Photochemical Synthesis and Catalytic Applications of Gold Nanoplates Fabricated Using Quercetin Diphosphate Macromolecules.

The demand for safer design and synthesis of gold nanoparticles (AuNPs) is on the increase with the ultimate goal of producing clean nanomaterials for biological applications. We hereby present a rapid, greener, and photochemical synthesis of gold nanoplates with sizes ranging from 10 to 200 nm using water-soluble quercetin diphosphate (QDP) macromolecules. The synthesis was achieved in water without the use of surfactants, reducing agents, or polymers. The edge length of the triangular nanoplates ranged from 50 to 1200 nm. Furthermore, the reduction of methylene blue was used to investigate the catalytic activity of AuNPs. The catalytic activity of triangular AuNPs was three times higher than that of the spherical AuNPs based on kinetic rate constants (k). The rate constants were 3.44 × 10-2 and 1.11 × 10-2 s-1 for triangular and spherical AuNPs, respectively. The X-ray diffraction data of gold nanoplates synthesized by this method exhibited that the nanocrystals were mainly dominated by (111) facets which are in agreement to the nanoplates synthesized by using thermal and chemical approaches. The calculated relative diffraction peak intensity of (200), (220), and (311) in comparison with (111) was found to be 0.35, 0.17, and 0.15, respectively, which were lower than the corresponding standard values (JCPDS 04-0784). For example, (200)/(111) = 0.35 compared to 0.52 obtained from the standard (JCPDS 04-0784), indicating that the gold nanoplates are dominated by (111) facets. The calculated lattice from selected area electron diffraction data of the as-synthesized and after 1 year nanoplates was 4.060 and 4.088 Å, respectively. Our calculations were found to be in agreement with 4.078 Å for face-centered cubic gold (JCPDS 04-0784) and literature values of 4.07 Å. The computed QDP-Au complex demonstrated that the reduction process took place in the B ring of QDP. This approach contributes immensely to promoting the ideals of sustainable nanotechnology by eradicating the use of hazardous and toxic organic solvents.

Introduction

The synthesis of gold nanotriangles is an exciting research area because of the unique optical, magnetic, and electrical properties. These properties have proved to be strongly dependent on size, shape, and morphology of the nanoparticles.[1−6] In recent years, different shapes of gold nanoparticles (AuNPs) have been reported including nanospheres, nanotriangles, nanocubes, nanorods, nanowires, nanobelts, rectangular, and nanoplates.[7−19] Emphases have been directed toward adopting better synthetic methods which can be used to fine-tune the size and shape of AuNPs to enhance these unique properties including the use of them as sensing devices for cyanide.[20] Notable synthetic approaches have been adopted including chemical, electrochemical, and physical methods. However, the chemical methods require the use of organic reducing agents, such as NaBH4, which are toxic. Furthermore, the formation of uniform and controlled shape of AuNPs is normally achieved by using capping and stabilizing agents, such as cetyltrimethylammonium bromide, although it is very toxic and hence affects the environment. Photochemical methods have been used successfully in the synthesis of AuNPs with interesting shapes.[9−19] Recent research has shown that naturally derived flavonoids acted as reducing, capping, and stabilizing agents in the synthesis of nanoparticles.[21−24]

Gold nanoplates have been synthesized using different approaches such as thermal-controlled techniques,[25,26] use of polymers,[27,28] and surfactants as well as using reducing organic ligands.[29−31,32a] However, high heat energy is generally required to attain the Au nanoplates. For example, the synthesis of gold nanoplates was reported to have been attained by carrying the reaction in a water bath at 196 °C for 18 min.[32b] Studies have reported the synthesis of gold nanoplates using biological[33,34] and plant-mediated approaches.[35] Starch-mediated gold triangular nanoplates of 200 nm size were successfully synthesized in the presence of sunlight.[36] Furthermore, biological synthesis of single crystalline gold nanoplates has been reported.[37]

