeds with longer thermal aging time led to more red-shifted LSPR peak along with the appearance of additional peak (Figure 2 a). For example, seeds with shorter aging time (90 min in present case), only one LSPR band at 533 nm was observed. However, when we switched to seeds with increased duration of thermal treatment (240 min) the LSPR band showed appearance of a shoulder in addition to the main peak and that of 400 min thermally aged seed showed the appearance of two well separated LSPR bands having peak positions at 525 nm and 593 nm. Presence of two separated LSPR bands generally arise either from structural properties of the one dimensional anisotropic nanostructures 39 or solutions having varied size nanostructures. To investigate this, we performed TEM analysis of the three different growth samples which were prepared using seeds at 90 min, 240 min and 400 min of thermal treatment. At shorter aging point i.e. 90 min, 80 % structure showed clear pentatwinned crystal structure having decahedral morphology and rest of the 20 % were like pseudo-spheres or some other random structures (Figure 2 b). For 240 min aged seeds, we observed few elongated decahedrons having bead like structure (Figure 2 c). Interestingly, for the 400 min heat treated


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seeds, we observed that almost all particles (above 90 %) changed their shape from decahedron to anisotropic pentatwinned beads like morphology (Figure 2 d). Thus, the two resolved peak observed in the UV visible spectrum of these particles could be ascribed to the transverse and longitudinal LSPR modes of these bead like nanostructure.

Figure 2. (a) UV-vis spectra of growth samples prepared using seeds at 90 min, 240 min and 400 min of heating. (b) TEM image of growth sample prepared using (b) 90 min, (c) 240 min and (d) 400 min thermally treated seeds. Scale bar of 2 (b), (c) and (d) corresponds to 50 nm, 100 nm and 200 nm respectively.

HRTEM images of Au bead is shown in Figure S2 a-c. The presence of twin planes along the longitudinal axis of the Au beads were clearly observed in the HRTEM images (Figure S2b). The top view of the Au bead also clearly showed pentatwinning. On the basis of these observations,


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we conclude that Au beads have penta-twinned morphology with {111}-faceted ends

17,40.

Polycrystalline nature of the Au bead was also confirmed from the corresponding SAED pattern (figure S2d).

Effect of seed concentration on the growth of Au beads was also evaluated by varying the seed volume of 400 min thermally treated seeds, while other factors were kept constant. It is to be noted here that thermal treatment of 400 min was important to achieve a uniform distribution of anisotropic Au nanobeads. When we decreased the volume of seed in the growth solution from 0.10 mL to 0.075 mL, 0.06 mL and 0.045 mL, the longitudinal LSPR shifted from 593 nm to 605 nm, 615 nm and 635 nm respectively (Figure 3 a). On the other hand, we did not find any observable change in its transverse LSPR band (Figure 3 a). Transmission electron microscopy revealed increase in the size of the Au beads with decrease in seed volume (Figure 3b and 3c). For larger seed volume i.e. 0.10 mL, the size of Au-bead was l = 67.4 ± 3.0 nm and b = 42.9 ± 2.5 nm (aspect ratio = 1.6 ± 0.10) and for the lowest seed volume used, i.e. 0.045 mL, the size was l = 90.7 ± 4.1 nm and b = 45.8 ± 3.8 nm (aspect ratio = 2.0 ± 0.20). This suggested that the present methodology can be used for the silver free synthesis of size tuneable Au beads, simply by varying the amount of thermally treated seed in the reaction.


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Figure 3. (a) UV-vis spectra of growth samples prepared using seeds volume of 0.1 mL, 0.075 ml, 0.06 mL and 0.045 mL. (b) TEM image of growth sample prepared using (b) 0.10 mL of seed volume (size, l = 67.4 ± 3.0 nm and b = 42.9 ± 2.5 nm; aspect ratio = 1.6 ± 0.10) and (c) 0.045 ml of seed volume (size, l = 90.7 ± 4.1 nm and b = 45.8 ± 3.8 nm; aspect ratio = 2.0 ± 0.20). Scale bar of 3 (b) and 3 (c) corresponds to 200 nm and 100 nm respectively.

Next, we used this pentatwinned Au bead (aspect ratio. = 1.6) as core template to prepare bimetallic core shell Au beads@Ag nanorod. Pentatwinned nanostructures are generally surrounded by both low-index end facets {111} and high index side facets

1,17,22,41.

