nanomaterials Article Sculpting Windows onto AuAg Hollow Cubic Nanocrystals Javier Patarroyo 1 , Neus G. Bastús 1,2, * and Victor Puntes 1,2,3,4, * 1 Institut Català de Nanociència i Nanotecnologia (ICN2), CSIC, The Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, 08193 Barcelona, Spain; javier.patarroyo@icn2.cat 2 CIBER en Bioingeniería, Biomateriales y Nanomedicina, CIBER-BBN, 28029 Madrid, Spain 3 Vall d’Hebron Institut de Recerca (VHIR), 08035 Barcelona, Spain 4 Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain * Correspondence: neus.bastus@icn2.cat (N.G.B.); victor.puntes@icn2.cat (V.P.) Abstract: Using surfactants in the galvanic replacement reaction (GRR) offers a versatile approach to modulating hollow metal nanocrystal (NC) morphology and composition. Among the various surfactants available, quaternary ammonium cationic surfactants are commonly utilised. However, understanding how they precisely influence morphological features, such as the size and void distribution, is still limited. In this study, we aim to uncover how adding different surfactants—CTAB, CTAC, CTApTS, and PVP—can fine-tune the morphological characteristics of AuAg hollow NCs synthesised via GRR at room temperature. Our findings reveal that the halide counterion in the surfactant significantly controls void formation within the hollow structure. When halogenated surfactants, such as CTAB or CTAC, are employed, multichambered opened nanoboxes are formed. In contrast, with non-halogenated CTApTS, single-walled closed nanoboxes with irregularly thick walls form. Furthermore, when PVP, a polymer surfactant, is utilised, changes in concentration lead to the production of well-defined single-walled closed nanoboxes. These observations highlight the role of surfactants in tailoring the morphology of hollow NCs synthesised through GRR. Keywords: hollow nanocrystals; galvanic replacement reaction; surfactant effect 1. Introduction Citation: Patarroyo, J.; Bastús, N.G.; Embracing the forms of porous nanocages and nanoframes, bimetallic AuAg hollow Puntes, V. Sculpting Windows onto nanocrystals (NCs) are garnering significant interest across diverse domains, including AuAg Hollow Cubic Nanocrystals. nanocatalysis [1–5], biomedical imaging [6,7], controlled drug delivery [8], chemical sens- Nanomaterials 2023, 13, 2590. https:// ing [9], and photothermal applications [10]. In these systems, the essence of their applica- doi.org/10.3390/nano13182590 bility lies in the precision with which the engineering of the hollow systems is achieved, as it fundamentally determines their optical, catalytic, and structural properties. One of the Academic Editor: Antonios Kelarakis most employed techniques for producing them is the galvanic replacement reaction (GRR). Received: 16 August 2023 GRR involves the spontaneous transmetalation redox reaction between two different Revised: 13 September 2023 metal ions driven by their difference in reduction potentials. A more reactive metal cation in Accepted: 14 September 2023 solution (Au) takes electrons from a less reactive metal in a sacrificial template (Ag), leading Published: 19 September 2023 to its oxidation and dissolution. In contrast, the noble metal is reduced and deposited onto the surface of the template [11]. In the process, as time advances or as the Au/Ag ratio increases, single or multiple voids open at the NC surface, which serve as channels through which the template’s core is removed, forming hollow nanostructures. In the Copyright: © 2023 by the authors. initial stages of the process, as the more noble metal is deposited and covers the template, Licensee MDPI, Basel, Switzerland. the chemical potential between the deposited and the corroded metals increases. This This article is an open access article leads to the formation of a pinhole at the interface, allowing a flux of cations to escape and distributed under the terms and preventing its closure until the core of the template is voided. The number and position of conditions of the Creative Commons these pinholes determine the final structure of multichambered hollow NCs [12]. Typically, Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ the pinhole closes once the corrosion of the template is complete, mainly when the reaction 4.0/). is conducted at high temperatures [13,14]. Once the hollow NC is formed, for high Au/Ag Nanomaterials 2023, 13, 2590. https://doi.org/10.3390/nano13182590 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2023, 13, 2590 2 of 13 ratios, further corrosion triggers a de-alloying process, forming pores in the hollow NC surface. As more vacancies collapse, these pores gradually develop, small and polydisperse in size and shape, distributed across the surface. While the shape of the resulting hollow NCs is primarily determined by the starting template, wall thickness, composition, and porosity are controlled by the interfacial alloying and de-alloying processes associated with the GRR [15–17]. Despite the effort, it remains challenging today to precisely control the position of the void within the hollow structure, especially its size and location [18]. In the pursuit of optimising performance [19], significant efforts have been focused on creating a variety of hollow NCs with increasingly sophisticated surfaces and inner structures via the coupling GRR with sequentially deposited templates [14,20,21], the Kirkendall effect [12,22] and combining co-reduction and co-corrosion processes [23,24]. Following these strategies, different configurations have led to a plethora of hollow NCs. However, one challenge remains