In this study, we were able to integrate principles of green chemistry in the synthesis of AuNPs by using a suitable solvent such as water; nontoxic reducing, capping, and stabilizing agent; and less energy. The ultimate goals of green chemistry are always attained by adopting the 12 fundamental principles of green chemistry which include avoidance of toxic solvents and reagents, minimizing toxic waste, elimination of unnecessary steps, and minimization of energy usage.[38] It is important to note that the green and sustainable synthesis of nanoparticles should aim at using environmentally benign solvents such as water, renewable materials, nonoxic chemicals,[39] and adopting synthetic protocols that require less energy, thereby reducing the cost of production.[40,41] Again in our protocol, we are using water as the solvent to dissolve our modified quercetin molecule and using sunlight as the energy source for the formation of the nanoparticles. The synthesis of metal alloy nanoparticles with emphasis on environmentally-benign approach by using water as a solvent, glucose as reducing agent and starch as protecting agent has been reported.[42] We are reporting for the first time an economical and eco-friendly photochemical synthesis of gold nanoplates using quercetin diphosphate (QDP) macromolecules in the presence of sunlight. Although, literature indicates that the photochemical synthesis of spherical AuNPs and nanorods has been successfully attained. This synthesis was achieved by using an organic surfactant, sodium dodecylsulfonate, as the protective agent.[43]

We hereby report the synthesis of gold nanoplates by the reduction of Au3+ ions to Au0 using QDP in the presence of sunlight for 55 min and using only water as a solvent without using organic solvents, capping agents, or stabilizing agents, thereby promoting the ideals of green chemistry and sustainable nanotechnology. Starch-mediated photochemical synthesis of gold nanoplates has been achieved by carrying out a reaction of starch and gold(III) chloride in the presence of sunlight for 5 days.[36] In our synthetic protocol, QDP acted as both reducing and capping agent in the presence of sunlight to produce gold triangular nanoplates with well-refined edges. These have the potential to be used in the surface-enhanced Raman spectroscopy detection platform, catalysis, and electronic and photonic applications. We have demonstrated from the computed density functional theory (DFT) calculations that QDP–Au complex with the lowest energy required for the complexation reaction of QDP and Au to take place in the B ring between the 3-OH and 4′-phosphate group.

Results and Discussion

Synthesis and Characterization of Gold Nanoplates

In this work, the use of sunlight to provide energy for the synthesis of gold nanoplates was explored. In Figure a(B–D), samples were exposed to sunlight for 2, 11, and 55 min, respectively, and the color changes that occurred depicted the formation of AuNPs.

Figure 1

(a) (A) UV–vis spectra of the formation of gold nanoplates when 4.5 × 10–4 M HAuCl4 were reacted with 5 × 10–3 M QDP in a mole ratio of 1:1, 1:2, 1:3, 1:4, 1:6, and 1:8 for P, Q, R, S, T, and U, respectively, and then all of the samples were exposed to direct sunlight for 55 min. Pictogram showing color changes when the reaction was carried out in the presence of sunlight for 2 (B), 11 (C), and 55 min (D). (b) TEM images showing the morphology of Au nanoplates (A–F) of gold nanoplates formed after 55 min from samples P, Q, R, S, T, and U, respectively. EDS spectrum showing the formation of AuNPs obtained from QDP (G).

The UV–vis spectrum, as shown in Figure a(A), clearly demonstrates the shift in absorption band and the red shift from 550 to around 760 nm. The intensity of absorption band at NIR increased steadily with an increase in concentration of QDP when exposed to sunlight for 55 min. Concentration of QDP influenced the size and morphology of the gold nanoplates, which is in agreement with literature studies.[34,37]

Increase in irradiation time led to increase in nanoparticle sizes as demonstrated by transmission electron microscopy (TEM) images in Figures b and S3 in the Supporting Information section. It has been established that the optical properties of nanoparticles depend entirely on size and shape. The UV–vis spectrum in Figure a(A),b(A) clearly demonstrates the anisotropic nature of the gold nanoplates. Energy-dispersive X-ray spectroscopy (EDS) [Figure b(G)] confirmed the formation of gold nanoplates. The formation of two surface plasmon resonance (SPR) peaks is more pronounced after 90 min than after 55 min. According to Mie theory,[55,56] spherical nanoparticles only exhibit one SPR band, whereas anisotropic nanoparticles may show two or three SPR bands based on the shape of the nanoparticles. The concentration of QDP influenced the morphology of the gold nanoplates with average edge lengths ranging from 73 to 1200 nm.[57] The nanoplates exhibited sharp edges with increase in concentration of QDP and sunlight exposure time. An absorption band at around 550 and 745–849 nm was observed for the nanoplates (Figure ). Blue shift in SPR peaks of band 1 from 550 to 529 nm, whereas the second band exhibited a red shift from 700 to 849 nm. The TEM analysis (Figure ) shows the morphology of hexagonal gold nanostructures. Large nanoprisms are also observed. TEM images clearly demonstrate the sintering of nanoparticles coupled by their adherence to nanoprisms. This observation was also reported in literature.[58]Figure shows blunt-angled nanoprisms, and this could be mainly attributed to the fact that nanoprisms possess high energy and hence suffers a shrinking process which leads to the formation of blunt edges to reduce the surface energy.[59] The experimental results reveal that by increasing the sunlight exposure time, large nanoplates with different sizes and shapes will be obtained, as shown in Figure , which agree very well with the nanoplates formed by other methods.[60,61]