Due to this

property, multi-twinned nanostructures have more active sites which supports easy deposition of other metals on its various facets 41. Firstly, BDAC was replaced with CTAC as the surface capping agent of Au bead. Next, we followed a stepwise seed mediated process for the silver overgrowth on Au bead (aspect ratio = 1.6 ± 0.1) in the presence of CTAC as a capping agent, AgNO3 as silver precursor and ascorbic acid as the reducing agent.

Here, constant magnetic stirring and

temperature during the Ag overgrowth process plays a crucial role in obtaining a uniform monodisperse final product

38.

Larger sized rods generally accelerate flocculation-induced

sedimentation process. To overcome this, we employed a constant magnetic stirring and temperature of 60 oC during the Ag overgrowth process which yielded highly uniform products (Figure 4). This method produced high quality nanostructures in terms of uniformity and monodispersed Ag nanorods, with core Au bead located at the central position. We were also able to prepare Au bead@Ag nanorods of varied aspect ratio from 2.8 ± 0.2 to 6.5 ± 0.4 simply by


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increasing the volume of AgNO3 and ascorbic acid in the growth solution (Figure S3), keeping other parameters constant. As indicated by the UV-Vis-NIR spectra in Figure 4 a, by increasing the volume of AgNO3 (2 mM) from 0.8 mL to 1.8 mL leads to shift in the longitudinal surface plasmon peak from 760 nm to 1104 nm. On the other hand, we observed a negligible shift in its transverse LSPR mode. This indicated the directional overgrowth of silver along the longitudinal axis of the rod. Liu et al. have explained the mechanism of silver overgrowth on Au decahedron. In their work, they considered decahedral Au seed for growth of nanobipyramids and rods and proposed that each decahedral seed have 10 {111} facets and 5 small {100} facets which led to the growth of Ag shell along the direction. Similarly, during overgrowth of Ag on pentatwinned Au-bead, deposition of Ag-based species leads to the construction of {100} facets on the side surfaces of Au bead and the {111} facets at the two ends. Along with this, the presence of halide surfactant complexes increases the stability of {100} facets. In addition, the packing density of the surfactant CTAC is lower at the ends of the Au bead, leaving the {111} facet more open for reactions with Ag species. This leads to selective deposition of Ag on {111} along with the very fast growth in the direction creating one dimensional products

1,17.

As our

pentatwinned Au bead which is used as seed is the elongated form of the decahedron, therefore, the growth of Ag on Au-bead is expected to follow a similar mechanism of growth along the longitudinal axis thereby creating one dimensional Au bead@Ag nanorods (Figure 4 b-f). This growth mechanism is also consistent with that of Ag-Au-Ag heterometallic nanorods proposed by Seo et al. 42. The outer shell of this bimetallic Au-bead@Ag rod possess pentagonal symmetry.


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Figure 4. (a) UV-vis-NIR spectra of Au-bead@Ag nanorods of different aspect ratios. TEM image of Au-bead@Ag nanorod (b) aspect ratio: 2.8 ± 0.2, (c) aspect ratio: 4.5 ± 0.3 (d) aspect ratio: 5.3 ± 0.3, (e) aspect ratio: 6.0 ± 0.3 and (f) aspect ratio: 6.5 ± 0.4. Scale bar corresponds to 4 (b) 100 nm, 4 (c) 200 nm, 4 (d) 200 nm, 4 (e) 200 nm and 4 (f) 200 nm. Finally, we prepared Au nanocapsules having a solid Au bead core and hollow and porous Au shell giving it a capsular structure through galvanic replacement reaction (GRR) by titrating the bimetallic Au-bead@Ag rods with HAuCl4 solution. The rationale behind synthesizing these structures is to achieve a hollow porous structure having increased surface area, intrinsic EM hotspots and tuneable LSPR across wide range, spanning from visible to NIR region, which finds potential application in catalysis, photothermal therapy, drug delivery, bioimaging, and surface enhanced Raman scattering 2,4-6,9. GRR involves the exchange of metal atom from the nanoparticle in colloidal solution with another metal precursor having higher reduction potential 33. Here, the standard reduction potential of the AuCl4-/ Au redox pair (0.99 V vs SHE) is higher than the that of Ag+/Ag redox pair (0.80 V vs SHE) which results in the replacement of Ag from the Au@Ag


