Synthesis of Citrate-Coated Penta-twinned Palladium Nanorods and Ultrathin Nanowires with a Tunable Aspect Ratio.

Green and scalable methodologies for the preparation of metal nanoparticles with fine control of shape and size are of high interest in many areas including catalysis, nanomedicine, and nanodiagnostics. In this contribution, we describe a new synthetic method for the production of palladium (Pd) penta-twinned nanowires and nanorods utilizing sodium citrate, formic acid, ascorbic acid, and potassium bromide (KBr) in water, without the use of surfactants or polymers. The synthesis is green, fast, and without the need of complex setups. Interestingly, a microwave-assisted scale-up process has been developed. The combination of a synthetic protocol for seeds and the seed-mediated growth process allows us to synthesize nanorods and nanowires by modulating the concentration of KBr. The synthesized nanomaterials have been physicochemically characterized. High-resolution transmission electron microscopy shows that the nanorods and nanowires have a penta-twinned structure enclosed by {100} lateral facets. Moreover, the absence of sticky molecules or toxic byproducts guarantees the biocompatibility of the nanomaterials, while leaving the surface clean to perform enzymatic activities.

Introduction

Accurate control on the shape of the nanocrystals has proven to be a powerful way to modulate the properties of metal nanocrystals and hence their performance in a broad range of applications, including catalysis, sensing, nanodiagnostics, and nanomedicine.[1−14] For palladium (Pd) nanocrystals, the shape and size play a key role as they regulate their catalytic and enzymatic activity[10,15−18] as the surface structure greatly influences the catalytic properties.[19−21]

Pd nanomaterials with a high aspect ratio, such as nanowires and nanorods, have attracted great interest because their one-dimensional (1D) morphology guarantees the presence of largely pristine surfaces with extended crystal facets.[22−24] In particular, penta-twinned Pd nanorods and nanowires exhibit extended {100} lateral facets and lattice strain at the twin boundaries (TBs). The presence of {100} facets is highly advantageous as these facets have been demonstrated to be 1 order of magnitude more effective in the oxygen reduction reaction compared to {111} facets.[25] Also, lattice strain has been proven to increase the activity in oxygen reduction reaction and formic acid oxidation by modifying the surface electronic structures.[25]

These features greatly enhance the performance of nanorods and nanowires in catalysis and electrocatalysis by strongly reducing Ostwald ripening, sintering, and detachment from the support and providing improved resistance against dissolution.[21]

However, despite their promising features and potential applications, there are very few articles on methods to obtain Pd nanowires and nanorods, as 1D growth is difficult to achieve, being highly disfavored energetically. Like other face-centered cubic (fcc) metals, Pd does not spontaneously form anisotropic structures in an isotropic medium. To minimize surface energy, Pd nanoparticles (NPs) tend to have an isotropic (quasi-spherical) shape with a mixture of several facets on the surface.[26,27] Therefore, it is challenging to synthesize Pd nanorods and nanowires as uniform samples with high yield. The most promising reports in the literature use hydrothermal processes to achieve Pd 1D morphologies.[1,28] For these methods, it has been clearly established that oxidative etching provoked by the coupling of O2 and halides and the seed morphology are two crucial parameters to be strictly controlled to favor anisotropic growth.[21] In a colloidal solution of Pd particles, halides (Br–, I–, and Cl–) can chemisorb on the Pd surfaces. The chemisorption efficiency of halides on the Pd surface decreases in the order I– > Br– > Cl–.[27] Moreover, halides have been demonstrated to favor the preferential formation of {100} facets.[29−31] The reduction rate and, in general, the reaction kinetics, pivotal parameters to guarantee anisotropic features, are strongly influenced by the concentration of halide ions.[27,29,32] Despite the progress made with 1D syntheses of Pd, there is still a need for an easy, fast, and green method.