Figure 2

(A) UV–vis spectra of formation of gold nanoplates when 4.5 × 10–4 M HAuCl4 was reacted with 5 × 10–3 M QDP in mole ratios of 1:1, 1:2, 1:3, 1:4, 1:6, and 1:8 for P, Q, R, S, T, and U, respectively, and exposed direct sunlight for 90 min. TEM images showing morphology of Au nanoplates (B–I), whereas the pictorial color changes depicted by (J,K).

In dark conditions, AuNPs were not formed even after a period of 10 days.

No color change was observed; thus, it could be deduced that sunlight (light energy) plays a significant role in the formation of gold nanoplates. This is in agreement to the literature report.[36]

Powder X-ray diffraction (XRD) was used to confirm the crystalline nature of gold nanoplates, as depicted by the XRD pattern in Figure A. Four peaks observed at 2θ = 38.29°, 44.51°, 64.80°, and 77.70° can be assigned to (111), (200), (220), and (311) planes, respectively, for face-centered cubic (fcc) which represents the characteristic diffraction of elemental gold metal (Au0) (JCPDS 04-0784), thereby confirming the formation of crystalline AuNPs.

Figure 3

XRD crystallography pattern of gold nanoplates obtained from the reduction of Au3+ by QDP (A), SAED pattern of gold nanoplates (B), high-resolution TEM (HRTEM) of gold nanoplates (C), and model of fcc gold nanotriangle in (220) orientations (D).

Furthermore, relative diffraction peak intensity of (200), (220), and (311) in comparison with (111) determined to be 0.35, 0.17, and 0.15, respectively. The calculated values are lower than the corresponding standard values (JCPDS 04-0784). For example, (200)/(111) = 0.35 compared to 0.52 obtained from the standard (JCPDS 04-0784), thereby depicting that the gold nanoplates are majorly dominated by (111) facets. Hence, the (111) planes tend to be preferentially oriented parallel to the surface of the substrate.[24,28] From our results, it can be deduced that the nanoplates are majorly composed of (111) lattice planes which are in agreement with literature reports of XRD characterization of gold nanoplates synthesized by chemical and thermal processes.[4,24,26,28,33,37] Literature reports indicate that the formation of nanoplates with preferential (111) facets may be attributed to lower free energy of (111) planes relative to other planes such as (200), (222), and (311).[44] The selected area electron diffraction (SAED) pattern which depicted highly oriented (111) crystalline lattice (Figure B) was used to calculate a lattice constant of 4.060 Å which is in agreement with 4.078 Å for the fcc gold (JCPDS 04-0784). The calculated lattice constant is also in agreement with the lattice constant of 4.070 Å calculated from the XRD analysis of hexagonal and triangular gold nanoplates, synthesized from the chemical method using organic dye molecules.[61]Figure C demonstrates that the growth of the nanoplate was predominantly along the (111) plane. The results from XRD data were used to model the nanotriangles based on (111), (200), (220), and (311) orientations. The crystal maker software was used to generate fcc models by using a d-spacing of 0.228 nm; nanotriangles generated based on Bragg’s reflection indices (111), (200), (220), and (311), as shown in Figure D.

Figure A–C shows TEM images of the gold nanoplates of sample S in which the mole ratio of Au3+/QDP was 1:6, left in sunlight for 90 min, and the sample was kept for 1 year.

Figure 4

Typical TEM images [(A–C) taken after 1 year] of gold nanoplates obtained from reduction of Au3+ by QDP in the ratio of 1:6 for 90 min and stored for 1 year and SAED pattern of gold nanoplates (D).

