Highly Uniform AuPt Bimetallic Nanoplates and Nanorings with Tunable Optical Properties and Enhanced Photothermal Conversion Performance in NIR-II Window

Highly asymmetric bimetal nanostructures, such as AuPt nanorings and Au–Ag nanoplates, possess superior plasmonic properties owing to various synergistic effects between different components and diversified morphologies. Herein, we report a controllable growth of asymmetric Pt-Au nanoplates and Au@Pt nanorings with excellent photothermal conversion efficiency. Hexagonal Au nanoplates are used as templates, which is achieved through a transformation reaction from Au nanotriangles. Pt-Au nanoplates are prepared by a site-selective growth of Pt on the rim of obtained hexagonal Au nanoplates. Subsequently, Pt nanorings are obtained by a selective etching of Au, and a regrowth of Au on the Pt nanorings leads to bimetallic Au@Pt nanorings. The evolution of extinction spectra during the whole process is carefully studied. Under irradiation by 1064-nm laser located in the second near-infrared bio-window, the Pt-Au nanoplates exhibit excellent photothermal conversion, better than that of initial Au nanotriangles and hexagonal Au nanoplates with same mass concentration. The improvement of photothermal effect can be ascribed to the strong surface plasmon resonances and coupling between Au and Pt.


Introduction
Near-infrared (NIR) photothermal conversion materials have attracted much attention in many applications like nanoscale heat source [1], photothermal cancer therapy [2], and drug release traceability [3]. The NIR window is often divided into NIR-I (750-950 nm), NIR-II (1000-1350 nm), and NIR-III (1450-1870 nm) [4,5]. The NIR light with longer wavelength has lower photon scattering and deeper penetration depth in biological tissue [6,7]. In recent years, various NIR-II photothermal agents have been developed for efficient heat production, including plasmonic metal nanoparticles [8], bimetal nanostructures [9], and metal-semiconductor heterostructures [10]. The plasmon-induced field enhancement and charge separation effect have been applied to improve the photothermal conversion efficiency [11]. Au and Ag nanoparticles with localized surface plasmon resonances (LSPRs) usually exhibit significantly enhanced light-to-heat conversion efficiency due to their excellent plasmonic light harvesting performance [12][13][14][15]. Bi and co-workers reported spiky gold nanoparticles with a record light-to-heat conversion efficiency of 78.8% under irradiation by 980-nm light [16]. Lindley and co-workers prepared bumpy hollow gold nanospheres by a facile pH modification with a maximum photothermal conversion efficiency of 99% [17]. Compared with plasmonic Au and Ag nanocrystals, Pt and Pd nanocrystals have larger plasmon damping and lower Fermi level. The combination of Au or Ag with Pt or Pd could efficiently utilize the harvested light energy for photocatalytic activity and photothermal conversion [18][19][20]. For instance, Tang and co-workers reported that Au@Pt nanostructures exhibited better photothermal efficiency compared to Au nanorods [21]. Yang and co-workers synthesized Au@Pt nanoparticles with dendritic structure and reported the photothermal conversion efficiency of 44.2% [22]. The combination of plasmonic nanocrystals with other photothermal materials has also attracted broad attention. Leng and co-workers constructed gold nanorods-copper sulfide heterostructures, offering significant photothermal conversion efficiency up to 62% [23].
The optical properties of plasmonic metal nanostructures are also dependent on their morphology [24][25][26][27]. In recent years, the preparation of plasmonic metal nanostructure with various morphologies has been well developed, including spherical [28], cubic [29], rod-like [30,31], and plate-like [32][33][34]. Among these different shapes of metal nanostructures, 2D nanoplates aroused particularly interest due to their unique optical properties. Au nanoplates with sharp corner and straight edge give rise to rich LSPRs including in-plane dipolar, quadrupolar, and out-of-plane plasmon modes [35]. Moreover, derivatives of nanoplates exhibit intriguing morphology and optical properties. For instance, using Au nanoprisms as template, Park's group synthesized Au@Pt nanorings and split nanorings with controlled morphologies [36].
Herein, we report a facile and controllable approach to synthesize Pt-Au nanoplates. Au nanotriangles with uniform size and shape were prepared at first step. Then, the Au nanotriangles were transferred to hexagonal Au nanoplates by an overgrowth of Au via reducing HAuCl 4 with ascorbic acid (AA). Followed by selectively growing Pt onto the rim of hexagonal Au nanoplates, the Pt-Au nanoplates were prepared. Whereafter, Pt nanorings were obtained by etching Au from the Pt-Au nanoplates. Interestingly, Au could be regrown onto the rim of Pt nanorings, forming Au@Pt nanorings. Accompanying with the morphology revolution, the optical properties of these bimetallic nanostructures exhibit highly tunability in NIR region. We investigated the optical properties and photothermal conversion performance of these synthesized nanoplates and nanorings systematically. Under laser irradiation of 1064 nm in NIR-II window, the Pt-Au nanoplates exhibit excellent photothermal conversion, better than that of initial Au nanotriangles and hexagonal Au nanoplates with same mass concentration. The enhanced photothermal effect of Pt-Au nanoplates is mainly attributed to the hexagonal Au nanoplates as light-harvesting unit with size-dependent LSPRs, and the Pt nanocrystals as plasmonic damping unit were deposited on the rim of hexagonal Au nanoplates.