Current protocols rely on the use of various surfactants and polymers as shape-directing or capping agents, often complicated by the use of a multiphase setup or by the use of soft and hard templates.[33−36] This represents a limit to applications of these nanomaterials as the most commonly used shape-directing agents, that is, poly(vinylpyrrolidone) (PVP), tetradecyl trimethyl ammonium bromide, cetyltrimethylammonium bromide (CTAB), and hexadecylpyridinium bromide, are detrimental to many catalytic reactions because they alter the surface properties of the material.[37−41] To overcome this problem, several protocols have been adopted to eliminate the organic coating from the surfaces of the NPs. However, the majority of these treatments do not guarantee complete removal of the coatings together with being expensive, time consuming, and, most importantly, harsh for the surface structure of the materials.[19]

In this work, we report a “green” and fast synthetic method in which we use sodium citrate, ascorbic acid, formic acid, and potassium bromide (KBr) to produce Pd nanostructures in an aqueous environment without the use of a complex setup. This protocol produces different shapes of citrate-capped Pd NPs simply by combining a seed synthetic procedure with the concentration-dependent effect of KBr. Penta-twinned nanorods of different lengths, penta-twinned nanowires with high aspect ratios, and quasi-decahedron/truncated octahedron NPs have been synthesized. These nanostructures are stabilized by citrate molecules, overcoming the issues related to the use of polymers, surfactants, and difficult-to-remove organic capping agents. In addition, their catalytic properties as antioxidant nanozymes have been investigated, together with their toxicological profile.

Materials and Methods

Materials

Palladium(II) chloride 99.999%, acetic acid puriss. p.a., ACS reagent, reag. ISO, reag. Ph. Eur., ≥99.8%, formic acid puriss. p.a., ACS reagent, reag. Ph. Eur., ≥98%, l-ascorbic acid BioXtra, sodium citrate tribasic dehydrate BioUltra, potassium bromide BioXtra, ≥99.0%, sodium borohydride, and citric acid anhydrous were bought from Merck/Sigma-Aldrich and used as received. Deionized water with a resistivity of 18.2 MΩ·cm was used throughout the experiment.

Pd Seed Synthetic Procedure

The precursor solution was obtained by dissolving PdCl2 in 1 M CH3COOH aqueous solution at a concentration of 56.4 mM. All the NP syntheses were carried out in a sealed glass container (ACE glass pressure reactor with a Teflon cap).

Pd seeds were synthesized by adding 80 μL of PdCl2 acidic solution (56.4 mM) (Sigma-Aldrich) to 130 mL of MilliQ water at 20 °C, immediately followed by a quick addition of 8.8 mL solution containing sodium citrate and citric acid (at 0.03 M and 2 mM concentration, respectively) and 550 μL of freshly prepared NaBH4 (0.02 M). The vessel was moved to an oil bath already at 105 °C to obtain a quick reduction of the Pd ions, and, hence, seeds of size below 3 nm. The reaction time was 10 min (under magnetic stirring at a moderate rate). After removal from the glycerol bath, the glass container was left to cool under stirring for another hour.

10 nm Quasi-Decahedron/Truncated Octahedron Pd Citrate-Capped Nanocrystal Synthetic Procedure

Pd nanocrystal seeds (8 mL, synthesized following the protocol described in the previous paragraph) were added to 60 mL of MilliQ water at room temperature together with 159 μL of Pd (II) acetate (0.05 M) and 1 mL of a solution containing 0.34 M sodium citrate, 0.2 M formic acid, and 0.5 mM l-ascorbic acid. After being sealed, the vessel was placed in a glycerol bath at 20 °C and brought to 105 °C in a time equal to 5 min. The reaction time was 10 min (under stirring at a moderate rate). The vessel was then lifted from the glycerol bath and brought to room temperature under stirring for 1 h.

After cooling, the NPs were purified using 10K Amicon Ultra centrifugal filters and stored at 4 °C.

Pd Citrate-Capped Nanorod Synthetic Procedure

38 nm Pd nanorods: Pd seed solution (4 mL) was added to 60 mL of MilliQ water at room temperature together with 159 μL of Pd (II) acetate (0.05 M) and 1 mL of a solution containing 0.34 M sodium citrate, 0.2 M formic acid, and 0.5 mM l-ascorbic acid. After 1 min, 1.8 mL of 0.3 M KBr was added in the solution. The vessel was then sealed, placed in a glycerol bath at room temperature, and brought to 105 °C in 10 min. The reaction time was 10 min (under stirring at a moderate rate). The vessel was then removed from the glycerol bath and gradually cooled to room temperature under stirring for 1 h. The products were purified using 3K Amicon Ultra centrifugal filters and stored at 4 °C.

