Unveiling an unexpected potential of beetroot waste in green synthesis of single crystalline gold nanoplates: A mechanistic study

1 Introduction

Since the dawn of nanoscience, metal nanoparticles have achieved massive interest because of their distinctive properties. Au nanomaterials show wide applications in multiple fields due to their excellent catalytic activity and biocompatibility, non-toxicity and offer a favourable environment for biomolecules (Rosi and Mirkin, 2005; Schmid, 1992). The last few years have witnessed remarkable advancement by gaining the significant attention in the synthesis of a variety of metal nanostructures due to their distinctive physical and chemical properties. These are clearly dissimilar with their bulk solid, and widely used in catalysis (Yin et al., 2011), Photonics (Sardar et al., 2009), biosensing (Pingarrón et al., 2008), electronics (Jeong et al., 2008) and nanomedicines (Chen et al., 2007). The range of methods for variety of structures includes nanocrystals (Hao et al., 2004), nanorods, nanocubes, nanowires, nanoplates, nanoflower (Al-Akraa et al., 2017), nanoclusters, nanobelts (Beeram and Zamborini, 2010; Osberg et al., 2012; Payne et al., 2014; Tsung et al., 2006) and so on. Besides the variety of morphologies, nanoplates of gold achieved noticeable interest due to their superior localized surface plasmon resonance (LSPR) properties having distinct dipolar and quadrupolar plasmon resonances consigned from their sharp corners and edges. Additionally, plate-like nanostructures exhibit higher surface to bulk atom’s ratio than other nanostructures and provide wide applications in nanodevices, bio-imaging and surface-enhanced Raman scattering (SERS), electrochemical sensing (Deckert-Gaudig and Deckert, 2009; Huang et al., 2010; Li et al., 2014; Seo et al., 2011), gas sensors (Ankamwar et al., 2005) near-infrared (NIR) light absorbers for cancer hyperthermia (Shankar et al., 2005) and high-resolution scanning tunnelling microscopy (Dahanayaka et al., 2006).

Importantly, Au nanoplates synthesized using common synthetic approaches including seeded growth (Millstone et al., 2008, 2006; Scarabelli et al., 2014), electrochemical and photocatalytic approach (Miranda et al., 2010), thermal reduction (Chu et al., 2006), polyol (Xiong et al., 2006).Amongst these the seeded growth method is still the widely accepted due to its easy setup and comparatively higher throughput. This method used the surfactant as shape-directing agent. In recent times, another report showed the potential of sodium thiosulfate for triangular Au nanoplates synthesis and in this work polyethylene glycol (PEG) used as a stabilizing agent. PEG contain carboxylic acid and thiol end groups as a non-hazardous alternative for CTAB (Pelaz et al., 2012). The challenges of using natural products for controlled synthesis over crystallinity, size and shape is still an indefinable topic.

However, chemical methods have numerous drawbacks including harmful nature of solvents, reagents and the reducing environment that might result in the contamination in end product or may generate some risky by-product. Therefore, in mid of past decade the challenge of an incorporation of PRODUCTIVELY (Dahl et al., 2007; Tang et al., 2005) principles, for the synthesis of gold nanostructures, met with limited success. Some efforts include biosynthesis of triangular nanoplates using Neem leaf broth and lemongrass leaf (Shankar et al., 2004a,b), bovine serum albumin (Xie et al., 2007a,b), and an amino acid aspartate (Shao et al., 2004). There are reports on the crystal morphology and size regulation using biomolecules (Allen et al., 2002). Some primary research revealed the ability of biomolecules and organisms to particularly identify the inorganic surfaces or else used as matrices for inorganic growth and nucleation (Cha et al., 2000). For instance peptides (Cha et al., 2000) and Bacteria (Cha et al., 2000), had used for functionalization of silver nanoparticles and the morphology of Au crystals had still manipulated using polypeptides (Brown et al., 2000). It is usual that the organisms or component from bio macromolecules must create a beginning for indisputable biological nanofabrication. No doubt that “Green synthesis” approaches advances over the other synthetic methods by means of the solvent system, reducing and capping agents (Shankar et al., 2005). Apart from the application of these principles, the variety of dominant factors like pH, reaction time and the temperature needs further optimization. Besides these facts, the challenging task is to rationally design optimized synthetic strategy for single crystalline triangular nanoplates in green approach. More significantly, the formation mechanism of the nanoplates also unveiled scientifically.