the rational control of the pore locations within these hollow structures. Often, pores are small and randomly distributed across surfaces [8,25]. Attempts to directly enlarge the pores often result in nanoframes forming with huge voids, risking structural collapse [14]. In this context, it has been observed that localising the pores at the more reactive sites, such as the corners of the cube-shaped NCs, allows for enlarging the pores without compromising the overall structural integrity of the NCs. Xia and co-workers, in 2006, motivated the use of Ag nanocubes with truncated corners to obtain AuAg nanocages with pores confined to the corners [26]. Later on, Sun et al. selectively deposited Ag2 O patches at the corners of Ag nanocubes to chemically control the removal of Ag2 O deposits at the corner sides and the Ag core, forming Au-based nanoboxes with well-defined openings at corners [27]. Lu et al. recently developed a one-step process to produce hollow NCs with well-defined openings at the corners by employing cetylpyridinium chloride [28]. The method is based on the well-known fact that molecular surfactants, when absorbed, can modify the surface energy of the NC, thereby influencing—favouring or hindering—the deposition of atoms on specific crystalline facets, ultimately affecting the growth kinetics and morphology of the NC [29]. Moreover, surfactants interact with the precursors and intermediate species, governing whole NC growth kinetics [30]. This has been extensively explored and exploited for anisotropic crystal growth since Peng and Alivisatos’ seminal work on CdSe nanorods [31]. However, understanding surfactant effects in the formation and evolution of NCs during reverse growth reactions, i.e., by removing ions from a template rather than adding them, as in GRRs, remains poorly described and understood. Built upon the previous work of Gonzalez and co-authors [12] on the production of complex hollow NCs at room temperature, we herein study the role of the surfactant in shaping and controlling the morphological characteristics of hollow AuAg NCs, particularly the pore distributions at its surface. To this end, several surfactants, including three quaternary ammonium salt cationic surfactants, cetyltrimethylammonium bromide (CTAB), chloride (CTAC), and p-toluenesufonate (CTApTS), along with Polyvinylpyrrolidone (PVP), were tested. As a typical cationic surfactant, CTAB has turned out to be very effective in the shape-controlled synthesis of gold nanorods [32,33] and for the synthesis of complex hollow NCs [12]. CTAB and CTAC present the same alkyl chain but a different halide counterion. A non-halogenated cationic surfactant CTApTS was used to study the effect of the counterion on the final morphology. Additionally, a non-cationic polymer such as PVP was used to evaluate the role of the ionic surfactant. Understanding the influence of diverse molecular components provides valuable information for the rational design and synthesis of complex hollow NCs with tailored structures and functionalities by adding different surfactant amounts. Results show that the halide counterion controls the void formation in the hollow structure. When halogenated surfactants, such as CTAB or CTAC, are employed, multichambered opened nanoboxes are formed. The main difference is that, when using CTAB, a pinhole forms at the centre of the nanocube face, while, when utilising CTAC, the pinhole emerges non-centred, away from the face’s centre, edges, and corners. On the other hand, when CTApTS was used, Nanomaterials Nanomaterials 2023, 13,2023, 2590 13, x FOR PEER REVIEW 3 of3 of 14 13 centre of the nanocube only single-walled face, while,with closed nanoboxes whenanutilising CTAC, irregular the thick pinhole wall wereemerges non-cen- obtained. Finally, tred, away from the face’s centre, edges, and corners. On the other hand, when PVP was used, well-defined single-walled closed nanoboxes were obtained. when CTApTS was used, only single-walled closed nanoboxes with an irregular thick wall were obtained. Finally, when PVP was used, well-defined single-walled closed nanoboxes were obtained. 2. Results The production of AuAg hollow nanocrystals (NCs) is a two-step process which in- 2. Results volves the synthesis of monodisperse Ag nanocubes, which are later used as sacrificial The production of AuAg hollow nanocrystals (NCs) is a two-step process which in- templates for preparing hollow AuAg nanoboxes by GRR at room temperature in the volves the synthesis of monodisperse Ag nanocubes, which are later used as sacrificial presence of the different surfactants [5,16,22,34] (Figure S1). Ag NCs with well-defined templates for preparing hollow AuAg nanoboxes by GRR at room temperature in the pres- shapes, such as different ence of the nanospheres, nanocubes, surfactants nanorods, [5,16,22,34] (Figure nanowires, S1). Ag NCs and withnanoprisms, are com- well-defined shapes, monly such as nanospheres, nanocubes, nanorods, nanowires, and nanoprisms, are commonlyby used as sacrificial templates. Among them, we used cubic Ag NCs produced polyolused reduction as sacrificial as sacrificial templates templates. Among due them,towetheir anisotropic used cubic Ag NCs shape, interesting produced optical by polyol properties, and widely described applicability [17]. reduction as sacrificial templates due to their anisotropic shape, interesting optical prop- Representative erties, and widely transmission electron microscopy described applicability [17]. (TEM) images of the as-obtained samples (Figure 1A) show Representative the systematic transmission formation