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nanoparticle. This process further leads to the creation of hollow structure with porous shells on subsequent dealloying process. This is achieved by the removal of Ag atoms by AuCl4- which generates lattice vacancies, creating small holes in the shells. The size of the holes gets bigger as the dealloying process continues yielding hollow and porous nanostructures.33,34. We employed this GRR method for all the five different sets of Au bead@Ag nanorods having different sizes to study the effect of GRR on its final morphology. UV-vis-NIR spectroscopy was used to monitor the GRR process, where we observed the disappearance of the peaks corresponding to the plasmon modes of Ag shell around the bead along with the appearance of two redshifted broadened peak due to the creation of thin porous anisotropic Au shell (Figure 5a). The plasmon peak at NIR II region appeared due to the longitudinal plasmon mode of porous Au shell formed by GRR validating the replacement of Ag shell by Au. It is noteworthy that all the synthesized plasmonic nanocapsules (Set 1 - 5) possessed extinction peak in both NIR I and II regions. With increase in size of the Au nanocapsules from set 1 to set 5, a redshift in the longitudinal LSPR modes are observed which is attributed to the increased size of the nanocapsule in the direction. The experimental spectra (Figure 5a) could be reproduced by the simulations. An exact match could be found for the transversal mode (530 nm) whereas the multiple longitudinal are slightly blueshifted (Figure S4). However, this difference is not surprising, because the simulation only takes one nanoparticle into account while the experiment is an average over many nanoparticles (spectral broadening). We have studied this influence of spectral broadening in an earlier publication at gold nanoframes 43. The TEM image clearly revealed the hollow porous structure of the Au nanocapsules with Au beads at its core and a thin porous Au shell. The shell thickness of the nanocapsules was approximately 5 nm for all the different sets. Monodisperisity and uniformity can be clearly seen


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from the TEM images of all the samples. The SEM images of the Au nanocapsule (Figure S5) clearly revealed the presence of pores in the nanostructure with solid visible core. The SEM image of the Au-bead@Ag nanorods after partial GRR demonstrated clear pentagonal symmetry of the structure (Figure S6).


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Figure 5. (a) Represents the UV-vis-NIR spectra of Au nanocapsules after galvanic replacement reaction of Au-bead@Ag nanorods of different aspect ratios. (b), (c), (d), (e) and (f) corresponds to TEM images of Au nanocapsules of different aspect ratios. Scale bar corresponds to (b) 100 nm, (c) 200 nm, (d) 200 nm, (e) 200 nm and (f) 200 nm.

Noble metals (Au, Ag) are very well known for their excellent surface enhanced Raman scattering (SERS) activity. SERS activity of the metal nanostructures are highly dependent on their shape, surface roughness, and presence of edges or corners

2,5,7,9,10,22.

Here the synthesized Au

nanocapsules resembles to the nanorattles which have a core shell morphology. Apart from this, it has surface roughness due to the presence of porous outer shell. Most importantly, nanorattle structure offers a distinct feature of plasmonic coupling between the core and outer porous shell 9 and also at the pore sites. This plasmonic coupling leads to the creation of intense electromagnetic hotspots within the nanorattles on impingement of light of suitable wavelength44. So, presence of these three special features, (i) high roughened surface, (ii) intrinsic electromagnetic hotspots and (iii) presence of edges or corners in the synthesized Au nanocapsules provides an excellent platform to be used for SERS applications. To evaluate the SERS performance of different sized Au nanocapsules in comparison with intact Au-bead, we used 2-NT (2-napthalnethiol) as a model Raman reporter molecule 45. A fixed amount of 2-NT was exposed to the different sized plasmonic nanocapsules on a silicon substrate and its SERS was recorded at identical conditions using 785 nm NIR laser. In bulk 2-NT SERS spectra, the Raman band which corresponds to C-H bending at 1069 cm-1 (Figure S7) shifts to 1059 cm-1 in all SERS spectra indicating the binding of 2-NT with metal surface9,46. We obtained four prominent Raman bands at 1059 cm-1, 1373 cm-1, 1448 cm-1