Synthesis of Pt-Au Nanoplates
The CTAB-stabilized Au nanotriangles were prepared by a seed-mediated growth method, as reported in previous work [18]. Whereafter, 0.07 mL HAuCl 4 (0.01 M), 0.035 mL AA (0.1 M), 1.6 mL H 2 O, and 0.5 mL Au nanotriangles were added into 0.25 mL CTAB (0.1 M) aqueous solution in sequence. In this reaction, the Au nanotriangles were transferred into hexagonal Au nanoplates. The hexagonal Au nanoplates were centrifuged at 8000 rpm for 8 min and then redispersed in water for further use.

Preparation of Pt Nanorings
In a 50-mL vial, 10 mL CTAB (0.05 M), 0.5 mL NaI (0.01 mM), 0.5 mL HAuCl 4 (2 mM), and 1 mL of as prepared Pt-Au nanoplates were mixed and heated to 50 °C for half an hour. The product of highly uniform Pt nanorings was centrifugated and washed by water for morphology characterization.

Preparation of Au@Pt Nanorings
For the regrowth of Au onto the Pt nanorings, 1.25 mL AA (5 mM) was added into the uncentrifuged solution of Pt-Au nanorings. Then, the solution was kept undisturbed in an oven at 30 °C until the color was changed to black. Finally, the sample was centrifuged at 8000 rpm for 8 min and washed with water.

Photothermal Conversion Measurement
To study the photothermal effect of these nanorings and nanoplates, a 1064-nm NIR laser beam with a power density of 1 W/cm 2 was used in the experiment. The mass concentration of these samples was kept same as 300 μg/mL. The temperatures were recorded using an FORTRIC225 IR thermographic camera (Shanghai Thermal Image Electromechanical Technology Co. Ltd., China).

Characterizations
The morphology of Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanorings, and Au@Pt nanorings were investigated by using a scanning electron microscope (SEM, Hitachi S-4800) operating at 200 kV accelerating voltage. The absorption spectra were recorded by using an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co. Ltd., and Agilent Cary 5000).

Growth of Bimetallic Pt-Au Nanoplates and Their Optical Properties
The preparation process of Pt-Au nanoplates is shown in Fig. 1a. In this case, a seed-mediated growth method was applied to prepare Au nanotriangles, using CTAB as structure-directing agent, HAuCl 4 as gold source, and AA as reductant. The Au nanotriangles with high-dimensional uniformity were obtained through a selective sedimentation mechanism [37]. Figure 1b shows the typical SEM image of purified Au nanotriangles. The average edge length of these Au nanotriangles is 180 nm. The energy-dispersive X-ray spectroscopic (EDS) analysis in Fig. S1a confirms the presence of Au. Under the mild reduction condition in the presence of HAuCl 4 and AA, the abundant AuCl 4 − ions could be used to etch the tips of these nanotriangles, leading a transformation of these Au nanotriangles to hexagonal Au nanoplates (see Fig. 1c). The transformation process is dependent on the concentration of HAuCl 4 , which is reflected by SEM images and extinction spectra (Fig. S2). As shown in Fig. 1i and Fig. S2e-f, the main peak of in-plane dipolar plasmon resonance for the purified Au nanotriangles is presented at around 1188 nm. Accompanying with the increase of HAuCl 4 , the corresponding main peak is blueshifted gradually. When the amount of HAuCl 4 solution reaches to 70 μL, highly uniform hexagonal Au nanoplates with average edge size of 86 nm are obtained (Fig. 1c). The corresponding main peak is located at 799 nm.
In the presence of CTAB, NaI, and AgNO 3 , Pt was grown onto the hexagonal Au nanoplates with potassium chloroplatinate and ascorbic acid as platinum source and reductant, respectively. Figure 1d shows the SEM image of Pt-Au nanoplates with dipolar LSPR at 1036 nm. It is clearly that Pt atoms are preferentially deposited on the rim of hexagonal Au nanoplates. There also have some Pt nanoparticles deposited onto the flat surface of Au nanoplates. The EDS mapping shown in Fig. 1f-h displays the element distributions of an individual Pt-Au nanoplate, which matches well with the morphology observation in SEM. In the reaction of Pt deposition, AgNO 3 and NaI were added into the reaction solution. AgI may be precipitated, and Ag atoms may be deposited onto the Au nanoplates [25]. Halide ions strongly affect the anisotropic growth due to the selective binding of iodide ions onto specific surfaces of metal [38]. Due to the existence of I − ions, Ag layer, and AgI precipitation, Pt atoms are preferentially deposited on the rim of hexagonal Au nanoplates [39]. After the deposition of Pt, the main peak of LSPRs is redshifted from 799 to 1036 nm, as shown in Fig. 1i. The corresponding photograph image of Au and Pt-Au nanoplates is exhibited in Fig. 1e.