electron of highly microscopy (TEM) monodisperse images of theAg nanocubes, as-obtained whose samples crystals(Figure exhibit1A) show length an edge of 45.4 ±formation the systematic 5.1 nm inofthehighly monodisperse sides (Figure Ag 1B), which nanocubes, falls within whose crystals the typical range of exhibit sizesan edge length employed in of GRR45.4studies. ± 5.1 nm The in theUV–Vis sides (Figure 1B), absorption which spectra falls within of colloidal Ag the typical range nanocubes displayof sizes employed distinct in GRR and intense studies. surface The UV–Vis plasmon ab- resonance (SPR)sorption peaks atspectra ~440 nmof colloidal and ~350 Ag nm, nanocubes display indicative distinct of their and intense narrow surface plasmon size distribution cubic shaperesonance (SPR) peaks [35–37] (Figure at ~440 1C). The nmdiffraction X-ray and ~350 nm, indicative (XRD) patterns of their revealnarrow size distribu- four distinct diffrac- tion cubic shape [35–37] (Figure 1C). The X-ray diffraction (XRD) patterns tion peaks at 2θ values of 38, 44, 64, and 76◦ , which can be attributed to the (111), (200), reveal four dis- (220), tinct diffraction peaks at 2θ values of 38, 44, 64, and 76°, which can be attributed to the and (311) fcc Ag crystal planes, respectively (JCPDS file No. 04-0783). The overdominance (111), (200), (220), and (311) fcc Ag crystal planes, respectively (JCPDS file No. 04-0783). of the {200} reflection in the XRD pattern confirms the flat faces of the NCs and their cubic The overdominance of the {200} reflection in the XRD pattern confirms the flat faces of the shape (Figure 1D) [38]. NCs and their cubic shape (Figure 1D) [38]. Figure 1. Synthesis of Ag nanocubes. (A,B) Representative transmission electron microscopy (TEM) Figure 1. Synthesis of Ag nanocubes. (A,B) Representative transmission electron microscopy (TEM) images of theofhighly images monodisperse the highly Ag Ag monodisperse nanocubes with nanocubes withananedge edgelength lengthof 45.4 ± of 45.4 5.1nm. ± 5.1 nm.(C) (C) UV–Vis UV–Vis absorption spectra exhibit strong SPR peaks at ~440 nm and ~350 nm, indicating the uniform size distribution of the nanocubes. The inset shows a colloidal sample of the AgNCs. (D) XRD patterns of Ag nanocubes with distinct diffraction peaks corresponding to specific crystallographic planes and JCPDS file No. 04-0783. Nanomaterials 2023, 13, 2590 4 of 13 These synthesised Ag nanocubes were utilised as sacrificial templates for producing AuAg hollow nanoboxes. In a typical experimental procedure, the Ag nanocubes were dis- persed in a reaction medium containing various surfactants, ascorbic acid (AA) employed as a mild reducing agent, and an aqueous HAuCl4 solution, which was slowly added into the reaction mixture using a syringe pump. The presence of AA in the reaction system enables the reduction of AuCl4 − into AuCl2 − . This co-reduction process competes with the transmetalation redox process. It modifies the redox potential of Au cations, reducing their etching power and, ultimately, allowing for tailoring the structural features of the resulting NCs at both its morphology and roughness and porosity of the walls. The reaction temperature plays a crucial role in forming bimetallic AuAg nanostructures. The GRR rate is faster at higher temperatures, often leading to more aggressive corrosion processes, affecting the final structure’s roughness and porosity [39], pinhole sealing [12], and loss of reproducibility. Thus, to have better control over the GRR’s kinetics and the products’ morphology, all our experiments were performed at room temperature (RT). The structural evolution of the Ag nanocubes in the presence of CTAB (14 mM) with the addition of different amounts of HAuCl4 (1 mM) was investigated. CTAB is a cationic surfactant widely used in the shape-controlled synthesis of noble metal NCs. It acts as a stabiliser, structure-directing agent, and complexing ligand, effectively reducing the reduction rate of the metal precursor while facilitating its epitaxial deposition [33]. During the reaction, the morphological and compositional changes of Ag nanocubes were studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF- STEM), energy-dispersive X-ray spectroscopy analysis (EDS), and UV–Vis spectroscopy. When substoichiometric amounts of HAuCl4 (25 µL, 6.25 × 10−4 mmol) were added to the Ag nanocube solution, a thin layer of Au was initially deposited on their surfaces (Figure 2A). This deposition occurred through the direct reduction of Au cations to Au0 in solution, facilitated by the action of AA [40]. As more Au precursor was added (50 µL, 1.25 × 10−3 mmol), a small pinhole formed at the centre of the faces of the nanocubes (Figure 2B), indicating that the reaction is initiated locally at the high-energy sites of the crystal surface rather than randomly across its entire surface. As the reaction proceeded (75 µL, 1.87 × 10−3 mmol), this small hole provided access to the Ag core, the anode, where Ag is oxidised, and electrons stripped and transported from the core through the solid to the NC surface where Au cations are reduced (Figure 2C). With the addition of Au (100 µL, 2.5 × 10−3 mmol), the conversion of the Ag solid nanocubes into hollow AuAg nanoboxes is achieved (Figure 2D). When higher amounts of HAuCl4 solution were used (150 µL, 3.2 × 10−3 mmol), double-walled opened structures were formed (Figure 2E–H), which exhibited the homogeneous distribution of Ag (31.3 ± 7.9%, red) and Au (68.7 ± 7.8% Au, green) (Figure 2I). As expected, its size (51.0 ± 4.7 nm) is more significant than that of the precursor Ag