Herein, we report simplistic route for the synthesis of single crystalline triangular gold nanoplates using an unexpected potential of aqueous extract from the waste of red beetroot as an excellent reducing, capping and shape-directing agent first time. Beet juice had a potential of plasmonic noble metal nanoparticles synthesis (Kou and Varma, 2012a,b) while in our earlier work, crystallinity dependent study of Au nanoparticles carried out using beetroot peel extract (Deokar and Ingale, 2017). Excluding these efforts, to the best of our knowledge, there is no information available on the mechanistic study of the green synthesis of single crystalline gold nanoplates using beetroot peel aqueous extract. Adding to the novelty, this is the first time report of setting a novel green synthetic approach for resolving the synthesis mechanism of single crystalline triangular Au nanoplates synthesis using a red beetroot peel extract.

2 Material and methods

3 Results

It is commonly accepted that different shapes specifically exhibit different SPR peaks. The UV/Vis spectrums recorded at increasing reaction time, shown in Fig. 1 certainly confirm the Au nanoparticles with the spherical shape at 525 nm and indicate the presence of less anisotropic particles. The presence of a broad peak at 595 nm is due to truncated icosahedrons. The SPR peak at 600–613 nm which is clearly distinguishable for the icosahedral structure it is in agreement with previous reports (Kim et al., 2004; Xu et al., 2008). Under these circumstances, the peaks look like linked structures. The SPR peak at >700 nm showed the existence of triangular nanostructures in a large amount (Hong et al., 2011). Still there are numerous open issues concerning the formation mechanism, however, depending upon existing peaks a preliminary mechanism is proposed which involves the concurrent induction of Au nanosphere and its transformation into icosahedrons and triangles. The underlying principle for this mechanism could be the temperature is increasing from cooling to the room temperature and pH is stable. The mechanism further validated using electron microscopy.

Close

Fig. 1 UV Vis spectra of green synthesized nanostructures at different time interval.

Export to PPT

Fig. 2 provides a comparison through attenuated total reflectance Fourier transform-infrared (ATR-FTIR) spectroscopy of the biomolecules present in extract and the Au nanosuspension respectively. In case of extract the strong and broad peak existing at 3280 cm⁻¹ possibly attributed to the O–H stretching vibrations in alcoholic hydroxyl functional groups and N—H stretch vibrations in the amides I, II and amines, (Kou and Varma, 2012b; Abdel-Raouf et al., 2017; Kanchi et al., 2018; Dzimitrowicz et al., 2016) also a strong peak at 1631 cm⁻¹ may indicate the stretching vibrations of C⚌O group in aldehydes, ketone or even in amide (Kumar et al., 2011; Xie et al., 2007a,b). The existence of bands at 2934 and 1463 cm⁻¹ may be occurring due to C—H stretching and C—H bending vibrations in the hydrocarbon chains. The consecutive bands at 1214 and 1305 cm⁻¹ were coming up from aliphatic amines and C—O stretching in the ether, respectively. The presence of a medium band at 2821 cm⁻¹ may be attributed to C—H stretch off C⚌O in aldehydes. The sharp peak at 739 cm⁻¹ arises due to out-of-plane N—H bending in amide. In this study except for few bands, all of FTIR bands match with the earlier studies on beet juice and serum albumin protein and sweet sorghum syrup (Kou and Varma, 2012b; Kumar et al., 2011; Xie et al., 2007a,b). Comparatively few bands were disappeared and/or shifted in ATR-FTIR spectrum of Au nanosuspension, which encourage stating that the biomolecules in BRW acting as a reducing and functionalizing agent for Au nanostructures. Furthermore, there is a presence of the pectin in beetroot peel (Neelwarne, 2013) which is acting as a surfactant/stabilizing agent.