Morphology and Optical Properties of Pt-Au Nanoplates with Varied Amounts of Pt
As for the Pt deposition onto the Au nanoplates, the amount of potassium source would influence the morphology and optical properties of bimetallic Pt-Au nanoplates. By varying the amount of potassium chloroplatinate (5 mM K 2 PtCl 6 ) from 40 to 480 μL, the SEM images and extinction spectra of Pt-Au nanoplates are shown in Fig. 2. In this case, the initial hexagonal Au nanoplates have a dipolar LSPR at 799 nm. When 40 μL of K 2 PtCl 6 solution is added, Pt is preferentially deposited on the rim of hexagonal Au nanoplates. As the volume of K 2 PtCl 6 solution is increased, the thickness of Pt increases obviously. Gradually, Pt is also deposited both on the top and bottom flat surfaces of nanoplates. The sample shows erythrocyte-like when the amount of K 2 PtCl 6 solution reaches to 480 μL.
The extinction spectra of Pt-Au nanoplates with different volumes of K 2 PtCl 6 solution are shown in Fig. 2g. For better illustrating the spectral evolution, the wavelength and linewidth of main peak are plotted in Fig. 2 h. With the increase of Pt precursor amount, the main peak is red-shifted from 799 to 1036 nm. Meanwhile, the width of main peak is broadened from 196 to 368 nm. Interestingly, the high-order mode near 600 nm gradually disappeared. The evolution of extinction spectra demonstrates the influence of Pt deposition on the LSPRs of Au nanoplates. The damping effect of Pt causes the peak red-shift and broadening of dipolar plasmon mode [24].

Growth of Bimetallic Au@Pt Nanorings and Their Optical Properties
Theoretical and experimental investigations show that hollow plasmonic nanostructures have attractive optical properties in terms of field enhancement and polarizationdependent LSPR bands [40][41][42]. In this work, the Pt-Au nanoplates can be easily transformed to hollow ring-shaped 1 3 nanostructures through etching and regrowth progresses. Figure 3a exhibits the schematic progresses of etching and regrowth, which lead to the formation of Pt nanorings and Au@Pt nanorings with high uniformity. Monodisperse Pt-Au nanoplates are used as templates (Fig. 3b). In the presence of iodide ions, the etching process is obtained after adding HAuCl 4 and AA. As previously reported, HAuCl 4 could be used to etch Au nanocrystals, and halide ions were often used as Au etchants [24]. As shown in Fig. 3c, the central region of Pt-Au nanoplates was selectively etched, resulting in Pt nanorings. The average width of Pt nanorings is 36 nm. Subsequently, due to the reduction HAuCl 4 by AA, Au is regrown onto the Pt nanorings, forming Au@Pt nanorings. As shown in Fig. 3d, the Au@Pt nanorings with average side length of 135 nm are observed. The side width of nanorings is around 50 nm. The EDS analysis in Fig. S1c-d confirms the component materials of these two nanorings.
The optical properties of AuPt bimetallic nanostructures are closely related to the morphology and therefore tuned by the etching and regrowth processes. As shown in Fig. 3e, the Pt-Au nanoplates before etching exhibit an in-plane dipolar plasmon peak located at 1036 nm, with a weak side band around 718 nm. The plasmon band disappeared when Au is removed after the etching reaction. However, the LSPR is