NCs (45.4 ± 5.1 nm), accounting for the deposition of the Au layer onto the sacrificial template structure. One of the most exciting features of these bimetallic AuAg hollow nanostructures is the strong dependence of the colour of the colloidal solution on the degree of voiding of the NCs, which can be directly controlled by the amount of HAuCl4 added to the reaction. Thus, by increasing the content of HAuCl4 , the solution gradually turns from yellow, dark yellow, orange, red, red-violet, purple, and, finally, blue (Figure 2F). This colour evolution directly correlates with the position of the SPR band, allowing the precise tuning of the SPR peak to virtually any position within the visible range by controlling the volume of the added HAuCl4 (Figure 2G). Accordingly, while Ag nanocubes exhibit a narrow SPR band peaking at 440 nm, the double-walled opened nanoboxes display a broader band centred at ~680 nm. While the redshift is directly attributed to the formation of hollow structures with increasing Au content [41], the broadness of the peak can be ascribed to both phase retardation effects in larger NCs at lower energies [42]. The purity of the final structures, where single- and double-walled AuAg nanoboxes with different edge lengths, thickness, and corner sharpness coexist in the sample, can be observed in Figure 2E. This remarkable tunability of both colour and SPR peak position in the visible region highlights Nanomaterials 2023, 13, 2590 5 of 13 the unique optical properties of the bimetallic multichambered opened AuAg nanoboxes, Nanomaterials 2023, 13, x FOR PEER REVIEW opportunities for diverse applications in fields such as sensing, imaging,5and providing of 14 plasmon-mediated chemical reactions. Figure 2. Effect of CTAB on the structural evolution of the Ag nanocubes to multichambered opened Figure AuAg 2. Effect of CTAB nanoboxes on the structural after adding evolution different amounts of of the Ag. nanocubes HAuCl to multichambered opened 4 Representative transmission electron AuAg nanoboxes after adding different amounts of HAuCl4. Representative transmission electron microscopy (TEM) images of AuAg nanoboxes synthesised in the presence of CTAB with the addition microscopy (TEM) images of AuAg nanoboxes synthesised in the presence of CTAB with the addi- of various amounts of HAuCl4 1 mM: (A) 25 µL (6.25 × 10−4 mmol), (B) 50 µL (1.25 × 10−3 mmol), tion of various amounts of HAuCl 4 1 mM: (A) 25 µL (6.25 × 10−4 mmol), (B) 50 µL (1.25 × 10−3 mmol), (C) 75 µL (1.87 × 10−3 mmol), (D) 100 µL (2.50 × 10−3 mmol), and (E) 125 µL (3.12 × 10−3 mmol). (C) 75 µL (1.87 × 10−3 mmol), (D) 100 µL (2.50 × 10−3 mmol), and (E) 125 µL (3.12 × 10−3 mmol). (F) (F) Variation Variation of theof colour the colour ofcolloidal of the the colloidal solution solution uponupon increasing increasing amounts amounts of HAuCl of HAuCl 4 1 added 4 1 mM mM added into intovolume a fixed a fixed ofvolume Ag NCs of in Agthe NCs in the presence presence of CTAB of 14 CTAB mM. (G)14 UV–Vis mM. (G)spectra UV–Visofspectra of Ag Ag nanocubes nanocubes (black), (black), A (red), A (red), B (blue), B (blue), C (green), DC (green),and (purple), D (purple), and(H) E (yellow). E (yellow). HR-TEM(H) HR-TEM images images of multicham- of multichambered bered opened AuAg openednanoboxesAuAg nanoboxes as shown as shown in (D). (I) EDSinmapping (D). (I) EDS mapping ofnanoboxes of double-wall double-wall (red for Ag, green nanoboxes forfor (red AuAg, andgreen composite). for Au and composite). In our One system, of the most the critical exciting role ofofCTAB features these assisting bimetallicthe controlled AuAg hollowtransmetallation nanostructures is process is revealed when its concentration was systematically varied the strong dependence of the colour of the colloidal solution on the degree by threeof orders voidingofof magnitude, from 0.14 mM to 140 mM, while keeping constant the amount of the NCs, which can be directly controlled by the amount of HAuCl4 added to the reaction. HAuCl 4 and Thus, by increasing the content of HAuCl4, the solution gradually turns from yellow,low Ag templates added to the reaction mixture. When the concentration of CTAB was kept dark (0.14–1.4 mM), the process was less controlled, leading to poorly defined nanoboxes with yellow, orange, red, red-violet, purple, and, finally, blue (Figure 2F). This colour evolution compromised structural integrity and agglomerated small Au nanoparticles. In contrast, directly correlates with the position of the SPR band, allowing the precise tuning of the increasing the CTAB concentration to 140 mM facilitated a more controlled and efficient SPR peak to virtually any position within the visible range by controlling the volume of process, forming well-defined nanocrystals with single face-centred holes. (Figure S2). the added HAuCl4 (Figure 2G). Accordingly, while Ag nanocubes exhibit a narrow SPR The halide ions also play essential roles in directing the anisotropic growth of solid band metal NCs. Toatinvestigate peaking 440 nm, the double-walled their effect on the opened nanoboxesofdisplay final morphology hollowaNCs, broader band we sub- centred at ~680 nm. While the redshift is directly attributed to the formation stituted CTAB with CTAC at the same concentration (14 mM) while keeping all other of hollow structures experimentalwithparameters increasingunchanged. Au content Halide [41], the broadness ions, such as of Cl the andpeak can Br, not be modulate only ascribed to both phase retardation effects in larger NCs at lower energies [42]. The purity the redox potentials of metal ions but