Introduction

Nanostructured noble metals with diverse size, shape, morphology, and crystal orientation have attracted much attention due to a porous nature, high and favorable surface areas, and unique physical and chemical properties different from their bulk counterparts [1]. In this regard, noble metal nanorods [2], nanowires [3], nanoflowers [4], nanorings [5], nanobelts [6], nanocubes [7], nanoprisms [8], and nanoplates [9] were synthesized. These nanostructures have potential applications due to special electronic, optical, thermal, catalytic, or magnetic functions [1, 10]. Among the noble metals, gold nanostructures are of importance due to their unique physicochemical properties arising from size and collective effects [11], shape [1, 12], atomic arrangement [13], electronic properties [14], and the local dielectric property [15]. Therefore, the precise control of size and shape is the key parameter to better understand and control the physicochemical properties of gold nanostructures [16]. Gold nanostructures have extensive applications in catalysis and electrocatalysis [17], sensing and biosensing [18], electronic and optical detecting systems [19], surface-enhanced Raman spectroscopy [20], chemical analysis [21], fuel cells [22], photothermal therapy [23], antibacterial nanomedicine [24], and radiotherapy [25].

Up to now, different methods have been developed for the synthesis of gold nanostructures including simple chemical reduction, seed mediation, photoreduction, sonochemical, template-based method, solvothermal, hydrothermal, electrodeposition, galvanic replacement, layer-by-layer self-assembly, selectively de-alloying, and lithography [17, 26]. Using these methods, various sizes and shapes of zero-to-three-dimensional gold nanostructures including nanoparticles [27, 28], hollow nanotubes [29], nanorods [30], nanoflowers [31], nanodendrites [32], hollow spheres [33], anisotropic nanoparticles and plates [34], nanopyramidal, nanorod-like, and spherical nanostructures [35, 36], nanoblooms [37], nanospears [38], nanoleaves and nanoleaflets [39, 40], nanostars [41], porous textile-like sheet arrays [42], nanodumbbells, nanopods, and nanodendrites [26] were synthesized.

Electrodeposition synthesis of nanostructured materials (including gold nanostructures) is a potentially superior method due to advantages of having a high degree of controllability, being single-step process and easy control, having effective controllable of size and shape of the electrodeposits, being easy to anchor securely on the substrate, producing uniform and high pure deposits, being environmentally friend, and providing more opportunities for the design and fabrication of different devices [26–32, 35–40, 43].

In the present study, one-step, green, and template-less electrodeposition methods were developed to fabricate gold nanostructures with different sizes and shapes.

Experimental section

Reagents and chemicals

All chemicals were of analytical grade form Merck (Germany) or Sigma (USA) and were used without further purification. All solutions were prepared with doubly distilled water.

Apparatus

Electrochemical experiments were carried out in a conventional three-electrode cell containing a supporting electrolyte (including synthesis solutions or else) powered by a μ-Autolab type III potentiostat/galvanostat (The Netherlands). An Ag/AgCl, saturated KCl, a glassy carbon rod, and a gold disk (Au) electrode were used as the reference, counter, and working electrodes, respectively. The system was run on a PC through GPES 4.9 software.

In order to obtain information about the morphology and size of the electrodeposited gold nanostructures, field emission scanning electron microscopy (FESEM) was performed by the instrument Zeiss, Sigma-IGMA/VP (Germany).

Procedures

In order to synthesize different gold nanostructures, electrodeposition method at various experimental conditions was employed. To this purpose, potentiostatic electrodeposition was performed from an AuCl4 −-containing solution. Different supporting electrolytes and different additives were added to this solution (the synthesis solution). The color of the synthesis solutions was all bright yellow. In some procedures, ultrasound wave of 45-W power was irradiated to the synthesis solution and also the Au electrode surface. Before electrodeposition, the Au electrode was polished by sand papers and then on a polishing pad with 50-nm alumina powder lubricated by glycerin. Polishing was continued to attain a mirror-like surface. The electrode was then cleaned by immersion in a 1:3 water/ethanol mixture and ultrasonication for 5 min in an ultrasound bath. The electrode was further electropolished by immersion in a 0.5-mol L−1 H2SO4 solution and applying potential in the range of cathodic and anodic edges of the electrolyte stability in a regime of cyclic voltammetry for 25 consecutive cycles. Upon this pretreatment, clean and stable Au electrode surface was attained. The Au electrode was then placed in the cell containing the synthesis solutions. Electrodeposition was done at desired potentials and times. The nanostructures gold electrodeposited-Au electrodes were then rinsed thoroughly with distilled water.