71 nm Pd nanorods were obtained following the same procedure described for the 38 nm Pd nanorods with 1.8 mL of 0.5 M KBr and a reaction time of 20 min. The products were centrifuged for 45 min and then washed as reported in the previous paragraph.

Pd Citrate-Capped Nanowire Synthetic Procedure

280 nm Pd nanowires: Pd seed solution (4 mL) was added to 60 mL of MilliQ water at room temperature together with 159 μL of Pd (II) acetate (0.05 M) and 1 mL of a solution containing 0.34 M sodium citrate, 0.2 M formic acid, and 0.5 mM l-ascorbic acid. After 1 min, 1.8 mL of 1 M KBr was added in the solution. The vessel was then sealed, placed in a glycerol bath at room temperature, and brought to 105 °C in 10 min. The reaction time was 20 min (under stirring at a moderate rate). The vessel was then lifted and cooled to 20 °C under stirring for 60 min.

The products were centrifuged for 45 min and washed as reported in the previous paragraph.

470 nm Pd nanowires were synthesized following the same procedure described for the 280 nm nanowires by increasing the reaction time to 1 h. The vessel was then lifted and cooled to 20 °C under stirring for at least 60 min. The products were centrifuged for 35 min and washed as reported in the previous paragraph.

Scale-up of Nanocrystal Synthesis Using a Flexiwave Microwave Reactor

All the synthetic processes previously described were scaled up in a microwave reactor (Flexiwave microwave reactor), using the multivessel setup (15 vessels). This machine controls the power of microwave irradiation using a temperature control (fiber glass) immersed in one of the vessels. This precise temperature control of the Flexiwave microwave reactor allows us to replicate the reaction conditions developed in the round bottom flask without further changes. Further details are reported in the Supporting Information.

Transmission Electron Microscopy Analysis

Bright-field transmission electron microscopy (BF-TEM) overview images of the samples were acquired using a JEOL JEM-1011 microscope with a thermionic source (W filament) and operated at 100 kV. High-resolution transmission electron microscopy (HR-TEM) analyses were carried out using an image-CS-corrected JEOL JEM-2200FS microscope with a Schottky emitter and operated at 200 kV. For HR-TEM analyses, drop-casting of the nanocrystals in suspension (after concentration and purification with 3K Amicon Ultra centrifugal filters) was carried out onto a carbon-coated (ultrathin film on a holey film) Cu grid for the seeds and onto a holey-carbon-coated Cu grid for the nanowires. An average background subtraction filter was applied to the HR-TEM images of the seeds[42] in order to improve the visibility of the crystalline features because of the small size of the seeds and to the amorphous carbon support film. Thickness and length of the nanocrystals were obtained by imposing a threshold on the BF-TEM images and, then, by measuring with ImageJ software.[43]

Cytotoxicity Assays

Human cervix epithelioid carcinoma cells (HeLa) were cultured at 37 °C in a humidified incubator containing 5% CO2 in high glucose Dulbecco’s modified Eagle’s medium (DMEM, Sigma-Aldrich) supplemented with 10% (v/v) fetal bovine serum (FBS, Sigma-Aldrich), 100 U/mL penicillin, and 100 mg/mL streptomycin (Sigma-Aldrich).

Cell Viability Assessment by MTS Assay

HeLa cell viability was measured with MTS assay (Cell-Titer96 Aqueous One Solution, Promega). The assay was carried out following the protocol provided by the manufacturer and previously published protocols.[4] DMEM supplemented with 10% FBS solutions containing Pd nanowires and nanorods at concentrations of 5, 10, and 25 μg/mL were used. After incubation for 24 and 48 h, viability was measured. All experiments were performed in quadruplicates.