also form insoluble compounds with Ag, exhibiting of the final a structures, where single- and double-walled AuAg nanoboxes with different edge lengths, thickness, and corner sharpness coexist in the sample, can be observed in Figure 2E. This remarkable tunability of both colour and SPR peak position in the visible region highlights the unique optical properties of the bimetallic multichambered opened AuAg nanoboxes, providing opportunities for diverse applications in fields such as sensing, imaging, and Nanomaterials 2023, 13, 2590 6 of 13 strong affinity for the {111} crystal planes rather than the {001} planes. This preference leads to the effective passivation of the corners and edges of the Ag nanocubes by the formation of AgBr compounds. The morphological evolution of the Ag nanocubes with increasing amounts of HAuCl4 added to the reaction media is shown in Figure 3A–D. The use of CTAC resulted in notable morphological differences, especially in the initial stages of the reaction (50 µL, 1.25 × 10−3 mmol). Thus, the pinhole appears non-centred, away from the nanocube’s centre, edges, and corners (Figure 3A–E). EDS maps of a single non-centred pinhole NC show that Au is homogeneously distributed throughout the crystal, forming an outer layer of approximately ~4 nm (Figure 3F). In this case, by using higher amounts of HAuCl4 solution (125 µL, 3.2 × 10−3 mmol), we also observed the formation of double-walled opened nanoboxes (Figure 3D). Impor- tantly, the non-centred position of the pinhole is maintained. When multiple pinholes are formed (Figure S3), multichambered opened nanoboxes are produced (Figure 3G). The average length of the resulting nanoboxes was 54.2 ± 4.3 nm, with a wall thickness of 7.0 ± 1.4 nm (Figure S4), almost the same size as the nanoboxes obtained with CTAB. Remarkably, the inner cage is not centred, as observed with CTAB. Instead, its position corresponds to the pinhole’s initial location, which impacts the absorbance profile of the NCs (Figure 3I) [43]. EDS maps of a double-walled nanobox reveal a homogeneous dis- tribution of Ag and Au throughout the crystal (Figure 3H). This observation is consistent with line-scanning profiles, where the nanoboxes consist of both Ag and Au, with Ag being more abundant than Au. Furthermore, both hollow cavities are distinctly visible in the HAADF-STEM image and the EDS profile, where a drop in the Ag signal confirms their presence (Figure 3J,K). We hypothesise that, in the presence of Cl-, the compact ordering of AgCl (note that Br− is significantly larger than Cl− ) loosens in density as it approaches corners and edges. As a result, far from the centre, the area better protected by the AgCl flat layer, and close but not at the corners or edges, for which higher reactivity leads to faster passivation, is at these distances where GRR better occurs. This ultimately leads to pinholes opening away from the centre of the crystal face and closer to the corners and edges. This contrasts with hollow NCs synthesised at higher temperatures [11] or using PVP as a stabiliser, where the pinhole tends to close. As previously mentioned, carrying out the reaction at low temperatures is desirable to preserve the open pinhole and avoid surface reconstruction. Notably, if the reaction is performed more aggressively (rapid addition of more HAuCl4 ), multiple pinholes may appear, consistently positioned away from the corners and the center. Four windows can be opened in each cube face in such cases, leading to square-cross 3D morphologies. The optical properties of the synthesised AgAu nanoboxes were investigated using UV–Vis spectroscopy (Figure 3I). The SPR peak exhibited a redshift, ranging from ~440 nm to ~650 nm as the concentration of HAuCl4 solution was increased (1.25 × 10−3 mmol to 3.12 × 10−3 mmol). As mentioned earlier, this redshift results from the increasing Au content in the nanostructure and the enlargement of the void size. Similar to the observa- tions with CTAB, the broadness of the SPR band is attributed to both phase retardation effects in Au-rich structures and the formation of various products during the reaction, including single-, double-, or multi-walled nanoboxes, as well as the presence of different morphologies. Nanomaterials 2023, 13, 2590 7 of 13 Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 14 Figure 3. Effect Figure of CTAC 3. Effect on the of CTAC onstructural evolution the structural of theof evolution Agthenanocubes to multichambered Ag nanocubes opened to multichambered opened AuAg nanoboxes after adding different amounts of HAuCl 4. Representative transmission electron AuAg nanoboxes after adding different amounts of HAuCl4 . Representative transmission electron microscopy (TEM) images of AuAg bimetallic nanoboxes synthesised in the presence of CTAC with microscopy (TEM) images of AuAg bimetallic nanoboxes synthesised in the presence of−3CTAC the addition of various amounts of HAuCl4 1 mM: (A) 50 µL (1.25 × 10−3 mmol), (B) 75−µL (1.87 × 10 with the addition of various amounts of HAuCl4 1 mM: (A)−3 50 µL (1.25 × 10 3 mmol), (B) 75 µL mmol), (C) 100 µL (2.50 × 10−3 mmol), and (D) 125 µL (3.12 × 10 mmol). (E) HR-TEM images of (1.87 AuAg pinhole 3 mmol), (C) 100 µL (2.50 × 10−3 mmol), and (D) 125 µL (3.12 × 10−3 mmol). (E) HR- × 10−bimetallic nanoboxes as shown in (A). (F) HAADF-STEM images EDS mapping of TEMAuAg pinhole images of pinhole bimetallic NCsAuAg shown bimetallic nanoboxes in (A) (red as shown for Ag, green in (A). for Au and (F) HAADF-STEM composite). images EDS (G) HR-TEM images of multichambered mapping of pinhole AuAgopened AuAg