Photothermal Performance
The aforementioned discussion indicates that the Pt deposition would damp the LSPR of Au. Due to the efficient light-harvesting of LSPRs, the plasmon damping by Pt deposition would possibly lead to an improvement of the photothermal conversion. The photothermal conversion efficiency is important for further successful photothermal therapy applications of plasmonic metal nanostructures. For investigating the photothermal effect of these bimetallic AuPt nanostructures, the LSPRs of Pt-Au nanoplates and Au@Pt nanorings were tuned to match 1064-nm laser in NIR-II window. Under 5-min laser irradiation with wavelength of 1064 nm and power intensity of 1 W/ cm 2 , the temperature of various samples was monitored and recorded by NIR image camera every 30 s. During the measurements, the concentration of all the samples was kept same as 300 μg/mL. Figure 4a exhibits the temperature curves along time of purified Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanoplates, and Au@Pt nanorings, respectively. The temperature increment ΔT of purified Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanoplates, and Au@Pt nanorings is 23.4 °C, 13.7 °C, 30.4 °C, and 25.6 °C, respectively. A remarkable maximum temperature of 60.1 °C for the Pt-Au nanoplates is obtained. b Photothermal temperature curves of Pt-Au nanoplates with varied amount of platinum source (40-480 μL of 5 mM K 2 PtCl 6 ). c Photothermal temperature curves of Pt-Au nanoplates with different concentrations. d Photothermal cycling test of Pt-Au nanoplates (300 μg/ mL)

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To systematically illustrate the photothermal conversion performance of Pt-Au nanoplates, the samples prepared with different amounts of Pt source were compared. As shown in Fig. 4b, with the increase of platinum source, the deposited Pt shells on the rim of hexagonal Au nanoplates are grown thicker, and the maximum temperature increment ΔT is increased. For the Pt-Au nanoplates prepared with 5 mM K 2 PtCl 6 , the dependence of ΔT on sample concentration is shown in Fig. 4c. With the increase of sample concentration, the maximum temperature increment ΔT is also increased. When the sample concentration reached to 300 μg/mL, the maximum temperature increment ΔT is 1.3 times that with concentration of 150 μg/mL. In additional, for clarifying the photothermal stability, six repeated cycles of laser heating were measured and shown in Fig. 4d. Any noticeable decrease is not observed in the temperature elevation. We also investigated the photothermal effect with varied power densities of 1064-nm NIR laser. The temperature increment ΔT is nearly linear related to the laser power. These results indicate that the Pt-Au nanoplates have excellent photothermal stability during laser irradiation.
The photothermal conversion efficiency of these nanostructures under same experimental condition is summarized in Table 1. Compared to the hexagonal Au nanoplates, the photothermal conversion performance of Pt-Au nanoplates is enhanced. The strong LSPRs of Au nanoplates lead to an efficient light-harvesting in NIR-II window. The enhanced photothermal conversion performance of Pt-Au nanoplates is attributed to the damping effect of Pt deposition, which promotes the photothermal conversion and heat generation. Furthermore, the Pt deposition produces rough surface and brings abundant active sites for catalytic reactions. It is expected that these bimetallic nanoplates and nanorings would show excellent performance in many photocatalytic applications.

Discussion and Conclusions
In summary, highly uniform monodisperse Au nanotriangles, hexagonal Au nanoplates, Pt-Au nanoplates, and Au@Pt nanorings were successfully prepared. The bimetallic Pt-Au nanoplates were synthesized through a selective growth mechanism, and that Pt nanocrystals were deposited on the rim of hexagonal Au nanoplates. Followed by etching and regrowth of Au, the hollow ring-shaped Pt and Au@Pt nanorings could be produced. The optical properties and photothermal effect of these bimetallic nanostructures were carefully investigated. The bimetallic Pt-Au nanoplates and Au@Pt nanorings exhibit intense and tunable LSPRs in NIR region, therefore excellent photothermal conversion performance. This study highlights the advantages of using bimetallic nanostructures for controllable photothermal conversion, photocatalytic, and biomedical application. Data Availability Informed consent was obtained from all subjects involved in the study. The data presented in this study are available on request from the corresponding author.