Membrane Integrity Assay

The plasma membrane integrity was measured by detecting lactate dehydrogenase (LDH) release with CytoTox-ONE (Promega), following the protocol previously published.[4] Cells were incubated with a solution containing nanorods and nanowires at increasing concentrations (from 5 to 25 μg/mL) for 24 and 48 h, and the release of LDH was then measured.

Peroxidase-like Activity

3,3′,5,5′-Tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) were chosen as chromogenic substrates to evaluate the peroxidase-like activity of nanorods and nanowires.

The TMB oxidation reaction kinetics has been characterized by UV–vis spectroscopy measurement (absorbance peak at 652 nm). The reaction kinetics was monitored during the time (5 min).

The horseradish peroxidase (HRP)-like activity test was performed using 10 mM acetate buffer at pH 4.7, together with 100 mM H2O2, at room temperature. The catalytic activities of 10 pM (1 ppm) citrate-coated and PVP-coated (10 and 100 ppm) nanowires were assessed. The same procedure was used for assessing the catalytic activities for 35 pM (1 ppm) nanorods. Citrate-coated Pt NPs (120 pM, 0.1 ppm) were used as a control in the experiment.

Results and Discussion

Synthetic Methodology

In this work, we present a new protocol for the synthesis of citrate-capped Pd nanowires, nanorods, and quasi-decahedron/truncated octahedron NPs in an aqueous environment in the absence of polymers, surfactants, and organic solvents and only using “green” biocompatible compounds. Fine control of the size and shape is achieved by finely tuning the synthetic parameters, the seed morphology, and the concentrations of KBr (Figure ).

Figure 1

Scheme illustrating the synthetic method.

The synthesis produces highly monodisperse nanomaterials (Figures and 3). Moreover, the length of the 1D structures can be modulated from 38 to 470 nm by increasing the concentration of KBr and the time of the reaction, while maintaining constant thickness.

Figure 2

Morphological characterization by BF-TEM of the Pd nanowires with a length of 470 nm (A) and with a length of 280 nm (C) and the corresponding size distribution analysis of length and thickness in (B) and (D), respectively.

Figure 3

Morphological characterization by BF-TEM of Pd nanorods with different lengths in (A) and (C) and size distribution analysis of the length and thickness in (B) and (D), respectively.

Table gives a summary of the parameters of Pd nanorods, nanowires, and quasi-decahedron/truncated octahedron NPs obtained with different concentrations of KBr and different reaction times, including the average values for the diameter, thickness, length, and aspect ratio. The presence of Br– promotes the growth of nanowires with an aspect ratio as high as 67.2. We therefore established a clear dependence between the length of the rods/wires and the concentration of KBr, while maintaining a constant thickness of the nanomaterials.

Table 1

Dimension of Pd Quasi-Decahedra/Truncated Octahedra, Nanorods, and Nanowires Obtained at Different Concentrations of KBr and Reaction Times

Pd NPs	KBr (mM)	diameter/thickness (nm)	length (nm)	reaction time (min)	aspect ratio
nanowires	27	7	470	60	67.2
nanowires	27	7.5	280	20	37.3
nanorods	13.5	7.4	71	20	9.5
nanorods	8	6.5	38	10	5.8
quasi-decahedron/truncated octahedron	0	8.9	8.9	20	1

The seed-mediated growth method has been used as it decouples the growth of the nanomaterial from seed production, therefore allowing the selection of the protocols for the seed syntheses which favor specific geometries. Indeed, it has been proven in the literature that the specific surface atomic structure of the seeds controls the final shape of nanomaterials.[21]

Pd seeds with well-defined defect structures (single-crystal, single-twinned, multiple-twinned, or stacking fault-lined) can be formed by manipulating the reduction rate of Pd ions in a synthesis together with the other physical parameters.[23]

We tested different protocols for synthesis of the seeds, but the majority provides a high polydispersity of seed morphologies and lead to high polydispersity of shapes of the nanomaterial (Figure S1a). On the other hand, by increasing the reaction rate, we were most likely able to form a higher amount of multiply twinned seeds (Figure S1c).

HR-TEM analyses (Figure ) confirm that our protocol for the synthesis of seeds produces a small fraction of multiply twinned seeds, together with a more abundant population of few nanometers, single-crystalline particles.