nanoboxes bimetallic NCs shown NCs as shown in (A) in Ag, (red for (D).green (H) HAADF-STEM im- for Au and composite). ages and (G) EDS mapping HR-TEM imagesofof multichambered multichambered opened openedAuAg AuAgnanoboxes nanoboxesNCs NCsshown in D as shown in(red (D).for (H)Ag, HAADF- greenSTEMfor images Au andandcomposite). (I) UV–Vis EDS mapping spectra of Agopened of multichambered nanocubes AuAg (black), A (blue), nanoboxes NCsBshown (green), C (red in (D) (purple), and D (yellow). (J) HAADF-STEM image of multichambered opened AuAg nanoboxes for Ag, green for Au and composite). (I) UV–Vis spectra of Ag nanocubes (black), A (blue), B (green), shown in (D). (K) EDS line scanning through the green/blue arrow of nanostructure presented in C (purple), and D (yellow). (J) HAADF-STEM image of multichambered opened AuAg nanoboxes (J). shown in (D). (K) EDS line scanning through the green/blue arrow of nanostructure presented in (J). Nanomaterials 2023, 13, 2590 8 of 13 Next, we investigated the effect of a non-halogenated cationic surfactant, specifi- cally p-toluenesulfonate (CTApTS) (Figure 4). When CTApTS was used as the surfactant, single-walled closed nanoboxes with an average edge size of 53.1 ± 5.0 nm and irregular thick walls of 8.3 ± 1.7 nm were obtained (Figure S5). Notably, the characteristic pinhole previously observed with other surfactants containing halogenated counterions was not seen in this case. This suggests that galvanic corrosion occurs rapidly during the early stages of the reaction. Single-walled and partially corroded nanoboxes further support this observation. As more gold precursor was added, the formation of poorly controlled single-walled nanoboxes increased in abundance. The UV–Vis spectra evolution with the addition of different amounts of Au precursor is depicted in Figure 4E. Initially, upon adding small volumes of the Au precursor, the SPR band redshifts until approximately ~550 nm. As more gold precursor is added, the spectra transform into a flat line with a Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 14 slight bump around ~750 nm. This noticeable change in the absorption spectra is attributed to the aggregation of the resulting nanostructures. Figure 4. Effect of CTApTS on the structural evolution of the Ag nanocubes to single-walled closed Figure 4. Effect AuAg of CTApTS nanoboxes on the structural after adding evolution different amounts of of the Ag: (A) HAuCl nanocubes to single-walled 50 µL (1.25 × 10−3 mmol),closed (B) 75 µL 4 AuAg nanoboxes − 3 after adding different amounts − 3 of HAuCl4: (A) 50 µL (1.25 × 10−3 mmol), − 3 (B) 75 µL (1.87 −3× 10 mmol), (C) 100 µL (2.50 −3× 10 mmol), and (D) 125 µL (3.12 −3 × 10 mmol). (E) UV–Vis (1.87 × 10 mmol), (C) 100 µL (2.50 × 10 mmol), and (D) 125 µL (3.12 × 10 mmol). (E) UV–Vis spectra of Ag NCs (black), A (blue), B (green), C (purple), and D (yellow). (F) HR-TEM of pinhole spectra of Ag NCs (black), A (blue), B (green), C (purple), and D (yellow). (F) HR-TEM of pinhole AuAg bimetallic AuAg bimetallic nanoboxes nanoboxes as shown as shown in (D). in (D). The obtained results indicate that the presence of halogens provided by the surfactant formsobtained The a layer ofresults AgBr indicate (Ksp: 5.4that the × 10 −13presence ) or AgClof(Ksp: halogens 1.8 ×provided 10−10 ) onby thethe surfac- surface of the tant Ag forms a layer of nanocubes AgBr with (Ksp:densities, varying 5.4 × 10 which −13 ) or AgCl (Ksp: plays 1.8 × 10 a crucial role )inon −10 the surface hindering fastof the corrosion Ag nanocubes and modifying with the varying densities, reaction which kinetics. plays a crucial Specifically, AgBr orrole in hindering AgCl fast corro- acts as a physical barrier, sion interfering and modifying the reaction kinetics. Specifically, AgBr or AgCl acts as a physical with the galvanic reaction. Consequently, it exerts control over the diffusion bar- of rier, the interfering with the Au precursor, galvanicthe favouring reaction. formation Consequently, of complex it exerts control[44]. nanostructures over the diffu- sion of the Finally, the effect of a non-ionic stabiliser, PVP, was also investigated [44]. Au precursor, favouring the formation of complex nanostructures as it plays a piv- Finally, otal thea effect role as of a non-ionic stabilising stabiliser, agent and shape-directing PVP, was also investigated in synthesising as it plays noble metal a NPs [45,46]. pivotal role as a stabilising and shape-directing agent in synthesising noble The influence of varying PVP concentrations was examined to ascertain the optimal pre- metal NPs [45,46]. Theamount cursor influencetoofinduce varyingthePVP concentrationsreaction, transmetallation was examined akin totothe ascertain earlier the opti- experiments mal precursor amount to induce the transmetallation reaction, akin to the earlier experi- ments with CTAB. PVP with a molecular weight of 55,000 was utilised at different con- centrations while keeping the Au precursor concentration constant (100 µL of HAuCl4 1 mM). Obtained results show that the concentration of PVP significantly influences the fi- Nanomaterials 2023, 13, 2590 9 of 13 with CTAB. PVP with a molecular weight of 55,000 was utilised at different concentra- tions while keeping the Au precursor concentration constant (100 µL of HAuCl4 1 mM). Obtained results show that the concentration of PVP significantly influences the final nanostructure morphology, allowing the precise control of single-walled nanoboxes with high yield (Figure 5). Thus, when no PVP was employed (Figure 5A), the rapid oxidation of the Ag template, combined