The nanoplates were hexagonal, triangular, and irregular spheres and truncated triangles (Figure A–C). The lattice constant of gold nanoplates determined from the SAED pattern with very bright spots (Figure D) taken after 1 year was found to be 4.088 Å which was close to the standard lattice constant of 4.078 Å (JCPDS 04-0784). The control experiment was carried out at room temperature (Figure ) to validate the results of the effect of sunlight. The results from Figure demonstrate that the AuNPs formed exhibited smaller sizes clearly demonstrating that the gold nanoplates could only be formed in the presence of sunlight possibly because of rapid radical formation, leading to faster reaction between the gold(III) chloride complex and QDP. The UV–vis spectrum (Figure A) shows the change from isotropic to anisotropic from sample A to D, which is supported by TEM (Figure B–G) images being transformed from spherical to nanoprisms. An increase in the concentration of QDP at room temperature led to an increase in the number of nanoprisms with higher number of hexagonals compared to triangular AuNPs.

Figure 5

Effect of concentration of QDP on the formation of AuNPs when 5 × 10–3 M HAuCl4·3H2O with 5 × 10–3 M QDP at room temperature in mole ratios of 1:1, 1:2, 1:3, and 1:4 for A, B, C, and D, respectively. TEM images of (B) for sample A, (C,D) for sample B, (E) for C, and (F,G) for sample D. HRTEM image of sample B (H) clearly depicting fringes.

Comparison of Catalytic Activity of Spherical and Triangular Nanoparticles

The reduction of methylene blue (MB) is commonly evaluated to test the catalytic activity of metallic nanoparticles because the reduction of MB can be easily monitored using UV–vis absorption.[62,63] The addition of AuNPs and excess of 32 mM NaBH4 into MB solution at room temperature produced a color change from blue to colorless. MB exhibits a peak maxima at 658 nm, which corresponds to n−π* transition and a shoulder at 614 nm (Figure D).[62,64] In the presence of NaBH4 alone, the reaction takes an extensive amount of time to react, which can be shown by the observation of the absorption band at 658 nm (Figure C). The absorbance peak at 658 nm barely decreases even after 45 min of reaction time. However, in the presence of a catalyst, MB undergoes reduction to form leucomethylene blue (Figure A,B), and the MB absorption peak at 664 nm decreases rapidly. The catalytic activity, as measured by the value of the activation energy, correlates with the fraction of surface atoms. Research has found out that anisotropic nanoparticles have more surface atoms located on the corners, edges, and crystallographic facets.[57] This idea explains why there is a higher catalytic activity of triangular nanoparticles compared to spherical nanoparticles.

Figure 6

Time-dependent UV–vis absorption spectra of the reduction of MB by NaBH4 in the presence of spherical (A) and triangular AuNPs (B). Time-dependent UV–vis absorption spectra of the reduction of MB by NaBH4 in the absence of AuNPs (C). UV–vis response of MB (D). Plot of ln(A/A0) as a function of time for the reaction catalyzed by spherical and triangular AuNPs (E).

The mechanism of how the nanoparticles work as a catalyst has been described previously.[65] Na+ and BH4– will dissociate in solution from one another. Therefore, BH4– will act as a donor in the reduction process. We know then that MB will act as an acceptor because the nanoparticles in solution will not. Therefore, since no reduction occurs without the nanoparticle catalysts, it can be thought that the nanoparticles accept the electrons easily from BH4– and then transfer the electrons to MB to cause the reduction process to occur catalytically.[65]

Because this reaction was conducted under a large excess of NaBH4, the reaction rate will be considered to be independent of the NaBH4 concentration, and consequently the kinetics can be modeled by a quasi-first-order process with respect to the concentration of MB.[62] Therefore, the rate constant for the reduction processes was determined by measuring the change in absorbance at the operational wavelength as a function of time. Figure E depicts the plot of ln(A/A0) as a function of reaction time (t) for both spherical and triangular AuNPs. A and A0 are the absorption intensities at 658 nm at time t and 0, respectively. The kinetic rate constants (k) were calculated to be 3.44 × 10–2 and 1.11 × 10–2 s–1 for triangular and spherical AuNPs, respectively. This confirms that the catalytic activity of triangular AuNPs is greater than spherical AuNPs. In fact, our results suggest that the catalytic activity is over three times higher than that of spherical AuNPs.