Figure 4

HR-TEM analysis of the two main types of Pd seeds. (a) Image of a multiply twinned Pd particle, with three distinguishable domains, separated by TBs. Twinning is confirmed by (b) the corresponding fast Fourier transform (FFT). (c) Single-crystal Pd particle, (d) exhibiting [011] orientation in the FFT. The observed patterns have been compared with the Pd fcc structure (ICSD 41517) and indexed accordingly.

Therefore, the analysis of HR-TEM further supports the theory that multiply twinned seed morphology is a preliminary condition to grow rods and wires combined with the stabilization of lateral {100} surfaces.[21,23,44,45] It should be further emphasized that the presence of multiply twinned seeds is a necessary but not sufficient condition to obtain 1D Pd nanomaterials. The synergistic interplay of all the synthetic parameters (weak reducing agents, fixed amount of oxygen, KBr and seed morphology) is fundamental to favor the formation of nanorods and nanowires.

Role of Bromide

To favor the formation of rods and nanowires over icosahedra, decahedra, and octahedra, Pd seeds and the PdCl42– precursor have been exposed to an excess amount of Br–, with the consequent formation of PdBr42–. This exchange leads to a more stable complex, which can modify the chemical equilibrium and then slow down the reduction kinetics. In addition, chemisorption of Br– to {100} facets has been clearly shown to promote anisotropic growth of Pd nanostructures.[21,27,28]

Hence, we choose Br– to achieve selective chemisorption on the growing {100} facets. This is coupled with the use of relatively weak reducing agents, that is, sodium citrate, formic acid, and l-ascorbic acid, which allow to further slow down the reaction kinetics. Moreover, Br– not only affects growth but also acts at the seed level, by favoring localized oxidative etching of seeds in synergy with oxygen.[23]

Transmission electron microscopy (TEM) images show the different shapes obtained by harnessing the ability of KBr to favor anisotropic growth. We demonstrate that with a concentration of 27 mM of KBr, we obtained the formation of 470 nm and 280 nm nanowires with a high aspect ratio (Figure ). Using a concentration of 8 mM KBr, we obtained 38 nm nanorods, while with a concentration of 13.5 mM, we obtained 70 nm nanorods (Figure ). All the samples were centrifuged to remove small NPs and then imaged by TEM. This step is fundamental to purify the sample from the other small NPs formed during the synthesis (Figures S2–S4).

Our results confirm that it is possible to modulate anisotropic growth by tuning the concentration of KBr. As reported by Xia et al.,[45] at high concentrations of Br–, Pd{100} facets are more efficiently capped by Br– ions in the growth phase, and thus, the growth is quicker on the {111} facets than on the {100} facets.[20,21] The result is axial growth along the ⟨110⟩ direction during synthesis, thus resulting in the formation of ultrathin nanowires. On the other hand, we found experimentally that fine tuning of the amount of KBr is necessary to control the growth. At an extremely high concentration of KBr, the reduction rate of the Pd(II) precursor was nearly suppressed, thus losing the ability to control the shape and size of the Pd NPs (Figures S6 and S7). The excess of Br could block the growth along the 1D direction on seed surfaces, thus promoting the self-nucleation of unshaped NPs.[46] Indeed, from the point of view of thermodynamics, the excess of Br– ions present in the solution renders Pd (II) complexes less prone to reduction, because of the shift of the chemical equilibrium, a dominant factor responsible for the slow reduction kinetics.[47] On the other hand, a small concentration of KBr is not sufficient to promote the growth of Pd 1D morphologies, probably because there is an insufficient amount of Br– ions during the growth to efficiently cap the nanomaterial surface.[48,49]

Moreover, in the absence of KBr, we obtained NPs with a quasi-decahedron/truncated octahedron shape and an average diameter of 9 nm (Figure ). Citrate acts as a capping agent, favoring the formation of {111} facets in the truncated octahedral particles (Figure ). It should be noted that we only achieved anisotropic growth (Figure S5) by fine-tuning the concentrations of all the reagents and reaction conditions.