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and 1620 cm-1 (Figure 6 g – l). Out of these, 1059 cm-1 band correspond to the ring C-H bending and other three corresponds to ring stretch vibrations of 2-NT molecule 45. Only two bands i.e. 1059 cm-1 and 1373 cm-1 have intense Raman signal and hence, these two Raman bands were monitored to identify the best SERS performer. The SEM image of the SERS substrate prepared by depositing the different nanomaterials on Si wafer, is shown in Figure 6 a  f and the corresponding SERS spectra obtained from 5 different spots from each of the substrate with different nanomaterials is given in Figure 6 g  l. For Raman band at 1059 cm-1 (band 1), the average SERS intensity (in counts) of Au-bead and Au nanocapsules of different aspect ratios 2.8 (set-1), 4.5 (set-2), 5.3 (set-3), 6.0 (set-4) and 6.5 (set-5) were observed to be 2767 ± 590, 36450 ± 4703, 6418 ± 1190, 9553 ± 1964, 10178 ± 941 and 13367 ± 1414, respectively. The order of SERS counts value for band 1 was set-1 > set-5 > set-4 > set-3 > set-2 > Au-bead (Figure 6 m). The same order of SERS counts value was also observed for Raman band 2 at 1373 cm-1 (Figure 6n). Here, the Au nanocapsules which have least size (aspect ratio = 2.8) shows the highest SERS intensity value as compared to other sizes and nearly 11 times more intense than the Au-bead nanoparticles. The distance between the solid core and porous shell affects plasmons coupling, which is thought to be the possible reason for the variation in the observed SERS intensity. Since, in case of set-1 of Au nanocapsules, the gap between the solid core and the outer shell is least amongst the investigated system, this particle possesses much intense EM field and thus enables it to display the maximum SERS enhancement. On the other hand, the order of SERS intensity counts of rest of Au nanocapsules are set-5 > set-4 > set-3 > set-2. Here, we observe an increase in the SERS intensity of both the bands 1 and 2 with increase in size of the nanocapsules. The possible reason behind this opposite behaviour is the increase in size which also affects the EM field along with the presence of more pores on bigger


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particles 47. Further, Au-bead showed least SERS intensity because of the absence of plasmonic coupling and surface roughness/porosity.


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Figure 6. (a - e) FESEM image of Au beads and different sets of Au nanocapsules deposited in Si substrate which was used for SERS measurements. (f- l) Corresponding SERS spectra obtained from five different points of the substrate deposited with Au beads and Au nanocapsules of different aspect ratios following adsorption of 2-NT. Average SERS intensity plot of different nanostructures (m) for 1059 cm-1 band and (n) 1373 cm-1 band.

FDTD simulations were also carried out to map the EM field intensity distribution in the different nanostructures. All nanocapsules are excited at one higher order longitudinal mode (see Figure S4). To be specific, Set-1 shows an exact match with the second order longitudinal mode, whereas Set-2 to Set-5 shows an excitation at the fourth order longitudinal mode (right shoulder). The simulated electric field distribution is shown in Figure 7 a and b which clearly demonstrates that the Au nanocapsules of the smallest size (set -1) possess the most intense electric field with maximum intensity in the gap between the solid core and shell. Whereas, for rest of the sets 2 – 5, there was increase in the electric field intensity with increase in the size of the nanocapsules. Further, the Au bead displayed the least electric field intensity along its surface. This FDTD simulation results exactly match with the experimental data where set 1 showed to have the maximum SERS intensity while Au-beads the least (Table S1). Therefore, we can say that Au nanocapsules which have lowest aspect ratio (i.e. 2.8, Set-1) have the highest SERS signals as opposed to other sizes of Au nanocapsules and can suitably be employed as a SERS candidate for the detection of trace analytes.


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Figure 7. FDTD simulations showing the unpolarized electric field distribution in Au beads and Au nanocapsules of different sizes at an excitation wavelength of 785 nm. (a) Electric field distribution plotted (a) in x-z plane (y=0) and (b) as slice plot. All scale bars between 0 and 10 |V/m|².