The shape of nanoparticles and their catalytic activity have been studied extensively in recent years. Gangapuram et al. synthesized AuNPs from Annona squamosa L peel extract. The nanoparticles synthesized had an average size of 5 ± 2 nm and were spherical.[64] The rate constant was found to be 0.038 s–1 at room temperature against MB.[64] Gangapuram et al. also synthesized 5–20 nm spherical AuNPs using Salmalia malabarica gum.[65,66] The catalytic activity was tested for these nanoparticles against MB as well. The rate constant found in this experiment was 0.00402 s–1.[65] Chowdhury et al. showed that synthesizing an alloy nanoparticle of gold and palladium can increase the catalytic activity.[67] The shape of the nanoparticle was hexagonal with a range of 11–30 nm in size, however. These particles had a rate constant of 0.230 s–1 for the reduction of MB.[67] Shaik et al. tested the catalytic activity of Au triangular nanoplates versus spherical nanoparticles inside of mesoporous hollow silica shells.[68] The rate constants for the two nanoparticles were 2.6 × 10–3 and 1.03 × 10–3 s–1. This is worth noting because triangular nanoparticles were close to 2.5 times as effective compared to spherical nanoparticles.[68] In our study, the triangular nanoparticles were slightly over three times as effective in comparison with the spherical nanoparticles that were synthesized. The catalytic activity that was found in this work was close to what has been previously published.[68]

Computational Analysis of QDP Conformers

Figure illustrates the lowest energy conformation of the QDP monomer. Favorable hydrogen bonding (noncovalent) interactions between the partial positive hydrogen from the phosphate group stabilize this conformer (P=O–H distance 1.78 Å, C=O–H distance 1.59 Å). The two phosphate groups exist in either above or below the QDP monomer. However, they cannot be on one side simultaneously. This is because of the electrostatic repulsion created on one side of the molecule. It should be noted that up to seven different conformers (Figure S1) lie within 1.5 kcal/mol of each other. This is a result of the orientation of the phosphate groups; hence, several structures are easily accessible as presented in the Supporting Information (Figure S1). Several complexations of gold and QDP were suggested and computed, and Figure shows the lowest possible conformer where gold interacts with the phosphate group and hydroxyl group in ring B. Other conformers that are higher in energy are presented in the Supporting Information (Figure S2). Furthermore, interactions between gold and hydroxyl groups were found to be much higher in energy. Interestingly, another structure similar to conformer 1 was generated from the rotation of the phosphate group in ring B (conformer F). This conformer was found to have two phosphate groups on the same side, and an increase of 0.5 kcal/mol in energy was observed.

Figure 7

(A) Optimized structures of the QDP monomer at the B3LYP/6-31G** level of theory with selected distances in angstroms. All energies (kcal/mol) are computed at the B3LYP/6-31G** level of theory and are related to conformer 1. (B) Lowest possible conformations of the QDP compound elucidated from NMR characterization.

Figure 8

(A) Optimized structures of the QDP–gold monomer at the B3LYP-D3/SDD (Au)--6-31+G** (H, C, O, and P) level of theory with selected distances in angstroms. All energies (kcal/mol) are computed relative to conformer 1A. (B) Lowest possible conformations of the QDP complex with gold.

The computed QDP–Au complex demonstrated that the lowest energy required for complexation with Au took place in the B ring between the 3-OH and 4′-phosphate group, as depicted in Figure .

The main difference between conformer 1A and F is that the latter lacks hydrogen bonding between the phosphate group and hydroxyl groups in ring B (Figure S2).

Conclusions

We have synthesized gold nanoplates using water-soluble QDP in the presence of sunlight. This is a clear demonstration of green synthesis of gold nanoplates by using environmentally safe reducing and capping agents. This is the first green synthesis we have attained because the choice of water-soluble QDP acted as reducing and capping agents and sunlight as the source of energy. The catalytic activity of triangular AuNPs was three times higher than that of the spherical AuNPs based on kinetic rate constants (k) of 3.44 × 10–2 and 1.11 × 10–2 s–1 for triangular and spherical AuNPs, respectively.

Material and Methods

Materials

All reagents purchased were of analytical or reagent grade purity and were used as purchased prior to experimentation. Anhydrous N,N dimethylformamide (99.8%) was obtained from Acros Organics, a division of Thermo Fischer Scientific. Buffers for the calibration of the pH electrode were from Thermo Scientific, Waltham MA. Quercetin was purchased from INDOFINE Chemicals Inc. (Hillsborough, NJ). 4-Dimethyl aminopyridine, hydrogen tetrachloroaurate (HAuCl4·3H2O), and trimethylsilyl-bromide were purchased from Sigma-Aldrich, Milwaukee, WI. Dimethyl sulfoxide-d6 was from Cambridge Isotope Laboratories, Inc. MA. Dibenzyl phosphite, acetonitrile, carbon tetrachloride (CCl4), N,N-diisopropylethylamine, and dichloro methane were purchased from Sigma (St. Louis, MO). Methanol, hexane, ethyl acetate, sodium chloride (NaCl), anhydrous sodium sulfate (Na2SO4), and potassium dihydrogen phosphate (KH2PO4) were purchased from Fisher Scientific, Pittsburg, PA. Nanopure water with a specific resistivity of 18 MΩ was used in the preparation of reagents. All of the reactions involving moisture or air-sensitive reagents were carried out under Ar or N2 atmosphere. All chemicals were of analytical or reagent grade and were used without further purification.