Figure 5

Morphological characterization by BF-TEM imaging of Pd quasi-decahedron/truncated octahedron NPs prepared without KBr (a), HR-TEM image of a Pd truncated octahedron NP (b) with the corresponding FFT analysis (c), compared with the Pd fcc structure (ICSD 41517) and indexed accordingly.

Formic acid is the main reducing agent that promotes the growth of anisotropic Pd. Without formic acid, ultrasmall NPs form (Figure S12). Formic acid has the required reducing power to drive the formation of nanowires with a controlled growth kinetics in which the bromide ions can exert their directing agent function (Figures S13 and S14). Moreover, the synergy of formic acid with ascorbic acid and sodium citrate guarantees higher quality and lower polydispersity of the nanowires (Figures S9 and S10).

Oxidative Etching

Oxidative etching was controlled and limited using a closed vessel environment and by working at a relatively low temperature (slightly above 100 °C). As observed by Xia and co-workers,[48] temperatures above 150 °C and a high concentration of oxidants such as O2 can fracture the newly formed wires and favor the formation of smaller irregular NPs. In the case of an open vessel, we observed the formation of NPs that are polydisperse in shape and size, likely because of the high excess of oxygen (Figure S8). Therefore, it is important to work in an environment in which the amount of oxygen is controlled and kept constant. Moreover, we introduce citrate molecules in the reaction solution, as they could adsorb onto Pd NP surfaces. Once present on the surface, citrate molecules can compete with oxygen adsorption on the NP surfaces, thus reducing the quantity of oxygen, while, simultaneously, they can react and, consequently, exhaust the adsorbed oxygen.[23] Therefore, our synthetic protocol harnesses these factors (closed vessels and presence of citrate molecules) to disfavor the fracture of Pd nanowires, hence promoting the anisotropic growth of the nanomaterial.

On the other hand, a controlled and limited amount of O2 is beneficial at the beginning of the reaction as it likely favors the formation of multiply twinned seed morphology through oxidative etching, improving the yield of the synthesis. Indeed, it has been reported in the literature that oxygen in synergy with Br– acts at the seed level, by favoring localized oxidative etching of seeds (as reported by Xia’s group).[21,23,45] As a further proof of this mechanism, the reaction performed under a nitrogen atmosphere produces a lower amount of nanowires with higher polydispersity (Figure S11).

HR-TEM Analysis of Nanowires

Pd nanowires drop-casted onto a porous carbon support film were found to adhere to the contours of the pores (Figure ). They thus demonstrated partial flexibility, while keeping homogeneous crystallographic orientation along their whole length. The wires exhibit TBs, parallel to the ⟨110⟩ elongation direction, running throughout the nanowire. At the two sides of the boundary, [11̅0]- and [11̅1]-oriented regions are observed, in close agreement with the well-known penta-twinned structure fully enclosed by {100} lateral facets, reported for several 1D metal nanostructures.[45,50,51]

Figure 6

HR-TEM study of the crystal structure of Pd nanowires. (a) Overview BF-TEM image and (b) HR-TEM image of a nanowire, partly suspended on a hole in the carbon support film. The nanowire exhibits two regions, separated by a twin boundary, identified by their (c,d) FFTs as [11̅0] and [11̅1] zone-axis-oriented Pd (ICSD 41517).

Microwave Scale-up

Reaction scale-up has been achieved using a microwave reactor with a multivessel setup. This allows the simultaneous synthesis of more than 1 L solution, while maintaining the control of all the synthetic parameters, including low exposure to ambient oxygen and strict temperature control.

Toxicological Profile

In order to exploit the potential of citrate-coated Pd nanorods and nanowires as a novel tool in nanomedicine, it is pivotal to have a clear toxicological profile. Therefore, we performed cytotoxicity studies by monitoring mitochondrial activity and membrane integrity after exposure to the nanomaterial. A careful toxicity assessment of Pd nanowires and nanorods is fundamental because there has been a long controversy regarding the toxicity of these asymmetric shapes in the literature. For instance, the toxicological profile of gold nanorods has not been clearly established until pivotal studies demonstrated that CTAB molecules, present in gold nanorod solutions, were playing a crucial role in cellular cytotoxicity.[52] On the other hand, recent studies on the toxicity of the material per se show that Pd NPs synthesized without polymers, surfactants, or toxic stabilizers do not cause cellular damage in vitro, while having beneficial antioxidant properties.[4] Moreover, there is growing evidence from in vitro and in vivo studies that pristine Pd NPs do not modify cellular metabolism.[5,53−55]