with the swift deposition of Au, resulted in the formation of sponge-like nanostructures. However, fixing the PVP concentration at 0.01 mM pro- duced a mixture of sponge-like nanostructures and single-walled nanoboxes (pinholed or not) (Figure 5B). Notably, adjusting the PVP concentration to 0.1 and 1 mM (Figure 5C,D) yielded predominantly single-walled nanoboxes with high efficiency. As the PVP concen- Nanomaterials 2023, 13, x FOR PEER REVIEW trations were further increased to 10 mM and 100 mM (Figure 5E,F), an inhomogeneous 10 of 14 mixture of single-walled nanoboxes and partially corroded nanostructures was obtained. This phenomenon can be attributed to the PVP binding to the surface of Ag nanocubes, acting as thereby a physical slowing thebarrier that kinetics ofhinders GRR [30].Au precursor HR-TEM deposition–reduction, images (Figure 5G) of thereby slow- single-walled ing the kinetics of GRR [30]. HR-TEM images (Figure 5G) of single-walled nanoboxes nanoboxes shown in Figure 5D reveal the high monodispersity and homogeneity of the shown in Figure 5D reveal the high monodispersity and homogeneity of the product. The product. The EDS elemental mapping (Figure 5H) shows the distribution of the constitu- EDS elemental mapping (Figure 5H) shows the distribution of the constituent elements ent elements in the sample, revealing that the wall of the nanobox consisted of 37.8 ± 6.2% in the sample, revealing that the wall of the nanobox consisted of 37.8 ± 6.2% Ag and Ag and 62.2 ± 62.2 6.2%±Au,6.2% Au, the while while thepart inner inner part consisted consisted of 36.2 ±of 9.2% 36.2 ±Ag 9.2% andAg63.8and 63.8 ±Au. ± 9.2% 9.2% Au. Remarkably, the walls and inner parts exhibited similar chemical compositions, Remarkably, the walls and inner parts exhibited similar chemical compositions, enabling ena- bling theirtheir presentation presentation as an as an overall overall result:result: 37.8 ±37.8 7.1%± Ag7.1% Ag62.2 and and±62.2 7.1%± Au. 7.1%TheAu.UV–Vis The UV– Vis spectra of the colloidal solutions obtained at increasing concentrations spectra of the colloidal solutions obtained at increasing concentrations of PVP are shown of PVPinare shown Figurein5I.Figure 5I. Unexpectedly, Unexpectedly, theobtained the structures structures obtained at higher PVPatconcentrations higher PVP concentrations displayed a displayed a lower lower redshift (670redshift nm) than(670 nm)obtained those than those obtained at lower PVPat lower PVP concentrations concentrations (up to 700 nm).(up toThis 700 phenomenon can be attributed nm). This phenomenon can beto the presence attributed ofpresence to the nanostructures that are not that of nanostructures fullyare corroded. not fully corroded. Figure 5. Effect of PVP on the structural evolution of the Ag nanocubes to single-walled closed AuAg Figure 5. Effect of PVP on the structural evolution of the Ag nanocubes to single-walled closed AuAg nanoboxes.TEM nanoboxes. TEMimages images of of AuAg AuAg bimetallic bimetallic nanoboxes nanoboxessynthesised synthesisedininthe presence the presenceof increasing of increasing concentrations of PVP (by repeating unit) after adding 100 µL of concentrations of PVP (by repeating unit) after adding 100 µL of HAuCl HAuCl 1 mM (2.50 4 4 1 mM (2.50 × 10×−10 3 mmol): −3 mmol): (A)No (A) NoPVP. PVP.(B) (B) 0.01 0.01 mM, mM, (C) (C) 0.1 0.1 mM, mM,(D)(D)11mM, mM,(E)(E)1010mM, mM,andand(F)(F) 100100mM. mM. (G)(G) HAADF-STEM HAADF-STEM imageand image and(H) (H)EDS EDSmapping mapping ofof aa single-walled single-walled nanobox nanobox shown shownin in(D) (redfor D (red forAg, Ag, green green for for Au). Au). (I) (I) UV–Vis UV–Vis spectra spectra of Ag of Ag NCs NCs (black),AA(red), (black), (red),BB(blue), (blue), C C (green), (green), DD(purple), (purple),E E(yellow), (yellow),and F (cyan). and F (cyan). Interestingly, by employing even higher concentrations of PVP (180 mM) and a slow addition of the Au precursor (10 µL/min), we achieved the synthesis of highly porous nanoboxes (Figure 6A), which resulted from the de-alloying and dissolution of Ag. Se- lected TEM tilt images of the same nanobox (+50° to −45°) confirm the highly porous mor- phology of the nanoboxes (Figure 6B), further accentuating their unique structural prop- Nanomaterials 2023, 13, 2590 10 of 13 Interestingly, by employing even higher concentrations of PVP (180 mM) and a slow addition of the Au precursor (10 µL/min), we achieved the synthesis of highly porous nanoboxes (Figure 6A), which resulted from the de-alloying and dissolution of Ag. Se- lected TEM tilt images of the same nanobox (+50◦ to −45◦ ) confirm the highly porous morphology of the nanoboxes (Figure 6B), further accentuating their unique structural properties. Remarkably, these nanoboxes exhibited a robust SPR band in the near-infrared region at 1000 nm (Figure 6C), rendering them particularly promising as contrast agents Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 14 for photoacoustic tomography [47,48] and photothermal therapy [49]. Figure6.6. Highly Figure Highlyporous porousAuAg AuAg nanoboxes. nanoboxes. (A) (A) TEM TEM images images of ofAuAg AuAg nanoboxes nanoboxes synthesised synthesised in in the the presence of PVP 180 mM (by repeating unit) after the slow addition of 250 µL of HAuCl4 1 mM (10 presence of PVP 180 mM (by repeating unit) after the slow addition of 250 µL of HAuCl4 1 mM µL/min). (B) TEM images of the same nanobox tilted −45°, −30°, −15°, 0°, 15°, 30°, 45°, and 50°. The (10 µL/min). (B) TEM images of the same nanobox tilted −45◦ , −30◦ , −15◦ , 0◦ , 15◦ , 30◦ , 45◦ , and scale bar represents 20 nm for all images. (C) UV–Vis spectra of Ag NCs (black) and AuAg nano- ◦ . The scale bar represents 20 nm for all images. (C) UV–Vis spectra of Ag NCs (black) and AuAg 50 boxes (green). nanoboxes (green). 