Close

Fig. 2 FTIR spectra of aqueous extract of red beet peel and as-synthesized Au nanostructures.

Export to PPT

The XPS investigations disclosed that the as-synthesized nanoplates be composed of elemental gold. The Au 4f XPS spectrum composed of doublet peaks corresponds to the 4f_7/2 and 4f_5/2 at 83.4 and 87.07 eV BEs, respectively, which could be assigned to metallic Au⁰ (Yin et al., 2016; Yang et al., 2015) (Fig. 3a). The high-resolution C1s XPS spectrum was fitted with three complex peaks that could be decomposed into three chemically varied components (Fig. 3b). The high flying peak at 288.23 eV BE assigned to the electron emission from carbonyl carbons (C⚌O) in aldehydes or ketone (Kumar et al., 2003; Miyama and Yonezawa, 2004), whereas the medium peak centered at 286.16 eV BE to the Cα of the carbonyl carbon (O⚌CR) (Wang et al., 2014), hydroxyl carbon (C—OH) (Huang et al., 2011). The BEs of carbons united with a carbonyl functional group is eminently influenced by the I effect (Shankar et al., 2005). This is presumably due to the co-ordination bonds between Au nanostructure and functional groups (O—H, C—O) of carbohydrates (Deokar and Ingale, 2016), primarily present in BRW extract. The last C 1s deconvoluted peak at 284.80 eV BE corresponds to the hydrocarbon (C—H/C—C) (Dhayal and Ratner, 2009). The peaks at 531.30 and 532.60 eV BEs attributed to O1s (Fig. 3c), which probably due to the presence of the oxygen in ketonic carbonyl (C⚌O) (He et al., 2014),(Wu et al., 2015),(Chehimi and Delamar, 1990), C—OH of alcohol (Schulze and Hardegree, 1989), C—O—C of acetal and hemiacetal (Rouxhet et al., 2008),(Briggs and Beamson, 1993), respectively. The deconvolution at 399.87 eV BE ascribed to the amide of proteins (Fig. 3d).

Close

Fig. 3 XPS high resolution spectra in the Au (4f) (a), C 1s (b), O 1s (c) and N 1s (d).

Export to PPT

First FE SEM image (Fig. 4a) revealed the spherical nanoparticles with slight size variation. Lower magnification image (Fig. 4b) clearly shows the appearance of triangular nanoplates whereas in higher magnification (Fig. 4c), both nanoparticle and truncated triangular nanoplate were clearly seen. While in Fig. 4d we found a single triangular nanoplate which seems to be detached from any solid structure specifically from icosahedrons. In Fig. 4e single nanoplate with an irregular width was observed, the widths vary from 50 nm to 65 nm. In the last image (Fig. 4f) EDS analysis verified the existence of elemental gold by matching the sturdy signal at inimitable 2.120 keV and 9.712 keV of elemental gold. In addition, the strong signal of oxygen, nitrogen and carbon peak was also detected. These signals were originated from the X-ray emissions from biomolecules bound to surface of Au nanoparticles. This clearly suggested the formation of approximately pure nanoplates of Au, dominantly triangular in shape, by the room temperature reduction of HAuCl₄·3H₂O by red BRW aqueous extract. The presence of elemental Au readily supported the emergence of SPR peak in absorption spectrum and XPS analysis mentioned earlier.

Close

Fig. 4 Representative FESEM images: spherical (a), triangular nanoplates (b) and (c), single truncated nanotriangle (d), irregular nanoplates (e) and EDX spectra of Au nanostructures (f).