Next, we fabricated a flexible SERS substrate simply by loading the set-1 plasmonic Au nanocapsules on cellulose fibre membrane. Presence of hydroxyl groups on this cellulose surface enables the electrostatic adsorption of positively charged Au nanocapsules 9,48. SEM image of the loaded cellulose substrate revealed the uniform adsorption of Au Nanocapsules on the cellulose fibres (Figure 8b.) whereas, smooth morphology of the cellulose fibrils is clearly observed in the SEM image of unloaded substrate (Figure 8a). The magnified SEM image (Figure S8) of the loaded


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substrate clearly reveals the presence of the Au nanocapsules on the cellulose fibres. The density of these plasmonic nanocapsules on cellulose surface as calculated from SEM images and it was found to be ~ 40 ± 4/ µm2. After adsorption of Au nanocapsules onto the surface of cellulose fibres, it can assist as a simple and flexible SERS substrate for the detection of trace level analytes. Next, we investigated the SERS sensing efficiency of the fabricated substrate by exposing the substrate to different concentrations of 2-NT (from 1 mM to 1 fM) (Figure 8c and d). Here, we got shift in the Raman band from 1059 cm-1 to 1061 cm-1 due to the adsorption of nanocapsules to cellulose fibres instead of Si wafer 9,46. We monitored the Raman band at 1061 cm-1 to determine the sensing ability of the substrate. From figure 8c, we can clearly observe increase in the SERS signal with increase in the concentration of analyte. Whereas, no SERS signal was detected in case where the 2-NT molecules were not loaded, demonstrating that the signals are arising from the analyte molecule and not from the fabricated substrate itself. Figure 8d represents the magnified SERS data for the three lowest concentration of the analyte used along with the negative control in the study. We can clearly see distinguishable peak of 1061 cm-1 even for 1 fM concentration of the analyte (S/N ratio > 4). The analytical enhancement factor of the fabricated SERS substrate was calculated to be 4.08 × 1013 (see supporting information for details). The semi-log plot between 2NT concentration vs 1061 cm-1 Raman band showed increase in intensity with respect to the concentration of analyte molecule (Figure 8e). The inset of figure 8e represents the digital images of the loaded and unloaded SERS substrate used in the present study. The dark grey colour of the loaded substrate is due to the adsorption of the nanocapsules onto the cellulose substrate. Thus, we were able to detect 1 fM analyte molecule using the fabricated SERS substrate.


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Figure 8. (a) SEM image of the unloaded cellulose substrate, (b) SEM image showing the uniform adsorption of Au nanocapsules on a cellulose substrate, (c) SERS spectra obtained from flexible plasmonic cellulose substrate after exposing it to varying concentration of 2-NT, (d) zoomed SERS spectra at low concentrations, and (e) Semi-log plot showing the concentration vs intensity of the 1061 cm−1 Raman band demonstrating monotonic increase in the peak intensity with increasing concentration. Inset shows the photograph of the cellulose substrate before and after adsorption of Au nanocapsules.

Conclusion We have successfully demonstrated a facile approach for the synthesis of Au nanocapsules with a solid pentatwined Au bead as core and thin porous rod-shaped Au shell having LSPR peak both in


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NIR I and NIR II region. The synthesis process is simple, reproducible, and convenient, and affords Au nanocapsules in high yield. Au nanocapsules are excellent building blocks for SERS substrate due to presence of intense intrinsic EM hot spots. FDTD simulations data represents outstanding agreement with the experimentally observed size dependent SERS performance of the Au nanocapsules. In addition, the Au nanocapsules showed superior SERS sensing ability in detecting molecules of interest (2-NT as a model analyte) with distinguishable Raman band with a detection limit of 1 fM (S/N ratio > 4). This nanostructure developed in this work is believed to be extendable for a broad range of biomedical applications such as SERS based bioimaging, photothermal therapy and synergistic photothermal + chemotherapy.

Supporting Information: TEM images, SAED pattern and size distribution of thermally twinned Au seeds, TEM images and SAED pattern of Au beads, effect of Ag+ ions concentration on aspect ratio of Au bead@Ag nanorods, SEM images of Au nanocapsules and fabricated cellulose SERS substrate. The Supporting Information is available free of charge on the ACS Publications website.

Notes: The authors declare no competing financial interest.

Acknowledgement AJ acknowledges the support from BioX centre and Advanced Materials Research Centre (AMRC), Indian Institute of Technology Mandi for research and infrastructure facility. Financial support from Department of Biotechnology (DBT), Government of India, under project number: BT/PR14749/NNT/28/954/2015 and Department of Atomic Energy – Board of Research in Nuclear Sciences (DAE-BRNS) under the project number 37(2)/20/29/2016-BRNS/37260 is also


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acknowledged. This project was also financially supported by the Volkswagen Foundation through a Freigeist Fellowship to TAFK. PS would like to acknowledge MHRD for providing doctoral fellowship.

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NIR-Active Plasmonic Gold Nanocapsules Synthesized Using

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