Ecofriendly ultrasound-assisted rapid synthesis of gold nanoparticles using Calothrix algae

Gold chloride (${{{\rm{AuCl}}}_{4}}^{-},$ 99.0%) and 4-nitrophenol (>99.5%, 4-NP) were purchased from Spectrum, USA. Sodium borohydride (NaBH₄, >99.5%) was purchased from Sigma Aldrich, USA. Calothrix algae was kindly provided by Dr Ever Morales Avendaño, Universidad de las Fuerzas Armadas ESPE, Sangolqui, Ecuador. For ecofriendly synthesis of AuNPs, 3 mg of Calothrix algae was added to 10 ml of 0.1 mm AuCl₄⁻ solution and the reaction mixture were incubated for 3 d at 22 °C–25 °C. Then, 3 d incubated AuNPs was ultrasonicated for 10 min and color of the mixture solution had become a light pink color due to the formation of the AuNPs. Ultrasound assisted synthesis was performed with ultrasonic processors (DAIGGER GE 505, 500 W, 20 kHz) immersed directly into the reaction solution. The operating condition was at 30 s pulse on/30 s pulse off time with amplitude of 72% at 25 °C for (5 × 2) min. The synthesized AuNPs were characterized further using different analytical instruments.

To test the catalytic efficiency of AuNPs, reduction of 4-NP was carried out as a model reaction and monitored as follows: about 0.2 ml of 4-NP (2 mm), 0.2 ml of as synthesized AuNPs and 3.4 ml of H₂O were mixed in a 4 ml standard quartz cuvette. To this reaction mixture, 0.2 ml of NaBH₄ (0.05 mm) solution was added and UV–vis absorption spectra were recorded at 1 min intervals, leading to a color change from pale yellow to colorless. Control reactions were also monitored for 15 min in the absence of AuNPs or NaBH₄ keeping other parameters constant. The observed rate constant (k) of the reduction process was determined by analyzing the change in an intensity of the peaks at 400 nm as a function of time. The concentration of 4-NP at time t is denoted as C_t, and the initial concentration of 4-NP at t = 0 is regarded as C₀. The C₀/C_t is measured from the relative intensity of absorbance (A₀/A_t). The linear relationship of ln(A₀/A_t) versus time indicates that the reduction of 4-NP by AuNPs follows the first order kinetics [20].

The UV–vis spectrum was measured using a Thermo Spectronic, GENESYS^TM8, England spectrophotometer. The particle size distributions of nanoparticles were determined using the dynamic light scattering, DLS (HORIBA, Version LB-550 program). Size and selective area electron diffraction (SAED) pattern of nanoparticles are studied on transmission electron microscopy, TEM (FEI, TECNAI, G2 spirit twin). X-ray diffraction (XRD) studies on thin films of the nanoparticle were carried out using a PANalytical brand θ–2θ configuration (generator-detector) x-ray tube copper λ = 1.54 Å and EMPYREAN diffractometer.

The change of color is a significant evidence for the synthesis of AuNPs under the reducing and capping action of the chemicals generated by the Calothrix algae. Figure 1 shows the visual and UV–vis spectroscopy of AuNPs prepared at room temperature and ultrasound technique. In figures 1(a) and (b), it clearly indicated that the synthesis of AuNPs is enhanced under the assistance of ultrasound and the color of AuNPs rapidly changes to light pink. This observation was confirmed by the UV–vis spectrum (figures 1(c) and (d)) and the appearance of the surface plasmon resonance (SPR) peak of the colloidal AuNPs at λ_max = 550 nm [21]. However, there is no SPR peak observed for the AuNPs incubated for 3 d at room temperature.

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Figure 2 depicted the DLS size distribution histogram for AuNPs synthesized at room temperature for 3 d and ultrasound irradiation for 10 min. The average particle size of the AuNPs synthesized before and after ultrasound treatment was found to be 3789.1 ± 1173 nm and 238.9 ± 275.4 nm, respectively. It was observed that, the size of as-synthesized AuNPs at room temperature was bigger than ultrasound assisted method. It may be either due to the intact of AuNPs inside the algal cells or less synthesis of AuNPs. But after ultrasound treatment, the synthesis of AuNPs was enhanced and observed size was small, ranging from 40 to 300 nm.

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The morphology, size and crystallinity of the as-synthesized AuNPs were investigated by TEM and SAED measurements. Figures 3(a) and (b) show that the AuNPs synthesized using the Calothrix algae under the influence of ultrasound irradiation are anisotropic in nature and size ranging from 30 to 120 nm. In addition to truncated and spherical shape AuNPs, some triangular shape AuNPs (figure 4(a)) also observed. The partial concentric rings of the SAED spots clearly verifies the existence of polycrystalline nature of AuNPs (figure 3(c)). However, the square lattice pattern (figure 4(b)) of the triangular AuNPs represents the monocrystalline nature and crystal growth towards the (111) plane [22]. The observed size of AuNPs in TEM analysis is smaller than DLS analysis, it may be either due to the involvement of algal phytochemicals on the surface of AuNPs or screening of smaller AuNPs by a bigger one [23].

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As shown in figure 5, the Bragg reflection peak at 38.23° corresponds to the (111) lattice planes, which confirms the face centered cubic structure of the formed AuNPs (ICSD No. 98-005-3763). The broadening of Bragg's peaks also indicates the formation of nanoparticles and the size of as-formed AuNPs was estimated by using Scherrer's equation by determining the width of the (111) Bragg reflection. The observed size of AuNPs by XRD analysis was calculated to be around ∼57 nm and agreed with the result of TEM images corresponding to the truncated AuNPs.

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Figure 6(a) shows the catalytic activity of AuNPs for the formation of 4-AP from 4-NP in the presence of NaBH₄ and the reduction reaction is easily detected by UV–vis spectroscopic methods. It showed that the concentration of 4-NP in the form of 4-nitrophenolate ions at λ_max = 400 nm gradually decreased with the concomitant appearance of a new peak at 298 nm due to the reduction of 4-NP to 4-AP [24] and more than 63% of 4-NP reduction observed within 15 min at room temperature, respectively. In figure 6(b), the plots of ln(A₀/A_t) versus time yielded good linear correlations revealing that the NaBH₄ reduction of 4-NP catalyzed by AuNPs followed the first-order kinetics. The observed values of first-order rate constant (k) and correlation coefficients (R²) were 0.072 4752 min⁻¹ and 0.9931, respectively. The cause behind the catalytic reduction of 4-NP by AuNPs may be either due to interfacial electron transfer or the rate of electron transfer at the AuNPs surface can be influenced by the diffusion of 4-NP to the metal surface, and the diffusion of 4-AP away from the surface [25, 26].

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