Electrochemical measurements to obtain the real surface area and calculation of roughness factor for the electrodeposited surface were performed as follows. After electrodeposition of gold nanostructures, the electrode was transferred to a solution of KCl (0.5 mol L−1) containing K4Fe(CN)6 (0.5 mmol L−1) as a redox probe and cyclic voltammograms at different potential sweep rates were recorded. The real surface areas were obtained from the Randles-Sevcik equation [44] and the value of 7.60 × 10−6 cm s−1 for the diffusion coefficient of Fe(CN)6 4− [45].

Results

The first gold nanostructure was electrodeposited on the Au electrode surface in the presence of arginine (20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 arginine), at 0 V, and the electrodeposition duration was 600 s (Au/nano-Au-Arg). Figure 1a shows FESEM images of Au/nano-Au-Arg electrode surface with different magnifications. At the low magnification, spear-like gold nanostructures with small sawteeth are observed. Higher magnification images show that these nanospears comprised nanowedges of 150–300 nm mean length with sawtooth-shaped surfaces. When electrodeposition was performed in the presence of aspartic acid (20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 aspartic acid) at 0 V for 600 s (Au/nano-Au-Asp), gold was electrodeposited as spherical and highly smooth surface nanoparticles with a mean diameter of 162 ± 5 nm. FESEM images of Au/nano-Au-Asp are shown in Fig. 1b.

Fig. 1
figure 1

a FESEM images of Au/nano-Au-Arg electrode surface with different magnifications. The electrodeposition solution contained 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 arginine. The electrodeposition potential was 0 V, and the electrodeposition duration was 600 s. b A FESEM image of Au/nano-Au-Asp electrode surface. The electrodeposition solution contained 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 aspartic acid. The electrodeposition potential was 0 V, and the electrodeposition duration was 600 s

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Electrodeposition of gold nanostructure in the presence of histidine (20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 histidine) at 0 V for 600 s (Au/nano-Au-Hist) resulted in the morphology of dendrite with different sizes of hyperbranched pine-like structures (Fig. 2). The dendrites, however, comprise the building units of about 150- to 200-nm nanoparticles. Therefore, the dendritic nanostructure electrodeposited in the presence of histidine has a hierarchical structure.

Fig. 2
figure 2

FESEM images of Au/nano-Au-Hist electrode surface with different magnifications. The electrodeposition solution contained 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 histidine. The electrodeposition potential was 0 V, and the electrodeposition duration was 600 s

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Electrodeposition of gold in the presence of lysine (20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 lysine) at the potential of 0 V for 600 s (Au/nano-Au-Lys) formed long dendrites; each dendrite is also hierarchical nanostructure consisting of an array of parallel arranged pyramidal nanoparticles (Fig. 3).

Fig. 3
figure 3

FESEM images of Au/nano-Au-Lys electrode surface with different magnifications. The electrodeposition solution contained 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 lysine. The electrodeposition potential was 0 V, and the electrodeposition duration was 600 s

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Electrodeposition in the presence of glucosamine (20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 glucosamine) at 0 V with an electrodeposition time of 600 s (Au/nano-Au-Glua) resulted in the formation of nanoparticles with different shapes and sizes. A FESEM image of the Au/nano-Au-Glua electrode is presented in Fig. 4.

Fig. 4
figure 4

A FESEM image of Au/nano-Au-Glua electrode surface. The electrodeposition solution contained 20 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 150 mmol L−1 glucosamine. The electrodeposition potential was 0 V, and the electrodeposition duration was 600 s

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Electrodeposited gold structure without using amino compounds in the presence of PVP (5 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 1.0 g L−1 PVP, average molecular weight of 40,000) at 300 mV with an electrodeposition time of 500 s (Au/nano-Au-PVP) was nanocubes with a mean length of 104 ± 4 nm (Fig. 5).

Fig. 5
figure 5

FESEM images of Au/nano-Au-PVP electrode surface with different magnifications. The electrodeposition solution contained 5 mmol L−1 HAuCl4 + 0.5 mol L−1 H2SO4 + 1.0 g L−1 PVP. The electrodeposition potential was 300 mV, and the electrodeposition duration was 500 s

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If no additive was employed in the electrodeposition solution (5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl) at 300 mV for 500 s (Au/nano-Au), the resultant nanostructure obtained in these conditions was oblong-shaped and polyangular rods. Figure 6 shows FESEM images of the Au/nano-Au electrode surface with two different magnifications. Each rod comprises two pyramids and appears as bipyramidal oblongs.