MTS and LDH assays (Figures S16 and S17) proved that Pd nanorods and nanowires do not alter metabolic activity and do not affect viability of HeLa cells, even at high concentrations, such as 50 μg/mL. It should be further stressed that the lack of LDH release indicates that these nanomaterials are not altering cellular membrane integrity, demonstrating that the shape is not playing a role in exerting damages to the cellular environment.

Enzymatic Activity

Pd nanomaterials have remarkable catalytic and enzymatic properties. We studied the antioxidant nanozyme abilities of Pd nanorods and nanowires, namely their peroxidase (HRP)-like antioxidant activity.

To investigate HRP-mimicking activity, TMB and hydrogen peroxide (H2O2) were chosen as HRP substrates. The peroxidase-like activity of 35 pM rods and 10 pM wire NPs (both 9.4 μM in terms of Pd atoms) was assessed by UV–vis spectroscopy. The Pd NP activity was directly compared to Pt NPs. Interestingly, Pd nanowires and nanorods efficiently catalyze the oxidation of TMB, despite their low concentration, as clearly visible in Figure . Pt NPs have higher catalytic activity compared to the Pd nanomaterial, thanks to the higher surface available for catalysis, also because of their diameter value (5 nm). These data show a remarkable peroxidase performance of the Pd nanomaterial and highlight their potential application as biomimetic nanozymes.

Figure 7

Time-dependent absorption curves at 652 nm of the TMB–H2O2 reaction system catalyzed by (A) Pd nanowires at a concentration of 10 pM (1 ppm) coated with different concentrations of PVP in comparison with spherical Pt NPs at a concentration of 120 pM (0.1 ppm); (B) Pd nanorods at a concentration of 35 pM (1 ppm) coated with different concentrations of PVP in comparison with spherical Pt NPs at a concentration of 120 pM (0.1 ppm).

Moreover, the catalytic activity of citrate-coated nanomaterials is higher than that of PVP-coated ones, thanks to the absence of sticky molecules, as it is clear in Figure . Indeed, the presence of PVP on the surface of nanowires and nanorods decreases the catalytic turnover rate in the oxidation of the chromogenic substrate in a PVP concentration-dependent manner.

Discussion

Because of the difficulty in achieving the anisotropic growth in an isotropic medium, there are few reports in the literature about monometallic Pd nanorods and nanowires.[45,56−58] Some of these approaches employ soft or hard templates, such as step edges on a solid surface, mesostructures assembled from surfactants or polymers, channels within a porous material, or an existing nanostructure of different materials.[33−35,59] In addition, biological macromolecules, such as proteins, DNA, and viruses, have been proposed as biotemplates. However, the majority of biotemplates produce 1D materials with a high polydispersity of surface facets, while requiring a complex setup for synthesis.[60−62] Moreover, when templates are employed during synthesis, it is necessary to selectively remove them using expensive and time-consuming postsynthesis treatment to obtain clean surfaces suitable for catalytic applications. In contrast, our approach is a solution-phase process that is free of templates.

Of the previously reported solution-phase protocols, the majority produce 1D structures by random aggregation and orientated attachment, resulting in a high polydispersity of surface facets.[33,34,63] Here, we presented a new method that produces highly tunable, monometallic penta-twinned Pd nanorods and ultrathin nanowires with a high percentage of {100} facets.

Our synthesis method is based on the use of citrate-coated Pd seeds. The seeds are made of the same Pd material as the final nanowires and nanorods. Monometallic materials have several advantages, such as high selectivity and durability together with a longer lifespan in specific applications compared to bimetallic systems.[64] Our method also takes advantage of the synergistic interplay of KBr, weak reducing agents, low temperature, and a reduced amount of oxidative etching to obtain anisotropic growth. The synthesis does not require a complex setup, contrary to many reports in the literature, which propose multiphase procedures and vapor or freeze-dry treatment for the synthesis of the 1D material.[34−36,65] In addition, this method is fast, as Pd nanowires can be produced in 20 min for a length of up to 280 nm, while nanorods can be produced in around 10 min.