Export to PPT

A sturdy overwhelming peak observed at 38.2^° assigned to {1 1 1} of fcc Au. Three weakly resolved peaks were also detected at 44.39^°, 64.44^° and 77.5^° and assigned to {2 0 0}, {2 2 0}, {3 1 1} respectively (Fig. 5). It is significantly noted that the ratio intensity of {2 0 0} to {1 1 1} diffraction peaks is 0.131 which is lower than the well known standard value (0.131 versus 0.52) from JCPDS No. 04-0784. Nonetheless, other remaining intensity ratios, 0.094 and 0.090 also revealed much lower value than the corresponding values of bulk Au: 0.32 and 0.36, respectively. These annotations signifying that the nanoplates were mainly dominated by {1 1 1} planes and it tends to be preferentially oriented parallel to the surface of the supporting substrate. Additionally, appearance of the SPR at 540 nm clearly indicates the existence of the less anisotropic nanostructures (Balasubramanian et al., 2016), evidenced with the mismatch of the ratio intensities of the {1 1 1} and {2 0 0} diffraction peaks which appeared much lower than the standard value obtained from the icosahedra i.e. 2.69 (Xu et al., 2008).

Close

Fig. 5 PXRD pattern of the Au nanostructure.

Export to PPT

TEM and HRTEM were used to investigate the crystallization of the Au nanoplates. It is important, not to ignore that, the TEM images were captured directly from the original product neither size selection nor any purification process were applied. Fig. 6a and 6b revealed that there is a presence of spherical as well as pentagonal nanoparticles simultaneously, supported that the SPR peak in absorption spectra (Fig. 1). It would be interesting to know that while magnified on the Pentagon, in fact, it was an icosahedral structure (Fig. 6c), further it was also noted that the structure disassembling or fragmenting into triangular nanoplates. Fig. 6d and e demonstrated the nanoplates formed with different sizes and edge lengths; one of the reasons for this could be, the plates were fragmented from icosahedral nanostructure and the location of the respective plate varies in the icosahedrons. Moreover, truncated triangular image (Fig. 6d), suggested that there is a presence of planar defects primarily stacking faults. These faults were validated through the selected area electron diffraction pattern (Fig. 6f) by arranging the electron beam upright to the triangular face of a nanoplate which confirms the plates were single crystalline. SAED pattern showed a series of diffraction spots with a sixfold rotational symmetry, which is a distinguishing feature of {1 1 1} oriented single-crystal nanoplates and it could be assigned to regular Bragg’s diffraction spots of {2 2 0} (boxed) and to the kinematically fractional (forbidden) 1/3{4 2 2} (circled) diffractions from fcc gold. The emerging fcc forbidden 1/3{4 2 2} diffraction pattern seen on a plate-like structures of gold or silver while the stacking faults parallel to the {1 1 1} plane run throughout the nanoplate (Abdel-Hamid et al., 1999; Geng et al., 2017; Kirkland et al., 1993). The HRTEM investigations of triangular nanoplate, Fig. 6 g pointed out that, the lattice distance is 0.234 nm, which coincides with bulk Au {1 1 1} spacing (0.236 nm) suggesting the{1 1 1} planar facet (Xu et al., 2008). Significantly, the findings suggested that the surface of the Au nanoplates is atomically flat with fcc {1 1 1} plane (Jin et al., 2003, 2001; Liu et al., 2005). The last HRTEM image (Fig. 6 h) showed the presence of distinct moiré fringes when the triangular nanoplates stacked against each other (Auzary et al., 1998; Xie et al., 2007a,b; Yamanaka et al., 1994). That is an indication of the Au nanoplates have mixed or diverse orientation at an overlapped region.

Close

Fig. 6 TEM image of Au nanostructure synthesized using beetroot aqueous extract at room temperature: spherical nanoparticle (a), regular icosahedrons vertex (b), single icosahedron (c), truncated triangle (d) and (e), SAED pattern of single nanotriangle (f), lattice fringe (g) and stacking faults (h).

Export to PPT

4 Discussion

Few different mechanisms have been suggested in the literature for nanoplate formation. One of the reports revealed that the carbonyl-containing compounds like citric acid and indole acetic acid stabilized {1 1 1} surface by adsorbing on the same plane and allowed the growth of other planes, resultant of {1 1 1} facets rich plane. The remarkable significance of capping agent for a shape direction, stabilization and their engrossment on to the {1 1 1} planes as is well known. Some other investigations propose that during the nucleation phase the formation of stacking faults may develop the asymmetric growth of the particle (Lofton and Sigmund, 2005). Several other reports suggest that the other factors such as a thermodynamic driving force of the reaction (Viswanath et al., 2009) as well as the impact of the rate of nucleation versus growth (Xia et al., 2009).