Fig. 6
figure 6

FESEM images of Au/nano-Au electrode surface with different magnifications. The electrodeposition solution contained 5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl. The electrodeposition potential was 300 mV, and the electrodeposition duration was 500 s

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For sonoelectrodeposition of gold without any additive with a positive potential of 300 mV for 500 s, the synthesis solution (5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl) and also the Au electrode surface were irradiated by ultrasound wave (Au/nano-Au-us3). The morphology of Au/nano-Au-us3 is similar to that of Au/nano-Au (Fig. 7a). The results indicate that the ultrasound irradiation of the synthesis solution and the depositing surface did not affect the morphology of the surface. On the other hand, sonoelectrodeposition without any additive (5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl) at a highly negative potential of −1800 mV for 300 s while the synthesis solution and also the Au electrode surface were irradiated by ultrasound wave was performed (Au/nano-Au-us18). Figure 7b shows FESEM images of Au/nano-Au-us18 with different magnifications. The nanostructure has a complex morphology and at low magnification comprises clung ribbons which are partly covered by blooms. At higher magnifications, FESEM images show nanoparticles of gold deposited on the connected smooth surface nanoribbons.

Fig. 7
figure 7

a FESEM images of Au/nano-Au-us3 electrode surface with different magnifications. The electrodeposition solution contained 5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl. The electrodeposition potential was 300 mV, and the electrodeposition duration was 500 s. The synthesis solution and also the Au electrode surface were irradiated by ultrasound wave. b FESEM images of Au/nano-Au-us18 with different magnifications. The electrodeposition solution contained 5 mmol L−1 HAuCl4 + 0.5 mol L−1 KCl. The electrodeposition potential was −1800 mV, and the electrodeposition duration was 300 s. The synthesis solution and also the Au electrode surface were irradiated by ultrasound wave

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Real surface areas of the synthesized gold nanostructures were electrochemically determined using the redox probe of ferrocyanide. Cyclic voltammograms of Fe(CN)6 4− recorded at different potential sweep rates using the gold-electrodeposited Au electrodes (Supplementary materials) and the real surface area of the nanostructures were determined, as reported in Table 1. Electrochemical activity of the synthesized gold nanostructures was evaluated in the course of electrooxidation/electroreduction of some biologically important compounds of ascorbic acid, glucose, and hydrogen peroxide (Supplementary materials). The gold nanostructures represented different activities toward different compounds (Table 1).

Table 1 Characteristics of different synthesized gold nanostructures
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Discussion

Affinity of amino acid to bind with gold surface has been investigated both theoretically [46–49] and experimentally [48, 50, 51]. Amino acids have also been employed as soft templates for the synthesis of gold nanostructures [52]. Gold has also affinity to amine functional group [48, 51, 53, 54]. During the electrodeposition of gold, nucleation, adsorption (of additives), branching, and growth are the dominant steps. From these steps, kinetics of the growth step is slow and controls the total process [55]. At the early stage of the electrodeposition, AuCl4− was quickly reduced to gold atoms, followed by distribution of gold nuclei at the surface. If an additive (amino acids, glucosamine, or PVP) is present in the solution, it is rapidly and selectively adsorbed (mainly via amine groups) on the specific planes of gold crystal. This adsorption prevents the newly generated gold atoms to be aggregated with the previously deposited ones. On the other hand, the additives act as shape-directing agents and depending on their chemical structures facilitate the gold crystal growth at a specific direction(s). If the electrodeposition is performed at highly negative potentials, the hydrogen co-evolution process would be the main shape-directing agent.

Regarding the real surface areas, gold nanospears with small sawteeth had the highest and gold pine-like hyperbranched nanodendrites and gold nanoparticles had the lowest real surface areas. There was a wide range of roughness factor, and different shapes of gold provide huge different surface areas. Electrochemical activity of the gold structures was also different depending on the shape and size from both kinetic and thermodynamic points of view. The different peak currents for the electroreduction/electrooxidation of the analytes can be related to the active surface areas of the gold nanostructures and different shapes and sizes causing the reactions to occur at different potentials. It depended on both the entity of the analyte and the shape and size of the gold nanostructures; the best gold nanostructure can be selected for a special analyte to electroreduce/electrooxidize.

Conclusion

This study showed that electrodeposition method can provide a variety of gold nanostructures with diverse size, shape, and electrochemical activity. The green additives, dc potential, and ultrasound irradiation played the major roles in the fabrication of these nanostructures. The surface of the nanostructures had different roughness and electrochemical activities related to the size and shape. The method can be extendable to the synthesis of other gold nanostructures and similar noble metal nanostructures.