3. Conclusions 3. Conclusions Weherein We hereinstudied studiedthe the effect effect of of different different surfactants surfactants on synthesis on the the synthesis of AuAgof AuAg nano- nanoboxes, boxes, showing showing how the how the halide halide counterion counterion (Cl vs.(Cl Br)vs.has Br)anhas an apparent apparent impactimpact in shaping in shaping the the final final morphology morphology of hollow of hollow NCs,NCs, revealing revealing thethe significant significant roleofofthe role thehalide halidecounterion counterion (Cl vs. (Cl vs. Br) Br)inincontrolling controlling thetheformation formation of voids within of voids the structure. within The choice the structure. of surfac- The choice of tant and the amount of Au precursor added to the reaction were surfactant and the amount of Au precursor added to the reaction were critical in shaping critical in shaping the nanostructures the nanostructures andand determining determining thethefinal finalmorphology morphologyofofthe the resulting nanoboxes and resulting nanoboxes and their optical their optical properties. properties. With With CTAB, CTAB, the the SPR SPR band band exhibited exhibited aa redshift redshift from from 440440 nmnm (Ag (Ag nanocubes) to nanocubes) to 680 680 nm nm (nanoboxes) as the amount of HAuCl HAuCl44increased. increased.CTAC CTACshowed showeda asimilar similarprocess process but butat at a faster a fasterrate, owing rate, owingto the weaker to the interaction weaker strength interaction of AgCl strength com- of AgCl pared to AgBr. compared On On to AgBr. the the other hand, other using hand, CTApTS using CTApTS resulted in the resulted polydisperse in the polydisperse etching of etching Ag templates and non-homogeneous deposition of Au, leading of Ag templates and non-homogeneous deposition of Au, leading to optical absorption to optical absorption ex- tending across extending acrossthe theentire entirevisible visiblespectrum, spectrum,with withaaslight slight bump bump around 750 750 nm. nm. Employing Employing high concentrations high concentrations of of PVP PVP resulted resulted in in highly highly porous porous nanoboxes nanoboxes exhibiting exhibiting aa robust robust SPR SPR band in band inthe thenear-infrared near-infraredregion regionat at1000 1000nm. nm. Results show Results show that that the the halide halide counterion counterion controls controls thethe void void formation formation in in the the hollow hollow structure, structure, which can be beattributed attributedtotohow howhalide halide ionsions modulate modulate metal metal ionion redoxredox poten- potentials tials and form insoluble compounds with Ag, selectively passivating the surface of Ag and form insoluble compounds with Ag, selectively passivating the surface Ag nanocubes. nanocubes. Thus, Thus, thethe presence presence of ofClCl−- induced induced a looser packing packing of of surfactant surfactant molecules molecules near nearcorners, corners,promoting promotingreactivity reactivityand andleading leading toto pinholes pinholes opening opening away away from from thethe centre of centre of the crystal face, ultimately forming multichambered structures. In contrast, using CTApTS as a surfactant produced single-walled closed nanoboxes with irregular thick walls. At the same time, PVP yielded well-defined single-walled closed nanoboxes, which evolved into highly porous structures by Ag de-alloying and dissolution. Nanomaterials 2023, 13, 2590 11 of 13 the crystal face, ultimately forming multichambered structures. In contrast, using CTApTS as a surfactant produced single-walled closed nanoboxes with irregular thick walls. At the same time, PVP yielded well-defined single-walled closed nanoboxes, which evolved into highly porous structures by Ag de-alloying and dissolution. Overall, these findings highlight the critical role of surfactant and gold precursor amounts in tailoring the properties of AuAg nanoboxes, offering a versatile approach to en- gineering these nanostructures for specific applications. This effect highlights the significant influence of halogen-containing surfactants in shaping the morphology and composition of the resulting nanocrystals, offering promising prospects for tailored nanomaterial design. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13182590/s1, Figure S1. Structure of surfactant used in this study. Figure S2. Effect of CTAB concentration. Figure S3. HAADF-STEM image of a multiple pinholed Nanocrystals and EDS line scanning analysis. Figure S4. TEM images of double-walled nanoboxes in the presence of CTAC and EDS mapping analysis. Figure S5. TEM images of single walled nanoboxes synthesized in the presence of CTApTS. Author Contributions: Investigation, J.P., N.G.B. and V.P.; Data curation, J.P.; Writing—original draft, J.P., N.G.B. and V.P.; Supervision, N.G.B. and V.P.; Funding acquisition, V.P. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge the financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MCIU) (RTI2018-099965-B-I00, AEI/FEDER,UE) proyectos de I+D+i de programación conjunta internacional MCIN/AEI (CONCORD, PCI2019-103436), cofunded by the European Union and Generalitat de Catalunya (2021-SGR-00878). ICN2 is supported by the Severo Ochoa program from Spanish MINECO (SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Informed Consent Statement: Not applicable. 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