Even if solving the detailed mechanism of formation of every Au nanostructure by the green method is quite difficult, a successive attempt has been carried out. To figure out the synthetic mechanism for Au nanoplate, a systematic HR-TEM study has been performed. This determined three distinctive steps in formation of triangular nanoplates: synthesis, transformation, and fragmentation. Briefly, in the first step, the formation of Au nanospheres takes place with the help of BRW extract as reducing agent for the HAuCl₄·3H₂O The formed nanospheres then transformed into icosahedrons in the second step and lastly the nanotriangles fragmented from icosahedral structures. It is believed that the biomolecules present in the BRW aqueous extract responsible for the nanospheres formation. Later the transformation step is the outcome of oxidative etching by air born oxygen and chloride from reaction solution. Lastly the fragmentation of icosahedrons into triangular nanoplate is result of the increasing reaction temperature. In this environment the icosahedra cannot maintain its structure intact, because the increasing reaction temperature interferes. Increasing temperature may act a unique shape-transforming role without affecting the crystallinity of the {1 1 1} plane. Accordingly the single crystalline growth of the nanoplates is the result of the oxidative etching as stated by previous reports of oxidative etching by polyaniline (Guo et al., 2008) but the exact reaction mechanism is not known. It is noted that the triangular nanoplates are not anisotropic one of the reason for this could be the absence of the iodide ions which specially bind to the Au {1 1 1} facet and advance the synthesis of anisotropic nanostructures along with removal of different shape impurities by chemical etching (Chen et al., 2014). Additionally, for the growth on the {1 1 1} plane the capping agents playing a key role in many previous reports, such capping agents are polymers (Kan et al., 2010; Xiong et al., 2006), biomolecules (Liu et al., 2005), halide ions (Chen et al., 2014), and surfactants (Chu et al., 2006). In this green method the biomolecules present in the BRW extract acting as capping and shape-directing agent as evidenced by FTIR and XPS analysis mechanistically adsorbing on a specific surface and avoiding the further addition of the gold ions on that particular surface, so that the growth is restricted in planar direction. It’s noteworthy that in SPR analysis, the presence of spherical nanoparticles evident as time increases, including the icosahedral and triangular shapes simultaneously. The mechanism derived from TEM studies excitedly supported by respective SPR peaks in absorption spectrum. The limitation of this method could be, the three steps will require very short time for the nanoplate synthesis. Characteristically in a very shorter period of time we are unable to distinguish these three steps of the mechanism. For better differentiation of three steps of mechanism we suggest that supplementary concentration-dependent study is essential which may perhaps split the steps.

5 Conclusion

In summary, one step, simplistic way towards single crystalline triangular Au nanoplates was demonstrated with the probable mechanism. The appearance of assorted peaks in UV–Vis spectrum clearly evidenced that the variety of nanostructures present in the suspension. The TEM studies showed this variety of structures signifies as spherical, icosahedral and triangular. The nano triangular structure exhibited flat surface, referred as plates, and was of single crystalline, oriented in the {1 1 1} lattice planes, supported by HRTEM, XRD and SAED pattern studies. It was believed that the biomolecules in BRW extract were responsible for the reduction of chloroauric acid and oxidative etching results transformation of nanosphere into icosahedrons following the fragmentation of single crystalline triangular nanoplates by increasing reaction temperature in the green synthetic method. To the best of our knowledge, we reported the mechanistic approach for the synthesis of single crystalline Au nanoplates through the use of red beetroot waste for the first time. This novel eco-friendly route will not only enlighten the potential of waste in the synthesis of single crystalline Au nanoplates but also draw attention towards the plenty of research in the field of anisotropic noble metal nanostructures. This cost-effective approach may be extended to attained sustainable synthesis for many practical purposes. Comprehensively the work cover up three out of the twelve fundamental principles of green chemistry and one out of twelve principles of green engineering.