Synthesis of QDP

The synthesis of QDP followed the procedure we reported in literature,[45] and the solubility of QDP in water increased 32-fold over parent quercetin.

Instrumentation

UV–vis absorption spectra were carried out on a HP 8453 UV–visible diode array spectrophotometer. TEM analysis of AuNPs was carried out by adding a drop of samples to the carbon-coated copper grid and then dried. TEM measurements were carried out on a JEOL TEM 2100F. The TLC analyses were performed using 0.25 mm EM Silica Gel 60 F250 plates visualized by UV irradiation (254 nm). Flash chromatography CombiFlash Companion/TS model serial 207L20329, Teledyne Isco, Inc. was used for the purification of the products. 1H, 13C, and 31P NMR spectra were obtained using 600 MHz Bruker AVANCE. The XRD study was done using D8 ADVANCE 800234-X-ray (9729) Bruker at 40 kV and 40 mA.

Computational Methods

Stationary points of the QDP compound were optimized and characterized by frequency analysis using hybrid DFT (B3LYP)[46,47] and the 6-31G**[48−50] basis set, as implemented in Gaussian 98.[51] Enthalpies, ΔH°298, and free energies, ΔG⧧298, were computed for the gas phase, and the solvation energies, ΔG⧧298, for QDP were computed using a polarizable continuum model with a permittivity of acetonitrile, the solvent used in the experiments. These calculations involve the solvation model PCM[52] as implemented in Gaussian 98. Stability analyses were performed in addition to analytical frequency calculations on all stationary points to ensure that geometries correspond to local minima (all positive eigenvalues). All reported energies are corrected for zero-point vibrational energy, whereas free energies are quoted at 298.15 K and 1 atm.

Starting with the initial B3LYP/6-31G** optimized structure, possible conformations of the QDP-AuNP were optimized using a Stuttgart basis set[53] SDD which was employed for gold and the Pople basis set[54] 6-31+G** was used for all other atoms. A polarizable continuum model for the organic reaction solvent (acetonitrile) was also applied during optimizations.[55]

Photochemical Synthesis of Gold Nanoparticles

In atypical experiment, 4.5 × 10–4 M HAuCl4 was reacted with 5 × 10–3 M QDP in mole ratios of 1:1, 1:2, 1:3, 1:4, 1:6, and 1:8 for P, Q, R, S, T, and U, respectively. Then, all of the samples were exposed to direct sunlight for 55 min. A repeat of reactions of P, Q, R, S, and T were exposed to sunlight for 90 min to investigate the effect of exposure time on the morphology of gold nanoplates. Control experiment using the same concentration setup for P, Q, R, S, and T was conducted in a dark room. The effect of concentration of QDP on the formation of AuNPs was investigated by reacting 5 × 10–3 M HACl4·3H2O with 5 × 10–3 M QDP in mole ratios of 1:1, 1:2, 1:3, and 1:4 for A, B, C, and D, respectively.

Reduction of MB Catalyzed by AuNPs

The catalytic ability of spherical and triangular AuNPs was evaluated using a standard MB reduction reaction. The same procedure was followed as Kariuki et al.[62] Three reactions were set up and monitored using UV–visible spectrophotometer in the visible region. All of the reactions contained 1 mL of MB (1 × 10–4 M) and an excess of 32 mM NaBH4. The first setup was for the determination of reduction potential of the NaBH4 by itself. The second setup involved the addition of 300 μL of spherical AuNPs (5 × 10–5 M). The third setup involved the addition of 300 μL triangular AuNPs (5 × 10–5 M). The timing of each reaction did vary. The first setup was monitored for 45 min, the second setup was monitored for 4 min, and the third setup was only monitored for 2 min. The last two setups were only analyzed for such a short time because of the catalytic ability of the AuNPs added to the reaction mixture.