A major advantage of this new method is the removal of PVP and other large surfactant molecules from the synthetic process. Studies have reported the synthesis of penta-twinned Pd nanowires or nanorods using surfactants, such as CTAB, DTAB, TOAB, and/or dodecylamine.[25,36,57,66] Although these surfactants favor anisotropic growth, they represent a problem after synthesis, as they are extremely difficult to remove.[67] PVP is also a widely used polymer for the preparation of 1D Pd nanostructures.[21] Although PVP confers stability to the nanowires, it covers the surface and deteriorates their catalytic properties.[25] The removal of polymers from the synthetic environment does not affect the quality of the surface facets and the level of monodispersity obtained with our method, as we demonstrate that these features are of high quality compared to the methods present in the literature. Moreover, we showed that the absence of difficult-to-remove polymers guarantees better catalytic performance in the oxidation of TMB.

Our approach is based on the ability of KBr to act as a shape-directing agent on seed-mediated growth. However, the presence of Br– is not sufficient to favor the formation of nanowires as the synergistic interplay between KBr, citric acid, formic acid, seed morphology, and limited oxidative etching is necessary to obtain anisotropic growth. The ability of KBr to promote anisotropic growth in combination with PVP in the organic solvent has been demonstrated by Xiong et al.[44] in their seminal work. However, we have established a clear relationship between the concentration of KBr and the length of the 1D nanomaterial, paving the way to a deeper understanding and, hence, control of the growth mechanism.

Another advantage of our method is that synthesis is carried out in water and does not require an organic solvent. Moreover, only biogenic and easy-to-remove small molecules are used to achieve the growth of nanowires and nanorods, guaranteeing a green approach to the production of these nanomaterials. To the best of our knowledge, there is only one report in the literature in which an aqueous synthesis of Pd nanowires is described without the use of a polymer or surfactant.[68] However, TEM images show an intricate network of wires that are wavy and polydisperse in length and thickness. Analysis of these images suggests that nanowire formation is due to random aggregation and orientated attachment that lead to high polydispersity of the surface facets.[24,25] Our protocol guarantees the complete absence of surfactants on the surface of the material as the reaction is conducted using sodium citrate, ascorbic acid, formic acid, and KBr, that is, reagents that are biocompatible, biogenic, and easier to remove compared to surfactants. This represents a great advantage for applications in catalysis where clean surfaces are required. Moreover, the reaction maintains the thickness of the nanowires at 7 nm, independent of the length, retaining the ultrathin characteristic of the material. This represents a great advantage as the ratio between the available surface and mass of the nanomaterial remains high even for long nanowires. To the best of our knowledge, the only report in the literature that allows the production of nanowires with a comparable thickness relies on the use of an organic solvent and high quantity of PVP to achieve this ultrathin feature.[45]

Microwave scale-up of our method has been performed, allowing the synthesis of more than 1 L of solution simultaneously without compromising the control of the synthetic parameters. To the best of our knowledge, our method is one of the few present in the literature that allows a scale-up of the production without compromising the quality of the nanomaterials.

Conclusions

In conclusion, we have developed a fast synthetic procedure to synthesize tunable ultrathin, penta-twinned Pd nanorods and nanowires with a thickness of 7 nm and length ranging from 38 to 470 nm. The synthesis was performed in an aqueous solution and with the use of green, biogenic, biocompatible, and easy to remove compounds. Because of the synergy between Br– ions, citrate molecules, reduced oxidative etching, and the seed geometry, the protocol achieves tunability, speed, and high quality of the material. In particular, a high fraction of {100} facets is present on the surface, as proven by HR-TEM. The protocol does not require any soft or hard template.

Moreover, the as-prepared Pd nanorods and nanowires show a high level of biocompatibility and interesting enzymatic properties because of the absence of sticky molecules together with the high quality of the surface, paving the way to their application in catalysis, sensing, nanodiagnostics, and nanomedicine.